Association between leukotriene B4-induced phospholipase D activation and degranulation of human neutrophils

PLD1 rather than PLD2 regulates phorbol-ester-, adhesion-dependent and Fc{gamma}-receptor-stimulated ROS production in neutrophils

The signalling lipid phosphatidic acid (PA) is generated by the hydrolysis of phosphatidylcholine (PC), which is catalysed by phospholipase D (PLD) enzymes. Neutrophils, important cells of the innate immune system, maintain the body's defence against infection. Previous studies have implicated PLD-generated PA in neutrophil function; these have relied heavily on the use of primary alcohols to act as inhibitors of PA production. The recent development of isoform-selective small molecule inhibitors and the generation of a knockout mouse model provide us with accurate tools to study the role of PLDs in neutrophil responses. We show that PLD1 is a regulator of phorbol-ester-, chemoattractant, adhesion-dependent and Fcγ-receptor-stimulated production of reactive oxygen species (ROS) in neutrophils. Significantly we found that this role of PLD is isoform specific: the absence of PLD2 does not negatively affect these processes. Contrary to expectation, other functions required for an efficient immune response operate effectively in Pld2-deficient neutrophils or when both isoforms are inhibited pharmacologically. We conclude that although PLD1 does have important regulatory roles in neutrophils, the field has been confused by the use of primary alcohols; now that gold standard Pld-knockout mouse models are available, previous work might need to be reassessed.

Crystal-induced neutrophil activation. IX. Syk-dependent activation of class Ia phosphatidylinositol 3-kinase

The deposition of monosodium urate (MSU) crystals in the joints of humans leads to an extremely acute, inflammatory reaction, commonly known as gout, characterized by a massive infiltration of neutrophils. Direct interactions of MSU crystals with human neutrophils and inflammatory mediators are crucial to the induction and perpetuation of gout attacks. The intracellular signaling events initiated by the physical interaction between MSU crystals and neutrophils depend on the activation of specific tyrosine kinases (Src and Syk, in particular). In addition, PI-3Ks may be involved. The present study investigates the involvement of the PI-3K family in the mediation of the responses of human neutrophils to MSU crystals. The results obtained indicate that the interaction of MSU crystals with human neutrophils leads to the stimulation of class Ia PI-3Ks by a mechanism that is dependent on the tyrosine kinase Syk. We also found an increase in the amount of p85 associated with the Nonidet P-40-insoluble fraction derived from MSU crystal-stimulated human neutrophils. Furthermore, MSU crystals induce the formation of a complex containing p85 and Syk, which is mediated by the Src family kinases. Finally, evidence is also obtained indicating that the activation of PI-3Ks by MSU crystals is a critical element regulating phospholipase D activation and degranulation of human neutrophils. The latter response is likely to be involved in the joint and tissue damage that occurs in gouty patients.

Lipid mediators as agonists for the resolution of acute lung inflammation and injury

Resolution of acute lung inflammation and injury is an active process; it is not merely the absence of proinflammatory signals. Restoration of homeostasis is coordinated by specific mediators and cellular events. In response to injury and inflammatory stimuli, infiltrating leukocytes and tissue-resident cells interact to generate lipoxins (LXs), which are bioactive eicosanoids derived from arachidonic acid. In contrast to proinflammatory leukotrienes and prostaglandins, LXs display potent antiinflammatory actions. LXA(4) interacts with a G protein-coupled receptor, termed ALX, that transduces counter-regulatory signals in part via intracellular polyisoprenyl phosphate remodeling. Presqualene diphosphate (PSDP) is a polyisoprenyl phosphate in human neutrophils that is rapidly converted to presqualene monophosphate (PSMP) upon cell activation. PSDP, but not PSMP, directly inhibits phospholipase D, phosphoinositol-3 kinase, and superoxide anion generation. LXs block PSDP turnover in neutrophil membranes to prevent proinflammatory responses. Hence, LX and polyisoprenyl phosphate signaling provide a counter-regulatory circuit to promote resolution of acute lung inflammation. LXA(4) and PSDP mimetics have been prepared with potent protective actions in murine models of asthma and acute lung injury.

Effect of priming on activation and localization of phospholipase D-1 in human neutrophils

Phospholipase D (PLD) plays a major role in the activation of the neutrophil respiratory burst. However, the repertoire of PLD isoforms present in these primary cells, the precise mechanism of activation, and the impact of cell priming on PLD activity and localization remain poorly defined. RT-PCR analysis showed that both PLD1 and PLD2 isoforms are expressed in human neutrophils, with PLD1 expressed at a higher level. Endogenous PLD1 was detected by immunoprecipitation and Western blotting, and was predominantly membrane-associated under control and primed/stimulated conditions. Immunofluorescence showed that PLD had a punctate distribution throughout the cell, which was not altered after stimulation by soluble agonists. In contrast, PLD localized to the phagolysosome membrane after ingestion of nonopsonized zymosan particles. We also demonstrate that tumour necrosis factor alpha greatly potentiates agonist-stimulated PLD activation, myeloperoxidase release, and superoxide anion generation, and that PLD activation occurs via a phosphatidylinositol 3-kinase-sensitive and brefeldin-sensitive ADP-ribosylation factor GTPase-regulated mechanism. Moreover, propranolol, which causes an increase in PLD-derived phosphatidic acid accumulation, caused a selective increase in agonist-stimulated myeloperoxidase release. Our results indicate that priming is a critical regulator of PLD activation, that the PLD-generated lipid products exert divergent effects on neutrophil functional responses, that PLD1 is the major PLD isoform present in human neutrophils, and that PLD1 actively translocates to the phagosomal wall after particle ingestion.

Selenium deficiency alters the formation of eicosanoids and signal transduction in rat lymphocytes

Previous reports have shown that selenium (Se) nutrition alters the lipoxygenase pathway and mitogenic responses in bovine lymphocytes. In order to further understand how Se may alter lymphocyte function, we examined the effects of Se nutrition on arachidonic acid (AA) metabolism and phospholipase D (PLD) activation. Lymphocytes were isolated from the lymph nodes of rats fed either Se-deficient diet (-Se) or Se-supplemented diet (+Se) for 12 weeks. Our results revealed that calcium ionophore A23187-stimulated lymphocytes derived from -Se rats produced significantly less prostaglandins (PGs) than those obtained from +Se rats. Phospholipase D (PLD) activation by 12-O-tetradecanoylphorbol-13-acetate (TPA) was significantly lower in lymphocytes obtained from -Se rats when compared to cells from +Se rats. Furthermore, the addition of PGE2, PGD2 or PGF2alpha to suspended lymphocytes from -Se rats significantly enhanced PLD activity. The effects of TPA and PGE2 on PLD activation were additive. However, the addition of PGE2 abolished the significant difference in PLD activation between -Se and +Se cells observed in response to TPA alone. Based on these results, we postulate that dietary Se status plays an important role in the regulation of AA metabolism that subsequently affects PLD activation.

Selective induction of phospholipase D1 in pathogen-activated human monocytes

Phospholipase D (PLD) activation is part of the complex signalling cascade induced during phagocyte activation. Two PLD isoforms have been cloned, but their role in phagocyte functions is still poorly defined. We report that resting fresh circulating human monocytes expressed PLD1. PLD1 protein expression was rapidly down-regulated during cell culture. Lipopolysaccharide and pathogen-derived agonists (Candida albicans, arabinoside-terminated lipoarabinomannan and Gram-positive bacteria, but not mannose-capped lipoarabinomannan or double-stranded RNA) strongly induced PLD1 expression at both the mRNA and protein levels. Pro-inflammatory cytokines [interleukin (IL)-1beta and tumour necrosis factor alpha] had only a weak effect, whereas immune cytokines (IL-6 and interferon gamma), anti-inflammatory cytokines (IL-13 and IL-10) and chemoattractants (fMet-Leu-Phe and macrophage chemoattractant protein 1) were inactive. None of the agonists tested induced significant changes in the basal expression of PLD2 mRNA. Consistent with PLD1 up-regulation was the observation that PLD enzymic activity was higher in monocytes treated with active-pathogen-derived agonists than in control cells, when stimulated with PMA or with chemotactic agonists (fMet-Leu-Phe and C5a). Thus PLD2 seems to be a constitutive enzyme in circulating monocytes. Conversely, PLD1 is an inducible protein, rapidly regulated during culture conditions and selectively induced during cell activation. Therefore PLD1 might have a relevant role in immune responses against pathogens and in chronic inflammation.

A novel polyisoprenyl phosphate signaling cascade in human neutrophils

Activation of neutrophil (PMN) surface receptors can evoke inflammation and tissue injury via aberrant release of excess effectors. The molecular mechanisms involved in host protection and control of PMN responses have yet to be defined. As Billah and coworkers (1989), and Exton (1997), for example, have pointed out, phospholipase D (PLD) signaling is known to play a pivotal role in PMN activation. Here, we determined the relationship between polyisoprenyl phosphate (PIPP) remodeling and PLD signaling and their impact in activation of PMN receptors by "pro-inflammatory" (leukotriene B4), and "anti-inflammatory" (aspirin-triggered lipoxinA4) ligands. Activation of the leukotriene B4 receptor initiated a rapid (within seconds) decrement in presqualene disphosphate (PSDP), activation of PLD and production of superoxide anions. This contrasts with activation of the LXA4 receptor by an aspirin-triggered lipoxin A4 mimetic that before leukotriene B4 gave an inverse relationship with rapidly increasing PSDP levels, and inhibition of both PLD activity and superoxide generation. PSDP proved to be a potent and direct-acting inhibitor of PLD (rhPLD1b:Ki = 5.9 nM), a property not shared by structurally related endogenous lipids. This PIPP also interacted with Src homology domains, selectively targeting SH2 and not SH3 domains. These results indicate a role for ligand-driven rapid PIPP remodeling as an early switch and "stop" signaling event that controls PMN. Moreover, they indicate that PSDP directly down-regulates PMN signaling events via select protein-target interactions controlling intracellular responses relevant in inflammation.

Novel effects of propranolol. Release of internal Ca(2+) followed by activation of capacitative Ca(2+) entry in Madin Darby canine kidney cells

The effect of propranolol on Ca(2+) signalling in Madin Darby canine kidney (MDCK) cells was investigated by using fura-2 as a Ca(2+) probe. Propranolol increased cytosolic free Ca(2+) levels ([Ca(2+)](i)) in a concentration-dependent manner between 0.1 and 1 mM. The response was partly inhibited by external Ca(2+) removal. In Ca(2+)-free medium pretreatment with 0.2 mM propranolol partly inhibited the [Ca(2+)](i) rise induced by 1 microM thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+) pump; but pretreatment with thapsigargin abolished propranolol-induced Ca(2+) release. Addition of 3 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment with 0.2 mM propranolol in Ca(2+)-free medium. Propranolol (0.2 mM) inhibited 25% of thapsigargin-induced capacitative Ca(2+) entry. Suppression of 1,4,5-trisphosphate (IP(3)) formation by 2 microM U73122, a phospholipase C inhibitor, did not alter 0.2 mM propranolol-induced internal Ca(2+) release. Propranolol (1 mM) also increased [Ca(2+)](i) in human neutrophils. Collectively, we have found that 0.2 mM propranolol increased [Ca(2+)](i) in MDCK cells by releasing Ca(2+) from thapsigargin-sensitive Ca(2+) stores in an IP(3)-independent manner, followed by Ca(2+) influx from external space. Independently, propranolol was able to inhibit thapsigargin-induced capacitative Ca(2+) entry.

Polyisoprenyl phosphate (PIPP) signaling regulates phospholipase D activity: a 'stop' signaling switch for aspirin-triggered lipoxin A4

It is of wide interest to understand how opposing extracellular signals (positive or negative) are translated into intracellular signaling events. Receptor-ligand interactions initiate the generation of bioactive lipids by human neutrophils (PMN), which serve as signals to orchestrate cellular responses important in host defense and inflammation. We recently identified a novel polyisoprenyl phosphate (PIPP) signaling pathway and found that one of its components, presqualene diphosphate (PSDP), is a potent negative intracellular signal in PMN that regulates superoxide anion generation by several stimuli, including phosphatidic acid. We determined intracellular PIPP signaling by autocoids with opposing actions on PMN: leukotriene B4 (LTB4), a potent chemoattractant, and lipoxin A4 (LXA4), a 'stop signal' for recruitment. LTB4 receptor activation initiated a rapid decrease in PSDP levels concurrent with activation of PLD and cellular responses. In sharp contrast, activation of the LXA4 receptor reversed LTB4-initiated PSDP remodeling, leading to an accumulation of PSDP and potent inhibition of both PLD and superoxide anion generation. Thus, an inverse relationship was established for PSDP levels and PLD activity with two PMN ligands that evoke opposing responses. In addition, PSDP directly inhibited both isolated human recombinant (Ki = 6 nM) and plant (Ki = 20 nM) PLD. Together, these findings link PIPP remodeling to intracellular regulation of PMN function and suggest a role for PIPPs as lipid repressors in signal transduction, a novel mechanism that may also explain aspirin's suppressive actions in vivo in cell signaling.

Adhesion-Dependent Release of Elastase From Human Neutrophils in a Novel, Flow-Based Model: Specificity of Different Chemotactic Agents

Neutrophils must adhere to the vessel wall, migrate, and degranulate in an ordered manner to perform their protective function. Disruption of these processes may be pathogenic. Current knowledge of the degranulation process is derived almost exclusively from studies on neutrophils in suspension, in which priming with the nonphysiological agent cytochalasin B is necessary to obtain elastase release in response to activating agents. To avoid this, we have adopted a different approach. Using a novel flow-based adhesion system, we have been able to quantify the release of elastase from the primary granules of activated neutrophils adherent to immobilized platelets or purified receptors without priming. Comparing stimuli, formyl tripeptide (fMLP), interleukin-8 (IL-8), activated complement fragment C5a, and platelet-activating factor (PAF) all induced rapid conversion to CD11b/CD18 (MAC-1) -mediated stationary adhesion when perfused over neutrophils already rolling on platelet monolayers or purified P-selectin. However, fMLP, C5a, and IL-8, but not PAF, induced release of elastase from the adherent cells in minutes. Neutrophils stimulated in suspension showed little degranulation. Treatment of neutrophils with an inhibitor of 5-lipoxygenase–activating protein (MK886) and thus synthesis of leukotrienes (LTs) or with an antagonist of the LTB4 receptor (LY223982) blocked the release of elastase. This indicated that endogenous synthesis of 5-lipoxygenase products such as LTs and autocrine activation of neutrophils was required for fMLP-driven elastase release. We hypothesize that the differential ability of PAF and fMLP to induce elastase release from surface-adherent neutrophils could arise from differential ability to generate leukotrienes, such as LTB4, and would be an appropriate mechanism for the control of elastase release during inflammation in vivo, where it is important that cytotoxic agents are not released until activated neutrophils have migrated into the extravascular tissues.

Adhesion-Dependent Release of Elastase From Human Neutrophils in a Novel, Flow-Based Model: Specificity of Different Chemotactic Agents

Neutrophils must adhere to the vessel wall, migrate, and degranulate in an ordered manner to perform their protective function. Disruption of these processes may be pathogenic. Current knowledge of the degranulation process is derived almost exclusively from studies on neutrophils in suspension, in which priming with the nonphysiological agent cytochalasin B is necessary to obtain elastase release in response to activating agents. To avoid this, we have adopted a different approach. Using a novel flow-based adhesion system, we have been able to quantify the release of elastase from the primary granules of activated neutrophils adherent to immobilized platelets or purified receptors without priming. Comparing stimuli, formyl tripeptide (fMLP), interleukin-8 (IL-8), activated complement fragment C5a, and platelet-activating factor (PAF) all induced rapid conversion to CD11b/CD18 (MAC-1) -mediated stationary adhesion when perfused over neutrophils already rolling on platelet monolayers or purified P-selectin. However, fMLP, C5a, and IL-8, but not PAF, induced release of elastase from the adherent cells in minutes. Neutrophils stimulated in suspension showed little degranulation. Treatment of neutrophils with an inhibitor of 5-lipoxygenase–activating protein (MK886) and thus synthesis of leukotrienes (LTs) or with an antagonist of the LTB4 receptor (LY223982) blocked the release of elastase. This indicated that endogenous synthesis of 5-lipoxygenase products such as LTs and autocrine activation of neutrophils was required for fMLP-driven elastase release. We hypothesize that the differential ability of PAF and fMLP to induce elastase release from surface-adherent neutrophils could arise from differential ability to generate leukotrienes, such as LTB4, and would be an appropriate mechanism for the control of elastase release during inflammation in vivo, where it is important that cytotoxic agents are not released until activated neutrophils have migrated into the extravascular tissues.

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

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

ADP-ribosylation-factor-regulated phospholipase D activity localizes to secretory vesicles and mobilizes to the plasma membrane following N-formylmethionyl-leucyl-phenylalanine stimulation of human neutrophils

Phospholipase D (PLD) is responsible for the hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. Human neutrophils contain PLD activity which is regulated by the small GTPases, ADP-ribosylation factor (ARF) and Rho proteins. In this study we have examined the subcellular localization of the ARF-regulated PLD activity in non-activated neutrophils and cells 'primed' with N-formylmethionyl-leucyl-phenylalanine (fMetLeuPhe). We report that PLD activity is localized at the secretory vesicles in control cells and is mobilized to the plasma membrane upon stimulation with fMetLeuPhe. We conclude that the ARF-regulated PLD activity is translocated to the plasma membrane by secretory vesicles upon stimulation of neutrophils with fMetLeuPhe in inflammatory/priming doses. We propose that this relocalization of PLD is important for the subsequent events occurring during neutrophil activation.

Effect of wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) on N-formyl-methionyl-leucyl-phenylalanine-induced phospholipase D activation in differentiated HL60 cells: possible involvement of phosphatidylinositol 3-kinase in phospholipase D activation

Phospholipase D (PLD) plays an important role in neutrophil activation. However, despite various proposed mechanisms, its detailed regulatory mechanism is not fully understood. The functional coupling between phosphatidylinositol 3-kinase (PI 3-kinase) and PLD was investigated in N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated human promyelocytic leukemia HL60 cells, using wortmannin, a fungal metabolite that is known as a selective inhibitor for phosphatidylinositol 3-kinase. Treatment of cells with this drug inhibited the formation of both phosphatidylinositol 3,4,5-trisphosphate (PIP3), a product of PI 3-kinase, and phosphatidylbutanol (PBut), the specific product of transphosphatidylation due to PLD in the presence of butanol, with similar concentration dependence (IC50 = 30-70 nM). Another PI 3-kinase inhibitor, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) also inhibited PBut formation in a concentration-dependent manner. However, wortmannin failed to inhibit phorbol 12-myristate 13-acetate-induced PLD activation in whole cells and membrane PLD activity in an in vitro assay system, indicating that inhibition of fMLP-induced PLD activation by wortmannin was not due to its direct effect on PLD activity. These results suggest that a major part of inhibition of PLD activation by wortmannin might be mediated through its effect on PI 3-kinase.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Phosphatidic acid and lysophosphatidic acid induce haptotactic migration of human monocytes

The present study was aimed at defining the chemotactic activity of phosphatidic acid, which is rapidly produced by phagocytes in response to chemotactic agonists. Exogenously added phosphatidic acid induced human monocyte directional migration across polycarbonate filters with an efficacy (number of cell migrated) comparable to that of "classical" chemotactic factors. In lipid specificity studies, activity of phosphatidic acid decreased with increasing acyl chain length but was restored by introducing unsaturation in the acyl chain with the most active form being the natural occurring 18:0,20:4-phosphatidic acid. Lysophosphatidic acid was also active in inducing monocyte migration. No other phospholipid and lysophospholipid tested was effective in this response. Monocyte migration was regulated by a gradient of phosphatidic acid and lysophosphatidic acid bound to the polycarbonate filter, in the absence of detectable soluble chemoattractant. Migration was also observed if phospholipids were bound to fibronectin-coated polycarbonate filters. Thus, phosphatidic acid and lysophosphatidic acid, similarly to other physiological chemoattractants (e.g. C5a and interleukin-8), induce cell migration by an haptotactic mechanism. Phosphatidic acid caused a rapid increase of filamentous actin and, at higher concentrations, induced a rise of intracellular calcium concentration. Monocyte migration to phosphatidic acid and lysophosphatidic acid, but not to diacylglycerol, was inhibited in a concentration-dependent manner by Bordetella pertussis toxin, while cholera toxin was ineffective. In the chemotactic assay, phosphatidic acid and lysophosphatidic acid induced a complete homologous desensitization and only partially cross-desensitized one with each other, or with diacyl-glycerol and monocyte chemotactic protein-1. Suramine inhibited monocyte chemotaxis with a different efficiency phosphatidic acid > lysophosphatidic acid" diacyl-glycerol On the contrary, monocyte chemotactic protein-1-induced chemotaxis was not affected by the drug. Collectively, these data show that phosphatidic acid induces haptotactic migration of monocytes that is at least in part receptor-mediated. These results support a role for phosphatidic acid and lysophosphatidic acid in the regulation of leukocyte accumulation into tissues.

Early signalling events implicated in leukotriene B4-induced activation of the NADPH oxidase in eosinophils: role of Ca2+, protein kinase C and phospholipases C and D

The early signalling events that may ultimately contribute to the assembly and subsequent activation of the NADPH oxidase in guinea-pig peritoneal eosinophils were investigated in response to leukotriene B4 (LTB4). LTB4 promoted a rapid, transient and receptor-mediated increase in the rate of H2O2 generation that was potentiated by R 59 022, a diradylglycerol (DRG) kinase inhibitor, implicating protein kinase C (PKC) in the genesis of this response. This conclusion was supported by the finding that the PKC inhibitor, Ro 31-8220, attenuated (by about 30%) the peak rate of LTB4-induced H2O2 generation under conditions where the same response evoked by 4 beta-phorbol 12,13-dibutyrate (PDBu) was inhibited by more than 90%. Paradoxically, Ro 31-8220 doubled the amount of H2O2 produced by LTB4 which may relate to the ability of PKC to inhibit cell signalling through phospholipase C (PLC). Indeed, Ro 31-8220 significantly enhanced LTB4-induced Ins(1,4,5)P3 accumulation and the duration of the Ca2+ transient in eosinophils. Experiments designed to assess the relative importance of DRG-mobilizing phospholipases in LTB4-induced oxidase activation indicated that phospholipase D (PLD) did not play a major role. Thus, although H2O2 generation was abolished by butan-1-ol, this was apparently unrelated to the inhibition of PLD, as LTB4 failed to stimulate the formation of Ptd[3H]BuOH in [3H]butan-1-ol-treated eosinophils. Rather, the inhibition was probably due to the ability of butan-1-ol to increase the eosinophil cyclic AMP content. In contrast, Ca(2+)- and PLC-driven mechanisms were implicated in H2O2 generation, as LTB4 elevated the Ins(1,4,5)P3 content and intracellular free Ca2+ concentration in intact cells, and cochelation of extracellular and intracellular Ca2+ significantly attenuated LTB4-induced H2O2 generation. Pretreatment of eosinophils with wortmannin did not affect LTB4-induced H2O2 production at concentrations at which it abolished the respiratory burst evoked by formylmethionyl-leucylphenylalanine in human neutrophils. Collectively, these data suggest that LTB4 activates the NADPH oxidase in eosinophils by PLD- and PtdIns 3-kinase-independent mechanisms that involve Ca2+, PLC and PKC. Furthermore, the activation of additional pathways that do not require Ca2+ is also suggested by the finding that LTB4 evoked a significant respiratory burst in Ca(2+)-depleted cells.

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.

Eicosanoids, fatty acids and neutrophils: their relevance to the pathophysiology of disease

PUFA and their eicosanoid metabolites are potent biological modifiers. They have beneficial effects in a number of diseases, which may result in part from their direct actions on neutrophils as well as from their ability to modulate eicosanoid biosynthesis. A consideration of their interactions with other cell types, e.g. lymphocytes and macrophages, is beyond the scope of this review. Small alterations in structure can result in large changes in the neutrophil response. This will have important implications for the further development and use of fatty acids for therapeutic purposes.

Tamoxifen stimulates phospholipase D activity by an estrogen receptor-independent mechanism

The effects of tamoxifen (TAM), a widely used agent in the treatment of breast cancer, were examined on phospholipase D (PLD)-mediated phospholipid hydrolysis. In drug-sensitive MCF-7 human breast carcinoma cells TAM, similar to several well-established activators of PLD, had no effect on phospholipid hydrolysis. In an estrogen receptor-deficient multidrug-resistant subline of MCF-7 cells, TAM preferentially stimulated the hydrolysis of phosphatidylethanolamine; two-fold stimulation required 2.5 or 5 microM TAM in the absence or presence of serum, respectively. In NIH 3T3 fibroblasts significant (4- to 4.8-fold) stimulation of phosphatidylethanolamine and phosphatidylcholine hydrolysis in the presence of serum required 10 microM TAM. These data establish that TAM can stimulate PLD activity by an estrogen receptor-independent mechanism.

CoA-independent transacylase activity is increased in human neutrophils after treatment with tumor necrosis factor alpha

CoA-independent transacylase (CoA-IT) appears to play a critical role in lipid mediator generation by rapidly moving arachidonate (AA) between phospholipid pools during cell activation. Tumor necrosis factor-alpha (TNF) pretreatment of human neutrophils increases agonist-induced production of inflammatory mediators. The current study tested if the TNF-induced increase in lipid mediator production may be, in part, due to altered CoA-IT activity. Neutrophils were treated with TNF (250 U/ml, 30 min), homogenates prepared, and CoA-IT activity measured by the ability of these homogenates to acylate 1-[3H]alkyl-2-lyso-sn-glycero-3-phosphocholine (GPC). There was an increased CoA-IT activity, from 9.1 +/- 1.1 to 13.7 +/- 1.4 pmol/mg per min in control vs. TNF-treated samples, respectively. Varying the concentration of 1-alkyl-2-lyso-GPC revealed an increased CoA-IT activity in microsomes that was due to an increased Vmax, from 26 to 54 pmol/mg per min. The ability of TNF to increase CoA-IT activity was concentration-dependent, with maximal response observed at 25 U/ml. This effect on CoA-IT appears to be specific, in that TNF treatment of neutrophils had no effect on CoA-dependent acylation of 1-acyl-2-lyso-sn-glycero-3-phosphocholine, using either AA-CoA or linolenoyl-CoA as substrates. In the intact cell, the movement of [3H]AA from other phospholipids into PE in fMLP-stimulated neutrophils was greatly enhanced after TNF treatment, demonstrating a functional consequence of increased CoA-IT activity. In addition, TNF treatment doubled platelet-activating factor production in response to the chemotactic peptide fMLP, as measured by [3H]acetate incorporation, while the response to A23187 remained unchanged. Taken together, these results provide the first evidence of modulation of CoA-IT activity by a proinflammatory cytokine and suggest that one mechanism for augmented lipid mediator formation is through increases in CoA-IT activity.

Phorbol ester selectively stimulates the phospholipase D-mediated hydrolysis of phosphatidylethanolamine in multidrug-resistant MCF-7 human breast carcinoma cells

The phospholipase D (PLD)-mediated synthesis of phosphatidylethanol (PtdEtOH) and the hydrolysis of phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho) were examined in drug-sensitive and multidrug-resistant lines of MCF-7 human breast carcinoma cells. In drug-sensitive (MCF-7/WT) cells, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) failed to enhance either the synthesis of PtdEtOH or the hydrolysis of either phospholipid. In the drug-resistant (MCF-7/MDR) cells, 100 nM PMA greatly enhanced both the synthesis of PtdEtOH (approximately 21-fold) and the hydrolysis of PtdEtn (approximately 29-fold), but had no effect on the hydrolysis of PtdCho. The PLD activators sphingosine and H2O2 were found to elicit only a slight (1.28-1.4-fold) stimulatory effect on PtdCho hydrolysis in both the MCF-7/WT and MCF-7/MDR cell types, and had only a small effect on PtdEtn hydrolysis in the MCF-7/WT cells as well. However, these agents significantly (approximately 2.6-3.5-fold) stimulated PtdEtn hydrolysis in the MCF-7/MDR cells. These data indicate that MCF-7/MDR cells contain a PtdEtn-specific PLD activity which can be selectively stimulated by PMA, sphingosine and H2O2.