Characterization of the Dose–Response of CYP1B1, CYP1A1, and CYP1A2 in the Liver of Female Sprague–Dawley Rats Following Chronic Exposure to 2,3,7,8-Tetrachlorodibenzo-p-dioxin☆

Abstract One of the current knowledge gaps in the evaluation of risk for human exposure to 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) is the relationship between gene expression induced by TCDDmore complex biological responses such as altered growth, differentiation, and neoplasia. This study investigates the dose-dependent expression of CYP1A1, CYP1A2,CYP1B1 in the livers of female Sprague–Dawley rats chronically exposed to TCDD. Animals were treated biweekly for 30 weeks with daily averaged doses of 0 to 125 ng TCDD/kg/day. Immunoblot analysis showed that protein levels for CYP1B1, CYP1A1,CYP1A2 exhibited a dose-dependent induction by TCDD. However, CYP1A1 and CYP1A2 protein levels were approximately 100-fold higher than CYP1B1, which could not be detected by either immunoblot analysis or immunohistochemistry in the livers of rats treated with TCDD for 30 weeks at a dose-equivalent less than 35.7 ng/kg/day. In control animals, CYP1A1CYP1A2 RNA levels, measured by quantitative RT-PCR, were 1100-15,000-fold higher than that of CYP1B1, respectively. TCDD induced CYP1B1 RNA levels at all doses, although absolute TCDD-induced levels of CYP1A1CYP1A2 at the highest dose (125 ng/kg/day) were more than 40-fold higher than that of CYP1B1. While the liver concentration of TCDD required for half-maximal induction of CYP1A1, CYP1A2,CYP1B1 RNA levels was similar, the shaping parameter (Hill coefficient) of the dose–response curve for CYP1B1 was significantly higher than that for CYP1A1 or CYP1A2. The low level of TCDD-induced CYP1B1 expression in the liver relative to that of the CYP1A1CYP1A2 suggest that, if CYP1B1 is involved in TCDD-induced hepatocarcinogenesis, its endogenous function is likely to be uniquenot overlapping with that of CYP1A1 or CYP1A2.