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 µ/kg) increased estradiol 2-hydroxylase (E₂OHase)activity 2-fold in liver microsomes of female rats but hadno effect on E₂OHase activity in hepatic microsomes of malerats. In contrast, TCDD increased P-450d (an enzyme which hasa high turnover number for E₂OHase in a reconstituted enzymesystem) 10- to 20-fold in livers of both male and female rats.The discrepancy between the increases in P-450d and E₂OHaseactivity in liver microsomes of TCDD-induced rats was abolishedby the addition of exogenous purified P-450 reductase to themicrosomal assays for E₂OHase, suggesting that reductase waslimiting in the in vitro assays. When E₂OHase activity was assayedin the presence of exogenous reductase, TCDD increased E₂OHase2-fold and 4-fold respectively in liver microsomes of male andfemale rates. Antibody to P-450d completely inhibited the increasein E₂OHase activity in liver microsomes of TCDD-treated maleand female rats, but had little effect on E₂OHase activity inliver microsomes of untreated male or female rats. These dataindicate that P-450d is responsible for the increase in E₂OHaseactivity in TCDD-treated rats, but other P-450 isozymes areresponsible for constitutive E₂OHase activity. Biweekly administrationof 1.4 µg/kg of TCDD for 30 weeks as a potential promoterof hepatocarcinogenesis increased the volume of the liver occupiedby -glutamyl transpeptidase (GGT)-positive foci in livers offemale rats given a necrogenic dose of diethylnitrosamine (DEN)(200 mg/kg) as the initiator. Biweekly doses of 0.14–1.4µg/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 effectsof the low dose of TCDD on both hepatic microsomal P-450d andE₂OHase activity.