Role of Ca2+-independent phospholipase A2 on arachidonic acid release induced by reactive oxygen species

Previous studies have shown that reactive oxygen species (ROS) enhance arachidonic acid (AA) release and the subsequent AA metabolism in macrophages. The purpose of this study was determined the implication of phospholipases A2 (PLA2s) in these events. Our results show that oxidative stress induced by exogenous adding of hydrogen peroxide or superoxide anion in macrophage RAW 264.7 and mouse peritoneal macrophage cultures caused a marked enhancement of calcium-independent PLA2 (iPLA2) activity,whereas the increment of secreted PLA2 (sPLA2) and calcium-dependent cytosolic PLA2 (cPLA2) activities were slight. This increase of iPLA2 activity by ROS was rapid and dose-dependent. ROS also induced a significant [3H] arachidonic acid (AA) release. The iPLA2 selective inhibitor, bromoenol lactone, almost completely suppressed the mobilization of [3H]AA induced by ROS whereas antisense oligonucleotide against cPLA2 did not have any appreciable effect. Thus, our data show that iPLA2 activity is involved in the mechanism by which ROS increases the availability of free AA in macrophages RAW 264.7. Moreover, the protein kinase C (PKC) inhibitor, calphostin C, and calcium chelators had no effect on the [3H]AA release induced by ROS, suggesting this is a regulatory role of iPLA2.     

Decrease in cytosolic phospholipase A2alpha mRNA levels by reactive oxygen species via MAP kinase pathways in PC12 cells: effects of dopaminergic neurotoxins

Excess production of reactive oxygen species (ROS), including H2O2, leads to neuronal death in pathological conditions. Although ROS stimulates alpha-type cytosolic phospholipase A2 (cPLA2alpha) activity, their role in cPLA2alpha expression has not been elucidated. We investigated the effect of ROS on cPLA2alpha mRNA levels and signaling pathways in rat pheochromocytoma PC12 cells. Treatment with H2O2 and xanthine-xanthine oxidase (X/XO) for 4 h decreased cPLA2alpha mRNA levels without changing the mRNA levels of other tested proteins. H2O2 and X/XO caused cell toxicity not after 4 h but 24 h after their addition. The H2O2-induced decrease in cPLA2alpha mRNA levels was inhibited in cells treated with N-acetyl-cysteine and selective inhibitors of mitogen-activated protein kinase (MAPK) pathways (extracellular signal-regulated kinase and p38 MAPK). Treatment with dopaminergic neurotoxins, including 1,2,3,4-tetrahydroisoquinoline (TIQ)-inducing ROS formation, decreased cPLA2alpha mRNA levels. These findings suggest that ROS decreases cPLA2alpha mRNA levels via MAPK pathways in PC12 cells.     

Naegleria fowleri amoebae express a membrane-associated calcium-independent phospholipase A(2)

Naegleria fowleri, a free-living amoeba, is the causative agent of primary amoebic meningoencephalitis. Previous reports have demonstrated that N. fowleri expresses one or more forms of phospholipase A(2) (PLA(2)) and that a secreted form of this enzyme is involved in pathogenesis. However, the molecular nature of these phospholipases remains largely unknown. This study was initiated to determine whether N. fowleri expresses analogs of the well-characterized PLA(2)s that are expressed by mammalian macrophages. Amoeba cell homogenates contain a PLA(2) activity that hydrolyzes the substrate that is preferred by the 85 kDa calcium-dependent cytosolic PLA(2), cPLA(2). However, unlike the cPLA(2) enzyme in macrophages, this activity is largely calcium-independent, is constitutively associated with membranes and shows only a modest preference for phospholipids that contain arachidonate. The amoeba PLA(2) activity is sensitive to inhibitors that block the activities of cPLA(2)-alpha and the 80 kDa calcium-independent PLA(2), iPLA(2), that are expressed by mammalian cells. One of these compounds, methylarachidonyl fluorophosphonate, partially inhibits the constitutive release of [(3)H]arachidonic acid from pre-labeled amoebae. Together, these data suggest that N. fowleri expresses a constitutively active calcium-independent PLA(2) that may play a role in the basal phospholipid metabolism of these cells.     

Polyunsaturated fatty acids potentiate interleukin-1-stimulated arachidonic acid release by cells overexpressing type IIA secretory phospholipase A2.

By analyzing human embryonic kidney 293 cell transfectants stably overexpressing various types of phospholipase A2 (PLA2), we have shown that polyunsaturated fatty acids (PUFAs) preferentially activate type IIA secretory PLA2 (sPLA2-IIA)-mediated arachidonic acid (AA) release from interleukin-1 (IL-1)-stimulated cells. When 293 cells prelabeled with 13H]AA were incubated with exogenous PUFAs in the presence of IL-1 and serum, there was a significant increase in [3H]AA release (in the order AA > linoleic acid > oleic acid), which was augmented markedly by sPLA2-IIA and modestly by type IV cytosolic PLA2 (cPLA2), but only minimally by type VI Ca2(+)-independent PLA2, overexpression. Transfection of cPLA2 into sPLA2-IIA-expressing cells produced a synergistic increase in IL-1-dependent [3H]AA release and subsequent prostaglandin production. Our results support the proposal that prior production of AA by cPLA2 in cytokine-stimulated cells destabilizes the cellular membranes, thereby rendering them more susceptible to subsequent hydrolysis by sPLA2-IIA.     

Cross-talk between cytosolic phospholipase A2 alpha (cPLA2 alpha) and secretory phospholipase A2 (sPLA2) in hydrogen peroxide-induced arachidonic acid release in murine mesangial cells: sPLA2 regulates cPLA2 alpha activity that is responsible for arachidonic acid release

Oxidant stress and phospholipase A2 (PLA2) activation have been implicated in numerous proinflammatory responses of the mesangial cell (MC). We investigated the cross-talk between group IValpha cytosolic PLA2 (cPLA2alpha) and secretory PLA2s (sPLA2s) during H2O2-induced arachidonic acid (AA) release using two types of murine MC: (i). MC+/+, which lack group IIa and V PLA2s, and (ii). MC-/-, which lack groups IIa, V, and IValpha PLA2s. H2O2-induced AA release was greater in MC+/+ compared with MC-/-. It has been argued that cPLA2alpha plays a regulatory role enhancing the activity of sPLA2s, which act on phospholipids to release fatty acid. Group IIa, V, or IValpha PLA2s were expressed in MC-/- or MC+/+ using recombinant adenovirus vectors. Expression of cPLA2alpha in H2O2-treated MC-/- increased AA release to a level approaching that of H2O2-treated MC+/+. Expression of either group IIa PLA2 or V PLA2 enhanced AA release in MC+/+ but had no effect on AA release in MC-/-. When sPLA2 and cPLA2alpha are both present, the effect of H2O2 is manifested by preferential release of AA compared with oleic acid. Inhibition of the ERK and protein kinase C signaling pathways with the MEK-1 inhibitor, U0126, and protein kinase C inhibitor, GF 1092030x, respectively, and chelating intracellular free calcium with 1,2-bis(2-aminophenoyl)ethane-N,N,N',N'-tetraacetic acid-AM, which also reduced ERK1/2 activation, significantly reduced H2O2-induced AA release in MC+/+ expressing either group IIa or V PLA2s. By contrast, H2O2-induced AA release was not enhanced when ERK1/2 was activated by infection of MC+/+ with constitutively active MEK1-DD. We conclude that the effect of group IIa and V PLA2s on H2O2-induced AA release is dependent upon the presence of cPLA2alpha and the activation of PKC and ERK1/2. Group IIa and V PLA2s are regulatory and cPLA2alpha is responsible for AA release.     

Phospholipase A2, reactive oxygen species, and lipid peroxidation in cerebral ischemia

Ischemic stroke is caused by obstruction of blood flow to the brain, resulting in energy failure that initiates a complex series of metabolic events, ultimately causing neuronal death. One such critical metabolic event is the activation of phospholipase A2 (PLA2), resulting in hydrolysis of membrane phospholipids and release of free fatty acids including arachidonic acid, a metabolic precursor for important cell-signaling eicosanoids. PLA2 enzymes have been classified as calcium-dependent cytosolic (cPLA2) and secretory (sPLA2) and calcium-independent (iPLA2) forms. Cardiolipin hydrolysis by mitochondrial sPLA2 disrupts the mitochondrial respiratory chain and increases production of reactive oxygen species (ROS). Oxidative metabolism of arachidonic acid also generates ROS. These two processes contribute to formation of lipid peroxides, which degrade to reactive aldehyde products (malondialdehyde, 4-hydroxynonenal, and acrolein) that covalently bind to proteins/nucleic acids, altering their function and causing cellular damage. Activation of PLA2 in cerebral ischemia has been shown while other studies have separately demonstrated increased lipid peroxidation. To the best of our knowledge no study has directly shown the role of PLA2 in lipid peroxidation in cerebral ischemia. To date, there are very limited data on PLA2 protein by Western blotting after cerebral ischemia, though some immunohistochemical studies (for cPLA2 and sPLA2) have been reported. Dissecting the contribution of PLA2 to lipid peroxidation in cerebral ischemia is challenging due to multiple forms of PLA2, cardiolipin hydrolysis, diverse sources of ROS arising from arachidonic acid metabolism, catecholamine autoxidation, xanthine oxidase activity, mitochondrial dysfunction, activated neutrophils coupled with NADPH oxidase activity, and lack of specific inhibitors. Although increased activity and expression of various PLA2 isoforms have been demonstrated in stroke, more studies are needed to clarify the cellular origin and localization of these isoforms in the brain, their responses in cerebral ischemic injury, and their role in oxidative stress.     

Enzymatic properties of human cytosolic phospholipase A(2)gamma

The enzymatic properties of cytosolic phospholipase A(2)gamma (cPLA(2)gamma), an isoform of 85-kDa group IV cPLA(2)alpha (cPLA(2)alpha) were studied in vitro and when the enzyme was expressed in cells. cPLA(2)gamma expressed in Sf9 cells is associated with membrane. Membranes isolated from [(3)H]arachidonic acid-labeled Sf9 cells expressing cPLA(2)gamma, constitutively release [(3)H]arachidonic acid. The membrane-associated activity is inhibited by the group IV PLA(2) inhibitor methylarachidonyl fluorophosphonate, but not effectively by the group VI PLA(2) inhibitor (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one. cPLA(2)gamma has higher lysophospholipase activity than PLA(2) activity. Purified His-cPLA(2)gamma does not exhibit phospholipase A(1) activity, but sequentially hydrolyzes fatty acid from the sn-2 and sn-1 positions of phosphatidylcholine. cPLA(2)gamma overexpressed in HEK293 cells is constitutively active in isolated membranes, releasing large amounts of oleic, arachidonic, palmitic, and stearic acids; however, basal fatty acid release from intact cells is not increased. cPLA(2)gamma overexpressed in lung fibroblasts from cPLA(2)alpha-deficient mice is activated by mouse serum resulting in release of arachidonic, oleic, and palmitic acids, whereas overexpression of cPLA(2)alpha results primarily in arachidonic acid release.     

Human group IVC phospholipase A2 (cPLA2gamma). Roles in the membrane remodeling and activation induced by oxidative stress

To create the unique properties of a certain cellular membrane, both the composition and the metabolism of membrane phospholipids are key factors. Phospholipase A(2) (PLA(2)), with hydrolytic enzyme activities at the sn-2 position in glycerophospholipids, plays critical roles in maintaining the phospholipid composition as well as producing bioactive lipid mediators. In this study we examined the contribution of a Ca(2+)-independent group IVC PLA(2) isozyme (cPLA(2)gamma), a paralogue of cytosolic PLA(2)alpha (cPLA(2)alpha), to phospholipid remodeling. The enzyme was localized in the endoplasmic reticulum and Golgi apparatus, as seen using green fluorescence fusion proteins. Electrospray ionization mass spectrometric analysis of membrane extracts revealed that overexpression of cPLA(2)gamma increased the proportion of polyunsaturated fatty acids in phosphatidylethanolamine, suggesting that the enzyme modulates the phospholipid composition. We also found that H(2)O(2) and other hydroperoxides induced arachidonic acid release in cPLA(2)gamma-transfected human embryonic kidney 293 cells, possibly through the tyrosine phosphorylation pathway. Thus, we propose that cPLA(2)gamma is constitutively expressed in the endoplasmic reticulum and plays important roles in remodeling and maintaining membrane phospholipids under various conditions, including oxidative stress.     

PLA(2) dependence of diaphragm mitochondrial formation of reactive oxygen species.

Contraction-induced respiratory muscle fatigue and sepsis-related reductions in respiratory muscle force-generating capacity are mediated, at least in part, by reactive oxygen species (ROS). The subcellular sources and mechanisms of generation of ROS in these conditions are incompletely understood. We postulated that the physiological changes associated with muscle contraction (i.e., increases in calcium and ADP concentration) stimulate mitochondrial generation of ROS by a phospholipase A(2) (PLA(2))-modulated process and that sepsis enhances muscle generation of ROS by upregulating PLA(2) activity. To test these hypotheses, we examined H(2)O(2) generation by diaphragm mitochondria isolated from saline-treated control and endotoxin-treated septic animals in the presence and absence of calcium and ADP; we also assessed the effect of PLA(2) inhibitors on H(2)O(2) formation. We found that 1) calcium and ADP stimulated H(2)O(2) formation by diaphragm mitochondria from both control and septic animals; 2) mitochondria from septic animals demonstrated substantially higher H(2)O(2) formation than mitochondria from control animals under basal, calcium-stimulated, and ADP-stimulated conditions; and 3) inhibitors of 14-kDa PLA(2) blocked the enhanced H(2)O(2) generation in all conditions. We also found that administration of arachidonic acid (the principal metabolic product of PLA(2) activation) increased mitochondrial H(2)O(2) formation by interacting with complex I of the electron transport chain. These data suggest that diaphragm mitochondrial ROS formation during contraction and sepsis may be critically dependent on PLA(2) activation.     

Redistribution of cPLA(2) in rat renal tubular cell cultures in response to PCBs

The influence of different polychlorinated biphenyls (PCBs) upon the cytosolic phospholipase A(2) (cPLA(2)) redistribution to the particulate fraction has been investigated in rat renal proximal tubule culture cells. Treatment with Aroclor 1248 increased PLA(2) activity in the particulate fraction in a concentration-dependent manner using two radioactive substrates. However, the activity of PLA(2) in the cytosolic fraction decreased. This work also shows that 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) (a di-ortho-substituted nonplanar congener) can increase the activity of PLA(2) in the particulate fraction and decrease the enzyme activity in the cytosolic fraction. The exposure of cell cultures to 3,3',4,4'-tetrachlorobiphenyl (TCB) (a non-ortho-subtituted planar congener) does not alter PLA(2) activity. These results suggest that PCBs, depending on their planar or nonplanar structures, cause a translocation of the enzyme from the cytosol to membranes. To evaluate this possibility, the contents of immunoreactive cPLA(2) were examined by immunoblot analysis in the high-speed supernatant and the particulate fraction of treated cell cultures. The increases/decreases in the amounts of cPLA(2) protein agree with the increases/decreases of PLA(2) activity previously cited. These data demonstrate that the PCB-stimulated redistribution of cPLA(2) to membranes is associated, at least in part, with the changes detected in the activity of the enzyme.     

Biochemical properties and pathophysiological roles of cytosolic phospholipase A2s

Phospholipase A(2) (PLA(2)) (EC 3.1.1.4) catalyzes hydrolysis of the sn-2 ester bond of glycerophospholipids. The enzyme is essential for the production of two classes of lipid mediators, fatty acid metabolites and lysophospholipid-related lipids, as well as being involved in the remodeling of membrane phospholipids. Among many mammalian PLA(2)s, cytosolic PLA(2)alpha (cPLA(2)alpha) plays a critical role in various physiological and pathophysiological conditions through generating lipid mediators. Here, we summarize the in vivo significance of cPLA(2)alpha, revealed from the phenotypes of cPLA(2)alpha-null mice, and properties of newly discovered cPLA(2) family enzymes. We also briefly introduce a quantitative lipidomics strategy using liquid chromatography-mass spectrometry, a powerful tool for the comprehensive analysis of lipid mediators.     

Stimulation of phospholipase A2 by xanthine oxidase in the rat corpus luteum.

The ability of the superoxide radical (SOR) generated by xanthine oxidase to activate phospholipase A2 (PLA2) was examined in microsomes prepared from luteinized rat ovaries. Treatment of microsomes with xanthine oxidase resulted in a rapid burst in SOR formation followed by an increase in PLA2 activity. Stimulation of PLA2 activity was dose related and similar in microsomes prepared from control or prostaglandin F2 alpha (PGF2 alpha)-treated rats. Activation was inhibited by the antioxidants, vitamin E and nordihydroguaiaretic acid, and by superoxide dismutase and catalase, which metabolize SOR and H2O2 to remove reactive oxygen species from the cell. The stimulation of PLA2 activity by xanthine oxidase was dependent upon the addition of calcium ions, and it was highest in samples in which cytosol was added to membranes. These results indicate that the SOR and/or H2O2 may mediate PLA2 activation, which may be involved in the luteolytic process.     

The role of cytosolic phospholipase A2-alfa in regulation of phagocytic functions

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2alpha was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2alpha also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2alpha-deficient PLB cell model. The novel dual subcellular localization of cPLA2alpha in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2alpha in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.     

Prostaglandin E2 production in astrocytes: regulation by cytokines, extracellular ATP, and oxidative agents

Upregulation and activation of phospholipases A2 (PLA2) and cyclooxygenases (COX) leading to prostaglandin E2(PGE2) production have been implicated in a number of neurodegenerative diseases. In this study, we investigated PGE2 production in primary rat astrocytes in response to agents that activate PLA2 including pro-inflammatory cytokines (IL-1beta, TNFalpha and IFNgamma), the P2 nucleotide receptor agonist ATP, and oxidants (H2O2 and menadione). Exposure of astrocytes to cytokines resulted in a time-dependent increase in PGE2 production that was marked by increased expression of secretory sPLA2 and COX-2, but not COX-1 and cytosolic cPLA2. Although astrocytes responded to ATP or phorbol ester (PMA) with increased cPLA2 phosphorylation and arachidonic acid release, ATP or PMA only caused a small increase in levels of PGE2. However, when astrocytes were first treated with cytokines, further exposure to ATP or PMA, but not H2O2 or menadione, markedly increased PGE2 production. These results suggest that ATP release during neuronal excitation or injury can enhance the inflammatory effects of cytokines on PGE2 production and may contribute to chronic inflammation seen in Alzheimer's disease.     

Amyloid-beta peptide induces temporal membrane biphasic changes in astrocytes through cytosolic phospholipase A2

Oligomeric amyloid-beta peptide (Abeta) is known to induce cytotoxic effects and to damage cell functions in Alzheimer's disease. However, mechanisms underlying the effects of Abeta on cell membranes have yet to be fully elucidated. In this study, Abeta 1-42 (Abeta(42)) was shown to cause a temporal biphasic change in membranes of astrocytic DITNC cells using fluorescence microscopy of Laurdan. Abeta(42) made astrocyte membranes became more molecularly-disordered within the first 30 min to 1 h, but gradually changed to more molecularly-ordered after 3 h. However, Abeta(42) caused artificial membranes of vesicles made of rat whole brain lipid extract to become more disordered only. The trend for more molecularly-ordered membranes in astrocytes induced by Abeta(42) was abrogated by either an NADPH oxidase inhibitor, apocynin, or an inhibitor of cytosolic phospholipase A(2) (cPLA(2)), but not by an inhibitor of calcium-independent PLA(2) (iPLA(2)). Apocynin also suppressed the increased production of superoxide anions (O(2)(-)) and phosphorylation of cPLA(2) induced by Abeta(42). In addition, hydrolyzed products of cPLA(2), arachidonic acid (AA), but not lysophosphatidylcholine (LPC) caused astrocyte membranes to become more molecularly-ordered. These results suggest (1) a direct interaction of Abeta(42) with cell membranes making them more molecularly-disordered, and (2) Abeta(42) also indirectly makes membranes become more molecularly-ordered by triggering the signaling pathway involving NADPH oxidase and cPLA(2) in astrocytes.     

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications.The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases.The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.     

Hydrogen peroxide activation of Ca(2+)-independent phospholipase A(2) in uterine stromal cells

In rat uterine stromal cells (U(III) cells), an oxidative stress induced by H(2)O(2) caused a dose-dependent release of arachidonic acid (AA) that was independent of intracellular Ca(2+) concentration and was not inhibited by Ca(2+)-dependent phospholipase A(2) (cPLA(2)) inhibitors, nor by protein kinase C (PKC) inhibitors or by PKC down-regulation. H(2)O(2) treatment did not impair AA esterification but significantly increased Ca(2+)-independent PLA(2) (iPLA(2)) activity. Since iPLA(2) specific inhibitor bromoenollactone almost completely suppressed the release of AA induced by H(2)O(2), we conclude that iPLA(2) activity represents the major mechanism by which H(2)O(2) increases the availability of non-esterified AA in U(III) cells. Moreover, PKC inhibitors sphingosine and calphostin C markedly potentiated the release of AA trigger by H(2)O(2), suggesting a regulatory mechanism of iPLA(2) by PKC that remains to be clarified.     

IL-1beta signaling in cat lower esophageal sphincter circular muscle

In a cat model of acute experimental esophagitis, resting in vivo lower esophageal sphincter (LES) pressure and in vitro tone are lower than in normal LES, and the LES circular smooth muscle layer contains elevated levels of IL-1beta that decrease the LES tone of normal cats. We now examined the mechanisms of IL-1beta-induced reduction in LES tone. IL-1beta significantly reduced acetylcholine-induced Ca(2+) release in Ca(2+)-free medium, and this effect was partially reversed by catalase, demonstrating a role of H(2)O(2) in these changes. IL-1beta significantly increased the production of H(2)O(2), and the increase was blocked by the p38 MAPK inhibitor SB-203580, by the cytosolic phospholipase A(2) (cPLA(2)) inhibitor AACOCF3, and by the NADPH oxidase inhibitor apocynin, but not by the MEK1 inhibitor PD-98059. IL-1beta significantly increased the phosphorylation of p38 MAPK and cPLA(2). IL-1beta-induced cPLA(2) phosphorylation was blocked by SB-203580 but not by AACOCF3, suggesting sequential activation of p38 MAPK-phosphorylating cPLA(2). The IL-1beta-induced reduction in LES tone was partially reversed by AACOCF3 and by the Ca(2+)-insensitive PLA(2) inhibitor bromoenol lactone (BEL). IL-1beta significantly increased cyclooxygenase (COX)-2 and PGE(2) levels. The increase in PGE(2) was blocked by SB-203580, AACOCF3, BEL, and the COX-2 inhibitor NS-398 but not by PD-98059 or the COX-1 inhibitor valeryl salicylate. The data suggested that IL-1beta reduces LES tone by producing H(2)O(2), which may affect Ca(2+)-release mechanisms and increase the synthesis of COX-2 and PGE(2). Both H(2)O(2) and PGE(2) production depend on sequential activation of p38 MAPK and cPLA(2). cPLA(2) activates NADPH oxidases, producing H(2)O(2), and may produce arachidonic acid, converted to PGE(2) via COX-2.     

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.