In Vivo Mutagenicity and Carcinogenicity of Heterocyclic Amines in Transgenic Ro

2-氨基- 3,8 -二甲基咪唑[4,5-f]喹诺啉（MeIQx）和2-氨基-1-甲基-6-苯咪唑[4,5-b]吡啶是在熟肉中发现的致突变和致癌的杂环胺。在携带lacZ或lacI突变报告基因的小鼠和大鼠（Muta小鼠和Big Blue大鼠）中检测了这些化合物的体内诱变性。过表达c-myc癌基因的双转基因小鼠也进行了检测。饲养C57Bl/ lacZ和双基因c-myc（白蛋白启动子）/ lacZ小鼠，断奶时饲喂含有0.06% (w/w) MeIQx的AIN-76型饲粮或对照组饲粮。在给药30周后，只有服用MeIQx的雄性双转基因小鼠发生了肝细胞癌（100%的发生率），这表明c-myc过表达与MeIQx之间存在协同作用。40周时，给予MeIQx（或对照组）饮食的雄性/雌性c-myc/ lacZ和C57Bl/ lacZ小鼠的肝脏肿瘤发生率分别为100% /75%（17%/0%）和44%/17%(0%/0%)，表明MeIQx或c-myc过表达最终诱导了肝脏肿瘤。在任何一个时间点，在meiqx处理的小鼠中，lacZ基因的突变频率至少比任一品系的对照小鼠高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。32p标记后分析发现，在meiqx处理的小鼠中，突变频率升高还伴随着meiqx鸟嘌呤加合物的形成。对来自雄性小鼠肝脏的lacZ突变体的序列分析表明，主要的序列改变是单个鸟嘌呤碱基的取代。然而，腺嘌呤突变仅在控制饮食的动物中检测到。含有c-myc致癌基因的meiqx喂养小鼠的lacZ基因突变频率比未携带转基因的小鼠高1.4-2.6倍。虽然c-myc小鼠的加合物水平有升高的趋势，但在饮食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，除了MeIQx-DNA加合物水平外，其他因素，如与c-myc过表达相关的增殖率增强，可能是c-myc小鼠中较高突变频率的原因。在对照组中，c-myc/ lacZ小鼠的lacZ突变频率显著高于C57Bl/ lacZ小鼠。这些发现与c-myc过表达与突变增加有关的观点是一致的。c-myc过表达似乎通过MeIQx协同增强基因突变和肝癌致癌性。

2-Amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine are mutagenic and carcinogenic heterocyclic amines found in cooked meat. The in vivo mutagenicity of these compounds were examined in mice and rats harboring the lacZ or lacI mutation reporter gene (Muta Mice and Big Blue rats). Bitransgenic mice over-expressing the c-myc oncogene were also examined. 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.