2-Amino-9H-pyrido[2,3-b]indole: a potential colorectal carcinogen formed in tobac

描述（由申请人提供）：流行病学研究表明，吸烟是结直肠癌的一个危险因素，但与这种风险相关的化学物质尚不清楚。我们报道了一种啮齿类致癌物杂环芳香胺，2-氨基- 9h -吡啶- [2,3-b]吲哚（AaC），一种啮齿类致癌物，在上海队列研究的男性受试者尿液中存在高频率。尿中AaC浓度与吸烟频率密切相关。我们推测，吸烟者通过香烟大量接触AaC，以及AaC在肝脏和结直肠中表达的酶进行生物活化的倾向，为结直肠癌的发生提供了一种生化机制。我们的长期目标是确定烟草烟雾中的AaC是否与吸烟者的结直肠癌有因果关系，并阐明这种接触-癌症关系的机制途径。这个应用程序有三个目标。在Aim 1中，将在临床试验环境下，通过测量参与戒烟研究的吸烟者尿液和粪便中的AaC水平来确定AaC暴露的程度。在Aim 2中，AaC在C57BL/6N小鼠结直肠中的遗传毒性将通过测量DNA加合物和异常隐窝灶（作为肿瘤的早期生物标志物）来确定。这项研究将使我们更清楚地了解AaC在该小鼠品系中的致癌潜力。AaC在小鼠结直肠中具有器官特异性的基因毒性作用，这对我们关于AaC在烟草诱导的结直肠癌中的因果作用的假设至关重要。在Aim 3中，AaC代谢的主要途径将在小鼠和人类中进行表征，特别是它们与靶组织中DNA加合物的形成有关。这些研究将在肝细胞、结肠直肠细胞质和小鼠模型中进行，其中在肝脏或小肠和大肠中选择性地删除了NADPH-细胞色素P450还原酶基因。给药将通过口服或吸入来评估给药途径对结肠DNA损伤的影响。这些研究将揭示跨物种酶生物激活AaC的能力，以及肝脏和结肠中p450在诱导结肠DNA损伤中的相对作用。这项拟议的研究与NIH在公共卫生方面的使命相关，并被基因、环境和健康倡议所强调，该倡议旨在解决本申请中提出的问题。我们关于非饮食来源的HAA对人类结直肠癌风险的贡献的假设是非常新颖和创新的。拟议的研究将填补AaC暴露和生化毒理学的关键空白，并促进我们对主流烟草烟雾中最丰富的芳香胺的遗传毒性的理解，这是结直肠癌的潜在危险因素。

DESCRIPTION (provided by applicant): Epidemiologic studies have shown that tobacco smoking is a risk factor for colorectal cancer, but the chemicals associated with this risk are unknown. We reported that the heterocyclic aromatic amine, 2-amino-9H-pyrido- [2,3-b]indole (AaC), a rodent carcinogen, is present at high frequency in urine of male subjects of the Shanghai Cohort Study. The urinary concentration of AaC was strongly related to the frequency of cigarette smoking. We hypothesize that the high exposure to AaC through cigarettes, and the propensity of AaC to undergo bioactivation by enzymes expressed in liver and colorectum, provides a biochemical mechanism for the development of colorectal cancer in smokers. Our long-term goal is to determine if AaC in tobacco smoke is causally related to colorectal cancer in smokers, and to elucidate the mechanistic pathways underlying this exposure-cancer relationship. There are three objectives of this application. In Aim 1, the extent of exposure to AaC will be determined, by measuring the levels of AaC in urine and stool of tobacco smokers participating in a smoking cessation study, under a clinical trial setting. In Aim 2, the genotoxicity of AaC in the colorectum of C57BL/6N mice will be determined, by measurement of DNA adducts and aberrant crypt foci, as early biomarkers of neoplasia. This study will provide us with a clearer idea about the carcinogenic potential of AaC in this mouse strain. The demonstration of an organ-specific genotoxic effect of AaC in the colorectum of mice is critical for our hypothesis of a causal role for AaC in tobacco-induced colorectal cancer. In Aim 3, the major pathways of AaC metabolism will be characterized in mice and humans, especially as they relate to DNA adduct formation at the target tissue. These studies will be done with hepatocytes, colorectal cytosols, and in a mouse model where the NADPH- cytochrome P450 reductase gene has been selectively deleted in either the liver or in the small and large intestines. Dosing will be done orally or by inhalation to assess the impact of dose routes on DNA damage in the colon. These studies will reveal the capacities of enzymes across species to bioactivate AaC and the relative roles of P450s in liver and colon to induce DNA damage in the colon. This proposed research is relevant to NIH's mission on public health and highlighted by the Genes, Environment, and Health Initiative, which is designed to address the very questions raised in this application. Our hypothesis of the contribution of an HAA from a non-dietary source to human colorectal cancer risk is highly novel and innovative. The proposed research will fill critical gaps on exposure and the biochemical toxicology of AaC and advance our understanding of the genotoxicity of the most abundant aromatic amine present in mainstream tobacco smoke that is a potential risk factor of colorectal cancer.