Biotransformation and DNA Repair in 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone-Induced Pulmonary Carcinogenesis
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Studies described in this thesis were at aimed at characterizing the mechanisms involved in the pulmonary carcinogenicity of the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), by addressing two critical determinants of carcinogenicity; biotransformation and DNA repair. The contributions of cytochrome P450 (CYP) 2A13 and CYP2A6 to NNK biotransformation in human lung microsomes were investigated. Based on total bioactivation and detoxification of NNK and its keto-reduced metabolite, 4 (methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), subjects could be classified as either high or low bioactivators and detoxifiers. Data from all of 29 individuals revealed no correlations between levels of CYP2A mRNA, enzyme activity or immunoinhibition and the degree of total NNK bioactivation or detoxification. However, subgroups were identified for whom CYP2A13 mRNA correlated with total NNK and NNAL bioactivation (n=4) and NNAL detoxification (n=5). Although results do not support CYP2A13 or CYP2A6 as predominant contributors to NNK metabolism in lung of all individuals, CYP2A13 appears to be important in some. The involvement of nucleotide excision repair (NER) in the repair of NNK-induced DNA pyridyloxobutylation was assessed. Extracts from NER-deficient cells were less active at repairing pyridyloxobutyl (POB) adducts on plasmid DNA than were extracts from normal cells, and NER-deficient cells were more susceptible to 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc)-induced cytotoxicity, demonstrating the participation of NER in the repair of POB-DNA adducts. The role of DNA repair in contributing to inter-organ susceptibility to NNK-induced carcinogenesis was investigated. POB adduct repair was greater in extracts from mouse liver than lung, and activities in lungs of NNK-treated mice were lower than those of saline-treated mice, while repair was 3 times higher in livers of NNK-treated mice relative to control. NNK treatment decreased incision of POB adducts by 92 % in lung extracts and increased incision by 169 % in liver extracts. In addition, NNK altered the levels and binding to POB damage of key incision proteins. These results suggest that lower NER incision activity and NNK-mediated alterations in levels and activities of incision proteins contribute to the relative susceptibility of mouse lung to NNK-induced carcinogenesis.