TCA Cycle and Mitochondrial Membrane Potential Are Necessary for Diverse Biological Functions.

Mitochondrial metabolism is necessary for the maintenance of oxidative TCA cycle function and mitochondrial membrane potential. Previous attempts to decipher whether mitochondria are necessary for biological outcomes have been hampered by genetic and pharmacologic methods that simultaneously disrupt multiple functions linked to mitochondrial metabolism. Here, we report that inducible depletion of mitochondrial DNA (ρ(ο) cells) diminished respiration, oxidative TCA cycle function, and the mitochondrial membrane potential, resulting in diminished cell proliferation, hypoxic activation of HIF-1, and specific histone acetylation marks. Genetic reconstitution only of the oxidative TCA cycle function specifically in these inducible ρ(ο) cells restored metabolites, resulting in re-establishment of histone acetylation. In contrast, genetic reconstitution of the mitochondrial membrane potential restored ROS, which were necessary for hypoxic activation of HIF-1 and cell proliferation. These results indicate that distinct mitochondrial functions associated with respiration are necessary for cell proliferation, epigenetics, and HIF-1 activation.

Mitochondrial DNA damage as driver of cellular outcomes.

Mitochondria are primarily involved in energy production through the process of oxidative phosphorylation (OXPHOS). Increasing evidence has shown that mitochondrial function impacts a plethora of different cellular activities, including metabolism, epigenetics, and innate immunity. Like the nucleus, mitochondria own their genetic material, but this organellar genome is circular, present in multiple copies, and maternally inherited. The mitochondrial DNA (mtDNA) encodes 37 genes that are solely involved in OXPHOS. Maintenance of mtDNA, through replication and repair, requires the import of nuclear DNA-encoded proteins. Thus, mitochondria completely rely on the nucleus to prevent mitochondrial genetic alterations. As most cells contain hundreds to thousands of mitochondria, it follows that the shear number of organelles allows for the buffering of dysfunction-at least to some extent-before tissue homeostasis becomes impaired. Only red blood cells lack mitochondria entirely. Impaired mitochondrial function is a hallmark of aging and is involved in a number of different disorders, including neurodegenerative diseases, diabetes, cancer, and autoimmunity. Although alterations in mitochondrial processes unrelated to OXPHOS, such as fusion and fission, contribute to aging and disease, maintenance of mtDNA integrity is critical for proper organellar function. Here, we focus on how mtDNA damage contributes to cellular dysfunction and health outcomes.

Emerging mitochondrial signaling mechanisms in cardio-oncology: beyond oxidative stress.

Many anticancer therapies (CTx) have cardiotoxic side effects that limit their therapeutic potential and cause long-term cardiovascular complications in cancer survivors. This has given rise to the field of cardio-oncology, which recognizes the need for basic, translational, and clinical research focused on understanding the complex signaling events that drive CTx-induced cardiovascular toxicity. Several CTx agents cause mitochondrial damage in the form of mitochondrial DNA deletions, mutations, and suppression of respiratory function and ATP production. In this review, we provide a brief overview of the cardiovascular complications of clinically used CTx agents and discuss current knowledge of local and systemic secondary signaling events that arise in response to mitochondrial stress/damage. Mitochondrial oxidative stress has long been recognized as a contributor to CTx-induced cardiotoxicity; thus, we focus on emerging roles for mitochondria in epigenetic regulation, innate immunity, and signaling via noncoding RNAs and mitochondrial hormones. Because data exploring mitochondrial secondary signaling in the context of cardio-oncology are limited, we also draw upon clinical and preclinical studies, which have examined these pathways in other relevant pathologies.

Mitochondrial nicotinamide adenine dinucleotide reduced (NADH) oxidation links the tricarboxylic acid (TCA) cycle with methionine metabolism and nuclear DNA methylation.

Mitochondrial function affects many aspects of cellular physiology, and, most recently, its role in epigenetics has been reported. Mechanistically, how mitochondrial function alters DNA methylation patterns in the nucleus remains ill defined. Using a cell culture model of induced mitochondrial DNA (mtDNA) depletion, in this study we show that progressive mitochondrial dysfunction leads to an early transcriptional and metabolic program centered on the metabolism of various amino acids, including those involved in the methionine cycle. We find that this program also increases DNA methylation, which occurs primarily in the genes that are differentially expressed. Maintenance of mitochondrial nicotinamide adenine dinucleotide reduced (NADH) oxidation in the context of mtDNA loss rescues methionine salvage and polyamine synthesis and prevents changes in DNA methylation and gene expression but does not affect serine/folate metabolism or transsulfuration. This work provides a novel mechanistic link between mitochondrial function and epigenetic regulation of gene expression that involves polyamine and methionine metabolism responding to changes in the tricarboxylic acid (TCA) cycle. Given the implications of these findings, future studies across different physiological contexts and in vivo are warranted.

Locus coeruleus neurons are most sensitive to chronic neuroinflammation-induced neurodegeneration.

Parkinson's disease (PD) develops over decades through spatiotemporal stages that ascend from the brainstem to the forebrain. The mechanism behind this caudo-rostral neurodegeneration remains largely undefined. In unraveling this phenomenon, we recently developed a lipopolysaccharide (LPS)-elicited chronic neuroinflammatory mouse model that displays sequential losses of neurons in brainstem, substantia nigra, hippocampus and cortex. In this study, we aimed to investigate the mechanisms of caudo-rostral neurodegeneration and focused our efforts on the earliest neurodegeneration of vulnerable noradrenergic locus coeruleus (NE-LC) neurons in the brainstem. We found that compared with neurons in other brain regions, NE-LC neurons in untreated mice displayed high levels of mitochondrial oxidative stress that was severely exacerbated in the presence of LPS-elicited chronic neuroinflammation. In agreement, NE-LC neurons in LPS-treated mice displayed early reduction of complex IV expression and mitochondrial swelling and loss of cristae. Mechanistically, the activation of the superoxide-generating enzyme NADPH oxidase (NOX2) on NE-LC neurons was essential for their heightened vulnerability during chronic neuroinflammation. LPS induced early and high expressions of NOX2 in NE-LC neurons. Genetic or pharmacological inactivation of NOX2 markedly reduced mitochondrial oxidative stress and dysfunction in LPS-treated mice. Furthermore, inhibition of NOX2 significantly ameliorated LPS-induced NE-LC neurodegeneration. More importantly, post-treatment with NOX2 inhibitor diphenyleneiodonium when NE-LC neurodegeneration had already begun, still showed high efficacy in protecting NE-LC neurons from degeneration in LPS-treated mice. This study strongly supports that chronic neuroinflammation and NOX2 expression among vulnerable neuronal populations contribute to caudo-rostral degeneration in PD.

Mitochondria and telomeres: the promiscuous roles of TIN2.

Telomeric proteins are best known for their role in maintenance of telomere function. In this issue, Chen et al. (2012) demonstrate that the telomeric protein TIN2 can specifically localize to the mitochondria, where it can regulate metabolism and ROS production.

Critical Role for Telomerase in the Mechanism of Flow-Mediated Dilation in the Human Microcirculation.

RATIONALE: Telomerase is a nuclear regulator of telomere elongation with recent reports suggesting a role in regulation of mitochondrial reactive oxygen species. Flow-mediated dilation in patients with cardiovascular disease is dependent on the formation of reactive oxygen species. OBJECTIVE: We examined the hypothesis that telomerase activity modulates microvascular flow-mediated dilation, and loss of telomerase activity contributes to the change of mediator from nitric oxide to mitochondrial hydrogen peroxide in patients with coronary artery disease (CAD). METHODS AND RESULTS: Human coronary and adipose arterioles were isolated for videomicroscopy. Flow-mediated dilation was measured in vessels pretreated with the telomerase inhibitor BIBR-1532 or vehicle. Statistical differences between groups were determined using a 2-way analysis of variance repeated measure (n≥4; P<0.05). L-NAME (N(ω)-nitro-L-arginine methyl ester; nitric oxide synthase inhibitor) abolished flow-mediated dilation in arterioles from subjects without CAD, whereas polyethylene glycol-catalase (PEG-catalase; hydrogen peroxide scavenger) had no effect. After exposure to BIBR-1532, arterioles from non-CAD subjects maintained the magnitude of dilation but changed the mediator from nitric oxide to mitochondrial hydrogen peroxide (% max diameter at 100 cm H2O: vehicle 74.6±4.1, L-NAME 37.0±2.0*, PEG-catalase 82.1±2.8; BIBR-1532 69.9±4.0, L-NAME 84.7±2.2, PEG-catalase 36.5±6.9*). Conversely, treatment of microvessels from CAD patients with the telomerase activator AGS 499 converted the PEG-catalase-inhibitable dilation to one mediated by nitric oxide (% max diameter at 100 cm H2O: adipose, AGS 499 78.5±3.9; L-NAME 10.9±17.5*; PEG-catalase 79.2±4.9). Endothelial-independent dilation was not altered with either treatment. CONCLUSIONS: We have identified a novel role for telomerase in re-establishing a physiological mechanism of vasodilation in arterioles from subjects with CAD. These findings suggest a new target for reducing the oxidative milieu in the microvasculature of patients with CAD.

Mitochondrial membrane potential regulates nuclear DNA methylation and gene expression through phospholipid remodeling.

Maintenance of the mitochondrial inner membrane potential (ΔΨM) is critical for many aspects of mitochondrial function, including mitochondrial protein import and ion homeostasis. While ΔΨM loss and its consequences are well studied, little is known about the effects of increased ΔΨM. In this study, we used cells deleted of ATPIF1, a natural inhibitor of the hydrolytic activity of the ATP synthase, as a genetic model of mitochondrial hyperpolarization. Our data show that chronic ΔΨM increase leads to nuclear DNA hypermethylation, regulating transcription of mitochondria, carbohydrate and lipid metabolism genes. Surprisingly, remodeling of phospholipids, but not metabolites or redox changes, mechanistically links the ΔΨM to the epigenome. These changes were also observed upon chemical exposures and reversed by decreasing the ΔΨM, highlighting them as hallmark adaptations to chronic mitochondrial hyperpolarization. Our results reveal the ΔΨM as the upstream signal conveying the mitochondrial status to the epigenome to regulate cellular biology, providing a new framework for how mitochondria can influence health outcomes in the absence of canonical dysfunction.

Mitochondrial genome instability and ROS enhance intestinal tumorigenesis in APC(Min/+) mice.

Alterations in mitochondrial oxidative phosphorylation have long been documented in tumors. Other types of mitochondrial dysfunction, including altered reactive oxygen species (ROS) production and apoptosis, also can contribute to tumorigenesis and cancer phenotypes. Furthermore, mutation and altered amounts of mitochondrial DNA (mtDNA) have been observed in cancer cells. However, how mtDNA instability per se contributes to cancer remains largely undetermined. Mitochondrial transcription factor A (TFAM) is required for expression and maintenance of mtDNA. Tfam heterozygous knock-out (Tfam(+/-)) mice show mild mtDNA depletion, but have no overt phenotypes. We show that Tfam(+/-) mouse cells and tissues not only possess less mtDNA but also increased oxidative mtDNA damage. Crossing Tfam(+/-) mice to the adenomatous polyposis coli multiple intestinal neoplasia (APC(Min/+)) mouse cancer model revealed that mtDNA instability increases tumor number and growth in the small intestine. This was not a result of enhancement of Wnt/ß-catenin signaling, but rather appears to involve a propensity for increased mitochondrial ROS production. Direct involvement of mitochondrial ROS in intestinal tumorigenesis was shown by crossing APC(Min/+) mice to those that have catalase targeted to mitochondria, which resulted in a significant reduction in tumorigenesis in the colon. Thus, mitochondrial genome instability and ROS enhance intestinal tumorigenesis and Tfam(+/-) mice are a relevant model to address the role of mtDNA instability in disease states in which mitochondrial dysfunction is implicated, such as cancer, neurodegeneration, and aging.

Answer to Gerber et al. "Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy".

Gal et al address the issues raised by Gerber et al and reiterate that patients in their study showed decreased Misato homolog 1 (MSTO1) mRNA and protein levels, but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene. Whether MSTO2p variant contributes to the observed decrease in MSTO1 levels in patients remains unclear.

Photochemical Targeting of Mitochondria to Overcome Chemoresistance in Ovarian Cancer †.

Ovarian cancer is the most lethal gynecologic malignancy with a stubborn mortality rate of ~65%. The persistent failure of multiline chemotherapy, and significant tumor heterogeneity, has made it challenging to improve outcomes. A target of increasing interest is the mitochondrion because of its essential role in critical cellular functions, and the significance of metabolic adaptation in chemoresistance. This review describes mitochondrial processes, including metabolic reprogramming, mitochondrial transfer and mitochondrial dynamics in ovarian cancer progression and chemoresistance. The effect of malignant ascites, or excess peritoneal fluid, on mitochondrial function is discussed. The role of photodynamic therapy (PDT) in overcoming mitochondria-mediated resistance is presented. PDT, a photochemistry-based modality, involves the light-based activation of a photosensitizer leading to the production of short-lived reactive molecular species and spatiotemporally confined photodamage to nearby organelles and biological targets. The consequential effects range from subcytotoxic priming of target cells for increased sensitivity to subsequent treatments, such as chemotherapy, to direct cell killing. This review discusses how PDT-based approaches can address key limitations of current treatments. Specifically, an overview of the mechanisms by which PDT alters mitochondrial function, and a summary of preclinical advancements and clinical PDT experience in ovarian cancer are provided.

Methods to Evaluate Changes in Mitochondrial Structure and Function in Cancer.

Mitochondria are regulators of key cellular processes, including energy production and redox homeostasis. Mitochondrial dysfunction is associated with various human diseases, including cancer. Importantly, both structural and functional changes can alter mitochondrial function. Morphologic and quantifiable changes in mitochondria can affect their function and contribute to disease. Structural mitochondrial changes include alterations in cristae morphology, mitochondrial DNA integrity and quantity, and dynamics, such as fission and fusion. Functional parameters related to mitochondrial biology include the production of reactive oxygen species, bioenergetic capacity, calcium retention, and membrane potential. Although these parameters can occur independently of one another, changes in mitochondrial structure and function are often interrelated. Thus, evaluating changes in both mitochondrial structure and function is crucial to understanding the molecular events involved in disease onset and progression. This review focuses on the relationship between alterations in mitochondrial structure and function and cancer, with a particular emphasis on gynecologic malignancies. Selecting methods with tractable parameters may be critical to identifying and targeting mitochondria-related therapeutic options. Methods to measure changes in mitochondrial structure and function, with the associated benefits and limitations, are summarized.

NRF2 Alters Mitochondrial Gene Expression in Neonate Mice Exposed to Hyperoxia.

Approximately 1 in 10 newborns are born preterm and require supplemental oxygen (O2) in an extrauterine environment following birth. Supplemental O2 can induce oxidative stress that can impair mitochondrial function, resulting in lung injury and increased risk in early life pulmonary diseases. The nuclear factor-erythroid 2 related factor 2 (NRF2) protects the cells from oxidative stress by regulating the expression of genes containing antioxidant response elements and many mitochondrial-associated genes. In this study, we compared Nrf2-deficient (Nrf2-/-) and wild-type (Nrf2+/+) mice to define the role of NRF2 in lung mitochondrial genomic features in late embryonic development in mice (embryonic days, E13.5 and E18.5) versus birth (postnatal day 0, PND0). We also determined whether NRF2 protects lung mitochondrial genome parameters in postnatal mice exposed to a 72 h hyperoxia environment. We found Nrf2-/- embryonic lungs were characterized by decreases in mtDNA copies from E13.5 to E18.5. Interestingly, Nrf2-/- heteroplasmy frequency was significantly higher than Nrf2+/+ at E18.5, though this effect reversed at PND0. In postnatal mice exposed to hyperoxia, we identified three- to four-fold increases in mitochondria-encoded mitochondrial genes, which regulate oxidative phosphorylation. Overall, our findings demonstrate a potentially critical role of NRF2 in mediating long-term effects of hyperoxia on mitochondrial function.

A brain-specific pgc1α fusion transcript affects gene expression and behavioural outcomes in mice.

PGC1α is a transcriptional coactivator in peripheral tissues, but its function in the brain remains poorly understood. Various brain-specific Pgc1α isoforms have been reported in mice and humans, including two fusion transcripts (FTs) with non-coding repetitive sequences, but their function is unknown. The FTs initiate at a simple sequence repeat locus ∼570 Kb upstream from the reference promoter; one also includes a portion of a short interspersed nuclear element (SINE). Using publicly available genomics data, here we show that the SINE FT is the predominant form of Pgc1α in neurons. Furthermore, mutation of the SINE in mice leads to altered behavioural phenotypes and significant up-regulation of genes in the female, but not male, cerebellum. Surprisingly, these genes are largely involved in neurotransmission, having poor association with the classical mitochondrial or antioxidant programs. These data expand our knowledge on the role of Pgc1α in neuronal physiology and suggest that different isoforms may have distinct functions. They also highlight the need for further studies before modulating levels of Pgc1α in the brain for therapeutic purposes.

Mitochondrial DNA lesions and copy number are strain dependent in endurance-trained mice.

In this pilot work, we selected two inbred strains that respond well to endurance training (ET) (FVB/NJ, and SJL/J strains), and two strains that respond poorly (BALB/cByJ and NZW/LacJ), to determine the effect of a standardized ET treadmill program on mitochondrial and nuclear DNA (nucDNA) integrity, and mitochondrial DNA (mtDNA) copy number. DNA was isolated from plantaris muscles (n = 37) and a gene-specific quantitative PCR-based assay was used to measure DNA lesions and mtDNA copy number. Mean mtDNA lesions were not different within strains in the sedentary or exercise-trained states. However, mtDNA lesions were significantly higher in trained low-responding NZW/LacJ mice (0.24 ± 0.06 mtDNA lesions/10 Kb) compared to high-responding strains (mtDNA lesions/10 Kb: FVB/NJ = 0.11 ± 0.01, p = .049; SJL/J = 0.04 ± 0.02; p = .003). ET did not alter mean mtDNA copy numbers for any strain, although both sedentary and trained FVB/NJ mice had significantly higher mtDNA copies (99,890 ± 4,884 mtDNA copies) compared to low-responding strains (mtDNA copies: BALB/cByJ = 69,744 ± 4,675; NZW/LacJ = 65,687 ± 5,180; p < .001). ET did not change nucDNA lesions for any strain, however, SJL/J had the lowest mean nucDNA lesions (3.5 ± 0.14 nucDNA lesions/6.5 Kb) compared to all other strains (nucDNA lesions/6.5 Kb: FVB/NJ = 4.4 ± 0.11; BALB/cByJ = 4.7 ± 0.09; NZW/LacJ = 4.4 ± 0.11; p < .0001). Our results demonstrate strain differences in plantaris muscle mtDNA lesions in ET mice and, independent of condition, differences in mean mtDNA copy and nucDNA lesions between strains.

Single Nucleotide Resolution Analysis Reveals Pervasive, Long-Lasting DNA Methylation Changes by Developmental Exposure to a Mitochondrial Toxicant.

Mitochondrial-driven alterations of the epigenome have been reported, but whether they are relevant at the organismal level remains unknown. The viable yellow agouti mouse (Avy) is a powerful epigenetic biosensor model that reports on the DNA methylation status of the Avy locus, which is established prior to the three-germ-layer separation, through the coat color of the animals. Here we show that maternal exposure to rotenone, a potent mitochondrial complex I inhibitor, not only changes the DNA methylation status of the Avy locus in the skin but broadly affects the liver DNA methylome of the offspring. These effects are accompanied by altered gene expression programs that persist throughout life, and which associate with impairment of antioxidant activity and mitochondrial function in aged animals. These pervasive and lasting genomic effects suggest a putative role for mitochondria in regulating life-long gene expression programs through developmental nuclear epigenetic remodeling.

Mitochondrial acetyl-CoA reversibly regulates locus-specific histone acetylation and gene expression.

The impact of mitochondrial dysfunction in epigenetics is emerging, but our understanding of this relationship and its effect on gene expression remains incomplete. We previously showed that acute mitochondrial DNA (mtDNA) loss leads to histone hypoacetylation. It remains to be defined if these changes are maintained when mitochondrial dysfunction is chronic and if they alter gene expression. To fill these gaps of knowledge, we here studied a progressive and a chronic model of mtDNA depletion using biochemical, pharmacological, genomics, and genetic assays. We show that histones are primarily hypoacetylated in both models. We link these effects to decreased histone acetyltransferase activity unrelated to changes in ATP citrate lyase, acetyl coenzyme A synthetase 2, or pyruvate dehydrogenase activities, which can be reversibly modulated by altering the mitochondrial pool of acetyl-coenzyme A. Also, we determined that the accompanying changes in histone acetylation regulate locus-specific gene expression and physiological outcomes, including the production of prostaglandins. These results may be relevant to the pathophysiology of mtDNA depletion syndromes and to understanding the effects of environmental agents that lead to physical or functional mtDNA loss.

Mitochondrial Oxidative Phosphorylation defect in the Heart of Subjects with Coronary Artery Disease.

Coronary artery disease (CAD) is a leading cause of death worldwide and frequently associated with mitochondrial dysfunction. Detailed understanding of abnormalities in mitochondrial function that occur in patients with CAD is lacking. We evaluated mitochondrial damage, energy production, and mitochondrial complex activity in human non-CAD and CAD hearts. Fresh and frozen human heart tissue was used. Cell lysate or mitochondria were isolated using standard techniques. Mitochondrial DNA (mtDNA), NAD + and ATP levels, and mitochondrial oxidative phosphorylation capacity were evaluated. Proteins critical to the regulation of mitochondrial metabolism and function were also evaluated in tissue lysates. PCR analysis revealed an increase in mtDNA lesions and the frequency of mitochondrial common deletion, both established markers for impaired mitochondrial integrity in CAD compared to non-CAD patient samples. NAD+ and ATP levels were significantly decreased in CAD subjects compared to Non-CAD (NAD+ fold change: non-CAD 1.00 ± 0.17 vs. CAD 0.32 ± 0.12* and ATP fold change: non-CAD 1.00 ± 0.294 vs. CAD 0.01 ± 0.001*; N = 15, P < 0.005). We observed decreased respiration control index in CAD tissue and decreased activity of complexes I, II, and III. Expression of ETC complex subunits and respirasome formation were increased; however, elevations in the de-active form of complex I were observed in CAD. We observed a corresponding increase in glycolytic flux, indicated by a rise in pyruvate kinase and lactate dehydrogenase activity, indicating a compensatory increase in glycolysis for cellular energetics. Together, these results indicate a shift in mitochondrial metabolism from oxidative phosphorylation to glycolysis in human hearts subjects with CAD.

A Leveraged Signal-to-Noise Ratio (LSTNR) Method to Extract Differentially Expressed Genes and Multivariate Patterns of Expression From Noisy and Low-Replication RNAseq Data.

To life scientists, one important feature offered by RNAseq, a next-generation sequencing tool used to estimate changes in gene expression levels, lies in its unprecedented resolution. It can score countable differences in transcript numbers among thousands of genes and between experimental groups, all at once. However, its high cost limits experimental designs to very small sample sizes, usually N = 3, which often results in statistically underpowered analysis and poor reproducibility. All these issues are compounded by the presence of experimental noise, which is harder to distinguish from instrumental error when sample sizes are limiting (e.g., small-budget pilot tests), experimental populations exhibit biologically heterogeneous or diffuse expression phenotypes (e.g., patient samples), or when discriminating among transcriptional signatures of closely related experimental conditions (e.g., toxicological modes of action, or MOAs). Here, we present a leveraged signal-to-noise ratio (LSTNR) thresholding method, founded on generalized linear modeling (GLM) of aligned read detection limits to extract differentially expressed genes (DEGs) from noisy low-replication RNAseq data. The LSTNR method uses an agnostic independent filtering strategy to define the dynamic range of detected aggregate read counts per gene, and assigns statistical weights that prioritize genes with better sequencing resolution in differential expression analyses. To assess its performance, we implemented the LSTNR method to analyze three separate datasets: first, using a systematically noisy in silico dataset, we demonstrated that LSTNR can extract pre-designed patterns of expression and discriminate between "noise" and "true" differentially expressed pseudogenes at a 100% success rate; then, we illustrated how the LSTNR method can assign patient-derived breast cancer specimens correctly to one out of their four reported molecular subtypes (luminal A, luminal B, Her2-enriched and basal-like); and last, we showed the ability to retrieve five different modes of action (MOA) elicited in livers of rats exposed to three toxicants under three nutritional routes by using the LSTNR method. By combining differential measurements with resolving power to detect DEGs, the LSTNR method offers an alternative approach to interrogate noisy and low-replication RNAseq datasets, which handles multiple biological conditions at once, and defines benchmarks to validate RNAseq experiments with standard benchtop assays.

Switch of Mitochondrial Superoxide Dismutase into a Prooxidant Peroxidase in Manganese-Deficient Cells and Mice.

Superoxide radical anion (O2⋅‒) and other reactive oxygen species are constantly produced during respiration. In mitochondria, the dismutation of O2⋅‒ is accelerated by the mitochondrial superoxide dismutase 2 (SOD2), an enzyme that has been traditionally associated with antioxidant protection. However, increases in SOD2 expression promote oxidative stress, indicating that there may be a prooxidant role for SOD2. Here we show that SOD2, which normally binds manganese, can incorporate iron and generate an alternative isoform with peroxidase activity. The switch from manganese to iron allows FeSOD2 to utilize H2O2 to promote oxidative stress. We found that FeSOD2 is formed in cultured cells and in vivo. FeSOD2 causes mitochondrial dysfunction and higher levels of oxidative stress in cultured cells and in vivo. We show that formation of FeSOD2 converts an antioxidant defense into a prooxidant peroxidase that leads to cellular changes seen in multiple human diseases.