Cadmium-induced DNA strand damage in cultured liver cells: Reduction in cadmium genotoxicity following zinc pretreatment

Abstract

It is well established that zinc can decrease the carcinogenicity and toxicity of cadmium. In some tissues this may be due to the induction of metallothionein (MT). Therefore, in the present investigation, the effect of zinc pretreatment on cadmium-induced DNA strand damage was determined. The alkaline elution technique was used to assess DNA single strand damage (SSD) in cultured cells derived from rat hepatocytes (TRL-1215), a cell line previously shown to have an active MT gene. The ability to detect SSD in TRL-1215 was established following exposure to γ-irradiation. Exposure to increasing doses of γ-irradiation (150–600 rad) resulted in a dose-dependent increase in SSD. Exposure of TRL-1215 cells to CdCl 2 for 1 hr at 37°C, using concentrations from 5 to 250 μ m, failed to induce detectable SSD in these cells; however, exposure to 500 μ m CdCl 2 resulted in significant SSD. A time-dependent increase in SSD was demonstrated following a 2 hr continuous exposure to 500 μ m CdCl 2. Pretreatment of cells with 80 μ m zinc acetate, 18 hr prior to exposure to 500 μ m CdCl 2, resulted in markedly reduced SSD when compared to non-pretreated cells. Zinc pretreatment increased the level of MT gene expression as well as MT protein production. The decrease in DNA strand damage associated with cadmium exposure was not due to a decrease in cadmium accumulation by zinc pretreated cells. In fact, cellular cadmium burden was increased over 2-fold following zinc pretreatment. In addition to protection against cadmium genotoxicity, zinc pretreatment also reduced the cytotoxicity associated with a 2-hr, 500 μ m cadmium exposure. These data indicate that zinc pretreatment reduces cadmium genotoxicity, possibly through alterations in MT gene expression.