PAHs: Balance of Detoxication vs Metabolic Activation

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DESCRIPTION (provided by applicant): Polycyclic aromatic hydrocarbons (PAHs), nitrosamines, and N-heterocyclics are present in combustion products, e.g. grilled foods & cigarette smoke. Cytochromes P450 1A1 & 1B1 (CYP1A1, CYP1B1) are responsible for the metabolism of numerous PAHs, the prototype of which is benzo[a]pyrene (BaP). CYP1A2 is responsible for metabolizing nitrosamines and N-heterocyclics, but also PAHs (in particular, BaP) to a lesser extent. Using Cyp1a1(-/-) and Cyp1b1(-/-) knockout mice, we have shown that CYP1A1 is more important in detoxication than metabolic activation, whereas CYP1B1 causes metabolic activation of BaP to unwanted reactive intermediates. In other words, CYP1A1 is more good than bad in the intact mouse ingesting BaP, and CYP1B1 is more bad than good in the intact mouse administered PAHs by various routes. The importance of mesenteric lymphatics vs. the portal system (mesenteric blood vessels, liver, bile) is not known for oral BaP. This lab now has seven-all three single, all three double, and the one triple-Cyp1 knockout mouse lines. Our hypothesis is: lymph BaP uptake and CYP1B1 in distal tissues (e.g. immune cells, spleen, and bone marrow) are the principal causes of oral BaP toxicity, whereas inducible CYP1A1 in liver and intestine is the principal cause of BaP detoxication. In this proposed project, we therefore will: [a] identify and determine the amounts of metabolites vs. unchanged parent BaP in mesenteric lymph, portal vein blood, liver, and bile in wild-type and all seven Cyp1 knockout mouse lines, and the role and mechanism of chylomicrons in delivering BaP to target organs; [b] generate liver- and intestinal epithelium-specific Cyp1a1 conditional knockout lines; [c] replace the Cyp1b1 gene (in the genome) with the Cyp1a1 gene, and vice versa; and [d] repeat our BaP pharmacokinetics studies (see [a]) in these four newly generated mouse lines. Understanding the tissue- specific roles for each of the three CYP1 enzymes in the intact mouse receiving oral BaP will provide us with a greater understanding of BaP detoxification vs. metabolic activation. We expect this knowledge will provide a blueprint for understanding the mechanisms of elimination vs. dissemination of ingested BaP and will be informative in clinical studies in which we would determine which haplotypes of these three human genes might be associated with resistance vs. sensitivity to PAH-induced toxicity and cancer.