Binding of the thyroid hormone receptor to a negative element in the basal growth hormone promoter is associated with histone acetylation

Reversing thyroid-hormone-mediated repression of a HSV-1 promoter via computationally guided mutagenesis

Thyroid hormones (THs) and their DNA-binding nuclear receptors (TRs) direct transcriptional regulation in diverse ways depending on the host cell environment and specific promoter characteristics of TH-sensitive genes. This study sought to elucidate the impact on transcriptional repression of nucleotide sequence or orientation within TR binding sites - the TH response elements (TREs) of TH-sensitive promoters - to better understand ligand-dependent transcriptional repression of wild-type promoters. Computational analysis of the HSV-1 thymidine kinase (TK) gene TRE bound by TR and retinoid X receptor (RXR) revealed a single TRE point mutation sufficient to reverse the TRE orientation. In vitro experiments showed that the TRE point mutation had distinct impacts on promoter activity, sufficient to reverse the TH-dependent negative regulation in neuroendocrine differentiated cells. This point mutation altered the promoter's regulatory mechanism by discrete changes in transcription factor TR occupancy and altered enrichment of the repressive chromatin modification of histone-3-lysine-9-trimethyl (H3K9Me3). Insights relating to this negative TRE (nTRE) mechanism aids our understanding of other nTREs and TRE mutations associated with TH and herpes diseases.

The Mechanism of Negative Transcriptional Regulation by Thyroid Hormone: Lessons From the Thyrotropin β Subunit Gene

Thyroid hormone (T3) activates (positive regulation) or represses (negative regulation) target genes at the transcriptional level. The molecular mechanism of the former has been elucidated in detail; however, the mechanism for negative regulation has not been established. The best example of the gene that is negatively regulated by T3 is the thyrotropin (thyroid-stimulating hormone) β subunit (TSHβ) gene. Analogous to the T3-responsive element (TRE) in positive regulation, a negative TRE (nTRE) has been postulated in the TSHβ gene. However, TSHβ promoter analysis, performed in the presence of transcription factors Pit1 and GATA2, which are determinants of thyrotroph differentiation in the pituitary, revealed that the nTRE is dispensable for inhibition by T3. We propose a tethering model in which the T3 receptor is tethered to GATA2 via protein-protein interaction and inhibits GATA2-dependent transactivation of the TSHβ gene in a T3-dependent manner.

Mechanisms involved in glucocorticoid induction of pituitary GH expression during embryonic development

Glucocorticoid hormones are involved in functional differentiation of GH-producing somatotrophs. Glucocorticoid treatment prematurely induces GH expression in mammals and birds in a process requiring protein synthesis and Rat sarcoma (Ras) signaling. The objective of this study was to investigate mechanisms through which glucocorticoids initiate GH expression during embryogenesis, taking advantage of the unique properties of chicken embryos as a developmental model. We determined that stimulation of GH expression occurred through transcriptional activation of GH, rather than enhancement of mRNA stability, and this process requires histone deacetylase activity. Through pharmacological inhibition, we identified the ERK1/2 pathway as a likely downstream Ras effector necessary for glucocorticoid stimulation of GH. However, we also found that chronic activation of ERK1/2 activity with a constitutively active mutant or stimulatory ligand reduced initiation of GH expression by glucocorticoid treatment. Corticosterone treatment of cultured embryonic pituitary cells increased ERK1/2 activity in an apparent cyclical manner, with a rapid increase within 5 minutes, followed by a reduction to near-basal levels at 3 hours, and a subsequent increase again at 6 hours. Therefore, we conclude that ERK1/2 signaling must be strictly controlled for maximal glucocorticoid induction of GH to occur. These results are the first in any species to demonstrate that Ras- and ERK1/2-mediated transcriptional events requiring histone deacetylase activity are involved in glucocorticoid induction of pituitary GH during embryonic development. This report increases our understanding of the molecular mechanisms underlying glucocorticoid recruitment of somatotrophs during embryogenesis and should provide insight into glucocorticoid-induced developmental changes in other tissues and cell types.

The thyroid hormone triiodothyronine controls macrophage maturation and functions: protective role during inflammation

The endocrine system participates in regulating macrophage maturation, although little is known about the modulating role of the thyroid hormones. In vitro results demonstrate a negative role of one such hormone, triiodothyronine (T3), in triggering the differentiation of bone marrow-derived monocytes into unpolarized macrophages. T3-induced macrophages displayed a classically activated (M1) signature. A T3-induced M1-priming effect was also observed on polarized macrophages because T3 reverses alternatively activated (M2) activation, whereas it enhances that of M1 cells. In vivo, circulating T3 increased the content of the resident macrophages in the peritoneal cavity, whereas it reduced the content of the recruited monocyte-derived cells. Of interest, T3 significantly protected mice against endotoxemia induced by lipopolysaccharide i.p. injection; in these damaged animals, decreased T3 levels increased the recruited (potentially damaging) cells, whereas restoring T3 levels decreased recruited and increased resident (potentially beneficial) cells. These data suggest that the anti-inflammatory effect of T3 is coupled to the modulation of peritoneal macrophage content, in a context not fully explained by the M1/M2 framework. Thyroid hormone receptor expression analysis and the use of different thyroid hormone receptor antagonists suggest thyroid hormone receptor β1 as the major player mediating T3 effects on macrophages. The novel homeostatic link between thyroid hormones and the pathophysiological role of macrophages opens new perspectives on the interactions between the endocrine and immune systems.

Effects of perinatal hypothyroidism on regulation of reelin and brain-derived neurotrophic factor gene expression in rat hippocampus: Role of DNA methylation and histone acetylation

Thyroid hormones have long been known to play important roles in the development and functions of the central nervous system, however, the precise molecular mechanisms that regulate thyroid hormone-responsive gene expression are not well understood. The present study investigated the role of DNA methylaion and histone acetylation in the effects of perinatal hypothyroidism on regulation of reelin and brain-derived neurotrophic factor (BDNF) gene expression in rat hippocampus. The findings indicated that the activities of DNA methyltransferase (DNMT), methylated reelin and BDNF genes were up-regulated, whereas, the activities of histone acetylases (HAT), the levels of global acetylated histone 3 (H3) and global acetylated histone 4 (H4), and acetylated H3, acetylated H4 at reelin promoter and at BDNF gene promoter for exon II were down-regulated in the hippocampus at the developmental stage of the hypothyroid animals. These results suggest that epigenetic modification of chromatin might underlie the mechanisms of hypothyroidism-induced down-regulation of reelin and BDNF gene expression in developmental rat hippocampus.

Residues K128, 132, and 134 in the thyroid hormone receptor-alpha are essential for receptor acetylation and activity

The thyroid hormone receptor (TR)-alpha is a nuclear receptor that mediates both transrepression and ligand-dependent transactivation. Here we show that TRalpha is posttranslationally modified by acetylation in response to its own ligand (T(3)). Acetylation increases binding to DNA. Using mutagenesis, we identified three conserved lysine residues in the carboxi-terminal extension (CTE) of the DNA binding domain that are targets of the cAMP-response element-binding protein acetyltransferase. Substitution of these lysines by arginines in TRalpha decreased ligand binding affinity and precluded ligand-dependent release of corepressors and recruitment of coactivators. The acetylation TRalpha mutant lost the ability to transactivate even at high T(3) concentrations and acts as a dominant-negative inhibitor of wild-type TR activity. In addition, whereas native TRalpha interferes with AP-1 function, the mutant is unable to mediate transrepression. Finally, TRalpha suppresses NIH-3T3 fibroblast transformation by the Ras oncogene both in a ligand-dependent and -independent manner, but the CTE mutant is unable to mediate ligand-dependent repression of transformation. These results reveal a key role for the CTE region on acetylation, ligand affinity, transactivation, transrepression, and antitransforming properties of TRalpha.

Physiological replacement of T3 improves left ventricular function in an animal model of myocardial infarction-induced congestive heart failure

BACKGROUND

Patients with congestive heart failure (CHF) often have low serum triiodothyronine (T(3)) concentrations. In a rodent model of myocardial infarction-induced CHF and low serum T(3), we hypothesized that replacing T(3) to euthyroid levels would improve left ventricular function without producing untoward signs of thyrotoxicosis.

METHODS AND RESULTS

Adult male Sprague-Dawley rats were subjected to left anterior descending coronary artery ligation (myocardial infarction). One week post-myocardial infarction, left ventricular fractional shortening was significantly reduced to 22+/-1% in CHF animals versus 38+/-1% for sham-operated controls (P<0.001). Serum T(3) concentration was also significantly reduced (80+/-3 versus 103+/-6 ng/dL; P<0.001), in CHF animals versus Shams. At 9 weeks post-myocardial infarction, systolic function (+dP/dt max) was significantly attenuated in CHF animals (4773+/-259 versus 6310+/-267 mmHg/s; P<0.001) as well as diastolic function measured by half time to relaxation (15.9+/-1.2 versus 11.1+/-0.3 ms; P<0.001). alpha-myosin heavy chain expression was also significantly reduced by 77% (P<0.001), and beta-myosin heavy chain expression was increased by 21%. Continuous T(3) replacement was initiated 1 week post-myocardial infarction with osmotic mini-pumps (6 microg/kg/d), which returned serum T(3) concentrations to levels similar to Sham controls while resting conscious heart rate, arterial blood pressure and the incidence of arrhythmias were not different. At 9 weeks, systolic function was significantly improved by T(3) replacement (6279+/-347 mmHg/s; P<0.05) and a trend toward improved diastolic function (12.3+/-0.6 ms) was noted. T(3) replacement in CHF animals also significantly increased alpha- and reduced beta-MHC expression, (P<0.05).

CONCLUSIONS

These data indicate that T(3) replacement to euthyroid levels improves systolic function and tends to improve diastolic function, potentially through changes in myocardial gene expression.

To bind or not to bind - how to down-regulate target genes by liganded thyroid hormone receptor?

The terrain is well explored regarding genes whose gene expression is up-regulated upon binding of thyroid hormone (TH) to its nuclear receptor. This regulation mechanism has been intensively studied and is well understood. In contrast, a lot of white spots remain on the map when it comes to target genes whose expression is down-regulated upon binding of TH to the thyroid hormone receptor (TR). Since no consistent mechanism has been proposed to explain ligand-dependent down-regulation of target gene transcription several working hypotheses favour different molecular mechanisms. Some working theories suggest a direct binding of TR to regulatory elements of target genes. Others favour models that are independent of a direct DNA binding event. However recent data suggested that a direct binding of TR to DNA is dispensable for TH-dependent negative gene transcription.

Thyroid hormone regulates endogenous amyloid-beta precursor protein gene expression and processing in both in vitro and in vivo models

Thyroid hormone negatively regulates the amyloid-beta precursor protein (APP) gene in thyroid hormone receptor (TR)-transfected neuroblastoma cells. A negative thyroid hormone response element (nTRE) that mediates this regulation has been identified in the first exon of the APP gene. We demonstrate in an in vivo system that expression of APP mRNA, APP protein, and APP secretase cleavage products in mouse brain is influenced by thyroid status. Adult female mice were made hyperthyroid or hypothyroid for 3 weeks and compared to euthyroid mice. APP gene product expression was increased in hypothyroid mouse brain and reduced in hyperthyroid mouse brain, when compared to euthyroid controls. We observed similar effects of thyroid hormone on endogenous APP gene expression in human neuroblastoma cells. The incidence of hypothyroidism increases with age, and localized hypothyroidism of central nervous system has been reported in some patients with Alzheimer's disease (AD). Reduced action of thyroid hormone on the APP gene may contribute to AD pathology by increasing APP expression and the levels of processed APP products. These findings may be an underlying mechanism contributing to the association of hypothyroidism with AD in the elderly, as well as identifying a potential therapeutic target. Pharmacologic supplementation of thyroid hormone, or its analogs, may reduce APP gene expression and beta amyloid peptide accumulation.

Effect of serum triiodothyronine on regulation of cardiac gene expression: role of histone acetylation

Thyroid hormone regulates the transcription of several important cardiac genes. Although the thyroid gland produces predominantly thyroxine (T(4)), it is triiodothyronine (T(3)) that is transported across the sarcolemma and binds to nuclear thyroid hormone receptor proteins; yet various studies suggest that serum T(3) levels do not accurately reflect cellular T(3) action. To address this question, we studied the dose-response relationship of T(3) administered by constant infusion in hypothyroid animals with the simultaneous in vivo transcription rate of the cardiac-specific alpha-myosin heavy chain (MHC) gene, measured by quantitating alpha-MHC heteronuclear (hn)RNA content. Constant infusion of 4 mug T(3) x kg body wt(-1) x day(-1) for 3 days normalized serum T(3) and restored transcription to euthyroid levels; in contrast, daily injections of the same dose increased alpha-MHC transcription by only 55% of that obtained by infusion. Although infusion of T(3) at 1.25 microg T(3) x kg body wt(-1) x day(-1) was not sufficient to restore serum T(3) to normal, it was capable of restoring transcription to normal at 3 days, but when administered for 12 days, transcription of alpha-MHC was found to be 50% of euthyroid levels, demonstrating a decreased sensitivity to T(3) over time. Treatment with trichostatin A (TSA) to inhibit histone deacetylation increased levels of total nuclear acetylated histone H4 by almost 50% but was without effect on the real-time PCR measures of alpha-MHC hnRNA. TSA administered together with T(3) (10 mug T(3)/kg body wt) significantly increased transcription of alpha-MHC after 30 h, thus demonstrating a potential role for histones as cofactors in the T(3) regulation of cardiac alpha-MHC transcription.

Aberrant dynamics of histone deacetylation at the thyrotropin-releasing hormone gene in resistance to thyroid hormone

Histone acetylation status influences transcriptional activity, and the mechanism of negative gene regulation by thyroid hormone remains unclear, although its impairment by a mutant thyroid hormone receptor (TR) is critical for resistance to thyroid hormone (RTH). We found a novel RTH mutant, F455S, that exhibited impaired repression of the TRH gene and had a strong dominant-negative effect on the gene. F455S strongly interacted with nuclear receptor corepressor (NCoR) and was hard to dissociate from it. To analyze the dynamics of histone acetylation status in vivo, we established cell lines stably expressing the TRH promoter and wild-type or F455S TR. Treatment with a histone deacetylase (HDAC) inhibitor completely abolished the repression of the gene by T3. The histones H3 and H4 at the TRH promoter were acetylated, and addition of T3 caused recruitment of HDACs 2 and 3 within 15 min, resulting in a transient deacetylation of the histone tails. TR and NCoR were located on the promoter, and T3 caused NCoR dissociation and steroid receptor coactivator-1 recruitment. In the presence of F455S, the histones were hyperacetylated, and HDAC recruitment and histone deacetylation were significantly impaired. This is the first report demonstrating the direct involvement of aberrant dynamics of chromatin modification in RTH.