DNA damage and metallothionein synthesis in human hepatoma cells (HepG2) exposed to cadmium

Cadmium is an important heavy metal environmental toxicant, which is classified as a human carcinogen. The comet assay was used to evaluate the levels of DNA damage in a metabolically competent HepG2 cell line after treatment with low, non-cytotoxic and physiologically relevant concentrations of cadmium, alone and in combination with the dietary mutagen 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) and with the environmental mutagen benzo[a]pyrene (B(a)P). After exposure of the cells to 10, 100 and 1000 nM CdCl2, a dose- and time-dependent increase of DNA damage was detected. Maximal damage was found after 12 h of treatment, but declined with further incubation with CdCl2. The increased synthesis of metallothioneins on exposure to CdCl2 up to 12 h suggests that they are responsible for the adaptation of HepG2 cells to the DNA damaging effects of CdCl2. Co-treatment of the cells with CdCl2 (10–1000 nm) and IQ (300 μm) induced a dose-dependent increase of DNA damage compared to cells treated with IQ alone. Co-genotoxic activity was also observed by increased formation of micronuclei in cells exposed to IQ and 1000 nm CdCl2; at this concentration, CdCl2 alone also induced micronuclei in HepG2 cells. Our results support the hypothesis that direct and indirect mechanisms are involved in cadmium-induced DNA damage.