Abstract 5465: Role of CYP2A13 in NNK bioactivation and lung tumorigenesis: In vivo studies using a CYP2A13-humanized mouse model

Abstract

Abstract The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung procarcinogen. Previously, we have demonstrated that NNK-induced lung tumorigenesis depends on target tissue bioactivation by pulmonary P450 enzymes (Weng et al., 2007, Cancer Res 67:7825-7832). However, the specific P450 enzyme(s) responsible for NNK bioactivation in the lung have not been identified. The aim of the present study was to test the hypotheses that 1) mouse CYP2A5 plays an essential role in NNK bioactivation and tumorigenesis in mouse lung, and 2) transgenic expression of human CYP2A13, known to be selectively expressed in the respiratory tract and be the most efficient enzyme for NNK bioactivation in vitro, can increase the rates of NNK bioactivation and incidence of tumorigenesis in the mouse lung. Kinetic parameters of microsomal NNK bioactivation in vitro, tissue levels of O6-methylguanine (O6-mG) DNA adducts formed in vivo, and incidence of NNK-induced lung tumors were determined for wild-type, Cyp2a5-null, and CYP2A13-humanized (CYP2A13-transgenic/Cyp2a5-null) mice. The results indicated that, in both liver and lung microsomes, the loss of CYP2A5 resulted in significant increases in the Km value for the formation of 4-oxo-4-(3-pyridyl)-butanal (OPB), which represents the reactive intermediate that can lead to the formation of O6-mG DNA adducts; but, the gain of CYP2A13 led to recovery of the activity in the lung, but not in the liver. The levels of O6-mG, the DNA adduct highly correlated with lung tumorigenesis, were significantly higher in the lungs of the wild-type mice and CYP2A13-humanized mice, than in the Cyp2a5-null mice. Furthermore, lung tumor bioassays, performed for the three mouse strains on A/J background, and using NNK at various doses (30 -200 mg/kg; single i.p. dose), revealed that lung tumor multiplicity determined at 4 months post-treatment was significantly greater in wild-type and CYP2A13-humanized mice than in Cyp2a5-null mice, for all NNK doses tested. The difference in lung tumor multiplicity between CYP2A13-humanized mice and Cyp2a5-null mice was greater at low (30 mg/kg) NNK dose (∼70%) than at high (200 mg/kg) NNK dose (∼30%). In control experiments, rates of systemic clearance of NNK and its major circulating metabolite, NNAL, were confirmed to be not different among the three mouse strains. These results indicated that CYP2A13 is a low-Km enzyme in catalyzing NNK bioactivation in vivo, and they provide strong support for the idea that genetic polymorphisms of CYP2A13 can influence the risks of tobacco-induced lung tumorigenesis in humans. Moreover, the CYP2A13-humanized mouse model will be valuable for testing new, CYP2A13-targeted chemopreventive agents for lung carcinogenesis. (Supported in part by NIH grant CA092596) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5465. doi:1538-7445.AM2012-5465