Modulation of ethylnitrosourea-induced toxicity and mutagenicity in human cells by O6-benzylguanine.

We have examined the contributions of O6-alkylguanine-DNA alkyl-transferase (AGT) and nucleotide excision repair to the protection of human cells from the toxic and mutagenic effects of ethylnitrosourea. Three human lymphoblastoid cell lines were used: one which possesses both of these DNA repair pathways; one derived from a xeroderma pigmentosum complementation group A patient, which expresses AGT but is deficient in nucleotide excision repair; and a third which does not express AGT but is capable of excision repair. The level of active AGT in the cells was further modulated with the use of the AGT inhibitor, O6-benzylguanine. These cells were exposed to ethylnitrosourea in both the presence and absence of O6-benzylguanine, and population survival, growth, and mutagenesis at the hypoxanthine-guanine phosphoribosyl-transferase locus were measured. The results for all three measurements indicated that the lack of either AGT or nucleotide excision repair significantly impairs the ability of human cells to withstand DNA ethylation damage. Furthermore, the inhibition of AGT in xeroderma pigmentosum group A cells did not increase toxicity or mutagenicity, suggesting that AGT and nucleotide excision repair cooperate in the removal of DNA ethyl adducts. Related studies in our laboratory have shown that AGT and nucleotide excision repair are both necessary for the efficient removal of O6-ethyldeoxyguanosine.