Elucidating the molecular mechanisms of nuclear receptor-mediated transcriptional repression

The estrogen, progesterone, and thyroid hormone receptors (ER, PR, and TR) are members of the nuclear receptor superfamily of ligand-activated transcription factors. The principal roles of TR are in metabolism and development while the principal roles of ER and PR are in maintaining female reproductive function. In a simplistic view, the activity of these receptors is enhanced by coactivator proteins like ACTR and repressed by corepressor proteins like N-CoR. In reality, however, the regulation of nuclear receptor activity is much more complex and as such, not well understood. Elucidating the mechanisms involved regulating nuclear receptor function is essential for determining how the receptors can be manipulated for therapeutic intervention in cancer and other disease states.; Towards this end, we have studied nuclear receptor-mediated transcriptional repression through a series of molecular biology techniques, such as phage display screening, in vitro binding assays, immunoprecipitations, and cell culture studies. In evaluating the role of the corepressor N-CoR in the actions of unliganded TR, we have discovered and confirmed a novel interaction between the corepressor N-CoR and the coactivator ACTR. By binding to both unliganded TR and ACTR, N-CoR is important in facilitating the transition between TR repression and activation.; To further characterize the interplay between transcriptional repression and activation, we have examined PR-mediated repression of the estrogen receptor, ER. We discovered that the A isoform of PR interferes with the formation of stable transcription complexes on the promoters of ER target genes and thus represses ER activity. However, the role of PR A in transcriptional regulation is often confounded by the presence of the nearly identical B isoform of PR, so studying the functions of each isoform has been difficult. To circumvent this problem, we have developed peptide antagonists of PR that discriminate between the intertwined roles of PR A and B in transcriptional activation and repression.; Cumulatively, this study has shown us that repressive protein complexes can communicate with and in some cases, disrupt the assembly of activating complexes in order to control nuclear receptor activity. The evolution of such complex regulatory mechanisms allows nuclear receptor activity to be tightly controlled and as we learn more about these mechanisms, will facilitate therapeutic intervention of disease states.