Pyk2 and Syk participate in functional activation of granulocytic HL-60 cells in a different manner

Complement dependent TNFα production in neutrophil-like HL60 cells

Neutrophils develop in the bone marrow (BM) from hematopoietic stem cells (HSCs) through a series of progenitor cells and mature neutrophils play a critical role in the human immune system. Previous studies revealed that tumor necrosis factor α (TNFα) produced by immature neutrophils contributes to HSCs development and vascular regeneration in the BM niche. However, the precise mechanism of TNFα production in immature neutrophils remains unclear. This study aims to assess the relationship between complement C3 activation and TNFα production from immature neutrophils. We investigated the regulatory mechanism of TNFα production by complement components in neutrophil-like HL60 cells. Flow cytometric analysis showed that C3a receptor (C3aR) and C3bi receptor (CR3, Mac-1, CD11b/CD18, integrin αMβ2) are expressed on the surface of neutrophil-like HL60 cells. We found that zymosan-treated human serum leads to TNFα production in neutrophil-like HL60 cells, but not in human polymorphonuclear cells (PMNs). A C3-convertase inhibitor, compstatin suppresses TNFα production. These data suggest that the TNFα production is mediated by complement C3 activation. Furthermore, the TNFα production is enhanced by Ca²⁺ elevating agents, thapsigargin (TG), but is suppressed by treatment with Ca²⁺ chelators, EGTA, or BAPTA-AM. In addition, the soluble TNFα production is suppressed by treatment with immobilized-fibrinogen or -fibronectin. Thus, the TNFα production is enhanced by intracellular Ca²⁺ elevation and is negatively regulated by the interaction between the neutrophil-like HL60 cells and fibrinogen or fibronectin.

Myeloid Poldip2 Contributes to the Development of Pulmonary Inflammation by Regulating Neutrophil Adhesion in a Murine Model of Acute Respiratory Distress Syndrome

Background Lung injury, a severe adverse outcome of lipopolysaccharide-induced acute respiratory distress syndrome, is attributed to excessive neutrophil recruitment and effector response. Poldip2 (polymerase δ-interacting protein 2) plays a critical role in regulating endothelial permeability and leukocyte recruitment in acute inflammation. Thus, we hypothesized that myeloid Poldip2 is involved in neutrophil recruitment to inflamed lungs. Methods and Results After characterizing myeloid-specific Poldip2 knockout mice, we showed that at 18 hours post-lipopolysaccharide injection, bronchoalveolar lavage from myeloid Poldip2-deficient mice contained fewer inflammatory cells (8 [4-16] versus 29 [12-57]×10⁴/mL in wild-type mice) and a smaller percentage of neutrophils (30% [28%-34%] versus 38% [33%-41%] in wild-type mice), while the main chemoattractants for neutrophils remained unaffected. In vitro, Poldip2-deficient neutrophils responded as well as wild-type neutrophils to inflammatory stimuli with respect to neutrophil extracellular trap formation, reactive oxygen species production, and induction of cytokines. However, neutrophil adherence to a tumor necrosis factor-α stimulated endothelial monolayer was inhibited by Poldip2 depletion (225 [115-272] wild-type [myePoldip2^+/+] versus 133 [62-178] myeloid-specific Poldip2 knockout [myePoldip2^-/-] neutrophils) as was transmigration (1.7 [1.3-2.1] versus 1.1 [1.0-1.4] relative to baseline transmigration). To determine the underlying mechanism, we examined the surface expression of β2-integrin, its binding to soluble intercellular adhesion molecule 1, and Pyk2 phosphorylation. Surface expression of β2-integrins was not affected by Poldip2 deletion, whereas β2-integrins and Pyk2 were less activated in Poldip2-deficient neutrophils. Conclusions These results suggest that myeloid Poldip2 is involved in β2-integrin activation during the inflammatory response, which in turn mediates neutrophil-to-endothelium adhesion in lipopolysaccharide-induced acute respiratory distress syndrome.

The Non-receptor Tyrosine Kinase Pyk2 in Brain Function and Neurological and Psychiatric Diseases

Pyk2 is a non-receptor tyrosine kinase highly enriched in forebrain neurons. Pyk2 is closely related to focal adhesion kinase (FAK), which plays an important role in sensing cell contacts with extracellular matrix and other extracellular signals controlling adhesion and survival. Pyk2 shares some of FAK’s characteristics including recruitment of Src-family kinases after autophosphorylation, scaffolding by interacting with multiple partners, and activation of downstream signaling pathways. Pyk2, however, has the unique property to respond to increases in intracellular free Ca²⁺, which triggers its autophosphorylation following stimulation of various receptors including glutamate NMDA receptors. Pyk2 is dephosphorylated by the striatal-enriched phosphatase (STEP) that is highly expressed in the same neuronal populations. Pyk2 localization in neurons is dynamic, and altered following stimulation, with post-synaptic and nuclear enrichment. As a signaling protein Pyk2 is involved in multiple pathways resulting in sometimes opposing functions depending on experimental models. Thus Pyk2 has a dual role on neurites and dendritic spines. With Src family kinases Pyk2 participates in postsynaptic regulations including of NMDA receptors and is necessary for specific types of synaptic plasticity and spatial memory tasks. The diverse functions of Pyk2 are also illustrated by its role in pathology. Pyk2 is activated following epileptic seizures or ischemia-reperfusion and may contribute to the consequences of these insults whereas Pyk2 deficit may contribute to the hippocampal phenotype of Huntington’s disease. Pyk2 gene, PTK2B, is associated with the risk for late-onset Alzheimer’s disease. Studies of underlying mechanisms indicate a complex contribution with involvement in amyloid toxicity and tauopathy, combined with possible functional deficits in neurons and contribution in microglia. A role of Pyk2 has also been proposed in stress-induced depression and cocaine addiction. Pyk2 is also important for the mobility of astrocytes and glioblastoma cells. The implication of Pyk2 in various pathological conditions supports its potential interest for therapeutic interventions. This is possible through molecules inhibiting its activity or increasing it through inhibition of STEP or other means, depending on a precise evaluation of the balance between positive and negative consequences of Pyk2 actions.

Focal Adhesion Genes Refine the Intermediate-Risk Cytogenetic Classification of Acute Myeloid Leukemia

In recent years, several attempts have been made to identify novel prognostic markers in patients with intermediate-risk acute myeloid leukemia (IR-AML), to implement risk-adapted strategies. The non-receptor tyrosine kinases are proteins involved in regulation of cell growth, adhesion, migration and apoptosis. They associate with metastatic dissemination in solid tumors and poor prognosis. However, their role in haematological malignancies has been scarcely studied. We hypothesized that PTK2/FAK, PTK2B/PYK2, LYN or SRC could be new prognostic markers in IR-AML. We assessed PTK2, PTK2B, LYN and SRC gene expression in a cohort of 324 patients, adults up to the age of 70, classified in the IR-AML cytogenetic group. Univariate and multivariate analyses showed that PTK2B, LYN and PTK2 gene expression are independent prognostic factors in IR-AML patients. PTK2B and LYN identify a patient subgroup with good prognosis within the cohort with non-favorable FLT3/NPM1 combined mutations. In contrast, PTK2 identifies a patient subgroup with poor prognosis within the worst prognosis cohort who display non-favorable FLT3/NPM1 combined mutations and underexpression of PTK2B or LYN. The combined use of these markers can refine the highly heterogeneous intermediate-risk subgroup of AML patients, and allow the development of risk-adapted post-remission chemotherapy protocols to improve their response to treatment.

Intracellular β2 integrin (CD11/CD18) interacting partners in neutrophil trafficking

BACKGROUND

Neutrophil recruitment during acute inflammation critically depends on the spatial and temporal regulation of β₂ integrins (CD11/CD18). This regulation occurs by inside-out and outside-in signalling via interaction of cytoplasmic proteins with the intracellular domains of the integrin α- and β-subunits. The underlying molecular mechanisms regulating β₂ integrins in neutrophils are still incompletely understood.

AIM

This review provides a comprehensive overview of our current knowledge on proteins interacting with the cytoplasmic tail of CD18, the conserved β-subunit of β₂ integrins, their regulation and their functional importance for neutrophil trafficking during acute inflammation.

RESULTS

A total of 22 proteins including Talin, Kindlin 3 and Coronin 1A have been reported to interact with the CD18 cytoplasmic tail. Here, proteins binding to the cytoplasmic domain of CD18 in experiments using purified, recombinant proteins or peptides in, for example, pull-down assays, are defined as direct interactors. Proteins that have been shown to interact with the cytoplasmic domain of CD18 using whole cell lysates in, for example, pull-down experiments are claimed as interacting proteins without evidence for direct interaction. In summary, β₂ integrin activation and signalling depend on a specific subset of proteins interacting with CD18 and their precise regulation. If disturbed, profound defects of neutrophil recruitment and activation become evident compromising the innate immune response.

CONCLUSIONS

The knowledge of proteins interacting with β₂ integrins and their regulation during neutrophil trafficking does not only improve our basic understanding of innate immunity but may pave the way to novel therapeutic strategies in the treatment of inflammatory diseases.

GBF1 bears a novel phosphatidylinositol-phosphate binding module, BP3K, to link PI3Kγ activity with Arf1 activation involved in GPCR-mediated neutrophil chemotaxis and superoxide production

In neutrophils, Arf1 is activated upon GPCR stimulation. GBF1, a GEF for Arf, is primarily responsible for Arf1 activation upon GPCR stimulation and is important for chemotaxis and superoxide production. GBF1 also binds to products of PI3Kγ . The results indicate a novel mechanism that links PI3Kγ with chemotaxis and superoxide production.

Most chemoattractants for neutrophils bind to the Gα_i family of heterotrimeric G protein–coupled receptors (GPCRs) and release Gβγ subunits to activate chemotaxis and superoxide production. GIT2, a GTPase-activating protein for Arf1, forms a complex with Gβγ and is integral for directional sensing and suppression of superoxide production. Here we show that GBF1, a guanine nucleotide exchanging factor for Arf-GTPases, is primarily responsible for Arf1 activation upon GPCR stimulation and is important for neutrophil chemotaxis and superoxide production. We find that GBF1 bears a novel module, namely binding to products of phosphatidyl inositol 3-kinase (PI3K), which binds to products of PI3Kγ. Through this binding, GBF1 is translocated from the Golgi to the leading edge upon GPCR stimulation to activate Arf1 and recruit p22phox and GIT2 to the leading edge. Moreover, GBF1-mediated Arf1 activation is necessary to unify cell polarity during chemotaxis. Our results identify a novel mechanism that links PI3Kγ activity with chemotaxis and superoxide production in GPCR signaling.

Hematopoietic Pyk2 regulates migration of differentiated HL-60 cells

Background

Pyk2 is a non-receptor cytoplasmic tyrosine kinase that belongs to the focal adhesion kinase family and has been implicated in neutrophil spreading and respiratory burst activity caused by TNF-α. However, the role of Pyk2 in neutrophil migration is incompletely defined. In this study, we tested the hypothesis that Pyk2 regulates the migration of neutrophil-like differentiated HL-60 cells subsequent to β2-integrin mediated cell adhesion.

Methods

HL-60 cells were induced to differentiate into neutrophil-like cells (dHL60) by incubation in medium containing 1.25% DMSO for up to 4 days. Pyk2 expression and tyrosine phosphorylation was measured by Western blot analysis. Adhesion of dHL60 cells to plated fibrinogen was measured by residual myeloperoxidase activity. dHL60 cell migration was evaluated using a 96-well chemoTx chamber.

Results

Western blot analysis demonstrated that hematopoietic Pyk2 was predominantly expressed after HL60 cell differentiation. Pyk2 was tyrosine phosphorylated upon adhesion of dHL60 cells to plated fibrinogen in the presence of fMLP. By contrast, tyrosine phosphorylation of Pyk2 was insignificant in dHL60 cells treated in suspension with fMLP. Antibodies against CD18 blocked both phosphorylation of Pyk2 and adhesion of dHL60 cells to fibrinogen, demonstrating that phosphorylation of Pyk2 was β₂-integrin dependent. TAT-Pyk2-CT, a dominant negative fusion protein in which the TAT protein transduction domain was fused to the c-terminal Pyk2, attenuated fMLP-stimulated spreading, migration and phosphorylation of endogenous Pyk2 without blocking adhesion of dHL-60 cells to fibrinogen. Similarly, silencing of Pyk2 expression by siRNA in dHL60 cells also attenuated dHL60 cell migration caused by fMLP. Phospho-Pyk2 was evenly distributed around cell membrane circumferentially in unstimulated dHL-60 cells adherent to plated fibrinogen. In dHL60 cells treated with fMLP to cause cell spreading and polarization, Pyk2 was concentrated at the leading edge of pseudopods or at the trailing edge of uropods during migration of neutrophilic dHL-60 cells.

Conclusions

We conclude that Pyk2 is activated by β2-integrin adhesion. The activated concentration of Pyk2 and colocalization with F-actin in pseudopodia suggests that Pyk2 may regulate cell spreading and migration in dHL60 cells.

The CysLT1 ligand leukotriene D4 supports alpha4beta1- and alpha5beta1-mediated adhesion and proliferation of CD34+ hematopoietic progenitor cells

Cytokines and chemokines control hematopoietic stem and progenitor cell (HPC) proliferation and trafficking. However, the role of nonpeptide mediators in the bone marrow microenvironment has remained elusive. Particularly CysLT(1), a G protein-coupled receptor recognizing inflammatory mediators of the cysteinyl leukotriene family, is highly expressed in HPCs. We therefore analyzed the effects of its ligands on human CD34(+) HPCs. The most potent CysLT(1) ligand, LTD(4), rapidly and significantly up-regulated alpha(4)beta(1) and alpha(5)beta(1) integrin-dependent adhesion of both primitive and committed HPC. LTD(4)-triggered adhesion was inhibited by specific CysLT(1) antagonists. The effects of other CysLT(1) ligands were weak (LTC(4)) or absent (LTE(4)). In serum-free liquid cultures supplemented with various hematopoietic cytokines including IL-3, only LTD(4) significantly augmented the expansion of HPCs in a dose-dependent manner comparable to that of peptide growth factors. LTC(4) and LTE(4) were less effective. In CD34(+) cell lines and primary HPCs, LTD(4) induced phosphorylation of p44/42 ERK/MAPK and focal adhesion kinase-related tyrosine kinase Pyk2, which is linked to integrin activation. Bone marrow stromal cells produced biologically significant amounts of cysteinyl leukotrienes only when hematopoietic cells were absent, suggesting a regulatory feedback mechanism in the hematopoietic microenvironment. In contrast to antagonists of the homing-related G protein-coupled receptor CXCR4, administration of a CysLT(1) antagonist failed to induce human CD34(+) HPC mobilization in vivo. Our results suggest that cysteinyl leukotriene may contribute to HPC retention and proliferation only when cysteinyl leukotriene levels are increased either systemically during inflammation or locally during marrow aplasia.

Protein tyrosine kinase, syk: a key player in phagocytic cells

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase expressed in a wide range of haematopoietic cells. At the initial stage of investigation, main exploring was toward its functions in platelets and in classical immunoreceptor signalling. However, Syk has now been recognized as a key player in both innate and adaptive immunity. Especially, in phagocytosis, Syk plays essential roles in signalling evoked by various types of receptors such as FcgammaR, CR3, Dectin-1 and apoptotic cell-recognizing receptor. A variety of upstream immunoreceptor tyrosine-based activation motif-like molecules have been found and are still in the course of new studies. On the contrary, downstream effectors to explain diverse function of Syk are still under exploration. As its novel function, we propose the role of Syk in the regulation of alpha-tubulin acetylation. Further investigation on the effectors of Syk would give us more information in relation to therapeutic molecular targets.

The Src family kinases Hck and Fgr regulate neutrophil responses to N-formyl-methionyl-leucyl-phenylalanine

The chemotactic peptide formyl-methionyl-leucyl-phenilalanine (fMLP) triggers intracellular protein tyrosine phosphorylation leading to neutrophil activation. Deficiency of the Src family kinases Hck and Fgr have previously been found to regulate fMLP-induced degranulation. In this study, we further investigate fMLP signaling in hck-/-fgr-/- neutrophils and find that they fail to activate a respiratory burst and display reduced F-actin polymerization in response to fMLP. Additionally, albeit migration of both hck-/-fgr-/-mouse neutrophils and human neutrophils incubated with the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) through 3-microm pore size Transwells was normal, deficiency, or inhibition, of Src kinases resulted in a failure of neutrophils to migrate through 1-microm pore size Transwells. Among MAPKs, phosphorylation of ERK1/2 was not different, phosphorylation of p38 was only partially affected, and phosphorylation of JNK was markedly decreased in fMLP-stimulated hck-/-fgr-/- neutrophils and in human neutrophils incubated with PP2. An increase in intracellular Ca(2+) concentration and phosphorylation of Akt/PKB occurred normally in fMLP-stimulated hck-/-fgr-/- neutrophils, indicating that activation of both phosphoinositide-specific phospholipase C and PI3K is independent of Hck and Fgr. In contrast, phosphorylation of the Rho/Rac guanine nucleotide exchange factor Vav1 and the Rac target p21-activated kinases were markedly reduced in both hck-/-fgr-/- neutrophils and human neutrophils incubated with a PP2. Consistent with these findings, PP2 inhibited Rac2 activation in human neutrophils. We suggest that Hck and Fgr act within a signaling pathway triggered by fMLP receptors that involves Vav1 and p21-activated kinases, leading to respiratory burst and F-actin polymerization.

The Vav binding site of the non-receptor tyrosine kinase Syk at Tyr 348 is critical for beta2 integrin (CD11/CD18)-mediated neutrophil migration

Leukocyte adhesion via beta(2) integrins (CD11/CD18) activates the tyrosine kinase Syk. We found that Syk was enriched at the lamellipodium during N-formyl-Met-Leu-Phe-induced migration of neutrophil-like differentiated HL-60 cells. Here, Syk colocalized with Vav, a guanine nucleotide exchange factor for Rac and Cdc42. The enrichment of Syk at the lamellipodium and its colocalization with Vav were absent upon expression of a Syk kinase-dead mutant (Syk K402R) or a Syk mutant lacking the binding site of Vav (Syk Y348F). Live cell imaging revealed that both mutations resulted in excessive lamellipodium formation and severely compromised migration compared with control cells. Similar results were obtained upon down-regulation of Syk by RNA interference (RNAi) technique as well as in Syk(-/-) neutrophils from wild-type mice reconstituted with Syk(-/-) bone marrow. A pivotal role of Syk in vivo was demonstrated in the Arthus reaction, where neutrophil extravasation, edema formation, and hemorrhage were profoundly diminished in Syk(-/-) bone marrow chimeras compared with those in control animals. In the inflamed cremaster muscle, Syk(-/-) neutrophils revealed a defect in adhesion and migration. These findings indicate that Syk is critical for beta(2) integrin-mediated neutrophil migration in vitro and plays a fundamental role in neutrophil recruitment during the inflammatory response in vivo.

Effects of Peripheral Cannabinoid Receptor Ligands on Motility and Polarization in Neutrophil-like HL60 Cells and Human Neutrophils

The possible role of the peripheral cannabinoid receptor (CB2) in neutrophil migration was investigated by using human promyelocytic HL60 cells differentiated into neutrophil-like cells and human neutrophils isolated from whole blood. Cell surface expression of CB2 on HL60 cells, on neutrophil-like HL60 cells, and on human neutrophils was confirmed by flow cytometry. Upon stimulation with either of the CB2 ligands JWH015 and 2-arachidonoylglycerol (2-AG), neutrophil-like HL60 cells rapidly extended and retracted one or more pseudopods containing F-actin in different directions instead of developing front/rear polarity typically exhibited by migrating leukocytes. Activity of the Rho-GTPase RhoA decreased in response to CB2 stimulation, whereas Rac1, Rac2, and Cdc42 activity increased. Moreover, treatment of cells with RhoA-dependent protein kinase (p160-ROCK) inhibitor Y27632 yielded cytoskeletal organization similar to that of CB2-stimulated cells. In human neutrophils, neither JWH015 nor 2-AG induced motility or morphologic alterations. However, pretreatment of neutrophils with these ligands disrupted N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced front/rear polarization and migration and also substantially suppressed fMLP-induced RhoA activity. These results suggest that CB2 might play a role in regulating excessive inflammatory response by controlling RhoA activation, thereby suppressing neutrophil migration.

Protein-tyrosine kinase Syk is required for pathogen engulfment in complement-mediated phagocytosis

The protein tyrosine kinase Syk plays a central role in Fcgamma receptor-mediated phagocytosis in the adaptive immune system. We show here that Syk also plays an essential role in complement-mediated phagocytosis in innate immunity. Macrophage-like differentiated HL60 cells and C3bi-opsonized zymosan comprised the pathogen-phagocyte system. C3bi-opsonized zymosan particles promptly attached to the cells and were subsequently engulfed via complement receptor 3. During this process, Syk became tyrosine phosphorylated and accumulated around the nascent phagosomes. The transfer of Syk-siRNA or dominant-negative Syk (DN-Syk) into HL60 cells resulted in impaired phagocytosis. Quenching assays using fluorescent zymosan revealed that most of the attached zymosan particles were located inside parental HL60 cells, whereas few were ingested by the mutant cells. These data indicated that Syk is required for the engulfment of C3bi-opsonized zymosan. During C3bi-zymosan-induced phagocytosis, actin accumulation occurred around phagosomes and was followed by depolymerization, and further RhoA was activated together with tyrosine phosphorylation of Vav. These responses including the actin remodeling were suppressed in Syk-siRNA- or DN-Syk-expressing cells. Our results demonstrated that Syk plays an indispensable role in complement-mediated phagocytosis by regulating both actin dynamics and the RhoA activation pathway and that these functions of Syk lead to phagosome formation and pathogen engulfment.

Generation and application of a monoclonal antibody that detects a wide variety of protein tyrosine kinases

To investigate expression profiles of the entire family of protein tyrosine kinases (PTKs), we attempted to generate an antibody that detects a variety of PTKs. For production of the antibody, antigenic peptides corresponding to amino acid sequences of a highly conserved region (subdomain VIB) of PTKs were synthesized and immunized to BALB/c mice. Among various antigens, a peptide with 11 amino acids, CYVHRDLRAAN, efficiently produced a polyclonal antibody with a broad cross-reactivity to PTKs. We established a hybridoma cell line producing a monoclonal antibody, YK34, which appeared to cross-react with at least 68 PTKs in the human genome, as evidenced by its reactivity with the recombinant Src tyrosine kinases whose subdomain VIB had been replaced by those of the other PTKs. When differentiation of HL-60 cells was induced by 12-O-tetradecanoylphorbol-13-acetate, on Western blotting we observed significant changes in immunoreactive bands with YK34 in HL-60 cell extracts along with changes in the morphology of the cells. These results suggest that the YK34 antibody will be a powerful tool for analysis of a variety of cellular PTKs.

Chemoattractant induces LFA-1 associated PI 3K activity and cell migration that are dependent on Fyn signaling

The activation state of integrins on leukocytes is tightly controlled by the association of intracellular molecules to the cytoplasmic domain of the integrin. To identify signaling intermediates involved in chemoattractant receptor-induced integrin activation, we analyzed the ability of LFA-1 integrin to associate with members of Src tyrosine kinase and phosphatidylinositol 3Kinase (PI 3K) families following fMLP stimulation of a lymphoid T cell line stably expressing the fMLP receptor (fPR). fMLP-induced cell activation resulted in a rapid increase of integrin-associated PI 3Kinase activity in G protein and Src kinase dependent manner. We show, upon fMLP-stimulation, a rapid and transient association of the Src tyrosine kinase Fyn with LFA-1. Also, by transiently expressing mutant forms of this kinase, we demonstrated that Fyn is required for the integrin associated PI 3K activity. Furthermore, overexpression of the mutant form of Fyn resulted in a significant impairment of fMLP-induced cell migration through ICAM-1-coated filters, while ICAM-1 independent migration was not affected. Our observations indicate that chemoattractant receptor engagement induces Fyn-dependent PI 3K activation in association with LFA-1 and suggests that Fyn is necessary to initiate and/or to regulate chemoattractant-mediated LFA-1 activation to promote directional migration.

The non-receptor tyrosine kinase Syk regulates lamellipodium formation and site-directed migration of human leukocytes

The tyrosine kinase Syk is associated with CD18, the beta-subunit of the leukocyte adhesion molecules of the beta(2) integrin family (CD11/CD18), and becomes activated upon beta(2) integrin-mediated adhesion. In this study, we elucidated the role of Syk in polarization and site-directed migration of neutrophil-like differentiated HL-60 cells and monocytic THP-1 cells. By means of confocal microscopy, we detected a homogenous distribution of Syk in unstimulated cells in suspension. The stimulation of HL-60 cells by formyl-methionyl-leucyl-phenylalanine (fMLP, 100 nM) or the activation of THP-1 cells by monocyte chemoattractant protein-1 (10 ng/ml) induced beta(2) integrin-mediated cell adhesion and polarization on immobilized fibrinogen which was associated with an enrichment of Syk at the lamellipodium forming site. This effect was abolished by function blocking anti-CD18 antibody or by treatment of the cells with the Syk inhibitor piceatannol (30 microM) suggesting that the redistribution of Syk required both, beta(2) integrin-mediated adhesion and Syk activation. Moreover, the inhibition of Syk by piceatannol or the downregulation of Syk by antisense technique resulted in an excessive formation of lamellipodia indicating that Syk may act as a negative regulator that limits lamellipodium formation. The analysis of chemotaxis revealed that the inhibition of Syk impaired the ability of the cells to follow a chemotactic gradient whereas random migration was intact. Taken together, our data suggest a novel role for Syk in the maintenance of a bipolar phenotype by regulating lamellipodium formation, which is a critical prerequisite for site-directed migration of leukocytes.

A Template-Assembled Synthetic Protein Surface Mimetic of the von Willebrand Factor A1 domain Inhibits Botrocetin-Induced Platelet Aggregation

Platelet adhesion, the initial step of platelet activation, is mediated by the interaction of von Willebrand factor (VWF) with its platelet receptor, the GPIb-IX complex. The binding of VWF to GPIb-IX is induced either by increased shear stress or by exogenous modulators, such as botrocetin. At a molecular level, this interaction takes place between the A1 domain of VWF and the GPIb alpha chain of the GPIb-IX complex. We report here the design and functional characteristics of a VWF template-assembled synthetic protein (TASP), a chimeric four-helix-bundle TASP scaffold mimicking the surface of the A1 domain. Twelve residues located on helices alpha 3 and alpha 4 in the native A1 domain were grafted onto a surface formed by two neighboring helices of the TASP. VWF TASP was found to inhibit specifically botrocetin-induced platelet aggregation and to bind both botrocetin and GPIb alpha. However, in contrast to the native A1 domain, VWF TASP did not bind simultaneously to both ligands. Modeling studies revealed that the relative orientation of the alpha helices in VWF TASP led to a clash of bound botrocetin and GPIb alpha. These results demonstrate that a chimeric four-helix-bundle TASP as a scaffold offers a suitable surface for presenting crucial residues of the VWF A1 domain; the potential of the TASP approach for de novo protein design and mimicry is thereby illustrated.

Critical role of the carboxyl terminus of proline-rich tyrosine kinase (Pyk2) in the activation of human neutrophils by tumor necrosis factor: separation of signals for the respiratory burst and degranulation

Transduction of Tat-tagged fusion proteins confirmed a hypothesis based on pharmacologic inhibitors (Fuortes, M., M. Melchior, H. Han, G.J. Lyon, and C. Nathan. 1999. J. Clin. Invest. 104:327-335) that proline-rich tyrosine kinase (Pyk2) plays a critical role in the activation of adherent human neutrophils, and allowed an analysis of individual Pyk2 domains not possible with chemical inhibitors. Acting as a dominant negative, the COOH terminus of Pyk2 fused to a Tat peptide (Tat-CT), but not other regions of Pyk2, specifically inhibited the respiratory burst of cells responding to tumor necrosis factor (TNF), Salmonella, or Listeria, while sparing responses induced by phorbol ester. Tat-CT suppressed TNF-triggered cell spreading and the phosphorylation of endogenous Pyk2 and the associated tyrosine kinase Syk without blocking the ability of neutrophils to degranulate and kill bacteria. Thus, separate signals control the respiratory burst and degranulation, and a normal rate of killing of some bacteria can be sustained by granule products in conjunction with a minimal residual respiratory burst. Inhibition of select inflammatory functions without impairment of antibacterial activity may commend the Pyk2 pathway as a potential target for antiinflammatory therapy.

Sustained activation of MEK1-ERK1/2 pathway in membrane skeleton occurs dependently on cell adhesion in megakaryocytic differentiation

A human megakaryoblastic cell line, CMK, was treated with 12-o-tetradecanoylphorbol-13-acetate (TPA) for differentiation-induction. We examined TPA-induced activation of the MEK1-ERK1/2 pathway in the 100,000g Triton X-insoluble fraction of CMK cells as the membrane skeleton and researched the relation of the MEK1-ERK1/2 activation with integrin expression. We found that this activation was divided into two phases: the first activation occurred transiently in the membrane skeleton fraction of the suspended cell status and diminished after 1h; and the second sustained activation was maintained by cell adhesion. TPA-treated CMK cells revealed increased expression of integrins alphaIIb and beta3 only when the cell adhesion persisted, regardless of the difference of culture substratum. Sustained activation of the MEK1-ERK1/2 pathway is generated in the membrane skeleton by continuous cell adhesion and seems to be essential to TPA-induced megakaryocytic differentiation of CMK cells.

Chemotactic factor-induced recruitment and activation of Tec family kinases in human neutrophils. Implication of phosphatidynositol 3-kinases

The importance of the tyrosine phosphorylation cascades in the initiation and regulation of the functional responsiveness of human neutrophils is well established. On the other hand, the link between the G protein-coupled receptors (to which the receptors for chemotactic factors belong) and the activation of tyrosine kinases is very poorly characterized. Based on previous observations indicating that the stimulation of tyrosine phosphorylation was sensitive to inhibition by the phosphatidylinositol 3-kinase inhibitor wortmannin and the recent description of pleckstrin homology domain-containing tyrosine kinases (the Tec family), we have examined the potential implication of the latter in the responses of human neutrophils to chemotactic factors. The results obtained indicate firstly that several members of the Tec family of tyrosine kinases are expressed in human neutrophils, including Tec, Btk, and Bmx. Stimulation of the cells with fMet-Leu-Phe led to a rapid activation of Tec as indicated by its translocation to a membrane fraction and to increases in its in situ level of tyrosine phosphorylation and its capacity to tyrosine phosphorylate itself or an exogenous substrate (SAM68-GST) in in vitro kinase assays. The activation of Tec was inhibited by pertussis toxin as well as by wortmannin. The results of this study provide direct evidence for the implication of Tec family kinases in the responses of human neutrophils to chemotactic factors. They also suggest that one of the links between G protein-coupled receptors and tyrosine kinases depends on the activation of phosphatidylinositol 3-kinase and the generation of phosphatidylinositol 3,4,5-trisphosphate.

Adhesion via CD43 induces Syk activation and cell proliferation in TF-1 cells

The effect of adhesion via CD43 (leukosialin, sialophorin) on cell proliferation and phosphorylation signaling were examined in a growth factor-dependent hematopoietic progenitor cell line, TF-1. TF-1 cells promptly resulted in death after withdrawal of growth factors. However, the viable cell number increased when TF-1 cells were cultured on anti-CD43 monoclonal antibody-coated plates. In this case, sustained activation of protein tyrosine kinase Syk and extracellular signal-regulated kinase (Erk) 1/2 were detected. Overexpression of exogenous Syk on TF-1 cells by the adenovirus vector system induced enhancement of the cell proliferation accompanied with enhancement of the Erk activation by a dominant-positive effect. The signal transducer and activator of transcription (STAT) 5 seemed not to be associated with the CD43-mediated cell proliferation. These results indicated that adhesion via CD43 induces the proliferation of TF-1 cells in the absence of growth factors in part by Syk-dependent Erk 1/2 signaling.

Pertussis toxin activates tyrosine kinase signaling cascade in myelomonocytic cells: a mechanism for cell adhesion

Pertussis toxin (PTX) has recently been shown to specifically bind to CD14 to promote myelomonocytic cell adhesion to serum. The present study investigated the signaling mechanisms responsible for PTX-induced differentiated U937 cell adhesion. PTX-induced myelomonocytic cell adhesion was blocked by genistein or tyrphostin-47 (two protein tyrosine kinase inhibitors), LY294002 (a phosphatidylinositol 3-kinase (PI3K) inhibitor), or PD098059 (a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor). PTX induced a rapid tyrosine phosphorylation of several discrete cytoplasmic proteins, which could be inhibited by genistein or tyrphostin 47. In addition, PTX induced phosphorylation of Akt and of ERK2, which could be completely blocked by LY294002 and PD098059, respectively, and by genistein or tyrphostin 47 as well. All of these PTX-induced signaling events could be reproduced using purified PTX B-oligomer (PTX-B) alone. Our data show that PTX can activate tyrosine kinase signaling cascade, including the downstream PI3K and ERK/MAPK pathways, in myelomonocytic cells to induce cell adhesion to serum.