| Summary: | 博士 === 國防醫學院 === 生命科學研究所 === 100 === Cigarette smoking is an important environmental factor in development of human lung cancer. In thousands of cigarette-related chemicals, the tobacco-specific nitrosamines (TSNAs) have been demonstrated to be the most potent carcinogens of the cigarette smoking-induced human lung cancer. The carcinogenicity of TSNAs mainly depends on the cytochrome P450 enzymes (CYPs)-mediated metabolic activation of TSNAs. Human CYP2A6 and CYP2A13, with a 93.5% of amino-acid identity, are the two major CYPs involved in the metabolic activation of TSNAs. In epidemiological studies, humans with a lower CYP2A6 or CYP2A13 activity are associated with a lower risk of the cigarette smoking-induced lung cancer occurrence. The present study aims to explore the roles of CYP2A6 and CYP2A13 in the underlying mechanism of the TSNAs-induced pulmonary mutagenesis/carcinogenesis. First, a recombinant adenovirus-mediated CYP2A6/CYP2A13 expression system is established to determine the specific effect of CYP2A6/CYP2A13 on the TSNAs-induced genotoxicity by using the tyrosine suppressor tRNA gene (supF) mutagenesis assay. The study reveals that both CYP2A6 and CYP2A13 significantly enhance the TSNAs (i.e., NNK and NNN)-induced mutagenesis, in which the mutagenic effect of CYP2A13 is markedly higher than that of CYP2A6. Analysis of the NNK-related DNA adduct formation indicates that, in the presence of CYP2A13, NNK treatments cause marked increases in O6-methylguanine (O6-MeG). The in vitro evidence demonstrates the predominant role of CYP2A13 in the TSNAs-induced mutagenesis. Second, two specific CYP2A6 and CYP2A13 antibodies are generated to reveal the differential distribution of CYP2A6 and CYP2A13 in human respiratory tract with immunohistochemistry (IHC) analysis. The results show that both of CYP2A6 and CYP2A13 are detected in the epithelial cells of trachea and bronchus and only CYP2A6 is expressed in the bronchiolar epithelial cells of peripheral lung. The differential distribution patterns of CYP2A6 and CYP2A13 in the respiratory tract are of importance in considering the pulmonary susceptibility to carcinogens and the following lung cancer development. Third, analysis of non-small cell lung carcinoma (NSCLC) tissue array with the CYP2A13-specific antibody indicates that the CYP2A13 expression is detected in ≥70% of NSCLC cancerous tissues and is significantly associated with the early occurrence of NSCLC, suggesting the possible involvement of CYP2A13 in the initiation step of the TSNAs-induced lung cancer development. In sum, the present study demonstrates the metabolic effects of CYP2A6 and CYP2A13 on the TSNAs-induced mutagenesis, the differential distribution of CYP2A6 and CYP2A13 in human respiratory tissues, and the association between the CYP2A13 expression in cancerous tissues and the early occurrence of NSCLC.
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