Cells of the Uterine Myometrium Predicts Agonism of Xenoestrogens in Normal and Neoplastic Estrogen Receptor Activation via Activation Function

The possible contribution of endocrine disrupters to human disease, particularly those compounds that modulate the estrogen receptor (ER), has recently drawn considerable attention. The tissue specificity of effects mediated by the ER is well recognized, although the mechanism of this specificity is not understood sufficiently to predict the effects of a particular ligand in different target tissues. Although the divergence of ERmediated effects in the breast, bone, and uterine endometrium has been described, a frequently overlooked site of estrogen action is the smooth muscle of the uterus. The uterine myometrium is the tissue of origin of an extremely common hormone-responsive tumor, uterine leiomyoma, a tumor with a significant impact on women’s health and a possible environmental influence. This report describes an in vitro/in vivo system for identifying the effects of ER ligands in the myometrium and elucidating their mechanism of action. Several natural and synthetic xenoestrogens were evaluated at the cellular and molecular level for their ability to mimic estrogen action in uterine myometrial tissues. Diethylstilbestrol, coumestrol, genistein, naringenin, and endosulfan were able to activate the AF2 function of the ER in vitro and demonstrated agonist activity in estrogen-responsive myometrial cells, as determined by induction of proliferation and increased message levels of progesterone receptor. Compounds that could not activate AF2 function (4-hydroxy-tamoxifen, LY117018, and LY317783) did not act as estrogen agonists. For agonists, rank order of potency was predicted by receptor affinity; however, endosulfan displayed a surprising degree of activity, despite negligible receptor binding. Additionally, diethylstilbestrol and tamoxifen demonstrated prototypical agonist and antagonist effects, respectively, in the intact myometrium of sexually mature rats. The results presented here suggest that some exogenous ER ligands may mimic the effects of endogenous estrogens on uterine leiomyoma and may contribute to a complex hormonal milieu that impacts both normal and neoplastic myometrium.