Differential effects of TR ligands on hormone dissociation rates: Evidence for multiple ligand entry/exit pathways

Some nuclear receptor (NR) ligands promote dissociation of radiolabeled bound hormone from the buried ligand binding cavity (LBC) more rapidly than excess unlabeled hormone itself. This result was interpreted to mean that challenger ligands bind allosteric sites on the LBD to induce hormone dissociation, and recent findings indicate that ligands bind weakly to multiple sites on the LBD surface. Here, we show that a large fraction of thyroid hormone receptor (TR) ligands promote rapid dissociation (T_1/2 < 2 h) of radiolabeled T₃ vs. T₃ (T_1/2 ≈ 5-7 h). We cannot discern relationships between this effect and ligand size, activity or affinity for TRβ. One ligand, GC-24, binds the TR LBC and (weakly) to the TRβ-LBD surface that mediates dimer/heterodimer interaction, but we cannot link this interaction to rapid T₃ dissociation. Instead, several lines of evidence suggest that the challenger ligand must interact with the buried LBC to promote rapid T₃ release. Since previous molecular dynamics simulations suggest that TR ligands leave the LBC by several routes, we propose that a subset of challenger ligands binds and stabilizes a partially unfolded intermediate state of TR that arises during T₃ release and that this effect enhances hormone dissociation.