Transient targeting of phosphoinositide 3-kinase acts as a roadblock in mast cells' route to allergy

Human placental extract suppresses mast cell activation and induces mast cell apoptosis

BACKGROUND

Human placental extract (HPE) has been documented to facilitate the healing of certain disorders including allergy. However, the effects of HPE on the functionality of mast cells, a critical cell type in allergic diseases, have not been reported.

METHODS

To investigate the effects of HPE on the regulation of allergy with respect to the biological functions of mast cells, the mast cell line C57 or HMC-1 cells were treated with HPE followed by the assessment of cell proliferation, apoptosis, activation, chemotaxis and phagocytosis. Mouse peritoneal mast cells were also investigated for their responses to induction of apoptosis by HPE in vivo. Furthermore, the effect of HPE on mast cell degranulation was confirmed using the passive cutaneous anaphylaxis (PCA) assay, an acute allergy model.

RESULTS

HPE was capable of suppressing mast cell proliferation and inducing mast cell apoptosis. Mast cell degranulation in response to compound 48/80- or anti-DNP IgE and DNP-mediated activation was suppressed. In addition, treatment with HPE compromised the production of cytokines by mast cells and cell chemotaxis. These observations were consistent with the dampened passive cutaneous anaphylaxis (PCA) assay following treatment with HPE.

CONCLUSION

This study revealed a suppressive effect of HPE on overall mast cell activities, suggesting a potential regulatory role of HPE on the alleviation of allergic diseases through mast cells.

The role of PI3Kγ in the immune system: new insights and translational implications

Over the past two decades, new insights have positioned phosphoinositide 3-kinase-γ (PI3Kγ) as a context-dependent modulator of immunity and inflammation. Recent advances in protein structure determination and drug development have allowed for generation of highly specific PI3Kγ inhibitors, with the first now in clinical trials for several oncology indications. Recently, a monogenic immune disorder caused by PI3Kγ deficiency was discovered in humans and modelled in mice. Human inactivated PI3Kγ syndrome confirms the immunomodulatory roles of PI3Kγ and strengthens newly defined roles of this molecule in modulating inflammatory cytokine release in macrophages. Here, we review the functions of PI3Kγ in the immune system and discuss how our understanding of its potential as a therapeutic target has evolved.

PI3K Isoforms in Cell Signalling and Innate Immune Cell Responses

Phosphoinositide-3-kinases (PI3Ks) are enzymes involved in signalling and modification of the function of all mammalian cells. These enzymes phosphorylate the 3-hydroxyl group of the inositol ring of phosphatidylinositol, resulting in lipid products that act as second messengers responsible for coordinating many cellular functions, including activation, chemotaxis, proliferation and survival. The identification of the functions that are mediated by a specific PI3K isoform is complex and depends on the specific cell type and inflammatory context. In this chapter we will focus on the role of PI3K isoforms in the context of innate immunity, focusing on the mechanisms by which PI3K signalling regulates phagocytosis, the activation of immunoglobulin, chemokine and cytokines receptors, production of ROS and cell migration, and how PI3K signalling plays a central role in host defence against infections and tissue injury.

Targeting Phosphoinositide 3-Kinase - Five Decades of Chemical Space Exploration

Phosphoinositide 3-kinase (PI3K) plays a key role in a plethora of physiologic processes and controls cell growth, metabolism, immunity, cardiovascular and neurological function, and more. The discovery of wort-mannin as the first potent PI3K inhibitor (PI3Ki) in the 1990s provided rapid identification of PI3K-dependent processes, which drove the discovery of the PI3K/protein kinase B (PKB/Akt)/target of rapamycin (mTOR) pathway. Genetic mouse models and first PI3K isoform-specific inhibitors pinpointed putative therapeutic applications. The recognition of PI3K as target for cancer therapy drove subsequently drug development. Here we provide a brief journey through the emerging roles of PI3K to the development of preclinical and clinical PI3Ki candidates.

A Review of Phosphocreatine 3 Kinase δ Subtype (PI3Kδ) and Its Inhibitors in Malignancy

Most cancer deaths are caused by metastasis. The phosphocreatine 3-kinase (PI3K) family includes the I–III classes, with class I divided into 4 subtypes (α, β, γ, δ); and PI3K signaling participates in the regulatory processes of cell proliferation, differentiation, apoptosis, and glucose transport. Moreover, PI3Ks are modulators of cellular membrane lipids involved in signaling and trafficking events. The PI3Kdelta isoform (PI3Kδ), which is not only specifically expressed in hematopoietic cells, but also in different tumor cell lines, is expressed extensively. The increase in PI3Kδ activity is often associated with a variety of cancers. Currently, the strategy of tumor therapy based on PI3Kδ and its related signaling pathway is developing. Besides its established role in controlling functions in autoimmunity and inflammation, the role of PI3Kδ in tumor and metastasis is not clearly elucidated, with the effects of inhibiting PI3Kδ in several types of tumors also remaining unexplored. In addition, the specific inhibitor of PI3Kδ in tumor progression and metastasis and its underlying mechanism need to be further studied. The purpose of this review is to rationalize the existing functions and mechanisms of PI3Kδ in tumor metastasis and the relationship with hematopoietic cells in cancers as well cross-talking with miRNA, which provides a new theoretical basis and potential therapeutic target for the drug therapy of tumor metastasis.

Disease-related mutations in PI3Kγ disrupt regulatory C-terminal dynamics and reveal a path to selective inhibitors

Class I Phosphoinositide 3-kinases (PI3Ks) are master regulators of cellular functions, with the class IB PI3K catalytic subunit (p110γ) playing key roles in immune signalling. p110γ is a key factor in inflammatory diseases and has been identified as a therapeutic target for cancers due to its immunomodulatory role. Using a combined biochemical/biophysical approach, we have revealed insight into regulation of kinase activity, specifically defining how immunodeficiency and oncogenic mutations of R1021 in the C-terminus can inactivate or activate enzyme activity. Screening of inhibitors using HDX-MS revealed that activation loop-binding inhibitors induce allosteric conformational changes that mimic those in the R1021C mutant. Structural analysis of advanced PI3K inhibitors in clinical development revealed novel binding pockets that can be exploited for further therapeutic development. Overall, this work provides unique insights into regulatory mechanisms that control PI3Kγ kinase activity and shows a framework for the design of PI3K isoform and mutant selective inhibitors.

PI3Kγ Regulatory Protein p84 Determines Mast Cell Sensitivity to Ras Inhibition-Moving Towards Cell Specific PI3K Targeting?

Mast cells are the major effector cells in immunoglobulin E (IgE)-mediated allergy. The high affinity IgE receptor FcεRI, as well as G protein-coupled receptors (GPCRs) on the mast cell surface signals to phosphoinositide 3-kinase γ (PI3Kγ) to initiate degranulation, cytokine release, and chemotaxis. PI3Kγ is therefore considered as a target for treatment of allergic disorders. However, leukocyte PI3Kγ is key to many functions in innate and adaptive immunity, and attenuation of host defense mechanisms is an expected adverse effect that complicates treatment of chronic illnesses. PI3Kγ operates as a p110γ/p84 or p110γ/p101 complex, where p110γ/p84 requires Ras activation. Here we investigated if modulation of Ras-isoprenylation could target PI3Kγ activity to attenuate PI3Kγ-dependent mast cell responses without impairment of macrophage functions. In murine bone marrow-derived mast cells, GPCR stimulation triggers activation of N-Ras and H-Ras isoforms, which is followed by the phosphorylation of protein kinase B (PKB/Akt) relayed through PI3Kγ. Although K-Ras is normally not activated in Ras wild-type cells, it is able to compensate for genetically deleted N- and H-Ras isoforms. Inhibition of Ras isoprenylation with farnesyltransferase inhibitor FTI-277 leads to a significant reduction of mast cell degranulation, cytokine production, and migration. Complementation experiments expressing PI3Kγ adaptor proteins p84 or p101 demonstrated a differential sensitivity towards Ras-inhibition depending on PI3Kγ complex composition. Mast cell responses are exclusively p84-dependent and were effectively controlled by FTI-277. Similar results were obtained when GTP-Ras was inactivated by overexpression of the GAP-domain of Neurofibromin-1 (NF-1). Unlike mast cells, macrophages express p84 and p101 but are p101-dominated and thus remain functional under treatment with FTI-277. Our work demonstrates that p101 and p84 have distinct physiological roles, and that Ras dependence of PI3Kγ signaling differs between cell types. FTI-277 reduces GPCR-activated PI3Kγ  responses in p84-expressing but not p101-containing bone marrow derived cells. However, prenylation inhibitors have pleiotropic effects beyond Ras and non-tolerable side-effects that disfavor further clinical validation. Statins are, however, clinically well-established drugs that have previously been proposed to block mast cell degranulation by interference with protein prenylation. We show here that Simvastatin inhibits mast cell degranulation, but that this does not occur via Ras-PI3Kγ pathway alterations.

Roles of PI3K pan-inhibitors and PI3K-δ inhibitors in allergic lung inflammation: a systematic review and meta-analysis

Meta-analysis can be applied to study the effectiveness of the summary estimates for experimental papers, producing objective and unbiased results. We investigated the effects of phosphoinositide-3-kinase (PI3K) on the inflammatory profile in allergic mouse models, which are currently under development in signal transduction materials. PubMed, EMBASE and Web of Science databases were searched for relevant literature using the search terms “ PI3K inhibitor” and “allergy” or “asthma”. Cochrane Review Manager and R were used for handling continuous variables. The primary outcomes of the inflammatory profile were divided into cell counts and inflammatory cytokines. We used a random effects model to draw a forest plot. Through the database search and subsequent selection, 17 articles were identified. Regarding the cell counts, both the PI3K pan-inhibitors and PI3K-δ inhibitors effectively reduced the total cell counts, eosinophils, neutrophils and lymphocytes. In contrast to PI3K-δ inhibitors, PI3K pan-inhibitors effectively reduced macrophages. Regarding the inflammatory cytokines, PI3K pan-inhibitors and PI3K-δ inhibitors effectively reduced total IgE, IL-4, IL-5, IL-13, TNF-α, IL-1β, VEGF and had no effect on IL-6. Compared to the PI3K pan-inhibitors, which block all pathways, selective PI3K-δ inhibitors are expected to be relatively less toxic. Regarding the efficacy, PI3K-δ inhibitors have at least the same or better efficacy than PI3K pan-inhibitors in effector cells and inflammatory mediators.

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

Phosphatidlyinositol-3-kinase C2 beta (PI3KC2β) is a potential new target to treat IgE mediated disease

Cross linking of the IgE receptor (FcεRI) on mast cells plays a critical role in IgE-dependent allergy including allergic rhinitis, asthma, anaphylaxis, and delayed type hypersensitivity reactions. The Ca2+ activated K+ channel, KCa3.1, plays a critical role in IgE-stimulated Ca2+ entry and degranulation in mast cells by helping to maintain a negative membrane potential, which provides an electrochemical gradient to drive Ca2+ influx. Of the 3 classes of PI3K, the class II PI3Ks are the least studied and little is known about the roles for class II PI3Ks in vivo in the context of the whole organism under normal and pathological conditions. Studying bone marrow derived mast cells (BMMC) isolated from PI3KC2β-/- mice, we now show that the class II PI3KC2β is critical for FcεRI stimulated KCa3.1 channel activation and the subsequent activation of mast cells. We found FcεRI-stimulated Ca2+ entry, cytokine production, and degranulation are decreased in BMMC isolated from PI3KC2β-/- mice. In addition, PI3KC2β-/- mice are markedly resistant to both passive cutaneous and passive systemic anaphylaxis. These findings identify PI3KC2β as a new pharmacologic target to treat IgE-mediated disease.

Sorafenib stimulates human skin type mast cell degranulation and maturation

BACKGROUND

Sorafenib is a multi-kinase inhibitor for treating advanced hepatocellular and renal cell carcinomas by targeting various types of receptors and signaling molecules, including vascular endothelial growth factor receptors, platelet-derived growth factor receptor, and Raf-1. Sorafenib may cause diverse cutaneous adverse reactions, including hand-foot reaction, facial and scalp eruptions, alopecia and pruritus. However, the mechanism of these adverse effects has not been well-investigated.

OBJECTIVE

Mast cells (MCs) are reported to be associated with various types of skin diseases. To investigate the mechanism of sorafenib-induced cutaneous adverse effects, we focused on MCs in situ.

METHODS

We evaluated skin samples of organ cultured normal human skin treated with sorafenib using c-Kit, tryptase, and stem cell factor (SCF), Ki-67, and TUNEL immunohistochemistry as well as quantitative real-time polymerase chain reaction to evaluate MC number, degranulation, proliferation, and apoptosis in situ.

RESULTS

Sorafenib significantly increased the number and degranulation of skin-type MCs compared with the vehicle-treated control group in situ. However, sorafenib did not affect MC proliferation and apoptosis, suggesting that it stimulated MC maturation from resident precursors. Furthermore, sorafenib increased SCF expression in situ. The increase in MC number by sorafenib was abrogated by co-administration of SCF neutralizing antibody or the phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, but not the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor, PD98059. This suggests that SCF is involved in sorafenib-induced MC maturation. In addition, the compensatory upregulation of PI3K-signaling from inhibition of MAPK signaling by sorafenib might stimulate MC maturation in situ. We also evaluated MCs within the skin samples from patients with drug eruptions by sorafenib administration. The total and degranuated MCs number as well as SCF expression was significantly increased compared to healthy individuals.

CONCLUSION

Our results contribute to a better understanding of the mechanism by which sorafenib induces adverse cutaneous reactions via activation of skin-type MC degranulation and maturation. This activation appears to be related to PI3K signaling and SCF production, which could be a new targets for treating sorafenib-induced adverse reactions.

Role of stem cell growth factor/c-Kit in the pathogenesis of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional bowel disease with a complicated etiopathogenesis, often characterized by gastrointestinal motility disorder and high visceral sensitivity. IBS is a comprehensive multi-systemic disorder, with the interaction of multiple factors, such as mental stress, intestinal function and flora, heredity, resulting in the disease. The existence of a common mechanism underlying the aforementioned factors is currently unknown. The lack of therapies that comprehensively address the disease symptoms, including abdominal pain and diarrhea, is a limitation of current IBS management. The current review has explored the role of the SCF/c-Kit receptor/ligand system in IBS. The SCF/c-Kit system constitutes a classical ligand/receptor tyrosine kinase signaling system that mediates inflammation and smooth muscle contraction. Additionally, it provides trophic support to neural crest-derived cell types, including the enteric nervous system and mast cells. The regulation of SCF/c-Kit on the interstitial cells of Cajal (ICC) suggest that it may play a key role in the aberrant intestinal dynamics and high visceral sensitivity observed in IBS. The role of the SCF/c-Kit system in intestinal motility, inflammation and nerve growth has been reported. From the available biomedical evidence on the pathogenesis of IBS, it has been concluded that the SCF-c-Kit system is a potential therapeutic target for rational drug design in the treatment of IBS.

Tie2 Signaling Enhances Mast Cell Progenitor Adhesion to Vascular Cell Adhesion Molecule-1 (VCAM-1) through α4β1 Integrin

Mast cell (MC) activation contributes considerably to immune responses, such as host protection and allergy. Cell surface immunoreceptors expressed on MCs play an important role in MC activation. Although various immunoreceptors on MCs have been identified, the regulatory mechanism of MC activation is not fully understood. To understand the regulatory mechanisms of MC activation, we used gene expression analyses of human and mouse MCs to identify a novel immunoreceptor expressed on MCs. We found that Tek, which encodes Tie2, was preferentially expressed in the MCs of both humans and mice. However, Tie2 was not detected on the cell surface of the mouse MCs of the peritoneal cavity, ear skin, or colon lamina propria. In contrast, it was expressed on mouse bone marrow–derived MCs and bone marrow MC progenitors (BM-MCps). Stimulation of Tie2 by its ligand angiopoietin-1 induced tyrosine phosphorylation of Tie2 in MEDMC-BRC6, a mouse embryonic stem cell-derived mast cell line, and enhanced MEDMC-BRC6 and mouse BM-MCp adhesion to vascular cell adhesion molecule-1 (VCAM-1) through α4β1 integrin. These results suggest that Tie2 signaling induces α4β1 integrin activation on BM-MCps for adhesion to VCAM-1.

PI3K signaling in arterial diseases: Non redundant functions of the PI3K isoforms

Cardiovascular diseases are the most common cause of death around the world. This includes atherosclerosis and the adverse effects of its treatment, such as restenosis and thrombotic complications. The development of these arterial pathologies requires a series of highly-intertwined interactions between immune and arterial cells, leading to specific inflammatory and fibroproliferative cellular responses. In the last few years, the study of phosphoinositide 3-kinase (PI3K) functions has become an attractive area of investigation in the field of arterial diseases, especially since inhibitors of specific PI3K isoforms have been developed. The PI3K family includes 8 members divided into classes I, II or III depending on their substrate specificity. Although some of the different isoforms are responsible for the production of the same 3-phosphoinositides, they each have specific, non-redundant functions as a result of differences in expression levels in different cell types, activation mechanisms and specific subcellular locations. This review will focus on the functions of the different PI3K isoforms that are suspected as having protective or deleterious effects in both the various immune cells and types of cell found in the arterial wall. It will also discuss our current understanding in the context of which PI3K isoform(s) should be targeted for future therapeutic interventions to prevent or treat arterial diseases.

PI3K inhibitors in inflammation, autoimmunity and cancer

The healthy immune system protects against infection and malignant transformation without causing significant damage to host tissues. Immune dysregulation results in diverse pathologies including autoimmune disease, chronic inflammatory disorders, allergies as well as immune deficiencies and cancer. Phosphoinositide 3-kinase (PI3K) signalling has been shown to be a key pathway in the regulation of the immune response and continues to be the focus of intense research. In recent years we have gained detailed understanding of PI3K signalling, and saw the development of potent and highly selective small molecule inhibitors, of which several are currently in clinical trials for the treatment of immune-related disorders and cancer. The role of PI3K signalling in the immune response has been the subject of detailed reviews; here we focus on relevant recent progress in pre-clinical and clinical development of PI3K inhibitors.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

Mast cell plasticity and sphingosine-1-phosphate in immunity, inflammation and cancer

Mast cells (MC) are found in all vascularized tissues at homeostasis and, until recently, were viewed only as effector cells of allergic reactions via degranulation, the canonical process through which MC release mediators, including histamine and pre-formed proteases and cytokines such as TNF. Cross-linking of IgE bound to surface high affinity receptors for IgE (FcɛRI) by a specific antigen (Ag) triggers signaling events leading to degranulation. We and others have reported the concomitant production and export of an influential multifaceted sphingolipid mediator, sphingosine-1-phosphate (S1P) transported outside of MC by ATP-binding cassettes (ABC) transporters, i.e., independently of degranulation. Indeed, the MC horizon expanded by the discovery of their unique ability to selectively release mediators depending upon the stimulus and receptors involved. Aside from degranulation and transporter usage, MC are also endowed with piecemeal degranulation, a slower process during which mediator release occurs with minor morphological changes. The broad spectrum of pro- and anti-inflammatory bioactive substances MC produce and release, their amounts and delivery pace render these cells bona fide fine-tuners of the immune response. In this viewpoint article, MC developmental, phenotypic and functional plasticity, its modulation by microRNAs and its relevance to immunity, inflammation and cancer will be discussed.

PI3K signalling in inflammation

PI3Ks regulate several key events in the inflammatory response to damage and infection. There are four Class I PI3K isoforms (PI3Kα,β,γ,δ), three Class II PI3K isoforms (PI3KC2α, C2β, C2γ) and a single Class III PI3K. The four Class I isoforms synthesise the phospholipid 'PIP3'. PIP3 is a 'second messenger' used by many different cell surface receptors to control cell movement, growth, survival and differentiation. These four isoforms have overlapping functions but each is adapted to receive efficient stimulation by particular receptor sub-types. PI3Kγ is highly expressed in leukocytes and plays a particularly important role in chemokine-mediated recruitment and activation of innate immune cells at sites of inflammation. PI3Kδ is also highly expressed in leukocytes and plays a key role in antigen receptor and cytokine-mediated B and T cell development, differentiation and function. Class III PI3K synthesises the phospholipid PI3P, which regulates endosome-lysosome trafficking and the induction of autophagy, pathways involved in pathogen killing, antigen processing and immune cell survival. Much less is known about the function of Class II PI3Ks, but emerging evidence indicates they can synthesise PI3P and PI34P2 and are involved in the regulation of endocytosis. The creation of genetically-modified mice with altered PI3K signalling, together with the development of isoform-selective, small-molecule PI3K inhibitors, has allowed the evaluation of the individual roles of Class I PI3K isoforms in several mouse models of chronic inflammation. Selective inhibition of PI3Kδ, γ or β has each been shown to reduce the severity of inflammation in one or more models of autoimmune disease, respiratory disease or allergic inflammation, with dual γ/δ or β/δ inhibition generally proving more effective. The inhibition of Class I PI3Ks may therefore offer a therapeutic opportunity to treat non-resolving inflammatory pathologies in humans. This article is part of a Special Issue entitled Phosphoinositides.

miRNA-155 controls mast cell activation by regulating the PI3Kγ pathway and anaphylaxis in a mouse model

BACKGROUND

Mast cells (MCs) play a central role in allergic and inflammatory disorders by rapid degranulation and release of inflammatory mediators upon antigen-driven engagement of the FcεRI. Receptor-mediated MC responses are controlled by the activation of different isoforms of phosphoinositide-3-kinase (PI3K) and the downstream signaling processes. Recent evidence suggests that miRNAs are important molecular players regulating the PI3K/Akt pathway.

METHODS

The role of miR-155 in the regulation of MC functions in vivo was studied in the passive cutaneous anaphylaxis (PCA) MC-dependent model. WT and miR-155(-/-) mice were injected intradermally with anti-DNP-IgE and intravenously with the antigen DNP-HSA. Ear swelling was assessed to evaluate the anaphylactic response. All investigations, to characterize miR-155 specific activities in MCs, were conducted comparing WT and miR-155(-/-) bone marrow-derived MCs (BMMCs).

RESULTS

We report that miR-155(-/-) mice display enhanced anaphylaxis reactions. Although miR-155(-/-) BMMCs show normal development, proliferation, and survival, miR-155 deficiency enhances FcεRI-mediated degranulation and release of TNF-α, IL-13, and IL-6. Interestingly, the level of Akt phosphorylation on both of its regulatory residues Thr308 and Ser473 was increased in miR-155(-/-) compared to WT BMMCs. Gene expression profiling showed that miR-155(-/-) BMMCs exhibited significantly increased expression of the adapter PI3Kγ subunits Pik3r5 (p101) and Pik3r6 (p84, p87(PIKAP) ). Furthermore, selective blockade of the PI3Kγ pathway inhibited degranulation in miR-155(-/-) BMMCs.

CONCLUSIONS

Thus, we suggest that miR-155 plays a critical role in FcεRI-mediated MC responses by modulating components of the PI3Kγ pathway. This newly identified mechanism of miRNA-controlled MC activation may affect the initiation and maintenance of allergic disorders.

Mast cell progenitors: origin, development and migration to tissues

Mast cells in tissues are developed from mast cell progenitors emerging from the bone marrow in a process highly regulated by transcription factors. Through the advancement of the multicolor flow cytometry technique, the mast cell progenitor population in the mouse has been characterized in terms of surface markers. However, only cell populations with enriched mast cell capability have been described in human. In naïve mice, the peripheral tissues have a constitutive pool of mast cell progenitors. Upon infections in the gut and in allergic inflammation in the lung, the local mast cell progenitor numbers increase tremendously. This review focuses on the origin and development of mast cell progenitors. Furthermore, the evidences for cells and molecules that govern the migration of these cells in mice in vivo are described.

PKCβ phosphorylates PI3Kγ to activate it and release it from GPCR control

All class I phosphoinositide 3-kinases (PI3Ks) associate tightly with regulatory subunits through interactions that have been thought to be constitutive. PI3Kγ is key to the regulation of immune cell responses activated by G protein-coupled receptors (GPCRs). Remarkably we find that PKCβ phosphorylates Ser582 in the helical domain of the PI3Kγ catalytic subunit p110γ in response to clustering of the high-affinity IgE receptor (FcεRI) and/or store-operated Ca²⁺- influx in mast cells. Phosphorylation of p110γ correlates with the release of the p84 PI3Kγ adapter subunit from the p84-p110γ complex. Ser582 phospho-mimicking mutants show increased p110γ activity and a reduced binding to the p84 adapter subunit. As functional p84-p110γ is key to GPCR-mediated p110γ signaling, this suggests that PKCβ-mediated p110γ phosphorylation disconnects PI3Kγ from its canonical inputs from trimeric G proteins, and enables p110γ to operate downstream of Ca²⁺ and PKCβ. Hydrogen deuterium exchange mass spectrometry shows that the p84 adaptor subunit interacts with the p110γ helical domain, and reveals an unexpected mechanism of PI3Kγ regulation. Our data show that the interaction of p110γ with its adapter subunit is vulnerable to phosphorylation, and outline a novel level of PI3K control.