| It is important to know whether cytochrome P450 (P450 or CYP)-mediated target tissue metabolic activation is essential for chemical-induced toxicity and carcinogenicity in extrahepatic tissues, in order to design effective chemopreventive strategies. However, because of lack of appropriate experimental approaches, direct evidence for a critical role of target tissue activation has not been obtained for most, if not all, xenobiotic compounds. In this study, a novel triple transgenic (3-tg) mouse model with doxycycline-inducible, lung-selective deletion of the NADPH-cytochrome P450 reductase (Cpr ) gene, which is essential for the function of all microsomal CYP enzymes, was generated and characterized. The doxycycline-induced Cpr deletion in the lungs of the 3-tg mice was restricted to subsets of airway epithelial cells and type II alveolar cells. To investigate the impact of the site-specific loss of CPR expression on chemical carcinogenesis in the lung, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a known lung carcinogen, was chosen as a model compound. We found that in the doxycycline-treated 3-tg mice on an A/J background, following a single i.p. injection of NNK, systemic clearance of NNK and its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and hepatic levels of O6-methylguanine, were not changed, whereas pulmonary levels of O6-methylguanine and lung tumor multiplicity were ∼63--52% and ∼57% lower, respectively, compared to those in similarly treated control littermates. In contrast, in transgenic mice with liver-specific deletion of Cpr on an A/J background, following a single i.p. injection of NNK, hepatic levels of O6-metheylguanine was ∼95% lower, whereas areas under the curve for plasma NNK and NNAL were increased by 1.8 and 2.4 fold, respectively, moreover pulmonary levels of O6-methylguanine and the lung tumor multiplicity were increased by 1.5 and 2.4fold, respectively, compared to those in similarly treated wild-type littermates. Collectively, these data strongly support our hypothesis that pulmonary microsomal CYPs play a major role in chemical-induced lung carcinogenesis by generating reactive metabolites that are essential for initiating the carcinogenic process, whereas hepatic CYPs play a major role in systemic clearance of NNK, thereby reducing the susceptibility to NNK-induced lung carcinogenesis by decreasing the bioavailability of the pro-carcinogens in the lung. |
