The
estrogen receptor (ER) is a
hormone-regulated
transcription factor that binds, as a dimer, to
estrogens and to specific DNA sequences. To explore at a fundamental level the geometric and topological features of bivalent-
ligand binding to the ER dimer, dimeric ER crystal structures were used to rationally design nonsteroidal bivalent
estrogen ligands. Guided by this structure-based
ligand design, we prepared two series of bivalent
ligands (agonists and antagonists) tethered by flexible spacers of varying lengths (7-47 Å) and evaluated their ER-binding affinities for the two ER subtypes and their biological activities in cell lines. Bivalent
ligands based on the agonist
diethylstilbestrol (DES) proved to be poor candidates, but bivalent
ligands based on the antagonist
hydroxytamoxifen (OHT) were well suited for intensive study. Binding affinities of the OHT-based bivalent
ligands were related to spacer length in a distinctive fashion, reaching two maximum values at 14 and 29 Å in both ER subtypes. These results demonstrate that the bivalent concept can operate in determining ER-
ligand binding affinity and suggest that two distinct modes operate for the binding of bivalent
estrogen ligands to the ER dimers, an intermolecular as well as an intramolecular mode. Our insights, particularly the possibility of intramolecular bivalent binding on a single ER monomer, may provide an alternative strategy for preparing more selective and active ER antagonists for endocrine
therapy of
breast cancer.