SPECIFIC MUTATIONS IN THE ESTROGEN-RECEPTOR CHANGE THE PROPERTIES OF ANTIESTROGENS TO FULL AGONISTS

The estrogen receptor (ER) stimulates transcription of target genes by means of its two transcriptional activation domains, AF-1 in the N-te rminal part of the receptor and AF-2 in its ligand-binding domain. AF- 2 activity is dependent upon a putative amphipathic alpha-helix betwee n residues 538 and 552 in the mouse ER Point mutagenesis of conserved hydrophobic residues within this region reduces estrogen-dependent tra nscriptional activation without affecting hormone and DNA binding sign ificantly. Here we show that these mutations dramatically alter the ph armacology of estrogen antagonists. Both tamoxifen and ICI 164,384 beh ave as strong agonists in HeLa cells expressing the ER mutants. In con trast to the wild-type ER, the mutant receptors maintain nuclear local ization and DNA-binding activity after ICI 164,384 treatment. Structur al alterations in AF-2 caused by gene mutations such as those describe d herein or by estrogen-independent signaling pathways may account for the insensitivity of some breast cancers to tamoxifen treatment.