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

The mechanism underlying defective Fcgamma receptor-mediated phagocytosis by HIV-1-infected human monocyte-derived macrophages

Clearance of IgG-opsonized erythrocytes is impaired in HIV-1-infected patients, suggesting defective FcgammaR-mediated phagocytosis in vivo. We have previously shown defective FcgammaR-mediated phagocytosis in HIV-1-infected human monocyte-derived macrophages (MDM), establishing an in vitro model for defective tissue macrophages. Inhibition was associated with decreased protein expression of FcR gamma-chain, which transduces immune receptor signals via ITAM motifs. FcgammaRI and FcgammaRIIIa signal via gamma-chain, whereas FcgammaRIIa does not. In this study, we showed that HIV-1 infection inhibited FcgammaRI-, but not FcgammaRIIa-dependent Syk activation in MDM, showing that inhibition was specific for gamma-chain-dependent signaling. HIV-1 infection did not impair gamma-chain mRNA levels measured by real-time PCR, suggesting a posttranscriptional mechanism of gamma-chain depletion. HIV-1 infection did not affect gamma-chain degradation (n = 7, p = 0.94) measured in metabolic labeling/chase experiments, whereas gamma-chain biosynthesis was inhibited (n = 12, p = 0.0068). Using an enhanced GFP-expressing HIV-1 strain, we showed that FcgammaR-mediated phagocytosis inhibition is predominantly due to a bystander effect. Experiments in which MDM were infected in the presence of the antiretroviral drug 3TC suggest that active viral replication is required for inhibition of phagocytosis in MDM. These data suggest that HIV-1 infection may affect only gamma-chain-dependent FcgammaR functions, but that this is not restricted to HIV-1-infected cells.

Syk is downstream of intercellular adhesion molecule-1 and mediates human rhinovirus activation of p38 MAPK in airway epithelial cells

The airway epithelium is the primary target of inhaled pathogens such as human rhinovirus (HRV). Airway epithelial cells express ICAM-1, the major receptor for HRV. HRV binding to ICAM-1 mediates not only viral entry and replication but also a signaling cascade that leads to enhanced inflammatory mediator production. The specific signaling molecules and pathways activated by HRV-ICAM-1 interactions are not well characterized, although studies in human airway epithelia implicate a role for the p38 MAPK in HRV-induced cytokine production. In the current study, we report that Syk, an important immunoregulatory protein tyrosine kinase, is highly expressed by primary and cultured human airway epithelial cells and is activated in response to infection with HRV16. Biochemical studies revealed that ICAM-1 engagement by HRV and cross-linking Abs enhanced the coassociation of Syk with ICAM-1 and ezrin, a cytoskeletal linker protein. In polarized airway epithelial cells, Syk is diffusely distributed in the cytosol under basal conditions but, following engagement of ICAM-1 by cross-linking Abs, is recruited to the plasma membrane. The enhanced Syk-ICAM-1 association following HRV exposure is accompanied by Syk phosphorylation. ICAM-1 engagement by HRV and cross-linking Abs also induced phosphorylation of p38 in a Syk-dependent manner, and conversely, knockdown of Syk by short interfering (si)RNA substantially diminished p38 activation and IL-8 gene expression. Taken together, these observations identify Syk as an important mediator of the airway epithelial cell inflammatory response by modulating p38 phosphorylation and IL-8 gene expression following ICAM-1 engagement by HRV.

Genomic and expression profiling identifies the B-cell associated tyrosine kinase Syk as a possible therapeutic target in mantle cell lymphoma

Among B-cell lymphomas mantle cell lymphoma (MCL) has the worst prognosis. By using a combination of genomic and expression profiling (Affymetrix GeneChip Mapping 10k Xba131 and U133 set), we analysed 26 MCL samples to identify genes relevant to MCL pathogenesis and that could represent new therapeutic targets. Recurrent genomic deletions and gains were detected. Genes were identified as overexpressed in regions of DNA gain on 3q, 6p, 8q, 9q, 16p and 18q, including the cancer genes BCL2 and MYC. Among the transcripts with high correlation between DNA and RNA, we identified SYK, a tyrosine kinase involved in B-cell receptor signalling. SYK was amplified at DNA level, as validated by fluorescence in situ hybridisation (FISH) analysis, and overexpressed at both RNA and protein levels in the JeKo-1 cell line. Low-level amplification, with protein overexpression of Syk was demonstrated by FISH in a small subset of clinical samples. After treatment with low doses of the Syk inhibitor piceatannol, cell proliferation arrest and apoptosis were induced in the cell line overexpressing Syk, while cells expressing low levels of Syk were much less sensitive. A combination of genomic and expression profiling suggested Syk inhibition as a new therapeutic strategy to be explored in lymphomas.

The Syk tyrosine kinase localizes to the centrosomes and negatively affects mitotic progression

We showed previously that the spleen tyrosine kinase Syk is expressed by mammary epithelial cells and that it suppresses malignant growth of breast cancer cells. The exact molecular mechanism of its tumor-suppressive activity remains, however, to be identified. Here, we show that Syk colocalizes and copurifies with the centrosomal component gamma-tubulin and exhibits a catalytic activity within the centrosomes. Moreover, its centrosomal localization depends on its intact kinase activity. Centrosomal Syk expression is persistent in interphase but promptly drops during mitosis, obviously resulting from its ubiquitinylation and proteasomal degradation. Conversely, unrestrained exogenous expression of a fluorescently tagged Discosoma sp. red fluorescent protein (DsRed)-Syk chimera engenders abnormal cell division and cell death. Transient DsRed-Syk overexpression triggers an abrupt cell death lacking hallmarks of classic apoptosis but reminiscent of mitotic catastrophe. Surviving stable DsRed-Syk-transfected cells exhibit multipolar mitotic spindles and contain multiple abnormally sized nuclei and supernumerary centrosomes, revealing anomalous cell division. Taken together, these results show that Syk is a novel centrosomal kinase that negatively affects cell division. Its expression is strictly controlled in a spatiotemporal manner, and centrosomal Syk levels need to decline to allow customary progression of mitosis.

Syk tyrosine kinase mediates Epstein-Barr virus latent membrane protein 2A-induced cell migration in epithelial cells

Although spleen tyrosine kinase (Syk) is known to be important in hematopoietic cell development, the roles of Syk in epithelial cells have not been well studied. Limited data suggest that Syk plays alternate roles in carcinogenesis under different circumstances. In breast cancer, Syk has been suggested to be a tumor suppressor. In contrast, Syk is essential for murine mammary tumor virus-mediated transformation. However, the roles of Syk in tumor migration are still largely unknown. Nasopharyngeal carcinoma, an unusually highly metastatic tumor, expresses Epstein-Barr virus LMP2A (latent membrane protein 2A) in most clinical specimens. Previously, we demonstrated LMP2A triggers epithelial cell migration. LMP2A contains an immunoreceptor tyrosine-based activation motif, which is important for Syk kinase activation in B cells. In this study, we explored whether Syk is important for LMP2A-mediated epithelial cell migration. We demonstrate that LMP2A expression can activate endogenous Syk activity. The activation requires the tyrosine residues in LMP2A ITAM but not YEEA motif, which is important for Syk activation by Lyn in B cells. LMP2A interacts with Syk as demonstrated by coimmunoprecipitation and confocal microscopy. Furthermore, LMP2A-induced cell migration is inhibited by a Syk inhibitor and short interfering RNA. Tyrosines 74 and 85 in the LMP2A immunoreceptor tyrosine-based activation motif are essential for both Syk activation and LMP2A-mediated cell migration, indicating the involvement of Syk in LMP2A-triggered cell migration. The LMP2A-Syk pathway may provide suitable drug targets for treatment of nasopharyngeal carcinoma.

Syk is required for monocyte/macrophage chemotaxis to CX3CL1 (Fractalkine)

CX3CL1 (fractalkine), the only member of the delta subclass of chemokines, is a known chemotactic factor for monocytes/macrophages as well as NK cells and T lymphocytes. In several pathologies, excessive production of CX3CL1 at specific sites leads primarily to monocyte/macrophage recruitment, which causes tissue and vascular damage. Despite their clinical relevance, the mechanisms underlying monocyte/macrophage chemotaxis to CX3CL1 remain poorly documented. The present report addresses this issue and identifies cell signaling crucial for this process. Using the murine monocyte/macrophage RAW cell line, we show that CX3CL1 treatment elicits a rapid and transient increase in F-actin and the formation of F-actin-enriched cell protrusions. CX3CL1 also triggers tyrosine phosphorylation of proteins localized in those protrusions. The protein tyrosine kinase Syk is activated upon CX3CL1 treatment, and reduction of Syk expression using RNA-mediated interference results in a specific and massive impairment of RAW cell migration to CX3CL1. Similar results are obtained using the Syk inhibitor, piceatannol. Cells with reduced Syk expression also exhibit a major defect in CX3CL1-induced cytoskeletal remodeling. These data suggest that in monocytes/macrophages, Syk is essential for proper reorganization of the actin cytoskeleton in response to CX3CL1 and is therefore required for cell chemotaxis to CX3CL1.

Protein Flexibility and Ligand Rigidity: A Thermodynamic and Kinetic Study of ITAM-Based Ligand Binding to Syk Tandem SH2

The Syk tandem Src homology 2 domain (Syk tSH2) constitutes a flexible protein module involved in the regulation of Syk kinase activity. The Syk tSH2 domain is assumed to function by adapting the distance between its two SH2 domains upon bivalent binding to diphosphotyrosine ligands. A thermodynamic and kinetic analysis of ligand binding was performed by using surface plasmon resonance (SPR). Furthermore, the effect of binding on the Syk tSH2 structural dynamics was probed by hydrogen/deuterium exchange and electrospray mass spectrometry (ESI-MS). Two ligands were studied: 1, a flexible peptide derived from the tSH2 recognition ITAM sequence at the gamma chain of the FcepsilonRI-receptor, and 2, a ligand in which the amino acids between the two SH2 binding motifs in ligand 1 have been replaced by a rigid linker of comparable length. Both ligands display comparable affinity for Syk tSH2 at 25 degrees C, yet a major difference in thermodynamics is observed. Upon binding of the rigid ligand, 2, the expected entropy advantage is not realized. On the contrary, 2 binds with a considerably higher entropy price of approximately 9 kcal mol-1, which is attributed to a further decrease in protein flexibility upon binding to this rigid ligand. The significant reduction in deuterium incorporation in the Syk tSH2 protein upon binding of either 1 or 2, as monitored by ESI-MS, indicates a major reduction in protein dynamics upon binding. The results are consistent with a two-step binding model: after an initial binding step, a rapid structural change of the protein occurs, followed by a second binding step. Such a bivalent binding model allows high affinity and fast dissociation kinetics, which are very important in transient signal-transduction processes.

Dectin-1 activates Syk tyrosine kinase in a dynamic subset of macrophages for reactive oxygen production

Dectin-1 is a lectin receptor for beta-glucan that is important for innate macrophage recognition of fungi and contributes to phagocytosis, reactive oxygen production, and induction of inflammatory cytokines. The mechanisms by which Dectin-1 mediates intracellular signaling are just beginning to be defined. Spleen tyrosine kinase (Syk) is a protein tyrosine kinase that is critical for adaptive immune responses where it mediates signaling through B-cell receptors, T-cell receptors, and Fc receptors. Here we report that Dectin-1 activates Syk in macrophages and is important for Dectin-1-stimulated reactive oxygen production, but not for phagocytosis. Syk activation is restricted to a subpopulation of macrophages that is in equilibrium with cells that cannot activate the pathway. The proportion of macrophages using this signaling pathway can be modulated by cytokine treatment. Thus, Dectin-1 signaling reveals dynamic macrophage heterogeneity in inflammatory activation potential.

A cyclic adenosine 3',5'-monophosphate-induced tyrosine phosphorylation of Syk protein tyrosine kinase in the flagella of boar spermatozoa

A cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein tyrosine phosphorylation is involved in the expression of fertilizing ability in mammalian spermatozoa. However, there are only limited data concerning the identification of protein tyrosine kinase (PTK) that is activated by the cAMP signaling. In this study, we have shown data supporting that boar sperm flagellum possesses a unique cAMP-protein kinase A (PKA) signaling cascade leading to phosphorylation of Syk PTK at the tyrosine residues of the activation loop. Ejaculated spermatozoa were washed and then incubated in a modified Krebs-Ringer HEPES medium (mKRH) containing polyvinyl alcohol (PVA) plus 0.1 mM cBiMPS (a cell-permeable cAMP analog), 0.25 mM sodium orthovanadate (Na3VO4) (a protein tyrosine phosphatase (PTP) inhibitor) or both at 38.5 degrees C for 180 min. Aliquots of the sperm suspensions were recovered before and after incubation and then used to detect sperm tyrosine-phosphorylated proteins by Western blotting and indirect immunofluorescence. In the Western blotting, the anti-phosphotyrosine monoclonal antibody (4G10) recognized several bands including 72-kDa protein in the protein extracts from spermatozoa that were incubated solely with cBiMPS. The tyrosine phosphorylation in these sperm proteins was dependent on cBiMPS and enhanced by the addition of Na3VO4. The 72-kDa tyrosine-phosphorylated protein was apparently reacted with the anti-phospho-Syk antibody (Tyr525/526). Indirect immunofluorescence revealed that the connecting and principal pieces of spermatozoa incubated with cBiMPS and Na3VO4 were stained with the anti-phospho-Syk antibody. However, the reactivity of the 72-kDa protein with the anti-phospho-Syk antibody was reduced by the addition of H-89 (a PKA inhibitor, 0.01-0.1 mM) to the sperm suspensions but not affected by the pretreatment of spermatozoa with BAPTA-AM (an intracellular Ca2+ chelator, 0.1 mM). Fractionation of phosphorylated proteins from the spermatozoa with a detergent Nonidet P-40 suggested that the 72-kDa tyrosine-phosphorylated protein might be a cytoskeletal component. Based on these findings, we have concluded that the cAMP-PKA signaling is linked to the Ca2+-independent tyrosine phosphorylation of Syk in the connecting and principal pieces of boar spermatozoa.

A Novel Mode of Gleevec Binding Is Revealed by the Structure of Spleen Tyrosine Kinase

Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase required for signaling from immunoreceptors in various hematopoietic cells. Phosphorylation of two tyrosine residues in the activation loop of the Syk kinase catalytic domain is necessary for signaling, a phenomenon typical of tyrosine kinase family members. Syk in vitro enzyme activity, however, does not depend on phosphorylation (activation loop tyrosine --> phenylalanine mutants retain catalytic activity). We have determined the x-ray structure of the unphosphorylated form of the kinase catalytic domain of Syk. The enzyme adopts a conformation of the activation loop typically seen only in activated, phosphorylated tyrosine kinases, explaining why Syk does not require phosphorylation for activation. We also demonstrate that Gleevec (STI-571, Imatinib) inhibits the isolated kinase domains of both unphosphorylated Syk and phosphorylated Abl with comparable potency. Gleevec binds Syk in a novel, compact cis-conformation that differs dramatically from the binding mode observed with unphosphorylated Abl, the more Gleevec-sensitive form of Abl. This finding suggests the existence of two distinct Gleevec binding modes: an extended, trans-conformation characteristic of tight binding to the inactive conformation of a protein kinase and a second compact, cis-conformation characteristic of weaker binding to the active conformation. Finally, the Syk-bound cis-conformation of Gleevec bears a striking resemblance to the rigid structure of the nonspecific, natural product kinase inhibitor staurosporine.

Syk tyrosine kinase participates in beta1-integrin signaling and inflammatory responses in airway epithelial cells

The protein tyrosine kinase Syk is critically involved in immunoreceptor signaling in hematopoietic cells. Recent studies demonstrate Syk expression in nonhematopoietic cells, including fibroblasts, endothelial cells, hepatocytes, and breast epithelium. However, the role of Syk in these cells is uncertain. We hypothesized that Syk is expressed in respiratory epithelial cells (EC) and that it functions as a signaling molecule involved in inflammatory responses in the epithelium. With the use of immunohistochemistry, Western blot, PCR, and laser scanning confocal microscopy, Syk was detected in human, rat, and mouse bronchial epithelium in situ and in cultured human bronchial EC in primary cells and the cell lines HS-24 and BEAS-2B. Syk-dependent signaling pathways in EC were initiated by engagement of beta1-integrin receptors. Stimulation of beta1-integrin receptors by fibronectin or antibody cross-linking caused redistribution of Syk from a cytoplasmic to plasma membrane localization. In stimulated cells, Syk and beta1-integrin colocalized. In addition, following beta1-integrin receptor engagement, tyrosine phosphorylation of Syk was observed. Expression of the intercellular adhesion molecule-1 (ICAM-1) and production of IL-6, both important molecules in lung inflammation, was downregulated in EC treated with Syk small interfering RNA or Syk inhibitor piceatannol. We propose that Syk is involved in signaling pathways induced by integrin engagement in airway EC. Syk-mediated signaling regulates IL-6 and ICAM-1 expression and may be important in the pathophysiology of lung inflammation.

Phospholipase D1 regulates high-affinity IgE receptor-induced mast cell degranulation

To investigate the role of phospholipase D (PLD) in FcepsilonRI signaling, the wild-type or the catalytically inactive forms of PLD1 or PLD2 were stably overexpressed in RBL-2H3 mast cells. FcepsilonRI stimulation resulted in the activation of both PLD1 and PLD2. However, PLD1 was the source of most of the receptor-induced PLD activity. There was enhanced FcepsilonRI-induced degranulation only in cells that overexpressed the catalytically inactive PLD1. This dominant-negative PLD1 enhanced FcepsilonRI-induced tyrosine phosphorylations of early signaling molecules such as the receptor subunits, Syk and phospholipase C-gamma which resulted in faster release of Ca(2+) from intracellular sources. Therefore, PLD1 negatively regulates signals upstream of the Ca(2+) response. However, FcepsilonRI-induced PLD activation required Syk and was downstream of the Ca(2+)response, suggesting that basal PLD1 activity rather than that activated by cell stimulation controlled these early signaling events. Dominant-negative PLD1 reduced the basal phosphatidic acid formation in unstimulated cells, which was accompanied by an increase in FcepsilonRI within the lipid rafts. These results indicate that constitutive basal PLD1 activity by regulating phosphatidic acid formation controls the early signals initiated by FcepsilonRI aggregation that lead to mast cell degranulation.

The complexity of complexes in signal transduction

Many activities of cells are controlled by cell-surface receptors, which in response to ligands, trigger intracellular signaling reactions that elicit cellular responses. A hallmark of these signaling reactions is the reversible nucleation of multicomponent complexes, which typically begin to assemble when ligand-receptor binding allows an enzyme, often a kinase, to create docking sites for signaling molecules through chemical modifications, such as tyrosine phosphorylation. One function of such docking sites is the co-localization of enzymes with their substrates, which can enhance both enzyme activity and specificity. The directed assembly of complexes can also influence the sensitivity of cellular responses to ligand-receptor binding kinetics and determine whether a cellular response is up- or downregulated in response to a ligand stimulus. The full functional implications of ligand-stimulated complex formation are difficult to discern intuitively. Complex formation is governed by conditional interactions among multivalent signaling molecules and influenced by quantitative properties of both the components in a system and the system itself. Even a simple list of the complexes that can potentially form in response to a ligand stimulus is problematic because of the number of ways signaling molecules can be modified and combined. Here, we review the role of multicomponent complexes in signal transduction and advocate the use of mathematical models that incorporate detail at the level of molecular domains to study this important aspect of cellular signaling.

Glycoproteins VI and Ib-IX-V stimulate tyrosine phosphorylation of tyrosine kinase Syk and phospholipase Cgamma2 at distinct sites

Glycoproteins GPVI and GPIb-IX-V stimulate robust tyrosine phosphorylation of Syk and PLCg2 (phospholipase Cg2) in washed platelets, but only the former stimulates pronounced activation of phospholipase. Using phospho-specific antibodies, we demonstrate that GPVI, but not GPIb-IX-V, stimulates significant tyrosine phosphorylation of Syk at the autophosphorylation site pY525/526, a marker of Syk activity. In addition, GPVI stimulates tyrosine phosphorylation of PLCg2 at Tyr753 and Tyr759, whereas GPIb-IX-V only induces significant phosphorylation at Tyr753. Both receptors stimulate tyrosine phosphorylation of Btk at the regulatory Tyr223 and Tyr551. Syk and Btk phosphorylate peptides from PLCg2 containing Tyr753 and Tyr759 respectively, suggesting that they may stimulate phosphorylation at these sites in phospholipase. Studies using PLCg2-deficient platelets demonstrated that phospholipase is not required for the activation of integrin aIIbb3 by GPIb-IX-V. Our results demonstrate fundamental differences between GPVI and GPIb-IX-V in the regulation of tyrosine phosphorylation of Syk and PLCg2 consistent with the functional impairment of phospholipase in signalling by GPIb-IX-V.

Activation of Syk protein tyrosine kinase in response to osmotic stress requires interaction with p21-activated protein kinase Pak2/gamma-PAK

The p21-activated serine/threonine protein kinase Pak2/gamma-PAK and the nonreceptor type of protein tyrosine kinase Syk are known to be activated when the cells are exposed to osmotic stress. The purpose of the present study was to examine whether Pak2 and Syk functionally cooperate in cellular signaling. Cotransfection studies revealed that Pak2 associates with Syk in COS cells. The constitutively active form of Cdc42 increases the association of Pak2 with Syk. Pak2 coexpressed with an inactive form of Cdc42 or kinase-inactive Pak2 interacts to a lesser extent with Syk, suggesting that Pak2-Syk association is enhanced by Pak2 activation. Interaction with Pak2 enhances the intrinsic kinase activity of Syk. This is supported by in vitro studies showing that Pak2 phosphorylates and activates Syk. Treatment of cells with sorbitol to induce hyperosmolarity results in the translocation of Pak2 and Syk to the region surrounding the nucleus and in dramatic enhancement of their association. Furthermore, cotransfection of Pak2 and Syk leads to the activation of c-Jun N-terminal kinase (JNK) under hyperosmotic conditions. Pak2 short interfering RNA suppresses sorbitol-mediated activation of endogenous Syk and JNK, thus identifying a novel pathway for JNK activation by Cdc42. These results demonstrate that Pak2 and Syk positively cooperate to regulate cellular responses to stress.

A role for Syk-kinase in the control of the binding cycle of the beta2 integrins (CD11/CD18) in human polymorphonuclear neutrophils

A fine control of beta(2) integrin (CD11/CD18)-mediated firm adhesion of human neutrophils to the endothelial cell monolayer is required to allow ordered emigration. To elucidate the molecular mechanisms that control this process, intracellular protein tyrosine signaling subsequent to beta(2) integrin-mediated ligand binding was studied by immunoprecipitation and Western blotting techniques. The 72-kDa Syk-kinase, which was tyrosine-phosphorylated upon adhesion, was found to coprecipitate with CD18, the beta-subunit of the beta(2) integrins. Moreover, inhibition of Syk-kinase by piceatannol enhanced adhesion and spreading but diminished N-formyl-Met-Leu-Phe-induced chemotactic migration. The enhancement of adhesiveness was associated with integrin clustering, which results in increased integrin avidity. In contrast, piceatannol had no effect on the surface expression or on the affinity of beta(2) integrins. Altogether, this suggests that Syk-kinase controls alternation of beta(2) integrin-mediated ligand binding with integrin detachment.

Adaptor protein 3BP2 is a potential ligand of Src homology 2 and 3 domains of Lyn protein-tyrosine kinase

Adaptor protein 3BP2, a c-Abl-Src homology 3 (SH3) domain-binding protein, is known to play a regulatory role in T-cell receptor-mediated transcriptional activation of nuclear factor of activated T cell and activator protein 1 by interacting with Syk/ZAP-70 protein-tyrosine kinase. We have previously demonstrated that aggregation of high affinity IgE receptor (FcepsilonRI) induces tyrosine phosphorylation of 3BP2, and overexpression of the 3BP2-SH2 domain suppresses antigen-induced degranulation in rat basophilic leukemia RBL-2H3 mast cell line. In this report, we attempt to analyze the biological relevance of 3BP2 tyrosine phosphorylation. By using the transient expression system in COS-7 cells, we have demonstrated that 3BP2 was predominantly phosphorylated on Tyr174, Tyr183, and Tyr446 when it was coexpressed with Syk. An in vitro binding study revealed that phosphorylation of Tyr446 by Syk was likely to create a binding site for the Lyn-SH2 domain in RBL-2H3 cells. In addition, proline-rich region of 3BP2 bound to the Lyn-SH3 domain. Conformational microscopic analysis showed that Lyn and 3BP2 are constitutively colocalized in RBL-2H3 cells. Overexpression of 3BP2 in RBL-2H3 cells resulted in an enhancement of Lyn autophosphorylation. These results suggest that the adaptor protein 3BP2 is a potential regulator of Lyn protein-tyrosine kinase as a ligand of its SH3/SH2 domains in FcepsilonRI-mediated signaling in mast cells.

Cbl-b negatively regulates B cell antigen receptor signaling in mature B cells through ubiquitination of the tyrosine kinase Syk

Members of the Cbl family of molecular adaptors play key roles in regulating tyrosine kinase-dependent signaling in a variety of cellular systems. Here we provide evidence that in B cells Cbl-b functions as a negative regulator of B cell antigen receptor (BCR) signaling during the normal course of a response. In B cells from Cbl-b-deficient mice cross-linking the BCRs resulted in sustained phosphorylation of Igalpha, Syk, and phospholipase C (PLC)-gamma2, leading to prolonged Ca2+ mobilization, and increases in extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal protein kinase (JNK) phosphorylation and surface expression of the activation marker, CD69. Image analysis following BCR cross-linking showed sustained polarization of the BCRs into large signaling-active caps associated with phosphorylated Syk in Cbl-b-deficient B cells in contrast to the BCRs in Cbl-b-expressing B cells that rapidly proceeded to form small, condensed, signaling inactive caps. Significantly, prolonged phosphorylation of Syk correlated with reduced ubiquitination of Syk indicating that Cbl-b negatively regulates BCR signaling by targeting Syk for ubiquitination.

Investigation of early events in Fc epsilon RI-mediated signaling using a detailed mathematical model

Aggregation of Fc epsilon RI on mast cells and basophils leads to autophosphorylation and increased activity of the cytosolic protein tyrosine kinase Syk. We investigated the roles of the Src kinase Lyn, the immunoreceptor tyrosine-based activation motifs (ITAMs) on the beta and gamma subunits of Fc epsilon RI, and Syk itself in the activation of Syk. Our approach was to build a detailed mathematical model of reactions involving Fc epsilon RI, Lyn, Syk, and a bivalent ligand that aggregates Fc(epsilon)RI. We applied the model to experiments in which covalently cross-linked IgE dimers stimulate rat basophilic leukemia cells. The model makes it possible to test the consistency of mechanistic assumptions with data that alone provide limited mechanistic insight. For example, the model helps sort out mechanisms that jointly control dephosphorylation of receptor subunits. In addition, interpreted in the context of the model, experimentally observed differences between the beta- and gamma-chains with respect to levels of phosphorylation and rates of dephosphorylation indicate that most cellular Syk, but only a small fraction of Lyn, is available to interact with receptors. We also show that although the beta ITAM acts to amplify signaling in experimental systems where its role has been investigated, there are conditions under which the beta ITAM will act as an inhibitor.

Essential role of Src-family protein tyrosine kinases in NF-kappaB activation during B cell development

The nature of signals that govern the development of immunoglobulin heavy chain-dependent B cells is largely unknown. Using mice deficient for the B cell-expressed Src-family protein tyrosine kinases (SFKs) Blk, Fyn and Lyn, we show an essential role of these kinases in pre-B cell receptor (pre-BCR)- mediated NF-kappaB activation and B cell development. This signaling defect is SFK specific, as a deficiency in Syk, which controls pre-B cell development, does not affect NF-kappaB induction. Impaired NF-kappaB induction was overcome by the activation of protein kinase C (PKC)-lambda, thus suggesting the involvement of PKC-lambda in pre-BCR-mediated SFK-dependent activation of NF-kappaB. Our data show the existence of a functionally distinct SFK signaling module responsible for pre-BCR-mediated NF-kappaB activation and B cell development.

Differential signaling via surface IgM is associated with VH gene mutational status and CD38 expression in chronic lymphocytic leukemia

The mutational status of tumor immunoglobulin V(H) genes is providing a powerful prognostic marker for chronic lymphocytic leukemia (CLL), with patients having tumors expressing unmutated V(H) genes being in a less favorable subset. However, the biologic differences correlating with V(H) gene status that could determine the clinical course of the disease are unknown. Here we show that differing responses to IgM ligation are closely associated with V(H) gene status. Specifically, 80% of cases with unmutated V(H) genes showed increased global tyrosine phosphorylation following IgM ligation, whereas only 20% of samples with mutated V(H) genes responded (P =.0002). There was also an association between response to IgM ligation and expression of CD38 (P =.015). The Syk kinase, critical for transducing B-cell receptor (BCR)- derived signals, was constitutively present in all CLL samples, and there was a perfect association between global phosphorylation and induction of phosphorylation/activation of Syk. Nonresponsiveness to anti-IgM could be circumvented by ligation of IgD (10 of 15 samples tested) or the BCR-associated molecule CD79alpha (12 of 15 samples tested). These results suggest that multiple mechanisms underlie nonresponsiveness to anti-IgM in CLL and that retained responsiveness to anti-IgM contributes to the poor prognosis associated with the unmutated subset of CLL. The prognostic power of the in vitro response to IgM ligation remains to be determined in a large series, but the simple technology involved may present an alternative or additional test for predicting clinical course.

Mitochondrial reactive oxygen species regulate spatial profile of proinflammatory responses in lung venular capillaries

Cytokine-induced lung expression of the endothelial cell (EC) leukocyte receptor P-selectin initiates leukocyte rolling. To understand the early EC signaling that induces the expression, we conducted real-time digital imaging studies in lung venular capillaries. To compare receptor- vs nonreceptor-mediated effects, we infused capillaries with respectively, TNF-alpha and arachidonate. At concentrations adjusted to give equipotent increases in the cytosolic Ca(2+), both agents increased reactive oxygen species (ROS) production and EC P-selectin expression. Blocking the cytosolic Ca(2+) increases abolished ROS production; blocking ROS production abrogated P-selectin expression. TNF-alpha, but not arachidonate, released Ca(2+) from endoplasmic stores and increased mitochondrial Ca(2+). Furthermore, Ca(2+) depletion abrogated TNF-alpha responses partially, but arachidonate responses completely. These differences in Ca(2+) mobilization by TNF-alpha and arachidonate were reflected in spatial patterning in the capillary in that the TNF-alpha effects were localized at branch points, while the arachidonate effects were nonlocalized and extensive. Furthermore, mitochondrial blockers inhibited the TNF-alpha- but not the arachidonate-induced responses. These findings indicate that the different modes of Ca(2+) mobilization determined the spatial patterning of the proinflammatory response in lung capillaries. Responses to TNF-alpha revealed that EC mitochondria regulate the proinflammatory process by generating ROS that activate P-selectin expression.

Phosphorylation of Tyr342 in the linker region of Syk is critical for Fc epsilon RI signaling in mast cells

The linker region of Syk and ZAP70 tyrosine kinases plays an important role in regulating their function. There are three conserved tyrosines in this linker region; Tyr317 of Syk and its equivalent residue in ZAP70 were previously shown to negatively regulate the function of Syk and ZAP70. Here we studied the roles of the other two tyrosines, Tyr342 and Tyr346 of Syk, in Fc epsilon RI-mediated signaling. Antigen stimulation resulted in Tyr342 phosphorylation in mast cells. Syk with Y342F mutation failed to reconstitute Fc epsilon RI-initiated histamine release. In the Syk Y342F-expressing cells there was dramatically impaired receptor-induced phosphorylation of multiple signaling molecules, including LAT, SLP-76, phospholipase C-gamma2, but not Vav. Compared to wild-type Syk, Y342F Syk had decreased binding to phosphorylated immunoreceptor tyrosine-based activation motifs and reduced kinase activity. Surprisingly, mutation of Tyr346 had much less effect on Fc epsilon RI-dependent mast cell degranulation. An anti-Syk-phospho-346 tyrosine antibody indicated that antigen stimulation induced only a very minor increase in the phosphorylation of this tyrosine. Therefore, Tyr342, but not Tyr346, is critical for regulating Syk in mast cells and the function of these tyrosines in immune receptor signaling appears to be different from what has been previously reported for the equivalent residues of ZAP70.

Determination of protein phosphorylation in Fc epsilon RI-activated human mast cells by immunoblot analysis requires protein extraction under denaturing conditions

The advent of activation state antibodies has greatly facilitated studies aimed at understanding the intracellular signaling cascade following occupancy and/or aggregation of surface receptors. As part of an ongoing study investigating the signal transduction cascade initiated following aggregation of the high affinity receptor for IgE (Fc epsilon RI) in human mast cells, we observed substantial differences in responses monitored by these antibodies when cells were extracted either under nonreducing or reducing conditions. This was true even in the presence of high concentrations of protease inhibitors. Although the activation of some proteins such as those of the MAP kinase pathway appeared to be unaffected by the extraction conditions, other signals, including overall tyrosine phosphorylation and activation of phospholipase Cgamma(1), were substantially different. This was due to the significant proteolysis in samples extracted under nondenaturing conditions. When the signaling proteins were extracted rapidly under denaturing conditions, however, there was little evidence of proteolysis of the signaling proteins with a resulting improved recovery of signal. Thus, accurate determination of signaling events utilizing activation state-specific antibodies in human mast cells requires protein extraction under denaturing conditions. The data presented in this report would be applicable to other cell types where high concentrations of proteases are present.

Modeling the early signaling events mediated by FcepsilonRI

We present a detailed mathematical model of the phosphorylation and dephosphorylation events that occur upon ligand-induced receptor aggregation, for a transfectant expressing FcepsilonRI, Lyn, Syk and endogenous phosphatases that dephosphorylate exposed phosphotyrosines on FcepsilonRI and Syk. Through model simulations we show how changing the ligand concentration, and consequently the concentration of receptor aggregates, can change the nature of a cellular response as well as its amplitude. We illustrate the value of the model in analyzing experimental data by using it to show that the intrinsic rate of dephosphorylation of the FcepsilonRI gamma immunoreceptor tyrosine-based activation motif (ITAM) in rat basophilic leukemia (RBL) cells is much faster than the observed rate, provided that all of the cytosolic Syk is available to receptors.

Protein tyrosine kinase Syk in mast cell signaling

The tyrosine kinase Syk is essential for signaling from FcrepsilonRI in mast cells. The Src homology domain mediated binding of Syk to the phosphorylated immunoreceptor tyrosine-based motif (ITAM) of the receptor subunits results in a conformational change and activation. Studies in Syk deficient mast cells have defined the pathways that are activated upstream and downstream of Syk and have demonstrated the functional importance of the linker region of Syk in signaling in mast cells.

A novel Syk-family tyrosine kinase from Schistosoma mansoni which is preferentially transcribed in reproductive organs

The complete coding deoxyribonucleic acid for a novel tyrosine kinase (TK) of the human parasite Schistosoma mansoni has been cloned and characterized. The molecule was designated TK4. The sequence predicts a translation product of about 140 kDa containing two Src homology 2 domains and a tyrosine kinase domain. Data base analyses indicate that TK4 belongs to the Syk family of TKs which has not been identified in schistosomes or other Acoelomata yet. The presence of a member of the Syk family in this phylum supports previous findings demonstrating that TK subclasses were established early in evolution. Although Northern blot and reverse transcription polymerase chain reaction analyses show transcription of TK4 in larval stages and adult schistosomes of both genders, TK4 is more abundantly transcribed in males. In situ hybridization data demonstrate the gender-independent occurrence of TK4 transcripts in parenchymatic cells. Significant signals were detected in the oocytes of the female and in the spermatocytes of the male suggesting that TK4, among other functions, may play a role in germ cell development. This is an unexpected finding considering that Syk-family TKs of invertebrates and vertebrates described so far are not involved in the differentiation of the gonads.

The adapter molecule Gab2 regulates Fc epsilon RI-mediated signal transduction in mast cells

The recently cloned scaffolding molecule Gab2 can assemble multiple molecules involved in signaling pathways. Bone marrow-derived mast cells isolated from Gab2(-/-) mice have defective signaling probably due to the lack of the activation of phosphatidylinositol-3 kinase (PI3-kinase). In this study, we investigated the role of Gab2 using the rat basophilic leukemia 2H3 cell line mast cells. Fc epsilon RI aggregation induced the tyrosine phosphorylation of Gab2 and translocation of a significant fraction of it from the cytosol to the plasma membrane. As in other cells, Gab2 was found to associate with several signaling molecules including Src homology 2-containing protein tyrosine phosphatase 2, Grb2, Lyn, and phospholipase C gamma (PLC gamma). The association of Gab2 with Lyn and PLC gamma were enhanced after receptor aggregation. Overexpression of Gab2 in rat basophilic leukemia 2H3 cell line cells inhibited the Fc epsilon RI-induced tyrosine phosphorylation of the subunits of the receptor, and the phosphorylation and/or activation of Syk and mitogen-activated protein kinase. Downstream events such as calcium mobilization, degranulation, and induction of TNF-alpha and IL-6 gene transcripts were decreased in Gab2 overexpressing cells, although Akt phosphorylation as a measure of PI3-kinase activation was unaffected. These results suggest that in addition to the positive effects mediated by PI3-kinase that are apparent in Gab2(-/-) mast cells, Gab2 by interacting with Lyn and PLC gamma may have negative regulatory effects on Fc epsilon RI-induced mast cell signaling and functions.

Deoxygenation of sickle cells stimulates Syk tyrosine kinase and inhibits a membrane tyrosine phosphatase

Polymerization of hemoglobin S in sickle red cells, in deoxygenated conditions, is associated with K+ loss and cellular dehydration. It was previously reported that deoxygenation of sickle cells increases protein tyrosine kinase (PTK) activity and band 3 tyrosine phosphorylation and that PTK inhibitors reduce cell dehydration. Here, the study investigates which PTKs are involved and the mechanism of their activation. Deoxygenation of sickle cells induced a 2-fold increase in Syk activity, measured by autophosphorylation in immune complex assays, but had no effect on Lyn. Syk was not stimulated by deoxygenation of normal red cells, and stimulation was partly reversible on reoxygenation of sickle cells. Syk activation was independent of the increase in intracellular Ca++ and Mg2+ associated with deoxygenation. Lectins that promote glycophorin or band 3 aggregation did not activate Syk. In parallel to Syk stimulation, deoxygenation of sickle cells, but not of normal red cells, decreased the activity of both membrane-associated protein tyrosine phosphatase (PTPs) and membrane protein thiol content. In vitro pretreatment of Syk immune complexes with membrane PTP inhibited Syk autophosphorylation. It is suggested that Syk activation in vivo could be mediated by PTP inhibition, itself resulting from thiol oxidation, as PTPs are known to be inhibited by oxidants. Altogether these data indicate that Syk could be involved in the mechanisms leading to sickle cell dehydration.

Inhibition of beta 2 integrin receptor and Syk kinase signaling in monocytes by the Src family kinase Fgr

While beta 2 integrin ligand-receptor recognition interactions are well characterized, less is known about how these events trigger signal transduction cascades to regulate the transition from tethering to firm adhesion, spreading, and transendothelial migration. We have identified critical positive and negative regulatory components of this cascade in monocytes. Whereas the Syk tyrosine kinase is essential for beta 2 integrin signaling and cell spreading, the Src family kinase Fgr is a negative regulator of this pathway. Fgr selectively inhibits beta 2 but not beta 1 integrin signaling and Syk kinase function via a direct association between the Fgr SH2 domain and Syk tyrosine Y342. The inhibitory effects of Fgr are independent of its kinase activity, are dose dependent, and can be overcome by chemokines and inflammatory mediators.