Estrogen receptor (ER) β regulates ERα expression in stromal cells derived from ovarian endometriosis

Context: Estradiol and its nuclear receptors, estrogen receptor (ER) α and ERβ, play critical roles in endometrium and endometriosis. Levels of ERβ, due to pathological hypomethylation of its promoter, are significantly higher in endometriotic vs. endometrial tissue and stromal cells, whereas ERα levels are lower in endometriosis. Estradiol regulates ERα gene expression via its alternatively used promoters A, B, and C. Objective: The aim of the study was to determine whether high levels of ERβ in endometriotic stromal cells from ovarian endometriomas regulate ERα gene expression. Results: ERβ knockdown significantly increased ERα mRNA and protein levels in endometriotic stromal cells. Conversely, ERβ overexpression in endometrial stromal cells decreased ERα mRNA and protein levels. ERβ knockdown significantly decreased proliferation of endometriotic stromal cells. Chromatin immunoprecipitation assays demonstrated that estradiol enhanced ERβ binding to nonclassical activator protein 1 and specificity protein 1 motifs in the ERα gene promoters A and C and a classic estrogen response element in promoter B in endometriotic stromal cells. Conclusions: High levels of ERβ suppress ERα expression and response to estradiol in endometrial and endometriotic stromal cells via binding to classic and nonclassic DNA motifs in alternatively used ERα promoters. ERβ also regulates cell cycle progression and might contribute to proliferation of endometriotic stromal cells. We speculate that a significantly increased ratio of ERβ:ERα in endometriotic tissues may also suppress progesterone receptor expression and contribute to progesterone resistance. Thus, ERβ may serve as a significant therapeutic target for endometriosis.