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

描述(申请人提供)：流行病学研究表明，吸烟是结直肠癌的风险因素，但与这种风险相关的化学物质尚不清楚。我们报道了杂环芳胺，2-氨基-9H-吡啶-[2，3-b]吲哚(AAC)，这是一种啮齿动物致癌物质，在上海队列研究的男性受试者尿中出现的频率很高。AAC的尿药浓度与吸烟次数密切相关。我们推测，香烟对AAC的高暴露，以及AAC被肝脏和结肠中表达的酶激活的倾向，为吸烟者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存在因果关系，并阐明这种暴露与癌症关系的机制。这个应用程序有三个目标。在目标1中，将通过测量参与戒烟研究的烟草吸烟者的尿液和粪便中的AAC水平，在临床试验环境下确定接触AAC的程度。在目的2中，将通过测量作为肿瘤早期生物标志物的DNA加合物和异常隐窝病灶来确定AAC在C57BL/6N小鼠结肠直肠中的遗传毒性。这项研究将为我们更清楚地了解AAC在该小鼠品系中的致癌潜力。AAC在小鼠结肠中的器官特异性遗传毒性效应的证明对于我们关于AAC在烟草诱导的结直肠癌中的因果作用的假说至关重要。在目标3中，AAC代谢的主要途径将在小鼠和人类中得到表征，特别是当它们与靶组织的DNA加合物形成有关时。这些研究将在肝细胞、结直肠胞浆和小鼠模型中进行，在小鼠模型中，NADPH-细胞色素P450还原酶基因在肝脏或小肠和大肠中被选择性地删除。剂量将通过口服或吸入进行，以评估剂量途径对结肠DNA损伤的影响。这些研究将揭示不同物种的酶激活AAC的能力，以及P450在肝脏和结肠中诱导结肠DNA损伤的相对作用。这项拟议的研究与NIH的公共卫生使命相关，并由基因、环境和健康倡议强调，该倡议旨在解决在这一申请中提出的问题。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。这项拟议的研究将填补AAC暴露和生化毒理学方面的关键空白，并促进我们对主流烟草烟雾中存在的最丰富的芳香胺的遗传毒性的理解，该芳香胺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.