An essential role for phospholipase D in the activation of protein kinase C and degranulation in mast cells

PKCε controls the fusion of secretory vesicles in mast cells in a phosphatidic acid-dependent mode

PKCε is highly expressed in mast cells and plays a fundamental role in the antigen-triggered activation of the allergic reaction. Although its regulation by diacylglycerols has been described, its regulation by acidic phospholipids and how this regulation leads to the control of downstream vesicle secretion is barely known. Here, we used structural and evolutionary studies to find the molecular mechanism that explains the selectivity of the C1B domain of PKCε by Phosphatidic Acid (PA). This resided in a collection of Arg residues that form a specific rim on the outer surface of the C1B domain, around the diacylglycerol binding cleft. In RBL-2H3 cells, this basic rim allowed the kinase to respond specifically to phosphatidic acid signals that induced its translocation to the plasma membrane and subsequent activation. Further experiments in cells that overexpress PKCε and a mutant of the PA binding site, showed that PA-dependent PKCε activation increased vesicle degranulation in RBL-2H3 cells, and this correlated with increased SNAP23 phosphorylation. Over-expression of PKCε in these cells also induced an increase in the number of docked vesicles containing SNAP23, when stimulated with PA. This accumulation could be attributed to the stabilizing effect of phosphorylation on the formation of the SNARE complex, which ultimately led to increased release of content in the presence of Ca²⁺ during the fusion process. Therefore, these findings reinforce the importance of PA signaling in the activation of PKCε, which could be an important target to inhibit the exacerbated responses of these cells in the allergic reaction.

Antimicrobial agent triclosan suppresses mast cell signaling via phospholipase D inhibition

Humans are exposed to the antimicrobial agent triclosan (TCS) through use of TCS-containing products. Exposed tissues contain mast cells, which are involved in numerous biological functions and diseases by secreting various chemical mediators through a process termed degranulation. We previously demonstrated that TCS inhibits both Ca²⁺ influx into antigen-stimulated mast cells and subsequent degranulation. To determine the mechanism linking the TCS cytosolic Ca²⁺ depression to inhibited degranulation, we investigated the effects of TCS on crucial signaling enzymes activated downstream of the Ca²⁺ rise: protein kinase C (PKC; activated by Ca²⁺ and reactive oxygen species [ROS]) and phospholipase D (PLD). We found that TCS strongly inhibits PLD activity within 15 minutes post-antigen, a key mechanism of TCS mast cell inhibition. In addition, experiments using fluorescent constructs and confocal microscopy indicate that TCS delays antigen-induced translocations of PKCβII, PKCδ and PKC substrate myristoylated alanine-rich C-kinase. Surprisingly, TCS does not inhibit PKC activity or overall ability to translocate, and TCS actually increases PKC activity by 45 minutes post-antigen; these results are explained by the timing of both TCS inhibition of cytosolic Ca²⁺ (~15+ minutes post-antigen) and TCS stimulation of ROS (~45 minutes post-antigen). These findings demonstrate that it is incorrect to assume that all Ca²⁺ -dependent processes will be synchronously inhibited when cytosolic Ca²⁺ is inhibited by a toxicant or drug. The results offer molecular predictions of the effects of TCS on other mammalian cell types, which share these crucial signal transduction elements and provide biochemical information that may underlie recent epidemiological findings implicating TCS in human health problems.

Altered Metabolism of Phospholipases, Diacylglycerols, Endocannabinoids, and N-Acylethanolamines in Patients with Mastocytosis

Background

Mastocytosis is a condition characterized by the expansion and accumulation of mast cells (MCs) in various organs. The symptoms are related to the increased release of MC-derived mediators that exert local and distant effects. MCs are a source and target of phospholipase enzymes (PLs), which catalyze the cleavage of membrane phospholipids releasing lipid mediators (e.g., diacylglycerols (DAGs) and the endocannabinoid (EC) 2-arachidonoylglycerol (2-AG)). To date, there are no data on the role of these lipid mediators in mastocytosis. Here, we analyzed plasma levels of PLA₂, PLC, DAG, ECs, and EC-related N-acylethanolamines in patients with mastocytosis.

Methods

In 23 patients with mastocytosis and 23 healthy individuals, we measured plasma PLA₂ and PLC activities, DAG, 2-AG, anandamide (AEA), palmitoylethanolamide (PEA), and oleoylethanolamide (OEA).

Results

Plasma PLA₂ and PLC activities were increased in mastocytosis patients compared to controls. Concentrations of DAG (18:1 20:4 and 18:0 20:4), two second messengers produced by PLC, were higher in mastocytosis compared to controls, whereas the concentrations of their metabolite, 2-AG, were not altered. AEA was decreased in mastocytosis patients compared to controls; by contrast, AEA congener, PEA, was increased. PLA₂ and PLC activities were increased only in patients with mediator-related symptoms. Moreover, PLC activity was positively correlated with disease severity and tryptase concentrations. By contrast, AEA was negatively correlated with tryptase concentrations.

Conclusions

PLs and some lipid mediators are altered in patients with mastocytosis. Our results may pave the way for investigating the functions of these mediators in the pathophysiology of mastocytosis and provide new potential biomarkers and therapeutic targets.

Anti-Allergic and Anti-Inflammatory Effects of Kuwanon G and Morusin on MC/9 Mast Cells and HaCaT Keratinocytes

Atopic dermatitis (AD) is a prevalent chronic inflammatory skin disease. The use of immunomodulatory corticosteroids in AD treatment causes adverse side effects. Therefore, novel natural anti-inflammatory therapeutics are needed. The aim of the present study was to investigate the anti-allergic and anti-inflammatory activities of kuwanon G and morusin. To investigate the effect of kuwanon G and morusin on skin inflammation, enzyme-linked immunosorbent assays (ELISA) to quantitate secreted (RANTES/CCL5), thymus- and activation-regulated chemokine (TARC/CCL17), and macrophage-derived chemokine (MDC/CCL22) were performed, followed by Western blotting to measure the phosphorylation of signal transducer and activator of transcription 1 (STAT1) and nuclear transcription factor-κB (NF-κB) p65 in tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ)-stimulated HaCaT keratinocytes. In order to evaluate the anti-allergic effects, ELISA to quantify histamine and leukotriene C₄ (LTC₄) production and Western blotting to measure 5-lipoxygenase (5-LO) activation were performed using PMA and A23187-stimulated MC/9 mast cells. Kuwanon G reduced the release of RANTES/CCL5, TARC/CCL17, and MDC/CCL22 via down-regulation of STAT1 and NF-κB p65 signaling in TNF-α and IFN-γ-stimulated HaCaT keratinocytes. Kuwanon G also inhibited histamine production and 5-LO activation in PMA and A23187-stimulated MC/9 mast cells. Morusin inhibited RANTES/CCL5 and TARC/CCL17 secretion via the suppression of STAT1 and NF-κB p65 phosphorylation in TNF-α and IFN-γ-stimulated HaCaT keratinocytes, and the release of histamine and LTC₄ by suppressing 5-LO activation in PMA and A23187-stimulated MC/9 mast cells. Kuwanon G and morusin are potential anti-inflammatory mediators for the treatment of allergic and inflammatory skin diseases such as AD.

Arsenic inhibits mast cell degranulation via suppression of early tyrosine phosphorylation events

Exposure to arsenic is a global health concern. We previously documented an inhibitory effect of inorganic Arsenite on IgE-mediated degranulation of RBL-2H3 mast cells (Hutchinson et al., 2011; J. Appl. Toxicol. 31: 231-241). Mast cells are tissue-resident cells that are positioned at the host-environment interface, thereby serving vital roles in many physiological processes and disease states, in addition to their well-known roles in allergy and asthma. Upon activation, mast cells secrete several mediators from cytoplasmic granules, in degranulation. The present study is an investigation of Arsenite's molecular target(s) in the degranulation pathway. Here, we report that arsenic does not affect degranulation stimulated by either the Ca(2) (+) ionophore A23187 or thapsigargin, which both bypass early signaling events. Arsenic also does not alter degranulation initiated by another non-IgE-mediated mast cell stimulant, the G-protein activator compound 48/80. However, arsenic inhibits Ca(2) (+) influx into antigen-activated mast cells. These results indicate that the target of arsenic in the degranulation pathway is upstream of the Ca(2) (+) influx. Phospho-Syk and phospho-p85 phosphoinositide 3-kinase enzyme-linked immunosorbent assays data show that arsenic inhibits early phosphorylation events. Taken together, this evidence indicates that the mechanism underlying arsenic inhibition of mast cell degranulation occurs at the early tyrosine phosphorylation steps in the degranulation pathway. Copyright © 2016 John Wiley & Sons, Ltd.

Differential Roles of Phospholipase D Proteins in FcεRI-Mediated Signaling and Mast Cell Function

Phospholipase D (PLD) proteins are enzymes that catalyze the hydrolysis of phosphatidylcholine to generate an important signaling lipid, phosphatidic acid. Phosphatidic acid is a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Previous studies using inhibitors and overexpression of PLD proteins indicate that PLD1 and PLD2 play positive roles in FcεRI-mediated signaling and mast cell function. We used mice deficient in PLD1, PLD2, or both to study the function of these enzymes in mast cells. In contrast to published studies, we found that PLD1 deficiency impaired FcεRI-mediated mast cell degranulation; however, PLD2 deficiency enhanced it. Biochemical analysis showed that PLD deficiency affected activation of the PI3K pathway and RhoA. Furthermore, our data indicated that, although PLD1 deficiency impaired F-actin disassembly, PLD2 deficiency enhanced microtubule formation. Together, our results suggested that PLD1 and PLD2, two proteins that catalyze the same enzymatic reaction, regulate different steps in mast cell degranulation.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Vesicular trafficking and signaling for cytokine and chemokine secretion in mast cells

Upon activation mast cells (MCs) secrete numerous inflammatory compounds stored in their cytoplasmic secretory granules by a process called anaphylactic degranulation, which is responsible for type I hypersensitivity responses. Prestored mediators include histamine and MC proteases but also some cytokines and growth factors making them available within minutes for a maximal biological effect. Degranulation is followed by the de novo synthesis of lipid mediators such as prostaglandins and leukotrienes as well as a vast array of cytokines, chemokines, and growth factors, which are responsible for late phase inflammatory responses. While lipid mediators diffuse freely out of the cell through lipid bilayers, both anaphylactic degranulation and secretion of cytokines, chemokines, and growth factors depends on highly regulated vesicular trafficking steps that occur along the secretory pathway starting with the translocation of proteins to the endoplasmic reticulum. Vesicular trafficking in MCs also intersects with endocytic routes, notably to form specialized cytoplasmic granules called secretory lysosomes. Some of the mediators like histamine reach granules via specific vesicular monoamine transporters directly from the cytoplasm. In this review, we try to summarize the available data on granule biogenesis and signaling events that coordinate the complex steps that lead to the release of the inflammatory mediators from the various vesicular carriers in MCs.

Mast cell activation: a complex interplay of positive and negative signaling pathways

Mast cells regulate the immunological responses causing allergy and autoimmunity, and contribute to the tumor microenvironment through generation and secretion of a broad array of preformed, granule-stored and de novo synthesized bioactive compounds. The release and production of mast cell mediators is the result of a coordinated signaling machinery, followed by the FcεRI and FcγR antigen ligation. In this review, we present the latest understanding of FcεRI and FcγR signaling, required for the canonical mast cell activation during allergic responses and anaphylaxis. We then describe the cooperation between the signaling of FcR and other recently characterized membrane-bound receptors (i.e., IL-33R and thymic stromal lymphopoietin receptor) and their role in the chronic settings, where mast cell activation is crucial for the development and the sustainment of chronic diseases, such as asthma or airway inflammation. Finally, we report how the FcR activation could be used as a therapeutic approach to treat allergic and atopic diseases by mast cell inactivation. Understanding the magnitude and the complexity of mast cell signaling is necessary to identify the mechanisms underlying the potential effector and regulatory roles of mast cells in the biology and pathology of those disease settings in which mast cells are activated.

Negative control of mast cell degranulation and the anaphylactic response by the phosphatase lipin1

Mast cells play a critical role in the pathogenesis of allergic diseases; however, how mast cell function is regulated is still not well understood. Both phosphatidic acid (PA) and diacylglycerol (DAG) are important secondary messengers involved in mast cell activ-ation. Lipin1 is a phosphatidate phosphatase that hydrolyzes PA to produce DAG, but the role of lipin1 in mast cell function has been thus far unknown. Here we show that lipin1 is an important and selective inhibitor of mast cell degranulation. Lipin1 deficiency enhanced FcεRI-mediated β-hexosaminidase and prostaglandin D2 release from mast cells in vitro and exacerbated the passive systemic anaphylaxis reaction in vivo. Lipin1 deficiency, however, did not exert obvious effects on IL-6 or TNF-α production following FcεRI engagement. FcεRI-induced PKC and SNAP-23 phosphorylation were augmented in the lipin1-deficient mast cells. Moreover, inhibition of PKC activity reduced SNAP-23 phosphorylation and mast cell degranulation in lipin1-deficient mast cells. Together, our findings suggest that lipin1 may negatively control mast cell degranulation and the anaphylactic response through inhibiting the PKC-SNAP-23 pathway.

Sequestration of phosphoinositides by mutated MARCKS effector domain inhibits stimulated Ca(2+) mobilization and degranulation in mast cells

A new strategy for interfering with phosphoinositide-dependent processes at the plasma membrane uses high-avidity association of the polybasic MARCKS effector domain with negatively charged phospholipids to provide new insights into roles for phosphoinositides in IgE receptor signaling leading to exocytosis.

Protein kinase C β (PKCβ) participates in antigen-stimulated mast cell degranulation mediated by the high-affinity receptor for immunoglobulin E, FcεRI, but the molecular basis is unclear. We investigated the hypothesis that the polybasic effector domain (ED) of the abundant intracellular substrate for protein kinase C known as myristoylated alanine-rich protein kinase C substrate (MARCKS) sequesters phosphoinositides at the inner leaflet of the plasma membrane until MARCKS dissociates after phosphorylation by activated PKC. Real-time fluorescence imaging confirms synchronization between stimulated oscillations of intracellular Ca²⁺ concentrations and oscillatory association of PKCβ–enhanced green fluorescent protein with the plasma membrane. Similarly, MARCKS-ED tagged with monomeric red fluorescent protein undergoes antigen-stimulated oscillatory dissociation and rebinding to the plasma membrane with a time course that is synchronized with reversible plasma membrane association of PKCβ. We find that MARCKS-ED dissociation is prevented by mutation of four serine residues that are potential sites of phosphorylation by PKC. Cells expressing this mutated MARCKS-ED SA4 show delayed onset of antigen-stimulated Ca²⁺ mobilization and substantial inhibition of granule exocytosis. Stimulation of degranulation by thapsigargin, which bypasses inositol 1,4,5-trisphosphate production, is also substantially reduced in the presence of MARCKS-ED SA4, but store-operated Ca²⁺ entry is not inhibited. These results show the capacity of MARCKS-ED to regulate granule exocytosis in a PKC-dependent manner, consistent with regulated sequestration of phosphoinositides that mediate granule fusion at the plasma membrane.

Regulation of C3a receptor signaling in human mast cells by G protein coupled receptor kinases

Background

The complement component C3a activates human mast cells via its cell surface G protein coupled receptor (GPCR) C3aR. For most GPCRs, agonist-induced receptor phosphorylation leads to receptor desensitization, internalization as well as activation of downstream signaling pathways such as ERK1/2 phosphorylation. Previous studies in transfected COS cells overexpressing G protein coupled receptor kinases (GRKs) demonstrated that GRK2, GRK3, GRK5 and GRK6 participate in agonist-induced C3aR phosphorylation. However, the roles of these GRKs on the regulation of C3aR signaling and mediator release in human mast cells remain unknown.

Methodology/Principal Findings

We utilized lentivirus short hairpin (sh)RNA to stably knockdown the expression of GRK2, GRK3, GRK5 and GRK6 in human mast cell lines, HMC-1 and LAD2, that endogenously express C3aR. Silencing GRK2 or GRK3 expression caused a more sustained Ca²⁺ mobilization, attenuated C3aR desensitization, and enhanced degranulation as well as ERK1/2 phosphorylation when compared to shRNA control cells. By contrast, GRK5 or GRK6 knockdown had no effect on C3aR desensitization, but caused a significant decrease in C3a-induced mast cell degranulation. Interestingly, GRK5 or GRK6 knockdown rendered mast cells more responsive to C3a for ERK1/2 phosphorylation.

Conclusion/Significance

This study demonstrates that GRK2 and GRK3 are involved in C3aR desensitization. Furthermore, it reveals the novel finding that GRK5 and GRK6 promote C3a-induced mast cell degranulation but inhibit ERK1/2 phosphorylation via C3aR desensitization-independent mechanisms. These findings thus reveal a new level of complexity for C3aR regulation by GRKs in human mast cells.

The mechanisms of exocytosis in mast cells

Upon activation through high affinity IgE receptors (FcεRI), mast cells (MCs) can release up to 100% of their content of preformed mediators stored in cytoplasmic secretory granules by compound exocytosis. This causes Type I immediate hypersensitivity reactions and, in the case of inappropriate activation by allergens, the symptoms of allergy. Recent work has uncovered a central role of SNARE (Soluble N-ethylmaleimide-Sensitive Factor (NSF) Attachment Protein (SNAP) Receptors) proteins in regulating the numerous membrane fusion events during exocytosis. This has defined a series of new molecular actors in MC exocytosis that participate in the regulation of membrane fusion and the connection of the fusion machinery with early signaling events. The purpose of this chapter is to describe these proteins and provide a brief overview on their mechanism of action.

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Stimulus-secretion coupling by high-affinity IgE receptor: new developments

Stimulation of mast cells through their high affinity IgE receptor (FcεRI) leads to the secretion of pre- and neoformed soluble mediators through vesicular carriers. This process is highly regulated in order to adapt the secretion of these potentially dangerous factors to the physiological needs. This regulation requires numerous essential effectors that are necessary to transmit the initial signal of FcεRI aggregation and couple it to the sophisticated secretory machinery of membrane fusion. Studies in recent years have led to the discovery of a series of new effector molecules that link FcεRI to secretion. We describe here some of the new developments that have allowed to obtain a clearer picture of stimulus/secretion coupling in mast cells.

Inhibition of IgE-induced mast cell activation by ethyl tertiary-butyl ether, a bioethanol-derived fuel oxygenate

Objectives

The effect of ethyl tertiary-butyl ether (ETBE), which is widely used as a fuel oxygenate commonly produced from bioethanol, on immunoglobulin (Ig)E-dependent mast cell activation was investigated.

Methods

The rat mast cell line RBL2H3 sensitised with monoclonal anti-ovalbumin IgE was challenged with ovalbumin in the presence or absence of ETBE, tert-butanol (TBA), which is the main metabolite of ETBE in humans, and ethanol. Degranulation of RBL2H3 was examined by the release of β-hexosaminidase. To understand the mechanisms responsible for regulating mast cell function, the effects of ETBE, TBA and ethanol on the levels of intracellular calcium, phosphorylation of Akt (as a marker of phosphatidylinositol 3-kinase) and global tyrosine phosphorylation were also measured as indicators of mast cell activation.

Key findings

In the presence of ETBE, TBA or ethanol, IgE-induced release of β-hexosaminidase was decreased. These compounds also attenuated the IgE-mediated increase in the levels of intracellular Ca2+, phosphorylation of Akt and global tyrosine phosphorylation in RBL2H3 cells.

Conclusions

ETBE, TBA and ethanol inhibited mast cell degranulation by inhibiting the increase in intracellular calcium ion concentration and activation of phosphatidylinositol 3-kinase and protein tyrosine kinase activation, suggesting that exposure to ETBE might affect immune responses, particularly in allergic diseases.

Phospholipase d promotes lipid microdomain-associated signaling events in mast cells

Initial IgE-dependent signaling events are associated with detergent-resistant membrane microdomains. Following Ag stimulation, the IgE-receptor (Fc(epsilon)RI ) accumulates within these domains. This facilitates the phosphorylation of Fc(epsilon)RI subunits by the Src kinase, Lyn, and the interaction with adaptor proteins, such as the linker for activation of T cells. Among the phospholipases (PL) subsequently activated, PLD is of interest because of its presence in lipid microdomains and the possibility that its product, phosphatidic acid, may regulate signal transduction and membrane trafficking. We find that in Ag-stimulated RBL-2H3 mast cells, the association of Fc(epsilon)RI with detergent-resistant membrane fractions is inhibited by 1-butanol, which subverts production of phosphatidic acid to the biologically inert phosphatidylbutanol. Furthermore, the knockdown of PLD2, and to a lesser extent PLD1 with small inhibitory RNAs, also suppressed the accumulation of Fc(epsilon)RI and Lyn in these fractions as well as the phosphorylation of Src kinases, Fc(epsilon)RI , linker for activation of T cells, and degranulation. These effects were accompanied by changes in distribution of the lipid microdomain component, ganglioside 1, in the plasma membrane as determined by binding of fluorescent-tagged cholera toxin B subunit and confocal microscopy in live cells. Collectively, these findings suggest that PLD activity plays an important role in promoting IgE-dependent signaling events within lipid microdomains in mast cells.

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

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

Diacylglycerol kinases in immune cell function and self-tolerance

Both diacylglycerol (DAG) and phosphatidic acid (PA) are important second messengers involved in signal transduction from many immune cell receptors and can be generated and metabolized through multiple mechanisms. Recent studies indicate that diacylglycerol kinases (DGKs), the enzymes that catalyze phosphorylation of DAG to produce PA, play critical roles in regulating the functions of multiple immune cell lineages. In T cells, two DGK isoforms, alpha and zeta, inhibit DAG-mediated signaling following T-cell receptor engagement and prevent T-cell hyperactivation. DGK alpha and zeta synergistically promote T-cell anergy and are critical for T-cell tolerance. In mast cells, DGKzeta plays differential roles in their activation by promoting degranulation but attenuating cytokine production following engagement of the high affinity receptor for immunoglobulin E. In dendritic cells and macrophages, DGKzeta positively regulates Toll-like receptor-induced proinflammatory cytokine production through its product PA and is critical for host defense against Toxoplasma gondii infection. These studies demonstrate pivotal roles of DGKs in regulating immune cell function by acting both as signal terminator and initiator.

A HPLC-fluorescence detection method for determination of phosphatidic acid phosphohydrolase activity: application in human myocardium

Phosphatidic acid phosphohydrolase (PAP) catalyzes the dephosphorylation of phosphatidic acid (PA) to diacylglycerol, the second messenger responsible for activation of protein kinase C. Despite the crucial role of PAP lipid signaling, there are no data on PAP signaling function in the human heart. Here we present a nonradioactive assay for the investigation of PAP activity in human myocardium using a fluorescent derivative of PA, 2-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphate (BODIPY-PA), as substrate in an in vitro PAP-catalyzed reaction. Unreacted BODIPY-PA was resolved from the PAP products by a binary gradient HPLC system and BODIPY-diacylglycerol was detected by fluorimetry. The reaction proceeded at a linear rate for up to 60 min and increased linearly with increasing amounts of cardiac protein in a range of 0.25 to 8.0 microg. This assay proved to be sensitive for accurate quantitation of total PAP activity, PAP-1 activity, and PAP-2 activity in human atrial tissue and right ventricular endomyocardial biopsies. Total PAP activity was approximately fourfold higher in ventricular myocardium than in atrial tissue. There was negligible PAP-1 activity in atrial myocardium compared with ventricular myocardium, indicating regional differences in activities and distribution pattern of PAP-1 and PAP-2 in the human heart.

Src family kinases and lipid mediators in control of allergic inflammation

The Src family kinases Fyn and Lyn are important modulators of the molecular events initiated by engagement of the high-affinity IgE receptor (Fc epsilon RI). These kinases control many of the early signaling events and initiate the production of several lipid metabolites that have an important role in regulating mast cell responses. The intracellular level of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), which is produced by phosphatidylinositol 3-OH kinase, plays an important role in determining the extent of a mast cells response to a stimulus. Enhanced levels lead to a hyperdegranulating phenotype (as seen in SHIP-1(-/-) and Lyn(-/-) mast cells), whereas decreased levels cause hypodegranulation (as seen in Fyn(-/-) mast cells). Downregulation of mast cell phosphatase and tensin homologue deleted on chromosone 10 expression, a phosphatase that reduces cellular levels of PIP(3), caused constitutive cytokine production, demonstrating that this response is particularly sensitive to PIP(3) levels. Lyn and Fyn are also intimately linked to other lipid kinases, like sphingosine kinases (SphK). By producing sphingosine-1-phosphate (S1P), SphKs contribute to mast cell chemotaxis and degranulation. In vivo studies now reveal that circulating S1P as well as that found within the mast cell is important in determining mast cell responsiveness. These studies demonstrate the connection between Src protein tyrosine kinases and lipid second messengers that control mast cell function and allergic responses.

Phospholipase D1 Production of Phosphatidic Acid at the Plasma Membrane Promotes Exocytosis of Large Dense-core Granules at a Late Stage

Substantial efforts have recently been made to demonstrate the importance of lipids and lipid-modifying enzymes in various membrane trafficking processes, including calcium-regulated exocytosis of hormones and neurotransmitters. Among bioactive lipids, phosphatidic acid (PA) is an attractive candidate to promote membrane fusion through its ability to change membrane topology. To date, however, the biosynthetic pathway, the dynamic location, and actual function of PA in secretory cells remain unknown. Using a short interference RNA strategy on chromaffin and PC12 cells, we demonstrate here that phospholipase D1 is activated in secretagogue-stimulated cells and that it produces PA at the plasma membrane at the secretory granule docking sites. We show that phospholipase D1 activation and PA production represent key events in the exocytotic progression. Membrane capacitance measurements indicate that reduction of endogenous PA impairs the formation of fusion-competent granules. Finally, we show that the PLD1 short interference RNA-mediated inhibition of exocytosis can be rescued by exogenous provision of a lipid that favors the transition of opposed bi-layer membranes to hemifused membranes having the outer leaflets fused. Our findings demonstrate that PA synthesis is required during exocytosis to facilitate a late event in the granule fusion pathway. We propose that the underlying mechanism is related to the ability of PA to alter membrane curvature and promote hemi-fusion.

The potential for phospholipase D as a new therapeutic target

Mammalian phospholipase D (PLD), a signal transduction-activated enzyme, hydrolyzes phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA) and choline. Genetic and pharmacological methods have implicated PLD and its product PA in a wide variety of cellular processes including vesicle trafficking, receptor signaling, cell proliferation and survival. Dysregulation of these cell biologic processes occurs in a diverse range of illnesses including cancer. This review summarizes PLD regulation and function and highlights its potential as a therapeutic target in disease settings.

Different approaches to study mast cell functions

Mast cells have long been known to play a detrimental role in the pathogenesis of IgE-associated allergic disorders by their ability to release a wide variety of pro-inflammatory mediators. A number of studies, however, have demonstrated that mast cells play a beneficial role in innate host defense against bacterial infections. Since mast cells clearly play both physiological and pathophysiological functions in the body, it is important to learn about the components of mast cells that drive these responses. The functional roles of mast cell in vivo have been principally characterized by comparing the biological responses in mast cell-deficient mice (WBB6F(1)-W/W(v)), their normal wild-type littermates (WBB6F(1)-+/+) and mast cell deficient mice reconstituted locally or systemically with mast cells cultured from the bone marrow cells of WBB6F(1)-+/+ mice (WBB6F(1)-W/W(v)+MC). Recently investigators have demonstrated that mast cell-deficient mice (WBB6F(1)-W/W(v)) can be reconstituted with mast cells derived in vitro from the bone marrow cells of certain gene knock-out mice or genetically-manipulated embryonic stem cells. This novel approach of analyzing the biological consequences of gene mutations in mast cells will help us to better understand the role of individual gene products in mast cell responses. In this review, we discuss these new approaches to investigate the functions of mast cells in vivo.

Mechanism of vitamin D3-induced transcription of phospholipase D1 in HaCat human keratinocytes

1alpha,25-Dihydroxyvitamin D(3) (VitD(3)) increases protein and gene expression of phospholipase D1 (PLD1), but not PLD2, in HaCaT human keratinocytes. We show that VitD(3) increases PLD1 gene expression through a vitamin D responsive element (VDRE) on the 5' PLD1 promoter (-260 bp to -246 bp from exon 1). Similar results were obtained by transfecting VitD(3) receptor (VDR) into HEK293 cells, which are originally VitD(3)-unresponsive. Electrophoresis mobility shift assays (EMSA) and chromatin immunoprecipitation (CHIP) assays showed that the complex of VitD(3), VDR and retinoid X receptor alpha (RXRalpha) binds to the VDRE and increases PLD1 gene expression in HaCaT cells.

Essential role of GATA transcriptional factors in the activation of mast cells

Mast cells are pivotal effector cells in IgE-mediated allergic reactions. GATA transcriptional factors such as GATA-1 and GATA-2 are expressed in mast cells, and recent studies have revealed that both GATA-1 and GATA-2 are required for mast cell development. However, the role of GATA transcriptional factors in differentiated mast cells has remained largely unknown. In this study, we repressed the activity of GATA-1 and GATA-2 by using three different approaches (inducible overexpression of a dominant-negative form of GATA, pharmacological inactivation, or small interfering RNA technology), and analyzed the molecular mechanisms of GATA transcriptional factors in the activation of mast cells. Surprisingly, the repression of GATA activity in differentiated mast cells led to the impairment of cell survival, IgE-induced degranulation, and cytokine production. Signal transduction and histone modification in the chromatin related to protein kinase Cbeta were defective in these cells. These results identify that GATA has a critical role in the activation of mast cell.

Interaction of PLD1b with actin in antigen-stimulated mast cells

Phosphatidic acid, the product of phospholipase D catalysed phosphatidylcholine hydrolysis is an important signalling molecule that has been implicated in regulation of actin cytoskeleton remodelling and secretion from mast cells. We show that human PLD1b (hPLD1b) is an actin-binding protein and the N-terminus is predominantly involved in this interaction. Protein kinase C (PKC) is a major upstream regulator of PLD activity and PKC phosphorylation sites have been identified within the N-terminus of PLD1b at serine 2 and threonine 147. Over-expression of wild type hPLD1b in mast cells showed that antigen stimulation significantly enhanced co-localisation of PLD1b with actin structures. Mutation of serine 2 to alanine abolished antigen-induced co-localisation whereas mutation of threonine 147 had less dramatic effects on co-localisation. The absence of co-localisation of PLD1b (S2A) with actin coincides with a significant decrease in PLD activity in cells expressing the PLD1b (S2A) mutant. In resting RBL-2H3 cells, mutation of serine 2 to aspartate resulted in constitutive co-localisation of PLD with the actin cytoskeleton, coincident with restored PLD activity. These results reveal that serine 2 is an important regulatory site involved in controlling PLD enzyme activity and the interaction between PLD and actin.

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

Antigen receptor signalling: a distinctive role for the p110delta isoform of PI3K

The activation of antigen receptors triggers two important signalling pathways originating from phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P₂]. The first is phospholipase Cγ (PLCγ)-mediated hydrolysis of PtdIns(4,5)P₂, resulting in the activation of Ras, protein kinase C and Ca²⁺ flux. This culminates in profound alterations in gene expression and effector-cell responses, including secretory granule exocytosis and cytokine production. By contrast, phosphoinositide 3-kinases (PI3Ks) phosphorylate PtdIns(4,5)P₂ to yield phosphatidylinositol(3,4,5)-trisphosphate, activating signalling pathways that overlap with PLCγ or are PI3K-specific. Pathways that are PI3K-specific include Akt-mediated inactivation of Foxo transcription factors and transcription-independent regulation of glucose uptake and metabolism. The p110δ isoform of PI3K is the main source of PI3K activity following antigen recognition by B cells, T cells and mast cells. Here, we review the roles of p110δ in regulating antigen-dependent responses in these cell types.

Phospholipid signalling through phospholipase D and phosphatidic acid

Phospholipase D (PLD) hydrolyzes the phosphodiester bond of the predominant membrane phospholipid, phosphatidylcholine producing phosphatidic acid and free choline. This activity can participate in signal transduction pathways and impact on vesicle trafficking for secretion and endocytosis, as well as receptor signalling. Phospholipids can regulate PLD activity directly, through specific intermolecular interactions, or indirectly, through their effect on the localization or activity of PLD's protein effectors. This short review highlights these various phospholipid inputs into the regulation of PLD activity and also reviews potential roles for PLD-generated phosphatidic acid, particularly a mechanism by which the phospholipid may participate in the process of vesicular trafficking.

Phospholipase D2 acts as an essential adaptor protein in the activation of Syk in antigen-stimulated mast cells

Mast cells are responsible for IgE-mediated allergic reactions. Phospholipase D1 (PLD1) and PLD2 regulate mast cell activation, but the mechanisms remain unclear. Here we show that PLD2 associates with and promotes activation of Syk, a key enzyme in mast cell activation. Antigen stimulation resulted in increased association and colocalization of Syk with PLD2 on the plasma membrane as indicated by coimmunoprecipitation and confocal microscopy. This association was dependent on tyrosine phosphorylation of Syk but not on PLD2 activity. In vitro, PLD2 interacted via its Phox homology (PX) domain with recombinant Syk to induce phosphorylation and activation of Syk. Furthermore, overexpression of PLD2 or catalytically inactive PLD2K758R enhanced antigen-induced phosphorylations of Syk and its downstream targets, the adaptor proteins LAT and SLP-76, while expression of a PLD2 siRNA blocked these phosphorylations. Apparently, the interaction of PLD2 with Syk is an early critical event in the activation of mast cells.

Impaired degranulation but enhanced cytokine production after Fc epsilonRI stimulation of diacylglycerol kinase zeta-deficient mast cells

Calcium and diacylglycerol are critical second messengers that together effect mast cell degranulation after allergen cross-linking of immunoglobulin (Ig)E-bound FcepsilonRI. Diacylglycerol kinase (DGK)zeta is a negative regulator of diacylglycerol-dependent signaling that acts by converting diacylglycerol to phosphatidic acid. We reported previously that DGKzeta-/- mice have enhanced in vivo T cell function. Here, we demonstrate that these mice have diminished in vivo mast cell function, as revealed by impaired local anaphylactic responses. Concordantly, DGKzeta-/- bone marrow-derived mast cells (BMMCs) demonstrate impaired degranulation after Fc epsilonRI cross-linking, associated with diminished phospholipase Cgamma activity, calcium flux, and protein kinase C-betaII membrane recruitment. In contrast, Ras-Erk signals and interleukin-6 production are enhanced, both during IgE sensitization and after antigen cross-linking of Fc epsilonRI. Our data demonstrate dissociation between cytokine production and degranulation in mast cells and reveal the importance of DGK activity during IgE sensitization for proper attenuation of Fc epsilonRI signals.

Regulation of IL-1-induced selective IL-6 release from human mast cells and inhibition by quercetin

Mast cells are involved in allergic reactions, but also in innate immunity and inflammation. Crosslinkage of mast cell Fc immunoglobulin E receptors (FcvarepsilonRI) by multivalent antigen triggers secretion of granule-stored mediators, as well as de novo synthesis of cytokines, including interleukin (IL)-6. We showed recently that the proinflammatory cytokine IL-1 stimulates human leukemic mast cells (HMC-1) and human umbilical cord blood-derived cultured mast cells (hCBMCs) to release newly synthesized IL-6 without tryptase in the absence of degranulation. Here, we investigated several signal-transduction pathways activated by IL-1 leading to IL-6 production by HMC-1 and hCBMCs. We also investigated the effect of the flavonol quercetin that was recently shown to strongly inhibit IL-6 secretion in response to allergic stimulation from hCBMCs.IL-1 stimulated p38, but did not activate extracellular signal-regulated kinase (ERK) or c-jun N-terminal kinase (JNK); it also did not activate protein kinase C (PKC) isozymes alpha, beta, mu and zeta, except for PKC-theta, which was phosphorylated. The p38 inhibitor SB203580 and the PKC inhibitors Calphostin C and Gö6976 completely inhibited IL-1-induced IL-6 production. Quercetin 1-100 microM inhibited IL-1-induced IL-6 secretion, p38 and PKC-theta phosphorylation in a dose-dependent manner. These results indicate that IL-1-stimulated IL-6 production from human mast cells is regulated by biochemical pathways distinct from IgE-induced degranulation and that quercetin can block both IL-6 secretion and two key signal transduction steps involved.