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List of abbreviation:
DTNB- 5,5- dithiobis,2-nitrobenzoic acid
t-BHP- Tertiary butyl hydroperoxide
GSH-Reduced glutathione
GSSG- Oxidised glutathione
Objective: The study was taken up to see the effect of melatonin on the alteration of reduced glutathione level in red blood cells.
Pineal melatonin is involved in many physiological functions, the most important among them being sleep promotion and circadian regulation. This pineal product exhibits characteristic diurnal rhythm of synthesis and secretion, which attains its peak at night followed by a gradual decrease during the daytime. Melatonin detoxifies highly toxic hydroxyl and peroxyl radicals in vitro, scavenges hydrochlorous acid, as well as peroxynitrite. It has also been reported to increase the synthesis of glutathione and of several antioxidant enzymes [1].
Method: The present study was undertaken to understand the modulation of intracellular reduced glutathione (GSH) by melatonin in human red blood cells according to the oscillatory circadian changes in levels of this hormone.We have also studied the dose-dependent effect of melatonin on GSH in erythrocytes obtained from blood at two different times, subjected to oxidative stress by incubating with tert-butyl hydroperoxide (t-BHP) [2]. We used t-BHP as pro-oxidant [3]. Erythrocyte GSH was measured following the method of Beutler [4]. The method was based on the ability of the –SH group to reduce 5,5- dithiobis,2-nitrobenzoic acid (DTNB) and form a yellow coloured anionic product whose OD is measured at 412 nm.
A suspension of packed red blood cells in phosphate-buffered saline (PBS) containing glucose was treated with melatonin taken at different concentrations. A stock solution (10mM) of melatonin was prepared in absolute ethanol; further dilutions (100 uM–10 nM) were done with PBS. The concentration of ethanol was alwaysThe in vitro effect of melatonin was evaluated by incubating erythrocytes with melatonin at different doses (10 uM –1 nM final concentration) of melatonin for 30 minutes at 37°C. After washing the erythrocytes with the buffer, to remove any amount of the compound, and finally, packed erythrocytes were used for the assay of GSH. In parallel control experiments, blood was incubated with ethanol (final concentration not more than 0.01% (v/v)) but without melatonin.Oxidative stress was induced in vitro by using tert-butyl hydroperoxide both in presence and absence of melatonin. Use of TBHP is in accordance with the published reports [5].
Results and Discussion: The experiment demonstrated that erythrocyte GSH level increased in nocturnal samples which highlights the role of endogenous melatonin in the circadian changes in cellular glutathione level. Exogenous melatonin demonstrated a protective effect against t-BHP-induced peroxidative damage in both diurnal and nocturnal samples, the effect being more pronounced in aliquots containing very low concentration of melatonin (10 nM – 1 nM) [6]. Melatonin was found to inhibit GSH oxidation in a dose-dependent manner.
Melatonin has been found to upregulate cellular glutathione level to check lipid peroxidation in brain cells [7]. We may say that the incubation of the red cells with melatonin for an extended period (more than 30 minutes) may not have the same effects on the level of glutathione in these cells [12]. Melatonin may act as pro-oxidant in the cells exposed to the indoleamine for longer time. Also, the half-life period of pineal melatonin is for 30 to 60 minutes, as reviewed by Karasek and Winczyk [11].The recycling of glutathione in the cells depends on an NADPH-dependent glutathione enzyme system which includes glutathione peroxidise, glutathione reductase, and γ-glutamyl-cysteine synthase forming a meshwork of an antioxidative system. The stimulatory effect of melatonin on the regulation of the antioxidant enzymes has been reported [8].Since melatonin has an amphiphilic nature, its antioxidative efficiency crosses the cellular membrane barriers in a non-receptor-mediated mechanism. Another explanation of melatonin’s antioxidative activity may be based on its role in the upregulation of some antioxidant enzymes directly. Blanco et al had reported that glutathione reductase and glutathione peroxidase, the major constituents of the glutathione-redox system being stimulated by melatonin [9]. The plasma GSH/GSSG redox state is controlled by multiple processes, which includes synthesis of GSH from its constitutive amino acids, cyclic oxidation and reduction involving GSH peroxidase and GSSG reductase, transport of GSH into the plasma, and the degradation of GSH and GSSG by γ-glutamyltranspeptidase. The increase in erythrocyte GSH concentration after melatonin administration can be related Blanco et al’s report on the known stimulation of γ-glutamylcysteine synthase,a rate-limiting enzyme in reduced glutathione synthesis, by melatonin [10].
Conclusion: On the basis of our study, we may conclude that melatonin affects the glutathione level in red blood cells in a circadian manner. The rhythmic pattern of glutathione level confirms the relationship between physiological melatonin and erythrocyte GSH level and pharmacological dosage of the drug. The role of melatonin as an antioxidant and its activity in relation to these biomarkers has been studied in the above experiments.
AMPK Is a Negative Regulator of the Warburg Effect and Suppresses Tumor Growth In Vivo
Reporter-Curator: Stephen J. Williams, Ph.D.
Word Cloud by Daniel Menzin
There has been a causal link between alterations in cellular metabolism and the cancer phenotype. Reorganization of cellular metabolism, marked by a shift from oxidative phosphorylation to aerobic glycolysis for cellular energy requirements (Warburg effect), is considered a hallmark of the transformed cell. In addition, if tumors are to survive and grow, cancer cells need to adapt to environments high in metabolic stress and to avoid programmed cell death (apoptosis). Recently, a link between cancer growth and metabolism has been supported by the discovery that the LKB1/AMPK signaling pathway as a tumor suppressor axis[1].
LKB1/AMPK/mTOR Signaling Pathway
The Liver Kinase B1 (LKB1)/AMPK AMP-activated protein kinase/mammalian Target of Rapamycin Complex 1 (mTORC1) signaling pathway links cellular metabolism and energy status to pathways involved in cell growth, proliferation, adaption to energy stress, and autophagy. LKB1 is a master control for 14 other kinases including AMPK, a serine-threonine kinase which senses cellular AMP/ATP ratios. In response to cellular starvation, AMPK is allosterically activated by AMP, leading to activation of ATP-generating pathways like fatty acid oxidation and blocking anabolic pathways, like lipid and cholesterol synthesis (which consume ATP). In addition, AMPK regulates cell growth, proliferation, and autophagy by regulating the mTOR pathway. AMPK activates the tuberous sclerosis complex 1/2, which ultimately inhibits mTORC1 activity and inhibits protein translation. This mTOR activity is dis-regulated in many cancers.
LKB1/AMPK in Cancer
Somatic mutations of the STK11 gene encoding LKB1 are detected in lung and cervical cancers
Therefore LKB1 may be a strong tumor suppressor
Pharmacologic activation of LKB1/AMPK with metformin can suppress cancer cell growth
In a recent Cell Metabolism paper[2], Brandon Faubert and colleagues describe how AMPK activity reduces aerobic glycolysis and tumor proliferation while loss of AMPK activity promotes tumor proliferation by shifting cells to aerobic glycolysis and increasing anabolic pathways in a HIF1-dependent manner.
The paper’s major findings were as follows:
Loss of AMPKα1 cooperates with the Myc oncogene to accelerate lymphomagenesis
Inhibiting HIF-1α reverses the metabolic effects of AMPKα loss
HIF-1α mediates the growth advantage of tumors with reduced AMPK signaling
Summary
AMPK is a metabolic sensor that helps maintain cellular energy homeostasis. Despite evidence linking AMPK with tumor suppressor functions, the role of AMPK in tumorigenesis and tumor metabolism is unknown. Here we show that AMPK negatively regulates aerobic glycolysis (the Warburg effect) in cancer cells and suppresses tumor growth in vivo. Genetic ablation of the α1 catalytic subunit of AMPK accelerates Myc-induced lymphomagenesis. Inactivation of AMPKα in both transformed and nontransformed cells promotes a metabolic shift to aerobic glycolysis, increased allocation of glucose carbon into lipids, and biomass accumulation. These metabolic effects require normoxic stabilization of the hypoxia-inducible factor-1α (HIF-1α), as silencing HIF-1α reverses the shift to aerobic glycolysis and the biosynthetic and proliferative advantages conferred by reduced AMPKα signaling. Together our findings suggest that AMPK activity opposes tumor development and that its loss fosters tumor progression in part by regulating cellular metabolic pathways that support cell growth and proliferation.
Below is the graphical abstract of this paper.
(Photo credit reference(2; Faubert et. al) permission from Elsevier)
However, this regulation of tumor promotion by AMPK may be more complicated and dependent on the cellular environment.
Nissam Hay from the University of Illinois College of Medicine, Chicago, Illinois, USA and his co-workers Sang-Min Jeon and Navdeep Chandel were investigating the mechanism through which LKB1/AMPK regulate the balance between cancer cell growth and apoptosis under energy stress[3]. In their system, the loss of function of either of these proteins makes cells more sensitive to apoptosis in low glucose environments, and cells deficient in either AMPK or LKB1 were shown to be resistant to oncogenic transformation. Whereas previous studies showed (as above) AMPK opposes tumor proliferation in a HIF1-dependent manner, their results showed AMPK could promote tumor cell survival during periods of low glucose or altered redox status.
The researchers incubated LKB1-deficient cancer cells in the presence of either glucose or one of the non-metabolizable glucose analogues 2-deoxyglucose (2DG) and 5-thioglucose (5TG), and found that 2DG, but not 5TG, induced the activation of AMPK and protected the cells from apoptosis, even in cells that were deficient in LKB1.
The authors demonstrated that glucose deprivation depleted NADPH levels, increased H2O2 levels and increased cell death, and that this was accelerated in cells deficient in the enzyme glucose-6-phosphate dehydrogenase. Anti-oxidants were also found to inhibit cell death in cells deficient in either AMPK or LKB1.
Knockdown or knockout of either LKB1 or AMPK in cancer cells significantly increased levels of H2O2 but not of peroxide (O2–) during glucose depletion. The glucose analogue 2DG was able to activate AMPK and maintain high levels of NADPH and low levels of H2O2 in these cells.
The nucleotide coenzyme NADPH is generated in the pentose phosphate pathway and mitochondrial metabolism, and consumed in H2O2 elimination and fatty acid synthesis. If glucose is limited mitochondrial metabolism becomes the major source of NADPH, supported by fatty acid oxidation. AMPK is known to be a regulator of fatty acid metabolism through inhibition of two acetyl-CoA carboxylases, ACC1 and ACC2.
Short interfering RNAs (siRNAs) to knock down levels of both ACC1 and ACC2 in A549 cancer cells and found that only ACC2 knockdown significantly increased peroxide accumulation and apoptosis, while over-expression of mutant ACC1 and ACC2 in LKB1-proficient cells increased H2O2 and apoptosis.
Therefore, it was concluded AMPK acts to promote early tumor growth and prevent apoptosis in conditions of energy stress through inhibiting acetyl-CoA carboxylase activity, thus maintaining NADPH levels and preventing the build-up of peroxide in glucose-deficient conditions.
This may appear to be conflicting with the previous report in this post however, it is possible that these reports reflect differences in the way cells respond to various cellular stresses, be it hypoxia, glucose deprivation, or changes in redox status. Therefore a complex situation may arise:
AMPK promotes tumor progression under glucose starvation
AMPK can oppose tumor proliferation under a normoxic, HIF1-dependent manner
Could AMPK regulation be different in cancer stem cells vs. non-stem cell?
References:
1. Green AS, Chapuis N, Lacombe C, Mayeux P, Bouscary D, Tamburini J: LKB1/AMPK/mTOR signaling pathway in hematological malignancies: from metabolism to cancer cell biology. Cell Cycle 2011, 10(13):2115-2120.
2. Faubert B, Boily G, Izreig S, Griss T, Samborska B, Dong Z, Dupuy F, Chambers C, Fuerth BJ, Viollet B et al: AMPK is a negative regulator of the Warburg effect and suppresses tumor growth in vivo. Cell metabolism 2013, 17(1):113-124.
3. Jeon SM, Chandel NS, Hay N: AMPK regulates NADPH homeostasis to promote tumour cell survival during energy stress. Nature 2012, 485(7400):661-665.
Other posts on this site related to Warburg Effect and Cancer include:
This post is in continuation to Part 1 by the same title.
In part one I covered the basics of role of redox chemistry in immune reactions, the phagosome cauldron, and how bacteria bacteria, virus and parasites trigger the complex pathway of NO production and its downstream effects. While we move further in this post, the previous post can be accessed here.
Regulation of the redox immunomodulators—NO/RNS and ROS
In addition to eradicating pathogens, NO/RNS and ROS and their chemical interactions act as effective immunomodulators that regulate many cellular metabolic pathways and tissue repair and proinflammatory pathways. Figure 3 shows these pathways.
Figure 3. Schematic overview of interactive connections between NO and ROS-mediated metabolic pathways. Credit: (Wink et al., 2011)
Regulation of iNOS enzyme activity is critical to NO production. Factors such as the availability of arginine, BH4, NADPH, and superoxide affect iNOS activity and thus NO production. In the absence of arginine and BH4 iNOS becomes a O2_/H2O2 generator (Vásquez-Vivar et al., 1999). Hence metabolic pathways that control arginine and BH4 play a role in determining the NO/superoxide balance. Arginine levels in cells depend on various factors such as type of uptake mechanisms that determine its spatial presence in various compartments and enzymatic systems. As shown in Fig3 Arginine is the sole substrate for iNOS and arginase. Arginase is another key enzyme in immunemodulation. AG is also regulated by NOS and NOX activities. NOHA, a product of NOS, inhibits AG, and O2–increases AG activity. Importantly, high AG activity is associated with elevated ROS and low NO fluxes. NO antagonises NOX2 assembly that in turn leads to reduction in O2_ production. NO also inhibits COX2 activity thus reducing ROS production. Thus, as NO levels decline, oxidative mechanisms increase. Oxidative and nitrosative stress can also decrease intracellular GSH (reduced form) levels, resulting in a reduced antioxidant capability of the cell.
Immune-associated redox pathways regulate other important metabolic cell functions that have the potential for widespread impact on cells, organs, and organisms. These pathways, such as mediated via methionine and polyamines, are critical for DNA stabilization, cell proliferation, and membrane channel activity, all of which are also involved in immune-mediated repair processes.
NO levels dictate the immune signaling pathway
NO/RNS and ROS actively control innate and adaptive immune signaling by participating in induction, maintenance, and/or termination of proinflammatory and anti-inflammatory signaling. As in pathogen eradication, the temporal and spatial concentration profiles of NO are key factors in determining immune-mediated processes.
Brune and coworkers (Messmer et al., 1994) first demonstrated that p53 expression was associated with the concentrations of NO that led to apoptosis in macrophages. Subsequent studies linked NO concentration profiles with expression of other key signaling proteins such as HIF-1α and Akt-P (Ridnour et al., 2008; Thomas et al., 2008). Various levels of NO concentrations trigger different pathways and expectedly this concentration-dependent profile varies with distance from the NO source.NO is highly diffucible and this characteristic can result in 1000 fold reduction in concentration within one cell length distance travelled from the source of production. Time course studies have also shown alteration in effects of same levels of NO over time e.g. NO-mediated ERK-P levels initially increased rapidly on exposure to NO donors and then decreased with continued NO exposure (Thomas et al., 2004), however HIF-1α levels remained high as long as NO levels were elevated. Thus some of the important factors that play critical role in NO effects are: distance from source, NO concentrations, duration of exposure, bioavailability of NO, and production/absence of other redox molecules.
Figure and legend credits: (Wink et al., 2011)
Fig 4: The effect of steady-state flux of NO on signal transduction mechanisms.
This diagram represents the level of sustained NO that is required to activate specific pathways in tumor cells. Similar effects have been seen on endothelial cells. These data were generated by treating tumor or endothelial cells with the NO donor DETANO (NOC-18) for 24 h and then measuring the appropriate outcome measures (for example, p53 activation). Various concentrations of DETANO that correspond to cellular levels of NO are: 40–60 μM DETANO = 50 nM NO; 80–120 μM DETANO = 100 nM NO; 500 μM DETANO = 400 nM NO; and 1 mM DETANO = 1 μM NO. The diagram represents the effect of diffusion of NO with distance from the point source (an activated murine macrophage producing iNOS) in vitro (Petri dish) generating 1 μM NO or more. Thus, reactants or cells located at a specific distance from the point source (i.e., iNOS, represented by star) would be exposed to a level of NO that governs a specific subset of physiological or pathophysiological reactions. The x-axis represents the different zone of NO-mediated events that is experienced at a specific distance from a source iNOS producing >1 μM. Note: Akt activation is regulated by NO at two different sites and by two different concentration levels of NO.
Species-specific NO production
The relationship of NO and immunoregulation has been established on the basis of studies on tumor cell lines or rodent macrophages, which are readily available sources of NO. However in humans the levels of protein expression for NOS enzymes and the immune induction required for such levels of expression are quite different than in rodents (Weinberg, 1998). This difference is most likely due to the human iNOS promotor rather than the activity of iNOS itself. There is a significant mismatch between the promoters of humans and rodents and that is likely to account for the notable differences in the regulation of gene induction between them. The combined data on rodent versus human NO and O2– production strongly suggest that in general, ROS production is a predominant feature of activated human macrophages, neutrophils, and monocytes, and the equivalent murine immune cells generate a combination of O2– and NO and in some cases, favor NO production. These differences may be crucial to understanding how immune responses are regulated in a species-specific manner. This is particularly useful, as pathogen challenges change constantly.
The next post in this series will cover the following topics:
The impact of NO signaling on an innate immune response—classical activation
NO and proinflammatory genes
NO and regulation of anti-inflammatory pathways
NO impact on adaptive immunity—immunosuppression and tissue-restoration response
Coronary Artery Disease – Medical Devices Solutions: From First-In-Man Stent Implantation, via Medical Ethical Dilemmas to Drug Eluting Stents August 13, 2012
Cardiovascular Disease (CVD) and the Role of agent alternatives in endothelial Nitric Oxide Synthase (eNOS) Activation and Nitric Oxide Production July 19, 2012
Curator and Research Study Originator: Aviva Lev-Ari, PhD, RN
Macrovascular Disease – Therapeutic Potential of cEPCs: Reduction Methods for CV Risk
July 2, 2012
An Investigation of the Potential of circulating Endothelial Progenitor Cells (cEPCs) as a Therapeutic Target for Pharmacological Therapy Design for Cardiovascular Risk Reduction: A New Multimarker Biomarker Discovery
Inhibition of ET-1, ETA and ETA-ETB, Induction of NO production, stimulation of eNOS and Treatment Regime with PPAR-gamma agonists (TZD): cEPCs Endogenous Augmentation for Cardiovascular Risk Reduction – A Bibliography
Mitochondrial Damage and Repair under Oxidative Stress
Curator: Larry H Bernstein, MD, FCAP
Keywords: Mitochondria, mitochondrial dysfunction, electron transport chain, mtDNA, oxidative stress, oxidation-reduction, NO, DNA repair, lipid peroxidation, thiols, ROS, RNS, sulfur,base excision repair, ferredoxin. Summary: The mitochondrion is the energy source for aerobic activity of the cell, but it also has regulatory functions that will be discussed. The mitochondrion has been discussed in other posts at this site. It has origins from organisms that emerged from an anaerobic environment, such as the bogs and marshes, and may be related to the chloroplast. The aerobic cell was an advance in evolutionary development, but despite the energetic advantage of using oxygen, the associated toxicity of oxygen abundance required adaptive changes. Most bacteria that reduce nitrate (producing nitrite, nitrous oxide or nitrogen) are called facultative anaerobes use electron acceptors such as ferric ions, sulfate or carbon dioxide which become reduced to ferrous ions, hydrogen sulfide and methane, respectively, during the oxidation of NADH (reduced nicotinamide adenine dinucleotide is a major electron carrier in the oxidation of fuel molecules).
The underlying problem we are left with is oxidation-reduction reactions that are necessary for catabolic and synthetic reactions, and that cumulatively damage the organism associated with cancer, cardiovascular disease, neurodegerative disease, and inflammatory overload. Aerobic organisms tolerate have evolved mechanisms to repair or remove damaged molecules or to prevent or deactivate the formationof toxic species that lead to oxidative stress and disease. However, the normal balance between production of pro-oxidant species and destruction by the antioxidant defenses is upset in favor of overproduction of the toxic species, which leads to oxidative stress and disease. How this all comes together is the topic of choice.
The transformation of energy is central to mitochondrial function. The system of energetics includes:
the enzymes of the Kreb’s citric acid or TCA cycle,
some of the enzymes involved in fatty acid catabolism (β-oxidation), and
the proteins needed to help regulate these systems,
central to mitochondrial physiology through the production of reducing equivalents. Reducing equivalents are also used for anabolic reactions. Electron Transport Chain
It also houses the protein complexes involved in the electron transport component of oxidative phosphorylation and proteins involved in substrate and ion transport. The chemical energy contained in both fats and amino acids can also be converted into NADH and FADH2 through mitochondrial pathways. The major mechanism for harvesting energy from fats is β-oxidation; the major mechanism for harvesting energy from amino acids and pyruvate is the TCA cycle. Once the chemical energy has been transformed into NADH and FADH2, these compounds are fed into the mitochondrial respiratory chain.
Under physiological conditions, electrons generally enter either through complex I (NADH-mediated, examined in vitro using substrates such as glutamate/malate) or complex II (FADH2-mediated, examined in vitro using succinate).
Electrons are then sequentially passed through a series of electron carriers.
The progressive transfer of electrons (and resultant proton pumping) converts the chemical energy stored in carbohydrates, lipids, and amino acids into potential energy in the form of the proton gradient. The potential energy stored in this gradient is used to phosphorylate ADP forming ATP. Redox-Cycling
In redox cycling the reductant is continuously regenerated, thereby providing substrate for the “auto-oxidation” reaction.
When partially oxidized compounds are enzymatically reduced, the auto-oxidative generation of superoxide and other ROS to start again. Several enzymes
NADPH-cytochrome P450 reductase,
NADPH-cytochrome b5 reductase [EC 1.6.2.2]
NADPH-ubiquinone oxidoreductase [EC 1.6.5.3], and
xanthine oxidase [EC 1.2.3.2]),
can reduce quinones into semiquinones in a single electron process.
The semiquinone can then reduce dioxygen to superoxide during its oxidation to a quinone.
Redox cycling is thought to play a role in carcinogenesis. The naturally occurring estrogen metabolites (the catecholestrogens) have been implicated in hormone-induced cancer, possibly as a result of their redox cycling and production of ROS. It is thought that diethylstilbestrol causes the production of the mutagenic lesion 8-hydroxy-2’deoxyguanosine. It can also cause DNA strand breakage.
Another oxidative reaction that is associated with H2O2 is a significant problem for living organisms as a consequence of the reaction between hydrogen peroxide and oxidizable metals, the Fenton reaction [originally described in the oxidation of an α-hydroxy acid to an α-keto acid in the presence of hydrogen peroxide (or hypochlorite) and low levels of iron salts (Fenton (1876, 1894)). Chemical Reactions and Biological Significance
The hydroxyl free radical is so aggressive that it will react within 5 (or so) molecular diameters from its site of production. The damage caused by it, therefore, is very site specific. Biological defenses have evolved that reduce the chance that the hydroxyl free radical will be produced to repair damage. An antioxidant would have to occur at the site of hydroxyl free radical production and be at sufficient concentration to be effective.
Some endogenous markers have been proposed as a useful measures of total “oxidative stress” e.g., 8-hydroxy-2’deoxyguanosine in urine. The ideal scavenger
must be non-toxic,
have limited or no biological activity,
readily reach the site of hydroxyl free radical production,
react rapidly with the free radical, be specific for this radical, and
neither the scavenger nor its product(s) should undergo further metabolism.
Unlike oxygen, nitrogen does not possess unpaired electrons and is therefore considered diamagnetic. Nitrogen does not possess available d orbitals so it is limited to a valency of 3. In the presence of oxygen, nitrogen can produce Nitric oxide which occurs physiologically with the immune system which, when activated, can produce large quantities of nitric oxide.
Nitric oxide is produced by stepwise oxidation of L-arginine catalyzed by nitric oxide synthase (NOS). Nitric oxide is formed from the guanidino nitrogen of the L-arginine in a reaction that
consumes five electrons and
requires flavin adenine dinucleotide (FAD),
flavin mononucleotide (FMN) tetrahydrobiopterin (BH4), and
iron protoporphyrin IX as cofactors.
The primary product of NOS activity may be the nitroxyl anion that is then converted to nitric oxide by electron acceptors.
NOS cDNAs show homology with the cytochrome P450 reductase family. Based on molecular genetics there appears to be at least three distinct forms of NOS:
A Ca2+/calmodulin-requiring constitutive enzyme (c-NOS; ncNOS or type I)
A calcium-independent inducible enzyme (i-NOS; type II), which is primarily involved in the mediation of the cellular immune response; and
A second Ca2+/calmodulin-requiring constitutive enzyme found in aortic and umbilical endothelia (ec-NOS or type III)
This has been discussed extensively in this series of posts. Recently, a mitochondrial form of the enzyme, which appears to be similar to the endothelial form, has been found in brain and liver tissue. Although the exact role of nitric oxide in the mitochondrion remains elusive, it may play a role in the regulation of cytochrome oxidase. Nitric Oxide
Nitric oxide appears to regulate its own production through a negative feedback loop. The binding of nitric oxide to the heme prosthetic group of NOS inhibits this enzyme, and c-NOS and ec-NOS are much more sensitive to this regulation than i-NOS. It appears that in the brain, NO can regulate its own synthesis and therefore the neurotransmission process.
On the one hand, inhibition of ec-NOS will prevent the cytotoxicity associated with excessive nitric oxide production.
On the other, the insensitivity of i-NOS to nitric oxide will enable high levels of nitric oxide to be produced for cytotoxic effects.
Endogenous inhibitors of NOS (guanidino-substituted derivatives of arginine) occur in vivo as a result of post-translational modification of protein contained arginine residues by S-adenosylmethionine. The dimethylarginines (NG,NG-dimethyl-L-arginine and NG,N’G-dimethyl-L-arginine) occurs in tissue proteins, plasma, and urine of humans and they are thought to act as both regulators of NOS activity and reservoirs of arginine for the synthesis of nitric oxide.
It has been calculated that even though membrane makes up about 3% of the total tissue volume, 90% of the reaction of nitric oxide with oxygen occurs within this compartment. Thus the membrane is an important site for nitric oxide chemistry.
There are two major aspects to nitric oxide chemistry.
It can undergo single electron oxidation and reduction reactions producing nitrosonium and nitroxyl
Having a single unpaired electron in its π*2p molecular orbital it will react readily with other molecules that also have unpaired electrons, such as free radicals and transition metals.
Examples of the reaction of nitric oxide with radical species include:
Nitric oxide will react with oxygen to form the peroxynitrite (nitrosyldioxyl) radical (ONO2)
and with superoxide to form the powerful oxidizing and nitrating agent, peroxynitrite anion (ONO2-). Peroxynitrite causes damage to many important biomolecules
Importance:
nitrosothiols that are important in the regulation of blood pressure terminates lipid peroxidation
3-nitrosotyrosine and/or 4-O-nitrosotyrosine can affect the activity of enzymes that utilize tyrosyl radicals
rapidly reacts with oxyhemoglobin, the primary route of its destruction in vivo
the reaction between nitric oxide and transition metal complexes
During the last reaction a “ligand” bond is formed (the unpaired electron of nitric oxide is partially transferred to the metal cation),
resulting in a nitrosated (nitrosylated) complex.
For example, such complexes can be formed with free iron ions,
iron bound to heme or iron located in iron-sulfur clusters.
Ligand formation allows nitric oxide to act as a signal, activating some enzymes while inhibiting others. Thus, the binding of nitric oxide to the Fe (II)-heme of guanylate (guanalyl) cyclase [GTP-pyrophosphate lyase: cyclizing] is the signal transduction mechanism. Guanylate cyclase exists as cytosolic and membrane-bound isozymes. Thiol-Didulfide Redox Couple
The thiol-disulfide redox couple is very important to oxidative metabolism. For example, GSH is a reducing cofactor for glutathione peroxidase, an antioxidant enzyme responsible for the destruction of hydrogen peroxide.
The importance of the antioxidant role of the thiol-disulfide redox couple:
Thiols and disulfides can readily undergo exchange reactions, forming mixed disulfides. Thiol-disulfide exchange is biologically very important. For example,
GSH can react with protein cystine groups and influence the correct folding of proteins.
GSH may also play a direct role in cellular signaling through thiol-disulfide exchange reactions with membrane bound receptor proteins
the insulin receptor complex)
transcription factors (e.g., nuclear factor κB)
and regulatory proteins in cells
Conditions that alter the redox status of the cell can have important consequences on cellular function.
The generation of ROS by redox cycling is only one possible explanation for the action of many drugs. Rifamycin not only owes its activity to ROS generation but also to its ability to block bacterial RNA synthesis as well. Quinones (and/or semiquinones) can also form adducts with nucleophiles, especially thiols. These adducts may act as toxins directly or indirectly through the inhibition of key enzymes (e.g., by reacting with essential cysteinyl residues) or the depletion of GSH. DNA Adduct Formation
By far the most intense research in this field has been directed towards the chemistry and biology of DNA adduct formation. Attack of the free bases and nucleosides by pro-oxidants can yield a wide variety of adducts and DNA-protein cross-links. Such attack usually occurs
at the C-4 and C-8 position of purines and
C-5 and C-6 of pyrimidines.
Hydroxyl free radical-induced damage to purine bases and nucleosides can proceed through a C-8-hydroxy N-7 radical intermediate, and then either undergo oxidation with the production of an 8-hydroxy purine, or reduction, probably by cellular thiols, followed by ring opening and the formation of FAPy (formamido-pyrimidine) metabolites (hydroxyl free radical-induced damage to guanosine). Although most research has focused on 8-hydroxy-purine adducts a growing number of publications are attempting to measure the FAPy derivative.
Nitrosation of the Amines of the Nucleic Acid Bases.
Primary aromatic amines produce deaminated products, while secondary amines form N-nitroso compounds. Formation of Peroxynitrite from Nitric Oxide.
Peroxynitrite shows complex reactivity
with DNA initiating DNA strand breakage, oxidation (e.g., formation of 8-hydroxyguanine, 8-OH2’dG, (5-hydroxymethyl)-uracil, and FAPyGua),
nitration (e.g., 8-nitroguanine), and
deamination of bases.
Peroxynitrite can also promote the production of lipid peroxidation related active carbonyls and cause the activation of NAD+ ADP-ribosyltransferase.
Modification of Guanine
Although all DNA bases can be oxidatively damaged, it is the modification of guanine that is the most frequent. 8OH2’dG is the most abundant DNA adduct. This can affect its hydrogen bonding between base-pairs. These base-pair substitutions are usually found clustered into areas called “hot spots”. Guanine normally binds to cytosine.
8OH2’dG, however, can form hydrogen bonds with adenine. The formation of 8OH2’dG in DNA can therefore result in a G→T transversion.
8-Hydroxyguanine was also shown to induce codon 12 activation of c-Ha-ras and K-ras in mammalian systems. G→T transversions are also the most frequent hot spot mutations found in the p53 supressor gene which is associated with human tumors.
Other mechanisms by which ROS/RNS can lead to mutations have been
proposed. Direct mechanisms include:
conformational changes in the DNA template that reduces the accuracy of replication by DNA polymerases
altered methylation of cytosine that affects gene control
Indirect mechanisms include:
Oxidative damage to proteins, including DNA polymerases and repair enzymes.
Damage to lipids causes the production of mutagenic carbonyl compounds
Misalignment mutagenesis (“slippery DNA”)
DNA Mismatch Repair 5 (Photo credit: Allen Gathman)
Repair of ROS/RNS-induced DNA Damage
The repair of damaged DNA is an ongoing and continuous process involving a
number of repair enzymes. Damaged DNA appears to be mended by two major mechanisms:
base excision repair (BER) and
nucleotide excision repair (NER)
Isolated DNA is found to contain low levels of damaged bases, so it appears that these repair processes are not completely effective. Base Excision Repair
BER is first started by DNA glycosylases which recognize specific base
modifications (e.g., 8OH2’dG). For example,
Formamido-pyrimidine-DNA glycosylase (Fpg protein) recognizes damaged purines such as 8-oxoguanine and FAPyGua.
Damaged pyrimidines are recognized by endonuclease III, which acts as both a glycosylase and AP endonuclease.
Glycosylases cleave the N-glycosylic bond between the damaged base and the sugar
Following the glycosylase step, AP endonucleases then remove the 3′-deoxyribose moiety by cleavage of the phosphodiester bonds thereby generating a 3’-hydroxyl group that can then be extended by DNA polymerase.
The final step in mending damaged DNA is the rejoining of the free ends of DNA by a DNA ligase. It also appears that the presence of 8-oxoguanine modified bases in DNA is not only a result of ROS attack on this macromolecule. Oxidized nucleosides and nucleotides from free cellular pools can also be incorporated into DNA by polymerases and cause AT to CG base substitution mutations.
Mitochondrial DNA Repair
The mitochondrion genome encodes the various complexes of the electron transport chain, but contains no genetic information for DNA repair enzymes. These enzymes must be obtained from the nucleus. As mitochondria are continuously producing DNA damaging pro-oxidant species, effective DNA repair mechanisms must exist within the mitochondrial matrix in order for these organelles to function. Mitochondria have a short existence, and excessively damaged mitochondria will be quickly removed. Mitochondria contain many BER enzymes and are proficient at repair, but they do not appear to repair damaged DNA by NER mechanisms.
Single Strand DNA Damage and PARP Activation
Single strand DNA breakage activates NAD+ ADP-ribosyltransferase (PARP). PARP is a protein-modifying, nucleotide-polymerizing enzyme and is found at high levels in the nucleus. Activated PARP
cleaves NAD+ into ADP-ribose and nicotinamide
then attaches the ADP-ribose units to a variety of nuclear proteins (including histones and its own automodification domain).
then polymerizes the initial ADP-ribose modification with other ADP-ribose units to form the nucleic acid-like polymer, poly (ADP) ribose.
PARP only appears to be involved with BER and not NER. In BER PARP does not appear to play a direct role but rather it probably helps by keeping the chromatin in a conformation that enables other repair enzymes to be effective. It may also provide temporary protection to DNA molecules while it is being repaired. Conflicting evidence suggests that PARP may not be an important DNA repair enzyme as cells from a PARP knockout mouse model have normal repair characteristics.
Activation of PARP can be dangerous to the cell. For each mole of ADP-ribose transferred, one mole of NAD+ is consumed, and through the regeneration of NAD+ four ATP molecules are wasted. Thus the activation of PARP can rapidly deplete a cell’s energy store and even lead to cell death. Some researchers suggest that this may be one mechanism whereby cells with excessive DNA damage are effectively removed. However, a variety of diseases may involve PARP overactivation including
circulatory shock,
CNS injury,
diabetes,
drug-induced cytotoxicity, and
inflammation.
The Indirect Pathway.
This (mutation) pathway does not involve oxidative damage to the protein per se. This process involves oxidative damage to the DNA molecule encoding the protein. Thus pro-oxidants can cause changes in the base sequence of the DNA molecule. If such base modification is in a coding region of DNA (exon) and not corrected, the DNA molecule may be transcribed incorrectly. Translation of the mutant mRNA can result in a mutant protein containing a wrong amino acid in its primary sequence. If this modified amino acid occurs in an essential part of the protein (e.g., the active site of an enzyme or a portion that alters folding), the function of that protein may be impaired. Fortunately, unlike modified DNA
that can pass from cell to cell during mitosis thereby continuing the production of mutant protein, damage to a protein is non-replicating and stops with its destruction.
The Direct Pathway
This (post-translational) pathway involves the action of a pro-oxidant on a protein resulting in
modification of amino acid residues,
the formation of carbonyl adducts,
cross-linking and
polypeptide chain fragmentation.
Such changes often result in altered protein conformation and/or activity. Proteins will produce a variety of carbonyl products when exposed to metal-based systems (metal/ascorbate and metal/hydrogen peroxide) in vitro. For example, histidine yields aspartate, asparagine and 2-oxoimidazoline, while proline produces glutamate, pyroglutamate, 4-hydroxyproline isomers, 2-pyrrolidone and γ-aminobutyric acid. Metal-based systems and other pro-oxidant conditions can oxidize methionine to its sulfoxide.
This portion of the presentation is endebted to THE HANDBOOK OF REDOX
BIOCHEMISTRY, Ian N. Acworth, August 2003, esa. (inacworth@esainc.com).
We shall now identify more recent work related to this presentation.
Oxygen and Oxidative Stress
The reduction of oxygen to water proceeds via one electron at a time. In the mitochondrial respiratory chain, Complex IV (cytochrome oxidase) retains all partially reduced intermediates until full reduction is achieved. Other redox centres in the electron transport chain, however, may leak electrons to oxygen, partially reducing this molecule to superoxide anion (O2_•). Even though O2_• is not a strong oxidant, it is a precursor of most other reactive oxygen species, and it also becomes involved in the propagation of oxidative chain reactions. Despite the presence of various antioxidant defences, the mitochondrion appears to be the main intracellular source of these oxidants. This review describes the main mitochondrial sources of reactive species and the antioxidant defences that evolved to prevent oxidative damage in all the mitochondrial compartments.
Reactive oxygen species (ROS) is a phrase used to describe a variety of molecules and free radicals (chemical species with one unpaired electron) derived from molecular oxygen. Molecular oxygen in the ground state is a bi-radical, containing two unpaired electrons in the outer shell (also known as a triplet state).
Since the two single electrons have the same spin, oxygen can only react with one electron at a time and therefore it is not very reactive with the two electrons in a chemical bond.
On the other hand, if one of the two unpaired electrons is excited and changes its spin, the resulting species (known as singlet oxygen) becomes a powerful oxidant as the two electrons with opposing spins can quickly react with other pairs of electrons, especially double bonds.
The formation of OH• is catalysed by reduced transition metals, which in turn may be re-reduced by O2 -•, propagating this process. In addition, O2-• may react with other radicals including nitric oxide (NO•) in a reaction controlled by the rate of diffusion of both radicals. The product, peroxynitrite, is also a very powerful oxidant. The oxidants derived from NO• have been recently called reactive nitrogen species (RNS).
‘Oxidative stress’ is an expression used to describe various deleterious processes resulting from an imbalance between the excessive formation of ROS and/or RNS and limited antioxidant defences.
Whilst small fluctuations in the steady-state concentration of these oxidants may actually play a role in intracellular signalling,
uncontrolled increases in the steady-state concentrations of these oxidants lead to free radical mediated chain reactions
which indiscriminately target
proteins,
lipids,
polysaccharides.
In vivo, O2-• is produced both enzymatically and nonenzymatically.
Enzymatic sources include
NADPH oxidases located on the cell membrane of
polymorphonuclear cells,
macrophages and
endothelial cells and
cytochrome P450-dependent oxygenases.
The proteolytic conversion of xanthine dehydrogenase to xanthine oxidase provides another enzymatic source of both O2 -• and H2O2 (and therefore constitutes a source of OH•) and has been proposed to mediate deleterious processes in vivo.
Given the highly reducing intramitochondrial environment, various respiratory components, including flavoproteins, iron–sulfur clusters and ubisemiquinone, are thermodynamically capable of transferring one electron to oxygen. Moreover, most steps in the respiratory chain involve single-electron reactions, further favouring the monovalent reduction of oxygen. On the other hand, the mitochondrion possesses various antioxidant defences designed to eliminate both O2- • and H2O2.
The rate of O2 -• formation by the respiratory chain is controlled primarily by mass action, increasing both when electron flow slows down (increasing the concentration of electron donors, R•) and when the concentration of oxygen increases (eqn (1); Turrens et al. 1982).
d[O2]/dt = k [O2] [R•].
The energy released as electrons flow through the respiratory chain is converted into a H+ gradient through the inner mitochondrial membrane (Mitchell, 1977). This gradient, in turn, dissipates through the ATP synthase complex (Complex V) and is responsible for the turning of a rotor-like protein complex required for ATP synthesis. In the absence of ADP,
the movement of H+ through ATP synthase ceases and
the H+ gradient builds up
causing electron flow to slow down and
the respiratory chain to become more reduced (State IV respiration).
Mitochondrial Antioxidant Defences
The deleterious effects resulting from the formation of ROS in the mitochondrion are, to a large extent, prevented by various antioxidant systems. Superoxide is enzymatically converted to H2O2 by a family of metalloenzymes called superoxide dismutases (SOD). Since O2-• may either reduce transition metals, which in turn can react with H2O2 producing OH• or spontaneously react with NO• to produce peroxynitrite, it is important to maintain the steady-state concentration of O2-• at the lowest possible level. Thus, although the dismutation of O2-• to H2O2 and O2 can also occur spontaneously, the role of SODs is to increase the rate of the reaction to that of a diffusion-controlled process.
The mitochondrial matrix contains a specific form of SOD, with manganese in the active site, which eliminates the O2 -• formed in the matrix or on the inner side of the inner membrane. The expression of MnSOD is further induced by agents that cause oxidative stress, including radiation and hyperoxia, in a process mediated by the oxidative activation of the nuclear transcription factor NFkB .
Reactive oxygen species (ROS) are products of normal metabolism and xenobiotic exposure, and depending on their concentration, ROS can be beneficial or harmful to cells and tissues.
At physiological low levels, ROS function as “redox messengers” in intracellular signaling and regulation, whereas
excess ROS induce oxidative modification of cellular macromolecules, inhibit protein function, and promote cell death.
Additionally, various redox systems, such as
the glutathione,
thioredoxin, and
pyridine nucleotide redox couples,
NADPH and antioxidant defense
NAD+ and the function of sirtuin proteins
participate in cell signaling and modulation of cell function, including apoptotic cell death. Cell apoptosis is initiated by extracellular and intracellular signals via two main pathways,
the death receptor and
the mitochondria-mediated pathways.
ROS and JNK-mediated apoptotic signaling
GSH redox status and apoptotic signaling
Various pathologies can result from oxidative stress-induced apoptotic signaling that is consequent to
ROS increases and/or antioxidant decreases,
disruption of intracellular redox homeostasis, and
irreversible oxidative modifications of lipid, protein, or DNA.
We focus on several key aspects of ROS and redox mechanisms in apoptotic signaling and highlight the gaps in knowledge and potential avenues for further investigation. A full understanding of the redox control of apoptotic initiation and execution could underpin the development of therapeutic interventions targeted at oxidative stress-associated disorders.
Iron-sulfur (FeyS) cluster-containing proteins catalyze a number of electron transfer and metabolic reactions. The components and molecular mechanisms involved in the assembly of the FeyS clusters have been identified only partially. In eukaryotes, mitochondria have been proposed to execute a crucial task in the generation of intramitochondrial and extramitochondrial FeyS proteins. Herein, we identify the essential ferredoxin Yah1p of Saccharomyces cerevisiae mitochondria as a central component of the FeyS protein biosynthesis machinery. Depletion of Yah1p by regulated gene expression resulted in a
30-fold accumulation of iron within mitochondria,
similar to what has been reported for other components involved in FeyS protein biogenesis. Yah1p was shown to be required for the assembly of FeyS proteins both inside mitochondria and in the cytosol. Apparently, at least one of the steps of FeyS cluster biogenesis within mitochondria requires reduction by ferredoxin. Our findings lend support to the idea of a primary function of mitochondria in the biosynthesis of FeyS proteins outside the organelle. To our knowledge, Yah1p is the first member of the ferredoxin family for which a function in FeyS cluster formation has been established. A similar role may be predicted for the bacterial homologs that are encoded within iron-sulfur cluster assembly (isc) operons of prokaryotes.
H Lange, A Kaut, G Kispal, and R Lill. A mitochondrial ferredoxin is essential for biogenesis of cellular iron-sulfur proteins. PNAS 2000; 97(3): 1050–1055.
DNA Charge Transport
Damaged bases in DNA are known to lead to errors in replication and transcription, compromising the integrity of the genome. The authors proposed a model where repair proteins containing redoxactive [4Fe-4S] clusters utilize DNA charge transport (CT) as a first step in finding lesions. In this model, the population of sites to search is reduced by a localization of protein in the vicinity of lesions. Here, we examine this model using single-molecule atomic force microscopy (AFM). XPD, a 5′-3′ helicase involved in nucleotide
excision repair, contains a [4Fe-4S] cluster and exhibits a DNA bound redox potential that is physiologically relevant.
In AFM studies, they observe the redistribution of XPD onto kilobase DNA strands containing a single base mismatch, which is not a specific substrate for XPD but, like a lesion, inhibits CT. They also provide evidence for DNA-mediated signaling between XPD and Endonuclease III (EndoIII), a base excision repair glycosylase that also contains a [4Fe-4S] cluster.
When XPD and EndoIII are mixed together, they coordinate in relocalizing onto the mismatched strand.
However, when a CT-deficient mutant of either repair protein is combined with the CT-proficient repair partner, no relocalization occurs.
The data presented here indicate that XPD, an archaeal protein from the NER pathway, may cooperate with other proteins that are proficient at DNA CT to localize in the vicinity of damage. XPD, a superfamily 2 DNA helicase with 5′-3′ polarity, is a component of TFIIH that is essential for repair of bulky lesions generated by exogenous sources such as UV light and chemical carcinogens. XPD contains a conserved [4Fe-4S] cluster suggested to be conformationally controlled by ATP binding and hydrolysis.
Mutations in the iron-sulfur domain of XPD can lead to diseases including TTD and XP, yet the function of the [4Fe-4S] cluster appears to be unknown.
Electrochemical studies have shown that when BER proteins MutY and EndoIII bind to DNA, their [4Fe-4S] clusters are activated toward one electron oxidation. XPD exhibits a DNA-bound midpoint potential similar to that of EndoIII and MutY when bound to DNA (approximately 80 mV vs. NHE), indicative of a possible role for the [4Fe-4S] cluster in DNA-mediated CT.
For EndoIII we have also already determined a direct correlation between the ability of proteins to redistribute in the vicinity of mismatches as measured by AFM, and the CT proficiency of the proteins measured electrochemically. Thus, we may utilize single-molecule AFM as a tool to probe the redistribution of proteins in the vicinity of base lesions and in so doing, the proficiency of the protein to carry out DNA CT.
Here we show that, like the BER protein EndoIII, XPD, involved both in transcription and NER, redistributes in the vicinity of a lesion. Importantly, this ability to relocalize is associated with the ability of XPD to carry out DNA CT. The mutant L325V is defective in its ability to carry out DNA CTand this XPD mutant also does not redistribute effectively onto the mismatched strand.
These data not only indicate a general link between the ability of a repair protein to carry out DNA CT and its ability to redistribute onto DNA strands near lesions but also provide evidence for coordinated DNA CT between different repair proteins in their search for damage in the genome. These data also provide evidence that two different repair proteins, each containing a [4Fe-4S] cluster at similar DNA bound potential, can communicate with one another through DNA-mediated CT.
The signaling function of mitochondria is considered with a special emphasis on their role in the regulation of redox status of the cell, possibly determining a number of pathologies including cancer and aging. The review summarizes the transport role of mitochondria in energy supply to all cellular compartments (mitochondria as an electric cable in the cell), the role of mitochondria in plastic metabolism of the cell including synthesis of
heme,
steroids,
iron-sulfur clusters, and
reactive oxygen and nitrogen species.
Mitochondria also play an important role in the Ca2+-signaling and the regulation of apoptotic cell death. Knowledge of mechanisms responsible for apoptotic cell death is important for the strategy for prevention of unwanted degradation of postmitotic cells such as cardiomyocytes and neurons.
In accordance with P. Mitchell’s chemiosmotic concept, vectorial transmembrane transfer of electrons and protons is accompanied by generation of electrochemical difference of proton electrochemical potential on the inner mitochondrial membrane; its utilization by ATP synthase induces conformational rearrangements resulting in ATP synthesis from ADP and inorganic phosphate. Details of the mechanism responsible for ATP synthesis are given elsewhere.
Membrane potential (DY) generated across the inner mitochondrial membrane is the component of the transmembrane electrochemical potential of H+ ions (DμH+), which provides ATP synthesis together with the concentration component (DpH). Maintenance of constant membrane potential is a vitally important precondition for functioning of mitochondria and the cell. Under conditions of limited supply of the cell with oxygen (hypoxia) and inability to carry out aerobic ATP synthesis, mitochondria become ATP consumers (rather than generators) and ATP is hydrolyzed by mitochondrial ATPase, and this is accompanied by generation of membrane potential.
Redox homeostasis, i.e. the sum of redox components (including proteins, low molecular weight redox components such as NAD/NADH, flavins, coenzymes Q, oxidized and reduced substrates, etc.) is one of important preconditions for normal cell functioning.
Single-strand and double-strand DNA damage (Photo credit: Wikipedia)
Mitochondria generate such potent regulators of redox potential as
superoxide anion,
hydrogen peroxide,
nitric oxide,
peroxynitrite, etc.
They are actively involved in regulation of cell redox potential and consequently
control proteolysis,
activation of transcription,
changes in mitochondrial DNA (mDNA),
cell metabolism, and
cell differentiation.
Zorov DB, Isaev NK, Plotnikov EY, Zorova LD, et al. The Mitochondrion as Janus Bifrons. Biochemistry (Moscow) 2007; 72(10): 1115-1126. ISSN 0006-2979.
DOI: 10.1134/S0006297907100094
Structure of the human mitochondrial genome. (Photo credit: Wikipedia)
Gene Expression Associated with Oxidoreduction and Mitochondria
The naked mole-rat (Heterocephalus glaber) is a long-lived, cancer resistant rodent and there is a great interest in identifying the adaptations responsible for these and other of its unique traits. We employed RNA sequencing to compare liver gene expression profiles between naked mole-rats and wild-derived mice. Our results indicate that genes associated with oxidoreduction and mitochondria were expressed at higher relative levels in naked mole-rats. The largest effect is nearly
300-fold higher expression of epithelial cell adhesion molecule (Epcam), a tumour-associated protein.
Also of interest are the
protease inhibitor, alpha2-macroglobulin (A2m), and the
mitochondrial complex II subunit Sdhc,
both ageing-related genes found strongly over-expressed in the naked mole-rat.
These results hint at possible candidates for specifying species differences in ageing and cancer, and in particular suggest complex alterations in mitochondrial and oxidation reduction pathways in the naked mole-rat. Our differential gene expression analysis obviated the need for a reference naked mole-rat genome by employing a combination of Illumina/Solexa and 454 platforms for transcriptome sequencing and assembling transcriptome contigs of the non-sequenced species. Overall, our work provides new research foci and methods for studying the naked mole-rat’s fascinating characteristics.
The complete set of viable deletion strains in Saccharomyces cerevisiae was screened for sensitivity of mutants to five oxidants to identify cell functions involved in resistance to oxidative stress. This screen identified a unique set of mainly constitutive functions providing the first line of defense against a particular oxidant; these functions are very dependent on the nature of the oxidant. Most of these functions are distinct from those involved in repair and recovery from damage, which are generally induced in response to stress, because there was little correlation between mutant sensitivity and the reported transcriptional response to oxidants of the relevant gene. The screen identified 456 mutants sensitive to at least one of five different types of oxidant, and these were ranked in order of sensitivity. Many genes identified were not previously known to have a role in resistance to reactive oxygen species. These encode functions including
protein sorting,
ergosterol metabolism,
autophagy, and
vacuolar acidification.
two mutants were sensitive to all oxidants examined, 12 were sensitive to at least four,
Different oxidants had very different spectra of deletants that were sensitive. These findings highlight the specificity of cellular responses to different oxidants:
No single oxidant is representative of general oxidative stress.
Mitochondrial respiratory functions were overrepresented in mutants sensitive to H2O2, and
vacuolar protein-sorting mutants were enriched in mutants sensitive to diamide.
Core functions required for a broad range of oxidative-stress resistance include
Isolated complex I deficiency is the most common enzymatic defect of the oxidative phosphorylation (OXPHOS) system, causing a wide range of clinical phenotypes. Th authers reported before that the rates at which reactive oxygen species (ROS)-sensitive dyes are converted into their fluorescent oxidation products are markedly increased in cultured skin fibroblasts of patients with nuclear-inherited isolated complex I deficiency.
Using videoimaging microscopy we show here that these cells also display a marked increase in NAD(P)H autofluorescence. Linear regression analysis revealed a negative correlation with the residual complex I activity and a positive correlation with the oxidation rates of the ROS sensitive dyes (5-(and-6)-chloromethyl-2′,7′-dichlorodihydrofluorescein and hydroethidine for a large cohort of 10 patient cell lines.
On the other hand, video-imaging microscopy of cells selectively expressing reduction-oxidation sensitive GFP1 in either the mitochondrial matrix or cytosol showed the absence of any detectable change in thiol redox state. In agreement with this result, neither the glutathione nor the glutathione disulfide content differed significantly between patient and healthy fibroblasts.
Finally, video-rate confocal microscopy of cells loaded with C11-BODIPY581/591 demonstrated that the extent of lipid peroxidation, which is regarded as a measure of oxidative damage, was not altered in patient fibroblasts. Our results indicate that fibroblasts of patients with isolated complex I deficiency maintain their thiol redox state despite marked increases in ROS production.
Mitochondrial research has recently been driven by the
identification of mitochondria-associated diseases and the role of mitochondria in apoptosis.
Moreover, mitochondria have been implicated in the process of carcinogenesis because of their vital role in
energy production,
nuclear-cytoplasmic signal integration and
control of metabolic pathways.
At some point during neoplastic transformation, there is an increase in reactive oxygen species (ROS), which damage the mitochondrial genome. This accelerates the somatic mutation rate of mitochondrial DNA.
Mitochondrial characteristics
There are several biological characteristics which cast mitochondria and, in particular, the mitochondrial genome, as a biological tool for early detection and monitoring of neoplasia and its potential progression. These vital characteristics are important in cancer research, as not all neoplasias become malignant. Mitochondria are archived in the cytoplasm of the ovum and as such do not recombine.
This genome has an accelerated mutation rate, by comparison with the nucleus, and accrues somatic mutations in tumour tissue. Moreover, mitochondrial DNA (mtDNA) has a high copy number in comparison with the nuclear archive of DNA. There are potentially thousands of mitochondrial genomes per cell, which enables detection of important biomarkers, even at low levels. In addition, mtDNA can be heteroplasmic, which means that disease-associated mutations occur in a subset of the genomes.
The presence of heteroplasmy is an indication of disease and is found in many human tumours. Identification of low levels of heteroplasmy may allow unprecedented early identification and monitoring of neoplastic progression to malignancy.
Coding for just 13 enzyme complex subunits, 22 transfer RNAs and two ribosomal RNAs, the mitochondrial genome is packaged in a compact 16,569 base pair (bp) circular molecule. These products participate in the critical electron transport process of ATP production. Collectively, mitochondria generate 80 per cent of the chemical fuel which fires cellular metabolism.
As a result, nuclear investment in the mitochondria is high — that is, several thousand nuclear genes control this organelle in order to accomplish the complex interactions required to maintain a network of pathways, which coordinate energy demand and supply.
It has been proposed that these mutations may serve as an early indication of potential cancer development and may represent a means for tracking tumour progression.
Does this provide a potential utility in that these mutations may be used for the identification and monitoring of neoplasia and malignant transformation where appropriate body fluids or non-invasive tissue access is available for mtDNA recovery? Specifically discussed are:
prostate,
breast,
colorectal,
skin and
lung cancers
There are many important questions yet to be addressed: such as
the relationship between mtDNA and the actual disease;
are mutations causative or merely a reflection of nuclear instability?
And, are these processes independent events?
Alterations in the non-coding D-loop suggest genome instability;
however, as studies focus more on the coding regions of the
mitochondrial genome,
Particularly in the case of nonsynonymous mutations in the genes
contributing products to the electron transport process, metabolic
implications are evident. Moreover, mutations in mitochondrial transfer RNAs indicate the possibility of a global mitochondrial translational shut down.
RL Parr, GD Dakubo, RE Thayer, K McKenney, MA Birch-Machin. Mitochondrial DNA as a potential tool for early cancer detection. HUMAN GENOMICS 2006; 2(4). 252–257.
Mitochondrial DNA (mtDNA) is particularly prone to oxidation due to the lack of histones and a deficient mismatch repair system. This explains an increased mutation rate of mtDNA that results in heteroplasmy, e.g., the coexistence of the mutant and wild-type mtDNA molecules within the same mitochondrion. Hyperglycemia is a key risk factor not only for diabetes-related disease, but also for cardiovascular and all-cause mortality. One can assume an increase in the risk of cardiovascular disease by 18% for each unit (%) glycated hemoglobin HbA1c. In the Glucose Tolerance in Acute Myocardial Infarction study of patients with acute coronary syndrome, abnormal glucose tolerance was the strongest independent predictor of subsequent cardiovascular complications and death. In the Asian Pacific Study, fasting plasma glucose was shown to be an independent predictor of cardiovascular events up to a level of 5.2 mmol/L.
Glucose level fluctuations and hyperglycemia are triggers for inflammatory responses via increased mitochondrial superoxide production and endoplasmic reticulum stress. Inflammation leads to insulin resistance and β-cell dysfunction, which further aggravates hyperglycemia. The molecular pathways that integrate hyperglycemia, oxidative stress, and diabetic vascular complications have been most clearly described in the pathogenesis of endothelial dysfunction, which is considered as the first step in atherogenesis according to the response to injury hypothesis.
In diabetes mellitus,
glycotoxicity,
advanced oxidative stress,
collagen cross-linking, and
accumulation of lipid peroxides
in foam macrophage cells and arterial wall cells may significantly
decrease the mutation threshold,
endothelial dysfunction,
promoting atherosclerosis.
Alterations in mitochondrial DNA (mtDNA), known as homoplasmic and heteroplasmic mutations, may influence mitochondrial OXPHOS capacity, and in turn contribute to the magnitude of oxidative stress in micro- and macrovascular networks in diabetic patients.
The authors critically consider the impact of mtDNA mutations on the pathogenesis of cardiovascular diabetic complications.
Mutation Threshhold
Although cells may harbor mutant mtDNA, the expression of disease is dependent on the percent of alleles bearing mutations. Modeling confirms that an upper threshold level might exist for mutations beyond which the mitochondrial population collapses, with a subsequent decrease in ATP. This decrease in ATP results in the phenotypic expression of disease. It is estimated that in many patients with clinical manifestations of mitochondrial disorders, the proportion of mutant DNA exceeds 50%.
For the MELAS (mitochondrial encephalopathy, lactic acidosis and stroke-like syndrome)-causing mutation m.3243 A>G in the mitochondrial gene encoding tRNALeu, which is also associated with diabetes plus deafness, a strong correlation between the level of mutational heteroplasmy and documented disease has been found. Increased percentages of mutant mtDNA in muscle cells (up to 71%) can lead to mitochondrial myopathy. Levels of heteroplasmy of over 80% may lead to recurrent stroke and mutation levels of 95% have been associated with MELAS.
Regardless of the type of mutation or the level of heteroplasmy in affected mitochondria, unrepaired damage leads to a decrease in ATP, which in turn causes the phenotypic manifestation of disease. The manifestation of disease not only depends on the ATP level but also on the tissue affected. Various tissues have differing levels of demand on OXPHOS capacity. To evaluate a tissue threshold, Leber’s hereditary optic neuropathy can be used as a model for mitochondrial neurodegenerative disease. For neural and skeletal muscle tissues, the tissue threshold should be as high as or higher than 90% of
damaged (mutated) mtDNA. To induce mitochondrial malfunctions, the tissue threshold of the cardiac muscle is estimated to be significantly lower (approximately 64%-67%). In chronic vascular disease such as atherosclerosis, a mutation threshold in the affected vessel wall (e.g., in the postmortem aortic atherosclerotic plaques) was observed to be significantly lower. For example, for mutations m.3256 C>T, m.12315 G>A, m.15059 G>A, and m.15315 G>A, the heteroplasmy range of 18%-66% in the atherosclerotic lesions was 2-3.5-fold that in normal vascular tissue.
Mitochondrial stress and insulin resistance
Mitochondrial damage precedes the development of atherosclerosis and tracks the extent of the lesion in apoE-null mice, and
mitochondrial dysfunction caused by heterozygous deficiency of a superoxide dismutase increases atherosclerosis and vascular mitochondrial damage in the same model.
Blood vessels destined to develop atherosclerosis may be characterized by inefficient ATP production due to the uncoupling of respiration and OXPHOS. Blood vessels have regions of hypoxia, which lower the ratio of state 3 (phosphorylating) to state 4 (nonphosphorylating) respiration. Human atherosclerotic lesions have been known for decades to be deficient in essential fatty acids, a condition that causes respiratory uncoupling and atherosclerosis.
The finding by Kokaze et al. helps to explain, at least in part, the anti-atherogenic effect of the allele m. 5178A due to its relation with the favorable lipid profile. The nucleotide change causes leucine-to-methionine substitution at codon 237 (Leu-237Met) of the NADH dehydrogenase subunit 2 located in the loop between 7th and 8th transmembrane domains of the mitochondrial protein. Given that this methionine residue is exposed at the surface of respiratory Complex I, this residue may be available as an efficient oxidant scavenger. Complex I
accepts electrons from NADH,
transfers them to ubiquinone, and
uses the energy released to pump protons across the mitochondrial inner membrane.
Thus, the Leu237Met replacement in the ND2 subunit might have a protective effect against oxidative damage to mitochondria.
Most fatty acid oxidation, which is promoted by peroxisome proliferator-activated receptor α (PPARα) activation, occurs in the mitochondria. Mitochondrial effects could explain why PPARα- deficient mice are protected from diet-induced insulin resistance and atherosclerosis as well as glucocorticoid induced insulin resistance and hypertension. Caloric restriction,
Nitric Oxide and Sepsis, Hemodynamic Collapse, and the Search for Therapeutic Options
Curator, Reporter, EAW: Larry H Bernstein, MD, FCAP
This document explores the current understanding of sepsis as a cascade of events that involves the microcirculation unevenly because of a differential effect on the large and contiguous intestinal epithelium, secondary effects on cardiopulmonary blood flows and cardiac output, and the role of Nitric Oxide in the emergence of beneficial and potentially deleterious effects. This leads to a substantial body of work on therapeutic targets, either aimed at total inhibition or selective inhibition of NO synthase, and the special role of iNOS. This is another of a series of discussions on the metabolic and regulatory role of NO in health and disease.
Introduction
Antioxidants are essential, and are involved in several important biological processes such as immunity, protection against tissue damage, reproduction, growth and development. Antioxidants preserve adequate function of cells against homeostatic disturbances such as those caused by septic shock, aging and, in general, processes involving oxidative stress. This review focuses on the involvement of reactive oxygen and nitrogen species.
The presence of free radicals in biological materials was discovered about 50 years ago. Today, there is a large body of evidence indicating that patients in hospital intensive care units (ICUs) are exposed to excessive free radicals from drugs and other substances that alter cellular reduction -oxidation (redox) balance, and disrupt normal biological functions. However, low levels of free radicals are also vital for many cell signaling events and are essential for proper cell function.
Excess free radicals can result from a variety of conditions such as tissue damage and hypoxia (limiting oxygen levels), overexposure to environmental factors (tobacco smoke, ultraviolet radiation, and pollutants), a lack of antioxidants, or destruction of free radical scavengers. When the production of damaging free radicals exceeds the capacity of the body’s antioxidant defenses to detoxify them, a condition known as oxidative stress occurs.
A free radical can be described as any atom or a group of atoms or molecules in which there is at least one unpaired electron in the outermost shell . These free radicals are very reactive with adjacent molecules such as lipids, proteins, and carbohydrates and can cause cellular damage. Paradoxically, free radicals can also be produced by many cells as a protective mechanism, for example neutrophils produce free radicals to attack and destroy pathogens, while the liver uses free radicals for detoxification. However, the presence of free radicals within the body can also have a significant role in the development and progression of many disease processes for example heart disease, hypertension, cerebrovascular accidents, and diabetic complications. Any free radical involving O2 is referred to as a reactive oxygen species (ROS).
Normal cellular metabolism involves the production of ROS, and in humans, superoxide (O2 -) is the most commonly produced free radical. Phagocytic cells such as macrophages and neutrophils are prominent sources of O2 -. During an inflammatory response, these cells generate free radicals that attack invading pathogens such as bacteria and, because of this, the production of O2- by activated phagocytic cells in response to inflammation is one of the most studied free radical producing systems. The majority of the H2O2 is broken down to O2 and water by the antioxidant enzyme catalase. In addition to catalase, glutathione peroxidase can also break down H2O2 and also any peroxides that form on lipids within the body. When O2 – reacts with nitric oxide (NO), the toxic product peroxynitrite (ONOO-) is formed.
Cellular ROS originate from O2- generated as a by-product of oxidative phosphorylation (mitochondrial respiration), they differ in their mechanism of production, necessary cofactors, diffusion range, hydrophobicity, biological targets, detoxification pathways and breakdown products. O-2 damaging reactions largely involve disassembly of iron-sulphur clusters in proteins. H2O2 or O-2 alone lacked reactivity toward iron regulatory protein-1 (IRP-1), but a combined action of the two species induced reversible inactivation of IRP-1. Such an effect was attributed to direct interactions of O-2 and H2O2 with a preformed pool of IRP-1, resulting in reversible modifications of -SH residues; in fact, its action would be limited to removing only iron atoms, an effect sufficient to abolish enzyme activity.
The hydroxyl radical (.OH) is the most reactive of the free radical molecules. OH- damages cell membranes and lipoproteins by a process termed lipid peroxidation. In fact, lipid peroxidation can be defined as the process whereby free radicals “steal” electrons from the lipids in our cell membranes, resulting in cell damage and increased production of ROS. This process takes place in 3 stages:
Initiation: In a peroxide-free lipid system, the initiation of a peroxidation sequence refers to the attack of an ROS (with sufficient reactivity) able to abstract a hydrogen (H) atom from a methylene group (- CH2-).
Propagation: A peroxyl radical is able to abstract H from another lipid molecule (adjacent fatty acid), especially in the presence of metals such as copper or iron, thus causing an autocatalytic chain reaction. The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide).
Termination: formation of a hydroperoxide. Lipid peroxidative damage to lipids in low-density lipoprotein (LDL) plays an important role in atherosclerosis [9]. To protect against oxidative damage, organisms have developed a variety of antioxidant defenses that include proteins, compounds such as vitamins, and specialized antioxidant enzymes.
Lipid-soluble antioxidants are located in the cellular membranes and lipoproteins, whereas the water-soluble antioxidants are present in the aqueous environments, such as fluids inside cells and in the blood. Preventative antioxidant enzymes inside the cell are an important defense against free radicals.
In humans, the highest levels of SOD are found in the liver, adrenal gland, kidney, and spleen. Catalase and glutathione peroxidase both work to detoxify O2-reactive radicals by catalyzing the formation of H2O2 derived from O2 -. The liver, kidney, and red blood cells possess high levels of catalase, which helps to detoxify chemicals in the body. The water-soluble tripeptide-thiol glutathione also plays an important role in a variety of detoxification processes. Glutathione is found in millimolar concentrations in the cell cytosol and other aqueous phases, and readily interacts with free radicals, especially the hydroxyl radical, by donating a hydrogen atom.
Sepsis and Signaling Pathways
Serious infections trigger systemic inflammatory response and can result in sepsis. It is believed that sepsis and therefore septic shock are due to the inappropriate increase in the innate immune response via circulating and tissue inflammatory cells, such as monocytes/macrophages and neutrophils. These cells normally exist in a nonactivated state but are rapidly activated in response to bacteria. Sepsis induces a dysfunction in immune cells that contributes to the development of injuries by producing mediators such as cytokines and ROS.
Lipopolysaccharide (Lps) Signaling
LPS of Gram-negative organisms induces macrophages to secrete cytokines, which in turn activate T, and B cells to upregulate the adaptive immune responses. Toll-like receptor 4 (TLR4) is the LPS receptor and its stimulation induces nuclear factor kB (NF-kB) activation. The activation of NF-kB involves phosphorylation and degradation of IkB, an inhibitor of NF-kB. The NF-kB/IkB system exerts transcriptional regulation on proinflammatory genes encoded for various adhesion molecules and cytokines. Activation of NF-kB leads to the induction of NF-kB binding elements in their promoter regions and also leads to the induction of NF-kB dependent effector genes, which produce modifications in blood flow, and aggregation of neutrophils, and platelets. This results in damaged endothelium and also coagulation abnormalities often seen in patients with sepsis and septic shock. Therefore, NF-kB is reported to be an O2 sensor in LPS-induced endotoxemia.
Free Radicals and Antioxidants In Sepsis
The sources of ROS during sepsis are:
the mitochondrial respiratory chain.
the metabolic cascade of arachidonic acid.
the protease-mediated enzyme xanthine oxidase.
granulocytes and other phagocytes activated by complement, bacteria, endotoxin, lysosomal enzymes, etc.
Other oxidases mainly NADPH oxidase.
Under normal physiological conditions, the majority of ROS are formed during cellular respiration and by activated phagocytic cells, including neutrophils, involved in the inflammatory response. ROS have physiologically essential roles in mitochondrial respiration, prostaglandin production pathways and host defense . The electron reduction of O2 occurs in the mitochondrial electron transport system of all aerobically respiring cells. The enzyme catalyzing this transition metals iron and copper in its active site. These ions can be paramagnetic and contain stable unpaired electrons. By using the unpaired electrons in these transition metals to control the O2 reactions, mitochondria prevent the unwanted release of ROS.
In sepsis, there are several potential sources of ROS, including the mitochondrial respiratory electron transport chain, xanthine oxidase activation as a result of ischemia and reperfusion, the respiratory burst associated with immune cell activation, arachidonic acid metabolism and NADPH oxidase.
In fact, activated immune cells produce O2 – as a cytotoxic agent as part of the respiratory burst via the action of membrane-bound NADPH oxidase on O2.
The increase of ROS after LPS challenge has been demonstrated in different models of septic shock in peritoneal macrophages and lymphocytes.
This disturbance in the balance between pro-oxidants (ROS) and antioxidants in favor of the former is characteristic of oxidative stress in immune cells in response to endotoxin. In this context,
a typical behavior of these cells under an oxidative stress situation implies changes in different immune functions such as an increase in adherence and phagocytosis and a decrease in chemotaxis.
Neutrophils play a crucial role in the primary immune defense against infectious agents,which includes phagocytosis and the production of ROS.
Antioxidant Defenses
Antioxidants are central to the redox balance in the human body. They do not act in isolation, but synergistically with other classes of molecules. Primary antioxidants prevent oxygen radical formation, by either removing free radical precursors or by inhibiting catalysis, e.g. the enzymes glutathione peroxidase and catalase. Secondary antioxidants react with ROS which have already been formed, either to remove or inhibit them, e.g. vitamins C and E. Endogenous antioxidant defenses exist in a number of locations, namely intracellularly, on the cell membrane and extracellularly. The immune system is highly reliant on accurate cell-cell communication for optimal function, and any damage to the signaling systems involved will result in an impaired immune responsiveness.
Oxidant-mediated tissue injury is a particular hazard to the immune system, since phagocyte cells produce ROS as part of the defense against infection.
Therefore, adequate amounts of neutralizing antioxidants are required to prevent damage to the immune cells themselves.
The SOD enzymes are a family of metalloenzymes which rapidly promote the conversion of O2- to H2O2. Three forms of SOD are recognized to be important: copper-zinc SOD (cytoplasmic-located), manganese SOD (mitochondrial-located) and extracellular SOD (extracellular matrix-located). Catalase and glutathione peroxidase, a selenium containig enzyme which requires the presence of reduced GSH for its action, both catalyze the conversion of H2O2 to H 2O. GSH also has direct antioxidant activity, through donation of hydrogen ions, to repair damaged DNA. Oxidative stress and modulation on GSH/GSSG (GSSG=oxidized GSH) levels also up-regulate gene expression of several other antioxidant proteins, such as manganese SOD, glutathione peroxidase, thioredoxin (Trx) and metallothionein.
Effects of Nitric Oxide
NO is synthesized from L-arginine by different isoenzymes of (NOS), and is implicated in a wide range of disease processes, exerting both detrimental and beneficial effects at the cellular and vascular levels. To date, three main isoforms of NOS are known:
neuronal NOS (NOS-1 or nNOS),
inducible NOS (NOS-2 or iNOS), and
endothelial NOS (NOS-3 or eNOS).
NO has been shown to play a key role in the pathogenesis of septic shock
Hyperproduction of NO induces
excessive vasodilation,
changes in vascular permeability, and
inhibition of noradrenergic nerve transmission,
all characteristics of human septic shock.
The recogniton of NO production by activated macrophages as part of the inflammatory process was an important milestone for assesing both the biological production of NO and the phenomenon of induction of NOS activity. The observation has been extended to neutrophils, lymphocytes, and other cell types. The role of NO in the pathophysiology of endotoxic shock was advanced by Thiemermann and Vane, who observed that administration of the specific NOS inhibitor N-methyl-L-arginine (L-NMMA) decreased the severe hypotension produced by administration of LPS. Other groups simultaneously reported similar results indicating that endotoxin increases NO production and prompted the idea that pharmacological inhibition of NOS may be useful in the treatment of inflammation and septic shock. However, clinical trials using L-NMMA failed to show a beneficial effect in septic shock patient. The major limitation for the use of NOS inhibitors in clinical studies is the development of pulmonary hypertension as a side effect of NOS blockade, which can be alleviated by the use of inhaled NO.
However, several compounds which modulate NO synthesis have been patented in recent years, such as various inflammatory mediators that have been implicated in the induction and activation of iNOS, particularly IFNg, TNFa, IL-1b, and platelet-activating factor (PAF) alone or synergistically. In addition to the activation of iNOS, cytokines and endotoxin may increase NO release by increasing arginine availability through the opening of the specific y+ channels and the expression of the cationic amino acid transporter (CAT), or by increasing tetrahydrobiopterin levels, a key cofactor in NO synthesis. Several experimental studies have demonstrated a decrease in NOS activity resulting in an impairment in endothelial-dependent relaxation during endotoxemia and experimental sepsis, possibly as the result of a cytokine-or hypoxia-induced shortened half-life of NOS mRNA, or of altered calcium mobilization.
NO exerts in vitro toxic effects including nuclear damage, protein and membrane phospholipid alterations, and the inhibition of mitochondrial respiration in several cell types. Mitochondrial impairment could also be considered as an adaptive phenomenon, decreasing cellular metabolism when the energy supply is limited. The toxicity of NO itself may be enhanced by the formation of ONOO- from the reaction of NO with O-2. Therefore, the multiple organ failure syndrome (MOFS) that often accompanies severe sepsis may be related to the cellular effects of excess NO or ONOO-.
Involvement of Nitrogen Species
NO reacts rapidly with ferrous iron, and at physiological concentrations, NO also binds to soluble guanylate cyclase and to another hemoprotein, cytochrome c oxidase (Complex IV), the terminal enzyme of the mitochondrial respiratory chain. NO can therefore control cellular functions via the reversible inhibition of respiration. There are a number of reactive NO species, such as
N2O3 and
ONOO-
that can also alter critical cellular components.
During the first hours after injury, iNOS-mediated NO production is upregulated, producing a burst of NO that far exceeds basal levels. This overabundance of NO produces significant cellular injury via several mechanisms.
NO may
directly promote overwhelming peripheral vasodilation, resulting in vascular decomposition;
NO may upregulate the transcription NF-kB initiating an inflammatory signaling pathway that, in turn,
triggers numerous inflammatory cytokines.
NO also interacts with the O-2 to yield ONOO-, a highly reactive compound that exacerbates the injury produced by either O-2 alone or NO alone.
The ONOO- generation which occurs during fluid resuscitation in the injured subject produces cellular death by enhancing DNA single strand breakage, activates the nuclear enzyme polyADP ribose synthetase (PARS), leading to cellular energy depletion and cellular necrosis. The detrimental effects of ONOO- in shock and resuscitation have been attributed to oxidation of sulfhydryl groups, the nitration of tyrosine, tryptophane, and guanine, as well as inhibition of the membrane sodium-potassium adenosine triphosphatase. PARS activation depletes NAD and thus alters electron transport, ATP synthesis, and glycolysis; and leads to DNA fragmentation and cellular apoptosis.
The activation of monocytes, macrophages and endothelial cells by LPS results in the expression of iNOS, and consequently increases the transformation of L-arginine to NO, which can combine with O2- to form ONOO-, causing tissue injury during shock, inflammation and ischemia reperfusion. NO stimulates H2O2 and O-2 production by mitochondria, increasing leakage of electrons from the respiratory chain. H2O2, in turn, participates in the upregulation of iNOS expression via NFkB activation. ONOO- has been shown to stimulate H2O2 production by isolated mitochondria. On the other hand, NO can decrease ROS-produced damage that occurs at physiological levels of NO. The high reactivity of NO with radicals might be beneficial in vivo by scavenging peroxyl radicals and inhibiting peroxidation. ONOO- may also be a signal transmitter and can mediate vasorelaxation, similarly to NO.
Local generation of RNS contributes to tissue injury. Recent studies have demonstrated that activation of the nuclear enzyme poly(ADP-ribose) polymerase-1 by RNS-mediated DNA damage is an important pathway of tissue injury in conditions associated with oxidative stress. Increased formation of RNS in response to endotoxin challenge is organ specific.
In sepsis, NO may exert direct and indirect effects on cardiac function. Sustained generation of NO occurs in systemic inflammatory reactions, such as septic shock with involvement in circulatory failure. In fact, myocardial iNOS activity has been reported in response to endotoxin and cytokines and inversely correlated with myocardial performance. Low-to-moderate doses of iNOS inhibitors restore myocardial contractility in hearts exposed to proinflammatory cytokines, whereas at higher doses, the effects are reversed. This finding may indicate that small amounts of NO produced by iNOS may be necessary to maintain contractility and can be cardio-protective in experimental sepsis.
Nitric oxide in Septic Shock
A list of effects of NO in sepsis is as follows.
Inhibition of nitric oxide synthesis causes myocardial ischemia in endotoxemic rats
Nitric oxide causes dysfunction of coronary autoregulation in endotoxemic rats
Prolonged inhibition of nitric oxide synthesis in severe septic shock
Effect of L-NAME, an inhibitor of nitric oxide synthesis, on cardiopulmonary function in human septic shock
Pulmonary hypertension and reduced cardiac output during inhibition of nitric oxide synthesis in human septic shock
Effect of L-NAME, an inhibitor of nitric oxide synthesis, on plasma levels of IL-6, IL-8, TNF-u and nitrite/nitrate in human septic shock
Endothelin-1 and blood pressure after inhibition of nitric oxide synthesis in human septic shock
Distribution and metabolism of NO-nitro-L-arginine methyl ester in patients with septic shock
The possible involvement of the L-arginine-NO pathway in both the vascular and cellular processes seen in sepsis has been supported by numerous in vitro and in vivo studies. iNOS appears to be expressed in a wide array of cell types during sepsis, including immune cells (such as macrophages, neutrophils, T lymphocytes), as well as cells outside the classical immune system (for example, hepatocytes, Kuppfer cells, vascular smooth muscle cells, endothelial cells, and fibroblasts). Expression of iNOS is regulated, both positively and negatively, by a number of mediators present during infection and inflammation. The main stimuli for iNOS induction indude lipopolysaccharide (LPS), interferon-y, interleukin (IL)-10, and tumor necrosis factor (TNF)-a; inhibitory cytokines, such as transforming growth factor-5, IL-4 and IL-10, as well as glucocorticoids, can prevent this induction. The expression of iNOS in response to these agents differs among cell types, but a maximal inducing effect is generally obtained by the combination of microbial products and cytokines acting synergistically.
iNOS activity is also regulated by substrate and cofactor availability. Tetrahydrobiopterin (BH4), an essential cofactor for the enzyme, is coinduced with iNOS in cytokine-stimulated vascular smooth muscle cells.
NO is a simple molecule, but its widespread production in sepsis, coupled with its effects on a variety of intracellular and extracellular target molecules, results in a complex array of biologic roles. Interaction of NO with the metalloproteins in a number of key enzymes can modulate their activity. Many of the signaling actions of ‘NO are mediated by soluble guanylate cyclase. By binding the iron on the heme component of soluble guanylate cyclase, NO is able to activate the enzyme leading to cyclic guanosine monophosphate (cGMP) formation.
Increased cGMP levels account for several of the important cellular actions of NO, including
smooth muscle relaxation,
platelet aggregation and adherence, as well as
neutrophil chemotaxis.
However, *NO can adversely affect cellular metabolism through its disruption of iron-sulfur clusters in essential energy-generating enzymes involved in mitochondrial electron transport, glycolysis, and the Krebs cycle. Further, high concentrations of induced macrophage produced NO can directly interfere with DNA in target cells, resulting in fragmentation.
Another critical reaction that ‘NO undergoes during inflammation is with the superoxide anion radical (02j, yielding peroxynitrite (OONO-). OONO- is a potent oxidant that can decay under acidic conditions to produce a powerful hydoxyl-like free radical. This reaction between *NO and O2 can have a protective or damaging consequence, depending on the individual sites and rates of production of the free radicals, and the redox status of both the generating cells as well as the target cells. OONO- formation can initiate adverse effects such as lipid peroxidation of membranes, and modification of structural proteins through nitration of tyrosine residues (14). Indeed, increased levels of 3-nitrotyrosine have been detected in the lungs of patients with sepsis and animals with acute lung injury. However, OONO- can also S-nitrosylate glutathione and other thiol-containing substances to form S-nitrosothiols, which have marked cardioprotective and cytoprotective effects.
The damaging effect of NOS inhibition may be, in part, mediated by oxygen radicals and platelet deposition, suggesting a cytoprotective role of NO in preventing microvascular thrombosis and as a free radical scavenger. In addition, ‘NO has a protective role in hepatic microcirculatory dysfunction during sepsis through its effect on leukocyte adherence to sinusoidal walls. ‘NO may also protect against circulatory vasoconstrictors during inflammation, as enhanced ‘NO synthesis counteracted phenylephrine-induced increases in intrahepatic resistance in endotoxin-treated rats. Finally, we have recently demonstrated that different types of NOS inhibitors resulted in detectable apoptosis in the liver following LPS injection. This increase in apoptosis was present even with L-N-iminoethyl-lysine (L-NIL), a rather specific inhibitor of iNOS, revealing another important protective role of NO as an antiapoptotic agent in sepsis.
Even though overproduction of *NO in the vasculature contributes to the vasodilatation seen in septic shock, iNOS expression during inflammation also represents a beneficial, adaptive response in some organ systems. Moreover, different tissues can react dissimilarly to the effects of ‘NO cytotoxicity. In this setting, global nonselective inhibition of NOS, including the potentially undesirable consequences of eNOS inhibition, would be harmful. If confirmed, this would suggest that use of isoform-specific inhibitors of NOS within the vascular bed would be more appropriate.
Pulmonary Hypertension and Reduced Cardiac Output
Pulmonary hypertension and reduced cardiac output can be major side effects of continuous NO synthase inhibition. Pulmonary vasoconstriction is undesirable because it may compromise pulmonary gas exchange and because it increases the workload on the right ventricle. In cases where strain already exists on the right ventricle (e.g. sepsis or PEEP ventilation) or in cases where right sided cardiac reserve is minimal, such increase in workload may lead to right ventricular failure, reduced cardiac output and compromised tissue perfusion.
Blood pressure and systemic vascular resistance increased during infusion of the NO synthase inhibitor L-NAME, and the dosage of catecholamines was reduced. The vasoconstrictive response to L-NAME most likely was the result of blocking the NO system . In addition to the systemic effects of L-NAME, severe pulmonary vasoconstriction was observed with L-NAME. Analogous to these findings, in patients with Adult Respiratory Distress Syndrome (ARDS), inhalation of NO is reported to be beneficial by causing local vasodilation in bronchial and pulmonary circulation which results in reduced pulmonary vascular resistance and improved oxygenation. This suggests that the pulmonary circulation is sensitive to the vasodilating effects of both endogenous and exogenous NO. Pulmonary vasoconstriction is not, therefore, unexpected with systemic inhibition of NO synthesis. With a continuous infusion of L-NAME, pulmonary vascular resistance increased five-fold, whereas systemic vascular resistance “only” doubled. pulmonary hypertension was reversible after stopping L-NAME infusion. In prior experiments with a lower dose of LNAME, pulmonary vasoconstriction was less pronounced and did not result in pulmonary hypertension.’ Thus, pulmonary hypertension is a dose-related effect of L-NAME that can probably be attributed to overdosing of the drug. Reduced cardiac output may have directly resulted from the extreme increase in pulmonary vascular resistance compromising venous return and left ventricular preload and/or a reflex reduction in heart rate by the increase in vascular resistance and blood pressure. S-methyl-isothiourea, a relatively selective inhibitor of iNOS activity, decreased pulmonary leak and improved survival in endotoxemia. However, because of the tissue-protective and antiapoptotic effects of NO, even selective iNOS inhibitors may be detrimental in certain tissues during sepsis.Combining the salutary effects of site-specific local donors that exploit the cytoprotective actions of ‘NO with specific agents that combat the deleterious hypotensive and tissue-damaging effects of ‘NO overproduction may be needed to treat septic shock. In this regard, inhaled ‘NO gas has shown promise as a selective pulmonary vasodilator.
Role of Nitric Oxide in Inflammation and Tissue Injury
Since the discovery that nitric oxide (‘NO) accounts for the biologic activity of endothelial-derived relaxing factor, a torrent of research over the last decade has focused on its role, protective or detrimental, in myriad pathophysiologic conditions. Recently, increasing attention has focused on ‘NO as a possible mediator of the severe hypotension and impaired vasoreactivity characteristic of circulatory failure. Experimental and clinical studies have suggested NOS inhibition might have therapeutic potential in circulatory shock, and other studies have demonstrated the beneficial nature of iNOS expression in modulating tissue perfusion and mediating cytotoxicity. However, inhibition of ‘NO synthesis in experimental and clinical studies of shock has yielded mixed, sometimes contradictory, results. Overproduction of ‘NO in the vasculature may result in systemic vasodilatation, but still ‘NO synthesis has a beneficial role in regulating organ perfusion and mediating cytotoxicity.
Diminished *NO Production Occurs with Hemorrhage
These findings are consistent with those in trauma patients, where nitrite and nitrate levels were reduced for prolonged periods after injury. This impairment of ‘NO production in victims of hemorrhagic hypotension may be due to impairment of eNOS, and indeed, several investigators have demonstrated decreased vasodilatory activity in vascular rings taken from hemorrhaged animals in response to agonists that stimulate endothelial ‘NO production. In studies of hemorrhagic shock no iNOS expression could be detected until the very late irreversible phase of HS. The hemodynamic instability associated with decompensation occurred well before NOS induction.
Using either the selective inhibitor L-NIL or iNOS knockout mice, iNOS inhibition or deficiency not only prevented the upregulation of the inflammatory cytokines IL-6 and granulocyte colony-stimulating factor following resuscitation from HS but also produced a marked reduction in lung and liver injury. Furthermore, the activation of the proinflammatory transcriptional factors nuclear factor kappa B and signal transducer and activator of transcription 3 was also reduced, suggesting iNOS upregulation has a key role in proinflammatory signaling and the subsequent activation of inflammatory cascades. Recent studies have implicated a possible redox-sensitive mechanism. ‘NO activates the critical signaling enzyme p21ras through S-nitrosylation.
Vascular quenching of ‘NO using scavengers may again provide an alternative to NOS inhibition as a means to achieve the goal of reducing ‘NO levels. Use of ‘NO scavengers after HS and resuscitation may serve to supplement a possibly depleted antioxidant defense system and limit the harmful effects of free radicals such as OONO- and hydoxyl radicals. Removal of ‘NO by this method is complicated by the extreme rapidity of the reaction between ‘NO and 02’- .
iNOS upregulation also has a beneficial protective role in several organ systems. In conditions where excess NO production results in maladaptive damaging consequences with disruption of homeostasis, the therapeutic strategy should be to remove this surplus ‘NO without adversely affecting the cytoprotective actions of *NO. Interfering with the physiologic and microcirculatory role of eNOS through nonselective, global inhibition of NOS is undesirable in shock.
Effects of nitric oxide in endotoxemia and hemorrhagic shock and proposed therapeutic strategies for manipulation of nitric oxide production.
Endotoxemia Hemorrhagic shock
Effects of NO Beneficial Beneficial
by eNOS -maintains perfusion; -maintains perfusion;
cytoprotective cytoprotective
iNOS Beneficial and toxic- Beneficial and toxic-
depending on site can induce tissue damage and of production and promote inflammation with microenvironment sustained shock
Therapeutic strategy
Inhibition of eNOS Avoid Avoid
Inhibition of iNOS Possibly desirable- Probably desirable-
to reduce to limit exaggerated inflammatory
cytotoxicity and response and development of
combat hypotension MODS
NO scavengers Probably desirable-quench Probably desirable-quench
extracellular NO without extracellular NO without
inhibition of eNOS or iNOS; inhibition of eNOS or iNOS;
supplement antioxidant defenses supplement antioxidant defenses
NO donors Possibly desirable-site- Possibly desirable-site-
specific donors without adverse specific donors without adverse
systemic side effects; systemic side effects;
limited availability limited availability
Therapeutic Outlook
LINCS: L-NAME (a NO synthase inhibitor)
Patients were randomized to supportive care alone (n=15, control group) or to supportive care in addition to L-NAME (1 mg/Kg bolus and 1 mg/Kg/h continuous IV drip for 5 h n=15). Death at one month was 27% in the L-NAME group vs. 67% in the control group (p=0.008). Time on IABP and time on mechanical ventilation were significantly shorter in the L-NAME group. The results of this study indicate that NO synthase inhibitors are beneficial in the treatment of patients with refractory cardiogenic shock. Inducible Nitric Oxide Synthase Inhibitors
Inducible nitric oxide synthase (iNOS)-dependent production of nitric oxide (NO) plays an important role in inflammation. The effects of various naturally occurring furanocoumarins on NO production in lipopolysaccharide (LPS)-activated RAW 264.7 macrophage cells were evaluated in vitro. The results showed that angelicin, pimpinellin, sphondin, byakangelicol, oxypeucedanin, oxypeucedanin hydrate, xanthotoxin, and cnidilin are potential NO production inhibitors, and their IC50 values for inhibition of nitrite production were 19.5, 15.6, 9.8, 16.9, 16.8, 15.8, 16.6, and 17.7 mg/mL, respectively.
Distinct structure activity relationships were also revealed for the NO production inhibitory activities of these furanocoumarins. Activities of the angelicin type such as pimpinellin and sphondin were more potent than those of the psoralen type. Presence of a methoxy at the C6 position in the angelicin type seemed to be essential to augment the activity. Western blot analysis demonstrated that only sphondin dose-dependently inhibited the expression of the iNOS protein at 2.5±20 mg/mL. However, iNOS enzyme activity was stimulated with LPS for 12 h and sphondin was administered (20 mg/mL) for 24 h, which did not reasonably inhibit iNOS enzyme activity. l-NAME (100 mM), a known specific inhibitor of iNOS, was employed as a positive control with the same protocol and showed more than 50% inhibition activity. The results demonstrate that the NO production inhibitory activity of sphondin is due to the effect of iNOS expression, but not by direct inhibition of iNOS enzyme activity. Thus, sphondin may act as a potent inhibitor of NO production under tissue-damaging inflammatory conditions. S-Methylisothiourea Sulfate, A Potent And Selective Inhibitor Of Inducible Nitric Oxide Synthase
Non-isoform-selective inhibition of NO formation, however, may lead to side effects by inhibiting the constitutive isoform of NOS and, thus, the various physiological actions of NO. S-Methylisothiourea sulfate (SMT) is at least 10- to 30-fold more potent as an inhibitor of inducible NOS (iNOS) in immuno-stimulated cultured macrophages (EC50, 6 ,AM) and vascular smooth muscle cells (EC50, 2 ,uM) than NG-methyl-L-arginine (MeArg) or any other NOS inhibitor yet known. The effect of SMT on iNOS activity can be reversed by excess L-arginine in a concentration-dependent manner.
Enhanced formation of NO following the induction of iNOS contributes importantly to the circulatory failure (hypotension and vascular hyporeactivity to vasoconstrictor agents) in circulatory shock of various etiologies.
SMT dose-dependently reverses (0.01-3 mg/kg) the hypotension and the vascular hyporeactivity to vasoconstrictor agents caused by endotoxin [bacterial lipopolysaccharide]
SMT, a potent and selective inhibitor of iNOS, may have considerable value in the therapy of circulatory shock of various etiologies and other pathophysiological conditions associated with induction of iNOS. SMT, or other iNOS-selective inhibitors, are likely to have fewer side effects which are related to the inhibition of eNOS, such as excessive vasoconstriction and organ ischemia), increased platelet and neutrophil adhesion and accumulation, and microvascular leakage.
Iron Chelates Bind Nitric Oxide
Nitric oxide (NO), a short-lived potent vasodilator, was first described as the endothelium-derived relaxation factor (EDRF). The formation of NO from the guanidine nitrogen group of L-arginine is catalyzed by a group of enzymes termed constitutive (cNOs) and inducible (iNOs) NO synthases. The inducible form is not present constitutively in mammalian cells but is induced by proinflammatory stimuli such as bacterial lipopolysaccharide (LPS), Corynebacterium parvum, and the cytokines tumor necrosis factor-a, interleukin-1, or interferon-y, individually or in combination. Excess production of NO is reported to be associated with the development of hypotension associated with endotoxemia and sepsis.
Electrochemical studies show that FeIII-(DTPA)2- binds NO stoichiometrically upon reduction to iron(II) at biologically relevant potentials to form a stable NO adduct. In contrast, FeI”I(HDFB)+ is a stable and efficient electrocatalyst for the reduction of NO to N20 at biologically relevant potentials. These results suggest that the mechanism of protection against death by septic shock involves NO scavenging and that particularly effective drugs that operate a low dosages may be designed based on the principle of redox catalysis. These complexes constitute a new family of drugs that rely on the special ability of transition metals to activate small molecules.
Iron complexes could act as general NO scavengers and provide protection against septic shock. Iron complexes are capable of forming relatively stable NO adducts. Metal complexes, and in particular iron chelators, could act as “molecular sponges,” mopping up the excess NO produced during septic shock. Iron chelators can sequester and (as for 2) catalyze conversion of NO to benign products. Demonstration of mechanistic aspects of septic shock protection in vivo, including interaction with other free radicals, may be hampered by the detection limits of current analytical techniques. To detect the NO Fe-DETC complex formation in livers of LPS-treated mice by the electron paramagnetic resonance.
After screening a library of metal chelators and chelates [Fe(III)(H2DTPA)] and [Fe(III)(HDFB)]+ offered the highest mortality decrease in an experimental model of septic shock. The Fe(II) form of both complexes can bind NO, which appears to be related to their biological function.
Survival was greatly enhanced by the administration of 4 or 2 either 2 h before and at the time of or 30 min after LPS. In contrast, the Fe3+-free ligands of these compounds, 3 and 1, were less protective when administered before and at the time of LPS and virtually ineffective when administered after LPS. The clear advantage of 4 over 2 when administered after LPS was observed over a large number of experiments [76% survival with 4 (n = 102 mice) and 38% survival with 2 (n =64 mice)].
The hydroxamic acid siderophore ferrioxamine B [Fe”‘(HDFB)+] and the iron complex of diethylene-triamine-pentaacetic acid [FeI”(DTPA)2i] protected mice against death by septic shock induced by Corynebacterium parvum + lipopolysaccharide. Although Fem(DTPA)2- was somewhat more effective than FeI”(HDFB)+, the iron-free ligand H4DFB+ was significantly more effective than DTPA. The hydroxamic acid chelator has a much higher iron affinity than the amine carboxylate, allowing for more efficient formation of the FeI”(HDFB)+ complex upon administration of the iron-free ligand.
Efficacy of Treatment With the Iron (III) Complex of Diethylenetriamine Pentaacetic Acid
Bacteremia and septic shock also are associated with overproduction of free radicals such as hydroxyl, superoxide, and carbon- and oxygen-centered radicals. In addition, nitric oxide (NO) overproduction is at least partly responsible for the vasodilation that causes a reduction in mean systemic arterial pressure (MSAP) and organ perfusion pressure during septic shock. This overproduction of NO likely results from early activation of the endothelial constitutive form of NO synthase followed by induction of the inducible form of NO synthase via TNF and IL-1.
The simultaneous increase and further reaction of NO with superoxide, which yields the oxidant peroxynitrite anion, occurs in cellular systems in response to inflammatory mediators. In addition, in sepsis-associated adult respiratory distress syndrome (ARDS), the presence of nitrotyrosine residues (formed by reaction of peroxynitrite and the tyrosine residues of proteins) are apparent throughout the lung.
Administration of the iron (III) complex of diethylenetriamine pentaacetic acid (DTPA iron (III), prevented death in Corynebacterium parvum 1 LPS-treated mice. Using electrochemistry, the binding of NO to DTPA iron (II) is confirmed. The DTPA iron (II) form can be easily formed by common biological reductants, because the potential for the iron (III/II) couple is E = 0.22.
Treatment with DTPA iron (III) resulted in a significant decrease in mortality compared to the untreated controls. The efficacy of DTPA iron (III) increased when given to mice 2 h or more after infection. The best results were observed when DTPA iron (III) was given 5 h after infection.
The iron (III) complex of diethylenetriamine pentaacetic acid (DTPA iron [III]) protected mice and baboons from the lethal effects of an infusion with live LD 100 Escherichia coli. In mice, optimal results were obtained when DTPA iron (III) was administered two or more hours after infection. Prevention of death occurred in spite of the fact that the adverse effects of TNFa were well underway in the mouse model.
In septic baboons, survival was observed after administration of two doses of DTPA iron (III) at 2.125 mg/kg, the first one given before, or as late as 2 h after, severe hypotension. Administration of DTPA iron (III) did not alter mean systemic arterial pressure, but did protect baboons in the presence of high levels of TNFa and free radical overproduction. Furthermore, exaggerated production of nitric oxide was attenuated. Because of its ability to interact in vitro with free radicals, its poor cell permeability, and its short half-life, we postulate that DTPA iron (III) and/or its reduced form may have protected the mice and baboons by sequestration and subsequent elimination of free radicals (including nitric oxide) from their systems. (J. Clin. Invest.1996. 98:192–198.) Inhibitor of Poly(Adenosine 5′-Diphosphate-Ribose) Synthetase
Poly(adenosine 5′-diphosphate [ADP]-ribose) synthetase (PARS) is a nuclear enzyme which, when activated by DNA singlestrand breaks, initiates an energy-consuming, inefficient metabolic cycle by transferring ADP-ribose units to nuclear proteins. The result of this process is a rapid depletion of intracellular oxidized nicotinamide adenine dinucleotide and adenosine 5′-triphosphate energetic pools, which slows the rate of glycolysis and mitochondrial respiration, leading to cellular dysfunction and death.
Reactive oxygen-centered radicals (superoxide, hydroxyl radicals, singlet oxygen, and hydrogen peroxide) and peroxynitrite (a reactive oxidant produced from the reaction of superoxide and nitric oxide) are powerful triggers of DNA single strand breakage, and they induce activation of a cell suicide cycle governed by PARS in various cell types in vitro.
Multiple reports implicated a role of PARS activation in the pathophysiology of endotoxic shock, hemorrhagic shock, and various forms of ischemia-reperfusion injury.
Twenty pigs were chronically instrumented with intracardiac transducers to measure left ventricular pressure, sonomicrometer crystals in the left ventricle to measure short axis diameter, an ultrasonic flow meter to measure cardiac output, and catheters in the pulmonary artery and aorta to measure blood pressures and collect samples. By using a randomized study design, either the novel potent PARS inhibitor PJ34 (10 mg/kg for 1 hr, 2 mg·kg 1·hr 1 for 96 hrs) or placebo to pigs immediately before intraperitoneal implantation of Escherichia coli 0111.B4 (2.3 0.1 1010 colony-forming units/kg)-laden fibrin clots to produce peritonitis and bacteremia.
PJ34 treatment significantly attenuated this cytokine response. The formation of peroxynitrite and the activation of PARS were confirmed in hearts and lungs of the septic pigs by the immunohistochemical detection of nitrotyrosine and poly(ADP-ribose), respectively. Inhibition of PARS with PJ34 abolished poly(ADP-ribose) formation in septic animals.
Cardiac inotropicity was evaluated by analysis of percentage of short axis diameter shortening (one-dimensional ejection fraction). Bacteremia induced a rapid and progressive loss of inotropy until death in vehicle-treated pigs. A similar decline was observed in the first 6 hrs in PJ34 pigs. This decline was reversed on all subsequent days. Control pigs exhibited rapid and significant increases in systemic vascular (SVR) and pulmonary vascular (PVR) resistances.
This experimental model mimics many aspects of the human sepsis syndrome. Therefore, the positive survival benefit of PARS inhibition suggests a potential utility of PARS inhibitors in human sepsis management. PARS activation is triggered by DNA single-strand breakage.
The current work, demonstrating increased poly(ADP-ribose) staining in the heart of septic pigs, may point toward the importance of a myocardial, PARS dependent cardiodepressive mechanism in the current model of shock. This hypothesis is supported by the following findings:
free radicals cause myocardial dysfunction and injury in a PARS dependent fashion in vitro;
in the current study, pharmacologic inhibition of PARS with PJ34 markedly improved myocardial function; and
in prior studies, pharmacologic inhibition of PARS markedly improved myocardial contractile function in hypoxic-reoxygenated hearts as well as in a porcine model of hemorrhagic shock.
Treatment with a potent PARS inhibitor improved survival and cardiovascular status and attenuated an important mediator component of the inflammatory response in a lethal porcine model of sepsis. (Crit Care Med 2002; 30:974 –980).
Decrease of the inflammatory response and induction of the Akt/protein kinase B pathway by poly-(ADP-ribose) polymerase 1 inhibitor
The lack of efficacy of anti-inflammatory drugs, anti-coagulants, anti-oxidants, etc. in critically ill patients has shifted interest towards developing alternative treatments. Since inhibitors of the nuclear enzyme poly-(ADP-ribose) polymerase (PARP) were found to be beneficial in many pathophysiological conditions associated with oxidative stress and PARP-1 knock-out mice proved to be resistant to bacterial lipopolysaccharide (LPS)-induced septic shock, PARP. The mechanism of the protective effect of a potent PARP-1 inhibitor, PJ34 was studied in LPS-induced (20 mg/kg, i.p.) septic shock in mice.
We demonstrated a significant inflammatory response by magnetic resonance imaging in the dorsal subcutaneous region, in the abdominal regions around the kidneys and in the inter-intestinal cavities. We have found necrotic and apoptotic histological changes as well as obstructed blood vessels in the liver and small intestine. Additionally, we have detected elevated tumor necrosis factor-a levels in the serum and nuclear factor kappa B activation in liver of LPS-treated mice.
Pre-treating the animals with PJ34 (10 mg/kg, i.p.), before the LPS challenge, besides rescuing the animals from LPS-induced death, attenuated all these changes presumably by activating the phosphatidylinositol 3-kinase–Akt/protein kinase B cytoprotective pathway.
PJ34, a novel, potent PARP-1 inhibitor was found to protect against LPS induced tissue damage. PARP inhibitors protected Langendorff-perfused hearts against ischemia-reperfusion induced damages by activating the PI3-kinase–Akt pathway. The importance of the PI3-kinase–Akt pathway in LPS induced inflammatory mechanisms has gained support, raising the question whether this pathway was involved in the effect of PJ34 on LPS-induced septic shock.
Among all the observed LPS-induced inflammatory responses, we found the most characteristic and most pronounced increases in the gastro-intestinal tract, but no signal increase could be observed inside the kidneys and in skeletal muscle, in the paravertebral or in the femoral muscle. All increases in signal intensities were significantly attenuated in mice treated with PJ34.
Effect of PJ34 on survival of LPS-treated mice
PARP-1 inhibitor significantly protected the animals against LPS-induced death, with 86 and 43% surviving mice, respectively.
PJ34 treatment itself did not induce death or any obvious damage.
Effect of PJ34 on LPS-induced NF-kB activation
LPS treatment in the lung caused a significant increase in NF-kB activation that was slightly but not statistically significantly attenuated by PJ34 pre-treatment.
in contrast to the lung, NF-kB activation in the liver was prevented by PJ34 pre-treatment
The other tissue with observable LPS-induced pathological changes was the small intestine. Atrophy of villi may reflect the diarrhea observed in the LPS-treated animals and is in agreement with the results of Abreu et al. who found a Fas-mediated apoptosis in intestinal epithelial cells that was sensitised by inhibitors of PI3-kinase and opposed by expressing constitutively active Akt.
Pre-treatment of the animals with a novel, potent PARP-1 inhibitor, PJ34, diminished the thoracic and abdominal inflammatory responses as revealed by T2 imaging, and abolished the above mentioned pathological changes.
The protective role of PARP inhibitors in septic shock is likely to be more complex than merely the regulation of NF-kB/Rel-dependent gene expression.
Activation of the PI3-kinase–Akt/protein kinase B cytoprotective pathway is likely to contribute to the protective effects of PARP inhibitors in shock and inflammation.
Carboxy-PTIO On Hemodynamic And Blood Gas Changes
Infusion of LPS caused a marked decrease in mean arterial pressure (MAP), metabolic acidosis, and hypoxia. These effects were reversed by co-administration of carboxy-PTIO, without affecting other hemodynamic parameters. In control animals, neither hemodynamic nor blood gas parameters changed with or without carboxy-PTIO.
These results indicate that carboxy-PTIO attenuates
LPS-induced hypotension,
metabolic acidosis, and
hypoxia
by scavenging excess NO from the circulation without affecting NO synthase (NOS) activity. An NO scavenger, carboxy-PTIO, may be preferable to non-selective NOS inhibitors for the treatment of human septic shock.
Asymmetrical Dimethyl Arginine Levels
Overwhelming infection with resultant multiple organ failure, which has been termed the ‘sepsis syndrome’ , is a devastating illness with an incidence of 3 per 1,000 population per annum. It has been characterised as a dysregulation of inflammation in response to infection attributable to a combination of
excessive inflammation,
disseminated coagulopathy and
disruption of the integrity of microvascular endothelium.
Asymmetrical dimethyl arginine (ADMA) is an endogenous non-selective inhibitor of nitric oxide synthase that may influence the severity of organ failure and the occurrence of shock secondary to an infectious insult. Levels may be genetically determined by a promoter polymorphism in a regulatory gene encoding dimethylarginine dimethylaminohydrolase II (DDAH II).
A prospective observational study was designed, and 47 intensive care unit (ICU) patients with severe sepsis and 10 healthy controls were enrolled. Serum ADMA and IL-6 were assayed on admission to the ICU and seven days later. Allelic variation for a polymorphism at position -449 in the DDAH II gene was assessed in each patient.
ADMA levels and Sequential Organ Failure Assessment scores were directly associated on day one (p = 0.0001) and day seven (p = 0.002). The degree of acidaemia and lactaemia was directly correlated with ADMA levels at both time points (p < 0.01). On day seven, IL-6 was directly correlated with ADMA levels (p = 0.006). The variant allele with G at position -449 in the DDAH II gene was associated with increased ADMA concentrations at both time points (p < 0.05). Severity of organ failure, inflammation and presence of early shock in severe sepsis are associated with increased ADMA levels. ADMA concentrations may be influenced by a polymorphism in the DDAH II gene.
Several studies have added to the confusion surrounding the role of NO by demonstrating no effect of NO or NOS inhibition on the myocardium or on b-adrenergic responsiveness. Nevertheless, in most studies,
low-to-moderate doses of iNOS inhibitors restore myocardial contractility in hearts exposed to proinflammatory cytokines, whereas
at higher doses, the effects are reversed.
This finding may indicate that small amounts of NO produced by iNOS may be necessary to maintain contractility and can be cardio-protective in experimental sepsis.
Major cellular sources of ROS in living cells. Novo and Parola Fibrogenesis & Tissue Repair 2008 1:5 doi:10.1186/1755-1536-1-5 (Photo credit: Wikipedia)
endogenously produced VIP and PACAP are participants of the natural anti-inflammatory machinery.
VIP and PACAP are two attractive candidates for the development of therapies against acute and chronic inflammatory diseases, septic shock, and autoimmune diseases
Nitric oxide (NO), reactive nitrogen species (RNS) and reactive oxygen species (ROS) perform dual roles as immunotoxins and immunomodulators. An incoming immune signal initiates NO and ROS production both for tackling the pathogens and modulating the downstream immune response via complex signaling pathways. The complexity of these interactions is a reflection of involvement of redox chemistry in biological setting (fig. 1)
Fig 1. Image credit: (Wink et al., 2011)
Previous studies have highlighted the role of NO in immunity. It was shown that macrophages released a substance that had antitumor and antipathogen activity and required arginine for its production (Hibbs et al., 1987, 1988). Hibbs and coworkers further strengthened the connection between immunity and NO by demonstrating that IL2 mediated immune activation increased NO levels in patients and promoted tumor eradication in mice (Hibbs et al., 1992; Yim et al., 1995).
In 1980s a number of authors showed the direct evidence that macrophages made nitrite, nitrates and nitrosamines. It was also shown that NO generated by macrophages could kill leukemia cells (Stuehr and Nathan, 1989). Collectively these studies along with others demonstrated the important role NO plays in immunity and lay the path for further research in understanding the role of redox molecules in immunity.
NO is produced by different forms of nitric oxide synthase (NOS) enzymes such as eNOS (endothelial), iNOS (inducible) and nNOS (neuronal). The constitutive forms of eNOS generally produce NO in short bursts and in calcium dependent manner. The inducible form produces NO for longer durations and is calcium independent. In immunity, iNOS plays a vital role. NO production by iNOS can occur over a wide range of concentrations from as little as nM to as much as µM. This wide range of NO concentrations provide iNOS with a unique flexibility to be functionally effective in various conditions and micro-environements and thus provide different temporal and concentration profiles of NO, that can be highly efficient in dealing with immune challenges.
Redox reactions in immune responses
NO/RNS and ROS are two categories of molecules that bring about immune regulation and ‘killing’ of pathogens. These molecules can perform independently or in combination with each other. NO reacts directly with transition metals in heme or cobalamine, with non-heme iron, or with reactive radicals (Wink and Mitchell, 1998). The last reactivity also imparts it a powerful antioxidant capability. NO can thus act directly as a powerful antioxidant and prevent injury initiated by ROS (Wink et al., 1999). On the other hand, NO does not react directly with thiols or other nucleophiles but requires activation with superoxide to generate RNS. The RNS species then cause nitrosative and oxidative stress (Wink and Mitchell, 1998).
The variety of functions achieved by NO can be understood if one looks at certain chemical concepts. NO and NO2 are lipophilic and thus can migrate through cells, thus widening potential target profiles. ONOO-, a RNS, reacts rapidly with CO2 that shortens its half life to <10 ms. The anionic form and short half life limits its mobility across membranes. When NO levels are higher than superoxide levels, the CO2-OONO–intermediate is converted to NO2 and N2O3 and changes the redox profile from an oxidative to a nitrosative microenvironment. The interaction of NO and ROS determines the bioavailability of NO and proximity of RNS generation to superoxide source, thus defining a reaction profile. The ROS also consumes NO to generate NO2 and N2O3 as well as nitrite in certain locations. The combination of these reactions in different micro-environments provides a vast repertoire of reaction profiles for NO/RNS and ROS entities.
The Phagosome ‘cauldron’
The phagosome provides an ‘isolated’ environment for the cell to carry out foreign body ‘destruction’. ROS, NO and RNS interact to bring about redox reactions. The concentration of NO in a phagosome can depend on the kind of NOS in the vicinity and its activity and other localised cellular factors. NO and is metabolites such as nitrites and nitrates along with ROS combine forces to kill pathogens in the acidic environment of the phagosome as depicted in the figure 2 below.
Fig 2. The NO chemistry of the phagosome. (image credit: (Wink et al., 2011)
This diagram depicts the different nitrogen oxide and ROS chemistry that can occur within the phagosome to fight pathogens. The presence of NOX2 in the phagosomes serves two purposes: one is to focus the nitrite accumulation through scavenging mechanisms, and the second provides peroxide as a source of ROS or FA generation. The nitrite (NO2−) formed in the acidic environment provides nitrosative stress with NO/NO2/N2O3. The combined acidic nature and the ability to form multiple RNS and ROS within the acidic environment of the phagosome provide the immune response with multiple chemical options with which it can combat bacteria.
Bacteria
There are various ways in which NO combines forces with other molecules to bring about bacterial killing. Here are few examples
E.coli: It appears to be resistant to individual action of NO/RNS and H2O2 /ROS. However, when combined together, H2O2 plus NO mediate a dramatic, three-log increase in cytotoxicity, as opposed to 50% killing by NO alone or H2O2 alone. This indicates that these bacteria are highly susceptible to their synergistic action.
Staphylococcus: The combined presence of NO and peroxide in staphylococcal infections imparts protective effect. However, when these bacteria are first exposed to peroxide and then to NO there is increased toxicity. Hence a sequential exposure to superoxide/ROS and then NO is a potent tool in eradicating staphylococcal bacteria.
Mycobacterium tuberculosis: These bacterium are sensitive to NO and RNS, but in this case, NO2 is the toxic species. A phagosome microenvironment consisting of ROS combined with acidic nitrite generates NO2/N2O3/NO, which is essential for pathogen eradication by the alveolar macrophage. Overall, NO has a dual function; it participates directly in killing an organism, and/or it disarms a pathway used by that organism to elude other immune responses.
Parasites
Many human parasites have demonstrated the initiation of the immune response via the induction of iNOS, that then leads to expulsion of the parasite. The parasites include Plasmodia(malaria), Leishmania(leishmaniasis), and Toxoplasma(toxoplasmosis). Severe cases of malaria have been related with increased production of NO. High levels of NO production are however protective in these cases as NO was shown to kill the parasites (Rockett et al., 1991; Gyan et al., 1994). Leishmania is an intracellualr parasite that resides in the mamalian macrophages. NO upregulation via iNOS induction is the primary pathway involved in containing its infestation. A critical aspect of NO metabolism is that NOHA inhibits AG activity, thereby limiting the growth of parasites and bacteria including Leishmania, Trypanosoma, Schistosoma, Helicobacter, Mycobacterium, and Salmonella, and is distinct from the effects of RNS. Toxoplasma gondii is also an intracellular parasite that elicits NO mediated response. INOS knockout mice have shown more severe inflammatory lesions in the CNS that their wild type counterparts, in response to toxoplasma exposure. This indicates the CNS preventative role of iNOS in toxoplasmosis (Silva et al., 2009).
Virus
Viral replication can be checked by increased production of NO by induction of iNOS (HIV-1, coxsackievirus, influenza A and B, rhino virus, CMV, vaccinia virus, ectromelia virus, human herpesvirus-1, and human parainfluenza virus type 3) (Xu et al., 2006). NO can reduce viral load, reduce latency and reduce viral replication. One of the main mechanisms as to how NO participates in viral eradication involves the nitrosation of critical cysteines within key proteins required for viral infection, transcription, and maturation stages. For example, viral proteases or even the host caspases that contain cysteines in their active site are involved in the maturation of the virus. The nitrosative stress environment produced by iNOS may serve to protect against some viruses by inhibiting viral infectivity, replication, and maturation.
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Inhibition of ET-1, ETA and ETA-ETB, Induction of NO production, stimulation of eNOS and Treatment Regime with PPAR-gamma agonists (TZD): cEPCs Endogenous Augmentation for Cardiovascular Risk Reduction – A Bibliography
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