Secretory phospholipase A2 enzymes in atherogenesis

Alveolar macrophage-derived gVPLA2 promotes ventilator-induced lung injury via the cPLA2/PGE2 pathway

BACKGROUND

Ventilator-induced lung injury (VILI) is a clinical complication of mechanical ventilation observed in patients with acute respiratory distress syndrome. It is characterized by inflammation mediated by inflammatory cells and their secreted mediators.

METHODS

To investigate the mechanisms underlying VILI, a C57BL/6J mouse model was induced using high tidal volume (HTV) mechanical ventilation. Mice were pretreated with Clodronate liposomes to deplete alveolar macrophages or administered normal bone marrow-derived macrophages or Group V phospholipase A2 (gVPLA2) intratracheally to inhibit bone marrow-derived macrophages. Lung tissue and bronchoalveolar lavage fluid (BALF) were collected to assess lung injury and measure Ca2 + concentration, gVPLA2, downstream phosphorylated cytoplasmic phospholipase A2 (p-cPLA2), prostaglandin E2 (PGE2), protein expression related to mitochondrial dynamics and mitochondrial damage. Cellular experiments were performed to complement the animal studies.

RESULTS

Depletion of alveolar macrophages attenuated HTV-induced lung injury and reduced gVPLA2 levels in alveolar lavage fluid. Similarly, inhibition of alveolar macrophage-derived gVPLA2 had a similar effect. Activation of the cPLA2/PGE2/Ca2 + pathway in alveolar epithelial cells by gVPLA2 derived from alveolar macrophages led to disturbances in mitochondrial dynamics and mitochondrial dysfunction. The findings from cellular experiments were consistent with those of animal experiments.

CONCLUSIONS

HTV mechanical ventilation induces the secretion of gVPLA2 by alveolar macrophages, which activates the cPLA2/PGE2/Ca2 + pathway, resulting in mitochondrial dysfunction. These findings provide insights into the pathogenesis of VILI and may contribute to the development of therapeutic strategies for preventing or treating VILI.

Lysophosphatidylcholine in phospholipase A2-modified LDL triggers secretion of angiopoietin 2

BACKGROUND AND AIMS

Secretory phospholipase A₂ (PLA₂) hydrolyzes LDL phospholipids generating modified LDL particles (PLA₂-LDL) with increased atherogenic properties. Exocytosis of Weibel-Palade bodies (WPB) releases angiopoietin 2 (Ang2) and externalizes P-selectin, which both play important roles in vascular inflammation. Here, we investigated the effects of PLA₂-LDL on exocytosis of WPBs.

METHODS

Human coronary artery endothelial cells (HCAECs) were stimulated with PLA₂- LDL, and its uptake and effect on Ang2 release, leukocyte adhesion, and intracellular calcium levels were measured. The effects of PLA₂-LDL on Ang2 release and WPB exocytosis were measured in and ex vivo in mice.

RESULTS

Exposure of HCAECs to PLA₂-LDL triggered Ang2 secretion and promoted leukocyte-HCAEC interaction. Lysophosphatidylcholine was identified as a critical component of PLA₂-LDL regulating the WPB exocytosis, which was mediated by cell-surface proteoglycans, phospholipase C, intracellular calcium, and cytoskeletal remodeling. PLA₂-LDL also induced murine endothelial WPB exocytosis in blood vessels in and ex vivo, as evidenced by secretion of Ang2 in vivo, P-selectin translocation to plasma membrane in intact endothelial cells in thoracic artery and tracheal vessels, and reduced Ang2 staining in tracheal endothelial cells. Finally, in contrast to normal human coronary arteries, in which Ang2 was present only in the endothelial layer, at sites of advanced atherosclerotic lesions, Ang2 was detected also in the intima, media, and adventitia.

CONCLUSIONS

Our studies reveal PLA₂-LDL as a potent agonist of endothelial WPB exocytosis, resulting in increased secretion of Ang2 and translocation of P-selectin. The results provide mechanistic insight into PLA₂-LDL-dependent promotion of vascular inflammation and atherosclerosis.

Polyunsaturated Phospholipid Modified Membrane Degradation Catalyzed by a Secreted Phospholipase A2

To optimize the compositions of the lipid-based nanomedicine and to advance understanding of the roles of polyunsaturated phospholipids in biological membranes, this study examined the effects of polyunsaturated phospholipids on the degradation of giant unilamellar vesicles catalyzed by a secreted phospholipase A2 (sPLA₂) using fluorescence microscopy. Molecular interfacial packing, interaction, and degradation of the films containing various mixing ratios of saturated and polyunsaturated phospholipids were quantified using a Langmuir trough integrated with synchrotron X-ray surface scattering techniques. It was found that a high molar fraction (0.63 and above) of polyunsaturated phospholipids not only enhanced the rate of sPLA₂-catalyzed vesicle degradation but also changed the vesicle deformation process and degradation product morphology. Hydrolysis of the saturated phospholipids generated highly ordered liquid crystal domains, which was reduced or prohibited by the presence of the polyunsaturated phospholipids in the reactant film.

Molecular interactions of phospholipid monolayers with a model phospholipase

The intrinsic overexpression of secretory phospholipase A2 (sPLA2) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA2 (from bee venom) with the DPPC monolayer exhibited Ca2+ dependence. DPPC molecules at the interface without Ca2+ retained a monolayer organization; upon adsorption of sPLA2 to the monolayer the packing became tighter. In contrast, sPLA2-catalyzed degradation of DPPC occurred in the presence of Ca2+, leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA2. A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

Structural Insight into Binding Mode of 9-Hydroxy Aristolochic Acid, Diclofenac and Indomethacin to PLA2

Phospholipase A₂ (PLA₂) catalyzes the hydrolysis of phospholipids into arachidonic acid and lysophospholipids. Arachidonic acid is modified by cyclooxygenases into active compounds called eicosanoids that act as signaling molecules in a number of physiological processes. Excessive production of eicosanoids leads to several pathological conditions such as inflammation. In order to block the inflammatory effect of these compounds, upstream enzymes such as PLA₂ are valid targets. In the present contribution, molecular dynamic analysis was performed to evaluate the binding of diclofenac, 9-hydroxy aristolochic acid (9-HAA) and indomethacin to PLA₂. Obtained results revealed that 9-HAA could form a more stable complex with PLA₂ when compared to diclofenac and indomethacin. Furthermore, analysis of intermolecular binding energy components indicated that hydrophobic interactions were dominant in binding process. On the basis of obtained data, inhibitors bearing fused rings with hydrogen acceptor/donor substituent(s) interacted with His48 and Asp49 residues of the active site. More affinity toward PLA₂ might be envisaged through negatively charged moieties via interaction with Trp31, Lys34 and Lys69.

Going into labor and beyond: phospholipase A2 in pregnancy

The phospholipase A2(PLA2) family is a very diverse group of enzymes, all serving in the cleavage of phospholipids, thereby releasing high amounts of arachidonic acid (AA) and lysophospholipids. AA serves as a substrate for prostaglandin production, which is of special importance in pregnancy for the onset of parturition. Novel research demonstrates that PLA2action affects the immune response of the mother toward the child and is therefore probably implied in the tolerance of the fetus and prevention of miscarriage. This review presents data on the biochemical and enzymatic properties of PLA2during gestation with a special emphasis on its role for the placental function and development of the fetus. We also critically discuss the possible pathophysiological significance of PLA2alterations and its possible functional consequences. These alterations are often associated with pregnancy pathologies such as preeclampsia and villitis or pregnancy complications such as obesity and diabetes in the mother as well as preterm onset of labor.

Diverse activity of human secretory phospholipases A2 on the migration of human vascular smooth muscle cells

OBJECTIVE

Investigation of the diversity of human secretory phospholipases A2 (sPLA2) on the migration of human vascular smooth muscle cells (VSMC).

MATERIAL

We investigated the impact of sPLA2 IIA, V, and X and of oleic acid, linoleic acid and lysophosphatidylcholine on the migration of human VSMC.

METHODS

Recombinant human sPLA2's and Boyden's chamber method were applied.

RESULTS

sPLA2, IIA but not V or X enhanced migration of VSMC in a dose/time dependent manner. Oleic and linoleic acids, and lysophosphatidylcholine markedly enhanced migration.

CONCLUSIONS

These results imply that sPLA2 IIA, which is known to be present in the arterial wall in the vicinity of VSMC, as well as products of lipid hydrolysis induced by sPLA2, enhance the migration of VSMC, and thus may contribute to atherogenic process.

Ubiquitin activates patatin-like phospholipases from multiple bacterial species

Phospholipase A2 enzymes are ubiquitously distributed throughout the prokaryotic and eukaryotic kingdoms and are utilized in a wide array of cellular processes and physiological and immunological responses. Several patatin-like phospholipase homologs of ExoU from Pseudomonas aeruginosa were selected on the premise that ubiquitin activation of this class of bacterial enzymes was a conserved process. We found that ubiquitin activated all phospholipases tested in both in vitro and in vivo assays via a conserved serine-aspartate catalytic dyad. Ubiquitin chains versus monomeric ubiquitin were superior in inducing catalysis, and ubiquitin-like proteins failed to activate phospholipase activity. Toxicity studies in a prokaryotic dual-expression system grouped the enzymes into high- and low-toxicity classes. Toxicity measured in eukaryotic cells also suggested a two-tiered classification but was not predictive of the severity of cellular damage, suggesting that each enzyme may correspond to unique properties perhaps based on its specific biological function. Additional studies on lipid binding preference suggest that some enzymes in this family may be differentially sensitive to phosphatidyl-4,5-bisphosphate in terms of catalytic activation enhancement and binding affinity. Further analysis of the function and amino acid sequences of this enzyme family may lead to a useful approach to formulating a unifying model of how these phospholipases behave after delivery into the cytoplasmic compartment.

Dyslipidemia, but not hyperglycemia and insulin resistance, is associated with marked alterations in the HDL lipidome in type 2 diabetic subjects in the DIWA cohort: impact on small HDL particles

In this study we have used mass spectrometry in order to characterize the HDL lipidome in three groups of women from the DIWA cohort; one control group, plus two groups with type 2 diabetes with insulin resistance; one dyslipidemic and one normolipidemic. The aim was to investigate whether dyslipidemia is required in addition to insulin resistance for the occurrence of an altered HDL lipidome, which in turn might impact HDL functionality. The dyslipidemic type 2 diabetic subjects were distinguished by obesity, hypertriglyceridemia with elevated apoC3, low HDL-cholesterol and chronic low grade inflammation. In a stepwise multivariate linear regression analysis, including biomarkers of dyslipidemia and insulin resistance as independent variables, only dyslipidemia showed a significant correlation with HDL lipid classes. Small HDL-particles predominated in dyslipidemic subjects in contrast to the normolipidemic diabetic and control groups, and were enriched in lysophosphatidylcholine (+13%), a product of proinflammatory phospholipases, and equally in two core lipids, palmitate-rich triacylglycerols and diacylglycerols (+77 %), thereby reflecting elevated CETP activity. Dyslipidemic small HDL particles were further distinguished not only as the primary carrier of ceramides, which promote inflammation and insulin resistance, but also by a subnormal plasmalogen/apoAI ratio, consistent with elevated oxidative stress typical of type 2 diabetes. From these data we conclude that in type 2 diabetes, dyslipidemia predominates relative to hyperglycemia for the occurrence of an altered HDL lipidome. Furthermore, dyslipidemia alters the cargo of bioactive lipids, with implications for HDL function.

Phospholipases: an overview

Phospholipids are present in all living organisms. They are a major component of all biological membranes, along with glycolipids and cholesterol. Enzymes aimed at cleaving the various bonds in phospholipids, namely phospholipases, are consequently widespread in nature, playing very diverse roles from aggression in snake venom to signal transduction, lipid mediators production, and digestion in humans. Although all phospholipases target phospholipids as substrates, they vary in the site of action on the phospholipids molecules, physiological function, mode of action, and their regulation. Significant studies on phospholipases characterization, physiological role, and industrial potential have been conducted worldwide. Some of them have been directed for biotechnological advances, such as gene discovery and functional enhancement by protein engineering. Others reported phospholipases as virulence factors and major causes of pathophysiological effects. In this introductory chapter, we provide brief details of different phospholipases.

Secreted phospholipase A2 revisited

Phospholipase A(2) (PLA(2)) catalyses the hydrolysis of the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. So far, more than 30 enzymes that possess PLA(2) or related activity have been identified in mammals. About one third of these enzymes belong to the secreted PLA(2) (sPLA(2)) family, which comprises low molecular weight, Ca(2+) requiring, secreted enzymes with a His/Asp catalytic dyad. Individual sPLA(2)s display distinct localizations and enzymatic properties, suggesting their specialized biological roles. However, in contrast to intracellular PLA(2)s, whose roles in signal transduction and membrane homoeostasis have been well documented, the biological roles of sPLA(2)s in vivo have remained obscure until recently. Over the past decade, information fuelled by studies employing knockout and transgenic mice as well as specific inhibitors, in combination with lipidomics, has clarified when and where the different sPLA(2) isoforms are expressed, which isoforms are involved in what types of pathophysiology, and how they exhibit their specific functions. In this review, we highlight recent advances in PLA(2) research, focusing mainly on the physiological functions of sPLA(2)s and their modes of action on 'extracellular' phospholipid targets versus lipid mediator production.

Secreted phospholipase A2, lipoprotein hydrolysis, and atherosclerosis: integration with lipidomics

Phospholipase A₂ (PLA₂) is a group of enzymes that hydrolyze the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. Of many PLA₂s or related enzymes identified to date, secreted PLA₂s (sPLA₂s) comprise the largest family that contains 10 catalytically active isozymes. Besides arachidonic acid released from cellular membranes for eicosanoid synthesis, several if not all sPLA₂s have recently been implicated in hydrolysis of phospholipids in lipoprotein particles. The sPLA₂-processed low-density lipoprotein (LDL) particles contain a large amount of lysophospholipids and exhibit the property of “small-dense” or “modified” LDL, which facilitates foam cell formation from macrophages. Transgenic overexpression of these sPLA₂s leads to development of atherosclerosis in mice. More importantly, genetic deletion or pharmacological inhibition of particular sPLA₂s significantly attenuates atherosclerosis and aneurysm. In this article, we will give an overview of current understanding of the role of sPLA₂s in atherosclerosis, with recent lipidomics data showing the action of a subset of sPLA₂s on lipoprotein phospholipids.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Biliary cholesterol secretion: More than a simple ABC

Biliary cholesterol secretion is a process important for 2 major disease complexes, atherosclerotic cardiovascular disease and cholesterol gallstone disease. With respect to cardiovascular disease, biliary cholesterol secretion is regarded as the final step for the elimination of cholesterol originating from cholesterol-laden macrophage foam cells in the vessel wall in a pathway named reverse cholesterol transport. On the other hand, cholesterol hypersecretion into the bile is considered the main pathophysiological determinant of cholesterol gallstone formation. This review summarizes current knowledge on the origins of cholesterol secreted into the bile as well as the relevant processes and transporters involved. Next to the established ATP-binding cassette (ABC) transporters mediating the biliary secretion of bile acids (ABCB11), phospholipids (ABCB4) and cholesterol (ABCG5/G8), special attention is given to emerging proteins that modulate or mediate biliary cholesterol secretion. In this regard, the potential impact of the phosphatidylserine flippase ATPase class I type 8B member 1, the Niemann Pick C1-like protein 1 that mediates cholesterol absorption and the high density lipoprotein cholesterol uptake receptor, scavenger receptor class B type I, is discussed.

Varespladib (A-002), a secretory phospholipase A2 inhibitor, reduces atherosclerosis and aneurysm formation in ApoE-/- mice

The family of secretory phospholipase A2 (sPLA2) enzymes has been associated with inflammatory diseases and tissue injury including atherosclerosis. A-001 is a novel inhibitor of sPLA2 enzymes discovered by structure-based drug design, and A-002 is the orally bioavailable prodrug currently in clinical development. A-001 inhibited human and mouse sPLA2 group IIA, V, and X enzymes with IC50 values in the low nM range. A-002 (1 mg/kg) led to high serum levels of A-001 and inhibited PLA2 activity in transgenic mice overexpressing human sPLA2 group IIA in C57BL/6J background. In addition, the effects of A-002 on atherosclerosis in 2 ApoE mouse models were evaluated using en face analysis. (1) In a high-fat diet model, A-002 (30 and 90 mg/kg twice a day for 16 weeks) reduced aortic atherosclerosis by 50% (P < 0.05). Plasma total cholesterol was decreased (P < 0.05) by 1 month and remained lowered throughout the study. (2) In an accelerated atherosclerosis model, with angiotensin II-induced aortic lesions and aneurysms, A-002 (30 mg/kg twice a day) reduced aortic atherosclerosis by approximately 40% (P < 0.05) and attenuated aneurysm formation (P = 0.0096). Thus, A-002 was effective at significantly decreasing total cholesterol, atherogenesis, and aneurysm formation in these 2 ApoE mouse models.

Emerging roles of secreted phospholipase A2 enzymes: Lessons from transgenic and knockout mice

Among the emerging phospholipase A(2) (PLA(2)) superfamily, the secreted PLA(2) (sPLA(2)) family consists of low-molecular-mass, Ca(2+)-requiring extracellular enzymes with a His-Asp catalytic dyad. To date, more than 10 sPLA(2) enzymes have been identified in mammals. Individual sPLA(2)s exhibit unique tissue and cellular localizations and enzymatic properties, suggesting their distinct pathophysiological roles. Despite numerous enzymatic and cell biological studies on this enzyme family in the past two decades, their precise in vivo functions still remain largely obscure. Recent studies using transgenic and knockout mice for several sPLA(2) enzymes, in combination with lipidomics approaches, have opened new insights into their distinct contributions to various biological events such as food digestion, host defense, inflammation, asthma and atherosclerosis. In this article, we overview the latest understanding of the pathophysiological functions of individual sPLA(2) isoforms fueled by studies employing transgenic and knockout mice for several sPLA(2)s.

Bioactive products generated by group V sPLA(2) hydrolysis of LDL activate macrophages to secrete pro-inflammatory cytokines

OBJECTIVE

Previous studies have established that hydrolysis of LDL by Group V secretory phospholipase A(2) (GV sPLA(2)) generates a modified particle capable of inducing macrophage foam cell formation. The aim of the present study was to determine whether GV sPLA(2)-hydrolyzed LDL (GV-LDL) produces pro-atherogenic effects in macrophages independent of cholesterol accumulation.

METHODS AND RESULTS

J-774 cells incubated with GV-LDL produced more TNF-alpha and IL-6 compared to cells incubated with control-LDL. Indirect immunofluorescence showed that GV-LDL but not control-LDL induced nuclear translocation of NFkappaB. Inhibitors of NFkappaB activation, effectively blocked cytokine production induced by GV-LDL. Control-LDL and GV-LDL were separated from albumin present in reaction mixtures by ultracentrifugation. The albumin fraction derived from GV-LDL contained 80% of the FFA generated and was more potent than the re-isolated GV-LDL in inducing pro-inflammatory cytokine secretion. Linoleic acid (18:2) and oleic acid (18:1) were the most abundant FFAs generated, whereas newly formed lyso-PCs contained 14:0 (myristic), 16:1 (palmitic), and 18:2 fatty acyl groups. Experiments with synthetic FFA showed that 18:1 induced J-774 cells to secrete TNF-alpha and IL-6.

CONCLUSIONS

These results indicate that in addition to promoting atherosclerotic lipid accumulation in macrophages, GV sPLA(2) hydrolysis of LDL leads to activation of NFkappaB, a key regulator of inflammation.

Current and emerging paradigms in the therapeutic management of atherosclerosis

BACKGROUND

The pathogenesis of atherosclerosis lies in abnormalities in lipoprotein metabolism leading to pathological interactions with vessel walls and the release of inflammatory components, which further aggravate the disease condition.

OBJECTIVE

To elucidate current and emerging trends in drug discovery towards the development of new entities regulating lipoprotein metabolism and inflammatory components to combat the progression of atherosclerosis.

METHODS

Research/review articles in the public domain and press releases were employed.

RESULTS/CONCLUSION

With the recent failure of torcetrapib and succinobucol, drug discovery and development efforts towards the treatment of atherosclerosis have received a big jolt and have been slowed down to a certain extent [corrected]. But this could be a starting point for several new mechanisms that are emerging to discover new drugs to combat the disease.

The capacity of group V sPLA2 to increase atherogenicity of ApoE-/- and LDLR-/- mouse LDL in vitro predicts its atherogenic role in vivo

OBJECTIVE

In vitro data indicate that human LDL modified by Group V secretory phospholipase A(2) (GV sPLA(2)) is proatherogenic. Consistent with this, gain and loss of function studies demonstrated that GV sPLA(2) promotes atherosclerosis in LDLR(-/-) mice. The current study investigates whether GV sPLA(2) promotes atherosclerotic processes in apoE(-/-) mice.

METHODS AND RESULTS

LDL (d=1.019 to 1.063) from apoE(-/-) and LDLR(-/-) mice fed chow or Western diet were hydrolyzed by GV sPLA(2). Phosphatidylcholine on LDL from LDLR(-/-) mice fed either a chow or Western diet was hydrolyzed to a greater extent (61.1+/-0.4% and 45.3+/-4.6%) than the corresponding fractions from apoE(-/-) mice (41.7+/-3.6% and 39.4+/-1.2%). ApoE(-/-) LDL induced macrophage foam cell formation in vitro without modification by GV sPLA(2), whereas hydrolysis of LDLR(-/-) LDL was a prerequisite for foam cell formation. In contrast to findings in LDLR(-/-) mice, GV sPLA(2) deficiency did not significantly reduce atherosclerosis in apoE(-/-) mice, although collagen content was significantly reduced in lesions of apoE(-/-) mice lacking GV sPLA(2).

CONCLUSIONS

The ability of GV sPLA(2) to promote atherosclerotic lipid deposition in apoE(-/-) and LDLR(-/-) mice may be related to its ability to increase the atherogenic potential of LDL from these mice as assessed in vitro.

Scavenger receptor BI-mediated selective uptake is required for the remodeling of high density lipoprotein by endothelial lipase

Endothelial lipase (EL) is a negative regulator of high density lipoprotein (HDL) cholesterol plasma levels, and scavenger receptor BI (SR-BI) is involved in remodeling of HDL. The present study investigates the requirement of SR-BI for the effects of EL-mediated phospholipid hydrolysis on HDL metabolism in vivo. In vitro, selective uptake from EL-modified HDL was 129% higher than selective uptake from control HDL in SR-BI-overexpressing cells (p=0.01). In vivo overexpression of human EL by means of recombinant adenovirus decreased HDL plasma levels significantly (p<0.01). Fast protein liquid chromatography analysis and agarose gel electrophoresis revealed that EL expression resulted in the generation of small pre-beta HDL particles in wild-type mice, whereas in SR-BI-/- mice small HDL were preferentially removed. In kinetic experiments the fractional catabolic rate (FCR) of HDL cholesteryl ester increased by 110% (p<0.001), and the FCR of HDL apolipoproteins increased by 64% (p<0.001) in response to EL overexpression in wild-type mice. In SR-BI-/- mice a similar increase in the HDL apolipoprotein FCR occurred (p<0.001); however, there was no further increase in HDL cholesteryl ester catabolism. The apparent whole body selective uptake was increased 3-fold by EL in wild-type mice (p<0.001), whereas there was no selective uptake in SR-BI knock-out mice. EL overexpression increased hepatic selective uptake as well as holoparticle uptake (each p<0.01) in wild-type mice, whereas in SR-BI knock-out mice only holoparticle uptake increased (p<0.01). Our results indicate that SR-BI-mediated selective uptake of HDL cholesteryl ester is essential for the remodeling of large alpha-migrating HDL particles by EL.

Syndecan-4 mediates macrophage uptake of group V secretory phospholipase A2-modified LDL

We previously reported that LDL modified by group V secretory phospholipase A2 (GV-LDL) promotes macrophage foam cell formation through a mechanism independent of scavenger receptors SR-A and CD36, and dependent on cellular proteoglycans. This study investigates the role of syndecans, a family of cell surface proteoglycans known to mediate endocytosis through macropinocytosis, in macrophage uptake of GV-LDL. LY 294002, a phosphatidylinositol 3-kinase inhibitor, significantly reduced internalization of (125)I-labeled GV-LDL in J-774 macrophages, consistent with a macropinocytic uptake pathway. Using small, interfering RNA-directed gene silencing, we demonstrated a direct relationship between (125)I-labeled GV-LDL binding and the level of syndecan-3 and syndecan-4 expression in J-774 cells. However, (125)I-labeled GV-LDL uptake was significantly reduced only when syndecan-4 expression was suppressed. Peritoneal macrophages from syndecan-4-deficient mice exhibited markedly reduced uptake of fluorescently labeled GV-LDL compared with wild-type cells. Furthermore, cholesteryl ester accumulation induced by GV-LDL was dependent on syndecan-4 expression. Syndecan-4 expression and GV-LDL binding were significantly increased in J-774 cells treated with lipopolysaccharide, suggesting that GV-LDL uptake via this pathway may be enhanced during inflammation. Taken together, our data point to a novel role for syndecan-4 in mediating the uptake of GV-LDL, a process implicated in atherosclerotic lesion progression.

The secretory phospholipase A2 group IIA: a missing link between inflammation, activated renin-angiotensin system, and atherogenesis?

Inflammation, lipid peroxidation and chronic activation of the rennin – angiotensin system (RAS) are hallmarks of the development of atherosclerosis. Recent studies have suggested the involvement of the pro-inflammatory secretory phospholipase A₂ (sPLA₂)-IIA in atherogenesis. This enzyme is produced by different cell types through stimulation by pro-inflammatory cytokines. It is detectable in the intima and in media smooth muscle cells, not only in atherosclerotic lesions but also in the very early stages of atherogenesis. sPLA₂-IIA can hydrolyse the phospholipid monolayers of low density lipoproteins (LDL). Such modified LDL show increased affinity to proteoglycans. The modified particles have a greater tendency to aggregate and an enhanced ability to insert cholesterol into cells. This modification may promote macrophage LDL uptake leading to the formation of foam cells. Furthermore, sPLA₂-IIA is not only a mediator for localized inflammation but may be also used as an independent predictor of adverse outcomes in patients with stable coronary artery disease or acute coronary syndromes. An interaction between activated RAS and phospholipases has been indicated by observations showing that inhibitors of sPLA₂ decrease angiotensin (Ang) II-induced macrophage lipid peroxidation. Meanwhile, various interactions between Ang II and oxLDL have been demonstrated suggesting a central role of sPLA₂-IIA in these processes and offering a possible target for treatment. The role of sPLA₂-IIA in the perpetuation of atherosclerosis appears to be the missing link between inflammation, activated RAS and lipidperoxidation.

Simplified YM-26734 inhibitors of secreted phospholipase A2 group IIA

Simplified analogs of YM-26734, a known inhibitor of secreted phospholipase A(2) (sPLA(2)) group IIA, were synthesized and found to also display potent inhibition at low nanomolar concentrations. Analogs were based on the didodecanoylphloroglucinol portion of YM-26734 which contains the predicted active site calcium binding group.

Biology of secretory phospholipase A2

Introduction

The secretory phospholipase A₂ (sPLA₂) family provides a seemingly endless array of potential biological functions that is only beginning to be appreciated. In humans, this family comprises 9 different members that vary in their tissue distribution, hydrolytic activity, and phospholipid substrate specificity. Through their lipase activity, these enzymes trigger various cell-signaling events to regulate cellular functions, directly kill bacteria, or modulate inflammatory responses. In addition, some sPLA₂’s are high affinity ligands for cellular receptors.

Objective

This review merely scratches the surface of some of the actions of sPLA₂s in innate immunity, inflammation, and atherosclerosis. The goal is to provide an overview of recent findings involving sPLA₂s and to point to potential pathophysiologic mechanisms that may become targets for therapy.

Biochemistry and physiology of mammalian secreted phospholipases A2

Phospholipases A(2) (PLA2s) are esterases that hydrolyze the sn-2 ester of glycerophospholipids and constitute one of the largest families of lipid hydrolyzing enzymes. The mammalian genome contains 10 enzymatically active secreted PLA2s (sPLA2s) and two sPLA2-related proteins devoid of lipolytic enzymatic activity. In addition to the well-established functions of one of these enzymes in digestion of dietary phospholipids and another in host defense against bacterial infections, accumulating evidence shows that some of these sPLA2s are involved in arachidonic acid release from cellular phospholipids for the biosynthesis of eicosanoids, especially during inflammation. More speculative results suggest the involvement of one or more sPLA2s in promoting atherosclerosis and cancer. In addition, the mammalian genome encodes several types of sPLA2-binding proteins, and mounting evidence shows that sPLA2s may have functions related to binding to cellular target proteins in a manner independent of their lipolytic enzymatic activity.

Modulation of enzymatic activities by n-3 polyunsaturated fatty acids to support cardiovascular health

Epidemiological evidence from Greenland Eskimos and Japanese fishing villages suggests that eating fish oil and marine animals can prevent coronary heart disease. Dietary studies from various laboratories have similarly indicated that regular fish oil intake affects several humoral and cellular factors involved in atherogenesis and may prevent atherosclerosis, arrhythmia, thrombosis, cardiac hypertrophy and sudden cardiac death. The beneficial effects of fish oil are attributed to their n-3 polyunsaturated fatty acid (PUFA; also known as omega-3 fatty acids) content, particularly eicosapentaenoic acid (EPA; 20:5, n-3) and docosahexaenoic acid (DHA; 22:6, n-3). Dietary supplementation of DHA and EPA influences the fatty acid composition of plasma phospholipids that, in turn, may affect cardiac cell functions in vivo. Recent studies have demonstrated that long-chain omega-3 fatty acids may exert beneficial effects by affecting a wide variety of cellular signaling mechanisms. Pathways involved in calcium homeostasis in the heart may be of particular importance. L-type calcium channels, the Na+-Ca2+ exchanger and mobilization of calcium from intracellular stores are the most obvious key signaling pathways affecting the cardiovascular system; however, recent studies now suggest that other signaling pathways involving activation of phospholipases, synthesis of eicosanoids, regulation of receptor-associated enzymes and protein kinases also play very important roles in mediating n-3 PUFA effects on cardiovascular health. This review is therefore focused on the molecular targets and signaling pathways that are regulated by n-3 PUFAs in relation to their cardioprotective effects.

Enhanced protein expression in mammalian cells using engineered SUMO fusions: secreted phospholipase A2

SUMOylation, the covalent attachment of SUMO (small ubiquitin-like modifier), is a eukaryotic post-translational event that has been demonstrated to play a critical role in several biological processes. When used as an N-terminal tag or fusion partner, SUMO has been shown to enhance functional protein production significantly by improving folding, solubility, and stability. We have engineered several SUMOs and, through their fusion, developed a system for enhancing the expression and secretion of complex proteins. To demonstrate the fidelity of this fusion technology, secreted phospholipase A(2) proteins (sPLA(2)) were produced using HEK-293T and CHO-K1 cells. Five mouse sPLA(2) homologs were expressed and secreted in mammalian cell cultures using SUMO or SUMO-derived, N-terminal fusion partners. Mean and median increases of 43- and 18-fold, respectively, were obtained using novel SUMO mutants that are resistant to digestion by endogenous deSUMOylases.

Translating molecular discoveries into new therapies for atherosclerosis

Atherosclerosis is characterized by the thickening of the arterial wall and is the primary cause of coronary artery disease and cerebrovascular disease, two of the most common causes of illness and death worldwide. Clinical trials have confirmed that certain lipoproteins and the renin-angiotensin-aldosterone system are important in the pathogenesis of atherosclerotic cardiovascular disease, and that interventions targeted towards these are beneficial. Furthermore, efforts to understand how risk factors such as high blood pressure, dysregulated blood lipids and diabetes contribute to atherosclerotic disease, as well as to understand the molecular pathogenesis of atherosclerotic plaques, are leading to new targets for therapy.

Release of arachidonic acid induced by tumor necrosis factor-alpha in the presence of caspase inhibition: evidence for a cytosolic phospholipase A2alpha-independent pathway

Stimulation of L929 cells with tumor necrosis factor-alpha (TNFalpha) caused cell death accompanied by a release of arachidonic acid (AA). Although the inhibition of caspases has been shown to cause necrosis in TNFalpha-treated L929 cells, its role in the TNFalpha-induced release of AA has not been elucidated. The release of AA is tightly regulated by phospholipase A(2) (PLA(2)). To find out the mechanisms underlying the TNFalpha-induced release of AA, we investigated the relationship between TNFalpha stimulation and PLA(2) regulation with and without zVAD, an inhibitor of caspases. In the present study, we found that treatment with TNFalpha and zVAD stimulated release of AA and cell death in C12 cells (a variant of L929 cells lacking alpha type of cytosolic PLA(2) (cPLA(2)alpha)). Stimulation with TNFalpha/zVAD also caused the release of AA from L929-cPLA(2)alpha-siRNA cells. Treatment with pyrrophenone (a selective inhibitor of cPLA(2)alpha) completely inhibited the TNFalpha-induced release of AA, but only partially inhibited the TNFalpha/zVAD-induced response in L929 cells. The TNFalpha/zVAD-induced release of AA from C12 and L929-cPLA(2)alpha-siRNA cells was pyrrophenone-insensitive, but inhibited by treatment with butylated hydroxyanisole (BHA, an antioxidant). Treatment with dithiothreitol, which inactivates secretory PLA(2) activity, decreased the amount of AA released by TNFalpha/zVAD. TNFalpha/zVAD appears to stimulate release of AA from C12 cells in a cPLA(2)alpha-independent, BHA-sensitive manner. The possible roles of secretory PLA(2) and reactive oxygen species from different pools in the release of AA and cell death were discussed.

Secretory phospholipase A2 increases SR-BI-mediated selective uptake from HDL but not biliary cholesterol secretion

High density lipoprotein cholesterol represents a major source of biliary cholesterol. Secretory phospholipase A2 (sPLA2) is an acute phase enzyme mediating decreased plasma HDL cholesterol levels. Clinical studies reported a link between increased sPLA2 expression and the presence of cholesterol gallstones. The aim of our study was to investigate whether the overexpression of human sPLA2 in transgenic mice affects biliary cholesterol secretion and gallstone formation. Liver weight (P < 0.01) and hepatic cholesterol content (P < 0.01) were significantly increased in sPLA2 transgenic mice compared with controls as a result of increased scavenger receptor class B type I (SR-BI)-mediated hepatic selective uptake of HDL cholesterol (P < 0.01), whereas hepatic SR-BI expression remained unchanged. However, biliary cholesterol secretion as well as fecal neutral sterol and fecal bile salt excretion remained unchanged in sPLA2 transgenic mice. Furthermore, gallstone prevalence in response to a lithogenic diet was identical in both groups. These data demonstrate that i) increased flux of cholesterol from HDL into the liver via SR-BI as a result of phospholipase modification of the HDL particle translates neither into increased biliary and fecal sterol output nor into increased gallstone formation, and ii) increased sPLA2 expression in patients with cholesterol gallstones might be a consequence rather than the underlying cause of the disease.

Tagging SNP haplotype analysis of the secretory PLA2-V gene, PLA2G5 , shows strong association with LDL and oxLDL levels, suggesting functional distinction from sPLA2-IIA: results from the UDACS study

Animal and human studies suggest that both secretory PLA2 (sPLA2)-V and sPLA2-IIA (encoded, respectively, by the neighbouring PLA2G5 and PLA2G2A genes) contribute to atherogenesis. Elevated plasma sPLA2-IIA predicts coronary heart disease (CHD) risk, but no mass assay for sPLA2-V is available. We previously reported that tagging single nucleotide polymorphism (tSNP) haplotypes of PLA2G2A are strongly associated with sPLA2-IIA mass, but not lipid levels. Here, we use tSNPs of the sPLA2-V gene to investigate the association of PLA2G5 with CHD risk markers. Seven PLA2G5 tSNPs genotypes, explaining >92% of the locus genetic variability, were determined in 519 patients with Type II diabetes (in whom PLA2G2A tSNP data was available), and defined seven common haplotypes (frequencies >5%). PLA2G5 and PLA2G2A tSNPs showed linkage disequilibrium (LD). Compared to the common PLA2G5 haplotype, H1 (frequency 34.9%), haplotypes H2-7 were associated with overall higher plasma LDL (P < 0.00004) and total cholesterol (P < 0.00003) levels yet lower oxLDL/LDL (P = 0.006) and sPLA2-IIA mass (P = 0.04), probably reflecting LD with PLA2G2A. Intronic tSNP (rs11573248), unlikely itself to be functional, distinguished H1 from LDL-raising haplotypes and may mark a functional site. In conclusion, PLA2G5 tSNP haplotypes demonstrate an association with total and LDL cholesterol and oxLDL/LDL, not seen with PLA2G2A, thus confirming distinct functional roles for these two sPLA2s.

Group V Secretory Phospholipase A2Promotes Atherosclerosis

OBJECTIVE

Group V secretory phospholipase A2 (GV sPLA2) has been detected in both human and mouse atherosclerotic lesions. This enzyme has potent hydrolytic activity towards phosphatidylcholine-containing substrates, including lipoprotein particles. Numerous studies in vitro indicate that hydrolysis of high density lipoproteins (HDL) and low density lipoproteins (LDL) by GV sPLA2 leads to the formation of atherogenic particles and potentially proinflammatory lipid mediators. However, there is no direct evidence that this enzyme promotes atherogenic processes in vivo.

METHODS AND RESULTS

We performed gain-of-function and loss-of-function studies to investigate the role of GV sPLA2 in atherogenesis in LDL receptor-deficient mice. Compared with control mice, animals overexpressing GV sPLA2 by retrovirus-mediated gene transfer had a 2.7 fold increase in lesion area in the ascending region of the aortic root. Increased atherosclerosis was associated with an increase in lesional collagen deposition in the same region. Mice deficient in bone marrow-derived GV sPLA2 had a 36% reduction in atherosclerosis in the aortic arch/thoracic aorta.

CONCLUSIONS

Our data in mouse models provide the first in vivo evidence that GV sPLA2 contributes to atherosclerotic processes, and draw attention to this enzyme as an attractive target for the treatment of atherosclerotic disease.

A continuous fluorescence assay for the determination of calcium-dependent secretory phospholipase A2 activity in serum

BACKGROUND

Calcium-dependent secretory phospholipase A(2)-IIA (sPLA(2)-IIA) in the circulation is a marker of inflammation, associated with acute and chronic disease processes. We describe a quick, sensitive and reliable microplate continuous fluorescence assay for determining sPLA(2) activity in serum.

METHODS

Liposomes composed of a fluorescent probe and varying amounts of L-alpha-phosphatidylglycerol (PG) and 1,2-dioleoyl-L-alpha-phosphatidylcholine (DOPC) were used as substrates to determine the optimal protocol for sPLA(2) activity determination without interference from serum albumin and lipoproteins.

RESULTS

Hydrolysis of the labeled substrate by sPLA(2)-IIA, characterized by increase in fluorescence intensity (FI) and confirmed by end-product analysis, occurred in a time-, calcium-, and protein-dependent manner. Liposomes containing 100% PG were most suitable for measurement of sPLA(2) activity without interference from serum components; LDL produced a Ca(2+)-independent increase in FI when liposomes containing DOPC were used. The assay determined that sPLA(2) activity in serum spiked with sPLA(2)-IIA and illustrated that endogenous sPLA(2) activity was markedly higher in sera from patients with sepsis than in healthy subjects. Intra-assay and inter-assay CVs were in the ranges of 1.6-8.8% and 3.0-11.5%, respectively.

CONCLUSIONS

The described method has potential for rapid and sensitive screening of sPLA(2) activity in both clinical and research settings.

Phospholipase A 2 –Modified Low-Density Lipoprotein Activates the Phosphatidylinositol 3-Kinase-Akt Pathway and Increases Cell Survival in Monocytic Cells

Objective— Monocyte survival is an important determinant in the development of atherosclerotic lesions. We investigated the influence of phospholipase A 2 -modified LDL (PLA-LDL), a pro-atherogenic factor, on activation of the pro-survival kinase Akt and cell death in monocytic cells.

Methods and Results— PLA-LDL induced robust phosphorylation and activation of Akt in THP1 cells. It also attenuated oxidative stress-induced cell death, an effect abolished by phosphatidylinositol 3-kinase (PI3K) inhibition. In addition, PLA-LDL increased survival of human monocytes. We noticed that lipid products derived from LDL phospholipolysis are mediators of PLA-LDL–induced Akt activation. Arachidonic acid, which is released on phospholipase treatment of LDL, induced Akt phosphorylation and increased cell survival, whereas lysophosphatidylcholine, another compound generated by LDL phospholipolysis, induced only transient Akt phosphorylation and was cytotoxic.

Conclusions— Our data indicate that PLA-LDL induces activation of the PI3K-Akt pathway and promotes monocytic cell survival, which may contribute to the pro-atherogenic effects of phospholipase A 2 -modified LDL.

Distinctiveness of secretory phospholipase A2 group IIA and V suggesting unique roles in atherosclerosis

Clinical observations strongly support an association of circulating levels of secretory phospholipases A(2) (sPLA(2)) 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 sPLA(2)-IIA and sPLA(2)-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.

Atherogenic properties of LDL particles modified by human group X secreted phospholipase A2 on human endothelial cell function

Increasing evidence suggests that secreted phospholipases A2 (sPLA2s) play an important role in the pathophysiology of atherosclerosis. Among sPLA2s, the human group X (hGX) enzyme has the highest catalytic activity toward phosphatidylcholine, one of the major phospholipid species of cell membranes and low-density lipoprotein (LDL). Our study examined the presence of hGX sPLA2 in human atherosclerotic lesions and investigated the ability of hGX modified LDL to alter human endothelial cell (HUVEC) function. Our results show that hGX sPLA2 is present in human atherosclerotic lesions and that the hydrolysis of LDL by hGX sPLA2 results in a modified particle that induces lipid accumulation in human monocyte-derived macrophages. Acting on endothelial cells, hGX-modified LDL activates the MAP kinase pathway, which leads to increased arachidonic acid release, increased expression of adhesion molecules on the surface of HUVEC, and increased adhesion of monocytes to HUVEC monolayers. Together, our data suggest that LDL modified by hGX, rather than hGX itself may have strong proinflammatory and proatherogenic properties, which could play an important role in the propagation of atherosclerosis.

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.