Evidence that human thyroid cells express uncleaved, single-chain thyrotropin receptors on their surface

The Mysterious Universe of the TSH Receptor

The thyroid-stimulating hormone receptor (TSH-R) is predominantly expressed in the basolateral membrane of thyrocytes, where it stimulates almost every aspect of their metabolism. Several extrathyroidal locations of the receptor have been found including: the pituitary, the hypothalamus, and other areas of the central nervous system; the periorbital tissue; the skin; the kidney; the adrenal; the liver; the immune system cells; blood cells and vascular tissues; the adipose tissue; the cardiac and skeletal muscles, and the bone. Although the functionality of the receptor has been demonstrated in most of these tissues, its physiological importance is still a matter of debate. A contribution to several pathological processes is evident in some cases, as is the case of Grave's disease in its multiple presentations. Conversely, in the context of other thyroid abnormalities, the contribution of the TSH-R and its ligand is still a matter of debate. This article reviews the several different sites of expression of the TSH-R and its potential role in both physiological and pathological processes.

The Molecular Function and Clinical Role of Thyroid Stimulating Hormone Receptor in Cancer Cells

The thyroid stimulating hormone (TSH) and its cognate receptor (TSHR) are of crucial importance for thyrocytes to proliferate and exert their functions. Although TSHR is predominantly expressed in thyrocytes, several studies have revealed that functional TSHR can also be detected in many extra-thyroid tissues, such as primary ovarian and hepatic tissues as well as their corresponding malignancies. Recent advances in cancer biology further raise the possibility of utilizing TSH and/or TSHR as a therapeutic target or as an informative index to predict treatment responses in cancer patients. The TSH/TSHR cascade has been considered a pivotal modulator for carcinogenesis and/or tumor progression in these cancers. TSHR belongs to a sub-group of family A G-protein-coupled receptors (GPCRs), which activate a bundle of well-defined signaling transduction pathways to enhance cell renewal in response to external stimuli. In this review, recent findings regarding the molecular basis of TSH/TSHR functions in either thyroid or extra-thyroid tissues and the potential of directly targeting TSHR as an anticancer strategy are summarized and discussed.

Regulation of TSHR Expression in the Thyroid and Thymus May Contribute to TSHR Tolerance Failure in Graves' Disease Patients via Two Distinct Mechanisms

Graves' disease (GD) involves the presence of agonistic auto-antibodies against the thyrotropin receptor (TSHR), which are responsible for the clinical symptoms. While failure of TSHR tolerance is central to GD pathogenesis, the process leading to this failure remains poorly understood. Two mechanisms intimately linked to tolerance have been proposed to explain the association of SNPs located in TSHR intron 1 to GD: (1) differential alternative splicing in the thyroid; and (2) modulation of expression in the thymus. To elucidate the relative contribution to these two mechanisms to GD pathogenesis, we analyzed the level of full-length and ST4 and ST5 isoform expression in the thyroid (n = 49) and thymus (n = 39) glands, and the influence of intron 1-associated SNPs on such expression. The results show that: (1) the level of flTSHR and ST4 expression in the thymus was unexpectedly high (20% that of the thyroid); (2) while flTSHR is the predominant isoform, the levels are similar to ST4 (ratio flTSHR/ST4 = 1.34 in the thyroid and ratio flTSHR/ST4 in the thymus = 1.93); (3) next-generation sequencing confirmed the effect of the TSHR intron 1 polymorphism on TSHR expression in the thymus with a bias of 1.5 ± 0.2 overexpression of the protective allele in the thymus compared to the thyroid; (4) GD-associated intron 1 SNPs did not influence TSHR alternative splicing of ST4 and ST5 in the thyroid and thymus; and (5) three-color confocal imaging showed that TSHR is associated with both thymocytes, macrophages, and dendritic cells in the thymus. Our findings confirm the effect of intron 1 polymorphisms on thymic TSHR expression and we present evidence against an effect on the relative expression of isoforms. The high level of ST4 expression in the thymus and its distribution within the tissue suggest that this would most likely be the isoform that induces central tolerance to TSHR thus omitting most of the hinge and transmembrane portion. The lack of central tolerance to a large portion of TSHR may explain the relatively high frequency of autoimmunity related to TSHR and its clinical consequence, GD.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

Thyrotropin Stimulates Differentiation Not Proliferation of Normal Human Thyrocytes in Culture

Although TSH has been suggested to be a proliferative agent for thyrocytes, the effect of TSH on human thyroid cells remains controversial. In particular, most of the reported studies relied primarily on changes in DNA synthesis but have not included measurement of the number of cells. We argue that only a direct count of cell number, demonstrating classical exponential expansion, serves as a valid measurement of proliferation. Thus, although some data support TSH as a proliferative agent, most do not provide conclusive evidence. To generate conclusive evidence with regard to a proliferative effect of TSH in human thyrocytes, we performed various experiments using primary cultures of human thyrocytes. In contrast to previous reports, TSH [±insulin-like growth factor 1 (IGF-1)] did not induce proliferation of thyrocytes under a variety of different conditions. However, TSH/IGF-1 cotreatment did upregulate thyroid-specific gene expression including thyroglobulin (TG) and TSHR in a manner consistent with cellular differentiation. Evidence for a proliferative effect of TSH has been used to inform the American Thyroid Association's guidelines for the management of thyroid cancer patients, which include TSH suppression. While these recommendations are admittedly based on low- to moderate-quality evidence, TSH suppression is still widely used. We present data that question the consensus view that TSH promotes proliferation of human thyrocytes (upon which the American Thyroid Association's guidelines are based) and suggest that additional studies, including randomized controlled trials, are warranted to address this important clinical question.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

Targeting the thyrotropin receptor in thyroid disease

INTRODUCTION

The thyrotropin receptor (TSHR) is essential for thyroid growth and for the production of thyroid hormones. It is unique among the glycoprotein hormone receptors, in that some of the TSHRs undergo cleavage and shedding of the alpha subunit.

AREAS COVERED

This review discusses the structure and function of the TSHR, followed by an evaluation of its role in thyroid disease. Possible limitations of the TSHR as a therapeutic target are also discussed.

EXPERT OPINION

The TSHR is involved in a number of hereditary and acquired disorders of the thyroid making it of potential importance as a therapeutic target in thyroid disease. Expression of the TSHR in several non-thyroidal tissues and the development of systemic manifestations of thyroid disease suggest that the TSHR is also of interest as a therapeutic target outside the thyroid.

The cleavage of thyroid-stimulating hormone receptor is dependent on cell-cell contacts and regulates the hormonal stimulation of phospholipase c

Thyroid-stimulating hormone receptor (TSHR) consists of a hormone-binding extracellular subunit and a seven-transmembrane spanning subunit that interacts with the G proteins G(alphas) and G(alphaq). The two subunits, generated by proteolytic cleavage of a single polypeptide chain, are held together by disulphide bridges. The receptor is completely cleaved in thyroid tissue, while in cultured cells (thyrocytes and non-thyroid cells) the cleaved and uncleaved forms coexist. The reasons for these divergent data are not understood. Here we provide an explanation by showing that cleavage depends on cell-cell contacts. An almost complete cleavage was observed in confluent cells, while in sparse cells most of the receptor was in the uncleaved form. We also show that coupling of TSHR to G(alphaq) (as measured by inositolphosphate generation) is markedly reduced when the receptor is not cleaved. In contrast, coupling to G(alphas) [as measured by cyclic adenosine 3',5'-monophosphate (cAMP) synthesis] is unaffected by cleavage of the receptor. These results suggest that the cell-cell contacts are necessary for cleavage of the receptor, which acts as a regulatory step in inositolphosphate production via phospholipase C activation. The latter observation was confirmed using cells that express the uncleavable mutant TSHR-delta50-NET, for which the TSH-stimulated inositolphosphate production was completely abolished.

The hinge region: an important receptor component for GPHR function

Glycoprotein hormone receptors (GPHRs) are members of the seven-transmembrane-spanning receptor family characterized by a large ectodomain. The hinge region belongs to a part of the GPHR ectodomain for which the three-dimensional structure has not yet been deciphered, leaving important questions unanswered concerning ligand binding and GPHR activation. Recent publications indicate that specific residues of the hinge region mediate hormone binding, receptor activation and/or intramolecular signaling for the three GPHRs, emphasizing the importance of this region. Based on these findings, the hinge region is involved at least in part in hormone binding and receptor activation. This review summarizes functional data regarding the hinge region, demonstrating that this receptor portion represents a link between ligand binding and subsequent GPHR activation.

The thyroid-stimulating hormone receptor: impact of thyroid-stimulating hormone and thyroid-stimulating hormone receptor antibodies on multimerization, cleavage, and signaling

The thyroid-stimulating hormone receptor (TSHR) has a central role in thyrocyte function and is also one of the major autoantigens for the autoimmune thyroid diseases. We review the post-translational processing, multimerization, and intramolecular cleavage of TSHR, all of which may modulate its signal transduction. The recent characterization of monoclonal antibodies to the TSHR, including stimulating, blocking, and neutral antibodies, have also revealed unique biologic insights into receptor activation and the variety of these TSHR antibodies may help explain the multiple clinical phenotypes seen in autoimmune thyroid diseases. Knowledge of the structure/function relationship of the TSHR is beginning to provide a greater understanding of thyroid physiology and thyroid autoimmunity.

Thyrotropin and homologous glycoprotein hormone receptors: structural and functional aspects of extracellular signaling mechanisms

The TSH receptor (TSHR) together with the homologous lutropin/choriogonadotropin receptor and the follitropin receptor are glycoprotein hormone receptors (GPHRs). They constitute a subfamily of the rhodopsin-like G protein-coupled receptors with seven transmembrane helices. GPHRs and their corresponding hormones are pivotal proteins with respect to a variety of physiological functions. The identification and characterization of intra- and intermolecular signaling determinants as well as signaling mechanisms are prerequisites to gaining molecular insights into functions and (pathogenic) dysfunctions of GPHRs. Knowledge about activation mechanisms is fragmentary, and the specific aspects have still not been understood in their entirety. Therefore, here we critically review the data available for these receptors and bring together structural and functional findings with a focus on the important large extracellular portion of the TSHR. One main focus is the particular function of structural determinants in the initial steps of the activation such as: 1) hormone binding at the extracellular site; 2) hormone interaction at a second binding site in the hinge region; 3) signal regulation via sequence motifs in the hinge region; and 4) synergistic signal amplification by cooperative effects of the extracellular loops toward the transmembrane region. Comparison and consolidation of data from the homologous glycoprotein hormone receptors TSHR, follitropin receptor, and lutropin/choriogonadotropin receptor provide an overview of extracellular mechanisms of signal initiation, conduction, and regulation at the TSHR and homologous receptors. Finally, we address the issue of structural implications and suggest a refined scenario for the initial signaling process on GPHRs.

Cleavage of the human thyrotropin receptor by ADAM10 is regulated by thyrotropin

The thyrotropin receptor (TSHR) has a unique 50 residue (317-366) ectodomain insertion that sets it apart from other glycoprotein hormone receptors (GPHRs). Other ancient members of the leucine-rich repeat G protein-coupled receptor (GPCR) (LGR) family do exhibit ectodomain insertions of variable lengths and sequences. The TSHR-specific insert is digested, apparently spontaneously, to release the ectodomain (A-subunit) leaving the balance of the ectodomain attached to the serpentine (B-subunit). Despite concerted efforts for the last 12 years by many laboratories, the enzyme involved in TSHR cleavage has not been identified and a physiologic role for this process remains unclear. Several lines of evidence had suggested that the TSHR protease is likely a member of the a disintegrin and metalloprotease (ADAM) family of metalloproteases. We show here that the expression of ADAM10 was specific to the thyroid by specially designed DNA microarrays. We also show that TSH increases TSHR cleavage in a dose-dependent manner. To prove that ADAM10 is indeed the TSHR cleavage enzyme, we investigated the effect of TSH-induced cleavage by a peptide based on a motif (TSHR residues 334-349), shared with known ADAM10 substrates. TSH increased dose dependently TSHR ectodomain cleavage in the presence of wild-type peptide but not a scrambled control peptide. Interestingly, TSH increased the abundance of non-cleaved single chain receptor, as well higher molecular forms of the A-subunit, despite their enhancement of the appearance of the fully digested A-subunit. This TSH-related increase in TSHR digested forms was further increased by wild-type peptide. We have identified for the first time ADAM10 as the TSHR cleavage enzyme and shown that TSH regulates its activation.

The thyrotropin receptor in Graves' disease

The application of molecular biology to the study of the thyrotropin receptor (TSHR) has led to major advances in our understanding of its structure, function, and relationship to the pathogenesis of Graves' disease. This review summarizes many of these features and also provides a personal perspective, questioning some assumptions and general concepts, as well as describing remaining challenges. Among the issues raised are the limits in our understanding of the spatial orientation of the structural domains of the TSHR, including the enigmatic hinge region. We review the phenomenon of TSHR intramolecular cleavage, the shedding of the A-subunit component of the ectodomain, and the importance of the latter in generating thyroid-stimulating antibodies. The epitopes of thyroid-stimulating and -blocking autoantibodies have been a confusing and controversial subject that requires review and evaluation of available data. Finally, we address the potential physiological or pathophysiological significance of TSHR multimerization in TSHR. Taken together, this review will, hopefully, convey the fascination and excitement that molecular biology has contributed to the study of the TSHR, especially as it relates to the pathogenesis of Graves' disease.