Chinese hamster ovary cells deficient or proficient in O6-alkylguanine-DNA alkyltransferase activity are equally sensitive to X-rays

In mammalian cells, under aerobic conditions, ionizing radiations and radiomimetic chemical agents can induce an enzymatic activity involved in DNA repair, O⁶-alkylguanine-DNA alkyltransferase (O⁶-AT). This catalytic protein is active against alkyl-radical-induced DNA damages. This induction was proposed to be linked to the formation of hydroxyl radicals. The possible involvement of O⁶-AT in the defense mechanism of the cell against aerobic radiation damage was investigated by comparing the X-ray sensitivity of two Chinese hamster ovary (CHO) cell lines, the first deficient (CHO mex^-) and the second proficient by transfection of O⁶-AT (CHO mex⁺). The colony-forming ability after X-irradiation was appreciably reduced in CHO mex^- in comparison to CHO mex⁺ cells. Nevertheless, pretreatment of proficient cells with O⁶-methylguanine, a specific inhibitor of O⁶-AT, reduced the DNA repair activity but did not modify the degree of sensitivity to X-rays of the CHO mex⁺ cells. Since the glutathione concentrations as well as the DNA damage amounts induced by X-irradiation were comparable in the variously treated cell lines, these results suggest that the observed induction of O⁶-AT by ionizing radiation in aerobic conditions could be a generalized rather than a specific response to damage by radicals.