Estrogenic potency, receptor interactions, and metabolism of catechol estrogens in the immature rat uterus in vitro

To determine the inherent estrogenic potencies of the catechol estrogens 2-hydroxyestradiol [1,3,5-(10)estratriene-2,3,17β-triol (2-OHE₂)] and 2-hydroxyestrone [2,3-dihydroxy-l,3,5-(10)estratrien-17-one (2-OHE₁)] and to relate these to their affinities for the estrogen receptor, we have examined in parallel the metabolism of these compounds and their ability to stimulate the synthesis of the uterine induced protein (IP) in vitro. The catechol estrogens were able to stimulate IP synthesis maximally in immature rat uteri incubated in vitro. Depletion of cytosolic receptors by 2-OHE₂ was as complete as that with estradiol (E₂) and 2-OHE₂ fully translocated receptors to the nucleus. The relative binding affinities of the catechol estrogens for the uterine cytosolic estrogen receptor were: E₂, 100% 2-OHE₂, 23% and 2-OHE₁ 3%. The rank order of their potency in stimulating IP synthesis paralleled their relative binding affinity for the receptor. However, the potencies of 2-OHE₂ and 2-OHE₁were less than would be predicted from their respective receptor affinities. With blockade of the metabolism of 2-OHE₂ by an inhibitor of catechol-O-methyltransferase, U-0521 (3,4-dihydroxy-2-methylpropiophenone), stimulation of IP synthesis by 2-OHE₂ was potentiated and was brought closer to the value predicted from its receptor affinity. The ability of 2-0HE₂ to translocate receptors to the nucleus was also increased. Metabolism of 2-OHE₂ and 2-OHE₂ occurred mainly by methylation, which was blocked by U-0521. In the absence of U-0521, maximum tissue levels of the catechol estrogens were considerably lower than those of E₂. Inhibition of O-methylation decreased, but did not eliminate, these differences. Interestingly, U-0521 was itself found to have a low affinity for the estrogen receptor (0.001% that of E₂) and weak estrogenic activity at concentrations higher than those used to inhibit O-methylation of the catechol estrogens. We conclude that the catechol estrogens 2-OHE₁ and 2-OHE₁ are full estrogen agonists in this bioassay system and interact with the uterine estrogen receptor system in a manner similar to that of E₂ Metabolism by Omethylation significantly alters the potency of catechol estrogens in vitro. However, metabolism does not fully account for the discrepancy between their predicted potency, on the basis of cytosol receptor affinity, and their observed potency in vitro.