Benzo[a]pyrene-Induced Toxicity: Paradoxical Protection in Cyp1a1(−/−) Knockout Mice Having Increased Hepatic BaP–DNA Adduct Levels

Abstract

Previous studies have shown that cytochrome P450 1A1 (CYP1A1), CYP1B1, and prostaglandin-endoperoxide synthase (PTGS2) are inducible by benzo[a]pyrene (BaP) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin), and all three metabolize BaP to reactive DNA-binding intermediates and excreted products. Because these three enzymes show differing patterns of basal levels, inducibility, and tissue-specific expression, animal studies are necessary to delineate the role of CYP1A1 in BaP-mediated toxicity. In mice receiving large daily doses of BaP (500 mg/kg i.p.), Cyp1a1(−/−) knockout mice are protected by surviving longer than Cyp1a1(+/−) heterozygotes. We found that a single 500 mg/kg dose of BaP induces hepatic CYP1A1 mRNA, protein, and enzyme activity in Cyp1a1(+/−) but not in Cyp1a1(−/−) mice; TCDD pretreatment increases further the CYP1A1 in Cyp1a1(+/−) but not Cyp1a1(−/−) mice. Although a single 500 mg/kg dose of BaP was toxic to Cyp1a1(+/−) mice (serum liver enzyme elevated about 2-fold above control levels at 48 h), Cyp1a1(−/−) mice displayed no hepatotoxicity. Unexpectedly, we found 4-fold higher BaP–DNA adduct levels in Cyp1a1(−/−) than in Cyp1a1(+/−) mice; TCDD pretreatment lowered the levels of BaP–DNA adducts in both genotypes, suggesting the involvement of other TCDD-inducible detoxification enzymes. BaP was cleared from the blood much faster in Cyp1a1(+/−) than Cyp1a1(−/−) mice. Our results suggest that absence of the CYP1A1 enzyme protects the intact animal from BaP-mediated liver toxicity and death, by decreasing the formation of large amounts of toxic metabolites, whereas much slower metabolic clearance of BaP in Cyp1a1(−/−) mice leads to greater formation of BaP–DNA adducts.