N-nitrosodimethylamine-mediated formation of oxidized and methylated DNA bases in a cytochrome P450 2E1 expressing cell line

The metabolic activation of N-nitrosodimethylamine (NDMA) to reactive metabolites is a critical step for the expression of its toxic and carcinogenic potential. We have previously reported that a P450 2E1 expressing cell line, GM2E1, can metabolize NDMA to toxic reactive metabolites and cause apoptotic cell death. To investigate whether DNA is a critical target for the reactive metabolites of NDMA, we measured the levels of DNA adducts in untreated and NDMA-treated GM2E1 cells. 8-Hydroxydeoxyguanosine (8-OHdG), a biomarker for oxidative DNA damage, was analyzed following enzymatic hydrolysis of DNA. 7-Methylguanine (7-mGua), the most suitable marker for the DNA adducts formed by methylating agents, was released by thermal depurination of DNA. The modified guanine adducts were separated by HPLC and quantified using electrochemical detection. The levels of 8-OHdG and 7-mGua in GM2E1 cells treated with NDMA increased up to approximately 4- and 100-fold over those in the untreated cells, respectively. The addition of ascorbic acid, an antioxidant, to the NDMA-treated cells resulted in a significant decrease in the cytotoxicity with a concomitant decrease in the levels of 8-OHdG, but not the levels of 7-mGua. Our results demonstrate that the metabolism of NDMA in GM2E1 cells causes both DNA methylation and oxidation and support the hypothesis that NDMA-mediated DNA damage may play an important role in its toxic effects.