Estriol and estradiol interactions with the estrogen receptor in vivo and in vitro

The cytosolic estrogen receptor (calf uterus) bound to estradiol (E2) at 0 degrees C changes from a state with fast into a state with slow E2 dissociation rates when placed at 28 degrees C. This temperature accelerated transition in receptor affinity for its ligand takes place within 10 min at 28 degrees C. Similarly, receptor bound to estriol (E3) at 0 degrees C changes, when heated, from a state with fast into a state with slow E3 dissociation. The main difference between RE2 and RE3 was that E3 dissociates from unheated 8S RE3 and heat-transformed 5S RE3 at a much faster rate than E2 from RE2 . In the mature ovariectomized rat a slow dissociating 5S receptor estrogen complex is found in nuclei 1 h after injection of [3H]E2 or [3H]E3. In vitro dissociation of these 2 estrogens from this nuclear bound receptor formed in vivo takes place at rates similar to those from heat-transformed cytosolic RE2 or RE3 complexes. Addition of pyridoxal 5'-phosphate (PLP) to the slow-dissociating heat-transformed 5S estrogen receptor complexes causes rapid dissociation of E2 or E3; this effect is dose-dependent and is not due to disruption of 5S dimers, since after PLP addition RE2 and RE3 sediment unchanged as 5S dimers. The presence of a large excess of non-radioactive 4S RE3 does not interfere with the temperature induced rapid transition of 4S R[3H]E2 complexes from the state with fast into a state with slow E2 dissociation kinetics. A model is presented to explain the temperature induced biphasic estrogen dissociation from the receptor. It is proposed that the low affinity 4S RE2 monomer undergoes a temperature and estrogen dependent conformation change, such that the ligand is "locked" into the receptor's binding site. This conformational change results in the formation of a high affinity 4S monomer from which estrogen dissociates at a slower rate. This reaction is independent from subsequent 4S to 5S dimerization (transformation). The different rates of ligand dissociation from the low and high affinity 4S receptors reflect the different interactions (hydrophobic and hydrogen bonding) of E2 and E3 with the estrogen binding domain.