INHIBITORY CROSS-TALK BETWEEN STEROID-HORMONE RECEPTORS - DIFFERENTIAL TARGETING OF ESTROGEN-RECEPTOR IN THE REPRESSION OF ITS TRANSCRIPTIONAL ACTIVITY BY AGONIST-OCCUPIED AND ANTAGONIST-OCCUPIED PROGESTIN RECEPTORS

Although estrogen receptor (ER) and progestin receptor (PR) are member s of different steroid hormone receptor subfamilies, there is consider able biological evidence for cross-talk between the estrogen and proge stin hormone-receptor signaling pathways. We have developed a model sy stem to analyze the mechanisms underlying this cross-talk, specificall y the repression of PR-mediated transcriptional activity by PR complex ed with agonistic or antagonistic ligands. Estrogen- and progestin-res ponsive reporter vectors containing a variety of promoters were transf ected into primary cultures of rat uterine cells and 3T3 mouse fibrobl asts with expression vectors for PR (the A and/or B isoforms) as well as ER Our results demonstrate that both PR isoforms can act as potent ligand-dependent repressors of ER activity. The magnitude of the repre ssion was dependent on the PR isoform (i.e., PR A or PR B), ligand typ e (i.e., agonist or antagonist), PR levels, and ligand concentration b ut was unaffected by the ER levels. The promoter context was important in determining both the magnitude and PR isoform specificity of the r epression for agonist-occupied PR but not for antagonist-occupied PR. Ligand-occupied PR A was a stronger repressor of PR-mediated transcrip tional activity than was ligand-occupied PR B, and antagonist-occupied PR was a more effective repressor than agonist-occupied PR. Mechanist ic studies suggest that liganded PR represses ER activity by interferi ng with its ability to interact productively with the transcriptional machinery, a process known as quenching. The data do not support compe titive repression, direct repression, or squelching as the mechanism o f PR's inhibitory effect. Experiments with ER mutants demonstrated tha t the N-terminal portion of ER was required for repression by agonist occupied PR but not by antagonist-occupied PR. These results, as well as other differences between the two PR-ligand complexes, suggest that they differentially target ER when repressing ER transcriptional acti vity. These findings underscore the mounting evidence for the importan ce of interactions between members of the steroid hormone receptor fam ily.