Purified group X secretory phospholipase A(2) induced prominent release of arachidonic acid from human myeloid leukemia cells

Phospholipase A2 enzymes

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

Total direct chemical synthesis and biological activities of human group IIA secretory phospholipase A2

Human group IIA secretory phospholipase A(2) (hGIIA sPLA(2)) is reported to be involved in inflammation, since its expression level is enhanced under various inflammatory conditions. In this work, we report the total chemical synthesis of this enzyme (124 amino acids) by solid-phase method. The identity of the protein, in denatured or folded (7 disulphide bonds) forms, was confirmed by electrospray MS. Synthetic sPLA(2) possesses the same circular dichroism spectrum, enzymic activity in hydrolysing different phospholipid substrates, and inhibitory effect in thrombin formation from prothrombinase complex as the recombinant sPLA(2). Furthermore, LY311727, a reported specific hGIIA sPLA(2) inhibitor, is able to inhibit the synthetic and the recombinant enzymes with the same efficiency. This study demonstrates that chemically continuous solid phase synthesis is an alternative and less time-consuming approach to producing small, structurally folded and fully active proteins of up to 124 amino acids, such as hGIIA sPLA(2). Moreover, this technique provides more flexibility in analogue synthesis to elucidate their physiological functions and pathological effects.

Arachidonic acid release and prostaglandin F2alpha formation induced by phenylarsine oxide in PC12 cells: possible involvement of secretory phospholipase A2 activity

Activation of phospholipase A2 (PLA2) causing arachidonic acid (AA) release is involved in neuronal cell functions. Previously, we reported AA release and prostaglandin F(2alpha) (PGF(2alpha)) formation via activation of cytosolic PLA2 by orthovanadate (Na3VO4), an inhibitor of tyrosine phosphatases, in rat pheochromocytoma PC12 cells. We investigated the effects of phenylarsine oxide (PAO), which reacts with sulfhydryl groups of proteins and thus acts as an inhibitor of tyrosine phosphatases, on AA release and PGF(2alpha) formation in PC12 cells. PAO stimulated [3H]AA release from the prelabeled cells and PGF(2alpha) formation. The PAO responses were dependent upon the concentrations used (10 microM to 0.5mM) and on extracellular CaCl2. [3H]AA release induced by PAO was decreased significantly by inhibition of secretory, but not cytosolic, PLA2. [3H]AA release by PAO was not reversed by washing the cells, but the addition of dithiol compounds such as 2,3-dimercapto-1-propanol decreased the release from the PAO-treated cells. The existence of mRNA of types IB and IIC secretory PLA2 in PC12 cells was detected by reverse transcriptase-polymerase chain reaction using specific primers. Addition of secretory PLA2 from bee venom to the assay mixture stimulated [3H]AA release, and PAO enhanced the response synergistically. The addition of 0.1mM PAO directly enhanced the activity of secretory PLA2 from bee venom. These findings suggest that PAO stimulates AA release and PGF(2alpha) formation probably via activation of secretory PLA2 in PC12 cells.

Endotoxin induces respiratory failure and increases surfactant turnover and respiration independent of alveolocapillary injury in rats

Although endotoxin-induced acute lung injury is associated with inflammation, alveolocapillary injury, surfactant dysfunction, and altered lung mechanics, the precise sequence of these changes is polemic. We have studied the early pathogenesis of acute lung injury in spontaneously breathing anesthetized rats after intravenous infusion of Salmonella abortus equi endotoxin. The animals became hypoxic, and airway resistance, tissue resistance, lung elastance, and static compliance all deteriorated well before any change in alveolar neutrophils, macrophages, lung fluid (99mTc-labeled diethylenetriamine pentaacetic acid), or 125I-albumin flux, which were only appreciably increased at 8.5 hours. Lung elastance deteriorated before airway resistance, indicating that the compliance change was specific rather than caused by reduced lung volume. The subcellular and alveolar content of surfactant proteins A and B, cholesterol, disaturated phospholipids, and phospholipid classes remained normal in the face of a dramatic increase in the synthesis and turnover of 3H-disaturated phosphatidylcholine. Our findings indicate that the increase in surfactant disaturated phospholipid turnover reflects, at least in part, an approximately five-fold increase in "sigh frequency." We suggest that endotoxin has direct effects on tissue resistance and lung elastance independent of surfactant composition and that the initial respiratory failure results primarily from endotoxin-induced ventilation/perfusion mismatch independent of edema or alveolocapillary injury per se.

Protein kinase Calpha-dependent increase in Ca2+-independent phospholipase A2 in membranes and arachidonic acid liberation in zymosan-stimulated macrophage-like P388D1 cells

We previously reported that zymosan-stimulated, protein kinase C (PKC)-dependent arachidonic acid liberation occurs with association of Ca2+-independent phospholipase A2 (iPLA2) with the membranes of macrophage-like P388D1 cells. In the present study, the possible involvement of PKC isoforms (alpha, beta, delta, and epsilon) on the increase in iPLA2 was examined. Stimulation of P388D1 cells with zymosan induced increases in iPLA2 activity and protein in the membranes and liberation of arachidonic acid. In the stimulated cells, PKCalpha, PKCdelta, and PKCepsilon, but not PKCbeta, were increased in the membranes. The zymosan-induced increase in iPLA2 activity was suppressed by pretreatment with 4beta-phorbol 12-myristate 13-acetate for 10 hr, by which PKCalpha and PKCdelta, but not PKCbeta and PKCepsilon, were depleted, and by Gö6976, a PKCalpha inhibitor, but not rottlerin, a PKCdelta inhibitor. The zymosan-induced release of arachidonic acid was also reduced by the PKC depletion and Gö6976. However, stimulation with 4beta-phorbol 12-myristate 13-acetate alone did not increase iPLA2 activity in the membranes. Furthermore, the depletion of intracellular Ca2+ also impaired the zymosan-induced increase in iPLA2 activity in the membranes. However, no increase in iPLA2 activity was observed upon stimulation with Ca2+-mobilizing agents (ionomycin or thapsigargin). Cytochalasin D, an inhibitor of actin polymerization, suppressed the zymosan-induced increases in iPLA2 activity and protein in the membranes and the release of arachidonic acid. These results suggest that zymosan stimulates an increase in iPLA2 in the membranes of P388D1 cells probably through activation of PKCalpha in concert with cytochalasin D-sensitive events.

Group IID heparin-binding secretory phospholipase A(2) is expressed in human colon carcinoma cells and human mast cells and up-regulated in mouse inflammatory tissues

Group IID secretory phospholipase A(2) (sPLA(2)-IID), a heparin-binding sPLA(2) that is closely related to sPLA(2)-IIA, augments stimulus-induced cellular arachidonate release in a manner similar to sPLA(2)-IIA. Here we identified the residues of sPLA(2)-IID that are responsible for heparanoid binding, are and therefore essential for cellular function. Mutating four cationic residues in the C-terminal portion of sPLA(2)-IID resulted in abolition of its ability to associate with cell surface heparan sulfate and to enhance stimulus-induced delayed arachidonate release, cyclooxygenase-2 induction, and prostaglandin generation in 293 cell transfectants. As compared with several other group II subfamily sPLA(2)s, which were equally active on A23187- and IL-1-primed cellular membranes, sPLA(2)-IID showed apparent preference for A23187-primed membranes. Several human colon carcinoma cell lines expressed sPLA(2)-IID and sPLA(2)-X constitutively, the former of which was negatively regulated by IL-1. sPLA(2)-IID, but not other sPLA(2) isozymes, was expressed in human cord blood-derived mast cells. The expression of sPLA(2)-IID was significantly altered in several tissues of mice with experimental inflammation. These results indicate that sPLA(2)-IID may be involved in inflammation in cell- and tissue-specific manners under particular conditions.

Cellular arachidonate-releasing function and inflammation-associated expression of group IIF secretory phospholipase A2

Here we report the cellular arachidonate (AA)-releasing function of group IIF secretory phospholipase A(2) (sPLA(2)-IIF), a sPLA(2) enzyme uniquely containing a longer C-terminal extension. sPLA(2)-IIF increased spontaneous and stimulus-dependent release of AA, which was supplied to downstream cyclooxygenases and 5-lipoxygenase for eicosanoid production. sPLA(2)-IIF also enhanced interleukin 1-stimulated expression of cyclooxygenase-2 and microsomal prostaglandin E synthase. AA release by sPLA(2)-IIF was facilitated by oxidative modification of cellular membranes. Cellular actions of sPLA(2)-IIF occurred independently of the heparan sulfate proteoglycan glypican, which acts as a functional adaptor for other group II subfamily sPLA(2)s. Confocal microscopy revealed the location of sPLA(2)-IIF on the plasma membrane. The unique C-terminal extension was crucial for its plasma membrane localization and optimal cellular functions. sPLA(2)-IIF expression was increased in various tissues from lipopolysaccharide-treated mice and in ears of mice with experimental atopic dermatitis. In human rheumatoid arthritic joints, sPLA(2)-IIF was detected in synovial lining cells, capillary endothelial cells, and plasma cells. These results suggest that sPLA(2)-IIF is a potent regulator of AA metabolism and participates in the inflammatory process under certain conditions.

Characterization of a novel inhibitor of cytosolic phospholipase A2alpha, pyrrophenone

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha), one of the three subtypes of cPLA(2) (alpha, beta and gamma), is thought to be a rate-limiting enzyme in eicosanoid biosynthesis. We developed a novel and potent cPLA(2)alpha inhibitor with an optically active pyrrolidine, termed pyrrophenone, and characterized this compound in detail using enzyme and cellular assay systems. Pyrrophenone, which shows strong inhibition of cPLA(2)alpha activity, is one of the most potent cPLA(2)alpha inhibitors reported to date. Similar inhibitory potencies for cPLA(2)alpha were obtained from three different assays. The inhibitory activity of pyrrophenone is two or three orders of magnitude more potent than arachidonyl trifluoromethyl ketone (AACOCF(3)) under the same assay conditions. Pyrrophenone shows reversible inhibition of cPLA(2)alpha and displays no characteristics of the slow-binding inhibition observed for AACOCF(3). Pyrrophenone also inhibited the esterase and lysophospholipase activities of cPLA(2)alpha. However, the inhibition by pyrrophenone of 14 kDa secretory PLA(2)s, types IB and IIA, was over two orders of magnitude less potent than that for cPLA(2)alpha. Pyrrophenone strongly inhibited arachidonic acid release in calcium ionophore (A23187)-stimulated human monocytic cells (THP-1 cells) in a dose-dependent manner with an IC(50) value of 0.024 microM, followed by suppression of eicosanoid synthesis, and also showed dose-dependent inhibition for interleukin-1-induced prostaglandin E(2) synthesis in human renal mesangial cells with an IC(50) value of 0.0081 microM. The mechanism of inhibition of eicosanoid synthesis in these cell-based assays was due to inhibition of only one step of arachidonic acid release without any effect on cyclo-oxygenase or lipoxygenase pathways. These results suggest that pyrrophenone could be a potential therapeutic agent for inflammatory diseases.

Identification of a soluble form phospholipase A2 receptor as a circulating endogenous inhibitor for secretory phospholipase A2

Venomous snakes have various types of phospholipase A(2) inhibitory proteins (PLIs) in their circulatory system to protect them from attack by their own phospholipase A(2)s (PLA(2)s). Here we show the first evidence for the existence of circulating PLI against secretory PLA(2)s (sPLA(2)s) in mammals. In mouse serum, we detected specific binding activities of group IB and X sPLA(2)s, which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA(2) receptor (PLA(2)R), a type I transmembrane glycoprotein related to the C-type animal lectin family. Western blot analysis after partial purification with group IB sPLA(2) affinity column confirmed the identity of serum sPLA(2)-binding protein as a soluble form of PLA(2)R (sPLA(2)R) that retained all of the extracellular domains of the membrane-bound receptor. Both purified sPLA(2)R and the recombinant soluble receptor having all of the extracellular portions blocked the biological functions of group X sPLA(2), including its potent enzymatic activity and its binding to the membrane-bound receptor. Protease inhibitor tests with PLA(2)R-overexpressing Chinese hamster ovary cells suggested that sPLA(2)R is produced by cleavage of the membrane-bound receptor by metalloproteinases. Thus, sPLA(2)R is the first example of circulating PLI that acts as an endogenous inhibitor for enzymatic activities and receptor-mediated functions of sPLA(2)s in mice.

Role of group IIa and group V secretory phospholipases A(2) in the metabolism of lipoproteins. Substrate specificities of the enzymes and the regulation of their activities by sphingomyelin

Although many isoforms of secretory phospholipases A(2) (sPLA(2)) are known to be secreted by various inflammatory cells, and are present in plasma, their role in lipoprotein metabolism is unknown. We studied the in vitro hydrolysis of lipoprotein phospholipids by group IIa and group V sPLA(2), two structurally related enzymes with differing phospholipid specificities. The group V sPLA(2) was about 30 times more efficient than the group IIa enzyme in the hydrolysis of lipoprotein phosphatidylcholine (PC), and both enzymes were more active on high density liporotein (HDL) than on low density lipoprotein (LDL). The lower activity on LDL appears to be due to the higher sphingomyelin (SPH) concentration in this lipoprotein. PC hydrolysis in lipoproteins was stimulated significantly by enzymatic depletion of their SPH. The hydrolysis of PC in liposomes was inhibited by the incorporation of SPH, and this inhibition was reversed by treatment with sphingomyelinase. The incorporation of ceramide, on the other hand, stimulated the sPLA(2) activity significantly. Unlike most sPLA(2), which show no fatty acid preference, group V sPLA(2) released disproportionately more linoleate, and less arachidonate from lipoproteins. These studies show that group V sPLA(2) is physiologically more important than group IIa enzyme in lipoprotein metabolism, that the sPLA(2) activities are regulated by sphingomyelin and ceramide, and that the pathological effects of sPLA(2) may not be mediated through stimulation of eicosanoid synthesis.

Arachidonate release and eicosanoid generation by group IIE phospholipase A(2)

The heparin-binding group II subfamily of secretory phospholipase A(2)s (sPLA(2)s), such as sPLA(2)-IIA and -IID, augments stimulus-induced arachidonic acid (AA) release through the cellular heparan sulfate proteoglycan (HSPG)-dependent pathway when transfected into HEK293 cells. Here we show that the closest homolog, sPLA(2)-IIE, also promotes stimulus-induced AA release and prostaglandin (PG) production similar to those elicited by HSPG-dependent sPLA(2)s. Confocal laser microscopic analysis demonstrates the location of sPLA(2)-IIE in cytoplasmic punctate compartments. sPLA(2)-IIE also enhances leukotriene (LT) production and granule exocytosis by RBL-2H3 mastocytoma cells. Expression of sPLA(2)-IIE was highly upregulated in mice injected with lipopolysaccharide (LPS) and in mice with experimental atopic dermatitis. These observations suggest that this enzyme plays a role in the inflammatory process, as proposed for other group II subfamily sPLA(2)s.

Groups IV, V, and X phospholipases A2s in human neutrophils: role in eicosanoid production and gram-negative bacterial phospholipid hydrolysis

The bacterial tripeptide formyl-Met-Leu-Phe (fMLP) induces the secretion of enzyme(s) with phospholipase A(2) (PLA(2)) activity from human neutrophils. We show that circulating human neutrophils express groups V and X sPLA(2) (GV and GX sPLA(2)) mRNA and contain GV and GX sPLA(2) proteins, whereas GIB, GIIA, GIID, GIIE, GIIF, GIII, and GXII sPLA(2)s are undetectable. GV sPLA(2) is a component of both azurophilic and specific granules, whereas GX sPLA(2) is confined to azurophilic granules. Exposure to fMLP or opsonized zymosan results in the release of GV but not GX sPLA(2) and most, if not all, of the PLA(2) activity in the extracellular fluid of fMLP-stimulated neutrophils is due to GV sPLA(2). GV sPLA(2) does not contribute to fMLP-stimulated leukotriene B(4) production but may support the anti-bacterial properties of the neutrophil, because 10-100 ng per ml concentrations of this enzyme lead to Gram-negative bacterial membrane phospholipid hydrolysis in the presence of human serum. By use of a recently described and specific inhibitor of cytosolic PLA(2)-alpha (group IV PLA(2)alpha), we show that this enzyme produces virtually all of the arachidonic acid used for the biosynthesis of leukotriene B(4) in fMLP- and opsonized zymosan-stimulated neutrophils, the major eicosanoid produced by these pro-inflammatory cells.

Clearance of group X secretory phospholipase A(2) via mouse phospholipase A(2) receptor

Given the potent hydrolyzing activity toward phosphatidylcholine, group X secretory phospholipase A(2) (sPLA(2)-X) elicits a marked release of arachidonic acid linked to the potent production of lipid mediators in various cell types. We have recently shown that sPLA(2)-X can also act as a ligand for mouse phospholipase A(2) receptor (PLA(2)R). Here, we found that sPLA(2)-X was internalized and degraded via binding to PLA(2)R associated with the diminished prostaglandin E(2) (PGE(2)) formation in PLA(2)R-expressing Chinese hamster ovary (CHO) cells compared to CHO cells. Indirect immunocytochemical analysis revealed that internalized sPLA(2)-X was co-localized with PLA(2)R in the punctate structures in PLA(2)R-expressing CHO cells. Moreover, in mouse osteoblastic MC3T3-E(1) cells that endogenously express the PLA(2)R, the internalized sPLA(2)-X was localized in lysosomes. These findings demonstrate that PLA(2)R acts as a clearance receptor for sPLA(2)-X to suppress its strong enzymatic activity.

p38 MAPK regulates group IIa phospholipase A2 expression in interleukin-1beta -stimulated rat neonatal cardiomyocytes

Group IIa phospholipase A(2) (GIIa PLA(2)) is released by some cells in response to interleukin-1beta. The purpose of this study was to determine whether interleukin-1beta would stimulate the synthesis and release of GIIa PLA(2) from cardiomyocytes, and to define the role of p38 MAPK and cytosolic PLA(2) in the regulation of this process. Whereas GIIa PLA(2) mRNA was not identified in untreated cells, exposure to interleukin-1beta resulted in the sustained expression of GIIa PLA(2) mRNA. Interleukin-1beta also stimulated a progressive increase in cellular and extracellular GIIa PLA(2) protein levels and increased extracellular PLA(2) activity 70-fold. In addition, interleukin-1beta stimulated the p38 MAPK-dependent activation of the downstream MAPK-activated protein kinase, MAPKAP-K2. Treatment with the p38 MAPK inhibitor, SB202190, decreased interleukin-1beta stimulated MAPKAP-K2 activity, GIIa PLA(2) mRNA expression, GIIa PLA(2) protein synthesis, and the release of extracellular PLA(2) activity. Infection with an adenovirus encoding a constitutively active form of MKK6, MKK6(Glu), which selectively phosphorylates p38 MAPK, induced cellular GIIa PLA(2) protein synthesis and the release of GIIa PLA(2) and increased extracellular PLA(2) activity 3-fold. In contrast, infection with an adenovirus encoding a phosphorylation-resistant MKK6, MKK6(A), did not result in GIIa PLA(2) protein synthesis or release by unstimulated cardiomyocytes. In addition, infection with an adenovirus encoding MKK6(A) abrogated GIIa PLA(2) protein synthesis and release by interleukin-1beta-stimulated cells. These results provide direct evidence that p38 MAPK activation was necessary for interleukin-1beta-induced synthesis and release of GIIa PLA(2) by cardiomyocytes.

Microenvironment produced by acute myeloid leukemia cells prevents T cell activation and proliferation by inhibition of NF-kappaB, c-Myc, and pRb pathways

Tumors produce a variety of immunosuppressive factors which can prevent the proliferation and maturation of a number of normal hemopoietic cell types. We have investigated whether primary acute myeloid leukemia (AML) cells have an effect on normal T cell function and signaling. Tumor cell supernatant (TSN) from AML cells inhibited T cell activation and Th1 cytokine production and also prevented activated T cells from entering the cell cycle. These effects occurred in the absence of AML cell-T cell contact. We have demonstrated that AML TSN contained none of the immunosuppressors described to date, namely gangliosides, nitric oxide, TGF-beta, IL-10, vascular endothelial growth factor, or PGs. Furthermore, IL-2 did not overcome the block, despite normal IL-2R expression. However, the effect was overcome by preincubation with inhibitors of protein secretion and abolished by trypsinization, indicating that the active substance includes one or more proteins. To determine the mechanism of inhibition, we have studied many of the major pathways involved in T cell activation and proliferation. We show that nuclear translocation of NFATc and NF-kappaB are markedly reduced in T cells activated in the presence of primary AML cells. In contrast, calcium mobilization and activation of other signal transduction pathways, namely extracellular signal-regulated kinase1/2, p38, and STAT5 were unaffected, but activation of c-Jun N-terminal kinase 1/2 was delayed. Phosphorylation of pRb by cyclin-dependent kinase 6/4-cyclin D and of p130 did not occur and c-Myc, cyclin D3, and p107 were not induced, consistent with cell cycle inhibition early during the transition from G(0) to G(1). Our data indicate that TSN generated by AML cells induces T cell immunosuppression and provides a mechanism by which the leukemic clone could evade T cell-mediated killing.

Coupling between cyclooxygenase, terminal prostanoid synthase, and phospholipase A2

We have recently shown that two distinct prostaglandin (PG) E(2) synthases show preferential functional coupling with upstream cyclooxygenase (COX)-1 and COX-2 in PGE(2) biosynthesis. To investigate whether other lineage-specific PG synthases also show preferential coupling with either COX isozyme, we introduced these enzymes alone or in combination into 293 cells to reconstitute their functional interrelationship. As did the membrane-bound PGE(2) synthase, the perinuclear enzymes thromboxane synthase and PGI(2) synthase generated their respective products via COX-2 in preference to COX-1 in both the -induced immediate and interleukin-1-induced delayed responses. Hematopoietic PGD(2) synthase preferentially used COX-1 and COX-2 in the -induced immediate and interleukin-1-induced delayed PGD(2)-biosynthetic responses, respectively. This enzyme underwent stimulus-dependent translocation from the cytosol to perinuclear compartments, where COX-1 or COX-2 exists. COX selectivity of these lineage-specific PG synthases was also significantly affected by the concentrations of arachidonate, which was added exogenously to the cells or supplied endogenously by the action of cytosolic or secretory phospholipase A(2). Collectively, the efficiency of coupling between COXs and specific PG synthases may be crucially influenced by their spatial and temporal compartmentalization and by the amount of arachidonate supplied by PLA(2)s at a moment when PG production takes place.

Phospholipase A(2) in vascular disease

Secretory phospholipase A(2) (PLA(2)) can be proatherogenic both in the circulation and in the arterial wall. In blood plasma, PLA(2) can modify the circulating lipoproteins and so induce formation of small dense LDL particles, which are associated with increased risk for cardiovascular disease. In the arterial wall, PLA(2) can hydrolyze lipoproteins. The PLA(2)-modified lipoproteins bind tightly to extracellular proteoglycans, which may lead to their enhanced retention in the arterial wall. The modified lipoproteins may also aggregate and fuse, which can lead to accumulation of their lipids within the extracellular matrix. The PLA(2)-modified particles are more susceptible to further modifications by other enzymes and agents and can be taken up by macrophages, leading to accumulation of intracellular lipids. In addition, lysophospholipids and free fatty acids, the hydrolysis products of PLA(2), promote atherogenesis. Thus, these lipid mediators can be carried, either by the PLA(2)-modified lipoproteins themselves or by albumin, into the arterial cells, which then undergo functional alterations. This may, in turn, lead to specific changes in the extracellular matrix, which increase the retention and accumulation of lipoproteins within the matrix. In the present article, we discuss the possible actions of PLA(2) enzymes, especially PLA(2)-IIA, in the arterial wall during atherogenesis.

Expression of members of the phospholipase A2 family of enzymes in human nasal mucosa

Phospholipase A2 (PLA2) is a family of enzymes thought to play a key role in inflammation by releasing arachidonic acid for the synthesis of eicosanoids and lysophospholipid for the synthesis of platelet-activating factor. However, the precise contribution of different PLA2 types to the formation of inflammatory lipid mediators in the upper airways is not known and the expression of different PLA2 genes in the human nasal mucosa has not been examined. This study therefore investigated the occurrence of messenger ribonucleic acids (mRNAs) for different PLA2 forms (IB, IIA, IID, IIE, III, IVA, IVB, IVC, V, VI, VII, X, acid calcium-independent (aiPLA2), and calcium-independent membrane bound PLA2, (iPLA2-2)) in the nasal mucosa of five healthy human subjects. Using reversed transcription-polymerase chain reaction (RT-PCR) techniques it was found that all these PLA2 types except PLA2 V were expressed in all subjects, whereas PLA2 V was detected in only one individual on one single occasion. The relative abundance of the different PLA2 transcripts were aiPLA2 > X approximately = IVA > IIA approximately = IIE approximately = IVB approximately = VI > IB approximately = IID approximately = III approximately = IVC approximately = VII approximately = iPLA2-2. To further quantify the mRNA-expression of PLA2 X, IVA and IIA, the samples were reanalysed with a quantitative PCR-technique utilizing competitive deoxyribonucleic acid (DNA) mimics as references. The amounts of PLA2 X, IVA and IIA mRNA were then estimated to 0.9 +/- 0.2, 1.1 +/- 0.7, and 0.0025 +/- 0.0021 amol (mean +/- SE), respectively, confirming the relative abundance of these PLA2 transcripts and indicating that the recently described PLA2 X form is relatively strongly expressed. These findings demonstrate that a large number of PLA2 types are expressed in the normal human nasal mucosa. Moreover, this investigation demonstrates, for the first time, the presence of the newly discovered phospholipase A2 forms IID, IIE, III, IVB, IVC, X and calcium-independent membrane bound phospholipase A2 in the human nasal mucosa and raises the possibility that one or several of these may be involved in inflammatory reactions in the nose.

Secretory phospholipase A2 mediates cooperative prostaglandin generation by growth factor and cytokine independently of preceding cytosolic phospholipase A2 expression in rat gastric epithelial cells

Transforming growth factor (TGF)-alpha and interleukin (IL)-1beta are responsible for the healing of gastric lesions through, in part, prostaglandin (PG) generation. We examined the contribution of cytosolic and secretory phospholipase A(2)s (cPLA(2) and sPLA(2)) to the PG generation by rat gastric epithelial cells in response to both stimuli. Stimulation with TGF-alpha for 24 h increased cPLA(2) and cyclooxygenase (COX)-2 markedly, PGE(2) slightly, and type IIA sPLA(2) and COX-1 not at all, whereas IL-1beta increased sPLA(2) only. Both stimuli synergistically increased PGE(2), sPLA(2), and the two COXs but not cPLA(2). The onset of the PGE(2) generation paralleled the sPLA(2) release but was apparently preceded by increases in cPLA(2) and the two COXs. The increase in PGE(2) was impaired by inhibitors for sPLA(2) and COX-2 but not COX-1. cPLA(2) inhibitors suppressed PGE(2) generation by TGF-alpha alone but not augmentation of PGE(2) generation or sPLA(2) release by IL-1beta in combination with TGF-alpha. Furthermore, despite an increase in cPLA(2) including its phosphorylated form (phosphoserine), -induced arachidonic acid liberation was impaired in the TGF-alpha/IL-1beta-stimulated cells, in which p11, a putative cPLA(2) inhibitory molecule, was also increased and co-immunoprecipitated with cPLA(2). These results suggest that synergistic stimulation of sPLA(2) and COX-2 expression by TGF-alpha and IL-1beta results in an increase in PGE(2). Presumably, the preceding cPLA(2) expression is not involved in the PGE(2) generation, because of impairment of its hydrolytic activity in the stimulated cells.

Mechanism of human group V phospholipase A2 (PLA2)-induced leukotriene biosynthesis in human neutrophils. A potential role of heparan sulfate binding in PLA2 internalization and degradation

Human group V phospholipase A(2) (hVPLA(2)) has been shown to have high activity to elicit leukotriene production in human neutrophils (Han, S. K., Kim, K. P., Koduri, R., Bittova, L., Munoz, N. M., Leff, A. R., Wilton, D. C., Gelb, M. H., and Cho, W. (1999) J. Biol. Chem. 274, 11881-11888). To determine the mechanism by which hVPLA(2) interacts with cell membranes to induce leukotriene formation, we mutated surface cationic residues and a catalytic residue of hVPLA(2) and measured the interactions of mutants with model membranes, immobilized heparin, and human neutrophils. These studies showed that cationic residues, Lys(7), Lys(11), and Arg(34), constitute a part of the interfacial binding surface of hVPLA(2), which accounts for its moderate preference for anionic membranes. Additionally, hVPLA(2) binds heparin with high affinity and has a well defined heparin-binding site. The site is composed of Arg(100), Lys(101), Lys(107), Arg(108), and Arg(111), and is spatially distinct from its interfacial binding surface. Importantly, the activities of the mutants to hydrolyze cell membrane phospholipids and induce leukotriene biosynthesis, when enzymes were added exogenously to neutrophils, correlated with their activities on phosphatidylcholine membranes but not with their affinities for anionic membranes and heparin. These results indicate that hVPLA(2) acts directly on the outer plasma membranes of neutrophils to release fatty acids and lysophospholipids. Further studies suggest that products of hVPLA(2) hydrolysis trigger the cellular leukotriene production by activating cellular enzymes involved in leukotriene formation. Finally, the temporal and spatial resolution of exogenously added hVPLA(2) and mutants suggests that binding to cell surface heparan sulfate proteoglycans is important for the internalization and clearance of cell surface-bound hVPLA(2).

Therapeutic time-window of a group IIA phospholipase A2 inhibitor in rabbit acute lung injury: Correlation with lung surfactant protection

OBJECTIVE

We attempted to determine whether group IIA secretory phospholipase A2 (sPLA2-IIA) blockade after the onset of lung injury exerted therapeutic efficacy in the treatment of oleic acid (OA)-induced acute lung injury by using S-5920/LY315920Na, a novel specific inhibitor of sPLA2-IIA, with special interest in the changes of lung surfactant.

DESIGN

Prospective animal study.

SETTING

University laboratory.

SUBJECTS

Forty Japanese white rabbits.

INTERVENTIONS

The rabbits, under anesthesia, were endotracheally intubated and mechanically ventilated and then were divided into the following groups: OA + vehicle groups, intravenous infusion of OA for the first 2 hrs (0.1 mL x kg(-1) x hr(-1)) with the addition of vehicle (1 or 2 hrs after OA administration, each n = 9, total 18 rabbits); OA + S-5920/LY315920Na groups, treated identically to the OA control with the addition of S-5920/LY315920Na (1 mg/kg bolus followed by infusion at 0.5 mg x kg(-1) x hr(-1)) after OA (1 or 2 hrs after OA administration, each n = 9, total 18 rabbits); saline control groups, treated with saline instead of OA with the addition of vehicle (1 hr after OA administration, 4 rabbits). Arterial blood gas, lung mechanics, lung inflammation, lung surfactant phospholipids, and production of inflammatory mediators in the lung were measured.

MEASUREMENTS AND MAIN RESULTS

Treatment with S-5920/LY315920Na 1 hr after OA infusion, but not 2 hrs after infusion, significantly attenuated the lung injury, as estimated by hypoxemia, decreased lung compliance, pulmonary edema, and vascular permeability. The therapeutic efficacy was similar to that found in our previous pretreatment study. The treatment after 1 hr dramatically inhibited OA-induced surfactant degradation in the bronchoalveolar lavage fluid (BALF), without affecting the concentrations of thromboxane A2, leukotriene B4, and interleukin-8 in BALF. The degree of surfactant degradation in BALF paralleled well with the severity of the lung injury. Furthermore, recombinant human sPLA2-IIA reproduced the similar hydrolysis pattern of isolated surfactant in vitro, which was inhibited by S-5920/LY315920Na.

CONCLUSIONS

Our results indicate that therapeutic blockade of sPLA2-IIA ameliorated lung dysfunction via protection of surfactant degradation in an animal model of acute lung injury, and they suggest a new strategy in treating clinical acute lung injury.

Distinct arachidonate-releasing functions of mammalian secreted phospholipase A2s in human embryonic kidney 293 and rat mastocytoma RBL-2H3 cells through heparan sulfate shuttling and external plasma membrane mechanisms

We analyzed the ability of a diverse set of mammalian secreted phospholipase A(2) (sPLA(2)) to release arachidonate for lipid mediator generation in two transfected cell lines. In human embryonic kidney 293 cells, the heparin-binding enzymes sPLA(2)-IIA, -IID, and -V promote stimulus-dependent arachidonic acid release and prostaglandin E(2) production in a manner dependent on the heparan sulfate proteoglycan glypican. In contrast, sPLA(2)-IB, -IIC, and -IIE, which bind weakly or not at all to heparanoids, fail to elicit arachidonate release, and addition of a heparin binding site to sPLA(2)-IIC allows it to release arachidonate. Heparin nonbinding sPLA(2)-X liberates arachidonic acid most likely from the phosphatidylcholine-rich outer plasma membrane in a glypican-independent manner. In rat mastocytoma RBL-2H3 cells that lack glypican, sPLA(2)-V and -X, which are unique among sPLA(2)s in being able to hydrolyze phosphatidylcholine-rich membranes, act most likely on the extracellular face of the plasma membrane to markedly augment IgE-dependent immediate production of leukotriene C(4) and platelet-activating factor. sPLA(2)-IB, -IIA, -IIC, -IID, and -IIE exert minimal effects in RBL-2H3 cells. These results are also supported by studies with sPLA(2) mutants and immunocytostaining and reveal that sPLA(2)-dependent lipid mediator generation occur by distinct (heparanoid-dependent and -independent) mechanisms in HEK293 and RBL-2H3 cells.

Pyrrolidine inhibitors of human cytosolic phospholipase A2. Part 2: synthesis of potent and crystallized 4-triphenylmethylthio derivative 'pyrrophenone'

We synthesized a potent and crystallized human cytosolic phospholipase A2alpha inhibitor, pyrrophenone (6) which inhibits the isolated enzyme with an IC50 value of 4.2 nM. Pyrrophenone shows potent inhibition of arachidonic acid release, prostaglandin E2, thromboxane B2, and leukotriene B4 formation in human whole blood. The magnitudes of prostaglandin E2 and thromboxane B2 inhibition are the same as those of indomethacin.

Group X secretory phospholipase A(2) induces potent productions of various lipid mediators in mouse peritoneal macrophages

We have previously shown the expression of group X secretory phospholipase A(2) (sPLA(2)-X) in mouse splenic macrophages and its powerful potency for releasing fatty acids from various intact cell membranes. Here, we examined the potency of sPLA(2)-X in the production of lipid mediators in murine peritoneal macrophages. Mouse sPLA(2)-X was found to induce a marked release of fatty acids including arachidonic acid and linoleic acid, which contrasted with little, if any, release by the action of group IB and IIA sPLA(2)s. In resting macrophages, sPLA(2)-X elicited a modest production of prostaglandin E(2) and thromboxane A(2). After the induction of cyclooxygenase-2 (COX-2) by pretreatment with lipopolysaccharide, a dramatic increase in the production of these eicosanoids was observed in sPLA(2)-X-treated macrophages, which was completely blocked by the addition of either the specific sPLA(2) inhibitor indoxam or the COX inhibitor indomethacin. In accordance with its higher hydrolyzing activity toward phosphatidylcholine, mouse sPLA(2)-X induced a potent production of lysophosphatidylcholine. These findings strongly suggest that sPLA(2)-X plays a critical role in the production of various lipid mediators from macrophages. These events might be relevant to the progression of various pathological states, including chronic inflammation and atherosclerosis.

Characterization of group X phospholipase A(2) as the major enzyme secreted by human keratinocytes and its regulation by the phorbol ester TPA

HaCaT as well as human primary keratinocytes constitutively expressed mRNA of the human secreted phospholipase A(2) subtype groups X, V, IIA, and IID. A similar expression pattern was also found in human skin biopsies. Protein analysis showed that under serum-free conditions only group X secreted phospholipase A(2) is secreted into cell culture supernatants of HaCaT as well as human primary keratinocytes, whereas the other secreted phospholipases A(2) were not detectable at protein level. HaCaT keratinocytes constitutively released secreted phospholipase A(2) activity into the cell culture supernatant, being reflected by a constant release of fatty acids. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate, which is a potent inducer of inflammation in skin, drastically reduced the mRNA level of group X secreted phospholipase A(2) and other secreted phospholipase A(2) subtypes as well as secreted phospholipase A(2) activity in cell culture supernatants. This suggests that inhibition of secreted phospholipase A(2) expression and activity as well as of fatty acid release by 12-O-tetradecanoylphorbol-13-acetate treatment might be a critical step impairing the integrity of the epidermis during phorbol-ester-induced pathologic processes in skin. The results show that group X secreted phospholipase A(2) represents the major secreted phospholipase A(2) subtype in human keratinocytes and thus may indicate a physiologic role for this enzyme in epidermis in vivo.

[Functional analysis of phospholipase A2 receptor by gene knockout studies]

Phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family and mediates a variety of biological responses elicited by group IB secretory phospholipase A2 (sPLA2-IB). In the present study, we have shown the evidence that a novel type of sPLA2, sPLA2-X, also acts as one of the high-affinity ligands for mouse PLA2R. We then generated PLA2R-deficient mice and found that the knockout mice exhibited the resistance to an endotoxic shock with reduced plasma levels of proinflammatory cytokines, such as TNF-alpha and IL-1 beta. In situ hybridization analysis revealed that the expression of PLA2R transcript was markedly enhanced in type II alveolar epithelial cells and a subset of splenic lymphocytes in accordance with the elevated expression of sPLA2-IB and TNF-alpha mRNAs during endotoxic shock. In addition, the elevated expression level of TNF-alpha transcript was significantly reduced by the deficiency of PLA2R, suggesting that PLA2R plays a role in the regulation of TNF-alpha expression in these cell types. We then synthesized a specific sPLA2 inhibitor, indoxam, which blocked the binding of sPLA2-IB and X to PLA2R. Indoxam was found to suppress the elevation of the plasma level of TNF-alpha and prolonged the survival of endotoxin-challenged mice. The inhibitory effects of indoxam were abolished by the deficiency of PLA2R, demonstrating the involvement of PLA2R in the progression of endotoxic shock. We also detected and characterized a soluble form of PLA2R protein in the plasma of mouse with anti-PLA2R antibody, and showed its potential role as an endogenous sPLA2 inhibitor. Taken together, a series of studies with PLA2R-knockout mice have elucidated a critical role of PLA2R in the regulation of the development of endotoxic shock.

Potential role of group X secretory phospholipase A(2) in cyclooxygenase-2-dependent PGE(2) formation during colon tumorigenesis

Although the cyclooxygenase-2 (COX-2) pathway of the arachidonic acid cascade has been suggested to play an important role in colon carcinogenesis, there is little information concerning the identity of phospholipase A(2) (PLA(2)) involved in the arachidonic acid release in colon tumors. Here, we compared the potencies of three types of secretory PLA(2)s (group IB, IIA and X sPLA(2)s) for the arachidonic acid release from cultured human colon adenocarcinoma cells, and found that group X sPLA(2) has the most powerful potency in the release of arachidonic acid leading to COX-2-dependent prostaglandin E(2) (PGE(2)) formation. Furthermore, immunohistological analysis revealed the elevated expression of group X sPLA(2) in human colon adenocarcinoma neoplastic cells in concert with augmented expression of COX-2. These findings suggest a critical role of group X sPLA(2) in the PGE(2) biosynthesis during colon tumorigenesis.

Cloning and recombinant expression of human group IIF-secreted phospholipase A(2)

Mammalian-secreted phospholipases A(2) (sPLA(2)) form a diverse family of at least nine enzymes that hydrolyze phospholipids to release free fatty acids and lysophospholipids. We report here the cloning and characterization of human group IIF sPLA(2) (hGIIF sPLA(2)). The full-length cDNA codes for a signal peptide of 20 amino acid followed by a mature protein of 148 amino acids containing all of the structural features of catalytically active group II sPLA(2)s. hGIIF sPLA(2) gene is located on chromosome 1 and lies within a sPLA(2) gene cluster of about 300 kbp that also contains the genes for group IIA, IIC, IID, IIE, and V sPLA(2)s. In adult tissues, hGIIF is highly expressed in placenta, testis, thymus, liver, and kidney. Finally, recombinant expression of hGIIF sPLA(2) in Escherichia coli shows that the enzyme is Ca(2+)-dependent, maximally active at pH 7-8, and hydrolyzes phosphatidylglycerol versus phosphatidylcholine with a 15-fold preference.

Structure, function, and regulation of group V phospholipase A(2)

The hydrolysis of membrane phospholipid by phospholipase A(2) (PLA(2)) is a key step in the production of inflammatory eicosanoids. Recent cell studies have shown that secretory group V PLA(2) (gVPLA(2)) is involved in agonist-induced eicosanoid biosynthesis in mouse P388D1 cell line, mast cells, and transfected HEK 293 cells. gVPLA(2) is homologous to other group II PLA(2) family members but has distinctive enzymatic properties, including its activity to effectively hydrolyze phosphatidylcholine (PC) vesicles and the outer plasma membrane of mammalian cells. Mutational studies showed that gVPLA(2) has a unique structure that allows effective binding to PC membranes and efficient catalysis of an active-site-bound PC substrate. Thanks to this unique structure and activity, exogenously added gVPLA(2) can induce the eicosanoid biosynthesis in unstimulated inflammatory cells, including human neutrophils and eosinophils, suggesting that it might be able to trigger inflammatory responses under certain physiological conditions. Extensive structure-function and cell studies showed that gVPLA(2) could act directly on the outer plasma membranes of neutrophils and eosinophils. The release of fatty acids and lysophospholipids from the cell surfaces induces the translocation and activation of cytosolic PLA(2) and 5-lipoxygenase, resulting in the leukotriene synthesis. In case of neutrophils, induction of leukotriene B(4) synthesis by gVPLA(2) leads to the phosphorylation of cytosolic PLA(2) by a leukotriene B(4) receptor and MAP kinase-mediated mechanism. Finally, heparan sulfate proteoglycans in neutrophils appear to play a role of internalizing and degrading the cell surface-bound gVPLA(2) to protect the cells from extensive lipolytic damage.

The expanding superfamily of phospholipase A(2) enzymes: classification and characterization

The phospholipase A(2) (PLA(2)) superfamily consists of a broad range of enzymes defined by their ability to catalyze the hydrolysis of the middle (sn-2) ester bond of substrate phospholipids. The hydrolysis products of this reaction, free fatty acid and lysophospholipid, have many important downstream roles, and are derived from the activity of a diverse and growing superfamily of PLA(2) enzymes. This review updates the classification of the various PLA(2)'s now described in the literature. Four criteria have been employed to classify these proteins into one of the 11 Groups (I-XI) of PLA(2)'s. First, the enzyme must catalyze the hydrolysis of the sn-2 ester bond of a natural phospholipid substrate, such as long fatty acid chain phospholipids, platelet activating factor, or short fatty acid chain oxidized phospholipids. Second, the complete amino acid sequence of the mature protein must be known. Third, each PLA(2) Group should include all of those enzymes that have readily identifiable sequence homology. If more than one homologous PLA(2) gene exists within a species, then each paralog should be assigned a Subgroup letter, as in the case of Groups IVA, IVB, and IVC PLA(2). Homologs from different species should be classified within the same Subgroup wherever such assignments are possible as is the case with zebra fish and human Group IVA PLA(2) orthologs. The current classification scheme does allow for historical exceptions of the highly homologous Groups I, II, V, and X PLA(2)'s. Fourth, catalytically active splice variants of the same gene are classified as the same Group and Subgroup, but distinguished using Arabic numbers, such as for Group VIA-1 PLA(2) and VIA-2 PLA(2)'s. These four criteria have led to the expansion or realignment of Groups VI, VII and VIII, as well as the addition of Group XI PLA(2) from plants.

Increasing molecular diversity of secreted phospholipases A(2) and their receptors and binding proteins

Secreted phospholipases A(2) (sPLA(2)s) form a large family of structurally related enzymes which are widespread in nature. Snake venoms are known for decades to contain a tremendous molecular diversity of sPLA(2)s which can exert a myriad of toxic and pharmacological effects. Recent studies indicate that mammalian cells also express a variety of sPLA(2)s with ten distinct members identified so far, in addition to the various other intracellular PLA(2)s. Furthermore, scanning of nucleic acid databases fueled by the different genome projects indicates that several sPLA(2)s are also present in invertebrate animals like Drosophila melanogaster as well as in plants. All of these sPLA(2)s catalyze the hydrolysis of glycerophospholipids at the sn-2 position to release free fatty acids and lysophospholipids, and thus could be important for the biosynthesis of biologically active lipid mediators. However, the recent identification of a variety of membrane and soluble proteins that bind to sPLA(2)s suggests that the sPLA(2) enzymes could also function as high affinity ligands. So far, most of the binding data have been accumulated with venom sPLA(2)s and group IB and IIA mammalian sPLA(2)s. Collectively, venom sPLA(2)s have been shown to bind to membrane and soluble mammalian proteins of the C-type lectin superfamily (M-type sPLA(2) receptor and lung surfactant proteins), to pentraxin and reticulocalbin proteins, to factor Xa and to N-type receptors. Venom sPLA(2)s also associate with three distinct types of sPLA(2) inhibitors purified from snake serum that belong to the C-type lectin superfamily, to the three-finger protein superfamily and to proteins containing leucine-rich repeats. On the other hand, mammalian group IB and IIA sPLA(2)s can bind to the M-type receptor, and group IIA sPLA(2)s can associate with lung surfactant proteins, factor Xa and proteoglycans including glypican and decorin, a mammalian protein containing a leucine-rich repeat.

Cellular components that functionally interact with signaling phospholipase A(2)s

Accumulating evidence has suggested that cytosolic phospholipase A(2) (cPLA(2)) and several secretory PLA(2) (sPLA(2)) isozymes are signaling PLA(2)s that are functionally coupled with downstream cyclooxygenase (COX) isozymes for prostaglandin (PG) biosynthesis. Arachidonic acid (AA) released by cPLA(2) and sPLA(2)s is supplied to both COX-1 and COX-2 in the immediate, and predominantly to COX-2 in the delayed, PG-biosynthetic responses. Vimentin, an intermediate filament component, acts as a functional perinuclear adapter for cPLA(2), in which the C2 domain of cPLA(2) associates with the head domain of vimentin in a Ca(2+)-sensitive manner. The heparin-binding signaling sPLA(2)-IIA, IID and V bind the glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan glypican, which plays a role in sorting of these isozymes into caveolae and perinuclear compartments. Phospholipid scramblase, which facilitates transbilayer movement of anionic phospholipids, renders the cellular membranes more susceptible to signaling sPLA(2)s. There is functional cooperation between cPLA(2) and signaling sPLA(2)s in that prior activation of cPLA(2) is required for the signaling sPLA(2)s to act properly. cPLA(2)-derived AA is oxidized by 12/15-lipoxygenase, the products of which not only augment the induction of sPLA(2) expression, but also cause membrane perturbation, leading to increased cellular susceptibility to the signaling sPLA(2)s. sPLA(2)-X, a heparin-non-binding sPLA(2) isozyme, is capable of releasing AA from intact cells in the absence of cofactors. This property is attributed to its ability to avidly hydrolyze zwitterionic phosphatidylcholine, a major phospholipid in the outer plasma membrane. sPLA(2)-V can also utilize this route in several cell types. Taken together, the AA-releasing function of sPLA(2)s depends on the presence of regulatory cofactors and interfacial binding to membrane phospholipids, which differ according to cell type, stimuli, secretory processes, and subcellular distributions.

Internalization and degradation of type IIA phospholipase A(2) in mast cells

Whereas exogenous types IB and X secretory phospholipase A(2) (sPLA(2)) elicited prostaglandin D(2) (PGD(2)) production in mouse bone marrow-derived mast cells (BMMC), sPLA(2)-IIA was unable to do so. In search of a mechanism underlying this cellular refractoriness to exogenous sPLA(2)-IIA, we now report that this isozyme is promptly associated with cell surfaces, internalized, and then degraded in BMMC. Adsorption of sPLA(2)-IIA to BMMC was prevented by addition of heparin to the medium. Moreover, a heparin-nonbinding sPLA(2)-IIA mutant did not bind to BMMC. These results indicate that this sPLA(2)-IIA inactivation process depends on its rapid binding to heparan sulfate proteoglycan (HSPG) on BMMC surfaces. Thus, the present observations represent a particular situation in which cell surface HSPG exhibit a negative regulatory effect on cellular function of sPLA(2)-IIA, and argue that HSPG does not always act as a functional adapter for heparin-binding sPLA(2)s in mammalian cells as has been demonstrated before.

Mouse group X secretory phospholipase A2 induces a potent release of arachidonic acid from spleen cells and acts as a ligand for the phospholipase A2 receptor

Group X secretory phospholipase A2 (sPLA2-X) has recently been shown to possess a powerful potency for releasing arachidonic acid from cell membrane phospholipids. Here, we report the purification of mouse pro- and mature forms of sPLA2-X, as well as its expression and biological functions. Purified pro-sPLA2-X was found to possess a propeptide of 11 amino acid residues attached at the NH2-terminals of the mature protein, and showed as little as 8% of the PLA2 activity of the mature form. Limited proteolysis of pro-sPLA2-X with trypsin resulted in the appearance of the mature form with a concomitant increase in PLA2 activity, suggesting a requirement of proteolytic removal of the propeptide for the optimal activity. The expression of sPLA2-X mRNA was detected in various tissues including the lung, thymus, and spleen, and immunohistochemical analysis revealed its expression in splenic macrophages. In the spleen cells, mature sPLA2-X elicited a prompt release of arachidonic acid with significant production of prostaglandin E2 more efficiently than group IB and IIA sPLA2s. In addition, sPLA2-X was identified as a high-affinity ligand for both native and recombinant form of mouse PLA2 receptor (PLA2R). However, there was no significant difference in the sPLA2-X-induced arachidonic acid release responses in the spleen cells between wild-type and PLA2R-deficient mice. These findings strongly suggest that sPLA2-X possesses two distinct biological functions in mice: it elicits a marked release of arachidonic acid from membrane phospholipids leading to the production of lipid mediators based on its enzymatic potency, and it acts as a natural ligand for the PLA2R that has been shown to play a critical role in the production of inflammatory cytokines during endotoxic shock.

Secretory phospholipase A2 receptor-mediated activation of cytosolic phospholipase A2 in murine bone marrow-derived mast cells

The current study examined the signal transduction steps involved in the selective release of arachidonic acid (AA) induced by the addition of secretory phospholipase A2 (sPLA2) isotypes to bone marrow-derived mast cells (BMMC). Overexpression of sPLA2 receptors caused a marked increase in AA and PGD2 release after stimulation of BMMC, implicating sPLA2 receptors in this process. The hypothesis that the release of AA by sPLA2 involved activation of cytosolic PLA2 (cPLA2) was next tested. Addition of group IB PLA2 to BMMC caused a transient increase in cPLA2 activity and translocation of this activity to membrane fractions. Western analyses revealed that these changes in cPLA2 were accompanied by a time-dependent gel shift of cPLA2 induced by phosphorylation of cPLA2 at various sites. A noncatalytic ligand of the sPLA2 receptor, p-amino-phenyl-alpha-D-mannopyranoside BSA, also induced an increase in cPLA2 activity in BMMC. sPLA2 receptor ligands induced the phosphorylation of p44/p42 mitogen-activated protein kinase. Additionally, an inhibitor of p44/p42 mitogen-activated protein kinase (PD98059) significantly inhibited sPLA2-induced cPLA2 activation and AA release. sPLA2 receptor ligands also increased Ras activation while an inhibitor of tyrosine phosphorylation (herbimycin) inhibited the increase in cPLA2 activation and AA release. Addition of partially purified sPLA2 from BMMC enhanced cPLA2 activity and AA release. Similarly, overexpression of mouse groups IIA or V PLA2 in BMMC induced an increase in AA release. These data suggest that sPLA2 mediate the selective release of AA by binding to cell surface receptors and then inducing signal transduction events that lead to cPLA2 activation.

Small peptides Do not inhibit human non-pancreatic secretory phospholipase-A(2) (Type IIA)

Seven small peptides, that are among the most potent reported inhibitors of secreted mammalian phospholipases A(2), were found not to inhibit processing of a small phospholipid substrate by human non-pancreatic secretory phospholipase A(2) (type IIa), under conditions where certain non-peptides are potent inhibitors at nanomolar concentrations.

Identification of group X secretory phospholipase A(2) as a natural ligand for mouse phospholipase A(2) receptor

Phospholipase A(2) receptor (PLA(2)R) mediates various biological responses elicited by group IB secretory phospholipase A(2) (sPLA(2)-IB). The recently cloned group X sPLA(2) (sPLA(2)-X) possesses several structural features characteristic of sPLA(2)-IB. Here, we detected a specific binding site of sPLA(2)-X in mouse osteoblastic MC3T3-E(1) cells. Cross-linking experiments demonstrated its molecular weight (180 kDa) to be similar to that of PLA(2)R. In fact, sPLA(2)-X was found to bind the recombinant PLA(2)R expressed in COS-7 cells, and its specific binding detected in mouse lung membranes was abolished by the deficiency of PLA(2)R. These findings demonstrate sPLA(2)-X to be one of the high-affinity ligands for mouse PLA(2)R.

Cell-specific expression of group X and group V secretory phospholipases A(2) in human lung airway epithelial cells

Secretory phospholipase A(2) (sPLA(2)) enzymes contribute to inflammatory injury in human lungs by several mechanisms, including eicosanoid production and hydrolytic damage to surfactant phospholipids. Several distinct sPLA(2) genes have been described in human tissue but little is known regarding their presence, localization, or function(s) within lungs. We hypothesized that sPLA(2)s would have cell-specific distributions within lung. We used reverse transcriptase/polymerase chain reaction to identify sPLA(2) messenger RNAs (mRNAs) in adult human lung tissue. Resulting complementary DNA (cDNA) sequences indicated that total lung extracts contained mRNA for Groups IB, IIA, V, and X sPLA(2). An epithelial cell line, BEAS cells, expressed only Groups IIA, V, and X. We used these cDNAs to clone these enzymes, especially the recently described Group X and Group V enzymes. Digoxigenin-labeled complementary RNA probes were used to determine localization of each sPLA(2) by in situ hybridization of human lung. Hybridization was strongly positive for Group X and Group V in airway epithelial cells, which failed to hybridize Group IB or IIA probes. Although four known mammalian sPLA(2) isotypes were expressed in lung, only Group X and Group V sPLA(2) mRNAs appear uniquely expressed in airway epithelium, suggesting they could provide a mechanism of pulmonary surfactant hydrolysis during lung injury.

Enhanced tissue expression and elevated circulating level of phospholipase A(2) receptor during murine endotoxic shock

Phospholipase A(2) receptor (PLA(2)R) mediates a variety of biological responses elicited by mammalian secretory phospholipase A(2) (sPLA(2)). In mice, group IB sPLA(2) (sPLA(2)-IB) acts as an endogenous ligand of PLA(2)R, and analysis of PLA(2)R-deficient mice has demonstrated a critical role of the sPLA(2)-IB/PLA(2)R system in the production of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha) in the development of endotoxic shock. Here, we generated specific antibodies against a recombinant soluble form of PLA(2)R and examined its expression in the lung and spleen where a remarkable elevation of TNF-alpha expression has been observed during endotoxemia. Immunohistochemical analysis revealed the expression of PLA(2)R in type II alveolar epithelial cells and a subset of splenic lymphocytes, and its expression levels were markedly enhanced at 1 h after endotoxin challenge. Analysis with a newly developed sandwich enzyme-linked immunosorbent assay system revealed the presence of a soluble form of PLA(2)R in plasma of wild-type mice compared with its absence in plasma of PLA(2)R-deficient mice. After exposure to endotoxin, its circulating level was significantly elevated to the maximum level at 2-3 h after the treatment. These results suggest that tissue expression and the circulating level of PLA(2)R are elevated during murine endotoxemia, which might be relevant to its potential roles in the production of proinflammatory mediators during the development of inflammatory conditions.

Structures, enzymatic properties, and expression of novel human and mouse secretory phospholipase A(2)s

Mammalian secretory phospholipase A(2)s (sPLA(2)s) form a family of structurally related enzymes that are involved in a variety of physiological and pathological processes via the release of arachidonic acid from membrane phospholipids or the binding to specific membrane receptors. Here, we report the cloning and characterization of a novel sPLA(2) that is the sixth isoform of the sPLA(2) family found in humans. The novel human mature sPLA(2) consists of 123 amino acids (M(r) = 14,000) and is most similar to group IIA sPLA(2) (sPLA(2)-IIA) with respect to the number and positions of cysteine residues as well as overall identity (51%). Therefore, this novel sPLA(2) should be categorized into group II and called group IIE (sPLA(2)-IIE) following the recently identified group IID sPLA(2) (sPLA(2)-IID). The enzymatic properties of recombinant human sPLA(2)-IIE were almost identical to those of sPLA(2)-IIA and IID in terms of Ca(2+) requirement, optimal pH, substrate specificity, as well as high susceptibility to the sPLA(2) inhibitor indoxam. Along with the biochemical properties of proteins, genetic and evolutional similarities were also observed among these three types of group II sPLA(2)s as to the chromosomal location of the human gene (1p36) and the exon/intron organization. The expression of sPLA(2)-IIE transcripts in humans was restricted to the brain, heart, lung, and placenta in contrast to broad expression profiles for sPLA(2)-IIA and -IID. In sPLA(2)-IIA-deficient mice, the expression of sPLA(2)-IIE was markedly enhanced in the lung and small intestine upon endotoxin challenge, which contrasted with the reduced expression of sPLA(2)-IID mRNA. In situ hybridization analysis revealed elevation of sPLA(2)-IIE mRNA at alveolar macrophage-like cells in the lung of endotoxin-treated mice. These findings suggest a distinct functional role of novel sPLA(2)-IIE in the progression of inflammatory processes.