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Dynamic inhibition of nuclear receptor activation by corepressor binding
Young-Chang Sohn1, Seung-Whan Kim, Seunghee Lee
1Department of Life Science, Pohang University of Science and Technology, Pohang 790-784, Korea.
Abstract:
Nuclear receptors adopt dramatically different conformations in the presence or absence of ligand, and such liganded (holo) and unliganded (apo) receptors are specifically recognized by transcriptional coactivators and corepressors, respectively. These two states likely exist in dynamic equilibrium, contrary to the conventional model of static off and on conformations. First, corepressor SMRT [for silencing mediator of thyroid hormone receptor (TR) and retinoic acid receptor (RAR)] inhibits the interaction of coactivator steroid receptor coactivator-1 with liganded TR/RAR. Second, SMRT enables receptors to adopt apo-form even in the presence of ligand, as demonstrated with limited proteolyses and decreased binding of radiolabeled retinoid to RAR. Finally, chromatin immunoprecipitation results indicate that SMRT and steroid receptor coactivator-1 dynamically compete for receptor bindings in vivo in the presence of ligand. These results suggest that corepressor binding can drive receptors to adopt the apo-state, even in the presence of ligand, and inhibit activated liganded (holo) nuclear receptors in vivo.
Insights
Nuclear receptors dynamically switch between ligand-bound (holo) and unbound (apo) states. Corepressors like SMRT can force nuclear receptors into the apo-state, even with ligands present, inhibiting their activity.
Area of Science:
- Molecular Biology
- Endocrinology
- Gene Regulation
Background:
- Nuclear receptors (NRs) undergo conformational changes upon ligand binding, influencing interactions with transcriptional regulators.
- Traditionally, NRs are viewed as static in either ligand-bound (holo) or unbound (apo) states, but dynamic equilibrium is increasingly recognized.
Purpose of the Study:
- To investigate the dynamic interplay between nuclear receptors, corepressors, and coactivators.
- To determine if corepressors can induce an apo-like state in NRs even in the presence of ligands.
Main Methods:
- Limited proteolysis assays to assess receptor conformation.
- Radioligand binding assays to measure retinoid binding to RAR.
- Chromatin immunoprecipitation (ChIP) to evaluate in vivo protein-DNA interactions.
Main Results:
- The corepressor SMRT (silencing mediator of thyroid hormone receptor and retinoic acid receptor) inhibited coactivator SRC-1 interaction with liganded TR/RAR.
- SMRT promoted an apo-like conformation in receptors, even with ligand present, evidenced by altered protease sensitivity and reduced retinoid binding.
- ChIP assays demonstrated dynamic in vivo competition between SMRT and SRC-1 for receptor binding.
Conclusions:
- Corepressor binding can actively drive nuclear receptors into an apo-state, irrespective of ligand presence.
- This mechanism allows corepressors to inhibit the activity of liganded, holo nuclear receptors in vivo.