RasGRP4, a new mast cell-restricted Ras guanine nucleotide-releasing protein with calcium- and diacylglycerol-binding motifs. Identification of defective variants of this signaling protein in asthma, mastocytosis, and mast cell leukemia patients and demonstration of the importance of RasGRP4 in mast cell development and function

RasGRP1 confers the phorbol ester-sensitive phenotype to EL4 lymphoma cells

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to the tumor promoter phorbol 12-myristate 13-acetate (PMA). In sensitive EL4 cells, PMA causes robust Erk mitogen-activated protein kinase activation that results in growth arrest. In resistant cells, PMA induces minimal Erk activation, without growth arrest. PMA stimulates IL-2 production in sensitive, but not resistant, cells. The role of RasGRP1, a PMA-activated guanine nucleotide exchange factor for Ras, in EL4 phenotype was examined. Endogenous RasGRP1 protein is expressed at much higher levels in sensitive than in resistant cells. PMA-induced Ras activation is observed in sensitive cells but not in resistant cells lacking Ras-GRP1. PMA induces down-regulation of RasGRP1 protein in sensitive cells but increases RasGRP1 in resistant cells. Transfection of RasGRP1 into resistant cells enhances PMA-induced Erk activation. In the reverse experiment, introduction of small interfering RNA (siRNA) for RasGRP1 suppresses PMA-induced Ras and Erk activations in sensitive cells. Sensitive cells incubated with siRNA for RasGRP1 exhibit the PMA-resistant phenotype, in that they are able to proliferate in the presence of PMA and do not secrete IL-2 when stimulated with PMA. These studies indicate that the PMA-sensitive phenotype, as previously defined for the EL4 cell line, is conferred by endogenous expression of RasGRP1 protein.

Ras activation in response to phorbol ester proceeds independently of the EGFR via an unconventional nucleotide-exchange factor system in COS-7 cells

Ras is a major mediator of PE (phorbol ester) effects in mammalian cells. Various mechanisms for PE activation of Ras have been reported [Downward, Graves, Warne, Rayter and Cantrell (1990) Nature (London) 346, 719-723; Shu, Wu, Mosteller and Broek (2002) Mol. Cell. Biol. 22, 7758-7768; Roose, Mollenauer, Gupta, Stone and Weiss (2005) Mol. Cell. Biol. 25, 4426-4441; Grosse, Roelle, Herrlich, Höhn and Gudermann (2000) J. Biol. Chem. 275, 12251-12260], including pathways that target GAPs (GTPase-activating proteins) for inactivation and those that result in activation of GEFs (guanine nucleotide-exchange factors) Sos (son of sevenless homologue) or RasGRP (RAS guanyl releasing protein). However, a biochemical link between PE and GAP inactivation is missing and GEF stimulation is hard to reconcile with the observation that dominant-negative S17N-Ras does not compromise Ras-dependent ERK (extracellular-signal-regulated kinase) activation by PE. We have addressed this controversy and carried out an in-depth biochemical study of PE-induced Ras activation in COS-7 cells. Using a cell-permeabilization approach to monitor nucleotide exchange on Ras, we demonstrate that PE-induced Ras-GTP accumulation results from GEF stimulation. Nucleotide exchange stimulation by PE is prevented by PKC (protein kinase C) inhibition but not by EGFR [EGF (epidermal growth factor) receptor] blockade, despite the fact that EGFR inhibition aborts basal and PE-induced Shc (Src homology and collagen homology) phosphorylation and Shc-Grb2 (growth-factor-receptor-bound protein 2) association. In fact, EGFR inhibition ablates basal nucleotide exchange on Ras in growth-arrested COS-7 cells. These data disclose the existence of two separate GEF systems that operate independently from each other to accomplish PE-dependent formation of Ras-GTP and to maintain resting Ras-GTP levels respectively. We document that COS-7 cells do not express RasGRP and present evidence that the PE-responsive GEF system may involve PKC-dependent phosphorylation of Sos. More fundamentally, these observations shed new light on enigmatic issues such as the inefficacy of S17N-Ras in blocking PE action or the role of the EGFR in heterologous agonist activation of the Ras/ERK pathway.

The plasma membrane shuttling of CAPRI is related to regulation of mast cell activation

The Ca(2+)-promoted Ras inactivator (CAPRI), a Ras GTPase-activating protein, is involved in the inactivation of mitogen-activated protein kinase pathway. However, a precise role of CAPRI in immune responses is still unknown. Here we showed that overexpression of CAPRI suppresses antigen-induced degranulation and cytokine production in mast cells (RBL cells). Antigen elicited the translocation of CAPRI to the plasma membrane from the cytoplasm, which was concomitant with the increase in the intracellular Ca(2+) concentration. The nuclear import of extracellular signal-regulated kinase 2 (ERK2) occurred after the re-localization of CAPRI to the cytoplasm in the mast cells, suggesting that the early phase of ERK2 activation is eliminated. A mutant of GAP-related domain, CAPRI(R472S), showed a feeble translocation to the plasma membrane but did not affect the degranulation, ERK2 activation, and cytokine production. The results suggested that the translocation of CAPRI to the plasma membranes regulates crucially cellular responses in mast cells.

Impaired degranulation but enhanced cytokine production after Fc epsilonRI stimulation of diacylglycerol kinase zeta-deficient mast cells

Calcium and diacylglycerol are critical second messengers that together effect mast cell degranulation after allergen cross-linking of immunoglobulin (Ig)E-bound FcepsilonRI. Diacylglycerol kinase (DGK)zeta is a negative regulator of diacylglycerol-dependent signaling that acts by converting diacylglycerol to phosphatidic acid. We reported previously that DGKzeta-/- mice have enhanced in vivo T cell function. Here, we demonstrate that these mice have diminished in vivo mast cell function, as revealed by impaired local anaphylactic responses. Concordantly, DGKzeta-/- bone marrow-derived mast cells (BMMCs) demonstrate impaired degranulation after Fc epsilonRI cross-linking, associated with diminished phospholipase Cgamma activity, calcium flux, and protein kinase C-betaII membrane recruitment. In contrast, Ras-Erk signals and interleukin-6 production are enhanced, both during IgE sensitization and after antigen cross-linking of Fc epsilonRI. Our data demonstrate dissociation between cytokine production and degranulation in mast cells and reveal the importance of DGK activity during IgE sensitization for proper attenuation of Fc epsilonRI signals.

Involvement of p38 MAP kinase and Smad3 in TGF-beta-mediated mast cell functions

Transforming growth factor-beta (TGF-beta) modulates functions of bone marrow-derived cultured mast cells (BMMCs); cell maturation (up-regulation of mouse mast cell proteases (mmcps)), growth arrest and migration. We investigated the roles of p38 MAP kinase and Smad3 in TGF-beta-mediated cell responses in BMMCs. Treating BMMCs with TGF-beta induced the phosphorylation of p38 within 2 h and persisted for 24 h. The involvement of p38 in TGF-beta-induced cell responses depended upon mast cell functions; it was necessary for up-regulation of mmcp-1 and migration, but not for up-regulation of mmcp-7 and inhibition of metabolic activity. New protein synthesis was required for the up-regulation of mmcp-1 but not mmcp-7 in response to TGF-beta treatment, and stabilization of mRNA was partially responsible for the increase in gene transcript of mmcp-1. The decrease in metabolic activity in response to TGF-beta treatment was smaller in Smad3-deficient BMMCs compared to wild-type BMMCs. Maximal migration was detected at a TGF-beta concentration of 40 fM in wild-type BMMCs, whereas TGF-beta-induced migration was absent in Smad3-deficient BMMCs. Thus, the roles of p38 and Smad3 are different among TGF-beta-mediated cell responses in BMMCs.

Responses during cell preparation for functional analyses in mouse bone marrow-derived cultured mast cells

Murine bone marrow-derived cultured mast cells (BMMCs) are most widely used in in vitro experiments for evaluation of mast cell functions. The present study has shown that cell preparation procedure, i.e., cell collection by centrifugation and the subsequent adjustment and culture of cell density at the desired concentrations, transiently induced gene expression of plasminogen activator inhibitor-1 (PAI-1) and the AP-1 components (c-fos, c-jun, and junB). The level of PAI-1 gene transcript was closely related to the cell density and the gene expression was enhanced by pretreatment with okadaic acid, an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A). The cell preparation procedure also caused dephosphorylation of MAP kinases, i.e., ERK, p38, and JNK, resulting from PP1/PP2A activation. In view of the cell responses to the cell preparation procedure itself, care is needed in the interpretation of in vitro data using BMMCs.

Positioning prostanoids of the D and J series in the immunopathogenic scheme

Prostaglandin D(2) (PGD(2)) is produced by a variety of immune and non-hematopoietic cells and appears to function in both an inflammatory and homeostatic capacity. Two genetically distinct PGD(2)-synthesizing enzymes have been identified to date, including hematopoietic- and lipocalin-type PGD synthases (H-PGDS and L-PGDS, respectively). Though the inter-species expression profiles of these two enzymes vary widely, H-PGDS is generally localized to the cytosolic aspect of immune and inflammatory cells, whereas L-PGDS is more resigned to tissue-based expression. PGD(2) activity is principally mediated through two unique G protein-coupled receptors (GPCR), designated DP(1) and DP(2). These receptors exhibit overlapping binding profiles, yet their respective agonists elicit generally distinctive responses. Additional to DP receptors, the PGD(2) metabolite 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) binds the nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) and has the facility to initiate a variety of anti-inflammatory phenotypes either through or independent of PPARgamma association. This review highlights the collective relevance of PGD(2) and its respective synthases, receptors, and metabolites in immunopathologic responses.

RasGRP1 and RasGRP3 regulate B cell proliferation by facilitating B cell receptor-Ras signaling

The RasGRPs are a family of Ras activators that possess diacylglycerol-binding C1 domains. In T cells, RasGRP1 links TCR signaling to Ras. B cells coexpress RasGRP1 and RasGRP3. Using Rasgrp1 and Rasgrp3 single and double null mutant mice, we analyzed the role of these proteins in signaling to Ras and Erk in B cells. RasGRP1 and RasGRP3 both contribute to BCR-induced Ras activation, although RasGRP3 alone is responsible for maintaining basal Ras-GTP levels in unstimulated cells. Surprisingly, RasGRP-mediated Ras activation is not essential for B cell development because this process occurs normally in double-mutant mice. However, RasGRP-deficient mice do exhibit humoral defects. Loss of RasGRP3 led to isotype-specific deficiencies in Ab induction in immunized young mice. As reported previously, older Rasgrp1-/- mice develop splenomegaly and antinuclear Abs as a result of a T cell defect. We find that such mice have elevated serum Ig levels of several isotypes. In contrast, Rasgrp3-/- mice exhibit hypogammaglobulinemia and show no signs of splenomegaly or autoimmunity. Double-mutant mice exhibit intermediate serum Ab titers, albeit higher than wild-type mice. Remarkably, double-mutant mice exhibit no signs of autoimmunity or splenomegaly. B cell proliferation induced by BCR ligation with or without IL-4 was found to be RasGRP1- and RasGRP3-dependent. However, the RasGRPs are not required for B cell proliferation per se, because LPS-induced proliferation is unaffected in double-mutant mice.

Activation of Ras Proteins by Ras Guanine Nucleotide Releasing Protein Family Members

Ras guanine nucleotide releasing proteins (RasGRPs) function as guanine nucleotide exchange factors for Ras proteins. Thus, RasGRPs are direct activators of Ras proteins and contribute an important role in various cell-signaling pathways that are regulated by the activation state of Ras proteins. RasGRPs are regulated by the second messengers diacylglycerol and intracellular calcium and are also known as CalDAG-GEFs or calcium and diacylglycerol-regulated guanine nucleotide exchange factors. RasGRPs couple signaling events that generate these second messengers in the cell into activation of signaling pathways that are regulated by Ras. RasGRPs, therefore, increase the repertoire of extracellular stimuli that lead to activation of Ras. Analyzing the regulation of RasGRP activity should continue to play an important role in understanding the mechanisms by which signal transduction pathways use RasGRP proteins to activate Ras proteins in cells.

Signaling interplay in Ras superfamily function

Ras proteins function as signaling hubs that are activated by convergent signaling pathways initiated by extracellular stimuli. Activated Ras in turn regulates a diversity of downstream cytoplasmic signaling cascades. Ras proteins are founding members of a large superfamily of small GTPases that have significant sequence and biochemical similarities. Recent observations have established a complex signaling interplay between Ras and other members of the family. A key biochemical mechanism facilitating this crosstalk involves guanine nucleotide exchange factors (GEFs), which serve as regulators and effectors, as well as signaling integrators, of Ras signaling.

Multiple roles of Rap1 in hematopoietic cells: complementary versus antagonistic functions

Small G proteins serve as critical control points in signal transduction, integrating a wide range of stimuli to dictate discrete cellular outcomes. The outcomes of small G-protein signaling can both potentiate and antagonize one another. Studies in hematopoietic cells have uncovered multiple functions for the small G protein, Rap1 (Ras-proximate-1). Because Rap1 can regulate cell proliferation, differentiation, and adhesion through distinct mechanisms, it serves as a paradigm for the need for tight cellular control of small G-protein function. Rap1 has received recent attention for its role in enhancing integrin-dependent signals. This action of Rap1 augments a variety of processes that characterize hematopoietic-cell function, including aggregation, migration, extravasation, and homing to target tissues. Rap1 may also regulate cellular differentiation and proliferation via pathways that are distinct from those mediating adhesion, and involve regulation of the mitogen-activated protein (MAP) kinase or ERK (extracellular signal-regulated kinase) cascade. These actions of Rap1 occur in selected cell types to enhance or diminish ERK signaling, depending on the expression pattern of the MAP kinase kinase kinases of the Raf family: Raf-1 and B-Raf. This review will examine the functions of Rap1 in hematopoietic cells, and focus on 3 cellular scenarios where the multiple actions of Rap1 function have been proposed. Recent studies implicating Rap1 in the maturation of megakaryocytes, the pathogenesis of chronic myelogenous leukemia (CML), and activation of peripheral T cells will receive particular attention.

A diacylglycerol-protein kinase C-RasGRP1 pathway directs Ras activation upon antigen receptor stimulation of T cells

Ras GTPases are on/off switches regulating numerous cellular responses by signaling to various effector molecules. In T lymphocytes, Ras can be activated by two Ras exchange factors, SOS and RasGRP1, which are recruited through the adapters Grb2 and LAT and via the second-messenger diacylglycerol (DAG), respectively. Mitogen-activated protein (MAP) kinase phosphorylation patterns induced by active Ras can vary and contribute to distinct cellular responses. The different consequences of Ras activation by either guanine exchange factor are unknown. DAG also recruits and activates the kinase protein kinase Ctheta (PKCtheta) turning on the Erk MAP kinase pathway, but the biochemical mechanism responsible is unclear. We generated T-cell clones deficient in phorbol myristate acetate (a surrogate for DAG)-induced Ras activation. Analysis of a RasGRP1-deficient Jurkat T-cell clone and RasGRP1 RNA interference in wild-type cells revealed that RasGRP1 is required for optimal, antigen receptor-triggered Ras-Erk activation. RasGRP1 relies on its DAG-binding domain to selectively activate Erk kinases. Activation of Erk correlates with the phosphorylation of threonine residue 184 in RasGRP1. This phosphorylation event requires the activities of novel PKC kinases. Conversely, active PKCtheta depends on RasGRP1 sufficiency to effectively trigger downstream events. Last, DAG-PKC-RasGRP1-driven Ras-Erk activation in T cells is a unique signaling event, not simply compensated for by SOS activity.

Mast cells as "tunable" effector and immunoregulatory cells: recent advances

This review focuses on recent progress in our understanding of how mast cells can contribute to the initiation, development, expression, and regulation of acquired immune responses, both those associated with IgE and those that are apparently expressed independently of this class of Ig. We emphasize findings derived from in vivo studies in mice, particularly those employing genetic approaches to influence mast cell numbers and/or to alter or delete components of pathways that can regulate mast cell development, signaling, or function. We advance the hypothesis that mast cells not only can function as proinflammatory effector cells and drivers of tissue remodeling in established acquired immune responses, but also may contribute to the initiation and regulation of such responses. That is, we propose that mast cells can also function as immunoregulatory cells. Finally, we show that the notion that mast cells have primarily two functional configurations, off (or resting) or on (or activated for extensive mediator release), markedly oversimplifies reality. Instead, we propose that mast cells are "tunable," by both genetic and environmental factors, such that, depending on the circumstances, the cell can be positioned phenotypically to express a wide spectrum of variation in the types, kinetics, and/or magnitude of its secretory functions.

Diacylglycerol kinase iota regulates Ras guanyl-releasing protein 3 and inhibits Rap1 signaling

To study the physiological function of diacylglycerol (DAG) kinase iota (DGKiota), which converts DAG to phosphatidic acid, we deleted this gene in mice. In contrast to previous studies showing that DGK isoforms decrease Ras activity, signaling downstream of Ras in embryonic fibroblasts was significantly reduced in cells lacking DGKiota. DGKs regulate Ras signaling by attenuating the function of the DAG-dependent Ras guanyl nucleotide-releasing proteins (RasGRPs). We tested whether DGKiota inhibited the four known RasGRPs and found that it inhibited only RasGRP3. In addition to activating Ras, RasGRP3 also activates Rap1, which in some cases can antagonize the function of Ras. We demonstrate that DGKiota bound to RasGRP3 and inhibited its activation of Rap1 by metabolizing DAG. This inhibition consequently affected Ras signaling. We tested the physiological consequence of deleting DGKiota by crossing wild-type or DGKiota-deficient mice with mice carrying a v-Ha-Ras transgene, and then we assessed tumor formation. We observed significantly fewer tumors in DGKiota-deficient mice. Because Rap1 can antagonize the function of Ras, our data are consistent with a model in which DGKiota regulates RasGRP3 with a predominant effect on Rap1 activity. Additionally, we found that DGKzeta, which is structurally similar to DGKiota, inhibited RasGRPs 1, 3, and 4 and predominantly affected Ras signaling. Thus, type IV DGKs regulate RasGRPs, but the downstream effects differ depending on the DGK.

RasGRP3 mediates phorbol ester-induced, protein kinase C-independent exocytosis

Phorbol esters are involved in neurotransmitter release and hormone secretion via activation of protein kinase C (PKC). In addition, it has been recently reported to enhance neurotransmitter release in a PKC-independent manner. However, the exocytotic machinery is not fully clarified. Nowadays members of the RasGRP family are being identified as novel molecules binding to diacylglycerol and calcium, representing a new class of guanine nucleotide exchange factor that activates small GTPases including Ras and Rap1. In the present study, we demonstrated that RasGRP3 is expressed in endocrine tissues and mediates phorbol ester-induced exocytosis. Furthermore, the effects were partially blocked by PKC inhibitor but not mitogen-activated protein kinase kinase inhibitor, although both significantly suppressed the phorbol ester-induced phosphorylation of extracellular signal-regulated kinase 1/2. These results indicate that RasGRP3 is implicated in phorbol ester-induced, PKC-independent exocytosis.

Phosphorylation of RasGRP3 on threonine 133 provides a mechanistic link between PKC and Ras signaling systems in B cells

B-cell receptor (BCR) signaling activates a number of intracellular signaling molecules including phospholipase C-gamma2 (PLC-gamma2), which generates membrane diacylglycerol (DAG). DAG recruits both protein kinase C (PKC) and RasGRP family members to the membrane and contributes to their activation. We have hypothesized that membrane colocalization facilitates activation of RasGRP3 by PKC. Here we demonstrate that PKC phosphorylates RasGRP3 on Thr133 in vitro, as determined by mass spectrometry. RasGRP3 with a Thr133Ala substitution is a poor PKC substrate in vitro and a poor Ras activator in vivo. Antiphosphopeptide antibodies recognize Thr133-phosphorylated RasGRP3 in B cells after BCR stimulation or DAG analog treatment, but much less so in resting cells. PKC inhibitors block RasGRP3 Thr133 phosphorylation and Ras-extracellular signal-related kinase (Erk) signaling with a similar pattern. After stimulation of T-cell receptor (TCR) or DAG analog treatment of T cells, PKC-catalyzed phosphorylation of RasGRP1 occurs on the homologous residue, Thr184. These studies shed light on the proposed "PKC-Ras pathway" and support the hypothesis that RasGRP phosphorylation by PKC is a mechanism that integrates DAG signaling systems in T and B cells. PKC-mediated regulation of RasGRPs in lymphocytes may generate cooperative signaling in response to increases in DAG. The mast- and myeloid-selective family member RasGRP4 is regulated by different means.

A vascular gene trap screen defines RasGRP3 as an angiogenesis-regulated gene required for the endothelial response to phorbol esters

We identified Ras guanine-releasing protein 3 (RasGRP3) as a guanine exchange factor expressed in blood vessels via an embryonic stem (ES) cell-based gene trap screen to identify novel vascular genes. RasGRP3 is expressed in embryonic blood vessels, down-regulated in mature adult vessels, and reexpressed in newly formed vessels during pregnancy and tumorigenesis. This expression pattern is consistent with an angiogenic function for RasGRP3. Although a loss-of-function mutation in RasGRP3 did not affect viability, RasGRP3 was up-regulated in response to vascular endothelial growth factor (VEGF) stimulation of human umbilical vein endothelial cells, placing RasGRP3 regulation downstream of VEGF signaling. Phorbol esters mimic the second messenger diacylglycerol (DAG) in activating both protein kinase C (PKC) and non-PKC phorbol ester receptors such as RasGRP3. ES cell-derived wild-type blood vessels exposed to phorbol myristate acetate (PMA) underwent extensive aberrant morphogenesis that resulted in the formation of large endothelial sheets rather than properly branched vessels. This response to PMA was completely dependent on the presence of RasGRP3, as mutant vessels were refractory to the treatment. Taken together, these findings show that endothelial RasGRP3 is up-regulated in response to VEGF stimulation and that RasGRP3 functions as an endothelial cell phorbol ester receptor in a pathway whose stimulation perturbs normal angiogenesis. This suggests that RasGRP3 activity may exacerbate vascular complications in diseases characterized by excess DAG, such as diabetes.

Divergent and convergent signaling by the diacylglycerol second messenger pathway in mammals

Diacylglycerol is an essential second messenger in mammalian cells. The most prominent intracellular targets of diacylglycerol and the functionally analogous phorbol esters belong to the protein kinase C family, but at least five alternative types of high affinity diacylglycerol/phorbol ester receptors are known: protein kinase D, diacylglycerol kinases alpha, beta, and gamma, RasGRPs, chimaerins, and Munc13s. These function independently of protein kinase C isozymes, and form a network of signaling pathways in the diacylglycerol second messenger system that regulates processes as diverse as gene transcription, lipid signaling, cytoskeletal dynamics, intracellular membrane trafficking, or neurotransmitter release.

MITF is necessary for generation of prostaglandin D2 in mouse mast cells

Mast cells generate eicosanoids that are linked to asthma and other inflammatory diseases. A basic-helix-loop-helix leucine zipper transcription factor termed MITF is essential for the development of mast cells. Although other substances also linked to inflammatory reactions (such as various proteases and serotonin) require MITF for their expression, the role of MITF in eicosanoid generation has not been studied. We examined eicosanoid generation in bone marrow-derived mast cells (BMMCs) of tg/tg mice that lack MITF. Most eicosanoids generated by BMMCs are either prostaglandin (PG) D2 or leukotriene C4. The former is synthesized via the cyclooxygenase pathway, whereas the latter is synthesized via the 5-lipoxygenase pathway. In response to stimulation with IgE and antigens, BMMCs of tg/tg mice synthesized leukotriene C4 normally. However, neither immediate nor delayed PGD2 production was detected in these BMMCs. This indicates that MITF is a transcription factor that specifically activates the cyclooxygenase pathway, but not the 5-lipoxygenase pathway. Significant decreases in expression of hematopoietic PGD2 synthase (hPGDS, a terminal synthase for PGD2) were observed at both mRNA and protein levels in tg/tg BMMCs. MITF transactivated the hPGDS gene via a CACCTG motif located in the promoter region. MITF appeared to be essential for generation of PGD2 by enhancing expression of the hPGDS gene in BMMCs.

CalDAG-GEFI integrates signaling for platelet aggregation and thrombus formation

Signaling through the second messengers calcium and diacylglycerol (DAG) is a critical element in many biological systems. Integration of calcium and DAG signals has been suggested to occur primarily through protein kinase C family members, which bind both calcium and DAG. However, an alternative pathway may involve members of the CalDAG-GEF/RasGRP protein family, which have structural features (calcium-binding EF hands and DAG-binding C1 domains) that suggest they can function in calcium and DAG signal integration. To gain insight into the signaling systems that may be regulated by CalDAG-GEF/RasGRP family members, we have focused on CalDAG-GEFI, which is expressed preferentially in the brain and blood. Through genetic ablation in the mouse, we have found that CalDAG-GEFI is crucial for signal integration in platelets. Mouse platelets that lack CalDAG-GEFI are severely compromised in integrin-dependent aggregation as a consequence of their inability to signal through CalDAG-GEFI to its target, the small GTPase Rap1. These results suggest that analogous signaling defects are likely to occur in the central nervous system when CalDAG-GEFI is absent or compromised in function.

Non-kinase second-messenger signaling: new pathways with new promise

Intercellular signaling by growth factors, hormones and neurotransmitters produces second messenger molecules such as cyclic adenosine monophosphate (cAMP) and diacylglycerol (DAG). Protein Kinase A and Protein Kinase C are the principal effector proteins of these prototypical second messengers in certain cell types. Recently, novel receptors for cAMP and DAG have been identified. These proteins, designated EPAC (Exchange Protein directly Activated by cAMP) or cAMP-GEF (cAMP regulated Guanine nucleotide Exchange Factor) and CalDAG-GEF (Calcium and Diacylglycerol regulated Guanine nucleotide Exchange Factor) or RasGRP (Ras Guanine nucleotide Releasing Protein) are able to mediate some of the physiologic effects of the second messengers in a protein-kinase-independent fashion. These proteins are exchange factors for Ras family GTPases that operate in pathways that run parallel to the classic kinase-dependent pathways. The rapidly emerging recognition of the functions of these "non-kinase" effectors in diverse processes such as insulin secretion, thymocyte development, asthma and malignant transformation creates new opportunities for discovery and identifies potential new therapeutic targets.

PKCδ Associates with and Is Involved in the Phosphorylation of RasGRP3 in Response to Phorbol Esters

RasGRP is a family of guanine nucleotide exchange factors that activate small GTPases and contain a C1 domain similar to the one present in protein kinase C (PKC). In this study, we examined the interaction of RasGRP3 and PKC in response to the phorbol ester PMA. In Chinese hamster ovary or LN-229 cells heterologously expressing RasGRP3, phorbol 12-myristate 13-acetate (PMA) induced translocation of RasGRP3 to the perinuclear region and a decrease in the electrophoretic mobility of RasGRP3. The mobility shift was associated with phosphorylation of RasGRP3 on serine residues and seemed to be PKCdelta-dependent because it was blocked by the PKCdelta inhibitor rottlerin as well as by a PKCdelta kinase-dead mutant. Using coimmunoprecipitation, we found that PMA induced the physical association of RasGRP3 with PKCdelta and, using in situ methods, we showed colocalization of PKCdelta and RasGRP3 in the perinuclear region. PKCdelta phosphorylated RasGRP3 in vitro. Previous studies suggest that ectopic expression of RasGRP3 increases activation of Erk1/2. We found that overexpression of either PKCdelta or RasGRP3 increased the activation of Erk1/2 by PMA. In contrast, coexpression of PKCdelta and RasGRP3 yielded a level of phosphorylation of Erk1/2 similar to that of control vector cells. Our results suggest that PKCdelta may act as an upstream kinase associating with and phosphorylating RasGRP3 in response to PMA. The interaction between RasGRP3 and PKCdelta points to the existence of complex cross-talk between various members of the phorbol ester receptors which can have important impact on major signal transduction pathways and cellular processes induced by phorbol esters or DAG

RasGRP1 sensitizes an immature B cell line to antigen receptor-induced apoptosis

RasGRP1 is a guanine nucleotide exchange factor that activates Ras GTPases and is activated downstream of antigen receptors on both T and B lymphocytes. Ras-GRP1 provides signals to immature T cells that confer survival and proliferation, but RasGRP1 also promotes T cell receptor-mediated deletion of mature T cells. We used the WEHI-231 cell line as an experimental system to determine whether RasGRP1 can serve as a quantitative modifier of B cell receptor-induced deletion of immature B cells. A 2-fold elevation in RasGRP1 expression markedly increased apoptosis of WEHI-231 cells following B cell receptor ligation, whereas a dominant negative mutant of RasGRP1 suppressed B cell receptor-induced apoptosis. Activation of ERK1 or ERK2 kinases was not required for RasGRP1-mediated apoptosis. Instead, elevated RasGRP1 expression caused down-regulation of NF-kappaB and Bcl-x(L), which provide survival signals counter-acting apoptosis induction by B cell receptor. Inhibition of NF-kappaB was sufficient to enhance B cell receptor-induced apoptosis of WEHI-231 cells, and ligation of co-stimulatory receptors that activate NF-kappaB suppressed the ability of RasGRP1 to promote B cell receptor-induced apoptosis. These experiments define a novel apoptosis-promoting pathway leading from B cell receptor to the inhibition of NF-kappaB and demonstrate that differential expression of RasGRP1 has the potential to modulate the sensitivities of B cells to negative selection following antigen encounter.

Requirement for Ras guanine nucleotide releasing protein 3 in coupling phospholipase C-gamma2 to Ras in B cell receptor signaling

Two important Ras guanine nucleotide exchange factors, Son of sevenless (Sos) and Ras guanine nucleotide releasing protein (RasGRP), have been implicated in controlling Ras activation when cell surface receptors are stimulated. To address the specificity or redundancy of these exchange factors, we have generated Sos1/Sos2 double- or RasGRP3-deficient B cell lines and determined their ability to mediate Ras activation upon B cell receptor (BCR) stimulation. The BCR requires RasGRP3; in contrast, epidermal growth factor receptor is dependent on Sos1 and Sos2. Furthermore, we show that BCR-induced recruitment of RasGRP3 to the membrane and the subsequent Ras activation are significantly attenuated in phospholipase C-gamma2-deficient B cells. This defective Ras activation is suppressed by the expression of RasGRP3 as a membrane-attached form, suggesting that phospholipase C-gamma2 regulates RasGRP3 localization and thereby Ras activation.

Diacylglycerols Containing Omega 3 and Omega 6 Fatty Acids Bind to RasGRP and Modulate MAP Kinase Activation

We elucidated the effects of different diacylglycerols (DAGs), i.e. 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG), 1-stearoyl-2-docosahexaenoyl-sn-glycerol (SDG), and 1-stearoyl-2-eicosapentaenoyl-sn-glycerol (SEG), on [3H]PDBu binding to RasGRP. The competition studies with these DAGs on [3H]PDBu binding to RasGRP revealed different Ki values for these DAG molecular species. Furthermore, we transfected human Jurkat T cells by a plasmid containing RasGRP and assessed the implication of endogenous DAGs on activation of MAP kinases ERK1/ERK2, induced by phorbol-12-myristate-13-acetate (PMA). In control cells, GF109203X, a protein kinase C inhibitor, inhibited ERK1/ERK2 activation. However, this agent curtailed but failed to completely diminish ERK1/ERK2 phosphorylation in RasGRP-overexpressing cells, though calphostin C, a DAG binding inhibitor, suppressed the phosphorylation of MAP kinases in these cells. In cells incubated with arachidonic acid (AA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), PMA induced the production of endogenous DAGs containing these fatty acids, respectively: DAG-AA, DAG-DHA, and DAG-EPA. The inhibition of production of DAG-AA and DAG-DHA significantly inhibited MAP kinase activation in RasGRP overexpressing, but not in control, cells. Our study demonstrates that three DAG molecular species bind to RasGRP, but only DAG-AA and DAG-DHA participate in the modulation of RasGRP-mediated activation of MAP kinases in Jurkat T cells.

The four distal tyrosines are required for LAT-dependent signaling in FcepsilonRI-mediated mast cell activation

The linker for activation of T cells (LAT) is an adaptor protein critical for Fc epsilon RI-mediated mast cell activation. LAT is a substrate of the tyrosine kinases activated after TCR and Fc epsilon RI engagement. After phosphorylation of the cytosolic domain of LAT, multiple signaling molecules such as phospholipase C-gamma1, Grb2, and Gads associate with phosphorylated LAT via their SH2 domains. The essential role of the four distal tyrosines in TCR-mediated signaling and T cell development has been demonstrated by experiments using LAT-deficient cell lines and genetically modified mice. To investigate the role of these four tyrosines of LAT in Fc epsilon RI-mediated mast cell activation, bone marrow-derived mast cells from LAT-deficient mice were infected with retroviral vectors designed to express wild-type or mutant LAT. Examination of bone marrow-derived mast cells expressing various tyrosine to phenylalanine mutants in LAT demonstrates a differential requirement for these different binding sites. In these studies, assays of biochemical pathways, degranulation, and cytokine and chemokine release were performed. Finally, the role of these tyrosines was also evaluated in vivo using genetically modified animals. Deletion of all four distal tyrosines, and in particular, loss of the primary phospholipase C-gamma-binding tyrosine had a significant effect on antigen-induced histamine release.

Integration of DAG signaling systems mediated by PKC-dependent phosphorylation of RasGRP3

Members of the RasGRP family of Ras activators have C1 domains that bind diacylglycerol (DAG) and DAG analogs such as the tumor-promoting phorbol esters. RasGRP members could be responsible for some of the DAG signaling processes that have previously been attributed to protein kinase C (PKC). We found that RasGRP3 is selectively expressed in B cells, suggesting that RasGRP3 might function downstream of the B-cell receptor (BCR). Indeed, stimulation of Ramos B cells with the DAG analog phorbol ester myristate (PMA) results in the association of RasGRP3 with the membrane fraction. However, we also made the unexpected observation that RasGRP3 is phosphorylated, coincident with Ras activation after stimulation. When inhibitors of PKC are present, Ras activation is attenuated, and this attenuation correlates with an inhibition of RasGRP3 phosphorylation. RasGRP3 is phosphorylated in vitro by PKC-theta and PKC-beta2. When ectopically coexpressed in HEK-293 cells, a dominant-activated mutant of PKC-theta phosphorylates RasGRP3 and enhances Ras-Erk signaling. These results provide the first indication for a functional interaction between a RasGRP family member and a dissimilar DAG binding protein. A convergent DAG signaling system could be important in fine-tuning Ras signaling during B-cell development or during the humoral immune response.

Hematopoietic prostaglandin D synthase

The biological actions of prostaglandin (PG) D(2) include vasodilatation, bronchoconstriction, inhibition of platelet aggregation, and recruitment of inflammatory cells. Characterization of DP receptor null mice in which antigen-induced airway and inflammatory responses are attenuated and identification of CRTH2 as a novel PGD(2) receptor have shed light on the role of PGD(2) in the immune and inflammatory responses. Hematopoietic PGD synthase (H-PGDS) is a cytosolic enzyme that isomerizes PGH(2), a common precursor for all PGs and thromboxanes, to PGD(2) in a glutathione-dependent manner. H-PGDS is expressed in mast cells, antigen-presenting cells, and Th2 cells, and is the only mammalian member of the Sigma class of cytosolic glutathione S-transferases. In this review, we focus on the molecular biology of H-PGDS, the determination of its three-dimensional structure, characterization of the regulation of its gene expression, and information gleaned from transgenic animals.

Mast cells in airway hyporesponsive C3H/HeJ mice express a unique isoform of the signaling protein Ras guanine nucleotide releasing protein 4 that is unresponsive to diacylglycerol and phorbol esters

cDNAs were recently isolated from BALB/c mouse mast cells (MCs) that encode the new signaling protein mouse Ras guanine nucleotide releasing protein 4 (mRasGRP4). The present study evaluates the expression pattern and biological activity of mRasGRP4 in a variety of mouse strains. As assessed immunohistochemically and by RNA analysis, mRasGRP4 is not coordinately expressed with any of its family members. Normally, mRasGRP4 is an MC-restricted protein in tissues, and kinetic studies revealed that mRasGRP4 is expressed relatively early in developing MCs. The expression of mRasGRP4 in the fetus before granulated MCs become abundant supports the conclusion that RasGRP4 participates in MC-specific differentiation pathways. Functional studies conducted with recombinant material revealed that mRasGRP4 is a cation-dependent, diacylglycerol (DAG)-regulated, guanine nucleotide exchange factor. Immunoelectron microscopic studies revealed that mRasGRP4 resides in either the cytosol or inner leaflet of the plasma membrane of the MC, implying that DAG controls the intracellular movement of this signaling protein in c-kit-stimulated MCs. The mRasGRP4 gene resides on chromosome 7B1 within a site that is prominently linked to baseline airway reactivity in backcrossed C3H/HeJ and A/J mice. A truncated isoform of mRasGRP4 that lacks its DAG-regulatory domain was isolated from C3H/HeJ mouse MCs. Sequence analysis showed that this isoform is the result of defective splicing of the precursor transcript. MCs play a central role in allergic inflammation. The discovery of a novel isoform of mRasGRP4 in hyporesponsive mice suggests that airway reactivity is influenced by RasGRP4-dependent signaling events in pulmonary MCs.

Structural requirements of SLP-76 in signaling via the high-affinity immunoglobulin E receptor (Fc epsilon RI) in mast cells

The adapter SLP-76 plays an essential role in Fc epsilon RI signaling, since SLP-76(-/-) bone marrow-derived mast cells (BMMC) fail to degranulate and release interleukin-6 (IL-6) following Fc epsilon RI ligation. To define the role of SLP-76 domains and motifs in Fc epsilon RI signaling, SLP-76(-/-) BMMC were retrovirally transduced with SLP-76 and SLP-76 mutants. The SLP-76 N-terminal and Gads binding domains, but not the SH2 domain, were critical for Fc epsilon RI-mediated degranulation and IL-6 secretion, whereas all three domains are essential for T-cell proliferation following T-cell receptor (TCR) ligation. Unexpectedly, the three tyrosine residues in SLP-76 critical for TCR signaling, Y112, Y128, and Y145, were not essential for IL-6 secretion, but were required for degranulation and mitogen-activated protein kinase activation. Furthermore, a Y112/128F SLP-76 mutant, but not a Y145F mutant, strongly reconstituted mast cell degranulation, suggesting a critical role for Y145 in Fc epsilon RI-mediated exocytosis. These results point to important differences in the function of SLP-76 between T cells and mast cells.

RasGRP4 regulates the expression of prostaglandin D2 in human and rat mast cell lines

Mast cells (MCs) are a major source of prostaglandin (PG) D(2) in connective tissues, and the expression of this eicosanoid has been linked to asthma and other inflammatory disorders. While it is known that the surface receptor c-kit controls PGD(2) expression in MCs by regulating the levels of a synthase that converts PGH(2) to PGD(2), the intracellular signaling proteins that act downstream of c-kit in this cyclooxygenase pathway have not been identified. We recently cloned a new cation-dependent, guanine nucleotide exchange factor/phorbol ester receptor (designated RasGRP4) that is required for the efficient expression of granule proteases in the human MC line HMC-1. GeneChip analysis of approximately 12,600 transcripts in RasGRP4(-) and RasGRP4(+) HMC-1 cells revealed a >100-fold difference in the levels of hematopoietic PGD(2) synthase mRNA. No other transcript in the eicosanoid pathway was influenced by RasGRP4 in a comparable manner. As assessed by SDS-PAGE immunoblot analysis, RasGRP4(+) HMC-1 cells contained substantial amounts of PGD(2) synthase protein. RasGRP4(+) MCs also produced approximately 15-fold more PGD(2) than did RasGRP4(-) MCs when both cell populations were activated by calcium ionophore. The induced transcript is therefore translated, and substantial amounts of functional PGD(2) synthase accumulate in RasGRP4(+) MCs. In support of the conclusion that RasGRP4 controls PGD(2) expression in MCs, inhibition of RasGRP4 expression in the rat MC line RBL-2H3 using a siRNA approach resulted in low levels of PGD(2) synthase protein.

Transgenic expression of RasGRP1 induces the maturation of double-negative thymocytes and enhances the production of CD8 single-positive thymocytes

RasGRP1 is a guanine nucleotide exchange factor for Ras that is required for the efficient production of both CD4 and CD8 single-positive thymocytes. We found that RasGRP1 expression is rapidly up-regulated in double-negative thymocytes following pre-TCR ligation. Transgenic overexpression of RasGRP1 compensated for deficient pre-TCR signaling in vivo, enabling recombinase-activating gene 2(-/-) double-negative thymocytes to mature to the double-positive stage. RasGRP1 transgenic mice had a 4-fold increase in CD8 single-positive thymocytes, most of which had atypically low levels of CD3. The RasGRP1 transgene lowered the threshold of TCR signaling needed to initiate proliferation of single-positive thymocytes, with this effect being particularly evident among CD8 single-positive cells. In 3-day cultures, TCR stimulation via anti-CD3 caused a 10-fold increase in the ratio of CD8 to CD4 thymocytes among RasGRP1 transgenic vs nontransgenic thymocytes. These results demonstrate that in addition to driving the double-negative to double-positive transition, increased expression of RasGRP1 selectively increases CD8 single-positive thymocyte numbers and enhances their responsiveness to TCR signaling.

Move over protein kinase C, you've got company: alternative cellular effectors of diacylglycerol and phorbol esters

Diacylglycerol is an essential second messenger in mammalian cells. The most prominent intracellular targets of diacylglycerol and of the functionally analogous phorbol esters belong to the protein kinase C (PKC) family. However, at least five alternative types of high-affinity diacylglycerol/phorbol-ester receptor are known: chimaerins, protein kinase D, RasGRPs, Munc13s and DAG kinase gamma. Recent evidence indicates that these have functional roles in diacylglycerol second messenger signalling in vivo and that several cellular processes depend on these targets rather than protein kinase C isozymes. These findings contradict the still prevalent view according to which all diacylglycerol/phorbol-ester effects are caused by the activation of protein kinase C isozymes. RasGRP1 (in Ras/Raf/MEK/ERK signalling) and Munc13-1 (in neurotransmitter secretion) are examples of non-PKC diacylglycerol/phorbol-ester receptors that mediate diacylglycerol and phorbol-ester effects originally thought to be caused by PKC isozymes. In the future, pharmacological studies on PKC must be complemented with alternative experimental approaches to allow the separation of PKC-mediated effects from those caused by alternative targets of the diacylglycerol second messenger pathway. The examples of RasGRP1 and Munc13-1 show that detailed genetic analyses of C(1)-domain-containing non-PKC diacylglycerol/phorbol-ester receptors in mammals are ideally suited to achieve this goal.

Cloning of rat Ras guanine nucleotide releasing protein 4, and evaluation of its expression in rat mast cells and their bone marrow progenitors

We recently cloned a new mast cell (MC) restricted, Ras guanine nucleotide releasing protein (designated mRasGRP4) from IL-3-developed, mouse bone marrow-derived MCs that can activate varied members of the Ras superfamily of small GTP-binding proteins. We now describe the rat ortholog of this MC-specific guanine exchange factor. Using the mRasGRP4 gene and transcript in a homology-based cloning approach, the relevant transcript was isolated and sequenced from the spleen and lungs of Sprague-Dawley rats. Evidence for differential splicing of the rRasGRP4 transcript was obtained in the spleen. The rat basophilic leukemia 1 MC line was found to express rRasGRP4, as well as the MC-committed progenitors residing in the bone marrow and the mature MCs residing in varied tissues of Sprague-Dawley rats. Based on its deduced amino acid sequence, rRasGRP4 is 93% identical to mRasGRP4. rRasGRP4 contains all of the functional domains present in the RasGRP family of guanine nucleotide exchange factors. Like its mouse ortholog, rRasGRP4 is a MC-restricted guanine exchange factor that contains Ca(2+) and phorbol ester/diacylglycerol-binding domains C-terminal of its CDC25-like catalytic domain.