Sensitivity to Cadmium-Induced Genotoxicity in Rat Testicular Cells Is Associated with Minimal Expression of the Metallothionein Gene

Cadmium is a carcinogenic metal. Although the mechanism of tumor induction is unknown, DNA/metal interactions may be involved. Metallothionein can protect against cadmium toxicity and in our previous work it was shown to reduce cadmium genotoxicity in cultured cells. To extend these results, the genotoxicity of cadmium was studied in R2C cells, a rat testicular Leydig cell line. The R2C cells were very sensitive to cadmium-induced single-strand DNA damage (SSD), as measured by alkaline elution. SSD occurred in R2C cells after treatment with 25 and 50 μM CdCl 2 for 2 hr. Prior work showed other cells required much higher levels of cadmium (~500 μM) to induce genotoxicity. The genotoxic levels of cadmium (25-50 μM) were not cytotoxic in R2C cells as assessed by a metabolic activity (MTT) assay. Pretreatment of R2C cells with a low cadmium dose (2 μM, 24 hr) had no effect on cadmium-induced SSD, in contrast to prior work in other cells where such pretreatments reduced SSD through metallothionein gene activation. In fact, cadmium or zinc treatments resulted in little or no increase in metallothionein gene expression in R2C cells as determined by Northern blot analysis for metallothionein mRNA using cDNA or oligonucleotide probes and radioimmunoassay for metallothionein protein production. Basal metallothionein mRNA was essentially nondetectable. Induction of a cadmium-binding protein in R2C cells did occur, as determined by Cd-heme assay, but did not induce tolerance to SSD. In vivo, the Leydig cell is a target for cadmium carcinogenicity and its cadmium-binding protein is thought not to be a true metallothionein. These results indicate that R2C cells are sensitive to cadmium-induced genotoxicity and that this sensitivity is associated with minimal expression of the metallothionein gene.