Molecular Mechanism of the Inhibition of Estradiol-Induced Endometrial Epithelial Cell Proliferation by Clomiphene Citrate

We examined the molecular mechanisms of the antiestrogenic effects of clomiphene citrate (CC) in the endometrium using two types of cell lines, Ishikawa and EM-E6/E7/hTERT cells. CC or ICI182780 inhibited 17β-estradiol (E2)-induced endometrial cell proliferation and transcriptional activation of the estrogen response element (ERE) gene. We directly visualized the ligand-estrogen receptor (ER)α interaction using green fluorescent protein (GFP)-tagged ERα in a single living cell. Whereas E2 changed the nuclear localization of GFP-ERα to a punctate distribution within 5 min, CC or ICI182780 changed the slower and less mobilization of GFP-ERα compared with E2. Pretreatment with CC or ICI182780 partly prevented the E2-induced nuclear redistribution of GFP-ERα. Fluorescence recovery after photobleaching revealed that GFP-ERα mobility treated with E2 was more rapid than that treated by CC or ICI182780. As coactivator recruitment to the ER is essential for ER-dependent transcription, we examined the interaction between ERα and steroid receptor coactivator-1 (SRC-1). The complex formation between ERα and SRC-1 was significantly increased by E2 but was prevented in the presence of CC or ICI182780 by coimmunoprecipitation. Moreover, the E2-induced colocalization of GFP-ERα and SRC-1 was prevented in the presence of CC or ICI182780 according to an immunofluorescence assay. We also observed that the reduction of SRC-1 using small interfering RNA for SRC-1 resulted in the inhibition of E2-induced cell proliferation and transcriptional activation of the ERE gene. Collectively, these results suggest that CC may inhibit E2-induced endometrial epithelial cell proliferation and ERE transactivation by inhibiting the recruitment of SRC-1 to ERα.