Background: Examining the TR␤1 cistrome in mouse liver is critical to understanding thyroid hormone signaling. Results: Novel mechanisms of positive versus negative regulation by biotinylated TR␤1 were identified. Conclusion: TR␤1 regulates transcription by changes in relative binding and use of preferred binding motifs. Significance: This study demonstrates that a mechanism other than differential co-regulator recruitment is involved in transcriptional regulation by TR␤1

c NCoR1 (nuclear receptor corepressor) and SMRT (silencing mediator of retinoid and thyroid hormone receptors; NCoR2) are well-recognized coregulators of nuclear receptor (NR) action. However, their unique roles in the regulation of thyroid hormone (TH) signaling in specific cell types have not been determined. To accomplish this we generated mice that lacked function of either NCoR1, SMRT, or both in the liver only and additionally a global SMRT knockout model. Despite both corepressors being present in the liver, deletion of SMRT in either euthyroid or hypothyroid animals had little effect on TH signaling. In contrast, disruption of NCoR1 action confirmed that NCoR1 is the principal mediator of TH sensitivity in vivo. Similarly, global disruption of SMRT, unlike the global disruption of NCoR1, did not affect TH levels. While SMRT played little role in TH-regulated pathways, when disrupted in combination with NCoR1, it greatly accentuated the synthesis and storage of hepatic lipid. Taken together, these data demonstrate that corepressor specificity exists in vivo and that NCoR1 is the principal regulator of TH action. However, both corepressors collaborate to control hepatic lipid content, which likely reflects their cooperative activity in regulating the action of multiple NRs including the TH receptor (TR).T hyroid hormone (TH) is one of the most important metabolic regulators in humans, and its actions in the liver include the regulation of cholesterol and lipid metabolism. Thyroxine (T4) is the major circulating form of thyroid hormone, and it is converted to its active form, triiodothyronine (T3), by a family of deiodinases (1). T3 regulates metabolic processes via thyroid hormone receptor (TR) isoforms that are expressed in all peripheral tissues, including liver. The TRs mediate target gene regulation by recruiting a constellation of coregulators that include corepressors (CoRs) and coactivators depending upon the presence of T3 (2). While classical models suggest that the corepressors are mainly recruited by the unliganded TR, the mechanisms by which T3 coordinates specific signaling in the liver both positively and negatively are still unknown (3). Indeed, recent work by our laboratory and others has demonstrated that the corepressors appear to play a critical role in mediating ligand sensitivity regardless of the concentration of ligand (4-7).The two principal corepressors that are suggested to be involved in mediating TH action are nuclear receptor corepressor 1 (NCoR1) and silencing mediator of retinoid and thyroid hormone receptors (SMRT) (4). They are highly homologous modular proteins and have three similar nuclear receptor (NR) interaction domains (NRIDs) at their C termini (8). Importantly, whole-body gene knockouts (KOs) of NCoR1 or SMRT result in embryonic lethality, but mutation or deletion of only the NRIDs allows for full development (6, 9, 10). In NCoR⌬ID mice, which express an altered NCoR1 allele that lacks the N3 and N2 NRIDs, there is evidence for increased thyroid hormone sensitivity...

TSH is the most important biomarker in the interpretation of thyroid function in man. Its levels are determined by circulating thyroid hormone (TH) levels that feed back centrally to regulate the expression of the subunits that comprise TSH from the pituitary. The nuclear corepressor 1 (NCoR1), is a critical coregulator of the TH receptor (TR) isoforms. It has been established to play a major role in the control of TSH secretion, because mice that express a mutant NCoR1 allele (NCoRΔID) that cannot interact with the TR have normal TSH levels despite low circulating TH levels. To determine how NCoR1 controls TSH secretion, we first developed a mouse model that allowed for induction of NCoRΔID expression postnatally to rule out a developmental effect of NCoR1. Expression of NCoRΔID postnatally led to a drop in TH levels without a compensatory rise in TSH production, indicating that NCoR1 acutely controls both TH production and feedback regulation of TSH. To demonstrate that this was a cell autonomous function of NCoR1, we expressed NCoRΔID in the pituitary using a Cre driven by the glycoprotein α-subunit promoter (P-ΔID mice). Importantly, P-ΔID mice have low TH levels with decreased TSH production. Additionally, the rise in TSH during hypothyroidism is blunted in P-ΔID mice. Thus, NCoR1 plays a critical role in TH-mediated regulation of TSH in the pituitary by regulating the repressive function of the TR. Furthermore, these studies suggest that endogenous NCoR1 levels in the pituitary could establish the set point of TSH secretion.