The small GTP-binding protein, Rhes, regulates signal transduction from G protein-coupled receptors

The Ras homolog enriched in striatum, Rhes, is the product of a thyroid hormone-regulated gene during brain development. Rhes and the dexamethasone-induced Dexras1 define a novel distinct subfamily of proteins within the Ras family, characterized by an extended variable domain in the carboxyl terminal region. We have carried this study because there is a complete lack of knowledge on Rhes signaling. We show that in PC12 cells, Rhes is targeted to the plasma membrane by farnesylation. We demonstrate that about 30% of the native Rhes protein is bound to GTP and this proportion is unaltered by typical Ras family nucleotide exchange factors. However, Rhes is not transforming in murine fibroblasts. We have also examined the role of Rhes in cell signaling. Rhes does not stimulate the ERK pathway. By contrast, it binds to and activates PI3K. On the other hand, we demonstrate that Rhes impairs the activation of the cAMP/PKA pathway by thyroid-stimulating hormone, and by an activated beta2 adrenergic receptor by a mechanism that suggests uncoupling of the receptor to its cognate heterotrimeric complex. Overall, our results provide the initial insights into the role in signal transduction of this novel Ras family member.

Upon thyrotropin binding the thyrotropin receptor is internalized and localized to endosome

To study the fate of TSH receptor (TSHR) on TSH binding, we constructed a chimeric cDNA that encodes a yellow fluorescent protein (YFP) fused to the carboxyl terminus of human TSHR. The protein expression in transfected cells was confirmed using flow cytometry. The functionality of the chimeric protein was determined by its ability to transduce signal leading to activation of cAMP in a TSH dose-dependent manner. The levels of cAMP produced by these cells were comparable with the levels seen in cells transfected with unfused TSHR without the YFP. Using deconvolution microscopy, we observed that the receptor is largely expressed on the cell surface, but on addition of TSH, some of the receptors were rapidly internalized. This conclusion was supported by several independent observations involving different cells expressing either native or recombinant TSHR. On TSH treatment, we observed internalization of human TSHR-YFP and human TSHR, expressed on 293 and CHO cells, respectively. This was further substantiated when we observed colocalization of rhodamine-labeled TSH with TSHR-YFP within the cell and by the uptake of radiolabeled TSH. Furthermore, shortly after ligand binding, there was a profound change in the morphology of the cells and some of the receptors accumulated in the perinuclear region of the cell. The TSHR-YFP was colocalized with RhoB-cyan fluorescent protein, indicating that it accumulated within the endosomes. These results indicate that the receptor internalization might in part be responsible for TSHR desensitization on TSH binding.

[Analysis of costimulatory molecules (CD28-CTLA-4/B7) expression on chosen mononuclear cells in adolescents with Graves' disease during methimazole therapy]

CD28 and CTLA-4 are glycoprotein molecules providing the potent costimulatory signal for T cells activation and proliferation via interactions with their ligands B7/BB1 molecule, present on the surface of Ag-presenting cells (APC). The present study was performed to elucidate the relationship between CTLA-4/CD28 molecules and stimulating (TSAb) or blocking (TBAb) antibodies to the TSH-receptor in Graves' disease. The aim of the study was to estimate the expression of CTLA-4 (cytolitic T lymphocyte associated antigen-4, CD152), CD28, B7.1 (CD80) and CD7.2 (CD86) molecules on peripheral blood cells in patients with Graves' disease (GD) (n=28, mean age 15.4), in patients with nontoxic nodular goiter (NTNG) (n=28, mean age 15.6 years) in comparison with sex- and age-matched healthy control subjects (n=28, mean age 15.9 years). The expression of the costimulatory molecules on mononuclear cells was analyzed by the three-color flow cytometry using a Coulter EPICS XL cytometer. Detection of stimulating and blocking antibodies to the TSH-receptor using JPO9 CHO cells in unfractionated serum was measured by a highly sensitive commercial radioimmunoassay. In untreated Graves' patients we observed a significant increase of CD152+ (p<0.004, p<0.004, p<0.001) and CD28+ (p<0.02, p<0.02, p<0.02) T lymphocytes in comparison to the non-toxic nodular goiter patients, healthy control subjects and euthyroid Graves' patients. After 2-6 months of methimazole therapy, the percentages of these cells in the peripheral blood of hyperthyroid patients returned to normal values. The analysis of CD3+ T lymphocytes co-expressing CD152 and CD28 antigens on peripheral blood revealed increased percentages of CTLA-4/CD28 positive cells in patients with Graves' disease (p<0.004, p<0.04) compared to the controls and euthyroid Graves' patients, while B7.1 (CD80) and B7.2 (CD86) molecules were detected only in some hyperthyroid patients on activated monocytes. In addition, 75% of children with untreated hyperthyroidism had positive TSAbs, whereas TBAbs were measured in 3 out of 7 TSAb negative patients with Graves' disease. In untreated Graves' patients a correlation between percentage of CD152+ T cells and serum level of stimulating and blocking antibodies to the TSH-receptor was found, while no such correlation was detected in relation to CD28+ T cells. We conclude that the changes of the expression of costimulatory molecules on peripheral blood mononuclear cells could be an important marker of activity of an autoimmune process in children and adolescents with Graves' disease and that their levels are modulated by thyroestatic treatment.

Evidence for two discontinuous regions on the thyrotropin receptor involved in the binding of the monoclonal antibody 34A

The human thyrotropin receptor (hTSHR) is a major autoantigen in thyroid autoimmunity. The aim of this study was to localize the discontinuous epitope recognized by the anti-hTSHR monoclonal antibody (mAb) 34A. We used the phage-displayed peptide technology and selected thirteen 34A-specific mimotopes which could be grouped into four families according to their sequence homologies. Mimotope sequence alignments on the primary sequence of hTSHR allowed us to identify regions 88-100 (family I homologous motif Tx(8)FYNL) and 276-281 (family IV homologous motif DxSYPS) as being putative parts of the discontinuous epitope recognized by the mAb 34A. Interestingly, by using the Spot method, TSH was also found to interact with peptides bearing amino acids from these two regions, suggesting their involvement in TSH/TSHR interaction. Moreover these data are in agreement with the ability of the mAb 34 to displace TSH binding to its receptor. In addition, purified IgG from nine patients with Graves' disease were able to specifically recognize family I-specific mimotopes, whereas those from healthy donors did not. Taken together, our data suggest the involvement of regions 88-100 and 276-281 in the epitope recognized by mAb 34A as well as purified IgG from patients' sera and provide a basis for rational guided mutagenesis.

Does thyrotropin cleave its cognate receptor?

A recent report of major pathophysiological significance, and opposed to present concepts, is that TSH (but not MS-1, a hamster monoclonal thyroid-stimulating antibody), cleaves the single-chain TSH receptor (TSHR) on the cell surface into its two-subunit form. We reassessed the issue using two approaches. First we wished to confirm the flow-cytometric assay previously used to quantitate TSHR cleavage. We used CHO cell lines expressing large (TSHR-10,000 cells) or conventional (TSHR-0 cells) numbers of TSHR. Cells were preincubated (16 h) in either control medium or medium supplemented with TSH (5 x 10(-8) m) or MS-1 (10 microg/ml). After stringent washing to maximize removal of residual ligand, we performed flow cytometry with two antibodies, one recognizing only the single-chain TSHR, the other recognizing all (cleaved and uncleaved) TSHRs. TSH pretreatment did not appear to increase TSHR cleavage. Instead we observed ligand occupancy of the TSHR (with MS-1) or fewer receptors on the cell surface (down-regulation), particularly with the TSHR-0 cells. Second, we covalently cross-linked [125I]TSH to monolayers of these cells, an unequivocal method to determine directly the proportion of single-chain and two-subunit TSHR forms. Pretreatment of TSHR-10,000 and TSHR-0 cells with TSH had no effect on the degree of TSHR cleavage. MS-1 slightly reduced spontaneous cleavage. In conclusion, in contrast to a recent report, we show that TSH does not alter the subunit structure of its cognate receptor, and we provide insight into the difficulties associated with the flow-cytometric assay for TSHR cleavage.

TSH, the bone suppressing hormone

The skeleton is a dynamic organ whose structural integrity depends on constant remodeling, controlled by many local and systemic factors. In this issue of Cell, identify thyroid-stimulating hormone (TSH) as an important regulator of this process.

Pitfalls in the use of transfected overexpression systems to study membrane proteins function: the case of TSH receptor and PRA1

Thyrotropin (TSH) stimulates thyroid cell differentiation and proliferation by binding to its G-protein coupled receptor (GPCR) receptor, the thyrotropin receptor (TSHR). To understand some differences in transduction between human and dog cells a search for partners proteins was performed by a two-hybrid screen with the dog C-terminal tail of the receptor as bait. Prenylated Rab acceptor 1 (PRA1) was identified by this way. Pursuing with the characterization of the biochemical interaction, we performed co-transfection experiments in COS-7 cells and we observed, by measurement of the intracellular cAMP produced and by FACS experiments, that the co-expression of two membrane proteins (PRA1, NIS, LH/CGR with TSHR) alters their expression. This effect seems to be restricted to co-expression of two-membrane proteins because the expression does not decrease when the membrane protein is co-expressed with a cytosolic protein (Rab9, Cyclin D3). This pitfall should be considered in such expression experiments.

Kinetics of thyrotropin-stimulating hormone (TSH) and thyroid-stimulating antibody binding and action on the TSH receptor in intact TSH receptor-expressing CHO cells

The kinetics of TSH binding and the effects of TSH and thyroid-stimulating antibody (TSAb) on cAMP accumulation have been measured in TSH receptor-expressing CHO cells (CHO-TSHR cells). The parallel kinetics of TSH binding to its receptor and of cell cAMP concentration after the addition and withdrawal of TSH show that in the case of this receptor, signal generation and concentration are at all times proportional to occupancy. In physiological ionic medium, TSAb, but not TSH, action is slowed and in some cases almost nonexistent. The kinetics of cAMP disappearance after washout of TSAb is also slower. cAMP accumulation is faster for Fabs than for the TSAb from which they derive. Analysis of the data suggest that 1) serum TSAb are oligoclonal antibodies sets, at low concentrations, with a high affinity for the TSH receptor; 2) ionic interactions are involved in the action of TSAb on the TSH receptor; and 3) TSAb activation of the TSH receptor is at least a two-step process. Among others, a possible explanation is that the full activation of the receptor requires the binding of two or more different antibody molecules on different sites of the same TSH receptor. This analysis provides a benchmark for studies of experimentally induced monoclonal antibodies activating the TSH receptor.

Localization and regulation of thyrotropin receptors within lipid rafts

The TSH receptor (TSHR) is a prototypic G protein-coupled receptor with a large extracellular domain. We have previously demonstrated homophilic interactions of TSHRs and their existence as constitutive oligomers. However, we have also shown that TSH itself promotes the formation of receptor monomers. We hypothesized, therefore, that TSHR monomers induced by TSH ligand may move into lipid rafts before effective TSH-induced signaling by bringing the cognate signaling molecules resident in such rafts together with the TSHRs. Thus, we aimed to determine whether the TSHRs would partition into these lipid rafts. The B subunit of cholera toxin (CTxB) binds to lipid raft-enriched GM1 ganglioside and has been widely exploited to visualize lipid rafts. Using such a method, we demonstrated the presence of these GM1-enriched lipid microdomains in Chinese hamster ovary cells by using CTxB labeled with a red dye (Alexa 594). To provide evidence for the presence of TSHRs in lipid rafts, we stained Chinese hamster ovary cells expressing TSHRGFP with labeled CTxB. Our results demonstrated that the TSHRGFP complexes localized to GM1-enriched lipid raft microdomains as evidenced by colocalization of the green fluorescent protein tag with the labeled CTxB. Hence, we concluded that a significant proportion of TSHRs were constitutively associated with lipid rafts. Furthermore, upon activation of these stained raft-receptor complexes with increasing concentrations of TSH, we observed that the raft-receptor complexes decreased significantly. The relevance of such receptor movement out of the rafts suggested that these may be the receptors critical in the initiation of signal transduction

TSH Is a Negative Regulator of Skeletal Remodeling

The established function of thyroid stimulating hormone (TSH) is to promote thyroid follicle development and hormone secretion. The osteoporosis associated with hyperthyroidism is traditionally viewed as a secondary consequence of altered thyroid function. We provide evidence for direct effects of TSH on both components of skeletal remodeling, osteoblastic bone formation, and osteoclastic bone resorption, mediated via the TSH receptor (TSHR) found on osteoblast and osteoclast precursors. Even a 50% reduction in TSHR expression produces profound osteoporosis (bone loss) together with focal osteosclerosis (localized bone formation). TSH inhibits osteoclast formation and survival by attenuating JNK/c-jun and NFkappaB signaling triggered in response to RANK-L and TNFalpha. TSH also inhibits osteoblast differentiation and type 1 collagen expression in a Runx-2- and osterix-independent manner by downregulating Wnt (LRP-5) and VEGF (Flk) signaling. These studies define a role for TSH as a single molecular switch in the independent control of both bone formation and resorption.

In vitro assay of thyroid disruptors affecting TSH-stimulated adenylate cyclase activity

Several natural or synthetic chemicals have been indicated as potential thyroid disruptors. The development of in vitro assays has been recommended to comprehensively assess the potential thyroid disrupting activity of a substance or a complex mixture. In this study, 12 substances suspected for acting as thyroid disruptors were tested for their ability to inhibit TSH-stimulated cAMP production in vitro. Those substances producing an inhibition were further studied to establish the level at which they interfere with this step of thyroid cell function. Using Chinese hamster ovary cells (CHO) transfected with the recombinant human TSH receptor, a dose-dependent inhibition of TSH-stimulated adenylate cyclase activity was produced by 1,1-bis-(4-chlorphenyl)-2,2,2-trichloroethan (DDT), Aroclor 1254 and Melissa Officinalis. All three substances also inhibited the cAMP production stimulated by TSH receptor antibody. Melissa Officinalis produced a significant inhibition of TSH binding to its receptor and of antibody binding to TSH, while no significant changes were produced by Aroclor 1254 or DDT in these assays. These data suggest that principles contained in Melissa Officinalis may block the binding of TSH to its receptor by acting both on the hormone and the receptor itself, while DDT and Aroclor 1254 affect cAMP production mainly at post-receptor step. In conclusion, we have developed a set of in vitro assays that allow investigation into the effect of thyroid disruptors on the TSH-mediated activation of the cAMP cascade. These assays may be useful to identify the mechanism of action of thyroid disruptors, coming beside and supporting animal studies or epidemiological surveys.

Correlation between the loss of thyroglobulin iodination and the expression of thyroid-specific proteins involved in iodine metabolism in thyroid carcinomas

Progress in biotechnology has provided useful tools for tracing proteins involved in thyroid hormone synthesis in vivo. Mono- or polyclonal antibodies are now available to detect on histological sections the Na(+)/I(-) symporter (NIS) at the basolateral pole of the cell, the putative iodide channel (pendrin) at the apical plasma membrane, thyroperoxidase (TPO), and members of the NADPH-oxidase family, thyroid oxidase 1 and 2 (ThOXs), part of the H(2)O(2)-generating system. The aim of this study was to correlate thyroglobulin (Tg) iodination with the presence of these proteins. Tg, T(4)-containing Tg, NIS, pendrin, TPO, ThOXs, and TSH receptor (TSHr) were detected by immunohistochemistry on tissue sections of normal thyroids and various benign and malignant thyroid disorders. Tg was present in all cases. T(4)-containing Tg was found in the adenomas, except in Hurthle cell adenomas. It was never detected in carcinomas. NIS was reduced in all types of carcinomas, whereas it was detected in noncancerous tissues. Pendrin was not expressed in carcinomas, except in follicular carcinomas, where weak staining persisted. TPO expression was present in insular, follicular carcinomas and in follicular variants of papillary carcinomas, but in a reduced percentage of cells. It was below the level of detection in papillary carcinomas. The H(2)O(2)-generating system, ThOXs, was found in all carcinomas and was even increased in papillary carcinomas. Its staining was apical in normal thyroids, whereas it was cytoplasmic in carcinomas. The TSHr was expressed in all cases, but the intensity of the staining was decreased in insular carcinomas. In conclusion, our work shows that all types of carcinomas lose the capacity to synthesize T(4)-rich, iodinated Tg. In follicular carcinomas, this might be due to a defect in iodide transport at the basolateral pole of the cell. In papillary carcinomas, this defect seems to be coupled to an altered apical transport of iodide and probably TPO activity. The TSHr persists in virtually all cases.

Proper targeting and activity of a nonfunctioning thyroid-stimulating hormone receptor (TSHr) combining an inactivating and activating TSHr mutation in one receptor

Activating mutations of the thyroid-stimulating hormone receptor (TSHr) have been identified as a cause of toxic adenomas. Germline-inactivating TSHr mutations have been described as a cause of congenital hypothyroidism. The effects of combining activating and inactivating mutations within a single receptor was studied. The double mutant T477I/P639S contained an activating TSHr mutation (P639S) together with an inactivating one (T477I). The other one (I486M/P639S) contained two activating mutations. Constructs were expressed in COS-7 cells and basal and TSH-stimulated cyclic AMP (cAMP) accumulation and inositol phosphate (IP) production were determined. The expression at the cell surface was studied both with binding and fluorescence-activated cell scanning analysis. Our results show that the effect of combining the two activating mutations is an increase in the constitutive activity only for the cAMP pathway and not for the IP pathway suggesting that different mutations result in receptor conformations with different relative abilities to couple to Gs-alpha or Gq-alpha. Surprisingly the double mutant containing the T477I behaves as an activating receptor with constitutive activity both for the cAMP and IP pathways. These data show that an inactive form of the TSHr which is trapped inside a cell after transfection is able to gain the membrane surface when combined with an activated form of the receptor.

Biological activity of activating thyroid-stimulating hormone receptor mutants depends on the cellular context

Activating TSH receptor (TSHR) mutations are a major cause of toxic thyroid adenoma and familial hyperthyroidism, and more than 37 such mutations have been described. Previously their functional activity had been assessed in terms of cAMP and inositol phosphate production and predominantly in transiently transfected COS-7 (monkey embryonic kidney cells), a model that does not reflect effects on thyrocyte proliferation and function. Here we have performed a systematic comparison of wild-type and seven gain-of-function TSHR mutants, introduced into rat FRTL-5 and human thyrocytes, using retroviral vectors. Our results show that 1) biological potency of TSHR mutants in thyroid cells does not correlate with their cAMP levels in transfected COS cells, highlighting the importance of cellular context and level of expression when assessing biological effects of oncogenic mutations; 2) dissociation between stimulation of function and growth occurs with thyrocyte differentiated functions more readily stimulated than growth; 3) TSHR mutants show a similar order of potency in FRTL-5 cells and human thyrocytes; 4) mutants inducing the highest stimulation of adenylyl cyclase may paradoxically fail to induce proliferation; and 5) biological effects of cAMP activating TSHR mutants are attenuated by complex counterregulatory mechanisms at least at the level of phosphodiesterases and cAMP regulatory element modulator isoforms.

Role of cleavage and shedding in human thyrotropin receptor function and trafficking

The thyrotropin receptor (TSHR) undergoes a cleavage at the cell membrane, leading to a heterodimer, comprising an alpha extracellular and a beta-transmembrane and intracellular subunits, held together by disulfide bonds. Moreover, part of the alpha-subunit of the receptor is shed from thyroid and transfected L cells. To understand the role of cleavage and shedding, we constructed deletion mutants starting, respectively, at the most N-terminal (S314), and C-terminal (L378) cleavage sites previously mapped, corresponding to free beta1 or beta2-subunits without further modification of receptor structure. Functional studies performed in COS-7 cells showed that both mutants display an increased basal activation of the cAMP pathway when compared with the wild-type receptor. By contrast, deletion of almost the entire extracellular domain of the receptor (TM409 mutant) totally impairs receptor function, thus confirming a role of the juxtamembrane extracellular region in receptor function. The beta1 mutant receptor exhibited an increased internalization when compared with the hormone-activated holoreceptor. Furthermore, no recycling was observed in the case of the beta1 mutant receptor. These observations strongly argue for a different conformation between the receptor activated by cleavage and shedding on the one hand, and the receptor activated by the ligand on the other hand. Cleavage and shedding of a receptor already activated by a transmembrane activating mutation M453T further increase its activity, showing that the extracellular domain still exerts a negative effect in the M453T holoreceptor. An increased internalization of the M453T receptor was observed when compared with the wild-type receptor, which was increased further in the corresponding truncated beta1-M453T receptor. Thus cleavage and shedding yield TSHR activation but also increase internalization of the free beta-subunits of the receptor, the latter mechanism limiting simultaneously excessive receptor signaling. The combined effects may be responsible for the limited basal constitutive activation of the cAMP pathway that is detected for the TSHR.

Generation of a transgenic animal model of hyperthyroid Graves' disease

Graves' disease (GD) is an organ-specific autoimmune disease characterized by hyperthyroidism. Agonistic anti-thyrotropin receptor antibodies (thyroid-stimulating antibodies, TSAb), which mimic the thyrotropin (TSH) action, are thought to cause GD. The precise immunological mechanism of TSAb production, however, remains elusive. Previous immunization approaches using TSH receptor led to transient hyperthyroidism, but did not seem sufficient for comprehensive understanding of the development of autoimmune responses. To create GD-related autoimmunity in mice, we here generated TSAb-transgenic mice in which a patient-derived TSAb is expressed in B cells. Expression of the human TSAb in mice resulted in various manifestations of hyperthyroidism including increased free thyroxine levels with concomitantly decreased TSH levels, increased thyroid uptake of technetium pertechnetate, hyperthermia and thyroid hyperplasia. We found a correlation between the serum levels of human TSAb immunoglobulin and free thyroxine. In addition, conventional B cells expressing the TSAb were partially deleted in the periphery while B1 cells expressing the TSAb persisted and accumulated in the peritoneal cavity, a finding consistent with previous demonstrations that the maintenance of B1 cells plays an important role in the development of autoimmune diseases. Thus, our transgenic mouse may provide a novel and useful animal model for elucidating the pathogenesis and pathophysiology of GD.

Thyrotropin receptor autoantibodies are associated with continued thyrotropin suppression in treated euthyroid Graves' disease patients

Antithyroid treatment effectively restores euthyroidism in patients with Graves' hyperthyroidism. After a few months of treatment, patients are clinically euthyroid with normal levels of thyroid hormones, but in many patients TSH levels remain suppressed. We postulated that TSH receptor autoantibodies could directly suppress TSH secretion, independently from thyroid hormone levels, via binding to the pituitary TSH receptor. To test this hypothesis, we prospectively followed 45 patients with Graves' hyperthyroidism who were treated with antithyroid drugs. Three months after reaching euthyroidism, blood was drawn for the analysis of thyroid hormones, TSH, and TSH binding inhibitory Ig (TBII) levels. After 6.7 +/- 1.5 months since start of antithyroid treatment, 20 patients still had detectable TBII levels, and 25 had become TBII negative. The two groups had similar levels of free T(4) and T(3), but TBII-positive patients had lower TSH values than TBII-negative patients: median 0.09 (range < 0.01-4.30) mU/liter vs. 0.84 (0.01-4.20; P = 0.015). In addition, TSH levels correlated only with TBII titers (r = -0.424; P = 0.004), and not with free T(4) or T(3) values. Our findings suggest that TBII suppress TSH secretion independently of thyroid hormone levels, most likely by binding to the pituitary TSH receptor.

Evidence that the C terminus of the A subunit suppresses thyrotropin receptor constitutive activity

The TSH receptor (TSHR), unlike the LH receptor (LHR), has considerable ligand-independent adenylyl cyclase activity, a feature of pathophysiological importance. The TSHR ectodomain partially suppresses constitutive activity, an effect reversed by trypsin treatment of intact cells. Localizing the functional site of trypsin action would provide insight into how the TSHR ectodomain exerts its constraint. For this purpose, we examined the effect of trypsin on intact cells expressing a series of modified TSHR. Trypsin did not increase cAMP production by a chimeric TSH-LH receptor involving substitution of TSHR residues 261-418 (the ectodomain C terminus). In contrast, with the wild-type TSHR, trypsin enhanced constitutive activity despite mutation of the following potential tryptic cleavage sites [arginine (R) and lysine (K) residues]: 1) K565, K651, K660 in the extracellular loops of the serpentine region; 2) B subunit juxtamembrane residues K371, K401, K415; 3) A subunit residues R310, R312, K313. We previously excluded K337 and K339 from being implicated in TSHR tryptic activation. By exclusion, only one R/K cluster remains as a possible target for the functional effect of trypsin, namely K287, K290, K291, and R293. Mutation of this cluster is incompatible with TSHR cell surface expression. However, tryptic clipping at this locus would reproduce a previously demonstrated structural effect of trypsin on the TSHR, removal of about a 2-kDa polypeptide fragment extending downstream from the locus to the C terminus of the A subunit. Taken together, these data suggest that the C terminus of the A subunit functions as a suppressor of TSHR constitutive activity.

Functional assessment of the thyrotropin receptor-beta subunit

Posttranslational processing of the TSH receptor (TSHR) involves proteolysis of a single chain holoreceptor into TSHR-alpha (or A) and TSHR-beta (or B) subunits, which remain associated via disulfide bonds and which may then form oligomers. As both uncleaved and cleavage-derived forms of this receptor have been reported to bind TSH and transduce signals, reasons for this cleavage into alpha- and beta-subunits have remained enigmatic. Recently we suggested that TSHR cleavage was related to receptor oligomerization and now we have asked if cleavage influenced the binding of G proteins to this receptor. Furthermore, as TSHR-alpha subunits are subject to shedding from the cell surface membrane, we have examined whether the remaining TSHR-beta subunits could mediate signaling themselves, either constitutively and /or ligand-induced. We found that only the cleaved form of the TSHR in transfected Chinese hamster ovary cells was able to bind Gsalpha protein, suggesting that cleavage of the native TSH receptor was associated with receptor activation. We also found that independently expressed TSHR-beta subunits on stable cell lines were unable to mediate either constitutive or TSH-induced signaling, as monitored by their inability to induce cAMP accumulation. These data suggested that receptor cleavage was intimately associated with receptor activation in the wild-type TSH receptor and that the residual TSHR-beta subunits left on the thyroid cell membrane, after TSHR cleavage and subsequent TSHR-alpha shedding, were essentially silent and did not participate in signal transduction.

Kinetics of iodide uptake and efflux in various human thyroid cancer cells by expressing sodium iodide symporter gene via a recombinant adenovirus

We evaluated the potential of radioiodide therapy in human sodium iodide symporter (hNIS)-defective thyroid cancer cells via exogenous hNIS expression. Three human thyroid cancer cells (ARO, FRO and NPA) of different origin were transduced by a recombinant adenovirus encoding hNIS expression cassette (Rad-hNIS). The cells were efficiently transduced by a recombinant adenovirus in a virus dose-dependent manner. Consequently, the hNIS protein could be readily detected by Western blot analysis 48-h post-infection at 10 infectious virus particles per cell. These hNIS-transduced cells actively transported iodide into the cytoplasm at the level of 11635.3, 61571.6, and 19367.5 pmoles/10(6) cells in ARO, FRO, and NPA, respectively. However, a significant amount of iodide was eluted to an iodide-free media within 60 min in all the cell lines. RT-PCR analysis revealed that the expression of genes related to iodide trapping (Tg, TSHR and TPO) was dramatically downregulated in these cells. The present study indicates that functional hNIS can be efficiently expressed and is responsible for active transport of iodide in hNIS-negative human thyroid cancer cells by a recombinant adenovirus. However, the human thyroid cancer cells, along with downregulation of iodide metabolism-related gene expression, lose the ability to maintain iodide. Therefore, these kinetic characteristics of iodide uptake and efflux may limit the therapeutic potential of hNIS/radioiodide-based treatment following exogenous hNIS expression in human thyroid cancer.

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.