Proline-rich tyrosine kinase 2 and Rac activation by chemokine and integrin receptors controls NK cell transendothelial migration

Specific targeting of glioblastoma with an oncolytic virus expressing a cetuximab-CCL5 fusion protein via innate and adaptive immunity

Chemokines such as C-C motif ligand 5 (CCL5) regulate immune cell trafficking in the tumor microenvironment (TME) and govern tumor development, making them promising targets for cancer therapy. However, short half-lives and toxic off-target effects limit their application. Oncolytic viruses (OVs) have become attractive therapeutic agents. Here, we generate an oncolytic herpes simplex virus type 1 (oHSV) expressing a secretable single-chain variable fragment of the epidermal growth factor receptor (EGFR) antibody cetuximab linked to CCL5 by an Fc knob-into-hole strategy that produces heterodimers (OV-Cmab-CCL5). OV-Cmab-CCL5 permits continuous production of CCL5 in the TME, as it is redirected to EGFR+ glioblastoma (GBM) tumor cells. OV-Cmab-CCL5 infection of GBM significantly enhances the migration and activation of natural killer cells, macrophages and T cells; inhibits tumor EGFR signaling; reduces tumor size; and prolongs survival of GBM-bearing mice. Collectively, our data demonstrate that OV-Cmab-CCL5 offers a promising approach to improve OV therapy for solid tumors.

The Adhesome Network: Key Components Shaping the Tumour Stroma

Beyond the conventional perception of solid tumours as mere masses of cancer cells, advanced cancer research focuses on the complex contributions of tumour-associated host cells that are known as "tumour microenvironment" (TME). It has been long appreciated that the tumour stroma, composed mainly of blood vessels, cancer-associated fibroblasts and immune cells, together with the extracellular matrix (ECM), define the tumour architecture and influence cancer cell properties. Besides soluble cues, that mediate the crosstalk between tumour and stroma cells, cell adhesion to ECM arises as a crucial determinant in cancer progression. In this review, we discuss how adhesome, the intracellular protein network formed at cell adhesions, regulate the TME and control malignancy. The role of adhesome extends beyond the physical attachment of cells to ECM and the regulation of cytoskeletal remodelling and acts as a signalling and mechanosensing hub, orchestrating cellular responses that shape the tumour milieu.

CD20 is dispensable for B-cell receptor signaling but is required for proper actin polymerization, adhesion and migration of malignant B cells

Surface protein CD20 serves as the critical target of immunotherapy in various B-cell malignancies for decades, however its biological function and regulation remain largely elusive. Better understanding of CD20 function may help to design improved rational therapies to prevent development of resistance. Using CRISPR/Cas9 technique, we have abrogated CD20 expression in five different malignant B-cell lines. We show that CD20 deletion has no effect upon B-cell receptor signaling or calcium flux. Also B-cell survival and proliferation is unaffected in the absence of CD20. On the contrary, we found a strong defect in actin cytoskeleton polymerization and, consequently, defective cell adhesion and migration in response to homeostatic chemokines SDF1α, CCL19 and CCL21. Mechanistically, we could identify a reduction in chemokine-triggered PYK2 activation, a calcium-activated signaling protein involved in activation of MAP kinases and cytoskeleton regulation. These cellular defects in consequence result in a severely disturbed homing of B cells in vivo.

Inhibition of natural killer cell cytotoxicity by interleukin-6: implications for the pathogenesis of macrophage activation syndrome

OBJECTIVE

Systemic juvenile idiopathic arthritis (JIA) is associated with high levels of interleukin-6 (IL-6) in the serum and synovial fluid, and impairment of natural killer (NK) cell function is often observed. This study was undertaken to evaluate a possible link between these 2 biologic findings and whether they may be associated with the development of macrophage activation syndrome, a condition frequently observed in systemic JIA.

METHODS

Splenocytes from wild-type (WT) or IL-6-transgenic (Tg) mice were evaluated for NK cell cytotoxicity using a (51) Cr-release assay. Numbers of NK cells and expression of perforin, granzyme B, CD69, and CD107a were evaluated by flow cytometry. Human peripheral blood mononuclear cells (PBMCs) isolated from healthy donors were treated with IL-6 and cultured in the presence or absence of tocilizumab (TCZ), an IL-6 receptor blocker. Human polyclonal NK cells from healthy donor PBMCs were evaluated for cell cytotoxicity and expression of perforin, granzyme B, and CD107a. PBMCs harvested from patients with systemic JIA during periods of active or inactive disease were left untreated or treated with IL-6 in combination with soluble IL-6 receptor and analyzed for the expression of perforin and granzyme B.

RESULTS

Splenic NK cell cytotoxicity was reduced in IL-6-Tg mice compared to WT mice. Levels of CD69 and CD107a showed no significant differences, whereas expression of perforin and granzyme B was impaired in NK cells from IL-6-Tg mice. Exposure of human peripheral blood NK cells to IL-6 led to reduced expression of perforin and granzyme B. Culturing human polyclonal NK cells in the presence of TCZ significantly increased cell cytotoxicity, and also increased expression of perforin and granzyme B. In patients with systemic JIA, a reduction in IL-6 plasma levels during disease remission correlated with the rescue of perforin and granzyme B expression in NK cells from these patients.

CONCLUSION

In both mice and humans, IL-6 down-modulated the cytotoxic activity of NK cells. This decrease was associated with reduced perforin and granzyme B levels in the absence of altered granule exocytosis.

Positive and negative regulation by SLP-76/ADAP and Pyk2 of chemokine-stimulated T-lymphocyte adhesion mediated by integrin α4β1

Stimulation by chemokines of integrin α4β1-dependent T-lymphocyte adhesion is a crucial step for lymphocyte trafficking. The adaptor Vav1 is required for chemokine-activated T-cell adhesion mediated by α4β1. Conceivably, proteins associating with Vav1 could potentially modulate this adhesion. Correlating with activation by the chemokine CXCL12 of T-lymphocyte attachment to α4β1 ligands, a transient stimulation in the association of Vav1 with SLP-76, Pyk2, and ADAP was observed. Using T-cells depleted for SLP-76, ADAP, or Pyk2, or expressing Pyk2 kinase-inactive forms, we show that SLP-76 and ADAP stimulate chemokine-activated, α4β1-mediated adhesion, whereas Pyk2 opposes T-cell attachment. While CXCL12-promoted generation of high-affinity α4β1 is independent of SLP-76, ADAP, and Pyk2, the strength of α4β1-VCAM-1 interaction and cell spreading on VCAM-1 are targets of regulation by these three proteins. GTPase assays, expression of activated or dominant-negative Rac1, or combined ADAP and Pyk2 silencing indicated that Rac1 activation by CXCL12 is a common mediator response in SLP-76-, ADAP-, and Pyk2-regulated cell adhesion involving α4β1. Our data strongly suggest that chemokine-stimulated associations between Vav1, SLP-76, and ADAP facilitate Rac1 activation and α4β1-mediated adhesion, whereas Pyk2 opposes this adhesion by limiting Rac1 activation.

Functions of the FAK family kinases in T cells: beyond actin cytoskeletal rearrangement

T cells control the focus and extent of adaptive immunity in infectious and pathological diseases. The activation of T cells occurs when the T cell antigen receptor (TCR) and costimulatory and/or adhesion receptors are engaged by their ligands. This process drives signaling that promotes cytoskeletal rearrangement and transcription factor activation, both of which regulate the quality and magnitude of the T cell response. However, it is not fully understood how different receptor-induced signals combine to alter T cell activation. The related non-receptor tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) are phosphorylated downstream of the TCR and several costimulatory and adhesion receptors. FAK family proteins integrate receptor-mediated signals that influence actin cytoskeletal rearrangement and effector T cell responses. In this review, we summarize the receptor-specific roles that FAK and Pyk2 control to influence T cell development and activation.

Effect of the disintegrin eristostatin on melanoma-natural killer cell interactions

Malignant melanoma is difficult to treat due to its resistance to chemotherapeutic regimens. Discovery of new pharmaceuticals with inhibitory potential can be helpful in the development of novel treatments. The snake venom disintegrin eristostatin, from the viper Eristicophis macmahoni, caused immunodeficient mice to be significantly protected from development of lung colonization when melanoma cells and the disintegrin were co-injected in vivo into the lateral tail vein compared to vehicle controls. Cytotoxicity assays suggested that eristostatin makes the melanoma cells a better target for lysis by human natural killer cells. Direct binding assays using atomic force microscopy showed eristostatin does specifically bind the surface of the six melanoma cell lines tested. Eristostatin binding was partially inhibited by the addition of soluble RGDS peptide, suggesting an integrin as one likely, but not the sole, binding partner. Studies done with melanoma cells on a culture dish and natural killer cells attached to a cantilever tip in atomic force microscopy showed four major populations of interactions which exhibited altered frequency and unbinding strength in the presence of eristostatin.

Small molecule inhibitors of the Pyk2 and FAK kinases modulate chemoattractant-induced migration, adhesion and Akt activation in follicular and marginal zone B cells

B-lymphocytes produce protective antibodies but also contribute to autoimmunity. In particular, marginal zone (MZ) B cells recognize both microbial components and self-antigens. B cell trafficking is critical for B cell activation and is controlled by chemoattactants such as CXCL13 and sphingosine 1-phosphate (S1P). The related tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase (Pyk2) regulate cell migration and adhesion but their roles in B cells are not fully understood. Using a novel Pyk2-selective inhibitor described herein (PF-719), as well as a FAK-selective inhibitor, we show that both Pyk2 and FAK are important for CXCL13- and S1P-induced migration of B-2 cells and MZ B cells. In contrast, LFA-1-mediated adhesion required only Pyk2 whereas activation of the Akt pro-survival kinase required FAK but not Pyk2. Thus Pyk2 and FAK mediate critical processes in B cells and these inhibitors can be used to further elucidate their functions in B cells.

The phosphatase PTP-PEST promotes secondary T cell responses by dephosphorylating the protein tyrosine kinase Pyk2

PTP-PEST (encoded by Ptpn12) is an intracellular protein tyrosine phosphatase belonging to the same family as LYP. LYP inhibits secondary T cell responses by suppressing Src family protein tyrosine kinases and is implicated in human autoimmunity. To determine the function of PTP-PEST in T cells, we generated mice with a conditionally deleted allele of Ptpn12. By removing PTP-PEST in T cells, we determined that PTP-PEST was not necessary for T cell development or primary responses. However, PTP-PEST was required for secondary T cell responses, anergy prevention, and autoimmunity induction. PTP-PEST specifically regulated the phosphorylation of Pyk2, a substrate of the Src family kinase Fyn. It also promoted the formation of T cell homoaggregates, which are known to enhance T cell activation. Thus, PTP-PEST controls Pyk2 activity and is a positive regulator of secondary T cell activation. These data illustrate the critical role of protein tyrosine phosphatases in T cell regulation.

Signal transduction during activation and inhibition of natural killer cells

Natural killer (NK) cells are important for early immune responses to viral infections and cancer. Upon activation, NK cells secrete cytokines and chemokines, and kill sensitive target cells by releasing the content of cytolytic granules. This unit is focused on the signal transduction pathways that regulate NK cell activities in response to contact with other cells. We will highlight signals regulating NK cell adhesion to target cells and describe the induction of cellular cytotoxicity by the engagement of different NK cell activation receptors. Negative signaling induced by inhibitory receptors opposes NK cell activation and provides an important safeguard from NK cell reactivity toward normal, healthy cells. We will discuss the complex integration of the different signals that occur during interaction of NK cells with target cells.

Suppressor of cytokine signaling 2 regulates IL-15-primed human NK cell function via control of phosphorylated Pyk2

NK cells are capable of killing virus-infected or tumor cells and producing IFN-gamma. Resting NK cells, however, have only minimal cytolytic activity and secrete a low level of IFN-gamma. The cytokine IL-15 can promote the expression of effector functions by resting NK cells. In this study, we demonstrate that suppressor of cytokine signaling 2 (SOCS2) has a novel role in IL-15-primed human NK cell function. SOCS2 expression was upregulated in NK cells following stimulation with IL-15. During IL-15-mediated NK cell priming, SOCS2 interacted with phosphorylated proline-rich tyrosine kinase 2 (Pyk2) at tyrosine 402 (p-Pyk2(Tyr402)) and induced the proteasome-mediated degradation of p-Pyk2(Tyr402) via ubiquitination. Knockdown of SOCS2 resulted in the accumulation of p-Pyk2(Tyr402) and blocked NK cell effector functions. In addition, NK cell cytolytic activity and IFN-gamma production were inhibited by overexpression of the wild-type of Pyk2 but not by the overexpression of tyrosine 402 mutant of Pyk2. These results suggest that SOCS2 regulates human NK cell effector functions via control of phosphorylated Pyk2 depending on IL-15 existence.

Impaired NK-cell migration in WAS/XLT patients: role of Cdc42/WASp pathway in the control of chemokine-induced beta2 integrin high-affinity state

We analyzed the involvement of Wiskott-Aldrich syndrome protein (WASp), a critical regulator of actin cytoskeleton remodeling, in the control of natural killer (NK)-cell migration. NK cells derived from patients with Wiskott-Aldrich syndrome/X-linked thrombocytopenia (WAS/XLT), carrying different mutations in the WASP coding gene, displayed reduced migration through intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), or endothelial cells in response to CXCL12/stromal cell-derived factor-1 and CX3CL1/fractalkine. Inhibition of WAS/XLT NK-cell migration was associated with reduced ability of these cells to up-regulate the expression of CD18 activation neoepitope and to adhere to ICAM-1 or VCAM-1 following chemokine stimulation. Moreover, chemokine receptor or beta1 or beta2 integrin engagement on NK cells rapidly resulted in Cdc42 activation and WASp tyrosine phosphorylation as well as in WASp association with Fyn and Pyk-2 tyrosine kinases. NK-cell pretreatment with wiskostatin, to prevent Cdc42/WASp association, impaired chemokine-induced NK-cell migration through ICAM-1 and beta2 integrin activation-dependent neoepitope expression. These results show that the Cdc42/WASp pathway plays a crucial role in the regulation of NK-cell migration by acting as a critical component of the chemokine-induced inside-out signaling that regulates lymphocyte function-associated antigen-1 function and suggest that after integrin or chemokine receptor engagement WASp function is regulated by the coordinate action of both Cdc42 and tyrosine kinases.

NK cells and chemokines

Natural killer (NK) cells belong to a distinct lineage of lymphocytes that play an important role in the early phase of immune responses against certain microbial pathogens by exhibiting cytotoxic functions and secreting a number of cytokines and chemokines. NK cells develop from a common lymphoid precursor resident in the bone marrow (BM) that is considered the main site of their generation. The BM microenvironment provides a rich source of cytokines and growth factors and allows intimate contact between developing NK cells and stromal cells, which is required for their full maturation. Individual NK cell subsets displaying unique functional features, and tissue locations have been identified both in mouse and humans. Involvement of chemokines in the regulation of DC-mediated NK cell priming and effector functions has also been documented and should be taken into account when analyzing the role of chemokines in NK cell-dependent immune responses. Studies in man and mouse have shown that NK cells are distributed in several organs under normal conditions. Their frequency is comparatively high in nonlymphoid organs such as the lung, the liver and the mucosal tissue of maternal uterus, and rare in thymus and lymph nodes. Chemotactic factors, including chemokines, play critical roles in the regulation of NK cell migration across endothelium and into the tissues. The differences in chemokine receptor expression together with distinct adhesive properties of different NK cell subsets as well as activated NK cells, imply that they have multiple routes of circulation and trafficking patterns. Besides their role in the regulation of NK cell trafficking, chemotactic molecules can also affect NK cell effector functions by regulating their priming and their ability to kill and secrete cytokines.

Vav1 regulates the migration and adhesion of dendritic cells

Dendritic cells (DCs) are the most potent APCs for activating naive T cells, a process facilitated by the ability of immature DCs to mature and home to lymph nodes after encountering an inflammatory stimulus. Proteins involved in cytoskeletal rearrangement play an important role in regulating the adherence and motility of DCs. Vav1, a guanine nucleotide exchange factor for Rho family GTPases, mediates cytoskeletal rearrangement in hematopoietic cells following integrin ligation. We show that Vav1 is not required for the normal maturation of DCs in vitro; however, it is critical for DC binding to fibronectin and regulates the distribution but not the formation of podosomes. We also found that DC Vav1 was an important component of a signaling pathway involving focal adhesion kinase, phospholipase C-gamma2, and ERK1/2 following integrin ligation. Surprisingly, Vav1(-/-) DCs had increased rates of migration in vivo compared with wild-type control DCs. In vitro findings show that the presence of adhesive substrates such as fibronectin resulted in inhibition of migration. However, there was less inhibition in the absence of Vav1. These findings suggest that DC migration is negatively regulated by adhesion and integrin-mediated signaling and that Vav1 has a central role in this process.

Minimal requirement for induction of natural cytotoxicity and intersection of activation signals by inhibitory receptors

Natural killer (NK) cells provide innate control of infected and neoplastic cells. Multiple receptors have been implicated in natural cytotoxicity, but their individual contribution remains unclear. Here, we studied the activation of primary, resting human NK cells by Drosophila cells expressing ligands for receptors NKG2D, DNAM-1, 2B4, CD2, and LFA-1. Each receptor was capable of inducing inside-out signals for LFA-1, promoting adhesion, but none induced degranulation. Rather, release of cytolytic granules required synergistic activation through coengagement of receptors, shown here for NKG2D and 2B4. Although engagement of NKG2D and 2B4 was not sufficient for strong target cell lysis, collective engagement of LFA-1, NKG2D, and 2B4 defined a minimal requirement for natural cytotoxicity. Remarkably, inside-out signaling induced by each one of these receptors, including LFA-1, was inhibited by receptor CD94/NKG2A binding to HLA-E. Strong inside-out signals induced by the combination of NKG2D and 2B4 or by CD16 could overcome CD94/NKG2A inhibition. In contrast, degranulation induced by these receptors was still subject to inhibition by CD94/NKG2A. These results reveal multiple layers in the activation pathway for natural cytotoxicity and that steps as distinct as inside-out signaling to LFA-1 and signals for granule release are sensitive to inhibition by CD94/NKG2A.

B cell receptor-induced phosphorylation of Pyk2 and focal adhesion kinase involves integrins and the Rap GTPases and is required for B cell spreading

Signaling by the B cell receptor (BCR) promotes integrin-mediated adhesion and cytoskeletal reorganization. This results in B cell spreading, which enhances the ability of B cells to bind antigens and become activated. Proline-rich tyrosine kinase (Pyk2) and focal adhesion kinase (FAK) are related cytoplasmic tyrosine kinases that regulate cell adhesion, cell morphology, and cell migration. In this report we show that BCR signaling and integrin signaling collaborate to induce the phosphorylation of Pyk2 and FAK on key tyrosine residues, a modification that increases the kinase activity of Pyk2 and FAK. Activation of the Rap GTPases is critical for BCR-induced integrin activation as well as for BCR- and integrin-induced reorganization of the actin cytoskeleton. We now show that Rap activation is essential for BCR-induced phosphorylation of Pyk2 and for integrin-induced phosphorylation of Pyk2 and FAK. Moreover Rap-dependent phosphorylation of Pyk2 and FAK required an intact actin cytoskeleton as well as actin dynamics, suggesting that Rap regulates Pyk2 and FAK via its effects on the actin cytoskeleton. Importantly B cell spreading induced by BCR/integrin co-stimulation or by integrin engagement was inhibited by short hairpin RNA-mediated knockdown of either Pyk2 or FAK expression and by treatment with PF-431396, a chemical inhibitor that blocks the kinase activities of both Pyk2 and FAK. Thus Pyk2 and FAK are downstream targets of the Rap GTPases that play a key role in regulating B cell morphology.

A Pyk2-Vav1 complex is recruited to beta3-adhesion sites to initiate Rho activation

Integrin alphavbeta3-mediated adhesion of haemopoietic cells to vitronectin results in beta3 tyrosine phosphorylation and Rho activation which is necessary for adhesion. Previously, we have shown that the RhoGEF (Rho guanine-nucleotide-exchange factor) Vav1 could associate indirectly with alphavbeta3 during leucocyte adhesion to vitronectin. In the present study, we have identified the non-receptor tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) as the adaptor protein that links Vav1 with alphavbeta3. The association of Pyk2 and Vav1 with beta3 relies on the presence of Tyr747 in beta3, the primary site of beta3 phosphorylation. However, association of Pyk2 with Vav1 is independent of beta3 tyrosine phosphorylation. Formation of a Pyk2-Vav1 complex occurs upon cell adhesion and Pro717 of Pyk2 plays a key role in Pyk2 interaction with Vav1. Utilizing purified recombinant proteins, we confirmed the direct interaction between Pyk2 and Vav1 In vitro. Cells transfected with GFP (green fluorescent protein)-Pyk2-P717A demonstrated severely suppressed cytoskeletal reorganization, impaired Vav1 recruitment, decreased Rho GTPase activation and loss of cell adhesion. Using siRNA (small interfering RNA) to specifically reduce Pyk2 levels in cells resulted in disrupted association between Vav1 and beta3 and impaired cell adhesion. These results indicate that Pyk2 is a critical signalling molecule downstream of beta3 integrin tyrosine phosphorylation and mediates Vav1 recruitment to accomplish actin reorganization necessary for adhesion.

Proline-rich tyrosine kinase 2 regulates spreading and migration of eosinophils after beta2-integrin adhesion

We examined the role of proline-rich tyrosine kinase (Pyk) 2 in the spreading and migration of human blood eosinophils after beta(2)-integrin ligation. Western blot analysis showed that Pyk2 was activated by phosphorylation at Y402 after eosinophil adhesion to BSA-coated plates after activation with IL-5, platelet-activating factor (PAF), formyl-met-leu-phe (fMLP), or Mn(2)(+). To determine the role of Pyk2 in regulating eosinophil migration, we used a transducable dominant-negative inhibitor of Pyk2, TAT-mediated protein transduction of dominant-negative C-terminal Pyk2 (TAT-Pyk2-CT), a fusion protein in which TAT peptide was fused to the C-terminal Pyk2. TAT-Pyk2-CT blocked tyrosine phosphorylation of Pyk2 caused by beta(2)-integrin adhesion, but did not block adhesion of eosinophils to plated BSA. TAT-Pyk2-CT also blocked subsequent spreading and migration of eosinophils caused by IL-5, PAF, or fMLP. Spreading eosinophils stained with FITC-conjugated phalloidin showed elongation and formation of multiple fillopodia and lamellipodia, whereas nonspreading eosinophils were smaller and round. Treatment of eosinophils with TAT-Pyk2-CT had no effect on the initial cell polarization, but blocked the formation of fillopodia and lamellipodia in adherent cells. Migration of eosinophils through Transwell plates caused by IL-5, PAF, or fMLP was blocked significantly after inhibition of Pyk2. These data indicate that Pyk2, although not involved in beta(2)-integrin adhesion, causes eosinophil spreading and regulates subsequent chemotactic migration after beta(2)-integrin ligation to endothelial counter ligands. We conclude that Pyk2 is activated by beta(2)-integrin adhesion and is a required signal for eosinophil spreading and subsequent chemotactic migration.

Recruitment of circulating NK cells through decidual tissues: a possible mechanism controlling NK cell accumulation in the uterus during early pregnancy

During early pregnancy, uterine mucosa decidualization is accompanied by a drastic enrichment of CD56(high)CD16(-) natural killer (NK) cells. Decidual NK (dNK) cells differ from peripheral blood NK (pbNK) cells in several ways, but their origin is still unclear. Our results demonstrate that chemokines present in the uterus can support pbNK cell migration through human endothelial and stromal decidual cells. Notably, we observed that pregnant women's pbNK cells are endowed with higher migratory ability compared with nonpregnant women's or male donors' pbNK cells. Moreover, NK cell migration through decidual stromal cells was increased when progesterone-cultured stromal cells were used as substrate, and this correlated with the ability of progesterone to up-regulate stromal cell chemokine expression. Furthermore, we demonstrate that dNK cells migrate through stromal cells using a distinct pattern of chemokines. Finally, we found that pbNK cells acquire a chemokine receptor pattern similar to that of dNK cells when they contact decidual stromal cells. Collectively these results strongly suggest that pbNK cell recruitment to the uterus contributes to the accumulation of NK cells during early pregnancy; that progesterone plays a crucial role in this event; and that pbNK cells undergo reprogramming of their chemokine receptor profile once exposed to uterine microenvironment.

Ephrin-A1 stimulates migration of CD8+CCR7+ T lymphocytes

We have previously demonstrated that binding of ephrin-A1 to Eph receptors on human CD4+ T cells stimulates migration. Here, we show that a distinct population of CD8+ T lymphocytes, expressing the chemokine receptor CCR7, also binds ephrin-A1 and is stimulated to migrate after binding. The Eph receptor signaling pathway taking part in the migration event was here investigated. Induced tyrosine phosphorylation of several proteins was seen after ephrin-A1 binding. In particular, induced phosphorylation and kinase activity of the Src kinase family member Lck was observed. An Lck inhibitor inhibited ephrin-A1-induced migration, indicating the involvement of Lck in the migration event. In addition, we observed an induced association of the focal adhesion-like kinase proline-rich tyrosine kinase 2 (Pyk2) and the guanidine exchange factor Vav1 with Lck. PI3K inhibitors also inhibited migration, and studies in transfectants indicate an association of PI3K with EphA1. Further, ephrin-A1-induced migration could be related to the activation of Rho GTPases. This was also observed by using an inhibitor of the Rho-associated kinase ROCK, a downstream effector of Rho. Our results suggest that stimulation of Eph receptors on CD8+CCR7+ T cells leads to migration involving activation of Lck, Pyk2, PI3K, Vav1 and Rho GTPase.

Phospholipase C-gamma1 potentiates integrin-dependent cell spreading and migration through Pyk2/paxillin activation

Phospholipase C-gamma1 (PLC-gamma1), which generates two second messengers, namely, inositol-1, 4, 5-trisphosphate and diacylglycerol, is implicated in growth factor-mediated chemotaxis. However, the exact role of PLC-gamma1 in integrin-mediated cell adhesion and migration remains poorly understood. In this study, we demonstrate that PLC-gamma1 is required for actin cytoskeletal organization and cell motility through the regulation of Pyk2 and paxillin activation. After fibronectin stimulation, PLC-gamma1 directly interacted with the cytoplasmic tail of integrin beta1. In PLC-gamma1-silenced cells, integrin-induced Pyk2 and paxillin phosphorylation were significantly reduced and PLC-gamma1 potentiated the integrin-induced Pyk2/paxillin activation in its enzymatic activity-dependent manner. In addition, specific knock-down of PLC-gamma1 resulted in a failure to form focal adhesions dependent on fibronectin stimulation, which appeared to be caused by the suppression of Pyk2 and paxillin phosphorylation. Interestingly, PLC-gamma1 potentiated the activations of Rac, thus integrin-induced lamellipodia formation was up-regulated. Consequently, the strength of cell-substratum interaction and cell motility were profoundly up-regulated by PLC-gamma1. Taken together, these results suggest that PLC-gamma1 is a key player in integrin-mediated cell spreading and motility achieved by the activation of Pyk2/paxillin/Rac signaling.

Activation, coactivation, and costimulation of resting human natural killer cells

Natural killer (NK) cells possess potent perforin- and interferon-gamma-dependent effector functions that are tightly regulated. Inhibitory receptors for major histocompatibility complex class I display variegated expression among NK cells, which confers specificity to individual NK cells. Specificity is also provided by engagement of an array of NK cell activation receptors. Target cells may express ligands for a multitude of activation receptors, many of which signal through different pathways. How inhibitory receptors intersect different signaling cascades is not fully understood. This review focuses on advances in understanding how activation receptors cooperate to induce cytotoxicity in resting NK cells. The role of activating receptors in determining specificity and providing redundancy of target cell recognition is discussed. Using Drosophila insect cells as targets, we have examined the contribution of individual receptors. Interestingly, the strength of activation is not determined simply by additive effects of parallel activation pathways. Combinations of signals from different receptors can have different outcomes: synergy, no enhancement over individual signals, or additive effects. Cytotoxicity requires combined signals for granule polarization and degranulation. The integrin leukocyte function-associated antigen-1 contributes a signal for polarization but not for degranulation. Conversely, CD16 alone or in synergistic combinations, such as NKG2D and 2B4, signals for phospholipase-C-gamma- and phosphatidylinositol-3-kinase-dependent degranulation.

Signaling protein SWAP-70 is required for efficient B cell homing to lymphoid organs

The migration of B cells into secondary lymphoid organs is required for the generation of an effective immune response. Here we analyzed the involvement of SWAP-70, a Rac-interacting protein involved in actin rearrangement, in B cell entry into lymph nodes. We noted reduced migration of Swap70-/- B cells into lymph nodes in vivo. Swap70-/- B cells rolled and adhered, yet accumulated in lymph node high endothelial venules. This defect was not due to impaired integrin expression or chemotaxis. Instead, Swap70-/- B cells aberrantly regulated integrin-mediated adhesion. During attachment, Swap70-/- B cells showed defective polarization and did not form uropods or stabilize lamellipodia at a defined region. Thus, SWAP-70 selectively regulates processes essential for B cell entry into lymph nodes.

RAFTK/Pyk2 mediates LPA-induced PC12 cell migration

The phospholipid lysophosphatidic acid (LPA) is a normal constituent of serum that functions as a lipid growth factor and intracellular signaling molecule. In this report, we have investigated the signaling mechanism and function of the tyrosine kinase RAFTK/Pyk2 in LPA-induced cell migration. Analysis of tyrosine phosphorylation upon LPA stimulation in neuroendocrine PC12 cells revealed 6 major tyrosine-phosphorylated proteins with estimated sizes of 180, 120, 115, 68, 44, and 42 kDa. These proteins were identified as epidermal growth factor receptor (EGFR), focal adhesion kinase, RAFTK/Pyk2, paxillin, Erk 1, and Erk 2, respectively. Using specific pharmacological inhibitors, we found that the tyrosine phosphorylation of RAFTK/Pyk2 was intracellular Ca2+-dependent, but not EGFR-dependent, during LPA stimulation of these cells. Moreover, the cytoskeletal and signal scaffolding protein, paxillin, associated with and was regulated by RAFTK/Pyk2 in a Ca2+-dependent manner. Characterization of LPA receptors showed that LPA1 (Edg2) and LPA2 (Edg4) are major receptors for LPA, while LPA3 receptor (Edg7) expression was limited. Upon using the LPA1/LPA3 receptor-specific antagonist VPC 32179, we observed that inhibition of the LPA1/LPA3 receptors had no effect on the LPA-induced phosphorylation of RAFTK, strongly suggesting that the LPA2 receptor is a key mediator of RAFTK phosphorylation. Furthermore, LPA induced PC12 cell migration, which was subsequently blocked by the dominant-negative form of FAK, FRNK. Expression of a dominant-negative form of the small GTPase Ras also blocked LPA-induced cell migration and RAFTK phosphorylation. Taken together, these results indicate that RAFTK is a key signaling molecule that mediates LPA-induced PC12 cell migration in a Ras-dependent manner.

Activation pathways of α4β1 integrin leading to distinct T-cell cytoskeleton reorganization, Rac1 regulation and Pyk2 phosphorylation

Alpha4beta1 integrin is highly expressed in lymphocytes and is essential in hematopoiesis, extravasation, and the inflammatory response. Alpha4beta1 can be activated by intracellular signals elicited upon T-cell activation by phorbol esters, CD3 crosslinking, or certain chemokine/receptor interactions (inside-out activation). Divalent cations or certain anti-beta1 mAbs (i.e., TS2/16) can also bind and activate integrins directly (outside-in activation). In both cases, activation results in increased adhesion and/or affinity for ligands. It is not known if these various stimuli produce the same or different post-adhesion events. To address this, we have studied the cytoskeleton organization and intracellular signaling following activation of 41 in Jurkat cells and in human T-lymphoblasts. Treatment with Mn2+, alpha-CD3 mAb or the chemokine SDF-1alpha followed by attachment to the fibronectin fragment H89 or the endothelial molecule VCAM-1 (alpha4beta1 ligands), resulted in cell polarization and migration. In contrast, activation with PMA or TS2/16 induced cell spreading and strong adherence. Video microscopy and Transwell analyses confirmed these results, which correlated with different resistance to detachment under flow. Activation of the small GTPase RhoA or transfection with the constitutively active mutants V14RhoA or V12Rac1, abolished the alpha4beta1-induced cell polarization but did not affect cell spreading. Moreover, Rac1 activity was distinctly modulated by agents that induce a polarized or spread phenotype. The tyrosine kinase Pyk2 was highly phosphorylated upon induction of cell polarity but not during cell spreading. These results reveal novel properties of alpha4beta1 integrin, namely the ability to trigger two types of T-cell cytoskeletal response with different signaling requirements.

Integrin alpha4beta1 promotes focal adhesion kinase-independent cell motility via alpha4 cytoplasmic domain-specific activation of c-Src

The fibronectin binding integrins alpha5beta1 and alpha4beta1 generate signals pivotal for cell migration through distinct yet undefined mechanisms. For alpha5beta1, beta1-mediated activation of focal adhesion kinase (FAK) promotes c-Src recruitment to FAK and the formation of a FAK-Src signaling complex. Herein, we show that FAK expression is essential for alpha5beta1-stimulated cell motility and that exogenous expression of human alpha4 in FAK-null fibroblasts forms a functional alpha4beta1 receptor that promotes robust cell motility equal to the alpha5beta1 stimulation of wild-type and FAK-reconstituted fibroblasts. alpha4beta1-stimulated FAK-null cell spreading and motility were dependent on the integrity of the alpha4 cytoplasmic domain, independent of direct paxillin binding to alpha4, and were not affected by PRNK expression, a dominant-negative inhibitor of Pyk2. alpha4 cytoplasmic domain-initiated signaling led to a approximately 4-fold activation of c-Src which did not require paxillin binding to alpha4. Notably, alpha4-stimulated cell motility was inhibited by catalytically inactive receptor protein-tyrosine phosphatase alpha overexpression and blocked by the p50Csk phosphorylation of c-Src at Tyr-529. alpha4beta1-stimulated cell motility of triple-null Src(-/-), c-Yes(-/-), and Fyn(-/-) fibroblasts was dependent on c-Src reexpression that resulted in p130Cas tyrosine phosphorylation and Rac GTPase loading. As p130Cas phosphorylation and Rac activation are common downstream targets for alpha5beta1-stimulated FAK activation, our results support the existence of a novel alpha4 cytoplasmic domain connection leading to c-Src activation which functions as a FAK-independent linkage to a common motility-promoting signaling pathway.

Insights into seven and single transmembrane-spanning domain receptors and their signaling pathways in human natural killer cells

Human natural killer (NK) cells are important cells of the innate immune system. These cells perform two prominent functions: the first is recognizing and destroying virally infected cells and transformed cells; the second is secreting various cytokines that shape up the innate and adaptive immune re-sponses. For these cells to perform these activities, they express different sets of receptors. The receptors used by NK cells to extravasate into sites of injury belong to the seven transmembrane (7TM) family of receptors, which characteristically bind heterotrimeric G proteins. These receptors allow NK cells to sense the chemotactic gradients and activate second messengers, which aid NK cells in polarizing and migrating toward the sites of injured tissues. In addition, these receptors determine how and why human resting NK cells are mainly found in the bloodstream, whereas activated NK cells extravasate into inflammatory sites. Receptors for chemokines and lysophospholipids belong to the 7TM family. On the other hand, NK cells recognize invading or transformed cells through another set of receptors that belong to the single transmembrane-spanning domain family. These receptors are either inhibitory or activating. Inhibitory receptors contain the immune receptor tyrosine-based inhibitory motif, and activating receptors belong to either those that associate with adaptor molecules containing the immune receptor tyrosine-based activating motif (ITAM) or those that associate with adaptor molecules containing motifs other than ITAM. This article will describe the nature of these receptors and examine the intracellular signaling pathways induced in NK cells after ligating both types of receptors. These pathways are crucial for NK cell biology, development, and functions.

The tyrosine kinase pyk2 promotes migration and invasion of glioma cells

Glioblastoma multiforme is extraordinarily aggressive due to the propensity of cells to migrate away from the tumor core into the surrounding normal brain. In this report, we investigated the role of proline-rich tyrosine kinase 2 (Pyk2) and FAK with regard to influencing glioma cell phenotypes. Expression of Pyk2 stimulated glioma cell migration, whereas expression of FAK inhibited glioma cell migration and stimulated cell cycle progression. Pyk2 autophosphorylation was necessary, but not sufficient, to stimulate cellular migration. The N-terminal domain of Pyk2 is required for stimulation of migration as an N-terminally deleted variant of Pyk2 failed to stimulate migration, whereas expression of an autonomous Pyk2 N-terminal domain inhibited cell migration. Substitution of the C-terminal domain of Pyk2 with the corresponding domain of FAK stimulated cell migration as effectively as wild-type Pyk2; however, substitution of the N-terminal domain of Pyk2 with that of FAK inhibited cell migration, substantiating that the N-terminal domain of Pyk2 was required to stimulate migration. Silencing of Pyk2 expression by RNA interference significantly inhibited glioma migration. Cell migration was restored on re-expression of Pyk2, but expression of FAK in Pyk2 knockdown cells failed to restore migration. We conclude that Pyk2 plays a central role in the migratory behavior of glioblastomas.

Mouse cytomegalovirus M33 is necessary and sufficient in virus-induced vascular smooth muscle cell migration

Mouse cytomegalovirus (MCMV) encodes two potential seven-transmembrane-spanning proteins with homologies to cellular chemokine receptors, M33 and M78. While these virus-encoded chemokine receptors are necessary for the in vivo pathogenesis of MCMV, the function of these proteins is unknown. Since vascular smooth muscle cell (SMC) migration is of critical importance for the development of atherosclerosis and other vascular diseases, the ability of M33 to promote SMC motility was assessed. Similar to human CMV, MCMV induced the migration of mouse aortic SMCs but not mouse fibroblasts. To demonstrate whether M33 was required for MCMV-induced SMC migration, we employed interfering-RNA technology to specifically knock down M33 expression in the context of viral infection. The knockdown of M33 resulted in the specific reduction of M33 protein expression and ablation of MCMV-mediated SMC migration but failed to reduce viral growth in cultured cells. Adenovirus vector expression of M33 was sufficient to promote SMC migration, which was enhanced in the presence of recombinant mouse RANTES (mRANTES). In addition, M33 promoted the activation of Rac1 and extracellular signal-related kinase 1/2 upon stimulation with mRANTES. These findings demonstrate that mRANTES is a ligand for this chemokine receptor and that the activation of M33 occurs in a ligand-dependent manner. Thus, M33 is a functional homologue of US28 that is required for MCMV-induced vascular SMC migration.

Critical role of proline-rich tyrosine kinase 2 in reversion of the adhesion-mediated suppression of reactive oxygen species generation by human neutrophils

Neutrophils act as the first line of innate immune defense against invading microorganisms during infection and inflammation. The tightly regulated production of reactive oxygen species (ROS) through activation of NADPH oxidase is a major weapon used by neutrophils and other phagocytic leukocytes to combat such pathogens. Cellular adhesion signals play important physiological roles in regulating the activation of NADPH oxidase and subsequent ROS formation. We previously showed that the initial suppression of the oxidase response of chemoattractant-stimulated adherent neutrophils is mediated via inhibition of Vav1-induced activation of the NADPH oxidase regulatory GTPase Rac2 by adhesion signals. In this study we show that prior exposure of neutrophils to a number of cytokines and inflammatory mediators, including TNF-alpha, GM-CSF, and platelet-activating factor, overcomes the adhesion-mediated suppression of ROS formation. Proline-rich tyrosine kinase 2 (pyk2) activity is enhanced under these conditions, correlating with the restoration of Vav1 and Rac2 activities. Both dominant negative pyk2 and a pyk2-selective inhibitor prevented restoration of ROS production induced by TNF-alpha, GM-CSF, and platelet-activating factor, and this loss of pyk2 activity resulted in decreased Vav1 tyrosine phosphorylation and subsequent Rac2 activation. Our studies identify pyk2 as a critical regulatory component and a molecular switch to overcome the suppression of leukocyte oxidant generation by cell adhesion. This activity constitutes a mechanism by which cytokines might lead to rapid elimination of invading pathogens by adherent neutrophils under normal conditions or enhance tissue damage in pathological states.

Ephrin-A1 binding to CD4+ T lymphocytes stimulates migration and induces tyrosine phosphorylation of PYK2

Eph receptors, the largest subfamily of receptor tyrosine kinases, and their ephrin ligands are important mediators of cell-cell communication regulating cell attachment, shape, and mobility. Here we demonstrate that CD4+ T lymphocytes express the EphA1 and EphA4 receptors and that these cells bind the ligand ephrin-A1. Further we show ephrin-A1 expression in vivo on high endothelial venule (HEV) endothelial cells. Ephrin-A1 binding to CD4+ T cells stimulates both stromal cell-derived factor 1α (SDF-1α)- and macrophage inflammatory protein 3β (MIP3β)-mediated chemotaxis. In line with the increased chemotactic response, increased actin polymerization is observed in particular with the combination of ephrin-A1 and SDF-1α. Signaling through EphA receptors induces intracellular tyrosine phosphorylation. In particular, proline-rich tyrosine kinase 2 (PYK2) is phosphorylated on tyrosine residues 402 and 580. Ephrin-A1-induced chemotaxis and intracellular tyrosine phosphorylation, including EphA1 and Pyk2, was inhibited by Tyrphostin-A9. In conclusion, ligand engagement of EphA receptors on CD4+ T cells stimulates chemotaxis, induces intracellular tyrosine phosphorylation, and affects actin polymerization. This, together with our finding that ephrin-A1 is expressed by HEV endothelial cells, suggests a role for Eph receptors in transendothelial migration.

The Chemokine Receptor CCR7 Activates in Dendritic Cells Two Signaling Modules That Independently Regulate Chemotaxis and Migratory Speed

CCR7 is necessary to direct dendritic cells (DCs) to secondary lymphoid nodes and to elicit an adaptative immune response. Despite its importance, little is known about the molecular mechanisms used by CCR7 to direct DCs to lymph nodes. In addition to chemotaxis, CCR7 regulates the migratory speed of DCs. We investigated the intracellular pathways that regulate CCR7-dependent chemotaxis and migratory speed. We found that CCR7 induced a G(i)-dependent activation of MAPK members ERK1/2, JNK, and p38, with ERK1/2 and p38 controlling JNK. MAPK members regulated chemotaxis, but not the migratory speed, of DCs. CCR7 induced activation of PI3K/Akt; however, these enzymes did not regulate either chemotaxis or the speed of DCs. CCR7 also induced activation of the GTPase Rho, the tyrosine kinase Pyk2, and inactivation of cofilin. Pyk2 activation was independent of G(i) and Src and was dependent on Rho. Interference with Rho or Pyk2 inhibited cofilin inactivation and the migratory speed of DCs, but did not affect chemotaxis. Interference with Rho/Pyk2/cofilin inhibited DC migratory speed even in the absence of chemokines, suggesting that this module controls the speed of DCs and that CCR7, by activating its components, induces an increase in migratory speed. Therefore, CCR7 activates two independent signaling modules, one involving G(i) and a hierarchy of MAPK family members and another involving Rho/Pyk2/cofilin, which control, respectively, chemotaxis and the migratory speed of DCs. The use of independent signaling modules to control chemotaxis and speed can contribute to regulate the chemotactic effects of CCR7.

Trading spaces: Rap, Rac, and Rho as architects of transendothelial migration

PURPOSE OF REVIEW

This review focuses on recent developments in understanding regulation of leukocyte transendothelial migration by small GTPase signaling.

RECENT FINDINGS

New studies are refining the model for GTPase regulation of leukocyte-endothelial cell interactions that occur during leukocyte transmigration. An emerging theme is that the endothelial cell is an active participant in this process; an example of this is the identification of a novel leukocyte docking structure. The role of second messengers such as reactive oxygen species downstream and the involvement of kinases such as Pyk2 and Tec kinases upstream of GTPase activation is becoming appreciated. In the leukocyte, finer distinctions between closely related GTPases like Rac1 and Rac2 are being made, and a new role for RhoH has been characterized. Finally, the focus on Rap1 as a key regulator of leukocyte integrin-dependent adhesion is expanding to include roles in endothelial cell-cell adhesion and junctional regulation during transmigration.

SUMMARY

Understanding the complex series of events involved in cell-cell interactions during leukocyte transendothelial migration is a prerequisite for designing novel therapies to treat clinical conditions in which an inappropriate inflammatory response leads to disease. A discussion is provided of recent developments in the molecular regulation of leukocyte recruitment.

Inhibition of interleukin-8 (CXCL8/IL-8) responses by repertaxin, a new inhibitor of the chemokine receptors CXCR1 and CXCR2

Repertaxin is a new non-competitive allosteric blocker of interleukin-8 (CXCL8/IL-8) receptors (CXCR1/R2), which by locking CXCR1/R2 in an inactive conformation prevents receptor signaling and human polymorphonuclear leukocyte (PMN) chemotaxis. Given the unique mode of action of repertaxin it was important to examine the ability of repertaxin to inhibit a wide range of biological activities induced by CXCL8 in human leukocytes. Our results show that repertaxin potently and selectively blocked PMN adhesion to fibrinogen and CD11b up-regulation induced by CXCL8. Reduction of CXCL8-mediated PMN adhesion by repertaxin was paralleled by inhibition of PMN activation including secondary and tertiary granule release and pro-inflammatory cytokine production, whereas PMN phagocytosis of Escherichia coli bacteria was unaffected. Repertaxin also selectively blocked CXCL8-induced T lymphocyte and natural killer (NK) cell migration. These data suggest that repertaxin is a potent and specific inhibitor of a wide range of CXCL8-mediated activities related to leukocyte recruitment and functional activation in inflammatory sites.

LFA-1 contributes an early signal for NK cell cytotoxicity

Cytotoxicity of human NK cells is activated by receptors that bind ligands on target cells, but the relative contribution of the many different activating and inhibitory NK cell receptors is difficult to assess. In this study, we describe an experimental system that circumvents some of the difficulties. Adhesion through beta2 integrin LFA-1 is a common requirement of CTLs and NK cells for efficient lysis of target cells. However, the contribution of LFA-1 to activation signals for NK cell cytotoxicity, besides its role in adhesion, is unclear. The role of LFA-1 was evaluated by exposing NK cells to human ICAM-1 that was either expressed on a Drosophila insect cell line, or directly coupled to beads. Expression of ICAM-1 on insect cells was sufficient to induce lysis by NK cells through LFA-1. Coexpression of peptide-loaded HLA-C with ICAM-1 on insect cells blocked the LFA-1-dependent cytotoxicity of NK cells that expressed HLA-C-specific inhibitory receptors. Polarization of cytotoxic granules in NK cells toward ICAM-1- and ICAM-2-coated beads showed that engagement of LFA-1 alone is sufficient to initiate activation signals in NK cells. Thus, in contrast to T cells, in which even adhesion through LFA-1 is dependent on signals from other receptors, NK cells receive early activation signals directly through LFA-1.

Impaired natural and CD16-mediated NK cell cytotoxicity in patients with WAS and XLT: ability of IL-2 to correct NK cell functional defect

In this study we show that Wiskott-Aldrich syndrome protein (WASp), a critical regulator of actin cytoskeleton that belongs to the Scar/WAVE family, plays a crucial role in the control of natural killer (NK) cell cytotoxicity. Analysis of NK cell numbers and cytotoxic activity in patients carrying different mutations in the WASP coding gene indicated that although the percentage of NK cells was normal or increased, natural cytotoxicity and antibody-mediated NK cell cytotoxicity were inhibited in all patients with the classical WAS phenotype and in most patients carrying mutations associated with the X-linked thrombocytopenia (XLT) phenotype. The inhibition of NK cell-mediated cytotoxicity was associated with the reduced ability of WAS and XLT NK cells to form conjugates with susceptible target cells and to accumulate F-actin on binding. Treatment with interleukin-2 (IL-2) corrected the functional defects of NK cells by affecting their ability to bind to sensitive target cells and to accumulate F-actin. In addition, we provide information on the molecular mechanisms that control WASp function, demonstrating that binding of NK cells to sensitive targets or triggering through CD16 by means of reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly activates Cdc42. We also found that WASp undergoes tyrosine phosphorylation upon CD16 or β2-integrin engagement on NK cells. (Blood. 2004;104:436-443)

Actin cytoskeletal reorganizations and coreceptor-mediated activation of rac during human immunodeficiency virus-induced cell fusion

The membrane fusion events which initiate human immunodeficiency virus type 1 (HIV-1) infection and promote cytopathic syncytium formation in infected cells commence with the binding of the HIV envelope glycoprotein (Env) to CD4 and an appropriate coreceptor. Here, we show that HIV Env-coreceptor interactions activate Rac-1 GTPase and stimulate the actin filament network reorganizations that are requisite components of the cell fusion process. Disrupting actin filament dynamics with jasplakinolide or latrunculin A arrested fusion at a late step in the formation of Env-CD4-coreceptor complexes. Time-lapse confocal microscopy of living cells revealed vigorous activity of actin-based, target cell membrane extensions at the target cell-Env-expressing cell interface. The expression of dominant-negative forms of actin-regulating Rho-family GTPases established that HIV Env-mediated syncytium formation relies on Rac-1 but not on Cdc42 or Rho activation in target cells. Similar dependencies were found when cell fusion was induced by Env expressed on viral or cellular membranes. Additionally, Rac activity was specifically upregulated in a coreceptor-dependent manner in fusion reaction cell lysates. These results define a role for HIV Env-coreceptor interactions in activating the cellular factors essential for virus-cell and cell-cell fusion and provide evidence for the participation of pertussis toxin-insensitive signaling pathways in HIV-induced membrane fusion.

Integrating an integrin: a direct route to actin

Integrins were so named for their ability to link the extracellular and intracellular skeletons. Now almost 20 years into integrin research, numerous questions remain as to how this interaction is accomplished and how it is modified to achieve a desired phenotype. As the cell adhesion and actin assembly fields are merging in combined approaches, novel actin assembly mechanisms are being uncovered. Some of the earliest identified cytoplasmic linker molecules, believed to mediate integrin-actin binding, are once again the subject of scrutiny as potential dynamic mediators of cell anchorage. It seems plausible that each unique cellular morphology occurs as the result of activation of distinct actin assembly systems that are either stabilized by unique bundling and linker proteins or modified for progression to a new phenotype. While this research initiative is likely to continue rapidly in a forward fashion, it remains to be clarified how integrins assemble the most stable and basic cytoskeletal phenotype, the adherent cell with prominent stress fibers. Recent investigations point towards a shift in the current model of anchoring at the cell periphery by providing both mechanisms and evidence for de novo actin assembly orchestrated by the adhesion site. Lacking a complete pathway from integrin ligation to an integrated extracellular-intracellular skeleton in any single system, this review proposes a simple model of integrin-mediated stress fiber integration by drawing from work in multiple systems.

Key role of proline-rich tyrosine kinase 2 in interleukin-8 (CXCL8/IL-8)-mediated human neutrophil chemotaxis

The signalling pathways leading to CXCL8/IL-8-induced human neutrophil migration have not been fully characterized. The present study demonstrates that CXCL8 induces tyrosine phosphorylation as well as enzymatic activity of proline-rich tyrosine kinase 2 (Pyk2), a non-receptor protein tyrosine kinase (PTK), in human neutrophils. Induction of Pyk2 tyrosine phosphorylation by CXCL8 is regulated by Src PTK activation, whereas it is unaffected by phosphatidylinositol 3-kinase activation. Inhibition of Pyk2 activation by PP1, a Src PTK inhibitor, is paralleled by the inhibition of CXCL8-mediated neutrophil chemotaxis. Among CXCL8 receptors, Src protein tyrosine kinase activation selectively regulates CXCR1-mediated polymorphonuclear neutrophil (PMN) chemotaxis. Overexpression of PykM, the kinase-dead mutant of Pyk2, blocks CXCL8-induced chemotaxis of HL-60-derived PMN-like cells, thus pinpointing the key role of Pyk2 in CXCL8-induced chemotaxis.

β3 integrin phosphorylation is essential for Arp3 organization into leukocyte αVβ3-vitronectin adhesion contacts

Integrins play a pivotal role in self-regulated hematopoietic adhesion and migration. Leukocyte αVβ3 integrin-mediated adhesion to vitronectin requires protein kinase C activation and phosphorylation on tyrosine 747 of the β3 cytoplasmic tail. We have previously shown that β3 phosphorylation is required for Rho activation. In this study, an antibody specific to phosphorylated β3 tyrosine 747 was used to localize phosphorylated αVβ3 in vitronectin adhesive structures. Early adhesion contacts containing phosphorylated β3 preceded actin stress fiber formation. β3 phosphorylation decreased progressively throughout the course of adhesion coincident with the appearance of actin stress fibers. Time-dependent increases in colocalization of β3 with tyrosine 402 phosphorylated Pyk2 in similar adhesive structures was observed, providing evidence for downstream signaling complex formation. Surprisingly, Arp3 organized into similar adhesion contacts in cells expressing wild-type β3 but not in those expressing a nonphosphorylatable mutant of β3, suggesting that β3 phosphorylation is required for sequestration of Arp3 to adhesion complexes. Suppression of actin stress fiber formation by an inhibitor to Rho kinase disrupted Arp3 organization while prolonging β3 phosphorylation throughout the adhesion time course. These data confirm a requirement for β3 phosphorylation in αVβ3-mediated adhesion to vitronectin and suggest that β3 phosphorylation permits signaling complex assembly at the adhesion site necessary for actin stress fiber formation in leukocytes.

Pyk2 amplifies epidermal growth factor and c-Src-induced Stat3 activation

Signal transducers and activators of transcription factors (STATs) mediate many of the cellular responses that occur following cytokine, growth factor, and hormone signaling. STATs are activated by tyrosine and serine phosphorylation, which normally occurs as a tightly regulated process. Dysregulated STAT activity may facilitate oncogenesis, as constitutively activated STATs have been found in many human tumors as well as in v-abl- and v-src-transformed cell lines. Pyk2 is a member of the focal adhesion kinase family and can be activated by c-Src, epidermal growth factor receptor (EGFR), Janus kinase 1, tyrosine kinases, and G-protein-coupled receptor signaling. Although Pyk2 has been implicated in Janus kinase-dependent activation of MAPK and Stat1, no role for Pyk2 in the activation of other STAT proteins has been ascribed. Here, we provide evidence that Pyk2, along with c-Src, facilitates EGFR-mediated Stat3 activation. Pyk2 expression in HeLa cells induces Stat3 reporter gene activation and Stat3 phosphorylation on amino acid residues Tyr-705 and Ser-727. Together Pyk2 and c-Src potently activate Stat3, and Pyk2 enhances Stat3-induced cell proliferation. Moreover, the expression of a dominant negative version of Pyk2 impairs c-Src-induced Stat3 activation and cell proliferation. The treatment of A431 cells with EGF results in the recruitment of c-Src, Pyk2, and Stat3 to the EGFR and the phosphorylation of c-Src, Pyk2, and Stat3. Expression of constructs for dominant negative forms of either Pyk2 or c-Src impair EGF-induced Stat3 phosphorylation. These results indicate that Pyk2 facilitates EGFR- and c-Src-mediated Stat3 activation, thereby implicating Pyk2 activation as a potential co-mediator in triggering Stat3-induced oncogenesis.

The Rap GTPases regulate integrin-mediated adhesion, cell spreading, actin polymerization, and Pyk2 tyrosine phosphorylation in B lymphocytes

Integrin-mediated adhesion plays an important role in B cell development and activation. Signaling initiated by antigens, chemokines, or phorbol esters can rapidly convert integrins to an activated adhesion-competent state. The binding of integrins to their ligands can then induce actin-dependent cell spreading, which can facilitate cell-cell adhesion or cell migration on extracellular matrices. The signaling pathways involved in integrin activation and post-adhesion events in B cells are not completely understood. We have previously shown that anti-Ig antibodies, the chemokine stromal cell-derived factor-1 (SDF-1; CXCL12), and phorbol esters activate the Rap1 and Rap2 GTPases in B cells and that Rap activation is essential for SDF-1-induced B cell migration (McLeod, S. J., Li, A. H. Y., Lee, R. L., Burgess, A. E., and Gold, M. R. (2002) J. Immunol. 169, 1365-1371; Christian, S. L., Lee, R. L., McLeod, S. J., Burgess, A. E., Li, A. H. Y., Dang-Lawson, M., Lin, K. B. L., and Gold, M. R. (2003) J. Biol. Chem. 278, 41756-41767). We show here that preventing Rap activation by expressing Rap-specific GTPase-activating protein II (RapGAPII) significantly decreased lymphocyte function-associated antigen-1- and alpha(4) integrin-dependent binding of murine B cell lines to purified adhesion molecules and to other cells. Conversely, augmenting Rap activation by expressing a constitutively active form of Rap2 enhanced B cell adhesion, showing for the first time that Rap2 can promote integrin activation. We also show that blocking Rap activation inhibited anti-Ig-induced cell spreading and phorbol ester-induced actin polymerization as well as anti-Ig- and SDF-1-induced phosphorylation of Pyk2, a tyrosine kinase involved in morphological changes and chemokine-induced B cell migration. Thus, the Rap GTPases regulate integrin-mediated B cell adhesion as well as processes that control B cell morphology and migration.

The molecular basis for adhesion-mediated suppression of reactive oxygen species generation by human neutrophils

Human neutrophil adherence to ECMs induces an initial inhibition of stimulated reactive oxygen species (ROS) formation, followed by an enhanced phase of oxidant production. The initial integrin-mediated suppression of ROS constitutes a mechanism to prevent inappropriate tissue damage as leukocytes migrate to inflammatory sites. The Rac2 guanosine 5'-triphosphatase (GTPase) is a critical regulatory component of the phagocyte NADPH oxidase. We show that activation of Rac2 is inhibited in adherent neutrophils, correlating with inhibition of ROS formation. Conversely, NADPH oxidase components p47 and p67 assemble normally, suggesting a specific action of adhesion on the Rac2 molecular switch. Reconstitution with activated Rac2 restored rapid NADPH oxidase activation kinetics to adherent neutrophils, establishing that inhibition was due to defective Rac2 activity. We provide evidence that integrins inhibit Rac2 activation via a membrane-associated guanine nucleotide exchange factor, likely to be Vav1. Activation of Vav1, but not its upstream activator, Syk, is suppressed by cell adhesion. Vav1 activity is inhibited due to dephosphorylation of the regulatory Tyr174 via enhanced tyrosine phosphatase activity in adherent cells. These studies identify an integrin-mediated pathway in which Vav1 is as a strong candidate for the critical regulatory point in suppression of Rac2 activation and ROS generation during inflammatory responses.