Effects of hydrogen peroxide and hypochlorite on membrane potential of mitochondria in situ in rat heart cells

Role of mitochondrial calcium in hypochlorite induced oxidative damage of cells

Hypochlorite (HOCl) is one of the most important mediators of inflammatory processes. Recent evidence demonstrates that changes in intracellular calcium pool play a significant role in the damaging effects of hypochlorite and other oxidants. Mitochondria are shown to be one of the intracellular targets of hypochlorite. But little is known about the mitochondrial calcium pool changes in HOCl-induced mitochondrial dysfunction. Using isolated rat liver mitochondria, we showed the oxidative damage of mitochondria (GSH oxidation and mixed protein-glutathione formation without membrane lipid peroxidation) and alterations in the mitochondrial functional parameters (decrease of respiratory activity and efficiency of oxidative phosphorylation, NADH and FADH coenzyme levels, and membrane potential) under hypochlorite action (50-300 μM). Simultaneously, the mitochondrial calcium release and swelling were demonstrated. In the presence of EGTA, the damaging effects of HOCl were less pronounced, reflecting direct involvement of mitochondrial Ca²⁺ in mechanisms of oxidant-induced injury. Furthermore, exposure of HeLa cells to hypochlorite resulted in a considerable increase in cytoplasmic calcium concentrations and a decrease in mitochondrial ones. Applying specific inhibitors of calcium transfer systems, we demonstrated that mitochondria play a key role in the redistribution of cytoplasmic Ca²⁺ ions under hypochlorite action and act as mediators of calcium release from the endoplasmic reticulum into the cytoplasm.

Is REDD1 a metabolic double agent? Lessons from physiology and pathology

The Akt/mechanistic target of rapamycin (mTOR) signaling pathway governs macromolecule synthesis, cell growth, and metabolism in response to nutrients and growth factors. Regulated in development and DNA damage response (REDD)1 is a conserved and ubiquitous protein, which is transiently induced in response to multiple stimuli. Acting like an endogenous inhibitor of the Akt/mTOR signaling pathway, REDD1 protein has been shown to regulate cell growth, mitochondrial function, oxidative stress, and apoptosis. Recent studies also indicate that timely REDD1 expression limits Akt/mTOR-dependent synthesis processes to spare energy during metabolic stresses, avoiding energy collapse and detrimental consequences. In contrast to this beneficial role for metabolic adaptation, REDD1 chronic expression appears involved in the pathogenesis of several diseases. Indeed, REDD1 expression is found as an early biomarker in many pathologies including inflammatory diseases, cancer, neurodegenerative disorders, depression, diabetes, and obesity. Moreover, prolonged REDD1 expression is associated with cell apoptosis, excessive reactive oxygen species (ROS) production, and inflammation activation leading to tissue damage. In this review, we decipher several mechanisms that make REDD1 a likely metabolic double agent depending on its duration of expression in different physiological and pathological contexts. We also discuss the role played by REDD1 in the cross talk between the Akt/mTOR signaling pathway and the energetic metabolism.

MitoLoc: A method for the simultaneous quantification of mitochondrial network morphology and membrane potential in single cells

Mitochondria assemble into flexible networks. Here we present a simple method for the simultaneous quantification of mitochondrial membrane potential and network morphology that is based on computational co-localisation analysis of differentially imported fluorescent marker proteins. Established in, but not restricted to, Saccharomyces cerevisiae, MitoLoc reproducibly measures changes in membrane potential induced by the uncoupling agent CCCP, by oxidative stress, in respiratory deficient cells, and in ∆fzo1, ∆ref2, and ∆dnm1 mutants that possess fission and fusion defects. In combination with super-resolution images, MitoLoc uses 3D reconstruction to calculate six geometrical classifiers which differentiate network morphologies in ∆fzo1, ∆ref2, and ∆dnm1 mutants, under oxidative stress and in cells lacking mtDNA, even when the network is fragmented to a similar extent. We find that mitochondrial fission and a decline in membrane potential do regularly, but not necessarily, co-occur. MitoLoc hence simplifies the measurement of mitochondrial membrane potential in parallel to detect morphological changes in mitochondrial networks. Marker plasmid open-source software as well as the mathematical procedures are made openly available.

•

MitoLoc is a workflow for measuring changes mitochondrial membrane potential and morphology in parallel.

•

Pixel-by pixel co-localisation of two fluorescent reporters is used to detect changes in mitochondrial membrane potential.

•

Automated 3D reconstruction of super resolution images to calculate mitochondrial morphological classifiers

•

We make fluorescent markers, mathematical procedures and software openly available.

•

We demonstrate the application of MitoLoc on several examples.

Comparative reactivity of myeloperoxidase-derived oxidants with mammalian cells

Myeloperoxidase is an important heme enzyme released by activated leukocytes that catalyzes the reaction of hydrogen peroxide with halide and pseudo-halide ions to form various hypohalous acids. Hypohalous acids are chemical oxidants that have potent antibacterial, antiviral, and antifungal properties and, as such, play key roles in the human immune system. However, increasing evidence supports an alternative role for myeloperoxidase-derived oxidants in the development of disease. Excessive production of hypohalous acids, particularly during chronic inflammation, leads to the initiation and accumulation of cellular damage that has been implicated in many human pathologies including atherosclerosis, neurodegenerative disease, lung disease, arthritis, inflammatory cancers, and kidney disease. This has sparked a significant interest in developing a greater understanding of the mechanisms involved in myeloperoxidase-derived oxidant-induced mammalian cell damage. This article reviews recent developments in our understanding of the cellular reactivity of hypochlorous acid, hypobromous acid, and hypothiocyanous acid, the major oxidants produced by myeloperoxidase under physiological conditions.

Evaluation of respiratory chain failure in mitochondrial cardiomyopathy by assessments of 99mTc-MIBI washout and 123I-BMIPP/99mTc-MIBI mismatch

Cardiomyopathy is one of the main features that determines prognosis in patients with mitochondrial encephalomyopathy. We investigated respiratory chain failure using 99mTc-MIBI- and 123I-BMIPP-SPECT in vivo in five patients with mitochondrial cardiomyopathty. With the lowering of cardiac function, the 99mTc-MIBI-washout rate (WOR) increased, and the 99mTc-MIBI-uptake decreased, conversely. In patients who showed severe cardiac involvement, 99mTc-MIBI-uptake was markedly reduced, and by contrast, 123I-BMIPP-uptake increased (123I-BMIPP/99mTc-MIBI mismatch). There were significant correlations between the WOR on 99mTc-MIBI-SPECT and interventricular septal thickness (IVST) on echocardiography and between WOR and left ventricular ejection fraction (LVEF) on 99mTc-MIBI-SPECT. The increased WOR and decreased uptake of 99mTc-MIBI were reflected by the lowered mitochondrial membrane potential created by mitochondrial respiratory chain whereas 123I-BMIPP/99mTc-MIBI mismatch may be created by the enhanced triglyceride-pool. These nuclear medicine techniques are the potential tools to evaluate the energy state in mitochondrial cardiomyopathy.

Noxious effects of oxygen reactive species on energy-coupling processes in Ehrlich ascites tumor mitochondria and the protection by pantothenic acid

Irradiation of Ehrlich ascites tumor cells with ultraviolet light or exposure to the Fenton reaction results in lesions in the mitochondrial energy-coupling system. Formation of the membrane potential and its utilization for ATP synthesis are more affected than the respiratory chain. Preincubation of the cells with pantothenic acid or its derivatives which can serve as precursors of CoA largely protects against the damage of mitochondrial energetics by oxygen reactive species formed by UV light or the Fenton reaction. Incubation of Ehrlich ascites tumor cells with pantothenic acid increases their content of glutathione (most of which is present in the reduced form) by 40%. It is concluded that the protective effect of precursors of CoA against lesions of the mitochondrial energy-coupling system by oxygen reactive species is mainly due to removal of free radicals and peroxides by glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase.

Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): nonperoxidative purine and pyrimidine nucleotide depletion

Hydrogen peroxide (H2O2) overload may contribute to cardiac ischemia-reperfusion injury. We report utilization of a previously described cardiomyocyte model (J. Cell. Physiol., 149:347, 1991) to assess the effect of H2O2-induced oxidative stress on heart-muscle purine and pyrimidine nucleotides and high-energy phosphates (ATP, phosphocreatine). Oxidative stress induced by bolus H2O2 elicited the loss of cardiomyocyte purine and pyrimidine nucleotides, leading to eventual de-energization upon total ATP and phosphocreatine depletion. The rate and extent of ATP and phosphocreatine loss were dependent on the degree of oxidative stress within the range of 50 microM to 1.0 mM H2O2. At the highest H2O2 concentration, 5 min was sufficient to elicit appreciable cardiomyocyte high-energy phosphate loss, the extent of which could be limited by prompt elimination of H2O2 from the culture medium. Only H2O2 dismutation completely prevented ATP loss during H2O2-induced oxidative stress, whereas various free-radical scavengers and metal chelators afforded no significant ATP preservation. Exogenously-supplied catabolic substrates and glycolytic or tricarboxylic acid-cycle intermediates did not ameliorate the observed ATP and phosphocreatine depletion, suggesting that cardiomyocyte de-energization during H2O2-induced oxidative stress reflected defects in substrate utilization/energy conservation. Compromise of cardiomyocyte nucleotide and phosphocreatine pools during H2O2-induced oxidative stress was completely dissociated from membrane peroxidative damage and maintenance of cell integrity. Cardiomyocyte de-energization in response to H2O2 overload may constitute a distinct nonperoxidative mode of injury by which cardiomyocyte energy balance could be chronically compromised in the post-ischemic heart.

Effects of hydrogen peroxide on mitochondrial enzyme function studied in situ in rat heart myocytes

Our previous work indicated that energy transduction, as measured by myocyte respiration, was inhibited by hydrogen peroxide, but the mitochondrial membrane potential was relatively unaffected. Therefore, we determined in the present study the critical steps in mitochondrial energy transduction by measuring the sensitivity to hydrogen peroxide of NADH-CoQ reductase, ATP synthase, and adenine nucleotide translocase in situ in myocytes. Adult rat heart cells were isolated using collagenase and incubated in the presence of 0.1-10 mM hydrogen peroxide for 30 min. Activities of NADH-CoQ reductase and oligomycin-sensitive ATP synthase were assayed enzymatically with sonicated myocytes, and adenine nucleotide translocase activities were determined by atractyloside-inhibitable [14C]ADP uptake of myocytes, permeabilized by saponin. The NADH-CoQ reductase and ATP synthase activities were inhibited to 77% and 67% of control, respectively, following an exposure to 10 mM hydrogen peroxide for 30 min. The adenine nucleotide translocase activities were inhibited in a concentration- and time-dependent manner and by 10 mM hydrogen peroxide to 44% of control. The dose-response relationship indicated that the translocase was the most susceptible to hydrogen peroxide among the three enzymes studied. Combined treatment of myocytes with 3-amino-1,2,4-triazole, 1,3-bis(2-chloroethyl)-1-nitrosourea and diethyl maleate (to inactivate catalase, to inhibit glutathione reductase activity, and to deplete glutathione, respectively) enhanced the sensitivity of translocase to hydrogen peroxide, supporting the view that the cellular defense mechanism is a significant factor in determining the toxicity of hydrogen peroxide. The results indicate that hydrogen peroxide can cause dysfunction in mitochondrial energy transduction, principally as the result of inhibition of adenine nucleotide translocase.

Role of cellular defense against hydrogen peroxide-induced inhibition of myocyte respiration

Hydrogen peroxide (H2O2) serves as a precursor for highly reactive oxygen intermediates. However, the respiratory function of myocytes is relatively resistant to exogenously administered H2O2. In this study, we examined whether or not the reduction of cellular defense increases the toxicity of H2O2. Rat heart myocytes were isolated by collagenase digestion. Respiratory rates of myocytes, suspended in a medium containing sucrose, 3-N-morpholino-propanesulfonic acid, EGTA and bovine serum albumin, were determined polarographically in the presence of pyruvate and malate with or without 2,4-dinitrophenol (DNP). Mitochondrial membrane potentials were measured by using [3H]triphenylmethylphosphonium+. Cellular defense was attenuated by i) inhibiting the catalase activity by 3-amino-1,2,4-triazole (AT), ii) reducing the glutathione concentration by diethyl maleate (DEM) or ethacrinic acid (EA), and iii) permeabilizing the sarcolemmal membrane by saponin. The dose-response relationship between H2O2 (0.1-5 mM) and mitochondrial membrane potential was not greatly affected by these experimental conditions. Myocyte respiration was inhibited by 5 mM H2O2, particularly that measured in the presence of DNP (48% of control). DEM treatment did not significantly affect the respiratory inhibition by H2O2, whereas the degree of inhibition was somewhat greater following EA or AT treatment. By contrast, the sensitivity of cellular respiration to H2O2 was potentiated approximately two orders of magnitude by the permeabilization of sarcolemmal membrane; thus, 100 microM H2O2 inhibited both DNP-stimulated and unstimulated respiration to 17% and 35% of control, respectively. The results indicate that factors existing in the sarcolemma and/or in the cytosol, which become ineffective and/or are diluted, respectively, following permeabilization with saponin, are important cellular defense mechanisms in alleviating the toxic effect of exogenous H2O2 on the respiration of mitochondria in situ in myocytes.