Mechanism of benzo[a]pyrene-induced Cyp1a-1 gene expression in mouse Hepa 1c1c7 cells: role of the nuclear 6 s and 4 s proteins.
Treatment of wild-type (wt) aryl hydrocarbon (Ah)-responsive mouse Hepa 1c1c7 cells with benzo[a]pyrene (B[a]P) caused a concentration-dependent induction of ethoxyresorufin O-deethylase (EROD) activity. In contrast, B[a]P was inactive as an inducer in Ah nonresponsive class 1 and class 2 mutant cell lines. In parallel experiments, the nuclear fractions from wt cells treated with 10(-7) M [3H]B[a]P contained both the 4 s carcinogen binding protein and the 6 s (Ah receptor) complexes, whereas only the 4 s complex was present in the nuclear fraction of the class 2 mutant cells. The results obtained from cotreatment of wt Hepa 1c1c7 cells with 10(-6) or 10(-7) M B[a]P and 5 x 10(-7) or 10(-7) M 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) showed that MCDF inhibited the induction of EROD activity and Cyp1a-1 mRNA levels by B[a]P. Moreover, using 10(-7) M [3H]B[a]P and unlabeled MCDF, it was shown that MCDF not only inhibited the induction response but also caused a concentration-dependent decrease in levels of the nuclear 6 s complex but not the 4 s complex. In contrast, in situ competition studies with unlabeled 10(-6) M benzo[ghi]-perylene (B[ghi]P) resulted in the elimination of the nuclear [3H]B[a]P 4 s complex (but not the 6 s complex); however, the EROD activity and Cyp1a-1 mRNA levels in cells treated with 10(-7) M B[a]P in the presence or absence of 10(-6) M B[ghi]P were not significantly different. These results indicate that the 4 s binding protein is not required for the induction of Cyp1a-1 gene expression in Hepa 1c1c7 cells and suggest that B[a]P and 2,3,7,8-tetrachlorodibenzo-p-dioxin induce Cyp1a-1 gene expression via a common mechanism which involves binding to the Ah receptor.
Merchant
M
M
Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station 77843.
Wang
X
X
Kamps
C
C
Rosengren
R
R
Morrison
V
V
Safe
S
S
D000818:Animals; D001483:Base Sequence; D001564:Benzo(a)pyrene / Q000494:pharmacology*; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002460:Cell Line; D019363:Cytochrome P-450 CYP1A1; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics*; D004790:Enzyme Induction; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D050938:Glycine N-Methyltransferase; D008114:Liver Neoplasms, Experimental / Q000201:enzymology / Q000235:genetics*; D008780:Methyltransferases; D051379:Mice; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D010088:Oxidoreductases / Q000096:biosynthesis / Q000235:genetics*; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug / Q000187:drug effects
D002352:Carrier Proteins; D009687:Nuclear Proteins; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug; D001564:Benzo(a)pyrene; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D019363:Cytochrome P-450 CYP1A1; D008780:Methyltransferases; D050938:Glycine N-Methyltransferase