Tamoxifen (1.0 μM) was found to inhibit the expression of a thymidine kinase (TK) promoter-reporter gene, lacking an estrogen response element (ERE), in transiently transfected BeWo cells, suggesting that inhibition of TK promoter activity was linked to secondary estrogen-dependent effects on BeWo cell function. Estradiol (0.05–0.45 μM) stimulated BeWo cell proliferation and increased the percentage of S-phase cells. Tamoxifen (1.35–4.05 μM) inhibited BeWo cell growth and antagonized the stimulatory actions of 0.15 μMestradiol. Reverse transcription-polymerase chain reaction and Western analyses confirmed the presence of estrogen receptor (ER) transcripts and the 67-kD ER in BeWo cells. The BeWo cell ER binds to an ERE consensus sequence and the ER-ERE complex is supershifted by antibodies directed against the ER. We conclude that BeWo cells express a functional ER that is important for the control of BeWo cell proliferation, suggesting a potential role for estrogens in mediating placental trophoblast growth and deve
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