Mitochondrial calcium release induced by prooxidants

Peroxynitrite stimulates the pyridine nucleotide-linked Ca2+ release from intact rat liver mitochondria

Rat liver mitochondria contain a specific Ca2+ release pathway which operates when oxidized mitochondrial pyridine nucleotides are hydrolyzed in a Ca2+-dependent manner to ADP-ribose and nicotinamide. We have previously shown that NAD+ hydrolysis is inhibited by cyclosporin A and is possible only when some vicinal thiols are cross-linked. Here we report that the thiol oxidant peroxynitrite (ONOO-), which can form from nitric oxide (nitrogen monoxide, NO.) and superoxide anion (O2-), at low concentrations stimulates the specific Ca2+ release pathway. Both peroxynitrite-induced pyridine nucleotide hydrolysis and Ca2+ release are inhibited by cyclosporin A, and peroxynitrite is ineffective when pyridine nucleotides are kept reduced. Ca2+ release induced by peroxynitrite occurs with maintenance of the mitochondrial membrane potential and is not accompanied by entry of sucrose into mitochondria. The results suggest that peroxynitrite stimulates the specific Ca2+ release from intact mitochondria by modifying critical mitochondrial thiols other than glutathione in such a way that hydrolysis of oxidized pyridine nucleotides is achieved. These findings provide further insight into the regulation of Ca2+ release from mitochondria by nitric oxide and its congeners.

The permeability transition pore as a mitochondrial calcium release channel: a critical appraisal

Mitochondria from a variety of sources possess an inner membrane channel, the permeability transition pore. The pore is a voltage-dependent channel, activated by matrix Ca2+ and inhibited by matrix H+, which can be blocked by cyclosporin A, presumably after binding to mitochondrial cyclophilin. The physiological function of the permeability transition pore remains unknown. Here we evaluate its potential role as a fast Ca2+ release channel involved in mitochondrial and cellular Ca2+ homeostasis. We (i) discuss the theoretical and experimental reasons why mitochondria need a fast, inducible Ca2+ release channel; (ii) analyze the striking analogies between the mitochondrial permeability transition pore and the sarcoplasmic reticulum ryanodine receptor-Ca2+ release channel; (iii) argue that the permeability transition pore can act as a selective release channel for Ca2+ despite its apparent lack of selectivity for the transported species in vitro; and (iv) discuss the importance of mitochondria in cellular Ca2+ homeostasis, and how disruption of this function could impinge upon cell viability, particularly under conditions of oxidative stress.

Nuclear architecture and ultrastructural distribution of poly(ADP-ribosyl)transferase, a multifunctional enzyme

A monospecific autoimmune serum for poly(ADP-ribosyl)transferase (pADPRT) was used to localise the enzyme in ultrastructural cellular compartments. We detected enzyme in mitochondria of HeLa and Sertoli cells. Within the nucleoplasm the enzyme concentration was positively correlated with the degree of chromatin condensation, with interchromatin spaces being virtually free of pADPRT. During spermatogenesis we observed a gradual increase of the chromatin associated pADPRT that parallelled chromatin condensation. The highest concentration was seen in the late stages of sperm differentiation, indicating the existence of a storage form in transcriptionally inactive nuclei. In nucleoli pADPRT is accumulated in foci within the dense fibrillar component. Such foci are seen in close spatial relationship to sites of nucleolar transcription as revealed by high resolution immunodetection of bromouridine uptake sites. It is suggested that nucleolar pADPRT plays a role in preribosome processing via the modification of nucleolus specific proteins that bind to nascent transcripts and hence indirectly regulates polymerase I activity. The persisting binding of pADPRT to ribonucleoproteins may explain the observed disperse enzyme distribution at lower concentrations in the granular component. The fibrillar centres seem to contain no pADPRT. We conclude that known compounds of fibrillar centres like polymerase I are unlikely candidates for modification via direct covalent ADP-ribosylation.

Oxidation of pyridine nucleotides is an early event in the lethality of allyl alcohol

The involvement of altered pyridine nucleotide concentrations in the cytolethality of allyl alcohol was studied in isolated rat hepatocytes. NAD+, NADH, NADP+, NADPH and viability loss (leakage of lactate dehydrogenase into the medium) were measured in cells incubated with 0.5 mM allyl alcohol with or without the addition of 2 mM dithiothreitol at 30 min. Exposure to allyl alcohol increased NADH levels in the first 15 min of incubation. A sharp drop in NADH and NADPH with an accumulation of NADP+ occurred between 30 and 60 min of incubation with allyl alcohol, indicating an oxidation and interconversion of pyridine nucleotides. Dithiothreitol prevented the oxidation of pyridine nucleotides, but not their reduction or interconversion, and protected against cell killing by allyl alcohol. The results suggest that pyridine nucleotide oxidation might be important for allyl alcohol-induced cytotoxicity; however, a causal relationship between pyridine nucleotide oxidation and cell killing is yet to be demonstrated.

A detailed analysis of hydrogen peroxide-induced cell death in primary neuronal culture

A variety of neurodegenerative disease states have been associated with oxidative damage or stress. Such stress is thought to be mediated by excessive exposure of cells to reactive oxygen species such as free radicals, which can be generated following cell lysis, oxidative burst (as part of the immune response) or by the presence of an excess of free transition metals. Since the neuronal death observed in neurodegenerative diseases may be related to free radical damage, we were interested in developing a model system to investigate the mechanisms by which reactive oxygen species may damage or kill neurons. To this end, we have recently reported that brief exposure of cultured cortical neurons to H2O2 can induce neuronal death that proceeds via an apoptotic cell suicide pathway. The studies reported here investigate H2O2-induced cell death in more detail. Our data suggest that exposure of cultured cortical neurons to H2O2 can induce apoptotic cell death within 3 h, as assessed by cell viability, morphological and ultrastructural measures. In addition, experiments presented show that exposure to high concentrations of H2O2 (100 microM) causes increases in intracellular free calcium within 3 h, suggesting that increased intracellular calcium may be associated with some aspects of H2O2-induced cell death. However, at intermediate concentrations of H2O2 (30 microM), intracellular calcium remained stable during a 3 h exposure, during which time membrane blebbing was observed in ultrastructural studies. This suggests that some aspects of apoptotic cell death induced by H2O2 may not be associated with increased intracellular free calcium. Thus, this model appears valuable for studies of the mechanism(s) by which oxidative injury may induce apoptotic cell death and damage to neurons in the CNS.

Channel-specific induction of the cyclosporine A-sensitive mitochondrial permeability transition by menadione

It is well established that menadione, 2-methyl-1,4-naphthoquinone, impairs the ability of rat liver mitochondria to accumulate and retain calcium. However, it remains unclear whether this reflects inhibition of mitochondrial calcium uptake or stimulation of calcium release by menadione. The purpose of the current investigation was to determine whether interference with mitochondrial calcium homeostasis by menadione reflects a selective activation of the cyclosporine A-sensitive pore, independent of actions on other mitochondrial calcium channels. Mitochondrial calcium flux was monitored using the metallochromic dye arsenazo III. Treatment of mitochondria with menadione caused a concentration-dependent decrease in net calcium accumulation followed by a delayed release of the accumulated calcium and concurrent mitochondrial swelling. Both the maximum steady-state accumulation of calcium and the delay preceding calcium release decreased as a function of calcium concentration. The release of calcium did not occur via the Na+/Ca2+ antiport or reversal of the uptake uniport, as neither diltiazem nor ruthenium red prevented the menadione-stimulated calcium release. In contrast, cyclosporine A, a potent inhibitor of the permeability transition pore, completely inhibited menadione-induced calcium release and the associated swelling. Furthermore, the menadione-induced inhibition of calcium accumulation was completely prevented in the presence of cyclosporine A, indicating a selective stimulation of calcium release by menadione, rather than inhibition of calcium uptake. These data provide the first definitive description of a specific action of menadione to stimulate mitochondrial calcium release through a cyclosporine A-sensitive pathway, independent of altering the regulation of other recognized calcium channels associated with the inner mitochondrial membrane.

Influence of metabolic inhibitors on mitochondrial permeability transition and glutathione status

Treatment of isolated mitochondria with Ca2+ and inorganic phosphate (Pi) induces an inner membrane permeability that appears to be mediated through a cyclosporin A (CsA)-inhibitable Ca(2+)-dependent pore. Isolated mitochondria during inner membrane permeability undergo rapid efflux of matrix solutes such as glutathione as GSH and Ca2+, loss of coupled functions, and large amplitude swelling. Permeability transition without large amplitude swelling, a parameter often used to assess inner membrane permeability, has been observed. The addition of either oligomycin, antimycin, or sulfide to incubation buffer containing Ca2+ and Pi abolished large amplitude swelling of mitochondria. The GSH status during a Ca(2+)- and Pi-dependent mechanism of mitochondrial GSH release in isolated mitochondria was influenced significantly by metabolic inhibitors of the respiratory chain but did not prevent inner membrane permeability as demonstrated by the release of mitochondrial GSH and Ca2+. The release of GSH was inhibited by the addition of CsA, a potent inhibitor of permeability transition. Under these conditions we did not find GSSG; however, rapid oxidation of pyridine nucleotides and depletion of ATP and ADP with conversion to AMP occurred. The addition of CsA, prevented the oxidation of pyridine nucleotides and depletion of ATP and ADP. Since NADH and NADPH were extensively oxidized, protection against oxidative stress is reflected in maintenance of GSH and not observable lipid peroxidation. Evidence from transmission electron microscopy analysis, combined with the GSH release data, indicate that permeability transition can be observed in the absence of large amplitude swelling.

Inactivation of the poly(ADP-ribose) polymerase gene affects oxygen radical and nitric oxide toxicity in islet cells

Activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) is an early response of cells exposed to DNA-damaging compounds such as nitric oxide (NO) or reactive oxygen intermediates (ROI). Excessive poly-(ADP-ribose) formation by PARP has been assumed to deplete cellular NAD+ pools and to induce the death of several cell types, including the loss of insulin-producing islet cells in type I diabetes. In the present study we used cells from mice with a disrupted and thus inactivated PARP gene to provide direct evidence for a causal relationship between PARP activation, NAD+ depletion, and cell death. We found that mutant islet cells do not show NAD+ depletion after exposure to DNA-damaging radicals and are more resistant to the toxicity of both NO and ROI. These findings directly prove that PARP activation is responsible for most of the loss of NAD+ following such treatment. The ADP-ribosylation inhibitor 3-aminobenzamide partially protected islet cells with intact PARP gene but not mutant cells from lysis following either NO or ROI treatment. Hence the protective action of 3-aminobenzamide must be due to inhibition of PARP and does not result from its other pharmacological properties such as oxygen radical scavenging. Finally, the use of mutant cells an alternative pathway of cell death was discovered which does not require PARP activation and NAD+ depletion. In conclusion, the data prove the causal relationship of PARP activation and subsequent islet cell death and demonstrate the existence of an alternative pathway of cell death independent of PARP activation and NAD+ depletion.

Is the beneficial effect of calcium channel blockers against cyclosporine A toxicity related to a restoration of ATP synthesis?

ATP synthesis inhibited by Cyclosporine A is restored by calcium channel blockers: nifedipine, verapamil, bepridil, diltiazem. ATP synthesis was estimated using liver mitochondria by measuring the rate of respiration during state 3 and a measure of the yield of ATP synthesis, the P/O ratio. The study of calcium fluxes through mitochondrial membrane indicates that calcium channel blockers counteract the mitochondrial calcium storage induced by cyclosporine A. If the restoration of ATP synthesis observed in vitro also occurred in vivo, the increase in ATP pool might contribute to a better functioning of the Ca2+ extrusion pumps of the cells, thereby maintaining the cytosolic calcium concentration (Cai), in the normal range. The nephrotoxicity of cyclosporine A appears to be due to a vasoconstrictive effect related to an increased Cai. This result may account for the reduction of clinical cyclosporine A toxicity by calcium channel blockers. Verapamil appears to be the most efficient in restoring ATP synthesis.

Mitochondrial Ca2+ overload in primary cultures of rat renal cortical epithelial cells by cytotoxic concentrations of cyclosporine: a digitized fluorescence imaging study

Cyclosporine (CsA) has been reported to disrupt Ca2+ efflux from mitochondria, which suggests that CsA interference with Ca2+ homeostasis may be related to its nephrotoxicity. Therefore, the purpose of this study was (1) to determine intracellular free Ca2+ concentration ([Ca2+]i) and mitochondrial free Ca2+ concentration ([Ca2+]m) after primary cultures of rat renal cortical epithelial cells were exposed to cytotoxic concentrations of CsA; and (2) to explore the role of disruption of intracellular and mitochondrial Ca2+ homeostasis in CsA-induced cytotoxicity. [Ca2+]i in single kidney cells was examined by digitized fluorescence imaging (DFI) of the Ca2+ fluorescent probe, fura-2, and [Ca2+]m in single cells was observed by DFI of fura-2 entrapped in mitochondria after selective permeabilization of plasma membrane and other non-mitochondrial organelles by digitonin. Mitochondrial membrane potential (delta psi) in single kidney cells was examined by rhodamine 123 (Rh-123) with DFI. Intracellular ATP in kidney cells was determined by a HPLC method. CsA resulted in an elevation in [Ca2+]i and [Ca2+]m, dissipation of delta psi and depletion of ATP in a dose- and time-dependent manner. The elevation of [Ca2+]i and [Ca2+]m and depletion of ATP preceded CsA-induced cytotoxicity in kidney cells as measured by lactate dehydrogenase (LDH) leakage. We conclude that CsA-induced alterations in mitochondrial Ca2+ homeostasis and a subsequent loss of energy supply may play a key role in CsA-induced cytotoxicity in primary cultures of rat renal cortical epithelial cells.

Cyanide-induced lipid peroxidation in different organs: subcellular distribution and hydroperoxide generation in neuronal cells

To evaluate hydroperoxide generation as a potential mechanism of cyanide neurotoxicity, mice were treated with KCN (7 mg/kg, subcutaneously (s.c.)) and the level of lipid peroxidation (expressed as conjugated dienes) was measured later in various organs. Brain showed elevated conjugated diene levels after cyanide but the liver, which is not considered a target for cyanide toxicity, showed no increase. The heart also showed no increase, whereas kidney conjugated dienes slowly increased to a peak 1 h after cyanide. In vitro studies show elevation of peroxidized lipids in mouse brain cortical slices following incubation with KCN (0.1 mM). Omission of calcium from the medium or pretreatment of brain slices with diltiazem (a calcium channel blocker) prevented formation of conjugated dienes by KCN. Calcium thus appears to play a critical role in cyanide-induced generation of peroxidized lipids in neuronal cells. Subcellular fractionation of brains from mice treated with cyanide showed that lipid peroxidation increased in the microsomal fraction but not in the mitochondrial fraction. Fluorescent studies using 2,7-dichlorofluorescein (a hydroperoxide sensitive fluorescent dye) show that hydroperoxides are generated rapidly after cyanide treatment of PC12 cells, a neuron-like cell, and hydroperoxide levels remain elevated for many minutes in the presence of cyanide. These results suggest that hydroperoxide generation with subsequent peroxidation of lipids may lead to changes in structure and function of certain membranes and contribute to the neurotoxic damage produced by cyanide.