Intracellular distribution of the endogenous and transfected beta form of thyroid hormone nuclear receptor visualized by the use of domain-specific monoclonal antibodies

Distribution of thyroid hormone and thyrotropin receptors in reproductive tissues of adult female rabbits

PURPOSE

Thyroid dysfunctions are related to anovulation, miscarriages, and infertility in women and laboratory animals. Mechanisms associated with these effects are unknown, although indirect or direct actions of thyroid hormones and thyrotropin could be assumed. The present study aimed to identify the distribution of thyroid hormones (TRs) and thyrotropin (TSHR) receptors in reproductive organs of female rabbits.

MATERIAL AND METHODS

Ovary of virgin and pregnant rabbits, as well as the oviduct, uterus, and vagina of virgin rabbits were excised, histologically processed, and cut. Slices from these organs were used for immunohistochemical studies for TRα1-2, TRß1, and TSHR.

RESULTS

The presence of TRs and TSHR was found in the primordial, primary, secondary, tertiary, and Graafian follicles of virgin rabbits, as well as in the corpora lutea, corpora albicans, and wall of hemorrhagic cysts of pregnant rabbits. Oviductal regions (fimbria-infundibulum, ampulla, isthmus, and utero-tubal junction), uterus (endometrium and myometrium), and vagina (abdominal, pelvic, and perineal portions) of virgin rabbits showed anti-TRs and anti-TSHR immunoreactivity. Additionally, the distal urethra, paravaginal ganglia, levator ani and iliococcygeus muscles, dorsal nerve and body of the clitoris, perigenital skin, and prostate had TRs and TSHR.

CONCLUSIONS

The wide presence of TRs and TSHR in female reproductive organs suggests varied effects of thyroid hormones and thyrotropin in reproduction.

Oncogenic mutations of thyroid hormone receptor β

The C-terminal frame-shift mutant of the thyroid hormone receptor TRβ1, PV, functions as an oncogene. An important question is whether the oncogenic activity of mutated TRβ1 is uniquely dependent on the PV mutated sequence. Using four C-terminal frame-shift mutants—PV, Mkar, Mdbs, and AM—we examined that region in the oncogenic actions of TRβ1 mutants. Remarkably, these C-terminal mutants induced similar growth of tumors in mouse xenograft models. Molecular analyses showed that they physically interacted with the p85α regulatory subunit of PI3K similarly in cells. In vitro GST-binding assay showed that they bound to the C-terminal Src-homology 2 (CSH2) of p85α with markedly higher avidity. The sustained association of mutants with p85α led to activation of the common PI3K-AKT-ERK/STAT3 signaling to promote cell proliferation and invasion and to inhibit apoptosis. Thus, these results argue against the oncogenic activity of PV being uniquely dependent on the PV mutated sequence. Rather, these four mutants could favor a C-terminal conformation that interacted with the CSH2 domain of p85α to initiate activation of PI3K to relay downstream signaling to promote tumorigenesis. Thus, we propose that the mutated C-terminal region of TRβ1 could function as an “onco-domain” and TRβ1 is a potential therapeutic target.

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.

Src-dependent phosphorylation at Y406 on the thyroid hormone receptor β confers the tumor suppressor activity

Association studies suggest that the thyroid hormone receptor β1 (TRβ1) could function as a tumor suppressor in cancer cells. However, the underlying molecular mechanisms remain to be elucidated. We explored how TRβ1 acted as a tumor suppressor in breast cancer MDA cells. Proliferation and invasiveness were markedly inhibited in cells stably expressing TRβ1 (MDA-TRβ1 cells). cSrc-phosphorylated TRβ1 at Y406 signaled T3-induced degradation. Mutation of Y406 to Phe (TRβ1Y406F) did not affect T3 binding affinity, but blocked T3-induced degradation in cells. Importantly, cell-based studies showed that TRβ1Y406F lost the inhibitory effects by TRβ1 on cell proliferation and invasion. Consistently, in xenograft models, MDA-TRβ1 cells exhibited significantly slower tumor growth rates than those of Neo control cells. In contrast, the tumor growth rates of MDA-TRβ1Y406F cells were indistinguishable from those of Neo control cells. We further showed that markedly more TRβ1Y406F than TRβ1 was physically associated with cSrc in cells, leading to constitutive activation of cSrc-FAK-ERK signaling. In contrast, degradation of T3-bound TRβ1 complexed with cSrc attenuated signaling to decrease cell proliferation and invasiveness, thus confirming TRβ1 as a tumor suppressor. Thus, the present studies suggested that TRβ1 could be tested as a novel potential therapeutic target.

Role of TSH in the spontaneous development of asymmetrical thyroid carcinoma in mice with a targeted mutation in a single allele of the thyroid hormone-β receptor

Mutations of the thyroid hormone receptor-β gene (THRB) cause resistance to thyroid hormone (RTH). A mouse model of RTH harboring a homozygous thyroid hormone receptor (TR)-β mutation known as PV (Thrb(PV/PV) mouse) spontaneously develops follicular thyroid cancer (FTC). Similar to RTH patients with mutations of two alleles of the THRB gene, the Thrb(PV/PV) mouse exhibits elevated thyroid hormones accompanied by highly nonsuppressible TSH. However, the heterozygous Thrb(PV/+) mouse with mildly elevated TSH (~2-fold) does not develop FTC. The present study examined whether the mutation of a single allele of the Thrb gene is sufficient to induce FTC in Thrb(PV/+) mice under stimulation by high TSH. Thrb(PV/+) mice and wild-type siblings were treated with propylthiouracil (PTU) to elevate serum TSH. Thrb(PV/+)mice treated with PTU (Thrb(PV/+)-PTU) spontaneously developed FTC similar to human thyroid cancer, but wild-type siblings treated with PTU did not. Interestingly, approximately 33% of Thrb(PV/+)-PTU mice developed asymmetrical thyroid tumors, as is frequently observed in human thyroid cancer. Molecular analyses showed activation of the cyclin 1-cyclin-dependent kinase-4-transcription factor E2F1 pathway to increase thyroid tumor cell proliferation of Thrb(PV/+)-PTU mice. Moreover, via extranuclear signaling, the PV also activated the integrin-Src-focal adhesion kinase-AKT-metalloproteinase pathway to increase migration and invasion of tumor cells. Therefore, mutation of a single allele of the Thrb gene is sufficient to drive the TSH-simulated hyperplastic thyroid follicular cells to undergo carcinogenesis. The present study suggests that the Thrb(PV/+)-PTU mouse model potentially could be used to gain insights into the molecular basis underlying the association between thyroid cancer and RTH seen in some affected patients.

Advanced bone formation in mice with a dominant-negative mutation in the thyroid hormone receptor β gene due to activation of Wnt/β-catenin protein signaling

Thyroid hormone (T(3)) acts in chondrocytes and bone-forming osteoblasts to control bone development and maintenance, but the signaling pathways mediating these effects are poorly understood. Thrb(PV/PV) mice have a severely impaired pituitary-thyroid axis and elevated thyroid hormone levels due to a dominant-negative mutant T(3) receptor (TRβ(PV)) that cannot bind T(3) and interferes with the actions of wild-type TR. Thrb(PV/PV) mice have accelerated skeletal development due to unknown mechanisms. We performed microarray studies in primary osteoblasts from wild-type mice and Thrb(PV/PV) mice. Activation of the canonical Wnt signaling in Thrb(PV/PV) mice was confirmed by in situ hybridization analysis of Wnt target gene expression in bone during postnatal growth. By contrast, T(3) treatment inhibited Wnt signaling in osteoblastic cells, suggesting that T(3) inhibits the Wnt pathway by facilitating proteasomal degradation of β-catenin and preventing its accumulation in the nucleus. Activation of the Wnt pathway in Thrb(PV/PV) mice, however, results from a gain of function for TRβ(PV) that stabilizes β-catenin despite the presence of increased thyroid hormone levels. These studies demonstrate novel interactions between T(3) and Wnt signaling pathways in the regulation of skeletal development and bone formation.

Regulation of beta-catenin by a novel nongenomic action of thyroid hormone beta receptor

We previously created a knock-in mutant mouse harboring a dominantly negative mutant thyroid hormone receptor beta (TRbeta(PV/PV) mouse) that spontaneously develops a follicular thyroid carcinoma similar to human thyroid cancer. We found that beta-catenin, which plays a critical role in oncogenesis, was highly elevated in thyroid tumors of TRbeta(PV/PV) mice. We sought to understand the molecular basis underlying aberrant accumulation of beta-catenin by mutations of TRbeta in vivo. Cell-based studies showed that thyroid hormone (T3) induced the degradation of beta-catenin in cells expressing TRbeta via proteasomal pathways. In contrast, no T3-induced degradation occurred in cells expressing the mutant receptor (TRbetaPV). In vitro binding studies and cell-based analyses revealed that beta-catenin physically associated with unliganded TRbeta or TRbetaPV. However, in the presence of T3, beta-catenin was dissociated from TRbeta-beta-catenin complexes but not from TRbetaPV-beta-catenin complexes. beta-Catenin signaling was repressed by T3 in TRbeta-expressing cells through decreasing beta-catenin-mediated transcription activity and target gene expression, whereas sustained beta-catenin signaling was observed in TRbetaPV-expressing cells. The stabilization of beta-catenin, via association with a mutated TRbeta, represents a novel activating mechanism of the oncogenic protein beta-catenin that could contribute to thyroid carcinogenesis in TRbeta(PV/PV) mice.

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.

Quantification of hormone-induced atrophy of large myotubes from C2C12and L6 cells: atrophy-inducible and atrophy-resistant C2C12myotubes

Myofiber atrophy is the final outcome of muscle wasting induced by catabolic factors such as glucocorticoids and thyroid hormones. We set up an in vitro system to define the catabolic reaction based on myotube atrophy. Both mouse C2C12and rat L6 cells were used. C2C12myotube formation was improved by replacing horse serum with the serum substitute Ultroser G. A new method was developed to quantify size changes of large (0.5–1 mm) myotubes only, excluding remaining myoblasts and small myotubes. Dexamethasone reduced myotube size by 30% in L6 but not in C2C12myotubes. Expression of the glucocorticoid receptor was twofold higher in L6 myotubes than in C2C12myotubes. In both cell lines, 3,3′,5-triiodo-l-thyronine (T3) did not induce a significant size reduction. Expression of the major T3receptor (T3Rβ1) was higher in L6 myotubes. We investigated whether the changes in myotube size are related to changes in atrogin-1 expression, as this enzyme is thought to be a key factor in the initiation of muscle atrophy. Dexamethasone induced a twofold increase of atrogin-1 mRNA; again, only L6 myotubes were susceptible. Interestingly, atrogin-1 expression in Ultroser G-fused C2C12myotubes was lower than that in horse serum-fused myotubes. Furthermore, dexamethasone treatment increased atrogin-1 expression only in horse serum-fused myotubes but not in Ultroser G-fused myotubes. Ultroser G-induced fusion may result in atrophy-resistant C2C12myotubes. Therefore, C2C12myotubes offer an ideal system in which to study skeletal muscle atrophy because, depending on differentiation conditions, C2C12cells produce atrophy-inducible and atrophy-resistant myotubes.

Thyroid status affects number and localization of thyroid hormone receptor expressing mast cells in bone marrow

Thyroid hormone (T(3)) plays a key role in endochondral ossification. The process relies on the coordinated synthesis and degradation of cartilage matrix and is disrupted in juvenile hypothyroidism, leading to abnormal skeletal development. Mast cells synthesize and store matrix-degrading enzymes. We examined whether thyroid status influences skeletal mast cell distribution in growing rats to determine whether they might modulate the actions of T(3) in bone. Tibiae were collected for histological, histochemical, immunohistochemical, and immunofluorescence analysis. Mast cells were increased throughout the bone marrow in hypothyroid rats compared with euthyroid, thyrotoxic, and hypothyroid-thyroxine replaced animals. Large numbers were present in metaphyseal marrow adjacent to the growth plate in hypothyroid animals and cells were distributed evenly throughout the marrow. Very few mast cells were present in metaphyseal marrow in other groups, but their numbers increased with increasing distance from the growth plate. T(3) receptor alpha1 (TRalpha1) was expressed in the nucleus and cytoplasm of skeletal mast cells, whereas TRalpha2 and TRbeta1 were restricted to the cytoplasm. Localization of TRs was not affected by altered thyroid status. Thus, disrupted endochondral ossification in hypothyroidism may be mediated in part by skeletal mast cells, which express TR proteins and may function as T(3) target cells.

Regulation of thyroid hormone receptor isoforms in physiological and pathological cardiac hypertrophy

Physiological and pathological cardiac hypertrophy have directionally opposite changes in transcription of thyroid hormone (TH)-responsive genes, including alpha- and beta-myosin heavy chain (MyHC) and sarcoplasmic reticulum Ca(2+)-ATPase (SERCA), and TH treatment can reverse molecular and functional abnormalities in pathological hypertrophy, such as pressure overload. These findings suggest relative hypothyroidism in pathological hypertrophy, but serum levels of TH are usually normal. We studied the regulation of TH receptors (TRs) beta1, alpha1, and alpha2 in pathological and physiological rat cardiac hypertrophy models with hypothyroid- and hyperthyroid-like changes in the TH target genes, alpha- and beta-MyHC and SERCA. All 3 TR subtypes in myocytes were downregulated in 2 hypertrophy models with a hypothyroid-like mRNA phenotype, phenylephrine in culture and pressure overload in vivo. Myocyte TRbeta1 was upregulated in models with a hyperthyroid-like phenotype, TH (triiodothyronine, T3), in culture and exercise in vivo. In myocyte culture, TR overexpression, or excess T3, reversed the effects of phenylephrine on TH-responsive mRNAs and promoters. In addition, TR cotransfection and treatment with the TRbeta1-selective agonist GC-1 suggested different functional coupling of the TR isoforms, TRbeta1 to transcription of beta-MyHC, SERCA, and TRbeta1, and TRalpha1 to alpha-MyHC transcription and increased myocyte size. We conclude that TR isoforms have distinct regulation and function in rat cardiac myocytes. Changes in myocyte TR levels can explain in part the characteristic molecular phenotypes in physiological and pathological cardiac hypertrophy.

Hormone binding induces rapid proteasome-mediated degradation of thyroid hormone receptors

The thyroid hormone 3,3',5-triiodo-l-thyronine (T3) is essential for growth, differentiation, and development. Its biological activities are mediated by T3 nuclear receptors (TRs). At present, how T3 regulates TR proteins and the resulting functional consequences are still unknown. Immunofluorescence analyses of endogenous TR in the growth hormone-producing GC cells showed that the T3-induced rapid degradation of TR was specifically blocked by lactacystin, a selective inhibitor of the ubiquitin-proteasome degradation pathway. Immunoblots demonstrated that the transfected TRbeta1 was ubiquitinated and that the ubiquitination was T3 independent. Studies with a series of truncated TRbeta1 showed that the hormone-binding domain was sufficient for the T3-induced rapid degradation of TRbeta1 by the proteasome degradation pathway. T3 also induced rapid degradation of TRbeta2 and TRalpha1. In contrast, the stability of the non-T3-binding TRalpha2 and naturally occurring TRbeta1 mutants that do not bind T3 was not affected by T3 treatment, indicating that hormone binding to receptor was essential for the degradation of the wild-type receptors. In the presence of proteasome protease inhibitors, the levels of both total and ubiquitinated TRbeta1 protein increased, yet T3-dependent transcriptional activation and the expression of the growth hormone gene were diminished, suggesting that proteasome-mediated degradation played a novel role in modulating transcriptional activation by TR. The present study reveals a role of T3 in modulating the functions of TR by regulating its receptor level via the ubiquitin-proteasome degradation pathway.

Molecular characterization of myocardial fibrosis during hypothyroidism: evidence for negative regulation of the pro-alpha1(I) collagen gene expression by thyroid hormone receptor

The purpose of this study was to gain insights into the underlying mechanism of myocardial fibrosis during hypothyroidism. Treatment of cardiac fibroblasts with a medium lacking thyroid hormone led to a 47% increase in [3H]thymidine incorporation into the cell nuclei compared with that in untreated cells. Northern blot analysis of RNA from cardiac fibroblasts grown in a thyroid hormone depleted medium resulted in a 38% increase in the abundance of mRNA for pro-alpha1(I) collagen. At the protein level, the amount of type I collagen, as determined by immunoprecipitation, was increased either in the cell lysate (46%) of cardiac fibroblasts grown in a thyroid hormone depleted medium or in the medium (44%). The chimeric plasmid, ColCAT 3.6, contains the 5'-flanking region of the rat pro-alphal(I) collagen gene (from bases -3520 to +115) fused to the chloramphenicol acetyltransferase (CAT) gene. The plasmid was cotransfected with thyroid hormone receptor (TR) expression plasmid into rat cardiac fibroblasts and COS-l cells (monkey mesangial cells). Cells transfected with the ColCAT plasmid in the presence of thyroid hormone (100 nM T(3)) had a significant decrease (39% in fibroblasts, P<0.01; 52% in COS-1 cells, P<0.001) in CAT activity when compared to cells not exposed to thyroid hormone. Transient co-transfection of TR with various pro-alphal(I) collagen/CAT deletion constructs showed that T(3)-dependent repression was preserved with the deletion from 3520 bp of the flanking sequence to a 5' end point at position -224, indicating that a thyroid hormone-response element (TRE) was localized at the region -224 to +115. The TR-DNA binding assays demonstrated binding of the human TRbeta1 to a fragment containing a proposed TRE located between position -35 and +115 in the 5'-flanking region of the rat pro-alphal(I) collagen gene.

The orphan nuclear receptor Ear-2 is a negative coregulator for thyroid hormone nuclear receptor function

Thyroid hormone (T3) nuclear receptors (TR) are ligand-dependent transcription factors which regulate growth, differentiation, and development. One emerging hypothesis suggests that TR mediate these diverse effects via a large network of coregulators. Recently, we found that TR-mediated transcriptional responses varied in six cell lines derived from different tissues. We therefore used human TR subtype beta1 (TRbeta1) as bait to search for coregulators in human colon carcinoma RKO cells with a yeast two-hybrid system. RKO cells exhibited T3-dependent and -independent transcriptional activation. One of the three positive clones was identified as Ear-2, which is a distant member of the chick ovalbumin upstream promoter-transcription factors of the orphan nuclear receptor family. The physical interaction between Ear-2 and TRbeta1 was further confirmed by specific binding of Ear-2 to glutathione S-transferase-TRbeta1. In addition, Ear-2 was found to associate with TRbeta1 in cells. As a result of this physical interaction, binding of TRbeta1 to the T3 response elements was inhibited. Using reporter systems, we found that both the basal activation and the T3-dependent activation mediated by TRbeta1 were repressed by Ear-2 in CV1 cells. In RKO cells, however, the T3-independent transcriptional activity was more sensitive to the repression effect of Ear-2 than the T3-dependent transcriptional activity. The repression effect of Ear-2 was reversed by steroid hormone receptor coactivator 1. These results suggest that TR-mediated responses reflect a balance of corepressors and coactivators in cells. These findings further strengthen the hypothesis that the diverse activities of TR are achieved via a large network of coregulators that includes Ear-2.

Role of thyroid hormones and their receptors in peripheral nerve regeneration

After peripheral nerve injury in adult mammals, reestablishment of functional connections depends on several parameters including neurotrophic factors, the extracellular matrix, and hormones. However, little is known about the contribution of hormones to peripheral nerve regeneration. Thyroid hormones, which are required for the development and maturation of the central nervous system, are also important for the development of peripheral nerves. The action of triiodothyronine (T3) on responsive cells is mediated through nuclear thyroid hormone receptors (TRs) which modulate the expression of specific genes in target cells. Thus, to study the effect of T3, it is first necessary to know whether the target tissues possess TRs. The fact that sciatic nerve cells possess functional TRs suggests that these cells can respond to T3 and, as a consequence, that thyroid hormone may be involved in peripheral nerve regeneration. The silicone nerve guide model provides an excellent system to study the action of local administration of T3. Evidence from such studies demonstrate that animals treated locally with T3 at the level of transection have more complete regeneration of sciatic nerve and better functional recovery. Among the possible regulatory mechanisms by which T3 enhances peripheral nerve regeneration is rapid action on both axotomized neurons and Schwann cells which, in turn, produce a lasting and stimulatory effect on peripheral nerve regeneration. It is probable that T3 up- or down-regulates gene expression of one or more growth factors, extracellular matrix, or cell adhesion molecules, all of which stimulate peripheral nerve regeneration. This could explain the greater effect of T3 on nerve regeneration compared with the effect of any one growth factor or adhesion molecule.

Tissue-specific differential repression of gene expression by a dominant negative mutant of thyroid hormone beta1 receptor

Resistance to thyroid hormone (RTH) is a genetic disease caused by the mutations of the thyroid hormone beta receptor (TRbeta) gene, producing receptors with a dominant negative action. The present study addressed the question as to whether tissue-specific factors modulate the dominant negative function in different tissues. We prepared stably transfected pituitary GH3 (GH3-PV) and liver SK-Hep-1 (SK-Hep-1-PV) cell lines with a potent dominant negative mutant, PV. The growth hormone (GH) and the malic enzyme genes (ME) in GH3 and SK-Hep-1, respectively, are directly regulated by the thyroid hormone, 3,3,'5-triiodo-L-thyronine (T3). The ratio of the expressed PV/endogenous TRbeta1 proteins was approximately 20 and 5 for GH3-PV and SK-Hep-1-PV cells, respectively. However, the T3-activated expression of the GH gene in GH3-PV and ME gene in SK-Hep-1-PV was repressed by approximately 30% and 90%, respectively, indicating the lack of correlation of PV/TRpbeta1 protein ratio with the dominant negative potency of mutant PV. Furthermore, the synergistic effect of the pituitary-specific factor 1 on the TR-mediated GH promoter activity was not repressed by mutant PV. Taken together, these results suggest that the dominant negative effect of mutant TR is variable in the tissues studied.

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

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors that regulate growth, differentiation, and development. To understand the role of the hormone, 3,3', 5-triiodo-L-thyronine (T3), in the nuclear translocation and targeting of TRs to the regulatory sites in chromatin, we appended green fluorescent protein (GFP) to the human TR subtype beta1 (TRbeta1). The fusion of GFP to the amino terminus of TRbeta1 protein did not alter T3 binding or transcriptional activities of the receptor. The subcellular localization of GFP-TRbeta1 in living cells was visualized by laser-scanning confocal microscopy. In the presence of T3, the expressed GFP-TRbeta1 was predominately localized in the nucleus, exhibiting a nuclear/cytoplasmic ratio of approximately 5.5. No GFP-TRbeta1 was detected in the nucleolus. In the absence of T3, more GFP-TRbeta1 was present in the cytoplasm, exhibiting a nuclear/cytoplasmic ratio of approximately 1.5. In these cells, cytoplasmic GFP-TRbeta1 could be induced to enter the nucleus by T3. The T3-induced translocation was blocked when Lys184-Arg185 in domain D of TRbeta1 was mutated to Ala184-Ala185. Furthermore, the inability of the mutant TR to translocate to the nucleus correlated with the loss of most of its transcriptional activity. These results suggest that TR functions may, in part, be regulated by T3-induced nuclear entry.

Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRaplha and TRbeta, indicates that TRbeta is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.

The expression of thyroid hormone receptors in human bone

The mechanism of action of thyroid hormones on bone is poorly understood. Thyroid hormones may act on bone cells either indirectly by increasing secretion of growth hormone (GH) and insulin-like growth factor-1 (IGF-1), or directly by influencing target genes via specific nuclear receptors. The presence of thyroid hormone receptors (TRs) has been demonstrated in human and rodent osteoblast-like cells and cell lines and recently in osteoclasts derived from an osteoclastoma in vitro. However, their presence in human bone in situ has not been reported. We have used specific polyclonal antibodies to TR-alpha 1, -alpha 2, and -beta 1 to investigate the expression of these receptors in sections of human osteophytes and heterotopic bone. Osteoblasts and osteoclasts were identified by alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP), respectively, whereas chondrocytes were identified morphologically. At sites of endochondral and intramembranous bone formation, TR-beta 1 and the splice variant -alpha 2 were widely expressed by proliferating, mature, and hypertrophic chondrocytes and also in cells within the fibrous tissue and at the bone forming surfaces, respectively. They were also detected in osteoblasts, osteoclasts, and a few osteocytes at sites of bone remodeling. In contrast, TR-alpha 1 was the least expressed and was present mainly in osteoblasts at remodeling sites and in a few mature and undifferentiated chondrocytes. Our results show, for the first time, the presence and distribution of TRs in human bone in situ and suggest that the skeletal actions of thyroid hormones may be mediated via these receptors. Further studies are required to define the role of the individual receptor isoforms in bone metabolism.

Critical role of glutamine 252 in the hormone-dependent transcriptional activity of the thyroid hormone beta1 nuclear receptor

To understand the molecular basis of the ligand-dependent transcriptional activity of thyroid hormone nuclear receptors (TRs), we investigated the effect of mutation of glutamine 252 (Q252) on the function of human TR subtype beta1 (wTRbeta1). Q252 is conserved in TRs in all species and is located in a region of the hormone binding domain that has been shown to undergo 3,3',5-triiodo-L-thyronine (T3) induced conformational changes. Q252 was mutated to Gly (Q252G) or Asn (Q252N) and their immunoreactivity, hormone, and DNA binding activities were characterized. Mutants Q252G and Q252N bound to T3 with similar affinity as the wTRbeta1. However, they failed to interact with a monoclonal anti-wTRbeta1 antibody whose epitope is located in the region of amino acids 248-256, suggesting that mutation of Q252 to Gly or Asn resulted in local structural alteration in TRbeta1. In addition, mutation of Glu to Gly or Asn led to increases in their binding to the thyroid hormone response elements (TREs) as homodimers and as heterodimers with the retinoid X receptor. Mutants Q252G and Q252N were more effective as repressors in the absence of T3, while both had a 1.5-2-fold higher T3-dependent transcriptional activity mediated by three TREs than the wTRbeta1. The increases in the transcriptional activity were not due to an increase in the expression of the mutant receptor proteins because the in vivo expression level of the mutant receptor proteins was identical to that of the wTRbeta1. Our data indicate that the T3-dependent transcriptional activity is not entirely dependent on the T3 binding activity of the receptor. The interplay of ligand and DNA binding domains plays a pivotal role in the transcriptional activity of the TRs.

The differential hormone-dependent transcriptional activation of thyroid hormone receptor isoforms is mediated by interplay of their domains

Human thyroid hormone nuclear receptor isoforms (TRalpha1 and TRbeta1) express differentially in a tissue-specific and development-dependent manner. It is unclear whether these two isoforms have differential functions. We analyzed their interaction with a thyroid hormone response element with half-site binding motifs arranged in an everted repeat separated by six nucleotides (F2). Despite extensive sequence homologies, the two isoforms bound to F2 with different affinities and ratios of homodimer/monomer. Using F2-containing reporter gene, we found that the transcriptional activity of TRbeta1 was approximately 6-fold higher than that of TRalpha1. The lower activity of TRalpha1 was not due to differences in expression of the two isoforms because similar nuclear localization patterns were observed. To understand the structural determinants responsible for these differences, we constructed chimeric receptors in which hinge regions (domain D), hormone binding domains (domain E), and domains (D + E) were sequentially interchanged and their activities were compared. Chimeric TRs containing the domains D, E or (D + E) of TRbeta1 showed increased propensities to form homodimers and mediated higher transactivation activities than TRalpha1. Thus, differential transactivation activities of TR isoforms are mediated by interplay of their domains and could serve as an important regulatory mechanism to achieve diversity and specificity of pleiotropic T3 effect.

Tissue-specific stabilization of the thyroid hormone beta1 nuclear receptor by phosphorylation

The present study evaluated the expression and regulation of endogenous thyroid hormone receptors (TRs) in cultured cells. In COS-1 cells, the endogenous TR, subtype beta1 (TRbeta1), but not subtype beta2 or alpha1, was induced to express by okadaic acid (OA) in a concentration-dependent manner. The induced TRbeta1 had immunoreactivity and partial V8 proteolytic maps similar to those of the transfected and in vitro translated human TRbeta1 (h-TRbeta1). The OA-induced expression of endogenous TRbeta1 was, however, not observed in a variety of other cultured cell lines tested, indicating that the induction was cell type-dependent. TRbeta1 induced by OA was a multisite phosphorylated protein, in which serine and threonine in a ratio of 10:1 were phosphorylated. The induced TRbeta1 was functional as it could mediate the thyroid hormone-dependent transcriptional activity via several thyroid hormone response elements. The induction of endogenous TRbeta1 expression by OA was not accompanied by an increase in mRNA levels but was the result of an increase in the stability of the TRbeta1 protein. This is the first report to indicate that one of the mechanisms by which the TR isoforms are differentially expressed is via the tissue-specific stabilization of the TR isoform proteins. Furthermore, this selective stability of TRbeta1 could be conferred by phosphorylation.

Exclusive homodimerization of the orphan receptor hepatocyte nuclear factor 4 defines a new subclass of nuclear receptors

Hepatocyte nuclear factor 4 (HNF-4), a highly conserved member of the steroid hormone receptor superfamily critical for development and liver-specific gene expression, is very similar to another superfamily member, retinoid X receptor alpha (RXR alpha), in overall amino acid sequence and DNA binding specificity. Since RXR alpha is known to heterodimerize with many other nuclear receptors, the formation of heterodimers between HNF-4 and RXR alpha was examined. With the electrophoretic mobility shift assay, coimmunoprecipitation, and transient transfection assays, it is shown that, unlike other nuclear receptors, HNF-4 does not form heterodimers with RXR alpha either in the presence or in the absence of DNA. We also show that in vitro-translated HNF-4 does not form heterodimeric complexes on DNA with a number of other receptors, including RXR beta, RXR gamma, retinoic acid receptor alpha, or thyroid hormone receptor alpha. To investigate the hypothesis that the lack of heterodimerization between HNF-4 and RXR alpha is due to a strong homodimerization activity of HNF-4, glycerol gradient sedimentation and kinetic analysis were used to show that HNF-4 is in fact a stable homodimer in solution. Finally, immunohistochemistry is used to show that the HNF-4 protein is found exclusively in the nuclei in both HepG2 cells, which express endogenous HNF-4, and transfected COS cells, which overexpress HNF-4. These findings lead us to propose that HNF-4 defines a new subclass of nuclear receptors which reside primarily in the nucleus and which bind DNA and regulate transcription as homodimers.

Nuclear and cytoplasmic triiodothyronine-binding sites in primary sensory neurons and Schwann cells: radioautographic study during development

The effects of the thyroid hormones on target cells are mediated through nuclear T3 receptors. In the peripheral nervous system, nuclear T3 receptors were previously detected with the monoclonal antibody 2B3 mAb in all the primary sensory neurons throughout neuronal life and in peripheral glia at the perinatal period only (Eur. J. Neurosci. 5, 319, 1993). To determine whether these nuclear T3 receptors correspond to functional ones able to bind T3, cryostat sections and in vitro cell cultures of dorsal root ganglion (DRG) or sciatic nerve were incubated with 0.1 nM [125I]-labeled T3, either alone to visualize the total T3-binding sites or added with a 10(3) fold excess of unlabeled T3 to estimate the part due to the non-specific T3-binding. After glutaraldehyde fixation, radioautography showed that the specific T3-binding sites were largely prevalent. The T3-binding capacity of peripheral glia in DRG and sciatic nerve was restricted to the perinatal period in vivo and to Schwann cells cultured in vitro. In all the primary sensory neurons, specific T3-binding sites were disclosed in foetal as well as adult rats. The detection of the T3-binding sites in the nucleus indicated that the nuclear T3 receptors are functional. Moreover the concomitant presence of both T3-binding sites and T3 receptors alpha isoforms in the perikaryon of DRG neurons infers that: 1) [125I]-labeled T3 can be retained on the T3-binding 'E' domain of nascent alpha 1 isoform molecules newly-synthesized on the perikaryal ribosomes; 2) the alpha isoforms translocated to the nucleus are modified by posttranslational changes and finally recognized by 2B3 mAb as nuclear T3 receptor. In conclusion, the radioautographic visualization of the T3-binding sites in peripheral neurons and glia confirms that the nuclear T3 receptors are functional and contributes to clarify the discordant intracellular localization provided by the immunocytochemical detection of nuclear T3 receptors and T3 receptor alpha isoforms.

Stimulation of proliferation by 3,3',5-triiodo-L-thyronine in poorly differentiated human hepatocarcinoma cells overexpressing beta 1 thyroid hormone receptor

To understand the role of thyroid hormone nuclear receptors (TRs) in hepatocarcinogenesis, we characterized the TRs in nine human hepatocarcinoma cell lines. The expression of TR proteins is receptor subtype- and cell type-dependent. TR alpha 1 protein expresses similarly at a low level in each of the nine cell lines. In contrast, TR beta 1 is overexpressed in hepatocarcinoma cells which are poorly differentiated. Furthermore, thyroid hormone was found to stimulate the proliferation of cells in which TR beta 1 is overexpressed. These results suggest that TR beta 1 is most likely involved in the differentiation and proliferation of hepatocarcinoma cells. Our studies have shed new light in the understanding of the role of TRs in liver carcinogenesis.

Phosphorylation enhances the target gene sequence-dependent dimerization of thyroid hormone receptor with retinoid X receptor

To understand the molecular basis of the phosphorylation-enhanced transcriptional activity of human thyroid hormone nuclear receptor subtype beta 1 (hTR beta 1), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta). In vitro, the extent of hTR beta 1.RXR beta heterodimer bound to various thyroid hormone response elements (TREs) was compared before and after phosphorylation of hTR beta 1. Without phosphorylation, hTR beta 1.RXR beta heterodimer was barely detectable under the experimental conditions. After phosphorylation of hTR beta 1, heterodimer bound to (i) the chicken lysozyme gene TRE, (ii) a TRE consisting of direct repeats of half-site binding motifs separated by four gaps, and (iii) a palindromic TRE was enhanced by approximately 10-, 7-, and 6-fold, respectively. The effect of phosphorylation on hTR beta 1.RXR beta heterodimerization was reversible. Dephosphorylation of the phosphorylated hTR beta 1 by alkaline phosphatase led to loss of the ability of hTR beta 1 to form a heterodimer with RXR beta in either the absence or the presence of DNA. These results indicate that the heterodimerization is enhanced by phosphorylation. To evaluate the effect of phosphorylation on the interaction of hTR beta 1 with RXR beta in vivo, we cotransfected hTR beta 1, RXR beta and TRE-chloramphenicol acetyltransferase (CAT) expression plasmids into CV-1 cells. CAT activity was assessed in the presence or absence of okadaic acid. Okadaic acid is a potent inhibitor of phosphatases 1 and 2A and increases the in vivo phosphorylation of hTR beta 1 by approximately 10-fold. Using the CAT reporter gene under control of the TRE from the malic enzyme gene, we found that RXR beta increased the okadaic acid-enhanced hTR beta 1-mediated CAT activity by 2- to 3-fold in the presence of 3,3',5-triiodo-L-thyronine. However, 9-cis-retinoic acid did not enhance the effect of okadaic acid. Our results indicate that phosphorylation is essential for the interaction of hTR beta 1 with RXR beta. Thus, phosphorylation plays a pivotal role in the gene-regulating activity of hTR beta 1.

One-step immunoaffinity purification of human beta 1 thyroid hormone receptor with DNA and hormone binding activity

An efficient and versatile method to purify large amounts of active human beta 1 thyroid hormone receptor (h-TR beta 1) was developed. Using a T7 expression system, h-TR beta 1 was overexpressed in Escherichia coli. Approx. 80% of the expressed receptor protein was concentrated in the insoluble inclusion bodies and approximately 20% was in the soluble form (h-TR beta 1-S). h-TR beta 1-S was conveniently purified by one immunoaffinity chromatographic step. From 1 l of cell culture, approx. 0.1 mg of purified h-TR beta 1-S was obtained. The purified h-TR beta 1-S binds to 3,3',5-triiodo-L-thyronine with a Ka = 2 x 10(9) M-1 and exhibits analog specificity. The purified h-TR beta 1-S also binds to T3 response elements (TRE) with different orientation in the half-sites with differential activity. In addition, binding of h-TR beta 1-S to TREs was enhanced by retinoid X receptor. These results indicate that the purified h-TR beta 1-S retains its hormone and DNA binding activity. The purified h-TR beta 1-S is suitable for structural and functional studies. This method could be used to purify h-TR beta 1 or rat TR beta 1 expressed in insect cells or yeast.

Phosphorylation stimulates the transcriptional activity of the human beta 1 thyroid hormone nuclear receptor

The role of phosphorylation on the gene activation activity of the human beta 1 thyroid hormone nuclear receptor (h-TR beta 1) was examined. h-TR beta 1 was found to be a phosphoprotein when expressed in COS-1 cells, with serine, threonine, and tyrosine (85:10:5) as the phosphorylation sites. Okadaic acid (a potent inhibitor of phosphatases 1 and 2A) at 0.1, 0.25, and 0.5 microM increased the phosphorylation of h-TR beta 1 by 3-, 7-, and 11-fold, respectively. The increase in phosphorylation was accompanied by a concomitant increase in phosphorylation was accompanied by a concomitant increase in receptor-mediated transcription in transient transfection assays. h-TR beta 1 purified from Escherichia coli was phosphorylated in vitro by the endogenous kinase from cellular extracts. Serine, threonine, and tyrosine were phosphorylated in a similar ratio to that found in COS-1 cells. The in vitro phosphorylation was stimulated by okadaic acid. Phosphorylation did not affect the binding of h-TR beta 1 to 3,3',5-triiodo-L-thyronine. However, phosphorylation of h-TR beta 1 resulted in an increase of its binding to DNA and conferred on it the ability to bind to nuclear accessory proteins. The results indicate that phosphorylation plays an important role in the transcriptional activity of h-TR beta 1.

Cutaneous manifestations of thyroid disease

Cutaneous manifestations of thyroid disease are protean in nature and affect all age groups. This review focuses on normal thyroid gland physiology, specific cutaneous/thyroid lesions such as the thyroglossal duct cyst and metastatic thyroid malignancies, nonspecific cutaneous alterations of the hyperthyroid and hypothyroid states, and the numerous associations of thyroid disease with other cutaneous and/or systemic disorders.