Ah receptor binding properties of indole carbinols and induction of hepatic estradiol hydroxylation

Abstract

The effect of route of administration on the ability of indole-3-carbinol (I3C), an anticarcinogen present in cruciferous vegetables, to induce estradiol 2-hydroxylase (EH) in female rat liver microsomes was investigated and compared to that of its main gastric conversion product, 3,3'-diindolylmethane (DIM). This dimer was more potent than I3C after either oral or intraperitoneal administration and was also a better in vitro inhibitor of EH in control and I3C-induced hepatic microsomes. The induction of both CYP1A1 and 1A2 in about equal amounts by I3C and DIM as well as of CYP2B1/2 was demonstrated using monoclonal antibodies. DIM, isosafrole, β-naphthoflavone, 3-methylcholanthrene and naringenin added in vitro inhibited EH strongly in induced microsomes but gestodene was a better inhibitor of estrogen 2-hydroxylation in liver microsomes from untreated female rats. The binding affinities of I3C and DIM to the Ah receptor were compared to that of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) by competition studies, and the IC 50 values were shown to be 2.0 × 10 −9 M, 5.0 × 10 −5 M and 2.3 × 10 −3 M for TCDD, DIM and I3C, respectively. The ability of I3C or DIM to cause in vitro transformation of the Ah receptor to a form able to bind to the dioxinresponsive element-3 (DRE3) was compared to that of TCDD and shown to parallel their abilities to compete for binding of [ 3H]TCDD to the Ah receptor. These experiments confirm and extend the proposals that dietary indoles induce specific cytochrome P450s in rat liver by a mechanism possibly involving the Ah receptor. The induced monooxygenases, in turn, increase the synthesis of 2-hydroxylated estrogens in the competing pathways of 2- and 16α--hydroxylation which decreases the levels of 16α-hydroxyestrone able to form stable covalent adducts with proteins including the estrogen receptor. Such steroid-protein interaction has been correlated with mammary carcinogenesis.