Reduced Cd2+ accumulation and elevated metallothionein levels in a Cd2+ and Zn2+-resistant clonal CHO-K1 cell line

A clonal cell line, R40F, was selected from a heterogenous population of Cd2+ and Zn2+-resistant CHO-K1 cells. These R40F cells demonstrated resistance to 120- and 4-fold higher concentrations of Cd2+ and Zn2+, respectively, than did wild type CHO-K1 cells. When cultured in the presence of low concentrations of Cd2+ (0.5-1.0 microM), the accumulation of intracellular Cd2+ in R40F cells appears to be significantly less than in wild type cells. Since R40F cells maintained in medium containing high concentrations of Cd2+ (200 microM) retain levels of Cd2+ equivalent to the intracellular concentration observed in wild type cells exhibiting cytotoxicity, it is assumed that reduced Cd2+ transport alone is unlikely to account for the resistance to Cd2+ toxicity. Exposure of R40F cells to non-toxic (2 microM or 100 microM) or toxic (200 microM) Zn2+ levels resulted in an accumulation of Zn2+ equal to, or greater than, that observed in the wild type cell. When compared to the basal level in uninduced wild type cells, metallothionein levels were elevated 14- and 23-fold, respectively, in R40F cells cultured in the presence of 0.5 microM Cd2+ and 100 microM Zn2+. These results are consistent with the hypothesis that R40F cells express Cd2+ and Zn2+ resistance as a consequence of a reduction in unbound intracellular Cd2+ levels and an elevation of metallothionein synthesis.