Comparison of transcriptional synergy of estrogen receptors alpha and beta from multiple tandem estrogen response elements.

Estrogen receptors alpha and beta (ERalpha and ERbeta) act as ligand-dependent transcriptional enhancers. We reported that ERalpha induces synergistic activation of luciferase reporter gene activity in response to E(2) from three or four tandem copies of a consensus estrogen response element (ERE) in transiently transfected MCF-7 cells. Here we addressed three questions: (1) is the synergistic activation of reporter gene activity from multiple tandem EREs by ERalpha restricted to MCF-7 cells?; (2) does ERbeta induce synergistic activation of reporter activity from multiple tandem EREs?; and (3) does ERbeta bind cooperatively to multiple tandem EREs? To address the first two questions, ER-negative CHO-K1 cells were co-transfected with ERalpha or ERbeta and ERE-driven reporter plasmids. Both ERalpha and ERbeta activated ERE-driven luciferase gene activity in an estradiol-dependent manner. Induction by ERbeta was lower than ERalpha from each ERE. We demonstrate that both ERalpha and ERbeta induce transcriptional synergy with three or four, but not two, tandem copies of an ERE. Electrophoretic mobility shift assays (EMSA) indicated an increase in ER-ERE binding affinity associated with cooperative binding of ERalpha and ERbeta to multiple EREs that may be responsible for transcriptional synergy in transiently transfected cells. We also postulate that interaction of ERalpha and ERbeta with coactivators may also play a role in transcriptional synergy.