Differential fMet-Leu-Phe- and platelet-activating factor-induced signaling toward Ral activation in primary human neutrophils

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Systematic single cell analysis of migration and morphological changes of human neutrophils over stimulus concentration gradients

To compare the responses of individual neutrophils to chemoattractants, migration pathway data were obtained using TAXIScan, an optically accessible/horizontal apparatus in which a concentration gradient is established reproducibly for a given stimulus. The observed linear-mode trajectory pattern of neutrophils toward N-formyl-methionyl-leucyl-phenylalanine (fMLP) or Interleukin (IL)-8/CXCL8 was distinguished from random migration patterns toward leukotriene (LT) B4 or platelet activating factor (PAF). The median values of velocity and directionality calculated for individual cells toward fMLP and IL-8 were both relatively similar and high, whereas the values toward LTB4 and PAF were widely dispersed over a lower range of directionality and from low to high ranges of velocity. The different patterns between the groups may be explained by unique morphology with single polarity toward fMLP and IL-8, and unstable morphology with multiple polarities toward LTB4 and PAF. Unique morphologies toward fMLP and IL-8 were not affected by coexisting LTB4 or PAF. On the other hand, the addition of suboptimum concentrations of fMLP or IL-8 to LTB4 or PAF induced a nearly maximum chemotactic response in most cells. These data suggest that exogenous formyl peptides and endogenous chemokines augment neutrophil accumulation at inflammation sites, whereas lipid mediators may play a role in supporting activation of the inflammatory cells for recruitment.

An integrated stochastic model of "inside-out" integrin activation and selective T-lymphocyte recruitment

The pattern of T-lymphocyte homing is hypothesized to be controlled by combinations of chemokine receptors and complementary chemokines. Here, we use numerical simulation to explore the relationship among chemokine potency and concentration, signal transduction, and adhesion. We have developed a form of adhesive dynamics-a mechanically accurate stochastic simulation of adhesion-that incorporates stochastic signal transduction using the next subvolume method. We show that using measurable parameter estimates derived from a variety of sources, including signaling measurements that allow us to test parameter values, we can readily simulate approximate time scales for T-lymphocyte arrest. We find that adhesion correlates with total chemokine receptor occupancy, not the frequency of occupation, when multiple chemokine receptors feed through a single G-protein. A general strategy for selective T-lymphocyte recruitment appears to require low affinity chemokine receptors. For a single chemokine receptor, increases in multiple cross-reactive chemokines can lead to an overwhelming increase in adhesion. Overall, the methods presented here provide a predictive framework for understanding chemokine control of T-lymphocyte recruitment.

Control of secondary granule release in neutrophils by Ral GTPase

Neutrophil (polymorphonuclear leukocyte; PMN) inflammatory functions, including cell adhesion, diapedesis, and phagocytosis, are dependent on the mobilization and release of various intracellular granules/vesicles. In this study, we found that treating PMN with damnacanthal, a Ras family GTPase inhibitor, resulted in a specific release of secondary granules but not primary or tertiary granules and caused dysregulation of PMN chemotactic transmigration and cell surface protein interactions. Analysis of the activities of Ras members identified Ral GTPase as a key regulator during PMN activation and degranulation. In particular, Ral was active in freshly isolated PMN, whereas chemoattractant stimulation induced a quick deactivation of Ral that correlated with PMN degranulation. Overexpression of a constitutively active Ral (Ral23V) in PMN inhibited chemoattractant-induced secondary granule release. By subcellular fractionation, we found that Ral, which was associated with the plasma membrane under the resting condition, was redistributed to secondary granules after chemoattractant stimulation. Blockage of cell endocytosis appeared to inhibit Ral translocation intracellularly. In conclusion, these results demonstrate that Ral is a critical regulator in PMN that specifically controls secondary granule release during PMN response to chemoattractant stimulation.

Amantadine inhibits platelet-activating factor induced clathrin-mediated endocytosis in human neutrophils

Receptor signaling is integral for adhesion, emigration, phagocytosis, and reactive oxygen species production in polymorphonuclear neutrophils (PMNs). Priming is an important part of PMN emigration, but it can also lead to PMN-mediated organ injury in the host. Platelet-activating factor (PAF) primes PMNs through activation of a specific G protein-coupled receptor. We hypothesize that PAF priming of PMNs requires clathrin-mediated endocytosis (CME) of the PAF receptor (PAFr), and, therefore, amantadine, known to inhibit CME, significantly antagonizes PAF signaling. PMNs were isolated by standard techniques to >98% purity and tested for viability. Amantadine (1 mM) significantly inhibited the PAF-mediated changes in the cellular distribution of clathrin and the physical colocalization [fluorescence resonance energy transfer positive (FRET+)] of early endosome antigen-1 and Rab5a, known components of CME and similar to hypertonic saline, a known inhibitor of CME. Furthermore, amantadine had no effect on the PAF-induced cytosolic calcium flux; however, phosphorylation of p38 MAPK was significantly decreased. Amantadine inhibited PAF-mediated changes in PMN physiology, including priming of the NADPH oxidase and shape change with lesser inhibition of increases in CD11b surface expression and elastase release. Furthermore, rimantadine, an amantadine analog, was a more potent inhibitor of PAF priming of the N-formyl-methionyl-leucyl-phenylalanine-activated oxidase. PAF priming of PMNs requires clathrin-mediated endocytosis that is inhibited when PMNs are pretreated with either amantadine or rimantadine. Thus, amantadine and rimantadine have the potential to ameliorate PMN-mediated tissue damage in humans.

Functional analysis of RalGPS2, a murine guanine nucleotide exchange factor for RalA GTPase

RalGPS2 is a murine guanine nucleotide exchange factor of the RalGPS family; it contains a Cdc25-like GEF domain and does not exhibit a Ras-binding domain. The main characteristic of RalGPS2 is its pleckstrin homology (PH) domain, present at the C terminus, that preferentially binds phosphatidylinositol-4,5-biphosphate and in HEK 293 cells localized in membranes, causing ruffling and vesiculation. Moreover, RalGPS2 contains a PxxP motif in the central part of the molecule. This motif binds in vitro and in vivo SH3 domains of Grb2 and PLCgamma. RalGPS2 and its GEF domain activate RalA in vivo while the PH-PxxP domains inhibited it behaving as a dominant negative for the RalA pathway; this activation was not inhibited by co-expression of a dominant negative Ras. RalGPS2 is physiologically expressed in testis and brain; when overexpressed, the whole RalGPS2 causes considerable morphological changes in HEK 293 cells, suggesting its possible role on cytoskeleton reorganization. This is further strengthened by data obtained in NIH3T3 cells where expression of PH-PxxP domain promotes actin depolymerization. Finally, RalGPS2 and its GEF domain induce Ras-independent transcriptional activation of the c-fos promoter in NIH3T3 cells.

E-selectin permits communication between PAF receptors and TRPC channels in human neutrophils

The selectin family of molecules (L-, P-, and E-selectin) mediates adhesive interactions between leukocytes and endothelial cells required for recruitment of leukocytes to inflammatory sites. Soluble E-selectin levels are elevated in inflammatory diseases and act to promote neutrophil β2-integrin–mediated adhesion by prolonging Ca2+ mobilization. Although soluble E-selectin alone was unable to initiate Ca2+ signaling, it allowed a novel “permissive” store-operative calcium entry (SOCE) following the initial platelet-activating factor (PAF)–induced release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)–sensitive stores. This induction of permissive SOCE in response to soluble E-selectin and PAF was shown to act through a G protein-coupled receptor (GPCR) coupled to pertussis toxin-insensitive Gq/11. Furthermore, we demonstrated that permissive SOCE was mediated by canonical transient receptor potential channel (TRPC) due to its sensitivity to specific inhibition by MRS1845 and Gd3+ and that TRPC6 was the principal TRPC family member expressed by human neutrophils. In terms of mechanism, we demonstrated that soluble E-selectin activated Src family tyrosine kinases, an effect that was upstream of phosphatidylinositol 3′-kinase in a signaling pathway that regulates permissive SOCE following exposure of neutrophils to PAF. In summary, this report provides the first evidence for communication between an inflammatory mediator and adhesion receptors at a molecular level, through selectin receptor ligation allowing permissive SOCE to occur following PAF stimulation of human neutrophils.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Small GTP-binding protein-coupled receptors

Heterotrimeric GPCRs (G-protein-coupled receptors) form the largest group of integral membrane receptor proteins and mediate diverse physiological processes. In addition to signalling via heterotrimeric G-proteins, GPCRs can also signal by interacting with various small G-proteins to regulate downstream effector pathways. The small G-protein superfamily is structurally classified into at least five families: the Ras, Rho/Rac/cdc42, Rab, Sar1/Arf and Ran families. They are monomeric G-proteins with molecular masses over the range 20-30 kDa, which function as molecular switches to control many eukaryotic cell functions. Several studies have provided evidence of crosstalk between GPCRs and small G-proteins. It is well documented that GPCR signalling through heterotrimeric G-proteins can lead to the activation of Ras and Rho GTPases. In addition, RhoA, Rabs, ARFs and ARF GEFs (guanine nucleotide-exchange factors) can associate directly with GPCRs, and GPCRs may also function as GEFs for small GTPases. In this review, we summarize the recent progress made in understanding the interaction between GPCRs and small GTPases, focusing on understanding how the association of small G-proteins with GPCRs and GPCR-regulatory proteins may influence GPCR signalling and intracellular trafficking.

Impaired interleukin-8- and GROα-induced phosphorylation of extracellular signal-regulated kinase result in decreased migration of neutrophils from patients with myelodysplasia

Patients with myelodysplasia suffer from recurrent bacterial infections as a result of differentiation defects of the myeloid lineage and a disturbed functioning of neutrophilic granulocytes. Important physiological activators of neutrophils are the cytokines interleukin-8/CXC chemokine ligand 8 (IL-8/CXCL8), which activates CXC chemokine receptor 1 and 2 (CXCR1 and CXCR2), and growth-related oncogene (GROalpha)/CXCL1, which stimulates only CXCR2. In this study, we show that migration toward IL-8/GROalpha gradients is decreased in myelodysplastic syndrome (MDS) neutrophils compared with healthy donors. We investigated the signal transduction pathways involved in IL-8/GROalpha-induced migration and showed that specific inhibitors for extracellular signal-regulated kinase (ERK)1/2 and phosphatidylinositol-3 kinase (PI-3K) abrogated neutrophil migration toward IL-8/GROalpha. In accordance with these results, we subsequently showed that IL-8/GROalpha-stimulated activation of ERK1/2 was substantially diminished in MDS neutrophils. Activation of the PI-3K downstream target protein kinase B/Akt was disturbed in MDS neutrophils when cells were activated with IL-8 but normal upon GROalpha stimulation. IL-8 stimulation resulted in higher migratory behavior and ERK1/2 activation than GROalpha stimulation, suggesting a greater importance of CXCR1. We then investigated IL-8-induced activation of the small GTPase Rac implicated in ERK1/2-dependent migration and found that it was less efficient in neutrophils from MDS patients compared with healthy donors. In contrast, IL-8 triggered a normal activation of the GTPases Ras and Ral, indicating that the observed defects were not a result of a general disturbance in CXCR1/2 signaling. In conclusion, our results demonstrate a disturbed CXCR1- and CXCR2-induced neutrophil chemotaxis in MDS patients, which might be the consequence of decreased Rac-ERK1/2 and PI-3K activation within these cells.

Dual signaling and effector pathways mediate human eosinophil activation by platelet-activating factor

Platelet-activating factor (PAF) induces various cellular functions in eosinophils including chemotaxis, adhesion, superoxide anion (O2-) production, and degranulation. While PAF shares many biological effects with other chemotactic factors such as N-formyl-methionyl-leucyl-phenylalanine, complement fragments, and lipid mediators, PAF is unique in that its action is relatively resistant to pertussis toxin (PTX), and in activating eosinophils more strongly than neutrophils. In this review we consider how PAF might activate human eosinophils in preference to neutrophils, and discuss possible mechanisms of PAF-induced activation of human eosinophils via two distinct signaling and effector pathways. Recently we analyzed O2- production by eosinophils using a sensitive, real-time chemiluminescence method. Our results showed that in human eosinophils PAF activates two distinct signaling and effector pathways coupled to the PAF receptor: one linked to PTX-sensitive G protein(s) and another to PTX-resistant G protein(s), phosphatidylinositol 3-kinase, and cellular adhesion. This activation of two different G proteins by the eosinophil PAF receptor may explain the strong and diverse biological responses of human eosinophils to PAF.

Disturbed granulocyte macrophage-colony stimulating factor priming of phosphatidylinositol 3,4,5-trisphosphate accumulation and Rac activation in fMLP-stimulated neutrophils from patients with myelodysplasia

The production of reactive oxygen species (ROS) by human neutrophils is imperative for their bactericidal activity. Proinflammatory agents such as granulocyte macrophage-colony stimulating factor (GM-CSF) can prime ROS production in response to chemoattractants such as N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP). In neutrophils from patients suffering from Myelodysplastic syndromes (MDS), a clonal, hematological disorder characterized by recurrent bacterial infections, this GM-CSF priming is severely impaired. In this study, we set out to further delineate the defects in neutrophils from MDS patients. We examined the effect of GM-CSF priming on fMLP-triggered activation of Rac, a small GTPase implicated in neutrophil ROS production. In contrast to healthy neutrophils, activation of Rac in response to fMLP was not enhanced by GM-CSF pretreatment in MDS neutrophils. Furthermore, activation of Rac was attenuated by pretreatment of neutrophils with the phosphatidylinositol 3-kinase (PI-3K) inhibitor LY294002. Unlike healthy neutrophils, fMLP-induced accumulation of the PI-3K lipid product PI(3,4,5)trisphosphate was not increased by GM-CSF pretreatment in MDS neutrophils. The disturbed Rac and PI-3K activation observed in MDS neutrophils did not appear to reflect a general GM-CSF or fMLP receptor-signaling defect, as fMLP-triggered Ras activation could be primed by GM-CSF in MDS and healthy neutrophils. Moreover, fMLP-induced activation of the GTPase Ral was also normal in neutrophils from MDS patients. Taken together, our data suggest that in neutrophils from MDS patients, a defect in priming of the PI-3K-Rac signaling pathway, located at the level of PI-3K, results in a decreased GM-CSF priming of ROS production.

Constitutive NF-kappaB DNA-binding activity in AML is frequently mediated by a Ras/PI3-K/PKB-dependent pathway

In the present study, we aimed to elucidate the mechanism responsible for constitutive NF-kappaB DNA-binding activity in AML cells. Intervening in aberrant signaling pathway provides a rational approach for in vivo targeting of AML cells. Constitutive NF-kappaB DNA-binding activity was observed in 16 of 22 (73%) investigated AML cases and was, in general, associated with resistance to spontaneous apoptosis. Indeed, inhibition of NF-kappaB activity by the NF-kappaB inhibitor SN-50 peptide resulted in enhanced chemotherapy-induced apoptosis. In the majority of cases, constitutive NF-kappaB activity was mediated by a Ras/PI3 kinase (PI3-K)/protein kinase B (PKB)-mediated pathway. The PI3-K inhibitor Ly294002 and the Ras inhibitor L-744832 both inhibited PKB phosphorylation and NF-kappaB DNA-binding activity. The constitutive activation of Ras GTP-ase was caused by mutations in the gene encoding for N-Ras in 29% of the cases. The constitutive NF-kappaB activity could so far not be ascribed to the autocrine production of growth factors or to mutations in the Flt3 receptor, since anti-GM-CSF, -IL-1, -IL6, -TNFalpha or the tyrosine kinase inhibitor AG1296 did not affect the NF-kappaB DNA-binding activity. The present study demonstrates that Ras activation is an important pathway for triggering the NF-kappaB pathway in AML cells.

Novel chemoattractant peptides for human leukocytes

Phospholipase A(2) plays a key role in phagocytic cell functions. By screening a synthetic hexapeptide combinatorial library, we identified 24 novel peptides based on their ability to stimulate arachidonic acid release associated with cytosolic phospholipase A(2) activity in differentiated HL60 cells. The identified peptides, that contain the consensus sequence (K/R/M)KYY(P/V/Y)M, also induce intracellular calcium release in a pertussis toxin-sensitive manner showing specific action on phagocytic leukocytes, but not on other cells. Functionally, the peptides stimulate superoxide generation and chemotactic migration in human neutrophils and monocytes. Four of the tested active peptides were ligands for formyl peptide receptor like 1. Among these, two peptides with the consensus sequence (R/M)KYYYM can induce intracellular calcium release in undifferentiated HL60 cells that do not express formyl peptide receptor like 1, indicating usage of other receptor(s). A study of intracellular signaling in differentiated HL60 cells induced by the peptides has revealed that four of the novel peptides can induce extracellular signal-regulated protein kinase activation via shared and distinct signaling pathways, based on their dependence of phospatidylinositol-3-kinase, protein kinase C, and MEK. These peptides provide previously unavailable tools for study of differential signaling in leukocytes.

Sphingosine 1-phosphate, a diffusible calcium influx factor mediating store-operated calcium entry

Store-operated calcium entry (SOCE) is a fundamental mechanism of calcium signaling. The mechanisms linking store depletion to SOCE remain controversial, hypothetically involving both diffusible messengers and conformational coupling of stores to channels. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that can signal via cell surface G-protein-coupled receptors, but S1P can also act as a second messenger, mobilizing calcium directly via unknown mechanisms. We show here that S1P opens calcium entry channels in human neutrophils (PMNs) and HL60 cells without prior store depletion, independent of G-proteins and of phospholipase C. S1P-mediated entry has the typical divalent cation permeability profile and inhibitor profile of SOCE in PMNs, is fully inhibited by 1 microm Gd3+, and is independent of [Ca2+]i. Depletion of PMN calcium stores by thapsigargin induces S1P synthesis. Inhibition of S1P synthesis by dimethylsphingosine blocks thapsigargin-, ionomycin-, and platelet-activating factor-mediated SOCE despite normal store depletion. We propose that S1P is a "calcium influx factor," linking calcium store depletion to downstream SOCE.

Ca2+/calmodulin binds and dissociates K-RasB from membrane

We have investigated the interaction of calmodulin (CaM) with Ras-p21 and the significance of this association. All Ras-p21 isoforms tested (H-, K-, and N-Ras) were detected in the particulate fraction of human platelets and MCF-7 cells, a human breast cancer cell line. In MCF-7 cells, H- and N-Ras were also detected in the cytosolic fraction. K-RasB from platelet and MCF-7 cell lysates was found to bind CaM in a Ca2+ -dependent but GTPgammaS-independent manner. The yeast two-hybrid analysis demonstrated that K-RasB binds to CaM in vivo. Incubation of isolated membranes from platelet and MCF-7 cells with CaM caused dissociation of only K-RasB from membranes in a Ca2+ -dependent manner. CaM antagonist, W7, inhibited dissociation of K-RasB. Addition of platelet or MCF-7 cytosol alone to isolated platelet membranes did not cause dissociation of K-RasB and only addition of exogenous CaM caused dissociation. The results suggest a potential role for Ca2+/CaM in the regulation of K-RasB function.

Modulation of neutrophil function by the tripeptide feG

BACKGROUND

Neutrophils are critical in the defense against potentially harmful microorganisms, but their excessive and inappropriate activation can contribute significantly to tissue damage and a worsening pathology. Through the release of endocrine factors submandibular glands contribute to achieving a balance in neutrophil function by modulating the state of activation and migratory potential of circulating neutrophils. A putative hormonal candidate for these effects on neutrophils was identified as a heptapeptide named submandibular gland peptide T (SGP-T; sequence = TDIFEGG). Since the tripeptide FEG, derived from SGP-T, and its D-amino acid analogue feG had similar inhibitory effects on inflammatory reactions, we investigated the effects of feG on human and rat neutrophil function.

RESULTS

With human neutrophils feG had no discernible effect on oxidative burst or phagocytosis, but in picomolar amounts it reduced PAF-induced neutrophil movement and adhesion, and the binding of CD11b by 34% and that of CD16b close to control values. In the rat feG (10-11M) reduced the binding of CD11b and CD16 antibodies to PAF-stimulated circulating neutrophils by 35% and 43%, respectively, and at 100 micrograms/kilograms intraperitoneally feG reduced neutrophil in vivo migration by 40%. With ovalbumin-sensitized rats that were challenged with antigen, feG inhibited binding of antibodies against CD16b but not CD11b, on peritoneal leukocytes.

CONCLUSIONS

The inhibitory effect of feG on neutrophil movement may be mediated by alterations in the co-stimulatory molecules CD11b and CD16.

Identification of a JAK2-independent pathway regulating growth hormone (GH)-stimulated p44/42 mitogen-activated protein kinase activity. GH activation of Ral and phospholipase D is Src-dependent

We have demonstrated here that growth hormone (GH) stimulates the formation of the active GTP-bound form of both RalA and RalB in NIH-3T3 cells. Full activation of RalA and RalB by GH required the combined activity of c-Src and JAK2, both kinases activated by GH independent of the other. Activation of RalA and RalB by growth hormone did not require the activity of JAK2 per se. Ras was also activated by GH and was required for the GH-stimulated formation of GTP-bound RalA and RalB. Activation of RalA by GH subsequently resulted in increased phospholipase D activity and the formation of its metabolite, phosphatidic acid. GH-stimulated RalA-phospholipase D-dependent formation of phosphatidic acid was required for activation of p44/42 MAPK and subsequent Elk-1-mediated transcription stimulated by GH. Thus we report the identification of a JAK2-independent pathway regulating GH-stimulated p44/42 MAPK activity.

Platelet-activating factor activates two distinct effector pathways in human eosinophils

In granulocytes, platelet-activating factor (PAF) shares many of its biological effects with other chemotactic factors, such as FMLP, complement fragments, and lipid mediators. Two unique effects are that PAF is relatively resistant to pertussis toxin (PTX) and that PAF activates the inflammatory functions of eosinophils more strongly than it activates those of neutrophils. To investigate the molecular mechanisms of the responses of eosinophils to PAF, we analyzed superoxide anion production by a chemiluminescence method that provides real-time kinetic data for the cellular responses. We found that PAF induced bimodal superoxide anion production in human eosinophils, consisting of an intense, but transient, first phase and a larger and sustained second phase. In contrast, PAF induced essentially a transient unimodal response in human neutrophils. The two phases of eosinophil response were mediated by distinct cellular mechanisms: the second phase was highly dependent on cellular adhesion and beta(2) integrins, but the first phase was independent of both adhesion and beta(2) integrins. The upstream signaling mechanisms were also different: the second phase was mediated by PTX-resistant G-protein(s) and through activation of phosphatidylinositol 3-kinase, while the first phase was mediated by PTX-sensitive G-protein(s). Furthermore, the second-phase response was approximately 100-fold more resistant to inhibition by a competitive PAF receptor antagonist than the first phase. Thus, eosinophils and neutrophils react differently to PAF, and PAF activates two separate and distinct effector pathways in human eosinophils. These two activation pathways may explain the eosinophils' strong and diverse biological responses to PAF.

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.

Calmodulin binds RalA and RalB and is required for the thrombin-induced activation of Ral in human platelets

Ral GTPases may be involved in calcium/calmodulin-mediated intracellular signaling pathways. RalA and RalB are activated by calcium, and RalA binds calmodulin in vitro. It was examined whether RalA can bind calmodulin in vivo, whether RalB can bind calmodulin, and whether calmodulin is functionally involved in Ral activation. Yeast two-hybrid analyses demonstrated both Rals interact directly but differentially with calmodulin. Coimmunoprecipitation experiments determined that calmodulin and RalB form complexes in human platelets. In vitro pull-down experiments in platelets and in vitro binding assays showed endogenous Ral and calmodulin interact in a calcium-dependent manner. Truncated Ral constructs determined in vitro and in vivo that RalA has an additional calmodulin binding domain to that previously described, that although RalB binds calmodulin, its C-terminal region is involved in partially inhibiting this interaction, and that in vitro RalA and RalB have an N-terminal calcium-independent and a C-terminal calcium-dependent calmodulin binding domain. Functionally, in vitro Ral-GTP pull-down experiments determined that calmodulin is required for the thrombin-induced activation of Ral in human platelets. We propose that differential binding of calmodulin by RalA and RalB underlies possible functional differences between the two proteins and that calmodulin is involved in the regulation of the activation of Ral-GTPases.

Beta-arrestins regulate a Ral-GDS Ral effector pathway that mediates cytoskeletal reorganization

beta-Arrestins are important in chemoattractant receptor-induced granule release, a process that may involve Ral-dependent regulation of the actin cytoskeleton. We have identified the Ral GDP dissociation stimulator (Ral-GDS) as a beta-arrestin-binding protein by yeast two-hybrid screening and co-immunoprecipitation from human polymorphonuclear neutrophilic leukocytes (PMNs). Under basal conditions, Ral-GDS is localized to the cytosol and remains inactive in a complex formed with beta-arrestins. In response to formyl-Met-Leu-Phe (fMLP) receptor stimulation, beta-arrestin Ral-GDS protein complexes dissociate and Ral-GDS translocates with beta-arrestin from the cytosol to the plasma membrane, resulting in the Ras-independent activation of the Ral effector pathway required for cytoskeletal rearrangement. The subsequent re-association of beta-arrestin Ral-GDS complexes is associated with the inactivation of Ral signalling. Thus, beta-arrestins regulate multiple steps in the Ral-dependent processes that result in chemoattractant-induced cytoskeletal reorganization.

RalGEF2, a pleckstrin homology domain containing guanine nucleotide exchange factor for Ral

Ral is a ubiquitously expressed Ras-like small GTPase. Several guanine nucleotide exchange factors for Ral have been identified, including members of the RalGDS family, which exhibit a Ras binding domain and are regulated by binding to RasGTP. Here we describe a novel type of RalGEF, RalGEF2. This guanine nucleotide exchange factor has a characteristic Cdc25-like catalytic domain at the N terminus and a pleckstrin homology (PH) domain at the C terminus. RalGEF2 is able to activate Ral both in vivo and in vitro. Deletion of the PH domain results in an increased cytoplasmic localization of the protein and a corresponding reduction in activity in vivo, suggesting that the PH domain functions as a membrane anchor necessary for optimal activity in vivo.

Mechanism of regulation of the Epac family of cAMP-dependent RapGEFs

Epac1 (cAMP-GEFI) and Epac2 (cAMP-GEFII) are closely related guanine nucleotide exchange factors (GEFs) for the small GTPase Rap1, which are directly regulated by cAMP. Here we show that both GEFs efficiently activate Rap2 as well. A third member of the family, Repac (GFR), which lacks the cAMP dependent regulatory sequences, is a constitutive activator of both Rap1 and Rap2. In contrast to Epac1, Epac2 contains a second cAMP binding domain at the N terminus, as does the Epac homologue from Caenorhabditis elegans. Affinity measurements show that this distal cAMP binding domain (the A-site) binds cAMP with much lower affinity than the cAMP binding domain proximal to the catalytic domain (the B-site), which is present in both Epac1 and Epac2. Deletion mutant analysis shows that the high affinity cAMP binding domains are sufficient to regulate the GEFs in vitro. Interestingly, isolated fragments containing the B-sites of either Epac1 or Epac2, but not the A-site from Epac2, inhibit the catalytic domains in trans. This inhibition is relieved by the addition of cAMP. In addition to the cAMP binding domains, both Epac1 and Epac2 have a DEP domain. Deletion of this domain does not affect regulation of Epac1 activity but affects membrane localization. From these results, we conclude that all three members of the Epac family regulate both Rap1 and Rap2. Furthermore, we conclude that the catalytic activity of Epac1 is constrained by a direct interaction between GEF and high affinity cAMP binding domains in the absence of cAMP. Epac1 becomes activated by a release of this inhibition when cAMP is bound.

Involvement of Ras and Ral in chemotactic migration of skeletal myoblasts

In skeletal myoblasts, Ras has been considered to be a strong inhibitor of myogenesis. Here, we demonstrate that Ras is involved also in the chemotactic response of skeletal myoblasts. Expression of a dominant-negative mutant of Ras inhibited chemotaxis of C2C12 myoblasts in response to basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), and insulin-like growth factor 1 (IGF-1), key regulators of limb muscle development and skeletal muscle regeneration. A dominant-negative Ral also decreased chemotactic migration by these growth factors, while inhibitors for phosphatidylinositol 3-kinase and mitogen-activated protein kinase kinase (MEK) showed no effect. Activation of the Ras-Ral pathway by expression of an activated mutant of either Ras, the guanine-nucleotide dissociation stimulator for Ral, or Ral resulted in increased motility of myoblasts. The ability of Ral to stimulate motility was reduced by introduction of a mutation which prevents binding to Ral-binding protein 1 or phospholipase D. These results suggest that the Ras-Ral pathway is essential for the migration of myoblasts. Furthermore, we found that Ras and Ral are activated in C2C12 cells by bFGF, HGF and IGF-1 and that the Ral activation is regulated by the Ras- and the intracellular Ca(2+)-mediated pathways. Taken together, our data indicate that Ras and Ral regulate the chemotactic migration of skeletal muscle progenitors.

An EGF receptor/Ral-GTPase signaling cascade regulates c-Src activity and substrate specificity

c-Src is a membrane-associated tyrosine kinase that can be activated by many types of extracellular signals, and can regulate the function of a variety of cellular protein substrates. We demonstrate that epidermal growth factor (EGF) and beta-adrenergic receptors activate c-Src by different mechanisms leading to the phosphorylation of distinct sets of c-Src substrates. In particular, we found that EGF receptors, but not beta(2)-adrenergic receptors, activated c-Src by a Ral-GTPase-dependent mechanism. Also, c-Src activated by EGF treatment or expression of constitutively activated Ral-GTPase led to tyrosine phosphorylation of Stat3 and cortactin, but not Shc or subsequent Erk activation. In contrast, c-Src activated by isoproterenol led to tyrosine phosphorylation of Shc and subsequent Erk activation, but not tyrosine phosphorylation of cortactin or Stat3. These results identify a role for Ral-GTPases in the activation of c-Src by EGF receptors and the coupling of EGF to transcription through Stat3 and the actin cytoskeleton through cortactin. They also show that c-Src kinase activity can be used differently by individual extracellular stimuli, possibly contributing to their ability to generate unique cellular responses.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.