Hormone-induced translocation of thyroid hormone receptors in living cells visualized using a receptor green fluorescent protein chimera

Selective Inhibition of Genomic and Non-Genomic Effects of Thyroid Hormone Regulates Muscle Cell Differentiation and Metabolic Behavior

Thyroid hormones (THs) are key regulators of different biological processes. Their action involves genomic and non-genomic mechanisms, which together mediate the final effects of TH in target tissues. However, the proportion of the two processes and their contribution to the TH-mediated effects are still poorly understood. Skeletal muscle is a classical target tissue for TH, which regulates muscle strength and contraction, as well as energetic metabolism of myofibers. Here we address the different contribution of genomic and non-genomic action of TH in skeletal muscle cells by specifically silencing the deiodinase Dio2 or the β3-Integrin expression via CRISPR/Cas9 technology. We found that myoblast proliferation is inversely regulated by integrin signal and the D2-dependent TH activation. Similarly, inhibition of the nuclear receptor action reduced myoblast proliferation, confirming that genomic action of TH attenuates proliferative rates. Contrarily, genomic and non-genomic signals promote muscle differentiation and the regulation of the redox state. Taken together, our data reveal that integration of genomic and non-genomic signal pathways finely regulates skeletal muscle physiology. These findings not only contribute to the understanding of the mechanisms involved in TH modulation of muscle physiology but also add insight into the interplay between different mechanisms of action of TH in muscle cells.

Dityrosine suppresses the cytoprotective action of thyroid hormone T3 via inhibiting thyroid hormone receptor-mediated transcriptional activation

Dityrosine (Dityr) is the most common oxidized form of tyrosine. In the previous studies of mice treated with dityrosine, cell death in the pancreas, kidneys, and liver was detected in the presence of enhanced plasma triiodothyronine (T3) content. Due to its structural similarity with the thyroid hormone T3, we hypothesized that dityrosine might disrupt T3-dependent endocrine signaling. The cytotoxic effect of dityrosine was studied in C57BL/6 mice by gavage with a dityrosine dose of 320 μg per kg per day for 10 weeks. Cell death in the liver was detected in the presence of enhanced plasma thyroid hormone content in mice treated with dityrosine. The antagonistic effect of dityrosine on T3 biofunction was studied using HepG2 cells. Dityrosine incubation reduced T3 transport ability and attenuated the T3-mediated cell survival via regulation of the PI3k/Akt/MAPK pathway. Furthermore, dityrosine inhibited T3 binding to thyroid hormone receptors (TRs) and suppressed the TR-mediated transcription. Dityrosine also downregulated the expressions of T3 action-related factors. Taken together, this study demonstrates that dityrosine inhibits T3-dependent cytoprotection by competitive inhibition, resulting in downstream gene suppression. Our findings offer insights into how dityrosine acts as an antagonist of T3. These findings shed new light on cellular processes underlying the energy metabolism disorder caused by dietary oxidized protein, thus contributing to a better understanding of the diet-health axis at a cellular level.

Thyroid hormone receptor localization in target tissues

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. We have identified multiple nuclear localization signals and nuclear export signals within TRα1 and TRβ1 that interact with importins and exportins, respectively, to mediate translocation across the nuclear envelope. More recently, enigmatic cytoplasmic functions have been ascribed to other TR subtypes, expanding the diversity of the cellular response to thyroid hormone. By integrating data on localization signal motifs, this review provides an overview of the complex interplay between TR's dynamic transport pathways and thyroid hormone signaling activities. We examine the variation in TR subtype response to thyroid hormone signaling, and what is currently known about regulation of the variety of tissue-specific localization patterns, including targeting to the nucleus, the mitochondria and the inner surface of the plasma membrane.

Toxic effects of triclosan on a zebrafish (Danio rerio) liver cell line, ZFL

Triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy) phenol) is an antimicrobial agent widely used in personal care products. It has been detected in surface water, soil, aquatic species, and even humans. In this study, we used zebrafish (Danio rerio) as a model to test the hypothesis that TCS exhibits toxic effects by interacting with thyroid hormone receptor β (TRβ) and aryl hydrocarbon receptor (AhR) and by inducing the transcription of thyroid hormone (TH)-associated genes and affecting phase I and phase II enzymes. The median lethal concentrations (LC₅₀) of TCS in zebrafish embryos/larvae and a zebrafish liver cell line (ZFL) were first determined. Hatched larvae were most sensitive to TCS exposure, with LC₅₀ values ranging from 1.26 to 1.46μM for 96h after hatching exposure. The major effect of TCS was delayed hatching which occurred from 1.13μM. The constructed GFP-zfTRβ fusion protein revealed the subcellular location of zfTRβ as the nucleus in both T3-induced and uninduced states, adding to the difficulty of studying TCS action on thyroid hormone receptors in ZFL cells. TCS had neither agonistic nor antagonistic effects on zfTRβLBD or AhR from the reporter gene systems. Ethoxyresorufin-o-deethylase (EROD) assay suggested that TCS is a weak P4501a (Cyp1a) agonist at 5μM and that it inhibits cytochrome Cyp1a activity induced by benzo(a)pyrene (BaP). In time course-based mRNA profiling in ZFL cells, 4-h exposure to TCS caused a significant (up to 37.5-fold) inhibition of Cyp1a at 2.5μM. An overall inhibition of liver phase I and II gene transcription at 4h exposure indicates the possible quick catabolism of TCS. Our findings suggest that TCS is not a TH mimic that affects TH-related gene expression. The impairment of Cyp1a mRNA expression could be due to stimulation by other stressors such as oxidative stress, warranting further investigation into the underlying mechanism in zebrafish.

Effects of dietary oxidized tyrosine products on insulin secretion via the thyroid hormone T3-regulated TRβ1–Akt–mTOR pathway in the pancreas

Oxidized tyrosine products (OTPs) have been detected in commercial foods with high protein content.

Thyroid hormones inhibit TGF-β signaling and attenuate fibrotic responses

TGF-β, the most potent profibrogenic factor, acts by activating SMAD (mothers against decapentaplegic) transcription factors, which bind to SMAD-binding elements in target genes. Here, we show that the thyroid hormone triiodothyronine (T3), through binding to its nuclear receptors (TRs), is able to antagonize transcriptional activation by TGF-β/SMAD. This antagonism involves reduced phosphorylation of SMADs and a direct interaction of the receptors with SMAD3 and SMAD4 that is independent of T3-mediated transcriptional activity but requires residues in the receptor DNA binding domain. T3 reduces occupancy of SMAD-binding elements in response to TGF-β, reducing histone acetylation and inhibiting transcription. In agreement with this transcriptional cross-talk, T3 is able to antagonize fibrotic processes in vivo. Liver fibrosis induced by carbon tetrachloride is attenuated by thyroid hormone administration to mice, whereas aged TR knockout mice spontaneously accumulate collagen. Furthermore, skin fibrosis induced by bleomycin administration is also reduced by the thyroid hormones. These findings define an important function of the thyroid hormone receptors and suggest TR ligands could have beneficial effects to block the progression of fibrotic diseases.

Nuclear import of the thyroid hormone receptor α1 is mediated by importin 7, importin β1, and adaptor importin α1

The thyroid hormone receptor α1 (TRα1) is a nuclear receptor for thyroid hormone that shuttles rapidly between the nucleus and cytoplasm. Our prior studies showed that nuclear import of TRα1 is directed by two nuclear localization signals, one in the N-terminal A/B domain and the other in the hinge domain. Here, we showed using in vitro nuclear import assays that TRα1 nuclear localization is temperature and energy-dependent and can be reconstituted by the addition of cytosol. In HeLa cells expressing green fluorescent protein (GFP)-tagged TRα1, knockdown of importin 7, importin β1 and importin α1 by RNA interference, or treatment with an importin β1-specific inhibitor, significantly reduced nuclear localization of TRα1, while knockdown of other importins had no effect. Coimmunoprecipitation assays confirmed that TRα1 interacts with importin 7, as well as importin β1 and the adapter importin α1, suggesting that TRα1 trafficking into the nucleus is mediated by two distinct pathways.

Molecular characterization of human thyroid hormone receptor β isoform 4

Thyroid hormone exerts a pleiotropic effect on development, differentiation, and metabolism through thyroid hormone receptor (TR). A novel thyroid hormone receptor β isoform (TRβ4) was cloned using PCR from a human pituitary cDNA library as a template. We report here the characterization of TRβ4 from a molecular basis. Temporal expression of TRβ4 during the fetal period is abundant in the brain and kidney, comparable with the adult pattern. Western blot analysis revealed that TRs are ubiquitination labile proteins, while TRβ1 is potentially stable. TRβ1, peroxisome proliferator-activated receptors (PPAR), and vitamin D receptor (VDR), which belong to class II transcription factors that function via the formation of heterodimeric complexes with retinoid X receptor (RXR), were suppressed by TRβ4 in a dose-dependent manner. Thus, TRβ4 exhibits ligand-independent transcriptional silencing, possibly as a substitute for dimerized RXR. In this study, TRβ1 and TRβ4 transcripts were detected in several cell lines. Quantitative RT-PCR assay showed that the expression of TRβ4 in human embryonic carcinoma cells of the testis was suppressed by sex hormone in a reciprocal manner to TRβ1. In contrast, TRβ4 was expressed under a high dose of triiodothyronine (T3) in a reciprocal manner to TRβ1. Finally, in transiently transfected NIH-3T3 cells, green fluorescence protein (GFP)-tagged TRβ4 was mostly nuclear in both the absence and the presence of T3. By mutating defined regions of both TRβs, we found that both TRβ1 and TRβ4 had altered nuclear/cytoplasmic distribution as compared with wild-type, and different to T3 and the nuclear receptor corepressor (NCoR). Thus, site-specific DNA binding is not essential for maintaining TRβs within the nucleus.

Intracellular Protein Shuttling: A Mechanism Relevant for Myelin Repair in Multiple Sclerosis?

A prominent feature of demyelinating diseases such as multiple sclerosis (MS) is the degeneration and loss of previously established functional myelin sheaths, which results in impaired signal propagation and axonal damage. However, at least in early disease stages, partial replacement of lost oligodendrocytes and thus remyelination occur as a result of resident oligodendroglial precursor cell (OPC) activation. These cells represent a widespread cell population within the adult central nervous system (CNS) that can differentiate into functional myelinating glial cells to restore axonal functions. Nevertheless, the spontaneous remyelination capacity in the adult CNS is inefficient because OPCs often fail to generate new oligodendrocytes due to the lack of stimulatory cues and the presence of inhibitory factors. Recent studies have provided evidence that regulated intracellular protein shuttling is functionally involved in oligodendroglial differentiation and remyelination activities. In this review we shed light on the role of the subcellular localization of differentiation-associated factors within oligodendroglial cells and show that regulation of intracellular localization of regulatory factors represents a crucial process to modulate oligodendroglial maturation and myelin repair in the CNS.

A rapid cytoplasmic mechanism for PI3 kinase regulation by the nuclear thyroid hormone receptor, TRβ, and genetic evidence for its role in the maturation of mouse hippocampal synapses in vivo

Several rapid physiological effects of thyroid hormone on mammalian cells in vitro have been shown to be mediated by the phosphatidylinositol 3-kinase (PI3K), but the molecular mechanism of PI3K regulation by nuclear zinc finger receptor proteins for thyroid hormone and its relevance to brain development in vivo have not been elucidated. Here we show that, in the absence of hormone, the thyroid hormone receptor TRβ forms a cytoplasmic complex with the p85 subunit of PI3K and the Src family tyrosine kinase, Lyn, which depends on two canonical phosphotyrosine motifs in the second zinc finger of TRβ that are not conserved in TRα. When hormone is added, TRβ dissociates and moves to the nucleus, and phosphatidylinositol (3, 4, 5)-trisphosphate production goes up rapidly. Mutating either tyrosine to a phenylalanine prevents rapid signaling through PI3K but does not prevent the hormone-dependent transcription of genes with a thyroid hormone response element. When the rapid signaling mechanism was blocked chronically throughout development in mice by a targeted point mutation in both alleles of Thrb, circulating hormone levels, TRβ expression, and direct gene regulation by TRβ in the pituitary and liver were all unaffected. However, the mutation significantly impaired maturation and plasticity of the Schaffer collateral synapses on CA1 pyramidal neurons in the postnatal hippocampus. Thus, phosphotyrosine-dependent association of TRβ with PI3K provides a potential mechanism for integrating regulation of development and metabolism by thyroid hormone and receptor tyrosine kinases.

Adjunctive input to the nuclear thyroid hormone receptor from the cell surface receptor for the hormone

At thyroid hormone response elements on specific genes, complexes of nuclear thyroid hormone receptors (TRs) and 3,5,3'-triiodo-L-thyronine (T(3)), coactivator or corepressor nucleoproteins, and histone acetylases or deacetylases mediate genomic effects of the hormone. Nongenomic effects of the hormone are those whose initiation does not primarily depend upon formation of the TR-T(3) complex. Among the nongenomic effects of thyroid hormone are a set of actions initiated at a cell surface receptor on integrin αvβ3 that are relevant to a) intracellular trafficking of proteins, including TRβ1, b) serine phosphorylation and acetylation of this nuclear receptor, c) assembly within the nucleus of complexes of coactivators and corepressor, and d) transcription of specific genes, including that for TRβ1. These actions initiated at αvβ3 are reviewed here and appear to be adjunctive to the genomic actions of the TR-T(3) complex.

Rapid responses to reverse T₃ hormone in immature rat Sertoli cells: calcium uptake and exocytosis mediated by integrin

There is increasing experimental evidence of the nongenomic action of thyroid hormones mediated by receptors located in the plasma membrane or inside cells. The aim of this work was to characterize the reverse T₃ (rT₃) action on calcium uptake and its involvement in immature rat Sertoli cell secretion. The results presented herein show that very low concentrations of rT₃ are able to increase calcium uptake after 1 min of exposure. The implication of T-type voltage-dependent calcium channels and chloride channels in the effect of rT₃ was evidenced using flunarizine and 9-anthracene, respectively. Also, the rT₃-induced calcium uptake was blocked in the presence of the RGD peptide (an inhibitor of integrin-ligand interactions). Therefore, our findings suggest that calcium uptake stimulated by rT₃ may be mediated by integrin αvβ₃. In addition, it was demonstrated that calcium uptake stimulated by rT₃ is PKC and ERK-dependent. Furthermore, the outcomes indicate that rT₃ also stimulates cellular secretion since the cells manifested a loss of fluorescence after 4 min incubation, indicating an exocytic quinacrine release that seems to be mediated by the integrin receptor. These findings indicate that rT₃ modulates the calcium entry and cellular secretion, which might play a role in the regulation of a plethora of intracellular processes involved in male reproductive physiology.

Thyroid hormone promotes postnatal rat pancreatic β-cell development and glucose-responsive insulin secretion through MAFA

Neonatal β cells do not secrete glucose-responsive insulin and are considered immature. We previously showed the transcription factor MAFA is key for the functional maturation of β cells, but the physiological regulators of this process are unknown. Here we show that postnatal rat β cells express thyroid hormone (TH) receptor isoforms and deiodinases in an age-dependent pattern as glucose responsiveness develops. In vivo neonatal triiodothyronine supplementation and TH inhibition, respectively, accelerated and delayed metabolic development. In vitro exposure of immature islets to triiodothyronine enhanced the expression of Mafa, the secretion of glucose-responsive insulin, and the proportion of responsive cells, all of which are effects that were abolished in the presence of dominant-negative Mafa. Using chromatin immunoprecipitation and electrophoretic mobility shift assay, we show that TH has a direct receptor-ligand interaction with the Mafa promoter and, using a luciferase reporter, that this interaction was functional. Thus, TH can be considered a physiological regulator of functional maturation of β cells via its induction of Mafa.

Identification of thyroid hormone response elements in vivo using mice expressing a tagged thyroid hormone receptor α1

TRα1 (thyroid hormone receptor α1) is well recognized for its importance in brain development. However, due to the difficulties in predicting TREs (thyroid hormone response elements) in silico and the lack of suitable antibodies against TRα1 for ChIP (chromatin immunoprecipitation), only a few direct TRα1 target genes have been identified in the brain. Here we demonstrate that mice expressing a TRα1–GFP (green fluorescent protein) fusion protein from the endogenous TRα locus provide a valuable animal model to identify TRα1 target genes. To this end, we analysed DNA–TRα1 interactions in vivo using ChIP with an anti-GFP antibody. We validated our system using established TREs from neurogranin and hairless, and by verifying additional TREs from known TRα1 target genes in brain and heart. Moreover, our model system enabled the identification of novel TRα1 target genes such as RNF166 (ring finger protein 166). Our results demonstrate that transgenic mice expressing a tagged nuclear receptor constitute a feasible approach to study receptor–DNA interactions in vivo, circumventing the need for specific antibodies. Models like the TRα1–GFP mice may thus pave the way for genome-wide mapping of nuclear receptor-binding sites, and advance the identification of novel target genes in vivo.

Multiple novel signals mediate thyroid hormone receptor nuclear import and export

Thyroid hormone receptor (TR) is a member of the nuclear receptor superfamily that shuttles between the cytosol and nucleus. The fine balance between nuclear import and export of TR has emerged as a critical control point for modulating thyroid hormone-responsive gene expression; however, sequence motifs of TR that mediate shuttling are not fully defined. Here, we characterized multiple signals that direct TR shuttling. Along with the known nuclear localization signal in the hinge domain, we identified a novel nuclear localization signal in the A/B domain of thyroid hormone receptor α1 that is absent in thyroid hormone receptor β1 and inactive in the oncoprotein v-ErbA. Our prior studies showed that thyroid hormone receptor α1 exits the nucleus through two pathways, one dependent on the export factor CRM1 and the other CRM1-independent. Here, we identified three novel CRM1-independent nuclear export signal (NES) motifs in the ligand-binding domain as follows: a highly conserved NES in helix 12 (NES-H12) and two additional NES sequences spanning helix 3 and helix 6, respectively. Mutations predicted to disrupt the α-helical structure resulted in a significant decrease in NES-H12 activity. The high degree of conservation of helix 12 suggests that this region may function as a key NES in other nuclear receptors. Furthermore, our mutagenesis studies on NES-H12 suggest that altered shuttling of thyroid hormone receptor β1 may be a contributing factor in resistance to thyroid hormone syndrome. Taken together, our findings provide a detailed mechanistic understanding of the multiple signals that work together to regulate TR shuttling and transcriptional activity, and they provide important insights into nuclear receptor function in general.

Overlapping nongenomic and genomic actions of thyroid hormone and steroids

Nuclear receptors for thyroid hormone and steroids are members of a receptor superfamily with similar molecular organization, but discrete transcriptional functions that define genomic actions of these nonpeptide hormones. Nongenomic actions of thyroid hormone and estrogens and androgens are initiated outside the nucleus, at receptors in the plasma membrane or in cytoplasm; these actions are largely regarded to be unique to the respective hormones. However, there is an increasing number of descriptions of overlapping nongenomic and genomic effects of thyroid hormone and estrogens and testosterone. These effects are concentrated in tumor cells, where, for example, estrogens and thyroid hormone have similar mitogen-activate protein kinase (MAPK)-dependent proliferative actions on ERα-positive human breast cancer cells, and where dihydrotestosterone also can stimulate proliferation. Steroids and thyroid hormone have similar anti-apoptotic effects in certain tumors. But thyroid hormone and steroids also have overlapping or interacting nongenomic and genomic actions in heart and brain cells. These various effects of thyroid hormone and estrogens and androgens are reviewed here and their possible clinical consequences are enumerated.

Membrane-initiated actions of thyroid hormones on the male reproductive system

The presence of specific nuclear receptors to thyroid hormones, described in prepubertal Sertoli cells, implies the existence of an early and critical influence of these hormones on testis development. Although the mechanism of action thyroid hormones has been classically established as a genomic action regulating testis development, our research group has demonstrated that these hormones exert several effects in Sertoli cells lacking nuclear receptor activation. These findings led to the identification of non-classical thyroid hormone binding elements in the plasma membrane of testicular cells. Through binding to these sites, thyroid hormones could exert nongenomic effects, including those on ion fluxes at the plasma membrane, on signal transduction via kinase pathways, on amino acid accumulation, on modulation of extracellular nucleotide levels and on vimentin cytoskeleton. The evidence of the participation of different K(+), Ca(2+) and Cl(-) channels in the mechanism of action of thyroid hormones, characterizes the plasma membrane as an important microenvironment able to coordinate strategic signal transduction pathways in rat testis. The physiological responses of the Sertoli cells to hormones are dependent on continuous cross-talking of different signal transduction pathways. Apparently, the choice of the signaling pathways to be activated after the interaction of the hormone with cell surface binding sites is directly related to the physiological action to be accomplished. Yet, the enormous complexity of the nongenomic actions of thyroid hormones implies that different specific binding sites located on the plasma membrane or in the cytosol are believed to initiate specific cell responses.

Membrane receptor for thyroid hormone: physiologic and pharmacologic implications

Plasma membrane integrin αvβ3 is a cell surface receptor for thyroid hormone at which nongenomic actions are initiated. L-thyroxine (T₄) and 3,3',5-triiodo-L-thyronine (T₃) promote angiogenesis and tumor cell proliferation via the receptor. Tetraiodothyroacetic acid (tetrac), a deaminated T₄ derivative, blocks the nongenomic proliferative and proangiogenic actions of T₄ and T₃. Acting at the integrin independently of T₄ and T₃, tetrac and a novel nanoparticulate formulation of tetrac that acts exclusively at the cell surface have oncologically desirable antiproliferative actions on multiple tumor cell survival pathway genes. These agents also block the angiogenic activity of vascular growth factors. Volume and vascular support of xenografts of human pancreatic, kidney, lung, and breast cancers are downregulated by tetrac formulations. The integrin αvβ3 receptor site for thyroid hormone selectively regulates signal transduction pathways and distinguishes between unmodified tetrac and the nanoparticulate formulation. The receptor also mediates nongenomic thyroid hormone effects on plasma membrane ion transporters and on intracellular protein trafficking.

Degradation of thyroid hormone receptor beta 1: existence of stable and unstable forms

BACKGROUND

The degradation of many nuclear receptors is controlled by ligand-binding and mediated by the ubiquitin-proteasome pathway. However, the mechanisms implicated in thyroid hormone receptor (TR) degradation remain unclear. Our objective was to define the kinetics, mechanisms, and sub-cellular fractions involved in TRs degradation.

METHODS

We used pulse-chase analyses, time-course experiments carried out in presence of cycloheximide (to inhibit new protein synthesis), and biochemical fractionation with Western blot analyses to determine the kinetics of the degradation of the TRβ isoform, TRβ1, in transiently transfected QBI-HEK 293A cells.

RESULTS

We observed that TRβ1 degradation is mediated by the proteasome pathway. Also, the kinetics of TRβ1 degradation is atypical due to the co-existence of more than one TRβ1 population, located in different cellular compartments and having different stability profiles. Moreover, TRβ1 degradation was unaffected by a mutation in its putative PEST motif, which confers turnover of other proteins.

CONCLUSION

Our findings introduce novel evidence suggesting that stable and unstable forms of TRβ1, which might have distinct functions, co-exist in cells.

Molecular basis for gene-specific transactivation by nuclear receptors

Nuclear receptors (NRs) are key transcriptional regulators of metazoan physiology and metabolism. Different NRs bind to similar or even identical core response elements; however, they regulate transcription in a highly receptor- and gene-specific manner. These differences in gene activation can most likely be accounted for by mechanisms involving receptor-specific interactions with DNA as well as receptor-specific interactions with protein complexes binding to adjacent and distant DNA sequences. Here, we review key molecular aspects of transactivation by NRs with special emphasis on the recent advances in the molecular mechanisms responsible for receptor- and gene-specific transcriptional activation. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

Receptors for thyroid-stimulating hormone and thyroid hormones in human ovarian tissue

Dysfunction in thyroid regulation can cause menstrual and ovulatory disturbances, the mechanism of which is not clear. The distribution and activity of the thyroid-stimulating hormone (TSHR), and the thyroid hormone receptors (TR) alpha1, alpha2 and beta1 in human ovarian tissue and in granulosa cells was studied using immunohistochemistry, reverse-transcriptase polymerase chain reaction (RT-PCR), quantitative PCR and immunoassays. Strong immunostaining of TSHR, TRalpha1 and TRbeta1 was observed in ovarian surface epithelium and in oocytes of primordial, primary and secondary follicles, with minimal staining in granulosa cells of secondary follicles. Granulosa cells of antral follicles expressed TSHR, TRalpha1 and TRbeta1 proteins. Messenger RNA for all receptors was present in ovarian tissue. Mature human granulosa cells expressed transcripts for 5' deiodinases types 2 and 3, but not type 1, indicating the possibility of conversion of peripheral thyroid hormone thyroxin (T(4)). Granulosa cells stimulated with TSH showed a significant increase in cAMP concentrations after 2 h of culture (P = 0.047), indicating activation through TSHR. Stimulation with T(4) resulted in increased extracellular signal-regulated kinase 1 and 2 activation after 10, 30, 60 min and 24 h. These data demonstrate that TSH and thyroid hormone receptors may participate in the regulation of ovarian function.

Cytoplasm-to-nucleus shuttling of thyroid hormone receptor-beta1 (Trbeta1) is directed from a plasma membrane integrin receptor by thyroid hormone

INTRODUCTION

In CV-1 cells, shuttling from cytoplasm to nucleus of the nuclear thyroid hormone receptor-beta1 (TRbeta1, TR) is shown in this report to be regulated by extracellular thyroid hormone at a hormone receptor on cell surface integrin alphav3.

METHODS

The receptor was introduced into cells as a GFP-TR1 chimera and intracellular movement of the receptor was monitored by confocal microscopy of cells treated with L-thyroxine (T(4)).

RESULTS AND DISCUSSION

TR-GFP translocation in the presence of T(4) requires activation of extracellular-regulated protein kinases 1/2 (ERK1/2). Inhibition of T(4)-binding to alphavbeta3 with anti-alphavbeta3 or Arg-Gly-Asp (RGD) peptide blocks T(4)-stimulated GFP-TR nuclear translocation, as do the hormone-binding inhibitor tetraiodothyroacetic acid (tetrac) and the ERK1/2 inhibitor, PD98059. TR1 is an ERK1/2 substrate.

CONCLUSIONS

Via a nongenomic mechanism initiated at plasma membrane integrin v3, T(4)-activated ERK1/2 and TR1 move transiently in an immunoprecipitable complex to the nuclei of T(4)-treated cells.

Mechanisms of nongenomic actions of thyroid hormone

The nongenomic actions of thyroid hormone require a plasma membrane receptor or nuclear receptors located in cytoplasm. The plasma membrane receptor is located on integrin alphaVbeta3 at the Arg-Gly-Asp recognition site important to the binding by the integrin of extracellular matrix proteins. l-Thyroxine (T(4)) is bound with greater affinity at this site than 3,5,3'-triiodo-l-thyronine (T(3)). Mitogen-activated protein kinase (MAPK; ERK1/2) transduces the hormone signal into complex cellular/nuclear events including angiogenesis and tumor cell proliferation. Acting at the integrin receptor and without cell entry, thyroid hormone can foster ERK1/2-dependent serine phosphorylation of nuclear thyroid hormone receptor-beta1 (TRbeta1) and de-repress the latter. The integrin receptor also mediates actions of the hormone on intracellular protein trafficking and on plasma membrane ion pumps, including the sodium/protein antiporter. Tetraiodothyroacetic (tetrac) is a T(4) analog that inhibits binding of iodothyronines to the integrin receptor and is a probe for the participation of this receptor in cellular actions of the hormone. Tetrac blocks thyroid hormone effects on angiogenesis and cancer cell proliferation. Acting on a truncated form of nuclear TRalpha1 (TRDeltaalpha1) located in cytoplasm, T(4) and 3,3',5'-triiodothyronine (reverse T(3)), but not T(3), cause conversion of soluble actin to fibrous (F) actin that is important to cell motility, e.g., in cells such as glia and neurons. Normal development of the central nervous system requires such motility. TRbeta1 in cytoplasm mediates action of T(3) on expression of certain genes via phosphatidylinositol 3-kinase (PI 3-K) and the protein kinase B/Akt pathway. PI 3-K and, possibly, cytoplasmic TRbeta1 are involved in stimulation by T(3) of insertion of Na,K-ATPase in the plasma membrane and of increase in activity of this pump. Because ambient thyroid hormone levels are constant in the euthyroid intact organism, these nongenomic hormone actions are likely to be contributors to basal rate-setting of transcription of certain genes and of complex cellular events such as angiogenesis and cancer cell proliferation.

Thyroid hormone receptor alpha 1-beta 1 expression in epididymal epithelium from euthyroid and hypothyroid rats

The objectives of the present work were to assess whether epithelial cells from the different segments of epididymis express TR alpha 1-beta 1 isoforms, to depict its subcellular immunolocalization and to evaluate changes in their expression in rats experimentally submitted to a hypothyroid state by injection of 131I. In euthyroid and hypothyroid groups, TR protein was expressed in epididymal epithelial cells, mainly in the cytoplasmic compartment while only a few one showed a staining in the nucleus as well. A similar TR immunostaining pattern was detected in the different segments of the epididymis. In hypothyroid rats, the number of TR-immunoreactive epithelial cells as well as the intensity of the cytoplasmic staining significantly increased in all sections analyzed. In consonance to the immunocytochemical analysis, the expression of TR alpha 1-beta 1 isoforms, assessed by Western blot revealed significantly higher levels of TR in cytosol compared to the nuclear fractions. Furthermore, TR expression of both alpha 1 and beta 1 isoforms and their mRNA levels were increased by the hypothyroid state. The immuno-electron-microscopy showed specific reaction for TR in principal cells associated with eucromatin, cytosolic matrix and mitochondria. The differences in expression levels assessed in control and thyroidectomized rats ascertain a specific function of TH on this organ.

Control of dendritic cell maturation and function by triiodothyronine

Accumulating evidence indicates a functional crosstalk between immune and endocrine mechanisms in the modulation of innate and adaptive immunity. However, the impact of thyroid hormones (THs) in the initiation of adaptive immune responses has not yet been examined. Here we investigated the presence of thyroid hormone receptors (TRs) and the impact of THs in the physiology of mouse dendritic cells (DCs), specialized antigen-presenting cells with the unique capacity to fully activate naive T cells and orchestrate adaptive immunity. Both immature and lipopolysaccharide-matured bone marrow-derived DCs expressed TRs at mRNA and protein levels, showing a preferential cytoplasmic localization. Remarkably, physiological levels of triiodothyronine (T3) stimulated the expression of DC maturation markers (major histocompatibility complex II, CD80, CD86, and CD40), markedly increased the secretion of interleukin-12, and stimulated the ability of DCs to induce naive T cell proliferation and IFN-gamma production in allogeneic T cell cultures. Analysis of the mechanisms involved in these effects revealed the ability of T3 to influence the cytoplasmic-nuclear shuttling of nuclear factor-kappaB on primed DCs. Our study provides the first evidence for the presence of TRs on bone marrow-derived DCs and the ability of THs to regulate DC maturation and function. These results have profound implications in immunopathology, including cancer and autoimmune manifestations of the thyroid gland at the crossroads of the immune and endocrine systems.

Thyroid hormone (T3)-modification of polyethyleneglycol (PEG)-polyethyleneimine (PEI) graft copolymers for improved gene delivery to hepatocytes

Targeting of gene vectors to liver hepatocytes could offer the opportunity to cure various acquired and inherited diseases. Efficient gene delivery to the liver parenchyma has been obscured from efficient targeting of hepatocytes. Here we show that the thyroid hormone, triiodothyronine (T3), can be used to improve the gene transfer efficiency of nonviral gene vectors to hepatocytes in vitro and to the liver of mice in vivo. T3 conjugated to the distal ends of fluorescent labeled PEG-g-dextran resulted in T3-specific cellular endosomal uptake into the hepatocellular cell line HepG2. PEG-g-PEI graft copolymers with increasing molar PEG-ratios were synthesized, complexed with plasmid DNA, and transfected into HepG2 or HeLa cells. Gene transfer efficiency decreased as the number of PEG blocks increased. T3 conjugation to PEI and the distal ends of PEG blocks resulted in T3 specific gene transfer in HepG2 cells as evidenced by reduction of gene transfer efficiency after pre-incubation of cells with excess of T3. In vivo application of T3-PEG-g-PEI based gene vectors in mice after tail vein injection resulted in a significantly 7-fold increase of gene expression in the liver compared with PEG-g-PEI based gene vectors.

Thyroid hormone receptor mutations and disease: insights from knock-in mouse models

Thyroid hormone nuclear receptors (TRs) mediate thyroid hormone's activities in growth, differentiation, and development. Two TR genes (α and β ) encode four thyroid hormone-binding receptors that regulate target gene expression. Mutations of the TRβ gene cause the genetic syndrome of resistance to thyroid hormone. Studies indicate a close association between TRβ mutations and several human cancers, suggesting their oncogenic role. A TRβ gene knock-in mutant mouse (TRβ^PV/PV mouse) that spontaneously develops thyroid cancer allows elucidation of the oncogenic functions in vivo. TRβPV is a potent dominant negative mutant identified in a resistance to thyroid hormone patient. Molecular studies indicate that the PV mutant mediates its oncogenic activities via nucleus-initiated transcription and novel extranuclear actions. Thus, the deleterious effects of the gene mutations go beyond resistance to thyroid hormone and are more severe and extensive than previously envisioned. This newly identified oncogene exerts its tumorigenic effects via multiple signaling mechanisms.

Cell-surface receptor for thyroid hormone and tumor cell proliferation

Integrin αVβ3 is a structural protein of the plasma membrane that transduces signals from extracellular matrix proteins and has recently been shown to contain a novel receptor for thyroid hormone. Thyroid hormone signals are converted by αVβ3 into mitogen-activated protein kinase (MAPK) (ERK1/2) activation and downstream intracellular events in the cell nucleus. The latter include post-translational modification of the nuclear thyroid hormone receptor (TRβ1) and complex cellular or tissue responses, such as hormone-induced angiogenesis via basic fibroblast growth factor release. The integrin receptor for thyroid hormone has been shown to mediate proliferative effects of the hormone on certain tumor cell lines, including murine glioma/glioblastoma cells and human breast cancer (MCF-7) cells. More than one mechanism may account for this hormonal action, but in vitro studies indicate a direct hormonal action on cellular proliferation. Other possible mechanisms involve indirect actions via the release of tumor growth factors and effects on cell migration. In the intact organism, support of tumor growth by thyroid hormone is postulated to include angiogenesis. Crosstalk between the integrin thyroid hormone receptor and the epidermal growth factor receptor on the plasma membrane may be another mechanism by which thyroid hormone may modify tumor cell growth. Tetraiodothyroacetic acid (tetrac) is an iodothyronine analog that has no agonist activity at the integrin receptor, but inhibits binding of l-thyroxine and 3,5,3´-triiodo-l-thyronine to the receptor, preventing MAPK activation and consequent actions downstream of MAPK. In vitro studies and a preliminary in vivo experiment indicate that tetrac blocks the action of thyroid hormone on tumor cell proliferation. Both unmodified tetrac and tetrac reformulated as a nanoparticle that does not gain access to the cell interior are under investigation in animal models as anticancer agents. Also under study is the susceptibility of other human cancer cell lines to induction of proliferation by physiological concentrations of thyroid hormone.

Non-DNA binding, dominant-negative, human PPARgamma mutations cause lipodystrophic insulin resistance

PPARgamma is essential for adipogenesis and metabolic homeostasis. We describe mutations in the DNA and ligand binding domains of human PPARgamma in lipodystrophic, severe insulin resistance. These receptor mutants lack DNA binding and transcriptional activity but can translocate to the nucleus, interact with PPARgamma coactivators and inhibit coexpressed wild-type receptor. Expression of PPARgamma target genes is markedly attenuated in mutation-containing versus receptor haploinsufficent primary cells, indicating that such dominant-negative inhibition operates in vivo. Our observations suggest that these mutants restrict wild-type PPARgamma action via a non-DNA binding, transcriptional interference mechanism, which may involve sequestration of functionally limiting coactivators.

Genomic-independent action of thyroid hormones on NTPDase activities in Sertoli cell cultures from congenital hypothyroid rats

The Sertoli cells play an essential role in the maintenance and control of spermatogenesis. The ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) and 5'-nucleotidase activities can modulate the extracellular adenine nucleotide levels, controlling nucleotide-mediated signaling events in Sertoli cells. Since thyroid hormones (TH) and adenine nucleotides and nucleosides play important modulatory roles in Sertoli cell proliferation and differentiation, the aim of our study was to investigate the effect of hypothyroidism upon the NTPDase and 5'-nucleotidase activities in Sertoli cell cultures, as well as to verify whether these effects may be reversed by short and long-term supplementation with TH. Congenital hypothyroidism was induced by adding 0.02% methimazole in the drinking water from day 9 of gestation and continually until 18 days of age. Hypothyroidism significantly decreased the extracellular ATP and ADP hydrolysis and this effect was significantly reversed when cell cultures were supplemented with 1 microM T3 or 0.1 microM T4 for 30 min. In contrast, AMP hydrolysis was not altered by hypothyroidism, but was increased by T4 supplementation for 24 h. The presence of the enzymes NTPDase 1, 2 and 3 was detected by RT-PCR in Sertoli cell cultures, however, hypothyroidism was not able to alter the expression of these enzymes. These findings demonstrate that TH modify NTPDase activities in hypothyroid Sertoli cells, probably via nongenomic mechanisms and, consequently, may influence the reproductive function throughout development.

Intracellular localization of RORα is isoform and cell line-dependent

The retinoid-related orphan receptor alpha (RORalpha) belongs to the nuclear receptor superfamily and comprises four isoforms generated by different promotor usage and alternative splicing. To better understand its function, the subcellular distribution of RORalpha was investigated. We could show that subcellular distribution of RORalpha is cell line and isoform-dependent. Isoform specific differences were mediated by the A/B domains which with the exception of RORalpha1 contain a signal that mediates cytoplasmic localization. The lack of this signal in RORalpha1 results in a complete nuclear localization and prevents cell membrane association observed for RORalpha2, 3, and 4. The region responsible for membrane association was identified as the C-terminal alpha-helix 12. Furthermore, the hinge region/ligand binding domain mediates nuclear localization. Our results show that isoform specific activity of RORalpha is not only regulated by different expression and DNA binding affinities but also by different subcellular distribution. Different access to the nucleus reveals an important mechanism regulating the activity of this constitutively active nuclear receptor.

Functional neuroanatomy of thyroid hormone feedback in the human hypothalamus and pituitary gland

A major change in thyroid setpoint regulation occurs in various clinical conditions such as critical illness and psychiatric disorders. As a first step towards identifying determinants of these setpoint changes, we have studied the distribution and expression of thyroid hormone receptor (TR) isoforms, type 2 and type 3 deiodinase (D2 and D3), and the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the human hypothalamus and anterior pituitary. Although the post-mortem specimens used for these studies originated from patients who had died from many different pathologies, the anatomical distribution of these proteins was similar in all patients. D2 enzyme activity was detectable in the infundibular nucleus/median eminence (IFN/ME) region coinciding with local D2 immunoreactivity in glial cells. Additional D2 immunostaining was present in tanycytes lining the third ventricle. Thyrotropin-releasing hormone (TRH) containing neurons in the paraventricular nucleus (PVN) expressed MCT8, TRs as well as D3. These findings suggest that the prohormone thyroxine (T4) is taken up in hypothalamic glial cells that convert T4 into the biologically active triiodothyronine (T3) via the enzyme D2, and that T3 is subsequently transported to TRH producing neurons in the PVN. In these neurons, T3 may either bind to TRs or be metabolized into inactive iodothyronines by D3. By inference, local changes in thyroid hormone metabolism resulting from altered hypothalamic deiodinase or MCT8 expression may underlie the decrease in TRH mRNA reported earlier in the PVN of patients with critical illness and depression. In the anterior pituitary, D2 and MCT8 immunoreactivity occurred exclusively in folliculostellate (FS) cells. Both TR and D3 immunoreactivity was observed in gonadotropes and to a lesser extent in thyrotropes and other hormone producing cell types. Based upon these neuroanatomical findings, we propose a novel model for central thyroid hormone feedback in humans, with a pivotal role for hypothalamic glial cells and pituitary FS cells in processing and activation of T4. Production and action of T3 appear to occur in separate cell types of the human hypothalamus and anterior pituitary.

Short-Term Effects of Thyroid Hormones on Cytoskeletal Proteins Are Mediated by GABAergic Mechanisms in Slices of Cerebral Cortex from Young Rats

: Thyroid hormones play important roles in brain function. However, few information is available about the effect of 3,5,3'-triiodo-L-thyronine (T(3)) or thyroxine (T(4)) on the in vitro phosphorylation of intermediate filament (IF) proteins from cerebral cortex of rats. In this study we investigated the involvement of GABAergic mechanisms mediating the effects of T(3) and T(4) on the in vitro incorporation of (32)P into IF proteins from cerebral cortex of 10-day-old male rats. Tissue slices were incubated with or without T(3), T(4), gamma-aminobutiric acid (GABA), kinase inhibitors or specific GABA antagonists and (32)P-orthophosphate for 30 min. The IF-enriched cytoskeletal fraction was extracted in a high salt Triton-containing buffer and the in vitro (32)P incorporation into IF proteins was measured. We first observed that 1 microM T(3) and 0.1 microM T(4) significantly increased the in vitro incorporation of (32)P into the IF proteins studied through the PKA and PKCaMII activities. A similar effect on IF phosphorylation was achieved by incubating cortical slices with GABA. Furthermore, by using specific GABA antagonists, we verified that T(3) induced a stimulatory effect on IF phosphorylation through noncompetitive mechanisms involving GABA(A), beyond GABA(B) receptors. In contrast, T(4) effects were mediated mainly by GABA(B) mechanisms. In conclusion, our results demonstrate a rapid nongenomic action of T(3) and T(4) on the phosphorylating system associated to the IF proteins in slices of cerebral cortex of 10 day-old male rats and point to GABAergic mechanisms mediating such effects.

Thyroid hormone receptor β1 in normal colon and colorectal cancer–association with differentiation, polypoid growth type and K-ras mutations

The precursors for colorectal cancer include polypoid (conventional), flat and serrated adenomas. Polypoid growth in polypoid adenomas and serrated adenomas is associated with K-ras mutations. The regulation of polypoid or nonpolypoid growth is not well known, but could be related to trophic stimuli, such as thyroid hormones. Hence, we investigated the expression pattern of thyroid hormone receptor TRbeta1 in colorectal mucosa and in colorectal tumours and its relationship to tumour growth type. One hundred fourteen colorectal carcinoma specimens were evaluated for TRbeta1. Normal mucosa, adjacent adenomatous component (N = 46) and lymph node metastases (N = 28) were analysed when present, and the results were confirmed by Western blot analysis in selected cases. Nuclear TRbeta1 was almost always present in normal epithelium (96%), but less frequent in adenomas (83%) and in cancer (68%; p < 0.001 and p < 0.001, respectively). TRbeta1 was associated with polypoid growth, presence of K-ras mutations and also with a higher WHO histological grade and advanced Dukes' stage. Cytoplasmic expression of TRbeta1 was observed in nonneoplastic and neoplastic epithelium. In Western blot analysis, a 58 kDa band corresponding to TRbeta1 was expressed in normal mucosa and in colorectal cancer specimens with positive immunohistochemistry. Association of TRbeta1 expression with growth pattern and the presence of K-ras mutations suggest that abnormalities in thyroid hormone signalling involving TRbeta1 play a role in the development of some types of colorectal adenocarcinomas.

Macrophages and neurons are targets of retinoic acid signaling after spinal cord contusion injury

The physiological reactions after spinal cord injury are accompanied by local synthesis of the transcriptional activator retinoic acid (RA). RA exerts its effects by binding to retinoic acid receptors (RAR) which heterodimerize with retinoid X receptors (RXR) and then act as ligand-activated transcription factors. To identify possible cellular targets of RA we investigated protein levels and cellular distribution of retinoid receptors in the rat spinal cord at 4, 7, 14 and 21 days after a contusion injury. In the nonlesioned spinal cord, immunoreactivity for RARalpha, RXRalpha, RXRbeta and RXRgamma was localized in the cytosol of neurons, that of RXRalpha and RXRbeta in astrocytes and that of RARalpha, RXRalpha and RXRgamma in some oligodendrocytes. After contusion injury RARalpha and all RXRs appeared in the cell nuclei of reactive microglia and macrophages. This nuclear staining began at 4 days, was most prominent at 7 and 14 days and had decreased at 21 days after injury. A similar nuclear translocation was also observed for the RARalpha, RXRalpha and RXRbeta staining in neurons situated around the border of the contusion. These observations suggest that RA participates as a signal for the physiological responses of microglia and neurons after CNS injury.

Hypothalamic thyroid hormone feedback in health and disease

The role of the human hypothalamus in the neuroendocrine response to illness has only recently begun to be explored. Extensive changes in the hypothalamus-pituitary-thyroid (HPT) axis occur within the framework of critical illness. The best-documented change in the HPT axis is a decrease in serum concentrations of the biologically active thyroid hormone triiodothyronine (T3). From studies in post-mortem human hypothalamus it appeared that low serum T3 and thyrotropin (TSH) during illness (nonthyroidal illness, NTI) are paralleled by decreased thyrotropin-releasing hormone (TRH)mRNA expression in the hypothalamic paraventricular nucleus (PVN), pointing to a major alteration in HPT axis setpoint regulation. A strong decrease in TRHmRNA expression is also present in the PVN of patients with major depression as well as in glucocorticoid-treated patients. By inference, hypercortisolism in hospitalized patients with severe depression or in critical illness may induce down-regulation of the HPT axis at the level of the hypothalamus. In order to start defining the determinants and mechanisms of these setpoint changes in various clinical conditions, it is important to note that an increasing number of hypothalamic proteins appears to be involved in central thyroid hormone metabolism. In recent studies, we have investigated the distribution and expression of thyroid hormone receptor (TR) isoforms, type 2 and type 3 deiodinase (D2 and D3), and the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the human hypothalamus by a combination of immunocytochemistry, mRNA in situ hybridization and enzyme activity assays. Both D2 and D3 enzyme activities are detectable in the mediobasal hypothalamus. D2 immunoreactivity is prominent in glial cells of the infundibular nucleus/median eminence region and in tanycytes lining the third ventricle. Combined D2, D3, MCT8 or TR immunocytochemistry and TRHmRNA in situ hybridization indicates that D3, MCT8 and TRs are all expressed by TRH neurons in the PVN, whereas D2 is not. Taken together, these results suggest that the prohormone thyroxine (T4) is taken up in glial cells that convert T4 into the biologically active T3 via the enzyme D2; T3 is subsequently transported to TRH producing neurons in the PVN where it may bind to TRs and/or may be degraded into inactive iodothyronines by D3. This model for thyroid hormone action in the human hypothalamus awaits confirmation in future experimental studies.

Regulation of thyroid hormone receptor alpha2 RNA binding and subcellular localization by phosphorylation

Thyroid hormone receptor alpha2 (TRalpha2) is an alternative splice product of the TRalpha primary transcript whose unique carboxyl terminus does not bind T3 or activate transcription. The physiological function of TRalpha2 is unknown. We have found that TRalpha2 is a single stranded RNA binding protein and that the RNA binding domain localizes to a 41 amino acid region immediately distal to the second zinc finger. TRalpha2 contains a single protein kinase CK2 phosphorylation site in its amino terminus and potentially nine CK2 sites in its unique carboxyl terminus. In vitro CK2 treatment of TRalpha2 eliminated its RNA binding. Mutational analysis indicated that phosphorylations at the N- and C-terminal sites both contribute to this inhibitory effect. Cellular localization studies demonstrated that phosphorylated TRalpha2 is primarily cytoplasmic, whereas unphosphorylated TRalpha2 is primarily nuclear. Since RNA binding is a property of unphosphorylated TRalpha2, the TRalpha2-RNA interaction likely represents a nuclear function of TRalpha2.

Membrane receptors mediating thyroid hormone action

A recently identified thyroid hormone cell surface receptor on the extracellular domain of integrin alphaVbeta3 leads in human cell lines to activation of the mitogen-activated protein kinase (MAPK) signal transduction cascade. Examples of MAPK-dependent thyroid hormone actions are plasma membrane ion pump stimulation and specific nuclear events. These events include serine phosphorylation of the nuclear thyroid hormone receptor, leading to coactivator protein recruitment and complex tissue responses, such as thyroid hormone-induced angiogenesis or tumor cell growth. The existence of this cell surface receptor means that the activity of administered hormone could be limited through structural modification of the molecule to reproduce or inhibit only those hormone actions initiated at the cell surface. Examples of such modifications are provided.

FRAP analysis of nucleocytoplasmic dynamics of the vitamin D receptor splice variant VDRB1: preferential targeting to nuclear speckles

Although the key components of the cellular nuclear transport machinery have largely been characterized through extensive efforts in recent years, in vivo measurements of the kinetics of nuclear protein import/export are patently few. The present study applies the approach of FRAP (fluorescence recovery after photobleaching) to examine the nucleocytoplasmic flux of a novel human VDRB1 (vitamin D receptor B1) isoform in living cells. Through an N-terminal extension containing a consensus nuclear targeting sequence, VDRB1 is capable of localizing in nuclear speckles adjacent to SC-35 (35 kDa splicing component)-containing speckles as well as in the nucleoplasm, dependent on ligand. Investigation of VDRB1 nucleocytoplasmic transport using FRAP indicates for the first time that the VDRB1 has a serum-modulated, active nuclear import mechanism. There is no evidence of an efficient, active export mechanism for VDRB1, probably as a result of nuclear retention. VDRB1 nuclear import in the absence of serum occurred more rapidly and to a greater extent to nuclear speckles compared with import to other nuclear sites. This preferential transport from the cytoplasm to and accumulation within nuclear speckles is consistent with the idea that the latter represent dynamic centres of VDRB1 interaction with other nuclear proteins. The results are consistent with the existence of specialized pathways to target proteins to nuclear subdomains.

General Receptor for Phosphoinositides 1, a Novel Repressor of Thyroid Hormone Receptor Action that Prevents Deoxyribonucleic Acid Binding

Thyroid hormone receptors (TRs) bind to response elements (TREs) located in the promoter region of target genes and modulate their transcription. The effects of TRs require the presence of coregulators that act as adaptor molecules between TRs and complexes that are involved in chromatin remodeling or that directly contact the basal transcription machinery. Using the yeast two-hybrid system, we identified a new interacting partner for TRs: GRP1 (general receptor for phosphoinositides-1), a nucleotide exchange factor, which had never been shown to interact with nuclear receptors. We reconfirmed the interaction between TRs and GRP1 in yeast and glutathione-S-transferase pull-down assays, and determined the areas of TRs and GRP1 involved in the interaction. Coimmunoprecipitation studies demonstrated that the interaction between GRP1 and TRs takes place in the cytoplasm and the nucleus of mammalian cells. To assess functional consequences of the interaction, we used transient transfection of CV-1 cells with TR and GRP1 expression vectors and luciferase reporter genes. On positive TREs, GRP1 decreased activation by 45-60%. On the negative TREs it increased repression by blunting the activation in the absence of T3, except for TRbeta2, which was not affected. Using EMSA, we have determined that addition of GRP1 diminishes the formation of TR/TR homodimers and TR/retinoid X receptor heterodimers on TREs, which could explain the effect of GRP1 on transcription. Furthermore, protein interaction assays using increasing concentrations of double-stranded TREs show a dose-dependent decrease of the interaction between GRP1 and TRs. The homo/heterodimers formed by TRs and retinoic X receptor-alpha were not influenced by the presence of GRP1, also suggesting that GRP1 interferes directly with DNA binding. Taken together, these data provide evidence that GRP1 is a new corepressor for TRs, which modulates both positive and negative regulation by T3 by decreasing TR-complex formation on TREs.

Involvement of calcium-dependent mechanisms in T3-induced phosphorylation of vimentin of immature rat testis

Thyroid hormones have been shown to act at extra nuclear sites, inducing target cell responses by several mechanisms, frequently involving intracellular calcium concentration. It has also been reported that cytoskeletal proteins are a target for thyroid and steroid hormones and cytoskeletal rearrangements are observed during hormone-induced differentiation and development of rat testes. However, little is known about the effect of 3,5,3'-triiodo-L-thyronine (T3) on the intermediate filament (IF) vimentin in rat testes. In this study we investigated the immunocontent and in vitro phosphorylation of vimentin in the cytoskeletal fraction of immature rat testes after a short-term in vitro treatment with T3. Gonads were incubated with or without T3 and 32P orthophosphate for 30 min and the intermediate filament-enriched cytoskeletal fraction was extracted in a high salt Triton-containing buffer. Vimentin immunoreactivity was analyzed by immunoblotting and the in vitro 32P incorporation into this protein was measured. Results showed that 1 microM T3 was able to increase the vimentin immunoreactivity and in vitro phosphorylation in the cytoskeletal fraction without altering total vimentin immunocontent in immature rat testes. Besides, these effects were independent of active protein synthesis. The involvement of Ca2+-mediated mechanisms in vimentin phosphorylation was evident when specific channel blockers (verapamil and nifedipine) or chelating agents (EGTA and BAPTA) were added during pre-incubation and incubation of the testes with T3. The effect of T3 was prevented when Ca2+ influx was blocked or intracellular Ca2+ was chelated. These results demonstrate a rapid nongenomic Ca2+-dependent action of T3 in phosphorylating vimentin in immature rat testes.

Role of hsp90 and the hsp90-binding immunophilins in signalling protein movement

The ubiquitous protein chaperone hsp90 has been shown to regulate more than 100 proteins involved in cellular signalling. These proteins are called 'client proteins' for hsp90, and a multiprotein hsp90/hsp70-based chaperone machinery forms client protein.hsp90 heterocomplexes in the cytoplasm and the nucleus. In the case of signalling proteins that act as transcription factors, the client protein.hsp90 complexes also contain one of several TPR domain immunophilins or immunophilin homologs that bind to a TPR domain binding site on hsp90. Using several intracellular receptors and the tumor suppressor p53 as examples, we review evidence that dynamic assembly of heterocomplexes with hsp90 is required for rapid movement through the cytoplasm to the nucleus along microtubular tracks. The role of the immunophilin in this system is to connect the client protein.hsp90 complex to cytoplasmic dynein, the motor protein for retrograde movement toward the nucleus. Upon arrival at the nuclear pores, the receptor.hsp90.immunophilin complexes are transferred to the nuclear interior by importin-dependent facilitated diffusion. The unliganded receptors then distribute within the nucleus to diffuse patches from which they proceed in a ligand-dependent manner to discrete nuclear foci where chromatin binding occurs. We review evidence that dynamic assembly of heterocomplexes with hsp90 is required for movement to these foci and for the dynamic exchange of transcription factors between chromatin and the nucleoplasm.

Thyroid hormone induces cardiac myocyte hypertrophy in a thyroid hormone receptor alpha1-specific manner that requires TAK1 and p38 mitogen-activated protein kinase

Alterations in TR [thyroid hormone (TH) receptor]1 isoform expression have been reported in models of both physiologic and pathologic cardiac hypertrophy as well as in patients with heart failure. In this report, we demonstrate that TH induces hypertrophy as a direct result of binding to the TRalpha1 isoform and, moreover, that overexpression of TRalpha1 alone is also associated with a hypertrophic phenotype, even in the absence of ligand. The mechanism of TH and TRalpha1-specific hypertrophy is novel for a nuclear hormone receptor and involves the transforming growth factor beta-activated kinase (TAK1) and p38. Mitigating TRalpha1 effects, both TRalpha2 and TRbeta1 attenuate TRalpha1-induced myocardial growth and gene expression by diminishing TAK1 and p38 activities, respectively. These findings refine our previous observations on TR expression in the hypertrophied and failing heart and suggest that manipulation of thyroid hormone signaling in an isoform-specific manner may be a relevant therapeutic target for altering the pathologic myocardial program.

Thyroid hormone can increase estrogen-mediated transcription from a consensus estrogen response element in neuroblastoma cells

Thyroid hormones (T) and estrogens (E) are nuclear receptor ligands with at least two molecular mechanisms of action: (i) relatively slow genomic effects, such as the regulation of transcription by cognate T receptors (TR) and E receptors (ER); and (ii) relatively rapid nongenomic effects, such as kinase activation and calcium release initiated at the membrane by putative membrane receptors. Genomic and nongenomic effects were thought to be disparate and independent. However, in a previous study using a two-pulse paradigm in neuroblastoma cells, we showed that E acting at the membrane could potentiate transcription from an E-driven reporter gene in the nucleus. Because both T and E can have important effects on mood and cognition, it is possible that the two hormones can act synergistically. In this study, we demonstrate that early actions of T via TRalpha1 and TRbeta1 can potentiate E-mediated transcription (genomic effects) from a consensus E response element (ERE)-driven reporter gene in transiently transfected neuroblastoma cells. Such potentiation was reduced by inhibition of mitogen-activated protein kinase. Using phosphomutants of ERalpha, we also show that probable mitogen-activated protein kinase phosphorylation sites on the ERalpha, the serines at position 167 and 118, are important in TRbeta1-mediated potentiation of ERalpha-induced transactivation. We suggest that crosstalk between T and E includes potential interactions through both nuclear and membrane-initiated molecular mechanisms of hormone signaling.