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Posts Tagged ‘NFkB’


Manipulate Signaling Pathways

Writer and Curator: Larry H Bernstein, MD, FCAP 

 

7.6  Manipulate Signaling Pathways

7.6.1 The Dynamics of Signaling as a Pharmacological Target

7.6.2 A Protein-Tagging System for Signal Amplification in Gene Expression and Fluorescence Imaging

7.6.3 IQGAPs choreograph cellular signaling from the membrane to the nucleus

7.6.4 Signaling cell death from the endoplasmic reticulum stress response

7.6.5 An Enzyme that Regulates Ether Lipid Signaling Pathways in Cancer Annotated by Multidimensional Profiling

7.6.6 Peroxisomes – A Nexus for Lipid Metabolism and Cellular Signaling

7.6.7 A nexus for cellular homeostasis- the interplay between metabolic and signal transduction pathways

7.6.8 Mechanisms-of-intercellular-signaling

7.6.9 Cathepsin B promotes colorectal tumorigenesis, cell invasion, and metastasis

 

 

7.6.1 The Dynamics of Signaling as a Pharmacological Target

Marcelo Behar, Derren Barken, Shannon L. Werner, Alexander Hoffmann
Cell  10 Oct 2013; 155(2):448–461
http://dx.doi.org/10.1016/j.cell.2013.09.018

Highlights

  • Drugs targeting signaling hubs may block specific dynamic features of the signal
  • Specific inhibition of dynamic features may introduce pathway selectivity
  • Phase space analysis reveals principles for drug targeting signaling dynamics
  • Based on these principles, NFκB dynamics can be manipulated with specificity

Summary

Highly networked signaling hubs are often associated with disease, but targeting them pharmacologically has largely been unsuccessful in the clinic because of their functional pleiotropy. Motivated by the hypothesis that a dynamic signaling code confers functional specificity, we investigated whether dynamic features may be targeted pharmacologically to achieve therapeutic specificity. With a virtual screen, we identified combinations of signaling hub topologies and dynamic signal profiles that are amenable to selective inhibition. Mathematical analysis revealed principles that may guide stimulus-specific inhibition of signaling hubs, even in the absence of detailed mathematical models. Using the NFκB signaling module as a test bed, we identified perturbations that selectively affect the response to cytokines or pathogen components. Together, our results demonstrate that the dynamics of signaling may serve as a pharmacological target, and we reveal principles that delineate the opportunities and constraints of developing stimulus-specific therapeutic agents aimed at pleiotropic signaling hubs.

http://www.cell.com/cms/attachment/2021777732/2041663648/fx1.jpg

Intracellular signals link the cell’s genome to the environment. Misregulation of such signals often cause or exacerbate disease (Lin and Karin, 2007 and Weinberg, 2007) (so-called “signaling diseases”), and their rectification has been a major focus of biomedical and pharmaceutical research (Cohen, 2002Frelin et al., 2005 and Ghoreschi et al., 2009). For the identification of therapeutic targets, the concept of discrete signaling pathways that transmit intracellular signals to connect cellular sensor/receptors with cellular core machineries has been influential. In this framework, molecular specificity of therapeutic agents correlates well with their functional or phenotypic specificity. However, in practice, clinical outcomes for many drugs with high molecular specificity has been disappointing (e.g., inhibitors of IKK, MAPK, and JNK; Berger and Iyengar, 2011DiDonato et al., 2012Röring and Brummer, 2012 and Seki et al., 2012).

Many prominent signaling mediators are functionally pleiotropic, playing roles in multiple physiological functions (Chavali et al., 2010 and Gandhi et al., 2006). Indeed, signals triggered by different stimuli often travel through shared network segments that operate as hubs before reaching the effectors of the cellular response (Bitterman and Polunovsky, 2012 and Gao and Chen, 2010). Hubs’ inherent pleiotropy means that their inhibition may have broad and likely undesired effects (Karin, 2008Berger and Iyengar, 2011,Force et al., 2007Oda and Kitano, 2006 and Zhang et al., 2008); this is a major obstacle for the efficacy of drugs targeting prominent signaling hubs such as p53, MAPK, or IKK.

Recent studies have begun to address how signaling networks generate stimulus-specific responses (Bardwell, 2006Haney et al., 2010Hao et al., 2008 and Zalatan et al., 2012). For example, the activity of some pleiotropic kinases may be steered to particular targets by scaffold proteins (Park et al., 2003,Schröfelbauer et al., 2012 and Zalatan et al., 2012). Alternatively, or in addition, some signaling hubs may rely on stimulus-specific signal dynamics to activate selective downstream branches in a stimulus-specific manner in a process known as temporal or dynamic coding or multiplexing (Behar and Hoffmann, 2010,Chalmers et al., 2007Hoffmann et al., 2002Kubota et al., 2012Marshall, 1995 and Purvis et al., 2012;Purvis and Lahav, 2013Schneider et al., 2012 and Werner et al., 2005).

Although the importance of signaling scaffolds and their pharmacological promise is widely appreciated (Klussmann et al., 2008 and Zalatan et al., 2012) and isolated studies have altered the stimulus-responsive signal dynamics (Purvis et al., 2012Park et al., 2003Sung et al., 2008 and Sung and Simon, 2004), the capacity for modulating signal dynamics for pharmacological gain has not been addressed in a systematic manner. In this work, we demonstrate by theoretical means that, when signal dynamics are targeted, pharmacological perturbations can produce stimulus-selective results. Specifically, we identify combinations of signaling hub topology and input-signal dynamics that allow for pharmacological perturbations with dynamic feature-specific or input-specific effects. Then, we investigate stimulus-specific drug targeting in the IKK-NFκB signaling hub both in silico and in vivo. Together, our work begins to define the opportunities for pharmacological targeting of signaling dynamics to achieve therapeutic specificity.

Dynamic Signaling Hubs May Be Manipulated to Mute Specific Signals

Previous work has shown how stimulus-specific signal dynamics may allow a signaling hub to selectively route effector functions to different downstream branches (Behar et al., 2007). Here, we investigated the capacity of simple perturbations to kinetic parameters (caused for example by drug treatments) to produce stimulus-specific effects. For this, we examined a simple model of an idealized signaling hub (Figure 1A), reminiscent of the NFκB p53 or of MAPK signaling modules. The hub X reacts with strong but transient activity to stimulus S1 and sustained, slowly rising activity to stimulus S2. These stimulus-specific signaling dynamics are decoded by two effector modules, regulating transcription factors TF1 and TF2. TF1, regulated by a strongly adaptive negative feedback, is sensitive only to fast-changing signals, whereas TF2, regulated by a slowly activating two-state switch, requires sustained signals for activation (Figure 1B). We found it useful to characterize the X, TF1, and TF2 responses in terms of two dynamic features, namely the maximum early amplitude (“E,” time < 15′) and the average late amplitude (“L,” 15′ < t < 6 hr). These features, calculated using a mathematical model of the network (see Experimental Procedures) show good fidelity and specificity (Komarova et al., 2005) (Figure 1C), as S1 causes strong activation of TF1 with minimal crosstalk to TF2, and vice versa for S2.

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Figure 1. Pharmacologic Perturbations with Stimulus-Specific Effects

(A) A negative-feedback module transduces input signals S1 and S2, producing outputs that are decoded by downstream effectors circuits that may distinguish between different dynamics.

(B) Unperturbed dynamics of X, TF1, and TF2 in response to S1 (red) and S2 (blue). Definition of early (E) and late (L) parts of the signal is indicated.

(C) Specificity and fidelity of E and L for TF1 and TF2, as defined in Komarova et al., 2005).

(D) Partial inhibition of X activation (A) abolishes the response to S1, but not S2, whereas a perturbation targeting the feedback regulator (FBR) suppresses the response to S2, but not S1.

(E) Perturbation phenotypes defined as difference between unperturbed and perturbed values of the indicated quantities (arbitrary scales for X, TF1, and TF2). Perturbation A inhibits E and TF1, but not TF2; perturbation FBR inhibits L and TF2, but not TF1.

(F) Virtual screening pipeline showing the experimental design and the two analysis branches for characterizing feature- and input-specific effects.

See also in Experimental Procedures and Table S1.

Seeking simple (affecting a single reaction) perturbations that selectively inhibit signaling by S1 or S2, we found that perturbation A, partially inhibiting the activation of X, was capable of suppressing hub activity in response to a range of S1 amplitudes while still allowing for activity in response to S2 (Figure 1D). Consequently, this perturbation significantly reduced TF1 activity in response to S1 but had little effect on TF2 activity elicited by S2. We also found that the most effective way to inhibit S2 signaling was by targeting the deactivation of negative feedback regulator Y (FBR). This perturbation caused almost complete abrogation of late X activity yet allows for significant levels of early activity. As a result, TF2 was nearly completely abrogated in response to S2, but stimulus S1 still produced a solid TF1 response. The early (E) and late (L) amplitudes could be used to quantify the input-signal-specific effects of these perturbations (Figure 1E).

This numerical experiment showed that it is possible to selectively suppress transient or sustained dynamic signals transduced through a common negative-feedback-containing signaling hub. Moreover, the dynamic features E and L could be independently inhibited. To study how prevalent such opportunities for selective inhibition are, we established a computational pipeline for screening reaction perturbations within multiple network topologies and in response to multiple dynamic input signals; the simulation results were analyzed to identify cases of either “input-signal-specific” inhibition or “dynamic feature-specific” inhibition (Figure 1F).

A Computational Screen to Identify Opportunities for Input-Signal-Specific Inhibition

The computational screen involved small libraries of one- and two-component regulatory modules and temporal profiles of input signals (Figure 2A), both commonly found in intracellular signaling networks. All modules (M1–M7, column on left) contained a species X that, upon stimulation by an input signal, is converted into an active form X (the output) that propagates the signal to downstream effectors. One-component modules included a reversible two-state switch (M1) and a three-state cycle with a refractory state (M2). Two-component modules contained a species Y that, upon activation via a feedback (M3 and M5) or feedforward (M4 and M6) loop, either deactivates X (M3 and M4) or inhibits (M5 and M6) its activation. We also included the afore-described topology that mimics the IκB-NFκB or the Mdm2-p53 modules (M7). Mathematical descriptions may be found in the Experimental Procedures. Although many biological signaling networks may conform to one of these simple topologies, others may be abstracted to one that recapitulates the physiologically relevant emergent properties

Figure 2. A Virtual Screen for Stimulus Specificity in Pharmacologic Perturbations

(A) Signaling modules (left) and input library (top) used in the screen. Dotted lines indicate enzymatic reactions (perturbation names indicated in letter code). Time courses of hub activity for each module/input combination for the unperturbed (black) and perturbed cases (blue indicates a decrease, red an increase in parameter value).

(B) Relative sensitivity of the stimulus response to the indicated perturbation (defined as the perturbation’s effect on the area under the curve), normalized per row.

See also Experimental ProceduresFigure S1, and Tables S2 and S3.

The library of stimuli (S1–S10; Figure 2A, top row) comprises ten input functions with different combinations of “fast” and “slow” initiation and decay phases (see Experimental Procedures). The virtual screen was performed by varying the kinetic parameter for each reaction over a range of values, thereby modeling simple perturbations of different strengths and recording the temporal profile of X abundance. To quantify stimulus-specific inhibition, we measured the area under the normalized dose-response curves (time average of X versus perturbation dose) for each module-input combination (Experimental ProceduresFigure 2B, and Figure S1 available online).

Phase Space Analysis Reveals Underlying Regulatory Principles

To understand the origin of dynamic feature-specific inhibition, we investigated the perturbation effects analytically on each module’s phase space, i.e., the space defined by X∗ and Y∗ quasi-equilibrium surfaces (Figures 4 and S4). These surfaces (“q.e. surfaces”) represent the dose response of X∗ as a function of Y∗ and a stationary input signal S (“X surface”) and the dose response of Y∗ as a function of X∗ and S (“Y surface”) (Figure 4A). The points at which the surfaces intersect correspond to the concentrations of X∗ and Y∗ in equilibrium for a given value of S. In the basal state, when S is low, the system is resting at an equilibrium point close to the origin of coordinates. When S increases, the concentrations of X∗ and Y∗ adjust until the signal settles at some stationary value (Figure 4A). Gradually, changing input signals cause the concentrations to follow trajectories close to the q.e. surfaces (quasi-equilibrium dynamics), following the line defined by the intersection of the surfaces (“q.e. line”) in the extreme of infinitely slow inputs. Fast-changing stimuli drive the system out of equilibrium, causing the trajectories to deviate markedly from the q.e. surfaces.

Two main principles emerged: (1) perturbations that primarily affect the shape of a q.e. surface tend to affect steady-state levels or responses that evolve close to quasi-equilibrium, and (2) perturbations that primarily affect the balance of timescales (X, Y activation, and S) tend to affect transient out-of-equilibrium parts of the response. These principles reflect the fact that out-of-equilibrium parts of a signal are largely insensitive to the precise shape of the underlying dose-response surfaces (they may still be bounded by them) but depend on the balance between the timescales of the biochemical processes involved. Perturbation of these balances affects how a system approaches steady state (thus affecting out-of-equilibrium and quasi-equilibrium dynamics), but not steady-state levels. To illustrate these principles, we present selected results for modules M3 and M4 and discuss additional cases in the supplement (Figure S3).

Detailed Analysis of Modules M3 and M4, Related to Figure 4

Time courses and projections of the phase space for modules M3 and M4. Color coding similar to Figure 4.

In the feedback-based modules (M3 and M5), the early peak of activity in response to rapidly changing signals is an out-of-equilibrium feature that occurs when the timescale of Y activation is significantly slower than that of X. Under these conditions, the concentration of X increases rapidly (out of equilibrium) before decaying along the X surface (in quasi-equilibrium) as more Y gets activated (Figure 4A, parameters modified to better illustrate the effects being discussed; see Table S2). For input signals that settle at some stationary level of S, Y activation eventually catches up and the concentration of X settles at the equilibrium point where the X and Y curves intersect. Gradually changing signals allow X and Yactivation to continuously adapt, and the system evolves closer to the q.e. line.

In such modules, perturbation A (X activation) changes both the shape of the q.e. surface for X and the kinetics of activation. When in the unperturbed system Y saturates, perturbation A primarily reduces Xsteady-state level (Figures 4B and 4C, left and center). When Y does not saturate in the unperturbed system, the primary effect is the reduced activation kinetics. Thus the perturbation affects the out-of-equilibrium peak (Figures 4B and 4C, center and right), with only minor reduction of steady-state levels (especially when Y’s dose response respect to X is steep). The transition from saturated to not-saturated feedback (as well as the perturbation strength) underlies the dose-dependent switch from L to E observed in the screen. In both saturated and unsaturated regimes, the shift in the shape of the surfaces does change the q.e. line and thus affects responses occurring in quasi-equilibrium. In contrast, perturbation of the feedback recovery (FBR) shifts the Y surface vertically (Figure 4D), specifically affecting the steady-state levels and late signaling; the effect on Y kinetics is limited because the reaction is relatively slow. Perturbation FBA also shifts the Y surface, but the net effect is less specific because the associated increase in the rate of Y activation tends to equalize X and Y kinetics affecting also the out-of-equilibrium peak.

In resting cells, NFκB is held inactive through its association with inhibitors IκBα, β, and ε. Upon stimulation, these proteins are phosphorylated by the kinase IKK triggering their degradation. Free nuclear NFκB activates the expression of target genes, including IκB-encoding genes, which thereby provide negative feedback (Figure 5A). The IκB-NFκB-signaling module is a complex dynamic system; however, by abstracting the control mechanism to its essentials, we show below that the above-described principles can be applied profitably.

IκB-NFκB signaling module

IκB-NFκB signaling module

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Figure 5. Modulating NFκB Signaling Dynamics

(A) The IκB-NFκB signaling module.

(B) Equilibrium dose-response relationship for NFκB versus IKK.

(C) Three IKK curves representative of three stimulation regimes; TNFc (red), TNFp (green), and LPS (blue) function as inputs into the model, which computes the corresponding NFκB activity dynamics (bottom). The quasi-equilibrium line (black) was obtained by transforming the IKK temporal profiles by the dose response in (B). Deviation from the quasi-equilibrium line for the TNF response indicates out-of-equilibrium dynamics.

(D) Coarse-grained model of the IκB-NFκB module and predicted effects of perturbations.

(E) Selected perturbations with specific effects on out-of-equilibrium (top three) or steady state (bottom two). (Left to right) Feature maps in the E-L space (E: t < 60 ′, L: 120′ < t < 300′), tangent angle at the unperturbed point (θ > 0 indicates L is more suppressed than E and vice versa), and time courses (green, TNF chronic; red, TNF pulse; blue, LPS). Only inhibitory perturbations are shown. Additional perturbations are shown in Figure S4.

See also Experimental Procedures and Table S7.

Here, we delineate the potential of achieving stimulus-specific inhibition when targeting molecular reactions within pleiotropic signaling hubs. We found that it is theoretically possible to design perturbations that (1) selectively attenuate signaling in response to one stimulus but not another, (2) selectively attenuate undesirable features of dynamic signals or enhance desirable ones, or (3) remodulate output signals to fit a dynamic profile normally associated with a different stimulus.

These opportunities—not all of them possible for every signaling module topology or biological scenario—are governed by two general principles based on timescale and dose-response relationships between upstream signal dynamics and intramodule reaction kinetics (Figure 4 and Table S4). In short, a steady-state or quasi-equilibrium part of a response may be selectively affected by perturbations that introduce changes in the relevant dose-response surfaces. Out-of-equilibrium responses that are not sensitive to the precise shape of a dose-response curve may be selectively attenuated by perturbations that modify the relative timescales. Dose responses and timescales cannot, in general, be modified independently by simple perturbations (combination treatments are required), but as we show, in some cases, one effect dominates resulting in feature or stimulus specificity.

The degree to which specific dynamic features of a signaling profile or the dynamic responses to specific stimuli can be selectively inhibited depends on how distinctly they rely on quasi-equilibrium and out-of-equilibrium control. Signals that contain both features may be partially inhibited by both types of perturbation, limiting the specific inhibition achievable by simple perturbations. In practice, this limited the degree to which NFκB signaling could be inhibited in a stimulus-specific manner (Figure 5) and the associated therapeutic dose window (Figure 6). The most selective stimulus-specific effects can be introduced when a signal is heavily dependent on a particular dynamic feature; for example, suppression of out-of-equilibrium transients will abrogate the response to stimuli that produce such transients. For a selected group of target genes, this specificity at the signal level translated directly to expression patterns (Figure 6B, middle). More generally, selective inhibition of early or late phases of a signal may allow for specific control of early and late response genes (Figure 6C), a concept that remains to be studied at genomic scales. Though the principles are general, how they apply to specific signaling pathways depends not only on the regulatory topology, but also on the dynamic regime determined by the parameters. As demonstrated with the IκB-NFκB module, analysis of a coarse-grained topology in terms of the principles may allow the prediction of perturbations with a desired specificity.

 

7.6.2 A Protein-Tagging System for Signal Amplification in Gene Expression and Fluorescence Imaging

Marvin E. Tanenbaum, Luke A. Gilbert, Lei S. Qi, Jonathan S. Weissman, Ronald D. Vale
Cell 23 Oct 2014; 159(3): 635–646
http://dx.doi.org/10.1016/j.cell.2014.09.039

Highlights

  • SunTag allows controlled protein multimerization on a protein scaffold
  • SunTag enables long-term single-molecule imaging in living cells
  • SunTag greatly improves CRISPR-based activation of gene expression

Summary

Signals in many biological processes can be amplified by recruiting multiple copies of regulatory proteins to a site of action. Harnessing this principle, we have developed a protein scaffold, a repeating peptide array termed SunTag, which can recruit multiple copies of an antibody-fusion protein. We show that the SunTag can recruit up to 24 copies of GFP, thereby enabling long-term imaging of single protein molecules in living cells. We also use the SunTag to create a potent synthetic transcription factor by recruiting multiple copies of a transcriptional activation domain to a nuclease-deficient CRISPR/Cas9 protein and demonstrate strong activation of endogenous gene expression and re-engineered cell behavior with this system. Thus, the SunTag provides a versatile platform for multimerizing proteins on a target protein scaffold and is likely to have many applications in imaging and controlling biological outputs.

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SunTag, which can recruit multiple copies of an antibody-fusion protein
Development of the SunTag, a System for Recruiting Multiple Protein Copies to a Polypeptide Scaffold Protein multimerization on a single RNA or DNA template is made possible by identifying protein domains that bind with high affinity to a relatively short nucleic acid motif. We therefore sought a protein-based system with similar properties, specifically a protein that can bind tightly to a short peptide sequence (Figures 1A and1B).Antibodies arecapable ofbindingto short,unstructured peptide sequences with high affinity and specificity, and, importantly, peptide epitopes can be designed that differ from naturally occurring sequences in the genome. Furthermore, whereas antibodies generally do not fold properly in the cytoplasm, single-chain variable fragment (scFv) antibodies, in which the epitope-binding regions of the light and heavy chains of the antibody are fused to forma single polypeptide, have been successfully expressed in soluble form in cells (Colby et al., 2004; Lecerf et al., 2001; Wo ¨rn et al., 2000).
We expressed three previously developed single-chain antibodies (Colby et al., 2004; Lecerf et al., 2001; Wo ¨rn et al., 2000) fused to EGFP in U2OS cells and coexpressed their cognate peptides (multimerized in four tandem copies) fused to the cytoplasmic side of the mitochondrial protein mitoNEET (Colca et al., 2004) (referred to here as Mito, Figure S1A). We then assayed whether the antibody-GFP fusion proteins would be recruited to the mitochondria by fluorescence microscopy, which would indicate binding between antibody and peptide (Figure 1B). Of the three antibody-peptide pairs tested, only the GCN4 antibody-peptide pair showed robust and specific binding while not disrupting normal mitochondrial morphology (Figures 1C and S1B). Thus, we focused our further efforts on the GCN4 antibody-peptide pair. The GCN4 antibody was optimized to allow intracellular expression in yeast (Wo ¨rn et al., 2000). In human cells, however, we still observed some protein aggregates of scFv-GCN4-GFP at high expression levels (Figure S2A). To improve scFv-GCN4 stability, we added a variety of N- and C-terminal fusion proteins known to enhance protein solubility and found that fusion of superfolder-GFP (sfGFP) alone
(Pe’delacq et al., 2006) or along with the small solubility tag GB1 (Gronenborn et al., 1991) to the C terminus of the GCN4 antibody almost completely eliminated protein aggregation, even at high expression levels (Figure S2A). Thus, we performed all further experiments with scFv-GCN4-sfGFP-GB1 (hereafter referred to as scFvGCN4-GFP). Very tight binding of the antibody-peptide pair in vivo is critical fortheformation ofmultimersonaproteinscaffoldbackbone.To determine the dissociation rate of the GCN4 antibody-peptide interaction, we performed fluorescence recovery after photobleaching (FRAP) experiments on scFv-GCN4-GFP bound to the mitochondrial-localized mito-mCherry-4xGCN4pep. After photobleaching, very slow GFP recovery was observed (halflife of 5–10 min [Figures 2A and 2B]), indicating that the antibody bound very tightly to the peptide. It is also important to optimize the spacing of the scFv-GCN4 binding sites within the protein scaffold so that they could be saturated by scFvGCN4 because steric hindrance of neighboring peptide binding sites is a concern. We varied the spacing between neighboring GCN4 peptides and quantified the antibody occupancy on the peptide array.

Figure 1. Identification of an Antibody-Peptide Pair that Binds Tightly In Vivo (A) Schematic of the antibody-peptide labeling strategy. (B) Schematic of the experiment described in (C) in which the mitochondrial targeting domain of mitoNEET (yellow box, mito) fused to mCherry and four tandem copies of a peptide recruits a GFP-tagged intracellular antibody to mitochondria. (C) ScFv-GCN4-GFP was coexpressed with either mito-mCherry-4xGCN4peptide (bottom) or mito-mCherry-FKBP as a control (top) in U2OS cells, and cells were imaged using spinning-disk confocal microscopy. Scale bars, 10 mm. See also Figure S1.

Figure 2. Characterization of the Off Rate and Stoichiometry of the Binding Interaction between the scFv-GCN4 Antibody and the GCN4 Peptide Array In Vivo (A) Mito-mCherry-24xGCN4pep was cotransfected with scFv-GCN4-GFP in HEK293 cells, and their colocalization on mitochondria in a single cell is shown (10 s). At 0 s, the mitochondria-localized GFP signal was photobleached in a single z plane using a 472 nm laser, and fluorescence recovery was followed by time-lapse microscopy. Scale bar, 5 mm. (B) The FRAP was quantified for 20 cells. (C–E) Indicated constructs were transfected in HEK293 cells, and images were acquired 24 hr after transfection with identical image acquisition settings. Representative images are shown in (C). Note that the GFP signal intensity in the mito-mCherry-24xGCN4pep + scFv-GCN4-GFP is highly saturated when the same scaling is used as in the other panels. Bottom row shows a zoom of a region of interest: dynamic scaling was different for the GFP and mCherry signals, so that both could be observed. Scale bars, 10 mm. (D and E) Quantifications of the GFP:mCherry fluorescence intensity ratio on mitochondria after normalization. Eachdot represents a single cell, and dashed lines indicates the average value. See also Figure S2.

Figure 3. The SunTag Allows Long-Term Single-Molecule Fluorescence Imaging in the Cytoplasm (A–H) U2OS cells were transfected with indicated SunTag24x constructs together with the scFv-GCN4-GFP-NLS and were imaged by spinning-disk confocal microscopy 24 hr after transfection. (A) A representative image of SunTag24x-CAAX-GFP is shown (left), as well as the fluorescence intensities quantification of the foci (right, blue bars). As a control, U2OS were transfected with sfGFP-CAAX and fluorescence intensities of single sfGFP-CAAX molecules were also quantified (red bars). The average fluorescence intensity of the single sfGFP-CAAX was set to 1. Dotted line marks the outline of the cell (left). Scale bar, 10 mm. (B) Cells expressing K560-SunTag24x-GFP were imaged by spinning disk confocal microscopy (image acquisition every 200 ms). Movement is revealed by a maximum intensity projection of 50 time points (left) and a kymograph (right). Scale bar, 10 mm. (C and D) Cells expressing both EB3-tdTomato and K560-SunTag24x-GFP were imaged, and moving particles were tracked manually. Red and blue tracks (bottom) indicate movement toward the cell interior and periphery, respectively (C). The duration of the movie was 20 s. Scale bar, 5 mm. Dots in (D) represent individual cells with between 5 and 20 moving particles scored per cell. The mean and SD are indicated. (E and F) Cells expressing Kif18b-SunTag24x-GFP were imaged with a 250 ms time interval. Images in (E) show a maximum intensity projection (50 time- points, left) and a kymograph (right). Speeds of moving molecules were quantified from ten different cells (F). Scale bar, 10 mm. (G and H) Cells expressing both mCherry-a-tubulin and K560rig-SunTag24x-GFP were imaged with a 600 ms time interval.The entire cell is shown in (G), whereas H shows zoomed-instills of atime series from the same cell. Open circlestrack two foci on the same microtubule,which is indicated bythe dashed line. Asterisks indicate stationary foci. Scale bars, 10 and 2 mm (G and H), respectively. See also Figure S3 and Movies S1, S2, S3, S4, S5, and S6.
The GCN4 peptide contains many hydrophobic residues (Figure 4B) and is largely unstructured in solution (Berger et al., 1999); thus, the poor expression of the peptide array could be due to its unstructured and hydrophobic nature. To test this idea, we designed several modified peptide sequence that were predicted to increase a-helical propensity and reduce hydrophobicity. One of these optimized peptides (v4, Figure 4B) was expressed moderately well as a 243 peptide array, and even higher expression was achieved with a 103 peptide array (Figure 4C). Importantly, fluorescence imaging revealed that thescFv-GCN4antibody robustlyboundto theGCN4v4peptide array in vivo and FRAP analysis suggests that the scFv-GCN4 antibody dissociates with a similar slow off rate from the GCN4
v4 peptide array as the original peptide (Figures 4D and 4E). Furthermore, K560 motility could be observed when it was tagged with the optimized v4 243 peptide array, indicating that the optimized v4 peptide array did not interfere with protein function (Movie S7). Together, these results identify a second version of the peptide array that can be used for applications requiring higher expression.
Activation of Gene Transcription Using Cas9-SunTag Because the SunTag system could be used for amplification of a fluorescence signal, we tested whether it also could be used to amplify regulatory signals involved in gene expression. Transcription of a gene is strongly enhanced by recruiting multiple copies of transcriptional activators to endogenous or artificial gene promoters (Anderson and Freytag, 1991; Chen et al., 1992; Pettersson and Schaffner, 1990). Thus, we thought that robust, artificial activation of gene transcription might also be achieved by recruiting multiple copies of a synthetic transcriptional activator to a gene using the SunTag.

Figure 4. An Optimized Peptide Array for High Expression (A) Indicated constructs were transfected in HEK293 cells and imaged 24 hr after transfection using wide-field microscopy. All images were acquired using identical acquisition parameters. Representative images are shown (left), and fluorescence intensities were quantified (n = 3) (right). (B) Sequence of the first and second generation GCN4 peptide (modified or added residues are colored blue, hydrophobic residues are red, and linker residues are yellow). (C–E) Indicated constructs were transfected in HEK293 cells and imaged 24 hr after transfection using wide-field (C) or spinning-disk confocal (D and E) microscopy. (C) Representative images are shown (left), and fluorescence intensities were quantified (n = 3) (right). (D and E) GFP signal on mitochondria was photobleached, and fluorescence recovery was determined over time. The graph (E) represents an average of six cells per condition. (E) shows an image of a representative cell before photobleaching. Scale bars in (A) and (C), 50 mm; scale bars in (D) and (E), 10 mm. Error bars in (A) and (C) represent SDs. See also Movie S7.

Figure 5. dCas9-SunTag Allows Genetic Rewiring of Cells through Activation of Endogenous Genes (A) Schematic of gene activation by dCas9-VP64 and dCas9-SunTag-VP64. dCas9 binds to a gene promoter through its sequence-specific sgRNA (red line). Direct fusion of VP64 to dCas9 (top) results in a single VP64 domain at the promoter, which poorly activates transcription of the downstream gene. In contrast, recruitment of many VP64 domains using the SunTag potently activates transcription of the gene (bottom). (B–D) K562 cells stably expressing dCas9-VP64 or dCas9-SunTag-VP64 were infected with lentiviral particles encoding indicated sgRNAs, as well as BFP and a puromycin resistance gene and selected with 0.7 mg/ml puromycin for 3 days to kill uninfected cells. (B and C) Cells were stained for CXCR4 using adirectlylabeleda-CXCR4 antibody, and fluorescence was analyzed by FACS. (D) Trans-well migration assays (see Experimental Procedures) were performed with indicated sgRNAs. Results are displayed as the fold change in directional migrating cells over control cell migration. (E) dCas9-VP64 or dCas9-SunTag-VP64 induced transcription of CDKN1B with several sgRNAs. mRNA levels were quantified by qPCR. (F) Doubling timeofcontrolcells orcells expressing indicated sgRNAs was determined (see Experimental Procedures section). Graphs in (C), (D), and (F) are averages of three independent experiments. Graph in (E) is average of two biological replicates, each with two or three technical replicates. All error bars indicate SEM. See also Figure S4

 

7.6.3 IQGAPs choreograph cellular signaling from the membrane to the nucleus

Jessica M. Smith, Andrew C. Hedman, David B. Sacks
Trends Cell Biol Mar 2015; 25(3): 171–184
http://dx.doi.org/10.1016/j.tcb.2014.12.005

Highlights

  • IQGAP proteins scaffold diverse signaling molecules.
  • IQGAPs mediate crosstalk between signaling pathways.
  • IQGAP1 regulates nuclear processes, including transcription.

Since its discovery in 1994, recognized cellular functions for the scaffold protein IQGAP1 have expanded immensely. Over 100 unique IQGAP1-interacting proteins have been identified, implicating IQGAP1 as a critical integrator of cellular signaling pathways. Initial research established functions for IQGAP1 in cell–cell adhesion, cell migration, and cell signaling. Recent studies have revealed additional IQGAP1 binding partners, expanding the biological roles of IQGAP1. These include crosstalk between signaling cascades, regulation of nuclear function, and Wnt pathway potentiation. Investigation of the IQGAP2 and IQGAP3 homologs demonstrates unique functions, some of which differ from those of IQGAP1. Summarized here are recent observations that enhance our understanding of IQGAP proteins in the integration of diverse signaling pathways.

http://ars.els-cdn.com/content/image/1-s2.0-S0962892414002153-gr2.sml

 

7.6.4 Signaling cell death from the endoplasmic reticulum stress response

Shore GC1, Papa FR, Oakes SA
Curr Opin Cell Biol. 2011 Apr; 23(2):143-9
http://dx.doi.org/10.1016%2Fj.ceb.2010.11.003

Inability to meet protein folding demands within the endoplasmic reticulum (ER) activates the unfolded protein response (UPR), a signaling pathway with both adaptive and apoptotic outputs. While some secretory cell types have a remarkable ability to increase protein folding capacity, their upper limits can be reached when pathological conditions overwhelm the fidelity and/or output of the secretory pathway.

The lumen of the ER is a unique cellular environment optimized to carry out the three primary tasks of this organelle:

  1. calcium storage and release,
  2. protein folding and secretion, and
  3. lipid biogenesis [1].

A range of cellular disturbances lead to accumulation of misfolded proteins in the ER, including

  • point mutations in secreted proteins that disrupt their proper folding,
  • sustained secretory demands on endocrine cells,
  • viral infection with ER overload of virus-encoding protein, and
  • loss of calcium homeostasis with detrimental effects on ER-resident calcium-dependent chaperones [24].

 

The tripartite UPR consists of three ER transmembrane proteins (IRE1α, PERK, ATF6) that

  • alert the cell to the presence of misfolded proteins in the ER and
  • attempt to restore homeostasis in this organelle through increasing ER biogenesis,
  1. decreasing the influx of new proteins into the ER,
  2. promoting the transport of damaged proteins from the ER to the cytosol for degradation, and
  3. upregulating protein folding chaperones [5].

The adaptive responses of the UPR can markedly expand the protein folding capacity of the cell and restore ER homeostasis [6]. However, if these adaptive outputs fail to compensate because ER stress is excessive or prolonged, the UPR induces cell death.

The cell death pathways collectively triggered by the UPR include both caspase-dependent apoptosis and caspase-independent necrosis. While many details remain unknown, we are beginning to understand how cells determine when ER stress is beyond repair and communicate this information to the cell death machinery. For the purposes of this review, we focus on the apoptotic outputs triggered by the UPR under irremediable ER stress.

Connections from the UPR to the Mitochondrial Apoptotic Pathway

Connections from the UPR to the Mitochondrial Apoptotic Pathway

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3078187/bin/nihms256554f1.jpg

Figure 1 Connections from the UPR to the Mitochondrial Apoptotic Pathway

Under excessive ER stress, the ER transmembrane sensors IRE1α and PERK send signals through the BCL-2 family of proteins to activate the mitochondrial apoptotic pathway. In response to unfolded proteins, IRE1α oligomerizes and induces endonucleolytic decay of hundreds of ER-localized mRNAs, depleting ER protein folding components and leading to worsening ER stress. Phosphorylated IRE1α also recruits TNF receptor-associated factor 2 (TRAF2) and activates apoptosis signaling kinase 1 (ASK1) and its downstream target c-Jun NH2-terminal kinase (JNK). JNK then activates pro-apoptotic BIM and inhibits anti-apoptotic BCL-2. These conditions result in dimerization of PERK and activation of its kinase domain to phosphorylate eukaryotic translation initiation factor 2α (eIF2α), which causes selective translation of activating transcription factor-4 (ATF4). ATF4 upregulates expression of the CHOP/GADD153 transcription factor, which inhibits the gene encoding anti-apoptotic BCL-2 while inducing expression of pro-apoptotic BIM. ER stress also promotes p53-dependent transcriptional upregulation of Noxa and Puma, two additional pro-apoptotic BH3-only proteins. Furthermore, high levels of UPR signaling induce initiator caspase-2 to proteolytically cleave and activate pro-apoptotic BID upstream of the mitochondrion. In addition to antagonizing pro-survival BCL-2 members, cleaved BID, BIM and PUMA activate Bax and/or Bak. Hence, in response to excessive UPR signaling, the balance of BCL-2 family proteins shifts in the direction of apoptosis and leads to the oligomerization of BAX and BAK, two multi-domain pro-apoptotic BCL-2 family proteins that then drive the permeabilization of the outer mitochondrial membrane, apoptosome formation and activation of executioner caspases such as Caspase-3. Figure adapted with permission from the Journal of Cell Science [58].

The proximal unfolded protein response sensors

UPR signaling is initiated by three ER transmembrane proteins:

  1. IRE1α,
  2. PERK, and

The most ancient ER stress sensor, IRE1α, contains

  1. an ER lumenal domain,
  2. a cytosolic kinase domain and
  3. a cytosolic RNase domain [9,10].

In the presence of unfolded proteins, IRE1α’s ER lumenal domains homo-oligomerize, leading

  • first to kinase trans-autophosphorylation and
  • subsequent RNase activation.

Dissociation of the ER chaperone BiP from IRE1α’s lumenal domain in order to engage unfolded proteins may facilitate IRE1α oligomerization [11]; alternatively, the lumenal domain may bind unfolded proteins directly [12]. PERK’s ER lumenal domain is thought to be activated similarly [13,14]. The ATF6 activation mechanism is less clear. Under ER stress, ATF6 translocates to the Golgi and is cleaved by Site-1 and Site-2 proteases to generate the ATF6(N) transcription factor [15].

All three UPR sensors have outputs that attempt to tilt protein folding demand and capacity back into homeostasis. PERK contains a cytosolic kinase that phosphorylates eukaryotic translation initiation factor 2α (eIF2α), which impedes translation initiation to reduce the protein load on the ER [16]. IRE1α splices XBP1mRNA, to produce the homeostatic transcription factor XBP1s [17,18]. Together with ATF6(N), XBP1s increases transcription of genes that augment ER size and function[19]. When eIF2α is phosphorylated, the translation of the activating transcription factor-4 (ATF4) is actively promoted and leads to the transcription of many pro-survival genes [20]. Together, these transcriptional events act as homeostatic feedback loops to reduce ER stress. If successful in reducing the amount of unfolded proteins, the UPR attenuates.

However, when these adaptive responses prove insufficient, the UPR switches into an alternate mode that promotes apoptosis. Under irremediable ER stress, PERK signaling can induce ATF-4-dependent upregulation of the CHOP/GADD153 transcription factor, which inhibits expression of the gene encoding anti-apoptotic BCL-2 while upregulating the expression of oxidase ERO1α to induce damaging ER oxidation [21,22]. Sustained IRE1α oligomerization leads to activation of apoptosis signal-regulating kinase 1 (ASK1) and its downstream target c-Jun NH2-terminal kinase (JNK) [23,24]. Phosphorylation by JNK has been reported to both activate pro-apoptotic BIM and inhibit anti-apoptotic BCL-2 (see below). Small molecule modulators of ASK1 have been shown to protect cultured cells against ER stress-induced apoptosis, emphasizing the importance of the IRE1α-ASK1-JNK output as a death signal in this pathway [25]. In response to sustained oligomerization, the IRE1α RNase also causes endonucleolytic decay of hundreds of ER-localized mRNAs [26]. By depleting ER cargo and protein folding components, IRE1α-mediated mRNA decay may worsen ER stress, and could be a key aspect of IRE1α’s pro-apoptotic program [27]. Recently, inhibitors of IRE1α’s kinase pocket have been shown to conformationally activate its adjacent RNase domain in a manner that enforces homeostatic XBP1s without causing destructive mRNA decay [27], a potentially exciting strategy for preventing ER stress-induced cell loss.

The BCL-2 family and the Mitochondrial Apoptotic Pathway

A wealth of genetic and biochemical data argues that the intrinsic (mitochondrial) apoptotic pathway is the major cell death pathway induced by the UPR, at least in most cell types. This apoptotic pathway is set in motion when several toxic proteins (e.g., cytochrome c, Smac/Diablo) are released from mitochondria into the cytosol where they lead to activation of downstream effector caspases (e.g., Caspase-3) [30]. The BCL-2 family, a large class of both pro- and anti- survival proteins, tightly regulates the intrinsic apoptotic pathway by controlling the integrity of the outer mitochondrial membrane [31]. This pathway is set in motion when cell injury leads to the transcriptional and/or post-translational activation of one or more BH3-only proteins that share sequence similarity in a short alpha helix (~9–12 a.a.) known as the Bcl-2 homology 3 (BH3) domain [32]. Once activated, BH3-only proteins lead to loss of mitochondrial integrity by disabling mitochondrial protecting proteins that drive the permeabilization of the outer mitochondrial membrane.

ER stress has been reported to activate at least four distinct BH3-only proteins (BID, BIM, NOXA, PUMA) that then signal the mitochondrial apoptotic machinery (i.e., BAX/BAK) [3335]. Each of these BH3-only proteins is activated by ER stress in a unique way. Cells individually deficient in any of these BH3-only proteins are modestly protected against ER stress-inducing agents, but not nearly as resistant as cells null for their common downstream targets BAX and BAK [36]—the essential gatekeepers to the mitochondrial apoptotic pathway. Moreover, cells genetically deficient in both Bim andPuma are more protected against ER stress-induced apoptosis than Bim or Puma single knockout cells [37].

The ER stress sensor that signals these BH3-only proteins is known in a few cases (i.e., BIM is downstream of PERK); however, we do not yet understand how the UPR communicates with most of the BH3-only proteins. Moreover, it is not known if all of the above BH3-only proteins are simultaneously set in motion by all forms of ER stress or if a subset is activated under specific pathological stimuli that injure this organelle. Understanding the molecular details of how ER damage is communicated to the mitochondrial apoptotic machinery is critical if we want to target disease specific apoptotic signals sent from the ER.

Initiator and Executor Caspases

Caspases, or cysteine-dependent aspartate-directed proteases, play essential roles in both initiating apoptotic signaling (initiator caspases- 2, 4, 8, 12) and executing the final stages of cell demise (executioner caspases- 3, 7, 9) [38]. It is not surprising that the executioner caspases (casp-3,7,9) are critical for cell death resulting from damage to this organelle. Caspase 12 was the first caspase reported to localize to the ER downstream of BAX/BAK-dependent mitochondrial permeabilization becomes activated by UPR signaling in murine cells [39],but humans fail to express a functional Caspase 12 [41. Genetic knockdown or pharmacological inhibition of caspase-2 confers resistance to ER stress-induced apoptosis [42]. How the UPR activates caspase-2 and whether other initiator caspasesare also involved remains to be determined.

Calcium and Cell Death

Although an extreme depletion of ER luminal Ca2+ concentrations is a well-documented initiator of the UPR and ER stress-induced apoptosis or necrosis, it represents a relatively non-physiological stimulus. Ca2+ signaling from the ER is likely coupled to most pathways leading to apoptosis. UPR-induced activation of ERO1-α via CHOP in macrophages results in stimulation of inositol 1,4,5-triphosphate receptor (IP3R) [43]. All three sub-groups of the Bcl-2 family at the ER regulate IP3R activity. A significant fraction of IP3R is a constituent of highly specialized tethers that physically attach ER cisternae to mitochondria (mitochondrial-associated membrane) and regulate local Ca2+ dynamics at the ER-mitochondrion interface [4546]. This results in propagation of privileged IP3R-mediated Ca2+ oscillations into mitochondria. In an extreme scenario, massive transmission of Ca2+ into mitochondria results in Ca2+ overload and cell death by caspase-dependent and –independent means [46,47]. More refined transmission regulated by the Bcl-2 axis at the ER can influence cristae junctions and the availability of cytochrome c for its release across the outer mitochondrial membrane [48]. Finally, such regulated Ca2+transmission to mitochondria is a key determinant of mitochondrial bioenergetics [49].

ER Stress-Induced Cell Loss and Disease

Mounting evidence suggests that ER stress-induced apoptosis contributes to a range of human diseases of cell loss, including diabetes, neurodegeneration, stroke, and heart disease, to name a few (reviewed in REF [50]). The cause of ER stress in these distinct diseases varies depending on the cell type affected and the intracellular and/or extracellular conditions that disrupt proteostasis. Both mutant SOD1 and mutant huntingtin proteins aggregate, exhaust proteasome activity, and result in secondary accumulations of misfolded proteins in the ER [5152].

In the case of IRE1α, it may be possible to use kinase inhibitors to activate its cytoprotective signaling and shut down its apoptotic outputs [27]. Whether similar strategies will work for PERK and/or ATF6 remains to be seen. Alternatively, blocking the specific apoptotic signals that emerge from the UPR is perhaps a more straightforward strategy to prevent ER stress-induced cell loss. To this end, small molecular inhibitors of ASK and JNK are currently being tested in a variety preclinical models of ER stress [5253,5657]. This is just the beginning, and much work needs to be done to validate the best drugs targets in the ER stress pathway.

Conclusions

The UPR is a highly complex signaling pathway activated by ER stress that sends out both adaptive and apoptotic signals. All three transmembrane ER stress sensors (IRE1α, PERK, AFT6) have outputs that initially decrease the load and increase capacity of the ER secretory pathway in an effort to restore ER homeostasis. However, under extreme ER stress, continuous engagement of IRE1α and PERK results in events that simultaneously exacerbate protein misfolding and signal death, the latter involving caspase-dependent apoptosis and caspase-independent necrosis. Advances in our molecular understanding of how these stress sensors switch from life to death signaling will hopefully lead to new strategies to prevent diseases caused by ER stress-induced cell loss.

7.6.5 An Enzyme that Regulates Ether Lipid Signaling Pathways in Cancer Annotated by Multidimensional Profiling

Chiang KP, Niessen S, Saghatelian A, Cravatt BF.
Chem Biol. 2006 Oct; 13(10):1041-50.
http://dx.doi.org/10.1016/j.chembiol.2006.08.008

Hundreds, if not thousands, of uncharacterized enzymes currently populate the human proteome. Assembly of these proteins into the metabolic and signaling pathways that govern cell physiology and pathology constitutes a grand experimental challenge. Here, we address this problem by using a multidimensional profiling strategy that combines activity-based proteomics and metabolomics. This approach determined that KIAA1363, an uncharacterized enzyme highly elevated in aggressive cancer cells, serves as a central node in an ether lipid signaling network that bridges platelet-activating factor and lysophosphatidic acid. Biochemical studies confirmed that KIAA1363 regulates this pathway by hydrolyzing the metabolic intermediate 2-acetyl monoalkylglycerol. Inactivation of KIAA1363 disrupted ether lipid metabolism in cancer cells and impaired cell migration and tumor growth in vivo. The integrated molecular profiling method described herein should facilitate the functional annotation of metabolic enzymes in any living system.

Elucidation of the metabolic and signaling networks that regulate health and disease stands as a principal goal of postgenomic research. The remarkable complexity of these molecular pathways has inspired the advancement of “systems biology” methods for their characterization [1]. Toward this end, global profiling technologies, such as DNA microarrays 2 and 3 and mass spectrometry (MS)-based proteomics 4 and 5, have succeeded in generating gene and protein signatures that depict key features of many human diseases. However, extricating from these associative relationships the roles that specific biomolecules play in cell physiology and pathology remains problematic, especially for proteins of unknown biochemical or cellular function.

The functions of certain proteins, such as adaptor or scaffolding proteins, can be gleaned from large-scale protein-interaction maps generated by technologies like yeast two-hybrid 6 and 7, protein microarrays [8], and MS analysis of immunoprecipitated protein complexes 9 and 10. In contrast, enzymes contribute to biological processes principally through catalysis. Thus, elucidation of the activities of the many thousands of enzymes encoded by eukaryotic and prokaryotic genomes requires knowledge of their endogenous substrates and products. The functional annotation of enzymes in prokaryotic systems has been facilitated by the clever analysis of gene clusters or operons 11 and 12, which correspond to sets of genes adjacently located in the genome that encode for enzymes participating in the same metabolic cascade. The assembly of eukaryotic enzymes into metabolic pathways is more problematic, however, as their corresponding genes are not, in general, physically organized into operons, but rather are scattered randomly throughout the genome.

We hypothesized that the determination of endogenous catalytic activities for uncharacterized enzymes could be accomplished directly in living systems by the integrated application of global profiling technologies that survey both the enzymatic proteome and its primary biochemical output (i.e., the metabolome). Here, we have tested this premise by utilizing multidimensional profiling to characterize an integral membrane enzyme of unknown function that is highly elevated in human cancer.

Development of a Selective Inhibitor for the Uncharacterized Enzyme KIAA1363

Previous studies using the chemical proteomic technology activity-based protein profiling (ABPP) 15, 16 and 17 have identified enzyme activity signatures that distinguish human cancer cells based on their biological properties, including tumor of origin and state of invasiveness [18]. A primary component of these signatures was the protein KIAA1363, an uncharacterized integral membrane hydrolase found to be upregulated in aggressive cancer cells from multiple tissues of origin. To investigate the role that KIAA1363 plays in cancer cell metabolism and signaling, a selective inhibitor of this enzyme was generated by competitive ABPP 20 and 21.

Previous competitive ABPP screens that target the serine hydrolase superfamily identified a set of trifluoromethyl ketone (TFMK) inhibitors that showed activity in mouse brain extracts [20]. These TFMK inhibitors showed only limited activity in living human cells (data not shown). We postulated that the activity of KIAA1363 inhibitors could be enhanced by replacing the TFMK group with a carbamate, which inactivates serine hydrolases via a covalent mechanism (Figure S1; see the Supplemental Data available with this article online). Carbamate AS115 (Figure 1A) was synthesized and tested for its effects on the invasive ovarian cancer cell line SKOV-3 by competitive ABPP (Figure 1B). AS115 was found to potently and selectively inactivate KIAA1363, displaying an IC50 value of 150 nM, while other serine hydrolase activities were not affected by this agent (IC50 values > 10 μM) (Figures 1B and 1C). AS115 also selectively inhibited KIAA1363 in other aggressive cancer cell lines that possess high levels of this enzyme, including the melanoma lines C8161 and MUM-2B (Figure S2B).

Figure 1. Characterization of AS115, a Selective Inhibitor of the Cancer-Related Enzyme KIAA1363

Profiling the Metabolic Effects of KIAA1363 Inactivation in Cancer Cells

We next compared the global metabolite profiles of SKOV-3 cells treated with AS115 to identify endogenous small molecules regulated by KIAA1363, using a recently described, untargeted liquid chromatography-mass spectrometry (LC-MS) platform for comparative metabolomics [22]. AS115 (10 μM, 4 hr) was found to cause a dramatic reduction in the levels of a specific set of lipophilic metabolites (m/z 317, 343, and 345) in SKOV-3 cells ( Figure 2A). These metabolites did not correspond to any of the typical lipid species found in cells, none of which were significantly altered by AS115 treatment ( Table S1). High-resolution MS of the m/z 317 metabolite provided a molecular formula of C19H40O3 ( Figure 2B), which suggests that this compound might represent a monoalkylglycerol ether bearing a C16:0 alkyl chain (C16:0 MAGE).  This structure assignment was corroborated by tandem MS and LC analysis, in which the endogenous m/z 317 product and synthetic C16:0 MAGE displayed equivalent fragmentation and migration patterns, respectively ( Figure S3). By extension, the m/z 343 and 345 metabolites were interpreted to represent the C18:1 and C18:0 MAGEs, respectively. A control carbamate inhibitor, URB597, which targets other hydrolytic enzymes [23], but not KIAA1363, did not affect MAGE levels in cancer cells ( Figure S4).

Pharmacological Inhibition of KIAA1363 Reduces Monoalkylglycerol Ether, MAGE, Levels in Human Cancer Cells

Pharmacological Inhibition of KIAA1363 Reduces Monoalkylglycerol Ether, MAGE, Levels in Human Cancer Cells

http://ars.els-cdn.com/content/image/1-s2.0-S1074552106003000-gr2.jpg

Figure 2. Pharmacological Inhibition of KIAA1363 Reduces Monoalkylglycerol Ether, MAGE, Levels in Human Cancer Cells

(A) Global metabolite profiling of AS115-treated SKOV-3 cells (10 μM AS115, 4 hr) with untargeted LC-MS methods [22]revealed a specific reduction in a set of structurally related metabolites with m/z values of 317, 343, and 345 (p < 0.001 for AS115- versus DMSO-treated SKOV-3 cells). Results represent the average fold change for three independent experiments. See Table S1for a more complete list of metabolite levels.

(B) High-resolution MS analysis of the sodium adduct of the purified m/z 317 metabolite provided a molecular formula of C19H40O3, which, in combination with tandem MS and LC analysis ( Figure S3), led to the determination of the structure of this small molecule as C16:0 monoalkylglycerol ether (C16:0 MAGE).

Biochemical Characterization of KIAA1363 as a 2-Acetyl MAGE Hydrolase

The correlation between KIAA1363 inactivation and reduced MAGE levels suggests that these lipids are products of a KIAA1363-catalyzed reaction. A primary route for the biosynthesis of MAGEs has been proposed to occur via the enzymatic hydrolysis of their 2-acetyl precursors 24 and 25. This 2-acetyl MAGE hydrolysis activity was first detected in cancer cell extracts over a decade ago [25], but, to date, it has eluded molecular characterization. To test whether KIAA1363 functions as a 2-acetyl MAGE hydrolase, this enzyme was transiently transfected into COS7 cells. KIAA1363-transfected cells possessed significantly higher 2-acetyl MAGE hydrolase activity compared to mock-transfected cells, and this elevated activity was blocked by treatment with AS115 (Figure 3A). In contrast, KIAA1363- and mock-transfected cells showed no differences in their respective hydrolytic activity for 2-oleoyl MAGE, monoacylglycerols, or phospholipids (e.g., platelet-activating factor [PAF], phosphatidylcholine) (Figure S5A). These data indicate that KIAA1363 selectively catalyzes the hydrolysis of 2-acetyl MAGEs to MAGEs.

KIAA1363 Regulates an Ether Lipid Signaling Network that Bridges Platelet-Activating Factor and the Lysophospholipids

Examination of the Kyoto Encyclopedia of Genes and Genomes (KEGG) database [26] suggests that the KIAA1363-MAGE pathway might serve as a unique metabolic node linking the PAF [27] and lysophospholipid [28] signaling systems in cancer cells (Figure 4A). Consistent with a direct pathway leading from MAGEs to these lysophospholipids, addition of 13C-MAGE to SKOV-3 cells resulted in the formation of 13C-labeled alkyl-LPC and alkyl-LPA (Figure 4C).
Conversely, the levels of 2-acetyl MAGE in SKOV-3 cells, as judged by metabolic labeling experiments, were significantly stabilized by treatment with AS115, which, in turn, led to an accumulation of PAF (Figure 4D).  A comparison of the metabolite profiles of SKOV-3 and OVCAR-3 cells revealed significantly higher levels of MAGE, alkyl-LPC, and alkyl-LPA in the former line (Figure 4E). These data indicate that the lysophospholipid branch of the MAGE network is elevated in aggressive cancer cells, and that this metabolic shift is regulated by KIAA1363.

Figure 4. KIAA1363 Serves as a Key Enzymatic Node in a Metabolic Network that Connects the PAF and Lysophospholipid Families of Signaling Lipids

Stable Knockdown of KIAA1363 Impairs Tumor Growth In Vivo

Figure 6. KIAA1363 Contributes to Ovarian Tumor Growth and Cancer Cell Migration

The decrease in tumorigenic potential of shKIAA1363 cells was not associated with a change in proliferation potential in vitro (Figure S8). shKIAA1363 cells were, however, impaired in their in vitro migration capacity compared to control cells (Figure 6B). Neither MAGE nor alkyl-LPC impacted cancer cell migration at concentrations up to 1 μM (Figure 6B). In contrast, alkyl-LPA (10 nM) completely rescued the reduced migratory activity of shKIAA1363 cells. Collectively, these results indicate that KIAA1363 contributes to the pathogenic properties of cancer cells in vitro and in vivo, possibly through regulating the levels of the bioactive lipid LPA.

We have determined by integrated enzyme and small-molecule profiling that KIAA1363, a protein of previously unknown function, is a 2-acetyl MAGE hydrolase that serves as a key regulator of a lipid signaling network that contributes to cancer pathogenesis. Although we cannot yet conclude which of the specific metabolites regulated by KIAA1363 supports tumor growth in vivo, the rescue of the reduced migratory phenotype of shKIAA1363 cancer cells by LPA is consistent with previous reports showing that this lipid signals through a family of G protein-coupled receptors to promote cancer cell migration and invasion 2829 and 30. LPA is also an established biomarker in ovarian cancer, and the levels of this metabolite are elevated nearly 10-fold in ascites fluid and plasma of patients with ovarian cancer [31]. Our results suggest that additional components in the KIAA1363-ether lipid network, including MAGE, alkyl LPC, and KIAA1363 itself, might also merit consideration as potential diagnostic markers for ovarian cancer. Consistent with this premise, our preliminary analyses have revealed highly elevated levels of KIAA1363 in primary human ovarian tumors compared to normal ovarian tissues (data not shown). The heightened expression of KIAA1363 in several other cancers, including breast 18 and 32, melanoma [18], and pancreatic cancer [33], indicates that alterations in the KIAA1363-ether lipid network may be a conserved feature of tumorigenesis. Considering further that reductions in KIAA1363 activity were found to impair tumor growth of both ovarian and breast cancer cells, it is possible that inhibitors of this enzyme may prove to be of value for the treatment of multiple types of cancer.

 

7.6.6 Peroxisomes – A Nexus for Lipid Metabolism and Cellular Signaling

Lodhi IJ, Semenkovich CF
Cell Metab. 2014 Mar 4; 19(3):380-92
http://dx.doi.org/10.1016%2Fj.cmet.2014.01.002

Peroxisomes are often dismissed as the cellular hoi polloi, relegated to cleaning up reactive oxygen chemical debris discarded by other organelles. However, their functions extend far beyond hydrogen peroxide metabolism. Peroxisomes are intimately associated with lipid droplets and mitochondria, and their ability to carry out fatty acid oxidation and lipid synthesis, especially the production of ether lipids, may be critical for generating cellular signals required for normal physiology. Here we review the biology of peroxisomes and their potential relevance to human disorders including cancer, obesity-related diabetes, and degenerative neurologic disease.

Peroxisomes are multifunctional organelles present in virtually all eukaryotic cells. In addition to being ubiquitous, they are also highly plastic, responding rapidly to cellular or environmental cues by modifying their size, number, morphology, and function (Schrader et al., 2013). Early ultrastructural studies of kidney and liver cells revealed cytoplasmic particles enclosed by a single membrane containing granular matrix and a crystalline core (Rhodin, 1958). These particles were linked with the term “peroxisome” by Christian de Duve, who first identified the organelle in mammalian cells when enzymes such as oxidases and catalases involved in hydrogen peroxide metabolism co-sedimented in equilibrium density gradients (De Duve and Baudhuin, 1966). Based on these studies, it was originally thought that the primary function of these organelles was the metabolism of hydrogen peroxide. Novikoff and colleagues observed a large number of peroxisomes in tissues active in lipid metabolism such as liver, brain, intestinal mucosa, and adipose tissue (Novikoff and Novikoff, 1982;Novikoff et al., 1980). Peroxisomes in different tissues vary greatly in shape and size, ranging from 0.1-0.5 μM in diameter. In adipocytes, peroxisomes tend to be small in size and localized in the vicinity of lipid droplets. Notably, a striking increase in the number of peroxisomes was observed during differentiation of adipogenic cells in culture (Novikoff and Novikoff, 1982). These findings suggest that peroxisomes may be involved in lipid metabolism.

Lazarow and de Duve hypothesized that peroxisomes in animal cells were capable of carrying out fatty acid oxidation. This was confirmed when they showed that purified rat liver peroxisomes contained fatty acid oxidation activity that was robustly increased by treatment of animals with clofibrate (Lazarow and De Duve, 1976). In a series of experiments, Hajra and colleagues discovered that peroxisomes were also capable of lipid synthesis (Hajra and Das, 1996). Over the past three decades, multiple lines of evidence have solidified the concept that peroxisomes play fundamentally important roles in lipid metabolism. In addition to removal of reactive oxygen species, metabolic functions of peroxisomes in mammalian cells include β-oxidation of very long chain fatty acids, α-oxidation of branched chain fatty acids, and synthesis of ether-linked phospholipids as well as bile acids (Figure 1). β-oxidation also occurs in mitochondria, but peroxisomal β-oxidation involves distinctive substrates and complements mitochondrial function; the processes of α-oxidation and ether lipid synthesis are unique to peroxisomes and important for metabolic homeostasis.

Structure and functions of peroxisomes

Structure and functions of peroxisomes

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3951609/bin/nihms-555068-f0001.jpg

Figure 1 Structure and functions of peroxisomes

The peroxisome is a single membrane-enclosed organelle that plays an important role in metabolism. The main metabolic functions of peroxisomes in mammalian cells include β-oxidation of very long chain fatty acids, α-oxidation of branched chain fatty acids, synthesis of bile acids and ether-linked phospholipids and removal of reactive oxygen species. Peroxisomes in many, but not all, cell types contain a dense crystalline core of oxidative enzymes.

Here we highlight the established role of peroxisomes in lipid metabolism and their emerging role in cellular signaling relevant to metabolism. We describe the origin of peroxisomes and factors involved in their assembly, division, and function. We address the interaction of peroxisomes with lipid droplets and implications of this interaction for lipid metabolism. We consider fatty acid oxidation and lipid synthesis in peroxisomes and their importance in brown and white adipose tissue (sites relevant to lipid oxidation and synthesis) and disease pathogenesis.

peroxisomal biogenesis and protein import

peroxisomal biogenesis and protein import

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3951609/bin/nihms-555068-f0002.jpg

Potential pathways to peroxisomal biogenesis. Peroxisomes are generated autonomously through division of pre-existing organelles (top) or through a de novo process involving budding from the ER followed by import of matrix proteins (bottom). B. Peroxisomal membrane protein import. Peroxisomal membrane proteins (PMPs) are imported post-translationally to the peroxisomal membrane. Pex19 is a soluble chaperone that binds to PMPs and transports them to the peroxisomal membrane, where it docks with a complex containing Pex16 and Pex3. Following insertion of the PMP, Pex19 is recycled back to the cytosol.

Regardless of their origin, peroxisomes require a group of proteins called peroxins for their assembly, division, and inheritance. Over 30 peroxins, encoded by Pex genes, have been identified in yeast (Dimitrov et al., 2013). At least a dozen of these proteins are conserved in mammals, where they regulate various aspects of peroxisomal biogenesis, including factors that control assembly of the peroxisomal membrane, factors that interact with peroxisomal targeting sequences allowing proteins to be shuttled to peroxisomes, and factors that act as docking receptors for peroxisomal proteins.

At least three peroxins (Pex3, Pex16 and Pex19) appear to be critical for assembly of the peroxisomal membrane and import of peroxisomal membrane proteins (PMPs) (Figure 2B). Pex19 is a soluble chaperone and import receptor for newly synthesized PMPs (Jones et al., 2004). Pex3 buds from the ER in a pre-peroxisomal vesicle and functions as a docking receptor for Pex19 (Fang et al., 2004). Pex16 acts as a docking site on the peroxisomal membrane for recruitment of Pex3 (Matsuzaki and Fujiki, 2008). Peroxisomal matrix proteins are translated on free ribosomes in the cytoplasm prior to their import. These proteins have specific peroxisomal targeting sequences (PTS) located either at the carboxyl (PTS1) or amino (PTS2) terminus (Gould et al., 1987Swinkels et al., 1991).

 

7.6.7 A nexus for cellular homeostasis- the interplay between metabolic and signal transduction pathways

Ana P Gomes, John Blenis
Current Opinion in Biotechnology Aug 2015; 34:110–117
http://dx.doi.org/10.1016/j.copbio.2014.12.007

Highlights

  • Signaling networks sense intracellular and extracellular cues to maintain homeostasis.
  • PI3K/AKT and Ras/ERK signaling induces anabolic reprogramming.
  • mTORC1 is a master node of signaling integration that promotes anabolism.
  • AMPK and SIRT1 fine tune signaling networks in response to energetic status.

In multicellular organisms, individual cells have evolved to sense external and internal cues in order to maintain cellular homeostasis and survive under different environmental conditions. Cells efficiently adjust their metabolism to reflect the abundance of nutrients, energy and growth factors. The ability to rewire cellular metabolism between anabolic and catabolic processes is crucial for cells to thrive. Thus, cells have developed, through evolution, metabolic networks that are highly plastic and tightly regulated to meet the requirements necessary to maintain cellular homeostasis. The plasticity of these cellular systems is tightly regulated by complex signaling networks that integrate the intracellular and extracellular information. The coordination of signal transduction and metabolic pathways is essential in maintaining a healthy and rapidly responsive cellular state.

AMPK and SIRT1 fine tune signaling networks in response to energetic status

AMPK and SIRT1 fine tune signaling networks in response to energetic status

 

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AMPK and SIRT1 fine tune signaling networks in response to energetic status

 

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mTORC1 is a master node of signaling integration that promotes anabolism.

 

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Fine-tuning signaling networks

 PI3K/Akt signaling-induced anabolic reprogramming

Growth factors and other ligands activate PI3K signaling upon binding and consequent activation of their cell surface receptors, such as receptor tyrosine kinases (RTKs) and G protein-coupled
receptors (GPCRs). This leads to the phosphorylation of membrane phosphatidylinositiol lipids and the recruitment and activation of several protein kinases, which perpetuate the extracellular
signals to modulate intracellular processes [3,4]. One of the most crucial signal propagators regulated by PI3K signaling is protein kinase B/Akt [3,4]. Indeed, Akt rewires metabolism in response
to environmental cues by three distinct means;
(i) by the direct phosphorylation and regulation of metabolic enzymes,
(ii) by activating/inactivating metabolism altering transcriptional factors, and
(iii) by modulating other kinases that themselves regulate metabolism [5].
Akt regulates glucose metabolism, inducing both glucose uptake and glycolytic flux by increasing the expression of the glucose transporter genes and regulating the activity of glycolytic enzymes,
respectively [6–8]. Moreover, the ability of Akt to induce glycolysis is also mediated by the regulation of Hexokinase (HK). HK performs the first step of glycolysis.

Figure 1 Anabolic rewiring induced by PI3K/Akt, Ras/ERK and mTORC1 signaling.
Extracellular signals activate two major signaling cascades controlled by the activation of PI3K and Ras. PI3K and Ras regulate Akt and ERK, which in turn induce changes in intermediate metabolism
to promote anabolic processes. In addition, they also induce the activation of  mTORC1, thus further supporting the rewiring of cellular metabolism towards anabolic processes. Through various mechanisms
Akt, ERK and mTORC1 stimulate mRNA translation, aerobic glycolysis, glutamine anaplerosis, lipid synthesis, the pentose phosphate and pyrimidine synthesis, thus producing the major components
necessary for cell growth and proliferation.

Figure 2. Regulation of intermediate metabolism by nutrient and energy sensors.
Nutrient and energy-responsive pathways fine-tune the output of signaling cascades, allowing for the correct balance between the availability of nutrients and the cellular capacity to use them effectively.
AMPK and SIRT1 respond to the energy status of the cells through sensing of AMP and NAD+ levels respectively. When energy is scarce, these sensors are activated inducing a rewiring of intermediate
metabolism to catabolic processes in order to produce energy and restore homeostasis. When nutrients (such as glucose and amino acids) and energy are available, AMPK, SIRT1, SIRT3 and SIRT6 are
repressed and mTORC1 is active, thus promoting a shift towards anabolic processes and energy production. These networks of signaling cascades, their interconnection and regulation allow the cells
to maintain energetic balance and allow for the physiological adaptation to the ever-changing environment.

 

7.6.8 Mechanisms-of-intercellular-signaling

7.6.8.1 Activation and signaling of the p38 MAP kinase pathway

Tyler Zarubin1 and Jiahuai Han
Cell Research (2005) 15, 11–18
http://dx.doi.org:/10.1038/sj.cr.7290257

The family members of the mitogen-activated protein (MAP) kinases mediate a wide variety of cellular behaviors in response to extracellular stimuli. One of the four main sub-groups, the p38 group of MAP kinases, serve as a nexus for signal transduction and play a vital role in numerous biological processes. In this review, we highlight the known characteristics and components of the p38 pathway along with the mechanism and consequences of p38 activation. We focus on the role of p38 as a signal transduction mediator and examine the evidence linking p38 to inflammation, cell cycle, cell death, development, cell differentiation, senescence and tumorigenesis in specific cell types. Upstream and downstream components of p38 are described and questions remaining to be answered are posed. Finally, we propose several directions for future research on p38.

Cellular behavior in response to extracellular stimuli is mediated through intracellular signaling pathways such as the mitogen-activated protein (MAP) kinase pathways 1. MAP kinases are members of discrete signaling cascades and serve as focal points in response to a variety of extracellular stimuli. Four distinct subgroups within the MAP kinase family have been described:

  • extracellular signal-regulated kinases (ERKs),
  • c-jun N-terminal or stress-activated protein kinases (JNK/SAPK),
  • ERK/big MAP kinase 1 (BMK1), and
  • the p38 group of protein kinases.

The focus of this review will be to highlight the characteristics of

  • the p38 kinases,
  • components of this kinase cascade,
  • activation of this pathway, and
  • the biological consequences of its activation.

p38 (p38) was first isolated as a 38-kDa protein rapidly tyrosine phosphorylated in response to LPS stimulation 23. p38 cDNA was also cloned as a molecule that binds puridinyl imidazole derivatives which are known to inhibit biosynthesis of inflammatory cytokines such as interleukin-1 (IL-1) and tumor-necrosis factor (TNF) in LPS stimulated monocytes 4. To date, four splice variants of the p38 family have been identified: p38, p38 5, p38 (ERK6, SAPK3) 67, and p38(SAPK4) 89. Of these, p38 and p38 are ubiquitously expressed while p38 and p38 are differentially expressed depending on tissue type. All p38 kinases can be categorized by a Thr-Gly-Tyr (TGY) dual phosphorylation motif 10. Sequence comparisons have revealed that each p38 isoform shares 60% identity within the p38 group but only 40–45% to the other three MAP kinase family members.

Mammalian p38s activation has been shown to occur in response to extracellular stimuli such as UV light, heat, osmotic shock, inflammatory cytokines (TNF- & IL-1), and growth factors (CSF-1) 13151617,18192021. This plethora of activators conveys the complexity of the p38 pathway and this matter is further complicated by the observation that activation of p38 is not only dependent on stimulus, but on cell type as well. For example, insulin can stimulate p38 in 3T3-L1 adipocytes 22, but downregulates p38 activity in chick forebrain neuron cells 23. The activation of p38 isoforms can be specifically controlled through different regulators and coactivated by various combinations of upstream regulators 2426.

Like all MAP kinases, p38 kinases are activated by dual kinases termed the MAP kinase kinases (MKKs). However, despite conserved dual phosphorylation sites among p38 isoforms, selective activation by distinct MKKs has been observed. There are two main MAPKKs that are known to activate p38, MKK3 and MKK6. It is proposed that upstream kinases can differentially regulate p38 isoforms as evidenced by the inability of MKK3 to effectively activate p38 while MKK6 is a potent activator despite 80% homology between these two MKKs 27. Also, it has been shown that MKK4, an upstream kinase of JNK, can aid in the activation of p38 and p38 in specific cell types 8. This data suggests then, that activation of p38 isoforms can be specifically controlled through different regulators and coactivated by various combinations of upstream regulators. Furthermore, substrate selectivity may be a reason why each MKK has a distinct function. In addition to the activation by upstream kinases, there is a MAPKK-independent mechanism of p38 MAPK activation involving TAB1 (transforming growth factor–activated protein kinase 1 (TAK1)-binding protein) 28. The activation of p38 in this pathway is achieved by the autophosphorylation of p38 after interaction with TAB1.

The activation of p38 in response to the wide range of extracellular stimuli can be seen in part by the diverse range of MKK kinases (MAP3K) that participate in p38 activation. These include TAK1 33, ASK1/MAPKKK5 34, DLK/MUK/ZPK 3536, and MEKK4 353738. Overexpression of these MAP3Ks leads to activation of both p38 and JNK pathways which is possibly one reason why these two pathways are often co-activated. Also contributing to p38 activation upstream of MAPK kinases are low molecular weight GTP-binding proteins in the Rho family such as Rac1 and Cdc42 4041. Rac1 can bind to MEKK1 or MLK1 while Cdc42 can only bind to MLK1 and both result in activation of p38 via MAP3Ks 3542.

Dephosphorylation, would seem to play a major role in the downregulation of MAP kinase activity. Many dual-specificity phosphatases have been identified that act upon various members of the MAP kinase pathway and are grouped as the MAP kinase phosphatase (MKP) family 45. Several members can efficiently dephosphorylate p38 and p38 4647; however, p38 and p38 are resistant to all known MKP family members.

The first p38 substrate identified was the MAP kinase-activated protein kinase 2 (MAPKAPK2 or MK2) 11552. This substrate, along with its closely related family member MK3 (3pk), were both shown to activate various substrates including small heat shock protein 27 (HSP27) 53, lymphocyte-specific protein 1 (LSP1) 54, cAMP response element-binding protein (CREB) 55, transcription factor ATF1 55, SRF 56, and tyrosine hydroxylase 57. p38 regulated/activated kinase (PRAK) is a p38 and/or p38activated kinase that shares 20-30% sequence identity to MK2 and is thought to regulate heat shock protein 27 (HSP27) 61. Mitogen- and stress-activated protein kinase-1 (MSK1) can be directly activated by p38 and ERK, and may mediate activation of CREB 626364.

Another group of substrates that are activated by p38 comprise transcription factors. Many transcription factors encompassing a broad range of action have been shown to be phosphorylated and subsequently activated by p38. Examples include activating transcription factor 1, 2 & 6 (ATF-1/2/6), SRF accessory protein (Sap1), CHOP (growth arrest and DNA damage inducible gene 153, or GADD153), p53, C/EBP, myocyte enhance factor 2C (MEF2C), MEF2A, MITF1, DDIT3, ELK1, NFAT, and high mobility group-box protein 1 (HBP1) 175566676869707172,73747576. An important cis-element, AP-1 appears to be influenced by p38 through several different mechanisms.  Taken together, all the data suggest that the p38 pathway has a wide variety of functions.

Abundant evidence for p38 involvement in apoptosis exists to date and is based on concomitant activation of p38 and apoptosis induced by a variety of agents such as NGF withdrawal and Fas ligation 959697. Cysteine proteases (caspases) are central to the apoptotic pathway and are expressed as inactive zymogens 98,99. Caspase inhibitors then can block p38 activation through Fas cross-linking, suggesting p38 functions downstream of caspase activation 97100. However, overexpression of dominant active MKK6b can also induce caspase activity and cell death thus implying that p38 may function both upstream and downstream of caspases in apoptosis 101102. It must be mentioned that the role of p38 in apoptosis is cell type and stimulus dependent. While p38 signaling has been shown to promote cell death in some cell lines, in different cell lines p38 has been shown to enhance survival, cell growth, and differentiation.

p38 now seems to have a role in tumorigenesis and sensescence. There have been reports that activation of MKK6 and MKK3 led to a senescent phenotype dependent upon p38 MAPK activity. Also, p38 MAPK activity was shown responsible for senescence in response to telomere shortening, H2O2 exposure, and chronic RAS oncogene signaling 117118119. A common feature of tumor cells is a loss of senescence and p38 may be linked to tumorigenesis in certain cells. It has been reported that p38 activation may be reduced in tumors and that loss of components of the p38 pathway such as MKK3 and MKK6 resulted in increased proliferation and likelihood of tumorigenic conversion regardless of the cell line or the tumor induction agent used in these studies 29.

Although all research done on the p38 pathway cannot be reviewed here, certain conclusions can still be made regarding the operation of p38 as a signal transduction mediator. The p38 family (,,,) is activated by both stress and mitogenic stimuli in a cell dependent manner and certain isoforms can either directly or indirectly target proteins to control pre/post transcription. p38 MAPKs also have the ability to activate other kinases and consequently regulate numerous cellular responses. Because p38 signaling has been implicated in cellular responses including inflammation, cell cycle, cell death, development, cell differentiation, senescence, and tumorigenesis, emphasis must be placed on p38 function with respect to specific cell types.

Regulation of the p38 pathway is not an isolated cascade and many different upstream signals can lead to p38 activation. These signals may be p38 specific (MKK3/6), general MAPKKs (MKK4), or MAPKK independent signals (TAB1). Downstream signaling pathways of p38 are quite divergent and each component may interact with other cellular components, both upstream and downstream, to coordinate cellular processes such as feedback mechanisms. Furthermore, in vivo p38 is not an isolated event and exists in the presence of other MAP kinases and a plethora of other signaling pathways. The subcellular location of p38 activation may also play a critical role determining the resulting effect and may add yet another order of complexity to the investigation of p38 function.

 

7.6.8.2 Mitogen-Activated Protein Kinase Pathways Mediated by ERK, JNK, and p38 Protein Kinases

Gary L. Johnson and Razvan Lapadat
Science 6 Dec 2002; 298: 1911-1912.

Multicellular organisms have three well-characterized subfamilies of mitogen activated protein kinases (MAPKs) that control a vast array of physiological processes. These enzymes are regulated by a characteristic phosphorelay system in which a series of three protein kinases phosphorylate and activate one another. The extracellular signal–regulated kinases (ERKs) function in the control of cell division, and inhibitors of these enzymes are being explored as anticancer agents. The c-Jun amino-terminal kinases ( JNKs) are critical regulators of transcription, and JNK inhibitors may be effective in control of rheumatoid arthritis. The p38 MAPKs are activated by inflammatory cytokines and environmental stresses.

Protein kinases are enzymes that covalently attach phosphate to the side chain of either serine, threonine, or tyrosine of specific proteins inside cells. Such phosphorylation of proteins can control their enzymatic activity, their interaction with other proteins and molecules, their location in the cell, and their propensity for degradation by proteases. Mitogen-activated protein kinases (MAPKs) compose a family of protein kinases whose function and regulation have been conserved during evolution from unicellular organisms such as brewers’ yeast to complex organisms including humans (1). MAPKs phosphorylate specific serines and threonines of target protein substrates and regulate cellular activities ranging from gene expression, mitosis, movement, metabolism, and programmed death. Because of the many important cellular functions controlled by MAPKs, they have been studied extensively to define their roles in physiology and human disease. MAPK-catalyzed phosphorylation of substrate proteins functions as a switch to turn on or off the activity of the substrate protein.

MAPKs are part of a phosphorelay system composed of three sequentially activated kinases, and, like their substrates, MAPKs are regulated by phosphorylation (Fig. 1) (2). MKK-catalyzed phosphorylation activates the MAPK and increases its activity in catalyzing the phosphorylation of its own substrates. MAPK phosphatases reverse the phosphorylation and return the MAPK to an inactive state. MKKs are highly selective in phosphorylating specific MAPKs. MAPK kinase kinases (MKKKs) are the third component of the phosphorelay system. MKKKs phosphorylate and activate specific MKKs. MKKKs have distinct motifs in their sequences that selectively confer their activation in response to different stimuli.

Fig. 1. MAPK phosphorelay systems.

The modules shown are representative of pathway connections for the respective MAPK phosphorelay systems.There are multiple component MKKKs, MKKs, and MAPKs for each system.For example, there are three Raf proteins (c-Raf1, B-Raf, A-Raf), two MKKs (MKK1 and MKK2), and two ERKs (ERK1 and ERK2) that can compose MAPK phosphorelay systems responsive to growth factors.The ERK, JNK, and p39 pathways in the STKE Connections Map demonstrate the potential complexity of these systems.

ERKs 1 and 2 are both components of a three-kinase phosphorelay module that includes the MKKK c-Raf1, B-Raf, or A-Raf, which can be activated by the proto-oncogene Ras. Mutations that convert Ras to an activated oncogene are common oncogenic mutations in many human tumors. Oncogenic Ras persistently activates the ERK1 and ERK2 pathways, which contributes to the increased proliferative rate of tumor cells. For this reason, inhibitors of the ERK pathways are entering clinical trials as potential anticancer agents.

Regulation of the JNK pathway is extremely complex and is influenced by many MKKKs. As depicted in the STKE JNK Pathway Connections Map, there are 13 MKKKs that regulate the JNKs. This diversity of MKKKs allows a wide range of stimuli to activate this MAPK pathway. JNKs are important in controlling programmed cell death or apoptosis (9). The inhibition of JNKs enhances chemotherapy-induced inhibition of tumor cell growth, suggesting that JNKs may provide a molecular target for the treatment of cancer. The pharmaceutical industry is bringing JNK inhibitors into clinical trials.

Recently, a major paradigm shift for MAPK regulation was developed for p38. The p38 enzyme is activated by the protein TAB1 (12), but TAB1 is not a MKK. Rather, TAB1 appears to be an adaptor or scaffolding protein and has no known catalytic activity. This is the first demonstration that another mechanism exists for the regulation of MAPKs in addition to the MKKK-MKKMAPK regulatory module.

The importance of MAPKs in controlling cellular responses to the environment and in regulating gene expression, cell growth, and apoptosis has made them a priority for research related to many human diseases. The ERK, JNK, and p38 pathways are all molecular targets for drug development, and inhibitors of MAPKs will undoubtedly be one of the next group of drugs developed for the treatment of human disease (13).

7.6.9 Cathepsin B promotes colorectal tumorigenesis, cell invasion, and metastasis

B Bian, S Mongrain, S Cagnol, Marie-Josée Langlois, J Boulanger, et al.
Molec Carcinogen 25 Mar 2015; 54(5). http://dx.doi.org:/10.1002/mc.22312

Cathepsin B is a cysteine proteinase that primarily functions as an endopeptidase within endolysosomal compartments in normal cells. However, during tumoral expansion, the regulation of cathepsin B can be altered at multiple levels, thereby resulting in its overexpression and export outside of the cell. This may suggest a possible role of cathepsin B in alterations leading to cancer progression. The aim of this study was to determine the contribution of intracellular and extracellular cathepsin B in growth, tumorigenesis, and invasion of colorectal cancer (CRC) cells. Results show that mRNA and activated levels of cathepsin B were both increased in human adenomas and in CRCs of all stages. Treatment of CRC cells with the highly selective and non-permeant cathepsin B inhibitor Ca074 revealed that extracellular cathepsin B actively contributed to the invasiveness of human CRC cells while not essential for their growth in soft agar. Cathepsin B silencing by RNAi in human CRC cells inhibited their growth in soft agar, as well as their invasion capacity, tumoral expansion, and metastatic spread in immunodeficient mice. Higher levels of the cell cycle inhibitor p27Kip1 were observed in cathepsin B-deficient tumors as well as an increase in cyclin B1. Finally, cathepsin B colocalized with p27Kip1 within the lysosomes and efficiently degraded the inhibitor. In conclusion, the present data demonstrate that cathepsin B is a significant factor in colorectal tumor development, invasion, and metastatic spreading and may, therefore, represent a potential pharmacological target for colorectal tumor therapy

Colorectal cancer (CRC),a major malignancy worldwide and the second leading cause of cancer death in North America, develops through multiple steps. The ability of cancers to invade and metastasize depends on the action of proteases actively taking center stage in extracellular proteolysis [2]. Of all the proteases, the cysteine protease cathepsin B is of significant importance [3]. Cathepsin B primarily functions as an endopeptidase within endolysosomal compartments in normal cells. However, during malignant transformation cathepsin B can be upregulated [3, 4]. Cathepsin B in tumors can either be secreted, bound to the cell membrane or released by shedding vesicles [4]. Expression and redistribution of active cathepsin B to the basal plasma membrane occurs in late colon adenomas [5, 6] coincident with the activation of KRAS [1]. In line with these results, Cavallo-Medved et al. [7] have demonstrated that trafficking of cathepsin B to caveolae and its secretion are regulated by active KRAS in CRC cells in culture. Accordingly, secretion of cathepsin B, increased in the extracellular environment of CRC [8, 9], is suspected to play an essential role in disrupting extracellular matrix barriers, facilitating invasion and metastasis [10-12]. These data are consistent with the link between cathepsin B protein expression in colorectal carcinomas and shortened patient survival [6].

In a recent prospective cohort study of 558 men and women with colonic tumors [13] 82% of patients had tumors that expressed cathepsin B, irrespective of stage, while the remaining 18% had tumors that did not express cathepsin B. Other studies have suggested that cathepsin B expression or activity may actually peak during early stage cancer and subsequently decline with advanced disease [14, 15]. This points to a possible role of cathepsin B in both early and late alterations leading to colonic cancer.

This study used two strategies to specifically counteract the action of cathepsin B. The first involved the use of RNA interference (RNAi) to inhibit the expression of cathepsin B protein into CRC cells while the second approach employed the highly selective cathepsin inhibitor Ca074 to block extracellular cathepsin B activity. Results suggest that extracellular cathepsin B is involved in cell invasion whereas intracellular cathepsin B controls malignant properties of CRC cells. Further, biochemical analysis suggests that intracellular cathepsin B regulates tumorigenesis by degrading the p27Kip1 cell cycle inhibitor.

mRNA and Activated Levels of Cathepsin B Are Increased in Adenomas and in Colorectal Tumors of All Stages

Cathepsin B expression was analyzed at both the mRNA and protein levels in a series of human paired specimens at various tumor stages. As shown in Figure 1A, increased transcript levels of cathepsin B were observed in colorectal tumors, regardless of tumor stage, including in adenomas. Of note, increased cathepsin B expression was more prominent in tumors exhibiting APC mutations. By contrast, there did not appear to be a significant difference relative to KRAS mutations (Figure 1B). To establish whether these increased mRNA levels could be correlated with increased cathepsin B protein levels and more importantly with increased activity, expression of the active processed forms of the protease (25 and 30 kDa) was analyzed by Western blot. Both pro-cathepsin B and active cathepsin B were also increased in colorectal tumors compared to normal tissues (Figure 1C and D). These data hence suggest that increased transcription contributes to a greater expression of active cathepsin B in CRC.

Extracellular Cathepsin B Contributes to Invasiveness of Human CRC Cells but is Dispensable for Their Growth in Soft Agar

Cathepsin B protein levels were next examined in lysates obtained from various human CRC cell lines. As shown in Figure 2A, the proactive and catalytically active processed forms of cathepsin B were detected at various levels in CRC cell lines. Selected cathepsin B presence was also confirmed in conditioned culture medium of CRC cells, again at various levels (Figure 2A, lower panel). However, while the pro-form of cathepsin B was readily observed in conditioned culture medium of all CRC cells, the catalytically-active processed forms of cathepsin B were not detected in Western blot analyses. Additionally, using a fluorescence-based enzymatic assay, no cathepsin B enzyme activity was detected in conditioned medium. Since the pro-protease form might be activated under acidic pH conditions (peri- or extracellular) and by extracellular components of the extracellular matrix, the impact of extracellular inhibition of cathepsin B activation on CRC cell invasion was verified using Biocoat Matrigel chambers. HT-29, DLD1, and SW480 CRC cell lines secreting different levels of pro-cathepsin B (Figure 2A) were tested. Experiments were performed using the highly selective and non-permeant inhibitor Ca074 to reduce extracellular cathepsin B activity. At 10 μM, Ca074 produced a >99% inhibition of recombinant cathepsin B levels while barely reducing intracellular cathepsin B, that is, 5–8%, even upon 12 h exposure to the inhibitor (data not shown). Of note, treatment with 10 μM Ca074 significantly inhibited Matrigel invasion by approximately 45–60% in HT29, DLD1, and SW480 CRC cell lines (Figure 2B). By contrast, treatment with Ca074 had no significant effect on their capacity to form colonies in soft agarose (Figure 2C).

Cathepsin B Silencing in Human CRC Cells Inhibits Tumorigenicity and Metastasis in Immunodeficient Mice

Suppression of cathepsin B expression was found to significantly attenuate the metastatic potential of CRC cells in vivo in experimental metastasis assays. Indeed, immunodeficient mice injected with control CRC cells into the tail vein showed extensive lung metastasis within 28 d, whereas cells expressing shRNA against cathepsin B exhibited reduced lung colonization (Figure 4A). Cathepsin B silencing also altered the capacity of CRC cells to form tumors in mice as assessed by subcutaneous xenograft assays. HT29 cells induced palpable tumors with a short latency period of 9 d after their injection while downregulation of cathepsin B expression in these cells severely impaired their capacity to grow as tumors (Figure 4B).

Cathepsin B Silencing in Human CRC Cells Inhibits Growth in Soft Agar and Invasion Capacity

Recombinant lentiviruses encoding anti-cathepsin B short hairpin RNA (shRNA) were developed in order to stably suppress cathepsin B expression in CRC cells. As shown in Figure 3A, intracellular cathepsin B mRNA and protein levels were decreased in HT29 and DLD1 cells in comparison to a control shRNA which had no effect. Reduction of cathepsin B expression modestly slowed the proliferation rate of HT29 and DLD1 populations in 2D cell culture (Figure 3B). Conversely, cathepsin B silencing significantly reduced the ability of HT29 and DLD1 cells to form colonies in soft agarose (Figure 3C). This indicates that intracellular cathepsin B controls anchorage-independent growth of CRC cells given the absence of Ca074 effect (Figure 2C). Moreover, cathepsin B silencing also reduced the number of invading HT29 and DLD1 cells to a similar extent as Ca074 treatment (Figure 3D vs. Figure 2B).

Cathepsin B Silencing in Human CRC Cells Inhibits Tumorigenicity and Metastasis in Immunodeficient Mice

Suppression of cathepsin B expression was found to significantly attenuate the metastatic potential of CRC cells in vivo in experimental metastasis assays. Indeed, immunodeficient mice injected with control CRC cells into the tail vein showed extensive lung metastasis within 28 d, whereas cells expressing shRNA against cathepsin B exhibited reduced lung colonization (Figure 4A). Cathepsin B silencing also altered the capacity of CRC cells to form tumors in mice as assessed by subcutaneous xenograft assays. HT29 cells induced palpable tumors with a short latency period of 9 d after their injection while downregulation of cathepsin B expression in these cells severely impaired their capacity to grow as tumors (Figure 4B).

Cathepsin B Cleaves the Cell Cycle Inhibitor p27Kip1

In order to verify whether p27Kip1 is in fact a substrate for cathepsin B, both proteins were first overexpressed in 293 T cells and cells subsequently lysed 2 d later for Western blot analysis of their respective expression. As shown in Figure 5A, forced expression of cathepsin B in 293 T cells dose-dependently reduced p27Kip1 protein levels. Next, to determine whether p27Kip1 could be degraded by cathepsin B in vitro, lysates from 293 T cells overexpressing HA-tagged p27Kip1 were incubated with purified cathepsin B and analyzed by Western blot. Figure 5B and C shows that cathepsin B degraded p27Kip1 in a time-dependent manner as visualized by the accumulation of three lower molecular mass species (26, 20, and 12 kDa) in addition to the full-length p27Kip1 protein (see arrows versus arrowhead).

Cathepsin B is capable of endopeptidase, peptidyl-dipeptidase, and carboxydipeptidase activities [18-20]. Cathepsin B also possesses a basic amino acid in the catalytic subsite in position S2 enabling the protease to preferentially split its substrates after Arg–Arg or Lys–Arg or Arg–Lys sequences. At least five of these sequences can be found within the human p27Kip1 sequence (Figure 5D). Therefore, the first amino acid of these doublets was mutated into alanine to test whether it would affect the degradation by cathepsin B. Mutation of arginine 58 (Figure 5E) and lysine 189 (Figure 5F) did not alter the cleavage profile of p27Kip1 by cathepsin B. Mutation of lysine 165 and arginine 194 also had no altering effect (not shown). On the other hand, mutation of arginine 152 into alanine markedly reduced the detection of the 20-kDa fragment (Figure 5E).

The protein stability of wild-type p27Kip1 was then compared to that of the p27Kip1 R152A/Δ189–198 mutant, which is more resistant to cathepsin B cleavage. 293T cells were transiently transfected with either wild-type p27Kip1 or p27Kip1 mutant and subsequently treated with cycloheximide to inhibit protein neosynthesis. Thereafter, cells were lysed at different time intervals in order to analyze protein expression levels of p27Kip1 forms. As shown in Figure 6A, following cycloheximide treatment, protein levels of the p27Kip1 mutant decreased much more slowly than that of wild-type protein. Specifically, 10 h after cycloheximide addition, expression of p27Kip1 protein was clearly decreased while expression of the p27Kip1 mutant remained at control (time 0) levels. Of note, forced expression of cathepsin B in 293 T cells dose-dependently reduced the wild-type form of p27Kip1 protein levels while expression of p27Kip1 R152A/Δ189–198 mutant was only very slightly affected (Figure 6B).

Colocalization of Endogenous p27Kip1 With Cathepsin B Into Lysosomes

As shown in Figure 7A, the anti-cathepsin B antibody confirmed the colocalization of cathepsin B (in green) with the lysosomal acidotropic probe LysoTracker (in red). As expected, most of p27Kip1 staining (in green) was observed in the cell nucleus (Figure 7B). However, certain areas of colocalization were observed between endogenous p27Kip1 (in green) and cathepsin B (in red) (Figure 7B, asterisks). Moreover, Western blot analyses revealed the presence of p27Kip1 protein in lysosome-enriched fractions obtained from differential centrifugation of Caco-2/15 and SW480 cell lysates (Figure 7C and D). These lysosomal fractions were enriched in lysosome-associated membrane protein 1 (LAMP1) and exhibited very low or undetectable levels of the nuclear lamin B protein.

The most extensive literature to date regarding cathepsin B highlights a key role of this protease in the invasiveness and metastasis of various carcinoma cells [3, 8, 10-12]. The present findings demonstrate that cathepsin B has not only a role in facilitating CRC invasion and metastasis, but also in mediating early premalignant processes. Results herein show that cathepsin B promotes anchorage-independent CRC cell growth, which translates in vivo to enhanced tumor growth. In addition, cathepsin B was identified as a new protease capable of proteolytic cleavage of the cell cycle inhibitor p27Kip1. This is especially relevant since the loss of p27Kip1 expression has been strongly associated with aggressive tumor behavior and poor clinical outcome in CRC [22, 23].

These data are reminiscent of the immunohistochemistry data reported by Chan et al. [13] showing that cathepsin B protein was expressed in the vast majority of colon cancers analyzed (558 tumors), which was also independent of tumor stage. The present data also revealed that increased transcription of cathepsin B was associated with the presence of mutations in APC but not in KRAS, thus emphasizing the fact that cathepsin B gene expression is already deregulated in early stages of colorectal carcinoma. Indeed, most CRCs acquire loss-of-function mutations in both copies of the APC gene, resulting in inefficient breakdown of intracellular β-catenin and enhanced nuclear signaling [27]. Given the importance of the Wnt/APC/β-catenin pathway in human tumorigenesis initiation, the present data showing an association between cathepsin B expression and APC mutations are particularly noteworthy.

 

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The Delicate Connection:  IDO (Indolamine 2, 3 dehydrogenase) and Cancer Immunology

Author and Curator: Demet Sag, PhD, CRA, GCP      

Table of Contents:

  1. Abstract
  2. Dual role for IDO
  3. Immune System and IDO
  4. Autoimmune disorders and IDO
  5. Cancer and Ido
  6. Clinical Interventions
  7. Clinical Trials
  8. Future Actions for Molecular Dx and Targeted Therapies:
  9. Conclusion
  10. References

TABLE 1- IDO Clinical Trials

TABLE 2- Kyn induced Genes

TABLE 3 Possible biomarkers and molecular diagnostics targets

TABLE 4: Current Interventions ______________________________________________________________________________________________________________

ABSTRACT:

Overall purpose is to find a method to manipulate IDO for clinical applications, mainly the focus of this review is is cancer prevention and treatment.  The first study proving the connection between IDO and immune response came from, a very natural event, a protection of pregnancy in human. This led to discover that high IDO expression is a common factor in cancer tumors. Thus, attention promoted investigations on IDO’s role in various disease states, immune disorders, transplantation, inflammation, women health, mood disorders.
Many approaches, vaccines and adjuvants are underway to find new immunotherapies by combining the power of DCs in immune response regulation and specific direction of siRNA.  As a result, with this unique qualities of IDO, DCs and siRNA, we orchestrated a novel intervention for immunomodulation of IDO by inhibiting with small interference RNA, called siRNA-IDO-DCvax.  Proven that our DCvax created a delay and regression of tumor growth without changing the natural structure and characterization of DCs in melanoma and breast cancers in vivo. (** The shRNA IDO- DCvax is developed by Regen BioPhrama, San Diego, CA ,  Thomas Ichim, Ph.D, CSO. and David Koos, CEO)

______________________________________________________________________________________________________________

Double-Edged Sword of IDO: The Good and The Bad for Clinical intervention and Developments

IDO almost has a dual role. There is a positive side of high expression of IDO during pregnancy (29; 28; 114), transplants (115; 116; 117; 118; 119), infectious diseases (96) and but this tolerance is negative during autoimmune-disorders (120; 121; 122), tumors of cancer (123; 124; 117; 121; 125; 126; 127) (127), and mood disorders (46). The increased IDO expression has a double-edged sword in human physiology provides a positive role during protection of fetus and grafts after transplantations but becomes a negative factor during autoimmune disorders, cancer, sepsis and mood disorders.

Prevention of allogeneic fetal rejection is possible by tryptophan metabolism (26) rejecting with lack of IDO but allocating if IDO present (29; 28; 114). These studies lead to find “the natural regulation mechanism” for protecting the transplants from graft versus host disease GVHD (128) and getting rid of tumors.

The plasticity of  mammary and uterus during reproduction may hold some more answers to prevent GVHD and tumors of cancer with good understanding of IDO and tryptophan mechanism (129; 130). After allogeneic bone marrow transplants the risk of solid tumor development increased about 80% among 19,229 patients even with a greater risk among patients under 18 years old (117).  The adaptation of tolerance against host mechanism is connected to the IDO expression (131). During implantation and early pregnancy IDO has a role by making CD4+CD25+Foxp3+ regulatory T cells (Tregs) and expressing in DCs and  MQs  (114; 132; 133).

Clonal deletion mechanism prevents mother to react with paternal products since female mice accepted the paternal MHC antigen-expressing tumor graft during pregnancy and rejected three weeks after delivery (134). CTLA-4Ig gene therapy alleviates abortion through regulation of apoptosis and inhibition of spleen lymphocytes (135).  

 Immune System and IDO DCs are the orchestrator of the immune response (56; 57; 58) with list of functions in uptake, processing, and presentation of antigens; activation of effector cells, such as T-cells and NK-cells; and secretion of cytokines and other immune-modulating molecules to direct the immune response. The differential regulation of IDO in distinct DC subsets is widely studied to delineate and correct immune homeostasis during autoimmunity, infection and cancer and the associated immunological outcomes. Genesis of antigen presenting cells (APCs), eventually the immune system, require migration of monocytes (MOs), which is originated in bone marrow. Then, these MOs move from bloodstream to other tissues to become macrophages and DCs (59; 60).

Initiation of immune response requires APCs to link resting helper T-cell with the matching antigen to protect body. DCs are superior to MQs and MOs in their immune action model. When DCs are first described (61) and classified, their role is determined as a highly potent antigen-presenting cell (APC) subset with 100 to 1000-times more effective than macrophages and B-cells in priming T-cells. Both MQs and monocytes phagocytize the pathogen, and their cell structure contains very large nucleus and many internal vesicles. However, there is a nuance between MQ and DCs, since DCs has a wider capacity of stimulation, because MQs activates only memory T cells, yet DCs can activate both naïve and memory T cells.

DCs are potent activators of T cells and they also have well controlled regulatory roles. DC properties determine the regulation regardless of their origin or the subset of the DCs. DCs reacts after identification of the signals or influencers for their inhibitory, stimulatory or regulatory roles, before they express a complex repertoire of positive and negative cytokines, transmembrane proteins and other molecules. Thus, “two signal theory” gains support with a defined rule.  The combination of two signals, their interaction with types of cells and time are critical.

In short, specificity and time are matter for a proper response. When IDO mRNA expression is activated with CTL40 ligand and IFNgamma, IDO results inhibition of T cell production (4).  However, if DCs are inhibited by 1MT, an inhibitor of IDO, the response stop but IgG has no affect (10).  In addition, if the stimulation is started by a tryptophan metabolite, which is downstream of IDO, such as 3-hydroxyantranilic or quinolinic acids, it only inhibits Th1 but not Th2 subset of T cells (62).

Furthermore, inclusion of signal molecules, such as Fas Ligand, cytochrome c, and pathways also differ in the T cell differentiation mechanisms due to combination, time and specificity of two-signals.  The co-culture experiments are great tool to identify specific stimuli in disease specific microenvironment (63; 12; 64) for discovering the mechanism and interactions between molecules in gene regulation, biochemical mechanism and physiological function during cell differentiation.

As a result, the simplest differential cell development from the early development of DCs impact the outcome of the data. For example, collection of MOs from peripheral blood mononuclear cells (PBMCs) with IL4 and GM-CSF leads to immature DCs (iDCs). On next step, treatment of iDCs with tumor necrosis factor (TNF) or other plausible cytokines (TGFb1, IFNgamma, IFNalpha,  IFNbeta, IL6 etc.) based on the desired outcome differentiate iDCs  into mature DCs (mDCs). DCs live only up to a week but MOs and generated MQs can live up to a month in the given tissue. B cells inhibit T cell dependent immune responses in tumors (65).

AutoImmune Disorders:

The Circadian Clock Circuitry and the AHR

The balance of IDO expression becomes necessary to prevent overactive immune response self-destruction, so modulation in tryptophan and NDA metabolisms maybe essential.  When splenic IDO-expressing CD11b (+) DCs from tolerized animals applied, they suppressed the development of arthritis, increased the Treg/Th17 cell ratio, and decreased the production of inflammatory cytokines in the spleen (136).

The role of Nicotinamide prevention on type 1 diabetes and ameliorates multiple sclerosis in animal model presented with activities of  NDAs stimulating GPCR109a to produce prostaglandins to induce IDO expression, then these PGEs and PGDs converted to the anti-inflammatory prostaglandin, 15d-PGJ(2) (137; 138; 139).  Thus, these events promotes endogenous signaling mechanisms involving the GPCRs EP2, EP4, and DP1 along with PPARgamma. (137).

Modulating the immune response at non-canonical at canonocal pathway while keeping the non-canonical Nf-KB intact may help to mend immune disorders. As a result, the targeted blocking in canonical at associated kinase IKKβ and leaving non-canonocal Nf-kB pathway intact, DCs tips the balance towards immune supression. Hence, noncanonical NF-κB pathway for regulatory functions in DCs required effective IDO induction, directly or indirectly by endogenous ligand Kyn and negative regulation of proinflammatory cytokine production. As a result, this may help to treat autoimmune diseases such as rheumatoid arthritis, type 1 diabetes, inflammatory bowel disease, and multiple sclerosis, or allergy or transplant rejection.

While the opposite action needs to be taken during prevention of tumors, that is inhibition of non-canonical pathway.  Inflammation induces not only relaxation of veins and lowering blood pressure but also stimulate coagulopathies that worsen the microenvironment and decrease survival rate of patients after radio or chemotherapies.Cancer Generating tumor vaccines and using adjuvants underway (140).

Clinical correlation and genetic responses also compared in several studies to diagnose and target the system for cancer therapies (127; 141; 131).  The recent surveys on IDO expression and human cancers showed that IDO targeting is a candidate for cancer therapy since IDO expression recruiting Tregs, downregulates MHC class I and creating negative immune microenvironment for protection of development of tumors (125; 27; 142).  Inhibition of IDO expression can make advances in immunotherapy and chemotherapy fields (143; 125; 131; 144).

IDO has a great importance on prevention of cancer development (126). There are many approaches to create the homeostasis of immune response by Immunotherapy.  However, given the complexity of immune regulations, immunomodulation is a better approach to correct and relieve the system from the disease.  Some of the current IDO targeted immunotherapy or immmunomodulations with RNA technology for cancer prevention (145; 146; 147; 148; 149; 150) or applied on human or animals  (75; 151; 12; 115; 152; 9; 125) or chemical, (153; 154) or  radiological (155).  The targeted cell type in immune system generally DCs, monocytes (94)T cells (110; 156)and neutrophils (146; 157). On this paper, we will concentrate on DCvax on cancer treatments.

 T-reg, regulatory T cells; Th, T helper; CTLA-4, cytotoxic T lymphocyte-associated antigen 4; TCR, T cell receptor; IDO, indoleamine 2,3-dioxygenase. (refernece: http://www.pnas.org/content/101/28/10398/suppl/DC)

T-reg, regulatory T cells; Th, T helper; CTLA-4, cytotoxic T lymphocyte-associated antigen 4; TCR, T cell receptor; IDO, indoleamine 2,3-dioxygenase. (refernece: http://www.pnas.org/content/101/28/10398/suppl/DC)

IDO and the downstream enzymes in tryptophan pathway produce a series of immunosuppressive tryptophan metabolites that may lead into Tregs proliferation or increase in T cell apoptosis (62; 16; 27; 158), and some can affect NK cell function (159).

The interesting part of the mechanism is even without presence of IDO itself, downstream enzymes of IDO in the kynurenine tryptophan degradation still show immunosuppressive outcome (160; 73) due to not only Kyn but also TGFbeta stimulated long term responses. DC vaccination with IDO plausible (161) due to its power in immune response changes and longevity in the bloodstream for reversing the system for Th17 production (162).

Clinical Interventions are taking advantage of the DC’s central role and combining with enhancing molecules for induction of immunity may overcome tolerogenic DCs in tumors of cancers (163; 164).

The first successful application of DC vaccine used against advanced melanoma after loading DCs with tumor peptides or autologous cell lysate in presence of adjuvants keyhole limpet hematocyanin (KLH) (165).  Previous animal and clinical studies show use of DCs against tumors created success (165; 166; 167) as well as some problems due to heterogeneity of DC populations in one study supporting tumor growth rather than diminishing (168).

DC vaccination applied onto over four thousand clinical trial but none of them used siRNA-IDO DC vaccination method. Clinical trials evaluating DCs loaded ex vivo with purified TAAs as an anticancer immunotherapeutic interventions also did not include IDO (Table from (169). This table presented the data from 30 clinical trials, 3 of which discontinued, evaluating DCs loaded ex vivo with TAAs as an anticancer immunotherapy for 12 types of cancer [(AML(1), Breast cancer (4), glioblastoma (1), glioma (2), hepatocellular carcinoma (1), hematological malignancies (1), melanoma (6), neuroblastoma sarcoma (2), NSCLC (1), ovarian cancer (3), pancreatic cancer (3), prostate cancer (10)] at phase I, II or I/II.

Tipping the balance between Treg and Th17 ratio has a therapeutic advantage for restoring the health that is also shown in ovarian cancer by DC vaccination with adjuvants (161).  This rebalancing of the immune system towards immunogenicity may restore Treg/Th17 ratio (162; 170) but it is complicated. The stimulation of IL10 and IL12 induce Treg produce less Th17 and inhibiting CTL activation and its function (76; 171; 172) while animals treated with anti-TGFb before vaccination increase the plasma levels of IL-15 for tumor specific T cell survival in vivo (173; 174) ovarian cancer studies after human papilloma virus infection present an increase of IL12 (175).

Opposing signal mechanism downregulates the TGFb to activate CTL and Th1 population with IL12 and IL15 expression (162; 173).  The effects of IL17 on antitumor properties observed by unique subset of CD4+ T cells (176) called also CD8+ T cells secrete even more IL17 (177).

Using cytokines as adjuvants during vaccination may improve the efficacy of vaccination since cancer vaccines unlike infections vaccines applied after the infection or disease started against the established adoptive immune response.  Adjuvants are used to improve the responses of the given therapies commonly in immunotherapy applications as a combination therapy (178).

Enhancing cancer vaccine efficacy via modulation of the microenvironment is a plausible solution if only know who are the players.  Several molecules can be used to initiate and lengthen the activity of intervention to stimulate IDO expression without compromising the mechanism (179).  The system is complicated so generally induction is completed ex-vivo stimulation of DCs in cell lysates, whole tumor lysates, to create the microenvironment and natural stimulatory agents. Introduction of molecules as an adjuvants on genetic regulation on modulation of DCs are critical, because order and time of the signals, specific location/ tissue, and heterogeneity of personal needs (174; 138; 180). These studies demonstrated that IL15 with low TGFb stimulates CTL and Th1, whereas elevated TGFb with IL10 increases Th17 and Tregs in cancer microenvironments.

IDO and signaling gene regulation

For example Ret-peptide antitumor vaccine contains an extracellular fragment of Ret protein and Th1 polarized immunoregulator CpG oligonucleotide (1826), with 1MT, a potent inhibitor of IDO, brought a powerful as well as specific cellular and humoral immune responses in mice (152).

The main idea of choosing Ret to produce vaccine in ret related carcinomas fall in two criterion, first choosing patients self-antigens for cancer therapy with a non-mutated gene, second, there is no evidence of genetic mutations in Ret amino acids 64-269. Demonstration of proliferating hemangiomas, benign endothelial tumors and often referred as hemangiomas of infancy appearing at head or neck, express IDO and slowly regressed as a result of immune mediated process.

After large scale of genomic analysis show insulin like growth factor 2 as the key regulator of hematoma growth (Ritter et al. 2003). We set out to develop new technology with our previous expertise in immunotherapy and immunomodulation (181; 182; 183; 184), correcting Th17/Th1 ratio (185), and siRNA technology (186; 187).  We developed siRNA-IDO-DCvax. Patented two technologies “Immunomodulation using Altered DCs (Patent No: US2006/0165665 A1) and Method of Cancer Treatments using siRNA Silencing (Patent No: US2009/0220582 A1).

In melanoma cancer DCs were preconditioned with whole tumor lysate but in breast cancer model pretreatment completed with tumor cell lysate before siRNA-IDO-DCvax applied. Both of these studies was a success without modifying the autanticity of DCs but decreasing the IDO expression to restore immunegenity by delaying tumor growth in breast cancer (147) and in melanoma (188).  Thus, our DCvax specifically interfere with Ido without disturbing natural structure and content of the DCs in vivo showed that it is possible to carry on this technology to clinical applications.

Furthermore, our method of intervention is more sophisticated since it has a direct interaction mechanism with ex-vivo DC modulation without creating long term metabolism imbalance in Trp/Kyn metabolite mechanisms since the action is corrective and non-invasive.

There were several reasons.

First, prevention of tumor development studies targeting non-enzymatic pathway initiated by pDCs conditioned with TGFbeta is specific to IDO1 (189).

Second, IDO upregulation in antigen presenting cells allowing metastasis show that most human tumors express IDO at high levels (123; 124).

Third, tolerogenic DCs secretes several molecules some of them are transforming growth factor beta (TGFb), interleukin IL10), human leukocyte antigen G (HLA-G), and leukemia inhibitory factor (LIF), and non-secreted program cell death ligand 1 (PD-1 L) and IDO, indolamine 2.3-dioxygenase, which promote tumor tolerance. Thus, we took advantage of DCs properties and Ido specificity to prevent the tolerogenicity with siRNA-IDO DC vaccine in both melanoma and breast cancer.

Fourth, IDO expression in DCs make them even more potent against tumor antigens and create more T cells against tumors. IDOs are expressed at different levels by both in broad range of tumor cells and many subtypes of DCs including monocyte-derived DCs (10), plasmacytoid DCs (142), CD8a+ DCs (190), IDO compotent DCs (17), IFNgamma-activated DCs used in DC vaccination.  These DCs suppress immune responses through several mechanisms for induction of apoptosis towards activated T cells (156) to mediate antigen-specific T cell anergy in vivo (142) and for enhancement of Treg cells production at sites of vaccination with IDO-positive DCs+ in human patients (142; 191; 192; 168; 193; 194). If DCs are preconditioned with tumor lysate with 1MT vaccination they increase DCvax effectiveness unlike DCs originated from “normal”, healthy lysate with 1MT in pancreatic cancer (195).  As a result, we concluded that the immunesupressive effect of IDO can be reversed by siRNA because Treg cells enhances DC vaccine-mediated anti-tumor-immunity in cancer patients.

Gene silencing is a promising technology regardless of advantages simplicity for finding gene interaction mechanisms in vitro and disadvantages of the technology is utilizing the system with specificity in vivo (186; 196).  siRNA technology is one of the newest solution for the treatment of diseases as human genomics is only producing about 25,000 genes by representing 1% of its genome. Thus, utilizing the RNA open the doors for more comprehensive and less invasive effects on interventions. Thus this technology is still improving and using adjuvants. Silencing of K-Ras inhibit the growth of tumors in human pancreatic cancers (197), silencing of beta-catenin in colon cancers causes tumor regression in mouse models (198), silencing of vascular endothelial growth factor (VGEF) decreased angiogenesis and inhibit tumor growth (199).

Combining siRNA IDO and DCvax from adult stem cell is a novel technology for regression of tumors in melanoma and breast cancers in vivo. Our data showed that IDO-siRNA reduced tumor derived T cell apoptosis and tumor derived inhibition of T cell proliferation.  In addition, silencing IDO made DCs more potent against tumors since treated or pretreated animals showed a delay or decreased the tumor growth (188; 147)

 

Clinical Trials:

First FDA approved DC-based cancer therapies for treatment of hormone-refractory prostate cancer as autologous cellular immunotherapy (163; 164).  However, there are many probabilities to iron out for a predictive outcome in patients.

Table 2 demonstrates the current summary of clinical trials report.  This table shows 38 total studies specifically Ido related function on cancer (16), eye (3), surgery (2), women health (4), obesity (1), Cardiovascular (2), brain (1), kidney (1), bladder (1), sepsis shock (1), transplant (1),  nervous system and behavioral studies (4), HIV (1) (Table 4).  Among these only 22 of which active, recruiting or not yet started to recruit, and 17 completed and one terminated.

Most of these studies concentrated on cancer by the industry, Teva GTC ( Phase I traumatic brain injury) Astra Zeneca (Phase IV on efficacy of CRESTOR 5mg for cardiovascular health concern), Incyte corporation (Phase II ovarian cancer) NewLink Genetics Corporation Phase I breast/lung/melanoma/pancreatic solid tumors that is terminated; Phase II malignant melanoma recruiting, Phase II active, not recruiting metastatic breast cancer, Phase I/II metastatic melanoma, Phase I advanced malignancies) , HIV (Phase IV enrolling by invitation supported by Salix Corp-UC, San Francisco and HIV/AIDS Research Programs).

Many studies based on chemotherapy but there are few that use biological methods completed study with  IDO vaccine peptide vaccination for Stage III-IV non-small-cell lung cancer patients (NCT01219348), observational study on effect of biological therapy on biomarkers in patients with untreated hepatitis C, metastasis melanoma, or Crohn disease by IFNalpha and chemical (ribavirin, ticilimumab (NCT00897312), polymorphisms of patients after 1MT drug application in treating patients with metastatic or unmovable refractory solid tumors by surgery (NCT00758537), IDO expression analysis on MSCs (NCT01668576), and not yet recruiting intervention with adenovirus-p53 transduced dendric cell vaccine , 1MT , radiation, Carbon C 11 aplha-methyltryptophan- (NCT01302821).

Among the registered clinical trials some of them are not interventional but  observational and evaluation studies on Trp/Kyn ratio (NCT01042847), Kyn/Trp ratio (NCT01219348), Kyn levels (NCT00897312, NCT00573300),  RT-PCR analysis for Kyn metabolism (NCT00573300, NCT00684736, NCT00758537), and intrinsic IDO expression of mesenchymal stem cells in lung transplant with percent inhibition of CD4+ and CD8+ T cell proliferation toward donor cells (NCT01668576), determining polymorphisms (NCT00426894). These clinical trials/studies are immensely valuable to understand the mechanism and route of intervention development with the data collected from human populations   

Future Actions for Molecular Dx and Targeted Therapies:

Viable tumor environment. Tumor survival is dependent upon an exquisite interplay between the critical functions of stromal development and angiogenesis, local immune suppression and tumor tolerance, and paradoxical inflammation. TEMs: TIE-2 expressing monocytes; “M2” TAMs: tolerogenic tumor-associated macrophages; MDSCs: myeloid-derived suppressor cells; pDCs: plasmacytoid dendritic cells; co-stim.: co-stimulation; IDO: indoleamine 2,3-dioxygenase; VEGF: vascular endothelial growth factor; EGF: epidermal growth factor; MMP: matrix metaloprotease; IL: interleukin; TGF-β: transforming growth factor-beta; TLRs: toll-like receptors.  (reference: http://www.hindawi.com/journals/cdi/2012/937253/fig1/)

Viable tumor environment. Tumor survival is dependent upon an exquisite interplay between the critical functions of stromal development and angiogenesis, local immune suppression and tumor tolerance, and paradoxical inflammation. TEMs: TIE-2 expressing monocytes; “M2” TAMs: tolerogenic tumor-associated macrophages; MDSCs: myeloid-derived suppressor cells; pDCs: plasmacytoid dendritic cells; co-stim.: co-stimulation; IDO: indoleamine 2,3-dioxygenase; VEGF: vascular endothelial growth factor; EGF: epidermal growth factor; MMP: matrix metaloprotease; IL: interleukin; TGF-β: transforming growth factor-beta; TLRs: toll-like receptors. (reference: http://www.hindawi.com/journals/cdi/2012/937253/fig1/)

Current survival or response rate is around 40 to 50 % range.  By using specific cell type, selected inhibition/activation sequence based on patient’s genomic profile may improve the efficacy of clinical interventions on cancer treatments. Targeted therapies for specific gene regulation through signal transduction is necessary but there are few studies with genomics based approach.

On the other hand, there are surveys, observational or evaluations (listed in clinical trials section) registered with www.clinicaltrials.gov that will provide a valuable short-list of molecules.  Preventing stimulation of Ido1 as well as Tgfb-1gene expression by modulating receptor mediated phosphorylation between TGFb/SMAD either at Mad-Homology 1 (MH1) or Mad-Homology 1 (MH2) domains maybe possible (79; 82; 80). Within Smads are the conserved Mad-Homology 1 (MH1) domain, which is a DNA binding module contains tightly bound Zinc atom.

Smad MH2 domain is well conserved and one the most diverse protein-signal interacting molecule during signal transduction due to two important Serine residues located extreme distal C-termini at Ser-Val-Ser in Smad 2 or at pSer-X-PSer in RSmads (80). Kyn activated orphan G protein–coupled receptor, GPR35 with unknown function with a distinct expression pattern that collides with IDO sites since its expression at high levels of the immune system and the gut (63) (200; 63).  

The first study to connect IDO with cancer shows that group (75).  The directly targeting to regulate IDO expression is another method through modulating ISREs in its promoter with RNA-peptide combination technology. Indirectly, IDO can be regulated through Bin1 gene expression control over IDO since Bin1 is a negative regulator of IDO and prevents IDO expression.  IDO is under negative genetic control of Bin1, BAR adapter–encoding gene Bin1 (also known as Amphiphysin2). Bin1 functions in cancer suppression since attenuation of Bin1 observed in many human malignancies (141; 201; 202; 203; 204; 205; 206) .  Null Bin-/- mice showed that when there is lack of Bin1, upregulation of IDO through STAT1- and NF-kB-dependent expression of IDO makes tumor cells to escape from T cell–dependent antitumor immunity.

This pathway lies in non-enzymatic signal transducer function of IDO after stimulation of DCs by TGFb1.  The detail study on Bin1 gene by alternative spicing also provided that Bin1 is a tumor suppressor.  Its activities also depends on these spliced outcome, such as  Exon 10, in muscle, in turn Exon 13 in mice has importance in role for regulating growth when Bin1 is deleted or mutated C2C12 myoblasts interrupted due to its missing Myc, cyclinD1, or growth factor inhibiting genes like p21WAF1 (207; 208).

On the other hand alternative spliced Exon12A contributing brain cell differentiation (209; 210). Myc as a target at the junction between IDO gene interaction and Trp metabolism.  Bin1 interacts with Myc either early-dependent on Myc or late-independent on Myc, when Myc is not present. This gene regulation also interfered by the long term signaling mechanism related to Kynurenine (Kyn) acting as an endogenous ligand to AHR in Trp metabolite and TGFb1 and/or IFNalpha and IFNbeta up regulation of DCs to induce IDO in noncanonical pathway for NF-kB and myc gene activations (73; 74).  Hence, Trp/Kyn, Kyn/Trp, Th1/Th17 ratios are important to be observed in patients peripheral blood. These direct and indirect gene interactions place Bin1 to function in cell differentiation (211; 212; 205).

Regulatory T-cel generation via reverse and non-canonical signaliing to pDCs

Table 3 contains the microarray analysis for Kyn affect showed that there are 25 genes affected by Kyn, two of which are upregulated and 23 of them downregulated (100). This list of genes and additional knowledge based on studies creating the diagnostics panel with these genes as a biomarker may help to analyze the outcomes of given interventions and therapies. Some of these molecules are great candidate to seek as an adjuvant or co-stimulation agents.  These are myc, NfKB at IKKA, C2CD2, CREB3L2, GPR115, IL2, IL8, IL6, and IL1B, mir-376 RNA, NFKB3, TGFb, RelA, and SH3RF1. In addition, Lip, Fox3P, CTLA-4, Bin1, and IMPACT should be monitored.

In addition, Table 4 presents the other possible mechanisms. The highlights of possible target/biomarkers are specific TLRs, conserved sequences of IDO across its homologous structures, CCR6, CCR5, RORgammat, ISREs of IDO, Jak, STAT, IRFs, MH1 and MH2 domains of Smads. Endothelial cell coagulation activation mechanism and pDC maturation or immigration from lymph nodes to bloodstream should marry to control not only IDO expression but also genesis of preferred DC subsets. Stromal mesenchymal cells are also activated by these modulation at vascular system and interferes with metastasis of cancer. First, thrombin (human factor II) is a well regulated protein in coagulation hemostasis has a role in cell differentiation and angiogenesis.

Protein kinase activated receptors (PARs), type of GPCRs, moderate the actions. Second, during hematopoietic response endothelial cells produce hematopoietic growth factors (213; 214). Third, components of bone marrow stroma cells include monocytes, adipocytes, and mesenchymal stem cells (215). As a result, addressing this issue will prevent occurrence of coagulapathologies, namely DIC, bleeding, thrombosis, so that patients may also improve response rate towards therapies. Personal genomic profiles are powerful tool to improve efficacy in immunotherapies since there is an influence of age (young vs. adult), state of immune system (innate vs. adopted or acquired immunity). Table 5 includes some of the current studies directly with IDO and indirectly effecting its mechanisms via gene therapy, DNA vaccine, gene silencing and adjuvant applications as an intervention method to prevent various cancer types.

CONCLUSION

IDO has a confined function in immune system through complex interactions to maintain hemostasis of immune responses. The genesis of IDO stem from duplication of bacterial IDO-like genes.  Inhibition of microbial infection and invasion by depleting tryptophan limits and kills the invader but during starvation of trp the host may pass the twilight zone since trp required by host’s T cells.  Thus, the host cells in these small pockets adopt to new microenvironment with depleted trp and oxygen poor conditions. Hence, the cell metabolism differentiate to generate new cellular structure like nodules and tumors under the protection of constitutively expressed IDO in tumors, DCs and inhibited T cell proliferation.

On the other hand, having a dichotomy in IDO function can be a potential limiting factor that means is that IDOs impact on biological system could be variable based on several issues such as target cells, IDO’s capacity, pathologic state of the disease and conditions of the microenvironment. Thus, close monitoring is necessary to analyze the outcome to prevent conspiracies since previous studies generated paradoxical results.

Current therapies through chemotherapies, radiotherapies are costly and effectiveness shown that the clinical interventions require immunotherapies as well as coagulation and vascular biology manipulations for a higher efficacy and survival rate in cancer patients. Our siRNA and DC technologies based on stem cell modulation will provide at least prevention of cancer development and hopefully prevention in cancer.

11.       References

1. Biochemistry of tryptophan in health and disease. BenderDA. 1983, Mol Aspects Med , pp. 6:101–197.

2. Molecular insights into substrate recognition and catalysis by indolamine 2,3-dioxygenase. Forouhar, F., Anderson, R., Mowat, C.F, et al. 2006, PNAS, pp. vol. 104, no:2, 473-478.

3. Importance of the Two Interferon-stimulated Response Element. Konan KV, Taylor, MW. 1996, J. Biol. Chem.-, pp. 19140-5.

4. Induction of indolamine 2,3 dioxygenase: A mechanism of the anti-tumor activity of interferon gamma. Ozaki, Y., Edelstein, M.P., Duch, D.S. 1998, PNAS USA., pp. vol:85, 1242-1246.

5. Localization of the human indoleamine 2,3-dioxygenase (IDO) gene to the pericentromeric region of human chromosome . Burkin, D. J., Kimbro, K. S., Barr, B. L., Jones, C., Taylor, M. W., Gupta, S. L. 1993, Genomics , pp. 17: 262-263.

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Liver Endoplasmic Reticulum Stress and Hepatosteatosis

Larry H Bernstein, MD, FCAP

 

1. Absence of adipose triglyceride lipase protects from hepatic endoplasmic reticulum stress in mice.

Fuchs CD, Claudel T, Kumari P, Haemmerle G, et al.
LabExpMol Hepatology, Medical Univ of Graz, Austria.
Hepatology. 2012 Jul;56(1):270-80.   http://dx.doi.org/10.1002/hep.25601. Epub 2012 May 29.

Nonalcoholic fatty liver disease (NAFLD) is characterized by

  • triglyceride (TG) accumulation and
  • endoplasmic reticulum (ER) stress.

Fatty acids (FAs) may trigger ER stress, therefore,

  •  the absence of adipose triglyceride lipase (ATGL/PNPLA2)-
    • the main enzyme for intracellular lipolysis,
  • releasing FAs, and
  • closest homolog to adiponutrin (PNPLA3)

recently implicated in the pathogenesis of NAFLD-

  • could protect against hepatic ER stress.

Wild-type (WT) and ATGL knockout (KO) mice

  •  were challenged with tunicamycin (TM) to induce ER stress.

Markers of hepatic

  •  lipid metabolism,
  • ER stress, and
  • inflammation were explored
    • for gene expression by
    •  serum biochemistry,
    • hepatic TG and FA profiles,
    • liver histology,
    • cell-culture experiments were performed in Hepa1.6 cells
  • after the knockdown of ATGL before FA and TM treatment.

TM increased hepatic TG accumulation in ATGL KO, but not in WT mice. Lipogenesis and β-oxidation
were repressed at the gene-expression level
(sterol regulatory element-binding transcription factor 1c,
fatty acid synthase, acetyl coenzyme A carboxylase 2, and carnitine palmitoyltransferase 1 alpha) in
both WT and ATGL KO mice. Genes for very-low-density lipoprotein (VLDL) synthesis (microsomal
triglyceride transfer protein and apolipoprotein B)

  •  were down-regulated by TM in WT
  • and even more in ATGL KO mice,
  • which displayed strongly reduced serum VLDL cholesterol levels.

ER stress markers were induced exclusively in TM-treated WT, but not ATGL KO, mice:

  •  glucose-regulated protein,
  • C/EBP homolog protein,
  • spliced X-box-binding protein,
  • endoplasmic-reticulum-localized DnaJ homolog 4, and
  • inflammatory markers Tnfα and iNos.

Total hepatic FA profiling revealed a higher palmitic acid/oleic acid (PA/OA) ratio in WT mice.
Phosphoinositide-3-kinase inhibitor-

  • known to be involved in FA-derived ER stress and
  • blocked by OA-
  • was increased in TM-treated WT mice only.

In line with this, in vitro OA protected hepatocytes from TM-induced ER stress. Lack of ATGL may protect from
hepatic ER stress through alterations in FA composition. ATGL could constitute a new therapeutic strategy
to target ER stress in NAFLD.
PMID: 22271167 Diabetes Obes Metab. 2010 Oct;12 Suppl 2:83-92.
http://dx.doi.org/10.1111/j.1463-1326.2010.01275.x.

2. Hepatic steatosis: a role for de novo lipogenesis and the transcription factor SREBP-1c.
Ferré P, Foufelle F. INSERM, and Université Pierre et Marie Curie-Paris, Paris, France.    PMID: 21029304

Excessive availability of plasma fatty acids and lipid synthesis from glucose (lipogenesis) are important determinants of steatosis.
Lipogenesis is an insulin- and glucose-dependent process that is under the control of specific transcription factors,

Insulin induces the maturation of SREBP-1c in the endoplasmic reticulum (ER).

  • SREBP-1c in turn activates glycolytic gene expression,
    • allowing glucose metabolism, and
    • lipogenic genes in conjunction with ChREBP.

Lipogenesis activation in the liver of obese markedly insulin-resistant steatotic rodents is then paradoxical.
It appears the activation of SREBP-1c and thus of lipogenesis is

  •  secondary in the steatotic liver to an ER stress.

The ER stress activates the

  •  cleavage of SREBP-1c independent of insulin,
  • explaining the paradoxical stimulation of lipogenesis
  • in an insulin-resistant liver.

Inhibition of the ER stress in obese rodents

  •  decreases SREBP-1c activation and lipogenesis and
  • improves markedly hepatic steatosis and insulin sensitivity.
  • ER is thus worth considering as a potential therapeutic target for steatosis and metabolic syndrome.

3. SREBP-1c transcription factor and lipid homeostasis: clinical perspective
Ferré P, Foufelle F
Inserm, Centre de Recherches Biomédicales des Cordeliers, Paris, France.
Horm Res. 2007;68(2):72-82. Epub 2007 Mar 5. PMID:17344645

Insulin has long-term effects on glucose and lipid metabolism through its control on the expression of specific genes.
In insulin sensitive tissues and particularly in the liver,

  •  the transcription factor sterol regulatory element binding protein-1c (SREBP-1c) transduces the insulin signal, which is
  • synthetized as a precursor in the membranes of the endoplasmic reticulum
  • which requires post-translational modification to yield its transcriptionally active nuclear form.

Insulin activates the transcription and the proteolytic maturation of SREBP-1c, which induces the

  •  expression of a family of genes
  • involved in glucose utilization and fatty acid synthesis and
  • can be considered as a thrifty gene.

Since a high lipid availability is

  •  deleterious for insulin sensitivity and secretion,
  • a role for SREBP-1c in dyslipidaemia and type 2 diabetes
  • has been considered in genetic studies.

SREBP-1c could also participate in

  •  hepatic steatosis observed in humans
  • related to alcohol consumption and
  • hyperhomocysteinemia
  • concomitant with a ER-stress and
  • insulin-independent SREBP-1c activation.

4. Hepatic steatosis: a role for de novo lipogenesis and the transcription factor SREBP-1c
Ferré P, Foufelle F
INSERM, Centre de Recherches des Cordeliers and Université Pierre et Marie Curie-Paris, Paris, France.
Diabetes Obes Metab. 2010 Oct;12 Suppl 2:83-92. PMID: 21029304
http://dx.doiorg/10.1111/j.1463-1326.2010.01275.x.

Lipogenesis in liver steatosis is

  •  an insulin- and glucose-dependent process
  • under the control of specific transcription factors,
  • sterol regulatory element binding protein 1c (SREBP-1c),
  • activated by insulin and carbohydrate response element binding protein (ChREBP)

Insulin induces the maturation of SREBP-1c in the endoplasmic reticulum (ER).
SREBP-1c in turn activates glycolytic gene expression, allowing –

  •  glucose metabolism in conjunction with ChREBP.

activation of SREBP-1c and lipogenesis is secondary in the steatotic liver to ER stress, which

  •  activates the cleavage of SREBP-1c independent of insulin,
  • explaining the stimulation of lipogenesis in an insulin-resistant liver.
  • Inhibition of the ER stress in obese rodents decreases SREBP-1c activation and improves
  • hepatic steatosis and insulin sensitivity.

ER is thus a new partner in steatosis and metabolic syndrome

5. Pharmacologic ER stress induces non-alcoholic steatohepatitis in an animal model
Jin-Sook Leea, Ze Zhenga, R Mendeza, Seung-Wook Hac, et al.
Wayne State University SOM, Detroit, MI
Toxicology Letters 20 May 2012; 211(1):29–38      http://dx.doi.org/10.1016/j.toxlet.2012.02.017

Endoplasmic reticulum (ER) stress refers to a condition of

  •  accumulation of unfolded or misfolded proteins in the ER lumen, which is known to
  • activate an intracellular stress signaling termed
  • Unfolded Protein Response (UPR).

A number of pharmacologic reagents or pathophysiologic stimuli

  •  can induce ER stress and activation of the UPR signaling,
  • leading to alteration of cell physiology that is
  • associated with the initiation and progression of a variety of diseases.

Non-alcoholic steatohepatitis (NASH), characterized by hepatic steatosis and inflammation, has been considered the
precursor or the hepatic manifestation of metabolic disease. In this study, we delineated the

  • toxic effect and molecular basis
  • by which pharmacologic ER stress,
  • induced by a bacterial nucleoside antibiotic tunicamycin (TM),
  • promotes NASH in an animal model.

Mice of C57BL/6J strain background were challenged with pharmacologic ER stress by intraperitoneal injection of TM. Upon TM injection,

  •  mice exhibited a quick NASH state characterized by
  • hepatic steatosis and inflammation.

TM-treated mice exhibited an increase in –

  •  hepatic triglycerides (TG) and a –
  • decrease in plasma lipids, including
  • plasma TG,
  • plasma cholesterol,
  • high-density lipoprotein (HDL), and
  • low-density lipoprotein (LDL),

In response to TM challenge,

  •  cleavage of sterol responsive binding protein (SREBP)-1a and SREBP-1c,
  •  the key trans-activators for lipid and sterol biosynthesis,
  • was dramatically increased in the liver.

Consistent with the hepatic steatosis phenotype, expression of

  •  some key regulators and enzymes in de novo lipogenesis and lipid droplet formation was up-regulated,
  • while expression of those involved in lipolysis and fatty acid oxidation was down-regulated
  • in the liver of mice challenged with TM.

TM treatment also increased phosphorylation of NF-κB inhibitors (IκB),

  •  leading to the activation of NF-κB-mediated inflammatory pathway in the liver.

Our study not only confirmed that pharmacologic ER stress is a strong “hit” that triggers NASH, but also demonstrated

  •  crucial molecular links between ER stress,
  • lipid metabolism, and
  • inflammation in the liver in vivo.

Highlights
► Pharmacologic ER stress induced by tunicamycin (TM) induces a quick NASH state in vivo.
► TM leads to dramatic increase in cleavage of sterol regulatory element-binding protein in the liver.
► TM up-regulates lipogenic genes, but down-regulates the genes in lipolysis and FA oxidation.
► TM activates NF-κB and expression of genes encoding pro-inflammatory cytokines in the liver.
Abbreviations
ER, endoplasmic reticulum; TM, tunicamycin; NASH, non-alcoholic steatohepatitis; NAFLD,
non-alcoholic fatty liver disease; TG, triglycerides; SREBP, sterol responsive binding protein;
NF-κB, activation of nuclear factor-kappa B; IκB, NF-κB inhibitor
Keywords: ER stress; Non-alcoholic steatohepatitis; Tunicamycin; Lipid metabolism; Hepatic inflammation
Figures and tables from this article:

Fig. 1. TM challenge alters lipid profiles and causes hepatic steatosis in mice. (A) Quantitative real-time RT-PCR analysis of liver mRNA isolated from mice challenged with TM or vehicle control. Total liver mRNA was isolated at 8 h or 30 h after injection with vehicle or TM (2 μg/g body weight) for real-time RT-PCR analysis. Expression values were normalized to β-actin mRNA levels. Fold changes of mRNA are shown by comparing to one of the control mice. Each bar denotes the mean ± SEM (n = 4 mice per group); **P < 0.01. Edem1, ER degradation enhancing, mannosidase alpha-like 1. (B) Oil-red O staining of lipid droplets in the livers of the mice challenged with TM or vehicle control (magnification: 200×). (C) Levels of TG in the liver tissues of the mice challenged with TM or vehicle control. (D) Levels of plasma lipids in the mice challenged with TM or vehicle control. TG, triglycerides; TC, total plasma cholesterol; HDL, high-density lipoproteins; VLDL/LDL, very low and low density lipoproteins. For C and D, each bar denotes mean ± SEM (n = 4 mice per group); *P < 0.05; **P < 0.01.

 Fhttp://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr1.jpgigure options

Fig. 2. TM challenge leads to a quick NASH state in mice. (A) Histological examination of liver tissue sections of the mice challenged with TM (2 μg/g body weight) or vehicle control. Upper panel, hematoxylin–eosin (H&E) staining of liver tissue sections; the lower panel, Sirius staining of collagen deposition of liver tissue sections (magnification: 200×). (B) Histological scoring for NASH activities in the livers of the mice treated with TM or vehicle control. The grade scores were calculated based on the scores of steatosis, hepatocyte ballooning, lobular and portal inflammation, and Mallory bodies. The stage scores were based on the liver fibrosis. Number of mice examined is given in parentheses. Mean ± SEM values are shown. P-values were calculated by Mann–Whitney U-test.

 http://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr2.jpg

Fig. 3. TM challenge significantly increases levels of cleaved/activated forms of SREBP1a and SREBP1c in the liver. Western blot analysis of protein levels of SREBP1a (A) and SREBP1c (B) in the liver tissues from the mice challenged with TM (2 μg/g body weight) or vehicle control. Levels of GAPDH were included as internal controls. For A and B, the values below the gels represent the ratios of mature/cleaved SREBP signal intensities to that of SREBP precursors. The graph beside the images showed the ratios of mature/cleaved SREBP to precursor SREBP in the liver of mice challenged with TM or vehicle. The protein signal intensities shown by Western blot analysis were quantified by NIH imageJ software. Each bar represents the mean ± SEM (n = 3 mice per group); **P < 0.01. SREBP-p, SREBP precursor; SREBP-m, mature/cleaved SREBP.

 http://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr3.jpg

Fig. 4. TM challenge up-regulates expression of genes involved in lipogenesis but down-regulates expression of genes involved in lipolysis and FA oxidation. Quantitative real-time RT-PCR analysis of liver mRNAs isolated from the mice challenged with TM (2 μg/g body weight) or vehicle control, which encode regulators or enzymes in: (A) de novo lipogenesis: PGC1α, PGC1β, DGAT1 and DGAT2; (B) lipid droplet production: ADRP, FIT2, and FSP27; (C) lipolysis: ApoC2, Acox1, and LSR; and (D) FA oxidation: PPARα. Expression values were normalized to β-actin mRNA levels. Fold changes of mRNA are shown by comparing to one of the control mice. Each bar denotes the mean ± SEM (n = 4 mice per group); **P < 0.01. (E and F) Isotope tracing analysis of hepatic de novo lipogenesis. Huh7 cells were incubated with [1-14C] acetic acid for 6 h (E) or 12 h (F) in the presence or absence of TM (20 μg/ml). The rates of de novo lipogenesis were quantified by determining the amounts of [1-14C]-labeled acetic acid incorporated into total cellular lipids after normalization to cell numbers.

 http://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr4.jpg

Fig. 5. TM activates the inflammatory pathway through NF-κB, but not JNK, in the liver. Western blot analysis of phosphorylated Iκ-B, total Iκ-B, phosphorylated JNK, and total JNK in the liver tissues from the mice challenged with TM (2 μg/g body weight) or vehicle control. Levels of GAPDH were included as internal controls. The values below the gels represent the ratios of phosphorylated protein signal intensities to that of total proteins.

 http://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr5.jpg

Fig. 6. TM induces expression of pro-inflammatory cytokines and acute-phase responsive proteins in the liver. Quantitative real-time RT-PCR analyses of liver mRNAs isolated from the mice challenged with TM (2 μg/g body weight) or vehicle control, which encode: (A) pro-inflammatory cytokine TNFα and IL6; and (B) acute-phase protein SAP and SAA3. Expression values were normalized to β-actin mRNA levels. Fold changes of mRNA are shown by comparing to one of the control mice. (C–E) ELISA analyses of serum levels of TNFα, IL6, and SAP in the mice challenged with TM or vehicle control for 8 h ELISA. Each bar denotes the mean ± SEM (n = 4 mice per group); *P < 0.05, **P < 0.01.

http://ars.els-cdn.com/content/image/1-s2.0-S0378427412000732-gr6.jpg

Corresponding author at: Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, 540 E. Canfield Avenue, Detroit, MI 48201, USA. Tel.: +1 313 577 2669; fax: +1 313 577 5218.

The SREBP regulatory pathway. Brown MS, Goldst...

The SREBP regulatory pathway. Brown MS, Goldstein JL (1997). “The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor”. Cell 89 (3) : 331–340. doi:10.1016/S0092-8674(00)80213-5. PMID 9150132. (Photo credit: Wikipedia)

English: Structure of the SREBF1 protein. Base...

English: Structure of the SREBF1 protein. Based on PyMOL rendering of PDB 1am9. (Photo credit: Wikipedia)

The SREBP regulatory pathway

The SREBP regulatory pathway (Photo credit: Wikipedia)

English: Diagram of rough endoplasmic reticulu...

English: Diagram of rough endoplasmic reticulum by Ruth Lawson, Otago Polytechnic. (Photo credit: Wikipedia)

Micrograph demonstrating marked (macrovesicula...

Micrograph demonstrating marked (macrovesicular) steatosis in non-alcoholic fatty liver disease. Masson’s trichrome stain. (Photo credit: Wikipedia)

 

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 Curator: Ritu Saxena, Ph.D.

Vitamin C or Ascorbic acid (AA) or Ascorbate

Biochemical role: AA serves a basic biochemical role of accelerating hydroxylation in several biochemical reactions. It provides electrons to metal ions, the reduced forms of which are required for the full enzymatic activity of some enzymes. Most emphasized role of AA is as a cofactor for the enzyme required for the biosynthesis of collagen.

Molecular structure and the oxidized form of AA, dihydroascorbic acid, bear similarity to that of glucose.

Biological role: AA is an essential vitamin for humans and its deficiency leads to disease called Scurvy characterized by initial symptoms of malaise and lethargy, followed by formation of spots on the skin, spongy gums, and bleeding from the mucous membranes. As scurvy advances, there can be open, suppurating wounds, loss of teeth, jaundice, fever, neuropathy and death. AA is water soluble and found in high concentrations in several tissues including eye lens, WBCs, adrenal glad and pituitary gland. Some of the roles of ascorbate include:

  1. Carnitine synthesis from lysine
  2. Neurotransmitter synthesis,
  3. Cytochrome P-450 activity,
  4. Cholesterol metabolism,
  5. Detoxification of exogenous compounds,
  6. Antioxidant
  7. Possibly an ergogenic aid (Ergogenic aids are substances, devices, or practices that enhance an individual’s energy use, production, or recovery.)

Vitamin C and Cancer

As early as in 1949, vitamin C was implicated in cancer therapy. Since then, several research articles have been published exploring the role of ascorbate in cancer therapy. Among the plethora of literature discussing the relationship between vitamin C and cancer, one of the very significant and comprehensive reviews was published in 1979 in Cancer Research (2).

Mechanisms of action of AA (1) with respect to cancer have been divided and subdivided into the following:

  1. Primary mechanisms
  2. Secondary mechanisms
  • Preventive mechanism

Ascorbate acts as a cancer preventive agent by virtue of its strong antioxidant activities. Being one of the strongest reductants and radical scavenger, it absorbs unstable oxygen, nitrogen, and sulphur-centered radicals. AA can prevent biomembranes from peroxidative damage from peroxyl radicals. Ascorbate can trap peroxyl radicals and lead to their peroxidation in the aqueous phase before they reach the lipid rich biomembranes and cause damage. Ascorbate has been speculated to have a biomembrane protective action by its synergistic antioxidant activity with vitamin E (tocopherol).  Vitamin E is lipid-soluble and tocopheroxyl radical is generated in the cell membranes as a result of its antioxidant activity.  Ascorbate reacts with the tocopheroxyl radical and regenerates tocopherol transferring the oxidative challenge to the aqueous phase. At this point, the less active ascorbate radical might be reduced to AA by an NADP-dependent system. The probably mechanism might explain the reduction of nitrates via ascorbate to prevent the formation of carcinogenic nitrosamines.

  • Anticancer mechanisms

1. Primary anticancer mechanisms

i.     Oxidative, oxidant and pro-oxidant properties: Ascorbate has been reported to be cytotoxic at high concentrations, which has been demonstrated in a number of malignant cell lines. Transcription factor NFkB is potentially activated via ascorbate and its radicals leading to the inhibition of cell growth. Also, ascorbate inhibits certain prostaglandins leading to decrease in cell proliferation.

ii.     Hydrogen peroxide: On oxidation with oxygen, ascorbate produces a hydrogen peroxide, a reactive oxygen species. Hydrogen peroxide can generate several other reactive species and can have several damaging effects on cells including decrease in cell viability by damaging cell membranes of malignant cells. The amount of these reactive species produced via oxidation is limited in healthy cells unlike that in malignant cells where they exist in large amounts. The amount of hydrogen peroxide generated has been correlated to the amount of ascorbate in the cells. The reactive species can lead to multiple negative effects on cells including DNA strand breaks, lipid peroxidation leading to membrane function disruption, cellular ATP depletion.

Authors state that “the failure to maintain high ATP production may be a consequence of oxidative inactivation of key enzymes especially those related to the Krebs cycle and the electron transport chain.” This might result in alteration of transmembrane potential and distortion of mitochondrial function, suggestive of the important role of mitochondria in the process of carcinogenesis. In this paper, vitamin C has been correlated with cancer with the involvement of altered mitochondrial function. In addition, ascorbate has been detected in mitochondria where it is also regenerated. Different aspects of mitochondrial involvement in cancer have been discussed in several posts published earlier (3-8).

iii.     Other oxidation products of AA: Other oxidation products of AA include 2,3-diketoglutonic acid, and 5-methyl 1-3, 4-dehydrotetrone and other degradation products, have demonstrated antitumor activity. Additionally, some degradation and oxidation products of AA, gamma-cronolactone and 3-hydroxyl-2-pyrone, have been found to inhibit tumor growth. The mechanism of their antitumor actions is complex and might involve multitude of steps, including generation of reactive oxygen species, lipid peroxidation, inducing structural changes in important cellular proteins, inhibition of mitosis and so on.

iv.     Intracellular transport of ascorbate and its tumor specificity: Oxidized ascorbate, dihydroascorbic acid, is transported intracellularly where it is reduced back to ascorbate. Owing to its structural similarity with glucose, dihydroascorbic transport is facilitated via glucose transporters (GLUTs). Ascrobate in its reduced form is transported through a sodium-dependent cotransporter in some cells. Tumor cells require large amounts of glucose, which leads to an increase in the number of GLUTs, hence, resulting in an increase in ascorbate concentration within cancer cells. Because of this selective increased uptake of ascorbate and its cytotoxic effects in cancer cells (generation of hydrogen peroxide, DNA damage, other cytotoxic effects), AA has become a selective, nontoxic chemotherapeutic agent. The difference in the levels of catalase enzyme has been found to lead to intracellular tumor selectivity in cancer cells.

Ascorbate induced cytotoxicity in cancer cells involves its final electron acceptor, oxygen, which interferes with the anaerobic respiration within malignant cells. This gives an important clue for the involvement of mitochondria in malignant cells.

v.     Intravenous AA: High concentrations of AA in plasma (>200mg/dL) have been found to be cytotoxic to cancer cells. Clinically high plasma concentrations of AA can be achieved by its intravenous administration. It was observed that 60g infusion of AA given to cancer patients for 60 minutes followed by 20g given over the next 60 minutes resulted in a 240 minutes high plasma AA concentration of >400mg/dL, that is known to be cytotoxic.

Lipoic acid when administered with AA, is able to reduce the high-dose requirement of AA for its cytotoxic activity reducing it from 700mg/dL to 120mg/dL. Lipoic acid can recycle vitamin C, mediate the reduction of dihydroascorbic acid and improves mitochondrial function. Thus, energy intermediates such as coenzyme Q, vitamin K3, B-complex vitamins, alpha-ketoglutarate aspartate, magnesium might aid in cancer therapy by intercting with ascorbate, directly or indirectly, thereby stimuating/interacting/correcting aerobic mitochondrial respiration.

Hence, the pro-oxidant activity of vitamin C is being referred to as the primary mechanism of anticancer action.

2. Secondary anticancer mechanisms

i.     AA and intracellular matrix: Collagen is an important constituent of the matrix and its concentration determines the strength of the tissue along with its resistance to the infiltration of malignant cancer cells. In Scurvy, a disease resulting from a chronic deficiency of vitamin C, there is generalized tissue disintegration, dissolution of intercellular ground substance and the disruption of collagen bundles. This disintegration leads to ulceration; bacterial colonization and general undifferentiated cellular proliferation with specialized cells reverting back to their primitive form, very much like cancer.  Lack of ascorbate causes a reduction in the hydroxylation of prolyl and lysyl residues into hydroxyproline and hydroxylysine, leading to instability of the collagen triple helix, a common feature in scurvy and also in cancer. Thus, a secondary mechanism of ascorbic acid anticancer mechanism would be to repair these sites, which is emphasized by its role in wound healing, including surgical recovery and other traumatic injuries.

ii.     Ascorbate and immunocompetence: Ascorbate plays several roles for the efficient functioning of immune system in ways that are invoved in both humoral and cell-mediated.  Ascorbate provides humoral immunocompetence as it is essential for immunoglobulin synthesis. In addition, lymphocytes, seminal cells involved in cell-mediated immunity have been found to contain high concentrations of ascorbate. Other immune system roles include, aid in active phagocytosis and enhancing of interferon production.

Classical vitamin C and Cancer controversy-A possible explanation

Conflicting results were obtained from the studies performed by Pauling (Pauling Institute) and Cameron (Mayo Clinic) with vitamin C and its effect on cancer, the issue was debated a few decades ago. Both the studies, however, used oral doses of ascorbate (10g). Gonzalez et al, authors of the review on which the post is based, analyzed and expressed their views on the controversy. They state that the plasma concentration cannot be replicated when the dose is given orally as opposed to when the dose is given intravenously. According to their research, when AA is administered intravenously, higher plasma levels of ascorbate are achieved that could be retained for longer time periods. Also, the authors advocate the use of substantially higher doses (25-200g) to be given intravenously for selective toxicity towards cancer cells.

Modern vitamin C and Cancer controversy-Chemotherapy and radiation

A recent concern regarding the antioxidants like vitamin C is that they might reduce the effectiveness of chemotherapy and radiation by reducing the potency of free radicals necessary for killing cells. A publication by Agus et al (13) has a major role to play in this misconception. The authors describe how cancer cells acquire and concentrate vitamin C providing malignant cells with metabolic advantage. However, details or explanations regarding the theory are missing. Some studies, on the other hand, explain that high concentrations of AA in cancer cells is cytotoxic and is achieved because of similarity in structure between AA and glucose. Cancer cells uptake AA derivative, dehydroascorbic acid via glucose transporters (GLUTs).

In a case report published in PNAS in 1985 (12), two patients with ovarian cancer stage IIIC were found to respond positively to chemotherapy along with high-dose of antioxidants. Antioxidant, AA was administered intravenously to maintain a high plasma dose of 200 mg/dL. The two patients didn’t show disease recurrence after three years of chemotherapy and vitamin C administration. Vast literature exists on the topic indicating that antioxidants, including ascorbate, provide beneficial effects in several cancers without reducing the efficacy of chemotherapy or radiation during treatment of these cancers. Some data, in fact, suggests increase in effectiveness of chemotherapy when supplemented with antioxidants along with an increase in adverse effects. The topic has been summarized and discussed in a series of articles by Lawson and Brignall (9-11).

REFERENCES

The post is primarily based on the following two review articles:

1. González MJ et al. Orthomolecular oncology review: ascorbic acid and cancer 25 years later.  Integr Cancer Ther. 2005 Mar;4(1):32-44.

2. Cameron E, Pauling L, Leibovitz B. Ascorbic acid and cancer: a review. Cancer Res. 1979 Mar;39(3):663-81.

Other articles  on Mitochondria and Cancer were published on this Open Source Online Scientific Journal

3. Ritu Saxena. Mitochondria and Cancer: An overview of mechanisms

4. Ritusaxena. β Integrin emerges as an important player in mitochondrial dysfunction associated Gastric Cancer.

5. Larry H Bernstein. Mitochondria: Origin from oxygen free environment, role in aerobic glycolysis, metabolic adaptation

6. Ritu Saxena. Mitochondria and Cancer: An overview of mechanisms

7. Larry H Bernstein. Mitochondrial Damage and Repair under Oxidative Stress

8. Larry H Bernstein. What can we expect of tumor therapeutic response?

Research articles:

9. Lamson DW, Brignall MS. Antioxidants and cancer, part 3: quercetin. Altern Med Rev. 2000 Jun;5(3):196-208. Review.

10. Lamson DW, Brignall MS. Antioxidants and cancer therapy II: quick reference guide. Altern Med Rev. 2000 Apr;5(2):152-63.

11. Lamson DW, Brignall MS. Antioxidants in cancer therapy; their actions and interactions with oncologic therapies. Altern Med Rev. 1999 Oct;4(5):304-29.

12. Bensch KG, Fleming JE, Lohman W. The role of ascorbic acid in senile cataracts. Proc Natl Acad Sci USA 1985;82:7193-7196.

13. Agus DB, Vera JG, Golde DW. Stand allocation: a mechanism by which tumors obtain vitamin C. Cancer Res. 1999;59:4555-4558.

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Ubiquinin-Proteosome pathway, autophagy, the mitochondrion, proteolysis and cell apoptosis


Original description - :Cartoon representation...

Original description – :Cartoon representation of ubiquitin protein, highlighting the secondary structure. α-helices are coloured in blue and the β-sheet in green. The normal attachment point for a further ubiquitin molecule in polyubiquitin chain formation, lysine 48, is shown in pink. :Image was created using PyMOL (Photo credit: Wikipedia)

Ubiquinin-Proteosome pathway, autophagy, the mitochondrion, proteolysis and cell apoptosis

Larry H Bernstein, MD, FACP, Curator, Reporter, AEW

The work reviewed follows a seminal contribution by two Israeli and an American molecular biologists who shared the Nobel Prize in Chemistry in 2004.

The Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry for 2004 “for the discovery of ubiquitin-mediated protein degradation” jointly to Aaron Ciechanover Technion – Israel Institute of Technology, Haifa, Israel, Avram Hershko Technion – Israel Institute of Technology, Haifa, Israel and Irwin Rose – University of California, Irvine, USA.

Aaron Ciechanover, born 1947 (57 years) in Haifa, Israel (Israeli citizen) received a Doctor’s degree in medicine in 1975 at Hebrew University of Jerusalem, and in biology in 1982 at the Technion (Israel Institute of Technology), Haifa. He is a Distinguished Professor at the Center for Cancer and Vascular Biology, and the Rappaport Faculty of Medicine and Research Institute at the Technion, Haifa,
Israel.

Avram Hershko, born 1937 (67 years) in Karcag, Hungary (Israeli citizen) earned the Doctor’s degree in medicine in 1969 at the Hadassah and the Hebrew University Medical School, Jerusalem.  He is a Distinguished Professor at the Rappaport Family Institute for Research in Medical Sciences at the Technion (Israel Institute of Technology), Haifa, Israel.

Irwin Rose, born 1926 (78 years) in New York, USA (American citizen) achieved a Doctor’s degree in 1952 at the University of Chicago, USA. Specialist at the Department of Physiology and Biophysics, College of Medicine, University of California, Irvine, USA.

Proteins labelled for destruction
Proteins build up all living things: plants, animals and therefore us humans. In the past few decades biochemistry has come a long way towards explaining how the cell produces all its various proteins. But as to the breaking down of proteins, not so many researchers were interested. Aaron Ciechanover, Avram Hershko and Irwin Rose went against the stream and at the beginning of the 1980s discovered one of the cell’s most important cyclical processes, regulated protein degradation. For this, they are being rewarded
with the 2004 Nobel Prize in Chemistry.

The label consists of a molecule called ubiquitin. This fastens to the protein to be destroyed, accompanies it to the proteasome where it is recognised as the key in a lock, and signals that a protein is on the way for disassembly. Shortly before the protein is squeezed into the proteasome, its ubiquitin label is disconnected for re-use.

Aaron Ciechanover, Avram Hershko and Irwin Rose have brought us to realise that the cell functions as a highly-efficient checking station where proteins are built up and broken down at a furious rate. The degradation is not indiscriminate but takes place through a process that is controlled in detail so that the proteins to be broken down at any given moment are given a molecular label, a ‘kiss of death’, to be dramatic. The labelled proteins are then fed into the cells’ “waste disposers”, the so called proteasomes, where they are chopped into small pieces and destroyed.

Animation (Plug in requirement: Flash Player 6)

Thanks to the work of the three Laureates it is now possible to understand at  molecular level how the cell controls a number of central processes by breaking down certain proteins and not others. Examples of processes governed by ubiquitin-mediated protein degradation are cell division, DNA repair, quality control of newly-produced proteins, and important parts of the immune defence. When the degradation does not work correctly, we fall ill. Cervical cancer and cystic fibrosis are two examples. Knowledge of
ubiquitin-mediated protein degradation offers an opportunity to develop drugs against these diseases and others.

Aaron Ciechanover and Ronen Ben-Saadon. N-terminal ubiquitination: more protein substrates join in. TRENDS in Cell Biology 2004; 14 (3):103-106.

The ubiquitin–proteasome system (UPS) is involved in selective targeting of innumerable cellular proteins through a complex pathway that plays important roles in a broad array of processes. An important step in the proteolytic cascade is specific recognition of the substrate by one of many ubiquitin ligases, E3s, which is followed by generation of the polyubiquitin degradation signal. For most substrates, it is believed that the first ubiquitin moiety is conjugated, through its C-terminal Gly76 residue, to an 1-NH2 group of an internal Lys residue. Recent findings indicate that, for several proteins, the first ubiquitin moiety is fused linearly to the a-NH2 group of the N-terminal residue.

The ubiquitin–proteasome system (UPS). Ubiquitin is first activated to a high-energy intermediate by E1. It is then transferred to a member of the E2 family of enzymes. From E2 it can be transferred directly to the substrate (S, red) that is bound specifically to a member of the ubiquitin ligase family of proteins, E3

  • (a). This occurs when the E3 belongs to the RING finger family of ligases. In the case of a HECT-domain-containing ligase
  • (b), the activated ubiquitin is transferred first to the E3 before it is conjugated to the E3-bound substrate . Additional ubiquitin moieties are added successively to the previously conjugated moiety to generate a polyubiquitin chain.
  • The polyubiquitinated substrate binds to the 26S proteasome complex (comprising 19S and 20S sub-complexes): the substrate is degraded to short peptides, and free and reusable ubiquitin is released through the activity of de-ubiquitinating enzymes (DUBs).

Ubiquitination on an internal lysine and on the N-terminal residue of the target substrate.

  • (a) The first ubiquitin moiety is conjugated, through its C-terminal Gly76 residue, to the 1-NH2 group of an internal lysine residue of the target substrate (Kn).
  • (b) The first ubiquitin moiety is conjugated to a free a-NH2 group of the N-terminal residue, X.
  • In both cases, successive addition of activated ubiquitin moieties to internal Lys48 on the previously conjugated ubiquitin moiety leads to the synthesis of a  polyubiquitin chain that serves as the degradation signal for the 26S proteasome

 

A UPS Autophagy Review

Summary: This discussion is another in a series discussing mitochondrial metabolism, energetics and regulatory function, and dysfunction, and the process leading to apoptosis and a larger effect on disease, with a specific targeting of neurodegeneration. Why neurological and muscle damage are more sensitive than other organs is not explained easily, but recall in the article on mitochondrial oxidation-reduction reactions and repair that there are organ specific differences in the rates of organelle mutation errors and in the rates of repair. In addition, consider the effect of iron-binding in the function of the cell, and Ca2+ binding in the creation of the mechanic work or signal transmission carried out by the neuromuscular system. We target the previously mentioned role of ubiquitin-proteosome, and interaction with autophagy, mitophagy, and disease.

Keywords: autophagy, ubiquitin-proteosome, UPS, protein degradation, defective organelle removal, selective degradation, E3, neurodegenerative disease, mitochondria, mitophagy, proteolysis, ribosomes, apoptosis, Ca++, rapamycin, TORC1, atg1p kinase, ubiqitization, trafficking pathways, unfolded protein response (UPS), p52/sequestrome, IC3, nitrogen starvation, acetaldehyde dehydrogenase (Ald6p), Ut1hp, toxisomes, Pex3/14 proteins, Bax, E3 Ligase, TRAP1, TNF-a, NFkB.

Ubiquitin-Proteosome Pathway
Three recent papers, describing three apparently independent biological processes, highlight the role of the ubiquitin-proteasome system as a major, however selective, proteolytic and regulatory pathway. Using specific inhibitors to the proteasome, Rock et al. (1994) demonstrate a role for this protease in the degradation of the major bulk of cellular proteins. They also showed that antigen processing requires the ubiquitin-activating enzyme, El. This indicates that antigen processing is both ubiquitin dependent and proteasome dependent. Furthermore, inhibitors to the proteasome prevent tumor necrosis factor a (TNFa)-induced activation of mature NFKB and its entry into the nucleus. The two studies clearly demonstrate that the ubiquitin-proteasome system is involved not only in complete destruction of its protein substrates, but also in limited proteolysis and posttranslational processing in which biologically active peptides or fragments are generated. In addition, the unstable c-Jut but not the stable v-Jun, is multiubiquitinated and degraded. The escape of the oncogenic v-Jun from ubiquitin-dependent degradation suggests a novel route to malignant transformation. Presented here is a review of the components, mechanisms of action, and cellular physiology of the ubiquitin-proteasome pathway.

Experimental evidence implicates the ubiquitin system in the degradation of

  • mitotic cyclins,
  • oncoproteins,
  • the tumor suppressor protein p53,
  • several cell surface receptors,
  • transcriptional regulators, and
  • mutated and damaged proteins.

Some of the proteolytic processes occur throughout the cell cycle, whereas others are tightly programmed and occur following cell cycle-dependent posttranslational modifications of the components involved. Signaling and degradation of other proteins (cell surface receptors, for example) may occur only following structural changes or modification(s) in the target molecule that results from ligand binding. Cell cycle-and modification-dependent degradation, as well the ability of the system to destroy completely or only partially its protein substrates, reflects the complexity involved in regulated intracellular protein degradation.

Enzymes of the System
The reaction occurs in two distinct steps:

  1. signaling of the protein by covalent attachment of multiple ubiquitin molecules and
  2. degradation of the targeted protein with the release of free and reutilizable ubiquitin.

Conjugation of ubiquitin to proteins destined for degradation proceeds, in general, in a three-step mechanism.

  1. Initially, the C-terminal Gly of ubiquitin is activated by ATP to a high energy thiol ester intermediate in a reaction catalyzed by the ubiquitin-activating enzyme, El.
  2. Following activation, E2 (ubiquitin carrier protein or ubiquitin-conjugating enzyme [USC]) transfers ubiquitin from El to the substrate that is bound to a ubiquitin-protein ligase, E3.
  3. Here an isopeptide bond is formed between the activated C-terminal Gly of ubiquitin and an c-NH2 group of a Lys residue of the substrate.

As E3 enzymes specifically synthesized by processive transfer of ubiquitin moieties to Lys-48 of the previous (and already conjugated) ubiquitin molecule. In many cases, E2 transfers activated ubiquitin directly to the protein substrate. Thus, E2 enzymes also play an important role in substrate recognition, although, in most cases, this modification is of the monoubiquitin type.

The Ubiquitin-Mediated Proteolytic Pathway
(1) Activation of ubiquitin by El and E2.
(2) Binding of the protein substrate to E3.
(3) EP dependent but EM independent monoubiquitination.
(4) EP-dependent but EM independent polyubiquitination?
(5) Ed-dependent polyubiquitination.
(6) Degradation of ubiquitin-protein conjugate by the 26s protease.
(7) “Correction” function of C-terminal hydrolase(s).
(6) Release of ubiquitin from terminal proteolytic products by &terminal hydrolase(s).

It is essential for the system that ubiquitin recycles. This function is carried out by ubiquitin C-terminal hydrolases (isopeptidases). In protein degradation, hydrolase(s) is required to release ubiquitin from isopeptide linkage with Lys residues of the protein substrate at the final stage of the proteolytic process. A ubiquitin C-terminal hydrolytic activity is also required to disassemble polyubiquitin chains linked to the protein substrate, following or during the degradative process. A “proofreading” function has been proposed for hydrolases to release free protein from “incorrectly” ubiquitinated proteins. Another possibility is that ubiquitin C-terminal hydrolases are required for trimming polyubitin chains.

Hydrolases are probably required for the processing of biosynthetic precursors of ubiquitin, since most ubiquitin genes are arranged either in linear polyubiquitin arrays or are fused to ribosomal proteins. Yet another hydrolase may be required for the removal of extra amino acid residues that are encoded by certain genes at the C-termini of some polyubiquitin molecules. Ubiquitin C-terminal hydrolases may have other functions as well. High energy El-ubiquitin and E2-ubiquitin thiol esters may react with intracellular nucleophiles (such as glutathione or polyamines). Such reactions may lead to rapid depletion of free ubiquitin unless such side products are rapidly cleaved.

Recognition of Substrates
Short-lived proteins contain a region enriched with Pro, Glu, Ser, and Thr (PEST region). However, it has not been shown that this region indeed serves as a consensus proteolysis targeting signal. An interesting problem involves the evolution of the N-end rule pathway and its physiological roles. Proteins that are derived from processing of polyproteins (Sindbis virus RNA polymerase, for example) may contain destabilizing N-termini and thus are proteolyzed via the N-end rule pathway.

Using a “synthetic lethal” screen, Ota and Varshavsky attempted to isolate a mutant that requires the N-end rule pathway for viability. They characterized an extragenic suppressor of the mutation and found that it encodes a protein with a strong correlation to protein phosphotyrosine phosphatase. The target protein or the connection between dephosphorylation of phosphotyrosine and the N-end rule pathway is still obscure. In an additional study, these researchers have shown that a missense mutation in SLNI, a member of a two-component signal transduction system in yeast, is lethal in the absence, but not in the presence, of the N-end rule pathway. Further studies are required to isolate the target protein and identify the signal transduction pathway.

Two recent studies have shed light on the role of the ubiquitin system and the proteasome in the process. Michalek et al. (1993) have shown that a mutant cell that harbors a thermolabile El cannot present peptides derived from ovalbumin following inactivation of the enzyme. In contrast, presentation of a minigene-expressed antigene peptide or presentation of exogenous similar peptide was not perturbed at the nonpermissive temperature. The important conclusion of the researchers is that the processing of the protein to peptides requires the complete ubiquitin pathway. In a complementary study, Rock et al. (1994) have shown that inhibitors that block the chymotryptic activity of the proteasome also block antigen presentation, most probably by inhibiting proteolysis of the antigen (ovalbumin). Thus, it appears that processing of MHC restricted class I antigens requires both ubiquitination and subsequent degradation by the proteasome. It is likely that the proteasome catalyzes processing of these antigens as part of the 26s protease complex.
Ciechanover A. The Ubiquitin-Proteasome Proteolytic Pathway. Cell 1994; 79:13-21.
Regulation of autophagy
The protein content of the cell is determined by the balance between protein synthesis and protein degradation. At constant intracellular protein concentration, i.e. at steady state, rates of protein synthesis and degradation are equal. Although turnover of protein results in energy dissipation, regulation at the level of protein degradation effectively controls protein levels.
Intracellular proteins to be degraded in the lysosomes can get access to these organelles by the following processes:

  • macroautophagy,
  • microautophagy,
  • crinophagy and selective,
  • chaperonin mediated, direct uptake of proteins.

Overview of the involvement of signal transduction in the regulation of macroautophagic proteolysis by amino acids and cell swelling.

  1. Amino acids (AA) stimulate a protein kinase cascade via a plasma membrane receptor.
  2. Receptor activation results in activation of PtdIns 3-kinase (PI3K), possibly via a heterotrimeric Gái3 protein.
  3. followed by activation of PKC-æ, PKB/Akt, p70S6 kinase (p70S6k) and finally phosphorylation of ribosomal protein S6 (S6P).
  4. The GDP-bound form of Gái3 is required for autophagic sequestration, whereas the GTP-bound form is inhibitory.
  5. The constitutively formed phosphatidylinositol 3-phosphate (PI3P) is also required for autophagic sequestration. Therefore,

inhibition of PtdIns 3-kinase activity by

  • wortmannin (W),
  • LY294002 (LY) or
  • 3-methyladenine (3MA) prevents autophagic sequestration.

Activation of PKC-æ and PKB/Akt is mediated by the 3,4- and 3,4,5-phosphate forms of phosphatidylinositol (PI3,4P2 and PI3,4,5P3) that are produced upon activation of PtdIns 3-kinase.

As a result of this, the first step of the macroautophagic pathway is

  • inhibited by components of the cascade that are downstream of PtdIns 3-kinase.
  • inhibition of this downstream cascade by rapamycin (RAPA) accelerates autophagic sequestration.
  • cell swelling potentiates the effect of amino acids via a change in the receptor owing to membrane stretch.

Furthermore, the site of action of the different effectors of the cytoskeleton (okadaic acid, cytochalasin, nocodazole, vinblastin and colchicine) are indicated.

  • AVi,
  • initial autophagic vacuole;
  • AVd,
  • mature degradative autophagic vacuole,
  • ER, endoplasmic reticulum.

The rate of proteolysis , an important determinant of the intracellular protein content, and part of its degradation occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include

  • amino acids,
  • cell swelling and
  • insulin.

In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role.

  • Activation of a signal transduction pathway, ultimately
  • leading to phosphorylation of ribosomal protein S6,
  • accompanies Inhibition of macroautophagy.

Components of this pathway may include

  • a heterotrimeric Gi3-protein,
  • phosphatidylinositol 3-kinase and
  • p70S6 kinase.

Selectivity of Autophagy
It has been assumed for a long time that macroautophagy is a non-selective process, in which macromolecules are randomly degraded in the same ratio as they occur in the cytoplasm . However, recent observations strongly suggest that this may not always be the case, and that macroautophagy can be selective. Lysosomal protein degradation can selectively occur via ubiquitin-dependent and -independent pathways. In the perfused liver, although autophagic breakdown of protein and RNA (mainly ribosomal RNA) is sensitive to inhibition by amino acids and insulin, glucagon accelerates proteolysis but has no effect on RNA degradation.

Another example of selective autophagy is the degradation of superfluous peroxisomes in hepatocytes from clofibrate-treated rats. When hepatocytes from these rats, in which the number of peroxisomes is greatly increased, are incubated in the absence of amino acids to ensure maximal flux through the macroautophagic pathway, peroxisomes are degraded at a relative rate that exceeds that of any other component in the liver cell. The accelerated degradation of peroxisomes was sensitive to inhibition by 3-methyladenine, a specific autophagic sequestration inhibitor. Interestingly, the accelerated removal of peroxisomes was prevented by long-chain but not short-chain fatty acids. Since long-chain fatty acids are substrates for peroxisomal â-oxidation, this indicates that these organelles are removed by autophagy when they are functionally redundant.  Our hypothesis is that acylation (palmitoylation?) of a peroxisomal membrane protein protects the peroxisome against autophagic sequestration.

Under normal conditions macroautophagy may be largely unselective and serves, for example, to produce amino acids for gluconeogenesis and the synthesis of essential proteins in starvation. When cell structures are functionally redundant or when they become damaged, the autophagic system is able to recognize this and is able to degrade the structure concerned. As yet, nothing is known about the recognition signals. A possibility is that ubiquitination of membrane proteins is required to mark the structure to be degraded for autophagic sequestration.

Ubiquitin may be involved in macroautophagy
Ubiquitin not only contributes to extralysosomal proteolysis but is also involved in autophagic protein degradation. Thus, in fibroblasts ubiquitin–protein conjugates can be found in the lysosomes, as shown by immunohistochemistry and immunogold electron microscopy. Free ubiquitin can also be found inside lysosomes. Accumulations of ubiquitin–protein conjugates in filamentous, presumably lysosomal, structures are also found in a large number of neurodegenerative diseases. Mallory bodies in the liver of alcoholics also contain ubiquitin–protein conjugates.

This presence of ubiquitin–protein conjugates in filamentous inclusions in neurons and other cells can be caused by a defect in the extralysosomal ubiquitin-dependent proteolytic pathway. However, it is also possible that these filamentous inclusions represent an attempt of the cell to get rid of unwanted material (proteins, organelles) via autophagy. Direct evidence that ubiquitin may be involved in the control of macroautophagy came from experiments with CHO cells with a temperature-sensitive mutation in the ubiquitin-activating enzyme E1. Wild-type cells increased their rate of proteolysis in response to stress (amino acid depletion, increased temperature). This was prevented by the acidotropic agent ammonia or by the autophagic sequestration inhibitor 3-methyladenine, indicating that the accelerated proteolysis occurred by autophagy. In the mutant cells, there was no such increase in proteolysis in response to stress at the restrictive temperature.

Autophagy and carcinogenesis
In cancer development, cell growth is mainly induced by inhibition of protein degradation, since differences in the rate of protein synthesis between tumorigenic cells and their normal counterparts are rather small. A striking example of how reduced autophagic proteolysis can contribute to cell growth can be found in the development of liver carcinogenesis. This decrease in autophagic flux results from a decrease in the rate of autophagic sequestration and is already detectable in the early preneoplastic stage. Autophagic flux is then hardly inhibitable by amino acids nor is it inducible by catabolic stimuli
and declines in the more advanced stage of cancer development to a rate of less than 20% of that seen in normal hepatocytes. The fact that the addition of 3-methyladenine to hepatocytes from normal rats increased hepatocyte viability to the same level as observed for the tumour cells strongly suggests that the fall in autophagic proteolysis contributes to the rapid growth rate of these cells and gives them a selective advantage over the normal hepatocytes.

Underlying control mechanisms for autophagy are gradually being unravelled. It is perhaps not surprising that protein phosphorylation and signal transduction are key elements in these mechanisms. The discovery of an amino acid receptor in the plasma membrane of the hepatocyte with a signal transduction pathway coupled to it may have important repercussions, not only for the control of macroautophagy but also for the control of other pathways.

It remains to be seen whether the details of the mechanisms controlling the process in yeast are similar to those in mammalian cells. For example, it is not known whether amino acids are able to control the process as they do in mammalian cells.

Blommaart EFC, Luiken JJFP, Meijer AJ. Autophagic proteolysis: control and specificity. Histochemical Journal (1997); 29:365–385.
A Novel Type of Selective Autophagy
Eukaryotic cells use autophagy and the ubiquitin–proteasome system (UPS) as their major protein degradation pathways. Whereas the UPS is required for the rapid degradation of proteins when fast adaptation is needed, autophagy pathways selectively remove protein aggregates and damaged or excess organelles. However, little is known about the targets and mechanisms that provide specificity to this process. Here we show that mature ribosomes are rapidly degraded by autophagy upon nutrient starvation in Saccharomyces cerevisiae. Surprisingly, this degradation not only occurs by a nonselective mechanism, but also involves a novel type of selective autophagy, which we term ‘ribophagy’. A genetic screen revealed that selective degradation of ribosomes requires catalytic activity of the Ubp3p/Bre5p ubiquitin protease. Although Ubp3p and Bre5p cells strongly accumulate 60S ribosomal particles upon starvation, they are proficient in starvation sensing and in general trafficking and autophagy pathways. Moreover, ubiquitination of several ribosomal subunits and/or ribosome associated proteins was specifically enriched in Ubp3p cells, suggesting that the regulation of ribophagy by ubiquitination may be direct. Interestingly, Ubp3p cells are sensitive to rapamycin and nutrient starvation, implying that selective degradation of ribosomes is functionally important in vivo. Taken together, our results suggest a link between ubiquitination and the regulated degradation of mature ribosomes by autophagy.
Kraft C, Deplazes A, Sohrmann M,Peter M. Mature ribosomes are selectively degraded upon starvation by an autophagy pathway requiring the Ubp3p/Bre5p ubiquitin protease. Nature Cell Biology 2008; 10(5): 603-609. DOI: 10.1038/ncb1723.  www.nature.com/naturecellbiology

Mitochondrial Failure and Protein Degradation

Progressive mitochondrial failure is tightly associated with the the development of most age-related human diseases including neurodegenerative diseases, cancer, and type 2 diabetes.

This tight connection results from the double-edged sword of mitochondrial respiration, which is responsible for generating both ATP and ROS, as well as from risks that are inherent to mitochondrial biogenesis. To prevent and treat these diseases, a precise understanding of the mechanisms that maintain functional mitochondria is necessary. Mitochondrial protein quality control is one of the mechanisms that protect mitochondrial integrity, and increasing evidence implicates the cytosolic ubiquitin/proteasome system (UPS) as part of this surveillance network. In this review, we will discuss our current understanding of UPS-dependent mitochondrial protein degradation, its roles in diseases progression, and insights into future studies.

While mitochondria have their own genome, about 99% of the roughly 1000 mitochondrial proteins are encoded in the nuclear genome. Most mitochondrial proteins are therefore

  • synthesized in the cytoplasm,
  • unfolded,
  • transported across one or both mitochondrial membranes,
  • then refolded and/or assembled into complexes (Tatsuta, 2009).

Failure of this complex series of events generates unfolded or misfolded proteins within mitochondria, often disrupting critical functions.

Mitochondrial oxidative phosphorylation generates usable cellular energy in the form of ATP, but also produces reactive oxygen species (ROS) . ROS tend to react quickly, so their predominant sites of damage are mitochondrial macromolecules that are localized nearby the source of ROS production.

Exposure to oxidative stress facilitates misfolding and aggregation of these mitochondrial proteins, leading to disassembly of protein complexes and eventual loss of mitochondrial integrity.

The clearance of misfolded and aggregated proteins is constantly needed to maintain functional mitochondria.
There are several systems promoting this turnover.

  1. Mitophagy, a selective mitochondrial autophagy, mediates a bulk removal of damaged mitochondria.
  2. mitochondria intrinsically contain proteases in each of their compartments and these proteases recognize misfolded mitochondrial proteins and mediate their degradation.

Accumulating evidence shows that the ubiquitin proteasome system (UPS) plays an important role in mitochondrial protein degradation. At various cellular sites, the UPS is involved in protein degradation. With the help of ubiquitin E1–E2–E3 enzyme cascades, target proteins destined for destruction are marked by conjugation of K48-linked poly-ubiquitin chain. This poly-ubiquitinated protein is then targeted to the proteasome for degradation.

Cells treated with proteasome inhibitors exhibit elevated levels of ubiquitinated mitochondrial proteins, suggesting the potentially important roles of the proteasome on mitochondrial protein degradation. Studies have also identified mitochondrial substrates of the UPS.

  • Fzo1, an outer mitochondrial membrane (OMM) protein involved in mitochondrial fusion, is partially dependent on the proteasome for its degradation in yeast.
  • The F box protein Mdm30 mediates ubiquitination of Fzo1 by Skp1-Cullin-F-boxMdm30 ligase, which leads to proteasomal degradation.

The UPS has also been implicated in mitochondrial protein degradation in higher organisms. In mammals,

  • the OMM proteins mitofusin 1 and 2 (Mfn1/2; the mammalian orthologs of Fzo1) and Mcl1 are polyubiquitinated and degraded by the proteasome.
  • VDAC1, Tom20 and Tom70 were also suggested as targets of proteasomal degradation as they are stabilized by proteasome inhibition.
  •  inactivation of the proteasome also induces accumulation of intermembrane space (IMS) proteins and, consistent with this, the proteasome plays a role in degradation of the IMS protein, Endonuclease G.

Turnover of some inner mitochondrial membrane (IMM) proteins is also dependent upon the proteasome. Uncoupling proteins (UCPs) 2 and 3 exhibit an unusually short half-life compared with other IMM proteins, and Brand and colleagues showed that inactivation of the proteasome prevents their turnover in vivo and in a reconstituted in vitro system. Finally, mitochondrial matrix proteins can also be degraded by the proteasome.

Cdc48/p97 is involved in many cellular processes through its role in protein degradation and is targeted to different subcellular sites by adaptor proteins. For example, Cdc48/p97 is recruited to the endoplasmic reticulum with the help of two adaptor proteins, Npl4 and Ufd1. This implies the existence of specific adaptors that recruit Cdc48/p97 to mitochondria. Consistent with this notion, the authors recently identified a mitochondrial adaptor protein for Cdc48, which we named Vms1 (VCP/Cdc48-associated mitochondrial stress responsive 1). Vms1 interacts with Cdc48/p97 and Npl4, but not with Ufd1, which indicates that the Cdc48/p97–Npl4–Ufd1 complex functions in ER protein degradation while the Vms1–Cdc48/p97–Npl4 complex acts in mitochondria. In agreement with this notion, overexpression of Cdc48 or Npl4 rescues the Vms1 mutant phenotype while Ufd1 has no effect.

Normally, Vms1 is cytoplasmic. Upon mitochondrial stress, however, Vms1 recruits Cdc48 and Npl4 to mitochondria. In agreement with the role of Cdc48/p97 in OMM protein degradation, loss of the Vms1 system results in accumulation of ubiquitin-conjugated proteins in purified mitochondria as well as stabilization of Fzo1 under mitochondrial stress conditions. Accumulation of damaged and misfolded mitochondrial proteins disturbs the normal physiology of the mitochondria, leading to mitochondrial dysfunction. As expected, the Vms1 mutants progressively lose mitochondrial respiratory activity, eventually leading to cell death. The VMS1 gene is broadly conserved in eukaryotes, implying an important functional role in a wide range of organisms. The C. elegans Vms1 homolog exhibits a similar pattern of mitochondrial stress responsive translocation and is required for normal lifespan. Additionally, mammalian Vms1 also forms a stable complex with p97. Combining these observations, the authors conclude that Vms1 is a conserved component of the UPS-dependent mitochondrial protein quality control system.

The UPS regulates mitochondrial dynamics and initiation of mitophagy
The UPS regulates mitochondrial dynamics. Major proteins involved in mitochondrial fission or fusion (e.g. Mfn1/2, Drp1 and Fis1) are degraded by the UPS.  MITOL, a mitochondrial E3 ubiquitin ligase, is required for Drp1-dependent mitochondrial fission as depletion or inactivation of MITOL blocks mitochondrial fragmentation. Moreover, knockdown of USP30, an OMM-localized deubiquitinating enzyme, induces an elongated mitochondrial morphology, suggesting a defect in fission. Through this regulatory process, the UPS controls mitochondrial dynamics. Parkin, an E3 ligase involved in mitophagy, utilizes the UPS to enhance mitochondrial fission through degradation of components of the fusion machinery. By facilitating fragmentation of damaged mitochondria, which is essential for initiation of mitophagy, Parkin stimulates mitophagy. The underlying mechanisms linking the UPS to the regulation of mitochondrial dynamics remain unclear.

Accumulation of aberrant proteins and human diseases
In neurodegenerative diseases wherein aberrant pathological proteins accumulate throughout the cell, including sites in mitochondria. Amyloid precursor protein (APP), a protein associated with Alzheimer’s disease, accumulates within mitochondria and is implicated in blockade of mitochondrial protein import. A, a neurotoxic APP cleavage product, can also facilitate the formation of the mitochondrial permeability transition pore (mPTP) by binding to mPTP components VDAC1, CypD and ANT, which provokes cell death.
-Synuclein, a protein associated with the development of Parkinson’s disease, is targeted to the IMM where it binds to the mitochondrial respiratory complex I and impairs its function. -Synuclein interferes with mitochondrial dynamics as its unique interaction with the mitochondrial membrane disturbs the fusion process. Finally, in Huntington’s disease, increased association of the mutant huntingtin protein with mitochondria can impair mitochondrial trafficking. Moreover, accumulation of mutant huntingtin protein disrupts cristae structure and it facilitates mitochondrial fragmentation by activation of Drp1. These examples demonstrate the crucial importance of prompt removal of dysfunctional and/or aberrant proteins in maintaining functional mitochondria.

UPS-mediated mitochondrial protein degradation.
Misfolded and/or damaged mitochondrial proteins destined for proteasomal degradation in the cytosol are recruited to the outer mitochondrial membrane (OMM) from each mitochondrial compartment by unknown mechanisms. Upon reaching the OMM, these proteins are presented to the proteasome through a series of events. They are K48 polyubiquitinated by the cytoplasmic (e.g. Parkin) or mitochondrial ubiquitin E3 ligases. For proteasomal degradation, polyubiquitinated mitochondrial substrate proteins need to be retrotranslocated to the cytoplasm, probably, either by the proteasome per se or by the help of UPS components such as Vms1, Cdc48/p97 and Npl4. Following dislocation to the cytoplasm, these substrate proteins are degraded by the proteasome.

Treatment of diseases that arise from defects in protein quality control will depend on greater depth in our understanding of this process, which could contribute to the development of novel therapeutic approaches. For instance, both mutant SOD1, a misfolded mitochondrial protein associated with the onset of amyotrophic lateral sclerosis, and polyglutamine expanded ataxin-3, a pathogenic protein causing Machado-Joseph disease, are ubiquitinated by MITOL and then degraded by the proteasome. Facilitating the proteasomal degradation of these aberrant proteins might therefore efficiently control diseases progression and, eventually, cure the diseases. Answering these questions would partially unveil the mysterious physiology of mitochondria, which, in turn, would facilitate the development of therapeutics to prevent and cure devastating human diseases.

Heo JM, Rutter J. Ubiquitin-dependent mitochondrial protein degradation. The International Journal of Biochemistry & Cell Biology 2011; 43:1422– 1426. http://www.elsevier.com/locate/biocel
UPS Inhibitors and Apoptotic Machinery
Over the past decade, the promising results of UPSIs (UPS inhibitors) in eliciting apoptosis in various cancer cells, and the approval of the first UPSI (Bortezomib/Velcade/PS-341) for the treatment of multiple myeloma have raised interest in assessing the death program activated upon proteasomal blockage. Several reports indicate that UPSIs stimulate apoptosis in malignant cells by operating at multiple levels, possibly by inducing different types of cellular stress. Normally cellular stress signals converge on the core elements of the apoptotic machinery to trigger the cellular demise. In addition to eliciting multiple stresses, UPSIs can directly operate on the core elements of the apoptotic machinery to control their abundance. Alterations in the relative levels of anti and pro-apoptotic factors can render cancer cells more prone to die in response to other anti-cancer treatments. Aim of the present review is to discuss those core elements of the apoptotic machinery that are under the control of the UPS.

The UPS (Ubquitin-Proteasome System)
To fulfill the protein-degradation process two branches, operating at different levels, principally comprise the UPS.

  • The first branch is formed by the enzymatic activities responsible for delivering the substrate to the degradative machinery: the targeting branch.
  • The second branch is represented by the proteolytic machinery, which ultimately fragments the protein substrate into small oligopeptides.

Oligopeptides are further digested to single amino acids by cytosolic proteases.
It is important to remember that conjugation of ubiquitin to a specific protein is not sufficient to determine its degradation. In fact, mono-ubiquitination or poly-monoubiquitination and in certain cases also poly-ubiquitination of proteins are post-translational modifications related to various cellular functions including DNA repair or membrane trafficking . To deliver polypeptides for proteasomal degradation poly-ubiquitin chains of more than 4 ubiquitins must be assembled through lysine-48 linkages.

There are 3 catalytic sites for each polyubiquitin chain. These sites show specific requirements in terms of substrate specificities and catalytic activities, and they are identified as

  1. trypsin-like, which prefer to cleave after hydrophobic bonds, chymotrypsin-like, which cleave at basic residues and
  2. postglutamyl peptide hydrolase-like or
  3. caspase-like activities, which cut after acidic amino acid.

Each proteasome active site uses the side chain hydroxyl group of an NH2-terminal threonine as the catalytic nucleophile, a mechanism that distinguishes the proteasome from other cellular proteases. The presence of substrate proteolysis small size peptides ranging from 3 to 22 residues are generated. Alternative catalytic sites guarantees the efficient processing of several different substrates.

UPS Inhibitors
By UPS inhibitors (UPSI) we mean small molecules that share the ability to target and inhibit specific activities of the UPS, causing the accumulation of poly-ubiquitinated proteosomal substrates. UPSIs are heterogeneous compounds and among them bortezomib is the only one used in clinical practice.

PR-171, a modified peptide related to the natural product epoxomicin, is composed of two key elements:

  1. a peptide portion that selectively binds with high affinity in the substrate binding pocket(s) of the proteasome and
  2. an epoxyketone pharmacophore that stereospecifically interacts with the catalytic threonine residue and irreversibly inhibits enzyme activity.

In comparison to bortezomib, PR-171 exhibits equal potency, but greater selectivity, for the chymotrypsin-like activity of the proteasome. In cell culture PR-171 is more cytotoxic than bortezomib. In mice PR-171 is well tolerated and shows stronger anti-tumor activity when compared with bortezomib . Clinical studies are in progress to test the safety of PR-171 at different dose levels on some hematological cancers.

Cell Death by UPSI
In vitro experiments have unambiguously established that incubation of neoplastic cells with UPSIs including bortezomib triggers their death. Apoptosis or type I cell death relies on the timed activation of caspases, a group of cysteine proteases, which cleave selected cellular substrates after aspartic residues. Two main apoptotic pathways keep in check caspase activation.

The turnover of a large number of cellular proteins is under the control of the UPS. Thus in principle any proteosomal substrate could contribute directly or indirectly to the cell death phenotype. This is perfectly exemplified by two master regulators of cell life and death, p53 and NFkB.  UPSIs cause

  • NF-kB inhibition through reduced IkB degradation and,
  • in opposition; they promote stabilization and accumulation of p53.

c-FLIP is the most important element of the extrinsic pathway under the direct control of the UPS. Two different FLIP isoforms exist:

  1. c-FLIPL (Long) and
  2. c-FLIPS (Short).

c-FLIPL is highly homologus to caspase-8 and contains two tandem repeat Death Effector Domains (DED) and a catalytically inactive caspase-like domain. Both FLIPs can be degraded by the UPS; however they display distinct half-lives and the unique C terminus of c-FLIPS possesses a destabilizing function. The regulation of c-FLIP levels in response to UPSIs is rather controversial. Some reports indicate that UPSIs can reduce c-FLIP levels and in this manner synergize with TRAIL to promote apoptosis.

UPSIs activate multiple cellular responses and different stress signals that ultimately cause cell death. For this reason they represent broad inducers of apoptosis. In addition, since many of the available UPSIs alter the proteolytic activity of the proteasome, they represent non-specific modulators of the expression/activity of various components of the apoptotic machinery. Paradoxically they can simultaneously favor the accumulation of pro- and anti-apoptotic factors.
Brancolini C. Inhibitors of the Ubiquitin-Proteasome System and the Cell Death Machinery: How Many Pathways are Activated? Current Molecular Pharmacology, 2008; 1:24-37.

Mitochondrial Quality Control
The PINK1–Parkin pathway plays a critical role in mitochondrial quality control by selectively targeting damaged mitochondria for autophagy. The AAA-type ATPase p97 acts downstream of PINK1 and Parkin to segregate fusion-incompetent mitochondria for turnover. [Tanaka et al. (2010. J. Cell Biol. doi: 10.1083/jcb.201007013)]. p97 acts by targeting the mitochondrial fusion-promoting factor mitofusin for degradation through an endoplasmic reticulum–associated degradation (ERAD)-like mechanism.

Pallanck LJ. Culling sick mitochondria from the herd. J Cell Biol 2012;191(7):1225–1227. http://www.jcb.org/cgi/doi/10.1083/jcb.201011068

PINK1 and Parkin and Parkinson’s Disease

Studies of the familial Parkinson disease-related proteins PINK1 and Parkin have demonstrated that these factors promote the fragmentation and turnover of mitochondria following treatment of cultured cells with mitochondrial depolarizing agents. Whether PINK1 or Parkin influence mitochondrial quality control under normal physiological conditions in dopaminergic neurons, a principal cell type that degenerates in Parkinson disease, remains unclear. To address this matter, we developed a method to purify and characterize neural subtypes of interest from the adult Drosophila brain.

Using this method, we find that dopaminergic neurons from Drosophila parkin mutants accumulate enlarged, depolarized mitochondria, and that genetic perturbations that promote mitochondrial fragmentation and turnover rescue the mitochondrial depolarization and neurodegenerative phenotypes of parkin mutants. In contrast, cholinergic neurons from parkin mutants accumulate enlarged depolarized mitochondria to a lesser extent than dopaminergic neurons, suggesting that a higher rate of mitochondrial damage, or a deficiency in alternative mechanisms to repair or eliminate damaged mitochondria explains the selective vulnerability of dopaminergic neurons in Parkinson disease.

Our study validates key tenets of the model that PINK1 and Parkin promote the fragmentation and turnover of depolarized mitochondria in dopaminergic neurons. Moreover, our neural purification method provides a foundation to further explore the pathogenesis of Parkinson disease, and to address other neurobiological questions requiring the analysis of defined neural cell types.

Burmana JL, Yua S, Poole AC, Decala RB , Pallanck L. Analysis of neural subtypes reveals selective mitochondrial dysfunction in dopaminergic neurons from parkin mutants.

Autophagy in Parkinson’s Disease.
Parkinson’s disease is a common neurodegenerative disease in the elderly. To explore the specific role of autophagy and the ubiquitin-proteasome pathway in apoptosis, a specific proteasome inhibitor and macroautophagy inhibitor and stimulator were selected to investigate pheochromocytoma (PC12) cell lines transfected with human mutant (A30P) and wildtype (WT) -synuclein.

The apoptosis ratio was assessed by flow cytometry. LC3, heat shock protein 70 (hsp70) and caspase-3 expression in cell culture were determined by Western blot. The hallmarks of apoptosis and autophagy were assessed with transmission electron microscopy. Compared to the control group or the rapamycin (autophagy stimulator) group, the apoptosis ratio in A30P and WT cells was significantly higher after treatment with inhibitors of the proteasome and macroautophagy. The results of Western blots for caspase-3 expression were similar to those of flow cytometry; hsp70 protein was significantly higher in the proteasome inhibitor group than in control, but in the autophagy inhibitor and stimulator groups, hsp70 was similar to control. These findings show that inhibition of the proteasome and autophagy promotes apoptosis, and the macroautophagy stimulator rapamycin reduces the apoptosis ratio. And inhibiting or stimulating autophagy has less impact on hsp70 than the proteasome pathway.

In conclusion, either stimulation or inhibition of macroautophagy, has less impact on hsp70 than on the proteasome pathway. This study found that rapamycin decreased apoptotic cells in A30P cells independent of caspase-3 activity. Although several lines of evidence recently demonstrated crosstalk between autophagy and caspase-independent apoptosis, we could not confirm that autophagy activation protects cells from caspase-independent cell death. Undoubtedly, there are multiple connections between the apoptotic and autophagic processes.

Inhibition of autophagy may subvert the capacity of cells to remove damaged organelles or to remove misfolded proteins, which would favor apoptosis. However, proteasome inhibition activated macroautophagy and accelerated apoptosis. A likely explanation is inhibition of the proteasome favors oxidative reactions that trigger apoptosis, presumably through

1. a direct effect on mitochondria, and
2. the absence of NADPH2 and ATP

which may deinhibit the activation of caspase-2 or MOMP. Another possibility is that aggregated proteins induced by proteasome inhibition increase apoptosis.

Yang F, Yanga YP, Maoa CJ, Caoa BY, et al. Role of autophagy and proteasome degradation pathways in apoptosis of PC12 cells overexpressing human -synuclein. Neuroscience Letters 2009; 454:203–208. doi:10.1016/j.neulet.2009.03.027. http://www.elsevier.com/locate/neulet

Parkin-dependent Ubiquitination of Endogenous Bax 

Autosomal recessive loss-of-function mutations within the PARK2 gene functionally inactivate the E3 ubiquitin ligase parkin, resulting in neurodegeneration of catecholaminergic neurons and a familial form of Parkinson disease. Current evidence suggests both a mitochondrial function for parkin and a neuroprotective role, which may in fact be interrelated. The antiapoptotic effects of Parkin have been widely reported, and may involve fundamental changes in the threshold for apoptotic cytochrome c release, but the substrate(s) involved in Parkin dependent protection had not been identified. Here, we demonstrate the Parkin-dependent ubiquitination of endogenous Bax comparing primary cultured neurons from WT and Parkin KO mice and using multiple Parkin-overexpressing cell culture systems. The direct ubiquitination of purified Bax was also observed in vitro following incubation with recombinant parkin. The authors found that Parkin prevented basal and apoptotic stress induced translocation of Bax to the mitochondria. Moreover, an engineered ubiquitination-resistant form of Bax retained its apoptotic function, but Bax KO cells complemented with lysine-mutant Bax did not manifest the antiapoptotic effects of Parkin that were observed in cells expressing WT Bax. These data suggest that Bax is the primary substrate responsible for the antiapoptotic effects of Parkin, and provide mechanistic insight into at least a subset of the mitochondrial effects of Parkin.

Johnson BN, Berger AK, Cortese GP, and LaVoie MJ. The ubiquitin E3 ligase Parkin regulates the proapoptotic function of Bax. PNAS 2012, pp 6. http://www.pnas.org/cgi/doi/10.1073/pnas.1113248109
Parkin Promotes Mitochondrial Loss in Autophagy
Parkin, an E3 ubiquitin ligase implicated in Parkinson’s disease, promotes degradation of dysfunctional mitochondria by autophagy. Using proteomic and cellular approaches, we show that upon translocation to mitochondria, Parkin activates the ubiquitin–proteasome system (UPS) for widespread degradation of outer membrane proteins. This is evidenced by an increase in K48-linked polyubiquitin on mitochondria, recruitment of the 26S proteasome and rapid degradation of multiple outer membrane proteins. The degradation of proteins by the UPS occurs independently of the autophagy pathway, and inhibition of the 26S proteasome completely abrogates Parkin-mediated mitophagy in HeLa, SH-SY5Y and mouse cells. Although the mitofusins Mfn1 and Mfn2 are rapid degradation targets of Parkin, degradation of additional targets is essential for mitophagy. These results indicate that remodeling of the mitochondrial outer membrane proteome is important for mitophagy, and reveal a causal link between the UPS and autophagy, the major pathways for degradation of intracellular substrates.

Chan NC, Salazar AM, Pham AH, Sweredoski MJ, et al. Broad activation of the ubiquitin–proteasome system by Parkin is critical for mitophagy. Human Molecular Genetics 2011; 20(9): 1726–1737. doi:10.1093/hmg/ddr048.

TRAP1 and TBP7 Interaction in Refolding of Damaged Proteins
TRAP1 is a mitochondrial antiapoptotic heat shock protein. The information available on the TRAP1 pathway describes just a few well-characterized functions of this protein in mitochondria. However, our group’s use of mass spectrometry analysis identified TBP7, an AAA-ATPase of the 19S proteasomal subunit, as a putative TRAP1-interacting protein. Surprisingly, TRAP1 and TBP7 co-localize in the endoplasmic reticulum (ER), as demonstrated by biochemical and confocal/electron microscopy analyses, and directly interact, as confirmed by FRET analysis. This is the first demonstration of TRAP1 presence in this cellular compartment. TRAP1 silencing by shRNAs, in cells exposed to thapsigargin-induced ER stress, correlates with up-regulation of BiP/Grp78, thus suggesting a role of TRAP1 in the refolding of damaged proteins and in ER stress protection. Consistently, TRAP1 and/or TBP7 interference enhanced stress-induced cell death and increased intracellular protein ubiquitination. These experiments led us to hypothesize an involvement of TRAP1 in protein quality control for mistargeted/misfolded mitochondria-destined proteins, through interaction with the regulatory proteasome protein TBP7. Remarkably, the expression of specific mitochondrial proteins decreased upon TRAP1 interference as a consequence of increased ubiquitination. The proposed TRAP1 network has an impact in vivo, since it is conserved in human colorectal cancers, is controlled by ER-localized TRAP1 interacting with TBP7 and provides a novel model of ER-mitochondria crosstalk.

Amoroso MR, Matassa DS, Laudiero G, Egorova AV. TRAP1 AND THE PROTEASOME REGULATORY PARTICLE TBP7/Rpt3 INTERACT IN THE ENDOPLASMIC RETICULUM AND CONTROL CELLULAR UBIQUITINATION OF SPECIFIC MITOCHONDRIAL PROTEINS. Cell Death and Differentiation 2012; pp? DOI : 10.1038/cdd.2011.128

VMS1 and Mitochondrial Protein Degradation
We show that Ydr049 (renamed VCP/Cdc48-associated mitochondrial stress-responsive—Vms1), a member of an unstudied pan-eukaryotic protein family, translocates from the cytosol to mitochondria upon mitochondrial stress. Cells lacking Vms1 show progressive mitochondrial failure, hypersensitivity to oxidative stress, and decreased chronological life span. Both yeast and mammalian Vms1 stably interact with Cdc48/VCP/p97, a component of the ubiquitin/proteasome system with a well-defined role in endoplasmic reticulum-associated protein degradation (ERAD), wherein misfolded ER proteins are degraded in the cytosol. We show that oxidative stress triggers mitochondrial localization of Cdc48 and this is dependent on Vms1. When this system is impaired by mutation of Vms1,

  • ubiquitin-dependent mitochondrial protein degradation,
  • mitochondrial respiratory function,and
  • cell viability are compromised.

We demonstrate that Vms1 is a required component of an evolutionarily conserved system for mitochondrial protein degradation, which is
necessary to maintain

  • mitochondrial,
  • cellular, and
  • organismal viability.

Heo JM, Livnat-Levanon N, Taylor EB, Jones KT. A Stress-Responsive System
for Mitochondrial Protein Degradation. Molecular Cell 2010; 40:465–480.
DOI 10.1016/j.molcel.2010.10.021

Mitochondrial Protein Degradation
The biogenesis of mitochondria and the maintenance of mitochondrial functions depends on an autonomous proteolytic system in the organelle which is highly conserved throughout evolution. Components of this system include processing

  • peptidases and
  • ATP-dependent proteases, as well as
  • molecular chaperone proteins and
  • protein complexes with apparently regulatory functions.

While processing peptidases mediate maturation of nuclear-encoded mitochondrial preproteins, quality control within various subcompartments of mitochondria is ensured by ATP-dependent proteases which selectively remove non-assembled or misfolded polypeptides. Moreover, these proteases appear to control the activity- or steady-state levels of specific regulatory proteins and thereby ensure mitochondrial genome integrity, gene expression and protein assembly.

Kaser M and Langer T. Protein degradation in mitochondria. CELL & DEVELOPMENTAL BIOLOGY 2000; 11:181–190. doi: 10.1006/10.1006/scdb.2000.0166.

RING finger E3s

Ubiquitin-ligases or E3s are components of the ubiquitin proteasome system (UPS) that coordinate the transfer of ubiquitin to the target protein. A major class of ubiquitin-ligases consists of RING-finger domain proteins that include the substrate recognition sequences in the same polypeptide; these are known as single-subunit RING finger E3s. We are studying a particular family of RING finger E3s, named ATL, that contain a transmembrane domain and the RING-H2 finger domain; none of the member of the family contains any other previously described domain. Although the study of a few members in A. thaliana and O. sativa has been reported, the role of this family in the life cycle of a plant is still vague.

To provide tools to advance on the functional analysis of this family we have undertaken a phylogenetic analysis of ATLs in twenty-four plant genomes. ATLs were found in all the 24 plant species analyzed, in numbers ranging from 20–28 in two basal species to 162 in soybean. Analysis of ATLs arrayed in tandem indicates that sets of genes are expanding in a species-specific manner. To
get insights into the domain architecture of ATLs we generated 75 pHMM LOGOs from 1815 ATLs, and unraveled potential protein-protein interaction regions by means of yeast two-hybrid assays. Several ATLs were found to interact with DSK2a/ubiquilin through a region at the amino-terminal end, suggesting that this is a widespread interaction that may assist in the mode of action of ATLs; the region was traced to a distinct sequence LOGO. Our analysis provides significant observations on the evolution and expansion of the ATL family in addition to information on the domain structure of this class of ubiquitin-ligases that may be involved in plant adaptation to environmental stress.

Aguilar-Hernandez V, Aguilar-Henonin L, Guzman P. Diversity in the Architecture of ATLs, a Family of Plant Ubiquitin-Ligases, Leads to Recognition and Targeting of Substrates in Different Cellular Environments. PLoS ONE 2011; 6(8): e23934. doi:10.1371/journal.pone.0023934
UPS Proteolytic Function Inadequate in Proteinopathies
Proteinopathies are a family of human disease caused by toxic aggregation-prone proteins and featured by the presence of protein aggregates in the affected cells. The ubiquitin-proteasome system (UPS) and autophagy are two major intracellular protein degradation pathways. The UPS mediates the targeted degradation of most normal proteins after performing their normal functions as well as the removal of abnormal, soluble proteins. Autophagy is mainly responsible for degradation of defective organelles and the bulk degradation of cytoplasm during starvation. The collaboration between the UPS and autophagy appears to be essential to protein quality control in the cell.

UPS proteolytic function often becomes inadequate in proteinopathies which may lead to activation of autophagy, striving to remove abnormal proteins especially the aggregated forms. HADC6, p62, and FoxO3 may play an important role in mobilizing this proteolytic consortium. Benign measures to enhance proteasome function are currently lacking; however, enhancement of autophagy via pharmacological intervention and/or lifestyle change has shown great promise in alleviating bona fide proteinopathies in the cell and animal models. These pharmacological interventions are expected to be applied clinically to treat human proteinopathies in the near future.

Zheng Q, Li J, Wang X. Interplay between the ubiquitin-proteasome system and
autophagy in proteinopathies. Int J Physiol Pathophysiol Pharmacol 2009;1:127-142. http://www.ijppp.org/IJPPP904002

Ubiquitin-associated Protein-Protein Interactions

Applicability of in vitro biotinylated ubiquitin for evaluation of endogenous ubiquitin conjugation and analysis of ubiquitin-associated protein-protein interactions has been investigated. Incubation of rat brain mitochondria with biotinylated ubiquitin followed by affinity chromatography on avidin-agarose, intensive washing, tryptic digestion of proteins bound to the affinity sorbent and their mass spectrometry analysis resulted in reliable identification of 50 proteins belonging to mitochondrial and extramitochondrial compartments. Since all these proteins were bound to avidin-agarose only after preincubation of the mitochondrial fraction with biotinylated ubiquitin, they could therefore be referred to as specifically bound proteins. A search for specific
ubiquitination signature masses revealed several extramitochondrial and intramitochondrial ubiquitinated proteins representing about 20% of total number of proteins bound to avidin-agarose. The interactome analysis suggests that the identified non-ubiquitinated proteins obviously form tight complexes either with ubiquitinated proteins or with their partners and/or mitochondrial membrane components. Results of the present study demonstrate that the use of biotinylated ubiquitin may be considered as the method of choice for in vitro evaluation of endogenous ubiquitin-conjugating machinery in particular
subcellular organelles and changes in ubiquitin/organelle associated interactomes. This may be useful for evaluation of changes in interactomes induced by protein ubiquitination.

Buneeva OA, Medvedeva MV, Kopylov AT, Zgoda VG, Medvedev AE. Use of Biotinylated Ubiquitin for Analysis of Rat Brain Mitochondrial Proteome and Interactome. Int J Mol Sci 2012; 13: 11593-11609; doi:10.3390/ijms130911593
IL-6 regulation on mitochondrial remodeling/dysfunction

Muscle protein turnover regulation during cancer cachexia is being rapidly defined, and skeletal muscle mitochondria function appears coupled to processes regulating muscle wasting. Skeletal muscle oxidative capacity and the expression of proteins regulating mitochondrial biogenesis and dynamics are disrupted in severely cachectic ApcMin/+ mice. It has not been determined if these changes occur at the onset of cachexia and are necessary for the progression of muscle wasting. Exercise and anti-cytokine therapies have proven effective in preventing cachexia development in tumor bearing mice, while their effect on mitochondrial content, biogenesis and dynamics is not well understood.

The purposes of this study were to

1) determine IL-6 regulation on mitochondrial remodeling/dysfunction during the progression of cancer cachexia and
2) to determine if exercise training can attenuate mitochondrial dysfunction and the induction of proteolytic pathways during IL-6 induced cancer cachexia.

ApcMin/+ mice were examined during the progression of cachexia, after systemic interleukin (IL)-6r antibody treatment, or after IL-6 over-expression with or without exercise. Direct effects of IL-6 on mitochondrial remodeling were examined in cultured C2C12 myoblasts.

Mitochondrial content was not reduced during the initial development of cachexia, while muscle PGC-1α and fusion (Mfn1, Mfn2) protein expression was repressed.

With progressive weight loss mitochondrial content decreased, PGC-1α and fusion proteins were further suppressed, and fission protein (FIS1) was induced.

IL-6 receptor antibody administration after the onset of cachexia

  • improved mitochondrial content,
  • PGC-1α,
  • Mfn1/Mfn2 and
  • FIS1 protein expression.

IL-6 over-expression in pre-cachectic mice

  • accelerated body weight loss and muscle wasting, without reducing mitochondrial content,
  • while PGC-1α and Mfn1/Mfn2 protein expression was suppressed
  • and FIS1 protein expression induced.

Exercise normalized these IL-6 induced effects. C2C12 myotubes administered IL-6 had

  • increased FIS1 protein expression,
  • increased oxidative stress, and
  • reduced PGC-1α gene expression
  • without altered mitochondrial protein expression.

Altered expression of proteins regulating mitochondrial biogenesis and fusion are early events in the initiation of cachexia regulated by IL-6, which precede the loss of muscle mitochondrial content. Furthermore, IL-6 induced mitochondrial remodeling and proteolysis can be rescued with moderate exercise training even in the presence of high circulating IL-6 levels.

White JP, Puppa MJ, Sato S, Gao S. IL-6 regulation on skeletal muscle mitochondrial remodeling during cancer cachexia in the ApcMin/+ mouse. Skeletal Muscle 2012; 2:14-30.
http://www.skeletalmusclejournal.com/content/2/1/14

Starvation-induced Autophagy
Upon starvation cells undergo autophagy, a cellular degradation pathway important in the turnover of whole organelles and long lived proteins. Starvation-induced protein degradation has been regarded as an unspecific bulk degradation process. We studied global protein dynamics during amino acid starvation-induced autophagy by quantitative mass spectrometry and were able to record nearly 1500 protein profiles during 36 h of starvation. Cluster analysis of the recorded protein profiles revealed that cytosolic proteins were degraded rapidly, whereas proteins annotated to various complexes and organelles were degraded later at different time periods. Inhibition of protein degradation pathways identified the lysosomal/autophagosomal system as the main degradative route.

Thus, starvation induces degradation via autophagy, which appears to be selective and to degrade proteins in an ordered fashion and not completely arbitrarily as anticipated so far.

Kristensen AR, Schandorff S, Høyer-Hansen M, Nielsen MO, et al. Ordered Organelle Degradation during Starvation-induced Autophagy. Molecular & Cellular Proteomics 2008; 7:2419–2428.

Skeletal Muscle Macroautophagy
Skeletal muscles are the agent of motion and one of the most important tissues responsible for the control of metabolism. Coordinated movements are allowed by the highly organized structure of the cytosol of muscle fibers (or myofibers), the multinucleated and highly specialized cells of skeletal muscles involved in contraction. Contractile proteins are assembled into repetitive structures, the basal unit of which is the sarcomere, that are well packed into the myofiber cytosol. Myonuclei are located at the edge of the myofibers, whereas the various organelles such as mitochondria and sarcoplasmic reticulum are embedded among the myofibrils. Many different changes take place in the cytosol of myofibers during catabolic conditions:

  • proteins are mobilized
  • organelles networks are reorganized for energy needs
  • the setting of myonuclei can be modified.

Further,

  • strenuous physical activity,
  • improper dietary regimens and
  • aging

lead to mechanical and metabolic damages of myofiber organelles, especially mitochondria, and contractile proteins. During aging the protein turnover is slowed down, therefore it is easier to accumulate aggregates of dysfunctional proteins. Therefore, a highly dynamic tissue such as skeletal muscle requires a rapid and efficient system for the removal of altered organelles, the elimination of protein aggregates, and the disposal of toxic products.

The two major proteolytic systems in muscle are the ubiquitin-proteasome and the autophagy-lysosome pathways. The proteasome system requires

  • the transcription of the two ubiquitin ligases (atrogin-1 and MuRF1) and
  • the ubiquitination of the substrates.

Therefore, the ubiquitin-proteasome system can provide the rapid elimination of single proteins or small aggregates. Conversely, the autophagic system is able to degrade entire organelles and large proteins aggregates. In the autophagy-lysosome system, double-membrane vesicles named autophagosomes are able to engulf a portion of the cytosol and fuse with lysosomes, where their content is completely degraded by lytic enzymes.

The autophagy flux can be biochemicaly monitored following LC3 lipidation and p62 degradation. LC3 is the mammalian homolog of the yeast Atg8 gene, which is lipidated when recruited for the double-membrane commitment and growth. p62 (SQSTM-1) is a polyubiquitin-binding protein involved in the proteasome system and that can either reside free in the cytosol and nucleus or occur within autophagosomes and lysosomes. The GFP-LC3 transgenic mouse model allows easy detection of autophagosomes by simply monitoring the presence of bright GFP-positive puncta inside the myofibrils and beneath the plasma membrane of the myofibers, thus investigate the activation of autophagy in skeletal muscles with different contents of slow and fast-twitching myofibers and in response to stimuli such as fasting. For example, in the fast-twiching extensor digitorum longus muscle few GFP-LC3 dots were observed before starvation, while many small GFP-LC3 puncta appeared between myofibrils and in the perinuclear regions after 24 h starvation. Conversely, in the slow-twitching soleus muscle, autophagic puncta were almost absent in standard condition and scarcely induced after 24 h starvation.
Autophagy in Muscle Homeostasis
The autophagic flux was found to be increased during certain catabolic conditions, such as fasting, atrophy , and denervation , thus contributing to protein breakdown. Food deprivation is one of the strongest stimuli known to induce autophagy in muscle. Indeed skeletal muscle, after the liver, is the most responsive tissue to autophagy activation during food deprivation. Since muscles are the biggest reserve of amino acids in the body, during fasting autophagy has the vital role to maintain the amino acid pool by digesting muscular protein and organelles. In mammalian cells, mTORC1, which consists of

  • mTOR and
  • Raptor,

is the nutrient sensor that negatively regulates autophagy.

During atrophy, protein breakdown is mediated by atrogenes, which are under the forkhead box O (FoxO) transcription factors control, and activation of autophagy seems to aggravate muscle loss during atrophy. In vivo and in vitro studies demonstrated that several genes coding for components of the autophagic machinery, such as

  • LC3,
  • GABARAP,
  • Vps34,
  • Atg12 and
  • Bnip3,

are controlled by FoxO3 transcription factor. FoxO3 is able to regulate independently the ubiquitin-proteasome system and the autophagy-lysosome machinery in vivo and in vitro. Denervation is also able to induce autophagy in skeletal muscle, although at a slower rate than fasting. This effect is mediated by RUNX1, a transcription factor upregulated during autophagy; the lack of RUNX1 results in excessive autophagic flux in denervated muscle and leads to atrophy. The generation of Atg5 and Atg7 muscle-specific knockout mice have shown that with suppression of autophagy both models display muscle weakness and atrophy and a significant reduction of weight, which is correlated with the important loss of muscle tissue due to an atrophic condition. An unbalanced autophagy flux is highly detrimental for muscle, as too much induces atrophy whereas too little leads to muscle weakness and degeneration. Muscle wasting associated with autophagy inhibition becomes evident and symptomatic only after a number of altered proteins and dysfunctional organelles are accumulated, a condition that becomes evident after months or even years. On the other hand, the excessive increase of autophagy flux is able to induce a rapid loss of muscle mass (within days or weeks).
Alterations of autophagy are involved in the pathogenesis of several myopathies and dystrophies.

The maintenance of muscle homeostasis is finely regulated by the balance between catabolic and anabolic process. Macroautophagy (or autophagy) is a catabolic process that provides the degradation of protein aggregation and damaged organelles through the fusion between autophagosomes and lysosomes. Proper regulation of the autophagy flux is fundamental for the homeostasis of skeletal muscles during physiological situations and in response to stress. Defective as well as excessive autophagy is harmful for muscle health and has a pathogenic role in several forms of muscle diseases.
Grumati P, Bonaldo P. Autophagy in Skeletal Muscle Homeostasis and in Muscular Dystrophies. Cells 2012, 1, 325-345; doi:10.3390/cells1030325. ISSN 2073-4409. http://www.mdpi.com/journal/cells

Parkinson’s Disease Mutations
Mutations in parkin, a ubiquitin ligase, cause early-onset familial Parkinson’s disease (AR-JP). How Parkin suppresses Parkinsonism remains unknown. Parkin was recently shown to promote the clearance of impaired mitochondria by autophagy, termed mitophagy. Here, we show that Parkin promotes mitophagy by catalyzing mitochondrial ubiquitination, which in turn recruits ubiquitin-binding autophagic components, HDAC6 and p62, leading to mitochondrial clearance.

During the process, juxtanuclear mitochondrial aggregates resembling a protein aggregate-induced aggresome are formed. The formation of these “mito-aggresome” structures requires microtubule motor-dependent transport and is essential for efficient mitophagy. Importantly, we show that AR-JP–causing Parkin mutations are defective in supporting mitophagy due to distinct defects at

  • recognition,
  • transportation, or
  • ubiquitination of impaired mitochondria,

thereby implicating mitophagy defects in the development of Parkinsonism. Our results show that impaired mitochondria and protein aggregates are processed by common ubiquitin-selective autophagy machinery connected to the aggresomal pathway, thus identifying a mechanistic basis for the prevalence of these toxic entities in Parkinson’s disease.
Lee JY,Nagano Y, Taylor JP,Lim KL, and Yao TP. Disease-causing mutations in Parkin impair mitochondrial ubiquitination, aggregation, and HDAC6-dependent mitophagy. J Cell Biol 2010; 189(4):671-679. http://www.jcb.org/cgi/doi/10.1083/jcb.201001039

Drosophila Parkin Requires PINK1

Loss of the E3 ubiquitin ligase Parkin causes early onset Parkinson’s disease, a neurodegenerative disorder of unknown etiology. Parkin has been linked to multiple cellular processes including

  • protein degradation,
  • mitochondrial homeostasis, and
  • autophagy;

however, its precise role in pathogenesis is unclear. Recent evidence suggests that Parkin is recruited to damaged mitochondria, possibly affecting

  • mitochondrial fission and/or fusion,
  • to mediate their autophagic turnover.

The precise mechanism of recruitment and the ubiquitination target are unclear. Here we show in Drosophila cells that PINK1 is required to recruit Parkin to dysfunctional mitochondria and promote their degradation. Furthermore, PINK1 and Parkin mediate the ubiquitination of the profusion factor Mfn on the outer surface of mitochondria. Loss of Drosophila PINK1 or parkin causes an increase in Mfn abundance in vivo and concomitant elongation of mitochondria. These findings provide a molecular mechanism by which the PINK1/Parkin pathway affects mitochondrial fission/fusion as suggested by previous genetic interaction studies. We hypothesize that Mfn ubiquitination may provide a mechanism by which terminally damaged mitochondria are labeled and sequestered for degradation by autophagy.

Ziviani E, Tao RN, and Whitworth AJ. Drosophila Parkin requires PINK1 for mitochondrial translocation and ubiquitinates Mitofusin. PNAS 2010. Pp6 http://www.pnas.org/cgi/doi/10.1073/pnas.0913485107

Dynamin-related protein 1 (Drp1) in Parkinson’s
Mutations in Parkin, an E3 ubiquitin ligase that regulates protein turnover, represent one of the major causes of familial Parkinson’s disease (PD), a neurodegenerative disorder characterized by the loss of dopaminergic neurons and impaired mitochondrial functions. The underlying mechanism by which pathogenic parkin mutations induce mitochondrial abnormality is not fully understood. Here we demonstrate that Parkin interacts with and subsequently ubiquitinates dynamin-related protein 1 (Drp1), for promoting its proteasome-dependent degradation. Pathogenic mutation or knockdown of Parkin inhibits the ubiquitination and degradation of Drp1, leading to an increased level of Drp1 for mitochondrial fragmentation. These results identify Drp1 as a novel substrate of Parkin and suggest a potential mechanism linking abnormal Parkin expression to mitochondrial dysfunction in the pathogenesis of PD.

Wang H, Song P, Du L, Tian W. Parkin ubiquitinates Drp1 for proteasome-dependent degradation: implication of dysregulated mitochondrial dynamics in Parkinson’s disease.
JBC Papers in Press. Published on February 3, 2011 as Manuscript M110.144238. http://www.jbc.org/cgi/doi/10.1074/jbc.M110.144238

Pink1, Parkin, and DJ-1 Form a Complex
Mutations in the genes PTEN-induced putative kinase 1 (PINK1), PARKIN, and DJ-1 cause autosomal recessive forms of Parkinson disease (PD), and the Pink1/Parkin pathway regulates mitochondrial integrity and function. An important question is whether the proteins encoded by these genes function to regulate activities of other cellular compartments. A study in mice, reported by Xiong et al. in this issue of the JCI, demonstrates that Pink1, Parkin, and DJ-1 can form a complex in the cytoplasm, with Pink1 and DJ-1 promoting the E3 ubiquitin ligase activity of Parkin to degrade substrates via the proteasome (see the related article, doi:10.1172/ JCI37617).

This protein complex in the cytosol may or may not be related to the role of these proteins in regulating mitochondrial function or oxidative stress in vivo.
Three models for the role of the PPD complex. In this issue of the JCI, Xiong et al. report that Pink1, Parkin, and DJ-1 bind to each other and form a PPD E3 ligase complex in which Pink1 and DJ-1 modulate Parkin-dependent ubiquitination and subsequent degradation of substrates via the proteasome. Previous work suggests that the Pink1/Parkin pathway regulates mitochondrial integrity and promotes mitochondrial fission in Drosophila.

(A) Parkin and DJ-1 may be recruited to the mitochondrial outer membrane during stress and interact with Pink1. These interactions may facilitate the ligase activity of Parkin, thereby facilitating the turnover of molecules that regulate mitochondrial dynamics and mitophagy. The PPD complex may have other roles in the cytosol that result in degradative ubiquitination and/or relay information from mitochondria to other cellular compartments.
(B) Alternatively, Pink1 may be released from mitochondria after cleavage to interact with DJ-1 and Parkin in the cytosol.
A and B differ in the site of action of the PPD complex and the cleavage status of Pink1.
The complex forms on the mitochondrial outer membrane potentially containing full-length Pink1 in A, and in the cytosol with cleaved Pink1 in B.
Lack of DJ-1 function results in phenotypes that are distinct from the mitochondrial phenotypes observed in null mutants of Pink1 or Parkin in Drosophila. Thus, although the PPD complex is illustrated here as regulating mitochondrial fission, the role of DJ-1 in vivo remains to be clarified.
(C) It is also possible that the action occurs in the cytosol and is independent of the function of Pink1/Parkin in regulating mitochondrial integrity and function.

The Xiong et al. study offers an entry point for explorations of the role of Pink1, Parkin, and DJ-1 in the cytoplasm. It remains to be shown whether Parkin, in complex with Pink1 and DJ-1, carries out protein degradation in vivo.

Li H, and Guo M. Protein degradation in Parkinson disease revisited: it’s complex. commentaries. J Clin Invest.  doi:10.1172/JCI38619. http://www.jci.org

Xiong, H., et al. Parkin, PINK1, and DJ-1 form a ubiquitin E3 ligase complex promoting unfolded protein degradation. J. Clin. Invest. 2009; 119:650–660.

 Mitochondrial Ubiquitin Ligase, MITOL, protects neuronal cells

Nitric oxide (NO) is implicated in neuronal cell survival. However, excessive NO production mediates neuronal cell death, in part via mitochondrial dysfunction. Here, we report that the mitochondrial ubiquitin ligase, MITOL, protects neuronal cells from mitochondrial damage caused by accumulation of S-nitrosylated microtubule associated protein 1B-light chain 1 (LC1). S-nitrosylation of LC1 induces a conformational change that serves both to activate LC1 and to promote its ubiquination by MITOL, indicating that microtubule
stabilization by LC1 is regulated through its interaction with MITOL. Excessive NO production can inhibit MITOL, and MITOL inhibition resulted in accumulation of S-nitrosylated LC1 following stimulation of NO production by calcimycin and N-methyl-D-aspartate. LC1 accumulation under these conditions resulted in mitochondrial dysfunction and neuronal cell death. Thus, the balance between LC1 activation by S-nitrosylation and down-regulation by MITOL is critical for neuronal cell survival. Our findings may contribute significantly to an understanding of the mechanisms of neurological diseases caused by nitrosative stress-mediated mitochondrial dysfunction.

Yonashiro R, Kimijima Y, Shimura T, Kawaguchi K, et al. Mitochondrial ubiquitin ligase MITOL blocks S-nitrosylated MAP1B-light chain 1-mediated mitochondrial dysfunction and neuronal cell death. PNAS; 2012. pp 6. http://www.pnas.org/cgi/doi/10.1073/pnas.1114985109

Ubiquitin–Proteasome System in Neurodegeneration
A common histopathological hallmark of most neurodegenerative diseases is the presence of aberrant proteinaceous inclusions inside affected neurons. Because these protein aggregates are detected using antibodies against components of the ubiquitin–proteasome system (UPS), impairment of this machinery for regulated proteolysis has been suggested to be at the root of neurodegeneration. This hypothesis has been difficult to prove in vivo owing to the lack of appropriate tools. The recent report of transgenic mice with ubiquitous expression of a UPS-reporter protein should finally make it possible to test in vivo the role of the UPS in neurodegeneration.

Hernandez F, Dıaz-Hernandez M, Avila J and Lucas JJ. Testing the ubiquitin–proteasome hypothesis of neurodegeneration in vivo. TRENDS in Neurosciences 2004; 27(2): 66-68.

ALP in Parkinson’s
The ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway (ALP) are the two most important mechanisms that normally repair or remove abnormal proteins. Alterations in the function of these systems to degrade misfolded and aggregated proteins are being increasingly recognized as playing a pivotal role in the pathogenesis of many neurodegenerative disorders such as Parkinson’s disease. Dysfunction of the UPS has been already strongly implicated in the pathogenesis of this disease and, more recently, growing interest has been shown in identifying the role of ALP in neurodegeneration. Mutations of a-synuclein and the increase of intracellular concentrations of non-mutant a-synuclein have been associated with Parkinson’s disease phenotype.

The demonstration that a-synuclein is degraded by both proteasome and autophagy indicates a possible linkage between the dysfunction of the UPS or ALP and the occurrence of this disorder.The fact that mutant a-synucleins inhibit ALP functioning by tightly binding to the receptor on the lysosomal membrane for autophagy pathway further supports the assumption that impairment of the ALP may be related to the development of Parkinson’s disease.

In this review, we summarize the recent findings related to this topic and discuss the unique role of the ALP in this neurogenerative disorder and the putative therapeutic potential through ALP enhancement.

Pan Y, Kondo S, Le W, Jankovic J. The role of autophagy-lysosome pathway in
neurodegeneration associated with Parkinson’s disease. Brain 2008; 131: 1969-1978. doi:10.1093/brain/awm318.

Ubiquitin-Proteasome System in Parkinson’s

There is growing evidence that dysfunction of the mitochondrial respiratory chain and failure of the cellular protein degradation machinery, specifically the ubiquitin-proteasome system, play an important role in the pathogenesis of Parkinson’s disease. We now show that the corresponding pathways of these two systems are linked at the transcriptomic level in Parkinsonian substantia nigra. We examined gene expression in medial and lateral substantia nigra (SN) as well as in frontal cortex using whole genome DNA oligonucleotide microarrays. In this study, we use a hypothesis-driven approach in analysing microarray data to describe the expression of mitochondrial and ubiquitin-proteasomal system (UPS) genes in Parkinson’s disease (PD).

Although a number of genes showed up-regulation, we found an overall decrease in expression affecting the majority of mitochondrial and UPS sequences. The down-regulated genes include genes that encode subunits of complex I and the Parkinson’s-disease-linked UCHL1. The observed changes in expression were very similar for both medial and lateral SN and also affected the PD cerebral cortex. As revealed by “gene shaving” clustering analysis, there was a very significant correlation between the transcriptomic profiles of both systems including in control brains.

Therefore, the mitochondria and the proteasome form a higher-order gene regulatory network that is severely perturbed in Parkinson’s disease. Our quantitative results also suggest that Parkinson’s disease is a disease of more than one cell class, i.e. that it goes beyond the catecholaminergic neuron and involves glia as well.

Duke DC, Moran LB, Kalaitzakis ME, Deprez M, et al. Transcriptome analysis reveals link between proteasomal and mitochondrial pathways in Parkinson’s disease. Neurogenetics 2006; 7:139-148.
Bax Degradation a Novel Mechanism for Survival in Bcl-2 overexpressed cancer cells
The authors previously reported that proteasome inhibitors were able to overcome Bcl-2-mediated protection from apoptosis, and now show that inhibition of the proteasome activity in Bcl-2-overexpressing cells accumulates the proapoptotic Bax protein to mitochondrial cytoplasm, where it interacts to Bcl-2 protein. This event was followed by release of mitochondrial cytochrome c into the cytosol and activation of caspase-mediated apoptosis. In contrast, proteasome inhibition did not induce any apparent changes in Bcl-2 protein levels. In addition, treatment with a proteasome inhibitor increased levels of ubiquitinated forms of Bax protein, without any effects on Bax mRNA expression. They also established a cell-free Bax degradation assay in which an in vitro-translated, 35S-labeled Bax protein can be degraded by a tumor cell protein extract, inhibitable by addition of a proteasome inhibitor or depletion of the proteasome or ATP. The Bax degradation activity can be reconstituted in the proteasome-depleted supernatant by addition of a purified 20S proteasome or proteasome-enriched fraction. Finally, by using tissue samples of human prostate adenocarcinoma, they demonstrated that increased levels of Bax degradation correlated well with decreased levels of Bax protein and increased Gleason scores of prostate cancer. These studies strongly suggest that ubiquitin-proteasome-mediated Bax degradation is a novel survival mechanism in human cancer cells and that selective targeting of this pathway should provide a unique approach for treatment of human cancers, especially those overexpressing Bcl-2.
In the current study, These investigators report that

  • (i) proteasome inhibition results in Bax accumulation before release of cytochrome c and induction of apoptosis, which is associated with the ability of proteasome inhibitors to overcome Bcl-2-mediated antiapoptotic function;
  • (ii) Bax is regulated by an ATP-ubiquitin-proteasome-dependent degradation pathway; and
  • (iii) decreased levels of Bax protein correlate with increased levels of Bax degradation in aggressive human prostate cancer.

Li B and Dou QP. Bax degradation by the ubiquitin-proteasome-dependent pathway: Involvement in tumor survival and progression. PNAS 2000; 97(8): 3851-3855. http://www.pnas.org

p97 and DBeQ, ATP-competitive p97 inhibitor
A major limitation to current studies on the biological functions of p97/Cdc48 is that there is no method to rapidly shut off its ATPase activity. Given the range of cellular processes in which Cdc48 participates, it is difficult to determine whether any particular phenotype observed in the existing mutants is due to a direct or indirect effect. Moreover, inhibition of p97 activity in animal cells by siRNA or expression of a dominant-negative version is challenged by its high abundance and is not suited to evaluating proximal phenotypic effects of p97 loss of function.

A specific small-molecule inhibitor of p97 would provide an important tool to investigate diverse functions of this essential ATPase associated with diverse cellular activities (AAA) ATPase and to evaluate its potential to be a therapeutic target in human disease. Cancer cells may be particularly sensitive to killing by suppression of protein degradation mechanisms, because they may exhibit a heightened dependency on these mechanisms to clear an elevated burden of quality-control substrates. For example, some cancers produce high levels of a specific protein that is a prominent quality-control substrate (e.g., Ig light chains in multiple myeloma) or produce high levels of reactive oxygen species, which can result in excessive protein damage via oxidation. Therefore, a specific p97 inhibitor would be a valuable research tool to investigate p97 function in cells.

We carried out a high-throughput screen to identify inhibitors of p97 ATPase activity. Dual-reporter cell lines that simultaneously express p97-dependent and p97-independent proteasome substrates were used to stratify inhibitors that emerged from the screen. N2,N4-dibenzylquinazoline-2,4-diamine (DBeQ) was identified as a selective,potent, reversible, and ATP-competitive p97 inhibitor.

DBeQ blocks multiple processes that have been shown by RNAi to depend on p97, including degradation of ubiquitin fusion degradation and endoplasmic reticulum-associated degradation pathway reporters, as well as autophagosome maturation. DBeQ also potently inhibits cancer cell growth and is more rapid than a proteasome inhibitor at mobilizing the executioner caspases-3 and -7.

Simultaneous inhibition of proteasome and histone deacetylase 6 (HDAC6) [which is required for autophagy results in synergistic killing of multiple myeloma cells]. Interestingly, more than one dozen human clinical trials (www.clinicaltrials.gov) combine bortezomib with the broad-spectrum HDAC inhibitor vorinostat, which is active toward HDAC6. Targeting p97
may provide an alternative route to achieving the same objective. Our results provide a rationale for targeting p97 in cancer therapy. Future work will provide molecular insight into how inhibition of p97 activity by DBeQ results in apoptosis and could strengthen the rationale for a p97-targeted cancer therapeutic.

Chou TF, Brown SJ, Minond D, Nordin BE, et al. Reversible inhibitor of p97, DBeQ, impairs both ubiquitin-dependent and autophagic protein clearance pathways. PNAS 2011; pp 6 http://www.pnas.org/cgi/doi/10.1073/pnas.1015312108

The causes of various neurodegenerative diseases, particularly sporadic cases, remain unknown, but increasing evidence suggests that these diseases may share similar molecular and cellular mechanisms of pathogenesis. One prominent feature common to most neurodegenerative diseases is the accumulation of misfolded proteins in the form of insoluble protein aggregates or inclusion bodies. Although these aggregates have different protein compositions, they all contain ubiquitin and proteasome subunits, implying a failure of the ubiquitin-proteasome system (UPS) in the removal of misfolded proteins.

A direct link between UPS dysfunction and neurodegeneration has been
provided by recent findings that genetic mutations in UPS components cause several rare, familial forms of neurodegenerative diseases. Furthermore, it is becoming increasingly clear that oxidative stress, which results from aging or exposure to environmental toxins, can directly damage UPS components, thereby contributing to the pathogenesis of sporadic forms of neurodegenerative diseases.

Aberrations in the UPS often result in defective proteasome-mediated protein degradation, leading to accumulation of toxic proteins and eventually to neuronal cell death. Interestingly, emerging evidence has begun to suggest that impairment in substrate-specific components of the UPS, such as E3 ubiquitin-protein ligases, may cause aberrant ubiquitination and neurodegeneration in a proteasome-independent manner. This provides an overview of the molecular components of the UPS and their impairment in familial and sporadic forms of neurodegenerative diseases, and summarizes present knowledge about the pathogenic mechanisms of UPS dysfunction in neurodegeneration.

Molecular mechanisms of protein ubiquitination and degradation by the UPS. Ubiquitination involves a highly specific enzyme cascade in which

  • ubiquitin (Ub) is first activated by the ubiquitinactivating enzyme (E1),
  • then transferred to an ubiquitin-conjugating enzyme (E2), and
  • finally covalently attached to the substrate by an ubiquitin-protein ligase (E3).

Ubiquitination is a reversible posttranslational modification in which the removal of Ub is mediated by a deubiquitinating enzyme (DUB).

  • Substrate proteins can be either monoubiquitinated or polyubiquitinated through successive conjugation of Ub moieties to an internal lysine residue in Ub.
  • K48-linked poly-Ub chains are recognized by the 26S proteasome, resulting in degradation of the substrate and recycling of Ub.
  • Monoubiquitination or K63-linked polyubiquitination plays a number of regulatory roles in cells that are proteasome-independent.

Parkin

Loss-of-function mutations in parkin, a 465-amino-acid RING-type E3 ligase, were first identified as the cause for autosomal recessive juvenile Parkinsonism (AR-JP) and subsequently found to account for ~50% of all recessively transmitted early-onset PD cases. Interestingly, patients with parkin mutations do not exhibit Lewy body pathology.

Possible pathogenic mechanisms by which impaired UPS components cause neurodegeneration. Genetic mutations or oxidative stress from aging and/or exposure to environmental toxins have been shown to impair the ubiquitination machinery (particularly E3 ubiquitin-protein ligases) and deubiquitinating enzymes (DUBs), resulting in abnormal ubiquitination. Depending on the type of ubiquitination affected, the impairment could cause neurodegeneration through two different mechanisms.

In the first model, aberrant K48-linked polyubiquitination resulting from impaired E3s or DUBs alters protein degradation by the proteasome, leading to accumulation of toxic proteins and subsequent neurodegeneration. The proteasomes could be directly damaged by oxidative stress or might be inhibited by protein aggregation, which exacerbates the neurotoxicity.

In the second model, aberrant monoubiquitination or K63-linked polyubiquitination resulting from impaired E3s or DUBs alters crucial non-proteasomal functions, such as gene transcription and protein trafficking, thereby causing neurodegeneration without protein aggregation.

These two models are not mutually exclusive because a single E3 or DUB enzyme, such as parkin or UCH-L1, could regulate more than one type of ubiquitination. In addition, abnormal ubiquitination and neurodegeneration could also result from mutation or oxidative stress-induced structural changes in the protein substrates that alter their recognition and degradation by the UPS.

Lian Li and Chin LS. IMPAIRMENT OF THE UBIQUITIN-PROTEASOME SYSTEM: A COMMON PATHOGENIC MECHANISM IN NEURODEGENERATIVE DISORDERS. In The Ubiquitin Proteasome System…Chapter 23. (Eds: Eds: Mario Di Napoli and Cezary Wojcik) 553-577 © 2007 Nova Science Publishers, Inc. ISBN 978-1-60021-749-4.

filedesc Schematic diagram of the ubiquitylati...

filedesc Schematic diagram of the ubiquitylation system. Created by Roger B. Dodd (Photo credit: Wikipedia)

 

Current Noteworthy Work

Nassif M and Hetz C.  Autophagy impairment: a crossroad between neurodegeneration and tauopathies.  BMC Biology 2012; 10:78. http://www.biomedcentral.com/1741-7007/10/78

Impairment of protein degradation pathways such as autophagy is emerging as a consistent and transversal pathological phenomenon in neurodegenerative diseases, including Alzheimer´s, Huntington´s, and Parkinson´s disease. Genetic inactivation of autophagy in mice has demonstrated a key role of the pathway in maintaining protein homeostasis in the brain, triggering massive neuronal loss and the accumulation of abnormal protein inclusions.  A paper in Molecular Neurodegeneration from Abeliovich´s group now suggests a role for phosphorylation of Tau and the activation of glycogen synthase kinase 3β (GSK3β) in driving neurodegeneration in autophagy-deficient neurons. We discuss the implications of this study for understanding the factors driving neurofibrillary tangle formation in Alzheimer´s disease and tauopathies.

Cajee UF, Hull R and Ntwasa M. Modification by Ubiquitin-Like Proteins: Significance in Apoptosis and Autophagy Pathways. Int. J. Mol. Sci. 2012, 13, 11804-11831; doi:10.3390/ijms130911804

Ubiquitin-like proteins (Ubls) confer diverse functions on their target proteins. The modified proteins are involved in various biological processes, including DNA replication, signal transduction, cell cycle control, embryogenesis, cytoskeletal regulation,
metabolism, stress response, homeostasis and mRNA processing. Modifiers such as SUMO, ATG12, ISG15, FAT10, URM1, and UFM have been shown to modify proteins thus conferring functions related to programmed cell death, autophagy and regulation of
the immune system. Putative modifiers such as Domain With No Name (DWNN) have been identified in recent times but not fully characterized. In this review, we focus on cellular processes involving human Ubls and their targets.

Aloy P. Shaping the future of interactome networks. (A report of the third Interactome Networks Conference, Hinxton, UK, 29 August-1 September 2007). Genome Biology 2007; 8:316 (doi:10.1186/gb-2007-8-10-316)

Complex systems are often networked, and biology is no exception. Following on from the genome sequencing projects,
experiments show that proteins in living organisms are highly connected, which helps to explain how such great complexity
can be achieved by a comparatively small set of gene products. At a recent conference on interactome networks held outside
Cambridge, UK, the most recent advances in research on cellular networks were discussed. This year’s conference focused on
identifying the strengths and weaknesses of currently resolved interaction networks and the techniques used to determine
them – reflecting the fact that the field of mapping interaction networks is maturing.

Peroutka RJ, Orcutt SJ, Strickler JE, and Butt TR. SUMO Fusion Technology for Enhanced Protein Expression and Purification in Prokaryotes and Eukaryotes. Chapter 2. in T.C. Evans, M.-Q. Xu (eds.), Heterologous Gene Expression in E. coli, Methods in Molecular Biology 705:15-29. DOI 10.1007/978-1-61737-967-3_2, © Springer Science+Business Media, LLC 2011

The preparation of sufficient amounts of high-quality protein samples is the major bottleneck for structural proteomics. The use of recombinant proteins has increased significantly during the past decades. The most commonly used host, Escherichia coli, presents many challenges including protein misfolding, protein degradation, and low solubility. A novel SUMO fusion technology appears to enhance protein expression and solubility (www.lifesensors.com). Efficient removal of the SUMO tag by SUMO protease in vitro facilitates the generation of target protein with a native N-terminus. In addition to its physiological relevance in eukaryotes, SUMO can be used as a powerful biotechnology tool for enhanced functional protein expression in prokaryotes and eukaryotes.

Juang YC, Landry MC, et al. OTUB1 Co-opts Lys48-Linked Ubiquitin Recognition to Suppress E2 Enzyme Function. Molecular Cell 2012; 45: 384–397. DOI 10.1016/j.molcel.2012.01.011

Ubiquitylation entails the concerted action of E1, E2, and E3 enzymes. We recently reported that OTUB1, a deubiquitylase, inhibits the DNA damage response independently of its isopeptidase activity. OTUB1 does so by blocking ubiquitin transfer by UBC13, the cognate E2 enzyme for RNF168. OTUB1 also inhibits E2s of the UBE2D and UBE2E families. Here we elucidate the structural mechanism by which OTUB1 binds E2s to inhibit ubiquitin transfer. OTUB1 recognizes ubiquitin-charged E2s through contacts with both donor ubiquitin and the E2 enzyme. Surprisingly, free ubiquitin associates with the canonical distal ubiquitin-binding site on OTUB1 to promote formation of the inhibited E2 complex. Lys48 of donor ubiquitin lies near the OTUB1 catalytic site and the C terminus of free ubiquitin, a configuration that mimics the products of Lys48-linked ubiquitin chain cleavage. OTUB1 therefore co-opts Lys48-linked ubiquitin chain recognition to suppress ubiquitin conjugation and the DNA damage response.

Hunter T. The Age of Crosstalk: Phosphorylation, Ubiquitination, and Beyond. Molecular Cell  2007; 28:730-738. DOI 10.1016/ j.molcel.2007.11.019.

Crosstalk between different types of posttranslational modification is an emerging theme in eukaryotic biology. Particularly prominent are the multiple connections between phosphorylation and ubiquitination, which act either positively or negatively in both directions to regulate these processes.

Tu Y, Chen C, et al. The Ubiquitin Proteasome Pathway (UPP) in the regulation of cell cycle control and DNA damage repair and its implication in tumorigenesis. Int J Clin Exp Pathol 2012;5(8):726-738. www.ijcep.com /ISSN:1936-2625/IJCEP1208018

Accumulated evidence supports that the ubiquitin proteasome pathway (UPP) plays a crucial role in protein
metabolism implicated in the regulation of many biological processes such as cell cycle control, DNA damage
response, apoptosis, and so on. Therefore, alterations for the ubiquitin proteasome signaling or functional impairments
for the ubiquitin proteasome components are involved in the etiology of many diseases, particularly in cancer
development.The authors discuss the ubiquitin proteasome pathway in the regulation of cell cycle control and DNA
damage response, the relevance for the altered regulation of these signaling pathways in tumorigenesis, and finally
assess and summarize the advancement for targeting the ubiquitin proteasome pathway in cancer therapy.

Cebollero E , Reggiori F  and Kraft C.  Ribophagy: Regulated Degradation of Protein Production Factories. Int J Cell Biol. 2012; 2012: 182834. doi:  10.1155/2012/182834 (online).

During autophagy, cytosol, protein aggregates, and organelles are sequestered into double-membrane vesicles called autophagosomes and delivered to the lysosome/vacuole for breakdown and recycling of their basic components. In all eukaryotes this pathway is important for adaptation to stress conditions such as nutrient deprivation, as well as to regulate intracellular homeostasis by adjusting organelle number and clearing damaged structures. For a long time, starvation-induced autophagy has been viewed as a nonselective transport pathway; however, recent studies have revealed that autophagy is able to selectively engulf specific structures, ranging from proteins to entire organelles. In this paper, we discuss recent findings on the mechanisms and physiological implications of two selective types of autophagy: ribophagy, the specific degradation of ribosomes, and reticulophagy, the selective elimination of portions of the ER.

Lee JH, Yu WH,…, Nixon RA.  Lysosomal Proteolysis and Autophagy Require Presenilin 1 and Are Disrupted by Alzheimer-Related PS1 Mutations. Cell 2010; 141, 1146–1158. DOI 10.1016/j.cell.2010.05.008.

Macroautophagy is a lysosomal degradative pathway essential for neuron survival. Here, we show that macroautophagy requires the Alzheimer’s disease (AD)-related protein presenilin-1 (PS1). In PS1 null blastocysts, neurons from mice hypomorphic for PS1 or
conditionally depleted of PS1, substrate proteolysis and autophagosome clearance during macroautophagy are prevented as a result of a selective impairment of autolysosome acidification and cathepsin activation. These deficits are caused by failed PS1-dependent
targeting of the v-ATPase V0a1 subunit to lysosomes. N-glycosylation of the V0a1 subunit, essential for its efficient ER-to-lysosome delivery, requires the selective binding of PS1 holoprotein to the unglycosylated subunit and the  sec61alpha/ oligosaccharyltransferase complex. PS1 mutations causing early-onset AD produce a similar lysosomal/autophagy phenotype in
fibroblasts from AD patients. PS1 is therefore essential for v-ATPase targeting to lysosomes, lysosome acidification, and proteolysis during autophagy. Defective lysosomal proteolysis represents a basis for pathogenic protein accumulations and neuronal cell death in AD and suggests previously unidentified therapeutic targets.

Pohl C and Jentsch S. Midbody ring disposal by autophagy is a post-abscission event of cytokinesis. nature cell biology 2009; 11 (1): 65-70.  DOI: 10.1038/ncb1813.

At the end of cytokinesis, the dividing cells are connected by an intercellular bridge, containing the midbody along with a single,
densely ubiquitylated, circular structure called the midbody ring (MR). Recent studies revealed that the MR serves as a target
site for membrane delivery and as a physical barrier between the prospective daughter cells. The MR materializes in telophase,
localizes to the intercellular bridge during cytokinesis, and moves asymmetrically into one cell after abscission. Daughter
cells rarely accumulate MRs of previous divisions, but how these large structures finally disappear remains unknown.
Here, we show that MRs are discarded by autophagy, which involves their sequestration into autophagosomes and delivery to
lysosomes for degradation. Notably, autophagy factors, such as the ubiquitin adaptor p62 and the ubiquitin-related protein Atg8 , associate with the MR during abscission, suggesting that autophagy is coupled to cytokinesis. Moreover, MRs accumulate in cells of patients with lysosomal storage disorders, indicating that defective MR disposal is characteristic of these diseases. Thus our findings suggest that autophagy has a broader role than previously assumed, and that cell renovation by clearing from superfluous large macromolecular assemblies, such as MRs, is an important autophagic function.

 

Hanai JI, Cao P, Tanksale P, Imamura S, et al. The muscle-specific ubiquitin ligase atrogin-1/MAFbx mediates statin-induced muscle toxicity. The Journal of Clinical Investigation  2007; 117(12):3930-3951.    http://www.jci.org

Statins inhibit HMG-CoA reductase, a key enzyme in cholesterol synthesis, and are widely used to treat hypercholesterolemia.
These drugs can lead to a number of side effects in muscle, including muscle fiber breakdown; however, the mechanisms of muscle injury by statins are poorly understood. We report that lovastatin induced the expression of atrogin-1, a key gene involved in skeletal muscle atrophy, in humans with statin myopathy, in zebrafish embryos, and in vitro in murine skeletal muscle cells. In cultured mouse myotubes, atrogin-1 induction following lovastatin treatment was accompanied by distinct morphological changes, largely absent in
atrogin-1 null cells. In zebrafish embryos, lovastatin promoted muscle fiber damage, an effect that was closely mimicked by knockdown of zebrafish HMG-CoA reductase. Moreover, atrogin-1 knockdown in zebrafish embryos prevented lovastatin-induced muscle injury. Finally, overexpression of PGC-1α, a transcriptional coactivator that induces mitochondrial biogenesis and protects against the development of muscle atrophy, dramatically prevented lovastatin-induced muscle damage and abrogated atrogin-1 induction both in fish and in cultured mouse myotubes. Collectively, our human, animal, and in vitro findings shed light on the molecular mechanism of statin-induced myopathy and suggest that atrogin-1 may be a critical mediator of the muscle
damage induced by statins.

Inami Y, Waguri S, Sakamoto A, Kouno T, et al.  Persistent activation of Nrf2 through p62 in hepatocellular carcinoma cells. J. Cell Biol. 2011; 193(2): 275–284. http://www.jcb.org/cgi/doi/10.1083/jcb.201102031

Macroautophagy (hereafter referred to as autophagy) is a cellular degradation system in which cytoplasmic components, including
organelles, are sequestered by double membrane structures called autophagosomes and the sequestered materials are
degraded by lysosomal hydrolases for supply of amino acids and for cellular homeostasis. Although autophagy has generally been considered nonselective, recent studies have shed light on another indispensable role for basal autophagy in cellular homeostasis, which is mediated by selective degradation of a specific substrate(s).  p62 is a ubiquitously expressed cellular protein that is conserved in metazoa but not in plants and fungi, and recently it has been known as one of the selective substrates for autophagy.
This protein is localized at the autophagosome formation site  and directly interacts with LC3, an autophagosome localizing protein . Subsequently, the p62 is incorporated into the autophagosome and then degraded. Therefore, impaired autophagy is accompanied by
accumulation of p62 followed by the formation of p62 and ubiquitinated protein aggregates because of the nature of both self- oligomerization and ubiquitin binding of p62.

 

Kima K, Khayrutdinov BI, Leeb CK, et al. Solution structure of the Zβ domain of human DNA-dependent activator of IFN-regulatory factors and its binding modes to B- and Z-DNAs. PNAS 2010; Early Edition ∣ pp 6. www.pnas.org/cgi/doi/10.1073/pnas.1014898107

The DNA-dependent activator of IFN-regulatory factors (DAI), also known as DLM-1/ZBP1, initiates an innate immune response by binding to foreign DNAs in the cytosol. For full activation of the immune response, three DNA binding domains at the N terminus are required: two Z-DNA binding domains (ZBDs), Zα and Zβ, and an adjacent putative B-DNA binding domain. The crystal structure of the Zβ domain of human DAI (hZβDAI) in complex with Z-DNA revealed structural features distinct from other known Z-DNA binding proteins, and it was classified as a group II ZBD. To gain structural insights into the DNA binding mechanism of hZβDAI, the solution structure of the free hZβDAI was solved, and its bindings to B- and Z-DNAs were analyzed by NMR spectroscopy. Compared to the Z-DNA–bound structure, the conformation of free hZβDAI has notable alterations in the α3 recognition helix, the “wing,” and Y145, which are critical in Z-DNA recognition. Unlike some other Zα domains, hZβDAI appears to have conformational flexibility, and structural adaptation is required for Z-DNA binding. Chemical-shift perturbation experiments revealed that hZβDAI also binds weakly to B-DNA via a different binding mode. The C-terminal domain of DAI is reported to undergo a conformational change on B-DNA binding; thus, it is possible that these changes are correlated. During the innate immune response, hZβDAI is likely to play an active role in binding to DNAs in both B and Z conformations in the recognition of foreign DNAs.

 

Epicrisis

This extensive review leaves little left unopened. We have seen the central role that the UPS system plays in normal organelle proteolysis in concert with autophagy. Impaired ubiquitination occurs from aging, and/or toxins, under oxidative stress involving E3s or DUBs.

This leads to altered gene transcripton, altered protein trafficking, and plays a role in neurodegenative disease, muscle malfunction, and cancer as well.

English: A cartoon representation of a lysine ...

English: A cartoon representation of a lysine 48-linked diubiquitin molecule. The two ubiquitin chains are shown as green cartoons with each chain labelled. The components of the linkage are indicated and shown as orange sticks. Image was created using PyMOL from PDB id 1aar. (Photo credit: Wikipedia)

Different forms of protein ubiquitylation

Different forms of protein ubiquitylation (Photo credit: Wikipedia)

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