Influence of alpha-naphthoflavone on the metabolism and binding of ethinylestradiol in male Syrian hamster liver microsomes: possible role in hepatocarcinogenesis.

The influence of alpha-naphthoflavone (ANF) on the metabolism and binding of radiolabeled ethinylestradiol (EE2) has been examined both in vitro and in vivo in hamster liver microsomes. [14C]EE2 was metabolized extensively to seven major oxidative metabolites, 7 alpha-hydroxyEE2, 4-hydroxyEE2, 2-hydroxyEE2, D-homoestrone, monohydroxyEE2, and two dihydroxyEE2 metabolites identified as catechols with the additional hydroxy group on ring B or C, and a nonpolar fraction. The main EE2 metabolite found was 2-hydroxyEE2, and it represented 47% of the total metabolites formed. The total amount of EE2 catechol metabolites formed in untreated hamster liver microsomes was 65.5%. When ANF was added in vitro to these hepatic microsomes, there was a 27-45% decline in 2-hydroxyEE2 formation, a 98% reduction in dihydroxyEE2 (catechol-2), and a 56-66% reduction in the nonpolar fraction. The marked inhibition of EE2 metabolism by the in vitro addition of ANF is reflected by a corresponding decrease in the irreversible binding of radioactive hormone to hamster liver microsomal proteins. In contrast, a biphasic response of ANF on EE2 metabolism was observed after in vivo administration for 2.0 and 4.0 months. After 2.0 months of ANF treatment, there was a modest decline in all [14C]EE2 metabolites, except 2-hydroxyEE2 and dihydroxyEE2 (catechol 1). After 4.0 months of ANF treatment, the reduction in EE2 metabolism was even lower than after 2.0 months of treatment. Most interestingly, there was a 1.5-fold increase in 2-hydroxyEE2 after 4.0 months of ANF treatment, representing nearly 69% of the total EE2 metabolites formed. These results are consistent with an increase in irreversible binding of [14C]EE2 metabolites to hamster liver microsomal proteins after 4.0 months of ANF treatment. The relative 1.3-fold and the absolute 1.5-fold increases in 2-hydroxyEE2 after 2.0 and 4.0 months of ANF treatment in vivo, respectively, suggest that the elevation in this reactive EE2 metabolite precursor may contribute importantly to hepatotumorigenesis in the hamster following prolonged EE2 plus ANF treatment.