The interaction of TRbeta1-N terminus with steroid receptor coactivator-1 (SRC-1) serves a full transcriptional activation function of SRC-1

Role of co-regulators in metabolic and transcriptional actions of thyroid hormone

Thyroid hormone (TH) controls a wide range of physiological processes through TH receptor (TR) isoforms. Classically, TRs are proposed to function as tri-iodothyronine (T3)-dependent transcription factors: on positively regulated target genes, unliganded TRs mediate transcriptional repression through recruitment of co-repressor complexes, while T3 binding leads to dismissal of co-repressors and recruitment of co-activators to activate transcription. Co-repressors and co-activators were proposed to play opposite roles in the regulation of negative T3 target genes and hypothalamic-pituitary-thyroid axis, but exact mechanisms of the negative regulation by TH have remained elusive. Important insights into the roles of co-repressors and co-activators in different physiological processes have been obtained using animal models with disrupted co-regulator function. At the same time, recent studies interrogating genome-wide TR binding have generated compelling new data regarding effects of T3, local chromatin structure, and specific response element configuration on TR recruitment and function leading to the proposal of new models of transcriptional regulation by TRs. This review discusses data obtained in various mouse models with manipulated function of nuclear receptor co-repressor (NCoR or NCOR1) and silencing mediator of retinoic acid receptor and thyroid hormone receptor (SMRT or NCOR2), and family of steroid receptor co-activators (SRCs also known as NCOAs) in the context of TH action, as well as insights into the function of co-regulators that may emerge from the genome-wide TR recruitment analysis.

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.

Modulation of testicular receptor 4 activity by mitogen-activated protein kinase-mediated phosphorylation

Testicular receptor 4 (TR4) is an orphan member of the nuclear receptor superfamily. Despite the lack of identified ligands, its functional role has been demonstrated both in animals and cell cultures. However, it remains unclear how the biological activity of TR4 is regulated without specific ligands. In this study, we showed that in the absence of specific ligands the activity of TR4 could be modulated by mitogen-activated protein kinase (MAPK)-mediated phosphorylation of its activation function 1 (AF-1) domain. A mass spectrometry-based proteome analysis of TR4 expressed in insect cells revealed three phosphorylation sites in its AF-1 domain, specifically on Ser(19), Ser(55), and Ser(68). Site-directed mutagenesis studies demonstrated the functionality of phosphorylation on Ser(19) and Ser(68) but not Ser(55). We also demonstrated that MAPK-mediated phosphorylation of the AF-1 domain rendered TR4 a repressor, mediated through the preferential recruitment of corepressor RIP140. Dephosphorylation of its AF-1 made TR4 an activator due to its selective recruitment of coactivator, P300/cyclic AMP-responsive element binding protein-binding protein-associated factor (PCAF). The biological effects were validated by using the wild type TR4 and its constitutive negative (dephosphorylated) and constitutive positive (phosphorylated) mutants in the studies of regulation of its natural target gene, apoE. This study uncovered, for the first time, a ligand-independent mechanism underlying the biological activity of TR4 that was mediated by MAPK-mediated receptor phosphorylation of AF-1 domain.