Effect of Antibodies to Estrogen Receptor on the Binding of3H-Labeled Antiestrogens and Androstanediol in the Uterus*

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Abstract
We report here the effects of antibodies to estrogen receptor (ER) on the binding of tritiated 5α-androstane-3β-17β-diol and antiestrogens to receptor in calf uterine cytosol. The antibodies, prepared from a rabbit immunized with purified estradiol-receptor complex from calf uterine nuclei, were found to decrease the affinity, but not the number of binding sites, of ER for 5α-androstanediol and tamoxifen [α-(4β-N-dimethylaminoethoxy) phenyl-α′-ethyl-trans-stilbene]. This appeared to be due to the decrease in the association rate of the ligands, while the dissociation rate was not modified. The ER antibodies did not affect the binding of ER to estradiol or to the monohydroxylated metabolite of tamoxifen, nor did they affect the interaction of androgens with the androgen receptor. The formation of a ternary complex among [3H]hydroxytamoxifen [3H-labeled α-(4β-N-dimethylaminoethoxy)phenyl-4-hydroxyα′-ethyl-trans-stilbene], the ER, and the antibodies was demonstrated by sucrose gradient ultracentrifugation. For [3H]tamoxifen and [3H]5α-androstanediol, ternary complexes were also formed with ER bound to monoclonal antibodies linked to Sepharose. These results confirm that ER directly binds androstanediol, tamoxifen, and hydroxytamoxifen. The lower affinity of ligands for ER in the presence of antibodies was only observed for the low affinity ligands and did not appear to be correlated to their antiestrogenic activities.