TGF beta inhibits rat thyroid cell proliferation without alterations in the expression of TSH-induced cell cycle-related genes

TGF-β promotes proliferation of thyroid epithelial cells in IFN-γ(-/-) mice by down-regulation of p21 and p27 via AKT pathway

IFN-γ(-/-) NOD.H-2h4 mice develop an autoimmune disease characterized by hyperplasia and proliferation of thyroid epithelial cells (TEC H/P). Proliferating TECs produce TGF-β, and IFN-γ inhibits TEC H/P. In the present study, cultured TECs were used to directly determine the mechanisms by which these cytokines act on TECs to result in proliferation or inhibition of proliferation. With TECs from IFN-γ(-/-) NOD.H-2h4 mice or mice expressing the dominant negative TGF-β type II receptor on TECs, TGF-β was shown to promote TEC proliferation and IFN-γ was shown to inhibit TEC proliferation in vitro. TGF-β may promote TEC proliferation by down-regulating antiproliferative molecules p21 and p27, whereas IFN-γ may inhibit proliferation by up-regulating antiproliferative molecules p18 and p21 and down-regulating the pro-proliferative molecule cyclin D. Inhibition of AKT abolished the effect of TGF-β on p21 and p27, resulting in similar proliferation of TGF-β-treated and control TECs. Increased expression of proliferating cell nuclear antigen (PCNA), TGF-β, and p-AKT and decreased expression of p21 and p27 by proliferating TECs correlated with the proliferative state of TEC H/P. Taken together, the results suggest that TGF-β promotes TEC proliferation by down-regulating p21 and p27 via the AKT pathway in IFN-γ(-/-) NOD.H-2h4 mice, which may have significant implications for development of effective therapeutic strategies targeting the TGF-β and AKT pathways for treatment of hyperplasia and/or neoplasia.

Role of transforming growth factor beta in the regulation of thyroid function and growth

Transforming growth factor beta (TGF-beta) exists in nature as three isoforms. They exert their effects by binding to a type II receptor located at the cell membrane. The TGF-beta-type II receptor complex then recruits type I receptor, and this new complex stimulates the phosphorylation of Smads 2 and 3, which are subsequently transferred to the nucleus, where they regulate gene transcription. The thyroid gland expresses the TGF-beta1 gene mRNA and synthesizes the protein, which under physiologic conditions regulates thyroid growth and function. Different studies have demonstrated that TGF-beta1 inhibits cell proliferation and a number of functional parameters. These include cyclic adenosine monophosphate (AMP) formation, iodine uptake and organification, hormone secretion, and the expression of thyroglobulin, thyroid peroxidase, and Na(+)/I(-) symporter. The expression of the TGF-beta1 gene and protein may be stimulated by iodine under normal conditions. Since TGF-beta1 mimics some of the inhibitory actions of iodine, its participation in thyroid autoregulation has been proposed; however, this concept is still debated. In thyroid tumors, the inhibitory action of TGF-beta1 on cell proliferation is progressively lost as the tumor becomes more undifferentiated. The alterations in the signaling pathway of TGF-beta1 are not the same in tumors from different species. Even within the same species, such as the pig thyroid, the results may be different depending on whether monolayers or follicular suspensions are employed. The data suggest that it is not entirely possible to apply the results obtained in animal studies to normal or pathological human thyroid tissue. More studies are required to provide the information needed to develop treatments, based on targeting the signaling pathway of TGF-beta1, for undifferentiated thyroid cancer and other thyroid diseases.

TGF-beta control of rat thyroid follicular cells differentiation

TGF-beta1 is a potent inhibitor of growth and DNA synthesis in thyroid cells. It has also been shown that TGF-beta1 inhibits thyrocyte function. The functional inhibition is represented by a downregulation of thyroid specific genes, such as Na(+)/I(-) symporter (NIS), thyroglobulin (TG) and thyroperoxidase (TPO). The transcriptional control of these genes is mediated by thyroid-specific transcription factors: thyroid transcription factor-1 (TTF-1) and PAX-8. It has been shown that Smad proteins play a pivotal role in the intracellular signal transduction of the TGF-beta family members. In this paper, the functional relevance of Smad4, in the control of thyroid differentiation genes and thyroid-specific transcription factors, has been investigated. The data obtained provides, for the first time, evidence that D.N. Smad4-100T is capable of blocking TGF-beta1 action in the regulation of thyroid-specific genes expression. Such action is possible by blocking nuclear translocation of Smad4 and Smad2.

Influence of transforming growth factor beta1 (TGF-beta1) on the behaviour of porcine thyroid epithelial cells in primary culture through thrombospondin-1 synthesis

Transforming growth factor beta1 (TGF-beta1) is a secreted polypeptide that is thought to play a major role in the regulation of folliculogenesis and differentiation of thyroid cells. On porcine thyroid follicular cells cultured on plastic substratum, TGF-beta1, in a concentration-dependent way, promoted the disruption of follicles, cell spreading, migration and confluency by a mechanism that did not involve cell proliferation. TGF-beta1 strongly activated the production of thrombospondin-1 and (alpha)vbeta3 integrin in a concentration-dependent manner whereas the expression of thyroglobulin was unaffected. Anisomycin, an inhibitor of protein synthesis, inhibited the effect of TGF-beta1 on cell organization. Thrombospondin-1 reproduced the effect of TGF-beta1. In the presence of thrombospondin-1 cells did not organize in follicle-like structures but, in contrast, spreaded and reached confluency independently of cell proliferation. This effect is suppressed by an RGD-containing peptide. The adhesive properties of thrombospondin-1 for thyroid cells were shown to be mediated by both the amino-terminal heparin-binding domain and the RGD domain of thrombospondin-1. Adhesion was shown to involve (alpha)vbeta3 integrin. The results show that TGF-beta1 exerted an influence upon function and behaviour of follicle cells partly mediated by the synthesis of thrombospondin-1 and of its receptor (alpha)vbeta3 integrin.

Induction of apoptosis in porcine thyroid follicles by transforming growth factor beta1 and epidermal growth factor

For thyroid cells in culture DNA fragmentation and morphological changes related to apoptosis were first described in dog thyroid cells after deprivation of serum, epidermal growth factor or thyrotropin. With intact porcine thyroid follicles in three-dimensional culture, the effect of deprivation of growth factors and of incubation with transforming growth factor beta1 (TGF-beta1), epidermal growth factor (EGF), thyrotropin (TSH) or insulin-like growth factor I (IGF-I) on the incidence of apoptosis was studied. Thyroid follicles were embedded in growth factor-depleted Matrigel and cultured in serum-free medium with or without growth factors for 7 days followed by incubation for 4, 24 and 72 h with TGF-beta1 (2 or 5 ng/mL). The percentage of apoptotic cells was determined by direct counting in electron-microscopy. Approximately 1% of apoptotic bodies could be detected in unstimulated follicles. This was unchanged in the presence of TSH (1 mU/mL) or IGF (10 ng/mL) but significantly increased up to 3.99 +/- 1.24% with 2 ng/mL of EGF. After incubation with TGF-beta apoptosis increased dose-dependently to 4.05 +/- 0.67% with 2 ng/mL TGF-beta1 and 5.16 +/- 1.75% with 5 ng/mL TGF-beta1. The incidence of necrotic cells remained constant at about 1 to 2%. Preincubation of follicles with 2 ng/mL of EGF followed by incubation with 5 ng/mL TGF-beta1 increased the rate of apoptic bodies up to 13.19 +/- 1.9%. We conclude that growth factor depletion in thyroid follicles in three-dimensional culture does not lead to apoptosis. TGF-beta1, however, induces apoptosis even in quiescent thyroid follicular cells and is significantly more pronounced in growing thyroid cells. EGF, which is a dedifferentiating growth factor for thyroid cells, also induces apoptosis. As EGF enhances TGF-beta1 mRNA and protein in thyroid follicular cells, the induction of apoptosis by EGF might also be due to TGF-beta1.

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.

The phosphatase inhibitor okadaic acid stimulates the TSH-induced G1-S phase transition in thyroid cells

Protein phosphorylation plays an essential role in regulating many cellular processes in eukaryotes. Signal transduction mechanisms that are reversibly controlled by protein phosphorylation require also protein phosphatases (PPs). Okadaic acid (OA), which is a potent inhibitor of protein phosphatase 2A (PP2A) and protein phosphatase 1, elicits phosphorylation of many proteins in unstimulated cells and induces different cellular responses, including transcriptional activation, shape changes, and pseudomitotic state. In this study, the effects of OA on rat thyroid cells (FRTL-5 strain) were analyzed to evaluate the role of serine/threonine phosphatases in hormone-induced thyroid cell proliferation. OA at a concentration range between 0.1 and 1 nM stimulated thyroid cell growth. Furthermore, 0.25 nM OA increased about 3.5-fold the thyrotropin (TSH)-induced DNA synthesis in quiescent cells. OA treatment also stimulated cell proliferation induced by drugs that mimic TSH effect, such as 8Br-cAMP and cholera toxin, suggesting that PP2A activity was relevant in the cAMP pathway activated by the hormone. Flow cytometry experiments showed that OA significantly increased the fraction of TSH-stimulated quiescent cells entering the S phase. In order to define the mechanisms underlying the observed stimulatory effect of OA on thyroid cell growth, expression of genes relevant in the G1-S phase transition was evaluated. A 2-fold increase in the level of cyclin D1 mRNA expression was found by Northern blot analysis in OA-treated cells. Although cdk2 gene expression was not modulated by the same OA treatment, an increase in Cdk2 protein was revealed by immunoprecipitation experiments. Moreover, OA modifies the phosphorylation pattern of the tumor suppressor retinoblastoma protein, a key event in the G1-S phase transition. Therefore, these experiments reveal that PP2A phosphatases play an important role in thyroid cell growth and can act at multiple sites in the TSH pathways driving cells to S phase.