Platelet-activating factor stimulates a rapid accumulation of inositol (1,4,5)trisphosphate in guinea pig eosinophils: relationship to calcium mobilization and degranulation

GPR114/ADGRG5 is activated by its tethered peptide agonist because it is a cleaved adhesion GPCR

Family B2 or adhesion G protein-coupled receptors (AGPCRs) are distinguished by variable extracellular regions that contain a modular protease, termed the GPCR autoproteolysis-inducing domain that self-cleaves the receptor into an N-terminal fragment (NTF) and a C-terminal fragment (CTF), or seven transmembrane domain (7TM). The NTF and CTF remain bound after cleavage through noncovalent interactions. NTF binding to a ligand(s) presented by nearby cells, or the extracellular matrix anchors the NTF, such that cell movement generates force to induce NTF/CTF dissociation and expose the AGPCR tethered peptide agonist. The released tethered agonist (TA) binds rapidly to the 7TM orthosteric site to activate signaling. The orphan AGPCR, GPR114 was reported to be uncleaved, yet paradoxically capable of activation by its TA. GPR114 has an identical cleavage site and TA to efficiently cleave GPR56. Here, we used immunoblotting and biochemical assays to demonstrate that GPR114 is a cleaved receptor, and the self-cleavage is required for GPR114 TA-activation of Gs and no other classes of G proteins. Mutagenesis studies defined features of the GPR114 and GPR56 GAINA subdomains that influenced self-cleavage efficiency. Thrombin treatment of protease-activated receptor 1 leader/AGPCR fusion proteins demonstrated that acute decryption of the GPR114/56 TAs activated signaling. GPR114 was found to be expressed in an eosinophilic-like cancer cell line (EoL-1 cells) and endogenous GPR114 was efficiently self-cleaved. Application of GPR114 TA peptidomimetics to EoL-1 cells stimulated cAMP production. Our findings may aid future delineation of GPR114 function in eosinophil cAMP signaling related to migration, chemotaxis, or degranulation.

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.

A parallel signal-transduction pathway for eotaxin- and interleukin-5-induced eosinophil shape change

Interleukin-5 (IL-5) and eotaxin are the most important cytokines/chemokines responsible for regulating eosinophil locomotion and are known to play a co-operative role in the selective recruitment of eosinophils to inflamed tissues. Following exposure to chemoattractants, eosinophils undergo a series of events, including reorganization of actin filaments and subsequent rapid shape changes, culminating in chemotaxis. In this study we examined the signalling pathways for eosinophil shape change regulated by eotaxin and IL-5, primarily using a gated autofluorescence/forward-scatter assay. Eotaxin and IL-5 were able to elicit shape change with peaks at 10 and 60 min, respectively, and IL-5 triggered the shape change more efficiently than eotaxin. The pharmacological inhibitors of mitogen-activated protein kinase (MAP kinase) and p38 blocked both eotaxin- and IL-5-induced eosinophil shape change in a dose-dependent manner. In addition, depletion of intracellular Ca2+ and inhibition of protein kinase A (PKA) strongly reduced eosinophil shape change. In contrast, even when used at high concentrations, protein tyrosine kinase (PTK) inhibitors caused only a slight reduction in the ability to change shape. However, treatment with protein kinase C (PKC) inhibitors, such as GF109203X and staurosporine, resulted in a striking inhibition of eosinophil shape change by IL-5, but not eotaxin. Data from the inhibition of activation and chemotaxis of the extracellular signal-regulated kinases (ERK1/2) by the PKC inhibitors were also consistent with findings from the experiments on shape change. Collectively, two eosinophil-selective cytokines/chemokines probably regulate eosinophil shape change via a largely overlapping signalling pathway, with involvement of PKC restricted to the IL-5 signal alone.

Inhibition by fenoterol of human eosinophil functions including beta2-adrenoceptor-independent actions

Agonists at beta2 adrenoceptors are used widely as bronchodilators in treating bronchial asthma. These agents also may have important anti-inflammatory effects on eosinophils in asthma. We examined whether widely prescribed beta2-adrenoceptor agonists differ in ability to suppress stimulus-induced eosinophil effector functions such as superoxide anion (O2-) generation and degranulation. To examine involvement of cellular adhesion in such responses, we also investigated effects of beta2 agonists on cellular adhesion and on CD11b expression by human eosinophils. O2- was measured using chemiluminescence. Eosinophil degranulation and adhesion were assessed by a radioimmunoassay for eosinophil protein X (EPX). CD11b expression was measured by flow cytometry. Fenoterol inhibited platelet-activating factor (PAF)-induced O2- generation by eosinophils significantly more than salbutamol or procaterol. Fenoterol partially inhibited PAF-induced degranulation by eosinophils similarly to salbutamol or procaterol. Fenoterol inhibited phorbol myristate acetate (PMA)-induced O2- generation and degranulation by eosinophils, while salbutamol or procaterol did not. Fenoterol inhibition of PMA-induced O2- generation was not reversed by ICI-118551, a selective beta2-adrenoceptor antagonist. Fenoterol, but not salbutamol or procaterol, significantly inhibited PAF-induced eosinophil adhesion. Fenoterol inhibited O2- generation and degranulation more effectively than salbutamol or procaterol; these effects may include a component involving cellular adhesion. Inhibition also might include a component not mediated via beta2 adrenoceptors.

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

We examined the regulatory role of cytosolic phospholipase A(2) (cPLA(2)) and phosphatidylinositol (PI)-specific phospholipase C (PLC) in the degranulation of human eosinophils and leukotriene (LT) C(4) synthesis. Activation with formyl-Met-Leu-Phe + cytochalasin B (fMLP/B) caused a time-dependent release of eosinophil peroxidase (EPO) and LTC(4), which was inhibited by pertussis toxin. By immunoblotting, eosinophil PLC-beta2 and -gamma2 isoforms were identified, and PLC activation was measured as a function of inositol 1,4,5-trisphosphate concentration. Stimulated release of EPO and intracellular Ca(2+) concentration was inhibited by ET-18-OCH(3), a PI-PLC inhibitor, whereas trifluoromethylketone (TFMK), a cPLA(2) blocker, had no inhibitory effect. Both TFMK and ET-18-OCH(3) attenuated stimulated arachidonate release and LTC(4) secretion, suggesting that activation of both PLC and cPLA(2) is essential for LTC(4) synthesis caused by fMLP/B. The structurally unrelated protein kinase C inhibitors bisindolylmaleimide, Ro-31-8220, and Go-6976 all blocked fMLP/B-induced EPO release but not LTC(4) secretion. 1,2-bis(2-Aminophenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethyl ester, an intracellular Ca(2+) chelator, suppressed both EPO release and LTC(4) secretion. We found that fMLP/B-induced LTC(4) secretion from human eosinophils is regulated by PI-PLC through calcium-mediated activation of cPLA(2). However, cPLA(2) does not regulate eosinophil degranulation.

Characterization of platelet-activating factor-induced cytosolic calcium mobilization in human eosinophils

BACKGROUND

Activated eosinophils play an important role in the pathogenesis of bronchial asthma and other allergic diseases, and platelet-activating factor (PAF) is a potent activator of eosinophils.

OBJECTIVE

To characterize the cytosolic Ca2+ ([Ca2+]i) mobilization in human eosinophils in response to PAF.

METHODS

[Ca2+]i responses to PAF were examined in human eosinophils using a microscopic fura-2 fluorescence-ratio imaging system.

RESULTS

PAF caused a significant and dose-dependent increase in (Ca2+)i, which consisted of an initial rapid rise followed by a sustained elevation. This PAF-induced (Ca2+)i rise was inhibited by WEB 2086, a specific PAF receptor antagonist. The addition of 5 mM EGTA or 1 mM Ni2+ to a nominally Ca2+-free solution did not appreciably reduce the initial rise but significantly inhibited the sustained rise. The application of a protein kinase C inhibitor, Ro31-8220, augmented the sustained increase by PAF. Thapsigargin, a microsomal Ca2+ ATPase inhibitor, induced no appreciable change in a nominally Ca2+-free solution but induced a marked increase in (Ca2+)i when changed to a Ca2+-containing solution.

CONCLUSIONS

The initial rapid rise and the following sustained rise in (Ca2+)i by PAF depends on Ca2+ release from the intracellular Ca2+ stores and Ca2+ influx, respectively, which are regulated by protein kinase C in human eosinophils. Furthermore, the so called Ca2+-capacitative entry is possibly involved in the Ca2+ influx from the extracellular solution in human eosinophils.

ADP, adrenaline and serotonin stimulate inositol 1,4,5-trisphosphate production in human platelets

Although adenosine diphosphate (ADP) is a well-known stimulus of platelet aggregation, it is not the generally accepted view that ADP stimulates phosphatidylinositolbisphosphate (PtdIns(4,5)P2) hydrolysis. Using a very sensitive competitive receptor binding assay for inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), we have detected Ins(1,4,5)P3 production at early ( < 10 s) time points after stimulation of human platelets by the weak agonists ADP, adrenaline and serotonin (5-hydroxytryptamine, 5-HT). When adrenaline or 5-HT was combined with ADP in the presence of aspirin, there was a significant potentiation of ADP-induced platelet aggregation, but there was no potentiation of Ins(1,4,5)P3 generation. Also, the increases in intracellular calcium (Ca2+) concentrations stimulated by ADP were not potentiated by adrenaline in the presence of aspirin. Therefore, the synergism between the purinergic and adrenergic pathways of platelet activation occurs downstream from PtdIns(4,5)P2 hydrolysis and intracellular Ca2+ mobilization, although prior to platelet aggregation.

Signal transduction and activation of the NADPH oxidase in eosinophils

Activation of eosinophil NADPH oxidase and the subsequent release of toxic oxygen radicals has been implicated in the mechanism of parasite killing and inflammation. At present, little is known of the signal transduction pathway that govern agonist-induced activation of the respiratory burst and is the subject of this review. In particular, we focus on the ability of leukotrine B4 to activate the NADPH oxidase in guinea-pig peritoneal eosinophils which can be obtained in sufficient number and purity for detailed biochemical experiments to be performed.

Ligation of the beta2 integrin triggers activation and degranulation of human eosinophils

Evidence suggests that cellular adhesion is critical for eosinophil effector functions. Here, we tested the hypothesis that an adhesion molecule, specifically beta2 integrin, participates in intracellular signaling events of eosinophils. Eosinophils stimulated with interleukin (IL)-5 and adherent to protein-coated tissue culture plates via beta2 integrin (CD18) showed tyrosine phosphorylation of a number of proteins. Among these proteins, tyrosine phosphorylation of the 105 kD and 115 kD proteins and the product of the c-cbl protooncogene, Cbl, was specifically inhibited using soluble anti-CD18 monoclonal antibody (mAb) to block eosinophil cell adhesion. Furthermore, phosphoinositide turnover of IL-5-stimulated adherent eosinophils was also inhibited by anti-CD18 mAb, suggesting that cellular adhesion plays important roles in eosinophil signal transduction. alphaM beta2 (Mac-1, CD11b/18) was one of the beta2 integrins involved in eosinophil adhesion to protein-coated plates. We found that direct ligation of eosinophil alphaM beta2 with anti-CD11b mAb coupled to polystyrene microbeads induced tyrosine phosphorylation of a 115 kD protein and Cbl. Furthermore, anti-CD11b mAb microbeads induced increases in both phosphoinositide hydrolysis and the eosinophil degranulation response. Control antibodies, such as mouse myeloma IgG1 and anti-HLA class I antigen mAb, did not induce these cellular responses. These results suggest that engagement of beta2 integrin either by cell adhesion or by anti-CD11b mAb triggers activation of an intracellular signaling cascade, including protein tyrosine phosphorylation and phosphoinositide turnover, and subsequent cellular degranulation in human eosinophils. Tyrosine phosphorylation of a 115 kD protein and Cbl may play important roles in adhesion-dependent cellular functions of eosinophils.

Leukotriene B4 Activates the NADPH Oxidase in Eosinophils by a Pertussis Toxin-Sensitive Mechanism That Is Largely Independent of Arachidonic Acid Mobilization

Experiments were designed to investigate whether leukotriene (LTB4) receptors can couple directly to phospholipase A2 (PLA2) in guinea pig eosinophils and the role of endogenous arachidonic acid (AA) in LTB4-induced activation of the NADPH oxidase. LTB4 (EC50 ∼ 16 nM) and AA (EC50 ∼ 6 μM) generated hydrogen peroxide (H2O2) in a concentration-dependent manner and at an equivalent maximum rate (5–6 nmol/min/106 cells). LTB4 stimulated PLA2 over a similar concentration range that activated the NADPH oxidase, although kinetic studies revealed that the release of [3H]AA (t1/2 ∼ 2 s) preceded H2O2 generation (t1/2 &gt; 30 s). Pretreatment of eosinophils with pertussis toxin abolished the increase in inositol(1,4,5)trisphosphate mass, [Ca2+]c, [3H]AA release, and H2O2 generation evoked by LTB4. Qualitatively identical results were obtained in eosinophils in which phospholipase C (PLC) was desensitized by 4β-phorbol 12,13-dibutyrate with the exception that [3H]AA release was largely unaffected. Additional studies performed with the protein kinase C inhibitor, Ro 31-8220, and under conditions in which Ca2+ mobilization was abolished, provided further evidence that LTB4 released [3H]AA independently of signal molecules derived from the hydrolysis of phosphatidylinositol(4,5)bisphosphate by PLC. Pretreatment of eosinophils with the PLA2 inhibitor, mepacrine, abolished LTB4-induced [3H]AA release at a concentration that inhibited H2O2 by only 36%. Collectively, the results of this study indicate that agonism of LTB4 receptors on guinea pig eosinophils mobilizes AA by a mechanism that does not involve the activation of PLC. In addition, although LTB4 effectively stimulated PLA2, a central role for AA in the activation of the NADPH oxidase was excluded.

Pertussis Toxin Shows Distinct Early Signalling Events in Platelet-Activating Factor–, Leukotriene B4–, and C5a-Induced Eosinophil Homotypic Aggregation In Vitro and Recruitment In Vivo

The present study was performed to investigate the early signalling events responsible for eosinophil activation in response to platelet-activating factor (PAF ), C5a, and leukotriene B4 (LTB4 ). We evaluated the effect of pertussis toxin (PTX) on eosinophil aggregation in vitro and cutaneous eosinophil recruitment in vivo. Further studies using the protein kinase inhibitors Ro 31-8220 and staurosporine were performed in vitro to assess in more detail the early signalling events induced by these three mediators. Our results show that C5a and LTB4 signal predominantly or exclusively through a PTX-sensitive G protein that is negatively modulated by protein kinase C, possibly at the level of phospholipase C-β. In contrast, PAF activates eosinophils independent of Gi by a mechanism that is abolished by Ro 31-8220, a selective protein kinase C inhibitor. In addition, these results show for the first time that a receptor-operated event on the eosinophil is essential for chemoattractant-induced eosinophil recruitment in vivo.

Platelet-activating factor-induced inositol 1,4,5-trisphosphate generation in undifferentiated and differentiated U937 cells: role of tyrosine kinase

We compared the effect of platelet-activating factor (PAF) on inositol 1,4,5-trisphosphate (IP3) content in undifferentiated and differentiated U937 cells. In both cell types, PAF induced a rapid transient and concentration-dependent elevation of IP3 content. The production of IP3 in response to PAF was greater in differentiated than in undifferentiated cells. The increases in IP3 produced by PAF in both types of cell were inhibited by the PAF receptor antagonist, WEB 2086, as well as by the tyrosine kinase inhibitor, genistein. PAF also caused increased tyrosine phosphorylation of a 32 kDa protein substrate in both undifferentiated and differentiated cells. The magnitude of the phosphorylation was, however, greater in the differentiated cells. Genistein reduced the PAF-induced tyrosine phosphorylation of the substrate in both cell preparations. The specific binding of [3H]PAF was also markedly enhanced in differentiated cells. This effect was attenuated by genistein. The results indicate that PAF induces the production of IP3 in U937 cells via tyrosine kinase-mediated mechanisms and this process is augmented in differentiated cells.

Effect of a 5-lipoxygenase inhibitor, ZM 230487, on cutaneous allergic inflammation in the guinea-pig

1. Leukotrienes have potent biological effects in vitro and in vivo and are found in tissue and in biological fluids in various pathological conditions including allergic diseases. Leukotriene B4 (LTB4) is a potent stimulus for eosinophil accumulation and activation and there is much interest in determining its importance in mediating the accumulation of eosinophils at sites of allergic inflammation in vivo. In this study, we investigated the effects of a potent 5-lipoxygenase inhibitor, ZM 230487, on the accumulation of eosinophils and on local oedema formation in cutaneous inflammation in the guinea-pig. 2. The i.d. injection of increasing concentrations of arachidonic acid (AA) led to a dose-dependent accumulation of 111In-eosinophils but oedema formation was only significant at the top dose of AA tested (3 x 10(-8) mol per site). Co-injection of ZM 230487 with AA inhibited 111In-eosinophil accumulation up to 99% but the small oedema response to AA was only partially inhibited. AA-induced oedema formation was only effectively inhibited when a combination of a PAF antagonist, an antihistamine and ZM 230487 was used. 3. Local administration of the cyclo-oxygenase inhibitor, ibuprofen, partially inhibited AA-induced oedema formation suggesting that vasodilator prostaglandins may be released following i.d. injection of AA. AA-induced 111In-eosinophil accumulation was also partially inhibited by ibuprofen. 4. PAF-induced 111In-eosinophil accumulation was partially suppressed by local administration of ZM 230487. In contrast, LTB4-induced 111In-eosinophil accumulation was enhanced by ZM 230487. These data suggest that locally-released leukotrienes may modulate mediator-induced eosinophil accumulation. ZM 230487 had no effect on PAF- or LTB4-induced oedema formation. 5. ZM230487 significantly inhibited the accumulation of 111 In-eosinophils, but did not affect local oedema formation, in a passive cutaneous anaphylaxis (PCA) reaction. However, the PAF antagonist WEB 2086 either alone or in combination with ZM 230487 had no effect on "'In-eosinophil accumulation or oedema formation in the PCA reaction.6. In conclusion, it appears that a product of 5-lipoxygenase, probably LTB4, is important for the accumulation of "'In-eosinophils, but not local oedema formation, in the PCA reaction in guinea-pigskin. These data support a major role for LTB4 in allergic inflammation in the guinea-pig and make this animal (and the PCA model) suitable for studying the effects of inhibitors of leukotriene synthesis or action in vivo.

Platelet activating factor stimulates a receptor-coupled membrane GTPase in guinea pig eosinophils

The platelet activating factor (PAF) receptor of the guinea pig peritoneal eosinophil was characterized by radioligand ([3H]WEB 2086) binding and measurement of PAF-stimulated membrane GTPase activity. Specific binding of [3H]WEB 2086 was rapid, reversible and saturable, with an equilibrium KD of 20.4 nM at 0 degrees C, and was displaced competitively by unlabelled PAF with a KI of 3.26 nM and a pseudo-Hill coefficient significantly less than unity (0.44). The affinity of PAF for these binding sites was reduced by the nonhydrolysable GTP analogue 5'-guanylylimidodiphosphate (GppNHp), suggesting the coupling of PAF receptors to intracellular effectors through a guanine nucleotide-binding protein (G protein). PAF stimulated a membrane-associated GTPase, indicating the formation of a G protein alpha subunit-GTP complex upon agonist occupation of the PAF receptor. The EC50 for PAF stimulation was 25.5 nM and the Hill coefficient was significantly less than unity (0.56), while the response to 1 microM PAF was antagonised by WEB 2086 with an IC50 of 128 nM and a slope factor not significantly different from unity (0.91), suggesting the coupling of multiple classes of PAF receptors to the G protein. The activation of GTPase by PAF was insensitive to inhibition by cholera toxin; basal GTPase activity was increased by pertussis toxin and no further stimulation was attainable with PAF.

Cytokines, platelet activating factor and eosinophils in asthma

Understanding of the pathogenesis of asthma has increased considerably during the past few years. These advances were possible through scientific progress in three areas which contribute to this complex and multifaceted disease: (a) the much clearer understanding of eosinophil function; (b) the defining of lipid mediators in tissue inflammation and bronchial obstruction; and (c) the growing knowledge about the biological action of a new class of protein hormones, collectively called cytokines. In line with this, evidence has accumulated of how these components may interact with each other in providing the basis of inflammatory processes in asthma. Hence it seems appropriate to review the potential implications of this new information for the pathogenesis and therapy of this disease.