Cytosolic phospholipase A2-alpha is necessary for platelet-activating factor biosynthesis, efficient neutrophil-mediated bacterial killing, and the innate immune response to pulmonary infection: cPLA2-alpha does not regulate neutrophil NADPH oxidase activity

IRGM/Irgm1 deficiency inhibits neutrophil-platelet interactions and thrombosis in experimental atherosclerosis and arterial injury

BACKGROUND

Neutrophil extracellular traps (NETs) closely link inflammation and thrombosis. The immune-related GTPase family M protein (IRGM) and its ortholog of mouse IRGM1 are positively correlated with plaque rupture during atherosclerosis process. However, whether and how IRGM/IRGM1 affects NETs formation and atherosclerotic thrombosis remains unknown, which will further promote the development of antithrombotic treatment tools.

METHODS

The thrombi images, platelet activation makers and NETs makers were detected in the serum of STEMI patients and controls. To futher investigate IRGM/IRGM1 affects NETs formation and atherothrombosis in vivo, ApoE^-/-Irgm1^+/- and ApoE^-/- mice received diets rich in fat and 2.5% FeCl₃ was then used to induce experimental arterial thrombosis in an atherosclerosis background. In vitro, PMA and thrombin were used to stimulate neutrophils and platelets, respectively, and the expression of IRGM/IRGM1 were modified. To reveal the molecular mechanisms, MAPK-cPLA2 signals inhibitors were used.

RESULTS

Serum IRGM was positively correlated with PF4 and neutrophil elastase. Subsequently, Irgm1 deficient mice have a longer occlusion time and lower growth rate. In vitro, as expected, IRGM/Irgm1 deficiency inhibits platelet activation and platelet-neutrophil interaction. More importantly, IRGM promoted NETs production through activating MAPK-cPLA2 signals in PMA stimulated neuropils, whereas inhibiting the production of NETs eliminated the difference in platelet activation and thrombosis caused by IRGM/Irgm1 modification in vivo and vitro. Similarly, inhibition of platelet activation also eliminated the influence of IRGM/Irgm1 modification on NETs production.

CONCLUSIONS

Overall, our data indicate that IRGM/Irgm1 deficiency in neuropils inhibits the intense interaction between neutrophils and platelets, and ultimately inhibits thrombosis.

Characterization of the binding of cytosolic phospholipase A2 alpha and NOX2 NADPH oxidase in mouse macrophages

BACKGROUND

Previous studies have demonstrated that cytosolic phospholipase A₂α (cPLA₂α) is required for NOX2 NADPH oxidase activation in human and mouse phagocytes. Moreover, upon stimulation, cPLA₂α translocates to the plasma membranes by binding to the assembled oxidase, forming a complex between its C2 domain and the PX domain of the cytosolic oxidase factor, p47^phox in human phagocytes. Intravenous administration of antisense against cPLA₂α that significantly inhibited its expression in mouse peritoneal neutrophils and macrophages also inhibited superoxide production, in contrast to cPLA₂α knockout mice that showed normal superoxide production. The present study aimed to determine whether there is a binding between cPLA₂α-C2 domain and p47^phox-PX in mouse macrophages, to further support the role of cPLA₂α in oxidase regulation also in mouse phagocytes.

METHODS AND RESULTS

A significant binding of mouse GST-p47^phox-PX domain fusion protein and cPLA₂α in stimulated mouse phagocyte membranes was demonstrated by pull-down experiments, although lower than that detected by the human p47^phox-PX domain. Substituting the amino acids Phe98, Asn99, and Gly100 to Cys98, Ser99, and Thr100 in the mouse p47^phox-PX domain (present in the human p47^phox-PX domain) caused strong binding that was similar to that detected by the human p47^phox-PX domain CONCLUSIONS: The binding between cPLA₂α-C2 and p47^phox-PX domains exists in mouse macrophages and is not unique to human phagocytes. The binding between the two proteins is lower in the mice, probably due to the absence of amino acids Cys98, Ser 99, and Thr100in the p47^phox-PX domain that facilitate the binding to cPLA₂α.

Pegbovigrastim Treatment around Parturition Enhances Postpartum Immune Response Gene Network Expression of whole Blood Leukocytes in Holstein and Simmental Cows

Simple Summary

The innate and adaptive immune system of dairy cows is impaired during the transition period, leading to an increase in susceptibility to infectious disease. Pegbovigrastim is a recombinant form of a granulocyte colony-stimulating factor that stimulates differentiation of hemopoietic stem cells to granulocytes and shortens maturation time within the bone marrow and release in circulation. The objective of the present study was to explore the effect of pegbovigrastim on whole blood leukocytes by analyzing the expression of 34 genes involved in immune and inflammatory responses immediately after calving in Simmental, a dual-purpose cow breed selected for both meat and milk production, and Holstein, a cow breed highly specialized for milk production. This study provides insight into immune cell functions impacted by pegbovigrastim treatment. Treatment of cows with pegbovigrastim increased the mRNA abundance level of most genes investigated, suggesting a thorough activation of the immune machinery during the critical post-partum period.

Abstract

Pegbovigrastim is a commercial long-acting analog of bovine granulocyte colony-stimulating factor (rbG-CSF) that promotes the increased count and functionality of polymorphonuclear cells in dairy cows around the time of parturition. We hypothesized that pegbovigrastim administered to periparturient cows at approximately seven days before parturition and within 24 hours after calving could affect the profiles of gene networks involved in leukocyte function. Blood was collected on Day 3 after calving from treated groups (pegbovigrastim (PEG); 13 Simmental (seven multiparous and six primiparous) and 13 Holstein (seven multiparous and six primiparous) cows) that received pegbovigrastim (Imrestor; Elanco Animal Health) and controls (CTR; 13 Simmental (seven multiparous and six primiparous) and 13 Holstein (six multiparous and seven primiparous) cows) that received saline solution. Blood from all cows was sampled from the jugular vein in a PAXgene Blood RNA System tube (Preanalytix, Hombrechtikon, Switzerland) for RNA extraction. The RT-qPCR analysis was performed to investigate a panel of 34 genes of interest, representing recognition, immune mediation, migration, cell adhesion, antimicrobial strategies, inflammatory cascade, oxidative pattern, and leukotrienes in whole blood leukocytes. Normalized data were subjected to the MIXED model of SAS (ver. 9.4) with treatment, breed, parity, and their interaction as fixed effects. Compared with CTR, whole blood leukocytes of PEG cows had higher expression of genes involved in recognition and immune modulation (CD14, CD16, MYD88, TLR2, and TLR4), cell adhesion (ITGB2, ITGAL, TLN1, SELL, SELPLG, and CD44), antimicrobial activity (MMP9, LTF, and LCN2), and inflammatory cascade (CASP1, TNFRSF1A, IL1B, IL1R, IL18, IRAK1, NLRP3, and S100A8). This suggested an improvement of migration, adhesion, and antimicrobial ability and an enhanced inflammatory response, which in turn could trigger immune cell activation and enhance function. Expression of SOD2 and ALOX5 was also greater in the PEG group. In contrast, compared with CTR cows, PEG led to lower expression of RPL13A, ALOX15, IL8, and TNF. Overall, leukocytes from Simmental compared with Holstein cows had greater expression of IDO1, RPL13A, ALOX5, CD44, CX3CR1, ITGB2, and TNFA, whereas expression of CD16 and TLR2 was lower. Overall, compared with multiparous cows, primiparous cows had higher expression of IL1B, IL18, MYD88, SELL, and TLR2 and lower expression of MMP9. Simmental cows seemed more sensitive to induction of the immune system after calving, as revealed by the greater abundance of genes involved in immune system adaptation, regardless of pegbovigrastim treatment. Primiparous cows undergoing a new stress condition with respect to older cows were characterized by leukocytes with a higher inflammatory response. In conclusion, pegbovigrastim led to higher expression levels of most genes involved in the processes investigated, suggesting a thorough activation of the immune machinery during the critical post-partum period.

Intersecting Stories of the Phagocyte NADPH Oxidase and Chronic Granulomatous Disease

Neutrophils serve as the circulating cells that respond early and figure prominently in human host defense to infection and in inflammation in other settings. Optimal oxidant-dependent antimicrobial activity by neutrophils relies on the ability of stimulated phagocytes to utilize a multicomponent NADPH oxidase to generate oxidants. The frequent, severe, and often fatal infections experienced by individuals with chronic granulomatous disease (CGD), an inherited disorder in which one of the NADPH oxidase components is absent or dysfunctional, underscore the link between a functional phagocyte NADPH oxidase and robust host protection against microbial infection.The history of the discovery and characterization of the normal neutrophil NADPH oxidase and the saga of recognizing CGD and its underlying causes together illustrate how the observations of astute clinicians and imaginative basic scientists synergize to forge new understanding of both basic cell biology and pathogenesis of human disease.In this chapter, we review the events in the stepwise evolution of our understanding of the phagocyte NADPH oxidase, both in the context of normal human neutrophil function and in the setting of CGD. The phagocyte oxidase complex employs a heterodimeric transmembrane protein composed of gp91phox and p22phox to relay electrons from NADPH to molecular oxygen, while other cofactors contribute to localization and regulation of the activity of the assembled oxidase. The b-type cytochrome gp91phox, also known as NOX2, serves as the catalytic component of this multicomponent enzyme complex. Although many of the features of the composition and regulation of the phagocyte oxidase may apply as well to NOX2 expressed in non-phagocytes and to other members of the NOX protein family, exceptions exist and pose special challenges to investigators exploring the biology of NADPH oxidases.

Identification of Small Peptides that Inhibit NADPH Oxidase (Nox2) Activation

Nicotinamide adenine phosphate (NADPH) oxidase type 2 (Nox2), a major source of reactive oxygen species in lungs, plays an important role in tissue damage associated with acute inflammatory diseases. The phospholipase A₂ (PLA₂) activity of peroxiredoxin 6 (Prdx6), called aiPLA₂, is required for Nox2 activation through its role in the cellular generation of Rac, a key cytosolic component of the activation cascade. Lung surfactant protein A (SP-A) binds to Prdx6, inhibits its aiPLA₂ activity, and prevents activation of Nox2. Based on protein docking software, we previously identified a 16 amino acid (aa) peptide derived from rat SP-A as the Prdx6 binding motif. We now identify the minimal effective sequences of rat/mouse and human SP-A as 9-aa sequences that we have called PLA₂-inhibitory peptide (PIP).These sequences are PIP-1, rat/mouse; PIP-2, human; and PIP-3, a hybrid of PIPs 1&2. aiPLA₂ activity in vitro was inhibited by 50% with ~7⁻10 µg PIP/µg Prdx6. Inhibition of the aiPLA₂ activity and Nox2 activation of lungs in vivo was similar for intratracheal (IT) and intravenous (IV) administration of PIP-2, but required its incorporation into liposomes as a delivery vehicle; tissue ½ time for decrease of the in vivo inhibition of aiPLA₂ activity after PIP-2 administration was ~50 h. These properties suggest that PIP-2 could be an effective therapeutic agent to prevent tissue injury associated with lung inflammation.

Danger signals activate a putative innate immune system during regeneration in a filamentous fungus

The ability to respond to injury is a biological process shared by organisms of different kingdoms that can even result in complete regeneration of a part or structure that was lost. Due to their immobility, multicellular fungi are prey to various predators and are therefore constantly exposed to mechanical damage. Nevertheless, our current knowledge of how fungi respond to injury is scarce. Here we show that activation of injury responses and hyphal regeneration in the filamentous fungus Trichoderma atroviride relies on the detection of two danger or alarm signals. As an early response to injury, we detected a transient increase in cytosolic free calcium ([Ca²⁺]_c) that was promoted by extracellular ATP, and which is likely regulated by a mechanism of calcium-induced calcium-release. In addition, we demonstrate that the mitogen activated protein kinase Tmk1 plays a key role in hyphal regeneration. Calcium- and Tmk1-mediated signaling cascades activated major transcriptional changes early following injury, including induction of a set of regeneration associated genes related to cell signaling, stress responses, transcription regulation, ribosome biogenesis/translation, replication and DNA repair. Interestingly, we uncovered the activation of a putative fungal innate immune response, including the involvement of HET domain genes, known to participate in programmed cell death. Our work shows that fungi and animals share danger-signals, signaling cascades, and the activation of the expression of genes related to immunity after injury, which are likely the result of convergent evolution.

The idea of regenerating lost body parts has always fascinated humans due to its impact on human health. Recently, the study of the response to damage has gained importance not only in regenerative medicine, but also in organ transplantation. We have established a microbial model that given its relative simplicity and ease to work with, promises to accelerate the advance of our understanding of damage responses and regeneration. We show that activation of injury responses and hyphal regeneration in a filamentous fungus relies on the detection of danger (also called alarm) signals also used by plants and animals. Finally, we discovered a set of genes that become active in response to injury, including those putatively participating in the immune response.

The phospholipase A2 activity of peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a Ca²⁺-independent intracellular phospholipase A₂ (called aiPLA₂) that is localized to cytosol, lysosomes, and lysosomal-related organelles. Activity is minimal at cytosolic pH but is increased significantly with enzyme phosphorylation, at acidic pH, and in the presence of oxidized phospholipid substrate; maximal activity with phosphorylated aiPLA₂ is ∼2 µmol/min/mg protein. Prdx6 is a "moonlighting" protein that also expresses glutathione peroxidase and lysophosphatidylcholine acyl transferase activities. The catalytic site for aiPLA₂ activity is an S32-H26-D140 triad; S32-H26 is also the phospholipid binding site. Activity is inhibited by a serine "protease" inhibitor (diethyl p-nitrophenyl phosphate), an analog of the PLA₂ transition state [1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol (MJ33)], and by two naturally occurring proteins (surfactant protein A and p67^phox), but not by bromoenol lactone. aiPLA₂ activity has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids, in the repair of peroxidized cell membranes, and in the activation of NADPH oxidase type 2 (NOX2). The enzyme has been implicated in acute lung injury, carcinogenesis, neurodegenerative diseases, diabetes, male infertility, and sundry other conditions, although its specific roles have not been well defined. Protein mutations and animal models are now available to further investigate the roles of Prdx6-aiPLA₂ activity in normal and pathological physiology.

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.

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

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

Cytosolic Phospholipase A2α Promotes Pulmonary Inflammation and Systemic Disease during Streptococcus pneumoniae Infection

Pulmonary infection by Streptococcus pneumoniae is characterized by a robust alveolar infiltration of neutrophils (polymorphonuclear cells [PMNs]) that can promote systemic spread of the infection if not resolved. We previously showed that 12-lipoxygenase (12-LOX), which is required to generate the PMN chemoattractant hepoxilin A₃ (HXA₃) from arachidonic acid (AA), promotes acute pulmonary inflammation and systemic infection after lung challenge with S. pneumoniae As phospholipase A₂ (PLA₂) promotes the release of AA, we investigated the role of PLA₂ in local and systemic disease during S. pneumoniae infection. The group IVA cytosolic isoform of PLA₂ (cPLA₂α) was activated upon S. pneumoniae infection of cultured lung epithelial cells and was critical for AA release from membrane phospholipids. Pharmacological inhibition of this enzyme blocked S. pneumoniae-induced PMN transepithelial migration in vitro Genetic ablation of the cPLA₂ isoform cPLA₂α dramatically reduced lung inflammation in mice upon high-dose pulmonary challenge with S. pneumoniae The cPLA₂α-deficient mice also suffered no bacteremia and survived a pulmonary challenge that was lethal to wild-type mice. Our data suggest that cPLA₂α plays a crucial role in eliciting pulmonary inflammation during pneumococcal infection and is required for lethal systemic infection following S. pneumoniae lung challenge.

Neutrophil-Derived Cytosolic PLA2α Contributes to Bacterial-Induced Neutrophil Transepithelial Migration

Eicosanoids are a group of bioactive lipids that are shown to be important mediators of neutrophilic inflammation; selective targeting of their function confers therapeutic benefit in a number of diseases. Neutrophilic airway diseases, including cystic fibrosis, are characterized by excessive neutrophil infiltration into the airspace. Understanding the role of eicosanoids in this process may reveal novel therapeutic targets. The eicosanoid hepoxilin A3 is a pathogen-elicited epithelial-produced neutrophil chemoattractant that directs transepithelial migration in response to infection. Following hepoxilin A3-driven transepithelial migration, neutrophil chemotaxis is amplified through neutrophil production of a second eicosanoid, leukotriene B4 (LTB4). The rate-limiting step of eicosanoid generation is the liberation of arachidonic acid by phospholipase A2, and the cytosolic phospholipase A2 (cPLA2)α isoform has been specifically shown to direct LTB4 synthesis in certain contexts. Whether cPLA2α is directly responsible for neutrophil synthesis of LTB4 in the context of Pseudomonas aeruginosa-induced neutrophil transepithelial migration has not been explored. Human and mouse neutrophil-epithelial cocultures were used to evaluate the role of neutrophil-derived cPLA2α in infection-induced transepithelial signaling by pharmacological and genetic approaches. Primary human airway basal stem cell-derived epithelial cultures and micro-optical coherence tomography, a new imaging modality that captures two- and three-dimensional real-time dynamics of neutrophil transepithelial migration, were applied. Evidence from these studies suggests that cPLA2α expressed by neutrophils, but not epithelial cells, plays a significant role in infection-induced neutrophil transepithelial migration by mediating LTB4 synthesis during migration, which serves to amplify the magnitude of neutrophil recruitment in response to epithelial infection.

The intimate and controversial relationship between voltage-gated proton channels and the phagocyte NADPH oxidase

One of the most fascinating and exciting periods in my scientific career entailed dissecting the symbiotic relationship between two membrane transporters, the Nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase complex and voltage-gated proton channels (HV 1). By the time I entered this field, there had already been substantial progress toward understanding NADPH oxidase, but HV 1 were known only to a tiny handful of cognoscenti around the world. Having identified the first proton currents in mammalian cells in 1991, I needed to find a clear function for these molecules if the work was to become fundable. The then-recent discoveries of Henderson, Chappell, and colleagues in 1987-1988 that led them to hypothesize interactions of both molecules during the respiratory burst of phagocytes provided an excellent opportunity. In a nutshell, both transporters function by moving electrical charge across the membrane: NADPH oxidase moves electrons and HV 1 moves protons. The consequences of electrogenic NADPH oxidase activity on both membrane potential and pH strongly self-limit this enzyme. Fortunately, both consequences specifically activate HV 1, and HV 1 activity counteracts both consequences, a kind of yin-yang relationship. Notwithstanding a decade starting in 1995 when many believed the opposite, these are two separate molecules that function independently despite their being functionally interdependent in phagocytes. The relationship between NADPH oxidase and HV 1 has become a paradigm that somewhat surprisingly has now extended well beyond the phagocyte NADPH oxidase - an industrial strength producer of reactive oxygen species (ROS) - to myriad other cells that produce orders of magnitude less ROS for signaling purposes. These cells with their seven NADPH oxidase (NOX) isoforms provide a vast realm of mechanistic obscurity that will occupy future studies for years to come.

Peroxiredoxin 6 in the repair of peroxidized cell membranes and cell signaling

Peroxiredoxin 6 represents a widely distributed group of peroxiredoxins that contain a single conserved cysteine in the protein monomer (1-cys Prdx). The cys when oxidized to the sulfenic form is reduced with glutathione (GSH) catalyzed by the π isoform of GSH-S-transferase. Three enzymatic activities of the protein have been described:1) peroxidase with H2O2, short chain hydroperoxides, and phospholipid hydroperoxides as substrates; 2) phospholipase A2 (PLA2); and 3) lysophosphatidylcholine acyl transferase (LPCAT). These activities have important physiological roles in antioxidant defense, turnover of cellular phospholipids, and the generation of superoxide anion via initiation of the signaling cascade for activation of NADPH oxidase (type 2). The ability of Prdx6 to reduce peroxidized cell membrane phospholipids (peroxidase activity) and also to replace the oxidized sn-2 fatty acyl group through hydrolysis/reacylation (PLA2 and LPCAT activities) provides a complete system for the repair of peroxidized cell membranes.

Peroxiredoxin 6 (Prdx6) supports NADPH oxidase1 (Nox1)-based superoxide generation and cell migration

Nox1 is an abundant source of reactive oxygen species (ROS) in colon epithelium recently shown to function in wound healing and epithelial homeostasis. We identified Peroxiredoxin 6 (Prdx6) as a novel binding partner of Nox activator 1 (Noxa1) in yeast two-hybrid screening experiments using the Noxa1 SH3 domain as bait. Prdx6 is a unique member of the Prdx antioxidant enzyme family exhibiting both glutathione peroxidase and phospholipase A2 activities. We confirmed this interaction in cells overexpressing both proteins, showing Prdx6 binds to and stabilizes wild type Noxa1, but not the SH3 domain mutant form, Noxa1 W436R. We demonstrated in several cell models that Prdx6 knockdown suppresses Nox1 activity, whereas enhanced Prdx6 expression supports higher Nox1-derived superoxide production. Both peroxidase- and lipase-deficient mutant forms of Prdx6 (Prdx6 C47S and S32A, respectively) failed to bind to or stabilize Nox1 components or support Nox1-mediated superoxide generation. Furthermore, the transition-state substrate analogue inhibitor of Prdx6 phospholipase A2 activity (MJ-33) was shown to suppress Nox1 activity, suggesting Nox1 activity is regulated by the phospholipase activity of Prdx6. Finally, wild type Prdx6, but not lipase or peroxidase mutant forms, supports Nox1-mediated cell migration in the HCT-116 colon epithelial cell model of wound closure. These findings highlight a novel pathway in which this antioxidant enzyme positively regulates an oxidant-generating system to support cell migration and wound healing.

The phospholipase A2 activity of peroxiredoxin 6 modulates NADPH oxidase 2 activation via lysophosphatidic acid receptor signaling in the pulmonary endothelium and alveolar macrophages

Peroxiredoxin 6 (Prdx6) is essential for activation of NADPH oxidase type 2 (NOX2) in pulmonary microvascular endothelial cells (PMVECs), alveolar macrophages (AMs), and polymorphonuclear leukocytes. Angiotensin II and phorbol ester increased superoxide/H2O2 generation in PMVECs, AMs, and isolated lungs from wild-type (WT) mice, but had much less effect on cells or lungs from Prdx6-null or Prdx6-D140A-knock-in mice that lack the phospholipase A2 activity (PLA2) of Prdx6; addition of either lysophosphatidylcholine (LPC) or lysophosphatidic acid (LPA) to cells restored their oxidant generation. The generation of LPC by PMVECs required Prdx6-PLA2 We propose that Prdx6-PLA2 modulates NOX2 activation by generation of LPC that is converted to LPA by the lysophospholipase D activity of autotaxin (ATX/lysoPLD). Inhibition of lysoPLD with HA130 (cells,10 μM; lungs, 20 μM; IC50, 29 nM) decreased agonist-induced oxidant generation. LPA stimulates pathways regulated by small GTPases through binding to G-protein-coupled LPA receptors (LPARs). The LPAR blocker Ki16425 (cells, 10 μM; lungs, 25 μM; Ki, 0.34 μM) or cellular knockdown of LPAR type 1 decreased oxidant generation and blocked translocation of rac1 to plasma membrane. Thus, Prdx6-PLA2 modulates NOX2 activation through generation of LPC for conversion to LPA; binding of LPA to LPAR1 signals rac activation.-Vázquez-Medina, J. P., Dodia, C., Weng, L., Mesaros, C., Blair, I. A., Feinstein, S. I., Chatterjee, S., Fisher, A. B. The phospholipase A2 activity of peroxiredoxin 6 modulates NADPH oxidase 2 activation via lysophosphatidic acid receptor signaling in the pulmonary endothelium and alveolar macrophages.

Novel role of group VIB Ca2+-independent phospholipase A2γ in leukocyte-endothelial cell interactions: An intravital microscopic study in rat mesentery

BACKGROUND

Phospholipase A2 (PLA2) is associated with a variety of inflammatory processes related to polymorphonuclear neutrophil (PMN)-endothelial cell interactions. However, the cellular and molecular mechanisms underlying the interactions and the causative isoform(s) of PLA2 remain elusive. In addition, we recently showed that calcium-independent PLA2γ (iPLA2γ), but not cytosolic PLA2 (cPLA2), is responsible for the cytotoxic functions of human PMN including respiratory bursts, degranulation, and chemotaxis. We therefore hypothesized that iPLA2γ is a prerequisite for the PMN recruitment cascade into the site of inflammation. The aim of this study was to elucidate the roles of the three major phospholipases A2, iPLA2, cPLA2 and secretory PLA2, in leukocyte rolling and adherence and in the surface expression of β2-integrins in vivo and in vitro in response to well-defined stimuli.

METHODS

Male Wistar rats were pretreated with PLA2 inhibitors selective for iPLA2β, iPLA2γ, cPLA2, or secretory PLA2. Leukocyte rolling/adherence in the mesenteric venules superfused with platelet-activating factor (PAF) were quantified by intravital microscopy. Furthermore, isolated human PMNs or whole blood were incubated with each PLA2 inhibitor and then activated with formyl-methionyl-leucyl-phenylalanine (fMLP) or PAF. PMN adherence was assessed by counting cells bound to purified fibrinogen, and the surface expression of lymphocyte function-associated antigen 1 and macrophage antigen 1 (Mac-1) was measured by flow cytometry.

RESULTS

The iPLA2γ-specific inhibitor almost completely inhibited the fMLP/PAF-induced leukocyte adherence in vivo and in vitro and also decreased the fMLP/PAF-stimulated surface expression of Mac-1 by 60% and 95%, respectively. In contrast, the other inhibitors did not affect these cellular functions.

CONCLUSION

iPLA2γ seems to be involved in leukocyte/PMN adherence in vivo and in vitro as well as in the up-regulation of Mac-1 in vitro in response to PAF/fMLP. This enzyme is therefore likely to be a major regulator in the PMN recruitment cascade.

Pharmacological inhibition of eicosanoids and platelet-activating factor signaling impairs zymosan-induced release of IL-23 by dendritic cells

The engagement of the receptors for fungal patterns induces the expression of cytokines, the release of arachidonic acid, and the production of PGE2 in human dendritic cells (DC), but few data are available about other lipid mediators that may modulate DC function. The combined antagonism of leukotriene (LT) B4, cysteinyl-LT, and platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) inhibited IL23A mRNA expression in response to the fungal surrogate zymosan and to a lower extent TNFA (tumor necrosis factor-α) and CSF2 (granulocyte macrophage colony-stimulating factor) mRNA. The combination of lipid mediators and the lipid extract of zymosan-conditioned medium increased the induction of IL23A by LPS (bacterial lipopolysaccharide), thus suggesting that unlike LPS, zymosan elicits the production of mediators at a concentration enough for optimal response. Zymosan induced the release of LTB4, LTE4, 12-hydroxyeicosatetraenoic acid (12-HETE), and PAF C16:0. DC showed a high expression and detectable Ser663 phosphorylation of 5-lipoxygenase in response to zymosan, and a high expression and activity of LPCAT1/2 (lysophosphatidylcholine acyltransferase 1 and 2), the enzymes that incorporate acetate from acetyl-CoA into choline-containing lysophospholipids to produce PAF. Pharmacological modulation of the arachidonic acid cascade and the PAF receptor inhibited the binding of P-71Thr-ATF2 (activating transcription factor 2) to the IL23A promoter, thus mirroring their effects on the expression of IL23A mRNA and IL-23 protein. These results indicate that LTB4, cysteinyl-LT, and PAF, acting through their cognate G protein-coupled receptors, contribute to the phosphorylation of ATF2 and play a central role in IL23A promoter trans-activation and the cytokine signature induced by fungal patterns.

Discrete roles of intracellular phospholipases A2 in human neutrophil cytotoxicity

BACKGROUND

The role of calcium-independent phospholipase A2 (iPLA2), a component of the three major PLA2 families, in acute/chronic inflammatory processes remains elusive. Previous investigations have documented iPLA2-mediated respiratory burst of neutrophils (PMNs); however, the causative isoform of iPLA2 is unidentified. We also demonstrated that the iPLA2γ-specific inhibitor attenuates trauma/hemorrhagic shock-induced lung injury. Therefore, iPLA2γ may be implicated in acute inflammation. In addition, arachidonic acid (AA), which is primarily produced by cytosolic PLA2 (cPLA2), is known to display PMN cytotoxicity, although the relationship between AA and the cytotoxic function is still being debated on. We therefore hypothesized that iPLA2γ regulates PMN cytotoxicity via AA-independent signaling pathways. The study aim was to distinguish the role of intracellular phospholipases A2, iPLA2, and cPLA2, in human PMN cytotoxicity and explore the possibility of the presence of signaling molecule(s) other than AA.

METHODS

Isolated human PMNs were incubated with the PLA2 inhibitor selective for iPLA2β, iPLA2γ, or cPLA2 and then activated with formyl-methionyl-leucyl-phenylalanine (fMLP) or phorbol 12-myristate 13-acetate (PMA). Superoxide production was assayed according to the superoxide dismutase-inhibitable cytochrome c reduction method, and the degree of elastase release was measured using a p-nitroanilide-conjugated elastase-specific substrate. In addition, chemotaxis toward platelet activating factor/fMLP was determined with a modified Boyden chamber system.

RESULTS

The iPLA2γ-specific inhibitor reduced the fMLP/PMA-stimulated superoxide generation by 90% and 30%, respectively; in addition, the inhibitor completely blocked the fMLP/PMA-activated elastase release. However, the cPLA2-specific inhibitor did not abrogate these effects to any degree at all concentrations. Likewise, the inhibitor for iPLA2γ, but not iPLA2β or cPLA2, completely inhibited the platelet activating factor/fMLP-induced chemotaxis.

CONCLUSION

iPLA2 is involved in extracellular reactive oxygen species production, elastase release, and chemotaxis in response to well-defined stimuli. In addition, the ineffectiveness of the cPLA2 inhibitor suggests that AA may not be relevant to these cytotoxic functions.

Cytosolic phospholipase A₂: physiological function and role in disease

The group IV phospholipase A2 (PLA2) family is comprised of six intracellular enzymes (GIVA, -B, -C, -D, -E, and -F) commonly referred to as cytosolic PLA2 (cPLA2)α, -β, -γ, -δ, -ε, and -ζ. They contain a Ser-Asp catalytic dyad and all except cPLA2γ have a C2 domain, but differences in their catalytic activities and subcellular localization suggest unique regulation and function. With the exception of cPLA2α, the focus of this review, little is known about the in vivo function of group IV enzymes. cPLA2α catalyzes the hydrolysis of phospholipids to arachidonic acid and lysophospholipids that are precursors of numerous bioactive lipids. The regulation of cPLA2α is complex, involving transcriptional and posttranslational processes, particularly increases in calcium and phosphorylation. cPLA2α is a highly conserved widely expressed enzyme that promotes lipid mediator production in human and rodent cells from a variety of tissues. The diverse bioactive lipids produced as a result of cPLA2α activation regulate normal physiological processes and disease pathogenesis in many organ systems, as shown using cPLA2α KO mice. However, humans recently identified with cPLA2α deficiency exhibit more pronounced effects on health than observed in mice lacking cPLA2α, indicating that much remains to be learned about this interesting enzyme.

Lack of group X secreted phospholipase A₂ increases survival following pandemic H1N1 influenza infection

The role of Group X secreted phospholipase A2 (GX-sPLA2) during influenza infection has not been previously investigated. We examined the role of GX-sPLA2 during H1N1 pandemic influenza infection in a GX-sPLA2 gene targeted mouse (GX(-/-)) model and found that survival after infection was significantly greater in GX(-/-) mice than in GX(+/+) mice. Downstream products of GX-sPLA2 activity, PGD2, PGE2, LTB4, cysteinyl leukotrienes and Lipoxin A4 were significantly lower in GX(-/-) mice BAL fluid. Lung microarray analysis identified an earlier and more robust induction of T and B cell associated genes in GX(-/-) mice. Based on the central role of sPLA2 enzymes as key initiators of inflammatory processes, we propose that activation of GX-sPLA2 during H1N1pdm infection is an early step of pulmonary inflammation and its inhibition increases adaptive immunity and improves survival. Our findings suggest that GX-sPLA2 may be a potential therapeutic target during influenza.

Cryptogenic multifocal ulcerating stenosing enteritis associated with homozygous deletion mutations in cytosolic phospholipase A2-α

OBJECTIVE

Cryptogenic multifocal ulcerating stenosing enteritis (CMUSE) is an extremely rare, but devastating, disease of unknown aetiology. We investigated the genetic basis of this autosomal recessive condition in a pair of affected siblings who have 40-year histories of catastrophic gastrointestinal and extraintestinal disease.

DESIGN

Genome-wide single-nucleotide polymorphism homozygosity mapping in the two affected family members combined with whole-exome sequencing of one affected sibling. This was followed by confirmatory Sanger sequencing of the likely disease-causing sequence variant and functional studies in affected and unaffected family members.

RESULTS

Insertion/deletion variation analysis revealed the presence of a homozygous 4 bp deletion (g.155574_77delGTAA) in the PLA2G4A gene, located in the splice donor site directly after exon 17 (the penultimate exon) of the gene in both affected siblings. This introduces a frameshift of 10 amino acids before a premature stop codon (p.V707fsX10), which is predicted to result in the loss of 43 amino acids (residues 707-749) at the C-terminus of cytosolic phospholipase A2-α (cPLA(2)α). cPLA(2)α protein expression was undetectable in the gut of both siblings, with platelet aggregation and thromboxane A(2) production, as functional assays for cPLA(2)α activity, grossly impaired.

CONCLUSIONS

We have identified mutations in PLA2G4A as a cause of CMUSE in two affected siblings. Further studies are needed to determine if mutations in this gene are also responsible for disease of a similar phenotype in other cases.

The ubiquitin-CXCR4 axis plays an important role in acute lung infection-enhanced lung tumor metastasis

PURPOSE

Our goals were to test the effect of acute lung infection on tumor metastasis and to investigate the underlying mechanisms.

EXPERIMENTAL DESIGN

We combined bacteria-induced and lipopolysaccharide (LPS)-induced acute lung injury/inflammation (ALI) mouse models with mouse metastatic models to study the effect of acute inflammation on lung metastasis in mice. The mechanisms were investigated in ex vivo, in vitro, and in vivo studies.

RESULTS

Both bacteria- and LPS-induced ALI significantly enhanced lung metastasis of four tail vein-injected mouse tumor cell lines. Bacteria also enhanced lung metastasis when 4T1 cells were orthotopically injected. The bronchoalveolar lavage fluid (BALF) from LPS- or bacteria-injected mice stimulated migration of tumor cells. In vivo tracking of metastatic RM-9 cells showed that bacterial injection enhanced early dissemination of tumor cells to the lung. The majority of the BALF migratory activity could be blocked by AMD3100, a chemokine receptor 4 (CXCR4) inhibitor. All tested cell lines expressed CXCR4. The levels of extracellular ubiquitin, but not stromal cell-derived factor-1, in BALF were significantly increased by LPS. Ubiquitin was able to induce AMD3100-sensitive migration of tumor cells. Finally, the antibacterial agent amoxicillin and the CXCR4 inhibitor AMD3100 blocked the enhancement effect of bacterial infection on tumor metastasis.

CONCLUSIONS

Acute lung infection dramatically increased cancer cell homing to the lung and lung metastasis. This change may be due to an alteration of the lung microenvironment and preparation of a favorable metastatic "niche." This effect was seen in multiple cancer types and thus may have broad applications for cancer patients in prevention and/or treatment of metastasis.

A phosphatidylinositol species acutely generated by activated macrophages regulates innate immune responses

Activation of macrophages with stimuli of the innate immune response results in the intense remodeling of arachidonate-containing phospholipids, leading to the mobilization of large quantities of this fatty acid for conversion into biologically active eicosanoids. As a consequence of this process, the arachidonate levels in membrane phospholipids markedly decrease. We have applied mass spectrometry-based lipid profiling to study the levels of arachidonate-containing phospholipids under inflammatory activation of macrophages. We identify an unusual inositol phospholipid molecule, PI(20:4/20:4), the levels of which do not decrease but actually increase by 300% after activation of the macrophages. PI(20:4/20:4) is formed and degraded rapidly, suggesting a role for this molecule in regulating cell signaling events. Using a metabolipidomic approach consisting in exposing the cells to deuterium-labeled arachidonate at the time they are exposed to stimuli, we show that PI(20:4/20:4) biosynthesis occurs via the sequential incorporation of arachidonate, first into the sn-2 position of a preformed phosphatidylinositol (PI) molecule, followed by the rapid introduction of a second arachidonate moiety into the sn-1 position. Generation requires the participation of cytosolic phospholipase A2α and CoA-dependent acyltransferases. PI(20:4/20:4) formation is also detected in vivo in murine peritonitis exudates. Elevating the intracellular concentration of PI(20:4/20:4) by introducing the lipid into the cells results in enhancement of the microbicidal capacity of macrophages, as measured by reactive oxygen metabolite production and lysozyme release. These findings suggest that PI(20:4/20:4) is a novel bioactive inositol phospholipid molecule that regulates innate immune responses in macrophages.

Key role of group v secreted phospholipase A2 in Th2 cytokine and dendritic cell-driven airway hyperresponsiveness and remodeling

Background

Previous work has shown that disruption of the gene for group X secreted phospholipase A₂ (sPLA₂-X) markedly diminishes airway hyperresponsiveness and remodeling in a mouse asthma model. With the large number of additional sPLA₂s in the mammalian genome, the involvement of other sPLA₂s in the asthma model is possible – in particular, the group V sPLA₂ (sPLA₂-V) that like sPLA₂-X is highly active at hydrolyzing membranes of mammalian cells.

Methodology and Principal Findings

The allergen-driven asthma phenotype was significantly reduced in sPLA₂-V-deficient mice but to a lesser extent than observed previously in sPLA₂-X-deficient mice. The most striking difference observed between the sPLA₂-V and sPLA₂-X knockouts was the significant impairment of the primary immune response to the allergen ovalbumin (OVA) in the sPLA₂-V^−/− mice. The impairment in eicosanoid generation and dendritic cell activation in sPLA2-V^−/− mice diminishes Th2 cytokine responses in the airways.

Conclusions

This paper illustrates the diverse roles of sPLA₂s in the immunopathogenesis of the asthma phenotype and directs attention to developing specific inhibitors of sPLA₂-V as a potential new therapy to treat asthma and other allergic disorders.

New insights into the regulation of neutrophil NADPH oxidase activity in the phagosome: a focus on the role of lipid and Ca(2+) signaling

SIGNIFICANCE

Reactive oxygen species, produced by the phagosomal NADPH oxidase of neutrophils, play a significant physiological role during normal defense. Their role is not only to kill invading pathogens, but also to act as modulators of global physiological functions of phagosomes. Given the importance of NADPH oxidase in the immune system, its activity has to be decisively controlled by distinctive mechanisms to ensure appropriate regulation at the phagosome.

RECENT ADVANCES

Here, we describe the signal transduction pathways that regulate phagosomal NADPH oxidase in neutrophils, with an emphasis on the role of lipid metabolism and intracellular Ca(2+) mobilization.

CRITICAL ISSUES

The potential involvement of Ca(2+)-binding S100A8 and S100A9 proteins, known to interact with the plasma membrane NADPH oxidase, is also considered.

FUTURE DIRECTIONS

Recent technical progress in advanced live imaging microscopy will permit to focus more accurately on phagosomal rather than plasma membrane NADPH oxidase regulation during neutrophil phagocytosis.

Inhibition of phospholipase A(2) abrogates intracellular processing of NADPH-oxidase derived reactive oxygen species in human neutrophils

Upon activation of human neutrophils, superoxide can be produced at two cellular sites; either in the plasma membrane, giving extracellular release of oxidants, or in intracellular organelles, resulting in oxidants being retained in the cell. The involvement of phospholipase A(2) (PLA(2)) in phorbol myristate acetate (PMA)-induced activation of the two pools of NADPH-oxidase was investigated using a variety of PLA(2) inhibitors and the oxidase activity was measured by luminol/isoluminol-amplified chemiluminescence (CL). Two of the seven inhibitors were without effect, two inhibitors inhibited both intra- and extracellular ROS production equally, and three inhibitors inhibited intracellular but not extracellular CL. Using another technique to measure ROS, PHPA oxidation, we found that intracellular ROS production was unaltered with the three last inhibitors, indicating that PLA(2) is not involved in the NADPH-oxidase activity per se, but in the intracellular processing of the radicals necessary for the CL reaction to take place. The PLA(2) inhibitors did not abolish the activity of myeloperoxidase (MPO), an enzyme necessary for intracellular CL to occur. Instead, we suggest that these PLA(2) inhibitors block heterotypic granule fusion and prohibit the colocalization of ROS and MPO needed for intracellular CL activity.

Phox activity of differentiated PLB-985 cells is enhanced, in an agonist specific manner, by the PLA2 activity of Prdx6-PLA2

Peroxiredoxin 6-phospholipase A(2) (Prdx6-PLA(2) ) is a bi-functional enzyme with peroxi-redoxin (Prdx) and phospholipase A(2) (PLA(2) ) activities. To investigate its impact on phagocyte NADPH oxidase (phox) activity in a neutrophil model, the protein was knocked down in PLB-985 cells using stable expression of a small hairpin RNA (shRNA) and phox activity was monitored after cell differentiation. The knockdown cells had reduced oxidase activity in response to stimulation with the formylated peptide fMLF, but the response to the phorbol ester PMA was unchanged. Reintroduction of shRNA-resistant Prdx6-PLA(2) into the knockdown cells by stable transfection with a Prdx6-PLA(2) expression plasmid restored the fMLF response, as did reintroduction of Prdx6-PLA(2) mutated in the Prdx active site; reintroduction of PLA(2) active site mutants, however, failed to restore the response. Thus, the PLA(2) activity of Prdx6-PLA(2) in intact cells mediates its ability to enhance phox activity in response to fMLF. In combination with previous publications by other groups, our work indicates that various PLA(2) isoforms can enhance oxidase activity but they are differentially important in different cell types and in the response to different agonists.

Group V phospholipase A2 in bone marrow-derived myeloid cells and bronchial epithelial cells promotes bacterial clearance after Escherichia coli pneumonia

Group V-secreted phospholipase A(2) (GV sPLA(2)) hydrolyzes bacterial phospholipids and initiates eicosanoid biosynthesis. Here, we elucidate the role of GV sPLA(2) in the pathophysiology of Escherichia coli pneumonia. Inflammatory cells and bronchial epithelial cells both express GV sPLA(2) after pulmonary E. coli infection. GV(-/-) mice accumulate fewer polymorphonuclear leukocytes in alveoli, have higher levels of E. coli in bronchoalveolar lavage fluid and lung, and develop respiratory acidosis, more severe hypothermia, and higher IL-6, IL-10, and TNF-α levels than GV(+/+) mice after pulmonary E. coli infection. Eicosanoid levels in bronchoalveolar lavage are similar in GV(+/+) and GV(-/-) mice after lung E. coli infection. In contrast, GV(+/+) mice have higher levels of prostaglandin D(2) (PGD(2)), PGF(2α), and 15-keto-PGE(2) in lung and express higher levels of ICAM-1 and PECAM-1 on pulmonary endothelial cells than GV(-/-) mice after lung infection with E. coli. Selective deletion of GV sPLA(2) in non-myeloid cells impairs leukocyte accumulation after pulmonary E. coli infection, and lack of GV sPLA(2) in either bone marrow-derived myeloid cells or non-myeloid cells attenuates E. coli clearance from the alveolar space and the lung parenchyma. These observations show that GV sPLA(2) in bone marrow-derived myeloid cells as well as non-myeloid cells, which are likely bronchial epithelial cells, participate in the regulation of the innate immune response to pulmonary infection with E. coli.

Peroxiredoxin 6 phosphorylation and subsequent phospholipase A2 activity are required for agonist-mediated activation of NADPH oxidase in mouse pulmonary microvascular endothelium and alveolar macrophages

Peroxiredoxin 6 (Prdx6), a bifunctional enzyme with glutathione peroxidase and phospholipase A2 (PLA(2)) activities, participates in the activation of NADPH oxidase 2 (NOX2) in neutrophils, but the mechanism for this effect is not known. We now demonstrate that Prdx6 is required for agonist-induced NOX2 activation in pulmonary microvascular endothelial cells (PMVEC) and that the effect requires the PLA(2) activity of Prdx6. Generation of reactive oxygen species (ROS) in response to angiotensin II (Ang II) or phorbol 12-myristate 13-acetate was markedly reduced in perfused lungs and isolated PMVEC from Prdx6 null mice. Rac1 and p47(phox), cytosolic components of NOX2, translocated to the endothelial cell membrane after Ang II treatment in wild-type but not Prdx6 null PMVEC. MJ33, an inhibitor of Prdx6 PLA(2) activity, blocked agonist-induced PLA(2) activity and ROS generation in PMVEC by >80%, whereas inhibitors of other PLA(2)s were ineffective. Transfection of Prx6 null cells with wild-type and C47S mutant Prdx6, but not with mutants of the PLA(2) active site (S32A, H26A, and D140A), "rescued" Ang II-induced PLA(2) activity and ROS generation. Ang II treatment of wild-type cells resulted in phosphorylation of Prdx6 and its subsequent translocation from the cytosol to the cell membrane. Phosphorylation as well as PLA(2) activity and ROS generation were markedly reduced by the MAPK inhibitor, U0126. Thus, agonist-induced MAPK activation leads to Prdx6 phosphorylation and translocation to the cell membrane, where its PLA(2) activity facilitates assembly of the NOX2 complex and activation of the oxidase.

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.

Dual roles of endogenous platelet-activating factor acetylhydrolase in a murine model of necrotizing enterocolitis

Human preterm infants with necrotizing enterocolitis (NEC) have increased circulating and luminal levels of platelet-activating factor (PAF) and decreased serum PAF-acetylhydrolase (PAF-AH), the enzyme that inactivates PAF. Formula supplemented with recombinant PAF-AH decreases NEC in a neonatal rat model. We hypothesized that endogenous PAF-AH contributes to neonatal intestinal homeostasis and therefore developed PAF-AH mice using standard approaches to study the role of this enzyme in the neonatal NEC model. After exposure to a well-established NEC model, intestinal tissues were evaluated for histology, proinflammatory cytokine mRNA synthesis, and death using standard techniques. We found that mortality rates were significantly lower in PAF-AH pups compared with wild-type controls before 24 h of life but surviving PAF-AH animals were more susceptible to NEC development compared with wild-type controls. Increased NEC incidence was associated with prominent inflammation characterized by elevated intestinal mRNA expression of sPLA2, inducible NOS, and CXCL1. In conclusion, the data support a protective role for endogenous PAF-AH in the development of NEC, and because preterm neonates have endogenous PAF-AH deficiency, this may place them at increased risk for disease.

Eicosanoids in the innate immune response: TLR and non-TLR routes

The variable array of pattern receptor expression in different cells of the innate immune system explains the induction of distinct patterns of arachidonic acid (AA) metabolism. Peptidoglycan and mannan were strong stimuli in neutrophils, whereas the fungal extract zymosan was the most potent stimulus in monocyte-derived dendritic cells since it induced the production of PGE₂, PGD₂, and several cytokines including a robust IL-10 response. Zymosan activated κB-binding activity, but inhibition of NF-κB was associated with enhanced IL-10 production. In contrast, treatments acting on CREB (CRE binding protein), including PGE₂, showed a direct correlation between CREB activation and IL-10 production. Therefore, in dendritic cells zymosan induces il10 transcription by a CRE-dependent mechanism that involves autocrine secretion of PGE₂, thus unraveling a functional cooperation between eicosanoid production and cytokine production.

Localization and function of cytosolic phospholipase A2alpha at the Golgi

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

Diabetes-induced oxidative stress is mediated by Ca2+-independent phospholipase A2 in neutrophils

Neutrophils from people with poorly controlled diabetes present a primed phenotype and secrete excessive superoxide. Phospholipase A(2) (PLA(2))-derived arachidonic acid (AA) activates the assembly of NADPH oxidase to generate superoxide anion. There is a gap in the current literature regarding which PLA(2) isoform regulates NADPH oxidase activation. The aim of this study was to identify the PLA(2) isoform involved in the regulation of superoxide generation in neutrophils and investigate if PLA(2) mediates priming in response to pathologic hyperglycemia. Neutrophils were isolated from people with diabetes mellitus and healthy controls, and HL60 neutrophil-like cells were grown in hyperglycemic conditions. Incubating neutrophils with the Ca(2+)-independent PLA(2) (iPLA(2)) inhibitor bromoenol lactone (BEL) completely suppressed fMLP-induced generation of superoxide. The nonspecific actions of BEL on phosphatidic acid phosphohydrolase-1, p47(phox) phosphorylation, and apoptosis were ruled out by specific assays. Small interfering RNA knockdown of iPLA(2) inhibited superoxide generation by neutrophils. Neutrophils from people with poorly controlled diabetes and in vitro incubation of neutrophils with high glucose and the receptor for advanced glycation end products ligand S100B greatly enhanced superoxide generation compared with controls, and this was significantly inhibited by BEL. A modified iPLA(2) assay, Western blotting, and PCR confirmed that there was increased iPLA(2) activity and expression in neutrophils from people with diabetes. AA (10 microM) partly rescued the inhibition of superoxide generation mediated by BEL, confirming that NADPH oxidase activity is, in part, regulated by AA. This study provides evidence for the role of iPLA(2) in enhanced superoxide generation in neutrophils from people with diabetes mellitus and presents an alternate pathway independent of protein kinase C and phosphatidic acid phosphohydrolase-1 hydrolase signaling.

Regulatory role of cytosolic phospholipase A2alpha in NADPH oxidase activity and in inducible nitric oxide synthase induction by aggregated Abeta1-42 in microglia

In Alzheimer's disease, extracellular deposits of amyloid beta(1-42) (Abeta(1-42)) may induce activation of microglial cells by releasing proinflammatory factors that contribute to the neurodegeneration process. Since the activation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha) has been reported in inflammatory conditions, its role in primary rat microglial cell activated by aggregated Abeta(1-42) was elucidated. The results of the present study show that activation of microglia by 5 microM aggregated Abeta(1-42) (as evident by the amoeboid morphology and increased CD68 immunofluorescence reactivity) caused an immediate activation of cPLA(2)alpha, measured by its phosphorylated form and its specific activity, followed by a gradual elevation of its expression and activity during 24 h. Inhibition of cPLA(2)alpha expression and activity by the presence of 1 microM specific antisense resulted in a significant decrease in NADPH oxidase activity that releases superoxides, PGE(2) formation, iNOS expression, and NO production, indicating a major role for cPLA(2)alpha in the regulation of these inflammatory processes. NADPH oxidase activity, which is under cPLA(2)alpha regulation, was found to upregulate cPLA(2)alpha and COX-2 protein expression through the redox-sensitive NFkappaB activation as evident by its phosphorylation on Ser-536, resulting in increased PGE(2) formation. The secreted PGE(2) induced the synthesis of iNOS and the production of NO through the PKA-CREB pathway. Taken together, our results suggest that the response of cPLA(2)alpha to aggregated Abeta(1-42) is probably a key player in the oxidative stress present in AD, regulating potent oxidative agents: the production of superoxides by NADPH oxidase and NO formation by iNOS.

Involvement of Cytosolic Phospholipase A2α in MMP-9 Up-regulation

Matrix metalloproteinase-9 (MMP-9) is important in the pathogenesis of periodontitis. Cytosolic phospholipase A2α (cPLA2α) is involved in MMP-9 up-regulation in human monocytes. We tested the hypothesis that cPLA2α also regulates MMP-9 induction by Fusobacterium nucleatum and by phorbol 12-myristate-13-acetate (PMA) in gingival epithelial cells. While PMA induced MMP-9 expression considerably, F. nucleatum did so moderately. This time-course study demonstrated that MMP-9 mRNA up-regulation occurred at 3 hours, whereas MMP-9 secretion and activity in cell-free supernatants occurred at 12 hours. cPLA2α mRNA was constitutively expressed in gingival epithelial cells. Transient activation of cPLA2 by Ser505 phosphorylation was observed in the nuclei upon stimulation, suggesting its role as a transcription factor, while cPLA2 protein expression remained unchanged. Induction of MMP-9 expression and activity was significantly inhibited by 1 μM of the specific cPLA2α inhibitor (P < 0.01). These findings demonstrate the involvement of cPLA2α in MMP-9 up-regulation.

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

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

Physalin B inhibits Rhodnius prolixus hemocyte phagocytosis and microaggregation by the activation of endogenous PAF-acetyl hydrolase activities

The effects of physalin B (a natural secosteroidal chemical from Physalis angulata, Solanaceae) on phagocytosis and microaggregation by hemocytes of 5th-instar larvae of Rhodnius prolixus were investigated. In this insect, hemocyte phagocytosis and microaggregation are known to be induced by the platelet-activating factor (PAF) or arachidonic acid (AA) and regulated by phospholipase A(2) (PLA(2)) and PAF-acetyl hydrolase (PAF-AH) activities. Phagocytic activity and formation of hemocyte microaggregates by Rhodnius hemocytes were strongly blocked by oral treatment of this insect with physalin B (1mug/mL of blood meal). The inhibition induced by physalin B was reversed for both phagocytosis and microaggregation by exogenous arachidonic acid (10microg/insect) or PAF (1microg/insect) applied by hemocelic injection. Following treatment with physalin B there were no significant alterations in PLA(2) activities, but a significant enhancement of PAF-AH was observed. These results show that physalin B inhibits hemocytic activity by depressing insect PAF analogous (iPAF) levels in hemolymph and confirm the role of PAF-AH in the cellular immune reactions in R. prolixus.

Expression of phospholipases A2 in primary human lung macrophages: role of cytosolic phospholipase A2-alpha in arachidonic acid release and platelet activating factor synthesis

Macrophages are a major source of lipid mediators in the human lung. Expression and contribution of cytosolic (cPLA(2)) and secreted phospholipases A(2) (sPLA(2)) to the generation of lipid mediators in human macrophages are unclear. We investigated the expression and role of different PLA(2)s in the production of lipid mediators in primary human lung macrophages. Macrophages express the alpha, but not the zeta isoform of group IV and group VIA cPLA(2) (iPLA(2)). Two structurally-divergent inhibitors of group IV cPLA(2) completely block arachidonic acid release by macrophages in response to non-physiological (Ca(2+) ionophores and phorbol esters) and physiological agonists (lipopolysaccharide and Mycobacterium protein derivative). These inhibitors also reduce by 70% the synthesis of platelet-activating factor by activated macrophages. Among the full set of human sPLA(2)s, macrophages express group IIA, IID, IIE, IIF, V, X and XIIA, but not group IB and III enzymes. Me-Indoxam, a potent and cell impermeable inhibitor of several sPLA(2)s, has no effect on arachidonate release or platelet-activating factor production. Agonist-induced exocytosis is not influenced by cPLA(2) inhibitors at concentrations that block arachidonic acid release. Our results indicate that human macrophages express cPLA(2)-alpha, iPLA(2) and several sPLA(2)s. Cytosolic PLA(2)-alpha is the major enzyme responsible for lipid mediator production in human macrophages.

Lipid mediators and vector infection: Trypanosoma rangeli inhibits Rhodnius prolixus hemocyte phagocytosis by modulation of phospholipase A2 and PAF-acetylhydrolase activities

In this work we investigated the effects of Trypanosoma rangeli infection through a blood meal on the hemocyte phagocytosis in experiments using the 5th instar larvae of Rhodnius prolixus. Hemocyte phagocytic activity was strongly blocked by oral infection with the parasites. In contrast, hemocyte phagocytosis inhibition caused by T. rangeli infection was rescued by exogenous arachidonic acid (20 microg/insect) or platelet activating factor (PAF; 1 microg/insect) applied by hemocelic injection. Following the oral infection with the protozoan we observed significant attenuation of phospholipase A2 (PLA2) activities in R. prolixus hemocytes (cytosolic PLA2: cPLA2, secreted PLA2: sPLA2 and Ca+2-independent PLA2: iPLA2) and enhancement of sPLA2 activities in cell-free hemolymph. At the same time, the PAF-acetyl hydrolase (PAF-AH) activity in the cell-free hemolymph increased considerably. Our results suggest that T. rangeli infection depresses eicosanoid and insect PAF analogous (iPAF) pathways giving support to the role of PLA2 in the regulation of arachidonic acid and iPAF biosynthesis and of PAF-AH by reducing the concentration of iPAF in R. prolixus. This illustrates the ability of T. rangeli to modulate the immune responses of R. prolixus to favor its own multiplication in the hemolymph.

Intracellular pH regulates superoxide production by the macula densa

We hypothesized that elevated macula densa intracellular pH (pH(i)) during tubuloglomerular feedback enhances O(2)(-) production from NAD(P)H oxidase. Microdissected thick ascending limbs from rabbits with intact macula densa were cannulated and perfused with physiological saline. When luminal NaCl was switched from 10 to 80 mM, O(2)(-) production increased from 0.53 +/- 0.09 to 2.62 +/- 0.54 U/min (P < 0.01). To determine whether inhibiting the Na/H exchanger blocks O(2)(-) production, we used dimethyl amiloride (DMA) to block Na/H exchange. In the presence of DMA, O(2)(-) production induced by NaCl was blunted by 40%. To study the effect of pH(i) on O(2)(-) in intact macula densa cells, we measured O(2)(-) while pH(i) was changed by adjusting luminal pH. When the macula densa was perfused with 80 mM NaCl and the pH of the perfusate was switched to 6.8, 7.4, and 8.0, O(2)(-) production was significantly enhanced, but not at 10 mM NaCl. To ascertain the source of O(2)(-), we used the NAD(P)H oxidase inhibitor apocynin. In the presence of apocynin (10(-5) M), O(2)(-) production induced by elevating pH(i) was blocked. Finally, we measured the optimum pH for O(2)(-) production by the macula densa and found optimum extracellular pH is at 7.7 and optimum pH(i) is approximately 8 for O(2)(-) production. We found that elevated pH(i) enhances O(2)(-) production from NAD(P)H oxidase induced by increasing luminal NaCl when the lumen is perfused with 80 mM NaCl, not 10 mM, and O(2)(-) production is pH sensitive, with an optimum pH(i) of 8.

Costimulation of dectin-1 and DC-SIGN triggers the arachidonic acid cascade in human monocyte-derived dendritic cells

Inflammatory mediators derived from arachidonic acid (AA) alter the function of dendritic cells (DC), but data regarding their biosynthesis resulting from stimulation of opsonic and nonopsonic receptors are scarce. To address this issue, the production of eicosanoids by human monocyte-derived DC stimulated via receptors involved in Ag recognition was assessed. Activation of FcgammaR induced AA release, short-term, low-grade PG biosynthesis, and IL-10 production, whereas zymosan, which contains ligands of both the mannose receptor and the human beta-glucan receptor dectin-1, induced a wider set of responses including cyclooxygenase 2 induction and biosynthesis of leukotriene C(4) and IL-12p70. The cytosolic phospholipase A(2) inhibitor pyrrolidine 1 completely inhibited AA release stimulated via all receptors, whereas the spleen tyrosine kinase (Syk) inhibitors piceatannol and R406 fully blocked AA release in response to immune complexes, but only partially blocked the effect of zymosan. Furthermore, anti-dectin-1 mAb partially inhibited the response to zymosan, and this inhibition was enhanced by mAb against DC-specific ICAM-3-grabbing nonintegrin (SIGN). Immunoprecipitation of DC lysates showed coimmunoprecipitation of DC-SIGN and dectin-1, which was confirmed using Myc-dectin-1 and DC-SIGN constructs in HEK293 cells. These data reveal a robust metabolism of AA in human DC stimulated through both opsonic and nonopsonic receptors. The FcgammaR route depends on the ITAM/Syk/cytosolic phospholipase A(2) axis, whereas the response to zymosan involves the interaction with the C-type lectin receptors dectin-1 and DC-SIGN. These findings help explain the distinct functional properties of DC matured by immune complexes vs those matured by beta-glucans.