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Carcinogenesis Advance Access published online on December 13, 2006
Carcinogenesis, doi:10.1093/carcin/bgl244
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

7H-dibenzo[c,g]carbazole metabolism by the mouse and human CYP1 family of enzymes

Howard G. Shertzer*, Mary Beth Genter, Glenn Talaska, Christine P. Curran, Daniel W. Nebert and Timothy P. Dalton

Department of Environmental Health and Center for Environmental Genetics, University of Cincinnati Medical Center, P.O. Box 670056, Cincinnati, OH 45267-0056, USA

* Corresponding author 3223 Eden Avenue, Cincinnati, OH 45267-0056, Voice 513.558.0522; Fax 513.558.0925, email: shertzhg{at}ucmail.uc.edu

Found in tobacco smoke, fossil fuel and other organic combustion products, 7H-dibenzo[c,g]carbazole (DBC) is a potent mouse lung carcinogen and potential human carcinogen. Although the first hydroxylation is critical for determining activation versus detoxication, the enzymes responsible for site-specific hydroxylation of DBC are not known. We found that DBC-DNA adduct levels are significantly higher in aromatic hydrocarbon receptor null Ahr(-/-) mice, suggesting that the induction of AHR-regulated genes, such as those in the CYP1 family, decrease DBC genotoxicity. Using knockout mice for Cyp1a1, Cyp1a2 and Cyp1b1, we showed that the major CYP1 enzymes that metabolize DBC are CYP1A1 in ß-naphthoflavone-induced liver, CYP1A2 in non-induced liver, CYP1B1 and CYP1A1 in induced lung, and none in non-induced lung. DBC metabolism by the human CYP1 enzymes was examined in vitro using SupersomesTM. Each mouse CYP1, as well as each human CYP1, has a unique DBC metabolite profile. Comparison of the metabolite profile in BNF-induced mice suggested that CYP1A1 primarily generates 1-OH, 2-OH and (5+6)-OH-DBC, while CYP1A2 generates primarily (5+6)-OH-DBC, and CYP1B1 primarily generates 4-OH-DBC. This was similar to that observed in the human CYP1 enzymes. Most importantly, lung CYP1B1 is associated with forming 4-OH-DBC, the most potent metabolite leading to DBC-DNA adducts. These studies suggest that for non-pulmonary routes of exposure (i.e., skin, gastric, i.p.) low hepatic expression of CYP1A2 and CYP1A1, together with high expression levels of lung CYP1B1 and CYP1A1, may define a phenotype for high susceptibility to carcinogens such as DBC.


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