The orphan nuclear receptor Ear-2 is a negative coregulator for thyroid hormone nuclear receptor function

Thyroid hormone (T3) nuclear receptors (TR) are ligand-dependent transcript ion factors which regulate growth, differentiation, and development. One em erging hypothesis suggests that TR mediate these diverse effects via a larg e network of coregulators. Recently, we found that TR-mediated transcriptio nal responses varied in six cell lines derived from different tissues. We t herefore used human TR subtype beta 1 (TR beta 1) as bait to search for cor egulators in human colon carcinoma RKO cells with a yeast two-hybrid system . RKO cells exhibited T3-dependent and -independent transcriptional activat ion. One of the three positive clones was identified as Ear-2, which is a d istant member of the chick ovalbumin upstream promoter-transcription factor s of the orphan nuclear receptor family. The physical interaction between E ar-2 and TR beta 1 was further confirmed by specific binding of Ear-2 to gl utathione S-transferase-TR beta 1, In addition, Ear-2 was found to associat e with TR beta 1 in cells. As a result of this physical interaction, bindin g of TR beta 1 to the T3 response elements was inhibited. Using reporter sy stems, we found that both the basal activation and the T3-dependent activat ion mediated by TP beta 1 were repressed by Ear-2 in CV1 cells. In RKO cell s, however, the T3-independent transcriptional activity was more sensitive to the repression effect of Ear-2 than the T3-dependent transcriptional act ivity. The repression effect of Ear-2 was reversed by steroid hormone recep tor coactivator I. These results suggest that TR-mediated responses reflect a balance of corepressors and coactivators in cells. These findings furthe r strengthen the hypothesis that the diverse activities of TR are achieved via a large network of coregulators that includes Ear-2.