Nucleotide excision repair modulates the cytotoxic and mutagenic effects of N-n-butyl-N-nitrosourea in cultured mammalian cells as well as in mouse splenocytes in vivo

The butylating agent N-n-butyl-N-nitrosourea (BNU) was employed to study th e role of nucleotide excision repair (NER) in protecting mammalian cells ag ainst the genotoxic effects of monofunctional alkylating agents. The direct acting agent BNU was found to be mutagenic in normal and XPA mouse splenoc ytes after a single i.p. treatment in vivo. After 25 and 35 mg/kg BNU, but not after 75 mg/ kg, 2- to 3-fold more hprt mutants were detected in spleno cytes from XPA mice than from normal mice. Using O-6-alkylguanine-DNA alkyl transferase (AGT)-deficient hamster cells, it was found that NER-deficient CHO UV5 cells carrying a mutation in the ERCC-2 gene were 40% more mutable towards lesions induced by BNU when compared with parental NER-proficient C HO AA8 cells. UV5 cells were 1.4-fold more sensitive to the cytotoxic effec ts of BNU compared with AA8 cells. To investigate whether this increased se nsitivity of NER-deficient cells is modulated by AGT activity, cell surviva l studies were performed in human and mouse primary fibroblasts as well. BN U was 2.7-fold more toxic for mouse XPA fibroblasts compared with normal mo use fibroblasts. Comparable results were found for human fibroblasts. Taken together these data indicate that the role of NER in protecting rodent cel ls against the mutagenic and cytotoxic effects of the alkylating agent BNU depends on AGT.