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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Isoform-specific membrane insertion of secretory phospholipase A2 and functional implications

Despite increasing evidence that the membrane-binding mode of interfacial enzymes including the depth of membrane insertion is crucial for their function, the membrane insertion of phospholipase A(2) (PLA(2)) enzymes has not been studied systematically. Here, we analyze the membrane insertion of human group IB PLA(2) (hIBPLA(2)) and compare it with that of a structurally homologous V3W mutant of human group IIA PLA(2) (V3W-hIIAPLA(2)) and with a structurally divergent group III bee venom PLA(2) (bvPLA(2)). Increasing the anionic charge of membranes results in a blue shift of the fluorescence of Trp(3) of hIBPLA(2), a decrease in quenching by acrylamide, and an increase in enzyme activity, reflecting an enhancement in the membrane binding of PLA(2). Fluorescence quenching by brominated lipids indicates significant penetration of Trp(3) into fluid POPC/POPG membranes but little insertion into the solid DPPC/DPPG membranes. Increased membrane fluidity also supports hIBPLA(2) activity, suggesting that membrane insertion of hIBPLA(2) is controlled by membrane fluidity and is necessary for the full activity of the enzyme. Trp fluorescence quenching of the V3W-hIIAPLA(2) and bvPLA(2) by water- and membrane-soluble quenchers indicates substantial membrane insertion of Trp(3) of V3W-hIIAPLA(2), similar to that found for hIBPLA(2), and no insertion of tryptophans of bvPLA(2). Our results provide evidence that (a) structurally similar group IB and IIA PLA(2)s, but not structurally diverse group III PLA(2), significantly penetrate into membranes; (b) membrane insertion is controlled by membrane fluidity and facilitates activation of IB and IIA PLA(2)s; and (c) structurally distinct PLA(2) isoforms may employ different tactics of substrate accession/product release during lipid hydrolysis.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Differential inhibition of group IVA and group VIA phospholipases A2 by 2-oxoamides

Inhibitors of the Group IVA phospholipase A(2) (GIVA cPLA(2)) and GVIA iPLA(2) are useful tools for defining the roles of these enzymes in cellular signaling and inflammation. We have developed inhibitors of GVIA iPLA(2) building upon the 2-oxoamide backbone that are uncharged, containing ester groups. Although the most potent inhibitors of GVIA iPLA(2) also inhibited GIVA cPLA(2), there were three 2-oxoamide compounds that selectively and weakly inhibited GVIA iPLA(2). We further show that several potent 2-oxoamide inhibitors of GIVA cPLA(2) containing free carboxylic groups (Kokotos et al. J. Med. Chem. 2002, 45, 2891-2893) do not inhibit GVIA iPLA(2) and are, therefore, selective GIVA cPLA(2) inhibitors.

The role of phospholipases A2 in schizophrenia

A range of neurotransmitter systems have been implicated in the pathogenesis of schizophrenia based on the antidopaminergic activities of antipsychotic medications, and chemicals that can induce psychotic-like symptoms, such as ketamine or PCP. Such neurotransmitter systems often mediate their cellular response via G-protein-coupled release of arachidonic acid (AA) via the activation of phospholipases A2 (PLA2s). The interaction of three PLA2s are important for the regulation of the release of AA--phospholipase A2 Group 2 A, phospholipase A2 Group 4A and phospholipase A2 Group 6A. Gene variations of these three key enzymes have been associated with schizophrenia with conflicting results. Preclinical data suggest that the activity of these three enzymes are associated with monoaminergic neurotransmission, and may contribute to the differential efficacy of antipsychotic medications, as well as other biological changes thought to underlie schizophrenia, such as altered neurodevelopment and synaptic remodelling. We review the evidence and discuss the potential roles of these three key enzymes for schizophrenia with particular emphasis on published association studies.

Effect of d-Alanylation of (Lipo)Teichoic Acids ofStaphylococcus aureuson Host Secretory Phospholipase A2Action before and after Phagocytosis by Human Neutrophils

Invading bacteria such as Staphylococcus aureus induce mobilization of professional phagocytes (e.g., neutrophils) and extracellular antibacterial proteins (e.g., group IIA phospholipase A2 (gIIA PLA2)). Accumulation of gIIA PLA2 in inflammatory fluids confers potent extracellular antistaphylococcal activity and at lower concentrations promotes bacterial phospholipid degradation during phagocytosis of S. aureus by human neutrophils. D-alanylation of (lipo) teichoic acids of S. aureus increases bacterial resistance to gIIA PLA2 approximately 100-fold, raising the possibility that the resistance of ingested S. aureus to related gV and gX secretory PLA2 present in human neutrophil granules depends on D-alanylation mediated by the dlt operon. However, we show that isogenic wild-type and dltA S. aureus are equally resistant to gV/X PLA2 during phagocytosis and when exposed to the purified enzymes. The fates of wild-type and dltA S. aureus exposed to serum and human neutrophils differed significantly only when extracellular gIIA PLA2 was also present before phagocytosis. The extreme potency of the gIIA PLA2 toward dltA S. aureus suggests that even small amounts of this extracellular enzyme mobilized early in inflammation could contribute substantially to the overall cytotoxicity of acute inflammatory exudates toward S. aureus when D-alanylation of (lipo)teichoic acids is limiting.

Inhibition of secretory phospholipase A2. 2-Synthesis and structure–activity relationship studies of 4,5-dihydro-3-(4-tetradecyloxybenzyl)-1,2,4-4H-oxadiazol-5-one (PMS1062) derivatives specific for group II enzyme

We have recently reported the discovery of a series of specific inhibitors of human group IIA phospholipase A(2) (hGIIA PLA(2)) to display promising in vitro and in vivo properties. Here we describe the influence of different structural modifications on the specificity and potency against hGIIA PLA(2) versus porcine group IB PLA(2). The SAR results, as well as the logP and pK(a) values of oxadiazolone determined in this work, provide important information towards the comprehension of the mode of action of this kind of compounds.

Multiple forms of secretory phospholipase A2 in plants

Multiple secretory phospholipase A2 (sPLA2) genes have been identified in plants and encode isoforms with distinct regulatory and catalytic properties. Elucidation of this genetic and biochemical heterogeneity has provided important clues to the regulation and function of the individual enzymes. An increasing body of evidence shows that their lipid products, lysophospholipids and free fatty acids, mediate a variety of cellular responses, including plant growth, development, and responses to stress and defense. This review discusses the newly-acquired information on plant sPLA2s including the molecular and biochemical characteristics, and signaling functions of each isoform.

Catalytic features, regulation and function of myocardial phospholipase A2

Phospholipase A2 (PLA2) catalyzes the hydrolysis of sn-2 fatty acids from membrane phospholipids resulting in the production of several biologically active phospholipid metabolites such as lysophospholipids, arachidonic acid, eicosanoids and platelet-activating factor. The majority of myocardial PLA2 activity is membrane-associated and does not require Ca2+ for activity (iPLA2). Myocardial iPLA2 demonstrates unique characteristics when compared to other PLA2 isoforms described previously, including a selectivity for plasmalogen phospholipids and resistance to inhibition by methyl arachidonyl fluorophosphonate. Activation of myocardial iPLA2 results in the production of lysoplasmenylcholine and arachidonic acid, both of which can change the electrophysiologic properties of the myocardium. Arachidonic acid can modulate ion channel activity via protein kinase C activation and has been demonstrated to decrease gap junctional conductance. Lysoplasmenylcholine directly produces action potential derangements and alters calcium cycling in cardiac myocytes. Thus, inhibition of iPLA2 activity to block production of phospholipid metabolites that mediate pathologic changes in the myocardium would be of considerable benefit. However, there are situations where inhibition of PLA2 activity would be detrimental to the myocardium, in particular if iPLA2 acts as a phospholipid repair enzyme following oxidative damage. Although little is known regarding the function of cPLA2 or sPLA2 in the myocardium, it is possible that they may be important for signal transduction or may modulate the activity of iPLA2.

Lysophospholipid generation and phosphatidylglycerol depletion in phospholipase A(2)-mediated surfactant dysfunction

Pulmonary surfactant's complex mixture of phospholipids and proteins reduces the work of breathing by lowering alveolar surface tension during respiration. One mechanism of surfactant damage appears to be the hydrolysis of phospholipid by phospholipases activated in the inflamed lung. Humans have several candidate secretory phospholipase A(2) (sPLA(2)) enzymes in lung cells and infiltrating leukocytes that could damage extracellular surfactant. We considered two mechanisms of surfactant disruption by five human sPLA(2)s, including generation of lysophospholipids and the depletion of specific phospholipids. All five sPLA(2)s studied ultimately caused surfactant dysfunction. Each enzyme exhibited a different pattern of hydrolysis of surfactant phospholipids. Phosphatidylcholine, the major phospholipid in surfactant and the greatest potential source for generation of lysophospholipids, was susceptible to hydrolysis by group IB, group V, and group X sPLA(2)s, but not group IIA or IID. Group IIA hydrolyzed both phosphatidylethanolamine and phosphatidylglycerol, whereas group IID was active against only phosphatidylglycerol. Thus, with groups IB and X, the generation of lysophospholipids corresponded with surfactant dysfunction. However, hydrolysis of and depletion of phosphatidylglycerol had a greater correlation with surfactant dysfunction for groups IIA and IID. Surfactant dysfunction caused by group V sPLA(2) is less clear and may be the combined result of both mechanisms.

Comparative proteomics and subtyping of venom phospholipases A2 and disintegrins of Protobothrops pit vipers

To explore the venom diversity and systematics of pit vipers under the genus Protobothrops, the venom phospholipases A2 (PLA2s) of P. mangshanensis, P. elegans and P. tokarensis were purified and characterized for the first time. The results were compared with the corresponding venom data of other co-generic species including P. mucrosquamatus, P. flavoviridis and P. jerdonii. Based on sequence features at the N-terminal regions, we identified five PLA2 subtypes, i.e., the Asp49-PLA2s with N6, E6 or R6 substitution and the Lys49-PLA2. However, not all subtypes were expressed in each of the species. Venom N6-PLA2s from P. mangshanensis and P. tokarensis venom were weakly neurotoxic toward chick biventer cervicis tissue preparations. The venoms of P. tokarensis and P. flavoviridis contained identical PLA2 isoforms. In most Protobothrop disintegrins, sequences flanking the RGD-motif are conserved. Phylogenetic analyses based on amino acid sequences of both families of the acidic PLA2s and the disintegrins clarify that these species could belong to a monophyletic group.

Brain phospholipases A2: a perspective on the history

The phospholipases A2 (PLA2) belong to a large family of enzymes involved in the generation of several second messengers that play an important role in signal transduction processes associated with normal brain function. The phospholipase A2 family includes secretory phospholipase A2, cytosolic phospholipase A2, calcium-independent phospholipase A2, plasmalogen-selective phospholipase A2 and many other enzymes with phospholipase A2 activity that have not been classified. Few attempts have been made purify and characterize the multiple forms of PLA2 and none have been fully characterized and cloned from brain tissue. A tight regulation of phospholipase A2 isozymes is necessary for maintaining physiological levels of free fatty acids including arachidonic acid and its metabolites in the various types of neural cells. Under normal conditions, phospholipase A2 isozymes may be involved in neurotransmitter release, long-term potentiation, growth and differentiation, and membrane repair. Under pathological conditions, high levels of lipid metabolites generated by phospholipase A2 are involved in neuroinflammation, oxidative stress, and neural cell injury.

Effect of crotapotin on the biological activity of Asp49 and Lys49 phospholipases A(2) from Bothrops snake venoms

Myonecrosis, in addition to edema and other biological manifestations, are conspicuous effects of Bothrops snake venoms, some of them caused by phospholipases A(2) (PLA(2)s). Asp49-PLA(2)s are catalytically active, whereas Lys49-PLA(2)s, although highly toxic, have little or no enzymatic activity upon artificial substrates, due to a substitution of lysine for aspartic acid at position 49. Crotapotin (CA), the acidic counterpart of crotoxin PLA(2) (CB), is a PLA(2)-like protein from Crotalus durissus terrificus snake venom, and is considered a chaperone protein for CB, able to increase its lethality about ten fold, but to inhibit the formation of the rat paw edema induced by carrageenin and by snake venoms. In this study, we demonstrate that CA significantly inhibits the edema induced by BthTX-I (23% inhibition), BthTX-II (27%), PrTX-I (25%), PrTX-III (35%) and MjTX-II (10%) on the mouse paw. CK levels evoked by isolated Asp49 or Lys49-PLA(2)s were reduced by 40% to 54% in the presence of CA and, in all cases, the membrane damaging activity of the toxins was also reduced. Circular dichroism spectra of the PLA(2)s in the presence and absence of CA showed that there was not any detectable secondary structural modification due to association between CA and the myotoxins. However, Fourier Transformed Infrared (FT-IR) analysis indicated that ionic and hydrophobic contacts contributed to stabilize this interaction.

The catalytic activity, but not receptor binding, of sPLA2s plays a critical role for neurite outgrowth induction in PC12 cells

We previously showed that fungal secretory phospholipase A2 (sPLA2) induces neurite formation in PC12 cells in an L-type Ca2+ channel activity-dependent manner. In this study we compared neurite-inducing activity of different sPLA2s, including bee venom sPLA2 (bvPLA2), and found that it correlated with the ability of each sPLA2 to release fatty acids from live PC12 cells. Consistently, using several mutants of bvPLA2, we found that the enzymatic activity rather than the binding activity to the putative N-type receptor for neurotoxic sPLA2s is the critical determinant for the neuritogenic response. These results imply that the neurite outgrowth is elicited by the messenger(s) produced upon degradation of membrane phospholipids.

[Lipoprotein-associated phospholipase A2: importance and perspectives]

Type VII phospholipase A2 associated to low density lipoproteins (LDL), also known as platelet-activating factor acetylhydrolase, has been recently indicated as a new non traditional and independent risk factor of coronary disease. After the classification of phospholipase A2 family enzymes, a review is made of the recent physiologic and biochemical knowledges on A2 type VII phospholipase LDL lipoproteins-associated and the role developed in lipoproteins metabolism and atherogenesis. Finally, future therapeutic implications and perspectives depending on these knowledges are pointed out especially by using molecules inhibiting the activity of the enzyme in atherosclerosis therapy. The evaluation of circulating activity of the enzyme may be useful in the prevention and recognition of acute coronary syndromes.

Islet complex lipids: involvement in the actions of group VIA calcium-independent phospholipase A(2) in beta-cells

The beta-isoform of group VIA calcium-independent phospholipase A(2) (iPLA(2)beta) does not require calcium for activation, is stimulated by ATP, and is sensitive to inhibition by a bromoenol lactone suicide substrate. Several potential functions have been proposed for iPLA(2)beta. Our studies indicate that iPLA(2)beta is expressed in beta-cells and participates in glucose-stimulated insulin secretion but is not involved in membrane phospholipid remodeling. If iPLA(2)beta plays a signaling role in glucose-stimulated insulin secretion, then conditions that impair iPLA(2)beta functions might contribute to the diminished capacity of beta-cells to secrete insulin in response to glucose, which is a prominent characteristic of type 2 diabetes. Our recent studies suggest that iPLA(2)beta might also participate in beta-cell proliferation and apoptosis and that various phospholipid-derived mediators are involved in these processes. Detailed characterization of the iPLA(2)beta protein level reveals that beta-cells express multiple isoforms of the enzyme, and our studies involve the hypothesis that different isoforms have different functions.

New phospholipase A(2) isozymes with a potential role in atherosclerosis

PURPOSE OF REVIEW

Inflammation is an integral feature of atherosclerosis, in which inflammatory processes contribute to the initiation, progression and rupture of lipid-rich atherosclerotic plaques. Recent studies have suggested the involvement of the proinflammatory secretory phospholipase A2 (sPLA2)-IIA in the development of atherosclerosis. This enzyme has been proposed to hydrolyze phosphatidylcholine (PC) in lipoproteins to liberate lyso-PC and free fatty acids in the arterial wall, thereby facilitating the accumulation of bioactive lipids and modified lipoproteins in atherosclerotic foci. However, the recent discovery of several novel sPLA2 isozymes has raised the question of which types of sPLA2 truly contribute to the atherosclerotic process.

RECENT FINDINGS

Amongst the 10 mammalian sPLA2 isozymes, sPLA2-X, -V, -IIF and -III exhibit much more potent PC-hydrolyzing activity than do the others, and can release free fatty acids and lysophospholipids from the PC-rich outer leaflet of the cellular plasma membrane. In particular, sPLA2-X and sPLA2-V hydrolyze PC in lipoproteins far more efficiently than does sPLA2-IIA. Moreover, sPLA2-X promotes foam cell formation in vitro and is expressed in the atherosclerotic arterial walls of apolipoprotein E deficient mice in vivo.

SUMMARY

PC-hydrolyzing sPLA2 isozymes, particularly sPLA2-V and sPLA2-X, are attractive candidates for proatherosclerotic factors that may act in place of sPLA2-IIA. However, their expression in human atherosclerotic lesions requires confirmation by specific methods that can distinguish between the different sPLA2 isozymes.

Phospholipase A(2)

Considerable progress has been made in characterizing the individual participant enzymes and their relative contributions in the generation of eicosanoids, lipid mediators derived from arachidonic acid, such as prostaglandins and leukotrienes. However, the role of individual phospholipase (PL) A(2) enzymes in providing arachidonic acid to the downstream enzymes for eicosanoid generation in biologic processes has not been fully elucidated. In this review, we will provide an overview of the classification of the families of PLA(2) enzymes, their putative mechanisms of action, and their role(s) in eicosanoid generation and inflammation.

Lipides et inflammation cutanée : place des phospholipases A2

Phospholipases A2 (PLA2) are enzymes that catalyse the hydrolysis of glycerophospholipids at the sn-2 position, generating free fatty acids and lysophospholipids. At present, PLA2 family consists of 12 groups. PLA2 are involved in many pathophysiological processes such as barrier function, eicosanoid production, and inflammation. They are implicated in inflammatory diseases of the skin: psoriasis, eczema, atopy. The presence of PLA2 activity has been demonstrated several years ago, however the precise localization of all these PLA2 in the epidermis and its appendages has to be determined. Further studies have shown that these enzymes are expressed in various layers of epidermis. This differential localization suggests different roles for each PLA2 in skin physiology and during inflammation.

Secreted phospholipase A2 enzymes as therapeutic targets

Homology cloning through in silico database search analysis has led to the definition of ten structurally-related mammalian secreted phospholipase A(2) (sPLA(2)) enzyme forms at present, each expressed in a species-, genotype- and cell-type-specific manner and with different enzymatic properties. These studies have shown that models based on the premise that there is only one PLA(2) drug target are now inadequate. Type IIA sPLA(2) remains the most advanced clinical target, with rationally designed inhibitors in Phase II clinical trials. However, progress in our understanding of the functional role of the ten secreted enzymes in phospholipid (PL) metabolism and in eicosanoid-mediated disorders, together with their emerging activity-independent and receptor-mediated functions, is likely to significantly impact on current and future drug development efforts.

Papyriflavonol A from Broussonetia papyrifera inhibits the passive cutaneous anaphylaxis reaction and has a secretory phospholipase A2-inhibitory activity

Papyriflavonol A, a new prenylated flavonol isolated from Broussonetia papyrifera, selectively inhibits recombinant human secretory phospholipase A(2)s (sPLA(2)s). Papyriflavonol A was found to inhibit human group IIA and V sPLA(2)s potently and irreversibly in a dose-dependent manner, with respective IC(50) values of 3.9 and 4.5 microM. The inhibitory effects of papyriflavonol A against bovine group IB (IC(50) of 76.9 microM) and the human group X (IC(50) of 225 microM) sPLA(2)s were weaker than those against human group IIA and V sPLA(2)s, and human group IIF sPLA(2) was not inhibited. In addition, papyriflavonol A potently inhibited the stimulus-induced production of leukotriene C(4) with an IC(50) value of approximately 0.64 microM in mouse bone marrow-derived mast cells. In addition, papyriflavonol A significantly reduced IgE-dependent passive cutaneous anaphylaxis in rats. These results indicate that papyriflavonol A provides a basis for novel types of antiinflammatory drugs.

Phospolipase A2 and apoptosis

Phospolipase A(2) (PLA(2)) is the esterase activity that cleaves the sn-2 ester bond in glycerophospholipids, releasing free fatty acids and lysophospholipids. The PLA(2) activity is found in a variety of enzymes which can be divided in several types based on their Ca(2+) dependence for their activity; Ca(2+)-dependent secretory phosholipases (sPLA(2)s) and cytosolic phospholipases (cPLA(2)s), and Ca(2+)-independent phospholipase A(2)s (iPLA(2)s). These enzymes also show diverse size and substrate specificity (i.e., in the fatty acid chain length and extent of saturation). Among the fatty acids released by PLA(2), arachidonic acid (AA) is of particular biological importance, because it is subsequently converted to prostanoids and leukotrienes by cyclooxygenases (COX) and lipoxygenases (LOX), respectively. Free AA may also stimulate apoptosis through activation of sphingomyelinase. Alternatively, it is suggested that oxidized metabolites generated from AA by LOX induce apoptosis. Although the precise mechanisms remain to be elucidated, changes are observed in glycerolipid metabolism during apoptotic processes. In some cells induced to undergo apoptosis, AA is released concomitant with loss of cell viability, caspase activation and DNA fragmentation. Such AA releases appear to be mediated by activation of cPLA(2) and/or iPLA(2). For example, tumor necrosis factor-alpha (TNF-alpha)-induced cell death is mediated by cPLA(2), whereas Fas-induced apoptosis appears to be mediated by iPLA(2). Some discrepancies among early experimental results were probably caused by differences in the experimental conditions such as the serum concentration, inhibitors used that are not necessarily specific to a single-type enzyme, or differential expression of each PLA(2) in cells employed in the experiments. Recent studies eliminated such problems, by carefully defining the experimental conditions, and using multiple inhibitors that show different specificities. Accordingly, more convincing data are available that demonstrate involvement of some PLA(2)s in the apoptotic processes. In addition to cPLA(2) and iPLA(2), sPLA(2)s were recently found to play roles in apoptosis. Moreover, new proteins that appear to control PLA(2)s are being discovered. Here, the roles of PLA(2)s in apoptosis are discussed by reviewing recent reports.

Structural characterization and phylogenetic relationships of myotoxin II from Atropoides (Bothrops) nummifer snake venom, a Lys49 phospholipase A(2) homologue

In order to analyze its structure-function relationships, the complete amino acid sequence of myotoxin II from Atropoides (Bothrops) nummifer from Costa Rica was determined. This toxin is a Lys49-type phospholipase A(2) (PLA(2)) homologue, devoid of catalytic activity, structurally belonging to class IIA. In addition to the Asp49 --> Lys change in the (inactive) catalytic center, substitutions in the calcium-binding loop suggest that its lack of enzymatic activity is due to the loss of ability to bind Ca(2+). The toxin occurs as a homodimer of basic subunits of 121 residues. Its sequence has highest similarity to Lys49 PLA(2)s from Cerrophidion, Trimeresurus, Bothrops and Agkistrodon species, which form a subfamily of proteins that diverged early from Asp49 PLA(2)s present in the same species, as shown by phylogenetic analysis. The tertiary structure of the toxin was modeled, based on the coordinates of Cerrophidion godmani myotoxin II. Its exposed C-terminal region 115-129 shows several differences in comparison to the homologous sequences of other Lys49 PLA(2)s, i.e. from Agkistrodon p. piscivorus and Bothrops asper. Region 115-129 of the latter two proteins has been implicated in myotoxic activity, on the basis of the direct membrane-damaging of their corresponding synthetic peptides. However, peptide 115-129 of A. nummifer myotoxin II did not exert toxicity upon cultured skeletal muscle cells or mature muscle in vivo. Differences in several amino acid residues, either critical for toxicity, or influencing the conformation of free peptide 115-129 from A. nummifer myotoxin II, may account for its lack of direct membrane-damaging properties.

Increased gene expression of novel cytosolic and secretory phospholipase A(2) types in human airway epithelial cells induced by tumor necrosis factor-alpha and IFN-gamma

Phospholipase A(2) (PLA(2)) is a growing family of enzymes that may play a major role in inflammation. We investigated the effect of tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) on the gene expression of 19 different PLA(2) types (IB, IIA, IID, IIE, IIF, III, IVA, IVB, IVC, V, VIA, VIB, VIIA, VIIB, VIIIA, VIIIB, X, XII, and XIII) in human bronchoepithelial (BEAS-2B) and nasal epithelial (RPMI 2650) cells. The cells were stimulated with TNF-alpha or IFN-gamma for different lengths of time (1, 4, 18, and 48 h), and the mRNA levels of the different PLA(2) types were determined by reverse transcriptase-PCR (RT-PCR) and normalized to those of the housekeeping gene, GAPDH. In both cell lines, TNF-alpha increased the expression of PLA(2) IVA and IVC, and IFN-gamma increased the expression of PLA(2) IIA and IID. No influence on the gene expression of PLA(2)-activating protein (PLAP) was noted on cytokine stimulation. These findings indicate that TNF-alpha and IFN-gamma induce gene expression of two novel cytosolic and secretory PLA(2) types (IVC and IID, respectively) in human airway epithelial cells. The possibility that these PLA(2) types are involved in cytokine-mediated inflammation in the respiratory tract is inferred.

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.

A comparative analysis of invaded sequences from group IA phospholipase A(2) genes provides evidence about the divergence period of genes groups and snake families

Two phospholipase A(2) (PLA(2)) genes classified into group IA were cloned from the genomic library of the sea snake Laticauda semifasciata. Eight clones were obtained by PCR cloning procedure from genomic DNA of Laticauda laticaudata (four clones) and Laticauda colubrina (four clones). The genes were 3.6-4.4kbp in length. Intron and exon organization of the group IA PLA(2) genes was the same as that of Naja sputatrix group IA PLA(2) genes (four exons and three introns). There were two kinds of repetitive sequences in the first and second introns of all sequenced PLA(2) genes. The differences in the length of these genes were derived from the length of their repetitive sequences. The chicken repeat-1 (CR1)-like long interspersed repeated DNA (LINE) sequences, different from the above repetitive sequences, were also found in all sequenced Laticauda PLA(2) genes. A comparative analysis of groups IA, IA' and IIA PLA(2)s genes suggests a period of CR1-like LINE integration during molecular and family evolution. The integration of CR1-like LINE into PLA(2) genes occurred after the divergence of groups I and II PLA(2)s but before the divergence of groups, IA and IA' PLA(2)s. These integration events occurred before the family divergence of Naja and Laticauda. The presence of CR1-like LINE and a comparison of intron and exon organization showed that the divergence of Naja and Bungarus occurred before the divergence of Laticauda and Naja.

Cloning of a novel phospholipase A2 from the cnidarian Adamsia carciniopados

PLA2 catalytic activity was detected in homogenised tissues, including tentacles and acontia (structures for preying and defence, respectively), of the sea anemone Adamsia carciniopados. Nested reverse transcription polymerase chain reaction (RT PCR) with degenerate primers and rapid amplification of cDNA ends (RACE) were used to clone a novel phospholipase A2 from Adamsia carciniopados (AcPLA2). AcPLA2 contains a putative prepropeptide of 37 residues, ending with a basic doublet followed by a mature protein of 119 amino acids, including 12 cysteines. AcPLA2 displays only 30-42% similarity with other known secretory PLA2s (sPLA2). C-terminal extension, typical of groups II and X PLA2s, is absent. Predicted molecular weight and pI of the mature protein are 13.5 kDa and 9.1, respectively. Structural features and phylogenetic analysis set AcPLA2 apart from the known sPLA2s and define this molecule in the ancient metazoan phylum Cnidaria as a member of a new class of sPLA2s.

Phospholipase A2

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 (PGs) and leukotrienes (LTs). 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 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, host defense, and atherosclerosis. The cytosolic PLA2 (cPLA2) family consists of 3 enzymes, among which cPLA2alpha plays an essential role in the initiation of AA metabolism. Intracellular activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains 2 enzymes and may play a major role in membrane phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family represents a unique group of PLA2 that contains 4 enzymes exhibiting unusual substrate specificity toward PAF and/or oxidized phospholipids. In this review, we will overview current understanding of the properties and functions of each enzyme belonging to the sPLA2, cPLA2, and iPLA2 families, which have been implicated in signal transduction.

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.

Participation of phospholipase A2 isoforms in the control of calcium influx into electrically non-excitable cells

The participation of phospholipase A2 isoforms in capacitative store-operated Ca2+ influx into Jurkat leukemic T and MDCK cells was investigated. Preincubation of Jurkat cells with either bromophenacyl bromide (an inhibitor of secreted phospholipase A2, sPLA2) or Helss (an inhibitor of calcium independent phospholipase A2--iPLA2) resulted in a significant inhibition of the calcium influx. The extent of this inhibition depended on the pH of the extracellular millieu; it increased with alkalisation. The rate of Ca2+ influx into MDCK cells was reduced by bromophenacyl bromide. Preincubation of these cells with Helss resulted in the stimulation of the influx. These observations suggest the participation of different PLA2 isoforms in the regulation of Ca2+ influx. They also show that the extent that PLA2 isoforms control the influx depends on the pH of the medium. Finally, these data indicate that various phospholipase A2 isoforms may play a role in the control of Ca2+ influx in different cell lines.

Cloning and expression of group IB phospholipase A2 isoforms in the red sea bream, Pagrus major

Two cDNA encoding red sea bream DE-1 and DE-2 phospholipases A2 (PLA2) were cloned from the hepatopancreas of red sea bream, Pagrus (Chrysophrys) major. The cDNA of DE-1 PLA2 encoded a mature protein of 125 amino acid residues with an apparent signal peptide of 20 residues and propeptide of 5 residues, and that of DE-2 PLA2, a mature protein of 126 amino acid residues with an apparent signal peptide of 17 residues and propeptide of 6 residues. Comparison of the predicted amino acid sequences for mature DE-1 and DE-2 PLA2 showed that both proteins contain 14 cysteines including Cys 11 and 77 and a pancreatic loop, which are commonly conserved in group IB PLA2; however, the identity in amino acid sequence between DE-1 and DE-2 PLA2 was low (47%). A previous report concerning the cDNA cloning of red sea bream gill G-3 PLA2 and the present results represent the first cloning and sequencing of three distinct isoforms of group IB PLA2 in a single fish species, red sea bream. Reverse transcription-polymerase chain reaction analysis showed that DE-1 PLA2 mRNA was expressed in the hepatopancreas, pyloric ceca, intestine, spleen, gonad, stomach, and kidney, whereas gill G-3 PLA2 mRNA was expressed only in the gills and gonad. The expression of DE-2 PLA2 mRNA was detected in all of the tissues analyzed. These results indicate that three distinct isoforms of group IB PLA2, DE-1 and DE-2 PLA2 in hepatopanceas and gill G-3 PLA2, are expressed in a tissue-specific manner in red sea bream.

Absence of group II phospholipase A2, a Paneth cell marker, from the epididymis

Paneth cell-like metaplasia has been reported in the epithelium of the epididymis and prostatic adenocarcinomas. We studied the expression of group II phospholipase A2 (PLA2), a marker of Paneth cell differentiation, in six orchiectomy specimens with Paneth cell-like metaplasia. Both immunohistochemistry for group II PLA2 protein and in situ hybridization for the mRNA of group II PLA2 gave negative results in all six cases but positive reaction for lysozyme. The results show that the cells of the Paneth cell-like metaplasia are not true Paneth cells.

Deletion of cytosolic phospholipase A(2) suppresses Apc(Min)-induced tumorigenesis

Although nonsteroidal antiinflammatory drugs (NSAIDs) show great promise as therapies for colon cancer, a dispute remains regarding their mechanism of action. NSAIDs are known to inhibit cyclooxygenase (COX) enzymes, which convert arachidonic acid (AA) to prostaglandins (PGs). Therefore, NSAIDs may suppress tumorigenesis by inhibiting PG synthesis. However, various experimental studies have suggested the possibility of PG-independent mechanisms. Notably, disruption of the mouse group IIA secretory phospholipase A(2) locus (Pla2g2a), a potential source of AA for COX-2, increases tumor number despite the fact that the mutation has been predicted to decrease PG production. Some authors have attempted to reconcile the results by suggesting that the level of the precursor (AA), not the products (PGs), is the critical factor. To clarify the role of AA in tumorigenesis, we have examined the effect of deleting the group IV cytosolic phospholipase A(2) (cPLA(2)) locus (Pla2g4). We report that Apc(Min/+), cPLA(2)(-/-) mice show an 83% reduction in tumor number in the small intestine compared with littermates with genotypes Apc(Min/+), cPLA(2)(+/-) and Apc(Min/+), cPLA(2)(+/+). This tumor phenotype parallels that of COX-2 knockout mice, suggesting that cPLA(2) is the predominant source of AA for COX-2 in the intestine. The protective effect of cPLA(2) deletion is thus most likely attributed to a decrease in the AA supply to COX-2 and a resultant decrease in PG synthesis. The tumorigenic effect of sPLA(2) mutations is likely to be through a completely different pathway.

Roles of secretory phospholipases A(2) in inflammatory diseases and trauma

Six distinct secretory small molecular weight phospholipases A(2) (PLA(2)) have been cloned and characterized from human tissues. Two of them, pancreatic group IB PLA(2) (PLA(2)-IB) and synovial-type group IIA PLA(2) (PLA(2)-IIA) have been studied as to their association to various inflammatory diseases. PLA(2)-IB is a digestive enzyme synthesized by pancreatic acinar cells. In acute pancreatitis, which is characterized by destruction of pancreatic tissue, PLA(2)-IB is released into the circulation, but its role in pancreatic and other tissue damage is still hypothetical. The concentration of PLA(2)-IIA increases in blood plasma in generalized inflammatory response resulting from infections, chronic inflammatory diseases, acute pancreatitis, trauma and surgical operations. PLA(2)-IIA is synthesized in a number of gland cells and is present in cellular secretions on mucosal surfaces including Paneth cells of intestinal mucosa, prostatic gland cells and seminal plasma, and lacrimal glands and tears. PLA(2)-IIA is expressed in hepatoma-derived cells in vitro and hepatocytes in vivo. PLA(2)-IIA is regarded as an acute phase protein and seems to function as an antibacterial agent especially effective against Gram-positive bacteria. Other putative functions in the inflammatory reaction include hydrolysis of cell membrane phospholipids and release of arachidonic acid for prostanoid synthesis.

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.

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.

Sera of patients suffering from inflammatory diseases contain group IIA but not group V phospholipase A(2)

During recent years, the high phospholipase A(2) (PLA(2)) concentrations at sites of inflammation and in circulation in several life-threatening diseases, such as sepsis, multi-organ dysfunction and acute respiratory distress syndrome, has generally been ascribed to the non-pancreatic group IIA PLA(2). Recently the family of secreted low molecular mass PLA(2) enzymes has rapidly expanded. In some cases, a newly described enzyme appeared to be cross-reactive with antibodies against the group IIA enzyme. For this reason, reports describing the expression of group IIA PLA(2) during inflammatory conditions need to be reevaluated. Here we describe the identification of the PLA(2) activity in sera of acute chest syndrome patients and in sera of trauma victims. In both cases, the PLA(2) activity was identified as group IIA. This classification was based upon cross-reactivity with monoclonal antibodies against group IIA PLA(2) which do not recognize the recombinant human group V enzyme. Moreover, purification of the enzymatic activity from the two sera followed by N-terminal amino acid sequence analyses revealed only the presence of group IIA enzyme.

Redundant and segregated functions of granule-associated heparin-binding group II subfamily of secretory phospholipases A2 in the regulation of degranulation and prostaglandin D2 synthesis in mast cells

We herein demonstrate that mast cells express all known members of the group II subfamily of secretory phospholipase A2 (sPLA2) isozymes, and those having heparin affinity markedly enhance the exocytotic response. Rat mastocytoma RBL-2H3 cells transfected with heparin-binding (sPLA2-IIA, -V, and -IID), but not heparin-nonbinding (sPLA2-IIC), enzymes released more granule-associated markers (beta-hexosaminidase and histamine) than mock- or cytosolic PLA2alpha (cPLA2alpha)-transfected cells after stimulation with IgE and Ag. Site-directed mutagenesis of sPLA2-IIA and -V revealed that both the catalytic and heparin-binding domains are essential for this function. Confocal laser and electron microscopic analyses revealed that sPLA2-IIA, which was stored in secretory granules in unstimulated cells, accumulated on the membranous sites where fusion between the plasma membrane and granule membranes occurred in activated cells. These results suggest that the heparin-binding sPLA2s bind to the perigranular membranes through their heparin-binding domain, and lysophospholipids produced in situ by their enzymatic action may facilitate the ongoing membrane fusion. In contrast to the redundant role of sPLA2-IIA, -IID, and -V in the regulation of degranulation, only sPLA2-V had the ability to markedly augment IgE/Ag-stimulated immediate PGD2 production, which reached a level comparable to that elicited by cPLA2alpha. The latter observation reveals an unexplored functional segregation among the three related isozymes expressed in the same cell population.

Regulation of the expression of group IIA and group V secretory phospholipases A(2) in rat mesangial cells

Rat mesangial cells synthesize and secrete a secretory phospholipase A(2) upon stimulation of the cells with cytokines, like IL-1beta and TNF and with cAMP elevating agents like forskolin. This enzyme was previously characterized to belong to group IIA sPLA(2). The discovery of several other low molecular weight phospholipases, like group IIC in murine testis and group V in human and rat heart, prompted investigations on the presence of group IIC and group V sPLA(2) in rat mesangial cells. This was done by isolating the RNA from stimulated cells and performing RT-PCR, using primers specific for group IIC and V sPLA(2). The results indicate that rat mesangial cells upon stimulation express next to group IIA also group V sPLA(2). No indications were obtained for the expression of group IIC sPLA(2). The regulation of the expression of group V sPLA(2) at the mRNA level was further investigated by examining the time-dependent expression, the influence of dexamethasone and the signaling route of the IL-1beta stimulation. The results show that the IL-1beta induced expression of group V sPLA(2) mRNA was time dependent and, similar to that of group IIA sPLA(2) mRNA, involves activation of NF-kappaB. However, in contrast to the group IIA sPLA(2), the expression of group V sPLA(2) was not influenced by the presence of dexamethasone. The expression of both phospholipases was also examined at the protein level in stimulated mesangial cells. Western blot analysis shows that stimulated mesangial cells synthesize both group IIA and group V sPLA(2) protein but the expression of group V is lower compared to that of group IIA sPLA(2). In addition, the extent of secretion into the medium appears to be considerably higher for group IIA than for group V sPLA(2).

Role of type IIA secretory phospholipase A2 in arachidonic acid metabolism

Recent recognition of the rapidly growing sPLA2 family has led to a suggestion that some of the previously described functions of sPLA2-IIA need to be reevaluated, since studies based upon enzyme activities and using inhibitors or antibodies against sPLA2-IIA may not discriminate these sPLA2s. Our present studies reconfirm the involvement of sPLA2-IIA in biological responses, demonstrated significant crosstalk between the two Ca(2+)-dependent PLA2s (cPLA2 and sPLA2) where one enzyme is required for the induction of the other, and revealed segregated coupling of discrete PLA2 and COX enzymes in the different phases of PG biosynthesis. Based upon the analysis of cells derived from sPLA2-IIA "natural knock-out" mice, it is apparent that sPLA2-IIA is not essential for the initiation of delayed PGE2 biosynthesis. However, it is capable of contributing to the delayed response as an enhancer when appropriately induced by proinflammatory stimuli, leading to optimal COX-2-dependent PGE2 generation. Importantly, in order for sPLA2-IIA (or related sPLA2 isozymes) to attack the biological membranes, so-called "membrane rearrangement" should take place in activated, but not resting, cells. Membrane rearrangement also occurs when cells are undergoing apoptosis, during which acidic phospholipids, the preferred substrates for sPLA2-IIA, are exposed on the outer leaflet of the plasma membranes. Nonetheless, in view of the dramatically elevated levels of sPLA2-IIA in inflamed or ischemic sites, it is likely that this extracellular isozyme participates in the expansion of chronic tissue disorders by augmenting generation of proinflammatory eicosanoids or lysophospholipids, depending upon the states of the inflammatory response.

A Ca(2+)-independent activation of a type IV cytosolic phospholipase A(2) underlies the receptor stimulation of arachidonic acid-dependent noncapacitative calcium entry

The oscillatory [Ca(2+)](i) signals typically seen following physiologically relevant stimulation of phospholipase C-linked receptors are associated with a receptor-activated entry of Ca(2+), which plays a critical role in driving the oscillations and influencing their frequency. We have recently shown that this receptor-activated entry of Ca(2+) does not conform to the widely accepted "capacitative" model and, instead, reflects the activity of a distinct, novel Ca(2+) entry pathway regulated by arachidonic acid (Shuttleworth, T. J., and Thompson, J. L. (1998) J. Biol. Chem. 273, 32636-32643). We now show that the generation of arachidonic acid under these conditions results from the activity of a type IV cytosolic phospholipase A(2) (cPLA(2)). Although cPLA(2) activation commonly involves a Ca(2+)-dependent translocation to the membrane, at these low agonist concentrations cPLA(2) activation was independent of increases in [Ca(2+)](i), and no detectable translocation to the membrane occurs. Nevertheless, stimulation of cPLA(2) activity was confined to the membrane fraction, where an increase in phosphorylation of the enzyme was observed. We suggest that, at the low agonist concentrations associated with oscillatory [Ca(2+)](i) signals, cPLA(2) activation involves an increased phosphorylation of a discrete pool of the total cellular cPLA(2) that is already localized within the membrane fraction at resting [Ca(2+)](i).

Diphenyleneiodium chloride blocks inflammatory cytokine-induced up-regulation of group IIA phospholipase A(2) in rat mesangial cells

Inflammatory cytokines, interleukin 1beta and tumor necrosis factor-alpha, potently stimulate rat mesangial cells to express and secrete group IIA phospholipase A(2) (PLA(2)). Cytokine-induced up-regulation of PLA(2) has been blocked by inhibitors (antioxidants) of the transcription factor, nuclear factor-kappaB (NF-kappaB), suggesting a role for NF-kappaB in the regulation of group IIA PLA(2) expression. Reactive oxygen species such as H(2)O(2), which are elevated in mesangial cells after cytokine activation, can mimic cytokine-induced NF-kappaB activation. However, the source of reactive oxygen species generation in mesangial cells, produced by cytokine stimulation, has yet to be clarified. Recently, tumor necrosis factor-alpha has been demonstrated to increase superoxide radical generation in mesangial cells. Therefore, we hypothesized that a selective NADPH oxidase inhibitor, diphenyleneiodium chloride (DPI), could block cytokine-induced group IIA PLA(2) up-regulation by attenuating NF-kappaB binding. To test this hypothesis, we isolated rat mesangial cells and characterized them by ultrastructural and immunochemical methods. This homogeneous mesangial cell population was responsive to cytokine as evidenced by an increase in steady-state levels of group IIA PLA(2) mRNA and extracellular enzymatic activity over time. DPI (0.02-20 microM), added 90 min before cytokine activation, inhibited both group IIA PLA(2) mRNA and enzymatic activity in a concentration-dependent manner. By electrophoretic mobility shift analysis, cytokine activation also increased specific NF-kappaB binding to one of two NF-kappaB consensus elements in the rat group IIA PLA(2) promoter and also was suppressed by DPI pretreatment. Antibodies to NF-kappaB p65 (Rel A) and p50 (but not normal rabbit IgG) supershifted this retardation signal and verified the type of NF-kappaB species as the classical p50/p65 heterodimer.