Mitochondrial glutathione status during Ca2+ ionophore-induced injury to isolated hepatocytes

Renal Glutathione: Dual roles as antioxidant protector and bioactivation promoter

The tripeptide glutathione (GSH) possesses two key structural features, namely the nucleophilic sulfur and the γ-glutamyl isopeptide bond. The former allows GSH to serve as a critical antioxidant and anti-electrophile. The latter allows GSH to translocate throughout the systemic circulation without being degraded. The kidneys exhibit several unique processes for handling GSH. This includes the extraction of 80% of plasma GSH, in part by glomerular filtration but mostly by transport across the basolateral plasma membrane. Studies on the protective effect of exogenous GSH are summarized, showing the different inherent susceptibility of proximal tubular and distal tubular cells and the impact on pathological or disease states, including hypoxia, diabetic nephropathy, and compensatory renal growth associated with uninephrectomy. Studies on mitochondrial GSH transport show the coordination between the citric acid cycle and oxidative phosphorylation in generating driving forces for both plasma membrane and mitochondrial carriers. The strong protective effects of increasing expression and activity of these carriers against oxidants and mitochondrial toxicants are summarized. Although GSH plays a cytoprotective role in most situations, two distinct exceptions to this are presented. In contrast to expectations, overexpression of the mitochondrial 2-oxoglutarate carrier markedly increased cell death from exposure to the nephrotoxic chemotherapeutic drug cisplatin (CDDP). Another key example of GSH serving a bioactivation role in the kidneys, rather than a detoxification role, is the metabolism of halogenated alkenes such as trichloroethylene (TCE). Although considerable research has gone into this topic, unanswered questions and emerging topics remain and are discussed.

Mitochondrial glutathione transport: physiological, pathological and toxicological implications

Although most cellular glutathione (GSH) is in the cytoplasm, a distinctly regulated pool is present in mitochondria. Inasmuch as GSH synthesis is primarily restricted to the cytoplasm, the mitochondrial pool must derive from transport of cytoplasmic GSH across the mitochondrial inner membrane. Early studies in liver mitochondria primarily focused on the relationship between GSH status and membrane permeability and energetics. Because GSH is an anion at physiological pH, this suggested that some of the organic anion carriers present in the inner membrane could function in GSH transport. Indeed, studies by Lash and colleagues in isolated mitochondria from rat kidney showed that most of the transport (>80%) in that tissue could be accounted for by function of the dicarboxylate carrier (DIC, Slc25a10) and the oxoglutarate carrier (OGC, Slc25a11), which mediate electroneutral exchange of dicarboxylates for inorganic phosphate and 2-oxoglutarate for other dicarboxylates, respectively. The identity and function of specific carrier proteins in other tissues is less certain, although the OGC is expressed in heart, liver, and brain and the DIC is expressed in liver and kidney. An additional carrier that transports 2-oxoglutarate, the oxodicarboxylate or oxoadipate carrier (ODC; Slc25a21), has been described in rat and human liver and its expression has a wide tissue distribution, although its potential function in GSH transport has not been investigated. Overexpression of the cDNA for the DIC and OGC in a renal proximal tubule-derived cell line, NRK-52E cells, showed that enhanced carrier expression and activity protects against oxidative stress and chemically induced apoptosis. This has implications for development of novel therapeutic approaches for treatment of human diseases and pathological states. Several conditions, such as alcoholic liver disease, cirrhosis or other chronic biliary obstructive diseases, and diabetic nephropathy, are associated with depletion or oxidation of the mitochondrial GSH pool in liver or kidney.

Hepatitis C virus core protein inhibits mitochondrial electron transport and increases reactive oxygen species (ROS) production

Hepatitis C infection causes a state of chronic oxidative stress, which may contribute to fibrosis and carcinogenesis in the liver. Previous studies have shown that expression of the HCV core protein in hepatoma cells depolarized mitochondria and increased reactive oxygen species (ROS) production, but the mechanisms of these effects are unknown. In this study we examined the properties of liver mitochondria from transgenic mice expressing HCV core protein, and from normal liver mitochondria incubated with recombinant core protein. Liver mitochondria from transgenic mice expressing the HCV proteins core, E1 and E2 demonstrated oxidation of the glutathione pool and a decrease in NADPH content. In addition, there was reduced activity of electron transport complex I, and increased ROS production from complex I substrates. There were no abnormalities observed in complex II or complex III function. Incubation of control mitochondria in vitro with recombinant core protein also caused glutathione oxidation, selective complex I inhibition, and increased ROS production. Proteinase K digestion of either transgenic mitochondria or control mitochondria incubated with core protein showed that core protein associates strongly with mitochondria, remains associated with the outer membrane, and is not taken up across the outer membrane. Core protein also increased Ca(2+) uptake into isolated mitochondria. These results suggest that interaction of core protein with mitochondria and subsequent oxidation of the glutathione pool and complex I inhibition may be an important cause of the oxidative stress seen in chronic hepatitis C.

Role of Ca2+ in radiation-induced damage in murine splenocytes

PURPOSE

To address the links between calcium, peroxidation, cell damage and death and the response of the enzymes involved in free radical metabolism, in splenocytes of mice irradiated with gamma-rays.

MATERIALS AND METHODS

Splenocytes of Swiss albino mice were irradiated with various doses (0-7 Gy) of gamma-rays (60Co) at a dose-rate of 0.0575 Gy s(-1). Membrane peroxidation and fluidity were determined by the thiobarbituric acid-reactive substances (TBARS) method, and fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), respectively. Apoptosis was analysed by nucleosomal ladder formation and activity of NF-kappaB by electrophoretic mobility shift assay (EMSA). The specific activities of the antioxidant enzymes, lactate dehydrogenase (LDH), levels of nitric oxide (NO*) and glutathione were determined spectrophotometrically. Modulatory effects of Ca2+ were examined at 3 Gy using different concentrations (1, 3 and 5 mM) in the presence or absence of the ionophore A23187.

RESULTS

Irradiation of splenocytes resulted in enhanced peroxidative damage. membrane fluidity, apoptosis and DNA binding activity of NF-kappaB. The specific activities of LDH and antioxidant enzymes superoxide dismutase (SOD), DT-diaphorase (DTD), glutathione S-transferase (GST) and levels of glutathione (GSH) and NO* were increased with radiation dose up to 4Gy. Ca2+ augmented the radiation-induced responses. The presence of ionophore A23187 potentiated the modulatory effects of Ca2+.

CONCLUSIONS

These findings show that Ca2+ augments radiation damage and is more effective intracellularly. Ca2+, peroxidation, cellular damage and apoptosis are possibly interlinked through signals, as is evident from the increased activity of NF-kappaB and generation of NO*. The enhanced antioxidant status suggests an attempt made by the irradiated cells to maintain their normal functions.

Metabolic responses to epinephrine stimulation in goldfish hepatocytes: evidence for the presence of alpha-adrenoceptors

The effect of epinephrine on various aspects of cellular metabolism was studied in hepatocytes from the goldfish Carassius auratus. Epinephrine increased cytosolic free calcium ([Ca2+](i)) from a baseline value of 108 +/- 22 nM to a peak value of 577 +/- 127 nM in suspensions of hepatocytes. Responses of single cells ranged from a single spike (66% of hepatocytes) to variable oscillatory patterns (34%). The increase in [Ca(2+)](i) was independent of the presence of extracellular Ca2+ and was prevented by the alpha-adrenergic antagonist phentolamine. Cellular glucose release induced by epinephrine (1.7- to 3.2-fold) was significantly reduced in Ca2+-depleted cells and in the presence of phentolamine, providing evidence for the co-occurrence of alpha-adrenoceptors and a Ca2+-independent, presumably beta-adrenergic, system in these cells. Furthermore, epinephrine stimulated oxygen consumption in a Ca2+-dependent manner, which was not due to stimulated Na(+) pump activity. An increased rate of acid secretion of 50%, evoked by epinephrine, appears to be mediated by enhanced Na(+)/H(+) exchange but did not result in intracellular alkalization.

Biochemical changes in transplanted rat liver stored in University of Wisconsin and Euro-Collins solutions

BACKGROUND

Liver ischemia/reperfusion injury is a severe problem in transplantation, and preservation solutions could be critical for liver viability. The aim of our study was to evaluate the cytosolic and mitochondrial glutathione levels, the glyoxalase II activity, and the mitochondrial hydroperoxide contents of livers stored in different preservation solutions for 7 or 24 h and after transplantation.

MATERIALS AND METHODS

Orthotopic liver transplantation was performed without reconstruction of the hepatic artery. The livers were stored at 4 degrees C for 7 or 24 h in University of Wisconsin or Euro-Collins solutions. Portions of livers before and after transplantation were homogenized and mitochondria isolated.

RESULTS

Cytosolic glutathione levels were decreased in all stored livers and after transplantation. In livers stored with University of Wisconsin solution, mitochondrial glutathione was unchanged during cold storage and no significant decrease has been found after reperfusion, whereas in livers stored in Euro-Collins solution, mitochondrial glutathione was decreased and a further significant decrease was found 30 min after reperfusion. Mitochondrial hydroperoxides were higher in livers stored in Euro-Collins solution than in University of Wisconsin solution after 30 min of reperfusion. Mitochondrial glyoxalase II did not show any change by reperfusion.

CONCLUSION

We have demonstrated that rat liver stored in Euro-Collins solution suffered a severe depletion of mitochondrial GSH and a concomitant increase of hydroperoxides. The data obtained suggested that the livers stored with University of Wisconsin solution were probably less prone to ischemia/reperfusion injury after liver transplantation.

Role of voltage-dependent anion channels in glutathione transport into yeast mitochondria

Glutathione (GSH) is imported into mitochondria from the extra-mitochondrial cytoplasm. Translocation across the inner membrane of mitochondria is thought to occur via the dicarboxylate and 2-oxoglutarate carriers; however, the means by which GSH passes through the outer membrane is unknown. Disruption of the outer membrane of yeast mitochondria using either digitonin or osmotic shock did not alter GSH accumulation as compared with accumulation in intact mitochondria. These results suggested that passage across the outer membrane was not the rate-limiting step in GSH accumulation. Mitochondria isolated from yeast strains with a disruption in the major pore-forming protein of the outer membrane, VDAC1, accumulated GSH to a greater extent than mitochondria isolated from a wild-type strain. Disruption of the gene for VDAC2 did not affect GSH import. Thus, neither VDAC form is essential for GSH translocation into mitochondria, and the participation of another outer membrane channel in GSH import is possible.

Mitochondrial glutathione and oxidative stress: implications for pulmonary oxygen toxicity in premature infants

Administration of supplemental oxygen, despite being an important clinical therapy, can cause significant lung damage. Because they have underdeveloped lungs, prematurely born human infants frequently require supportive therapies that employ elevated oxygen concentrations, which put them at risk for developing pulmonary oxygen toxicity. This risk is made even greater by the immaturity of their cellular antioxidant defenses. Although the exact mechanisms of oxygen toxicity are still not fully defined, cellular damage is probably mediated by increased production of chemically reactive oxygen species (ROS) in the mitochondria. Cellular protection against ROS is provided by a variety of antioxidant molecules and enzymes, including the glutathione (GSH)-dependent antioxidant system. The GSH-dependent antioxidant enzyme system provides vital cellular protection against ROS, particularly hydrogen peroxide and certain organic hydroperoxides, under pathological and toxicological conditions, by using selenium-dependent and -independent peroxidases to reduce hydrogen peroxide or lipid peroxides to water or the respective alcohols, with the concurrent oxidation of GSH to glutathione disulfide (GSSG). In the mitochondria, limitations of GSH synthesis and transmembrane transport suggest that optimal functioning of the mitochondrial GSH system, and maintenance of adequate thiol-disulfide redox tone is essential to protect against the injurious effects of ROS. Manipulation of endogenous GSH concentrations can alter cellular responses to oxidant injury. Beneficial effects are evident when intracellular GSH concentrations are increased. In conditions that increase mitochondrial production of ROS, such as exposure to high concentrations of oxygen, therapies based on enhancing mitochondrial GSH concentrations could be highly beneficial.

Glutathione and ultrastructural changes in inflow occlusion of rat liver

BACKGROUND

Liver ischemia/reperfusion is frequently associated with organ injury to which reactive oxygen species contribute. The aim of our study was to evaluate cytosolic and mitochondrial glutathione levels and morphological changes in hepatocytes of rat liver in an experimental model of ischemia/reperfusion.

MATERIALS AND METHODS

The experimental procedure consisted of temporary interruption of blood flow to the left lateral and medial hepatic lobes for different lengths of time and, in some cases, subsequent reperfusion. Cytosolic and mitochondrial glutathione levels were evaluated and ultrastructural analysis was carried out for all samples.

RESULTS

Ischemic lobes showed ultrastructural changes in relationship with the increase in ischemia time. Total glutathione levels did not show variations in ischemic lobes and sham lobes with respect to control rats during ischemia only. Instead, during reperfusion, significant ultrastructural alterations of the hepatocytes and a significant depletion of glutatione in cytosolic and mitochondrial compartments were evident. The sham lobes also showed a significant glutathione decrement. Increased oxidized glutathione (GSSG) levels were found during ischemia both in ischemic lobes and in sham lobes. During reperfusion GSSG was found to a minor extent, in the cytosolic compartment. In mitochondria GSSG levels were also high during reperfusion.

CONCLUSIONS

We conclude that depletion of glutathione contributes to impaired liver after reperfusion following ischemia but depletion of glutathione alone does not induce changes in the morphology of the hepatocytes. Glutathione depletion and a greater quantity of GSSG, even in sham lobes, may indicate a metabolic alteration which spreads to compartments that are not involved in ischemia/reperfusion.

Enrichment and functional reconstitution of glutathione transport activity from rabbit kidney mitochondria: further evidence for the role of the dicarboxylate and 2-oxoglutarate carriers in mitochondrial glutathione transport

In previous studies, we provided evidence for uptake of glutathione (GSH) by the dicarboxylate and the 2-oxoglutarate carriers in rat kidney mitochondria. To investigate further the role of these two carriers, GSH transport activity was enriched from rabbit kidney mitochondria and functionally reconstituted into phospholipid vesicles. Starting with 200 mg of mitoplast protein, 2 mg of partially enriched proteins were obtained after Triton X-114 solubilization and hydroxyapatite chromatography. The reconstituted proteoliposomes catalyzed butylmalonate-sensitive uptake of [(14)C]malonate, phenylsuccinate-sensitive uptake of [(14)C]2-oxoglutarate, and transport activity with [(3)H]GSH. The initial rate of uptake of 5 mM GSH was approximately 170 nmol/min per mg protein, with a first-order rate constant of 0.3 min(-1), which is very close to that previously determined in freshly isolated rat kidney mitochondria. The enrichment procedure resulted in an approximately 60-fold increase in the specific activity of GSH transport. Substrates and inhibitors for the dicarboxylate and the 2-oxoglutarate carriers (i.e., malate, malonate, 2-oxoglutarate, butylmalonate, phenylsuccinate) significantly inhibited the uptake of [(3)H]GSH, whereas most substrates for the tricarboxylate and monocarboxylate carriers had no effect. GSH uptake exhibited an apparent K(m) of 2.8 mM and a V(max) of 260 nmol/min per mg protein. Analysis of mutual inhibition between GSH and the dicarboxylates suggested that the dicarboxylate carrier contributes a somewhat higher proportion to overall GSH uptake and that both carriers account for 70 to 80% of total GSH uptake. These results provide further evidence for the function of the dicarboxylate and 2-oxoglutarate carriers in the mitochondrial transport of GSH.

Energetics of trout hepatocytes during A23187-induced disruption of Ca2+ homeostasis

The impact of an increase of intracellular Ca2+ i on the energy metabolism of trout hepatocytes was assessed by applying the Ca2+ ionophore A23187 and studying the consequences of the ensuing elevation of Ca2+ i on various metabolic parameters. After application of A23187 no loss of viability occurred for 2 h, but glutathione content decreased by 46%. A concomitant decrease of [ATP] as well as of Na,K-ATPase activity by over 50% could be prevented by incubating the cells in a Ca2+-free medium. Upon addition of the ionophore cellular oxygen consumption more than doubled in a strictly Ca2+-dependent manner, with half of this increase being sensitive to ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter. This increase in oxygen consumption was transient in nature and at its peak it was similar in magnitude to that induced by 2,4-dinitrophenol. Similarly, oxygen consumption sensitive to the protein synthesis inhibitor cycloheximide was transiently increased by A23187, but returned to control levels within 30 min of incubation. These results suggest that elevation of intracellular Ca2+ leads to an energetic imbalance not related to stimulation of ATP consuming processes, but mainly due to impairment of mitochondrial function, possibly by the decoupling of oxidative phosphorylation and by inducing dissipative Ca2+ cycling.

Oxidative injury induced by synthetic humic acid polymer and monomer in cultured rabbit articular chondrocytes

Humic substance has been proposed as one of the causative factors of Kashin-Beck disease (KBD), an endemic osteoarthritic disorder with necrosis of chondrocytes widely prevalent in some regions of China. In order to exclude the complications of natural humic substance, here we prepared phenolic polymers of synthetic humic acid (SHA) by oxidation of phenolic monomer, the protocatechuic acid (PCA). The biological effects of SHA and PCA on primary culture of rabbit articular chondrocytes were investigated. We found that not only SHA but also PCA caused chondrocyte injury, as evidenced by the loss of cell viability measured with methylthiazol tetrazolium (MTT) assay and the increased release of intracellular lactate dehydrogenase (LDH). Both SHA and PCA could result in lipid peroxidation and glutathione (GSH) depletion in chondrocytes, indicating that oxidative stress may be involved in chondrocyte injury. Furthermore, a marked increase in intracellular calcium level ([Ca2+]i) occurred after chondrocytes treated with SHA or PCA. These results suggest that chondrocyte injury elicited by SHA or PCA may be mediated through the occurrence of oxidative stress and the disruption of intracellular Ca2+ homeostasis. Data also suggest that the monomeric phenolic acid may be considered one of the causative factors of KBD in addition to humic substance.

Glutathione depletion and oxidative damage in mitochondria following exposure to cadmium in rat liver and kidney

The dose-dependent effects of cadmium (Cd) on mitochondria and post-mitochondrial supernatant (PMS) of liver and kidney were investigated in adult male albino rats. Two groups of rats were injected intraperitoneally with 0.1 mg Cd/kg body weight and 1 mg/kg body weight, respectively, for a period of 3 months (5 days/week). This resulted in a significant decrease in total glutathione (GSH) levels, irrespective of the doses, in mitochondrial as well as in PMS fractions of liver and kidney. In contrast, end products of lipid and protein were significantly increased in a dose-dependent manner in subcellular fractions of liver and kidney. These results suggest that the depletion of tissue glutathione levels is not a primary reason of the observed oxidative damage in liver and kidney caused by Cd.

The mitochondrial permeability transition mediates both necrotic and apoptotic death of hepatocytes exposed to Br-A23187

A23187 and related Ca2+ ionophores are widely used to study Ca2+-dependent cell injury. Here, using laser scanning confocal microscopy and parameter-indicating fluorophores, we investigated the role of the mitochondrial permeability transition (MPT) in Br-A23187 toxicity to cultured rat hepatocytes. After 10 microM Br-A23187, over 60% of hepatocytes lost viability within 1 h. This necrotic cell killing was preceded by increased mitochondrial free Ca2+, mitochondrial depolarization, and onset of the MPT. Cyclosporin A (CsA), a blocker of the permeability transition pore, prevented the MPT and cell killing but had no effect on increased mitochondrial free Ca2+ and depolarization after Br-A23187. To determine whether Br-A23187-induced cell killing was linked to loss of cellular ATP supply, hepatocytes were incubated with fructose and oligomycin, a source of glycolytic ATP and an inhibitor of the uncoupler-stimulated mitochondrial ATPase, respectively. Fructose plus oligomycin prevented cell killing after Br-A23187 but not the MPT. When fructose plus oligomycin prevented necrotic cell killing, apoptosis developed after 10 h. When cells were treated additionally with CsA, these apoptotic changes were prevented. In conclusion, the MPT mediates Br-A23187 cytotoxicity. Acutely, the MPT causes mitochondrial uncoupling and profound ATP depletion, which leads to necrotic cell death. However, when glycolytic ATP generation is available, the MPT induces apoptosis. CsA blocks the MPT and prevents both necrotic and apoptotic cell killing after Br-A23187.

Loss of K+ homeostasis in trout hepatocytes during chemical anoxia: a screening study for potential causes and mechanisms

In isolated trout hepatocytes intoxication with CN- (chemical anoxia) leads to a rapid breakdown of K+ homeostasis. In the present study an attempt has been made to identify the causes and mechanisms underlying this phenomenon. Our results indicate that neither Ca2+ elevation nor cell swelling, both of which occurred during chemical anoxia and could be prevented by exposure to Ca2+ chelating agents or to hyperosmotic conditions, respectively, is solely responsible for the breakdown of K+ homeostasis. From a number of inhibitors of dissipative K+ fluxes tested, only BaCl2, an inhibitor of voltage-gated K+ channels, proved to be effective in significantly reducing K+ efflux during chemical anoxia. The KCl cotransporter known to be involved in regulatory volume decrease after hypoosmotic shock does not seem to be activated during CN(-)-induced cell swelling.

Toxicity of fotemustine in rat hepatocytes and mechanism-based protection against it

Fotemustine is a relatively novel DNA-alkylating 2-chloroethyl-substituted N-nitrosourea (CENU) drug, clinically used for the treatment of disseminated malignant melanoma in different visceral and non-visceral tissues. Thrombocytopenia has been observed in patients treated with fotemustine and liver and renal toxicities as well. In this study, firstly the metabolism of fotemustine was investigated in vitro and secondly the undesired cytotoxicity of fotemustine as well as different ways of protection against it. In rat hepatocytes, chosen as a model system, fotemustine was shown to cause lactate dehydrogenase (LDH) leakage, glutathione (GSH) depletion, GSSG-formation and lipid peroxidation (LPO). A reactive metabolite, DEP-isocyanate, is most likely responsible for these undesired cytotoxic effects. Based on the observed cytotoxicity mechanisms, chemoprotection with several sulfhydryl-containing nucleophiles and antioxidants was investigated. The sulfhydryl nucleophiles; GSH, N-acetyl-L-cysteine (NAC) and glutathione isopropylester (GSH-IP) protected almost completely against fotemustine-induced LDH-leakage and LPO. NAC and GSH protected partly against fotemustine-induced GSH-depletion. The antioxidant, vitamin E protected completely against fotemustine-induced LPO, but only partly against fotemustine-induced LDH-leakage and not against GSH-depletion. Ebselen, a peroxidase-mimetic organoselenium compound, did not show protective effects against the cytotoxicity of fotemustine, possibly because GSH is required for the bioactivation of ebselen. It is concluded that co-administration of sulfhydryl nucleophiles, in particular NAC and GSH-IP, possibly in combination with antioxidants, such as vitamin E, are effective against the toxicity of fotemustine in vitro. It might, therefore, be worthwhile to investigate the cytoprotective potency of these agents against undesired toxicities of fotemustine in vivo as well.

Apoptotic cell death and CPP32-like activation induced by thapsigargin and their prevention by nerve growth factor in PC12 cells

Thapsigargin, an endoplasmic reticular Ca2+-ATPase inhibitor, induced apoptotic cell death (chromatin condensation and DNA fragmentation) accompanied by the activation of CPP32-like protease, a member of the interleukin-1beta converting enzyme protease (ICE) family, but not the activation of ICE-like protease. Nerve growth factor (NGF) completely inhibited the cell death and CPP32-like activation induced by thapsigargin while Ac-Asp-Glu-Val-Asp-CHO, an inhibitor of CPP32-like protease, reduced the cell death. PD98059, a specific inhibitor of Map kinase kinase, did not reduce the protective effect of NGF on thapsigargin-induced cell death. These results suggest that calcium ion-induced apoptotic cell death was mediated by CPP32-like, but not ICE-like, protease and was regulated by a neurotrophic factor possibly, through the Map kinase cascade independent pathway.

Effects of antioxidants and Ca2+ in cisplatin-induced cell injury in rabbit renal cortical slices

Effects of antioxidants, reactive oxygen species (ROS) scavengers, and Ca2+ on cisplatin-induced renal cell injury were studied in rabbit renal cortical slices in vitro. Cisplatin induced LDH release and lipid peroxidation, inhibition of PAH uptake, and GSH depletion. These changes were significantly prevented by thiols (DTT and GSH), antioxidants (DPPD and BHA), and an iron chelator (deferoxamine). Superoxide dismutase partially reduced the cisplatin-induced LDH release without affecting the lipid peroxidation and the GSH depletion. Catalase did not affect the LDH release and the lipid peroxidation induced by cisplatin. Hydroxyl radical scavengers prevented the lipid peroxidation, whereas they did not alter the LDH release, the inhibition of PAH uptake, and the GSH depletion induced by cisplatin. Removal of Ca2+ or addition of EGTA to the incubation medium did not alter cisplatin effects on LDH release and lipid peroxidation. Buffering intracellular Ca2+ with quin-2/AM or inhibition of intracellular Ca2+ release with TMB-8 significantly reduced the cisplatin effect on LDH release without any effect on the lipid peroxidation and the GSH depletion. Ruthenium red attenuated the LDH release, the lipid peroxidation, and the inhibition of PAH uptake mediated by cisplatin. La3+ prevented the cisplatin effect on the LDH release, whereas it did not affect the lipid peroxidation, the inhibition of PAH uptake, and the GSH depletion by cisplatin. These results suggest that cisplatin induces a lethal cell injury by lipid peroxidation-dependent and -independent mechanisms and that the cell injury and the lipid peroxidation by cisplatin are iron-dependent. In addition, the data indicate that the Ca2+ released from intracellular stores, but not the Ca2+ moved from extracellular space, plays a role in the cisplatin-induced cell injury independent of lipid peroxidation.

Depletion of glutathione by benzo(a)pyrene metabolites, ionomycin, thapsigargin, and phorbol myristate in human peripheral blood mononuclear cells

Previous studies in this laboratory have shown that polycyclic aromatic hydrocarbons (PAHs) alter Ca2+ homeostasis and inhibit activation of both B and T lymphocytes obtained from rodents and humans. In the present studies, we demonstrate that alpha-naphthoflavone (ANF), an inhibitor of cytochrome P4501A activity, reduced the Ca2+ elevation produced by BaP in human peripheral blood mononuclear cell (HPBMC) lymphocytes. These results suggested that BaP metabolites may play a role in intracellular Ca2+ homeostasis in human lymphocytes. Reactive oxidative intermediates of BaP produced in HPMBC are known to be highly carcinogenic and have also been shown to be immunosuppressive. We examined the effects of benzo(a)pyrene (BaP), 7,12-dimethylbenz(a)anthracene (DMBA), benzo(e)pyrene (BeP), and anthracene, as well as certain BaP metabolites, on the levels of intracellular Ca2+ and glutathione in HPBMC. While BaP, DMBA, BeP, and anthracene did not cause a statistically significant decrease in GSH in HPBMC at concentrations of 1 or 10 microM following a 6-, 48-, or 72-hr exposure, reactive BaP metabolites including 4,5-epoxide BaP and 7,8-diol-9,10-epoxide BaP consistently produced a 20-30% depletion of glutathione in HPBMC following a 6-hr treatment period. These BaP metabolites also elevated intracellular Ca2+ in HPBMC during a 6-hr incubation. Results of these experiments suggest that metabolism of BaP to certain epoxide metabolites may be responsible for sulfhydryl damage leading to transient GSH depletion and Ca2+ elevation. These results are consistent with the hypothesis that sulfhydryl damage by certain PAH metabolites may lead to altered Ca2+ homeostasis, leading to inhibition of cell activation and proliferation in HPBMC.

Role of cellular thiol status in tocopheryl hemisuccinate cytoprotection against ethyl methanesulfonate-induced toxicity

Suspensions of rat hepatocytes treated with the alkylating agent ethyl methanesulfonate (EMS) exhibited extensive lipid peroxidation as well as rapid and near complete depletion of cellular reduced glutathione (GSH) levels prior to cell death. Pretreatment of hepatocytes with medium deficient in sulfur amino acids accelerated cell death induced by EMS, confirming the previously reported cytoprotective role for GSH in this toxic event. Nearly all of the cellular GSH lost following 50 mM EMS treatment was accounted for as S-ethyl glutathione (GS-Et). No significant formation of glutathione disulfide was observed. The GS-Et formed was not exported from the cell but remained at high intracellular concentrations throughout the course of the experiment. In addition, EMS treatment inhibited the efflux of intracellular GSH and inhibited the cellular accumulation of glutamate (Glu). Supplementation of hepatocytes with 25 microM d-alpha-tocopheryl hemisuccinate (TS) protected these cells against EMS-induced lipid peroxidation and cell death. Cytoprotection with TS had no effect on EMS-induced depletion of intracellular GSH or intracellular levels of GS-Et or Glu. However, TS supplementation did prevent EMS-induced depletion of cellular protein thiols. Interestingly, the pretreatment of hepatocytes with 1 mM dithiothreitol promoted EMS toxicity. The results of this study suggest that the cytoprotective abilities of TS are related to the prevention of both EMS-induced lipid peroxidation and protein thiol depletion. Thus, the onset of lipid peroxidation and the loss of protein thiols in hepatocytes appear to be critical cellular events leading to EMS-induced cell death.

Thiols metabolism is altered by the herbicides paraquat, dinoseb and 2,4-D: a study in isolated hepatocytes

This report is an extension and complement of a previous study reporting the effect of three herbicides (paraquat, dinoseb and 2,4-D) on cell viability, GSH oxidation, NADH and ATP depletion (Arch. Toxicol. 68:24-31, 1994). Here we report additional data and findings aimed at a better understanding of the toxicity mechanisms induced by these herbicides. Biochemical mechanisms of cytotoxicity induced by the herbicides paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride), dinoseb (2-sec-butyl-4,6-dinitrophenol) and 2,4-D (2,4-dichlorophenoxyacetic acid) were investigated in freshly isolated rat hepatocytes. Herbicide metabolism, especially paraquat and 2,4-D, rapidly depletes GSH and protein thiols. Paraquat and 2,4-D (1-10 mM) decrease the GSH/GSSG ratio, promote loss of protein thiol contents and induce lipid peroxidation. Dinoseb, the most effective cytotoxic compound under study (used in concentrations 1000-fold lower than paraquat and 2,4-D), had moderate effects upon the GSH/GSSG ratio and lipid peroxidation, causing a depletion of protein thiols of about 20%. The results indicate that the herbicides paraquat and 2,4-D are hepatotoxic and may induce cell death by decreasing cellular GSH/GSSG ratio and protein thiols, and by inducing lipid peroxidation. The cytotoxic action of dinoseb is likely to be related with the uncoupling of oxidative phosphorylation in mitochondria. Therefore, it is likely that liver damage observed during the first phase of herbicide-intoxication is related to these metabolic processes.

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.

Effects of extracellular Ca2+ and HCO3- on epidermal growth factor-induced DNA synthesis in cultured rat hepatocytes

BACKGROUND/AIMS

The elevation of cytosolic free calcium concentration ([Ca2+]i) and intracellular pH mediate the growth factor-initiated proliferation of many cells, but it is not known if they trigger mitosis in resting hepatocytes. The maintenance of [Ca2+]i and intracellular pH depends partly on extracellular calcium concentration ([Ca2+]e) and extracellular bicarbonate concentration ([HCO3-]e). Therefore, the effects of [Ca2+]e and [HCO3-]e on hepatocyte proliferation were examined.

METHODS

Epidermal growth factor induced proliferation in primary cultures of rat hepatocytes. [3H]thymidine incorporation into DNA and nuclear labeling indices were measured.

RESULTS

Between 0.2 and 0.9 mmol/L of [Ca2+]e, the proliferative response to epidermal growth factor increased, and total hepatocellular Ca2+ content was increased. Increasing [HCO3-]e also stimulated DNA synthesis in a concentration-dependent manner, maximal at 35 mmol/L. Using optimal [Ca2+]e (0.9 mmol/L) and [HCO3-]e (35 mmol/L), a synergistic stimulation of hepatocellular DNA synthesis was shown. Voltage-dependent Ca2+ channel blockers failed to inhibit hepatocyte proliferation when administered in concentrations that inhibit proliferation in other cell types.

CONCLUSIONS

[Ca2+]e and [HCO3-]e are both essential for hepatocyte proliferation, and their effects are synergistic. The entry of extracellular Ca2+ is critical for epidermal growth factor-induced DNA synthesis in hepatocytes, but this is not mediated by voltage-dependent Ca2+ channels.

Early cellular events couple covalent binding of reactive metabolites to cell killing by nephrotoxic cysteine conjugates

Addition of the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), to the LLC-PK1 line of renal epithelial cells leads to covalent binding of reactive intermediates followed by thiol depletion, lipid peroxidation, and cell death (Chen et al., 1990, J. Biol. Chem., 265:21603-21611). The present study was designed to determine if increased intracellular free calcium might play a role in this pathway of DCVC-induced toxicity by comparing the temporal relationships among increased intracellular free calcium, lipid peroxidation, and cytotoxicity. Intracellular free calcium increased 1 hr after DCVC treatment, long before LDH release occurred. The elevation of intracellular free calcium and cytotoxicity was prevented by inhibiting DCVC metabolism with AOA. The cell-permeable chelators, Quin-2AM and EGTA-AM, prevented the toxicity. Pretreatment of cells with a nontoxic concentration of ionomycin increased intracellular free calcium and potentiated DCVC-induced LDH release. However, the antioxidant, DPPD, which blocks lipid peroxidation and toxicity, did not affect the increase in intracellular free calcium, whereas buffering intracellular calcium with Quin-2AM or EGTA-AM blocked both lipid peroxidation and toxicity without preventing the depletion of nonprotein sulfhydryls by DCVC. Ruthenium red, an inhibitor of mitochondrial calcium uptake, also blocked cell death. We hypothesize that covalent binding of the reactive fragment from DCVC metabolism leads to deregulation of intracellular calcium homeostasis and elevation of intracellular free calcium. Increased intracellular free calcium may in turn be coupled to mitochondrial damage and the accumulation of endogenous oxidants which cause lipid peroxidation and cell death.

Cisplatin nephrotoxicity: decreases in mitochondrial protein sulphydryl concentration and calcium uptake by mitochondria from rat renal cortical slices

The effects of cisplatin on several aspects of the function of mitochondria isolated from the rat renal cortex have been investigated in vitro. Incubation of renal cortical slices with cisplatin (2 mM) caused a rapid loss of mitochondrial protein-SH followed by a substantial decrease in Ca2+ uptake by the mitochondria and a decline in the mitochondrial membrane potential, which was assessed by rhodamine 123 uptake by the slices. Dithiothreitol, a glutathione (GSH)-reducing agent, significantly reversed the alterations in protein-SH, Ca2+ accumulation and rhodamine 123 uptake. There was also a marked amelioration of cisplatin-induced cytotoxicity, as shown by the decreased leakage of several enzymes from the slices. Diethylmaleate, a GSH depletor, enhanced both the cisplatin-induced increase in toxicity, as assessed by enzyme leakage, and also the decreases in protein-SH, Ca2+ accumulation and rhodamine 123 uptake. The antioxidant N,N'-diphenylphenylenediamine substantially alleviated cisplatin toxicity but did not protect against cisplatin-induced alterations to protein-SH and Ca2+ uptake. In addition, the cytotoxicity caused by cisplatin was not affected by cyclosporin A, an inhibitor of Ca2+ release from mitochondria and ruthenium red, an inhibitor of the reuptake of Ca2+. It was concluded that loss of mitochondrial protein-SH and a decrease of Ca2+ uptake are implicated in the toxicity of cisplatin and that mitochondrial GSH is an important factor in relation to oxidative stress to mitochondria and cytotoxicity.

Effects of altered calcium homeostasis on the expression of glutathione S-transferase isozymes in primary cultured rat hepatocytes

The effects of altered Ca2+ homeostasis on glutathione S-transferase (GST) isozyme expression in cultured primary rat hepatocytes were examined. Isolated hepatocytes were cultured on Vitrogen substratum in serum-free modified Chee's essential medium and treated with Ca2+ ionophore A23187 at 120 hr post-plating. GST activity increased slightly, albeit significantly, in a concentration-dependent manner in A23187-treated hepatocytes relative to untreated controls. Western blot analysis using GST class alpha and mu specific antibodies showed an approximately 1.6- and 1.5-fold increase in the class alpha, Ya and Yc subunits, respectively, whereas no significant increase (approximately 1.2-fold) in class mu GST expression was observed following A23187 treatment. Northern blot analysis revealed an approximately 5-fold increase in GST class alpha and an approximately 7-fold increase in class mu GST mRNA levels in ionophore-treated hepatocytes compared to untreated cells. Results of the Western and Northern blot analyses of the ionophore-treated hepatocytes were compared with those obtained for tert-butyl hydroperoxide-treated cells. Immunoblot analysis showed a significant increase in the expression of GST class alpha, Ya and Yc subunits, approximately 1.8- and 1.7-fold, respectively, for tert-butyl hydroperoxide-treated hepatocytes as compared to controls, with little or no increase in class mu GSTs. Northern blot analysis showed approximately 3- and 2-fold increases, respectively, in class alpha and mu GST mRNA levels, following the tert-butyl hydroperoxide treatment. The results of the present investigation show that alterations in Ca2+ homeostasis produced by either Ca2+ ionophore A23187 or tert-butyl hydroperoxide treatment of hepatocytes enhanced the expression of GST isozymes in primary cultured rat hepatocytes.

Hepatic mitochondrial glutathione depletion and progression of experimental alcoholic liver disease in rats

Long-term ethanol feeding has been shown to selectively reduce hepatic mitochondrial glutathione content by impairing mitochondrial uptake of this thiol. In this study, we assessed the role of this defect in evolution of alcoholic liver disease by examining the mitochondrial glutathione pool and lipid peroxidation during progression of experimental alcoholic liver disease to centrilobular liver necrosis and fibrosis. Male Wistar rats were intragastrically infused with a high-fat diet plus ethanol for 3, 6 or 16 wk (the duration that resulted in induction of liver steatosis, necrosis and fibrosis, respectively). During this feeding period, the cytosolic pool of glutathione remained unchanged in the ethanol-fed animals compared with that in pair-fed controls. In contrast, the mitochondrial pool of glutathione selectively and progressively decreased in rats infused with ethanol for 3, 6 or 16 wk, by 39%, 61% and 85%, respectively. Renal mitochondrial glutathione level remained unaffected throughout the experiment. Serum ALT levels increased significantly in the ethanol-fed rats at 6 wk and remained elevated at 16 wk. In the mitochondria with severely depleted glutathione levels at 16 wk, enhanced lipid peroxidation was evidenced by increased malondialdehyde levels. Thus a progressive and selective depletion of mitochondrial glutathione is demonstrated in the liver in this experimental model of alcoholic liver disease and associated with mitochondrial lipid peroxidation and progression of liver damage.

Depletion of ATP but not of GSH affects viability of rat hepatocytes

The purpose of this study was to examine the role of glutathione depletion and alterations in the energy status in the induction of acute cytotoxicity to freshly isolated rat hepatocytes. Depletion of intracellular glutathione by diethyl maleate and phorone to levels below 5% of control did not induce loss of viability nor loss of intracellular ATP. Ethacrynic acid, a compound known to deplete mitochondrial GSH in addition to cytosolic GSH, induced cell killing after a depletion of ATP, next to GSH depletion. The results confirmed that depletion of intracellular glutathione alone does not necessarily result in cell killing. Only when glutathione depletion is succeeded by reduction in ATP levels, loss of cell viability is observed. The relationship between alterations in the energy status and the induction of cell death was further substantiated by inhibition of glycolytic and mitochondrial ATP generation. Treatment of hepatocytes either with iodoacetic acid to inhibit glycolysis (in hepatocytes from fed rats) or with potassium cyanide to inhibit mitochondrial respiration (in hepatocytes from both fed and fasted rats) revealed that depletion of intracellular ATP could lead to lethal cell injury. The susceptibility of cells to metabolic inhibition was better reflected by the rate of reduction in the energy charge than by the reduction of ATP alone. In conclusion, our results suggest that alterations of the energy status may be a critical event in the induction of irreversible cell injury. Depletion of cellular GSH is only cytotoxic when followed by a reduction of the energy charge.

Involvement of intracellular Ca2+ and K+ in dissipation of the mitochondrial membrane potential and cell death induced by extracellular ATP in hepatocytes

Isolated rat hepatocytes were incubated with extracellular ATP to induce a prolonged increase in intracellular Ca2+ ([Ca2+]i) and a loss of viability within 2 h. By using video-intensified fluorescence microscopy, the effects of exposure to extracellular ATP on [Ca2+]i, mitochondrial membrane potential (MMP) and cell viability were determined simultaneously in individual living hepatocytes. The increase in [Ca2+]i on exposure to ATP was followed by a decreasing MMP; there were big differences between individual cells. Complete loss of the MMP occurred before cell death was observed. Omission of K+ from the incubation medium decreased the cytotoxicity of ATP; under these conditions, intracellular K+ was decreased by more than 80%. Treatment with nigericin also depleted intracellular K+ and decreased ATP-induced toxicity. Protection against loss of viability by means of a decrease in intracellular [K+] was reflected by maintenance of the MMP. These observations suggest that ATP-induced cell death may be caused by a mechanism that has been described for isolated mitochondria: after an increase in Ca2+ levels, a K+ influx into mitochondria is induced, which finally disrupts the MMP and leads to cell death.

Lipid peroxidation--a common pathogenetic mechanism?

Lipid peroxidation is considered at present as one of the basic mechanisms involved in reversible and irreversible cell and tissue damage. The current knowledge about the role of peroxidative breakdown of polyunsaturated fatty acids in the pathogenesis of various diseases has been reviewed. Lipid peroxidation leads to degradation of the lipid membrane, interaction of degradation products with intra- and extracellular targets and to the production of new reactive oxygen species during the course of the chain reaction thus leading to damage of cells and tissues. According to our current view lipid peroxidation is implicated in the pathogenesis of cancer, inflammatory processes, atherosclerosis, toxic injury by xenobiotics and ischemic-reperfusion damage.

Mn2+ sequestration by mitochondria and inhibition of oxidative phosphorylation

Manganese is known to accumulate in mitochondria and in mitochondria-rich tissues in vivo. Although Ca2+ enhances mitochondrial Mn2+ uptake, ATP-bound Mn2+ is not sequestered by suspended rat brain mitochondria, and ATP binds Mn2+ even more tightly than it binds Mg2+. Physiological levels of the polyamine spermine enhanced 54 Mn2+ uptake at the low [Ca2+]s characteristic of unstimulated cells (approximately 100 nM). With succinate as substrate, Mn2+ inhibited oxygen consumption by suspensions of rat liver mitochondria after the addition of ADP but not after the addition of uncoupler. With glutamate/malate as substrate, Mn2+ inhibited ADP-stimulated respiration and also slightly inhibited uncoupler-stimulated respiration. State 4 (resting) respiration was unchanged in all cases, indicating that the inner membrane retained its impermeability to protons. These results suggest that Mn2+ was not oxidized and that it can interfere directly with oxidative phosphorylation, most likely by binding to the F1 ATPase. Mn2+ may also bind to the NADH dehydrogenase complex, but not strongly enough to affect electron transport in vivo. It is suggested that accumulation of manganese within the mitochondria of globus pallidus may help explain the distinctive pathology of manganism.

Interrelation of active oxygen species, membrane damage and altered calcium functions

Incubation of freshly isolated rat liver mitochondria in the presence of oxygen free radical generating hypoxanthine-xanthine oxidase system led to swelling of mitochondria as measured by the change in optical density, which was reversed by the addition of superoxide dismutase. O2- in the presence of CaCl2 enhanced the peroxidative decomposition of mitochondrial membrane lipids along with swelling of the organelle. Free radical generation led to enhancement of monoamine oxidase activity while glutathione peroxidase and cytochrome c oxidase were inhibited. Tert-butyl hydroperoxide (t-BHP) caused mitochondrial swelling through oxidative stress. Incorporation of ruthenium red, which is a Ca2+ transport blocker, during assay abolished peroxidative membrane damage and swelling. Dithiothreitol (DTT) accorded protection against t-BHP induced mitochondrial swelling. The above in vitro data suggest a possible interrelationship of active oxygen species, membrane damage and calcium dynamics.

Effect of extracellular phosphatidylinositol on c-myc gene-expressed human renal cancer cell line

Effect of exogenously added soybean phosphatidylinositol on c-myc gene expressed and unexpressed human cancer cell lines was investigated. When phosphatidylinositol liposomes were introduced into culture media, viability of c-myc unexpressed cells was reduced, while that of c-myc expressed cells was not. Death of c-myc unexpressed cells by phosphatidylinositol liposomes was found to be caused by abnormally accumulated intracellular Ca2+, and it seemed to be related to reduction of protein kinase C activity.

Effect of 2-acetylaminofluorene on intracellular free Ca2+ in isolated rat hepatocytes

The effect of 2-acetylaminofluorene (2-AAF) on the intracellular free Ca2+ ([Ca2+]i) and viability of isolated rat hepatocytes has been investigated using the fluorescent probes quin 2 and propidium iodide respectively. At the highest concentration tested (224 microM), 2-AAF produces an elevation of [Ca2+]i which shows a biphasic profile. A small initial increase is observed during the first 5 min; this is followed by a considerable rise which reaches up to 2.5 times the control value at 15 min. These changes in intracellular calcium are not accompanied by detectable alterations in cell viability. In order to determine the mechanisms by which this effect of 2-AAF takes place, three calcium antagonists, namely verapamil, TMB-8 (8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate) and ruthenium red (RuR), have been used. The results suggest that the first phase is dependent upon internal Ca2+ store mobilization, while the second phase seems to be related to Ca2+ entry from the extracellular space. The data obtained with RuR further indicate that mitochondria may be involved in the perturbation of calcium homeostasis caused by 2-AAF. In addition, in the experiments involving antagonists, no consistent pattern emerges that suggests a close relationship between intracellular Ca2+ levels and cell viability. The present study provides further information on the mechanisms by which these well-known hepatotoxin 2-AAF may interact with liver cells. It also shows that when these cells are exposed to a toxin, short-term changes in [Ca2+]i may not be accompanied by loss of cell viability, and conversely, that changes in cell viability may occur without alterations in [Ca2+]i.

Glutathione metabolism and its role in hepatotoxicity

Glutathione (GSH) fulfills several essential functions: Detoxification of free radicals and toxic oxygen radicals, thiol-disulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intense exposure to exogenous toxins such as the liver, kidney, lung and intestine. Within the cell mitochondrial GSH is the main defense against physiological oxidant stress generated by cellular respiration and may be a critical target for toxic oxygen and electrophilic metabolites. Glutathione homeostasis is a highly complex process, which is predominantly regulated by the liver, lung and kidney.

Involvement of glutathione in 1-naphthylisothiocyanate (ANIT) metabolism and toxicity to isolated hepatocytes

1-Naphthylisothiocyanate (ANIT) is a model compound which causes cholestasis in laboratory animals. Various biochemical and morphological changes including biliary epithelial and parenchymal cell necrosis occur in the liver of animals treated with ANIT. Although the mechanism(s) for these effects is not understood, a role for glutathione (GSH) in toxicity has been implicated. The possible role of GSH in hepatocellular toxicity caused by ANIT was investigated in this study. Treatment of freshly isolated rat hepatocytes with ANIT caused a concentration- and time-dependent depletion of cellular GSH that preceded lactate dehydrogenase (LDH) leakage. Analysis of the incubation medium indicated that the majority of the cellular GSH which was lost was present extracellularly as GSH or as a GSH-releasing compound. Mixing ANIT with GSH at pH 7.5 yielded a compound that was characterized by HPLC and fast atom bombardment-mass spectrometry (FAB-MS) S-(N-naphthyl-thiocarbamoyl)-L-glutathione (GS-ANIT). When dissolved in aqueous solutions at neutral pH, 95% of GS-ANIT dissociated to yield free ANIT and GSH. Under conditions designed to maximize formation and stability of GS-ANIT, GS-ANIT was found in the extracellular medium of hepatocytes treated with ANIT. Treatment of hepatocytes with the GS-ANIT caused GSH depletion and LDH leakage similar to that observed with equimolar amounts of ANIT. These data suggest that ANIT depletes hepatocytes of GSH through a reversible conjugation process. Such a process may play a role in the toxicity of ANIT.

Silica increases cytosolic free calcium ion concentration of alveolar macrophages in vitro

Rat alveolar macrophages were exposed to silica dust (quartz) suspended in culture medium (SiO2, dry particle size less than 5 microns in diameter) and fluctuation in their cytosolic free calcium content ([Ca2+]i) was detected in cell monolayers with a fluorescent calcium probe (Indo-1AM). Cytosolic free calcium content was correlated with lactate dehydrogenase (LDH) release, an index of cell damage. SiO2 induced a concentration- and time-dependent increase of cytosolic free Ca2+ ion concentration and LDH release. [Ca2+]i was increased about fivefold when cells were exposed to 200 micrograms of SiO2 per milliliter (3 ml per dish) for 2 hr. [Ca2+]i changed within 15 min of SiO2 treatment, whereas LDH release was measurably increased only after 30 min. Chelation of extracellular Ca2+ by 2 mM ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetate did not prevent SiO2-induced fluctuation of macrophage [Ca2+]i, but did partially prevent the SiO2-induced increase in LDH release (p less than 0.01). We conclude that a very early event in SiO2-induced damage of alveolar macrophages involves mobilization of intracellular calcium pools to increase [Ca2+]i. These results suggest that SiO2-induced macrophage damage, a key event in the development of silicosis, may involve perturbation of intracellular calcium homeostasis.

Calcium-dependent opening of a non-specific pore in the mitochondrial inner membrane is inhibited at pH values below 7. Implications for the protective effect of low pH against chemical and hypoxic cell damage

1. The rate of opening of the Ca(2+)-induced non-specific, cyclosporin A-inhibited, pore of the mitochondrial inner membrane of rat heart and liver mitochondria at pH 6.0 was less than 10% of that at pH 7.4. 2. The effect could not be explained by inhibition of Ca2+ uptake into the mitochondria, or of the matrix peptidyl-prolyl cis-trans isomerase (PPIase), or of the Ca(2+)-induced conformational change of the adenine nucleotide translocase. 3. It is suggested that the proposed interaction of matrix PPIase with the 'c' conformation of the adenine nucleotide carrier in the presence of Ca2+ [Griffiths & Halestrap (1991) Biochem. J. 274, 611-614] is inhibited by low pH. 4. The relevance of this to the protective effect of low pH on hypoxic and chemical-induced cell damage is discussed.

Manganese neurotoxicity: cellular effects and blood-brain barrier transport

The observations by Couper in 1837 are acknowledged as the earliest description of the toxic syndrome associated with chronic manganese (Mn) exposure. Since that time, many of the neurotoxic aspects of manganism have been described, yet, the primary basis for its neurotoxicity remains unknown. Recent evidence corroborates the original hypothesis by Maynard and Cotzias (82) which invokes the mitochondrion as the target organelle for Mn cytotoxicity which is primarily expressed as a perturbation in Ca2+ homeostasis. Despite recognition that excessive Mn exposure culminates in Mn accumulation in the CNS and a clinical picture dominated by neurological disturbances, the role of the blood-brain barrier in the CNS uptake of Mn has received little attention. Accordingly, the first part of this review summarizes the current understanding of the interaction of Mn with biologically active sites in the induction of Mn cytotoxicity. The second part of this review summarizes what is known about Mn transport across the blood-brain barrier, a major regulator of the CNS milieu, with the contention that the rate and extent of Mn transport across the blood-brain barrier modulates its neurotoxicity.

Uptake of glutathione by renal cortical mitochondria

Transport of GSH into renal cortical mitochondria was studied. Mitochondria were highly enriched with little contamination from other subcellular organelles (as assessed by marker enzymes), they exhibited coupled respiration (respiratory control ratio greater than 3.0), and they had initial GSH concentrations of 5.71 +/- 0.65 nmol/mg protein (n = 47). Incubation of mitochondria with GSH in a triethanolamine, pH 7.4, buffer containing sucrose, potassium phosphate, MgCl2, and KCl, produced time- and concentration-dependent increases in intramitochondrial GSH content. Uptake was linear versus time for at least 2 min and exhibited kinetics consistent with one low-affinity, high-capacity process (Km = 1.3 mM, Vmax = 5.59 nmol/min per mg protein), although the results cannot exclude the presence of other, less quantitatively significant pathways. The initial rate of uptake of 5 mM GSH was not significantly altered by uncouplers (0.1 mM 2,4-dinitrophenol and 25 microM carbonyl cyanide m-chlorophenylhydrazone) or by 1 mM ADP. In contrast, incubation with 1 mM ATP, 1 mM KCN, 0.1 mM or 1 mM CaCl2 inhibited uptake by 41, 39, 43, or 55%, respectively. GSH uptake was markedly inhibited by gamma-glutamylglutamate and by a series of S-alkyl GSH derivatives. Strong interactions (i.e., both cis and trans effects) were observed with other dicarboxylates (i.e., succinate, malate, glutamate) but not with monocarboxylates (i.e., lactate, pyruvate). Preincubation of mitochondria with GSH protected against tert-butyl hydroperoxide- or methyl vinyl ketone-induced inhibition of state 3 respiration. These results demonstrate uptake of GSH into renal cortical mitochondria that appears to involve electroneutral countertransport (exchange) with other dicarboxylates. Functionally, GSH uptake into mitochondria can protect these organelles from various forms of injury, such as oxidative stress.

Inhibition of iodoacetamide and t-butylhydroperoxide toxicity in LLC-PK1 cells by antioxidants: a role for lipid peroxidation in alkylation induced cytotoxicity

Previously we reported that thiol depletion and lipid peroxidation were associated with the cytotoxicity of nephrotoxic cysteine S-conjugates, a group of toxins which kill LLC-PK1 cells after metabolic activation and covalent binding. To determine if this is a general mechanism of cytotoxicity in these cells, we compared the effect of antioxidants, an iron chelator, and a thiol reducing agent on the toxicity of an alkylating agent, iodoacetamide (IDAM), and an organic peroxidant, t-butylhydroperoxide (TBHP). IDAM or TBHP toxicity was concentration (0.01 to 1.0 mM) and time (1 to 6 h) dependent. Both toxins caused lipid peroxidation which occurred prior to cell death as determined by leakage of lactate dehydrogenase (LDH). The alkylating agent IDAM bound to cellular macromolecules and depleted cellular non-protein thiols almost completely by 1 h, while LDH release occurred first at 2 to 3 h. The toxicity of IDAM and TBHP was inhibited by the antioxidants DPPD, BHA, BHQ, PGA, and BHT and the iron chelator deferoxamine. However, DPPD blocked TBHP- and IDAM-induced lipid peroxidation and toxicity without affecting binding and depletion of cellular nonprotein thiols. Furthermore, the thiol reducing agent dithiothreitol was able to block lipid peroxidation and toxicity. Therefore it is possible that with an alkylating agent, depletion of cellular nonprotein thiols cooperates with covalent binding and contributes to lipid peroxidation and cell death. There appear to be common elements in the toxicity of alkylating agents and organic peroxidants in LLC-PK1 cells.