RESUMO
Nuclear receptors are important targets for pharmaceuticals, but similarities between family members cause difficulties in obtaining highly selective compounds. Synthetic ligands that are selective for thyroid hormone (TH) receptor beta (TRbeta) vs. TRalpha reduce cholesterol and fat without effects on heart rate; thus, it is important to understand TRbeta-selective binding. Binding of 3 selective ligands (GC-1, KB141, and GC-24) is characterized at the atomic level; preferential binding depends on a nonconserved residue (Asn-331beta) in the TRbeta ligand-binding cavity (LBC), and GC-24 gains extra selectivity from insertion of a bulky side group into an extension of the LBC that only opens up with this ligand. Here we report that the natural TH 3,5,3'-triodothyroacetic acid (Triac) exhibits a previously unrecognized mechanism of TRbeta selectivity. TR x-ray structures reveal better fit of ligand with the TRalpha LBC. The TRbeta LBC, however, expands relative to TRalpha in the presence of Triac (549 A(3) vs. 461 A(3)), and molecular dynamics simulations reveal that water occupies the extra space. Increased solvation compensates for weaker interactions of ligand with TRbeta and permits greater flexibility of the Triac carboxylate group in TRbeta than in TRalpha. We propose that this effect results in lower entropic restraint and decreases free energy of interactions between Triac and TRbeta, explaining subtype-selective binding. Similar effects could potentially be exploited in nuclear receptor drug design.
Assuntos
Entropia , Receptores alfa dos Hormônios Tireóideos/metabolismo , Receptores beta dos Hormônios Tireóideos/metabolismo , Ácido Acético/química , Ácido Acético/metabolismo , Sítios de Ligação , Humanos , Ligação de Hidrogênio , Ligantes , Simulação de Dinâmica Molecular , Maleabilidade , Eletricidade Estática , Termodinâmica , Tri-Iodotironina/química , Tri-Iodotironina/metabolismo , ÁguaRESUMO
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)<2h) of radiolabeled T(3) vs. T(3) (T(1/2) approximately 5-7h). We cannot discern relationships between this effect and ligand size, activity or affinity for TRbeta. One ligand, GC-24, binds the TR LBC and (weakly) to the TRbeta-LBD surface that mediates dimer/heterodimer interaction, but we cannot link this interaction to rapid T(3) dissociation. Instead, several lines of evidence suggest that the challenger ligand must interact with the buried LBC to promote rapid T(3) 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 T(3) release and that this effect enhances hormone dissociation.
Assuntos
Receptores dos Hormônios Tireóideos/metabolismo , Hormônios Tireóideos/metabolismo , Dimerização , Cinética , LigantesRESUMO
Thyroid hormone nuclear receptors (TRs) bind to DNA and activate transcription as heterodimers with the retinoid X receptor (RXR) or as homodimers or monomers. RXR also binds to DNA and activates transcription as homodimers but can, in addition, self-associate into homotetramers in the absence of ligand and DNA templates. It is thought that homotetramer formation serves to sequester excess RXRs into an inactive pool within the cell. Here, we report systematic studies of the multimeric state of a recombinant human TRbeta1 truncation (hTRbeta1deltaAB) that encompasses the complete DNA binding domain and ligand binding domain in solution. Native gel electrophoresis, chemical crosslinking, gel filtration, and dynamic light scattering experiments reveal that hTRbeta1deltaAB forms a mixture of monomers, dimers, and tetramers. Like RXR, increasing protein concentration shifts the equilibrium between TR multimers toward tetramer formation, whereas binding of cognate thyroid hormone leads to dissociation of tetramers and increased formation of dimers. This work represents the first evidence that apo-hTRbeta1 forms homotetramers. The findings raise the possibility that tetramer formation provides an additional, and previously unsuspected, level of control of TR activity and that the capacity for homotetramer formation may be more widespread in the nuclear receptor family than previously thought.