Secretory and cytosolic phospholipases A2 are activated during TNF priming of human neutrophils

Radiation-induced reactive oxygen species partially assemble neutrophil NADPH oxidase

Neutrophils are key cells from the innate immune system that destroy invading bacteria or viruses, thanks mainly to the non-mitochondrial reactive oxygen species (ROS) generated by the enzyme NADPH oxidase. Our aim was to study the response of neutrophils to situations of oxidative stress with emphasis on the impact on the NADPH oxidase complex. To mimic oxidative stress, we used gamma irradiation that generated ROS (OH^•, O₂^•- and H₂O₂) in a quantitative controlled manner. We showed that, although irradiation induces shorter half-lives of neutrophil (reduced by at least a factor of 2), it triggers a pre-activation of surviving neutrophils. This is detectable by the production of a small but significant amount of superoxide anions, proportional to the dose (about 3 times that of sham). Investigations at the molecular level showed that this ROS increase was generated by the NADPH oxidase enzyme after neutrophils irradiation. The NADPH oxidase complex undergoes an incomplete assembly which includes p47^phox and p67^phox but excludes the G-protein Rac. Importantly, this irradiation-induced pre-activation is capable of considerably improving neutrophil reactivity. Indeed, we have observed that this leads to an increase in the production of ROS and the capacity of phagocytosis, leading to the conclusion that radiation induced ROS clearly behave as neutrophil primers.

Possible Involvement of Intracellular Calcium-Independent Phospholipase A2 in the Release of Secretory Phospholipases from Mast Cells-Increased Expression in Ileal Mast Cells of Crohn's Disease

Increased activity of secretory phospholipases A₂ (sPLA₂) type-II was previously observed in ileum of Crohn’s disease (CD). Our aims were to explore the involvement of calcium-independent (i)PLA₂β in the release of sPLA₂s from the human mast cell (MC) line (HMC-1) and investigate expressions of cytosolic (c)PLA₂α, iPLA₂β, sPLA₂-IIA and sPLA₂-V in MCs of CD ileum. The release of sPLA₂ was investigated in HMC-1 by immunocytochemistry and ELISA. The expression intensities of PLA₂s in mucosal MCs, and the proportion of PLA₂-positive MCs, were investigated in normal ileum and in ileum from patients with CD by immunohistochemistry. The calcium ionophore-stimulated release of sPLA₂-IIA and sPLA₂-V from HMC-1 was reduced by the iPLA₂-inhibitor bromoenol lactone. All four PLA₂s were detectable in mucosal MCs, both in normal ileum and in CD, but the proportion of iPLA₂β-containing mucosal MCs and the expression intensity of sPLA₂-IIA was increased in CD. Results indicate that iPLA₂β is involved in the secretion of sPLA₂s from HMC-1, and suggest that iPLA₂β-mediated release of sPLA₂ from intestinal MCs may contribute to CD pathophysiology. Ex vivo studies on isolated mucosal mast cells are however needed to clarify the precise role of MC PLA₂s in the inflammatory processes of CD.

Multiple Phenotypic Changes Define Neutrophil Priming

Exposure to pro-inflammatory cytokines, chemokines, mitochondrial contents, and bacterial and viral products induces neutrophils to transition from a basal state into a primed one, which is currently defined as an enhanced response to activating stimuli. Although, typically associated with enhanced generation of reactive oxygen species (ROS) by the NADPH oxidase, primed neutrophils show enhanced responsiveness of exocytosis, NET formation, and chemotaxis. Phenotypic changes associated with priming also include activation of a subset of functions, including adhesion, transcription, metabolism, and rate of apoptosis. This review summarizes the breadth of phenotypic changes associated with priming and reviews current knowledge of the molecular mechanisms behind those changes. We conclude that the current definition of priming is too restrictive. Priming represents a combination of enhanced responsiveness and activated functions that regulate both adaptive and innate immune responses.

The Neutrophil Response Induced by an Agonist for Free Fatty Acid Receptor 2 (GPR43) Is Primed by Tumor Necrosis Factor Alpha and by Receptor Uncoupling from the Cytoskeleton but Attenuated by Tissue Recruitment

Ligands with improved potency and selectivity for free fatty acid receptor 2 (FFA2R) have become available, and we here characterize the neutrophil responses induced by one such agonist (Cmp1) and one antagonist (CATPB). Cmp1 triggered an increase in the cytosolic concentration of Ca(2+), and the neutrophils were then desensitized to Cmp1 and to acetate, a naturally occurring FFA2R agonist. The antagonist CATPB selectively inhibited responses induced by Cmp1 or acetate. The activated FFA2R induced superoxide anion secretion at a low level in naive blood neutrophils. This response was largely increased by tumor necrosis factor alpha (TNF-α) in a process associated with a recruitment of easily mobilizable granules, but neutrophils recruited to an aseptic inflammation in vivo were nonresponding. Superoxide production induced by Cmp1 was increased in latrunculin A-treated neutrophils, but no reactivation of desensitized FFA2R was induced by this drug, suggesting that the cytoskeleton is not directly involved in terminating the response. The functional and regulatory differences between the receptors that recognize short-chain fatty acids and formylated peptides, respectively, imply different roles of these receptors in the orchestration of inflammation and confirm the usefulness of a selective FFA2R agonist and antagonist as tools for the exploration of the precise role of the FFA2R.

Secretory phospholipase A2-mediated depletion of phosphatidylglycerol in early acute respiratory distress syndrome

INTRODUCTION

Secretory phospholipases A2 (sPLA2) hydrolyze phospholipids in cell membranes and extracellular structures such as pulmonary surfactant. This study tests the hypothesis that sPLA2 are elevated in human lungs during acute respiratory distress syndrome (ARDS) and that sPLA2 levels are associated with surfactant injury by hydrolysis of surfactant phospholipids.

METHODS

Bronchoalveolar lavage (BAL) fluid was obtained from 18 patients with early ARDS (<72 hours) and compared with samples from 10 healthy volunteers. Secreted phospholipase A2 levels were measured (enzyme activity and enzyme immunoassay) in conjunction with ARDS subjects' surfactant abnormalities including surfactant phospholipid composition, large and small aggregates distribution and surface tension function.

RESULTS

BAL sPLA2 enzyme activity was markedly elevated in ARDS samples relative to healthy subjects when measured by ex vivo hydrolysis of both phosphatidylglycerol (PG) and phosphatidylcholine (PC). Enzyme immunoassay identified increased PLA2G2A protein in the ARDS BAL fluid, which was strongly correlated with the sPLA2 enzyme activity against PG. Of particular interest, the authors demonstrated an average depletion of 69% of the PG in the ARDS sample large aggregates relative to the normal controls. Furthermore, the sPLA2 enzyme activity against PG and PC ex vivo correlated with the BAL recovery of in vivo PG and PC, respectively, and also correlated with the altered distribution of the large and small surfactant aggregates.

CONCLUSIONS

These results support the hypothesis that sPLA2-mediated hydrolysis of surfactant phospholipid, especially PG by PLA2G2A, contributes to surfactant injury during early ARDS.

Omega-3 fatty acids cause dramatic changes in TLR4 and purinergic eicosanoid signaling

Dietary fish oil containing ω3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), elicit cardioprotective and anti-inflammatory effects through unresolved mechanisms that may involve competition and inhibition at multiple levels. Here, we report the effects of arachidonic acid (AA), EPA, and DHA supplementation on membrane incorporation, phospholipase A(2) catalyzed release, and eicosanoid production in RAW264.7 macrophages. Using a targeted lipidomics approach, we observed that Toll-like receptor 4 and purinergic receptor activation of supplemented cells leads to the release of 22-carbon fatty acids that potently inhibit cyclooxygenase pathways. This inhibition was able to shunt metabolism of AA to lipoxygenase pathways, augmenting leukotriene and other lipoxygenase mediator synthesis. In resident peritoneal macrophages, docosapentaenoic acid (DPA) was responsible for cyclooxygenase inhibition after EPA supplementation, offering fresh insights into how EPA exerts anti-inflammatory effects indirectly through elongation to 22-carbon DPA.

A phospholipase A₂ from Bothrops asper snake venom activates neutrophils in culture: expression of cyclooxygenase-2 and PGE₂ biosynthesis

In this study, the production of prostaglandin E₂ (PGE₂) and up-regulation in cyclooxygenase (COX) pathway induced by a phospholipase A₂ (PLA₂), myotoxin-III (MT-III), purified from Bothrops asper snake venom, in isolated neutrophils were investigated. The arachidonic acid (AA) production and the participation of intracellular PLA₂s (cytosolic PLA₂ and Ca(2+)-independent PLA₂) in these events were also evaluated. MT-III induced COX-2, but not COX-1 gene and protein expression in neutrophils and increased PGE₂ levels. Pretreatment of neutrophils with COX-2 and COX-1 inhibitors reduced PGE₂ production induced by MT-III. Arachidonyl trifluoromethyl ketone (AACOCF₃), an intracellular PLA₂ inhibitor, but not bromoenol lactone (BEL), an iPLA₂ inhibitor, suppressed the MT-III-induced AA and PGE₂ release. In conclusion, MT-III directly stimulates neutrophils inducing COX-2 mRNA and protein expression followed by production of PGE₂. COX-2 isoform is preeminent over COX-1 for production of PGE₂ stimulated by MT-III. PGE₂ and AA release by MT-III probably is related to cPLA₂ activation.

Regulation of arachidonate remodeling enzymes impacts eosinophil survival during allergic asthma

Although the role of arachidonic acid (AA) metabolism to eicosanoids has been well established in allergy and asthma, recent studies in neoplastic cells have revealed that AA remodeling through phospholipids impacts cell survival. This study tests the hypothesis that regulation of AA/phospholipid-remodeling enzymes, cytosolic phospholipase A(2) alpha(cPLA(2)-alpha, gIValphaPLA(2)) and CoA-independent transacylase (CoA-IT), provides a mechanism for altered eosinophil survival during allergic asthma. In vitro incubation of human eosinophils (from donors without asthma) with IL-5 markedly increased cell survival, induced gIValphaPLA(2) phosphorylation, and increased both gIValphaPLA(2) and CoA-IT activity. Furthermore, treatment of eosinophils with nonselective (ET18-O-CH(3)) and selective (SK&F 98625) inhibitors of CoA-IT triggered apoptosis, measured by changes in morphology, membrane phosphatidylserine exposure, and caspase activation, completely reversing IL-5-induced eosinophil survival. To determine if similar activation occurs in vivo, human blood eosinophils were isolated from either normal individuals at baseline or from subjects with mild asthma, at both baseline and 24 hours after inhaled allergen challenge. Allergen challenge of subjects with allergic asthma induced a marked increase in cPLA(2) phosphorylation, augmented gIValphaPLA(2) activity, and increased CoA-IT activity. These findings indicate that both in vitro and in vivo challenge of eosinophils activated gIValphaPLA(2) and CoA-IT, which may play a key role in enhanced eosinophil survival.

Pathophysiology of metabolic syndrome X and its links to the perinatal period

It is proposed that metabolic syndrome X is initiated in the perinatal period as a low-grade systemic inflammatory condition. Increased consumption of energy-dense diets by pregnant women and lactating mothers suppresses the activities of Delta-6 and Delta-5 desaturases not only in maternal tissues but also in fetal liver and the placenta, resulting in decreased plasma and tissue concentrations of long-chain polyunsaturated fatty acids omega-6 arachidonic acid (AA), omega-3 eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). EPA, DHA, and AA have negative feedback control on tumor necrosis factor-alpha and IL-6 synthesis. Hence, EPA, DHA, and AA deficiencies induced by an energy-dense diet increase generation of tumor necrosis factor-alpha and interleukin-6, markers of inflammation that in turn decrease production of endothelial nitric oxide and adiponectin to induce insulin resistance in maternal and fetal tissues. Increased concentrations of tumor necrosis factor-alpha and interleukin-6 enhance expression and activity of 11beta-hydroxysteroid dehydrogenase type 1 enzyme, which produces abdominal obesity, insulin resistance, hyperlipidemia, hyperphagia, and hyperleptinemia, characteristic features of metabolic syndrome X. Continued consumption of an energy-dense diet in childhood aggravates these molecular events. This implies that supplementation of long-chain polyunsaturated fatty acids (especially AA, EPA, and DHA in appropriate ratios) from the perinatal period through adulthood could prevent, arrest, or postpone development of metabolic syndrome X.

Cytochalasin B triggers a novel pertussis toxin sensitive pathway in TNF-alpha primed neutrophils

Background

Cytochalasin B does not directly activate the oxygen-radical-producing NADPH oxidase activity of neutrophils but transfers desensitized G-protein coupled receptors (GPCR) into an active signaling state by uncoupling GCPR from the cytoskeleton. The receptor uncoupling results in respiratory burst activity when signals generated by reactivated formyl peptide receptors trigger the NADPH-oxidase to produce superoxide anions.

Results

Tumor necrosis factor alpha (TNF-alpha) primes neutrophils for subsequent activation by cytochalasin B. Pretreatment with TNF-alpha induced mobilization of receptor-storing neutrophil organelles, suggesting that receptor up-regulation significantly contributes to the response, but the receptor mobilization was not sufficient for induction of the cytochalasin B sensitive state. The TNF-alpha primed state resembled that of the desensitized non-signaling state of agonist-occupied neutrophil formyl peptide receptors. The fact that the TNF-alpha primed, cytochalasin B-triggered activation process was pertussis toxin sensitive suggests that the activation process involves a GPCR. Based on desensitization experiments the unidentified receptor was found to be distinct from the C5a receptor as well as the formyl peptide receptor family members FPR and FPRL1. Based on the fact the occupied and desensitized receptors for interleukin-8 and platelet activating factor could not be reactivated by cytochalasin B, also these could be excluded as receptor candidates involved in the TNF-alpha primed state.

Conclusions

The TNF-alpha-induced priming signals could possibly trigger a release of an endogenous GPCR-agonist, amplifying the response to the receptor-uncoupling effect of cytochalasin B. However, no such substance could be found, suggesting that TNF-alpha can transfer G-protein coupled receptors to a signaling state independently of agonist binding.

Phospholipase A2, hydroxyl radicals, and lipid peroxidation in transient cerebral ischemia

Phospholipid degradation is an important promoter of neuronal death after transient cerebral ischemia. Phospholipid hydrolysis by phospholipase A2 (PLA2) after transient cerebral ischemia releases arachidonic acid. Arachidonic acid metabolism results in formation of reactive oxygen species, lipid peroxides, and toxic aldehydes (malondialdehyde, 4-hydroxynonenal, and acrolein). Citicoline (cytidine-5'-diphosphocholine), an intermediate in phosphatidylcholine synthesis, has undergone 13 phase III clinical trials for stroke, and is being evaluated for treatment of Alzheimer's and Parkinson's diseases. Here we examined the effect of citicoline on PLA2 activity in relationship to attenuating hydroxyl radical (OH*) generation and lipid peroxidation after transient forebrain ischemia in gerbil. High Ca2+ dependency (millimolar range) of PLA2 activity suggests that secretory PLA2 is the predominant isoform in membrane and mitochondria. Citicoline attenuated the increase in PLA2 activity in both membrane and mitochondrial fractions. In vitro, citicoline and its components choline and cytidine had no effect on the PLA2 activity. Thus, citicoline is not a "direct PLA2 inhibitor." Citicoline also significantly attenuated loss of cardiolipin and arachidonic acid release from phosphatidylcholine and phosphatidylethanolamine. Transient cerebral ischemia resulted in significant formation of OH* and malondialdehyde, and citicoline significantly attenuated their formation. These results suggest that citicoline provides neuroprotection by attenuating the stimulation of PLA2.

Citicoline decreases phospholipase A2 stimulation and hydroxyl radical generation in transient cerebral ischemia

Neuroprotection by citicoline (CDP-choline) in transient cerebral ischemia has been demonstrated previously. Citicoline has undergone several Phase III clinical trials for stroke, and is being evaluated for treatment of Alzheimer's and Parkinson's diseases. Phospholipid degradation and generation of reactive oxygen species (ROS) are major factors causing neuronal injury in CNS trauma and neurodegenerative diseases. Oxidative metabolism of arachidonic acid (released by the action of phospholipases) contributes to ROS generation. We examined the effect of citicoline on phospholipase A(2) (PLA(2)) activity in relation to the attenuation of hydroxyl radical (OH.) generation after transient forebrain ischemia of gerbil. PLA(2) activity (requires mM Ca(2+)) increased significantly (P < 0.05) in both membrane (50.2 +/- 2.2 pmol/min/mg protein compared to sham 35.9 +/- 3.2) and mitochondrial fractions (77.0 +/- 1.2 pmol/min/mg protein compared to sham 33.9 +/- 1.2) after cerebral ischemia and 2 hr reperfusion in gerbil, which was significantly attenuated (P < 0.01) by citicoline (membrane, 39.9. +/- 2.2 and mitochondria, 41.9 +/- 3.2 pmol/min/mg protein). In vitro, citicoline and its components cytidine and choline had no effect on PLA(2) activity, and thus citicoline as such is not a PLA(2) inhibitor. Ischemia/reperfusion resulted in significant OH. generation (P < 0.01) and citicoline significantly (P < 0.01) attenuated their formation (expressed as 2,3-dihydroxybenzoic acid/salicylate ratio; ischemia/24 hr reperfusion, 6.30 +/- 0.23; sham, 2.56 +/- 0.27; ischemia/24 hr reperfusion + citicoline, 4.85 +/- 0.35). These results suggest that citicoline affects PLA(2) stimulation and decreases OH. generation after transient cerebral ischemia.

Ex-vivo regulation of endotoxin-induced tissue factor in whole blood by eicosanoids

The influence of several eicosanoids of the lipoxygenase pathway was examined in an ex vivo system of human whole blood subjected to stimulation by lipopolysaccharide (LPS). Exogenously added leukotriene B4 [5(S),12(R)-dihydroxy-6,14-cis-8,10-trans-eicosatetraenoic acid (LTB4)] or 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) significantly (P<0.05) enhanced LPS-evoked expression of monocyte tissue factor (TF) activity in a concentration-dependent manner. 15(S)-HETE, on the other hand, exerted such activity only when added at certain concentrations, whereas 5(S)-HETE was devoid of any apparent activity. LPS-induced TF activity was inhibited by the lipoxygenase inhibitors nordihydroguaiaretic acid, CGS 23885 and ZM 230487, by 59, 32 and 88%, respectively. Furthermore, the production of LTB4 in LPS-stimulated whole blood was investigated, in the absence or presence of either tumor necrosis factor alpha (TNFalpha) or phorbol-12-myristate-13-acetate (PMA). LPS alone induced a moderate time-dependent and concentration-dependent release of LTB4, reaching the maximum concentration (1260 +/- 202 pg/ml) within 90 min at 5 ng/ml LPS. The prior and concurrent presence of PMA (5 ng/ml) or TNFalpha (10 ng/ml) further enhanced the LTB4 production approximately twofold (P < 0.05). TNFalpha added alone evoked approximately twice the LTB4 production seen when LPS (2200 +/- 243 versus 1260 +/- 203 pg/ml) was added alone. Considering these results, LPS and TNFalpha emerge as important agonists of LTB4 production in whole blood. LTB4 in turn appears to be of importance for the expression of TF in monocytes, potentially amplifying the thrombogenic potential of these cells.

Dopamine D(2) receptor-induced COX-2-mediated production of prostaglandin E(2) in D(2)-transfected Chinese hamster ovary cells without simultaneous administration of a Ca(2+)-mobilizing agent

We have earlier demonstrated that dopamine stimulates the liberation of the prostaglandin E(2) (PGE(2)) precursor, arachidonic acid, in Chinese hamster ovary cells transfected with the rat dopamine D(2) receptor (long isoform), also without concomitant administration of a Ca(2+)-releasing agent [Nilsson et al., Br J Pharmacol 1998;124:1651-8]. In the present report, we show that dopamine, under the same conditions, also induces a concentration-dependent increase in the production of PGE(2), with a maximal effect of 235% at approximately 100 microM, and with an EC(50) of 794 nM. The effect was counteracted by the D(2) antagonist eticlopride, pertussis toxin, the inhibitor of intracellular Ca(2+) release TMB-8, incubation in Ca(2+)-free experimental medium, and PKC desensitization obtained by chronic pretreatment with the phorbol ester TPA. It was also antagonized by the non-specific cyclooxygenase (COX) inhibitor, indomethacin, and by the selective COX-2 inhibitor, NS-398, but not by the specific COX-1 inhibitor, valeryl salicylate. Both the non-specific phospholipase A(2) inhibitor, quinacrine, and an inhibitor of cPLA(2) and iPLA(2), AACOF3, counteracted the effect; in contrast, a selective iPLA(2) inhibitor, BEL, and a selective sPLA(2) inhibitor, TAPC, were ineffective. No effects of dopamine were obtained in control cells mock-transfected with the p3C vector only. The results reinforce previous assumptions that dopamine may interact with eicosanoid metabolism by means of D(2) receptor activation, and implicate an involvement of cPLA(2) and COX-2 in this effect. It is suggested that measurement of dopamine-induced PGE(2) production may serve as a convenient way to study D(2) receptor function in vitro.

Enhancement of mast cell survival: a novel function of some secretory phospholipase A(2) isotypes

This study tested the hypothesis that certain secretory phospholipase A(2) (sPLA(2)) isotypes act in a cytokine-like fashion through cell surface receptors to influence mast cell survival. Initial experiments revealed that sPLA(2) activity and sPLA(2) receptor expression are increased, and mast cells lost their capacity to maintain membrane asymmetry upon cytokine depletion. Groups IB and III, but not group IIA PLA(2), prevented the loss of membrane asymmetry. Similarly, group IB prevented nucleosomal DNA fragmentation in mast cells. Providing putative products of sPLA(2) hydrolysis to cytokine-depleted mast cells did not influence survival. Furthermore, catalytic inactivation of sPLA(2) did not alter its capacity to prevent apoptosis. Inhibition of protein synthesis using cycloheximide or actinomycin reversed the antiapoptotic effect of sPLA(2). Additionally, both wild-type and catalytically inactive group IB PLA(2) induced IL-3 synthesis in mast cells. However, adding IL-3-neutralizing Ab did not change Annexin V(FITC) binding and only partially inhibited thymidine incorporation in sPLA(2)-supplemented mast cells. In contrast, IL-3-neutralizing Ab inhibited both Annexin V(FITC) binding and thymidine incorporation in mast cells maintained with IL-3. sPLA(2) enhanced phosphoinositide 3'-kinase activity, and a specific inhibitor of phosphoinositide 3'-kinase reversed the antiapoptotic effects of sPLA(2). Likewise, sPLA(2) increased the degradation of I-kappaBalpha, and specific inhibitors of nuclear factor kappa activation (NF-kappaB) reversed the antiapoptotic effects of sPLA(2). Together, these experiments reveal that certain isotypes of sPLA(2) enhance the survival of mast cells in a cytokine-like fashion by activating antiapoptotic signaling pathways independent of IL-3 and probably via sPLA(2) receptors rather than sPLA(2) catalytic products.

Mechanisms and modulation of formyl-methionyl-leucyl-phenylalanine (fMLP)-induced Ca2+ mobilization in human neutrophils

The effect of fMLP (N-formyl-methionyl-leucyl-phenylalanine), a neutrophil-stimulating bacterial peptide, on Ca2+ mobilization in human neutrophils was examined using fura-2 as a Ca2+ indicator. fMLP (10 nM-10 microM) increased [Ca2+]i concentration-dependently. The [Ca2+]i signal comprised an initial rise followed by a gradual decay and a sustained phase. External Ca2+ removal partly decreased the signal. La3+ (50 microM) pretreatment mimicked the effect of Ca2+ removal. In Ca(2+)-free medium, pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor) prevented 10 microM fMLP from increasing [Ca2+]i; whereas 1 microM thapsigargin still significantly increased [Ca2+]i after pretreatment with 10 microM fMLP. Addition of 3 mM Ca2+ induced a concentration-dependent [Ca2+]i increase after pretreatment with fMLP in Ca(2+)-free medium. This Ca2+ entry was partly inhibited by econazole (25 microM), SKF96365 (50 microM), and a phospholipase A2 inhibitor (aristolochic acid; 20 microM). The fMLP (10 microM)-induced Ca2+ release was abolished by inhibiting phospholipase C with 2 microM U73122. The fMLP-induced [Ca2+]i increase was inhibited by 25% by pretreatment with 10 nM phorbol ester to activate protein kinase C but was augmented by 27% by pretreatment with 2 microM GF 109203X to inactivate protein kinase C. We found that fMLP increase reactive oxygen intermediate (ROI) production in neutrophils, which can be suppressed by U73122 pretreatment. Collectively, this study shows that in human neutrophils, fMLP increased [Ca2+]i concentration-dependently by releasing Ca2+ from phospholipase C-coupled, thapsigargin-sensitive stores, accompanied by Ca2+ entry. The fMLP-induced [Ca2+]i rise was modulated by protein kinase C, and the fMLP-induced Ca2+ entry was abolished by La3+, and was reduced by econazole, SKF96365 and inhibition of phospholipase A2.

Activation and priming of neutrophil nicotinamide adenine dinucleotide phosphate oxidase and phospholipase A(2) are dissociated by inhibitors of the kinases p42(ERK2) and p38(SAPK) and by methyl arachidonyl fluorophosphonate, the dual inhibitor of cytosolic and calcium-independent phospholipase A(2)

Arachidonic acid (AA) generated by phospholipase A(2) (PLA(2)) is thought to be an essential cofactor for phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Both enzymes are simultaneously primed by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha). The possibility that either unprimed or cytokine-primed responses of PLA(2) or NADPH oxidase to the chemotactic agents formyl-methionyl-leucyl-phenylalanine (FMLP) and complement factor 5a (C5a) could be differentially inhibited by inhibitors of the mitogen-activated protein (MAP) kinase family members p42(ERK2) (PD98059) and p38(SAPK) (SB203580) was investigated. PD98059 inhibited the activation of p42(ERK2) by GM-CSF, TNF-alpha, and FMLP, but it did not inhibit FMLP-stimulated superoxide production in either unprimed or primed neutrophils. There was no significant arachidonate release from unprimed neutrophils stimulated by FMLP, and arachidonate release stimulated by calcium ionophore A23187 was not inhibited by PD98059. In contrast, PD98059 inhibited both TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP. On the other hand, SB203580 inhibited FMLP-superoxide responses in unprimed as well as TNF-alpha- and GM-CSF-primed neutrophils, but failed to inhibit TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP, and additionally enhanced A23187-stimulated arachidonate release, showing that priming and activation of PLA(2) and NADPH oxidase are differentially dependent on both the p38(SAPK) and p42(ERK2) pathways. Studies using C5a as an agonist gave similar results and confirmed the findings with FMLP. In addition, methyl arachidonyl fluorophosphonate (MAFP), the dual inhibitor of c and iPLA(2) enzymes, failed to inhibit superoxide production in primed cells at concentrations that inhibited arachidonate release. These data demonstrate that NADPH oxidase activity can be dissociated from AA generation and indicate a more complex role for arachidonate in neutrophil superoxide production.

Clara cell secretory protein (CC16): characteristics and perspectives as lung peripheral biomarker

Clara cell protein (CC16) is a 15.8-kDa homodimeric protein secreted in large amounts in airways by the non-ciliated bronchiolar Clara cells. This protein increasingly appears to protect the respiratory tract against oxidative stress and inflammation. In vitro, CC16 has been shown to modulate the production and/or the activity of various mediators of the inflammatory response including PLA2, interferon-gamma and tumour necrosis factor-alpha. CC16 has also been found to inhibit fibroblast migration or to bind various endogenous or exogenous substances such as polychlorobiphenyls (PCBs). This protective role is confirmed by studies on transgenic mice, showing that CC16 deficiency is associated with an increased susceptibility of the lung to viral infections and oxidative stress. In humans, a polymorphism of the CC16 gene, localized to a region linked to airway diseases, has recently been discovered in association with an increased risk of developing childhood asthma. Finally, CC16 also presents a major interest as a peripheral marker for assessing the integrity of the lung epithelium. The determination of CC16 in serum is a new non-invasive test to detect Clara cell damage or an increased epithelial permeability in various acute and chronic lung disorders.

Involvement of cytosolic phospholipase A2 and secretory phospholipase A2 in arachidonic acid release from human neutrophils

The purpose of this study was to define the role of secretory phospholipase A2 (sPLA2), calcium-independent PLA2, and cytosolic PLA2 (cPLA2) in arachidonic acid (AA) release from fMLP-stimulated human neutrophils. While fMLP induced the release of extracellular sPLA2 activity and AA, 70% of sPLA2 activity remained associated with the cell. Treatment with the cell-impermeable sPLA2 inhibitors DTT or LY311-727, or the anti-sPLA2 Ab 3F10 all inactivated extracellular sPLA2 activity, but had minimal effect on neutrophil AA mass release. In contrast, coincubation of streptolysin-O toxin-permeabilized neutrophils with DTT, LY311-727, or 3F10 all decreased [3H8]AA release from [3H8]AA-labeled, fMLP-stimulated cells. Exposure to fMLP resulted in a decrease in the electrophoretic mobility of cPLA2, a finding consistent with cPLA2 phosphorylation, and stimulated the translocation of cPLA2 from cytosolic to microsomal and nuclear compartments. The role of cPLA2 was further evaluated with the cPLA2 inhibitor methyl arachidonyl fluorophosphonate, which attenuated cPLA2 activity in vitro and decreased fMLP-stimulated AA mass release by intact neutrophils, but had no effect on neutrophil sPLA2 activity. Inhibition of calcium-independent PLA2 with haloenol lactone suicide substrate had no effect on neutrophil cPLA2 activity or AA mass release. These results indicate a role for cPLA2 and an intracellular or cell-associated sPLA2 in the release of AA from fMLP-stimulated human neutrophils.

Elevated cytosolic phospholipase A2 expression and activity in human neutrophils during sepsis

Sepsis is defined as the systemic inflammatory response to infection. Phospholipase A2 (PLA2) plays an important role in inflammation processes by initiating the production of inflammatory mediators. The role of cytosolic PLA (cPLA2) has not yet been identified in inflammatory and infectious disease clinical settings. The aim of the present research was to determine whether cPLA2 activity has a role during sepsis. Since neutrophil activation has been documented during sepsis, these cells were chosen as a model to evaluate the function of cPLA2 in this clinical setting. cPLA2 was studied at 3 levels: activity, protein expression, and messenger RNA (mRNA). Neutrophils from 32 septic patients with and without bacteremia were examined. cPLA2 activity was measured using labeled phosphatidyl choline vesicles as a substrate, and total PLA2 was determined by the release of labeled arachidonic acid from prelabeled cells. A significant increase in cPLA2activity, protein expression, and total PLA2 activity in neutrophils was detected during sepsis. mRNA levels, detected by reverse transcriptase–polymerase chain reaction, were significantly higher during sepsis, indicating that the increase in the amount of cPLA2 is regulated on the mRNA level. The significant elevation of cPLA2 activity and expression in neutrophils during sepsis suggests that this enzyme plays a major role in neutrophil function in this clinical setting.

Sepsis increases contraction-related generation of reactive oxygen species in the diaphragm

Recent work indicates that free radicals mediate sepsis-induced diaphragmatic dysfunction. These previous experiments have not, however, established the source of the responsible free radical species. In theory, this phenomenon could be explained if one postulates that sepsis elicits an upregulation of contraction-related free radical formation in muscle. The purpose of the present study was to test this hypothesis by examination of the effect of sepsis on contraction-related free radical generation [i.e. , formation of reactive oxygen species (ROS)] by the diaphragm. Rats were killed 18 h after injection with either saline or endotoxin. In vitro hemidiaphragms were then prepared, and ROS generation during electrically induced contractions (20-Hz trains delivered for 10 min) was assessed by measurement of the conversion of hydroethidine to ethidium. ROS generation was negligible in noncontracting diaphragms from both saline- and endotoxin-treated groups (2.0 +/- 0. 6 and 2.8 +/- 1.0 ng ethidium/mg tissue, respectively), but it was marked in contracting diaphragms from saline-treated animals (19.0 +/- 2.8 ng/mg tissue) and even more pronounced (30.0 +/- 2.8 ng/mg tissue) in diaphragms from septic animals (P < 0.01). This enhanced free radical generation occurred despite the fact that the force-time integral (i.e., the area under the curve of force vs. time) for control diaphragms was higher than that for the septic group. In additional studies, in which we altered the stimulation paradigm in control muscles to achieve a force-time integral similar to that achieved in septic muscles, an even greater difference between control and septic muscle ROS formation was observed. These data indicate that ROS formation during contraction is markedly enhanced in diaphragms from endotoxin-treated septic animals. We speculate that ROS generated in this fashion plays a central role in producing sepsis-related skeletal muscle dysfunction.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

Regulation and metabolism of arachidonic acid

The metabolism of AA reflects a carefully balanced series of biochemical pathways. The level of free arachidonate in a cells is controlled by de novo synthesis, dietary uptake, and transcellular metabolism. Lysophospholipids are key controlling substrates for a variety of acyl transferase and transacylase reactions, whose combined effect is to remodel cellular membranes placing AA in up to 20 different molecular species of phospholipids. PLA2 enzymes, both cytosolic and secretory, can release AA for subsequent metabolism via lipoxygenase, COX, and cytochrome P450 enzymes into a variety of eicosanoid products. Reactions are often tissue- and cell-specific, and provide a spectrum of inflammatory mediator release in which many of the molecular details remain to be elucidated.

Luteal phospholipase A2 activity increases during functional and structural luteolysis in pregnant rats

We determined cytosolic phospholipase A2 activity of the corpus luteum during luteolysis in pregnant and post-partum rats. Phospholipase A2 activity and its metabolite prostaglandin F2alpha in the corpus luteum remarkably increased just before parturition and further rose transiently during post-partum structural luteolysis. The absence of a pups' suckling stimulus delayed corpus luteum involution, being associated with an altered fluctuation in phospholipase A2 activity and depressed prostaglandin F2alpha levels. Exogenous prolactin had a reversal effect. Pharmacological and immunochemical characterization suggests multiple isoforms of phospholipase A2 in a pregnant corpus luteum. These results show the increased phospholipase A2 activity and its possible implication in luteolysis in pregnant rats.

Differential activation of human neutrophil cytosolic phospholipase A2 and secretory phospholipase A2 during priming by 1,2-diacyl- and 1-O-alkyl-2-acylglycerols

We have shown previously that both 1,2-diacylglycerol (AAG) and 1-O-alkyl-2-acylglycerol (EAG) prime neutrophil release of arachidonic acid via uncharacterized phospholipases A2. Therefore, we investigated the actions of EAG and AAG specifically on neutrophil cytosolic (cPLA2) and secretory (sPLA2) phospholipase A2s. We hypothesized that AAG as a protein kinase activator would activate cPLA2 via phosphorylation events. EAG is antagonistic to the AAG activation of PKC, thus it was not expected to act via phosphorylation of cPLA2. Neutrophils were primed with either AAG or EAG and then stimulated with fMLP. When neutrophils were primed with 5-20 microM 1,2-diacylglycerol, a shift was observed in cPLA2 migration on SDS-PAGE gels, consistent with phosphorylation of the protein. This gel shift was not seen after exposure to EAG. AAG also caused a parallel increase in enzymatic activity of cPLA2 that was not seen with EAG. We also investigated whether either diglyceride would cause similar priming or direct secretion of sPLA2. Both AAG and EAG directly caused significant secretion of neutrophil sPLA2. EAG also increased the release of sPLA2 in cells subsequently stimulated with fMLP. Thus, AAG activated cPLA2 and stimulated secretion of sPLA2. In contrast, EAG did not activate cPLA2, but directly activated secretion of sPLA2. We also demonstrated that human synovial fluid sPLA2 increased AA release from resting and fMLP-stimulated neutrophils. Given that diglycerides prime for release of AA, PAF, and LTB4, these current data support the hypothesis that such priming may be mediated by phosphorylation dependent (cPLA2) or phosphorylation independent (e.g. secretion of sPLA2) events.