CD95/CD95 ligand-independent potentiation of treosulfan cytotoxicity by BSO in malignant glioma cells in vitro and in vivo

The glutathione synthase inhibitor, buthionine sulfoximine (BSO), specifically enhances the cytotoxic effects of treosulfan in human glioma cells. BSO depletes glutathione and greatly enhances treosulfan cytotoxicity in all the 12 human malignant glioma cell lines examined. None of these cell lines showed enhanced susceptibility to CD95L- or tumor necrosis factor (TNF)-alpha-induced apoptosis when glutathione is depleted. The combination of serial systemic BSO applications (300 mg/kg) and a single systemic dose of treosulfan (2.5 g/kg) reduced the growth of intracranially growing rat C6 gliomas in vivo by 73% whereas treosulfan alone reduced tumor growth by 16% and BSO alone had no effect. BSO lowered glutathione levels to 25-30% in peripheral blood mononuclear cells (PBMC) and to 50% in the glioma tissue. The glutathione levels in the non-tumor-bearing contralateral hemisphere were unaffected by systemic BSO treatment. The main side effects of treosulfan, gastrointestinal and bone marrow toxicity, were not significantly enhanced by BSO.

Acrylamide-induced cellular transformation

Acrylamide is a monomer of polyacrylamide, whose products are used in biochemistry, the manufacture of paper, water treatment, and as a soil stabilizer. While polymeric acrylamide is nontoxic, the monomer can cause several toxic effects and has the potential for human occupational exposure. While acrylamide is not mutagenic in prokaryotic mutagenesis assays, chronic acrylamide treatment in rodents has been shown to produce tumors in both rats and mice. The mechanism for the induction of tumors by acrylamide is not known. In the present study, we examined the possibility that acrylamide might induce cellular transformation, using Syrian hamster embryo (SHE) cell morphological transformation as well as potential mechanisms for the cellular transformation. Results showed that treatment with 0.5 mM and higher concentrations of acrylamide continuously for 7 days induced morphological transformation. Cotreatment with acrylamide and N-acetyl-L-cysteine (NAC), a sulfhydryl group donor, resulted in the reduction of acrylamide-induced morphological transformation in SHE cells. Cotreatment with 1-aminobenzotriazole (ABT), a nonspecific P450 inhibitor, and acrylamide produced no change in morphological transformation when compared to acrylamide treatment only. Cotreatment with acrylamide and DL-buthionone-[S,R]-sulfoximine (BSO), a selective inhibitor of gamma-glutamylcysteine synthetase, increased the percent of morphologically transformed colonies compared to acrylamide treatment alone. Acrylamide reduced GSH levels in SHE cells, and cotreatment with acrylamide and NAC prevented the acrylamide-induced reduction of GSH. BSO treatment with acrylamide enhanced the depletion of GSH. These results suggest that acrylamide itself, but not oxidative P450 metabolites of acrylamide appear to be involved in acrylamide-induced cellular transformation and that cellular thiol status (possibly GSH) is involved in acrylamide-induced morphological transformation.

Toxicity by pyruvate in HepG2 cells depleted of glutathione: role of mitochondria

Several studies have shown that pyruvate can scavenge H(2)O(2) and protect from H(2)O(2)-mediated cell injury. Mitochondria are critical participants in the control of apoptotic and necrotic cell death. Mitochondrial GSH plays an important role in the maintenance of cell functions and viability by metabolism of oxygen free radicals generated by the respiratory chain. Since loss of GSH, especially mitochondrial GSH, is associated with increased production of reactive oxygen species and cell toxicity, the ability of pyruvate to protect against these actions was evaluated. Adding pyruvate to HepG2 cells depleted of GSH by treatment with l-buthionine sulfoximine (BSO) surprisingly caused loss of viability after 24 and 48 h of incubation. Anoxia, treatment with antioxidants, and infection with cytosolic catalase, and interestingly, catalase expressed in the mitochondrial compartment were able to rescue the HepG2 cells from this pyruvate plus BSO injury, suggesting a key role for H(2)O(2), and lipid peroxides as mediators in the cytotoxicity. This toxicity and cell death observed was linked to damage to the mitochondria as evidenced by the increased lipid peroxidation in total homogenate and mitochondrial fraction, loss of mitochondrial membrane potential, and a decrease in protein-sulfhydryl groups. The type of cell death observed under these conditions was a mixture of apoptosis and necrosis. These results suggest that the protective ability of pyruvate against oxidant damage requires a functional GSH pool, especially in the mitochondrial compartment, and that in the absence of GSH, pyruvate increases cell injury by damaging the mitochondria, presumably as a consequence of enhanced electron flow and reactive oxygen production by the respiratory chain.

p-Aminophenol-induced liver toxicity: tentative evidence of a role for acetaminophen

p-Aminophenol (PAP) is a widely used industrial chemical and a metabolite of analgesics, such as acetaminophen (APAP). It was found recently that PAP, a known nephrotoxicant, could cause acute hepatotoxicity in mice but not in rats. The mechanism of hepatotoxicity is not known. The aim of this study was to investigate the role of N-acetylation of PAP to APAP in PAP-induced toxicity. Male C57BL/6 mice injected intraperitoneally (i.p.) with various doses of PAP were sacrificed at 12 hours for measurement of serum glutamic-pyruvic transaminase (GPT) and sorbitol dehydrogenase (SDH) levels and determination of the extent of hepatic nonprotein sulfhydryl (NPSH) and glutathione (GSH) depletion. Plasma levels of APAP and its metabolites were measured by HPLC after PAP administration. p-Aminophenol depleted NPSH in a dose- and time-dependent manner. Depletion of NPSH in mouse liver occurred at PAP doses above 400 mg/kg. Buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, potentiated the PAP-induced hepatotoxicity. Ascorbate, a reducing agent, did not affect PAP-induced hepatotoxicity and NPSH depletion. After PAP treatment, APAP and its sulfate and glucuronide conjugates as well as GSH conjugates (APAP-cysteine and APAP-mercapturate) were detected in the plasma. The results suggest the roles of GSH and N-acetylation of PAP to APAP in PAP-induced hepatotoxicity.

Peptide transport by the multidrug resistance protein MRP1

Small hydrophobic peptides were studied as possible substrates of the multidrug resistance protein (MRP)-1 (ABCC1) transmembrane transporter molecule. As observed earlier for P-glycoprotein- (Pgp; ABCB1) overexpressing cells, MRP1-overexpressing cells, including cells stably transfected with the MRP1 cDNA, showed distinct resistance to the cytotoxic peptide N-acetyl-Leu-Leu-norleucinal (ALLN). Resistance to this peptide and another toxic peptide derivative, which is based on a Thr-His-Thr-Nle-Glu-Gly backbone conjugated to butyl and benzyl groups (4A6), could be reversed by MRP1 inhibitors. The reduced toxicity of 4A6 in MRP1-overexpressing cells was found to be associated with lower accumulation of a fluorescein-labeled derivative of this peptide. Glutathione (GSH) depletion had a clear effect on resistance to ALLN but hardly affected 4A6 resistance. In a limited structure-activity study using peptides that are analogous to 4A6, MRP1-overexpressing cells were found to be resistant to these peptides as well. Remarkably, when selecting A2780 ovarian cancer cells for resistance to ALLN, even in the absence of Pgp blockers, resulting cell lines had up-regulated MRP1, rather than any of the other currently known multidrug resistance transporter molecules including Pgp, MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCCS), and the breast cancer resistance protein ABCG2. ALLN-resistant, MRP1-overexpressing cells were found to be cross-resistant to 4A6 and the classical multidrug resistance drugs doxorubicin, vincristine, and etoposide. This establishes MRP1 as a transporter for small hydrophobic peptides. More extensive structure-activity relationship studies should allow the identification of clinically useful peptide antagonists of MRP1.

Glutathione monoethyl ester protects against glutathione deficiencies due to aging and acetaminophen in mice

Our previous results indicated that glutathione (GSH) and/or cysteine (Cys) deficiency occurs in many aging tissues and also after acetaminophen (APAP) administration. The aim of this study was to investigate whether GSH monoethyl ester (GSH-OEt) can correct these deficiencies. Mice of different ages (3-31 months) through the life span were sacrificed 2 h after i.p. injection of GSH-OEt (10 mmol/kg). In separate experiments, old mice (30-31 months) received the same dose of ester 30 min before the administration of APAP (375 mg/kg) or buthionine sulfoximine (BSO, 4 mmol/kg), an inhibitor of GSH synthesis. Liver and kidney samples were analyzed for GSH and Cys by HPLC. The hepatic GSH and renal cortical GSH and Cys concentrations were about 30% lower in old mice (30-31 months) compared to mature mice (12 months). GSH-OEt corrected these aging-related decreases. APAP decreased both hepatic and renal cortical GSH and Cys concentrations in old mice, but GSH-OEt prevented these decreases. GSH-OEt also prevented the BSO-induced decreases in hepatic and renal GSH concentrations. The results demonstrated that GSH-OEt protected against GSH deficiency due to biological aging as well as APAP-induced decreases in old mice.

Effect of 1,25-dihydroxyvitamin D(3) on cultured mesencephalic dopaminergic neurons to the combined toxicity caused by L-buthionine sulfoximine and 1-methyl-4-phenylpyridine

A decrease in intracellular glutathione content may be related to the primary event in Parkinson's disease, so increasing the glutathione level may have a therapeutic benefit. The biologically active form of vitamin D, 1,25-dihydroxyvitamin D(3) [1, 25-(OH)(2)D(3)] has been recently reported to enhance the intracellular glutathione concentration in the central nervous system. Exposing rat cultured mesencephalic neurons for 24 hr to a mixture of L-buthionine sulfoximine (BSO) and 1-methyl-4-phenylpyridium ions (MPP(+)) resulted in a relatively selective damage to dopaminergic neurons. This damage has been accompanied by a reduction of intracellular glutathione levels. Low doses, i.e., 1-100 nM, of 1,25-(OH)(2)D(3) protect cultured dopaminergic neurons against this toxicity, although higher concentrations of this active form of vitamin D have been found to enhance the toxic effect. Generation of reactive oxygen species (ROS) by this toxicity has been attenuated in cultures being pretreated with low concentrations of 1,25-(OH)(2)D(3). Because the hormone increases the intracellular glutathione content in cultures, determining how this hormone suppresses ROS generation may involve the enhancement of the antioxidative system. These data suggest that low doses of 1,25-(OH)(2)D(3) are able to protect mesencephalic dopaminergic neurons against BSO/MPP(+)-induced toxicity that causes a depletion in glutathione content.

Hepatocellular response to chemical stress in CD-1 mice: induction of early genes and gamma-glutamylcysteine synthetase

Exposure of cells to toxic chemical species can result in reduced glutathione (GSH) depletion, generation of free radicals, and/or binding to critical cell determinants. Chemical stress is usually followed by a concerted cellular response aimed at restoring homeostasis, although the precise initial stimulus for the response is unclear. We have focused on one component of this stress response, the up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS) and the preceding molecular events involved in its regulation in an in vivo mouse model. Male CD-1 mice received buthionine sulphoximine (BSO; 7.2 mmol/kg), diethyl maleate (DEM; 4.2 mmol/kg), paracetamol (APAP; 3.5 and 1.0 mmol/kg), or carbon tetrachloride (CCl(4); 1.0 and 0.2 mmol/kg). Biochemical (serum transaminase and hepatic GSH levels) and molecular (c-jun and c-fos messenger RNA [mRNA] levels and activator protein 1 [AP-1] DNA binding activity) parameters were measured, as well as the consequent effects on gamma-GCS levels and activity. All compounds produced GSH depletion, but only the higher doses of APAP and CCl(4) caused liver damage. DEM, APAP, and CCl(4) increased c-jun and c-fos mRNA levels, together with an increase in AP-1 binding; BSO failed to induce AP-1 despite an increase in c-fos. Interestingly, the effects on gamma-GCS varied markedly according to the compound: BSO and DEM increased gamma-GCS enzyme activity, although only DEM, but not BSO, resulted in an increase in gamma-GCS(h) mRNA and protein. In contrast, APAP and CCl(4) both increased gamma-GCS(h) mRNA and protein; however, there was a marked dose-dependent decrease in gamma-GCS activity. These data indicate that the effect of chemical stress on the liver is compound specific and is not merely dependent on depletion of GSH.

Mechanisms of antiapoptotic effects of estrogens in nigral dopaminergic neurons

Parkinson's disease is characterized by the mesencephalic dopaminergic neuronal loss, possibly by apoptosis, and the prevalence is higher in males than in females. The estrogen receptor (ER) subtype in the mesencephalon is exclusively ER beta, a recently cloned novel subtype. Bound with estradiol, it enhances gene transcription through the estrogen response element (ERE) or inhibits it through the activator protein-1 (AP-1) site. We demonstrated that 17beta-estradiol provided protection against nigral neuronal apoptosis caused by exposure to either bleomycin sulfate (BLM) or buthionine sulfoximine (BSO). BLM and BSO-induced nigral apoptosis was blocked by inhibitors for caspase-3 or c-Jun/AP-1. The antiapoptotic effect by estradiol was blocked by ICI 182,780, an antagonist for ER, but not by a synthesized peptide that inhibits binding of the ER to the ERE. Estradiol had no effects on caspase-3 activation and c-Jun NH(2)-terminal kinase (JNK), which were activated by BLM. It also suppressed apoptosis by serum deprivation, which was independent of caspase-3 activation. Therefore, the antiapoptotic neuroprotection by estradiol is mediated by transcription through AP-1 site downstream from JNK and caspase-3 activation. Furthermore, 17alpha-estradiol, a stereoisomer without female hormone activity, also provided an antiapoptotic effect. Therefore, the antiapoptotic effect is independent of female hormone activity.

Potentiation of benzo[a]pyrene-induced pulmonary and forestomach tumorigenesis in mice by D,L-buthionine-S,R-sulfoximine-mediated tissue glutathione depletion

In vitro studies have suggested that the glutathione (GSH) S-transferase (GST)-catalyzed GSH conjugation is an important mechanism for the detoxification of (+)-anti-7,8-dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE], which is the activated form of the widespread environmental pollutant benzo[a]pyrene (BP). However, in vivo experimental evidence for the importance of GSH/GST system in defense against carcinogenic effects of BP is lacking. We hypothesized that if GSH/GST were to play an important role in the detoxification of (+)-anti-BPDE, the tumorigenic activity of BP would be increased by depleting the levels of GSH, which is the required nucleophilic substrate for GST-catalyzed conjugation reactions. In the present study, we have tested the above hypothesis by determining the effect of D, L-buthionine-S,R-sulfoximine (BSO)-mediated tissue GSH depletion on BP-induced tumorigenesis of the lung and forestomach in female A/J mice. Treatment of mice with three i.p. injections of 2.5 mmol BSO/kg (12 h apart) plus 20 mM BSO in drinking water, resulted in a statistically significant reduction in hepatic, pulmonary and forestomach GSH levels. At the same time, BSO-administration caused a statistically significant increase in BP-induced pulmonary and forestomach tumor multiplicity. To the best of our knowledge, the present study is the first report that provides in vivo experimental evidence for the importance of GSH/GST system in cellular protection against carcinogenic effects of BP.

Dual role of glutathione in selenite-induced oxidative stress and apoptosis in human hepatoma cells

It is well known that glutathione, the major intracellular antioxidant, is closely involved in the metabolism and bioactivity of selenium. In the present study, glutathione was demonstrated to play a dual role on selenite (Se)-induced oxidative stress and apoptosis in human hepatoma HepG(2) cells. The experiment was carried out in two different modes to modulate intracellular reduced glutathione (GSH) content. In Mode A (pretreatment), cells were pretreated with N-acetylcysteine (NAC), buthionine sulfoximine (BSO), or GSH prior to Se exposure. In Mode B (simultaneous treatment), cells were treated with Se and NAC, BSO, or GSH simultaneously. It was found that Se-induced oxidative stress and apoptosis are closely related to the intracellular level of GSH. Both the increase and depletion of GSH content significantly enhanced Se-induced oxidative stress and apoptosis in HepG(2) cells. Results from this study clearly demonstrated that GSH has a dual role in the effects of Se on cancer cells: (i) GSH acts as a pro-oxidant, facilitating Se-induced oxidative stress, and (ii) GSH acts as an antioxidant, protecting against Se-induced oxidative stress and apoptosis. Understanding such a unique association between GSH and Se may help to explain the controversy in the literature over the complex relationship between selenium and glutathione, and ultimately the capability of selenium to prevent cancer.

Evidence for the involvement of N-methylthiourea, a ring cleavage metabolite, in the hepatotoxicity of methimazole in glutathione-depleted mice: structure-toxicity and metabolic studies

In mice depleted of GSH by treatment with buthionine sulfoximine (BSO), methimazole (2-mercapto-1-methylimidazole, MMI) causes liver injury characterized by centrilobular necrosis of hepatocytes and an increase in serum alanine transaminase (SALT) activity. MMI requires metabolic activation by both P450 monooxygenase and flavin-containing monooxygenase (FMO) before it produces the hepatotoxicity. MMI and its analogues were examined for the ability to increase SALT activity in GSH-depleted mice. Saturation of the C-4,5 double bond in MMI resulted in a complete loss of hepatotoxicity. Similarly, ring fusion of a benzene nucleus to the C-4,5 double bond, forming 2-mercapto-1-methylbenzimidazole, abolished the toxic potency. As for MMI, 2-mercapto-1,4,5-trimethylimidazole, and 2-mercapto-1-methyl-4, 5-di-n-propylimidazole, the toxic potency decreased with the increasing bulk of the 4- and 5-alkyl substituents. Furthermore, methylation of the thiol group of MMI totally reduced its toxicity. These structural requirements and the known toxicity of thiono-sulfur compounds led us to the hypothesis that MMI would undergo epoxidation of the C-4,5 double bond by P450 enzymes and, after being hydrolyzed, the resulting epoxide would be then decomposed to form N-methylthiourea, a proximate toxicant. Before N-methylthiourea would produce toxicity, it would be further biotransformed to its S-oxidized metabolites mainly by FMO. Evidence for this hypothesis was provided by the facts that N-methylthiourea and glyoxal as the accompanying fragment were identified as urinary metabolites in mice treated with MMI and that N-methylthiourea caused a marked increase in SALT activity when administered to mice in combination with BSO.

Inhibition of L-homocysteic acid and buthionine sulphoximine-mediated neurotoxicity in rat embryonic neuronal cultures with alpha-lipoic acid enantiomers

In the present report, we have set out to investigate the potential capacity of both the oxidised and reduced forms of RS-alpha-lipoic acid, and its separate R-(+) and S-(-)enantiomers, to prevent cell death induced with L-homocysteic acid (L-HCA) and buthionine sulphoximine (BSO) in rat primary cortical and hippocampal neurons. L-HCA induced a concentration-dependent neurotoxic effect, estimated by cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) reduction, in primary neurons, but was significantly more toxic for hippocampal (EC(50)=197 microM) compared with cortical neurons (EC(50)=1016 microM) whereas D-HCA demonstrated only moderate (<20%) toxicity. On the other hand, cortical and hippocampal cultures were equally susceptible (341 and 326 microM, respectively) to the neurotoxic action of BSO. Antioxidants including butylated hydroxyanisole, propyl gallate and vitamin E protected cells against the neurotoxic effect of L-HCA and BSO. However, N-acetyl-cysteine and tert-butylphenyl nitrone, although capable of abrogating L-HCA-mediated cell death showed no protective effect against BSO-mediated toxicity. RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid and the enantiomers R-alpha-lipoic acid and S-alpha-lipoic acid protected cells against L-HCA-mediated toxicity with EC(50) values between 3.1-8.3 microM in primary hippocampal neurons and 2.6-16.8 microM for cortical neurons. However, RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid, and S-alpha-lipoic acid failed to protect cells against the degeneration induced by prolonged exposure to BSO, whereas the natural form, R-alpha-lipoic, was partially active under the same conditions. The present results indicate a unique sensitivity of hippocampal neurons to the effect of L-HCA-mediated toxicity, and suggest that RS-alpha-lipoic acid, and in particular the R-alpha-enantiomeric form is capable of preventing oxidative stress-mediated neuronal cell death in primary cell culture.

Role of oxyradicals in mutagenicity and DNA damage induced by crocidolite asbestos in mammalian cells

Crocidolite, one of the most carcinogenic forms of asbestos, is mutagenic in cultured mammalian cells when assayed using a system that can detect multilocus deletions. In the present study, we examined the effect of buthionine sulfoximine (BSO) on mutation frequency and the formation of 8-hydroxydeoxyguanosine (8-OHdG) in human-hamster hybrid (A(L)) cells induced by crocidolite fibers in an attempt to determine the role of oxyradicals in mediating fiber mutagenesis. BSO, a competitive inhibitor of the enzyme gamma-glutamyl cysteine synthetase, depleted nonprotein sulfhydryls to <5% of control within 24 h at a nonmutagenic dose of 25 microM. In cells pretreated with BSO for 24 h, the mutation yield at the CD59 locus induced by a 4 microg/cm2 dose of crocidolite fibers was increased by more than 3-fold (P < 0.05). Using immunoperoxidase staining with a monoclonal antibody specific for 8-OHdG, we demonstrated that crocidolite fibers induced a dose-dependent increase in oxidative DNA damage in A(L) cells. Furthermore, addition of DMSO, a well-established hydroxyl radical (OH*) scavenger, dramatically suppressed 8-OHdG induction (P < 0.005). Our results definitely demonstrate that reactive oxygen species mediate fiber-induced DNA damage mutagenesis in A(L) cells in a concentration-dependent manner.

Arsenic trioxide sensitivity is associated with low level of glutathione in cancer cells

Arsenic trioxide (As2O3) is a novel anticancer agent, which has been found to induce remission in acute promyelocytic leukaemic patients following daily intravenous administration. The therapeutic value of As2O3 in other cancers is still largely unknown. Cytotoxic tests in a panel of cancer cell lines showed that bladder cancer, acute promyelocytic leukaemic and gastrointestinal cancer cells were the most sensitive to As2O3 among 17 cell lines tested. Cellular glutathione (GSH) system plays an important role in arsenic detoxification in mammalian cells. Cancer cells that were intrinsically sensitive to As2O3 contained lower levels of GSH, whereas resistant cancer cells contained higher levels of GSH. On the other hand, there was no association of glutathione-S-transferase-pi or multidrug resistance-associated protein 1 levels with arsenic sensitivity in these cancer cells. Multidrug-resistant cancer cells that were cross-resistant to arsenic contained higher levels of GSH or multidrug-resistance-associated protein 1 than their drug-sensitive parental cells. Cancer cells become more sensitive to arsenic after depletion of cellular GSH with L-buthionine sulphoximine. We concluded that cellular GSH level is the most important determinant of arsenic sensitivity in cancer cells. Cellular GSH level and its modulation by buthionine sulphoximine should be considered in designing clinical trials using arsenic in solid tumours.

Evidence for functional discrimination between leukemic cells overexpressing multidrug-resistance associated protein and P-glycoprotein

Multidrug-resistance (MDR), caused by overexpression of either P-glycoprotein (Pgp) or the multidrug-resistance associated protein (MRP), is characterised by a decreased cellular drug accumulation. One form of MDR is the sequestration of the drug inside cytoplasmic vesicles followed by an a exocytotic and/or efflux process. In some studies, increased intracellular glutathione (GSH) has been associated with MDR. In this study, we examined the effects of 7-chloro-4-nitrobenz-2-oxa-1,3-diazole or NBD (a H(+)-ATPase pump inhibitor) and buthionine sulphoximine or BSO (an inhibitor of GSH biosynthesis) on the subcellular distribution of daunorubicin or DNR in two leukemic homoharringtonine-resistant K562 cell lines, overexpressing MRP (K-H30) and Pgp (K-H300). DNR nuclear accumulation was carried out using microspectrofluorometry. Our results show that DNR nuclear accumulation and sensitivity of K-H30 cells were increased by NBD and BSO whereas in K-H300 cells, NBD and BSO were unable to increase the DNR nuclear accumulation and sensitivity of these cells. This study demonstrates clearly that even if vesicular sequestration can happen in cells overexpressing MRP and Pgp proteins, only the MRP protein is able to extrude the drug through intracellular vesicles and efflux. In addition, GSH plays an important part in the pathway of drug transport in cells overexpressing MRP. Data entrain also the notion of functional discrimination between the MDR and MRP phenotype.

Glial cells mediate toxicity in glutathione-depleted mesencephalic cultures

We have examined the role of glial cells in the toxicity that results from inhibition of reduced glutathione (GSH) synthesis by L-buthionine sulfoximine (BSO) in mesencephalic cell cultures. We show that GSH depletion, to levels that cause total cell loss in cultures containing neurons and glial cells, has no effect on cell viability in enriched neuronal cultures. An increase in the plating cell density sensitizes glia-containing cultures to GSH depletion-induced toxicity. This suggests that cell death in this model is the consequence of events that are induced by GSH depletion and are mediated by glial cells. The antioxidant ascorbic acid and the lipoxygenase (LOX) inhibitor nordihydroguaiaretic acid (1-10 microM) provide full protection from BSO toxicity, indicating that arachidonic acid metabolism through the LOX pathway and the generation of reactive oxygen species play a role in the loss of cell viability. In contrast, inhibition of nitric oxide (NO) synthase affords only partial protection from BSO toxicity, suggesting that increased NO production cannot entirely account for cell death in this model. Our data provide evidence that GSH depletion in the presence of glial cells leads to neuronal degeneration that can be prevented by inhibition of LOX. This may have relevance to the pathogenesis of Parkinson's disease, where glial activation and depletion of GSH have been found in the substantia nigra pars compacta.

Characterization of H+-ATPase-dependent activity of multidrug resistance-associated protein in homoharringtonine-resistant human leukemic K562 cells

Multidrug resistance (MDR), caused by overexpression of either P-glycoprotein or the multidrug resistance-associated protein (MRP), is characterized by a decreased cellular drug accumulation due to an enhanced drug efflux. Many studies on cells overexpressing MRP and/or Pgp, have shown a concentration of the drug inside cytoplasmic acidic vesicles followed by an exocytotic process. In this study, we examined the effects of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole or NBD (a H+-ATPase pump inhibitor), buthionine sulphoximine or BSO (an inhibitor of glutathione (GSH) biosynthesis) and verapamil or VPL (a calcium channel blocker) on the subcellular distribution of daunorubicin or DNR in K562 cells overexpressing MRP (K-H30) and Pgp (K-H300) and A549 cells overexpressing spontaneously MRP. Nucleo-cytoplasmic distribution of DNR was carried out using scanning confocal microspectrofluorometry. This technique allows determination of nuclear accumulation of anthracyclines. Our results show that nuclear accumulation of DNR in K-H30 and A549 cells was increased by NBD, BSO and VPL while in K-H300 cells, only VPL was able to increase nuclear accumulation of DNR. Similarly, NBD, BSO and VPL could reverse DNR resistance in K-H30 cells whereas, in K-H300 cells, only VPL increased the sensitivity of these cells. These data suggest a requirement for GSH in MRP-mediated resistance and suggest that even if vesicular sequestration can happen in cells overexpressing MRP and Pgp proteins, probably only the MRP protein is able to extrude the drug through intracellular vesicles and efflux. Finally, NBD and BSO might be a useful agents in facilitating discrimination between Pgp and MRP phenotypes and prognosis in patients.

Antileukemic action of buthionine sulfoximine: evidence for an intrinsic death mechanism based on oxidative stress

The glutathione-depleting agent buthionine sulfoximine (BSO) was found to be toxic to some AML blast populations. This toxicity was manifested as the appearance of high levels of reactive oxygen generation in GSH-depleted cells, and later by the loss of mitochondrial membrane potential and an increase in intracellular calcium. Striking heterogeneity in BSO sensitivity was observed in a series of four human AML cell lines, and in fresh leukemic blasts obtained from eight AML patients. In some cases, toxicity was seen at BSO concentrations as low as 1 microM; approximately 100-fold less than the plasma levels achieved in patients treated with BSO as a drug resistance reversing agent. Based on these results we propose that some AML blast populations are unusually dependent on GSH-based antioxidant mechanisms, due to high intrinsic rates of reactive oxygen generation. The mitochondrial respiratory chain is the most likely source of this reactive oxygen. Because toxicity is seen at clinically achievable concentrations of BSO, this agent might have antileukemic activity in patients.

Phosphatidylcholine-specific phospholipase C regulates glutamate-induced nerve cell death

Phosphatidylcholine-specific phospholipase C (PC-PLC) is a necessary intermediate in transducing apoptotic signals for tumor necrosis factor and Fas/Apo-1 ligands in nonneuronal cells. The data presented here show that PC-PLC also is required in oxidative glutamate-induced programmed cell death of both immature cortical neurons and a hippocampal nerve cell line, HT22. In oxidative glutamate toxicity, which is distinct from excitotoxicity, glutamate interferes with cystine uptake by blocking the cystine/glutamate antiporter, indirectly causing a depletion of intracellular glutathione. A PC-PLC inhibitor blocks oxidative glutamate toxicity, and exogenous PC-PLC potentiates glutamate toxicity. The inhibition of PC-PLC uncouples the cystine uptake from glutamate inhibition, allowing the maintenance of glutathione synthesis and cell viability. These data suggest that PC-PLC modulates neuronal cell death through a mechanism that is distinct from that involved in nonneuronal apoptosis.

Involvement of oxidative stress in paraquat-induced metallothionein synthesis under glutathione depletion

The inhibition of glutathione (GSH) synthesis by L-buthionine-SR-sulfoximine (BSO) causes aggravation of hepatotoxicity of paraquat (PQ), an oxidative-stress inducing substance, in mice. On the other hand, synthesis of metallothionein (MT), a cysteine-rich protein having radical scavenging activity, is induced by PQ, and the induction by PQ is significantly enhanced by pretreatment of mice with BSO. The purpose of present study is to examine whether generation of reactive oxygens is involved in the induction of MT synthesis by PQ under inhibition of GSH synthesis. Administration of PQ to BSO-pretreated mice increased hepatic lipid peroxidation and frequency of DNA single strand breakage followed by manifestation of the liver injury and induction of MT synthesis. Both vitamin E and deferoxamine prevented MT induction as well as lipid peroxidation in the liver of mice caused by administration of BSO and PQ. In cultured colon 26 cells, both cytotoxicity and the increase in MT mRNA level caused by PQ were significantly enhanced by pretreatment with BSO. Facilitation of PQ-induced reactive oxygen generation was also observed by BSO treatment. These results suggest that reactive oxygens generated by PQ under inhibition of GSH synthesis may stimulate MT synthesis. GSH depletion markedly increased reactive oxygen generation induced by PQ, probably due to the reduced cellular capability to remove the radical species produced.

Nerve growth factor potentiates the oxidative necrosis of striatal cholinergic neurons

We examined the effects of nerve growth factor (NGF) on free radical neurotoxicity in striatal cell cultures. Following exposure to 30 microM Fe2+ or 1 mM L-buthionine-[S,R]-sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase, striatal neurons underwent cell body swelling and then widespread death over the next 24 h. The degeneration was prevented by addition of 100 microM trolox, an antioxidant. Addition of 100 ng/ml BDNF beginning 12 h before Fe2+ or BSO potentiated necrosis of most striatal neurons after exposure to 10 microM Fe2+ or 1 mM BSO. In contrast, treatment with 100 ng/ml NGF selectively potentiated the oxidative degeneration of striatal cholinergic neurons. The present findings provide additional evidence that NGF, like other neurotrophins, can potentiate oxidative neuronal cell necrosis.

Increased cytotoxic sensitivity of cultured FHM fish cells by simultaneous treatment with sodium dodecyl sulfate and buthionine sulfoximine

A previously described neutral red uptake inhibition assay on cultured fathead minnow (FHM) fish cells revealed a good correlation with fish toxicity data (Brandao et al., 1992) for a series of 50 test compounds, belonging to completely different chemical classes. The major drawback was the lower sensitivity of the cytotoxicity assay. Aiming at a higher sensitivity the assay was adapted by reducing the cell number, by a longer treatment period, and by simultaneous treatment with sodium dodecyl sulfate and buthionine sulfoximine (BSO). The fluorimetrically measured protein content was chosen as the endpoint. The endogenous glutathione (GSH) content was reduced by 54% of the test chemicals, confirming the importance of GSH in detoxification processes. Higher sensitivity was, at least partly, obtained by treating the FHM cells with BSO, reducing the GSH content to 22% of that in control cells. The cytotoxicity of the 50 chemicals was measured using the modified more sensitive assay. Although some exceptions were observed, this new assay is at least one order of magnitude more sensitive. Toxicity values comparable with fish toxicity data were obtained, making the assay sensitive enough to measure the toxicity of environmental water samples.

Deprenyl and desmethylselegiline protect mesencephalic neurons from toxicity induced by glutathione depletion

Oxidative stress is thought to play an important role in the pathogenesis of Parkinson's disease (PD). Glutathione (GSH), a major cellular antioxidant, is decreased in the substantia nigra pars compacta of PD patients. The aim of the present study was to investigate whether deprenyl and its desmethyl metabolite, putative neuroprotective agents in the treatment of PD, could protect cultured rat mesencephalic neurons from cell death caused by GSH depletion due to treatment with L-buthionine-(S,R)-sulfoximine (BSO). BSO (10 microM) caused extensive cell death after 48 hr, as demonstrated by disruption of cellular integrity and release of lactate dehydrogenase into the culture medium. Both deprenyl and desmethylselegiline, at concentrations of 5 and 50 microM, significantly protected dopaminergic neurons from toxicity without preventing the BSO-induced loss in GSH. Protection was not associated with monoamine oxidase type B inhibition in that pargyline, a potent MAO inhibitor, was ineffective and pretreatment with pargyline did not prevent the protective effects of deprenyl. Protection was not associated with inhibition of dopamine uptake by deprenyl because the dopamine uptake inhibitor mazindol did not diminish BSO toxicity. The antioxidant ascorbic acid (200 microM) also protected against BSO-induced cell death, suggesting that oxidative events were involved. This study demonstrates that deprenyl and its desmethyl metabolite can diminish cell death associated with GSH depletion.

Characterization of antioxidant properties of natural killer-enhancing factor-B and induction of its expression by hydrogen peroxide

Natural killer-enhancing factor B (NKEF-B) belongs to a highly conserved family of recently discovered antioxidants. The role of NKEF-B as an antioxidant was demonstrated by its protection of transfected cells to oxidative damage by hydrogen peroxide. To further characterize the antioxidant properties of NKEF-B, we compared the sensitivity of a human endothelial cell line ECV304 and its transfectant, B/1 that hyperexpresses NKEF-B, to various oxidants. In addition, we investigated the changes in the expression of NKEF-B mRNA upon oxidative stress. We found that B/1 was significantly more resistant than the control cells to the oxidative stresses caused by t-butyl hydroperoxide (t-BHP) and methyl mercury (MeHg). In contrast, there was no difference in the sensitivity of B/1 and the control cells to sulfhydryl reactive agents, diethyl maleate and diamide. B/1 was also as sensitive as the control cells to buthionine sulfoximine. The expression of NKEF-B mRNA was induced when the parental cell line ECV304 was treated with 2 mm HP. The induction reached a maximum level around 2 hr and decreased to the basal level around 4 hr. NKEF-A mRNA was not induced by HP. These results demonstrate antioxidant activities of NKEF-B toward prooxidants such as alkyl hydroperoxide and MeHg. Together with its antioxidant activity, the induction of NKEF-B by HP indicates that NKEF-B is an important oxidative stress protein providing protection against a variety of xenobiotic toxic agents.