Roles of various phospholipases A2 in providing lysophospholipid acceptors for fatty acid phospholipid incorporation and remodelling

Prostaglandin production in human coronary artery endothelial cells is modulated differentially by selective phospholipase A(2) inhibitors

Atherosclerotic plaque formation is a dynamic process involving repeated injury and inflammation of the endothelium. We have demonstrated previously that thrombin and tryptase stimulation of human coronary artery endothelial cells (HCAEC) leads to increased phospholipase A(2) (PLA(2)) activity and generation of membrane phospholipid derived inflammatory metabolites, including eicosanoids and platelet activating factor. Thus, our hypothesis is that selective PLA(2) inhibitors have therapeutic potential as anti-inflammatory agents. Stimulation of confluent HCAEC monolayers with thrombin or tryptase resulted in a concentration and time-dependent increase in both prostaglandin E(2) (PGE(2)) and prostacyclin (PGI(2)) production. Pretreatment with PX-18 to inhibit secretory PLA(2) or BEL to inhibit calcium-independent PLA(2) prior to thrombin or tryptase stimulation resulted in a significant inhibition of both PGI(2) and PGE(2) release. However, pretreatment with methyl arachidonyl fluorophosphonate (MAFP), a widely used inhibitor of cytosolic PLA(2) isoforms, resulted in a significant potentiation of both thrombin and tryptase stimulated PGI(2) and PGE(2) release as a consequence of increased free arachidonic acid production. We conclude that the use of selective PLA(2) inhibitors may be of therapeutic benefit in the development and progression of atherosclerosis, however, the development of such an agent requires rigorous screening.

Phospholipase A2 in chamber angle of normal eyes and patients with primary open angle glaucoma and exfoliation glaucoma

PURPOSE

Phospholipase A2 (PLA2) is a growing family of lipolytic enzymes that play a key role in various biological processes including general lipid metabolism, membrane homeostasis, and in diseases such as atherosclerosis, arthritis, and acute pancreatitis. Oxidative stress as well as inflammation may be associated with glaucoma pathogenesis. Therefore, our aim was to examine the expression of group IIA secretory PLA2 (sPLA2-IIA), group V secretory PLA2 (sPLA2-V), calcium-independent PLA2 (iPLA2), and cytosolic PLA2 (cPLA2) type in the trabecular meshwork (TM) and the canal of Schlemm in normal eyes and in juxtacanalicular tissue samples from patients with primary open angle glaucoma (POAG) or exfoliation glaucoma (ExG).

METHODS

TM tissues were isolated from healthy donor eyes for corneal transplantation. Specimens of inner wall of the Schlemm's canal and the juxtacanalicular tissue were collected during deep sclerectomy from the eyes of patients who had POAG or ExG. Antibodies against PLA2s (sPLA2-IIA, sPLA2-V, iPLA2, and cPLA2) and a standard immunohistochemical procedure were used for the analysis. Quantification of immunoreactions was provided using a Photoshop-based image analysis. Double-staining immunofluorescence of macrophages and sPLA2-IIA was performed by using confocal microscopy.

RESULTS

sPLA2-IIA was not present in normal TM. In contrast, sPLA2-IIA levels were significantly higher in glaucoma patients than in controls. Furthermore, sPLA2-IIA expression was much higher in POAG when compared to ExG. iPLA2 was found to predominate in normal human TM, and it demonstrated strong labeling in the uveal and corneoscleral meshwork. The staining of juxtacanalicular meshwork was only moderate in density. In contrast, expression of the enzyme was significantly decreased in glaucoma patients, especially in ExG, when compared to normal controls or to POAG. In addition, strong regional differences were detected in sPLA2-IIA and iPLA2 levels in POAG, whereas immunostaining of these enzymes was much lower and rather uniform throughout ExG sample. In POAG, sPLA2-IIA staining was restricted to certain parts of the trabecular samples where sPLA2-IIA positive macrophages were also present. Immunostaining of sPLA2-V or cPLA2 was low, and no significant changes were found in levels of these enzymes between normal and glaucomatous samples.

CONCLUSIONS

sPLA2-IIA, an oxidative stress marker in atherosclerosis, is overexpressed especially in POAG. This result supports the hypothesis that oxidative stress may play a significant role in the pathogenesis of POAG. In ExG, a dramatic decrease in the expression level of iPLA2, a housekeeping enzyme in phospholipid remodeling, may indicate imbalance in phospholipid turnover and also inhibition of normal physiological functions in the TM. These findings may contribute to understanding the pathogenesis of POAG and ExG and may be important for the development of novel therapeutic strategies to different glaucomas.

Omega-3 fatty acid deficiency in major depressive disorder is caused by the interaction between diet and a genetically determined abnormality in phospholipid metabolism

Omega-3 fatty acids are a type of polyunsaturated fatty acid (PUFA). A growing body of evidence suggests that this form PUFA is a useful and well tolerated treatment for major depressive disorder, a common and serious mental illness. The efficacy of omega-3 PUFA is routinely explained as being due to a deficiency caused by inadequate dietary intake of this class of fatty acid. The hypothesis considered states that low omega-3 PUFA abundance in patients with major depressive and related disorders is due to an underlying genetically determined abnormality. The hypothesis can explain why although a specific and consistent deficit in omega-3, but not omega-6, PUFA occurs in major depressive and related disorders, the literature does not consistently support the notion that this is due to deficient dietary intake. Specifically it is hypothesized that having genetically determined low activity of fatty acid CoA ligase 4 and/or Type IV phospholipase A(2) combined with the low dietary availability of omega-3 PUFA results in reduced cellular uptake of omega-3 PUFA and constitutes a risk factor for depression. The hypothesis also has important consequences for the pharmacological treatment of depression in that it predicts that administering agents which enhance phospholipid synthesis, particularly those containing ethanolamine such as CDP-ethanolamine, should be effective antidepressants especially when co-administered with omega-3 PUFA.

Calcium-independent phospholipase A2 and apoptosis

Apoptosis or programmed cell death is associated with changes in glycerophospholipid metabolism. Cells undergoing apoptosis generally release free fatty acids including arachidonic acid, which parallels the reduction in cell viability. The involvement of cytosolic group IVA phospholipase A(2)alpha (cPLA(2)alpha) in apoptosis has been the subject of numerous studies but a clear picture of the role(s) played by this enzyme is yet to emerge. More recently, the importance of lipid products generated by the action of a second phospholipase A(2), the group VIA calcium-independent phospholipase A(2) (iPLA(2)-VIA) in apoptosis has begun to be unveiled. Current evidence suggests that iPLA(2)-VIA-derived lysophosphatidylcholine may play a prominent role in mediating the chemoattractant and recognition/engulfment signals that accompany the process of apoptotic cell death, and gives possibility to the efficient clearance of dying cells by circulating phagocytes. Other lines of evidence suggest that perturbations in the control of free arachidonic acid levels within the cells, a process that may implicate iPLA(2)-VIA as well, may provide important cellular signals for the onset of apoptosis.

Insulin secretory responses and phospholipid composition of pancreatic islets from mice that do not express Group VIA phospholipase A2 and effects of metabolic stress on glucose homeostasis

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A(2) (iPLA(2)beta) activity in pancreatic islets and insulinoma cells suggest that iPLA(2)beta participates in insulin secretion. It has also been suggested that iPLA(2)beta is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA(2)beta-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA(2)beta-null mice have impaired insulin secretory responses to D-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA(2)beta-null mice is virtually identical to that of wild-type mice, and no iPLA(2)beta mRNA was observed in any tissue from iPLA(2)beta-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA(2)beta-null and wild-type mice are essentially identical under normal circumstances, but iPLA(2)beta-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the beta-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA(2)beta-null mice than in wild-type mice, but PLA(2)beta-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA(2)beta-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

Differential Roles for Cytosolic and Microsomal Ca2+-Independent Phospholipase A2in Cell Growth and Maintenance of Phospholipids

Physiological roles of microsomal (iPLA(2)gamma) and cytosolic (iPLA(2)beta)Ca(2+)-independent phospholipase A(2) were determined in two different epithelial cell models. R- and S-enantiomers of the iPLA(2) inhibitor bromoenol lactone (BEL) were isolated and shown to selectively inhibit iPLA(2gamma) and iPLA(2beta), respectively. The effect of these enantiomers on cell growth was assessed in human embryonic kidney 293 and Caki-1 cells using 3-(4-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT). S-BEL (0-5.0 microM) decreased MTT staining 35% after 24 h compared with control cells, whereas treatment with either R-BEL or R/S-BEL induced 15% decreases. Neither R-BEL nor S-BEL induced cell death as determined by annexin V and propidium iodide staining. Transfection of cells with iPLA(2)beta siRNA reduced MTT staining approximately 35%, whereas transfection of cells with iPLA(2)gamma siRNA only decreased MTT staining 10 to 15% compared with control cells. The effect of iPLA(2)beta and iPLA(2)gamma siRNA on cell number and protein was also determined, and iPLA(2)beta siRNA decreased cell number and protein 25% compared with control cells. In contrast, iPLA(2)gamma siRNA decreased cell number, but not cellular protein, compared with control cells. Selective inhibition of iPLA(2)beta, but not iPLA(2)gamma, decreased several arachidonic acid-containing phospholipids, including 16:1-20:4, 16:0-20:4, 18:1-20:4, and 18:0-20:4 phosphatidylcholine, showing that the ability of iPLA(2)beta inhibitors to decrease cell growth correlates with their ability to decrease arachidonic acid-containing phospholipids. These data show that iPLA(2)beta inhibition results in greater decreases in cell growth and proliferation than iPLA(2)gamma, identifies specific phospholipids whose expressions are differentially regulated by iPLA(2)beta and iPLA(2)gamma, and suggests novel roles for iPLA(2)beta in cell growth.

Attenuated secretion of very low density lipoproteins from McA-RH7777 cells treated with eicosapentaenoic acid is associated with impaired utilization of triacylglycerol synthesized via phospholipid remodeling

In McA-RH7777 cells stably expressing human apolipoprotein (apo) B100, treatment with oleic acid (18:1(n-9)) promoted whereas treatment with eicosapentaenoic acid (EPA, 20:5(n-3)) attenuated assembly and secretion of VLDL. Under conditions where the cells were cultured in the presence of 20% serum, EPA (0.4 mM) had marginal effect on the secretion of total apoB100 (determined by pulse-chase analysis) but decreased (by 50%) secretion of triacylglycerol (TG), indicating that the inhibitory effect of EPA was exerted primarily on TG-rich VLDL. Analysis of phospholipid mass and species by tandem mass spectrometry showed increased phosphatidylethanolamine (PE) in EPA-treated cells, the increase was significant in the distal Golgi membranes (by 170%) and endoplasmic reticulum (by 116%). Lipid pulse-chase studies showed a major distinction between phospholipid species containing 20:5(n-3) and 18:1(n-9), which in turn was associated with distinct compartmentalization of TG containing 20:5(n-3) or 18:1(n-9) between cytosol and microsomes and their recruitment during VLDL assembly. Thus, 18:1-TG was secreted as VLDL but 20:5-TG was not. These results suggest that EPA attenuation of VLDL secretion is associated with impaired utilization of TG derived from phospholipid remodeling.

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

Effects of stable suppression of Group VIA phospholipase A2 expression on phospholipid content and composition, insulin secretion, and proliferation of INS-1 insulinoma cells

Studies involving pharmacologic inhibition or transient reduction of Group VIA phospholipase A2 (iPLA2beta) expression have suggested that it is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels, rates of arachidonate incorporation into phospholipids, and degradation of excess phosphatidylcholine (PC). In insulin-secreting islet beta-cells and some other cells, in contrast, iPLA2beta signaling functions have been proposed. Using retroviral vectors, we prepared clonal INS-1 beta-cell lines in which iPLA2beta expression is stably suppressed by small interfering RNA. Two such iPLA2beta knockdown (iPLA2beta-KD) cell lines express less than 20% of the iPLA2beta of control INS-1 cell lines. The iPLA2beta-KD INS-1 cells exhibit impaired insulin secretory responses and reduced proliferation rates. Electrospray ionization mass spectrometric analyses of PC and LPC species that accumulate in INS-1 cells cultured with arachidonic acid suggest that 18:0/20:4-glycerophosphocholine (GPC) synthesis involves sn-2 remodeling to yield 16:0/20:4-GPC and then sn-1 remodeling via a 1-lyso/20:4-GPC intermediate. Electrospray ionization mass spectrometric analyses also indicate that the PC and LPC content and composition of iPLA2beta-KD and control INS-1 cells are nearly identical, as are the rates of arachidonate incorporation into PC and the composition and remodeling of other phospholipid classes. These findings indicate that iPLA2beta plays signaling or effector roles in beta-cell secretion and proliferation but that stable suppression of its expression does not affect beta-cell GPC lipid content or composition even under conditions in which LPC is being actively consumed by conversion to PC. This calls into question the generality of proposed housekeeping functions for iPLA2beta in PC homeostasis and remodeling.

Dynamic lipidomics of the nucleus

Once nuclear envelope membranes have been removed from isolated nuclei, around 6% of mammalian cell phospholipid is retained within the nuclear matrix, which calculations suggest may occupy 10% of the volume of this subcellular compartment. It is now acknowledged that endonuclear phospholipid, largely ignored for the past 40 years, provides substrate for lipid-mediated signaling events. However, given its abundance, it likely also has other as yet incompletely defined roles. Endonuclear phosphatidylcholine is the predominant phospholipid comprising distinct and highly saturated molecular species compared with that of the whole cell. Moreover, this unusual composition is subject to tight homeostatic maintenance even under conditions of extreme dietary manipulation, presumably reflecting a functional requirement for highly saturated endonuclear phosphatidylcholine. Recent application of new lipidomic technologies exploiting tandem electrospray ionization mass spectrometry in conjunction with deuterium stable isotope labeling have permitted us to probe not just molecular species compositions but endonuclear phospholipid acquisition and turnover with unparalleled sensitivity and specificity. What emerges is a picture of a dynamic pool of endonuclear phospholipid subject to autonomous regulation with respect to bulk cellular phospholipid metabolism. Compartmental biosynthesis de novo of endonuclear phosphatidylcholine contrasts with import of phosphatidylinositol synthesized elsewhere. However, irrespective of the precise temporal-spatial-dynamic relationships underpinning phospholipid acquisition, derangement of endonuclear lipid-mediated signaling from these parental phospholipids halts cell growth and division indicating a pivotal control point. This in turn defines the manipulation of compartmentalized endonuclear phospholipid acquisition and metabolism as intriguing potential targets for the development of future antiproliferative strategies.

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.

Differential effects of eicosapentaenoic and docosahexaenoic acids upon oxidant-stimulated release and uptake of arachidonic acid in human lymphoma U937 cells

The use of n-3 polyunsaturated fatty acids, as found in fish-oil derived dietary supplements, as anti-inflammatory agents is supported by a variety of biochemical and physiological data. Recent studies investigating the therapeutic potential of long chain (>C20) n-3 fatty acids in mental illness have lead to the conclusion, however, that not all n-3 fatty acid types are equally efficacious. In particular eicosapentaeoic acid (EPA) appears to possess antidepressant and antipsychotic activity, while docosahexaenoic acid (DHA) does not, an effect suggested to be due to a differential ability to antagonize arachidonic acid (AA)-dependent cell signalling. In this study, we examine the effect of EPA and DHA supplementation upon uptake and release of arachidonic acid stimulated by tert-butyl hydroperoxide/Fe2+ in U937 cells. Oxidant-stimulated 3H-AA release from cells was enhanced by pre-treatment with EPA, DHA and AA, but not stearic or oleic acids for 18 days, with the order of effect magnitude being EPA > DHA = AA. Supplementation of cells for 1 day gave qualitatively similar results, although the effect magnitude was smaller. To determine whether enhanced release was due to decreased reuptake of AA, cells were cultured in the presence of 10 microM fatty acids. Pre-treatment of cells with EPA, and to a lesser extent AA, but not DHA, inhibited uptake of 3H-AA measured subsequent to the removal of unesterified fatty acids. This study suggests that, in U937 cells, EPA can alter the rate of uptake and release of AA from phospholipids in an exposure time-dependent manner, whereas DHA has no or little effect. Our results predict that EPA will have a more pronounced effect upon AA-dependent processes compared to DHA, and suggests that the relative amounts of EPA and DHA in fish oil supplements may modify their biochemical, and potentially, behavioural effects.

Localization and functional interrelationships among cytosolic Group IV, secreted Group V, and Ca2+-independent Group VI phospholipase A2s in P388D1 macrophages using GFP/RFP constructs

P388D(1) cells exposed to bacterial lipopolysaccharide (LPS) mobilize arachidonic acid (AA) for prostaglandin synthesis in two temporally distinct pathways. The "immediate pathway" is triggered within minutes by receptor agonists such as platelet-activating factor (PAF) but only if the cells have previously been primed with LPS for 1 h. The "delayed pathway" occurs in response to LPS alone over the course of several hours. We have now investigated the subcellular localization of both the Group IV cytosolic phospholipase A(2) (cPLA(2)) and the Group V secreted PLA(2) (sPLA(2)) during these two temporally distinct routes of AA release. We have prepared cells overexpressing fusion proteins of sPLA(2)-GFP and cPLA(2)-RFP. In the resting cells, cPLA(2)-RFP was uniformly located throughout the cytoplasm, and short-term treatment with LPS did not induce translocation to perinuclear and/or Golgi membranes. However, such a translocation occurred almost immediately after the addition of PAF to the cells. Long-term exposure of the cells to LPS led to the translocation of cPLA(2)-RFP to intracellular membranes after 3 h, and correlates with a significant release of AA in a cPLA(2)-dependent manner. At the same time period that the delayed association of cPLA(2) with perinuclear membranes is detected, an intense fluorescence arising from the sPLA(2)-GFP was found around the nucleus in the sPLA(2)-GFP stably transfected cells. In parallel with these changes, significant AA release was detected from the sPLA(2)-GFP transfectants in a cPLA(2)-dependent manner, which may reflect cross-talk between sPLA(2) and cPLA(2). The subcellular localization of the Group VIA Ca(2+)-independent PLA(2) (iPLA(2)) was also investigated. Cells overexpressing iPLA(2)-GFP showed no fluorescence changes under any activation condition. However, the iPLA(2)-GFP-expressing cells showed relatively high basal AA release, confirming a role for iPLA(2) in basal deacylation reactions. These new data illustrate the subcellular localization changes that accompany the distinct roles that each of the three kinds of PLA(2) present in P388D(1) macrophages play in AA mobilization.

Lysophospholipid acyltransferases in monocyte inflammatory responses and sepsis

Acyltransferases are important in the regulation of membrane phospholipid fatty acyl composition and together with phospholipase A2 enzymes control arachidonic acid incorporation and remodelling within phospholipids. In addition, monocyte and macrophage acyltransferase activity has been shown to respond to various inflammatory cytokines under conditions that can induce enhanced cellular responses. Work in our laboratory indicates that the enzyme lysophosphatidylcholine acyltransferase may mediate the priming reactions of monocytes to the cytokine interferon-gamma. Our recent studies suggest that this enzyme might also affect the responses of monocytes to the bacterial agent lipopolysaccharide that may be important in the development of sepsis. This article summarises the relationship between monocyte lysophosphatidylcholine acyltransferase, lipopolysaccharide and sepsis.

Regulation of group VIA phospholipase A2 expression by sterol availability

Several lines of evidence suggest that glycerophospholipid mass is maintained through the coordinate regulation of CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) and the group VIA calcium-independent phospholipase A2 (iPLA2). CTalpha expression is modulated by sterol and this is mediated in part through sterol regulatory element binding proteins (SREBP). In this report, we investigate the possibility that iPLA2 expression is controlled in a similar manner. When Chinese hamster ovary (CHO) cells were cultured under sterol-depleted conditions, iPLA2 catalytic activity, mRNA, and protein were induced by between two- and threefold. These inductions were suppressed when the cells were supplemented with exogenous sterols. Luciferase reporter assays indicated that sterol depletion induced transcription of iPLA2, an analysis of the 5' flanking region suggested that the iPLA2 gene contained a putative sterol regulatory element (SRE), and electrophoretic mobility shift assay (EMSA) analysis indicated that this element can bind SREBP-2. Notably, a mutant CHO cell line (SRD4) that constitutively generates mature SREBP proteins exhibited increased iPLA2 activity and expression compared to wild-type cells. These data suggest that iPLA2 expression is regulated in a manner consistent with other important genes in sterol and glycerophospholipid metabolism. Such coordinate regulation may be essential for maintaining the lipid composition of cell membranes.

Role of group VIA calcium-independent phospholipase A2 in arachidonic acid release, phospholipid fatty acid incorporation, and apoptosis in U937 cells responding to hydrogen peroxide

Group VIA calcium-independent phospholipase A2 (iPLA2) has been shown to play a major role in regulating basal phospholipid deacylation reactions in certain cell types. More recently, roles for this enzyme have also been suggested in the destruction of membrane phospholipid during apoptosis and after oxidant injury. Proposed iPLA2 roles have rested heavily on the use of bromoenol lactone as an iPLA2-specific inhibitor, but this compound actually inhibits other enzymes and lipid pathways unrelated to PLA2, which makes it difficult to define the contribution of iPLA2 to specific functions. In previous work, we pioneered the use of antisense technology to decrease cellular iPLA2 activity as an alternative approach to study iPLA2 functions. In the present study, we followed the opposite strategy and prepared U937 cells that exhibited enhanced iPLA activity by stably expressing a plasmid containing iPLA2 cDNA. Compared with control cells, the iPLA2 -overexpressing U937 cells showed elevated responses to hydrogen peroxide with regard to both arachidonic acid mobilization and incorporation of the fatty acid into phospholipids, thus providing additional evidence for the key role that iPLA2 plays in these events. Long-term exposure of the cells to hydrogen peroxide resulted in cell death by apoptosis, and this process was accelerated in the iPLA2-overexpressing cells. Increased phospholipid hydrolysis and fatty acid release also occurred in these cells. Unexpectedly, however, abrogation of U937 cell iPLA2 activity by either methyl arachidonyl fluorophosphonate or an antisense oligonucleotide did not delay or decrease the extent of apoptosis induced by hydrogen peroxide. These results indicate that, although iPLA2-mediated phospholipid hydrolysis occurs during apoptosis, iPLA2 may actually be dispensable for the apoptotic process to occur. Thus, beyond a mere destructive role, iPLA2 may play other roles during apoptosis.

Role of an endoplasmic reticulum Ca2+-independent phospholipase A2 in cisplatin-induced renal cell apoptosis

It has been demonstrated recently that rabbit renal proximal tubule cells (RPTC) express a novel Ca(2+)-independent phospholipase A(2) (iPLA(2)) whose activity localizes to the endoplasmic reticulum (ER-iPLA(2)) and is similar to group VIB PLA(2). In this study, the expression of group VIB PLA(2) was examined and the role of ER-iPLA(2) in cisplatin-induced apoptosis was determined. Cisplatin induced both time- and concentration-dependent RPTC apoptosis as determined by p53 nuclear localization, annexin V staining, caspase 3 activity, and chromatin condensation. Inhibition of ER-iPLA(2) with bromoenol lactone (5 microM) reduced cisplatin-induced annexin V binding 40%, chromatin condensation 55%, and caspase 3 activity 42%, but had no effect on p53 nuclear localization. Treatment of RPTC with the protein kinase C stimulator phorbol 12-myristate 13-acetate increased the activity of ER-iPLA(2) 2-fold and increased cisplatin-induced RPTC apoptosis. These studies demonstrate that group VIB PLA(2) is expressed in RPTC and suggest that RPTC ER-iPLA(2) is the rabbit homolog of group VIB PLA(2). These data also demonstrate that ER-iPLA(2) acts downstream of p53 and upstream of caspase 3 to mediate cisplatin-induced RPTC apoptosis. Finally, ER-iPLA(2) seems to be regulated by protein kinase C.

Amplification mechanisms of inflammation: paracrine stimulation of arachidonic acid mobilization by secreted phospholipase A2 is regulated by cytosolic phospholipase A2-derived hydroperoxyeicosatetraenoic acid

In macrophages and other major immunoinflammatory cells, two phospholipase A(2) (PLA(2)) enzymes act in concert to mobilize arachidonic acid (AA) for immediate PG synthesis, namely group IV cytosolic phospholipase A(2) (cPLA(2)) and a secreted phospholipase A(2) (sPLA(2)). In this study, the molecular mechanism underlying cross-talk between the two PLA(2)s during paracrine signaling has been investigated. U937 macrophage-like cells respond to Con A by releasing AA in a cPLA(2)-dependent manner, and addition of exogenous group V sPLA(2) to the activated cells increases the release. This sPLA(2) effect is abolished if the cells are pretreated with cPLA(2) inhibitors, but is restored by adding exogenous free AA. Inhibitors of cyclooxygenase and 5-lipoxygenase have no effect on the response to sPLA(2). In contrast, ebselen strongly blocks it. Reconstitution experiments conducted in pyrrophenone-treated cells to abolish cPLA(2) activity reveal that 12- and 15-hydroperoxyeicosatetraenoic acid (HPETE) are able to restore the sPLA(2) response to levels found in cells displaying normal cPLA(2) activity. Moreover, 12- and 15-HPETE are able to enhance sPLA(2) activity in vitro, using a natural membrane assay. Neither of these effects is mimicked by 12- or 15-hydroxyeicosatetraenoic acid, indicating that the hydroperoxy group of HPETE is responsible for its biological activity. Collectively, these results establish a role for 12/15-HPETE as an endogenous activator of sPLA(2)-mediated phospholipolysis during paracrine stimulation of macrophages and identify the mechanism that connects sPLA(2) with cPLA(2) for a full AA mobilization response.

Calcium-independent phospholipase A2 is required for lysozyme secretion in U937 promonocytes

As a part of their surveillance functions in the immune system, monocytes/macrophages secrete large amounts of the bactericidal enzyme lysozyme to the extracellular medium. We report here that lysozyme secretion in activated U937 promonocytes depends on a functional calcium-independent phospholipase A(2) (iPLA(2)). Inhibition of the enzyme by bromoenol lactone or by treatment with a specific antisense oligonucleotide results in a diminished capacity of the cells to secrete lysozyme to the extracellular medium. Calcium-independent PLA(2) is largely responsible for the maintenance of the steady state of lysophosphatidylcholine (lysoPC) levels within the cells, as manifested by the marked decrease in the levels of this metabolite in cells deficient in iPLA(2) activity. Reconstitution experiments reveal that lysoPC efficiently restores lysozyme secretion in iPLA(2)-deficient cells, whereas other lysophospholipids, including lysophosphatidic acid, lysophosphatidylserine, and lysophosphatidylethanolamine, are without effect. Arachidonic acid mobilization in activated U937 cells is under control of cytosolic phospholipase A(2) (cPLA(2)). Selective inhibition of cPLA(2) results in a complete abrogation of the arachidonate mobilization response, but has no effect on lysozyme secretion. These results identify iPLA(2)-mediated lysoPC production as a necessary component of the molecular machinery leading to lysozyme secretion in U937 cells and rule out a role for cPLA(2) in the response. Collectively, the results demonstrate distinct roles in inflammatory cell signaling for these two intracellular phospholipases.

Phospholipase A(2) regulation of arachidonic acid mobilization

Phospholipase A(2) (PLA(2)) constitutes a growing superfamily of lipolytic enzymes, and to date, at least 19 distinct enzymes have been found in mammals. This class of enzymes has attracted considerable interest as a pharmacological target in view of its role in lipid signaling and its involvement in a variety of inflammatory conditions. PLA(2)s hydrolyze the sn-2 ester bond of cellular phospholipids, producing a free fatty acid and a lysophospholipid, both of which are lipid signaling molecules. The free fatty acid produced is frequently arachidonic acid (AA, 5,8,11,14-eicosatetraenoic acid), the precursor of the eicosanoid family of potent inflammatory mediators that includes prostaglandins, thromboxanes, leukotrienes and lipoxins. Multiple PLA(2) enzymes are active within and surrounding the cell and these enzymes have distinct, but interconnected roles in AA release.

Involvement of calcium-independent phospholipase A2 in hydrogen peroxide-induced accumulation of free fatty acids in human U937 cells

Previous studies have demonstrated that U937 cells are able to mobilize arachidonic acid (AA) and synthesize prostaglandins in response to receptor-directed and soluble stimuli by a mechanism that involves the activation of Group IV cytosolic phospholipase A(2)alpha. In this paper we show that these cells also mobilize AA in response to an oxidative stress induced by H(2)O(2) through a mechanism that appears not to be mediated by cytosolic phospholipase A(2)alpha but by the calcium-independent Group VI phospholipase A(2) (iPLA(2)). This is supported by the following lines of evidence: (i) the response is essentially calcium-independent, (ii) it is inhibited by bromoenol lactone, and (iii) it is inhibited by an iPLA(2) antisense oligonucleotide. Enzyme assays conducted under a variety of conditions reveal that the specific activity of the iPLA(2) does not change as a result of H(2)O(2) exposure, which argues against the activation of a specific signaling cascade ending in the iPLA(2). Rather, the oxidant acts to perturb membrane homeostasis in a way that the enzyme susceptibility/accessibility to its substrate increases, and this results in altered fatty acid release. In support of this view, not only AA, but also other fatty acids, were found to be liberated in an iPLA(2)-dependent manner in the H(2)O(2)-treated cells. Collectively, these studies underscore the importance of the iPLA(2) in modulating homeostatic fatty acid deacylation reactions and document a potentially important route under pathophysiological conditions for increasing free fatty acid levels during oxidative stress.