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 (ERα and ERβ), are critically involved in the development of the mammary gland and breast cancer. An isoform of ERβ, ERβ2 (also called ERβcx), with an altered COOH-terminal region, is coexpressed with ERα in many human breast cancers. In this study, we generated a stable cell line from MCF7 breast cancer cells expressing an inducible version of ERβ2, along with endogenous ERα, and examined the effects of ERβ2 on the ERα protein levels and function. We showed that ERβ2 inhibited ERα-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 ERβ2 expression caused a significant reduction in the recruitment of ERα to both the pS2 and MMP-1 promoters. Furthermore, ERβ2 expression induced proteasome-dependent degradation of ERα. The inhibitory effects of ERβ2 on ERα activity were further confirmed in HEK293 cells that lack functional endogenous ERs. We also showed that ERβ2 can interact with ERα both in vitro and in mammalian cells, which is compatible with a model where ERβ2/ERα heterodimers are targeted to the proteasome. Finally, in human breast cancer samples, we observed that expression of ERβ2 significantly correlated with ERα-negative phenotype. Our data suggest that ERβ2 could influence ERα-mediated effects relevant for breast cancer development, including hormone responsiveness.