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

Flow cytometric measurement of lipid peroxidation in vital cells using parinaric acid

A method for measuring lipid peroxidation using time resolved flow cytometry is described. Because of its chemical nature, the naturally fluorescent fatty acid cis-parinaric acid is readily consumed in lipid peroxidation reactions. It could be loaded into Chinese hamster ovary cells in a time and concentration dependent manner at 37 degrees C, with 5 microM for 60' giving consistent, bright fluorescence without evidence of cytotoxicity. Examination of cells by fluorescence microscopy showed diffuse staining of surface and internal membranes. Cells were maintained at 37 degrees C while being examined in an Epics Elite flow cytometer equipped with a 325 nm HeCd laser, and parinaric acid fluorescence at 405 nm was measured over time. Addition of the oxidant tert-butyl hydroperoxide resulted in a burst of intracellular oxidation, shown by simultaneously loading the cells with dichlorofluorescein, and loss of parinaric fluorescence over time. This was followed by cell death, indicated by loss of forward light scatter and uptake of propidium iodide. Pretreatment of the cells with the antioxidant alpha-tocopherol, 200 microM, reduced the rate of loss of parinaric acid fluorescence and delayed the onset of cell death. Simultaneous biochemical determination of the lipid peroxidation breakdown product malondialdehyde confirmed a close temporal relationship with loss of parinaric acid fluorescence, both with and without alpha-tocopherol pretreatment and suggested that the flow cytometric assay for lipid peroxidation is of comparable sensitivity. The mitochondrial stain dodecyl acridine orange and the cyanine dye DiOC(6)3 were combined with cis-parinaric acid staining and could be excited by the latter using resonance energy transfer.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of glutathione and glutathione-dependent enzymes in the reversal of adriamycin resistance in colon carcinoma cell lines

Four human colon cancer cell lines (SW620, LS 180, DLD-I, and HCT-15) and sub-lines isolated in vitro by selection with Adriamycin were studied for reversal of intrinsic and acquired Adriamycin resistance, using buthionine sulfoximine (BSO) to deplete cellular glutathione alone and in combination with the P-glycoprotein antagonist verapamil. GSH levels varied among the parental cell lines but did not increase with resistance. In the parental SW620, DLD-I and HCT-15 and their drug-resistant derivatives, there was no relation between the effect of the glutathione-depleting agent BSO, the mRNA expression of both selenium-dependent glutathione peroxidase (GPx) and glutathione S-transferase pi (GST pi), bulk glutathione S-transferase (GST) activity, and the degree of resistance. However, in LS 180 and its derivative sub-lines, which do not principally rely on P-glycoprotein (Pgp) for Adriamycin resistance, treatment with BSO demonstrated a relatively diminished GSH depletion and enhanced recovery. In comparison with the other acquired cell lines, BSO specifically reversed acquired resistance in the LS 180 Adriamycin-resistant subline (LS 180 Ad150) after short-term drug exposure. Furthermore, the LS 180 Ad150 cells demonstrated an increase in both GPx and GST pi mRNA expression. These observations suggest that glutathione-mediated detoxification of Adriamycin may play a role in the resistance of this sub-line. Verapamil enhanced Adriamycin cytotoxicity 1.2- to 12-fold in the intrinsically resistant cells and as much as 15-fold in cell lines with acquired resistance. Combination of BSO with verapamil resulted in additive, but not synergistic, reversal of resistance. The results underscore the complex nature of Adriamycin resistance, and suggest a role for drug-resistance-modulating agents in the treatment of colon carcinoma.

A phase II trial of high-dose cisplatin and dacarbazine. Lack of efficacy of high-dose, cisplatin-based therapy for metastatic melanoma

Cisplatin and dacarbazine are used widely in the treatment of metastatic melanoma. To evaluate high-dose cisplatin and dacarbazine, 32 patients with metastatic melanoma were treated with cisplatin 50 mg/m2 and dacarbazine 350 mg/m2 daily for three days repeated at 28-day intervals. Their median age was 43.5 years (range, 25 to 73 years), and their median Karnofsky performance status was 80% (range, 70% to 100%). Measurable and evaluable disease sites (number of patients) included lymph nodes (22), lung (17), soft tissue (16), liver (13), bone (seven), spleen (four), adrenal gland (three), skin (three), and other sites (five). Patients received a median of two cycles of therapy (range, one to eight cycles). Thirty patients were evaluable for response. No complete responses were observed. Five patients had a partial response (17%; 95% confidence interval, 3% to 30%) for 16+, 12+, 7, 6.5, and 3 months. Responding sites of disease included lymph nodes (five of 22), lung (three of 17), and soft tissue (two of 16). Hematologic toxicity (Grade greater than or equal to 3) included neutropenia (16 of 32 patients, 30 of 90 cycles), thrombocytopenia (eight of 32 patients, 12 of 90 cycles), and anemia (five patients). Nine episodes of neutropenia and fever were seen in four patients; two had bacteremia. Nonhematologic toxicity (Grade greater than or equal to 3) included hypotension (two patients), nausea and vomiting (four), neuropathy (two), ototoxicity (four), and hypomagnesemia (nine). The low objective response rate and severe toxicity of this regimen preclude its standard use in patients with metastatic melanoma. A review of cisplatin-based therapy in metastatic melanoma suggests that there is no dose-response relationship. The use of high-dose cisplatin (greater than 100 mg/m2) in the treatment of metastatic melanoma is not recommended.

Increased paracellular permeability in intrahepatic cholestasis induced by carmustine (BCNU) in rats

Carmustine [i.e., 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)] is a drug with cholestatic potency both in experimental animals and in humans. To study the mechanisms involved in the development of the hepatic lesions, early changes in liver function in rats pretreated with the drug were investigated. Dosages and sampling times that did not result in hepatocellular injury, as indicated by release of marker enzymes, were applied. In isolated perfused livers from pretreated rats, bile flow and maximal secretion rate of taurocholate were decreased. An increase in biliary [14C]sucrose clearance suggested enhanced permeability of the bile tract and was correlated with increased inorganic phosphate concentration in bile. To assess the contribution of paracellular and transcellular pathways of sucrose, [14C]sucrose access into bile was analyzed by biliary off-kinetics after omission of the radioactive marker from the perfusion medium. An improved method was developed to quantitate the permeability of the bile tract by applying the classical flow equation to the paracellular portion of biliary sucrose clearance. With this method it was shown that pretreatment of rats with BCNU resulted in an increase in both diffusion and convection of paracellular sucrose from perfusate into bile. Accordingly, the fast access of horseradish peroxidase from perfusate into bile was facilitated in isolated perfused livers of BCNU-treated rats. The results indicate that an increase in paracellular permeability is an early alteration that may contribute to the development of hepatotoxic lesions caused by BCNU. It is shown that inert solute clearance can be used to assess paracellular permeability if the paracellular fraction is determined.

Role of selenium-dependent glutathione peroxidase in protecting against t-butyl hydroperoxide-induced damage in hepatocytes

The role of the selenoenzyme glutathione peroxidase (Se-GSHPx) in protecting against oxidative injury was studied in hepatocytes isolated from rats fed either a low-selenium (Se-) or a selenium-adequate (Se+, control) diet. In rats fed Se- diet for eight weeks the selenium content of plasma and liver was lowered to 15 and 8%, respectively. No Se-GSHPx and only 5% of total GSHPx activity was detected in Se- hepatocytes. However, the Se- hepatocytes were as resistant as the Se+ cells to oxidative injury by 0.8 mM tert-butyl hydroperoxide (t-BuOOH), or 0.2 mM t-BuOOH plus 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of oxidized glutathione (GSSG) reductase. Only at 1.5 mM t-BuOOH or at 0.5 mM t-BuOOH with BCNU were cell damage and lipid peroxidation more evident in Se- cells. At all t-BuOOH concentrations used the depletion of cellular glutathione (GSH) was similar in magnitude in Se- and Se+ cells, but Se+ cells released more glutathione (mainly GSSG), obviously due to their higher Se-GSHPx activity. These results suggest that hepatocytes devoid of Se-GSHPx activity maintain a high capacity to resist peroxidative attack, either via residual (non-Se)GSHPx activity or other compensatory GSH-associated detoxication mechanisms.

The toxicity of menadione (2-methyl-1,4-naphthoquinone) and two thioether conjugates studied with isolated renal epithelial cells

Menadione (2-methyl-1,4-naphthoquinone) was used as a model compound to test the hypothesis that thioether conjugates of quinones can be toxic to tissues associated with their elimination through a mechanism involving oxidative stress. Unlike menadione, the glutathione (2-methyl-3-(glutathion-S-yl)-1,4-naphthoquinone; MGNQ) and N-acetyl-L-cysteine (2-methyl-3-(N-acetylcysteine-S-yl)-1,4-naphthoquinone; M(NAC)NQ) thioether conjugates were not able to arylate protein thiols but were still able to redox cycle with cytochrome c reductase/NADH and rat kidney microsomes and mitochondria. Interestingly, menadione and M(NAC)NQ were equally toxic to isolated rat renal epithelial cells (IREC) while MGNQ was nontoxic. The toxicity of both menadione and M(NAC)NQ was preceded by a rapid depletion of soluble thiols and was associated with a depletion of soluble thiols and was associated with a depletion of protein thiols. Treatment of IREC with the glutathione reductase inhibitor, 1,3-bis(2-chloroethyl)-1-nitrosourea, potentiated the thiol depletion and toxicity observed with menadione and M(NAC)NQ indicating the involvement of oxidative stress in this model of renal cell toxicity. The lack of MGNQ toxicity can be attributed to an intramolecular cyclization reaction which destroys the quinone nucleus and therefore eliminates its ability to redox cycle. These findings have important implications with regard to our understanding of the toxic potential of quinone thioether conjugates and of quinone toxicity in general.

Exogenous glutathione increases endurance to muscle effort in mice

Many data suggest an involvement of toxic oxygen radicals in the termination of endurance to muscle fatigue. Being reduced glutathione (GSH), an efficient intracellular physiological antioxidant, experiments have been performed to discover whether exogenous GSH modifies endurance to exhaustive swimming in mice. GSH was administered to mice as a single dose (250, 500, 750 or 1000 mg/kg i.p.) or as repeated doses (250 mg/kg i.p. once a day during 7 days) 10 min before a swimming test to exhaustion. GSH 500, 750 and 1000 mg/kg, increased endurance to swimming by respectively 102.4%, 120.0% and 140.7%. GSH 250 mg/kg did not affect endurance when injected in a single dose but increased it by 103.7% when injected once a day for 7 days.

Molecular mechanisms of quinone cytotoxicity

Quinones are probably found in all respiring animal and plant cells. They are widely used as anticancer, antibacterial or antimalarial drugs and as fungicides. Toxicity can arise as a result of their use as well as by the metabolism of other drugs and various environmental toxins or dietary constituents. In rapidly dividing cells such as tumor cells, cytotoxicity has been attributed to DNA modification. However the molecular basis for the initiation of quinone cytotoxicity in resting or non-dividing cells has been attributed to the alkylation of essential protein thiol or amine groups and/or the oxidation of essential protein thiols by activated oxygen species and/or GSSG. Oxidative stress arises when the quinone is reduced by reductases to a semiquinone radical which reduces oxygen to superoxide radicals and reforms the quinone. This futile redox cycling and oxygen activation forms cytotoxic levels of hydrogen peroxide and GSSG is retained by the cell and causes cytotoxic mixed protein disulfide formation. Most quinones form GSH conjugates which also undergo futile redox cycling and oxygen activation. Prior depletion of cell GSH markedly increases the cell's susceptibility to alkylating quinones but can protect the cell against certain redox cycling quinones. Cytotoxicity induced by hydroquinones in isolated hepatocytes can be attributed to quinones formed by autoxidation. The higher redox potential benzoquinones and naphthoquinones are the most cytotoxic presumably because of their higher electrophilicty and thiol reactivity and/or because the quinones or GSH conjugates are more readily reduced to semiquinones which activate oxygen.

Historical aspects of glutathione and cancer chemotherapy

This presentation sets in historical context the impact of glutathione and its metabolism upon the efficacy of several anticancer drugs. The basic biochemistry of the tripeptide is reviewed briefly, highlighting its role in oxido-reduction and in the gamma-glutamyl cycle. The ability of selective modulators of glutathione metabolism, such as buthionine sulfoximine, as adjuncts to chemotherapy is also discussed.

Evidence for the participation of activated oxygen species and the resulting peroxidation of lipids in the killing of cultured hepatocytes by aryl halides

Primary cultures of rat hepatocytes were used to explore the mechanisms of the toxicity of aryl halides. The sensitivity of the hepatocytes to chloro-, bromo-, and iodobenzene was enhanced by inhibition of glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). In each case, the increased cell killing depended on the metabolism of the toxicant, a result shown by the protective effect of SKF-525A, an inhibitor of mixed function oxidation. BCNU decreased the metabolism of [14C]bromobenzene and the covalent binding of its metabolites by 20%. Chelation by deferoxamine of a cellular source of ferric iron prevented the cell killing in the presence or absence of BCNU. Deferoxamine had no effect on the metabolism or the covalent binding of [14C]bromobenzene. Similarly, the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD) reduced the cell killing and had no effect on the metabolism of [14C]bromobenzene. Thus, the toxicity of the three aryl halides was manipulated in ways that modify the sensitivity of hepatocytes to an oxidative stress, and the changes in cell killing occurred without parallel changes in the metabolism of [14C]bromobenzene or the covalent binding of its metabolites.

Enhanced depletion of lens reduced glutathione Adriamycin in riboflavin-deficient rats

The anticancer drug Adriamycin has photosensitizing properties which potentially may be detrimental to lens tissue. Since reduced glutathione (GSH) serves to protect lens from photo-oxidative stress and dietary riboflavin is required by glutathione reductase to regenerate GSH, we investigated whether Adriamycin intensifies the depletion of GSH levels in rat lens during dietary riboflavin deficiency. Three-week-old rats were divided into two groups. One group was fed a diet deficient in riboflavin (less than 1 ppm) and the other group was pair-fed a control diet containing adequate riboflavin (8.5 ppm). After 6-12 weeks of dietary treatment, half the animals in each dietary group received Adriamycin (8 mg/kg/day) intraperitoneally for 3 days. After killing the rats, lenses were removed, and GSH content and glutathione reductase activity were measured in freshly prepared homogenates. To determine the extent of systemic oxidative stress and the degree of riboflavin deficiency, glucose-6-phosphate dehydrogenase and glutathione reductase activities, respectively, were measured in erythrocytes. In lens of rats fed the riboflavin-sufficient diet, treatment with Adriamycin did not diminish GSH content or alter glutathione reductase activity. In confirmation of reports by others, lenses of animals fed the riboflavin-deficient diet had diminished GSH levels, lower basal glutathione reductase activity, and elevated glutathione reductase activity coefficients compared to those of animals pair-fed the control diet. The present study shows that in riboflavin-deficient rats, Adriamycin exacerbated the depletion of GSH but did not reduce further glutathione reductase activity. The implications of these findings are that nutritional deficiencies, in particular riboflavin deprivation, may pose a potential risk to lenticular tissue following Adriamycin treatment.

Menadione and cumene hydroperoxide induced cytotoxicity in biliary epithelial cells isolated from rat liver

Biliary epithelial cells (BEC) and parenchymal cells isolated from normal rat liver were exposed in vitro to a number of toxic compounds. BEC were found to be highly sensitive to concentrations of menadione (100 microM) and cumene hydroperoxide (10 microM) that are usually not effective as toxic agents in hepatocytes under normoxic conditions. On the other hand, BEC were not affected by concentrations of carbon tetrachloride or 7-ethoxycoumarin that are known to exert toxic effects on hepatocytes. The development of both menadione- and cumene hydroperoxide-induced toxic injury in BEC followed a common and time-correlated pattern, and included a strong depletion of GSH, depletion of protein thiols and an increase in the extent of cell death. The damage induced by cumene hydroperoxide was found to be independent of lipid peroxidative processes and was prevented by a pre-incubation with desferrioxamine. The cytotoxicity of menadione was further exacerbated by dicoumarol but was not prevented by desferrioxamine or promethazine. The mechanisms underlying BEC injury and death induced by the quinone and by the organic hydroperoxide are discussed in relation to the known biochemical characteristics of BEC.

Biochemical studies on bile duct epithelial cells isolated from rat liver

In the present paper we provide a basic enzymatic characterization of biliary epithelial cells (BEC) that have been isolated from normal rat liver. When compared with liver parenchymal cells, BEC display the following major features: (a) a very high specific activity of gamma-glutamyltranspeptidase (approx. 200-times higher than the value usually found in hepatocytes); (b) a lack of enzymes that are usually associated with the endoplasmic reticulum in hepatocytes such as cytochrome P-450, aminopyrine demethylase, glucose 6-phosphatase and NADPH cytochrome-c reductase; (c) the presence of enzymes related to the glutathione redox cycle (e.g., GSH-peroxidase, GSSG-reductase and different isozymes of GSH-transferase), but accompanied by a very low content in reduced glutathione. The enzyme pattern of BEC correlates well with histochemical and immunohistochemical studies, as well as with biochemical studies on bile ductular cells isolated from rat liver during cholestasis.

Quantitative analysis of glutathione in human brain tumors

Reduced glutathione (gamma-glutamylcysteinylglycine, GSH) plays an important role in the protection of cells against damage from free radicals and other electrophils and also influences cellular radiosensitivity, cellular response to hyperthermia, and cytotoxicity to some kinds of chemotherapeutic agents. The concentrations of GSH in 40 primary and metastatic brain tumors were quantitatively analyzed, and GSH was localized in these tumors by a novel o-phthalaldehyde histofluorescence method. The level of GSH was 195.2 +/- 57.1 micrograms/gm (mean +/- standard deviation) in glioblastomas multiforme, 444.1 +/- 105.1 micrograms/gm in normal brain tissues, and 614.4 +/- 237.4 micrograms/gm in meningiomas. The differences in GSH levels between glioblastomas and normal brain tissues and between glioblastomas and meningiomas were statistically significant (p less than 0.01). The mean GSH level in astrocytoma grades II and III was 321.9 +/- 11.8 micrograms/gm. The difference in the GSH level between glioblastomas and astrocytomas was statistically significant (p less than 0.05). Radiosensitive tumors, such as multiple myeloma, germinoma, and small-cell carcinoma, showed low GSH levels. These data suggest the possibility that the GSH may be a predictor for the efficacy of radiation therapy. The cytochemical study showed GSH localized in the cytoplasm; although it stained well in meningioma tissue, GSH was not well stained in sections of multiple myeloma. The endothelial proliferation did not stain well in glioblastoma, which seems to imply that this area is vulnerable to attack by free radicals from irradiation and/or chemotherapy.

Quantitative analysis of glutathione in rat central nervous system: comparison of GSH in infant brain with that in adult brain

Reduced glutathione (GSH) plays an important role in cellular protection against damage from free radicals and other electrophiles. GSH in the rat central nervous system was quantitatively analyzed. In adult brain, tissue from the cerebellum and cerebrum had higher GSH levels than that from the brainstem and spinal cord. Infant cerebrum and cerebellum contained lower levels of GSH than adult cerebrum and cerebellum. These data seem to suggest that the brainstem and spinal cord in adults, and the cerebrum and cerebellum in infants, are more vulnerable to injury by free radical-like radiation than the cerebrum and cerebellum in adults.

Enhanced in vitro cytotoxicity of 1,3-bis-(2-chloroethyl)-1-nitrosourea or buthionine sulfoximine combined with a reactive oxygen-generating enzyme immunotoxin

The glutathione inhibitor drugs, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and buthionine sulfoximine (BSO), were tested in vitro in order to assess their cytotoxic effectiveness when combined with an enzyme immunotoxin (eIT) composed of a T-cell reactive monoclonal antibody (mAb) 097 coupled to the reactive oxygen-generating enzyme, glucose oxidase (GO) (EC 1.1.3.4). As targets of this eIT we used mature human T-cells or leukemia cells that expressed the 097 epitope. We found that treatment of the cells with subtoxic amounts of mixtures of both a drug and the 097 eIT markedly potentiated cytotoxicity compared to either drug or eIT alone.

Potentiation of the cell specific toxicity of paraquat by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Implications for the heterogeneous distribution of glutathione (GSH) in rat lung

In order to study oxidative stress in the lung, we have developed a rat lung slice model with compromised oxidative defences. Lung slices with markedly inhibited glutathione reductase activity (approximately 80% inhibition) were prepared by incubating slices, with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) (100 microM) in an amino acid-rich medium for 45 min at 37 degrees. These lung slices had similar levels of GSH and ATP and polyamine uptake (a marker of alveolar epithelial type I and II cell function) to control rat lung slices. We have utilized these BCNU pretreated slices to study the effects of the herbicide, paraquat, in comparison to those of 2,3-dimethoxy-1,4-naphthoquinone, a potent redox cycler. Paraquat (10-100 microM) caused only minimal changes in the levels of GSH or ATP in control or compromised slices. In contrast, 2,3-dimethoxy-1,4-naphthoquinone caused a decrease in GSH in control slices but a markedly enhanced decrease in both GSH and ATP in compromised slices. Both compounds had only limited effects on putrescine and spermidine uptake in control slices. However, they caused a marked inhibition in compromised slices. Paraquat had little effect on 5-hydroxytryptamine uptake (a marker of endothelial cell function) in either control or compromised slices whereas the quinone inhibited uptake in the compromised slices. Thus, the lack of effect of paraquat on GSH and ATP does not support the involvement of oxidative stress in its toxicity. In contrast, using polyamine uptake, as a functional marker of alveolar epithelial cell damage, suggests a role for redox cycling. As paraquat is known to be accumulated primarily in alveolar type I and II cells (a small fraction of the lung cell population), our data suggest that only a small proportion of pulmonary GSH and ATP is present in alveolar epithelial type I and II cells but that much larger amounts may be present in endothelial cells. These studies highlight the problem of gross tissue measurements in heterogeneous tissues such as the lung.

Protection by exogenous glutathione against hypoxic and cyanide-induced damage to isolated perfused rat livers

In experiments with isolated perfused livers from fasted rats, addition of 2 mmol/l glutathione (GSH) to the perfusion medium protected against hepatic damage induced by cyanide or hypoxia and reoxygenation as evidenced by leakage of lactate dehydrogenase and hepatic calcium accumulation. In control experiments as well as in experiments with cyanide or hypoxia and reoxygenation, exogenous glutathione resulted in an augmentation of cellular glutathione content, indicating either direct uptake of GSH or stimulation of its intracellular synthesis. The protective effects of glutathione against hypoxic and cyanide-induced hepatotoxicity substantiate the role of oxidative stress in both types of injury.

Free radicals and anticancer drug resistance: oxygen free radicals in the mechanisms of drug cytotoxicity and resistance by certain tumors

Certain anticancer agents form free radical intermediates during enzymatic activation. Recent studies have indicated that free radicals generated from adriamycin and mitomycin C may play a critical role in their toxicity to human tumor cells. Furthermore, it is becoming increasingly apparent that reduced drug activation and or enhanced detoxification of reactive oxygen species may be related to the resistance to these anticancer agents by certain tumor cell lines. The purposes of this review are to summarize the evidence pointing toward the significance of free radicals formation in drug toxicity and to evaluate the role of decreased free radical formation and enhanced free radical scavenging and detoxification in the development of anticancer drug resistance by a spectrum of tumor cell types. Studies failing to support the participation of oxyradicals in the cytotoxicity and resistance of adriamycin are also discussed.

Role of the glutathione-glutathione peroxidase cycle in the cytotoxicity of the anticancer quinones

Recent studies have suggested that the selenoenzyme glutathione peroxidase, in the presence of reducing equivalents from the tripeptide glutathione, is responsible for detoxifying hydrogen peroxide and lipid hydroperoxides generated as a consequence of the cyclic reduction and oxidation of quinone-containing anticancer agents including doxorubicin, daunorubicin, mitomycin C, diaziquone, and menadione. Alterations in the intracellular levels of glutathione peroxidase or glutathione can significantly affect the activity of these drugs against human tumor cells and the expression of their normal tissue toxicity, especially with respect to the heart. Furthermore, augmentation of the glutathione peroxidase pathway appears to render certain human tumor cells relatively resistant to the anticancer quinones; therefore, the glutathione peroxidase system may, at least in part, modulate certain forms of acquired drug resistance in man. Thus, the glutathione peroxidase cycle appears to play a central role in maintaining intracellular peroxide homeostasis during quinone-induced oxidative stress.

Decrease of liver energy charge, ATP and glutathione at concomitant intraarterial administration of adriamycin and degradable starch microspheres in rat

Adriamycin (Adr) and degradable starch microspheres (DSM) were infused either combined or each separately into the hepatic artery in rats. Liver ATP, GTP, UDP-glucuronic acid, UDP-N-acetyl-hexosamine and energy charge and glutathione were decreased 20 min later with combined treatment but not by Adr or DSM when infused alone. the nucleotide levels were normalized 60 min after the combined treatment. After one week, the Adr rats showed a less weight gain than controls. The Adr + DSM rats lost weight. Only minor changes were found in the livers at microscopical examination at this time.

Doxorubicin (adriamycin): a critical review of free radical-dependent mechanisms of cytotoxicity

The antineoplastic drug doxorubicin is capable of generating a variety of free radical species in subcellular systems and this capacity has been considered critical for its antitumor action. However, for most tumor cell lines this mechanism of cytotoxicity does not appear to play a major role. Free radical-independent cytotoxicity mechanisms, taking place in the nuclear compartment of the cell, may more likely be involved in the antitumor effect of doxorubicin.

A role for the glutathione peroxidase/reductase enzyme system in the protection from paracetamol toxicity in isolated mouse hepatocytes

The role of the glutathione peroxidase/reductase (GSH-Px/GSSG-Rd) enzyme system in protection from paracetamol toxicity was investigated in isolated mouse hepatocytes in primary culture. The effect of inhibitors of these enzymes on the toxicity of paracetamol and on t-butylhydroperoxide (t-BOOH), used as a positive control, was determined. 1,3-Bis(chloroethyl)-1-nitrosourea (BCNU) was used to inhibit GSSG-Rd, and goldthioglucose (GTG) used to inhibit GSH-Px. Both these inhibitors increased cell membrane damage in response to oxidative stress initiated by t-BOOH. However, they also increased the susceptibility of hepatocytes to paracetamol toxicity, indicating that a component of paracetamol's toxic effect involves formation of species that are detoxified by the GSH-Px/GSSG-Rd enzymes. To further examine the role of these enzymes, age-related differences in their activity were exploited. Hepatocytes from two-week-old mice were less susceptible to both t-BOOH and paracetamol toxicity than were those from adult mice. This corresponds to higher activity of cytosolic GSH-Px/GSSG-Rd in this age group. However, after inhibition of GSSG-Rd with BCNU, hepatocytes from these postnatal mice were more susceptible to paracetamol toxicity. This suggests that the higher activity of GSH-Px/GSSG-Rd in hepatocytes from two-week-old mice is responsible for their reduced susceptibility to paracetamol toxicity. The data indicate that the GSH-Px/GSSG-Rd enzymes contribute to protection from paracetamol toxicity and suggest that formation of peroxides contributes to this drug's hepatotoxic effects.

The role of reductive and oxidative metabolism in the toxicity of mitoxantrone, adriamycin and menadione in human liver derived Hep G2 hepatoma cells

The cytotoxic properties of quinones, such as menadione, are mediated through one electron reduction to yield semi-quinone radicals which can subsequently enter redox cycles with molecular oxygen leading to the formation of reactive oxygen radicals. In this study the role of reduction and oxidation in the toxicity of mitoxantrone was studied and its toxicity compared with that of adriamycin and menadione. The acute toxicity of mitoxantrone was not mediated through one-electron reduction, since inhibition of the enzymes glutathione reductase and catalase, responsible for protecting the cells against oxidative damage, did not affect its toxicity. Adriamycin was the most potent inhibitor of protein and RNA synthesis of the three quinones. Menadione, at concentrations up to 25 microM, did not inhibit either protein or RNA synthesis unless dicoumarol, an inhibitor of DT-diaphorase, was also present. The two-electron reduction of menadione by DT-diaphorase is therefore a protective mechanism in the cell. This enzyme also protected against the toxicity of high concentrations (100 microM) of mitoxantrone. The inhibitory effect of mitoxantrone, but not of menadione or adriamycin, on cell growth was prevented by inhibiting the activity of cytochrome P450-dependent mixed function oxidase (MFO) system using metyrapone. This suggests that mitoxantrone is oxidised to a toxic intermediate by the MFO system.

Biochemical mechanism of GSH depletion induced by 1,2-dibromoethane in isolated rat liver mitochondria. Evidence of a GSH conjugation process

HPLC measurements of GSH and GSSG levels in isolated rat liver mitochondria, on addition of 1,2-dibromoethane (DBE), revealed the presence of a glutathione (GSH)-conjugating pathway of DBE. This process required the structural integrity of the mitochondrial matrix and inner membrane complex and was inhibited by the uncouplers of oxidative phosphorylation, particularly 2,4-dinitrophenol. On the other hand it was not affected by the energetic state of the mitochondria, since other mitochondrial inhibitors like KCN and oligomycin did not have any effect on it. This process also did not require the involvement of mitochondrial inner membrane transport systems, based on the measurement of the mitochondrial transmembrane potential. The involvement of mitochondrial GSH-S-transferases, located either in the matrix or in the intermembrane space, is discussed.

Lipid peroxidation and the metabolism of glutathione in rat liver and brain following ethylene oxide inhalation

The effects of repeated exposure to ethylene oxide on lipid peroxidation and glutathione metabolism in both rat liver and brain were examined. Increased levels of malondialdehyde (MDA) in the liver were observed after 6 and 13 weeks of exposure to ethylene oxide. The increased level of MDA observed in the hepatic homogenates of the treated rats reflected that of the microsomal fraction. On the other hand, no change in the level of MDA was detected in the brain of rats either at a 6- or 13-week treatment. Glutathione reductase activity was found to decrease at 6 or 13 weeks in liver and brain of treated rats. Both reduced and oxidized forms of glutathione in homogenates of liver and brain obtained from treated rats were, however, similar to those of the control at 40 h after the last exposure in individual experiments. To elucidate the cause of lipid peroxidation, the time course of glutathione content after exposure with ethylene oxide were studied in more detail. Significant decreases in both GSH and GSSG content in these organs were detected shortly after exposure to ethylene oxide and their levels recovered gradually with time and reached the control values at 40 h in the liver, although the changes were less significant in the brain as compared with those in the liver. These results suggest that enhancement of lipid peroxidation in the microsomal fraction of the liver after repeated exposure to ethylene oxide may possibly arise from repeated depletions of glutathione to certain critical levels and less removal of lipid peroxidation.

Influence of pH on the modification of thiols by carbamoylating agents and effects on glutathione levels in normal and neoplastic cells

In previous studies, we have suggested that the selective inhibitory effect of sodium cyanate (NaOCN) on hepatoma metabolism may be due to the lower pH observed in tumors relative to normal tissues. Lower pH might enhance the action of NaOCN by increasing the formation of isocyanic acid and carbamoylation of sulfhydryl groups. In the present work, studies were conducted on the effect of pH on the carbamoylation of sulfhydryl groups. The data indicated that carbamoylation of the sulfhydryl group of glutathione by NaOCN was enhanced by decreasing the pH from 7.4 to 6.6. A less pH-dependent response was observed with organic isocyanates. However, all reactions were reversible after the pH was increased by the addition of base. Kinetic studies showed that the rate of the reaction is very rapid, a maximal effect occurring within the first 10 min. Dose-dependent modifications of cellular glutathione by NaOCN and organic isocyanates were observed in human HT29 colon tumor cells, rat HTC hepatoma cells, and rat hepatocytes. The rate of carbamoylation of the glutathione sulfhydryl group in cells was similar to that of pure glutathione (GSH). The effect of buthionine sulfoxamine on GSH levels in cells was at least as great as that of sodium cyanate, but only the latter showed inhibitory effects on macromolecular synthesis; these were very rapid, pH-dependent, and reversible in tumor cells. Our results suggest that cellular sulfhydryl group(s) other than that of GSH might be involved in the effect of NaOCN on macromolecular synthesis.

Comparative study of demethoxydaunorubicin with other anthracyclines on generation of oxygen radicals and clonogenic survival of fibroblasts

Demethoxydaunorubicin was compared to other anthracyclines (daunorubicin, doxorubicin, epirubicin) on its ability to generate free oxygen radicals when mixed with Fe2+ in solution and on its ability to reduce clonogenic survival of fibroblasts in culture. Oxygen electrode measurements of free radical generation showed that most of the consumed oxygen entered the monovalet oxygen reduction pathway. Catalase and superoxide dismutase additions inhibited oxygen consumption for all tested anthracyclines and diethylenetriaminepentacetic acid (DTPA) was also inhibitory except for demethoxydaunorubicin. Demethoxydaunorubicin and epirubicin dose-dependently reduced the clonogenic survival of fibroblasts. Addition of catalase or superoxide dismutase was without effect, whereas metal chelators DPTA, desferrioxamine and EDTA all protected against epirubicin-induced toxicity. Of the chelators, only desferrioxamine protected against demethoxydaunorubicin toxicity. Tests in vivo will further elucidate whether demethoxydaunorubicin also differs from the other anthracyclines in therapeutic effect as well as in side effects such as myocardial toxicity.

Differential cytotoxicity of sodium arsenite in human fibroblasts and Chinese hamster ovary cells

Human skin fibroblast (HF) cells were approximately 10-fold more sensitive to sodium arsenite toxicity than Chinese hamster ovary (CHO) cells using the clonogenic assay. G1-phase CHO cells showed a 2-fold increase in the susceptibility to the toxic effects of sodium arsenite as compared to asynchronous CHO cells. The concentrations of sodium arsenite required to kill 50% of the cell population were correlated with the intracellular glutathione levels in asynchronous, G1-phase CHO, and asynchronous HF cells. Moreover, verapamil potentiated the cytotoxicity of sodium arsenite in CHO cells but not in HF cells. These results indicated that a verapamil-sensitive outward channel may be involved in detoxification of arsenic in CHO cells. Treatment with sodium arsenite resulted in a marked cell-cycle disturbance in CHO, but not in HF cells. Thus, CHO cells may take time to recover from sodium arsenite insult before progressing through the cell cycle. A different response of sodium arsenite in heat-shock protein synthesis in these 2 cell types was also revealed.

The toxicity of menadione and mitozantrone in human liver-derived Hep G2 hepatoma cells

The cytotoxic properties of quinone drugs such as menadione and adriamycin are thought to be mediated through one-electron reduction to semiquinone free radicals. Redox cycling of the semiquinones results in the generation of reactive oxygen species and in oxidative damage. In this study the toxicity of mitozantrone, a novel quinone anticancer drug, was compared with that of menadione in human Hep G2 hepatoma cells. Mitozantrone toxicity in these cells was not mediated by the one-electron reduction pathway. In support of this, inhibition of the enzymes glutathione reductase and catalase, responsible for protecting the cells from oxidative damage, did not affect the response of the Hep G2 cells to mitozantrone, whereas it exacerbated menadione toxicity. In addition, the toxicity of menadione was preceded by depletion of reduced glutathione which was probably due to oxidation of the glutathione. Mitozantrone did not cause glutathione depletion prior to cell death. DT-diaphorase activity and intracellular glutathione were found to protect the cells from the toxicity of both quinones. Inhibition of epoxide hydrolase potentiated mitozantrone toxicity but did not affect that of menadione. Our experiments indicate that mitozantrone toxicity may involve activation to an epoxide intermediate. Both quinone drugs inhibited cytochrome P-450-dependent mixed-function oxidase activity, although menadione was more potent in this respect.

Effect of 7-hydroxymethyl-12-methylbenz[a]anthracene and 1,3-bis-(2-chloroethyl)-1-nitrosourea on enzyme activities and oxidation of glutathione in cultured rat adrenal cells

1. The activities of enzymes participating in the regeneration of reduced glutathione (GSH), and their subcellular distribution were studied in cultured rat adrenal cells. 2. It has previously been shown that the adrenocorticolytic agent 7-hydroxymethyl-12-methylbenz[a]anthracene (7-hydroxymethyl-12-MBA) causes a drastic and selective oxidation of mitochondrial GSH in rat adrenal cells. Treatment of the adrenal cells with 7-hydroxymethyl-12-MBA, resulted in a minor decrease in the content of cytochrome c oxidase, nicotinamide nucleotide transhydrogenase, isocitrate dehydrogenase and cytosolic GSH reductase, whereas the activity of lactate dehydrogenase and citrate synthase was unaffected. None of these effects were considered to be responsible for the massive oxidation of mitochondrial GSH induced by 7-hydroxymethyl-12-MBA. 3. 1,3-Bis-(2-chloroethyl)-1-nitrosourea (BCNU) was used to obtain rat adrenal cells cultures with inactivated cytosolic and mitochondrial GSH reductase. The oxidation of mitochondrial GSH, induced by 7-hydroxymethyl-12-MBA, was not dramatically enhanced by the inactivation of GSH reductase, indicating that this enzyme was not rate-limiting in the regeneration of GSH. 4. Fractionation of rat adrenal cells with increasing concentrations of digitonin resulted in an earlier release of citrate synthase in cells treated with 7-hydroxymethyl-12-MBA compared with controls. These results may indicate damage to mitochondrial membranes as a result of 7-hydroxymethyl-12-MBA treatment.

[Biochemistry of thiol groups: the role of glutathione]

Glutathione (GSH) comprises the bulk of the pool of free thiol groups in biological systems. Since its first description as philothione 100 years ago, there have been repeated surprises in discoveries of novel functions. Just recently the important role of thioethers with products of the lipoxygenase reaction, i.e., the leukotrienes, was revealed as mediator of physiological and pathophysiological processes. Another major function resides in detoxication, GSH being cosubstrate in the GSH-peroxidase reaction for the reduction of hydroperoxides in the defense against oxidative stress. Interest also focuses on reactions of glutathionyl radicals in protection by thiols against DNA damage resulting from ionizing radiation.

Cell calcium, vitamin E, and the thiol redox system in cytotoxicity

The controversial role of extracellular Ca2+ in toxicity to in vitro hepatocyte systems is reviewed. Recent reports demonstrate that extracellular Ca2+-related cytotoxicity is dependent on Ca2+-influenced vitamin E (alpha-tocopherol) content of isolated hepatocytes. Based on a Ca2+-omission model of in vitro oxidative stress, the role of vitamin E in cytotoxicity is further explored. This model demonstrates the interdependence of the GSH redox system and vitamin E as protective agents during oxidative stress. Following chemical oxidant-induced depletion of intracellular GSH, cell morphology and viability are maintained by the continuous presence of cellular alpha-tocopherol above a threshold level of 0.6-1.0 nmol/10(6) cells. alpha-Tocopherol threshold-dependent cell viability is directly correlated with the prevention of the loss of cellular protein thiols in the absence of intracellular GSH. Potential mechanisms for this phenomenon are explored and include a direct reductive action of alpha-tocopherol on protein thiyl radicals, and the prevention of oxidation of protein thiols by scavenging of lipid peroxyl radicals by alpha-tocopherol. It is suggested that in light of the threshold phenomenon of vitamin E prevention of potentially severe oxidative stress-induced cytotoxicity, its use as a protective agent against an oxidative challenge in vivo should be reassessed.

The pulmonary toxicity of nitrosoureas

Drug-induced pulmonary fibrosis in cancer chemotherapy has become a more serious problem as treatment regimens are refined, larger total doses of cytotoxic drugs are administered and patient prognosis improves. The nitrosoureas are a class of chemotherapeutic agents whose clinical use is severely limited by drug-induced toxicities. The myelosuppression caused by nitrosourea administration can be managed clinically; however, the development of irreversible pulmonary fibrosis is a more serious clinical problem. Studies are needed to identify biochemical markers for early lung injury so that the amount of pulmonary tissue damage can be assessed and monitored. Additionally, considerable research is needed to understand the mechanisms by which these agents produce lung injury, so that therapeutic regimens can be developed to minimize or prevent lung toxicity. In understanding the mechanisms of toxicity, potentially the pulmonary injury caused by nitrosourea administration can be altered without affecting the clinical antitumor activity of this class of compounds. In the future, knowledge on mechanisms of drug-induced pulmonary injury can be used in the development of antineoplastic agents which are more disease specific and less pulmonary toxic.

Hepatic actions of levonorgestrel: correlations between biochemical and morphological findings

The influence of the synthetic sexual steroid levonorgestrel (LN) on rat liver in various doses and at different structural levels was investigated. A slight reactive hepatosis was found by histological examination after administration of LN in a dose of 10 mg per kg body wt. The same dose caused exclusively distinct lesions of the mitochondria, however, only in centrilobular parenchymal cells, whereas in the periportal hepatocytes only the lipid droplet content appears somewhat elevated. LN decreased the total glutathione content of the liver. The mitochondrial glutathione was decreased more intensively. One mg/kg body wt. of LN decreased the cytochrome P-450 content, but 10 mg/kg body wt. increased ethyl-morphine N-demethylation and 7-ethoxycoumarin O-deethylation activities. Distinct correlations could be shown between the biochemical changes and the ultrastructural findings.

Peroxidation-dependent and peroxidation-independent mechanisms by which acetaminophen kills cultured rat hepatocytes

Acetaminophen killed cultured hepatocytes prepared from male rats induced with 3-methylcholanthrene by two distinct mechanisms. With 0.5 to 5 mM acetaminophen, cell killing within 4 h depended on the inhibition of glutathione reductase by 1,3-bis(chloroethyl)-1-nitrosourea (BCNU) and was accompanied by the peroxidation of cellular lipids as assessed by the accumulation of malondialdehyde. The antioxidant diphenylphenylenediamine (DPPD) prevented both the peroxidation of lipids and the death of the cells. By contrast, DPPD had no effect on the metabolism of acetaminophen as assessed by the extent of the covalent binding of [3H]acetaminophen; by the rate and extent of the depletion of glutathione; and by the accumulation of acetaminophen metabolites in the culture medium. It is concluded that the peroxidation of the phospholipids of cellular membranes is the mechanism whereby 0.5 to 5 mM acetaminophen lethally injures cultured hepatocytes. With 10-20 mM acetaminophen, cell killing at 4 h still depended on BCNU. However, the amount of malondialdehyde in the cultures progressively decreased in parallel with the decreasing ability of DPPD to protect the cells. With 20 mM acetaminophen, there was no evidence of lipid peroxidation, and DPPD had no protective effect. Thus, a second mechanism of lethal cell injury with 10-20 mM acetaminophen is independent of lipid peroxidation and insensitive to antioxidants.

Potentiation in the intact rat of the hepatotoxicity of acetaminophen by 1,3-bis(2-chloroethyl)-1-nitrosourea

Studies of the killing of cultured hepatocytes by acetaminophen indicate that the cells are injured by an oxidative stress that accompanies the metabolism of the toxin (J. L. Farber et al. (1988) Arch. Biochem. Biophys. 267, 640-650). The present report documents that the essential features of the killing of cultured hepatocytes by acetaminophen are reproduced in the intact animal. Male rats had no evidence of liver necrosis 24 h after administration of up to 1000 mg/kg of acetaminophen. Induction of mixed function oxidase activity by 3-methylcholanthrene increased the hepatotoxicity of acetaminophen. Inhibition of glutathione reductase by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) potentiated the hepatotoxicity of acetaminophen in male rats induced with 3-methylcholanthrene. Whereas the pretreatment with BCNU reduced the GSH content by 40%, a comparable depletion of GSH by diethylmaleate did not potentiate the toxicity of acetaminophen. The antioxidant diphenylphenylenediamine (25 mg/kg) and the ferric iron chelator deferoxamine (1000 mg/kg) prevented the liver necrosis produced by 500 mg/kg acetaminophen in rats pretreated with BCNU. Neither protective agent prevented the fall in GSH produced by acetaminophen. It is concluded the conditions of the irreversible injury of cultured hepatocytes by acetaminophen previously reported are not necessarily different from those that obtain in the intact rat with this toxin.

Different cytotoxicity and metabolism of doxorubicin, daunorubicin, epirubicin, esorubicin and idarubicin in cultured human and rat hepatocytes

Both cytotoxicity and metabolism of five anthracyclines, namely doxorubicin, daunorubicin, epirubicin, esorubicin and idarubicin, were investigated in primary cultures of both rat and human adult hepatocytes and, for comparison, in a rat liver epithelial cell line. Toxicity was assessed by morphological examination and measurement of lactate dehydrogenase leakage after 24 hr of treatment. The rank order of toxicity for both rat and human hepatocytes was esorubicin greater than doxorubicin = epirubicin greater than or equal to idarubicin greater than daunorubicin, and for rat epithelial cells: esorubicin greater than or equal to epirubicin greater than idarubicin = daunorubicin = doxorubicin. Human cells were around 2-fold less sensitive than rat hepatocytes to all anthracyclines. Anthracyclines and their metabolites were analyzed by HPLC. Differences in both the percentages and routes of metabolism were demonstrated between rat and human hepatocytes. The main metabolite was the 13-dihydro-derivative (-ol derivative) in both species from daunorubicin, idarubicin and esorubicin. Glucuronides of epirubicin and epirubicinol were found only in human hepatocytes. In addition, several unidentified metabolites were detected of esorubicin, idarubicin and daunorubicin in rat hepatocytes. In human hepatocytes, only one unknown metabolite from daunorubicin and doxorubicin was found to be formed by cells from a different donor. In spite of variations between individuals, human hepatocytes generally metabolized anthracyclines more actively than did rat hepatocytes. Rat liver epithelial cells were only able to convert daunorubicin and idarubicin, the two molecules which have the best affinity for the non-specific NADPH-dependent aldoketoreductase system. Three compounds (doxorubicin, epirubicin and esorubicin) were present in large amounts in the cells as the parent drug, another (idarubicin) as the 13-dihydro-derivative. This comparative study on cytotoxicity and metabolism of five anthracyclines in rat and human hepatocyte cultures emphasises species differences and the importance of this in vitro model system for further analysis of the metabolism and effect of anthracyclines.

Adriamycin and its iron(III) and copper(II) complexes. Glutathione-induced dissociation; cytochrome c oxidase inactivation and protection; binding to cardiolipin

Some reactions of adriamycin (doxorubicin) and its Fe3+ and Cu2+ complexes were investigated with a view to understanding the mechanisms by which metal ion-adriamycin complexes damage cellular components. The ability of adriamycin in the presence of Cu2+ to inactivate the mitochondrial enzyme cytochrome c oxidase was effectively prevented by physiologic levels of glutathione. This result is explained by the observation that glutathione reacts with the Cu2+-adriamycin complex to produce free adriamycin. As sulfhydryl compounds are, in contrast, known to promote Fe3+-adriamycin-induced damage to cellular components, these results suggest that the response of a metal ion-adriamycin system to the presence of sulfhydryl compounds may be indicative of whether or not Cu2+-adriamycin is the damaging species. The partition of adriamycin into the octanol phase of an octanol-water two-phase system was greatly enhanced by the presence of cardiolipin. This result can be explained by the formation of a strong adriamycin-cardiolipin complex in the octanol phase which is one-half formed at an adriamycin concentration of 6 microM.

Alterations of surface morphology caused by the metabolism of menadione in mammalian cells are associated with the oxidation of critical sulfhydryl groups in cytoskeletal proteins

Incubation of freshly-isolated (rat hepatocytes) or cultured (HeLa, GH3, and McCoy) mammalian cells with menadione (2-methyl-1,4-naphthoquinone) resulted in the appearance of numerous cell surface protrusions. The perturbation of surface structure was associated with an increase in the amount of cytoskeletal protein and the oxidation of sulfhydryl groups in actin, leading to the formation of high-molecular weight aggregates sensitive to treatment with thiol reductants. Our findings indicate that the oxidation of thiol groups in cytoskeletal proteins may be responsible for menadione-induced cell surface abnormalities in mammalian cells.

Toxic consequence of the abrupt depletion of glutathione in cultured rat hepatocytes

Cultured hepatocytes were exposed to two chemicals, dinitrofluorobenzene (DNFB) and diethyl maleate (DEM), that abruptly deplete cellular stores of glutathione. Upon the loss of GSH, lipid peroxidation was evidenced by an accumulation of malondialdehyde in the cultures followed by the death of the hepatocytes. Pretreatment of the hepatocytes with a ferric iron chelator, deferoxamine, or the addition of an antioxidant, N,N'-diphenyl-p-phenylenediamine (DPPD), to the culture medium prevented both the lipid peroxidation and the cell death produced by either DNFB or DEM. However, neither deferoxamine nor DPPD prevented the depletion of GSH caused by either agent. Inhibition of glutathione reductase by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or inhibition of catalase by aminotriazole sensitized the hepatocytes to the cytotoxicity of DNFB. In a similar manner, pretreatment with BCNU potentiated the cell killing by DEM. DPPD and deferoxamine protected hepatocytes pretreated with BCNU and then exposed to DNFB or DEM. These data indicate that an abrupt depletion of GSH leads to lipid peroxidation and cell death in cultured hepatocytes. It is proposed that GSH depletion sensitizes the hepatocyte to its constitutive flux of partially reduced oxygen species. Such an oxidative stress is normally detoxified by GSH-dependent mechanisms. However, with GSH depletion these activated oxygen species are toxic as a result of the iron-dependent formation of a potent oxidizing species.

Characterization of the inhibition of glutathione reductase and the recovery of enzyme activity in exponentially growing murine leukemia (L1210) cells treated with 1,3-bis(2-chloroethyl)-1-nitrosourea

The inactivation of the enzyme glutathione reductase by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) was studied in exponentially growing murine leukemia cells. A 1-hr incubation with 1.6 +/- 0.2 microM BCNU resulted in a 50% inhibition of glutathione reductase, while 10 microM BCNU caused total inhibition of the enzyme. The time required for 50% inhibition of glutathione reductase by 10 microM BCNU was 7 min. The recovery of glutathione reductase activity was studied by incubating cells with 10 microM BCNU for 30 min to inhibit all glutathione reductase activity, washing the cells free of drug, and continuing the incubation in fresh medium. Fifty percent of the activity returned within 12 hr. Glutathione reductase activity recovered normally when cell growth and DNA synthesis were inhibited in the cells, but it failed to recover when protein synthesis was inhibited. Therefore, the inactivation of glutathione reductase appears irreversible, and the recovery of enzymatic activity is dependent on the synthesis of new protein. Continuous incubation with 19.8 +/- 0.4 microM BCNU resulted in a 50% inhibition of cell growth. A 1-hr incubation with 7.3 +/- 0.8 microM BCNU resulted in a 50% loss of viability as measured by a soft agar clonogenic assay. These experiments quantify the inhibition of glutathione reductase by BCNU and the recovery of enzyme activity in the context of the toxic effects of the compound. A clinically useful inhibitor of glutathione reductase will require a wider difference between the concentrations required for enzyme inhibition and cytotoxicity than BCNU provides.

Oxidative cell injury in the killing of cultured hepatocytes by allyl alcohol

The killing of cultured hepatocytes by allyl alcohol depended on the metabolism of this hepatotoxin by alcohol dehydrogenase to the reactive electrophile, acrolein. An inhibitor of alcohol dehydrogenase, pyrazole, prevented both the toxicity of allyl alcohol and the rapid depletion of GSH. Treatment of the hepatocytes with a ferric iron chelator, deferoxamine, or an antioxidant, N,N'-diphenyl-p-phenylenediamine (DPPD), prevented the cell killing but not the metabolism of allyl alcohol and the resulting depletion of GSH. Inhibition of glutathione reductase by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) sensitized the hepatocytes to allyl alcohol, an effect that was not attributable to the reduction in GSH with BCNU. The cell killing with allyl alcohol was preceded by the peroxidation of cellular lipids as evidence by an accumulation of malondialdehyde in the cultures. Deferoxamine and DPPD prevented the lipid peroxidation in parallel with their protection from the cell killing. These data indicate that acrolein produces an abrupt depletion of GSH that is followed by lipid peroxidation and cell death. Such oxidative cell injury is suggested to result from the inability to detoxify endogenous hydrogen peroxide and the ensuing iron-dependent formation of a potent oxidizing species. Oxidative cell injury more consistently accounts for the hepatotoxicity of allyl alcohol than does the covalent binding of acrolein to cellular macromolecules.

Physiological roles of nicotinamide nucleotide transhydrogenase

From the foregoing considerations, the energy-linked transhydrogenase reaction emerges as a powerful and flexible element in the network of redox and energy interrelationships that integrate mitochondrial and cytosolic metabolism. Its thermodynamic features make it possible for the reaction to respond readily to challenges, either on the side of NADPH utilization or on the side of energy depletion. Yet, the kinetic features are designed to prevent a wasteful input of energy when other sources of reducing equivalents to NADP are available, or to deplete the redox potential of NADPH in other than emergency conditions. By virtue of these characteristics, the energy-linked transhydrogenase can act as an effective buffer system, guarding against an excessive depletion of NADPH, preventing uncontrolled changes in key metabolites associated with NADP-dependent enzymes and calling on the supply of reducing equivalents from NAD-linked substrates only under conditions of high demand for NADPH. At the same time, it can provide an emergency protection against a depletion of energy, especially in situations of anoxia where a supply of reducing equivalents through NADP-linked substrates can be maintained. The flexibility of this design makes it possible that the functions of the energy-linked transhydrogenase vary from one tissue to another and are readily adjustable to different metabolic conditions.