title: Role of nitroreductases but not cytochromes P450 in the metabolic activation of 1-nitropyrene in the HepG2 human hepatoblastoma cell line.
abstract: 1-Nitropyrene is an environmental contaminant that is mutagenic in many prokaryotic and eukaryotic systems, including the hypoxanthine-guanosine phosphoribosyl transferase (HGPRT) locus in the human hepatoma cell line HepG2. Metabolism and DNA adduct formation of [3H]1-nitropyrene in the HepG2 were quantified to understand the role of nitroreduction and/or cytochrome P450-mediated C-oxidation of 1-nitropyrene in DNA adduct formation and mutagenicity. In uninduced HepG2 cells, 10 microM [3H]1-nitropyrene was metabolized principally by nitroreduction to 1-aminopyrene (516 pmol/24 hr/10(6) cells), and by cytochrome P450-mediated C-oxidation to K-region trans-dihydrodiols (37 pmol/24 hr/10(6) cells), 1-nitropyren-3-ol (51 pmol/24 hr/10(6) cells), and 1-nitropyren-6-ol and 1-nitropyren-8-ol (77 pmol/24 hr/10(6) cells). Pretreatment of the HepG2 cells for 24 hr with 5 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in a complete change in the metabolism of [3H]1-nitropyrene, with 1-nitropyren-6-ol and 1-nitropyren-8-ol formation (449 pmol/24 hr/10(6) cells) being 80-fold greater than 1-aminopyrene formation (6 pmol/24 hr/10(6) cells). This increase in C-oxidation of 1-nitropyrene was consistent with increased levels of cytochrome P450 1A. The only DNA adduct detected using the 32P-postlabeling assay in the HepG2 cells administered 1-nitropyrene was N-(2'-deoxyguanosin-8-yl)-1-aminopyrene (dG-C8-AP). Induction of C-oxidative metabolism through TCDD treatment resulted in a concomitant decrease in dG-C8-AP formation. DNA adducts for oxidized 1-nitropyrene metabolites were not detected in the TCDD-treated HepG2 cells administered 1-nitropyrene, which indicates that cytochrome P450-mediated C-oxidative pathways are detoxification pathways in HepG2 cells.
authors.affiliation: Department of Environmental Health Sciences, School of Medicine, Case Western Reserve University, Cleveland, OH 44106-4940, U.S.A.
authors.forename: K J
authors.initials: KJ
authors.lastname: Silvers
authors.forename: L H
authors.initials: LH
authors.lastname: Couch
authors.forename: E A
authors.initials: EA
authors.lastname: Rorke
authors.forename: P C
authors.initials: PC
authors.lastname: Howard
mesh_terms: D001711:Biotransformation; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D018736:DNA Adducts / Q000378:metabolism*; D006801:Humans; D008099:Liver / Q000378:metabolism; D009153:Mutagens / Q000378:metabolism*; D009601:Nitroreductases / Q000502:physiology*; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D011721:Pyrenes / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D018736:DNA Adducts; D009153:Mutagens; D000072317:Polychlorinated Dibenzodioxins; D011721:Pyrenes; D003577:Cytochrome P-450 Enzyme System; D009601:Nitroreductases; C032668:1-nitropyrene