Mitochondrial complex I, aconitase, and succinate dehydrogenase during hypoxia-reoxygenation: modulation of enzyme activities by MnSOD

Health position paper and redox perspectives on reactive oxygen species as signals and targets of cardioprotection

The present review summarizes the beneficial and detrimental roles of reactive oxygen species in myocardial ischemia/reperfusion injury and cardioprotection. In the first part, the continued need for cardioprotection beyond that by rapid reperfusion of acute myocardial infarction is emphasized. Then, pathomechanisms of myocardial ischemia/reperfusion to the myocardium and the coronary circulation and the different modes of cell death in myocardial infarction are characterized. Different mechanical and pharmacological interventions to protect the ischemic/reperfused myocardium in elective percutaneous coronary interventions and coronary artery bypass grafting, in acute myocardial infarction and in cardiotoxicity from cancer therapy are detailed. The second part keeps the focus on ROS providing a comprehensive overview of molecular and cellular mechanisms involved in ischemia/reperfusion injury. Starting from mitochondria as the main sources and targets of ROS in ischemic/reperfused myocardium, a complex network of cellular and extracellular processes is discussed, including relationships with Ca2+ homeostasis, thiol group redox balance, hydrogen sulfide modulation, cross-talk with NAPDH oxidases, exosomes, cytokines and growth factors. While mechanistic insights are needed to improve our current therapeutic approaches, advancements in knowledge of ROS-mediated processes indicate that detrimental facets of oxidative stress are opposed by ROS requirement for physiological and protective reactions. This inevitable contrast is likely to underlie unsuccessful clinical trials and limits the development of novel cardioprotective interventions simply based upon ROS removal.

New Iron Metabolic Pathways and Chelation Targeting Strategies Affecting the Treatment of All Types and Stages of Cancer

There is new and increasing evidence from in vitro, in vivo and clinical studies implicating the pivotal role of iron and associated metabolic pathways in the initiation, progression and development of cancer and in cancer metastasis. New metabolic and toxicity mechanisms and pathways, as well as genomic, transcription and other factors, have been linked to cancer and many are related to iron. Accordingly, a number of new targets for iron chelators have been identified and characterized in new anticancer strategies, in addition to the classical restriction of/reduction in iron supply, the inhibition of transferrin iron delivery, the inhibition of ribonucleotide reductase in DNA synthesis and high antioxidant potential. The new targets include the removal of excess iron from iron-laden macrophages, which affects anticancer activity; the modulation of ferroptosis; ferritin iron removal and the control of hyperferritinemia; the inhibition of hypoxia related to the role of hypoxia-inducible factor (HIF); modulation of the function of new molecular species such as STEAP4 metalloreductase and the metastasis suppressor N-MYC downstream-regulated gene-1 (NDRG1); modulation of the metabolic pathways of oxidative stress damage affecting mitochondrial function, etc. Many of these new, but also previously known associated iron metabolic pathways appear to affect all stages of cancer, as well as metastasis and drug resistance. Iron-chelating drugs and especially deferiprone (L1), has been shown in many recent studies to fulfill the role of multi-target anticancer drug linked to the above and also other iron targets, and has been proposed for phase II trials in cancer patients. In contrast, lipophilic chelators and their iron complexes are proposed for the induction of ferroptosis in some refractory or recurring tumors in drug resistance and metastasis where effective treatments are absent. There is a need to readdress cancer therapy and include therapeutic strategies targeting multifactorial processes, including the application of multi-targeting drugs involving iron chelators and iron-chelator complexes. New therapeutic protocols including drug combinations with L1 and other chelating drugs could increase anticancer activity, decrease drug resistance and metastasis, improve treatments, reduce toxicity and increase overall survival in cancer patients.

A CRISPR screen identifies redox vulnerabilities for KEAP1/NRF2 mutant non-small cell lung cancer

The redox regulator NRF2 is hyperactivated in a large percentage of non-small cell lung cancer (NSCLC) cases, which is associated with chemotherapy and radiation resistance. To identify redox vulnerabilities for KEAP1/NRF2 mutant NSCLC, we conducted a CRISPR-Cas9-based negative selection screen for antioxidant enzyme genes whose loss sensitized cells to sub-lethal concentrations of the superoxide (O2•-) -generating drug β-Lapachone. While our screen identified expected hits in the pentose phosphate pathway, the thioredoxin-dependent antioxidant system, and glutathione reductase, we also identified the mitochondrial superoxide dismutase 2 (SOD2) as one of the top hits. Surprisingly, β-Lapachone did not generate mitochondrial O2•- but rather SOD2 loss enhanced the efficacy of β-Lapachone due to loss of iron-sulfur protein function, loss of mitochondrial ATP maintenance and deficient NADPH production. Importantly, inhibition of mitochondrial electron transport activity sensitized cells to β-Lapachone, demonstrating that these effects may be translated to increase ROS sensitivity therapeutically.

Impact of Zinc on Oxidative Signaling Pathways in the Development of Pulmonary Vasoconstriction Induced by Hypobaric Hypoxia

Hypobaric hypoxia is a condition that occurs at high altitudes (>2500 m) where the partial pressure of gases, particularly oxygen (PO2), decreases. This condition triggers several physiological and molecular responses. One of the principal responses is pulmonary vascular contraction, which seeks to optimize gas exchange under this condition, known as hypoxic pulmonary vasoconstriction (HPV); however, when this physiological response is exacerbated, it contributes to the development of high-altitude pulmonary hypertension (HAPH). Increased levels of zinc (Zn2+) and oxidative stress (known as the “ROS hypothesis”) have been demonstrated in the vasoconstriction process. Therefore, the aim of this review is to determine the relationship between molecular pathways associated with altered Zn2+ levels and oxidative stress in HPV in hypobaric hypoxic conditions. The results indicate an increased level of Zn2+, which is related to increasing mitochondrial ROS (mtROS), alterations in nitric oxide (NO), metallothionein (MT), zinc-regulated, iron-regulated transporter-like protein (ZIP), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-induced protein kinase C epsilon (PKCε) activation in the development of HPV. In conclusion, there is an association between elevated Zn2+ levels and oxidative stress in HPV under different models of hypoxia, which contribute to understanding the molecular mechanism involved in HPV to prevent the development of HAPH.

The epigenetic and morphogenetic effects of molecular oxygen and its derived reactive species in development

The development of multicellular organisms involves the unpacking of a complex genetic program. Extensive characterization of discrete developmental steps has revealed the genetic program is controlled by an epigenetic state. Shifting the epigenome is a group of epigenetic enzymes that modify DNA and proteins to regulate cell type specific gene expression. While the role of these modifications in development has been established, the input(s) responsible for electing changes in the epigenetic state remains unknown. Development is also associated with dynamic changes in cellular metabolism, redox, free radical production, and oxygen availability. It has previously been postulated that these changes are causal in development by affecting gene expression. This suggests that oxygen is a morphogenic compound that impacts the removal of epigenetic marks. Likewise, metabolism and reactive oxygen species influence redox signaling through iron and glutathione to limit the availability of key epigenetic cofactors such as α-ketoglutarate, ascorbate, NAD⁺ and S-adenosylmethionine. Given the close relationship between these cofactors and epigenetic marks it seems likely that the two are linked. Here we describe how changing these inputs might affect the epigenetic state during development to drive gene expression. Combined, these cofactors and reactive oxygen species constitute the epigenetic landscape guiding cells along differing developmental paths.

Succinate Dehydrogenase (SDH)-subunit C Regulates Muscle Oxygen Consumption and Fatigability in an Animal Model of Pulmonary Emphysema

Patients with pulmonary emphysema often develop locomotor muscle dysfunction, which is independently associated with disability and higher mortality in that population. Muscle dysfunction entails reduced force-generation capacity which partially depends on fibers' oxidative potential, yet very little mechanistic research has focused on muscle respiration in pulmonary emphysema. Using a recently established animal model of pulmonary emphysema-driven skeletal muscle dysfunction, we found downregulation of succinate dehydrogenase (SDH) subunit C in association with lower oxygen consumption and fatigue-tolerance in locomotor muscles. Reduced SDH activity has been previously observed in muscles from patients with pulmonary emphysema and we found that SDHC is required to support respiration in cultured muscle cells. Moreover, in-vivo gain of SDH function in emphysema animals muscles resulted in better oxygen consumption rate (OCR) and fatigue tolerance. These changes correlated with a larger number of relatively more oxidative type 2-A and 2X fibers, and a reduced amount of 2B fibers. Our data suggests that SDHC is a key regulator of respiration and fatigability in pulmonary emphysema-driven skeletal muscles, which could be impactful to develop strategies aimed at attenuating this comorbidity.

Interval aerobic training improves bioenergetics state and mitochondrial dynamics of different brain regions in restraint stressed rats

Evidence has validated the prophylactic effects of exercising on different aspects of health. On the opposite side, immobilization may lead to various destructive effects causing neurodegeneration. Here, we investigated the association between exercising and mitochondrial quality for preventing the destructive effects of restraint stress in different rat brain regions. Twenty-four male Wistar rats, were randomized into four groups (n = 6), exercise, stress, exercise + stress, and control. The exercise procedure consisted of running on a rodent treadmill for 8 weeks, and rats in the stress group were immobilized for 6 h. Rats were then euthanized by decapitation and tricarboxylic acid (TCA) cycle enzyme activity, antioxidant levels, and mitochondrial biogenesis factors were assessed in the frontal, hippocampus, parietal and temporal regions using spectrophotometer and western blot technique. Based on our results, increased activity of TCA cycle enzymes in the exercised and exercise-stressed groups was detected, except for malate dehydrogenase which was decreased in exercise-stressed group, and fumarase that did not change. Furthermore, the level of antioxidant agents (superoxide dismutase and reduced glutathione), mitochondrial biogenesis factors (peroxisome proliferator-activated receptor gamma coactivator 1-alpha and mitochondrial transcription factor A), and dynamics markers (Mitofusin 2, dynamic related protein 1, PTEN induced putative kinase-1, and parkin) increased in both mentioned groups. Interestingly our results also revealed that the majority of the mitochondrial factors increased in the frontal and parietal lobes, which may be in relation with the location of motor and sensory areas. Exercise can be used as a prophylactic approach against bioenergetics and mitochondrial dysfunction.

Superoxide produced by mitochondrial site IQ inactivates cardiac succinate dehydrogenase and induces hepatic steatosis in Sod2 knockout mice

Superoxide produced by mitochondria has been implicated in numerous physiologies and pathologies. Eleven different mitochondrial sites that can produce superoxide and/or hydrogen peroxide (O₂^.-/H₂O₂) have been identified in vitro, but little is known about their contributions in vivo. We introduce novel variants of S1QELs and S3QELs (small molecules that suppress O₂^.-/H₂O₂ production specifically from mitochondrial sites I_Q and III_Qo, respectively, without compromising bioenergetics), that are suitable for use in vivo. When administered by intraperitoneal injection, they achieve total tissue concentrations exceeding those that are effective in vitro. We use them to study the engagement of sites I_Q and III_Qo in mice lacking functional manganese-superoxide dismutase (SOD2). Lack of SOD2 is expected to elevate superoxide levels in the mitochondrial matrix, and leads to severe pathologies and death about 8 days after birth. Compared to littermate wild-type mice, 6-day-old Sod2^-/- mice had significantly lower body weight, lower heart succinate dehydrogenase activity, and greater hepatic lipid accumulation. These pathologies were ameliorated by treatment with a SOD/catalase mimetic, EUK189, confirming previous observations. A 3-day treatment with S1QEL352 decreased the inactivation of cardiac succinate dehydrogenase and hepatic steatosis in Sod2^-/- mice. S1QEL712, which has a distinct chemical structure, also decreased hepatic steatosis, confirming that O₂^.- derived specifically from mitochondrial site I_Q is a significant driver of hepatic steatosis in Sod2^-/- mice. These findings also demonstrate the ability of these new S1QELs to suppress O₂^.- production in the mitochondrial matrix in vivo. In contrast, suppressing site III_Qo using S3QEL941 did not protect, suggesting that site III_Qo does not contribute significantly to mitochondrial O₂^.- production in the hearts or livers of Sod2^-/- mice. We conclude that the novel S1QELs are effective in vivo, and that site I_Q runs in vivo and is a significant driver of pathology in Sod2^-/- mice.

Riding the tiger - physiological and pathological effects of superoxide and hydrogen peroxide generated in the mitochondrial matrix

Elevated mitochondrial matrix superoxide and/or hydrogen peroxide concentrations drive a wide range of physiological responses and pathologies. Concentrations of superoxide and hydrogen peroxide in the mitochondrial matrix are set mainly by rates of production, the activities of superoxide dismutase-2 (SOD2) and peroxiredoxin-3 (PRDX3), and by diffusion of hydrogen peroxide to the cytosol. These considerations can be used to generate criteria for assessing whether changes in matrix superoxide or hydrogen peroxide are both necessary and sufficient to drive redox signaling and pathology: is a phenotype affected by suppressing superoxide and hydrogen peroxide production; by manipulating the levels of SOD2, PRDX3 or mitochondria-targeted catalase; and by adding mitochondria-targeted SOD/catalase mimetics or mitochondria-targeted antioxidants? Is the pathology associated with variants in SOD2 and PRDX3 genes? Filtering the large literature on mitochondrial redox signaling using these criteria highlights considerable evidence that mitochondrial superoxide and hydrogen peroxide drive physiological responses involved in cellular stress management, including apoptosis, autophagy, propagation of endoplasmic reticulum stress, cellular senescence, HIF1α signaling, and immune responses. They also affect cell proliferation, migration, differentiation, and the cell cycle. Filtering the huge literature on pathologies highlights strong experimental evidence that 30-40 pathologies may be driven by mitochondrial matrix superoxide or hydrogen peroxide. These can be grouped into overlapping and interacting categories: metabolic, cardiovascular, inflammatory, and neurological diseases; cancer; ischemia/reperfusion injury; aging and its diseases; external insults, and genetic diseases. Understanding the involvement of mitochondrial matrix superoxide and hydrogen peroxide concentrations in these diseases can facilitate the rational development of appropriate therapies.

Dietary Mg2+ Intake and the Na+/Mg2+ Exchanger SLC41A1 Influence Components of Mitochondrial Energetics in Murine Cardiomyocytes

Cardiomyocytes are among the most energy-intensive cell types. Interplay between the components of cellular magnesium (Mg) homeostasis and energy metabolism in cardiomyocytes is poorly understood. We have investigated the effects of dietary Mg content and presence/functionality of the Na+/Mg2+ exchanger SLC41A1 on enzymatic functions of selected constituents of the Krebs cycle and complexes of the electron transport chain (ETC). The activities of aconitate hydratase (ACON), isocitrate dehydrogenase (ICDH), α-ketoglutarate dehydrogenase (KGDH), and ETC complexes CI-CV have been determined in vitro in mitochondria isolated from hearts of wild-type (WT) and Slc41a1-/- mice fed a diet with either normal or low Mg content. Our data demonstrate that both, the type of Mg diet and the Slc41a1 genotype largely impact on the activities of enzymes of the Krebs cycle and ETC. Moreover, a compensatory effect of Slc41a1-/- genotype on the effect of low Mg diet on activities of the tested Krebs cycle enzymes has been identified. A machine-learning analysis identified activities of ICDH, CI, CIV, and CV as common predictors of the type of Mg diet and of CII as suitable predictor of Slc41a1 genotype. Thus, our data delineate the effect of dietary Mg content and of SLC41A1 functionality on the energy-production in cardiac mitochondria.

Regulation of Vascular Function and Inflammation via Cross Talk of Reactive Oxygen and Nitrogen Species from Mitochondria or NADPH Oxidase-Implications for Diabetes Progression

Oxidative stress plays a key role for the development of cardiovascular, metabolic, and neurodegenerative disease. This concept has been proven by using the approach of genetic deletion of reactive oxygen and nitrogen species (RONS) producing, pro-oxidant enzymes as well as by the overexpression of RONS detoxifying, antioxidant enzymes leading to an amelioration of the severity of diseases. Vice versa, the development and progression of cardiovascular diseases is aggravated by overexpression of RONS producing enzymes as well as deletion of RONS detoxifying enzymes. We have previously identified cross talk mechanisms between different sources of RONS, which can amplify the oxidative stress-mediated damage. Here, the pathways and potential mechanisms leading to this cross talk are analyzed in detail and highlighted by selected examples from the current literature and own data including hypoxia, angiotensin II (AT-II)-induced hypertension, nitrate tolerance, aging, and others. The general concept of redox-based activation of RONS sources via “kindling radicals” and enzyme-specific “redox switches” as well as the interaction with redox-sensitive inflammatory pathways are discussed. Here, we present evidence for the existence of such cross talk mechanisms in the setting of diabetes and critically assess their contribution to the severity of diabetic complications.

A neuroglobin-based high-affinity ligand trap reverses carbon monoxide-induced mitochondrial poisoning

Carbon monoxide (CO) remains the most common cause of human poisoning. The consequences of CO poisoning include cardiac dysfunction, brain injury, and death. CO causes toxicity by binding to hemoglobin and by inhibiting mitochondrial cytochrome c oxidase (CcO), thereby decreasing oxygen delivery and inhibiting oxidative phosphorylation. We have recently developed a CO antidote based on human neuroglobin (Ngb-H64Q-CCC). This molecule enhances clearance of CO from red blood cells in vitro and in vivo Herein, we tested whether Ngb-H64Q-CCC can also scavenge CO from CcO and attenuate CO-induced inhibition of mitochondrial respiration. Heart tissue from mice exposed to 3% CO exhibited a 42 ± 19% reduction in tissue respiration rate and a 33 ± 38% reduction in CcO activity compared with unexposed mice. Intravenous infusion of Ngb-H64Q-CCC restored respiration rates to that of control mice correlating with higher electron transport chain CcO activity in Ngb-H64Q-CCC-treated compared with PBS-treated, CO-poisoned mice. Further, using a Clark-type oxygen electrode, we measured isolated rat liver mitochondrial respiration in the presence and absence of saturating solutions of CO (160 μm) and nitric oxide (100 μm). Both CO and NO inhibited respiration, and treatment with Ngb-H64Q-CCC (100 and 50 μm, respectively) significantly reversed this inhibition. These results suggest that Ngb-H64Q-CCC mitigates CO toxicity by scavenging CO from carboxyhemoglobin, improving systemic oxygen delivery and reversing the inhibitory effects of CO on mitochondria. We conclude that Ngb-H64Q-CCC or other CO scavengers demonstrate potential as antidotes that reverse the clinical and molecular effects of CO poisoning.

Mitochondrial Superoxide Dismutase: What the Established, the Intriguing, and the Novel Reveal About a Key Cellular Redox Switch

Significance: Reactive oxygen species (ROS) are now widely recognized as central mediators of cell signaling. Mitochondria are major sources of ROS. Recent Advances: It is now clear that mitochondrial ROS are essential to activate responses to cellular microenvironmental stressors. Mediators of these responses reside in large part in the cytosol. Critical Issues: The primary form of ROS produced by mitochondria is the superoxide radical anion. As a charged radical anion, superoxide is restricted in its capacity to diffuse and convey redox messages outside of mitochondria. In addition, superoxide is a reductant and not particularly efficient at oxidizing targets. Because there are many opportunities for superoxide to be neutralized in mitochondria, it is not completely clear how redox cues generated in mitochondria are converted into diffusible signals that produce transient oxidative modifications in the cytosol or nucleus. Future Directions: To efficiently intervene at the level of cellular redox signaling, it seems that understanding how the generation of superoxide radicals in mitochondria is coupled with the propagation of redox messages is essential. We propose that mitochondrial superoxide dismutase (SOD2) is a major system converting diffusion-restricted superoxide radicals derived from the electron transport chain into highly diffusible hydrogen peroxide (H₂O₂). This enables the coupling of metabolic changes resulting in increased superoxide to the production of H₂O₂, a diffusible secondary messenger. As such, to determine whether there are other systems coupling metabolic changes to redox messaging in mitochondria as well as how these systems are regulated is essential.

Primer for Mainstreaming Mind-Body Techniques for Extreme Climates-Insights and Future Directions

Background: The deprivation of oxygen reaching the tissues (also termed as hypoxia) affects the normal functioning of the body. This results in development of many diseases like ischemia, glaucoma, MCI (Mild Cognitive Impairment), pulmonary and cerebral edema, stress and depression. There are no effective drugs that can treat such diseases. Despite such failure, alternative interventions such as mind-body techniques (MBTs) have not been adequately investigated. Methods: The first part of this review has been focused on philosophical aspects of various MBTs besides evolving an ayurgenomic perspective. The potential of MBTs as a preventive non-pharmacological intervention in the treatment of various general and hypoxic pathologies has been further described in this section. In the second part, molecular, physiological, and neuroprotective roles of MBTs in normal and hypoxic/ischemic conditions has been discussed. Results: In this respect, the importance of and in vivo studies has also been discussed. Conclusions: Although several studies have investigated the role of protective strategies in coping with the hypoxic environment, the efficacy of MBTs at the molecular level has been ignored.

Dysfunction of cortical synapse-specific mitochondria in developing rats exposed to lead and its amelioration by ascorbate supplementation

BACKGROUND

Lead (Pb) is a widespread environmental neurotoxin and its exposure even in minute quantities can lead to compromised neuronal functions. A developing brain is particularly vulnerable to Pb mediated toxicity and early-life exposure leads to permanent alterations in brain development and neuronal signaling and plasticity, culminating into cognitive and behavioral dysfunctions and elevated risk of neuropsychiatric disorders later in life. Nevertheless, the underlying biochemical mechanisms have not been completely discerned.

METHODS

Because of their ability to fulfill high energy needs and to act as calcium buffers in events of high intensity neuronal activity as well as their adaptive regulatory capability to match the requirements of the dynamicity of synaptic signaling, synapse-specific or synaptic mitochondria (SM) are critical for synaptic development, function and plasticity. Our aim for the present study hence was to characterize the effects of early-life Pb exposure on the functions of SM of prepubertal rats. For this purpose, employing a chronic model of Pb neurotoxicity, we exposed rat pups perinatally and postnatally to Pb and used a plethora of colorimetric and fluorometric assays for assessing redox and bioenergetic properties of SM. In addition, taking advantage of its ability as an antioxidant and as a metal chelator, we employed ascorbic acid (vitamin C) supplementation as an ameliorative therapeutic strategy against Pb-induced neurotoxicity and dysfunction of SM.

RESULTS

Our results suggest that early-life exposure to Pb leads to elevated oxidative stress in cortical SM with consequent compromises in its energy metabolism activity. Ascorbate supplementation resulted in significant recovery of Pb-induced oxidative stress and functional compromise of SM.

CONCLUSION

Alterations in redox status and bioenergetic properties of SM could potentially contribute to the synaptic dysfunction observed in events of Pb neurotoxicity. Additionally, our study provides evidence for suitability of ascorbate as a significant ameliorative agent in tacking Pb neurotoxicity.

Nanocurcumin-pyrroloquinoline formulation prevents hypertrophy-induced pathological damage by relieving mitochondrial stress in cardiomyocytes under hypoxic conditions

This study investigates the therapeutic effect of a nanocurcumin formulation (NCF) containing nanocurcumin (NC) and pyrroloquinoline quinone (PQQ) on ameliorating hypoxia-induced stress in hypertrophied primary human ventricular cardiomyocytes (HVCM) under hypoxic conditions, as validated in a Sprague-Dawley rat model of chronic hypobaric hypoxia (cHH)-induced right ventricular hypertrophy (RVH). Based on our previous findings, here, we analyzed the improvement in the protective efficacy of NCF against mitochondrial damage. The electron transport chain Complexes’ activities were analyzed as a chief operational center for mitochondrial homeostasis, along with key gene and protein markers for mitochondrial biogenesis, redox function, fatty acid oxidation, bio-energetic deficit and cell survival. NCF supplementation imparts cyto-protection from hypoxia-induced hypertrophy and damage in both in vitro and in vivo models while maintaining mitochondrial homeostasis better than NC and PQQ alone. This study proposes the use of NCF as a potential candidate molecule for imparting protection from high altitude-induced maladies in ascendants.

Thrombospondin-1, Free Radicals, and the Coronary Microcirculation: The Aging Conundrum

SIGNIFICANCE

Successful matching of cardiac metabolism to perfusion is accomplished primarily through vasodilation of the coronary resistance arterioles, but the mechanism that achieves this effect changes significantly as aging progresses and involves the contribution of reactive oxygen species (ROS). Recent Advances: A matricellular protein, thrombospondin-1 (Thbs-1), has been shown to be a prolific contributor to the production and modulation of ROS in large conductance vessels and in the peripheral circulation. Recently, the presence of physiologically relevant circulating Thbs-1 levels was proven to also disrupt vasodilation to nitric oxide (NO) in coronary arterioles from aged animals, negatively impacting coronary blood flow reserve.

CRITICAL ISSUES

This review seeks to reconcile how ROS can be successfully utilized as a substrate to mediate vasoreactivity in the coronary microcirculation as "normal" aging progresses, but will also examine how Thbs-1-induced ROS production leads to dysfunctional perfusion and eventual ischemia and why this is more of a concern in advancing age.

FUTURE DIRECTIONS

Current therapies that may effectively disrupt Thbs-1 and its receptor CD47 in the vascular wall and areas for future exploration will be discussed. Antioxid. Redox Signal. 27, 785-801.

TCA cycle rewiring fosters metabolic adaptation to oxygen restriction in skeletal muscle from rodents and humans

In mammals, hypoxic stress management is under the control of the Hypoxia Inducible Factors, whose activity depends on the stabilization of their labile α subunit. In particular, the skeletal muscle appears to be able to react to changes in substrates and O₂ delivery by tuning its metabolism. The present study provides a comprehensive overview of skeletal muscle metabolic adaptation to hypoxia in mice and in human subjects exposed for 7/9 and 19 days to high altitude levels. The investigation was carried out combining proteomics, qRT-PCR mRNA transcripts analysis, and enzyme activities assessment in rodents, and protein detection by antigen antibody reactions in humans and rodents. Results indicate that the skeletal muscle react to a decreased O₂ delivery by rewiring the TCA cycle. The first TCA rewiring occurs in mice in 2-day hypoxia and is mediated by cytosolic malate whereas in 10-day hypoxia the rewiring is mediated by Idh1 and Fasn, supported by glutamine and HIF-2α increments. The combination of these specific anaplerotic steps can support energy demand despite HIFs degradation. These results were confirmed in human subjects, demonstrating that the TCA double rewiring represents an essential factor for the maintenance of muscle homeostasis during adaptation to hypoxia.

Production of superoxide and hydrogen peroxide from specific mitochondrial sites under different bioenergetic conditions

Mitochondrial production of superoxide and hydrogen peroxide is potentially important in cell signaling and disease. Eleven distinct mitochondrial sites that differ markedly in capacity are known to leak electrons to oxygen to produce O₂^̇̄ and/or H₂O₂ We discuss their contributions to O₂^̇̄/H₂O₂ production under native conditions in mitochondria oxidizing different substrates and in conditions mimicking physical exercise and the changes in their capacities after caloric restriction. We review the use of S1QELs and S3QELs, suppressors of mitochondrial O₂^̇̄/H₂O₂ generation that do not inhibit oxidative phosphorylation, as tools to characterize the contributions of specific sites in situ and in vivo.

Stress-dependent opposing roles for mitophagy in aging of the ascomycete Podospora anserina

Mitochondrial dysfunction is causatively linked to organismal aging and the development of degenerative diseases. Here we describe stress-dependent opposing roles of mitophagy, the selective autophagic degradation of mitochondria, in aging and life-span control. We report that the ablation of the mitochondrial superoxide dismutase which is involved in reactive oxygen species (ROS) balancing, does not affect life span of the fungal aging model Podospora anserina, although superoxide levels are strongly increased and complex I-dependent respiration is impaired. This unexpected phenotype depends on functional autophagy, particularly mitophagy, which is upregulated during aging of this mutant. It identifies mitophagy as a prosurvival response involved in the control of mitohormesis, the well-known beneficial effect of mild mitochondrial oxidative stress. In contrast, excessive superoxide stress turns mitophagy to a prodeath pathway and leads to accelerated aging. Overall our data uncover mitophagy as a dynamic pathway that specifically responds to different levels of mitochondrial oxidative stress and thereby affects organismal aging.

Enhanced Th2 cell differentiation and function in the absence of Nox2

BACKGROUND

Patients with chronic granulomatous disease (CGD), whom inherit abnormal function of NADPH oxidase 2 (Nox2), suffer from hyperinflammatory responses in lung as well as bacterial and fungal infection. There have been studies to reveal the function of Nox2 in hyperinflammatory diseases, especially in asthma, but the exact role of Nox2 in asthma is still unclear and controversial. Therefore, we attempted to clarify the exact role of Nox2 in asthma, using various experimental asthma models.

METHODS

Asthma phenotypes were analyzed in response to various allergen-induced experimental asthma using Nox2-deficient mice and recombinase gene-activating-1-deficient mice. To understand the underlying mechanisms of exaggerated Th2 effector functions, we investigated the degree of T-cell activation, levels of activation-induced cell death (AICD), and regulatory T (Treg)-cell differentiation in Nox2-deficient T cells.

RESULTS

Asthma phenotypes were increased through enhanced Th2 differentiation and function in Nox2-null mice regardless of dose and route of various allergens. Nox2-deficient T cells also showed hyperactivation, reduced AICD, and diminished Treg-cell differentiation through increased AKT phosphorylation (T308/S473) and enhanced mitochondrial ROS production.

CONCLUSION

Our findings indicate that Nox2 deficiency results in exaggerated experimental asthma, which is caused by enhanced Th2 effector function in a T-cell-intrinsic manner.

Suppressors of Superoxide-H2O2 Production at Site IQ of Mitochondrial Complex I Protect against Stem Cell Hyperplasia and Ischemia-Reperfusion Injury

Using high-throughput screening we identified small molecules that suppress superoxide and/or H₂O₂ production during reverse electron transport through mitochondrial respiratory complex I (site I_Q) without affecting oxidative phosphorylation (suppressors of site I_Q electron leak, "S1QELs"). S1QELs diminished endogenous oxidative damage in primary astrocytes cultured at ambient or low oxygen tension, showing that site I_Q is a normal contributor to mitochondrial superoxide-H₂O₂ production in cells. They diminished stem cell hyperplasia in Drosophila intestine in vivo and caspase activation in a cardiomyocyte cell model driven by endoplasmic reticulum stress, showing that superoxide-H₂O₂ production by site I_Q is involved in cellular stress signaling. They protected against ischemia-reperfusion injury in perfused mouse heart, showing directly that superoxide-H₂O₂ production by site I_Q is a major contributor to this pathology. S1QELs are tools for assessing the contribution of site I_Q to cell physiology and pathology and have great potential as therapeutic leads.

Regulation of NADPH oxidases in skeletal muscle

The only known function of NAD(P)H oxidases is to produce reactive oxygen species (ROS). Skeletal muscles express three isoforms of NAD(P)H oxidases (Nox1, Nox2, and Nox4) that have been identified as critical modulators of redox homeostasis. Nox2 acts as the main source of skeletal muscle ROS during contractions, participates in insulin signaling and glucose transport, and mediates the myocyte response to osmotic stress. Nox2 and Nox4 contribute to skeletal muscle abnormalities elicited by angiotensin II, muscular dystrophy, heart failure, and high fat diet. Our review addresses the expression and regulation of NAD(P)H oxidases with emphasis on aspects that are relevant to skeletal muscle. We also summarize: i) the most widely used NAD(P)H oxidases activity assays and inhibitors, and ii) studies that have defined Nox enzymes as protagonists of skeletal muscle redox homeostasis in a variety of health and disease conditions.

Pharmacological targeting of mitochondrial reactive oxygen species counteracts diaphragm weakness in chronic heart failure

Diaphragm muscle weakness in chronic heart failure (CHF) is caused by elevated oxidants and exacerbates breathing abnormalities, exercise intolerance, and dyspnea. However, the specific source of oxidants that cause diaphragm weakness is unknown. We examined whether mitochondrial reactive oxygen species (ROS) cause diaphragm weakness in CHF by testing the hypothesis that CHF animals treated with a mitochondria-targeted antioxidant have normal diaphragm function. Rats underwent CHF or sham surgery. Eight weeks after surgeries, we administered a mitochondrial-targeted antioxidant (MitoTEMPO; 1 mg·kg(-1)·day(-1)) or sterile saline (Vehicle). Left ventricular dysfunction (echocardiography) pre- and posttreatment and morphological abnormalities were consistent with the presence of CHF. CHF elicited a threefold (P < 0.05) increase in diaphragm mitochondrial H2O2 emission, decreased diaphragm glutathione content by 23%, and also depressed twitch and maximal tetanic force by ∼20% in Vehicle-treated animals compared with Sham (P < 0.05 for all comparisons). Diaphragm mitochondrial H2O2 emission, glutathione content, and twitch and maximal tetanic force were normal in CHF animals receiving MitoTEMPO. Neither CHF nor MitoTEMPO altered the diaphragm protein levels of antioxidant enzymes: superoxide dismutases (CuZn-SOD or MnSOD), glutathione peroxidase, and catalase. In both Vehicle and MitoTEMPO groups, CHF elicited a ∼30% increase in cytochrome c oxidase activity, whereas there were no changes in citrate synthase activity. Our data suggest that elevated mitochondrial H2O2 emission causes diaphragm weakness in CHF. Moreover, changes in protein levels of antioxidant enzymes or mitochondrial content do not seem to mediate the increase in mitochondria H2O2 emission in CHF and protective effects of MitoTEMPO.

Effects of mild hyperhomocysteinemia on electron transport chain complexes, oxidative stress, and protein expression in rat cardiac mitochondria

Hyperhomocysteinemia (HHcy) is an independent risk factor of cardiovascular disease, but the mechanisms of tissue injury are poorly understood. In the present study, we investigated the effect of HHcy on rat heart function, activities electron transport chain (ETC) complexes, mitochondrial protein expression, and protein oxidative damage. HHcy was induced by subcutaneous injection of Hcy (0.45 μmol/g of body weight) twice a day for a period of 2 weeks. Performance of hearts excised after the Hcy treatment was examined according to the Langendorff method at a constant pressure. Left ventricular developed pressure, as well as maximal rates of contraction (+dP/dt) and relaxation (-dP/dt), was significantly depressed in HHcy rats. HHcy was accompanied by significant inhibition of ETC complexes II-IV, whereas activity of the complex I was unchanged. The decline in ETC activities was not associated with elevated protein oxidative damage, as indicated by unchanged protein carbonyl, thiol, and dityrosine contents. Moreover, the level of protein adducts with 4-hydroxynonenal was decreased in HHcy rats. Additionally, 2D-gel electrophoresis with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry did not show alterations in contents of inhibited ETC complexes. However, mass spectrometry analyses identified 8 proteins whose expression was significantly increased by HHcy. These proteins are known to play important roles in the cellular stress response, bioenergetics, and redox balance. Altogether, the results suggest that oxidative damage and altered protein expression are not possible causes of ETC dysfunction in HHcy rats. Increased expression of the other mitochondrial proteins indicates a protective response to Hcy-induced myocardial injury.

Reperfusion injury and reactive oxygen species: The evolution of a concept

Reperfusion injury, the paradoxical tissue response that is manifested by blood flow-deprived and oxygen-starved organs following the restoration of blood flow and tissue oxygenation, has been a focus of basic and clinical research for over 4-decades. While a variety of molecular mechanisms have been proposed to explain this phenomenon, excess production of reactive oxygen species (ROS) continues to receive much attention as a critical factor in the genesis of reperfusion injury. As a consequence, considerable effort has been devoted to identifying the dominant cellular and enzymatic sources of excess ROS production following ischemia-reperfusion (I/R). Of the potential ROS sources described to date, xanthine oxidase, NADPH oxidase (Nox), mitochondria, and uncoupled nitric oxide synthase have gained a status as the most likely contributors to reperfusion-induced oxidative stress and represent priority targets for therapeutic intervention against reperfusion-induced organ dysfunction and tissue damage. Although all four enzymatic sources are present in most tissues and are likely to play some role in reperfusion injury, priority and emphasis has been given to specific ROS sources that are enriched in certain tissues, such as xanthine oxidase in the gastrointestinal tract and mitochondria in the metabolically active heart and brain. The possibility that multiple ROS sources contribute to reperfusion injury in most tissues is supported by evidence demonstrating that redox-signaling enables ROS produced by one enzymatic source (e.g., Nox) to activate and enhance ROS production by a second source (e.g., mitochondria). This review provides a synopsis of the evidence implicating ROS in reperfusion injury, the clinical implications of this phenomenon, and summarizes current understanding of the four most frequently invoked enzymatic sources of ROS production in post-ischemic tissue.

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Reperfusion injury is implicated in a variety of human diseases and disorders.

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Evidence implicating ROS in reperfusion injury continues to grow.

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Several enzymes are candidate sources of ROS in post-ischemic tissue.

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Inter-enzymatic ROS-dependent signaling enhances the oxidative stress caused by I/R.

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Oxidised low density lipoprotein causes human macrophage cell death through oxidant generation and inhibition of key catabolic enzymes

Oxidised low density lipoprotein (oxLDL) is thought to be a significant contributor to the death of macrophage cells observed in advanced atherosclerotic plaques. Using human-derived U937 cells we have examined the effect of cytotoxic oxLDL on oxidative stress and cellular catabolism. Within 3h of the addition of oxLDL, there was a rapid, concentration dependent rise in cellular reactive oxygen species followed by the loss of cellular GSH, and the enzyme activity of both glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and aconitase. The loss of these catabolic enzymes was accompanied by the loss of cellular ATP and lower lactate generation. Addition of the macrophage antioxidant 7,8-dihydroneopterin inhibited the ROS generation, glutathione loss and catabolic inactivation. NOX was shown to be activated by oxLDL addition while apocynin inhibited the loss of GSH and cell viability. The data suggests that oxLDL triggers an excess of ROS production through NOX activation, and catabolic failure through thiol oxidation resulting in cell death.

Redox control of microglial function: molecular mechanisms and functional significance

Neurodegenerative diseases are characterized by chronic microglial over-activation and oxidative stress. It is now beginning to be recognized that reactive oxygen species (ROS) produced by either microglia or the surrounding environment not only impact neurons but also modulate microglial activity. In this review, we first analyze the hallmarks of pro-inflammatory and anti-inflammatory phenotypes of microglia and their regulation by ROS. Then, we consider the production of reactive oxygen and nitrogen species by NADPH oxidases and nitric oxide synthases and the new findings that also indicate an essential role of glutathione (γ-glutamyl-l-cysteinylglycine) in redox homeostasis of microglia. The effect of oxidant modification of macromolecules on signaling is analyzed at the level of oxidized lipid by-products and sulfhydryl modification of microglial proteins. Redox signaling has a profound impact on two transcription factors that modulate microglial fate, nuclear factor kappa-light-chain-enhancer of activated B cells, and nuclear factor (erythroid-derived 2)-like 2, master regulators of the pro-inflammatory and antioxidant responses of microglia, respectively. The relevance of these proteins in the modulation of microglial activity and the interplay between them will be evaluated. Finally, the relevance of ROS in altering blood brain barrier permeability is discussed. Recent examples of the importance of these findings in the onset or progression of neurodegenerative diseases are also discussed. This review should provide a profound insight into the role of redox homeostasis in microglial activity and help in the identification of new promising targets to control neuroinflammation through redox control of the brain.

Inhibition of akt phosphorylation diminishes mitochondrial biogenesis regulators, tricarboxylic acid cycle activity and exacerbates recognition memory deficit in rat model of Alzheimer's disease

3-Methyladenine (3-MA), as a PI3K inhibitor, is widely used for inhibition of autophagy. Inhibition of PI3K class I leads to inhibition of Akt phosphorylation, a central molecule involved in diverse arrays of intracellular cascades in nervous system. Accordingly, in the present study, we aimed to determine the alterations of specific mitochondrial biogenesis markers and mitochondrial function in 3-MA-injected rats following amyloid beta (Aβ) insult. Our data revealed that inhibition of Akt phosphorylation downregulates master regulator of mitochondrial biogenesis, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). Our data also showed that decrease in PGC-1α level presumably is due to decrease in the phosphorylation of cAMP-response element binding and AMP-activated kinase, two upstream activators of PGC-1α. As a consequence, the level of some mitochondrial biogenesis factors including nuclear respiratory factor-1, mitochondrial transcription factor A, and Cytochrome c decreased significantly. Also, activities of tricarboxylic acid cycle (TCA) enzymes such as Aconitase, a-ketoglutarate dehydrogenase, and malate dehydrogenase reduced in the presence of 3-MA with or without Aβ insult. Decrease in mitochondrial biogenesis factors and TCA enzyme activity in the rats receiving 3-MA and Aβ were more compared to the rats that received either alone; indicating the additive destructive effects of these two agents. In agreement with our molecular results, data obtained from behavioral test (using novel objective recognition test) indicated that inhibition of Akt phosphorylation with or without Aβ injection impaired novel recognition (non-spatial) memory. Our results suggest that 3-MA amplified deleterious effects of Aβ by targeting central molecule Akt.

Regulation of lung oxidative damage by endogenous superoxide dismutase in sepsis

Background

The purpose of this research is to study the relationship between superoxide dismutase (SOD) and lung redox state in an animal model of sepsis.

Methods

Sepsis was induced in rats by the cecal ligation and perforation model (CLP). After 3, 6, and 12 h, CLP protein content and expression of SOD1, SOD2, and SOD3 were evaluated, and SOD activity was assessed. Oxidative damage was determined by quantifying nitrotyrosine content. Lung localization of SOD3 was performed by immunohistochemistry. The protective effect of a SOD mimetic on oxidative damage, inflammation, and lung permeability was assessed 12 and 24 h after sepsis induction.

Results

Lung levels of SOD1 decreased 3 and 12 h after sepsis, but SOD2 and SOD3 increased, as well as SOD activity. These alterations were not associated with changes in sod gene expression. Nitrotyrosine levels increased 3 and 12 h after sepsis. The administration of a SOD mimetic decreased nitrotyrosine and proinflammatory cytokine levels and improved lung permeability.

Conclusions

SOD2 and SOD3 increased after sepsis induction, but this was insufficient to protect the lung. Treatments based on SOD mimetics could have a role in lung injury associated with sepsis.

Modulation of redox regulatory molecules and electron transport chain activity in muscle of air breathing fish Heteropneustes fossilis under air exposure stress

Responses of redox regulatory system to long-term survival (>18 h) of the catfish Heteropneustes fossilis in air are not yet understood. Lipid and protein oxidation level, oxidant (H2O2) generation, antioxidative status (levels of superoxide dismutase, catalase, glutathione peroxidase and reductase, ascorbic acid and non-protein sulfhydryl) and activities of respiratory complexes (I, II, III and IV) in mitochondria were investigated in muscle of H. fossilis under air exposure condition (0, 3, 6, 12 and 18 h at 25 °C). The increased levels of both H2O2 and tissue oxidation were observed due to the decreased activities of antioxidant enzymes in muscle under water deprivation condition. However, ascorbic acid and non-protein thiol groups were the highest at 18 h air exposure time. A linear increase in complex II activity with air exposure time and an increase up to 12 h followed by a decrease in activity of complex I at 18 h were observed. Negative correlation was observed for complex III and V activity with exposure time. Critical time to modulate the above parameters was found to be 3 h air exposure. Dehydration induced oxidative stress due to modulation of electron transport chain and redox metabolizing enzymes in muscle of H. fossilis was clearly observed. Possible contribution of redox regulatory system in muscle tissue of the fish for long-term survival in air is elucidated. Results of the present study may be useful to understand the redox metabolism in muscle of fishes those are exposed to air in general and air breathing fishes in particular.

Monitoring of neuronal loss in the hippocampus of Aβ-injected rat: autophagy, mitophagy, and mitochondrial biogenesis stand against apoptosis

In the present study, we tried to answer the following questions: which kind of defense pathways are activated after Aβ insult? How defense systems react against noxious effects of Aβ and whether they are able to deal against apoptosis or not? So, we traced some molecular pathways including autophagy, mitophagy, and mitochondrial biogenesis before reaching to the endpoint of apoptosis. Besides, we measured the function of mitochondria after injection of Aβ (1-42) in CA1 area of hippocampus as a model of Alzheimer's disease (AD). Based on our data, autophagy markers reached to their maximum level and returned to the control level as apoptotic markers started to increase. As a specialized form of autophagy, mitophagy markers followed the trend of autophagy markers. Whereas mitochondrial dynamic processes shifted toward fission, mitochondrial biogenesis was severely affected by Aβ and significantly decreased. Alongside suppression of mitochondrial biogenesis, activity of specific enzymes involved in antioxidant defense system, electron transport chain, and tricarboxylic acid cycle (TCA) decreased in response to the Aβ. Activity of antioxidant enzymes increased at first and then decreased significantly compared to the control. TCA enzymes aconitase and malate dehydrogenase activities reduced immediately while citrate synthase and fumarase activities did not change. Based on our finding, monitoring of the master molecules of intracellular cascades and determining their trends before the destructive function of Aβ could be the target of therapeutic issues for AD.

Role of mitochondrial oxidative stress in hypertension

Based on mosaic theory, hypertension is a multifactorial disorder that develops because of genetic, environmental, anatomical, adaptive neural, endocrine, humoral, and hemodynamic factors. It has been recently proposed that oxidative stress may contribute to all of these factors and production of reactive oxygen species (ROS) play an important role in the development of hypertension. Previous studies focusing on the role of vascular NADPH oxidases provided strong support of this concept. Although mitochondria represent one of the most significant sources of cellular ROS generation, the regulation of mitochondrial ROS generation in the cardiovascular system and its pathophysiological role in hypertension are much less understood. In this review, the role of mitochondrial oxidative stress in the pathophysiology of hypertension and cross talk between angiotensin II signaling, pathways involved in mechanotransduction, NADPH oxidases, and mitochondria-derived ROS are considered. The possible benefits of therapeutic strategies that have the potential to attenuate mitochondrial oxidative stress for the prevention/treatment of hypertension are also discussed.

Tissue-specific changes in fatty acid oxidation in hypoxic heart and skeletal muscle

Exposure to hypobaric hypoxia is sufficient to decrease cardiac PCr/ATP and alters skeletal muscle energetics in humans. Cellular mechanisms underlying the different metabolic responses of these tissues and the time-dependent nature of these changes are currently unknown, but altered substrate utilization and mitochondrial function may be a contributory factor. We therefore sought to investigate the effects of acute (1 day) and more sustained (7 days) hypoxia (13% O₂) on the transcription factor peroxisome proliferator-activated receptor α (PPARα) and its targets in mouse cardiac and skeletal muscle. In the heart, PPARα expression was 40% higher than in normoxia after 1 and 7 days of hypoxia. Activities of carnitine palmitoyltransferase (CPT) I and β-hydroxyacyl-CoA dehydrogenase (HOAD) were 75% and 35% lower, respectively, after 1 day of hypoxia, returning to normoxic levels after 7 days. Oxidative phosphorylation respiration rates using palmitoyl-carnitine followed a similar pattern, while respiration using pyruvate decreased. In skeletal muscle, PPARα expression and CPT I activity were 20% and 65% lower, respectively, after 1 day of hypoxia, remaining at this level after 7 days with no change in HOAD activity. Oxidative phosphorylation respiration rates using palmitoyl-carnitine were lower in skeletal muscle throughout hypoxia, while respiration using pyruvate remained unchanged. The rate of CO₂ production from palmitate oxidation was significantly lower in both tissues throughout hypoxia. Thus cardiac muscle may remain reliant on fatty acids during sustained hypoxia, while skeletal muscle decreases fatty acid oxidation and maintains pyruvate oxidation.

Mitochondrial NADH:ubiquinone oxidoreductase alterations are associated with endometriosis

Genetic alterations and aberrant expression of 'mitochondrial membrane complex I' (MMC-I) underlie several complex human disorders, but no reports are documented to date in endometriosis. Sequencing of mitochondrially encoded MMC-I subunits revealed 72 mutations of which 2 missense (G10398A; A13603A/G) mutations and 1 synonymous (T10400C) mutation showed higher prevalence in patients. In silico functional analysis predicted A13603A/G, a novel heteroplasmy as a 'damaging variant'. Our results indicate higher endometriosis risk for haplotype '10398A/10400C/13603AG' and haplogroup 'N'. Immunohistochemical analysis revealed elevated MMC-I expression in eutopic endometria of patients compared to controls. In conclusion, MMC-I alterations may constitute an inheritable risk factor for endometriosis.

Curbing cancer's sweet tooth: is there a role for MnSOD in regulation of the Warburg effect?

Reactive oxygen species (ROS), while vital for normal cellular function, can have harmful effects on cells, leading to the development of diseases such as cancer. The Warburg effect, the shift from oxidative phosphorylation to glycolysis, even in the presence of adequate oxygen, is an important metabolic change that confers many growth and survival advantages to cancer cells. Reactive oxygen species are important regulators of the Warburg effect. The mitochondria-localized antioxidant enzyme manganese superoxide dismutase (MnSOD) is vital to survival in our oxygen-rich atmosphere because it scavenges mitochondrial ROS. MnSOD is important in cancer development and progression. However, the significance of MnSOD in the regulation of the Warburg effect is just now being revealed, and it may significantly impact the treatment of cancer in the future.

Mitochondria as a target of environmental toxicants

Enormous strides have recently been made in our understanding of the biology and pathobiology of mitochondria. Many diseases have been identified as caused by mitochondrial dysfunction, and many pharmaceuticals have been identified as previously unrecognized mitochondrial toxicants. A much smaller but growing literature indicates that mitochondria are also targeted by environmental pollutants. We briefly review the importance of mitochondrial function and maintenance for health based on the genetics of mitochondrial diseases and the toxicities resulting from pharmaceutical exposure. We then discuss how the principles of mitochondrial vulnerability illustrated by those fields might apply to environmental contaminants, with particular attention to factors that may modulate vulnerability including genetic differences, epigenetic interactions, tissue characteristics, and developmental stage. Finally, we review the literature related to environmental mitochondrial toxicants, with a particular focus on those toxicants that target mitochondrial DNA. We conclude that the fields of environmental toxicology and environmental health should focus more strongly on mitochondria.

The peroxidase activity of mitochondrial superoxide dismutase

Manganese superoxide dismutase (MnSOD) is an integral mitochondrial protein known as a first-line antioxidant defense against superoxide radical anions produced as by-products of the electron transport chain. Recent studies have shaped the idea that by regulating the mitochondrial redox status and H(2)O(2) outflow, MnSOD acts as a fundamental regulator of cellular proliferation, metabolism, and apoptosis, thereby assuming roles that extend far beyond its proposed antioxidant functions. Accordingly, allelic variations of MnSOD that have been shown to augment levels of MnSOD in mitochondria result in a 10-fold increase in prostate cancer risk. In addition, epidemiologic studies indicate that reduced glutathione peroxidase activity along with increases in H(2)O(2) further increase cancer risk in the face of MnSOD overexpression. These facts led us to hypothesize that, like its Cu,ZnSOD counterpart, MnSOD may work as a peroxidase, utilizing H(2)O(2) to promote mitochondrial damage, a known cancer risk factor. Here we report that MnSOD indeed possesses peroxidase activity that manifests in mitochondria when the enzyme is overexpressed.

Organochalcogens inhibit mitochondrial complexes I and II in rat brain: possible implications for neurotoxicity

Organochalcogens, such as organoselenium and organotellurium compounds, can be neurotoxic to rodents. Since mitochondrial dysfunction plays a pivotal role in neurological disorders, the present study was designed to test the hypothesis that rat brain mitochondrial complexes (I, II, I-III, II-III and IV) could be molecular targets of organochalcogens. The results show that organochalcogens caused statistically significant inhibition of mitochondrial complex I activity, which was prevented by preincubation with NADH and fully blunted by reduced glutathione (GSH). Mitochondrial complex II activity remained unchanged in response to (PhSe)₂ treatment. Ebs and (PhTe)₂ caused a significant concentration-dependent inhibition of complex II that was also blunted by GSH. Mitochondrial complex IV activity was not modified by organochalcogens. Collectively, Ebs, (PhSe)₂ and (PhTe)₂ were more effective inhibitors of brain mitochondrial complex I than of complex II, whereas they did not affect complex IV. These observations are consistent with organochalcogens inducing mitochondrial complex I and II inhibition via their thiol-oxidase-like activity, with Ebs, (PhSe)₂ and (PhTe)₂ effectively oxidising critical thiol groups of these complexes.

Interaction of plant cell signaling molecules, salicylic acid and jasmonic acid, with the mitochondria of Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera is a polyphagous pest in Asia, Africa, and the Mediterranean Europe. Salicylic acid (SA) and jasmonic acid (JA) are the cell signaling molecules produced in response to insect attack in plants. The effect of these signaling molecules was investigated on the oxidative phosphorylation and oxidative stress of H. armigera. SA significantly inhibited the state III and state IV respiration, respiratory control index (RCI), respiratory complexes I and II, induced mitochondrial swelling, and cytochrome c release in vitro. Under in vivo conditions, SA induced state IV respiration as well as oxidative stress in time- and dose-dependent manner, and also inhibited the larval growth. In contrast, JA did not affect the mitochondrial respiration and oxidative stress. SA affected the growth and development of H. armigera, in addition to its function as signaling molecules involved in both local defense reactions at feeding sites and the induction of systemic acquired resistance in plants.

The effects of chromium(VI) on the thioredoxin system: implications for redox regulation

Hexavalent chromium [Cr(VI)] compounds are highly redox active and have long been recognized as potent cytotoxins and carcinogens. The intracellular reduction of Cr(VI) generates reactive Cr intermediates, which are themselves strong oxidants, as well as superoxide, hydrogen peroxide, and hydroxyl radical. These probably contribute to the oxidative damage and effects on redox-sensitive transcription factors that have been reported. However, the identification of events that initiate these signaling changes has been elusive. More recent studies show that Cr(VI) causes irreversible inhibition of thioredoxin reductase (TrxR) and oxidation of thioredoxin (Trx) and peroxiredoxin (Prx). Mitochondrial Trx2/Prx3 are more sensitive to Cr(VI) treatment than cytosolic Trx1/Prx1, although both compartments show thiol oxidation with higher doses or longer treatments. Thiol redox proteomics demonstrate that Trx2, Prx3, and Trx1 are among the most sensitive proteins in cells to Cr(VI) treatment. Their oxidation could therefore represent initiating events that have widespread implications for protein thiol redox control and for multiple aspects of redox signaling. This review summarizes the effects of Cr(VI) on the TrxR/Trx system and how these events could influence a number of downstream redox signaling systems that are influenced by Cr(VI) exposure. Some of the signaling events discussed include the activation of apoptosis signal regulating kinase and MAP kinases (p38 and JNK) and the modulation of a number of redox-sensitive transcription factors including AP-1, NF-κB, p53, and Nrf2.

Clair DK. Manganese superoxide dismutase: guardian of the powerhouse

The mitochondrion is vital for many metabolic pathways in the cell, contributing all or important constituent enzymes for diverse functions such as β-oxidation of fatty acids, the urea cycle, the citric acid cycle, and ATP synthesis. The mitochondrion is also a major site of reactive oxygen species (ROS) production in the cell. Aberrant production of mitochondrial ROS can have dramatic effects on cellular function, in part, due to oxidative modification of key metabolic proteins localized in the mitochondrion. The cell is equipped with myriad antioxidant enzyme systems to combat deleterious ROS production in mitochondria, with the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) acting as the chief ROS scavenging enzyme in the cell. Factors that affect the expression and/or the activity of MnSOD, resulting in diminished antioxidant capacity of the cell, can have extraordinary consequences on the overall health of the cell by altering mitochondrial metabolic function, leading to the development and progression of numerous diseases. A better understanding of the mechanisms by which MnSOD protects cells from the harmful effects of overproduction of ROS, in particular, the effects of ROS on mitochondrial metabolic enzymes, may contribute to the development of novel treatments for various diseases in which ROS are an important component.

Cross talk between mitochondria and NADPH oxidases

Reactive oxygen species (ROS) play an important role in physiological and pathological processes. In recent years, a feed-forward regulation of the ROS sources has been reported. The interactions between the main cellular sources of ROS, such as mitochondria and NADPH oxidases, however, remain obscure. This work summarizes the latest findings on the role of cross talk between mitochondria and NADPH oxidases in pathophysiological processes. Mitochondria have the highest levels of antioxidants in the cell and play an important role in the maintenance of cellular redox status, thereby acting as an ROS and redox sink and limiting NADPH oxidase activity. Mitochondria, however, are not only a target for ROS produced by NADPH oxidase but also a significant source of ROS, which under certain conditions may stimulate NADPH oxidases. This cross talk between mitochondria and NADPH oxidases, therefore, may represent a feed-forward vicious cycle of ROS production, which can be pharmacologically targeted under conditions of oxidative stress. It has been demonstrated that mitochondria-targeted antioxidants break this vicious cycle, inhibiting ROS production by mitochondria and reducing NADPH oxidase activity. This may provide a novel strategy for treatment of many pathological conditions including aging, atherosclerosis, diabetes, hypertension, and degenerative neurological disorders in which mitochondrial oxidative stress seems to play a role. It is conceivable that the use of mitochondria-targeted treatments would be effective in these conditions.

Training-induced modifications of circadian rhythmicity of peroxidative parameters in horses

The aim of this study was to evaluate the daily rhythms of peroxidative parameters in untrained and trained horses. Blood samples were collected every 4 h for a 48-h period for the determination of reactive oxygen metabolites test (d-ROMs), antioxidant barrier (Oxy-ads), thiol antioxidant barrier (SHp) and lipid peroxidation (LPO). Two-way anova showed a significant effect of time of day on all parameters studied, except on LPO. Higher values of Oxy-ads and SHp were observed in trained horses during both days of monitoring (p < 0.01). All studied parameters, except for LPO, showed robust daily rhythms. Their acrophases occurred during the day in untrained horses and in the evening in trained horses. Our results showed that training increases antioxidative capacity and demonstrated that the daily rhythms of antioxidative parameters can be modified by training.

The redox basis of epigenetic modifications: from mechanisms to functional consequences

Epigenetic modifications represent mechanisms by which cells may effectively translate multiple signaling inputs into phenotypic outputs. Recent research is revealing that redox metabolism is an increasingly important determinant of epigenetic control that may have significant ramifications in both human health and disease. Numerous characterized epigenetic marks, including histone methylation, acetylation, and ADP-ribosylation, as well as DNA methylation, have direct linkages to central metabolism through critical redox intermediates such as NAD(+), S-adenosyl methionine, and 2-oxoglutarate. Fluctuations in these intermediates caused by both normal and pathologic stimuli may thus have direct effects on epigenetic signaling that lead to measurable changes in gene expression. In this comprehensive review, we present surveys of both metabolism-sensitive epigenetic enzymes and the metabolic processes that may play a role in their regulation. To close, we provide a series of clinically relevant illustrations of the communication between metabolism and epigenetics in the pathogenesis of cardiovascular disease, Alzheimer disease, cancer, and environmental toxicity. We anticipate that the regulatory mechanisms described herein will play an increasingly large role in our understanding of human health and disease as epigenetics research progresses.

MnSOD deficiency results in elevated oxidative stress and decreased mitochondrial function but does not lead to muscle atrophy during aging

In a previous study, we reported that a deficiency in MnSOD activity (approximately 80% reduction) targeted to type IIB skeletal muscle fibers was sufficient to elevate oxidative stress and to reduce muscle function in young adult mice (TnIFastCreSod2(fl/fl) mice). In this study, we used TnIFastCreSod2(fl/fl) mice to examine the effect of elevated oxidative stress on mitochondrial function and to test the hypothesis that elevated oxidative stress and decreased mitochondrial function over the lifespan of the TnIFastCreSod2(fl/fl) mice would be sufficient to accelerate muscle atrophy associated with aging. We found that mitochondrial function is reduced in both young and old TnIFastCreSod2(fl/fl) mice, when compared with control mice. Complex II activity is reduced by 47% in young and by approximately 90% in old TnIFastCreSod2(fl/fl) mice, and was found to be associated with reduced levels of the catalytic subunits for complex II, SDHA and SDHB. Complex II-linked mitochondrial respiration is reduced by approximately 70% in young TnIFastCreSod2(fl/fl) mice. Complex II-linked mitochondrial Adenosine-Tri-Phosphate (ATP) production is reduced by 39% in young and was found to be almost completely absent in old TnIFastCreSod2(fl/fl) mice. Furthermore, in old TnIFastCreSod2(fl/fl) mice, aconitase activity is almost completely abolished; mitochondrial superoxide release remains > 2-fold elevated; and oxidative damage (measured as F(2) - isoprostanes) is increased by 30% relative to age-matched controls. These data show that despite elevated skeletal muscle-specific mitochondrial oxidative stress, oxidative damage, and complex II-linked mitochondrial dysfunction, age-related muscle atrophy was not accelerated in old TnIFastCreSod2(fl/fl) mice, suggesting mitochondrial oxidative stress may not be causal for age-related muscle atrophy.

Effect of long-term normobaric hyperoxia on oxidative stress in mitochondria of the guinea pig brain

Normobaric hyperoxia (NBO) is applied for treatment of various clinical conditions related to hypoxia, but it can potentially also induce generation of reactive oxygen species, causing cellular damage. In this study, we examined the effects of 60 h NBO treatment on lipid and protein oxidative damage and activity of superoxide dismutase (Mn-SOD) in brain mitochondria of guinea pigs. Despite significant stimulation of Mn-SOD expression and activity the NBO treatment resulted in accumulation of markers of oxidative lesions, including lipid peroxidation (conjugated dienes, thiobarbituric acid reactive substances) and protein modification (bityrosines, adducts with lipid peroxidation products, oxidized thiols). When inhaled O(2) was enriched with oxygen cation, O (2) (•+) , the Mn-SOD expression and activity were stimulated to similar extend, but lipid peroxidation and protein oxidation were prevented. These results suggest that long-term NBO treatment causes oxidative stress, but enrichment of inhaled oxygen by oxygen cation can protect the brain again adverse effects of hyperoxia.

The intracellular redox stress caused by hexavalent chromium is selective for proteins that have key roles in cell survival and thiol redox control

Hexavalent chromium [Cr(VI)] compounds (e.g. chromates) are strong oxidants that readily enter cells where they are reduced to reactive Cr intermediates that can directly oxidize some cell components and can promote the generation of reactive oxygen and nitrogen species. Inhalation is a major route of exposure which directly exposes the bronchial epithelium. Previous studies with non-cancerous human bronchial epithelial cells (BEAS-2B) demonstrated that Cr(VI) treatment results in the irreversible inhibition of thioredoxin reductase (TrxR) and the oxidation of thioredoxins (Trx) and peroxiredoxins (Prx). The mitochondrial Trx/Prx system is somewhat more sensitive to Cr(VI) than the cytosolic Trx/Prx system, and other redox-sensitive mitochondrial functions are subsequently affected including electron transport complexes I and II. Studies reported here show that Cr(VI) does not cause indiscriminant thiol oxidation, and that the Trx/Prx system is among the most sensitive of cellular protein thiols. Trx/Prx oxidation is not unique to BEAS-2B cells, as it was also observed in primary human bronchial epithelial cells. Increasing the intracellular levels of ascorbate, an endogenous Cr(VI) reductant, did not alter the effects on TrxR, Trx, or Prx. The peroxynitrite scavenger MnTBAP did not protect TrxR, Trx, Prx, or the electron transport chain from the effects of Cr(VI), implying that peroxynitrite is not required for these effects. Nitration of tyrosine residues of TrxR was not observed following Cr(VI) treatment, further ruling out peroxynitrite as a significant contributor to the irreversible inhibition of TrxR. Cr(VI) treatments that disrupt the TrxR/Trx/Prx system did not cause detectable mitochondrial DNA damage. Overall, the redox stress that results from Cr(VI) exposure shows selectivity for key proteins which are known to be important for redox signaling, antioxidant defense, and cell survival.