A bimodal switch model is widely used to describe transcriptional regulation by the thyroid hormone receptor (TR). In this model, the unliganded TR forms stable, chromatin-bound complexes with transcriptional co-repressors to repress transcription. Binding of hormone dissociates co-repressors and facilitates recruitment of co-activators to activate transcription. Here we show that in addition to hormone-independent TR occupancy, ChIP-seq against endogenous TR in mouse liver tissue demonstrates considerable hormone-induced TR recruitment to chromatin associated with chromatin remodelling and activated gene transcription. Genome-wide footprinting analysis using DNase-seq provides little evidence for TR footprints both in the absence and presence of hormone, suggesting that unliganded TR engagement with repressive complexes on chromatin is, similar to activating receptor complexes, a highly dynamic process. This dynamic and ligand-dependent interaction with chromatin is likely shared by all steroid hormone receptors regardless of their capacity to repress transcription in the absence of ligand.

The thyroid hormone 3,3,5-triiodo-L-thyronine (T3) is essential for growth, differentiation, and development. Its biological activities are mediated by T3 nuclear receptors (TRs). At present, how T3 regulates TR proteins and the resulting functional consequences are still unknown. Immunofluorescence analyses of endogenous TR in the growth hormone-producing GC cells showed that the T3-induced rapid degradation of TR was specifically blocked by lactacystin, a selective inhibitor of the ubiquitin-proteasome degradation pathway. Immunoblots demonstrated that the transfected TR␤1 was ubiquitinated and that the ubiquitination was T3 independent. Studies with a series of truncated TR␤1 showed that the hormone-binding domain was sufficient for the T3-induced rapid degradation of TR␤1 by the proteasome degradation pathway. T3 also induced rapid degradation of TR␤2 and TR␣1. In contrast, the stability of the non-T3-binding TR␣2 and naturally occurring TR␤1 mutants that do not bind T3 was not affected by T3 treatment, indicating that hormone binding to receptor was essential for the degradation of the wild-type receptors. In the presence of proteasome protease inhibitors, the levels of both total and ubiquitinated TR␤1 protein increased, yet T3-dependent transcriptional activation and the expression of the growth hormone gene were diminished, suggesting that proteasome-mediated degradation played a novel role in modulating transcriptional activation by TR. The present study reveals a role of T3 in modulating the functions of TR by regulating its receptor level via the ubiquitin-proteasome degradation pathway. T he thyroid hormone 3,3Ј,5-triiodo-L-thyronine (T3) is essential in metabolic-energetic homeostasis, development, and differentiation. Its actions are mediated by thyroid hormone nuclear receptors (TRs), which regulate the expression of T3-targeted genes. TRs belong to a superfamily of hormone nuclear receptors functioning as ligand-activated transcription factors, which include receptors for steroid hormones, vitamin D3, and the retinoids (1). TR consists of domains including the Nterminal A͞B domain, the DNA-binding domain C, and the hormone-binding domain (domains D and E). Recent studies indicate that the transcriptional activity of TR depends not only on the type of the thyroid hormone response elements located on the promoter regions of T3 target genes but also on a host of corepressors and coactivators (2). In the absence of T3, TR binds to corepressors, such as N-CoR. Binding of T3 leads to the release of N-CoR from TR and recruitment of coactivators leading to gene activation (2). In this model, how TR proteins are regulated and the role of T3 in this process are unknown.Using biochemical methods, Samuels and Casanova have previously reported that T3 down-regulates its endogenous TR in growth hormone (GH)-producing GC cells (3). However, the underlying molecular mechanisms by which T3 downregulates the TR have not been elucidated. In this report, we demonstrated that the T3-induced degradation of TR was via ...