Role of the conserved C-terminal region of thyroid hormone receptor-alpha in ligand-dependent transcriptional activation

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

The transforming acidic coiled coil (TACC1) protein modulates the transcriptional activity of the nuclear receptors TR and RAR

Background

The transcriptional activity of Nuclear hormone Receptors (NRs) is regulated by interaction with coactivator or corepressor proteins. Many of these cofactors have been shown to have a misregulated expression or to show a subcellular mislocalization in cancer cell lines or primary tumors. Therefore they can be factors involved in the process of oncogenesis.

Results

We describe a novel NR coregulator, TACC1, which belongs to the Transforming Acidic Coiled Coil (TACC) family. The interaction of TACC1 with Thyroid Hormone Receptors (TR) and several other NRs has been shown in a yeast two-hybrid screen and confirmed by GST pulldown, colocalization and co-immunoprecipitation experiments. TACC1 interacts preferentially with unliganded NRs. In F9 cells, endogenous TACC1 localized in the chromatin-enriched fraction of the nucleus and interacted with Retinoid Acid Receptors (RARα) in the nucleus. TACC1 depletion in the cell led to decreased RARα and TRα ligand-dependent transcriptional activity and to delocalization of TR from the nucleus to the cytoplasm.

Conclusions

From these experimental studies we propose that TACC1 might be a scaffold protein building up a transcriptional complex around the NRs we studied. This function of TACC1 might account for its involvement in several forms of tumour development.

Farnesyl Pyrophosphate Is a Novel Transcriptional Activator for a Subset of Nuclear Hormone Receptors

In silico docking of a chemical library with the ligand-binding domain of thyroid hormone nuclear receptor-beta (TRbeta) suggested that farnesyl pyrophosphate (FPP), a key intermediate in cholesterol synthesis and protein farnesylation, might function as an agonist. Surprisingly, addition of FPP to cells activated TR as well as the classical steroid hormone receptors but not peroxisome proliferative-activating receptors, farnesoid X receptor, liver X receptor, or several orphan nuclear receptors the ligands of which are unknown. FPP enhanced receptor-coactivator binding in vitro and in vivo, and elevation of FPP levels in cells by squalene synthetase or farnesyl transferase inhibitors leads to activation. The FPP effect was blocked by selective receptor antagonists, and in silico docking with 143 nuclear receptor ligand-binding domain structures revealed that FPP only docked with the agonist conformation of those receptors activated by FPP. Our results suggest that certain nuclear receptors maintain a common structural feature that may reflect an action of FPP on an ancient nuclear receptor or that FPP could function as a ligand for one of the many orphan nuclear receptors the ligands of which have not yet been identified. This finding also has potential interesting implications that may, in part, explain the pleotropic effects of statins as well as certain actions of farnesylation inhibitors in cells.

Nuclear hormone receptor coregulator: role in hormone action, metabolism, growth, and development

Nuclear hormone receptor coregulator (NRC) (also referred to as activating signal cointegrator-2, thyroid hormone receptor-binding protein, peroxisome proliferator activating receptor-interacting protein, and 250-kDa receptor associated protein) belongs to a growing class of nuclear cofactors widely known as coregulators or coactivators that are necessary for transcriptional activation of target genes. The NRC gene is also amplified and overexpressed in breast, colon, and lung cancers. NRC is a 2063-amino acid protein that harbors a potent N-terminal activation domain (AD1) and a second more centrally located activation domain (AD2) that is rich in Glu and Pro. Near AD2 is a receptor-interacting domain containing an LxxLL motif (LxxLL-1), which interacts with a wide variety of ligand-bound nuclear hormone receptors with high affinity. A second LxxLL motif (LxxLL-2) located in the C-terminal region of NRC is more restricted in its nuclear hormone receptor specificity. The intrinsic activation potential of NRC is regulated by a C-terminal serine, threonine, leucine-regulatory domain. The potential role of NRC as a cointegrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known transcriptional regulators including CBP/p300. Recent studies in mice indicate that deletion of both NRC alleles leads to embryonic lethality resulting from general growth retardation coupled with developmental defects in the heart, liver, brain, and placenta. NRC(-/-) mouse embryo fibroblasts spontaneously undergo apoptosis, indicating the importance of NRC as a prosurvival and antiapoptotic gene. Studies with 129S6 NRC(+/-) mice indicate that NRC is a pleiotropic regulator that is involved in growth, development, reproduction, metabolism, and wound healing.

Domain structure of the NRIF3 family of coregulators suggests potential dual roles in transcriptional regulation

The identification of a novel coregulator for nuclear hormone receptors, designated NRIF3, was recently reported (D. Li et al., Mol. Cell. Biol. 19:7191-7202, 1999). Unlike most known coactivators, NRIF3 exhibits a distinct receptor specificity in interacting with and potentiating the activity of only TRs and RXRs but not other examined nuclear receptors. However, the molecular basis underlying such specificity is unclear. In this report, we extended our study of NRIF3-receptor interactions. Our results suggest a bivalent interaction model, where a single NRIF3 molecule utilizes both the C-terminal LXXIL (receptor-interacting domain 1 [RID1]) and the N-terminal LXXLL (RID2) modules to cooperatively interact with TR or RXR (presumably a receptor dimer), with the spacing between RID1 and RID2 playing an important role in influencing the affinity of the interactions. During the course of these studies, we also uncovered an NRIF3-NRIF3 interaction domain. Deletion and mutagenesis analyses mapped the dimerization domain to a region in the middle of NRIF3 (residues 84 to 112), which is predicted to form a coiled-coil structure and contains a putative leucine zipper-like motif. By using Gal4 fusion constructs, we identified an autonomous transactivation domain (AD1) at the C terminus of NRIF3. Somewhat surprisingly, full-length NRIF3 fused to the DNA-binding domain of Gal4 was found to repress transcription of a Gal4 reporter. Further analyses mapped a novel repression domain (RepD1) to a small region at the N-terminal portion of NRIF3 (residues 20 to 50). The NRIF3 gene encodes at least two additional isoforms due to alternative splicing. These two isoforms contain the same RepD1 region as NRIF3. Consistent with this, Gal4 fusions of these two isoforms were also found to repress transcription. Cotransfection of NRIF3 or its two isoforms did not relieve the transrepression function mediated by their corresponding Gal4 fusion proteins, suggesting that the repression involves a mechanism(s) other than the recruitment of a titratable corepressor. Interestingly, a single amino acid residue change of a potential phosphorylation site in RepD1 (Ser(28) to Ala) abolishes its transrepression function, suggesting that the coregulatory property of NRIF3 (or its isoforms) might be subjected to regulation by cellular signaling. Taken together, our results identify NRIF3 as an interesting coregulator that possesses both transactivation and transrepression domains and/or functions. Collectively, the NRIF3 family of coregulators (which includes NRIF3 and its other isoforms) may play dual roles in mediating both positive and negative regulatory effects on gene expression.

A new family of nuclear receptor coregulators that integrate nuclear receptor signaling through CREB-binding protein

We describe the cloning and characterization of a new family of nuclear receptor coregulators (NRCs) which modulate the function of nuclear hormone receptors in a ligand-dependent manner. NRCs are expressed as alternatively spliced isoforms which may exhibit different intrinsic activities and receptor specificities. The NRCs are organized into several modular structures and contain a single functional LXXLL motif which associates with members of the steroid hormone and thyroid hormone/retinoid receptor subfamilies with high affinity. Human NRC (hNRC) harbors a potent N-terminal activation domain (AD1), which is as active as the herpesvirus VP16 activation domain, and a second activation domain (AD2) which overlaps with the receptor-interacting LXXLL region. The C-terminal region of hNRC appears to function as an inhibitory domain which influences the overall transcriptional activity of the protein. Our results suggest that NRC binds to liganded receptors as a dimer and this association leads to a structural change in NRC resulting in activation. hNRC binds CREB-binding protein (CBP) with high affinity in vivo, suggesting that hNRC may be an important functional component of a CBP complex involved in mediating the transcriptional effects of nuclear hormone receptors.

NRIF3 is a novel coactivator mediating functional specificity of nuclear hormone receptors

Many nuclear receptors are capable of recognizing similar DNA elements. The molecular event(s) underlying the functional specificities of these receptors (in regulating the expression of their native target genes) is a very important issue that remains poorly understood. Here we report the cloning and analysis of a novel nuclear receptor coactivator (designated NRIF3) that exhibits a distinct receptor specificity. Fluorescence microscopy shows that NRIF3 localizes to the cell nucleus. The yeast two-hybrid and/or in vitro binding assays indicated that NRIF3 specifically interacts with the thyroid hormone receptor (TR) and retinoid X receptor (RXR) in a ligand-dependent fashion but does not bind to the retinoic acid receptor, vitamin D receptor, progesterone receptor, glucocorticoid receptor, or estrogen receptor. Functional experiments showed that NRIF3 significantly potentiates TR- and RXR-mediated transactivation in vivo but has little effect on other examined nuclear receptors. Domain and mutagenesis analyses indicated that a novel C-terminal domain in NRIF3 plays an essential role in its specific interaction with liganded TR and RXR while the N-terminal LXXLL motif plays a minor role in allowing optimum interaction. Computer modeling and subsequent experimental analysis suggested that the C-terminal domain of NRIF3 directly mediates interaction with liganded receptors through an LXXIL (a variant of the canonical LXXLL) module while the other part of the NRIF3 protein may still play a role in conferring its receptor specificity. Identification of a coactivator with such a unique receptor specificity may provide new insight into the molecular mechanism(s) of receptor-mediated transcriptional activation as well as the functional specificities of nuclear receptors.