Fuminori Otsuka

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.

Silver staining for carboxymethylated metallothioneins in polyacrylamide gels.

A sensitive method for detecting metallothioneins (MTs) by use of silver staining after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of carboxymethylated MTs was developed. Carboxymethylation of metallothioneins is indispensable because it prevents their aggregation, thereby allowing each of them to be detected as a single band by SDS-PAGE. However, when the gel was subjected to the silver-staining method of C. R. Merril, D. Goldman, S. A. Sedman, and M. H. Ebert [(1981) Science 211, 1437-1438], the image of carboxymethylated purified MTs was totally negative. Pretreatment of the gel with 1% sodium thiosulfate just prior to the silver-staining procedure successfully reversed the negative image of carboxymethylated MTs. Further, they could be detected with a limit of nanogram levels per lane. This method can be applied to MTs in cell extracts from cultured cell lines treated with cadmium or to those from liver of cadmium-intoxicated mice.

Novel responses of ZRF, a variant of human MTF-1, to in vivo treatment with heavy metals

Heavy metal-dependent transcriptional activation of metallothionein (MT) genes is mediated by multiple enhancer sequences, metal responsive element (MRE), located in the upstream region of the genes. Previously, we have reported purification of a zinc-dependent MRE-binding protein, zinc regulatory factor (ZRF), from HeLa cells, and have pointed to the close relationship between ZRF and mouse MRE-binding transcription factor-1 (MTF-1) according to the analysis of partial amino acid sequences. By means of cDNA cloning and the product analyses, we show that ZRF is a variant of human MTF-1 (hMTF-1), which carries a single amino acid exchange in the zinc finger domain. Accordingly, ZRF is renamed hMTF-1b. Expression of hMTF-1b in HeLa cells is constitutive at both mRNA and protein levels, and is unaffected by treatment with cadmium (Cd). On the other hand, when cells were fractionated into nuclear extract and cytosol, a large part of the hMTF-1b was recovered in the cytosol fraction. A significant increase in the amount of nuclear hMTF-1b occurs when cells are treated with various heavy metals, including Cd, Zn, Cu and Ag, which is associated with concomitant decrease in the amount recovered in the cytosol fraction. Since immunocytochemical analysis revealed that intracellular distribution of hMTF-1b is restricted to the nucleus irrespective of the heavy metal treatment, such an increment in the nuclear extracts apparently results from promotion of nuclear retention of hMTF-1b by the heavy metal treatment. Analysis by native gel electrophoresis shows that the mobility of hMTF-1b significantly changes in association with Cd treatment, raising the possibility that a conformational change of hMTF-1b occurs in response to treatment with heavy metals in vivo.

Strain differences in cadmium-mediated suppression of lymphocyte proliferation in mice☆

Strain differences were investigated on the proliferative responses of splenic lymphocytes obtained from C3H/He, BALB/c, and DBA/2 mice that were treated with cadmium (Cd) for 5 days (0.5 or 1.0 mg Cd/kg/day, sc), and the results were compared with those of in vitro treatment of spleen cells with Cd. Following in vivo treatment, splenocytes from the C3H strain were significantly more susceptible to suppressive effects of Cd exposure on all indices for proliferative responses to mitogens (concanavalin A, phytohemagglutinin, and lipopolysaccharide) and allogeneic lymphocytes, while those from DBA and BALB strains were fairly resistant. Among the three strains, the highest Cd concentrations in plasma and spleen were obtained in the C3H strain with the lowest hepatic concentration of Cd. On the other hand, the Cd exposure hardly affected the splenic concentration of zinc in the C3H strain in contrast to its decrease in the others. When spleen cells obtained from normal mice were treated in vitro with Cd, the C3H strain was more resistant to the suppressive effect of Cd than the other strains. These results indicate that the mouse strain variations in Cd-mediated suppression of lymphocyte proliferation are not based on intrinsic lymphocyte sensitivities, but likely are due to differences in the metabolism of Cd, which is under genetic control.

Differential susceptibility of T- and B-lymphocyte proliferation to cadmium : relevance to zinc requirement in T-lymphocyte proliferation

Effect of zinc on an inhibitory action of cadmium to mitogen-induced lymphocyte proliferation was investigated. Cadmium at concentrations below 10 microM selectively inhibited concanavalin A-induced T-cell proliferation as compared with bacterial lipopolysaccharide-induced B-cell proliferation. Such differential susceptibility of T- and B-cell proliferation was not observed in the cases of other cations such as mercury, lead, nickel, molybdenum, chromium(VI) and arsenic (V). The inhibitory effect of 10 microM cadmium on T-cell proliferation was almost completely prevented by addition of 30 microM zinc to the culture medium, but was not by ferrous iron, nickel and copper. Further, cadmium exerted the same extent of inhibition even when it was added at 16 h after concanavalin A stimulation, and thereafter the inhibition gradually decreased. Correlated well with this observation, the protective effect of zinc was seen as far as it existed during the first 16 h of the mitogen stimulation. As intracellular cadmium content and a cadmium-induced metallothionein level were not changed by zinc addition, these observations strongly suggest that cadmium inhibits some zinc-dependent processes required for T-cell proliferation.

Detection of carboxymethylmetallothionein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

A sensitive method for detecting metallothioneins (MTs) by using silver staining and autoradiography after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of carboxymethylated MTs is described. Carboxymethylation of MTs is indispensable because it prevents their aggregation, thereby allowing each of them to appear as a single band using SDS-PAGE. Metallothioneins can be detected with a limit of nanogram levels per lane. This method can be applied to MTs induced in in vitro cultured cells and in in vivo tissues.

CHANGE IN ACTIVITY OF NUCLEAR POLY(ADP-RIBOSE) GLYCOHYDROLASE DURING THE HELA S3 CELL CYCLE

The change in activity of nuclear poly(ADP-ribose) glycohydrolase during the cell cycle of HeLa S3 cells was investigated. The poly(ADP-ribose) glycohydrolase activity was solubilized from HeLa S3 cell nuclei and chromosomes only by sonication at high ionic strength. The enzyme hydrolyzed poly(ADP-ribose) exoglycosidically, producing ADP-ribose. After release from mitosis, the activity of the solubilized nuclear poly(ADP-ribose) glycohydrolase per nucleus or per unit protein, assayed with [3H]poly(ADP-ribose) (average chain length, n = 15) as substrate, was lowest in the early G1 phase and highest in the late G1 phase. The specific activity in the late G1 phase was about two times that in the early G1 phase. The high activity remained constant during the S-G2-M phase. A similar change during the cell cycle was observed after release from hydroxyurea block. These results suggest that the activity of poly(ADP-ribose) glycohydrolase doubled during the G1 phase of the cell cycle of HeLa S3 cells.

A nuclear factor that interacts with metal responsive elements of a human metallothionein gene.

Metallothioneins (MTs) are low molecular weight heavy metal-binding proteins which are known to play a major role in heavy metal detoxification and understanding of their regulatory mechanism is toxicologically important. Expression of MT genes is induced by heavy metals and metal responsive elements (MREs) upstream of MT genes are essential for the transcriptional activation. By several types of mobility shift assay with 32P-labeled oligonucleotide probes, we detected HeLa cell nuclear as well as cytoplasmic factors that bind to MRE sequences of human MTIIA (hMTIIA) gene. One of the nuclear factors, which gives stronger signal than others, was further characterized. Competition experiments showed that the nuclear factor (named MREBP) specifically recognizes MREs of hMTIIA gene. EDTA abolished the binding of MREBP to MRE, suggesting that a divalent cation(s) is required for the complex formation. Also in blotting experiments with HeLa nuclear extract and the [32P]MRE probes an EDTA-sensitive 95k protein band, which possibly represents MREBP, was detected.

Modulation of the Immune Response by Trace Elements

The immune response comprizing multi-step processes is greatly susceptible to an alteration in nutritional and environmental factors including trace elements. Recently research concerns on the immune response and trace elements have been directed to clarify (a) their marginal levels in nutritional deficiency or excess environmental exposure, (b) immunological implications in pathogenesis of diseases caused in their deficiency or excess, and (c) their specific or critical significance in immune function. Current findings are briefly summarized from these aspects. Moreover Cd-induced autoantibody formation associated with its immunostimulative effects is described as an example for modulation of the immune response by direct interaction of a trace element with immune cells. Its mechanism is further discussed as follows: Cd as well as Hg may act on B lymphocytes, as a polyclonal activator and via involvement of T lymphocytes, consequently to induce production of autoantibodies such as anti-nuclear and anti-DNA antibodies.

Effect of 3-aminobenzamide on the process of ultraviolet-induced DNA excision repair

The effect of 3-aminobenzamide, a potent inhibitor of poly(ADP-ribosyl)ation, on UV-induced DNA excision repair was investigated. HeLa cells were treated with DNA replication inhibitors, hydroxyurea (HU) and 1-beta-D-arabinofuranosyl cytosine (araCyt), before and after ultraviolet light (UV) irradiation, to accumulate DNA single-strand breaks. The activity of poly(ADP-ribosyl)ation measured in the permeable cell system of HeLa cells was enhanced in a UV dose-dependent manner after the combined treatment with HU and araCyt in vivo. However, DNA repair synthesis in vitro was not affected by addition of 1 mM 3-aminobenzamide or nicotinamide, while incorporation of [3H]NAD in the same system was completely inhibited. Furthermore, neither the magnitude of UV-induced DNA single-strand breaks accumulated by the combined treatment of HU and araCyt nor the rate of their rejoining after release from the HU and araCyt block were influenced even in the presence of 10 mM 3-aminobenzamide. As the cytotoxicity of UV irradiation was significantly potentiated by 5 mM 3-aminobenzamide, these results suggest that poly(ADP-ribosyl)ation is involved in a process other than DNA excision repair induced by UV irradiation.