Analysis of in vivo conformation: full‐length estrogen receptor‐alpha dimers bound by estradiol, tamoxifen, raloxifene or ICI 182780

Abstract

Tamoxifen, raloxifene and ICI 182780 are clinically important selective estrogen receptor modulators (SERMs) that block estrogen activation of the estrogen receptor (ER) in some tissues, while activating ER in other tissues. The mechanisms by which SERM binding to the ligand binding domain (LBD) activates ER in some tissues are poorly understood. One hypothesis is that some SERMs and estrogens release ER transcriptional activation function-1 through a ligand-regulated shift in the position of the ER amino terminal domains (NTD). Here, we measured that ligand-specific conformational shift in living human breast cancer cells as the amount of energy transfer between fluorophores attached to the LBD and NTD of interacting ERα’s. We determined that the relative positions of the LBD and the NTD were indistinguishable in the tamoxifen and estradiol-bound ERα dimers, but different in the raloxifene and ICI 182780-bound ERα dimers. All four ligands promoted the formation of their characteristic ERα dimer conformations with equal, rapid temporal kinetics. Thus, each SERM rapidly promotes a unique conformation of the ERα dimer which may contribute to their unique, tissue-selective activity profiles. The ability to measure such changes in conformational and interaction in different cell types or under different conditions is crucial to understanding cell-specific activities and for the improvement of pharmacologic intervention.