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N, Kansra S. Epidermal growth factor receptor cross-talks with ligand-occupied estrogen receptor-␣ to modulate both lactotroph proliferation and prolactin gene expression. Am J Physiol Endocrinol Metab 297: E331-E339, 2009. First published May 26, 2009 doi:10.1152 doi:10. /ajpendo.00133.2009 and EGF regulate lactotrophs, and we recently demonstrated that EGF phosphorylates S118 on estrogen receptor-␣ (ER␣) and requires ER␣ to stimulate prolactin (PRL) release. However, the interactions between ligand-occupied ER␣ and activated ErbB1 and its impact on lactotroph function are unknown. Using rat GH3 lactotrophs, we found that both E2 and EGF independently stimulated proliferation and PRL gene expression. Furthermore, their combination resulted in an enhanced stimulatory effect on both cell proliferation and PRL gene expression. Inhibitors of ER as well as ErbB1 blocked the combined effects of E2 and EGF. Pretreatment with UO126 abolished the combined effects, demonstrating Erk1/2 requirement. Although bidirectionality in ER-ErbB1 cross-talk is a well-accepted paradigm, interestingly in lactotrophs, ErbB1 kinase inhibitor failed to block the effect of E2 on proliferation and stimulation of PRL gene expression, suggesting that ER does not require ErbB1 to mediate its effects. Furthermore, E2 did not affect the ability of EGF to induce c-Fos expression or modulate AP-1 activity. However, both E2 and EGF combine to enhance S118 phosphorylation of ER␣, leading to enhanced E2-mediated estrogen response element transactivation. Taken together, our results suggest that, in lactotrophs, activated ErbB1 phosphorylates ER␣ to enhance the stimulatory effect of E2, thereby providing the molecular basis by which EGF amplifies the response of E2. lactotrophs; cell proliferation ESTROGEN (E2) EXERTS A WIDE RANGE of biological activities, including rapid cell activation, reversible gene regulation, and chronic structural/proliferative alterations. Binding of E2 to the estrogen receptors (ERs) facilitates receptor binding to specific DNA target sequences called the estrogen response elements (ERE). The ability of ER to bind DNA is regulated by several coactivators or corepressors that enhance or suppress, respectively, the association of ER with the gene transcriptional apparatus (22,26). The transcriptional activity of ER also depends on its phosphorylation, which takes place within minutes of E2 binding. The pituitary lactotroph is a wellcharacterized E2 target cell, with estrogens stimulating prolactin (PRL) gene expression and release, upregulating genes such as TGF-␣, galanin, and VEGF, and increasing lactotroph proliferation (3, 11). The importance of ER(s) in lactotroph homeostasis is highlighted by the decreases in pituitary PRL content and lactotroph number in ovariectomized animals, whereas chronic exposure to E2 results in hyperprolactinemia and lactotroph hyperplasia (35).In the absence of a human pituitary cell line, investigators have relied on the rat pituitary lactotroph GH3 cell line as a model for studying lactot...

BackgroundWe recently reported that estrogen receptor α (ERα), even in absence of estrogen (E2), plays a critical role in lactotroph homeostasis. The anti-estrogen ICI 182780 (ICI), but not tamoxifen or raloxifene, rapidly promoted the degradation of ERα, and inhibited cell proliferation. However, all three ER antagonists suppressed PRL release, suggesting that receptor occupation is sufficient to inhibit prl gene expression whereas receptor degradation is required to suppress lactotroph proliferation. In this study our objective was to determine whether ERα degradation versus occupation, differentially modulates the biological outcome of anti-estrogens.Principal FindingsUsing the rat lactotroph cell line, GH3 cells, we report that ICI induced proteosome mediated degradation of ERα. In contrast, an ERα specific antagonist, MPP, that does not promote degradation of ERα, did not inhibit cell proliferation. Further, ICI, but not MPP, abolished anchorage independent growth of GH3 cells. Yet, both ICI and MPP were equally effective in suppressing prl expression and release, as well as ERE-mediated transcriptional activity.ConclusionTaken together, our results demonstrate that in lactotrophs, ERα degradation results in decreased cell proliferation, whereas ERα occupation by an antagonist that does not promote degradation of ERα is sufficient to inhibit prl expression.