Transcriptional activation by thyroid hormone receptor-beta involves chromatin remodeling, histone acetylation, and synergistic stimulation by p300 and steroid receptor coactivators

Thyroid Hormone Receptors as Tumor Suppressors in Cancer

Accumulated research has revealed the multifaceted roles of thyroid hormone receptors (TRs) as potent tumor suppressors across various cancer types. This review explores the intricate mechanisms underlying TR-mediated tumor suppression, drawing insights from preclinical mouse models and cancer biology. This review examines the tumor-suppressive functions of TRs, particularly TRβ, in various cancers using preclinical models, revealing their ability to inhibit tumor initiation, progression, and metastasis. Molecular mechanisms underlying TR-mediated tumor suppression are discussed, including interactions with oncogenic signaling pathways like PI3K-AKT, JAK-STAT, and transforming growth factor β. Additionally, this paper examines TRs' effect on cancer stem cell activity and differentiation, showcasing their modulation of key cellular processes associated with tumor progression and therapeutic resistance. Insights from preclinical studies underscore the therapeutic potential of targeting TRs to impede cancer stemness and promote cancer cell differentiation, paving the way for precision medicine in cancer treatment and emphasizing the potential of TR-targeted therapies as promising approaches for treating cancers and improving patient outcomes.

Thyroid Hormone-mediated Histone Modification Protects Cortical Neurons From the Toxic Effects of Hypoxic Injury

Context

Thyroid hormone has been shown to have a protective role in neuronal injury, although the mechanisms have not been established. The cellular response to stress that promotes adaptation and survival has been shown to involve epigenetic modifications.

Objective

We hypothesized that the neuroprotective role of thyroid hormone was associated with epigenetic modifications of histone proteins. We used hypoxic neurons as a model system for hypoxia-induced brain injury.

Methods

Mouse primary cortical neurons were exposed to 0.2% oxygen for 7 hours, with or without, treatment with triiodothyronine (T3). We analyzed the expression of histone-modifying enzymes by RNA-seq and the post-translationally modified histone 3 proteins by enzyme-linked immunosorbent assay (ELISA) and Western blot.

Results

We found that methylation of H3K27, associated with inactive promoters, was highly induced in hypoxic neurons, and this histone methylation was reduced by T3 treatment. H3K4 methylation is the hallmark of active promoters. The expression of 3 (Set1db, Kmta2c, and Kmt2e) out of 6 H3K4 methyltransferases was downregulated by hypoxia and expression was restored by T3 treatment. H3K4me3 protein, measured by ELISA, was increased 76% in T3-treated hypoxic neurons compared with the levels without T3 treatment. H3K56ac plays a critical role in transcription initiation and was markedly increased in T3-treated hypoxic neurons compared with those without T3 treatment, indicating stimulation of gene transcription. Additionally, T3 treatment restored hypoxia-induced downregulation of histone acetyltransferase, Kat6a, Kat6b, and Crebbp, which function as transcription factors.

Conclusion

These findings indicate that T3 treatment mitigates hypoxia-induced histone modifications and protects neurons from hypoxia-induced injury.

Thyroid hormone signaling in the intestinal stem cells and their niche

Several studies emphasized the function of the thyroid hormones in stem cell biology. These hormones act through the nuclear hormone receptor TRs, which are T3-modulated transcription factors. Pioneer work on T3-dependent amphibian metamorphosis showed that the crosstalk between the epithelium and the underlying mesenchyme is absolutely required for intestinal maturation and stem cell emergence. With the recent advances of powerful animal models and 3D-organoid cultures, similar findings have now begun to be described in mammals, where the action of T3 and TRα1 control physiological and cancer-related stem cell biology. In this review, we have summarized recent findings on the multiple functions of T3 and TRα1 in intestinal epithelium stem cells, cancer stem cells and their niche. In particular, we have highlighted the regulation of metabolic functions directly linked to normal and/or cancer stem cell biology. These findings help explain other possible mechanisms by which TRα1 controls stem cell biology, beyond the more classical Wnt and Notch signaling pathways.

Thyroid Hormone Receptor Beta as Tumor Suppressor: Untapped Potential in Treatment and Diagnostics in Solid Tumors

There is compelling evidence that the nuclear receptor TRβ, a member of the thyroid hormone receptor (TR) family, is a tumor suppressor in thyroid, breast, and other solid tumors. Cell-based and animal studies reveal that the liganded TRβ induces apoptosis, reduces an aggressive phenotype, decreases stem cell populations, and slows tumor growth through modulation of a complex interplay of transcriptional networks. TRβ-driven tumor suppressive transcriptomic signatures include repression of known drivers of proliferation such as PI3K/Akt pathway, activation of novel signaling such as JAK1/STAT1, and metabolic reprogramming in both thyroid and breast cancers. The presence of TRβ is also correlated with a positive prognosis and response to therapeutics in BRCA+ and triple-negative breast cancers, respectively. Ligand activation of TRβ enhances sensitivity to chemotherapeutics. TRβ co-regulators and bromodomain-containing chromatin remodeling proteins are emergent therapeutic targets. This review considers TRβ as a potential biomolecular diagnostic and therapeutic target.

Involvement of Thyroid Hormones in Brain Development and Cancer

The development and maturation of the mammalian brain are regulated by thyroid hormones (THs). Both hypothyroidism and hyperthyroidism cause serious anomalies in the organization and function of the nervous system. Most importantly, brain development is sensitive to TH supply well before the onset of the fetal thyroid function, and thus depends on the trans-placental transfer of maternal THs during pregnancy. Although the mechanism of action of THs mainly involves direct regulation of gene expression (genomic effects), mediated by nuclear receptors (THRs), it is now clear that THs can elicit cell responses also by binding to plasma membrane sites (non-genomic effects). Genomic and non-genomic effects of THs cooperate in modeling chromatin organization and function, thus controlling proliferation, maturation, and metabolism of the nervous system. However, the complex interplay of THs with their targets has also been suggested to impact cancer proliferation as well as metastatic processes. Herein, after discussing the general mechanisms of action of THs and their physiological effects on the nervous system, we will summarize a collection of data showing that thyroid hormone levels might influence cancer proliferation and invasion.

Multiple mechanisms regulate H3 acetylation of enhancers in response to thyroid hormone

Hormone-dependent activation of enhancers includes histone hyperacetylation and mediator recruitment. Histone hyperacetylation is mostly explained by a bimodal switch model, where histone deacetylases (HDACs) disassociate from chromatin, and histone acetyl transferases (HATs) are recruited. This model builds on decades of research on steroid receptor regulation of transcription. Yet, the general concept of the bimodal switch model has not been rigorously tested genome wide. We have used a genomics approach to study enhancer hyperacetylation by the thyroid hormone receptor (TR), described to operate as a bimodal switch. H3 acetylation, HAT and HDAC ChIP-seq analyses of livers from hypo- and hyperthyroid wildtype, TR deficient and NCOR1 disrupted mice reveal three types of thyroid hormone (T3)-regulated enhancers. One subset of enhancers is bound by HDAC3-NCOR1 in the absence of hormone and constitutively occupy TR and HATs irrespective of T3 levels, suggesting a poised enhancer state in absence of hormone. In presence of T3, HDAC3-NCOR1 dissociates from these enhancers leading to histone hyperacetylation, suggesting a histone acetylation rheostat function of HDACs at poised enhancers. Another subset of enhancers, not occupied by HDACs, is hyperacetylated in a T3-dependent manner, where TR is recruited to chromatin together with HATs. Lastly, a subset of enhancers, is not occupied directly by TR yet requires TR for histone hyperacetylation. This indirect enhancer activation involves co-association with TR bound enhancers within super-enhancers or topological associated domains. Collectively, this demonstrates various mechanisms controlling hormone-dependent transcription and adds significant details to the otherwise simple bimodal switch model.

Thyroid hormone (T3) is a central regulator of growth, thermogenesis, heart rate and metabolism. In the liver T3 binds thyroid hormone receptor beta (TRβ) controlling expression of genes involved in processes such as lipid and cholesterol metabolism. The molecular mechanisms controlling TR-dependent gene regulation are centred on a bimodal switch model. In the absence of T3 co-repressors bind TR reducing gene expression. When hormone binds TR, co-repressors dissociate, and co-activators are recruited inducing gene expression. This model predominates the current understanding of T3-regulated gene expression. However, only a few studies have tested this model by genome-wide approaches. We have quantified histone3 acetylation genome-wide in the liver of hypo- and hyperthyroid mice and identified gene regulatory regions regulated by T3. Probing TR and co-regulators at these regulatory regions, and analysing histone3 acetylation in mouse models for disrupted co-repressor and TR activity, reveal additional insights to the mechanisms regulating T3-dependent gene expression. We suggest a revision of the prevailing bimodal switch model which helps understanding T3-regulated gene expression in tissues such as liver. We hope that this study, together with future studies, will add new perspectives on nuclear receptor-mediated transcriptional regulation to reveal general principles.

Physiologic Significance of Epigenetic Regulation of Thyroid Hormone Target Gene Expression

BACKGROUND

In previous publications, we have reported our findings demonstrating that exposure to high maternal levels of thyroid hormones (TH) has life-long effects on the wild-type (WT, without THRB mutation) progeny of mothers with resistance to thyroid hormone beta (RTHβ). The mechanism of this epigenetic effect remains unclear.

OBJECTIVES

We reviewed the mechanisms involved in the epigenetic regulation of TH target genes and understand how they may explain the reduced sensitivity to TH in the WT progeny of RTHβ mothers.

METHODS

The availability of a large, formerly genotyped Azorean population with many individuals harboring the THRB mutation, R243Q, provided us a model to study the influence of fetal exposure to high maternal TH levels.

RESULTS

The thyroid-stimulating hormone (TSH) response in WT adults was less suppressible following the administration of L-triiodothyronine (L-T₃). This finding suggests reduced sensitivity to TH that is induced by an epigenetic mechanism resulting from exposure to high maternal levels of TH during pregnancy. The persistence of this effect across 3 generations of WT subjects favors transgenerational epigenetic inheritance. Based on preliminary studies in mice, we identified the naturally imprinted gene encoding deiodinase type 3, i.e., DIO3, as a possible mediator of this epigenetic effect through increased inactivation of TH.

CONCLUSION

Increased D3 expression and consequently increased T₃ degradation appear to be responsible for the reduced sensitivity of the anterior pituitary to administered L-T₃. The imprinted DIO3 gene may be a candidate gene that mediates the epigenetic effect induced by exposure to high maternal levels of TH. However, we cannot exclude the role of other TH-responsive genes.

Novel Insight Into the Epigenetic and Post-transcriptional Control of Cardiac Gene Expression by Thyroid Hormone

Thyroid hormone (TH) signaling is critically involved in the regulation of cardiovascular physiology. Even mild reductions of myocardial TH levels, as occur in hypothyroidism or low T3 state conditions, are thought to play a role in the progression of cardiac disorders. Due to recent advances in molecular mechanisms underlying TH action, it is now accepted that TH-dependent modulation of gene expression is achieved at multiple transcriptional and post-transcriptional levels and involves the cooperation of many processes. Among them, the epigenetic remodeling of chromatin structure and the interplay with non-coding RNA have emerged as novel TH-dependent pathways that add further degrees of complexity and broaden the network of genes controlled by TH signaling. Increasing experimental and clinical findings indicate that aberrant function of these regulatory mechanisms promotes the evolution of cardiac disorders such as post-ischemic injury, pathological hypertrophy, and heart failure, which may be reversed by the correction of the underlying TH dyshomeostasis. To encourage the clinical implementation of a TH replacement strategy in cardiac disease, here we discuss the crucial effect of epigenetic modifications and control of non-coding RNA in TH-dependent regulation of biological processes relevant for cardiac disease evolution.

Cardioprotection and Thyroid Hormones in the Clinical Setting of Heart Failure

Ischemic heart disease is the main cause of morbidity and mortality worldwide and is becoming more widespread with population aging. Cardioprotection is a dynamic process characterized by mechanisms related to myocardial damage and activation of protective factors. Targeting these processes could be attractive as a new therapeutic strategy in the evolution of post-ischemic heart failure (HF). In this context, the role of thyroid hormone (TH)-mediated cardioprotection is supported by a number of findings regarding the modulation of neuroendocrine systems, inflammatory and oxidative stress status, pro-survival intracellular pathways, and epigenetic factors, its effects on cardiac angiogenesis, structure, and function and on the preservation of mitochondrial function and morphology, and its beneficial effects on cell growth and redifferentiation. Moreover, the numerous effects of TH on the heart involve genomic mechanisms, which include cardiac differentiation during the perinatal period and non-genomic action, directed toward the maintenance of cardiovascular homeostasis. This evidence suggests that there is an opportunity to treat HF patients with TH. This review is mainly focused on the clinical evidence of the role of the thyroid system in the complex setting of HF.

RXRB Is an MHC-Encoded Susceptibility Gene Associated with Anti-Topoisomerase I Antibody-Positive Systemic Sclerosis

Systemic sclerosis is a systemic autoimmune and connective tissue disorder associated with the human leukocyte antigen locus. However, the functional relationship between human leukocyte antigen gene(s) and disease development remains unknown. To elucidate major histocompatibility complex-linked systemic sclerosis genetics, we performed genotyping of major histocompatibility complex-borne microsatellites and HLA-DPB1 alleles using DNA obtained from 318 anti-topoisomerase I antibody-positive patients and 561 healthy controls, all of Japanese descent. Those results revealed two major histocompatibility complex haplotypes associated with systemic sclerosis. Exome sequencing and targeted analysis of these risk haplotypes identified rs17847931 in RXRB as a susceptibility variant (P = 1.3 × 10^-15; odds ratio [OR] = 9.4) with amino acid substitution p.V95A on the risk haplotype harboring HLA-DPB1∗13:01. No identical variant in the other haplotype including DPB1*09:01 was observed, though that haplotype also showed a significant association (P = 8.5 × 10^-22; OR = 4.3) with systemic sclerosis. Furthermore, the number of risk factors was shown to be a predominant factor, as individuals with two factors had elevated risk (P = 6.7 × 10^-13; OR = 30.2). We concluded that RXRB may be involved in antifibrotic activity in skin and chromatin remodeling.

Acetylation of RNA polymerase II regulates growth-factor-induced gene transcription in mammalian cells

Lysine acetylation regulates transcription by targeting histones and nonhistone proteins. Here we report that the central regulator of transcription, RNA polymerase II, is subject to acetylation in mammalian cells. Acetylation occurs at eight lysines within the C-terminal domain (CTD) of the largest polymerase subunit and is mediated by p300/KAT3B. CTD acetylation is specifically enriched downstream of the transcription start sites of polymerase-occupied genes genome-wide, indicating a role in early stages of transcription initiation or elongation. Mutation of lysines or p300 inhibitor treatment causes the loss of epidermal growth-factor-induced expression of c-Fos and Egr2, immediate-early genes with promoter-proximally paused polymerases, but does not affect expression or polymerase occupancy at housekeeping genes. Our studies identify acetylation as a new modification of the mammalian RNA polymerase II required for the induction of growth factor response genes.

Requirement of protein l-isoaspartyl O-methyltransferase for transcriptional activation of trefoil factor 1 (TFF1) gene by estrogen receptor alpha

Lysine- and arginine-specific methyltransferases have been shown to act as either direct or secondary transcriptional co-activator of the estrogen receptor (ERα). However, little is known about the role of protein l-isoaspartyl O-methyltransferase (PIMT) on transcriptional regulation. Here, we show that PIMT acts as a co-activator for ERα-mediated transcription. Activation of the estrogen response element (ERE) promoter by β-estradiol (E(2)) was suppressed by knockdown of PIMT, and enhanced by overexpression of wild-type PIMT. However, the ERE promoter activity was resistant to E(2) stimulation in cells overexpressing a catalytically inactive PIMT mutant, G88A. Consistent with these results, the expression of the endogenous ERα response gene trefoil factor 1 (TFF1) by E(2) was completely abrogated by PIMT depletion and decreased to approximately 50% when PIMT mutant G88A was expressed. In addition, over-expression of PIMT significantly increased the levels of TFF1 mRNA in the presence or absence of E(2). Interestingly, PIMT interacted with ERα and was distributed to the cytosol and the nucleus. The chromatin immunoprecipitation analysis revealed that PIMT was recruited to the promoter of TFF1 gene together with ERα in an E(2)-dependent manner, which was accompanied by uploading of RNA polymerase II on the promoter. Taken together, the results suggest that PIMT may act as a co-activator in ERα-mediated transcription through its recruitment to the promoter via interacting with ERα.

Thyroid hormone regulates the expression of the sonic hedgehog signaling pathway in the embryonic and adult Mammalian brain

Thyroid hormone is important for development and plasticity in the immature and adult mammalian brain. Several thyroid hormone-responsive genes are regulated during specific developmental time windows, with relatively few influenced across the lifespan. We provide novel evidence that thyroid hormone regulates expression of the key developmental morphogen sonic hedgehog (Shh), and its coreceptors patched (Ptc) and smoothened (Smo), in the early embryonic and adult forebrain. Maternal hypo- and hyperthyroidism bidirectionally influenced Shh mRNA in embryonic forebrain signaling centers at stages before fetal thyroid hormone synthesis. Further, Smo and Ptc expression were significantly decreased in the forebrain of embryos derived from hypothyroid dams. Adult-onset thyroid hormone perturbations also regulated expression of the Shh pathway bidirectionally, with a significant induction of Shh, Ptc, and Smo after hyperthyroidism and a decline in Smo expression in the hypothyroid brain. Short-term T₃ administration resulted in a significant induction of cortical Shh mRNA expression and also enhanced reporter gene expression in Shh(+/LacZ) mice. Further, acute T₃ treatment of cortical neuronal cultures resulted in a rapid and significant increase in Shh mRNA, suggesting direct effects. Chromatin immunoprecipitation assays performed on adult neocortex indicated enhanced histone acetylation at the Shh promoter after acute T₃ administration, providing further support that Shh is a thyroid hormone-responsive gene. Our results indicate that maternal and adult-onset perturbations of euthyroid status cause robust and region-specific changes in the Shh pathway in the embryonic and adult forebrain, implicating Shh as a possible mechanistic link for specific neurodevelopmental effects of thyroid hormone.

The nuclear receptor corepressor (NCoR) controls thyroid hormone sensitivity and the set point of the hypothalamic-pituitary-thyroid axis

The role of nuclear receptor corepressor (NCoR) in thyroid hormone (TH) action has been difficult to discern because global deletion of NCoR is embryonic lethal. To circumvent this, we developed mice that globally express a modified NCoR protein (NCoRΔID) that cannot be recruited to the thyroid hormone receptor (TR). These mice present with low serum T(4) and T(3) concentrations accompanied by normal TSH levels, suggesting central hypothyroidism. However, they grow normally and have increased energy expenditure and normal or elevated TR-target gene expression across multiple tissues, which is not consistent with hypothyroidism. Although these findings imply an increased peripheral sensitivity to TH, the hypothalamic-pituitary-thyroid axis is not more sensitive to acute changes in TH concentrations but appears to be reset to recognize the reduced TH levels as normal. Furthermore, the thyroid gland itself, although normal in size, has reduced levels of nonthyroglobulin-bound T(4) and T(3) and demonstrates decreased responsiveness to TSH. Thus, the TR-NCoR interaction controls systemic TH sensitivity as well as the set point at all levels of the hypothalamic-pituitary-thyroid axis. These findings suggest that NCoR levels could alter cell-specific TH action that would not be reflected by the serum TSH.

Thyroid hormone controls the gene expression of HSV-1 LAT and ICP0 in neuronal cells

Various factors/pathways including hormonal regulation have been suggested to control HSV-1 latency and reactivation. Our computer analysis identified a DNA repeat containing thyroid hormone response elements (TRE) in the regulatory region of HSV-1 LAT. Thyroid hormone (T₃) exerts its function via its receptor (TR), a transcriptional factor. Present study investigated the roles of TR and T₃ on HSV-1 gene expression using cultured neuoroblastoma cell lines. We demonstrated that liganded TR activated LAT transcription but repressed ICP0 transcription in the presence of LAT TRE. The chromatin immunoprecipitation (ChIP) assays showed that TRs were recruited to LAT TREs independently of T₃ and hyperacetylated H4 was associated with promoters that were transcriptionally active. In addition, ChIP results showed that a chromatin insulator protein CTCF was enriched at the LAT TREs in the presence of TR and T₃. In addition, chromatin remodeling factor BRG1 complex is found to participate in the T₃/TR-mediated LAT activation since overexpression of BRG1 enhanced the LAT transcription and the dominant negative mutant K785R abolished the activation. This is the first report revealing that TR exerted epigenetic regulation on HSV-1 ICP0 expression in neuronal cells and could have a role in the complex processes of HSV-1 latency/reactivation.

Distinct and histone-specific modifications mediate positive versus negative transcriptional regulation of TSHalpha promoter

Background

Hormonally-regulated histone modifications that govern positive versus negative transcription of target genes are poorly characterized despite their importance for normal and pathological endocrine function. There have been only a few studies examining chromatin modifications on target gene promoters by nuclear hormone receptors. Moreover, these studies have focused on positively-regulated target genes.

TSHα, a heterodimer partner for thyrotropin (TSH), is secreted by the pituitary gland. T₃ negatively regulates TSHα gene expression via thyroid hormone receptors (TRs) which belong to the nuclear hormone receptor superfamily, whereas thyrotropin releasing hormone (TRH) positively regulates via the TRH receptor, a G protein-coupled receptor.

Methodology/Principal Findings

We studied regulation of the TSHα gene by cAMP and T₃ using chromatin immunoprecipitation (ChIP) assays in stably-transfected rat pituitary cells containing the human TSHα promoter. Interestingly, cAMP selectively increased histone H4 acetylation whereas, as previously reported, T₃ induced histone H3 acetylation. In particular, cAMP increased H4K5 and H4K8 acetylation and decreased H4K20 trimethylation, modifications associated with transcriptional activation. T₃ increased H3K9 and H3K18 acetylation and H3K4 trimethylation; however, it also decreased H3K27 acetylation and increased H3K27 trimethylation which are associated with transcriptional repression. Of note, cAMP recruited pCREB, CBP/p300, and PCAF to the promoter whereas T₃ caused dissociation of NCoR/SMRT and HDAC3. Overexpression of a dominant negative mutant thyroid hormone receptor (TR) from a patient with resistance to thyroid hormone (RTH) led to less T₃-dependent negative regulation and partially blocked histone H3 modifications of the TSHα promoter.

Conclusions/Significance

Our findings show that non-overlapping and specific histone modifications determine positive versus negative transcriptional regulation, and integrate opposing hormonal and intracellular signals at the TSHα promoter. A mutant TR from a patient with RTH exerted dominant negative activity by blocking the histone modifications induced by T₃ on the TSHα promoter and likely contributes to the inappropriate TSH production observed in RTH.

Reversible epigenetic modifications of the two cardiac myosin heavy chain genes during changes in expression

The two genes of the cardiac myosin heavy chain (MHC) locus-alpha-MHC (aMHC) and beta-MHC (bMHC)--are reciprocally regulated in the mouse ventricle during development and in adult conditions such as hypothyroidism and pathological cardiac hypertrophy. Their expressions are under the control of thyroid hormone T3 levels. To gain insights into the epigenetic mechanisms that underlie this inducible and reversible switching of the aMHC and bMHC isoforms, we have investigated the histone modification patterns that occur over the two cardiac MHC promoters during T3-mediated reversible switching of gene expression. Mice fed a diet of propylthiouracil (PTU, an inhibitor of T3 synthesis) for 2 weeks dramatically reduce aMHC mRNA expression and increase bMHC mRNA levels to high levels, while a subsequent withdrawal of PTU diet for 2 weeks completely reverses the T3-mediated changes in MHC expression. Using hearts from mice treated in this way, we carried out chromatin immunoprecipitation-qPCR assays with antibodies against acetylated histone H3 (H3ac) and trimethylated histone (H3K4me3)-two well-documented markers of activation. Our results show that the reexpression of bMHC is associated at the bMHC promoter with increased H3ac but not H3K4me3. In contrast, the silencing of aMHC is associated at its promoter with decreased H3K4me3, but not decreased H3ac. The epigenetic changes at the two MHC promoters are completely reversed when the gene expression returns to initial levels. These data indicate that during reciprocal and inducible gene expression H3ac parallels bMHC isoform expression while H3K4me3 parallels expression of the tightly linked aMHC isoform.

Administration of thyroid hormone increases reelin and brain-derived neurotrophic factor expression in rat hippocampus in vivo

Thyroid hormones play important roles in the maturation and function of the central nervous system. However, the underlying mechanism behind thyroid hormone-regulated gene expression in the adult brain is not well understood. Two genes critical for neuronal plasticity and implicated in psychiatric disorders, reelin and brain-derived neurotrophic factor (BDNF), were investigated in the present study. Triiodothyronine (T3), the active form of thyroid hormone was administered to young adult rats in two different manners: systemic injection or local brain infusion. Real time RT-PCR results revealed that T3 administration lead to a significant increase in reelin, total BDNF and exon-specific BDNF mRNA expression in the hippocampus. Furthermore, the association of transcriptional coactivators (including steroid receptor coactivator-1 (SRC-1), cAMP response element binding protein-binding protein (CBP), and thyroid hormone receptor associated protein 220 (TRAP 220)) and RNA polymerase II (RNA Pol II), with reelin and BDNF genes in the rat hippocampus displayed a distinct process following thyroid hormone administration. These findings suggest that association of transcriptional coactivators and RNA Pol II with gene promoters may be a possible mechanism explaining T3-induced reelin and BDNF expression in the hippocampus of young adult rats.

Biochemical analyses of nuclear receptor-dependent transcription with chromatin templates

Chromatin, the physiological template for transcription, plays important roles in gene regulation by nuclear receptors (NRs). It can (1) restrict the binding of NRs or the transcriptional machinery to their genomic targets, (2) serve as a target of regulatory posttranslational modifications by NR coregulator proteins with histone-directed enzymatic activities, and (3) function as a binding scaffold for a variety of transcription-related proteins. The advent of in vitro or "cell-free" systems that accurately recapitulate ligand-dependent transcription by NRs with chromatin templates has allowed detailed analyses of these processes. Biochemical studies have advanced our understanding of the mechanisms of gene regulation, including the role of ligands, coregulators, and nucleosome remodeling. In addition, they have provided new insights about the dynamics of NR-mediated transcription. This chapter reviews the current methodologies for assembling, transcribing, and analyzing chromatin in vitro, as well as the new information that has been gained from these studies.

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.

Mutations in the AF-2 region abolish ligand-induced intranuclear immobilization of the liver X receptor alpha

The liver X receptors (LXR) alpha and beta are ligand-induced transcription factors that regulate the expression of genes important for cholesterol metabolism, lipogenesis, and other metabolic pathways. Despite their high degree of similarity, LXRs have redundant as well as nonredundant functions. The regulation of LXRs' intranuclear mobility most likely plays a major role in the regulation of their transcriptional activities. In order to elucidate how ligand binding, receptor-protein and receptor-DNA interactions affect intranuclear receptor mobility, we expressed transcriptionally active yellow fluorescent protein (YFP)-LXR alpha and YFP-LXR beta in Cos-7 cells. We used the fluorescence recovery after photobleaching (FRAP) technique and confocal laser scanning microscopy as well as Triton X-100 permeabilization experiments and fluorescence microscopy to measure differences in the intranuclear mobility between LXR alpha and LXR beta. The image analyses revealed that after agonist binding, LXR alpha exhibits slower intranuclear trafficking and greater intranuclear immobilization compared with LXR beta. In addition, mutational analysis showed that the integrity of the Activation Function (AF)-2 region of LXR alpha is essential for its immobilization whereas the integrity of the DNA binding domain is not. These findings imply that specific protein interactions with the AF-2 region of LXR alpha play a role in its intranuclear immobilization.

Reshaping the way we view vitamin D signalling and the role of vitamin D in health

Although the biological requirement for vitamin D can be met by epidermal exposure to UV light, there are a number of conditions where this production does not occur or is not sufficient to meet biological needs. When this happens, vitamin D must be consumed and is a nutrient. However, two distinct observations have caused researchers to rethink certain dogma in vitamin D biology. First, it appears that in addition to the hormonally active form of 1,25 dihydroxyvitamin D (1,25(OH)2D), circulating levels of 25 hydroxyvitamin D have a critical importance for optimal human health. This and other data suggest that extra-renal production of 1,25(OH)2D contributes to Ca homeostasis and cancer prevention. Second, in addition to its role in the transcriptional activation of genes through the vitamin D receptor there is now compelling evidence that 1,25(OH)2D has a second molecular mode of action; the rapid activation of second-messenger and kinase pathways. The purpose of this second mode of action is only now being explored. The present review will discuss how these two areas are reshaping our understanding of vitamin D metabolism and action.

Thyroid hormone-regulated target genes have distinct patterns of coactivator recruitment and histone acetylation

Thyroid hormone receptors (TRs) are ligand-regulated transcription factors that bind to thyroid hormone response elements of target genes. Upon ligand binding, they recruit coactivator complexes that increase histone acetylation and recruit RNA polymerase II (Pol II) to activate transcription. Recent studies suggest that nuclear receptors and coactivators may have temporal recruitment patterns on hormone response elements, yet little is known about the nature of the patterns at multiple endogenous target genes. We thus performed chromatin immunoprecipitation assays to investigate coactivator recruitment and histone acetylation patterns on the thyroid hormone response elements of four endogenous target genes (GH, sarcoplasmic endoplasmic reticulum calcium-adenosine triphosphatase, phosphoenolpyruvate carboxykinase, and cholesterol 7alpha-hydroxylase) in a rat pituitary cell line that expresses TRs. We found that TRbeta, several associated coactivators (steroid receptor coactivator-1, glucocorticoid receptor interacting protein-1, and TR-associated protein 220), and RNA Pol II were rapidly recruited to thyroid hormone response elements as early as 15 min after T3 addition. When the four target genes were compared, we observed differences in the types and temporal patterns of recruited coactivators and histone acetylation. Interestingly, the temporal pattern of RNA Pol II was similar for three genes studied. Our findings suggest that thyroid hormone-regulated target genes may have distinct patterns of coactivator recruitment and histone acetylation that may enable highly specific regulation.

Coregulator Interactions with the Thyroid Hormone Receptor

The thyroid hormone receptor (TR) directly regulates the transcription of thyroid hormone-responsive genes in response to changing levels of thyroid hormone. Mechanistically TR utilizes a complex set of binding interactions, with hormone, response elements, and coregulatory proteins, to provide specific local control of patterns of transcriptional response that are partially responsible for inducing the tissue-selective responses to the circulating hormone. One of the apparently dominant phenomena in the regulation of thyroid hormone responses is the protein interactions between TR and its coregulators. This review summarizes the current state of knowledge with respect to the identity of these coregulators, their interaction with TR, and the consequences of those interactions.

Rapid, membrane-initiated actions of 1,25 dihydroxyvitamin D: what are they and what do they mean?

Vitamin D is a conditionally required nutrient traditionally thought to influence physiology as the metabolite 1,25-dihydroxyvitamin D [1,25(OH)(2) D] by binding to the vitamin D receptor (VDR) and stimulating the transcription of genes through direct VDR-DNA interactions. However, over the past 15 y research has demonstrated that 1,25(OH)(2) D, as well as other steroid hormones, can rapidly stimulate ion fluxes and activate protein kinases by transcription-independent mechanisms. This review summarizes recent research on the rapid actions of 1,25(OH)(2) D and identifies questions that remain to be answered in this area.

Acetylation of nuclear receptors in cellular growth and apoptosis

Post-translational modification of chromatin histones governs a key mechanism of transcriptional regulation. Histone acetylation, together with methylation, phosphorylation, ubiquitylation, sumoylation, glycosylation, and ADP ribosylation, modulate the activity of many genes by modifying both core histones and non-histone transcription factors. Epigenetic protein modification plays an important role in multiple cellular processes including DNA repair, protein stability, nuclear translocation, protein-protein interactions, and in regulation of cellular proliferation, differentiation and apoptosis. Histone acetyltransferases modify histones, coactivators, nuclear transport proteins, structural proteins, cell cycle components and transcription factors including p53 and nuclear receptors. The estrogen, PPARgamma and androgen receptor are members of the nuclear receptor (NR) superfamily. The androgen receptor (AR) and estrogen receptor alpha (ERalpha) are directly acetylated by histone acetyltransferases at a motif that is conserved between species and other NR. Point mutations at the lysine residue within the acetylation motif of the AR and ERalpha have been identified in prostate cancer as well as in breast cancer tissue. Acetylation of the NR governs ligand sensitivity and hormone antagonist responses. The AR is acetylated by p300, P/CAF and TIP60 and acetylation of the AR regulates co-regulator recruitment and growth properties of the receptors in cultured cells and in vivo. AR acetylation mimic mutants convey reduced apoptosis and enhanced growth properties correlating with altered promoter specificity for cell-cycle target genes. Cell-cycle control proteins, including cyclins, in turn alter the access of transcription factors and nuclear receptors to the promoters of target genes.

Concerted Activation of ETS Protein ER81 by p160 Coactivators, the Acetyltransferase p300 and the Receptor Tyrosine Kinase HER2/Neu

Activator of thyroid and retinoic acid receptor (ACTR) is overexpressed in approximately 60% of primary human breast tumors and belongs to the p160 steroid receptor coactivator family. In this study, we identified a novel interaction partner of ACTR, the ETS transcription factor ER81 that is also heavily implicated in mammary tumor formation. ACTR and related p160 family members (steroid receptor coactivator-1 and glucocorticoid receptor-interacting protein-1 (GRIP-1)) augment ER81-mediated transcription. Although ACTR and GRIP-1 can acetylate ER81, this posttranslational modification of ER81 is not required for its stimulation by ACTR or GRIP-1. In addition, ACTR collaborates with the p300 coactivator, a joint interaction partner of ACTR and ER81, to stimulate ER81 function and the ability of p300 to acetylate ER81 is indispensable for this collaboration. Furthermore, the receptor tyrosine kinase HER2/Neu, an oncoprotein particularly found overexpressed in breast tumors, cooperates with both ACTR and p300 to stimulate ER81-mediated transcription. Thus, oncogenic HER2/Neu and ACTR may synergize to orchestrate mammary tumorigenesis through the dysregulation of the transcription factor ER81 and its target genes.

p300 regulates the synergy of steroidogenic factor-1 and early growth response-1 in activating luteinizing hormone-beta subunit gene

Tight regulation of luteinizing hormone-beta subunit (LHbeta) expression is critical for differentiation and maturation of mammalian sexual organs and reproductive function. Two transcription factors, steroidogenic factor-1 (SF-1) and early growth response-1 (Egr-1), play a central role in activating LHbeta promoter, and the synergy between these two factors is essential in mediating gonadotropin-releasing hormone stimulation of LHbeta promoter. Here we demonstrate that the transcriptional co-activator p300 regulates this synergy. Overexpression of p300 results in strong stimulation of LHbeta promoter but only in the presence of both SF-1 and Egr-1, and not in the presence of other Egr proteins. Mutation of the binding sites for either SF-1 or Egr-1 completely abolishes the synergy between these two factors, as well as the influence of p300. Importantly, LHbeta promoter is precipitated using p300 antibodies in a chromatin immunoprecipitation assay with LbetaT2 gonadotropes, and this effect is enhanced by gonadotropin-releasing hormone. The influence of p300 on LHbeta promoter is potentiated by steroid receptor co-activator, as well as by E1A proteins, and attenuated by Smad nuclear interacting protein 1. Taken together, these results suggest that p300 is recruited to LHbeta promoter where it coordinates the functional synergy between SF-1 and Egr-1.