Cells respond to environmental stressors through several key pathways, including response to reactive oxygen species (ROS), nutrient and ATP sensing, DNA damage response (DDR), and epigenetic ...alterations. Mitochondria play a central role in these pathways not only through energetics and ATP production but also through metabolites generated in the tricarboxylic acid cycle, as well as mitochondria-nuclear signaling related to mitochondria morphology, biogenesis, fission/fusion, mitophagy, apoptosis, and epigenetic regulation.
We investigated the concept of bidirectional interactions between mitochondria and cellular pathways in response to environmental stress with a focus on epigenetic regulation, and we examined DNA repair and DDR pathways as examples of biological processes that respond to exogenous insults through changes in homeostasis and altered mitochondrial function.
The National Institute of Environmental Health Sciences sponsored the Workshop on Mitochondria, Energetics, Epigenetics, Environment, and DNA Damage Response on 25-26 March 2013. Here, we summarize key points and ideas emerging from this meeting.
A more comprehensive understanding of signaling mechanisms (cross-talk) between the mitochondria and nucleus is central to elucidating the integration of mitochondrial functions with other cellular response pathways in modulating the effects of environmental agents. Recent studies have highlighted the importance of mitochondrial functions in epigenetic regulation and DDR with environmental stress. Development and application of novel technologies, enhanced experimental models, and a systems-type research approach will help to discern how environmentally induced mitochondrial dysfunction affects key mechanistic pathways.
Understanding mitochondria-cell signaling will provide insight into individual responses to environmental hazards, improving prediction of hazard and susceptibility to environmental stressors.
Pharmacological ascorbate has been proposed as a potential anti-cancer agent when combined with radiation and chemotherapy. The anti-cancer effects of ascorbate are hypothesized to involve the ...autoxidation of ascorbate leading to increased steady-state levels of H2O2; however, the mechanism(s) for cancer cell-selective toxicity remain unknown. The current study shows that alterations in cancer cell mitochondrial oxidative metabolism resulting in increased levels of O2⋅− and H2O2 are capable of disrupting intracellular iron metabolism, thereby selectively sensitizing non-small-cell lung cancer (NSCLC) and glioblastoma (GBM) cells to ascorbate through pro-oxidant chemistry involving redox-active labile iron and H2O2. In addition, preclinical studies and clinical trials demonstrate the feasibility, selective toxicity, tolerability, and potential efficacy of pharmacological ascorbate in GBM and NSCLC therapy.
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•High-dose ascorbate sensitizes NSCLC and GBM cells to radio-chemotherapy•O2⋅− and H2O2 increase labile iron causing cancer cell-selective ascorbate toxicity•Therapeutic levels of ascorbate are achievable and well tolerated in GBM and NSCLC•Cancer cell oxidative metabolism can be targeted with ascorbate for cancer therapy
Schoenfeld et al. show that cancer cells are selectively sensitive to ascorbate due to their altered redox-active iron metabolism. They present preclinical and clinical data demonstrating the feasibility, tolerability, and potential efficacy of pharmacological ascorbate for treating glioblastoma and non-small cell lung cancer.
Previously we have shown that nonsyndromic cleft lip with or without cleft palate (NSCL/P) (1) is strongly associated with SNPs in IRF6 (interferon regulatory factor 6) (2). Here, we use multispecies ...sequence comparisons to identify a common SNP (rs642961, G > A) in a newly identified IRF6 enhancer. The A allele is significantly overtransmitted (P = 1 x 10.sup.-11) in families with NSCL/P, in particular those with cleft lip but not cleft palate. Further, there is a dosage effect of the A allele, with a relative risk for cleft lip of 1.68 for the AG genotype and 2.40 for the AA genotype. EMSA and ChIP assays demonstrate that the risk allele disrupts the binding site of transcription factor AP-2α and expression analysis in the mouse localizes the enhancer activity to craniofacial and limb structures. Our findings place IRF6 and AP-2α in the same developmental pathway and identify a high-frequency variant in a regulatory element contributing substantially to a common, complex disorder.
Manganese superoxide dismutase (SOD2), encoded by the nuclear gene SOD2, is a critical mitochondrial antioxidant enzyme whose activity has broad implications in health and disease. Thirty years ago, ...Oberley and Buettner elegantly folded SOD2 into cancer biology with the free radical theory of cancer, which was built on the observation that many human cancers had reduced SOD2 activity. In the original formulation, the loss of SOD2 in tumor cells produced a state of perpetual oxidative stress, which, in turn, drove genetic instability, leading to cancer development.
In the past two decades, research has established that SOD2 transcriptional activity is controlled, at least in part, via epigenetic mechanisms at different stages in the development of human cancer. These mechanisms, which include histone methylation, histone acetylation, and DNA methylation, are increasingly recognized as being aberrantly regulated in human cancer. Indeed, the epigenetic progenitor model proposed by Henikoff posits that epigenetic events are central governing agents of carcinogenesis. Important recent advances in epigenetics research have indicated that the loss of SOD activity itself may contribute to changes in epigenetic regulation, establishing a vicious cycle that drives further epigenetic instability.
With these observations in mind, we propose an epigenetic revision to the free radical theory of cancer: that loss of SOD activity promotes epigenetic aberrancies, driving the epigenetic instability in tumor cells which produces broad phenotypic effects.
The development of next-generation sequencing technologies and novel approaches in systems biology and bioinformatics promise to make testing this exciting model a reality in the near future.
Many enzymes involved in xenobiotic metabolism, including cytochrome P450 (CYP) 1A1, are regulated by the aryl hydrocarbon receptor (AhR). 3,3',4,4',5-Penta chlorobiphenyl (PCB 126) is a potent ...ligand for AhR and can thus induce the expression of CYP1A1. Interestingly, we observed that human carcinoma cell lines derived from different types of epithelial cells displayed divergent degrees of CYP1A1 induction after exposure to PCB 126. Since epigenetic mechanisms are known to be involved in cell type-specific gene expression, we sought to assess the epigenetic determinants of CYP1A1 induction in these carcinoma cell lines. In contrast to HepG2 hepatocarcinoma cells, HeLa cervical carcinoma cells showed significantly lower levels of CYP1A1 mRNA expression following PCB 126 exposure. Our results show that the two cell lines maintained differences in the chromatin architecture along the CYP1A1 promoter region. Furthermore, treatment with the epigenetic modifiers, trichostatin A (TSA) and 5-aza-2'-deoxycytidine (5-Aza-dC), significantly increased the expression of CYP1A1 after PCB 126 treatment in HeLa cells. However, we did not observe apparent differences in methylation levels or specific location of CpG DNA methylation between the two cell lines in the analyzed CYP1A1 promoter region. Taken together, our findings suggest that the differences in CYP1A1 expression between HepG2 and HeLa cells are due to differences in the chromatin architecture of the CYP1A1 promoter and thus establish a role of epigenetic regulation in cell-specific CYP1A1 expression.
Cancer arises from normal cells that acquire a series of molecular changes; however, the founding events that create the clonogens from which a tumor will arise and progress have been the subject of ...speculation. Through the efforts of several generations of cancer biologists it has been established that the malignant phenotype is an amalgamation of genetic and metabolic alterations. Numerous theories have suggested that either, or both, of these elements might serve as the impetus for cancer formation. Recently, the epigenetic origins of cancer have been suggested as an additional mechanism giving rise to the malignant phenotype. When the discovery that the enzymes responsible for initiating and perpetuating epigenetic events is linked to metabolism by their cofactors, a new paradigm for the origins of cancer can be created. Here, we summarize the foundation of such a paradigm on the origins of cancer, in which metabolic alterations create an epigenetic progenitor that clonally expands to become cancer. We suggest that metabolic alterations disrupt the production and availability of cofactors such as S-adenosylmethionine, α-ketoglutarate, NAD+, and acetyl-CoA to modify the epigenotype of cells. We further speculate that redox biology can change epigenetic events through oxidation of enzymes and alterations in metabolic cofactors that affect epigenetic events such as DNA methylation. Combined, these metabolic and redox changes serve as the foundation for altering the epigenotype of normal cells and creating the epigenetic progenitor of cancer.
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► Cancer is a disease associated with changes in metabolism, redox, and gene expression. ► The mechanisms leading to epigenetic alterations during carcinogenesis are unknown. ► Epigenetic disruption of cancer genes is linked to aberrant mitochondrial metabolism. ► Metabolic redox changes alter cofactor availability to enzymes that alter chromatin. ► Aberrant metabolic and redox changes cause epigenetic instability in cancer.
Molecular oxygen is a Janus-faced electron acceptor for biological systems, serving as a reductant for respiration, or as the genesis for oxygen-derived free radicals that damage macromolecules. ...Superoxide is well known to perturb nonheme iron proteins, including Fe/S proteins such as aconitase and succinate dehydrogenase, as well as other enzymes containing labile iron such as the prolyl hydroxylase domain-containing family of enzymes; whereas hydrogen peroxide is more specific for two-electron reactions with thiols on glutathione, glutaredoxin, thioredoxin, and the peroxiredoxins.
Over the past two decades, familial cases of amyotrophic lateral sclerosis (ALS) have been shown to have an association with commonly altered superoxide dismutase 1 (SOD1) activity, expression, and protein structure. This has led to speculation that an altered redox balance may have a role in creating the ALS phenotype.
While SOD1 alterations in familial ALS are manifold, they generally create perturbations in the flux of electrons. The nexus of SOD1 between one- and two-electron signaling processes places it at a key signaling regulatory checkpoint for governing cellular responses to physiological and environmental cues.
The manner in which ALS-associated mutations adjust SOD1's role in controlling the flow of electrons between one- and two-electron signaling processes remains obscure. Here, we discuss the ways in which SOD1 mutations influence the form and function of copper zinc SOD, the consequences of these alterations on free radical biology, and how these alterations might influence cell signaling during the onset of ALS.
Elderly cancer patients treated with ionizing radiation (IR) or chemotherapy experience more frequent and greater normal tissue toxicity relative to younger patients. The current study demonstrates ...that exponentially growing fibroblasts from elderly (old) male donor subjects (70, 72, and 78 years) are significantly more sensitive to clonogenic killing mediated by platinum-based chemotherapy and IR (∼70%-80% killing) relative to young fibroblasts (5 months and 1 year; ∼10%-20% killing) and adult fibroblasts (20 years old; ∼10%-30% killing). Old fibroblasts also displayed significantly increased (2-4-fold) steady-state levels of O
, O
consumption, and mitochondrial membrane potential as well as significantly decreased (40%-50%) electron transport chain (ETC) complex I, II, IV, V, and aconitase (70%) activities, decreased ATP levels, and significantly altered mitochondrial structure. Following adenoviral-mediated overexpression of SOD2 activity (5-7-fold), mitochondrial ETC activity and aconitase activity were restored, demonstrating a role for mitochondrial O
in these effects. Old fibroblasts also demonstrated elevated levels of endogenous DNA damage that were increased following treatment with IR and chemotherapy. Most importantly, treatment with the small-molecule, superoxide dismutase mimetic (GC4419; 0.25 μmol/L) significantly mitigated the increased sensitivity of old fibroblasts to IR and chemotherapy and partially restored mitochondrial function without affecting IR or chemotherapy-induced cancer cell killing. These results support the hypothesis that age-associated increased O
and resulting DNA damage mediate the increased susceptibility of old fibroblasts to IR and chemotherapy that can be mitigated by GC4419.
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The seemingly disparate areas of oxygen toxicity, radiation exposure, and aging are now recognized to share a common feature-the aberrant production and/or removal of biologically derived free ...radicals and other reactive oxygen and nitrogen species (ROS/RNS). Advances in our understanding of the effects of free radicals in biology and medicine have been, and continue to be, actively translated into clinically tractable diagnostic and therapeutic applications. This issue is dedicated to recent advances, both basic discoveries and clinical applications, in the field of free radicals in biology and medicine. As more is understood about the proximal biological targets of aberrantly produced or removed reactive species, their sensors, and effectors of compensatory response, a great deal more will be learned about the commonalities in mechanisms underlying seemingly disparate disease states. Together with this deeper understanding, opportunities will arise to devise rational therapeutic interventions to decrease the incidence and severity of these diseases and positively impact the human healthspan.
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