Mutagenicity of cadmium in mammalian cells: implication of oxidative DNA damage

Cadmium and cadmium compounds are well established human carcinogens and are ubiquitously present in the environment. The carcinogenic mechanism(s) of cadmium remains largely unknown since direct mutagenic effect is weak in bacterial and in standard mammalian cell mutation assays. In this study, we show that when evaluated using the human-hamster hybrid A(L) cell mutation assay in which both intragenic and multilocus deletions can readily be detected, CdCl(2) is a strong mutagen that induces predominantly large deletion mutations. Concurrent treatment of A(L) cells with the oxyradical scavenger dimethyl sulfoxide significantly reduced the number of cadmium-induced mutations. In contrast, pre-treatment of cells with buthionine sulfoximine that depletes intracellular glutathione, increased cytotoxicity and mutagenicity of cadmium. These results demonstrate that reactive oxygen species mediate cadmium induced mutations in A(L) cells. With laser scanning confocal microscopy and the fluorescent probe 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, we demonstrated that cadmium induced a dose and time dependent formation of intracellular oxyradicals. Using immunoperoxidase staining coupled with a monoclonal antibody-specific for 8-OHdG adducts in DNA, we demonstrated that cadmium induced a dose dependent increase of 8-OHdG adducts, which accumulated with prolonged exposure. Furthermore, we showed that at low concentration, cadmium, attenuated removal of hydrogen peroxide induced 8-OHdG adducts. Thus, the carcinogenicity of cadmium can, in part, be explained by its mutagenic activity, which is mediated by reactive oxygen species induced DNA damage and by its interference with the repair of oxidative DNA damage.