Quantitative determination of Rap 1 activation in cyclic nucleotide-treated HL-60 leukemic cells: lack of Rap 1 activation in variant cells

Epac activation sensitizes rat sensory neurons through activation of Ras

Guanine nucleotide exchange factors directly activated by cAMP (Epacs) have emerged as important signaling molecules mediating persistent hypersensitivity in animal models of inflammation, by augmenting the excitability of sensory neurons. Although Epacs activate numerous downstream signaling cascades, the intracellular signaling which mediates Epac-induced sensitization of capsaicin-sensitive sensory neurons remains unknown. Here, we demonstrate that selective activation of Epacs with 8-CPT-2'-O-Me-cAMP-AM (8CPT-AM) increases the number of action potentials (APs) generated by a ramp of depolarizing current and augments the evoked release of calcitonin gene-related peptide (CGRP) from isolated rat sensory neurons. Internal perfusion of capsaicin-sensitive sensory neurons with GDP-βS, substituted for GTP, blocks the ability of 8CPT-AM to increase AP firing, demonstrating that Epac-induced sensitization is G-protein dependent. Treatment with 8CPT-AM activates the small G-proteins Rap1 and Ras in cultures of sensory neurons. Inhibition of Rap1, by internal perfusion of a Rap1-neutralizing antibody or through a reduction in the expression of the protein using shRNA does not alter the Epac-induced enhancement of AP generation or CGRP release, despite the fact that in most other cell types, Epacs act as Rap-GEFs. In contrast, inhibition of Ras through expression of a dominant negative Ras (DN-Ras) or through internal perfusion of a Ras-neutralizing antibody blocks the increase in AP firing and attenuates the increase in the evoked release of CGRP induced by Epac activation. Thus, in this subpopulation of nociceptive sensory neurons, it is the novel interplay between Epacs and Ras, rather than the canonical Epacs and Rap1 pathway, that is critical for mediating Epac-induced sensitization.

Regulatory T-cells and cAMP suppress effector T-cells independently of PKA-CREM/ICER: a potential role for Epac

cAMP signalling is both a major pathway as well as a key therapeutic target for inducing immune tolerance and is involved in Treg cell (regulatory T-cell) function. To achieve potent immunoregulation, cAMP can act through several downstream effectors. One proposed mechanism is that cAMP-mediated suppression, including immunosuppression by Treg cells, results from activation of PKA (protein kinase A) leading to the induction of the transcription factor ICER (inducible cAMP early repressor). In the present study, we examined CD4(+)CD25(-) Teff cell (effector T-cell) and CD4(+)CD25(+) Treg cell immune responses in Crem (cAMP-response-element modulator) gene-deficient mice which lack ICER (Crem(-/-)/ICER-deficient mice). ICER deficiency did not significantly alter the frequency or number of Treg cells and Teff cells. Treg cells or a pharmacological increase in cAMP suppressed Teff cells from Crem(+/+) and Crem(-/-)/ICER-deficient mice to an equivalent degree, demonstrating that ICER is dispensable in these functions. Additionally, activating the cAMP effector Epac (exchange protein directly activated by cAMP) suppressed Teff cells. Treg cells expressed low levels of all cyclic nucleotide Pde (phosphodiesterase) genes tested, but high levels of Epac. These data identify ICER as a redundant mediator of Treg cells and cAMP action on Teff cells and suggest that Epac may function as an alternative effector to promote cAMP-dependent Teff cell suppression.

Role of microtubules in attenuation of PepG-induced vascular endothelial dysfunction by atrial natriuretic peptide

Apart from control of circulating fluid, atrial natriuretic peptide (ANP) exhibits anti-inflammatory effects in the lung. However, molecular mechanisms of ANP anti-inflammatory effects are not well-understood. Peripheral microtubule (MT) dynamics is essential for agonist-induced regulation of vascular endothelial permeability. Here we studied the role of MT-dependent signaling in ANP protective effects against endothelial cell (EC) barrier dysfunction and acute lung injury induced by Staphylococcus aureus-derived peptidoglican-G (PepG). PepG-induced vascular endothelial dysfunction was accompanied by MT destabilization and disruption of MT network. ANP attenuated PepG-induced MT disassembly, NFκB signaling and activity of MT-associated Rho activator GEF-H1 leading to attenuation of EC inflammatory activation reflected by expression of adhesion molecules ICAM1 and VCAM1. ANP-induced EC barrier preservation and MT stabilization were linked to phosphorylation and inactivation of MT-depolymerizing protein stathmin. Expression of stathmin phosphorylation-deficient mutant abolished ANP protective effects against PepG-induced inflammation and EC permeability. In contrast, siRNA-mediated stathmin knockdown prevented PepG-induced peripheral MT disassembly and endothelial barrier dysfunction. ANP protective effects in a murine model of PepG-induced lung injury were associated with increased phosphorylation of stathmin, while exacerbated lung injury in the ANP knockout mice was accompanied by decreased pool of stable MT. Stathmin knockdown in vivo reversed exacerbation of lung injury in the ANP knockout mice. These results show a novel MT-mediated mechanism of endothelial barrier protection by ANP in pulmonary EC and animal model of PepG-induced lung injury via stathmin-dependent control of MT assembly.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Epac/Rap and PKA are novel mechanisms of ANP-induced Rac-mediated pulmonary endothelial barrier protection

Acute lung injury, sepsis, lung inflammation, and ventilator-induced lung injury are life-threatening conditions associated with lung vascular barrier dysfunction, which may lead to pulmonary edema. Increased levels of atrial natriuretic peptide (ANP) in lung circulation reported in these pathologies suggest its potential role in the modulation of lung injury. Besides well recognized physiological effects on vascular tone, plasma volume, and renal function, ANP may exhibit protective effects in models of lung vascular endothelial cell (EC) barrier dysfunction. However, the molecular mechanisms of ANP protective effects are not well understood. The recently described cAMP-dependent guanine nucleotide exchange factor (GEF) Epac activates small GTPase Rap1, which results in activation of small GTPase Rac-specific GEFs Tiam1 and Vav2 and Rac-mediated EC barrier protective responses. Our results show that ANP stimulated protein kinase A and the Epac/Rap1/Tiam/Vav/Rac cascade dramatically attenuated thrombin-induced pulmonary EC permeability and the disruption of EC monolayer integrity. Using pharmacological and molecular activation and inhibition of cAMP-and cGMP-dependent protein kinases (PKA and PKG), Epac, Rap1, Tiam1, Vav2, and Rac we linked ANP-mediated protective effects to the activation of Epac/Rap and PKA signaling cascades, which dramatically inhibited the Rho pathway of thrombin-induced EC hyper-permeability. These results suggest a novel mechanism of ANP protective effects against agonist-induced pulmonary EC barrier dysfunction via inhibition of Rho signaling by Epac/Rap1-Rac and PKA signaling cascades.

Signaling by small GTPases in the immune system

The Ras superfamily consists of over 50 low-molecular-weight proteins that cycle between an inactive guanosine diphosphate-bound state and an active guanosine triphosphate (GTP)-bound state. They are involved in a variety of signal transduction pathways that regulate cell growth, intracellular trafficking, cell migration, and apoptosis. Several methods have been devised to measure the activation state of Ras proteins, defined as the percent of Ras molecules in the active GTP-bound state. We have previously developed a quantitative biochemical method that can be applied to animal and human tissues and have used it to measure the activation state of Ras, Rap1, Rheb, and Rho proteins in cultured cells and in animal and human tumors. Ras, Rac, and Rho all play roles in regulating the functions of T and B lymphocytes and dendritic cells, and these proteins are clearly important in maintaining normal immune system function.

Drosophila TCTP is essential for growth and proliferation through regulation of dRheb GTPase

Cellular growth and proliferation are coordinated during organogenesis. Misregulation of these processes leads to pathological conditions such as cancer. Tuberous sclerosis (TSC) is a benign tumour syndrome caused by mutations in either TSC1 or TSC2 tumour suppressor genes. Studies in Drosophila and other organisms have identified TSC signalling as a conserved pathway for growth control. Activation of the TSC pathway is mediated by Rheb (Ras homologue enriched in brain), a Ras superfamily GTPase. Rheb is a direct target of TSC2 and is negatively regulated by its GTPase-activating protein activity. However, molecules required for positive regulation of Rheb have not been identified. Here we show that a conserved protein, translationally controlled tumour protein (TCTP), is an essential new component of the TSC-Rheb pathway. Reducing Drosophila TCTP (dTCTP) levels reduces cell size, cell number and organ size, which mimics Drosophila Rheb (dRheb) mutant phenotypes. dTCTP is genetically epistatic to Tsc1 and dRheb, but acts upstream of dS6k, a downstream target of dRheb. dTCTP directly associates with dRheb and displays guanine nucleotide exchange activity with it in vivo and in vitro. Human TCTP (hTCTP) shows similar biochemical properties compared to dTCTP and can rescue dTCTP mutant phenotypes, suggesting that the function of TCTP in the TSC pathway is evolutionarily conserved. Our studies identify TCTP as a direct regulator of Rheb and a potential therapeutic target for TSC disease.

Oncogenic Ras leads to Rho activation by activating the mitogen-activated protein kinase pathway and decreasing Rho-GTPase-activating protein activity

Transformation by oncogenic Ras requires signaling through Rho family proteins including RhoA, but the mechanism(s) whereby oncogenic Ras regulates the activity of RhoA is (are) unknown. We examined the effect of Ras on RhoA activity in NIH 3T3 cells either stably transfected with H-Ras(V12) under control of an inducible promoter or transiently expressing the activated H-Ras. Using a novel method to quantitate enzymatically the GTP bound to Rho, we found that expression of the oncogenic Ras increased Rho activity approximately 2-fold. Increased Rho activity was associated with increased plasma membrane binding of RhoA and decreased activity of the Rho/Ras-regulated p21(WAF1/CIP1) promoter. RhoA activation by oncogenic Ras could be explained by a decrease in cytosolic p190 Rho-GAP activity and translocation of p190 Rho-GAP from the cytosol to a detergent-insoluble cytoskeletal fraction. Pharmacologic inhibition of the Ras/Raf/MEK/ERK pathway prevented Ras-induced activation of RhoA and translocation of p190 Rho-GAP; expression of constitutively active Raf-1 kinase or MEK was sufficient to induce p190 Rho-GAP translocation. We conclude that in NIH 3T3 cells oncogenic Ras activates RhoA through the Raf/MEK/ERK pathway by decreasing the cytosolic activity and changing the subcellular localization of p190 Rho-GAP.

Cyclic AMP induces activation of extracellular signal-regulated kinases in HL-60 cells: role in cAMP-induced differentiation

It is well known that elevated intracellular cAMP induces growth arrest and the differentiation of HL-60 cells to neutrophil-like cells. The present study was designed to assess the regulation of the extracellular signal-regulated kinase (ERK) pathway by cAMP and its association with differentiation in HL-60 cells. We found that 8-bromoadenosine-3',5'-cyclic-monophosphate (8Br-cAMP)-induced the activation of ERK and mitogen-activated protein kinase (MEK), but inhibited B-Raf kinase via a protein kinase A (PKA)-mediated mechanism. Prolonged exposure to 8Br-cAMP increased the phorbol 12-myristate 13-acetate (TPA)-stimulated superoxide generation and CD14 expression that characterize the differentiation phenotype, which was blocked by MEK-1 inhibitor. These data suggest that cAMP-induced ERK activation is essential for the differentiation of HL-60 cells, independently of B-Raf.

cGMP-dependent protein kinase inhibits serum-response element-dependent transcription by inhibiting rho activation and functions

RhoA, in its active GTP-bound form, stimulates transcription through activation of the serum-response factor (SRF). We found that cGMP inhibited serum-induced Rho.GTP loading and transcriptional activation of SRF-dependent reporter genes in smooth muscle and glial cells in a cGMP-dependent protein kinase (G-kinase)-dependent fashion. Serum stimulation of the SRF target gene vinculin was also blocked by cGMP/G-kinase. G-kinase activation inhibited SRF-dependent transcription induced by upstream RhoA activators including Galpha(13) and p115RhoGEF, with Galpha(13)-induced Rho.GTP loading inhibited by G-kinase. G-kinase had no effect on the high activation levels of RhoA(63L) or the double mutant RhoA(63L,188A) but inhibited transcriptional activation by these two RhoA mutants to a similar extent, suggesting an effect downstream of RhoA and independent of RhoA Ser(188) phosphorylation. Constitutively active forms of the Rho effectors Rho kinase (ROK), PKN, and PRK-2 induced SRF-dependent transcription in a cell type-specific fashion with ROK being the most efficient; G-kinase inhibited transcription induced by all three effectors without affecting ROK catalytic activity. G-kinase had no effect on RhoA(63L)-induced morphological changes in glial cells, suggesting distinct transcriptional and cytoskeletal effectors of RhoA. We conclude that G-kinase inhibits SRF-dependent transcription by interfering with RhoA signaling; G-kinase acts both upstream of RhoA, inhibiting serum- or Galpha(13)-induced Rho activation, and downstream of RhoA, inhibiting steps distal to the Rho targets ROK, PKN, and PRK-2.

Rheb is in a high activation state and inhibits B-Raf kinase in mammalian cells

Rheb (Ras homolog enriched in brain) is a member of the Ras family of proteins, and is in the immediate Ras/Rap/Ral subfamily. We found in three different mammalian cell lines that Rheb was highly activated, to levels much higher than for Ras or Rap 1, and that Rheb's activation state was unaffected by changes in growth conditions. Rheb's high activation was not secondary to unique glycine to arginine, or glycine to serine substitutions at positions 14 and 15, corresponding to Ras residues 12 and 13, since Rheb R14G and R14G, S15G mutants had similarly high activation levels as wild type Rheb. These data are consistent with earlier work which showed that purified Rheb has similar GTPase activity as Ras, and suggest a relative intracellular deficiency of Rheb GTPase activating proteins (GAPs) compared to Rheb activators. Further evidence for relatively low intracellular GAP activity was that increased Rheb expression led to a marked increase in Rheb activation. Rheb, like Ras and Rap1, bound B-Raf kinase, but in contrast to Ras and Rap 1, Rheb inhibited B-Raf kinase activity and prevented B-Raf-dependent activation of the transcription factor Elk-1. Thus, Rheb appears to be a unique member of the Ras/Rap/Ral subfamily, and in mammalian systems may serve to regulate B-Raf kinase activity.

Crosstalk between cAMP and MAP kinase signaling in the regulation of cell proliferation

Hormonal stimulation of cyclic adenosine monophosphate (cAMP) and the cAMP-dependent protein kinase PKA regulates cell growth by multiple mechanisms. A hallmark of cAMP is its ability to stimulate cell growth in many cell types while inhibiting cell growth in others. In this review, the cell type-specific effects of cAMP on the mitogen-activated protein (MAP) kinase (also called extracellular signal-regulated kinase, or ERK) cascade and cell proliferation are examined. Two basic themes are discussed. First, the capacity of cAMP for either positive or negative regulation of the ERK cascade accounts for many of the cell type-specific actions of cAMP on cell proliferation. Second, there are several specific mechanisms involved in the inhibition or activation of ERKs by cAMP. Emerging new data suggest that one of these mechanisms might involve the activation of the GTPase Rap1, which can activate or inhibit ERK signaling in a cell-specific manner.

Role of cAMP, PKA and Rap1A in thyroid follicular cell survival

Cyclic AMP (cAMP) rescues cells from apoptosis stimulated by diverse insults. We examined the role of cAMP as a survival factor, and the signaling pathways through which cAMP affords protection. Rat thyroid cells were selected for these studies given the predominant role of cAMP in thyrotropin (TSH)-stimulated proliferation and as an oncogene in thyroid cells. Wistar rat thyroid (WRT) cells perished via apoptosis following sodium nitroprusside (SNP) treatment. Elevations in cAMP following treatment with forskolin, 8BrcAMP or IBMX rescued cells from SNP-induced cell death. Notably, TSH prevented apoptosis, implicating an important role for this hormone as a survival factor. Cyclic AMP activates multiple signaling pathways including those mediated through PKA, PI3K, p70S6k and the Ras-related small G protein, Rap1. Intriguingly, multiple pathways modulate thyroid cell survival. Interference with cAMP-stimulated p70S6k, but not PI3K, activity abrogated cell survival. Treatment with PKA inhibitors was sufficient to stimulate apoptosis in hormone-deprived cells and markedly enhanced cell death in response to SNP. Cells expressing an activated Rap1A mutant exhibited an enhanced sensitivity to SNP-induced apoptosis, while those expressing dominant negative Rap1A were resistant to SNP-initiated cell death. Together, these findings establish an important role for PKA and Rap1 in the control of thyroid cell survival.

Invited review: cGMP-dependent protein kinase signaling mechanisms in smooth muscle: from the regulation of tone to gene expression

cGMP is a second messenger that produces its effects by interacting with intracellular receptor proteins. In smooth muscle cells, one of the major receptors for cGMP is the serine/threonine protein kinase, cGMP-dependent protein kinase (PKG). PKG has been shown to catalyze the phosphorylation of a number of physiologically relevant proteins whose function it is to regulate the contractile activity of the smooth muscle cell. These include proteins that regulate free intracellular calcium levels, the cytoskeleton, and the phosphorylation state of the regulatory light chain of smooth muscle myosin. Other studies have shown that vascular smooth muscle cells (VSMCs) that are cultured in vitro may cease to express PKG and will, coincidentally, acquire a noncontractile, synthetic phenotype. The restoration of PKG expression to the synthetic phenotype VSMC results in the cells acquiring a more contractile phenotype. These more recent studies suggest that PKG controls VSMC gene expression that, in turn, regulates phenotypic modulation of the cells. Therefore, the regulation of PKG gene expression appears to be linked to phenotypic modulation of VSMC. Because several vascular disorders are related to the accumulation of synthetic, fibroproliferative VSMC in the vessel wall, it is likely that changes in the activity of the nitric oxide/cGMP/PKG pathway is involved the development of these diseases.

Cyclic AMP-mediated inhibition of cell growth requires the small G protein Rap1

In many normal and transformed cell types, the intracellular second messenger cyclic AMP (cAMP) blocks the effects of growth factors and serum on mitogenesis, proliferation, and cell cycle progression. cAMP exerts these growth-inhibitory effects via inhibition of the mitogen-activated protein (MAP) kinase cascade. Here, using Hek293 and NIH 3T3 cells, we show that cAMP's inhibition of the MAP kinase cascade is mediated by the small G protein Rap1. Activation of Rap1 by cAMP induces the association of Rap1 with Raf-1 and limits Ras-dependent activation of ERK. In NIH 3T3 cells, Rap1 is required not only for cAMP's inhibition of ERK activation but for inhibition of cell proliferation and mitogenesis as well.

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.

Cell type-specific regulation of B-Raf kinase by cAMP and 14-3-3 proteins

Cyclic AMP can either activate or inhibit the mitogen-activated protein kinase (MAPK) pathway in different cell types; MAPK activation has been observed in B-Raf-expressing cells and has been attributed to Rap1 activation with subsequent B-Raf activation, whereas MAPK inhibition has been observed in cells lacking B-Raf and has been attributed to cAMP-dependent protein kinase (protein kinase A)-mediated phosphorylation and inhibition of Raf-1 kinase. We found that cAMP stimulated MAPK activity in CHO-K1 and PC12 cells but inhibited MAPK activity in C6 and NB2A cells. In all four cell types, cAMP activated Rap1, and the 95- and 68-kDa isoforms of B-Raf were expressed. cAMP activation or inhibition of MAPK correlated with activation or inhibition of endogenous and transfected B-Raf kinase. Although all cell types expressed similar amounts of 14-3-3 proteins, approximately 5-fold less 14-3-3 was associated with B-Raf in cells in which cAMP was inhibitory than in cells in which cAMP was stimulatory. We found that the cell type-specific inhibition of B-Raf could be completely prevented by overexpression of 14-3-3 isoforms, whereas expression of a dominant negative 14-3-3 mutant resulted in partial loss of B-Raf activity. Our data suggest that 14-3-3 bound to B-Raf protects the enzyme from protein kinase A-mediated inhibition; the amount of 14-3-3 associated with B-Raf may explain the tissue-specific effects of cAMP on B-Raf kinase activity.