2-Amino-3,8-Dimethylimidazo-[4,5-f]Quinoxaline–Induced DNA Adduct Formation and Mutagenesis in DNA Repair–Deficient Chinese Hamster Ovary Cells Expressing Human Cytochrome P4501A1 and Rapid or Slow Acetylator N-Acetyltransferase 2

Abstract

2-Amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) is one of the most potent and abundant mutagens in the western diet. Bioactivation includes N-hydroxylation catalyzed by cytochrome P450s followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). In humans, NAT2*4 allele is associated with rapid acetylator phenotype, whereas NAT2*5B allele is associated with slow acetylator phenotype. We hypothesized that rapid acetylator phenotype predisposes humans to DNA damage and mutagenesis from MeIQx. Nucleotide excision repair-deficient Chinese hamster ovary cells were constructed by stable transfection of human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. CYP1A1 and NAT2 catalytic activities were undetectable in untransfected Chinese hamster ovary cell lines. CYP1A1 activity did not differ significantly (P > 0.05) among the CYP1A1-transfected cell lines. Cells transfected with NAT2*4 had 20-fold significantly higher levels of sulfamethazine N-acetyltransferase (P = 0.0001) and 6-fold higher levels of N-hydroxy-MeIQx O-acetyltransferase (P = 0.0093) catalytic activity than cells transfected with NAT2*5B. Only cells transfected with both CYP1A1 and NAT2*4 showed concentration-dependent cytotoxicity and hypoxanthine phosphoribosyl transferase mutagenesis following MeIQx treatment. Deoxyguanosine-C8-MeIQx was the primary DNA adduct formed and levels were dose dependent in each cell line and in the following order: untransfected < transfected with CYP1A1 < transfected with CYP1A1 and NAT2*5B < transfected with CYP1A1 and NAT2*4. MeIQx DNA adduct levels were significantly higher (P < 0.001) in CYP1A1/NAT2*4 than CYP1A1/NAT2*5B cells at all concentrations of MeIQx tested. MeIQx-induced DNA adduct levels correlated very highly (r2 = 0.88) with MeIQx-induced mutants. These results strongly support extrahepatic activation of MeIQx by CYP1A1 and a robust effect of human NAT2 genetic polymorphism on MeIQx-induced DNA adducts and mutagenesis. The results provide laboratory-based support for epidemiologic studies reporting higher frequency of heterocyclic amine-related cancers in rapid NAT2 acetylators.