Essential Ca(2+)-independent role of the group IVA cytosolic phospholipase A(2) C2 domain for interfacial activity

Phosphorylation of cPLA2α at Ser505 Is Necessary for Its Translocation to PtdInsP2-Enriched Membranes

Group IVA cytosolic phospholipase A₂α (cPLA₂α) is a key enzyme in physiology and pathophysiology because it constitutes a rate-limiting step in the pathway for the generation of pro- and anti-inflammatory eicosanoid lipid mediators. cPLA₂α activity is tightly regulated by multiple factors, including the intracellular Ca²⁺ concentration, phosphorylation reactions, and cellular phosphatidylinositol (4,5) bisphosphate levels (PtdInsP₂). In the present work, we demonstrate that phosphorylation of the enzyme at Ser⁵⁰⁵ is an important step for the translocation of the enzyme to PtdInsP₂–enriched membranes in human cells. Constructs of eGFP-cPLA₂ mutated in Ser⁵⁰⁵ to Ala (S505A) exhibit a delayed translocation in response to elevated intracellular Ca²⁺, and also in response to increases in intracellular PtdInsP₂ levels. Conversely, translocation of a phosphorylation mimic mutant (S505E) is fully observed in response to cellular increases in PtdInsP₂ levels. Collectively, these results suggest that phosphorylation of cPLA₂α at Ser⁵⁰⁵ is necessary for the enzyme to translocate to internal membranes and mobilize arachidonic acid for eicosanoid synthesis.

Platelet-Activating Factor Promotes the Development of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a multifaceted clinicopathological syndrome characterised by excessive hepatic lipid accumulation that causes steatosis, excluding alcoholic factors. Platelet-activating factor (PAF), a biologically active lipid transmitter, induces platelet activation upon binding to the PAF receptor. Recent studies have found that PAF is associated with gamma-glutamyl transferase, which is an indicator of liver disease. Moreover, PAF can stimulate hepatic lipid synthesis and cause hypertriglyceridaemia. Furthermore, the knockdown of the PAF receptor gene in the animal models of NAFLD helped reduce the inflammatory response, improve glucose homeostasis and delay the development of NAFLD. These findings suggest that PAF is associated with NAFLD development. According to reports, patients with NAFLD or animal models have marked platelet activation abnormalities, mainly manifested as enhanced platelet adhesion and aggregation and altered blood rheology. Pharmacological interventions were accompanied by remission of abnormal platelet activation and significant improvement in liver function and lipids in the animal model of NAFLD. These confirm that platelet activation may accompany a critical importance in NAFLD development and progression. However, how PAFs are involved in the NAFLD signalling pathway needs further investigation. In this paper, we review the relevant literature in recent years and discuss the role played by PAF in NAFLD development. It is important to elucidate the pathogenesis of NAFLD and to find effective interventions for treatment.

Deletion mutant of sPLA2 using CRISPR/Cas9 exhibits immunosuppression, developmental retardation, and failure of oocyte development in legume pod borer, Maruca vitrata

Phospholipase A₂ (PLA₂) catalyzes release of free fatty acids linked to phospholipids at sn-2 position. Some of these released free fatty acids are used to synthesize eicosanoids that mediate various physiological processes in insects. Although a large number of PLA₂s form a superfamily consisting of at least 16 groups, few PLA₂s have been identified and characterized in insects. Furthermore, physiological functions of insect PLA₂s remain unclear. Clustered regularly interspaced short parlindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) has been a useful research tool to validate gene function. This study identified and characterized a secretory PLA₂ (sPLA₂) from legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), and validated its physiological functions using CRISPR/Cas9. An open reading frame of M. vitrata sPLA₂ (Mv-sPLA₂) encoding 192 amino acids contained signal peptide, calcium-binding domain, and catalytic site. Phylogenetic analysis indicated that Mv-sPLA₂ was related to other Group III sPLA₂s. Mv-sPLA₂ was expressed in both larval and adult stages. It was inducible by immune challenge. RNA interference (RNAi) of Mv-sPLA₂ significantly suppressed cellular immunity and impaired larval development. Furthermore, RNAi treatment in female adults prevented oocyte development. These physiological alterations were also observed in a mutant line of M. vitrata with Mv-sPLA₂ deleted by using CRISPR/Cas9. Mv-sPLA₂ was not detected in the mutant line from western blot analysis. Addition of an eicosanoid, PGE₂, significantly rescued oocyte development of females of the mutant line. These results suggest that Mv-sPLA₂ plays crucial role in immune, developmental, and reproductive processes of M. vitrata.

Toll/IMD signal pathways mediate cellular immune responses via induction of intracellular PLA 2 expression

Phospholipase A₂ (PLA₂ ) hydrolyzes fatty acids from phospholipids at the sn-2 position. Two intracellular PLA₂ s, iPLA₂ A and iPLA₂ B, have been found in Spodoptera exigua. Both are calcium-independent cellular PLA₂ . Their orthologs have been found in other insects. These two iPLA₂ s are different in ankyrin motif of N terminal region. The objective of this study was to determine whether Toll/immune deficiency (IMD) signal pathways could mediate cellular immune responses via induction of iPLA₂ expression. Both iPLA ₂ s were expressed in all developmental stages of S. exigua, showing the highest expression in the adult stage. During larval stage, hemocyte is the main tissue showing expression of these iPLA₂ s. Both iPLA₂ s exhibited similar expression patterns after immune challenge with different microbial pathogens such as virus, bacteria, and fungi. Promoter component analysis of orthologs encoded in S. frugiperda indicated nuclear factor-κB- and Relish-responsible elements on their promoters, suggesting their expression in S. exigua under Toll/IMD immune signaling pathways. RNA interference (RNAi) of MyD88 or Pelle under Toll pathway suppressed inducible expression levels of both iPLA₂ s in response to Gram-positive bacteria containing Lys-type peptidoglycan or fungal infection. In contrast, RNAi against Relish under IMD pathway suppressed both iPLA₂ s in response to infection with Gram-negative bacteria. Under RNAi conditions, hemocytes significantly lost cellular immune response measured by nodule formation. However, addition of arachidonic acid (a catalytic product of PLA₂ ) rescued such immunosuppression. These results suggest that Toll/IMD signal pathways can mediate cellular immune responses via eicosanoid signaling by inducing iPLA₂ expression.

Structural basis of phosphatidylcholine recognition by the C2-domain of cytosolic phospholipase A2α

Ca2+-stimulated translocation of cytosolic phospholipase A2α (cPLA2α) to the Golgi induces arachidonic acid production, the rate-limiting step in pro-inflammatory eicosanoid synthesis. Structural insights into the cPLA2α preference for phosphatidylcholine (PC)-enriched membranes have remained elusive. Here, we report the structure of the cPLA2α C2-domain (at 2.2 Å resolution), which contains bound 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and Ca2+ ions. Two Ca2+ are complexed at previously reported locations in the lipid-free C2-domain. One of these Ca2+ions, along with a third Ca2+, bridges the C2-domain to the DHPC phosphate group, which also interacts with Asn65. Tyr96 plays a key role in lipid headgroup recognition via cation-π interaction with the PC trimethylammonium group. Mutagenesis analyses confirm that Tyr96 and Asn65 function in PC binding selectivity by the C2-domain and in the regulation of cPLA2α activity. The DHPC-binding mode of the cPLA2α C2-domain, which differs from phosphatidylserine or phosphatidylinositol 4,5-bisphosphate binding by other C2-domains, expands and deepens knowledge of the lipid-binding mechanisms mediated by C2-domains.

A non-venomous sPLA2 of a lepidopteran insect: Its physiological functions in development and immunity

Eicosanoids are oxygenated C20 polyunsaturated fatty acids that mediate various physiological processes in insects. Eicosanoid biosynthesis begins with a C20 precursor, arachidonic acid (5,8,11,14-eicosatetraenoic acid: AA). AA is usually released from phospholipids at sn-2 position by catalytic activity of phospholipase A₂ (PLA₂). Although various PLA₂s classified into 16 gene families (= Groups) are known in various biological systems, few PLA₂s are known in insects. Only two PLA₂s involved in intracellular calcium independent PLA₂ (iPLA₂) group have been identified in lepidopteran insects with well known eicosanoid physiology. This study reports the first secretory PLA₂ (sPLA₂) in lepidopteran insects. A partial open reading frame (ORF) of PLA₂ was obtained by interrogating Spodoptera exigua transcriptome. Subsequent 3'-RACE resulted in a full ORF (Se-sPLA2A) encoding 194 amino acid sequence containing signal peptide, calcium-binding domain, and catalytic site. Phylogenetic analysis indicated that Se-sPLA₂A was clustered with other Group III sPLA₂s. Se-sPLA₂A was expressed in most larval instars except late last instar. Its expression was inducible by immune challenge and juvenile hormone analog injection. RNA interference of Se-sPLA₂A significantly suppressed cellular immunity and impaired larval development. These results suggest that non-venomous sPLA₂ plays a crucial role in immune and developmental processes in S. exigua, a lepidopteran insect.

The structure of iPLA2β reveals dimeric active sites and suggests mechanisms of regulation and localization

Calcium-independent phospholipase A₂β (iPLA₂β) regulates important physiological processes including inflammation, calcium homeostasis and apoptosis. It is genetically linked to neurodegenerative disorders including Parkinson’s disease. Despite its known enzymatic activity, the mechanisms underlying iPLA₂β-induced pathologic phenotypes remain poorly understood. Here, we present a crystal structure of iPLA₂β that significantly revises existing mechanistic models. The catalytic domains form a tight dimer. They are surrounded by ankyrin repeat domains that adopt an outwardly flared orientation, poised to interact with membrane proteins. The closely integrated active sites are positioned for cooperative activation and internal transacylation. The structure and additional solution studies suggest that both catalytic domains can be bound and allosterically inhibited by a single calmodulin. These features suggest mechanisms of iPLA₂β cellular localization and activity regulation, providing a basis for inhibitor development. Furthermore, the structure provides a framework to investigate the role of neurodegenerative mutations and the function of iPLA₂β in the brain.

Calcium-independent phospholipase A₂β (iPLA₂β) is involved in many physiological and pathological processes but the underlying mechanisms are largely unknown. Here, the authors present the structure of dimeric iPLA₂β, providing insights into the regulation of its activity and cellular localization.

Lipidomics in translational research and the clinical significance of lipid-based biomarkers

Lipidomics is a rapidly developing field of study that focuses on the identification and quantitation of various lipid species in the lipidome. Lipidomics has now emerged in the forefront of scientific research due to the importance of lipids in metabolism, cancer, and disease. Using both targeted and untargeted mass spectrometry as a tool for analysis, progress in the field has rapidly progressed in the last decade. Having the ability to assess these small molecules in vivo has led to better understanding of several lipid-driven mechanisms and the identification of lipid-based biomarkers in neurodegenerative disease, cancer, sepsis, wound healing, and pre-eclampsia. Biomarker identification and mechanistic understanding of specific lipid pathways linked to a disease's pathologies can form the foundation in the development of novel therapeutics in hopes of curing human disease.

Lactosylceramide-Induced Phosphorylation Signaling to Group IVA Phospholipase A2 via Reactive Oxygen Species in Tumor Necrosis Factor-α-Treated Cells

The activity of α-type cytosolic phospholipase A₂ (cPLA₂ α, group IVA PLA₂ ), which releases arachidonic acid (AA), is mainly regulated by the Ca²⁺ -induced intracellular translocation/attachment of the enzyme to substrate membranes and its phosphorylation. We previously reported that tumor necrosis factor-α (TNFα) stimulated the formation of lactosylceramide (LacCer) in L929 fibroblast cells, and this lipid directly bound with and activated cPLA₂ α [Nakamura et al. [2013] J. Biol. Chem. 288:23264-23272]. We herein investigated the role of phosphorylation signaling in the TNFα/LacCer-induced activation of cPLA₂ α in cells. TNFα-treated L929 cells released AA via the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and cPLA₂ α, while a treatment with LacCer alone released AA in a similar manner. The TNFα-induced responses including release of AA were decreased by the inhibition of LacCer synthesis. The treatment with TNFα and LacCer increased the levels of reactive oxygen species (ROS), and the reduction/scavenging of ROS decreased the phosphorylation cascade and release of AA in TNFα/LacCer-treated L929 cells. In the cell line CHO, the treatment with LacCer stimulated the phosphorylation cascade and release of AA via the formation of ROS. Treatments with the anti-LacCer antibody and 4β-phorbol 12-myristate 13-acetate stimulated the phosphorylation cascade, but did not release AA by itself. When combined with the Ca²⁺ ionophore A23187, treatments with the anti-LacCer antibody and 4β-phorbol 12-myristate 13-acetate released AA. These results, including our previous findings, showed that LacCer alone simultaneously stimulates two processes to activate cPLA₂ α: a phosphorylation signal and attachment of the enzyme to substrate membranes. J. Cell. Biochem. 118: 4370-4382, 2017. © 2017 Wiley Periodicals, Inc.

Cellular Assays for Evaluating Calcium-Dependent Translocation of cPLA2α to Membrane

The group IVA phospholipase A₂, commonly called cytosolic phospholipase A₂α (cPLA₂α), is a widely expressed enzyme that hydrolyzes membrane phospholipid to produce arachidonic acid and lysophospholipids, which are precursors for a number of bioactive lipid mediators. Arachidonic acid is metabolized through the cyclooxygenase and lipoxygenase pathways for production of prostaglandins and leukotrienes that regulate normal physiological processes and contribute to disease pathogenesis. cPLA₂α is composed of an N-terminal C2 domain and a C-terminal catalytic domain that contains the Ser-Asp catalytic dyad. The catalytic domain contains phosphorylation sites and basic residues that regulate the catalytic activity of cPLA₂α. In response to cell stimulation, cPLA₂α is rapidly activated by posttranslational mechanisms including increases in intracellular calcium and phosphorylation by mitogen-activated protein kinases. In resting cells, cPLA₂α is localized in the cytosol but translocates to membrane including the Golgi, endoplasmic reticulum, and the peri-nuclear membrane in response to increases in intracellular calcium. Calcium binds to the C2 domain, which promotes the interaction of cPLA₂α with membrane through hydrophobic interactions. In this chapter, we describe assays used to study the calcium-dependent translocation of cPLA₂α to membrane, a regulatory step necessary for access to phospholipid and release of arachidonic acid.

Liberating Chiral Lipid Mediators, Inflammatory Enzymes, and LIPID MAPS from Biological Grease

In 1970, it was well accepted that the central role of lipids was in energy storage and metabolism, and it was assumed that amphipathic lipids simply served a passive structural role as the backbone of biological membranes. As a result, the scientific community was focused on nucleic acids, proteins, and carbohydrates as information-containing molecules. It took considerable effort until scientists accepted that lipids also "encode" specific and unique biological information and play a central role in cell signaling. Along with this realization came the recognition that the enzymes that act on lipid substrates residing in or on membranes and micelles must also have important signaling roles, spurring curiosity into their potentially unique modes of action differing from those acting on water-soluble substrates. This led to the creation of the concept of "surface dilution kinetics" for describing the mechanism of enzymes acting on lipid substrates, as well as the demonstration that lipid enzymes such as phospholipase A₂ (PLA₂) contain allosteric activator sites for specific phospholipids as well as for membranes. As our understanding of phospholipases advanced, so did the understanding that many of the lipids released by these enzymes are chiral information-containing signaling molecules; for example, PLA₂ regulates the generation of precursors for the biosynthesis of eicosanoids and other bioactive lipid mediators of inflammation and resolution underlying disease progression. The creation of the LIPID MAPS initiative in 2003 and the ensuing development of the lipidomics field have revealed that lipid metabolites are central to human metabolism. Today lipids are recognized as key mediators of health and disease as we enter a new era of biomarkers and personalized medicine. This article is my personal "reflection" on these scientific advances.

Membrane-dependent Activities of Human 15-LOX-2 and Its Murine Counterpart: IMPLICATIONS FOR MURINE MODELS OF ATHEROSCLEROSIS

The enzyme encoded by the ALOX15B gene has been linked to the development of atherosclerotic plaques in humans and in a mouse model of hypercholesterolemia. In vitro, these enzymes, which share 78% sequence identity, generate distinct products from their substrate arachidonic acid: the human enzyme, a 15-S-hydroperoxy product; and the murine enzyme, an 8-S-product. We probed the activities of these enzymes with nanodiscs as membrane mimics to determine whether they can access substrate esterified in a bilayer and characterized their activities at the membrane interface. We observed that both enzymes transform phospholipid-esterified arachidonic acid to a 15-S-product. Moreover, when expressed in transfected HEK cells, both enzymes result in significant increases in the amounts of 15-hydroxyderivatives of eicosanoids detected. In addition, we show that 15-LOX-2 is distributed at the plasma membrane when the HEK293 cells are stimulated by the addition Ca(2+) ionophore and that cellular localization is dependent upon the presence of a putative membrane insertion loop. We also report that sequence differences between the human and mouse enzymes in this loop appear to confer distinct mechanisms of enzyme-membrane interaction for the homologues.

Membrane and inhibitor interactions of intracellular phospholipases A2

Studying phospholipases A2 (PLA2s) is a challenging task since they act on membrane-like aggregated substrates and not on monomeric phospholipids. Multidisciplinary approaches that include hydrogen/deuterium exchange mass spectrometry (DXMS) and computational techniques have been employed with great success in order to address important questions about the mode of interactions of PLA2 enzymes with membranes, phospholipid substrates and inhibitors. Understanding the interactions of PLA2s is crucial since these enzymes are the upstream regulators of the eicosanoid pathway liberating free arachidonic acid (AA) and other polyunsaturated fatty acids (PUFA). The liberation of AA by PLA2 enzymes sets off a cascade of molecular events that involves downstream regulators such as cyclooxygenase (COX) and lipoxygenase (LOX) metabolites leading to inflammation. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) work by inhibiting COX, while Zileuton inhibits LOX and both rely on PLA2 enzymes to provide them with AA. That means PLA2 enzymes can potentially also be targeted to diminish inflammation at an earlier point in the process. In this review we describe extensive efforts reported in the past to define the interactions of PLA2 enzymes with membranes, substrate phospholipids and inhibitors using DXMS, molecular docking, and molecular dynamics (MD) simulations.

Malaria Parasite Proteins and Their Role in Alteration of the Structure and Function of Red Blood Cells

Malaria, caused by Plasmodium spp., continues to be a major threat to human health and a significant cause of socioeconomic hardship in many countries. Almost half of the world's population live in malaria-endemic regions and many of them suffer one or more, often life-threatening episodes of malaria every year, the symptoms of which are attributable to replication of the parasite within red blood cells (RBCs). In the case of Plasmodium falciparum, the species responsible for most malaria-related deaths, parasite replication within RBCs is accompanied by striking alterations to the morphological, biochemical and biophysical properties of the host cell that are essential for the parasites' survival. To achieve this, the parasite establishes a unique and extensive protein export network in the infected RBC, dedicating at least 6% of its genome to the process. Understanding the full gamut of proteins involved in this process and the mechanisms by which P. falciparum alters the structure and function of RBCs is important both for a more complete understanding of the pathogenesis of malaria and for development of new therapeutic strategies to prevent or treat this devastating disease. This review focuses on what is currently known about exported parasite proteins, their interactions with the RBC and their likely pathophysiological consequences.

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

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

Design, synthesis, and biological evaluation of 3-(1-Aryl-1H-indol-5-yl)propanoic acids as new indole-based cytosolic phospholipase A2α inhibitors

This article describes the design, synthesis, and biological evaluation of new indole-based cytosolic phospholipase A2α (cPLA2α, a group IVA phospholipase A2) inhibitors. A screening-hit compound from our library, (E)-3-{4-[(4-chlorophenyl)thio]-3-nitrophenyl}acrylic acid (5), was used to design a class of 3-(1-aryl-1H-indol-5-yl)propanoic acids as new small molecule inhibitors. The resultant structure-activity relationships studied using the isolated enzyme and by cell-based assays revealed that the 1-(p-O-substituted)phenyl, 3-phenylethyl, and 5-propanoic acid groups on the indole core are essential for good inhibitory activity against cPLA2α. Optimization of the p-substituents on the N1 phenyl group led to the discovery of 56n (ASB14780), which was shown to be a potent inhibitor of cPLA2α via enzyme assay, cell-based assay, and guinea pig and human whole-blood assays. It displayed oral efficacy toward mice tetradecanoyl phorbol acetate-induced ear edema and guinea pig ovalbumin-induced asthma models.

Lactosylceramide interacts with and activates cytosolic phospholipase A2α

Lactosylceramide (LacCer) is a member of the glycosphingolipid family and is known to be a bioactive lipid in various cell physiological processes. However, the direct targets of LacCer and cellular events mediated by LacCer are largely unknown. In this study, we examined the effect of LacCer on the release of arachidonic acid (AA) and the activity of cytosolic phospholipase A2α (cPLA2α). In CHO-W11A cells, treatment with 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP), an inhibitor of glucosylceramide synthase, reduced the glycosphingolipid level, and the release of AA induced by A23187 or platelet-activating factor was inhibited. The addition of LacCer reversed the PPMP effect on the stimulus-induced AA release. Exogenous LacCer stimulated the release of AA, which was decreased by treatment with an inhibitor of cPLA2α or silencing of the enzyme. Treatment of CHO-W11A cells with LacCer induced the translocation of full-length cPLA2α and its C2 domain from the cytosol to the Golgi apparatus. LacCer also induced the translocation of the D43N mutant of cPLA2α. Treatment of L929 cells with TNF-α induced LacCer generation and mediated the translocation of cPLA2α and AA release, which was attenuated by treatment with PPMP. In vitro studies were then conducted to test whether LacCer interacts directly with cPLA2α. Phosphatidylcholine vesicles containing LacCer increased cPLA2α activity. LacCer bound to cPLA2α and its C2 domain in a Ca(2+)-independent manner. Thus, we propose that LacCer is a direct activator of cPLA2α.

Assessing phospholipase A2 activity toward cardiolipin by mass spectrometry

Cardiolipin, a major component of mitochondria, is critical for mitochondrial functioning including the regulation of cytochrome c release during apoptosis and proper electron transport. Mitochondrial cardiolipin with its unique bulky amphipathic structure is a potential substrate for phospholipase A₂ (PLA₂) in vivo. We have developed mass spectrometric methodology for analyzing PLA₂ activity toward various cardiolipin forms and demonstrate that cardiolipin is a substrate for sPLA₂, cPLA₂ and iPLA₂, but not for Lp-PLA₂. Our results also show that none of these PLA₂s have significant PLA₁ activities toward dilyso-cardiolipin. To understand the mechanism of cardiolipin hydrolysis by PLA₂, we also quantified the release of monolyso-cardiolipin and dilyso-cardiolipin in the PLA₂ assays. The sPLA₂s caused an accumulation of dilyso-cardiolipin, in contrast to iPLA₂ which caused an accumulation of monolyso-cardiolipin. Moreover, cardiolipin inhibits iPLA₂ and cPLA₂, and activates sPLA₂ at low mol fractions in mixed micelles of Triton X-100 with the substrate 1-palmitoyl-2-arachidonyl-sn-phosphtidylcholine. Thus, cardiolipin functions as both a substrate and a regulator of PLA₂ activity and the ability to assay the various forms of PLA₂ is important in understanding its function.

Structure of the type III secretion effector protein ExoU in complex with its chaperone SpcU

Disease causing bacteria often manipulate host cells in a way that facilitates the infectious process. Many pathogenic gram-negative bacteria accomplish this by using type III secretion systems. In these complex secretion pathways, bacterial chaperones direct effector proteins to a needle-like secretion apparatus, which then delivers the effector protein into the host cell cytosol. The effector protein ExoU and its chaperone SpcU are components of the Pseudomonas aeruginosa type III secretion system. Secretion of ExoU has been associated with more severe infections in both humans and animal models. Here we describe the 1.92 Å X-ray structure of the ExoU–SpcU complex, a full-length type III effector in complex with its full-length cognate chaperone. Our crystallographic data allow a better understanding of the mechanism by which ExoU kills host cells and provides a foundation for future studies aimed at designing inhibitors of this potent toxin.

Roles of acidic phospholipids and nucleotides in regulating membrane binding and activity of a calcium-independent phospholipase A2 isoform

Phospholipase A(2) activity plays key roles in generating lipid second messengers and regulates membrane topology through the generation of asymmetric lysophospholipids. In particular, the Group VIA phospholipase A(2) (GVIA-iPLA(2)) subfamily of enzymes functions independently of calcium within the cytoplasm of cells and has been implicated in numerous cellular processes, including proliferation, apoptosis, and membrane transport steps. However, mechanisms underlying the spatial and temporal regulation of these enzymes have remained mostly unexplored. Here, we examine the subset of Caenorhabditis elegans lipases that harbor a consensus motif common to members of the GVIA-iPLA(2) subfamily. Based on sequence homology, we identify IPLA-1 as the closest C. elegans homolog of human GVIA-iPLA(2) enzymes and use a combination of liposome interaction studies to demonstrate a role for acidic phospholipids in regulating GVIA-iPLA(2) function. Our studies indicate that IPLA-1 binds directly to multiple acidic phospholipids, including phosphatidylserine, phosphatidylglycerol, cardiolipin, phosphatidic acid, and phosphorylated derivatives of phosphatidylinositol. Moreover, the presence of these acidic lipids dramatically elevates the specific activity of IPLA-1 in vitro. We also found that the addition of ATP and ADP promote oligomerization of IPLA-1, which probably underlies the stimulatory effect of nucleotides on its activity. We propose that membrane composition and the presence of nucleotides play key roles in recruiting and modulating GVIA-iPLA(2) activity in cells.

Omega-3 fatty acids cause dramatic changes in TLR4 and purinergic eicosanoid signaling

Dietary fish oil containing ω3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), elicit cardioprotective and anti-inflammatory effects through unresolved mechanisms that may involve competition and inhibition at multiple levels. Here, we report the effects of arachidonic acid (AA), EPA, and DHA supplementation on membrane incorporation, phospholipase A(2) catalyzed release, and eicosanoid production in RAW264.7 macrophages. Using a targeted lipidomics approach, we observed that Toll-like receptor 4 and purinergic receptor activation of supplemented cells leads to the release of 22-carbon fatty acids that potently inhibit cyclooxygenase pathways. This inhibition was able to shunt metabolism of AA to lipoxygenase pathways, augmenting leukotriene and other lipoxygenase mediator synthesis. In resident peritoneal macrophages, docosapentaenoic acid (DPA) was responsible for cyclooxygenase inhibition after EPA supplementation, offering fresh insights into how EPA exerts anti-inflammatory effects indirectly through elongation to 22-carbon DPA.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Functional characterization of mutations in inherited human cPLA₂ deficiency

Group IVA cytosolic phospholipase A(2) (cPLA(2)α) catalyzes the first step in the arachidonic acid cascade leading to the synthesis of important lipid mediators, the prostaglandins and leukotrienes. We previously described a patient deficient in cPLA(2)α activity, which was associated with mutations in both alleles encoding the enzyme. In this paper, we describe the biochemical characterization of each of these mutations. Using saturating concentrations of calcium, we showed that the R485H mutant was nearly devoid of any catalytic activity, that the S111P mutation did not affect the enzyme activity, and that the known K651R polymorphism was associated with activity slightly higher than that of the wild type. Using MDCK cells, we showed that translocation to the Golgi in response to serum activation was impaired for the S111P mutant but not for the other mutants. Using immortalized mouse lung fibroblasts lacking endogenous cPLA(2)α activity, we showed that both mutations S111P and R485H/K651R caused a profound defect in the enzyme catalytic activity in response to cell stimulation with serum. Taken together, our results show that the S111P mutation hampers calcium binding and membrane translocation without affecting the catalytic activity, and that the mutation R485H does not affect membrane translocation but blocks catalytic activity that leads to inactivation of the enzyme. Interestingly, our results show that the common K651R polymorphism confers slightly higher activity to the enzyme, suggesting a role of this residue in favoring a catalytically active conformation of cPLA(2)α. Our results define how the mutations negatively influence cPLA(2)α function and explain the inability of the proband to release arachidonic acid for eicosanoid production.

Protease-activated receptor signaling in platelets activates cytosolic phospholipase A2α differently for cyclooxygenase-1 and 12-lipoxygenase catalysis

OBJECTIVE

The rate-limiting step in the biosynthesis of thromboxane A(2) (TxA(2)) and 12-hydroxyeicosatetraenoic acid (12-HETE) by platelets is activation of cytosolic phospholipase A(2α) (cPLA(2α)), which releases arachidonic acid, which is the substrate for cyclooxygenase-1 (COX-1) and 12-lipoxygenase. We evaluated signaling via the human platelet thrombin receptors, protease-activated receptor (PAR) 1 and PAR4, to the activation of cPLA(2α), which provides a substrate for the biosynthesis of TxA(2) and 12-HETE.

METHODS AND RESULTS

Stimulating washed human platelets resulted in delayed biosynthesis of 12-HETE, which continues after maximal formation of TxA(2) is completed, suggesting that 12-HETE is not formed by the same pool of arachidonic acid that provides a substrate to COX-1. PAR1-induced formation of TxA(2) was inhibited by the phosphatidylinositol kinase inhibitor LY294002, whereas this inhibitor did not block 12-HETE biosynthesis. Both 1-butanol and propranolol also blocked TxA(2) biosynthesis but did not inhibit 12-HETE formation.

CONCLUSIONS

The concerted evidence indicates that the platelet thrombin receptors signal activation of cPLA(2α) coupled to COX-1 by a pathway different from that signaling activation of the cPLA(2α) coupled to 12-lipoxygenase.

Localization and function of cytosolic phospholipase A2alpha at the Golgi

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha, Group IVA phospholipase A(2)) is a central mediator of arachidonate release from cellular phospholipids for the biosynthesis of eicosanoids. cPLA(2)alpha translocates to intracellular membranes including the Golgi in response to a rise in intracellular calcium level. The enzyme's calcium-dependent phospholipid-binding C2 domain provides the targeting specificity for cPLA(2)alpha translocation to the Golgi. However, other features of cPLA(2)alpha regulation are incompletely understood such as the role of phosphorylation of serine residues in the catalytic domain and the function of basic residues in the cPLA(2)alpha C2 and catalytic domains that are proposed to interact with anionic phospholipids in the membrane to which cPLA(2)alpha is targeted. Increasing evidence strongly suggests that cPLA(2)alpha plays a role in regulating Golgi structure, tubule formation and intra-Golgi transport. For example, recent data suggests that cPLA(2)alpha regulates the transport of tight junction and adherens junction proteins through the Golgi to cell-cell contacts in confluent endothelial cells. However, there are now examples where data based on knockdown using siRNA or pharmacological inhibition of enzymatic activity of cPLA(2)alpha affects fundamental cellular processes yet these phenotypes are not observed in cells from cPLA(2)alpha deficient mice. These results suggest that in some cases there may be compensation for the lack of cPLA(2)alpha. Thus, there is continued need for studies employing highly specific cPLA(2)alpha antagonists in addition to genetic deletion of cPLA(2)alpha in mice.

Modulation of the activity of cytosolic phospholipase A2alpha (cPLA2alpha) by cellular sphingolipids and inhibition of cPLA2alpha by sphingomyelin

We examined the effect of the cellular sphingolipid level on the release of arachidonic acid (AA) and activity of cytosolic phospholipase A2alpha (cPLA2alpha) using two Chinese hamster ovary (CHO)-K1-derived mutants deficient in sphingolipid synthesis: LY-B cells defective in the LCB1 subunit of serine palmitoyltransferase for de novo synthesis of sphingolipid species, and LY-A cells defective in the ceramide transfer protein CERT for SM synthesis. When LY-B and LY-A cells were cultured in Nutridoma medium and the sphingolipid level was reduced, the release of AA stimulated by the Ca(2+) ionophore A23187 increased 2-fold and 1.7-fold, respectively, compared with that from control cells. The enhancement in LY-B cells was decreased by adding sphingosine and treatment with the cPLA2alpha inhibitor. When CHO cells were treated with an acid sphingomyelinase inhibitor to increase the cellular SM level, the release of AA induced by A23187 or PAF was decreased. In vitro studies were then conducted to test whether SM interacts directly with cPLA2alpha. Phosphatidylcholine vesicles containing SM reduced cPLA2alpha activity. Furthermore, SM disturbed the binding of cPLA2alpha to glycerophospholipids. These results suggest that SM at the biomembrane plays important roles in regulating the cPLA2alpha-dependent release of AA by inhibiting the binding of cPLA2alpha to glycerophospholipids.

JNK and ceramide kinase govern the biogenesis of lipid droplets through activation of group IVA phospholipase A2

The biogenesis of lipid droplets (LD) induced by serum depends on group IVA phospholipase A(2) (cPLA(2)alpha). This work dissects the pathway leading to cPLA(2)alpha activation and LD biogenesis. Both processes were Ca(2+)-independent, as they took place after pharmacological blockade of Ca(2+) transients elicited by serum or chelation with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester). The single mutation D43N in cPLA(2)alpha, which abrogates its Ca(2+) binding capacity and translocation to membranes, did not affect enzyme activation and formation of LD. In contrast, the mutation S505A did not affect membrane relocation of the enzyme in response to Ca(2+) but prevented its phosphorylation, activation, and the appearance of LD. Expression of specific activators of different mitogen-activated protein kinases showed that phosphorylation of cPLA(2)alpha at Ser-505 is due to JNK. This was confirmed by pharmacological inhibition and expression of a dominant-negative form of the upstream activator MEKK1. LD biogenesis was accompanied by increased synthesis of ceramide 1-phosphate. Overexpression of its synthesizing enzyme ceramide kinase increased phosphorylation of cPLA(2)alpha at Ser-505 and formation of LD, and its down-regulation blocked the phosphorylation of cPLA(2)alpha and LD biogenesis. These results demonstrate that LD biogenesis induced by serum is regulated by JNK and ceramide kinase.

The cationic cluster of group IVA phospholipase A2 (Lys488/Lys541/Lys543/Lys544) is involved in translocation of the enzyme to phagosomes in human macrophages

Group IVA cytosolic phospholipase A(2)alpha (cPLA(2)alpha) plays a role in the microbicidal machinery of immune cells by translocating to phagosomes to initiate the production of antimicrobial eicosanoids. In this work, we have studied the involvement of the cationic cluster of cPLA(2)alpha (Lys(488)/Lys(541)/Lys(543)/Lys(544)) in the translocation of the enzyme to the phagosomal cup in human macrophages responding to opsonized zymosan. Phagocytosis was accompanied by an increased mobilization of free arachidonic acid, which was strongly inhibited by pyrrophenone. In transfected cells, a catalytically active enhanced green fluorescent protein-cPLA(2)alpha translocated to the phagocytic cup, which was corroborated by frustrated phagocytosis experiments using immunoglobulin G-coated plates. However, a cPLA(2)alpha mutant in the polybasic cluster that cannot bind the anionic phospholipid phosphatidylinositol 4, 5-bisphosphate (PIP(2)) did not translocate to the phagocytic cup. Moreover, an enhanced yellow fluorescent protein (EYFP)-cPLA(2)alpha and an enhanced cyan fluorescent protein-pleckstrin homology (PH) domain of the phospholipase Cdelta1 (PLCdelta(1)) construct that specifically recognizes endogenous PIP(2) in the cells both localized at the same sites on the phagosome. High cellular expression of the PH domain inhibited EYFP-cPLA(2)alpha translocation. On the other hand, group V-secreted phospholipase A(2) and group VIA calcium-independent phospholipase A(2) were also studied, but the results indicated that neither of these translocated to the phagosome. Collectively, these data indicate that the polybasic cluster of cPLA(2)alpha (Lys(488)/Lys(541)/Lys(543)/Lys(544)) regulates the subcellular localization of the enzyme in intact cells under physiologically relevant conditions.

Localizing the membrane binding region of Group VIA Ca2+-independent phospholipase A2 using peptide amide hydrogen/deuterium exchange mass spectrometry

The Group VIA-2 Ca(2+)-independent phospholipase A(2) (GVIA-2 iPLA(2)) is composed of seven consecutive N-terminal ankyrin repeats, a linker region, and a C-terminal phospholipase catalytic domain. No structural information exists for this enzyme, and no information is known about the membrane binding surface. We carried out deuterium exchange experiments with the GVIA-2 iPLA(2) in the presence of both phospholipid substrate and the covalent inhibitor methyl arachidonoyl fluorophosphonate and located regions in the protein that change upon lipid binding. No changes were seen in the presence of only methyl arachidonoyl fluorophosphonate. The region with the greatest change upon lipid binding was region 708-730, which showed a >70% decrease in deuteration levels at numerous time points. No decreases in exchange due to phospholipid binding were seen in the ankyrin repeat domain of the protein. To locate regions with changes in exchange on the enzyme, we constructed a computational homology model based on homologous structures. This model was validated by comparing the deuterium exchange results with the predicted structure. Our model combined with the deuterium exchange results in the presence of lipid substrate have allowed us to propose the first structural model of GVIA-2 iPLA(2) as well as the interfacial lipid binding region.

Role of phosphorylation and basic residues in the catalytic domain of cytosolic phospholipase A2alpha in regulating interfacial kinetics and binding and cellular function

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) is regulated by phosphorylation and calcium-induced translocation to membranes. Immortalized mouse lung fibroblasts lacking endogenous cPLA(2)alpha (IMLF(-/-)) were reconstituted with wild type and cPLA(2)alpha mutants to investigate how calcium, phosphorylation, and the putative phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding site regulate translocation and arachidonic acid (AA) release. Agonists that elicit distinct modes of calcium mobilization were used. Serum induced cPLA(2)alpha translocation to Golgi within seconds that temporally paralleled the initial calcium transient. However, the subsequent influx of extracellular calcium was essential for stable binding of cPLA(2)alpha to Golgi and AA release. In contrast, phorbol 12-myristate 13-acetate induced low amplitude calcium oscillations, slower translocation of cPLA(2)alpha to Golgi, and much less AA release, which were blocked by chelating extracellular calcium. AA release from IMLF(-/-) expressing phosphorylation site (S505A) and PIP(2) binding site (K488N/K543N/K544N) mutants was partially reduced compared with cells expressing wild type cPLA(2)alpha, but calcium-induced translocation was not impaired. Consistent with these results, Ser-505 phosphorylation did not change the calcium requirement for interfacial binding and catalysis in vitro but increased activity by 2-fold. Mutations in basic residues in the catalytic domain of cPLA(2)alpha reduced activation by PIP(2) but did not affect the concentration of calcium required for interfacial binding or phospholipid hydrolysis. The results demonstrate that Ser-505 phosphorylation and basic residues in the catalytic domain principally act to regulate cPLA(2)alpha hydrolytic activity.

Phospholipase A2 structure/function, mechanism, and signaling

Tremendous advances in understanding the structure and function of the superfamily of phospholipase A2 (PLA2) enzymes has occurred in the twenty-first century. The superfamily includes 15 groups comprising four main types including the secreted sPLA2, cytosolic cPLA2, calcium-independent iPLA2, and platelet activating factor (PAF) acetyl hydrolase/oxidized lipid lipoprotein associated (Lp)PLA2. We review herein our current understanding of the structure and interaction with substrate phospholipids, which resides in membranes for a representative of each of these main types of PLA2. We will also briefly review the development of inhibitors of these enzymes and their roles in lipid signaling.

Phospholipase A2 biochemistry

The phospholipase A(2) (PLA(2)) superfamily consists of many different groups of enzymes that catalyze the hydrolysis of the sn-2 ester bond in a variety of different phospholipids. The products of this reaction, a free fatty acid, and lysophospholipid have many different important physiological roles. There are five main types of PLA(2): the secreted sPLA(2)'s, the cytosolic cPLA(2)'s, the Ca(2+)independent iPLA(2)'s, the PAF acetylhydrolases, and the lysosomal PLA(2)'s. This review focuses on the superfamily of PLA(2) enzymes, and then uses three specific examples of these enzymes to examine the differing biochemistry of the three main types of these enzymes. These three examples are the GIA cobra venom PLA(2), the GIVA cytosolic cPLA(2), and the GVIA Ca(2+)-independent iPLA(2).

C2 domain protein MIN1 promotes eyespot organization in Chlamydomonas reinhardtii

Assembly and asymmetric localization of the photosensory eyespot in the biflagellate, unicellular green alga Chlamydomonas reinhardtii requires coordinated organization of photoreceptors in the plasma membrane and pigment granule/thylakoid membrane layers in the chloroplast. min1 (mini-eyed) mutant cells contain abnormally small, disorganized eyespots in which the chloroplast envelope and plasma membrane are no longer apposed. The MIN1 gene, identified here by phenotypic rescue, encodes a protein with an N-terminal C2 domain and a C-terminal LysM domain separated by a transmembrane sequence. This novel domain architecture led to the hypothesis that MIN1 is in the plasma membrane or the chloroplast envelope, where membrane association of the C2 domain promotes proper eyespot organization. Mutation of conserved C2 domain loop residues disrupted association of the MIN1 C2 domain with the chloroplast envelope in moss cells but did not abolish eyespot assembly in Chlamydomonas. In min1 null cells, channelrhodopsin-1 (ChR1) photoreceptor levels were reduced, indicating a role for MIN1 in ChR1 expression and/or stability. However, ChR1 localization was only minimally disturbed during photoautotrophic growth of min1 cells, conditions under which the pigment granule layers are disorganized. The data are consistent with the hypothesis that neither MIN1 nor proper organization of the plastidic components of the eyespot is essential for localization of ChR1.

The endocannabinoid 2-arachidonoylglycerol activates human platelets through non-CB1/CB2 receptors

BACKGROUND

The endocannabinoid 2-arachidonoylglycerol (2-AG) is an endogenous lipid that acts through the activation of G-protein-coupled cannabinoid receptors and plays essential roles in many physiological contexts. In the cardiovascular system 2-AG is generated by both activated endothelial cells and platelets, and participates in the regulation of inflammation and thrombosis. Although human platelets actively metabolize endocannabinoids, 2-AG also binds to platelet surface and leads to cell activation.

OBJECTIVE

To investigate the biological consequence of 2-AG interactions with human platelets and to clarify the role of cannabinoid receptors.

METHODS

Gel-filtered platelets were stimulated with 2-AG in the presence or absence of various inhibitors. Platelet aggregation and secretion were measured in a lumiaggregometer. Calcium ion movements were measured in FURA-2 loaded platelets. Thromboxane A(2) (TxA(2)) generation was evaluated as Thromboxane B(2) accumulation with a commercial EIA assay.

RESULTS

2-AG induced platelet shape change, aggregation and secretion with a dose-dependent mechanism that required engagement of platelet TxA(2) receptors. 2-AG caused also cytosolic calcium increase; however, it was totally dependent on availability of TxA(2). Indeed 2-AG was able to induce a robust generation of TxA(2) through the cyclooxygenase pathway. Treatment of platelets with inhibitors of monoacylglycerol lipase and fatty acid amide hydrolase did not affect the activation induced by 2-AG. Moreover, neither CB(1) and CB(2) proteins nor CB(1)/CB(2) mRNAs were detected in platelets.

CONCLUSIONS

2-AG can be considered a new physiologic platelet agonist able to induce full platelet activation and aggregation with a non-CB(1)/CB(2) receptor-mediated mechanism.

A phospholipid substrate molecule residing in the membrane surface mediates opening of the lid region in group IVA cytosolic phospholipase A2

The Group IVA (GIVA) phospholipase A(2) associates with natural membranes in response to an increase in intracellular Ca(2+) along with increases in certain lipid mediators. This enzyme associates with the membrane surface as well as binding a single phospholipid molecule in the active site for catalysis. Employing deuterium exchange mass spectrometry, we have identified the regions of the protein binding the lipid surface and conformational changes upon a single phospholipid binding in the absence of a lipid surface. Experiments were carried out using natural palmitoyl arachidonyl phosphatidylcholine vesicles with the intact GIVA enzyme as well as the isolated C2 and catalytic domains. Lipid binding produced changes in deuterium exchange in eight different regions of the protein. The regions with decreased exchange included Ca(2+) binding loop one, which has been proposed to penetrate the membrane surface, and a charged patch of residues, which may be important in interacting with the polar head groups of phospholipids. The regions with an increase in exchange are all located either in the hydrophobic core underneath the lid region or near the lid and hinge regions from 403 to 457. Using the GIVA phospholipase A(2) irreversible inhibitor methyl-arachidonyl fluorophosphonate, we were able to isolate structural changes caused only by pseudo-substrate binding. This produced results that were very similar to natural lipid binding in the presence of a lipid interface with the exception of the C2 domain and region 466-470. This implies that most of the changes seen in the catalytic domain are due to a substrate-mediated, not interface-mediated, lid opening, which exposes the active site to water. Finally experiments carried out with inhibitor plus phospholipid vesicles showed decreases at the C2 domain as well as charged residues on the putative membrane binding surface of the catalytic domain revealing the binding sites of the enzyme to the lipid surface.

Inherited human cPLA(2alpha) deficiency is associated with impaired eicosanoid biosynthesis, small intestinal ulceration, and platelet dysfunction

Cytosolic phospholipase A2alpha (cPLA2alpha) hydrolyzes arachidonic acid from cellular membrane phospholipids, thereby providing enzymatic substrates for the synthesis of eicosanoids, such as prostaglandins and leukotrienes. Considerable understanding of cPLA2alpha function has been derived from investigations of the enzyme and from cPLA2alpha-null mice, but knowledge of discrete roles for this enzyme in humans is limited. We investigated a patient hypothesized to have an inherited prostanoid biosynthesis deficiency due to his multiple, complicated small intestinal ulcers despite no use of cyclooxygenase inhibitors. Levels of thromboxane B2 and 12-hydroxyeicosatetraenoic acid produced by platelets and leukotriene B4 released from calcium ionophore-activated blood were markedly reduced, indicating defective enzymatic release of the arachidonic acid substrate for the corresponding cyclooxygenase and lipoxygenases. Platelet aggregation and degranulation induced by adenosine diphosphate or collagen were diminished but were normal in response to arachidonic acid. Two heterozygous single base pair mutations and a known SNP were found in the coding regions of the patient's cPLA2alpha genes (p.[Ser111Pro]+[Arg485His; Lys651Arg]). The total PLA2 activity in sonicated platelets was diminished, and the urinary metabolites of prostacyclin, prostaglandin E2, prostaglandin D2, and thromboxane A2 were also reduced. These findings characterize what we believe is a novel inherited deficiency of cPLA2.

Cytosolic phospholipase A2alpha activation induced by S1P is mediated by the S1P3 receptor in lung epithelial cells

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) activation is a regulatory step in the control of arachidonic acid (AA) liberation for eicosanoid formation. Sphingosine 1-phosphate (S1P) is a bioactive lipid mediator involved in the regulation of many important proinflammatory processes and has been found in the airways of asthmatic subjects. We investigated the mechanism of S1P-induced AA release and determined the involvement of cPLA(2)alpha in these events in A549 human lung epithelial cells. S1P induced AA release rapidly within 5 min in a dose- and time-dependent manner. S1P-induced AA release was inhibited by the cPLA(2)alpha inhibitors methyl arachidonyl fluorophosphonate (MAFP) and pyrrolidine derivative, by small interfering RNA-mediated downregulation of cPLA(2)alpha, and by inhibition of S1P-induced calcium flux, suggesting a significant role of cPLA(2)alpha in S1P-mediated AA release. Knockdown of the S1P3 receptor, the major S1P receptor expressed on A549 cells, inhibited S1P-induced calcium flux and AA release. The S1P-induced calcium flux and AA release was associated with sphingosine kinase 1 (Sphk1) expression and activity. Furthermore, Rho-associated kinase, downstream of S1P3, was crucial for S1P-induced cPLA(2)alpha activation. Our data suggest that S1P acting through S1P3, calcium flux, and Rho kinase activates cPLA(2)alpha and releases AA in lung epithelial cells. An understanding of S1P-induced cPLA(2)alpha activation mechanisms in epithelial cells may provide potential targets to control inflammatory processes in the lung.

Novel translocation responses of cytosolic phospholipase A2alpha fluorescent proteins

Cytosolic phospholipase A2 (cPLA2)alpha responds to the rise in cytosolic Ca2+ ([Ca2+]i) attending cell stimulation by moving to intracellular membranes, releasing arachidonic acid (AA) from these membranes, and thereby initiating the synthesis of various lipid mediators. Under some conditions, however, cPLA2alpha translocation occurs without any corresponding changes in [Ca2+]i. The signal for such responses has not been identified. Using confocal microscopy to track fluorescent proteins fused to cPLA2alpha or cPLA2alpha's C2 domain, we find that AA mimics Ca2+ ionophores in stimulating cPLA(2)alpha translocations to the perinuclear ER and to a novel site, the lipid body. Unlike the ionophores, AA acted independently of [Ca2+](i) rises and did not translocate the proteins to the Golgi. AA's action did not involve its metabolism to eicosanoids or acylation into cellular lipids. Receptor agonists also stimulated translocations targeting lipid bodies. We propose that AA is a signal for Ca2+-independent cPLA2alpha translocation and that lipid bodies are common targets of cPLA2alpha and contributors to stimulus-induced lipid mediator synthesis.

Calcium binding rigidifies the C2 domain and the intradomain interaction of GIVA phospholipase A2 as revealed by hydrogen/deuterium exchange mass spectrometry

The GIVA phospholipase A(2) (PLA(2)) contains two domains: a calcium-binding domain (C2) and a catalytic domain. These domains are linked via a flexible tether. GIVA PLA(2) activity is Ca(2+)-dependent in that calcium binding promotes protein docking to the phospholipid membrane. In addition, the catalytic domain has a lid that covers the active site, presumably regulating GIVA PLA(2) activity. We now present studies that explore the dynamics and conformational changes of this enzyme in solution utilizing peptide amide hydrogen/deuterium (H/D) exchange coupled with liquid chromatography-mass spectrometry (DXMS) to probe the solvent accessibility and backbone flexibility of the C2 domain, the catalytic domain, and the intact GIVA PLA(2). We also analyzed the changes in H/D exchange of the intact GIVA PLA(2) upon Ca(2+) binding. The DXMS results showed a fast H/D-exchanging lid and a slow exchanging central core. The C2 domain showed two distinct regions: a fast exchanging region facing away from the catalytic domain and a slow exchanging region present in the "cleft" region between the C2 and catalytic domains. The slow exchanging region of the C2 domain is in tight proximity to the catalytic domain. The effects of Ca(2+) binding on GIVA PLA(2) are localized in the C2 domain and suggest that binding of two distinct Ca(2+) ions causes tightening up of the regions that surround the anion hole at the tip of the C2 domain. This conformational change may be the initial step in GIVA PLA(2) activation.

Ceramide-1-phosphate Binds Group IVA Cytosolic Phospholipase a2 via a Novel Site in the C2 Domain

Previously, ceramide-1-phosphate (C1P) was demonstrated to be a potent and specific activator of group IV cytosolic phospholipase A(2)alpha (cPLA(2)alpha) via interaction with the C2 domain. In this study, we hypothesized that the specific interaction site for C1P was localized to the cationic beta-groove (Arg(57), Lys(58), Arg(59)) of the C2 domain of cPLA(2)alpha. In this regard, mutants of this region of cPLA(2)alpha were generated (R57A/K58A/R59A, R57A/R59A, K58A/R59A, R57A/K58A, R57A, K58A, and R59A) and examined for C1P affinity by surface plasmon resonance. The triple mutants (R57A/K58A/R59A), the double mutants (R57A/R59A, K58A/R59A, and R57A/K58A), and the single mutant (R59A) demonstrated significantly reduced affinity for C1P-containing vesicles as compared with wild-type cPLA(2)alpha. Examining these mutants for enzymatic activity demonstrated that these five mutants of cPLA(2)alpha also showed a significant reduction in the ability of C1P to: 1) increase the V(max) of the reaction; and 2) significantly decrease the dissociation constant (K (A)(s)) of the reaction as compared with the wild-type enzyme. The mutational effect was specific for C1P as all of the cationic mutants of cPLA(2)alpha demonstrated normal basal activity as well as normal affinities for phosphatidylcholine and phosphatidylinositol-4,5-bisphosphate as compared with wild-type cPLA(2)alpha. This study, for the first time, demonstrates a novel C1P interaction site mapped to the cationic beta-groove of the C2 domain of cPLA(2)alpha.

TLR-4 and sustained calcium agonists synergistically produce eicosanoids independent of protein synthesis in RAW264.7 cells

Arachidonic acid is released by phospholipase A(2) and converted into hundreds of distinct bioactive mediators by a variety of cyclooxygenases (COX), lipoxygenases (LO), and cytochrome P450s. Because of the size and diversity of the eicosanoid class of signaling molecules produced, a thorough and systematic investigation of these biological processes requires the simultaneous quantitation of a large number of eicosanoids in a single analysis. We have developed a robust liquid chromatography/tandem mass spectrometry method that can identify and quantitate over 60 different eicosanoids in a single analysis, and we applied it to agonist-stimulated RAW264.7 murine macrophages. Fifteen different eicosanoids produced through COX and 5-LO were detected either intracellularly or in the media following stimulation with 16 different agonists of Toll-like receptors (TLR), G protein-coupled receptors, and purinergic receptors. No significant differences in the COX metabolite profiles were detected using the different agonists; however, we determined that only agonists creating a sustained Ca(2+) influx were capable of activating the 5-LO pathway in these cells. Synergy between Ca(2+) and TLR pathways was detected and discovered to be independent of NF-kappaB-induced protein synthesis. This demonstrates that TLR induction of protein synthesis and priming for enhanced phospholipase A(2)-mediated eicosanoid production work through two distinct pathways.

Role of phosphatidylinositol 4,5-bisphosphate in the activation of cytosolic phospholipase A2-α

Cytosolic phospholipase A(2)-alpha (cPLA(2)) plays an important role in the release of arachidonic acid and in cell injury. Activation of cPLA(2) is dependent on a rise in cytosolic Ca(2+) concentration, membrane association via the Ca(2+)-dependent lipid binding (CaLB) domain, and phosphorylation. This study addresses the activation of cPLA(2) via potential association with membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)), including the role of a "pleckstrin homology (PH)-like" region of cPLA(2) (amino acids 263-354). In cells incubated with complement, phorbol myristate acetate+the Ca(2+) ionophore, A23187, or epidermal growth factor+A23187, expression of the PH domain of phospholipase C-delta1 (which sequesters membrane PIP(2)) attenuated cPLA(2) activity. Stimulated cPLA(2) activity was also attenuated by the expression of cPLA(2) 135-366, or cPLA(2) 2-366, and expression of a PIP(2)-specific 5'-phosphatase. However, in a yeast-based assay that tests the ability of proteins to bind to membrane lipids, including PIP(2), with high affinity, only cPLA(2) 1-200 (CaLB domain) was able to interact with membrane lipids, whereas cPLA(2)s 135-366, 2-366, 201-648, and 1-648 were unable to do so. Therefore, cPLA(2) activity can be modulated by sequestration or depletion of cellular PIP(2), although the interaction of cPLA(2) with membrane PIP(2) appears to be indirect, or of weak affinity.

Interaction of human 15-lipoxygenase-1 with phosphatidylinositol bisphosphates results in increased enzyme activity

15-lipoxygenase-1 (15-LO-1) can oxygenate both free fatty acids and fatty acids bound to membrane phospholipids. The regulation of the activity of membrane associated 15-LO-1 is poorly understood. Here we demonstrate that calcium ionophore stimulates the translocation of 15-LO-1 to the plasma membrane in human dendritic cells. In a protein-lipid overlay assay, 15-LO-1 was capable of interacting with several phosphoinositides. In the presence of calcium, addition of phosphatidylinositol-4.5-bisphosphate (PI(4.5)P(2)) or PI(3.4)P(2) to the vesicles containing arachidonic acid, led to the formation of approximately three times more 15-HETE than vesicles without phosphoinositides and up to seven times more 15-HETE than vesicles without both calcium and phosphoinositides. The Vmax was unchanged but the apparent Km of 15-LO-1 towards arachidonic acid was significantly lower in the presence of PI(4.5)P(2) or PI(3.4)P(2) in the vesicles in comparison to vesicles with PC only. Taken together, this report demonstrates that human 15-LO-1 binds to PI(4.5)P(2) and PI(3.4)P(2) and that these phospholipids stimulate enzyme activity in the presence of calcium in a vesicle based assay.

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.

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

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