Estrogen receptor binding to estrogen response elements slows ligand dissociation and synergistically activates reporter gene expression

Abstract

Estradiol (E 2)-liganded estrogen receptor (ER) bound to three or four tandem copies of a consensus ERE (EREc38) in a cooperative manner. E 2-ER binding to one or two EREs was non-cooperative. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), ER-ERE binding was not cooperative, regardless of the number of EREs. Here we evaluated how binding to EREc38 affects ER conformation in the ligand binding domain (LBD) as reflected in the dissociation kinetics of [ 3H]ligand from the ER. Binding of ER to EREc38 slowed the rate of dissociation of either E 2 or 4-OHT, indicating that DNA allosterically modulates the LBD conformation creating a tighter fit between the ligand and the ER. Conformational differences in ER induced by E 2 versus antiestrogen were not reflected in differences in E 2 or 4-OHT dissociation parameters under these conditions. No difference in the association rate of E 2- versus 4-OHT-liganded ER binding to EREc38 was detected in electrophoretic mobility shift assay (EMSA). Synergistic, E 2-dependent activation of a reporter gene was detected from three and four, but not one or two, tandem copies of EREc38. These observations suggest that cooperative binding of E 2-ER to multiple copies of EREc38 is likely responsible for transcriptional synergy and that cooperativity may not involve direct interaction between the LBDs of ERE-bound ER. Since the number of copies of EREc38 did not alter E 2 dissociation kinetics, functional synergy must involve cellular factors in addition to the ER ligand.