Identification of cytochrome P450 enzymes critical for lung tumorigenesis by the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridy1)-1-butanone (NNK): insights from a novel Cyp2abfgs-null mouse

Cytochrome P450 (P450) enzymes encoded by the mouse Cyp2abfgs gene cluster are preferentially expressed in the respiratory tract. Previous studies have demonstrated that pulmonary P450-mediated bioactivation is necessary for lung tumorigenesis induced by the tobacco-specific lung procarcinogen 4-(methylnitrosam ino)-1-(3-pyridyI)-1-buta none (NNK), and that CYP2A5 mediates a noteworthy fraction, but not all, of NNK bioactivation in the lung. The aim of this study was to determine whether other P450s encoded by the Cyp2abfgs gene cluster also play significant roles in NNK lung tumorigenesis. A novel Cyp2ablgs-null mouse was generated, in which all Cyp2a, 2b, 2g, 2f and 2s genes are deleted. The Cyp2abfgs-null mouse was viable, fertile and without discernible physiological abnormalities or compensatory increases in the expression of other P450s. NNK bioactivation in vitro and NNK-induced DNA adduction and lung tumorigenesis in vivo were determined for wild-type (WT) and Cyp2abfgs-null mice; the results were compared with previous findings from Cyp2a5 -null mice. The Cyp2abfgs-null mice exhibited significantly lower rates of NNK bioactivation in lung and liver microsomes, compared with either INT or Cyp2a5-null mice. The levels of lung 06-methyl guanine DNA adduct were also substantially reduced in Cyp2abfgs-null mice, compared with either WT or C3p2a5-null mice. Moreover, the C3p2ablgs-null mice were largely- resistant to NNK-induced lung tumorigenesis at both low (50 mg/kg) and high (200 mg/kg) NNK doses, in contrast to the WT or Cyp2a5 -null mice. These results indicate for the first time that, collectively, the CYP2A, 2B, 2F, 26, and 2S enzymes are indispensable for NNK-induced lung tumorigenesis.