Embryonic and fetal beta-globin gene repression by the orphan nuclear receptors, TR2 and TR4

The TR2 and TR4 orphan nuclear receptors comprise the DNA-binding core of direct repeat erythroid definitive, a protein complex that binds to direct repeat elements in the embryonic and fetal beta-type globin gene promoters. Silencing of both the embryonic and fetal beta-type globin genes is delayed in definitive erythroid cells of Tr2 and Tr4 null mutant mice, whereas in transgenic mice that express dominant-negative TR4 (dnTR4), human embryonic epsilon-globin is activated in primitive and definitive erythroid cells. In contrast, human fetal gamma-globin is activated by dnTR4 only in definitive, but not in primitive, erythroid cells, implicating TR2/TR4 as a stage-selective repressor. Forced expression of wild-type TR2 and TR4 leads to precocious repression of epsilon-globin, but in contrast to induction of gamma-globin in definitive erythroid cells. These temporally specific, gene-selective alterations in epsilon- and gamma-globin gene expression by gain and loss of TR2/TR4 function provide the first genetic evidence for a role for these nuclear receptors in sequential, gene-autonomous silencing of the epsilon- and gamma-globin genes during development, and suggest that their differential utilization controls stage-specific repression of the human epsilon- and gamma-globin genes.

Loss of Testicular Orphan Receptor 4 Impairs Normal Myelination in Mouse Forebrain

Testicular orphan nuclear receptor 4 (TR4) has been suggested to play important roles in the development and functioning of the central nervous system (CNS). We find reduced myelination in TR4 knockout (TR4(-/-)) mice, which is particularly obvious in forebrains and in early developmental stages. Further analysis reveals that CC-1-positive (CC-1+) oligodendrocytes are decreased in TR4(-/-) forebrains. The O4+ signals are also reduced in TR4(-/-) forebrains when examined at postnatal d 7. However, the number and proliferation rate of platelet-derived growth factor receptor alpha-positive (PDGFalphaR+) oligodendrocyte precursor cells (OPCs) remain unaffected in these regions, suggesting that loss of TR4 interrupts oligodendrocyte differentiation. This is further supported by the observation that CC-1+ oligodendrocytes derived from 5-bromo-2'-deoxyuridine incorporating OPCs are significantly reduced in TR4(-/-) forebrains. We also find higher Jagged1 expression levels in axon fiber-enriched regions in TR4(-/-) forebrains, suggesting a more activated Notch signaling in these regions that correlates with previous reports showing that Notch activation inhibits oligodendrocyte differentiation. Together, our results suggest that TR4 is required for proper myelination in the CNS and is particularly important for oligodendrocyte differentiation and maturation in the forebrain regions. The altered Jagged1-Notch signaling in TR4(-/-) forebrain underlies a potential mechanism that contributes to the reduced myelination in the forebrain.

Orphan nuclear receptor TR2, a mediator of preadipocyte proliferation, is differentially regulated by RA through exchange of coactivator PCAF with corepressor RIP140 on a platform molecule GRIP1

Orphan nuclear receptor TR2 is a preadipocyte proliferator. Knockdown of TR2 in 3T3-L1 preadipocytes reduced their proliferation efficiency, whereas specific elevation of TR2 in these cells facilitated their proliferation. All-trans retinoic acid (RA) stimulates cellular proliferation in 3T3-L1 preadipocytes by activating TR2 through an IR0-type RA response element, which further activates c-Myc expression. In post-differentiated adipocytes, RA becomes a repressive signal for TR2 and rapidly down-regulates its expression. The biphasic effect of RA on TR2 expression in 3T3-L1 is mediated by differential RA-dependent coregulator recruitment to the receptor/Glucocorticoid Receptor-Interacting Protein 1 (GRIP1) complex that binds IR0 on the TR2 promoter. RA induces the recruitment of histone acetyl transferase-containing/GRIP1/p300/CBP-associated factor (PCAF) complex to the TR2 promoter in undifferentiated cells, whereas it triggers recruitment of histone deacetylase-containing/GRIP1/receptor-interacting protein 140 (RIP140) complex in differentiated cells. GRIP1 directly interacts with RIP140 through its carboxyl terminal AD2 domain. GRIP1 interacts with PCAF and RIP140 directly and differentially, functioning as a platform molecule to mediate differential RA-induced coregulator recruitment to TR2 promoter target. This results in a biphasic effect of RA on the expression of TR2 in undifferentiated and differentiated cells, which is required for RA-stimulated preadipocyte proliferation.

Thyroid hormone receptors in brain development and function

Thyroid hormones are important during development of the mammalian brain, acting on migration and differentiation of neural cells, synaptogenesis, and myelination. The actions of thyroid hormones are mediated through nuclear thyroid hormone receptors (TRs) and regulation of gene expression. The purpose of this article is to review the role of TRs in brain maturation. In developing humans maternal and fetal thyroid glands provide thyroid hormones to the fetal brain, but the timing of receptor ontogeny agrees with clinical data on the importance of the maternal thyroid gland before midgestation. Several TR isoforms, which are encoded by the THRA and THRB genes, are expressed in the brain, with the most common being TRalpha1. Deletion of TRalpha1 in rodents is not, however, equivalent to hormone deprivation and, paradoxically, even prevents the effects of hypothyroidism. Unliganded receptor activity is, therefore, probably an important factor in causing the harmful effects of hypothyroidism. Accordingly, expression of a mutant receptor with impaired triiodothyronine (T(3)) binding and dominant negative activity affected cerebellar development and motor performance. TRs are also involved in adult brain function. TRalpha1 deletion, or expression of a dominant negative mutant receptor, induces consistent behavioral changes in adult mice, leading to severe anxiety and morphological changes in the hippocampus.

Novel functions of thyroid hormone receptor mutants: beyond nucleus-initiated transcription

Study of molecular actions of thyroid hormone receptor beta (TRbeta) mutants in vivo has been facilitated by creation of a mouse model (TRbetaPV mouse) that harbors a knockin mutant of TRbeta (denoted PV). PV, which was identified in a patient with resistance to thyroid hormone, has lost T3 binding activity and transcription capacity. The striking phenotype of thyroid cancer exhibited by TRbeta(PV/PV) mice has allowed the elucidation of novel oncogenic activity of a TRbeta mutant (PV) [PAS1] beyond nucleus-initiated transcription. PV was found to physically interact with the regulatory p85alpha subunit of phosphatidylinositol 3-kinase (PI3K) in both the nuclear and cytoplasmic compartments. This protein-protein interaction activates the PI3K signaling by increasing phosphorylation of AKT, mammalian target of rapamycin (mTOR), and p70(S6K). PV, via interaction with p85alpha, also activates the PI3K-integrin-linked kinase-matrix metalloproteinase-2 signaling pathway in the extra-nuclear compartment. The PV-mediated PI3K activation results in increased cell proliferation, motility, migration, and metastasis. In addition to affecting these membrane-initiated signaling events, PV affects the stability of the pituitary tumor-transforming gene (PTTG) product. PTTG (also known as securin), a critical mitotic checkpoint protein, is physically associated with TRbeta or PV in vivo. Concomitant with T3-induced degradation of TRbeta, PTTG is degraded by the proteasome machinery, but no such degradation occurs when PTTG is associated with PV. The degradation of PTTG/TRbeta is activated by the direct interaction of the T3-bound TRbeta with the steroid receptor coactivator-3 (SRC-3) that recruits a proteasome activator (PA28gamma). PV that does not bind T3 cannot interact directly with SRC-3/PA28gamma to activate proteasome degradation, and the absence of degradation results in an aberrant accumulation of PTTG. The PV-induced failure of timely degradation of PTTG results in mitotic abnormalities. PV, via novel protein-protein interaction and transcription regulation, acts to antagonize the functions of wild-type TRs and contributes to the oncogenic functions of this mutation.

[Thyroid hormones and lipid metabolism]

Thyroid hormone (T3) and its receptor (TR) have the diverse effects on the lipid metabolism and hypothyroidism causes hypercholesterolaemia characterized by increased levels of low-density ripoproteins (LDL). There are multiple TR isoforms such as TRalpha1, TRbeta1 and TRbeta2, of which expressions are known to be tissue-specific. For example, TRbeta1 is the major TR in the liver while T3 action is mediated via TRalpha1 in the heart. The X-ray crystallography of the ligand-binding domain of TRs enabled the development of TRbeta isoform specific T3 analogues including GC1. Without tachycardia, GC1 selectively targets the TRbeta1 in the liver and decreases cholesterol levels with more potent efficacy than that of atorvastatin, a potent HMG-CoA reductase. However, the reduction of serum TSH by GC1 should be overcome in future. Current reports also describe the existence of the complex cross-talks in the lipid metabolism between TR and other nuclear hormone receptors including peroxisome proliferator -activated receptors (PPARs), liver X receptor alpha (LXRalpha) and farnesoid X receptors (FXRs). Understanding for the function of TRs and other nuclear factors may provide the new approach to the control of hypercholesterolaemia.

[Thyroid hormone and the cardiovascular system]

Thyroid hormone has many effects on the heart and vascular system. Many of the clinical manifestations of hyperthyroidism are due to the ability of thyroid hormone to alter cardiovascular hemodynamics. The hemodynamic effects of hypothyroidism are opposite to those of hyperthyroidism, although the clinical manifestations are less obvious. This review will integrate what is known about the mechanisms of thyroid hormone action on the heart with recent observations from both experimental and clinical studies of hyperthyroidism and hypothyroidism. Thyroid hormone has both direct and indirect actions on the cardiovascular system. Patients with thyroid disease, especially those with hyperthyroidism, often have symptoms and signs indicating changes in cardiovascular hemodynamics. Indeed, symptoms and signs referable to the cardiovascular system may be the only manifestations of thyroid dysfunction, and thyroid function should therefore be assessed by the measurement of serum thyrotropin concentrations in all patients with cardiovascular disease. Some suggest that the administration of triiodothyronine may benefit some patients with cardiovascular disease.

The orphan nuclear receptor, NOR-1, is a target of beta-adrenergic signaling in skeletal muscle

beta-Adrenergic receptor (beta-AR) agonists induce Nur77 mRNA expression in the C2C12 skeletal muscle cell culture model and elicit skeletal muscle hypertrophy. We previously demonstrated that Nur77 (NR4A1) is involved in lipolysis and gene expression associated with the regulation of lipid homeostasis. Subsequently it was demonstrated by another group that beta-AR agonists and cold exposure-induced Nur77 expression in brown adipocytes and brown adipose tissue, respectively. Moreover, NOR-1 (NR4A3) was hyperinduced by cold exposure in the nur77(-/-) animal model. These studies underscored the importance of understanding the role of NOR-1 in skeletal muscle. In this context we observed 30-480 min of beta-AR agonist treatment significantly and transiently increased expression of the orphan nuclear receptor NOR-1 in both mouse skeletal muscle tissue (plantaris) and C2C12 skeletal muscle cells. Specific beta(2)- and beta(3)-AR agonists had similar effects as the pan-agonist and were blocked by the beta-AR antagonist propranolol. Moreover, in agreement with these observations, isoprenaline also significantly increased the activity of the NOR-1 promoter. Stable exogenous expression of a NOR-1 small interfering RNA (but not the negative control small interfering RNA) in skeletal muscle cells significantly repressed endogenous NOR-1 mRNA expression and led to changes in the expression of genes involved in the control of lipid use and muscle mass underscored by a dramatic increase in myostatin mRNA expression. Concordantly the myostatin promoter was repressed by NOR-1 expression. In conclusion, NOR-1 is highly responsive to beta-adrenergic signaling and regulates the expression of genes controlling fatty acid use and muscle mass.

[Mechanisms of the intracellular activities of thyroid gland hormones]

Function of thyroid hormones affecting broad spectrum of various biochemical and molecular biology reactions in organisms is unimaginable without fully functional nuclear receptors. On account of this fact, research on the role and function of thyroid hormone receptors that play a role as thyroid hormone inducible transcription factors, belongs to dynamically developing branches of molecular endocrinology. In organism, full functionality of thyroid hormone receptors in the form of heterodimer with nuclear 9-cis retinoic acid receptor is essential for biological effects of 3,5,3'-triiodothyronine.

The N-CoR complex enables chromatin remodeler SNF2H to enhance repression by thyroid hormone receptor

Unliganded thyroid hormone receptor (TR) actively represses transcription via the nuclear receptor corepressor (N-CoR)/histone deacetylase 3 (HDAC3) complex. Although transcriptional activation by liganded receptors involves chromatin remodeling, the role of ATP-dependent remodeling in receptor-mediated repression is unknown. Here we report that SNF2H, the mammalian ISWI chromatin remodeling ATPase, is critical for repression of a genomically integrated, TR-regulated reporter gene. N-CoR and HDAC3 are both required for recruitment of SNF2H to the repressed gene. SNF2H does not interact directly with the N-CoR/HDAC3 complex, but binds to unacetylated histone H4 tails, suggesting that deacetylase activity of the corepressor complex is critical to SNF2H function. Indeed, HDAC3 as well as SNF2H are required for nucleosomal organization on the TR target gene. Consistent with these findings, reduction of SNF2H induces expression of an endogenous TR-regulated gene, dio1, in liver cells. Thus, although not apparent from studies of transiently transfected reporter genes, gene repression by TR involves the targeting of chromatin remodeling factors to repressed genes by the HDAC activity of nuclear receptor corepressors.

Zebrafish Trap230/Med12 is required as a coactivator for Sox9-dependent neural crest, cartilage and ear development

The vertebrate Sox9 transcription factor directs the development of neural crest, otic placodes, cartilage and bone. In zebrafish, there are two Sox9 orthologs, Sox9a and Sox9b, which together perform the functions of the single-copy tetrapod Sox9. In a large-scale genetic screen, we have identified a novel zebrafish mutant that strongly resembles the Sox9a/Sox9b double mutant phenotype. We show that this mutation disrupts the zebrafish Trap230/Med12 ortholog, a member of the Mediator complex. Mediator is a coactivator complex transducing the interaction of DNA-binding transcription factors with RNA polymerase II, and our results reveal a critical function of the Trap230 subunit as a coactivator for Sox9.

NABP1, a novel RORgamma-regulated gene encoding a single-stranded nucleic-acid-binding protein

RORgamma2 (retinoid-related orphan receptor gamma2) plays a critical role in the regulation of thymopoiesis. Microarray analysis was performed in order to uncover differences in gene expression between thymocytes of wild-type and RORgamma-/- mice. This analysis identified a novel gene encoding a 22 kDa protein, referred to as NABP1 (nucleic-acid-binding protein 1). This subsequently led to the identification of an additional protein, closely related to NABP1, designated NABP2. Both proteins contain an OB (oligonucleotide/oligosaccharide binding) motif at their N-terminus. This motif is highly conserved between the two proteins. NABP1 is highly expressed in the thymus of wild-type mice and is greatly suppressed in RORgamma-/- mice. During thymopoiesis, NABP1 mRNA expression is restricted to CD4+CD8+ thymocytes, an expression pattern similar to that observed for RORgamma2. These observations appear to suggest that NABP1 expression is regulated either directly or indirectly by RORgamma2. Confocal microscopic analysis showed that the NABP1 protein localizes to the nucleus. Analysis of nuclear proteins by size-exclusion chromatography indicated that NABP1 is part of a high molecular-mass protein complex. Since the OB-fold is frequently involved in the recognition of nucleic acids, the interaction of NABP1 with various nucleic acids was examined. Our results demonstrate that NABP1 binds single-stranded nucleic acids, but not double-stranded DNA, suggesting that it functions as a single-stranded nucleic acid binding protein.

TR3 nuclear orphan receptor prevents cyclic stretch-induced proliferation of venous smooth muscle cells

In coronary artery bypass surgery, the patency of arterial grafts is higher than that of venous grafts because of vein-graft disease, which involves excessive proliferation of venous smooth muscle cells (SMCs) and subsequent accelerated atherosclerosis. We studied the function of TR3 nuclear orphan receptor (TR3) in the early response of SMCs to mechanical strain, a major initiator of vein-graft disease. We demonstrate that TR3 expression is induced in human saphenous vein segments exposed ex vivo to whole-blood perfusion under arterial pressure. Cultured venous SMCs challenged by cyclic stretch displayed TR3 induction and enhanced DNA synthesis, whereas SMCs derived from the internal mammary artery remained quiescent. Small-interfering RNA-mediated knockdown of TR3 and adenovirus-mediated overexpression of TR3 in venous SMCs enhanced and abolished stretch-induced DNA synthesis, respectively. Accordingly, in organ cultures of wild-type murine vessel segments exposed to cyclic stretch, p27(Kip1) was down-regulated, whereas expression of this cell cycle inhibitor was unaffected by cyclic stretch in TR3-transgenic vessels, concordant with a lower proliferative response. Finally, stretch-mediated proliferation was inhibited by 6-mercaptopurine, an agonist of TR3. In conclusion, TR3 represents inhibitory mechanisms to restrict venous SMC proliferation and may contribute to prevention of vein-graft disease.

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.

Mediator is a transducer of Wnt/beta-catenin signaling

Signal transduction within the canonical Wnt/beta-catenin pathway drives development and carcinogenesis through programmed or unprogrammed changes in gene transcription. Although the upstream events linked to signal-induced activation of beta-catenin in the cytoplasm have been deciphered in considerable detail, much less is known regarding the mechanism by which beta-catenin stimulates target gene transcription in the nucleus. Here, we show that beta-catenin physically and functionally targets the MED12 subunit in Mediator to activate transcription. The beta-catenin transactivation domain bound directly to isolated MED12 and intact Mediator both in vitro and in vivo, and Mediator was recruited to Wnt-responsive genes in a beta-catenin-dependent manner. Disruption of the beta-catenin/MED12 interaction through dominant-negative interference- or RNA interference-mediated MED12 suppression inhibited beta-catenin transactivation in response to Wnt signaling. This study thus identifies the MED12 interface within Mediator as a new component and a potential therapeutic target in the Wnt/beta-catenin pathway.

3,5,3'-triiodothyronine (T3) is a survival factor for pancreatic beta-cells undergoing apoptosis

3,5,3'-triiodothyronine (T3) is essential for the growth and the regulation of metabolic functions, moreover, the growth-stimulatory effect of T3 has largely been demonstrated and the pathways via which T3 promotes cell growth have been recently investigated. Type 1 diabetes (T1D) is due to the destruction of beta-cells, which occurs even through apoptosis. Aim of our study was to analyze whether T3 could have an antiapoptotic effect on cultured beta-cells undergoing apoptosis. We have demonstrated that T3 promotes cell proliferation in islet beta-cell lines (rRINm5F and hCM) provoking an increment in cell number (up to 55%: rRINm5F and 45%: hCM), cell viability, and BrdU incorporation, and regulating the cell cycle-related molecules (cyc A, D1, E, and p27(kip1)). T3 inhibited the apoptotic process induced by streptozocin, S-Nitroso-N-Acetylpenicylamine (SNAP), and H2O2 via regulation of the pro- and anti-apoptotic factors Bcl-2, Bcl-XL, Bad, Bax, and Caspase 3. The T3 protective effect was PI-3 K-, but not MAPK- or PKA-mediated, involving pAktThr308. Thus, T3 could be considered a survival factor protecting islet beta-cells from apoptosis.

A developmental switch induced by thyroid hormone: Xenopus laevis metamorphosis

Thyroid hormone induces the complete metamorphosis of anuran tadpoles into juvenile frogs. Arguably, anuran metamorphosis is the most dramatic effect of a hormone in any vertebrate. Recent advances in pharmacology and molecular biology have made the study of this remarkable process in the frog Xenopus laevis attractive to developmental biologists and endocrinologists alike. In particular, the availability of a straightforward transgenesis assay and the near completion of the Xenopus tropicalis genome are enabling significant advances to be made in our understanding of the major remaining problems of metamorphosis: the extraordinary tissue specificity of responses, the precise timing of morphological changes, the degree of cell autonomy of hormone responses and developmental competence. We argue that X. laevis metamorphosis presents an exciting opportunity for understanding the role of thyroid hormone in vertebrate development.

Amphibian metamorphosis as a model for the developmental actions of thyroid hormone

Thyroid hormone (TH) elicits multiple physiological actions in vertebrates from fish to man. These actions can be divided into two broad categories: those where the hormone regulates developmental processes and those that involve actions in the adult organism. Amphibian metamorphosis is a most dramatic example of extensive morphological, biochemical and cellular changes occurring during post-embryonic development, which is obligatorily initiated and sustained by TH. It is, therefore, an ideal model system to understand the action of the hormone. Each tissue of the frog tadpole responds differently to TH, ranging from altered gene expression, morphogenesis, tissue re-structuring and extensive cell death, according to a developmental programme set in place before the thyroid gland begins to secrete the hormone. The key element determining the response to the hormone is the nuclear thyroid hormone receptor (TR). As in most vertebrates, there are two thyroid hormone receptors, TRalpha and TRbeta, which repress transcription in the absence of the ligand and whose concentration in the tissues is directly modulated by the hormone itself. In Xenopus, biochemical and in situ techniques have shown that the amount of TRbeta mRNA and protein are elevated 50-100 times during TH-induced metamorphic climax. This phenomenon of "autoinduction" of receptor is also seen with developmental or inductive processes regulated by other hormones acting through nuclear receptors. It is possible that receptor upregulation may be a pre-requisite for hormonal response. Recent molecular and cell biological studies have suggested that TRs function as multimeric complexes with other nuclear or chromatin proteins, such as co-repressors and co-activators, to regulate the structure of the chromatin, and thereby determine the transcription of the receptor-specified target gene. There is evidence that this may also be so for thyroid hormone regulated transcription during amphibian metamorphosis.

The mitochondrion as a primary site of action of steroid and thyroid hormones: presence and action of steroid and thyroid hormone receptors in mitochondria of animal cells

Mitochondria are key cellular organelles that regulate events related to energy production and apoptosis. These processes are modulated, in turn, by steroid and thyroid hormones in the course of their actions on metabolism, growth and development. In this context, a direct effect of these hormones on the mitochondrial-linked processes, possibly by way of cognate mitochondrial receptors, has been proposed. In this paper we review data from the literature and present new findings supporting this concept. Receptors for steroid hormones, glucocorticoids and estrogens, and for T(3), have been detected in mitochondria by immunofluorescence labeling and confocal laser microscopy, by Western blotting of mitochondrial proteins and by immunogold electron microscopy. Furthermore, the mitochondrial genome contains nucleotide sequences with high similarity to known hormone-responsive elements, which interact with the appropriate receptors to confer hormone-dependent activation of reporter genes in transfection experiments. Thus, thyroid hormone stimulates mitochondrial transcription mediated by the cognate receptor when added to an in organello mitochondrial system, capable of faithful transcription.

Thyroid hormone action at the cellular, genomic and target gene levels

Thyroid hormone (TH) plays important roles in metabolism, growth and differentiation. Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors that bind both TH and DNA enhancer sequences in the promoter region of target genes where they can interact with co-repressor and co-activator complexes. These interactons, in turn, have consequent effects on transcription. This review describes studies on TH action from our laboratory examining the cellular localization and motility of TRs using green fluorescent fusion proteins, gene expression profiles of TH in WT and TRalpha and TRbeta KO mice, as well as general transcription factor and co-activator recruitment on the promoters of target genes by TH in chromatin immunoprecipitation assays.

Molecular and developmental analyses of thyroid hormone receptor function in Xenopus laevis, the African clawed frog

The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.

Thyroid hormones and their receptors in the regulation of cell proliferation

In the present work, we have reviewed data showing that triiodothyronine and its nuclear receptors modify expression of different genes/proteins involved in cell cycle control beginning from growth factors (such as EGF and TGF-beta), to cell surface receptors (EGFR), as well as proteins acting at the cell membrane (Ras), various transcription factors (c-Fos, c-Myc, E2F1), cyclins, Cip/Kip family of cdk2 inhibitors, and p53 inhibitor Mdm2 (Table 1). We have shown how TRs are also able to modify the fate of a cell, thanks to their ability to form complexes with other transcription factors such as p53 - a key regulator of apoptosis and proliferation. Available data show that the function of thyroid hormones and of their receptors on cell proliferation is not homogenous. In fact, it strongly depends on the cell type, its developmental state (progenitor or differentiated), its patho-physiological state (normal or tumor cell), and the so-called 'cellular context'. Therefore, it is not possible to uniformly recommend T3 treatment or T3 depletion to stop or initiate proliferation of all cell types. Instead, a very individual and careful action should be considered.

Role of CCAAT/enhancer-binding protein, histone acetylation, and coactivator recruitment in the regulation of malic enzyme transcription by thyroid hormone

In chick embryo hepatocytes, activation of malic enzyme gene transcription by triiodothyronine (T3) is mediated by a T3 response unit (T3RU) that contains five T3 response elements (T3REs) plus five accessory elements that enhance T3 responsiveness conferred by the T3REs. Results from in vitro binding assays indicate that one of the accessory elements (region F) binds CCAAT/enhancer-binding protein-alpha (C/EBPalpha). Here, we investigated the role of C/EBPalpha in the regulation of malic enzyme transcription by T3. Transfection analyses demonstrated that the stimulation of T3RE function by region F did not require the presence of additional malic enzyme gene promoter sequences. Expression of a dominant negative C/EBP inhibited the ability of region F to stimulate T3 responsiveness. In chromatin immunoprecipitation assays, C/EBPalpha and TR associated with the malic enzyme T3RU in the absence and presence of T3 with the extent of the association being greater in the presence of T3. These observations indicate that C/EBPalpha interacts with TR on the malic enzyme T3RU to enhance T3 regulation of malic enzyme gene transcription. T3 treatment increased the acetylation of histones, decreased the recruitment of nuclear receptor corepressor and increased the recruitment of steroid receptor coactivator-1, CREB binding protein, and the thyroid hormone associated protein/mediator complex at the malic enzyme T3RU. In contrast, T3 treatment had no effect on the acetylation of histones and the recruitment of corepressors and coactivators at the T3RU that mediates the T3 activation of acetyl-CoA carboxylase-alpha gene transcription. We propose that differences between the malic enzyme T3RU and the ACCalpha T3RU in the ability of T3 to modulate histone acetylation and coregulatory protein recruitment are due to differences in the composition of the nuclear receptor complexes that bind these regulatory regions.

RORgammat recruits steroid receptor coactivators to ensure thymocyte survival

Thymocytes undergo apoptosis unless a functional TCR is assembled. Steroid receptor coactivators (SRCs) regulate nuclear receptor-mediated transcription by associated histone acetyltransferase activity. However, it has been a challenge to demonstrate the in vivo function of SRCs due to the overlapping functions among different members of SRCs. In this study, we show that recruitment of SRCs is required for thymic-specific retinoic acid-related orphan receptor gamma (RORgamma)t-regulated thymocyte survival in vivo. An activation function 2 domain, identified at the carboxyl terminus of RORgammat, is responsible for recruiting SRCs. A mutation in the activation function domain (Y479F) of RORgammat disrupted the interaction with SRCs and abolished RORgammat-mediated trans-activation but not its ability to inhibit transcription. Transgenes encoding the wild-type RORgammat, but not the mutant, restored thymocyte survival in RORgamma null mice. Our results thus clearly demonstrate that RORgammat recruits SRCs to impose a gene expression pattern required to expand the life span of thymocytes in vivo, which increases the opportunities for assembling a functional TCR.