Protective role of the glutathione redox cycle against adriamycin-mediated toxicity in isolated hepatocytes

The coupling of metabolic to secretory events in pancreatic islets. The possible role of glutathione reductase

The participation of glutathione reductase in the process of nutrient-stimulated insulin release was investigated in rat pancreatic islets exposed to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU caused a time-and dose-related, irreversible inhibition of glutathione reductase activity. This coincided with a fall in both GSH/GSSG ratio and the thiol content of the islets. Pretreatment of the islets with BCNU inhibited the oxidation of glucose and its stimulant action upon both 45Ca net uptake and insulin release. Although BCNU (up to 0.5 mM) failed to affect the oxidation of L-leucine and L-glutamine, it also caused a dose-related inhibition of insulin release evoked by the combination of these two amino acids. The latter inhibition was apparently not fully accounted for by the modest to negligible effects of BCNU upon 45Ca uptake, 45Ca efflux, 86Rb efflux and cyclic AMP production. Since BCNU failed to inhibit insulin release evoked by the association of Ba2+ and theophylline, these results support the view that glutathione reductase participates in the coupling of metabolic to secretory events in the process of nutrient-stimulated insulin release. However, the precise modality of such a participation, for example the control of intracellular Ca2+ distribution, remains to be elucidated.

Oxygen-mediated cell injury in the killing of cultured hepatocytes by acetaminophen

Sensitivity of cultured hepatocytes to acetaminophen was induced by pretreatment of the rat with 3-methylcholanthrene. Under these conditions, 10 uM B-naphthoflavone but not SKF-525A prevented the cell killing, indicating dependence on metabolism. Inhibition of glutathione reductase by 50 uM bis-chloro-nitrosourea, shown previously to increase the sensitivity of hepatocytes to an oxidative stress, potentiated the toxicity of acetaminophen without increasing the covalent binding of acetaminophen metabolites. Pretreatment of the hepatocytes with the ferric iron chelator deferoxamine, known to reduce the sensitivity of hepatocytes to an oxidative stress, prevented the cell killing without reducing covalent binding. Addition of ferric chloride to the culture medium restored the sensitivity of the cells to acetaminophen, again without effect on the extent of covalent binding. These data demonstrate that the toxicity of acetaminophen can be dissociated from the covalent binding of its metabolites and support the conclusion that the hepatocytes were lethally injured by an oxidative stress accompanying the mixed function oxidase-dependent biotransformation of acetaminophen.

Vitamin E reversal of the effect of extracellular calcium on chemically induced toxicity in hepatocytes

Isolated rat hepatocytes were incubated in the presence or absence of extracellular calcium and alpha-tocopherol succinate with three different toxic chemicals; namely, adriamycin in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea, ethyl methanesulfonate, and the calcium ionophore A23187. In the absence of extracellular calcium these three compounds were far more toxic to the cells than in its presence. The addition of vitamin E to calcium-free medium, however, protected hepatocytes against toxic injury, whereas cells incubated in medium containing calcium were not protected. Hepatocyte viability during each toxic insult correlated well with the cellular alpha-tocopherol content but not with the presence or absence of extracellular calcium. These results suggest that cellular alpha-tocopherol maintains the viability of the cell during a toxic insult and that the presence or absence of vitamin E in the incubation medium probably explains the conflicting reports on the role of extracellular calcium in toxic cell death.

A redox cycling mechanism of action for 2,3-dichloro-1,4-naphthoquinone with mitochondrial membranes and the role of sulfhydryl groups

The addition of 2,3-dichloro-1,4-naphthoquinone (CNQ) to substrate-depleted, GSH-supplemented rat liver mitochondria resulted in a dose-dependent depletion of reactable suflhydryl groups and a concomitant increase in mitochondrial disulfide content at a ratio of 2 thiols depleted/disulfide generated. The molar ratio of thiol depleted/CNQ added approached 20 at low CNQ concentrations and was unity at higher doses. The addition of CNQ to substrate-depleted mitochondrial suspensions resulted in O2 consumption which increased with increasing concentrations of mitochondria and was sensitive to N-ethylmaleimide (NEM) which establishes the ability of CNQ to interact with mitochondrial thiol redox centers. The CNQ-mediated large amplitude swelling of rat liver mitochondria was exacerbated by thiol oxidizing agents and depressed by disulfide reducing agents. A redox cycling mechanism between mitochondrial thiol groups, CNQ and oxygen was proposed to lower the matrix glutathione pool and make the mitochondria more susceptable to toxic oxygen radicals which induce swelling in isolated mitochondrial suspensions. In support of this mechanism, alpha-tocopherol was shown to prevent the CNQ-mediated swelling process. Beef heart mitochondrial NADH was oxidized by CNQ in a 1/1 molar ratio anaerobically and in a 3/1 molar ratio under aerobic conditions, whereas the fully reduced quinone, CNQH2, oxidized NADH aerobically but not anaerobically. Thus, CNQ is capable of interacting with NADH of the mitochondrial electron transport chain in a redox cycling fashion.

Oxidation of glutathione during hydroperoxide metabolism. A study using isolated hepatocytes and the glutathione reductase inhibitor 1,3-bis(2-chloroethyl)-1-nitrosourea

In the present study freshly isolated rat hepatocytes treated with the glutathione reductase inhibitor BCNU (1,3-bis(2-chloroethyl)-1-nitrosourea) were used to investigate the metabolism of tert-butyl hydroperoxide and of hydrogen peroxide formed in different intracellular compartments. Glycolate, benzylamine and hexobarbital were used to stimulate H2O2 production in the peroxisomal, mitochondrial and endoplasmic reticular/cytosolic compartments, respectively. Our results support previous findings that catabolism of H2O2 formed in the mitochondrial and cytosolic compartments occurs predominantly by the glutathione peroxidase system, whereas H2O2 generated within the peroxisomes is metabolized by catalase. They further reveal that the capacity of uninhibited glutathione reductase to reduce glutathione disulfide, formed during hydroperoxide metabolism by glutathione peroxidase, is high and that a decreased NADPH/NADP+ redox level, rather than insufficient reductase activity, is responsible for the accumulation and subsequent excretion of cellular glutathione disulfide observed during hydroperoxide metabolism. Finally, our results demonstrate that H2O2 generated during cytochrome P-450-mediated drug oxidation is metabolized primarily by the glutathione peroxidase system.

Enhancement of adriamycin toxicity by carboxymethylcellulose in mice

Carboxymethylcellulose (CMC) (1% in 0.9% NaCl, 0.2 ml/10 g ip) a common suspending agent, enhanced adriamycin (ADR) (15 mg/kg ip) toxicity when administered to mice 5 hr before the antibiotic. Compared with ADR alone, this combination treatment produced, after 7 days, an increase in lethality from 15 to 80%. The pathologic analysis of hearts, livers, kidneys, and small bowels was performed, revealing an increase in the incidence and severity of hepatic damage in mice receiving ADR + CMC. Furthermore, reduced glutathione (GSH) was measured in livers of all mice; the animals treated with CMC and ADR + CMC showed a significant (p less than 0.01) reduction of hepatic GSH in comparison with controls and ADR-alone-treated animals. These data further confirm a crucial protective role for GSH in ADR toxicity and prove that CMC exerts an important biochemical effect on hepatic GSH.

Hepatic redox homeostasis following acute adriamycin intoxication in rats

Adriamycin toxicity is postulated to result from cytochrome P-450 reductase-catalyzed univalent reduction of the quinone to the semiquinone free radical intermediate. Oxygen radicals generated during the nonenzymatic reoxidation of the semiquinone have been implicated in the deleterious modification of a variety of tissue macromolecules. Detoxification of reactive products, such as hydroperoxides, is proposed to involve the consumption of vital cellular reducing equivalents which may, in itself, represent the primary causative event in toxic tissue damage. The present investigation demonstrates that hepatic tissue has sufficient glutathione (GSH) reductase to prevent a decrease in GSH following acute adriamycin administration to rats. Similarly, except for a transient decrease in NAD, adriamycin intoxication caused minimal changes in the hepatic pyridine nucleotide content in vivo. It is concluded that species- and tissue-specific differences in the distribution of antioxidant defense mechanisms may be primary determinants of the relative insensitivity of liver and, in contrast, the rather selective cardiomyopathy resulting from adriamycin administration in vivo.

The effect of BCNU (carmustine) on tissue glutathione reductase activity

A single, 50 mg/kg i.p. dose of 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) to mice significantly inhibited glutathione reductase activity in liver within 10 min, and in lung and heart within 30 min. Maximal inhibition was reached at 4 h in all tissues and glutathione reductase activity remained significantly depressed for 48 h in liver and for 96 h in lung and heart. BCNU doses of less than 25 mg/kg had no effect on glutathione reductase activity. Increasing the BCNU dose to 100 mg/kg caused greater inhibition of glutathione reductase activity in all tissues. In spite of significantly decreased glutathione reductase activity, reduced glutathione (GSH) levels were unaffected in all tissues both 1 and 24 h after 50 mg/kg BNCU. These results suggest that lower than normal glutathione reductase activity is sufficient to maintain GSH levels. These results also demonstrate that BCNU inhibits tissue glutathione reductase activity for several days, but only at doses significantly greater than previously reported.

Depletion in vitro of mitochondrial glutathione in rat hepatocytes and enhancement of lipid peroxidation by adriamycin and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)

Treatment of isolated rat hepatocytes with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and adriamycin (ADR) produced a complete depletion of cellular glutathione accompanied by a significant increase in lactate dehydrogenase (LDH) leakage. Separation of the mitochondrial and cytoplasmic pools of glutathione by digitonin disruption showed that, although BCNU, a specific inhibitor of glutathione, completely depleted the cytoplasmic pool of glutathione, the mitochondrial supply was not entirely expended and LDH leakage was only moderately stimulated. Only after depletion of the mitochondrial supply of glutathione by ADR and BCNU did LDH leakage increase markedly. Measurement of lipid peroxidation, by monitoring malondialdehyde through the thiobarbituric acid procedure, showed that malondialdehyde accumulated more extensively and at a rate mirroring release of LDH from ADR/BCNU treated cells. The time of increase in LDH leakage and malondialdehyde production corresponded to the time of depletion of mitochondrial glutathione to less than 10% of the initial pool size. No such increase in LDH leakage was observed with BCNU or ADU treatment alone or when aminopyrine, an inhibitor of lipid peroxidation, was included. Aminopyrine was found to prevent, in a dose-dependent manner, both LDH leakage and malondialdehyde production stimulated by ADR/BCNU treatment. The protective effect peaked at 5 mM aminopyrine, and higher concentrations produced significant LDH leakage exhibiting LDH release kinetics different than those observed with ADR/BCNU. Although aminopyrine had no effect on the rate or extent of cytoplasmic glutathione depletion by ADR/BCNU treatment, the mitochondrial pool was conserved significantly in those cells protected by aminopyrine. These data suggest that enhanced hepatocyte damage observed after treatment with a combination of ADR and BCNU versus BCNU or ADR alone is due to the extensive depletion of mitochondrial glutathione supported by ADR after glutathione reductase inhibition. Further, enhancement of lipid peroxidation is strongly implicated in the mechanism of adriamycin toxicity.

Modifiers of free radicals inhibit in vitro the oncogenic actions of x-rays, bleomycin, and the tumor promoter 12-O-tetradecanoylphorbol 13-acetate

Using short-term cultures of hamster embryo cells, we have examined the effects of the free-radical scavenger superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) and the enzyme catalase (hydrogen-peroxide:hydrogenperoxide oxidoreductase, EC 1.11.1.6) on x-ray- and bleomycin-induced transformation and on the enhancement of radiogenic transformation by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA). We find that superoxide dismutase inhibits (i) transformation induced by x-ray and bleomycin and (ii) promotional action of TPA in vitro. The results suggest that the oncogenic action of x-rays and bleomycin and the enhancement of oncogenic transformation by TPA are mediated in part by free radicals. The findings also suggest that superoxide dismutase can serve as an inhibitor of oncogenesis and that its actions, as seen in this in vitro system, are most predominantly on inhibiting late events in the progression of cellular transformation--those associated with promotion.

Studies on glutathione metabolism in ventral prostate and chemically induced prostatic carcinoma in rats

Glutathione content and the activity of glutathione reductase were examined in ventral prostate and chemically induced 11095 squamous-cell prostatic carcinoma in rats. Castration produced a significant reduction in the levels of reduced (GSH) and oxidized (GSSG) glutathione and glutathione reductase activity in the prostate. Replacement of testosterone (50 mg/kg) daily for 7 days to castrated animals elevated the reduced glutathione level and the activity of glutathione reductase almost to normal limits. Squamous-cell carcinoma was implanted in castrated and intact animals. Tumor growth in normal rats produced a decrease of almost 30% in the weight of the ventral prostate at 21 days post-implantation, although the glutathione levels remained unaffected. Much greater activity of glutathione reductase was detected in the tumor in comparison to the values noted for the normal tissue. The tumor also showed significantly higher values for the GSH/GSSG ratio. No apparent difference could be found in the rate of the growth of tumors whether implanted in normal or castrated animals. The levels of reduced and oxidized glutathione and glutathione reductase activity also seemed identical in tumors obtained from both groups of animals. Administration of testosterone (50 mg/kg) or beta-estradiol (2 mg/kg) daily for 11 days to tumor-bearing castrated animals did not alter the levels of glutathione and glutathione reductase activity. A significantly higher level of blood reduced glutathione was found in tumor-bearing rats in comparison to that seen for the normal subjects. Our results demonstrate that androgen depletion and replacement therapy influence the metabolism of glutathione in rat ventral prostate. Squamous-cell carcinoma of the prostate appears to differ from the normal tissue with respect to the observed androgen effects. There is dissimilarity in the metabolism of glutathione in the two tissues since greater activity of glutathione reductase and lower values of reduced glutathione were seen in the tumor as compared to those of the ventral prostate. Treatment with beta-estradiol, an antiprostatic agent, does not seem to influence the growth or glutathione metabolism of squamous-cell carcinoma of the prostate. The observed changes in blood glutathione levels might prove to be useful as an index of rapid growth of the neoplastic tissue.

Inhibition of glutathione synthesis augments lysis of murine tumor cells by sulfhydryl-reactive antineoplastics

GSH plays an important role in cellular defense against a wide variety of toxic electrophiles via the formation of thioether conjugates. We studied the role of GSH in murine tumor cell defense against a novel class of sulfhydryl-reactive antineoplastics, the sesquiterpene lactones (SL). Incubation of P815 mastocytoma cells with any of the four SL tested (vernolepin, helenalin, elephantopin, and eriofertopin) for 1 h resulted in 70-97% depletion of GSH. The importance of GSH resynthesis upon exposure of tumor cells to SL was evaluated with the use of buthionine sulfoximine (BSO), a selective, nontoxic inhibitor of gamma-glutamylcysteine synthetase. Inhibition of GSH synthesis with 0.2 mM BSO markedly enhanced SL-mediated cytolysis of four murine tumor cell lines. A 6- to 34-fold reduction in the amount of SL causing 50% lysis was obtained with BSO. Addition of BSO to P815cells either during or immediately after a 1-h pulse with 10 micrograms/ml of vernolepin increased cytolysis from less than 3% to 78-82%. However, a 1.5-h delay in the addition of BSO to such cells, which allowed for substantial resynthesis of GSH, reduced cytolysis to 30%. Recovery of GSH synthetic capacity after BSO treatment correlated with loss of the synergistic effect of BSO on lysis by vernolepin. BSO did not augment cytolysis by six other antineoplastics (doxorubicin, mitomycin C, vinblastine, cytosine arabinoside, maytansine, and 1,3-bis-[2-chloroethyl]-1-nitrosourea [BCNU]). Of these, only BCNU depleted cellular GSH. Lysis by jatrophone, another GSH-depleting antitumor agent, was increased 21-fold by BSO. Since prolonged incubation with BSO alone results in near-complete GSH depletion without loss of cell viability, SL-mediated cytolysis is probably not a result of GSH depletion. We have demonstrated, however, a critical role for GSH synthetic capacity as a determinant of tumor cell susceptibility to cytolysis by SL. GSH also plays an important role in cellular defense against oxidative injury. Vernolepin, acting as a GSH-depleting agent, markedly sensitized tumor cells to lysis by H2O2 (greater than 6.5-fold increase with 20 micrograms/ml of vernolepin). These findings suggest the possibility that the coordinated deployment of sulfhydryl-reactive antitumor agents, BSO, and oxidative injury might constitute an effective chemotherapeutic strategy.

Regulation of intracellular calcium compartmentation: studies with isolated hepatocytes and t-butyl hydroperoxide

In suspensions of isolated hepatocytes, two intracellular Ca2+ pools were distinguished in the presence of the metallochrome indicator arsenazo III, first by treatment with the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and then with the Ca2+ ionophore A23187. The available evidence indicates that the two pools are of mitochondrial and extramitochondrial origin. Metabolism of t-butyl hydroperoxide by hepatocytes caused release of Ca2+ from both compartments concomitant with oxidation of cellular glutathione and NADPH, which was followed by characteristic alterations in cell surface structure. When NADPH oxidation was prevented by selective inactivation of glutathione reductase, t-butyl hydroperoxide metabolism was without effect on the mitochondrial Ca2+ pool, whereas the loss from the extramitochondrial pool was accelerated. Our results suggest that different regulatory mechanisms modulate mitochondrial (NADPH-dependent) and extramitochondrial (thiol-dependent) Ca2+ compartmentation and that disturbance of normal Ca2+ homeostasis may be critical in peroxide-induced cytotoxicity.

Involvement of glutathione reductase in selenite metabolism and toxicity, studied in isolated rat hepatocytes

Cellular lysis in freshly isolated hepatocytes incubated with varying concentrations of selenite could be related to the reductive metabolism of selenite. A decrease in intracellular GSH levels was observed concomitant with an increased rate of accumulation of oxidized glutathione in the incubation medium. Pretreatment of hepatocytes with an inhibitor of GSSG-reductase (1,3-bis(2-chloroethyl)-1-nitrosourea), prior to the addition of 50 microM selenite, resulted in substantially lower GSH-levels. The rate of GSSG reductase-catalyzed metabolism of selenite (50 microM) could be estimated to approximately 7 nmoles reduced/h per 10(6) cells. The results indicate that this was the major metabolic pathway for toxic concentrations of selenite in isolated hepatocytes. Furthermore, selenite considerably decreased cellular NADPH levels. In hepatocytes isolated from starved rats, the presence of alanine and glucose in the incubation medium protected against selenite-mediated cellular lysis. These observations suggest that an insufficient NADPH generation could be critical for selenite reduction and toxicity in isolated hepatocytes.