Differential effects of protein kinase A on Ras effector pathways

Quemeiteng granule relieves goiter by suppressing thyroid microvascular endothelial cell proliferation and angiogenesis via miR-217-5p-mediated targeting of FGF2-induced regulation of the ERK pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Goiters are enlargements of the thyroid gland and are a global public issue. Quemeiteng granule (QMTG) is a traditional Chinese medicine (TCM) formula used to treat goiter in Yunnan Province. However, the effectiveness and underlying mechanism of these treatments have not been fully elucidated.

AIM OF THE STUDY

This study aimed to investigate the therapeutic effects of QMTG on goiter and the downstream regulatory mechanisms.

MATERIALS AND METHODS

In this study, we first evaluated the antigoiter efficacy of QMTG through biochemical indices [body weight, thyroid coefficient, triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH)] and hematoxylin-eosin (HE) staining in a Propylthiouracil (PTU)-induced model. Based on microRNA sequencing (miRNA-seq) and bioinformatics analysis, key miRNA was screened out. A dual-luciferase reporter assay was performed to confirm the transcriptional regulation of the target gene by the miRNA. The viability of rat thyroid microvascular endothelial cells (RTMECs) and human thyroid microvascular endothelial cells (HTMECs) was assessed using the CCK-8 assays. The migration and angiogenesis of RTMECs and HTMECs were visualized through tube formation and wound scratch assays. Proteins involved in angiogenesis and the ERK pathway were assessed via Western blotting.

RESULTS

QMTG significantly increased body weight, decreased the thyroid coefficient, increased the levels of T3, T4, FT3 and FT4 and reduced TSH levels in rats with goiter. QMTG also promoted the morphological recovery of thyroid follicles. MiR-217-5p was identified as a key miRNA. Our studies revealed that miR-217-5p directly targets FGF2 and that QMTG promotes the recovery of thyroid hormone (TH) levels and morphological changes in the thyroid, suppresses thyroid microvascular endothelial cell vitality, tube formation and migration, and reduces the expression of VEGF, Ang-1 and VCAM-1 triggered by miR-217-5p, thereby inhibiting the Ras/MEK/ERK cascade through FGF2.

CONCLUSIONS

Our experiments demonstrated that the QMTG had therapeutic effects on goiter. These effects were attributed to the inhibition of ERK pathway-induced proliferation and angiogenesis through the targeting of FGF2 by miR-217-5p.

Expression of the ring ligase PRAJA2 in thyroid cancer

INTRODUCTION

In thyroid cells, binding of TSH to its receptor increases cAMP levels, sustaining thyrocytes growth and hormone production. The main cAMP effector enzyme is protein kinase A (PKA). Praja2 is a widely expressed RING (Really Interesting New Gene) ligase, which degrades the regulatory subunits of PKA, thus controlling the strength and duration of PKA signaling in response to cAMP. Differentiated thyroid cancer expresses a functional TSH receptor, and its growth and progression are positively regulated by TSH and cAMP signaling.

AIM

We aimed to analyze the expression of praja2 in a group of 36 papillary thyroid cancer (PTC), 14 benign nodules, and six anaplastic thyroid cancers (ATC).

METHODS

We measured praja2 mRNA levels by quantitative RT-PCR and praja2 expression by Western blot and immunohistochemistry. Possible association between praja2 mRNA and the presence of known mutations was evaluated.

RESULTS

We found a statistical significant increase of mRNA levels in PTC tissue samples, compared with benign nodules and ATC. In particular, mRNA levels were maximal in differentiated thyroid cancer (PTC), progressively decreasing in more aggressive tumors, ATC having the lowest amount of praja2 mRNA. Accordingly, higher levels of praja2 protein were detected in lysates from PTC, compared with ATC. By immunohistochemistry, in PTC sections we observed a marked increase of cytoplasmic praja2 signal, which significantly decreased in less differentiated thyroid tumors, completely disappearing in ATC. Studies in cultured cells stably expressing RET/PTC1 oncogene or mutant BRAF revealed a direct correlation between praja2 mRNA levels and malignant phenotype of transformed cells. Similar results were obtained using thyroid cancer tissues carrying the same mutations.

CONCLUSIONS

praja2 is markedly overexpressed in differentiated thyroid cancer, and its levels inversely correlate with the malignant phenotype of the tumor. Thus, praja2 is a novel cancer-related gene whose expression is linked to the histotype and mutational status of the thyroid tumor.

Signal control through Raf: in sickness and in health

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is the prototype mammalian mitogen-activated protein kinase (MAPK) signaling cascade that regulates a number of processes, including proliferation, differentiation, survival, migration, stress responses and apoptosis. How this seemingly linear cascade is modulated to achieve a specific cellular function has been a main focus of the field. In this review, we describe new as well as old findings in the regulation of the ERK1/2 pathway in normal and disease states via MAP3Ks.

Ral activation promotes melanomagenesis

Up to one-third of human melanomas are characterized by an oncogenic mutation in the gene encoding the small guanosine triphosphatase (GTPase) NRAS. Ras proteins activate three primary classes of effectors, namely, Rafs, phosphatidyl-inositol-3-kinases (PI3Ks) and Ral guanine exchange factors (RalGEFs). In melanomas lacking NRAS mutations, the first two effectors can still be activated through an oncogenic BRAF mutation coupled with a loss of the PI3K negative regulator PTEN. This suggests that Ras effectors promote melanoma, regardless of whether they are activated by oncogenic NRas. The only major Ras effector pathway not explored for its role in melanoma is the RalGEF-Ral pathway, in which Ras activation of RalGEFs converts the small GTPases RalA and RalB to an active guanosine triphosphate-bound state. We report that RalA is activated in several human melanoma cancer cell lines harboring an oncogenic NRAS allele, an oncogenic BRAF allele or wild-type NRAS and BRAF alleles. Furthermore, short hairpin RNA (shRNA)-mediated knockdown of RalA, and to a lesser extent of RalB, variably inhibited the tumorigenic growth of melanoma cell lines having these three genotypes. Thus, as is the case for Raf and PI3 K signaling, Rals also contribute to melanoma tumorigenesis.

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.

K-ras/PI3K-Akt signaling is essential for zebrafish hematopoiesis and angiogenesis

The RAS small GTPases orchestrate multiple cellular processes. Studies on knock-out mice showed the essential and sufficient role of K-RAS, but not N-RAS and H-RAS in embryonic development. However, many physiological functions of K-RAS in vivo remain unclear. Using wild-type and fli1:GFP transgenic zebrafish, we showed that K-ras-knockdown resulted in specific hematopoietic and angiogenic defects, including the impaired expression of erythroid-specific gene gata1 and ße3-hemoglobin, reduced blood circulation and disorganized blood vessels. Expression of either K-rasC40 that links to phosphoinositide 3-kinase (PI3K) activation, or Akt2 that acts downstream of PI3K, could rescue both hematopoietic and angiogenic defects in the K-ras knockdown. Consistently, the functional rescue by k-ras mRNA was significantly suppressed by wortmannin, a PI3K-specific inhibitor. Our results provide direct evidence that PI3K-Akt plays a crucial role in mediating K-ras signaling during hematopoiesis and angiogenesis in vivo, thus offering new targets and alternative vertebrate model for studying these processes and their related diseases.

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.

Analysis of Ras transformation of human thyroid epithelial cells

Activation of Ras oncogene by point mutations is an early frequent event in thyroid tumorigenesis. In this chapter, we describe the use of human primary thyroid follicular epithelial cells expressing oncogenic mutant Ras by means of retroviral transduction as a biological model of human cancer initiation that provides powerful insights into thyroid tumorigenesis. We describe protocols for manipulating primary epithelial cells and describe the use of this model to dissect the signaling pathways required for Ras-induced proliferation in these cells. We also highlight the importance of studying Ras signaling in an appropriate cell context, summarizing some of the key differences identified between more widespread experimental models based on fibroblasts or rodent cell lines and primary epithelial cells.

Downregulation of Rap1GAP contributes to Ras transformation

Although abundant in well-differentiated rat thyroid cells, Rap1GAP expression was extinguished in a subset of human thyroid tumor-derived cell lines. Intriguingly, Rap1GAP was downregulated selectively in tumor cell lines that had acquired a mesenchymal morphology. Restoring Rap1GAP expression to these cells inhibited cell migration and invasion, effects that were correlated with the inhibition of Rap1 and Rac1 activity. The reexpression of Rap1GAP also inhibited DNA synthesis and anchorage-independent proliferation. Conversely, eliminating Rap1GAP expression in rat thyroid cells induced a transient increase in cell number. Strikingly, Rap1GAP expression was abolished by Ras transformation. The downregulation of Rap1GAP by Ras required the activation of the Raf/MEK/extracellular signal-regulated kinase cascade and was correlated with the induction of mesenchymal morphology and migratory behavior. Remarkably, the acute expression of oncogenic Ras was sufficient to downregulate Rap1GAP expression in rat thyroid cells, identifying Rap1GAP as a novel target of oncogenic Ras. Collectively, these data implicate Rap1GAP as a putative tumor/invasion suppressor in the thyroid. In support of that notion, Rap1GAP was highly expressed in normal human thyroid cells and downregulated in primary thyroid tumors.

Ras Induces Chromosome Instability and Abrogation of the DNA Damage Response

Ras mutations are frequent in thyroid tumors, the most common endocrine malignancy. The ability of Ras to transform thyroid cells is thought to rely on its mitogenic activity. Unexpectedly, acute expression of activated Ras in normal rat thyroid cells induced a DNA damage response, followed by apoptosis. Notably, a subpopulation of cells evaded apoptosis and emerged with features of transformation, including the loss of epithelial morphology, dedifferentiation, and the acquisition of hormone- and anchorage-independent proliferation. Strikingly, the surviving cells showed marked chromosomal instability. Acutely, Ras stimulated replication stress as evidenced by the induction of ataxia telangiectasia mutated and Rad3-related protein kinase (ATR) activity (Chk1 phosphorylation) and of gammaH2A.X, a marker of DNA damage. Despite the activation of a checkpoint, cells continued through mitosis in the face of DNA damage, resulting in an increase in cells harboring micronuclei, an indication of defects in chromosome segregation and other forms of chromosome damage. Cells that survived exposure to Ras continued to exhibit replication stress (ATR activation) but no longer exhibited gammaH2A.X or full activation of p53. When rechallenged with Ras or DNA-damaging agents, the surviving cells were more resistant to apoptosis than parental cells. These data show that acute expression of activated Ras is sufficient to induce chromosomal instability in the absence of other signals, and suggest that Ras-induced chromosomal instability arises as a consequence of defects in the processing of DNA damage. Hence, abrogation of the DNA damage response may constitute a novel mechanism for Ras transformation.

The p85 regulatory subunit of PI3K mediates TSH-cAMP-PKA growth and survival signals

Phosphatidylinositol 3-kinase (PI3K) is necessary for thyroid stimulating hormone (TSH)-induced cell cycle progression. To determine the molecular mechanism linking PI3K to TSH, we have identified a serine residue in p85alpha(PI3K) phosphorylated by protein kinase A (PKA) in vitro and in vivo. Expression of an alanine mutant (p85A) abolished cyclic AMP/TSH-induced cell cycle progression and was lethal in thyroid cells (FRTL-5). The aspartic version of the p85alpha(PI3K) (p85D) inhibited apoptosis following TSH withdrawal. The p85alpha(PI3K) wild type not the p85A bound PKA regulatory subunit RIIbeta in cells stimulated with cAMP or TSH. The binding of the aspartic version of p85alpha(PI3K) to RIIbeta was independent of cAMP or TSH stimulation. Similarly, binding of PI3K to p21Ras and activation of AKT, a downstream PI3K target, were severely impaired in cells expressing the p85A mutant. Finally, we found that the catalytic activity of PI3K was stimulated by TSH in cells expressing the wild-type p85alpha(PI3K) but not in cells expressing p85A. This latter mutant did not affect the epidermal growth factor-stimulated PI3K activity. We suggest that (1) TSH-cAMP-induced PKA phosphorylates p85alpha(PI3K) at serine 83, (2) phosphorylated p85alpha(PI3K) binds RIIbeta-PKA and targets PKAII to the membrane, and (3) PI3K activity and p21Ras binding to PI3K increase and activate PI3K downstream targets. This pathway is essential for the transmission of TSH-cAMP growth signals.

HMG-CoA reductase inhibitors inhibit rat propylthiouracil-induced goiter by modulating the ras-MAPK pathway

The aim of this study was to evaluate in vivo the antiproliferative effect of an inhibitor of isoprenoids metabolism, lovastatin, in an experimental model of propylthiouracil-induced goiter. In thyroid cells, thyrotropin (TSH)-induced proliferation requires active isoprenoid synthesis, and the HMG-CoA reductase inhibitors have antiproliferative effects in vitro. Propylthiouracil treatment (PTU) of rats led to thyroid hypertrophy and hyperplasia by TSH-induced activation of the mitogen-activated protein kinase (MAPK) pathway. Immunohistochemistry showed an increased number of proliferating cell nuclear antigen (PCNA)-positive cells in the thyroid gland of PTU-treated rats. Moreover, the phosphorylation of ERK1 and ERK2 was increased in the extract from goiter tissue as compared with the thyroid tissue of untreated rats. To determine whether the inhibition of selected pro-survival pathways (i.e., p21ras-MAPK) was sufficient to affect goitrogenesis, thyroids from 12 PTU-treated rats were injected in vivo with an adenovirus transducing a dominant-negative ras gene (Rad-L61.S186) and another set of 12 rats were injected with a pharmacological inhibitor of MAPK (PD98059). Both Rad-L61.S186 and PD98059 were able to inhibit the PTU-induced goiter. It is interesting to note that lovastatin, when administered in drinking water, significantly prevented the thyroid gland enlargement. Therefore, lovastatin-treated thyroid glands were significantly smaller than those treated with PTU alone. In addition, the lovastatin-treated glands also showed a decreased expression of phosphorylated ERK1/2 and a number of PCNA-positive cells. Our data suggest that lovastatin is an efficient inhibitor of goitrogenesis and provide a rationale for innovative therapeutic strategies employing statins in the treatment of nodular goiter in humans.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

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.

Mutant ras-induced proliferation of human thyroid epithelial cells requires three effector pathways

Ras mutations occur as an early event in many human tumours of epithelial origin, including thyroid. Using primary human thyroid epithelial cells to model tumour initiation by Ras, we have shown previously that activation of both the MAP kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) effector pathways are necessary, but even when activated together are not sufficient, for Ras-induced proliferation. Here, we show that a third effector, RalGEF, is also activated by Ras in these cells, that this activation is necessary for Ras-induced proliferation, and furthermore that in combination with the MAPK and PI3K effectors, it is able to reproduce the proliferative effect of activated Ras. The requirement for three effector pathways indicates a more robust control of cell proliferation in this normal human epithelial cell type than has been displayed in previous similar studies using rodent and human cell lines. Our findings highlight the importance of the appropriate cellular context in models of Ras-induced tumour development.

TSH-activated signaling pathways in thyroid tumorigenesis

Thyrotropin (TSH) is considered the main regulator of thyrocyte differentiation and proliferation. Thus, the characterization of the different signaling pathways triggered by TSH on these cells is of major interest in order to understand the mechanisms implicated in thyroid pathology. In this review we focus on the different signaling pathways involved in TSH-mediated proliferation and their role in thyroid transformation and tumorigenesis. TSH mitogenic activities are mediated largely by cAMP, which in turn may activate protein kinase (PKA)-dependent and independent processes. We analyze the effects of increased cAMP levels and PKA activity during cell cycle progression and the role of this signaling pathway in thyroid tumor initiation. Alternative pathways to PKA in the cAMP-mediated proliferation appear to involve the small GTPases Rap1 and Ras. We analyze the Ras effectors (PI3K, RalGDS and Raf) that are thought to mediate its oncogenic activity, as well as the ability of Ras to induce apoptosis in thyrocytes. Finally, we discuss the activation of the PLC/PKC cascade by TSH in thyroid cells and the role of this signaling pathway in the TSH-mediated proliferation and tumorigenesis.

Responses of tumorigenic and non-tumorigenic mouse lung epithelial cell lines to electrophilic metabolites of the tumor promoter butylated hydroxytoluene

A model system to investigate the promotion phase of pulmonary carcinogenesis involves chronic exposure of carcinogen-initiated mice to the food additive, butylated hydroxytoluene (BHT). Previous studies strongly suggested that this activity is due to the cytochrome p450-catalyzed formation of quinone methides 2,6-di-tert-butyl-4-methylenecyclohexa-2,5-dienone (BHT-QM) and 6-tert-butyl-2-(1',1'-dimethyl-2'-hydroxy)ethyl-4-methylenecyclohexa-2,5-dienone (BHTOH-QM). The effects of these electrophiles on non-tumorigenic C10 and E10 epithelial cell lines derived from a normal mouse lung explant were compared with effects on their corresponding neoplastic siblings, the A5 and E9 spontaneous transformants, respectively. The tumorigenic cells were more resistant to cell killing, with LC(50) values of 165-180 microM for BHT-QM and 12-22 microM for BHTOH-QM, versus LC(50) values in the non-tumorigenic cells of 105-118 microM and 5.0-6.0 microM, respectively. Constitutive glutathione (GSH) concentrations were 12-20 nmol/10(6) cells, and BHT-QM toxicity was enhanced >2-fold by depleting GSH with buthionine sulfoximine (BSO). Formation of the GSH conjugate of BHT-QM accounted for a substantial fraction of the cellular GSH lost by quinone methide exposure. Enhanced lipid peroxidation and superoxide formation occurred in all cell lines treated with BHT-QM, but both tumorigenic lines contained higher levels of GSH S-transferase and superoxide dismutase (SOD) activities. These data suggest the possibility that BHT-derived quinone methides may exert their promoting effects by inducing oxidative stress; such stress is better tolerated by tumorigenic cells, which have higher levels of antioxidant enzymes. Normal cells are destroyed more readily which allows neoplastic cells to expand their proliferation.

New advances on prostate carcinogenesis and therapies: involvement of EGF-EGFR transduction system

The prostate cancers (PCs) are among the major causes of death because therapeutic treatments are not effective against advanced and metastatic forms of this cellular hyperproliferative disorder. In fact, although androgen-deprivation therapies permit to cure localized PC forms, the metastatic PC cells have acquired multiple functional features that confer to them resistance to ionizing radiations and anticarcinogenic drugs currently used in therapy. The present review describes last advances on molecular mechanisms that might be responsible for sustained growth and survival of PC cells. In particular, emphasis is on intracellular signaling cascades which are involved in the mitogenic and antiapoptotic effects of epidermal growth factor EGF-EGFR system. Of therapeutic interest, recent advances and prospects for development of new treatments against incurable forms of metastatic PC forms are also discussed.

Ras stimulates aberrant cell cycle progression and apoptosis in rat thyroid cells

Abundant evidence supports the ability of Ras to stimulate thyroid cell proliferation. Stable expression of activated Ras enhances the sensitivity of thyroid cells to apoptosis. We report that apoptosis is a primary and general response of rat thyroid cells to acute expression of activated Ras in the absence or presence of thyrotropin, insulin, and serum, survival factors for thyroid cells. Ras induced apoptosis in quiescent and cycling cells. Concomitantly, Ras stimulated S phase entry in quiescent cells and enhanced G1/S transition in cycling cells. Ras effects on the cell cycle were characterized by delayed progression through S phase and an apparent failure to proceed through G2/M phase. Unlike thyroid cell mitogens, Ras markedly decreased cyclin D1 expression. Although acute expression of Ras decreased cyclin D1 protein levels, cells selected to survive chronic Ras expression exhibited a selective increase in cyclin D1 expression. In summary, thyroid cells harbor an apoptotic program activated by Ras that outstrips the protective effects of thyrotropin, insulin, and serum. Apoptosis is accompanied by dysregulated cell cycle progression, suggesting that cell death may arise, at least in part, as a consequence of inappropriate proliferative cues.

cAMP inhibition of Akt is mediated by activated and phosphorylated Rap1b

Rap1b has been implicated in the transduction of the cAMP mitogenic signal. Rap1b is phosphorylated and activated by cAMP, and its expression in cells where cAMP is mitogenic leads to an increase in G(1)/S phase entry and tumor formation. The PCCL3 thyroid follicular cells represent a differentiated and physiologically relevant system that requires thyrotropin (TSH), acting via cAMP, for a full mitogenic response. In this model system, cAMP stimulation of DNA synthesis requires activation and phosphorylation of Rap1b by the cAMP-dependent protein kinase A (PKA). This scenario presents the challenge of identifying biochemical processes involved in the phosphorylation-dependent Rap1b mitogenic action. In thyroid cells, Akt has been implicated in the stimulation of cell proliferation by TSH and cAMP. However, the mechanism(s) by which cAMP regulates Akt activity remains unclear. In this study we show that in PCCL3 cells 1) TSH inhibits Akt activity via cAMP and PKA; 2) Rap1b is required for cAMP inhibition of Akt; and 3) transduction of the cAMP signal into Akt requires activation as well as phosphorylation of Rap1b by PKA.

Involvement of the protein kinase C pathway in thyrotropin-induced STAT3 activation in FRTL-5 thyroid cells

The binding of thyrotropin (TSH) to the TSH receptor (TSHR) activates two signaling pathways: the cAMP-protein kinase A (PKA) and the protein kinase C (PKC) systems. We have recently demonstrated that TSH activates the Janus kinases (JAK)/signal transducer and activator of transcription (STAT) pathway via TSHR. This study aimed to investigate whether the cAMP/PKA or the PKC system is involved in STAT3 activation in response to TSH. Treatment with TSH activated STAT3 phosphorylation in FRTL-5 thyrocytes and human TSHR-expressing Chinese hamster ovary cells. TSH-induced STAT3 activation was inhibited by a blocking antibody directed against TSHR that was isolated from patients with primary myxoedema. Increased intracellular cAMP activated STAT3 but inhibition of PKA did not affect STAT3 activation. On the other hand, the PKC stimulant PMA induced STAT3 phosphorylation and the PKC inhibitors inhibited it. Moreover, inhibition of PKC blocked STAT3 activation induced by a stimulator of cAMP. Our data suggest that TSH activates STAT3 via TSHR and cAMP- and PKC-dependent pathways, and provide evidence that PKC may be involved in the pathway downstream from cAMP.

Minireview: branded from the start-distinct oncogenic initiating events may determine tumor fate in the thyroid

Thyroid follicular neoplasms commonly have aneuploidy, presumably due to chromosomal instability. This property is associated with a greater malignant potential and worse prognosis. Recently, there has been considerable progress in our understanding of mechanisms that may account for chromosomal instability in cancer cells. Many tumors with chromosomal instability have abnormalities in the cell cycle checkpoint that monitors the fidelity of mitosis. Mutations of Bub1 or BubR1, genes coding for kinases involved in mitotic spindle assembly checkpoint signaling, are found in a small subset of aneuploid tumors. Other components of protein complexes responsible for attachment of kinetochores to microtubules, or for cohesion between sister chromatids, may also be subject to alterations during tumor progression. Here, we also discuss the evidence that certain oncogenic events, such as Ras mutations, may predispose cells to chromosomal instability by favoring inappropriate posttranslational changes in mitotic checkpoint components through activation of upstream kinases during tumor initiation or progression.

On the mitogenic properties of Rap1b: cAMP-induced G(1)/S entry requires activated and phosphorylated Rap1b

We have shown that the small GTPase Rap1b, a protein known to antagonize the mitogenic and transforming activity of Ras, is endowed with both mitogenic and tumorigenic properties. Rap1b can be activated by cAMP, an intracellular message known to either stimulate or inhibit cell proliferation. The oncogenic property of Rap1b was revealed in a model system in which cAMP stimulates cell proliferation and was linked to Rap's ability to promote S phase entry. We have now tested the significance of the mitogenic action of Rap1b in a physiologically relevant model, the differentiated thyroid follicular cells, a system that requires thyroid-stimulating hormone (TSH), acting via cAMP, to mediate a full mitogenic response. Here we report that cAMP-dependent hormonal stimulation of DNA synthesis requires Rap1b in a manner dependent on its phosphorylation by protein kinase A.

The RAS effector RIN1 directly competes with RAF and is regulated by 14-3-3 proteins

Activation of RAS proteins can lead to multiple outcomes by virtue of regulated signal traffic through alternate effector pathways. We demonstrate that the RAS effector protein RIN1 binds to activated RAS with an affinity (K(d), 22 nM) similar to that observed for RAF1. At concentrations close to their equilibrium dissociation constant values, RIN1 and RAF1 compete directly for RAS binding. RIN1 was also observed to inhibit cellular transformation by activated mutant RAS. This distinguishes RIN1 from other RAS effectors, which are transformation enhancing. Blockade of transformation was mediated by the RAS binding domain but required membrane localization. RIN1 recognizes endogenous RAS following transient activation by epidermal growth factor, and a portion of RIN1 fractionates to the cell membrane in a manner consistent with a reversible interaction. RIN1 also binds to 14-3-3 proteins through a sequence including serine 351. Mutation of this residue abolished the 14-3-3 binding capacity of RIN1 and led to more efficient blockade of RAS-mediated transformation. The mutant protein, RIN1(S351A), showed a shift in localization to the plasma membrane. Serine 351 is a substrate for protein kinase D (PKD [also known as PKCmu]) in vitro and in vivo. These data suggest that the normal localization and function of RIN1, as well as its ability to compete with RAF, are regulated in part by 14-3-3 binding, which in turn is controlled by PKD phosphorylation.

Thyrotropin activates mitogen-activated protein kinase pathway in FRTL-5 by a cAMP-dependent protein kinase A-independent mechanism

The involvement of mitogen-activated protein (MAP) kinases in the mitogenic effect of thyrotropin (TSH) is not fully elucidated. In FRTL-5 cells, we found that the MAP kinase kinase (MEK) inhibitors UO126 and PD98059 substantially decreased TSH-induced DNA synthesis, indicating that MAP kinases are involved in the TSH-stimulated proliferative response. Accordingly, TSH, forskolin (FSK) and 8-bromo-cAMP induced a rapid (3 min) and transient activation of ERK1/2, as assessed by phosphorylation of myelin basic protein and ERK1/2. This effect was cAMP-dependent and protein kinase A (PKA)-independent. The activation of Rap1 and B-Raf was involved in the mechanism of MAP kinase stimulation by TSH. TSH induced rapid (3 min) GDP/GTP exchange and activation of Rap1. After a 3-min exposure to FSK, B-Raf was recruited to a vesicular compartment, where it colocalized with Rap1. Both activation of Rap1 and translocation of B-Raf were PKA-independent. The Rap1 dominant negative Rap1N17 significantly reduced TSH-stimulated but not insulin-like growth factor 1-stimulated ERK1/2 phosphorylation, whereas the Ras dominant negative RasN17 inhibited the effect of both agonists. In conclusion, our results document that TSH increases intracellular cAMP, which rapidly stimulates MAP kinase cascade independent of PKA. This novel mechanism could integrate other pathways involved in TSH-stimulated proliferative response.

Enhanced sensitivity to apoptosis in Ras-transformed thyroid cells

Ras mutations occur at high frequency in thyroid cancer. In vitro, the effects of Ras in thyroid cells are pleiotropic in that expression of activated Ras has been reported to stimulate proliferation and apoptosis. An understanding of the factors that contribute to the survival versus demise of Ras-transformed cells is essential to our understanding of the contribution of Ras to thyroid neoplasia and other cancers. Constitutive expression of oncogenic H-Ras sensitized Wistar rat thyroid (WRT) cells to apoptosis stimulated by multiple insults. When deprived of matrix attachment, Ras-transformed cells perished by apoptotic cell death at a high frequency. In contrast, parental cells were more resistant to suspension-induced cell death. Ras effects on anchorage-independent cell death were reproduced by a mutant protein that signals selectively to Raf-1, but not by mutant Ras that preferentially binds to RalGDS. Expression of a Ras mutant that selectively activates PI3K resulted in substantial protection from detachment-induced cell death. MAPK activity was increased in adherent Ras12V- and Ras12V35S-expressing cells, but abolished upon detachment. Interestingly, impaired MAPK activity was sufficient to stimulate apoptosis in adherent Ras-transformed cells, but not in parental cells. Treatment with a PI3K inhibitor also stimulated apoptosis selectively in Ras-transformed cells. These results demonstrate that constitutive expression of activated Ras elicits differential effects on the survival of thyroid cells. Moreover, Ras expression results in a greater dependence of thyroid cells on MAPK and PI3K activity for their survival.

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.

G-protein-coupled receptors and signaling networks: emerging paradigms

G-protein-coupled receptors (GPCRs) constitute the largest family of cell-surface molecules involved in signal transmission. These receptors play key physiological roles and their dysfunction results in several diseases. Recently, it has been shown that many of the cellular responses mediated by GPCRs do not involve the sole stimulation of conventional second-messenger-generating systems, but instead result from the functional integration of an intricate network of intracellular signaling pathways. Effectors for GPCRs that are independent of G proteins have now also been identified, thus changing the conventional view of the GPCR-heterotrimeric-G-protein-associated effector. The emerging information is expected to help elucidate the most basic mechanism by which these receptors exert their numerous physiological roles, in addition to determining why the perturbation of their function results in many pathological conditions.

Regulation of lung epithelial cell morphology by cAMP-dependent protein kinase type I isozyme

Cell shape is mediated in part by the actin cytoskeleton and the actin-binding protein vinculin. These proteins in turn are regulated by protein phosphorylation. We assessed the contribution of cAMP-dependent protein kinase A isozyme I (PKA I) to lung epithelial morphology using the E10/E9 sibling cell lines. PKA I concentration is high in flattened, nontumorigenic E10 cells but low in their round E9 transformants. PKA I activity was lowered in E10 cells by stable transfection with a dominant negative RIα mutant of the PKA I regulatory subunit and was raised in E9 cells by stable transfection with a wild-type Cα catalytic subunit construct. Reciprocal changes in morphology ensued. E10 cells became rounder and grew in colonies, their actin microfilaments were disrupted, and vinculin localization at cell-cell junctions was diminished. The converse occurred in E9 cells on elevating their PKA I content. Demonstration that PKA I is responsible for the dichotomy in these cellular behaviors suggests that manipulating PKA I concentrations in lung cancer would provide useful adjuvant therapy.

cAMP signaling selectively influences Ras effectors pathways

Thyrotropin (TSH) stimulates survival and growth of thyroid cells via a seven transmembrane G protein-coupled receptor. TSH elevates the intracellular cyclic AMP (cAMP) levels activating protein kinase A (PKA). Recent evidence indicates that p21 Ras is required for TSH-induced mitogenesis, but the molecular mechanism(s) is not known. Here we report that Ras p21 activity is necessary for the Go- G1 transition in TSH induced cycle and that the downstream effector of Ras upon TSH signaling is p85-p110 PI3K. We show that PI3K inhibitors block TSH-induced DNA synthesis, cAMP-PKA stimulate the formation of the complex PI3K-p21 Ras and reduce the complex Ras-Raf1 in thyroid and other cells types. Moreover, PKA phosphorylates immunoprecipitated p85 and PKA phosphorylation of cell extracts significantly stimulates the formation of the complex PI3K-Ras. We suggest that PKA phosphorylates p85 and stabilizes the complex p110-p85, enhancing the interaction PI3K and p21 Ras. Simultaneously, cAMP inhibits Raf-1-ERK signaling by decreasing Raf1 availability to Ras. Under these circumstances PI3K signaling is favored. These results indicate that PI3K is an important mediator of Ras effects in cAMP-induced proliferation and illustrates how cAMP can selectively influence Ras effector pathways.

Coordinated regulation of Rap1 and thyroid differentiation by cyclic AMP and protein kinase A

Originally identified as an antagonist of Ras action, Rap1 exhibits many Ras-independent effects, including a role in signaling pathways initiated by cyclic AMP (cAMP). Since cAMP is a critical mediator of the effects of thyrotropin (TSH) on cell proliferation and differentiation, we examined the regulation of Rap1 by TSH in a continuous line of rat thyroid-like cells. Both cAMP and protein kinase A (PKA) contribute to the regulation of Rap1 activity and signaling by TSH. TSH activates Rap1 through a cAMP-mediated and PKA-independent mechanism. TSH phosphorylates Rap1 in a PKA-dependent manner. Interference with PKA activity blocked phosphorylation but not the activation of Rap1. Rather, PKA inhibitors prolonged Rap1 activation, as did expression of a Rap1A mutant lacking a PKA phosphorylation site. These results indicate that PKA elicits negative feedback regulation on cAMP-stimulated Rap1 activity in some cells. The dual regulation of Rap1 by cAMP and PKA extends to downstream effectors. The ability of TSH to stimulate Akt phosphorylation was markedly enhanced by the expression of activated Rap1A and was repressed in cells expressing a putative dominant-negative Rap1A mutant. Although the expression of activated Rap1A was sufficient to stimulate wortmannin-sensitive Akt phosphorylation, TSH further increased Akt phosphorylation in a phosphatidylinositol 3-kinase- and PKA-dependent manner. The ability of TSH to phosphorylate Akt was impaired in cells expressing a Rap1A mutant that could be activated but not phosphorylated. These findings indicate that dual signals, Rap1 activation and phosphorylation, contribute to TSH-stimulated Akt phosphorylation. Rap1 plays an essential role in cAMP-regulated differentiation. TSH effects on thyroid-specific gene expression, but not its effects on proliferation, were markedly enhanced in cells expressing activated Rap1A and repressed in cells expressing a dominant-negative Rap1A mutant. These findings reveal complex regulation of Rap1 by cAMP including PKA-independent activation and PKA-dependent negative feedback regulation. Both signals appear to be required for TSH signaling to Akt.

Estrogen promotes growth of human thyroid tumor cells by different molecular mechanisms

Thyroid tumors are about 3 times more frequent in females than in males. Epidemiological studies suggest that the use of estrogens may contribute to the pathogenesis of thyroid tumors. In a very recent study a direct growth stimulatory effect of 17beta-estradiol was demonstrated in FRTL-5 rat thyroid cells. In this work the presence of estrogen receptors alpha and beta in thyroid cells derived from human goiter nodules and in human thyroid carcinoma cell line HTC-TSHr was demonstrated. There was no difference between the expression levels of estrogen receptor alpha in males and females, but there was a significant increase in expression levels in response to 17beta-estradiol. Stimulation of benign and malignant thyroid cells with 17beta-estradiol resulted in an increased proliferation rate and an enhanced expression of cyclin D1 protein, which plays a key role in the regulation of G(1)/S transition in the cell cycle. In malignant tumor cells maximal cyclin D1 expression was observed after 3 h, whereas in benign cells the effect of 17beta-estradiol was delayed. ICI 182780, a pure estrogen antagonist, prevented the effects of 17beta-estradiol. In addition, 17beta-estradiol was found to modulate activation of mitogen-activated protein (MAP) kinase, whose activity is mainly regulated by growth factors in thyroid carcinoma cells. In response to 17beta-estradiol, both MAP kinase isozymes, extracellular signal-regulated protein kinases 1 and 2, were strongly phosphorylated in benign and malignant thyroid cells. Treatment of the cells with 17beta-estradiol and MAP kinase kinase 1 inhibitor, PD 098059, prevented the accumulation of cyclin D1 and estrogen-mediated mitogenesis. Our data indicate that 17beta-estradiol is a potent mitogen for benign and malignant thyroid tumor cells and that it exerts a growth-promoting effect not only by binding to nuclear estrogen receptors, but also by activation of the MAP kinase pathway.

Adrenergic regulation of mitogen-activated protein kinase in rat pinealocytes: opposing effects of protein kinase A and protein kinase G

The role of adrenergic stimulation in the regulation of mitogen-activated protein kinase (MAPK) in rat pinealocytes was investigated by measuring phosphorylated MAPK using Western blot analysis and a MAPK enzymatic assay. Stimulation with the endogenous neurotransmitter, norepinephrine (NE; a mixed alpha- and beta-adrenergic agonist), concentration dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of NE was blocked by PD98059 and U0126 (two inhibitors of MEK). Treatment with prazosin or propranolol significantly reduced the effect of NE on MAPK phosphorylation, suggesting the involvement of both alpha- and beta-adrenergic receptors. Investigation into the intracellular mechanisms of NE action revealed that the increase in MAPK phosphorylation was blocked by KT5823 (a protein kinase G inhibitor), but was enhanced by H89 (a protein kinase A inhibitor). Calphostin C (a protein kinase C inhibitor) and KN93 (a Ca2+/calmodulin-dependent protein kinase inhibitor) also attenuated NE-mediated MAPK activation, but to a lesser degree. Furthermore, inhibition of MAPK phosphorylation by (Bu)2cAMP was effective in reducing MAPK activation by (Bu)2cGMP, an active phorbol ester or ionomycin. These results indicate that the effect of NE on MAPK phosphorylation represents mainly the integration of two signaling mechanisms, protein kinase A and protein kinase G, each having an opposite effect on MAPK phosphorylation.

Conditional apoptosis induced by oncogenic ras in thyroid cells

Mutations of ras are tumor-initiating events for many cell types, including thyrocytes. To explore early consequences after oncogenic Ras activation, we developed a doxycycline-inducible expression system in rat thyroid PCCL3 cells. Beginning 3-4 days after H-Ras(v12) expression, cells underwent apoptosis. The H-Ras(v12) effects on apoptosis were decreased by a mitogen-activated protein kinase kinase (MEK1) inhibitor and recapitulated by doxycycline-inducible expression of an activated MEK1 mutant (MEK1(S217E/S221E)). As reported elsewhere, acute expression of H-Ras(v12) also induces mitotic defects in PCCL3 cells through ERK (extracellular ligand-regulated kinase) activation, suggesting that apoptosis may be secondary to DNA damage. However, acute activation of SAPK/JNK (stress-activated protein kinase/Jun N-terminal kinase) through acute expression of Rac1(v12) also triggered apoptosis, without inducing large-scale genomic abnormalities. H-Ras(v12)-induced apoptosis was dependent on concomitant activation of cAMP by either TSH or forskolin, in a protein kinase A-independent manner. Thus, coactivation of cAMP-dependent pathways and ERK or JNK (either through H-Ras(v12), Rac1(v12), or MEK1(S217E/S221E)) is inconsistent with cell survival. The fate of thyrocytes within the first cell cycles after expression of oncogenic Ras is dependent on ambient TSH levels. If both cAMP and Ras signaling are simultaneously activated, most cells will die. Those that survive will eventually lose TSH responsiveness and/or inactivate the apoptotic cascade through secondary events, thus enabling clonal expansion.

Role of the cAMP and MAPK pathways in the transformation of mouse 3T3 fibroblasts by a TSHR gene constitutively activated by point mutation

Constitutive activating mutations of the TSHR gene, have been detected in about 30 per cent of hyperfunctioning human thyroid adenomas and in a minority of differentiated thyroid carcinomas. The mutations activating the TSHR gene(s) in the thyroid carcinomas, were located at the codon 623 changing an Ala to a Ser (GCC-->TCC) or in codon 632 changing a Thr to Ala or Ile (ACC-->GCC or ACC-->ATC). In order to study if the constitutively activated TSHR gene(s) has played a role in the determination of the malignant phenotype presented by these tumors, we investigated: (1) the transforming capacity after transfection of mouse 3T3 cells, of a TSHR cDNA activated by an Ala-->Ser mutation in codon 623 or an Thr-->Ile mutation in codon 632 and (2) the pathway(s) eventually responsible(s) for the malignant phenotype of the cells transformed by these constitutively activated TSHR cDNAs. Our results show that (1) the TSHR(M623) or (M632) cDNAs give rise to 3T3 clones presenting a fully neoplastic phenotype (growth in agar and nude mouse tumorigenesis); this phenotype was weaker in the cells transformed by the 632 cDNA; (2) suggest that the fully transformed phenotype of our 3T3 cells, may be the consequence of the additive effect of the activation of at least two different pathways: the cAMP pathway through G(alpha)s and the Ras dependent MAPK pathway through G(beta)gamma and PI3K and (3) show that the PI3K isoform playing a key role as an effector in the MAPK pathway activation in our 3T3-transformed cells is PI3Kgamma. Signaling from PI3Kgamma to MAPK appears to require in our murine cellular system a tyrosine kinase (still not characterized), Shc, Grb2, Sos, Ras and Raf. It is proposed that the constitutively activated TSHR genes detected in the thyroid carcinomas, may have played an oncogenic role, participating in their development through these two pathways.

The RAS oncogene induces genomic instability in thyroid PCCL3 cells via the MAPK pathway

Activating mutations of RAS are thought to be early events in the evolution of thyroid follicular neoplasms. We used a doxycycline-inducible expression system to explore the acute effects of H-RAS12 on genomic stability in thyroid PCCL3 cells. At 2-3 days (first or second cell cycle) there was a significant increase in the frequency of micronucleation. Treatment of cells with YVAD-CHO inhibited RAS-induced apoptosis, but had no effect on micronucleation. The effects of H-RAS(V12) were mediated by activation of MAPK, as treatment with PD98059 at concentrations verified to selectively inhibit MEK1 reduced the frequency of prevalence of cells with micronuclei. In addition, doxycycline-inducible expression of a constitutively active MEK1, but not of a mutant RAC1, mimicked the effects of H-RAS(V12). The effects of H-RAS(V12) on genome destabilization were apparent even though the sequence of p53 in PCCL3 cells was confirmed to be wild-type. Acute activation of H-RAS(V12) evoked a proportional increase in both CREST negative and CREST positive micronuclei, indicating that both clastogenic and aneugenic effects were involved. H-RAS(V12) and activated MEK1 also induced centrosome amplification, and chromosome misalignment. Evidence that acute expression of constitutively activated RAS destabilizes the genome of PCCL3 cells is consistent with a mode of tumor initiation in which this oncogene promotes phenotypic progression by predisposing to large scale genomic abnormalities.

Cyclic AMP activates Ras

In addition to protein kinase A (PKA), cAMP regulates the activity of cAMP-gated channels and Rap1-specific guanine nucleotide exchange factors. We tested the hypothesis that the targets of cAMP might also include regulators of the Ras protooncogene. In rat thyroid cells, thyrotropin (TSH) stimulates proliferation through a cAMP-mediated pathway that requires Ras activity. Interference with Ras impairs DNA synthesis stimulated by TSH as well as cAMP elevating agents and analogs, demonstrating that the requirement for Ras lies down-stream of cAMP. Although cAMP stimulates proliferation, microinjection of the purified PKA catalytic subunit failed to do so, suggesting that factors in addition to PKA are required for cAMP-stimulated cell cycle progression. When added to thyroid cells expressing human Ha-Ras, TSH rapidly and markedly increased the proportion of GTP-bound Ras. Ras activity was increased within 1 min of TSH addition, maximal at 5-15 min, and declined to basal levels 30-60 min after hormone treatment. Cyclic AMP elevating agents elicited similar effects on Ras, indicating that TSH activates Ras through a cAMP-mediated pathway. Although cAMP-mediated, Ras activation by TSH and cAMP was independent of PKA activity. Moreover, cAMP-stimulated Ras activation was not impaired by tyrosine kinase inhibitors. These results indicate that cAMP activates targets in addition to PKA in thyroid cells, and that these targets may include regulators of Ras. The ability of cAMP elevating agents to activate Ras in addition to PKA may explain the inability of the PKA catalytic subunit to stimulate DNA synthesis in thyroid cells.

TSH and cAMP do not signal mitogenesis through Ras activation

Ras activation by receptor tyrosine kinases or serpentine receptors is generally considered to be essential for G1 phase progression and mitogenesis. In the physiologically relevant model of primary dog thyrocytes, the accumulation of the GTP-bound form of Ras constituted an early convergence point of various mitogenic or comitogenic stimuli including EGF, HGF, phorbol esters, insulin and carbachol. By contrast, the basal level of GTP-Ras was slightly reduced by TSH and forskolin and did not increase during the TSH/cAMP-dependent progression into G1 phase. This rules out a role for the activation of Ras as a signal in the mitogenesis elicited by TSH via cAMP in these cells.

Somatostatin interferes with thyrotropin-induced G1-S transition mediated by cAMP-dependent protein kinase and phosphatidylinositol 3-kinase. Involvement of RhoA and cyclin E x cyclin-dependent kinase 2 complexes

cAMP-mediated cell proliferation is a complex process that involves multiple pathways. Using a cAMP-dependent cell system, FRTL-5 thyroid cells, we have previously demonstrated the existence of a precise autocrine loop in the control of cell proliferation that involves the positive effector thyrotropin (TSH) and the general inhibitor somatostatin. In search of the regulatory mechanisms responsible for the TSH and somatostatin control of cell proliferation, we analyzed the cell cycle regulatory proteins and the cellular pathways involved in the action of both signals. The results show that specific inhibition of cAMP-dependent protein kinase (PKA) and phosphatidylinositol (PI) 3-kinase blocks independently TSH-induced FRTL-5 cell proliferation and that somatostatin interferes with both signals. Each pathway activates different proteins required for G(1)/S progression. Thus, PKA is responsible for the TSH-induction of 3-hydroxy-3-methylglutaryl-CoA reductase mRNA levels, RhoA activation, and down-regulation of p27(kip1). These correlated events are necessary for FRTL-5 cell proliferation after TSH stimulation. Moreover, TSH through PKA pathway increases cyclin-dependent kinase 2 levels, whereas PI 3-kinase signaling increases cyclin E levels. Together, both pathways finally converge, increasing the formation and activation of cyclin E x cyclin-dependent kinase 2 complexes and the phosphorylation of the retinoblastoma protein, two important steps in the transition from G(1) to S phase in growth-stimulated cells. Somatostatin exerts its antiproliferative effect inhibiting more upstream the TSH stimulation of PKA and PI 3-kinase, interfering with the TSH-mediated increases of intracellular cAMP levels by inactivation of adenylyl cyclase activity. Together, these results suggest the existence of a PKA-dependent pathway and a new PKA-independent PI 3-kinase pathway in the TSH/cAMP-mediated proliferation of FRTL-5 thyroid cells.

PI-3-kinase is an essential anti-apoptotic effector in the proliferative response of primary human epithelial cells to mutant RAS

In contrast to its growth-inhibitory effect on primary mesenchymal cells, RAS oncogene activation induces a proliferative phenotype in normal human thyroid epithelial cells in vitro, consistent with its putative role in tumour initiation. Using this model, we previously showed that activation of the MAP kinase (MAPK) pathway is necessary, but not sufficient for the proliferative response to mutant (V12) H-RAS. Here we extend this work to show that another major RAS effector-- phosphatidylinositol-3-kinase (PI-3-K)--while also insufficient alone, is able to synergize with MAPK activation to mimic the effect of mutant RAS, albeit at reduced efficiency. Furthermore we show that PI-3-K is an absolute requirement for the proliferative response to RAS in these cells, acting via suppression of RAS-induced apoptosis. These data extend our understanding of RAS signalling in a clinically-relevant cell context and point to the use of PI-3-K inhibitors as potential therapeutic agents for targetting human cancers induced by RAS mutation.

Ras signaling through PI3K confers hormone-independent proliferation that is compatible with differentiation

Hormones are specialized mitogens that stimulate proliferation in their differentiated target cells. Thyrotropin (TSH), the physiologic regulator of thyroid cells, stimulates cAMP-mediated proliferation and thyroid-specific gene expression. The mitogenic effects of TSH require Ras, therefore Ras activation should be compatible with the maintenance of thyroid differentiation. However, expression of activated Ras extinguishes the differentiated phenotype of thyroid cells. One explanation for this apparent paradox is the selective utilization of Ras effector pathways. We tested the hypothesis that Ras signaling through PI3K mediates the mitogenic effects of TSH in cells which retain their differentiated character. Expression of a Ras effector mutant (RasV12S35) that signals preferentially through Raf-1, although sufficient to confer TSH-independent proliferation, abolished hormone-regulated expression of thyroglobulin and the sodium/iodide symporter. In contrast, expression of a Ras mutant (RasV12C40) that binds selectively to PI3K conferred TSH-independent proliferation without marked effects on thyroid-specific gene expression. Unlike the inhibitory effects of TSH on the proliferation of RasV12S35-expressing cells, TSH enhanced RasV12C40-stimulated proliferation by further increasing the activity of p70s6k, an important mediator of the mitogenic effects of TSH and RasV12C40. These results demonstrate that channeling Ras-dependent signals to PI3K confers TSH with the ability to stimulate proliferation in differentiated cells. Oncogene (2000) 19, 924 - 932.

RAS oncogene activation induces proliferation in normal human thyroid epithelial cells without loss of differentiation

Neoplastic transformation of rodent thyroid epithelial cell lines by mutant RAS genes has been widely studied as an experimental model of oncogene-induced loss of tissue-specific differentiation. However, separate evidence strongly implicates RAS mutation as an early event in human thyroid tumour development at a stage prior to loss of differentiation. To resolve this controversy we examined the short- and long-term responses of normal human thyroid epithelial cells to mutant RAS introduced by micro-injection and retroviral transduction respectively. In both cases, expression of RAS at a level sufficient to induce rapid proliferation did not lead to loss of differentiation as shown by expression of cytokeratin 18, E-cadherin, thyroglobulin, TTF-1 and Pax-8 proteins. Indeed, RAS was able to prevent, and to reverse, the loss of thyroglobulin expression which occurs normally in TSH-deficient culture medium. These responses were partially mimicked by activation of RAF, a major RAS effector, indicating involvement of the MAP Kinase signal pathway. The striking contrast between the effect of mutant RAS on differentiation in primary human, compared to immortalized rodent, epithelial cultures is most likely explained by the influence of additional co-operating abnormalities in the latter, and highlights the need for caution in extrapolating from cell line data.

Differential effects of acute and chronic exposure to interferon-gamma on cyclic adenosine 3',5'-monophosphate response element-regulated gene expression

TSH stimulates proliferation and maintains differentiated function in thyroid follicular cells. The mitogenic activity and the stimulatory effects of TSH on thyroid-specific gene expression are impaired by interferon-gamma (IFNgamma); however, the mechanisms for these effects have not been elucidated in detail. We examined the effects of IFNgamma on acute responses to TSH in rat thyroid cells. IFNgamma did not impair TSH-stimulated p70/p85 ribosomal protein S6 kinase (p70/p85s6k) activity or cAMP response element (CRE)-regulated gene expression, although it inhibited DNA synthesis and thyroglobulin expression, effects measured over a more prolonged time course than those on kinase activity and reporter gene expression. Unexpectedly, when cells were chronically exposed to IFNgamma, CRE-lacZ promoter activity was decreased, whereas other cAMP-mediated signals, such as p70/p85s6k activity and CRE-binding protein phosphorylation, were unaffected. Activating protein-1-regulated promoters were also impaired by IFNgamma treatment, but with kinetics that differed from those of CRE-regulated promoters. Neither acute nor chronic treatment with interleukin-1beta impaired cAMP signaling, indicating that the effects of IFNgamma are specific. These studies identify CRE- and activating protein-1-regulated promoters as targets of IFNgamma in thyroid cells and fibroblasts. IFNgamma-mediated inhibition of these promoters, in addition to those containing thyroid-specific transcription factor-1-binding sites, may contribute to the profound effects of IFNgamma on thyroid cells.

A novel regulatory mechanism of MAP kinases activation and nuclear translocation mediated by PKA and the PTP-SL tyrosine phosphatase

Protein tyrosine phosphatase PTP-SL retains mitogen-activated protein (MAP) kinases in the cytoplasm in an inactive form by association through a kinase interaction motif (KIM) and tyrosine dephosphorylation. The related tyrosine phosphatases PTP-SL and STEP were phosphorylated by the cAMP-dependent protein kinase A (PKA). The PKA phosphorylation site on PTP-SL was identified as the Ser(231) residue, located within the KIM. Upon phosphorylation of Ser(231), PTP-SL binding and tyrosine dephosphorylation of the MAP kinases extracellular signal-regulated kinase (ERK)1/2 and p38alpha were impaired. Furthermore, treatment of COS-7 cells with PKA activators, or overexpression of the Calpha catalytic subunit of PKA, inhibited the cytoplasmic retention of ERK2 and p38alpha by wild-type PTP-SL, but not by a PTP-SL S231A mutant. These findings support the existence of a novel mechanism by which PKA may regulate the activation and translocation to the nucleus of MAP kinases.

Protein kinase A-dependent and -independent signaling pathways contribute to cyclic AMP-stimulated proliferation

The effects of cyclic AMP (cAMP) on cell proliferation are cell type specific. Although the growth-inhibitory effects of cAMP have been well studied, much less is known regarding how cAMP stimulates proliferation. We report that cAMP stimulates proliferation through both protein kinase A (PKA)-dependent and PKA-independent signaling pathways and that phosphatidylinositol 3-kinase (PI3K) is required for cAMP-stimulated mitogenesis. In cells where cAMP is a mitogen, cAMP-elevating agents stimulate membrane ruffling, Akt phosphorylation, and p70 ribosomal S6 protein kinase (p70s6k) activity. cAMP effects on ruffle formation and Akt were PKA independent but sensitive to wortmannin. In contrast, cAMP-stimulated p70s6k activity was repressed by PKA inhibitors but not by wortmannin or microinjection of the N-terminal SH2 domain of the p85 regulatory subunit of PI3K, indicating that p70s6k and Akt can be regulated independently. Microinjection of highly specific inhibitors of PI3K or Rac1, or treatment with the p70s6k inhibitor rapamycin, impaired cAMP-stimulated DNA synthesis, demonstrating that PKA-dependent and -independent pathways contribute to cAMP-mediated mitogenesis. Direct elevation of PI3K activity through microinjection of an antibody that stimulates PI3K activity or stable expression of membrane-localized p110 was sufficient to confer hormone-independent DNA synthesis when accompanied by elevations in p70s6k activity. These findings indicate that multiple pathways contribute to cAMP-stimulated mitogenesis, only some of which are PKA dependent. Furthermore, they demonstrate that the ability of cAMP to stimulate both p70s6k- and PI3K-dependent pathways is an important facet of cAMP-regulated cell cycle progression.

Activation of mitogen-activated protein kinase is necessary but not sufficient for proliferation of human thyroid epithelial cells induced by mutant Ras

Given the high frequency of ras oncogene activation in several common human cancers, its signal pathways are an important target for novel therapy. For practical reasons, however, these have been studied mainly in the context of transformation of established fibroblast cell lines, whereas ras acts at an earlier stage in human tumorigenesis and predominantly on epithelial cells. Here we have developed a more directly relevant model - human primary thyroid epithelial cells - which are a major target of naturally-occurring Ras mutation, and in which expression of mutant Ras in culture induces clonal expansion without morphological transformation, closely reproducing the phenotype of the corresponding tumour in vivo. Transient or stable expression of mutant H-ras (by scrapeloading or retroviral infection) at levels which stimulated proliferation induced sustained activation and translocation of MAP kinase (MAPK) in these cells. Inhibition of the MAPK pathway at the level of MAPKK, by expression of a dominant-negative mutant or by the pharmacological inhibitor PD98059, efficiently blocked the proliferative response. Conversely, selective activation of MAPK by a constitutively-active MAPKK1 mutant failed to mimic the action of Ras and, although this was achievable with activated Raf, micro-injection of anti-ras antibodies showed that this still required endogenous wild-type Ras function. In contrast to recent results obtained with a rodent thyroid cell line (WRT), therefore, activation of the MAPK pathway is necessary, but not sufficient, for the proliferogenic action of mutant Ras on primary human thyroid cells. These data emphasize the unreliability of extrapolation from cell lines and establish the feasibility of using a more representative human epithelial model for Ras signalling studies.

The gene expression of the amiloride-sensitive epithelial sodium channel alpha-subunit is regulated by antagonistic effects between glucocorticoid hormone and ras pathways in salivary epithelial cells

The functional expression of the amiloride-sensitive epithelial sodium channel (ENaC) in select epithelia is critical for maintaining electrolyte and fluid homeostasis. Although ENaC activity is strictly dependent upon its alpha-subunit expression, little is known about the molecular mechanisms by which cells modulate alpha-ENaC gene expression. Previously, we have shown that salivary alpha-ENaC expression is transcriptionally repressed by the activation of Raf/extracellular signal-regulated protein kinase pathway. Here, this work further investigates the molecular mechanism(s) by which alpha-ENaC expression is regulated in salivary epithelial Pa-4 cells. A region located between -1.5 and -1.0 kilobase pairs of the alpha-ENaC 5'-flanking region is demonstrated to be indispensable for the maximal and Ras-repressible reporter expression. Deletional analyses using heterologous promoter constructs reveal that a DNA sequence between -1355 and -1269 base pairs functions as an enhancer conferring the high level of expression on reporter constructs, and this induction effect is inhibited by Ras pathway activation. Mutational analyses indicate that full induction and Ras-mediated repression require a glucocorticoid response element (GRE) located between -1323 and -1309 base pairs. The identified alpha-ENaC GRE encompassing sequence (-1334/-1306) is sufficient to confer glucocorticoid receptor/dexamethasone-dependent and Ras-repressible expression on both heterologous and homologous promoters. This report demon- strates for the first time that the cross-talk between glucocorticoid receptor and Ras/extracellular signal-regulated protein kinase signaling pathways results in an antagonistic effect at the transcriptional level to modulate alpha-ENaC expression through the identified GRE. In summary, this study presents a mechanism by which alpha-ENaC expression is regulated in salivary epithelial cells.