Macrophage colony stimulating factor activates phosphatidylcholine hydrolysis by cytoplasmic phospholipase A2

Integrated Metabolomics and Network Pharmacology Study on Immunoregulation Mechanisms of Panax ginseng through Macrophages

Panax ginseng (PG) is a widely used functional food and herbal with immunoregulation activity. Currently, immunoregulation studies of PG mainly focused on the specific actions of individual constituents. However, the integral immunoregulation mechanisms of PG need further research. In this study, an integrated metabolomics and network pharmacology approach were used to investigate it. High-content screening was used to evaluate macrophage phagocytosis activity of PG. Untargeted metabolomics profiling of murine macrophage cells with UHPLC-Q-TOF-MS and a multivariate data method were performed to discover the potential biomarkers and metabolic pathways. Then, a macrophage phenotype related "ingredients-targets-metabolites" network of PG was constructed using network pharmacology for further research. As a result, PG can significantly enhance macrophage phagocytosis of GFP-E. coli. A total of twenty potential biomarkers and ten main pathways for which levels changed markedly upon treatment were identified, including glycerophospholipid metabolism, glutathione metabolism, choline metabolism, and taurine metabolism. Twenty compounds of PG associated with metabolomic changes were selected by the network pharmacology analysis, including ginsenoside Re, ginsenoside Rg1, frutinone A, and kaempferol. The network pharmacology results also showed that PG can polarize macrophages to both M1 and M2 phenotype but may be prone to M2 phenotype. In conclusion, our results indicated that PG may be prone to polarize macrophages to M2 phenotype by mainly regulating the glutathione and choline metabolism, which was related to twenty compounds of PG.

Corticotropin-releasing hormone receptors mediate apoptosis via cytosolic calcium-dependent phospholipase A₂ and migration in prostate cancer cell RM-1

Peripheral corticotropin-releasing hormone receptors (CRHRs) are G protein-coupled receptors that play different roles depending on tissue types. Previously, we discovered the mechanism of CRHR-mediated apoptosis of mouse prostate cancer cell line (RM-1) to be a change of Bcl-2:Bax ratio, and CRH was found to inhibit transforming growth factor β migration of breast cancer cells via CRHRs. In the present study, we investigated cytosolic calcium-dependent phospholipase A2 (cPLA2) bridging CRHR activations and Bcl-2:Bax ratio and the effect of CRHR activation on cell migration. Silencing of cPLA2 attenuated a CRHR1 agonist, CRH-induced apoptosis, and the decrease of the Bcl-2:Bax ratio, whereas silencing of cPLA2 aggravated CRHR2 agonist, Urocortin 2 (Ucn2)-inhibited apoptosis, and the increase of the Bcl-2:Bax ratio. CRH in a time- and concentration-dependent manner increased cPLA2 expression mainly through interleukin 1β (IL1β) upregulation. Ucn2 decreased cPLA2 expression through neither tumor necrosis factor α nor IL1β. CRH-suppressed decay of cPLA2 mRNA and Ucn2 merely suppressed its production. Overexpression of CRHR1 or CRHR2 in HEK293 cells correspondingly upregulated or downregulated cPLA2 expression after CRH or Ucn2 stimulation respectively. In addition, both CRH and Ucn2 induced migration of RM-1 cells. Our observation not only established a relationship between CRHRs and cell migration but also for the first time, to our knowledge, demonstrated that cPLA2 participates in CRHR1-induced apoptosis and CRHR2-inhibited apoptosis.

A distal enhancer controls cytokine-dependent human cPLA2α gene expression

Specific control of group IVA cytosolic phospholipase A2 (cPLA2α or PLA2G4A) expression modulates arachidonic acid production, thus tightly regulating the downstream effects of pro- and anti-inflammatory eicosanoids. The significance of this pathway in human disease is apparent in a range of pathologies from inflammation to tumorigenesis. While much of the regulation of cPLA2α has focused on posttranslational phosphorylation of the protein, studies on transcriptional regulation of this gene have focused only on proximal promoter regions. We have identified a DNase I hypersensitive site encompassing a 5' distal enhancer element containing a highly conserved consensus AP-1 site involved in transcriptional activation of cPLA2α by interleukin (IL)-1β. Chromatin immunoprecipitation (ChIP), knockdown, knockout, and overexpression analyses have shown that c-Jun acts both in a negative and positive regulatory role. Transcriptional activation of cPLA2α occurs through the phosphorylation of c-Jun in conjunction with increased association of C/EBPβ with the distal novel enhancer. The association of C/EBPβ with the transcriptional activation complex does not require an obvious DNA binding site. These data provide new and important contributions to the understanding of cPLA2α regulation at the transcriptional level, with implications for eicosanoid metabolism, cellular signaling, and disease pathogenesis.

Antioxidants inhibit the ability of lysophosphatidylcholine to regulate proteoglycan synthesis

OBJECTIVE

We previously have shown that lysophosphatidylcholine (lysoPC) regulates proteoglycan synthesis by vascular smooth muscle cells (SMCs). Given the accumulating evidence for reactive oxygen species (ROS) as mediators of a variety of effects of lysoPC, the present study evaluates the potential role of ROS as intermediate molecules in the regulation of proteoglycan synthesis by lysoPC.

METHODS AND RESULTS

LysoPC (10 micromol/L) was found to stimulate rapid and sustained generation of ROS by SMC, as indicated using a fluorescent probe for measuring intracellular oxidants and fluorescence-activated cell sorting. This was not associated with cytotoxicity, as evaluated by fluorescence microscopy using MitoTracker Red or propidium iodide, cell number, cell protein, or lactate dehydrogenase release. Pretreatment with catalase or superoxide dismutase, specific scavengers of hydrogen peroxide and superoxide, respectively, blocked the ability of lysoPC to stimulate both accumulation of ROS and proteoglycan synthesis. Most importantly, these enzymatic antioxidants prevented lysoPC from stimulating the synthesis of proteoglycans with enhanced lipoprotein-binding properties, as quantified by a gel shift binding assay.

CONCLUSIONS

These findings strongly suggest that ROS are key mediators in the ability of lysoPC to regulate proteoglycan synthesis and that these effects can be inhibited by antioxidants.

The role of TFIID, the initiator element and a novel 5' TFIID binding site in the transcriptional control of the TATA-less human cytosolic phospholipase A2-alpha promoter

Human cytosolic phospholipase A2-alpha (cPLA2-alpha) is a critical enzyme in the liberation of arachidonic acid (AA) from cellular membranes and the subsequent formation of prostaglandins (PGs), leukotrienes (LTs), hydroxyeicosatetraenoic acids (HETEs) and platelet activating factor in many different cell types. Much is known of the effect of posttranslational phosphorylation and calcium binding events on the enzymatic activity of cPLA2-alpha, but to date little is known about its specific transcriptional control. Through the use of reporter gene constructs and eletrophoretic mobility shift assays (EMSAs), this study determined the minimal promoter required for basal transcriptional activity of the human cPLA2-alpha promoter to include base pairs -40 through the transcription start site (TSS). In addition, it confirms the importance of an initiator (Inr) element at the TSS by deletion reporter gene analysis, and further identifies bases -3 (C) and -2 (T) as critical bases in the Inr function by mutation reporter gene analysis. Finally, this study describes a novel AAGGAG motif at -30 to -35 which is bound by TATA-box binding protein (TBP) and is critical for basal transcriptional activity.

Nerve growth factor-induced up-regulation of cytosolic phospholipase A2alpha level in rat PC12 cells

Nerve growth factor (NGF) regulates various types of gene transcription in neurons. One of the cytosolic phospholipase A(2)s, cPLA(2)alpha, which preferentially cleaves phospholipids at the sn-2 position to arachidonic acid (AA), is involved in neuronal responses including survival. We investigated the effect of NGF on cPLA(2)alpha expression and its signaling pathways in PC12 cells, which differentiate into neuronal-like cells with neurites by NGF treatment. Treatment with NGF increased cPLA(2)alpha mRNA level after 4h and its protein level 24h after NGF addition. The NGF-induced increase in cPLA(2)alpha mRNA was inhibited by actinomycin D. NGF caused phosphorylation of mitogen-activated protein kinases (MAPKs); sustained phosphorylation of extracellular-regulated kinases (ERK1/2) and transient phosphorylation of p38 MAPK. NGF responses (cPLA(2)alpha mRNA and its protein) were inhibited by selective inhibitors for the ERK1/2 pathway, p38 MAPK and c-Jun NH(2)-terminal kinase. Epidermal growth factor, which transiently activates ERK1/2, did not modify cPLA(2)alpha expression. Although phorbol 12-myristate 13-acetate, an activator of protein kinase C (PKC), alone showed no effect, NGF-induced cPLA(2)alpha mRNA expression decreased due to the inhibition of PKC. These findings suggest that NGF-induced cPLA(2)alpha expression is regulated by gene transcription via the ERK1/2, p38 MAPK and PKC pathways in PC12 cells.

Cytosolic phospholipase A2 is responsible for prostaglandin E2 and leukotriene B4 formation in phagocyte-like PLB-985 cells: studies of differentiated cPLA2-deficient PLB-985 cells

Our previously established model of cytosolic phospholipase A(2) (cPLA(2))-deficient, differentiated PLB-985 cells (PLB-D cells) was used to determine the physiological role of cPLA(2) in eicosanoid production. Parent PLB-985 (PLB) cells and PLB-D cells were differentiated toward the monocyte or granulocyte lineages using 5 x 10(-)(8) M 1,25 dihydroxyvitamin D(3) or 1.25% dimethyl sulfoxide, respectively. Parent monocyte- or granulocyte-like PLB cells released prostaglandin E(2) (PGE(2)) when stimulated by ionomycin, A23187, opsonized zymosan, phorbol 12-myristate 13-acetate, or formyl-Met-Leu-Phe (fMLP), and monocyte- or granulocyte-like PLB-D cells did not release PGE(2) with any of the agonists. The kinetics of cPLA(2) translocation to nuclear fractions in monocyte-like PLB cells stimulated with fMLP or ionomycin was in correlation with the kinetics of PGE(2) production. Granulocyte-like PLB cells, but not granulocyte-like PLB-D cells, secreted leukotriene B(4) (LTB(4)) after stimulation with ionomycin or A23187. Preincubation of monocyte-like parent PLB cells with 100 ng/ml lipopolysaccharide (LPS) for 16 h enhanced stimulated PGE(2) production, which is in correlation with the increased levels of cPLA(2) detected in these cells. LPS preincubation was less potent in increasing PGE(2) and LTB(4) secretion and did not affect cPLA(2) expression in granulocyte-like PLB cells, which may be a result of their lower levels of surface LPS receptor expression. LPS had no effect on monocyte- or granulocyte-like PLB-D cells. The lack of eicosanoid formation in stimulated, differentiated cPLA(2)-deficient PLB cells indicates that cPLA(2) contributes to stimulated eicosanoid formation in monocyte- and granulocyte-like PLB cells.

B-Myb-Dependent Regulation of c-Myc Expression by Cytosolic Phospholipase A2

Cytosolic phospholipase A(2) (cPLA(2)) cleaves membrane phospholipids to release arachidonic acid, initiating lipoxygenase and cyclooxygenase pathways. Mice lacking a gene for cPLA(2) suggested important roles of the protein in allergic responses, fertility, and neural cell death. Here we show that cPLA(2) negatively regulates c-Myc expression in a B-Myb-dependent manner. Overexpression of cPLA(2) protein but not a mutant cPLA(2) protein that lacks in vitro binding ability with B-Myb inhibits B-Myb-dependent c-myc gene expression. The inhibition was associated with physical interaction of B-Myb protein with cPLA(2) both in the cytoplasm and the nucleus. Binding site analysis demonstrated that both the N and C termini of cPLA(2) interact with B-Myb. Macrophage colony stimulating factor (MCSF) stimulated cPLA(2) redistribution into the nucleus and also association with B-Myb in human monocytes. Importantly, macrophages from mice with a disrupted cPLA(2) gene demonstrated significantly increased levels of c-Myc protein in the nucleus compared with cells from the wild-type mice, whereas B-Myb levels were similar in the cells from the cPLA(2)(+/+) and cPLA(2)(-/-) mice. Moreover, an introduction of cPLA(2) into cPLA(2)(-/-) mouse macrophages resulted in decreased c-Myc protein levels, and an inhibition of cPLA(2) expression by small interfering RNAs or antisense RNA increased the c-myc transcription in macrophage colony stimulating factor-activated human monocytes. These findings provide new insights into the function of cPLA(2) in B-Myb-dependent gene expression.

Mechanism of group IVA cytosolic phospholipase A(2) activation by phosphorylation

Group IVA cytosolic phospholipase A2 (cPLA2) has been shown to play a critical role in the agonist-induced release of arachidonic acid. To understand the mechanism by which phosphorylation of Ser505 and Ser727 activates cPLA2, we systematically analyzed the effects of S505A, S505E, S727A, S727E, S505A/S727A, S505A/S727E, and S505E/S727E mutations on its enzyme activity and membrane affinity. In vitro membrane binding measurements showed that S505A has lower affinity than the wild type or S505E for phosphatidylcholine membranes, which is exclusively due to faster desorption of the membrane-bound S505A. In contrast, neither S727A nor S727E mutation had a significant effect on the phosphatidylcholine vesicle binding affinity of cPLA2. The difference in in vitro membrane affinity between wild type (or S505E) and S505A increased with the decrease in Ca2+ concentration, reaching >60-fold at 2.5 microm Ca2+. When HEK293 cells transfected with cPLA2 and mutants were stimulated with ionomycin, the wild type and S505E translocated to the perinuclear region and caused the arachidonic acid release at 0.4 microm Ca2+, whereas S505A showed no membrane translocation and little activity to release arachidonic acid. Further mutational analysis of hydrophobic residues in the active site rim (Ile399, Leu400, and Leu552) indicate that a main role of the Ser505 phosphorylation is to promote membrane penetration of these residues, presumably by inducing a conformational change of the protein. These enhanced hydrophobic interactions allow the sustained membrane interaction of cPLA2 in response to transient calcium increases. On the basis of these results, we propose a mechanism for cPLA2 activation by calcium and phosphorylation.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Bovine papillomavirus oncoprotein E5 affects the arachidonic acid metabolism in cells

The bovine papillomavirus type 1 (BPV-1) oncoprotein encoded by the E5 ORF is a small highly hydrophobic protein, which is capable of inducing oncogenic transformation of cells. We studied the effect of the BPV-1 E5 protein expression on the arachidonic acid metabolism in monkey (COS1) and human (C33A) cells. At relatively low protein concentrations the phospholipase A(2) (PLA(2)) activity and the arachidonic acid (AA) metabolism are activated. E5 mutant proteins, lacking cysteines responsible for the dimerisation of the protein (C37S, C37SC39S), and truncated E5, lacking the C-terminal region, are non-transforming and unable to stimulate the PLA(2) activity and AA metabolism. The transformation-defective mutant D33V, which does not activate the platelet-derived growth factor receptor (PDGFR), activates AA metabolism like wt E5. Our data suggest that the BPV-1 E5 protein could stimulate the AA metabolism independently of PDGF receptor.

Mildly oxidized LDL induces expression of group IIa secretory phospholipase A(2) in human monocyte-derived macrophages

Phospholipase A(2)s (PLA(2)s) constitute a family of enzymes that hydrolyze fatty acids of membrane phospholipids, thus initiating the synthesis of proinflammatory mediators. Various PLA(2)s have been detected in human atherosclerotic arteries (advanced lesions); however, only the secretory group of PLA(2) has been shown to specifically hydrolyze low density lipoprotein (LDL)-associated phospholipids and, as such, may play a potential role in atherogenesis. In the present study, we investigated the expression pattern of group IIa, IV, and V PLA(2)s in human macrophages, which are the key cells involved in the onset and perpetuation of atherosclerosis. Immunohistochemical staining by double labeling showed that the secretory nonpancreatic PLA(2) (snpPLA(2)) is detectable in macrophages in the intima of early atherosclerotic lesions. Reverse transcription-polymerase chain reaction analysis of RNA extracted from human monocytes clearly showed that expression of group IV PLA(2) was enhanced during differentiation into macrophages, with an onset of induction at days 2 to 3 of differentiation. Group V snpPLA(2) was constitutively expressed on differentiation, whereas the detection of group IIa snpPLA(2) was dependent on both differentiation and subsequent stimulation of macrophages. Indeed, the transcription of group IIa snpPLA(2) in macrophages was induced by treatment with minimally modified or mildly oxidized LDL, whereas native, extensively oxidized, or acetylated LDL had no effect. To our knowledge, this is the first report describing induction of group IIa snpPLA(2) expression in human monocyte-derived macrophages. The mRNA levels of cytosolic PLA(2) group IV and snpPLA(2) group V remained unchanged on LDL treatment. Thus, our results show that the expression of distinct PLA(2) enzymes is regulated not only during differentiation of monocytes into macrophages but also on exposure of macrophages to distinct LDL species. Consequently, our results indicate a potential role for both cytosolic and secretory PLA(2) enzymes in inflammation and in macrophage functions related to atherosclerosis, with a specific role for group IIa snpPLA2 in LDL scavenging.

Transcriptional regulation and structural organization of the human cytosolic phospholipase A(2) gene

Cytokines are established regulators of the arachidonic acid cascade in lung cells. The levels of various arachidonic metabolites distinguish the normal and pathogenic states of the human lung. Arachidonyl-selective cytosolic phospholipase A(2) (cPLA(2)) is ubiquitously present in human lung and is most likely the rate-limiting step in eicosanoid generation. We therefore studied the regulation of this pivotal gene in human lung fibroblasts and epithelial cells by proinflammatory cytokines. We demonstrate a dose- and time-dependent induction of human cPLA(2) mRNA by interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma as well as the abrogation of this induction by glucocorticoids. Nuclear runoff studies demonstrate that de novo transcription of the cPLA(2) gene is required for cytokine induction. We have characterized the human cPLA(2) gene, which is encoded by 18 exons and spans in excess of 137 kb. Deletion analysis of a 3.4-kb fragment of the human promoter identified two regions responsible for basal expression of the cPLA(2) gene. Conversely, a CA-dinucleotide repeat in the proximal promoter appears to repress overall promoter activity. Understanding the molecular mechanisms associated with cytokine-dependent expression of the cPLA(2) gene should provide further insight into regulating the level of proinflammatory mediators in pulmonary diseases.

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

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

Interferon gamma priming is not critical for IL-12 production of murine spleen cells

Interferon gamma (IFN-gamma) priming is considered to be critical for interleukin 12 (IL-12) production of murine macrophages and human monocytes by lipopolysaccharide (LPS) stimulation. In our present experiments, freshly prepared spleen cells (f-spleen cells) were confirmed not to produce detectable level of IL-12 by LPS stimulation, although they produced significant amount of IL-12 by the stimulation with LPS plus IFN-gamma. However, the stimulation only with LPS induced IL-12 production of spleen cells preincubated in the absence of IFN-gamma. Findings on IL-12 p40 mRNA accumulation were consistent with their IL-12 production. Essentially the same results were obtained using spleen cells from IFN-gamma deficient mice. In the presence of anti-IL-10, f-spleen cells produced IL-12 upon LPS stimulation, indicating that the failure of f-spleen cells in IL-12 production is caused by IL-10 produced by themselves upon LPS stimulation. In addition, f-spleen cells produced IL-12 upon CD40 ligand stimulation, and the production was hardly affected by the presence of IFN-gamma or preincubation. These results indicate that IFN-gamma priming is not critical for IL-12 production of spleen cells stimulated with LPS or CD40 ligand, although IFN-gamma enhances the production, especially, in response to LPS stimulation.

Identification of a bipartite nuclear localization sequence necessary for nuclear import of 5-lipoxygenase

5-Lipoxygenase catalyzes the synthesis of leukotrienes from arachidonic acid. This enzyme can reside either in the cytoplasm or the nucleus; its subcellular distribution is influenced by extracellular factors, and its nuclear import correlates with changes in leukotriene synthetic capacity. To identify sequences responsible for the nuclear import of 5-lipoxygenase, we transfected NIH 3T3 cells and RAW 264.7 macrophages with expression vectors encoding various 5-lipoxygenase constructs fused to green fluorescent protein. Overexpression of wild type 5-lipoxygenase with or without fusion to green fluorescent protein resulted in a predominantly intranuclear pattern of fluorescence, similar to the distribution of native 5-lipoxygenase in primary alveolar macrophages. Within the 5-lipoxygenase protein is a sequence (Arg(638)-Lys(655)) that closely resembles a bipartite nuclear localization signal. Studies using deletion mutants indicated that this region was necessary for nuclear import of 5-lipoxygenase. Analysis of mutants containing specific amino acid substitutions within this sequence confirmed that it was this sequence that was necessary for nuclear import of 5-lipoxygenase and that a specific arginine residue was critical for this function. As nuclear import of 5-lipoxygenase may regulate leukotriene production, natural or induced mutations in this bipartite nuclear localization sequence may also be important in affecting leukotriene synthesis.

Competitive, reversible inhibition of cytosolic phospholipase A2 at the lipid-water interface by choline derivatives that partially partition into the phospholipid bilayer

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 2-(2'-benzyl-4-chlorophenoxy)ethyl-dimethyl-n-octadecyl-ammonium chloride (compound 1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 5 microM). It was over 70 times more selective for the cPLA2 as compared with the human nonpancreatic secreted phospholipase A2, and it did not inhibit other phospholipases. Additionally, it inhibited arachidonate production in N-formyl-methionyl-leucyl-phenylalanine-stimulated U937 cells. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn -glycero-3-phosphomethanol containing 6-10 mol % of 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition could be defined by kinetic equations describing competitive inhibition at the lipid-water interface. The apparent equilibrium dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.097 +/- 0.032 mol % versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.3 +/- 0.1 mol %. Thus, compound 1 represents a novel structural class of inhibitor of cPLA2 that partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site. Shorter n-alkyl-chained (C-4, C-6, C-8) derivatives of compound 1 were shown to have even smaller KI*app values. However, these short-chained analogs were less potent in terms of bulk inhibitor concentration needed for inhibition when using the [3H]arachidonate-labeled U937 membranes as substrate. This discrepancy was reconciled by showing that these shorter-chained analogs did not partition into the [3H]arachidonate-labeled U937 membranes as effectively as compound 1. The implications for in vivo efficacy that result from these findings are discussed.

Differentiation of U937 cells enables a phospholipase D-dependent pathway of cytosolic phospholipase A2 activation

Treatment with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line results in differentiation toward a monocyte/granulocyte-like cell. This differentiation enables the cell to activate cytosolic phospholipase A2 (cPLA2) to release arachidonate upon stimulation. In contrast, undifferentiated cells are unable to release arachidonate even when stimulated with calcium ionophores. In the present research, a role for phospholipase D (PLD) in the regulation of cPLA2 was shown based on a number of observations. First, the ionomycin- and fMLP-stimulated production of arachidonate in differentiated cells was sensitive to ethanol (2% (v/v)). Ethanol acts as an alternate substrate in place of water for PLD producing phosphatidylethanol (PEt) instead of phosphatidic acid. Indeed, ionomycin stimulation of differentiated cells produced a 14-fold increase in PEt levels. Further evidence for the involvement of PLD in the regulation of cPLA2 came from the observation that the stimulated production of diacylglycerol (for which phosphatidic acid is a major source) was greatly diminished in undifferentiated cells as compared to differentiated cells. Moreover, the normally deficient activation of cPLA2 in undifferentiated cells could be stimulated to release arachidonate if the cells were electroporated in the presence of GTP[gamma]S and MgATP. This treatment stimulates phosphatidylinositol-4,5-bisphosphate (PIP2) production which appears to activate PLD and cPLA2 in subsequent steps. The phosphatidic acid (and diacylglycerol derived from phosphatidic acid) appears to greatly regulate the action of cPLA2 by an unknown mechanism, and undifferentiated cells lack the ability to stimulate PLD activity due to a dysfunction of PIP2 production.

The size and curvature of anionic covesicle substrate affects the catalytic action of cytosolic phospholipase A2

Cytosolic phospholipase A2 (cPLA2) is normally located in the cytosol, but in response to cellular activation the enzyme binds to the membrane at the lipid/water interface where it catalyzes the hydrolysis of the sn-2 ester of arachidonate-containing phospholipids. Synthetic phospholipid vesicle systems have been used in kinetic and mechanistic analyses of cPLA2, but these systems result in a rapid loss of enzyme activity. In the present research, covesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM) containing </=10 mol% 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC) as substrate were used to show that this premature cessation of enzyme-catalyzed hydrolysis is dependent on vesicle size with 25-nm-diameter vesicles supporting little activity as compared to 100-, 200-, and 400-nm vesicles. This suggests that the curvature of the vesicle may shift a conformational equilibrium toward an enzyme state which does not support activity. Interestingly, the presence of 30% (v/v) glycerol greatly enhanced the activity of the enzyme, although vesicle size-dependent premature cessation of hydrolysis was still observed. While the premature cessation of hydrolysis in the absence of glycerol is accompanied by enzyme inactivation, little inactivation occured in the presence of glycerol, indicating that premature cessation and inactivation are not absolutely coupled. When using this covesicle substrate system under conditions (6-10 mM CaCl2) where the vesicles are fusing, no premature cessation of hydrolysis has been observed. This is despite a mean vesicle diameter of 400-450 nm under vesicle-fusing conditions, which is comparable to the largest vesicles used under nonfusing conditions (0.5 mM CaCl2) where considerable premature cessation of hydrolysis was observed. Since DMPM has an intrinsic active site dissociation constant at least 330 times larger than that of PAPC, the optimum conditions for conducting kinetic and mechanistic analyses of cPLA2 with this covesicle substrate is one in which cPLA2 is assayed in the presence of glycerol and with fusion-inducing concentrations of calcium. The use of 1,2-dioleoyl-sn-glycero-3-phosphomethanol (DOPM) instead of DMPM in this system supports much less activity and adds the complication of a strong affinity of DOPM for the active site.

Inhibition of cytoplasmic phospholipase A2 expression by glucocorticoids in rat intestinal epithelial cells

BACKGROUND & AIMS

Glucocorticoids are the most potent and widely accepted anti-inflammatory agents in the treatment of pathological conditions of the gastrointestinal tract in part by inhibiting the synthesis of proinflammatory prostanoids and leukotrienes. Multiple forms of phospholipase A2 may be associated with the production of these metabolites; this study focused on the molecular mechanism(s) by which glucocorticoids control expression of the arachidonyl-selective, cytosolic phospholipase A2 (cPLA2) in intestinal cells.

METHODS

Northern analysis, a transcriptional assay, and enzymatic evaluation were used to access expression of the cPLA2 gene in rat small intestinal epithelial and mouse fibroblast cell lines treated with dexamethasone.

RESULTS

Basal cPLA2 messenger RNA (mRNA) expression was repressed 75% in the presence of dexamethasone with a concomitant decrease in enzymatic activity. Nuclear runoff assays showed a marked decline in de novo cPLA2 RNA synthesis, implicating a transcriptional mechanism associated with the dexamethasone-mediated suppression of cPLA2. Induced expression of cPLA2 mRNA by several proinflammatory cytokines was blocked by cotreatment with dexamethasone.

CONCLUSIONS

Glucocorticoids are capable of markedly altering basal and cytokine-stimulated cPLA2 gene expression in intestinal epithelial cells, leading to a reduction in arachidonate pools in these cells. Dexamethasone-dependent inhibition occurs through a direct reduction of de novo cPLA2 gene transcription.

Macrophage Colony-Stimulating Factor Stimulates Synthesis and Secretion of a Mouse Homolog of a Human IgE-Dependent Histamine-Releasing Factor by Macrophages In Vitro and In Vivo

Treatment of murine resident peritoneal macrophages with macrophage-CSF (M-CSF) up-regulated the synthesis of a discrete set of proteins, including a 26-kDa protein (p26). The sequence of 20 NH2-terminal amino acids of the purified p26 was identical with the mouse homolog of a human IgE-dependent histamine-releasing factor (HRF). Among macrophage activators tested (M-CSF, granulocyte-macrophage-CSF, IL-3, TNF-α, IFN-γ, and LPS), only M-CSF could up-regulate the p26 HRF synthesis by cultured macrophages. M-CSF not only increased the levels of p26 HRF mRNA and protein, but also stimulated the secretion of an N-glycosylated p26 HRF with a m.w. of 30 kDa. Repeated injections of M-CSF into mouse peritoneal cavity for 4 days elicited macrophages expressing abundant p26 HRF. A single i.p. injection of M-CSF failed to increase the p26 HRF level in peritoneal macrophages of thioglycollate-, LPS-, or adjuvant-treated mice, while M-CSF challenge to OVA-immunized mice caused macrophage infiltration and overproduction of p26 HRF, similarly as did OVA challenge. The Ag-specific priming for enhanced synthesis and secretion of p26 HRF by M-CSF was also demonstrated in cultured macrophages prepared from OVA-immunized mice. An i.p. injection of M-CSF or recombinant p26 HRF triggered eosinophil recruitment, even in the absence of the Ag, in the sensitized mice, but not in normal mice. Furthermore, recombinant p26 HRF could induce eosinophilia without marked macrophage and lymphocyte infiltrations. Our results suggest that p26 HRF secreted by M-CSF-stimulated macrophages may be an important mediator for the late phase allergic inflammation.

Differential Regulation of Monocyte Matrix Metalloproteinase and TIMP-1 Production by TNF-α, Granulocyte-Macrophage CSF, and IL-1β Through Prostaglandin-Dependent and -Independent Mechanisms

Matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs) produced by monocytes are believed to be involved in the migration of these cells through the basement membrane and the ensuing destruction of connective tissue in chronic inflammatory lesions. Because monocytes encounter a variety of cytokines at these sites, we examined the effect of cytokines either alone or in combination on the production of monocyte MMPs and TIMP-1. TNF-α, granulocyte-macrophage-CSF (GM-CSF), or IL-1β when added individually enhanced the endogenous levels of 92-kDa gelatinase (MMP-9) and TIMP-1 but failed to induce interstitial collagenase (MMP-1). However, GM-CSF, when added with either TNF-α or IL-1β, induced MMP-1 and synergistically enhanced MMP-9 and TIMP-1. Th2 cytokines, such as IL-4, inhibited the induction of MMPs and TIMP-1 by TNF-α, GM-CSF, and IL-1. Cytokine stimulation of MMP-1 was due, at least in part, to an increase in the release of arachidonic acid and PG E2 (PGE2), because inhibition of MMP-1 by indomethacin could be reversed by exogenous PGE2. In contrast to MMP-1, cytokine stimulation of MMP-9 and TIMP-1 was unaffected by indomethacin. The PGE2-independent induction of monocyte MMP-9 and TIMP-1 by these cytokines differed from stimulation of MMP-9 and TIMP-1 by LPS, which is in large part PG-dependent. In addition, LPS stimulated higher levels of MMP-1 whereas cytokines induced higher levels of MMP-9 and TIMP-1. This is the first demonstration that monocyte MMP-1 can be induced by cytokines and that MMP-1, MMP-9, and TIMP-1 are differentially regulated by cytokines through PG-dependent and -independent mechanisms.

A beta-lactam inhibitor of cytosolic phospholipase A2 which acts in a competitive, reversible manner at the lipid/water interface

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 3,3-Dimethyl-6-(3-lauroylureido)-7-oxo-4-thia-1-azabicyclo[3,2,0] heptane-2-carboxylic acid (1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 72 microM). This beta-lactam did not inhibit other phospholipases, including the human nonpancreatic secreted phospholipase A2. The inhibition of cPLA2 was found not to be time-dependent. This, along with the observation that the degradation of the inhibitor was not catalyzed by the enzyme, demonstrates that the inhibition does not result from the formation of an acyl-enzyme intermediate with the active site serine residue. Moreover, the ring-opened form of 1 is also able to inhibit cPLA2 with near-equal potency. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol containing 6-10 mole percent of 1-palmitoyl-2-[1-14C]-arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition was defined by kinetic equations describing competitive inhibition at the lipid/water interface. The apparent dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.5 +/- 0.1 mole% versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.4 +/- 0.1 mole%. Thus, 1 represents a novel structural class of inhibitors of cPLA2 which partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site.

Biological consequences of a point mutation at codon 969 of the FMS gene

The FMS proto-oncogene encodes the cell surface receptor for colony stimulating factor-1 (CSF-1). Mutations of the FMS gene at codon 969, in the C-terminal region of the gene, have been detected in haematological malignancies. To ascertain the biological significance of a mutation at this codon, we have used a murine haematopoietic cell line, FDC-P1, containing a mutation at codon 969 that results in a phenylalanine replacing a tyrosine. FMS 969 mutant cells and v-fms transfected cells conferred interleukin 3 (IL-3) independent stimulation of FDC-P1 cells, whereas cells transfected with a wild-type FMS construct required exogenous IL-3 for growth. FDC-P1 cells containing a FMS 969 mutation and v-fms transfected cells were tumorigenic in nude mice. Binding studies with radioidonated CSF-1 revealed saturable specific binding in FMS wild-type cells with a Km of 0.9 mM; however, mutant FMS-containing cells did not display saturation kinetics, but instead exhibited a linear relationship between ligand concentration and amount bound. Constitutive expression of FOS was detected in 969 mutant cells in the absence of exogenous CSF-1, a phenotype that was only inducible in wild-type cells in response to CSF-1. FOS and JUNB expression by v-FMS transfected cells showed a similar pattern to FMS wild-type cells. This mutation has been detected in patients with haematological malignancies, and illustrates that the pathway of FMS 969 phenylalanine mutations and v-fms induced pathogenesis can be distinguished. These data indicate that there is a biological role for FMS codon 969 phenylalanine mutation which results in transformation of FDC-P1 cells.

Essential requirement of cytosolic phospholipase A2 for activation of the phagocyte NADPH oxidase

Arachidonic acid (AA) can trigger activation of the phagocyte NADPH oxidase in a cell-free assay. However, a role for AA in activation of the oxidase in intact cells has not been established, nor has the AA generating enzyme critical to this process been identified. The human myeloid cell line PLB-985 was transfected to express p85 cytosolic phospholipase A2 (cPLA2) antisense mRNA and stable clones were selected that lack detectable cPLA2. cPLA2-deficient PLB-985 cells differentiate similarly to control PLB-985 cells in response to retinoic acid or 1,25-dihydroxyvitamin D3, indicating that cPLA2 is not involved in the differentiation process. Neither cPLA2 nor stimulated [3H]AA release were detectable in differentiated cPLA2-deficient PLB-985 cells, demonstrating that cPLA2 is the major type of PLA2 activated in phagocytic-like cells. Despite the normal synthesis of NADPH oxidase subunits during differentiation of cPLA2-deficient PLB-985 cells, these cells fail to activate NADPH oxidase in response to a variety of soluble and particulate stimuli, but the addition of exogenous AA fully restores oxidase activity. This establishes an essential requirement of cPLA2-generated AA for activation of phagocyte NADPH oxidase.

Cytosolic 85-kDa phospholipase A2-mediated release of arachidonic acid is critical for proliferation of vascular smooth muscle cells

Recent evidence suggests that arachidonic acid (AA) may be involved in regulating cellular proliferation. The predominant mechanism of AA release from cellular phospholipids is via phospholipase A2 (PLA2) hydrolysis. The purpose of this study was to examine the roles of the distinct 14-kDa and 85-kDa PLA2 enzymes in human coronary artery vascular smooth muscle cell (hCAVSMC) proliferation. Cultured hCAVSMCs proliferate in the presence of growth medium with a typical doubling time of 30-40 h, grow at a slower proliferative rate upon reaching confluency (day 8), and eventually undergo contact inhibition of growth (day 10). Neither Type II 14-kDa PLA2 activity nor mass changed over a 10-day culture period. In contrast, 85-kDa PLA2 protein activity and mRNA decreased as time in culture progressed. This reduction in 85-kDa PLA2 correlated with reductions in DNA synthesis and suggested a possible association between 85-kDa PLA2 and proliferation. To directly evaluate the role of the 85-kDa PLA2 in proliferation we examined the effects of an 85-kDa PLA2 inhibitor (AACOCF3) and 85-kDa PLA2 antisense oligonucleotides on proliferation. Both reagents dose dependently inhibited proliferation, whereas a 14-kDa PLA2 inhibitor (SB203347), a calcium-independent PLA2 inhibitor (HELSS), an 85-kDa sense oligonucleotide, and a nonrelevant scrambled control oligonucleotide had no effect. The mechanism by which 85-kDa PLA2 influences cellular proliferation remains unclear. Inhibition of 85-kDa PLA2 activity produced neither phase-specific cell cycle arrest nor apoptosis (fluorescence-activated cell sorter analysis). Addition of AA (20 mu M) attenuated the effects of both AACOCF3 and 85-kDa antisense oligonucleotides implicating AA as a key mediator in cellular proliferation. However, although prostaglandin E2 (PGE2) was present in the culture medium, it peaked early (day 3) in culture, and indomethacin had no effect on cellular proliferation indicating that hCAVSMC proliferation was not mediated through PGE2. These data provide the first direct evidence that PLA2 is involved in control of VSMC proliferation and indicate that 85-kDa PLA2-mediated liberation of AA is critical for cellular proliferation.

Phosphorylation and calcium influx are not sufficient for the activation of cytosolic phospholipase A2 in U937 cells: requirement for a Gi alpha-type G-protein

Differentiation with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line toward a monocyte/granulocyte-like cell results in the cell acquiring an ability to release arachidonate upon stimulation. In contrast, the calcium ionophore ionomycin was able to stimulate phospholipase C, as measured by inositol 1,4,5-trisphosphate formation, to equal extents in both undifferentiated and dBcAMP-differentiated U937 cells. The role and regulation of cytosolic phospholipase A2 (cPLA2) in the production of arachidonate in these cells when either the chemotactic peptide fMLP or ionomycin are used as stimulus were investigated. The ionomycin- and fMLP-stimulated release of arachidonate were sensitive to the cPLA2 inhibitor arachidonyl trifluoromethylketone (IC50 values of 32 and 18 microM, respectively), but were not inhibited by E-6-(bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2 H-pyran-2-one, a bromoenol lactone inhibitor of the calcium-independent phospholipase A2. These results, coupled with the inhibition of ionomycin-induced arachidonate production by electroporation of differentiated cells to introduce an anti-cPLA2, demonstrate that the cPLA2 is the enzyme responsible for arachidonate release in differentiated cells. SDS-PAGE and immunoblot analysis of differentiated cells showed the cells to contain both phosphorylated and unphosphorylated forms of cPLA2 (ratio of about 2: 3). Surprisingly, undifferentiated cells contain 30% more enzyme than differentiated cells and contain a higher percentage (approximately 75%) of the phosphorylated in the absence of stimulation. The inability of undifferentiated cells to produce arachidonate is not due to insufficient intracellular calcium concentrations since ionomycin induces large (820-940 nM) influxes of intracellular calcium in both differentiated and undifferentiated cells. This demonstrates that phosphorylation of cPLA2 andan influx of intracellular calcium are not sufficient to activate the enzyme to produce arachidonate. Instead, activation of a pertussis toxin-sensitive Gi alpha-type G-protein is required as evidenced by the production of arachidonate in undifferentiated cells stimulated with mastoparan, an activator of Gi alpha subunits, in combination with ionomycin. This activation of a Gi alpha-type G-protein is independent of modulations of adenylyl cyclase activity since cellular cAMP levels were not modulated upon treatment with mastoparan and ionomycin.

Comparison of alkylacylglycerol vs. diacylglycerol as activators of mitogen-activated protein kinase and cytosolic phospholipase A2 in human neutrophil priming

In human neutrophils, the choline-containing phosphoglycerides contain almost equal amounts of alkylacyl- and diacyl-linked subclasses. In contrast to phosphatidylinositol hydrolysis which yields diacylglycerol, hydrolysis of choline-containing phosphoglycerides by phospholipase D coupled with phosphohydrolase yields both alkylacyl- and diacylglycerol. While diacylglycerol activates protein kinase C, alkylacylglycerol does not, and its role is unclear. Yet previous studies have shown that exogenous alkylacyl- and diacylglycerols can prime for the release of radiolabeled arachidonic acid (AA) in intact neutrophils stimulated by formyl-methionyl-leucyl-phenylalanine. We have now examined the effects of both diacylglycerol (1-oleoyl-2-acetylglycerol; OAG) and alkylacylglycerol (1-O-hexadecyl-2-acetylglycerol; EAG) on the activation of mitogen-activated protein (MAP) kinase and the 85-kDa cytosolic phospholipase A2 (cPLA2) in human neutrophils. We observed that while OAG could effectively activate p42 and p44 MAP kinases along with cPLA2 in a time- and concentration-dependent manner, EAG could not. A novel p40 MAP kinase isoform is also present and activated in response to OAG treatment; the behavior of this MAP kinase isoform is discussed. The activation of cPLA2 and MAP kinase by 20 microM OAG could be inhibited by pretreatment with 1 microM GF-109203X, a selective inhibitor of protein kinase C. Although only OAG activated cPLA2, both OAG and EAG primed for the release of AA mass as determined by gas chromatography/mass spectrometry. The priming of AA release by OAG may be explained by the phosphorylation of cPLA2 through the activation of protein kinase C linked to MAP kinase. However, priming by EAG appears to involve a separate mechanism that is dependent on a different PLA2. Our results support a role for phospholipase D-derived products modulating the activation of cPLA2, further supporting the idea of cross-talk among various phospholipases.

Post-transcriptional regulation of LTC4 synthase activity by retinoic acid in rat basophilic leukemia cells

Calcium ionophore-stimulated production of leukotriene (LT) C4 was enhanced by 16- to 26-h incubation with retinoic acid (RA) in rat basophilic leukemia-1 cells. Production of LTC4 by enzyme assay using cell lysates as the enzyme source and LTA4 as the substrate was also enhanced by RA-treatment. Production of LTB4 was not enhanced under these two experimental conditions, suggesting the preferential activation of LTC4 synthase activity. The RA-induced enhancement of LTC4 synthesis by the cells was suppressed by co-incubation with dexamethasone (DEX) or cyclosporine A (CSA). However, the expression of mRNA for LTC4 synthase was not affected by the exposure to RA, DEX or CSA. These results indicate that RA-induced enhancement of LTC4 production and its inhibition by DEX and CSA was determined by post-transcriptional regulation of LTC4 synthase.

P11, a unique member of the S100 family of calcium-binding proteins, interacts with and inhibits the activity of the 85-kDa cytosolic phospholipase A2

Using a two hybrid system screen of a human cDNA library, we have found that p11, a unique member of the S100 family of calcium-binding proteins, interacts with the carboxyl region of the 85-kDa cytosolic phospholipase A2 (cPLA2). p11 synthesized in a cell-free system interacts with cPLA2 in vitro. The p11-cPLA2 complex is detectable from a human bronchial epithelial cell line (BEAS 2B). Furthermore, p11 inhibits cPLA2 activity in vitro. Selective inhibition of p11 expression in the BEAS 2B cells by antisense RNA results in an increased PLA2 activity as well as an increased release of prelabeled arachidonic acid. This study demonstrates a novel mechanism for the regulation of cPLA2 by an S100 protein.

Lysophosphatidylcholine stimulates activator protein 1 and the c-Jun N-terminal kinase activity

Lysophosphatidylcholine (lyso-PC), a natural lipid generated through the action of phospholipase A2 on membrane phosphatidylcholine, has been implicated in atherogenesis and the inflammatory process. In vitro studies have established a role for lyso-PC in modulation of gene expression and other cellular responses including differentiation and proliferation. There is also evidence that lyso-PC may act as an intracellular second messenger transducing signals elicited from membrane-associated receptors. The mechanisms behind the diverse activities of lyso-PC are poorly understood. We report, in this study, that treatment of cultured cells with exogenous lyso-PC, at nontoxic concentrations, potently induced activator protein-1 (AP-1) DNA binding and transcriptional activity independent of well known AP-1 activators, protein kinase C or mitogen-activated protein kinases ERK1 and ERK2. Lyso-PC also activated the c-Jun N-terminal kinase (JNK/SAPK), a recently characterized member of the mitogen-activated protein kinase family, known to activate AP-1. The stimulated JNK and AP-1 activities probably mediate or contribute to some bioactive effects of lyso-PC.

Presence of glycerol masks the effects of phosphorylation on the catalytic efficiency of cytosolic phospholipase A2

Cytosolic phospholipase A2 catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. The enzymatic activity of cPLA2 is affected by several mechanisms, including substrate presentation and the phosphorylation state of the enzyme. Using covesicles of 1-palmitoy1-2-arachidonoyl-[arachidonoyl-1-14C]-8n-glycero-3 -phosphocholine and 1,2-dimyristoyl-phosphatidylmethanol as substrate, the effects of phosphorylation on the interfacial binding and catalytic constants were investigated. Phosphorylated and dephosphorylated enzyme forms were shown to have identical values of 2.6 microM for KMapp, an equilibrium dissociation constant which consists of the intrinsic dissociation constant from the lipid/water interface (Ks) and the dissociation constant for phospholipid from the active site (KM*). Moreover, the values of KM* for phosphorylated and dephosphorylated enzyme did not differ significantly (0.4 +/- 0.1 and 0.2 +/- 0.1, respectively). However, dephosphorylation of the enzyme reduced the value of kcat by 39%. The phosphorylation state of the enzyme had no effect on either the cooperativity shown by this enzyme or the thermal stability of the enzyme. Surprisingly, the presence of glycerol (4 M) masks the effect of phosphorylation on kcat. Instead, glycerol increased the value of kcat by 440% for the phosphorylated enzyme and by 760% for the dephosphorylated form. Moreover, addition of glycerol had only small effects on KMapp. the increase in the kcat upon addition of glycerol results from a substantial decrease in the activation energy from 29.4 to 14.8 kcal. mol-1. To determine whether the effects of phosphorylation of the enzyme or addition of glycerol are unique to this artificial substrate, membranes from U937 cells were isolated and used as substrate. With these membranes, the dephosphorylated enzyme was only 21% less active than the phosphorylated enzyme. In the presence of glycerol, there was no detectable difference the two enzyme forms, and the rate of hydrolysis was increased by 300-390% over that measured in the absence of glycerol. These results suggest that the catalytic efficiency of the phosphorylated enzyme is not particularly relevant to its activation in vivo. Moreover, it may be that glycerol is mimicking the effect of some unidentified factor which greatly enhances the catalytic efficiency of the enzyme.

Rapid import of cytosolic 5-lipoxygenase into the nucleus of neutrophils after in vivo recruitment and in vitro adherence

5-Lipoxygenase catalyzes the synthesis of leukotrienes from arachidonic acid. The subcellular distribution of 5-lipoxygenase is known to be cell type-dependent and is cytosolic in blood neutrophils. In this study, we asked whether neutrophil recruitment into sites of inflammation can alter the subcellular compartmentation of 5-lipoxygenase. In peripheral blood neutrophils from rats, 5-lipoxygenase was exclusively cytosolic, as expected. However, in glycogen-elicited peritoneal neutrophils, abundant soluble 5-lipoxygenase was in the nucleus. Upon activation with calcium ionophore A23187, intranuclear 5-lipoxygenase translocated to the nuclear envelope. Elicited neutrophils required a greater concentration of A23187 for activation than did blood neutrophils (half-maximal response, 160 versus 52 nM, respectively) but generated greater amounts of leukotriene B4 upon maximal stimulation (26.6 versus 7.68 ng/10(6) cells, respectively). Intranuclear 5-lipoxygenase was also evident in human blood neutrophils after adherence to a variety of surfaces, suggesting that adherence alone is sufficient to drive 5-lipoxygenase redistribution. These results demonstrate a physiologically relevant circumstance in which the subcellular distribution of 5-lipoxygenase can be rapidly altered in resting cells, independent of 5-lipoxygenase activation. Nuclear import of 5-lipoxygenase may be a universal accompaniment of neutrophil recruitment into sites of inflammation, and this may be associated with alterations in enzymatic function.

Epstein-Barr Virus Modulates 5-Lipoxygenase Product Synthesis in Human Peripheral Blood Mononuclear Cells

The effect of short-term coincubations of Epstein-Barr virus (EBV) with mononuclear cells on the synthesis of leukotrienes (LT) by monocytes was investigated. Although treatment of mononuclear cells with EBV alone had no significant effect on LT synthesis by monocytes, the preincubation of mononuclear cells with EBV before the further stimulation of the cells with either the ionophore A23187, the chemoattractant formyl-Met-Leu-Phe, or the phagocytic particles zymosan strikingly enhanced the formation of both LTB4 and LTC4 above the levels of synthesis observed with the stimuli alone. Such priming effect of EBV on LT synthesis was maximal after 15 minutes of preincubation of mononuclear cells with EBV and slowly declined at longer preincubation times; the priming effect of EBV was observed both in Hank's Balanced Salt Solution and plasma. The effect of EBV was abolished by prior treatment of viral particles by heat or by antibody raised against the glycoprotein gp350 of the viral envelope, but not by UV irradiation of the viral particles. Exposure of mononuclear cells to EBV was shown to strongly enhance the activation of the 5-lipoxygenase and the release of arachidonic acid induced upon cell stimulation with a second agonist. The release of arachidonic acid by the EBV-treated mononuclear cells was inhibitable by arachidonyl trifluoromethyl ketone, an inhibitor of the 80-kD cytosolic phospholipase A2 . Furthermore, EBV was shown to rapidly increase (maximum effect within 15 minutes) the levels of phosphorylated form of the cytosolic phospholipase A2 (as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis), a process related to the activation of this enzyme. These data show that the interaction of EBV with monocytes upregulates the formation of important lipid mediators of inflammation.

Arachidonic Acid Mediates Interferon-γ–Induced Sphingomyelin Hydrolysis and Monocytic Marker Expression in HL-60 Cell Line

The biochemical signaling mechanisms involved in transducing the effects of interferon-γ (IFN-γ) on human leukemia-derived HL-60 cell differentiation are not completely understood. Recent studies established the existence of a sphyngomyelin (SM) cycle that operates in response to the action of IFN-γ on HL-60 cells, but the mechanisms by which IFN-γ induces the SM hydrolysis remain unexplored. In this study, biochemical events mediating IFN-γ effects on SM turnover and their specificity and role in HL-60 differentiation were investigated. The activation of the SM cycle by IFN-γ occurred rapidly, with a decrease of approximately 20% in the SM level observed after 60 minutes with a concomitant increase in ceramide level. Treatment of HL-60 cells with IFN-γ did not influence the 1,2-diacylglycerol concentration, intracellular Ca2+ concentration, or phospholipase D activity. IFN-γ stimulated a rapid release of arachidonic acid (AA) from HL-60 cells; the effect was abolished by the pretreatment of cells with pertussis toxin, suggesting a role for a pertussis-toxin–sensitive G protein in IFN-γ–mediated activation of phospholipase A2 (PLA2 ). At 4 to 120 hours after the stimulation of the cells with IFN-γ, a significant increase in the particulate and soluble PLA2 activity was observed, corresponding to an increase in the level of immunoreactive cPLA2 in both cytosol and membrane fractions. The treatment of cells with tyrosine kinase inhibitor herbimycin A completely abolished the effect of IFN-γ on PLA2 activity in membrane and cytosolic fractions, but had no effect on IFN-γ–mediated early AA release suggesting dual mechanism of PLA2 activation. Melittin, potent activator of PLA2 , and AA mimicked the effect of IFN-γ on SM hydrolysis. Pretreatment of HL-60 cells with the PLA2 inhibitor, bromophenacyl bromide (BPB), or pertussis toxin abolished the effect of IFN-γ on SM hydrolysis; exogenous addition of AA overcame the effects of BPB and pertussis toxin. Long-term exposure (5 days) of HL-60 cells to IFN-γ caused an increase in nitroblue tetrazolium (NBT)-reducing and nonspecific esterase (NSE) activity and induced expression of FcγRI (CD64) without significant effects on cell number, adherence, or fagocytic activity. The treatment of cells with AA or melittin induced NBT, NSE, and CD64 expression to the level similar to that observed with IFN-γ, and no further increase was observed with the combination of IFN-γ and AA or IFN-γ and melittin. Treatment of HL-60 cells with indomethacin, an inhibitor of cyclo-oxygenase, and nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, had no effects on IFN-γ–mediated induction of CD64 expression. These studies indicate a key role for the phospholipase A2/AA pathway, as an early biochemical signal elicited by the occupation of IFN-γ–receptor, in mediating IFN-γ induction of the SM cycle and phenotypic changes associated with differentiation of HL-60 along monocytic lineage.

Arachidonic Acid Mediates Interferon-γ–Induced Sphingomyelin Hydrolysis and Monocytic Marker Expression in HL-60 Cell Line

The biochemical signaling mechanisms involved in transducing the effects of interferon-γ (IFN-γ) on human leukemia-derived HL-60 cell differentiation are not completely understood. Recent studies established the existence of a sphyngomyelin (SM) cycle that operates in response to the action of IFN-γ on HL-60 cells, but the mechanisms by which IFN-γ induces the SM hydrolysis remain unexplored. In this study, biochemical events mediating IFN-γ effects on SM turnover and their specificity and role in HL-60 differentiation were investigated. The activation of the SM cycle by IFN-γ occurred rapidly, with a decrease of approximately 20% in the SM level observed after 60 minutes with a concomitant increase in ceramide level. Treatment of HL-60 cells with IFN-γ did not influence the 1,2-diacylglycerol concentration, intracellular Ca2+ concentration, or phospholipase D activity. IFN-γ stimulated a rapid release of arachidonic acid (AA) from HL-60 cells; the effect was abolished by the pretreatment of cells with pertussis toxin, suggesting a role for a pertussis-toxin–sensitive G protein in IFN-γ–mediated activation of phospholipase A2 (PLA2 ). At 4 to 120 hours after the stimulation of the cells with IFN-γ, a significant increase in the particulate and soluble PLA2 activity was observed, corresponding to an increase in the level of immunoreactive cPLA2 in both cytosol and membrane fractions. The treatment of cells with tyrosine kinase inhibitor herbimycin A completely abolished the effect of IFN-γ on PLA2 activity in membrane and cytosolic fractions, but had no effect on IFN-γ–mediated early AA release suggesting dual mechanism of PLA2 activation. Melittin, potent activator of PLA2 , and AA mimicked the effect of IFN-γ on SM hydrolysis. Pretreatment of HL-60 cells with the PLA2 inhibitor, bromophenacyl bromide (BPB), or pertussis toxin abolished the effect of IFN-γ on SM hydrolysis; exogenous addition of AA overcame the effects of BPB and pertussis toxin. Long-term exposure (5 days) of HL-60 cells to IFN-γ caused an increase in nitroblue tetrazolium (NBT)-reducing and nonspecific esterase (NSE) activity and induced expression of FcγRI (CD64) without significant effects on cell number, adherence, or fagocytic activity. The treatment of cells with AA or melittin induced NBT, NSE, and CD64 expression to the level similar to that observed with IFN-γ, and no further increase was observed with the combination of IFN-γ and AA or IFN-γ and melittin. Treatment of HL-60 cells with indomethacin, an inhibitor of cyclo-oxygenase, and nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, had no effects on IFN-γ–mediated induction of CD64 expression. These studies indicate a key role for the phospholipase A2/AA pathway, as an early biochemical signal elicited by the occupation of IFN-γ–receptor, in mediating IFN-γ induction of the SM cycle and phenotypic changes associated with differentiation of HL-60 along monocytic lineage.

Phosphatidylcholine signaling in response to CSF-1

The formation of cell membrane following CSF-1 stimulation of a macrophage cell line is coordinated with cell cycle progression. The majority of membrane phospholipid accumulates during the S phase and results from cell-cycle dependent oscillations in the rates of phosphatidylcholine biosynthesis and degradation. Both synthesis and degradation are enhanced during the G1 phase, resulting in a high rate of phosphatidylcholine turnover. Degradation of phosphatidylcholine after CSF-1 stimulation is mediated by a phospholipase C, and the release of diacylglycerol during G1 phase is biphasic. The degradation essentially stops during the S phase, thus allowing biosynthesis to supply the necessary membrane for cell division and doubling. The degradation of phosphatidylcholine during G1 signals the downstream activation of c-fos and junB transcription and can be mimicked by incubation of the macrophage cells with exogenous bacterial phospholipase C. In contrast, the expression of c-myc transcripts normally associated with CSF-1 stimulation is severely compromised in phospholipase C-treated cells, indicating that the diacylglycerol signals a pathway distinct from the pathway that governs c-myc activation. Constitutive expression of c-myc complements phospholipase C activity and permits the growth of cells in the presence of exogenous bacterial enzyme and the absence of CSF-1. Protein kinase C is not required to mediate the diacylglycerol signal that supports cell growth. GTP exchange on Ras is not enhanced, and MAP kinase activity is not stimulated in response to phosphatidylcholine degradation by exogenous phospholipase C. The 85 kDa cytoplasmic phospholipase A2 is activated, however, as well as a novel protein we have called p96. Rapid serine phosphorylation of p96 follows stimulation of cells with either CSF-1 or exogenous phospholipase C. Analysis of the murine cDNA encoding p96 reveals an amino-terminal domain with significant similarity to the amino-terminal domain of the Drosophila-disabled gene product and a carboxy-terminal domain containing proline-rich sequences characteristic of SH3 binding regions. The sequence of p96 suggests an interactive role for this unique protein in the CSF-1 signal transduction cascade.

Quantitation of the cytosolic phospholipase A2 (type IV) in isolated human peripheral blood eosinophils by sandwich-ELISA

Sandwich enzyme-linked immunosorbent assay (sELISA) was developed for precise quantitation of cytosolic phospholipase A2 (cPLA2 type IV) concentration in isolated human peripheral blood eosinophils as an alternative to semiquantitative chemiluminescent assay employing immunoprecipitation/Western blot analysis. In this assay, monoclonal mouse anti-human cPLA2 antiserum was used as the capture antibody, polyclonal rabbit anti-human cPLA2 antiserum as the secondary antibody, and alkaline phosphatase-conjugated goat anti-rabbit IgG as the tertiary, reporter antibody. Purified human cPLA2 (0-1000 ng/ml) dissolved in Tris-HCl buffered saline was used as the standard protein. The detection limit for cPLA2 in 10(6) eosinophils was 0.109 ng/ml, and coefficients of inter- and intra-assay variation were 4.23% and 7.07%, respectively. There was no cross-reactivity with other (secretory) isoforms of PLA2 (sPLA2 types I-III) either from porcine pancreas, human synovial fluid, or bee venom. In separate studies, the recovery of cPLA2 was > 83% when eosinophil lysate was supplemented exogenously with two different concentrations of cPLA2. From a total protein content of 22.3 +/- 1.7 micrograms/10(6) cells, the baseline concentration of cPLA2 was 0.38 +/- 0.18 ng/10(6) cells in eosinophils obtained from mildly atopic donors. Immunoblotting studies confirmed the complete specificity for the type IV isoform as detected by sELISA. This sELISA method permits the precise quantitative assessment of cPLA2 in nanogram quantities per million cells, which has not previously been possible by immunoblotting analysis.

Type II secretory phospholipase A2 associated with cell surfaces via C-terminal heparin-binding lysine residues augments stimulus-initiated delayed prostaglandin generation

Type II secretory phospholipase A2 (sPLA2) has been shown to be induced by a variety of proinflammatory stimuli and, therefore, has been implicated in the inflammatory process. In order to determine whether association of sPLA2 with cell surfaces via heparan sulfate proteoglycan is important for its effects on cellular functions, we have identified the critical domain in sPLA2 for heparin and cell surface binding and examined its role in cellular prostaglandin (PG) biosynthesis. Replacement of several conserved Lys residues in the C-terminal region of mouse and rat sPLA2s by Glu resulted in a marked reduction of their capacities to bind to heparin and mammalian cell surfaces without affecting their enzymatic activities toward dispersed phospholipid as a substrate. CHO cells stably transfected with wild-type sPLA2 released about twice as much arachidonic acid (AA) during culture for 10 h with fetal calf serum and interleukin-1beta than cells transfected with vector alone, whereas the ability to enhance AA release was impaired in sPLA2 mutants incapable of binding to cell surfaces. AA released by wild-type sPLA2-transfected CHO cells was metabolized to prostaglandin E2 via prostaglandin endoperoxide H synthase (PGHS)-2 after IL-1beta stimulation, revealing a particular functional linkage of sPLA2 to PGHS-2. In contrast, A23187-initiated immediate AA release over 30 min was not affected by sPLA2 overexpression. Taken together, these results suggest that sPLA2 expressed endogenously and anchored on cell surfaces via its C-terminal heparin-binding domain is involved in the PGHS-2-dependent delayed PG biosynthesis initiated by growth factors and cytokines during long term culture.

A heparin-sensitive phospholipase A2 and prostaglandin endoperoxide synthase-2 are functionally linked in the delayed phase of prostaglandin D2 generation in mouse bone marrow-derived mast cells

BALB/cJ mouse bone marrow-derived mast cells (BMMC) developed with interleukin (IL)-3 can be stimulated by c-kit ligand (KL) in the presence of IL-10 and IL-1beta for sequential immediate and delayed generation of prostaglandin (PG) D2 through utilization of constitutive prostaglandin endoperoxide synthase (PGHS) -1 and induced PGHS-2, respectively (Murakami, M., Matsumoto, R., Austen, K. F., and Arm, J. P. (1994) J. Biol. Chem. 269, 22269-22275). We now report that BALB/cJ BMMC stimulated with KL + IL-10 + IL-1beta also exhibit the biphasic release of [3H]arachidonic acid with an immediate phase over the first 10 min followed by a delayed phase from 2 to 7 h. The delayed phase of arachidonic acid release and of PGD2 generation was inhibited by heparin, which concomitantly released a phospholipase (PL) A2 from the cells into the supernatant. Both dexamethasone and a type II PLA2 inhibitor, 12-epi-scalaradial, suppressed delayed-phase PGD2 generation at concentrations that did not affect immediate eicosanoid generation. Transcripts for type IIA PLA2, as assessed by reverse transcription-polymerase chain reaction, were progressively induced in BALB/cJ BMMC treated for 2 to 7 h with KL + IL-10 + IL-1beta; the induction of these transcripts was down-regulated by 10(-6) M dexamethasone. The expression of steady-state transcripts and protein for cytosolic PLA2 (cPLA2) did not change. PGHS-2-dependent delayed-phase PGD2 generation elicited by IgE-dependent activation of BALB/cJ BMMC primed with KL + IL-10 was also accompanied by the induction of type IIA PLA2 transcripts and was suppressed by heparin, with concomitant release of PLA2 into the supernatant. However, both the direct, cytokine-stimulated and the cytokine-primed, IgE-dependent, delayed-phase PGD2 generation occurred in BMMC from C57BL/6J mice, which have a natural disruption of the type IIA PLA2 gene. Thus, kinetic, pharmacologic, and genetic analyses suggest that an inducible, heparin-sensitive PLA2, rather than cPLA2, provides arachidonic acid to concomitantly induced PGHS-2 for delayed-phase PGD2 biosynthesis in activated BMMC. Furthermore, this heparin-sensitive PLA2 likely represents a novel PLA2 or a new function for a known low molecular weight PLA2.

Colocalization of cytosolic phospholipase A2, 5-lipoxygenase, and 5-lipoxygenase-activating protein at the nuclear membrane of A23187-stimulated human neutrophils

The distribution of cytosolic phospholipase A2 (cPLA2), arachidonate 5-lipoxygenase, and 5-lipoxygenase-activating protein (5-LAP) was investigated in subcellular fractions of human neutrophils disrupted by three techniques. As determined by immunoblot analysis, the bulk of cPLA2 and 5-lipoxygenase was detected in cytosolic fractions of unstimulated neutrophils disrupted by sonication or cavitation. After cell stimulation with the calcium ionophore A23187, both proteins accumulated primarily in nuclei-containing fractions; this accumulation was accompanied by a loss of these enzymes from cytosolic fractions. Further resolution of nuclear fractions revealed that 5-lipoxygenase and cPLA2 were localized in a fraction that contained nuclear membranes. In comparison, 5-LAP was localized to the nuclear-membrane fraction of resting and activated neutrophils, as determined by immunoblotting and photoaffinity labeling. In agreement with the immunoblot data, A23187 stimulation markedly enhanced 5-lipoxygenase enzymatic activity in the nuclear-membrane fraction, which was accompanied by decreased cytosolic 5-lipoxygenase activity. Similarly, neutrophil activation caused increased phosphorylation of cPLA2, a process that is known to result in enhanced catalytic activity. Our data demonstrate that in activated human neutrophils, the key proteins involved in leukotriene synthesis colocalize at the nuclear membrane, in a catalytically active state.

Interferon-alpha-induced phosphorylation and activation of cytosolic phospholipase A2 is required for the formation of interferon-stimulated gene factor three

Treatment of cells with interferon (IFN)-alpha caused phosphorylation and activation of cytosolic phospholipase A2 (cPLA2). The protein tyrosine kinase Jak1 was found to be necessary for the activation of cPLA2. Jak1 could be co-immunoprecipitated with cPLA2 from cell extracts, indicating that a close physical interaction occurs between these two proteins. The induction of IFN-stimulated gene factor three (ISGF3) by IFN-alpha, is blocked by cPLA2 inhibitors in cell cultures and in cell-free reconstituted systems. However, these inhibitors do not block IFN-alpha or gamma-induced binding of STAT1 to the inverted repeat (IR) element of the IFN regulatory factor 1 (IRF-1) gene. Thus, cPLA2 activations occurs as an early event in the IFN-alpha response and is selectively involved in ISGF3-dependent gene activation.

Identification of phosphorylation sites of human 85-kDa cytosolic phospholipase A2 expressed in insect cells and present in human monocytes

The phosphorylation sites on the human, 85-kDa cytosolic phospholipase A2 (cPLA2) were identified using recombinant cPLA2 expressed in Spodoptera frugiperda (Sf9) cells. Analysis by high performance liquid chromatography of tryptic digests of 32P-labeled recombinant cPLA2 showed four major peaks of radiolabeled phosphopeptides. The phosphorylated residues were identified as Ser-437, Ser-454, Ser-505, and Ser-727 using mass spectrometry and automated Edman sequencing. Sf9 cells infected with recombinant virus expressing cPLA2 exhibited a time-dependent release of arachidonic acid in response to the calcium ionophore A23187 or the protein phosphatase inhibitor okadaic acid, which was not observed in Sf9 cells infected with wild-type virus. Stimulation of Sf9 cells with A23187 and okadaic acid also increased the level of phosphorylation of cPLA2. Okadaic acid, but not A23187, induced a gel shift of cPLA2 and increased the level of phosphorylation of Ser-727 by 4.5-fold, whereas the level of phosphorylation of the other sites increased by 60% or less in response to both agonists. To determine whether the same sites on cPLA2 were phosphorylated in mammalian cells, human monocytes were studied. Okadaic acid stimulation of monocytes induced a gel shift of cPLA2, increased the release of arachidonic acid, and increased the level of phosphorylation of cPLA2 on serine residues. Comparison of two-dimensional peptide maps of tryptic digests of 32P-labeled recombinant cPLA2 and human monocyte cPLA2 demonstrated that the same peptides on cPLA2 were phosphorylated in mammalian cells as in insect cells. These results show that the Sf9-baculovirus expression system is useful for investigation of the phosphorylation sites on cPLA2. The results also suggest that phosphorylation of the cPLA2 by protein kinases other than mitogen-activated protein kinase may be important for the regulation of arachidonic acid release.

Inhibition of monocyte chemotaxis to C-C chemokines by antisense oligonucleotide for cytosolic phospholipase A2

Monocyte chemotactic protein (MCP)-1, a member of the C-C (or beta) branch of the chemokine superfamily, at chemotactic concentrations, induced a rapid release of [3H]arachidonic acid but not of [14C]oleic acid from prelabeled human monocytes. This effect was associated with an increase in the intensity of the immunoreactive band corresponding to the phosphorylated form of cytosolic phospholipase A2, (cPLA2). To address the role of cPLA2 in the induction of monocyte chemotaxis, cells were treated with a specific antisense oligonucleotide. Monocytes cultured in the presence of 10 microM antisense oligonucleotide for 48 h showed a marked decrease (57 +/- 5%; n = 4) of cPLA2 expression, as evaluated by Western blot analysis and a nearly complete inhibition (81.8 +/- 4.2%; n = 3) of [3H]arachidonic acid release in MCP-1-stimulated cells. Monocyte chemotaxis in response to MCP-l also was inhibited in a concentration-dependent manner by cPLA2 antisense oligonucleotide (IC50 = 1.9 +/- 1.1 microM; n = 3), with complete inhibition observed between 3 and 10 microM. No inhibition of chemotactic response was observed in monocytes treated with a control oligonucleotide. Monocyte migration in response to MCP-3, RANTES (regulated on activation normal T cells expressed and secreted), and MIP-1 alpha/LD78 also was inhibited (>70%) in antisense oligonucleotide-treated cells. On the contrary, the chemotactic response elicited by formyl-methionyl-leucyl-phenylalanine and C5a, two "classical" chemotactic agonists, was minimally affected (<20%) by antisense oligonucleotide treatment. These data show that cPLA2 plays a major role in [3H]arachidonic acid release by MCP-1 in human monocytes and provide direct evidence for the involvement of cPLA2 in C-C chemokine-induced monocyte chemotaxis.

Tumor necrosis factor-alpha induces the 85-kDa cytosolic phospholipase A2 gene expression in human bronchial epithelial cells

Phospholipase A2 (PLA2) activity has been suggested to mediate some of the tumor necrosis factor (TNF) induced cellular responses including cytotoxicity. We evaluated the induction of both the 85-kDa cytosolic phospholipase A2 (cPLA2) and non-pancreatic group II PLA2 gene expression by TNF-alpha in a human bronchial epithelial cell line (BEAS 2B cell). TNF-alpha (20 ng/ml) induced a significantly increased release of prelabeled [3H]arachidonic acid (AA) following 4-24 h incubation. Calcium ionophore A23187 (10(-5) M) further increased the [3H]AA release from the TNF-alpha-treated cells. In vitro activity assay revealed that TNF-alpha increased the dithiothreitol (DTT)-resistant PLA2 activity which was blocked by the cPLA2 inhibitor AACOCF3. Treatment with TNF-alpha for 4-24 h increased the cPLA2 protein and mRNA levels which were blocked by the broad inhibitor of protein kinases staurosporine, the protein kinase C (PKC) inhibitor calphostin C, and to a lesser extent the calcium/calmodulin-dependent protein kinase inhibitor W-7. Reverse transcription and polymerase chain reaction amplification of the group II PLA2 mRNA showed that it is expressed in human lung but not in the bronchial epithelial cell line. TNF-alpha failed to induce the expression of group II PLA2 in the BEAS 2B cells. These results demonstrate that the cPLA2 gene expression is up-regulated by TNF-alpha and this effect may contribute to the TNF-alpha stimulated AA release in airway epithelial cells.

Elevated cPLA2 levels as a mechanism by which the p70 TNF and p75 NGF receptors enhance apoptosis

The 70-kDa TNF cell surface receptor (p70TNFR) and the related 75-kDa nerve growth factor (NGF) receptor (p75NGFR) can enhance cell death. Expression of p70TNFR or p75NGFR in HeLa cells resulted in enhanced TNF-induced apoptotic cell death with a corresponding elevation of cytosolic phospholipase A2 (cPLA2) levels. This response was apparent by 24 h, did not occur with NGF treatment or when vector or TrkA NGFR were expressed and was reversed by dexamethasone pretreatment. These findings reveal a novel mechanism by which the p70TNFR and p75NGFR achieve enhanced TNFR-mediated programmed cell death by elevating cPLA2 levels.