The cooked-meat derived heterocyclic amines (HCAs) 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,8- dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AalphaC) are established mutagens in the Salmonella assay and hepatocarcinogens in mice. The current study uses transgenic mice to examine hepatic HCA-DNA adduct formation and mutagenesis in vivo and the impact of hepatic overexpression of the c-myc oncogene on HCA-induced mutagenesis. C57Bl/lacZ and c-myc/lacZ mice strains, produced by crossbreeding MutaTM mice (carrying the lacZ mutation target gene) with either C57Bl control or c-myc transgenic mice, respectively, were treated with ten daily doses of IQ, MeIQx, or AalphaC (20 mu g/g, p.o.). Four weeks after dosing, the frequency of mutations in the lacZ gene in liver of either C57Bl/lacZ or c-myc/lacZ mice was significantly higher in mice treated with any one of the three HCAs than in mice given vehicle only. In addition, all three HCAs formed hepatic DNA adducts, as measured by the 32P-postlabeling analysis 24 h after dosing. In both strains of mice, hepatic DNA adduct levels were 2-3 fold higher with AalphaC than with either IQ or MeIQx, although the mutant frequencies in the lacZ gene were 20-30% lower in mice dosed with AalphaC. These results suggest that AalphaC-DNA adducts may be less mutagenic in vivo than either IQ- or MeIQx-DNA adducts. The lacZ mutant frequencies observed with all three HCAs appeared to be influenced by c-myc transgene expression: after HCA treatment, transgenic mice carrying the c-myc gene showed a 30% higher lacZ mutant frequency than mice not carrying this transgene. Notably, lacZ mutant frequencies were not different among C57Bl/LacZ and c-myc/LacZ mice that received vehicle control. DNA adduct studies showed that the levels of IQ- and MeIQx-DNA adducts were 2-3-fold higher in c-myc/LacZ mice than in C57Bl/LacZ; however, AalphaC-DNA adducts were not statistically different between the two strains. In addition, phase I metabolic activation of these HCAs, as assessed by he patic microsomal mutagenic activation, was also similar in both strains of mice. These results support the notion that overexpression of the c-myc oncogene cooperates with the HCAs to enhance in vivo mutagenicity. Further studies are underway to assess the mechanisms of this cooperative effect and determine the effect of c-myc transgene overexpression on HCA-induced carcinogenesis.
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