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

Abstract

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 μM CdCl₂. Low‐concentration cadmium pretreatment (10 μM CdCl₂, 24 h) provided protection against the genotoxicity of high‐concentration cadmium (500 μM CdCl₂, 2 h). A 2‐h exposure to 500 μM 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‐1 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.