Thyrotropin via cyclic AMP induces insulin receptor expression and insulin Co-stimulation of growth and amplifies insulin and insulin-like growth factor signaling pathways in dog thyroid epithelial cells

A Brief Look at Hashimoto's Disease, Adrenal Incidentalomas, Obesity and Insulin Resistance-Could Endocrine Disruptors Be the Other Side of the Same Coin?

Hashimoto's disease (HD) is the most common cause of hypothyroidism in developed countries. The exact pathomechanism behind it has not been clearly established; however, an interplay of genetic susceptibility, environmental triggers (including diet) and epigenetic factors seems to be involved. Among the latter, increasingly more attention has been paid to some hormonally active substances, known as endocrine disruptors, which are commonly used worldwide. HD has become a condition widely reported in the media, acting as a culprit for inexplicable weight gain, chronic fatigue or weakness. Nevertheless, the recognition of HD is undeniably increasing and represents a major public health burden. At the same time, improving access to imaging tests has increased the number of incidentally diagnosed adrenal tumors. Above all, the widespread use of chest computed tomography (CT) due to the COVID-19 pandemic has contributed to frequent incidental detection of adrenal lesions. Fortunately, a vast majority of these findings are asymptomatic benign tumors with no excessive hormonal activity, and therefore, they are defined as adrenal incidentalomas (AIs). Interestingly, recent studies have indicated that patients with AIs are more prone to obesity and insulin resistance. Although mutual relationships between the thyroid and the adrenal glands have been studied widely, still, little is known about the possible pathophysiological associations between thyroid autoimmunity and the occurrence of adrenal incidentalomas. This article presents a brief review of the common endocrine disorders with a special focus on the frequently coexisting insulin resistance and/or obesity. Furthermore, in response to the recent growing interest in endocrine disruptors, with their transgenerational epigenetic effects that influence hormonal system function, a concise overview of the topic has also been included.

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.

Ex vivo model for elucidating the functional and structural differentiation of the embryonic mouse thyroid

Terminal thyroid gland differentiation - the last developmental step needed to enable thyroid hormone (T4) synthesis - involves profound structural and biochemical changes in the thyroid follicular cells (TFCs). We aimed to develop an ex vivo thyroid model of embryonic mouse thyroid that would replicate the in vivo TFC differentiation program. E13.5 thyroid explants were cultured ex vivo in chemically defined medium for 7 days. Immunostaining and qPCR of thyroid explants showed thyroglobulin production onset, follicle formation, and T4 synthesis onset in 1-, 3-, and 5-day-old cultures, respectively. Differentiation was maintained and follicular growth continued throughout the 7-day culture period. Pharmacological approaches to culture inhibition were performed successfully in the ex vivo thyroids. Our robust and well described ex vivo thyroid culture model replicates the sequence of thyroid differentiation to T4 synthesis seen in vivo. This model can be used to test the effects of pharmacological inhibitors on thyroid hormone production.

The increasing prevalence of chronic lymphocytic thyroiditis in papillary microcarcinoma

Although the incidence of some malignancy has decreased over the recent years, this is not the case of papillary thyroid microcarcinoma (PTMC), whose incidence has increased worldwide. Most PTMC are found incidentally after histological examination of specimens from surgery for benign thyroid disease. Hashimoto's thyroiditis, whose incidence has also increased, coexists in about one in three PTMC patients. Three different mechanisms have been proposed to clarify the association between chronic lymphocytic thyroiditis and PTMC, namely tumor development/growth by: (i) TSH stimulation, (ii) expression of certain proto-oncogenes, (iii) chemokines and other molecules produced by the lymphocytic infiltrate. Whether Hashimoto's thyroiditis protects against lymph node metastasis is debated. Overall, autommune thyroiditis seems to contribute to the favorable prognosis of PTMC. Major limitations of the studies so far performed include: (i) retrospective design, (ii) limited statistical power, (iii) high risk of selection bias, (iv) and predominant Asian ethnicity of patients. Full genetic profiling of both diseases and identification of environmental factors capable to trigger them, as well as well-powered prospective studies on different ethnical groups, may help understand their causal association and why their frequencies are continuing raising.

An assessment of the relationship between thyroid nodule characteristics, insulin resistance and arterial stiffness in euthyroid nodular goiter

OBJECTIVES

Publications suggesting that thyroid nodule might be associated with insulin resistance and metabolic syndrome are quite interesting. There is a need for studies assessing the relationship between nodule presence and cardiovascular risk in individuals with non-functioning nodular goiter. The purpose of the present study is to reveal whether or not insulin resistance, nodule presence, and nodule stiffness affect arterial stiffness, which is a reliable and valid cardiovascular risk indicator, in individuals with euthyroid nodular goiter using the pulse wave analysis (PWA).

MATERIALS AND METHODS

50 patients with euthyroid nodular goiter and 50 healthy volunteers were included in the study. All participants were examined by B-mode thyroid ultrasound, and the participants in the nodular goiter group were also examined by strain elastography (SE). The strain index of nodules was calculated according to the Rago scoring. Also, fasting plasma glucose (FPG) and insulin levels were measured, and HOMA-IR. Arterial stiffness measurements of the participants were performed using a PWA device which employs a cuff-based oscillometric method from the brachial artery.

RESULTS

PWV was found to be significantly higher in the euthyroid nodular goiter group (p < 0.001). PWV was found to be positively correlated with FPG and waist circumference. Fasting plasma glucose was found to be higher in the group with nodular goiter (p = 0.03). However, no difference was found between the groups in terms of HOMA-IR and insulin level. HOMA-IR was not correlated with thyroid volume, nodule volume, and nodule count. Also, HOMA-IR was not correlated with strain index value and PWA data.

CONCLUSION

We found that PWV was significantly higher in patients with euthyroid nodular goiter. This result suggests that these patients may be at risk for cardiovascular disease.

The Clinical Values of Insulin-Like Growth Factor-1 and Insulin-Like Growth Factor Binding Protein-3 Levels in Blood and Thyroid Nodules

AIM

Insulin-like growth factor-1 (IGF-1) is a potent mitogen for many cells. IGF-1 plays a role in the pathogenesis of various tumors with its mutagenic and antiapoptotic properties. The aim of this study was to determine both the serum and intranodular levels of IGF-1 and insulin-like growth factor binding protein-3 (IGFBP-3) in patients with nodular thyroid diseases.

MATERIALS AND METHODS

In this study, 80 subjects who performed fine-needle aspiration biopsy (FNAB) were required in order to investigate the effects of serum and intranodular IGF-1 and IGFBP-3 in the pathogenesis of nodules. After performing FNAB, IGF-1 and IGFBP-3 levels were determined in blood and aspiration samples.

RESULTS

The serum levels of IGF-1 (232.8 ± 12.9 ng/ml) and IGFBP-3 (4.8 μg/ml) were found significantly higher than that of the intranodular IGF-1 (39.1 ng/ml) and intranodular IGFBP-3 levels (0.173 μg/ml) (p < 0.01). Intranodular levels of IGF-1 and IGFBP-3 were higher in subjects with multinodular thyroid gland than those of subjects with solitary nodules (p = 0.043). A positive correlation between the nodule size and the serum IGFBP-3 levels was detected (p = 0.042, r = 0.23).

CONCLUSION

This study demonstrated the possible role of both IGF-1 and IGFBP-3 in the growth and the formation of multinodularity of thyroid nodules.

Protein kinase a-mediated cell proliferation in brown preadipocytes is independent of Erk1/2, PI3K and mTOR

The physiological agonist norepinephrine promotes cell proliferation of brown preadipocytes during the process of tissue recruitment. In a primary culture system, cAMP mediates these adrenergic effects. In the present study, we demonstrated that, in contrast to other systems where the mitogenic effect of cAMP requires the synergistic action of (serum) growth factors, especially insulin/IGF, the cAMP effect in brown preadipocytes was independent of serum and insulin. Protein kinase A, rather than Epac, mediated the cAMP mitogenic effect. The Erk 1/2 family of MAPK, the PI3K system and the mTOR complexes were all activated by cAMP, but these activations were not necessary for cAMP-induced cell proliferation; a protein kinase C isoform may be involved in mediating cAMP-activated cell proliferation. We conclude that the generally acknowledged cellular mediators for induction of cell proliferation are not involved in this process in the brown preadipocyte system; this conclusion may be of relevance both for examination of mechanisms for induction of brown adipose tissue recruitment but also for understanding the mechanism behind e.g. certain endocrine neoplasias.

In vitro silencing of the insulin receptor attenuates cellular accumulation of fibronectin in renal mesangial cells

Background

Insulin receptor (InsR) and insulin signaling proteins are widely distributed throughout the kidney cortex. Insulin signaling can act in the kidney in multiple ways, some of which may be totally independent of its primary role of the maintenance of whole-body glucose homeostasis. However, descriptions of the insulin signaling in renal glomerular mesangial cells (MCs) are quite limited and the roles of insulin signaling in MC functions have not been sufficiently elucidated.

Results

InsR silencing induced a unique phenotype of reduced fibronectin (FN) accumulation in renal glomerular MCs. Transcription level of FN was not significantly changed in the InsR silenced cells, suggesting the phenotype switching was caused by post-transcriptional modification. The decreased expression of InsR was associated with enhanced activity of insulin-like growth factor-1 receptor (IGF-1R)/PI3K/Akt signaling pathway which contributed in part to the attenuation of cellular FN accumulation. Formation of IGF-1R homodimer was increased in the InsR silenced cells. The InsR silenced cells also showed increased sensitivity to exogenous IGF-1, and increased PI3K activity was reversed significantly by incubating cells with IGF-1R specific antagonist, AG538. PI3K/Akt dependent activation of cAMP responsive element-binding protein (CREB)-1 induced expression of matrix metalloproteinase (MMP)-9 and suppressing MMP activity by doxycycline partially reversed FN accumulation in the InsR silenced cells.

Conclusions

The effects of InsR silencing on cellular FN accumulation in vitro are, at least partially, mediated by increased degradation of FN by MMPs which is induced by enhanced signaling sequence of IGF-1R/PI3K/Akt/CREB-1.

Serum thyrotropin as a risk factor for thyroid malignancy in euthyroid subjects with thyroid micronodule

BACKGROUND

An elevated thyroid stimulating hormone (TSH) level is a risk factor for differentiated thyroid cancer. The present study investigated whether TSH is a risk factor for thyroid microcarcinoma (TMC) in euthyroid subjects with a micronodule.

METHODS

In this cross-sectional study, 483 euthyroid subjects (77 males patients, 406 females patients) with a thyroid micronodule were enrolled and underwent ultrasound guided fine-needle aspiration biopsy. Serum TSH level was measured in every subject. The final diagnostic outcome (malignant or benign) was defined in conjunction with cytology and histology.

RESULTS

Compared with the patients below the population mean, patients above the population mean had a significantly higher malignancy rate (10.8% vs 4.8%, p = .025). Multiple logistic regression analysis revealed TSH as an independent risk factor for malignancy in euthyroid patients with a micronodule (odds ratio, 1.449; 95% confidence interval, 1.033-2.033; p = .031).

CONCLUSIONS

Measurement of TSH in subjects with a thyroid micronodule is helpful to estimate cancer risk.

Conditional deletion of insulin receptor in thyrocytes does not affect thyroid structure and function

Thyroid-stimulating hormone (TSH) is the primary regulator of thyroid growth and function acting via cyclic AMP signaling cascades. In cultured thyrocytes, insulin and/or insulin-like growth factor-1 (IGF-1) are required for mediating thyrocyte proliferation in concert with TSH. To determine the role of insulin signaling in thyroid, growth in vivo, mice with thyrocyte-selective ablation of the insulin receptor (IR) were generated by crossing mice homozygous for a floxed IR allele with transgenic mice in which thyrocyte-specific expression of Cre recombinase was driven by the human thyroid peroxidase (TPO) gene promoter. Immunohistochemistry and Western blot analysis confirmed near complete loss of IR expression in the thyroid of thyrocyte IR knockout mice. These mice are viable and have no obvious thyroid dysfunction and macro- and microscopic thyroid morphology was normal. Thus, insulin signaling in thyrocytes does not play an essential role in the architecture and function of the thyroid in vivo.

Is gender difference in postnatal thyroid growth associated with specific expression patterns of androgen and estrogen receptors?

Variations in sex steroids bioavailability were linked to the gender difference in the growth of thyroid glands of neonatal rats. In the present study we tested androgen receptor (AR) and estrogen receptor (ER) concentrations by ligand binding assay, and expression of their genes by RT-PCR and Western blot in the thyroid glands of neonatal rats. AR concentration remained elevated from postnatal day (PND) 10 onwards in males, whereas it decreased by PND 20 in females. AR mRNA and protein expressions were higher in males than females, which increased by PND 10, decreased after PND 15 and reached the nadir by PND 20. ER concentration increased by PND 10 and decreased thereafter in both sex. ERα mRNA expression diminished by PND 15 in both sex; while ERβ mRNA decreased by PND 15 to reach the nadir by PND 20 in males, it was augmented by PND 10 in females to reach the peak by PND 15 and diminished by PND 20. ERα protein expression increased by PND 10 and remained elevated till PND 20 in both sex. ERβ protein expression in males increased by PND 10 and decreased by PND 20, while it remained static up to PND 15 and decreased in females. Testosterone stimulated [(3)H]-thymidine uptake and the expression of IGF-1 and NIS genes in thyrocytes of both sex in vitro, while estradiol stimulated them in females but not in males. We conclude that androgens influence the growth and differentiation of thyrocytes through augmented expression of AR, IGF-1 and NIS in either sex, whereas estrogen imparts the gender difference, which may be at a level beyond the expression of ERs.

Hypothyroid phenotype of the Tpst2 mutant mouse is dependent upon genetic background

DW/J-grt is a congenital hypothyroid mouse model that is characterized by growth retardation, significantly lowered T3 and T4 levels, and severe thyroid hypoplasia related to TSH hyporesponsiveness. Previously, we identified the point mutation of the Tpst2 gene in DW/J-grt mice that causes a decrease in the enzymatic activity, and demonstrated that the Tpst2 transgene rescues the mutant phenotypes both in vitro and in vivo. The severity of hypothyroidism is highly variable indicating the influence of modifier genes in humans. In this study, to identify the modifier/resistant gene(s) to hypothyroidism, we produced congenic strains carrying this Tpst2(grt) mutation on the C57BL/6J and 129/SvJcl (129) genetic backgrounds and analyzed growth rate and thyroid function. Interestingly, the 129 congenic mice exhibited normal growth and thyroid function. The result suggests that 129 strain has the modifier(s) of attenuation of hypothyroidism. Therefore, the identification of the modifier loci of 129 mice will provide important new information about the gene(s) related to congenital hypothyroidism.

Signal transduction in the human thyrocyte and its perversion in thyroid tumors

The study of normal signal transduction pathways regulating the proliferation and differentiation of a cell type allows to predict and to understand the perversions of these pathways which lead to tumorigenesis. In the case of the human thyroid cell, three cascades are mostly involved in tumorigenesis: The pathways and genetic events affecting them are described. Caveats in the use of models and the interpretation of results are formulated and the still pending questions are outlined.

Phosphorylated Insulin Like Growth Factor-I Receptor Expression and Its Clinico-Pathological Significance in Histologic Subtypes of Human Thyroid Cancer

Overexpression of insulin-like growth factor-I receptor (IGF-IR) is seen in a multitude of human thyroid cancers and correlates with poor prognosis. However, recent studies suggest that low phospho-IGF-IR (pIGF-IR) expression rather than its overexpression may be an indicator of poorly differentiated disease. No previous study has evaluated the expression of pIGF-IR to determine if activation or loss of expression of this receptor is associated with thyroid tumor progression. Accordingly, a quantitative immunohistochemical (IHC) method was used to evaluate the clinico-pathological significance of pIGF-IR expression in archival samples of human thyroid carcinomas. Quantitative analysis of pIGF-IR levels revealed a significant difference in the median index of pIGF-IR between different histological subtypes of thyroid cancer ( P < 0.001). Specifically, the median pIGF-IR index of differentiated thyroid cancers was significantly higher than the median index of other poorly differentiated thyroid cancer ( P < 0.001). This was further confirmed in individual tumor sections of thyroid carcinoma where anaplastic and differentiated components co-existed. No significant difference was noted in the pIGF-IR index of tumors grouped by size or stage but a trend towards lower mean pIGF-IR index was noted in older patients. Our data indicates that pIGF-IR is upregulated in a majority of follicular thyroid carcinomas, suggesting it may be a potential target for therapy for patients with this disease. In addition, since low pIGF-IR expression was found to correlate with aggressive human thyroid carcinoma, it also suggests that IGF-IR may not be needed for progression of anaplastic thyroid carcinoma possibly because other cell signaling pathways are activated, obviating the need for IGF-IR signaling. However, more mechanistic studies would be necessary to substantiate the possibility that pIGF-IR may be important for differentiation of thyroid tissues and is lost with disease progression.

Reciprocal negative regulation between thyrotropin/3',5'-cyclic adenosine monophosphate-mediated proliferation and caveolin-1 expression in human and murine thyrocytes

The expression of caveolins is down-regulated in tissue samples of human thyroid autonomous adenomas and in the animal model of this disease. Because several cell types present in thyroid express caveolins, it remained unclear if this down-regulation occurs in thyrocytes and which are the mechanism and role of this down-regulation in the tumor context. Here we show that prolonged stimulation of isolated human thyrocytes by TSH/cAMP/cAMP-dependent protein kinase inhibits caveolins' expression. The expression of caveolins is not down-regulated by activators of other signaling pathways relevant to thyroid growth/function. Therefore, the down-regulation of caveolins' expression in autonomous adenomas is a direct consequence of the chronic activation of the TSH/cAMP pathway in thyrocytes. The down-regulation of caveolin-1 occurs at the mRNA level, with a consequent protein decrease. TSH/cAMP induces a transcription-dependent, translation-independent destabilization of the caveolin-1 mRNA. This effect is correlated to the known proliferative role of that cascade in thyrocytes. In vivo, thyrocytes of caveolin-1 knockout mice display enhanced proliferation. This demonstrates, for the first time, the in vivo significance of the specific caveolin-1 down-regulation by one mitogenic cascade and its relation to a human disease.

Normal human thyroid cells from the ARAMIS line follow the general concept of growth or differentiation: a study with thyroglobulin as a marker

A cell line of functional normal human adult thyroid cells was isolated 3 years ago. This cell line was used as a model to study human thyroglobulin (hTgb) production quantified in 5% fetal calf serum Click-RPMI medium and in the presence of insulin and thyrotropin (two hormones [2H]) to get maximal hTgb production. In this paper we demonstrate that these cells (line ARAMIS) follow the general dualistic opposition between growth and differentiation. Thyroglobulin cell production in 2H-stimulated cells is dependent on cell density and is not constant with time of culture in growing and in quiescent cells. High serum levels required for cell proliferation antagonize thyroglobulin cell production. Furthermore, in the absence of cell proliferation, the longer the cells stay in the stationary phase in 2H-medium, the better they produce thyroglobulin with time. The longer the cells stay in the stationary phase in 1H (insulin) medium, the higher will be the total thyroglobulin production and the initial rate in thyroglobulin production after TSH addition. The longer the cells stay in the stationary phase in 1H (insulin) medium, the higher are cyclic adenosine monophosphate (cAMP) levels after thyrotropin (TSH) stimulation. This is observed both for initial rates and total production. Neither insulin nor cell-cell interactions occurring during the stationary phase modify basal cAMP levels. Altogether the data demonstrate that in cell culture conditions that exclude proliferation, a gain in TSH sensitivity appears versus time in insulin-stimulated quiescent normal human thyroid cells during their stationary phase. This improved differentiating status appears to be TSH- and cAMP-independent. It could be an insulin or insulin-like growth factor-1 (IGF-1)-dependent trophic effect promoting an increase in TSH-receptor number or sensitivity. But once again and as mentioned previously, we cannot exclude from the data that cell to cell interactions between silent and "informed" cells (in other words, autocrine phenomena) could result, with time, in the recruitment of silent resting cells, explaining the gain in TSH sensitivity.

Sphingolipid-cholesterol domains (lipid rafts) in normal human and dog thyroid follicular cells are not involved in thyrotropin receptor signaling

Partition of signaling molecules in sphingolipid-cholesterol-enriched membrane domains, among which are the caveolae, may contribute to signal transduction efficiency. In normal thyroid, nothing is known about a putative TSH/cAMP cascade compartmentation in caveolae or other sphingolipid-cholesterol-enriched membrane domains. In this study we show for the first time that caveolae are present in the apical membrane of dog and human thyrocytes: caveolin-1 mRNA presence is demonstrated by Northern blotting in primary cultures and that of the caveolin-1 protein by immunohistochemistry performed on human thyroid tissue. The TSH receptor located in the basal membrane can therefore not be located in caveolae. We demonstrate for the first time by biochemical methods the existence of sphingolipid-cholesterol-enriched domains in human and dog thyroid follicular cells that contain caveolin, flotillin-2, and the insulin receptor. We assessed a possible sphingolipid-cholesterol-enriched domains compartmentation of the TSH receptor and the alpha- subunit of the heterotrimeric G(s) and G(q) proteins using two approaches: Western blotting on detergent-resistant membranes isolated from thyrocytes in primary cultures and the influence of 10 mm methyl-beta-cyclodextrin, a cholesterol chelator, on basal and stimulated cAMP accumulation in intact thyrocytes. The results from both types of experiments strongly suggest that the TSH/cAMP cascade in thyroid cells is not associated with sphingolipid-cholesterol-enriched membrane domains.

TSH-activated signaling pathways in thyroid tumorigenesis

Thyrotropin (TSH) is considered the main regulator of thyrocyte differentiation and proliferation. Thus, the characterization of the different signaling pathways triggered by TSH on these cells is of major interest in order to understand the mechanisms implicated in thyroid pathology. In this review we focus on the different signaling pathways involved in TSH-mediated proliferation and their role in thyroid transformation and tumorigenesis. TSH mitogenic activities are mediated largely by cAMP, which in turn may activate protein kinase (PKA)-dependent and independent processes. We analyze the effects of increased cAMP levels and PKA activity during cell cycle progression and the role of this signaling pathway in thyroid tumor initiation. Alternative pathways to PKA in the cAMP-mediated proliferation appear to involve the small GTPases Rap1 and Ras. We analyze the Ras effectors (PI3K, RalGDS and Raf) that are thought to mediate its oncogenic activity, as well as the ability of Ras to induce apoptosis in thyrocytes. Finally, we discuss the activation of the PLC/PKC cascade by TSH in thyroid cells and the role of this signaling pathway in the TSH-mediated proliferation and tumorigenesis.

Gene expression profile in thyroid of transgenic mice overexpressing the adenosine receptor 2a

Mutations of the TSH receptor leading to constitutive activation of the cAMP cascade are responsible for the development of hot nodules, if arising in a somatic cell, and nonautoimmune hyperthyroidism, when occurring in a germinal cell. An animal model of constitutive activation of the thyroid cAMP cascade has been obtained by generating transgenic mice expressing the adenosine receptor (Tg-A2aR) under the control of the thyroglobulin promoter. These mice develop huge goiters and die prematurely due to hyperthyroidism induced cardiac failure. To identify new genes involved in the tumorigenic pathway of the thyroid, we designed a protocol using microarray technology to study the differential expression, between normal and transgenic thyroid, of +/-13,000 genes. A total of 360 genes or expressed sequence tags showed a strong modulation with background corrected values of fluorescence superior to 2-fold change. The modulated genes were classified according to their proposed gene ontology functions. Approximately half of them were up-regulated. The function of the majority of these genes in thyroid physiology is still to be determined. Some of them, like IGF-I or IGF binding protein 3 or 5, may play an important role in the development of thyroid nodules through paracrine mechanisms. This study demonstrates the feasibility of sequentially following the cascade of events leading to the formation of benign tumors such as hot thyroid nodule or hyperfunctional goiter.

Isolation of a normal human thyroid cell line: hormonal requirement for thyroglobulin regulation

The long-term culture of functional follicular cells from normal adult human thyroid tissue has been obtained. They were expanded using a 1:2 split ratio until passage 28 (present status) in Click-RPMI medium enhanced with 5% fetal calf serum and diverse associations of hormones or components including porcine insulin and bovine thyrotropin. At passages 10 and 20, chromosome countings showed a normal diploid number and a normal karyotype. In calf serum containing media, cells are epithelial in the presence of thyrotropin (TSH) but present a slight elongated form in the absence of TSH. In serum-free media, 30 minutes after TSH stimulation, both epithelial and elongated cells changed in morphology to stellate-shaped, arborized forms, indicating the presence of functional TSH-receptors even in long term (18 months) TSH-free cultures. Cells produce thyroglobulin constitutively and large amounts of thyroglobulin are easily recovered in TSH-supplemented media, especially in the presence of insulin. Thyroglobulin production was increased versus days under TSH or insulin stimulation. Combination of the two hormones clearly resulted in a synergistic and not an additive effect. The other hormones present in the 6H components (transferrin, glycylhistidyl-lysine, somatostatin, and hydrocortisone) had no positive effect on thyroglobulin accumulation in media in our experimental conditions. Addition of TSH to hormone-free cultures or to insulin-, insulin plus hydrocortisone-, or 5H-containing cultures resulted in a clear increase in thyroglobulin production. Withdrawal of TSH from 6H cultures resulted in a decrease in thyroglobulin accumulation in media. Six months were required to select fibroblast-free cultures and to get passage 6. But only 17 months separated passage 6 to passage 28, indicating that the proliferative rate is increasing with in vitro cell adaptation. Such normal adult thyroid cells, thyroglobulin-producing, TSH, and insulin-sensitive, represent a new normal human thyroid cell line allowing comparative studies with cells originating from pathologic thyroid tissues.

The role of cyclic AMP and its effect on protein kinase A in the mitogenic action of thyrotropin on the thyroid cell

Cyclic AMP has been shown to inhibit cell proliferation in many cell types and to activate it in some. The latter has been recognized only lately, thanks in large part to studies on the regulation of thyroid cell proliferation in dog thyroid cells. The steps that led to this conclusion are outlined. Thyrotropin activates cyclic accumulation in thyroid cells of all the studied species and also phospholipase C in human cells. It activates directly cell proliferation in rat cell lines, dog, and human thyroid cells but not in bovine or pig cells. The action of cyclic AMP is responsible for the proliferative effect of TSH. It accounts for several human diseases: congenital hyperthyroidism, autonomous adenomas, and Graves' disease; and, by default, for hypothyroidism by TSH receptor defect. Cyclic AMP proliferative action requires the activation of protein kinase A, but this effect is not sufficient to explain it. Cyclic AMP action also requires the permissive effect of IGF-1 or insulin through their receptors, mostly as a consequence of PI3 kinase activation. The mechanism of these effects at the level of cyclin and cyclin-dependent protein kinases involves an induction of cyclin D3 by IGF-1 and the cyclic AMP-elicited generation and activation of the cyclin D3-CDK4 complex.

Somatic mutations in thyroid nodular disease

Thyroid nodules can be found in up to 50% of inhabitants of iodine-deficient areas and are classified as hot or cold thyroid nodules according to their scintigraphic characteristics. Studies of hot thyroid nodules with comparable mutation detection methods and screening at least exon 10 of the TSH receptor reported frequencies for somatic TSH-receptor mutations ranging from 20 to 82% in patients with similar iodine supply. We have recently screened 75 hot thyroid nodules for somatic TSH-receptor mutations with the more sensitive DGGE method and found somatic TSH-receptor mutations in 57% and Gsalpha mutations in 3%. As 50% of the mutation-negative nodules from female patients are of monoclonal origin when tested for X-chromosome inactivation somatic mutations in other genes are likely to cause the development of hot thyroid nodules. Scintigraphically nonsuppressible areas have been identified in up to 40% of euthyroid goiters in iodine-deficient areas. We recently identified somatic TSH-receptor mutations in microscopic autonomous areas with increased 125T uptake in euthyroid goiters studied by autoradiography 20 years ago. These constitutively activating somatic TSH-receptor mutations in minute autoradiographically hot areas of euthyroid goiters are very likely starting foci which most likely lead to toxic thyroid nodules in iodine-deficient goiters. Therefore iodine deficiency does not only lead to euthyroid goiters but also to thyroid autonomy. The latter is also suggested by epidemiologic studies. Similar mechanisms induced by iodine deficiency and the subsequent hyperplasia, mutagenesis, and selection of cell clones could also lead to cold thyroid nodules by somatic mutations that only initiate growth but not hyperfunction of the affected thyroid epithelial cell. Somatic ras mutations have frequently been detected in histologically characterized thyroid adenomas or adenomatous nodules. However, they seem to be rare in cold thyroid nodules. Since the majority of these latter nodules and 60% of the cold thyroid nodules are monoclonal other somatic mutations are likely in these nodules.

Low TSH requirement and goiter in transgenic mice overexpressing IGF-I and IGF-Ir receptor in the thyroid gland

Through the cAMP signaling pathway, TSH stimulates thyroid follicular cell proliferation, differentiation, and function. Although the autocrine production of IGF-I in the thyroid gland suggests an important physiological function for this factor in these processes, the exact role of the IGF-I/IGF-I receptor system in vivo remains unclear. Although the mitogenic action of TSH requires the presence of IGF-I or insulin in primary culture of dog and human thyroid cells, IGF-I has an effect equal to and independent of the effect of TSH on cell proliferation in rat thyroid cell lines and may even be the main growth regulator in this case. To investigate the in vivo function of the IGF-I/IGF-I receptor system, transgenic mice overexpressing human IGF-I, IGF-I receptor, or both in the thyroid were generated. Adult transgenic mice did not present external signs of thyroid dysfunction, but mice overexpressing both transgenes had significantly increased gland weight and follicular lumen area. A decreased TSH level together with a slightly increased serum T(4) concentration and increased thyroidal iodine uptake were also observed, suggesting that IGF-I and IGF-I receptor stimulate thyroid function to some extent in vivo.

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.

TSH induces insulin receptors that mediate insulin costimulation of growth in normal human thyroid cells

The mitogenic/goitrogenic effects of thyrotropin (TSH) on human thyrocytes in vitro and in vivo depend on permissive comitogenic effects of insulin-like growth factors (IGFs), which are mimicked in vitro by the low-affinity binding of high supraphysiological concentrations of insulin to IGF-I receptors. Contrary to general assumption, we show here that very low concentrations of insulin, acting through insulin receptors but not IGF-I receptors, can also support the stimulation of DNA synthesis by TSH in primary cultures of normal human thyrocytes. Moreover, TSH through cAMP increases the content of insulin receptors demonstrated by Western blotting and the cells' responsiveness to low insulin concentrations. These observations provide the first in vitro evidence in normal human thyroid cells of a functional interaction between TSH and insulin acting through its own receptor.

Excess iodine induces apoptosis in the thyroid of goitrogen-pretreated rats in vivo

Previously, we observed that excess iodide rapidly suppressed the elevated ornithine decarboxylase activity in the thyroid of propylthiouracil (PTU)-pretreated rats. Excess iodide also induces involution of goitrous thyroids. These findings led us to study effects of excess iodide on apoptosis of rat thyroids. When given to PTU-pretreated rats, excess potassium iodide (KI) (13 mg/kg body weight, 10 mg as iodine) induced DNA fragmentation in the thyroid at the first 3 hours after its treatment. The percentage of DNA fragmentation was also maximal at 3 hours after KI treatment. In methimazole-pretreated rats, the kinetic of DNA fragmentation was nearly the same; apoptosis increased for the first 6 hours and then decreased at 12 hours after KI administration. Other iodinated compounds such as amiodarone and diiodotyrosine have also shown apoptosis-inducing activity, but their effect was observed later than KI. Iopanoic acid had no such effect. Apoptotic changes were also observed with the use of flow cytometry. PTU or methimazole alone had some stimulatory effect on thyroid apoptosis. Iodine effect was not observed in rats treated with either perchlorate or thiocyanate. These results suggest that excess iodine induces thyroid involution in goitrogen-treated rats at least partially by apoptosis.

Cyclin D3 accumulation and activity integrate and rank the comitogenic pathways of thyrotropin and insulin in thyrocytes in primary culture

The proliferation of most normal cells depends on the synergistic interaction of several growth factors and hormones, but the cell cycle basis for this combined requirement remains largely uncharacterized. We have addressed the question of the requirement for insulin/IGF-1 also observed in many cell culture systems in the physiologically relevant system of primary cultures of dog thyroid epithelial cells stimulated by TSH, which exerts its mitogenic activity only via cAMP. The induction of cyclin A and cdc2, the phosphorylation of cdk2, the nuclear translocation of cdk4 and the assembly of cyclin D3-cdk4 complexes required the synergy of TSH and insulin. Cyclin D3 (the most abundant cyclin D) was necessary for the proliferation stimulated by TSH in the presence of insulin as shown by microinjection of a neutralizing antibody. Cyclin D3 accumulation and activity were differentially regulated by insulin and TSH, which points out this cyclin as an integrator that ranks these comitogenic pathways as supportive and activatory, respectively. Paradoxically TSH alone strongly repressed cyclin D3 accumulation. This inhibition was overridden by insulin, which markedly stimulated cyclin D3 mRNA and protein accumulation, but failed to assemble cyclin D3-cdk4 complexes in the absence of TSH. TSH unmasked the DCS-22 epitope of cyclin D3 and assembled cyclin D3-cdk4 in the presence of insulin. These data demonstrate that cyclin D synthesis and cyclin D-cdk assembly can be dissociated and complementarily regulated by different agents and signalling pathways.

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.

IGF-1 or insulin, and the TSH cyclic AMP cascade separately control dog and human thyroid cell growth and DNA synthesis, and complement each other in inducing mitogenesis

The regular doubling of cell mass, and therefore of cell protein content, is required for repetitive cell divisions. Preliminary observations have shown that in dog thyrocytes insulin induces protein accumulation but not DNA synthesis, while TSH does not increase protein accumulation but triggers DNA synthesis in the presence of insulin. We show here that EGF and phorbol myristate ester complement insulin action in the same way. HGF is the only factor activating both protein accumulation and DNA synthesis. The effects of insulin on protein accumulation and in permitting the TSH effect are reproduced by IGF-1 and are mediated, at least in part by the IGF-1 receptor. The concentration effect curves are similar for both effects. Similar results are obtained in human thyrocytes. They reflect true cell growth, as shown by increases in RNA content and cell size. Carbachol and fetal calf serum also stimulate protein synthesis and accumulation without triggering DNA synthesis, but they are not permissive for the mitogenic effects of TSH or of the general adenylate cyclase activator, forskolin. Moreover the mitogenic effect of TSH greatly decreased in cells deprived of insulin for 2 days although these cells remain hypertrophic. Hypertrophy may therefore be necessary for cell division, but it is not sufficient to permit it. Three different mechanisms can therefore be distinguished in the mitogenic action of TSH: (1) the increase of cell mass (hypertrophy) induced by insulin or IGF-1; (2) the permissive effect of insulin or IGF-1 on the mitogenic effect of TSH which may involve both the increase of cell mass and the induction of specific proteins such as cyclin D3 and (3) the mitogenic effect of the TSH cyclic AMP cascade proper.

A requirement for cyclin D3-cyclin-dependent kinase (cdk)-4 assembly in the cyclic adenosine monophosphate-dependent proliferation of thyrocytes

In different systems, cyclic adenosine monophosphate (cAMP) either blocks or promotes cell cycle progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (TSH). The cAMP-dependent mitogenic pathway is unique as it is independent of mitogen-activated protein kinase activation and differs from growth factor-dependent pathways at the level of the expression of several protooncogenes/transcription factors. This study examined the involvement of D-type G1 cyclins and their associated cyclin-dependent kinase (cdk4) in the cAMP-dependent G1 phase progression of dog thyroid cells. Unlike epidermal growth factor (EGF)+serum and other cAMP-independent mitogens, TSH did not induce the accumulation of cyclins D1 and D2 and partially inhibited the basal expression of the most abundant cyclin D3. However, TSH stimulation enhanced the nuclear detection of cyclin D3. This effect correlated with G1 and S phase progression. It was found to reflect both the unmasking of an epitope of cyclin D3 close to its domain of interaction with cdk4, and the nuclear translocation of cyclin D3. TSH and EGF+serum also induced a previously undescribed nuclear translocation of cdk4, the assembly of precipitable cyclin D3-cdk4 complexes, and the Rb kinase activity of these complexes. Previously, cdk4 activity was found to be required in the cAMP-dependent mitogenic pathway of dog thyrocytes, as in growth factor pathways. Here, microinjections of a cyclin D3 antibody showed that cyclin D3 is essential in the TSH/ cAMP-dependent mitogenesis, but not in the pathway of growth factors that induce cyclins D1 and D2. The present study (a) provides the first example in a normal cell of a stimulation of G1 phase progression occurring independently of an enhanced accumulation of cyclins D, (b) identifies the activation of cyclin D3 and cdk4 through their enhanced assembly and/or nuclear translocation, as first convergence steps of the parallel cAMP-dependent and growth factor mitogenic pathways, and (c) strongly suggests that this new mechanism is essential in the cAMP-dependent mitogenesis, which provides the first direct demonstration of the requirement for cyclin D3 in a G1 phase progression.