Estrogen receptors ERα and ERβ in proliferation in the rodent mammary gland

Most evidence supports the view that ERalpha is responsible for estrogen (ovarian estradiol, E-2)-induced proliferation in the epithelial cells of the mammary gland, but despite this, proliferating epithelial cells do not express ERalpha. We have examined this apparent paradox by studying the role of ERalpha and ERbeta in E-2-induced proliferation in mammary glands (measured by BrdUrd incorporation into DNA) in mice with intact ERbeta (WT mice) and those in which the ERbeta gene has been inactivated (ERbeta(-/-) mice). On treatment of ERbeta(-/-) mice with E-2 or ovariectomized WT mice with E-2, tamoxifen, or a specific ERbeta agonist (BAG), the number of BrdUrd-labeled cells in mammary glands increased from 3.4% in controls to 28-38% in the treated mice. This indicates that both ERalpha and ERbeta can mediate E-2-induced proliferation independently of each other. With specific antibodies, ERbeta was found in both epithelial and stromal cells, whereas ERalpha was strictly epithelial. Within 4 h of a single dose of E-2, ERalpha was lost from the nuclei of epithelial cells. In WT mice, ERalpha reappeared by 24 h, but in ERbeta(-/-) mice, return to the nucleus was delayed by 24 h. At 4 h after E-2, neither ERalpha nor progesterone receptor was detectable in BrdUrd-labeled nuclei but by 48 h after E-2, 29% of the BrdUrd-labeled cells expressed ERalpha, and 21-38% expressed progesterone receptor. During 3 weeks of continuous E-2 treatment, ERbeta remained in the nucleus, but there was no detectable ERalpha. With tamoxifen treatment, ERalpha remained in the nucleus, but ERbeta was lost. From these results, we conclude that ERalpha receives the proliferation signal from E-2, initiates DNA synthesis, and is then lost from cells. The subsequent steps in proliferation can proceed in the absence of either ERalpha or ERbeta. ERbeta facilitates the return of ERalpha to the nucleus and restores responsiveness to E-2. By down-regulating ERbeta, tamoxifen may prolong refractoriness to E-2 in mammary epithelium.