Tolerance to 06-Methylguanine and 6-Thioguanine Cytotoxic Effects: A Cross-resistant Phenotype in jY-Methylnitrosourea-resistant Chinese Hamster Ovary Cells1

The biochemical and genetic characteristics of a clone of Chinese hamster ovary cells displaying resistance to Vmrih>l-.Viiiiroviurra (MNU) and 6-thioguanine (6-TG) were analyzed. The initial level of 7methylguanine, 3-methyladenine, and 06-methylguanine formation and the repair rates for these alkylated bases were the same in the resistant and in the parental cell line, indicating that the resistance to alkylation damage is not due to differences in DNA alkylation. After exposure for 24 or 48 h to 6-TG (0.6 Mg/ml) in culture medium, the resistant clone incorporated 1.6-fold more 6-TG into DNA than the parental cells and, in contrast to them, was able to replicate the DNA containing the base analogue during the following 24 h. These data are in agreement with the hypothesis that resistant cells tolerate both 06-methylguanine and 6-TG present in DNA. The tolerance to MNU and 6-TG also included chro mosomal damage induced by these two agents, and MNU-resistant cells incurred less sister eliminatili exchanges after treatment with either MNU or 6-TG. 6-TG-resistant cells, selected by growth in 6-TG, exhib ited cross-resistance to MNU but not to methyl methanesulfonate, con firming that a common pathway of tolerance is responsible for resistance to 6-TG and O6-methylguanine.