Enhanced metallothionein gene expression is associated with protection from cadmium-induced genotoxicity in cultured rat liver cells.

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich proteins that appear to play an important role in the cellular defense system against cadmium toxicity. Although substantial evidence exists demonstrating a reduction in cadmium toxicity concomitant with MT induction, little is known about the possible effects of stimulation of MT synthesis on cadmium-induced genotoxicity. Thus, the alkaline elution technique was used to assess single-strand DNA damage (SSD) in TRL-1215 cells, a liver-derived cell line shown to have inducible MT gene expression. The SSD accumulated over a 2-h time period in a time-dependent manner following exposure to 500 microM CdCl2. Low-concentration cadmium pretreatment (10 microM CdCl2, 24 h) provided protection against the genotoxicity of high-concentration cadmium (500 microM CdCl2, 2 h). A 2-h exposure to 500 microM CdCl2 had no effect on viability, as assessed using a tetrazolium-dye based assay, in cells from either the pretreated or nonpretreated group. Metallothionein was induced in a time-dependent manner by low-concentration cadmium pretreatment: Exposure for 24 and 48 h resulted in 3.3- and 6.4-fold increases, respectively. In addition, a 24-h exposure to low-concentration cadmium resulted in an increase in MT-I gene expression. Cadmium accumulation was 2.6-fold greater in low-concentration cadmium-pretreated cells as compared to nonpretreated cells. These data demonstrate that low-concentration cadmium pretreatment provides protection against cadmium-induced single-strand DNA damage and support the hypothesis that this protection is due to stimulation of MT gene expression.