Effects of transforming growth factor alpha (TGF-alpha) on DNA synthesis and thyrotropin-induced iodine metabolism in cultured porcine thyroid cells

Prolonged effects of beta-estradiol 3-benzoate on thyroid tumorigenesis in gonadectomized rats pretreated with N-bis(2-hydroxypropyl)nitrosamine

The prolonged modulatory effects of beta-estradiol 3-benzoate (EB), a synthetic estrogenic compound, were investigated in a rat two-stage thyroid tumorigenesis model. One week after a single subcutaneous (s.c.) injection of N-bis(2-hydroxypropyl)nitrosamine, gonadectomized F344 rats of both sexes were s.c. implanted with fused pellets containing EB for 32 weeks. Doses of EB at 0, 0.004, 0.02 and 0.1mg were achieved by varying the ratio of EB to cholesterol in the pellet. Major organs including the thyroid, pituitary, liver, kidneys, uterus and brain were weighed and histopathological observation was performed. Serum was assayed for triiodothyronine (T(3)), thyroxine (T(4)) and thyroid-stimulating hormone (TSH). Thyroid weights were increased by the EB pellet implantation in a dose-dependent manner and significantly (P<0.05) elevated in the 0.1mg EB male group and in the 0.02 and 0.1mg EB female groups. The EB treatments dose-dependently suppressed serum T(4) levels and inversely elevated serum TSH levels in both sexes but without statistical significance in females. Histopathologically, EB increased the occurrence of thyroid proliferative lesions in males and showed a tendency for increase in females. Interestingly, the effect of EB was more intensive in males than in females, even the lowest dose inducing a follicular carcinoma in a male. These results, thus indicate the possible contribution of prolonged EB stimulation at lower doses to thyroid tumorigenesis without additional promotive condition.

Sodium iodide symporter in health and disease

Radioiodine-concentrating activity in thyroid tissues has allowed the use of radioiodine as a diagnostic and therapeutic agent for patients with thyroid disorders such as well-differentiated thyroid cancer. However, some extrathyroidal tissues also take up radioiodine, contributing to unwanted side effects of radioiodine therapy. Now that the molecule that mediates radioiodine uptake, the sodium iodide symporter (NIS), has been cloned and characterized, it may be possible to develop novel strategies to differentially modulate NIS expression and/or activity, enhancing it in target tissues and impeding it in others. In addition to restoring NIS expression/activity to ensure sufficient radioiodine uptake for the diagnosis and treatment of advanced thyroid cancers, we envision that it may be possible to selectively increase or confer NIS expression/activity in tumors of nonthyroidal tissues to facilitate the use of radioiodine in their diagnosis and treatment. We also consider the molecular basis of thyroid and nonthyroid disorders that may be complicated by NIS deregulation. Finally, we explore the use of NIS as an imaging reporter gene to monitor the expression profile of the transgene in transgenic mouse animal models and in patients undergoing gene therapy clinical trials.

Growth factor expression in cold and hot thyroid nodules

Hot thyroid nodules (HTNs) are predominantly caused by constitutively activating thyrotropin receptor (TSHR) mutations leading to an activation of the cyclic adenosine monophosphate (cAMP)-cascade that stimulates growth and function of thyroid epithelial cells and confers growth advantage. In contrast to HTNs, the molecular etiology of szintigraphically cold thyroid nodules (CTNs) is largely unknown. An increased prevalence of toxic multinodular goiters in iodine-deficient regions has been reported. Growth factors increase during early stages of iodine deficiency in rats. These growth factors could modulate the proliferation of thyrocytes. In order to determine if and which growth factors could modulate the increase in thyroid epithelial cell proliferation in late stages of CTNs and HTNs we investigated epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), and TGF-beta1 concentrations by enzyme-linked immunosorbant assay (ELISA) in CTNs (n = 7), HTNs (n = 9), and their normal surrounding tissue (ST). Insulin-like growth factor-1 (IGF-1) was determined in CTNs (n = 5) and HTNs (n = 10) and their surrounding tissues by radioimmunoassay (RIA). We found lower concentrations of all investigated growth factors and iodine in CTNs compared to surrounding normal tissues (ST). Only iodine showed a significant difference. Furthermore, we found significantly lower concentrations of EGF and TGF-beta1 concentration in HTNs compared to their STs. Differences of TGF-alpha and IGF-1 were not significant. In conclusion, low EGF, TGF-alpha, and IGF-1 concentrations in most CTNs in spite of low iodine concentrations argue against a pathophysiologic role of EGF, TGF-alpha, or IGF-1 in late stages of CTNs. The low EGF, TGF-alpha, and IGF-1 concentrations in HTNs irrespective of their clonal origin or the presence or absence of activating mutations argue for increased cAMP as the primary cause for thyroid epithelial cell proliferation in established HTNs. However, the pathophysiologic significance of low TGF-beta1 concentrations in CTNs and HTNs remains to be elucidated. It might be possible that growth factors like EGF, TGF-alpha, TGF-beta1, and IGF-1 play a more prominent role during early clonal expansion and that aberrant intrinsic signaling through a somatic mutation (e.g., TSHR for HTNs) confers the predominant selective growth advantage in later stages of HTNs or CTNs.

Estradiol decreases iodide uptake by rat thyroid follicular FRTL-5 cells

Estradiol has well-known indirect effects on the thyroid. A direct effect of estradiol on thyroid follicular cells, increasing cell growth and reducing the expression of the sodium-iodide symporter gene, has been recently reported. The aim of the present investigation was to study the effect of estradiol on iodide uptake by thyroid follicular cells, using FRTL-5 cells as a model. Estradiol decreased basal iodide uptake by FRTL-5 cells from control levels of 2.490 +/- 0.370 to 2.085 +/- 0.364 pmol I-/microg DNA at 1 ng/ml (P<0.02), to 1.970 +/- 0.302 pmol I-/microg DNA at 10 ng/ml (P<0.003), and to 2.038 +/- 0.389 pmol I-/microg DNA at 100 ng/ml (P<0.02). In addition, 4 ng/ml estradiol decreased iodide uptake induced by 0.02 mIU/ml thyrotropin from 8.678 +/- 0.408 to 7.312 +/- 0.506 pmol I-/microg DNA (P<0.02). A decrease in iodide uptake by thyroid cells caused by estradiol has not been described previously and may have a role in goiter pathogenesis.

Molecular pathogenesis of thyroid nodules and cancer

Tumours derived from the thyroid follicular epithelium represent an informative model for understanding the molecular pathogenesis of multistage tumourigenesis, which is the prevailing theory on cancer development and progression nowadays. The early stages of thyroid tumour development appear to be the consequence of the activation or 'de novo' expression of several proto-oncogenes or growth factor receptors, such as ras, ret, NTRK, met, gsp and the thyrotropin (TSH) receptor. Alterations in the expression pattern of these genes are associated with the development of differentiated neoplasms, ranging from benign toxic adenomas (gsp and TSH receptor), to follicular (ras) and papillary (ret/PTC, NTRK, met) carcinomas. They may all be considered to be early events of thyroid cell transformation and, for some, experimental evidence derived from gene transfer studies supports this hypothesis. Alterations in tumour suppressor genes (p53, Rb) are associated instead with the most aggressive and poorly differentiated forms of thyroid cancer, indicating that, in the thyroid tumourigenic process, they represent late genetic events. Specific environmental factors (iodine deficiency, ionizing radiations) have been shown to play a crucial role in promoting the development of thyroid cancer, influencing both its genotypic and phenotypic features. Interestingly, a high percentage of genetic lesions causing thyroid cancer originate from gene rearrangements and chromosomal translocations (ret/PTC, NTRK, Pax-8/PPARgamma) a finding which, being a rare event in most epithelial tumours, makes the molecular pathogenesis of thyroid cancer unique. The uninterrupted flow of information on the molecular genetics of thyroid nodules and cancer will broaden the correlation between genotype and phenotype and will also provide important information for the development of more accurate preoperative diagnostic tools and more efficient treatment choices for the different forms of thyroid cancer.

Estradiol increases proliferation and down-regulates the sodium/iodide symporter gene in FRTL-5 cells

Goiter (increased thyroid gland size) is more prevalent in women than men, even in areas where iodine levels in the diet are sufficient. We investigated a possible role of estrogen on thyroid follicular cell growth using rat FRTL-5 thyroid follicular cells as a model. Estrogen receptor-alpha (ERalpha) messenger RNA was present in FRTL-5 cells using a RT-PCR assay and was confirmed by Western blot analysis. An estrogen-responsive reporter gene was transfected into FRTL-5 cells to test the functionality of the endogenous ERs. Estradiol increased the activity of the reporter gene, and the antagonist, ICI182780, inhibited ER-dependent transcription. To extend this analysis, we examined the effect of estradiol on FRTL-5 cell growth. Estradiol increased FRTL-5 cell growth in a time- and concentration-dependent manner in either the absence or presence of TSH. Because iodine is known to inhibit thyroid cell growth, the effect of estradiol on the expression of the sodium/iodide symporter (NIS) was assessed as a potential target of estrogen action. Estradiol blocked TSH-induced NIS expression, and treatment of cells with estradiol and ICI182780 restored TSH-induced NIS expression to normal levels. These data demonstrate that FRTL-5 cells contain functional ERs that enhance cell growth and inhibit expression of the NIS. The demonstration of a direct effect of estradiol on thyroid follicular cells raises the possibility that it may play a role in the sexually dimorphic prevalence of goiter.

Regulation of transforming growth factor beta 1 messenger ribonucleic acid expression in porcine thyroid follicles in vitro by growth factors, iodine, or delta-iodolactone

Transforming growth factor beta 1 (TGF beta 1) is an autocrine growth factor for thyrocytes and is supposed to be the mediator of iodine-induced growth inhibition of thyroid epithelial cells, but this is still controversial. We further investigated this hypothesis using intact porcine thyroid follicles ex vivo in a three-dimensional culture system. In this culture system it has been shown previously that both iodide as well as delta-iodolactone, the putative iodocompound mediating thyroid cell proliferation, inhibit growth of these follicles. We measured the amount of TGF beta 1 mRNA expression in these follicles after treatment either with thyrotropin (TSH), epidermal growth factor (EGF), or transforming growth factor alpha (TGF alpha) for growth stimulation or with inorganic iodine or delta-iodolactone in concentrations known to inhibit growth. TGF beta 1-mRNA was detected by Northern blot analysis. The known major transcript of 2.5 kb was detected in a steady state level up to 48 hours in untreated thyroid follicles. EGF and TGF alpha (5 ng/mL each) enhanced TGF beta 1 mRNA about threefold within 4 and 8 hours. This increase of TGF beta 1 mRNA was slightly decreased by simultaneous incubation with delta-iodolactone (1 microM) or iodide (40 microM KI). In contrast, both TSH (1 mU/mL) and forskolin (16 microM) decreased TGF beta 1 mRNA expression to about 70%, and this effect was abolished when follicles were pretreated with iodide (40 microM KI) in a concentration known to inhibit TSH action on cyclic adenosine monophosphate (cAMP) formation and proliferation. Iodide or delta-iodolactone alone had no significant effect on basal TGF beta 1 mRNA expression. We conclude that the growth inhibitory effect of iodide as well as of delta-iodolactone is not mediated through TGF beta 1 in intact porcine thyroid follicles ex vivo. The stimulatory effect of EGF and TGF alpha on TGF beta 1 expression might be related to extracellular matrix modulation during proliferation.

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.