Different isozymes of protein kinase C mediate feedback inhibition of phospholipase C and stimulatory signals for exocytosis in rat RBL-2H3 cells

Diacylglycerol kinase is a keystone regulator of signaling relevant to the pathophysiology of asthma

Signal transduction by G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immunoreceptors converge at the activation of phospholipase C (PLC) for the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). This is a point for second-messenger bifurcation where DAG via protein kinase C (PKC) and IP3 via calcium activate distinct protein targets and regulate cellular functions. IP3 signaling is regulated by multiple calcium influx and efflux proteins involved in calcium homeostasis. A family of lipid kinases belonging to DAG kinases (DGKs) converts DAG to phosphatidic acid (PA), negatively regulating DAG signaling and pathophysiological functions. PA, through a series of biochemical reactions, is recycled to produce new molecules of PIP2. Therefore, DGKs act as a central switch in terminating DAG signaling and resynthesis of membrane phospholipids precursor. Interestingly, calcium and PKC regulate the activation of α and ζ isoforms of DGK that are predominantly expressed in airway and immune cells. Thus, DGK forms a feedback and feedforward control point and plays a crucial role in fine-tuning phospholipid stoichiometry, signaling, and functions. In this review, we discuss the previously underappreciated complex and intriguing DAG/DGK-driven mechanisms in regulating cellular functions associated with asthma, such as contraction and proliferation of airway smooth muscle (ASM) cells and inflammatory activation of immune cells. We highlight the benefits of manipulating DGK activity in mitigating salient features of asthma pathophysiology and shed light on DGK as a molecule of interest for heterogeneous diseases such as asthma.

Effects of Simvastatin on RBL-2H3 Cell Degranulation

Hypercholesterolemia is a major complication of arteriosclerosis. Mast cells in arteriosclerosis plaques induce inflammatory reactions and promote arterial sclerosis. In this study, we evaluated the pharmacological effects of simvastatin (SV)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitors on the degranulation of rat basophilic leukemia (RBL)-2H3 cells, which are commonly used as mast cell models. SV significantly decreased the degranulation induced by three types of stimulation: antigen antibody reaction (Ag-Ab), thapsigargin (Tg) serosal endoplasmic reticulum calcium ATPase (SERCA) inhibitor, and A23187 calcium ionophore. SV had a stronger inhibitory effect on degranulation induced by Ag-Ab stimulation than the other two stimulations. However, SV did not inhibit increase of intracellular Ca²⁺ concentrations. Mevalonate or geranylgeraniol co-treatment with SV completely prevented the inhibitory effect of SV on the degranulation induced by these stimulations. Immunoblotting results showed that SV inhibited protein kinase C (PKC) delta translocation induced by Ag-Ab but not by Tg or A23187. SV induced a reduction in active Rac1, and actin filament rearrangement. In conclusion, SV inhibits RBL-2H3 cell degranulation by inhibiting downstream signaling pathways, including the sequential degranulation pathway. These inhibitory effects were completely reversed by the addition of geranylgeraniol and might be induced by changes in the translocation of the small guanosine 5'-triphosphatase (GTPase) families Rab and Rho, which are related to vesicular transport PKC delta translocation and actin filament formation, respectively. These changes are caused by the inhibition of HMG-CoA reductase by SV following the synthesis of geranylgeranyl pyrophosphates, which play important roles in the activation of small GTPases, Rab.

Cetylpyridinium chloride (CPC) reduces zebrafish mortality from influenza infection: Super-resolution microscopy reveals CPC interference with multiple protein interactions with phosphatidylinositol 4,5-bisphosphate in immune function

The COVID-19 pandemic raises significance for a potential influenza therapeutic compound, cetylpyridinium chloride (CPC), which has been extensively used in personal care products as a positively-charged quaternary ammonium antibacterial agent. CPC is currently in clinical trials to assess its effects on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) morbidity. Two published studies have provided mouse and human data indicating that CPC may alleviate influenza infection, and here we show that CPC (0.1 μM, 1 h) reduces zebrafish mortality and viral load following influenza infection. However, CPC mechanisms of action upon viral-host cell interaction are currently unknown. We have utilized super-resolution fluorescence photoactivation localization microscopy to probe the mode of CPC action. Reduction in density of influenza viral protein hemagglutinin (HA) clusters is known to reduce influenza infectivity: here, we show that CPC (at non-cytotoxic doses, 5-10 μM) reduces HA density and number of HA molecules per cluster within the plasma membrane of NIH-3T3 mouse fibroblasts. HA is known to colocalize with the negatively-charged mammalian lipid phosphatidylinositol 4,5-bisphosphate (PIP2); here, we show that nanoscale co-localization of HA with the PIP2-binding Pleckstrin homology (PH) reporter in the plasma membrane is diminished by CPC. CPC also dramatically displaces the PIP2-binding protein myristoylated alanine-rich C-kinase substrate (MARCKS) from the plasma membrane of rat RBL-2H3 mast cells; this disruption of PIP2 is correlated with inhibition of mast cell degranulation. Together, these findings offer a PIP2-focused mechanism underlying CPC disruption of influenza and suggest potential pharmacological use of this drug as an influenza therapeutic to reduce global deaths from viral disease.

Triclosan disrupts immune cell function by depressing Ca2+ influx following acidification of the cytoplasm

Triclosan (TCS) is an antimicrobial agent that was effectively banned by the FDA from hand soaps in 2016, hospital soaps in 2017, and hand sanitizers in 2019; however, TCS can still be found in a few products. At consumer-relevant, non-cytotoxic doses, TCS inhibits the functions of both mitochondria and mast cells, a ubiquitous cell type. Via the store-operated Ca2+ entry mechanism utilized by many immune cells, mast cells undergo antigen-stimulated Ca2+ influx into the cytosol, for proper function. Previous work showed that TCS inhibits Ca2+ dynamics in mast cells, and here we show that TCS also inhibits Ca2+ mobilization in human Jurkat T cells. However, the biochemical mechanism behind the Ca2+ dampening has yet to be elucidated. Three-dimensional super-resolution microscopy reveals that TCS induces mitochondrial swelling, in line with and extending the previous finding of TCS inhibition of mitochondrial membrane potential via its proton ionophoric activity. Inhibition of plasma membrane potential (PMP) by the canonical depolarizer gramicidin can inhibit mast cell function. However, use of the genetically encoded voltage indicators (GEVIs) ArcLight (pH-sensitive) and ASAP2 (pH-insensitive), indicates that TCS does not disrupt PMP. In conjunction with data from a plasma membrane-localized, pH-sensitive reporter, these results indicate that TCS, instead, induces cytosolic acidification in mast cells and T cells. Acidification of the cytosol likely inhibits Ca2+ influx by uncoupling the STIM1/ORAI1 interaction that is required for opening of plasma membrane Ca2+ channels. These results provide a mechanistic explanation of TCS disruption of Ca2+ influx and, thus, of immune cell function.

In vitro anti-inflammatory effects of curcumin on mast cell-mediated allergic responses via inhibiting FcεRI protein expression and protein kinase C delta translocation

Allergy is a hypersensitivity reaction when exposed to certain environmental substances. It shows high relation between immunoglobulin E (IgE) binding to a specific receptor (FcεRI), pro-inflammatory cytokines, and mediators with allergic inflammation responses. Curcumin is a yellow pigment isolated from the turmeric. Curcumin possesses antioxidant and anti-inflammatory properties as well as exhibits significant chemopreventive activity. This study was aimed to investigate the in vitro assessment of the regulation of curcumin on allergic inflammatory responses on rat basophil leukemia (RBL)-2H3 and human pre-basophils (KU812) cell lines. Curcumin showed the activity against histamine and β-hexosaminidase releases from both IgE-mediated and A23187-induced cells degranulation. The morphological observation also confirmed that curcumin inhibits cells degranulation. IgE-mediated allergic responses and significantly induced mast cells intracellular reactive oxygen species (ROS) production. Curcumin reduced ROS production from IgE-mediated or A23187-induced cells degranulation. Curcumin also successfully reduced FcεRI expressions and some pro-inflammatory cytokines, such as interleukin (IL)-4 and IL-13. Furthermore, curcumin inhibited protein kinase C (PKC)-δ translocation from cytosolic to particulate. These results suggested that curcumin can alleviate both the IgE-mediated and calcium ionosphere A23187-stimulated allergic responses through reducing the release of the allergic mediators.

Diacylglycerol kinase ζ promotes allergic airway inflammation and airway hyperresponsiveness through distinct mechanisms

Asthma is a chronic allergic inflammatory airway disease caused by aberrant immune responses to inhaled allergens, which leads to airway hyperresponsiveness (AHR) to contractile stimuli and airway obstruction. Blocking T helper 2 (T_H2) differentiation represents a viable therapeutic strategy for allergic asthma, and strong TCR-mediated ERK activation blocks T_H2 differentiation. Here, we report that targeting diacylglycerol (DAG) kinase zeta (DGKζ), a negative regulator of DAG-mediated cell signaling, protected against allergic asthma by simultaneously reducing airway inflammation and AHR though independent mechanisms. Targeted deletion of DGKζ in T cells decreased type 2 inflammation without reducing AHR. In contrast, loss of DGKζ in airway smooth muscle cells decreased AHR but not airway inflammation. T cell-specific enhancement of ERK signaling was only sufficient to limit type 2 airway inflammation, not AHR. Pharmacological inhibition of DGK diminished both airway inflammation and AHR in mice and also reduced bronchoconstriction of human airway samples in vitro. These data suggest that DGK is a previously unrecognized therapeutic target for asthma and reveal that the inflammatory and AHR components of asthma are not as interdependent as generally believed.

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.

Antimicrobial agent triclosan disrupts mitochondrial structure, revealed by super-resolution microscopy, and inhibits mast cell signaling via calcium modulation

The antimicrobial agent triclosan (TCS) is used in products such as toothpaste and surgical soaps and is readily absorbed into oral mucosa and human skin. These and many other tissues contain mast cells, which are involved in numerous physiologies and diseases. Mast cells release chemical mediators through a process termed degranulation, which is inhibited by TCS. Investigation into the underlying mechanisms led to the finding that TCS is a mitochondrial uncoupler at non-cytotoxic, low-micromolar doses in several cell types and live zebrafish. Our aim was to determine the mechanisms underlying TCS disruption of mitochondrial function and of mast cell signaling. We combined super-resolution (fluorescence photoactivation localization) microscopy and multiple fluorescence-based assays to detail triclosan's effects in living mast cells, fibroblasts, and primary human keratinocytes. TCS disrupts mitochondrial nanostructure, causing mitochondria to undergo fission and to form a toroidal, "donut" shape. TCS increases reactive oxygen species production, decreases mitochondrial membrane potential, and disrupts ER and mitochondrial Ca²⁺ levels, processes that cause mitochondrial fission. TCS is 60 × more potent than the banned uncoupler 2,4-dinitrophenol. TCS inhibits mast cell degranulation by decreasing mitochondrial membrane potential, disrupting microtubule polymerization, and inhibiting mitochondrial translocation, which reduces Ca²⁺ influx into the cell. Our findings provide mechanisms for both triclosan's inhibition of mast cell signaling and its universal disruption of mitochondria. These mechanisms provide partial explanations for triclosan's adverse effects on human reproduction, immunology, and development. This study is the first to utilize super-resolution microscopy in the field of toxicology.

SNAP23-Dependent Surface Translocation of Leukotriene B4 (LTB4) Receptor 1 Is Essential for NOX2-Mediated Exocytotic Degranulation in Human Mast Cells Induced by Trichomonas vaginalis-Secreted LTB4

Trichomonas vaginalis is a sexually transmitted parasite that causes vaginitis in women and itself secretes lipid mediator leukotriene B₄ (LTB₄). Mast cells are important effector cells of tissue inflammation during infection with parasites. Membrane-bridging SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes are critical for fusion during exocytosis. Although T. vaginalis-derived secretory products (TvSP) have been shown to induce exocytosis in mast cells, information regarding the signaling mechanisms between mast cell activation and TvSP is limited. In this study, we found that SNAP23-dependent surface trafficking of LTB₄ receptor 1 (BLT1) is required for nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2)-mediated exocytotic degranulation of mast cells induced by TvSP. First, stimulation with TvSP induced exocytotic degranulation and reactive oxygen species (ROS) generation in HMC-1 cells. Next, TvSP-induced ROS generation and exocytosis were strongly inhibited by transfection of BLT1 small interfering RNA (siRNA). TvSP induced trafficking of BLT1 from the cytosol to the plasma membrane. We also found that knockdown of SNAP23 abrogated TvSP-induced ROS generation, exocytosis, and surface trafficking of BLT1 in HMC-1 cells. By coimmunoprecipitation, there was a physical interaction between BLT1 and SNAP23 in TvSP-stimulated HMC-1 cells. Taken together, our results suggest that SNAP23-dependent surface trafficking of BLT1 is essential for exocytosis in human mast cells induced by T. vaginalis-secreted LTB₄ Our data collectively demonstrate a novel regulatory mechanism for SNAP23-dependent mast cell activation of T. vaginalis-secreted LTB₄ involving surface trafficking of BLT1. These results can help to explain how the cross talk mechanism between parasite and host can govern deliberately tissue inflammatory responses.

Silver Nanoparticle-Directed Mast Cell Degranulation Is Mediated through Calcium and PI3K Signaling Independent of the High Affinity IgE Receptor

Engineered nanomaterial (ENM)-mediated toxicity often involves triggering immune responses. Mast cells can regulate both innate and adaptive immune responses and are key effectors in allergic diseases and inflammation. Silver nanoparticles (AgNPs) are one of the most prevalent nanomaterials used in consumer products due to their antimicrobial properties. We have previously shown that AgNPs induce mast cell degranulation that was dependent on nanoparticle physicochemical properties. Furthermore, we identified a role for scavenger receptor B1 (SR-B1) in AgNP-mediated mast cell degranulation. However, it is completely unknown how SR-B1 mediates mast cell degranulation and the intracellular signaling pathways involved. In the current study, we hypothesized that SR-B1 interaction with AgNPs directs mast cell degranulation through activation of signal transduction pathways that culminate in an increase in intracellular calcium signal leading to mast cell degranulation. For these studies, we utilized bone marrow-derived mast cells (BMMC) isolated from C57Bl/6 mice and RBL-2H3 cells (rat basophilic leukemia cell line). Our data support our hypothesis and show that AgNP-directed mast cell degranulation involves activation of PI3K, PLCγ and an increase in intracellular calcium levels. Moreover, we found that influx of extracellular calcium is required for the cells to degranulate in response to AgNP exposure and is mediated at least partially via the CRAC channels. Taken together, our results provide new insights into AgNP-induced mast cell activation that are key for designing novel ENMs that are devoid of immune system activation.

Mast cells and influenza a virus: association with allergic responses and beyond

Influenza A virus (IAV) is a widespread infectious agent commonly found in mammalian and avian species. In humans, IAV is a respiratory pathogen that causes seasonal infections associated with significant morbidity in young and elderly populations, and has a large economic impact. Moreover, IAV has the potential to cause both zoonotic spillover infection and global pandemics, which have significantly greater morbidity and mortality across all ages. The pathology associated with these pandemic and spillover infections appear to be the result of an excessive inflammatory response leading to severe lung damage, which likely predisposes the lungs for secondary bacterial infections. The lung is protected from pathogens by alveolar epithelial cells, endothelial cells, tissue resident alveolar macrophages, dendritic cells, and mast cells. The importance of mast cells during bacterial and parasitic infections has been extensively studied; yet, the role of these hematopoietic cells during viral infections is only beginning to emerge. Recently, it has been shown that mast cells can be directly activated in response to IAV, releasing mediators such histamine, proteases, leukotrienes, inflammatory cytokines, and antiviral chemokines, which participate in the excessive inflammatory and pathological response observed during IAV infections. In this review, we will examine the relationship between mast cells and IAV, and discuss the role of mast cells as a potential drug target during highly pathological IAV infections. Finally, we proposed an emerging role for mast cells in other viral infections associated with significant host pathology.

Immunopharmacological modulation of mast cells

Mast cells produce a wide spectrum of mediators and they have been implicated in several physiopathological conditions (e.g. allergic reactions and certain tumors). Pharmacologic agents that modulate the release of mediators from mast cells has helped to elucidate the biochemical mechanisms by which immunological and non-immunological stimuli activate these cells. Furthermore, the study of surface receptors and signaling pathways associated with mast cell activation revealed novel pharmacologic targets. Thus, the development of pharmacologic agents based on this new wave of knowledge holds promise for the treatment of several mast cell-mediated disorders.

Diacylglycerol kinase γ regulates antigen-induced mast cell degranulation by mediating Ca(2+) influxes

Diacylglycerol (DAG) is an important lipid that acts as a signaling messenger during mast cell degranulation after allergen cross-linking of immunoglobulin (Ig) E-bound FcεRI receptors. In this study, we determined the role of diacylglycerol kinase (DGK), which negatively regulates DAG-dependent signaling by converting DAG to phosphatidic acid (PA), in the regulation of mast cell degranulation. Treating RBL (rat basophilic leukemia)-2H3 mast cells with a type I DGK inhibitor significantly reduced antigen-induced degranulation and PA production. Among type I DGK isoforms, we observed that DGKα and DGKγ mRNAs were expressed in RBL-2H3 mast cells using reverse transcription polymerase chain reaction. DGKγ knockdown, but not DGKα, by isoform-specific short hairpin RNAs reduced mast cell degranulation and Ca(2+) influxes from the extracellular environment. These results suggest that DGKγ regulates mast cell degranulation after FcεRI cross-linking through mobilization of intracellular Ca(2+) through Ca(2+) influxes.

Novel PKCα-mediated phosphorylation site(s) on cofilin and their potential role in terminating histamine release

PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation, and the novel phosphorylation contributes to F-actin remodeling by regulating the ability of cofilin to bind to 14-3-3ζ and to depolymerize and/or sever F-actin. This highly regulated mechanism is necessary for the proper termination of degranulation.

Using specific inhibitors, kinase-negative mutants, and small interfering RNA against protein kinase Cα (PKCα) or PKCβI, we find that PKCβI positively regulates degranulation in rat basophilic leukemia–2H3 cells, whereas PKCα negatively regulates degranulation. Mass spectrometric and mutagenic analyses reveal that PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation. Overexpression of a nonphosphorylatable form (S23,24A), but not that of a mutant-mimicking phosphorylated form (S23,24E), increases degranulation. Furthermore, the S23,24A mutant binds to F-actin and retains its depolymerizing and/or cleavage activity; conversely, the S23,24E mutant is unable to sever actin filaments, resulting in F-actin polymerization. In addition, the S23,24E mutant preferentially binds to the 14-3-3ζ protein. Fluorescence-activated cell sorting analysis with fluorescein isothiocyanate–phalloidin and simultaneous observation of degranulation, PKC translocation, and actin polymerization reveals that during degranulation, actin polymerization is dependent on PKCα activity. These results indicate that a novel PKCα-mediated phosphorylation event regulates cofilin by inhibiting its ability to depolymerize F-actin and bind to 14-3-3ζ, thereby promoting F-actin polymerization, which is necessary for cessation of degranulation.

Cysteinyl leukotriene type I receptor desensitization sustains Ca2+-dependent gene expression

A universal mechanism to turn off a biological response is receptor desensitization, where the ability of a physiological trigger to activate a cell is lost despite the continued presence of the stimulus. Receptor desensitization of G protein-coupled receptors involves uncoupling of the receptor from its G protein/second messenger pathway, followed by receptor internalization¹. G protein-coupled cysteinyl leukotriene type I (CysLT1) receptors regulate immune cell function and the receptor is an established therapeutic target for allergies including asthma².

Desensitization of these receptors arises predominantly from protein kinase C-dependent phosphorylation of three serine residues in the receptor C-terminus³. Physiological concentrations of the receptor agonist LTC₄ evoke repetitive cytoplasmic Ca²⁺ oscillations, reflecting regenerative Ca²⁺ release from stores that is sustained by Ca²⁺ entry through store-operated CRAC channels⁴. CRAC channels are tightly linked to expression of the transcription factor c-fos⁵, a regulator of numerous genes important to cell growth and development⁶. Here we show that abolishing leukotriene receptor desensitization suppresses agonist-driven gene expression. Mechanistically, stimulation of non-desensitizing receptors evoked prolonged inositol trisphosphate-mediated Ca²⁺ release, which led to accelerated Ca²⁺-dependent slow inactivation of CRAC channels and a subsequent loss of excitation-transcription coupling. Rather than serving to turn off a biological response, reversible desensitization of a Ca²⁺ mobilizing receptor acts as an ‘on’switch, sustaining long-term signalling in the immune system.

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.

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.

Mast cell biology: introduction and overview

In recent years, the field of mast cell biology has expanded well beyond the boundaries of atopic disorders and anaphy laxis, on which it has been historically focused. The biochemical and signaling events responsible for the development and regulation of mast cells has been increasingly studied, aided in large part by novel breakthroughs in laboratory techniques used to study these cells. The result of these studies has been a more comprehensive definition of mast cells that includes added insights to their overall biology as well as the various disease states that can now be traced to defects in mast cells. This introductory chapter outlines and highlights the various topics of mast cell biology that will be discussed in further detail in subsequent chapters.

Aldehyde dehydrogenase activation prevents reperfusion arrhythmias by inhibiting local renin release from cardiac mast cells

BACKGROUND

Renin released by ischemia/reperfusion from cardiac mast cells activates a local renin-angiotensin system (RAS). This exacerbates norepinephrine release and reperfusion arrhythmias (ventricular tachycardia and fibrillation), making RAS a new therapeutic target in myocardial ischemia.

METHODS AND RESULTS

We investigated whether ischemic preconditioning (IPC) prevents cardiac RAS activation in guinea pig hearts ex vivo. When ischemia/reperfusion (20 minutes of ischemia/30 minutes of reperfusion) was preceded by IPC (two 5-minute ischemia/reperfusion cycles), renin and norepinephrine release and ventricular tachycardia and fibrillation duration were markedly decreased, a cardioprotective anti-RAS effect. Activation and blockade of adenosine A(2b)/A(3) receptors and activation and inhibition of protein kinase Cepsilon (PKCepsilon) mimicked and prevented, respectively, the anti-RAS effects of IPC. Moreover, activation of A(2b)/A(3) receptors or activation of PKCepsilon prevented degranulation and renin release elicited by peroxide in cultured mast cells (HMC-1). Activation and inhibition of mitochondrial aldehyde dehydrogenase type-2 (ALDH2) also mimicked and prevented, respectively, the cardioprotective anti-RAS effects of IPC. Furthermore, ALDH2 activation inhibited degranulation and renin release by reactive aldehydes in HMC-1. Notably, PKCepsilon and ALDH2 were both activated by A(2b)/A(3) receptor stimulation in HMC-1, and PKCepsilon inhibition prevented ALDH2 activation.

CONCLUSIONS

The results uncover a signaling cascade initiated by A(2b)/A(3) receptors, which triggers PKCepsilon-mediated ALDH2 activation in cardiac mast cells, contributing to IPC-induced cardioprotection by preventing mast cell renin release and the dysfunctional consequences of local RAS activation. Thus, unlike classic IPC in which cardiac myocytes are the main target, cardiac mast cells are the critical site at which the cardioprotective anti-RAS effects of IPC develop.

Protein phosphatase 2A carboxymethylation and regulatory B subunits differentially regulate mast cell degranulation

Asthma is characterised by antigen-mediated mast cell degranulation resulting in secretion of inflammatory mediators. Protein phosphatase 2A (PP2A) is a serine/threonine protein phosphatase composed of a catalytic (PP2A-C) subunit together with a core scaffold (PP2A-A) subunit and a variable, regulatory (PP2A-B) subunit. Previous studies utilising pharmacological inhibition of protein phosphatases have suggested a positive regulatory role for PP2A in mast cell degranulation. In support of this we find that a high okadaic acid concentration (1μM) inhibits mast cell degranulation. Strikingly, we now show that a low concentration of okadaic acid (0.1μM) has the opposite effect, resulting in enhanced degranulation. Selective downregulation of the PP2A-Cα subunit by short hairpin RNA also enhanced degranulation of RBL-2H3 mast cells, suggesting that the primary role of PP2A is to negatively regulate degranulation. PP2A-B subunits are responsible for substrate specificity, and carboxymethylation of the PP2A-C subunit alters B subunit binding. We show here that carboxymethylation of PP2A-C is dynamically altered during degranulation and inhibition of methylation decreases degranulation. Moreover downregulation of the PP2A-Bα subunit resulted in decreased MK2 phosphorylation and degranulation, whilst downregulation of the PP2A-B'δ subunit enhanced p38 MAPK phosphorylation and degranulation. Taken together these data show that PP2A is both a positive and negative regulator of mast cell degranulation, and this differential role is regulated by carboxymethylation and specific PP2A-B subunit binding.

Inhibitory effect of carotenoids on the degranulation of mast cells via suppression of antigen-induced aggregation of high affinity IgE receptors

Carotenoids have been demonstrated to possess antioxidative and anti-inflammatory effects. However, there is no report that the effects of carotenoids on degranulation of mast cell is critical for type I allergy. In this study, we focused on the effect of carotenoids on antigen-induced degranulation of mast cells. Fucoxanthin, astaxanthin, zeaxanthin, and beta-carotene significantly inhibited the antigen-induced release of beta-hexosaminidase in rat basophilic leukemia 2H3 cells and mouse bone marrow-derived mast cells. Those carotenoids also inhibited antigen-induced aggregation of the high affinity IgE receptor (Fc epsilonRI), which is the most upstream of the degranulating signals of mast cells. Furthermore, carotenoids inhibited Fc epsilonRI-mediated intracellular signaling, such as phosphorylation of Lyn kinase and Fyn kinase. It suggests that the inhibitory effect of carotenoids on the degranulation of mast cells were mainly due to suppressing the aggregation of Fc epsilonRI followed by intracellular signaling. In addition, those carotenoids inhibited antigen-induced translocation of Fc epsilonRI to lipid rafts, which are known as platforms of the aggregation of Fc epsilonRI. We assume that carotenoids may modulate the function of lipid rafts and inhibit the translocation of Fc epsilonRI to lipid rafts. This is the first report that focused on the aggregation of Fc epsilonRI to investigate the mechanism of the inhibitory effects on the degranulation of mast cells and evaluated the functional activity of carotenoids associated with lipid rafts.

Mast cells and epsilonPKC: a role in cardiac remodeling in hypertension-induced heart failure

Heart failure (HF) is a chronic syndrome in which pathological cardiac remodeling is an integral part of the disease and mast cell (MC) degranulation-derived mediators have been suggested to play a role in its progression. Protein kinase C (PKC) signaling is a key event in the signal transduction pathway of MC degranulation. We recently found that inhibition of epsilonPKC slows down the progression of hypertension-induced HF in salt-sensitive Dahl rats fed a high-salt diet. We therefore determined whether epsilonPKC inhibition affects MC degranulation in this model. Six week-old male Dahl rats were fed with a high-salt diet to induce systemic hypertension, which resulted in concentric left ventricular hypertrophy at the age of 11 weeks, followed by myocardial dilatation and HF at the age of 17 weeks. We administered epsilonV1-2, an epsilonPKC-selective inhibitor peptide (3 mg/kg/day), deltaV1-1, a deltaPKC-selective inhibitor peptide (3 mg/kg/day), TAT (negative control; at equimolar concentration; 1.6 mg/kg/day) or olmesartan (angiotensin receptor blocker [ARB] as a positive control; 3 mg/kg/day) between 11 weeks and 17 weeks. Treatment with epsilonV1-2 attenuated cardiac MC degranulation without affecting MC density, myocardial fibrosis, microvessel patency, vascular thickening and cardiac inflammation in comparison to TAT- or deltaV1-1-treatment. Treatment with ARB also attenuated MC degranulation and cardiac remodeling, but to a lesser extent when compared to epsilonV1-2. Finally, epsilonV1-2 treatment inhibited MC degranulation in isolated peritoneal MCs. Together, our data suggest that epsilonPKC inhibition attenuates pathological remodeling in hypertension-induced HF, at least in part, by preventing cardiac MC degranulation.

Regulation of M2-type pyruvate kinase mediated by the high-affinity IgE receptors is required for mast cell degranulation

BACKGROUND AND PURPOSE

M2-type pyruvate kinase (M2PK) was found to interact directly with the 'ITAM' region of the gamma chain of the high-affinity IgE receptor (FcvarepsilonRI). Our hypothesis was that mast cell degranulation might require the FcvarepsilonRI-mediated inhibition of M2PK activity.

EXPERIMENTAL APPROACH

In rat basophilic leukaemia (RBL-2H3) cells, the effects of directly inhibiting M2PK or preventing the FcvarepsilonRI-mediated inhibition of M2PK (disinhibition) on degranulation was measured by hexosaminidase release. Effects of blocking the FcvarepsilonRI-mediated inhibition of M2PK was also assessed in vivo in a mouse model of allergen-induced airway hyper-responsiveness.

KEY RESULTS

Activation of FcvarepsilonRI in RBL-2H3 cells caused the rapid phosphorylation of tyrosine residues in M2PK, associated with a decrease in M2PK enzymatic activity. There was an inverse correlation between M2PK activity and mast cell degranulation. FcvarepsilonRI-mediated inhibition of M2PK involved Src kinase, phosphatidylinositol 3-kinase, PKC and calcium. Direct inhibition of M2PK potentiated FcvarepsilonRI-mediated degranulation and prevention of the FcvarepsilonRI-mediated inhibition of M2PK attenuated mast cell degranulation. Transfection of RBL-2H3 cells with M1PK which prevents FcvarepsilonRI-induced inhibition of M2PK, markedly reduced their degranulation and exogenous M1PK (i.p.) inhibited ovalbumin-induced airway hyper-responsiveness in vivo.

CONCLUSIONS AND IMPLICATIONS

We have identified a new control point and a novel biochemical pathway in the process of mast cell degranulation. Our study suggests that the FcvarepsilonRI-mediated inhibition of M2PK is a crucial step in responses to allergens. Moreover, the manipulation of glycolytic processes and intermediates could provide novel strategies for the treatment of allergic diseases.

Regulation of the mast cell response to the type 1 Fc epsilon receptor

The type I Fc epsilon receptor (Fc epsilon RI) is one of the better understood members of its class and is central to the immunological activation of mast cells and basophils, the key players in immunoglobulin E (IgE)-dependent immediate hypersensitivity. This review provides background information on several distinct regulatory mechanisms controlling this receptor's stimulus-response coupling network. First, we review the current understanding of this network's operation, and then we focus on the inhibitory regulatory mechanisms. In particular, we discuss the different known cytosolic molecules (e.g. kinases, phosphatases, and adapters) as well as cell membrane proteins involved in negatively regulating the Fc epsilon RI-induced secretory responses. Knowledge of this field is developing at a fast rate, as new proteins endowed with regulatory functions are still being discovered. Our understanding of the complex networks by which these proteins exert regulation is limited. Although the scope of this review does not include addressing several important biochemical and biophysical aspects of the regulatory mechanisms, it does provide general insights into a central field in immunology.

PKC and cAMP positively modulate alkaline-induced exocytosis in the human mast cell line HMC-1

We study in HMC-1 the activation process, measured as histamine release. We know that ammonium chloride (NH(4)Cl) and ionomycin release histamine, and the modulatory role of drugs targeting protein kinase C (PKC), adenosine 3',5'-cyclic monophosphate (cAMP), tyrosine kinase (TyrK) and phosphatidylinositol 3-kinase (PI3K) on this effect. We used Gö6976 (100 nM) and low concentration of GF 109203X (GF) (50 nM) to inhibit Ca(2+)-dependent PKC isozymes. For Ca(2+)-independent isozymes, we used 500 nM GF and 10 microM rottlerin (specifically inhibits PKCdelta). Phorbol 12-myristate 13-acetate (PMA) (100 ng/ml) was used to stimulate PKC, and genistein (10 microM) and lavendustin A (1 microM) as unspecific TyrK inhibitors. STI571 10 microM was used to specifically inhibit the activity of Kit, the receptor for stem cell factor, and 10 nM wortmannin as a PI3K inhibitor. Activation of PKC with PMA enhances histamine release in response to NH(4)Cl and ionomycin. PMA increases NH(4)Cl-induced alkalinization and ionomycin-induced Ca(2+) entry. Inhibition of PKCdelta strongly inhibits Ca(2+) entry elicited by ionomycin, but failed to modify histamine release. The effect of cAMP-active drugs was explored with the adenylate cyclase activator forskolin (30 microM), the inhibitor SQ22,536 (1 microM), the cAMP analog dibutyryl cAMP (200 microM), and the PKA blocker H89 (1 microM). Forskolin and dibutyryl cAMP do increase NH(4)Cl-induced alkalinization, and potentiate histamine release elicited by this compound. Our data indicates that alkaline-induced exocytosis is modulated by PKC and cAMP, suggesting that pH could be a modulatory signal itself.

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.

B cell lymphoma 10 is essential for FcepsilonR-mediated degranulation and IL-6 production in mast cells

The adaptor protein B cell lymphoma 10 (Bcl10) plays an essential role in the functions of the AgRs in T and B cells. In this study, we report that Bcl10 also plays an important role in mast cells. Bcl10 is expressed in mast cells. Although Bcl10-deficient mast cells undergo normal development, we demonstrate that Bcl10 is essential for specific functions of FcepsilonR. Although Bcl10-deficient mast cells have normal de novo synthesis and release of the lipid mediator arachidonic acid, the mutant cells possess impaired FcepsilonR-mediated degranulation, indicated by decreased serotonin release, and impaired cytokine production, measured by release of IL-6. In addition, Bcl10-deficient mice display impaired IgE-mediated passive cutaneous anaphylaxis. Moreover, although Bcl10-deficient mast cells have normal FcepsilonR-mediated Ca(2+) flux, activation of PI3K, and activation of the three types of MAPKs (ERKs, JNK, and p38), the mutant cells have markedly diminished FcepsilonR-mediated activation of NF-kappaB and decreased activation of AP-1. Thus, Bcl10 is essential for FcepsilonR-induced activation of AP-1, NF-kappaB, degranulation, and cytokine production in mast cells.

Phosphorylation of nonmuscle myosin heavy chain IIA on Ser1917 is mediated by protein kinase C beta II and coincides with the onset of stimulated degranulation of RBL-2H3 mast cells

Dynamic remodeling of the actinomyosin cytoskeleton is integral to many biological processes. It is regulated, in part, by myosin phosphorylation. Nonmuscle myosin H chain IIA is phosphorylated by protein kinase C (PKC) on Ser(1917). Our aim was to determine the PKC isoform specificity of this phosphorylation event and to evaluate its potential role in regulated secretion. Using an Ab against the phosphorylated form of Ser(1917), we show that this site is not phosphorylated in unstimulated RBL-2H3 mast cells. The physiological stimulus, Ag, or the pharmacological activators, PMA plus A23187, induced Ser(1917) phosphorylation with a time course coincident with the onset of granule mediator secretion. Dephosphorylation at this site occurred as Ag-stimulated secretion declined from its peak, but dephosphorylation was delayed in cells activated with PMA plus A23187. Phosphate incorporation was also enhanced by PMA alone and by inhibition of protein phosphatase 2A. Gö6976, an inhibitor of conventional PKC isoforms, abolished secretion and Ser(1917) phosphorylation with similar dose dependencies consistent with involvement of either PKCalpha or PKCbeta. Phorbol ester-stimulated Ser(1917) phosphorylation was reconstituted in HEK-293 cells (which lack endogenous PKCbeta) by overexpression of both wild-type and constitutively active PKCbetaII but not the corresponding PKCbetaI or PKCalpha constructs. A similar selectivity for PKCbetaII overexpression was also observed in MIN6 insulinoma cells infected with recombinant PKC wild-type adenoviruses. Our results implicate PKC-dependent phosphorylation of myosin H chain IIA in the regulation of secretion in mast cells and suggest that Ser(1917) phosphorylation might be a marker of PKCbetaII activation in diverse cell types.

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.

A redundant role for PKC-epsilon in mast cell signaling and effector function

Protein kinase (PK) C-epsilon is strongly expressed in mast cells (MCs) and activated in response to antigen-mediated high-affinity receptor for IgE (Fc epsilonR1) engagement. A critical role of PKC-epsilon in antigen-triggered activation of various signaling pathways was observed in basophilic leukemia cells. To study the function of PKC-epsilon in MCs differentiated in vitro from murine bone marrow, we used our established PKC-epsilon null mice. Unexpectedly, we did not reveal any difference in antigen-induced activation of many central signaling molecules (PKB, mitogen-activated protein kinase, p38, Jun-N-terminal kinase, phospholipase C-gamma1, Bruton's tyrosine kinase, PKD, Fos and PKC-delta) in time-course as well as dose-response studies between PKC-epsilon-deficient and wild-type MCs. In correlation, antigen-triggered degranulation, release of arachidonic acid and secretion of IL-6 were unaltered by the loss of PKC-epsilon. Furthermore, stimulation of MCs via different receptor systems [Steel factor receptor (c-kit) and toll-like receptor 4] did not lead to differences in the measured responses between both cell types. These results strongly suggest that PKC-epsilon plays a redundant role in MCs stimulated by antigen as well as other well-known MC stimuli.

Protein kinase C α, βI, and βII isozymes regulate cytokine production in mast cells through MEKK2/ERK5-dependent and -independent pathways

Regulation of MAPK pathways by PKC isoforms was examined in murine bone marrow-derived mast cells (BMMCs). The PKCalpha, betaI, and betaII isoforms showed the most robust activation after FcepsilonR1-mediated stimulation by anti-ovalbumin specific IgE and ovalbumin (IgE-ova). PKCalpha, betaI, and betaII were all involved in activation of JNK, MEKK2, and ERK5, with differential relative contributions of each isoform to specific MAPK pathway components. BMMCs from mice lacking MEKK2 showed reduced production (50-60%) of IL-6, IL-13, and TNF-alpha after stimulation, demonstrating MEKK2-dependent and -independent pathways for cytokine production. Cytokine production was stimulated by over-expression of PKC in cells from MEKK2-deficient and wild-type mice. Activation of ERK5 did not occur in BMMCs lacking MEKK2, indicating that MEKK2-independent cytokine production was also ERK5-independent. Since MAPK modules differentially regulate mast cell functions, including degranulation and cytokine production, it is suggested that specific functions could be targeted by inhibiting specific PKC isoforms.

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

Activation of phospholipase D (PLD) and protein kinase C (PKC) as well as calcium mobilization are essential signals for degranulation of mast cells. However, the exact role of PLD in degranulation remains undefined. In this study we have tested the hypothesis that the PLD product, phosphatidic acid, and diacylglycerides generated therefrom might promote activation of PKC. Studies were conducted in two rodent mast cell lines that were stimulated with Ag via FcepsilonRI and a pharmacologic agent, thapsigargin. Diversion of production of phosphatidic acid to phosphatidylbutanol (the transphosphatidylation reaction) by addition of l-butanol suppressed both the translocation of diacylglyceride-dependent isoforms of PKC to the membrane and degranulation. Tertiary-butanol, which is not a substrate for the transphosphatidylation, had a minimal effect on PKC translocation and degranulation, and 1-butanol itself had no effect on PKC translocation when PKC was stimulated directly with phorbol ester, 12-O-tetradecanoylphorbol-13-acetate. Also, in cells transfected with small inhibitory RNAs directed against PLD1 and PLD2, activation of PLD, generation of diacylglycerides, translocation of PKC, and degranulation were all suppressed. Phorbol ester, which did not stimulate degranulation by itself, restored degranulation when used in combination with thapsigargin whether PLD function was disrupted with 1-butanol or the small inhibitory RNAs. However, degranulation was not restored when cells were costimulated with Ag and phorbol ester. These results suggested that the production of phosphatidic acid by PLD facilitates activation of PKC and, in turn, degranulation, although additional PLD-dependent processes appear to be critical for Ag-mediated degranulation.

Cross-Desensitization among CXCR1, CXCR2, and CCR5: Role of Protein Kinase C-ε

The IL-8 (or CXCL8) chemokine receptors, CXCR1 and CXCR2, activate protein kinase C (PKC) to mediate leukocyte functions. To investigate the roles of different PKC isoforms in CXCL8 receptor activation and regulation, human mononuclear phagocytes were treated with CXCL8 or CXCL1 (melanoma growth-stimulating activity), which is specific for CXCR2. Plasma membrane association was used as a measure of PKC activation. Both receptors induced time-dependent association of PKCalpha, -beta1, and -beta2 to the membrane, but only CXCR1 activated PKCepsilon. CXCL8 also failed to activate PKCepsilon in RBL-2H3 cells stably expressing CXCR2. DeltaCXCR2, a cytoplasmic tail deletion mutant of CXCR2 that is resistant to internalization, activated PKCepsilon as well as CXCR1. Expression of the PKCepsilon inhibitor peptide epsilonV1 in RBL-2H3 cells blocked PKCepsilon translocation and inhibited receptor-mediated exocytosis, but not phosphoinositide hydrolysis or peak intracellular Ca(2+) mobilization. epsilonV1 also inhibited CXCR1-, CCR5-, and DeltaCXCR2-mediated cross-regulatory signals for GTPase activity, Ca(2+) mobilization, and internalization. Peritoneal macrophages from PKCepsilon-deficient mice (PKCepsilon(-/-)) also showed decreased CCR5-mediated cross-desensitization of G protein activation and Ca(2+) mobilization. Taken together, the results indicate that CXCR1 and CCR5 activate PKCepsilon to mediate cross-inhibitory signals. Inhibition or deletion of PKCepsilon decreases receptor-induced exocytosis and cross-regulatory signals, but not phosphoinositide hydrolysis or peak intracellular Ca(2+) mobilization, suggesting that cross-regulation is a Ca(2+)-independent process. Because DeltaCXCR2, but not CXCR2, activates PKCepsilon and cross-desensitizes CCR5, the data further suggest that signal duration leading to activation of novel PKC may modulate receptor-mediated cross-inhibitory signals.

A comparative study of the FcepsilonRI molecule on human mast cell and basophil cell lines

BACKGROUND

Mast cells and basophils express the high-affinity IgE receptor FcepsilonRI. We have analysed the human mast cell line LAD2 and four subclones of the basophil cell line KU812 in order to reveal possible differences concerning the FcepsilonRI surface regulation, anti-IgE-triggered activation, FcepsilonRIalpha protein stability and the mRNA level of FcepsilonRIalpha-, beta- and the truncated beta-chain (beta(T)), and thereby determine the utility of these cell lines in investigations of the FcepsilonRI biology.

METHODS

The surface expression of FcepsilonRI was assessed by flow cytometry, using the monoclonal antibody CRA1. The FcepsilonRI-induced cellular activation (i.e. cross-linking of FcepsilonRI) was determined by changes in the intracellular level of Ca2+, which was measured by fluorescence of Fura-2. The level of the FcepsilonRIalpha protein was determined by a Western blot technique and by a radioimmunoassay. The mRNA level of FcepsilonRIalpha, beta- and beta(T)-chain was analysed using real-time PCR.

RESULTS

Two KU812 subclones and especially LAD2 had FcepsilonRI surface expression which was capable of inducing cellular activation. Both the FcepsilonRI expression and stability of the FcepsilonRIalpha protein were increased when IgE was present. All the cell lines expressed mRNA of FcepsilonRIalpha-, beta- and beta(T), with LAD2 tending to have the highest expression. However, a determination of the beta/beta(T) ratio demonstrated no difference between any of the cell clones.

CONCLUSION

These cell lines are important tools in the investigation of both the FcepsilonRI molecule and the effects induced by its activation.

Differential regulation of phospholipase Cgamma subtypes through FcepsilonRI, high affinity IgE receptor

The high affinity IgE receptor (FcepsilonRI) usually exists as a tetramer composed of alphabetagamma2 subunits. The COOH-tail of beta and gamma subunits contains consensus sequence termed 'immunoreceptor tyrosine-based activation motif' (ITAM). Tyrosine phosphorylated ITAM interacts with signaling proteins that contain the Src homology domain, forming a main amplifying and signaling route for FcepsilonRI. Unlike the COOH-tail, the functional role of NH(2)-tail of beta subunit in the signaling of FcepsilonRI is not clear because it lacks the ITAM sequences. To study the roles of NH(2)-tail of beta subunit, the cDNA library of RBL-2H3 cells was screened by yeast two-hybrid assay, and the NH(2)-tail of the beta subunit was found to interact with phospholipase Cgamma2 (PLCgamma2) but not with PLCgamma1. Since both PLCgamma1 and PLCgamma2 are expressed in RBL-2H3 cells and they possess identical cellular functions, the functional meaning of the protein-protein interaction between PLCgamma2 and NH(2)-tail of beta subunit was studied by comparing the regulatory pathways that control the FcepsilonRI-mediated tyrosine phosphorylation of the two enzymes. Our study shows that PI3-kinase and PMA-sensitive PKCs were required exclusively for the FcepsilonRI-mediated tyrosine phosphorylation of PLCgamma1. Also the FcepsilonRI-mediated tyrosine phosphorylation of PLCgamma1 was more sensitive to the inhibitors of Src and Syk kinases. These results therefore suggest that PLCgamma1 is involved in dynamic regulation of protein kinase C activity and inositol triphosphate levels in response to cellular needs. In contrast, PLCgamma2, through continuous interaction with the NH(2)-tail of beta subunit, co-localizes with FcepsilonRI in the same signaling domain, and maintains the basal cellular PLC activity.

Differential Role of Protein Kinase Cδ Isoform in Agonist-induced Dense Granule Secretion in Human Platelets

Several platelet agonists, including thrombin, collagen, and thromboxane A(2), cause dense granule release independently of thromboxane generation. Because protein kinase C (PKC) isoforms are implicated in platelet secretion, we investigated the role of individual PKC isoforms in platelet dense granule release. PKCdelta was phosphorylated in a time-dependent manner that coincided with dense granule release in response to protease-activated receptor-activating peptides SFLLRN and AYPGKF in human platelets. Only agonists that caused platelet dense granule secretion activated PKCdelta. SFLLRN- or AYPGKF-induced dense granule release and PKCdelta phosphorylation occurred at the same respective agonist concentration. Furthermore, AYPGKF and SFLLRN-induced dense granule release was blocked by rottlerin, a PKCdelta selective inhibitor. In contrast, convulxin-induced dense granule secretion was potentiated by rottlerin but was abolished by Go6976, a classical PKC isoform inhibitor. However, SFLLRN-induced dense granule release was unaffected in the presence of Go6976. Finally, rottlerin did not affect SFLLRN-induced platelet aggregation, even in the presence of dimethyl-BAPTA, indicating that PKCdelta has no role in platelet fibrinogen receptor activation. We conclude that PKCdelta and the classical PKC isoforms play a differential role in platelet dense granule release mediated by protease-activated receptors and glycoprotein VI. Furthermore, PKCdelta plays a positive role in protease-activated receptor-mediated dense granule secretion, whereas it functions as a negative regulator downstream of glycoprotein VI signaling.

The simultaneous production of phosphatidic acid and diacylglycerol is essential for the translocation of protein kinase Cepsilon to the plasma membrane in RBL-2H3 cells

To evaluate the role of the C2 domain in protein kinase Cepsilon (PKCepsilon) localization and activation after stimulation of the IgE receptor in RBL-2H3 cells, we used a series of mutants located in the phospholipid binding region of the enzyme. The results obtained suggest that the interaction of the C2 domain with the phospholipids in the plasma membrane is essential for anchoring the enzyme in this cellular compartment. Furthermore, the use of specific inhibitors of the different pathways that generate both diacylglycerol and phosphatidic acid has shown that the phosphatidic acid generated via phospholipase D (PLD)-dependent pathway, in addition to the diacylglycerol generated via phosphoinosite-phospholipase C (PLC), are involved in the localization of PKCepsilon in the plasma membrane. Direct stimulation of RBL-2H3 cells with very low concentrations of permeable phosphatidic acid and diacylglycerol exerted a synergistic effect on the plasma membrane localization of PKCepsilon. Moreover, the in vitro kinase assays showed that both phosphatidic acid and diacylglycerol are essential for enzyme activation. Together, these results demonstrate that phosphatidic acid is an important and essential activator of PKCepsilon through the C2 domain and locate this isoenzyme in a new scenario where it acts as a downstream target of PLD.

Gonadotropin-releasing hormone-desensitized LbetaT2 gonadotrope cells are refractory to acute protein kinase C, cyclic AMP, and calcium-dependent signaling

Sustained exposure of gonadotropes to GnRH causes a pronounced desensitization of gonadotropin release, but the mechanisms involved are poorly understood. It is known that desensitization is associated with decreased GnRH receptor and Gq/11 levels in alphaT3-1 cells, but it is not known whether downstream signaling is impaired. We have shown previously that chronic stimulation of signaling via expression of an active form of Galphaq causes GnRH resistance in LbetaT2 cells. In this study we investigated whether chronic GnRH treatment could down-regulate protein kinase C (PKC), cAMP, or Ca2+-dependent signaling in LbetaT2 cells. We found that chronic GnRH treatment desensitizes cells to acute GnRH stimulation not only by reducing GnRH receptor and Gq/11 expression but also by down-regulating PKC, cAMP, and calcium-dependent signaling. Desensitization was observed for activation of ERK and p38 MAPK and induction of c-fos and LHbeta protein expression. Activation of individual signaling pathways was able to partially mimic the desensitizing effect of GnRH on ERK, p38 MAPK, c-fos, and LHbeta but not on Gq/11. Chronic stimulation with phorbol esters reduced GnRH receptor expression to the same extent as chronic GnRH. Sustained GnRH also desensitized PKC signaling by down-regulating the delta, epsilon, and theta isoforms of PKC. We further show that chronic GnRH treatment causes heterologous desensitization of other Gq-coupled receptors.

Signaling mechanism for receptor-activated canonical transient receptor potential 3 (TRPC3) channels

Canonical transient receptor potential 3 (TRPC3) is a receptor-activated, calcium permeant, non-selective cation channel. TRPC3 has been shown to interact physically with the N-terminal domain of the inositol 1,4,5-trisphosphate receptor, consistent with a "conformational coupling" mechanism for its activation. Here we show that low concentrations of agonists that fail to produce levels of inositol 1,4,5-trisphosphate sufficient to induce Ca(2+) release from intracellular stores substantially activate TRPC3. By several experimental approaches, we demonstrate that neither inositol 1,4,5-trisphosphate nor G proteins are required for TRPC3 activation. However, diacylglycerols were sufficient to activate TRPC3 in a protein kinase C-independent manner. Surface receptor agonists and exogenously applied diacylglycerols were not additive in activating TRPC3. In addition, inhibition of metabolism of diacylglycerol slowed the reversal of receptor-dependent TRPC3 activation. We conclude that receptor-mediated activation of phospholipase C in intact cells activates TRPC3 via diacylglycerol production, independently of G proteins, protein kinase C, or inositol 1,4,5-trisphosphate.

Role of specific protein kinase C isozymes in mediating epidermal growth factor, thyrotropin-releasing hormone, and phorbol ester regulation of the rat prolactin promoter in GH4/GH4C1 pituitary cells

Epidermal growth factor (EGF) and TRH both produce enhanced prolactin (PRL) gene transcription and PRL secretion in GH4 rat pituitary tumor cell lines. These agents also activate protein kinase C (PKC) in these cells. Previous studies have implicated the PKCepsilon isozyme in mediating TRH-induced PRL secretion. However, indirect studies using phorbol ester down-regulation to investigate the role of PKC in EGF- and TRH-induced PRL gene transcription have been inconclusive. In the present study, we examined the role of multiple PKC isozymes on EGF- and TRH-induced activation of the PRL promoter by utilizing general and selective PKC inhibitors and by expression of genes for wild-type and kinase-negative forms of the PKC isozymes. Multiple nonselective PKC inhibitors, including staurosporine, bisindolylmaleimide I, and Calphostin C, inhibited both EGF and TRH induced rat PRL promoter activity. TRH effects were more sensitive to Calphostin C, a competitive inhibitor of diacylglycerol, whereas Go 6976, a selective inhibitor of Ca(2+)-dependent PKCs, produced a modest inhibition of EGF but no inhibition of TRH effects. Rottlerin, a specific inhibitor of the novel nPKCdelta isozyme, significantly blocked both EGF and TRH effects. Overexpression of genes encoding PKCs alpha, betaI, betaII, delta, gamma, and lambda failed to enhance either EGF or TRH responses, whereas overexpression of nPKCeta enhanced the EGF response. Neither stable nor transient overexpression of nPKCepsilon produced enhancement of EGF- or TRH-induced PRL promoter activity, suggesting that different processes regulate PRL transcription and hormone secretion. Expression of a kinase inactive nPKCdelta construct produced modest inhibition of EGF-mediated rPRL promoter activity. Taken together, these data provide evidence for a role of multiple PKC isozymes in mediating both EGF and TRH stimulated PRL gene transcription. Both EGF and TRH responses appear to require the novel isozyme, nPKCdelta, whereas nPKCeta may also be able to transmit the EGF response. Inhibitor data suggest that the EGF response may also involve Ca(2+)-dependent isozymes, whereas the TRH response appears to be more dependent on diacylglycerol.

Requirement of phosphatidylinositol 3-kinase activation and calcium influx for leukotriene B4-induced enzyme release

Leukotriene B(4) (LTB(4)) is a potent lipid mediator involved in host defense and inflammatory responses. It causes chemotaxis, generation of reactive oxygen species, and degranulation. However, only little is known of the molecular mechanisms by which LTB(4) induces these biological activities. To analyze the intracellular signaling pathways to mediate lysosomal enzyme release through the cloned LTB(4) receptor (BLT1), we transfected BLT1 to rat basophilic leukemia cells (RBL-2H3). LTB(4) dose-dependently released beta-hexosaminidase, and the release was mostly inhibited when the cells were pretreated with pertussis toxin, indicating that the degranulation is mediated by G(i) proteins. LTB(4) activated phosphatidylinositol 3-kinase (PI3-K) through G(i), and inhibition of PI3-K by wortmannin or LY290042 inhibited degranulation. Granulocytes from PI3-Kgamma-deficient mice showed reduced LTB(4)-induced degranulation, suggesting that this isozyme of PI3-K is involved in the degranulation. LTB(4) also caused calcium release from intracellular stores and calcium influx from the outside milieu through G(i), but only the calcium influx is critical for the lysosomal enzyme release. Calcium influx and PI3-K activation are both downstream events of G(i), since they were inhibited by pertussis toxin. These two events are in essence independent each other, because calcium depletion did not affect PI3-K, and inhibition of PI3-K did not attenuate calcium influx significantly. Thus, our results have clearly shown that LTB(4) binds BLT1 and activates G(i)-like protein, and both PI3-Kgamma activation and a sustained calcium elevation by calcium influx are necessary for enzyme release in these cells.

Real-time analysis of ligand-induced cell surface and intracellular reactions of living mast cells using a surface plasmon resonance-based biosensor

Surface plasmon resonance (SPR)-based sensors have been used to detect the binding between interactive molecules. We applied the SPR technology to the analysis of interactions between living cells and molecules reactive to the cells, using mast cells and mast cell-reactive antigens. The exposure of dinitrophenol-human serum albumin (DNP-HSA), an antigen that stimulates mast cells, to IgE-sensitized mast cells induced a robust and long-lasting SPR signal in a dose-dependent manner. The maximal increase in SPR signal induced by 100 ng/ml DNP-HSA was 0.200 +/- 0.120 angle (mean +/- SD, n = 37), about 1000 times larger than the theoretically expected increase for the simple binding of DNP-HSA to Fc(epsilon)RI, the high-affinity IgE receptor. A small, but similarly prolonged signal was observed when the cells were stimulated by an agonist of the adenosine A3 receptor. The signal induced by DNP-HSA was abolished by genistein, and partially inhibited by phorbol 12-myristate 13-acetate and wortmannin. Interestingly, the signal induced by DNP-HSA was only weakly inhibited by DNP-lysine, suggesting that DNP-lysine manifests its action not by inhibiting, but by modulating the crosslinking of Fc(epsilon)RI. We concluded that SPR sensors can detect biologically significant signals in a real-time manner from the interactions between cells and molecules reactive to the cells.

Protein phosphatase 2A and protein kinase Calpha are physically associated and are involved in Pseudomonas aeruginosa-induced interleukin 6 production by mast cells

Pulmonary infection with Pseudomonas aeruginosa is characterized by massive airway inflammation, which comprises significant cytokine production. Although mast cells are abundant in the lung and are potent sources of various cytokines, a role of mast cells in P. aeruginosa infection remains undefined, and P. aeruginosa-induced signaling mechanisms in mast cells have not been studied previously. Here we demonstrate that human cord blood-derived mast cells, mouse bone marrow-derived mast cells, and the mouse mast cell line MC/9 produce significant amounts of interleukin 6 (IL-6) in response to P. aeruginosa. This response was accompanied by a stimulation of protein kinase Calpha (PKCalpha) phosphorylation and PKC activity and was significantly blocked by the PKC inhibitors Ro 31-8220 and PKCalpha pseudosubstrate. Interestingly, mast cells treated with P. aeruginosa had reduced protein levels of phosphatase 2A catalytic unit (PP2Ac), which prompted us to determine whether a direct association between PKCalpha and PP2A occurs in mast cells. In mouse bone marrow-derived mast cells and MC/9 cells, as well as in the human mast cell line HMC-1, PP2A coimmunoprecipitated with PKCalpha either using PKCalpha- or PP2Ac-specific antibodies, suggesting that PKCalpha and PP2Ac are physically associated in mast cells. The PP2A inhibitor okadaic acid induced P. aeruginosa-like responses in mast cells including increased PKCalpha phosphorylation, stimulated PKC activity, and augmented IL-6 production, the last being blocked by the PKC inhibitor Ro 31-8220. Finally, okadaic acid potentiated the P. aeruginosa-induced IL-6 production. Collectively, these data provide, to our knowledge, the first evidence of both a direct physical association of PP2A and PKCalpha in mammalian cells and their coinvolvement in regulating mast cell activation in response to P. aeruginosa.

Interaction of chelerythrine with inositol phosphate metabolism

Chelerythrine, a potent inhibitor of protein kinase C (PKC), was evaluated for its effect on inositol phosphate (IP) metabolism in newborn rat cardiomyocytes in culture. In a first step, we evaluated the effect of chelerythrine on IP accumulation in basal conditions. For a 10(-4) M dose, 5-phosphatase activity (which dephosphorylates IP3 into IP2) was completely blocked and we observed a large increase in IP accumulation limited to IP2 without any increase in IP3, strongly suggesting that chelerythrine at this dose modifies IP metabolism. At a lower dose (10(-5) M) of chelerythrine, which did not modify IP accumulation and 5-phosphatase activity in basal conditions, the response to angiotensin II stimulation was completely abolished by the addition of chelerythrine. We conclude thus that chelerythrine, even at 10(-5) M, interacts markedly with IP metabolism, and caution should be exerted when interpreting the results obtained with this drug, which is still currently used at this dose.