Thyroid-stimulating hormone and insulin-like growth factor-1 synergize to elevate 1,2-diacylglycerol in rat thyroid cells. Stimulation of DNA synthesis via interaction between lipid and adenylyl cyclase signal transduction systems

Monoclonal antibodies in neuro-ophthalmology

Neuro-ophthalmologic diseases include a broad range of disorders affecting the afferent and efferent visual pathways. Recently, monoclonal antibody (mAb) therapies have emerged as a promising targeted approach in the management of several of these complex conditions. Here, we describe the mechanism-specific applications and advancements in neuro-ophthalmologic mAb therapies. The application of mAbs in neuro-ophthalmologic diseases highlights our increasing understanding of disease-specific mechanisms in autoimmune conditions such as neuromyelitis optica, thyroid eye disease, and myasthenia gravis. Due to the specificity of mAb therapies, applications in neuro-ophthalmologic diseases have yielded exceptional clinical outcomes, including both reduced rate of relapse and progression to disability, visual function preservation, and quality of life improvement. These advancements have not only expanded the range of treatable neuro-ophthalmologic diseases but also reduced adverse events and increased the response rate to treatment. Further research into neuro-ophthalmologic disease mechanisms will provide accurate and specific targeting of important disease mediators through applications of future mAbs. As our understanding of these diseases and the relevant therapeutic targets evolve, we will continue to build on our understanding of how mAbs interfere with disease pathogenesis, and how these changes improve clinical outcomes and quality of life for patients.

TSH receptor autoantibody levels post-total thyroidectomy in Graves' ophthalmopathy: a meta-analysis

BACKGROUND

TSH receptor autoantibodies (TRAbs) are pathognomonic for Graves' disease and are thought to also underly the pathogenesis of Graves' ophthalmopathy (GO). A decline in TRAb levels has been documented post-total thyroidectomy (TTx) in GO, however with conflicting correlations with disease outcomes. The aim of the study was to compare the effectiveness of TTx to other treatment modalities of Graves' disease and examine whether the lowering of TRAbs is associated with GO improvements.

METHOD

We searched electronic databases including Medline, Embase, Scopus, and Web of Science until 31 September 2022 using a broad range of keywords. Patients with GO undergoing TTx with measurements of both TRAbs and progression of the disease using a validated GO scoring system were included. Fourteen studies encompassing data from 1047 patients with GO met our eligibility criteria. The PRISMA guidelines were followed, and five studies had comparable data that were suitable for a meta-analysis.

RESULTS

The Cochrane Risk of Bias tool for RCTs showed low risk of bias across most domains. The pooled odds ratio showed that more patients significantly had normalized TRAb levels post-TTx as compared to other interventions (OR: 1.36, 95% CI: 1.02-1.81, p = 0.035). But, there was no significant difference in GO improvement post-TTx as compared with other intervention groups.

CONCLUSIONS

This meta-analysis shows that TRAb levels may decline largely post-TTx, but may not predict added improvements to the progression of GO. Thus, future studies with uniform designs are required to assess the minimal significant GO improvements.

It Takes Two to Tango: IGF-I and TSH Receptors in Thyroid Eye Disease

CONTEXT

Thyroid eye disease (TED) is a complex autoimmune disease process. Orbital fibroblasts represent the central orbital immune target. Involvement of the TSH receptor (TSHR) in TED is not fully understood. IGF-I receptor (IGF-IR) is overexpressed in several cell types in TED, including fibrocytes and orbital fibroblasts. IGF-IR may form a physical and functional complex with TSHR.

OBJECTIVE

Review literature relevant to autoantibody generation in TED and whether these induce orbital fibroblast responses directly through TSHR, IGF-IR, or both.

EVIDENCE

IGF-IR has traditionally been considered a typical tyrosine kinase receptor in which tyrosine residues become phosphorylated following IGF-I binding. Evidence has emerged that IGF-IR possesses kinase-independent activities and can be considered a functional receptor tyrosine kinase/G-protein-coupled receptor hybrid, using the G-protein receptor kinase/β-arrestin system. Teprotumumab, a monoclonal IGF-IR antibody, effectively reduces TED disease activity, proptosis, and diplopia. In addition, the drug attenuates in vitro actions of both IGF-I and TSH in fibrocytes and orbital fibroblasts, including induction of proinflammatory cytokines by TSH and TED IgGs.

CONCLUSIONS

Although teprotumumab has been proven effective and relatively safe in the treatment of TED, many questions remain pertaining to IGF-IR, its relationship with TSHR, and how the drug might be disrupting these receptor protein/protein interactions. Here, we propose 4 possible IGF-IR activation models that could underlie clinical responses to teprotumumab observed in patients with TED. Teprotumumab is associated with several adverse events, including hyperglycemia and hearing abnormalities. Underpinning mechanisms of these are being investigated. Patients undergoing treatment with drug must be monitored for these and managed with best medical practices.

Insulin-Like Growth Factor Pathway and the Thyroid

The insulin-like growth factor (IGF) pathway comprises two activating ligands (IGF-I and IGF-II), two cell-surface receptors (IGF-IR and IGF-IIR), six IGF binding proteins (IGFBP) and nine IGFBP related proteins. IGF-I and the IGF-IR share substantial structural and functional similarities to those of insulin and its receptor. IGF-I plays important regulatory roles in the development, growth, and function of many human tissues. Its pathway intersects with those mediating the actions of many cytokines, growth factors and hormones. Among these, IGFs impact the thyroid and the hormones that it generates. Further, thyroid hormones and thyrotropin (TSH) can influence the biological effects of growth hormone and IGF-I on target tissues. The consequences of this two-way interplay can be far-reaching on many metabolic and immunologic processes. Specifically, IGF-I supports normal function, volume and hormone synthesis of the thyroid gland. Some of these effects are mediated through enhancement of sensitivity to the actions of TSH while others may be independent of pituitary function. IGF-I also participates in pathological conditions of the thyroid, including benign enlargement and tumorigenesis, such as those occurring in acromegaly. With regard to Graves' disease (GD) and the periocular process frequently associated with it, namely thyroid-associated ophthalmopathy (TAO), IGF-IR has been found overexpressed in orbital connective tissues, T and B cells in GD and TAO. Autoantibodies of the IgG class are generated in patients with GD that bind to IGF-IR and initiate the signaling from the TSHR/IGF-IR physical and functional protein complex. Further, inhibition of IGF-IR with monoclonal antibody inhibitors can attenuate signaling from either TSHR or IGF-IR. Based on those findings, the development of teprotumumab, a β-arrestin biased agonist as a therapeutic has resulted in the first medication approved by the US FDA for the treatment of TAO. Teprotumumab is now in wide clinical use in North America.

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.

Insulin-like Growth Factor-I Receptor and Thyroid-Associated Ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is a complex disease process presumed to emerge from autoimmunity occurring in the thyroid gland, most frequently in Graves disease (GD). It is disfiguring and potentially blinding, culminating in orbital tissue remodeling and disruption of function of structures adjacent to the eye. There are currently no medical therapies proven capable of altering the clinical outcome of TAO in randomized, placebo-controlled multicenter trials. The orbital fibroblast represents the central target for immune reactivity. Recent identification of fibroblasts that putatively originate in the bone marrow as monocyte progenitors provides a plausible explanation for why antigens, the expressions of which were once considered restricted to the thyroid, are detected in the TAO orbit. These cells, known as fibrocytes, express relatively high levels of functional TSH receptor (TSHR) through which they can be activated by TSH and the GD-specific pathogenic antibodies that underpin thyroid overactivity. Fibrocytes also express insulin-like growth factor I receptor (IGF-IR) with which TSHR forms a physical and functional signaling complex. Notably, inhibition of IGF-IR activity results in the attenuation of signaling initiated at either receptor. Some studies suggest that IGF-IR-activating antibodies are generated in GD, whereas others refute this concept. These observations served as the rationale for implementing a recently completed therapeutic trial of teprotumumab, a monoclonal inhibitory antibody targeting IGF-IR in TAO. Results of that trial in active, moderate to severe disease revealed dramatic and rapid reductions in disease activity and severity. The targeting of IGF-IR with specific biologic agents may represent a paradigm shift in the therapy of TAO.

Forkhead Transcription Factor FOXO1 Is Regulated by Both a Stimulatory Thyrotropin Receptor Antibody and Insulin-Like Growth Factor-1 in Orbital Fibroblasts from Patients with Graves' Ophthalmopathy

BACKGROUND

Activation of thyrotropin receptor (TSHR) and/or insulin-like growth factor (IGF-1) receptor (IGF-1R) enhances HA production and adipogenesis in orbital fibroblasts from patients with Graves' ophthalmopathy (GO) and recapitulates the tissue remodeling characteristic of the orbit in GO. A functional relationship between TSHR and IGF-1R has long been postulated, and recently bidirectional crosstalk between the receptors in GO fibroblasts was demonstrated. Because the transcription factor Forkhead box O-1 (FOXO1) was recently shown to be a critical downstream mediator of TSH and IGF-1 effects on thyrocyte proliferation, studies were designed to determine whether FOXO1 might similarly act as a common mediator of M22, a stimulatory TSHR antibody (TSAb), and IGF-1 in GO orbital fibroblasts.

METHODS

FOXO1 mRNA and protein were measured in orbital tissue specimens derived from normal individuals and patients with GO. In addition, the control of FOXO1 cellular localization was investigated using quantitative Western blotting of fractionated cell lysates from orbital fibroblasts treated with M22 and/or IGF-1 with or without specific TSHR, IGF-1R, or PI3K/AKT1/2 inhibitors.

RESULTS

Significantly lower levels of both FOXO1 mRNA and protein were found in GO orbital tissue specimens compared with normal orbital tissues (M = 39%, p = 0.043; M = 46.4%; p = 0.028, respectively). In addition, treatment of GO orbital cultures with M22, IGF-1, or M22 plus IGF-1 increased cytoplasmic FOXO1 compared with control (1.63-fold, p = 0.008; 1.68-fold, p = 0.001; 1.61-fold, p ≤ 0.001, respectively) and decreased nuclear FOXO1 (M = 28%, p = 0.002; M = 38%, p ≤ 0.001; M = 35%, p = 0.007, respectively). These effects were inhibited by co-treatment with the respective, but not the opposite, receptor antagonist. AKT inhibition of M22 or IGF-1-treated cultures was found to increase nuclear (1.4-fold, p = 0.026; 1.3-fold, p = 0.001, respectively) and decrease cytoplasmic (24.2%, p = 0.001; 36%, p = 0.004, respectively) FOXO1 localization.

CONCLUSIONS

These data point to FOXO1 as an important mediator of TSAb and IGF-1 action via their cognate receptors in GO orbital fibroblasts. These findings provide a link between the low FOXO1 protein levels demonstrated in GO orbital tissue and the tissue remodeling characteristic of GO, and suggest novel therapy for GO aimed at increasing nuclear expression of FOXO1 in GO target cells.

Bidirectional TSH and IGF-1 receptor cross talk mediates stimulation of hyaluronan secretion by Graves' disease immunoglobins

CONTEXT

There is no pathogenetically linked medical therapy for Graves' ophthalmopathy (GO). Lack of animal models and conflicting in vitro studies have hindered the development of such therapy. Recent reports propose that Graves' Igs bind to and activate thyrotropin receptors (TSHRs) and IGF-1 receptors (IGF-1Rs) on cells in orbital fat, stimulating hyaluronan (HA) secretion, a component of GO.

OBJECTIVE

The objective of the study was to investigate potential cross talk between TSHRs and IGF-1Rs in the pathogenesis of GO using a sensitive HA assay.

DESIGN/SETTING/PARTICIPANTS

Orbital fibroblasts from GO patients were collected in an academic clinical practice and cultured in a research laboratory. Cells were treated with TSH, IGF-1, and a monoclonal Graves' Ig M22.

MAIN OUTCOME MEASURES

HA was measured by a modified ELISA.

RESULTS

Simultaneous activation by TSH and IGF-1 synergistically increased HA secretion from 320 ± 52 for TSH and 430 ± 65 μg/mL for IGF-1 alone, to 1300 ± 95 μg/mL. IGF-1 shifted the TSH EC50 19-fold to higher potency. The dose response to M22 was biphasic. An IGF-1R antagonist inhibited the higher potency phase but had no effect on the lower potency phase. M22 did not cause IGF-1R autophosphorylation. A TSHR antagonist abolished both phases of M22-stimulated HA secretion.

CONCLUSIONS

M22 stimulation of HA secretion by GO fibroblasts/preadipocytes involves cross talk between TSHR and IGF-1R. This cross talk relies on TSHR activation rather than direct activation of IGF-1R and leads to synergistic stimulation of HA secretion. These data propose a model for GO pathogenesis that explains previous contradictory results and argues for TSHR as the primary therapeutic target for GO.

Regulation of thyroid cell proliferation by TSH and other factors: a critical evaluation of in vitro models

TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G(1) phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.

Effect of the interaction of TSH and insulin on the stimulation of 2-deoxyglucose uptake in FRTL-5 cells

Since thyroid glycogen stores are low, the uptake of glucose is very important in order to maintain cell function (house-keeping). Previous studies have shown that TSH and insulin, independently, are regulators of this parameter. Since their corresponding mechanisms of action are different, we investigated the possible effect of the interaction between TSH and insulin on the stimulation of 2-deoxyglucose (2-DOG) uptake, a non metabolizable derivative of glucose. Confluent FRTL-5 cells were submitted to different treatments, usually for 72 h. In one series of experiments the concentration of TSH was kept constant, at 1 U/l, and the addition of insulin, from 0.16 to 1.6 micromol/l caused a progressive synergic increase in DOG uptake. When insulin concentration was kept constant, increasing amounts of TSH, from 0.5 to 10 U/l), also caused a synergic stimulation of DOG uptake. The effect of insulin was mimicked by IGF-1 (1-10 nmol/l), while that of TSH was mimicked by forskolin. Timecourse studies showed that TSH had a peak at 3 h of incubation, while insulin caused a progressive increase for up to 72 h. At short incubation times, up to 6 h, an additive effect of TSH and insulin was observed, while at longer times the interaction was synergic. The present results suggest that the interaction between the cAMP and the tyrosine kinase pathways on DOG uptake would involve two different mechanisms. At early times the effects of both hormones are additive, while in longer periods it becomes synergic.

Right ventricular outflow tract tachycardia due to a somatic cell mutation in G protein subunitalphai2

Idiopathic ventricular tachycardia is a generic term that describes the various forms of ventricular arrhythmias that occur in patients without structural heart disease and in the absence of the long QT syndrome. Many of these tachycardias are focal in origin, localize to the right ventricular outflow tract (RVOT), terminate in response to beta blockers, verapamil, vagal maneuvers, and adenosine, and are thought to result from cAMP-mediated triggered activity. DNA was prepared from biopsy samples obtained from myocardial tissue from a patient with adenosine-insensitive idiopathic ventricular tachycardia arising from the RVOT. Genomic sequences of the inhibitory G protein Galphai2 were determined after amplification by PCR and subcloning. A point mutation (F200L) in the GTP binding domain of the inhibitory G protein Galphai2 was identified in a biopsy sample from the arrhythmogenic focus. This mutation was shown to increase intracellular cAMP concentration and inhibit suppression of cAMP by adenosine. No mutations were detected in Galphai2 sequences from myocardial tissue sampled from regions remote from the origin of tachycardia, or from peripheral lymphocytes. These findings suggest that somatic cell mutations in the cAMP-dependent signal transduction pathway occurring during myocardial development may be responsible for some forms of idiopathic ventricular tachycardia.

Sphingosine 1-phosphate mobilizes sequestered calcium, activates calcium entry, and stimulates deoxyribonucleic acid synthesis in thyroid FRTL-5 cells

Sphingosine 1-phosphate (SPP) potently mobilizes sequestered calcium and is a mitogen in several cell types. In the present investigation, we have evaluated the effect of SPP on intracellular free calcium concentration ([Ca2+]i) and synthesis of DNA in thyroid FRTL-5 cells. SPP rapidly and transiently mobilized sequestered calcium and stimulated entry of extracellular calcium. The entry of calcium, but not the mobilization, was in part inhibited by pretreatment with pertussis toxin (Ptx), and by activation of protein kinase C. SPP did not stimulate the production of inositol 1,4,5-trisphosphate. SPP stimulated the incorporation of 3H-thymidine in a time- and dose-dependent manner. The effect was not inhibited by Ptx. Furthermore, SPP stimulated the activation of the proto-oncogene c-fos. SPP rapidly tyrosine-phosphorylated an approximately 66 kDa protein. This phosphorylation persisted for at least 1 h. Pretreatment of the cells with genistein abolished the SPP-evoked tyrosine phosphorylation, and attenuated the SPP-evoked increase in [Ca2+]i. Furthermore, the SPP-evoked activation of Na+-H+ exchange was inhibited by genistein. The phosphorylation was not attenuated by pretreatment of the cells with Ptx. SPP per se did not affect cellular cAMP levels but attenuated the TSH-evoked increase in cAMP. As the effect of SPP might be due to activation of phospholipase D, we tested whether phosphatidic acid (PA) mobilized calcium or stimulated the incorporation of 3H-thymidine. PA mobilized sequestered calcium but did not stimulate calcium entry. PA very modestly enhanced the incorporation of 3H-thymidine. Our results suggest, that SPP stimulates DNA synthesis and activates entry of calcium in FRTL-5 cells. The effect on calcium entry appears to be dependent, at least in part, on one or several tyrosine kinases.

Activation of phospholipase D in FRTL-5 thyroid cells by forskolin and dibutyryl-cyclic adenosine monophosphate

We demonstrated previously that TSH activates phospholipase D (PLD) via stimulation of protein kinase C (PKC) in Fischer rat thyroid line (FRTL)-5 thyroid cells. To examine the role of the cAMP pathway in the regulation of PLD, we studied the effects of forskolin (0-100 microM; 30 min) and dibutyryl cAMP (dbcAMP; 0-1 mM; 30 min) on PLD activation. FRTL-5 thyroid cells were labeled mainly in phosphatidylcholine with [3H]myristate followed by incubation with 200 mM ethanol before the addition of agonist. PLD was assessed by the measurement of [3H]phosphatidylethanol. Forskolin (100 nM to 100 microM) and dbcAMP (100 pM to 100 microM) increased PLD activity significantly. Maximal responses to forskolin and dbcAMP exceed the PLD responses produced by 100 microU/ml of TSH. To determine whether the effects of forskolin and dbcAMP on PLD occurred as a consequence of PKC activation, FRTL-5 thyroid cells were preincubated for 10 min with the PKC inhibitors, chelerythrine (1 microM) or calphostin C (1 microM), or they were pretreated for 24 h with phorbol myristate acetate (100 nM) to down-regulate PKC. Unlike TSH-mediated PLD activation, these treatments had no effect on PLD activation by cAMP agonists. Forskolin (10 microM; 30 min) had no effect on the subcellular distribution of PKC alpha-, epsilon-, or zeta-isoforms, confirming the lack of involvement of PKC. The protein kinase A (PKA) inhibitors, H-89 (10 microM; 30 min) and dideoxyadenosine (5 nM; 10 min) significantly decreased the forskolin- and dbcAMP-mediated PLD activation without any effect on the phorbol ester-mediated PLD response. Following pretreatment with H-89 or dideoxyadenosine, the TSH-mediated PLD response was also significantly reduced. These studies indicate that forskolin and dbcAMP stimulate PLD in FRTL-5 thyroid cells directly via PKA without involvement of PKC. Studies of cells in the presence and absence of ethanol revealed approximately 60% of the phosphatidate plus diacylglycerol produced via TSH occurs via PLD activation. Although TSH-mediated inositol phosphate generation occurred with similar concentrations of TSH that led to PLD activation, 10-fold higher TSH concentrations were required to increase intracellular Ca2+. These results and the lack of a rapid Ca2+ transient following physiological TSH concentrations suggest that alternatives to conventional hydrolysis of phosphatidylinositol 4,5-bisphosphate may initiate PKC activation. Thus, the two major signal transduction systems in the FRTL-5 thyroid cell (PKA and PKC) appear to converge on PLD activation. Stimulation of both of these pathways by TSH may be required for optimal physiological activation of PLD.

Adenosine inhibits DNA synthesis stimulated with TSH, insulin, and phorbol 12-myristate 13-acetate in rat thyroid FRTL-5 cells

Adenosine has been shown to modulate cell proliferation in FRTL-5 thyroid cells, although the mechanisms by which this interaction occurs is still unclear. In the present study we investigated the effects of adenosine on the 3H-thymidine incorporation, cell cycle kinetics, and expression of the transcription factor c-Fos in cells stimulated via three different mitogenic pathways, i.e., by thyroid stimulating hormone (TSH) [adenosine 3',5'-cyclic monophosphate(cAMP)], insulin (tyrosine kinase), or phorbol 12-myristate 13-acetate (protein kinase C). Addition of adenosine to cells grown in medium containing hormones and serum did not inhibit the incorporation of 3H-thymidine. If adenosine was added to hormone-deprived cells together with any of the tested mitogens, the stimulation of the 3H-thymidine incorporation was inhibited in a dose-dependent manner. The inhibition was significantly lower when the cells were preincubated with TSH or insulin for 48 h. Flow cytometric studies showed that adenosine evoked an inhibition of the cells in the G0/G1 phase. Submaximal doses of adenosine (10 nM-10 microM) were able to induce c-Fos expression in FRTL-5 cells. However, the mitogen-induced expression of c-Fos was not reduced by maximal dose of adenosine (100 microM). The effect of adenosine on DNA synthesis was not dependent on pertussis toxin-sensitive G-proteins. In addition, adenosine A1- or A2- receptor antagonists did not block the effect of adenosine. The effect of adenosine was abolished by treatment of the cells with adenosine deaminase, suggesting that the observed effect was not mediated by a metabolite of adenosine. The results suggest that adenosine is an effective blocker of mitogen-evoked DNA synthesis of FRTL-5 cells, provided that adenosine is administered simultaneously with the mitogen.

Purinergic agonist ATP is a comitogen in thyroid FRTL-5 cells

Several growth factors may stimulate proliferation of thyroid cells. This effect has, in part, been dependent on calcium entry. In the present study using FRTL-5 cells, we show that in addition to its effect on calcium fluxes, ATP acts as a comitogen in these cells. In medium containing 5% serum, but no TSH, ATP stimulated the incorporation of 3H-thymidine in a dose- and time-dependent manner in the cells. At least a 24-h incubation with ATP was necessary to observe the enhanced (30-50%) incorporation of 3H-thymidine and an increased (30%) cell number. The effect of ATP was dependent on insulin in the incubation medium. Furthermore, ATP enhanced the TSH-mediated incorporation of 3H-thymidine. The effect of ATP was apparently mediated via a G-protein dependent mechanism, as no stimulation of thymidine incorporation was observed in cells treated with pertussis toxin. The effect of ATP was not dependent on the activation of protein kinase C (PKC), as ATP was effective in cells with downregulated PKC. ATP rapidly phosphorylated mitogen activated protein (MAP) kinase in FRTL-5 cells. In addition, ATP stimulated the expression of a 62 kDa c-fos dependent protein in a dose- and time-dependent manner. Our results thus suggest that extracellular ATP, in the presence of insulin, may be a cofactor in the regulation of thyroid cell proliferation, probably by phosphorylating MAP kinase and stimulating the expression of c-fos.

Insulin-like growth factor I, a unique calcium-dependent stimulator of 1,25-dihydroxyvitamin D3 production. Studies in cultured mouse kidney cells

Previous in vivo and in vitro studies suggest that insulin-like growth factor (IGF-I) could be a regulator of the renal production of 1,25-(OH)2D3. In the present work, the local effect of low nanomolar concentrations of IGF-I on the 25-OH-D3-1 alpha-hydroxylase activity and the mechanism of its action have been investigated. To do so, an in vitro model of mouse proximal tubular cells in primary culture has been developed. These cells bear specific high affinity IGF-I binding sites (apparent Kd = 1.95 +/- 0.46 nM) and express the ability to convert [3H]25-(OH)D3 into [3H]1,25-(OH)2D3 (Km = 139 +/- 15.7 nM). Human recombinant IGF-I (10-100 ng/ml) stimulated both sodium-dependent phosphate uptake and 1,25-(OH)2D3 synthesis by these cells, in a time- and dose-dependent manner. IGF-I did not alter the apparent Michaelis constant but increased the maximum velocity of the 25-OH-D3-1 alpha-hydroxylase activity. This effect required protein synthesis. It was not affected by calphostin or GF109203X, two protein kinase C inhibitors, and was not mimicked by phorbol 12-myristate 13-acetate. In contrast, it was blocked by verapamil, a calcium channel blocker. Calcium depletion of the medium blunted the IGF-I effect but not that of human 1-34 parathyroid hormone 5 x 10(-8) M. IGF-I thus appears to be the first example of a physiological calcium-dependent regulator of the renal metabolism of vitamin D.

Insulin-like growth factor-1 is not mitogenic for the Walker-256 carcinosarcoma

This study was designed to determine whether intravenous infusion of recombinant human insulin-like growth factor 1 (IGF-1) stimulates tumor growth. In order to determine the potential interaction between nutrition and IGF-1 administration the study was conducted in fasting rats and during continuous feeding by total parenteral nutrition. Tumor cell cycle kinetics including labeling index, DNA synthesis time, cell cycle time in Go/G1, and G2/M in the total cell cycle, and potential doubling time were determined by flow cytometry after in vivo pulse labeling the rats bearing the Walker-256 Carcinosarcoma with 5'-bromo-2'-deoxyuridine (BrdUrd). The results show that IGF-1 treatment has no significant effects on the proliferative characteristics of the tumor model regardless of the feeding status of the animal. This study provides preliminary cell-cycle kinetics data on the short-term effect of IGF-1 on tumor growth. Failure to show a significant effect of IGF-1 on the proliferative characteristics of the tumor suggests that IGF-1 may be given to cancer patients in amounts sufficient to promote weight gain without deleterious stimulation of tumor proliferation.

Insulin-Like Growth Factor I in Human Thyroid Tissue: Specific Localization by Immunohistochemistry and In Situ Hybridization

Insulin-like growth factor I and II (IGF-l and IGF-ll) have been implicated in the replication of normal thyroid follicular cells in vitro. This study evaluates the distribution and abundance of immunoreactive IGF-l by histochemical analysis in human thyroid tissue with different histopathologic characteristics. We used two types of highly specific and sensitive polyclonal rabbit anti-IGF-l antibodies and one monoclonal antibody (MAb) with the immunoperoxidase technique on sections of 25 glands harboring adenomatous goiter; 11 glands with follicular adenoma (FA); 45 glands with thyroid carcinoma of papillary, follicular, and undifferentiated types; and 18 glands with Graves' disease. Immunoreactive IGF-l was present in some thyroid follicular cells of all thyroid tissues examined. The percentage of cells staining positively varies among the different processes, being lowest in normal thyroid tissues and highest in all thyroid carcinomas. The cytoplasmic pattern of IGF-l immunoreactivity also varied among the different thyroid conditions. Furthermore, using nonradioactive in situ hybridization (ISH) we detected IGF-l mRNA in the thyroid cells of adenomatous goiter. The expression was higher in the histologically hyperplastic areas. These findings provide further support for an autocrine and/or paracrine role of IGF-l in the function and/or growth of normal thyroid follicular cells and suggest that IGF-l may play a role in the dysfunctional growth of thyroid follicular cells in adenomatous goiter, thyroid carcinoma, and Graves' hyperthyroidism.

Interleukin 4 receptor signaling in human monocytes and U937 cells involves the activation of a phosphatidylcholine-specific phospholipase C: a comparison with chemotactic peptide, FMLP, phospholipase D, and sphingomyelinase

Interleukin 4 (IL-4) diminishes cytokine activation of human macrophage. IL-4 binding to monocyte IL-4R is associated with protein kinase C (PKC) translocation to a nuclear fraction. The cleavage of diacyglycerol (DAG), an activator of PKC, from membrane phospholipids was investigated to define the proximal events of IL-4R signaling. IL-4 induced a statistically significant time-and dose-dependent generation of DAG. The IL-4-triggered production of DAG was not derived from phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis, since neither cytosolic calcium flux nor liberation of inositol phosphates was detected in response to IL-4. Experiments were performed using [14C-methyl]choline-labeled U937 cells and monocytes to determine whether IL-4R activated phospholipase C (PLC), PLD, or PLA2 to use membrane phosphatidylcholine (PC) to form DAG. IL-4 induced a time- and dose-dependent increase of phosphocholine (pchol) with concomitant degradation of membrane PC (p < 0.05 compared with control). The finding that the peak reduction of PC was equivalent to peak production of pchol suggested that IL-4R signaling involved the activation of a PC-specific PLC. Changes in choline (chol) or lyso-PC and glycerolphosphocholine, the respective products of PC cleavage by PLD or PLA2, were not detected in IL-4-treated cells. In contrast, exogenous PLD induced an increase in chol and concomitant loss of membrane PC. Additional investigation suggested that IL-4R signaling does not involve PLD. In cells labeled with L-lyso-3-PC 1-[1-14C]palmitoyl, PLD but not IL-4, increased the production of phosphatidic acid (PA) and phosphatidyl-ethanol when pretreated with ethanol. Propranolol, an inhibitor of phosphatidate phosphohydrolase, and calyculin A, a phosphatase 1 and 2A inhibitor, blocked DAG production in response to FMLP but not to IL-4. In propranolol pretreated cells, PMA but not IL-4 triggered the production of PA and lowered the amount of DAG. Evidence that PLA2 is not coupled to IL-4R is the detection of arachidonate production in response to FMLP but not to IL-4. Furthermore, IL-4R is not coupled to sphingomyelinase (SMase) since IL-4, unlike exogenous SMase, did not generate ceramide but induced the hydrolysis of PC to pchol that was comparable to exogenous PLC. In summary, IL-4R signaling in monocytes and U937 cells involves PLC and not PLD, PLA2, or SMase, and it uses PC and not PIP2 to form DAG.

Evidence against roles for pertussis toxin sensitive G proteins or diacylglycerol generation in insulin-like growth factor-1 stimulated DNA synthesis in MG-63 osteosarcoma cells

A pertussis toxin-sensitive G protein has been reported to play a role in the mitogenic response to insulin-like growth factor-I (IGF-I) in mouse fibroblasts, and diacylglycerol generation has been shown to accompany growth stimulation by IGF-I of several cell lines. We have examined the roles of pertussis toxin sensitive G proteins and diacylglycerol generation in signaling by the insulin-like growth factor-I receptor in a cell line that is very responsive to IGF-I, the human osteosarcoma cell line, MG-63. Pertussis toxin failed to inhibit IGF-I induced [3H]-thymidine incorporation into DNA. Furthermore, the stable analog GTP gamma S had no effect on the binding of 125I-labelled IGF-I to MG-63 membrane preparations. Following addition of IGF-I to growth-arrested MG-63 cells there was no increase in diacylglycerol levels over 30 min. We conclude that the activated IGF-I receptor does not use pertussis toxin sensitive G proteins or diacylglycerol generation in a pathway leading to DNA synthesis in MG-63 cells.

Effect of sphingosine derivatives on calcium fluxes in thyroid FRTL-5 cells

The effects of sphingosine derivatives on Ca2+ fluxes were investigated in thyroid FRTL-5 cells labelled with Fura 2. Addition of sphingosylphosphocholine (SPC) or sphingosine (SP) increased intracellular free Ca2+ ([Ca2+]i) in a dose-dependent manner. At the highest dose tested (30 microM), the response was biphasic: a rapid transient increase in [Ca2+]i, followed by a new, elevated, level of [Ca2+]i. Both phases of the SPC-evoked increase in [Ca2+]i were dependent on extracellular Ca2+, whereas only the SP-evoked elevated level of [Ca2+]i was dependent on the influx of Ca2+. Both compounds released sequestered Ca2+ from thapsigargin- and inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pools. In addition, the increase in [Ca2+]i in response to SPC, but not to SP, was attenuated in cells treated with phorbol myristate acetate or with the putative Ca(2+)-channel blocker SKF 96365, and in cells pretreated with pertussis toxin for 24 h. SPC did not activate the production of IP3. Furthermore, both SPC and SP released sequestered Ca2+ from permeabilized cells. We observed that SPC, but not SP, stimulated release of [3H]arachidonate from cells prelabelled with [3H]arachidonate for 24 h. Both SPC and SP stimulated the incorporation of [3H]thymidine into DNA in cells grown in the absence of thyroid-stimulating hormone (TSH). The results suggest that sphingosine derivatives are putative regulators of Ca2+ fluxes in FRTL-5 cells, and that SP and SPC may act on [Ca2+]i via different mechanisms. Furthermore, both SP and SPC may be of importance in modulating thyroid-cell proliferation.

Enhancement of murine gustatory neural responses to D-amino acids by saccharin

Taste enhancing effects of sodium saccharin (Sac) on responses to particular sweet-tasting D-amino acids were found during the recording of mouse chorda tympani nerve responses to various taste stimuli in C57BL and BALB strains. In both strains, responses to D-tryptophan and D-histidine significantly increased (167.7-216.7% of control) after the stimulation with Sac as compared with those applied before Sac. In C57BL mice, the enhancement of Sac was also observed in response to D-phenylalanine (262.5% of control), but this was not the case for BALB mice, suggesting a prominent strain difference in response to D-phenylalanine, as shown previously. Responses to other sweet-tasting D- and L-amino acids and sugars were not enhanced by Sac. Enhancement of responses to these D-amino acids by Sac was also evident when responses to a mixture of D-amino acids and Sac were compared with the sum of responses to each component, although in this response analysis, the calculated magnitude of enhancement generally become smaller (135.7-180.5% of the sum) and enhancement of D-histidine responses disappeared. Except for Sac, various sweet-tasting amino acids and sugars and NaCl also tested showed no enhancing effect on D-phenylalanine responses in C57BL mice. Sac and D-amino acids, to which responses were enhanced by Sac, possess some common molecular features, namely ring structures. This structural similarity probably relates to the occurrence of the enhancement at the receptor sites.

Insulinlike growth factors and binding proteins in colon cancer

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

The effects of TSH receptor antibodies on protein kinase C in the thyroid

OBJECTIVE

There is increasing evidence that TSH activates a non-cyclic-AMP-dependent pathway in the thyroid resulting in protein kinase C activation. We have previously demonstrated that TSH activates protein kinase C by causing translocation of protein kinase C from an inactive cytosolic site to its active membrane-bound form in porcine thyroid cells. In addition, TSH can modify the protein kinase C translocation induced by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate. We tested six TSH receptor antibodies for their ability to activate protein kinase C in vitro.

DESIGN

Porcine thyroid cells were incubated with either TSH receptor antibody or immunoglobulin from pooled normal sera. Subsequently, cytosol and membrane compartments were separated and protein kinase C assessed in each compartment.

PATIENTS

The sera utilized in this study were obtained from patients with confirmed Graves' disease as defined by clinical evidence of thyrotoxicosis, increased free thyroxine index and suppressed TSH and increased radioactive iodine uptakes.

MEASUREMENTS

Protein kinase C was measured in the cytosol and membrane compartments of the porcine thyroid cells using an enzyme assay.

RESULTS

Compared to immunoglobulin from pooled normal sera, none of the six TSH receptor antibodies affected either cytosolic or membrane-bound protein kinase C in porcine thyroid cells. The tumour-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate, caused protein kinase C translocation. However, the presence of TSH receptor antibody did not modify the phorbol-induced protein kinase C translocation.

CONCLUSION

These results indicate that unlike TSH, TSH receptor antibodies neither cause translocation of protein kinase C nor modify phorbol-mediated protein kinase C translocation in porcine thyroid cells.

Characterization of Na+/H+ exchange in FRTL-5 thyroid cells. Evidence for dependence on activation of protein kinase C

Na+/H+ exchange activity was investigated in cultured rat thyroid follicular FRTL-5 cells using the pH sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Basal intracellular pH (pHi) was 7.13 +/- 0.10 in cells incubated in Hepes-buffered saline solution. The intracellular buffering capacity beta i was determined using the NH4Cl-pulse method, yielding a beta i value of 85 +/- 12 mM/pH unit. The relationship between extracellular Na+ and the initial rate of alkalinization of acid-loaded cells showed simple saturation kinetics, with an apparent Km value of 44 +/- 26 mM, and an Vmax value of 0.3 +/- 0.01 pH unit/min. The agonist-induced activation of Na+/H+ exchange was investigated in cells acidified with nigericin. Addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) or ATP induced rapid cytosolic alkalinization in acid-loaded cells. The action of both TPA and ATP was abolished by preincubating the cells with 100 microM amiloride, by substituting extracellular Na+ with equimolar concentrations of choline+, and by pretreating the cells with TPA for 24 h. Chelating extracellular Ca2+, or depleating intracellular Ca2+ pools did not affect the ATP-induced alkalinization. The results indicate, that FRTL-5 cells have a functional Na+/H+ exchange mechanism. Furthermore, stimulation of protein kinase C activity is of importance in activating the antiport.

Thyrotropin does not activate the phosphatidylinositol bisphosphate hydrolyzing phospholipase C in the dog thyroid

The effects of thyrotropin (TSH) and carbamylcholine (Cchol) on labeling by [3H]inositol of inositol lipids (i.e. total phosphoinositides (PI)) and inositol phosphates (IP) and on diacylglycerol (DAG) generation was studied in dog thyroid slices. Both agents (TSH 1-250 mU/ml, Cchol 10(-6) to 10(-4) M) increased the incorporation of [3H]inositol into PI and IP during 4 h labeling experiments; but the [3H]IP/[3H]PI ratio as compared to the control one was not modified by TSH (10 mU/ml: 1.03 +/- 0.24) while it was increased by Cchol (10(-5) M: 6.14 +/- 1.81). Slices prelabeled in the absence of agonists were then incubated in the presence or absence of 10 mM LiCl +/- 10(-4) M inositol. With LiCl alone, Cchol increased [3H]IP generation, while no such effect of TSH could be detected. However, in the absence of LiCl or in the presence of both LiCl and 10(-4) M inositol, TSH and Cchol both increased [3H]PI and [3H]IP, but IP and PI labeling remained strictly proportional with TSH (10 mU/ml: [3H]IP/[3H]PI ratio = 1.03 +/- 0.06 vs. control), while Cchol increased this ratio (10(-5) M = 2.44 +/- 0.24) with a preferential accumulation of IP. Both agonists stimulated DAG formation with similar kinetics and maximal effects (400% of control at 60 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential dependence of early and late increases in 1,2-diacylglycerol on the presence of catalytically active alpha-thrombin: evidence for regulation at the level of 1,2-diacylglycerol generation

alpha-Thrombin stimulates a biphasic increase in cellular 1,2-diacylglycerol mass in quiescent IIC9 fibroblasts. This report describes the use of hirudin, a high-affinity inhibitor of alpha-thrombin that renders it catalytically inactive, to investigate the dependence of elevated 1,2-diacylglycerol levels on the presence of catalytically active alpha-thrombin. When cultures were incubated in the presence of alpha-thrombin, 1,2-diacylglycerol levels remained elevated for greater than or equal to 4 h. Inactivation of alpha-thrombin after 15 s did not alter the kinetics of 1,2-diacylglycerol formation occurring over the next 1 h. However, sustained (1-4 h) increases in this lipid were eliminated. Inactivation of alpha-thrombin after 1 h of stimulation resulted in 1) an immediate and reversible decline in 1,2-diacylglycerol levels, 2) elimination of the sustained phase of 1,2-diacylglycerol production, 3) inhibition of the alpha-thrombin-stimulated generation of choline metabolites, and 4) a blunted mitogenic response to alpha-thrombin. These data indicate that early (0-1 h) and late (1-4 h) increases in 1,2-diacylglycerol are differentially dependent on the presence of catalytically active alpha-thrombin. Furthermore, sustained increases in 1,2-diacylglycerol in response to alpha-thrombin are regulated at least in part at the level of generation (via phosphatidylcholine hydrolysis). Our results also support a role for sustained 1,2-diacylglycerol levels in the mitogenic response.

Relationship between proliferation and cell cycle-dependent Ca2+ influx induced by a combination of thyrotropin and insulin-like growth factor-I in rat thyroid cells

The mechanism of cell proliferation by a combination of thyroid-stimulating hormone (TSH) and insulin-like growth factor-I (IGF-I) was studied in rat thyroid (FRTL-5) cells. IGF-I stimulated an approximately 3.5-fold increase in the rate of Ca2+ influx sustained for at least 6 h in TSH-pretreated cells but not in quiescent cells. The significant cell proliferation was observed when TSH-primed cells were incubated with IGF-I for 24 h but not for 12 h. IGF-I stimulated the rate of Ca2+ influx in a dose-dependent manner that was similar to that for induction of DNA synthesis. Both Ca2+ influx and DNA synthesis observed in response to IGF-I in TSH-primed cells were inhibited by cobalt. In addition, the stimulations of Ca2+ influx and DNA synthesis by IGF-I were dependent on extracellular Ca2+ in TSH-pretreated cells. When TSH-primed cells were pretreated with pertussis toxin, both IGF-I-induced Ca2+ influx and DNA synthesis were abolished. However, pertussis toxin did not block the priming action of TSH or forskolin. When calcium entry was induced by Bay K8644, it stimulated cell growth in TSH-primed cells but not in quiescent cells. Moreover, cobalt and lanthanum inhibited DNA synthesis even when added several hours after the addition of Bay K8644 but not when added 24 h after the growth factor in TSH-primed cells. These findings suggest that at least two important mechanisms may work in response to IGF-I only in the TSH-primed G1 phase of the cell cycle: first, IGF-I can activate directly or indirectly the Ca2+ channel via a pertussis toxin-sensitive substrate in TSH-primed cells; and second, a long lasting calcium entry by IGF-I may be a cell cycle-dependent mitogenic signal.

Two G protein oncogenes in human endocrine tumors

Somatic mutations in a subset of growth hormone (GH)-secreting pituitary tumors convert the gene for the alpha polypeptide chain (alpha s) of Gs into a putative oncogene, termed gsp. These mutations, which activate alpha s by inhibiting its guanosine triphosphatase (GTPase) activity, are found in codons for either of two amino acids, each of which is completely conserved in all known G protein alpha chains. The likelihood that similar mutations would activate other G proteins prompted a survey of human tumors for mutations that replace either of these two amino acids in other G protein alpha chain genes. The first gene so far tested, which encodes the alpha chain of Gi2, showed mutations that replaced arginine-179 with either cysteine or histidine in 3 of 11 tumors of the adrenal cortex and 3 of 10 endocrine tumors of the ovary. The mutant alpha i2 gene is a putative oncogene, referred to as gip2. In addition, gsp mutations were found in 18 of 42 GH-secreting pituitary tumors and in an autonomously functioning thyroid adenoma. These findings suggest that human tumors may harbor oncogenic mutations in various G protein alpha chain genes.

Control of thyroid cell and follicle growth: recent advances and current controversies

The regulation of cell proliferation within the thyroid follicle is a coordinated and finely balanced process involving integration of the action of pituitary thyrotropin with the effects of permissive and inhibitory growth factors of autocrine and paracrine origin. Our understanding of the cellular interactions and intracellular signalling processes involved in thyroid follicular growth control has been considerably assisted by in vitro cell culture techniques that enable thyroid follicular cells to be maintained and studied under conditions closely approximating those in vivo.

Development of the thyroid

The fetal hypothalamic-pituitary-thyroid axis develops autonomously of maternal influence. System ontogenesis begins with the appearance and histological development of the thyroid and pituitary glands followed by development of the hypothalamus and the pituitary portal vascular system. Hypothalamic-pituitary control of thyroid function matures during the last half of human fetal development. Thyroid hormones undergo several types of biochemical transformations in tissues, including deiodination, side-chain metabolism, and conjugation with sulphate or glucuronide. Enzyme-mediated monodeiodination is the most important pathway. The first step in T4 metabolism is either outer-ring monodeiodination to active T3 or innerring monodeiodination to inactive rT3. Most T4 is metabolized to rT3 in fetal tissues and/or placenta and rT3 is the major circulating T4 metabolite in the fetus. Selective tissues, such as brain, can monodeiodinate T4 to T3, and this T3 is available for local action. Nuclear thyroid hormone receptors mature at different times in different tissues. Receptors appear earlier in brain than in liver and local T3 production and action may be important in fetal brain development. Most thyroid hormone actions, however, appear in the perinatal period, and infants with thyroid agenesis appear normal at birth and develop normally with prompt neonatal diagnosis and treatment. Premature infants, particularly those less than 30-32 weeks' gestational age, have an immature thyroid system and manifest a state of transient hypothalamic-pituitary TSH deficiency. This does not require treatment. Infants with primary hypothyroidism, either due to thyroid dysgenesis or to thyroid dyshormonogenesis, by contrast, require prompt diagnosis and treatment. Rarely an infant is born with permanent TSH deficiency with or without other pituitary hormone deficiencies. These infants also require prompt treatment. Mothers with thyroid disease or a history of thyroid disease and with IgG autoantibodies to thyroid gland TSH receptors may deliver infants with hypothyroidism or hyperthyroidism due to transplacental passage of the receptor-blocking or receptor-stimulating autoantibodies. These infants also require careful evaluation and management.