The incorporation of arachidonic acid into triacylglycerol in P388D1 macrophage-like cells

Neutral Lipids Are Not a Source of Arachidonic Acid for Lipid Mediator Signaling in Human Foamy Monocytes

Human monocytes exposed to free arachidonic acid (AA), a secretory product of endothelial cells, acquire a foamy phenotype which is due to the accumulation of cytoplasmic lipid droplets with high AA content. Recruitment of foamy monocytes to the inflamed endothelium contributes to the development of atherosclerotic lesions. In this work, we investigated the potential role of AA stored in the neutral lipids of foamy monocytes to be cleaved by lipases and contribute to lipid mediator signaling. To this end, we used mass spectrometry-based lipidomic approaches combined with strategies to generate monocytes with different concentrations of AA. Results from our experiments indicate that the phospholipid AA pool in monocytes is stable and does not change upon exposure of the cells to the external AA. On the contrary, the AA pool in triacylglycerol is expandable and can accommodate relatively large amounts of fatty acid. Stimulation of the cells with opsonized zymosan results in the expected decreases of cellular AA. Under all conditions examined, all of the AA decreases observed in stimulated cells were accounted for by decreases in the phospholipid pool; we failed to detect any contribution of the triacylglycerol pool to the response. Experiments utilizing selective inhibitors of phospholipid or triacylglyerol hydrolysis confirmed that the phospholipid pool is the sole contributor of the AA liberated by stimulated cells. Thus, the AA in the triacylglycerol is not a source of free AA for the lipid mediator signaling during stimulation of human foamy monocytes and may be used for other cellular functions.

Simultaneous activation of p38 and JNK by arachidonic acid stimulates the cytosolic phospholipase A2-dependent synthesis of lipid droplets in human monocytes

Exposure of human peripheral blood monocytes to free arachidonic acid (AA) results in the rapid induction of lipid droplet (LD) formation by these cells. This effect appears specific for AA in that it is not mimicked by other fatty acids, whether saturated or unsaturated. LDs are formed by two different routes: (i) the direct entry of AA into triacylglycerol and (ii) activation of intracellular signaling, leading to increased triacylglycerol and cholesteryl ester formation utilizing fatty acids coming from the de novo biosynthetic route. Both routes can be dissociated by the arachidonyl-CoA synthetase inhibitor triacsin C, which prevents the former but not the latter. LD formation by AA-induced signaling predominates, accounting for 60-70% of total LD formation, and can be completely inhibited by selective inhibition of the group IVA cytosolic phospholipase A(2)α (cPLA(2)α), pointing out this enzyme as a key regulator of AA-induced signaling. LD formation in AA-treated monocytes can also be blocked by the combined inhibition of the mitogen-activated protein kinase family members p38 and JNK, which correlates with inhibition of cPLA(2)α activation by phosphorylation. Collectively, these results suggest that concomitant activation of p38 and JNK by AA cooperate to activate cPLA(2)α, which is in turn required for LD formation possibly by facilitating biogenesis of this organelle, not by regulating neutral lipid synthesis.

Phospholipid sources for adrenic acid mobilization in RAW 264.7 macrophages. Comparison with arachidonic acid

Cells metabolize arachidonic acid (AA) to adrenic acid (AdA) via 2-carbon elongation reactions. Like AA, AdA can be converted into multiple oxygenated metabolites, with important roles in various physiological and pathophysiological processes. However, in contrast to AA, there is virtually no information on how the cells regulate the availability of free AdA for conversion into bioactive products. We have used a comparative lipidomic approach with both gas chromatography and liquid chromatography coupled to mass spectrometry to characterize changes in the levels of AA- and AdA-containing phospholipid species in RAW 264.7 macrophage-like cells. Incubation of the cells with AA results in an extensive conversion to AdA but both fatty acids do not compete with each other for esterification into phospholipids. AdA but not AA, shows preference for incorporation into phospholipids containing stearic acid at the sn-1 position. After stimulation of the cells with zymosan, both AA and AdA are released in large quantities, albeit AA is released to a greater extent. Finally, a variety of phosphatidylcholine and phosphatidylinositol molecular species contribute to AA; however, AdA is liberated exclusively from phosphatidylcholine species. Collectively, these results identify significant differences in the cellular utilization of AA and AdA by the macrophages, suggesting non-redundant biological actions for these two fatty acids.

Altered arachidonate distribution in macrophages from caveolin-1 null mice leading to reduced eicosanoid synthesis

In this work we have studied the effect of caveolin-1 deficiency on the mechanisms that regulate free arachidonic acid (AA) availability. The results presented here demonstrate that macrophages from caveolin-1-deficient mice exhibit elevated fatty acid incorporation and remodeling and a constitutively increased CoA-independent transacylase activity. Mass spectrometry-based lipidomic analyses reveal stable alterations in the profile of AA distribution among phospholipids, manifested by reduced levels of AA in choline glycerophospholipids but elevated levels in ethanolamine glycerophospholipids and phosphatidylinositol. Furthermore, macrophages from caveolin-1 null mice show decreased AA mobilization and prostaglandin E(2) and LTB(4) production upon cell stimulation. Collectively, these results provide insight into the role of caveolin-1 in AA homeostasis and suggest an important role for this protein in the eicosanoid biosynthetic response.

Influence of cellular arachidonic acid levels on phospholipid remodeling and CoA-independent transacylase activity in human monocytes and U937 cells

The availability of free arachidonic acid (AA) constitutes a limiting step in the synthesis of biologically active eicosanoids. Free AA levels in cells are regulated by a deacylation/reacylation cycle of membrane phospholipids, the so-called Lands cycle, as well as by further remodeling reactions catalyzed by CoA-independent transacylase. In this work, we have comparatively investigated the process of AA incorporation into and remodeling between the various phospholipid classes of human monocytes and monocyte-like U937 cells. AA incorporation into phospholipids was similar in both cell types, but a marked difference in the rate of remodeling was appreciated. U937 cells remodeled AA at a much faster rate than human monocytes. This difference was found not to be related to the differentiation state of the U937 cells, but rather to the low levels of esterified arachidonate found in U937 cells compared to human monocytes. Incubating the U937 cells in AA-rich media increased the cellular content of this fatty acid and led to a substantial decrease of the rate of phospholipid AA remodeling, which was due to reduced CoA-independent transacylase activity. Collectively, these findings provide the first evidence that cellular AA levels determine the amount of CoA-independent transacylase activity expressed by cells and provide support to the notion that CoA-IT is a major regulator of AA metabolism in human monocytes.

Signaling role for lysophosphatidylcholine acyltransferase 3 in receptor-regulated arachidonic acid reacylation reactions in human monocytes

Cellular availability of free arachidonic acid (AA) is an important step in the production of pro- and anti-inflammatory eicosanoids. Control of free AA levels in cells is carried out by the action of phospholipase A2s and lysophospholipid acyltransferases, which are responsible for the reactions of deacylation and incorporation of AA from and into the sn-2 position of phospholipids, respectively. In this work, we have examined the pathways for AA incorporation into phospholipids in human monocytes stimulated by zymosan. Our data show that stimulated cells exhibit an enhanced incorporation of AA into phospholipids that is not secondary to an increased availability of lysophospholipid acceptors due to phospholipase A2 activation but rather reflects the receptor-regulated nature of the AA reacylation pathway. In vitro activity measurements indicate that the receptor-sensitive step of the AA reacylation pathway is the acyltransferase using lysophosphatidylcholine (lysoPC) as acceptor, and inhibition of the enzyme lysoPC acyltransferase 3 by specific small interfering RNA results in inhibition of the stimulated incorporation of AA into phospholipids. Collectively, these results define lysoPC acyltransferase 3 as a novel-signal-regulated enzyme that is centrally implicated in limiting free AA levels in activated cells.

Anti-inflammatory agent indomethacin reduces invasion and alters metabolism in a human breast cancer cell line

Hostile physiological environments such as hypoxia and acidic extracellular pH, which exist in solid tumors, may promote invasion and metastasis through inflammatory responses and formation of eicosanoids. Here, we have investigated the effects of the anti-inflammatory agent indomethacin on the invasion and metabolism of the human breast cancer cell line MDA-MB-435 in Dulbecco's Modified Eagles (DME)-based or Roswell Park Memorial Institute (RPMI)-based cell medium, using a magnetic resonance-compatible invasion assay. Indomethacin treatment significantly reduced the invasion of MDA-MB-435 cells independent of the culture and perfusion conditions examined. Significant changes were detected in levels of intracellular choline phospholipid metabolites and in triglyceride (TG) concentrations of these cells, depending on indomethacin treatment and basal cell medium used. Additionally, genetic profiling of breast cancer cells, grown and treated with low-dose indomethacin in cell culture using an RPMI-based medium, revealed the upregulation of several genes implicating cyclooxygenase-independent targets of indomethacin. These data confirm the ability of an anti-inflammatory agent to reduce breast cancer invasion and demonstrate, depending on cell culture and perfusion conditions, that the indomethacin-induced decrease in invasion is associated with changes in choline phospholipid metabolism, TG metabolism, and gene expression.

Blockade of arachidonic acid incorporation into phospholipids induces apoptosis in U937 promonocytic cells

Arachidonic acid (AA) participates in a reacylation/deacylation cycle of membrane phospholipids, the so-called Lands cycle, that serves to keep the concentration of this free fatty acid in cells at a very low level. To manipulate the intracellular AA level in U937 phagocytes, we have used several pharmacological strategies to interfere with the Lands cycle. We used inhibitors of the AA reacylation pathway, namely thimerosal and triacsin C, which block the conversion of AA into arachidonoyl-CoA, and a CoA-independent transacylase inhibitor that blocks the movement of AA within phospholipids. In addition, we used cells overexpressing group VIA phospholipase A(2), an enzyme with key roles in controlling basal fatty acid deacylation reactions in phagocytic cells. All of these different strategies resulted in the expected increase of cellular free AA but also in the induction of cell death by apoptosis. Moreover, when used in combination with any of the aforementioned drugs, AA itself was able to induce apoptosis at doses as low as 10 muM. Blocking cyclooxygenase or lipoxygenases had no effect on the induction of apoptosis by AA. Collectively, these results indicate that free AA levels within the cells may provide an important cellular signal for the onset of apoptosis and that perturbations of the mechanisms controlling AA reacylation, and hence free AA availability, may decisively affect cell survival.

Effect of dietary fat saturation on lipid metabolism, arachidonic acid turnover and peritoneal macrophage oxidative stress in mice

We investigated the effects of a saturated fat diet on lipid metabolism and arachidonic acid (AA) turnover in mouse resident peritoneal macrophages. The pro-oxidative effect of this diet was also studied. Female C57BL/6 mice were weaned at 21 days of age and assigned to either the experimental diet containing coconut oil (COCO diet), or the control diet containing soybean oil as fat source (10 mice per group). The fat content of each diet was 15% (w/w). Mice were fed for 6 weeks and then sacrificed. The concentration of total lipids, triglycerides, (LDL+VLDL)-cholesterol, thiobarbituric acid-reactive substances (TBARS) and reduced glutathione were increased in the plasma of mice fed the COCO diet, without changes in phospholipid or total cholesterol concentrations compared to control. The concentrations of total cholesterol, free and esterified cholesterol, triglycerides, and TBARS were increased in the macrophages of COCO-fed mice, while the content of total phospholipids did not change. The phospholipid composition showed an increase of phosphatidylcholine and a decrease of phosphatidylethanolamine. The [3H]-AA distribution in the phospholipid classes showed an increase in phosphatidylcholine and phosphatidylethanolamine. Incorporation of [3H]-cholesterol into the macrophages of COCO-fed mice and into the cholesterol ester fraction was increased. The COCO diet did not affect [3H]-AA uptake but induced an increase in [3H]-AA release. The COCO diet also enhanced AA mobilization induced by lipopolysaccharide. These results indicate that the COCO diet, high in saturated fatty acids, alters the lipid metabolism and AA turnover of peritoneal macrophages in female mice and also produces a significant degree of oxidative stress.

Role of group V phospholipase A2 in zymosan-induced eicosanoid generation and vascular permeability revealed by targeted gene disruption

Conclusions regarding the contribution of low molecular weight secretory phospholipase A2 (sPLA2) enzymes in eicosanoid generation have relied on data obtained from transfected cells or the use of inhibitors that fail to discriminate between individual members of the large family of mammalian sPLA2 enzymes. To elucidate the role of group V sPLA2, we used targeted gene disruption to generate mice lacking this enzyme. Zymosan-induced generation of leukotriene C4 and prostaglandin E2 was attenuated approximately 50% in peritoneal macrophages from group V sPLA2-null mice compared with macrophages from wild-type littermates. Furthermore, the early phase of plasma exudation in response to intraperitoneal injection of zymosan and the accompanying in vivo generation of cysteinyl leukotrienes were markedly attenuated in group V sPLA2-null mice compared with wild-type controls. These data provide clear evidence of a role for group V sPLA2 in regulating eicosanoid generation in response to an acute innate stimulus of the immune response both in vitro and in vivo, suggesting a role for this enzyme in innate immunity.

Transcriptional regulation of genes for enzymes of the prostaglandin biosynthetic pathway

Numerous studies over the years have demonstrated changes in prostaglandin (PG) levels in intrauterine tissues in association with labour, and PG administration has long been used to induce delivery. While it is now widely accepted that PGs play a major role in human parturition, the complex regulation of their levels is still being elucidated, with the focus on the transcriptional control of the enzymes responsible for the various steps in PG biosynthesis and catabolism.

Bromoenol lactone promotes cell death by a mechanism involving phosphatidate phosphohydrolase-1 rather than calcium-independent phospholipase A2

Originally described as a serine protease inhibitor, bromoenol lactone (BEL) has recently been found to potently inhibit Group VI calcium-independent phospholipase A2 (iPLA2). Thus, BEL is widely used to define biological roles of iPLA2 in cells. However, BEL is also known to inhibit another key enzyme of phospholipid metabolism, namely the magnesium-dependent phosphatidate phosphohydrolase-1 (PAP-1). In this work we report that BEL is able to promote apoptosis in a variety of cell lines, including U937, THP-1, and MonoMac (human phagocyte), RAW264.7 (murine macrophage), Jurkat (human T lymphocyte), and GH3 (human pituitary). In these cells, long term treatment with BEL (up to 24 h) results in increased annexin-V binding to the cell surface and nuclear DNA damage, as detected by staining with both DAPI and propidium iodide. At earlier times (2 h), BEL induces the proteolysis of procaspase-9 and procaspase-3 and increases cleavage of poly(ADP-ribose) polymerase. These changes are preceded by variations in the mitochondrial membrane potential. All these effects of BEL are not mimicked by the iPLA2 inhibitor methylarachidonyl fluorophosphonate or by treating the cells with a specific iPLA2 antisense oligonucleotide. However, propranolol, a PAP-1 inhibitor, is able to reproduce these effects, suggesting that it is the inhibition of PAP-1 and not of iPLA2 that is involved in BEL-induced cell death. In support of this view, BEL-induced apoptosis is accompanied by a very strong inhibition of PAP-1-regulated events, such as incorporation of [3H]choline into phospholipids and de novo incorporation of [3H]arachidonic acid into triacylglycerol. Collectively, these results stress the role of PAP-1 as a key enzyme for cell integrity and survival and in turn caution against the use of BEL in studies involving long incubation times, due to the capacity of this drug to induce apoptosis in a variety of cells.

In vivo phospholipase activity of the Pseudomonas aeruginosa cytotoxin ExoU and protection of mammalian cells with phospholipase A2 inhibitors

A number of clinical isolates of Pseudomonas aeruginosa are cytotoxic to mammalian cells due to the action of the 74-kDa protein ExoU, which is secreted into host cells by the type III secretion system and whose function is unknown. Here we report that the swift and profound cytotoxicity induced by purified ExoU or by an ExoU-expressing strain of P. aeruginosa is blocked by various inhibitors of cytosolic (cPLA2) and Ca2+ -independent (iPLA2) phospholipase A2 enzymes. In contrast, no cytoprotection is offered by inhibitors of secreted phospholipase A2 enzymes or by a number of inhibitors of signal transduction pathways. This suggests that phospholipase A2 inhibitors may represent a novel mode of treatment for acute P. aeruginosa infections. We find that 300-600 molecules of ExoU/cell are required to achieve half-maximal cell killing and that ExoU localizes to the host cell plasma membrane in punctate fashion. We also show that ExoU interacts in vitro with an inhibitor of cPLA2 and iPLA2 enzymes and contains a putative serine-aspartate catalytic dyad homologous to those found in cPLA2 and iPLA2 enzymes. Mutation of either the serine or the aspartate renders ExoU non-cytotoxic. Although no phospholipase or esterase activity is detected in vitro, significant phospholipase activity is detected in vivo, suggesting that ExoU requires one or more host cell factors for activation as a membrane-lytic and cytotoxic phospholipase.

Studies of insulin secretory responses and of arachidonic acid incorporation into phospholipids of stably transfected insulinoma cells that overexpress group VIA phospholipase A2 (iPLA2beta ) indicate a signaling rather than a housekeeping role for iPLA2beta

A cytosolic 84-kDa group VIA phospholipase A(2) (iPLA(2)beta) that does not require Ca(2+) for catalysis has been cloned from several sources, including rat and human pancreatic islet beta-cells and murine P388D1 cells. Many potential iPLA(2)beta functions have been proposed, including a signaling role in beta-cell insulin secretion and a role in generating lysophosphatidylcholine acceptors for arachidonic acid incorporation into P388D1 cell phosphatidylcholine (PC). Proposals for iPLA(2)beta function rest in part on effects of inhibiting iPLA(2)beta activity with a bromoenol lactone (BEL) suicide substrate, but BEL also inhibits phosphatidate phosphohydrolase-1 and a group VIB phospholipase A(2). Manipulation of iPLA(2)beta expression by molecular biologic means is an alternative approach to study iPLA(2)beta functions, and we have used a retroviral construct containing iPLA(2)beta cDNA to prepare two INS-1 insulinoma cell clonal lines that stably overexpress iPLA(2)beta. Compared with parental INS-1 cells or cells transfected with empty vector, both iPLA(2)beta-overexpressing lines exhibit amplified insulin secretory responses to glucose and cAMP-elevating agents, and BEL substantially attenuates stimulated secretion. Electrospray ionization mass spectrometric analyses of arachidonic acid incorporation into INS-1 cell PC indicate that neither overexpression nor inhibition of iPLA(2)beta affects the rate or extent of this process in INS-1 cells. Immunocytofluorescence studies with antibodies directed against iPLA(2)beta indicate that cAMP-elevating agents increase perinuclear fluorescence in INS-1 cells, suggesting that iPLA(2)beta associates with nuclei. These studies are more consistent with a signaling than with a housekeeping role for iPLA(2)beta in insulin-secreting beta-cells.

Identification of a third pathway for arachidonic acid mobilization and prostaglandin production in activated P388D1 macrophage-like cells

Previous studies have demonstrated that P388D(1) macrophages are able to mobilize arachidonic acid (AA) and synthesize prostaglandins in two temporally distinct phases. The first phase is triggered by platelet-activating factor within minutes, but needs the cells to be previously exposed to bacterial lipopolysaccharide (LPS) for periods up to 1 h. It is thus a primed immediate phase. The second, delayed phase occurs in response to LPS alone over long incubation periods spanning several hours. Strikingly, the effector enzymes involved in both of these phases are the same, namely the cytosolic group IV phospholipase A(2) (cPLA(2)), the secretory group V phospholipase A(2), and cyclooxygenase-2, although the regulatory mechanisms differ. Here we report that P388D(1) macrophages mobilize AA and produce prostaglandins in response to zymosan particles in a manner that is clearly different from the two described above. Zymosan triggers an immediate AA mobilization response from the macrophages that neither involves the group v phospholipase A(2) nor requires the cells to be primed by LPS. The group VI Ca(2+)-independent phospholipase A(2) is also not involved. Zymosan appears to signal exclusively through activation of the cPLA(2), which is coupled to the cyclooxygenase-2. These results define a secretory PLA(2)-independent pathway for AA mobilization in the P388D(1) macrophages, and demonstrate that, under certain experimental settings, stimulation of the cPLA(2) is sufficient to generate a prostaglandin biosynthetic response in the P388D(1) macrophages.

Cytosolic phospholipase A2 is required for macrophage arachidonic acid release by agonists that Do and Do not mobilize calcium. Novel role of mitogen-activated protein kinase pathways in cytosolic phospholipase A2 regulation

The 85-kDa cytosolic phospholipase A(2) (cPLA(2)) mediates agonist-induced arachidonic acid release and eicosanoid production. Calcium and phosphorylation on Ser-505 by mitogen-activated protein kinases (MAPKs) regulate cPLA(2). Arachidonic acid release and eicosanoid production induced by stimuli that do (A23187, zymosan) or do not (phorbol myristate acetate (PMA), okadaic acid) mobilize calcium were quantitatively suppressed in cPLA(2)-deficient mouse peritoneal macrophages. The contribution of MAPKs to cPLA(2)-mediated arachidonic acid release was investigated. Both extracellular signal-regulated kinases (ERKs) and p38 contributed to cPLA(2) phosphorylation on Ser-505. However, although ERK inhibition did not affect A23187-induced arachidonic acid release, it suppressed zymosan-, PMA-, and okadaic acid-induced arachidonic acid release under conditions where phosphorylation of cPLA(2) on Ser-505 was unaffected. This indicates an additional regulatory mechanism for the ERK pathway. A role for transcriptional regulation is suggested by data showing that cycloheximide and actinomycin D inhibited arachidonic acid release induced by zymosan, PMA and, okadaic acid but not by A23187. Our results show that MAPK pathways contribute to arachidonic acid release in macrophages through alternative mechanisms in addition to their ability to phosphorylate cPLA(2) on Ser-505 and suggest a role for new protein synthesis.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

Perturbations in the control of cellular arachidonic acid levels block cell growth and induce apoptosis in HL-60 cells

Our previous studies demonstrated that inhibitors of arachidonate-phospholipid remodeling [i.e. the enzyme CoA-independent transacylase (CoA-IT)] decrease cell proliferation and induce apoptosis in neoplastic cells. The goal of the current study was to elucidate the molecular events associated with arachidonate-phospholipid remodeling that influence cell proliferation and survival. Initial experiments revealed the essential nature of cellular arachidonate to the signaling process by demonstrating that HL-60 cells depleted of arachidonate were more resistant to apoptosis induced by CoA-IT inhibition. In cells treated with CoA-IT inhibitors a marked increase in free arachidonic acid and AA-containing triglycerides were measured. TG enrichment was likely due to acylation of arachidonic acid into diglycerides and triglycerides via de novo glycerolipid biosynthesis. To determine the potential of free fatty acids to affect cell proliferation, HL-60 cells were incubated with varying concentrations of free fatty acids; exogenously provided 20-carbon polyunsaturated fatty acids caused a dose-dependent inhibition of cell proliferation, whereas oleic acid was without effect. Blocking 5-lipoxygenase or cyclooxygenases had no effect on the inhibition of cell proliferation induced by arachidonic acid or CoA-IT inhibitors. An increase in cell-associated ceramides (mainly in the 16:0-ceramide fraction) was measured in cells exposed to free arachidonic acid or to CoA-IT inhibitors. This study, in conjunction with other recent studies, suggests that perturbations in the control of cellular arachidonic acid levels affect cell proliferation and survival.

Regulation of delayed prostaglandin production in activated P388D1 macrophages by group IV cytosolic and group V secretory phospholipase A2s

Group V secretory phospholipase A2 (sPLA2) rather than Group IIA sPLA2 is involved in short term, immediate arachidonic acid mobilization and prostaglandin E2 (PGE2) production in the macrophage-like cell line P388D1. When a new clone of these cells, P388D1/MAB, selected on the basis of high responsivity to lipopolysaccharide plus platelet-activating factor, was studied, delayed PGE2 production (6-24 h) in response to lipopolysaccharide alone occurred in parallel with the induction of Group V sPLA2 and cyclooxygenase-2 (COX-2). No changes in the level of cytosolic phospholipase A2 (cPLA2) or COX-1 were observed, and Group IIA sPLA2 was not detectable. Use of a potent and selective sPLA2 inhibitor, 3-(3-acetamide 1-benzyl-2-ethylindolyl-5-oxy)propanesulfonic acid (LY311727), and an antisense oligonucleotide specific for Group V sPLA2 revealed that delayed PGE2 was largely dependent on the induction of Group V sPLA2. Also, COX-2, not COX-1, was found to mediate delayed PGE2 production because the response was completely blocked by the specific COX-2 inhibitor NS-398. Delayed PGE2 production and Group V sPLA2 expression were also found to be blunted by the inhibitor methylarachidonyl fluorophosphonate. Because inhibition of Ca2+-independent PLA2 by an antisense technique did not have any effect on the arachidonic acid release, the data using methylarachidonyl fluorophosphonate suggest a key role for the cPLA2 in the response as well. Collectively, the results suggest a model whereby cPLA2 activation regulates Group V sPLA2 expression, which in turn is responsible for delayed PGE2 production via COX-2.

Selective Inhibitors of Cytosolic or Secretory Phospholipase A2 Block TNF-Induced Activation of Transcription Factor Nuclear Factor-κB and Expression of ICAM-1

TNF signaling mechanisms involved in activation of transcription factor NF-κB were studied in the human keratinocyte cell line HaCaT. We show that TNF-induced activation of NF-κB was inhibited by the well-known selective inhibitors of cytosolic phospholipase A2 (cPLA2): the trifluoromethyl ketone analogue of arachidonic acid (AACOCF3) and methyl arachidonyl fluorophosphate. The trifluoromethyl ketone analogue of eicosapentaenoic acid (EPACOCF3) also suppressed TNF-induced NF-κB activation and inhibited in vitro cPLA2 enzyme activity with a similar potency as AACOCF3. The arachidonyl methyl ketone analogue (AACOCH3) and the eicosapentanoyl analogue (EPACHOHCF3), which both failed to inhibit cPLA2 enzyme activity in vitro, had no effect on TNF-induced NF-κB activation. TNF-induced NF-κB activation was also strongly reduced in cells stimulated in the presence of the secretory PLA2 (sPLA2) inhibitors 12-epi-scalaradial and LY311727. Addition of excess arachidonic acid suppressed the inhibitory effect of 12-epi-scalaradial and LY311727. Moreover, both methyl arachidonyl fluorophosphate and 12-epi-scalaradial blocked TNF-mediated enhancement of expression of ICAM-1. Activation of NF-κB by IL-1β was markedly less sensitive to both cPLA2 and sPLA2 inhibitors. The results indicate that both cPLA2 and sPLA2 may be involved in the TNF signal transduction pathway leading to nuclear translocation of NF-κB and to NF-κB-activated gene expression in HaCaT cells.

Functional coupling between secretory phospholipase A2 and cyclooxygenase-2 and its regulation by cytosolic group IV phospholipase A2

Secretory phospholipase A2 (sPLA2) is the major effector involved in arachidonic acid (AA) mobilization and prostaglandin E2 (PGE2) production during stimulation of P388D1 macrophages with the inflammatory stimuli bacterial lipopolysaccharide and platelet-activating factor. We herein demonstrate that PGE2 in stimulated P388D1 cells is accounted for by the inducible cyclooxygenase (COX)-2. COX-1, though present, appears not to participate significantly in stimulus-induced PGE2 production in P388D1 macrophages. Reconstitution experiments utilizing exogenous recombinant sPLA2 demonstrate that activation of the sPLA2 at the plasma membrane is highly dependent on previous activation of the cytosolic phospholipase A2 (cPLA2). Collectively these results demonstrate (i) that functional coupling exists between sPLA2 and COX-2 in activated cells, (ii) the critical role that cPLA2 plays in lipid mediator production, and (iii) that there is crosstalk between cPLA2 and sPLA2 in the cell.

Effect of chronic ethanol exposure on mouse brain arachidonic acid specific phospholipase A2

The enzyme phospholipase A2 (PLA2), which catalyzes the hydrolysis of an ester bond at the sn-2 position of 1,2-sn-diacylglycerols, has been suggested to play an important role in regulating cellular functions. Although ethanol (EtOH)-induced activation of PLA2 activity was reported previously by us in mouse brain (Hungund et al., Neurochem Int 25: 321-325, 1994), its subcellular localization and biochemical properties have not been investigated. Therefore, in the present study, we examined the subcellular localization and characterization of EtOH-activated PLA2 activity in mouse brain. The results indicated that EtOH treatment decreased the specific activity of PLA2 for the first 48 hr, and then the activity increased and reached a peak level in both cytosol (1.6-fold) and membrane (1.7-fold) fractions at 96 hr of exposure. Specific activity was found to be higher in the membrane fraction than in the cytosol. Using differential density gradient centrifugation, subcellular localization of the membrane-associated PLA2 revealed that most of the EtOH-activated PLA2 specific activity was associated with the synaptic membrane (44%) followed by the nuclear membrane (13%). No significant increase in the PLA2 specific activity of mitochondrial and microsomal membranes was observed. No activity was detected in the myelin membrane. PLA2 specific activity of membranes from control and EtOH-exposed mouse brain exhibited preference for arachidonic acid over linoleic acid at the sn-2 position of glycero-3-phosphocholine (PC). No detectable PLA2 specific activity was found when PC containing oleic acid at the sn-2 position was used as a substrate. The present results also indicated that the PLA2 specific activity of membrane from control and EtOH-exposed mouse brain was insensitive to dithiothreitol, strongly stimulated by Ca2+, enhanced by glycerol, and inhibited by the cytosolic PLA2 (cPLA2) inhibitor methyl arachidonyl fluorophosphonate with an IC50 value of 3.33 microM. In summary, results suggest that the properties of EtOH-activated PLA2 activity found in mouse brain membrane fraction are similar to those of cPLA2 found in variety of cells, including mammalian brain.

Antisense inhibition of group VI Ca2+-independent phospholipase A2 blocks phospholipid fatty acid remodeling in murine P388D1 macrophages

A major issue in lipid signaling relates to the role of particular phospholipase A2 isoforms in mediating receptor-triggered responses. This has been difficult to study because of the lack of isoform-specific inhibitors. Based on the use of the Group VI Ca2+-independent phospholipase A2 (iPLA2) inhibitor bromoenol lactone (BEL), we previously suggested a role for the iPLA2 in mediating phospholipid fatty acid turnover (Balsinde, J., Bianco, I. D., Ackermann, E. J., Conde-Frieboes, K., and Dennis, E. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92: 8527-8531). We have now further evaluated the role of the iPLA2 in phospholipid remodeling by using antisense RNA technology. We show herein that inhibition of iPLA2 expression by a specific antisense oligonucleotide decreases both the steady-state levels of lysophosphatidylcholine and the capacity of the cell to incorporate arachidonic acid into membrane phospholipids. These effects correlate with a decrease in both iPLA2 activity and protein in the antisense-treated cells. Collectively these data provide further evidence that the iPLA2 plays a major role in regulating phospholipid fatty acyl turnover in P388D1 macrophages. In stark contrast, experiments with activated cells confirmed that the iPLA2 does not play a significant role in receptor-coupled arachidonate mobilization in these cells, as manifested by the lack of an effect of the iPLA2 antisense oligonucleotide on PAF-stimulated arachidonate release.

Inflammatory activation of arachidonic acid signaling in murine P388D1 macrophages via sphingomyelin synthesis

Ceramide has emerged as an important lipid messenger for many cellular processes triggered via surface receptors. In the present study, inflammatory activation of P388D1 macrophages with bacterial lipopolysaccharide (LPS) and platelet-activating factor (PAF) stimulated a transient accumulation of ceramide. Moreover, cell-permeable ceramide mimicked LPS/PAF in triggering arachidonate mobilization in these cells. LPS/PAF-induced ceramide synthesis did not result from sphingomyelinase activation but from increased de novo synthesis. Participation of this pathway in arachidonate signaling was detected since fumonisin B1, an inhibitor of de novo ceramide synthesis, was able to inhibit the LPS/PAF-induced response. These studies have uncovered a new role for sphingolipid metabolism in cellular signaling and constitute evidence that products of the sphingomyelin biosynthetic pathway may serve a specific role in signal transduction by influencing the activity of the novel Group V secretory phospholipase A2.

Bromoenol lactone inhibits magnesium-dependent phosphatidate phosphohydrolase and blocks triacylglycerol biosynthesis in mouse P388D1 macrophages

Bromoenol lactone (BEL) has previously been identified as a potent, irreversible, mechanism-based phospholipase A2 (PLA2) inhibitor that possesses greater than 1000-fold selectivity for inhibition of Ca2+-independent PLA2 (iPLA2) versus the Ca2+-dependent ones. Thus, this compound has been used as a selective tool for studies aimed at elucidating the role of iPLA2 in certain cellular functions. Herein we report that BEL also inhibits cellular phosphatidic acid phosphohydrolase (PAP) activity in intact P388D1 macrophages with an IC50 of about 8 microM, which is very similar to that previously found for inhibition of iPLA2 under the same experimental conditions. This results in the blockage of the incorporation of exogenous arachidonate and palmitate into diacylglycerol and triacylglycerol. Thus, inhibition of PAP by BEL blocks triacylglycerol biosynthesis in P388D1 cells due to decreased diacylglycerol availability. Because two forms of PAP activity exist in mammalian cells, differential assays were performed to identify which of these forms was inhibited by BEL. The results of these experiments revealed that BEL selectively inhibits the cytosolic, Mg2+-dependent enzyme. No apparent effect of BEL on the membrane-bound Mg2+-independent PAP form could be detected. Collectively, the results reported herein establish that BEL inhibits two cellular phospholipases, namely iPLA2 and Mg2+-dependent PAP, with similar potency. Therefore, the inhibitory effect of BEL on Mg2+-dependent PAP might explain several cellular functions previously attributed to iPLA2.