Vol. 27, Issue 2, 281-287, February 1999

Induction of Cytochrome P-450 Enzymes after Repeated Exposure to 4-Vinylcyclohexene in B6C3F₁ Mice

Julie K. Doerr-Stevens,¹ Juping Liu, Gregory J. Stevens,² James C. Kraner,³ Susan M. Fontaine, James R. Halpert, and I. Glenn Sipes

Department of Pharmacology and Toxicology, University of Arizona, Tucson, Arizona

4-Vinylcyclohexene (VCH), an ovarian toxicant in mice, is known to irreversibly deplete ovarian follicles as a consequence of VCH diepoxide formation. Because ovotoxicity requires repeated dosing of VCH, the effect of consecutive daily doses of VCH (7.5 mmol/kg/day) on mouse liver microsomal activities and VCH epoxidation was determined. Cytochromes P-450 2B and 2A (CYP2B and CYP2A), principle isoforms involved in the bioactivation of VCH, as well as CYP2E1 and CYP3A were evaluated. VCH exposure increased total cytochrome P-450 content (35-83% above control levels) after either 5, 10, or 15 days of treatment. Western blot analysis revealed an induction of CYP2A, CYP2B, and CYP2E1 at day 10. Elevated levels of CYP2A and CYP2B correlated with marker androstenedione and testosterone 16- and 16-hydroxylase activities. Microsomes prepared from mice pretreated with VCH for 10 days demonstrated an increase (2-fold) in the rate of VCH monoepoxide and diepoxide formation. Microsomal VCH epoxidation was increased to a similar extent by phenobarbital, acetone, and dexamethasone treatment. An increase in cytosolic glutathione S-transferase activity was observed after repeated VCH treatment, an enzyme potentially involved in detoxification of the VCH epoxides. Interestingly, preliminary studies indicated that circulating levels of the monoepoxide (vinylcyclohexene 1,2-monoepoxide) and diepoxide of VCH were elevated after repeated dosing of VCH. Overall, the results indicate that repeated exposure of VCH in mice induces cytochrome P-450-dependent activities, and in turn induction of its metabolism. Additional studies examining the toxicokinetics of VCH after repeated exposure are required to further delineate the relevance of induction in VCH-induced ovotoxicity.