Glycoprotein hormone receptors: determinants in leucine-rich repeats responsible for ligand specificity

Glycoprotein hormone receptors [thyrotropin (TSHr), luteinizing hormone/chorionic gonadotropin (LH/CGr), follicle stimulating hormone (FSHr)] are rhodopsin-like G protein-coupled receptors with a large extracellular N-terminal portion responsible for hormone recognition and binding. In structural models, this ectodomain is composed of two cysteine clusters flanking nine leucine-rich repeats (LRRs). The LRRs form a succession of beta-strands and alpha-helices organized into a horseshoe-shaped structure. It has been proposed that glycoprotein hormones interact with residues of the beta-strands making the concave surface of the horseshoe. Gain-of-function homology scanning of the beta-strands of glycoprotein hormone receptors allowed identification of the critical residues responsible for the specificity towards human chorionic gonadotropin (hCG). Substitution of eight or two residues of the LH/CGr into the TSHr or FSHr, respectively, resulted in constructs displaying almost the same affinity and sensitivity for hCG as wild-type LH/CGr. Molecular dynamics simulations and additional site-directed mutagenesis provided a structural rationale for the evolution of binding specificity in this duplicated gene family.

Thyrotropin stimulates production of procoagulant and vasodilative factors in human aortic endothelial cells

vasodilative Thyroid diseases have been associated with pathophysiological changes in the vasculature that may result from altered thyroid hormone production or to direct effect of elevated thyrotropin (TSH) levels on smooth muscle cells. A direct effect of TSH on vascular endothelium has not been considered. In the present study a strain of human aortic endothelial cells has been stimulated with TSH, and vascular parameters correlated with the atherosclerotic process have been analyzed. Addition of TSH induced an increase of cyclic AMP (cAMP) concentration in human aortic endothelial cells. Furthermore it induced a decrease of endothelin (from 30 +/- 2.5 to 13 +/- 1 fmol/mL) and of tissue plasminogen activator secretion (from 2,800 +/- 200 to 1,600 +/- 150 ng/mL). On the other hand, it increased nitric oxide (from 148 +/- 8 to 211 +/- 12 microM). TSH did not affect plasminogen activator inhibitor 1. Similar results were obtained when immunoglobulin Gs (IgGs) from Graves' disease patients were used. In conclusion, our findings suggest that TSH and IgGs from Graves' disease patients could stimulate endothelial cells, increasing the secretion of procoagulant and vasodilative factors, and that cAMP is involved in the transduction pathway. These findings are consistent with modifications of the fibrinolytic system reported in hypothyroidism and in Graves' disease. On the other hand, the increase of vascular resistance found in patients with hypothyroidism may be due to the altered thyroid hormone production and not to TSH directly, or to a different effect of TSH on peripheral vessels.

Autoantibodies against glutamic acid decarboxylase and 21-hydroxylase in Brazilian patients with type 1 diabetes or autoimmune thyroid diseases

Autoimmune thyroid diseases (ATD) are often associated with Type 1 diabetes mellitus (T1DM) and Addison's disease (AD), characterizing the autoimmune polyendocrine syndrome. We evaluated the frequency of autoantibodies against glutamic acid decarboxylase isoform 65 (GAD65Ab) and 21-hydroxylase (21OHAb) in the sera of 65 [58 females (F)/7 males (M), 17-70 yr] patients with Graves' disease (GD) and 47 (45 F/2 M, 12-77 yr) with Hashimoto's thyroiditis (HT), none of whom had either diabetes or AD. The sera of 30 recently diagnosed T1DM patients (16 M/14 F, 1-39 yr) and of 97 (54 F/43 M, 7-69 yr) healthy controls were also examined. GAD65Ab were detected in the sera of 18 (60%) T1DM, 8 (12%) GD and in none of the HT patients or the controls (p = 0.03 for GD vs HT, p = 0.002 for GD vs controls, and p < 0.001 for GD vs T1DM). 21OHAb were detected in the sera of 2 (3%) GD, 1 (2%) HT and in none of the T1DM patients or the controls. GAD65Ab levels were significantly lower in GD than in T1DM patients (median: -0.06 vs 0.28, p < 0.001). Six of the 8 GD GAD65Ab-positive patients submitted to an intravenous glucose tolerance test showed no diminished first phase insulin secretion. All 21OHAb positive patients had normal basal cortisol and adrenocorticotropin (ACTH), normal cortisol response after ACTH stimulation, but high plasma renin activity. In conclusion, despite the genetic diversity of the Brazilian population, the frequency of GAD65Ab and 21OHAb in our patients is similar to that observed in other countries. GAD65Ab were more prevalent in GD than in HT patients, suggesting a difference in the immune response between these disorders. Long-term follow-up is necessary to determine the clinical relevance of these autoantibodies in the Brazilian population.

Cloning and functional characterization of a testicular TSH receptor cDNA from the African catfish (Clarias gariepinus)

A cDNA encoding a putative thyroid-stimulating hormone receptor (cfTSH-R) was cloned from the testis of the African catfish (Clarias gariepinus). The cfTSH-R showed the highest amino acid sequence identity with the TSH-Rs of other fish species. In addition, an insertion of approximately 50 amino acids, specific for the TSH-R subfamily, was also present in the carboxy terminus of the amino-terminal extracellular domain of the cfTSH-R. Next to the testis and thyroid follicles, abundant cfTSH-R expression was detected in cerebellum, brain, ovary, seminal vesicles and pituitary, while weaker expression was found in muscle, stomach, intestine, head-kidney, liver, kidney and heart. HEK-T 293 cells, transiently expressing the cfTSH-R, significantly increased intracellular cAMP levels in response to human TSH. Catfish LH, human choriogonadotropin and human FSH were also able to induce this cfTSH-R-mediated response, although with considerably lower efficiency than human TSH. These results indicated that a functional cfTSH-R had been cloned from the testis of African catfish.

Targeted restoration of cleavage in a noncleaving thyrotropin receptor demonstrates that cleavage is insufficient to enhance ligand-independent activity

Two unusual features of the TSH receptor (TSHR) ectodomain are its intramolecular cleavage at the cell surface into disulfide-linked subunits and its constraint of ligand-independent (constitutive) activity inherent to the serpentine region. Whether ectodomain cleavage alters the level of TSHR constitutive activity is an important unanswered question. To address this issue, we used a TSHR engineered so as not to undergo spontaneous cleavage into subunits (deletion of amino acid residues 317-366 and GQE(367-369)NET substitution). Into this noncleaving TSHR (termed TSHR-D1-NET), we introduced thrombin recognition motifs (termed Thr 6 and Thr 18) at the site of spontaneous cleavage. Treatment of intact Chinese hamster ovary cells expressing TSHR-D1-NET-Thr 6 and -Thr 18 with thrombin induced cleavage into A and B subunits, as determined by (125)I-TSH covalent cross-linking. Nevertheless, constitutive activity of the thrombin-cleaved TSHR was unaltered. The level of TSHR constitutive activity was, therefore, fully dissociated from intramolecular cleavage into subunits. Trypsin treatment of the same cells expressing the noncleaving TSHR also generated disulfide-linked A and B subunits but, in contrast to thrombin, enhanced TSHR constitutive activity. Therefore, the activating effect of trypsin appears to involve clipping at an additional, as-yet unidentified, site. In summary, our data demonstrate that TSHR cleavage is, by itself, insufficient to reduce TSHR ectodomain constraint on ligand-independent constitutive activity. These data are consistent with other evidence that A subunit shedding consequent to TSHR cleavage is a critical factor in enhancing TSHR constitutive activity.

TSH-R expression and cytokine profile in orbital tissue of active vs. inactive Graves' ophthalmopathy patients

OBJECTIVE

From in vitro studies using cultures of orbital fibroblasts, it has become clear that cytokines play an important role in the orbital inflammation in Graves' ophthalmopathy (GO). Orbital fibroblasts seem to be the key target cells of the autoimmune attack, and they are able to express the TSH receptor (TSH-R). In vivo data on the presence of cytokines in orbital tissues are sparse, and mostly limited to samples obtained from patients with endstage, inactive GO; the same holds true for the presence of the TSH-R. The aim of the present study was to determine whether the cytokine profile and TSH-R expression differ in the active vs. the inactive stage of GO.

DESIGN AND MEASUREMENTS

Orbital fat/connective tissue was obtained from six patients with active, untreated GO undergoing emergency orbital decompression, and from 11 patients with inactive GO subjected to rehabilitative decompressive surgery. The mRNA levels of various cytokines and the TSH-R were assessed by real-time polymerase chain reaction (PCR) using the LightCycler. Data are expressed as ratios (unknown mRNA/beta-actin mRNA).

RESULTS

Active GO patients had much higher TSH-R expression than inactive patients: 4/0-24 (median value/range) vs. 0/0-9, P = 0.01. TSH-R expression was related to the Clinical Activity Score (r = 0.595, P = 0.015). Patients with active GO compared to those with inactive GO had higher mRNA levels of the proinflammatory cytokines interleukin-1beta (IL-1beta) (445/153-877 vs. 0/0-455, P = 0.001), IL-6 (1583/968-18825 vs. 559/0-7181, P = 0.01), IL-8 (1422/38-7579 vs. 32/0-1081, P = 0.046) and IL-10 (145/58-318 vs. 27/0-189, P = 0.002). In active GO there also existed a trend towards a predominance of T helper 1 (Th1)-derived cytokines as evident from higher IL-2 (37/0-158 vs. 0/0-68, P = 0.043), interferon-gamma (IFN-gamma) (20/0-79 vs. 0/0-16, P = 0.12) and IL-12 (2.3/0-14.8 vs. 0/0-1.6, P = 0.10) mRNAs. IL-1 receptor agonist (IL-1RA), IL-2 receptor (IL-2R), IL-3, IL-4, IL-5, IL-13, IL-18 and tumour necrosis factor-alpha (TNF-alpha) mRNAs were similar in both groups.

CONCLUSIONS

These data show that at the mRNA level, TSH-R expression is largely present only during the active stages of GO. The active phase is characterized by the presence of proinflammatory and Th1-derived cytokines, whereas other cytokines, among them Th2-derived cytokines, do not seem to be linked to a specific stage of GO.

The reaction of antibodies with the native and deglycosylated thyrotropin receptor obtained from transfected insect cells

The role of glycan moieties in thyrotropin receptor molecule in binding of antibodies is a subject of intense debate. To approach the function of sugars in recognition by antibody of the extracellular part of the receptor (ETSHR) we studied the reaction of the HPLC purified ETSHR from insect cells in the reaction with autoantibodies and antibodies of animal origin. None of the autoantibodies from Graves' patients sera bound to ETSHR. In contrast, each of the animal antibodies: three monoclonal, five polyclonal antireceptor and two polyclonal anti peptide corresponding to the amino acid sequence present in the receptor, became bound to the native receptor from insect cells as well as to its deglycosylated form. The shape of the dilution curves of particular antibodies in the reaction with either form of the receptor was almost the same. The coefficients of correlation was about 0.9. It seems that the correct receptor glycosylation is not crucial for binding of animal origin antibodies.

Functional significance of the thyrotropin receptor germline polymorphism D727E

In a toxic thyroid adenoma we identified a novel somatic mutation that constitutively activates the thyrotropin receptor (TSHR). Two heterozygous point mutations at adjacent nucleotides led to a substitution of alanine with asparagine at codon 593 (A593N) in the fifth transmembrane helix of TSHR. This somatic mutation resided on the same TSHR allele with the germline polymorphism D727E. The functional characteristics of the single TSHR mutants A593N and D727E and of the double mutant A593N/D727E were studied in transiently transfected COS-7 cells. The TSHR mutants A593N and A593N/D727E constitutively activated the cAMP cascade, whereas the D727E mutant did not differ from the wild-type TSHR. Surprisingly, the double mutant's specific constitutive activity was 2.3-fold lower than the A593N mutant. Thus, the polymorphism significantly ameliorates G(alphas) protein activation in the presence of the gain-of-function mutation A593N, although it is functionally inert in the context of the wild-type TSHR.

Specificity of cognate ligand-receptor interactions: fusion proteins of human chorionic gonadotropin and the heptahelical receptors for human luteinizing hormone, thyroid-stimulating hormone, and follicle-stimulating hormone

The family of glycoprotein hormones and their homologous heptahelical receptors represent an excellent system for comparative structure-function studies. We have engineered single chain molecules of human chorionic gonadotropin (hCG) fused to its cognate receptor, LH receptor (LHR), and to the noncognate receptors, TSH receptor (TSHR) and FSH receptor (FSHR; N-beta-alpha-receptor-C), to create the yoked (Y) complexes YCG/LHR, YCG/TSHR, and YCG/FSHR. The expression and bioactivity of these fusion proteins were examined in transiently transfected HEK 293 cells. Western blot analysis and antibody binding assays demonstrated that each of the proteins was expressed. In the case of YCG/LHR, minimal binding of exogenous hormone was observed due to the continued occupation of receptor by the fused ligand. The presence of hCG in the YCG/TSHR and YCG/FSHR, however, did not prevent binding of exogenous cognate ligand, presumably due to the lower affinity of hCG. The basal cAMP levels in cells expressing the YCG/LHR complex was approximately 20-fold higher than that in cells expressing LHR. Increases in basal cAMP production were also observed with YCG/TSHR and YCG/FSHR, e.g. 13- and 4-fold increases, respectively. Whereas the affinity and specificity of hCG for LHR are extraordinarily high, the hormone is capable of binding to and activating both TSHR and FSHR under these conditions that mimic high ligand concentrations. These findings were confirmed by adding high concentrations of hCG to cells expressing TSHR and FSHR. Although the functional interaction of hCG and TSHR has been recognized in gestational hyperthyroidism, there are no reports linking hCG to FSHR activation. This study, however, suggests that such a functional interaction is capable of occurring under conditions of high circulating levels of hCG, e.g. the first trimester of pregnancy and in patients with hCG-secreting tumors.

Thyroid-stimulating monoclonal antibodies

Thyrotropin (TSH) receptor monoclonal antibodies (TSHR mAbs) were obtained from cDNA-immunized NMRI mice. Three mAb immunoglobulin Gs (IgGs) (TSmAbs 1-3) that had distinct V(H )and V(L) region sequences stimulated cyclic adenosine monophosphate (cAMP) production in isolated porcine thyroid cells greater than 10x basal and as little as 20 ng/mL (0.13 nmol/L) of TSmAb 1 IgG caused a 2x basal stimulation. TSmAb 1 and 2 Fab fragments were also effective stimulators and thyroid-stimulating activities of the IgGs and Fabs were confirmed using TSHR transfected Chinese hamster ovary (CHO) cells. The TSmAbs also inhibited (125)I-labeled TSH binding to TSHR-coated tubes by 50% or more at concentrations of 1 microg/mL or less and gave 15%-20% inhibition at 20-50 ng/mL. (125)I-labeled TSmAbs bound to TSHR-coated tubes with high affinity (approximately 10(10) L/mol) and this binding was inhibited by TSHR autoantibodies with both TSH agonist and antagonist activities. Inhibition of labeled TSmAb binding by Graves' sera correlated well with inhibition of TSH binding (r = 0.96; n = 18; p < 0.001 for TSmAb 2). The TSmAbs have considerable potential as (1) new probes for TSHR structure-function studies, (2) reagents for new assays for TSHR autoantibodies, and (3) alternatives to recombinant TSH in various in vivo applications.

Characterization of the thyrotropin binding pocket

A panel of monoclonal antibodies (mAbs) to the thyrotropin receptor (TSHR) was prepared using three different immunization strategies. The mAbs obtained (n = 138) reacted with linear epitopes covering most of the TSHR extracellular domain and with conformational epitopes. mAbs that bound to five different regions of the TSHR (amino acids [aa] 32-41, aa 36-42, aa 246-260, aa 277-296, and aa 381-385) were able to inhibit (125)I-labeled thyrotropin (TSH) binding to solubilized TSHR preparations. Fab and immunoglobulin G (IgG) preparations were similarly effective inhibitors for mAbs reactive with aa 246-260, aa 277-291 and aa 381-385 suggesting that these three regions of the TSHR are involved in TSH binding. In contrast mAbs reactive with aa 32-41 and aa 36-42 were not effective at inhibiting TSH binding when Fab preparations were used, suggesting that these N terminal regions of the TSHR were less critical for TSH binding. Our studies suggest that three distinct and discontinuous regions of the TSHR (aa 246-260 and 277-296 on the TSHR A subunit) and aa 381-385 (on the TSHR B subunit) fold together to form a complex TSH binding pocket. Alignment of the aa sequences of these three regions in TSHRs from different species indicates that they are highly conserved.

A monoclonal thyroid-stimulating antibody

The thyrotropin receptor, also known as the thyroid-stimulating hormone receptor (TSHR), is the primary antigen of Graves disease. Stimulating TSHR antibodies are the cause of thyroid overstimulation and were originally called long-acting thyroid stimulators due to their prolonged action. Here we report the successful cloning and characterization of a monoclonal antibody (MS-1) with TSHR-stimulating activity. The thyroid-stimulating activity of MS-1 was evident at IgG concentrations as low as 20 ng/ml. MS-1 also competed for radiolabeled TSH binding to the native TSHR and was able to compete for TSH-induced stimulation. MS-1 recognized a conformational epitope within the TSHR alpha (or A) subunit but excluding the receptor cleavage region. Using an assay measuring loss of antibody recognition after cleavage we demonstrated that MS-1, in contrast to TSH, was unable to enhance TSHR posttranslational cleavage. Since receptor cleavage is followed by alpha subunit shedding and receptor degradation, the functional half-life of the receptor may be extended. The isolation and characterization of MS-1 provides a novel explanation for the prolonged thyroid stimulation in this disease which may be secondary to the lack of receptor cleavage in addition to the prolonged half-life of IgG itself.

Ligand-dependent inhibition of oligomerization at the human thyrotropin receptor

Recently, several studies have reported oligomerization of G protein-coupled receptors, although the functional implications of this phenomenon are still unclear. Using fluorescence resonance energy transfer (FRET) and coimmunoprecipitation (COIP), we previously reported that the human thyrotropin (TSH) receptor tagged with green fluorescent protein (TSHR(GFP)) and expressed in a heterologous system was present as oligomeric complexes on the cell surface. Here, we have extended this biophysical and biochemical approach to study the regulation of such oligomeric complexes. Co-expression of TSHR(GFP) and TSHR(Myc) constructs in Chinese hamster ovary cells resulted in FRET-positive cells. The specificity of the FRET signal was verified by the absence of energy transfer in individually transfected TSHR(GFP) and TSHR(Myc):Cy3 cells cultured together and also by acceptor photobleaching. Occupation of the receptor molecule by the ligand (TSH) resulted in a dose-dependent decrease in the FRET index from 20% in the absence of TSH to <1% with 10(3) microunits/ml of TSH. Such reduction in oligomeric forms was also confirmed by coimmunoprecipitation. Exposure of TSHR(GFP/Myc) cells to forskolin or cytochalasin D caused no change in the FRET index, confirming that the decrease in the oligomeric complexes was a receptor-dependent phenomenon and free of energy or microtuble requirements. The TSH-induced decrease in TSHR oligomers was found to be secondary to dissociation of the TSHR complexes as evidenced by an increase in fluorescent intensity of photobleached spots of GFP fluorescence with 10(3) microunits/ml of TSH. These data indicated that the less active conformation of the TSHR was comprised of receptor complexes and that such complexes were dissociated on the binding of ligand. Such observations support the concept of a constitutively active TSHR dimer or monomer that is naturally inhibited by the formation of higher order complexes. Inhibition of these oligomeric forms by ligand binding returns the TSHR to an activated state.

Differential responses of an invariant region in the ectodomain of three glycoprotein hormone receptors to mutagenesis and assay conditions

The glycoprotein hormone receptors-luteinizing hormone receptor (LHR), follicle-stimulating hormone receptor (FSHR), and thyroid-stimulating hormone receptor (TSHR)--are G-protein-coupled receptors with an invariant 10-amino acid residue sequence in the ectodomain proximal to transmembrane helix 1. A Glu-Asp, located at the midpoint of this conserved sequence, has been suggested to be important in ligand-mediated signaling of LHR and/or receptor expression or stability, but not binding. One goal of this study was to expand the studies on LHR and determine whether the invariant Glu and Asp residues were functional in FSHR and TSHR as well. Another goal was to investigate systematically the role of ionic strength, particularly Na+, which appears to have enigmatic functions in the three receptors regarding ligand binding and receptor activation, and to ascertain whether any of the purported effects of Na+ could involve the conserved pair of acidic side chains in the ectodomain. COS-7 cells were transiently transfected with cDNAs to the wild-type (WT) receptor (rat) and identical single and double mutants of each (Glu --> Ala, Asp; Asp --> Ala, Glu; and Glu-Asp--> Asp-Glu), followed by characterization of cognate ligand binding and signaling (basal and hormone mediated) in two commonly used buffer systems: Waymouth's medium, containing a near-physiologic concentration of Na+ (132 mM); a low ionic strength buffer with a 1 mM concentration of Na+. The three receptors exhibited differential responses to mutagenesis and the two buffers. Notably, a comparison of basal cyclic adenosine monophosphate (cAMP) production showed that the buffer of lower ionic strength resulted in increased basal cAMP production in WT TSHR but not LHR and FSHR; that the maximal ligand-mediated cAMP production was greatest in the buffer of higher ionic strength for the three WT receptors; that functionality of the conserved Glu and Asp residues in ligand-mediated signaling was buffer dependent in LHR, whereas it did not appear to be particularly important in FSHR and TSHR signaling; and that apparent ligand binding in WT and mutant TSHRs seemed to be particularly diminished in the buffer of higher ionic strength. These results demonstrate that identical amino acid residues in homologous receptors can exhibit distinct functions; moreover, the role of ionic strength (Na+) on signaling differs in the three receptors.

Association of the thyrotropin receptor with calnexin, calreticulin and BiP. Efects on the maturation of the receptor

The thyrotropin receptor (TSHR) is a member of the G protein-coupled receptor superfamily. It has by now been clearly established that the maturation of the glycoproteins synthesized in the endoplasmic reticulum involves interactions with molecular chaperones, which promote the folding and assembly of the glycoproteins. In this study, we investigated whether calnexin (CNX), calreticulin (CRT) and BiP, three of the main molecular chaperones present in the endoplasmic reticulum, interact with the TSHR and what effects these interactions might have on the folding of the receptor. In the first set of experiments, we observed that in a K562 cell line expressing TSHR, about 50% of the receptor synthesized was degraded by the proteasome after ubiquitination. In order to determine whether TSHR interact with CNX, CRT and BiP, coimmunoprecipitation experiments were performed. TSHR was found to be associated with all three molecular chaperones. To study the role of the interactions between CNX and CRT and the TSHR, we used castanospermine, a glucosidase I and II inhibitor that blocks the interactions between these chaperones and glycoproteins. In K562 cells expressing the TSHR, these drugs led to a faster degradation of the receptor, which indicates that these interactions contribute to stabilizing the receptor after its synthesis. The overexpression of calnexin and calreticulin in these cells stabilizes the receptor during the first hour after its synthesis, whereas the degradation of TSHR increased in a cell line overexpressing BiP and the quantity of TSHR able to acquire complex type oligosaccharides decreased. These results show that calnexin, calreticulin and BiP all interact with TSHR and that the choice made between these two chaperone systems is crucial because each of them has distinct effects on the folding and stability of this receptor at the endoplasmic reticulum level.

TSH signaling and cell survival in 3T3-L1 preadipocytes

Thyroid-stimulating hormone (TSH) action in adipose tissue remains largely unknown. Our previous work indicates that human preadipocytes express functional TSH receptor (TSHR) protein, demonstrated by TSH activation of p70 S6 kinase (p70 S6K). We have now studied murine 3T3-L1 preadipocytes to further characterize TSH signaling and cellular action. Western blot analysis of 3T3-L1 preadipocyte lysate revealed the 100-kDa mature processed form of TSHR. TSH activated p70 S6K and protein kinase B (PKB/Akt), as measured by immunoblot analysis. Preincubation with wortmannin or LY-294002 completely blocked TSH activation of p70 S6K and PKB/Akt, implicating phosphoinositide 3-kinase (PI3K) in their regulation. TSH increased phosphotyrosine protein(s) in the 125-kDa region and augmented the associated PI3K activity fourfold. TSH had no effect on cAMP levels in 3T3-L1 preadipocytes, suggesting that adenylyl cyclase is not involved in TSH activation of the PI3K-PKB/Akt-p70 S6K pathway. 3T3-L1 preadipocyte cell death was reduced by 29-76% in serum-deprived (6 h) preadipocytes treated with 1-20 microM TSH. In the presence of 20 microM TSH, an 88% reduction in terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL)-positive cells was observed in serum-starved (3 h) 3T3-L1 preadipocytes as well as a 93% reduction in the level of cleaved activated caspase 3. In summary, TSH acts as a survival factor in 3T3-L1 preadipocytes. TSH does not stimulate cAMP accumulation in these cells but instead activates a PI3K-PKB/Akt-p70 S6K pathway.

Involvement of the protein kinase C pathway in thyrotropin-induced STAT3 activation in FRTL-5 thyroid cells

The binding of thyrotropin (TSH) to the TSH receptor (TSHR) activates two signaling pathways: the cAMP-protein kinase A (PKA) and the protein kinase C (PKC) systems. We have recently demonstrated that TSH activates the Janus kinases (JAK)/signal transducer and activator of transcription (STAT) pathway via TSHR. This study aimed to investigate whether the cAMP/PKA or the PKC system is involved in STAT3 activation in response to TSH. Treatment with TSH activated STAT3 phosphorylation in FRTL-5 thyrocytes and human TSHR-expressing Chinese hamster ovary cells. TSH-induced STAT3 activation was inhibited by a blocking antibody directed against TSHR that was isolated from patients with primary myxoedema. Increased intracellular cAMP activated STAT3 but inhibition of PKA did not affect STAT3 activation. On the other hand, the PKC stimulant PMA induced STAT3 phosphorylation and the PKC inhibitors inhibited it. Moreover, inhibition of PKC blocked STAT3 activation induced by a stimulator of cAMP. Our data suggest that TSH activates STAT3 via TSHR and cAMP- and PKC-dependent pathways, and provide evidence that PKC may be involved in the pathway downstream from cAMP.

Receptor-mediated adenylyl cyclase activation through XLalpha(s), the extra-large variant of the stimulatory G protein alpha-subunit

XLalpha(s), the large variant of the stimulatory G protein alpha subunit (Gsalpha), is derived from GNAS1 through the use of an alternative first exon and promoter. Gs(alpha) and XLalpha(s) have distinct amino-terminal domains, but are identical over the carboxyl-terminal portion encoded by exons 2-13. XLalpha(s) can mimic some functions of Gs(alpha), including betagamma interaction and adenylyl cyclase stimulation. However, previous attempts to demonstrate coupling of XLalpha(s) to typically Gs-coupled receptors have not been successful. We now report the generation of murine cell lines that carry homozygous disruption of Gnas exon 2, and are therefore null for endogenous XLalpha(s) and Gs(alpha) (Gnas(E2-/E2-)). Gnas(E2-/E2-) cells transfected with plasmids encoding XLalpha(s) and different heptahelical receptors, including the beta2-adrenergic receptor and receptors for PTH, TSH, and CRF, showed agonist-mediated cAMP accumulation that was indistinguishable from that observed with cells transiently coexpressing Gs(alpha) and these receptors. Our findings thus indicate that XLalpha(s) is capable of functionally coupling to receptors that normally act via Gs(alpha).

Thyroid-stimulating autoantibodies in Graves disease preferentially recognize the free A subunit, not the thyrotropin holoreceptor

Graves disease is directly caused by thyroid-stimulating autoantibodies (TSAb's) that activate the thyrotropin receptor (TSHR). We observed upon flow cytometry using intact cells that a mouse mAb (3BD10) recognized the TSHR ectodomain with a glycosidylphosphatidylinositol (ECD-GPI) anchor approximately tenfold better than the same ectodomain on the wild-type TSHR, despite the far higher level of expression of the latter. The 3BD10 epitope contains the N-terminal cysteine cluster critical for TSAb action. Consequently, we hypothesized and confirmed that TSAb (but not thyrotropin-blocking autoantibodies [TBAb's]) also poorly recognize the wild-type TSHR relative to the ECD-GPI. Despite poor recognition by TSAb of the holoreceptor, soluble TSHR A subunits (known to be shed from surface TSHR) fully neutralized autoantibody-binding activity. These data indicate that the epitope(s) for TSAb's, but not for TBAb's, are partially sterically hindered on the holoreceptor by the plasma membrane, the serpentine region of the TSHR, or by TSHR dimerization. However, the TSAb epitope on the soluble A subunit is freely accessible. This observation, as well as other evidence, supports the concept that A subunit shedding either initiates or amplifies the autoimmune response to the TSHR, thereby causing Graves disease in genetically susceptible individuals.

Expression and activity of g protein-coupled receptor kinases in differentiated thyroid carcinoma

Most of the TSH effects on the proliferation and differentiation of thyroid cells are mediated by cAMP via an adenylyl cyclase-activating Gs protein. TSH receptor responsiveness in cell cultures, is regulated by G protein-coupled receptor kinase (GRK) 2 and 5. To determine whether an alteration in activity and expression of GRKs might be associated with variable levels of TSH receptor desensitization in vivo, we studied human thyroid tissues including 21 normal tissues and 18 differentiated carcinomas. GRK activity was assayed by rhodopsin phosphorylation, and GRK protein and mRNA expressions assessed by immunoblotting and real-time quantitative RT-PCR, respectively. GRK2 and GRK5 were found as the predominant isoforms in the human thyroid. GRK5 protein expression was significantly decreased in differentiated thyroid carcinoma (P < 0.02) and paralleled a decrease in GRK mRNA expression (P < 0.02). In contrast, no difference in protein and mRNA levels of GRK2 were observed between normal and cancerous thyroid tissues. Although GRK2 protein levels correlated with GRK activities, we demonstrated a significant increase in GRK activity in differentiated thyroid carcinoma (P < 0.02). Less TSH receptor desensitization occurred in differentiated carcinoma than in normal thyroid tissue, as judged by TSH-stimulated cAMP response in human thyroid cells in primary culture. In conclusion, this study indicates that GRK2 activity and GRK5 expression have opposite regulations in cancer cells. Furthermore, the decrease in GRK5 expression may underlie the reduction in homologous desensitization of the TSH receptor in differentiated thyroid carcinoma, contributing to explain the increased cAMP levels in these tumors.

Expression of G(alpha)(s) proteins and TSH receptor signalling in hyperfunctioning thyroid nodules with TSH receptor mutations

OBJECTIVE

Constitutively activating mutations of the thyrotrophin receptor (TSHR) are the main molecular cause of hyperfunctioning thyroid nodules (HTNs). The G protein coupling is an important and critical step in the TSHR signalling which mainly includes G(alpha)(s), G(alpha)(i) and G(alpha)(q)/11 proteins.

DESIGN

We investigated the in vitro consequences of overexpressing G(alpha) proteins on signalling of the wild-type (WT) or mutated TSHR. Moreover, we investigated whether changes in G(alpha) protein expression are pathophysiologically relevant in HTNs or cold thyroid nodules (CTNs).

METHODS

Wild-type TSH receptor and mutated TSH receptors were coexpressed with G(alpha)(s), G(alpha)(i) or G(alpha)(q)/11, and cAMP and inositol phosphate (IP) production was measured after stimulation with TSH. The expression of G(alpha)(s), G(alpha)(i) and G(alpha)(q)/11 proteins was examined by Western blotting in 28 HTNs and 14 CTNs.

RESULTS

Coexpression of G(alpha)(s) with the WT TSH receptor in COS 7 cells significantly increased the basal and TSH-stimulated cAMP accumulation while coexpression of the G(alpha)(q) or G(alpha)11 protein significantly increased the production of cAMP and inositol triphosphate (IP(3)). The coexpression of the TSH receptor mutants (I486F, DEL613-621), known to couple constitutively to G(alpha)(s) and G(alpha)(q) with G(alpha)(s) and G(alpha)(q)/11, significantly increased the basal and stimulated cAMP and IP(3) accumulation. Coexpression of the TSH receptor mutant V556F with G(alpha)(s) only increased the basal and stimulated cAMP production while its coexpression with G(alpha)(q)/11 increased the basal and stimulated IP(3) signalling. The expression of G(alpha)(s) protein subunits determined by Western blotting was significantly decreased in 14 HTNs with a constitutively activating TSH receptor mutation in comparison with the corresponding surrounding tissue, while in 14 HTNs without TSH receptor or G(alpha)(s) protein mutation and in 14 CTNs the expression of G(alpha)(s) protein was not different compared with the surrounding tissue. The expression of G(alpha)(i) and G(alpha)(q)/11 proteins in HTNs or CTNs was not significantly different compared with the surrounding tissue.

CONCLUSIONS

The reduced expression of G(alpha)(s) protein subunits in HTNs with TSHR mutations could act as a feedback mechanism to desensitise the chronically stimulated cAMP cascade. As G(alpha) protein expression was not significantly increased in the majority of CTNs and HTNs an influence of G(alpha) overexpression on TSH signalling could be excluded in these nodules.

Thyrostimulin, a heterodimer of two new human glycoprotein hormone subunits, activates the thyroid-stimulating hormone receptor

Human thyrotropin (TSH), luteotropin (LH), follitropin (FSH), and chorionic gonadotropin are members of the heterodimeric glycoprotein hormone family. The common alpha subunit forms noncovalent heterodimers with different beta subunits. Two novel human glycoprotein hormonelike genes, alpha2 (A2) and beta5 (B5), recently have been identified. Using a yeast two-hybrid assay, the two subunits were found as potential heterodimerization partners. Immunological analyses confirmed the heterodimerization of A2 and B5 in transfected cells and their colocalization in the anterior pituitary. Recombinant A2/B5 heterodimeric glycoproteins, purified using cation exchange and size fractionation chromatography, activated human TSH receptors, but not LH and FSH receptors, and showed high affinity to TSH receptors in a radioligand receptor assay. The heterodimer also stimulated cAMP production and thymidine incorporation by cultured thyroid cells and increased serum thyroxine levels in TSH-suppressed rats in vivo. This new heterodimeric glycoprotein hormone was named as thyrostimulin based on its thyroid-stimulating activity. The expression of thyrostimulin in the anterior pituitary known to express TSH receptors suggested a paracrine mechanism. The present discovery of a new ligand based on genomic approaches could facilitate the understanding of the physiological roles of extra-thyroid TSH receptor systems and the structural-functional basis of receptor signaling by related glycoprotein hormones.

TGFbeta1 effects on functional activity of porcine thyroid cells cultured in suspension

Thyrotropin (TSH) and transforming growth factor beta 1 (TGFbeta1) have major roles in the regulation of folliculogenesis and differentiation in thyroid cells. Isolated porcine thyroid cells cultured in the presence of TSH on a plastic surface recover a follicular architecture and exhibit normal functional properties. The addition of TGFbeta1 to the culture medium induces important morphological changes and extracellular matrix remodelling. Similarly, thyroid cells lose their ability to organify iodine and their responsiveness to adenylate cyclase. The aim of this study was to analyse the influence of TGFbeta1 on the functional activity of thyrocytes in suspension culture, independent of follicle disruption. In this system, we demonstrate that TGFbeta1 inhibits expression of thyroperoxidase, NADPH oxidase activity, iodine uptake and, consequently, iodine organification. Moreover, TGFbeta1 decreases basal and TSH-stimulated cAMP production and TSH receptor expression. Taken together, these data converge to demonstrate an essential role of TGFbeta1 in the regulation of the thyroid cell function.