Abstract

Exposure of female August-Copenhagen Irish (ACI) rats for 28 weeks to 3 mg of estradiol (E₂) contained in cholesterol pellets elevated blood E₂ levels and caused palpable mammary tumors in all animals. Coadministration of phenobarbital (PB) in their drinking water reduced the incidence, number, and size of mammary tumors (MTs) but did not reduce blood E₂ levels. Inhibition of MTs by PB was accompanied by significant changes in total hepatic metabolism of E₂ measured in vitro. PB treatment caused approximately a 4-fold increase in hepatic metabolism of E₂ in control and E₂-treated rats. The major NAD(P)H-dependent metabolites of E₂ were 2-OH-E₂ and estrone (E₁). PB, either alone or together with E₂, increased microsomal 2-hydroxylation of E₂; formation of E₁ was either unaffected or decreased slightly. PB also increased microsomal metabolism of E₂ to minor metabolites (4-OH-E₂, 6α-OH-E₂, 6β-OH-E₂, 14α-OH-E₂, 6-keto E₁, and 2-OH-E₁) and reduced the formation of the E₂-17β-oleoyl ester and the E₂ 3- and 17-glucuronides. In contrast, when given in combination with E₂, PB increased the formation of both glucuronides. Cotreatment of animals with PB and E₂ increased activities of NAD(P)H:quinone oxidoreductase and glutathione S-transferase to a greater extent than either compound alone. Collectively, these results show that the multiple actions of PB on hepatic metabolism of E₂, including induction of E₂ hydroxylation, glucuronidation, and antioxidant defense enzymes along with inhibition of E₂ esterification in livers of female ACI rats, accompany a marked reduction of E₂-dependent mammary tumors in this model.