title: O6-methyltransferase-deficient and -proficient CHO cells differ in their responses to ethyl- and methyl-nitrosourea-induced DNA alkylation.
abstract: 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.
authors.affiliation: Istituto Superiore di Sanita, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Bignami
authors.forename: E
authors.initials: E
authors.lastname: Dogliotti
authors.forename: G
authors.initials: G
authors.lastname: Aquilina
authors.forename: A
authors.initials: A
authors.lastname: Zijno
authors.forename: C P
authors.initials: CP
authors.lastname: Wild
authors.forename: R
authors.initials: R
authors.lastname: Montesano
mesh_terms: D000478:Alkylation; D000818:Animals; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D006224:Cricetinae; D003412:Cricetulus; D004247:DNA / Q000187:drug effects / Q000378:metabolism*; D005038:Ethylnitrosourea / Q000494:pharmacology*; D005260:Female; D007700:Kinetics; D008770:Methylnitrosourea / Q000494:pharmacology*; D008780:Methyltransferases / Q000172:deficiency / Q000378:metabolism*; D009154:Mutation; D019853:O(6)-Methylguanine-DNA Methyltransferase; D010053:Ovary; D012854:Sister Chromatid Exchange / Q000187:drug effects
chemical_list: D008770:Methylnitrosourea; D004247:DNA; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005038:Ethylnitrosourea