Estrogen receptor β: the guardian of the endometrium

The endometrium is the primary target organ for the 'female' sex steroid hormone estrogen, which exerts effects in the endometrium via two main classical estrogen receptor (ER) isoforms, ERα and ERβ. The main function of the endometrium, embryo implantation, appears unperturbed in ERβ knockout mice, which has led researchers to disregard other potentially important functional roles that ERβ may have in endometrium. This review focuses on ERβ in the human endometrium and its protective role from the undesired effects of ERα.We conducted a systematic search using PubMed and Ovid for publications between January 1996 and February 2014. All studies that examined ERβ expression or function in non-pregnant endometrium or cells derived from the endometrium were considered, including human and animal studies.Studies of the basic function of ERβ isoforms in restraining ERα-mediated cell-specific trophic/mitotic responses to estrogen in other tissues has allowed appreciation of the important potential role of ERβ in the regulation of cell fate in the human endometrium. Our current understanding of ERβ expression and function in endometrium is, however, incomplete. ERβ is dynamically expressed in healthy premenstrual endometrium, persists in post-menopausal atrophic endometrium and may play an important role in endometrial disease. All endometrial cell types express ERβ and aberrations in ERβ expression have been reported in almost all benign and malignant endometrial proliferative disease.The collective evidence suggests that ERβ has an important role in normal endometrial function and also in most, if not all, benign and malignant endometrial diseases. However, the conduct of studies of endometrial ERβ expression needs to be standardized: agreement is needed regarding the most appropriate control tissue for endometrial cancer studies as well as development of standardized methods for the quantification of ERβ immunohistochemical data, similar to those scoring systems employed for other hormonally regulated tissues such as breast cancer, since these data may have direct clinical implications in guiding therapy.