Increases in cytochrome P-450 mediated 17 beta-estradiol 2-hydroxylase activity in rat liver microsomes after both acute administration and subchronic administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin in a two-stage hepatocarcinogenesis model.

Administration of single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (10 micrograms/kg) increased estradiol 2-hydroxylase (E2OHase) activity approximately 2-fold in liver microsomes of female rats but had no effect on E2OHase activity in hepatic microsomes of male rats. In contrast, TCDD increased P-450d (an enzyme which has a high turnover number for E2OHase in a reconstituted enzyme system) 10- to 20-fold in livers of both male and female rats. The discrepancy between the increases in P-450d and E2OHase activity in liver microsomes of TCDD-induced rats was abolished by the addition of exogenous purified P-450 reductase to the microsomal assays for E2OHase, suggesting that reductase was limiting in the in vitro assays. When E2OHase activity was assayed in the presence of exogenous reductase, TCDD increased E2OHase 2-fold and 4-fold respectively in liver microsomes of male and female rates. Antibody to P-450d completely inhibited the increase in E2OHase activity in liver microsomes of TCDD-treated male and female rats, but had little effect on E2OHase activity in liver microsomes of untreated male or female rats. These data indicate that P-450d is responsible for the increase in E2OHase activity in TCDD-treated rats, but other P-450 isozymes are responsible for constitutive E2OHase activity. Biweekly administration of 1.4 micrograms/kg of TCDD for 30 weeks as a potential promoter of hepatocarcinogenesis increased the volume of the liver occupied by gamma-glutamyl transpeptidase (GGT)-positive foci in livers of female rats given a necrogenic dose of diethylnitrosamine (DEN) (200 mg/kg) as the initiator. Biweekly doses of 0.14-1.4 micrograms/kg TCDD in this model also increased P-450d (7-fold) and E2OHase activity maximally (4-fold) in DEN-initiated rats. Moreover, initiation with DEN substantially enhanced the effects of the low dose of TCDD on both hepatic microsomal P-450d and E2OHase activity.