Tyrosine phosphorylation is an early signaling event common to Fc receptor crosslinking in human neutrophils and rat basophilic leukemia cells (RBL-2H3)

A PKC-SHP1 signaling axis desensitizes Fcγ receptor signaling by reducing the tyrosine phosphorylation of CBL and regulates FcγR mediated phagocytosis

BACKGROUND

Fcγ receptors mediate important biological signals in myeloid cells including the ingestion of microorganisms through a process of phagocytosis. It is well-known that Fcγ receptor (FcγR) crosslinking induces the tyrosine phosphorylation of CBL which is associated with FcγR mediated phagocytosis, however how signaling molecules coordinate to desensitize these receptors is unclear. An investigation of the mechanisms involved in receptor desensitization will provide new insight into potential mechanisms by which signaling molecules may downregulate tyrosine phosphorylation dependent signaling events to terminate important signaling processes.

RESULTS

Using the U937IF cell line, we observed that FcγR1 crosslinking induces the tyrosine phosphorylation of CBL, which is maximal at 5 min. followed by a kinetic pattern of dephosphorylation. An investigation of the mechanisms involved in receptor desensitization revealed that pretreatment of U937IF or J774 cells with PMA followed by Fcγ receptor crosslinking results in the reduced tyrosine phosphorylation of CBL and the abrogation of downstream signals, such as CBL-CRKL binding, Rac-GTP activation and the phagocytic response. Pretreatment of J774 cells with GF109203X, a PKC inhibitor was observed to block dephosphorylation of CBL and rescued the phagocytic response. We demonstrate that the PKC induced desensitization of FcγR/ phagocytosis is associated with the inactivation of Rac-GTP, which is deactivated in a hematopoietic specific phosphatase SHP1 dependent manner following ITAM stimulation. The effect of PKC on FcγR signaling is augmented by the transfection of catalytically active SHP1 and not by the transfection of catalytic dead SHP1 (C124S).

CONCLUSIONS

Our results suggest a functional model by which PKC interacts with SHP1 to affect the phosphorylation state of CBL, the activation state of Rac and the negative regulation of ITAM signaling i.e. Fcγ receptor mediated phagocytosis. These findings suggest a mechanism for Fcγ receptor desensitization by which a serine-threonine kinase e.g. PKC downregulates tyrosine phosphorylation dependent signaling events via the reduced tyrosine phosphorylation of the complex adapter protein, CBL.

Effects of the protein tyrosine phosphatase CD45 on FcgammaRIIa signaling and neutrophil function

OBJECTIVE

Neutrophil receptors for the Fc portion of IgG (FcgammaR) trigger immune responses following cross-linking by IgG-coated foreign particles or immune complexes. Membrane-associated CD45, a protein tyrosine phosphatase termed leukocyte common antigen, has been shown to be essential for antigen receptor kinase mediated signaling in lymphocytes, and we hypothesized that CD45 may play a similar role in FcgammaR-mediated signaling and immune function in human neutrophils.

METHODS

The experimental approach was that of cell surface molecule ligation via cross-linking with specific antibodies. Antibody dependent cellular cytotoxicity (ADCC) was assessed using a single-cell plaque assay and IL-6 production measured using ELISA. Tyrosine phosphorylation levels were assessed with anti-phospho-tyrosine blots and F-actin polymerization by flow cytometry and confocal microscopy.

RESULTS

Neutrophils pretreated with anti-CD45 had a reduced ability to perform ADCC compared to untreated neutrophils. FcgammaRIIa cross-linking resulted in significantly increased concentrations of secreted IL-6 compared to untreated neutrophils, and IL-6 production was further enhanced by cocross-linking CD45 with FcgammaRIIa. Cross-linking CD45 alone also induced IL-6 production. FcgammaRIIa cross-linking resulted in increased protein tyrosine phosphorylation and F-actin polymerization in neutrophils. Cocross-linking CD45 with FcgammaRIIa resulted in abrogation of FcgammaRIIa mediated tyrosine phosphorylation and F-actin polymerization.

CONCLUSIONS

These data provide evidence that CD45 can regulate or enhance the stimulation and function of human neutrophils mediated through FcgammaR(s). In addition, CD45 ligation may play an essential role in cytokine induction pathways that lead to inflammatory reactions in vivo.

Regulation of polymorphonuclear leukocyte phagocytosis by myosin light chain kinase after activation of mitogen-activated protein kinase

Polymorphonuclear leukocyte (PMNL) phagocytosis mediated by FcγRII proceeds in concert with activation of the mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase ERK2. We hypothesized that myosin light chain kinase (MLCK) could be phosphorylated and activated by ERK, thereby linking the MAP kinase pathway to the activation of cytoskeletal components required for pseudopod formation. To explore this potential linkage, PMNLs were challenged with antibody-coated erythrocytes (EIgG). Peak MLCK activity, 3-fold increased over controls, occurred at 4 to 6 minutes, corresponding with the peak rate of target ingestion and ERK2 activity. The MLCK inhibitor ML-7 (10 μmol/L) inhibited both phagocytosis and MLCK activity to basal values, thereby providing further support for the linkage between the functional response and the requirement for MLCK activation. The MAPK kinase (MEK) inhibitor PD098059 inhibited phagocytosis, MLCK activity, and ERK2 activity by 80% to 90%. To directly link ERK activation to MLCK activation, ERK2 was immunoprecipitated from PMNLs after EIgG ingestion. The isolated ERK2 was incubated with PMNL cytosol as a source of unactivated MLCK and with MLCK substrate; under these conditions ERK2 activated MLCK, resulting in phosphorylation of the MLCK substrate or of the myosin light chain itself. Because MLCK activates myosin, we evaluated the effect of directly inhibiting myosin adenosine triphosphatase using 2,3-butanedione monoxime (BDM) and found that phagocytosis was inhibited by more than 90% but MLCK activity remained unaffected. These results are consistent with the interpretation that MEK activates ERK, ERK2 then activates MLCK, and MLCK activates myosin. MLCK activation is a critical step in the cytoskeletal changes resulting in pseudopod formation.

Regulation of polymorphonuclear leukocyte phagocytosis by myosin light chain kinase after activation of mitogen-activated protein kinase

Polymorphonuclear leukocyte (PMNL) phagocytosis mediated by FcγRII proceeds in concert with activation of the mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase ERK2. We hypothesized that myosin light chain kinase (MLCK) could be phosphorylated and activated by ERK, thereby linking the MAP kinase pathway to the activation of cytoskeletal components required for pseudopod formation. To explore this potential linkage, PMNLs were challenged with antibody-coated erythrocytes (EIgG). Peak MLCK activity, 3-fold increased over controls, occurred at 4 to 6 minutes, corresponding with the peak rate of target ingestion and ERK2 activity. The MLCK inhibitor ML-7 (10 μmol/L) inhibited both phagocytosis and MLCK activity to basal values, thereby providing further support for the linkage between the functional response and the requirement for MLCK activation. The MAPK kinase (MEK) inhibitor PD098059 inhibited phagocytosis, MLCK activity, and ERK2 activity by 80% to 90%. To directly link ERK activation to MLCK activation, ERK2 was immunoprecipitated from PMNLs after EIgG ingestion. The isolated ERK2 was incubated with PMNL cytosol as a source of unactivated MLCK and with MLCK substrate; under these conditions ERK2 activated MLCK, resulting in phosphorylation of the MLCK substrate or of the myosin light chain itself. Because MLCK activates myosin, we evaluated the effect of directly inhibiting myosin adenosine triphosphatase using 2,3-butanedione monoxime (BDM) and found that phagocytosis was inhibited by more than 90% but MLCK activity remained unaffected. These results are consistent with the interpretation that MEK activates ERK, ERK2 then activates MLCK, and MLCK activates myosin. MLCK activation is a critical step in the cytoskeletal changes resulting in pseudopod formation.

Activation of complement receptor 3 on human monocytes by cross-linking of very-late antigen-5 is mediated via protein tyrosine kinases

Bordetella pertussis interacts with very-late antigen-5 (VLA-5) receptors on the human monocyte resulting in cross-linking of these receptors followed by activation of complement receptor 3 (CR3) and firm adhesion of B. pertussis to these monocytes. In the present study we investigated whether protein tyrosine kinases are involved in the activation of CR3 on monocytes, which was assessed by the binding of C3bi-coated erythrocytes (EC3bi). Pre-incubation of monocytes with tyrphostin-A47, a specific protein tyrosine kinase inhibitor, before adherence of the cells to an anti-VLA-5 monoclonal antibody-coated surface, or addition of tyrphostin-A47 within 10 min of the adherence to such surface, reduced the binding of EC3bi to monocytes significantly. Pre-incubation of monocytes with tyrphostin-A47 reduced the binding of B. pertussis to such monocytes as well. Inhibitors of protein kinase A and/or C had no effect on EC3bi binding to monocytes. Cross-linking of VLA-5 on monocytes resulted in tyrosine phosphorylation of several proteins. Together, these results indicate that protein tyrosine kinases are involved in the VLA-5-induced activation of CR3 on human monocytes.

Yops of Yersinia enterocolitica inhibit receptor-dependent superoxide anion production by human granulocytes

The virulence plasmid-borne genes encoding Yersinia adhesin A (YadA) and several Yersinia secreted proteins (Yops) are involved in the inhibition of phagocytosis and killing of Yersinia enterocolitica by human granulocytes. One of these Yops, YopH, dephosphorylates multiple tyrosine-phosphorylated proteins in eukaryotic cells and is involved in the inhibition of phagocytosis of Y. enterocolitica by human granulocytes. We investigated whether antibody- and complement-opsonized plasmid-bearing (pYV+) Y. enterocolitica inhibits O2- production by human granulocytes in response to various stimuli and whether YopH is involved. Granulocytes were preincubated with mutant strains unable to express YadA or to secrete Yops or YopH. O2- production by granulocytes during stimulation was assessed by measuring the reduction of ferricytochrome c. PYV+ Y. enterocolitica inhibited O2- production by granulocytes incubated with opsonized Y. enterocolitica or N-formyl-Met-Leu-Phe (f-MLP). This inhibitory effect mediated by pYV did not affect receptor-independent O2- production by granulocytes in response to phorbol myristate acetate, indicating that NADPH activity remained unaffected after activation of protein kinase C. The inhibition of f-MLP-induced O2- production by granulocytes depends on the secretion of Yops and not on the expression of YadA. Insertional inactivation of the yopH gene abrogated the inhibition of phagocytosis of antibody- and complement-opsonized Y. enterocolitica by human granulocytes but not of the f-MLP-induced O2- production by granulocytes or tyrosine phosphorylation of granulocyte proteins. These findings suggest that the specific targets for YopH are not present in f-MLP receptor-linked signal transduction and that other Yop-mediated mechanisms are involved.

Sphingosine Blocks Human Polymorphonuclear Leukocyte Phagocytosis Through Inhibition of Mitogen-Activated Protein Kinase Activation

In the present study, we investigated the mechanism by which sphingosine and its analogues, dihydrosphingosine and phytosphingosine, inhibit polymorphonuclear leukocyte (PMN) phagocytosis of IgG-opsonized erythrocytes (EIgG) and inhibit ERK1 and ERK2 phosphorylation. We used antibodies that recognized the phosphorylated forms of ERK1 (p44) and ERK2 (p42) (extracellular signal-regulated protein kinases 1 and 2). Sphingoid bases inhibited ERK1 and ERK2 activation and phagocytosis of EIgG in a concentration-dependent manner. Incubation with glycine, N,N′-[1,2-ethanediylbis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]]-bis[(acetyloxy)methyl]ester (BAPTA,AM), an intracellular chelator of calcium, failed to block either phagocytosis or ERK1 and ERK2 phosphorylation, consistent with the absence of a role for a calcium-dependent protein kinase C (PKC) in ERK1 and ERK2 phosphorylations. Western blotting demonstrated that sphingosine inhibited the translocation of Raf-1 and PKCδ from PMN cytosol to the plasma membrane during phagocytosis. These data are consistent with the interpretation that sphingosine regulates ERK1 and ERK2 phosphorylation through inhibition of PKCδ, and this in turn leads to inhibition of Raf-1 translocation to the plasma membrane. Consistent with this interpretation, the sphingosine-mediated inhibition of phagocytosis, ERK2 activation, and PKCδ translocation to the plasma membrane could be abrogated with a cell-permeable diacylglycerol analog. The increase in the diacylglycerol mass correlated with the translocation of PKCδ and Raf-1 to the plasma membrane by 3 minutes after the initiation of phagocytosis. Additionally, the diacylglycerol analog enhanced phagocytosis by initiating activation of PKCδ and its translocation to the plasma membrane. Because PMN generate sufficient levels of sphingosine by 30 minutes during phagocytosis of EIgG to inhibit phagocytosis, it appears that sphingosine can serve as an endogenous regulator of EIgG-mediated phagocytosis by downregulating ERK activation.

Sphingosine Blocks Human Polymorphonuclear Leukocyte Phagocytosis Through Inhibition of Mitogen-Activated Protein Kinase Activation

In the present study, we investigated the mechanism by which sphingosine and its analogues, dihydrosphingosine and phytosphingosine, inhibit polymorphonuclear leukocyte (PMN) phagocytosis of IgG-opsonized erythrocytes (EIgG) and inhibit ERK1 and ERK2 phosphorylation. We used antibodies that recognized the phosphorylated forms of ERK1 (p44) and ERK2 (p42) (extracellular signal-regulated protein kinases 1 and 2). Sphingoid bases inhibited ERK1 and ERK2 activation and phagocytosis of EIgG in a concentration-dependent manner. Incubation with glycine, N,N′-[1,2-ethanediylbis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]]-bis[(acetyloxy)methyl]ester (BAPTA,AM), an intracellular chelator of calcium, failed to block either phagocytosis or ERK1 and ERK2 phosphorylation, consistent with the absence of a role for a calcium-dependent protein kinase C (PKC) in ERK1 and ERK2 phosphorylations. Western blotting demonstrated that sphingosine inhibited the translocation of Raf-1 and PKCδ from PMN cytosol to the plasma membrane during phagocytosis. These data are consistent with the interpretation that sphingosine regulates ERK1 and ERK2 phosphorylation through inhibition of PKCδ, and this in turn leads to inhibition of Raf-1 translocation to the plasma membrane. Consistent with this interpretation, the sphingosine-mediated inhibition of phagocytosis, ERK2 activation, and PKCδ translocation to the plasma membrane could be abrogated with a cell-permeable diacylglycerol analog. The increase in the diacylglycerol mass correlated with the translocation of PKCδ and Raf-1 to the plasma membrane by 3 minutes after the initiation of phagocytosis. Additionally, the diacylglycerol analog enhanced phagocytosis by initiating activation of PKCδ and its translocation to the plasma membrane. Because PMN generate sufficient levels of sphingosine by 30 minutes during phagocytosis of EIgG to inhibit phagocytosis, it appears that sphingosine can serve as an endogenous regulator of EIgG-mediated phagocytosis by downregulating ERK activation.

Activation of three classes of nonreceptor tyrosine kinases following Fc gamma receptor crosslinking in human monocytes

Fc gamma receptors on monocytes/macrophages play an important role in both host defense and autoimmune disorders. Fc gamma receptor signaling can lead to such downstream events as phagocytosis and the release of intracellular cytokines and reactive oxygen species. Freshly isolated human monocytes express two major classes of Fc gamma receptor proteins, Fc gamma RI (CD64) and Fc gamma RII (CD32). Crosslinking of Fc gamma RI and Fc gamma RII gives rise to rapid and transient phosphorylation of multiple monocyte intracellular proteins including proteins of 40, 68-72, 75-85, 95, and 115-165 kDa. A 72-kDa protein was earlier identified as the tyrosine kinase Syk. Here we identify one of the proteins in the 115- to 165-kDa cluster as FAK, a protein tyrosine kinase localized to focal adhesions. A 68-kDa phosphoprotein was identified as paxillin, a cytoskeleton associated substrate for tyrosine kinases, and a 95-kDa protein was found to be the proto-oncogene product Vav. The Src family protein tyrosine kinase Fgr (p58) also displayed enhanced tyrosine phosphorylation after Fc gamma RI and Fc gamma RII crosslinking. Although Fc gamma RIIA utilizes tyrosines within its own cytoplasmic domain for signaling while Fc gamma RI utilizes the cytoplasmic tyrosines of its associated gamma subunit, our results indicate sharing of several proteins for signaling in monocytes by these Fc receptors. These molecules include three distinct classes of tyrosine kinases, Syk, FAK, and Fgr, and the functionally diverse proteins Vav and paxillin.

Interaction of IgG immunoglobulins with the guinea pig peritoneal macrophage Fc gamma receptors. Effect on the association of the receptors with the membrane skeleton and the cytoskeleton

Binding of ligands to cell surface receptors may induce an interaction of the receptors with the cytoskeleton and/or membrane skeleton and decrease the solubility of the receptors in nonionic detergents. Cytochalasins, reagents affecting the structure of microfilaments, inhibit some cell functions induced by cross-linking of the receptors with ligands. Information concerning the function of the cytoskeleton in insolubilization of Fc gamma receptors (Fc gamma R) and in Fc gamma R-mediated signal transmission is rather limited. The aim of this work was to investigate the effect of binding of homologous (guinea pig IgG1 and IgG2) and heterologous (rabbit IgG) immunoglobulins to guinea pig peritoneal macrophages on association of the macrophage Fc gamma receptors with the membrane skeleton and cytoskeleton. Cross-linking the macrophage Fc gamma receptors with immunoglobulin ligands induced insolubilization of the receptors in nonionic detergents suggesting association of the receptors with the membrane skeleton and the cytoskeleton. The ligands showed differential effects depending on a subclass and origin of the IgG used. The process of association of the Fc gamma receptors with the skeletons was fast and did not depend on temperature. Treatment of insoluble complexes with cytochalasin D, DNAse I or colchicine showed that actin microfilaments and microtubules play a role, at least partially, in insolubilization of the cross-linked macrophage Fc gamma receptors. Inhibition of insolubilization of the macrophage Fc gamma receptors by genistein indicated that tyrosine kinases are involved in the process of insolubilization. The association with the skeletons might be a part of the process of transduction of a signal which depended on the subclass and origin of IgG used and on the type of the Fc gamma receptor.

Pharmacological comparison of UTP- and thapsigargin-induced arachidonic acid release in mouse RAW 264.7 macrophages

1. Although stimulation of mouse RAW 264.7 macrophages by UTP elicits a rapid increase in intracellular free Ca2+ ([Ca2+]i), phosphoinositide (PI) turnover, and arachidonic acid (AA) release, the causal relationship between these signalling pathways is still unclear. In the present study, we investigated the involvement of phosphoinositide-dependent phospholipase C (PI-PLC) activation, Ca2+ increase and protein kinase activation in UTP-induced AA release. The effects of stimulating RAW 264.7 cells with thapsigargin, which cannot activate the inositol phosphate (IP) cascade, but results in the release of sequestered Ca2+ and an influx of extracellular Ca2+, was compared with the effects of UTP stimulation to elucidate the multiple regulatory pathways for cPLA2 activation. 2. In RAW 264.7 cells UTP (100 microM) and thapsigargin (1 microM) caused 2 and 1.2 fold increases, respectively, in [3H]-AA release. The release of [3H]-AA following treatment with UTP and thapsigargin were non-additive, totally abolished in the Ca2+-free buffer, BAPTA (30 microM)-containing buffer or in the presence of the cPLA2 inhibitor MAFP (50 microM), and inhibited by pretreatment of cells with pertussis toxin (100 ng ml(-1)) or 4-bromophenacyl bromide (100 microM). By contrast, aristolochic acid (an inhibitor of sPLA2) had no effect on UTP and thapsigargin responses. 3. U73122 (10 microM) and neomycin (3 mM), inhibitors of PI-PLC, inhibited UTP-induced IP formation (88% and 83% inhibition, respectively) and AA release (76% and 58%, respectively), accompanied by a decrease in the [Ca2+]i rise. 4. Wortmannin attenuated the IP response of UTP in a concentration-dependent manner (over the range 10 nM-3 microM), and reduced the UTP-induced AA release in parallel. RHC 80267 (30 microM), a specific diacylglycerol lipase inhibitor, had no effect on UTP-induced AA release. 5. Short-term treatment with PMA (1 microM) inhibited the UTP-stimulated accumulation of IP and increase in [Ca2+]i, but had no effect on the release of AA. In contrast, the AA release caused by thapsigargin was increased by PMA. 6. The role of PKC in UTP- and thapsigargin-mediated AA release was shown by the blockade of these effects by staurosporine (1 microM), Ro 31-8220 (10 microM), Go 6976 (1 microM) and the down-regulation of PKC. 7. Following treatment of cells with SK&F 96365 (30 microM), thapsigargin-, but not UTP-, induced Ca2+ influx, and the accompanying AA release, were down-regulated. 8. Neither PD 98059 (100 microM), MEK a inhibitor, nor genistein (100 microM), a tyrosine kinase inhibitor, had any effect on the AA responses induced by UTP and thapsigargin. 9. We conclude that UTP-induced cPLA2 activity depends on the activation of PI-PLC and the sustained elevation of intracellular Ca2+, which is essential for the activation of cPLA2 by UTP and thapsigargin. The [Ca2+]i-dependent AA release that follows treatment with both stimuli was potentiated by the activity of protein kinase C (PKC). A pertussis toxin-sensitive pathway downstream of the increase in [Ca2+]i was also shown to be involved in AA release.

The anti-Fc gamma RIII mAb 3G8 induces neutrophil activation via a cooperative actin of Fc gamma RIIIb and Fc gamma RIIa

Human neutrophils express two types of Fc gamma receptors, the transmembrane Fc gamma RIIa and the glycan-phosphatidylinositol-anchored Fc gamma RIIIb, that show synergism in provoking a cellular response. To analyse further the requirements for this synergism to occur we used the monoclonal antibody 3G8, directed against Fc gamma RIII. This antibody is able to induce neutrophil activation, as measured by an increase in the intracellular free Ca2+ concentration and homotypic neutrophil aggregation, but only when the Fc part of the antibody is able to interact with Fc gamma RIIa. We observed that binding of the Fab parts of 3G8 mAb to two Fc gamma RIIIb molecules and binding of the Fc part to one Fc gamma RIIa molecule is required, because a bispecific antibody, 2B1, in which only one 3G8 Fab is present, did not induce neutrophil activation. Moreover, engagement of one Fc gamma RIIa molecule and two Fc gamma RIIb molecules on the same cell is instrumental to achieve activation of the mAb 3G8. The activation of neutrophils by the 3G8 antibody represents a further example of synergistic activation of neutrophils via Fc gamma receptors.

Persistence of tyrosine-phosphorylated FcepsilonRI in deactivated cells

Engagement of the high affinity IgE receptor (FcepsilonRI) with a multimeric antigen leads to immediate tyrosine phosphorylation of its beta and gamma subunits, recruitment, and activation of the tyrosine kinase Syk, and later to cell degranulation. Monovalent hapten treatment reverses these events, resulting in receptor dephosphorylation and an abrupt arrest of cell degranulation. Thus far, it has been assumed that there is a direct linkage between receptor tyrosine phosphorylation, Syk activation and phosphorylation, and cell degranulation. However, we show here that when FcepsilonRI receptors are cross-linked for extended periods of time, hapten-mediated receptor dephosphorylation is delayed. These receptors, which remain tyrosine-phosphorylated despite the addition of hapten, are progressively targeted to a Triton X-100-insoluble fraction, suggesting their progressive association with the membrane skeleton. In contrast to FcepsilonRI receptors, hapten-induced Syk dephosphorylation and the consequent arrest of degranulation are not affected by prolonged cross-linking. Thus, some tyrosine-phosphorylated receptors persist in deactivated cells. We propose that, with time, some tyrosine-phosphorylated receptors become unaccessible to phosphatases and, in addition, unable to activate Syk. This inactive status of tyrosine-phosphorylated FcepsilonRI may be the result of membrane skeleton compartmentalization. However, another population of clustered receptors that includes the ones most recently formed is still immediately sensitive to hapten deactivation. This latter population is critical in maintaining Syk activity and cell degranulation. The shift from a transiently active state of phosphorylated receptors toward an inactive state could be a general mechanism of desensitization also utilized by other antigen receptors.

Na+/H+ exchange activity during phagocytosis in human neutrophils: role of Fcgamma receptors and tyrosine kinases

In neutrophils, binding and phagocytosis facilitate subsequent intracellular killing of microorganisms. Activity of Na+/H+ exchangers (NHEs) participates in these events, especially in regulation of intracellular pH (pHi) by compensating for the H+ load generated by the respiratory burst. Despite the importance of these functions, comparatively little is known regarding the nature and regulation of NHE(s) in neutrophils. The purpose of this study was to identify which NHE(s) are expressed in neutrophils and to elucidate the mechanisms regulating their activity during phagocytosis. Exposure of cells to the phagocytic stimulus opsonized zymosan (OpZ) induced a transient cytosolic acidification followed by a prolonged alkalinization. The latter was inhibited in Na+-free medium and by amiloride analogues and therefore was due to activation of Na+/H+ exchange. Reverse transcriptase PCR and cDNA sequencing demonstrated that mRNA for the NHE-1 but not for NHE-2, 3, or 4 isoforms of the exchanger was expressed. Immunoblotting of purified plasma membranes with isoform-specific antibodies confirmed the presence of NHE-1 protein in neutrophils. Since phagocytosis involves Fcgamma (FcgammaR) and complement receptors such as CR3 (a beta2 integrin) which are linked to pathways involving alterations in intracellular [Ca2+]i and tyrosine phosphorylation, we studied these pathways in relation to activation of NHE-1. Cross-linking of surface bound antibodies (mAb) directed against FcgammaRs (FcgammaRII > FcgammaRIII) but not beta2 integrins induced an amiloride-sensitive cytosolic alkalinization. However, anti-beta2 integrin mAb diminished OpZ-induced alkalinization suggesting that NHE-1 activation involved cooperation between integrins and FcgammaRs. The tyrosine kinase inhibitors genistein and herbimycin blocked cytosolic alkalinization after OpZ or FcgammaR cross-linking suggesting that tyrosine phosphorylation was involved in NHE-I activation. An increase in [Ca2+]i was not required for NHE-1 activation because neither removal of extracellular Ca2+ nor buffering of changes in [Ca2+]i inhibited alkalinization after OpZ or Fc-gammaR cross-linking. In summary, Fc-gammaRs and beta2 integrins cooperate in activation of NHE-1 in neutrophils during phagocytosis by a signaling pathway involving tyrosine phosphorylation.

Activation of SRC family kinases in human neutrophils. Evidence that p58C-FGR and p53/56LYN redistributed to a Triton X-100-insoluble cytoskeletal fraction, also enriched in the caveolar protein caveolin, display an enhanced kinase activity

Protein tyrosine phosphorylation is one of the signals involved in stimulation of neutrophil (PMN) functions. We found that phorbol myristate acetate (PMA) activates the src family tyrosine kinases p58c-fgr and p53/56lyn in suspended PMNs. Moreover, we found that up to about 20% of p58c-fgr and p53/56lyn redistribute to a Triton X-100-insoluble fraction after PMA stimulation, and it is this fraction of the two kinases which diplays an increased activity. These changes of p58c-fgr and p53/56lyn distribution and activity correlate with tyrosine phosphorylation of endogenous substrates. In fact, in PMA-stimulated PMNs tyrosine phosphorylated proteins are mostly recovered in a Triton-insoluble cell fraction. To separate cytoskeletal from caveolar structures, which both display Triton X-100-insolubility, we used the detergent n-octyl beta-D-glucopyranoside (OGP) which solubilises components of caveolae. We found that the caveolae marker protein, caveolin, as well as the cytoskeletal protein alph-actinin and p58c-fgr and p53/56lyn, is insoluble in OGP. These findings suggest that PMA stimulation promotes the formation of multimolecular complexes containing cytoskeletal proteins, caveolin-containing structures and src family protein tyrosine kinases. Moreover, they show that p58c-fgr and p53/56lyn associated with this multimolecular complex display an enhanced kinase activity.

Temporal activation of nontransmembrane protein-tyrosine kinases following mast cell Fc epsilon RI engagement

One of the primary responses observed following antigen-induced cross-linking in mast cells is an increase in the phosphorylation of certain cellular proteins on tyrosine residues. Stimulation of protein-tyrosine kinase activity appears to be necessary for induction of downstream responses such as degranulation. The role of nonreceptor protein-tyrosine kinases in the signal transduction pathway initiated by Fc epsilon RI engagement in an interleukin-3-dependent mast cell line has been examined. The results presented here show that the enzymatic activity of Lyn is increased within seconds of receptor engagement. Syk activity also undergoes a rapid and transient increase, reaching a peak at approximately 30 s. Similarly, the activity of Fer, representing a third class of nontransmembrane protein-tyrosine kinase increases as well, with its activity peak reached at 1 min poststimulation. The enzymatic activities of Syk and Fer were found to correspond to anti-phosphotyrosine antibody reactivity. Phosphorylation of tyrosine residues of the beta and gamma chains of Fc epsilon RI increased concomitant with increased protein-tyrosine kinase activity. These results indicate that at least three classes of nontransmembrane protein-tyrosine kinases are involved in mast cell FceRI signaling and that the activation of these classes of enzymes is temporally regulated.

Activation of the high-affinity immunoglobulin E receptor Fc epsilon RI in RBL-2H3 cells is inhibited by Syk SH2 domains

Antigen-mediated aggregation of the high-affinity receptor for immunoglobulin E, Fc epsilon RI, results in the activation of multiple signaling pathways, leading to the release of mediators of the allergic response. One of the earliest responses to receptor stimulation is the tyrosine phosphorylation of the beta and gamma subunits of Fc epsilon RI and the association of the tyrosine kinase Syk with the phosphorylated receptor. This association is mediated by the SH2 domains of Syk and is believed to be critical for activating signaling pathways resulting in mediator release. To examine the importance of the interaction of Syk with Fc epsilon RI in signaling events following receptor activation, we introduced a protein containing the SH2 domains of Syk into streptolysin O-permeabilized RBL-2H3 cells. The Syk SH2 domains completely inhibited degranulation and leukotriene production following receptor aggregation, and they blocked the increase in protein tyrosine phosphorylation observed after receptor activation. Inhibition was specific for Fc epsilon RI-mediated signaling, since degranulation of cells activated by alternative stimuli was not blocked by the Syk SH2 domains. A protein containing a point mutation in the carboxy-terminal SH2 domain which abolishes phosphotyrosine binding was not inhibitory. In addition, inhibition of degranulation was reversed by pretreatment of the SH2 domains with a tyrosine phosphorylated peptide corresponding to the tyrosine-based activation motif found in the gamma subunit of Fc epsilon RI, the nonphosphorylated peptide had no effect. The association of Syk with the tyrosine-phosphorylated gamma subunit of the activated receptor was blocked by the Syk SH2 domains, and deregulation in cells activated by clustering of Syk directly without Fc epsilon RI aggregation was not affected by the Syk SH2 domains. These results demonstrate that the association of Syk with the activated Fc epsilon RI is critical for both early and late events following receptor activation and confirm the key role Syk plays in signaling through the high-affinity IgE receptor.

Distinct tyrosine kinase activation and Triton X-100 insolubility upon Fc gamma RII or Fc gamma RIIIB ligation in human polymorphonuclear leukocytes. Implications for immune complex activation of the respiratory burst

Two tyrosine kinase-dependent pathways exist for activation of the respiratory burst by polymorphonuclear leukocyte (PMN) immunoglobulin G Fc receptors. Direct ligation of Fc gamma RII activates the respiratory burst, but ligation of the glycan phosphoinositol-linked Fc gamma RIIIB does not. Instead, this receptor and the integrin complement receptor CR3 synergize in activation of the respiratory burst (Zhou, M.-J., and Brown, E. J. (1994) J. Cell Biol. 125, 1407-1416). Here we show that direct ligation of Fc gamma RII leads to activation and Triton X-100 insolubility of the Src family kinase Fgr, without effect on the related myeloid Src family member Hck. In contrast, adhesion of PMN via Fc gamma RIIIB leads to activation and Triton X-100 insolubility of Hck but not Fgr. The exclusive association of Fc gamma RIIIB with Hck activation and Triton insolubility is not solely a result of its glycan phosphoinositol anchor, since decay accelerating factor (CD55), another prominent glycan phosphoinositol-anchored PMN protein, is associated with Fgr insolubility to a greater extent than Hck. Ligation of decay accelerating factor, with or without coligation of CR3, does not activate the PMN respiratory burst. Coligation of Fc gamma RIIIB with Fc gamma RII overcomes the pertussis toxin inhibition of H2O2 production in response to direct ligation of Fc gamma RII. These data support the hypothesis that activation of Hck upon Fc gamma RIIIB ligation has a role in generation of the synergistic respiratory burst.

Molecular cloning of rodent p72Syk. Evidence of alternative mRNA splicing

Northern blot analysis of polyadenylated RNA prepared from RBL-2H3 cells revealed the presence of three distinct mRNAs encoding p72Syk, a protein-tyrosine kinase previously shown to be associated with the high affinity IgE receptor present on the surface of these cells (Hutchcroft, J. E., Geahlen, R. L., Deanin, G. G., and Oliver, J. M. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 9107-9111). Here we report the full-length nucleotide sequence of two of these messages, as well as the complete predicted amino acid sequence of the rodent p72Syk protein-tyrosine kinase. In addition, we report evidence indicating alternative splicing of p72Syk mRNAs within RBL-2H3 cells. This splicing event results in the expression of two distinct protein isoforms that differ with respect to the presence of a 23-amino acid insert located within the region of the protein that separates the two SH2 domains from the catalytic domain. Both mRNAs arising from this splicing event appear to encode functional protein-tyrosine kinases, as expression of the corresponding cDNAs in COS cells results in the production of proteins of the expected sizes that possess intrinsic tyrosine specific kinase activity.

Tyrosine phosphorylation induced by cross-linking of Fc gamma-receptor type II in human neutrophils

Neutrophils express several receptors for the Fc region of IgG molecules. Specific cross-linking of the type II receptor (Fc gamma RII) can be achieved by treating neutrophils with the Fab fragment of a specific monoclonal antibody IV.3 against the receptor followed by goat anti-mouse IgG F(ab')2 fragment. Such treatment initiates a number of neutrophil responses including the release of O2-. and increased protein tyrosine phosphorylation. The increase in tyrosine phosphorylation is rapid and transient and correlates with O2-. release. Both responses are inhibited by pretreatment of neutrophils with a protein tyrosine kinase inhibitor, genistein. The increase in protein tyrosine phosphorylation is not inhibited by pretreatment of neutrophils with pertussis toxin or an intracellular Ca2+ chelator, but is enhanced by a phosphoprotein phosphatase inhibitor, okadaic acid. The activity of a neutrophil Ca2+/calmodulin-dependent protein kinase II (CAMPKII) is also stimulated by cross-linking Fc gamma RII. The increase in CAMPKII activity is inhibited by pretreatment with either genistein or Ca2+ chelator. The results suggest that the increase in protein tyrosine phosphorylation induced by cross-linking of Fc gamma RII requires neither pertussis-toxin-sensitive G-proteins nor a rise in intracellular Ca2+ but can be regulated by protein phosphatases. Furthermore, protein tyrosine phosphorylation may be an early signal functionally linked to Fc gamma RII-mediated signal transduction leading to CAMPKII activation and O2-. release in human neutrophils.

Phagocytosis

Phagocytosis is the process of recognition and engulfment of microorganisms or tissue debris that accumulate during infection, inflammation or wound repair. This ingestion, which is performed most efficiently by migrating, bone marrow-derived cells called 'professional phagocytes', is essential for successful host defense. Ingestion results when an invading microorganism is recognized by specific receptors on the phagocyte surface and requires multiple, successive interactions between the phagocyte and the target. Each of these interactions results in a signal transduction event, which is confined to the membrane and cytoskeleton around the ligated receptor and which is required for successful phagocytosis. Many molecules found at sites of inflammation or infection stimulate phagocytosis, so that efficient ingestion is confined to the site of infection or inflammation, which in turn limits the proinflammatory and tissue-destructive processes that accompany phagocytosis. This review summarizes current understanding of this critical component of host defense and of its regulation.

Bacterial stimulators of macrophages

Our current understanding of the interaction between bacteria and macrophages, cells of the immune system that play a major role in the defense against infection, is summarized. Cell-surface structures of Gram-negative and Gram-positive bacteria that account for these interactions are described in detail. Besides surface structures, soluble bacterial molecules, toxins that are derived from pathogenic bacteria, are also shown to modulate macrophage functions. In order to affect macrophage functions, bacterial surface structures have to be recognized by the macrophage and toxins have to be taken up. Subsequently, signal transduction mechanisms are initiated that enable the macrophage to respond to the invading bacteria. To destroy bacteria, macrophages employ many strategies, among which antigen processing and presentation to T cells, phagocytosis, chemotaxis, and different bactericidal mechanisms are considered to be the main weapons.

Involvement of tyrosine kinase in the induction of cyclo-oxygenase and nitric oxide synthase by endotoxin in cultured cells

1. Cyclo-oxygenase (COX) and nitric oxide synthase (NOS) are two enzymes which have distinct cytokine-inducible isoforms (COX-2 and iNOS). Many cytokine receptors have an intracellular tyrosine kinase domain. Here we have used the tyrosine kinase inhibitors, erbstatin and genistein, to investigate the potential role of tyrosine kinase activation in the induction on COX-2 and iNOS caused by endotoxin (lipopolysaccharide; LPS) in bovine aortic endothelial cells (BAEC) and J774.2 macrophages. 2. The main COX metabolites, 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) (for BAEC) and PGF2 alpha (for 774.2 macrophages) were measured by radioimmunossay: (i) accumulation of COX metabolites from endogenous arachidonic acid was measured at 24 h after addition of LPS (1 microgram ml-1); (ii) in experiments designed to measure 'COX activity', COX metabolites generated by BAEC or J774.2 macrophages activated with LPS were assayed (at 12 h after LPS administration) after incubation of the washed cells with exogenous arachidonic acid (30 microM for 15 min). Western blot analysis with a specific antibody to COX-2 was used to determine the expression of COX-2 protein caused by LPS in cell extracts. Accumulation of nitrite (measured by the Griess reaction) was used as an indicator of NO formation and, hence, iNOS activity. 3. Erbstatin (0.05 to 5 micrograms ml-1) or genistein (0.5 to 50 micrograms ml-1) caused a dose-dependent inhibition of the accumulation of COX metabolites in the supernatant of BAEC or J774.2 macrophages activated with LPS. Erbstatin or genistein also caused a dose-dependent inhibition of 'COX activity' in both cell types. Western blot analysis showed that erbstatin (5 ig ml1') or genistein (50gg ml-') inhibited the expression of COX-2 protein in BAEC and J774.2 macrophages activated with LPS (lLgml-' for 24 h).4. Erbstatin or genistein also caused a dose-dependent inhibition of nitrite accumulation in J774.2 macrophages activated with LPS (1 sg ml-' for 24 h). In contrast to J774.2 macrophages, BAECstimulated with LPS (1 pg ml-' for 24 h) did not produce detectable amounts (<1PiM) of nitrite.5. These results suggest that tyrosine phosphorylation is part of the signal transduction mechanism that mediates (i) the induction of COX-2 and iNOS elicited by LPS in J774.2 macrophages, and (ii) the induction of COX-2 by LPS in BAEC.

Tyrosine phosphorylation and inositol phosphate production: are early events in human eosinophil activation stimulated by immobilized secretory IgA and IgG?

Eosinophils are implicated as major inflammatory cells in parasite infection and allergic reactions. Among various mediators, eosinophil granule cationic proteins play an important role in the pathophysiology of diseases. However, little is known about the actual physiologic stimuli for eosinophil degranulation and the signaling events for triggering eosinophil degranulation. A series of in vivo and in vitro studies suggest that the interaction between antibody coated parasites and Fc receptors on eosinophils is one of the most effective triggers for eosinophil degranulation. Similarly, eosinophil degranulation can be induced in vitro by Sepharose beads coated with human sIgA or IgG. Eosinophil degranulation induced by these stimuli is mediated by PTX-sensitive membrane-bound heterotrimeric G protein(s), and is accompanied by the rapid turnover of inositol phosphates. The production of inositol phosphates is inhibited by PTX. Eosinophil activation by sIgA and IgG also involves tyrosine phosphorylation of several proteins and is inhibited by tyrosine kinase inhibitors. Thus phospholipase C-coupled G protein(s) and tyrosine kinases are key molecules in early signal transduction of eosinophil activation induced by sIgA and IgG. Although further studies are needed to identify which tyrosine kinase(s) is specifically involved in the eosinophil degranulation mechanism, these molecules could be a target for therapies of human diseases where eosinophils are involved.

Tyrosine phosphorylation and activation of NADPH oxidase in human neutrophils: a possible role for MAP kinases and for a 75 kDa protein

Challenge of neutrophils with concanavalin A (ConA), formyl-methionyl-leucyl-phenylalanine (FMLP), and phorbol 12-myristate 13-acetate (PMA) induced the tyrosine phosphorylation of several proteins. Among these proteins we have identified two mitogen-activated protein kinase (MAPK) isoforms of 43 kDa (p43 MAPK) and 45 kDa (p45 MAPK) molecular mass. Moreover here we show that: (1) FMLP induced the tyrosine phosphorylation of the p43 MAPK, and ConA that of p45 MAPK, while PMA induced the tyrosine phosphorylation of both p43 and p45 MAPK; all these agonists induced the tyrosine phosphorylation of a 75 kDa protein (p75). (2) With FMLP or ConA as agonists, tyrosine phosphorylations of MAPK and p75 can be involved in the process of NADPH oxidase activation. On the contrary, PMA can activate the respiratory burst independently of these phosphorylations. (3) In Ca(2+)-depleted neutrophils, where phospholipid hydrolysis did not take place, ConA or FMLP did not activate the respiratory burst, but while ConA induced the tyrosine phosphorylation of p45 MAPK and p75, FMLP was not able to phosphorylate p43 MAPK and p75. (4) As previously observed in our laboratory, a double stimulation of Ca(2+)-depleted neutrophils with ConA plus FMLP induced a respiratory burst in the absence of activation of second messengers derived from phospholipase C, D and A2 activity. This respiratory burst was accompanied by tyrosine phosphorylation of both p43 and p45 MAPKs. These results indicate that when FMLP is the agonist, both the tyrosine phosphorylation of p43 MAPK and p75, and the activation of NADPH oxidase, are coupled to Ca(2+)-dependent mechanisms. On the contrary, ConA can induce the tyrosine phosphorylation of p45 MAPK and p75 independently of calcium, but an unknown Ca(2+)-dependent mechanism is necessary for the activation of NADPH oxidase by this agonist. This mechanism could be substituted by the induction of tyrosine phosphorylation of both p43 MAPK and p45 MAPK when Ca(2+)-depleted neutrophils are stimulated with ConA plus FMLP.

Protein tyrosine kinase activity is essential for Fc gamma receptor-mediated intracellular killing of Staphylococcus aureus by human monocytes

Our previous study revealed that the intracellular killing of Staphylococcus aureus by human monocytes after cross-linking Fc gamma receptor I (Fc gamma RI) or Fc gamma RII is a phospholipase C (PLC)-dependent process. The aim of the present study was to investigate whether protein tyrosine kinase (PTK) activity plays a role in the Fc gamma R-mediated intracellular killing of bacteria and activation of PLC in these cells. The results showed that phagocytosis of bacteria by monocytes was not affected by the PTK inhibitors genistein and tyrphostin-47. The intracellular killing of S. aureus by monocytes after cross-linking Fc gamma RII or Fc gamma RII with anti-Fc gamma R monoclonal antibody and a bridging antibody or with human immunoglobulin G (IgG) was inhibited by these compounds in a dose-dependent fashion. The production of O2- by monocytes after stimulation with IgG or IgG-opsonized S. aureus was almost completely blocked by the PTK inhibitor. These results indicate that inhibition of PTK impairs the oxygen-dependent bactericidal mechanisms of monocytes. Genistein and tyrphostin-47, which do not affect the enzymatic activity of purified PLC, prevented activation of PLC after cross-linking Fc gamma RI or Fc gamma RII, measured as an increase in the intracellular inositol 1,4,5-trisphosphate concentration. Cross-linking Fc gamma RI or Fc gamma RII induced rapid tyrosine phosphorylation of several proteins in monocytes, one of which was identified as PLC-gamma 1, and the phosphorylation could be completely blocked by PTK inhibitors, leading to the conclusion that activation of PLC after cross-linking Fc gamma R in monocytes is regulated by PTK activity. Together, these results demonstrate that PTK activity is essential for the activation of PLC which is involved in the Fc gamma R-mediated intracellular killing of S. aureus by human monocytes.

Tyrosine phosphorylation and activation of Bruton tyrosine kinase upon Fc epsilon RI cross-linking

Tyrosine phosphorylation of several cellular proteins is one of the earliest signaling events induced by cross-linking of the high-affinity receptor for immunoglobulin E (Fc epsilon RI) on mast cells or basophils. Tyrosine kinases activated during this process include the Src family kinases, Lyn, c-Yes, and c-Src, and members of another subfamily, Syk and PTK72 (identical or highly related to Syk). Recently, some of us described two novel tyrosine kinases, Emb and Emt, whose expression was limited to subsets of hematopoietic cells, including mast cells. Emb turned out to be identical to Btk, a gene product defective in human X-linked agammaglobulinemia and in X-linked immunodeficient (xid) mice. Here we report that Fc epsilon RI cross-linking induced rapid phosphorylation on tyrosine, serine, and threonine residues and activation of Btk in mouse bone marrow-derived mast cells. A small fraction of Btk translocated from the cytosol to the membrane compartment following receptor cross-linking. Tyrosine phosphorylation of Btk was not induced by either a Ca2+ ionophore (A23187), phorbol 12-myristate 13-acetate, or a combination of the two reagents. Co-immunoprecipitation between Btk and receptor subunit beta or gamma was not detected. The data collectively suggest that Btk is not associated with Fc epsilon but that its activation takes place prior to protein kinase C activation and plays a novel role in the Fc epsilon RI signaling pathway.

Tyrosine phosphorylation and activation of Bruton tyrosine kinase upon Fc epsilon RI cross-linking

Tyrosine phosphorylation of several cellular proteins is one of the earliest signaling events induced by cross-linking of the high-affinity receptor for immunoglobulin E (Fc epsilon RI) on mast cells or basophils. Tyrosine kinases activated during this process include the Src family kinases, Lyn, c-Yes, and c-Src, and members of another subfamily, Syk and PTK72 (identical or highly related to Syk). Recently, some of us described two novel tyrosine kinases, Emb and Emt, whose expression was limited to subsets of hematopoietic cells, including mast cells. Emb turned out to be identical to Btk, a gene product defective in human X-linked agammaglobulinemia and in X-linked immunodeficient (xid) mice. Here we report that Fc epsilon RI cross-linking induced rapid phosphorylation on tyrosine, serine, and threonine residues and activation of Btk in mouse bone marrow-derived mast cells. A small fraction of Btk translocated from the cytosol to the membrane compartment following receptor cross-linking. Tyrosine phosphorylation of Btk was not induced by either a Ca2+ ionophore (A23187), phorbol 12-myristate 13-acetate, or a combination of the two reagents. Co-immunoprecipitation between Btk and receptor subunit beta or gamma was not detected. The data collectively suggest that Btk is not associated with Fc epsilon but that its activation takes place prior to protein kinase C activation and plays a novel role in the Fc epsilon RI signaling pathway.

CR3 (Mac-1, alpha M beta 2, CD11b/CD18) and Fc gamma RIII cooperate in generation of a neutrophil respiratory burst: requirement for Fc gamma RIII and tyrosine phosphorylation

Cooperation among plasma membrane receptors in activating signal transduction cascades is not well understood. For almost 20 years, it has been clear that when a particulate foreign body is opsonized with complement as well as IgG, the efficiency of IgG effector functions is markedly enhanced. However, the molecular mechanisms involved in cooperation between IgG Fc receptors and complement receptors have not been elucidated. In this work, we show that when human neutrophils (PMN) are plated on a surface coated with both anti-CR3 and anti-Fc gamma RIII antibodies, the respiratory burst which occurs is equivalent to that stimulated by anti-Fc gamma RII. The CR3 ligand iC3b is as effective as anti-CR3 for cooperating with anti-Fc gamma RIII in generation of a respiratory burst. The synergy between CR3 and Fc gamma RIII for activating the NADPH oxidase is abolished by Fab of anti-Fc gamma RII. Nonetheless, the observed synergy is not an artifact of unintended Fc gamma RII ligation, since (a) only this combination of antibodies works to generate H2O2; (b) coating plates with either of the antibodies alone cannot activate the respiratory burst at any dose; (c) LAD (CR3 deficient) cells, which are perfectly competent to mount a respiratory burst when Fc gamma RII is engaged, are incapable of activating the respiratory burst when adherent to wells coated with anti-Fc gamma RIII and anti-CR3; (d) direct engagement of Fc gamma RII activates the respiratory burst by a pathway pharmacologically distinguishable from the synergistic respiratory burst. Fc gamma RIII/CR3 synergy is abolished by cytochalasin B and herbimicin, suggesting that both the actin cytoskeleton and tyrosine phosphorylation are necessary for activation of the synergistic respiratory burst. Further analysis shows that CR3 and Fc gamma RIII have distinct roles in activation of this Fc gamma RII-dependent assembly of the NADPH oxidase. Ligation of CR3 is sufficient to lead to Fc gamma RII association with the actin cytoskeleton on the adherent PMN surface. Coligation of Fc gamma RIII is required for tyrosine phosphorylation of Fc gamma RII. These data are consistent with a model in which phosphorylation of Fc gamma RII or a closely associated substrate initiates activation of a signal transduction pathway leading to oxidase assembly. These are the first data to demonstrate a molecular mechanism for synergy between IgG Fc and complement receptors in activation of phagocyte effector functions.

Bromophenacyl bromide binding to the actin-bundling protein l-plastin inhibits inositol trisphosphate-independent increase in Ca2+ in human neutrophils

Ligation of IgG Fc receptors on polymorphonuclear leukocytes causes an increase in the concentration of free intracytoplasmic Ca2+ ([Ca2+]i) which arises from release of intracellular stores but is independent of inositol 1,4,5-trisphosphate. We found that bromophenacyl bromide (BPB), an alkylating agent which inhibits leukocyte degranulation, adherence, and phagocytosis, inhibited IgG-stimulated increases in [Ca2+]i with an IC50 of 0.2 microM. In contrast, BPB had no effect on inositol 1,4,5-trisphosphate-dependent [Ca2+]i increases induced by fMet-Leu-Phe, complement fragment C5a, ATP, or platelet-activating factor. Using a monoclonal antibody specific for BPB, we identified in polymorphonuclear leukocytes a single cytosolic protein of 66 kDa and isoelectric point pH 5.6 which bound BPB when intact cells were treated with the alkylating agent. This BPB-binding protein was identified as l-plastin, a Ca(2+)-regulated actin-bundling protein. l-Plastin was found associated with the Triton X-100-insoluble cytoskeleton in polymorphonuclear leukocytes adherent to immune complexes, suggesting that BPB affects Fc receptor-mediated signal transduction by altering the actin cytoskeleton. Consistent with this hypothesis, both cytochalasin B and cytochalasin D inhibited the IgG-dependent increase in [Ca2+]i, without any effect on fMet-Leu-Phe-induced Ca2+ release. These data suggest that the actin cytoskeleton is essential for signal transduction from plasma membrane Fc receptors and that l-plastin has a critical role in activation of this pathway.

Stimulation of the intracellular killing of Staphylococcus aureus by human monocytes mediated by Fc gamma receptors I and II

Previous studies have shown that intracellular killing of bacteria by monocytes is stimulated by interaction between IgG and Fc gamma receptors (Fc gamma R) in the membrane of these cells. In the present study anti-Fc gamma R monoclonal antibodies (mAb) were used to investigate the relative contributions of the various classes of Fc gamma R to the intracellular killing of Staphylococcus aureus by human monocytes and the biochemical pathways involved. Anti-Fc gamma RI or anti-Fc gamma RII mAb, but not anti-Fc gamma RIII mAb, efficiently stimulated the intracellular killing of bacteria by monocytes. Cross-linking Fc gamma RI or Fc gamma RII, but not Fc gamma RIII, on monocytes with mouse anti-Fc gamma R mAb followed by bridging with F(ab')2 fragments of goat anti-mouse IgG enhanced this process. Since the NADPH oxidase inhibitor diphenyleneiodonium blocked the Fc gamma R-mediated intracellular killing of S. aureus, oxygen-dependent bactericidal mechanisms are most probably involved. Cross-linking Fc gamma RI or Fc gamma RII but not binding of the mAb to the Fc gamma R on monocytes activated phospholipase C, as demonstrated by the increase in the intracellular concentration of inositol-(1,4,5)-triphosphate. The enhanced intracellular killing stimulated by cross-linking Fc gamma R on monocytes was completely blocked by U-73122, an inhibitor of phospholipase C-dependent processes. Protein kinase C activity, but not the rise in the cytosolic free Ca++ concentration or pertussis toxin-sensitive G proteins, is essential for the Fc gamma R-mediated intracellular killing of bacteria by monocytes. Together, these results demonstrate that cross-linking Fc gamma RI or Fc gamma RII is equally effective in stimulating the intracellular killing of bacteria by monocytes and that this stimulation is a phospholipase C-dependent process.

Crystal-induced neutrophil activation. IV. Specific inhibition of tyrosine phosphorylation by colchicine

We recently demonstrated that pathologically relevant inflammatory microcrystals, namely triclinic monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals, potently stimulate a characteristic protein tyrosine phosphorylation pattern in human neutrophils that differed from that observed in response to other soluble or particulate agonists. In this study, the effects of colchicine on protein tyrosine phosphorylation induced by MSU and CPPD crystals in human blood neutrophils were investigated. Immunoblot analysis with antiphosphotyrosine antibodies demonstrated that colchicine dose-dependently inhibited the tyrosine phosphorylation of all the proteins phosphorylated in response to MSU and CPPD crystals. Other microtubule-disruptive agents such as vinblastine, nocodazole, and colcemid also inhibited crystal-induced protein tyrosine phosphorylation while lumicolchicine and trimethylcolchicinic acid were without effect. Indomethacin and phenylbutazone were similarly without effect on microcrystal-induced tyrosine phosphorylation. Colchicine, as well as the other active alkaloids, failed to inhibit the protein tyrosine phosphorylation elicited by FMLP, C5a, leukotriene B4, and unopsonized zymosan. Overall, these results demonstrate that colchicine specifically and significantly inhibits the protein tyrosine phosphorylation induced by MSU and CPPD crystals and suggest that its effects are associated, at least in part, with its interaction with microtubules. Furthermore, the use of microtubule-disrupting drugs demonstrate that the mechanisms implicated in the induction of protein tyrosine phosphorylation by microcrystals differed from those involved in response to other soluble or particulate agonists.

Association of immunoglobulin G Fc receptor II with Src-like protein-tyrosine kinase Fgr in neutrophils

The interaction of Fc receptors with antibody-antigen complexes activates multiple biological functions in hematopoietic cells. Recently, protein-tyrosine phosphorylation has been suggested to be involved in Fc receptor-mediated cell signaling. Here we show that the Src-like protein-tyrosine kinase Fgr, which is specifically expressed in mature myelomonocytic cells, coimmunoprecipitates with IgG Fc receptor II (Fc gamma RII), but not with Fc gamma RIII from detergent lysates of human peripheral neutrophils. Crosslinking of Fc gamma RII induced a rapid increase in the tyrosine kinase activity and comodulation of Fgr. These results suggest that Fgr is physically and functionally associated with Fc gamma RII and involved in Fc gamma RII-mediated signal transduction pathways.

Phospholipase A2 enzymes: regulation and inhibition

The phospholipase A2 enzymes are important components of the cellular machinery that responds to inflammatory stimuli and maintains cell homeostasis by membrane remodelling. Their role as the rate-limiting step in the production of pro-inflammatory lipid mediators makes these enzymes an important therapeutic target for the treatment of inflammatory disorders. Keith Glaser and colleagues explain how the two major groups of phospholipase A2, the secretory and cytosolic forms, are very different both structurally and enzymatically. Understanding the relative contributions of these different forms of phospholipase A2 to physiological and pathological conditions requires greater insight into their cellular regulation and the development of selective inhibitors.

Regulation of eicosanoid biosynthesis in the macrophage. Involvement of protein tyrosine phosphorylation and modulation by selective protein tyrosine kinase inhibitors

The protein tyrosine kinase (PTK) inhibitor genistein has been demonstrated to inhibit platelet-activating factor-stimulated prostaglandin E2 (PGE2) production in lipopolysaccharide (LPS)-primed P388D1 macrophage-like cells (Glaser et al., J Biol Chem 265: 8658-8664, 1990). Therefore, the role of PTK in eicosanoid biosynthesis was investigated in murine resident peritoneal macrophages using genistein and tyrphostin-25, selective PTK inhibitors. Genistein, a competitive inhibitor of ATP binding on PTK, inhibited PGE2 production (IC50 = 20 microM) in response to zymosan, calcium ionophore A23187, and phorbol myristate acetate stimulation. Genistein also inhibited leukotriene C4 (LTC4) production in response to zymosan and calcium ionophore A23187 (IC50 = 10 and 15 microM, respectively) stimulation. Tyrphostin-25, a competitive inhibitor of substrate binding on PTK, inhibited zymosan-stimulated PGE2 and LTC4 production, IC50 = 20 and 7 microM, respectively. Neither genistein nor tyrophostin-25 had any effect on human synovial fluid phospholipase A2 (PLA2) activity in vitro or on cyclooxygenase activity in the intact macrophage; however, tyrphostin-25 did affect 5-lipoxygenase activity (determined from the metabolism of exogenously applied arachidonic acid). These results suggest PTK-mediated phosphorylation as a common event in the signal transduction mechanisms of different stimuli which activate PLA2 for arachidonic acid release and subsequent eicosanoid biosynthesis. Immunoblot analyses of zymosan-stimulated peritoneal exudate cells with the phosphotyrosine monoclonal antibody clone 4G10 demonstrated an increase in protein phosphotyrosine levels in eight major protein bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis: p59, 71, 76, 90, 100, 112, 125 and 150. Maximal phosphorylation of these protein substrates occurred after 1-2 min stimulation. Zymosan and LPS stimulation of peritoneal exudate cells produced similar patterns of protein tyrosine phosphorylation. Zymosan-stimulated tyrosine phosphorylation was inhibited by tyrphostin-25 in a concentration-dependent manner between 10 and 60 microM, demonstrating a similar concentration response between effects on tyrosine phosphorylation and eicosanoid biosynthesis in the murine peritoneal macrophage. The use of selective PTK inhibitors suggests a common role for PTK and tyrosine phosphorylation in eicosanoid biosynthesis in the murine peritoneal macrophage.

Tyrosine phosphorylation is required for Fc receptor-mediated phagocytosis in mouse macrophages

Although Fc receptor-mediated phagocytosis is accompanied by a variety of transmembrane signaling events, not all signaling events are required for particle ingestion. For example, Fc receptor-mediated phagocytosis in mouse inflammatory macrophages (Di Virgilio, F., B. C. Meyer, S. Greenberg, and S. C. Silverstein. 1988. J. Cell Biol. 106:657; Greenberg, S., J. El Khoury, F. Di Virgilio, and S. C. Silverstein. 1991. J. Cell Biol. 113:757) and neutrophils (Della Bianca, V., M. Grzeskowiak, and F. Rossi. 1990. J. Immunol. 144:1411) occurs in the absence of cytosolic calcium transients. We sought to identify transmembrane signaling events that are essential for phagocytosis. Here we show that tyrosine phosphorylation is an early event after Fc receptor ligation in mouse inflammatory macrophages, and that the formation of tyrosine phosphoproteins coincides temporally with the appearance of F-actin beneath phagocytic cups. The distribution of tyrosine phosphoproteins that accumulated beneath phagocytic cups was punctate and corresponded to areas of high ligand density on the surface of the antibody-coated red blood cells, which provided the phagocytic stimulus. A tyrosine kinase inhibitor, genistein, but not several inhibitors of protein kinase C, blocked the appearance of tyrosine phosphoproteins as assessed by immunofluorescence, the focal accumulation of F-actin beneath immunoglobulin G-opsonized particles, and the ingestion of these particles as well. We suggest that tyrosine phosphorylation is a critical signaling event that underlies Fc receptor-mediated phagocytosis in mouse macrophages, and is necessary for the engulfment per se.

Adhesion-dependent protein tyrosine phosphorylation in neutrophils treated with tumor necrosis factor

Human neutrophils (PMN) respond to tumor necrosis factor (TNF) by releasing their granules, reorganizing their cytoskeleton, and massively secreting hydrogen peroxide. This response is dependent on adhesion to extracellular matrix proteins and expression of CD11b/CD18 integrins (Nathan, C., S. Srimal, C. Farber, E. Sanchez, L. Kabbash, A. Asch, J. Gailit, and S. D. Wright. 1989. J. Cell Biol. 109:1341-1349). We investigated the role of tyrosine phosphorylation in the response of PMN to TNF. PMN adherent to protein-coated surfaces but not suspended PMN showed tyrosine phosphorylation of several proteins (approximately 150, approximately 115, approximately 75, and approximately 65 kD) in response to TNF. Tyrosine phosphorylation was evident 5 min after addition of TNF and lasted at least 2 h. The tyrosine kinase inhibitors K252a, genistein and ST638 suppressed tyrosine phosphorylation and blocked hydrogen peroxide production in a reversible manner at low concentrations. Tyrosine kinase inhibitors also blocked the spreading of PMN in response to TNF. Dihydrocytochalasin B did not inhibit tyrosine phosphorylation, but in its presence phosphorylation was rapidly reversed. By immunocytochemistry, the majority of tyrosine phosphoproteins were localized to focal adhesions. Thus TNF-induced tyrosine phosphorylation depends on adhesion of PMN to extracellular matrix proteins, and participates in the transduction of the signals that direct the cells to spread on a biological surface and undergo a respiratory burst.

Transmembrane signaling by the high-affinity IgE receptor on membrane preparations

Aggregating the receptor with high affinity for IgE (Fc epsilon RI) stimulates a variety of phenomena in mast cells. Previous efforts to reproduce some of these events in broken-cell preparations such as isolated membranes have had limited success, possibly because the phenomena being monitored were too distal from the initial events. One of the earliest responses is now known to be the phosphorylation of tyrosine residues on several proteins, including the beta and gamma subunits of Fc epsilon RI. We show that in cell sonicates or on partially purified membranes derived from tumor mast cells, aggregating Fc epsilon RI stimulates phosphorylation of receptor tyrosine residues. As in the intact cells, receptor-mediated phosphorylation occurs only on receptors that are themselves aggregated. Because even in the unfractionated sonicates the phosphorylation of other cellular components was not detectably enhanced, and because the evidence is against the receptor itself being a kinase, our results suggest that phosphorylation of Fc epsilon RI is one of the earliest events stimulated by the receptor--an event that can now be investigated on simpler biological preparations than previously available.

Phosphorylation/dephosphorylation of high-affinity IgE receptors: a mechanism for coupling/uncoupling a large signaling complex

Engagement of high-affinity IgE receptors leads to activation of tyrosine and serine/threonine kinases and the immediate phosphorylation of receptor beta (serine and tyrosine) and gamma (threonine and tyrosine) chains. Receptor disengagement leads to dephosphorylation of beta and gamma chains via the action of undefined phosphatases. Here we have identified five distinct polypeptides associated with the high-affinity IgE-receptor tetrameric complex, which apparently become phosphorylated and dephosphorylated in sequence with the beta and gamma chains. Like beta chain, polypeptides pp180, pp48, pp42, and pp28 are phosphorylated on serine and tyrosine, whereas pp125 is only phosphorylated on serine. The phosphorylation of each of these receptor-associated polypeptides is antigen-dose dependent and is restricted to activated receptor complexes. Furthermore the physical association between pp125 and the receptor is quantitatively affected by receptor phosphorylation and dephosphorylation, indicating a coupling-uncoupling mechanism. Finally, in vitro kinase experiments show that activated receptor complexes are also physically associated with tyrosine and serine/threonine kinases as part of a larger complex containing the phosphorylated polypeptides.

Fc epsilon RI-mediated tyrosine phosphorylation and activation of the 72-kDa protein-tyrosine kinase, PTK72, in RBL-2H3 rat tumor mast cells

In RBL-2H3 rat tumor mast cells, cross-linking the high-affinity IgE receptor Fc epsilon RI causes tyrosine phosphorylation of multiple proteins. These phosphoproteins include phospholipase C gamma 1, the beta and gamma subunits of the Fc epsilon RI, the Src family protein-tyrosine kinase Lyn, and a 72-kDa protein that coimmunoprecipitates from lysates of antigen-stimulated cells with antibody to the receptor beta subunit. We now present evidence that the 72-kDa Fc epsilon RI-associated protein is the protein-tyrosine kinase PTK72 that forms part of the antigen receptor complex in B lymphocytes. The identification is based on immunoreactivity with anti-PTK72 antiserum, chromatographic profiles on the affinity resin heparin/agarose, and one-dimensional phosphopeptide mapping studies. Enzymatic activity of the kinase is increased in anti-PTK72 immune complexes prepared from lysates of antigen-activated RBL-2H3 cells. The 72-kDa protein-tyrosine kinase is the principal substrate for in vitro tyrosine phosphorylation in anti-phosphotyrosine immunoprecipitates of RBL-2H3 cells. The discovery that RBL-2H3 mast cells share a receptor-activated protein-tyrosine kinase, PTK72, with B lymphocytes provides additional support for the existence of common signaling pathways initiated by multichain immune recognition receptors.

Fc epsilon R1-mediated tyrosine phosphorylation of multiple proteins, including phospholipase C gamma 1 and the receptor beta gamma 2 complex, in RBL-2H3 rat basophilic leukemia cells

In basophils, mast cells, and the RBL-2H3 tumor mast cell line, cross-linking the high-affinity immunoglobulin E receptor (Fc epsilon R1) stimulates a series of responses, particularly the activation of phospholipase C (PLC), that lead to allergic and other immediate hypersensitivity reactions. The mechanism of activation of PLC, however, is not clear. Here, we show that cross-linking Fc epsilon R1 on RBL-2H3 cells causes the tyrosine phosphorylation of at least 12 cellular proteins, including PLC gamma 1 (PLC gamma 1) and the receptor beta and gamma subunits. 32P-labeled PLC gamma 1 can be detected by anti-phosphotyrosine antibody as early as 10 s after the addition of antigen. The tyrosine-phosphorylated 33-kDa beta subunit and 9- to 11-kDa gamma subunit of the Fc epsilon R1 are additionally phosphorylated on serine and theonine residues, respectively, and are found as complexes with other phosphotyrosine-containing proteins in antigen-stimulated cells. Our results indicate a means by which the Fc epsilon R1 may control PLC activity in RBL-2H3 cells and raise the possibility that other receptor-mediated signalling events in mast cells may also be controlled through protein tyrosine phosphorylation.

Fc epsilon R1-mediated tyrosine phosphorylation of multiple proteins, including phospholipase C gamma 1 and the receptor beta gamma 2 complex, in RBL-2H3 rat basophilic leukemia cells

In basophils, mast cells, and the RBL-2H3 tumor mast cell line, cross-linking the high-affinity immunoglobulin E receptor (Fc epsilon R1) stimulates a series of responses, particularly the activation of phospholipase C (PLC), that lead to allergic and other immediate hypersensitivity reactions. The mechanism of activation of PLC, however, is not clear. Here, we show that cross-linking Fc epsilon R1 on RBL-2H3 cells causes the tyrosine phosphorylation of at least 12 cellular proteins, including PLC gamma 1 (PLC gamma 1) and the receptor beta and gamma subunits. 32P-labeled PLC gamma 1 can be detected by anti-phosphotyrosine antibody as early as 10 s after the addition of antigen. The tyrosine-phosphorylated 33-kDa beta subunit and 9- to 11-kDa gamma subunit of the Fc epsilon R1 are additionally phosphorylated on serine and theonine residues, respectively, and are found as complexes with other phosphotyrosine-containing proteins in antigen-stimulated cells. Our results indicate a means by which the Fc epsilon R1 may control PLC activity in RBL-2H3 cells and raise the possibility that other receptor-mediated signalling events in mast cells may also be controlled through protein tyrosine phosphorylation.

Protein-tyrosine phosphorylation: an essential component of Fc epsilon RI signaling

Mast cell and basophil activation can be achieved by antigen-mediated aggregation of cell surface Fc epsilon RI molecules. At least two signaling pathways are triggered by this activation. Both involve tyrosine phosphorylation. This aspect of Fc epsilon RI signaling is examined here in detail and its position in a complex network of post-binding events assessed.

Tyrosine phosphorylation of vav proto-oncogene product containing SH2 domain and transcription factor motifs

Activation of receptor-linked and cytoplasmic protein tyrosine kinases is crucial in the control of normal and abnormal cell growth and differentiation. Some substrates of protein tyrosine kinases such as phospholipase C gamma and ras GTPase-activating protein (GAP) contain sequences homologous to the src protein domains SH2 and SH3 (refs 3-9). The proto-oncogene vav is expressed in haematopoietic cells and its product Vav contains sequence motifs commonly found in transcription factors, such as helix-loop-helix, leucine-zipper and zinc-finger motifs and nuclear localization signals, as well as a single SH2 and two SH3 domains. Here we show that stimulation of T-cell antigen receptor on normal human peripheral blood lymphocytes or on human leukaemic T cells, and the crosslinking of IgE receptors on rat basophilic leukaemia cells, both promote the phosphorylation of tyrosine residues in Vav. Moreover, activation of the receptor for epidermal growth factor leads to marked tyrosine phosphorylation of Vav in cells transiently expressing vav, and Vav associates with the receptor through its SH2 domain. We propose that vav encodes a new class of substrates whose tyrosine phosphorylation may provide a mechanism for direct signal transduction linking receptors at the cell surface to transcriptional control.