DIFFERENT REGIONS IN ACTIVATION FUNCTION-1 OF THE HUMAN ESTROGEN-RECEPTOR REQUIRED FOR ANTIESTROGEN-DEPENDENT AND ESTRADIOL-DEPENDENT TRANSCRIPTION ACTIVATION

The human estrogen receptor (ER) is a ligand-inducible transcription f actor that contains two transcriptional activation functions, one loca ted in the NH2-terminal region of the protein (AF-1) and the second in the COOH-terminal region (AF-2). Antiestrogens, such as trans-hydroxy tamoxifen (TOT), have partial agonistic activity in certain cell types , and studies have implied that this agonism is AF-1-dependent. We hav e made progressive NH2-terminal and other segment deletions and ligati ons in the A/B domain, and studied the transcriptional activity of the se mutant ERs in ER-negative MDA-MB-231 human breast cancer and HEC-1 human endometrial cancer cells. Using several estrogens and several pa rtial agonist/antagonist antiestrogens, we find that estrogens and ant iestrogens require different regions of AF-1 for transcriptional activ ation. Deletion of the first 40 amino acids has no effect on receptor activity. Antiestrogen agonism is lost upon deletion to amino acid 87, while estrogen agonism is not lost until deletions progress to amino acid 109. Antiestrogen agonism has been further defined to require ami no acids 41-64, as deletion of only these amino acids results in an ER that exhibits 100% activity with E(2), but no longer shows an agonist response to TOT. With A/B-modified receptors in which antiestrogens l ose their agonistic activity, the antiestrogens then function as pure estrogen antagonists. Our studies show that in these cellular contexts , hormone-dependent transcription utilizes a range of the amino acid s equence within the A/B domain. Furthermore, the agonist/antagonist bal ance and activity of antiestrogens such as TOT are determined by speci fic sequences within the A/B domain and thus may be influenced by diff erences in levels of specific factors that interact with these regions of the ER.