The in vivo role of nuclear receptor corepressors in thyroid hormone action.

BACKGROUND: The thyroid hormone receptor (TR) isoforms interact with a variety of coregulators depending upon the availability of T3 to mediate their transcriptional effect. Classically, in the absence of ligand, the TRs recruit the nuclear corepressors, NCoR and SMRT, to mediate transcriptional repression on positively regulated TR target genes. However, new insight into the roles of NCoR and SMRT using in vivo models have better defined the role of nuclear corepressors both in the absence and presence of T3. SCOPE OF REVIEW: This review will place the variety of in vivo nuclear corepressor mouse models developed to date in context of thyroid hormone action. Based on these models, we will also discuss how corepressor availability together with the levels of endogenous nuclear receptor ligands including T3 controls multiple signaling pathways. MAJOR
CONCLUSIONS: Nuclear corepressors mediate repression of positive TR targets in the absence of T3in vivo. Even more importantly they attenuate activation of these targets at the normal physiological levels of ligands by TR and other nuclear receptors. While the role of corepressors in the regulation of negative TR targets and HPT axis remains poorly understood, lack of corepressor recruitment to TR in the animals leads to a compensatory change in the set point of HPT axis that allows to balance the increased sensitivity to T3 action in other tissues. GENERAL SIGNIFICANCE: Available data indicate that targeting specific interactions between corepressors and TR or other nuclear receptors presents a new therapeutic strategy for endocrine and metabolic disorders. This article is part of a Special Issue entitled Thyroid hormone signalling.