Positive and negative regulation of mast cell activation by Lyn via the FcepsilonRI

Phospholipase C-β in immune cells

Great progress has recently been made in structural and functional research of phospholipase C (PLC)-β. We now understand how PLC-β isoforms (β1-β4) are activated by GTP-bound Gαq downstream of G protein-coupled receptors. Numerous studies indicate that PLC-βs participate in the differentiation and activation of immune cells that control both the innate and adaptive immune systems. The PLC-β3 isoform also interplays with tyrosine kinase-based signaling pathways, to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1, with which PLC-β3 and Stat5 form a multi-molecular signaling platform, named SPS complex. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation.

Immunoglobulin E induces VEGF production in mast cells and potentiates their pro-tumorigenic actions through a Fyn kinase-dependent mechanism

Background

High concentrations of plasmatic IgE have been related to distinct systemic inflammatory conditions that frequently predispose individuals to hypersensitivity reactions. Although effects of IgE have been suggested to relay on the low-intensity activation of distinct effector elements of the immune system, such as mast cells (MC), experimental evidence on the role of IgE-induced production of inflammatory mediators on specific pathologies is scarce. MC are an important component in tumor microenvironment where they seem to secrete a number of immunomodulatory and angiogenic mediators, such as the Vascular Endothelial Growth Factor (VEGF) by not well-described mechanisms. In this work, we investigated the effect of monomeric IgE (in the absence of antigen) on the production of VEGF in MC, analyzed if monomeric IgE could exacerbate the pro-tumorigenic properties of that cell type and characterized some of the molecular mechanisms behind the effects of IgE on VEGF production and tumor growth.

Methods

For in vitro studies, murine bone marrow-derived mast cells (BMMCs) were used. Pharmacological inhibitors and phosphorylation of key elements controlling VEGF secretion and protein translation were used to characterize the mechanism of VEGF production triggered by IgE.

In vivo, the effect of a single i.v. administration of monomeric IgE on B16 melanoma tumor weight, intratumoral blood vessel formation and tumor-associated MC was assessed in four groups of mice: MC-proficient (WT), MC-deficient (Wsh), Wsh reconstituted with MC derived from WT mice (Wsh Rec WT) and Wsh reconstituted with MC derived from Fyn −/− mice (Wsh Rec Fyn −/−).

Results

Monomeric IgE induced VEGF secretion through a Fyn kinase-dependent mechanism and modulated de novo protein synthesis modifying the activity of the translational regulator 4E-BP1 in BMMCs. In vivo, monomeric IgE increased melanoma tumor growth, peritumoral MC and blood vessel numbers in WT but not in Wsh mice. The positive effects of IgE on melanoma tumor growth were reproduced after reconstitution of Wsh mice with WT but not with Fyn −/− BMMCs.

Conclusion

Our data suggest that monomeric IgE, in the absence of antigen, induces VEGF production in MC and in vivo contributes to melanoma tumor growth through a Fyn kinase-dependent mechanism.

Hydrogen inhalation ameliorated mast cell-mediated brain injury after intracerebral hemorrhage in mice

OBJECTIVE

Hydrogen inhalation was neuroprotective in several brain injury models. Its mechanisms are believed to be related to antioxidative stress. We investigated the potential neurovascular protective effect of hydrogen inhalation especially effect on mast cell activation in a mouse model of intracerebral hemorrhage.

DESIGN

Controlled in vivo laboratory study.

SETTING

Animal research laboratory.

SUBJECTS

One hundred seventy-one 8-week-old male CD-1 mice were used.

INTERVENTIONS

Collagenase-induced intracerebral hemorrhage model in 8-week-old male CD-1 mice was used. Hydrogen was administrated via spontaneous inhalation. The blood-brain barrier permeability and neurologic deficits were investigated at 24 and 72 hours after intracerebral hemorrhage. Mast cell activation was evaluated by Western blot and immuno-staining. The effects of hydrogen inhalation on mast cell activation were confirmed in an autologous blood injection model intracerebral hemorrhage.

MEASUREMENT AND MAIN RESULTS

At 24 and 72 hours post intracerebral hemorrhage, animals showed blood-brain barrier disruption, brain edema, and neurologic deficits, accompanied with phosphorylation of Lyn kinase and release of tryptase, indicating mast cell activation. Hydrogen treatment diminished phosphorylation of Lyn kinase and release of tryptase, decreased accumulation and degranulation of mast cells, attenuated blood-brain barrier disruption, and improved neurobehavioral function.

CONCLUSION

Activation of mast cells following intracerebral hemorrhage contributed to increase of blood-brain barrier permeability and brain edema. Hydrogen inhalation preserved blood-brain barrier disruption by prevention of mast cell activation after intracerebral hemorrhage.

Single-cell measurements of IgE-mediated FcεRI signaling using an integrated microfluidic platform

Heterogeneity in responses of cells to a stimulus, such as a pathogen or allergen, can potentially play an important role in deciding the fate of the responding cell population and the overall systemic response. Measuring heterogeneous responses requires tools capable of interrogating individual cells. Cell signaling studies commonly do not have single-cell resolution because of the limitations of techniques used such as Westerns, ELISAs, mass spectrometry, and DNA microarrays. Microfluidics devices are increasingly being used to overcome these limitations. Here, we report on a microfluidic platform for cell signaling analysis that combines two orthogonal single-cell measurement technologies: on-chip flow cytometry and optical imaging. The device seamlessly integrates cell culture, stimulation, and preparation with downstream measurements permitting hands-free, automated analysis to minimize experimental variability. The platform was used to interrogate IgE receptor (FcεRI) signaling, which is responsible for triggering allergic reactions, in RBL-2H3 cells. Following on-chip crosslinking of IgE-FcεRI complexes by multivalent antigen, we monitored signaling events including protein phosphorylation, calcium mobilization and the release of inflammatory mediators. The results demonstrate the ability of our platform to produce quantitative measurements on a cell-by-cell basis from just a few hundred cells. Model-based analysis of the Syk phosphorylation data suggests that heterogeneity in Syk phosphorylation can be attributed to protein copy number variations, with the level of Syk phosphorylation being particularly sensitive to the copy number of Lyn.

Activation/Inhibition of mast cells by supra-optimal antigen concentrations

Mast cells (MCs) are tissue resident cells of hemopoietic origin and are critically involved in allergic diseases. MCs bind IgE by means of their high-affinity receptor for IgE (FcεRI). The FcεRI belongs to a family of multi-chain immune recognition receptors and is activated by cross-linking in response to multivalent antigens (Ags)/allergens. Activation of the FcεRI results in immediate release of preformed granular substances (e.g. histamine, heparin, and proteases), generation of arachidonic acid metabolites, and production of pro-inflammatory cytokines. The FcεRI shows a remarkable, bell-shaped dose-response behavior with weak induction of effector responses at both low and high (so-called supra-optimal) Ag concentrations. This is significantly different from many other receptors, which reach a plateau phase in response to high ligand concentrations. To explain this unusual dose-response behavior of the FcεRI, scientists in the past have drawn parallels to so-called precipitin curves resulting from titration of Ag against a fixed concentration of antibody (Ab) in solution (a.k.a. Heidelberger curves). Thus, for high, supra-optimal Ag concentrations one could assume that every IgE-bound FcεRI formed a monovalent complex with “its own Ag”, thus resulting in marginal induction of effector functions due to absence of receptor cross-linking. However, this was never proven to be the case. More recently, careful studies of FcεRI activation and signaling events in MCs in response to supra-optimal Ag concentrations have suggested a molecular explanation for the descending part of this bell-shaped curve. It is obvious now that extensive FcεRI/IgE/Ag clusters are formed and inhibitory molecules and signalosomes are engaged in response to supra-optimal cross-linking (amongst them the Src family kinase Lyn and the inositol-5′-phosphatase SHIP1) and they actively down-regulate MC effector responses. Thus, the analysis of MC signaling triggered by supra-optimal crosslinking holds great potential for identifying novel targets for pharmacologic therapeutic intervention to benefit patients with acute and chronic allergic diseases.

Nonredundant roles of Src-family kinases and Syk in the initiation of B-cell antigen receptor signaling

When a BCR on a mature B cell is engaged by its ligand, the cell becomes activated, and the Ab-mediated immune response can be triggered. The initiation of BCR signaling is orchestrated by kinases of the Src and Syk families. However, the proximal BCR-induced phosphorylation remains incompletely understood. According to a model of sequential activation of kinases, Syk acts downstream of Src family kinases (SFKs). In addition, signaling independent of SFKs and initiated by Syk has been proposed. Both hypotheses lack sufficient evidence from relevant B cell models, mainly because of the redundancy of Src family members and the importance of BCR signaling for B cell development. We addressed this issue by analyzing controlled BCR triggering ex vivo on primary murine B cells and on murine and chicken B cell lines. Chemical and Csk-based genetic inhibitor treatments revealed that SFKs are required for signal initiation and Syk activation. In addition, ligand and anti-BCR Ab-induced signaling differ in their sensitivity to the inhibition of SFKs.

The interaction between Lyn and FcεRIβ is indispensable for FcεRI-mediated human mast cell activation

BACKGROUND

FcεRIβ reportedly functions as an amplifier of the FcεRIγ-mediated activation signal using a reconstitution system. However, the amplification mechanisms in human mast cells (MCs) are poorly understood. We previously reported the hyperexpression of FcεRIβ of MCs in giant papillae from vernal keratoconjunctivitis patients, compared with that in conjunctivae from nonallergic conjunctivitis patients. Elucidation of the molecular mechanisms of the amplification induced by FcεRIβ should provide new targets for novel therapeutic interventions. The aim is to understand in greater details the function of FcεRIβ in human MC FcεRI expression and signaling.

METHODS

FcεRIβ and Lyn expression was reduced using a lentiviral shRNA silencing technique. Localization of Lyn and FcεRIβ in cultured MCs was examined by confocal microscopic analysis. Mediators were measured by ELISAs.

RESULTS

The diminution of FcεRIβ significantly downregulated cell surface FcεRI expression and FcεRI-mediated mediator release/production. The downregulation of FcεRI-mediated degranulation was not only due to the decrease in FcεRI expression. The diminution of FcεRIβ inhibited the redistribution of Lyn within the cell membrane following IgE sensitization. The diminution of Lyn in MCs significantly downregulated FcεRI-mediated degranulation. The recombinant cell-penetrating forms of phosphorylated FcεRIβ immunoreceptor tyrosine-based activation motif (ITAM) for intracellular delivery disturbed the interaction between Lyn and phosphorylated endogenous FcεRIβ ITAM, resulted in inhibiting IgE-dependent histamine release from MCs in vitro and from giant papillae specimens ex vivo.

CONCLUSION

The interaction between Lyn and FcεRIβ is indispensable for FcεRI-mediated human MC activation, and specific inhibition of the interaction may represent a new therapeutic strategy for the treatment of human allergic diseases.

Fine-Tuning of Mast Cell Activation by FcεRIβ Chain

Mast cells play a key role in allergic reaction and disorders through the high affinity receptor for IgE (Fc_εRI) which is primarily activated by IgE and antigen complex. In humans, mast cells express two types of Fc_εRI on the cell surface, tetrameric αβγ₂ and trimeric αγ₂, whereas in mice, the tetrameric αβγ₂ type is exclusively expressed. In human allergic inflammation lesions, mast cells increase in number and preferentially express the αβγ₂ type Fc_εRI. By contrast, in the lesion of non-allergic inflammation, mast cells mainly express the αγ₂type. Since the β chain amplifies the expression and signaling of FcεRI, mast cell effector functions and allergic reaction in vivo are enhanced in the presence of the β chain. In contrast, a truncated β chain-isoform (βT) inhibits FcεRI surface expression. The human Fc_εRIβ gene contains seven exons and a repressor element located in the forth intron, through which Fc_εRIβ transcription is repressed in the presence of GM-CSF. Regarding the additional signal regulatory function of the β chain, the β chain ITAM has dual (positive and negative) functions in the regulation of the mast cell activation. Namely, the Fc_εRIβ chain ITAM enhances the mast cell activation signal triggered by a low-intensity (weak) stimulation whereas it suppresses the signal triggered by high-intensity (strong) stimulation. In an oxazolone-induced mouse CHS model, IgE-mediated mast cell activation is required and the β chain ITAM is crucially involved. Adenosine receptor, one of the GPCRs, triggers a synergistic degranulation response with FcεRI in mast cells, for which the β chain ITAM critically plays positive role, possibly reflecting the in vivo allergic response. These regulatory functions of the FcεRIβ ITAM finely tune FcεRI-induced mast cell activation depending on the stimulation strength, enabling the Fc_εRIβ chain to become a potential molecular target for the development of new strategies for therapeutic interventions for allergies.

The Fyn-STAT5 Pathway: A New Frontier in IgE- and IgG-Mediated Mast Cell Signaling

Mast cells are central players in immune surveillance and activation, positioned at the host–environment interface. Understanding the signaling events controlling mast cell function, especially those that maintain host homeostasis, is an important and still less understood area of mast cell-mediated disease. With respect to allergic disease, it is well established that IgE and its high affinity receptor FcεRI are major mediators of mast cell activation. However, IgG-mediated signals can also modulate mast cell activities. Signals elicited by IgG binding to its cognate receptors (FcγR) are the basis for autoimmune disorders such as lupus and rheumatoid arthritis. Using knowledge of IgE-mediated mast cell signaling, recent work has begun to illuminate potential overlap between FcεRI and FcγR signal transduction. Herein we review the importance of Src family kinases in FcεRI and FcγR signaling, the role of the transcription factor STAT5, and impingement of the regulatory cytokines IL-4, IL-10, and TGFβ1 upon this network.

Lyn but not Fyn kinase controls IgG-mediated systemic anaphylaxis

Anaphylaxis is a rapid, life-threatening hypersensitivity reaction. Until recently, it was mainly attributed to histamine released by mast cells activated by allergen crosslinking (XL) of FcεRI-bound allergen-specific IgE. However, recent reports established that anaphylaxis could also be triggered by basophil, macrophage, and neutrophil secretion of platelet-activating factor subsequent to FcγR stimulation by IgG/Ag complexes. We have investigated the contribution of Fyn and Lyn tyrosine kinases to FcγRIIb and FcγRIII signaling in the context of IgG-mediated passive systemic anaphylaxis (PSA). We found that mast cell IgG XL induced Fyn, Lyn, Akt, Erk, p38, and JNK phosphorylation. Additionally, IgG XL of mast cells, basophils, and macrophages resulted in Fyn- and Lyn-regulated mediator release in vitro. FcγR-mediated activation was enhanced in Lyn-deficient (knockout [KO]) cells, but decreased in Fyn KO cells, compared with wild-type cells. More importantly, Lyn KO mice displayed significantly exacerbated PSA features whereas no change was observed for Fyn KO mice, compared with wild-type littermates. Intriguingly, we establish that mast cells account for most serum histamine in IgG-induced PSA. Taken together, our findings establish pivotal roles for Fyn and Lyn in the regulation of PSA and highlight their unsuspected functions in IgG-mediated pathologies.

Glucocorticosteroids rescue basophils from dasatinib-augmented immunoglobulin E-mediated histamine release

BACKGROUND

Dasatinib is a multikinase inhibitor active against several tyrosine kinases including ABL, KIT, Lyn and Btk. Apart from its known antileukemic activity, the drug produces several side effects including edemas and pleural effusions, which are supposedly triggered by activated immune cells. Effusion formation can be treated effectively by glucocorticosteroids. We have recently shown that low concentrations of dasatinib (<0.1 µM) promote IgE-dependent secretion of histamine in basophils, especially in allergic individuals. In the current study, we asked whether glucocorticosteroids inhibit dasatinib-induced activation of basophils.

METHODS

Basophils were preincubated with dexamethasone, prednisolone and hydrocortisone for 24 h, and were then exposed to an anti-IgE antibody (normal basophils) or the allergens Bet v 1 and Phl p 5 (allergic patients) with or without low concentrations of dasatinib (0.025 µM). After incubation, basophils were examined for histamine release and expression of CD63 and CD203c.

RESULTS

All three glucocorticosteroids were found to counteract IgE-dependent and dasatinib-enhanced histamine release in basophils in nonallergic and allergic individuals. In addition, glucocorticosteroids were found to inhibit anti-IgE-induced upregulation of CD63 and CD203c in the presence or absence of dasatinib. The inhibitory effects of glucocorticosteroids were dose-dependent (effective range: 1-10 µM) and seen in all donors examined.

CONCLUSIONS

Glucocorticosteroids rescue IgE receptor cross-linked basophils from additional costimulatory effects of low-dose dasatinib which may have clinical implications in dasatinib-treated patients.

Lyn kinase controls TLR4-dependent IKK and MAPK activation modulating the activity of TRAF-6/TAK-1 protein complex in mast cells

Mast cells (MCs) control allergic reactions and contribute to protective innate immune responses through TLR4 activation. The tyrosine kinase Lyn is important to the high affinity IgE receptor (FcεRI) signal transduction system in MCs, but its role on the TLR4 signalling cascade is still elusive. Here, we characterized several TLR4-triggered responses in bone marrow-derived mast cells (BMMCs) from wild-type (WT) and Lyn(-/-) mice. We found that Lyn(-/-) MCs secreted lower amounts of TNF-α after LPS challenge when compared with WT cells. Lyn(-/-) BMMCs showed less MAPK, IκB phosphorylation and NF-κB nuclear translocation after TLR-4 triggering than WT cells. LPS-induced MAPK and inhibitor of IκB kinase (IKK) phosphorylation were importantly reduced in the absence of Lyn. A constitutive interaction between TNF receptor associated factor 6 (TRAF-6) and phosphorylated TGF-β-activated kinase (TAK-1) was observed in Lyn(-/-) BMMCs and this complex was insensitive to LPS addition. Lyn kinase was activated and associated to TRAF-6 shortly after LPS addition in WT MCs. Analyzing two local MC-dependent innate immune responses in vivo, we found that Lyn positively controls early TNF-α production and immune cell recruitment after an intraperitoneal injection of LPS. Our results indicate that Lyn plays a positive role in TLR4-induced production of TNF-α in MCs controlling the activity of the TRAF-6/TAK-1 protein complex.

KIT-D816V–independent oncogenic signaling in neoplastic cells in systemic mastocytosis: role of Lyn and Btk activation and disruption by dasatinib and bosutinib

Systemic mastocytosis (SM) either presents as a malignant neoplasm with short survival or as an indolent disease with normal life expectancy. In both instances, neoplastic mast cells (MCs) harbor D816V-mutated KIT, suggesting that additional oncogenic mechanisms are involved in malignant transformation. We here describe that Lyn and Btk are phosphorylated in a KIT-independent manner in neoplastic MCs in advanced SM and in the MC leukemia cell line HMC-1. Lyn and Btk activation was not only detected in KIT D816V-positive HMC-1.2 cells, but also in the KIT D816V-negative HMC-1.1 subclone. Moreover, KIT D816V did not induce Lyn/Btk activation in Ba/F3 cells, and deactivation of KIT D816V by midostaurin did not alter Lyn/Btk activation. siRNAs against Btk and Lyn were found to block survival in neoplastic MCs and to cooperate with midostaurin in producing growth inhibition. Growth inhibitory effects were also obtained with 2 targeted drugs, dasatinib which blocks KIT, Lyn, and Btk activation in MCs, and bosutinib, a drug that deactivates Lyn and Btk without blocking KIT activity. Together, KIT-independent signaling via Lyn/Btk contributes to growth of neoplastic MCs in advanced SM. Dasatinib and bosutinib disrupt Lyn/Btk-driven oncogenic signaling in neoplastic MC, which may have clinical implications and explain synergistic drug interactions.

Immunoglobulin E receptor signaling and asthma

Elevated IgE levels and increased IgE sensitization to allergens are central features of allergic asthma. IgE binds to the high-affinity Fcε receptor I (FcεRI) on mast cells, basophils, and dendritic cells and mediates the activation of these cells upon antigen-induced cross-linking of IgE-bound FcεRI. FcεRI activation proceeds through a network of signaling molecules and adaptor proteins and is negatively regulated by a number of cell surface and intracellular proteins. Therapeutic neutralization of serum IgE in moderate-to-severe allergic asthmatics reduces the frequency of asthma exacerbations through a reduction in cell surface FcεRI expression that results in decreased FcεRI activation, leading to improved asthma control. Our increasing understanding of IgE receptor signaling may lead to the development of novel therapeutics for the treatment of asthma.

The Src family kinase Fgr is critical for activation of mast cells and IgE-mediated anaphylaxis in mice

Mast cells are critical for various allergic disorders. Mast cells express Src family kinases, which relay positive and negative regulatory signals by Ag. Lyn, for example, initiates activating signaling events, but it also induces inhibitory signals. Fyn and Hck are reported to be positive regulators, but little is known about the roles of other Src kinases, including Fgr, in mast cells. In this study, we define the role of Fgr. Endogenous Fgr associates with FcεRI and promotes phosphorylation of Syk, Syk substrates, which include linkers for activation of T cells, SLP76, and Gab2, and downstream targets such as Akt and the MAPKs in Ag-stimulated mast cells. As a consequence, Fgr positively regulates degranulation, production of eicosanoids, and cytokines. Fgr and Fyn appeared to act in concert, as phosphorylation of Syk and degranulation are enhanced by overexpression of Fgr and further augmented by overexpression of Fyn but are suppressed by overexpression of Lyn. Moreover, knockdown of Fgr by small interfering RNAs (siRNAs) further suppressed degranulation in Fyn-deficient bone marrow-derived mast cells. Overexpression of Fyn or Fgr restored phosphorylation of Syk and partially restored degranulation in Fyn-deficient cells. Additionally, knockdown of Fgr by siRNAs inhibited association of Syk with FcεRIγ as well as the tyrosine phosphorylation of FcεRIγ. Of note, the injection of Fgr siRNAs diminished the protein level of Fgr in mice and simultaneously inhibited IgE-mediated anaphylaxis. In conclusion, Fgr positively regulates mast cell through activation of Syk. These findings help clarify the interplay among Src family kinases and identify Fgr as a potential therapeutic target for allergic diseases.

Phospholipase C-β3 regulates FcɛRI-mediated mast cell activation by recruiting the protein phosphatase SHP-1

Mast cells are major effectors in high-affinity IgE receptor (FcɛRI)-dependent allergic reactions. Here we show that phospholipase C (PLC)-β3 is crucial for FcɛRI-mediated mast cell activation. Plcb3(-/-) mice showed blunted FcɛRI-dependent late-phase, but not acute, anaphylactic responses and airway inflammation. Accordingly, FcɛRI stimulation of Plcb3(-/-) mast cells exhibited reduced cytokine production but normal degranulation. Reduced cytokine production in Plcb3(-/-) cells could be accounted for by increased activity of the negative regulatory Src family kinase Lyn and reduced activities of the positive regulatory protein kinases MAPKs. Mechanistically, PLC-β3 constitutively interacts with FcɛRI, Lyn, and SHP-1 (protein phosphatase). SHP-1 probably recognizes its substrates Lyn and MAPKs via the recently described kinase tyrosine-based inhibitory motif, KTIM. Consistent with PLC-β3- and SHP-1-mediated repression of Lyn activity by dephosphorylation at Tyr396, FcɛRI-mediated phenotypes were similar in Plcb3(-/-) and SHP-1 mutant mast cells. Thus, we have defined a PLC-β3- and SHP-1-mediated signaling pathway for FcɛRI-mediated cytokine production.

The PH domain adaptor protein Bam32/DAPP1 functions in mast cells to restrain FcɛRI-induced calcium flux and granule release

Mast cell activation triggered by IgE binding to its high affinity receptor FcɛRI is highly dependent on signaling via phosphoinositde 3-kinases (PI3K). The phosphoinositide phosphatase SHIP controls mast cell activation by regulating accumulation of D3 phosphoinositide second messengers generated by PI3K. The PH domain adaptor protein Bam32/DAPP1 binds specifically to the D3 phosphoinositides PI(3,4,5)P3 and PI(3,4)P2 (the substrate and product of SHIP respectively). In B cells, Bam32 is phosphorylated by Src family kinases including Lyn, and is required for antigen receptor-induced activation; however the function of Bam32 in mast cells is unknown. Here we report that Bam32 is expressed in mast cells, is recruited to the plasma membrane upon stimulation and functions in FcɛRI signaling. Examination of bone marrow-derived mast cells (BMMC) isolated from Bam32-deficient mice revealed enhanced FcɛRI-induced degranulation and IL-6 production, indicating that Bam32 may function to restrain signaling via FcɛRI. These enhanced degranulation responses were PI3K-dependent, as indicated by blockade with PI3K inhibitors wortmannin or IC87114. While Bam32-deficient BMMC showed reduced FcɛRI-induced activation of mitogen-activated protein kinases ERK and JNK, FcɛRI-induced calcium flux and phosphorylation of PLCγ1 and Akt were increased. Bam32-deficient BMMC showed significantly reduced phosphorylation of Lyn and SHIP, indicating reduced activity of inhibitory signaling pathways. Together our results identify Bam32 as a novel regulator of mast cell activation, potentially functioning in membrane-proximal integration of positive and negative signaling pathways.

PTPalpha activates Lyn and Fyn and suppresses Hck to negatively regulate FcepsilonRI-dependent mast cell activation and allergic responses

Mast cell activation via FcεRI involves activation of the Src family kinases (SFKs) Lyn, Fyn, and Hck that positively or, in the case of Lyn, negatively regulate cellular responses. Little is known of upstream activators of these SFKs in FcεRI-dependent signaling. We investigated the role of receptor protein tyrosine phosphatase (PTP)α, a well-known activator of SFKs in diverse signaling systems, FcεRI-mediated mast cell activation, and IgE-dependent allergic responses in mice. PTPα(-/-) bone marrow-derived mast cells hyperdegranulate and exhibit increased cytokine and cysteinyl leukotriene secretion, and PTPα(-/-) mice display enhanced IgE-dependent anaphylaxis. At or proximal to FcεRI, PTPα(-/-) cells have reduced IgE-dependent activation of Lyn and Fyn, as well as reduced FcεRI and SHIP phosphorylation. In contrast, Hck and Syk activation is enhanced. Syk hyperactivation correlated with its increased phosphorylation at positive regulatory sites and defective phosphorylation at a negative regulatory site. Distal to FcεRI, we observed increased activation of PI3K and MAPK pathways. These findings demonstrate that PTPα activates the FcεRI-coupled kinases Lyn and Fyn and suppresses Hck activity. Furthermore, the findings indicate that hyperactivation of PTPα(-/-) mast cells and enhanced IgE-dependent allergic responses of PTPα(-/-) mice are due to the ablated function of PTPα as a critical regulator of Lyn negative signaling.

Lyn- and PLC-beta3-dependent regulation of SHP-1 phosphorylation controls Stat5 activity and myelomonocytic leukemia-like disease

Hyperactivation of the transcription factor Stat5 leads to various leukemias. Stat5 activity is regulated by the protein phosphatase SHP-1 in a phospholipase C (PLC)-β3-dependent manner. Thus, PLC-β3-deficient mice develop myeloproliferative neoplasm, like Lyn (Src family kinase)- deficient mice. Here we show that Lyn/PLC-β3 doubly deficient lyn(-/-);PLC-β3(-/-) mice develop a Stat5-dependent, fatal myelodysplastic/myeloproliferative neoplasm, similar to human chronic myelomonocytic leukemia (CMML). In hematopoietic stem cells of lyn(-/-);PLC-β3(-/-) mice that cause the CMML-like disease, phosphorylation of SHP-1 at Tyr(536) and Tyr(564) is abrogated, resulting in reduced phosphatase activity and constitutive activation of Stat5. Furthermore, SHP-1 phosphorylation at Tyr(564) by Lyn is indispensable for maximal phosphatase activity and for suppression of the CMML-like disease in these mice. On the other hand, Tyr(536) in SHP-1 can be phosphorylated by Lyn and another kinase(s) and is necessary for efficient interaction with Stat5. Therefore, we identify a novel Lyn/PLC-β3-mediated regulatory mechanism of SHP-1 and Stat5 activities.

Cooperation of adapter molecules in proximal signaling cascades during allergic inflammation

Activation of mast cells through their high-affinity immunoglobulin E receptor (FcepsilonRI) plays an important role in allergic disorders. Other mast cell-activating stimuli, such as Toll-like receptor (TLR) ligands, synergize with FcepsilonRI to enhance allergic inflammation. Thus, there is much interest in understanding how signaling occurs downstream of these receptors. One key event for FcepsilonRI-mediated mast cell activation is the inducible formation of multimolecular proximal signaling complexes. These complexes are nucleated by adapter proteins, scaffolds that localize various signaling molecules through their multiple molecule-binding domains. Here we review recent findings in proximal signaling cascades with an emphasis on how adapter molecules cooperate with each other to generate an optimal signal in mast cells, and we discuss how signals crosstalk between FcepsilonRI and TLRs in enhancing mast cell activation. Deciphering the molecular mechanisms leading to mast cell activation will hopefully bring new ideas for the development of novel therapeutics to control allergic diseases.

Inhibitory effects of whisky congeners on IgE-mediated degranulation in rat basophilic leukemia RBL-2H3 cells and passive cutaneous anaphylaxis reaction in mice

Whisky is matured in oak casks. Many nonvolatile substances (whisky congeners, WC) seep from the oak cask during the maturing process. In this study, three antiallergic agents (syringaldehyde, SA; lyoniresinol, Lyo; and ellagic acid, EA) were isolated from WC. Treatment with SA, Lyo, and EA reduced the elevation of intracellular free Ca(2+) concentration ([Ca(2+)]i) and intracellular ROS production caused by FcepsilonRI activation. The inhibitions of the elevation of [Ca(2+)]i and intracellular ROS production by SA and Lyo were mainly due to the suppression of the NADPH oxidase activity and scavenging of the produced radical, respectively. On the other hand, EA inactivated spleen tyrosine kinase and led to the inhibition of the elevation of [Ca(2+)]i and intracellular ROS production. Furthermore, it was found that WC strongly inhibited IgE binding to the FcepsilonRIalpha chain, whereas SA, Lyo, and EA did not indicate this inhibitory effect. These results suggest that WC inhibits allergic reactions through multiple mechanisms. To disclose the in vivo effects of WC, SA, Lyo, and EA, these compounds were administered to type I allergic model mice, and the passive cutaneous anaphylaxis (PCA) reaction was measured. These compounds remarkably suppressed the PCA reaction. Taken together, these findings suggest that WC seemed to be beneficial to ameliorate allergic reactions.

Tyrosine phosphorylation of SHIP promotes its proteasomal degradation

OBJECTIVE

The activity of the SH2-containing-phosphatidylinositol-5'-phosphatase (SHIP, also known as SHIP1), a critical hematopoietic-restricted negative regulator of the PI3 kinase (PI3K) pathway, is regulated in large part via its protein levels. We sought to determine the mechanism(s) involved in its downregulation by BCR-ABL and by interleukin (IL)-4.

MATERIALS AND METHODS

We used Ba/F3(p210-tetOFF) cells to study the downregulation of SHIP by BCR-ABL and bone marrow-derived macrophages to study SHIP's downregulation by IL-4.

RESULTS

We show herein that BCR-ABL downregulates SHIP, but not SHIP2 or PTEN, and this can be blocked with the Src kinase inhibitor PP2, which inhibits the tyrosine phosphorylation of SHIP, or with the proteasomal inhibitor MG-132. We also show, using anti-SHIP immunoprecipitates, that c-Cbl and Cbl-b are associated with SHIP and that BCR-ABL induces SHIP's polyubiquitination. This ubiquitination can be blocked with PP2, consistent with the tyrosine phosphorylation of SHIP acting as a signal for its ubiquitination. In bone marrow-derived macrophages, IL-4 also leads to the proteasomal degradation of SHIP but, unlike in Ba/F3(p210-tetOFF) cells, SHIP2 is also proteasomally degraded and the degradation of both inositol phosphatases can be prevented with PP2 or MG-132.

CONCLUSION

Our results suggest that SHIP protein levels can be reduced via BCR-ABL and/or Src family member-induced tyrosine phosphorylation of SHIP because this triggers its polyubiquitination and degradation within the proteasome.

Advances in mast cell biology: new understanding of heterogeneity and function

Mast cells are classically viewed as effector cells of IgE-mediated allergic diseases. However, over the last decade our understanding has been enriched about their roles in host defense, innate and adaptive immune responses, and in homeostatic responses, angiogenesis, wound healing, tissue remodeling, and immunoregulation. Despite impressive progress, there are large gaps in our understanding of their phenotypic heterogeneity, regulatory mechanisms involved, and functional significance. This review summarizes our knowledge of mast cells in innate and acquired immunity, allergic inflammation and tissue homeostasis, as well as some of the regulatory mechanisms that control mast cell development, phenotypic determination, and function, particularly in the context of mucosal surfaces.

The protein tyrosine kinase Tec regulates mast cell function

Mast cells play crucial roles in a variety of normal and pathophysiological processes and their activation has to be tightly controlled. Here, we demonstrate that the protein tyrosine kinase Tec is a crucial regulator of murine mast cell function. Tec was activated upon Fc epsilon RI stimulation of BM-derived mast cells (BMMC). The release of histamine in the absence of Tec was normal in vitro and in vivo; however, leukotriene C(4) levels were reduced in Tec(-) (/) (-) BMMC. Furthermore, the production of IL-4 was severely impaired, and GM-CSF, TNF-alpha and IL-13 levels were also diminished. Finally, a comparison of WT, Tec(-) (/) (-), Btk(-) (/) (-) and Tec(-) (/) (-)Btk(-) (/) (-) BMMC revealed a negative role for Btk in the regulation of IL-4 production, while for the efficient production of TNF-alpha, IL-13 and GM-CSF, both Tec and Btk were required. Our results demonstrate a crucial role for Tec in mast cells, which is partially different to the function of the well-characterized family member Btk.

A minor catalytic activity of Src family kinases is sufficient for maximal activation of mast cells via the high-affinity IgE receptor

Src family kinases (SFK) are critical for initiating and regulating the response of mast cells activated by engagement of the high-affinity IgE receptor, FcepsilonRI. Lyn is the predominant SFK in mast cells and has been ascribed both positive and negative roles in regulating mast cell activation. We analyzed the mast cell phenotype of WeeB, a recently described mouse mutant that expresses a Lyn protein with profoundly reduced catalytic activity. Surprisingly, we found that this residual activity is sufficient for wild-type levels of cytokine production and degranulation in bone marrow-derived mast cells after low-intensity stimulation with anti-IgE. High-intensity stimulation of lyn(-/-) bone marrow-derived mast cells with highly multivalent Ag resulted in enhanced cytokine production as previously reported, and WeeB cells displayed an intermediate phenotype. Under this latter condition, SFK inhibition using PP2 increased cytokine production in wild-type and WeeB but not lyn(-/-) cells, resulting in substantially higher levels in the PP2-treated WeeB than in lyn(-/-) cells. Restoration of wild-type and WeeB lyn alleles in lyn(-/-) cells generated activation phenotypes similar to those in nontransduced wild-type and WeeB cells, respectively, whereas a kinase-dead allele resulted in a phenotype similar to that of empty-vector-transduced cells. These data indicate that inhibition of Lyn and/or SFK activity can result in higher levels of mast cell activation than simple deletion of lyn and that only near-complete inhibition of Lyn can impair its positive regulatory functions. Furthermore, the data suggest that both positive and negative regulatory functions of Lyn are predominantly carried out by its catalytic activity and not an adaptor function.

SH2 domain-containing phosphatase-2 protein-tyrosine phosphatase promotes Fc epsilon RI-induced activation of Fyn and Erk pathways leading to TNF alpha release from bone marrow-derived mast cells

Clustering of the high affinity IgE receptor (Fc(epsilon)RI) in mast cells leads to degranulation and production of numerous cytokines and lipid mediators that promote allergic inflammation. Initiation of FFc(epsilon)RI signaling involves rapid tyrosine phosphorylation of Fc(epsilon)RI and membrane-localized adaptor proteins that recruit additional SH2 domain-containing proteins that dynamically regulate downstream signaling. SH2 domain-containing phosphatase-2 (SHP2) is a protein-tyrosine phosphatase implicated in Fc(epsilon)RI signaling, but whose function is not well defined. In this study, using a mouse model allowing temporal shp2 inactivation in bone marrow-derived mast cells (BMMCs), we provide insights into SHP2 functions in the Fc(epsilon)RI pathway. Although no overt defects in Fc(epsilon)RI-induced tyrosine phosphorylation were observed in SHP2 knock-out (KO) BMMCs, several proteins including Lyn and Syk kinases displayed extended phosphorylation kinetics compared with wild-type BMMCs. SHP2 was dispensable for Fc(epsilon)RI-induced degranulation of BMMCs, but was required for maximal activation of Erk and Jnk mitogen-activated protein kinases. SHP2 KO BMMCs displayed several phenotypes associated with reduced Fyn activity, including elevated phosphorylation of the inhibitory pY531 site in Fyn, impaired signaling to Grb2-associated binder 2, Akt/PKB, and IkappaB kinase, and decreased TNF-alpha release compared with control cells. This is likely due to elevated Lyn activity in SHP2 KO BMMCs, and the ability of Lyn to antagonize Fyn activity. Overall, our study identifies SHP2 as a positive effector of Fc(epsilon)RI-induced activation of Fyn/Grb2-associated binder 2/Akt and Ras/Erk pathways leading to TNF-alpha release from mast cells.

Intracellular receptor for human host defense peptide LL-37 in monocytes

The human cationic host defense peptide LL-37 has a broad range of immunomodulatory, anti-infective functions. A synthetic innate defense regulator peptide, innate defense regulator 1 (IDR-1), based conceptually on LL-37, was recently shown to selectively modulate innate immunity to protect against a wide range of bacterial infections. Using advanced proteomic techniques, ELISA, and Western blotting procedures, GAPDH was identified as a direct binding partner for LL-37 in monocytes. Enzyme kinetics and mobility shift studies also indicated LL-37 and IDR-1 binding to GAPDH. The functional relevance of GAPDH in peptide-induced responses was demonstrated by using gene silencing of GAPDH with small interfering RNA (siRNA). Previous studies have established that the induction of chemokines and the anti-inflammatory cytokine IL-10 are critical immunomodulatory functions in the anti-infective properties of LL-37 and IDR-1, and these functions are modulated by the MAPK p38 pathway. Consistent with that, this study demonstrated the importance of the GAPDH interactions with these peptides since gene silencing of GAPDH resulted in impaired p38 MAPK signaling, downstream chemokine and cytokine transcriptional responses induced by LL-37 and IDR-1, and LL-37-induced cytokine production. Bioinformatic analysis, using InnateDB, of the major interacting partners of GAPDH indicated the likelihood that this protein can impact on innate immune pathways including p38 MAPK. Thus, this study has demonstrated a novel function for GAPDH as a mononuclear cell receptor for human cathelicidin LL-37 and immunomodulatory IDR-1 and conclusively demonstrated its relevance in the functioning of cationic host defense peptides.

Understanding the mechanisms of anaphylaxis

PURPOSE OF REVIEW

The present review considers recent reports that identify the roles of key intermediate signaling components and mediators during and after mast cell activation and degranulation leading to anaphylaxis.

RECENT FINDINGS

Mechanisms of anaphylaxis are becoming better understood as the interaction of several regulatory systems in the mast cell activation and degranulation signaling cascade. Multiple tyrosine kinases, activated after immunoglobulin E binding to the high-affinity receptors for immunoglobulin E (FcepsilonRI), exert both positive and negative regulation on the signaling cascade, which may vary with genetic background or mutations in signaling proteins. Calcium influx, the essential, proximal intracellular event leading to mast cell degranulation, is controlled also by both negative and positive regulation through calcium channels. Sphingosine-1-phosphate is emerging as a newly realized mediator of anaphylaxis, acting as a signaling component within the mast cell and as a circulating mediator.

SUMMARY

Anaphylaxis is a systemic reaction involving multiple organ systems, but it is believed that it may be influenced by cellular events in mast cells and basophils resulting in the release of mediators. Therefore, understanding the mechanisms of mast cell activation and degranulation is critical to understanding the mechanisms of anaphylaxis. Recent reports have identified important regulatory components of the signaling cascade and, consequently, potential targets for therapeutic intervention.

The tyrosine kinase network regulating mast cell activation

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

Multiple roles of Lyn kinase in myeloid cell signaling and function

Lyn is an Src family kinase present in B lymphocytes and myeloid cells. In these cell types, Lyn establishes signaling thresholds by acting as both a positive and a negative modulator of a variety of signaling responses and effector functions. Lyn deficiency in mice results in the development of myeloproliferation and autoimmunity. The latter has been attributed to the hyper-reactivity of Lyn-deficient B cells due to the unique role of Lyn in downmodulating B-cell receptor activation, mainly through phosphorylation of inhibitory molecules and receptors. Myeloproliferation results, on the other hand, from the enhanced sensitivity of Lyn-deficient progenitors to a number of colony-stimulating factors (CSFs). The hyper-sensitivity to myeloid growth factors may also be secondary to poor inhibitory receptor phosphorylation, leading to impaired recruitment/activation of tyrosine phosphatases and reduced downmodulation of CSF signaling responses. Despite these observations, the overall role of Lyn in the modulation of myeloid cell effector functions is much less well understood, as often both positive and negative roles of this kinase have been reported. In this review, we discuss the current knowledge of the duplicitous nature of Lyn in the modulation of myeloid cell signaling and function.

Src-like adaptor protein (SLAP) is upregulated in antigen-stimulated mast cells and acts as a negative regulator

Our studies in the RBL-2H3 mast cell line suggest that responses to antigen (Ag) are negatively modulated through upregulation of Src-like adaptor protein (SLAP). Ag stimulation of RBL-2H3 cells leads to increased levels of SLAP (but not SLAP2) transcripts and protein over a period of several hours. The effects of pharmacologic inhibitors indicate that the upregulation of SLAP is dependent on multiple signaling pathways. Knockdown of SLAP with anti-SLAP siRNA is associated with enhanced phosphorylation of Syk, the linker for activation of T cells (LAT), phospholipase C gamma, MAP kinases, and various transcription factors. Production of IL-3 and MCP-1, but not degranulation, is also enhanced. The upregulation of SLAP may thus serve to limit the duration of cytokine production in Ag-stimulated cells.

Lyn kinase controls basophil GATA-3 transcription factor expression and induction of Th2 cell differentiation

T helper 1 (Th1)-Th2 cell balance is key to host defense and its dysregulation has pathophysiological consequences. Basophils are important in Th2 cell differentiation. However, the factors controlling the onset and extent of basophil-mediated Th2 cell differentiation are unknown. Here, we demonstrate that Lyn kinase dampened basophil expression of the transcription factor GATA-3 and the initiation and extent of Th2 cell differentiation. Lyn-deficient mice had a marked basophilia, a constitutive Th2 cell skewing that was exacerbated upon in vivo challenge of basophils, produced antibodies to a normally inert antigen, and failed to appropriately respond to a Th1 cell-inducing pathogen. The Th2 cell skewing was dependent on basophils, immunoglobulin E, and interleukin-4, but was independent of mast cells. Our findings demonstrate that basophil-expressed Lyn kinase exerts regulatory control on Th2 cell differentiation and function.

L-type Ca2+ channels in mast cells: activation by membrane depolarization and distinct roles in regulating mediator release from store-operated Ca2+ channels

Store-operated Ca(2+) channels (SOCs) are considered to be the principal route of Ca(2+) influx in non-excitable cells. We have previously shown that in mast cells IgE+antigen (Ag) induces a dihydropyridine (DHP)-sensitive Ca(2+) influx independently of Ca(2+) store depletion. Since the DHP receptor is the alpha subunit of L-type Ca(2+) channels (LTCCs), we examined the possible role of LTCCs in mast cell activation. Mast cells exhibited substantial expression of the alpha(1C) (Ca(V)1.2) subunit mRNA and protein on their cell surface. IgE+Ag-induced Ca(2+) influx was substantially reduced by the LTCC inhibitor nifedipine, and enhanced by the LTCC activator (S)-BayK8644, whereas these agents had minimal effects on thapsigargin (TG)-induced Ca(2+) influx. These LTCC-modulating agents regulated IgE+Ag-induced cell activation but not TG-induced cell activation. Inhibition of SOCs by 2-aminoethoxydiphenyl borate reduced both degranulation and production of cytokines, including interleukin-13 and tumor necrosis factor-alpha, whereas LTCC modulation reciprocally regulated degranulation and cytokine production. IgE+Ag, but not TG, induced substantial plasma membrane depolarization, which stimulated a DHP-sensitive Ca(2+) response. Moreover, IgE+Ag-, but not TG-induced mitochondrial Ca(2+) increase was regulated by LTCC modulators. Finally, gene silencing analyses using small interfering RNA revealed that the alpha(1C) (Ca(V)1.2) LTCC mediated the pharmacological effects of the LTCC-modulating agents. These results demonstrate that mast cells express LTCCs, which becomes activated by membrane depolarization to regulate cytosolic and mitochondrial Ca(2+), thereby controlling mast cell activation in a distinct manner from SOCs.

IgE-induced mast cell survival requires the prolonged generation of reactive oxygen species

We show in this study that the ability of five different monomeric IgEs to enhance murine bone marrow-derived mast cell (BMMC) survival correlates with their ability to stimulate extracellular calcium (Ca(2+)) entry. However, whereas IgE+Ag more potently stimulates Ca(2+) entry, it does not enhance survival under our conditions. Exploring this further, we found that whereas all five monomeric IgEs stimulate a less robust Ca(2+) entry than IgE+Ag initially, they all trigger a more prolonged Ca(2+) influx, generation of reactive oxygen species (ROS), and ERK phosphorylation. These prolonged signaling events correlate with their survival-enhancing ability and positively feedback on each other to generate the prosurvival cytokine, IL-3. Interestingly, the prolonged ERK phosphorylation induced by IgE appears to be regulated by a MAPK phosphatase rather than MEK. IgE-induced ROS generation, unlike that triggered by IgE+Ag, is not mediated by 5-lipoxygenase. Moreover, ROS inhibitors, which block both IgE-induced ROS production and Ca(2+) influx, convert the prolonged ERK phosphorylation induced by IgE into the abbreviated phosphorylation pattern observed with IgE+Ag and prevent IL-3 generation. In support of the essential role that IgE-induced ROS plays in IgE-enhanced BMMC survival, we found the addition of H(2)O(2) to IgE+Ag-stimulated BMMCs leads to IL-3 secretion.

Contributions of F-BAR and SH2 domains of Fes protein tyrosine kinase for coupling to the FcepsilonRI pathway in mast cells

This study investigates the roles of Fer-CIP4 homology (FCH)-Bin/amphiphysin/Rvs (F-BAR) and SH2 domains of Fes protein tyrosine kinase in regulating its activation and signaling downstream of the high-affinity immunoglobulin G (IgE) receptor (FcepsilonRI) in mast cells. Homology modeling of the Fes F-BAR domain revealed conservation of some basic residues implicated in phosphoinositide binding (R113/K114). The Fes F-BAR can bind phosphoinositides and induce tubulation of liposomes in vitro. Mutation of R113/K114 to uncharged residues (RK/QQ) caused a significant reduction in phosphoinositide binding in vitro and a more diffuse cytoplasmic localization in transfected COS-7 cells. RBL-2H3 mast cells expressing full-length Fes carrying the RK/QQ mutation show defects in FcepsilonRI-induced Fes tyrosine phosphorylation and degranulation compared to cells expressing wild-type Fes. This correlated with reduced localization to Lyn kinase-containing membrane fractions for the RK/QQ mutant compared to wild-type Fes in mast cells. The Fes SH2 domain also contributes to Fes signaling in mast cells, via interactions with the phosphorylated FcepsilonRI beta chain and the actin regulatory protein HS1. We show that Fes phosphorylates C-terminal tyrosine residues in HS1 implicated in actin stabilization. Thus, coordinated actions of the F-BAR and SH2 domains of Fes allow for coupling to FcepsilonRI signaling and potential regulation the actin reorganization in mast cells.

Positive and negative regulation of high affinity IgE receptor signaling by Src homology region 2 domain-containing phosphatase 1

Src homology region 2 domain-containing phosphatase 1 (SHP-1), a cytoplasmic protein tyrosine phosphatase, plays an important role for the regulation of signaling from various hematopoietic cell receptors. Although SHP-1 is shown to be a negative signal modulator in mast cells, its precise molecular mechanisms are not well defined. To elucidate how SHP-1 regulates mast cell signaling, we established bone marrow-derived mast cells from SHP-1-deficient motheaten and wild-type mice and analyzed downstream signals induced by cross-linking of high affinity IgE receptor, Fc epsilonRI. Upon Fc epsilonRI ligation, motheaten-derived bone marrow-derived mast cells showed enhanced tyrosine phosphorylation of Src homology region 2 domain-containing leukocyte protein of 76 kDa (SLP-76) and linker for activation of T cells, activation of mitogen-activated protein kinases and gene transcription and production of cytokine. Because the activity of Syk, responsible for the phosphorylation of SLP-76 and linker for activation of T cells, is comparable irrespective of SHP-1, both molecules might be substrates of SHP-1 in mast cells. Interestingly, the absence of SHP-1 expression disrupted the association between SLP-76 and phospholipase Cgamma, which resulted in the decreased phospholipase Cgamma phosphorylation, calcium mobilization, and degranulation. Collectively, these results suggest that SHP-1 regulates Fc epsilonRI-induced downstream signaling events both negatively and positively by functioning as a protein tyrosine phosphatase and as an adaptor protein contributing to the formation of signaling complex, respectively.