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

Abstract

Hydrogen peroxide-resistant Chinese hamster ovary (CHO R) cells were developed by exposing parental (CHO P) cells to sequential increases in H 2O 2 concentration. Cytotoxicity as well as DNA single-strand breaks induced by Na 2CrO 4 were then compared in CHO R and CHO P cell lines. Using the colony-forming assay, it was found that the cytotoxicity caused by Na 2CrO 4 did not differ in the parent and resistant cells. However, alkaline elution studies showed that the production of DNA single-strand breaks in CHO R cells treated with Na 2CrO 4 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 CHO R cells during treatment with Na 2CrO 4 was about 50% that in CHO P cells. CHO R cells were also found to be cross-resistant to the cytotoxicity and DNA breaks caused by other toxic metals such as CdCl 2 and HgCl 2. 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 α-tocopherol were slightly decreased in CHO R cells, suggesting that the resistance in CHO R 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.