Control of protein synthesis by thyrotropin and epidermal growth factor in human thyrocytes: role of morphological changes

Differential involvement of the actin cytoskeleton in differentiation and mitogenesis of thyroid cells: inactivation of Rho proteins contributes to cyclic adenosine monophosphate-dependent gene expression but prevents mitogenesis

In thyroid epithelial cells, TSH via cAMP induces a rounding up of the cells associated with actin stress fiber disruption, expression of differentiation genes and cell cycle progression. Here we have evaluated the role of small G proteins of the Rho family and their impact on the actin cytoskeleton in these different processes in primary cultures of canine thyrocytes. TSH and forskolin, but not growth factors, rapidly inactivated RhoA, Rac1, and Cdc42, as assayed by detection of GTP-bound forms. Using toxins that inactivate Rho proteins (toxin B, C3 exoenzyme) or activate them [cytotoxic necrotizing factor 1 (CNF1)], in comparison with disruption of the actin cytoskeleton by dihydrocytochalasin B (DCB) or latrunculin, two unexpected conclusions were reached: 1) inactivation of Rho proteins by cAMP, by disorganizing actin microfilaments and inducing cell retraction, could be necessary and sufficient to mediate at least part of the cAMP-dependent induction of thyroglobulin and thyroid oxidases, but only partly necessary for the induction of Na(+)/I(-) symporter and thyroperoxidase; 2) as indicated by the effect of their inhibition by toxin B and C3, some residual activity of Rho proteins could be required for the induction by cAMP-dependent or -independent mitogenic cascades of DNA synthesis and retinoblastoma protein (pRb) phosphorylation, through mechanisms targeting the activity, but not the stimulated assembly, of cyclin D3-cyclin-dependent kinase 4 complexes. However, at variance with current concepts mostly derived from fibroblast models, DNA synthesis induction and cyclin D3-cyclin-dependent kinase 4 activation were resistant to actin depolymerization by dihydrocytochalasin B in canine thyrocytes, which provides a first such example in a normal adherent cell.

Molecular pathogenesis of thyroid cancer

A number of molecular abnormalities have been described in association with the progression from normal thyroid tissue to benign adenomas to well-differentiated and finally anaplastic epithelial thyroid cancer. These include upregulation of proliferative factors, such as growth hormones and oncogenes, downregulation of apoptotic and cell-cycle inhibitory factors, such as tumor suppressors, disruption of normal cell-to-cell interactions, and cellular immortalization. The progression model for thyroid carcinoma has not been proven, but evidence suggests that an evolutionary molecular process is involved, especially in the development of follicular thyroid cancers for which there are distinct intermediate phenotypes. We present a comprehensive evaluation of factors involved in thyroid tumorigenesis and attempt to describe preliminary attributes of a progression model. The organization of this model should also provide a template for the incorporation of new information as it is derived from large-scale genomic studies.

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.

Thyroid stimulating hormone upregulates secretion of cathepsin B from thyroid epithelial cells

Constant levels of thyroid hormones in the blood are principal requirements for normal vertebrate development. Their release depends on the regulated proteolysis of thyroglobulin which is extracellularly stored in the follicle lumen under resting conditions. Thyroglobulin is proteolytically degraded to a major part in lysosomes, but in part also extracellularly leading to the release of thyroxine. Extracellularly occurring lysosomal enzymes are most probably involved in the proteolytic release of thyroxine. In this study we have analyzed the secretion of cathepsin B by thyroid follicle cells (primary cells as well as FRTL-5 cells) and its regulation by thyroid stimulating hormone, which stimulated the secretory release of the proenzyme as well as of mature cathepsin B. Within one to two hours of stimulation with thyroid stimulating hormone, the cathepsin B activity associated with the plasma membrane increased significantly. This increase correlated closely with the localization of lysosomes in close proximity to the plasma membrane of cultured thyrocytes as well as with the thyroxine liberating activity of thyrocyte secretion media. These observations indicate that thyroid stimulating hormone induces the secretion of cathepsin B, which contributes to the extracellular release of thyroxine by thyrocytes.

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.

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.

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.

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.

Effects of thyroid-stimulating hormone on the replicative cycle of vesicular stomatitis virus in primary cultured sheep thyroid cells

Sheep thyroid cells in primary culture are highly sensitive to thyroid stimulating hormone (TSH). We infected thyroid cells with vesicular stomatitis virus (VSV) in the course of studies on cell polarity, and we found that TSH augmented the speed of the replicative cycle of VSV but did not affect the final yield of the virus. Three hours post-infection, at a multiplicity of infection of 10, the virus was detected in the cell layer of the cultures incubated with TSH but not in those without TSH. Five hours post-infection, there was a 100-fold increase in the medium in the yield of VSV and a 60-fold increase in the cell-associated virus in the TSH-treated cells compared with the cells without TSH. We found that the early stages of infection were accelerated by TSH. This effect appears to be due, at least in part, to increased processing in the lysosomes, thus allowing deposition of the transcriptionally-active nucleocapsid into the cytoplasm. These studies show that TSH is critically involved in the infectivity of VSV and that by manipulating cell culture conditions, an increased rate of virus production can be achieved.

Nitric oxide donors inhibit iodide transport and organification and induce morphological changes in cultured bovine thyroid cells

Nitric oxide (NO) has been proposed as an intracellular signal in the thyroid. The NO effect on function and morphology of bovine thyroid follicles in culture was analyzed by using the NO donors sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO). Both NO donors induced a concentration-dependent NO release measured by the nitrite accumulation in the culture medium. The SNP (10 to 500 micromol/L) treatment for 24 hours significantly inhibited the uptake, organification and transport of iodide in a concentration-dependent manner. When SNP (50 micromol/L) was withdrawn from the culture medium after 24 hours' incubation, iodide uptake and organification were partially recovered at 24 hours and reached the control value at 48 hours, indicating a reversible effect of SNP. A possible involvement of cyanide in the SNP inhibitory effect was excluded because incubation of follicles with potassium cyanide (KCN) at concentrations estimated to be present in the medium (40 and 80 micromol/L) for 24 hours did not modify iodide uptake and organification. The GSNO (10 to 500 micromol/L) treatment for 24 hours also reduced the iodide uptake, organification and transport in a concentration-dependent manner. A significant inhibition of iodide organification was induced after incubation with 1000 micromol/L of N2, 2'-O-dibutyrylguanosine 3':5'-cyclic monophosphate ([Bu]2cGMP). Morphological evaluation by light microscopy revealed that the incubation with NPS or GSNO (500 micromol/L) produced cellular dispersion with loss of follicular cell aggregates that was evident at 96 hours exposure. Cell viability was not altered by 10-500 micromol/L SNP or GSNO (80% to 85%). We concluded that long-term NO exposure induces functional and morphological modifications compatible with a loss of differentiation in thyroid follicles. These observations further support a role of NO in the regulation of the thyroid function.

Expression of the proliferating cell nuclear antigen (PCNA) in the rat thyroid gland after exposure to bromide

Analysis of expression of the proliferating cell nuclear antigen (PCNA) was used to determine the presumed hyperplastic character of morphological changes in the rat thyroid evoked by bromide administration. Male rats fed by a standard diet with determined iodine and bromine content were given potassium bromide. Control animals received no bromide. Experimental animals were given 10, 50 or 100 mg Br- per 11 drinking water for 16 and 66 days, or 100, 200, 400 mg Br-/l drinking water for 133 days. The thyroids of treated animals showed activation of growth of the epithelial follicular component as well as diffuse and focal microfollicular rearrangement of the parenchyma with higher follicular cells accompanied by a decrease of the amount of colloid even at low bromine concentrations (10-100 mg Br-/l drinking water). Using the PCNA-LI index (PCNA-positive nuclei.100/total number of follicular cell nuclei in the section), immunohistochemical analysis of PCNA in the nuclei of the follicular cells was carried out in parrafin sections. The index was significantly higher in bromide exposed animals (P < 0.01) and correlated well with the histological changes, with bromide concentration and with a increased mitotic activity of the follicular cells. PCNA analysis showed that morphological changes resembling a parenchymatic goitre reflect a microfollicular rearrangement of the thyroid of rats exposed to bromide and have the character of hyperplasia owing to the increased mitotic activity of the follicular epithelium.

Protein tyrosine phosphorylation influences adhesive junction assembly and follicular organization of cultured thyroid epithelial cells

The follicular histoarchitecture of the thyroid forms the anatomical basis for thyroid physiology and is commonly disturbed in diseases of the thyroid. We have used cultured porcine thyroid cells to study thyroid epithelial morphogenesis and its regulation. When cultured in the presence of TSH, freshly isolated thyroid cells reorganize to form follicles within three-dimensional cell aggregates. However, when established follicles are washed into TSH-free medium, thyroid cells spread and migrate to convert follicles into confluent epithelioid monolayers, activating morphogenetic mechanisms, such as cell locomotility, that may be relevant to thyroid inflammation and tumor invasiveness. The phenomenon of follicle to monolayer conversion, therefore, provides an opportunity to identify morphogenetic mechanisms that 1) must be tonically inhibited to maintain follicular organization and 2) may contribute to pathogenetic disturbances of follicular architecture when functioning aberrantly. In this study we found that follicle to monolayer conversion is associated with an increase in cellular phosphotyrosine. This was particularly evident at nascent focal adhesions (cell-substrate adhesive junctions) and later at cell-cell junctions. Focal adhesion assembly was accompanied by reorganization of the actin cytoskeleton, with the appearance of prominent stress fibers. Genistein, a potent inhibitor of protein tyrosine kinases, inhibited the accumulation of phosphotyrosine, focal adhesion assembly, and follicle to monolayer conversion. We conclude that tyrosine phosphorylation exerts an important influence on thyroid epithelial organization in culture, at least partly mediated through regulation of focal adhesion assembly.

Stimulation of proliferation and inhibition of function of xenotransplanted human thyroid tissue by epidermal growth factor

A stimulation of thyroid epithelial cell proliferation by epidermal growth factor (EGF) has been repeatedly reported in different in vitro systems. Furthermore, a suppression of thyroid epithelial cell function by EGF has been described in vitro. In order to investigate the effects of EGF on the thyroid in vivo, human Graves' disease tissue was transplanted to 59 nu/nu mice. EGF was given once, and over a period of 7 days 7 times intermittently or continuously by osmotic mini pumps to mice. 3-H-thymidine histoautoradiography of transplants showed an increased 3-H-thymidine incorporation of thyroid epithelial cells and mesenchymal cells, following each form of EGF application. Thyroid epithelial cell nuclear volume, which has previously been shown to be a parameter for thyroid epithelial cell function showed a decrease following EGF application. There was a tendency to a more intensive proliferation and differentiation following intermittent EGF application compared to continuous stimulation. These results demonstrate that EGF does stimulate proliferation of thyroid epithelial as well as mesenchymal cells in vivo. The growth stimulating effect of EGF is linked with a concomitant decrease of thyroid function in vivo. The latter is most likely due to the dedifferentiating action of EGF previously shown in in vitro systems.

p53 protein, PCNA staining, and DNA content in follicular neoplasms of the thyroid gland

Forty-nine follicular adenomas and 11 follicular carcinomas of the thyroid were investigated by immunohistochemistry for the expression of p53 protein and proliferating cell nuclear antigen (PCNA). The DNA ploidy and the S-phase fraction (SPF) of the neoplasms were analysed by flow cytometry. Twelve adenomas (24 per cent) and six carcinomas (55 per cent) were DNA non-diploid (P = 0.07). The carcinomas had a higher proliferation rate than the adenomas when assessed either by SPF size (median 9.9 per cent vs. 2.9 per cent, P = 0.0003) or by PCNA staining intensity (P < 0.0001). Some scattered nuclei in two (4 per cent) adenomas and in three (27 per cent) carcinomas stained positively for p53 (P = 0.04). The two adenomas with positive staining for p53 were subserially sectioned, but no signs of invasion were found; both patients are alive and well 6 and 7 years after surgery. One of the two adenomas showing positive p53 nuclear staining was DNA aneuploid, and both were positive in PCNA staining, but their SPFs were low (2.1 and 3.3 per cent). We conclude that p53 protein expression is not confined to follicular carcinomas; scattered p53-positive cells may also be present in histologically and clinically benign follicular adenomas. Because both follicular adenomas and carcinomas may be DNA aneuploid and their SPF and PCNA staining distributions overlap, the distinction between follicular adenoma and carcinoma should still be based on histological criteria.

In the thyroid cells proliferation, differentiated and metabolic functions are under the control of different steps of the cyclic AMP cascade

In the course of studies to elucidate the complex network of interactions controlling FRTL5 cell proliferation, thyroid stimulating hormone (TSH)-independent mutants (M cells), have been obtained from FRTL5 cells by chemical mutagenesis. In the present studies, the role of TSH on the proliferation and on differentiated and metabolic functions in these mutant cells have been investigated and compared to their response to insulin-like growth factor I (IGF-I). The addition of IGF-I to M cells leads to normal stimulation of DNA synthesis. However, inspite of the fact that mutant cells display normal TSH receptors, TSH is unable to stimulate the proliferation of the M cells. Nevertheless, TSH is able to increase intracellular levels of cAMP leading to regulation of TSH function in the M cells. On the other hand, TSH does not influence iodide transport and actin filaments depolimerization in these cells. However, aminoacid transport, stimulated in wild-type FRTL5 cells by both TSH and IGFs, is under the control of IGFs but not of TSH in the mutant cells. Neither TSH or IGF-I modified the expression of c-fos proto-oncogene in the M cells, probably because of high constitutive expression. These data suggest that a crucial signalling step(s) required for TSH induced mitogenesis is impaired in the M cells, and that this signalling step is not required for IGF-I induced mitogenesis.

General inhibition by transforming growth factor beta 1 of thyrotropin and cAMP responses in human thyroid cells in primary culture

Transforming growth factor beta 1 (TGF beta 1) mRNA has previously been identified in human thyroid cells and this agent has been shown to inhibit DNA synthesis in thyroid cells of some other species. In normal human thyroid cells in primary culture, TGF beta 1 inhibited inconstantly the low basal DNA synthesis and strongly the stimulation of DNA synthesis by epidermal growth factor (EGF) and serum, and by thyroid-stimulating hormone (TSH) acting through cAMP. This inhibition, by TGF beta 1, of the TSH and cAMP-dependent DNA synthesis was associated with an inhibition of PCNA (proliferating cell nuclear antigen) synthesis. TGF beta 1 almost completely abolished the cAMP induced stimulation of iodide uptake and thyroperoxidase synthesis. It thus, like EGF, also acts as a dedifferentiating agent. Investigation of the pattern of protein synthesis by two-dimensional gel electrophoresis revealed that while TGF beta 1, by itself, increased the synthesis of only one protein, a tropomyosin isoform, it inhibited most of the effects of cAMP on protein synthesis (35 out of 45 cAMP-regulated proteins were affected). It also reversed the effect of cAMP on the morphology of the thyrocytes. The fact that TGF beta 1 did not affect the increase in cAMP provoked by TSH in human thyroid cells while inhibiting most of the effects of dibutyryl cAMP in these cells suggests an action at a step distal to cAMP generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of thyroid cell-cell and cell-substrate adhesion by thyrotropin

Preservation of cell aggregation is necessary for thyroid follicular differentiation in vitro and requires stimulation by thyrotropin (TSH). We have tested the hypothesis that TSH preferentially increases thyroid cell-cell adhesion relative to cell-substrate adhesion. Cell-cell adhesion was measured in short-term suspension cultures by the decrease in the fraction of single cells remaining in culture (free cell ratio, FCR). When incubated in medium alone freshly isolated cells showed a progressive fall in FCR but this was accelerated by TSH and the cyclic AMP analog, 8-(4-chlorophenylthio)cyclic AMP. Aggregation was dependent upon extracellular Ca2+ and also promoted by a cell-free membrane extract. In contrast, attachment of cells to plastic dishes treated for tissue culture was not affected by TSH. We conclude that thyroid cells possess a TSH-sensitive cell adhesion system. The preferential increase in cell-cell adhesion may be one mechanism by which TSH stimulates the formation and preservation of follicles in vitro.

Activation of the cyclic AMP cascade as an oncogenic mechanism: the thyroid example

Three cascades activate thyroid cell proliferation: the EGF-protein tyrosine kinase pathway, the phorbol ester-protein kinase C pathway and the thyrotropin-cyclic AMP pathway. While the first 2 cascades converge early, they remain distinct from the cyclic AMP cascade until very late in G1. The cyclic AMP cascade is characterized by an early and transient expression of c-myc, which may explain why it induces proliferation and differentiation expression. Constitutive activation of this cascade causes growth and hyperfunction, ie, hyperfunctioning adenomas. The various possible defects that could lead to such a constitutive activation are discussed.