Analysis of expression of secreted phospholipases A2 in mouse tissues at protein and mRNA levels

Secreted phospholipase PLA2G2E contributes to regulation of T cell immune response against influenza virus infection

The involvement of secreted phospholipase A2s in respiratory diseases, such as asthma and respiratory viral infections, is well-established. However, the specific role of secreted phospholipase A2 group IIE (PLA2G2E) during influenza virus infection remains unexplored. Here, we investigated the role of PLA2G2E during H1N1 influenza virus infection using a targeted mouse model lacking Pla2g2e gene (Pla2g2e-/-). Our findings demonstrated that Pla2g2e-/- mice had significantly lower survival rates and higher viral loads in lungs compared to wild-type mice following influenza virus infection. While Pla2g2e-/- mice displayed comparable innate and humoral immune responses to influenza virus challenge, the animals showed impaired influenza-specific cellular immunity and reduced T cell-mediated cytotoxicity. This indicates that PLA2G2E is involved in regulating specific T cell responses during influenza virus infection. Furthermore, transgenic mice expressing the human PLA2G2E gene exhibited resistance to influenza virus infection along with enhanced influenza-specific cellular immunity and T cell-mediated cytotoxicity. Pla2g2e deficiency resulted in perturbation of lipid mediators in the lung and T cells, potentially contributing to its impact on the anti-influenza immune response. Taken together, these findings suggest that targeting PLA2G2E could hold potential as a therapeutic strategy for managing influenza virus infections.IMPORTANCEThe influenza virus is a highly transmissible respiratory pathogen that continues to pose a significant public health concern. It effectively evades humoral immune protection conferred by vaccines and natural infection due to its continuous viral evolution through the genetic processes of antigenic drift and shift. Recognition of conserved non-mutable viral epitopes by T cells may provide broad immunity against influenza virus. In this study, we have demonstrated that phospholipase A2 group IIE (PLA2G2E) plays a crucial role in protecting against influenza virus infection through the regulation of T cell responses, while not affecting innate and humoral immune responses. Targeting PLA2G2E could therefore represent a potential therapeutic strategy for managing influenza virus infection.

Multimodal regulation of the osteoclastogenesis process by secreted group IIA phospholipase A2

Increasing evidence points to the involvement of group IIA secreted phospholipase A₂ (sPLA₂-IIA) in pathologies characterized by abnormal osteoclast bone-resorption activity. Here, the role of this moonlighting protein has been deepened in the osteoclastogenesis process driven by the RANKL cytokine in RAW264.7 macrophages and bone-marrow derived precursor cells from BALB/cJ mice. Inhibitors with distinct selectivity toward sPLA₂-IIA activities and recombinant sPLA₂-IIA (wild-type or catalytically inactive forms, full-length or partial protein sequences) were instrumental to dissect out sPLA₂-IIA function, in conjunction with reduction of sPLA₂-IIA expression using small-interfering-RNAs and precursor cells from Pla2g2a knock-out mice. The reported data indicate sPLA₂-IIA participation in murine osteoclast maturation, control of syncytium formation and resorbing activity, by mechanisms that may be both catalytically dependent and independent. Of note, these studies provide a more complete understanding of the still enigmatic osteoclast multinucleation process, a crucial step for bone-resorbing activity, uncovering the role of sPLA₂-IIA interaction with a still unidentified receptor to regulate osteoclast fusion through p38 SAPK activation. This could pave the way for the design of specific inhibitors of sPLA₂-IIA binding to interacting partners implicated in osteoclast syncytium formation.

Old but New: Group IIA Phospholipase A2 as a Modulator of Gut Microbiota

Among the phospholipase A₂ (PLA₂) superfamily, the secreted PLA₂ (sPLA₂) family contains 11 mammalian isoforms that exhibit unique tissue or cellular distributions and enzymatic properties. Current studies using sPLA₂-deficient or -overexpressed mouse strains, along with mass spectrometric lipidomics to determine sPLA₂-driven lipid pathways, have revealed the diverse pathophysiological roles of sPLA₂s in various biological events. In general, individual sPLA₂s exert their specific functions within tissue microenvironments, where they are intrinsically expressed through hydrolysis of extracellular phospholipids. Recent studies have uncovered a new aspect of group IIA sPLA₂ (sPLA₂-IIA), a prototypic sPLA₂ with the oldest research history among the mammalian PLA₂s, as a modulator of the gut microbiota. In the intestine, Paneth cell-derived sPLA₂-IIA acts as an antimicrobial protein to shape the gut microbiota, thereby secondarily affecting inflammation, allergy, and cancer in proximal and distal tissues. Knockout of intestinal sPLA₂-IIA in BALB/c mice leads to alterations in skin cancer, psoriasis, and anaphylaxis, while overexpression of sPLA₂-IIA in Pla2g2a-null C57BL/6 mice induces systemic inflammation and exacerbates arthritis. These phenotypes are associated with notable changes in gut microbiota and fecal metabolites, are variable in different animal facilities, and are abrogated after antibiotic treatment, co-housing, or fecal transfer. These studies open a new mechanistic action of this old sPLA₂ and add the sPLA₂ family to the growing list of endogenous factors capable of affecting the microbe–host interaction and thereby systemic homeostasis and diseases.

Gowda SG, Ikeda K, Arita M, Murakami M. Group IIA secreted phospholipase A2 controls skin carcinogenesis and psoriasis by shaping the gut microbiota

Besides promoting inflammation by mobilizing lipid mediators, group IIA secreted phospholipase A₂ (sPLA₂-IIA) prevents bacterial infection by degrading bacterial membranes. Here, we show that, despite the restricted intestinal expression of sPLA₂-IIA in BALB/c mice, its genetic deletion leads to amelioration of cancer and exacerbation of psoriasis in distal skin. Intestinal expression of sPLA₂-IIA is reduced after treatment with antibiotics or under germ-free conditions, suggesting its upregulation by gut microbiota. Metagenome, transcriptome, and metabolome analyses have revealed that sPLA₂-IIA deficiency alters the gut microbiota, accompanied by notable changes in the intestinal expression of genes related to immunity and metabolism, as well as in the levels of various blood metabolites and fecal bacterial lipids, suggesting that sPLA₂-IIA contributes to shaping of the gut microbiota. The skin phenotypes in Pla2g2a^–/– mice are lost (a) when they are cohoused with littermate WT mice, resulting in the mixing of the microbiota between the genotypes, or (b) when they are housed in a more stringent pathogen-free facility, where Pla2g2a expression in WT mice is low and the gut microbial compositions in both genotypes are nearly identical. Thus, our results highlight a potentially new aspect of sPLA₂-IIA as a modulator of gut microbiota, perturbation of which affects distal skin responses.

The interaction of secreted phospholipase A2-IIA with the microbiota alters its lipidome and promotes inflammation

Secreted phospholipase A₂-IIA (sPLA₂-IIA) hydrolyzes phospholipids to liberate lysophospholipids and fatty acids. Given its poor activity toward eukaryotic cell membranes, its role in the generation of proinflammatory lipid mediators is unclear. Conversely, sPLA₂-IIA efficiently hydrolyzes bacterial membranes. Here, we show that sPLA₂-IIA affects the immune system by acting on the intestinal microbial flora. Using mice overexpressing transgene-driven human sPLA₂-IIA, we found that the intestinal microbiota was critical for both induction of an immune phenotype and promotion of inflammatory arthritis. The expression of sPLA₂-IIA led to alterations of the intestinal microbiota composition, but housing in a more stringent pathogen-free facility revealed that its expression could affect the immune system in the absence of changes to the composition of this flora. In contrast, untargeted lipidomic analysis focusing on bacteria-derived lipid mediators revealed that sPLA₂-IIA could profoundly alter the fecal lipidome. The data suggest that a singular protein, sPLA₂-IIA, produces systemic effects on the immune system through its activity on the microbiota and its lipidome.

Group IIA secreted phospholipase A2 (PLA2G2A) augments adipose tissue thermogenesis

Group IIA secreted phospholipase A2 (PLA2G2A) hydrolyzes glycerophospholipids at the sn-2 position resulting in the release of fatty acids and lysophospholipids. C57BL/6 mice do not express Pla2g2a due to a frameshift mutation (wild-type [WT] mice). We previously reported that transgenic expression of human PLA2G2A in C57BL/6 mice (IIA+ mice) protects against weight gain and insulin resistance, in part by increasing total energy expenditure. Additionally, we found that brown and white adipocytes from IIA+ mice have increased expression of mitochondrial uncoupling markers, such as uncoupling protein 1 (UCP1), peroxisome proliferator-activated receptor-gamma coactivator, and PR domain containing 16, suggesting that the energy expenditure phenotype might be due to an increased thermogenic capacity in adipose tissue. Here, we further characterize the impact of PLA2G2A on thermogenic mechanisms in adipose tissue. Metabolic analysis of WT and IIA+ mice revealed that even when housed within their thermoneutral zone, IIA+ mice have elevated energy expenditure compared to WT littermates. Increased energy expenditure in IIA+ mice is associated with increased citrate synthase activity in brown adipose tissue (BAT) and increased mitochondrial respiration in both brown and white adipocytes. We also observed that direct addition of recombinant PLA2G2A enzyme to in vitro cultured adipocytes results in the marked induction of UCP1 protein expression. Finally, we report that PLA2G2A induces the expression of numerous transcripts related to energy substrate transport and metabolism in BAT, suggestive of an increase in substrate flux to fuel BAT activity. These data demonstrate that PLA2G2A enhances adipose tissue thermogenesis, in part, through elevated substrate delivery and increased mitochondrial content in BAT.

Role of human group IIA secreted phospholipase A2 in malaria pathophysiology: Insights from a transgenic mouse model

We previously showed that injection of recombinant human group IIA secreted phospholipase A₂ (hGIIA sPLA₂) to Plasmodium chabaudi-infected mice lowers parasitaemia by 20%. Here, we show that transgenic (TG) mice overexpressing hGIIA sPLA₂ have a peak of parasitaemia about 30% lower than WT littermates. During infection, levels of circulating sPLA₂, enzymatic activity and plasma lipid peroxidation were maximal at day-14, the peak of parasitaemia. Levels of hGIIA mRNA increased in liver but not in spleen and blood cells, suggesting that liver may contribute as a source of circulating hGIIA sPLA₂. Before infection, baseline levels of leukocytes and pro-inflammatory cytokines were higher in TG mice than WT littermates. Upon infection, the number of neutrophils, lymphocytes and monocytes increased and were maximal at the peak of parasitaemia in both WT and TG mice, but were higher in TG mice. Similarly, levels of the Th1 cytokines IFN-γ and IL-2 increased in WT and TG mice, but were 7.7- and 1.7-fold higher in TG mice. The characteristic shift towards Th2 cytokines was observed during infection in both WT and TG mice, with increased levels of IL-10 and IL-4 at day-14. The current data are in accordance with our previous in vitro findings showing that hGIIA kills parasites by releasing toxic lipids from oxidized lipoproteins. They further show that hGIIA sPLA₂ is induced during mouse experimental malaria and has a protective in vivo role, lowering parasitaemia by likely releasing toxic lipids from oxidized lipoproteins but also indirectly by promoting a more sustained innate immune response.

Macrophage polarization is linked to Ca2+-independent phospholipase A2β-derived lipids and cross-cell signaling in mice

Phospholipases A₂ (PLA₂s) catalyze hydrolysis of the sn-2 substituent from glycerophospholipids to yield a free fatty acid (i.e., arachidonic acid), which can be metabolized to pro- or anti-inflammatory eicosanoids. Macrophages modulate inflammatory responses and are affected by Ca²⁺-independent phospholipase A₂ (PLA₂)β (iPLA₂β). Here, we assessed the link between iPLA₂β-derived lipids (iDLs) and macrophage polarization. Macrophages from WT and KO (iPLA₂β-/-) mice were classically M1 pro-inflammatory phenotype activated or alternatively M2 anti-inflammatory phenotype activated, and eicosanoid production was determined by ultra-performance LC ESI-MS/MS. As a genotypic control, we performed similar analyses on macrophages from RIP.iPLA₂β.Tg mice with selective iPLA₂β overexpression in β-cells. Compared with WT, generation of select pro-inflammatory prostaglandins (PGs) was lower in iPLA₂β^-/- , and that of a specialized pro-resolving lipid mediator (SPM), resolvin D2, was higher; both changes are consistent with the M2 phenotype. Conversely, macrophages from RIP.iPLA₂β.Tg mice exhibited an opposite landscape, one associated with the M1 phenotype: namely, increased production of pro-inflammatory eicosanoids (6-keto PGF₁α, PGE₂, leukotriene B₄) and decreased ability to generate resolvin D2. These changes were not linked with secretory PLA₂ or cytosolic PLA₂α or with leakage of the transgene. Thus, we report previously unidentified links between select iPLA₂β-derived eicosanoids, an SPM, and macrophage polarization. Importantly, our findings reveal for the first time that β-cell iPLA₂β-derived signaling can predispose macrophage responses. These findings suggest that iDLs play critical roles in macrophage polarization, and we posit that they could be targeted therapeutically to counter inflammation-based disorders.

Pancreatic and mucosal enzymes in choline phospholipid digestion

The digestion of choline phospholipids is important for choline homeostasis, lipid signaling, postprandial lipid and energy metabolism, and interaction with intestinal bacteria. The digestion is mediated by the combined action of pancreatic and mucosal enzymes. In the proximal small intestine, hydrolysis of phosphatidylcholine (PC) to 1-lyso-PC and free fatty acid (FFA) by the pancreatic phospholipase A₂ IB coincides with the digestion of the dietary triacylglycerols by lipases, but part of the PC digestion is extended and must be mediated by other enzymes as the jejunoileal brush-border phospholipase B/lipase and mucosal secreted phospholipase A₂ X. Absorbed 1-lyso-PC is partitioned in the mucosal cells between degradation and reacylation into chyle PC. Reutilization of choline for hepatic bile PC synthesis, and the reacylation of 1-lyso-PC into chylomicron PC by the lyso-PC-acyl-CoA-acyltransferase 3 are important features of choline recycling and postprandial lipid metabolism. The role of mucosal enzymes is emphasized by sphingomyelin (SM) being sequentially hydrolyzed by brush-border alkaline sphingomyelinase (alk-SMase) and neutral ceramidase to sphingosine and FFA, which are well absorbed. Ceramide and sphingosine-1-phosphate are generated and are both metabolic intermediates and important lipid messengers. Alk-SMase has anti-inflammatory effects that counteract gut inflammation and tumorigenesis. These may be mediated by multiple mechanisms including generation of sphingolipid metabolites and suppression of autotaxin induction and lyso-phosphatidic acid formation. Here we summarize current knowledge on the roles of pancreatic and mucosal enzymes in PC and SM digestion, and its implications in intestinal and liver diseases, bacterial choline metabolism in the gut, and cholesterol absorption.

Epithelial-Cell-Derived Phospholipase A2 Group 1B Is an Endogenous Anthelmintic

Immunity to intestinal helminth infections has been well studied, but the mechanism of helminth killing prior to expulsion remains unclear. Here we identify epithelial-cell-derived phospholipase A₂ group 1B (PLA₂g1B) as a host-derived endogenous anthelmintic. PLA₂g1B is elevated in resistant mice and is responsible for killing tissue-embedded larvae. Despite comparable activities of other essential type-2-dependent immune mechanisms, Pla2g1b^−/− mice failed to expel the intestinal helminths Heligmosomoides polygyrus or Nippostrongylus brasiliensis. Expression of Pla2g1b by epithelial cells was dependent upon intestinal microbiota, adaptive immunity, and common-gamma chain-dependent signaling. Notably, Pla2g1b was downregulated in susceptible mice and inhibited by IL-4R-signaling in vitro, uncoupling parasite killing from expulsion mechanisms. Resistance was restored in Pla2g1b^−/− mice by treating infective H. polygyrus L3 larvae with PLA₂g1B, which reduced larval phospholipid abundance. These findings uncover epithelial-cell-derived Pla2g1b as an essential mediator of helminth killing, highlighting a previously overlooked mechanism of anti-helminth immunity.

•

Pla2g1b expression correlated with resistance to intestinal helminth infection

•

PLA₂g1B is essential for resistance to intestinal helminth infection in mice

•

PLA₂g1B directly reduces phospholipid abundance in infective larvae

•

Pla2g1b is expressed by epithelial cells and is negatively regulated by IL-4Rα

Preparation of the Full Set of Recombinant Mouse- and Human-Secreted Phospholipases A2

A family of 14-20kDa, disulfide-rich, calcium-dependent secreted phospholipases A₂ (sPLA₂s) that release fatty acids from the sn-2 position of glycerophospholipids can be found in mammals. They have a diverse array of tissue distribution and biological functions. In this chapter we provide detailed protocols for production of nearly all of the mouse and human sPLA₂s mainly by expression in bacteria and in vitro refolding or by expression in insect cells. High-resolution mass spectrometry and enzymatic assays were, respectively, used to show that all disulfides are formed and that the enzymes are active, strongly suggesting that each sPLA₂ was prepared in the structurally native form. The availability of these proteins has allowed kinetic studies to be carried out, to prepare highly selective antisera, to screen for selective inhibitors, to study receptor binding, and to study the action of each enzyme on mammalian cell membranes and their in vivo biological roles.

Secreted Phospholipases A2 Are Intestinal Stem Cell Niche Factors with Distinct Roles in Homeostasis, Inflammation, and Cancer

The intestinal stem cell niche provides cues that actively maintain gut homeostasis. Dysregulation of these cues may compromise intestinal regeneration upon tissue insult and/or promote tumor growth. Here, we identify secreted phospholipases A2 (sPLA2s) as stem cell niche factors with context-dependent functions in the digestive tract. We show that group IIA sPLA2, a known genetic modifier of mouse intestinal tumorigenesis, is expressed by Paneth cells in the small intestine, while group X sPLA2 is expressed by Paneth/goblet-like cells in the colon. During homeostasis, group IIA/X sPLA2s inhibit Wnt signaling through intracellular activation of Yap1. However, upon inflammation they are secreted into the intestinal lumen, where they promote prostaglandin synthesis and Wnt signaling. Genetic ablation of both sPLA2s improves recovery from inflammation but increases colon cancer susceptibility due to release of their homeostatic Wnt-inhibitory role. This "trade-off" effect suggests sPLA2s have important functions as genetic modifiers of inflammation and colon cancer.

Identification of a Novel Heart-Liver Axis: Matrix Metalloproteinase-2 Negatively Regulates Cardiac Secreted Phospholipase A2 to Modulate Lipid Metabolism and Inflammation in the Liver

BACKGROUND

Endocrine functions of the heart have been well established. We investigated the hypothesis that cardiac secretion of a unique phospholipase A2 recently identified by our laboratory (cardiac secreted phospholipase A2 [sPLA2]) establishes a heart-liver endocrine axis that is negatively regulated by matrix metalloproteinase-2 (MMP-2).

METHODS AND RESULTS

In Mmp2(-/-) mice, cardiac (but not hepatic) sPLA2 was elevated, leading to hepatic inflammation, immune cell infiltration, dysregulation of the sterol regulatory element binding protein-2 and liver X receptor-α pathways, abnormal transcriptional responses to dietary cholesterol, and elevated triglycerides in very low-density lipoprotein and in the liver. Expression of monocyte chemoattractant protein-3, a known MMP-2 substrate, was elevated at both mRNA and protein levels in the heart. Functional studies including in vivo antibody neutralization identified cardiac monocyte chemoattractant protein 3 as a possible agonist of cardiac sPLA2 secretion. Conversely, systemic sPLA2 inhibition almost fully normalized the cardiohepatic phenotype without affecting monocyte chemoattractant protein-3. Finally, wild-type mice that received high-performance liquid chromatography-isolated cardiac sPLA2 from Mmp2(-/-) donors developed a cardiohepatic gene expression profile similar to that of Mmp2(-/-) mice.

CONCLUSIONS

These findings identified the novel MMP-2/cardiac sPLA2 pathway that endows the heart with important endocrine functions, including regulation of inflammation and lipid metabolism in the liver. Our findings could also help explain how MMP2 deficiency leads to cardiac problems, inflammation, and metabolic dysregulation in patients.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Study of the role of cytosolic phospholipase A2 alpha in eicosanoid generation and thymocyte maturation in the thymus

The thymus is a primary lymphoid organ, home of maturation and selection of thymocytes for generation of functional T-cells. Multiple factors are involved throughout the different stages of the maturation process to tightly regulate T-cell production. The metabolism of arachidonic acid by cyclooxygenases, lipoxygenases and specific isomerases generates eicosanoids, lipid mediators capable of triggering cellular responses. In this study, we determined the profile of expression of the eicosanoids present in the mouse thymus at different stages of thymocyte development. As the group IVA cytosolic phospholipase A₂ (cPLA₂α) catalyzes the hydrolysis of phospholipids, thereby generating arachidonic acid, we further verified its contribution by including cPLA₂α deficient mice to our investigations. We found that a vast array of eicosanoids is expressed in the thymus, which expression is substantially modulated through thymocyte development. The cPLA₂α was dispensable in the generation of most eicosanoids in the thymus and consistently, the ablation of the cPLA₂α gene in mouse thymus and the culture of thymuses from human newborns in presence of the cPLA₂α inhibitor pyrrophenone did not impact thymocyte maturation. This study provides information on the eicosanoid repertoire present during thymocyte development and suggests that thymocyte maturation can occur independently of cPLA₂α.

On the role of protein disulfide isomerase in the retrograde cell transport of secreted phospholipases A2

Following the finding that ammodytoxin (Atx), a neurotoxic secreted phospholipase A₂ (sPLA₂) in snake venom, binds specifically to protein disulfide isomerase (PDI) in vitro we show that these proteins also interact in living rat PC12 cells that are able to internalize this group IIA (GIIA) sPLA₂. Atx and PDI co-localize in both differentiated and non-differentiated PC12 cells, as shown by fluorescence microscopy. Based on a model of the complex between Atx and yeast PDI (yPDI), a three-dimensional model of the complex between Atx and human PDI (hPDI) was constructed. The Atx binding site on hPDI is situated between domains b and b’. Atx interacts hPDI with an extensive area on its interfacial binding surface. The mammalian GIB, GIIA, GV and GX sPLA₂s have the same fold as Atx. The first three sPLA₂s have been detected intracellularly but not the last one. The models of their complexes with hPDI were constructed by replacement of Atx with the respective mammalian sPLA₂ in the Atx—hPDI complex and molecular docking of the structures. According to the generated models, mammalian GIB, GIIA and GV sPLA₂s form complexes with hPDI very similar to that with Atx. The contact area between GX sPLA₂ and hPDI is however different from that of the other sPLA₂s. Heterologous competition of Atx binding to hPDI with GV and GX sPLA₂s confirmed the model-based expectation that GV sPLA₂ was a more effective inhibitor than GX sPLA₂, thus validating our model. The results suggest a role of hPDI in the (patho)physiology of some snake venom and mammalian sPLA₂s by assisting the retrograde transport of these molecules from the cell surface. The sPLA₂–hPDI model constitutes a valuable tool to facilitate further insights into this process and into the (patho)physiology of sPLA₂s in relation to their action intracellularly.

Detection and quantification of microparticles from different cellular lineages using flow cytometry. Evaluation of the impact of secreted phospholipase A2 on microparticle assessment

Microparticles, also called microvesicles, are submicron extracellular vesicles produced by plasma membrane budding and shedding recognized as key actors in numerous physio(patho)logical processes. Since they can be released by virtually any cell lineages and are retrieved in biological fluids, microparticles appear as potent biomarkers. However, the small dimensions of microparticles and soluble factors present in body fluids can considerably impede their quantification. Here, flow cytometry with improved methodology for microparticle resolution was used to detect microparticles of human and mouse species generated from platelets, red blood cells, endothelial cells, apoptotic thymocytes and cells from the male reproductive tract. A family of soluble proteins, the secreted phospholipases A₂ (sPLA₂), comprises enzymes concomitantly expressed with microparticles in biological fluids and that catalyze the hydrolysis of membrane phospholipids. As sPLA₂ can hydrolyze phosphatidylserine, a phospholipid frequently used to assess microparticles, and might even clear microparticles, we further considered the impact of relevant sPLA2 enzymes, sPLA2 group IIA, V and X, on microparticle quantification. We observed that if enriched in fluids, certain sPLA₂ enzymes impair the quantification of microparticles depending on the species studied, the source of microparticles and the means of detection employed (surface phosphatidylserine or protein antigen detection). This study provides analytical considerations for appropriate interpretation of microparticle cytofluorometric measurements in biological samples containing sPLA2 enzymes.

Pseudomonas aeruginosa eradicates Staphylococcus aureus by manipulating the host immunity

Young cystic fibrosis (CF) patients' airways are mainly colonized by Staphylococcus aureus, while Pseudomonas aeruginosa predominates in adults. However, the mechanisms behind this infection switch are unclear. Here, we show that levels of type-IIA-secreted phospholipase A2 (sPLA2-IIA, a host enzyme with bactericidal activity) increase in expectorations of CF patients in an age-dependent manner. These levels are sufficient to kill S. aureus, with marginal effects on P. aeruginosa strains. P. aeruginosa laboratory strains and isolates from CF patients induce sPLA2-IIA expression in bronchial epithelial cells from CF patients (these cells are a major source of the enzyme). In an animal model of lung infection, P. aeruginosa induces sPLA2-IIA production that favours S. aureus killing. We suggest that sPLA2-IIA induction by P. aeruginosa contributes to S. aureus eradication in CF airways. Our results indicate that a bacterium can eradicate another bacterium by manipulating the host immunity.

Group X secreted phospholipase A(2) induces lipid droplet formation and prolongs breast cancer cell survival

Background

Alterations in lipid metabolism are inherent to the metabolic transformations that support tumorigenesis. The relationship between the synthesis, storage and use of lipids and their importance in cancer is poorly understood. The human group X secreted phospholipase A₂ (hGX sPLA₂) releases fatty acids (FAs) from cell membranes and lipoproteins, but its involvement in the regulation of cellular FA metabolism and cancer is not known.

Results

Here we demonstrate that hGX sPLA₂ induces lipid droplet (LD) formation in invasive breast cancer cells, stimulates their proliferation and prevents their death on serum deprivation. The effects of hGX sPLA₂ are shown to be dependent on its enzymatic activity, are mimicked by oleic acid and include activation of protein kinase B/Akt, a cell survival signaling kinase. The hGX sPLA₂-stimulated LD biogenesis is accompanied by AMP-activated protein kinase (AMPK) activation, up-regulation of FA oxidation enzymes and the LD-coating protein perilipin 2, and suppression of lipogenic gene expression. Prolonged activation of AMPK inhibited hGX sPLA₂-induced LD formation, while etomoxir, an inhibitor of FA oxidation, abrogated both LD formation and cell survival. The hGX sPLA₂-induced changes in lipid metabolism provide a minimal immediate proliferative advantage during growth under optimal conditions, but they confer to the breast cancer cells a sustained ability to resist apoptosis during nutrient and growth factor limitation.

Conclusion

Our results identify hGX sPLA₂ as a novel modulator of lipid metabolism that promotes breast cancer cell growth and survival by stimulating LD formation and FA oxidation.

Group X Secreted Phospholipase A2 Limits the Development of Atherosclerosis in LDL Receptor–Null Mice

Objective—

Several secreted phospholipases A2 (sPLA2s), including group IIA, III, V, and X, have been linked to the development of atherosclerosis, which led to the clinical testing of A-002 (varespladib), a broad sPLA2 inhibitor for the treatment of coronary artery disease. Group X sPLA2 (PLA2G10) has the most potent hydrolyzing activity toward phosphatidylcholine and is believed to play a proatherogenic role.

Methods and Results—

Here, we show that Ldlr –/– mice reconstituted with bone marrow from mouse group X–deficient mice ( Pla2g10 –/– ) unexpectedly display a doubling of plaque size compared with Pla2g10 +/+ chimeric mice. Macrophages of Pla2g10 –/– mice are more susceptible to apoptosis in vitro, which is associated with a 4-fold increase of plaque necrotic core in vivo. In addition, chimeric Pla2g10 –/– mice show exaggerated T lymphocyte (Th)1 immune response, associated with enhanced T-cell infiltration in atherosclerotic plaques. Interestingly, overexpression of human PLA2G10 in murine bone marrow cells leads to significant reduction of Th1 response and to 50% reduction of lesion size.

Conclusion—

PLA2G10 expression in bone marrow cells controls a proatherogenic Th1 response and limits the development of atherosclerosis. The results may provide an explanation for the recently reported inefficacy of A-002 (varespladib) to treat patients with coronary artery disease. Indeed, A-002 is a nonselective sPLA2 inhibitor that inhibits both proatherogenic (groups IIA and V) and antiatherogenic (group X) sPLA2s. Our results suggest that selective targeting of individual sPLA2 enzymes may be a better strategy to treat cardiovascular diseases.

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.

Drastic changes in the tissue-specific expression of secreted phospholipases A2 in chicken pulmonary disease

Infectious bronchitis is one of the most important diseases in poultry and it causes major economic losses. Infectious bronchitis is an acute, highly contagious, viral disease of chickens, characterized by rales, coughing, and sneezing.

Because secreted phospholipases A2 (sPLA2) are involved in inflammatory processes, the gene expressions of sPLA2s were investigated in both healthy chickens and chickens with infectious bronchitis and lung inflammation. The draft chicken genome was first scanned using human sPLA2 sequences to identify chicken sPLA2s (ChPLA2), chicken total mRNA were isolated and RT-PCR experiments were performed to amplify and then sequence orthologous cDNAs. Full-length cDNA sequences of ChPLA2-IB, -IIA, -IIE, -V and -X were cloned. The high degree of sequence identity of 50–70% between the avian and mammalian (human and mouse) sPLA2 orthologs suggests a conservation of important enzymatic functions for these phospholipases.

Quantitation by qPCR of the transcript levels of ChPLA2-IB, -IIA, -IIE, -V and -X in several tissues from healthy chicken indicated that the expression patterns and mRNA levels diverged among the phospholipases tested. In chicken with infectious bronchitis, an over expression of ChPLA2-V was observed in lungs and spleen in comparison with healthy chicken. These findings suggest that ChPLA2-V could be a potential biomarker for lung inflammation. Conversely, a down regulation of ChPLA2-IB, -IIA and -X was observed in lungs and spleen in case of infectious bronchitis. A significant increase in the expression level of ChPLA2-X and ChPLA2-IB was also noticed in pancreas. No or minor changes have been detected in the expression of ChPLA2-IIE in lungs and small intestine, but it shows a significant increase in several infected tissues.

Physiological roles of group X-secreted phospholipase A2 in reproduction, gastrointestinal phospholipid digestion, and neuronal function

Although the secreted phospholipase A(2) (sPLA(2)) family has been generally thought to participate in pathologic events such as inflammation and atherosclerosis, relatively high and constitutive expression of group X sPLA(2) (sPLA(2)-X) in restricted sites such as reproductive organs, the gastrointestinal tract, and peripheral neurons raises a question as to the roles played by this enzyme in the physiology of reproduction, digestion, and the nervous system. Herein we used mice with gene disruption or transgenic overexpression of sPLA(2)-X to clarify the homeostatic functions of this enzyme at these locations. Our results suggest that sPLA(2)-X regulates 1) the fertility of spermatozoa, not oocytes, beyond the step of flagellar motility, 2) gastrointestinal phospholipid digestion, perturbation of which is eventually linked to delayed onset of a lean phenotype with reduced adiposity, decreased plasma leptin, and improved muscle insulin tolerance, and 3) neuritogenesis of dorsal root ganglia and the duration of peripheral pain nociception. Thus, besides its inflammatory action proposed previously, sPLA(2)-X participates in physiologic processes including male fertility, gastrointestinal phospholipid digestion linked to adiposity, and neuronal outgrowth and sensing.

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.

Gastroduodenal reflux induces group IIa secretory phospholipase A(2) expression and activity in murine esophagus

Exposure of esophageal epithelium to gastric and duodenal contents results in the histologic changes of hyperproliferation and mucosal thickening. We have previously shown that presence of secretory phospholipase A(2) (sPLA(2)) is necessary to produce these histologic changes in a murine model of gastroduodenal reflux. We sought to determine the influence of gastroduodenal reflux (GDR) on sPLA(2) protein and mRNA levels as well as enzyme activity in esophageal tissue. BALB/c (sPLA(2)(+/+)) mice (n= 28) underwent side-to-side surgical anastomosis of the first portion of the duodenum and GE junction (DGEA) resulting in continuous exposure of esophageal mucosa to mixed gastric and duodenal contents. Sham control mice (n= 14) underwent laparotomy, esophagotomy and closure. Real-time RT PCR was used to quantitate the influence of GDR on group IIa sPLA(2) expression. Immunofluorescent staining was quantitated by digital microscopy using a specific antibody to identify and locate sPLA(2) protein. A colorimetric assay was used to quantify total sPLA(2) activity after standardization of protein levels. Statistical analysis was conducted using Student's t-test. Group IIa sPLA(2) mRNA and protein levels were increased at 4 and 8 weeks compared with sham controls. This increase occurred in a time-dependent manner and correlated with esophageal mucosal thickness. Furthermore, sPLA(2) enzyme activity was increased significantly at 4 and 8 weeks compared with untreated controls. The expression of group IIa sPLA(2) as well as sPLA(2) activity is induced by GDR. This novel finding indicates that sPLA(2) may play a role in the development of the histologic changes produced by GDR in esophageal mucosa.

Group X phospholipase A2 is released during sperm acrosome reaction and controls fertility outcome in mice

Ejaculated mammalian sperm must undergo a maturation process called capacitation before they are able to fertilize an egg. Several studies have suggested a role for members of the secreted phospholipase A2 (sPLA2) family in capacitation, acrosome reaction (AR), and fertilization, but the molecular nature of these enzymes and their specific roles have remained elusive. Here, we have demonstrated that mouse group X sPLA2 (mGX) is the major enzyme present in the acrosome of spermatozoa and that it is released in an active form during capacitation through spontaneous AR. mGX-deficient male mice produced smaller litters than wild-type male siblings when crossed with mGX-deficient females. Further analysis revealed that spermatozoa from mGX-deficient mice exhibited lower rates of spontaneous AR and that this was associated with decreased in vitro fertilization (IVF) efficiency due to a drop in the fertilization potential of the sperm and an increased rate of aborted embryos. Treatment of sperm with sPLA2 inhibitors and antibodies specific for mGX blocked spontaneous AR of wild-type sperm and reduced IVF success. Addition of lysophosphatidylcholine, a catalytic product of mGX, overcame these deficiencies. Finally, recombinant mGX triggered AR and improved IVF outcome. Taken together, our results highlight a paracrine role for mGX during capacitation in which the enzyme primes sperm for efficient fertilization and boosts premature AR of a likely phospholipid-damaged sperm subpopulation to eliminate suboptimal sperm from the pool available for fertilization.

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.

Structural basis of the significant calmodulin-induced increase in the enzymatic activity of secreted phospholipases A(2)

Ammodytoxin (Atx), a neurotoxic secreted phospholipase A(2) (sPLA(2)), forms a high-affinity complex with calmodulin (CaM). The latter substantially increases the enzymatic activity of Atx under both non-reducing and reducing conditions, and the activity enhancement was accompanied, but not caused, by conformational stabilization of the enzyme. In this work, the energetically most favorable model of the complex was generated, making use of interaction site mapping, mutagenesis data and protein-docking algorithms. The model explains, in structural terms, the observed effects of stabilization and activity enhancement of the neurotoxic sPLA(2) by CaM. The structures of four mammalian sPLA(2) isoforms, groups IB, IIA, V and X, having the same fold as Atx, were superimposed on the structure of Atx in the complex with CaM. According to the generated models, the group V and X sPLA(2)s, but not the group IB and IIA enzymes, form stable complexes with CaM, which should also result in the augmentation of their enzymatic activity. By confirming the latter, the presented model is validated as a valuable tool to investigate the as yet unexplained role of CaM in the pathophysiology of snake venom and mammalian sPLA(2)s.

Group X phospholipase A2 stimulates the proliferation of colon cancer cells by producing various lipid mediators

Among mammalian secreted phospholipases A2 (sPLA(2)s), the group X enzyme has the most potent hydrolyzing capacity toward phosphatidylcholine, the major phospholipid of cell membrane and lipoproteins. This enzyme has recently been implicated in chronic inflammatory diseases such as atherosclerosis and asthma and may also play a role in colon tumorigenesis. We show here that group X sPLA(2) [mouse (m)GX] is one of the most highly expressed PLA(2) in the mouse colon and that recombinant mouse and human enzymes stimulate proliferation and mitogen-activated protein kinase activation of various colon cell lines, including Colon-26 cancer cells. Among various recombinant sPLA(2)s, mGX is the most potent enzyme to stimulate cell proliferation. Based on the use of sPLA(2) inhibitors, catalytic site mutants, and small interfering RNA silencing of cytosolic PLA(2)alpha and M-type sPLA(2) receptor, we demonstrate that mGX promotes cell proliferation independently of the receptor and via its intrinsic catalytic activity and production of free arachidonic acid and lysophospholipids, which are mitogenic by themselves. mGX can also elicit the production of large amounts of prostaglandin E2 and other eicosanoids from Colon-26 cells, but these lipid mediators do not play a role in mGX-induced cell proliferation because inhibitors of cyclooxygenases and lipoxygenases do not prevent sPLA(2) mitogenic effects. Together, our results indicate that group X sPLA(2) may play an important role in colon tumorigenesis by promoting cancer cell proliferation and releasing various lipid mediators involved in other key events in cancer progression.

Secretory phospholipase A2-IID is an effector molecule of CD4+CD25+ regulatory T cells

Suppression by natural CD4(+)CD25(+) regulatory T cells (Tregs) is one mechanism by which tolerance is maintained. However, the way in which Tregs mediate suppression is not well understood. Here, we show that secreted phospholipase A2 (sPLA2)-IID is selectively produced by Tregs. sPLA2-IID is a potent mediator of Treg function, because it strongly suppressed proliferation of CD4(+) and CD8(+) T cells in vitro and in vivo in a manner independent of its catalytic activity. Furthermore, sPLA2-IID promoted the differentiation of Tregs, presumably via attenuating signaling through the PI3K/Akt/mammalian target of rapamycin pathway. Importantly, administration of a sPLA2-IID-Fc fusion protein inhibited disease development in murine models of colitis and multiple sclerosis, suggesting that sPLA2-IID's immunosuppressive function might be exploited therapeutically.

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).

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.

Role of secretory phospholipases in atherogenesis

Elevated circulating levels of secretory phospholipase A(2) (sPLA(2)) are associated with atherosclerotic cardiovascular disease. sPLA(2) can contribute to atherogenesis by hydrolyzing phospholipids of circulating lipoproteins and lipoproteins entrapped in the arterial wall and/or in cells that reside in the intima and that participate in the inflammatory response to lipoprotein deposition. This article reviews differences and similarities between sPLA(2)-IIA, sPLA(2)-V, and sPLA(2)-X, all of which are members of this family of enzymes with reported potential proatherogenic features. Published data suggest that each of the enzymes has a distinct profile characterized by differences in tissue expression and localization, capacity to act on phospholipids of cell membranes and lipoproteins, and their interaction with arterial proteoglycans. In addition, the article discusses results from the authors' laboratory showing that diet-induced or gene-induced hyperlipidemia in mice enhances the expression of sPLA(2)-V in different tissues, but not sPLA(2)-IIA. Such differences indicate that these enzymes may have different roles in atherosclerotic cardiovascular disease through their distinct profiles.

Secretory phospholipase A2 is required to produce histologic changes associated with gastroduodenal reflux in a murine model

OBJECTIVE

The earliest response of esophageal mucosa to gastric reflux is the development of oxidative damage and inflammation. These processes contribute to the development of metaplasia known as Barrett's esophagus, as well as the progression to malignancy. Secretory phospholipase A(2) is a mediator of inflammation with levels that are increased in Barrett's metaplasia and carcinoma when compared with levels in normal samples. Our goal is to determine the role of secretory phospholipase A(2) in the development of reflux-associated changes in the esophageal mucosa.

METHODS

Secretory phospholipase A(2)-deficient mice (C57BL/6, n = 5) and mice known to express high levels of secretory phospholipase A(2) (BALB/c, n = 5) underwent side-to-side surgical anastomosis of the first portion of the duodenum and gastroesophageal junction, allowing exposure of esophageal mucosa to duodenal and gastric contents duodeno-gastroesophageal anastomosis. Control animals (n = 5) of each strain underwent laparotomy with esophagotomy and repair. Tissue was frozen in embedding medium. Hematoxylin and eosin staining and Ki67 and secretory phospholipase A(2) immunohistochemistry were used to evaluate esophageal tissue and its response to duodeno-gastroesophageal anastomosis.

RESULTS

Immunofluorescent staining confirmed the absence of secretory phospholipase A(2) in C57BL/6 mice and its presence in BALB/c mice. Hematoxylin and eosin staining demonstrated significant thickening of the esophageal mucosa in response to gastroesophageal reflux in the presence of secretory phospholipase A(2). Mice known to express high levels of secretory phospholipase A(2) also demonstrated increased numbers of proliferating cells. Secretory phospholipase A(2)-deficient mice were immune to the early changes induced by mixed reflux.

CONCLUSIONS

The presence of secretory phospholipase A(2) appears necessary for early histologic changes produced by exposure of the esophagus to gastroduodenal contents. This enzyme is identified as a promising target for evaluation of mechanisms of carcinogenesis and chemoprevention of esophageal carcinoma.