BBA - Molecular and Cell Biology of Lipids (v.1761, #11)

Special issue on phospholipase A2 by Santosh Nigam; Takao Shimizu; David C. Wilton (1245).

The phospholipase A2 superfamily and its group numbering system by Ralph H. Schaloske; Edward A. Dennis (1246-1259).
The superfamily of phospholipase A2 (PLA2) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA2s (sPLA2), the cytosolic PLA2s (cPLA2), the Ca2+ independent PLA2s (iPLA2), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA2s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA2 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 PLA2s have been discovered and are now included. Additionally, tools for the investigation of PLA2s and approaches for distinguishing between the different Groups are described.
Keywords: Phospholipase A2; Arachidonic acid; Platelet activating factor acetylhydrolase; Prostaglandin; Lipid mediator; Eicosanoid;

The interfacial binding surface of phospholipase A2s by Jason M. Winget; Ying H. Pan; Brian J. Bahnson (1260-1269).
For membrane-associated enzymes, which access substrate from either a monolayer or bilayer of the aggregate substrate, the partitioning from the aqueous phase to this phospholipid interface is critical for catalysis. Despite a large and expanding body of knowledge regarding interfacial enzymes, the biophysical steps involved in interfacial recognition and adsorption remain relatively poorly understood. The surface of the enzyme that contacts the phospholipid surface is referred to as its interfacial binding surface, or more simply, its i-face. The interaction of a protein's i-face with the aggregate substrate may simply control access to substrate. However, it can be more complex, and this interaction often serves to allosterically activate the enzyme on this surface. First we briefly review what is currently known about i-face structure and function for a prototypical interfacial enzyme, the secreted Phospholipase A2 (PLA2). Then we develop, characterize, compare, and discuss models of the PLA2 i-face across a subset of five homologous PLA2 family members, groups IA, IB, IIA, V, and X. A homology model of human group-V is included in this comparison, suggesting that a similar approach could be used to explore interfacial function of any of the PLA2 family members. Despite moderate sequence identity, structural homology and sequence similarity are well conserved. We find that the residues predicted to be interfacial, while conserved structurally, are not highly conserved in sequence. Implications for this divergence on interfacial selectivity are discussed.
Keywords: Interfacial enzyme; i-face; Membrane associated protein; Homology model; Phospholipase A2; PLA2; Calcium esterase;

A new hat for an old enzyme: Waste management by Taylor J. Brueseke; John D. Bell (1270-1279).
The history of research regarding secretory phospholipase A2 (sPLA2) has often focused in one of two directions. Originally, the enzyme was studied biophysically in terms of its fundamental structure, enzymology, and the relationship between membrane physics and catalytic activity. More recently, a large and growing body of information has accumulated concerning regulatory factors, tissue distribution, and physiological/pathological roles of sPLA2. Evidence is presented that suggests an additional function for the protein in which it helps to clear dead and damaged cells while avoiding digestion of those that are healthy. Apparently, the ability of the enzyme to discriminate between susceptible and resistant cells depends on physical properties of membrane lipids related to order, distribution, and neighbor/neighbor interactions. Investigations into this action of the enzyme offer the rare opportunity to apply biophysical approaches and principles to a physiological setting.
Keywords: Enzyme kinetics; Fluorescence; Two-photon microscopy; Laurdan; Binding; Apoptosis;

Group V sPLA2: Classical and novel functions by Barbara Balestrieri; Jonathan P. Arm (1280-1288).
Group V sPLA2 is unique among the family of secretory sPLA2 enzymes in being able to bind to cell membranes through both interfacial-binding and through binding to proteoglycan. The function of group V sPLA2 as an enzyme and its cross-talk with cPLA2α in initiating eicosanoid generation is well documented. Evidence, though, is emerging on the ability of this molecule to act as a regulator of several intracellular and extracellular pathways independently of its ability to provide arachidonic acid for eicosanoid generation, acting within the cell or as a secreted enzyme. In this article we will provide an overview of the properties of the enzyme and how they relate to our current understanding of its function.
Keywords: Group V sPLA2; Gene; Structure; Eicosanoid; Arachidonic acid; Macrophage; Mast cell; Neutrophil; Eosinophil; Foam cell; Anti--bacterial property;

Activation of human inflammatory cells by secreted phospholipases A2 by Massimo Triggiani; Francescopaolo Granata; Annunziata Frattini; Gianni Marone (1289-1300).
Secreted phospholipases A2 (sPLA2s) are enzymes detected in serum and biological fluids of patients with various inflammatory, autoimmune and allergic disorders. Different isoforms of sPLA2s are expressed and released by human inflammatory cells, such as neutrophils, eosinophils, T cells, monocytes, macrophages and mast cells. sPLA2s generate arachidonic acid and lysophospholipids thus contributing to the production of bioactive lipid mediators in inflammatory cells. However, sPLA2s also activate human inflammatory cells by mechanisms unrelated to their enzymatic activity. Several human and non-human sPLA2s induce degranulation of mast cells, neutrophils and eosinophils and activate exocytosis in macrophages. In addition some, but not all, sPLA2 isoforms promote cytokine and chemokine production from macrophages, neutrophils, eosinophils, monocytes and endothelial cells. These effects are primarily mediated by binding of sPLA2s to specific membrane targets (heparan sulfate proteoglycans, M-type, N-type or mannose receptors) expressed on effector cells. Thus, sPLA2s may play an important role in the initiation and amplification of inflammatory reactions by at least two mechanisms: production of lipid mediators and direct activation of inflammatory cells. Selective inhibitors of sPLA2-enzymatic activity and specific antagonists of sPLA2 receptors are current being tested for pharmacological treatment of inflammatory and autoimmune diseases.
Keywords: Secreted PLA2; Inflammation; Cytokine; Chemokine; Signal transduction;

Distinctiveness of secretory phospholipase A2 group IIA and V suggesting unique roles in atherosclerosis by Birgitta Rosengren; Ann-Cathrine Jönsson-Rylander; Helena Peilot; German Camejo; Eva Hurt-Camejo (1301-1308).
Clinical observations strongly support an association of circulating levels of secretory phospholipases A2 (sPLA2) in atherosclerotic cardiovascular disease (ACVD). Two modes of action can provide causal support for these statistical correlations. One is the action of the enzymes on circulating lipoproteins and the other is direct action on the lipoproteins once in the arterial extracellular intima. In this review we discuss results suggesting a distinct profile of characteristics related to localization, action on plasma lipoproteins and interaction with arterial proteoglycans for sPLA2-IIA and sPLA2-V. The differences observed indicate that these enzymes may contribute to atherosclerosis through dissimilar pathways. Furthermore, we comment on recent animal studies from our laboratory indicating that the expression of type V enzyme is up-regulated by genetically and nutritionally-induced dyslipidemias but not the group type IIA enzyme, which is well known to be up-regulated by acute inflammation. The results suggest that if similar up-regulation occurs in humans in response to hyperlipidemia, it may create a distinctive link between the group V enzyme and the disease.

Atherosclerosis is a progressive inflammatory disease that takes place in the intima of the arterial wall. It is characterized by activation of endothelial cells, proliferation of smooth muscle cells and macrophages, accumulation of lipoproteins, deposition of extracellular matrix components and enhanced lipolytic enzyme activity. Phospholipase A2 (PLA2) has been postulated to play an important role in the inflammatory process of atherosclerosis, but its molecular mechanism is uncertain. The secretory PLA2 is expressed at increased levels in an atherosclerotic plaque and may hydrolyze low-density lipoproteins (LDL). This action promotes the production of pro-inflammatory lipids such as lysophospholipids, unsaturated fatty acids and eicosanoids. The current review highlights recent findings on how LDL-derived lipid mediators, generated by sPLA_2 modification of LDL, regulate pro-inflammatory activation and intracellular signaling in macrophages. Moreover, the review discusses how PLA_2 enzymes regulate signalling that promotes collagen accumulation and fibrotic plaque development. PLA_2 could therefore function as a connector between inflammation and fibrosis, the latter being an endpoint of chronic inflammation.
Keywords: PLA2; LDL; ECM; Inflammation; Atherosclerosis; Lysophosphatidylcholine;

Biochemical properties and pathophysiological roles of cytosolic phospholipase A2s by Yoshihiro Kita; Takayo Ohto; Naonori Uozumi; Takao Shimizu (1317-1322).
Phospholipase A2 (PLA2) (EC catalyzes hydrolysis of the sn-2 ester bond of glycerophospholipids. The enzyme is essential for the production of two classes of lipid mediators, fatty acid metabolites and lysophospholipid-related lipids, as well as being involved in the remodeling of membrane phospholipids. Among many mammalian PLA2s, cytosolic PLA2α (cPLA2α) plays a critical role in various physiological and pathophysiological conditions through generating lipid mediators. Here, we summarize the in vivo significance of cPLA2α, revealed from the phenotypes of cPLA2α-null mice, and properties of newly discovered cPLA2 family enzymes. We also briefly introduce a quantitative lipidomics strategy using liquid chromatography-mass spectrometry, a powerful tool for the comprehensive analysis of lipid mediators.
Keywords: Phospholipase A2; cPLA2α; cPLA2β; cPLA2γ; cPLA2δ; cPLA2ε; cPLA2ζ; Lipid mediator; Lipidomics;

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2α was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2α also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2α-deficient PLB cell model. The novel dual subcellular localization of cPLA2α in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2α in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.
Keywords: cPLA2; Neutrophil; Monocyte; PLB cell; Eicosanoid; NADPH oxidase;

Roles of cPLA2α and arachidonic acid in cancer by Masako Nakanishi; Daniel W. Rosenberg (1335-1343).
Phospholipase A2s (PLA2s) are key enzymes that catalyze the hydrolysis of membrane phospholipids to release bioactive lipids that play an important role in normal cellular homeostasis. Under certain circumstances, disrupted production of key lipid mediators may adversely impact physiological processes, leading to pathological conditions such as inflammation and cancer. In particular, cytosolic PLA2α (cPLA2α) has a high selectivity for liberating arachidonic acid (AA) that is subsequently metabolized by a panel of downstream enzymes for eicosanoid production. Although concentrations of free AA are maintained at low levels in resting cells, alterations in AA production, often resulting from dysregulation of cPLA2α activity, are observed in transformed cells. In this review, we summarize recent evidence that cPLA2α plays a role in the pathogenesis of many human cancers. Much of this evidence has been accumulated from functional studies using cPLA2α-deficient mice, as well as mechanistic studies in cell culture. We also discuss the potential contribution of cPLA2α and AA to apoptosis, and the regulatory mechanisms leading to aberrant expression of cPLA2α.
Keywords: Phospholipase A2; Arachidonic acid; Colon cancer; Apoptosis; Eicosanoid; Sphingomyelinase;

Calcium-independent phospholipase A2 and apoptosis by Jesús Balsinde; Rebeca Pérez; María A. Balboa (1344-1350).
Apoptosis or programmed cell death is associated with changes in glycerophospholipid metabolism. Cells undergoing apoptosis generally release free fatty acids including arachidonic acid, which parallels the reduction in cell viability. The involvement of cytosolic group IVA phospholipase A2α (cPLA2α) in apoptosis has been the subject of numerous studies but a clear picture of the role(s) played by this enzyme is yet to emerge. More recently, the importance of lipid products generated by the action of a second phospholipase A2, the group VIA calcium-independent phospholipase A2 (iPLA2-VIA) in apoptosis has begun to be unveiled. Current evidence suggests that iPLA2-VIA-derived lysophosphatidylcholine may play a prominent role in mediating the chemoattractant and recognition/engulfment signals that accompany the process of apoptotic cell death, and gives possibility to the efficient clearance of dying cells by circulating phagocytes. Other lines of evidence suggest that perturbations in the control of free arachidonic acid levels within the cells, a process that may implicate iPLA2-VIA as well, may provide important cellular signals for the onset of apoptosis.
Keywords: Phospholipase A2; Apoptosis; Arachidonic Acid; Calcium-independent; Free fatty acid; Eicosanoid;

Plasma PAF-acetylhydrolase: An unfulfilled promise? by Sonia-Athina Karabina; Ewa Ninio (1351-1358).
Plasma Platelet-activating-Factor (PAF)-acetylhydrolase (PAF-AH also named lipoprotein-PLA2 or PLA2G7 gene) is secreted by macrophages, it degrades PAF and oxidation products of phosphatidylcholine produced upon LDL oxidation and/or oxidative stress, and thus is considered as a potentially anti-inflammatory enzyme. Cloning of PAF-AH has sustained tremendous promises towards the use of PAF-AH recombinant protein in clinical situations. The reason for that stems from the numerous animal models of inflammation, atherosclerosis or sepsis, where raising the levels of circulating PAF-AH either through recombinant protein infusion or through the adenoviral gene transfer showed to be beneficial. Unfortunately, neither in human asthma nor in sepsis the recombinant PAF-AH showed sufficient efficacy. One of the most challenging questions nowadays is as to whether PAF-AH is pro- or anti-atherogenic in humans, as PAF-AH may possess a dual pro- and anti-inflammatory role, depending on the concentration and the availability of potential substrates. It is equally possible that the plasma level of PAF-AH is a diagnostic marker of ongoing atherosclerosis.
Keywords: PAF-AH; Lp-PLA2; LDL-PLA2; PLA2; Oxidized phospholipid; Inflammation; Atherogenesis;

Plasma platelet-activating factor (PAF)-acetylhydrolase (PAF-AH), which is characterized by tight association with plasma lipoproteins, degrades not only PAF but also phospholipids with oxidatively modified short fatty acyl chain esterified at the sn-2 position. Production and accumulation of these phospholipids are associated with the onset of inflammatory diseases and preventive role of this enzyme has been evidenced by many recent studies including prevalence of the genetic deficiency of the enzyme in the patients and therapeutic effects of treatment with recombinant protein or gene transfer. With respect to the atherosclerosis, however, it is not fully cleared whether this enzyme plays an anti-atherogenic role or pro-atherogenic role because plasma PAF-AH also might produce lysophosphatidylcholine (LysoPC) and oxidatively modified nonesterified fatty acids with potent pro-inflammatory and pro-atherogenic bioactivities. These dual roles of plasma PAF-AH might be regulated by the altered distribution of the enzyme between low density lipoprotein (LDL) and high density lipoprotein (HDL) particles because HDL-associated enzymes are considered to contribute to the protection of LDL from oxidative modification. This review focuses on the recent findings which address the role of this enzyme in the human diseases especially including asthma, septic shock and atherosclerosis.
Keywords: Platelet-activating factor; PAF; Platelet-activating factor-acetylhydrolase; PAF-AH; Atherosclerosis; Asthma; Shock; Inflammation; Lipoprotein; Atherogenesis; Oxidized phospholipid;

Control of phospholipase A2 activities for the treatment of inflammatory conditions by Saul Yedgar; Yuval Cohen; David Shoseyov (1373-1382).
Phospholipase-A2 (PLA2) enzymes hydrolyze cell membrane phospholipids to produce arachidonic acid (AA) and lyso-phospholipids (LysoPL), playing a key role in the production of inflammatory lipid mediators, mainly eicosanoids. They are therefore considered pro-inflammatory enzymes and their inhibition has long been recognized as a desirable therapeutic target. However, attempts to develop suitable PLA2 inhibitors for the treatment of inflammatory diseases have yet to succeed. This is due to their functional and structural diversity, and their homeostatic and even anti-inflammatory roles in certain circumstances. In the present review we outline the diversity and functions of PLA2 isoforms, and their interplay in the induction and inhibition of inflammatory processes, with emphasis on discussing approaches for therapeutic manipulation of PLA2 activities.
Keywords: Phospholipase A2; Arachidonic acid; Lyso-phospholipid;

The non-venom insect phospholipases A2 by David Stanley (1383-1390).
Phospholipases A2 (PLA2s) are responsible for releasing the fatty acid moiety from the sn-2 position of phospholipids. These enzymes are virtually ubiquitous proteins known from all major biological taxa. Various PLA2s act in a wide array of biological processes, including digestion of dietary lipids, cellular homeostasis, intra- and intercellular signaling, host defense and at least a few ecological interactions. PLA2 activities have been recorded in a small number of insect species, which can be taken to represent the broad group, Insecta. Within insects, PLA2s act in functions expected from the background on these enzymes. So far, we know PLA2s act in lipid digestion, cellular host defense signaling, reproduction and in organismal-level metabolism. Additional PLA2 actions are certain to emerge. This is the first article devoted to assembling the known information on insect PLA2s. I review the scant information available on the biological actions of PLA2s in insects, relate new findings on insect pathogens that disrupt insect immune functions by inhibiting PLA2s and mention the few reports of sequence information on insect PLA2s. Finally, I offer a brief prospectus on future research into insect PLA2s. There are two overarching points in this paper. One, there remains a great deal to learn about insect PLA2s and two, some of the findings on insect PLA2s will have meaningful practical significance.
Keywords: Insect; Phospholipase A2; Cellular immunity; Reproduction; Metabolism;

Phospholipase A2 and Phospholipase B activities in fungi by Gerwald A. Köhler; Audrey Brenot; Eric Haas-Stapleton; Nina Agabian; Rupal Deva; Santosh Nigam (1391-1399).
As saprophytes or disease causing microorganisms, fungi acquire nutrients from dead organic material or living host organisms. Lipids as structural components of cell membranes and storage compartments play an important role as energy-rich food source. In recent years, it also has become clear that lipids have a wide range of bioactive properties including signal transduction and cell to cell communication. Thus, it is not surprising that fungi possess a broad range of hydrolytic enzymes that attack neutral lipids and phospholipids. Especially during infection of a mammalian host, phospholipase A2 (PLA2) enzymes released by fungi could play important roles not only for nutrient acquisition and tissue invasion, but for intricate modulation of the host's immune response. Sequencing of fungal genomes has revealed a wide range of genes encoding PLA2 activities in fungi. We are just beginning to become aware of the significance these enzymes could have for the fungal cells and their interaction with the host.
Keywords: Phospholipase A2; PLB gene; Cryptcoccus neoformans; Aspergillus fumigatus; Candida albicans;