The co-chaperone XAP2 is required for activation of hypothalamic thyrotropin-releasing hormone transcription in vivo (original) (raw)
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Physiological regulation of hypothalamic TRH transcription in vivo is T3 receptor isoform specific
FASEB journal : official publication of the Federation of American Societies for Experimental Biology, 1998
Thyroid hormone (tri-iodo-thyronine, T3) exerts transcriptional effects on target genes in responsive cells. These effects are determined by DNA/protein interactions governed by the type of T3 receptors (TRs) in the cell. As TRs show tissue and developmental variations, regulation is best addressed in an integrated in vivo model. We examined TR subtype effects on thyrotropin-releasing hormone (TRH) transcription and on the pituitary/thyroid axis end point: thyroid hormone secretion. Polyethylenimine served to transfect a TRH-luciferase construct containing 554 bp of the rat TRH promoter into the hypothalami of newborn mice. Transcription from the TRH promoter was regulated in a physiologically faithful manner, being significantly increased in hypothyroidism and decreased in T3-treated animals. Moreover, when various ligand binding forms of mouse or chicken TRbeta and TRalpha were expressed with TRH-luciferase, all forms of TRbeta gave T3-dependent regulation of TRH transcription, wh...
Endocrine Journal, 2018
Mutations in TBL1X, a component of the nuclear receptor co-repressor (N-CoR) and silencing mediator of retinoic acid and thyroid hormone receptor co-repressor complexes, have recently been implicated in isolated central hypothyroidism (CeH). However, the mechanisms by which TBL1X mutations affect negative feedback regulation in the hypothalamuspituitary-thyroid axis remain unclear. N-CoR was previously reported to paradoxically enhance the ligand-independent stimulation of TRH and TSHβ gene promoters by thyroid hormone receptors (TR) in cell culture systems. We herein investigated whether TBL1X affects the unliganded TR-mediated stimulation of the promoter activities of genes negatively regulated by T3 in cooperation with N-CoR. In a hypothalamic neuronal cell line, the unliganded TR-mediated stimulation of the TRH gene promoter was significantly enhanced by co-transfected TBL1X, and the co-transfection of TBL1X with N-CoR further enhanced promoter activity. In contrast, the knockdown of endogenous Tbl1x using short interfering RNA significantly attenuated the N-CoR-mediated enhancement of promoter activity in the presence of unliganded TR. The co-transfection of N365Y or Y458C, TBL1X mutants identified in CeH patients, showed impaired co-activation with N-CoR for the ligandindependent stimulation of the TRH promoter by TR. In the absence of T3, similar or impaired enhancement of the TSHβ gene promoter by the wild type or TBL1X mutants, respectively, was observed in the presence of co-transfected TR and N-CoR in CV-1 cells. These results suggest that TBL1X is needed for the full activation of TRH and TSHβ gene promoters by unliganded TR. Mutations in TBL1X may cause CeH due to the impaired up-regulation of TRH and/or TSHβ gene transcription despite low T3 levels.
Journal of Molecular Endocrinology, 2002
Thyroid hormone receptors (TRs) often modulate transcriptional activity of target genes by heterodimerization with the 9-cis retinoic acid receptor (RXR). On positive thyroid response elements (TREs), RXR favors binding of the TR-RXR complex to DNA and stimulates transcription. RXR action on negative TREs is unclear. Furthermore, the single half-site configuration of many negative TREs does not favor the binding of a classic TR-RXR heterodimer. In a comparative study using CV-1 cells (relatively RXR- and TR-deficient) and JEG-3 cells (relatively TR-deficient), we demonstrate the importance of RXR in the negative transcriptional regulation of genes of the hypothalamo-pituitary axis by tri-iodothyronine. While RXR has variable effects on ligand-independent activation produced by TRs, it was required for efficient ligand-dependent repression of the TRH gene for TRalpha1 and TRbeta1 and of the TSH genes by all TRs. Using different RXR constructs we also observed the importance of the C-...
Regulation of gene transcription by the thyroid hormone receptors
2006
Thyroid hormone (TH) has important effects on postnatal development as well as on adult metabolic homeostasis. TH meditates its effects through different isoforms of the thyroid hormone receptors (TRs) encoded by the TRalpha and TRbeta genes, respectively. TRs belong to the nuclear hormone receptor superfamily. These receptors are ligand-modulated transcription factors that bind to specific DNA elements located in the regulatory regions of target genes. The nature of these elements determines the effect TR will have on transcription. On what is called a positive TH response element (pTRE), the un-liganded receptor binds to DNA and represses transcription. Hormone binding induces a conformational change in the receptor, allowing for activation of transcription. In contrast, on a negative TH response element (nTRE), activation of transcription is mediated by the un-liganded receptor, whereas the liganded receptor represses transcription. The mechanism behind the regulation of a pTRE i...
Molecular Endocrinology, 2009
Currently, little is known about histone modifications and molecular mechanisms of negatively regulated transcription. In pituitary cells, thyroid hormone (T3) decreased transcription, and surprisingly increased histone acetylation, of TSHα promoter. This increase was mediated directly by thyroid hormone receptor. Histone acetylation of H3K9 and H3K18 sites, two modifications usually associated with transcriptional activation, occur in negative regulation of TSHα promoter. T3 also caused release of a corepressor complex composed of histone deacetylase 3 (HDAC3), transducin β-like protein 1, and nuclear receptor coprepressor (NCoR)/ silencing mediator for retinoic and thyroid hormone receptor from TSHα promoter in chromatin immunoprecipitation assays. NCoR and HDAC3 overexpression selectively increased ligand-independent basal transcription. Two histone acetyltransferase inhibitors increased overall transcription but did not abrogate negative regulation or NCoR/HDAC3 complex release ...
Proceedings of the National Academy of Sciences, 1991
Regulation of human thyrotropin beta subunit gene (TSHB) expression by thyrotropin-releasing hormone (TRH) was examined in a clonal rat pituitary-cell line (GH3). Transient expression studies were done with various 5'-flanking DNA sequences of TSHB coupled to reporter gene chloramphenicol acetyltransferase. Deletion analysis defined two discrete regions (-128 to -92 base pairs and -28 to +8 base pairs) that each mediated an approximately 2-fold TRH induction. The upstream site contains a DNA sequence with close homology to the DNA-binding site for a pituitary-specific transcriptional factor Pit-1/GHF-1. DNase I footprinting analysis of mouse thyrotropic tumor extract as well as DNA-transfection studies using an expression vector containing an N-terminal deletion of Pit-1/GHF-1 cDNA suggest that Pit-1/GHF-1 or a closely related protein in the thyrotroph mediates TRH responsiveness of this gene. In addition, the downstream site overlaps with the recently characterized thyroid horm...
New insights into thyroid hormone action
Best Practice & Research Clinical Endocrinology & Metabolism, 2007
Thyroid hormones (THs) have important effects on cellular development, growth, and metabolism. They bind to thyroid hormone receptors (TRs), TRa and TRb, which belong to the nuclear hormone receptor superfamily. These receptors also bind to enhancer elements in the promoters of target genes, and can regulate both positive and negative transcription. Recent emerging evidence has characterized some of the molecular mechanisms by which THs regulate transcription as co-repressors, and co-activators have been identified and their effects on histone acetylation examined. THs also have rapid effects that do not require transcription. These can occur via TRs or other cellular proteins, and typically occur outside the nucleus. It appears that THs regulate multiple cellular functions using a diverse array of receptors and signaling systems. TR isoform-or pathway-specific drugs might provide the therapeutic benefits of TH action such as decreasing obesity or lowering cholesterol levels without some of the side effects of hyperthyroidism.
TR Alpha 2 Exerts Dominant Negative Effects on Hypothalamic Trh Transcription In Vivo
PLoS ONE, 2014
Mammalian thyroid hormone receptors (TRs) have multiple isoforms, including the bona fide receptors that bind T 3 (TRa1, TRb1 and TRb2) and a non-hormone-binding variant, TRa2. Intriguingly, TRa2 is strongly expressed in the brain, where its mRNA levels exceed those of functional TRs. Ablation of TRa2 in mice results in over-expression of TRa1, and a complex phenotype with low levels of free T 3 and T 4 , without elevated TSH levels, suggesting an alteration in the negative feedback at the hypothalamic-pituitary level. As the hypothesis of a potential TRH response defect has never been tested, we explored the functional role of TRa2 in negative feedback on transcription of hypothalamic thyrotropin, Trh. The in vivo transcriptional effects of TRa2 on hypothalamic Trh were analysed using an in vivo reporter gene approach. Effects on Trhluc expression were examined to that of two, T 3 positively regulated genes used as controls. Applying in vivo gene transfer showed that TRa2 over-expression in the mouse hypothalamus abrogates T 3 -dependent repression of Trh and T 3 activation of positively regulated promoters, blocking their physiological regulation. Surprisingly, loss of function studies carried out by introducing a shTRa2 construct in the hypothalamus also blocked physiological T 3 dependent regulation. Thus, modulating hypothalamic TRa2 expression by either gain or loss of function abrogated T 3 dependent regulation of Trh transcription, producing constant transcriptional levels insensitive to feedback. This loss of physiological regulation was reflected at the level of the endogenous Trh gene, were gain or loss of function held mRNA levels constant. These results reveal the as yet undescribed dominant negative role of TRa2 over TRa1 effect on hypothalamic Trh transcription.