IN VIVO MUTAGENICITY AND CARCINOGENICITY OF HETEROCYCLIC AMINES IN TRANSGENIC MIC

2-氨基-3，8-二甲基咪唑并[4，5-f]喹恶啉(MeIQx)是一种存在于熟肉中的致突变致癌杂环胺。用携带LacZ突变报告基因的小鼠(MutaTM小鼠)和高表达c-myc癌基因的双转基因小鼠检测MeIQx的体内致突变性和肝癌致癌性。C57BL/LacZ和双基因c-myc(白蛋白启动子)/LacZ小鼠分别饲喂含0.06%(w/w)MeIQx的AIN-76基础日粮和对照日粮。在饮食30周后，只有服用MeIQx的雄性双转基因小鼠发生肝细胞癌(100%发生率)，表明c-myc过表达与MeIQx之间存在协同作用。40周时，雌雄c-myc/LacZ和C57BL/LacZ小鼠肝脏肿瘤发生率分别为100%/75%(17%/0%)和44%/17%(0%/0%)，提示MeIQx或c-myc单独过表达可最终诱发肝肿瘤。在任何一个时间点，用MeIQx治疗的小鼠的LacZ基因突变频率都至少是两个品系的对照小鼠的40倍。这些发现表明，MeIQx诱导的肝癌发生与MeIQx诱导的突变有关。用~(32)P后标记试验检测到，在用MeIQx处理的小鼠中，突变频率也随着MeIQx-鸟嘌呤加合物的形成而增加。无论品系或饮食，对雄性小鼠肝脏LacZ突变体的序列分析表明，主要的序列变化是单一的鸟嘌呤碱基替换。然而，只有在对照饮食的动物中才能检测到腺嘌呤突变。用MeIQx喂养的携带c-myc癌基因的小鼠的LacZ基因突变频率是未携带该基因的小鼠的1.4-2.6倍。虽然c-myc小鼠有升高加合物水平的趋势，但饮食30周后，c-myc/LacZ和C57BL/LacZ小鼠的MeIQx-DNA加合物水平没有显著差异。因此，似乎除了MeIQx-DNA加合物水平之外，诸如与c-myc过度表达相关的增殖率增强等因素可能是c-myc小鼠突变频率较高的原因。在对照组中，c-myc/LacZ小鼠的LacZ突变频率显著高于C57BL/LacZ小鼠。这一发现与c-myc过度表达与突变增加有关的观点是一致的。C-myc的过度表达似乎协同增强了MeIQx的基因突变和肝癌致癌性。-c-myc，杂环胺，LacZ，突变，报告基因，转基因小鼠，肝癌发生

2-Amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is a mutagenic and carcinogenic heterocyclic amine found in cooked meat. The in vivo mutagenicity and hepatocarcinogenicity of MeIQx were examined in mice harboring the lacZ mutation reporter gene (MutaTM Mice) and bitransgenic mice over-expressing the c-myc oncogene. C57Bl/ lacZ and bitransgenic c-myc (albumin promoter)/ lacZ mice were bred and weaned onto an AIN-76 based diet containing 0.06% (w/w) MeIQx or onto control diet. After 30 weeks on diet, only male bitransgenic mice on MeIQx developed hepatocellular carcinoma (100% incidence) indicating that there was synergism between c-myc over-expression and MeIQx. By 40 weeks, hepatic tumor incidence was 100 %/75% (17%/0%) and 44%/17% (0%/0%) in male/female c-myc/ lacZ and C57Bl/ lacZ mice given MeIQx (or control) diet, respectively, indicating that either MeIQx or c-myc over-expression alone eventually induced hepatic tumors. 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 hepatocarcinogenesis is associated with MeIQx-induced mutations. Elevated mutant frequency in MeIQx- treated mice also occurred concomitant with the formation of MeIQx- guanine adducts as detected by the 32P-postlabeling assay. Irrespective of strain or diet, sequence analysis of the lacZ mutants from male mouse liver showed that the principal sequence alteration was a single guanine-base substitution. Adenine mutations, however, were detected only in animals on control diet. MeIQx-fed mice harboring the c-myc oncogene 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 adduct levels in c-myc mice, MeIQx-DNA adduct levels were not significantly different between c-myc/ lacZ and C57Bl/ lacZ mice after 30 weeks on diet. Thus, it appeared that factors in addition to MeIQx- DNA adduct levels, such as the enhanced rate of proliferation associated with c-myc over-expression, 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/ lacZ mice than in C57Bl/ lacZ mice. The findings are consistent with the notion that c- myc over-expression is associated with an increase in mutagenesis. The c-myc over-expression appears to synergistically enhance gene mutations and hepatocarcinogenicity by MeIQx. - c-myc, Heterocyclic Amines, LacZ, Mutagenesis, Reporter gene, Transgenic Mice, Hepatocarcinogenesis,