Nickel-induced renal lipid peroxidation in different strains of mice: concurrence with nickel effect on antioxidant defense systems

Effect of Cu and Ni on cellular energy allocation in Enchytraeus albidus

Effects of nickel and copper on Enchytraeus albidus (Oligochaeta) were investigated using the cellular energy allocation approach. This methodology is used to evaluate the energetic status of an organism and is indicative of its overall condition. Enchytraeids were exposed to the reproduction Effect Concentrations (EC₅₀ and EC₉₀), and the parameters measured were the total energy reserves available (protein, carbohydrate and lipid budgets) and the energy consumption [based on electron transport system activity] which were further integrated to obtain the cellular energy allocation over different periods of exposure (0-2, 2-4 and 4-8 days). Carbohydrates (in comparison to lipids and proteins) were the only energy source mobilized in the case of nickel within 8 days of exposure. For copper exposure, protein budgets were also strongly reduced. Energy consumption increased in a time and dose-dependent way for copper and in the longer exposure period (4-8 days) at the EC₉₀ for Ni exposure, indicating that this is a good biomarker for effects of short-time metal exposure, while cellular energy allocation was only significantly reduced for the EC₉₀ of copper (4-8 days) and EC₅₀ of nickel (2-4 days). The effects of nickel at concentrations causing 50 and 90 % decrease in reproduction were likely not due to the changes in cellular energy allocation within 8 days of exposure.

Nickel Chloride (NiCl2) Induces Histopathological Lesions via Oxidative Damage in the Broiler's Bursa of Fabricius

The purpose of this study was to investigate the histopathological lesions, oxidative damage, changes of immunoglobulin G (IgG), immunoglobulin M (IgM), and immunoglobulin A (IgA) contents in the bursa of Fabricius and serum immunoglobulins (IgG, IgM, IgA) induced by dietary nickel chloride (NiCl2). Two hundred and eighty-one-day-old broilers were randomly divided into four groups and fed on a control diet and three experimental diets supplemented with 300, 600, and 900 mg/kg of NiCl2 for 42 days. Lesions were observed in the NiCl2-treated groups. Histopathologically, lymphocytes were decreased in lymphoid follicles with thinner cortices and wider medullae. Concurrently, the activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and the ability to inhibit hydroxyl radical and glutathione (GSH) contents were significantly (p < 0.05 or p < 0.01) decreased, while malondialdehyde (MDA) contents were increased in the NiCl2-treated groups. The serum IgG, IgM, and bursa IgG and IgM contents were significantly (p < 0.05 or p < 0.01) lower in the NiCl2-treated groups than those in the control group. The above-mentioned results show that dietary NiCl2 in excess of 300 mg/kg can cause histopathological lesions via oxidative damage, which finally impairs the function of the bursa of Fabricius and reduces IgG and IgM contents of the serum and the bursa of Fabricius. The study is aimed to provide helpful materials for studies on Ni- or Ni compounds-induced B cell toxicity in both human and other animals in the future.

Effects of ascorbic acid on carcinogenicity and acute toxicity of nickel subsulfide, and on tumor transplants growth in gulonolactone oxidase knock-out mice and wild-type C57BL mice

The aim of this study was to test a hypothesis that ascorbate depletion could enhance carcinogenicity and acute toxicity of nickel. Homozygous L-gulono--lactone oxidase gene knock-out mice (Gulo-/- mice) unable to produce ascorbate and wild-type C57BL mice (WT mice) were injected intramuscularly with carcinogenic nickel subsulfide (Ni₃S₂), and observed for the development of injection site tumors for 57 weeks. Small pieces of one of the induced tumors were transplanted subcutaneously into separate groups of Gulo-/- and WT mice and the growth of these tumors was measured for up to 3 months. The two strains of mice differed significantly with regard to (1) Ni₃S₂ carcinogenesis: Gulo-/- mice were 40% more susceptible than WT mice; and (2) transplanted tumors development: Gulo-/- mice were more receptive to tumor growth than WT mice, but only in terms of a much shorter tumor latency; later in the exponential phase of growth, the growth rates were the same. And, with adequate ascorbate supplementation, the two strains were equally susceptible to acute toxicity of Ni₃S₂. Statistically significant effects of dietary ascorbate dosing levels were the following: (1) reduction in ascorbate supplementation increased acute toxicity of Ni₃S₂ in Gulo-/- mice; (2) ascorbate supplementation extended the latency of transplanted tumors in WT mice. In conclusion, the lack of endogenous ascorbate synthesis makes Gulo-/- mice more susceptible to Ni₃S₂ carcinogenesis. Dietary ascorbate tends to attenuate acute toxicity of Ni₃S₂ and to extend the latency of transplanted tumors. The latter effects may be of practical importance to humans and thus deserve further studies.

Oxidative damage effects in the copepod Tigriopus japonicus Mori experimentally exposed to nickel

Tigriopus japonicus Mori has been recognized as a good model for toxicological testing of marine pollutants. Recently, a large number of genes have been identified from this copepod, and their mRNA expression has been studied independently against exposure to marine pollutants; however, biochemical-response information is relatively scarce. The response of T. japonicus to nickel (Ni) additions was examined under laboratory-controlled conditions in 12 days exposure. Superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione-S-transferase (GST), acetylcholinesterase (AchE), reduced glutathione (GSH), the ratio of reduced to oxidized glutathione (GSH/GSSG) and metallothionein (MT) were analyzed for Ni treatments (0, 0.125, 0.25, 0.75 and 3.0 mg/L) after 1, 4, 7 and 12 days. The thiobarbituric reactive species assay was used to evaluate lipid peroxidation (LPO) level in copepods after exposure. The results showed that Ni remarkably affected the biochemical parameters (SOD, GPx, GST, GSH, and GSH/GSSG) after certain exposure durations. However, the copepod's LPO level was significantly decreased under metal treatments after exposure, hinting that the factors involved in LPO might not significantly depend on the operations and functions in the antioxidant system. Ni exhibited the neurotoxicity to copepods, because its use obviously elevated AchE activity. During exposure, Ni initially displayed an inhibition effect but induced MT synthesis in T. japonicus by day 12, probably being responsible for metal detoxification. Thus, Ni had intervened in the detoxification process and antioxidant system of this copepod, and it could be used as a suitable bioindicator of Ni exposure via measuring SOD, GPx, GST, and MT as biomarkers.

In vivo genotoxic effect of nickel chloride in mice leukocytes using comet assay

DNA damage induced by nickel chloride (NiCl2) in leucocytes of Swiss albino mice has been studied in vivo. The comet assay or the alkaline single cell gel electrophoresis (SCGE) assay was used to measure the DNA damage. The mice were administered orally with acute doses of 3.4, 6.8, 13.6, 27.2, 54.4 and 108.8 mg/kg body weight (b.wt.) NiCl2. Samples of whole blood were collected at 24, 48 and 72 h, first week and second week post-treatment for alkaline SCGE assay to study single/double strand breaks in DNA. A significant increase in mean comet tail length indicating DNA damage was observed with NiCl2 at 24, 48 and 72 h post-treatment (P<0.05). A gradual decrease in the mean tail length was observed at 72 h post-treatment indicating repair of the damaged DNA. The mean tail length showed a dose-related increase and time dependent decrease after treatment with NiCl2 when compared to controls. The study also confirms that the comet assay is a sensitive and rapid method to detect DNA damage caused by heavy metals like nickel (Ni).

Nickel-induced oxidative stress and effect of antioxidants in human lymphocytes

The purpose of this study was to evaluate the oxidative effect in human lymphocytes after acute nickel (Ni) treatment for 1 h; levels of intracellular reactive oxygen species (ROS), lipid peroxidation (LPO) and hydroxyl radicals ((*)OH) were examined in isolated lymphocytes. The potential effects of antioxidants were also examined. After acute treatment, NiCl(2) (0-10 mM) significantly decreased the viability of lymphocytes. NiCl(2) appear to increase the degree of dichlorofluorescein (DCF) fluorescence and the levels of thiobarbituric acid-reactive substances (TBARS) in human lymphocytes in vitro in a concentration-dependent manner. The level of (*)OH was quantified by two main hydroxylated derivates, 2,3- and 2,5-dihydroxybenzate (DHB). Levels of 2,3- and 2,5-DHB were significantly higher in the Ni-treated group than in controls. Catalase partially reduced the NiCl(2)-induced elevation of oxidants and TBARS, whereas superoxide dismutase (SOD) enhanced the level of oxidants and TBARS. Both NiCl(2)-induced fluorescence and LPO were prevented significantly by glutathione (GSH) and mannitol. NiCl(2)-induced increase in generation of (*)OH was prevented significantly by catalase, GSH and mannitol, but not by SOD. These results suggest that NiCl(2)-induced lymphocyte toxicity may be mediated by oxygen radical intermediates, for which the accelerated generation of (*)OH may plays an important role in Ni-induced oxidative damage of human lymphocytes. Catalase, GSH and mannitol each provides protection against the oxidative stress induced by Ni.

Toxicology of dietary nickel in lake whitefish (Coregonus clupeaformis)

The sublethal toxicity associated with exposure of adult lake whitefish (Coregonus clupeaformis) to diets containing 0, 10, 100, and 1000 microg Ni per g for 10, 31, and 104 days was assessed through the measurement of responses, through a range of levels of biological organization. The accumulation and distribution of Ni in these fish are described in the previous manuscript (Ptashynski and Klaverkamp, 2001. Aquat. Toxicol. in press). Hematological parameters, including concentrations of glucose and hemoglobin and hematocrit, were not different between control and treated fish. Organ and whole organism parameters, including LSI, growth, and condition factor, were also unaffected. Histopathological lesions in kidney and liver proved to be the most sensitive and reliable indicators of Ni exposure. In livers of treated fish, areas of focal necrosis and altered bile ducts were observed. Histological alterations were observed throughout the posterior kidneys, in glomeruli, tubules, collecting ducts, and hematopoietic tissue, in fish fed medium and high dose diets. In whitefish kidneys, the frequency (%) of altered distal tubules and fields of views with alterations increased with the dose and duration of exposure. Significant increases in metallothionein concentrations were observed in intestine of whitefish fed the high dose diet on day 10, but these increases were not sustained. Significant increases in lipid peroxide concentrations were also observed in plasma of whitefish fed the high dose diet on day 31, but were not observed on day 104. These biochemical responses to dietary Ni require further evaluation. To evaluate exposure of natural populations of fish to Ni, analysis of Ni in kidney and liver and assessment of renal and hepatic histopathology are recommended for use in field bio-monitoring programs.

Nickel-induced plasma lipid peroxidation and effect of antioxidants in human blood: involvement hydroxyl radical formation and depletion of alpha-tocopherol

To provide evidence for the oxidative effect of nickel (Ni) treatment on blood, lipid peroxidation (LPO) and hydroxyl radical (*OH) generation were examined in human plasma. Nickel chloride induced LPO in plasma of human blood in vitro in a concentration-dependent (0-10 mM) and time-dependent (0-2 h) manner. The *OH production in plasma was quantified by measurement of conversion of salicylic acid (SA) into its hydroxylated products, 2,3- and 2,5-dihydroxybenzoate (DHB). The concentrations of 2,3- and 2,5-DHB in plasma increased in a concentration-dependent manner after Ni treatment for 1 h. Furthermore, a decreasing trend in alpha-tocopherol levels in plasma was observed after Ni treatment. Concurrent incubation with gluthathione (GSH), catechin (CTCH), and mannitol decreased lipid peroxidation and reduced *OH formation induced by Ni, but exacerbation of the decrease of alpha-tocopherol in plasma occurred with catechin.

Induction of 8-hydroxy-2'-deoxyguanosine by cobalt(II) and hydrogen peroxide in vitro

In order to test the effects of cobalt on oxidative DNA damage, we measured the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the presence of cobalt in calf thymus DNA and in DNA of human diploid fibroblasts. Treatment of calf thymus DNA with Co(II) at 500 microM hydrogen peroxide resulted in a dose-dependent increase in 8-OHdG, which culminated at 25 microM Co(II) (62.6 modified/10(5) dG) and declined at higher Co(II) concentrations [9.6 modified/10(5) dG at 250 microM Co(II)]. Incubation of calf thymus DNA with Co(II) alone did not cause an increase in 8-OHdG. Treatment of calf thymus DNA with H2O2, (0.25-2 mM) caused only a slight generation of 8-OHdG (2.7/10(5) dG, at 2 mM H2O2). Exposure of human diploid fibroblasts to Co(II) at 5-250 microM did not result in an increase in 8-OHdG in a dose-dependent manner, although treated cells showed significantly higher 8-OHdG levels than untreated controls (2.026 +/- 0.695 vs. 1.395 +/- 0.433 8-OHdG/10(5) dG) at all concentrations. Our data indicate that in the presence of H2O2, Co(II) stimulates the in vitro formation of 8-OHdG.

Role of oxidative stress and thiol antioxidant enzymes in nickel toxicity and resistance in strains of the green alga Scenedesmus acutus f. alternans

Treatment with Ni(NO3)2 leads to the formation of reactive oxygen species (ROS) in the green alga Scenedesmus acutus f. alternans, causing lipid peroxidation. This effect was stronger in a Ni-sensitive strain, UTEX72, than in a Ni-resistant strain, B4. In the resistant strain, Ni induced an increased ratio of reduced to oxidized glutathione (GSH:GSSG), whereas it caused a lowered ratio in the sensitive strain. Enzymes involved in the control of ROS were studied in these strains as well as two others that have shown different degrees of nickel resistance. The resistant strain, B4, which grows while containing large amounts of internal Ni, had much higher levels of glutathione reductase and catalase than the other strains. The sensitive strain, UTEX72, had higher levels of glutathione peroxidase, superoxide dismutase, and glucose-6-phosphate dehydrogenase than did strain B4. The resistant strains, Ni-Tol and Cu-Tol, derived from strain UTEX72, which are partly able to exclude Ni, had enzyme profiles that resembled that of UTEX72 more closely than that of B4. Treatment with 10 and 100 microM Ni for 4 or 22 h had complex effects on enzyme levels in all four strains. Ni decreased glutathione reductase in B4, slightly increased it in Ni-Tol and Cu-Tol, and did not affect the low levels of this enzyme in UTEX72. Ni lowered glutathione peroxidase in B4 and either did not affect it or slightly raised it in the other strains. Ni lowered catalase in B4 and did not affect the other strains. Superoxide dismutase was raised in B4 and Ni-Tol and lowered in Cu-Tol and UTEX72, and glucose-6-phosphate dehydrogenase was lowered in all four strains. These results suggest that one major mechanism of Ni resistance, especially in strain B4, may be the ability to combat the formation of ROS when exposed to this metal, likely by maintaining a high GSH:GSSG ratio.

Redox-modulated pathways in inflammatory skin diseases

Inflammatory skin diseases account for a large proportion of all skin disorders and constitute a major health problem worldwide. Contact dermatitis, atopic dermatitis, and psoriasis represent the most prevalent inflammatory skin disorders and share a common efferent T-lymphocyte mediated response. Oxidative stress and inflammation have recently been linked to cutaneous damage in T-lymphocyte mediated skin diseases, particularly in contact dermatitis. Insights into the pathophysiology responsible for contact dermatitis can be used to better understand the mechanism of other T-lymphocyte mediated inflammatory skin diseases, and may help to develop novel therapeutic approaches. This review focuses on redox sensitive events in the inflammatory scenario of contact dermatitis, which comprise for example, several kinases, transcription factors, cytokines, adhesion molecules, dendritic cell surface markers, the T-lymphocyte receptor, and the cutaneous lymphocyte-associated antigen (CLA). In vitro and animal studies clearly point to a central role of several distinct but interconnected redox-sensitive pathways in the pathogenesis of contact dermatitis. However, clinical evidence that modulation of the skin's redox state can be used therapeutically to modulate the inflammatory response in contact dermatitis is presently not convincing. The rational for this discrepancy seems to be multi-faceted and complex and will be discussed.

Genetic susceptibility to irritant-induced acute lung injury in mice

Recent studies suggest that genetic variability can influence irritant-induced lung injury and inflammation. To begin identifying genes controlling susceptibility to inhaled irritants, seven inbred mouse strains were continuously exposed to nickel sulfate (NiSO(4)), polytetrafluoroethylene, or ozone (O(3)), and survival time was recorded. The A/J (A) mouse strain was sensitive, the C3H/He (C3) strain was intermediate, and the C57BL/6 (B6) strain was resistant to NiSO(4)-induced acute lung injury. The B6AF(1) offspring were also resistant. The strain sensitivity pattern for NiSO(4) exposure was similar to that of polytetrafluoroethylene or ozone (O(3)). Pulmonary pathology was comparable for A and B6 mice. In the A strain, 15 microg/m(3) of NiSO(4) produced 20% mortality. The strain sensitivity patterns for lavage fluid proteins (B6 > C3 > A) and neutrophils (A >/= B6 > C3) differed from those for acute lung injury. This phenotype discordance suggests that these traits are not causally linked (i.e., controlled by independent arrays of genes). As in acute lung injury, B6C3F(1) offspring exhibited phenotypes (lavage fluid proteins and neutrophils) resembling those of the resistant parental strain. Agreement of acute lung injury strain sensitivity patterns among irritants suggested a common mechanism, possibly oxidative stress, and offspring resistance suggested that sensitivity is inherited as a recessive trait.

Inhibition of antimutagenic enzymes, 8-oxo-dGTPases, by carcinogenic metals. Recent developments

Nickel, cadmium, cobalt, and copper are carcinogenic to humans and/or animals, but the underlying mechanisms are poorly understood. Our studies have been focused on one such mechanism involving mediation by the metals of promutagenic oxidative damage to DNA bases. The damage may be inflicted directly in DNA or in the deoxynucleotide pool, from which the damaged bases are incorporated into DNA. Such incorporation is prevented in cells by 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphatases (8-oxo-dGTPases). Thus, inhibition of these enzymes should enhance carcinogenesis. We have studied effects of Cd(II), Cu(II), Co(II), and Ni(II) on the activity of isolated bacterial and human 8-oxo-dGTPases. Cd(II) and Cu(II) were strongly inhibitory, while Ni(II) and Co(II) were much less suppressive. After developing an assay for 8-oxo-dGTPase activity, we confirmed the inhibition by Cd(II) in cultured cells and in the rat testis, the target organ for cadmium carcinogenesis. 8-Oxo-dGTPase inhibition was accompanied by an increase in the 8-oxo-dG level in testicular DNA.

Antioxidant protection from solar-simulated radiation-induced suppression of contact hypersensitivity to the recall antigen nickel sulfate in human skin

Solar radiation induces suppression of the local effector mechanisms involved in immune responses to recall antigens. By using a low-dose solar-simulated radiation protocol, we investigated whether oral supplementation of the antioxidants RRR-alpha-tocopherol combined with L-ascorbic acid prevented radiation-induced suppression of the contact hypersensitivity response to nickel sulfate. In a prospective, randomized study, nickel-sensitive individuals were given RRR-alpha-tocopherol 2 g/d oral supplements combined with L-ascorbic acid 3 g/d for 50 d (group 1). Individuals in the control group were given a placebo (group 2). The reaction to a standardized patch test with serial dilutions of nickel sulfate and the irritant skin reaction to sodium lauryl sulfate were assessed by visual grading and by reflectance spectrophotometry in radiation-exposed and nonexposed skin 50 days after supplementation. Results showed that the contact hypersensitivity response to the recall antigen nickel sulfate was significantly suppressed in the radiation-exposed skin of those who took the placebo. Supplementation with RRR-alpha-tocopherol combined with L-ascorbic acid significantly protected against the radiation-induced suppression of the contact hypersensitivity response to nickel sulfate. The irritant reaction to sodium lauryl sulfate was not suppressed by radiation, and antioxidant supplementation did not modulate this response. In conclusion, a combination therapy of systemic high-dose RRR-alpha-tocopherol combined with L-ascorbic acid prevented solar-simulated radiation-induced suppression of the local immune response to the recall antigen nickel sulfate in human skin. This immunoprotective effect of combined RRR-alpha-tocopherol and L-ascorbic acid could be exploited for the prevention of solar radiation-induced skin cancer in an antioxidant intervention study.

Relative susceptibilities of C57BL/6, (C57BL/6 x C3H/He)F1, and C3H/He mice to acute toxicity and carcinogenicity of nickel subsulfide

The aim of this study was to compare susceptibility of mice of different strains to the toxicity and carcinogenicity of nickel subsulfide (Ni3S2), a water insoluble compound suspected to damage cells through oxidative mechanisms. Groups of 30 male mice of each strain, C57BL/6 (C57BL), (C57BL x C3H/He)F1 (B6C3F1), and C3H/He (C3H), were injected with single doses of 0.5-10 mg of Ni3S2/site into the thigh muscle and observed for up to 78 weeks. The highest Ni3S2 dose was lethal within 1 week to C57BL (93%) > B6C3F1 (80%) > C3H (53%) mice. The most susceptible C57BL mice also had the most severe necrotic/inflammatory kidney damage, compared with that in the other mice. The final incidence of local sarcomas at the 5 mg Ni3S2 dose was: C3H (97%) > B6C3F1 (76%) > C57BL (40% of mice at risk, i.e. those surviving at least 25 week; P < 0.05 or better as compared to the incidence in C3H mice). The relatively highest acute toxicity of Ni3S2 in C57BL mice observed in the present study, concurred with the weakest antioxidant response to systemic water-soluble nickel(II), resulting in reduction in glutathione (GSH) level and increased lipid peroxidation (LPO) in their kidneys and livers, two main targets of acute nickel toxicity, reported by us previously (Toxicol. Lett. 1991, 57, 269; ibid. 1991, 58, 121). Ni3S2 deposited in the muscle, therefore, constitutes a source of soluble nickel that can reach and damage distant organs. THe strongest tumor response in C3H mice relative to other strains did, in turn, concur with the lowest base levels of GSH and highest LPO in their muscle, the target for Ni3S2 carcinogenesis. Thus, the acute toxicity and carcinogenicity of Ni3S2 and Ni3S2-derived soluble nickel(II) in mice seem to depend, at least in part, on antioxidant capacity of target organs, which varies among different strains.

Up-regulation of glutathione synthesis in rat kidney by methyl mercury. Relationship to mercury-induced oxidative stress

Prolonged exposure of rats to methyl mercury hydroxide (MMH) results, during the initial phase of exposure, in the rapid accumulation of mercury as Hg2+ by kidney cortex and in a significant increase in oxidative stress, as characterized by the rate of formation of thiobarbituric acid reactive substances (TBARS) by renal mitochondria. These events are accompanied by a progressive increase in steady-state levels of the mRNA encoding gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in glutathione (GSH) synthesis and a 2- to 3-fold elevation in renal cortical GSH levels. The present study showed that the increase in GSH content was accompanied by a concomitant decrease in the rate of TBARS formation. Subsequent to these initial phase events, continued MMH exposure was characterized by equilibration in the rate of renal Hg2+ accumulation, a sharp decrease in both the TBARS formation rate and GCS mRNA level, but sustained elevation of renal cortical GSH content. Depletion of GSH with buthionine sulfoximine subsequent to the decline in the rate of TBARS formation did not result in a rebound of the TBARS formation rate. These findings suggest that oxidative stress during the initial phase of MMH exposure is derived from the transformation of CH3Hg+ to Hg2+, which, in turn, induces the synthesis of Hg(2+)- and/or oxidant-scavenging GSH molecules via the up-regulation of renal GCS mRNA. The findings also suggest that resistance to Hg(2+)-mediated oxidative stress may be more closely associated with the capacity for up-regulation of GSH synthesis than with elevated GSH levels per se.

Possible role of oxidative damage in metal-induced carcinogenesis

This review presents and evaluates evidence relevant to the mechanisms of metal carcinogenicity with special emphasis on the emerging hypothesis of the oxidative nature of metals' effect on DNA. The carcinogenic transition metals are capable of in vivo binding with the cell nucleus and causing promutagenic damage that includes DNA base modifications, inter- and intramolecular crosslinking of DNA and proteins, DNA strand breaks, rearrangements, and depurination. The chemistry of that damage and the resulting mutations observed in vitro and in metal-induced tumors are both characteristic for oxidative attack on DNA. The underlying mechanism involves various kinds of active oxygen and other radical species arising from metal-catalyzed redox reactions of O2, H2O2, lipid peroxides, and others, with certain amino acids, peptides, and proteins. Other metal-mediated pathogenic effects, such as enhancement of lipid peroxidation, stimulation of inflammation, inhibition of cellular antioxidant defenses, and inhibition of DNA repair, may also contribute to that mechanism. Thus far, published data revealing the oxidative character of metal-induced promutagenic DNA alterations are particularly strong for two of the most powerful human metal carcinogens, chromium and nickel. However, without excluding contribution of other effects, the promotion of oxidative damage tends to take the leading role in explaining mechanisms of carcinogenicity and acute toxicity of certain other metals as well.

Effect of nickel(II) and tetraglycine on hydroxylation of the guanine moiety in 2'-deoxyguanosine, DNA, and nucleohistone by hydrogen peroxide

The purpose of this study was to determine whether the Ni(II)-tetraglycine complex system (NiG4) that is known to disproportionate H2O2 at pH > or = 8 can catalyze oxidation of the guanine residues in 2'-deoxyguanosine (dG), calf thymus DNA, and calf thymus nucleohistone (NH) by H2O2 at physiological pH. Incubation of dG with H2O2 in the presence of NiG4 at 37 degrees C, produced two effects: (a) formation of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) and (b) decomposition of dG to 2,6-diamino-4-hydroxy-5-formamidopyrimidine and several low molecular weight unidentified products. The magnitude of both effects depended on incubation time (1-48 h), H2O2 concentration (7.5-40 mM), NiG4 concentration (0.1 or 1 mM), and pH (6.0-8.0). The effects were not detected below pH 6 and above pH 8.0. For 0.1 mM NiG4 and 7.5 mM H2O2, production of 8-OH-dG from dG (0.75 mM) during 24 h at 37 degrees C was significantly lower than from NH (1 mg/ml) or DNA (0.5 mg/ml), indicating possible specific effects that might be related to the strength of interaction of NiG4 with dG, NH, or DNA. The results indicate production of hydroxyl radical or other oxidizing species in the reaction of H2O2 with NiG4 at pH 7-8. Reactions like this may be relevant to the mechanisms of Ni(II)-mediated oxidative damage, observed in vitro and in vivo, which may contribute to the toxic and carcinogenic effects of this metal.

Effect of Ni(II) on tissue hydrogen peroxide content in mice as inferred from glutathione and glutathione disulfide measurements

Studies on Ni(II)-induced carcinogenesis have suggested that oxidative damage caused by Ni(II) may in part be due to increased tissue H2O2 formation. However, there is lack of evidence in vivo. Because of limitations of available methods for direct measurement of the in vivo rate of H2O2 formation in animals, Ni(II)-induced production of H2O2 was estimated from changes in the rate of glutathione disulfide (GSSG) formation. Male B6C3F1 mice (6-8 wk old) were injected i.p. with 170 mumol NiAc2/kg. Biliary efflux and liver, kidney, and lung levels of glutathione (GSH and GSSG) were determined 0-2 h after treatment. In spite of slight increases in tissue GSSG levels by Ni(II), there was no significant change in the biliary efflux of GSSG. Pretreatment with 50 mg/kg (i.p.) of bis-chloroethyl-nitrosourea (BCNU), an inhibitor of GSSG reductase, did not augment the effects of Ni(II) on GSSG formation significantly. Based on these observations, it was apparent that Ni(II) did not change the concentration of H2O2 significantly in vivo.

Generation of free radicals in reactions of Ni(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides

The generation of free radicals from reactions of nickel(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides was investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Incubation of nickel(II) [Ni(II)] with cysteine in an aerobic environment generated hydroxyl (.OH) radical, which then reacted with cysteine to generate a carbon-centered alkyl (.R) radical. Radical generation was inhibited under a nitrogen atmosphere. Model lipid hydroperoxides, cumene hydroperoxide, and t-butyl hydroperoxide enhanced the yield of these radicals and also generated an alkoxyl (.OR) radical. Radical yield decreased by approximately half under a nitrogen atmosphere. Although histidine did not cause radical formation in the reaction between Ni(II) and cumene hydroperoxide under aerobic conditions, the addition of histidine to a mixture containing Ni(II), cysteine, and cumene hydroperoxide under the same experimental conditions increased the yield of .R radical but lowered the yield of .OR and .OH radical adducts. It thus appears that histidine caused the .OH attack to be more site-specific. Similar results were obtained utilizing t-butyl hydroperoxide. Penicillamine or N-acetylcysteine yielded similar results except that under aerobic conditions, reaction between Ni(II) and N-acetylcysteine without hydroperoxide did not generate a significant concentration of free radicals. Under the same experimental conditions, cystine did not generate any detectable free radicals, suggesting an important role of the -SH group in Ni(II)-mediated free radical generation. The results indicate that free radical generation from the reaction of Ni(II)-thiol complexes and molecular oxygen, and/or lipid hydroperoxides, may play an important role in the mechanism(s) of Ni(II) toxicity and carcinogenesis.

Enhancement of gamma-glutamylcysteine synthetase mRNA in rat kidney by methyl mercury

Glutathione (GSH), a major cellular antioxidant, is elevated 2- to 3-fold in kidneys of rats during prolonged treatment with mercury as methyl mercury hydroxide (MMH). Increased renal GSH is accompanied by a dose- and time-related elevation in the relative abundance of mRNA hybridizable to a cDNA probe which encodes renal gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis. Renal GCS mRNA is maximally elevated 4.4-fold at 3 weeks following initiation of MMH treatment. Enhancement of GSH and GCS mRNA content corresponds to a relative sparing of renal cells from oxidative tissue damage during MMH exposure. These observations suggest that increased synthesis of GSH at the genetic level occurs as an initial adaptive response to mercury-induced oxidative stress in kidney cells.

Carcinogen-mediated oxidant formation and oxidative DNA damage

This article reviews the experimental data that points to formation of reactive oxygen species (ROS) and oxidative DNA base damage as being important contributors to cancer development. Particular emphasis is placed on the role they play in genetic changes occurring during tumor promotion. A number of structurally different anticarcinogenic agents inhibit ROS production and oxidative DNA damage as they inhibit inflammation and tumor promotion. This underlines the importance of ROS and oxidative genetic damage to the carcinogenic process. It also points to the possibility that some types of cancer may be preventable if the cycles of tumor promotion can be interrupted.