Autocrine enhancement of leukotriene synthesis by endogenous leukotriene B4 and platelet-activating factor in human neutrophils

Self-extinguishing relay waves enable homeostatic control of human neutrophil swarming

Neutrophils collectively migrate to sites of injury and infection. How these swarms are coordinated to ensure the proper level of recruitment is unknown. Using an ex vivo model of infection, we show that human neutrophil swarming is organized by multiple pulsatile chemoattractant waves. These waves propagate through active relay in which stimulated neutrophils trigger their neighbors to release additional swarming cues. Unlike canonical active relays, we find these waves to be self-terminating, limiting the spatial range of cell recruitment. We identify an NADPH-oxidase-based negative feedback loop that is needed for this self-terminating behavior. We observe near-constant levels of neutrophil recruitment over a wide range of starting conditions, revealing surprising robustness in the swarming process. This homeostatic control is achieved by larger and more numerous swarming waves at lower cell densities. We link defective wave termination to a broken recruitment homeostat in the context of human chronic granulomatous disease.

Type 3 secretion system induced leukotriene B4 synthesis by leukocytes is actively inhibited by Yersinia pestis to evade early immune recognition

Subverting the host immune response to inhibit inflammation is a key virulence strategy of Yersinia pestis. The inflammatory cascade is tightly controlled via the sequential action of lipid and protein mediators of inflammation. Because delayed inflammation is essential for Y. pestis to cause lethal infection, defining the Y. pestis mechanisms to manipulate the inflammatory cascade is necessary to understand this pathogen's virulence. While previous studies have established that Y. pestis actively inhibits the expression of host proteins that mediate inflammation, there is currently a gap in our understanding of the inflammatory lipid mediator response during plague. Here we used the murine model to define the kinetics of the synthesis of leukotriene B4 (LTB4), a pro-inflammatory lipid chemoattractant and immune cell activator, within the lungs during pneumonic plague. Furthermore, we demonstrated that exogenous administration of LTB4 prior to infection limited bacterial proliferation, suggesting that the absence of LTB4 synthesis during plague contributes to Y. pestis immune evasion. Using primary leukocytes from mice and humans further revealed that Y. pestis actively inhibits the synthesis of LTB4. Finally, using Y. pestis mutants in the Ysc type 3 secretion system (T3SS) and Yersinia outer protein (Yop) effectors, we demonstrate that leukocytes recognize the T3SS to initiate the rapid synthesis of LTB4. However, several Yop effectors secreted through the T3SS effectively inhibit this host response. Together, these data demonstrate that Y. pestis actively inhibits the synthesis of the inflammatory lipid LTB4 contributing to the delay in the inflammatory cascade required for rapid recruitment of leukocytes to sites of infection.

Self-extinguishing relay waves enable homeostatic control of human neutrophil swarming

Neutrophils exhibit self-amplified swarming to sites of injury and infection. How swarming is controlled to ensure the proper level of neutrophil recruitment is unknown. Using an ex vivo model of infection, we find that human neutrophils use active relay to generate multiple pulsatile waves of swarming signals. Unlike classic active relay systems such as action potentials, neutrophil swarming relay waves are self-extinguishing, limiting the spatial range of cell recruitment. We identify an NADPH-oxidase-based negative feedback loop that is needed for this self-extinguishing behavior. Through this circuit, neutrophils adjust the number and size of swarming waves for homeostatic levels of cell recruitment over a wide range of initial cell densities. We link a broken homeostat to neutrophil over-recruitment in the context of human chronic granulomatous disease.

5-lipoxygenase-dependent biosynthesis of novel 20:4 n-3 metabolites with anti-inflammatory activity

5-lipoxygenase (5-LO) catalyzes the conversion of arachidonic acid (AA) into pro-inflammatory leukotrienes. N-3 PUFA like eicosapentaenoic acid are subject to a similar metabolism and are precursors of pro-resolving mediators. Stearidonic acid (18:4 n-3, SDA) is a plant source of n-3 PUFA that is elongated to 20:4 n-3, an analogue of AA. However, no 5-LO metabolites of 20:4 n-3 have been reported. In this study, control and 5-LO-expressing HEK293 cells were stimulated in the presence of 20:4 n-3. Metabolites were characterized by LC-MS/MS and their anti-inflammatory properties assessed using AA-induced autocrine neutrophil stimulation and leukotriene B₄-mediated chemotaxis. 8‑hydroxy‑9,11,14,17-eicosatetraenoic acid (Δ¹⁷-8-HETE) and 8,15-dihydroxy-9,11,13,17-eicosatetraenoic acid (Δ¹⁷-8,15-diHETE) were identified as novel metabolites. Δ¹⁷-8,15-diHETE production was inhibited by the leukotriene A₄ hydrolase inhibitor SC 57461A. Autocrine neutrophil leukotriene stimulation and neutrophil chemotaxis, both BLT1-dependent processes, were inhibited by Δ¹⁷-8,15-diHETE at low nM concentrations. These data support an anti-inflammatory role for Δ¹⁷-8,15-diHETE, a novel 5-LO product.

Impact of myeloperoxidase-derived oxidants on the product profile of human 5-lipoxygenase

Human 5-lipoxygenase (5-LOX) oxidizes arachidonic acid to 5S-hydroperoxy-6 E,8 Z,11 Z,14 Z-eicosatetraenoic acid (5-HpETE) and leukotriene (LT) A4. In neutrophils, LTA4 is further converted to the potent chemoattractant LTB4. These cells also contain the heme enzyme myeloperoxidase (MPO), which produces several potent oxidants such as hypochlorous acid (HOCl), which are involved in pathogen defense and immune regulation. Here, we addressed the question whether MPO-derived oxidants are able to affect the activity of 5-LOX and the product profile of this enzyme. Human 5-LOX was incubated with increasing amounts of HOCl or HOBr. Afterward, arachidonic acid metabolites of 5-LOX were analyzed by reverse-phase high-performance liquid chromatography as well as by liquid chromatography-electrospray ionization-tandem mass spectrometry. The incubation of 5-LOX with the MPO-derived oxidants significantly changed the product profile of 5-LOX. Thereby, HOCl and HOBr increased the ratio of 5-H(p)ETE to 6-trans-LTB4 in a concentration-dependent manner. At low oxidant concentrations, there was a strong decrease in the yield of 6-trans-LTB4, whereas 5-HpETE did not change or increased. Additionally, the formation of 8-HpETE and 12-HpETE by 5-LOX rose slightly with increasing HOCl and HOBr. Comparable results were obtained with the MPO-H2O2-Cl(-) system when glucose oxidase and glucose were applied as a source of H2O2. This was necessary because of a strong impairment of 5-LOX activity by H2O2. In summary, MPO-derived oxidants showed a considerable impact on 5-LOX, impairing the epoxidation of 5-HpETE, whereas the hydroperoxidation of arachidonic acid was unaffected. Apparently, this was caused by an oxidative modification of critical amino acid residues of 5-LOX. Further work is necessary to assess the specific type and position of oxidation in the substrate-binding cavity of 5-LOX and to specify whether this interaction between 5-LOX and MPO-derived oxidants also takes place in stimulated neutrophils.

Impairment of neutrophil migration to remote inflammatory site during lung histoplasmosis

Histoplasma capsulatum (Hc) induces a pulmonary disease in which leukotrienes promote activation and recruitment of effectors cells. It is also well-recognized that leukotriene B₄ (LTB₄) and platelet-activating factor (PAF) induce leukocyte recruitment to inflammatory sites. We investigated the impact of pulmonary Hc infection on PMN migration to a remote inflammatory site. Our results show that pulmonary Hc infection impairs LTB₄- or PAF-stimulated PMN recruitment to air pouch. Yet, remote inflammation did not modify PMN numbers in the bronchoalveolar lavage fluid (BALF) of Hc-infected mice. Interestingly, the concomitant administration of PAF and LTB₄ receptor antagonists inhibited PMN recruitment to both BALF and the remote site, demonstrating cooperation between both mediators. Along that line, our results show that PAF-elicited PMN chemotaxis was abrogated in 5-lipoxygenase-deficient animals. These results suggest caution in the indiscriminate use of anti-inflammatory drugs during infectious diseases.

Cooperative role of endogenous leucotrienes and platelet-activating factor in ischaemia-reperfusion-mediated tissue injury

Insufficient oxygen delivery to organs leads to tissue dysfunction and cell death. Reperfusion, although vital to organ survival, initiates an inflammatory response that may both aggravate local tissue injury and elicit remote organ damage. Polymorphonuclear neutrophil (PMN) trafficking to remote organs following ischaemia/reperfusion (I/R) is associated with the release of lipid mediators, including leucotriene (LT) B₄, cysteinyl-LTs (CysLTs) and platelet-activating factor (PAF). Yet, their potentially cooperative role in regulating I/R-mediated inflammation has not been thoroughly assessed. The present study aimed to determine the cooperative role of lipid mediators in regulating PMN migration, tissue oedema and injury using selective receptor antagonists in selected models of I/R and dermal inflammation. Our results show that rabbits, pre-treated orally with BIIL 284 and/or WEB 2086 and MK-0571, were protected from remote tissue injury following I/R or dermal inflammation in an additive or synergistic manner when the animals were pre-treated with two drugs concomitantly. The functional selectivity of the antagonists towards their respective agonists was assessed in vitro, showing that neither BIIL 284 nor WEB 2086 prevented the inflammatory response to IL-8, C5a and zymosan-activated plasma stimulation. However, these agonists elicited LTB₄ biosynthesis in isolated rabbit PMNs. Similarly, a cardioprotective effect of PAF and LTB₄ receptor antagonists was shown following myocardial I/R in mice. Taken together, these results underscore the intricate involvement of LTB₄ and PAF in each other’s responses and provide further evidence that targeting both LTs and PAF receptors provides a much stronger anti-inflammatory effect, regulating PMN migration and oedema formation.

Group X secretory PLA2 in neutrophils plays a pathogenic role in abdominal aortic aneurysms in mice

Group X secretory PLA(2) (sPLA(2)-X) is expressed in neutrophils and plays a role in the pathogenesis of neutrophil-mediated tissue inflammation and injury. This study tested the hypothesis that sPLA(2)-X in neutrophils may contribute to the pathogenesis of abdominal aortic aneurysms (AAA) using sPLA(2)-X(-/-) mice. AAA was created by application of CaCl(2) to external surface of aorta. As a result, the aortas of sPLA(2)-X(-/-) mice had smaller diameters (percent increase from baseline; 24.8 ± 3.5% vs. 49.9 ± 9.1%, respectively; P < 0.01), a reduced grade of elastin degradation, and lower activities of elastase and gelatinase (26% and 19% lower, respectively) after CaCl(2) treatment compared with sPLA(2)-X(+/+) mice. In sPLA(2)-X(+/+) mice, immunofluorescence microscopic images showed that the immunoreactivity of sPLA(2)-X was detected only in neutrophils within aortic walls 3 days, 1, 2, and 6 wk after CaCl(2) treatment, whereas the immunoreactivity was not detected in macrophages or mast cells in aortic walls. sPLA(2)-X immunoreactivity also was colocalized in cells expressing matrix metalloproteinase (MMP)-9. Neutrophils isolated from sPLA(2)-X(-/-) mice had lower activities of elastase, gelatinase, and MMP-9 in response to stimuli compared with sPLA(2)-X(+/+) mice. The attenuated release of elastase and gelatinase from sPLA(2)-X(-/-) neutrophils was reversed by exogenous addition of mouse sPLA(2)-X protein. The adoptive transfer of sPLA(2)-X(+/+) neutrophils days 0 and 3 after CaCl(2) treatment reversed aortic diameters and elastin degradation grades in the lethally irradiated sPLA(2)-X(+/+) mice reconstituted with sPLA(2)-X(-/-) bone marrow to an extent similar to that seen in sPLA(2)-X(+/+) mice. In conclusion, sPLA(2)-X in neutrophils plays a pathogenic role in AAA in a mice model.

Protein kinase C promotes restoration of calcium homeostasis to platelet activating factor-stimulated human neutrophils by inhibition of phospholipase C

BACKGROUND

The role of protein kinase C (PKC) in regulating the activity of phospholipase C (PLC) in neutrophils activated with the chemoattractant, platelet-activating factor (PAF, 20 and 200 nM), was probed in the current study using the selective PKC inhibitors, GF10903X (0.5 - 1 muM) and staurosporine (400 nM).

METHODS

Alterations in cytosolic Ca2+, Ca2+ influx, inositol triphosphate (IP3), and leukotriene B4 production were measured using spectrofluorimetric, radiometric and competitive binding radioreceptor and immunoassay procedures, respectively.

RESULTS

Activation of the cells with PAF was accompanied by an abrupt increase in cytosolic Ca2+ followed by a gradual decline towards basal levels. Pretreatment of neutrophils with the PKC inhibitors significantly increased IP3 production with associated enhanced Ca2+ release from storage vesicles, prolongation of the peak cytosolic Ca2+ transients, delayed clearance and exaggerated reuptake of the cation, and markedly increased synthesis of LTB4. The alterations in Ca2+ fluxes observed with the PKC inhibitors were significantly attenuated by U73122, a PLC inhibitor, as well as by cyclic AMP-mediated upregulation of the Ca2+-resequestering endomembrane ATPase.Taken together, these observations are compatible with a mechanism whereby PKC negatively modulates the activity of PLC, with consequent suppression of IP3 production and down-regulation of Ca2+ mediated pro-inflammatory responses of PAF-activated neutrophils.

CONCLUSION

Although generally considered to initiate and/or amplify intracellular signalling cascades which activate and sustain the pro-inflammatory activities of neutrophils and other cell types, the findings of the current study have identified a potentially important physiological, anti-inflammatory function for PKC, at least in neutrophils.

Toll-like receptor ligands induce polymorphonuclear leukocyte migration: key roles for leukotriene B4 and platelet-activating factor

Activation of toll-like receptors (TLRs) and polymorphonuclear leukocyte (PMN) accumulation at infection sites are critical events of host defense. The involvement of leukotriene (LT) B(4) and platelet-activating factor (PAF) in TLR ligand-induced activation of inflammatory cell functions is essentially unknown. Using an in vitro model of human PMN migration through human endothelial cell monolayers, we demonstrate that prototypic ligands of TLR1/2, 2/6, 3, 4, 5, and 7/8 promote PMN migration, an effect markedly inhibited by 3 LTB(4) receptor antagonists (70-80% inhibition at 100 nM compared to vehicle-treated cells), 3 PAF receptor antagonists (20-50% inhibition at 10 nM), 3 LT biosynthesis inhibitors (75-85% inhibition at 100 nM), and 1 cytosolic phospholipase A(2)alpha (cPLA(2)alpha) inhibitor (90% inhibition at 1 microM). Accordingly, selected TLR ligands caused Ser-505-phosphorylation of cPLA(2)alpha and measurable LTB(4) and PAF biosynthesis in the transmigration assay. As negative controls, interleukin-8- and formyl-methionyl-leucyl-phenylalanine-elicited migration in vitro was not inhibited either by an LTB(4) receptor antagonist or by the cPLA(2)alpha inhibitor. Finally, LTB(4) and PAF receptor antagonists inhibited (up to approximately 65% at optimal doses) TLR ligand-induced PMN infiltration in the mouse air-pouch model. These studies unravel the critical involvement of de novo LTB(4) and PAF biosynthesis in PMN migration elicited by TLR ligands.

Pharmacological control of neutrophil-mediated inflammation: strategies targeting calcium handling by activated polymorphonuclear leukocytes

Unlike most other effector cells of the innate, as well as the adaptive immune systems, the neutrophil is a relatively undiscerning aggressor with scant regard for damage limitation. Although this highly combative, professional phagocyte has become increasingly implicated in the immunopathogenesis of many acute and chronic inflammatory disorders, of both infective and noninfective origin, effective pharmacological strategies to counter neutrophil aggression have remained elusive. Activation of neutrophils results in rapid mobilization of both stored and extracellular Ca(2+), resulting in abrupt, usually transient increases in cytosolic Ca(2+), which precede, and are a prerequisite for activation of the Ca(2+)-dependent pro-inflammatory activities of these cells. Mobilization of Ca(2+) by, and restoration of Ca(2+) homeostasis to activated neutrophils are multistep processes which present a number of potential targets, some well recognized and others novel and unconventional, for the pharmacological control of neutrophil-mediated inflammation. Uncovering these targets represents the primary focus of this review.

Reduction in Myocardial Ischemia/Reperfusion Injury in Group X Secretory Phospholipase A 2 –Deficient Mice

Background— Group X secretory phospholipase A 2 (sPLA 2 -X) has the most potent hydrolyzing activity toward phosphatidylcholine and elicits a marked release of arachidonic acid among several types of sPLA 2 . sPLA 2 -X is expressed in neutrophils, but its pathogenic role remains unclear.

Methods and Results— We generated mice that lack sPLA 2 -X and studied their response to myocardial ischemia/reperfusion. The sPLA 2 -X −/− mice had a significant reduction in myocardial infarct size and a decrease in myocardial myeloperoxidase activity compared with sPLA 2 -X +/+ mice. Myocardial infarct size was also significantly reduced in lethally irradiated sPLA 2 -X +/+ mice reconstituted with sPLA 2 -X −/− bone marrow compared with sPLA 2 -X +/+ bone marrow. The extent of myocardial ischemia/reperfusion injury was comparable between sPLA 2 -X −/− and sPLA 2 -X +/+ mice in Langendorff experiments using isolated hearts and blood-free perfusion buffer, supporting a potential role of sPLA 2 -X in blood in myocardial ischemia/reperfusion injury. In the infarcted myocardium of sPLA 2 -X +/+ mice, sPLA 2 -X was released from neutrophils but not myocardial tissues and platelets and was undetectable in the peripheral serum. The sPLA 2 -X −/− mice had lower accumulation of neutrophils in ischemic myocardium, and the isolated sPLA 2 -X −/− neutrophils had lower release of arachidonic acid and attenuated cytotoxic activities including respiratory burst compared with sPLA 2 -X +/+ neutrophils. The attenuated functions of sPLA 2 -X −/− neutrophils were reversible by the exogenous addition of sPLA 2 -X protein. Furthermore, administration of a sPLA 2 inhibitor reduced myocardial infarct size and suppressed the cytotoxic activity of sPLA 2 -X +/+ neutrophils.

Conclusions— Myocardial ischemia/reperfusion injury was attenuated in sPLA 2 -X −/− mice partly through the suppression of neutrophil cytotoxic activities.

Involvement of endogenous leukotriene B4 and platelet-activating factor in polymorphonuclear leucocyte recruitment to dermal inflammatory sites in rats

A critical role for leukotriene B(4) (LTB(4)) and/or platelet-activating factor (PAF) in regulating polymorphonuclear cell (PMN) trafficking to inflammatory sites has been reported in a number of experimental inflammatory models. In vitro, newly synthesized LTB(4) and PAF were shown to act in an autocrine/paracrine or intracrine fashion to enhance intracellular arachidonic acid availability and leukotriene biosynthesis. This suggested potentially cooperative effects of these lipid mediators in regulating PMN extravasation. The present study aimed to elucidate whether endogenous LTB(4) and PAF may both act to regulate plasma extravasation and PMN trafficking to inflammatory sites in experimental inflammation. With this aim, we have used selective and potent PAF and LTB(4) receptor antagonist pretreatments in dermal and pulmonary inflammation models in rats. Our results show additive inhibitory effects of dual LTB(4) and PAF receptor blockade in either PAF- or LTB(4)-elicited cutaneous PMN accumulation compared to single-drug administration. Furthermore, the combined administration of the drugs inhibited the PMN accumulation induced by the chemically unrelated soluble agonists tumour necrosis factor-alpha and C5a. Finally, in a model of pulmonary inflammation induced by the intravenous injection of Sephadex beads, lung neutrophilia was reduced by 63% following the administration of LTB(4) and PAF antagonists, in contrast with the lack of effect of single drug administration. Our results strongly support a role of both endogenous LTB(4) and PAF in regulating PMN trafficking to inflammatory sites in various experimental conditions.

Inhibition of platelet-activating factor biosynthesis by adenosine and histamine in human neutrophils: involvement of cPLA2alpha and reversal by lyso-PAF

Leukotrienes (LT) and platelet-activating factor (PAF) are important lipid mediators of inflammation. We and others reported previously that autacoids such as adenosine, histamine, prostaglandin E2, and beta-adrenergic agents inhibit LT biosynthesis in activated human polymorphonuclear leukocytes (PMN). In this study, we demonstrate that CGS-21680 (a selective agonist of the adenosine A2A receptor) and histamine also potently inhibit PAF biosynthesis in agonist [formyl Met-Leu-Phe (fMLP)]- and thapsigargin-activated human PMN. The observed inhibitions of PAF biosynthesis were reversed effectively by exogenous 1-O-alkyl-lyso-sn-glyceryl-3-phosphocholine (lyso-PAF), suggesting that these effects of CGS-21680 and histamine implicate the blockade of cytosolic phospholipase A2alpha (cPLA2alpha) activity and lyso-PAF release and that the acetyl-coenzyme A/lyso-PAF acetyl transferase is not inhibited by the autacoids. Accordingly, the cPLA2alpha inhibitor pyrrophenone completely blocked PAF formation, and lyso-PAF similarly prevented this effect of pyrrophenone. The inhibitory effects of CGS-21680 and histamine on PAF biosynthesis were prevented by the protein kinase A inhibitor H-89, supporting roles for the Gs -coupled receptors A2A and H2, respectively, and cyclic adenosine monophosphate in the inhibitory mechanism. The fMLP-induced phosphorylations of p38 and extracellular signal-regulated kinase 1/2 were not altered significantly by the CGS-21680, indicating that inhibition of these kinases is not involved in the inhibitory effect of the adenosine A2A receptor ligand on LT and PAF biosynthesis. These data further emphasize the multiple and potent inhibitory effects of adenosine and histamine on leukocyte functions, in particular, on the biosynthesis of two classes of important lipid mediators and their putative regulatory roles in immune processes in health and diseases.

Non-specific effects of leukotriene synthesis inhibitors on HeLa cell physiology

We examined the effects of various leukotriene synthesis inhibitors on calcium signalling in HeLa cells, before and after transfection with BLT(1). All of the inhibitors studied were found to reduce increases in intracellular calcium concentration induced by BLT(1), but also by an ionophore or activation of various G-protein coupled receptors, regardless of BLT(1) expression. In order to explore the mechanism of these apparently general effects we examined HeLa cell expression of leukotriene receptors and biosynthetic enzymes and found that the genes for key leukotriene synthesis enzymes and all of the leukotriene receptors were not expressed. Leukotrienes are involved in the pathology of a variety of cancers, and for HeLa cells leukotrienes have been reported to be important for aspects of the carcinogenic phenotype. We find that leukotriene synthesis inhibitors have non-specific effects, so careful controls are necessary to avoid interpreting non-specific effects as evidence for leukotriene involvement.

Leukotriene modifiers as potential therapeutics for cardiovascular disease

Owing to their anti-inflammatory properties, leukotriene modifiers have been the primary therapeutics in asthma management for several years. Although blocking the inflammatory component of human disease is a long-standing and established concept, the use of leukotriene modifiers in treating the inflammatory component of cardiovascular disease encompassing atherosclerosis, myocardial infarction, stroke and aortic aneurysm has, surprisingly, only been seriously contemplated in the past few years. As reviewed here, several exciting studies have recently contributed to this expanding area of interest, and so far one leukotriene modifier has entered Phase II clinical trials to assess its potential for reducing the risk of heart attacks.

Distinct pathways of lipopolysaccharide priming of human neutrophil respiratory burst: role of lipid mediator synthesis and sensitivity to interleukin-10

OBJECTIVE

Exposure of neutrophils to low doses of bacterial lipopolysaccharides enhances their readiness to respond with inflammatory mediator generation including oxygen radical formation to a subsequently applied inflammatory stimulus ("priming"). In the present study, we investigated the role of lipid mediator synthesis and the impact of the anti-inflammatory cytokine interleukin-10 on the lipopolysaccharide-dependent priming of human neutrophils in response to N-formyl-methionyl-leucyl-phenylalanine.

DESIGN

Prospective, experimental study.

SETTING

Research laboratory at a university hospital.

SUBJECTS

Isolated neutrophils from healthy volunteers.

INTERVENTIONS

Incubation of isolated neutrophils with endotoxin.

MEASUREMENTS AND MAIN RESULTS

Evidence for two distinct priming mechanisms was obtained. The first was strictly serum component dependent, proceeded via CD14, and was not inhibited by even high concentrations of interleukin-10. The second priming mechanism was serum component independent but nevertheless proceeded via CD14. It was linked with neutrophil synthesis of the platelet activating factor and resulted in the appearance of leukotrienes, in particular leukotriene B4, as far as exogenous arachidonic acid was provided. The employment of a platelet-activating factor receptor antagonist (WEB 2086) blocked leukotriene synthesis, and both WEB 2086 and a 5-lipoxygenase inhibitor (MK-886) suppressed the respiratory burst linked with this second priming pathway. This sequence of priming events was inhibited by interleukin-10, when this cytokine was coadministered with the priming agent lipopolysaccharide, whereas late interleukin-10 admixture was ineffective.

CONCLUSIONS

We conclude that two mechanisms of lipopolysaccharide priming of human neutrophil respiratory burst can be differentiated. One displays serum component dependence, is independent of neutrophil lipid mediator generation, and is not affected by interleukin-10. The other is serum independent although being operated via CD14, employs autocrine loops of platelet-activating factor and leukotriene B4 synthesis, and is sensitive to the inhibitory capacity of interleukin-10. These features may be relevant when the goal is to pharmacologically modify neutrophil functions in septic events.

Group V phospholipase A2 induces leukotriene biosynthesis in human neutrophils through the activation of group IVA phospholipase A2

We reported previously that exogenously added human group V phospholipase A(2) (hVPLA(2)) could elicit leukotriene B(4) (LTB(4)) biosynthesis in human neutrophils (Han, S. K., Kim, K. P., Koduri, R., Bittova, L., Munoz, N. M., Leff, A. R., Wilton, D. C., Gelb, M. H., and Cho, W. (1999) J. Biol. Chem. 274, 11881-11888). To determine the mechanism of the hVPLA(2)-induced LTB(4) biosynthesis in neutrophils, we thoroughly examined the effects of hVPLA(2) and their lipid products on the activity of group IVA cytosolic PLA(2) (cPLA(2)) and LTB(4) biosynthesis under different conditions. As low as 1 nm exogenous hVPLA(2) was able to induce the release of arachidonic acid (AA) and LTB(4). Typically, AA and LTB(4) were released in two phases, which were synchronized with a rise in intracellular calcium concentration ([Ca(2+)](i)) near the perinuclear region and cPLA(2) phosphorylation. A cellular PLA(2) assay showed that hVPLA(2) acted primarily on the outer plasma membrane, liberating fatty acids and lysophosphatidylcholine (lyso-PC), whereas cPLA(2) acted on the perinuclear membrane. Lyso-PC and polyunsaturated fatty acids including AA activated cPLA(2) and 5-lipoxygenase by increasing [Ca(2+)](i) and inducing cPLA(2) phosphorylation, which then led to LTB(4) biosynthesis. The delayed phase was triggered by the binding of secreted LTB(4) to the cell surface LTB(4) receptor, which resulted in a rise in [Ca(2+)](i) and cPLA(2) phosphorylation through the activation of mitogen-activated protein kinase, extracellular signal-regulated kinase 1/2. These results indicate that a main role of exogenous hVPLA(2) in neutrophil activation and LTB(4) biosynthesis is to activate cPLA(2) and 5-lipoxygenase primarily by liberating from the outer plasma membrane lyso-PC that induces [Ca(2+)](i) increase and cPLA(2) phosphorylation and that hVPLA(2)-induced LTB(4) production is augmented by the positive feedback activation of cPLA(2) by LTB(4).

Leukotriene B(4) induces nitric oxide synthesis in Trypanosoma cruzi-infected murine macrophages and mediates resistance to infection

The production of nitric oxide (NO) by gamma interferon (IFN-gamma)-activated macrophages is a major effector mechanism during experimental Trypanosoma cruzi infection. In addition to IFN-gamma, chemoattractant molecules, such as platelet-activating factor (PAF) and CC chemokines, may also activate macrophages to induce NO and mediate the killing of T. cruzi in an NO-dependent manner. Here we investigated the ability of leukotriene B(4) (LTB(4)) to induce the production of NO by macrophages infected with T. cruzi in vitro and whether NO mediated LTB(4)-induced parasite killing. The activation of T. cruzi-infected but not naive murine peritoneal macrophages with LTB(4) induced the time- and concentration-dependent production of NO. In addition, low concentrations of LTB(4) acted in synergy with IFN-gamma to induce NO production. The NO produced mediated LTB(4)-induced microbicidal activity in macrophages, as demonstrated by the inhibitory effects of an inducible NO synthase inhibitor. LTB(4)-induced NO production and parasite killing were LTB(4) receptor dependent and were partially blocked by a PAF receptor antagonist. LTB(4) also induced significant tumor necrosis factor alpha (TNF-alpha) production, and blockade of TNF-alpha suppressed LTB(4)-induced NO release and parasite killing. A blockade of LTB(4) or PAF receptors partially inhibited IFN-gamma-induced NO and TNF-alpha production but not parasite killing. Finally, daily treatment of infected mice with CP-105,696 was accompanied by a significantly higher level of blood parasitemia, but not lethality, than that seen in vehicle-treated animals. In conclusion, our results suggest a role for LTB(4) during experimental T. cruzi infection. Chemoattractant molecules such as LTB(4) not only may play a major role in leukocyte migration into sites of inflammation in vivo but also, in the event of an infection, may play a relevant role in the activation of recruited leukocytes to kill the invading microorganism in an NO-dependent manner.

Decreased leukotriene release from neutrophils after severe trauma: role of immature cells

Polymorphonuclear granulocytes (PMN) play a key role in host defense against microbial infections. After severe trauma PMN show cellular dysfunctions including chemotactic migration, phagocytosis, and bacterial killing. In these settings the contribution of the cellular maturation stage compared to functional activities has not been investigated. Polymorphonuclear granulocytes are potent producers of lipid mediators via the 5-lipoxygenase (5-LO) pathway (leukotrienes, LTs) which exert important proinflammatory and immunoregulatory activities. We analyzed leukotriene generation from PMN-fractions (N = 23) of 15 polytrauma patients in comparison to 17 healthy donor cell fractions and correlated this lipid mediator release to the hematopoietic maturation stage of respective PMN. Polymorphonuclear granulocytes were isolated from EDTA-anticoagulated peripheral blood employing a one step procedure based on a discontinuous double Ficoll-gradient. Cells (5 x 10(6)/500 microl phosphate-buffered saline) were stimulated for 20 min at 37 degrees C with 1 microM Ca-ionophor A23187 in the presence of 1 mM Ca++ and 0.5 mM Mg++. Leukotrienes were analyzed by reversed-phase HPLC. Expression of 5-lipoxygenase (5-LO) was additionally determined by Western blot. Maturation stage of PMN was quantitated by Pappenheim-staining of cell smears. After polytrauma the generation of leukotrienes from PMN was individually diminished. Synthesis of enzymatically formed metabolites (LTB4, OH-LTB4 and COOH-LTB4) was concomitantly reduced. The decresaed leukotriene synthesis strongly correlated (r2 = 0.907, P < 0.0001) to the occurrence of immature PMN (mostly band cells). The expression of 5-lipoxygenase in PMN fractions consisting mainly of band cells was decreased. Our results provide evidence that posttraumatic granulocyte dysfunction is partly due to immature functional cell capacities.

Structure, function, and regulation of group V phospholipase A(2)

The hydrolysis of membrane phospholipid by phospholipase A(2) (PLA(2)) is a key step in the production of inflammatory eicosanoids. Recent cell studies have shown that secretory group V PLA(2) (gVPLA(2)) is involved in agonist-induced eicosanoid biosynthesis in mouse P388D1 cell line, mast cells, and transfected HEK 293 cells. gVPLA(2) is homologous to other group II PLA(2) family members but has distinctive enzymatic properties, including its activity to effectively hydrolyze phosphatidylcholine (PC) vesicles and the outer plasma membrane of mammalian cells. Mutational studies showed that gVPLA(2) has a unique structure that allows effective binding to PC membranes and efficient catalysis of an active-site-bound PC substrate. Thanks to this unique structure and activity, exogenously added gVPLA(2) can induce the eicosanoid biosynthesis in unstimulated inflammatory cells, including human neutrophils and eosinophils, suggesting that it might be able to trigger inflammatory responses under certain physiological conditions. Extensive structure-function and cell studies showed that gVPLA(2) could act directly on the outer plasma membranes of neutrophils and eosinophils. The release of fatty acids and lysophospholipids from the cell surfaces induces the translocation and activation of cytosolic PLA(2) and 5-lipoxygenase, resulting in the leukotriene synthesis. In case of neutrophils, induction of leukotriene B(4) synthesis by gVPLA(2) leads to the phosphorylation of cytosolic PLA(2) by a leukotriene B(4) receptor and MAP kinase-mediated mechanism. Finally, heparan sulfate proteoglycans in neutrophils appear to play a role of internalizing and degrading the cell surface-bound gVPLA(2) to protect the cells from extensive lipolytic damage.

Activation of leukotriene synthesis in human neutrophils by exogenous arachidonic acid: inhibition by adenosine A(2a) receptor agonists and crucial role of autocrine activation by leukotriene B(4)

We report here that the apparent inability of isolated human polymorphonuclear leukocytes (PMNs) to efficiently transform arachidonic acid (AA) is the consequence of A(2a) receptor engagement by endogenous adenosine accumulating in incubation media. Indeed, when adenosine is eliminated from PMN suspensions by the addition of adenosine deaminase, or when cells are incubated with adenosine A(2a) receptor antagonists, important quantities (40-80 pmol/10(6) cells) of 5-lipoxygenase products are synthesized by PMN incubated with 1 to 5 microM exogenous AA. The selective A(2a) receptor agonist CGS21680 was a very potent inhibitor of the AA-induced leukotriene (LT) synthesis, showing an IC(50) of approximately 1 nM. The mechanism of AA-induced stimulation of LT synthesis observed in the absence of extracellular adenosine was investigated. In adenosine deaminase-treated PMN, exogenous AA induced Ca(2+) mobilization and the translocation of 5-lipoxygenase to nuclear structures. A time lag of 20 to 60 s (variable between PMN preparations) was observed consistently between the addition of AA and the elevation of intracellular Ca(2+) concentration (and LT synthesis), indicating that AA itself did not trigger the Ca(2+) mobilization in PMN. This AA-induced Ca(2+) mobilization, as well as the corresponding 5-lipoxygenase translocation and stimulation of LT synthesis, was blocked efficiently by the LT synthesis inhibitor MK0591, the LTB(4) receptor antagonists CP105696 and LY223982, and the LTA(4) hydrolase inhibitor SC57461A. These data demonstrate that AA is a highly potent and effective activator of LT synthesis and acts through a mechanism that requires an autocrine stimulatory loop by LTB(4).

Role of arachidonic acid and its metabolites in the priming of NADPH oxidase in human polymorphonuclear leukocytes by peritoneal dialysis effluent

Peritoneal dialysis effluent (PDE) contains a low-molecular-weight solute that will activate and prime the NADPH oxidase of human neutrophils via a phospholipase A2 (PLA2)-dependent mechanism. Since the products of PLA2 are known to activate and prime the oxidase we have investigated their role in the dialysis effluent-mediated activation and priming of human neutrophils. NADPH oxidase activity of PDE-primed and -unprimed neutrophils was measured by lucigenin-enhanced chemiluminescence in the presence of known inhibitors of the arachidonic acid cascade. Incubation of neutrophils with the nonselective PLA2 inhibitor quinacrine (0 to 100 microM) reduced oxidase activity in both primed and unprimed cells. Furthermore, primed cells were more sensitive to the action of quinacrine than were unprimed cells. We were unable to determine the relative roles of secretory PLA2 (sPLA2) and cytosolic PLA2 (cPLA2) since the selective sPLA2 inhibitor scalaradial (0 to 100 microM) inhibited oxidase activity in both groups of cells by similar degrees, while the specific cPLA2 inhibitor AACO-CF3 (0 to 50 microM) failed to affect activity in either group. Inhibition of platelet-activating factor (PAF), cycloxygenase, and 5-lipoxygenase-activating protein by hexanolamino-PAF (0 to 25 microM), flurbiprofen (0 to 25 microM), and MK886 (0 to 5 microM), respectively, had no effect upon oxidase activity. However, the direct inhibition of 5-lipoxygenase by caffeic acid or lipoxin A4 resulted in a similar concentration-dependent attenuation of oxidase activity in both primed and unprimed cells. Leukotriene B4 (LTB4) release from primed neutrophils was comparable to that from unprimed cells with the exception of phorbol myristate acetate-stimulated cells, which released fivefold more LTB4 than control. Taken together, these results suggest that it is arachidonic acid per se, and not its metabolites, that is important in priming of the neutrophil NADPH oxidase by dialysis effluent.

Platelets, neutrophils, and vasoconstriction after arterial injury by angioplasty in pigs: effects of MK-886, a leukotriene biosynthesis inhibitor

1 Leukotrienes constitute a class of potent bioactive mediators known to play a pivotal role in inflammation. Since their biosynthesis has been shown to be enhanced by platelet-neutrophil interactions, leukotrienes may be involved in these interactions and the arterial response to injury. Therefore, we investigated the effects of the selective leukotriene biosynthesis inhibitor 3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2- dimethylpropanoic acid (MK-886) on the acute thrombotic and vasomotor responses after arterial injury by angioplasty. 2 Carotid arterial injury was produced by balloon dilatation in control (molecusol vehicle; n=10) and treated (MK-886, 10 mg kg(-1), i.v.; n=9) pigs. The acute thrombotic reaction following deep arterial wall injury was quantified with 51Cr labelled platelets and 111In labelled neutrophils, and the vasomotor response was determined angiographically. 3 Platelet deposition at the site of deep arterial wall injury averaged 56.4+/-11.0x10(6) platelets cm(-2) in the control group, and was significantly reduced to 18.2+/-3.8x10(6) platelets cm(-2) (P<0.005) by treatment with MK-886. Neutrophil deposition was also decreased by MK-886, from 242.8+/-36.8 to 120.9+/-20.3x10(3) neutrophils cm(-2) (P<0.01). MK-886-treated animals had a significant decrease in the postangioplasty vasoconstrictive response at the site of endothelial injury distally, from 37.5+/-3.1% in the control group to 13.5+/-2.5% (P<0.001). 4 The effects of MK-886 were associated with a profound inhibition of ex vivo leukotriene B4 (LTB4) synthesis in blood stimulated by the calcium ionophore A23187 and a significant reduction of neutrophil aggregation in whole blood (P<0.01), whereas neutrophil superoxide anion production, serum thromboxane B2 and platelet aggregation in whole blood were not influenced. 5 The relevant effects of MK-886 are primarily related to inhibition of neutrophil function and suggest an important modulatory role for leukotrienes in the pathophysiological response associated with platelet and neutrophil interactions following arterial injury in vivo.

Leukotriene B4 stimulates the release of arachidonate in human neutrophils via the action of cytosolic phospholipase A2

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation and is involved in the receptor-mediated activation of a number of leukocyte responses including degranulation, superoxide formation, and chemotaxis. In the present research, stimulation of unprimed polymorphonuclear leukocytes (neutrophils) with LTB4 results in the transient release of arachidonate as measured by mass. This release of arachidonate was maximal at an LTB4 concentration of 50-75 nM and peaked at 45 s after stimulation with LTB4. The transient nature of this release can be attributed, in part, to a fast (< 60 s) metabolism of the added LTB4. Moreover, the inhibition of the reacylation of the released arachidonate with thimerosal results in greater than 4-times as much arachidonate released. Thus, a rapid reacylation of the released arachidonate also contributes to the transient nature of its measured release. Multiple additions of LTB4, which would be expected to more closely resemble the situation in vivo where the cell may come into contact with an environment where LTB4 is in near constant supply, yielded a more sustained release of arachidonate. No release of [3H]arachidonate was observed when using [3H]arachidonate-labeled cells. This indicates that the release of arachidonate as measured by mass is most probably the result of hydrolysis of arachidonate-containing phosphatidylethanolamine within the cell since the radiolabeled arachidonate is almost exclusively incorporated into phosphatidylcholine and phosphatidylinositol pools under the non-equilibrium radiolabeling conditions used. Consistent with the role of cytosolic phospholipase A2 (cPLA2) in the release of arachidonate, potent inhibition of the LTB4-stimulated release was observed with methylarachidonylfluorophosphonate, an inhibitor of cPLA2 (IC50 of 1 microM). The bromoenol lactone of the calcium-independent phosphospholipase A2. failed to affect LTB4-stimulated release of arachidonate in these cells.

Leukotriene B4 costimulates 5-lipoxygenase activity in neutrophils via increased 5-lipoxygenase translocation

The goal of this investigation was to assess the effect of leukotriene B4 (LTB4) on 5-lipoxygenase activity and to examine the possible mechanisms of this effect. Exogenous LTB4 significantly increased the release of endogenous LTB4 from A-23187-stimulated neutrophils. The 5-lipoxygenase product release from A-23187-stimulated neutrophils decreased in the presence of an LTB4 receptor antagonist, suggesting that LTB4 has a receptor-mediated, autocrine effect on 5-lipoxygenase activity. Neutrophil 5-lipoxygenase activity increased significantly as cell density increased. In the presence of exogenous LTB4, no significant change in [14C]arachidonic acid release from neutrophils was observed. Exogenous LTB4 increased the amount of immunoreactive 5-lipoxygenase protein detected in the nuclear fraction of disrupted cells. LTB4 receptor antagonism decreased the amount of immunoreactive 5-lipoxygenase detected in the nuclear fraction. Thus LTB4 exerts an autocrine, receptor-mediated, costimulatory effect on 5-lipoxygenase activity. This feedback appears to have biological significance and involves enhanced 5-lipoxygenase translocation to the nuclear membrane.

Leukotriene B4 and platelet activating factor production in permeabilized human neutrophils: role of cytosolic PLA2 in LTB4 and PAF generation

The specific type of phospholipase A2 (PLA2) involved in formation of leukotriene B4 (LTB4) and platelet activating factor (PAF) in inflammatory cells has been controversial. In a recent report we characterized activation of the 'cytosolic' form of PLA2 (cPLA2) in human neutrophils (PMN) permeabilized with Staphylococcus aureus alpha-toxin under conditions where the secretory form of PLA2 (sPLA2) was inactive. In the current study, generation of both LTB4 and PAF in porated PMN are demonstrated. PMN, prelabeled with [3H]arachidonic acid (3H-AA, to assess AA release and LTB4 production) or with 1-O-[9',10'-3H]hexadecyl-2-lyso-glycero-3-phosphocholine (3H-lyso-PAF, for determination of lyso-PAF and PAF formation), were permeabilized with alpha-toxin in a 'cytoplasmic' buffer supplemented with acetyl CoA. Maximum production of both PAF and LTB4 required addition of 500 nM Ca2+, G-protein activation induced with 10 microM GTP gamma S, and stimulation with the chemotactic peptide, N-formyl-Met-Leu-Phe (FMLP, 1 microM); LTB4 production was confirmed by radioimmunoassay. Removal of acetyl CoA from the system had little effect on LTB4 generation but blocked PAF production with a concomitant increase in lyso-PAF formation LTB4 and PAF production occurred in parallel over time and at differing ATP and Ca2+ concentrations. Further work demonstrated that: (i) maximum production of both inflammatory mediators required a hydrolyzable form of ATP; (ii) blocking phosphorylation with staurosporin inhibited production of both; (iii) the reducing agent, dithiotreitol, had little affect on LTB4 formation but slightly enhanced PAF generation. This study clearly shows that cPLA2 activation can provide precursors for both LTB4 and PAF, that maximum PAF and LTB4 formation occur under conditions that induced optimal cPLA2 activation, that a close coupling between LTB4 and PAF formation exists, and that, after substrate generation, no additional requirements are necessary for LTB4 and PAF generation in the permeabilized PMN system.

Release of leukotriene A4 versus leukotriene B4 from human polymorphonuclear leukocytes

The reactive intermediate formed by 5-lipoxygenase metabolism of arachidonic acid, leukotriene A4, is known to be released from cells and subsequently taken up by other cells for biochemical processing. The objective of this study was to determine the relative amount of leukotriene A4 synthesized by human polymorphonuclear leukocytes (PMNL) that is available for transcellular biosynthetic processes. This was accomplished by diluting cell suspensions and measuring the relative amounts of enzymatic versus nonenzymatic leukotriene A4-derived metabolites after challenge with the Ca2+ ionophore A23187. Nonenzymatic leukotriene A4-derived metabolites were used as a quantitative index of the amount of leukotriene A4 released into the extracellular milieu. The results obtained demonstrated that in human PMNL, the relative amounts of nonenzymatic versus enzymatic leukotriene A4-derived metabolites increased with decreasing cell concentrations. After a 20-fold dilution of PMNL in cell preparations, a doubling in the amount of nonenzymatic leukotriene A4-derived metabolites was observed following challenge (from 53.9 +/- 1.3 to 110.4 +/- 8.9 pmol/10(6) PMNL, p < 0.01). Reduction of possible cell-cell interactions by dilution suggested that over 50% of leukotriene A4 synthesized is released from the PMNL. These data provide evidence that, in human PMNL preparations, transfer of leukotriene A4 to neighboring PMNL is taking place, resulting in additional formation of leukotriene B4 and its omega-oxidized metabolites 20-hydroxy- and 20-carboxy-leukotriene B4. Neutrophil reuptake of extracellular leukotriene A4 leads to an underestimation of the fraction of leukotriene A4 that is in fact available for transcellular metabolism when tight cell-cell interactions occur, such as during PMNL adhesion to the microvascular endothelium and diapedesis.

Inhibition of leukotriene formation and IL-8 release by the paf-receptor antagonist SM-12502

We analyzed the effect of the PAF receptor antagonist (+)-cis-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-12502) on the release of leukotriene B4 and IL-8 from human leukocytes. Peripheral blood from healthy donors was separated in two different fractions: polymorphonuclear leukocytes (PMN) and a lymphocyte, monocyte and basophil granulocyte cell fraction (LMB). After incubation of the cell population with different concentrations of SM-12502 the cells were subsequently stimulated with either the Ca ionophore A23187, the bacterial derived peptide fMLP, or with an activator of heterotrimetric G-proteins, the sodium fluoride (NaF, in the presence of Al3+). The PAF receptor antagonist led to a concentration and time dependent inhibition of LTB4 formation and IL-8 release from PMN and LMB. Our data clearly indicate an inhibitory effect of the PAF receptor antagonist SM-12502 on the formation of mediators of the lipoxygenase pathway and on the release of IL-8.

Effect of the PAF-receptor antagonist SM-12502 on human platelets

We analyzed the effect of the PAF receptor antagonist (+)-cis-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-12502) on human platelet aggregation as well as mediator release. After incubation of human platelet with different concentrations of SM-12502 the cells were subsequently stimulated with either the Ca ionophore A23187, with human thrombin, or with an activator of heterotrimeric G-proteins, sodium fluoride (NaF, in the presence of Al3+). Preincubation of platelets with the PAF receptor antagonist led to an inhibition of 12-lipoxygenase derived 12(S)-HETE and cyclooxygenase derived 12(S)-HHT. Pretreatment of platelets with the PAF receptor antagonist SM-12502 prior to activation with the Ca ionophore A23187 or PAF also inhibited platelet aggregation. Our data clearly indicate an inhibitory effect of the new PAF receptor antagonist SM-12502 on the formation of platelet derived inflammatory mediators of the lipoxygenase pathway as well as of the cyclooxygenase pathway, and furtherone, treatment with the PAF receptor antagonist diminished platelet aggregation after subsequent specific and unspecific activation.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) promotes phosphorylation and an increase in the activity of cytosolic phospholipase A2 in human neutrophils

Incubation of human neutrophils with 500 pM granulocyte-macrophage colony-stimulating factor (GM-CSF) results in a rapid and time-dependent increase in the phosphorylation of cytosolic phospholipase A2 (cPLA2), which was reflected in a slower electrophoretic mobility of the enzyme. The GM-CSF-induced phosphorylation of cPLA2 was accompanied by a parallel and time-dependent increase in the enzyme activity. Preincubation of neutrophils with the tyrosine kinase inhibitor genistein caused inhibition of the GM-CSF-stimulated phosphorylation and activity of cPLA2. Immunoprecipitation of the enzyme following incubation of neutrophils with [32P]Pi shows that cPLA2 is phosphorylated by GM-CSF. Potato acid phosphatase caused dephosphorylation of the enzyme, indicating that cPLA2 is indeed phosphorylated by GM-CSF. The subcellular distribution of cPLA2 in GM-CSF-stimulated neutrophils revealed that the enzyme resides almost completely in the cytosolic fraction. Addition of Ca2+ to the lysis buffer before homogenization results in the translocation of the phosphorylated and the dephosphorylated forms of the enzyme to the membranes. Translocation of cPLA2 was also achieved after incubation with 0.1 microM N-formylmethionyl-leucyl-phenyl-alanine (fMLP) after GM-CSF stimulation and when neutrophils were challenged with the Ca2+ ionophore A23187. EDTA and EGTA were unable to solubilize the translocated enzyme from the neutrophil membranes, indicating that cPLA2 is attached to the membranes by strong bonds and not merely due to ionic forces exerted by Ca2+. The inability of GM-CSF to promote arachidonic acid mobilization is probably due to the fact that GM-CSF does not cause an increase in intracellular Ca2+, which is necessary for the translocation of the enzyme to the membranes where its substrate(s) reside.

5-Lipoxygenase products modulate the activity of the 85-kDa phospholipase A2 in human neutrophils

Addition of submicromolar concentrations of arachidonic acid (AA) to human neutrophils induced a 2-fold increase in the activity of a cytosolic phospholipase A2 (PLA2) when measured using sonicated vesicles of 1-stearoyl-2-[14C]arachidonoylphosphatidylcholine as substrate. A similar increase in cytosolic PLA2 activity was induced by stimulation of neutrophils with leukotriene B4 (LTB4), 5-oxoeicosatetraenoic acid, or 5-hydroxyeicosatetraenoic acid (5-HETE). LTB4 was the most potent of the agonists, showing maximal effect at 1 nM. Inhibition of 5-lipoxygenase with either eicosatetraynoic acid or zileuton prevented the AA-induced increase in PLA2 activity but had no effect on the response induced by LTB4. Furthermore, pretreatment of neutrophils with a LTB4-receptor antagonist, LY 255283, blocked the AA- and LTB4-induced activation of PLA2 but did not influence the action of 5-HETE. Treatment of neutrophils with pancreatic PLA2 also induced an increase in the activity of the cytosolic PLA2; this response was inhibited by both eicosatetraynoic acid or LY 255283. The increases in PLA2 activity in response to stimulation correlated with a shift in electrophoretic mobility of the 85-kDa PLA2, as determined by Western blot analysis, suggesting that phosphorylation of the 85-kDa PLA2 likely underlies its increase in catalytic activity. Although stimulation of neutrophils with individual lipoxygenase metabolites did not induce significant mobilization of endogenous AA, they greatly enhanced the N-formylmethionyl-leucyl-phenylalanine-induced mobilization of AA as determined by mass spectrometry analysis. Our findings support a positive-feedback model in which stimulus-induced release of AA or exocytosis of secretory PLA2 modulate the activity of the cytosolic 85-kDa PLA2 by initiating the formation of LTB4. The nascent LTB4 is then released to act on the LTB4 receptor and thereby promote further activation of the 85-kDa PLA2. Since 5-HETE and LTB4 are known to prime the synthesis of platelet-activating factor, the findings suggest that 85-kDa PLA2 plays a role in platelet-activating factor synthesis.