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 carcinogenic activity in rodent bioassays; however, the fjord region diol epoxides of B[c]Ph, B[c]Ph-3,4-diol 1,2-epoxides (B[c]PhDE), are potent 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 cell lines were treated with the potent Ah receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prior to exposure to B[c]Ph for 24 h. Mice were treated topically with 1 microg of TCDD or vehicle (control) for 73 h and then with 2 micromol of B[c]Ph for 24 h. In MCF-7 cells, TCDD exposure increased B[c]PhDE-DNA adduct levels more than 3-fold with a 10-fold increase in the (-)-B[c]PhDE-2-dA(t) adduct. Treatment of HepG2 cells with TCDD prior to B[c]Ph application did not increase B[c]PhDE-DNA binding. Total B[c]PhDE-DNA adducts increased 3-fold in TCDD-treated mouse epidermis: the majority of the increase resulted from (+)-B[c]PhDE-1-dA adducts. Analysis of P450 enzymes by Western blotting detected a large increase of P4501B1 but almost no increase in P4501A1 in MCF-7 cells exposed to 10 microM B[c]Ph for 24 or 48 h. In HepG2 cells, there were no detectable levels of P4501A1 or P4501B1 after treatment with 10 microM B[c]Ph for 24 h. In contrast, topical application of 2 micromol 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 of 7,12-dimethylbenz[a]anthracene (DMBA) was analyzed in microsomes prepared from MCF-7 and HepG2 cells exposed to 0.1% DMSO, 10 microM B[c]Ph, or 10 nM TCDD for 24 or 48 h and from mouse epidermis treated with 1 microg of TCDD, or vehicle control for 72 h, or 2 micromol of B[c]Ph for 48 h. The levels of DMBA metabolites were low or undetectable in microsomes from B[c]Ph-treated MCF-7 and HepG2 cells, but a metabolite pattern consistent with P4501A1 metabolism of DMBA was present in B[c]Ph-exposed mouse epidermal microsomes. TCDD-treated MCF-7 cells, HepG2 cells, and mouse epidermis had DMBA metabolism patterns characteristic of P4501A1 activity. Microsomes from TCDD-treated human cells formed a higher proportion of the proximate carcinogenic metabolite DMBA-3,4-dihydrodiol (16% of total identified metabolites) than TCDD-treated mouse epidermis (2%). In mouse epidermis, the weak ability of B[c]Ph to increase hydrocarbon-metabolizing activity and the increase in mainly P4501A1, leading to formation of the less carcinogenic stereoisomer B[c]PhDE-1, may explain the low carcinogenic activity of B[c]Ph. In a human mammary carcinoma cell line, treatment with B[c]Ph increases mainly P4501B1 and results in formation of a higher proportion of the more carcinogenic B[c]PhDE-2. This indicates that cells in which B[c]Ph treatment increases P4501B1 levels effectively activate B[c]Ph to potent carcinogenic metabolites.
Einolf
H J
HJ
Department of Medicinal Chemistry & Molecular Pharmacology, Purdue University, West Lafayette, Indiana 47907, USA.
Story
W T
WT
Marcus
C B
CB
Larsen
M C
MC
Jefcoate
C R
CR
Greenlee
W F
WF
Yagi
H
H
Jerina
D M
DM
Amin
S
S
Park
S S
SS
Gelboin
H V
HV
Baird
W M
WM
D000818:Animals; D001711:Biotransformation; D001943:Breast Neoplasms; D002273:Carcinogens / Q000493:pharmacokinetics*; D006528:Carcinoma, Hepatocellular; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000502:physiology; D004790:Enzyme Induction / Q000502:physiology; D000078404:Epidermal Cells; D004817:Epidermis / Q000187:drug effects / Q000201:enzymology*; D005260:Female; D006801:Humans; D008113:Liver Neoplasms; D051379:Mice; D018632:Mice, Inbred SENCAR; D010616:Phenanthrenes / Q000493:pharmacokinetics*; D014407:Tumor Cells, Cultured
D002273:Carcinogens; D010616:Phenanthrenes; D003577:Cytochrome P-450 Enzyme System; C002615:benzo(c)phenanthrene