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

Regulation of Trafficking and Signaling of the High Affinity IgE Receptor by FcεRIβ and the Potential Impact of FcεRIβ Splicing in Allergic Inflammation

Mast cells are tissue-resident immune cells that function in both innate and adaptive immunity through the release of both preformed granule-stored mediators, and newly generated proinflammatory mediators that contribute to the generation of both the early and late phases of the allergic inflammatory response. Although mast cells can be activated by a vast array of mediators to contribute to homeostasis and pathophysiology in diverse settings and contexts, in this review, we will focus on the canonical setting of IgE-mediated activation and allergic inflammation. IgE-dependent activation of mast cells occurs through the high affinity IgE receptor, FcεRI, which is a multimeric receptor complex that, once crosslinked by antigen, triggers a cascade of signaling to generate a robust response in mast cells. Here, we discuss FcεRI structure and function, and describe established and emerging roles of the β subunit of FcεRI (FcεRIβ) in regulating mast cell function and FcεRI trafficking and signaling. We discuss current approaches to target IgE and FcεRI signaling and emerging approaches that could target FcεRIβ specifically. We examine how alternative splicing of FcεRIβ alters protein function and how manipulation of splicing could be employed as a therapeutic approach. Targeting FcεRI directly and/or IgE binding to FcεRI are promising approaches to therapeutics for allergic inflammation. The characteristic role of FcεRIβ in both trafficking and signaling of the FcεRI receptor complex, the specificity to IgE-mediated activation pathways, and the preferential expression in mast cells and basophils, makes FcεRIβ an excellent, but challenging, candidate for therapeutic strategies in allergy and asthma, if targeting can be realized.

The Src-family Kinase Lyn in Immunoreceptor Signaling

Effective regulation of immune-cell activation is critical for ensuring that the immune response, and inflammation generated for the purpose of pathogen elimination, are limited in space and time to minimize tissue damage. Autoimmune disease can occur when immunoreceptor signaling is dysregulated, leading to unrestrained inflammation and organ damage. Conversely, tumors can coopt the tissue healing and immunosuppressive functions of hematopoietic cells to promote metastasis and evade therapy. The Src-family kinase Lyn is an essential regulator of immunoreceptor signaling, initiating both proinflammatory and suppressive signaling pathways in myeloid immune cells (eg, neutrophils, dendritic cells, monocytes, macrophages) and in B lymphocytes. Defects in Lyn signaling are implicated in autoimmune disease, but mechanisms by which Lyn, expressed along with a battery of other Src-family kinases, may uniquely direct both positive and negative signaling remain incompletely defined. This review describes our current understanding of the activating and inhibitory contributions of Lyn to immunoreceptor signaling and how these processes contribute to myeloid and B-cell function. We also highlight recent work suggesting that the 2 proteins generated by alternative splicing of lyn, LynA and LynB, differentially regulate both immune and cancer-cell signaling. These principles may also extend to other Lyn-expressing cells, such as neuronal and endocrine cells. Unraveling the common and cell-specific aspects of Lyn function could lead to new approaches to therapeutically target dysregulated pathways in pathologies ranging from autoimmune and neurogenerative disease to cancer.

Novel phospho-specific monoclonal antibodies reveal differential regulation of tyrosine phosphorylation within the immunoreceptor tyrosine-based activation motif of the Fc receptor γ subunit leading to fine tuning of Syk activation

The cytoplasmic region of the γ chain of the high-affinity receptor for IgE (FcεRI) contains a consensus sequence termed the immunoreceptor tyrosine-based activation motif (ITAM). Phosphorylation of the two tyrosine residues (N-terminal Y47 and C-terminal Y58) in the ITAM sequence is crucial for the recruitment and activation of Syk, a cytoplasmic tyrosine kinase with central signaling roles in mast cells. Using a reconstitution system in which individual tyrosine-to-phenylalanine substituted γ chains were expressed in γ-chain-deficient mast cells, we previously reported differential dephosphorylation of these tyrosines. Herein, we developed monoclonal antibodies highly specific to the phosphorylated Y47 and Y58 residues, which enables monitoring their phosphorylation under more physiological conditions. Using these antibodies, preferential dephosphorylation of Y58 following FcεRI stimulation was confirmed. Furthermore, Y58 is potentially more susceptible to phosphorylation than is Y47. Consistent with this, an in vitro kinase assay using these phospho-specific antibodies demonstrated that the Src family kinase Lyn, which is primarily responsible for ITAM phosphorylation, phosphorylates Y58 more efficiently than Y47. These results indicate that Y58 is more susceptible to dephosphorylation and phosphorylation than is Y47. Because a phosphate group on Y58 is more important for Syk binding than is a phosphate group on Y47, the preferential phosphorylation and dephosphorylation of Y58 may contribute to the fine tuning of Syk activity by promoting rapid recruitment and reducing excessive activation.

IgE and IgG Antibodies as Regulators of Mast Cell and Basophil Functions in Food Allergy

Food allergy is a major health issue, affecting the lives of 8% of U.S. children and their families. There is an urgent need to identify the environmental and endogenous signals that induce and sustain allergic responses to ingested allergens. Acute reactions to foods are triggered by the activation of mast cells and basophils, both of which release inflammatory mediators that lead to a range of clinical manifestations, including gastrointestinal, cutaneous, and respiratory reactions as well as systemic anaphylaxis. Both of these innate effector cell types express the high affinity IgE receptor, FcϵRI, on their surface and are armed for adaptive antigen recognition by very-tightly bound IgE antibodies which, when cross-linked by polyvalent allergen, trigger degranulation. These cells also express inhibitory receptors, including the IgG Fc receptor, FcγRIIb, that suppress their IgE-mediated activation. Recent studies have shown that natural resolution of food allergies is associated with increasing food-specific IgG levels. Furthermore, oral immunotherapy, the sequential administration of incrementally increasing doses of food allergen, is accompanied by the strong induction of allergen-specific IgG antibodies in both human subjects and murine models. These can deliver inhibitory signals via FcγRIIb that block IgE-induced immediate food reactions. In addition to their role in mediating immediate hypersensitivity reactions, mast cells and basophils serve separate but critical functions as adjuvants for type 2 immunity in food allergy. Mast cells and basophils, activated by IgE, are key sources of IL-4 that tilts the immune balance away from tolerance and towards type 2 immunity by promoting the induction of Th2 cells along with the innate effectors of type 2 immunity, ILC2s, while suppressing the development of regulatory T cells and driving their subversion to a pathogenic pro-Th2 phenotype. This adjuvant effect of mast cells and basophils is suppressed when inhibitory signals are delivered by IgG antibodies signaling via FcγRIIb. This review summarizes current understanding of the immunoregulatory effects of mast cells and basophils and how these functions are modulated by IgE and IgG antibodies. Understanding these pathways could provide important insights into innovative strategies for preventing and/or reversing food allergy in patients.

Spleen Tyrosine Kinase as a Target Therapy for Pseudomonas aeruginosa Infection

Spleen tyrosine kinase (SYK) is a nonreceptor tyrosine kinase which associates directly with extracellular receptors, and is critically involved in signal transduction pathways in a variety of cell types for the regulation of cellular responses. SYK is expressed ubiquitously in immune and nonimmune cells, and has a much wider biological role than previously recognized. Several studies have highlighted SYK as a key player in the pathogenesis of a multitude of diseases. Pseudomonas aeruginosa is an opportunistic gram-negative pathogen, which is responsible for systemic infections in immunocompromised individuals, accounting for a major cause of severe chronic lung infection in cystic fibrosis patients and subsequently resulting in a progressive deterioration of lung function. Inhibition of SYK activity has been explored as a therapeutic option in several allergic disorders, autoimmune diseases, and hematological malignancies. This review focuses on SYK as a therapeutic target, and describes the possibility of how current knowledge could be translated for therapeutic purposes to regulate the immune response to the opportunistic pathogen P. aeruginosa.

Maximal aggregation of polynomial dynamical systems

Ordinary differential equations (ODEs) with polynomial derivatives are a fundamental tool for understanding the dynamics of systems across many branches of science, but our ability to gain mechanistic insight and effectively conduct numerical evaluations is critically hindered when dealing with large models. Here we propose an aggregation technique that rests on two notions of equivalence relating ODE variables whenever they have the same solution (backward criterion) or if a self-consistent system can be written for describing the evolution of sums of variables in the same equivalence class (forward criterion). A key feature of our proposal is to encode a polynomial ODE system into a finitary structure akin to a formal chemical reaction network. This enables the development of a discrete algorithm to efficiently compute the largest equivalence, building on approaches rooted in computer science to minimize basic models of computation through iterative partition refinements. The physical interpretability of the aggregation is shown on polynomial ODE systems for biochemical reaction networks, gene regulatory networks, and evolutionary game theory.

Differential mast cell outcomes are sensitive to FcεRI-Syk binding kinetics

Cross-linking of immunoglobulin E-bound FcεRI triggers multiple cellular responses, including degranulation and cytokine production. Signaling is dependent on recruitment of Syk via docking of its dual SH2 domains to phosphorylated tyrosines within the FcεRI immunoreceptor tyrosine-based activation motifs. Using single-molecule imaging in live cells, we directly visualized and quantified the binding of individual mNeonGreen-tagged Syk molecules as they associated with the plasma membrane after FcεRI activation. We found that Syk colocalizes transiently to FcεRI and that Syk-FcεRI binding dynamics are independent of receptor aggregate size. Substitution of glutamic acid for tyrosine between the Syk SH2 domains (Syk-Y130E) led to an increased Syk-FcεRI off-rate, loss of site-specific Syk autophosphorylation, and impaired downstream signaling. Genome edited cells expressing only Syk-Y130E were deficient in antigen-stimulated calcium release, degranulation, and production of some cytokines (TNF-a, IL-3) but not others (MCP-1, IL-4). We propose that kinetic discrimination along the FcεRI signaling pathway occurs at the level of Syk-FcεRI interactions, with key outcomes dependent upon sufficiently long-lived Syk binding events.

From IgE to Omalizumab

IgE is the least abundant Ig isotype, yet it plays a critical role in allergic reactions and host protection from helminth infection. Although IgE was discovered 50 years ago, the ultimate evidence for its role in human allergic diseases was obtained by the efficacy of anti-IgE therapy in many clinical trials on asthma and other allergic diseases. Beginning from the discovery of IgE 50 y ago, followed by studies of IgE receptors and activation mechanisms, this review provides a historic perspective of allergy research that has led to the development of anti-IgE therapy and other strategies targeting IgE and its receptors. Current IgE studies toward future precision medicine are also reviewed.

OxLDL receptor chromatography from live human U937 cells identifies SYK(L) that regulates phagocytosis of oxLDL

The binding and activation of macrophages by microscopic aggregates of oxLDL in the intima of the arteries may be an important step towards atherosclerosis leading to heart attack and stroke. Microbeads coated with oxLDL were used to activate, capture and isolate the oxLDL receptor complex from the surface of live cells. Analysis of the resulting tryptic peptides by liquid chromatography and tandem mass spectrometry revealed the Spleen Tyrosine Kinase (SYK), and many of SYK's known interaction network including Fc receptors (FCGR2A, FCER1G and FCGR1A) Toll receptor 4 (TLR4), receptor kinases like EGFRs, as well as RNA binding and metabolism proteins. High-intensity precursor ions (∼9*E3 to 2*E5 counts) were correlated to peptides and specific phosphopeptides from long isoform of SYK (SYK-L) by the SEQUEST, OMSSA and X!TANDEM algorithms. Peptides or phosphopeptides from SYK were observed with the oxLDL-microbeads. Pharmacological inhibitors of SYK activity significantly reduced the engulfment of oxLDL microbeads in the presence of serum factors, but had little effect on IgG phagocytosis. Anti SYK siRNA regulated oxLD engulfment in the context of serum factors and or SYK-L siRNA significantly inhibited engulfment of oxLDL microbeads, but not IgG microbeads.

Interaction of key pathways in sorafenib-treated hepatocellular carcinoma based on a PCR-array

This study aimed to identify the key pathways and to explore the mechanism of sorafenib in inhibiting hepatocellular carcinoma (HCC). The gene expression profile of GSE33621, including 6 sorafenib treated group and 6 control samples, was downloaded from the GEO (Gene Expression Omnibus) database. The differentially expressed genes (DEGs) in HCC samples were screened using the ΔΔCt method with the homogenized internal GAPDH. Also, the functions and pathways of DEGs were analyzed using the DAVID. Moreover, the significant pathways of DEGs that involved in HCC were analyzed based on the Latent pathway identification analysis (LPIA). A total of 44 down-regulated DEGs were selected in HCC samples. Also, there were 84 biological pathways that these 44 DEGs involved in. Also, LPIA showed that Osteoclast differentiation and hsa04664-Fc epsilon RI signaling pathway was the most significant interaction pathways. Moreover, Apoptosis, Toll-like receptor signaling pathway, Chagas disease, and T cell receptor signaling pathway were the significant pathways that interacted with hsa04664. In addition, DEGs such as AKT1 (v-akt murine thymoma viral oncogene homolog 1), TNF (tumor necrosis factor), SYK (spleen tyrosine kinase), and PIK3R1 (phosphoinositide-3-kinase, regulatory subunit 1 (alpha)) were the common genes that involved in the significant pathways. Several pathway interaction pairs that caused by several downregulated genes such as SYK, PI3K, AKT1, and TNF, were identified play curial role in sorafenib treated HCC. Sorafenib played important inhibition roles in HCC by affecting a complicate pathway interaction network.

Destabilization of TNF-α mRNA by Rapamycin

Stimulation of mast cells through the high affinity IgE receptor (FcεRI) induces degranulation, lipid mediator release, and cytokine secretion leading to allergic reactions. Although various signaling pathways have been characterized to be involved in the FcεRI-mediated responses, little is known about the precious mechanism for the expression of tumor necrosis factor-α (TNF-α) in mast cells. Here, we report that rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), reduces the expression of TNF-α in rat basophilic leukemia (RBL-2H3) cells. IgE or specific antigen stimulation of RBL-2H3 cells increases the expression of TNF-α and activates various signaling molecules including S6K1, Akt and p38 MAPK. Rapamycin specifically inhibits antigen-induced TNF-α mRNA level, while other kinase inhibitors have no effect on TNF-α mRNA level. These data indicate that mTOR signaling pathway is the main regulation mechanism for antigen-induced TNF-α expression. TNF-α mRNA stability analysis using reporter construct containing TNF-α adenylate/uridylate-rich elements (AREs) shows that rapamycin destabilizes TNF-α mRNA via regulating the AU-rich element of TNF-α mRNA. The antigen-induced activation of S6K1 is inhibited by specific kinase inhibitors including mTOR, PI3K, PKC and Ca²⁺chelator inhibitor, while TNF-α mRNA level is reduced only by rapamycin treatment. These data suggest that the effects of rapamycin on the expression of TNF-α mRNA are not mediated by S6K1 but regulated by mTOR. Taken together, our results reveal that mTOR signaling pathway is a novel regulation mechanism for antigen-induced TNF-α expression in RBL-2H3 cells.

The scaffold protein prohibitin is required for antigen-stimulated signaling in mast cells

The protein prohibitin (PHB) is implicated in diverse cellular processes, including cell signaling, transcriptional control, and mitochondrial function. We found that PHB was abundant in the intracellular granules of mast cells, which are critical for allergic responses to antigens. Thus, we investigated whether PHB played a role in signaling mediated by the high-affinity receptor for antigen-bound immunoglobulin E (IgE), FcεRI. PHB-specific small interfering RNAs (siRNAs) inhibited antigen-mediated signaling, degranulation, and cytokine secretion by mast cells in vitro. Knockdown of PHB inhibited the antigen-dependent association of the tyrosine kinase Syk with FcεRI and inhibited the activation of Syk. Fractionation studies revealed that PHB translocated from intracellular granules to plasma membrane lipid rafts in response to antigen, and knockdown of PHB suppressed the movement of FcεRIγ and Syk into lipid rafts. Tyrosine phosphorylation of PHB by Lyn was observed early after exposure to antigen, and point mutations in PHB indicated that Tyr(114) and Tyr(259) were required for the recruitment of Syk to FcεRIγ and mast cell activation. In mice, PHB-specific siRNAs inhibited antigen-initiated mast cell degranulation, passive cutaneous anaphylaxis, and passive systemic anaphylaxis. Together, these results suggest that PHB is essential for FcεRI-mediated mast cell activation and allergic responses in vivo, raising the possibility that PHB might serve as a therapeutic target for the treatment of allergic diseases.

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.

Activation of mTORC1 signaling pathway in AIDS-related lymphomas

Using immunohistochemistry with antibodies against the phosphoserine residues in both S6rp and 4E binding protein 1, we identified the activation of the mammalian target of rapamycin (mTORC)1 pathway in 29 cases of AIDS-related lymphoma. These cases represented a diverse spectrum of histological types of non-Hodgkin lymphoma (24 cases) and classic Hodgkin lymphoma (five cases). mTORC1 was also activated in the hyperplastic but not involuted follicles of HIV-associated lymphadenopathy in eight cases, supporting the notion that mTORC1 activation is a common feature of transformed lymphocytes irrespective of either their reactive or malignant phenotype. We also found that in B-cell lines that represent diffuse large B-cell lymphoma, Burkitt lymphoma, Epstein-Barr virus-infected lymphocytes, and human herpesvirus 8-positive primary effusion lymphoma, inhibitors of Syk, MEK, and, seemingly, phosphoinositide 3 kinases suppressed mTORC1 activation, in particular when these inhibitors were used in combination. These findings indicate that AIDS-related lymphoma and other histologically similar types of lymphomas that are derived from transformed B lymphocytes may display clinical responses to inhibitors that directly target mTORC1 or, possibly, upstream activators of the mTORC1 pathway.

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.

Immune-complex assays for tyrosine protein kinases

Tyrosine protein kinases (TPKs) represent a diverse group of enzymes that contribute to cellular signal transduction. The generally low abundance of TPKs, coupled with their rapid activation and deactivation, usually precludes their purification through conventional biochemical means. Using immune-complex protein kinase assays, the presence or absence of a given TPK can be established and an estimation of its functional state obtained. In the Basic Protocol of this unit, TPKs are immunoprecipitated, allowed to autophosphorylate in the presence of labeled ATP, run out on an SDS-PAGE gel, and detected by autoradiography. Alternate protocols are provided for the assessment of the functional state of TPKs by providing a potential substrate along with the labeled ATP in the reaction mixture. In the first alternate protocol, the exogenous substrate is a protein, permitting simultaneous assessment of autophosphorylation and exogenous substrate phosphorylation. The second alternate protocol utilizes a peptide substrate, resulting in a rapid, high-throughput assay that evaluates only exogenous substrate phosphorylation.

Quantitative time-resolved phosphoproteomic analysis of mast cell signaling

Mast cells play a central role in type I hypersensitivity reactions and allergic disorders such as anaphylaxis and asthma. Activation of mast cells, through a cascade of phosphorylation events, leads to the release of mediators of the early phase allergic response. Understanding the molecular architecture underlying mast cell signaling may provide possibilities for therapeutic intervention in asthma and other allergic diseases. Although many details of mast cell signaling have been described previously, a systematic, quantitative analysis of the global tyrosine phosphorylation events that are triggered by activation of the mast cell receptor is lacking. In many cases, the involvement of particular proteins in mast cell signaling has been established generally, but the precise molecular mechanism of the interaction between known signaling proteins often mediated through phosphorylation is still obscure. Using recently advanced methodologies in mass spectrometry, including automation of phosphopeptide enrichments and detection, we have now substantially characterized, with temporal resolution as short as 10 s, the sites and levels of tyrosine phosphorylation across 10 min of FcepsilonRI-induced mast cell activation. These results reveal a far more extensive array of tyrosine phosphorylation events than previously known, including novel phosphorylation sites on canonical mast cell signaling molecules, as well as unexpected pathway components downstream of FcepsilonRI activation. Furthermore, our results, for the first time in mast cells, reveal the sequence of phosphorylation events for 171 modification sites across 121 proteins in the MCP5 mouse mast cell line and 179 modification sites on 117 proteins in mouse bone marrow-derived mast cells.

Negative autoregulation of Src homology region 2-domain-containing phosphatase-1 in rat basophilic leukemia-2H3 cells

Src homology region 2-domain-containing phosphatase-1 (SHP-1) plays an important role in the regulation of signaling from various receptors in hematopoietic cells. In mast cells, SHP-1 has been shown to negatively regulate the initial signaling triggered by high-affinity receptor for IgE (FcepsilonRI) and positively regulate downstream outputs. To clarify the molecular mechanisms of SHP-1 in mast cells, we determined substrates for SHP-1 by using the substrate-trapping approach. When phosphatase-inactive SHP-1 was over-expressed in rat basophilic leukemia (RBL)-2H3 cells, tyrosine phosphorylation of a 68-kDa protein was enhanced before and after FcepsilonRI aggregation. Immunoprecipitation and western blot analyses revealed that this protein is SHP-1, either endogenous or ectopically expressed. FcepsilonRI-induced activation of Lyn and Syk was comparable between cells expressing wild-type (wt) and phosphatase-inactive SHP-1. In vitro phosphatase assay and combined transfection, immunoprecipitation and immunoblot analyses showed that tyrosine 536 of SHP-1 was potent phosphorylation site and that SHP-1 could dephosphorylate this site that had been phosphorylated by Lyn. Furthermore, the phosphatase activity of SHP-1 immunoprecipitated from cells expressing a phosphatase-inactive SHP-1 was increased compared with that from vector-transfected or wt SHP-1-expressing cells. Finally, expression of phosphatase-inactive SHP-1 resulted in decreased activation of mitogen-activated protein kinases and suppressed transcription of cytokine genes, whereas wt SHP-1 enhanced these processes. Taken collectively, these results suggest that SHP-1 may be a physiological substrate of SHP-1 in RBL-2H3 cells and that dephosphorylation of SHP-1 leads to a decrease in its catalytic activity and an enhancement of downstream signaling. A negative autoregulatory circuit of SHP-1 may contribute to mast cell regulation.

Spleen tyrosine kinase (Syk) as a novel target for allergic asthma and rhinitis

Allergic asthma and rhinitis are prevalent diseases in the modern world, both marked by inflammation of the airways. The spleen tyrosine kinase (Syk) plays a critical role in the regulation of such immune and inflammatory responses. Although Syk is best known as a key component of immunoreceptor signalling complexes in leukocytes, recent studies demonstrated Syk expression in cells outside the haematopoietic lineage. Moreover, in recent years, it has been established that Syk is involved in various signalling cascades including those originating from integrin and cytokine receptors. Thus, Syk likely has a much wider biological role than previously recognised. Specific inhibition of Syk using aerosolised antisense oligonucleotides in liposome complexes significantly decreased lung inflammatory responses in experimental asthma and acute lung injury models. In addition, pharmacological inhibitors of Syk have been recently developed with potential for use as therapeutics. However, in the development and the rational delivery of drugs targeting Syk, it is important to consider the multiple cell types that express this kinase and the potential effects of its inhibition on various physiological functions. This review focuses on the recent data and the emerging ideas about Syk as a therapeutic target.

Inhibition of spleen tyrosine kinase prevents mast cell activation and airway hyperresponsiveness

RATIONALE

Spleen tyrosine kinase (Syk) is important for Fc and B-cell receptor-mediated signaling.

OBJECTIVE

To determine the activity of a specific Syk inhibitor (R406) on mast cell activation in vitro and on the development of allergen-induced airway hyperresponsiveness (AHR) and inflammation in vivo.

METHODS

AHR and inflammation were induced after 10 d of allergen (ovalbumin [OVA]) exposure exclusively via the airways and in the absence of adjuvant. This approach was previously established to be IgE, FcepsilonRI, and mast cell dependent. Alternatively, mice were passively sensitized with OVA-specific IgE, followed by limited airway challenge. In vitro, the inhibitor was added to cultures of IgE-sensitized bone marrow-derived mast cells (BMMCs) before cross-linking with allergen.

RESULTS

The inhibitor prevented OVA-induced degranulation of passively IgE-sensitized murine BMMCs and inhibited the production of interleukin (IL)-13, tumor necrosis factor alpha, IL-2, and IL-6 in these sensitized BMMCs. When administered in vivo, R406 inhibited AHR, which developed in BALB/c mice exposed to aerosolized 1% OVA for 10 consecutive d (20 min/d), as well as pulmonary eosinophilia and goblet cell metaplasia. A similar inhibition of AHR was demonstrated in mice passively sensitized with OVA-specific IgE and exposed to limited airway challenge.

CONCLUSION

This study delineates a functional role for Syk in the development of mast cell- and IgE-mediated AHR and airway inflammation, and these results indicate that inhibition of Syk may be a target in the treatment of allergic asthma.

Regulation of highly cytokinergic IgE-induced mast cell adhesion by Src, Syk, Tec, and protein kinase C family kinases

Mast cells play a critical role in IgE-dependent immediate hypersensitivity. Recent studies have shown that, contrary to the traditional view, binding of monomeric IgE to Fc epsilon RI results in a number of biological outcomes in mast cells, including survival. However, IgE molecules display heterogeneity in inducing cytokine production; highly cytokinergic (HC) IgEs cause extensive Fc epsilon RI aggregation, which leads to potent enhancement of survival and other activation events, whereas poorly cytokinergic (PC) IgEs can do so inefficiently. The present study demonstrates that HC, but not PC, IgEs can efficiently induce adhesion and spreading of mouse mast cells on fibronectin-coated plates in slow and sustained kinetics. HC IgE-induced adhesion through beta1 and beta7 integrins promotes survival, IL-6 production, and DNA synthesis. Importantly, we have identified Lyn and Syk as requisite tyrosine kinases and Hck, Btk, and protein kinase C theta as contributory kinases in HC IgE-induced adhesion and spreading, whereas protein kinase C epsilon plays a negative role. Consistent with these results, Lyn, Syk, and Btk are activated in HC IgE-stimulated cells in a slower but more sustained manner, compared with cells stimulated with IgE and Ag. Thus, binding of HC IgEs to Fc epsilon RI induces adhesion of mast cells to fibronectin by modulating cellular activation signals in a unique fashion.

Inhibition of syk activity and degranulation of human mast cells by flavonoids

To investigate the effect of flavonoids on the activation of p72(syk) (Syk) protein tyrosine kinase which plays a pivotal role in the high affinity IgE receptor-mediated degranulation of mast cells, we picked out 10 flavonoids, classified them into 4 series, and examined their effects on the activation of Syk and on the degranulation of human mast cells. Flavones and flavonols showed clear inhibition, whereas flavanones and isoflavones had either weak or no effect on Syk enzymatic activity induced by amino acid peptide corresponding to the activation loop domain and on IgE-dependent degranulation of human cultured mast cells (HCMC). On the basis of calculated logP (ClogP) values as a prediction of compound lipophilicity, some flavonoids were speculated to have low lipophilicity, the reason for poor cell permeability. A significant relationship was observed between the inhibition of Syk activity and HCMC degranulation attributable to flavonoids when the ClogP values of the compounds were taken into account (r(2)=0.89). These results suggested that the impairment of mast cell degranulation by several flavonoids classified into flavones and flavonols might be mediated via inhibition of the intracellular activation of Syk.

p28, a novel IgE receptor-associated protein, is a sensor of receptor occupation by its ligand in mast cells

Mast cells express the high affinity receptor for IgE (FcepsilonRI). Aggregation of this receptor by IgE and antigen leads to a signaling cascade resulting in the secretion of histamine, in the synthesis of other pro-inflammatory mediators such as leukotrienes and prostaglandins, and in the production of various cytokines, all of which participate in the development of the allergic reaction. In the last years, growing evidence accumulated that binding of IgEs to FcepsilonRI in itself induces active signals leading to mast cell survival, increased expression of FcepsilonRI, transient induction of histidine decarboxylase synthesis, and increased cell adhesion. The mechanisms underlying monomeric IgE signaling in the absence of receptor aggregation are still poorly understood. Here, we show that a protein of 28 kDa (p28) is physically and constitutively associated with FcepsilonRI in mast cells. Coimmunoprecipitation studies from (125)I surface-labeled cells demonstrated that this association involves at least 50% of membrane-expressed FcepsilonRI. After the addition of monomeric IgE to the cells, the p28.FcepsilonRI complex dissociates almost completely in less than 2 min. This dissociation is temperature-sensitive and is not due to the recruitment of additional proteins to the complex. Stripping bound IgE from the cells by acidic treatment promotes a rapid reassociation between p28 and FcepsilonRI. Altogether, these data are consistent with a conformational regulation of the complex. Thus, p28 is a sensor for FcepsilonRI occupation by IgE on mast cells, and its dissociation from the receptor could represent an early step of monomeric IgE signaling.

The FcR gamma subunit and Syk kinase replace the CD3 zeta-chain and ZAP-70 kinase in the TCR signaling complex of human effector CD4 T cells

The TCR-mediated signals required to activate resting T cells have been well characterized; however, it is not known how TCR-coupled signals are transduced in differentiated effector T cells that coordinate ongoing immune responses. Here we demonstrate that human effector CD4 T cells up-regulate the expression of the CD3zeta-related FcRgamma signaling subunit that becomes part of an altered TCR/CD3 signaling complex containing CD3epsilon, but not CD3zeta. The TCR/CD3/FcRgamma complex in effector cells recruits and activates the Syk, but not the ZAP-70, tyrosine kinase. This physiologic switch in TCR signaling occurs exclusively in effector, and not naive or memory T cells, suggesting a potential target for manipulation of effector responses in autoimmune, malignant, and infectious diseases.

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

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

Phospholipase C gamma 2 is essential for specific functions of Fc epsilon R and Fc gamma R

Phospholipase Cgamma2 (PLCgamma2) plays a critical role in the functions of the B cell receptor in B cells and of the FcRgamma chain-containing collagen receptor in platelets. Here we report that PLCgamma2 is also expressed in mast cells and monocytes/macrophages and is activated by cross-linking of Fc(epsilon)R and Fc(gamma)R. Although PLCgamma2-deficient mice have normal development and numbers of mast cells and monocytes/macrophages, we demonstrate that PLCgamma2 is essential for specific functions of Fc(epsilon)R and Fc(gamma)R. While PLCgamma2-deficient mast cells have normal mitogen-activated protein kinase activation and cytokine production at mRNA levels, the mutant cells have impaired Fc(epsilon)R-mediated Ca(2+) flux and inositol 1,4,5-trisphosphate production, degranulation, and cytokine secretion. As a physiological consequence of the effect of PLCgamma2 deficiency, the mutant mice are resistant to IgE-mediated cutaneous inflammatory skin reaction. Macrophages from PLCgamma2-deficient mice have no detectable Fc(gamma)R-mediated Ca(2+) flux; however, the mutant cells have normal Fc(gamma)R-mediated phagocytosis. Moreover, PLCgamma2 plays a nonredundant role in Fc(gamma)R-mediated inflammatory skin reaction.

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

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

Regulation of mast-cell and basophil function and survival by IgE

Mast cells and basophils are important effector cells in T helper 2 (T(H)2)-cell-dependent, immunoglobulin-E-associated allergic disorders and immune responses to parasites. The crosslinking of IgE that is bound to the high-affinity receptor Fc epsilon RI with multivalent antigen results in the aggregation of Fc epsilon RI and the secretion of products that can have effector, immunoregulatory or autocrine effects. This response can be enhanced markedly in cells that have been exposed to high levels of IgE, which results in the increased surface expression of Fc epsilon RI. Moreover, recent work indicates that monomeric IgE (in the absence of crosslinking) can render mast cells resistant to apoptosis induced by growth-factor deprivation in vitro and, under certain circumstances, can induce the release of cytokines. So, the binding of IgE to Fc epsilon RI might influence mast-cell and basophil survival directly or indirectly, and can also regulate cellular function.

Proteasome-dependent regulation of Syk tyrosine kinase levels in human basophils

BACKGROUND

In human basophils, FcepsilonRI signal initiation, leading to histamine release, relies on activation of Syk protein tyrosine kinase. Basophils from approximately 10% of unselected donors do not degranulate in response to FcepsilonRI cross-linking. Their unresponsiveness has been linked to the absence of Syk protein despite apparently normal levels of Syk mRNA.

OBJECTIVE

The aim of this study was to explore pathways of Syk protein degradation as a possible posttranslational mechanism for downregulating Syk protein levels in human basophils and other leukocytes.

METHODS

Highly purified basophils, lymphocytes, and monocytes were incubated in the presence or absence of a panel of cell-permeable inhibitors of proteolytic degradation pathway(s). Subsequently, the protein level of Syk tyrosine kinase was determined by means of Western blotting. In vitro assays were conducted through use of immunoprecipitated basophil Syk and a rabbit reticulocyte lysate system.

RESULTS

Three inhibitors of proteasome-mediated degradation-PSI, lactacystin, and ALLN-substantially increased Syk levels in releaser basophils and restored Syk expression in nonreleaser basophils. Caspase inhibitors were less effective, and inhibitors of calpain-mediated proteolysis had no effect. Among other leukocytes tested, only naive CD4(+) T cells had more Syk after proteasome inhibitor treatment. In vitro ubiquitination assays demonstrated that Syk is readily ubiquitinated in vitro and also that Syk ubiquitination is associated with a substantial decrease in total levels of Syk protein.

CONCLUSION

These data provide evidence for a ubiquitin/proteasome-dependent mechanism that contributes to Syk regulation in human basophils and might also be relevant to naive T cells. Understanding this regulatory pathway might lead to strategies for suppressing allergic inflammation while preserving essential Syk-mediated functions in other hematopoietic cells.

FcepsilonRI signaling observed from the inside of the mast cell membrane

Crosslinking the high affinity IgE receptor, FcrepsilonRI, on basophils and mast cells initiates cascades of biochemical events leading to degranulation, membrane ruffling and other physiological responses. Downstream of FcepsilonRI and its coupled tyrosine kinases, Lyn and Syk, scores of different proteins and lipids are implicated in these signaling cascades and new players are being identified continuously. Here, we use immunogold probes to label receptors and signaling proteins on the cytoplasmic face of membrane sheets prepared from RBL-2H3 mast cells and transmission electron microscopy to examine their distributions in relationship to each other and to features of the membrane. New topographical data are integrated with existing knowledge of the biochemistry of FcepsilonRI signaling and of cell shape during signaling to implicate at least two distinct membrane domains in FcepsilonRI signaling. "Primary signaling domains", also called osmiophilic patches, are recognized by their dark staining with osmium, adjacency to coated pits (previously mapped to planar membrane between lamellae) and by the characteristic presence of receptor, Syk and PLCgamma2, but not Lyn. "Secondary signaling domains" are characterized by the presence of large elliptical linker for activation of T cells (LAT) rafts and of PLCgamma1 (previously mapped to lamellae) but not receptor. The signaling proteins, Vav, Grb2, Cbl and Gab2, and the endocytic proteins, AP2 and clathrin, all map to the primary domains, while the p85 regulatory subunit of phosphatidylinositol 3 (PI 3)-kinase maps to both domains. Recognition that FcepsilonRI signaling is controlled not only by which chemical species are available for interaction, but also by where the interactions occur, may provide new opportunities for the modeling of signaling cascades and new targets for the development of drugs to treat allergies and asthma.

Inhibition of allergic inflammation in the airways using aerosolized antisense to Syk kinase

Activation of the protein tyrosine kinase Syk is an early event that follows cross-linking of Fc gamma R and Fc epsilon R, leading to the release of biologically active molecules in inflammation. We reported previously that aerosolized Syk antisense oligodeoxynucleotides (ASO) depresses Syk expression in inflammatory cells, the release of mediators from alveolar macrophages, and pulmonary inflammation. To study the effect of Syk ASO in allergic inflammation and airway hyperresponsiveness, we used the Brown Norway rat model of OVA-induced allergic asthma. Syk ASO, delivered in a liposome, carrier/lipid complex by aerosol to rats, significantly inhibited the Ag-induced inflammatory cell infiltrate in the bronchoalveolar space, decreasing both neutrophilia and eosinophilia. The number of eosinophils in the lung parenchyma was also diminished. Syk ASO also depressed up-regulation of the expression of beta(2) integrins, alpha(4) integrin, and ICAM-1 in bronchoalveolar lavage leukocytes and reversed the Ag-induced decrease in CD62L expression on neutrophils. Furthermore, the increase in TNF levels in bronchoalveolar lavage following Ag challenge was significantly inhibited. Syk ASO also suppressed Ag-mediated contraction of the trachea in a complementary model. Thus, aerosolized Syk ASO suppresses many of the central components of allergic asthma and inflammation and may provide a new therapeutic approach.

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

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

Structure-function analysis of Lyn kinase association with lipid rafts and initiation of early signaling events after Fcepsilon receptor I aggregation

The first step in immunoreceptor signaling is represented by ligand-dependent receptor aggregation, followed by receptor phosphorylation mediated by tyrosine kinases of the Src family. Recently, sphingolipid- and cholesterol-rich plasma membrane microdomains, called lipid rafts, have been identified and proposed to function as platforms where signal transduction molecules may interact with the aggregated immunoreceptors. Here we show that aggregation of the receptors with high affinity for immunoglobulin E (FcepsilonRI) in mast cells is accompanied by a co-redistribution of the Src family kinase Lyn. The co-redistribution requires Lyn dual fatty acylation, Src homology 2 (SH2) and/or SH3 domains, and Lyn kinase activity, in cis or in trans. Palmitoylation site-mutated Lyn, which is anchored to the plasma membrane but exhibits reduced sublocalization into lipid rafts, initiates the tyrosine phosphorylation of FcepsilonRI subunits, Syk protein tyrosine kinase, and the linker for activation of T cells, along with an increase in the concentration of intracellular Ca(2+). However, Lyn mutated in both the palmitoylation and myristoylation sites does not anchor to the plasma membrane and is incapable of initiating FcepsilonRI phosphorylation and early signaling events. These data, together with our finding that a constitutively tyrosine-phosphorylated FcepsilonRI does not exhibit an increased association with lipid rafts, suggest that FcepsilonRI phosphorylation and early activation events can be initiated outside of lipid rafts.

Piceatannol is an effective inhibitor of IgE-mediated secretion from human basophils but is neither selective for this receptor nor acts on syk kinase at concentrations where mediator release inhibition occurs

BACKGROUND

Syk kinase is probably an early necessary tyrosine kinase involved in IgE-mediated secretion from human basophils. Causal testing of the role of syk kinase in the secretion requires a selective pharmacological agent. Piceatannol has previously been used to demonstrate the causal role of syk in secretion but its selectively has recently come into question.

OBJECTIVE

To determine whether piceatannol inhibits IgE-mediated signalling events in a manner consistent with its putative inhibitory effects on syk kinase and at concentrations relevant to its inhibition of mediator release.

METHODS

Human basophils were examined for the effects of piceatannol on mediator release or various signalling steps.

RESULTS

We show that while piceatannol has an IC50 for inhibition of IgE-mediated histamine release of 3-5 microm, these same concentrations inhibit secretion of phorbol 12-myristate 13-acetate (PMA)-induced histamine release (as previously shown) and leukotriene C (LTC)4 release induced by fMLP. Concentrations of piceatannol up to 100 microm also did not inhibit IgE-mediated phosphorylation of shc, a immediate downstream target of syk kinase. Similar concentrations also did not inhibit IgE-mediated cytosolic calcium elevations, another downstream signal thought to be dependent on syk kinase. In contrast, piceatannol did modify the cytosolic calcium response that follows stimulation with formyl methionyl-leucyl-phenylalanine (fMLP).

CONCLUSION

Taken together with published studies using other cell types, we conclude that piceatannol does not inhibit secretion from human basophils by inhibiting the activity of syk kinase.

Overcoming the signaling defect of Lyn-sequestering, signal-curtailing FcepsilonRI dimers: aggregated dimers can dissociate from Lyn and form signaling complexes with Syk

Clustering the tetrameric (alphabetagamma(2)) IgE receptor, FcepsilonRI, on basophils and mast cells activates the Src-family tyrosine kinase, Lyn, which phosphorylates FcepsilonRI beta and gamma subunit tyrosines, creating binding sites for the recruitment and activation of Syk. We reported previously that FcepsilonRI dimers formed by a particular anti-FcepsilonRI alpha mAb (H10) initiate signaling through Lyn activation and FcepsilonRI subunit phosphorylation, but cause only modest activation of Syk and little Ca(2+) mobilization and secretion. Curtailed signaling was linked to the formation of unusual, detergent-resistant complexes between Lyn and phosphorylated receptor subunits. Here, we show that H10-FcepsilonRI multimers, induced by adding F(ab')(2) of goat anti-mouse IgG to H10-treated cells, support strong Ca(2+) mobilization and secretion. Accompanying the recovery of signaling, H10-FcepsilonRI multimers do not form stable complexes with Lyn and do support the phosphorylation of Syk and phospholipase Cgamma2. Immunogold electron microscopy showed that H10-FcepsilonRI dimers colocalize preferentially with Lyn and are rarely within the osmiophilic "signaling domains" that accumulate FcepsilonRI and Syk in Ag-treated cells. In contrast, H10-FcepsilonRI multimers frequently colocalize with Syk within osmiophilic patches. In sucrose gradient centrifugation analyses of detergent-extracted cells, H10-treated cells show a more complete redistribution of FcepsilonRI beta from heavy (detergent-soluble) to light (Lyn-enriched, detergent-resistant) fractions than cells activated with FcepsilonRI multimers. We hypothesize that restraints imposed by the particular orientation of H10-FcepsilonRI dimers traps them in signal-initiating Lyn microdomains, and that converting the dimers to multimers permits receptors to dissociate from Lyn and redistribute to separate membrane domains that support Syk-dependent signal propagation.

Gi-mediated activation of the Syk kinase by the receptor mimetic basic secretagogues of mast cells: role in mediating arachidonic acid/metabolites release

Syk kinase is essential for FcepsilonRI-mediated signaling and release of inflammatory mediators from mast cells. We now show that activation of rat peritoneal mast cells by the nonimmunological, G(i)-mediated pathway also results in the activation of Syk. We show that compound 48/80 (c48/80), a receptor analogue that activates directly G proteins, activates Syk in a pertussis toxin-sensitive fashion. We further show that Syk activation by c48/80 is blocked by the protein kinase C inhibitor GF109203X, by the phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002, by EGTA, and by the selective src-like kinase inhibitor PP1. These results suggest that in the nonimmunological, G(i)-mediated pathway, Syk is located downstream from phospholipase C and phosphatidylinositol 3-kinase. However, in common with the FcepsilonRI-mediated pathway, activation of Syk by c48/80 is dependent on a src-like protein tyrosine kinase. Finally, we show that in the nonimmunological pathway, Syk plays a central role in the release of arachidonic acid/eicosanoid metabolites, but not in the release of prestored mediators such as histamine.

Exposure of RBL-2H3 mast cells to Ag(+) induces cell degranulation and mediator release

There is a growing need to understand the impact of environmental sulfhydryl group-reactive heavy metals on the immune system. Here we show that Ag(+) induces mast cell degranulation, as does the aggregation of the high affinity immunoglobulin E receptor (FcepsilonRI). Micromolar quantities of Ag(+) specifically induced degranulation of mast cell model rat basophilic leukemia (RBL-2H3) cells without showing cytotoxicity. The Ag(+)-mediated degranulation could be observed as rapidly as 5 min after the addition of the ions. Ag(+) also induced a rapid change in tyrosine phosphorylation of multiple cellular proteins including the focal adhesion kinase but not Syk kinase. The Syk-selective inhibitor piceatannol and the Src family-selective tyrosine kinase inhibitor PP1 dose-dependently inhibited FcepsilonRI-mediated degranulation, whereas neither compound inhibited the Ag(+)-mediated degranulation. Furthermore, likewise FcepsilonRI aggregation, Ag(+) also induced leukotriene secretion. These results show that Ag(+) activates RBL-2H3 mast cells through a tyrosine phosphorylation-linked mechanism, which is distinct from that involved in FcepsilonRI-mediated activation.

Peptoid - peptide hybrids that bind Syk SH2 domains involved in signal transduction

Peptoid-peptide hybrids are oligomeric peptidomimetics that contain one or more N-substituted glycine residues. In these hybrids, the side chains of one or several amino acids are "shifted" from the alpha-carbon atom to the amide nitrogen atom. A library of phosphorylated peptoid-peptide hybrids derived from the sequence pTyr-Glu-Thr-Leu was synthesized and tested for binding to the tandem SH2 domain of the protein tyrosine kinase Syk. A considerable influence of the side chain position was observed. Compounds 19-21, 24, and 25 comprising a peptoid NpTyr and/or NGlu residue did not show any binding. Compounds 22, 23, and 26 containing an NhThr (hThr=homothreonine) and/or NLeu peptoid residue showed binding with IC(50) values that were only five to eight times higher than that of the tetrapeptide lead compound 18. These data show that side chain shifting is possible with retention of binding capacity, but only at the two C-terminal residues of the tetramer. This method of a peptoid scan using peptoid-peptide hybrids appears to be very useful to explore to what extent a peptide sequence can be transformed into a peptoid while retaining its affinity.

Tyrosine phosphorylation of the linker for activator of T cells in mast cells by stimulation with the high affinity IgE receptor

Aggregation of the high affinity IgE receptors (FcepsilonRI) on basophils and mast cells, members of the immune receptor family, initiates a cascade of events that results in the release of inflammatory mediators. This pathway involves the activation of several protein-tyrosine kinases, including Lyn, Syk, Btk, and Fak that induce the tyrosine phosphorylation of various proteins. The linker for activation of T cells (LAT), was originally found as a ZAP-70 tyrosine kinase substrate that linked T cell receptors to cellular activation, and was expressed in T cells, NK cells and mast cells. Here we show that LAT expressed in the RBL-2H3 rat mast cell line is tyrosine-phosphorylated after aggregation of FcepsilonRI. The tyrosine phosphorylation of the LAT was dramatically enhanced after receptor aggregation. Furthermore, a tyrosine-phosphorylated 80-kDa protein associated with LAT transiently after receptor aggregation. GST fusion proteins containing parts of PLCgamma or PI3 kinase can bind LAT. These results suggest that LAT plays an important role not only in T cell, but also in mast cell activation, and that the association among these signaling molecules is critical for FcepsilonRI-mediated intracellular signal transduction in mast cells.

Pharmacology of IgE-mediated desensitization of human basophils: effects of protein kinase C and src-family kinase inhibitors

IgE-mediated down-regulation of secretion from basophils and mast cells is an important component of the overall cellular response that determines the ultimate extent of mediator release. The down-regulatory process that occurs during active secretion has also been associated with the methodological phenomenon called desensitization, but the mechanisms underlying desensitization are not understood. A variety of studies have suggested that activation of protein kinase C (PKC) results in down-regulation of IgE-mediated secretion so we have examined the effect of the PKC inhibitors Ro-31-8220 (3-[1-[3-amidinothio)propyl-1H-indol-3-yl]-3-(1-methyl-1H-indol-3- yl) maleimide) and bis-indolylmaleimide II on desensitization in human basophils. At concentrations that have been shown previously to inhibit PKC-mediated functions in basophils completely, these two drugs had no effect on IgE-mediated desensitization. We did find, however, that the src-family kinase inhibitors PP1 [4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] and PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3, 4-d]pyrimidine] inhibited desensitization as well as secretion. These data suggest that PKC has little role in down-regulating the IgE-mediated basophil response. However, like the activation signaling cascade, the desensitization process is dependent on the activation of src family kinases.

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

Syk is an important protein-tyrosine kinase in immunoreceptor signaling. FcepsilonRI aggregation in mast cells induces tyrosine phosphorylation and increased enzymatic activity of Syk. The two adjacent tyrosines in the Syk activation loop are thought to be important for the propagation of FcepsilonRI signaling. To evaluate the phosphorylation of these tyrosines in vivo and further understand the relationship of Syk tyrosine phosphorylation with its function, an antibody was developed specific for phosphorylated tyrosines in the activation loop of Syk. FcepsilonRI aggregation on mast cells induced the phosphorylation of both tyrosine residues of the activation loop. The kinase activity of Syk played the major role in phosphorylating its activation loop tyrosines both in vivo and in vitro. In FcepsilonRI-stimulated mast cells, the total Syk tyrosine phosphorylation paralleled the phosphorylation of its activation loop tyrosines and downstream propagation of signals for histamine release. In contrast, the cell surface binding of anti-ganglioside monoclonal antibody AA4 induced only strong general tyrosine phosphorylation of Syk and minimal histamine release and weak phosphorylation of activation loop tyrosines. These results demonstrate that phosphorylation of the activation loop tyrosines is important for mediating receptor signaling and is a better marker of Syk function than is total Syk tyrosine phosphorylation.

Diphenyleneiodonium prevents reactive oxygen species generation, tyrosine phosphorylation, and histamine release in RBL-2H3 mast cells

Mast cells play a central role in immediate allergic reactions mediated by immunoglobulin E. It has recently been reported that mast cells generate intracellular reactive oxygen species (ROS) in response to stimulation with divergent physiologically relevant stimulants. However, the physiological role of ROS is poorly understood. Here we demonstrate that mast cell model rat basophilic leukemia (RBL-2H3) cells generate ROS in response to antigen and the calcium-ionophore A23187 via activation of diphenyleneiodonuim (DPI)-sensitive enzyme and that blockade of ROS generation by DPI suppresses histamine release induced by either stimulant. Increased tyrosine phosphorylation of pp125(FAK) and a 77-kDa protein coprecipitating specifically with the kinase occurred in parallel with the secretion, and blockade of ROS generation by DPI also suppressed the tyrosine phosphorylation of both proteins. These findings suggest that ROS generated by a flavoenzyme-dependent mechanism may be involved in histamine release through the pp125(FAK) pathway.

Akt-dependent cytokine production in mast cells

Cross-linking of FcepsilonRI induces the activation of three protein tyrosine kinases, Lyn, Syk, and Bruton's tyrosine kinase (Btk), leading to the secretion of a panel of proinflammatory mediators from mast cells. This study showed phosphorylation at Ser-473 and enzymatic activation of Akt/protein kinase B, the crucial survival kinase, upon FcepsilonRI stimulation in mouse mast cells. Phosphorylation of Akt is regulated positively by Btk and Syk and negatively by Lyn. Akt in turn can regulate positively the transcriptional activity of interleukin (IL)-2 and tumor necrosis factor (TNF)-alpha promoters. Transcription from the nuclear factor kappaB (NF-kappaB), nuclear factor of activated T cells (NF-AT), and activator protein 1 (AP-1) sites within these promoters is under the control of Akt activity. Accordingly, the signaling pathway involving IkappaB-alpha, a cytoplasmic protein that binds NF-kappaB and inhibits its nuclear translocation, appears to be regulated by Akt in mast cells. Catalytic activity of glycogen synthase kinase (GSK)-3beta, a serine/threonine kinase that phosphorylates NF-AT and promotes its nuclear export, seems to be inhibited by Akt. Importantly, Akt regulates the production and secretion of IL-2 and TNF-alpha in FcepsilonRI-stimulated mast cells. Altogether, these results revealed a novel function of Akt in transcriptional activation of cytokine genes via NF-kappaB, NF-AT, and AP-1 that contributes to the production of cytokines.

Negative regulation of FcepsilonRI signaling by FcgammaRII costimulation in human blood basophils

BACKGROUND

Signaling through the antigen receptors of human B and T cells and the high-affinity IgE receptor FcepsilonRI of rodent mast cells is decreased by cross-linking these receptors to the low-affinity IgG receptor FcgammaRII. The inhibition is thought to involve the tyrosine phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the FcgammaRIIB cytoplasmic tail, creating binding sites for SH2-containing protein (Src homology domain containing protein tyrosine phosphatase 1 and 2 [SHP-1, SHP-2]) and/or lipid (SH2 domain-containing polyphosphatidyl-inositol 5-phosphatase) phosphatases that oppose activating signals from the costimulated antigen receptors.

OBJECTIVE

In human basophils and mast cells FcepsilonRI signaling generates mediators and cytokines responsible for allergic inflammation. We proposed to determine whether FcepsilonRI signaling is inhibited by FcgammaRII costimulation in human basophils and to explore the underlying mechanism as an approach to improving the treatment of allergic inflammation.

METHODS

FcgammaR expression on human basophils was examined using flow cytometry and RT-PCR analysis. FcgammaRII/FcepsilonRI costimulation was typically accomplished by priming cells with anti-dinitrophenol (DNP) IgE and anti-DNP IgG and stimulating with DNP-BSA. Phosphatases were identified by Western blotting, and their partitioning between membrane and cytosol was determined by cell fractionation. Biotinylated synthetic peptides and phosphopeptides corresponding to the FcgammaRIIB ITIM sequence were used for adsorption assays.

RESULTS

We report that peripheral blood basophils express FcgammaRII (in both the ITIM-containing FcgammaRIIB and the immunoreceptor tyrosine-based activation motif-containing FcgammaRIIA forms) and that costimulating FcgammaRII and FcepsilonRI inhibits basophil FcepsilonRI-mediated histamine release, IL-4 production, and Ca(2+) mobilization. The inhibition of basophil FcepsilonRI signaling by FcgammaRII/FcepsilonRI costimulation is linked to a significant decrease in Syk tyrosine phosphorylation. Human basophils express all 3 SH2-containing phosphatases.

CONCLUSIONS

Evidence that FcgammaRII/FcepsilonRI costimulation induces SHP-1 translocation from the cytosolic to membrane fractions of basophils and that biotinylated synthetic peptides corresponding to the phosphorylated FcgammaRIIB ITIM sequence specifically recruit SHP-1 from basophil lysates particularly implicates this protein phosphatase in the negative regulation of FcepsilonRI signaling by costimulated FcgammaRII.

Immunologically mediated signaling in basophils and mast cells: finding therapeutic targets for allergic diseases in the human FcvarepsilonR1 signaling pathway

The high affinity IgE receptor, FcvarepsilonRI, plays key roles in an array of acute and chronic human allergic reactions including asthma, allergic rhinitis, atopic dermatitis, urticaria and anaphylaxis. In humans and rodents, this receptor is found at high levels on basophils and mast cells where its activation by IgE and multivalent antigen produces mediators and cytokines responsible for FcvarepsilonRI-dependent acute inflammation. Mast cells can additionally contribute to sustained inflammatory responses by internalizing antigen bound to IgE-FcvarepsilonRI complexes for processing to peptides and presentation to T cells. In humans, the FcvarepsilonRI is also expressed, at lower density, on monocytes, macrophages and dendritic cells (DC) where its likely functions again include both signaling to mediator and cytokine production and antigen presentation. Our laboratories have focused on defining the earliest steps in the FcvarepsilonRI signaling cascade in basophils and mast cells and on developing new routes to control allergic inflammation based on inhibiting these events. Here, we describe novel strategies to limit antigen-stimulated FcvarepsilonRI signaling by: (1) sequestering the FcvarepsilonRI-associated protein-tyrosine kinase, Lyn, that initiates FcvarepsilonRI signaling; (2) eliminating; or (3) inactivating the protein-tyrosine kinase, Syk, that propagates FcvarepsilonRI signaling; and (4) establishing inhibitory crosstalk between FcvarepsilonRI and a co-expressed receptor, FcgammaRII, that again limits FcvarepsilonRI-mediated Syk activation. These strategies may form the basis for new therapies for allergic inflammation.

Inhibition of calcium release-activated calcium current by Rac/Cdc42-inactivating clostridial cytotoxins in RBL cells

Using large clostridial cytotoxins as tools, the role of Rho GTPases in activation of RBL 2H3 hm1 cells was studied. Clostridium difficile toxin B, which glucosylates Rho, Rac, and Cdc42 and Clostridium sordellii lethal toxin, which glucosylates Rac and Cdc42 but not Rho, inhibited the release of hexosaminidase from RBL cells mediated by the high affinity antigen receptor (FcepsilonRI). Additionally, toxin B and lethal toxin inhibited the intracellular Ca(2+) mobilization induced by FcepsilonRI-stimulation and thapsigargin, mainly by reducing the influx of extracellular Ca(2+). In patch clamp recordings, toxin B and lethal toxin inhibited the calcium release-activated calcium current by about 45%. Calcium release-activated calcium current, the receptor-stimulated Ca(2+) influx, and secretion were inhibited neither by the Rho-ADP-ribosylating C3-fusion toxin C2IN-C3 nor by the actin-ADP-ribosylating Clostridium botulinum C2 toxin. The data indicate that Rac and Cdc42 but not Rho are not only involved in late exocytosis events but are also involved in Ca(2+) mobilization most likely by regulating the Ca(2+) influx through calcium release-activated calcium channels activated via FcepsilonRI receptor in RBL cells.