Actions of epidermal growth factor and its receptor in the thyroid

Switching from MAPK-dependent to MAPK-independent repression of the sodium-iodide symporter in 2D and 3D cultured normal thyroid cells

Loss of sodium-iodide symporter (NIS) expression in thyroid tumour cells primarily caused by constitutive MAPK pathway activation is often refractory to small molecule MAPK inhibitors. Suggested mechanisms are rebound MAPK signalling and activation of alternative signalling pathways. Here we provide evidence that failure to recover down-regulated NIS by MEK inhibition is not specific to tumour cells. NIS mRNA levels remained repressed in TSH-stimulated primary thyroid cells co-treated with epidermal growth factor (EGF) and pan-MEK inhibitor U0126 in the presence of 5% fetal bovine serum or, independently of serum, in 3D cultured thyroid follicles. This led to inhibited iodide transport and iodination. In contrast, U0126 restituted thyroglobulin synthesis in EGF-treated follicular cells. Serum potentiated TSH-stimulated NIS expression in 2D culture. U0126 blocked down-regulation of NIS only in serum-starved cells with a diminished TSH response. Together, this suggests that morphogenetic signals modify the expression of NIS and recovery response to MEK inhibition.

Relevance of iodine intake as a reputed predisposing factor for thyroid cancer

Iodine is a trace element that is essential for the synthesis of thyroid hormone. Both chronic iodine deficiency and iodine excess have been associated with hypertrophy and hyperplasia of follicular cells, attributed to excessive secretion of TSH. This may be associated to thyroid cancer risk, particularly in women. Experimental studies have documented thyroid cancer induction by elevation of endogenous TSH, although in a small number of animals. Iodine deficiency associated with carcinogenic agents and chemical mutagens will result in a higher incidence of thyroid malignancy. Inadequate low iodine intake will result in increased TSH stimulation, increased thyroid cell responsiveness to TSH, increased thyroid cell EGF-induced proliferation, decreased TGFbeta 1 production and increased angiogenesis, all phenomena related to promotion of tumor growth. Epidemiological studies associating iodine intake and thyroid cancer led to controversial and conflicting results. There is no doubt that introduction of universal iodine prophylaxis in population previously in chronic iodine-deficiency leads to a changing pattern of more prevalent papillary thyroid cancer and declining of follicular thyroid cancer. Also anaplastic thyroid cancer is practically not seen after years of iodine supplementation. Iodine excess has also been indicated as a possible nutritional factor in the prevalence of differentiated thyroid cancer in Iceland, Hawaii and, more recently, in China.

IN CONCLUSION

available evidence from animal experiments, epidemiological studies and iodine prophylaxis has demonstrated a shift towards a rise in papillary carcinoma, but no clear relationship between overall thyroid cancer incidence and iodine intake.

Transforming growth factor-beta and epidermal growth factor synergistically stimulate epithelial to mesenchymal transition (EMT) through a MEK-dependent mechanism in primary cultured pig thyrocytes

Enhancement of tumor cell growth and invasiveness by transforming growth factor-beta (TGF-beta) requires constitutive activation of the ras/MAPK pathway. Here we have investigated how MEK activation by epidermal growth factor (EGF) influences the response of fully differentiated and growth-arrested pig thyroid epithelial cells in primary culture to TGF-beta1. The epithelial tightness was maintained after single stimulation with EGF or TGF-beta1 (both 10 ng/ml) for 48 hours. In contrast, co-stimulation abolished the transepithelial resistance and increased the paracellular flux of [(3)H]inulin within 24 hours. Reduced levels of the tight junction proteins claudin-1 and occludin accompanied the loss of barrier function. N-cadherin, expressed only in few cells of untreated or single-stimulated cultures, was at the same time increased 30-fold and co-localised with E-cadherin at adherens junctions in all cells. After 48 hours of co-stimulation, both E- and N-cadherin were downregulated and the cells attained a fibroblast-like morphology and formed multilayers. TGF-beta1 only partially inhibited EGF-induced Erk phosphorylation. The MEK inhibitor U0126 prevented residual Erk phosphorylation and abrogated the synergistic responses to TGF-beta1 and EGF. The observations indicate that concomitant growth factor-induced MEK activation is necessary for TGF-beta1 to convert normal thyroid epithelial cells to a mesenchymal phenotype.

Nuclear localization of epidermal growth factor and epidermal growth factor receptors in human thyroid tissues

Epidermal growth factor (EGF) has widespread growth effects, and in some tissues proliferation is associated with the nuclear localization of EGF and epidermal growth factor receptor (EGFR). In the thyroid, EGF promotes growth but differs from thyrotropin (TSH) in inhibiting rather than stimulating functional parameters. We have therefore studied the occurrence and cellular distribution of EGF and EGFR in normal thyroid, in Graves' disease, where growth is mediated through the thyrotropin receptor (TSHR), and in a variety of human thyroid tumors. In the normal gland the staining was variable, but largely cytoplasmic, for both EGF and EGFR. In Graves' disease there was strong cytoplasmic staining for both EGF and EGFR, with frequent positive nuclei. Nuclear positivity for EGF and particularly for EGFR was also a feature of both follicular adenomas and follicular carcinomas. Interestingly, nuclear staining was almost absent in papillary carcinomas. These findings document for the first time the presence of nuclear EGF and EGFR in thyroid. Their predominant occurrence in tissues with increased growth (Graves' disease, follicular adenoma, and carcinoma) may indicate that nuclear EGF and EGFR play a role in growth regulation in these conditions. The absence of nuclear EGF and EGFR in papillary carcinomas would suggest that the role played by EGF in growth control differs between papillary carcinoma and follicular adenomas/carcinomas of the thyroid.

The role of insulin dissociation from its endosomal receptor in insulin degradation

Mechanisms that terminate signals from activated receptors have potential to influence the magnitude and nature of cellular responses to insulin. The aims of this study were to determine in rat liver endosomes (the subcellular site of insulin signal termination) whether dissociation of insulin from its receptor was a pre-requisite for ligand degradation and whether the state of receptor phosphorylation influenced the dissociation and hence endosomal degradation of insulin and/or receptor recycling. Following in vivo administration of 125I-[A14]-insulin or analogues (native, X10 or H2, relative binding affinities 1:7:67) livers were removed and endosomes prepared. In the endosomal preparations a significantly greater percentage of both analogues were receptor-bound than native insulin with concomitantly less ligand degradation. When rats were injected with protein-tyrosine phosphatase inhibitors (peroxovanadium compounds bpV(phen) or bpV(pic)) before insulin, endosomal insulin receptor phosphotyrosine content, assessed by Western blotting, was increased as was receptor-bound 125I-[A14]-insulin, whilst insulin degradation was decreased. Peroxovanadiums also completely inhibited recycling of insulin receptors from endosomes. However, treatment of freshly isolated endosomes with acid phosphatase which completely dephosphorylated the insulin receptor, did not return the rate of insulin dissociation and degradation to control values, suggesting that peroxovanadium compounds elicit their effect on binding and degradation via a mechanism other than as protein-tyrosine phosphatase inhibitors. We conclude that promotion of sustained receptor binding decreases endosomal insulin degradation and extends the half-life of the activated endosomal receptor, which in turn would be expected to potentiate insulin signalling from this intracellular compartment.

Determinants of thyroid volume in healthy French adults participating in the SU.VI.MAX cohort

OBJECTIVE

To study the relative importance of determinants of thyroid volume.

DESIGN

Cross-sectional study on a sample of subjects issued from the SU.VI.MAX cohort.

SUBJECTS

2987 French subjects (1713 women aged 35-60 years and 1274 men aged 45-60 years). None of them had previous or present thyroid disease.

MEASUREMENTS

Thyroid volume was determined by ultrasound. Serum TSH and free thyroxine (fT4) were measured in duplicate. Urinary iodine and urinary thiocyanate were assayed in random morning urine samples.

RESULTS

For both sexes, thyroid volume (ml) was positively correlated with weight, height, body mass index and body surface area (P = 0.0001) and negatively with age for females (P = 0.0009). When the urinary iodine concentration was adjusted for urinary thiocyanate concentration and their interaction, the thyroid volume was negatively correlated with urinary iodine (males P = 0.02, females P = 0.006) and positively correlated with urinary thiocyanate (males P = 0.0001, females P = 0.004). Mean thyroid volume was greater among active smokers than non-smokers (males P < 0.0001, females P = 0.0004) and was greater among former smokers than among non-smokers (males P = 0.0001, females = 0.004). Free T4 and thyroid volume were positively correlated for both sexes (P = 0. 0001). TSH was negatively correlated with thyroid volume for both groups (P = 0.0001). Female users of oral contraception (aged 35-45 years) had a smaller thyroid volume than non-users (P = 0.0009).

CONCLUSIONS

The state of borderline iodine deficiency observed in France, in association with a slightly goitrogenic environment, may result in sustained stimulation of the thyroid, independently of TSH level, and is of paramount importance in the formation of goitre. Smoking may affect the thyroid, inducing marked long-lasting thyroid enlargement.

Molecular and cellular mechanisms of transepithelial iodide transport in the thyroid

Thyroid hormone is an essential regulator of developmental growth and metabolism in vertebrates. Iodine is a necessary constituent of thyroid hormone. Due to the scarcity and uneven distribution of iodine on the Earth's crust, the structure of the thyroid gland is adjusted to collect and store this element in order to secure a continuous supply of thyroid hormone throughout life. Still, disease resulting from hypothyroidism due to iodine deficiency is a global health problem, illustrating the great biological significance that iodine saving mechanisms have evolved. Iodide is accumulated together with prohormone (thyroglobulin) in the lumen of the thyroid follicles. The rate-limiting step of this transport is the sodium/iodide symporter located in the basolateral plasma membrane of the thyroid follicular cells. Iodide is also transferred across the apical plasma membrane into the lumen where hormonogenesis takes place. In this review, recent progress in the understanding of transepithelial iodide transport in the thyroid is summarized.

Functional retinoid and thyroid hormone receptors in human thyroid-carcinoma cell lines and tissues

Thyroid carcinomas no longer accessible to radio-iodide or TSH-suppressive T4 therapy, due to loss of thyroid-specific functions, might be sufficiently re-differentiated by retinoic acid (RA) to be treated by conventional methods again. To help evaluate the feasibility of RA re-differentiation therapy in thyroid carcinomas, we examined the functionality of RA receptors (RARs/RXRs), central RA signal mediators, in human thyroid-carcinoma cell lines as model systems. [3H]-RA binding assays with nuclear extracts from follicular thyroid-carcinoma cell lines FTC-133 and -238 revealed high-affinity binding sites for RA. Electrophoretic mobility shift and super-shift assays using a DR2 ("direct repeat" 2) RA response element demonstrated DNA-binding of RARalpha, RARgamma, RXRalpha and RXRbeta in nuclear extracts of FTC-133 and anaplastic HTh74 cells. Use of a DR5 RA response element revealed no difference in DNA binding. In supershift assays with a DR4 T3 response element, we found DNA-binding by TRalpha1, TRalpha2, and TRbeta. Northern-blot analysis showed low expression of RXRbeta mRNA in FTC-133 and of TRalpha1 mRNA in FTC-133 and FTC-238 cells. Using RT-PCR, we detected mRNA for RARalpha, RARbeta, RARgamma, RXRalpha, and RXRbeta in the 4 cell lines and in human thyroid-carcinoma samples. RARbeta mRNA was reduced in FTC-238 cells and RXRbeta mRNA was decreased in anaplastic C643 cells and 9 of 12 tumor samples. Differential RA regulation of RA-receptor-mRNA expression was observed in the various cell lines. Thus, RA and T3 nuclear receptors are present in thyroid-carcinoma cell lines or tissues, albeit with cell-line and tumor-dependent variations; in the cell lines, they were shown to be functional with respect to DNA and/or ligand binding.

Searching for pathogenic epitopes in thyroglobulin: parameters and caveats

Last year marked the 40th anniversary of the discovery that thyroglobulin (Tg) is a major autoantigen in autoimmune thyroiditis. The Tg molecule presents unique challenges for epitope mapping owing to its large size and extensive iodination. Consequently, pathogenic determinants have only recently been identified. Here, George Carayanniotis and Varada Rao summarize the approaches used to determine pathogenic Tg T-cell epitopes and discuss caveats in this unusual quest.

Cytokines and thyroid function

Cytokines play a crucial role in autoimmune thyroid disease (ATD) through various mechanisms. They are produced in the thyroid by intrathyroidal inflammatory cells, in particular lymphocytes, as well as by the thyroid follicular cells (TFC) themselves and may thus act in a cascade to enhance the autoimmune process (Fig. 1). Cytokines upregulate the inflammatory reaction through stimulation of both T and B cells, resulting in antibody production and tissue injury. In addition, intrathyroidal cytokines induce immunological changes in TFC including enhancement of both major histocompatibility complex (MHC) class I and class II molecule expression, and upregulation of adhesion and complement regulatory molecule expression. Cytokines can also modulate both growth and function of TFC and have a role in extrathyroidal complications of ATD, most importantly thyroid-associated ophthalmopathy (TAO), where they induce fibroblast proliferation and enhance the production of glycosaminoglycans (GAG), resulting in proptosis and the other clinical features of the disease. In addition to these effects, exogenous administration of cytokines has been associated with impairment of thyroid function ranging from the appearance of autoantibodies alone to the development of frank thyroid dysfunction. Cytokines have also been implicated in subacute thyroiditis (SAT) and amiodarone-induced thyroid dysfunction, as well as in thyroid function abnormalities occurring in patients with non-thyroidal illnesses (NTI). Genetic variations in cytokine genes represent potential risk factors for ATD, and disease associations have been described for polymorphisms in IL-1ra and TNF beta genes. Recent experimental evidence suggests the possibility of novel cytokine-based therapeutic approaches for ATD and its complications, in particular TAO.