Thyroid-stimulating hormone receptor (TSHR) as a target for imaging differentiated thyroid cancer.

BACKGROUND: Of the half a million cases of thyroid cancer diagnosed annually, 95% are differentiated thyroid cancers. Although clinical guidelines recommend surgical resection followed by radioactive iodine ablation, loss of sodium-iodine symporter expression causes up to 20% of differentiated thyroid cancers to become radioactive iodine refractory. For patients with radioactive iodine refractory disease, there is an urgent need for new diagnostic and therapeutic approaches. We evaluated the thyroid-stimulating hormone receptor as a potential target for imaging of differentiated thyroid cancer. METHODS: We immunostained tissue microarrays containing 52 Hurthle cell carcinomas to confirm thyroid-stimulating hormone receptor expression. We radiolabeled chelator deferoxamine conjugated to recombinant human thyroid-stimulating hormone analog superagonist TR1402 with 89Zr (t1/2 = 78.4 h, ß+ =22.7%) to produce [89Zr]Zr-TR1402. We performed in vitro uptake assays in high-thyroid-stimulating hormone receptor and low-thyroid-stimulating hormone receptor-expressing THJ529T and FTC133 thyroid cancer cell lines. We performed in vivo positron emission tomography/computed tomography and biodistribution studies in male athymic nude mice bearing thyroid-stimulating hormone receptor-positive THJ529T tumors. RESULTS: Immunohistochemical analysis revealed 62% of patients (27 primary and 5 recurrent) were thyroid-stimulating hormone receptor membranous immunostain positive. In vitro uptake of 1nM [89Zr]Zr-TR1402 was 38 ± 17% bound/mg in thyroid-stimulating hormone receptor-positive THJ529T thyroid cancer cell lines compared to 3.2 ± 0.5 in the low-expressing cell line (P < .01), with a similar difference seen in FTC133 cell lines (P < .0001). In vivo and biodistribution studies showed uptake of [89Zr]Zr-TR1402 in thyroid-stimulating hormone receptor-expressing tumors, with a mean percentage of injected dose/g of 1.9 ± 0.4 at 3 days post-injection. CONCLUSION: Our observation of thyroid-stimulating hormone receptor expression in tissue microarrays and [89Zr]Zr-TR1402 accumulation in thyroid-stimulating hormone receptor-positive thyroid cancer cells and tumors suggests thyroid-stimulating hormone receptor is a promising target for imaging of differentiated thyroid cancer.

Autoantibody mimicry of hormone action at the thyrotropin receptor.

Thyroid hormones are vital in metabolism, growth and development1. Thyroid hormone synthesis is controlled by thyrotropin (TSH), which acts at the thyrotropin receptor (TSHR)2. In patients with Graves' disease, autoantibodies that activate the TSHR pathologically increase thyroid hormone activity3. How autoantibodies mimic thyrotropin function remains unclear. Here we determined cryo-electron microscopy structures of active and inactive TSHR. In inactive TSHR, the extracellular domain lies close to the membrane bilayer. Thyrotropin selects an upright orientation of the extracellular domain owing to steric clashes between a conserved hormone glycan and the membrane bilayer. An activating autoantibody from a patient with Graves' disease selects a similar upright orientation of the extracellular domain. Reorientation of the extracellular domain transduces a conformational change in the seven-transmembrane-segment domain via a conserved hinge domain, a tethered peptide agonist and a phospholipid that binds within the seven-transmembrane-segment domain. Rotation of the TSHR extracellular domain relative to the membrane bilayer is sufficient for receptor activation, revealing a shared mechanism for other glycoprotein hormone receptors that may also extend to other G-protein-coupled receptors with large extracellular domains.

In Vivo Imaging of Thyroid Cancer with 99mTc-TR1401 and 99mTc-TR1402: A Comparison Study in Dogs.

Differentiated thyroid cancer (DTC) cells may lose NIS expression and iodine uptake, but usually express TSH receptors (TSHR). Therefore, the aim of our study was to compare two radiolabeled superagonist TSH analogues for DTC imaging. These analogues (namely TR1401 and TR1402) have a higher TSHR binding affinity than recombinant human TSH (Thyrogen®). Radiolabeling was performed with technetium-99m using an indirect method via HYNIC conjugation and was followed by in vitro quality controls and binding assay on TSHR-positive cell lines (ML-1). An in vitro binding assay was also performed and compared with radiolabeled human recombinant TSH. In vivo imaging was performed in four dogs with spontaneous follicular thyroid carcinoma with solid poorly differentiated areas with 99mTc-TR1401 SPECT/CT, 99mTc-TR1402 SPECT/CT, and [18F]FDG PET/CT on different days within 2 weeks. TR1401 and TR1402 were labeled with high specific activity (8.3 ± 1.2 MBq/µg) and retention of their biological activity and structural integrity. Both agonists were able to efficiently bind TSHR receptors expressed by cell lines with dissociation constants (Kd) of 2.7 nM for 99mTc-TR1401 and 0.5 nM for 99mTc-TR1402 compared with 99mTc-Thyrogen (Kd = 8.4 nM). In tumor-targeting experiments, a focal uptake was observed in dogs with spontaneous intraglandular thyroid carcinoma, in which TSHR expression was confirmed by immunohistochemistry. 99mTc-TR1402 provided higher T/B than 99mTc-TR1401 and [18F]FDG (12.9 ± 1.3, 10.2 ± 0.7, and 3.8 ± 0.6, respectively; all p < 0.001). Given these results, 99mTc-TR1402 appears to be a useful tool for in vivo imaging of thyroid cancer.

Radiolabeling of VEGF165 with 99mTc to evaluate VEGFR expression in tumor angiogenesis.

Angiogenesis is the main process responsible for tumor growth and metastatization. The principal effector of such mechanism is the vascular endothelial growth factor (VEGF) secreted by cancer cells and other components of tumor microenvironment. Radiolabeled VEGF analogues may provide a useful tool to noninvasively image tumor lesions and evaluate the efficacy of anti-angiogenic drugs that block the VEGFR pathway. Aim of the present study was to radiolabel the human VEGF165 analogue with 99mTechnetium (99mTc) and to evaluate the expression of VEGFR in both cancer and endothelial cells in the tumor microenvironment. 99mTc-VEGF showed in vitro binding to HUVEC cells and in vivo to xenograft tumors in mice (ARO, K1 and HT29). By comparing in vivo data with immunohistochemical analysis of excised tumors we found an inverse correlation between 99mTc-VEGF165 uptake and VEGF histologically detected, but a positive correlation with VEGF receptor expression (VEGFR1). Results of our studies indicate that endogenous VEGF production by cancer cells and other cells of tumor microenvironment should be taken in consideration when performing scintigraphy with radiolabeled VEGF, because of possible false negative results due to saturation of VEGFRs.

New Frontier in Glycoprotein Hormones and Their Receptors Structure-Function.

Last two decades of structure-function studies performed in numerous laboratories provided substantial progress in understanding basic science, physiological, pathophysiological, pharmacological, and comparative aspects of glycoprotein hormones (GPHs) and their cognate receptors. Multiple concepts and models developed based on experimental data in the past stood the test of time and have been, at least in part, confirmed and/or remained compatible with the new structures resolved at the atomic level. Major advances in understanding of the ligand-receptor relationships are heralding the dawn of a new era for GPHs and their receptors, although many basic questions still remain unanswered. This article examines retrospectively several basic science aspects of GPH super-agonists and related "biosuperiors" in a broader context of the advances in the ligand-receptor structure-function relationships and new mechanistic models generated based on the structure elucidation. Due to selective focus of my comments and perspectives in certain parts, the reader is directed to the most relevant publications and reviews in the field for more comprehensive analyses.

(99m)Tc-labeled-rhTSH analogue (TR1401) for imaging poorly differentiated metastatic thyroid cancer.

BACKGROUND: Differentiated thyroid carcinomas originating from thyroid follicular cells are frequent tumors of the thyroid with relatively good prognosis due to improved surgical techniques and follow-up procedures. Poorly differentiated thyroid cancers, which lose iodine uptake ability, in most cases still express thyrotropin (TSH) receptors (TSHR). Therefore, the aim of this study was to radiolabel a superagonist recombinant human TSH (rhTSH) analogue for imaging poorly differentiated thyroid cancer. METHODS: The TSHR superagonist, TR1401, was labeled with (99m)Tc using an indirect method via succinimidyl-6-hydrazinonicotinate hydrochloride conjugation. In vitro quality controls included SDS-PAGE, cysteine challenge, and cell-binding assay on TSHR positive cell lines (JP09 and ML-1). In vivo studies included tumor targeting experiments in athymic nude CD-1 mice xenografted with several different TSHR positive cells (JP09, K1, and ML-1) and TSHR negative cells (JP02) as control. RESULTS: The superagonist rhTSH analogue TR1401 was labeled with high labeling efficiency (>95%) and high specific activity (9250 MBq/mg). The labeled molecule retained its biologic activity and structural integrity. In tumor targeting experiments, a focal uptake of radiolabeled TR1401 was observed in TSHR positive cells but not in TSHR negative cells. The same observation was made in a dog with spontaneous intraglandular thyroid cancer. CONCLUSIONS: We were able to radiolabel the rhTSH superagonist analogue TR1401 with (99m)Tc efficiently with retention of in vitro and in vivo binding capacity to TSHR. The relative role of such novel radiopharmaceutical versus (131)I scanning of thyroid cancer will require future histopathologic and clinical studies, but it may open new perspectives for presurgical staging of thyroid cancer, and diagnosis of radioiodine negative local relapses and/or distant metastases.

Identification of novel TSH interaction sites by systematic binding analysis of the TSHR hinge region.

In which ways the binding of the thyroid stimulating hormone to the extracellular domain of its receptor leads to activation of the thyroid-stimulating hormone receptor (TSHR) is currently only incompletely understood. It is known that TSH binding to the TSHR depends on the interaction with the leucine-rich repeat and sulfation at Y385 of the hinge region. Recently it was also shown that electrostatic interactions between positive charges of bovine (b) TSH and the residues E297, E303, and D382 of the hinge region contribute to hormone-TSHR binding. After the identification of these first TSH binding sites in the hinge region, it was apparent that multiple positions in this region remained to be characterized for their roles in hormone binding. The goal of this study was therefore to clarify whether further contact points of TSH exist in the structurally undefined hinge region. Therefore, we systematically analyzed 41 uncharacterized residues of the TSHR hinge region as single mutants regarding differences between cell surface expression and bTSH binding. Indeed, we identified further amino acids of the hinge region with influence on bTSH binding. Some of these contribute to a new binding domain from human TSHR position F381 to D386. These hinge mutants with influence on bTSH binding were also analyzed for binding of the superagonistic human TSH analog TR1401 demonstrating that these positions also have an impact on TR1401 binding. Moreover, side chain variations revealed that different amino acid properties like the negative charge, aromatic as well as hydrophilic characteristics, contribute to maintain the hormone-TSHR hinge interaction.

Effects of recombinant human thyroid-stimulating hormone superagonists on thyroidal uptake of 18F-fluorodeoxyglucose and radioiodide.

BACKGROUND: Superagonist analogs of human thyroid-stimulating hormone (hTSH) may stimulate the uptake of (131)I-iodide and (18)F-fluorodeoxyglucose ((18)F-FDG) in thyroid carcinomas to a greater degree than hTSH. We herein report the potency and efficacy of two hTSH analogs, TR1401 and TR1402, to stimulate radioiodide and (18)F-FDG uptake in FRTL-5 cells and compared the effects of hTSH and TR1401 on radioiodide uptake in the thyroid in vivo in mice. METHODS: The effects of hTSH analogs on intracellular levels of cAMP, uptake of (131)I-iodide, and (18)F-FDG were studied in FRTL-5 cells to determine the stimulatory potency and efficacy of the compounds by calculating half-maximum effective concentration (EC(50)) values and maximal stimulatory effects (E(max)). Biodistribution studies (n = 96) and positron emission tomography/computed tomography imaging studies (single animals) on thyroid (125)I/(124)I-iodide uptake were performed with T3-suppressed CD-1 mice in a dose-dependent manner (3, 10, and 30 µg/animal). RESULTS: The EC(50) values of TR1401 and TR1402 demonstrated a 90-fold or 800-fold higher potency for their capacity to increase intracellular cAMP levels in comparison with hTSH (p < 0.05). Similar results were demonstrated for the stimulation of (18)F-FDG uptake. Bovine TSH, TR1401, and TR1402 were 85%-490% more potent to increase iodide uptake than hTSH (p < 0.05). TR1402 was 30% more efficacious to stimulate iodide uptake than hTSH. The agonist-induced increase in radiotracer uptake was paralleled by increases in NIS and GLUT-1 expression. Ex vivo biodistribution studies showed an increased iodide uptake in the thyroid of TR1401-treated mice at the low dose of 3 µg/animal in comparison with hTSH-treated mice (n = 16, p < 0.05). Positron emission tomography/computed tomography imaging studies confirmed the increased thyroidal iodide uptake in TR1401-treated mice in vivo. CONCLUSIONS: TR1401 and TR1402 have considerably higher potency than hTSH to stimulate thyroidal iodide and (18)F-FDG uptake in vitro. Moreover, in vivo studies indicated that at low but not higher doses, TR1401 induced an enhanced ability for the thyroid to concentrate iodide compared with hTSH. These properties makes TR1401 and TR1402 interesting candidates for use in humans to enhance uptake of radioiodine and (18)F-FDG by metastases and recurrences of thyroid carcinoma.

The superagonistic activity of bovine thyroid-stimulating hormone (TSH) and the human TR1401 TSH analog is determined by specific amino acids in the hinge region of the human TSH receptor.

Bovine TSH (bTSH) has a higher affinity to the human TSHR (hTSHR) and a higher signaling activity than human TSH (hTSH). The molecular reasons for these phenomena are unknown. Distinct negatively charged residues (Glu297, Glu303, and Asp382) in the hinge region of the hTSHR are known to be important for bTSH binding and signaling. To investigate the potential relevance of these positions for differences between bTSH and hTSH in the interaction to the hTSHR, we determined bTSH- and hTSH-mediated cAMP production of several substitutions at these three hinge residues. To examine specific variations of hTSH, we also investigated the superagonistic hTSH analog TR1401 (TR1401), whose sequence differs from hTSH by four additional positively charged amino acids that are also present in bTSH. To characterize possible interactions between the acidic hTSHR positions Glu297, Glu303, or Asp382 and the additional basic residues of TR1401, we investigated TR1401 binding and signaling properties. Our data reveal increased cAMP signaling of the hTSHR using TR1401 and bTSH compared with hTSH. Whereas Asp382 seems to be important for bTSH- and TR1401-mediated but not for hTSH-mediated signaling, the substitution E297K exhibits a decreased signaling for all three TSH variants. Interestingly, bTSH and TR1401 showed only a slightly different binding pattern. These observations imply that specific residues of the hinge region are mediators of the superagonistic activity of bTSH and TR1401 in contrast to hTSH. Moreover, the simultaneous localization of binding components in the glycoprotein hormone molecule and the receptor hinge region permits important reevaluation of interacting hormone receptor domains.

Challenges and opportunities of trapping ligands.

Because gonadotropin-releasing hormone (GnRH) analogs constitute an important class of therapeutics for various reproductive and hormone-dependent disorders, many novel compounds have been discovered and studied. Several orally active nonpeptide GnRH antagonists have recently gained increased attention. In the study published in this issue of Molecular Pharmacology, Kohout et al. (p. 238) used small-molecule TAK-013 (sufugolix; developed previously by Takeda Chemical Industries) as a tool to elucidate the mechanism of its insurmountable antagonism. On the basis of receptor mutagenesis combined with molecular modeling, the authors hypothesized that certain amino acid sequences uniquely present in the human GnRH receptor amino terminus and extracellular loop 2 may form a "trap door" retarding dissociation of TAK-013. Such a trapping mechanism could be both ligand- and receptor species-specific. Although analogous models were previously proposed for other G protein-coupled receptors, the study by Kohout et al. (2007) provides an important advance in the GnRH antagonists field and an illustration of the fact that preclinical studies using animal models with nonhuman receptors may have very limited value in predicting drug efficacy in human disease. There are many examples showing that high-affinity protein, peptide, or nonpeptide agonists or antagonists have also enhanced clinical efficacy. However, there are also numerous studies indicating that very high receptor binding affinity is not a guarantee of drug efficacy and that other factors, including pharmacokinetic profile, ligand-induced receptor desensitization, and "trafficking," are critical in design and development of optimal drugs.

Recombinant human thyrotropins of the twenty-first century.

In recent years, many new recombinant protein therapeutics have been developed and tested in clinical trials [1]. Current and future clinical uses of recombinant human thyroid-stimulating hormone (rhTSH; Thyrogen, Genzyme) in thyroid diseases are discussed in the review published in this issue of Expert Opinion on Pharmacotherapy [2]. As Thyrogen is a wild-type rhTSH produced in Chinese hamster ovary cells, it has relatively low affinity to the human TSH receptor. Such low affinity and weak intrinsic bioactivity of rhTSH, compared to the bovine or rodent TSH, may help to explain the results of several studies indicating limited clinical efficacy of Thyrogen. TSH analogues with largely increased receptor affinity, potency and efficacy, are expected to provide not only more effective than currently used diagnostic methods, but should also serve as indispensable second-generation thyrotropins for the diagnosis and treatment of thyroid carcinomas with a largely limited number of TSH receptors.

Human alpha-subunit analogs act as partial agonists to the thyroid-stimulating hormone receptor: differential effects of free and yoked subunits.

The alpha-subunit is common to the heterodimeric glycoprotein hormones and has been highly conserved throughout vertebrate evolution. In an effort to determine if wild-type and engineered human alpha analogs can serve as agonists or antagonists to the human thyroid-stimulating hormone (TSH) receptor (TSHR), a potent alpha mutant, obtained by replacing four amino acid residues with lysine (alpha4K), was assayed and compared with the wild-type alpha-subunit. When added to CHO cells expressing TSHR, alpha4K, and to a very limited extent the fused homodimer, alpha4K-alpha4K, but not alpha, exhibited agonist activity as judged by cAMP production. When yoked to TSHR to yield fusion proteins, neither alpha, alpha4K, alpha-alpha, nor alpha4K-alpha4K activated TSHR, although yoked alpha4K and alpha4K-alpha4K were weak inhibitors of TSH binding to TSHR. The yoked subunit-receptor complexes were, however, functional as evidenced by increased cAMP production in cells co-expressing human TSHbeta and alpha-TSHR, alpha4K-TSHR, alpha-alpha-TSHR, and alpha4K-alpha4K-TSHR. These results demonstrate that agonists to TSHR can be obtained with alpha-subunit analogs and suggest that rational protein engineering may lead to more potent alpha-based derivatives. The differences found between the experimental paradigms of adding free alpha analogs to TSHR and covalent attachment are attributed to con-formational constraints imposed by fusion of the alpha-subunit analog and receptor, and may suggest an important role for a free (C-terminal) alpha-carboxyl in the absence of the beta-subunit.

Minireview: structural and functional evolution of the thyrotropin receptor.

TSH receptor (TSHR) is a member of the leucine-rich repeat-containing G protein-coupled receptors. Both TSHR and its ligand TSH have evolved to acquire specificity, minimize cross-reaction to other glycoprotein hormone receptors, and modulate cognate interaction (and thereby thyrotropic activity). TSHR sequences available from two life orders, teleost and mammals, were analyzed. Teleost TSHRs with low affinity are expressed in many nonthyroidal tissues and show a tendency to gene duplication. In some teleosts, TSHR has limited specificity, and in others extremely high constitutive activity, suggesting the possibility of ligand-independent receptor function. Although mammalian TSHR, in contrast to other glycoprotein hormone receptors, maintains relatively high constitutive activity, the thyrotropic activity of TSH appears to decline in hominoids including humans, probably as part of metabolic adaptation to the changing environment. Critical TSHR residues that determine hormone specificity have been identified in the leucine-rich repeats, and others within the cysteine-rich C-flanking region that determines hormonal activation as well as receptor silencing. Transmembrane (TM) helices, particularly the TM5 and TM6, are likely involved in receptor homodimerization and a unique motif in TM7 appears essential to receptor silencing and internalization. Surprisingly, ternary structures in the intracellular domain as opposed to specific sequence motifs are critical for intracellular TSHR trafficking. It is evident that progress in understanding structure-function relationships of TSHR and its ligand can be further stimulated by inclusion of evolutionary analysis of their primary, secondary and tertiary structure. Such an integrated approach should also contribute to the rational design of highly efficacious therapeutics with either agonistic or antagonistic properties.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships.

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.