ROLE OF NITROREDUCTASES BUT NOT CYTOCHROMES P450 IN THE METABOLIC-ACTIVATION OF 1-NITROPYRENE IN THE HEPG2 HUMAN HEPATOBLASTOMA CELL-LINE

1-Nitropyrene is an environmental contaminant that is mutagenic in man y prokaryotic and eukaryotic systems, including the hypoxanthine-guano sine phosphoribosyl transferase (HGPRT) locus in the human hepatoma ce ll line HepG2. Metabolism and DNA adduct formation of [H-3]1-nitropyre ne in the HepG2 were quantified to understand the role of nitroreducti on and/or cytochrome P450-mediated C-oxidation of l-nitropyrene in DNA adduct formation and mutagenicity. In uninduced HepG2 cells, 10 mu M [H-3]1-nitropyrene was metabolized principally by nitroreduction to l- aminopyrene (516 pmol/24 hr/10(6) cells), and by cytochrome P450-media ted C-oxidation to K-region trans dihydrodiols (37 pmol/24 hr/10(6) ce lls), 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 o f the HepG2 cells for 24 hr with 5 nM 2,3,7,8-tetrachlorodibenzo-p-dio xin (TCDD) resulted in a complete change in the metabolism of [H-3]1-n itrupyrene, with 1-nitropyren-6-ol and 1-nitropyren-8-ol formation (44 9 pmol/24 r/10(6) cells) being 80-fold greater than l-aminopyrene form ation (6 pmol/24 hr/10(6) cells). This increase in C-oxidation of l-ni tropyrene was consistent with increased levels of cytochrome P450 1A. The only DNA adduct detected using the P-32-postlabeling assay in the HepG2 cells administered I-nitropyrene was N-(2'-deoxyguanosin-8-yl)-1 -aminopyrene (dG-C8-AP). Induction of C-oxidative metabolism through T CDD treatment resulted in a concomitant decrease in dG-C8-AP formation . DNA adducts for oxidized l-nitropyrene metabolites were not detected in the TCDD-treated HepG2 cells administered l-nitropyrene, which ind icates that cytochrome P450-mediated C-oxidative pathways are detoxifi cation pathways in HepG2 cells. (C) 1997 Elsevier Science Inc.