ROLE OF CYTOCHROME-P450 ENZYME-INDUCTION IN THE METABOLIC-ACTIVATION OF BENZO[C]PHENANTHRENE IN HUMAN CELL-LINES AND MOUSE EPIDERMIS

The environmental contaminant benzo[c]phenanthrene (B[c]Ph) has weak c arcinogenic activity in rodent bioassays; however, the fjord region di ol epoxides of B[c]Ph, B[e]Ph-3,4-diol 1,2-epoxides (B[c]PhDE), are po tent carcinogens. To determine the role of cytochrome P450 isozymes in the activation of B[c]Ph in MCF-7 cells and the low activation of B[c ]Ph in mouse skin, cells of the MCF-7 and the human hepatoma HepG2 cel l lines were treated with the potent Ah receptor agonist 2,3,7,8-tetra chlorodibenzo-p-dioxin (TCDD) prior to exposure to B[c]Ph for 24 h. Mi ce were treated topically with 1 mu g of TCDD or vehicle (control) for 73 h and then with 2 mu mol of B[c]Ph for 24 h. In MCF-7 cells, TCDD exposure increased B[c]PhDE-DNA adduct levels more than S-fold with a 10-fold increase in the (-)-B[c]PhDE-2-dA(t) adduct. Treatment of HepG 2 cells with TCDD prior to B[c]Ph application did not increase B[c]PhD E-DNA binding. Total B[c]PhDE-DNA adducts increased 3-fold in TCDD-tre ated mouse epidermis: the majority of the increase resulted from (+)-B [c]PhDE-1-dA adducts. Analysis of P450 enzymes by Western blotting det ected a large increase of P4501B1 but almost no increase in P4501A1 in MCF-7 cells exposed to 10 mu M B[c]Ph for 24 or 48 h. In HepGa cells, there were no detectable levels of P4501A1 or P4501B1 after treatment with 10 mu M B[c]Ph for 24 h. In contrast, topical application of 2 m u mol of B[c]Ph to mouse skin for 48 or 72 h increased P4501A1, but no P4501B1 was detected. As a measure of P450 activity, the metabolism o f 7,12-dimethylbenz[a]anthracene (DMBA) was analyzed in microsomes pre pared from MCF-7 and HepG2 cells exposed to 0.1% DMSO, 10 mu M B[c]Ph, or 10 mu M TCDD for 24 or 48 h and from mouse epidermis treated with 1 mu g of TCDD, or vehicle control for 72 h, or 2 mu mol of B[c]Ph for 48 h. The levels of DMBA metabolites were low or undetectable in micr osomes from B[c]Ph-treated MCF-7 and HepG2 cells, but a metabolite pat tern consistent with P4501A1 metabolism of DMBA was present in B[c]Ph- exposed mouse epidermal microsomes. TCDD-treated MCF-7 cells, HepG2 ce lls, and mouse epidermis had DMBA metabolism patterns characteristic o f P4501A1 activity. Microsomes from TCDD-treated human cells formed a higher proportion of the proximate carcinogenic metabolite DMBA-3,4-di hydrodiol (16% of total identified metabolites) than TCDD-treated mous e epidermis (2%). In mouse epidermis, the weak ability of B[c]Ph to in crease hydrocarbon-metabolizing activity and the increase in mainly P4 501A1, leading to formation of the less carcinogenic stereoisomer B[c] PhDE-1, may explain the low carcinogenic activity of B[c]Ph. In a huma n mammary carcinoma cell line, treatment with B[c]Ph increases mainly P4501B1 and results in formation of a higher proportion of the more ca rcinogenic B[c]PhDE-2. This indicates that cells in which B[c]Ph treat ment increases P4501B1 levels effectively activate B[c]Ph to potent ca rcinogenic metabolites.