On the expression of cytosolic calcium-independent phospholipase A2 (88kDa) in immature and mature myeloid cells and its role in leukotriene synthesis in human granulocytes

iPLA2 Activation Mediates Granular Exocytosis and Corrects Microbicidal Defects in ROS-Deficient and CGD Human Neutrophils

PURPOSE

The ubiquitous calcium-independent phospholipase A2 enzyme (iPLA2) is inhibited by calmodulin binding and known to be responsible for phospholipid remodeling housekeeping functions including granule exocytosis-associated membrane fusion in normal human neutrophils. We evaluate in human neutrophils the iPLA2 secretagogue effects using normal neutrophils, where reactive oxygen species (ROS) generation has been blocked by diphenyleneiodonium, as well as in neutrophils from chronic granulomatous disease (CGD) patients.

METHODS

Neutrophils were pretreated with W7, a calmodulin inhibitor known to activate iPLA2 and exocytosis of granules, and vesicles as well as intra- and extra-microbicidal activity against Staphylococcus aureus and Aspergillus fumigatus were evaluated.

RESULTS

W7 increases exocytosis of primary, secondary, and tertiary granules and vesicles and improves neutrophil microbicidal activity against S. aureus and A. fumigatus.

CONCLUSIONS

In neutrophils, calmodulin-mediated iPLA2 inhibition controls granule and vesicle exocytosis in the phagosome and in the extracellular microenvironment. Relieving iPLA2 inhibition results in increased exocytosis of primary, secondary, and tertiary granules and secretory vesicles with correction of defective intracellular and extracellular microbicidal activity. In CGD patients presenting ROS defective production, this increase in the non-oxidative killing pathway partially corrects their microbicidal defects.

A novel calcium-independent cellular PLA2 acts in insect immunity and larval growth

Phospholipase A2 (PLA2) catalyzes the position-specific hydrolysis of fatty acids linked to the sn-2 position of phospholipids (PLs). PLA2s make up a very large superfamily, with more than known 15 groups, classified into secretory PLA2 (sPLA2), Ca(2+)-dependent cellular PLA2 (sPLA2) and Ca(2+)-independent cellular PLA2 (iPLA2). Only a few insect sPLA2s, expressed in venom glands and immune tissues, have been characterized at the molecular level. This study aimed to test our hypothesis that insects express iPLA2, using the beet armyworm, Spodoptera exigua, our model insect. Substantial PLA2 activities under calcium-free condition were recorded in several larval tissue preparations. The PLA2 activity was significantly reduced in reactions conducted in the presence of a specific iPLA2 inhibitor, bromoenol lactone (BEL). Analysis of a S. exigua hemocyte transcriptome identified a candidate iPLA2 gene (SeiPLA2-A). The open reading frame encoded 816 amino acid residues with a predicted molecular weight of 90.5 kDa and 6.15 pI value. Our phylogenetic analysis clustered SeiPLA2-A with the other vertebrate iPLA2s. SeiPLA2-A was expressed in all tissues we examined, including hemocytes, fat body, midgut, salivary glands, Malpighian tubules and epidermis. Heterologous expression in Sf9 cells indicated that SeiPLA2-A was localized in cytoplasm and exhibited significant PLA2 activity, which was independent of Ca(2+) and inhibited by BEL. RNA interference (RNAi) of SeiPLA2-A using its specific dsRNA in the fifth instar larvae significantly suppressed iPLA2 expression and enzyme activity. dsSeiPLA2-A-treated larvae exhibited significant loss of cellular immune response, measured as nodule formation in response to bacterial challenge, and extended larval-to-pupal developmental time. These results support our hypothesis, showing that SeiPLA2-A predicted from the transcriptome analysis catalyzes hydrolysis of fatty acids from cellular PLs and plays crucial physiological roles in insect immunity and larval growth.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

B2 receptor-mediated dual effect of bradykinin on proximal tubule Na+ -ATPase: sequential activation of the phosphoinositide-specific phospholipase Cbeta/protein kinase C and Ca2+ -independent phospholipase A2 pathways

In a previous paper we showed that bradykinin (BK), interacting with its B2 receptor, inhibits proximal tubule Na+ -ATPase activity but does not change (Na+ +K+)ATPase activity. The aim of this paper was to investigate the molecular mechanisms involved in B2-mediated modulation of proximal tubule Na+ -ATPase by BK. To abolish B1 receptor-mediated effects, all experiments were carried out in the presence of (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Leu), des-Arg9-[Leu8]-BK (DALBK), a specific antagonist of B1 receptor. A dual effect on the Na+ -ATPase activity through the B2 receptor was found: short incubation times (1-10 min) stimulate the enzyme activity; long incubation times (10-60 min) inhibit it. The stimulatory effect of BK is mediated by activation of phosphoinositide-specific phospholipase C beta (PI-PLCbeta)/protein kinase C (PKC); its inhibitory action is mediated by Ca2+ -independent phospholipase A2 (iPLA2). Prior activation of the PI-PLCbeta/PKC pathway is required to activate the iPLA2-mediated inhibitory phase. These results reveal a new mechanism by which BK can modulate renal sodium excretion: coupling between B2 receptor and activation of membrane-associated iPLA2.

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.

Activation of PLA2 isoforms by cell swelling and ischaemia/hypoxia

Phospholipase A2 (PLA2) activity is increased in mammalian cells in response to numerous stimuli such as osmotic challenge, oxidative stress and exposure to allergens. The increased PLA2 activity is seen as an increased release of free, polyunsaturated fatty acids, e.g. arachidonic acid and membrane-bound lysophospholipids. Even though arachidonic acid acts as a second messenger in its own most mammalian cells seem to rely on oxidation of the fatty acid into highly potent second messengers via, e.g. cytochrome P450, the cyclo-oxygenase, or the lipoxygenase systems for downstream signalling. Here, we review data that illustrates that stress-induced PLA2 activity involves various PLA2 subtypes and that the PLA2 in question is determined by the cell type and the physiological stress condition.

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.

Polychlorinated biphenyls induce arachidonic acid release in human platelets in a tamoxifen sensitive manner via activation of group IVA cytosolic phospholipase A2-alpha

Polychlorinated biphenyls (PCBs) are stable compounds commonly found in nature as environmental pollutants. PCBs can affect the endocrine function of hormones such as steroid-hormones. Also, PCBs are known to be inducers of arachidonic acid release in various cells. We report, here, the effects of PCBs on eicosanoid formation, arachidonic acid release and cytosolic phospholipase A2-alpha (cPLA2-alpha) activation in human platelets. Ortho-substituted PCBs induced a time and dose-dependent release of arachidonic acid and the concomitant formation of 12(S)-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid (12-HETE) and 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid (12-HHT) in human platelets. The release of arachidonic acid and the formation of 12-HETE was completely blocked by the cPLA2-alpha inhibitors AACOCF3 or pyrrolidine-1. PCB-treatment of platelets demonstrated that the cPLA2-alpha protein as well as PLA2 activity translocated to the membrane fraction, independent of a rise in intracellular Ca2+. Furthermore, electrophoretic gel mobility shift analysis of cPLA2-alpha on SDS-PAGE demonstrated a PCB-dependent phosphorylation of cPLA2-alpha. The effects of 17beta-estradiol and two structurally unrelated anti-estrogens, nafoxidin and tamoxifen on PCB-induced arachidonic acid release in platelets were also investigated. Both nafoxidin and tamoxifen inhibited PCB-induced arachidonic acid release as well as 12-HETE and 12-HHT formation. Interestingly, platelets incubated with PCBs did not aggregate despite the fact that robust release of arachidonic acid was observed. In summary, these results demonstrate that certain PCBs induce activation of cPLA2-alpha independent of a rise in intracellular calcium and a robust release of arachidonic acid release with resulting eicosanoid formation in human platelets.

Spinal phospholipase A2 in inflammatory hyperalgesia: role of group IVA cPLA2

Current work has shown the importance of spinal cyclooxygenase (COX) products in facilitatory processes leading to tissue injury induced hyperalgesia. This cascade must originate with free arachidonic acid (AA) released by the activity of spinal phospholipase A2s (PLA2). In the present work, we studied the role of PLA2s in spinal sensitization. We first demonstrate the presence of constitutive mRNA in the spinal cord for PLA2 Groups IB, IIA, IIC, IVA, V and VI by reverse transcription-polymerase chain reaction (RT-PCR) and sequencing. Using quantitative-PCR, we found that Group IVA cPLA2 and Group VI iPLA2 are the predominant PLA2 messages in the spinal cord. Western blotting and activity assays specific for Group IVA cPLA2 and Group VI iPLA2 verified the presence of these enzymes. PLA2 activity in spinal cord homogenates was suppressed by methyl arachidonyl fluorophosphonate (MAFP) and arachidonyl trifluoromethylketone (AACOCF3), mixed inhibitors of Group IVA cPLA2 and Group VI iPLA2 as well as by bromoenol lactone (BEL), a Group VI iPLA2 inhibitor. The spinal expression of PLA2 mRNA or protein was not altered in the face of peripheral inflammation. Secondly, we showed that intrathecal (i.t.) administration of MAFP and AACOCF3, but not BEL, dose-dependently prevented thermal hyperalgesia induced by intraplantar carrageenan as well as formalin-induced flinching. Finally, i.t. injection of AACOCF3, at antihyperalgesic doses, decreased the release of prostaglandin E2 (PGE2) into spinal dialysate evoked by i.t. NMDA, while i.t. injection of BEL had no effect. Taken together, this work points to a role for constitutive Group IVA cPLA2 in spinal nociceptive processing.

Late-Gestation Rat Myometrial Cells Express Multiple Isoforms of Phospholipase A2 That Mediate PCB 50-Induced Release of Arachidonic Acid with Coincident Prostaglandin Production

Previous reports have shown that ortho-substituted polychlorinated biphenyls (PCBs) are uterotonic and activate phospholipase A2 to release arachidonic acid (AA) from membrane phospholipids. AA serves as the precursor to various eicosanoids, which, in addition to AA itself, are capable of modulating uterine function. To examine whether PCBs stimulate phospholipase A2 (PLA2) to mobilize arachidonic acid from late-gestation rat uterus, primary cultures of gestation day 20 (gd20) rat myometrial cells (RMC) were labeled with 0.5 microCi 3H-AA prior to a 10-, 20-, or 30-min exposure to 2,2',4,6-tetrachlorobiphenyl (PCB 50) (1-50 microM) or 0.1% DMSO (solvent control). PCB 50 stimulated the release of 3H-AA from gd20 RMC in concentration- and time-dependent manners (p < 0.05). PCB 50 stimulation of RMC was attenuated with ethylene glycol bis(2-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA) and nifedipine, suggesting that AA release was dependent on the influx of extracellular calcium through L-type voltage-operated calcium channels. PCB 50-induced release of AA from RMC was also attenuated with the PLA2-specific inhibitors methyl arachidonyl fluorophosphonate (MAFP), bromoenol lactone (BEL), and manoalide (p < 0.05). Stimulation of PLA2 enzymes in response to PCB exposure occurred via p38 mitogen activated protein kinase (MAPK) activation as indicated by the significant attenuation of PCB 50-induced AA release from RMC in the presence of SB 202190. In addition to stimulating AA release, PCB 50 induced a significant production of prostaglandins from gd20 RMC compared with controls (p < 0.05). These results suggest that myometrial cells express multiple PLA2 isoforms that may serve as a target and effector for ortho-substituted PCB-mediated stimulation of uterine function through arachidonic acid and prostaglandin release.

Cell Cycle Dependence of Group VIA Calcium-independent Phospholipase A2 Activity

Homeostasis of phosphatidylcholine (PC) is regulated by the opposing actions between CTP:phosphocholine cytidylyltransferase (CT) and the group VIA Ca(2+)-independent phospholipase A(2) (iPLA(2)). We investigated this process during the cell cycle. PC mass doubles during late G(1) and early S phase when its rate of catabolism is lowest. We show that iPLA(2) activity is cell cycle-dependent with peak activity during G(2)/M and late S phase. iPLA(2) activity declines during G(1) and is lowest at the G(1)/S transition and early S phase. The accumulation of PC correlates with decreased iPLA(2) activity, suggesting that regulation of this enzyme contributes to phospholipid accumulation. The levels of 80 kDa iPLA(2) protein do not change and thus cannot account for changes in enzyme activity. Reverse transcriptase and real-time PCR experiments show that splice variant iPLA(2) mRNAs are preferentially expressed during G(2)/M. Immunoblot analyses with an antibody directed against the N terminus of iPLA(2) revealed a approximately 50 kDa protein that is of appropriate size to be the truncated protein encoded by the ankyrin-iPLA(2)-1 splice variant mRNA. The levels of truncated iPLA(2) protein were high in cells in late G(1) and S phase cells that had low iPLA(2) activity and low in G(2)/M cells that had high iPLA(2) activity. The truncated protein co-immunoprecipitated with full-length iPLA(2), indicating a physical interaction between the two proteins. Together, these data suggest that truncated iPLA(2) proteins associate with active iPLA(2) and down-regulate its activity during G(1). This down-regulation may contribute to phospholipid accumulation during the cell cycle.

Regulation of arachidonic acid availability for eicosanoid production

Mammalian cells have developed specific pathways for the incorporation, remodeling, and release of arachidonic acid. Acyltransferase and transacylase pathways function to regulate the levels of esterified arachidonic acid in specific phospholipid pools. There are several distinct, differentially regulated phospholipases A2in cells that mediate agonist-induced release of arachidonic acid. These pathways are important in controlling cellular levels of free arachidonic acid. Both arachidonic acid and its oxygenated metabolites are potent bioactive mediators that regulate a myriad of physiological and pathophysiological processes.Key words: phospholipase A2, arachidonic acid, eicosanoid, phospholipid.

Cellular Function of Calcium-Independent Phospholipase A2

The catalytic activity of calcium-independent phospholipase A2 (iPLA2), which is classified as a group VI PLA2, is regulated by protein kinase C, calmodulin, and others such as reactive oxygen species. Numerous findings have shown that iPLA2 is involved in stimulus-induced arachidonic acid release and lysophospholipid generation, although the participation is dependent upon the cell type and stimulus. The catalytic action of iPLA2 is known to be responsible for phospholipid remodeling as a housekeeping function. However, it has been widely accepted that arachidonic acid and lysophospholipid generated by iPLA2 act as a signaling molecule in cellular functions. Those include eicosanoid production, glucose-induced insulin secretion, Fas-induced apoptosis, cellular proliferation, membrane traffic in fusion, contribution to myocardial ischemia, and others. In this review, the functional role of iPLA2 in cellular responses upon stimulation is the focus.

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

In the present study the lysophospholipid sources for arachidonic (AA) and eicosapentaenoic acid (EPA) incorporation into and redistribution within the phospholipids of phorbol-ester-differentiated U937 cells was investigated. Initially, AA incorporated primarily into choline glycerophospholipids (PC), whereas EPA incorporated mainly into ethanolamine glycerophospholipids (PE). Bromoenol lactone (BEL), an inhibitor of the Group VI Ca2+-independent phospholipase A2 (iPLA2), diminished both lysophosphatidylcholine levels and the incorporation of AA into phospholipids. However BEL had little effect on EPA incorporation. In concanavalin A-activated cells, EPA, but not AA, incorporation was also affected by methyl arachidonyl fluorophosphonate (MAFP), suggesting an additional role for the group IV cytosolic phospholipase A2. In the activated cells AA and EPA did not compete with each other for incorporation, indicating that the pathways for AA and EPA incorporation are partially different. The AA and EPA initially incorporated into PC slowly moved to PE in a process that took several hours. The transfer of AA and EPA from PC to PE was not inhibited by BEL, MAFP or LY311727 [3-(3-acetamide 1-benzyl-2-ethylindolyl-5-oxy)propanesulphonic acid], raising the possibility that an as-yet-undetermined phospholipase A2 may be involved in fatty acid phospholipid remodelling. A strong candidate to be involved in these reactions is a novel Ca2+-independent phospholipase A2 that, unlike all known iPLA2s, is resistant to inhibition by BEL and also to MAFP and LY311727. The enzyme activity cleaves both PC and PE and is thus able to provide the lysoPC and lysoPE acceptors required for the fatty acid acylation reactions.

Role of phospholipases A(2) in growth-dependent changes in prostaglandin release from 3T6 fibroblasts

Previously, we reported a growth-dependent change in prostaglandin production as a consequence of a marked growth-dependent alteration in arachidonic acid (AA) mobilization from phospholipids. Our present results show that fetal calf serum (FCS) and 4 beta-phorbol-12-myristate acetate (PMA) caused an enhancement of phospholipase A(2) (PLA(2)) activity in the membrane fraction of non-confluent cells allowing PLA(2) access to its substrate and the release of AA. Western blot analysis has shown that FCS and PMA increased secreted PLA(2) (sPLA(2)) expression in non-confluent 3T6 fibroblast cultures. Moreover, FCS and PMA induced dithiothreitol-sensitive and bromoenol lactone-sensitive PLA(2) activities in cytosol and membrane fraction. However, these stimuli did not modify significantly the PLA(2) activity in both fractions when 3T6 fibroblasts reached a high cell density. This could be associated with the impairment of AA mobilization in these cell culture conditions. On the other hand, we observed that FCS and PMA induced the same prostaglandin H synthase-2 induction in non-confluent and confluent culture conditions. Moreover, the prostaglandin E(2) levels reached in cell culture supernatants were independent of the degree of confluence when AA was added exogenously. These results suggest that the changes of intracellular distribution of PLA(2) activity of sPLA(2) and iPLA(2) stimulated by exogenous stimuli may be controlled by cell density conditions which constitute an important mechanism in the regulation of prostaglandin release.

The molecular biology of the group VIA Ca2+-independent phospholipase A2

The group VIA PLA2 is a member of the PLA2 superfamily. This enzyme, which is cytosolic and Ca2+-independent, has been designated iPLA2beta to distinguish it from another recently cloned Ca2+-independent PLA2. Features of iPLA2beta molecular structure offer some insight into possible cellular functions of the enzyme. At least two catalytically active iPLA2beta isoforms and additionalsplicing variants are derived from a single gene that consists of at least 17 exons located on human chromosome 22q13.1. Potential tumor suppressor genes also reside at or near this locus. Structural analyses reveal that iPLA2beta contains unique structural features that include a serine lipase consensus motif (GXSXG), a putative ATP-binding domain, an ankyrin-repeat domain, a caspase-3 cleavage motif DVTD138Y/N, a bipartite nuclear localization signal sequence, and a proline-rich region in the human long isoform. iPLA2beta is widely expressed among mammalian tissues, with highest expression in testis and brain. iPLA2beta prefers to hydrolyze fatty acid at the sn-2 fatty acid substituent but also exhibits phospholipase A1, lysophospholipase, PAF acetylhydrolase, and transacylase activities. iPLA2beta may participate in signaling, apoptosis, membrane phospholipid remodeling, membrane homeostasis, arachidonate release, and exocytotic membrane fusion. Structural features and the existence of multiple splicing variants of iPLA2beta suggest that iPLA2beta may be subject to complex regulatory mechanisms that differ among cell types. Further study of its regulation and interaction with other proteins may yield insight into how its structural features are related to its function.

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

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

Studies of phospholipid metabolism, proliferation, and secretion of stably transfected insulinoma cells that overexpress group VIA phospholipase A2

A cytosolic 84 kDa Group VIA phospholipase A2 (iPLA2beta) that does not require Ca2+ for catalysis was cloned from Chinese hamster ovary (CHO) cells, murine P388D1 cells, pancreatic islet beta-cells, and other sources. Proposed iPLA2beta functions include participation in phosphatidylcholine (PC) homeostasis by degrading excess PC generated in CHO cells that overexpress CTP:phosphocholine cytidylyltransferase (CT), which catalyzes the rate-limiting step in PC biosynthesis; participation in biosynthesis of arachidonate-containing PC species in P388D1 cells by generating lysophosphatidylcholine (LPC) acceptors for arachidonate incorporation; and participation in signaling events in insulin secretion from islet beta-cells. To further examine iPLA2beta functions in beta-cells, we prepared stably transfected INS-1 insulinoma cell lines that overexpress iPLA2beta activity eightfold compared to parental INS-1 cells or to INS-1 cells transfected with an empty retroviral vector that did not contain iPLA2beta cDNA. The iPLA2beta-overexpressing cells exhibit a twofold increase in CT activity compared to parental cells but little change in rates of [3H]choline incorporation into or disappearance from PC. Electrospray ionization (ESI) tandem mass spectrometric measurements indicate that iPLA2beta-overexpressing cells have 1.5-fold higher LPC levels than parental INS-1 cells but do not exhibit increased rates of [3H]arachidonate incorporation into phospholipids, and incorporation is unaffected by a bromoenol lactone (BEL) suicide substrate inhibitor of iPLA2beta. The rate of appearance of arachidonate-containing phosphatidylethanolamine species visualized by ESI mass spectrometry is also similar in iPLA2beta-overexpressing and parental INS-1 cells incubated with supplemental arachidonic acid, and this process is unaffected by BEL. Compared to parental INS-1 cells, iPLA2beta-overexpressing cells proliferate more rapidly and exhibit amplified insulin secretory responses to a protein kinase C-activating phorbol ester, glucose, and a cAMP analog. These findings suggest that iPLA2beta plays a signaling role in beta-cells that differs from housekeeping functions in PC biosynthesis and degradation in P388D1 and CHO cells.

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.

Activation and priming of neutrophil nicotinamide adenine dinucleotide phosphate oxidase and phospholipase A(2) are dissociated by inhibitors of the kinases p42(ERK2) and p38(SAPK) and by methyl arachidonyl fluorophosphonate, the dual inhibitor of cytosolic and calcium-independent phospholipase A(2)

Arachidonic acid (AA) generated by phospholipase A(2) (PLA(2)) is thought to be an essential cofactor for phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Both enzymes are simultaneously primed by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha). The possibility that either unprimed or cytokine-primed responses of PLA(2) or NADPH oxidase to the chemotactic agents formyl-methionyl-leucyl-phenylalanine (FMLP) and complement factor 5a (C5a) could be differentially inhibited by inhibitors of the mitogen-activated protein (MAP) kinase family members p42(ERK2) (PD98059) and p38(SAPK) (SB203580) was investigated. PD98059 inhibited the activation of p42(ERK2) by GM-CSF, TNF-alpha, and FMLP, but it did not inhibit FMLP-stimulated superoxide production in either unprimed or primed neutrophils. There was no significant arachidonate release from unprimed neutrophils stimulated by FMLP, and arachidonate release stimulated by calcium ionophore A23187 was not inhibited by PD98059. In contrast, PD98059 inhibited both TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP. On the other hand, SB203580 inhibited FMLP-superoxide responses in unprimed as well as TNF-alpha- and GM-CSF-primed neutrophils, but failed to inhibit TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP, and additionally enhanced A23187-stimulated arachidonate release, showing that priming and activation of PLA(2) and NADPH oxidase are differentially dependent on both the p38(SAPK) and p42(ERK2) pathways. Studies using C5a as an agonist gave similar results and confirmed the findings with FMLP. In addition, methyl arachidonyl fluorophosphonate (MAFP), the dual inhibitor of c and iPLA(2) enzymes, failed to inhibit superoxide production in primed cells at concentrations that inhibited arachidonate release. These data demonstrate that NADPH oxidase activity can be dissociated from AA generation and indicate a more complex role for arachidonate in neutrophil superoxide production.

Human calcium-independent phospholipase A2 mediates lymphocyte proliferation

Activation of lymphocytes induces blastogenesis and cell division which is accompanied by membrane lipid metabolism such as increased fatty acid turnover. To date little is known about the enzymatic mechanism(s) regulating this process. Release of fatty acids such as arachidonic acid requires sn-2-deacylation catalyzed by a class of enzymes known as phospholipases A(2) (PLA(2), EC ). Herein, we confirm that human peripheral blood B or T lymphocytes (PBL) do not possess measurable levels of 85-kDa PLA(2) as assessed by Western immunoblot. Low levels of 14-kDa PLA(2) protein and activity were detectable in the particulate fraction of PBL and Jurkat cells. Western immunoblot analysis indicates that PBLs possess the calcium-independent PLA(2) (iPLA(2)) protein. Calcium-independent sn-2-acylhydrolytic activity was measurable in PBL cytosols and could be inhibited by the selective iPLA(2) inhibitor bromoenol lactone. Mitogen activation of PBLs resulted in maintenance of activity levels which remained constant over 72 h suggesting an important role for iPLA(2) in this proliferative process. Indeed, evaluation of iPLA(2) activity in cell cycle-arrested Jurkat T cell fractions revealed the highest iPLA(2) levels occurring at the G(2)/M phase. Addition of the iPLA(2) inhibitors, bromoenol lactone, or arachidonyl trifluoromethyl ketone (AAOCF(3)), inhibited both mitogen-induced PBL as well as Jurkat T cell proliferation. Moreover, specific depletion of iPLA(2) protein by antisense treatment also resulted in marked suppression of cell division. Inhibition of Jurkat cell proliferation was not associated with arrest at a particular phase of the cell cycle nor was it associated with apoptosis as assessed by flow cytometry. These findings provide the first evidence that iPLA(2) plays a key role in the lymphocyte proliferative response.

Calcium-independent phospholipase A(2): structure and function

The classical Ca(2+)-independent phospholipase A(2) enzyme, now known as Group VIA PLA(2), was initially purified and characterized from the P388D(1) macrophage-like cell line. The corresponding cDNA was subsequently cloned from a variety of sources, and it is now known that multiple splice variants of the enzyme are expressed, some of which may act as negative regulators of the active enzyme. Group VIA PLA(2) has a consensus lipase motif (GTSTG) containing the catalytic serine, is 85-88 kDa, and exists in an aggregated form. The enzyme contains multiple ankyrin repeats, which may play a role in oligomerization. The Group VIA enzyme exhibits lysophospholipase activity as well as phospholipase A(2) activity, and it is capable of hydrolyzing a wide variety of phospholipid substrates. A major function of Group VIA PLA(2) is to mediate phospholipid remodeling, but the enzyme may play other roles as well. Other Ca(2+)-independent PLA(2) enzymes have more recently been identified, and it may be possible to discriminate between the various Ca(2+)-independent PLA(2) enzymes based on sequence or inhibitor-sensitivity. However, the physiological functions of the newly identified enzymes have yet to be elucidated.

Electrospray ionization/mass spectrometric analyses of human promonocytic U937 cell glycerolipids and evidence that differentiation is associated with membrane lipid composition changes that facilitate phospholipase A2 activation

Upon differentiation, U937 promonocytic cells gain the ability to release a large fraction of arachidonate esterified in phospholipids when stimulated, but the mechanism is unclear. U937 cells express group IV phospholipase A(2) (cPLA(2)), but neither its level nor its phosphorylation state increases upon differentiation. A group VI PLA(2) (iPLA(2)) that is sensitive to a bromoenol lactone inhibitor catalyzes arachidonate hydrolysis from phospholipids in some cells and facilitates arachidonate incorporation into glycerophosphocholine (GPC) lipids in others, but it is not known whether U937 cells express iPLA(2). We confirm that ionophore A23187 induces substantial [(3)H]arachidonate release from differentiated but not control U937 cells, and electrospray ionization mass spectrometric (ESI/MS) analyses indicate that differentiated cells contain a higher proportion of arachidonate-containing GPC species than control cells. U937 cells express iPLA(2) mRNA and activity, but iPLA(2) inhibition impairs neither [(3)H]arachidonate incorporation into nor release from U937 cells. Experiments with phosphatidate phosphohydrolase (PAPH) and phospholipase D (PLD) inhibitors coupled with ESI/MS analyses of PLD-PAPH products indicate that differentiated cells gain the ability to produce diacylglycerol (DAG) via PLD-PAPH. DAG promotes arachidonate release by a mechanism that does not require DAG hydrolysis, is largely independent of protein kinase C, and requires cPLA(2) activity. This may reflect DAG effects on cPLA(2) substrate state.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

The human calcium-independent phospholipase A2 gene multiple enzymes with distinct properties from a single gene

Recently, we reported the human 88-kDa calcium-independent phospholipase A2 (iPLA2) cDNA sequence, as well as extensive alternative splicing of the iPLA2 mRNA. In this report we identified the gene coding for iPLA2, which was localized on chromosome 22q13.1. The gene consists of at least 17 exons spanning > 69 kb. Based on the iPLA2 gene organization the splice variants can be explained. The putative promotor for the iPLA2 gene lacks a TATA-box and contains a CpG island as well as several potential Sp-1-binding sites. Furthermore, the 5'-flanking region also contains one medium reiteration frequency repeat (MER53) and an Alu repetitive sequence. Northern blot analysis of iPLA2 mRNA in various human tissues demonstrated tissue-specific expression of four distinct iPLA2 transcripts. The native human 3.2-kb iPLA2 transcript was predominantly expressed in heart, brain, skeletal muscle, prostate, testis, thyroid and spinal cord, and to a lesser extent in peripheral blood leucocytes, stomach, trachea and bone marrow. Studies on the subcellular localization of the native iPLA2 protein were performed in COS-7 cells overexpressing this enzyme. The cytosolic fraction of untransfected and cells overexpressing iPLA2 contained equal amounts of calcium-independent PLA2 activity. However, the membrane fraction displayed a 5.5-fold increased activity in iPLA2 overexpressing cells. This increased calcium-independent PLA2 activity correlated with the presence of iPLA2 immunoreactive protein in the membrane fraction, indicating that this form of iPLA2 protein was membrane associated. Studies of iPLA2 in rat vascular smooth muscle cells verified the membrane association of this form of iPLA2. The major difference between this form of iPLA2 enzyme and the soluble forms of iPLA2 studied previously is the presence of 54 additional amino acid residues derived from exon 9. We suggest that the addition of these 54 amino acids leads to a membrane-associated protein. In summary, these results demonstrate that alternative splicing of the human iPLA2 transcript generates multiple iPLA2 isoforms with distinct tissue distribution and cellular localization.

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.

Human pancreatic islets express mRNA species encoding two distinct catalytically active isoforms of group VI phospholipase A2 (iPLA2) that arise from an exon-skipping mechanism of alternative splicing of the transcript from the iPLA2 gene on chromosome 22q13.1

An 85-kDa Group VI phospholipase A2 enzyme (iPLA2) that does not require Ca2+ for catalysis has recently been cloned from three rodent species. A homologous 88-kDa enzyme has been cloned from human B-lymphocyte lines that contains a 54-amino acid insert not present in the rodent enzymes, but human cells have not previously been observed to express catalytically active iPLA2 isoforms other than the 88-kDa protein. We have cloned cDNA species that encode two distinct iPLA2 isoforms from human pancreatic islet RNA and a human insulinoma cDNA library. One isoform is an 85-kDa protein (short isoform of human iPLA2 (SH-iPLA2)) and the other an 88-kDa protein (long isoform of human iPLA2 (LH-iPLA2)). Transcripts encoding both isoforms are also observed in human promonocytic U937 cells. Recombinant SH-iPLA2 and LH-iPLA2 are both catalytically active in the absence of Ca2+ and inhibited by a bromoenol lactone suicide substrate, but LH-iPLA2 is activated by ATP, whereas SH-iPLA2 is not. The human iPLA2 gene has been found to reside on chromosome 22 in region q13.1 and to contain 16 exons represented in the LH-iPLA2 transcript. Exon 8 is not represented in the SH-iPLA2 transcript, indicating that it arises by an exon-skipping mechanism of alternative splicing. The amino acid sequence encoded by exon 8 of the human iPLA2 gene is proline-rich and shares a consensus motif of PX5PX8HHPX12NX4Q with the proline-rich middle linker domains of the Smad proteins DAF-3 and Smad4. Expression of mRNA species encoding two active iPLA2 isoforms with distinguishable catalytic properties in two different types of human cells demonstrated here may have regulatory or functional implications about the roles of products of the iPLA2 gene in cell biologic processes.