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Ah Receptor Regulation of CYP1B1 Expression in Primary Mouse Embryo-derived Cells¹

Environmental Toxicology Center [D. L. A., S. E. E., C. R. J.] and Department of Pharmacology [C. R. J.], University of Wisconsin, Madison, Wisconsin 53705

Cytochrome P4501B1 is highly active in the bioactivation of polycyclic aromatic hydrocarbons (PAHs) to mutagenic metabolites. The C3H mouse embryo fibroblast cell line, C3H10T1/2, and primary mouse embryo fibroblasts (MEFs) express CYP1B1 as the predominant cytochrome P-450 form. This is constitutively expressed and induced by 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) but also, to a greater extent, by PAH. To establish the role of the aryl hydrocarbon receptor (AhR) in the induction responses to PAH and TCDD in MEFs, we measured CYP1B1 expression in primary MEFs generated from congenic C57b/6 mice that differ only at the AhR locus. These MEF cells express either b-1 or d-type Ah receptors, with high and low affinities for AhR agonists, respectively. Both types of MEFs express constitutive CYP1B1 to a similar extent, as measured by expression of mRNA, protein, and activity (PAH metabolism). Induction of CYP1B1 in responsive b-1 MEFs exhibited a 5-fold lower EC₅₀ for TCDD, as compared with MEFs expressing d-type AhR. This is fully consistent with AhR mediation of the induction. Maximal induction of CYP1B1 by 10^-8 M TCDD was comparable in each MEF type. Very low levels of CYP1A1 mRNA, detected by reverse transcriptase-PCR, show a similar dependence on AhR phenotype in these MEFs. The expression of CYP1B1 was highly dependent upon the passage number of the primary MEF cultures. Paralleling decreased proliferation of these cells after passage 7 are changes in cell morphology, the appearance of increasing numbers of differentiated cell types, and the complete loss of CYP1B1 expression. Colonies of these MEFs, which have escaped senescence, proliferate at much later passages, regain CYP1B1 expression, and exhibit the same AhR-dependent CYP1B1 induction responses, as observed in early-passage MEFs and the C3H10T1/2 cell line. We conclude that CYP1B1 expression, including induction via the AhR, parallels a proliferative state for MEF cells.

¹ This work was supported by NIH Grant CA16265. This work was presented at the Annual Meeting of the Society of Toxicology, held March 5–9, 1995, in Seattle, WA, and at the 10th International Symposium on Microsomes and Drug Oxidations, held July 21–24, 1994, in Toronto, Canada.

² To whom requests for reprints should be addressed, at Department of Pharmacology, University of Wisconsin, 1300 University Avenue, Madison, Wisconsin 53705. Phone: (608) 263-3128; Fax: (608) 262-1257; E-mail: dalexanl@students.wisc.edu. Environmental Toxicology Center Publication No. 299.

Received 4/ 7/97. Accepted 8/13/97.

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