TSH Binding Research Articles

Calmodulin purified from human and porcine thyroids inhibits thyrotropin binding to porcine thyroid cells.

A thyrotropin (TSH) binding inhibiting protein (TBIP) that inhibits TSH binding to the TSH receptor, as determined by the TSH receptor assay, was purified from human and porcine thyroid. The soluble fraction (100,000 x g supernatant of Graves' thyroid homogenate) was precipitated with ammonium sulfate between 1.75 to 2.5 mol/L. TBIP was eluted by 0.5 mol/L sodium chloride (NaCl) containing 20 mmol/L Tris buffer, pH 7.5 from a Q-sepharose column. The unbound fraction from concanavalin A (Con A) and blue-sepharose was gel-filtered using sephadex G-100, and finally purified by Resource Q column chromatography. Purified TBIP was confirmed as a single protein band of 17 kDa. The TBI activity in the purified TBIP was significantly decreased by either etnylene glycol tetraacetate (EGTA) (1 mmol/L) or antibody to calmodulin (CaM) in the TSH receptor assay. The TBIP was confirmed immunologically as CaM by the Ouchterlony method using antibody for CaM. These findings demonstrated that the TBIP purified from human and porcine thyroids was, in fact, CaM. We examined the effects of TBIP purified from human thyroid on bovine TSH (bTSH) or thyroid stimulating antibody (TSAb)-stimulated cyclic adenosine monophosphate (cAMP) production in porcine thyroid cells (PTC). TBIP itself did not increase basal levels of cAMP production, but inhibited bTSH (100 mU/L)-stimulated cAMP production. However, TBIP did not inhibit cAMP production stimulated by TSAb-IgG and various thyroid stimulators (GTPgammaS, forskolin and pituitary adenylate cyclase-activating polypeptide [PACAP, 27 and 38 amino acids]). Authentic CaM purified from bovine brain behaved in a manner similar to that of TBIP. These data showed that CaM differentially affects thyroid stimulation by TSH and TSAb in intact thyroid cell experiments.

Understanding the thyrotropin receptor function—structure relationship

The thyrotropin (TSH) receptor (TSHR) is a key protein in the control of thyroid function and a major thyroid autoantigen. Recently, molecular cloning of the receptor has been carried out and we now review the impact of this work on our understanding of the physiology and pathophysiology of the TSHR. Analysis of recombinant TSHR proteins expressed in prokaryotic and eukaryotic systems has indicated that post-translational processing is important for the formation of active receptors. Studies of TSHR glycosylation have shown that a 'mature' form of the receptor containing mainly complex-type sugar residues is principally involved in TSH and TSHR autoantibody (TRAb) binding. In addition, the processing of the TSHR peptide chain into two subunits observed with native TSHR has been confirmed using recombinant TSHR. However, despite considerable efforts in many laboratories, the binding site(s) for TSH and TRAb on the TSHR have not been well characterized as yet and lessons learned from the discovery of naturally occurring amino acid mutations of the TSHR confirm the complexity of the hormone and autoantibody binding sites. Future progress in producing large amounts of pure TSHR as well as monoclonal TRAbs, followed by crystallographic analysis of TSHR-TSH complexes and TSHR-TRAb complexes, should be helpful in providing a better insight into the relationship between TSHR structure and function.

The human thyrotropin (TSH) receptor in a TSH binding inhibition assay for TSH receptor autoantibodies.

Seven years after the molecular cloning of the human TSH receptor (TSHR), the porcine TSHR remains in general use in the TSH binding inhibition (TBI) assay for autoantibodies to the TSHR. We compared porcine and recombinant human TSHR in two types of TBI assays: one using intact Chinese hamster ovary cells expressing the recombinant human TSHR on their surface, and the other using soluble receptors extracted from these cells with detergent. In the intact cell TBI assay, monolayers expressing large numbers of TSHR were less effective than cells expressing few receptors. These findings are consistent with the very low concentration of TSHR autoantibodies in serum. Binding of [125I]human TSH was about 5-fold lower than that of [125I]bovine TSH to the intact cells. Nevertheless, TBI values with the two ligands were similar for most sera. However, a few sera produced greater inhibition of human than of bovine TSH binding. In the solubilized human TSHR TBI assay, in contrast to the intact cell TBI assay, cells expressing very large number of TSHR were an excellent source for detergent extraction of soluble human TSHR, but only if the cells were extracted while still on the dish and not after scraping. A 10-cm diameter dish of cells provided TSHR for 100-200 replicate determinations when substituted for solubilized porcine TSHR in a commercial TBI kit. TBI values in serum from 30 individuals with suspected Graves' disease correlated closely when tested with solubilized human and porcine TSHR (r = 0.954; P < 0.001). However, 2 sera that were negative with the porcine TSHR were positive with the human TSHR. TBI and thyroid-stimulating activity in these sera correlated weakly regardless of whether the TBI used human or porcine TSHR. These findings open the way to a practical TBI assay using recombinant human TSHR.

Aspartate-474 in the first exoplasmic loop of the thyrotropin receptor is crucial for receptor activation

Asp-474 in the first exoplasmic loop of the thyrotropin receptor (TSHR), which is conserved among all glycoprotein hormone receptors, was mutated to Glu which is similarly charged but is longer by one methylene group and expressed in Cos-7 cells. Cells expressing this mutant receptor showed markedly impaired TSH- and TSAb (thyroid stimulating antibody)-stimulated cAMP responses with no effect on TSH binding affinity when compared with cells expressing a similar number of wild-type receptors. These results suggest the importance of Asp-474 in TSHR in receptor activation as demonstrated for LHR (lutropin receptor), but this, unlike LHR, is not due to the electrostatic interaction of this Asp residue with the α-subunit Lys-91 of the hormone.

Thyrotropin (TSH) receptor autoantibodies do not appear to bind to the TSH receptor produced in an in vitro transcription/translation system.

An in vitro transcription/translation (TnT) system was used to produce 35S-labeled full-length TSH receptor (TSHR) and TSHR extracellular domain (TSHRex). The interaction of the labeled proteins with TSHR autoantibodies in Graves' sera was then studied using an immunoprecipitation assay. In the assay, 35S-labeled TSHR or TSHRex were incubated with test sera, and any immune complexes formed were precipitated with protein A-Sepharose (in the case of mouse monoclonal antibodies, antimouse IgG-agarose was used). Rabbit antibodies to the TSHR and a mouse monoclonal antibody precipitated as much as 50% of the 35S-labeled TSHR preparations compared with about 2% for normal rabbit serum and 4% for a control monoclonal antibody. However, none of 34 Graves' sera (TSHR autoantibody levels ranging from 14-95% inhibition of [125I]TSH binding) were able specifically to immunoprecipitate 35S-labeled TSHR or TSHRex. These negative findings were confirmed by analysis of the immunoprecipitates on SDS-PAGE followed by autoradiography. Our results indicate that the TnT system is not useful for producing labeled TSHR preparations that can bind TSHR autoantibodies well. This is in contrast to TnT produced 35S-labeled glutamic acid decarboxylase, thyroid peroxidase, and 21-hydroxylase, which react well with their respective autoantibodies. One main difference between these 3 autoantigens and the TSHR is that the receptor is highly glycosylated, and this extensive glycosylation may be of critical importance for correct folding of the receptor. Consequently, the inability of the TnT system to glycosylate proteins could explain in part why TnT-produced 35S-labeled TSHR and TSHRex do not bind TSHR autoantibodies.

Differential reactivities of recombinant glycosylated ectodomains of mouse and human thyrotropin receptors with patient autoantibodies.

We expressed the extracellular domain of the mouse TSH receptor (mET-gp) using the baculovirus expression system. The recombinant protein was identified as mET-gp by immunoblotting and N-terminal amino acid sequencing. Carbohydrate analysis of the recombinant protein showed that the protein is glycosylated. Experimental antibodies raised against the extracellular domain of the human TSHr (ETSHr) were assayed for reactivity against mET-gp and glycosylated human ETSHr (ETSHr-gp) in an ELISA and found to be comparable. Similarly, both mET-gp and ETSHr-gp proteins neutralized the TSH binding inhibitory immunoglobulin (TBII) activity of rabbit anti-ETSHr antibodies in a RRA. However, when these proteins were compared for their ability to neutralize TBII and blocking activities (TSBAb) of IgG from patients with thyroid autoimmune disorders, only ETSHr-gp was able to neutralize these activities. In contrast, mET-gp partially neutralized, whereas ETSHr-gp completely neutralized the stimulatory (TSAb) activities of IgG from patients. Analyses of reactivities of these two proteins against a panel of antipeptide and monoclonal antibodies and their protein sequences showed differences in some specific epitopes. These data showed that in spite of significant homology between the two proteins, they exhibit specific epitope differences that are sufficient to cause divergence in their ability to react with patient autoantibodies to TSHr. This suggests that the two proteins might differ in their three-dimensional structure.

Thyrotropin receptor autoantibodies in serum are present at much lower levels than thyroid peroxidase autoantibodies: analysis by flow cytometry.

Using Chinese hamster ovary (CHO) cells that express high numbers of TSH receptor (TSHR) on their surface, we studied the feasibility of detecting directly by flow cytometry the binding of autoantibodies in patients' sera to the native TSHR. After using a serum (BBI) with high potency in the TSH binding inhibition (TBI) assay to establish the protocol, we studied an additional 38 sera: 10 without TBI activity (1-4.2% inhibition), 10 with moderately high TBI values (17.3-39.4% inhibition), 10 with high TBI levels (52-95.1% inhibition), 4 from normal individuals without autoimmune thyroid disease, and 4 from patients with systemic lupus erythematosus. We observed that a number of sera, including some without thyroid autoantibodies, contain antibodies against unknown antigens on CHO cells. Preadsorption with untransfected CHO cells before addition to the TSHR-10,000 cells eliminated or greatly reduced this nonspecific background. None of the sera from normal individuals, subjects with negative TBI values, or patients with systemic autoimmunity generated a positive signal on flow cytometry with TSHR-10,000 cells relative to the signal on untransfected cells. Remarkably, only 4 of 21 TBI-positive sera (including BBI) unequivocally recognized the TSHR on flow cytometry. In contrast, when thyroid peroxidase (TPO) autoantibodies in the same sera were studied using CHO cells overexpressing TPO on their surface, all 20 sera with TPO autoantibodies clearly elicited positive net fluorescence relative to untransfected cells. Study of the potent serum, BBI, revealed similar fluorescence (approximately 250 U) for TPO autoantibodies and TSHR autoantibodies at dilutions of 1:1000 and 1:10, respectively. Thus, by flow cytometry, the titer of TPO autoantibodies in the BBI serum is about 100-fold higher than that for TSHR autoantibodies in the same serum. In conclusion, the present data provide the strongest support for the idea that TSHR autoantibodies in the sera of patients with autoimmune thyroid disease are present at much lower levels than are TPO autoantibodies. This finding has important implications for the diagnostic detection of TSHR autoantibodies and for understanding the pathogenesis of Graves' disease.

Demonstration of thyroid stimulating activity within H chain fragments of TSAb-IgG by protease digestion and reduction.

Whether or not the distribution of biologically active fragments in TSAb-IgG molecules parallels antigen-binding activity in other anti-thyroidal antibodies was examined. Both the thyroid stimulating (TS) activity (cAMP production in thyroid cells) and TSH binding inhibition (TBI) activity (determined by TSH receptor assay) were examined by measuring the reduction in TSAb-IgG followed by gel-filtration on Sephadex G-100. Two forms of IgG [IgG(kappa) and IgG(lambda)] were separated from TSAb-IgG by column chromatography using Protein L-Sepharose (specifically binds to kappa chain). The IgG(kappa) was reduced with dithiothreitol (DTT) and the unbound fraction (UF) (the free heavy (H) chain) and the bound fraction (BF) (the free kappa chain and the non-reduced IgG(kappa)) were separated using Protein L-Sepharose. The Fab fragment was separated by Protein A-Sepharose after papain hydrolysis of TSAb-IgG, then separated into two Fab(kappa) and Fab(lambda) forms by Protein L-Sepharose. The Fd fragment (or fragment containing Fd) was prepared from Fab(kappa) by DTT reduction followed by Protein L column. The free H chain fraction showed TS and TBI activity, but neither anti-thyroglobulin (Tg) nor anti-thyroid peroxidase (TPO) antibody activities. The free light (L) chain was not biologically active. Similar TS and TBI activities were found not only in IgG(kappa) and IgG(lambda) but also in the Fab(kappa) and Fab(lambda) fractions. Fd fragment that was not contaminated with free kappa chain had TS and weak TBI activities. The thyroid stimulating activity in 5 TSAb-IgG samples was found in IgG(kappa), IgG(lambda), Fab(kappa), Fab(lambda), H chain and Fd separated by papain digestion and reduction. These results showed that TSAb was polyclonal and that the Fd fragment was important as the biologically active site.

Insight into Screening Immunoglobulin Gene Combinatorial Libraries in a Phage Display Vector: A Tale of Two Antibodies

Combinatorial libraries of immunoglobulin genes in “phage display” vectors are a powerful tool for obtaining antigen-specific antibody fragments. To date, this approach has been used to isolate abundant, but not rare, human autoantibodies of IgC class. We have compared the relative efficiencies of panning pComb3 libraries made from intrathyroidal plasma cells for abundant human autoantibodies to thyroid peroxidase (TPO) and rare autoantibodies to the thyrotropin receptor (TSHR). TPO-specific Fab were readily obtained from a library using three different forms of recombinant antigen, (i) purified TPO, (ii) impure TPO in culture medium and, (iii) TPO expressed on the surface of CHO cells. In contrast, TSHR-specific Fab were not isolated. This was the case despite repeated pannings of six libraries from three optimal patients (IgG/kappa and IgG/lambda libraries for each patient). Both purified recombinant TSHR and CHO cells expressing TSHR on their surface were used. Library enrichment was observed on some screenings. However, Fab expressed by individual clones or from enriched libraries were not specific as determined by (i) binding to purified, radio-labeled antigen, (ii) FACS analysis of TSHR on intact CHO cells and, (iii) inhibition of radiolabeled TSH binding. Remarkably, in screening for both TPO- and TSHR-specific Fab, neither library enrichment nor the retention of cDNA inserts of the correct size correlated with obtaining Fab with the antigenic specificity sought. Indeed, excellent enrichment could be observed with conditioned medium from untransfected cells. Our data suggest that the key to isolating rare antibodies from phage display libraries is not the creation of vast libraries of greater diversity or even the development of more stable vectors. Rather, success in this endeavor appears to require reducing the “noise” of non-specific clones in a moderately sized library.

Characterisation of the Antibody Response to the Extracellular Region of Recombinant Thyrotropin Receptor

Autoantibodies to the human thyrotropin receptor (TSH-R) are pathogenic in a number of autoimmune thyroid diseases including Graves' disease. We have characterised polyclonal antisera to TSH-R for antibodies which may mimic those present in autoimmune thyroid disease. For immunisations, recombinant extracellular region of human TSH-R which does not interact with its ligand TSH was used. The induced antibodies react with the full length membrane receptor in transfected mammalian cells by flow cytometry showing the presence of antibody capable” of recognising the native functional receptor. The properties of the generated antibodies have been compared after two injections or following a multiple immunisation protocol with the receptor in adjuvant. High titre antisera were readily generated after the short injection protocol and further immunisations did not lead to any change in antibody titers. Analysis of the epitopes recognised using synthetic peptides confirmed previous observations that the immunodominant determinants localise to the amino and the carboxyl terminal part of the extracellular region of the receptor. Antisera from both rabbits contain TSH blocking antibody as assessed by inhibition of TSH mediated cAMP stimulation. There was an increase in TSH binding inhibitory immunoglobulin (TBII) activity with multiple injections. Furthermore, the increase in TBII activity was not related to spreading of the antibody response to new determinants on TSH-R. Our results support previous observations on the difficulties in reproducing, by adjuvant immunisation with recombinant TSH-R preparations, the fine specificity of antibodies to TSH-R present in autoimmune disorders such as Graves' disease or primary myxoedema.

Molecular mechanism of LH/CG receptor activation

It is known that the N-terminal half of the LH/CG receptor is responsible for high hCG binding whereas the C-terminal half is capable of receptor activation. Our results suggest that initial hCG binding at the high affinity site in the N-half receptor induces conformational adjustments. This leads to low affinity secondary contacts of the complex of hCG/the N-half receptor with the C-half receptor. This low affinity secondary contact is responsible for activating the receptor. This is based on the following observations. The C-terminal tail of hCGα is known to be involved in activation of the LH/CG receptor. In addition to hCG, we examined the C-terminal three residues (His 90-Lys 91-Ser 92) of the common α subunit of FSH and TSH. The results show their differential roles in the three hormones. Ser 92 is important for binding and cAMP induction of TSH but not for hCG and FSH. Lys 91 is important for binding and cAMP induction of hCG, and cAMP induction but not binding of FSH. It is not important for binding or cAMP induction of TSH. His 90 is important for all three hormones. When all three residues were truncated, FSH and TSH lose their affinity for binding and cAMP induction, whereas hCG is still capable of binding but not cAMP induction. Therefore, the three amino acids contribute differently in receptor binding and cAMP induction of hCG, FSH and TSH. Our data also indicate that the evolution of the α subunit has been constrained in order not to impair any one of the hormones. This suggests that each hormone can be independently engineered to improve the potency. To chemically identify the contact site of the α C-tail of hCG in the LH/>CG receptor, a decamer peptide corresponding to the α subunit sequence from His 83 to Ser 92 (peptide α 83–92) was derivatized with UV sensitive reagent, ABG and radio-iodinated. The resulting ABG- 125I-peptide α 83–92 was capable of binding and activating the LH/CG receptor. Furthermore, it specifically photoaffinity-labeled the LH/CG receptor. In addition, the amino group of αLys 91 of peptide α 83–92 is crosslinked to a carboxyl group of the receptor, an indication of close association. Reciprocal mutagenesis of αLys 91 and Asp 397 in exoloop 1 of the LH/CG receptor suggests the complementary of this pair in receptor activation but not the high affinity interaction of hCG and the receptor. In addition, Lys 583 of exoloop 3 is also crucial for receptor activation. To test the conformational adjustment, ABG was attached to hCGα and reassociated with untreated β to produce ABG- 125I - α β . The extent of inter-subunit crosslinking of ABG- 125I - α β bound to the receptor was two to three fold less than unbound ABG- 125I - α β . This result indicates structural change at the subunit interface in response to hCG binding to the receptor.

Epitope-Tagging of a Functional Thyrotropin Receptor: Detection of the Native Receptor on Intact Cells

To facilitate immunological detection of thyrotropin receptor (TSHR), we inserted ac-mycepitope within the unique, 50 amino acid segment of the ectodomain (TSHRmyc). When stably expressed in 293 human embryonal kidney (HEK) cells, TSHRmyc demonstrated high affinity TSH binding and the ability to produce cAMP in response to TSH. Binding of themycmonoclonal antibody 9E10 to 293-TSHRmyc cells could be detected with [125I] anti-mouse IgG. No competition was observed between TSH and 9E10 binding to 293-TSHRmyc. Immunoprecipitation by 9E10 of TSHRmyc revealed TSHR forms of ∼95 and ∼100 kDa. Endoglycosidase digestion identified the ∼95 kDa species as the single chain precursor with high mannose carbohydrate. The ∼100 kDa single chain receptor contained mature, complex carbohydrate. No smaller species of TSHR subunits or proteolytic fragments was observed. Again TSH did not inhibit immunoprecipitation of TSHRmyc by 9E10. These data demonstrate that the normally functioningc-mycepitope-tagged TSHR can be detected directly and in native form with a readily available anti-myc9E10 and without the need for prior affinity capture. Lack of competition between 9E10 and TSH suggests that at least part of the 50 amino acid segment in TSHR ectodomain is not a TSH binding site. This epitope-tagged TSHR will be valuable for further studies on the synthesis and trafficking of TSHR.

Evidence for negative cooperativity among human thyrotropin receptors overexpressed in mammalian cells.

The complementary DNA for the human TSH receptor (TSHR) translated region was amplified in the genome of stably transfected Chinese hamster ovary (CHO) cells using a dihydrofolate reductase minigene. Immunoprecipitation of TSHR in whole cells precursor-labeled with [35S]methionine and [35S]cysteine revealed an approximately 10-fold increase in TSHR expression in cells stabilized in 10,000 nM methotrexate (TSHR-10,000 cells) compared to cells with the same gene not subjected to amplification (TSHR-0 cells). Similarly, [125I]TSH cross-linking to the surface of intact CHO cells revealed a progressive increase in TSH-binding sites with dihydrofolate reductase minigene amplification, with a 12.8-fold increase in TSHR in TSHR-10,000 vs. TSHR-0 cells. Based on the known number of TSHR expressed by TSHR-0 cells, TSHR-10,000 express approximately 1.9 x 10(6) TSHR on their surface. Two ligand-TSHR complexes were evident under reducing conditions, representing the single chain holoreceptor of about 115 kDa and a dissociated A subunit of about 60 kDa. In the absence of TSH, basal cAMP levels in TSHR-10,000 cells were greater than those in TSHR-0 cells (6-fold in isotonic medium and 18.5-fold in hypotonic medium), indicating that the unliganded TSHR has significant constitutive activity. We assessed the kinetics of TSH binding to CHO cells overexpressing the TSHR using [125I]TSH in the presence of increasing concentrations of unlabeled TSH as well as by attempted saturation with labeled ligand. Surprisingly, in contrast to TSHR-0 cells (Kd = approximately 5 x 10(-10) M), we observed progressively lower affinities for TSH binding by TSHR-800 cells (Kd = approximately 10(-9) M) and TSHR-10,000 cells (Kd = approximately 2 x 10(-9) M). In summary, we report a high level of expression of TSHR in CHO cells and confirm the high constitutive activity of the TSHR in the absence of ligand as well as the binding of TSH to the single subunit, uncleaved TSHR. Moreover, we found that a high level of expression is associated with apparent negative cooperativity among the TSHR in terms of their affinity for ligand.

Molecular analysis of stimulatory anti-thyrotropin receptor antibodies (TSAbs) involved in Graves' disease. Isolation and reconstruction of antibody genes, and production of monoclonal TSAbs.

Anti-thyrotropin (TSH) receptor autoantibodies (TRAbs) have been known to be involved in Graves' disease. To understand the molecular mechanism for pathogenesis of TSAbs in Graves' disease, we isolated and reconstituted the Ig genes of EBV-transformed B cell clones producing monoclonal thyroid stimulating Ab (TSAb) obtained from patients with Graves' disease. The V region genes of Ig heavy (H) and light (L) chains of two TSAb clones, IgG clone B6B7 and IgM clone 101-2, were isolated by the PCR. Nucleotide sequencing analysis revealed that germ-line VH and VK segments widely used for autoantibodies including the previously isolated TRAbs were utilized in the two clones. A significant number of somatic mutations were found in V regions of both clones, indicating the involvement of somatic mutations for the TSAb specificity. Reconstituted Ig H and L chain genes of the two clones were stably introduced into myeloma cells for IgG1 production. IgGs purified from cultured supernatants of both transfectants exhibited significant TSAb activities, while they did not inhibit TSH binding to the receptor. The successful expression of recombinant TSAbs in eukaryotic cells will provide opportunities to apply them to various pathophysiologic, diagnostic and therapeutic investigations in autoimmune thyroid diseases.

Multimeric complex formation by the thyrotropin receptor in solubilized thyroid membranes.

The TSH receptor (TSHR) has a large glycosylated ectodomain comprising the amino-terminal half of the molecule (394 of 743 residues) implicated in TSH binding, as well as autoantibody recognition in Graves' disease. In this study we employed antibodies specific for the amino-terminus (Ab1), midportion (Ab2), and carboxyl-terminus (Ab3) of the TSHR-ectodomain, previously mapped using recombinant receptor proteins, to detect the natural receptor present in detergent-solubilized porcine thyroid cell membranes via immunoblotting. Several forms of the receptor were detected. In reduced samples Ab1 detected full-length holoreceptors present in both nonglycosylated and glycoslylated forms of apparent molecular masses 80 and 90 kDa, respectively, as well as apparent dimeric nonglycosylated and dimeric glycosylated holoreceptor forms resistant to reduction. Also detected by Ab1 were a glycosylated amino-terminal 47- to 52-kDa fragment of the holoreceptor (gly alpha-subunit), reduced to 42 kDa (alpha-subunit) by Endo F deglycosylation. Ab2 detected all of the same forms. Ab3 detected primarily a carboxy-terminal, nonglycosylated fragment of 35 kDa (beta-subunit). In unreduced samples, the recognition pattern was unchanged with Ab1. Ab2 detected monomeric and dimeric beta-subunits, as well as higher order complexes. The different TSHR forms present in unreduced preparations were resolved by ammonium sulfate precipitation, confirming their autonomy. The data demonstrate the presence of multiple forms of the natural TSHR. Their roles in TSH action and TSHR autoimmunity require further exploration.

Thyrotropin (TSH) receptor antibodies (TSHrAb) can inhibit TSH-mediated cyclic adenosine 3',5'- monophosphate production in thyroid cells by either blocking TSH binding or affecting a step subsequent to TSH binding.

In the present study, rabbit antibodies that possess thyroid stimulation-blocking activity were used to investigate potential mechanisms by which TSH receptor antibodies can inhibit thyroid cell function. The antibodies were produced against two synthetic peptides corresponding to amino acids 357-372 (p357) and 367-386 (p367) of the human TSHr (hTSHr). By enzyme-linked immunosorbent assay, both antisera (alpha 357 and alpha 367) had high titers ( > 1:100,000) of IgG against their respective peptides and recombinant extracellular TSHr protein (ETSHr); alpha 357 had a low IgG titer to p367 (1:800), and alpha 367 had a low IgG titer to p357 ( < 1:200). Based on competitive inhibition studies, alpha 357 and alpha 367 displayed similar relative binding affinities for their respective peptides and for recombinant ETSHr. When tested by commercial RRA, alpha 357 did not block (TSH binding inhibition index, -3.7%), whereas alpha 367 blocked TSH binding to TSHr (TSH binding inhibition index, 53.9%). The blocking effect of alpha 367 could be reversed by incubating the antiserum with p367 before assay. When applied alone to FRTL-5 cells, IgG from alpha 357 inhibited [compared to normal rabbit IgG (NRI); P < 0.01] based cAMP production by the cells, whereas IgG from alpha 367 did not. IgG from both alpha 357 and alpha 367, however, were able to inhibit (P < 0.001) TSH-mediated cAMP production by FRTL-5 cells [bovine (b) TSH, 2.5 x 10(-10) M; cAMP (mean +/- SD; picomoles per ml): NRI, 62.5 +/- 6.1; alpha 357, 12.2 +/- 2.4; alpha 367, 36.2 +/- 3.5]. Alpha 357 continued to inhibit (P < 0.05) cAMP production by FRTL-5 cells in 10(-8) M bTSH, whereas alpha 367 no longer inhibited cAMP production at bTSH concentrations above 5 x 10(-10) M. Compared to NRI, both alpha 357 and alpha 367 were also able to inhibit (P < 0.001) Graves' IgG-mediated cAMP production by FRTL-5 cells. When IgG were tested on FRTL-5 cells in the presence of 10(-7) M forskolin, only alpha 357 inhibited (P < 0.001) cAMP production (NRI, 75.1 +/- 4.8; alpha 357, 52.3 +/- 4.5; alpha 367, 77.2 +/- 1.4). To determine whether the inhibitory effect of alpha 357 on forskolin-mediated stimulation was thyroid cell dependent, IgG were tested on Chinese hamster ovary (CHO) cells transfected with the complementary DNA of the hTSHr (CHO-R). Again, alpha 357 inhibited (P < 0.005) cAMP production mediated by forskolin (at 10(-7) M; NRI, 68.7 +/- 4.4; alpha 357, 36.8 +/- 5.7; alpha 367, 64.6 +/- 8.5). alpha 357 did not inhibit forskolin-mediated cAMP production by untransfected CHO cells (CHO-N), indicating that the inhibitory effect of alpha 357 on forskolin stimulation was TSHr dependent. In addition, alpha 357 inhibited (P < 0.01) basal cAMP production by CHO-R cells, but not by CHO-N cells. alpha 367 had no effect on the basal cAMP production in either CHO-R or CHO-N cells. Neither alpha 357 nor alpha 367 inhibited cholera toxin-mediated cAMP production in FRTL-5 cells. In all relevant bioassays, the inhibitory effects of alpha 357 and alpha 367 could be reversed by preincubating the IgG with the respective peptides. From these data, we conclude that 1) alpha 367 binds to the ETSHr and blocks TSH-mediated cAMP production by inhibiting TSH from binding to its receptor; 2) alpha 357 binds to the TSHr and, without blocking TSH binding, inhibits TSH-mediated cAMP production at a step(s) subsequent to ligand binding that affects adenylate cyclase activity; and 3) forskolin-mediated cAMP production by thyroid cells can be inhibited by IgG that bind directly to the TSHr.

Dissociation of thyrotropin receptor function and thyrotropin dependency in rat thyroid tumour cell lines derived from FRTL-5.

Spontaneously transformed somatic thyrocyte mutants, FRTL-5/TA and FRTL-5/TP, are thyrotropin (TSH) independent for growth and show loss of the thyroid-specific phenotype, with absent thyroglobulin and thyroid peroxidase gene expression. To investigate the role of TSH-receptor (TSH-R) activation in rat thyroid growth and function, binding of TSH and TSH-induced cAMP production were measured in intact cells under identical assay conditions. TSH binding did not differ in terms of affinity and receptor number and presence of 5.6 kb and 3.3 kb mRNA rat TSH-R transcripts was determined in all variants. By contrast, basal cAMP was 11-fold lower in FRTL-5/TA and 6-fold lower in FRTL-5/TP than in wild-type FRTL-5 (1.1 +/- 0.4; P < 0.01). Maximal cAMP production was similar between wild-type and cell variants and stimulation by bovine, rat and recombinant human TSH revealed normal activation patterns. Therefore, a dissociation was present between the loss of TSH control on growth and function, and the presence of a normally functioning TSH-R. Subsequent to TSH incubation FRTL-5/TP and FRTL-5/TA cells showed a different expression pattern of TSH-R and the proto oncogenes c-myc and fos than FRTL-5 wild-type. The data indicated that the cause of the TSH-independency is located down-stream of the cAMP cascade, influencing genes that control the expression of cell cycle-related proto-oncogenes and thyroid-specific genes.

Precise determination of TSH receptor antibody activity in serum containing bovine TSH (bTSH) binding antibody by absorption using denatured bTSH or sheep FSH.

A previous report demonstrated that sera with bovine TSH (bTSH) binding antibody showed abnormally negative TSH receptor antibody (TRAb) activity in the standard TRAb assay method. The corrected TRAb activity calculated by the determination of the nonspecific binding of the labeled bTSH for each test serum [NSB(T)] resulted in positive TRAb activity. However, the precise calculation was difficult because NSB(T) level was significantly higher than the nonspecific binding in normal pool serum [NSB(N)] level. In the present experiment the determination of the TRAb activity was performed after absorption of bTSH binding antibodies by the heat-denatured bTSH to obtain more precise TRAb activity. In addition, absorption by sheep FSH (sFSH) was performed because almost all bTSH binding antibodies showed specific binding to the alpha-subunit of mammalian pituitary glycoprotein hormones in our previous study. Three days absorption of test serum using 1 mU of the heat-denatured bTSH (100 degrees C for 1 h) or 5 mU of sFSH was chosen as optimal because NSB(T) decreased remarkably to NSB(N) levels. The corrected TRAb determined after these absorptions decreased significantly compared to the corrected TRAb activity without the absorption. When the complete absorption of bTSH binding antibody was performed by the decrease of the NSB(T) level to the NSB(N) level, the TRAb activity determined by these two different absorptions was almost similar (difference was less than 10%). However, it was difficult to obtain the precise TRAb activity in the cases with extremely high bTSH binding antibody, because the NSB(T) level was higher than the NSB(N) level by the incomplete absorption of bTSH binding antibody.

Thyrotropin (TSH) receptor antibodies (TSHrAb) can inhibit TSH-mediated cyclic adenosine 3',5'- monophosphate production in thyroid cells by either blocking TSH binding or affecting a step subsequent to TSH binding

Thyrotropin (TSH) receptor antibodies (TSHrAb) can inhibit TSH-mediated cyclic adenosine 3',5'- monophosphate production in thyroid cells by either blocking TSH binding or affecting a step subsequent to TSH binding

Increased cyclic adenosine 3',5'-monophosphate inhibits G protein-coupled activation of phospholipase C in rat FRTL-5 thyroid cells.

Thyroid cell growth and function are regulated by several hormones and growth factors that bind to cell surface receptors coupled via G proteins, Gs and Gq, to stimulation of adenylyl cyclase and phospholipase C (PLC), respectively. We created a permanently transfected FRTL-5 cell line (TG8) in which the thyroglobulin gene promoter directs expression of the cholera toxin (CT) A1 subunit (CTA1). CTA1 catalyzes ADP ribosylation of Gs alpha, which results in persistent activation of Gs alpha. Activated Gs alpha causes constitutive stimulation of adenylyl cyclase and increases levels of intracellular cAMP. Because G protein-coupled signaling pathways exhibit cross-talk, we compared TG8 cells to FRTL-5 cells transfected with the neomycin resistance gene (TG4) to determine whether constitutive stimulation of adenylyl cyclase influences the PLC pathway. PLC activity was assessed by measuring levels of total inositol phosphates (IPs) in TG4 and TG8 cells that had been preincubated with myo-[3H]inositol for 2 days. Baseline values of [3H]IP production were similar for the two cell lines. Incubation of TG4 control cells with 10(-8) M TSH, 300 microM ATP, and 100 microM norepinephrine for 60 min stimulated 2.5-, 8.1-, and 3.4-fold increases, respectively, in [3H]IP production over the control value. By contrast, there was no [3H]IP response to any of these ligands in TG8 cells. TG8 cells exhibit a decrease in [35S]adenosine 5'-(gamma-thio)triphosphate binding to their cell surface compared to TG4 control cells counterparts, but no decrease in [125I]TSH binding. Treatment of TG4 cells with 100 ng/ml CT, 50 microM forskolin, or 1 mM 8-bromo-cAMP for 2 days reproduced the loss of ligand-stimulated [3H]IP synthesis present in TG8 cells. Although levels of immunoreactive Gq alpha and Gq alpha 11 were normal in TG8 cells, sodium fluoride-induced [3H]IP production was also inhibited. Levels of immunoreactive PLC beta 3, the dominant subtype of PLC beta in FRTL-5 cells, were not altered in TG8 cells or by CT treatment of TG4 cells. These data indicate that elevated levels of cAMP can inhibit the activity of G protein-coupled PLC. Further study of this model will elucidate our understanding of the exact mechanism responsible for this interaction.