DNA single-strand breaks and cytotoxicity induced by sodium chromate(VI) in hydrogen peroxide-resistant cell lines

Hydrogen peroxide-resistant Chinese hamster ovary (CHOR) cells were developed by exposing parental (CHO(P)) cells to sequential increases in H2O2 concentration. Cytotoxicity as well as DNA single-strand breaks induced by Na2CrO4 were then compared in CHOR and CHO(P) cell lines. Using the colony-forming assay, it was found that the cytotoxicity caused by Na2CrO4 did not differ in the parent and resistant cells. However, alkaline elution studies showed that the production of DNA single-strand breaks in CHOR cells treated with Na2CrO4 was reduced by about 50% as compared with that in CHO(P) cells. Similarly, electron spin resonance (ESR) studies revealed that the level of chromium(V) in CHOR cells during treatment with Na2CrO4 was about 50% that in CHO(P) cells. CHOR cells were also found to be cross-resistant to the cytotoxicity and DNA breaks caused by other toxic metals such as CdCl2 and HgCl2. Catalase activity in resistant cells was 2-fold and the cellular content of glutathione was 3-fold that in parental cells. However, no obvious differences were seen in superoxide dismutase and glutathione reductase activity, although the contents of ascorbic acid or alpha-tocopherol were slightly decreased in CHOR cells, suggesting that the resistance in CHOR cells may be associated with the increase in both catalase activity and glutathione contents in cells. These results indicate that chromate-induced DNA breaks appear to be mediated by a different mechanism than that for the cytotoxicity of this metal, and also suggest that the formation of active oxygen species and/or chromium(V) during reduction of chromium(VI) inside cells might be associated with the induction of the DNA strand breaks caused by the metal.