Regulation of eosinophil function by phosphatidylinositol-specific PLC and cytosolic PLA(2)

P2X7 receptor-dependent increase in endocannabinoid 2-arachidonoyl glycerol production by neuronal cells in culture: Dynamics and mechanism

BACKGROUND AND PURPOSE

Neurotransmission and neuroinflammation are controlled by local increases in both extracellular ATP and the endocannabinoid 2-arachidonoyl glycerol (2-AG). While it is known that extracellular ATP stimulates 2-AG production in cells in culture, the dynamics and molecular mechanisms that underlie this response remain poorly understood. Detection of real-time changes in eCB levels with the genetically encoded sensor, GRABeCB2.0, can address this shortfall.

EXPERIMENTAL APPROACH

2-AG and arachidonoylethanolamide (AEA) levels in Neuro2a (N2a) cells were measured by LC-MS, and GRABeCB2.0 fluorescence changes were detected using live-cell confocal microscopy and a 96-well fluorescence plate reader.

KEY RESULTS

2-AG and AEA increased GRABeCB2.0 fluorescence in N2a cells with EC50 values of 81 and 58 nM, respectively; both responses were reduced by the cannabinoid receptor type 1 (CB1R) antagonist SR141617 and absent in cells expressing the mutant-GRABeCB2.0. ATP increased only 2-AG levels in N2a cells, as measured by LC-MS, and induced a transient increase in the GRABeCB2.0 signal within minutes primarily via activation of P2X7 receptors (P2X7R). This response was dependent on diacylglycerol lipase β activity, partially dependent on extracellular calcium and phospholipase C activity, but not controlled by the 2-AG hydrolysing enzyme, α/β-hydrolase domain containing 6 (ABHD6).

CONCLUSIONS AND IMPLICATIONS

Considering that P2X7R activation increases 2-AG levels within minutes, our results show how these molecular components are mechanistically linked. The specific molecular components in these signalling systems represent potential therapeutic targets for the treatment of neurological diseases, such as chronic pain, that involve dysregulated neurotransmission and neuroinflammation.

Emerging Role of Phospholipase-Derived Cleavage Products in Regulating Eosinophil Activity: Focus on Lysophospholipids, Polyunsaturated Fatty Acids and Eicosanoids

Eosinophils are important effector cells involved in allergic inflammation. When stimulated, eosinophils release a variety of mediators initiating, propagating, and maintaining local inflammation. Both, the activity and concentration of secreted and cytosolic phospholipases (PLAs) are increased in allergic inflammation, promoting the cleavage of phospholipids and thus the production of reactive lipid mediators. Eosinophils express high levels of secreted phospholipase A2 compared to other leukocytes, indicating their direct involvement in the production of lipid mediators during allergic inflammation. On the other side, eosinophils have also been recognized as crucial mediators with regulatory and homeostatic roles in local immunity and repair. Thus, targeting the complex network of lipid mediators offer a unique opportunity to target the over-activation and 'pro-inflammatory' phenotype of eosinophils without compromising the survival and functions of tissue-resident and homeostatic eosinophils. Here we provide a comprehensive overview of the critical role of phospholipase-derived lipid mediators in modulating eosinophil activity in health and disease. We focus on lysophospholipids, polyunsaturated fatty acids, and eicosanoids with exciting new perspectives for future drug development.

Functional role of phosphatidylcholine-specific phospholipase C in regulating leukotriene synthesis and degranulation in human eosinophils

Phosphatidylinositol-specific phospholipase C (PI-PLC) and cytosolic phospholipase A₂ (cPLA₂) regulate both eosinophil degranulation and leukotriene (LT) synthesis via PI-PLC-mediated calcium influx and cPLA₂ activation. Phosphatidylcholine-specific phospholipase C (PC-PLC) likely plays a key role in cellular signaling, including the eosinophilic allergic inflammatory response. This study examined the role of PC-PLC in eosinophil LT synthesis and degranulation using tricyclodecan-9-yl-xanthogenate (D609), a PC-specific PLC inhibitor. D609 inhibited N-formyl-met-leu-phe + cytochalasin B (fMLP/B)-induced arachidonic acid (AA) release and leukotriene C₄ (LTC₄) secretion. However, at concentrations that blocked both AA release and LTC₄ secretion, D609 had no significant inhibitory effect on stimulated cPLA₂ activity. D609 also partially blocked fMLP/B-induced calcium influx, indicating that inhibition of AA release and LTC₄ secretion by D609 is due to inhibition of calcium-mediated cPLA₂ translocation to intracellular membranes, not inhibition of cPLA₂ activity. In addition, D609 inhibited fMLP/B-stimulated eosinophil peroxidase release, indicating that PC-PLC regulates fMLP/B-induced eosinophil degranulation by increasing the intracellular calcium concentration ([Ca²⁺]_i). Overall, our results showed that PC-PLC is critical for fMLP/B-stimulated eosinophil LT synthesis and degranulation. In addition, degranulation requires calcium influx, while PC-PLC regulates LTC₄ synthesis through calcium-mediated cPLA₂ activation.

Eosinophil cysteinyl leukotriene synthesis mediated by exogenous secreted phospholipase A2 group X

Secreted phospholipase A(2) group X (sPLA(2)-X) has recently been identified in the airways of patients with asthma and may participate in cysteinyl leukotriene (CysLT; C(4), D(4), and E(4)) synthesis. We examined CysLT synthesis and arachidonic acid (AA) and lysophospholipid release by eosinophils mediated by recombinant human sPLA(2)-X. We found that recombinant sPLA(2)-X caused marked AA release and a rapid onset of CysLT synthesis in human eosinophils that was blocked by a selective sPLA(2)-X inhibitor. Exogenous sPLA(2)-X released lysophospholipid species that arise from phospholipids enriched in AA in eosinophils, including phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine as well as plasmenyl phosphatidylcholine and phosphatidylethanolamine. CysLT synthesis mediated by sPLA(2)-X but not AA release could be suppressed by inhibition of cPLA(2)α. Exogenous sPLA(2)-X initiated Ser(505) phosphorylation of cPLA(2)α, an intracellular Ca(2+) flux, and translocation of cPLA(2)α and 5-lipoxygenase in eosinophils. Synthesis of CysLTs in response to sPLA(2)-X or lysophosphatidylcholine was inhibited by p38 or JNK inhibitors but not by a MEK 1/2 inhibitor. A further increase in CysLT synthesis was induced by the addition of sPLA(2)-X to eosinophils under conditions of N-formyl-methionyl-leucyl-phenylalanine-mediated cPLA(2)α activation. These results indicate that sPLA(2)-X participates in AA and lysophospholipid release, resulting in CysLT synthesis in eosinophils through a mechanism involving p38 and JNK MAPK, cPLA(2)α, and 5-lipoxygenase activation and resulting in the amplification of CysLT synthesis during cPLA(2)α activation. Transactivation of eosinophils by sPLA(2)-X may be an important mechanism leading to CysLT formation in the airways of patients with asthma.

The signaling mechanism of eosinophil activation

Eosinophils play an important role in certain aspects of asthma pathogenesis. This review focuses on the mechanism of activation of eosinophils by the growth factor interleukin-5 and the CC chemokine receptor-3. Interleukin-5 activates members of the Janus and Src family of kinases. The latter kinases are largely responsible for the generation of initial signaling events. CC chemokine receptor-3, in contrast, signals through heterotrimeric G-proteins. Subsequently, various signaling pathways are activated, which converge on four major pathways - the mitogen-activated protein kinase pathway, the phosphoinositide-3 kinase pathway, the calcium signaling pathway and the Janus-signal transducer and activator of transcription signaling pathway. The biologic consequences of many of these signaling pathways are also discussed.

Cytosolic group IVa phospholipase A2 mediates IL-8/CXCL8-induced transmigration of human polymorphonuclear leukocytes in vitro

Background

Cytosolic gIVaPLA₂ is a critical enzyme in the generation of arachidonate metabolites and in induction of β₂-integrin adhesion in granulocytes. We hypothesized that gIVaPLA₂ activation also is an essential downstream step for post adhesive migration of PMN in vitro.

Methods

Migration of PMNs caused by IL-8/CXCL8 was assessed using a transwell migration chamber. PMNs were pretreated with two structurally unrelated inhibitors of gIVaPLA₂, arachidonyl trifluoromethylketone (TFMK) or pyrrophenone, prior to IL-8/CXCL8 exposure. The fraction of migrated PMNs present in the lower chamber was measured as total myeloperoxidase content. GIVaPLA₂ enzyme activity was analyzed using [¹⁴C-PAPC] as specific substrate F-actin polymerization and cell structure were examined after rhodamine-phalloidin staining.

Results

IL-8/CXCL8-induced migration of PMNs was elicited in concentration- and time-dependent manner. Time-related phosphorylation and translocation of cytosolic gIVaPLA₂ to the nucleus was observed for PMNs stimulated with IL-8/CXCL8 in concentration sufficient to cause upstream phosphorylation of MAPKs (ERK-1/2 and p38) and Akt/PKB. Inhibition of gIVaPLA₂ corresponded to the magnitude of blockade of PMN migration. Neither AA nor LTB₄ secretion was elicited following IL-8/CXCL8 activation. In unstimulated PMNs, F-actin was located diffusely in the cytosol; however, a clear polarized morphology with F-actin-rich ruffles around the edges of the cell was observed after activation with IL-8/CXCL8. Inhibition of gIVaPLA₂ blocked change in cell shape and migration caused by IL-8/CXCL8 but did not cause F-actin polymerization or translocation of cytosolic F-actin to inner leaflet of the PMN membrane.

Conclusion

We demonstrate that IL-8/CXCL8 causes a) phosphorylation and translocation of cytosolic gIVaPLA₂ to the nucleus, b) change in cell shape, c) polymerization of F-actin, and d) chemoattractant/migration of PMN in vitro. Inhibition of gIVaPLA₂ blocks the deformability and subsequent migration of PMNs caused by IL-8/CXCL8. Our data suggest that activation of gIVaPLA₂ is an essential step in PMN migration in vitro.

Regulation of eosinophil adhesion by lysophosphatidylcholine via a non-store-operated Ca2+ channel

We examined the mechanism by which lysophosphatidylcholine (LPC) regulates beta2-integrin-mediated adhesion of eosiniophils. Eosinophils were isolated from blood of mildly atopic volunteers by negative immunomagnetic selection. beta2-integrin-dependent adhesion of eosinophils to plated bovine serum albumin (BSA) was measured by residual eosinophil peroxidase activity. LPC caused maximal adhesion of eosinophils to plated BSA at 4 microM. Lysophosphatidylinositol, which has a similar molecular shape, mimicked the effect of LPC on eosinophil adhesion, while neither lysophosphatidylserine nor lysophosphatidylethanolamine had any effect. Phosphatidylethanolamine, a lipid that has a molecular orientation that is the inverse of LPC, blocked eosinophil adhesion caused by LPC. Unlike platelet-activating factor, a G-protein-coupled receptor agonist, LPC did not cause Ca2+-store depletion, but caused increased Ca2+ influx upon addition of Ca2+ to extracellular medium. This influx was not inhibited by U73122, a phospholipase C inhibitor, demonstrating independence from the G protein-activated phospholipase C pathway. Ca2+ influx was inhibited by either preincubation of phosphotidylethanolamine or La3+, a broad spectrum blocker of cation channels. LPC induced up-regulation of the active conformation of CD11b, which was blocked by preincubation with phosphatidylethanolamine. These data suggest that LPC causes a non-store-operated Ca2+ influx into eosinophils, which subsequently activates CD11b/CD18 to promote eosinophil adhesion.

Activation of group IV cytosolic phospholipase A2 in human eosinophils by phosphoinositide 3-kinase through a mitogen-activated protein kinase-independent pathway

Activation of group IV cytosolic phospholipase A(2) (gIV-PLA(2)) is the essential first step in the synthesis of inflammatory eicosanoids and in integrin-mediated adhesion of leukocytes. Prior investigations have demonstrated that phosphorylation of gIV-PLA(2) results from activation of at least two isoforms of mitogen-activated protein kinase (MAPK). We investigated the potential role of phosphoinositide 3-kinase (PI3K) in the activation of gIV-PLA(2) and the hydrolysis of membrane phosphatidylcholine in fMLP-stimulated human blood eosinophils. Transduction into eosinophils of Deltap85, a dominant negative form of class IA PI3K adaptor subunit, fused to an HIV-TAT protein transduction domain (TAT-Deltap85) concentration dependently inhibited fMLP-stimulated phosphorylation of protein kinase B, a downstream target of PI3K. FMLP caused increased arachidonic acid (AA) release and secretion of leukotriene C(4) (LTC(4)). TAT-Deltap85 and LY294002, a PI3K inhibitor, blocked the phosphorylation of gIV-PLA(2) at Ser(505) caused by fMLP, thus inhibiting gIV-PLA(2) hydrolysis and production of AA and LTC(4) in eosinophils. FMLP also caused extracellular signal-related kinases 1 and 2 and p38 MAPK phosphorylation in eosinophils; however, neither phosphorylation of extracellular signal-related kinases 1 and 2 nor p38 was inhibited by TAT-Deltap85 or LY294002. Inhibition of 1) p70 S6 kinase by rapamycin, 2) protein kinase B by Akt inhibitor, or 3) protein kinase C by Ro-31-8220, the potential downstream targets of PI3K for activation of gIV-PLA(2), had no effect on AA release or LTC(4) secretion caused by fMLP. We find that PI3K is required for gIV-PLA(2) activation and hydrolytic production of AA in activated eosinophils. Our data suggest that this essential PI3K independently activates gIV-PLA(2) through a pathway that does not involve MAPK.

Eosinophils and cysteinyl leukotrienes

Eosinophils are the main source of the cysteinyl leukotrienes, LTC(4)/D(4)/E(4), which are lipid mediators that play major roles in the pathogenesis of asthma and other forms of allergic inflammation. Here, we review the mechanisms governing eosinophil LTC(4) synthesis, focusing on the distinct intracellular domains that regulate eicosanoid formation and function within eosinophils. Cysteinyl leukotrienes exert their actions by engaging specific receptors. As recently shown, eosinophils express CysLT1 and CysLT2, the only cloned receptors for cysteinyl leukotrienes. Therefore, here we also present some of the new findings regarding the paracrine/autocrine activation of these CysLT receptors on eosinophils, and discuss some data on novel intracrine effects of LTC(4) triggered by a putative third CysLT receptor expressed intracellularly within eosinophils.

Actin assembly is a crucial factor for superoxide anion generation from adherent human eosinophils

BACKGROUND

Cellular adhesion is crucial for eosinophil effector functions.

OBJECTIVE

We sought to elucidate the role of the actin cytoskeleton in cellular adhesion and superoxide anion generation by human eosinophils.

METHODS

Eosinophils were stimulated with platelet-activating factor (PAF) or complement component 5a on human serum albumin-coated plates with or without an actin-polymerization inhibitor, cytochalasin B (CB), or cytochalasin D (CD). Superoxide anion generation was measured on the basis of reduction of absorbance associated with cytochrome c.2 Eosinophil adhesion was assessed on the basis of eosinophil protein X content in adherent cells. Transient stimulus-induced increase of intracellular calcium and translocation of protein kinase C (PKC) betaII, PKC delta, PKC zeta, and p47 phagocyte oxidase (a component of nicotinamide adenine dinucleotide phosphate oxidase) were also investigated.

RESULTS

CB, CD, or antibodies against CD18 (the beta2 chain of integrin, alphaMbeta2) inhibited stimulus-induced eosinophil superoxide anion generation. Stimulus-induced eosinophil adhesion was unaltered by CB, whereas it was significantly suppressed by CD or anti-CD18 antibodies. Transient PAF-induced intracellular calcium increase was also unaffected by CB or CD, but stimulus-induced eosinophil shape changes and translocation of PKCs and p47 phagocyte oxidase to the cell membrane region were completely inhibited by CB. PAF-induced eosinophil degranulation was inhibited by CB, CD, or anti-CD18 antibodies, whereas complement component 5-induced degranulation was not suppressed by CB.

CONCLUSION

By itself, beta2 integrin-dependent cellular adhesion is not sufficient for promoting eosinophil effector function. Adequate actin assembly is required for eosinophil adhesion and also for full superoxide anion generation in eosinophils.

Induction of cyclooxygenase-2 in Ras-transformed human mammary epithelial cells undergoing apoptosis

COX-2 expression has been reported to be elevated in several forms of human cancer. The presence of oncogenic ras has been associated with constitutive induction of COX-2, which confers resistance to apoptosis. Contrary to the above notion, we have found that H-ras-transformed human breast epithelial (MCF10A-ras) cells treated with the anticancer drug ET-18-O-CH(3) exhibit an increased expression of COX-2, while they still undergo apoptosis. To determine whether the induction of COX-2 by ET-18-O-CH(3) could contribute to apoptosis in MCF10A-ras cells, the selective COX-2 inhibitor celecoxib (SC-58635) was used. Celecoxib treatment attenuated ET-18-O-CH(3)-induced cell death. Taken together, the above findings suggest that COX-2 up-regulation does not necessarily confer the resistance to apoptosis in ras-transformed cells, but rather may sensitize these cells to apoptotic death.

The cellular biology of eosinophil eicosanoid formation and function

Eosinophils are capable of generating eicosanoid derivatives of arachidonic acid by means of cyclooxygenase and the 5- and 15-lipoxygenase (LO) pathways. Moreover, eosinophils, because of their expression of leukotriene (LT) C(4) synthase, are a major source of 5-LO-derived cysteinyl LTs, which are potent paracrine mediators of bronchial obstruction and inflammation pertinent to asthma. The regulation of eicosanoid formation within eosinophils involves activation of key enzymes at specific intracellular sites. Calcium ionophore-elicited translocation of 5-LO to the membranes of the nuclear envelope is associated with LTC(4) formation. In addition, lipid bodies, the formation of which is initiated by specific receptor-mediated signaling pathways, are sites of cyclooxygenase- and LO-pathway eicosanoid formation. Newly formed LTC(4) can be immunolocalized at perinuclear membranes in ionophore-activated eosinophils and at lipid bodies in CCR3 agonist (eg, eotaxin) chemokine-stimulated eosinophils. The local generation of eicosanoids at distinct sites within eosinophils may be important for the roles of these eicosanoids, both as paracrine mediators pertinent to inflammation and as intracrine signal-transducing mediators that help regulate cellular responses of eosinophils.