Retinoic Acid receptor-related orphan receptor alpha-enhanced thyroid hormone receptor-mediated transcription requires its ligand binding domain which is not, by itself, sufficient: possible direct interaction of two receptors.

BACKGROUND: Natural mutant staggerer (sg) mice harbor a mutated retinoic acid receptor-related orphan receptor alpha (RORalpha). A genetic deletion corresponding to the ligand-binding domain (LBD) of RORalpha results in aberrant cerebellar development in the sg mice. These mice show similar neurotrophin expression to that seen in perinatal hypothyroid animals. RORalpha augments thyroid hormone receptor (TR)-mediated transcription, which may be partly responsible for the similar cerebellar abnormalities between sg and hypothyroid animals. The objective of this study is to examine further the mechanisms of augmentation of TR action by RORalpha. We examined whether TR directly binds to ROR and which regions of TR or ROR are required for the TR-ROR interaction.

METHODS: A transient transfection-based reporter gene assay was performed to measure the activity of TR-mediated transcription in CV-1 cells. To examine TR-RORalpha binding mammalian two-hybrid and glutathione-S-transferase (GST) pull-down assays were carried out.

RESULTS: Although full-length RORalpha augmented TRalpha1- or beta1-mediated transcription, such augmentation was not observed with sg-type mutant RORalpha (RORsg) that contained the RORalpha N-terminal and DNA-binding domain (DBD) and a part of the LBD. On the other hand, the transcription of Gal4-DBD-fused TRbeta1-LBD was suppressed by RORalpha, indicating that RORalpha does not interact with TR-LBD. Full-length TRbeta1 bound to RORalpha or RORsg in GST pull-down assays; however, RORalpha-LBD did not bind to TRalpha1 or beta1.

CONCLUSION: The full-length forms of both RORalpha and TR are essential for the augmentation of TR-mediated transcription by RORalpha.