Estrogen receptor beta exerts growth-inhibitory effects on human mammary epithelial cells

Estrogen receptor beta (ER beta) is widely expressed in mammary epithelium. ER beta expression is reported to decline during carcinogenesis of the breast and other tissues. In this study, we examined the consequences of a loss of ER beta expression in mammary epithelial cells. We knocked down ER beta transcript levels in human mammary epithelial MCF-10A cells and in MCF-7 breast cancer cells by means of stable transfection with a specific shRNA plasmid. ER beta knockdown resulted in a significant growth increase of both cell types in a ligand-independent manner. This effect was accompanied by elevated cyclin A2 expression in MCF-10A cells and by decreased expression of growth-inhibitory p21/WAF and epithelial cell marker cytokeratine 8 in both cell lines. Transfection of ER beta shRNA did not alter the absent proliferative estrogen response of MCF-10A cells, but conferred sensitivity to selective estrogen receptor modulator tamoxifen to this cell line. In contrast, ER beta knockdown diminished estrogen responsiveness of MCF-7 breast cancer cells and also weakened the effect of tamoxifen on this cell line. These ligand-dependent effects only observed in MCF-7 cells exhibiting a high ER alpha/beta ratio were accompanied by smaller estrogenic repression of p21/WAF expression, an impaired tamoxifen-triggered induction of this gene and by relative downregulation of ER alpha and cyclin A2 transcript levels. Our data suggest that ER beta exerts antiproliferative effects both on MCF-10A and MCF-7 cells in a ligand- and ER alpha-independent manner by regulation of p21/WAF or cyclin A2 gene expression. Knockdown of ER beta in both cell types was sufficient to significantly decrease transcript levels of epithelial cell marker cytokeratin 8. The results of this study support the hypothesis that ER beta acts as a tumor suppressor in mammary epithelium.