Platelet-activating factor: a phospholipid autacoid with diverse actions

An inside out journey: biogenesis, ultrastructure and proteomic characterisation of the ectoparasitic flatworm Sparicotyle chrysophrii extracellular vesicles

BACKGROUND

Helminth extracellular vesicles (EVs) are known to have a three-way communication function among parasitic helminths, their host and the host-associated microbiota. They are considered biological containers that may carry virulence factors, being therefore appealing as therapeutic and prophylactic target candidates. This study aims to describe and characterise EVs secreted by Sparicotyle chrysophrii (Polyopisthocotyla: Microcotylidae), a blood-feeding gill parasite of gilthead seabream (Sparus aurata), causing significant economic losses in Mediterranean aquaculture.

METHODS

To identify proteins involved in extracellular vesicle biogenesis, genomic datasets from S. chrysophrii were mined in silico using known protein sequences from Clonorchis spp., Echinococcus spp., Fasciola spp., Fasciolopsis spp., Opisthorchis spp., Paragonimus spp. and Schistosoma spp. The location and ultrastructure of EVs were visualised by transmission electron microscopy after fixing adult S. chrysophrii specimens by high-pressure freezing and freeze substitution. EVs were isolated and purified from adult S. chrysophrii (n = 200) using a newly developed ultracentrifugation-size-exclusion chromatography protocol for Polyopisthocotyla, and EVs were characterised via nanoparticle tracking analysis and tandem mass spectrometry.

RESULTS

Fifty-nine proteins involved in EV biogenesis were identified in S. chrysophrii, and EVs compatible with ectosomes were observed in the syncytial layer of the haptoral region lining the clamps. The isolated and purified nanoparticles had a mean size of 251.8 nm and yielded 1.71 × 108 particles · mL-1. The protein composition analysis identified proteins related to peptide hydrolases, GTPases, EF-hand domain proteins, aerobic energy metabolism, anticoagulant/lipid-binding, haem detoxification, iron transport, EV biogenesis-related, vesicle-trafficking and other cytoskeletal-related proteins. Several identified proteins, such as leucyl and alanyl aminopeptidases, calpain, ferritin, dynein light chain, 14-3-3, heat shock protein 70, annexin, tubulin, glutathione S-transferase, superoxide dismutase, enolase and fructose-bisphosphate aldolase, have already been proposed as target candidates for therapeutic or prophylactic purposes.

CONCLUSIONS

We have unambiguously demonstrated for the first time to our knowledge the secretion of EVs by an ectoparasitic flatworm, inferring their biogenesis machinery at a genomic and transcriptomic level, and by identifying their location and protein composition. The identification of multiple therapeutic targets among EVs' protein repertoire provides opportunities for target-based drug discovery and vaccine development for the first time in Polyopisthocotyla (sensu Monogenea), and in a fish-ectoparasite model.

Platelet-activating factor contracts guinea pig esophageal muscularis mucosae by stimulating extracellular Ca2+ influx through diltiazem-insensitive Ca2+ channels

Platelet-activating factor (PAF) is expected to increase esophageal motility. However, to the best of our knowledge, this has not been examined. Thus, we investigated the contractile effects of PAF on guinea pig (GP) esophageal muscularis mucosae (EMM) and the extracellular Ca2+ influx pathways responsible. PAF (10-9-10-6 M) contracted EMM in a concentration-dependent manner. PAF (10-6 M)-induced contractions were almost completely suppressed by apafant (a PAF receptor antagonist, 3 × 10-5 M). In EMM strips, PAF receptor and PAF-synthesizing/degrading enzyme mRNAs were detected. PAF (10-6 M)-induced contractions were abolished by extracellular Ca2+ removal but were not affected by diltiazem [a voltage-dependent Ca2+ channel (VDCC) inhibitor, 10-5 M]. PAF (10-6 M)-induced contractions in the presence of diltiazem were significantly suppressed by LOE-908 [a receptor-operated Ca2+ channel (ROCC) inhibitor, 3 × 10-5 M], SKF-96365 [an ROCC and store-operated Ca2+ channel (SOCC) inhibitor, 3 × 10-5 M], and LOE-908 plus SKF-96365. Among the tested ROCC/SOCC-related mRNAs, Trpc3, Trpc6, and Trpv4/Orai1, Orai3, and Stim2 were abundantly expressed in EMM strips. These results indicate that PAF potently induces GP EMM contractions that are dependent on extracellular Ca2+ influx through ROCCs/SOCCs, and VDCCs are unlikely to be involved.

Platelet Activating Factor (PAF): A Mediator of Inflammation

Platelet-activating factor (PAF) is a phospholipid-derived mediator with an established role in multiple inflammatory states. PAF is synthesized and secreted by multiple cell types and is then rapidly hydrolyzed and degraded to an inactive metabolite, lyso-PAF, by the enzyme PAF acetylhydrolase. In addition to its role in platelet aggregation and activation, PAF contributes to allergic and nonallergic inflammatory diseases such as anaphylaxis, sepsis, cardiovascular disease, neurological disease, and malignancy as demonstrated in multiple animal models and, increasingly, in human disease states. Recent research has demonstrated the importance of the PAF pathway in multiple conditions including the prediction of severe pediatric anaphylaxis, effects on blood-brain barrier permeability, effects on reproduction, ocular diseases, and further understanding of its role in cardiovascular risk. Investigation of PAF as both a biomarker and a therapeutic target continues because of the need for directed management of inflammation. Collectively, studies have shown that therapies focused on the PAF pathway have the potential to provide targeted and effective treatments for multiple inflammatory conditions.

PAF signaling plays a role in obesity-induced adipose tissue remodeling

BACKGROUND/OBJECTIVES

Platelet-activating factor receptor (PAFR) activation controls adipose tissue (AT) expansion in animal models. Our objective was twofold: (i) to check whether PAFR signaling is involved in human obesity and (ii) investigate the PAF pathway role in hematopoietic or non-hematopoietic cells to control adipocyte size.

MATERIALS/SUBJECTS AND METHODS

Clinical parameters and adipose tissue gene expression were evaluated in subjects with obesity. Bone marrow (BM) transplantation from wild-type (WT) or PAFR^-/- mice was performed to obtain chimeric PAFR-deficient mice predominantly in hematopoietic or non-hematopoietic-derived cells. A high carbohydrate diet (HC) was used to induce AT remodeling and evaluate in which cell compartment PAFR signaling modulates it. Also, 3T3-L1 cells were treated with PAF to evaluate fat accumulation and the expression of genes related to it.

RESULTS

PAFR expression in omental AT from humans with obesity was negatively correlated to different corpulence parameters and more expressed in the stromal vascular fraction than adipocytes. Total PAFR^-/- increased adiposity compared with WT independent of diet-induced obesity. Differently, WT mice receiving PAFR^-/--BM exhibited similar adiposity gain as WT chimeras. PAFR^-/- mice receiving WT-BM showed comparable augmentation in adiposity as total PAFR^-/- mice, demonstrating that PAFR signaling modulates adipose tissue expansion through non-hematopoietic cells. Indeed, the PAF treatment in 3T3-L1 adipocytes reduced fat accumulation and expression of adipogenic genes.

CONCLUSIONS

Therefore, decreased PAFR signaling may favor an AT accumulation in humans and animal models. Importantly, PAFR signaling, mainly in non-hematopoietic cells, especially in adipocytes, appears to play a significant role in regulating diet-induced AT expansion.

Compound Kushen Injection Induces Immediate Hypersensitivity Reaction Through Promoting the Production of Platelet-Activating Factor via de Novo Pathway

Compound Kushen Injection (CKI) is a bis-herbal formulation extracted from Kushen (Radix Sophorae Flavescentis) and Baituling (Rhizoma Heterosmilacis Yunnanensis). Clinically, it is used as the adjuvant treatment of cancer. However, with the increased application, the cases of immediate hypersensitivity reactions (IHRs) also gradually rise. In this study, we investigated the underlying mechanism(s) and active constituent(s) for CKI-induced IHRs in experimental models. The obtained results showed that CKI did not elevate serum total IgE (tIgE) and mouse mast cell protease 1 (MMCP1) after consecutive immunization for 5 weeks, but could induce Evans blue extravasation (local) and cause obvious hypothermia (systemic) after a single injection. Further study showed that alkaloids in Kushen, especially matrine, were responsible for CKI-induced IHRs. Mechanism study showed that various platelet-activating factor (PAF) receptor antagonists could significantly counter CKI-induced IHRs locally or systemically. In cell system, CKI was able to promote PAF production in a non-cell-selective manner. In cell lysate, the effect of CKI on PAF production became stronger and could be abolished by blocking de novo pathway. In conclusion, our study identifies, for the first time, that CKI is a PAF inducer. It causes non-immunologic IHRs, rather than IgE-dependent IHRs, by promoting PAF production through de novo pathway. Alkaloids in Kushen, especially matrine, are the prime culprits for IHRs. Our findings may provide a potential approach for preventing and treating CKI-induced IHRs.

Simultaneous silencing of lysophosphatidylcholine acyltransferases 1-4 by nucleic acid nanoparticles (NANPs) improves radiation response of melanoma cells

Radiation induces the generation of platelet-activating factor receptor (PAF-R) ligands, including PAF and oxidized phospholipids. Alternatively, PAF is also synthesized by the biosynthetic enzymes lysophosphatidylcholine acyltransferases (LPCATs) which are expressed by tumor cells including melanoma. The activation of PAF-R by PAF and oxidized lipids triggers a survival response protecting tumor cells from radiation-induced cell death, suggesting the involvement of the PAF/PAF-R axis in radioresistance. Here, we investigated the role of LPCATs in the melanoma cell radiotherapy response. LPCAT is a family of four enzymes, LPCAT1-4, and modular nucleic acid nanoparticles (NANPs) allowed for the simultaneous silencing of all four LPCATs. We found that the in vitro simultaneous silencing of all four LPCAT transcripts by NANPs enhanced the therapeutic effects of radiation in melanoma cells by increasing cell death, reducing long-term cell survival, and activating apoptosis. Thus, we propose that NANPs are an effective strategy for improving radiotherapy efficacy in melanomas.

Plasmalogens, platelet-activating factor and beyond - Ether lipids in signaling and neurodegeneration

Glycerol-based ether lipids including ether phospholipids form a specialized branch of lipids that in mammals require peroxisomes for their biosynthesis. They are major components of biological membranes and one particular subgroup, the plasmalogens, is widely regarded as a cellular antioxidant. Their vast potential to influence signal transduction pathways is less well known. Here, we summarize the literature showing associations with essential signaling cascades for a wide variety of ether lipids, including platelet-activating factor, alkylglycerols, ether-linked lysophosphatidic acid and plasmalogen-derived polyunsaturated fatty acids. The available experimental evidence demonstrates links to several common players like protein kinase C, peroxisome proliferator-activated receptors or mitogen-activated protein kinases. Furthermore, ether lipid levels have repeatedly been connected to some of the most abundant neurological diseases, particularly Alzheimer's disease and more recently also neurodevelopmental disorders like autism. Thus, we critically discuss the potential role of these compounds in the etiology and pathophysiology of these diseases with an emphasis on signaling processes. Finally, we review the emerging interest in plasmalogens as treatment target in neurological diseases, assessing available data and highlighting future perspectives. Although many aspects of ether lipid involvement in cellular signaling identified in vitro still have to be confirmed in vivo, the compiled data show many intriguing properties and contributions of these lipids to health and disease that will trigger further research.

Critical lipids link breastfeeding to healthy adipose tissue in infancy and adulthood

The study of beige adipose tissue (BeAT) has recently gained popularity because of its potential as a therapeutic target for the treatment of obesity and other metabolic disorders. While BeAT regulation is well understood in adults, the critical signals regulating BeAT during infant development need to be better defined. The bioactive components in breast milk have been primarily studied in the context of immunity. In this issue of the JCI, Yu and Dilbaz et al. identify how a class of breast milk-specific lipid mediators referred to as alkylglycerols (AKGs) maintain BeAT in infants and prevent the transdifferentiation of BeAT into lipid-storing white adipose tissue (WAT).

Direct Growth Inhibitory Effect of Platelet Activating Factor C-16 and Its Structural Analogs on Mycobacteria

Mycobacterium tuberculosis, the causative agent of tuberculosis, is one of the leading causes of human deaths due to a single infectious agent. M. tuberculosis infection of the host initiates a local inflammatory response, resulting in the production of a range of inflammatory factors at the site of infection. These inflammatory factors may come in direct contact with M. tuberculosis and immune cells to activate different signaling pathways. One such factor produced in excess during inflammation is a phospholipid compound, Platelet Activating Factor C-16 (PAF C-16). In this study, PAF C-16 was shown to have a direct inhibitory effect on the growth of Mycobacterium bovis BCG (M. bovis BCG) and Mycobacterium smegmatis (M. smegmatis) in a dose- and time-dependent manner. Use of a range of PAF C-16 structural analogs, including the precursor form Lyso-PAF, revealed that small modifications in the structure of PAF C-16 did not alter its mycobacterial growth inhibitory properties. Subsequent experiments suggested that the attachment of aliphatic carbon tail via ether bond to the glycerol backbone of PAF C-16 was likely to play a vital role in its growth inhibition ability against mycobacteria. Fluorescence microscopy and flow cytometry using Propidium iodide (PI) indicated that PAF C-16 treatment had a damaging effect on the cell membrane of M. bovis BCG and M. smegmatis. Furthermore, the growth inhibitory effect of PAF C-16 was partially mitigated by treatment with membrane-stabilizing agents, α-tocopherol and Tween-80, which further suggests that the growth inhibitory effect of PAF C-16 was mediated through bacterial cell membrane damage.

Markers of anaphylaxis - a systematic review

Anaphylaxis is defined as severe, life-threatening, systemic or general, immediate reaction of hypersensitivity, with repeatable symptoms caused by the dose of stimulus which is well tolerated by healthy persons. The proper diagnosis, immediate treatment and differential diagnosis are crucial for saving patient's life. However, anaphylaxis is relatively frequently misdiagnosed or confused with other clinical entities. Thus, there is a continuous need for identifying detectable markers improving the proper diagnosis of anaphylaxis. Here we presented currently known markers of anaphylaxis and discussed in more detail the most clinically valuable ones: tryptase, platelet activacting factor (PAF), PAF-acethylhydrolase, histamine and its metabolites.

Phospholipase A2 catalysis and lipid mediator lipidomics

Phospholipase A₂ (PLA₂) enzymes are the upstream regulators of the eicosanoid pathway liberating free arachidonic acid from the sn-2 position of membrane phospholipids. Free intracellular arachidonic acid serves as a substrate for the eicosanoid biosynthetic enzymes including cyclooxygenases, lipoxygenases, and cytochrome P450s that lead to inflammation. The Group IVA cytosolic (cPLA₂), Group VIA calcium-independent (iPLA₂), and Group V secreted (sPLA₂) are three well-characterized human enzymes that have been implicated in eicosanoid formation. In this review, we will introduce and summarize the regulation of catalytic activity and cellular localization, structural characteristics, interfacial activation and kinetics, substrate specificity, inhibitor binding and interactions, and the downstream implications for eicosanoid biosynthesis of these three important PLA₂ enzymes.

Inflammation, not Cholesterol, Is a Cause of Chronic Disease

Since the Seven Countries Study, dietary cholesterol and the levels of serum cholesterol in relation to the development of chronic diseases have been somewhat demonised. However, the principles of the Mediterranean diet and relevant data linked to the examples of people living in the five blue zones demonstrate that the key to longevity and the prevention of chronic disease development is not the reduction of dietary or serum cholesterol but the control of systemic inflammation. In this review, we present all the relevant data that supports the view that it is inflammation induced by several factors, such as platelet-activating factor (PAF), that leads to the onset of cardiovascular diseases (CVD) rather than serum cholesterol. The key to reducing the incidence of CVD is to control the activities of PAF and other inflammatory mediators via diet, exercise, and healthy lifestyle choices. The relevant studies and data supporting these views are discussed in this review.

Requirement and Synergistic Contribution of Platelet-Activating Factor Acetylhydrolase Sse and Streptolysin S to Inhibition of Neutrophil Recruitment and Systemic Infection by Hypervirulent emm3 Group A Streptococcus in Subcutaneous Infection of Mice

Hypervirulent group A streptococcus (GAS) can inhibit neutrophil recruitment and cause systemic infection in a mouse model of skin infection. The purpose of this study was to determine whether platelet-activating factor acetylhydrolase Sse and streptolysin S (SLS) have synergistic contributions to inhibition of neutrophil recruitment and systemic infection in subcutaneous infection of mice by MGAS315, a hypervirulent genotype emm3 GAS strain. Deletion of sse and sagA in MGAS315 synergistically reduced the skin lesion size and GAS burden in the liver and spleen. However, the mutants were persistent at skin sites and had similar growth factors in nonimmune blood. Thus, the low numbers of Δsse ΔsagA mutants in the liver and spleen were likely due to their reduction in the systemic dissemination. Few intact and necrotic neutrophils were detected at MGAS315 infection sites. In contrast, many neutrophils and necrotic cells were present at the edge of Δsse mutant infection sites on day 1 and at the edge of and inside Δsse mutant infection sites on day 2. ΔsagA mutant infection sites had massive numbers of and few intact neutrophils at the edge and center of the infection sites, respectively, on day 1 and were full of intact neutrophils or necrotic cells on day 2. Δsse ΔsagA mutant infection sites had massive numbers of intact neutrophils throughout the whole infection site. These sse and sagA deletion-caused changes in the histological pattern at skin infection sites could be complemented. Thus, the sse and sagA deletions synergistically enhance neutrophil recruitment. These findings indicate that both Sse and SLS are required but that neither is sufficient for inhibition of neutrophil recruitment and systemic infection by hypervirulent GAS.

Organophosphorus Xenobiotic Toxicology

Originally, organophosphorus (OP) toxicology consisted of acetylcholinesterase inhibition by insecticides and chemical threat agents acting as phosphorylating agents for serine in the catalytic triad, but this is no longer the case. Other serine hydrolases can be secondary OP targets, depending on the OP structure, and include neuropathy target esterase, lipases, and endocannabinoid hydrolases. The major OP herbicides are glyphosate and glufosinate, which act in plants but not animals to block aromatic amino acid and glutamine biosynthesis, respectively, with safety for crops conferred by their expression of herbicide-tolerant targets and detoxifying enzymes from bacteria. OP fungicides, pharmaceuticals including calcium retention agents, industrial chemicals, and cytochrome P450 inhibitors act by multiple noncholinergic mechanisms, often with high potency and specificity. One type of OP-containing fire retardant forms a highly toxic bicyclophosphate γ-aminobutyric acid receptor antagonist upon combustion. Some OPs are teratogenic, mutagenic, or carcinogenic by known mechanisms that can be avoided as researchers expand knowledge of OP chemistry and toxicology for future developments in bioregulation.

Phospholipids of Animal and Marine Origin: Structure, Function, and Anti-Inflammatory Properties

In this review paper, the latest literature on the functional properties of phospholipids in relation to inflammation and inflammation-related disorders has been critically appraised and evaluated. The paper is divided into three sections: Section 1 presents an overview of the relationship between structures and biological activities (pro-inflammatory or anti-inflammatory) of several phospholipids with respect to inflammation. Section 2 and Section 3 are dedicated to the structures, functions, compositions and anti-inflammatory properties of dietary phospholipids from animal and marine sources. Most of the dietary phospholipids of animal origin come from meat, egg and dairy products. To date, there is very limited work published on meat phospholipids, undoubtedly due to the negative perception that meat consumption is an unhealthy option because of its putative associations with several chronic diseases. These assumptions are addressed with respect to the phospholipid composition of meat products. Recent research trends indicate that dairy phospholipids possess anti-inflammatory properties, which has led to an increased interest into their molecular structures and reputed health benefits. Finally, the structural composition of phospholipids of marine origin is discussed. Extensive research has been published in relation to ω-3 polyunsaturated fatty acids (PUFAs) and inflammation, however this research has recently come under scrutiny and has proved to be unreliable and controversial in terms of the therapeutic effects of ω-3 PUFA, which are generally in the form of triglycerides and esters. Therefore, this review focuses on recent publications concerning marine phospholipids and their structural composition and related health benefits. Finally, the strong nutritional value of dietary phospholipids are highlighted with respect to marine and animal origin and avenues for future research are discussed.

Class A CpG Oligonucleotide Priming Rescues Mice from Septic Shock via Activation of Platelet-Activating Factor Acetylhydrolase

Sepsis is a life-threatening, overwhelming immune response to infection with high morbidity and mortality. Inflammatory response and blood clotting are caused by sepsis, which induces serious organ damage and death from shock. As a mechanism of pathogenesis, platelet-activating factor (PAF) induces excessive inflammatory responses and blood clotting. In this study, we demonstrate that a Class A CpG oligodeoxynucleotide (CpG-A₁₅₈₅) strongly induced PAF acetylhydrolase, which generates lyso-PAF. CpG-A₁₅₈₅ rescued mice from acute lethal shock and decreased fibrin deposition, a hallmark of PAF-induced disseminated intravascular coagulation. Furthermore, CpG-A₁₅₈₅ improved endotoxin shock induced by lipopolysaccharide, which comprises the cell wall of Gram-negative bacteria and inhibits inflammatory responses induced by cytokines such as interleukin-6 and tumor necrosis factor-α. These results suggest that CpG-A₁₅₈₅ is a potential therapeutic target to prevent sepsis-related induction of PAF.

Alternative Anaphylactic Routes: The Potential Role of Macrophages

Anaphylaxis is an acute, life-threatening, multisystem syndrome resulting from the sudden release of mediators from effector cells. There are two potential pathways for anaphylaxis. The first one, IgE-dependent anaphylaxis, is induced by antigen (Ag) cross-linking of Ag-specific IgE bound to the high-affinity IgE receptor (FcεRI) on mast cells and basophils. The second one, IgG-dependent anaphylaxis is induced by Ag cross-linking of Ag-specific IgG bound to IgG receptors (FcγRI, FcγRIIA, FcγRIIB, FcγRIIC, and FcγRIIIA) on macrophages, neutrophils, and basophils. Macrophages exhibit a huge functional plasticity and are capable of exerting their scavenging, bactericidal, and regulatory functions under a wide variety of tissue conditions. Herein, we will review their potential role in the triggering and development of anaphylaxis. Thereby, macrophages, among other immune cells, play a role in both anaphylactic pathways (1) by responding to anaphylactic mediators secreted by mast cells after specific IgE cross-linking or (2) by acting as effector cells in the anaphylactic response mediated by IgG. In this review, we will go over the cellular and molecular mechanisms that take place in the above-mentioned anaphylactic pathways and will discuss the clinical implications in human allergic reactions.

AMP-activated protein kinase mediates the effects of lipoprotein-associated phospholipase A2 on endothelial dysfunction in atherosclerosis

The present study aimed to investigate the effects of lipoprotein-associated phospholipase A2 (Lp-PLA2) on endothelial dysfunction in an in vitro cell model of atherosclerosis, and to determine whether AMP-activated protein kinase (AMPK) mediates the effects of Lp-PLA2 on endothelial dysfunction. A total of 392 patients with coronary artery disease (CAD), including various sub-conditions, were recruited, and the plasma concentrations of Lp-PLA2 were evaluated. In addition, an in vitro model of atherosclerosis was established by exposing human umbilical vein endothelial cells (HUVECs) to oxidized low-density lipoprotein (oxLDL). SB-435495 was used to inhibit Lp-PLA2, and compound C was used to suppress AMPK expression. Lp-PLA2, AMPKα and phosphorylated-AMPKα (T172) expression in HUVECs were evaluated using western blot analysis. The concentrations of nitric oxide (NO), endothelin 1 (ET-1), intercellular adhesion molecule 1 (ICAM-1) and platelet/endothelial cell adhesion molecule 1 (PECAM-1) in cell culture supernatant were determined using commercially available ELISA kits. MTT assays were employed to indicate changes in cell viability. The current study found the plasma Lp-PLA2 levels were elevated in the CAD patients with stable angina pectoris, unstable angina pectoris, acute coronary syndromes and acute myocardial infarction, compared with a healthy control population. In addition, the in vitro results showed that Lp-PLA2 expression levels were elevated in oxLDL-exposed HUVECs. Lp-PLA2 suppression could increase cell viability, induce the production of NO and decrease the secretion of ET-1, in addition to suppressing the expression of cell adhesion molecules, including ICAM-1 and PECAM-1 in oxLDL-exposed HUVECs. The expression of AMPKα and phosphorylated-AMPKα (T172) was regulated by Lp-PLA2, and AMPK suppression was able to reverse the effects of Lp-PLA2 with regard to cell viability, endothelial vasorelaxation capacity and the secretion of adhesion molecules in oxLDL-exposed HUVECs. In conclusion, the present study provides initial evidence that Lp-PLA2 is able to cause endothelial dysfunction in an in vitro model of atherosclerosis, and the effects of Lp-PLA2 on endothelial dysfunction was at least partially a result of the downregulation of AMPKα, thus contributing to the progression of atherosclerosis.

Platelets in the immune response: Revisiting platelet-activating factor in anaphylaxis

Anaphylaxis is an acute, severe, life-threatening multisystem allergic reaction resulting from the sudden systemic release of biochemical mediators and chemotactic substances. Release of both preformed granule-associated mediators and newly generated lipid-derived mediators contributes to the amplification and prolongation of anaphylaxis. Platelet-activating factor (PAF) is a potent phospholipid-derived mediator the central role of which has been well established in experimental models of both immune-mediated and non-immune mediated anaphylaxis. It is produced and secreted by several types of cells, including mast cells, monocytes, tissue macrophages, platelets, eosinophils, endothelial cells, and neutrophils. PAF is implicated in platelet aggregation and activation through release of vasoactive amines in the inflammatory response, resulting in increased vascular permeability, circulatory collapse, decreased cardiac output, and various other biological effects. PAF is rapidly hydrolyzed and degraded to an inactive metabolite, lysoPAF, by the enzyme PAF acetylhydrolase, the activity of which has shown to correlate inversely with PAF levels and predispose to severe anaphylaxis. In addition to its role in anaphylaxis, PAF has also been implicated as a mediator in both allergic and nonallergic inflammatory diseases, including allergic rhinitis, sepsis, atherosclerotic disease, and malignancy, in which PAF signaling has an established role. The therapeutic role of PAF antagonism has been investigated for several diseases, with variable results thus far. Further investigation of its role in pathology and therapeutic modulation is highly anticipated because of the pressing need for more selective and targeted therapy for the management of severe anaphylaxis.

Platelet-activating factor receptor-mediated PI3K/AKT activation contributes to the malignant development of esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most common malignancies worldwide and occurs at a relatively high frequency in China, yet the mechanisms underlying its devastating outcome remain unclear. Here we report that platelet-activating factor receptor (PAFR), a type of G-protein-coupled receptor, was upregulated in ESCC tumors and cell lines, compared with controls; PAFR levels were positively correlated with ESCC clinical stages and survival time. Overexpression of PAFR promoted the malignant development of ESCC in vitro and in vivo, whereas depletion of PAFR suppressed these effects. Interestingly, PAFR was observed to activate PI3K/AKT (phosphatidylinositol 3-kinase/AKT) through the upregulation of FAK kinase activity. AKT-triggered nuclear factor-κB transcriptionally activated PAFR expression. This mutual positive regulation between PAFR and AKT was required for the aggressiveness of ESCC cells both in vitro and in vivo. Furthermore, treating mice bearing ESCC tumors with cholesterol-conjugated PAFR small interfering RNA effectively inhibited tumor progression and the expression of AKT-mediated oncogenic proteins. Taken together, we made the first demonstration that dysregulation of PAFR and the positive regulatory loop between PAFR and pAKT contribute to malignant progression of ESCC.

The extended phenotypes of marine symbioses: ecological and evolutionary consequences of intraspecific genetic diversity in coral-algal associations

Reef-building corals owe much of their success to a symbiosis with dinoflagellate microalgae in the genus Symbiodinium. In this association, the performance of each organism is tied to that of its partner, and together the partners form a holobiont that can be subject to selection. Climate change affects coral reefs, which are declining globally as a result. Yet the extent to which coral holobionts will be able to acclimate or evolve to handle climate change and other stressors remains unclear. Selection acts on individuals and evidence from terrestrial systems demonstrates that intraspecific genetic diversity plays a significant role in symbiosis ecology and evolution. However, we have a limited understanding of the effects of such diversity in corals. As molecular methods have advanced, so too has our recognition of the taxonomic and functional diversity of holobiont partners. Resolving the major components of the holobiont to the level of the individual will help us assess the importance of intraspecific diversity and partner interactions in coral-algal symbioses. Here, we hypothesize that unique combinations of coral and algal individuals yield functional diversity that affects not only the ecology and evolution of the coral holobiont, but associated communities as well. Our synthesis is derived from reviewing existing evidence and presenting novel data. By incorporating the effects of holobiont extended phenotypes into predictive models, we may refine our understanding of the evolutionary trajectory of corals and reef communities responding to climate change.

Lipoprotein-associated phospholipase A2 regulates macrophage apoptosis via the Akt and caspase-7 pathways

AIM

Mutations in lipoprotein-associated phospholipase A2 (Lp-PLA2) are related to atherosclerosis. However, the molecular effects of Lp-PLA2 on atherosclerosis have not been fully investigated. Therefore, this study attempted to elucidate this issue.

METHODS

Monocytes were isolated from randomly selected healthy male volunteers according to each Lp-PLA2 genotype (wild-type Lp-PLA2 [Lp-PLA2 (V/V)], the heterozygous V279F mutation [LpPLA2 (V/F)] and the homozygous V279F mutation [Lp-PLA2 (F/F)]) and differentiated into macrophages. The level of apoptosis in the macrophages following incubation without serum was measured using the annexin V/propidium iodide double staining method, and the underlying mechanisms were further examined using a culture cell line.

RESULTS

The average plasma Lp-PLA2 concentration [Lp-PLA2 (V/V): 129.4 ng/mL, Lp-PLA2 (V/F): 70.7 ng/mL, Lp-PLA2 (F/F): 0.4 ng/mL] and activity [Lp-PLA2 (V/V): 164.3 nmol/min/mL, LpPLA2 (V/F): 100.9 nmol/min/mL, Lp-PLA2 (F/F): 11.6 nmol/min/mL] were significantly different between each genotype, although the basic clinical characteristics were similar. The percentage of apoptotic cells was significantly higher among the Lp-PLA2 (F/F) macrophages compared with that observed in the Lp-PLA2 (V/V) macrophages. This induction of apoptosis was independent of the actions of acetylated low-density lipoproteins. In addition, the transfection of the expression plasmid of V279F mutant Lp-PLA2 into Cos-7 cells or monocyte/macrophage-like U937 cells promoted apoptosis. The knockdown of Lp-PLA2 also increased the number of apoptotic cells. Among the cells expressing mutant Lp-PLA2, the caspase-7 activity was increased, while the activated Akt level was decreased.

CONCLUSIONS

The V279F mutation of Lp-PLA2 positively regulates the induction of apoptosis in macrophages and Cos-7 cells. An increase in the caspase-7 activity and a reduction in the activated Akt level are likely to be involved in this phenomenon.

A Penicillium sp. F33 metabolite and its synthetic derivatives inhibit acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and carrageenan-induced paw edema in mice

Acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine (lyso-PAF) acetyltransferase is a key enzyme in the biosynthesis of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) in inflammatory cells. Substances which inhibit this enzyme are of therapeutic interest. In this study, we screened for new inhibitors of lyso-PAF acetyltransferase with anti-inflammatory effects. In a metabolite from Penicillium sp. F33, we isolated an acetyltransferase inhibitor identified as dihydrofumigatin (2-methoxy-1,3,4-trihydroxy-5-methylbenzene) from high resolution mass spectrometer and NMR data. Dihydrofumigatin had strong acetyltransferase inhibitory activity, but was not stable in aqueous solution. Thus, we chemically synthesized its oxidized form fumigatin (3-hydroxy-2-methoxy-5-methyl-1,4-benzoquinone) and derivatives thereof, and evaluated their inhibitory effects. Strong inhibitory activity was observed for saturated fatty acid esters of fumigatin; the order of inhibition was 3-decanoyloxy-2-methoxy-5-methyl-1,4-benzoquinone (termed FUD-7, IC₅₀ = 3 μM)>2-methoxy-5-methyl-3-tetradecanoyloxy-1,4-benzoquinone (termed FUD-8, IC₅₀ = 20 μM)>3-hexanoyloxy-2-methoxy-5-methyl-1,4-benzoquinone (IC₅₀ = 139 μM). Interestingly, these compounds also significantly suppressed the gene expression of lyso-PAF acetyltransferase/LPCAT2 in mouse bone marrow-derived macrophages stimulated by lipopolysaccharide (LPS). We further evaluated the effect of these substances on anti-inflammatory activity in vivo using the carrageenan-induced mouse paw edema test. FUD-7 and FUD-8 at 2.5 mg/kg showed significant, 47.9-51.7%, inhibition stronger than that of prednisolone at 10 mg/kg (41.9%). These results suggest that FUD-7 and FUD-8 are potent inhibitors with anti-inflammatory activity.

Characterization of streptococcal platelet-activating factor acetylhydrolase variants that are involved in innate immune evasion

Human pathogen group A streptococcus (GAS) has developed mechanisms to subvert innate immunity. We recently reported that the secreted esterase produced by serotype M1 GAS (SsE(M1)) reduces neutrophil recruitment by targeting platelet-activating factor (PAF). SsE(M1) and SsE produced by serotype M28 GAS (SsE(M28)) have a 37% sequence difference. This study aims at determining whether SsE(M28) is also a PAF acetylhydrolase and participates in innate immune evasion. We also examined whether SsE evolved to target PAF by characterizing the PAF acetylhydrolase (PAF-AH) activity and substrate specificity of SsE(M1), SsE(M28), SeE, the SsE homologue in Streptococcus equi, and human plasma PAF-AH (hpPAF-AH). PAF incubated with SsE(M28) or SeE was converted into lyso-PAF. SsE(M1) and SsE(M28) had kcat values of 373 s(-1) and 467 s(-1), respectively, that were ≥ 30-fold greater than that of hpPAF-AH (12 s(-1)). The comparison of SsE(M1), SsE(M28), and hpPAF-AH in kcat and Km in hydrolyzing triglycerides, acetyl esters, and PAF indicates that the SsE proteins are more potent hydrolases against PAF and have high affinity for PAF. SsE(M28) possesses much lower esterase activities against triglycerides and other esters than SsE(M1) but have similar potency with SsE(M1) in PAF hydrolysis. Deletion of sse(M28) in a covS deletion mutant of GAS increased neutrophil recruitment and reduced skin infection, whereas in trans expression of SsE(M28) in GAS reduced neutrophil infiltration and increased skin invasion in subcutaneous infection of mice. These results suggest that the SsE proteins evolved to target PAF for enhancing innate immune evasion and skin invasion.

Leishmania parasites possess a platelet-activating factor acetylhydrolase important for virulence

Leishmania parasites are intracellular protozoans capable of salvaging and remodeling lipids from the host. To understand the role of lipid metabolism in Leishmania virulence, it is necessary to characterize the enzymes involved in the uptake and turnover of phospholipids. This study focuses on a putative phospholipase A2 (PLA2)/platelet-activating factor acetylhydrolase (PAF-AH) in Leishmania major. In mammals, PAF-AH is a subgroup of PLA2 catalyzing the hydrolysis/inactivation of platelet-activating factor (PAF), a potent mediator of many leukocyte functions. By immunofluorescence microscopy, L. major PLA2/PAF-AH is predominantly localized in the ER. While wild type L. major parasites are able to hydrolyze PAF, this activity is completely absent in the PLA2/PAF-AH-null mutants. Meanwhile, deletion of PLA2/PAF-AH had no significant effect on the turnover of common glycerophospholipids such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol. PLA2/PAF-AH is not required for the growth of L. major parasites in culture, or the production of GPI-anchored virulence factors. Nonetheless, it does play a key role in the mammalian host as the PLA2/PAF-AH null mutants exhibit attenuated virulence in BALB/c mice. In conclusion, these data suggest that Leishmania parasites possess a functional PAF-AH and the degradation of PAF or PAF-like lipids is an important step in infection.

Group A Streptococcus secreted esterase hydrolyzes platelet-activating factor to impede neutrophil recruitment and facilitate innate immune evasion

The innate immune system is the first line of host defense against invading organisms. Thus, pathogens have developed virulence mechanisms to evade the innate immune system. Here, we report a novel means for inhibition of neutrophil recruitment by Group A Streptococcus (GAS). Deletion of the secreted esterase gene (designated sse) in M1T1 GAS strains with (MGAS5005) and without (MGAS2221) a null covS mutation enhances neutrophil ingress to infection sites in the skin of mice. In trans expression of SsE in MGAS2221 reduces neutrophil recruitment and enhances skin invasion. The sse deletion mutant of MGAS5005 (Δsse^MGAS5005) is more efficiently cleared from skin than the parent strain. SsE hydrolyzes the sn-2 ester bond of platelet-activating factor (PAF), converting biologically active PAF into inactive lyso-PAF. K_M and k_cat of SsE for hydrolysis of 2-thio-PAF were similar to those of the human plasma PAF acetylhydrolase. Treatment of PAF with SsE abolishes the capacity of PAF to induce activation and chemotaxis of human neutrophils. More importantly, PAF receptor-deficient mice significantly reduce neutrophil infiltration to the site of Δsse^MGAS5005 infection. These findings identify the first secreted PAF acetylhydrolase of bacterial pathogens and support a novel GAS evasion mechanism that reduces phagocyte recruitment to sites of infection by inactivating PAF, providing a new paradigm for bacterial evasion of neutrophil responses.

GAS is a major human pathogen causing a variety of infections, including pharyngitis and necrotizing fasciitis. GAS pathogenesis is mediated by a large array of secreted and cell-surface virulence factors. However, the functions of many GAS virulence factors are poorly understood. Recently, we reported that the esterase secreted by GAS (SsE) is a CovRS (the control of virulence two component regulatory system)-regulated protective antigen and is critical for spreading in the skin and systemic dissemination of GAS in a mouse model of necrotizing fasciitis. This report presents three major findings regarding the function and functional mechanism of SsE: 1) SsE contributes to GAS inhibition of neutrophil recruitment; 2) SsE is a potent PAF acetylhydrolase and the first secreted bacterial PAF acetylhydrolase identified so far; and 3) the PAF receptor significantly contributes to neutrophil recruitment in skin GAS infection. These findings support a novel mechanism for evasion of the innate immune system by GAS that may be relevant to other infections.

Antioxidant and inflammatory aspects of lipoprotein-associated phospholipase A₂ (Lp-PLA₂): a review

The association of cardiovascular events with Lp-PLA₂ has been studied continuously today. The enzyme has been strongly associated with several cardiovascular risk markers and events. Its discovery was directly related to the hydrolysis of the platelet-activating factor and oxidized phospholipids, which are considered protective functions. However, the hydrolysis of bioactive lipids generates lysophospholipids, compounds that have a pro-inflammatory function. Therefore, the evaluation of the distribution of Lp-PLA₂ in the lipid fractions emphasized the dual role of the enzyme in the inflammatory process, since the HDL-Lp-PLA₂ enzyme contributes to the reduction of atherosclerosis, while LDL-Lp-PLA₂ stimulates this process. Recently, it has been verified that diet components and drugs can influence the enzyme activity and concentration. Thus, the effects of these treatments on Lp-PLA₂ may represent a new kind of prevention of cardiovascular disease. Therefore, the association of the enzyme with the traditional assessment of cardiovascular risk may help to predict more accurately these diseases.

Enhancement of endothelial permeability by coculture with peripheral blood mononuclear cells in the presence of HLA Class II antibody that was associated with transfusion-related acute lung injury

BACKGROUND

HLA Class II antibody-initiated activation of monocytes possessing the corresponding antigen is thought to participate in the pathogenesis of transfusion-related acute lung injury (TRALI). Pulmonary edema, a hallmark of TRALI, is caused by increasing vascular permeability.

STUDY DESIGN AND METHODS

To investigate the contribution of HLA Class II antibody and monocytes to the development of pulmonary edema in TRALI, we studied whether the permeability of human lung microvascular endothelial cells (HMVECs) could be enhanced by coculturing HMVECs with peripheral blood mononuclear cells (PBMNCs) in the presence of HLA Class II antibody-containing plasma, which was implicated in TRALI (anti-HLA-DR plasma). In addition, similar experiments were performed with human umbilical vein endothelial cells (HUVECs). The endothelial permeability to fluoresceinated dextran, which was added from the start of coculture, was measured.

RESULTS

The coculture of HMVECs or HUVECs with PBMNCs in the presence of anti-HLA-DR plasma resulted in the increase of endothelial permeability in the corresponding antigen-antibody-dependent manner. CV-3988, a platelet-activating factor (PAF) receptor antagonist, almost completely suppressed the increase in endothelial permeability. Neutralizing antibodies to tumor necrosis factor (TNF)-α alone and simultaneous addition of the antibodies to TNF-α and interleukin (IL)-1β to the coculture partially suppressed the permeability increase of HMVECs and HUVECs, respectively.

CONCLUSIONS

HLA Class II antibody and monocytes in the corresponding antigen-antibody combination caused the enhancement of endothelial permeability. PAF, TNF-α, and/or IL-1β might be involved in the endothelial permeability increase. HLA Class II antibody-initiated monocyte activation could lead to the development of pulmonary edema in TRALI.

Platelet-activating factor induces TLR4 expression in intestinal epithelial cells: implication for the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a leading cause of morbidity and mortality in neonatal intensive care units, however its pathogenesis is not completely understood. We have previously shown that platelet activating factor (PAF), bacteria and TLR4 are all important factors in the development of NEC. Given that Toll-like receptors (TLRs) are expressed at low levels in enterocytes of the mature gastrointestinal tract, but were shown to be aberrantly over-expressed in enterocytes in experimental NEC, we examined the regulation of TLR4 expression and signaling by PAF in intestinal epithelial cells using human and mouse in vitro cell lines, and the ex vivo rat intestinal loop model. In intestinal epithelial cell (IEC) lines, PAF stimulation yielded upregulation of both TLR4 mRNA and protein expression and led to increased IL-8 secretion following stimulation with LPS (in an otherwise LPS minimally responsive cell line). PAF stimulation resulted in increased human TLR4 promoter activation in a dose dependent manner. Western blotting and immunohistochemical analysis showed PAF induced STAT3 phosphorylation and nuclear translocation in IEC, and PAF-induced TLR4 expression was inhibited by STAT3 and NFκB Inhibitors. Our findings provide evidence for a mechanism by which PAF augments inflammation in the intestinal epithelium through abnormal TLR4 upregulation, thereby contributing to the intestinal injury of NEC.

Lp-PLA2 Inhibition-The Atherosclerosis Panacea?

Based on the complex pathophysiology of atherosclerosis, a large number of biomarkers that relate to lipids, inflammation, immunity, thrombosis and hemostasis, have been investigated experimentally, in epidemiologic studies and in clinical trials. Interest focuses on their potential role to aid in risk stratification, as possible surrogate markers of atherosclerosis, and potential targets for therapy. More recently, one lipid associated biomarker, lipoprotein-associated phospholipase A2 (Lp-PLA₂), has gained considerable interest. In addition to a plausible pathophysiological role by generating pro-inflammatory and pro-atherogenic compounds from oxidized LDL in the vessel wall, there is a large, fairly consistent epidemiological database indicating that increased levels of Lp-PLA₂ mass or activity are associated with increased risk for cardiovascular outcomes; such data further suggest that it might improve risk stratification. In addition, clinical studies indicate that increased Lp-PLA₂ levels are associated with endothelial dysfunction. Moreover, it may also serve as an interesting therapeutic target, since a specific inhibitor of the enzyme is available with promising animal data and initial positive data in humans. Recent experimental data from a hyperlipidemic diabetic pig model strongly suggest that increased Lp-PLA₂ in the vessel wall is associated with a more vulnerable plaque phenotype which can be modulated by inhibiting Lp-PLA₂ activity. A biomarker study in more than 1,000 patients with CHD over three months has demonstrated a positive effect on various inflammatory molecules. In addition, an imaging study using IVUS based modalities (greyscale, virtual histology, and palpography) together with a panel of biomarkers (IBIS-2) has been done in more than 300 patients with CHD treated over 12 months and results indicate that the progression of the necrotic core of the plaque can be retarded. Inhibition of the pro-atherogenic and pro-inflammatory effects of Lp-PLA₂ may therefore contribute to decrease the residual risk in high risk patients already on polypharmacotherapy. This hypothesis is now being tested in two large phase 3 clinical trials. Thus, Lp-PLA₂ indeed may represent a biomarker and a promising target for intervention.

Platelet-activating factor-induced NF-kappaB activation and IL-8 production in intestinal epithelial cells are Bcl10-dependent

BACKGROUND

Platelet-activating factor (PAF), a potent proinflammatory phospholipid mediator, has been implicated in inducing intestinal inflammation in diseases such as inflammatory bowel disease (IBD) and necrotizing enterocolitis (NEC). However, its mechanisms of inducing inflammatory responses are not fully understood. Therefore, studies were designed to explore the mechanisms of PAF-induced inflammatory cascade in intestinal epithelial cells.

METHODS

Nuclear factor kappa B (NF-kappaB) activation was measured by luciferase assay and enzyme-linked immunosorbent assay (ELISA), and interleukin 8 (IL-8) production was determined by ELISA. B-cell lymphoma 10 (Bcl10), caspase recruitment domain-containing membrane-associated guanylate kinase protein 3 (CARMA3), and mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) mRNA and protein levels were assessed by real-time reverse-transcription polymerase chain reaction (RT-PCR) and Western blot, respectively. siRNA silencing of Bcl10 was used to examine its role in PAF-induced NF-kappaB activation and IL-8 production. The promoter region of the Bcl10 gene was cloned with the PCR method and promoter activity measured by luciferase assay.

RESULTS

The adaptor protein Bcl10 appeared to play an important role in the PAF-induced inflammatory pathway in human intestinal epithelial cells. Bcl10 was required for PAF-induced I kappaB alpha phosphorylation, NF-kappaB activation, and IL-8 production in NCM460, a cell line derived from normal human colon, and Caco-2, a transformed human intestinal cell line. PAF also stimulated Bcl10 interactions with CARMA3 and MALT1, and upregulated Bcl10 expression in these cells via transcriptional regulation.

CONCLUSIONS

These findings highlight a novel PAF-induced inflammatory pathway in intestinal epithelial cells, requiring Bcl10 as a critical mediator and involving CARMA3/Bcl10/MALT1 interactions. The proinflammatory effects of PAF play prominent roles in the pathogenesis of IBD and this pathway may present important targets for intervention in chronic inflammatory diseases of the intestine.

Targeting abnormal airway vascularity as a therapeutical strategy in asthma

Asthma is a chronic inflammatory disease of airways, characterized by airway hyperresponsiveness and airflow limitation with acute bronchoconstriction, swelling of the airway wall, chronic mucus plug formation and airway wall remodelling. Functional and structural changes in the vasculature of asthmatic airways have been documented, and the signalling mechanisms are complex and have recently attracted much attention. The vascular changes may affect inflammatory cell recruitment, airway hyperresponsiveness and the regulation of airway calibre, and further, the level of disease control. Many critical factors are involved in the pathophysiological regulation of vascular changes in bronchial asthma, and the actions of these factors must be very carefully orchestrated. By better understanding the complicated actions of each factor, we may be able to advance further in asthma treatment.

Levels of gingival tissue platelet activating factor after conventional and regenerative periodontal surgery

The hypothesis, a relationship between gingival tissue platelet activating factor (PAF) levels and healing after periodontal surgery, was tested by measuring PAF levels in gingival tissues collected from sites that had undergone flap surgery and guided tissue regeneration (GTR) or flap surgery alone. Using a split-mouth design, 20 intrabony defects were randomly assigned to treatment with flap surgery and GTR (group 1) or with flap surgery alone (group 2). Gingival tissue samples were obtained at surgery (baseline) and at 6-month follow-up evaluation visit. One half of each sample was used for analysis of PAF levels by high-performance liquid chromatography, and the other half of the sample was used for histomorphometric analysis that included measurements of number and diameter of blood vessels. PAF levels and diameter of blood vessels were significantly decreased (p < 0.01), and the number of blood vessels was significantly increased (p < 0.05) in both groups after 6 months compared to the baseline values. Postoperative number of blood vessels were significantly higher in group 1 (p < 0.05), whereas there was no significant difference in postoperative PAF levels between the two groups (p > 0.05). Based on the reported results, it is suggested that a decrease in gingival PAF levels might be found after conventional and regenerative periodontal surgery.

On the presence of C2-ceramide in mammalian tissues: possible relationship to etherphospholipids and phosphorylation by ceramide kinase

C(2)-ceramide (N-acetyl-sphingenine) is often used as an analog to study ceramide-mediated cellular processes. According to Lee et al. [J. Biol. Chem. 271 (1996), 209-217], C(2)-ceramide is formed by an acetyl transfer from platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to sphingenine. To substantiate these unconfirmed findings, we (i) developed a method to quantify C(2)-ceramide and (ii) analyzed C(2)-ceramide levels in Pex5(-/-) mice, a model for Zellweger syndrome, in which the synthesis of ether lipids such as PAF is impaired. The presence of C(2)-ceramide could be established in brain (+/-10 pmol/g) and liver (+/-25 pmol/g) from control mice, and was approximately 5000-fold less than the main long-chain ceramide species. In Pex5(-/-) mice, C(2)-ceramide levels did not differ significantly compared to control tissues. Given the presence of a ceramide kinase in mammals, phosphorylation of C(2)-ceramide by human ceramide kinase (HsCERK) was tested. C(2)-ceramide appears to be a good substrate when albumin is used as carrier. In CHO cells overexpressing HsCERK, phosphorylation of exogenously added C(2)-ceramide could also be demonstrated. Our data indicate that C(2)-ceramide is present in mammalian tissues and can be converted to C(2)-ceramide-1-phosphate, in addition to other documented metabolic alterations, but does not seem to be linked to ether lipid metabolism.

Levels of platelet activating factor in gingival crevice fluid following periodontal surgical therapy

BACKGROUND AND OBJECTIVE

Elevated levels of platelet activating factor (PAF), a potent inflammatory phospholipid mediator, have been previously detected in gingival tissues and gingival crevice fluid (GCF) in periodontal disease. However, the role of this mediator during wound healing after periodontal surgery remains unclear. The hypothesis, a relationship between PAF levels and periodontal healing, was tested by measuring PAF levels in GCF samples collected from sites that had undergone guided tissue regeneration (GTR) or flap surgery.

MATERIAL AND METHODS

Using a split-mouth design, 30 intrabony defects were randomly assigned to treatment with GTR (group 1) or to flap surgery (group 2). GCF was sampled pre-operatively and at 6-, 12- and 24-wk follow-up evaluation visits. PAF levels in GCF were analyzed by high-performance liquid chromatography (HPLC).

RESULTS

Both treatment modalities significantly reduced the probing pocket depth and improved the clinical attachment level (p < 0.01). Compared with pre-operative values, the GCF volume and PAF levels were significantly decreased at postoperative weeks 6, 12 and 24 in both groups (p < 0.01). There were also significant differences in GCF volume and PAF levels at all time points up to 24 wks in both groups (p < 0.01). No statistically significant differences were observed in any of the parameters investigated between the two groups (p > 0.05).

CONCLUSION

PAF is detectable in GCF by HPLC and showed a continuous decrease at all the time points monitored following periodontal surgical therapy. This suggests that changes in the levels of this mediator in GCF might be useful for monitoring the progress of periodontal repair and regeneration.

The role of cytosolic phospholipase A2-alfa in regulation of phagocytic functions

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2alpha was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2alpha also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2alpha-deficient PLB cell model. The novel dual subcellular localization of cPLA2alpha in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2alpha in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.

The role of PAF, TLR, and the inflammatory response in neonatal necrotizing enterocolitis

The pathogenesis of neonatal necrotizing enterocolitis remains poorly understood. Recent evidence suggests that PAF (platelet activating factor) and human toll-like receptors (TLRs) contribute to the pro-inflammatory response that is characteristic of NEC pathology. Understanding the regulation of these molecular interactions may provide new approaches for prevention or treatment of this dreaded condition.

Role of histamine and platelet-activating factor in allergic rhinitis

This review is focused on the effects of histamine and platelet-activating factor (PAF) in allergic rhinitis and the plausible implications for therapy. Rhinitis is defined as a heterogeneous disorder resulting from an IgE-mediated reaction associated with nasal inflammation of variable intensity. Two phases of response are triggered by an IgE/allergen cross-linking event: the first is the release of preformed mediators such as histamine or interleukins from mast cells and basophils; the second begins when cells start producing lipid-derived mediators. One of these mediators is PAF. Apart from leukotrienes, PAF is perhaps the most potent inflammatory mediator in allergic rhinitis for inducing vascular leakage, a response that may contribute to the appearance of rhinorrhea and nasal congestion.

cDNA Cloning and Expression of Biologically Active Platelet Activating Factor-Acetylhydrolase (PAF-AH) from Bovine Mammary Gland

Platelet activating factor (PAF)-acetylhydrolase (PAF-AH) is an enzyme that hydrolyzes the acetyl ester at the sn-2 position of PAF, and converts it to the inactive metabolite, lyso PAF. This enzyme is distributed widely in the intracellular as well as the extracellular matrix and is believed to be a defense mechanism that protects the host against the toxic effects of PAF and other biologically active oxidized phospholipids. Purification and expression of cDNA cloning of the intracellular and extracellular types of PAF-AH from several sources from different species have been reported. In this study, the cDNA for PAF-AH was cloned by reverse transcription (RT)-PCR from total RNA of bovine mammary gland. The complete amino acid sequences from the cDNA contains 444 amino acids and was identical to that of the PAF-AH isolated from the bovine spleen cDNA library except for two mismatches of amino acid residues (Thr-247 to Met and Ile-431 to Thr). Recombinant PAF-AH was expressed in HEK 293 cells, which exhibited enzyme activity in the in vitro assay system. Furthermore, recombinant bovine PAF-AH was identified by western blot using human plasma PAF-AH antibody as a monomeric polypeptide with a molecular weight of approximately 43 kDa. This protein can be applied to in vivo models to test its protective role against the deleterious PAF actions.

Platelet-activating factor-induced chemokine gene expression requires NF-kappaB activation and Ca2+/calcineurin signaling pathways. Inhibition by receptor phosphorylation and beta-arrestin recruitment

Previously, we reported that platelet-activating factor (PAF) stimulates higher G protein activation and a more robust Ca2+ mobilization in RBL-2H3 cells expressing carboxyl terminus deletion, phosphorylation-deficient mutant of PAF receptor (mPAFR) when compared with the wild-type receptor (PAFR). However, PAF did not provide sufficient signal for CC chemokine receptor ligand 2 (CCL2) production in cells expressing mPAFR. Based on these findings, we hypothesized that receptor phosphorylation provides a G protein-independent signal that synergizes with Ca2+ mobilization to induce CCL2 production. Here, we show that a mutant of PAFR (D289A), which does not couple to G proteins, was resistant to agonist-induced receptor phosphorylation. Unexpectedly, we found that when this mutant was coexpressed with mPAFR, it restored NF-kappaB activation and CCL2 production. PAF caused translocation of beta-arrestin from the cytoplasm to the membrane in cells expressing PAFR but not a phosphorylation-deficient mutant in which all Ser/Thr residues were replaced with Ala (DeltaST-PAFR). Interestingly, PAF induced significantly higher NF-kappaB and nuclear factor of activated T cells (NFAT)-luciferase activity as well as CCL2 production in cells expressing DeltaST-PAFR than those expressing PAFR. Furthermore, a Ca2+/calcineurin inhibitor completely inhibited PAF-induced NFAT activation and CCL2 production but not NF-kappaB activation. These findings suggest that the carboxyl terminus of PAFR provides a G protein-independent signal for NF-kappaB activation, which synergizes with G protein-mediated Ca2+/calcineurin activation to induce CCL2 production. However, receptor phosphorylation and beta-arrestin recruitment inhibit CCL2 production by blocking both NF-kappaB activation and Ca2+/calcineurin-dependent signaling pathways.

Sponge-induced angiogenesis and inflammation in PAF receptor-deficient mice (PAFR-KO)

1. To determine biological functions of platelet-activating factor (PAF) in chronic inflammation, we have investigated the kinetics of angiogenesis, inflammatory cells recruitment and cytokine production in sponge-induced granuloma in wild type and PAF receptor-deficient mice (PAFR-KO). 2. Angiogenesis as determined by morphometric analysis and hemoglobin content was significantly higher in the implants of PAFR-KO mice at all time points. Treatment with PAF receptor antagonist UK74505 (30 mg kg(-1)) also increased angiogenesis in sponge implants. 3. Neutrophils and macrophages accumulation, as determined by myeloperoxidase and N-acetylglucosaminidase activities in the supernatant of implanted sponges were markedly decreased in PAFR-KO mice. Surprisingly, the levels of the proinflammatory chemokines, keratinocyte-derived chemokine and chemokine monocyte chemoattractant protein 1 were higher in the implants of the transgenic animals. 4. We have shown that angiogenesis was stimulated in PAFR-KO mice whereas inflammation was decreased, indicating that PAF is an endogenous regulator of new blood vessels formation in the inflammatory microenvironment induced by the sponge implant.