Thyrotropin Causes Dose-dependent Biphasic Regulation of cAMP Production Mediated by Gs and Gi/o Proteins.

The thyrotropin (TSH) receptor (TSHR) signals via G proteins of all four classes and ß-arrestin 1. Stimulation of TSHR leads to increasing cAMP production that has been reported as a monotonic dose-response curve that plateaus at high TSH doses. In HEK 293 cells overexpressing TSHRs (HEK-TSHR cells), we found that TSHR activation exhibits an "inverted U-shaped dose-response curve" with increasing cAMP production at low doses of TSH and decreased cAMP production at high doses (>1 mU/ml). Since protein kinase A inhibition by H-89 and knockdown of ß-arrestin 1 or ß-arrestin 2 did not affect the decreased cAMP production at high TSH doses, we studied the roles of TSHR downregulation and of Gi/Go proteins. A high TSH dose (100 mU/ml) caused a 33% decrease in cell-surface TSHR. However, because inhibiting TSHR downregulation with combined expression of a dominant negative dynamin 1 and ß-arrestin 2 knockdown had no effect, we concluded that downregulation is not involved in the biphasic cAMP response. Pertussis toxin, which inhibits activation of Gi/Go, abolished the biphasic response with no statistically significant difference in cAMP levels at 1 and 100 mU/ml TSH. Concordantly, co-knockdown of Gi/Go proteins increased cAMP levels stimulated by 100 mU/ml TSH from 55% to 73% of the peak level. These data show that biphasic regulation of cAMP production is mediated by Gs and Gi/Go at low and high TSH doses, respectively, which may represent a mechanism to prevent overstimulation in TSHR-expressing cells. SIGNIFICANCE STATEMENT: We demonstrate biphasic regulation of TSH-mediated cAMP production involving coupling of the TSH receptor (TSHR) to Gs at low TSH doses and to Gi/o at high TSH doses. We suggest that this biphasic cAMP response allows the TSHR to mediate responses at lower levels of TSH and that decreased cAMP production at high doses may represent a mechanism to prevent overstimulation of TSHR-expressing cells. This mechanism could prevent chronic stimulation of thyroid gland function.

Is There Evidence for IGF1R-Stimulating Abs in Graves' Orbitopathy Pathogenesis?

In this review, we summarize the evidence against direct stimulation of insulin-like growth factor 1 receptors (IGF1Rs) by autoantibodies in Graves' orbitopathy (GO) pathogenesis. We describe a model of thyroid-stimulating hormone (TSH) receptor (TSHR)/IGF1R crosstalk and present evidence that observations indicating IGF1R's role in GO could be explained by this mechanism. We evaluate the evidence for and against IGF1R as a direct target of stimulating IGF1R antibodies (IGF1RAbs) and conclude that GO pathogenesis does not involve directly stimulating IGF1RAbs. We further conclude that the preponderance of evidence supports TSHR as the direct and only target of stimulating autoantibodies in GO and maintain that the TSHR should remain a major target for further development of a medical therapy for GO in concert with drugs that target TSHR/IGF1R crosstalk.

De novo triiodothyronine formation from thyrocytes activated by thyroid-stimulating hormone.

The thyroid gland secretes primarily tetraiodothyronine (T4), and some triiodothyronine (T3). Under normal physiological circumstances, only one-fifth of circulating T3 is directly released by the thyroid, but in states of hyperactivation of thyroid-stimulating hormone receptors (TSHRs), patients develop a syndrome of relative T3 toxicosis. Thyroidal T4 production results from iodination of thyroglobulin (TG) at residues Tyr5 and Tyr130, whereas thyroidal T3 production may originate in several different ways. In this study, the data demonstrate that within the carboxyl-terminal portion of mouse TG, T3 is formed de novo independently of deiodination from T4 We found that upon iodination in vitro, de novo T3 formation in TG was decreased in mice lacking TSHRs. Conversely, de novo T3 that can be formed upon iodination of TG secreted from PCCL3 (rat thyrocyte) cells was augmented from cells previously exposed to increased TSH, a TSHR agonist, a cAMP analog, or a TSHR-stimulating antibody. We present data suggesting that TSH-stimulated TG phosphorylation contributes to enhanced de novo T3 formation. These effects were reversed within a few days after removal of the hyperstimulating conditions. Indeed, direct exposure of PCCL3 cells to human serum from two patients with Graves' disease, but not control sera, led to secretion of TG with an increased intrinsic ability to form T3 upon in vitro iodination. Furthermore, TG secreted from human thyrocyte cultures hyperstimulated with TSH also showed an increased intrinsic ability to form T3 Our data support the hypothesis that TG processing in the secretory pathway of TSHR-hyperstimulated thyrocytes alters the structure of the iodination substrate in a way that enhances de novo T3 formation, contributing to the relative T3 toxicosis of Graves' disease.

Discovery of a Positive Allosteric Modulator of the Thyrotropin Receptor: Potentiation of Thyrotropin-Mediated Preosteoblast Differentiation In Vitro.

Recently, we showed that TSH-enhanced differentiation of a human preosteoblast-like cell model involved a ß-arrestin 1 (ß-Arr 1)-mediated pathway. To study this pathway in more detail, we sought to discover a small molecule ligand that was functionally selective toward human TSH receptor (TSHR) activation of ß-Arr 1. High-throughput screening using a cell line stably expressing mutated TSHRs and mutated ß-Arr 1 (DiscoverX1 cells) led to the discovery of agonists that stimulated translocation of ß-Arr 1 to the TSHR, but did not activate Gs-mediated signaling pathways, i.e., cAMP production. D3-ßArr (NCGC00379308) was selected. In DiscoverX1 cells, D3-ßArr stimulated ß-Arr 1 translocation with a 5.1-fold greater efficacy than TSH and therefore potentiated the effect of TSH in stimulating ß-Arr 1 translocation. In human U2OS-TSHR cells expressing wild-type TSHRs, which is a model of human preosteoblast-like cells, TSH upregulated the osteoblast-specific genes osteopontin (OPN) and alkaline phosphatase (ALPL). D3-ßArr alone had only a weak effect to upregulate these bone markers, but D3-ßArr potentiated TSH-induced upregulation of ALPL and OPN mRNA levels 1.6-fold and 5.5-fold, respectively, at the maximum dose of ligands. Furthermore, the positive allosteric modulator effect of D3-ßArr resulted in an increase of TSH-induced secretion of OPN protein. In summary, we have discovered the first small molecule positive allosteric modulator of TSHR. As D3-ßArr potentiates the effect of TSH to enhance differentiation of a human preosteoblast in an in vitro model, it will allow a novel experimental approach for probing the role of TSH-induced ß-Arr 1 signaling in osteoblast differentiation.

Taltirelin alleviates fatigue-like behavior in mouse models of cancer-related fatigue.

Fatigue affects most cancer patients and has numerous potential causes, including cancer itself and cancer treatment. Cancer-related fatigue (CRF) is not relieved by rest, can decrease quality of life, and has no FDA-approved therapy. Thyrotropin-releasing hormone (TRH) has been proposed as a potential novel treatment for CRF, but its efficacy against CRF remains largely untested. Thus, we tested the TRH analog, taltirelin (TAL), in mouse models of CRF. To model fatigue, we used a mouse model of chemotherapy, a mouse model of radiation therapy, and mice bearing colon 26 carcinoma tumors. We used the treadmill fatigue test to assess fatigue-like behavior after treatment with TAL. Additionally, we used wild-type and TRH receptor knockout mice to determine which TRH receptor was necessary for the actions of TAL. Tumor-bearing mice displayed muscle wasting and all models caused fatigue-like behavior, with mice running a shorter distance in the treadmill fatigue test than controls. TAL reversed fatigue-like behavior in all three models and the mouse TRH1 receptor was necessary for the effects of TAL. These data suggest that TAL may be useful in alleviating fatigue in all cancer patients and provide further support for evaluating TAL as a potential therapy for CRF in humans.

ß-Arrestin-1 mediates thyrotropin-enhanced osteoblast differentiation.

Thyrotropin (TSH) activation of the TSH receptor (TSHR), a 7-transmembrane-spanning receptor (7TMR), may have osteoprotective properties by direct effects on bone. TSHR activation by TSH phosphorylates protein kinases AKT1, p38α, and ERK1/2 in some cells. We found TSH-induced phosphorylation of these kinases in 2 cell lines engineered to express TSHRs, human embryonic kidney HEK-TSHR cells and human osteoblastic U2OS-TSHR cells. In U2OS-TSHR cells, TSH up-regulated pAKT1 (7.1±0.5-fold), p38α (2.9±0.4-fold), and pERK1/2 (3.1±0.2-fold), whereas small molecule TSHR agonist C2 had no or little effect on pAKT1 (1.8±0.08-fold), p38α (1.2±0.09-fold), and pERK1/2 (1.6±0.19-fold). Furthermore, TSH increased expression of osteoblast marker genes ALPL (8.2±4.6-fold), RANKL (21±5.9-fold), and osteopontin (OPN; 17±5.3-fold), whereas C2 had little effect (ALPL, 1.7±0.5-fold; RANKL, 1.3±0.6-fold; and OPN, 2.2±0.7-fold). ß-Arrestin-1 and -2 can mediate activatory signals by 7TMRs. TSH stimulated translocation of ß-arrestin-1 and -2 to TSHR, whereas C2 failed to translocate either ß-arrestin. Down-regulation of ß-arrestin-1 by siRNA inhibited TSH-stimulated phosphorylation of ERK1/2, p38α, and AKT1, whereas down-regulation of ß-arrestin-2 increased phosphorylation of AKT1 in both cell types and of ERK1/2 in HEK-TSHR cells. Knockdown of ß-arrestin-1 inhibited TSH-stimulated up-regulation of mRNAs for OPN by 87 ± 1.7% and RANKL by 73 ± 2.4%, and OPN secretion by 74 ± 10%. We conclude that TSH enhances osteoblast differentiation in U2OS cells that is, in part, caused by activatory signals mediated by ß-arrestin-1.

Platelet-derived growth factor BB mimics serum-induced dispersal of pancreatic epithelial cell clusters.

We showed previously that proliferating human islet-derived de-differentiated cells (DIDs) exhibit many characteristics of mesenchymal stem cells. Dispersed DIDs can be induced by serum deprivation to undergo mesenchymal-to-epithelial transition and aggregate into epithelial cell clusters (ECCs). Conversely, ECCs can be induced to disperse and undergo epithelial-to-mesenchymal transition (EMT) by re-addition of mammalian sera. In this study, we show that platelet-derived growth factor BB (PDGF-BB) mimics and mediates serum-induced ECCs' dispersal accompanied by accumulation of cytoplasmic ß-catenin and a decrease in the levels of insulin and glucagon mRNAs. Moreover, we show that PDGF-BB-induced dispersal of ECCs is a more general phenomenon that occurs also with bone marrow mesenchymal stem cells (BM-MSCs) and dermal fibroblasts (DFs). In DIDs, BM-MSCs, and DFs, PDGF decreased the levels of DKK1 mRNA, suggesting involvement of the Wnt signaling pathway. PDGF-BB stimulated a significant increase in S473 phosphorylation of Akt and the PI3K specific inhibitor (PIP828) partially inhibited PDGF-BB-induced ECC dispersal. Lastly, the PDGF-receptor (PDGF-R) antagonist JNJ-10198409 inhibited both PDGF-BB--and serum-induced ECC dispersal. Epidermal growth factor (EGF), which shares most of the PDGF signaling pathway, did not induce dispersal and only weakly stimulated Akt phosphorylation. Our data suggest that PDGF-BB mediates serum-induced DIDs dispersal, correlated with the activation of the PI3K-Akt pathway.

Heritability of fat accumulation in white adipocytes.

Since individual cells from freshly isolated white adipose tissue (WAT) exhibit variable levels of fat accumulation, we attempted to determine which factor(s) cause this variation. We used primary WAT cells from adult mice and the mouse 3T3-L1 cell-line of preadipocytes for these studies. Cells were labeled with BODIPY (boron-dipyrromethene) lipid probe, a marker for fat accumulation in live cells, and sorted on a fluorescence-activated cell sorter into two populations exhibiting low or high BODIPY fluorescence intensity. After more than 12 doublings as dedifferentiated cells in growth medium, the sorted populations were exposed to adipogenic medium for 7 days and analyzed for BODIPY accumulation and mRNA expression of adipogenic markers. WAT-derived cells initially sorted to have low or high BODIPY fluorescence intensity maintained a similar low or high lipid phenotype after redifferentiation. Cell surface TSH receptor expression, which is known to increase when preadipocytes are differentiated, correlated with BODIPY staining in all states. mRNA levels of Pparγ, Srebp1c, aP2, and Pref1, key regulators of adipogenesis, and leptin, Glut4, Fasn, and Tshr, markers of adipocyte differentiation, correlated with the levels of fat accumulation. Overexpression of Pparγ in 3T3-L1 cells, as expected, caused cells from low- and high-BODIPY populations to accumulate more fat. More importantly, prior to differentiation, the endogenous Pparγ promoter exhibited higher levels of acetylated histone H3, an activatory modification, in high-BODIPY- compared with low-BODIPY-derived populations. We conclude that fat accumulation is a heritable trait in WAT and that epigenetic modification on the Pparγ promoter contributes to this heritability.

An Orally Efficacious Thyrotropin Receptor Ligand Inhibits Growth and Metastatic Activity of Thyroid Cancers.

CONTEXT: Thyroid-stimulating hormone (or thyrotropin) receptor (TSHR) could be a selective target for small molecule ligands to treat thyroid cancer (TC). OBJECTIVE: We report a novel, orally efficacious ligand for TSHR that exhibits proliferation inhibitory activity against human TC in vitro and in vivo, and inhibition of metastasis in vivo. METHODS: A35 (NCATS-SM4420; NCGC00241808) was selected from a sublibrary of >200 TSHR ligands. Cell proliferation assays including BrdU incorporation and WST-1, along with molecular docking studies were done. In vivo activity of A35 was assessed in TC cell-derived xenograft (CDX) models with immunocompromised (NSG) mice. Formalin-fixed, paraffin-embedded sections of tumor and lung tissues were observed for the extent of cell death and metastasis. RESULTS: A35 was shown to stimulate cAMP production in some cell types by activating TSHR but not in TC cells, MDA-T32, and MDA-T85. A35 inhibited proliferation of MDA-T32 and MDA-T85 in vitro and in vivo, and pulmonary metastasis of MDA-T85F1 in mice. In vitro, A35 inhibition of proliferation was reduced by a selective TSHR antagonist. Inhibition of CDX tumor growth without decreases in mouse weights and liver function showed A35 to be efficacious without apparent toxicity. Lastly, A35 reduced levels of Ki67 in the tumors and metastatic markers in lung tissues. CONCLUSION: We conclude that A35 is a TSHR-selective inhibitor of TC cell proliferation and metastasis, and suggest that A35 may be a promising lead drug candidate for the treatment of differentiated TC in humans.

Persistent signaling by thyrotropin-releasing hormone receptors correlates with G-protein and receptor levels.

G-protein-coupled receptors with dissociable agonists for thyrotropin, parathyroid hormone, and sphingosine-1-phosphate were found to signal persistently hours after agonist withdrawal. Here we show that mouse thyrotropin-releasing hormone (TRH) receptors, subtypes 2 and 1(TRH-R2 and TRH-R1), can signal persistently in HEK-EM293 cells under appropriate conditions, but TRH-R2 exhibits higher persistent signaling activity. Both receptors couple primarily to Gα(q/11). To gain insight into the mechanism of persistent signaling, we compared proximal steps of inositolmonophosphate (IP1) signaling by TRH-Rs. Persistent signaling was not caused by slower dissociation of TRH from TRH-R2 (t(1/2)=77 ± 8.1 min) compared with TRH-R1 (t(1/2)=82 ± 12 min) and was independent of internalization, as inhibition of internalization did not affect persistent signaling (115% of control), but required continuously activated receptors, as an inverse agonist decreased persistent signaling by 60%. Gα(q/11) knockdown decreased persistent signaling by TRH-R2 by 82%, and overexpression of Gα(q/11) induced persistent signaling in cells expressing TRH-R1. Lastly, persistent signaling was induced in cells expressing high levels of TRH-R1. We suggest that persistent signaling by TRHRs is exhibited when sufficient levels of agonist/receptor/G-protein complexes are established and maintained and that TRH-R2 forms and maintains these complexes more efficiently than TRH-R1.

Knock-down of plasminogen-activator inhibitor-1 enhances expression of E-cadherin and promotes epithelial differentiation of human pancreatic adenocarcinoma cells.

High levels of plasminogen activator inhibitor-1 (PAI-1), which is produced by stromal, endothelial, and cancer cells and has multiple complex effects on cancers, correlate with poor cancer prognosis. To more definitively study the role of endogenously produced PAI-1 in human pancreatic adenocarcinoma (PAC) PANC-1 cell line biology, we used anti-PAI-1 shRNA to create stable PAI-1 deficient cells (PD-PANC-1s). PD-PANC-1s exhibited a heterogeneous morphology. While the majority of cells exhibited a cuboidal shape similar to the parental PANC-1 or the vector-infected control cells, numerous large cells with long filopodia and a neuronal-like appearance were observed. Although both Vector-control cells and PD-PANC-1s expressed mRNAs that are characteristic of mesenchymal, neural, and epithelial phenotypes, epithelial marker RNAs were up-regulated (e.g., E-cadherin, 32-fold) whereas mesenchymal marker RNAs were down-regulated (e.g., Thy1, ninefold) in PD-PANC-1s, suggesting mesenchymal-to-epithelial transition. Neural markers exhibited both up- and down-regulation. Immunocytochemistry indicated that epithelial-like PD-PANC-1s expressed E-cadherin and ß-catenin in significantly more cells, while neural-like cells exhibited robust expression of organized ß-3-tubulin. PAI-1 and E-cadherin were rarely co-expressed in the same cells. Indeed, examination of PAI-1 and E-cadherin mRNAs expression in additional cell lines yielded clear inverse correlation. Indeed, infection of Colo357 PAC cells (that exhibit high expression of E-cadherin) with PAI-1-expressing adenovirus led to a marked decrease in E-cadherin expression and to enhanced migration of cells from clusters. Our results suggest that endogenous PAI-1 suppresses expression of E-cadherin and differentiation in PAC cells in vitro, supporting its negative impact on tumor prognosis.

Occupancy of both sites on the thyrotropin (TSH) receptor dimer is necessary for phosphoinositide signaling.

The thyroid-stimulating hormone (TSH) receptor signals via G(s) to produce cAMP and via G(q/11) to produce inositol-1,4,5-trisphosphate, which is degraded to inositol monophosphate (IP1; phosphoinositide signaling). The potency of TSH for cAMP signaling is higher than for phosphoinositide signaling, and it was suggested that there are "spare receptors" for cAMP signaling. In a human embryonic kidney macrophage scavenger receptor-expressing (HEK-EM) 293 model system, there are no spare receptors, but the cells still exhibited 100-fold differences in potencies. Dose responses for TSH-stimulated dissociation of prebound (125)I-TSH (negative cooperativity; EC(50)=70 mU/ml), which requires TSH binding to both sites of the TSH receptor (TSHR) homodimer, and TSH-stimulated IP1 production (EC(50)=50 mU/ml) were indistinguishable. Fluorescence resonance energy transfer (FRET) using tagged receptors showed that TSHR formed homodimers and heterodimers with two binding-deficient mutant TSHRs, L252P and C41S. When L252P or C41S was expressed with TSHR, that is, when TSHR/L252P or TSHR/C41S heterodimers could only bind one TSH, TSH-stimulated IP1 production was decreased relative to cAMP production. The slopes of linear regression analyses comparing fold stimulation by TSH of IP1 vs. cAMP production were 0.044 ± 0.0047, 0.0043 ± 0.0041, and 0.0059 ± 0.0014 for cells expressing TSHR alone, TSHR and L252P, or TSHR and C41S, respectively. We suggest that TSHR coupling to phosphoinositide signaling is dependent on binding 2 molecules of TSH to TSHR homodimer, causing a conformational change allowing coupling to G(q/11).

Mutations that silence constitutive signaling activity in the allosteric ligand-binding site of the thyrotropin receptor.

The thyrotropin receptor (TSHR) exhibits elevated cAMP signaling in the basal state and becomes fully activated by thyrotropin. Previously we presented evidence that small-molecule ligands act allosterically within the transmembrane region in contrast to the orthosteric extracellular hormone-binding sites. Our goal in this study was to identify positions that surround the allosteric pocket and that are sensitive for inactivation of TSHR. Homology modeling combined with site-directed mutagenesis and functional characterization revealed seven mutants located in the allosteric binding site that led to a decrease of basal cAMP signaling activity. The majority of these silencing mutations, which constrain the TSHR in an inactive conformation, are found in two clusters when mapped onto the 3D structural model. We suggest that the amino acid positions identified herein are indicating locations where small-molecule antagonists, both neutral antagonists and inverse agonists, might interfere with active TSHR conformations.

Small-molecule agonists for the thyrotropin receptor stimulate thyroid function in human thyrocytes and mice.

Seven-transmembrane-spanning receptors (7TMRs) are prominent drug targets. However, small-molecule ligands for 7-transmembrane-spanning receptors for which the natural ligands are large, heterodimeric glycoprotein hormones, like thyroid-stimulating hormone (TSH; thyrotropin), have only recently been reported, and none are approved for human use. We have used quantitative high-throughput screening to identify a small-molecule TSH receptor (TSHR) agonist that was modified to produce a second agonist with increased potency. We show that these agonists are highly selective for human TSHR versus other glycoprotein hormone receptors and interact with the receptor's serpentine domain. A binding pocket within the transmembrane domain was defined by docking into a TSHR homology model and was supported by site-directed mutagenesis. In primary cultures of human thyrocytes, both TSH and the agonists increase mRNA levels for thyroglobulin, thyroperoxidase, sodium iodide symporter, and deiodinase type 2, and deiodinase type 2 enzyme activity. Moreover, oral administration of the agonist stimulated thyroid function in mice, resulting in increased serum thyroxine and thyroidal radioiodide uptake. Thus, we discovered a small molecule that activates human TSHR in vitro, is orally active in mice, and could be a lead for development of drugs to use in place of recombinant human TSH in patients with thyroid cancer.

Proximity ligation assay to study TSH receptor homodimerization and crosstalk with IGF-1 receptors in human thyroid cells.

Proximity ligation assay (PLA) is a methodology that permits detection of protein-protein closeness, that is, proteins that are within 40 nanometers of each other, in cells or tissues at endogenous protein levels or after exogenous overexpression. It detects the protein(s) with high sensitivity and specificity because it employs a DNA hybridization step followed by DNA amplification. PLA has been used successfully with many types of proteins. In this methods paper, we will describe the workings of PLA and provide examples of its use to study TSH/IGF-1 receptor crosstalk in Graves' orbital fibroblasts (GOFs) and TSH receptor homodimerization in primary cultures of human thyrocytes.

Opposing Effects of EGF Receptor Signaling on Proliferation and Differentiation Initiated by EGF or TSH/EGF Receptor Transactivation.

Regulation of thyroid cells by thyrotropin (TSH) and epidermal growth factor (EGF) has been known but different effects of these regulators on proliferation and differentiation have been reported. We studied these responses in primary cultures of human thyroid cells to determine whether TSH receptor (TSHR) signaling may involve EGF receptor (EGFR) transactivation. We confirm that EGF stimulates proliferation and de-differentiation whereas TSH causes differentiation in the absence of other growth factors. We show that TSH/TSHR transactivates EGFR and characterize it as follows: (1) TSH-induced upregulation of thyroid-specific genes is inhibited by 2 inhibitors of EGFR kinase activity, AG1478 and erlotinib; (2) the mechanism of transactivation is independent of an extracellular EGFR ligand by showing that 2 antibodies, cetuximab and panitumumab, that completely inhibited binding of EGFR ligands to EGFR had no effect on transactivation, and by demonstrating that no EGF was detected in media conditioned by thyrocytes incubated with TSH; (3) TSH/TSHR transactivation of EGFR is different than EGFR activation by EGF by showing that EGF led to rapid phosphorylation of EGFR whereas transactivation occurred in the absence of receptor phosphorylation; (4) EGF caused downregulation of EGFR whereas transactivation had no effect on EGFR level; (5) EGF and TSH stimulation converged on the protein kinase B (AKT) pathway, because TSH, like EGF, stimulated phosphorylation of AKT that was inhibited by EGFR inhibitors; and (6) TSH-induced upregulation of thyroid genes was inhibited by the AKT inhibitor MK2206. Thus, TSH/TSHR causes EGFR transactivation that is independent of extracellular EGFR ligand and in part mediates TSH regulation of thyroid hormone biosynthetic genes.

TSH stimulation of human thyroglobulin and thyroid peroxidase gene transcription is partially dependent on internalization.

The TSH receptor (TSHR) is the major regulator of thyroid hormone biosynthesis in human thyrocytes by regulating the transcription of a number of genes including thyroglobulin (TG) and thyroperoxidase (TPO). Until recently, it was thought that TSHR initiated signal transduction pathways only at the cell-surface and that internalization was primarily involved in TSHR desensitization and downregulation. Studies primarily in mouse cells showed that TSHR internalization regulates gene transcription at an intracellular site also. However, this has not been shown for genes involved in thyroid hormone biosynthesis in human thyrocytes. We used human thyrocytes in primary culture. In these cells, the dose-response to TSH for gene expression is biphasic with low doses upregulating gene expression and higher doses decreasing gene expression. We used two approaches to inhibit internalization. In the first, we used inhibitors of dynamins, dynasore and dyngo-4a. Pretreatment with dynasore or dyngo-4a markedly inhibited TSH upregulation of TG and TPO mRNAs, as well as TG secretion. In the second, we used knockdown of dynamin 2, which is the most abundant dynamin in human thyrocytes. We showed that dynamin 2 knockdown inhibited TSHR internalization and decreased the TSH-stimulated levels of TG and TPO mRNAs and proteins. Lastly, we showed that the level of the activatory transcription factor phosphorylated cAMP response element binding protein (pCREB) in the cell nuclei was reduced by 68% when internalization was inhibited. We conclude that upregulation of genes involved in thyroid hormone synthesis in human thyrocytes is, in part, dependent on internalization leading to nuclear localization of an activated transcription factor(s).

Graves' Autoantibodies Exhibit Different Stimulating Activities in Cultures of Thyrocytes and Orbital Fibroblasts Not Reflected by Clinical Assays.

Background: The pathogenesis of Graves' hyperthyroidism (GH) and associated Graves' orbitopathy (GO) appears to involve stimulatory autoantibodies (thyrotropin receptor [TSHR]-stimulating antibodies [TSAbs]) that bind to and activate TSHRs on thyrocytes and orbital fibroblasts. In general, measurement of circulating TSHR antibodies by clinical assays correlates with the status of GH and GO. However, most clinical measurements of TSHR antibodies use competitive binding assays that do not distinguish between TSAbs and antibodies that bind to but do not activate TSHRs. Moreover, clinical assays for TSAbs measure stimulation of only one signaling pathway, the cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) pathway, in engineered cells that are not thyrocytes or orbital fibroblasts. We determined whether measuring TSAbs by a cAMP-PKA readout in engineered cells accurately reveals the efficacies of stimulation by these antibodies on thyrocytes and orbital fibroblasts. Methods: We measured TSAb stimulation of normal human thyrocytes and orbital fibroblasts from patients with GO in primary cultures in vitro. In thyrocytes, we measured secretion of thyroglobulin (TG) and in orbital fibroblasts secretion of hyaluronan (hyaluronic acid [HA]). We also measured stimulation of cAMP production in engineered TSHR-expressing cells in an assay similar to clinical assays. Furthermore, we determined whether there were differences in stimulation of thyrocytes and orbital fibroblasts by TSAbs from patients with GH alone versus from patients with GO understanding that patients with GO have accompanying GH. Results: We found a positive correlation between TSAb stimulation of cAMP production in engineered cells and TG secretion by thyrocytes as well as HA secretion by orbital fibroblasts. However, TSAbs from GH patients stimulated thyrocytes more effectively than TSAbs from GO patients, whereas TSAbs from GO patients were more effective in activating orbital fibroblasts than TSAbs from GH patients. Conclusions: Clinical assays of stimulation by TSAbs measuring activation of the cAMP-PKA pathway do correlate with stimulation of thyrocytes and orbital fibroblasts; however, they do not distinguish between TSAbs from GH and GO patients. In vitro, TSAbs exhibit selectivity in activating TSHRs since TSAbs from GO patients were more effective in stimulating orbital fibroblasts and TSAbs from GH patients were more effective in stimulating thyrocytes.

Inhibition of TSH/IGF-1 Receptor Crosstalk by Teprotumumab as a Treatment Modality of Thyroid Eye Disease.

CONTEXT: We previously presented evidence that TSH receptor (TSHR)-stimulating autoantibodies (TSAbs) bind to and activate TSHRs but do not bind to IGF1 receptors (IGF1Rs). Nevertheless, we showed that IGF1Rs were involved in thyroid eye disease (TED) pathogenesis because TSAbs activated crosstalk between TSHR and IGF1R. Teprotumumab, originally generated to inhibit IGF1 binding to IGF1R, was recently approved for the treatment of TED (Tepezza). OBJECTIVE: To investigate the role of TSHR/IGF1R crosstalk in teprotumumab treatment of TED. DESIGN: We used orbital fibroblasts from patients with TED (TEDOFs) and measured stimulated hyaluronan (HA) secretion as a measure of orbital fibroblast activation by TED immunoglobulins (TED-Igs) and monoclonal TSAb M22. We previously showed that M22, which does not bind to IGF1R, stimulated HA in a biphasic dose-response with the higher potency phase dependent on TSHR/IGF1R crosstalk and the lower potency phase independent of IGF1R. Stimulation by TED-Igs and M22 was measured in the absence or presence of teprotumumab biosimilar (Tepro) or K1-70, an antibody that inhibits TSHR. RESULTS: We show: (1) Tepro dose-dependently inhibits stimulation by TED-Igs; (2) Tepro does not bind to TSHRs; (3) Tepro inhibits IGF1R-dependent M22-induced HA production, which is mediated by TSHR/IGF1R crosstalk, but not IGF1R-independent M22 stimulation; and (4) ß-arrestin 1 knockdown, which blocks TSHR/IGF1R crosstalk and prevents Tepro inhibition of HA production by M22 and by a pool of TED-Igs. CONCLUSION: We conclude that Tepro inhibits HA production by TEDOFs by inhibiting TSHR/IGF1R crosstalk and suggest that inhibition of TSHR/IGF1R crosstalk is the mechanism of its action in treating TED.

Thyrotropin receptor stimulates internalization-independent persistent phosphoinositide signaling.

The thyrotropin [thyroid-stimulating hormone (TSH)] receptor (TSHR) is known to acutely and persistently stimulate cAMP signaling and at higher TSH concentrations to acutely stimulate phosphoinositide signaling. We measured persistent signaling by stimulating TSHR-expressing human embryonic kidney-EM293 cells with TSH and measuring cAMP or inositol monophosphate (IP1) production, a measure of phosphoinositide signaling, 60 min or longer after TSH removal. In contrast to persistent cAMP production, persistent IP1 production increased progressively when TSH exposure was increased from 1 to 30 min, whereas the rates of decay of persistent signaling were similar. A small-molecule agonist and a thyroid-stimulating antibody also caused persistent IP1 and cAMP signaling. A small-molecule inverse agonist and a neutral antagonist inhibited TSH-stimulated persistent IP1 production, whereas the inverse agonist but not the neutral antagonist inhibited persistent cAMP production. As with persistent cAMP production, persistent IP1 production was not affected when TSHR internalization was inhibited or enhanced. Moreover, Alexa546-TSH-activated TSHR internalization was not accompanied by Gα(q) coupling protein internalization. Thus, transient exposure to high concentrations of TSH causes persistent phosphoinositide and cAMP signaling that is not dependent on internalization. To our knowledge, this is the first demonstration of persistent activation by any G protein-coupled receptor (GPCR) via the Gα(q) pathway and of two G protein-mediated pathways by any GPCR.