Phosphorylation of Syk activation loop tyrosines is essential for Syk function. An in vivo study using a specific anti-Syk activation loop phosphotyrosine antibody

Stromal CYR61 Confers Resistance to Mitoxantrone via Spleen Tyrosine Kinase Activation in Human Acute Myeloid Leukaemia

Approximately 50% of children with acute myeloid leukaemia (AML) relapse, despite aggressive chemotherapy. The bone marrow stromal environment protects leukaemia cells from chemotherapy (i.e., stroma-induced chemoresistance), eventually leading to recurrence. Our goal is to delineate the mechanisms underlying stroma-mediated chemoresistance in AML. We used two human bone marrow stromal cell lines, HS-5 and HS-27A, which are equally effective in protecting AML cells from chemotherapy-induced apoptosis in AML-stromal co-cultures. We found that CYR61 was highly expressed by stromal cells, and was upregulated in AML cells by both stromal cell lines. CYR61 is a secreted matricellular protein and is associated with cell-intrinsic chemoresistance in other malignancies. Here, we show that blocking stromal CYR61 activity, by neutralization or RNAi, increased mitoxantrone-induced apoptosis in AML cells in AML-stromal co-cultures, providing functional evidence for its role in stroma-mediated chemoresistance. Further, we found that spleen tyrosine kinase (SYK) mediates CYR61 signalling. Exposure to stroma increased SYK expression and activation in AML cells, and this increase required CYR61. SYK inhibition reduced stroma-dependent mitoxantrone resistance in the presence of CYR61, but not in its absence. Therefore, SYK is downstream of CYR61 and contributes to CYR61-mediated mitoxantrone resistance. The CYR61-SYK pathway represents a potential target for reducing stroma-induced chemoresistance.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

Targeting the Spleen Tyrosine Kinase with Fostamatinib as a Strategy against Waldenström Macroglobulinemia

PURPOSE

Waldenström macroglobulinemia (WMG) is a lymphoproliferative disorder characterized by good initial responses to standard therapeutics, but only a minority of patients achieve complete remissions, and most inevitably relapse, indicating a need for novel agents. B-cell receptor signaling has been linked to clonal evolution in WMG, and Spleen tyrosine kinase (Syk) is overexpressed in primary cells, suggesting that it could be a novel and rational target.

EXPERIMENTAL DESIGN

We studied the impact of the Syk inhibitor fostamatinib on BCWM.1 and MWCL-1 WMG-derived cell lines both in vitro and in vivo, as well as on primary patient cells.

RESULTS

In WMG-derived cell lines, fostamatinib induced a time- and dose-dependent reduction in viability, associated with activation of apoptosis. At the molecular level, fostamatinib reduced activation of Syk and Bruton's tyrosine kinase, and also downstream signaling through MAPK kinase (MEK), p44/42 MAPK, and protein kinase B/Akt. As a single agent, fostamatinib induced tumor growth delay in an in vivo model of WMG, and reduced viability of primary WMG cells, along with inhibition of p44/42 MAPK signaling. Finally, fostamatinib in combination with other agents, including dexamethasone, bortezomib, and rituximab, showed enhanced activity.

CONCLUSIONS

Taken together, these data support the translation of approaches targeting Syk with fostamatinib to the clinic for patients with relapsed and possibly even newly diagnosed WMG.

Adenine suppresses IgE-mediated mast cell activation

Nucleobase adenine is produced by dividing human lymphoblasts mainly from polyamine synthesis and inhibits immunological functions of lymphocytes. We investigated the anti-allergic effect of adenine on IgE-mediated mast cell activation in vitro and passive cutaneous anaphylaxis (PCA) in mice. Intraperitoneal injection of adenine to IgE-sensitized mice attenuated IgE-mediated PCA reaction in a dose dependent manner, resulting in a median effective concentration of 4.21 mg/kg. In mast cell cultures, only adenine among cytosine, adenine, adenosine, ADP and ATP dose-dependently suppressed FcɛRI (a high affinity receptor for IgE)-mediated degranulation with a median inhibitory concentration of 1.6mM. It also blocked the production of LTB4, an inflammatory lipid mediator, and inflammatory cytokines TNF-α and IL-4. In addition, adenine blocked thapsigargin-induced degranulation which is FcɛRI-independent but shares FcɛRI-dependent signaling events. Adenine inhibited the phosphorylation of signaling molecules important to FcɛRI-mediated allergic reactions such as Syk, PLCγ2, Gab2, Akt, and mitogen activated protein kinases ERK and JNK. From this result, we report for the first time that adenine inhibits PCA in mice and allergic reaction by inhibiting FcɛRI-mediated signaling events in mast cells. Therefore, adenine may be useful for the treatment of mast cell-mediated allergic diseases. Also, the upregulation of adenine production may provide another mechanism for suppressing mast cell activity especially at inflammatory sites.

Selective detection of tyrosine-containing proximally phosphorylated motifs using an antenna-free Tb3+ luminescent sensor

Tb(iii) can be used for sensing proximally phosphorylated tyrosine-containing peptide sequences.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Src family kinases and Syk are required for neutrophil extracellular trap formation in response to β-glucan particles

We report that particles of β-glucan, one of the surface components of yeasts, are powerful inducers of neutrophil extracellular trap (NET) formation in human neutrophils. β-Glucan triggered a prolonged phosphorylation of Src family kinases and Syk that were suppressed by the Src family inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3, 4-d] pyrimidine (PP2) and a novel Syk inhibitor, PRT-060318, respectively. PP2 and PRT-060318 also inhibited β-glucan-induced NET formation and reactive oxygen species (ROS) generation, suggesting that both responses are triggered by a Src/Syk-regulated signaling pathway. Given that the NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI) markedly inhibited NET formation, our findings suggest that ROS are required for the full-blown formation of NETs in response to β-glucan particles. Contrary to β-glucan, ROS generation triggered by phorbol myristate acetate (PMA) was unaffected by PP2 and PRT-060318, but these compounds, as well as DPI, suppressed Src/Syk phosphorylation triggered by PMA. Whereas PP2 had no effect on PMA-induced NET formation, PRT-060318 had a significant, albeit partial, inhibitory effect, thus suggesting that ROS induce NET formation in part via activation of Syk. These findings were substantiated by the evidence that neutrophils from mice with the conditional deletion of Syk were defective in formation of NETs in response to β-glucan.

An Interaction Library for the FcεRI Signaling Network

Antigen receptors play a central role in adaptive immune responses. Although the molecular networks associated with these receptors have been extensively studied, we currently lack a systems-level understanding of how combinations of non-covalent interactions and post-translational modifications are regulated during signaling to impact cellular decision-making. To fill this knowledge gap, it will be necessary to formalize and piece together information about individual molecular mechanisms to form large-scale computational models of signaling networks. To this end, we have developed an interaction library for signaling by the high-affinity IgE receptor, FcεRI. The library consists of executable rules for protein–protein and protein–lipid interactions. This library extends earlier models for FcεRI signaling and introduces new interactions that have not previously been considered in a model. Thus, this interaction library is a toolkit with which existing models can be expanded and from which new models can be built. As an example, we present models of branching pathways from the adaptor protein Lat, which influence production of the phospholipid PIP₃ at the plasma membrane and the soluble second messenger IP₃. We find that inclusion of a positive feedback loop gives rise to a bistable switch, which may ensure robust responses to stimulation above a threshold level. In addition, the library is visualized to facilitate understanding of network circuitry and identification of network motifs.

p85-RhoGDI2, a novel complex, is required for PSGL-1-induced β1 integrin-mediated lymphocyte adhesion to VCAM-1

P-selectin glycoprotein ligand-1 and β1 integrin play essential roles in T cell trafficking during inflammation. E-selectin and vascular cell adhesion molecule-1 are their ligands expressed on inflammation-activated endothelium. During the tethering and rolling of lymphocytes on endothelium, P-selectin glycoprotein ligand-1 binds E-selectin and induces signals. Subsequently, β1 integrin is activated and mediates stable adhesion. However, the intracellular signal pathways from PSGL-1 to β1 integrin have not yet been fully understood. Here, we find that p85, a regulatory subunit of phosphoinositide 3-kinase, forms a novel complex with Rho-GDP dissociation inhibitor-2, a lymphocyte-specific RhoGTPases dissociation inhibitor. Phosporylations of the p85-bound Rho-GDP dissociation inhibitor-2 on 130 and 153 tyrosine residues by c-Abl and Src were required for the complex to be recruited to P-selectin glycoprotein ligand-1 and thereby regulate β1 integrin-mediated T cell adhesion to vascular cell adhesion molecule-1. Both shRNAs to Rho-GDP dissociation inhibitor-2 and p85 and over-expression of Rho-GDP dissociation inhibitor-2 Y130F and Y153F significantly reduced the above-mentioned adhesion. Although Rho-GDP dissociation inhibitor-2 in the p85-Rho-GDP dissociation inhibitor-2 complex was also phosphorylated on 24 tyrosine residue by Syk, the phosphorylation is not required for the adhesion. Taken together, we find that specific phosphorylations on 130 and 153 tyrosine residues of p85-bound Rho-GDP dissociation inhibitor-2 are pivotal for P-selectin glycoprotein ligand-1-induced β1 integrin-mediated lymphocyte adhesion to vascular cell adhesion molecule-1. This will shed new light on the mechanisms that connect leukocyte initial rolling with subsequent adhesion.

Phosphorylation of the activation loop tyrosine 823 in c-Kit is crucial for cell survival and proliferation

The receptor tyrosine kinase c-Kit, also known as the stem cell factor receptor, plays a key role in several developmental processes. Activating mutations in c-Kit lead to alteration of these cellular processes and have been implicated in many human cancers such as gastrointestinal stromal tumors, acute myeloid leukemia, testicular seminomas and mastocytosis. Regulation of the catalytic activity of several kinases is known to be governed by phosphorylation of tyrosine residues in the activation loop of the kinase domain. However, in the case of c-Kit phosphorylation of Tyr-823 has been demonstrated to be a late event that is not required for kinase activation. However, because phosphorylation of Tyr-823 is a ligand-activated event, we sought to investigate the functional consequences of Tyr-823 phosphorylation. By using a tyrosine-to-phenylalanine mutant of tyrosine 823, we investigated the impact of Tyr-823 on c-Kit signaling. We demonstrate here that Tyr-823 is crucial for cell survival and proliferation and that mutation of Tyr-823 to phenylalanine leads to decreased sustained phosphorylation and ubiquitination of c-Kit as compared with the wild-type receptor. Furthermore, the mutated receptor was, upon ligand-stimulation, quickly internalized and degraded. Phosphorylation of the E3 ubiquitin ligase Cbl was transient, followed by a substantial reduction in phosphorylation of downstream signaling molecules such as Akt, Erk, p38, Shc, and Gab2. Thus, we propose that activation loop tyrosine 823 is crucial for activation of both the MAPK and PI3K pathways and that its disruption leads to a destabilization of the c-Kit receptor and decreased survival of cells.

Cross-talk between tetraspanin CD9 and transmembrane adaptor protein non-T cell activation linker (NTAL) in mast cell activation and chemotaxis

Chemotaxis, a process leading to movement of cells toward increasing concentrations of chemoattractants, is essential, among others, for recruitment of mast cells within target tissues where they play an important role in innate and adaptive immunity. Chemotaxis is driven by chemoattractants, produced by various cell types, as well as by intrinsic cellular regulators, which are poorly understood. In this study we prepared a new mAb specific for the tetraspanin CD9. Binding of the antibody to bone marrow-derived mast cells triggered activation events that included cell degranulation, Ca(2+) response, dephosphorylation of ezrin/radixin/moesin (ERM) family proteins, and potent tyrosine phosphorylation of the non-T cell activation linker (NTAL) but only weak phosphorylation of the linker for activation of T cells (LAT). Phosphorylation of the NTAL was observed with whole antibody but not with its F(ab)(2) or Fab fragments. This indicated involvement of the Fcγ receptors. As documented by electron microscopy of isolated plasma membrane sheets, CD9 colocalized with the high-affinity IgE receptor (FcεRI) and NTAL but not with LAT. Further tests showed that both anti-CD9 antibody and its F(ab)(2) fragment inhibited mast cell chemotaxis toward antigen. Experiments with bone marrow-derived mast cells deficient in NTAL and/or LAT revealed different roles of these two adaptors in antigen-driven chemotaxis. The combined data indicate that chemotaxis toward antigen is controlled in mast cells by a cross-talk among FcεRI, tetraspanin CD9, transmembrane adaptor proteins NTAL and LAT, and cytoskeleton-regulatory proteins of the ERM family.

Secreted aspartic proteases of Candida albicans activate the NLRP3 inflammasome

In a recent report, we demonstrated that distinct members of the secreted aspartic protease (Sap) family of Candida albicans are able to induce secretion of proinflammatory cytokines by human monocytes, independently of their proteolytic activity and specific pH optima. In particular, C. albicans Sap2 and Sap6 potently induced IL-1β, TNF-α, and IL-6 production. Here, we demonstrate that Sap2 and Sap6 proteins trigger IL-1β and IL-18 production through inflammasome activation. This occurs via NLRP3 and caspase-1 activation, which cleaves pro-IL-1β into secreted bioactive IL-1β, a cytokine that was induced by Saps in monocytes, in monocyte-derived macrophages and in dendritic cells. Downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL-1β. Inflammasome activation required Sap internalization via a clathrin-dependent mechanism, intracellular induction of K(+) efflux, and ROS production. Inflammasome activation of monocytes induced by Sap2 and Sap6 differed from that induced by LPS-ATP in several aspects. Our data reveal novel immunoregulatory mechanisms of C. albicans and suggest that Saps contribute to the pathogenesis of candidiasis by fostering rather than evading host immunity.

Tau protein kinases: involvement in Alzheimer's disease

Tau phosphorylation is regulated by a balance between tau kinase and phosphatase activities. Disruption of this equilibrium was suggested to be at the origin of abnormal tau phosphorylation and thereby might contribute to tau aggregation. Thus, understanding the regulation modes of tau phosphorylation is of high interest in determining the possible causes at the origin of the formation of tau aggregates in order to elaborate protection strategies to cope with these lesions in Alzheimer's disease. Among the possible and specific interventions that reverse tau phosphorylation is the inhibition of certain tau kinases. Here, we extensively reviewed tau protein kinases, their physiological roles and regulation, their involvement in tau phosphorylation and their relevance to AD. We also reviewed the most common inhibitory compounds acting on each tau kinase.

Mycophenolic acid regulates spleen tyrosine kinase to repress tumour necrosis factor-alpha-induced monocyte chemotatic protein-1 production in cultured human aortic endothelial cells

Atherosclerosis develops from cascades of inflammatory processes. Spleen tyrosine kinase (Syk) and monocyte chemotatic protein-1 (MCP-1) play important roles in the pathogenesis of atherosclerosis. Mycophenolic acid (MPA) has an anti-inflammatory effect. We have investigated whether MPA regulates Syk to repress tumour necrosis factor-α (TNF-α)-induced MCP-1 production in cultured human aortic endothelial cells. Expression of MCP-1 mRNA and its protein were measured by real time RT-PCR and ELISA, respectively. Reactive oxygen species (ROS) production were measured using 2'7'-dichlorofluorescein diacetate. Activation of AP-1 and NF-κB were assessed by electrophoretic mobility shift assay. Tyrosine phosphorylation of Syk was examined by Western blot analysis. TNF-α increased MCP-1 at both mRNA and protein levels. TNF-α-induced MCP-1 mRNA expression was inhibited by N-acetylcysteine (NAC), Syk inhibitor, Syk-siRNA and MPA. TNF-α-induced MCP-1 protein production was also inhibited by Syk inhibitor and MPA. TNF-α increased DNA binding activity of AP-1 and NF-κB, whereas both AP-1 and NF-κB decoy oligodeoxynucleotides downregulated TNF-α-induced MCP-1 mRNA expression. TNF-α increased ROS generation, which was inhibited by NAC and MPA, but not by Syk inhibitor. TNF-α increased tyrosine phosphorylation of Syk, which was attenuated by NAC and MPA. MPA and Syk inhibitor attenuated TNF-α-induced DNA binding activity of NF-κB and AP-1. TNF-α induced MCP-1 expression via activation of AP-1 and NF-κB. AP-1 and NF-κB were mediated through ROS, followed by Syk. MPA exerts anti-inflammatory effect by inhibiting MCP-1 expression via suppression of ROS and Syk.

Structural and biophysical characterization of the Syk activation switch

Syk is an essential non-receptor tyrosine kinase in intracellular immunological signaling, and the control of Syk kinase function is considered as a valuable target for pharmacological intervention in autoimmune or inflammation diseases. Upon immune receptor stimulation, the kinase activity of Syk is regulated by binding of phosphorylated immune receptor tyrosine-based activating motifs (pITAMs) to the N-terminal tandem Src homology 2 (tSH2) domain and by autophosphorylation with consequences for the molecular structure of the Syk protein. Here, we present the first crystal structures of full-length Syk (fl-Syk) as wild type and as Y348F,Y352F mutant forms in complex with AMP-PNP revealing an autoinhibited conformation. The comparison with the crystal structure of the truncated Syk kinase domain in complex with AMP-PNP taken together with ligand binding studies by surface plasmon resonance (SPR) suggests conformational differences in the ATP sites of autoinhibited and activated Syk forms. This hypothesis was corroborated by studying the thermodynamic and kinetic interaction of three published Syk inhibitors with isothermal titration calorimetry and SPR, respectively. We further demonstrate the modulation of inhibitor binding affinities in the presence of pITAM and discuss the observed differences of thermodynamic and kinetic signatures. The functional relevance of pITAM binding to fl-Syk was confirmed by a strong stimulation of in vitro autophosphorylation. A structural feedback mechanism on the kinase domain upon pITAM binding to the tSH2 domain is discussed in analogy of the related family kinase ZAP-70 (Zeta-chain-associated protein kinase 70). Surprisingly, we observed distinct conformations of the tSH2 domain and the activation switch including Tyr348 and Tyr352 in the interdomain linker of Syk in comparison to ZAP-70.

Abl family kinases regulate FcγR-mediated phagocytosis in murine macrophages

Phagocytosis of Ab-coated pathogens is mediated through FcγRs, which activate intracellular signaling pathways to drive actin cytoskeletal rearrangements. Abl and Arg define a family of nonreceptor tyrosine kinases that regulate actin-dependent processes in a variety of cell types, including those important in the adaptive immune response. Using pharmacological inhibition as well as dominant negative and knockout approaches, we demonstrate a role for the Abl family kinases in phagocytosis by macrophages and define a mechanism whereby Abl kinases regulate this process. Bone marrow-derived macrophages from mice lacking Abl and Arg kinases exhibit inefficient phagocytosis of sheep erythrocytes and zymosan particles. Treatment with the Abl kinase inhibitors imatinib and GNF-2 or overexpression of kinase-inactive forms of the Abl family kinases also impairs particle internalization in murine macrophages, indicating Abl kinase activity is required for efficient phagocytosis. Further, Arg kinase is present at the phagocytic cup, and Abl family kinases are activated by FcγR engagement. The regulation of phagocytosis by Abl family kinases is mediated in part by the spleen tyrosine kinase (Syk). Loss of Abl and Arg expression or treatment with Abl inhibitors reduced Syk phosphorylation in response to FcγR ligation. The link between Abl family kinases and Syk may be direct, as purified Arg kinase phosphorylates Syk in vitro. Further, overexpression of membrane-targeted Syk in cells treated with Abl kinase inhibitors partially rescues the impairment in phagocytosis. Together, these findings reveal that Abl family kinases control the efficiency of phagocytosis in part through the regulation of Syk function.

The transcription factor Zeb2 regulates signaling in mast cells

Mast cell activation results in the release of stored and newly synthesized inflammatory mediators. We found that Zeb2 (also named Sip1, Zfhx1b), a zinc finger transcription factor, regulates both early and late mast cell responses. Transfection with small interfering RNA (siRNA) reduced Zeb2 expression and resulted in decreased FcεRI-mediated degranulation, with a parallel reduction in receptor-induced activation of NFAT and NF-κB transcription factors, but an enhanced response to the LPS-mediated activation of NF-κB. There was variable and less of a decrease in the Ag-mediated release of the cytokines TNF-α, IL-13, and CCL-4. This suggests that low Zeb2 expression differentially regulates signaling pathways in mast cells. Multiple phosphorylation events were impaired that affected molecules both at early and late events in the signaling pathway. The Zeb2 siRNA-treated mast cells had altered cell cycle progression, as well as decreased expression of several molecules including cell surface FcεRI and its β subunit, Gab2, phospholipase-Cγ1, and phospholipase-Cγ2, all of which are required for receptor-induced signal transduction. The results indicate that the transcription factor Zeb2 controls the expression of molecules thereby regulating signaling in mast cells.

Key role of integrin α(IIb)β (3) signaling to Syk kinase in tissue factor-induced thrombin generation

The fibrin(ogen) receptor, integrin α_IIbβ₃, has a well-established role in platelet spreading, aggregation and clot retraction. How α_IIbβ₃ contributes to platelet-dependent coagulation is less well resolved. Here, we demonstrate that the potent suppressing effect of clinically used α_IIbβ₃ blockers on tissue factor-induced thrombin generation is linked to diminished platelet Ca²⁺ responses and phosphatidylserine (PS) exposure. The same blockers suppress these responses in platelets stimulated with collagen and thrombin receptor agonists, whereas added fibrinogen potentiates these responses. In platelets spreading on fibrinogen, outside-in α_IIbβ₃ signaling similarly enhances thrombin-induced Ca²⁺ rises and PS exposure. These responses are reduced in α_IIbβ₃-deficient platelets from patients with Glanzmann’s thrombasthenia. Furthermore, the contribution of α_IIbβ₃ to tissue factor-induced platelet Ca²⁺ rises, PS exposure and thrombin generation in plasma are fully dependent on Syk kinase activity. Tyrosine phosphorylation analysis confirms a key role of Syk activation, which is largely but not exclusively dependent on α_IIbβ₃ activation. It is concluded that the majority of tissue factor-induced procoagulant activity of platelets relies on Syk activation and ensuing Ca²⁺ signal generation, and furthermore that a considerable part of Syk activation relies on α_IIbβ₃ signaling. These results hence point to a novel role of Syk in integrin-dependent thrombin generation.

Spleen tyrosine kinase mediates high glucose-induced transforming growth factor-β1 up-regulation in proximal tubular epithelial cells

The role of spleen tyrosine kinase (Syk) in high glucose-induced intracellular signal transduction has yet to be elucidated. We investigated whether Syk is implicated in high glucose-induced transforming growth factor-β1 (TGF-β1) up-regulation in cultured human proximal tubular epithelial cells (HK-2 cell). High glucose increased TGF-β1 gene expression through Syk, extracellular signal-regulated kinase (ERK), AP-1 and NF-κB. High glucose-induced AP-1 DNA binding activity was decreased by Syk inhibitors and U0126 (an ERK inhibitor). Syk inhibitors suppressed high glucose-induced ERK activation, whereas U0126 had no effect on Syk activation. High glucose-induced NF-κB DNA binding activity was also decreased by Syk inhibitors. High glucose increased nuclear translocation of p65 without serine phosphorylation of IκBα and without degradation of IκBα, but with an increase in tyrosine phosphorylation of IκBα that may account for the activation of NF-κB. Both Syk inhibitors and Syk-siRNA attenuated high glucose-induced IκBα tyrosine phosphorylation and p65 nuclear translocation. Depletion of p21-activated kinase 2 (Pak2) by transfection of Pak2-siRNA abolished high glucose-induced Syk activation. In summary, high glucose-induced TGF-β1 gene transcription occurred through Pak2, Syk and subsequent ERK/AP-1 and NF-κB pathways. This suggests that Syk might be implicated in the diabetic kidney disease.

The high affinity IgE receptor (FcεRI) expression and function in airway smooth muscle

The airway smooth muscle (ASM) is no longer considered as merely a contractile apparatus and passive recipient of growth factors, neurotransmitters and inflammatory mediators signal but a critical player in the perpetuation and modulation of airway inflammation and remodeling. In recent years, a molecular link between ASM and IgE has been established through Fc epsilon receptors (FcεRs) in modulating the phenotype and function of these cells. Particularly, the expression of high affinity IgE receptor (FcεRI) has been noted in primary human ASM cells in vitro and in vivo within bronchial biopsies of allergic asthmatic subjects. The activation of FcεRI on ASM cells suggests a critical yet almost completely ignored network which may modulate ASM cell function in allergic asthma. This review is intended to provide a historical perspective of IgE effects on ASM and highlights the recent updates in the expression and function of FcεRI, and to present future perspectives of activation of this pathway in ASM cells.

An osteoclastic protein-tyrosine phosphatase regulates the β3-integrin, syk, and shp1 signaling through respective src-dependent phosphorylation in osteoclasts

This study utilized the glutathione transferase (GST) pull-down assay to identify novel substrates of an osteoclastic protein-tyrosine phosphatase, PTP-oc. Consistent with the previous findings that the phosphorylated tyr-527 (pY527) of Src is a substrate of PTP-oc, the major protein pulled down with the phosphatase-deficient (PD)-PTP-oc-GST trapping mutant in RAW264.7 cells was Src. The GST-PD-PTP-oc also pulled down pY-Syk and pY-β(3)-integrin, but not after PP2 pretreatment. However, PTP-oc transgenic osteoclasts or PTP-oc-overexpressing RAW264.7 cells had elevated, and not reduced, levels of pY525/526-Syk and pY759-β(3) integrin, and the PTP-oc siRNA treatment drastically reduced levels of pY525/526 Syk and pY759-β(3)-integrin in RAW264.7 cells. These findings are incompatible with the premise that they are substrates of PTP-oc. The PTP-oc-dependent increases in pY525/526-Syk and pY759-β(3)-integrin levels were completely blocked by PP2, indicating that these effects are secondary to PTP-oc-mediated activation of the Src protein-tyrosine kinase (PTK). Overexpression of PTP-oc increased, and siRNA-mediated suppression of PTP-oc reduced, pY160-Vav1, pY173-Vav3, and pY783-PLCγ levels, and Rac1 activation, which are downstream mediators of the ITAM/Syk signaling. Overexpression of PTP-oc also increased, and PTP-oc siRNA treatment decreased, the pY-Shp1 levels, which were blocked by PP2. Since Shp1 is a negative regulator of osteoclast activity and is a key mediator of the ITIM signaling, these findings suggest that PTP-oc is an upstream suppressor of the ITIM/Shp1 signaling through PTP-oc-induced Src-dependent Shp1 phosphorylation. In summary, PTP-oc plays a central regulatory role in the concerted regulation of the β(3)-integrin, the ITAM/Syk, and the ITIM/Shp1 signaling indirectly through activation of Src PTK.

Modeling and simulation of aggregation of membrane protein LAT with molecular variability in the number of binding sites for cytosolic Grb2-SOS1-Grb2

The linker for activation of T cells (LAT), the linker for activation of B cells (LAB), and the linker for activation of X cells (LAX) form a family of transmembrane adaptor proteins widely expressed in lymphocytes. These scaffolding proteins have multiple binding motifs that, when phosphorylated, bind the SH2 domain of the cytosolic adaptor Grb2. Thus, the valence of LAT, LAB and LAX for Grb2 is variable, depending on the strength of receptor activation that initiates phosphorylation. During signaling, the LAT population will exhibit a time-varying distribution of Grb2 valences from zero to three. In the cytosol, Grb2 forms 1∶1 and 2∶1 complexes with the guanine nucleotide exchange factor SOS1. The 2∶1 complex can bridge two LAT molecules when each Grb2, through their SH2 domains, binds to a phosphorylated site on a separate LAT. In T cells and mast cells, after receptor engagement, receptor phosphoyrlation is rapidly followed by LAT phosphorylation and aggregation. In mast cells, aggregates containing more than one hundred LAT molecules have been detected. Previously we considered a homogeneous population of trivalent LAT molecules and showed that for a range of Grb2, SOS1 and LAT concentrations, an equilibrium theory for LAT aggregation predicts the formation of a gel-like phase comprising a very large aggregate (superaggregate). We now extend this theory to investigate the effects of a distribution of Grb2 valence in the LAT population on the formation of LAT aggregates and superaggregate and use stochastic simulations to calculate the fraction of the total LAT population in the superaggregate.

Activation of spleen tyrosine kinase is required for TNF-α-induced endothelin-1 upregulation in human aortic endothelial cells

Endothelin-1 (ET-1) promotes atherosclerosis. We tested whether spleen tyrosine kinase (Syk) mediates tumor necrosis factor-α (TNF-α)-induced ET-1 upregulation in human aortic endothelial cells (HAECs) and sought to identify the signal pathways involved. TNF-α-induced reactive oxygen species (ROS) activated Syk and phosphatidylinositol 3-kinase (PI3K), which was required for the activation of AP-1 and subsequent ET-1 gene transcription. ROS mediated c-Jun NH(2)-terminal kinase (JNK) is also required for AP-1 activation, but Syk and PI3K regulated AP-1 activation independently of JNK. Through regulation of ET-1 production, Syk could be implicated in atherosclerosis.

Once phosphorylated, tyrosines in carboxyl terminus of protein-tyrosine kinase Syk interact with signaling proteins, including TULA-2, a negative regulator of mast cell degranulation

Activation of the high affinity IgE-binding receptor (FcεRI) results in the tyrosine phosphorylation of two conserved tyrosines located close to the COOH terminus of the protein-tyrosine kinase Syk. Synthetic peptides representing the last 10 amino acids of the tail of Syk with these two tyrosines either nonphosphorylated or phosphorylated were used to precipitate proteins from mast cell lysates. Proteins specifically precipitated by the phosphorylated peptide were identified by mass spectrometry. These included the adaptor proteins SLP-76, Nck-1, Grb2, and Grb2-related adaptor downstream of Shc (GADS) and the protein phosphatases SHIP-1 and TULA-2 (also known as UBASH3B or STS-1). The presence of these in the precipitates was further confirmed by immunoblotting. Using the peptides as probes in far Western blots showed direct binding of the phosphorylated peptide to Nck-1 and SHIP-1. Immunoprecipitations suggested that there were complexes of these proteins associated with Syk especially after receptor activation; in these complexes are Nck, SHIP-1, SLP-76, Grb2, and TULA-2 (UBASH3B or STS-1). The decreased expression of TULA-2 by treatment of mast cells with siRNA increased the FcεRI-induced tyrosine phosphorylation of the activation loop tyrosines of Syk and the phosphorylation of phospholipase C-γ2. There was parallel enhancement of the receptor-induced degranulation and activation of nuclear factor for T cells or nuclear factor κB, indicating that TULA-2, like SHIP-1, functions as a negative regulator of FcεRI signaling in mast cells. Therefore, once phosphorylated, the terminal tyrosines of Syk bind complexes of proteins that are positive and negative regulators of signaling in mast cells.

Phosphorylated Syk expression is enhanced in Nasu-Hakola disease brains

Nasu-Hakola disease (NHD) is a rare autosomal recessive disorder, characterized by progressive presenile dementia and formation of multifocal bone cysts, caused by a loss-of-function mutation of DNAX-activation protein 12 (DAP12) or triggering receptor expressed on myeloid cells 2 (TREM2). TREM2 and DAP12 constitute a receptor/adaptor complex on myeloid cells. The post-receptor signals are transmitted via rapid phosphorylation of the immunoreceptor tyrosine-based activating motif (ITAM) of DAP12, mediated by Src protein tyrosine kinases, followed by binding of phosphorylated ITAM to Src homology 2 (SH2) domains of spleen tyrosine kinase (Syk), resulting in autophosphorylation of the activation loop of Syk. To elucidate the molecular mechanism underlying the pathogenesis of NHD, we investigated Syk expression and activation in the frontal cortex and the hippocampus of three NHD and eight control brains by immunohistochemistry. In NHD brains, the majority of neurons expressed intense immunoreactivities for Syk and Y525/Y526-phosphorylated Syk (pSyk) chiefly located in the cytoplasm, while more limited populations of neurons expressed Src. The levels of pSyk expression were elevated significantly in NHD brains compared with control brains. In both NHD and control brains, substantial populations of microglia and macrophages expressed pSyk, while the great majority of reactive astrocytes and myelinating oligodendrocytes did not express pSyk, Syk or Src. These observations indicate that neuronal expression of pSyk was greatly enhanced in the cerebral cortex and the hippocampus of NHD brains, possibly via non-TREM2/DAP12 signaling pathways involved in Syk activation.

Regulation and function of syk tyrosine kinase in mast cell signaling and beyond

The protein tyrosine kinase Syk plays a critical role in FcεRI signaling in mast cells. Binding of Syk to phosphorylated immunoreceptor tyrosine-based activation motifs (p-ITAM) of the receptor subunits results in conformational changes and tyrosine phosphorylation at multiple sites that leads to activation of Syk. The phosphorylated tyrosines throughout the molecule play an important role in the regulation of Syk-mediated signaling. Reconstitution of receptor-mediated signaling in Syk^−/− cells by wild-type Syk or mutants which have substitution of these tyrosines with phenylalanine together with in vitro assays has been useful strategies to understand the regulation and function of Syk.

Syk kinase is phosphorylated in specific areas of the developing nervous system

An increasing number of data involve immunoreceptors in brain development, synaptic plasticity and behavior. However it has yet to be determined whether these proteins in fact transmit an immunoreceptor-like signal in non-hematopoietic neuronal cells. The recruitment and activation of the Syk family tyrosine kinases, Syk and ZAP-70, being a critical step in this process, we conducted a thorough analysis of Syk/ZAP-70 expression pattern in nervous tissues. Syk/ZAP-70 is present in neurons of different structures including the cerebellum, the hippocampus, the visual system and the olfactory system. During the olfactory system ontogeny the protein is detected from the 16th embryonic day and persists in adulthood. Importantly, Syk was phosphorylated on tyrosine residues representative of an active form of the kinase in specialized neuronal subpopulations comprising rostral migratory stream neuronal progenitor cells, hippocampal pyramidal cells, retinal ganglion cells and cerebellar granular cells. Phospho-Syk staining was also observed in synapse-rich regions such as the olfactory bulb glomeruli and the retina inner plexiform layer. Furthermore, our work on cultured primary hippoccampal neurons indicates that as for hematopoietic cells, Syk phosphorylation is readily induced upon pervanadate treatment. Therefore, Syk appears to be a serious candidate in connecting immunoreceptors to downstream adaptor/effector molecules in neurons.

Curcumin inhibits GPVI-mediated platelet activation by interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2

Turmeric (Curcuma longa), a herbal remedy and culinary spice, has been used in traditional Indian culture for millennia. An active ingredient found in turmeric is curcumin (diferuloylmethane). In the current study, we investigated the antiplatelet properties of this naturally occurring compound. Curcumin inhibited human platelet aggregation and dense granule secretion induced by GPVI agonist convulxin in a concentration-dependent manner. At 50 microM, it effectively inhibited the maximal extent of aggregation and dense granule secretion to as much as 75%. It also dramatically inhibited the activation-dependent tyrosine phosphorylation of Y753 and Y759 on PLCgamma2, but did not affect the phosphorylation of Y145 residue on the cytosolic adaptor protein SLP-76. Interestingly, curcumin had no significant effect on the phosphorylation of Y525/Y526 present on the activation loop of Syk (spleen tyrosine kinase), but had a significant inhibitory effect on in vitro Syk kinase activity. Moreover, the inhibitory action of curcumin is not due to an inhibition of thromboxane generation because all our studies were performed using aspirin-treated platelets. We conclude that curcumin inhibits platelet activation induced by GPVI agonists through interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2.

Shaping the response: the role of FcεRI and Syk expression levels in mast cell signaling

Many receptor systems initiate cell signaling through ligand-induced receptor aggregation. For bivalent ligands binding to mono- or bivalent receptors, a plot of the equilibrium concentration of receptors in aggregates against the log of the free ligand concentration, the cross-linking curve, is symmetric and bell shaped. However, steady state cellular responses initiated through receptor cross-linking may have a different dependence on ligand concentration than the aggregated receptors that initiate and maintain these responses. The authors illustrate by considering the activation of the protein kinase Syk that rapidly occurs after high affinity receptors for IgE, FcεRI, are aggregated on the surface of mast cells and basophils. Using a mathematical model of Syk activation the authors investigate two effects, one straightforward and one less so, that result in Syk activation not qualitatively following the cross-linking curve. Model predictions show that if the mechanism by which Syk is fully activated involves the transphosphorylation of Syk by Syk, then Syk activation curves can be either bell shaped or double humped, depending on the cellular concentrations of Syk and FcεRI. The model also predicts that the Syk activation curve can be non-symmetric with respect to the ligand concentration. The cell can exhibit differential Syk activation at two different ligand concentrations that produce identical distributions of receptor aggregates that form and dissociate at the same rates. The authors discuss how, even though it is only receptor aggregates that trigger responses, differences in total ligand concentration can lead to subtle kinetic effects that yield qualitative differences in the levels of Syk activation.

Cutting edge: NKp80 uses an atypical hemi-ITAM to trigger NK cytotoxicity

The human NK cell receptor NKp80 stimulates cytotoxicity upon engagement of its genetically linked ligand AICL. However, the mechanisms underlying NKp80-mediated signaling are unknown. In this study, we dissected NKp80 signaling using the NK cell line NK92MI. We demonstrated that NKp80, but not NKp80 mutated at tyrosine 7 (NKp80/Y7F), is tyrosine phosphorylated. Accordingly, NKp80/Y7F, but not NKp80/Y30F or NKp80/Y37F, failed to induce cytotoxicity. NKp80 phosphopeptides comprising the hemi-ITAM-like sequence surrounding tyrosine 7 bound Lck- and Syk-family kinases; accordingly, cross-linking of NKp80, but not NKp80/Y7F, induced Syk phosphorylation. Moreover, inhibition of Syk kinase, but not ZAP-70 kinase, impaired cytotoxic responses through NKp80. Atypical residues in the hemi-ITAM-like motif of NKp80 cause an altered stoichiometry of phosphorylation but did not substantially affect NK cytotoxicity. Altogether, these results show that NKp80 uses an atypical hemi-ITAM and Syk kinase to trigger cellular cytotoxicity.

A detailed mathematical model predicts that serial engagement of IgE-Fc epsilon RI complexes can enhance Syk activation in mast cells

The term serial engagement was introduced to describe the ability of a single peptide, bound to a MHC molecule, to sequentially interact with TCRs within the contact region between a T cell and an APC. In addition to ligands on surfaces, soluble multivalent ligands can serially engage cell surface receptors with sites on the ligand, binding and dissociating from receptors many times before all ligand sites become free and the ligand leaves the surface. To evaluate the role of serial engagement in Syk activation, we use a detailed mathematical model of the initial signaling cascade that is triggered when FcepsilonRI is aggregated on mast cells by multivalent Ags. Although serial engagement is not required for mast cell signaling, it can influence the recruitment of Syk to the receptor and subsequent Syk phosphorylation. Simulating the response of mast cells to ligands that serially engage receptors at different rates shows that increasing the rate of serial engagement by increasing the rate of dissociation of the ligand-receptor bond decreases Syk phosphorylation. Increasing serial engagement by increasing the rate at which receptors are cross-linked (for example by increasing the forward rate constant for cross-linking or increasing the valence of the ligand) increases Syk phosphorylation. When serial engagement enhances Syk phosphorylation, it does so by partially reversing the effects of kinetic proofreading. Serial engagement rapidly returns receptors that have dissociated from aggregates to new aggregates before the receptors have fully returned to their basal state.

Mast cell signaling: the role of protein tyrosine kinase Syk, its activation and screening methods for new pathway participants

The aggregation by antigen of the IgE bound to its high affinity receptor on mast cells initiates a complex series of biochemical events that result in the release of inflammatory mediators. The essential role of the protein tyrosine kinase Syk has been appreciated for some time, and newer results have defined the mechanism of its activation. The use of siRNA has defined the relative contribution of Syk, Fyn and Gab2 to signaling and has made possible a screening study to identify previously unrecognized molecules that are involved in these pathways.

Tyrosines in the carboxyl terminus regulate Syk kinase activity and function

The Syk tyrosine kinase family plays an essential role in immunoreceptor tyrosine-based activation motif (ITAM) signaling. The binding of Syk to tyrosine-phosphorylated ITAM subunits of immunoreceptors, such as FcepsilonRI on mast cells, results in a conformational change, with an increase of enzymatic activity of Syk. This conformational change exposes the COOH-terminal tail of Syk, which has three conserved Tyr residues (Tyr-623, Tyr-624, and Tyr-625 of rat Syk). To understand the role of these residues in signaling, wild-type and mutant Syk with these three Tyr mutated to Phe was expressed in Syk-deficient mast cells. There was decreased FcepsilonRI-induced degranulation, nuclear factor for T cell activation and NFkappaB activation with the mutated Syk together with reduced phosphorylation of MAP kinases p38 and p42/44 ERK. In non-stimulated cells, the mutated Syk was more tyrosine phosphorylated predominantly as a result of autophosphorylation. In vitro, there was reduced binding of mutated Syk to phosphorylated ITAM due to this increased phosphorylation. This mutated Syk from non-stimulated cells had significantly reduced kinase activity toward an exogenous substrate, whereas its autophosphorylation capacity was not affected. However, the kinase activity and the autophosphorylation capacity of this mutated Syk were dramatically decreased when the protein was dephosphorylated before the in vitro kinase reaction. Furthermore, mutation of these tyrosines in the COOH-terminal region of Syk transforms it to an enzyme, similar to its homolog ZAP-70, which depends on other tyrosine kinases for optimal activation. In testing Syk mutated singly at each one of the tyrosines, Tyr-624 but especially Tyr-625 had the major role in these reactions. Therefore, these results indicate that these tyrosines in the tail region play a critical role in regulating the kinase activity and function of Syk.

Formation of a mast cell synapse: Fc epsilon RI membrane dynamics upon binding mobile or immobilized ligands on surfaces

Fc epsilonRI on mast cells form a synapse when presented with mobile, bilayer-incorporated Ag. In this study, we show that receptor reorganization within the contacting mast cell membrane is markedly different upon binding of mobile and immobilized ligands. Rat basophilic leukemia mast cells primed with fluorescent anti-DNP IgE were engaged by surfaces presenting either bilayer-incorporated, monovalent DNP-lipid (mobile ligand), or chemically cross-linked, multivalent DNP (immobilized ligand). Total internal reflection fluorescence imaging and electron microscopy methods were used to visualize receptor reorganization at the contact site. The spatial relationships of Fc epsilonRI to other cellular components at the synapse, such as actin, cholesterol, and linker for activation of T cells, were also analyzed. Stimulation of mast cells with immobilized polyvalent ligand resulted in typical levels of degranulation. Remarkably, degranulation also followed interaction of mast cells, with bilayers presenting mobile, monovalent ligand. Receptors engaged with mobile ligand coalesce into large, cholesterol-rich clusters that occupy the central portion of the contacting membrane. These data indicate that Fc epsilonRI cross-linking is not an obligatory step in triggering mast cell signaling and suggest that dense populations of mobile receptors are capable of initiating low-level degranulation upon ligand recognition.

The novel Syk inhibitor R406 reveals mechanistic differences in the initiation of GPVI and CLEC-2 signaling in platelets

BACKGROUND

Syk is a key mediator of signaling pathways downstream of several platelet surface receptors including GPVI/FcRgamma collagen receptor, the C-type lectin receptor CLEC-2, and integrin alphaIIbbeta3. A recent study identified the novel small molecule R406 as a selective inhibitor of Syk.

OBJECTIVES

The present study evaluates the role of Syk in human platelets using the novel inhibitor R406.

METHODS

Agonist-induced GPVI and CLEC-2 signaling were assessed using aggregometry, immunoprecipitation and western blotting to determine the effects of R406 on platelet activation.

RESULTS

We demonstrate R406 to be a powerful inhibitor of Syk in human platelets. R406 abrogated shape change and aggregation induced by activation of GPVI and CLEC-2, and reduced platelet spreading on fibrinogen. The inhibitory effect of R406 was associated with inhibition of tyrosine phosphorylation of signaling proteins that lay downstream of Syk for all three receptors, including PLCgamma2. Strikingly, R406 markedly inhibited tyrosine phosphorylation of CLEC-2 and Syk downstream of CLEC-2 activation, whereas phosphorylation of Syk downstream of GPVI and integrin alphaIIbbeta3 was unaffected.

CONCLUSIONS

The inhibitory effect of R406 provides direct evidence of a role for Syk in GPVI, CLEC-2 and integrin alphaIIbbeta3 signaling in human platelets. Further, the results demonstrate a critical role for Syk in mediating tyrosine phosphorylation of CLEC-2, suggesting a novel model in which both Src and Syk kinases regulate tyrosine phosphorylation of the C-type lectin receptor leading to platelet activation.

The lymphoma-associated fusion tyrosine kinase ITK-SYK requires pleckstrin homology domain-mediated membrane localization for activation and cellular transformation

ITK-SYK, a novel fusion tyrosine kinase (FTK) resulting from a recurrent t(5;9)(q33;q22), was recently identified in a poorly understood subset of peripheral T-cell lymphomas. However, the biochemical and functional properties of ITK-SYK are unknown. Here we demonstrate that ITK-SYK is a catalytically active tyrosine kinase that is sensitive to an established inhibitor of SYK. The expression of ITK-SYK, but not SYK, transformed NIH3T3 cells, inducing loss of contact inhibition and formation of anchorage-independent colonies in soft agar, in a kinase activity-dependent manner. ITK-SYK is unusual among FTKs in having an N-terminal phosphatidylinositol 3,4,5-trisphosphate-binding pleckstrin homology (PH) domain. Introduction of a well characterized loss-of-function mutation (R29C) into the PH domain of ITK-SYK inhibited its phosphorylation, markedly reduced its catalytic activity, and abrogated its ability to activate the ERK signaling pathway and to transform NIH3T3 cells. Although ITK-SYK was membrane-associated, ITK-SYK-R29C was not. However, each of these properties could be recovered by retargeting ITK-SYK-R29C back to the plasma membrane by the addition of an N-terminal myristylation sequence. Consistent with a model in which ITK-SYK requires PH domain-mediated binding to phosphatidylinositol 3,4,5-trisphosphate generated by phosphatidylinositol 3-kinase (PI3K), ITK-SYK activity was reduced by pharmacological inhibition of PI3K and increased by co-expression with a constitutively active form of PI3K. Together, these findings identify ITK-SYK as an active, transforming FTK dependent upon PH domain-mediated membrane localization, identify a novel mechanism for activation of an oncogenic FTK, and suggest ITK-SYK as a rational therapeutic target for t(5;9)(q33;q22)-positive lymphomas.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.

Syk and pTyr'd: Signaling through the B cell antigen receptor

The B cell receptor (BCR) transduces antigen binding into alterations in the activity of intracellular signaling pathways through its ability to recruit and activate the cytoplasmic protein-tyrosine kinase Syk. The recruitment of Syk to the receptor, its activation and its subsequent interactions with downstream effectors are all regulated by its phosphorylation on tyrosine. This review discusses our current understanding of how this phosphorylation regulates the activity of Syk and its participation in signaling through the BCR.

Molecular mechanism of the Syk activation switch

Many immune signaling pathways require activation of the Syk tyrosine kinase to link ligation of surface receptors to changes in gene expression. Despite the central role of Syk in these pathways, the Syk activation process remains poorly understood. In this work we quantitatively characterized the molecular mechanism of Syk activation in vitro using a real time fluorescence kinase assay, mutagenesis, and other biochemical techniques. We found that dephosphorylated full-length Syk demonstrates a low initial rate of substrate phosphorylation that increases during the kinase reaction due to autophosphorylation. The initial rate of Syk activity was strongly increased by either pre-autophosphorylation or binding of phosphorylated immune tyrosine activation motif peptides, and each of these factors independently fully activated Syk. Deletion mutagenesis was used to identify regions of Syk important for regulation, and residues 340-356 of the SH2 kinase linker region were identified to be important for suppression of activity before activation. Comparison of the activation processes of Syk and Zap-70 revealed that Syk is more readily activated by autophosphorylation than Zap-70, although both kinases are rapidly activated by Src family kinases. We also studied Syk activity in B cell lysates and found endogenous Syk is also activated by phosphorylation and immune tyrosine activation motif binding. Together these experiments show that Syk functions as an "OR-gate" type of molecular switch. This mechanism of switch-like activation helps explain how Syk is both rapidly activated after receptor binding but also sustains activity over time to facilitate longer term changes in gene expression.

Sustained activation of the tyrosine kinase Syk by antigen in mast cells requires local Ca2+ influx through Ca2+ release-activated Ca2+ channels

Mast cell activation involves cross-linking of IgE receptors followed by phosphorylation of the non-receptor tyrosine kinase Syk. This results in activation of the plasma membrane-bound enzyme phospholipase Cgamma1, which hydrolyzes the minor membrane phospholipid phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol and inositol trisphosphate. Inositol trisphosphate raises cytoplasmic Ca2+ concentration by releasing Ca2+ from intracellular stores. This Ca2+ release phase is accompanied by sustained Ca2+ influx through store-operated Ca2+ release-activated Ca2+ (CRAC) channels. Here, we find that engagement of IgE receptors activates Syk, and this leads to Ca2+ release from stores followed by Ca2+ influx. The Ca2+ influx phase then sustains Syk activity. The Ca2+ influx pathway activated by these receptors was identified as the CRAC channel, because pharmacological block of the channels with either a low concentration of Gd3+ or exposure to the novel CRAC channel blocker 3-fluoropyridine-4-carboxylic acid (2',5'-dimethoxybiphenyl-4-yl)amide or RNA interference knockdown of Orai1, which encodes the CRAC channel pore, all prevented the increase in Syk activity triggered by Ca2+ entry. CRAC channels and Syk are spatially close together, because increasing cytoplasmic Ca2+ buffering with the fast Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis failed to prevent activation of Syk by Ca2+ entry. Our results reveal a positive feedback step in mast cell activation where receptor-triggered Syk activation and subsequent Ca2+ release opens CRAC channels, and the ensuing local Ca2+ entry then maintains Syk activity. Ca2+ entry through CRAC channels therefore provides a means whereby the Ca2+ and tyrosine kinase signaling pathways can interact with one another.

Platelets undergo phosphorylation of Syk at Y525/526 and Y352 in response to pathophysiological shear stress

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

Spleen tyrosine kinase: a novel target for therapeutic intervention of rheumatoid arthritis

BACKGROUND

In the last few years, significant progress has been made in understanding the pathogenic mechanisms and in defining the role of relevant cells and molecules in the pathophysiology of rheumatoid arthritis (RA). Various therapies, both biological (anti-TNF, anti-interleukins [e.g., IL-1]) and small molecule inhibitors have been explored for the treatment of RA.

OBJECTIVE

To date, no single signaling pathway inhibitor as wide acting as the corticosteroids, is known. However, treatment with corticosteroids is also associated with allied side effects. Despite a lot of efforts in the category of small molecule inhibitors, no inhibitor is available to deal with RA at both fronts (inflammation and tissue damage), without causing immense side effects.

METHOD

This present review explores the role of spleen tyrosine kinase (Syk) in the pathogenesis of RA and also discusses how it may meet the present day therapeutic requirements for the treatment of RA. This review gives an in-depth discussion on the role of Syk signaling in RA, the possibilities of using Syk as a target and also discusses the possible side effects that could be associated with its inhibition.

CONCLUSION

We propose Syk inhibition as a potential therapeutic approach for the treatment of RA.

Overexpression of Syk tyrosine kinase in peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) are fatal in the majority of patients and novel treatments, such as protein tyrosine kinase (PTK) inhibition, are needed. The recent finding of SYK/ITK translocations in rare PTCLs led us to examine the expression of Syk PTK in 141 PTCLs. Syk was positive by immunohistochemistry (IHC) in 133 PTCLs (94%), whereas normal T cells were negative. Western blot on frozen tissue (n=6) and flow cytometry on cell suspensions (n=4) correlated with IHC results in paraffin. Additionally, western blot demonstrated that Syk-positive PTCLs show tyrosine (525/526) phosphorylation, known to be required for Syk activation. Fluorescence in situ hybridization showed no SYK/ITK translocation in 86 cases. Overexpression of Syk, phosphorylation of its Y525/526 residues and the availability of orally available Syk inhibitors suggest that Syk merits further evaluation as a candidate target for pharmacologic PTK inhibition in patients with PTCL.

High glucose-induced NF-kappaB activation occurs via tyrosine phosphorylation of IkappaBalpha in human glomerular endothelial cells: involvement of Syk tyrosine kinase

Activation of nuclear factor-kappaB (NF-kappaB) occurs by dissociation from IkappaB after serine or tyrosine phosphorylation of IkappaBalpha, but the way of NF-kappaB activation by high glucose has not been defined. High glucose is known to activate NF-kappaB via protein kinase C and reactive oxygen species (ROS). In this study, we investigated how high glucose activates NF-kappaB for CC chemokine ligand 2 production in cultured human glomerular endothelial cells. High glucose increased nuclear translocation of p65 and also increased NF-kappaB DNA binding activity. High glucose-induced NF-kappaB activation occurred without degradation of IkappaBalpha. In agreement with this, there was no increase in serine phosphorylation of IkappaBalpha, while tyrosine phosphorylation of IkappaBalpha was increased by high glucose. High glucose increased the generation of ROS, whereas both alpha-lipoic acid and N-acetylcysteine scavenged the ROS and decreased high glucose-induced tyrosine phosphorylation of IkappaBalpha, nuclear translocation of p65, and NF-kappaB DNA binding activity. Protein kinase C pseudosubstrate inhibited high glucose-induced ROS production, tyrosine phosphorylation of IkappaBalpha, and nuclear translocation of p65. Both BAY 61-3606, a specific inhibitor of Syk protein-tyrosine kinase, and small interfering RNA directed against Syk inhibited high glucose-induced tyrosine phosphorylation of IkappaBalpha as well as p65 nuclear translocation. High glucose increased tyrosine phosphorylation of Syk, while it was inhibited by alpha-lipoic acid and protein kinase C pseudosubstrate. In summary, high glucose-induced NF-kappaB activation occurred not by serine phosphorylation of IkappaBalpha. Our data suggest that ROS-mediated tyrosine phosphorylation of IkappaBalpha is the mechanism for high glucose-induced NF-kappaB activation, and Syk may play a role in tyrosine phosphorylation of IkappaBalpha.