Binding of antiestrogens exposes an occult antigenic determinant in the human estrogen receptor

Treatment of human breast cancer cytosol with tamoxifen (Tam) or 4-monohydroxytamoxifen (MHT) enhances the immunoreactivity of the estrogen receptor toward monoclonal antibody H222 but not monoclonal antibodies D547 or D75. This effect is evident from an increase in the apparent receptor content measured by the Abbott enzyme immunoassay, which uses peroxidase-labeled H222 as the chromogenic marker, and in the rate and size of the sedimentation peak of the immune complex of the receptor with radiolabeled H222. In contrast, MHT shows no effect in reversed immunoassay systems that use peroxidase-labeled D547 or D75 as chromogenic markers, nor does it affect the sedimentation peak of the complex of D547 with the receptor. MHT can exert its action on receptor bound to immobilized antibody. These results indicate that reaction with antiestrogens causes a change, probably conformational, in the receptor protein that exposes an occult antigenic determinant recognized uniquely by H222. Since this can occur in cytosol previously treated with excess estradiol in the cold, it appears to result from an interaction of antiestrogens with a region of the receptor distinct from the estrogen-binding site, suggesting that agonist and antagonist actions may involve different parts of the receptor molecule.