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Posts Tagged ‘ROS’


Metabolic insight into cancer cell survival

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Revised 4/20/2016

AACR 2016: Novel Epigenetic Drug Therapeutics Revealed

http://www.genengnews.com/gen-news-highlights/aacr-2016-epigenetic-drug-therapeutics/81252634/

As the 2016 American Association for Cancer Research meeting begins to downshift toward a close, the presentation sessions certainly did not suffer from a lack of enthusiasm from attendees or high-quality research from presenters. Of particular note was a major symposium that discussed next-generation epigenetic therapeutics.

In the past several years, there have been a variety of epigenetic targets exploited by newly developed drug compounds, many of which have already progressed into clinical trials. Often these compounds will target specific classes of epigenetic regulators like acetylases and histone demethylases, for instance, the small-molecule inhibitors of protein interacting bromodomains—implicated in a diverse range of cancers—and methyltransferase inhibitors, such as lysyl demethylases (KDM).

However, for all of their recently achieved success, researchers are continually searching for increasingly rapid methods to validate epigenetic drug targets. Session Chairperson Udo Oppermann, Ph.D., principal investigator at the University of Oxford, stressed that open access research and continued investigator cooperation were key factors for driving rapid development of novel therapeutics in the field. Anecdotally, Dr. Oppermann noted that if biologists were a bit more like the international cooperative teams of physicists that discovered the Higgs boson or gravitational waves, many biological endeavors would advance rather quickly.

After providing the audience with a brief introduction to the symposium’s topic, Dr. Oppermann described his current research on histone demethylase inhibitors in multiple myeloma and the connection to metabolic pathways. He surmised that tricarboxylic acid (TCA) cycle-derived metabolites can link cellular metabolism to cancer—impacting epigenetic landscapes. Specifically, the TCA intermediates are inhibitors of KDMs, ultimately controlling epigenetic and metabolic regulation.

Furthermore, Dr. Oppermann’s group was able to show that treatment of myeloma cell lines with the potent and specific histone demethylase inhibitor GSK-J4 was able to reverse the Myc-driven metabolic dependency, forcing a selected amino acid depletion. This deficiency led to the integrated stress response and the activation of proapoptotic genes. This work helps to solidify further the potent nature of GSK-J4 in cancer while simultaneously uncovering the metabolic mechanisms that cancer cells employ to keep their overproliferative phenotypes progressing forward.

Next, Tomasz Cierpicki, Ph.D., assistant professor at the University of Michigan, described his work on targeting leukemic stem cells with small-molecule inhibitors of the protein regulator of cytokinesis 1 (PRC1). Dr. Cierpicki took the audience through his research design, which was to target BMI1, an oncogene that determines the proliferative capacity and self-renewal potential of normal and leukemic stem cells. BMI1 has been implicated in a variety of tumors and is essential for the Polycomb Repressive Complex 1 (PRC1). Moreover, BMI1 interacts with the RING1B protein to form an active E3 ubiquitin ligase that targets histone H2A, modifying epigenetic regulation mechanisms.

Dr. Cierpicki’s laboratory looked at inhibitors of the RING1B–BMI1 E3 ligase complex as potential therapeutic agents targeting cancer stem cells. Using an array of techniques from fragment screening to medicinal chemistry, the researchers were able to identify potent compounds that bound to RING1B–BMI1 and inhibit its E3 ubiquitin ligase activity with low micromolar affinities. When testing in vitro, the inhibitors revealed robust downregulation of H2A ubiquitination. Dr. Cierpicki and his colleagues found that the RING1–BMI1 inhibitor blocked the self-renewal capacity of the stem cells and induced cellular differentiation—validating RING ligases as a novel epigenetic drug target.

Finishing up the session was William Sellers, M.D., vice president and global head of oncology for the Novartis Institutes for BioMedical Research. Dr. Sellers’ research is focused on what genes are necessary for epigenetic regulation of cancer and how they are linked to essential metabolic processes. He and his colleagues accomplished their studies through the use of large-scale shRNA screening across a diverse set of 390 cancer cell lines.

Utilizing deep small hairpin RNA (shRNA) screening libraries, at 20 shRNAs per genome, provided the investigators with highly robust gene-level data, which resulted in the emergence of several distinct classes of cancer-dependent genes. For example, Dr. Sellers’ group found that several known oncogenes fell into the genetic dependence class, whereas other genes were sorted into lineage, paralog, and collateral synthetic lethality dependent classes.

An interesting example from Dr. Sellers’ work was the link his laboratory discovered between polyamine metabolism and salvage and the protein arginine methyltransferase 5 (PRMT5). In particular, the loss of methylthioadenosine phosphorylase (MTAP)—which has been observed in many solid tumors and hematologic malignancies—resulted in the accumulation of S-methyl-5′-thioadenosine (MTA), which specifically inhibited the epigenetic mechanisms of PRMT5. The culmination Dr. Sellers’ analysis led to the finding that PRMT5 is a novel target for therapeutic development in MTAP deleted cancers.

These three presentations represented some of the excellent, cutting-edge research that is not only looking to develop novel drug therapeutics but also trying to uncover the underlying molecular mechanisms of epigenetic regulation and cancer.

 

 

 

A Metabolic Twist that Drives Cancer Survival

http://www.technologynetworks.com/Metabolomics/news.aspx?ID=190262

A novel metabolic pathway that helps cancer cells thrive in conditions that are lethal to normal cells has been identified.

 

“It’s long been thought that if we could target tumor-specific metabolic pathways, it could lead to effective ways to treat cancer,” said senior author Dr. Ralph DeBerardinis, Associate Professor of CRI and Pediatrics, Director of CRI’s Genetic and Metabolic Disease Program, and Chief of the Division of Pediatric Genetics and Metabolism at UT Southwestern. “This study finds that two very different metabolic processes are linked in a way that is specifically required for cells to adapt to the stress associated with cancer progression.”

The study, available online today in the journal Nature, reveals that cancer cells use an alternate version of two well-known metabolic pathways called the pentose phosphate pathway (PPP) and the Krebs cycle to defend against toxins. The toxins are reactive oxygen species (ROS) that kill cells via oxidative stress.

This work builds on earlier studies by Dr. DeBerardinis’ laboratory that found the Krebs cycle, a series of chemical reactions that cells use to generate energy, could reverse itself under certain conditions to nourish cancer cells.

Dr. DeBerardinis said most normal cells and tumor cells grow by attaching to nutrient-rich tissue called a matrix. “They are dependent on matrix attachment to receive growth-promoting signals and to regulate their metabolism in a way that supports cell growth, proliferation, and survival,” he said.

Detachment from the matrix results in a sudden increase in ROS that is lethal to normal cells, he added. Cancer cells seem to have a workaround.

The destruction of healthy cells when detached from the matrix was reported in a landmark 2009 Nature study by Harvard Medical School cell biologist Dr. Joan Brugge. Intriguingly, that same study found that inserting an oncogene – a gene with the potential to cause cancer – into a normal cell caused it to behave like a cancer cell and survive detachment, said Dr. DeBerardinis, who also is affiliated with the Eugene McDermott Center for Human Growth & Development, holds the Joel B. Steinberg, M.D. Chair in Pediatrics, and is a Sowell Family Scholar in Medical Research at UT Southwestern.

“Another Nature study, this one from CRI Director Dr. Sean Morrison’s laboratory in November 2015, found that the rare skin cancer cells that were able to detach from the primary tumor and successfully metastasize to other parts of the body had the ability to keep ROS levels from getting dangerously high,” Dr. DeBerardinis said. Dr. Morrison, also a CPRIT Scholar in Cancer Research and a Howard Hughes Medical Institute Investigator, holds the Mary McDermott Cook Chair in Pediatrics Genetics at UT Southwestern.

Working under the premise that the two findings were pieces of the same puzzle, a crucial part of the picture seemed to be missing, he said.

It was known for decades that the PPP was a major source of NADPH, which provides a source of reducing equivalents (that is, electrons) to scavenge ROS; however, the PPP produces NADPH in a part of the cell called the cytosol, whereas the reactive oxygen species are generated primarily in another subcellular structure called the mitochondria.

“If you think of ROS as fire, then NADPH is like the water used by cancer cells to douse the flames,” Dr. DeBerardinis said. But how could NADPH from the PPP help deal with the stress of ROS produced in a completely different part of the cell? “What we did was to discover how this happens,” Dr. DeBerardinis said.

The current study in Nature demonstrates that cancer cells use a “piggybacking” system to carry reducing equivalents from the PPP into the mitochondria. This movement involves an unusual reaction in the cytosol that transfers reducing equivalents from NADPH to a molecule called citrate, similar to a reversed reaction of the Krebs cycle, he said. The citrate then enters the mitochondria and stimulates another pathway that results in the release of reducing equivalents to produce NADPH right at the location of ROS creation, allowing the cancer cells to survive and grow without the benefit of matrix attachment.

“We knew that both the PPP and Krebs cycle provide metabolic benefits to cancer cells.  But we had no idea that they were linked in this unusual fashion,” he said. “Strikingly, normal cells were unable to transport NADPH by this mechanism, and died as a result of the high ROS levels.”

Dr. DeBerardinis stressed that the findings were based on cultured cell models, and more research will be necessary to test the role of the pathway in living organisms. “We are particularly excited to test whether this pathway is required for metastasis, because cancer cells need to survive in a matrix-detached state in the circulation in order to metastasize,” he said.

CRI scientists find novel metabolic twist that drives cancer survival
http://www.utsouthwestern.edu/newsroom/news-releases/year-2016/april/cancer-metabolism.html

https://pharmaceuticalintelligence.com/2016/04/09/programmed-cell-death-and-cancer-therapy/5 days ago Cancer Cell Survival Driven by Novel Metabolic Pathway … This new study describes an alternate version of two wellknown metabolic pathways, the pentose phosphate pathway (PPP) and the Krebs cycle,…

http://www.nature.com/nature/journal/v481/n7381/abs/nature10642.html Jan 19, 2012 Nature | Letter …. DeBerardinis, R. J. et al. Beyond aerobic …. Andrew R. Mullen,; Pei-Hsuan Chen,; Tzuling Cheng &; Ralph J. DeBerardinis ..   

Haematopoietic stem cells require a highly regulated protein – Nature
http://www.nature.com/nature/journal/v509/n7498/abs/nature13035.html May 1, 2014 Nature | Article. Print; Share/ ….. synthesis. Nature Methods 6, 275–277 (2009) …. Robert A. J. Signer,; Jeffrey A. Magee &; Sean J. Morrison …       
http://www.nature.com/nature/journal/v527/n7577/abs/nature15726.html Nov 12, 2015 Nature | Article ….. Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and ….. Sean J. Morrison …     
Deep imaging of bone marrow shows non-dividing stem Nature
http://www.nature.com/nature/journal/v526/n7571/abs/nature15250.html   Oct 1, 2015 Nature | Letter. Print; Share/ …… Morrison, S. J. & Scadden, D. T. The bone marrow niche for ….. Kiranmai S. Kocherlakota &; Sean J. Morrison …
https://pharmaceuticalintelligence.com/category/cancer-biology-innovations-in-cancer-therapy/genomic-expression/Glutamine and cancer: cell biology, physiology, and clinical opportunities …. Metabolism of glutamine-derived α-ketoglutarate in the TCA cycle serves … known as hexosamines, that are used to glycosylate growth factor receptors and …… of two wellknown metabolic pathways, the pentose phosphate pathway (PPP )
The Mitochondrial Warburg Effect: A Cancer Enigma – IBC Journal
http://www.ibc7.org/article/file_down.php?pid=48&mode=article This feature of cancer cells is known as the Warburg effect, named … new paradigm of collaboration and a well-designed systemic approach will supply … Krebs cycle. … The pentose phosphate pathway uses glucose to produce ribose, which is used … glucose is taken up into cells, it is used in two main metabolic pathways

 

New paper offers intriguing insights into tumor metabolism     William G. Gilroy    August 19, 2009

Posted In: Research

A paper appearing in this week’s edition of the journal Nature by a team of researchers that includes University of Notre Dame biologist Zachary T. Schafer has important new implications for understanding the metabolism of tumors.

Schafer, an assistant professor of biological sciences and Coleman Junior Chair of Cancer Biology, points out that in the early stages of tumor formation some cells become detached from their normal cellular matrix. These “homeless” cells tend to develop certain defects that stop them from becoming cancerous. In a process known as apoptosis, these precancerous cells essentially kill themselves, allowing them to be destroyed by immune system cells.

The prevailing wisdom among researchers has been that apoptosis was the only way that cells could die.

In studies conducted prior to the research described in the Nature paper, it was found that even when apoptosis was inhibited in detached, precancerous cells, they still eventually died. Intrigued by these results, a team of researchers led by Joan S. Brugge, Louise Foote Pfieffer Professor of Cell Biology at Harvard Medical School, and Schafer decided to take a closer look.

They report in this week’s Nature paper that they found that even when apoptosis was inhibited in detached cells endowed with a cancer-causing gene, they still were incapable of absorbing glucose, their primary energy source. Additionally, the cells displayed signs of oxidative stress, which is a harmful accumulation of oxygen-derived molecules called reactive oxygen species (ROS). The research also revealed decreased ATP production, a key factor in energy transport in the cells.

Schafer notes that this combination of loss of glucose transport, decreased ATP production and heightened oxidative stress reveal a manner of cell death that hadn’t been previously demonstrated to play a role in this context.

In the next phase of the study, Schafer engineered the cells to express a high level of HER2, a gene known to be hyperactive in many breast cancer tumors. He also treated the cells with antioxidants to relieve oxidative stress.

Both approaches helped the cells survive. The HER2-treated cells regained glucose transport, avoided oxidative stress and recovered ATP levels.
Most surprisingly, the antioxidants restored metabolic activity in the cells by allowing fatty acids to be effectively used instead of glucose as an energy source, providing them with a chance to survive.

“Our results raise the possibility that antioxidant activity might allow early stage tumor cells to survive where they would otherwise die from these metabolic defects,” Schafer said.

He also cautions that while the antioxidant findings were surprising, their research was done solely in cell cultures and more research needs to be done before there are clear implications for individuals and their diets.

The paper does, however, offer important new clues about the metabolism of tumor cells and important information that may lead to drugs that can developed to target them.

 

Antioxidant and Oncogene Rescue of Metabolic Defects Caused by
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2931797/Aug 19, 2009
Nature. Author manuscript; available in PMC 2010 Sep 2. Published in … Y. Irie,1 Sizhen Gao,1 Pere Puigserver,1,2 and Joan S. Brugge1,*.

http://www.nature.com/cdd/journal/v10/n8/full/4401251a.html
Proteasome inhibitors have been shown to be effective in cancer treatment, an ability … a specific inhibitor of 26S proteasome, also reduced cell viability ( 80% with 10 mu …
be a consequence of the increased generation of ROS caused by MG132. …. vectors endowed with the wild type forms of RB or p53 genes (Figure 1f).

 

The Metastasis-Promoting Roles of Tumor-Associated Immune Cells

Tumor metastasis is driven not only by the accumulation of intrinsic alterations in malignant cells, but also by the interactions of cancer cells with various stromal cell components of the tumor microenvironment. In particular, inflammation and infiltration of the tumor tissue by host immune cells, such as tumor-associated macrophages, myeloid-derived suppressor cells, and regulatory T cells have been shown to support tumor growth in addition to invasion and metastasis. Each step of tumor development, from initiation through metastatic spread, is promoted by communication between tumor and immune cells via the secretion of cytokines, growth factors and proteases that remodel the tumor microenvironment. Invasion and metastasis requires neovascularization, breakdown of the basement membrane, and remodeling of the extracellular matrix for tumor cell invasion and extravasation into the blood and lymphatic vessels. The subsequent dissemination of tumor cells to distant organ sites necessitates a treacherous journey through the vasculature, which is fostered by close association with platelets and macrophages. Additionally, the establishment of the pre-metastatic niche and specific metastasis organ tropism is fostered by neutrophils and bone marrow-derived hematopoietic immune progenitor cells and other inflammatory cytokines derived from tumor and immune cells, which alter the local environment of the tissue to promote adhesion of circulating tumor cells. This review focuses on the interactions between tumor cells and immune cells recruited to the tumor microenvironment, and examines the factors allowing these cells to promote each stage of metastasis.

 

Once established, tumors are quite adept at preventing anti-tumor immune responses, and several defense mechanisms to circumvent immune detection have been described including antigen loss, down-regulation of major histocompatibility molecules (MHC), deregulation or loss of components of the endogenous antigen presentation pathway, and tumor-induced immune suppression mediated through cytokine secretion or direct interactions between tumor ligands and immune cell receptors [2]. These mechanisms contribute to the process of immunoediting in which tumor cell subpopulations susceptible to immune recognition are lysed and eliminated, while resistant tumor cells proliferate and increase their frequency in the developing neoplasia [3]. However, tumors not only effectively escape immune recognition, they also actively subvert the normal anti-tumor activity of immune cells to promote further tumor growth and metastasis.

During early stages of cancer development, infiltrating immune cell populations are primarily tumor suppressive, but depending on the presence of accessory stromal cells, the local cytokine milieu, and tumor-specific interactions, these immune cells can undergo phenotypic changes to enhance tumor cell dissemination and metastasis. For instance, CD4+ T cells, macrophages, and neutrophils have all been shown to possess opposing properties depending on the inflammatory state of the tumor environment, the tissue context, and other cellular stimuli intrinsic to the altered tumor cells [4, 5]. These features are dependent upon the inherent plasticity of immune cells in response to stimulatory or suppressive cytokines [6]. Notably, the switch from a Th1 tumor-suppressive phenotype such as CD4+ “helper” T cells, which aid cytotoxic CD8+ T cells in tumor rejection, to a Th2 tumor-promoting “regulatory” phenotype, which blocks CD8+ T-cell activity, is a characteristic outcome in the inflammatory, immune-suppressive tumor microenvironment [5, 7]. Likewise, M1 macrophages and N1 neutrophils are known to have pronounced anti-tumor activity; however, these immune cells are often subverted to a tumor-promoting M2 and N2 phenotype, respectively, in response to immune-suppressive cytokines secreted by tumor tissue [8].

 

The crosstalk that occurs between tumor and immune cells within the tumor microenvironment, the circulation, or at distant metastatic sites has been clearly shown to foster metastatic dissemination. Immune cells as well as the suppressive factors that they secrete represent potential targets for therapeutic intervention. Regardless of their source, cytokines, chemokines, proteases, and growth factors are some of the main factors contributing to immunosuppression and immune-mediated tumor progression. These molecules can be produced by immune, stromal, or malignant cells and can act in paracrine and autocrine fashion to promote each stage of tumor cell invasion and metastasis by enhancing inflammation, angiogenesis, tumor proliferation, and recruitment of additional immunosuppressive and tumor-promoting immune cells. These secreted factors provide the malignant cells with an abundant source of growth and survival signals that perpetuate a supportive microenvironment for tumor metastasis and represent some of the most attractive targets for directed anti-tumor therapy. Immune pathways provide numerous soluble targets for cancer treatment, and indeed, many drugs to target immune-suppressive molecules are moving forward in clinical trials. For instance, the anti-RANKL (Denosumab) antibody has been shown to effectively inhibit bone metastasis in prostate cancer patients [201], while a variety of neutralizing antibodies to IL-1β and IL-1 receptor have been shown to have efficacy in treating metastasis in pre-clinical animal models [202]. Several agents that target IL-1 or other immune-suppressive cytokines are already approved for the treatment of some inflammatory diseases and are prime candidates for human trails [202]. Additionally, other proteins involved in tumor progression that are induced directly or indirectly by immune cell populations, such as EMT-associated transcription factors, adhesion molecules, and tumor receptors and ligands which mediate immune suppression, could also be targeted with small molecules or blocking antibodies. Antibodies against two surface molecules expressed by suppressive lymphoid cells, anti-CTLA-4 (ipiliumimab) [203, 204] and anti-PD-1 have been recently gaining increasing support from clinical trials for their effective treatment for many forms of cancer including advanced melanoma and prostate cancer [205, 206]. Specifically, anti-CTLA-4 has been shown to be particularly efficacious in metastatic melanoma, while anti-PD-1 has only just begun a comprehensive evaluation in clinical trials [204, 207]. Likewise, non-steroidal anti-inflammatory drugs (NSAIDS) to prevent or treat chronic inflammation and lymphangiogenesis [208210], and anti-coagulants to prevent platelet aggregation on circulating tumor cells [211] are just two examples of a multitude of therapeutic agents that could be utilized to prevent immune-mediated tumor progression at unique stages of metastasis. Of course, new methods or biomarkers for the detection of patients at risk of tumor progression or metastasis are also desperately needed to tailor personalized therapy for patients to obtain the best possible clinical outcome.

 

  1. https://pharmaceuticalintelligence.com/category/cancer-and-therapeutics/Mar 26, 2016 This turns your immune systems ability to attack and kill cancer cells back on” …. the rare skin cancer cells that were able to detach from theprimary tumor and successfully metastasize to other parts of the body had the ability to keep ROS levels from getting dangerously high,” Dr. DeBerardinis remarked.

  2. https://pharmaceuticalintelligence.com/tag/histone-deacetylase-inhibitors-hdac/The HDAC-inhibiting agent romidepsin significantly increased T-cell tumor … skin cancer cells that were able to detach from the primary tumor and successfully … of the body had the ability to keep ROS levels from getting dangerously high,” Dr. …. Sensitivity for EGFR or KRAS was higher in patients with multiplemetastatic …

  3. https://pharmaceuticalintelligence.com/category/cancer-biology-innovations-in-cancer-therapy/genomic-expression/In a study involving 320 patients, the researchers were able to infer cell death in …. Glutamine and cancer: cell biology, physiology, and clinical opportunities … On the other hand, GLS2 expression is enhanced in some neuroblastomas, …… of the body had the ability to keep ROS levels from getting dangerously high,” Dr.

 

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Reactive Oxygen Species and Melanoma

Curator: Larry H Bernstein, MD, FCAP

Antioxidant Use May Promote Melanoma Metastasis

http://www.genengnews.com/gen-news-highlights/antioxidant-use-may-promote-melanoma-metastasis/81251856/

  • Click Image To Enlarge +
    Results from a new study suggest that cancer patients should not supplement their diet with large doses of antioxidants. [grThirteen/iStock]

    Decades ago Nobel laureate and American chemist Linus Pauling espoused the benefits of taking megadoses of vitamin C to prevent and treat various diseases. This controversial practice has led many to promote the taking of antioxidant supplements to prevent everything from the common cold to cancer and has become an almost engrained practice for individual healthcare—unfortunately, with little scientific evidence to support the ritual.

    Now, researchers at the Children’s Research Institute at UT Southwestern (CRI) has uncovered evidence that suggests cancer cells benefit more from antioxidants than normal cells, raising concerns about the use of dietary antioxidant supplements by patients with cancer.

    The investigators utilized a specialized mice model that had been transplanted with melanoma cells from patients, which previous work showed recapitulated metastasis of human melanoma cells and was predictive of their metastasis in patients. The CRI team found that when antioxidants were administered to the mice, the cancer spread more quickly than in mice that did not get antioxidants.

    “We discovered that metastasizing melanoma cells experience very high levels of oxidative stress, which leads to the death of most metastasizing cells,” explained senior author Sean Morrison, Ph.D., CRI director and chair in pediatric genetics at UT Southwestern Medical Center. “Administration of antioxidants to the mice allowed more of the metastasizing melanoma cells to survive, increasing metastatic disease burden.”

    The findings from this study were published online today in Nature through an article entitled “Oxidative stress inhibits distant metastasis by human melanoma cells.”

    It has long been known that the spread of cancer cells from one part of the body to another is an inefficient process in which the vast majority of cancer cells that enter the blood fail to survive, due to the highly oxidative environment and exposure to immune cells.

    “The idea that antioxidants are good for you has been so strong that there have been clinical trials done in which cancer patients were administered antioxidants,” noted Dr. Morrison. “Some of those trials had to be stopped because the patients getting the antioxidants were dying faster. Our data suggest the reason for this: cancer cells benefit more from antioxidants than normal cells do.”

    The CRI researchers were intrigued by their findings and acknowledged that although the study’s results have not yet been tested in people, they surmise that cancer should be treated with pro-oxidants and that cancer patients should not supplement their diet with large doses of antioxidants.

    “This finding also opens up the possibility that when treating cancer, we should test whether increasing oxidative stress through the use of pro-oxidants would prevent metastasis,” Dr. Morrison stated. “One potential approach is to target the folate pathway that melanoma cells use to survive oxidative stress, which would increase the level of oxidative stress in the cancer cells.”

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Larry H. Benstein, MD, FCAP, Gurator and writer

https://pharmaceuticalintelligence.com/7/8/2014/Update on mitochondrial function, respiration, and associated disorders

This is a condensed account of very recent published work on respiration and disturbed mitochondrail function.  We know that their is an equilibrium between respiration and autophagy in eukaryotic cells.  The Krebs Cycle produces 32 ATPs in oxidative phosphorylation, which is far more efficient than glycolysis.  There is also a different contribution of mitochondrial metabolism, in the balance, between tissues that are synthetic and those that are catabolic.  This is a subject long understood, essential for cellular energetics, and not adequately explored.

 

Gain-of-Function Mutant p53 Promotes Cell Growth and Cancer Cell Metabolism via Inhibition of AMPK Activation.

Zhou G1Wang J2Zhao M2Xie TX2Tanaka N2, et al.
Mol Cell. 
2014 Jun 19;54(6):960-974.   doi: 10.1016/j.molcel.2014.04.024. 

Many mutant p53 proteins (mutp53s) exert oncogenic gain-of-function (GOF) properties, but the mechanisms mediating these functions remain poorly defined.

We show here that GOF mutp53s inhibit AMP-activated protein kinase (AMPK) signaling in head and neck cancer cells.

Conversely, downregulation of GOF mutp53s enhances AMPK activation under energy stress, decreasing the activity of the anabolic factors acetyl-CoA carboxylase and ribosomal protein S6 and inhibiting aerobic glycolytic potential and invasive cell growth.

Under conditions of energy stress, GOF mutp53s, but not wild-type p53, preferentially bind to the AMPKα subunit and inhibit AMPK activation.

Given the importance of AMPK as an energy sensor and tumor suppressor that inhibits anabolic metabolism, our findings reveal that direct inhibition of AMPK activation is an important mechanism through which mutp53s can gain oncogenic function. PMID:24857548

Investigating and Targeting Chronic Lymphocytic Leukemia Metabolism with the HIV Protease Inhibitor Ritonavir and Metformin.

Adekola KUAydemir SDMa SZhou ZRosen STShanmugam M.
Leuk Lymphoma. 2014 May 14:1-23.

Chronic Lymphocytic Leukemia (CLL) remains fatal due to the development of resistance to existing therapies. Targeting abnormal glucose metabolism sensitizes various cancer cells to chemotherapy and/or elicits toxicity.

Examination of glucose dependency in CLL demonstrated variable sensitivity to glucose deprivation. Further evaluation of metabolic dependencies of CLL cells resistant to glucose deprivation revealed increased engagement of fatty acid oxidation upon glucose withdrawal.

Investigation of glucose transporter expression in CLL reveals up-regulation of glucose transporter GLUT4. Treatment of CLL cells with HIV protease inhibitor ritonavir, that inhibits GLUT4, elicits toxicity similar to that elicited upon glucose-deprivation.

CLL cells resistant to ritonavir are sensitized by co-treatment with metformin, potentially targeting compensatory mitochondrial complex 1 activity. Ritonavir and metformin have been administered in humans for treatment of diabetes in HIV patients, demonstrating the tolerance of this combination in humans. Our studies strongly substantiate further investigation of FDA approved ritonavir and metformin for CLL.

KEYWORDS:  Basic Biology; Chemotherapeutic approaches; Lymphoid Leukemia; Signal transduction             PMID: 24828872

Utilizing hydrogen sulfide as a novel anti-cancer agent by targeting cancer glycolysis and pH imbalance.

Lee ZW1Teo XYTay EYTan CHHagen TMoore PKDeng LW.
Br J Pharmacol. 2014 May 15.    doi: 10.1111/bph.12773

Many disparate studies have reported the ambiguous role of hydrogen sulfide (H2 S) in cell survival. The present study investigated the effect of H2 S on viability of cancer and non-cancer cells.

Cancer and non-cancer cells were exposed to H2 S (using sodium hydrosulfide, NaHS and GYY4137) and cell viability was examined by crystal violet assay. We then examined cancer cellular glycolysis process by in vitro enzymatic assays and pH regulator activity. Lastly, intracellular pH (pHi) was determined by ratiometric pHi measurement using BCECF staining.

Continuous, but not single, exposure to H2 S decreased cell survival more effectively in cancer cells, as compared to non-cancer cells. Slow H2 S-releasing donor, GYY4137, significantly increased glycolysis leading to overproduction of lactate. H2 S also decreased anion exchanger and sodium/proton exchanger activity. The combination of increased metabolic acid production and defective pH regulation resulted in an uncontrolled intracellular acidification leading to cancer cell death. In contrast, no significant intracellular acidification or cell death was observed in non-cancer cells.

Low and continuous exposure to H2 S targets metabolic processes and pH homeostasis in cancer cells, potentially serving as a novel and selective anti-cancer strategy.

KEYWORDS:  cancer cell death; cancer glucose metabolism; hydrogen sulfide; pH homeostasis          PMID: 24827113


Agonism of the 5-Hydroxytryptamine 1F Receptor Promotes Mitochondrial Biogenesis and Recovery from Acute Kidney Injury

Garrett SMWhitaker RMBeeson CC, and Schnellmann RG

Center for Cell Death, Injury, and Regeneration, Department of Drug Discovery and Biomedical Sciences, Medical University of South Carolina, Charleston, South Carolina (S.M.G., R.M.W., C.C.B., R.G.S.); and Ralph H. Johnson Veterans Affairs Medical Center, Charleston, South Carolina (R.G.S.)
Address correspondence to: Dr. Rick G. Schnellmann, Department of Drug Discovery and Biomedical Sciences, MUSC, Charleston, SC 29425.
E-mail: schnell@musc.edu

Many acute and chronic conditions, such as acute kidney injury, chronic kidney disease, heart failure, and liver disease, involve mitochondrial dysfunction. Although we have provided evidence that drug-induced stimulation of mitochondrial biogenesis (MB) accelerates mitochondrial and cellular repair, leading to recovery of organ function, only a limited number of chemicals have been identified that induce MB.

The goal of this study was to assess the role of the 5-hydroxytryptamine 1F (5-HT1F) receptor in MB. Immunoblot and quantitative polymerase chain reaction analyses revealed 5-HT1F receptor expression in renal proximal tubule cells (RPTC). A MB screening assay demonstrated that two selective 5-HT1F receptor agonists,

  1. LY334370 (4-fluoro-N-[3-(1-methyl-4-piperidinyl)-1H-indol-5-yl]benzamide) and
  2. LY344864 (N-[(3R)-3-(dimethylamino)-2,3,4,9-tetrahydro-1H-carbazol-6-yl]-4-fluorobenzamide; 1–100 nM)

increased carbonylcyanide-p-trifluoromethoxyphenylhydrazone–uncoupled oxygen consumption in RPTC, and

  • validation studies confirmed both agonists increased mitochondrial proteins  in vitro.
    [e.g., ATP synthase β, cytochrome c oxidase 1 (Cox1), and NADH dehydrogenase (ubiquinone) 1β subcomplex subunit 8 (NDUFB8)]

Small interfering RNA knockdown of the 5-HT1F receptor

  • blocked agonist-induced MB.

Furthermore, LY344864 increased

  • peroxisome proliferator–activated receptor (PPAR) coactivator 1-α, Cox1, and
  • NDUFB8 transcript levels and
  • mitochondrial DNA (mtDNA) copy number

in murine renal cortex, heart, and liver.

Finally, LY344864 accelerated recovery of renal function, as indicated by

  • decreased blood urea nitrogen and kidney injury molecule 1 and
  • increased mtDNA copy number

following ischemia/reperfusion-induced acute kidney injury (AKI).

In summary, these studies reveal that

  • the 5-HT1F receptor is linked to MB, 5-HT1F receptor agonism promotes MB in vitro and in vivo, and

5-HT1F receptor agonism promotes recovery from AKI injury.

Induction of MB through 5-HT1F receptor agonism represents a new target and approach to treat mitochondrial organ dysfunction.

Footnotes

  • Portions of this work have been presented previously: Garrett SM, Wills LP, and Schnellmann RG (2012) Serotonin (5-HT) 1F receptor agonism as a potential treatment for acceleration of recovery from acute kidney injury.American Society of Nephrology Annual Meeting; 2012 Nov 1–4; San Diego, CA.
  • dx.doi.org/10.1124/jpet.114.214700.

Ca2+ regulation of mitochondrial function in neurons.

Rueda CB1Llorente-Folch I1Amigo I1Contreras L1González-Sánchez P1Martínez-Valero P1Juaristi I1Pardo B1Del Arco A2Satrústegui J3

Biochim Biophys Acta. 2014 May 10. pii: S0005-2728(14)00126-1.
doi: 10.1016/j.bbabio.2014.04.010.

Calcium is thought to regulate respiration but it is unclear whether this is dependent on the increase in ATP demand caused by any Ca2+ signal or to Ca2+ itself.

[Na+]i, [Ca2+]i and [ATP]i dynamics in intact neurons exposed to different workloads in the absence and presence of Ca2+ clearly showed that

  • Ca2+-stimulation of coupled respiration is required to maintain [ATP]i levels.

Ca2+ may regulate respiration by

  1. activating metabolite transport in mitochondria from outer face of the inner mitochondrial membrane, or
  2. after Ca2+ entry in mitochondria through the calcium uniporter (MCU).

Two Ca2+-regulated mitochondrial metabolite transporters are expressed in neurons,

  1. the aspartate-glutamate exchanger ARALAR/AGC1/Slc25a12, a component of the malate-aspartate shuttle, with a Kd for Ca2+ activation of 300nM, and
  2. the ATP-Mg/Pi exchanger SCaMC-3/Slc25a23, with S0.5 for Ca2+ of 300nM and 3.4μM, respectively.

The lack of SCaMC-3 results in a smaller Ca2+-dependent stimulation of respiration only at high workloads, as caused by veratridine, whereas

  • the lack of ARALAR reduced by 46% basal OCR in intact neurons using glucose as energy source and the Ca2+-dependent responses to all workloads (veratridine, K+-depolarization, carbachol).

The lack of ARALAR caused a reduction of about 65-70% in the response to the high workload imposed by veratridine, and

  • completely suppressed the OCR responses to moderate (K+-depolarization) and small (carbachol) workloads,
  • effects reverted by pyruvate supply.

For K+-depolarization, this occurs in spite of the presence of large [Ca2+]mit signals and increased reduction of mitochondrial NAD(P)H.

These results show that ARALAR-MAS is a major contributor of Ca2+-stimulated respiration in neurons

  • by providing increased pyruvate supply to mitochondria.

In its absence and under moderate workloads, matrix Ca2+ is unable to stimulate pyruvate metabolism and entry in mitochondria suggesting a limited role of MCU in these conditions.

This article was invited for a Special Issue entitled: 18th European Bioenergetic Conference.    Copyright © 2014. Published by Elsevier B.V.

KEYWORDS:  ATP-Mg/Pi transporter; Aspartate–glutamate transporter; Calcium; Calcium-regulated transport; Mitochondrion; Neuronal respiration PMID: 24820519

 

Sestrin2 inhibits uncoupling protein 1 expression through suppressing reactive oxygen species.

Ro SH1Nam M2Jang I1Park HW1Park H1Semple IA1Kim M1et al.
Proc Natl Acad Sci U S A. 2014 May 27;111(21):7849-54.
doi: 10.1073/pnas.1401787111.

Uncoupling protein 1 (Ucp1), which is localized in the mitochondrial inner membrane of mammalian brown adipose tissue (BAT), generates heat by uncoupling oxidative phosphorylation. Upon cold exposure or nutritional abundance, sympathetic neurons stimulate BAT to express Ucp1 to induce energy dissipation and thermogenesis. Accordingly, increased Ucp1 expression reduces obesity in mice and is correlated with leanness in humans.

Despite this significance, there is currently a limited understanding of how Ucp1 expression is physiologically regulated at the molecular level. Here, we describe the involvement of Sestrin2 and reactive oxygen species (ROS) in regulation of Ucp1 expression. Transgenic overexpression of Sestrin2 in adipose tissues inhibited both basal and cold-induced Ucp1 expression in interscapular BAT, culminating in decreased thermogenesis and increased fat accumulation.

Endogenous Sestrin2 is also important for suppressing Ucp1 expression because BAT from Sestrin2(-/-) mice exhibited a highly elevated level of Ucp1 expression. The redox-inactive mutant of Sestrin2 was incapable of regulating Ucp1 expression, suggesting that Sestrin2 inhibits Ucp1 expression primarily through reducing ROS accumulation.

Consistently, ROS-suppressing antioxidant chemicals, such as butylated hydroxyanisole and N-acetylcysteine, inhibited cold- or cAMP-induced Ucp1 expression as well. p38 MAPK, a signaling mediator required for cAMP-induced Ucp1 expression, was inhibited by either Sestrin2 overexpression or antioxidant treatments.

Taken together, these results suggest that Sestrin2 and antioxidants inhibit Ucp1 expression through suppressing ROS-mediated p38 MAPK activation, implying a critical role of ROS in proper BAT metabolism.

KEYWORDS: aging; homeostasis; mouse; β-adrenergic signaling      PMID: 24825887     PMCID:  PMC4040599

Mitochondrial EF4 links respiratory dysfunction and cytoplasmic translation in Caenorhabditis elegans.

Yang F1Gao Y1Li Z2Chen L3Xia Z4Xu T5Qin Y6
Biochim Biophys Acta. 2014 May 15. pii: S0005-2728(14)00499-X.
doi: 10.1016/j.bbabio.2014.05.353.

How animals coordinate cellular bioenergetics in response to stress conditions is an essential question related to aging, obesity and cancer. Elongation factor 4 (EF4/LEPA) is a highly conserved protein that promotes protein synthesis under stress conditions, whereas its function in metazoans remains unknown.

Here, we show that, in Caenorhabditis elegans, the mitochondria-localized CeEF4 (referred to as mtEF4) affects mitochondrial functions, especially at low temperature (15°C).

At worms’ optimum growing temperature (20°C), mtef4 deletion leads to self-brood size reduction, growth delay and mitochondrial dysfunction.

Transcriptomic analyses show that mtef4 deletion induces retrograde pathways, including mitochondrial biogenesis and cytoplasmic translation reorganization.

At low temperature (15°C), mtef4 deletion reduces mitochondrial translation and disrupts the assembly of respiratory chain supercomplexes containing complex IV.

These observations are indicative of the important roles of mtEF4 in mitochondrial functions and adaptation to stressful conditions.

Copyright © 2014. Published by Elsevier B.V.

KEYWORDSC. elegans; EF4(LepA/GUF1); Mitochondrial dysfunction; Retrograde pathways; Translation    PMID:  24837196

The metabolite α-ketoglutarate extends lifespan by inhibiting ATP synthase and TOR.

Chin RM1Fu X2Pai MY3Vergnes L4Hwang H5Deng G6Diep S2, et al.
Nature  2014 Jun 19;509(7505):397-401. doi: 10.1038/nature13264. 

Metabolism and ageing are intimately linked. Compared with ad libitum feeding, dietary restriction consistently extends lifespan and delays age-related diseases in evolutionarily diverse organisms. Similar conditions of nutrient limitation and genetic or pharmacological perturbations of nutrient or energy metabolism also have longevity benefits.

Recently, several metabolites have been identified that modulate ageing; however, the molecular mechanisms underlying this are largely undefined. Here we show that α-ketoglutarate (α-KG), a tricarboxylic acid cycle intermediate, extends the lifespan of adult Caenorhabditis elegans.

ATP synthase subunit β is identified as a novel binding protein of α-KG using a small-molecule target identification strategy termed drug affinity responsive target stability (DARTS). The ATP synthase, also known as complex V of the mitochondrial electron transport chain, is the main cellular energy-generating machinery and is highly conserved throughout evolution.

Although complete loss of mitochondrial function is detrimental, partial suppression of the electron transport chain has been shown to extend C. elegans lifespan.

We show that α-KG inhibits ATP synthase and, similar to ATP synthase knockdown, inhibition by α-KG leads to reduced ATP content, decreased oxygen consumption, and increased autophagy in both C. elegans and mammalian cells.

We provide evidence that the lifespan increase by α-KG requires ATP synthase subunit β and is dependent on target of rapamycin (TOR) downstream.

Endogenous α-KG levels are increased on starvation and α-KG does not extend the lifespan of dietary-restricted animals, indicating that α-KG is a key metabolite that mediates longevity by dietary restriction.

Our analyses uncover new molecular links between a common metabolite, a universal cellular energy generator and dietary restriction in the regulation of organismal lifespan, thus suggesting new strategies for the prevention and treatment of ageing and age-related diseases.

PMID: 24828042

 

 

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Telling NO to Cardiac Risk

DDAH Says NO to ADMA(1); The DDAH/ADMA/NOS Pathway(2)

Author-Writer-Reporter:  Stephen J. Williams, PhD

Endothelium-derived nitric oxide (NO) has been shown to be vasoprotective.  Nitric oxide enhances endothelial cell survival, inhibits excessive proliferation of vascular smooth muscle cells, regulates vascular smooth muscle tone, and prevents platelets from sticking to the endothelial wall.  Together with evidence from preclinical and human studies, it is clear that impairment of the NOS pathway increases risk of cardiovascular disease (3-5).

This post contains two articles on the physiological regulation of nitric oxide (NO) by an endogenous NO synthase inhibitor asymmetrical dimethylarginine (ADMA) and ADMA metabolism by the enzyme DDAH(1,2).  Previous posts on nitric oxide, referenced at the bottom of the page, provides excellent background and further insight for this posting. In summary plasma ADMA levels are elevated in patients with cardiovascular disease and several large studies have shown that plasma ADMA is an independent biomarker for cardiovascular-related morbidity and mortality(6-8).

admacardiacrisk

admaeffects

Figure 1 A. Cardiac risks of ADMA B. Effects of ADMA (Photo credit: Wikipedia)

ADMA Production and Metabolism

Nuclear proteins such as histones can be methylated on arginine residues by protein-arginine methyltransferases, enzymes which use S-adenosylmethionine as methyl groups.  This methylation event is thought to regulate protein function, much in the way of protein acetylation and phosphorylation (9).  And much like phosphorylation, these modifications are reversible through methylesterases.   The proteolysis of these arginine-methyl modifications lead to the liberation of free guanidine-methylated arginine residues such as L-NMMA, asymmetric dimethylarginine (ADMA) and symmetrical methylarginine (SDMA).

The first two, L-NMMA and ADMA, have been shown to inhibit the activity of the endothelial NOS.  This protein turnover is substantial: for instance the authors note that each day 40% of constitutive protein in adult liver is newly synthesized protein. And in several diseases, such as muscular dystrophy, ischemic heart disease, and diabetes, it has been known since the 1970’s that protein catabolism rates are very high, with corresponding increased urinary excretion of ADMA(10-13).  Methylarginines are excreted in the urine by cationic transport.  However, the majority of ADMA and L-NMMA are degraded within the cell by dimethylaminohydrolase (DDAH), first cloned and purified in rat(14).

endogenous NO inhibitors from pubchem

Figure 2.  Endogenous inhibitors of NO synthase.  Chemical structures generated from PubChem.

DDAH

DDAH specifically hydrolyzes ADMA and L-NMMA to yield citruline and demethylamine and usually shows co-localization with NOS. Pharmacologic inhibition of DDAH activity causes accumulation of ADMA and can reverse the NO-mediated bradykinin-induced relaxation of human saphenous vein.

Two isoforms have been found in human:

  • DDAH1 (found in brain and kidney and associated with nNOS) and
  • DDAH2 (highly expressed in heart, placenta, and kidney and associated with eNOS).

DDAH2 can be upregulated by all-trans retinoic acid (atRA can increase NO production).  Increased reactive oxygen species and possibly homocysteine, a risk factor for cardiovascular disease, can decrease DDAH activity(15,16).

  • The importance of DDAH activity can also be seen in transgenic mice which overexpress DDAH, exhibiting increased NO production, increased insulin sensitivity, and reduced vascular resistance  (17).  Likewise,
  • Transgenic mice, null for the DDAH1, showed increase in blood pressure, decreased NO production, and significant increase in tissue and plasma ADMA and L-NMMA.

amdanosfigure

Figure 3.  The DDAH/ADMA/NOS cycle. Figure adapted from Cooke and Ghebremarian (1).

As mentioned in the article by Cooke and Ghebremariam, the authors state: the weight of the evidence indicates that DDAH is a worthy therapeutic target. Agents that increase DDAH expression are known, and 1 of these, a farnesoid X receptor agonist, is in clinical trials

http://www.interceptpharma.com

An alternate approach is to

  • develop an allosteric activator of the enzyme.  Although
  • development of an allosteric activator is not a typical pharmaceutical approach, recent studies indicate that this may be achievable aim(18).

References:

1.            Cooke, J. P., and Ghebremariam, Y. T. : DDAH says NO to ADMA.(2011) Arteriosclerosis, thrombosis, and vascular biology 31, 1462-1464

2.            Tran, C. T., Leiper, J. M., and Vallance, P. : The DDAH/ADMA/NOS pathway.(2003) Atherosclerosis. Supplements 4, 33-40

3.            Niebauer, J., Maxwell, A. J., Lin, P. S., Wang, D., Tsao, P. S., and Cooke, J. P.: NOS inhibition accelerates atherogenesis: reversal by exercise. (2003) American journal of physiology. Heart and circulatory physiology 285, H535-540

4.            Miyazaki, H., Matsuoka, H., Cooke, J. P., Usui, M., Ueda, S., Okuda, S., and Imaizumi, T. : Endogenous nitric oxide synthase inhibitor: a novel marker of atherosclerosis.(1999) Circulation 99, 1141-1146

5.            Wilson, A. M., Shin, D. S., Weatherby, C., Harada, R. K., Ng, M. K., Nair, N., Kielstein, J., and Cooke, J. P. (2010): Asymmetric dimethylarginine correlates with measures of disease severity, major adverse cardiovascular events and all-cause mortality in patients with peripheral arterial disease. Vasc Med 15, 267-274

6.            Kielstein, J. T., Impraim, B., Simmel, S., Bode-Boger, S. M., Tsikas, D., Frolich, J. C., Hoeper, M. M., Haller, H., and Fliser, D. : Cardiovascular effects of systemic nitric oxide synthase inhibition with asymmetrical dimethylarginine in humans.(2004) Circulation 109, 172-177

7.            Kielstein, J. T., Donnerstag, F., Gasper, S., Menne, J., Kielstein, A., Martens-Lobenhoffer, J., Scalera, F., Cooke, J. P., Fliser, D., and Bode-Boger, S. M. : ADMA increases arterial stiffness and decreases cerebral blood flow in humans.(2006) Stroke; a journal of cerebral circulation 37, 2024-2029

8.            Mittermayer, F., Krzyzanowska, K., Exner, M., Mlekusch, W., Amighi, J., Sabeti, S., Minar, E., Muller, M., Wolzt, M., and Schillinger, M. : Asymmetric dimethylarginine predicts major adverse cardiovascular events in patients with advanced peripheral artery disease.(2006) Arteriosclerosis, thrombosis, and vascular biology 26, 2536-2540

9.            Kakimoto, Y., and Akazawa, S.: Isolation and identification of N-G,N-G- and N-G,N’-G-dimethyl-arginine, N-epsilon-mono-, di-, and trimethyllysine, and glucosylgalactosyl- and galactosyl-delta-hydroxylysine from human urine. (1970) The Journal of biological chemistry 245, 5751-5758

10.          Inoue, R., Miyake, M., Kanazawa, A., Sato, M., and Kakimoto, Y.: Decrease of 3-methylhistidine and increase of NG,NG-dimethylarginine in the urine of patients with muscular dystrophy. (1979) Metabolism: clinical and experimental 28, 801-804

11.          Millward, D. J.: Protein turnover in skeletal muscle. II. The effect of starvation and a protein-free diet on the synthesis and catabolism of skeletal muscle proteins in comparison to liver. (1970) Clinical science 39, 591-603

12.          Goldberg, A. L., and St John, A. C.: Intracellular protein degradation in mammalian and bacterial cells: Part 2. (1976) Annual review of biochemistry 45, 747-803

13.          Dice, J. F., and Walker, C. D.: Protein degradation in metabolic and nutritional disorders. (1979) Ciba Foundation symposium, 331-350

14.          Ogawa, T., Kimoto, M., and Sasaoka, K.: Purification and properties of a new enzyme, NG,NG-dimethylarginine dimethylaminohydrolase, from rat kidney. (1989) The Journal of biological chemistry 264, 10205-10209

15.          Ito, A., Tsao, P. S., Adimoolam, S., Kimoto, M., Ogawa, T., and Cooke, J. P.: Novel mechanism for endothelial dysfunction: dysregulation of dimethylarginine dimethylaminohydrolase. (1999) Circulation 99, 3092-3095

16.          Stuhlinger, M. C., Tsao, P. S., Her, J. H., Kimoto, M., Balint, R. F., and Cooke, J. P. : Homocysteine impairs the nitric oxide synthase pathway: role of asymmetric dimethylarginine.(2001) Circulation 104, 2569-2575

17.          Sydow, K., Mondon, C. E., Schrader, J., Konishi, H., and Cooke, J. P.: Dimethylarginine dimethylaminohydrolase overexpression enhances insulin sensitivity. (2008) Arteriosclerosis, thrombosis, and vascular biology 28, 692-697

18.          Zorn, J. A., and Wells, J. A.: Turning enzymes ON with small molecules. (2010) Nature chemical biology 6, 179-188

Other research papers on Nitric Oxide and Cardiac Risk  were published on this Scientific Web site as follows:

The Nitric Oxide and Renal is presented in FOUR parts:

Part I: The Amazing Structure and Adaptive Functioning of the Kidneys: Nitric Oxide

Part II: Nitric Oxide and iNOS have Key Roles in Kidney Diseases

Part III: The Molecular Biology of Renal Disorders: Nitric Oxide

Part IV: New Insights on Nitric Oxide donors

Cardiac Arrhythmias: A Risk for Extreme Performance Athletes

What is the role of plasma viscosity in hemostasis and vascular disease risk?

Cardiovascular Risk Inflammatory Marker: Risk Assessment for Coronary Heart Disease and Ischemic Stroke – Atherosclerosis.

Endothelial Dysfunction, Diminished Availability of cEPCs, Increasing CVD Risk for Macrovascular Disease – Therapeutic Potential of cEPCs

Biochemistry of the Coagulation Cascade and Platelet Aggregation – Part I

Nitric Oxide Function in Coagulation

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Author and Curator: Ritu Saxena, Ph.D.

Introduction: Mitochondrial fission & fusion

Mitochondria, double membranous and semi-autonomous organelles, are known to convert energy into forms that are usable to the cell. Apart from being sites of cellular respiration, multiple roles of mitochondria have been emphasized in processes such as cell division, growth and cell death. Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes, and can make their own proteins. Mitochondria have been discussed in several posts published in the Pharmaceutical Intelligence blog.

Mitochondria do not exist as lone organelles, but are part of a dynamic network that continuously undergoes fusion and fission in response to various metabolic and environmental stimuli. Nucleoids, the assemblies of mitochondrial DNA (mtDNA) with its associated proteins, are distributed during fission in such a way that each mitochondrion contains at least one nucleoid. Mitochondrial fusion and fission within a cell is speculated to be involved in several functions including mtDNA DNA protection, alteration of cellular energetics, and regulation of cell division.

Proteins involved in mitochondrial fission & fusion

Multiple mitochondrial membrane GTPases that regulate mitochondrial networking have recently been identified. They are classified as fission and fusion proteins:

Fusion proteins: Members of dynamin family of protein, mitofusin 1 (Mfn-1) and mitofusin 2 (Mfn-2), are involved in fusion between mitochondria by tethering adjacent mitochondria. These proteins have two transmembrane segments that anchor them in the mitochondrial outer membrane. Mutations in Mitofusin proteins gives rise to fragmented mitochondria, but this can be reversed by mutations in mammalian Drp1. Mitochondrial inner membranes are fused by dynamin family members called Opa1.

Fission proteins: Another member of the dynamin family of proteins, dynamin-related protein 1 (Drp-1) mediates fission of mitochondria. Drp-1 is activated by phosphorylation. Drp-1 proteins are largely cytosolic, but cycle on and off of mitochondria as needed for fission. Fission is a complex process and involves a series of well-defined stages and proteins. Cytosolic Drp-1 is activated by calcineurin or other cytosolic signaling proteins after which it translocates to the mitochondrial tubules where it assembles into foci through its interaction with another protein, hFis1. Once Drp-1 rings assemble on the constricted spots, outer membrane of mitochondria undergoes fission through GTP hydrolysis. Drp-1 is now left bound to one of the newly formed mitochondrial ends after which it slowly disassembles before returning to the cytoplasm.

Control of mitochondrial fission & fusion

  • Mitochondrial fission and fusion are controlled by several regulatory mechanisms. Few of which are mentioned as follows:
  • Drp-1 activation by Cdk1/Cyclin B mediated phosphorylation during mitosis – triggers fission
  • Drp-1 inactivation by cAMP-dependent protein kinase (PKA) in quiescent cells- prevents fission
  • Drp-1 activation after reversal of PKA phosphorylation by Calcineurin- triggers fission
  • Ubiquination of fission and fusion proteins by E3 ubiquitin ligase- alters fission
  • Sumoylation of fission proteins – regulates fission

Imparied mitochondrial fission leads to loss of mtDNA

Mitochondrial fission plays an important role in mitochondrial and cellular homeostasis. It was reported by Parone et al (2008) that preventing mitochondrial fission by down-regulating expression of Drp-1 lead to loss of mtDNA and mitochondrial dysfunction. An increase in cellular reactive oxygen species (ROS) was observed. Other cellular implications included depletion of cellular ATP, inhibition of cell proliferation and autophagy. The observations were made in HeLa cells.

MicroRNA regulation of mitochondrial fission

Although several factors have been attributed to the regulation of mitochondrial fission, the mechanism still remains poorly understood. Recently, regulation of mitochondrial fission via miRNAs has become a topic of interest. Following miRNAs have been found to be involved in mitochondrial fission:

  • miR-484:  Wang et al (2012) demonstrated that miR-484 was able to regulate mitochondrial fission by suppressing the translation of a fission protein Fis1, leading to inhibition of Fis1-mediated fission and apoptosis in cardiomyocytes and in the adrenocortical cancer cells. The authors showed that Fis1 is necessary for mitochondrial fission and apoptosis, and is upregulated during anoxia, whereas miR-484 is downregulated. Underlying mechanism involved transactivation of miR-484 by a transcription factor, Foxo3a and miR-484 is able to attenuate Fis1 upregulation and mitochondrial fission, by binding to the amino acid coding sequence of Fis1 and inhibiting its translation.
  • miR-499: miR-499 was reported by Wang et al (2011) to be able to directly target both the α- and β-isoforms of the calcineurin catalytic subunit. Suppression of calcineurin-mediated dephosphorylation of  Drp-1 lead to inhibition of the fission machinery ultimately resulting in the inhibition of cardiomyocyte apoptosis. miR-499 levels, by altering mitochondrial fusion were able affect the severity of myocardial infarction and cardiac dysfunction induced by ischemia-reperfusion. Modulation of miR-499 expression could provide a therapeutic approach for myocardial infarction treatment.
  • miR-30: It was reported by Li et al (2010) that miR-30 family members were able to inhibit mitochondrial fission and also the resulting apoptosis. While exploring the underlying molecular mechanism, the authors identified that miR-30 family members can suppress p53 expression. When cell received apoptotic stimulation, p53 was found to transcriptionally activate the fission protein, Drp-1. Drp-1 was able to induce mitochondrial fission. miR-30 family members were observed to inhibit mitochondrial fission through attenuation of p53 expression and its downstream target Drp-1.

Mitochondrial fission & fusion as a therapeutic target

Since alteration of mitochondrial fission and fusion have been reported to affect various cellular processes including apoptosis, proliferation, ATP consumption, the proteins involved in the process of fission and fusion might be harnessed as therapeutic target.

Mentioned below is a description of research where dynamics of the mitochondrial organelle has been utilized as a therapeutic target:

Inhibition of mitochondrial fission prevents cell cycle progression in lung cancer

A recent article published by Rehman et al (2012) in the FASEB journal drew much attention after interesting observations were made in the mitochondria of lung adenocarcinoma cells. The mitochondrial network of these cells exhibited both impaired fusion and enhanced fission. It was also found that the fragmented phenotype in multiple lung adenocarcinoma cell lines was associated with both a down-regulation of the fusion protein, Mfn-2 and an upregulation of expression of fission protein, Drp-1. The imbalance of Drp-1/Mfn-2 expression in human lung cancer cell lines was reported to promote a state of mitochondrial fission. Similar increase in Drp-1 and decrease in Mfn-2 was observed in the tissue samples from patients compared to adjacent healthy lung. Authors used complementary approaches of Mfn-2 overexpression, Drp-1 inhibition, or Drp-1 knockdown and were able to observe reduction of cancer cell proliferation and an increase spontaneous apoptosis. Thus, the study identified mitochondrial fission and Drp-1 activation as a novel therapeutic target in lung cancer.

Image

Reference:

Research articles-

http://www.ncbi.nlm.nih.gov/pubmed/20556877

http://www.ncbi.nlm.nih.gov/pubmed?term=18806874

http://www.ncbi.nlm.nih.gov/pubmed/22510686

http://www.ncbi.nlm.nih.gov/pubmed/21186368

http://www.ncbi.nlm.nih.gov/pubmed?term=20062521

http://www.ncbi.nlm.nih.gov/pubmed?term=22321727

News brief:

http://www.uchospitals.edu/news/2012/20120221-mitochondria.html

http://news.uchicago.edu/article/2012/02/23/energy-network-within-cells-may-be-new-target-cancer-therapy

http://www.doctortipster.com/7881-mitochondria-could-represent-a-new-target-for-cancer-therapy-according-to-new-study.html

Related reading:

Reviewer: Larry H Bernstein, MD, FACP

https://pharmaceuticalintelligence.com/2012/10/28/mitochondrial-damage-and-repair-under-oxidative-stress/

Author and Curator: Larry H Bernstein, MD, FACP https://pharmaceuticalintelligence.com/2012/09/26/mitochondria-origin-from-oxygen-free-environment-role-in-aerobic-glycolysis-metabolic-adaptation/

Reporter and Editor: Larry H Bernstein, MD, FACP

https://pharmaceuticalintelligence.com/2012/09/16/nitric-oxide-has-a-ubiquitous-role-in-the-regulation-of-glycolysis-with-a-concomitant-influence-on-mitochondrial-function/

Author and Reporter: Ritu Saxena, PhD

https://pharmaceuticalintelligence.com/2012/09/10/%CE%B2-integrin-emerges-as-an-important-player-in-mitochondrial-dysfunction-associated-gastric-cancer/

Author: Ritu Saxena, PhD

https://pharmaceuticalintelligence.com/2012/09/01/mitochondria-and-cancer-an-overview/

Author and Reporter: Ritu Saxena, PhD

https://pharmaceuticalintelligence.com/2012/08/14/mitochondrial-mutation-analysis-might-be-1-step-away/

Reporter: Venkat S. Karra, PhD

https://pharmaceuticalintelligence.com/2012/08/14/detecting-potential-toxicity-in-mitochondria/

Reporter: Aviva Lev-Ari, PhD, RN https://pharmaceuticalintelligence.com/2012/08/01/mitochondrial-mechanisms-of-disease-in-diabetes-mellitus/

Author and Curator: Ritu Saxena, PhD; Consultants: Aviva Lev-Ari, PhD, RN and Pnina G. Abir-Am, PhD

https://pharmaceuticalintelligence.com/2012/07/09/mitochondria-more-than-just-the-powerhouse-of-the-cell/

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Curator/Reporter Aviral Vatsa PhD, MBBS

Based on: A review by (Wink et al., 2011)

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.

REDOX REGULATION OF IMMUNE FUNCTION

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

NO and revascularization

Acute versus chronic inflammatory disease

Bibliography

1. Wink, D. A. et al. Nitric oxide and redox mechanisms in the immune response. J Leukoc Biol 89, 873–891 (2011).

2. Vásquez-Vivar, J. et al. Tetrahydrobiopterin-dependent inhibition of superoxide generation from neuronal nitric oxide synthase. J. Biol. Chem. 274, 26736–26742 (1999).

3. Messmer, U. K., Ankarcrona, M., Nicotera, P. & Brüne, B. p53 expression in nitric oxide-induced apoptosis. FEBS Lett. 355, 23–26 (1994).

4. Ridnour, L. A. et al. Molecular mechanisms for discrete nitric oxide levels in cancer. Nitric Oxide 19, 73–76 (2008).

5. Thomas, D. D. et al. The chemical biology of nitric oxide: implications in cellular signaling. Free Radic. Biol. Med. 45, 18–31 (2008).

6. Thomas, D. D. et al. Hypoxic inducible factor 1alpha, extracellular signal-regulated kinase, and p53 are regulated by distinct threshold concentrations of nitric oxide. Proc. Natl. Acad. Sci. U.S.A. 101, 8894–8899 (2004).

7. Weinberg, J. B. Nitric oxide production and nitric oxide synthase type 2 expression by human mononuclear phagocytes: a review. Mol. Med. 4, 557–591 (1998).

Further reading on NO:

Nitric Oxide in bone metabolism July 16, 2012

Author: Aviral Vatsa PhD, MBBS

https://pharmaceuticalintelligence.com/2012/07/16/nitric-oxide-in-bone-metabolism/?goback=%2Egde_4346921_member_134751669

Nitric Oxide production in Systemic sclerosis July 25, 2012

Curator: Aviral Vatsa, PhD, MBBS

https://pharmaceuticalintelligence.com/2012/07/25/nitric-oxide-production-in-systemic-sclerosis/?goback=%2Egde_4346921_member_138370383

Nitric Oxide Signalling Pathways August 22, 2012 by

Curator/ Author: Aviral Vatsa, PhD, MBBS

https://pharmaceuticalintelligence.com/2012/08/22/nitric-oxide-signalling-pathways/?goback=%2Egde_4346921_member_151245569

Nitric Oxide: a short historic perspective August 5, 2012

Author/Curator: Aviral Vatsa PhD, MBBS

https://pharmaceuticalintelligence.com/2012/08/05/nitric-oxide-a-short-historic-perspective-7/

Nitric Oxide: Chemistry and function August 10, 2012

Curator/Author: Aviral Vatsa PhD, MBBS

https://pharmaceuticalintelligence.com/2012/08/10/nitric-oxide-chemistry-and-function/?goback=%2Egde_4346921_member_145137865

Nitric Oxide and Platelet Aggregation August 16, 2012 by

Author: Dr. Venkat S. Karra, Ph.D.

https://pharmaceuticalintelligence.com/2012/08/16/no-and-platelet-aggregation/?goback=%2Egde_4346921_member_147475405

The rationale and use of inhaled NO in Pulmonary Artery Hypertension and Right Sided Heart Failure August 20, 2012

Author: Larry Bernstein, MD

https://pharmaceuticalintelligence.com/2012/08/20/the-rationale-and-use-of-inhaled-no-in-pulmonary-artery-hypertension-and-right-sided-heart-failure/

Nitric Oxide: The Nobel Prize in Physiology or Medicine 1998 Robert F. Furchgott, Louis J. Ignarro, Ferid Murad August 16, 2012

Reporter: Aviva Lev-Ari, PhD, RN

https://pharmaceuticalintelligence.com/2012/08/16/nitric-oxide-the-nobel-prize-in-physiology-or-medicine-1998-robert-f-furchgott-louis-j-ignarro-ferid-murad/

Coronary Artery Disease – Medical Devices Solutions: From First-In-Man Stent Implantation, via Medical Ethical Dilemmas to Drug Eluting Stents August 13, 2012

Author: Aviva Lev-Ari, PhD, RN

https://pharmaceuticalintelligence.com/2012/08/13/coronary-artery-disease-medical-devices-solutions-from-first-in-man-stent-implantation-via-medical-ethical-dilemmas-to-drug-eluting-stents/

Nano-particles as Synthetic Platelets to Stop Internal Bleeding Resulting from Trauma

August 22, 2012

Reported by: Dr. V. S. Karra, Ph.D.

https://pharmaceuticalintelligence.com/2012/08/22/nano-particles-as-synthetic-platelets-to-stop-internal-bleeding-resulting-from-trauma/

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

https://pharmaceuticalintelligence.com/2012/07/19/cardiovascular-disease-cvd-and-the-role-of-agent-alternatives-in-endothelial-nitric-oxide-synthase-enos-activation-and-nitric-oxide-production/

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

Curator: Aviva Lev-Ari, PhD, RN

https://pharmaceuticalintelligence.com/2012/07/02/macrovascular-disease-therapeutic-potential-of-cepcs-reduction-methods-for-cv-risk/

Bone remodelling in a nutshell June 22, 2012

Author: Aviral Vatsa, Ph.D., MBBS

https://pharmaceuticalintelligence.com/2012/06/22/bone-remodelling-in-a-nutshell/

Targeted delivery of therapeutics to bone and connective tissues: current status and challenges- Part, September  

Author: Aviral Vatsa, PhD, September 23, 2012

https://pharmaceuticalintelligence.com/2012/09/23/targeted-delivery-of-therapeutics-to-bone-and-connective-tissues-current-status-and-challenges-part-i/

Calcium dependent NOS induction by sex hormones: Estrogen

Curator: S. Saha, PhD, October 3, 2012

https://pharmaceuticalintelligence.com/2012/10/03/calcium-dependent-nos-induction-by-sex-hormones/

Nitric Oxide and Platelet Aggregation,

Author V. Karra, PhD, August 16, 2012

https://pharmaceuticalintelligence.com/2012/08/16/no-and-platelet-aggregation/

Bystolic’s generic Nebivolol – positive effect on circulating Endothelial Progenitor Cells endogenous augmentation

Curator: Aviva Lev-Ari, PhD, July 16, 2012

https://pharmaceuticalintelligence.com/?s=Nebivolol

Endothelin Receptors in Cardiovascular Diseases: The Role of eNOS Stimulation

Author: Aviva Lev-Ari, PhD, 10/4/2012

https://pharmaceuticalintelligence.com/2012/10/04/endothelin-receptors-in-cardiovascular-diseases-the-role-of-enos-stimulation/

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

Curator: Aviva Lev-Ari, 10/4/2012.

https://pharmaceuticalintelligence.com/2012/10/04/inhibition-of-et-1-eta-and-eta-etb-induction-of-no-production-and-stimulation-of-enos-and-treatment-regime-with-ppar-gamma-agonists-tzd-cepcs-endogenous-augmentation-for-cardiovascular-risk-reduc/

Nitric Oxide Nutritional remedies for hypertension and atherosclerosis. It’s 12 am: do you know where your electrons are?

Author and Reporter: Meg Baker, 10/7/2012.

https://pharmaceuticalintelligence.com/2012/10/07/no-nutritional-remedies-for-hypertension-and-atherosclerosis-its-12-am-do-you-know-where-your-electrons-are/

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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.

Schematic diagram of the mitochondrial .

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

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:

  1. base excision repair (BER) and
  2. 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

  1. cleaves NAD+ into ADP-ribose and nicotinamide
  2. then attaches the ADP-ribose units to a variety of nuclear proteins (including histones and its own automodification domain).
  3. 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 .

Turrens JF. Mitochondrial formation of reactive oxygen species. J Physiol 2003; 552(2): 335–344. DOI: 10.1113/jphysiol.2003.049478. http://www.jphysiol.org

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Reactive Oxygen Species and Control of Apoptosis

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,

  1. the death receptor and
  2. 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.

Circu, M. L.; Aw, T. Y., Reactive oxygen species, cellular redox systems, and apoptosis, Free Radic. Biol. Med. 2010. FRB-10057; pp 14. doi:10.1016/j.freeradbiomed.2009.12.022

Assembly of Iron-sulfur (FeyS) Clusters

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.

Sontz PA, Mui TP, Fuss JO, Tainer JA, and Barton JK. DNA charge transport as a first step in coordinating the detection of lesions by repair proteins. PNAS 2012; 109(6):1856–1861. doi:10.1073/pnas.1120063109/-/ DCSupplemental. http://www.pnas.org/lookup/suppl/

Janus Bifron 

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

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.

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.

C Yu, Y Li, A Holmes, K Szafranski, CG Faulkes, et al. RNA Sequencing Reveals Differential Expression of Mitochondrial and Oxidation reduction Genes in the Long-Lived Naked Mole-Rat When Compared to Mice. PLoS ONE 2011; 6(11): 1-9. e26729. http://www.plosone.org

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

  • transcription,
  • protein trafficking, and
  • vacuolar function.

GW Thorpe, CS Fong, N Alic, VJ Higgins, and IW Dawes. Cells have distinct mechanisms to maintain protection against different reactive oxygen species: Oxidative-stress-response genes. PNAS 2004;101: 6564–6569. http://www.pnas.org cgi doi 10.1073 pnas.0305888101
Subcellular Thiol Redox State in Complex I Deficiency

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.

S Verkaart, WJH Koopman, J Cheek, SE van Emst-de Vries. Mitochondrial and cytosolic thiol redox state are not detectably altered in isolated human NADH:ubiquinone oxidoreductase deficiency. Biochimica et Biophysica Acta (BBA) – Molecular Basis of Disease 2007; 1772(9): 1041. DOI : 10.1016/j.bbadis.2007.05.004

  • Mitochodrial mtDNA and Cancer
  • 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,

  • improves features of insulin resistance,
  • increases mitochondrial biogenesis and, surprisingly,
  • enhances the efficiency of ATP production.

Dysfunctional mitochondria in cultured cells can be rescued by transfer of mitochondria from adult stem cells, raising the possibility of restoration of normal bioenergetics in the vasculature to treat atherosclerosis associated with insulin resistance.
Chistiakov DA, Sobenin IA, Bobryshev YV, Orekhov AN. Mitochondrial dysfunction and mitochondrial DNA mutations in atherosclerotic complications in diabetes. World J Cardiol 2012; 4(5): 148-156. ISSN 1949-8462 (online). doi:10.4330/wjc.v4.i5.148. http://www.wjgnet.com/1949-8462/full/v4/i5/148.htm

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