Nuclear receptor corepressors activate rather than suppress basal transcription of genes that are negatively regulated by thyroid hormone

Minireview: Thyrotropin-releasing hormone and the thyroid hormone feedback mechanism

Thyroid hormone (TH) plays a critical role in development, growth, and cellular metabolism. TH production is controlled by a complex mechanism of positive and negative regulation. Hypothalamic TSH-releasing hormone (TRH) stimulates TSH secretion from the anterior pituitary. TSH then initiates TH synthesis and release from the thyroid gland. The synthesis of TRH and TSH subunit genes is inhibited at the transcriptional level by TH, which also inhibits posttranslational modification and release of TSH. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback at the pituitary was thought to be the primary regulator of serum TSH levels. However, study of transgenic animals showed an unexpected, dominant role for TRH in regulating the hypothalamic-pituitary-thyroid axis and an unanticipated involvement of the thyroid hormone receptor ligand-dependent activation function (AF-2) domain in TH negative regulation. These results are summarized in the review.

Negative regulation of TSHalpha target gene by thyroid hormone involves histone acetylation and corepressor complex dissociation

Currently, little is known about histone modifications and molecular mechanisms of negatively regulated transcription. In pituitary cells, thyroid hormone (T(3)) decreased transcription, and surprisingly increased histone acetylation, of TSHalpha 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 TSHalpha promoter. T(3) also caused release of a corepressor complex composed of histone deacetylase 3 (HDAC3), transducin beta-like protein 1, and nuclear receptor coprepressor (NCoR)/ silencing mediator for retinoic and thyroid hormone receptor from TSHalpha 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 by T(3). Chromatin immunoprecipitation analyses of an endogenous positively regulated target gene showed increased histone acetylation and corepressor complex release with T(3) treatment. Finally, microarray analyses suggested there is a subset of negatively regulated genes with increased histone acetylation. These findings demonstrate the critical role of NCoR/HDAC3 complex in negative regulation of TSHalpha gene expression and show that similar complexes and overlapping epigenetic modifications can participate in both negative and positive transcriptional regulation.

A selective peroxisome proliferator-activated receptor-gamma modulator, telmisartan, binds to the receptor in a different fashion from thiazolidinediones

Angiotensin type 1 receptor blockers are widely used for the treatment of hypertension, and one angiotensin type 1 receptor blocker, telmisartan, specifically activates the peroxisome proliferator-activated receptor (PPAR)-gamma. We studied the impact of PPARgamma mutants on transcriptional control and interaction with cofactors to elucidate differences in the molecular mechanism between telmisartan and other PPARgamma agonists, thiazolidinediones (TZDs). We created several amino acid substitutions in the ligand binding domain of PPARgamma that, based on molecular modeling, may affect the binding of these agents. In transient expression experiments, wild-type PPARgamma-mediated transcription stimulated by telmisartan was more than one third of that stimulated by TZDs. The activation stimulated by TZDs was impaired, whereas activation stimulated by telmisartan was retained, in the H323Y, S342A, and H449A mutants. In the Y473A mutant, the TZD-induced activation was further impaired and lower than that of telmisartan-induced activation. Coexpression of coactivators enhanced the activation by both telmisartan and TZDs, but activation by telmisartan always exceeded that of TZDs in the Y473A mutant. Based on a mammalian two-hybrid assay, the interaction with corepressors was retained in Y473A. Telmisartan and TZDs, but not 9cis retinoic acid, dissociated corepressors from the wild-type PPARgamma. Telmisartan most effectively dissociated corepressors from Y473A. The interaction with coactivators was enhanced by TZD activation of wild-type PPARgamma and both telmisartan and TZD activation of Y473A. Thus, the Y473A mutant is selectively stimulated by telmisartan but not TZDs, suggesting that telmisartan and TZDs have differential effects on the transcriptional control. In conclusion, these PPARgamma mutants could be powerful tools for developing novel therapeutic agents that retain the metabolic efficacy of PPARgamma activation with fewer adverse effects, such as the increase in body weight associated with TZDs.

Coactivators and corepressors: what's in a name?

Nuclear receptor coregulators are molecules required for efficient function of nuclear receptors. The field of nuclear receptor coregulators has experienced remarkably rapid growth since its inception in the mid-1990s. With this growth has emerged a complex and often redundant nomenclature, which, although relatively familiar to those within the immediate field, has the potential to generate confusion as coactivators and corepressors come under increasing scrutiny in other, less familiar disciplines. We discuss this issue with reference to some specific examples and proffer some guidelines to the community for identifying these molecules.

MUC1 expression is repressed by protein inhibitor of activated signal transducer and activator of transcription-y

Mucin 1 (MUC1) is a transmembrane glycoprotein that modulates the interaction between the embryo and the uterine epithelial cell surface. MUC1 also is a tumor marker and has been implicated in the protection of cancer cells from immune cell attack as well as in cell signaling in some tumors. We and others have shown that MUC1 expression is activated by progesterone (P), TNF-alpha, and interferon-gamma (IFN-gamma). Here we demonstrate that MUC1 expression is down-regulated by overexpression of members of the protein inhibitor of activated signal transducer and activator of transcription (PIAS) family, PIAS1, PIAS3, PIASxalpha, PIASxbeta, and PIASy, in human uterine epithelial cell lines HES and HEC-1A and in a breast cancer cell line, T47D. Treatments with P, TNF-alpha, and IFN-gamma were unable to overcome the repression by PIASy. PIASy repression of basal, P-, and TNF-alpha-stimulated MUC1 promoter activity was not dependent on the PIASy sumoylation domain. In contrast, PIASy suppression of IFN-gamma-activated MUC1 promoter activity was dependent on the PIASy sumoylation domain. PIASy and P receptor B were localized to the nucleus upon P treatment, and small interfering RNA knockdown of PIASy resulted in an increase in P-mediated stimulation of MUC1 protein expression. Overexpression of PIASy did not affect P receptor B binding to the MUC1 promoter but surprisingly led to a loss of nuclear receptor corepressor (NCoR), which was recruited to the promoter in response to P. Collectively, these data indicate that PIASy may be a useful target for down-regulation of MUC1 expression in various contexts.

The Fas ligand intracellular domain is released by ADAM10 and SPPL2a cleavage in T-cells

Fas ligand (FasL) is a type II transmembrane protein belonging to the tumor necrosis factor family. Its binding to the cognate Fas receptor triggers the apoptosis that plays a pivotal role in the maintenance of immune system homeostasis. The cell death-inducing property of FasL has been associated with its extracellular domain, which can be cleaved off by metalloprotease activity to produce soluble FasL. The fate of the remaining membrane-anchored N-terminal part of the FasL molecule has not been determined. Here we show that post-translational processing of overexpressed and endogenous FasL in T-cells by the disintegrin and metalloprotease ADAM10 generates a 17-kDa N-terminal fragment, which lacks the receptor-binding extracellular domain. This FasL remnant is membrane anchored and further processed by SPPL2a, a member of the signal peptide peptidase-like family of intramembrane-cleaving proteases. SPPL2a cleavage liberates a smaller and highly unstable fragment mainly containing the intracellular FasL domain (FasL ICD). We show that this fragment translocates to the nucleus and is capable of inhibiting gene transcription. With ADAM10 and SPPL2a we have identified two proteases implicated in FasL processing and release of the FasL ICD, which has been shown to be important for retrograde FasL signaling.

New insights into thyroid hormone action

Thyroid hormones (THs) have important effects on cellular development, growth, and metabolism. They bind to thyroid hormone receptors (TRs), TRalpha and TRbeta, 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.

Antithyroid drugs inhibit thyroid hormone receptor-mediated transcription

CONTEXT

Methimazole (MMI) and propylthiouracil (PTU) are widely used as antithyroid drugs (ATDs) for the treatment of Graves' disease. Both MMI and PTU reduce thyroid hormone levels by several mechanisms, including inhibition of thyroid hormone synthesis and secretion. In addition, PTU decreases 5'-deiodination of T(4) in peripheral tissues. ATDs may also interfere with T(3) binding to nuclear thyroid hormone receptors (TRs). However, the effect of ATDs on the transcriptional activities of T(3) mediated by TRs has not been studied.

OBJECTIVE

The present study was undertaken to determine whether ATDs have an effect on the gene transcription regulated by T(3) and TRs in vitro.

METHODS

Transient gene expression experiments and GH secretion assays were performed. To elucidate possible mechanisms of the antagonistic action of ATDs, the interaction between TR and nuclear cofactors was examined.

RESULTS

In the transient gene expression experiments, both MMI and PTU significantly suppressed transcriptional activities mediated by the TR and T(3) in a dose-dependent manner. In mammalian two-hybrid assays, both drugs recruited one of the nuclear corepressors, nuclear receptor corepressor, to the TR in the absence of T(3). In addition, PTU dissociated nuclear coactivators, such as steroid receptor coactivator-1 and glucocorticoid receptor interacting protein-1, from the TR in the presence of T(3). Finally, MMI decreased the GH release that was stimulated by T(3).

CONCLUSIONS

ATDs inhibit T(3) action by recruitment of transcriptional corepressors and/or dissociation of coactivators. This is the first report to show that ATDs can modulate T(3) action at the transcriptional level.

Essential role of GATA2 in the negative regulation of thyrotropin beta gene by thyroid hormone and its receptors

Previously we reported that the negative regulation of the TSHbeta gene by T(3) and its receptor [thyroid hormone receptor (TR)] is observed in CV1 cells when GATA2 and Pit1 are introduced. Using this system, we further studied the mechanism of TSHbeta inhibition. The negative regulatory element (NRE), which had been reported to mediate T(3)-bound TR (T(3)-TR)-dependent inhibition, is dispensable, because deletion or mutation of NRE did not impair suppression. The reporter construct, TSHbeta-D4-chloramphenicol acetyltransferase, which possesses only the binding sites for Pit1 and GATA2, was activated by GATA2 alone, and this transactivation was specifically inhibited by T(3)-TR. The Zn finger region of GATA2 interacts with the DNA-binding domain of TR in a T(3)-independent manner. The suppression by T(3)-TR was impaired by overexpression of a dominant-negative type TR-associated protein (TRAP) 220, an N- and C-terminal deletion construct, indicating the participation of TRAP220. Chromatin immunoprecipitation assays with a thyrotroph cell line, TalphaT1, revealed that T(3) treatment recruited histone deacetylase 3, reduced the acetylation of histone H4, and caused the dissociation of TRAP220 within 15-30 min. The reduction of histone H4 acetylation was transient, whereas the dissociation of TRAP220 persisted for a longer period. In the negative regulation of the TSHbeta gene by T(3)-TR we report that 1) GATA2 is the major transcriptional activator of the TSHbeta gene, 2) the putative NRE previously reported is not required, 3) TR-DNA-binding domain directly interacts with the Zn finger region of GATA2, and 4) histone deacetylation and TRAP220 dissociation are important.

Peptide aptamers as guides for small-molecule drug discovery

Peptide aptamers are combinatorial protein reagents that bind to target proteins with a high specificity and a strong affinity. By so doing, they can modulate the function of their cognate targets. Because peptide aptamers introduce perturbations that are similar to those caused by therapeutic molecules, their use identifies and/or validates therapeutic targets with a higher confidence level than is typically provided by methods that act upon protein expression levels. The unbiased combinatorial nature of peptide aptamers enables them to 'decorate' numerous polymorphic protein surfaces, whose biological relevance can be inferred through characterization of the peptide aptamers. Bioactive aptamers that bind druggable surfaces can be used in displacement screening assays to identify small-molecule hits to the surfaces. The peptide aptamer technology has a positive impact on drug discovery by addressing major causes of failure and by offering a seamless, cost-effective process from target validation to hit identification.

The Lipoprotein Lipase Inhibitor ANGPTL3 Is Negatively Regulated by Thyroid Hormone

Whereas the role of thyroid hormone is clearly established in the regulation of cholesterol homeostasis, its involvement in the control of serum triglyceride (TG) levels remains largely debated. Angiopoietin-like proteins 3 and 4 have recently been characterized as potent lipoprotein lipase inhibitors and therefore as important components of plasma triglyceride homeostasis. In the present study, the role of thyroid hormone in the regulation of both ANGPTL4 and ANGPTL3 gene expression was investigated. In vivo studies revealed that thyroid hormone down-regulates ANGPTL3 but not ANGPTL4 gene expression in hypothyroid rats. Using thyroid hormone receptor (TR)-deficient mice, we show that thyroid hormone regulates ANGPTL3 gene expression in a TRbeta-dependent manner. Transfection studies revealed that this inhibition occurs at the transcriptional level in a DNA binding-independent fashion and requires the proximal (-171 to +66) region of the ANGPTL3 gene promoter. Moreover, site-directed mutagenesis experiments indicate that the HNF1 site within this proximal region mediates this TRbeta-dependent repression. Finally, co-transfection studies and electrophoretic mobility shift assays suggest that TRbeta antagonizes the HNF1alpha signaling pathway by inhibiting its transcriptional activity without interfering with its DNA-binding capacity. Taken together, our results lead to the identification of ANGPTL3 as a novel TRbeta target gene and provide a new potential mechanism to explain the hypotriglyceridemic properties of TRbeta agonists in vivo.

Deregulated activation of oncoprotein kinase Tpl2/Cot in HTLV-I-transformed T cells

Protein kinase Tpl2/Cot is encoded by a protooncogene that is cis-activated by retroviral insertion in murine T cell lymphomas. It has remained unclear whether this oncoprotein kinase is mutated or post-translationally activated in human cancer cells. We have shown here that Tpl2/Cot is constitutively activated in human leukemia cell lines transformed by the human T cell leukemia virus type I (HTLV-I). The kinase activity of Tpl2/Cot is normally suppressed through its physical interaction with an inhibitor, the NF-kappaB1 precursor protein p105. Interestingly, a large pool of Tpl2/Cot is liberated from p105 and exhibits constitutive kinase activity in HTLV-I-transformed T cells. In contrast to its labile property in normal cells, the pathologically activated Tpl2/Cot is remarkably stable. Further, whereas the physiological activation of Tpl2/Cot involves its long isoform, the HTLV-activated Tpl2/Cot is predominantly the short isoform. We have also shown that the HTLV-I-encoded Tax protein is able to activate Tpl2/Cot in transfected cells. Finally, Tpl2/Cot participates in the activation of NF-kappaB by Tax. These findings indicate that deregulated activation of Tpl2/Cot may occur in human cancer cells.

Glucocorticoid receptor regulates ATP-binding cassette transporter-A1 expression and apolipoprotein-mediated cholesterol efflux from macrophages

OBJECTIVE

The ATP-binding cassette transporter-A1 (ABCA1) regulates cholesterol efflux from cells and is involved in high-density lipoprotein metabolism and atherogenesis. The objective of this study was to investigate the effect of dexamethasone (Dex) and other glucocorticoid receptor (GR) ligands on apolipoprotein AI-mediated cholesterol efflux from macrophages and ABCA1 expression in them.

METHODS AND RESULTS

Dex, a GR agonist, decreased ABCA1 mRNA levels in a dose- and time-dependent fashion, and RU486, a GR antagonist, reversed the inhibitory effect of Dex. The effects of Dex and RU486 on ABCA1 protein levels and apolipoprotein AI-mediated cholesterol efflux from the macrophages were consistent with these changes in mRNA levels. Transfected RAW264.7, together with a human ABCA1 promoter-luciferase construct, inhibited transcriptional activity by Dex and overexpression of human GR. Transrepression by GR was not mediated by liver X receptor (LXR), because there were no differences in the effects of the GR ligands on promoter activity between a reporter construct with mutations at the LXR binding site and one without the mutations, and no changes were brought about in ABCG1 and ABCG4 expression by GR ligands.

CONCLUSIONS

Our results showed that GR ligands affected ABCA1 expression and cholesterol efflux from macrophages, which are regulated by GR through a LXR-independent mechanism.

Negative regulation by thyroid hormone receptor requires an intact coactivator-binding surface

Thyroid hormone (TH) action is mediated by TH receptors (TRs), which are members of the nuclear hormone receptor superfamily. In vitro studies have demonstrated that TR activity is regulated by interactions with corepressor and coactivator proteins (CoRs and CoAs, respectively). TH stimulation is thought to involve dissociation of CoRs and recruitment of CoAs to the liganded TR. In contrast, negative regulation by TH is thought to occur via recruitment of CoRs to the liganded TR. The physiological role of CoAs bound to TRs, however, has yet to be defined. In this study, we used gene-targeting techniques to mutate the TR-beta locus within its activation function-2 (AF-2) domain (E457A). This mutation was chosen because it completely abolished CoA recruitment in vitro, while preserving normal triiodothyronine (T3) binding and CoR interactions. As expected, TH-stimulated gene expression was reduced in homozygous E457A mice. However, these animals also displayed abnormal regulation of the hypothalamic-pituitary-thyroid axis. Serum thyroxine, T3, and thyroid-stimulating hormone (TSH) levels and pituitary Tshb mRNA levels were inappropriately elevated compared with those of WT animals, and L-T3 treatment failed to suppress serum TSH and pituitary Tshb mRNA levels. Therefore, the AF-2 domain of TR-beta is required for positive and, paradoxically, for negative regulation by TH in vivo.

E1A and a nuclear receptor corepressor splice variant (N-CoRI) are thyroid hormone receptor coactivators that bind in the corepressor mode

Unliganded thyroid hormone (TH) receptors (TRs) and other nuclear receptors (NRs) repress transcription of hormone-activated genes by recruiting corepressors (CoRs), such as NR CoR (N-CoR) and SMRT. Unliganded TRs also activate transcription of TH-repressed genes. Some evidence suggests that these effects also involve TR/CoR contacts; however, the precise reasons that CoRs activate transcription in these contexts are obscure. Unraveling these mechanisms is complicated by the fact that it is difficult to decipher direct vs. indirect effects of TR-coregulator contacts in mammalian cells. In this study, we used yeast, Saccharomyces cerevisiae, which lack endogenous NRs and NR coregulators, to determine how unliganded TRs can activate transcription. We previously showed that adenovirus 5 early-region 1A coactivates unliganded TRs in yeast, and that these effects are blocked by TH. We show here that human adenovirus type 5 early region 1A (E1A) contains a short peptide (LDQLIEEVL amino acids 20-28) that resembles CoR-NR interaction motifs (CoRNR boxes), and that this motif is required for TR binding and coactivation. Although full-length N-CoR does not coactivate TR in yeast, a naturally occurring N-CoR variant (N-CoR(I)) and an artificial N-CoR truncation (N-CoR(C)) that retain CoRNR boxes but lack N-terminal repressor domains behave as potent and direct TH-repressed coactivators for unliganded TRs. We conclude that E1A and N-CoR(I) are naturally occurring TR coactivators that bind in the typical CoR mode and suggest that similar factors could mediate transcriptional activation by unliganded TRs in mammals.

A Novel Mechanism of Thyroid Hormone-dependent Negative Regulation by Thyroid Hormone Receptor, Nuclear Receptor Corepressor (NCoR), and GAGA-binding Factor on the Rat CD44 Promoter

CD44 is an adhesion molecule in the extracellular matrix that shows various functions, including tumor genesis and metastasis. A recent study showed that CD44 expression level was strongly correlated with the generation of papillary thyroid carcinomas, the most prevalent malignancy of the thyroid gland. We report here that CD44 is negatively regulated by thyroid hormone (T(3)) through a novel mechanism. We demonstrate that nuclear receptor corepressor (NCoR) enhances thyroid hormone receptor (TR)-mediated basal transactivation by a weak TR.DNA interaction in the absence of T(3), which is repressed by T(3) through a transient TR .DNA interaction. Initially, we identified that CD44 was negatively directly transcriptionally T(3) -responsive. Deletion and mutation analysis indicated that both a weak TR and a GAGA-binding factor (GAF) binding sites on the CD44 promoter were required for negative regulation by T(3). The weak TR.DNA interaction was further confirmed by electrophoretic gel mobility shift assay, chromatin immunoprecipitation, and transfection assays using a non-DNA-binding TRalpha1 mutant. More interestingly, NCoR acted as a co-activator to enhance TR-mediated basal transactivation in the absence of T(3). This effect was eliminated by removal of TR or NCoR binding. Most strikingly, T(3) induced a remarkable increase in TR.DNA binding at 40-60 min after T(3) exposure that rapidly returned to basal levels, suggesting a T(3)-induced remodeling of chromatin structure at the early stage of T(3) stimulation resulting in repression. Therefore, we propose a mechanism by which NCoR, GAF, and TR interact with the CD44 negative T(3)-responsive element to enhance basal transactivation, whereas T(3) induces the remodeling of chromatin structure for repression.

Acute LiCl-treatment affects the cytoplasmic T4 availability and the expression pattern of thyroid hormone receptors in adult rat cerebral hemispheres

We have previously reported that short-term LiCl-treatment affects the kinetic characteristics of thyroid hormone binding in adult rat brain (Bolaris, S., Margarity, M., Valcana, T., 1995. Effects of LiCl on triiodothyronine (T3) binding to nuclei from rat cerebral hemispheres. Biol. Psychiatry 37, 106-111); however, the mechanism underlying the above effects of LiCl administration is yet to be determined. In this study, the effects of lithium within one day after its administration (5 mmol/kg BW) on the relative expression of thyroid hormone receptor isoforms and on the cytoplasmic and synaptosomal thyroid hormone availability in adult rat cerebral hemispheres were examined. Although short-term LiCl-treatment did not affect the levels of triiodothyronine either in the synaptosomal or in the cytoplasmic fraction 24 h after LiCl administration, the cytoplasmic availability of thyroxin was lower. In addition, 24 h after the administration of lithium the mRNA levels of the TRalpha1 isoform (T3 binding) increased while the relative expression of the TRalpha2 variant (non-T3 binding) was decreased. Notably, the decrease of the TRalpha2 mRNA levels was also observed 4h after LiCl administration. The expression levels of the TRbeta1 isoform were unaffected in any interval examined. The present study suggests that short-term lithium treatment regulates the relative expression of TRs in an isoform-specific manner and affects the cytoplasmic availability of thyroxin in adult rat brain.

Clinical and molecular features of a TSH-secreting pituitary microadenoma

We describe a case of a thyroid stimulating hormone (TSH)-secreting pituitary microadenoma, and report the systematic gene expression profile of the surgically- removed tumor. A 50-year-old woman was referred to our hospital because she had high TSH, free-T4, and free-T3 levels, and a pituitary tumor that was visualized with magnetic resonance imaging. Her basal TSH level was high even after a high T3 loading dose, and increased following administration of thyroid releasing hormone (TRH) even after administration of a high dose of exogenous T3. Her clinical symptoms and peripheral markers for T3 were responsive to exogenous T3. There was no thyroid hormone receptor (TR) beta gene mutation. The patient was diagnosed with a TSH-secreting pituitary adenoma, and trans-sphenoid surgery was performed. The histologic features and immunophenotype were consistent with a TSH-secreting pituitary adenoma. Reverse transcription-polymerase chain reaction analysis of pituitary hormones, pituitary-specific transcription factors, receptors, and transcriptional cofactors of clinical significance was performed on the removed tumor. The tumor expressed TSH, growth hormone, prolactin, alpha-subunit, pituitary transcription factor-1 (pit-1) but not proopiomelanocortin (POMC), prophet of pit-1 (prop-1) and pituitary cell-restricted T box factor (Tpit). TRbeta and TRH-receptor gene expression was normal. Three steroid receptor coactivators (SRC)-1, SRC-2, and SRC-3 were expressed. Nuclear receptor corepressor (N-CoR)2 was absent in the tumor, whereas nuclear receptor corepressor (N-CoR1) was expressed. Somatostatin receptor type 1 expression was significantly decreased, whereas type 4 receptor was expressed, which are unusual characteristics for pituitary tumors. The gene expression pattern in the tumor might have a role in the clinical features of this case.

A novel cell type-specific mechanism for thyroid hormone-dependent negative regulation of the human type 1 deiodinase gene

We have identified a cell type-specific, negative thyroid hormone-responsive element in the human type 1 iodothyronine deiodinase (hdio1) gene. This fragment, termed a JEG response element, bound tightly to a JEG-cell nuclear protein [JEG cell-specific transcription factor (JTF)] also present in placenta but not in COS-7, HeLa, or human embryonic kidney-293 cells. In JEG-3 cells, three copies of the JEG response element conferred a more than 40-fold transcriptional stimulation to the heterologous rat GH promoter which was further increased 2-fold by apo-thyroid hormone receptor (TR) and reduced 3-fold by T(3). Dimethyl sulfide footprinting showed overlapping contact sites for the high-affinity interaction of JTF and low-affinity binding of TR-retinoid X receptor. Expression of the same construct was unaffected by TR or T(3) in COS cells, indicating JTF was required for negative regulation by T(3)-TR. Mutations of the critical thyroid hormone responsive element binding P box amino acids EG to GS in TRalpha1 or TRbeta2 eliminated the apo-TR and T(3)-TR effects. These studies identify a novel mechanism for cell type-specific, promoter-independent negative regulation by T(3).

Thyroid-hormone-dependent negative regulation of thyrotropin beta gene by thyroid hormone receptors: study with a new experimental system using CV1 cells

The molecular mechanism involved in the liganded thyroid hormone receptor suppression of the TSHbeta (thyroid-stimulating hormone beta, or thyrotropin beta) gene transcription is undetermined. One of the main reasons is the limitation of useful cell lines for the experiments. We have developed an assay system using non-pituitary CV1 cells and studied the negative regulation of the TSHbeta gene. In CV1 cells, the TSHbeta-CAT (chloramphenicol acetyltransferase) reporter was stimulated by Pit1 and GATA2 and suppressed by T3 (3,3',5-tri-iodothyronine)-bound thyroid hormone receptor. The suppression was dependent on the amounts of T3 and the receptor. Unliganded receptor did not stimulate TSHbeta activity, suggesting that the receptor itself is not an activator. Analyses using various receptor mutants revealed that the intact DNA-binding domain is crucial to the TSHbeta gene suppression. Co-activators and co-repressors are not necessarily essential, but are required for the full suppression of the TSHbeta gene. Among the three receptor isoforms, beta2 exhibited the strongest inhibition and its protein level was the most predominant in a thyrotroph cell line, TalphaT1, in Western blotting. The dominant-negative effects of various receptor mutants measured on the TSHbeta-CAT reporter were not simple mirror images of those in the positive regulation under physiological T3 concentration.

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.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

A Response Unit in the First Exon of the β-Amyloid Precursor Protein Gene Containing Thyroid Hormone Receptor and Sp1 Binding Sites Mediates Negative Regulation by 3,5,3′-Triiodothyronine

Thyroid hormones repress expression of APP (beta-amyloid precursor protein) in cultured cells of neuronal origin. The effect involves binding to the nuclear thyroid hormone receptor (TR) and is mediated by DNA sequences located within the first exon of the gene. These sequences contain a thyroid hormone response element that is necessary, but not sufficient, to mediate the inhibitory effect of the thyroid hormone T(3). In this report, we show that repression by T(3) is mediated by a response unit composed by the thyroid hormone response element and 5'-flanking sequences that bind Sp1 and mediate stimulation by this transcription factor. In that unit, binding sites for TR and Sp1 overlap and a complex mechanism appears to account for the TR-mediated regulation of APP. Unliganded TR does not bind to DNA and allows Sp1 to bind to DNA and stimulate APP basal expression. Binding of ligand T(3), which increases affinity of TR by DNA, precludes binding of Sp1 to DNA and decreases the Sp1-dependent expression of APP.

Gene silencing by the thyroid hormone receptor

The thyroid hormone receptors (TR) are able to bind DNA and to repress transcription in the absence of thyroid hormone. This repression function is an important feature of TRs as aberrant silencing can lead to severe diseases and developmental abnormalities. TR utilizes different mechanisms to achieve repression of target genes including the recruitment of cofactors called corepressors and interference with the basal transcriptional machinery. Recent studies have revealed an important role of chromatin in TR silencing involving different histone modifications and the responsible enzymes. Furthermore, the transcriptional properties of TR depend on the type of the TR DNA-binding elements. This review will focus on the molecular basis of gene silencing by TR and diseases caused by aberrant functioning.

Cytoplasmic IkappaBalpha increases NF-kappaB-independent transcription through binding to histone deacetylase (HDAC) 1 and HDAC3

IkappaBalpha is an inhibitory molecule that sequesters NF-kappaB dimers in the cytoplasm of unstimulated cells. Upon stimulation, NF-kappaB moves to the nucleus and induces the expression of a variety of genes including IkappaBalpha. This newly synthesized IkappaBalpha also translocates to the nucleus, removes activated NF-kappaB from its target genes, and brings it back to the cytoplasm to terminate the phase of NF-kappaB activation. We show here that IkappaBalpha enhances the transactivation potential of several homeodomain-containing proteins such as HOXB7 and Pit-1 through a NF-kappaB-independent association with histone deacetylase (HDAC) 1 and HDAC3 but not with HDAC2, -4, -5, and -6. IkappaBalpha bound both HDAC proteins through its ankyrin repeats, and this interaction was disrupted by p65. Immunofluorescence experiments demonstrated further that IkappaBalpha acts by partially redirecting HDAC3 to the cytoplasm. At the same time, an IkappaBalpha mutant, which lacked a functional nuclear localization sequence, interacted very efficiently with HDAC1 and -3 and intensively enhanced the transactivation potential of Pit-1. Our results support the hypothesis that the NF-kappaB inhibitor IkappaBalpha regulates the transcriptional activity of homeodomain-containing proteins positively through cytoplasmic sequestration of HDAC1 and HDAC3, a mechanism that would assign a new and unexpected role to IkappaBalpha.

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

Nuclear thyroid hormone receptors (TRs) act as ligand-dependent activators, but paradoxically unliganded TRs can increase transcription of promoters containing negative response elements (nTRE), and hormone binding represses this activation. The rat growth hormone (GH) promoter contains a positive TRE and a nTRE. Ligand-dependent negative regulation mediated by the nTRE could play an important physiological role in restricting GH gene expression in non-pituitary cells that express TRs. With chromatin immunoprecipitation assays, we show here that the nTRE is responsible for binding of TR to the promoter in non-pituitary HeLa cells and that this element also governs transactivation by the unoccupied receptor and repression by triiodothyronine. Occupancy of the promoter by TR is concomitant with appearance of acetylated histone H3, and triiodothyronine causes release of the receptor as well as disappearance of the acetylated histone from the promoter. Although the nTRE overlaps the TATA box, the receptor does not exclude binding of TATA-binding protein, but could rather facilitate formation of the preinitiation complex. Furthermore, the proximal GH promoter is synergistically stimulated by unliganded TR and TATA-binding protein, whereas the ligand represses this cooperation. Constitutive receptor activity and synergism with TATA-binding protein require binding of corepressors. Furthermore, inhibitors of histone deacetylases enhance promoter activation by the unliganded receptor and reduce triiodothyronine-dependent repression, whereas expression of HDAC1 reverses promoter stimulation. This suggests that partitioning of histone acetylases and deacetylases between the receptors and basal transcription factors could be involved in regulation of the basal GH promoter by TRs.

Thyroid hormone action: a binding contract

Thyroid hormones are critical for differentiation, growth, and metabolism. A new study investigating the biological role of the TH receptor TR-beta has demonstrated that DNA binding is critical for most of its functions, but also suggests that novel mechanisms independent of DNA binding may contribute to regulation of auditory function by TR-beta.

Thyroid hormone action in the absence of thyroid hormone receptor DNA-binding in vivo

Thyroid hormone action is mediated by thyroid hormone receptors (TRs), which are members of the nuclear hormone receptor superfamily. DNA-binding is presumed to be essential for all nuclear actions of thyroid hormone. To test this hypothesis in vivo, the DNA-binding domain of TR-beta was mutated within its P-box (GS mutant) using gene targeting techniques. This mutation in vitro completely abolishes TR-beta DNA-binding, while preserving ligand (T3) and cofactor interactions with the receptor. Homozygous mutant (TR-betaGS/GS) mice displayed abnormal T3 regulation of the hypothalamic-pituitary-thyroid axis and retina identical to abnormalities previously observed in TR-beta KO (TR-beta-/-) mice. However, TR-betaGS/GS mutant mice maintained normal hearing at certain frequencies and did not display significant outer hair cell loss, in contrast to TR-beta-/- mice. DNA-binding, therefore, is essential for many functions of the TR, including retinal development and negative feedback regulation by thyroid hormone of the hypothalamic-pituitary-thyroid axis. Inner ear development, although not completely normal, can occur in the absence of TR DNA-binding, suggesting that an alternative and perhaps novel thyroid hormone-signaling pathway may mediate these effects.

Thyroid Hormone Action: Insight from Transgenic Mouse Models

Thyroid hormone receptors (TRs) are cellular homologues of the viral erythroblastic leukemia oncogene (v-erbA). TRs (c-crbA isoforms) are derived from two separate gene loci in mammals: a and p. Through a series of knockout experiments in mice in which one or several of the TR isoforms were deleted, it has been demonstrated that the TR-β isoforms control central regulation of thyroid-stimulating hormone. Of these isoforms, TR-β2 is the most important in mediating negative feedback control of the hypothalamic-pituitary-thyroid axis. Further analysis of TR knockout animals revealed, however, that they exhibited a much milder overall phenotype than hypothyroid animals, indicating that receptor loss was not equivalent to ligand loss in vivo. To understand this apparent paradox, we generated animals expressing a non-T3 binding receptor (Δ337T) from the TR-β allele. These mice displayed a complete hypothyroid phenotype, demonstrating that the unliganded TR mediates the effect of hypothyroidism. Because this mutant TR constitutively binds to nuclear coreprssors, it also suggests that this class of proteins is essential for mediating hypothyroidism in vivo.

Nuclear receptor corepressors

The ability of NR LBDs to transfer repression function to a heterologous DNA binding domain, and the cross-squelching of repression by untethered LBDs, has suggested that repression is mediated by interactions with putative cellular corepressor proteins. The yeast-two hybrid screen for protein interactors has proven to be the key to the isolation and characterization of corepressors. This short review will focus on N-CoR and SMRT.

Effects of ligand and thyroid hormone receptor isoforms on hepatic gene expression profiles of thyroid hormone receptor knockout mice

Little is known about the overall patterns of thyroid hormone (Th)-mediated gene regulation by the main Th receptor (Tr) isoforms, Tr-alpha and Tr-beta, in vivo. We used 48 complementary DNA microarrays to examine hepatic gene expression profiles of wild-type and Thra and Thrb knockout mice under different Th conditions: no treatment, treatment with 3,3',5-triiodothyronine (T(3)), Th-deprivation using propylthiouracil (PTU), and treatment with a combination of PTU and T(3). Hierarchical clustering analyses showed that positively regulated genes fit into three main expression patterns. In addition, only a subpopulation of target genes repressed basal transcription in the absence of ligand. Interestingly, Thra and Thrb knockout mice showed similar gene expression patterns to wild-type mice, suggesting that these isoforms co-regulate most hepatic target genes. Differences in the gene expression patterns of Thra/Thrb double-knockout mice and Th-deprived wild-type mice show that absence of receptor and of hormone can have different effects. This large-scale study of hormonal regulation reveals the functions of Th and of Tr isoforms in the regulation of gene expression patterns.

Retinoic acid repression of bone morphogenetic protein 4 in inner ear development

Bone morphogenetic protein 4 (BMP4) and retinoic acid are important for normal development of the inner ear, but whether they are linked mechanistically is not known. BMP4 antagonists disrupt semicircular canal formation, as does exposure to retinoic acid. We demonstrate that retinoic acid directly down-regulates BMP4 transcription in a mouse inner ear-derived cell line, and we identify a novel promoter in the second intron of the BMP4 gene that is a target of this regulation both in the cell line and in the mouse embryonic inner ear in vivo. The importance of this down-regulation is demonstrated in chicken embryos by showing that the retinoic acid effect on semicircular canal development can be overcome by exogenous BMP4.

Thyroid hormone receptor DNA binding is required for both positive and negative gene regulation

The beta isoform of thyroid hormone receptor (TR-beta) has a key role in the feedback regulation of the hypothalamic-pituitary-thyroid (H-P-T) axis. The mechanism of trans-repression of the hypothalamic thyrotropin-releasing hormone (TRH) and pituitary thyroid-stimulating hormone (TSH) subunit genes, however, remains poorly understood. A number of distinct mechanisms for TR-beta-mediated negative regulation by thyroid hormone have been proposed, including those that require and do not require DNA binding. To clarify the importance of DNA binding in negative regulation, we constructed a DNA-binding mutant of TR-beta in which two amino acids within the P box were altered (GSG for EGG) to resemble that found in the glucocorticoid receptor (GR). We termed this mutant GS125, and as expected, it displayed low binding affinities for positive and negative thyroid hormone-response element (pTRE and nTRE, respectively) in gel-mobility shift assays. In transient transfection assays, the GS125 mutant abolished transactivation on three classic pTREs (DR+4, LAP, and PAL) and all negatively regulated promoters in the H-P-T axis (TRH, TSH-beta, and TSH-alpha). However, GS125 TR-beta bound to a composite TR/GR-response element and was fully functional on this hybrid TR/GR-response element. Moreover, the GS125 TR-beta mutant displayed normal interactions with transcriptional cofactors in mammalian two-hybrid assays. These data do not support a DNA-binding independent mechanism for thyroid hormone negative regulation in the H-P-T axis.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

Corepressor SMRT functions as a coactivator for thyroid hormone receptor T3Ralpha from a negative hormone response element

Nuclear receptors are ligand-modulated transcription factors that transduce the presence of lipophilic ligands into changes in gene expression. Nuclear receptor activity is regulated by ligand-induced interactions with coactivator or corepressor molecules. From a positive hormone response element (pHRE) and in the absence of hormone, corepressors SMRT and N-CoR are bound to some nuclear receptors such as the thyroid hormone (T3Rs) and retinoic acid receptors and mediate inhibition of basal levels of transcription. Ligand binding results in dissociation of corepressors and association of coactivators, resulting in the reversal of inhibition and a net activation of transcription. However, the role of cofactors on the activity of nuclear receptors from negative HREs (nHREs) is poorly understood. Here we show that corepressor SMRT can act as a potent coactivator for T3Ralpha from a nHRE; N-CoR has a similar but significantly attenuated activity. Mutagenesis of residues in the hinge region of T3Ralpha that block binding of SMRT and N-CoR inhibits ligand-independent transcriptional activation by T3Ralpha from a nHRE. These mutations also abrogate SMRT-mediated increase in transcriptional activity by T3Ralpha at a nHRE without significantly affecting ligand-dependent activation at a pHRE. Partial protease digestion coupled to the mobility shift assay indicate differences in the conformation of T3Ralpha-SMRT complexes bound to a pHRE versus a nHRE. These results suggest that allosteric changes resulting from binding of T3Ralpha to different response elements, i.e. pHREs versus nHREs, dictate whether a cofactor will function as a coactivator or a corepressor. This, in turn, greatly expands the repertoire of mechanisms used in modulating transcription without the need for expression of new regulatory molecules.

Transcriptional regulation of the human glycoprotein hormone common alpha subunit gene by cAMP-response-element-binding protein (CREB)-binding protein (CBP)/p300 and p53

We have investigated the functional interactions between adenovirus early region 1A (AdE1A) protein, the co-activators cAMP-response-element-binding protein (CREB)-binding protein (CBP)/p300 and SUG1, and the transcriptional repressor retinoblastoma (Rb) in mediating T3-dependent repression. Utilizing the human glycoprotein hormone common alpha-subunit (alpha-subunit) promoter and AdE1A mutants with selective binding capacity to these molecules we have determined an essential role for CBP/p300. In normal circumstances, wild-type 12 S AdE1A inhibited alpha-subunit activity. In contrast, adenovirus mutants that retain both the SUG1- and Rb-binding sites, but lack the CBP/p300-binding site, were unable to repress promoter activity. We have also identified a role for the tumour-suppressor gene product p53 in regulation of the alpha-subunit promoter. Akin to 12 S AdE1A, exogenous p53 expression repressed alpha-subunit activity. This function resided in the ability of p53 to interact with CBP/p300; an N-terminal mutant incapable of interacting with CBP/p300 did not inhibit alpha-subunit activity. Stabilization of endogenous p53 by UV irradiation also correlated positively with reduced alpha-subunit activity. Intriguingly, T3 stimulated endogenous p53 transcriptional activity, implicating p53 in T3-dependent signalling pathways. These data indicate that CBP/p300 and p53 are key regulators of alpha-subunit activity.

Thyroid hormone action is disrupted by bisphenol A as an antagonist

Bisphenol A (BPA), a monomer of polycarbonate plastics, has been shown to possess estrogenic properties and act as an agonist for the estrogen receptors. Although an epidemiologically based investigation has suggested that some chemicals could disrupt thyroid function in animals, the effects on thyroid hormone receptors (TRs) are unknown. We show here that BPA inhibits TR-mediated transcription by acting as an antagonist. In the transient gene expression experiments, BPA suppressed transcriptional activity that is stimulated by thyroid hormone (T(3)) in a dose-dependent manner. The inhibitory effects were observed in the presence of physiological concentrations of T(3). In contrast, in the case of negatively regulated TSHalpha promoter, BPA activated the gene transcription that is suppressed by T(3). To elucidate possible mechanisms of the antagonistic action of BPA, the effects on T(3) binding and cofactor interaction with TR were examined. The K(i) value for BPA was 200 micro M when assessed by inhibition of [(125)I]T(3) binding to rat hepatic nuclear TRs. In a mammalian two-hybrid assay, BPA recruited the nuclear corepressor to the TR. These results suggest that BPA could displace T(3) from the TR and recruit a transcriptional repressor, resulting in gene suppression. This is the first report that BPA can antagonize T(3) action at the transcriptional level. BPA may disrupt the function of various types of nuclear hormone receptors and their cofactors to disturb our internal hormonal environment.

Regulation of gene promoters of hypothalamic peptides

In order to fulfill their roles in neuroendocrine regulation, specific hypothalamic neurons are devoted to produce and deliver biologically active peptides to the pituitary gland. The biosynthesis and release of peptides are strictly controlled by afferents to these hypothalamic neurons. Cell-specific expression and biosynthetic regulation largely relies on transcription from the gene promoter for which the 5(')-flanking regions of the peptidergic genes contain essential elements. Cell-specific transcription factors employ these regulatory elements to exert their control over the expression of the peptidergic gene. This article explores the properties of regulatory elements of the major hypothalamic peptides, somatostatin, growth hormone-releasing hormone, gonadotropin-releasing hormone, thyrotropin-releasing hormone, corticotropin-releasing hormone, vasopressin and oxytocin, and the transcription factors acting on them. These transcription factors are often endpoints of signal transduction pathways that can be activated by neurotransmitters or steroid hormones. Others are essential to provide cell-specific expression of the peptidergic gene during development and mature regulation.

Feedback on hypothalamic TRH transcription is dependent on thyroid hormone receptor N terminus

The beta thyroid hormone receptor (TRbeta), but not TRalpha1, plays a specific role in mediating T(3)-dependent repression of hypothalamic TRH transcription. To investigate the structural basis of isoform specificity, we compared the transcriptional regulation and DNA binding obtained with chimeric and N-terminally deleted TRs. Using in vivo transfection assays to follow hypothalamic TRH transcription in the mouse brain, we found that TRbeta1 and chimeras with the TRbeta1 N terminus did not affect either transcriptional activation or repression from the rat TRH promoter, whereas N-terminally deleted TRbeta1 impaired T(3)-dependent repression. TRalpha1 or chimeras with the TRalpha1 N terminus reduced T(3)-independent transcriptional activation and blocked T(3)-dependent repression of transcription. Full deletion of the TRalpha1 N terminus restored ligand-independent activation of transcription. No TR isoform specificity was seen after transcription from a positive thyroid hormone response element. Gel mobility assays showed that all TRs tested bound specifically to the main negative thyroid hormone response element in the TRH promoter (site 4). Addition of neither steroid receptor coactivator 1 nor nuclear extracts from the hypothalamic paraventricular nuclei revealed any TR isoform specificity in binding to site 4. Thus N-terminal sequences specify TR T(3)-dependent repression of TRH transcription but not DNA recognition, emphasizing as yet unknown neuron-specific contributions to protein-promoter interactions in vivo.

The N-CoR-HDAC3 nuclear receptor corepressor complex inhibits the JNK pathway through the integral subunit GPS2

The corepressors N-CoR and SMRT partner with histone deacetylases (HDACs) in diverse repression pathways. We report here that GPS2, a protein involved in intracellular signaling, is an integral subunit of the N-CoR-HDAC3 complex. We have determined structural motifs that direct the formation of a highly stable and active deacetylase complex. GPS2 and TBL1, another component of the N-CoR-HDAC3 complex, interact cooperatively with repression domain 1 of N-CoR to form a heterotrimeric structure and are indirectly linked to HDAC3 via an extended N-CoR SANT domain that also activates latent HDAC3 activity. More importantly, we show here that the N-CoR-HDAC3 complex inhibits JNK activation through the associated GPS2 subunit and thus could potentially provide an alternative mechanism for hormone-mediated antagonism of AP-1 function.

Stimulation of the alpha-fetoprotein promoter by unliganded thyroid hormone receptor in association with protein deacetylation

alpha-Fetoprotein (AFP) is a serum protein expressed during fetal life, the expression of which is shut off after birth. The activity of the mouse Afp gene promoter region comprised between -80 and -38 bp is regulated by the thyroid hormone receptor (T3R): negatively in the presence of T3 and positively in the absence of T3. The stimulating effect of unliganded T3R is, unexpectedly, antagonized by cofactors that have histone-acetyl-transferase activity, or by sodium butyrate, which inhibits histone acetylases (HDACs). The unliganded T3R stimulating activity effect is thus associated with protein deacetylation, contrary to the usual situation. In combination with previous results, our observations suggest that T3-mediated down regulation of the Afp promoter is due to T3-induced protein acetylation leading to loss of a nucleosomal structure (required for promoter activity) and chromatin opening.

Congenital hypothyroid Pax8(-/-) mutant mice can be rescued by inactivating the TRalpha gene

Mice devoid of all TRs are viable, whereas Pax8(-/-) mice, which lack the follicular cells producing T4 and T3 in the thyroid gland, die during the first weeks of postnatal life. A precise comparison between the two types of mutants reveals that their phenotypes are similar, but the defects in spleen, bone, and small intestine are more pronounced in Pax8(-/-) mice. This is interpreted as the result of a negative effect of the unliganded TR on thyroid hormone target genes expression in the Pax8(-/-) mutants. Pax8/TRalpha compound mutants can survive to adulthood, and the expression of target genes is partially restored. This demonstrates the importance of TRalpha aporeceptor activity in several aspects of postnatal development.

Retinoid-dependent antagonism of serum response factor transactivation mediated by transcriptional coactivator proteins

Transcriptional coactivators SRC-1 and p300 specifically interact with liganded-nuclear receptors and also modulate other transcription factors, including serum response factor (SRF). Here, we report that retinoids repress transactivation by SRF and specific interactions exist between the DNA binding domains of SRF and retinoic acid and retinoid X receptors. We further demonstrate that the repression may involve retinoid-dependent competition for a limiting amount of SRC-1 and p300 between SRF and retinoid receptors. We propose that the well-defined anti-proliferative action of retinoids could, at least in part, result from this novel transrepressive action on the mitogenic transcription factor SRF.

Chromatin remodeling by the thyroid hormone receptor in regulation of the thyroid-stimulating hormone alpha-subunit promoter

The chromatin architecture of a promoter is an important determinant of its transcriptional response. For most target genes, the thyroid hormone receptor (TR) activates gene expression in response to thyroid hormone (T(3)). In contrast, the thyroid-stimulating hormone alpha-subunit (TSH alpha) gene promoter is down-regulated by TR in the presence of T(3). Here we utilize the capacity for the Xenopus oocyte to chromatinize exogenous nuclear- injected DNA to analyze the chromatin architecture of the TSH alpha promoter and how this changes upon TR-mediated regulation. Interestingly, in the oocyte, the TSH alpha promoter was positively regulated by T(3). In the inactive state, the promoter contained six loosely positioned nucleosomes. The addition of TR/retinoid X receptor together had no effect on the chromatin structure, but the inclusion of T(3) induced strong positioning of a dinucleosome in the TSH alpha proximal promoter that was bordered by regions that were hypersensitive to cleavage by methidiumpropyl EDTA. We identified a novel thyroid response element that coincided with the proximal hypersensitive region. Furthermore, we examined the consequences of mutations in TR that impaired coactivator recruitment. In a comparison with the Xenopus TR beta A promoter, we found that the effects of these mutations on transactivation and chromatin remodeling were significantly more severe on the TSH alpha promoter.

Aberrant alternative splicing of thyroid hormone receptor in a TSH-secreting pituitary tumor is a mechanism for hormone resistance

Patients with TSH-secreting pituitary tumors (TSHomas) have high serum TSH levels despite elevated thyroid hormone levels. The mechanism for this defect in the negative regulation of TSH secretion is not known. We performed RT-PCR to detect mutations in TRbeta from a surgically resected TSHoma. Analyses of the RT-PCR products revealed a 135-bp deletion within the sixth exon that encodes the ligand-binding domain of TRbeta2. This deletion was caused by alternative splicing of TRbeta2 mRNA, as near-consensus splice sequences were found at the junction site and no deletion or mutations were detected in the tumoral genomic DNA. This TRbeta variant (TRbeta2spl) lacked thyroid hormone binding and had impaired T3-dependent negative regulation of both TSHbeta and glycoprotein hormone alpha-subunit genes in cotransfection studies. Furthermore, TRbeta2spl showed dominant negative activity against the wild-type TRbeta2. These findings strongly suggest that aberrant alternative splicing of TRbeta2 mRNA generated an abnormal TR protein that accounted for the defective negative regulation of TSH in the TSHoma. This is the first example of aberrant alternative splicing of a nuclear hormone receptor causing hormonal dysregulation. This novel posttranscriptional mechanism for generating abnormal receptors may occur in other hormone-resistant states or tumors in which no receptor mutation is detected in genomic DNA.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Transgenic targeting of a dominant negative corepressor to liver blocks basal repression by thyroid hormone receptor and increases cell proliferation

Unliganded thyroid hormone receptors (TRs) interact with corepressors and repress basal transcription of target genes in cotransfection and in vitro studies. Currently, little is known about the function of corepressors in vivo. We thus used a mouse albumin promoter to generate several transgenic mouse lines that overexpressed a dominant negative mutant corepressor, NCoRi, in liver. The transgenic mice had normal liver weight, appearance, and minimal changes in enzyme activity. To study the effects of NCoRi on transcription of hepatic target genes, we examined T3-regulated gene expression of hypo- and hyperthyroid transgenic mice. In hypothyroid mice, hepatic expression of Spot 14, Bcl-3, glucose 6-phosphatase, and 5'-deiodinase mRNA was higher in transgenic mice than littermate controls whereas these genes were induced to similar levels in T3-treated mice. Derepression was not observed for malic enzyme mRNA expression in hypothyroid mice. Thus, NCoRi selectively blocked basal transcription of several thyroid hormone-responsive genes but had no effect on ligand-mediated transcription. Additionally, compensatory increases in endogenous SMRT and NCoR mRNA were observed in hypothyroid transgenic mice. Interestingly, hepatocyte proliferation as detected by BrdUrd incorporation was increased in transgenic mice. The gene profile in transgenic mouse livers was studied by cDNA microarray, and several genes related to cell proliferation were induced. In summary, our studies show that NCoR plays important roles in mediating basal repression by TRs and may prevent cellular proliferation in vivo.

Critical role for thyroid hormone receptor beta2 in the regulation of paraventricular thyrotropin-releasing hormone neurons

Thyroid hormone thyroxine (T(4)) and tri-iodothyronine (T(3)) production is regulated by feedback inhibition of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) synthesis in the pituitary and hypothalamus when T(3) binds to thyroid hormone receptors (TRs) interacting with the promoters of the genes for the TSH subunit and TRH. All of the TR isoforms likely participate in the negative regulation of TSH production in vivo, but the identity of the specific TR isoforms that negatively regulate TRH production are less clear. To clarify the role of the TR-beta2 isoform in the regulation of TRH gene expression in the hypothalamic paraventricular nucleus, we examined preprothyrotropin-releasing hormone (prepro-TRH) expression in mice lacking the TR-beta2 isoform under basal conditions, after the induction of hypothyroidism with propylthiouracil, and in response to T(3) administration. Prepro-TRH expression was increased in hypothyroid wild-type mice and markedly suppressed after T(3) administration. In contrast, basal TRH expression was increased in TR-beta2-null mice to levels seen in hypothyroid wild-type mice and did not change significantly in response to induction of hypothyroidism or T(3) treatment. However, the suppression of TRH mRNA expression in response to leptin reduction during fasting was preserved in TR-beta2-null mice. Thus TR-beta2 is the key TR isoform responsible for T(3)-mediated negative-feedback regulation by hypophysiotropic TRH neurons.