Tyrosine kinase-dependent phosphatidylinostiol turnover and functional responses in the Fc epsilon R1 signalling pathway

Lyn Dissociation from Phosphorylated FcεRI Subunits: A New Regulatory Step in the FcεRI Signaling Cascade Revealed by Studies of FcεRI Dimer Signaling Activity

Cross-linking the heterotrimeric (αβγ2) IgE receptor, FcεRI, of mast cells activates two tyrosine kinases: Lyn, which phosphorylates β and γ subunit immunoreceptor tyrosine-based activation motifs, and Syk, which binds γ-phospho-immunoreceptor tyrosine-based activation motifs and initiates cellular responses. We studied three FcεRI-dimerizing mAbs that maintain similar dispersed distributions over the surface of RBL-2H3 mast cells but elicit very different signaling responses. Specifically, mAb H10 receptor dimers induce very little inositol 1,4,5-trisphosphate synthesis, Ca2+ mobilization, secretion, spreading, ruffling, and actin plaque assembly, whereas dimers generated with the other anti-FcεRI mAbs induce responses that are only modestly lower than that to multivalent Ag. H10 receptor dimers activate Lyn and support FcεRI β and γ subunit phosphorylation but are poor Syk activators compared with Ag and the other anti-FcεRI mAbs. H10 receptor dimers have two other distinguishing features. First, they induce stable complexes between activated Lyn and receptor subunits. Second, the predominant Lyn-binding phospho-β isoform found in mAb H10-treated cells is a less tyrosine phosphorylated, more electrophoretically mobile species than the predominant isoform in Ag-treated cells that does not coprecipitate with Lyn. These studies implicate Lyn dissociation from highly phosphorylated receptor subunits as a new regulatory step in the FcεRI signaling cascade required for Syk activation and signal progression.

Wortmannin-sensitive phosphorylation, translocation, and activation of PLCgamma1, but not PLCgamma2, in antigen-stimulated RBL-2H3 mast cells

In RBL-2H3 tumor mast cells, cross-linking the high affinity IgE receptor (FcepsilonRI) with antigen activates cytosolic tyrosine kinases and stimulates Ins(1,4,5)P3 production. Using immune complex phospholipase assays, we show that FcepsilonRI cross-linking activates both PLCgamma1 and PLCgamma2. Activation is accompanied by the increased phosphorylation of both PLCgamma isoforms on serine and tyrosine in antigen-treated cells. We also show that the two PLCgamma isoforms have distinct subcellular localizations. PLCgamma1 is primarily cytosolic in resting RBL-2H3 cells, with low levels of plasma membrane association. After antigen stimulation, PLCgamma1 translocates to the plasma membrane where it associates preferentially with membrane ruffles. In contrast, PLCgamma2 is concentrated in a perinuclear region near the Golgi and adjacent to the plasma membrane in resting cells and does not redistribute appreciably after FcepsilonRI cross-linking. The activation of PLCgamma1, but not of PLCgamma2, is blocked by wortmannin, a PI 3-kinase inhibitor previously shown to block antigen-stimulated ruffling and to inhibit Ins(1,4,5)P3 synthesis. In addition, wortmannin strongly inhibits the antigen-stimulated phosphorylation of both serine and tyrosine residues on PLCgamma1 with little inhibition of PLCgamma2 phosphorylation. Wortmannin also blocks the antigen-stimulated translocation of PLCgamma1 to the plasma membrane. Our results implicate PI 3-kinase in the phosphorylation, translocation, and activation of PLCgamma1. Although less abundant than PLCgamma2, activated PLCgamma1 may be responsible for the bulk of antigen-stimulated Ins(1,4,5)P3 production in RBL-2H3 cells.

Translocation of phospholipase C-gamma 2 induced by in vitro activation of protein tyrosine kinase activity in mast cell lysates

Aggregation of the high-affinity receptor for IgE (Fc eta RI) on the surface of intact or permeabilized rodent mast cells results in tyrosine phosphorylation of phospholipase C-gamma 1 (PLC gamma 1) and PLC gamma 2, and translocation of both isozymes to the particulate fraction. We report here that activation of resident tyrosine kinases by the addition of adenosine triphosphate (ATP), orthovanadate, and Mg2+ to rat basophilic leukemia cell (RBL) lysates induces an association of PLC gamma 2 with the Triton-insoluble particulate fraction, with a parallel increase in tyrosine phosphorylation of cellular proteins. Both PLC gamma 2 translocation and tyrosine phosphorylation are supported by millimolar Mg2+ or Mn2+ but not by Ca2+. Both tyrosine phosphorylation and PLC gamma 2 translocation are inhibited by genistein. These data suggest that in vitro activation of tyrosine kinase activity in broken cell preparations induces the formation of association between PLC gamma 2 and ligands with the Triton-insoluble fraction.

Inhibition of Fc epsilon-RI-mediated activation of rat basophilic leukemia cells by Clostridium difficile toxin B (monoglucosyltransferase)

Treatment of rat basophilic leukemia (RBL) 2H3-hm1 cells with Clostridium difficile toxin B (2 ng/ml), which reportedly depolymerizes the actin cytoskeleton, blocked [3H]serotonin release induced by 2,4-dinitrophenyl-bovine serum albumin, carbachol, mastoparan, and reduced ionophore A23187-stimulated degranulation by about 55-60%. In lysates of RBL cells, toxin B 14C-glucosylated two major and one minor protein. By using two-dimensional gel electrophoresis and immunoblotting, RhoA and Cdc42 were identified as protein substrates of toxin B. In contrast to toxin B, Clostridium botulinum transferase C3 that selectively inactivates RhoA by ADP-ribosylation did not inhibit degranulation up to a concentration of 150 microg/ml. Antigen-stimulated tyrosine phosphorylation of a 110-kDa protein was inhibited by toxin B as well as by the phosphatidylinositol 3-kinase inhibitor wortmannin. Depolymerization of the microfilament cytoskeleton of RBL cells by C. botulinum C2 toxin or cytochalasin D resulted in an increased [3H]serotonin release induced by antigen, carbachol, mastoparan, or by calcium ionophore A23187, but without affecting toxin B-induced inhibition of degranulation. The data indicate that toxin B inhibits activation of RBL cells by glucosylation of low molecular mass GTP-binding proteins of the Rho subfamily (most likely Cdc42) by a mechanism not involving the actin cytoskeleton.

Effect of Ca2+ influx on intracellular free Ca2+ responses in antigen-stimulated RBL-2H3 cells

We undertake a quantitative investigation of changes in intracellular free Ca2+ concentration ([Ca2+]i) in antigen-stimulated rat basophilic leukemia (RBL-2H3) cells, which include contributions of both Ca2+ store release and Ca2+ influx from the medium. Following Keizer and De Young (J. Keizer and G. De Young. Biophys. J. 61: 649-660, 1992), we develop a highly constrained mathematical model for [Ca2+]i oscillations in RBL-2H3 cells, which includes activation of the inositol trisphosphate receptor (IP3R) by inositol 1,4,5-trisphospate, indirect Ca2+ activation of the IP3R via Ca2+ -dependent activity of phospholipase C-gamma, slow inhibition of the IP3R by cytosolic Ca2+, refilling of Ca2+ stores by a Ca2+ -ATPase (SERCA)-type pump, and a simple representation of the dependence of plasma membrane (PM) fluxes on experimental conditions. Using this full (open cell) model, we simulate [Ca2+]i responses for protocols in which antigen concentration and external Ca2+ are manipulated and compare out calculations with experimental data. In protocol A, cells are stimulated in the presence of external Ca2+, in protocols B and C, cells are stimulated in the absence of external Ca2+, with external Ca2+ later reapplied in protocol C. We are able to reproduce quantitatively the important features of all three protocols, including the dose response of protocol B, the [Ca2+]i response to thapsigargin, and lag time results, and we provide qualitative explanations for the responses derived from our calculations. We also develop a simplified (closed cell) version of the model in which PM fluxes are neglected and total free Ca2+ concentration ([Ca2+]T) is a slowly varying parameter. This permits us to explain in a simple graphical fashion how PM fluxes may influence [Ca2+]i responses in RBH-2H3 cells through modulation of [Ca2+]T.

Wortmannin blocks lipid and protein kinase activities associated with PI 3-kinase and inhibits a subset of responses induced by Fc epsilon R1 cross-linking

We have investigated the effects of wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase), on antigen-mediated signaling in the RBL-2H3 mast cell model. In RBL-2H3 cells, the cross-linking of high affinity IgE receptors (Fc epsilon R1) activates at least two cytoplasmic protein tyrosine kinases, Lyn and Syk, and stimulates secretion, membrane ruffling, spreading, pinocytosis, and the formation of actin plaques implicated in increased cell-substrate adhesion. In addition, Fc epsilon R1 cross-linking activates PI 3-kinase. It was previously shown that wortmannin causes a dose-dependent inhibition of PI 3-kinase activity and also inhibits antigen-stimulated degranulation. We report that the antigen-induced synthesis of inositol(1,4,5)P3 is also markedly inhibited by wortmannin. Consistent with evidence in other cell systems implicating phosphatidylinositol(3,4,5)P3 in ruffling, pretreatment of RBL-2H3 cells with wortmannin inhibits membrane ruffling and fluid pinocytosis in response to Fc epsilon R1 cross-linking. However, wortmannin does not inhibit antigen-induced actin polymerization, receptor internalization, or the actin-dependent processes of spreading and adhesion plaque formation that follow antigen stimulation in adherent cells. Wortmannin also fails to inhibit either of the Fc epsilon R1-coupled tyrosine kinases, Lyn or Syk, or the activation of mitogen-activated protein kinase as measured by in vitro kinase assays. Strikingly, there is substantial in vitro serine/threonine kinase activity in immunoprecipitates prepared from Fc epsilon R1-activated cells using antisera to the p85 subunit of PI 3-kinase. This activity is inhibited by pretreatment of the cells with wortmannin or by the direct addition of wortmannin to the kinase assay, suggesting that PI 3-kinase itself is capable of acting as a protein kinase. We conclude that Fc epsilon R1 cross-linking activates both lipid and protein kinase activities of PI 3-kinase and that inhibiting these activities with wortmannin results in the selective block of a subset of Fc epsilon R1-mediated signaling responses.

Roles for Ca2+ stores release and two Ca2+ influx pathways in the Fc epsilon R1-activated Ca2+ responses of RBL-2H3 mast cells

Cross-linking the high affinity IgE receptor, Fc epsilon R1, with multivalent antigen induces inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-dependent release of intracellular Ca2+ stores, Ca2+ influx, and secretion of inflammatory mediators from RBL-2H3 mast cells. Here, fluorescence ratio imaging microscopy was used to characterize the antigen-induced Ca2+ responses of single fura-2-loaded RBL-2H3 cells in the presence and absence of extracellular Ca2+ (Ca2+o). As antigen concentration increases toward the optimum for secretion, more cells show a Ca2+ spike or an abrupt increase in [Ca2+]i and the lag time to onset of the response decreases both in the presence and the absence of Ca2+o. When Ca2+o is absent, fewer cells respond to low antigen and the lag times to response are longer than those measured in the presence of Ca2+o, indicating that Ca2+o contributes to Ca2+ stores release. Ins(1,4,5)P3 production is not impaired by the removal of Ca2+o, suggesting that extracellular Ca2+ influences Ca2+ stores release via an effect on the Ins(1,4,5)P3 receptor. Stimulation with low concentrations of antigen can lead, only in the presence of Ca2+o, to a small, gradual increase in [Ca2+]i before the abrupt spike response that indicates store release. We propose that this small, initial [Ca2+]i increase is due to receptor-activated Ca2+ influx that precedes and may facilitate Ca2+ stores release. A mechanism for capacitative Ca2+ entry also exists in RBL-2H3 cells. Our data suggest that a previously undescribed response to Fc epsilon R1 cross-linking, inhibition of Ca2+ stores refilling, may be involved in activating capacitative Ca2+ entry in antigen-stimulated RBL-2H3 cells, thus providing the elevated [Ca2+]i required for optimal secretion. The existence of both capacitative entry and Ca2+ influx that can precede Ca2+ release from intracellular stores suggests that at least two mechanisms of stimulated Ca2+ influx are present in RBL-2H3 cells.

Distinct functions of the Fc epsilon R1 gamma and beta subunits in the control of Fc epsilon R1-mediated tyrosine kinase activation and signaling responses in RBL-2H3 mast cells

In RBL-2H3 rat tumor mast cells, cross-linking the high affinity IgE receptor, Fc epsilon R1, activates the protein-tyrosine kinases Lyn and Syk and initiates a series of responses including protein-tyrosine phosphorylation, inositol 1,4,5-trisphosphate synthesis, Ca2+ mobilization, secretion, membrane ruffling, and actin plaque assembly. The development of chimeric receptors containing cytoplasmic domains of individual subunits of the heterotrimeric (alpha beta gamma 2) Fc epsilon R1 has simplified analyses of early signaling events in RBL-2H3 cells. Here, RBL-2H3 cells were transfected with cDNAs encoding the extracellular and transmembrane domains of the interleukin-2 receptor alpha subunit (the Tac antigen) joined to the C-terminal cytoplasmic domains of the Fc epsilon R1 gamma and beta subunits (TT gamma and TT beta). Both sequences contain tyrosine activation motifs implicated in antigen receptor signal transduction. TT gamma and TT beta are expressed independently of the native Fc epsilon R1, as demonstrated by the ability of Tac cross-linking agents to trigger the clustering and internalization through coated pits of both chimeric receptors without co-clustering the Fc epsilon R1. A full range of signaling activities is induced by TT gamma cross-linking; the TT gamma-induced responses are slower and, except for Lyn activation, smaller than the Fc epsilon R1-induced responses. In striking contrast, TT beta cross-linking elicits no tyrosine phosphorylation or signaling responses, it impairs basal activities measured in secretion and anti-PY (anti-phosphotyrosine antibody) immune complex kinase assays, and it antagonizes Fc epsilon R1-induced Lyn and Syk activation, protein-tyrosine phosphorylation, and signaling responses. We hypothesize that the isolated beta subunit binds a specific kinase or coupling protein(s) required for signaling activity, sequestering it from the signal-transducing gamma subunit. Binding the same kinase or coupling protein to the beta subunit of the intact Fc epsilon R1 may serve instead to present it to the adjacent gamma subunit, resulting in enhanced kinase activation and signaling responses.

Collagen type IV stimulates an increase in intracellular Ca2+ in pancreatic acinar cells via activation of phospholipase C

Intracellular Ca2+ responses to extracellular matrix molecules were studied in suspensions of pancreatic acinar cells loaded with Fura-2. Collagen type I, laminin, fibrinogen and fibronectin were unable to raise cytosolic free Ca2+ concentration ([Ca2+]i), whereas collagen type IV, at concentrations from 5 to 50 micrograms/ml, significantly increased it. The effect of collagen type IV was not due to possible contamination with type-I transforming growth factor beta or plasminogen, as neither of these agents was able to increase [Ca2+]i. Using highly specific mass assays, concentrations of inositol lipids, 1,2-diacylglycerol (DAG) and Ins(1,4,5) P3 were measured in pancreatic acinar cells stimulated with collagen type IV. A decrease in the concentrations of PtdIns(4,5) P2 and PtdIns4 P with a concomitant increase in the concentrations of DAG and InsP3 mass were observed, showing that collagen type IV increases [Ca2+]i by activation of phospholipase C. The observed [Ca2+]i signals had two components, the first resulting from Ca2+ release from the intracellular stores, and the second resulting from Ca2+ flux from the extracellular medium through the verapamil-insensitive channels. A tyrosine kinase inhibitor (tyrphostine) was able to block inositol lipid signalling caused by collagen type IV, which together with the insensitivity of this pathway to cholera toxin and pertussis toxin or to preactivation of protein kinase C, the longer duration of the increase in [Ca2+]i and a longer lag period needed for observation of increases in DAG and InsP3 concentration with collagen type IV than with carbachol (50 mM) suggest that activation of phospholipase C by collagen type IV is caused by tyrosine kinase activation. Inositol lipid signalling and increases in [Ca2+]i were also observed with Arg-Gly-Asp (RGD)-containing peptide but not with Arg-Asp-Gly (RDG)-containing peptide. Collagen type IV and RGD-containing peptide, but not carbachol, competed in increasing [Ca2+]i and DAG concentration, suggesting that the binding site of collagen type IV responsible for phospholipase C activation contains the RGD sequence. Together the present results suggest that, in pancreatic acinar cells, RGD sequence(s) within collagen type IV molecules cause activation of tyrosine kinase, probably through one of the integrin receptors, which then stimulates phospholipase C and increases [Ca2+]i.

7,12-Dimethylbenz[a]anthracene activates protein-tyrosine kinases Fyn and Lck in the HPB-ALL human T-cell line and increases tyrosine phosphorylation of phospholipase C-gamma 1, formation of inositol 1,4,5-trisphosphate, and mobilization of intracellular calcium

Previous studies have shown that the immunosuppressive and carcinogenic polycyclic aromatic hydrocarbon 7,12-dimethylbenz(a)anthracene (DMBA) impairs Ca(2+)-dependent transmembrane signaling in human and murine lymphocytes. The purpose of the present studies was to analyze potential mechanisms of immunosuppression by DMBA and to examine effects on Ca2+ homeostasis and antigen-receptor signaling in human T cells. DMBA produced a rapid and sustained increase in Ca2+ levels in HPB-ALL cells by release of cytoplasmic Ca2+. DMBA also inhibited anti-CD3/CD4 mobilization of Ca2+ in HPB-ALL cells, with half-maximal inhibition occurring at approximately 4 hr. Thus, the kinetics for initial Ca2+ mobilization and inhibition of the anti-CD3/CD4 response differed. The rapid rise in intracellular Ca2+ induced by DMBA alone was accompanied by a rapid but transient increase in inositol 1,4,5-trisphosphate and tyrosine phosphorylation of phospholipase C-gamma 1. The pattern of tyrosine phosphorylation induced by DMBA in HPB-ALL cells was remarkably similar to that induced by anti-CD3/CD4 activation. Thus, DMBA-induced phosphorylation may mimic antigen-receptor activation in T cells, which may lead to alterations in antigen responsiveness. The mechanism of DMBA-induced tyrosine phosphorylation of phospholipase C-gamma 1 may have been due to an increase in protein-tyrosine kinase activity, since it was found that DMBA produced a > 2-fold increase in the activity of the T-cell receptor-associated Src-family kinases Fyn and Lck. The kinetics of activation of protein-tyrosine kinases demonstrated that Fyn activity was increased within 10 min of exposure to DMBA, whereas maximal Lck activation required 30 min. Thus, it is likely that the Fyn kinase or other protein-tyrosine kinases may be responsible for the early tyrosine phosphorylation of phospholipase C-gamma 1, which results in inositol 1,4,5-trisphosphate release and mobilization of intracellular Ca2+.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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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.