Equilibrium binding analysis of estrogen agonists and antagonists: relation to the activation of the estrogen receptor.

The equilibrium binding kinetics of the interaction between the estrogen receptor and natural estrogens (estradiol, estriol and estrone), non-steroidal estrogen agonists (11 beta-chloromethyl-estradiol-17 beta, diethyl-stilbestrol, hexestrol) and non-steroidal antiestrogens (clomiphene, tamoxifen) have been characterized. It is proposed that positive cooperative binding of ligands by the estrogen receptor reflects conformational changes in the DNA binding domain of the receptor dimer which increase its affinity to estrogen responsive elements. Weak estrogens fail to induce maximal cooperativity and are less efficient in activating the receptor complex. Antiestrogens, that inhibit the [3H]estradiol-induced cooperative binding, suppress the activation of the receptor and inhibit its nuclear interactions. Another class of antiestrogens (e.g., 4-hydroxytamoxifen) interacts with the receptor in a manner that is indistinguishable from the cooperative interaction of estradiol, and the resulting complex may also exhibit increased affinity for estrogen responsive elements. However, these complexes cannot activate transcription, presumably due to an aberrant induction of transcription-activating domain in the receptor. We suggest that the positive cooperativity of the estrogen receptor results from conformational changes in the receptor that are transmitted also to the DNA binding domain. On the other hand, conformational changes in the transcription activating domain are not revealed by equilibrium binding kinetics. Thus, compounds that block the positive cooperative binding of [3H]estradiol by the receptor act as antiestrogens. Other compounds that interact cooperatively with the receptor can activate the receptor DNA binding domain, however, they may or may not induce the full array of conformational changes required for transactivation of transcription.