Estrogen receptor β2 negatively regulates the transactivation of estrogen receptor α in human breast cancer cells

Estrogens, by binding to and activating two estrogen receptors (ER alpha and ER beta), are critically involved in the development of the mammary gland and breast cancer. An isoform of ER beta, ER beta 2 (also called ER beta cx), with an altered COOH-terminal region, is coexpressed with ER alpha. in many human breast cancers. In this study, we generated a stable cell line from MCF7 breast cancer cells expressing an inducible version of ER beta 2, along with endogenous ER alpha, and examined the effects of ER beta 2 on the ER alpha protein levels and function. We showed that ER beta 2 inhibited ER alpha-mediated transactivation via estrogen response element and activator protein-1 sites of reporter constructs as well as the endogenous genes pS2 and MMP-1. Chromatin immunoprecipitation assays revealed that ER beta 2 expression caused a significant reduction in the recruitment of ER alpha to both the pS2 and MMP-1 promoters. Furthermore, ER beta 2 expression induced proteasome-dependent degradation of ER alpha. The inhibitory effects of ER beta 2 on ER alpha activity were further confirmed in HEK293 cells that lack functional endogenous ER alpha. We also showed that ER beta 2 can interact with ER alpha both in vitro and in mammalian cells' which is compatible with a model where ER beta 2/ER alpha heterodimers are targeted to the proteasome. Finally, in human breast cancer samples, we observed that expression of ER beta 2 significantly correlated with ER alpha-negative phenotype. Our data suggest that ER beta 2 could influence ER alpha-mediated effects relevant for breast cancer development, including hormone responsiveness.