O6-methyltransferase-deficient and -proficient CHO cells differ in their responses to ethyl- and methyl-nitrosourea-induced DNA alkylation.

The mutagenic and cytotoxic effects of N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) were compared in two isogenic Chinese hamster ovary (CHO) cell lines differing for the expression of the repair function for O6-methylguanine (O6-meGua), the O6-methyl-DNA-methyltransferase (MT). Survival and ouabain resistance (ouar) mutation frequency were similar in the two cell lines after treatment with ENU while both effects were strongly reduced in the MT-proficient (MT+) CHO cells after exposure to MNU. The slow repair kinetics of O6-ethylguanine (O6-etGua) when compared to O6-meGua, i.e. 25% versus 88% removal at 20 h after treatment, could still account for the similar mutational curves reported in the two cell lines after ENU treatment. The number of ENU-induced sister chromatid exchanges (SCE) was slightly reduced in the MT+ as compared to MT-deficient CHO cells suggesting a role for O6-etGua in SCE formation. Comparison of survival after exposure to ENU and MNU showed that, at similar levels of O6-alkylguanine on DNA, the ethyl- is more tolerated than the methyl-adduct. These data focus the attention on the importance of DNA damage processing in the cytotoxic response to alkylating agents.