In vivo mutagenicity and hepatocarcinogenicity of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in bitransgenic c-myc/lambda lacZ mice

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is a mutagenic and ca rcinogenic heterocyclic amine found in cooked meat, Hepatic DNA adduct form ation, in vivo mutagenicity, and hepatocarcinogenicity of MeIQx were examin ed in mice harboring the lacZ mutation reporter gene (Muta mice) and bitran sgenic mice overexpressing the c-myc oncogene. C57Bl/lambda lacZ and bitran sgenic c-myc (albumin promoter)/lambda lacZ mice were bred and weaned onto an American Institute of Nutrition-76-based diet containing 0.06% (w/w) MeI Qx or onto control diet. After 30 weeks on diet, only male bitransgenic mic e on MeIQx developed hepatocellular carcinoma (100% incidence). By 40 weeks , hepatic tumor incidence was 100%/75% (17%/0%) and 44%/17% (0%/0%) in male c-myc/lambda lacZ and C57Bl/lambda lacZ mice who were given MeIQx (or cont rol) diet, respectively, supporting a synergism between MeIQx and c-myc ove rexpression in hepatocarcinogenesis. At either time point, mutant frequency in the lacZ gene was at least 40-fold higher in MeIQx-treated mice than in control mice of either strain. These findings suggest that MeIQx-induced h epatocarcinogenesis is associated with MeIQx-induced mutations, Elevated mu tant frequency in MeIQx-treated mice also occurred concomitant with the for mation of MeIQx-guanine adducts, as detected by the P-32-postlabeling assay . Irrespective of strain or diet, sequence analysis of the lacZ mutants fro m male mouse liver showed that the principal sequence alterations were base substitutions at guanine bases. Adenine mutations, however, were detected only in animals on control diet. MeIQx-fed mice harboring the c-myc oncogen e showed, a 1.4-2.6-fold higher mutant frequency in the lacZ gene than mice not carrying the transgene, Although there was a trend toward higher adduc t levels in c-myc mice, MeIQx-DNA adduct levels were not significantly diff erent between c-myc/lambda lacZ and C57Bl/lambda lacZ mice after 30 weeks o n diet. Thus, it seemed that factors in addition to MeIQx-DNA adduct levels , such as the enhanced rate of proliferation associated with c-myc overexpr ession, may have accounted for a higher mutant frequency in c-myc mice. In the control diet groups, the lacZ mutant frequency was significantly higher in c-myc/lambda lacZ mice than in C57Bl/lambda lacZ mice. The findings are consistent with the notion that c-myc overexpression is associated with an increase in mutagenesis. The mechanism for the synergistic effects of c-my c overexpression on MeIQx hepatocarcinogenicity seems to involve an enhance d expression of MeIQx-induced mutations.