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

A-Kinase Anchor Protein 95 Is Involved in ERK1/2-Elk-1 Signal Transduction in Colon Cancer

Objectives

To assess A-kinase anchor protein 95 (AKAP95), B-Raf, extracellular regulated protein kinases 1/2 (ERK1/2), and Elk-1 expression in colon cancer tissue, and characterize AKAP95 associations with B-Raf, ERK1/2, Elk-1, and colon cancer clinicopathological indices.

Methods

The immunohistochemistry streptavidin-perosidase (SP) method was used to determine protein expression levels in 64 colon cancer and 32 para-carcinoma tissue specimens.

Results

(1) Positive AKAP95 expression rates in colon cancer tissue were higher when compared with para-carcinoma tissue (92.19% vs. 59.38%, P < 0.05). Similar findings were determined for B-Raf (76.56% vs. 25%, P < 0.05), ERK1/2 (90.63% vs. 31.25%, P < 0.05), and Elk-1 levels (92.19% vs. 40.63%, P < 0.05). (2) No significant associations were identified between AKAP95, B-Raf, ERK1/2, and Elk-1 protein expression and degree of differentiation, histological type, and lymph node metastasis in colon cancer samples (P > 0.05); however, in The Cancer Genome Atlas and Gene Expression Omnibus datasets, AKAP95 was closely related to immune infiltration, and highly expressed AKAP95 was negatively associated with overall survival and relapse free survival rates in colon cancer patients. (3) Correlations were observed between AKAP95 and ERK1/2, AKAP95 and Elk-1, B-Raf and ERK1/2, B-Raf and Elk-1, and ERK1/2 and Elk-1 (all P < 0.05), but no correlation was observed between AKAP95 and B-Raf (P > 0.05).

Conclusions

AKAP95 may affect immune infiltration levels in colon cancer by participating in ERK1/2-Elk-1 signal transduction.

Regulation of Transcriptional Activity of Merkel Cell Polyomavirus Large T-Antigen by PKA-Mediated Phosphorylation

Merkel cell polyomavirus (MCPyV) is the major cause of Merkel cell carcinoma (MCC), an aggressive skin cancer. MCPyV large T-antigen (LTag) and small T-antigen (sTag) are the main oncoproteins involved in MCPyV-induced MCC. A hallmark of MCPyV-positive MCC cells is the expression of a C-terminal truncated LTag. Protein kinase A (PKA) plays a fundamental role in a variety of biological processes, including transcription by phosphorylating and thereby regulating the activity of transcription factors. As MCPyV LTag has been shown to be phosphorylated and acts as a transcription factor for the viral early and late promoter, we investigated whether LTag can be phosphorylayted by PKA, and whether this affects the transcript activity of LTag. Using a phosphorylation prediction algorithm, serine 191, 203, and 265 were identified as putative phosphorylation sites for PKA. Mass spectrometry of in vitro PKA-phosphorylated peptides confirmed phosphorylation of S203 and S265, but not S191. Full-length LTag inhibited early and late promoter activity of MCPyV, whereas the truncated MKL2 LTag variant stimulated both promoters. Single non-phosphorylable, as well as phosphomimicking mutations did not alter the inhibitory effect of full-length LTag. However, the non-phosphorylable mutations abrogated transactivation of the MCPyV promoters by MKL2 LTag, whereas phosphomimicking substitutions restored the ability of MKL2 LTag to activate the promoters. Triple LTag and MKL2 LTag mutants had the same effect as the single mutants. Activation of the PKA signaling pathway did not enhance MCPyV promoter activity, nor did it affect LTag expression levels in MCPyV-positive Merkel cell carcinoma (MCC) cells. Our results show that phosphorylation of truncated LTag stimulates viral promoter activity, which may contribute to higher levels of the viral oncoproteins LTag and sTag. Interfering with PKA-induced LTag phosphorylation/activity may be a therapeutic strategy to treat MCPyV-positive MCC patients.

Expression of active B-Raf proto-oncogene in kidney collecting ducts induces cyst formation in normal mice and accelerates cyst growth in mice with polycystic kidney disease

Polycystic kidney disease (PKD) is characterized by the formation and progressive enlargement of fluid-filled cysts due to abnormal cell proliferation. Cyclic AMP agonists, including arginine vasopressin, stimulate ERK-dependent proliferation of cystic cells, but not normal kidney cells. Previously, B-Raf proto-oncogene (BRAF), a MAPK kinase kinase that activates MEK-ERK signaling, was shown to be a central intermediate in the cAMP mitogenic response. However, the role of BRAF on cyst formation and enlargement in vivo had not been demonstrated. To determine if active BRAF induces kidney cyst formation, we generated transgenic mice that conditionally express BRAFV600E, a common activating mutation, and bred them with Pkhd1-Cre mice to express active BRAF in the collecting ducts, a predominant site for cyst formation. Collecting duct expression of BRAFV600E (BRafCD) caused kidney cyst formation as early as three weeks of age. There were increased levels of phosphorylated ERK (p-ERK) and proliferating cell nuclear antigen, a marker for cell proliferation. BRafCD mice developed extensive kidney fibrosis and elevated blood urea nitrogen, indicating a decline in kidney function, by ten weeks of age. BRAFV600E transgenic mice were also bred to Pkd1RC/RC and pcy/pcy mice, well-characterized slowly progressive PKD models. Collecting duct expression of active BRAF markedly increased kidney weight/body weight, cyst number and size, and total cystic area. There were increased p-ERK levels and proliferating cells, immune cell infiltration, interstitial fibrosis, and a decline in kidney function in both these models. Thus, our findings demonstrate that active BRAF is sufficient to induce kidney cyst formation in normal mice and accelerate cystic disease in PKD mice.

Hormonal Regulation of Autophagy in Thyroid PCCL3 Cells and the Thyroids of Male Mice

Autophagy is an evolutionarily conserved catabolic process by which cells degrade intracellular proteins and organelles in the lysosomes and recycle their metabolites. We have recently demonstrated the crucial role for the basal level of autophagic activity in thyrocyte survival and homeostasis using the thyroid-specific autophagy knockout mice. Here, we first studied hormonal regulation of autophagy in thyrocytes in vitro using a rat thyroid cell line PCCl3 and in vivo with mice. In cultured PCCl3 cells, thyroxine decreased microtubule-associated protein 1 light chain 3 (LC3) puncta (a component of autophagosome) and increased p62 (an autophagy substrate) levels, showing thyroxine-suppression of autophagy. In contrast, TSH increased both LC3 puncta and p62 levels, but at the same time stabilized p62 protein by inhibiting p62 degradation, indicating TSH induction of autophagy. Our experiments with various inhibitors identified that both the cAMP-protein kinase (PK) A-cAMP response element binding protein/ERK and PKC signaling pathways regulates positively autophagic activity. The in vivo results obtained with wild-type mice treated with methimazole and perchlorate or thyroxine were consistent with in vitro results. Next, in thyroid-specific autophagy knockout mice treated with methimazole and perchlorate (that is, mice were placed under a stressed condition where enhanced autophagy was required) for 2 months, lower follicle sizes and lower thyroglobulin contents in thyrocytes were observed, suggesting impaired thyroglobulin production presumably from insufficient nutrient supply. We therefore conclude that TSH positively regulates autophagic activity through the cAMP-PKA-cAMP response element binding protein/ERK and PKC signaling pathways, whereas thyroid hormones inhibit its activity in thyrocytes. Metabolites produced by autophagy appear to be necessary for protein synthesis stimulated by TSH.

Metalloproteinase PAPP-A regulation of IGF-1 contributes to polycystic kidney disease pathogenesis

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic cause of end-stage renal disease (ESRD). The treatment options for ADPKD are limited. We observed an upregulation in several IGF-1 pathway genes in the kidney of Pkd1RC/RC mice, a model of ADPKD. Pregnancy-associated plasma protein A (PAPP-A), a metalloproteinase that cleaves inhibitory IGF binding proteins (IGFBPs), increasing the local bioactivity of IGF-1, was highly induced in the kidney of ADPKD mice. PAPP-A levels were high in cystic fluid and kidneys of humans with ADPKD. Our studies further showed that PAPP-A transcription in ADPKD was mainly regulated through the cAMP/CREB/CBP/p300 pathway. Pappa deficiency effectively inhibited the development of cysts in the Pkd1RC/RC mice. The role of PAPP-A in cystic disease appears to be regulation of the IGF-1 pathway and cellular proliferation in the kidney. Finally, preclinical studies demonstrated that treatment with a monoclonal antibody that blocks the proteolytic activity of PAPP-A against IGFBP4 ameliorated ADPKD cystic disease in vivo in Pkd1RC/RC mice and ex vivo in embryonic kidneys. These data indicated that the PAPP-A/IGF-1 pathway plays an important role in the growth and expansion of cysts in ADPKD. Our findings introduce a therapeutic strategy for ADPKD that involves the inhibition of PAPP-A.

Trace amine-associated receptor 1 (TAAR1) promotes anti-diabetic signaling in insulin-secreting cells

Pancreatic β-cell failure in type 2 diabetes mellitus is a serious challenge that results in an inability of the pancreas to produce sufficient insulin to properly regulate blood glucose levels. Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor expressed by β-cells that has recently been proposed as a potential target for improving glycemic control and suppressing binge eating behaviors. We discovered that TAAR1 is coupled to Gα_s-signaling pathways in insulin-secreting β-cells to cause protein kinase A (PKA)/exchange protein activated by cAMP (Epac)-dependent release of insulin, activation of RAF proto-oncogene, Ser/Thr kinase (Raf)-mitogen-activated protein kinase (MAPK) signaling, induction of cAMP response element-binding protein (CREB)-insulin receptor substrate 2 (Irs-2), and increased β-cell proliferation. Interestingly, TAAR1 triggered cAMP-mediated calcium influx and release from internal stores, both of which were required for activation of a MAPK cascade utilizing calmodulin-dependent protein kinase II (CaMKII), Raf, and MAPK/ERK kinase 1/2 (MEK1/2). Together, these data identify TAAR1/Gα_s-mediated signaling pathways that promote insulin secretion, improved β-cell function and proliferation, and highlight TAAR1 as a promising new target for improving β-cell health in type 2 diabetes mellitus.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Counterregulation of cAMP-directed kinase activities controls ciliogenesis

The primary cilium emanates from the cell surface of growth-arrested cells and plays a central role in vertebrate development and tissue homeostasis. The mechanisms that control ciliogenesis have been extensively explored. However, the intersection between GPCR signaling and the ubiquitin pathway in the control of cilium stability are unknown. Here we observe that cAMP elevation promotes cilia resorption. At centriolar satellites, we identify a multimeric complex nucleated by PCM1 that includes two kinases, NEK10 and PKA, and the E3 ubiquitin ligase CHIP. We show that NEK10 is essential for ciliogenesis in mammals and for the development of medaka fish. PKA phosphorylation primes NEK10 for CHIP-mediated ubiquitination and proteolysis resulting in cilia resorption. Disarrangement of this control mechanism occurs in proliferative and genetic disorders. These findings unveil a pericentriolar kinase signalosome that efficiently links the cAMP cascade with the ubiquitin-proteasome system, thereby controlling essential aspects of ciliogenesis.

Phosphorylation of Rap1 by cAMP-dependent Protein Kinase (PKA) Creates a Binding Site for KSR to Sustain ERK Activation by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1 have been proposed to mediate this activation. Using HEK293 cells as a model system, we have recently shown that both Ras and Rap1 are required for cAMP signaling to ERKs. However, cAMP-dependent Ras signaling to ERKs is transient and rapidly terminated by PKA phosphorylation of the Raf isoforms C-Raf and B-Raf. In contrast, cAMP-dependent Rap1 signaling to ERKs and Rap1 is potentiated by PKA. We show that this is due to sustained binding of B-Raf to Rap1. One of the targets of PKA is Rap1 itself, directly phosphorylating Rap1a on serine 180 and Rap1b on serine 179. We show that these phosphorylations create potential binding sites for the adaptor protein 14-3-3 that links Rap1 to the scaffold protein KSR. These results suggest that Rap1 activation of ERKs requires PKA phosphorylation and KSR binding. Because KSR and B-Raf exist as heterodimers within the cell, this binding also brings B-Raf to Rap1, allowing Rap1 to couple to ERKs through B-Raf binding to Rap1 independently of its Ras-binding domain.

Protein Kinase A-independent Ras Protein Activation Cooperates with Rap1 Protein to Mediate Activation of the Extracellular Signal-regulated Kinases (ERK) by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1, have been proposed to mediate this activation, with either Ras or Rap1 acting in distinct cell types. Using Hek293 cells, we show that both Ras and Rap1 are required for cAMP signaling to ERKs. The roles of Ras and Rap1 were distinguished by their mechanism of activation, dependence on the cAMP-dependent protein kinase (PKA), and the magnitude and kinetics of their effects on ERKs. Ras was required for the early portion of ERK activation by cAMP and was activated independently of PKA. Ras activation required the Ras/Rap guanine nucleotide exchange factor (GEF) PDZ-GEF1. Importantly, this action of PDZ-GEF1 was disrupted by mutation within its putative cyclic nucleotide-binding domain within PDZ-GEF1. Compared with Ras, Rap1 activation of ERKs was of longer duration. Rap1 activation was dependent on PKA and required Src family kinases and the Rap1 exchanger C3G. This is the first report of a mechanism for the cooperative actions of Ras and Rap1 in cAMP activation of ERKs. One physiological role for the sustained activation of ERKs is the transcription and stabilization of a range of transcription factors, including c-FOS. We show that the induction of c-FOS by cAMP required both the early and sustained phases of ERK activation, requiring Ras and Rap1, as well as for each of the Raf isoforms, B-Raf and C-Raf.

Control of βAR- and N-methyl-D-aspartate (NMDA) Receptor-Dependent cAMP Dynamics in Hippocampal Neurons

Norepinephrine, a neuromodulator that activates β-adrenergic receptors (βARs), facilitates learning and memory as well as the induction of synaptic plasticity in the hippocampus. Several forms of long-term potentiation (LTP) at the Schaffer collateral CA1 synapse require stimulation of both βARs and N-methyl-D-aspartate receptors (NMDARs). To understand the mechanisms mediating the interactions between βAR and NMDAR signaling pathways, we combined FRET imaging of cAMP in hippocampal neuron cultures with spatial mechanistic modeling of signaling pathways in the CA1 pyramidal neuron. Previous work implied that cAMP is synergistically produced in the presence of the βAR agonist isoproterenol and intracellular calcium. In contrast, we show that when application of isoproterenol precedes application of NMDA by several minutes, as is typical of βAR-facilitated LTP experiments, the average amplitude of the cAMP response to NMDA is attenuated compared with the response to NMDA alone. Models simulations suggest that, although the negative feedback loop formed by cAMP, cAMP-dependent protein kinase (PKA), and type 4 phosphodiesterase may be involved in attenuating the cAMP response to NMDA, it is insufficient to explain the range of experimental observations. Instead, attenuation of the cAMP response requires mechanisms upstream of adenylyl cyclase. Our model demonstrates that Gs-to-Gi switching due to PKA phosphorylation of βARs as well as Gi inhibition of type 1 adenylyl cyclase may underlie the experimental observations. This suggests that signaling by β-adrenergic receptors depends on temporal pattern of stimulation, and that switching may represent a novel mechanism for recruiting kinases involved in synaptic plasticity and memory.

Noradrenaline is a stress related molecule that facilitates learning and memory when released in the hippocampus. The facilitation of memory is related to modulation of synaptic plasticity, but the mechanisms underlying this modulation are not well understood. We utilize a combination of live cell imaging and computational modeling to discover how noradrenergic receptor stimulation interacts with other molecules, such as calcium, required for synaptic plasticity and memory storage. Though prior work has shown that noradrenergic receptors and calcium interact synergistically to elevate intracellular second messengers when combined simultaneously, our results demonstrate that prior stimulation of noradrenergic receptors inhibits the elevation of intracellular second messengers. Our results further demonstrate that the inhibition may be caused by the noradrenergic receptor switching signaling pathways, thereby recruiting a different set of memory kinases. This switching represents a novel mechanism for recruiting molecules involved in synaptic plasticity and memory.

Ras-mutant cancer cells display B-Raf binding to Ras that activates extracellular signal-regulated kinase and is inhibited by protein kinase A phosphorylation

The small G protein Ras regulates proliferation through activation of the mitogen-activated protein (MAP) kinase (ERK) cascade. The first step of Ras-dependent activation of ERK signaling is Ras binding to members of the Raf family of MAP kinase kinase kinases, C-Raf and B-Raf. Recently, it has been reported that in melanoma cells harboring oncogenic Ras mutations, B-Raf does not bind to Ras and does not contribute to basal ERK activation. For other types of Ras-mutant tumors, the relative contributions of C-Raf and B-Raf are not known. We examined non-melanoma cancer cell lines containing oncogenic Ras mutations and express both C-Raf and B-Raf isoforms, including the lung cancer cell line H1299 cells. Both B-Raf and C-Raf were constitutively bound to oncogenic Ras and contributed to Ras-dependent ERK activation. Ras binding to B-Raf and C-Raf were both subject to inhibition by the cAMP-dependent protein kinase PKA. cAMP inhibited the growth of H1299 cells and Ras-dependent ERK activation via PKA. PKA inhibited the binding of Ras to both C-Raf and B-Raf through phosphorylations of C-Raf at Ser-259 and B-Raf at Ser-365, respectively. These studies demonstrate that in non-melanocytic Ras-mutant cancer cells, Ras signaling to B-Raf is a significant contributor to ERK activation and that the B-Raf pathway, like that of C-Raf, is a target for inhibition by PKA. We suggest that cAMP and hormones coupled to cAMP may prove useful in dampening the effects of oncogenic Ras in non-melanocytic cancer cells through PKA-dependent actions on B-Raf as well as C-Raf.

RAP1GAP inhibits cytoskeletal remodeling and motility in thyroid cancer cells

The functional significance of decreased RAP1GAP protein expression in human tumors is unclear. To identify targets of RAP1GAP downregulation in the thyroid gland, RAP1 and RAP2 protein expression in human thyroid cells and in primary thyroid tumors were analyzed. RAP1GAP and RAP2 were co-expressed in normal thyroid follicular cells. Intriguingly, RAP1 was not detected in normal thyroid cells, although it was detected in papillary thyroid carcinomas, which also expressed RAP2. Both RAP proteins were detected at the membrane in papillary thyroid tumors, suggesting that they are activated when RAP1GAP is downregulated. To explore the functional significance of RAP1GAP depletion, RAP1GAP was transiently expressed at the lowest level that is sufficient to block endogenous RAP2 activity in papillary and anaplastic thyroid carcinoma cell lines. RAP1GAP impaired the ability of cells to spread and migrate on collagen. Although RAP1GAP had no effect on protein tyrosine phosphorylation in growing cells, RAP1GAP impaired phosphorylation of focal adhesion kinase and paxillin at sites phosphorylated by SRC in cells acutely plated on collagen. SRC activity was increased in suspended cells, where it was inhibited by RAP1GAP. Inhibition of SRC kinase activity impaired cell spreading and motility. These findings identify SRC as a target of RAP1GAP depletion and suggest that the downregulation of RAP1GAP in thyroid tumors enhances SRC-dependent signals that regulate cellular architecture and motility.

Impaired learning and memory performance in a subclinical hypothyroidism rat model induced by hemi-thyroid electrocauterisation

It is well known that clinical hypothyroidism (CH) can induce cognitive deficits, and the decision to start treatment for CH with thyroxine is usually straightforward. However, the relationship of cognition dysfunction with subclinical hypothyroidism (SCH) is inconsistent, and the decision concerning the need to treat SCH is controversial. In the present study, we induced a SCH rat model by hemi-thyroid electrocauterisation; then employed a serial of behavioural tests, including a beam balance, open field task and Morris water maze (MWM), to investigate the behaviour performance of SCH rats; and finally explored the protein expression of phosphorylated extracellular signal-regulated kinase (ERK)1/2 in the hippocampus by western blotting. The results demonstrated that hemi-thyroid electrocauterised rats had an elevated plasma thyrotrophin-stimulating hormone (TSH) level, with normal free thyroxine (fT4) and triiodothyronine (T3) concentrations, which defines SCH in humans. If rat SCH is diagnosed according to measurements of both plasma TSH higher than 97.5 percentile for the sham group and fT4 in the range 2.5-97.5 percentile for the sham group, the success rate of SCH modelling was 66.6%. SCH decreased exploratory behaviour but did not affect motor function in rats, showing a negative correlation of exploratory behaviour with plasma TSH concentration. Moreover, SCH rats displayed an impairment of learning and memory ability in the MWM task, with a longer escape latency in the acquisition phase and a shorter duration in the target quadrant in the test phase compared to that of sham rats. The mechanism for this might be related to the increased plasma TSH concentration, the decreased hippocampal T3 level and the enhanced expression of phosphorylated ERK1/2 in the hippocampus. The results of the present study, together with the results obtained in other studies, suggest that treatment is necessary for SCH.

Role of Epac and protein kinase A in thyrotropin-induced gene expression in primary thyrocytes

cAMP pathway activation by thyrotropin (TSH) induces differentiation and gene expression in thyrocytes. We investigated which partners of the cAMP cascade regulate gene expression modulations: protein kinase A and/or the exchange proteins directly activated by cAMP (Epac). Human primary cultured thyrocytes were analysed by microarrays after treatment with the adenylate cyclase activator forskolin, the protein kinase A (PKA) activator 6-MB-cAMP and the Epac-selective cAMP analog 8-pCPT-2'-O-Me-cAMP (007) alone or combined with 6-MB-cAMP. Profiles were compared to those of TSH. Cultures treated with the adenylate cyclase- or the PKA activator alone or the latter combined with 007 had profiles similar to those induced by TSH. mRNA profiles of 007-treated cultures were highly distinct from TSH-treated cells, suggesting that TSH-modulated gene expressions are mainly modulated by cAMP and PKA and not through Epac in cultured human thyroid cells. To investigate whether the Epac-Rap-RapGAP pathway could play a potential role in thyroid tumorigenesis, the mRNA expressions of its constituent proteins were investigated in two malignant thyroid tumor types. Modulations of this pathway suggest an increased Rap pathway activity in these cancers independent from cAMP activation.

Prostaglandin E2 induces matrix metalloproteinase 9 expression in dendritic cells through two independent signaling pathways leading to activator protein 1 (AP-1) activation

Dendritic Cells (DCs) play an important role in the initiation of the immune response by migrating to regional lymph nodes and presenting antigen processed at the inflammatory site to antigen-specific naïve T cells. Prostaglandin E2 (PGE2) has been reported to play an essential role in DC migration. We reported previously that PGE2 induces matrix metalloproteinase 9 (MMP-9) expression in DCs and that PGE2-induced MMP-9 is required for DC migration in vivo and in vitro. In this study, we investigated the signaling mechanisms involved in PGE2-induced MMP-9 expression in DCs. We show that PGE2-induced MMP-9 expression is mediated primarily through the EP2/EP4 → cAMP → protein kinase A (PKA)/PI3K → ERK signaling pathway, leading to c-Fos expression, and through JNK-mediated activation of c-Jun in a PKA/PI3K/ERK-independent manner. EP2 and EP4 receptor agonists, as well as cAMP analogs, mimic the up-regulation of MMP-9 by PGE2. PKA, PI3K, and ERK inhibitors abolished PGE2- and cAMP-induced c-Fos and MMP-9 up-regulation, and ERK activation was required for the binding of activator protein 1 (AP-1) transcription factor to the MMP-9 promoter. Our results describe a new molecular mechanism for the effect of PGE2 on MMP-9 production in DCs that could lead to future therapeutic approaches using ERK inhibitors to regulate DC migration.

cAMP antagonizes ERK-dependent antiapoptotic action of insulin

Insulin has antiapoptotic activity in various cell types. However, the signaling pathways underlying the antiapoptotic activity of insulin is not yet known. This study was conducted to determine if cAMP affects the antiapoptotic activity of insulin and the activity of PI3K and ERK in CHO cells expressing human insulin receptors (CHO-IR). Insulin-stimulated ERK activity was completely suppressed by cAMP-elevating agents like as pertussis toxin (Ptx) and cholera toxin (Ctx) after 4 h treatment. Insulin-stimulated PKB/Akt activity was not affected at all. Ptx treatment together with insulin increased the number of apoptotic cells and the degree of DNA fragmentation. Ctx or 8-brcAMP treatment also increased the number of apoptotic cells and stimulated the cleavage of caspase-3 and the hydrolysis of PARP. Taken together, cAMP antagonizes the antiapoptotic activity of insulin and the main target molecule of cAMP in this process is likely ERK, not PI3K-dependent PKB/Akt.

Rap1GAP impairs cell-matrix adhesion in the absence of effects on cell-cell adhesion

The significance of the widespread downregulation of Rap1GAP in human tumors is unknown. In previous studies we demonstrated that silencing Rap1GAP expression in human colon cancer cells resulted in sustained increases in Rap activity, enhanced spreading on collagen and the weakening of cell-cell contacts. The latter finding was unexpected based on the role of Rap1 in strengthening cell-cell adhesion and reports that Rap1GAP impairs cell-cell adhesion. We now show that Rap1GAP is a more effective inhibitor of cell-matrix compared to cell-cell adhesion. Overexpression of Rap1GAP in human colon cancer cells impaired Rap2 activity and the ability of cells to spread and migrate on collagen IV. Under the same conditions, Rap1GAP had no effect on cell-cell adhesion. Overexpression of Rap1GAP did not enhance the dissociation of cell aggregates nor did it impair the accumulation of β-catenin and E-cadherin at cell-cell contacts. To further explore the role of Rap1GAP in the regulation of cell-cell adhesion, Rap1GAP was overexpressed in non-transformed thyroid epithelial cells. Although the formation of cell-cell contacts required Rap1, overexpression of Rap1GAP did not impair cell-cell adhesion. These data indicate that transient, modest expression of Rap1GAP is compatible with cell-cell adhesion and that the role of Rap1GAP in the regulation of cell-cell adhesion may be more complex than is currently appreciated.

Histone deacetylase inhibitors upregulate Rap1GAP and inhibit Rap activity in thyroid tumor cells

Increases in Rap activity have been associated with tumor progression. Although activating mutations in Rap have not been described, downregulation of Rap1GAP is frequent in human tumors including thyroid carcinomas. In this study, we explored whether endogenous Rap1GAP expression could be restored to thyroid tumor cells. The effects of deacetylase inhibitors and a demethylating agent, individually and in combination, were examined in four differentiated and six anaplastic thyroid carcinoma (ATC) cell lines. Treatment with the structurally distinct histone deacetylase (HDAC) inhibitors, sodium butyrate and trichostatin A, increased Rap1GAP expression in all the differentiated thyroid carcinoma cell lines and in four of the six ATC cell lines. The demethylating agent, 5-aza-deoxycytidine, restored Rap1GAP expression in one anaplastic cell line and enhanced the effects of HDAC inhibitors in a second anaplastic cell line. Western blotting indicated that Rap2 was highly expressed in human thyroid cancer cells. Importantly, treatment with HDAC inhibitors impaired Rap2 activity in both differentiated and anaplastic tumor cell lines. The mechanism through which Rap activity is repressed appears to entail effects on the expression of multiple Rap regulators, including RapGEFs and RapGAPs. These results suggest that HDAC inhibitors may provide a tractable approach to impair Rap activity in human tumor cells.

Molecular regulation of thyroid gland function

PURPOSE OF REVIEW

For de-novo thyroid hormone synthesis ex vivo, thyroid follicular cells require a serum-free medium supplying nutrients, iodide, thyroid-stimulating hormone and insulin-like growth factor I (IGF-I) (or insulin). Under these conditions, T3 and T4 are secreted but so are other factors such as growth factors, plasminogen activators, their inhibitors known as serpins, and so on. What is the function of these factors? Do thyroid cells respond to them or are these paracrine/endocrine factors? The purpose of this review is to highlight the current developments in the identification and role of the signalling pathways that regulate thyroid growth and function and the putative role of endogenous thyroid proteases in regulating this.

RECENT FINDINGS

The roles of the mitogen-activated protein kinases and phosphoinositol 3 kinases and integrins in mediating growth and function in thyroid cancer cells and the roles of plasminogen activators, their receptors and the downstream signalling pathways they modulate have been developed. Discoveries of novel proteases, expressed in thyroid cancers, may be useful in diagnosis.

SUMMARY

The signalling pathways regulating thyroid activity are examined and the roles of follicular cell products in maintaining thyroid homeostasis evaluated. The possibility that thyroid cell products other than T3 and T4 may circulate and have extrathyroidal effects is proposed.

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.

cAMP/CREB-mediated transcriptional regulation of ectonucleoside triphosphate diphosphohydrolase 1 (CD39) expression

CD39 is a transmembrane enzyme that inhibits platelet reactivity and inflammation by phosphohydrolyzing ATP and ADP to AMP. Cyclic AMP (cAMP), an essential second messenger, is particularly important in regulating genes controlling vascular homeostasis. These experiments test the hypothesis that cAMP might positively regulate the expression of CD39 and thereby modulate important vascular homeostatic properties. Cd39 mRNA was induced by 13.8- fold in RAW cells treated with a membrane-permeant cAMP analogue (8-bromo-cyclic AMP; 8-Br-cAMP), stimulation of adenylate cyclase, or prostanoids known to drive cAMP response. Fluorescence-activated cell sorting, immunofluorescence, and TLC assays demonstrated that both CD39 protein expression and enzymatic activity were increased in cells treated with 8-Br-cAMP but not in cells transfected with short hairpin RNA against CD39. This analogue drove a significant increase in transcriptional activity at the Cd39 promoter although not when the promoter's cAMP-response element sites were mutated. Pretreatment with cAMP-dependent protein kinase (PKA), phosphoinositide 3-kinase (PI3K), or ERK inhibitors nearly obliterated the cAMP-driven increase in Cd39 mRNA, protein expression, and promoter activity. 8-Br-cAMP greatly increased the phosphorylation of CREB1 (Ser(133)) and ATF2 (Thr(71)) in a PKA-, PI3K-, and ERK-dependent fashion. Chromatin immunoprecipitation assays demonstrated that binding of phosphorylated CREB1 and ATF2 to cAMP-response element-like sites was significantly increased with 8-Br-cAMP treatment and that binding was reduced with PKA, PI3K, and ERK inhibition, whereas transfection of Creb1 and Atf2 overexpression constructs enhanced cAMP-driven Cd39 mRNA expression. Transfection of RAW cells with mutated Creb1 (S133A) reduced cAMP-driven Cd39 mRNA expression. Furthermore, the cAMP-mediated induction of Cd39 mRNA, protein, and phosphohydrolytic activity was replicated in primary peritoneal macrophages. These data identify cAMP as a crucial regulator of macrophage CD39 expression and demonstrate that cAMP acts through the PKA/CREB, PKA/PI3K/ATF2, and PKA/ERK/ATF2 pathways to control a key vascular homeostatic mediator.