Thyrotropin receptor stimulates internalization-independent persistent phosphoinositide signaling

Amelioration of Glutamate-induced Toxicity by a New Thyrotropin-releasing Hormone (TRH) Analogue PYR-l-(2,5-Dibromo)-His-l-ProNH2

Background

Glutamate has been implicated in the pathophysiology of central nervous system diseases, including stroke.

Purpose

In this study, the neuroprotective potential of the newly synthesised thyrotropin-releasing hormone (TRH) analogue [Pyr-l-(2,5-dibromo)-His-l-ProNH2; NP-2376] against glutamate-induced injury model were investigated.

Methods

Cortical neurons isolated from neonatal rats were used to evaluate the effects of the NP-2376. Cortical neurons were pre-treated with NP-2376 (6, 12 and 24 h) prior to glutamate (15 mM) exposure. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and neutral red uptake (NRU) assay, and oxidative stress by chloromethyl-2,7-dichlorodihydrofluorescein diacetate (DCFH-DA) and glutathione assays.

Results

NP-2376 protected against glutamate-induced cortical neuron death and oxidative stress in a dose-dependent manner.

Conclusions

This study demonstrates the neuroprotective potential of TRH analogue NP-2376 against glutamate-induced toxicity, which is attributed to a decrease in oxidative stress.

Thyrotropin receptor antagonists and inverse agonists, and their potential application to thyroid diseases

The thyrotropin receptor (TSHR) plays critical roles in thyroid growth and function and in the pathogenesis of several thyroid diseases including Graves' hyperthyroidism and ophthalmopathy, non-autoimmune hyperthyroidism and thyroid cancer. Several low-molecular weight compounds (LMWCs) and anti-TSHR monoclonal antibodies (mAbs) with receptor antagonistic and inverse agonistic activities have been reported. The former binds to the pocket formed by the receptor transmembrane bundle, and the latter to the extracellular TSH binding site. Both are effective inhibitors of TSH/thyroid stimulating antibody-stimulated cAMP and/or hyaluronic acid production in TSHR-expressing cells. Anti-insulin-like growth factor 1 inhibitors are also found to inhibit TSHR signaling. Each agent has advantages and disadvantages; for example, mAbs have a higher affinity and longer half-life but are more costly than LMWCs. At present, mAbs appear most promising, yet the development of more efficacious LMWCs is desirable. These agents are anticipated to be efficacious not only for the above-mentioned diseases but also for resistance to thyroid hormone and have utility for thyroid cancer radionuclide scintigraphy/therapy as a new theranostic.

Thyroid-Stimulating Hormone Receptor: the Role in the Development of Thyroid Pathology and Its Correction

Abstract

One of the key elements responsible for the thyroid response to thyroid-stimulating hormone (TSH) is the TSH receptor (TSHR), which belongs to the G protein-coupled receptor superfamily. Binding of TSH or stimulatory autoantibodies to the TSHR extracellular domain triggers multiple signaling pathways in target cells that are mediated through various types of G proteins and β-arrestins. Inhibitory autoantibodies, in contrast, suppress TSHR activity, inducing hypothyroid states. Activating mutations lead to constitutively active TSHR forms and can trigger cancer. Therefore, the TSHR is one of the key targets for the regulation of thyroid function and thyroid status, as well as correction of diseases caused by changes in TSHR activity (autoimmune hyper- and hypothyroidism, Graves’ ophthalmopathy, thyroid cancer). TSH preparations are extremely rarely used in medicine due to their immunogenicity and severe side effects. Most promising is the development of low-molecular allosteric TSHR regulators with an activity of full and inverse agonists and neutral antagonists, which are able to penetrate into the allosteric site located in the TSHR transmembrane domain and specifically bind to it, thus controlling the ability of the receptor to interact with G proteins and β-arrestins. Allosteric regulators do not affect the binding of TSH and autoantibodies to the receptor, which enables mild and selective regulation of thyroid function, while avoiding critical changes in TSH and thyroid hormone levels. The present review addresses the current state of the problem of regulating TSHR activity, including the possibility of using ligands of its allosteric sites.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

The Role of Inositol in Thyroid Physiology and in Subclinical Hypothyroidism Management

Myo-Inositol (MYO) is the most abundant stereoisomer of inositols' family, cyclic polyols with 6 hydroxyl groups. Myo-Inositol has a relevant role in thyroid function and autoimmune diseases, as a precursor of phosphoinositides that takes part in the phosphatidylinositol (PI) signal transduction pathway. Among phosphoinositides, phosphatidylinositol 4,5- bisphosphate (PIP2) is the precursor of inositol triphosphates (IP3), second messenger of several hormones including thyroid-stimulating hormone (TSH). As a second messenger in the phospholipase C (PLC)-dependent inositol phosphate Ca2+/DAG pathway, Myo-Inositol is essential to produce H2O2 required for the synthesis of thyroid hormones. Consequently, depletion of Myo-Inositol or impaired inositol dependent TSH signaling pathway may predispose to the development of some thyroid diseases, such as hypothyroidism. Many clinical studies have shown that after treatment with Myo-Inositol plus Selenium (MYO+Se), TSH levels significantly decreased in patients with subclinical hypothyroidism with or without autoimmune thyroiditis. The TSH reduction was accompanied by a decline of antithyroid autoantibodies. Moreover, Myo-Inositol supplementation seemed to be involved also in the management of thyroidal benign nodules, with a possible effect in the size reduction. This review proposes a summary of the role of inositol, especially of Myo-Inositol, in the thyroidal physiology and its contribution on the management of some thyroid diseases.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Structural-Functional Features of the Thyrotropin Receptor: A Class A G-Protein-Coupled Receptor at Work

The thyroid-stimulating hormone receptor (TSHR) is a member of the glycoprotein hormone receptors, a sub-group of class A G-protein-coupled receptors (GPCRs). TSHR and its endogenous ligand thyrotropin (TSH) are of essential importance for growth and function of the thyroid gland and proper function of the TSH/TSHR system is pivotal for production and release of thyroid hormones. This receptor is also important with respect to pathophysiology, such as autoimmune (including ophthalmopathy) or non-autoimmune thyroid dysfunctions and cancer development. Pharmacological interventions directly targeting the TSHR should provide benefits to disease treatment compared to currently available therapies of dysfunctions associated with the TSHR or the thyroid gland. Upon TSHR activation, the molecular events conveying conformational changes from the extra- to the intracellular side of the cell across the membrane comprise reception, conversion, and amplification of the signal. These steps are highly dependent on structural features of this receptor and its intermolecular interaction partners, e.g., TSH, antibodies, small molecules, G-proteins, or arrestin. For better understanding of signal transduction, pathogenic mechanisms such as autoantibody action and mutational modifications or for developing new pharmacological strategies, it is essential to combine available structural data with functional information to generate homology models of the entire receptor. Although so far these insights are fragmental, in the past few decades essential contributions have been made to investigate in-depth the involved determinants, such as by structure determination via X-ray crystallography. This review summarizes available knowledge (as of December 2016) concerning the TSHR protein structure, associated functional aspects, and based on these insights we suggest several receptor complex models. Moreover, distinct TSHR properties will be highlighted in comparison to other class A GPCRs to understand the molecular activation mechanisms of this receptor comprehensively. Finally, limitations of current knowledge and lack of information are discussed highlighting the need for intensified efforts toward TSHR structure elucidation.

Multiple Transduction Pathways Mediate Thyrotropin Receptor Signaling in Preosteoblast-Like Cells

It has been shown that the TSH receptor (TSHR) couples to a number of different signaling pathways, although the Gs-cAMP pathway has been considered primary. Here, we measured the effects of TSH on bone marker mRNA and protein expression in preosteoblast-like U2OS cells stably expressing TSHRs. We determined which signaling cascades are involved in the regulation of IL-11, osteopontin (OPN), and alkaline phosphatase (ALPL). We demonstrated that TSH-induced up-regulation of IL-11 is primarily mediated via the Gs pathway as IL-11 was up-regulated by forskolin (FSK), an adenylyl cyclase activator, and inhibited by protein kinase A inhibitor H-89 and by silencing of Gαs by small interfering RNA. OPN levels were not affected by FSK, but its up-regulation was inhibited by TSHR/Gi-uncoupling by pertussis toxin. Pertussis toxin decreased p38 MAPK kinase phosphorylation, and a p38 inhibitor and small interfering RNA knockdown of p38α inhibited OPN induction by TSH. Up-regulation of ALPL expression required high doses of TSH (EC50 = 395nM), whereas low doses (EC50 = 19nM) were inhibitory. FSK-stimulated cAMP production decreased basal ALPL expression, whereas protein kinase A inhibition by H-89 and silencing of Gαs increased basal levels of ALPL. Knockdown of Gαq/11 and a protein kinase C inhibitor decreased TSH-stimulated up-regulation of ALPL, whereas a protein kinase C activator increased ALPL levels. A MAPK inhibitor and silencing of ERK1/2 inhibited TSH-stimulated ALPL expression. We conclude that TSH regulates expression of different bone markers via distinct signaling pathways.

GPCR signaling along the endocytic pathway

Many G protein-coupled receptors (GPCRs) internalize after agonist-induced activation. While endocytosis has long been associated with homeostatic attenuation of cellular responsiveness, accumulating evidence from study of a wide range of eukaryotes reveals that the endocytic pathway also contributes to generating receptor-initiated signals themselves. Here we review recent progress in this area, discussing primarily but not exclusively GPCR signaling in mammalian cells.

Kinetics of 5-HT2B receptor signaling: profound agonist-dependent effects on signaling onset and duration

The kinetics of drug-receptor interactions can profoundly influence in vivo and in vitro pharmacology. In vitro, the potencies of slowly associating agonists may be underestimated in assays capturing transient signaling events. When divergent receptor-mediated signaling pathways are evaluated using combinations of equilibrium and transient assays, potency differences driven by kinetics may be erroneously interpreted as biased signaling. In vivo, drugs with slow dissociation rates may display prolonged physiologic effects inconsistent with their pharmacokinetic profiles. We evaluated a panel of 5-hydroxytryptamine2B (5-HT2B) receptor agonists in kinetic radioligand binding assays and in transient, calcium flux assays, and inositol phosphate accumulation assays; two functional readouts emanating from Gαq-mediated activation of phospholipase C. In binding studies, ergot derivatives demonstrated slow receptor association and dissociation rates, resulting in significantly reduced potency in calcium assays relative to inositol phosphate accumulation assays. Ergot potencies for activation of extracellular signal-regulated kinases 1 and 2 were also highly time-dependent. A number of ergots produced wash-resistant 5-HT2B signaling that persisted for many hours without appreciable loss of potency, which was not explained simply by slow receptor-dissociation kinetics. Mechanistic studies indicated that persistent signaling originated from internalized or sequestered receptors. This study provides a mechanistic basis for the long durations of action in vivo and wash-resistant effects in ex vivo tissue models often observed for ergots. The 5-HT2B agonist activity of a number of ergot-derived therapeutics has been implicated in development of cardiac valvulopathy in man. The novel, sustained nature of ergot signaling reported here may represent an additional mechanism contributing to the valvulopathic potential of these compounds.

Persistent cAMP signaling by TSH receptors revealed by phosphodiesterase inhibition

BACKGROUND

It is controversial whether persistent signaling by the thyrotropin (TSH) receptor (TSHR) is cell-type specific. We reported persistent TSHR signaling in human embryonic kidney 293 (HEK293) cells expressing human TSHRs (HEK-TSHRs), whereas another group reported persistent signaling in mouse thyroid follicles but not in HEK293 cells. Herein, we test this hypothesis directly.

METHODS

We used two methods to measure persistent signaling in HEK-TSHRs and confirm our previous observations. In Method 1, we used a chemiluminescent immunoassay to measure intracellular cAMP accumulation over 30-60 min by adding a phosphodiesterase inhibitor to the incubation medium. In Method 2, we used an intracellular biosensor to record cAMP levels continuously.

RESULTS

Using Method 1, we show that TSHR signals persistently in human thyrocytes and human osteosarcoma U2OS-TSHR cells. Using Method 1 in HEK-TSHRs, we show that after 5 min, the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) increases cAMP to 2.5 pmol/well, TSH increases cAMP to 1.6 pmol/well, but IBMX added 30 min after TSH withdrawal increases cAMP to 105 pmol/well. Using Method 2 in HEK-TSHRs, we confirm that without IBMX, TSH causes a transient increase in cAMP and 30 min after TSH withdrawal, IBMX increases cAMP in cells pretreated with TSH more rapidly and to a higher level than IBMX added to cells not pre-exposed to TSH. Lastly, using Method 2, we show that in HEK-TSHRs phosphodiesterases types 3 and 4 are involved in degrading cAMP as the specific inhibitors Rolipram and Milrinone expose persistent TSHR signaling.

CONCLUSIONS

We conclude that persistent TSHR activation occurs in human thyrocytes, U2OS-TSHR cells and HEK-TSHRs; it is not cell-type specific but is revealed by inhibiting phosphodiesterases.

A small molecule antagonist inhibits thyrotropin receptor antibody-induced orbital fibroblast functions involved in the pathogenesis of Graves ophthalmopathy

CONTEXT

Graves ophthalmopathy (GO) is an autoimmune disorder characterized by increased adipogenesis and hyaluronan (HA) production by orbital fibroblasts. Circulating autoantibodies (thyroid-stimulating antibodies [TSAbs]) directed at the thyrotropin receptor (TSHR) on these cells stimulate or augment these cellular processes. A recently developed drug-like small molecule inverse agonist of TSHR, NCGC00229600, termed 1, binds to TSHR and blocks basal and stimulated signal transduction.

OBJECTIVE

The purpose of this article was to determine whether 1 might inhibit HA production and relevant signaling pathways in orbital fibroblasts cultured in the presence of monoclonal TSAbs or bovine TSH (bTSH).

DESIGN

Primary cultures of undifferentiated GO orbital fibroblasts (n = 13) were untreated or treated with a TSAb (M22 or MS-1) or bTSH in serum-free medium, with or without 1 or a TSHR neutral antagonist, NCGC00242595, termed 2, which does not inhibit basal signaling but does inhibit stimulated signaling.

MAIN OUTCOME MEASURES

cAMP production, Akt phosphorylation (Ser473pAkt in media and immunoblotting for pAkt/total Akt), and HA production were analyzed.

RESULTS

Compound 1 inhibited basal cAMP, pAkt, and HA production and that stimulated by M22 in undifferentiated orbital fibroblasts. Inhibition of HA production was dose-dependent, with a half-maximal inhibitory dose of 830 nM. This compound also inhibited MS-1- and bTSH-stimulated cAMP, pAkt, and HA production. Compound 2 did not inhibit basal HA production but did inhibit M22-stimulated HA production.

CONCLUSIONS

Because cAMP, pAkt, and HA production are fibroblast functions that are activated via TSHR signaling and are important in the pathogenesis of GO, small molecule TSHR antagonists may prove to be effective in the treatment or prevention of the disease in the future.

Update in TSH receptor agonists and antagonists

The physiological role of the TSH receptor (TSHR) as a major regulator of thyroid function is well understood, but TSHRs are also expressed in multiple normal extrathyroidal tissues, and the physiological roles of TSHRs in these tissues are unclear. Moreover, TSHRs play a major role in several pathological conditions including hyperthyroidism, hypothyroidism, and thyroid tumors. Small molecule, "drug-like" TSHR agonists, neutral antagonists, and inverse agonists may be useful as probes of TSHR function in extrathyroidal tissues and as leads to develop drugs for several diseases of the thyroid. In this Update, we review the most recent findings regarding the development and use of these small molecule TSHR ligands.

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.

On the pulmonary toxicity of oxygen. 4. The thyroid arena

Normally developed thyroid function is critical to the transition from fetal to neonatal life with the onset of independent thermoregulation, the most conspicuous of the many ways in which thyroid secretions act throughout the body. A role for thyroid secretions in growth and maturation of the lungs as part of the preparation for the onset of breathing has been recognized for some time but how this contributes to tissue and cell processes and defenses under the duress of respiratory distress has not been well examined. Extensive archival autopsy material was searched for thyroid and adrenal weights, first by gestational age, and then for changes during the first hours after birth as ratios to body weight. After a gestational age of 22 weeks the fetal thyroid and adrenal glands at autopsy in those with hyaline membrane disease are persistently half the size of those in "normal" infants dying with other disorders. When the thyroid is examined shortly after birth it reveals a post natal loss of mass per body weight of similar orders of magnitude which does not occur in the control group. A clinical sample of premature infants with (12) and without (14) hyaline membrane disease was tested for T(4), TSH, TBG, and total serum protein. The results also demonstrate a special subset with lower birth weights at the same gestational age, and lower serum T(4) and total serum protein. Ventilatory distress in newborn rabbits was induced by bilateral cervical vagotomy at 24 h post natal following earlier injection of thyroxine (T(4)) or thyroid stimulating hormone (TSH) and comparisons were made with untreated animals and by dose. Early life thyroidectomy was performed followed by exposure to either air or 100% oxygen. A final experiment in air was vagotomy after thyroidectomy. Composite analysis of these methods indicates that thyroid factors are both operative and important in the newborn animal with ventilatory distress. This work and the archival data indicate those infants destined to develop hyaline membrane disease through respiratory distress are a distinct developmental and clinical subset with the point of departure from otherwise normal development and maturation in the second or early third trimester. This interval is known to be a period of marked variation in the overview indicators of fetal progress through gestational time. The initiating factor or circumstance which then separates this special subset from normal future development is placed by these observations firmly into the period when human fetal TSH dramatically rises 7-fold (17.5-25.5 weeks) followed by a lesser 3 to 4-fold increase in T(4) which is extended into the early third trimester. The earlier part of this interval is characterized by the thyrotrophic action of chorionic gonadotropin (hCG). The possibility that abnormalities in the intrauterine environment secondary to maternal infection play a role within this time frame is indicated by the demonstration that interleukin-2 (IL-2) induces an anterior pituitary release of TSH. Since IL-2 has this property and is not an acute phase cytokine, some form of chronic infection or an immunopathic process seems more likely as a possible active factor in pathogenesis.

cAMP: novel concepts in compartmentalised signalling

Cyclic adenosine 3,'5'-monophosphate (cAMP) is the archetypal second messenger produced at the membrane by adenylyl cyclase following activation of many different G protein-coupled receptor (GPCR) types. Although discovered over fifty years ago, the notion that cAMP responses were compartmentalised was born in the 1980s. Since then, modern molecular techniques have facilitated visualisation of cellular cAMP dynamics in real time and helped us to understand how a single, ubiquitous second messenger can direct receptor-specific functions in cells. The aim of this review is to highlight emerging ideas in the cAMP field that are currently developing the concept of compartmentalised cAMP signalling systems.