Inhibitory Cross-Talk between Steroid Hormone Receptors: Differential Targeting of Estrogen Receptor in the Repression of Its Transcriptional Activity by Agonist- and Antagonist-Occupied Progestin Receptors

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Abstract
Although estrogen receptor (ER) and progestin receptor (PR) are members of different steroid hormone receptor subfamilies, there is considerable biological evidence for cross-talk between the estrogen and progestin hormone-receptor signaling pathways. We have developed a model system to analyze the mechanisms underlying this cross-talk, specifically the repression of ER-mediated transcriptional activity by PR complexed with agonistic or antagonistic ligands. Estrogen- and progestin-responsive reporter vectors containing a variety of promoters were transfected into primary cultures of rat uterine cells and 3T3 mouse fibroblasts 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 repression was dependent on the PR isoform (i.e., PR A or PR B), ligand type (i.e., agonist or antagonist), PR levels, and ligand concentration but was unaffected by the ER levels. The promoter context was important in determining both the magnitude and PR isoform specificity of the repression for agonist-occupied PR but not for antagonist-occupied PR. Ligand-occupied PR A was a stronger repressor of ER-mediated transcriptional activity than was ligand-occupied PR B, and antagonist-occupied PR was a more effective repressor than agonist-occupied PR. Mechanistic studies suggest that liganded PR represses ER activity by interfering with its ability to interact productively with the transcriptional machinery, a process known as quenching. The data do not support competitive repression, direct repression, or squelching as the mechanism of PR's inhibitory effect. Experiments with ER mutants demonstrated that 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 activity. These findings underscore the mounting evidence for the importance of interactions between members of the steroid hormone receptor family.