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 μM [ 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 l-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 l-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.