Regulatory and signaling properties of the Vav family

Anatomy of a murder—signal transduction pathways leading to activation of natural killer cells

Natural killer (NK) cells control the early phases of viral infections, modulate antigen-specific immune responses, and participate in the rejection of tumours and bone marrow grafts. A fine balance between inhibitory and activating receptors tightly regulates NK cell activation. Biochemical studies in human cell lines and primary cells have revealed some of the activating NK cell signalling pathways, however animal models are instrumental to understand the physiological implications of these findings for immune responses in vivo. Gene targeting in mice and biochemical studies in cells are helping to dissect out the various signal transduction pathways that control NK cell activation. A clearer view of these pathways may eventually help designing more effective immune therapies based on the use of NK cells.

Vav2 as a Rac-GDP/GTP Exchange Factor Responsible for the Nectin-induced, c-Src- and Cdc42-mediated Activation of Rac

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules that form homo- and hetero-trans-dimers (trans-interactions). Nectins first form cell-cell contact and then recruit cadherins to the nectin-based cell-cell contact sites to form adherens junctions cooperatively with cadherins. In addition, the trans-interactions of nectins induce the activation of Cdc42 and Rac small G proteins, which enhances the formation of adherens junctions by forming filopodia and lamellipodia, respectively. The trans-interactions of nectins first recruit and activate c-Src at the nectin-based cell-cell contact sites. c-Src then phosphorylates and activates FRG, a Cdc42-GDP/GTP exchange factor (GEF) for Cdc42. The activation of both c-Src and Cdc42 by FRG is necessary for the activation of Rac, but the Rac-GEF responsible for this activation of Rac remains unknown. We showed here that the nectin-induced activation of Rac was inhibited by a dominant negative mutant of Vav2, a Rac-GEF. Nectins recruited and tyrosine-phosphorylated Vav2 through c-Src at the nectin-based cell-cell contact sites, whereas Cdc42 was not necessary for the nectin-induced recruitment of Vav2 or the nectin-induced, c-Src-mediated tyrosine phosphorylation of Vav2. Cdc42 activated through c-Src then enhanced the GEF activity of tyrosine-phosphorylated Vav2 on Rac1. These results indicate that Vav2 is a GEF responsible for the nectin-induced, c-Src-, and Cdc42-mediated activation of Rac.

PAK1 negatively regulates the activity of the Rho exchange factor NET1

Rho family small G-protein activity is controlled by guanine nucleotide exchange factors that stimulate the release of GDP, thus allowing GTP binding. Once activated, Rho proteins control cell signaling through interactions with downstream effector proteins, leading to changes in cytoskeletal organization and gene expression. The ability of Rho family members to modulate the activity of other Rho proteins is also intrinsic to these processes. In this work we show that the Rac/Cdc42hs-regulated protein kinase PAK1 down-regulates the activity of the RhoA-specific guanine nucleotide exchange factor NET1. Specifically, PAK1 phosphorylates NET1 on three sites in vitro: serines 152, 153, and 538. Replacement of serines 152 and 153 with glutamate residues down-regulates the activity of NET1 as an exchange factor in vitro and its ability to stimulate actin stress fiber formation in cells. Using a phospho-specific antibody that recognizes NET1 phosphorylated on serine 152, we show that PAK1 phosphorylates NET1 on this site in cells and that Rac1 stimulates serine 152 phosphorylation in a PAK1-dependent manner. Furthermore, coexpression of constitutively active PAK1 inhibits the ability of NET1 to stimulate actin polymerization only when serines 152 and 153 are present. These data provide a novel mechanism for the control of RhoA activity by Rac1 through the PAK-dependent phosphorylation of NET1 to reduce its activity as a guanine nucleotide exchange factor.

Signaling and membrane dynamics during phagocytosis: many roads lead to the phagos(R)ome

Phagocytosis is the mechanism used by specialized cells such as macrophages, dendritic cells and neutrophils to internalize, degrade and eventually present peptides derived from particulate antigens. This process relies on profound rearrangements of the actin cytoskeleton and the plasma membrane to engulf particles. Recent work has highlighted the early recruitment of internal membranes derived from endocytic compartments and from the endoplasmic reticulum to allow plasma membrane extension at the onset of phagocytosis. This ensures that the phagosome is rapidly provided with the machinery appropriate for later phagocytic functions, including particle degradation and antigen presentation.

Role of vav1 in the lipopolysaccharide-mediated upregulation of inducible nitric oxide synthase production and nuclear factor for interleukin-6 expression activity in murine macrophages

vav1 has been shown to play a key role in lymphocyte development and activation, but its potential importance in macrophage activation has received little attention. We have previously reported that exposure of macrophages to bacterial lipopolysaccharide (LPS) leads to increased activity of hck and other src-related tyrosine kinases and to the prompt phosphorylation of vav1 on tyrosine. In this study, we tested the role of vav1 in macrophage responses to LPS, focusing on the upregulation of nuclear factor for interleukin-6 expression (NF-IL-6) activity and inducible nitric oxide synthase (iNOS) protein accumulation in RAW-TT10 murine macrophages. We established a series of stable cell lines expressing three mutant forms of vav1 in a tetracycline-regulatable fashion: (i) a form producing a truncated protein, vavC; (ii) a form containing a point mutation in the regulatory tyrosine residue, vavYF174; and (iii) a form with an in-frame deletion of 6 amino acids required for the guanidine nucleotide exchange factor (GEF) activity of vav1 for rac family GTPases, vavGEFmt. Expression of the truncated mutant (but not the other two mutants) has been reported to interfere with T-cell activation. In contrast, we now demonstrate that expression of any of the three mutant forms of vav1 in RAW-TT10 cells consistently inhibited LPS-mediated increases in iNOS protein accumulation and NF-IL-6 activity. These data provide direct evidence for a role for vav1 in LPS-mediated macrophage activation and iNOS production and suggest that vav1 functions in part via activation of NF-IL-6. Furthermore, these findings indicate that the GEF activity of vav1 is required for its ability to mediate macrophage activation by LPS.

Ectopic expression of VAV1 reveals an unexpected role in pancreatic cancer tumorigenesis

Herein, we show that the hematopoietic-specific GEF VAV1 is ectopically expressed in primary pancreatic adenocarcinomas due to demethylation of the gene promoter. Interestingly, VAV1-positive tumors had a worse survival rate compared to VAV1-negative tumors. Surprisingly, even in the presence of oncogenic KRAS, VAV1 RNAi abrogates neoplastic cellular proliferation in vitro and in vivo, thus identifying Vav1 as a growth-stimulatory protein in this disease. Vav1 acts synergistically with the EGF receptor to stimulate pancreatic tumor cell proliferation. Mechanistically, the effects of Vav1 require its GEF activity and the activation of Rac1, PAK1, and NF-kappaB and involve cyclin D1 upregulation. Thus, the discovery of prooncogenic pathways regulated by Vav1 makes it an attractive target for therapeutic intervention.

Thymosin β10 Inhibits Angiogenesis and Tumor Growth by Interfering with Ras Function

Thymosin β10 is a monomeric actin sequestering protein that regulates actin dynamics. Previously, we and others have shown that thymosin β10 acts as an actin-mediated tumor suppressor. In this study, we show that thymosin β10 is not only a cytoskeletal regulator, but that it also acts as a potent inhibitor of angiogenesis and tumor growth by its interaction with Ras. We found that overexpressed thymosin β10 significantly inhibited vascular endothelial growth factor–induced endothelial cell proliferation, migration, invasion, and tube formation in vitro. Vessel sprouting was also inhibited ex vivo. We further show that thymosin β10 directly interacted with Ras. This interaction resulted in inhibition of the Ras downstream mitogen-activated protein kinase/extracellular signal-regulated kinase kinase signaling pathway, leading to decreased vascular endothelial growth factor production. Thymosin β10 injected into a xenograft model of human ovarian cancer in nude mice markedly inhibited tumor growth and reduced tumor vascularity. In contrast, a related thymosin family member, thymosin β4, did not bind to Ras and showed positive effects on angiogenesis. These findings show that the inhibition of Ras signal transduction by thymosin β10 results in antiangiogenic and antitumor effects, suggesting that thymosin β10 may be valuable in anticancer therapy.

Novel association of Vav2 and Nek3 modulates signaling through the human prolactin receptor

Prolactin (PRL) receptor activation contributes to the progression and motility of human breast cancer. This event activates multimeric signaling pathways, including the activation of the Vav family of guanine nucleotide exchange factors. To detect novel proteins interacting with Vav, yeast two-hybrid analysis was performed and demonstrated an interaction between the serine/threonine NIMA (never in mitosis A)-related family kinase p56Nek3 and Vav1. The PRL-dependent interaction of Nek3 with Vav1 and Vav2 was confirmed by coimmunoprecipitation analysis. PRL stimulation of T47D cells induced Nek3 kinase activity and the interaction of Vav2/Nek3 with the PRL receptor. Increased Nek3 levels up-regulated Vav2 serine and tyrosine phosphorylation, whereas knockdown of Nek3 resulted in a reduction of Vav2 phosphorylation. Activation of guanosine triphosphatase Rac-1 in Chinese hamster ovary transfectants required both Nek3 and Vav2 and was inhibited by the overexpression of a kinase inactivating Nek3 mutant. However, overexpression of either Nek3 or kinase-inactive Nek3 had no effect on Vav2-potentiated signal transducer and activator of transcription 5-mediated gene expression. Overexpression of kinase inactive Nek3 in T47D cells led to a 50% increase in apoptosis vs. controls. These data suggest that the PRL-mediated activation of Nek3 contributes differentially to Vav2 signaling pathways involving Rac1 and signal transducer and activator of transcription 5 and implicates Nek3 during PRL-mediated actions in breast cancer.

Control of lymphocyte shape and the chemotactic response by the GTP exchange factor Vav

Rho GTPases control many facets of cell polarity and migration; namely, the reorganization of the cellular cytoskeleton to extracellular stimuli. Rho GTPases are activated by GTP exchange factors (GEFs), which induce guanosine diphosphate (GDP) release and the stabilization of the nucleotide-free state. Thus, the role of GEFs in the regulation of the cellular response to extracellular cues during cell migration is a critical step of this process. In this report, we have analyzed the activation and subcellular localization of the hematopoietic GEF Vav in human peripheral blood lymphocytes stimulated with the chemokine stromal cell-derived factor-1 (SDF-1alpha). We show a robust activation of Vav and its redistribution to motility-associated subcellular structures, and we provide biochemical evidence of the recruitment of Vav to the membrane of SDF-1alpha-activated human lymphocytes, where it transiently interacts with the SDF-1alpha receptor CXCR4. Overexpression of a dominant negative form of Vav abolished lymphocyte polarization, actin polymerization, and migration. SDF-1alpha-mediated cell polarization and migration also were impaired by overexpression of an active, oncogenic Vav, although the mechanism appears to be different. Together, our data postulate a pivotal role for Vav in the transmission of the migratory signal through the chemokine receptor CXCR4.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

The actin cytoskeleton controls the efficiency of killer Ig-like receptor accumulation at inhibitory NK cell immune synapses

Killer cell Ig-like receptors (KIRs) are MHC class I-specific receptors expressed in NK and T lymphocytes. KIR antagonism of activation signals occurs at the immune synapse between the effector and target cells. The processes that regulate clustering of KIR are not well defined. We have expressed KIR-GFP receptor chimeras in two human NK-like lines, YTS and NK92. In this study, we show that the frequency of KIR enrichment at the synapse was decreased for a KIR that lacks a portion of the cytoplasmic tail. Strikingly, blocking actin polymerization with a high dose of cytochalasin D also substantially decreased clustering of KIR as well as KIR-induced clustering of HLA-C-GFP in target cells. However, the effect of inhibiting actin polymerization was only clearly evident at the earlier time points after cell mixing, and eventually clustering of KIR and HLA-C occurred independently of actin remodeling. Although treatment with anti-LFA-1 also decreased conjugate formation, the frequency of KIR clustering remained normal within the population of conjugates that did form, suggesting that the effect of cytochalasin D is not solely through LFA-1. Collectively, these data suggest that the actin cytoskeleton and the cytoplasmic tail of KIR regulate the efficiency by which KIR accumulates at inhibitory NK cell synapses.

Characterization of VIK-1: a new Vav-interacting Kruppel-like protein

Binding partners of the Src homology domains of Vav-1 were characterized by a two-hybrid screening of a Jurkat cell cDNA library. One of the isolated clones encoded a new protein named VIK that belongs to the Kruppel-like zinc-finger gene family. Genome mapping showed that a single gene positioned at chromosome 7q22.1 generated three possible isoforms containing alternative domains such as proline-rich and Kruppel-associated box A or B repressor domains. The isolated isoform, VIK-1, did not contain such motifs but presented six tandemly arranged zinc-fingers and consensus Kruppel H-C links. VIK-1 interacted both with Vav-1 and cyclin-dependent kinase 4 through two independent domains and corresponded to a Vav C-Src homology domain (SH)3 partner able to shuttle between the nucleus and the cytoplasm exhibiting functional nuclear addressing and export sequences. The results indicated a restricted expression of the protein during the G1 phase and its overexpression resulted in an inhibition of the cell-cycle progression that was reversed in the presence of Vav 1. Thus, this ubiquitous factor provides a first link between Vav-1 and the cell-cycle machinery.

Novel interaction partners of the TPR/MET tyrosine kinase

A large variety of biological processes is mediated by stimulation of the receptor tyrosine kinase MET. Screening a mouse embryo cDNA library, we were able to identify several novel, putative intracellular TPR/MET-substrates: SNAPIN, DCOHM, VAV-1, Sorting nexin 2, Death associated protein kinase 3, SMC-1, Centromeric protein C, and hTID-1. Interactions as identified by yeast two-hybrid analysis were validated in vitro and in vivo by mammalian two-hybrid studies, a far-western assay and coimmunoprecipitation. Participation in apoptosis-regulating mechanisms through interaction with DAPK-3 and cell cycle control via binding to nuclear proteins such as CENPC and SMC-1 are possible new aspects of intracellular MET signaling.

Nonsteroidal anti-inflammatory drugs inhibit a Fyn-dependent pathway coupled to Rac and stress kinase activation in TCR signaling

In addition to their anti-inflammatory properties, nonsteroidal anti-inflammatory drugs (NSAIDs) harbor immunosuppressive activities related to their capacity both to inhibit cyclooxygenases (COXs) and to act as peroxisome proliferator-activated receptor (PPAR) ligands. We have previously shown that the stress-activated kinase p38 is a selective target of NSAIDs in T cells. Here we have investigated the effect of NSAIDs on the signaling pathway triggered by the T-cell antigen receptor (TCR) and leading to stress kinase activation. The results show that nonselective and COX-1-selective NSAIDs also block activation of the stress kinase c-Jun N-terminal kinase (JNK) and that prostaglandin-E2 (PGE2) reverses this block and enhances TCR-dependent JNK activation. Analysis of the activation state of the components upstream of p38 and JNK showed that NSAIDs inhibit the serine-threonine kinase p21-activated protein kinase 1 (Pak1) and the small guanosine 5'-triphosphatase (GTPase) Rac, as well as the Rac-specific guanine nucleotide exchanger, Vav. Furthermore, activation of Fyn, which controls Vav phosphorylation, is inhibited by NSAIDs, whereas activation of lymphocyte-specific protein tyrosine kinase (Lck) and of the Lck-dependent tyrosine kinase cascade is unaffected. Accordingly, constitutively active Fyn reverses the NSAID-dependent stress kinase inhibition. The data identify COX-1 as an important early modulator of TCR signaling and highlight a TCR proximal pathway selectively coupling the TCR to stress kinase activation.

Vav proteins, masters of the world of cytoskeleton organization

Vav proteins are evolutionarily conserved from nematodes to mammals and play a pivotal role in many aspects of cellular signaling, coupling cell surface receptors to various effectors functions. In mammals, there are three family members; Vav1 is specifically expressed in the hematopoietic system, whereas Vav2 and Vav3 are more ubiquitously expressed. Vav proteins contain multiple domains that enable their function in various fashions. The participation of the Vav proteins in several processes that require cytoskeletal reorganization, such as the formation of the immunological synapse (IS), phagocytosis, platelet aggregation, spreading, and transformation will be discussed in this review. We will also cover how the Vav proteins succeed in controlling these processes by their function as guanine nucleotide exchange factors (GEFs) for the Rho/Rac family of GTPases. The contribution of the Vav proteins in a GEF-independent manner to the organization of the cytoskeleton will also be deliberated. The scope of this review is to highlight the numerous roles of the Vav signal transducer proteins in actin organization.

Vav1 and vav3 have critical but redundant roles in mediating platelet activation by collagen

Vav family proteins are guanine nucleotide exchange factors for the Rho/Rac family of small GTP-binding proteins. In addition, they have domains that mediate protein-protein interactions, including one Src homology 2 (SH2) and two Src homology 3 (SH3) domains. Vav1, Vav2, and Vav3 play a crucial role in the regulation of phospholipase C gamma (PLC gamma) isoforms by immuno-tyrosine-based activation motif (ITAM)-coupled receptors, including the T- and B-cell antigen receptors. We have reported in platelets, however, that Vav1 and Vav2 are not required for activation of PLC gamma 2 in response to stimulation of the ITAM-coupled collagen receptor glycoprotein VI (GPVI). Here we report that Vav3 is tyrosinephosphorylated upon activation of GPVI but that Vav3-deficient platelets also exhibit a normal response upon activation of the ITAM receptor. In sharp contrast, platelets deficient in both Vav1 and Vav3 show a marked inhibition of aggregation and spreading upon activation of GPVI, which is associated with a reduction in tyrosine phosphorylation of PLC gamma 2. The phenotype of Vav1/2/3 triple-deficient platelets is similar to that of Vav1/3 double-deficient cells. These results demonstrate that Vav3 and Vav1 play crucial but redundant roles in the activation of PLC gamma 2 by GPVI. This is the first time that absolute redundancy between two protein isoforms has been observed with respect to the regulation of PLC gamma 2 in platelets.

Stromal cell-derived factor-1alpha/CXCL12-induced chemotaxis of T cells involves activation of the RasGAP-associated docking protein p62Dok-1

Events mediating stromal cell-derived factor-1 (SDF-1alpha/CXCL12) chemotaxis of lymphocytes are not completely known. We evaluated intracellular signaling through RasGAP-associated protein p62Dok-1 (downstream of tyrosine kinase [Dok-1]) and associated proteins. SDF-1alpha/CXCL12 stimulated Dok-1 tyrosine phosphorylation and association with RasGAP, adaptor protein p46Nck, and Crk-L in Jurkat T cells. The phosphorylation of Dok-1 was blocked by pretreatment of cells with the src kinase inhibitor PP2. Src kinase family member Lck was implicated. SDF-1alpha/CXCL12 did not phosphorylate Dok-1 in J.CaM1.6 cells, a Jurkat derivative not expressing Lck, but did phosphorylate Dok-1 in J.CaM1.6 cells expressing Lck. SDF-1alpha/CXCL12 induced the tyrosine phosphorylation of Pyk2 and the association of Pyk2 with zeta chain-associated protein-70 kilodaltons (Zap-70) and Vav. SDF-1alpha/CXCL12 enhanced the association of RasGAP with Pyk2. CXCR4-expressing NIH3T3 and Baf3 cells transfected with full-length Dok-1 cDNA were suppressed in their responses to SDF-1alpha/CXCL12-induced chemotaxis; mitogen-activated protein (MAP) kinase activity was also decreased. Chemotaxis to SDF-1/CXCL12 was significantly enhanced in Dok-1(-/-) CD4+ and CD8+ splenic T cells. These results implicate Dok-1, Nck, Crk-L, and Src kinases-especially Lck, Pyk2, Zap-70, Vav, and Ras-GAP-in intracellular signaling by SDF-1alpha/CXCL12, and they suggest that Dok-1 plays an important role in SDF-1alpha/CXCL12-induced chemotaxis in T cells.

Granulocyte CEACAM3 is a phagocytic receptor of the innate immune system that mediates recognition and elimination of human-specific pathogens

Carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) are used by several human pathogens to anchor themselves to or invade host cells. Interestingly, human granulocytes express a specific isoform, CEACAM3, that participates together with CEACAM1 and CEACAM6 in the recognition of CEACAM-binding microorganisms. Here we show that CEACAM3 can direct efficient, opsonin-independent phagocytosis of CEACAM-binding Neisseria, Moraxella, and Haemophilus species. CEACAM3- but not CEACAM6-mediated uptake is blocked by dominant-negative versions of the small GTPase Rac. Moreover, CEACAM3 engagement triggers membrane recruitment and increased GTP loading of Rac that are not observed upon bacterial binding to CEACAM6. Internalization and Rac stimulation are also inhibited by compromising the integrity of an immunoreceptor tyrosine-based activation motif (ITAM)–like sequence in the cytoplasmic tail of CEACAM3 or by interference with Src family protein tyrosine kinases that phosphorylate CEACAM3. In contrast to interfering with CEACAM6, blockage of CEACAM3-mediated events reduces the ability of primary human granulocytes to internalize and eliminate CEACAM-binding bacteria, indicating an important role of CEACAM3 in the control of human-specific pathogens by the innate immune system.

RACK1 regulates G1/S progression by suppressing Src kinase activity

Cancer genes exert their greatest influence on the cell cycle by targeting regulators of a critical checkpoint in late G(1). Once cells pass this checkpoint, they are fated to replicate DNA and divide. Cancer cells subvert controls at work at this restriction point and remain in cycle. Previously, we showed that RACK1 inhibits the oncogenic Src tyrosine kinase and NIH 3T3 cell growth. RACK1 inhibits cell growth, in part, by prolonging G(0)/G(1). Here we show that RACK1 overexpression induces a partial G(1) arrest by suppressing Src activity at the G(1) checkpoint. RACK1 works through Src to inhibit Vav2, Rho GTPases, Stat3, and Myc. Consequently, cyclin D1 and cyclin-dependent kinases 4 and 2 (CDK4 and CDK2, respectively) are suppressed, CDK inhibitor p27 and retinoblastoma protein are activated, E2F1 is sequestered, and G(1)/S progression is delayed. Conversely, downregulation of RACK1 by short interference RNA activates Src-mediated signaling, induces Myc and cyclin D1, and accelerates G(1)/S progression. RACK1 suppresses Src- but not mitogen-activated protein kinase-dependent platelet-derived growth factor signaling. We also show that Stat3 is required for Rac1 induction of Myc. Our results reveal a novel mechanism of cell cycle control in late G(1) that works via an endogenous inhibitor of the Src kinase.

Integrins and T cell-mediated immunity

Integrin receptors mediate adhesive events that are critical for a specific and effective immune response to foreign pathogens. Integrin-dependent interactions of lymphocytes and antigen-presenting cells (APCs) to endothelium regulate the efficiency and specificity of trafficking into secondary lymphoid organs and peripheral tissue. Within these sites, integrins facilitate cell movement via interactions with the extracellular matrix, and promote and stabilize antigen-specific interactions between T lymphocytes and APCs that are critical for initiating T cell-activation events. In this review, we discuss the role of integrins in T cell-mediated immunity, with a focus on how these receptors participate in lymphocyte recirculation and T cell activation, how antigen stimulation regulates integrin activity, and how integrins define functionally unique subsets of T cells and APCs.

Role of the Sec14-like domain of Dbl family exchange factors in the regulation of Rho family GTPases in different subcellular sites

Mechanisms underlying subcellular region-specific regulation of Rho family GTPases through Dbl family guanine nucleotide exchange factors (GEFs) remain totally unknown. Here we show that the Sec14-like domain, which lies in the N-terminus of the Dbl family GEFs Dbl and Ost, directs the subcellular localization of these GEFs and also their substrate Cdc42. When coexpressed with Cdc42 in human adenocarcinoma HeLa cells, Dbl-I and Ost-I, which lack the Sec14-like domain, translocated Cdc42 to the plasma membrane, where Dbl-I or Ost-I was colocalized. In marked contrast, Dbl-II and Ost-II, which contain the Sec14-like domain, were colocalized with Cdc42 in endomembrane compartments. Furthermore, ruffle membrane formation upon epidermal growth factor treatment was mediated by Dbl-I or Ost-I, but neither Dbl-II nor Ost-II, supporting a notion that GEFs with or without the Sec14-like domain are linked to different upstream signals. By employing a novel method to detect the active GTP-bound form of Cdc42 in situ, we demonstrate that Dbl-I and Ost-I, but neither Dbl-II nor Ost-II, indeed activate colocalized Cdc42.

Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglycerol-dependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav- and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-gamma, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.

Vav GEFs are required for beta2 integrin-dependent functions of neutrophils

Integrin regulation of neutrophils is essential for appropriate adhesion and transmigration into tissues. Vav proteins are Rho family guanine nucleotide exchange factors that become tyrosine phosphorylated in response to adhesion. Using Vav1/Vav3-deficient neutrophils (Vav1/3^ko), we show that Vav proteins are required for multiple β₂ integrin-dependent functions, including sustained adhesion, spreading, and complement-mediated phagocytosis. These defects are not attributable to a lack of initial β₂ activation as Vav1/3^ko neutrophils undergo chemoattractant-induced arrest on intercellular adhesion molecule-1 under flow. Accordingly, in vivo, Vav1/3^ko leukocytes arrest on venular endothelium yet are unable to sustain adherence. Thus, Vav proteins are specifically required for stable adhesion. β₂-induced activation of Cdc42, Rac1, and RhoA is defective in Vav1/3^ko neutrophils, and phosphorylation of Pyk2, paxillin, and Akt is also significantly reduced. In contrast, Vav proteins are largely dispensable for G protein-coupled receptor–induced signaling events and chemotaxis. Thus, Vav proteins play an essential role coupling β₂ to Rho GTPases and regulating multiple integrin-induced events important in leukocyte adhesion and phagocytosis.

Eph receptor–ephrin bidirectional signals that target Ras and Rho proteins

The ability of cells to respond to their surrounding environment and relay signals to the cell interior is essential for numerous processes during the development and maintenance of tissues. Eph receptors and their membrane-bound ligands, the ephrins, are unique in the receptor tyrosine kinase family in that their signaling is bidirectional, through both the receptor and the ligand. Eph receptors and ephrins are essential for a variety of biological processes, and play a particularly important role in regulating cell shape and cell movement. Recent data have linked Eph receptor-ephrin signaling complexes to the Ras and Rho families of small molecular weight GTPases and also to heterotrimeric G proteins. Understanding the signaling networks involved is an important step to understand the molecular basis for normal and defective cell-cell communication through Eph receptors and ephrins.

Distinct Functions of Vav1 in JNK1 Activation in Jurkat T Cells Versus Non-Haematopoietic Cells

Vav1, the 95-kDa protein encoded by the vav1 proto-oncogene, is expressed exclusively in haematopoietic cells, where it becomes phosphorylated on tyrosine residues in response to antigen receptor ligation. Vav1 was found to act as a Rac1-specific guanine nucleotide exchange factor and to activate c-Jun N-terminal kinase (JNK1) in vitro and in ectopic expression systems using non-haematopoietic cells. Here, we studied the role of Vav1 in JNK1 activation in T cells versus non-haematopoietic cells. Vav1 overexpression activated JNK1 in COS7 and 293T cells but not in Jurkat T lymphocytes. In contrast, constitutively activated Rac1 efficiently stimulated JNK1 in both cell types under the same conditions. Vav1 did function in T cells because it clearly stimulated the activity of a nuclear factor of activated T-cell reporter plasmid in the same cells. Moreover, Vav1 induction of JNK1 in T cells required coexpression with calcineurin. This cooperation was cell type specific because it was not observed in COS7 or 293T cells. In contrast, Vav1 did not cooperate with calcineurin to activate either extracellular signal-regulated kinase 2 or p38. These findings demonstrate that Vav1 alone is a poor activator of the JNK1 pathway in T cells and emphasize the importance of studying the physiological functions of Vav1 in haematopoietic cells.

Genetic control of NKT cell numbers

NKT cells play a critical role in shaping the character and strength of a wide range of immune responses, including those against pathogens, tumours, allografts and autologous tissues. Because numbers of NKT cells affect clinical outcomes in a wide range of disease models, and this characteristic demonstrates allelic variation, the mapping of the locations and identification of the coding sequences of these genes has become a matter of significant importance. Here, we review the results to date that examine the effects of targeted deletion of a number of candidate genes, as well as the congenic and genetic linkage analyses that have attempted to localize allelic loci that affect NKT cell numbers. Although a number of candidate genes have been examined, there is no evidence that any of these contribute to variation in NKT cell numbers in natural populations. Two of the most important genetic regions controlling NKT cell numbers are Nkt1 on chromosome 1, which may contribute to lupus susceptibility, and Nkt2 on chromosome 2, which appears to contribute to diabetes susceptibility. Of great interest is a third locus on chromosome 18, identified in a novel congenic line, which can confer an absolute deficiency in this important immunoregulatory lymphocyte population.

mRNA expression profiling reveals a role of Helicobacter pylori vacuolating toxin in escaping host defense

AIM: To study the immune response of host to Helicobacter pylori VacA.

METHODS: The monocyte/macrophage-like U937 cells were infected with Helicobacter pylori vacA-positive strain NCTC 11638 or isogenic vacA-negative mutant. Differentially expressed genes were identified at 2, 6, 10, and 24 h post-infection by cDNA microarray. Differential expressions of some genes were confirmed by Northern blot.

RESULTS: More than 100 genes altered their mRNA expression at different time points respectively, many of which were identified to be related to immune evasion.

CONCLUSION: VacA is a crucial element for H pylori to escape from host immune defense by means of differentially regulating the expression of some related genes. These genes, previously known or unknown to be involved in the mechanism of immune evasion, deserve further investigation to unearth much more information complicated in the immune response.

F-actin-dependent Translocation of the Rap1 GDP/GTP Exchange Factor RasGRP2

RasGRPs constitute a new group of diacylglycerol-dependent GDP/GTP exchange factors that activate Ras subfamily GTPases. Despite a common structure, Ras-GRPs diverge in their GTPase specificity, subcellular distribution, and downstream biological effects. The more divergent family member is RasGRP2, a Rap1-specific exchange factor with low affinity toward diacylglycerol. The regulation of RasGRP2 during signal transduction has remained elusive up to now. In this report, we show that the subcellular localization of Ras-GRP2 is highly dependent on actin dynamics. Thus, the induction of F-actin by cytoskeletal regulators such as Vav, Vav2, Dbl, and Rac1 leads to the shift of RasGRP2 from the cytosol to membrane ruffles and its co-localization with F-actin. Treatment of cells with cytoskeletal disrupting drugs abolishes this effect, leading to an abnormal localization of RasGRP2 in cytoplasmic clusters of actin. The use of Rac1 effector mutants indicates that the RasGRP2 translocation is linked exclusively to actin polymerization and is independent of other pathways such as p21-activated kinase JNK, or superoxide production. Biochemical experiments demonstrate that the translocation of RasGRP2 to membrane ruffles is mediated by the direct association of this protein with F-actin, a property contained within its 150 first amino acids. Finally, we show that the RasGRP2/F-actin interaction promotes the regionalized activation of Rap1 in juxtamembrane areas of the cell. These results reveal a novel function of the actin cytoskeleton in mediating the spatial activation of Ras subfamily GTPases through the selective recruitment of GDP/GTP exchange factors.

Control of TCR-Mediated Activation of β1 Integrins by the ZAP-70 Tyrosine Kinase Interdomain B Region and the Linker for Activation of T Cells Adapter Protein

One of the earliest functional responses of T lymphocytes to extracellular signals that activate the Ag-specific CD3/TCR complex is a rapid, but reversible, increase in the functional activity of integrin adhesion receptors. Previous studies have implicated the tyrosine kinase zeta-associated protein of 70 kDa (ZAP-70) and the lipid kinase phosphatidylinositol 3-kinase, in the activation of beta(1) integrins by the CD3/TCR complex. In this report, we use human ZAP-70-deficient Jurkat T cells to demonstrate that the kinase activity of ZAP-70 is required for CD3/TCR-mediated increases in beta(1) integrin-mediated adhesion and activation of phosphatidylinositol 3-kinase. A tyrosine to phenylalanine substitution at position 315 in the interdomain B of ZAP-70 inhibits these responses, whereas a similar substitution at position 292 enhances these downstream signals. These mutations in the ZAP-70 interdomain B region also specifically affect CD3/TCR-mediated tyrosine phosphorylation of residues 171 and 191 in the cytoplasmic domain of the linker for activation of T cells (LAT) adapter protein. CD3/TCR signaling to beta(1) integrins is defective in LAT-deficient Jurkat T cells, and can be restored with expression of wild-type LAT. Mutant LAT constructs with tyrosine to phenylalanine substitutions at position 171 and/or position 191 do not restore CD3/TCR-mediated activation of beta(1) integrins in LAT-deficient T cells. Thus, these studies demonstrate that the interdomain B region of ZAP-70 regulates beta(1) integrin activation by the CD3/TCR via control of tyrosine phosphorylation of tyrosine residues 171 and 191 in the LAT cytoplasmic domain.

Signaling through the Leukocyte Integrin LFA-1 in T Cells Induces a Transient Activation of Rac-1 That Is Regulated by Vav and PI3K/Akt-1

Integrin LFA-1 is a receptor that is able to transmit multiple intracellular signals in leukocytes. Herein we show that LFA-1 induces a potent and transient increase in the activity of the small GTPase Rac-1 in T cells. Maximal Rac-1 activity peaked 10-15 min after LFA-1 stimulation and rapidly declined to basal levels at longer times. We have identified Vav, a guanine nucleotide exchange factor for Rac-1, and PI3K/Akt, as regulators of the activation and inactivation phases of the activity of Rac-1, respectively, in the context of LFA-1 signaling based on the following experimental evidence: (i) LFA-1 induced activation of Vav and PI3K/Akt with kinetics consistent with a regulatory role for these molecules on Rac-1, (ii) overexpression of a constitutively active Vav mutant induces activation of Rac independently of LFA-1 stimulation whereas overexpression of a dominant-negative Vav mutant blocks LFA-1-mediated Rac activation, (iii) pharmacological inhibition of PI3K/Akt prevented the fall in the activity of Rac-1 after its initial activation but had no effect on Vav activity, and (iv) overexpression of a dominant-negative or a constitutively active Akt-1 induced or inhibited, respectively, Rac-1 activity. Finally, we show that T cells with a sustained Rac activity have impaired capacity to elongate onto ICAM-1. These results demonstrate that down-regulation of the activity of this GTPase is a requirement for the regulation of T cell morphology and motility and highlight the importance of temporal regulation of the signaling triggered from this integrin.

Association of PI 3-K with tyrosine phosphorylated Vav is essential for its activity in neutrophil-like maturation of myeloid cells

The importance of the Vav family of signal transduction molecules in hematopoietic cells has long been acknowledged, even though its role and its regulatory mechanism are not completely understood. We have previously demonstrated that tyrosine-phosphorylated Vav, also located inside the nucleus of myeloid cells, is up-regulated during maturation of promyelocytic precursors induced by all-trans-retinoic acid (ATRA). Here, we report that the tyrosine phosphorylation of Vav during granulocytic maturation is dependent on the tyrosine kinase Syk and is essential for the morphological changes of the cell nucleus. These ATRA-induced events are independent on the guanine nucleotide exchange activity of Vav. We also found that, in differentiating cells, and in both cytoplasmic and nuclear compartments, tyrosine phosphorylated Vav associates with the regulatory subunit of phosphoinositide 3-kinase (PI 3-K). The Vav/p85 interaction is essential for the ATRA-induced PI 3-K activity and for association of PI 3-K with actin, particularly in the nucleus. Our data indicate an unprecedented crucial function for Vav in modulating the morphological maturation process of myeloid cells in a GDP-GTP exchange factor (GEF)-independent manner and suggest a role of Vav as an adaptor protein responsible of targeting PI 3-K to its intranuclear substrates.

Leishmania-induced inhibition of macrophage antigen presentation analyzed at the single-cell level

A number of studies have previously examined the capacity of intracellular Leishmania parasites to modulate the capacity of macrophages to process and present Ags to MHC class II-restricted CD4(+) T cells. However, the bulk culture approaches used for assessing T cell activation make interpretation of some of these studies difficult. To gain a more precise understanding of the interaction between Leishmania-infected macrophages and effector T cells, we have analyzed various parameters of T cell activation in individual macrophage-T cell conjugates. Leishmania-infected macrophages efficiently stimulate Ag-independent as well as Ag-dependent, TCR-mediated capping of cortical F-actin in DO.11 T cells. However, infected macrophages are less efficient at promoting the sustained TCR signaling necessary for reorientation of the T cell microtubule organizing center and for IFN-gamma production. A reduced ability to activate these T cell responses was not due to altered levels of surface-expressed MHC class II-peptide complexes. This study represents the first direct single-cell analysis of the impact of intracellular infection on the interaction of macrophages with T cells and serves to emphasize the subtle influence Leishmania has on APC function.

Role of vav1- and src-related tyrosine kinases in macrophage activation by CpG DNA

Macrophage activation by CpG DNA requires toll-like receptor 9 and the adaptor protein MyD88. Gram-negative bacterial lipopolysaccharide also activates macrophages via a toll-like receptor pathway (TLR-4), but we and others have reported that lipopolysaccharide also stimulates tyrosine phosphorylation in macrophages. Herein we report that exposure of RAW 264.7 murine macrophages to CpG DNA (but not non-CpG DNA) provoked the rapid tyrosine phosphorylation of vav1. PP1, a selective inhibitor of src-related tyrosine kinases, blocked both the CpG DNA-mediated tyrosine phosphorylation of vav1 and the CpG DNA-mediated up-regulation of macrophage tumor necrosis factor secretion and inducible nitric-oxide synthase protein accumulation. Furthermore, we found that the inducible expression of any of three dominant interfering mutants of vav1 (a truncated protein, vavC; a form containing a point mutation in the regulatory tyrosine residue, vavYF174; and a form with an in-frame deletion of six amino acids required for the guanidine nucleotide exchange factor (GEF) activity of vav1 for rac family GTPases, vavGEFmt) consistently inhibited CpG DNA-mediated up-regulation of tumor necrosis factor secretion and inducible nitric-oxide synthase protein accumulation in RAW-TT10 macrophages. Finally, we determined that CpG DNA-mediated up-regulation of NF-kappaB activity (but not mitogen-activated protein kinase activation) was inhibited by preincubation with PP1 or by expression of the truncated vavC mutant. Taken together, our results indicate that the tyrosine phosphorylation of vav1 by a src-related tyrosine kinase or kinases plays an important role in the macrophage response to CpG DNA.

Homo- and hetero-oligomerization of PDZ-RhoGEF, LARG and p115RhoGEF by their C-terminal region regulates their in vivo Rho GEF activity and transforming potential

PDZ-RhoGEF, LARG, and p115RhoGEF are members of a newly identified family of Rho-guanine nucleotide exchange factors (GEFs) exhibiting a unique structural feature consisting of the presence of an area of similarity to regulators of G protein signaling (RGS). This RGS-like (RGL) domain provides a functional motif by which Galpha(12) and Galpha(13) can bind and regulate the activity of these RhoGEFs, thus providing a direct link from these heterotrimeric G proteins to Rho. PDZ-RhoGEF and LARG can also be phosphorylated by tyrosine kinases, including FAK, and associate with Plexin B, a semaphorin receptor, which controls axon guidance during development, through their PDZ domain, thereby stimulating Rho. Interestingly, while characterizing a PDZ-RhoGEF antiserum, we found that a transfected PDZ-RhoGEF construct associated with the endogenous PDZ-RhoGEF. Indeed, we observed that PDZ-RhoGEF and LARG can form homo- and hetero-oligomers, whereas p115RhoGEF can only homo-oligomerize, and that this intermolecular interaction was mediated by their unique C-terminal regions. Deletion of the C-terminal tail of PDZ-RhoGEF had no significant effect on the GEF catalytic activity towards Rho in vitro, but resulted in a drastic increase in the ability to stimulate a serum response element reporter and the accumulation of the GTP-bound Rho in vivo. Furthermore, removal of the C-termini of each of the three RGL-containing GEFs unleashed their full transforming potential. Together, these findings suggest the existence of a novel mechanism controlling the activity of PDZ-RhoGEF, LARG, and p115RhoGEF, which involves homo- and hetero-oligomerization through their inhibitory C-terminal region.

The hemopoietic Rho/Rac guanine nucleotide exchange factor Vav1 regulates N-formyl-methionyl-leucyl-phenylalanine-activated neutrophil functions

Vav1 is a hemopoietic-specific Rho/Rac guanine nucleotide exchange factor that plays a prominent role in responses to multisubunit immune recognition receptors in lymphoid cells, but its contribution to regulation of neutrophil functions is unknown. Activated Rho family GTPases are critical participants in neutrophil signaling cascades initiated by binding of FMLP and other chemoattractants to their cognate G protein-coupled receptors. Therefore, we investigated whether Vav1 regulates chemoattractant-induced responses in neutrophils. We found that superoxide production elicited by FMLP in Vav1(-/-) murine neutrophils isolated from either bone marrow or from peritoneal exudates was substantially reduced compared with that of wild type. Filamentous actin generation in FMLP-stimulated Vav1(-/-) neutrophils was also markedly reduced, whereas it was normal in response to IL-8 or leukotriene B(4). FMLP induced tyrosine phosphorylation of Vav1, whereas IL-8 or leukotriene B(4) did not, correlating with the requirement for Vav1 in chemoattractant-stimulated filamentous actin generation. Neutrophil motility in vitro and neutrophil mobilization into peripheral blood in vivo elicited by FMLP were both decreased in Vav1(-/-) mice. Hence, this study defines a new role for Vav1 in regulating granulocytic leukocytes as well as linking Vav1 to specific cellular responses downstream of a seven transmembrane domain receptor.

Microtubule disassembly induces cytoskeletal remodeling and lung vascular barrier dysfunction: Role of Rho-dependent mechanisms

Barrier dysfunction of pulmonary endothelial monolayer is associated with dramatic cytoskeletal reorganization, activation of actomyosin contractility, and gap formation. The linkage between the microtubule (MT) network and the contractile cytoskeleton has not been fully explored, however, clinical observations suggest that intravenous administration of anti-cancer drugs and MT inhibitors (such as the vinca alkaloids) can lead to the sudden development of pulmonary edema in breast cancer patients. In this study, we investigated the crosstalk between MT and actomyosin cytoskeleton and characterized specific molecular mechanisms of endothelial cells (EC) barrier dysfunction induced by MT inhibitor nocodazole (ND). Our results demonstrate that MT disassembly by ND induced rapid decreases in transendothelial electrical resistance (TER) and actin cytoskeletal remodeling, indicating EC barrier dysfunction. These effects involved ND-induced activation of Rho GTPase. Rho-mediated activation of its downstream target, Rho-kinase, induced phosphorylation of Rho-kinase effector EC MLC phosphatase (MYPT1) at Thr(696) and Thr(850) resulting in MYPT1 inactivation. Phosphatase inhibition leaded to accumulation of diphospho-MLC, which induced acto-myosin polymerization, stress fiber formation and gap formation. Inhibition of Rho-kinase by Y27632 abolished ND-induced MYPT1 phosphorylation, MLC phosphorylation, and stress fiber formation. In addition, MT preservation via the MT stabilizer paclitaxel, Rho inhibition (via C3 exotoxin, or dominant negative (DN)-Rho, or DN-Rho-kinase) attenuated ND-induced TER decreases, stress fiber formation and MLC phosphorylation. Collectively, our results demonstrate a leading role for Rho-dependent mechanisms in crosstalk between the MT and actomyosin cytoskeleton, and suggest Rho-kinase and MYPT1 as major Rho effectors mediating pulmonary EC barrier disruption in response to ND-induced MT disassembly.

The Helicobacter pylori vacuolating toxin inhibits T cell activation by two independent mechanisms

Helicobacter pylori toxin, VacA, damages the gastric epithelium by erosion and loosening of tight junctions. Here we report that VacA also interferes with T cell activation by two different mechanisms. Formation of anion-specific channels by VacA prevents calcium influx from the extracellular milieu. The transcription factor NF-AT thus fails to translocate to the nucleus and activate key cytokine genes. A second, channel-independent mechanism involves activation of intracellular signaling through the mitogen-activated protein kinases MKK3/6 and p38 and the Rac-specific nucleotide exchange factor, Vav. As a consequence of aberrant Rac activation, disordered actin polymerization is stimulated. The resulting defects in T cell activation may help H. pylori to prevent an effective immune response leading to chronic colonization of its gastric niche.

HIV-1 Nef triggers Vav-mediated signaling pathway leading to functional and morphological differentiation of dendritic cells

The accessory HIV-1 Nef protein plays a key role in AIDS pathogenesis. We recently demonstrated that exogenous Nef triggers phenotypic and functional differentiation of immature dendritic cells (DCs). Here we investigated whether the Nef-induced DC differentiation occurs with morphological remodeling and have focused on the interference of Nef in the signaling pathways that regulates DC maturation. We found that exogenous Nef enters immature DCs, promoting their functional and morphological differentiation. Specifically, Nef promotes interleukin (IL) -12 release, which closely fits with nuclear factor (NF) -kappaB activation. Nef induces rearrangement of actin microfilaments, leading to uropod and ruffle formation. Moreover, Nef increases the capacity of DCs to form clusters with allogeneic CD4+ T cells, improving immunological synapse formation. Searching for molecules involved in Nef-triggered signaling pathways driving the DC maturation, we found that Nef targets Vav and promotes its tyrosine phosphorylation, associated with its nucleus-to-cytoplasm redistribution. This has a direct effect on Vav guanine nucleotide exchange factor activity for the small GTPase Rac1. We hypothesize that targeting Vav, Nef modulates both early signaling events (such as cytoskeletal rearrangement) and delayed responses (such as transcriptional regulation), promoting DC differentiation. Our results highlight how Nef may enhance T lymphocyte activation, thus fostering virus dissemination, manipulating the DC arm of the immune response.

Characterization of novel splicing variants of the mouse MCF-2 (DBL) proto-oncogene

MCF-2 (DBL) proto-oncogene is a prototype guanine nucleotide exchange factor (GEF) that modulates Rho GTPases such as Rho, Rac, and Cdc42. Although the partial sequence of mouse MCF-2 has been determined, its full-length cDNA and biochemical functions had not been elucidated. We isolated the complete mouse MCF-2 cDNA and obtained recombinant functional protein. Homology between the mouse and human MCF-2 (DBL) cDNAs is 75.08% identity and between the mouse and human amino acid sequences 74.52% identity. Analysis of tissue distribution showed that mouse MCF-2 mRNA is expressed in brain, kidney, intestine, and testis. The brain-specific transcript is an alternatively spliced derivative that omits the 48bp exon 11. A similar alternatively spliced mRNA product is also found in humans (DBL). Guanine nucleotide exchange activities of the testis-expressed mouse Mcf-2 and human Dbl were analyzed using RhoA, Rac1, and Cdc42 as substrates. RhoA and Cdc42 were activated similarly by both gene products, but Rac1 was activated only by the mouse product. The brain-specific Mcf-2 gene product, and its human counterpart, was less active than the respective testis-specific products. This indicates that the element encoded by the 48bp exon missing in the brain transcripts is necessary for full GEF activity. This report provides fundamental data on the structure of Mcf-2, which regulates a variety of cellular signaling pathways.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

DroVav, the Drosophila melanogaster homologue of the mammalian Vav proteins, serves as a signal transducer protein in the Rac and DER pathways

Mammalian Vav signal transducer proteins couple receptor tyrosine kinase signals to the activation of the Rho/Rac GTPases, leading to cell differentiation and/or proliferation. The unique and complex structure of mammalian Vav proteins is preserved in the Drosophila melanogaster homologue, DroVav. We demonstrate that DroVav functions as a guanine-nucleotide exchange factor (GEF) for DRac. Drosophila cells overexpressing wild-type (wt) DroVav exhibited a normal morphology. However, overexpression of a truncated DroVav mutant (that functions as an oncogene when expressed in NIH3T3 cells) results in striking changes in the actin cytoskeleton, resembling those usually visible following Rac activation. Dominant-negative DRac abrogated these morphological changes, suggesting that the effect of the truncated DroVav mutant is mediated by activation of DRac. In Drosophila cells, we find that stimulation of the Drosophila EGF receptor (DER) increases tyrosine phosphorylation of DroVav, which in turn associates with tyrosine-phosphorylated DER. In addition, the following results imply that DroVav participates in downstream DER signalling, such as ERK phosphorylation: (a) overexpression of DroVav induces ERK phosphorylation; and (b) 'knockout' of DroVav by RNA interference blocks ERK phosphorylation induced by DER stimulation. Unlike mammalian Vav proteins, DroVav was not found to induce Jnk phosphorylation under the experimental circumstances tested in fly cells. These results establish the role of DroVav as a signal transducer that participates in receptor tyrosine kinase pathways and functions as a GEF for the small RhoGTPase, DRac.

An SH2 domain-dependent, phosphotyrosine-independent interaction between Vav1 and the Mer receptor tyrosine kinase: a mechanism for localizing guanine nucleotide-exchange factor action

Mer belongs to the Mer/Axl/Tyro3 receptor tyrosine kinase family, which regulates immune homeostasis in part by triggering monocyte ingestion of apoptotic cells. Mutations in Mer can also cause retinitis pigmentosa, again due to defective phagocytosis of apoptotic material. Although, some functional aspects of Mer have been deciphered, how receptor activation lead to the physiological consequences is not understood. By using yeast two-hybrid assays, we identified the carboxyl-terminal region of the guanine nucleotide-exchange factor (GEF) Vav1 as a Mer-binding partner. Unlike similar (related) receptors, Mer interacted with Vav1 constitutively and independently of phosphotyrosine, yet the site of binding localized to the Vav1 SH2 domain. Mer activation resulted in tyrosine phosphorylation of Vav1 and release from Mer, whereas Vav1 was neither phosphorylated nor released from kinase-dead Mer. Mutation of the Vav1 SH2 domain phosphotyrosine coordinating Arg-696 did not alter Mer/Vav1 constitutive binding or Vav1 tyrosine phosphorylation but did retard Vav1 release from autophosphorylated Mer. Ligand-dependent activation of Mer in human monocytes led to Vav1 release and stimulated GDP replacement by GTP on RhoA family members. This unusual constitutive, SH2 domain-dependent, but phosphotyrosine-independent, interaction and its regulated local release and subsequent activation of Rac1, Cdc42, and RhoA may explain how Mer coordinates precise cytoskeletal changes governing the ingestion of apoptotic material by macrophages and pigmented retinal epithelial cells.

Structural basis for the signaling specificity of RhoG and Rac1 GTPases

RhoG is a new GTPase that has high sequence similarity with members of the Rac subfamily (Rac1, Rac2, and Rac3), including the regions involved in effector recognition and binding. To characterize its biological properties, we have compared the activity of RhoG and Rac1 in a number of experimental systems, including the study of their subcellular localization, oncogenic potential, activation of effectors, and effect on F-actin dynamics. Our study indicates that RhoG and Rac1 share overlapping, but not identical, signal transduction pathways. In contrast to previous results, we also provide evidence that RhoG works in parallel to Rac1 rather than as a Rac1 upstream activator. Using an extensive collection of Rho/Rac1 chimeras and point mutants, we demonstrate that the different biological properties of RhoG and Rac1 can be traced to specific amino acid variations in their switch I, beta2/beta3 hairpin, alpha5 helix, and C-terminal polybasic regions. Taken collectively, our results highlight the complexity of the signal transduction pathways activated by Rho/Rac GTPases and provide insight into the structural determinants that mediate the differential engagement of biological responses by GTPases of very similar structure.

Epidermal growth factor-dependent regulation of Cdc42 is mediated by the Src tyrosine kinase

Treatment of cells with epidermal growth factor (EGF) promotes the activation of the small GTP-binding protein Cdc42, as well as its phosphorylation in cells. The EGF-dependent phosphorylation of Cdc42 occurs at tyrosine 64 in the Switch II domain and appears to be mediated through the Src tyrosine kinase, because both the expression of a dominant-negative Src mutant (mouse Src(K297R)) and treatment of cells with the Src kinase inhibitor PP2 blocks the EGF-stimulated phosphorylation of Cdc42, whereas expression of an activated Src mutant (Src(Y529F)) promotes phosphorylation in the absence of EGF treatment. The EGF-stimulated phosphorylation of Cdc42 is not required for its activation, nor does it directly affect the interactions of activated Cdc42 with target/effector proteins including PAK, ACK, WASP, or IQGAP. However, the EGF-stimulated phosphorylation of Cdc42 is accompanied by an enhancement in the interaction of Cdc42 with the Rho-GDP dissociation inhibitor (RhoGDI). The EGF-stimulated activation of Cdc42 does require activated Src, as well as the Vav2 protein, a member of the Dbl family of guanine nucleotide exchange factors. Src catalyzes the tyrosine phosphorylation of Vav2, and overexpression of Vav2 together with activated Src (Src(Y529F)) can completely bypass the need for EGF to promote the activation of Cdc42. Thus, EGF signaling through Src appears to have dual regulatory effects on Cdc42: 1). it leads to the activation of Cdc42 as mediated by the Vav2 guanine nucleotide exchange factor, and 2). it results in the phosphorylation of Cdc42, which stimulates the binding of RhoGDI, perhaps to direct the movement of Cdc42 to a specific cellular site to trigger a signaling response, because Cdc42-RhoGDI interactions are essential for Cdc42-induced cellular transformation.

Rac1 function is required for Src-induced transformation. Evidence of a role for Tiam1 and Vav2 in Rac activation by Src

The proto-oncogene c-Src has been implicated in the development and progression of a number of human cancers including those of colon and breast. Accumulating evidence indicates that activated alleles of Src may induce cell transformation through Ras-ERK-dependent and -independent pathways. Here we show that Rac1 activity is strongly elevated in Src-transformed cells and that this small G protein is a critical component of the pathway connecting oncogenic Src with cell transformation. We further show that Vav2 and the ubiquitously expressed Rac1 guanine nucleotide exchange factor Tiam1 are phosphorylated in tyrosine residues in cells transfected with active and oncogenic Src. Moreover, phosphorylation of Tiam1 in cells treated with pervanadate, a potent inhibitor of tyrosine phosphatases, was partially inhibited by the Src inhibitor SU6656. Using truncated mutants of Tiam1, we demonstrate that multiple sites can be tyrosine-phosphorylated by Src. Furthermore, Tiam1 cooperated with Src to induce activation of Rac1 in vivo and the formation of membrane ruffles. Similarly, activation of JNK and the c-jun promoter by Src were also potently increased by Tiam1. Together, these results suggest that Vav2 and Tiam1 may act as downstream effectors of Src, thereby regulating Rac1-dependent pathways that participate in Src-induced cell transformation.

Cell growth and metastasis in pancreatic cancer: is Vav the Rho'd to activation?

The best-known family of low molecular weight GTP-binding proteins is Ras, owing to their high incidence of gain of function mutations in a variety of human cancers including pancreatic cancer. Unlike Ras, no activating mutations have been observed thus far for Rho family GTP-binding proteins in cancer, yet there is increasing evidence that overexpression of Rho family members and/or dysregulation of the GDP-->GTP cycle play an important role in cancer development and progression. The activation of Rho family GTPases downstream of cell surface receptors results in the induction of several intracellular signaling cascades that have been shown to impact on such diverse cellular responses as reorganization of the actin cytoskeleton, gene transcription, cell survival, and cell proliferation. One family of guanine nucleotide exchange factors (GEFs) that have the potential to couple the activation of Rho family members to upstream growth factor receptor tyrosine kinases (RTKs) is the Vav family of proto-oncogenes. Recent experimental evidence has implicated Vav in the regulation of numerous Rho-mediated pathways downstream of RTKs and other cell surface receptors. In this review, we will discuss our current understanding of how Vav proteins are regulated, and how Vav and their target GTP-binding proteins participate in tumorigenesis.

EGF receptor mediates adhesion-dependent activation of the Rac GTPase: a role for phosphatidylinositol 3-kinase and Vav2

Organization of the actin cytoskeleton in eucaryotic cells is controlled by small GTPases of the Rho family. Rac becomes activated by growth factor stimulation and integrin-mediated cell adhesion to extracellular matrix and is known to have a crucial role in lamellipodia formation, cell spreading and migration. At present, the intracellular pathways that connect cell surface receptors to Rac activation are poorly characterized. It has been reported previously that integrin-mediated cell attachment induces activation of the EGF receptor (EGFR) in the absence of EGF. We demonstrate here that this activation is instrumental for integrin-dependent Rac activation. Thus, we found that cells in which EGFR activity had been inhibited failed to spread and form lamellipodia on fibronectin. Failure to spread coincided with inhibition of adhesion-induced GTP loading of Rac and also with inhibition of the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Subsequent studies demonstrated that an activated form of PI 3-kinase restored Rac GTP loading in the presence of EGFR inhibition, while a dominant-negative form of PI 3-kinase blocked Rac GTP loading in fibronectin-adherent cells. Our further functional studies identified Vav2, a known exchange factor for Rac, as a crucial downstream component in EGFR- and PI 3-kinase-dependent Rac activation upon integrin-mediated cell adhesion. Our results provide a mechanistic insight into integrin-dependent Rac activation, and identify a novel role for EGFR, PI 3-kinase and Vav2 in this pathway.

Vav1 dephosphorylation by the tyrosine phosphatase SHP-1 as a mechanism for inhibition of cellular cytotoxicity

Here, we present data suggesting a novel mechanism for regulation of natural killer (NK) cell cytotoxicity through inhibitory receptors. Interaction of activation receptors with their ligands on target cells induces cytotoxicity by NK cells. This activation is under negative control by inhibitory receptors that recruit tyrosine phosphatase SHP-1 upon binding major histocompatibility class I on target cells. How SHP-1 blocks the activation pathway is not known. To identify SHP-1 substrates, an HLA-C-specific inhibitory receptor fused to a substrate-trapping mutant of SHP-1 was expressed in NK cells. Phosphorylated Vav1, a regulator of actin cytoskeleton, was the only protein detectably associated with the catalytic site of SHP-1 during NK cell contact with target cells expressing HLA-C. Vav1 trapping was independent of actin polymerization, suggesting that inhibition of cellular cytotoxicity occurs through an early dephosphorylation of Vav1 by SHP-1, which blocks actin-dependent activation signals. Such a mechanism explains how inhibitory receptors can block activating signals induced by different receptors.