Role of the different mitogen-activated protein kinase subfamilies in the stimulation of dog and human thyroid epithelial cell proliferation by cyclic adenosine 5'-monophosphate and growth factors

MiR-7-5p inhibits thyroid cell proliferation by targeting the EGFR/MAPK and IRS2/PI3K signaling pathways

The aberrant expression of miRNAs is often correlated to tumor development. MiR-7-5p is a recently discovered downregulated miRNA in thyroid papillary carcinoma (PTC). The goal of this project was to characterize its functional role in thyroid tumorigenesis and to identify the targeted modulated pathways. MiR-7-5p overexpression following transfection in TPC1 and HT-ori3 cells decreased proliferation of the two thyroid cell lines. Analysis of global transcriptome modifications showed that miR-7-5p inhibits thyroid cell proliferation by modulating the MAPK and PI3K signaling pathways which are both necessary for normal thyroid proliferation and play central roles in PTC tumorigenesis. Several effectors of these pathways are indeed targets of miR-7-5p, among which EGFR and IRS2, two upstream activators. We confirmed the upregulation of IRS2 and EGFR in human PTC and showed the existence of a negative correlation between the decreased expression of miR-7-5p and the increased expression of IRS2 or EGFR. Our results thus support a tumor-suppressor activity of miR-7-5p. The decreased expression of miR-7-5p during PTC tumorigenesis might give the cells a proliferative advantage and delivery of miR-7-5p may represent an innovative approach for therapy.

Switch in signaling control of mTORC1 activity after oncoprotein expression in thyroid cancer cell lines

CONTEXT

Thyroid growth is regulated by TSH and requires mammalian target of rapamycin (mTOR). Thyroid cancers frequently exhibit mutations in MAPK and/or phosphoinositol-3-kinase-related kinase effectors.

OBJECTIVE

The objective of the study was to explore the contribution of RET/PTC, RAS, and BRAF to mTOR regulation and response to mTOR inhibitors.

METHODS

PCCL3 cells conditionally expressing RET/PTC3, HRAS(G12V), or BRAF(V600E) and human thyroid cancer cells harboring mutations of these genes were used to test pathways controlling mTOR and its requirement for growth.

RESULTS

TSH/cAMP-induced growth of PCCL3 cells requires mTOR, which is stimulated via protein kinase A in a MAPK kinase (MEK)- and AKT-independent manner. Expression of RET/PTC3, HRAS(G12V), or BRAF(V600E) in PCCL3 cells induces mTOR but does not entirely abrogate the cAMP-mediated control of its activity. Acute oncoprotein-induced mTOR activity is regulated by MEK and AKT, albeit to differing degrees. By contrast, mTOR was not activated by TSH/cAMP in human thyroid cancer cells. Tumor genotype did not predict the effects of rapamycin or the mTOR kinase inhibitor AZD8055 on growth, with the exception of a PTEN-null cell line. Selective blockade of MEK did not influence mTOR activity of BRAF or RAS mutant cells. Combined MEK and mTOR kinase inhibition was synergistic on growth of BRAF- and RAS-mutant thyroid cancer cells in vitro and in vivo.

CONCLUSION

Thyroid cancer cells lose TSH/cAMP dependency of mTOR signaling and cell growth. mTOR activity is not decreased by the MEK or AKT inhibitors in the RAS or BRAF human thyroid cancer cell lines. This may account for the augmented effects of combining the mTOR inhibitors with selective antagonists of these oncogenic drivers.

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.

β-Arrestin-1 mediates thyrotropin-enhanced osteoblast differentiation

Thyrotropin (TSH) activation of the TSH receptor (TSHR), a 7-transmembrane-spanning receptor (7TMR), may have osteoprotective properties by direct effects on bone. TSHR activation by TSH phosphorylates protein kinases AKT1, p38α, and ERK1/2 in some cells. We found TSH-induced phosphorylation of these kinases in 2 cell lines engineered to express TSHRs, human embryonic kidney HEK-TSHR cells and human osteoblastic U2OS-TSHR cells. In U2OS-TSHR cells, TSH up-regulated pAKT1 (7.1±0.5-fold), p38α (2.9±0.4-fold), and pERK1/2 (3.1±0.2-fold), whereas small molecule TSHR agonist C2 had no or little effect on pAKT1 (1.8±0.08-fold), p38α (1.2±0.09-fold), and pERK1/2 (1.6±0.19-fold). Furthermore, TSH increased expression of osteoblast marker genes ALPL (8.2±4.6-fold), RANKL (21±5.9-fold), and osteopontin (OPN; 17±5.3-fold), whereas C2 had little effect (ALPL, 1.7±0.5-fold; RANKL, 1.3±0.6-fold; and OPN, 2.2±0.7-fold). β-Arrestin-1 and -2 can mediate activatory signals by 7TMRs. TSH stimulated translocation of β-arrestin-1 and -2 to TSHR, whereas C2 failed to translocate either β-arrestin. Down-regulation of β-arrestin-1 by siRNA inhibited TSH-stimulated phosphorylation of ERK1/2, p38α, and AKT1, whereas down-regulation of β-arrestin-2 increased phosphorylation of AKT1 in both cell types and of ERK1/2 in HEK-TSHR cells. Knockdown of β-arrestin-1 inhibited TSH-stimulated up-regulation of mRNAs for OPN by 87 ± 1.7% and RANKL by 73 ± 2.4%, and OPN secretion by 74 ± 10%. We conclude that TSH enhances osteoblast differentiation in U2OS cells that is, in part, caused by activatory signals mediated by β-arrestin-1.

The tyrosine kinase receptor c-met, its cognate ligand HGF and the tyrosine kinase receptor trasducers STAT3, PI3K and RHO in thyroid nodules associated with Hashimoto's thyroiditis: an immunohistochemical characterization

Hepatocyte growth factor (HGF) exerts proliferative activities in thyrocytes upon binding to its tyrosine kinase receptor c-met and is also expressed in benign thyroid nodules as well as in Hashimoto's thyroiditis (HT). The simultaneous expression of HGF/c-met and three trasducers of tyrosine kinase receptors (STAT3, PI3K, RHO) in both the nodular and extranodular tissues were studied by immunohistochemistry in 50 benign thyroid nodules (NGs), 25 of which associated with HT. The ligand/tyrosine kinase receptor pair HGF/c-met and the two trasducers PI3K and RHO were expressed in NGs, regardless of association with HT, with a higher positive cases percentage in HT-associated NGs compared to not HT-associated NGs (25/25 or 100% vs 7/25 or 28%; P<0.001). HGF, PI3K and RHO expression was only stromal (fibroblasts and endothelial cells), in all 32 reactive NGs, while c-met localization was consistently epithelial (thyrocyes). Immunoreactions for HGF, c-met, PI3K and RHO were also apparent in the extra-nodular tissue of HT specimens, where HGF and PI3K were expressed not only in stromal cells but also in thyrocyes along with the c-met. Finally, a positive correlation was observed between the proportion of HGF, c-met, PI3K follicular cells and the grade of lymphoid aggregates in HT. In conclusion, HGF, c-met, PI3K are much more frequently and highly expressed in HT compared to NGs, and among all NGs in those present in the context of HT. A paracrine effect of HFG/c-met on nodule development, based on the prevalent stromal expression, may be suggested along with a major role of HGF/c-met and PI3K in HT. Finally, the expression of such molecules in HT may be regulated by lymphoid infiltrate.

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.

[Renal cell carcinoma: antiangiogenic therapies and management of the complications. A case report]

Direct side effects of the inhibition of activation of VEGF receptors are well known and could be easily explained (HTA). The indirect toxicity of the inhibitors of tyrosinekinases is much less known and several hypotheses appear. Usually, the common side effects of the inhibitors of tyrosine-kinases can be easily managed and are reversible when the treatment is stopped. Their management is essentially based on prevention measures. It is necessary to stop definitively or temporarily the treatment in case of intensification of pre-existing comorbidities or side effects of rank 3 or 4. There is no predictive factor of treatment toxicity and, at the moment, there is thus no indication in a previous dose adaptation.

cAMP-dependent activation of mammalian target of rapamycin (mTOR) in thyroid cells. Implication in mitogenesis and activation of CDK4

How cAMP-dependent protein kinases [protein kinase A (PKA)] transduce the mitogenic stimulus elicited by TSH in thyroid cells to late activation of cyclin D3-cyclin-dependent kinase 4 (CDK4) remains enigmatic. Here we show in PC Cl3 rat thyroid cells that TSH/cAMP, like insulin, activates the mammalian target of rapamycin (mTOR)-raptor complex (mTORC1) leading to phosphorylation of S6K1 and 4E-BP1. mTORC1-dependent S6K1 phosphorylation in response to both insulin and cAMP required amino acids, whereas inhibition of AMP-activated protein kinase and glycogen synthase kinase 3 enhanced insulin but not cAMP effects. Unlike insulin, TSH/cAMP did not activate protein kinase B or induce tuberous sclerosis complex 2 phosphorylation at T1462 and Y1571. However, like insulin, TSH/cAMP produced a stable increase in mTORC1 kinase activity that was associated with augmented 4E-BP1 binding to raptor. This could be caused in part by T246 phosphorylation of PRAS40, which was found as an in vitro substrate of PKA. Both in PC Cl3 cells and primary dog thyrocytes, rapamycin inhibited DNA synthesis and retinoblastoma protein phosphorylation induced by TSH and insulin. Although rapamycin reduced cyclin D3 accumulation, the abundance of cyclin D3-CDK4 complexes was not affected. However, rapamycin inhibited the activity of these complexes by decreasing the TSH and insulin-mediated stimulation of activating T172 phosphorylation of CDK4. We propose that mTORC1 activation by TSH, at least in part through PKA-dependent phosphorylation of PRAS40, crucially contributes to mediate cAMP-dependent mitogenesis by regulating CDK4 T172-phosphorylation.

Protein kinase A and B-Raf mediate extracellular signal-regulated kinase activation by thyrotropin

Thyrotropin (TSH) regulates thyroid cell proliferation and function through cAMP-mediated signaling pathways that activate protein kinase A (PKA) and Epac/Rap1. The respective roles of PKA versus Epac/Rap1 in TSH signaling remain unclear. We set out to determine whether PKA and/or Rap1 mediate extracellular signal-regulated kinase (ERK) activation by TSH. Neither blocking Rap1 activity nor silencing the expression of Rap1 impaired TSH or forskolin-induced ERK activation in Wistar rat thyroid cells. Direct activation of Epac1 failed to stimulate ERK activity in starved cells, suggesting that Epac-induced Rap1 activity is not coupled to ERK activation in rat thyroid cells. By contrast, PKA activity was required for cAMP-stimulated ERK phosphorylation and was sufficient to increase ERK phosphorylation in starved cells. Expression of dominant-negative Ras inhibited ERK activation by TSH, forskolin, and N(6)-monobutyryl (6MB)-cAMP, a selective activator of PKA. Silencing the expression of B-Raf also inhibited ERK activation by TSH, forskolin, and 6MB-cAMP, but not that stimulated by insulin or serum. Depletion of B-Raf impaired TSH-induced DNA synthesis, indicating a functional role for B-Raf in TSH-regulated proliferation. Collectively, these results position PKA, Ras, and B-Raf as upstream regulators of ERK activation and identify B-Raf as a selective target of cAMP-elevating agents in thyroid cells. These data provide the first evidence for a functional role for B-Raf in TSH signaling.

Evidence for an association between thyroid-stimulating hormone and insulin-like growth factor 1 receptors: a tale of two antigens implicated in Graves' disease

Thyroid-stimulating hormone receptor (TSHR) plays a central role in regulating thyroid function and is targeted by IgGs in Graves' disease (GD-IgG). Whether TSHR is involved in the pathogenesis of thyroid-associated ophthalmopathy (TAO), the orbital manifestation of GD, remains uncertain. TSHR signaling overlaps with that of insulin-like grow factor 1 receptor (IGF-1R). GD-IgG can activate fibroblasts derived from donors with GD to synthesize T cell chemoattractants and hyaluronan, actions mediated through IGF-1R. In this study, we compare levels of IGF-1R and TSHR on the surfaces of TAO and control orbital fibroblasts and thyrocytes and explore the physical and functional relationship between the two receptors. TSHR levels are 11-fold higher on thyrocytes than on TAO or control fibroblasts. In contrast, IGF-1R levels are 3-fold higher on TAO vs control fibroblasts. In pull-down studies using fibroblasts, thyrocytes, and thyroid tissue, Abs directed specifically against either IGF-1Rbeta or TSHR bring both proteins out of solution. Moreover, IGF-1Rbeta and TSHR colocalize to the perinuclear and cytoplasmic compartments in fibroblasts and thyrocytes by confocal microscopy. Examination of orbital tissue from patients with TAO reveals similar colocalization to cell membranes. Treatment of primary thyrocytes with recombinant human TSH results in rapid ERK phosphorylation which can be blocked by an IGF-1R-blocking mAb. Our findings suggest that IGF-1R might mediate some TSH-provoked signaling. Furthermore, they indicate that TSHR levels on orbital fibroblasts are considerably lower than those on thyrocytes and that this receptor associates with IGF-1R in situ and together may comprise a functional complex in thyroid and orbital tissue.

TSH signalling and cancer

Thyroid cancers are the most frequent endocrine neoplasms and mutations in the thyrotropin receptor (TSHR) are unusually frequent. Here we present the state-of-the-art concerning the role of TSHR in thyroid cancer and discuss it in light of the cancer stem cell theory or the classical view. We briefly review the gene and protein structure updating the cancer related TSHR mutations database. Intriguingly, hyperfunctioning TSHR mutants characterise differentiated cancers in contrast to undifferentiated thyroid cancers which very often bear silenced TSHR. It remains unclear whether TSHR alterations in thyroid cancers play a role in the onset or they appear as a consequence of genetic instability during evolution, but the presence of functional TSHR is exploited in therapy. We outline the signalling network build up in the thyrocyte between TSHR/PKA and other proliferative pathways such as Wnt, PI3K and MAPK. This networks integrity surely plays a role in the onset/evolution of thyroid cancer and needs further research. Lastly, future investigation of epigenetic events occurring at the TSHR and other loci may give better clues for molecular based therapy of undifferentiated thyroid carcinomas. Targeted demethylating agents, histone deacetylase inhibitors combined with retinoids and specific RNAis may help treatment in the future.

Thyroid-stimulating hormone/cAMP-mediated proliferation in thyrocytes

Current research on thyrotropin-activated proliferation in the thyrocyte needs to be aimed at a better understanding of crosstalk and negative-feedback mechanisms with other proliferative pathways, especially the insulin/IGF-1-induced phosphoinositol-3 kinase pathway and the serum-induced MAPK or Wnt pathways. Convergence of proliferative pathways in mTOR is a hotspot of current research, and combined treatment using double class inhibitors for thyroid cancer may bring some success. New thyroid-stimulating hormone receptor (TSHR)-interacting proteins, a better picture of cAMP targets, a deeper knowledge of the action of the protein kinase A regulatory subunits, especially their interactions with the replication machinery, and a further understanding of mechanisms that lead to cell cycle progression through G₁/S and G₂/M checkpoints are areas that need further elucidation. Finally, massive information coming from microarray data analysis will prompt our understanding of thyroid-stimulating hormone-promoted thyrocyte proliferation in health and disease.

Cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinases, but not exchange proteins directly activated by cAMP (Epac), mediate thyrotropin/cAMP-dependent regulation of thyroid cells

TSH, mainly acting through cAMP, is the principal physiological regulator of thyroid gland function, differentiation expression, and cell proliferation. Both cAMP-dependent protein kinases [protein kinase A (PKA)] and the guanine-nucleotide-exchange factors for Rap proteins, exchange proteins directly activated by cAMP (Epac) 1 and Epac2, are known to mediate a broad range of effects of cAMP in various cell systems. In the present study, we found a high expression of Epac1 in dog thyrocytes, which was further increased in response to TSH stimulation. Epac1 was localized in the perinuclear region. Epac2 showed little or no expression. The TSH-induced activation of Rap1 was presumably mediated by Epac1 because it was mimicked by the Epac-selective cAMP analog (8-p-chloro-phenyl-thio-2'-O-methyl-cAMP) and not by PKA-selective cAMP analogs. Surprisingly, in view of the high Epac1 expression and its TSH responsiveness, all the cAMP-dependent functions of TSH in cultures or tissue incubations of dog thyroid, including acute stimulation of thyroid hormone secretion, H(2)O(2) generation, actin cytoskeleton reorganization, p70(S6K1) activity, delayed stimulation of differentiation expression, and mitogenesis, were induced only by PKA-selective cAMP analogs. The Epac activator 8-p-chloro-phenyl-thio-2'-O-methyl-cAMP, used alone or combined with PKA-selective cAMP analogs, had no measurable effect on any of these TSH targets. Therefore, PKA activation seems to mediate all the recognized cAMP-dependent effects of TSH and is thus presumably responsible for the pathological consequences of its deregulation. The role of Epac1 and TSH-stimulated Rap1 activation in thyrocytes is still elusive.

Molecular and genotypic characterization of human thyroid follicular cell carcinoma-derived cell lines

OBJECTIVE

Our aim was to characterize the molecular and genotypic profile of eight thyroid carcinoma-derived cell lines-TPC1, FB2, B-CPAP, K1, XTC-1, C643, 8505C, and Hth74-in order to use them as in vitro models of thyroid carcinogenesis.

DESIGN

We evaluated the expression of five thyroid-specific genes (Tg, TSHr, TPO, PAX8, and TTF-1) to establish the cell lineage and to assess the differentiation status of each of the cell lines. We screened for mutations in the most relevant oncogenes/tumor suppressor genes affected in thyroid carcinogenesis: RAS, BRAF, CTNNB1, and TP53 along with RET/PTC rearrangements. Considering the putative relevance in general carcinogenesis, we have also studied other molecules such as EGFR, PI3K, RAF-1, and THRB. To determine the genetic identity of the cell lines, we performed genotypic analysis.

MAIN OUTCOME

The panel of cell lines we have studied displayed activation of several oncogenes (BRAF, RAS, RET/PTC) and inactivation of tumor suppressor genes (TP53) known to be important for thyroid carcinogenesis. Two of the cell lines-TPC1 and FB2-shared the same genotypic profile, probably representing clones of an ancestor cell line (TPC1).

CONCLUSION

Due to their different molecular alterations, these cell lines represent a valuable tool to study the molecular mechanisms underlying thyroid carcinogenesis. We suggest that genotypic analyses should be included as a routine procedure to guarantee the uniqueness of each cell line used in research.

Long-term EGF/serum-treated human thyrocytes mimic papillary thyroid carcinomas with regard to gene expression

Constitutive activation of the RAS/RAF/MAPK pathway has been found in different tumor types including papillary thyroid carcinomas (PTCs). To get more insight into genes primarily regulated in the human tumor cells, an in vitro model was developed in which primary cultures of human thyrocytes were treated for different times with epidermal growth factor and serum (EGF/serum), which stimulate the MAPK cascade. Gene expression profiles were obtained by microarrays and compared to the expression profiles of PTCs. An evolution from short-term to long-term EGF/serum-treated cells was found, i.e., a program change showing a distinction between gene expression profiles of short-term and long-term EGF/serum-treated cells. The late pattern of EGF/serum stimulated cells converges to the pattern of PTCs. Comparison of these two types of cells with cAMP activated cells, from thyroid-stimulating hormone-treated thyrocytes and autonomous adenomas, showed distinct gene expression profiles for the two pathways. For the two models, an overlap was found in a number of genes which were early induced in vitro but down-regulated later in vitro and in the in vivo tumors. Thus, long-term stimulated human primary cultures demonstrate a clear relation with the tumor in vivo and could therefore be used as models for the disease.

Hardy–Weinberg Expectations in Canine Breeds: Implications for Genetic Studies

Hardy-Weinberg equilibrium (HWE) is a useful indicator of genotype frequencies within a population and whether they are based on a valid definition of alleles and a randomly mating sample. HWE assumes a stable population of adequate size without selective pressures and is used in human genetic studies as a guide to data quality by comparing observed genotype frequencies to those expected within a population. The calculation of genetic associations in case-control studies assume that the population is "in HWE." Canine breed populations deviate away from many of the criteria for HWE, and if genetic markers are not in HWE, conventional statistical analysis cannot be performed. To date, little attention has been paid as to whether genetic markers in dog breeds are distributed in compliance to HWE. In this study, 109 single-nucleotide polymorphisms (SNPs) were genotyped from 13 genes in a cohort of 894 dogs encompassing 33 breeds. Analysis of the entire cohort of dogs revealed a significant deviation away from HWE for all SNPs tested (P < 0.00001); analysis of the cohort stratified by breed and subbreed indicated that the majority of the markers complied with HWE expectation. This suggests that canine case-control association studies will be valid if performed within defined breeds.

Thyroid-stimulating hormone stimulates interleukin-6 release from 3T3-L1 adipocytes through a cAMP-protein kinase A pathway

OBJECTIVE

Thyroid-stimulating hormone (TSH) is a novel modulator of adipokine release from human and mouse adipocytes. The aim of our study was to identify the signal transduction pathways activated by TSH that stimulate interleukin (IL)-6 production.

RESEARCH METHODS AND PROCEDURES

Mouse 3T3-L1 preadipocyte and differentiated adipocyte cell cultures were studied. The effect of 0 to 1 microM TSH on IL-6 protein release into the medium over 0 to 24 hours was assessed. TSH signaling pathways responsible for regulating IL-6 were studied through the use of 1 muM forskolin, 100 microM 8-pCPT-2'-O-Me-cAMP, 10 microM H89, 50 microM PD98059, and 2 mug/mL actinomycin D.

RESULTS

TSH stimulated IL-6 release by 2.6-fold from 3T3-L1 adipocytes at concentrations as low as 0.01 microM but did not alter IL-6 production of corresponding preadipocytes. Forskolin (elevates intracellular cAMP) stimulated IL-6 release from 3T3-L1 adipocytes (n = 3, p < 0.005), and H89, an inhibitor of cAMP-dependent protein kinase A (PKA), reduced TSH-stimulated IL-6 release by 66% (n = 3, p < 0.01), indicating a requirement for cAMP-dependent PKA. Inhibition of the mitogen-activated protein kinase pathway with PD98059 did not affect TSH-stimulated IL-6 release. Activation of an alternate cAMP target, the exchange protein of cAMP, with 8-pCPT-2'-O-Me-cAMP, had no effect on IL-6 release. TSH raised the level of IL-6 mRNA, and blockade of transcription with actinomycin D abrogated IL-6 protein release by TSH (n = 3, p < 0.05).

DISCUSSION

TSH stimulates IL-6 release from differentiated 3T3-L1 adipocytes, but not preadipocytes, by signaling through cAMP-PKA to activate IL-6 gene transcription.

Hypothyroidism and hyperthyroidism modulates Ras-MAPK intracellular pathway in rat thyroids

Thyrotrophin induces proliferation and function in thyroid cells acting through a seven transmembrane G protein-coupled receptor. The proliferative pathways induced by thyrotropin (TSH) in thyrocytes in vivo are not completely understood yet. The aim of this work is to evaluate if Ras can be induced by TSH in rat thyroids, and whether extracellular regulated kinase (ERK) may be involved in the subsequent intracellular signalling cascade. We induced hypothyroidism in Wistar rats by methimazole (MMI) treatment (0.03% in the drinking water for 21 days). A subset of the hypothyroid rats received T4 (1 microg/100 g bw) during the last 10 days of MMI treatment. Hyperthyroidism was induced by subcutaneous injections of T4 (10 microg/100 g bw) during 10 days in another group of rats. Our data show that in the hypothyroid rats there is a clear positive Ras modulation, but a decrease in pERK. In contrast, thyroidal pERK increases in T4-induced hyperthyroidism, but without any change in RAS, although these changes did not reach statistical significance. Thus, while the rat thyroid proliferation induced by TSH may involve an increase in RAS signalling, the subsequent cascade does not involve ERK phosphorilation, which in fact, increases during T4-induced hyperthyroidism.

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.

Monoclonal pathogenic antibodies to the thyroid-stimulating hormone receptor in Graves' disease with potent thyroid-stimulating activity but differential blocking activity activate multiple signaling pathways

The thyroid target Ag for disease-inducing autoantibodies in Graves' disease is the receptor for thyroid-stimulating hormone (TSH), but little is known about the molecular basis of this pathogenic Ab response. We describe the characteristics of two high- affinity mAbs developed from an experimental murine model of hyperthyroid Graves' disease that exhibit potent thyroid-stimulating activity. Nanogram concentrations of the IgG mAbs KSAb1 and KSAb2 and their Fab induce full stimulation of the TSH receptor that is matched by the ligand TSH and, thus, act as full agonists for the receptor. However, KSAb1 and KSAb2 display differential activities in their ability to block TSH-mediated stimulation of the receptor, indicating subtle differences in their biological properties. In displacement studies, IgG and Fabs of KSAb1 and KSAb2 compete with Graves' disease autoantibodies as well as thyroid-blocking Abs present in some hypothyroid patients, indicating a close relationship between these autoimmune determinants on the receptor. In passive transfer studies, single injections of microgram quantities of KSAb1 or KSAb2 IgG led to rapid elevation of serum thyroxine and a hyperthyroid state that was maintained for a number of days. The thyroid glands showed evidence of cell necrosis, but there was no accompanying mononuclear cell infiltrate. In studying their receptor activation pathways, both KSAb1 and KSAb2 provoked phosphorylation of the intracellular ERK1/2 pathway in primary thyrocytes, indicating that multiple signaling pathways may participate in the pathogenesis of Graves' disease. In summary, our findings emphasize the similarities of the experimental mouse model in reproducing the human disorder and provide improved means for characterizing the molecular basis of this pathogenic response.

Extracellular signal-regulated kinase-dependent proliferation is mediated through the protein kinase A/B-Raf pathway in human uveal melanoma cells

Mutated B-Raf-mediated constitutive activation of ERK1/2 is involved in about 66% of cutaneous melanoma. By contrast, activating mutations in B-RAF are rare in ocular melanoma. This study aimed to determine the role of wild-type B-Raf ((WT)B-Raf) in uveal melanoma cell growth. We used cell lines derived from primary tumors of uveal melanoma to assess the role of (WT)B-Raf in cell proliferation and to characterize its upstream regulators and downstream effectors. Melanoma cell lines expressing (WT)B-Raf and (WT)Ras grew with similar proliferation rates, showed constitutive activation of ERK1/2, and had similar levels of B-Raf expression and B-Raf kinase activity as melanoma cell lines expressing the activating V600E mutation ((V600E)B-Raf). They were equally as sensitive to pharmacological inhibition of MEK1/2 for cell proliferation and transformation as (V600E)B-Raf cells. siRNA-mediated depletion of Raf-1 did not affect either ERK1/2 activation, whereas siRNA-mediated depletion of B-Raf reduced cell proliferation by up to 65% through the inhibition of ERK1/2 activation, irrespective of the mutational status of B-Raf. Pharmacological inhibition of cAMP-dependent protein kinase (PKA) and siRNA-mediated depletion of PKA greatly reduced B-Raf activity, ERK1/2 activation, and cell proliferation in (WT)B-Raf cells, whereas it did not affect (V600E)B-Raf cells, demonstrating a key role of PKA in mediating (WT)B-Raf/ERK signaling for uveal melanoma cell growth. Moreover, inactivation or depletion of PKA did not affect Rap-1 activity, and Rap-1 depletion did not affect either B-Raf activity or ERK1/2 activation. This ruled out a role for Rap1 in the PKA-mediated B-Raf/ERK activation in (WT)B-Raf cells. Finally, we demonstrated the importance of cyclin D1 in mediating PKA/(WT)B-Raf signaling for cell proliferation. Altogether, our results suggest that the PKA/B-Raf pathway is a potential target for therapeutic strategies against (WT)B-Raf-expressing uveal melanoma.

Gene expression in thyroid autonomous adenomas provides insight into their physiopathology

The purpose of this study was to use the microarray technology to define expression profiles characteristic of thyroid autonomous adenomas and relate these findings to physiological mechanisms. Experiments were performed on a series of separated adenomas and their normal counterparts on Micromax cDNA microarrays covering 2400 genes (analysis I), and on a pool of adenomatous tissues and their corresponding normal counterparts using microarrays of 18,000 spots (analysis II). Results for genes present on the two arrays corroborated and several gene regulations previously determined by Northern blotting or microarrays in similar lesions were confirmed. Five overexpressed and 24 underexpressed genes were also confirmed by real-time RT-PCR in some of the samples used for microarray analysis, and in additional tumor specimens. Our results show: (1) a change in the cell populations of the tumor, with a marked decrease in lymphocytes and blood cells and an increase in endothelial cells. The latter increase would correspond to the establishment of a close relation between thyrocytes and endothelial cells and is related to increased N-cadherin expression. It explains the increased blood flow in the tumor; (2) a homogeneity of tumor samples correlating with their common physiopathological mechanism: the constitutive activation of the thyrotropin (TSH)/cAMP cascade; (3) a low proportion of regulated genes consistent with the concept of a minimal deviation tumor; (4) a higher expression of genes coding for specific functional proteins, consistent with the functional hyperactivity of the tumors; (5) an increase of phosphodiesterase gene expression which explains the relatively low cyclic AMP levels measured in these tumors; (6) an overexpression of antiapoptotic genes and underexpression of proapoptotic genes compatible with their low apoptosis rate; (7) an overexpression of N-cadherin and downregulation of caveolins, which casts doubt about the use of these expressions as markers for malignancy.

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.

Integrating signals between cAMP and the RAS/RAF/MEK/ERK signalling pathways. Based on the anniversary prize of the Gesellschaft für Biochemie und Molekularbiologie Lecture delivered on 5 July 2003 at the Special FEBS Meeting in Brussels

One of the hallmarks of cAMP is its ability to inhibit proliferation in many cell types, but stimulate proliferation in others. Clearly cAMP has cell type specific effects and the outcome on proliferation is largely attributed to crosstalk from cAMP to the RAS/RAF/mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway. We review the crosstalk between these two ancient and conserved pathways, describing the molecular mechanisms underlying the interactions between these pathways and discussing their possible biological importance.

p38 mitogen-activated protein kinase contributes to cell cycle regulation by cAMP in FRTL-5 thyroid cells

OBJECTIVE

Thyrotropin activates the cAMP pathway in thyroid cells, and stimulates cell cycle progression in cooperation with insulin or insulin-like growth factor-I. Because p38 mitogen-activated protein kinases (p38 MAPKs) were stimulated by cAMP in the FRTL-5 rat thyroid cell line, we investigated (i) the effect of the specific inhibition of p38 MAPKs on FRTL-5 cell proliferation and (ii) the mechanism of action of p38 MAPKs on cell cycle control, by studying the expression and/or the activity of several cell cycle regulatory proteins in FRTL-5 cells.

METHODS

DNA synthesis was monitored by incorporation of [(3)H]thymidine into DNA and the cell cycle distribution was assessed by fluorescence-activated cell sorter analysis. Expression of cell cycle regulatory proteins was determined by Western blot analysis. Cyclin-dependent kinase 2 (Cdk2) activity associated to cyclin E was immunoprecipitated and was measured by an in vitro kinase assay.

RESULTS

SB203580, an inhibitor of alpha and beta isoforms of p38 MAPKs, but not its inactive analog SB202474, inhibited DNA synthesis and the G1-S transition induced by forskolin plus insulin. SB203580 inhibited specifically p38 MAPK activity but not other kinase activities such as Akt and p70-S6 kinase. Treatment of FRTL-5 cells with SB203580 decreased total and cyclin E-associated Cdk2 kinase activity stimulated with forskolin and insulin. However, inhibition of p38 MAPKs by SB203580 was without effect on total cyclin E and Cdk2 levels. The decrease in Cdk2 kinase activity caused by SB203580 treatment was not due to an increased expression of p21(Cip1) or p27(Kip1) inhibitory proteins. In addition, SB203580 affected neither Cdc25A phosphatase expression nor Cdk2 Tyr-15 phosphorylation. Inhibition of p38 MAPKs decreased Cdk2-cyclin E activation by regulating the subcellular localization of Cdk2 and its phosphorylation on Thr-160.

CONCLUSIONS

These results indicate that p38 MAPK activity is involved in the regulation of cell cycle progression in FRTL-5 thyroid cells, at least in part by increasing nuclear Cdk2 activity.

The cAMP signalling pathway activates CREB through PKA, p38 and MSK1 in NIH 3T3 cells

Cyclic adenosine 3',5'-monophosphate (cAMP) was originally shown to induce gene transcription through activation of cAMP-dependent protein kinase (PKA), and subsequent phosphorylation of the transcription factor cAMP response element-binding protein, CREB, at serine-133. However, elevated cAMP levels may activate multiple signalling pathways with protein kinases that can phosphorylate CREB at serine-133. We analysed the pathways involved in CREB phosphorylation and activation in NIH 3T3 cells exposed to the cAMP elevating agent forskolin. PKA represented the predominant pathway during the burst phase, while the mitogen-activated protein kinase p38 pathway became activated in a PKA-dependent fashion in forskolin treated cells. The phosphorylation kinetics of p38 was delayed compared to PKA activation. Activated p38 stimulated CREB-mediated transcription and potentiated the transcriptional strength of CREB provoked by forskolin. The p38-mediated activation of CREB was inhibited by dominant negative mutants of MSK-1 and by the PKA/MSK-1 inhibitor H89, but not by dominant negative mutants of MSK-2/RSK-B and MAPKAPK2. Our results suggest that forskolin-induced CREB phosphorylation and activation in NIH 3T3 cells is mediated directly by PKA and by a time-delayed PKA-dependent p38/MSK-1 pathway. This bifurcation and time-dependent regulation of the cAMP-responsive signalling pathways may enable the cell to endure and/or enforce a cellular response provoked by a cAMP-elevating stimulus.

Molecular Cloning of the Canine c-Met/HGF Receptor and Its Expression in Normal and Regenerated Liver

The c-Met proto-oncogene is the receptor for hepatocyte growth factor (HGF), which is a member of the tyrosine kinase family. Activation of the HGF/c-Met signal pathway leads to cell proliferation, motility, regeneration, and morphogenesis. In this study, the complete nucleotide sequence of complementary DNA (cDNA) of canine c-Met was cloned, and its distribution was determined in tissues. The canine c-Met cDNA clone had an open reading frame of 4419 bp that encoded a putative polypeptide of 1383 amino acids. The c-Met mRNA was expressed in a variety of canine tissues including peripheral blood mononuclear cells (PBMC), bone marrow, liver, kidney, lung, stomach, uterus, testis, thymus, lymph node, small intestine, colon, adrenal gland, thyroid gland, heart, muscle, skin, pancreas, ovary, prostate, spleen, fat, cerebrum, and cerebellum. In addition, the c-Met mRNA expression in normal and regenerated liver was examined. The levels of the mRNA increased 2-fold in regenerated liver compared to that found in normal liver, indicating that c-Met is involved in various functions including remodeling of canine hepatocytes.

Effects of extracellular nucleotides in the thyroid: P2Y2 receptor-mediated ERK1/2 activation and c-Fos induction in PC Cl3 cells

Aim of the present paper was to investigate the signaling pathways of P2Y2 in rat thyroid PC Cl3 cell line and its effects on proliferation. This study demonstrates that P2Y2 activation provoked: (a) a cytosol-to-membrane translocation of PKC-alpha, -betaI and -epsilon; (b) the phosphorylation of the extra cellular signal-regulated kinases 1 and 2 (ERK1/2); (c) the expression of c-Fos protein; (d) no effects on the G1/S progression and overall cell proliferation. The P2Y2-stimulated ERK1/2 phosphorylation was: (a) completely blocked by PD098059, a mitogen-activated protein kinase (MEK) inhibitor or by W-7, a Ca2+-calmodulin (CaM) antagonist; (b) reduced by GF109203X, inhibitor of PKCs, or AG1478, inhibitor of EGFR tyrosine kinase, or LY294002/wortmannin, inhibitors of phosphoinositide 3-kinases, or cytochalasin D, inhibitor of actin microfilament bundles polymerization. The c-Fos induction was greatly diminished by Go6976 or PD098059, and completely abolished when combined. In conclusion, data indicate that the P2Y2-induced phosphorylation of ERK1/2 and the induction of c-Fos are due to the operation of CaM, with PKC, PI3K, EGFR and receptor endocytosis mechanisms endorsing the signalling. On the other hand, no mitogenic effects of P2Y2 are whatsoever noticed in PC Cl3 cells.