Role of phospholipase D-derived diradylglycerol in the activation of the human neutrophil respiratory burst oxidase. Inhibition by phosphatidic acid phosphohydrolase inhibitors.

An agonist-activated phospholipase D/phosphatidic acid phosphohydrolase (PAH) pathway was recently demonstrated in human neutrophils, and evidence suggests that phosphatidic acid (PA) and/or diradylglycerol (DG) generated from this pathway participates in activation of the O2(-)-generating respiratory burst. We have used a series of cationic amphiphilic compounds (sphingosine, propranolol, chlorpromazine, and desipramine) and antibiotics (clindamycin, trimethoprim, and roxithromycin) all of which inhibit the respiratory burst, to investigate the role of the phospholipase D/PAH pathway in neutrophil activation. The phosphatidylcholine (PC) pool in intact cells was first labeled using [3H]-1-O-alkyl-lysoPC; released [3H]-PA and [3H]-DG were then quantified after the addition of either chemo-attractant or PMA. Using either agonist, all compounds showed a dose-dependent inhibition of [3H]-DG generation which correlated with inhibition of O2- generation, but compounds failed to inhibit directly the NADPH oxidase in a cell-free system. For either activator, a plot of the ID50 values for O2- generation vs those for DG generation was linear over four orders of magnitude. In many cases, inhibition of [3H]-DG generation corresponded to an increase in [3H]-PA, implicating PAH as the locus of inhibition. Superoxide generation was inhibited under conditions where PA was either elevated or minimally affected. Neither O2- release nor DG generation showed any selectivity for stereoisomers of propranolol, suggesting that this inhibition does not act via a specific binding site on PAH. No evidence was obtained for an effect of the inhibitors on PA mobility as monitored by electron spin resonance studies of spin-labeled PA in a model membrane system. Data are consistent with an effect of the inhibitors at the level of the interaction of PAH with the membrane and/or its substrate. These data imply that DG produced via the phospholipase D/PAH pathway functions in the activation or maintenance of the respiratory burst.     

Roles of phosphatidylinositol 3-kinase and phospholipase D in temporal activation of superoxide production in FMLP-stimulated human neutrophils

To determine the temporal roles of phosphatidylinositol 3-kinase (PI3-kinase) and phospholipase D (PLD) during human neutrophil activation stimulated by a chemotactic peptide, we examined the kinetics of these enzymes and related them to a neutrophil function (superoxide production). Both wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), potent and specific inhibitors of PI3-kinase, inhibit PI3-kinase activity in human neutrophils and significantly inhibit superoxide production from the early phase. Ethanol has no effect on PI3-kinase and markedly inhibits superoxide production at the late phase. Although these agents are inhibitory to different degrees, when neutrophils are simultaneously treated with ethanol and PI3-kinase inhibitors, superoxide is not produced. These results suggest that PI3-kinase and PLD play a pivotal role in the signal transduction pathway of the chemo-attractant-receptor involved neutrophil activation. These enzymes produce second messengers which are required for subsequent superoxide production in human neutrophils. NADPH oxidase is activated in a PI3-kinase-dependent manner at the early phase, and PLD activity follows it and is related to superoxide production at the late phase in human neutrophils by stimulation with FMLP.     

Contribution of CD54 to human eosinophil and neutrophil superoxide production.

We have reported that CD54 on eosinophils is involved in eosinophil degranulation. However, the role of CD54 in eosinophil and neutrophil superoxide production is still uncertain. We assessed the effect of CD54 on eosinophils and neutrophils in recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF)- or phorbol myristate acetate (PMA)-induced superoxide production through CD18. Anti-CD54 monoclonal antibody attenuated leukocyte aggregation and superoxide production of rGM-CSF- or PMA-stimulated neutrophils and PMA-stimulated eosinophils. Anti-CD18 monoclonal antibody or theophylline attenuated superoxide production of eosinophils and neutrophils stimulated by either stimuli. Flow cytometric analysis demonstrated CD54 expression on freshly isolated neutrophils but not on freshly isolated eosinophils. CD54 newly expressed on eosinophils reached its peak expression 30 min after PMA stimulation. The increase in CD18 and CD54 expression on neutrophils caused by rGM-CSF stimulation was partially inhibited by theophylline. These data demonstrated that CD54 and CD18 interaction of eosinophils or neutrophils is involved in superoxide production and that the inhibition of superoxide production by theophylline may be at least partly due to the inhibition of CD54 and CD18.     

The bioflavonoid quercetin inhibits neutrophil degranulation, superoxide production, and the phosphorylation of specific neutrophil proteins

Quercetin, a C-kinase antagonist, inhibits neutrophil degranulation and superoxide production induced by f-met-leu-phe, solid phase IgG, zymosan treated serum and a phorbol ester (PMA). Quercetin is more effective in inhibiting degranulation (IC50 = 20 uM) than superoxide production (IC50 = 80 microM). Neutrophil activation by PMA is accompanied by the phosphorylation of neutrophil proteins of 205, 170, 130, 91, 77, 67, 56, 47, 39, 34, 27, and 20 kilodaltons; quercetin also inhibits the phosphorylation of these proteins. Dose-response studies indicated that phosphorylation of the 67 kilodalton protein was particularly sensitive to inhibition by quercetin at concentrations that also inhibit neutrophil degranulation and superoxide production. These results suggest that phosphorylation of the 67 kilodalton protein may be an important intracellular reaction associated with neutrophil activation.     

Inhibition of neutrophil superoxide production by human plasma alpha 1-antitrypsin.

We report here that human plasma alpha 1-antitrypsin (alpha 1-AT) inhibited human neutrophil O2.- release elicited by a variety of stimulants. In comparison, the inhibitory capacities of two serine protease inhibitors, L-1-tosylamide 2-phenylethyl chloromethyl ketone (TPCK) and soybean trypsin inhibitor (SBTI), and the human recombinant alpha 1-AT mutant, alpha 1-AT-Arg358 were in the order: alpha 1-AT = TPCK much greater than alpha 1-AT-Arg358 greater than SBTI when cells were stimulated with concanavalin A plus cytochalasin E. These data suggest that, in human inflammatory fluids containing relatively high concentrations of alpha 1-AT (such as rheumatoid arthritis synovial fluid), (i) alpha 1-AT may down-regulate the inflammatory process by inhibiting the neutrophil respiratory burst and (ii) serpin oxidation by neutrophil-released reactive oxygen species is unlikely to occur.     

Direct relationship between intracellular calcium mobilization and phospholipase D activation in prostaglandin E-stimulated human erythroleukemia cells.

The relationship between calcium mobilization and phospholipase D (PLD) activation in response to E-series prostaglandins (PGEs) was investigated in human erythroleukemia cells. Intracellular free Ca2+ concentration ([Ca2+]i) was increased by PGE1 and PGE2 over the same concentration range at which PLD activation was seen. Pretreatment of cells with pertussis toxin greatly inhibited the PGE-stimulated increase in [Ca2+]i, implying that a G protein participates in the PGE receptor signaling process. The peak level and also the plateau level of Ca2+ mobilization stimulated by these prostaglandins were markedly decreased in Ca(2+)-depleted medium, indicating that both extracellular and intracellular Ca2+ stores contribute to the changes in [Ca2+]i. Likewise, activation of PLD by PGE1 and PGE2 was abolished by pertussis toxin pretreatment or incubation in Ca(2+)-depleted medium. U73122, a putative phospholipase C inhibitor, blocked both Ca2+ mobilization and PLD activation in PGE-stimulated cells. Furthermore, the intracellular loading of BAPTA, a Ca2+ chelator, inhibited both Ca2+ mobilization and PLD activation by PGE1 and PGE2 in a similar dose-dependent manner. Simultaneous measurement of [Ca2+]i and PLD activity in the same cell samples indicated that PLD activity increases as a function of [Ca2+]i in a similar fashion in cells stimulated either by PGEs or by the calcium ionophore ionomycin. Taken together, these findings suggest that a rise in [Ca2+]i is necessary for PGE-stimulated PLD activity in human erythroleukemia cells.     

Apolipoprotein A-I decreases neutrophil degranulation and superoxide production.

Neutrophils participate in the acute phase response and are often associated with tissue injury in a number of inflammatory disorders. The acute phase response is accompanied by alterations in the metabolism of apolipoprotein A-I and high density lipoprotein (HDL). Structural considerations led to studies investigating the effect of purified HDL and apolipoprotein A-I on neutrophil degranulation and superoxide production. Apolipoprotein A-I but not HDL inhibited IgG-induced neutrophil activation by about 60% as measured by degranulation and superoxide production. This suggests that the lipid-associating amphipathic helical domains of apolipoprotein A-I mediate this effect. In support of this was finding inhibitory effects with two synthetic model lipid-associating amphipathic helix peptide analogs. Apolipoprotein A-I, containing tandem repeating amphipathic helical domains, was approximately ten times more effective than the two peptide analogs and inhibited neutrophil activation at well below physiologic concentrations. Competitive binding studies indicate that resting neutrophils have approximately 190,000 (Kd = 1.7 x 10(-7)) binding sites per cell for apolipoprotein A-I, consistent with a ligand-receptor interaction. These observations suggest that apolipoprotein A-I may play an important role in regulating neutrophil function during the inflammatory response.     

Tumor necrosis factor alpha priming of phospholipase D in human neutrophils. Correlation between phosphatidic acid production and superoxide generation.

Tumor necrosis factor alpha (TNF) primes human neutrophils (PMN) for enhanced superoxide (O2-) production if cells are subsequently stimulated with the chemotactic peptide, n-formyl-Met-Leu-Phe (fMLP). fMLP activates phospholipase D to form phosphatidic acid (PA), and a correlation may exist between PA production and O2- generation in PMN. Therefore, we assessed the ability of TNF to prime phospholipase D activation in PMN stimulated with fMLP. TNF (100 units/ml) pretreatment primed enhanced PA production in PMN challenged with 1 microM fMLP, in the absence of cytochalasin B, as demonstrated by increased production of tritiated PA from PMN label with 1-O-[9',10'-3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine ([3H]LPAF) and by increased PA mass. PA was formed via activation of phospholipase D and occurred with minimal production of diglycerides. Production of O2- was also enhanced in identically treated cells, and we demonstrated a direct correlation between enhanced PA formation and O2- production. Conversely, ethanol inhibition of PA formation led to a comparable reduction in O2- generation. This report of priming of phospholipase D by physiological agonists is the only natural system where enhanced PA formation has been dissociated from diglyceride formation. Our results suggest a link between PA production and NADPH oxidase activation in human PMN.     

Role of extracellular superoxide in neutrophil activation: interactions between xanthine oxidase and TLR4 induce proinflammatory cytokine production

Reactive oxygen species (ROS) contribute to neutrophil activation and the development of acute inflammatory processes in which neutrophils play a central role. However, there is only limited information concerning the mechanisms through which extracellular ROS, and particularly cell membrane-impermeable species, such as superoxide, enhance the proinflammatory properties of neutrophils. To address this issue, neutrophils were exposed to superoxide generating combinations of xanthine oxidase and hypoxanthine or lumazine. Extracellular superoxide generation induced nuclear translocation of nuclear factor-kappaB (NF-kappaB) and increased neutrophil production of the NF-kappaB-dependent cytokines tumor necrosis factor-alpha (TNF-alpha) and macrophage inhibitory protein-2 (MIP-2). In contrast, there were no changes in TNF-alpha or MIP-2 expression when neutrophils lacking Toll-like receptor-4 (TLR4) were exposed to extracellular superoxide. Immunoprecipitation, confocal microscopy, and fluorescence resonance energy transfer (FRET) studies demonstrated association between TLR4 and xanthine oxidase. Exposure of neutrophils to heparin attenuated binding of xanthine oxidase to the cell surface as well as interactions with TLR4. Heparin also decreased xanthine oxidase-induced nuclear translocation of NF-kappaB as well as production of proinflammatory cytokines. These results demonstrate that extracellular superoxide has proinflammatory effects on neutrophils, predominantly acting through an TLR4-dependent mechanism that enhances nuclear translocation of NF-kappaB and increases expression of NF-kappaB-dependent cytokines.     

Role of different Ca2+ sources in the superoxide production of human neutrophil granulocytes.

The role of different Ca2+ sources in the activation of the NADPH oxidase was investigated in human neutrophil granulocytes. Selective depletion of the stimulus-responsive intracellular Ca2+ -pool and the consequent opening of the store-dependent Ca2+ channel of the plasma membrane was achieved with thapsigargin, an inhibitor of microsomal Ca2+ -ATPase. Low concentration (10-100 nM) of thapsigargin did not induce any O2*- -production, indicating that elevation of [Ca2+]ic to similar level and probably via similar route as following stimulation of chemotactic receptors, by itself is not sufficient to activate the NADPH oxidase. In significantly higher concentration (1-10 microM) thapsigargin did induce O2*- -generation but this effect was not the result of elevation of [Ca2+]ic. In the absence of external Ca2+ a gradual decrease of the responsive Ca2+ pool was accompanied by a gradual decrease of the rate and duration of the respiratory response stimulated by formyl-methionyl-leucyl-phenylalanin. Maximal extent of receptor-initiated O2*- -production could only be obtained when the intracellular [Ca2+] was higher than the resting level. Under this condition Ca2+ originating from intracellular or external source was equally effective in supporting the biological response.     

Differential inhibition of human neutrophil activation by cyclosporins A, D, and H. Cyclosporin H is a potent and effective inhibitor of formyl peptide-induced superoxide formation.

Cyclosporin (Cs)A but not CsH inhibits activation of human lymphocytes. We studied the effects of CsA, CsD, and CsH on human neutrophil activation induced by chemoattractants and by various substances that circumvent receptor stimulation. CsH inhibited superoxide (O2-) formation induced by the chemotactic peptide, FMLP (30 nM), with a half-maximal effect at 40 nM. O2- formation was abolished by CsH at 1 microM. CsH increased the concentration of FMLP causing half-maximal activation of O2- formation from 30 nM to 0.8 microM and substantially reduced the stimulatory effect of FMLP at supra-maximally effective concentrations. The inhibitory effect of CsH on O2- formation was evident immediately after addition to neutrophils. CsH also markedly inhibited the increase in cytosolic Ca2+ ([Ca2+]i), beta-glucuronidase, and lysozyme release and aggregation stimulated by FMLP. CsA and CsD were considerably less effective than CsH to inhibit FMLP-induced O2- formation. CsA and CsD were without effect on exocytosis, rises in [Ca2+]i, and aggregation induced by the chemotactic peptide. Cyclosporines inhibited FMLP-induced O2- formation in an additive manner, indicating that they acted through a mechanism they had in common. Cyclosporines only slightly inhibited O2- formation and lysozyme release induced by C5a. Aggregation and rises in [Ca2+]i stimulated by C5a were not affected by cyclosporines, and they did not inhibit O2- formation and exocytosis induced by platelet-activating factor and leukotriene B4. Cyclosporines partially inhibited O2- formations induced by NaF and gamma-hexachlorocyclohexane. CsA marginally inhibited PMA-induced O2- formation and lysozyme release. CsA, CsD, and CsH did not inhibit arachidonic acid-induced O2- formation and its potentiation by NaF or stable guanine nucleotides in a cell-free system from DMSO-differentiated HL-60 cells. CsH partially inhibited binding of FML [3H]P to formyl peptide receptors in membranes from DMSO- or dibutyryl cAMP-differentiated HL-60 cells. Our data show that: 1) cyclosporines differentially inhibit activation of human neutrophils; and 2) CsH is, indeed, not immunologically inactive but is a potent and effective inhibitor of FMLP-induced O2- formation. 3) CsH interferes with agonist binding to formyl peptide receptors and in addition, cyclosporines may also act at sites distal to chemoattractant receptors.     

Rac2 regulates neutrophil chemotaxis, superoxide production, and myeloid colony formation through multiple distinct effector pathways

Polymorphonuclear neutrophils (PMN) are an important component of the innate immune system. We have shown previously that migration and superoxide (O2*-) production, as well as some kinase signaling pathways are compromised in mice deficient in the Ras-related Rho GTPase Rac2. In this study, we demonstrate that Rac2 controls chemotaxis and superoxide production via distinct pathways and is critical for development of myeloid colonies in vitro. The Rac2 mutants V36A, F37A, and N39A all bind to both Pak1 and p67(phox), yet are unable to rescue superoxide production and chemotaxis when expressed in Rac2-/- PMN. In contrast, the N43A mutant, which binds to Por1 (Arfaptin 2), p67phox, and Pak1, is able to rescue superoxide production but not chemotaxis. The F37A mutant, demonstrated to have reduced binding to Por1, shows reduced rescue of fMLP-induced chemotaxis. Finally, the Rac2Y40C mutant that is defective in binding to all three potential downstream effectors (Pak1, p67phox, and Por1) is unable to rescue chemotaxis, motility, or superoxide production, but is able to rescue defective growth of myeloid colonies in vitro. These findings suggest that binding to any single effector is not sufficient to rescue the distinct cellular phenotypes of Rac2-/- PMN, implicating multiple, distinct, and potentially parallel effector pathways.     

Dissociation between aggregation and superoxide production in human granulocytes

Aggregation and the activation of the granulocyte (PMN) superoxide (O2-) generating system occur when certain stimuli are added to resting cells. It had previously been postulated that PMN aggregation is essential for maximal O2- production. This study was undertaken to test the hypothesis that PMN aggregation is required for full expression of PMN O2- production. We examined aggregation and O2- production induced by four stimuli; concanavalin A (Con A), phorbol myristate acetate (PMA), N-formylmethionyl-leucyl-phenylalanine (FMLP), and ionophore A23187. Cytochalasin B enhanced aggregation by all four stimuli but only enhanced the rate of O2- production by Con A; 2-deoxyglucose inhibited aggregation by all stimuli. Dissociation of PMN aggregation and O2- production was achieved by using NEM, TPCK, and divalent cations. NEM and TPCK prevent Con A-induced O2- production but have no effect on Con A-induced aggregation. PMA-stimulated PMN generate O2- in the presence or absence of Ca++ and Mg++. In contrast, PMA stimulated maximum PMN aggregation only in the presence of both Ca++ and Mg++. Thus PMN can generate O2- without aggregating, and PMN can aggregate without producing O2-. PMN from patients with chronic granulomatous disease do not generate O2- or undergo membrane potential depolarization in response to PMA. These PMN aggregated when stimulated with PMA, providing evidence that depolarization is not required for PMN aggregation. We conclude that aggregation and the activation of the O2- generating system, though temporally related, are not necessarily causally related.     

The temporal relationship between protein phosphatase, mitochondrial cytochrome c release, and caspase activation in apoptosis

Apoptosis is mediated by members of the caspase family of proteases which can be activated by release of mitochondrial cytochrome c. Additional members of the caspase family are activated at the cell surface in response to direct stimulus from the external environment such as by activation of the Fas receptor. It has been suggested that these upstream caspases directly activate the downstream caspases which would obviate a role for cytochrome c in apoptosis induced by the Fas receptor. We demonstrate that cytochrome c is released from mitochondria of Jurkat cells in response to both staurosporine and an agonistic anti-Fas antibody and that only the latter is inhibited by the caspase inhibitor z-VAD-FMK. This suggests that an upstream caspase such as caspase-8 is required for the Fas-mediated release of mitochondrial cytochrome c. The protein phosphatase inhibitor calyculin A prevented cytochrome c release and apoptosis induced by both agents, suggesting that release of cytochrome c is required in both models. Zinc, once thought of as an endonuclease inhibitor, has previously been shown to prevent the activation of caspase-3. We show that zinc prevents the activation of downstream caspases and apoptosis induced by both insults, yet does not prevent release of mitochondrial cytochrome c. The ability of calyculin A and zinc to prevent DNA digestion implies that the mitochondrial pathway is important for induction of apoptosis by both agents. These results do not support an alternative pathway in which caspase-8 directly activates caspase-3. These results also demonstrate that a critical protein phosphatase regulates the release of cytochrome c and apoptosis induced by both insults.     

Activation of NADPH oxidase and phospholipase D in permeabilized human neutrophils. Correlation between oxidase activation and phosphatidic acid production.

A major function of human neutrophils (PMN) during inflammation is formation of oxygen radicals through activation of the respiratory burst enzyme, NADPH oxidase. Stimulus-induced production of both phosphatidic acid (PA) and diglyceride (DG) has been suggested to mediate oxidase activity; however, transductional mechanisms and cofactor requirements necessary for activation are poorly defined. We have utilized PMN permeabilized with Staphylococcus aureus alpha-toxin to elucidate the signal pathway involved in eliciting oxidase activity and to investigate whether PA or DG act as second messengers. PMN were permeabilized in cytoplasmic buffer supplemented with ATP and EGTA for 15 min before addition of NADPH and various cofactors. Oxidase activation was assessed by superoxide dismutase inhibitable reduction of ferricytochrome C; PA and DG levels were measured by radiolabeled product formation or by metabolite mass formation. Both superoxide (O2-) and PA formation were initiated by 10 microM GTP gamma S; addition of cytosolic levels of calcium ions (Ca2+, 120 nM) enhanced O2- and PA formation 1.5-2 fold. DG levels showed little change during these treatments. PA formation preceded O2- production and varying GTP gamma S levels had parallel effects on O2- and PA formation. However, while PA formation and oxidase activation occurred in tandem at Ca2+ levels of < 1 microM, higher calcium enhanced PA formation but inhibited O2- production. Removal of ATP completely blocked O2- production but had little effect on PA formation; in contrast, if ATP was replaced with ATP gamma S, parallel production of PA and O2- occurred in the absence of other cofactors. Finally, while inhibition of PA production by ethanol pretreatment led to inhibition of O2- formation in PMN treated with GTP gamma S alone, in cells stimulated with a combination of GTP gamma S and Ca2+, ethanol continued to inhibit PA formation but had no effect on O2- production. Our results do not support a role for DG in the signal transduction path leading to oxidase activation and, while we show a close correlation between oxidase activation and PA production under many physiologic conditions, we also demonstrate that PA is not sufficient to induce oxidase activation and O2- formation can occur when PA production is inhibited.     

The activation of human neutrophil gelatinase

To study the mechanisms of activation of human neutrophil gelatinase, the enzyme has been purified using a combination of chromatography on a DEAE-Sephacel and a gelatin-peptide-Sepharose column. On reducing SDS-polyacrylamide-gel electrophoresis the purified gelatinase ran as a single band of about 94,000 Da, and had a specific activity of 5624.4 units/mg of enzyme protein. When latent gelatinase was treated with trypsin, cathepsin G, neutrophil elastase, HgCl2 or urea, its activity was enhanced and the enzyme was processed and converted into species of the lower molecular mass. Upon activation, the protein band of 94,000 Da of reduced latent gelatinase underwent a decrease of about 6,000-12,000 Da. Formation of the species of lower molecular mass during urea activation could be blocked by the addition of EDTA.     

Sequential phospholipase activation in the stimulation of the neutrophil NADPH oxidase

Abstract Stimulation of human neutrophils with the chemotactic peptide fMet-Leu-Phe results in activation of a rapid, transient burst of oxidant secretion, which reaches a maximal rate by about 1 min after stimulation. This phase of oxidant secretion is then followed by intracellular oxidant production, which is detected by luminol chemiluminescence but not by assays such as cytochrome c reduction or scopoletin oxidation. The rapid phase of oxidant secretion requires increases in intracellular free Ca²⁺ and phospholipase A₂ activity, but not the activities of phospholipase D or D or protein kinase C. In contrast, intracellular oxidant production requires the activities of phospholipase D and protein kinase C. A model is thus proposed suggesting the sequential activation of different phospholipases which activate oxidase molecules on the plasma membrane or else from the membranes of specific granules.     

The relationship between exhaled carbon monoxide and human neutrophil function in the Japanese general population

We have evaluated the relationship between exhaled carbon monoxide (CO) level and neutrophil‐related functions such as reactive oxygen species (ROS) production capability, phagocytic activity and serum opsonic activity in the general population. Serum opsonic activity was determined by measuring the effects of serum on neutrophil ROS production capability using lucigenin‐ and luminol‐dependent chemiluminescence (LgCL, LmCL). LgCL is associated with the detection of O₂^?, whereas LmCL mainly detects H₂O₂ and HOCl, which are higher reactive oxygen radicals. In females, exhaled CO level was found to have positive associations with ROS production capability and LgCL. However, the opposite tendency was seen between exhaled CO level and LmCL in both genders. This result suggests that neutrophil ROS production in females may have contributed to oxidative stress, which led to the increases in intrinsic CO and exhaled CO consequently. Such changes then may have inhibited the process of changing reactive oxygen radicals into higher oxidizing potential levels. Copyright © 2010 John Wiley & Sons, Ltd.