Zinc effects on glutathione metabolism relationship to zinc-induced protection from alkylating agents

Zinc supplementation protects against cadmium accumulation and cytotoxicity in Madin-Darby bovine kidney cells

Cadmium ions (Cd2+) have been reported to accumulate in bovine tissues, although Cd2+ cytotoxicity has not been investigated thoroughly in this species. Zinc ions (Zn2+) have been shown to antagonize the toxic effects of heavy metals such as Cd2+ in some systems. The present study investigated Cd2+ cytotoxicity in Madin-Darby bovine kidney (MDBK) epithelial cells, and explored whether this was modified by Zn2+. Exposure to Cd2+ led to a dose- and time-dependent increase in apoptotic cell death, with increased intracellular levels of reactive oxygen species and mitochondrial damage. Zn2+ supplementation alleviated Cd2+-induced cytotoxicity and this protective effect was more obvious when cells were exposed to a lower concentration of Cd2+ (10 μM), as compared to 50 μM Cd2+. This indicated that high levels of Cd2+ accumulation might induce irreversible damage in bovine kidney cells. Metallothioneins (MTs) are metal-binding proteins that play an essential role in heavy metal ion detoxification. We found that co-exposure to Zn2+ and Cd2+ synergistically enhanced RNA and protein expression of MT-1, MT-2, and the metal-regulatory transcription factor 1 in MDBK cells. Notably, addition of Zn2+ reduced the amounts of cytosolic Cd2+ detected following MDBK exposure to 10 μM Cd2+. These findings revealed a protective role of Zn2+ in counteracting Cd2+ uptake and toxicity in MDBK cells, indicating that this approach may provide a means to protect livestock from excessive Cd2+ accumulation.

Protective effect of zinc on N-methyl-N-nitrosourea and testosterone-induced prostatic intraepithelial neoplasia in the dorsolateral prostate of Sprague Dawley rats

Previous studies have suggested that zinc exerts anticarcinogenic and antiproliferative effects against prostate cancer both in vitro and in rat ventral prostate. Zinc accumulation diminishes early in the course of prostate malignancy and it inhibits the growth of several carcinoma cells through induction of cell cycle arrest and apoptosis. In this study, we have investigated the influence of zinc on N-methyl-N-nitrosourea (MNU) and testosterone (T)-induced prostatic intraepithelial neoplasia in the dorsolateral prostate of Sprague Dawley (SD) rats. The results indicate that zinc plays an important role in prostate carcinogenesis. Increased tumor incidence was accompanied by a decrease in prostatic acid phosphatase activity, citrate, zinc, glutathione-S-transferase, reduced glutathione, p53, B-cell lymphoma protein (Bcl-2)-associated X protein and caspase-3 levels in MNU + T-treated rats. On the contrary, significantly increased phase I drug metabolizing enzyme activities, lipid peroxide, hydrogen peroxide, proliferating cell nuclear antigen, Bcl-2 and Bcl-X(L) protein levels were observed in the dorsolateral prostate of MNU + T-treated rats. Simultaneous zinc supplementation significantly reversed these effects in MNU + T-treated rats. Signs of dysplasia, a characteristic of prostatic intraepithelial neoplasia, were evident in the dorsolateral prostatic tissue sections by MNU + T administration. However, zinc supplementation has reversed these effects in the dorsolateral prostatic histoarchitecture. These results suggest that zinc may act as an essential trace element against MNU and testosterone-induced prostatic preneoplastic progression in SD rats.

Protective Effects of Zinc on Oxidative Stress Enzymes in Liver of Protein-Deficient Rats

Persons afflicted with protein malnutrition are generally deficient in a variety of essential micronutrients like zinc, copper, iron, and selenium, which in turn affects number of metabolic processes in the body. To evaluate the protective effects of zinc on the enzymes involved in oxidative stress induced in liver of protein-deficient rats, the current study was designed. Zinc sulfate at a dose level of 227 mg/L zinc in drinking water was administered to female Sprague-Dawley normal control as well as protein-deficient rats for a total duration of 8 weeks. The effects of zinc treatment in conditions of protein deficiency were studied on rat liver antioxidant enzymes, which included catalase, glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), glutathione reduced (GSH), and glutathione-S-transferase (GST). Protein deficiency in normal rats resulted in a significant increase in hepatic activities of catalase, glutathione peroxidase, glutathione reductase, and glutathione-S-transferase and the levels of lipid peroxidation. A significant inhibition in the levels of reduced glutathione and the enzyme activity of superoxide dismutase has been observed after protein deficiency in normal rats. Interestingly, Zn treatment to protein-deficient animals lowered already raised activity catalase, glutathione peroxidase, and glutathione-S-transferase and levels of lipid peroxidation to significant levels when compared to protein-deficient animals. Also, Zn treatment to the protein-deficient animals resulted in a significant elevation in the levels of GSH and SOD activity as compared to their respective controls, thereby indicating its effectiveness in regulating their levels in adverse conditions. It has also been observed that concentrations of zinc, copper, iron, and selenium were found to be decreased significantly in protein-deficient animals. However, the levels of these elements came back to within normal limits when zinc was administrated to protein-deficient rats. This study concludes that zinc has the potential to regulate the activities of oxidative stress enzymes as well as essential hepatic elements.

Zinc deficiency increases platelet oxidative stress in nephrectomized rats

We previously reported that reduced platelet endogenous antioxidant enzymes activities are related to the low plasma zinc level in patients with end-stage renal failure (ESRF). In this study, we attempt to evaluate whether dietary zinc deprivation reduces the activities of endogenous antioxidant and then enhances oxidative stress in the unstimulated platelet of normal and 5/6 nephrectomized (Nx) rats because increased platelet oxidative stress is suggested to involve in the incidence of thrombotic and atherosclerotic diseases. Male Sprague-Dawley rats (n = 48) were fed a zinc-deficient diet and deionized distilled water for 1 week to induce reduction of plasma zinc level. Half of the rats continued on this diet for 4 weeks as zinc-deplete group, and the other half were maintained on the same diet but with zinc-supplemented water (120 mg/L zinc sulfate solution) to correct the reduction of plasma zinc level as zinc-replete group. Half of each group underwent 5/6 Nx, while the other half underwent sham operation. Another 12 normal rats were fed standard rat chow (containing 23.4% protein and 50 ppm zinc) and drank deionized distilled water as normal control rats. In zinc-deplete rats including sham-operated and 5/6 Nx rats exhibited lower endogenous antioxidant enzymes activities such as reduced glutathione (GSH), superoxide dismutase (SOD), and glutathione peroxidase (GPX) and higher malondialdehyde (MDA) levels than normal control rats in the unstimulated platelets. However, in zinc-replete rats including sham-operated and 5/6 Nx rats have a normal endogenous antioxidant enzymes activity and normal MDA levels in the unstimulated platelets. We suggest that in uremia, the low plasma zinc level may be a risk factor for thrombotic and atherosclerotic diseases because it reduces the activities of endogenous antioxidant enzymes and increases oxidative stress in the unstimulated platelet.

Protective effects of zinc on lipid peroxidation, antioxidant enzymes and hepatic histoarchitecture in chlorpyrifos-induced toxicity

The present study investigated the hepatoprotective role of zinc in attenuating the toxicity induced by chlorpyrifos in rat liver. Male Sprague-Dawley (SD) rats received either oral chlorpyrifos (13.5mg/kg body weight), zinc alone (227mg/l in drinking water) or combined chlorpyrifos plus zinc treatment for a total duration of 8 weeks. The effects of these treatments were studied on various parameters in rat liver, including lipid peroxidation, antioxidant enzymes, levels of metallothionein (MT) and hepatic histoarchitecture. Chlorpyrifos treatment resulted in a significant increase in hepatic lipid peroxidation and activities of superoxide dismutase (SOD), glutathione peroxidase (G-Px) and glutathione reductase (GR). On the contrary, chlorpyrifos intoxication caused a significant inhibition in the levels of reduced glutathione (GSH), catalase (CAT) and glutathione-S-transferase (GST) activities. However, zinc treatment to chlorpyrifos-intoxicated animals normalized the otherwise raised levels of lipid peroxidation to within normal limits. Moreover, zinc treatment to these animals resulted in an elevation in the levels of GSH, catalase and GST, as well as a significant decrease in the levels of SOD. Levels of MT were also found to be depressed in chlorpyrifos-treated animals, but tended to increase following co-administration of zinc. Additionally, chlorpyrifos-treated animals demonstrated increased vacuolization, necrosis and ballooning of the hepatocytes and dilatation of sinusoids as well as increase in the number of binucleated cells. However, zinc administration to chlorpyrifos-treated animals resulted in overall improvement in the hepatic histoarchitecture, emphasizing the protective potential of zinc. Hence, the present study suggests the protective potential of zinc in alleviating the hepatic toxicity induced by chlorpyrifos.

Protective role of zinc in nickel induced hepatotoxicity in rats

This study was planned to determine the protective role of zinc, if any, in attenuating the toxicity induced by nickel sulfate in rat liver. Female Sprague Dawley (SD) rats received either nickel alone in the dose of 800 mg/l in drinking water, zinc alone in the dose of 227 mg/l in drinking water, and nickel plus zinc or drinking water alone for a total duration of eight weeks. The effects of different treatments were studied on various parameters in rat liver which include antioxidant enzymes, levels of nickel and zinc and histoarchitecture at the light microscopic level. Further, the activities of hepatic marker enzymes AST and ALT were also studied in rat serum. Nickel treatment to the normal control animals, resulted in a significant increase in lipid peroxidation and enzyme activities of catalase and glutathione-S-transferase. On the contrary, nickel treatment to normal rats caused a significant inhibition in the levels of reduced glutathione. Superoxide dismutase activity was found to be decreased which however was not significant. Interestingly, when Zn was supplemented to nickel treated rats, the activities of catalase, and glutathione-S-transferase and the levels of GSH and lipid peroxidation came back to within normal limits. Activities of serum AST and ALT were increased significantly following nickel treatment to normal rats. Simultaneous zinc administration to nickel treated rats tended to restore the altered levels of AST and ALT. Normal control and zinc treated animals revealed normal histology of liver. On the other hand, nickel treated animals showed alterations in normal hepatic histoarchitecture which comprise of vacuolization of the hepatocytes and dilatation of sinusoids as well as increase in the number of bi-nucleated cells. Administration of zinc to nickel treated rats resulted in marked improvement in the structure of hepatocytes, thus emphasizing the protective potential of zinc in restoring the altered hepatic histoarchitecture. The nickel administration to normal rats indicated increased concentrations of nickel and decreased concentrations of zinc. However, zinc effectively brought the altered levels of nickel and zinc to within normal range. The study concludes that zinc has the potential in alleviating the toxic effects of nickel in rat liver because of its property to induce metallothionein (S-rich protein) as a free radical scavenger, or its indirect action in reducing the levels of oxygen reactive species.

Time-course of changes in hepatic lipid peroxidation and glutathione metabolism in rats with carbon tetrachloride-induced cirrhosis

1. The aims of the present study were to assess: (i) the temporal relationships between hepatic lipid peroxidation, changes in the glutathione detoxification system and the onset/development of cirrhosis in CCl4-treated rats; and (ii) the effects of oral zinc administration on these parameters. 2. Cirrhosis was induced in 120 rats by intraperitoneal injections of CCl4 twice a week over 9 weeks. One hundred and twenty additional animals were used as controls. Both groups were further subdivided to receive either a standard diet or one supplemented with zinc. Subsets of 10 animals each were killed at weeks 1, 2, 3, 5, 7 and 9 from the start of the study. 3. Induction of cirrhosis produced a decrease in the components of the hepatic glutathione anti-oxidant system: glutathione transferase activity decreased from week 1, the concentration of reduced glutathione (GSH) decreased from week 5 and glutathione peroxidase (GPx) activity decreased from week 7. This impairment was chronologically related to an increase in free radical generation. Hepatic lipid peroxidation was significantly correlated with GPx activity (r = -0.47; P < 0.001) in CCl4-treated rats. Zinc administration did not produce any significant improvement of the hepatic glutathione system. 4. In conclusion, cirrhosis induction in rats by CCl4 administration produced a decrease in the hepatic glutathione antioxidant system that was related to an increase in free radical production. Furthermore, zinc supplementation produced a reduction in the degree of hepatic injury and a normalization of lipid peroxidation, but not an improvement of the hepatic GSH anti-oxidant system.

The protective effects of zinc on diethyldithiocarbamate cytotoxicity on rat astrocytes in vitro

The neurotoxicity of diethyldithiocarbamate (DDC) has been documented for decades. In particular, cytotoxic effects against rat astrocytes have been noted by a decrease in cell viability and numerous ultrastructural defects. This study indicates an in vitro protective effect by zinc administration prior to DDC insult. Cell groups pre-treated with 50 microM ZnCl2 prior to the addition of 35 microg/ml DDC showed significant protection when compared with cells treated with DDC alone. Zinc reduced the DDC-mediated toxicity to astrocytes as indicated by an increase in cell adherence and viability. Morphological evaluation indicated a significant decrease in ultrastructural alterations. Metallothionein (MT), a metal regulatory protein known to be induced by zinc was studied to determine its role in this mechanism of protection. Immunocytochemistry and immunoblots showed increased presence of MT in all zinc treated groups. This suggests a protective effect against DDC cytotoxicity on rat astrocytes in vitro may be associated with an increase in MT concentration.

Radioprotective effect of zinc aspartate on mouse spermatogenesis: a flow cytometric evaluation

The radioprotective effect of zinc aspartate on spermatogonial cells of whole-body irradiated mice was studied using flow cytometry. Adult male Swiss albino mice were treated with 30 mg/kg body weight of zinc aspartate 30 min before exposure to 0, 0.5, 1, 2 and 3 Gy of gamma-radiation. The animals were killed 7 to 70 days postirradiation and the relative percentages of different germ cells were analyzed by flow cytometry. A significant increase (p<0.002, 0. 0001, 0.005 and 0.008 for 0.5, 1, 2 and 3 Gy, respectively) in the relative percentage of spermatogonial (2C) population was observed in mice treated with zinc aspartate before exposure to different doses of gamma-radiation, compared to the irradiated controls on day 35 posttreatment. Also mean of each radiation dose of all the intervals studied showed a significant (p<0.03) increase in the relative percentage of spermatogonia. Despite the increase in the relative percentage of spermatogonia, the relative percentage of tetraploid cells (4C) remained higher in the zinc aspartate treated mice, compared to the irradiated controls. However, there was no change in the haploid populations viz. round (1C) and elongated (HC) spermatids of the zinc aspartate pretreated animals compared to irradiated controls. These data suggests that zinc aspartate pretreatment protects spermatogonia and tetraploid cells from radiation-induced cell killing.

Involvement of zinc in intracellular oxidant/antioxidant balance

The effect of zinc (Zn) on cellular oxidative metabolism is complex and could be explained by multiple complementary interactions. In this study, we evaluated the impact of Zn on the pro-oxidant/antioxidant balance of HaCaT keratinocytes. Cells were submitted to a diffusible metal chelator able to induce intracellular Zn deprivation, TPEN, in combination or not with Zn chloride (ZnCl2), in the culture medium. The intracellular amount of Zn, copper (Cu), and iron (Fe) was determined, as well as CuZnSOD and MnSOD activities and glutathione reserves. The consequence of the modulation of Zn concentration on lipid peroxidation was also evaluated. TPEN induced a significant dose-dependent decrease in intracellular Zn and Cu (from 394-181 and 43-21 microg/g protein, respectively, after 6 h of TPEN 50 microM). No significant change in intracellular Fe concentration was found following TPEN exposure. The SOD activities were unchanged after 6 h of TPEN 50 microM application, either CuZnSOD or MnSOD. Cells exposure to TPEN induced a deep time- and dose-dependent decrease in their glutathione content (from 65-8 microM/g protein after 6 h of TPEN 50 microM), and a concomitant increase in glutathione in the cell-culture supernatants. No significant change in lipid peroxidation products was detected.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

Evidence for potential application of zinc as an antidote to acetaminophen-induced hepatotoxicity

The therapeutic application of zinc sulphate as an antidote to acetaminophen overdose was examined in ICR mice. Hepatotoxicity was induced by a single oral dose of acetaminophen (750 mg/kg). Various treatments (normal saline, 15 or 30 mg/kg zinc sulphate, 150 mg/kg N-acetylcysteine, 15 mg/kg zinc sulphate + 150 mg/kg N-acetylcysteine) were given i.p. 1 h after acetaminophen overdose. Serum alanine aminotransferase, hepatic glutathione and malondialdehyde levels were measured before experiments and at various intervals after the administration of acetaminophen. Serum acetaminophen levels were also measured at different different intervals. Zinc sulphate showed protection by dose-dependently reducing alanine aminotransferase and malondialdehyde levels. The drug also partially prevented the depletion of hepatic glutathione. These effects were not as good as those of N-acetylcysteine. However, the combination of zinc sulphate with N-acetylcysteine produced even better protective effects. Furthermore, drug treatments did not affect serum acetaminophen levels. It is concluded that both drugs attenuate acetaminophen-induced hepatic toxicity, and the action is likely to be mediated through replenishment of hepatic glutathione levels. The use of zinc sulphate alone or in combination with N-acetylcysteine could be another alternative for the treatment of acetaminophen overdose in view of possible side effects produced by N-acetylcysteine.

Glutathione mercaptides as transport forms of metals

Among the many cellular functions of GSH, the roles of this tripeptide in metal transport, storage, and metabolism have recently received considerable attention. Although these roles had often been overlooked, they are critical for normal cellular metabolism and for protection from xenobiotics. Indeed, a number of the protective and regulatory functions of GSH are related to its ability to chelate reactive metals. GSH functions in the mobilization and delivery of metals between ligands, in the transport of metals across cell membranes, as a source of cysteine for metal binding, and as a reductant or cofactor in redox reactions involving metals. However, the interaction between GSH and metals can also produce or exacerbate cell injury. For example, GSH appears to be involved in the renal accumulation and toxicity of a number of metals, and in the carcinogenicity of chromium. Additional work is clearly needed to identify the mechanisms involved, and to better define the roles of GSH in metal homeostasis.

Effect of zinc supplementation on resistance of cultured human skin fibroblasts toward oxidant stress

In purified system zinc has been shown to have an antioxidant role. Its effects on the resistance of cultured cells towards oxidative stress in vitro were examined. Diploid human skin fibroblasts were grown for 21 d in culture media (RPMI 1640 containing 15% fetal calf serum) added with different zinc (Zn) concentrations (100, 125, and 150 microM as Zinc chlorur ZnCl2). In comparison, cell controls were grown in standard culture media (6.5 microM Zn). The intracellular zinc levels of treated fibroblasts increased from 3- to 7-fold (2330 +/- 120 ng/mg protein in 150-microM Zn-treated cells versus 331 +/- 21 ng/mg protein in control cells). The intracellular copper increased 3- fold whereas the iron content slightly but not significantly decreased. The index of basal lipid peroxidation measured as thiobarbituric acid reactants (TBARs) of zinc-supplemented cells was lower than that of non zinc supplemented controls (0.89 mumol/g protein in 150 microM Zn-treated cells versus 1.59 mumol/g protein in controls). At these high doses of zinc, fibroblasts expressed lower antioxidant metalloenzymes activities. Diminished TBARs in Zn treated cells tends to support that Zn acts protectively against free radical mediated damage. However when the cells were challenged with extracellular oxidant stresses mediated by hypoxanthine/xanthine oxidase or hydrogen peroxide (H2O2), an increased toxicity in Zn-supplemented cells was observed. When we applied an intracellular oxidative stress as UV-B or UV-A radiation, Zn-treated fibroblasts were more resistant than cells grown in normal medium. If Zn has shown antioxidant effect in some in vitro or in vivo systems our observations clearly demonstrate that this role is not mediated by antioxidant metalloenzymes.

Protective elevations of glutathione and metallothionein in cadmium-exposed mesangial cells

Exposure of cultured rat mesangial cells to CdCl2 caused a dose- and time-dependent increase in intracellular glutathione that was significant at 0.5 microM and maximal at 1 microM Cd2+. The effect depended on glutathione synthesis and was masked by inhibiting synthesis with buthionine sulfoximine. The cells responded to slightly higher concentrations of Cd with a marked decrease in DNA synthesis, and reversible depletion of glutathione enhanced this sensitivity. Pre-induction of the thiol-rich protein metallothionein with ZnCl2 afforded a degree of protection to the glutathione-depleted cells. We conclude that the Cd-dependent elevation of glutathione in these cells may be protective at concentrations of Cd that can arise during acute environmental and occupational exposures.

Zinc deficiency, ethanol, and myocardial ischemia affect lipoperoxidation in rats

The production of oxygen free radicals can be stimulated by excess iron, cadmium, nickel, and the like. Inversely, copper, zinc, and selenium inhibit production, either via their own action or via antiradical metalloenzymes. The study involved determining the effect of zinc deficiency combined with chronic ethanol administration on the status of blood and tissue free radicals, as well as on cardiac function in isolated, perfused rats' hearts. Animals were fed a basic diet containing residual zinc at 0.2-0.3 ppm. Following a zinc deficiency lasting 5 wk, which during the last 4 wk was accompanied by chronic ethanol administration, hearts were submitted to ischemia for 30 min in vitro, followed by reperfusion. Biochemical analyses (zinc, superoxide dismutase, malondialdehyde, conjugated dienes, and so on) were performed in the blood and in the homogenates of different organs. The experimental zinc deficiency caused a slight decrease of superoxide dismutase activity, accompanied by increased production of peroxidated lipids. Ethanol administration appeared to increase the levels of peroxidated lipids in the heart. Finally, the combination of zinc deficiency and ethanol administration had very harmful effects, especially on lipid peroxidation and contractile function of the isolated, perfused heart in preischemic conditions.

The interaction of ethanol and zinc on hepatic glutathione and glutathione transferase activity in mice

The effects of ethanol and/or zinc sulphate on liver glutathione and glutathione transferase activity were studied in mice. Ethanol suppressed glutathione transferase activity and had no significant effect on glutathione levels in the organ. Zinc sulphate administration dose-dependently increased glutathione transferase activity but did not affect hepatic glutathione content. Furthermore, the depressive action of ethanol on glutathione transferase activity was prevented by zinc sulphate pretreatment. It is suggested that zinc sulphate can reactivate glutathione transferase which in turn increases the excretion of the active metabolites produced by ethanol, through conjugation with glutathione in the liver. This action of zinc may alleviate the hepatic toxicity of ethanol in mice.

The physiological role of zinc as an antioxidant

The purpose of this review is to consider whether an essential biochemical function of zinc (Zn) is to serve as an antioxidant. Zn has been shown to have an antioxidant role(s) in defined chemical systems. Two mechanisms have been elucidated; the protection of sulfhydryl groups against oxidation and the inhibition of the production of reactive oxygens by transition metals. Supraphysiological concentrations of Zn have antioxidant-like effects in organelle-based systems and isolated cell-based systems in vitro. Administration of pharmacological doses of Zn in vivo has a protective effect against general and liver-specific prooxidants. Dietary Zn deficiency causes increased susceptibility to oxidative damage in membrane fractions from some tissues suggesting that increased oxidative stress may be a small but significant component of the pathology observed in dietary Zn deficiency. However, the biochemical basis for Zn deficiency pathology remains unelucidated; critical antioxidant functions for Zn may still be uncovered.

Overexpression of metallothionein confers resistance to anticancer drugs

Resistance to antineoplastic agents is the major obstacle to curative therapy of cancer. Tumor cell lines with acquired resistance to the antineoplastic agent cis-diamminedichloroplatinum(II) overexpressed metallothionein and demonstrated cross-resistance to alkylating agents such as chlorambucil and melphalan. Human carcinoma cells that maintained high levels of metallothionein because of chronic exposure to heavy metals were resistant to cis-diamminedichloroplatinum(II), melphalan, and chlorambucil. Furthermore, cells transfected with bovine papilloma virus expression vectors containing DNA encoding human metallothionein-IIA were resistant to cis-diamminedichloroplatinum(II), melphalan, and chlorambucil but not to 5-fluorouracil or vincristine. Thus, overexpression of metallothionein represents one mechanism of resistance to a subset of clinically important anticancer drugs.

Glutathione and metallothioneins as cellular defense against cadmium toxicity in cultured Chinese hamster cells

To evaluate the protective role of cell glutathione (GSH) against the toxicity of cadmium, the effect of GSH depletion on the metal toxicity was investigated and the role of glutathione was compared with that of zinc-induced metallothioneins (MTs). A 6-h incubation of cultured Chinese hamster V79 cells with 0.2 mM L-buthionine-SR-sulfoximine (BSO), a selective inhibitor of gamma-glutamylcysteine synthetase, resulted in approx. 95% depletion of GSH in the cells. The depletion of GSH did not influence the rate of cell growth, the amount of cell protein or the chromosome structure during culture for at least 24 h. Cells depleted or not depleted of GSH were challenged with (1-5).10(-5) M CdCl2 for 2 h and subsequent cell growth was evaluated. The cytotoxicity of cadmium was enhanced with increasing duration of BSO pretreatment and was correlated with the decrease of cell GSH, indicating that GSH constitutes a cellular defense against toxicity by cadmium. Inducibility of MTs by zinc was investigated in cultured V79 cells. Incubation of the cells with 1.10(-4) M zinc acetate did not result in accumulation of MTs over the control values for up to 2 h. Thereafter, however, the synthesis of MTs increased with increasing duration of zinc treatment and an approx. 9-fold increase in the amount of MTs was observed 10 h after addition of zinc. Depletion of cell GSH by BSO did not much influence the increased accumulation of MTs by zinc. In contrast, zinc at the same concentration did not influence the level of cell glutathione up to 12 h. The cytotoxicity of cadmium was markedly reduced in the cells pretreated with zinc and the protective effect of zinc was dependent upon duration of pretreatment, being parallel with the increased accumulation of MTs. Protection of cells from cadmium toxicity by zinc pretreatment was as or a little more effective in the cells depleted of GSH as in those not depleted. Thus, glutathione appears to be an intrinsic protector against cadmium toxicity, while MTs serve as an induced cellular defense that is mobilized against heavy metal stress, but takes more than 2 h to accumulate in significant amounts. Accordingly, it is suggested that GSH and MTs have cooperative protective roles against cadmium toxicity, as an initial defense for the former and a second-stage defense for the latter.

The effect of cellular age on zinc levels in untreated and zinc-treated human diploid fibroblasts

Cellular aging is accompanied by increased cellular permeability to zinc(II). The intrinsic zinc content of human diploid fibroblast cells increases with cell age, so that it quadruples from early to late passage, on a Zn(II) per cell or per cell volume basis, but it remains constant on a Zn(II) per protein basis. When the cells are challenged with toxic concentrations (0.2 mM) of Zn(II), both the rate of zinc incorporation into the cells and the amount of zinc incorporated at equilibrium increases considerably with age (unless measured as zinc per protein). In terms of growth inhibition, Zn(II) is more toxic to the cell than Cu(II), Mn(II), or Mg(II).

Zinc-induced reduction in melphalan cytotoxicity: heterogeneity of response of cloned human tumor cells

Previous studies with cultured human normal fibroblasts indicated that pretreatment of the cells with zinc for 12 h prior to exposure to the alkylating agent melphalan increased survival by seven- to ninefold over survival values obtained in cultures treated with drug only. Comparable pretreatment of cells derived from a variety of human tumors resulted in an increase in survival of 1.7-fold or less. To determine whether the limited responsiveness to zinc represented a general property of tumor cells (which would be characterized by a lack of highly zinc-responsive subpopulations contained within the parental tumor populations), a series of clones was prepared from the A101D human melanoma line and the A549 human alveolar cell carcinoma line. Cells from each clone were then challenged with melphalan with and without zinc pretreatment. Twenty-five percent of the tumor clones exhibited increased resistance to melphalan following pretreatment with zinc (range of 2.1- to 5.2-fold increase in survival), indicating that the parental tumor lines were highly heterogenous in regard to inducibility to a state of reduced sensitivity to melphalan. There was no evidence of a relationship between zinc-induced reductions in toxicity and induced elevations in total intracellular glutathione content, indicating that the primary effect of zinc is not directed toward elevating intracellular levels of glutathione.

Metallothionein-mediated cisplatin resistance in human ovarian carcinoma cells

We have determined the ability of two human ovarian carcinoma cells to over-express metallothioneins (MTs) and the subsequent effect this elevation has on DDP cytotoxicity. Cells of 2008 and COLO 316 human ovarian carcinomas that were resistant to CdCl2 were obtained by stepwise selection and chronic culture in CdCl2 and ZnCl2. The 2008/MT cells were 3.2-fold resistant to CdCl2 and 4.1-fold resistant to DDP; they had 23-fold elevated MTs. The COLO/MT cells were 1.2-fold resistant to CdCl2 and 3.3-fold resistant to DDP, and they had 9-fold elevated MTs. Glutathione (GSH) was also elevated in the Cd-resistant sublines. However, four times more intracellular thiols were contributed by the MTs than by the GSH. 2008 and 2008/MT cells were examined in more detail to elucidate the mechanism of DDP resistance. Depletion of GSH with D,L-buthionine-S,R-sulfoximine (BSO) had no effect on the sensitivity of these cells to either CdCl2 or DDP. Uptake of [195m Pt]DDP in 2008 and 2008/MT cells was identical. Fractionation of the cytosol from [195mPt]DDP-exposed cells on Sephadex G-75 revealed that 17% of the total cellular Pt in 2008/MT cells was associated with the MT fraction, as against 4% in the parent 2008 cells. This increase corresponded to a concomitant loss of Pt from the particulate fraction. Fractionation of 2008 cells selected with DDP (2008/DDP cells) indicated that elevated MTs did not contribute to the DDP resistance of these cells. Only 2% of the total cellular Pt was in the MT fraction in 2008/DDP cells. These results showed that elevation of MTs may be one mechanism of DDP resistance in ovarian carcinoma; however, in vitro selection with DDP does not trigger this mechanism.

Molecular and cellular mechanisms of cadmium resistance in cultured cells

Heavy metal induction of the synthesis of metallothioneins (MTs) provides an ideal model system for basic mechanistic studies of gene expression. Cell lines varying in their resistance to heavy metals have been isolated through a regime of exposure to serially increasing levels of Cd followed by clonal isolation. These cell lines have been used to examine the role of methylation and amplification in the Cd-resistant (Cdr) phenotype. It is suggested that regulation of expression of the MT genes in Cdr Chinese hamster cells is modulated at both the transcriptional and translational levels. An analysis of the MT2 gene sequence has uncovered a potential alternative splice site in the first intron. Usage of this site would insert 3 or 12 additional amino acids between amino acids 9 and 10. Analysis of the splicing pattern of the MT2 gene transcript in cultured cells has indicated that the second intron is preferentially removed prior to first intron excision.

Mechanism of protection by zinc against mercuric chloride toxicity in rats: effects of zinc and mercury on glutathionine metabolism

To investigate the mechanism by which zinc suppresses mercury toxicity, the effects of zinc and mercury on glutathione (GSH) metabolism in the rat kidney were studied. When the time course of GSH level in the rat kidney was examined at 2, 6, and 12 h after treatment of rats with both metals, an increase of GSH was found and was apparently related to the activation of some GSH-associated enzymes. In the kidney of rats treated with both metals, the response of the protective function involving GSH and GSH-associated enzymes depended on the magnitude of mercury toxicity but appeared to be independent of the zinc dosage. The administration of diethyl maleate (DEM), which depletes GSH, increased lipid peroxidation and mercury toxicity concomitantly with a decrease of GSH level in the kidney of rats treated with zinc and mercury. In conclusion, the data suggest that an increased GSH level in the kidney resulting from the activation of GSH-associated enzymes plays a role in the protective effect of zinc against mercury toxicity.

The roles of intracellular glutathione in antineoplastic chemotherapy

Glutathione is a sulfhydryl containing tripeptide that participates in detoxification of xenobiotic compounds, including the alkylating agents melphalan, cyclophosphamide, and BCNU. The role of glutathione in the detoxification of these compounds, both in terms of initial tumor response, and drug-induced resistance to these alkylating agents is examined. Since glutathione disulfide and glutathione are a pivotal redox pair, the modulation of intracellular glutathione levels is shown to change the cytotoxicity of drugs dependent on the redox cycle, such as adriamycin and bleomycin, as well as the oxygen dependent drug neocarzinostatin. Areas of further research are discussed.

Cytotoxicity related changes in biochemical cell function following in vitro cadmium treatment

In an effort to examine cellular responses to cadmium insult a bovine kidney cell line was used to monitor select cell functions for toxicity related alterations. Cadmium concentrations used ranged between 0.2 and 2.5 microM CdCl2 and elicited 0-85% cytotoxicity (cell attachment); 24-h incubations were used for all studies. Toxicity related inhibition of leucine incorporation into cellular protein and thymidine incorporation into DNA was noted. Decreases in protein synthesis activity closely paralleled the cytotoxicity profile; DNA synthesis was a less sensitive indicator to toxicity. K+-dependent phosphatase (KP), acid phosphatase (AP) and succinate dehydrogenase (SDH) were monitored in surviving cells and in a cell-free system. Significant inhibitions were detected for all enzyme activities following a 24 h culture with cadmium. KP and AP were most sensitive. In the cell-free system KP was significantly inhibited with 0.1 microM cadmium; AP and SDH were either unchanged or sensitive only at concentrations of 100 microM cadmium or greater. Reduced glutathione (GSH) concentration in surviving cells was elevated up to 7-fold over control cultures. The elevation occurred in a progressive toxicity-related manner.

Effects of cadmium on glutathione metabolism in cadmium sensitive and cadmium resistant Chinese hamster cell lines

Three cadmium resistant sublines, Cdr20F4, Cdr30F9, and Cdr200T1, resistant to 26, 40, and 145 micron CdCl2, respectively, have been derived from the cadmium sensitive Chinese hamster cell line CHO (resistant to 0.2 microM CdCl2). The resistance appears to be largely a function of the increased ability of the variant cells to induce the synthesis of metallothioneins (MTs) in response to cadmium, as the incorporation of [35S]cysteine into MTs ranges from an undetectable level to nearly 60% of the total cysteine incorporation into proteins in the CHO and Cdr200T1 cell lines, respectively. Treatment of the cadmium resistant sublines and the parent line with maximal subtoxic levels of Cdcl2 produced increases in the concentration of glutathione and glutathione S-transferase activity. In the parent, cadmium-sensitive CHO cell, the glutathione concentration began to increase after 9 h of exposure to 0.22 microM CdCl2 to over 250% of control level by 12 h. In 1 cadmium resistant line (Cdr20F4) the increase, again at 9 h, was preceded by a decrease, possibly due to depletion of the cysteine pools by the cadmium-induced MT synthesis. The second cadmium-resistant cell line (Cdr30F9) displayed no decrease, and the most cadmium-resistant line (Cdr200T1) showed the decrease and a recovery, but no significant increase by 12 h. In all cell lines there was a small but significant increase in glutathione S-transferase activity by 9 or 12 h. These responses may be specific for the thiol-reactive metal, cadmium, or may represent more general responses to cellular toxicity.