Ligand-dependent heterodimerization of thyroid hormone receptor and retinoid X receptor

Abstract

The thyroid hormone receptors (TR) bind to cis-acting DNA elements as heterodimers with the retinoid X receptors (RXR). Those heterodimers display distinct specificities in mediating the hormonal response to target gene transcription. We characterized the interaction between TRα1 and RXRα via their ligand binding domains (LBDs) and the effect of ligands on the interaction using a yeast two-hybrid system. The DNA binding domain (BD) of yeast Gal4 fusion to the LBD of TRα1 had no transcriptional activity on its own, but when it was coexpressed with the activation domain (AD) of yeast Gal4 fusion to LBD of RXRα conferred activation to a reporter gene harboring a Gal4 binding site, indicating that LBDs of TRα1 and RXRα interact with each other in solution. Furthermore, T3 and 9-cis-RA increased the reporter activity, and an additive effect was observed when both ligands were added, indicating that the TRα1·RXRα heterodimerization is augmented by their respective ligands in vivo. Using an in vitro pull-down experiment, we confirmed the ligand-dependent interaction observed in the yeast system. Matrix-bound glutathione S-transferase-RXRα specifically coprecipitated the 35S-labeled TRα1 above the control, and associated 35S-labeled TRα1 was increased by the addition of T3 and 9-cis-RA. These results imply a complex, sensitive crosstalk in vivo among nuclear receptors and their respective ligands through distinct hormonal signaling pathways.