Identification of Estrogen Receptor Dimer Selective Ligands Reveals Growth-Inhibitory Effects on Cells That Co-Express ERα and ERβ

Abstract

Estrogens play essential roles in the progression of mammary and prostatic diseases. The transcriptional effects of estrogens are transduced by two estrogen receptors, ERa and ERb, which elicit opposing roles in regulating proliferation: ERa is proliferative while ERb is anti-proliferative. Exogenous expression of ERb in ERa-positive cancer cell lines inhibits cell proliferation in response to estrogen and reduces xenografted tumor growth in vivo, suggesting that ERb might oppose ERa's proliferative effects via formation of ERa/b heterodimers. Despite biochemical and cellular evidence of ERa/b heterodimer formation in cells co-expressing both receptors, the biological roles of the ERa/b heterodimer remain to be elucidated. Here we report the identification of two phytoestrogens that selectively activate ERa/b heterodimers at specific concentrations using a cell-based, two-step high throughput small molecule screen for ER transcriptional activity and ER dimer selectivity. Using ERa/b heterodimer-selective ligands at defined concentrations, we demonstrate that ERa/b heterodimers are growth inhibitory in breast and prostate cells which co-express the two ER isoforms. Furthermore, using Automated Quantitative Analysis (AQUA) to examine nuclear expression of ERa and ERb in human breast tissue microarrays, we demonstrate that ERa and ERb are co-expressed in the same cells in breast tumors. The co-expression of ERa and ERb in the same cells supports the possibility of ERa/b heterodimer formation at physio-and pathological conditions, further suggesting that targeting ERa/b heterodimers might be a novel therapeutic approach to the treatment of cancers which coexpress ERa and ERb.