ANALYSIS OF ESTROGEN-RECEPTOR TRANSCRIPTIONAL ENHANCEMENT BY A NUCLEAR HORMONE-RECEPTOR COACTIVATOR

The estrogen receptor (ER), a member of a large superfamily of nuclear hormone receptors, is a ligand-inducible transcription factor that re gulates the expression of estrogen-responsive genes, The ER, in common with other members of this superfamily, contains two transcription ac tivation functions (AFs)-one located in the amino-terminal region (AF- 1) and the second located in the carboxyl-terminal region (AF-2), In m ost cell contexts, the synergistic activity of AF-1 and AF-2 is requir ed for full estradiol (E(2))-stimulated activity. We have previously s hown that a ligand-dependent interaction between the two AF-containing regions of ER was promoted by E(2) and the antiestrogen trans-hydroxy tamoxifen (TOT). This interaction, however, was transcriptionally prod uctive only in the presence of E(2). To explore a possible role of ste roid receptor coactivators in transcriptional synergism between AF-1 a nd AF-2, we expressed the amino terminal (AF-1-containing) and carboxy l-terminal (AF-2-containing) regions of ER as separate polypeptides in mammalian cells, along with the steroid receptor coactivator-1 protei n (SRC-1). We demonstrate that SRC-1, which has been shown to signific antly increase ER transcriptional activity, enhanced the interaction, mediated by either E(2) or TOT, between the AF-1-containing and AF-2-c ontaining regions of the ER. However, this enhanced interaction result ed in increased transcriptional effectiveness only with E(2) and not w ith TOT, consistent with the effects of SRC-1 on the full-length recep tor. Our results suggest that after ligand binding, SRC-1 may act, in part, as an adapter protein that promotes the integration of amino- an d carboxyl-terminal receptor functions, allowing for full receptor act ivation, Potentially, SRC-1 may be capable of enhancing the transcript ional activity of related nuclear receptor superfamily members by faci litating the productive association of the two AF-containing regions i n these receptors.