A calcium-dependent acyltransferase that produces N-acyl phosphatidylethanolamines

Mast Cell-Derived Histamine Regulates Liver Ketogenesis via Oleoylethanolamide Signaling

The conversion of lipolysis-derived fatty acids into ketone bodies (ketogenesis) is a crucial metabolic adaptation to prolonged periods of food scarcity. The process occurs primarily in liver mitochondria and is initiated by fatty-acid-mediated stimulation of the ligand-operated transcription factor, peroxisome proliferator-activated receptor-α (PPAR-α). Here, we present evidence that mast cells contribute to the control of fasting-induced ketogenesis via a paracrine mechanism that involves secretion of histamine into the hepatic portal circulation, stimulation of liver H₁ receptors, and local biosynthesis of the high-affinity PPAR-α agonist, oleoylethanolamide (OEA). Genetic or pharmacological interventions that disable any one of these events, including mast cell elimination, deletion of histamine- or OEA-synthesizing enzymes, and H₁ blockade, blunt ketogenesis without affecting lipolysis. The results reveal an unexpected role for mast cells in the regulation of systemic fatty-acid homeostasis, and suggest that OEA may act in concert with lipolysis-derived fatty acids to activate liver PPAR-α and promote ketogenesis.

Perturbation-Based Proteomic Correlation Profiling as a Target Deconvolution Methodology

Molecular target identification of small molecules, so-called target deconvolution, is a major obstacle to phenotype-based drug discovery. Here, we developed an approach called perturbation-based proteomic correlation profiling (PPCP) utilizing the correlation between protein quantity and binding activity of compounds under cellular perturbation by gene silencing and successfully identified lanosterol synthase as a molecular target of TGF-β pathway inhibitor. This PPCP concept was extended to the use of a cell line panel and provides a new option for target deconvolution.

NAPE-PLD controls OEA synthesis and fat absorption by regulating lipoprotein synthesis in an in vitro model of intestinal epithelial cells

Oleoylethanolamide (OEA), a fatty acid ethanolamide (FAE), is a lipid mediator that controls food intake and lipid metabolism. Accumulating data imply the importance of intestinal OEA in controlling satiety in addition to gastrointestinal peptide hormones. Although the biochemical pathway of FAE production has been illustrated, the enzymes responsible for the cleavage of OEA from its precursor N-acyl-phosphatidylethanolamine (NAPE) must be identified among reported candidates in the gut. In this study, we assessed the involvement of NAPE-specific phospholipase D (NAPE-PLD), which can directly release FAEs from NAPE, in intestinal OEA synthesis and lipid metabolism. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPER-associated protein 9 (Cas9)-mediated deletion of the NAPE-PLD gene in intestinal epithelial-like Caco-2 cells reduced OEA levels, regardless of their differentiation states. Transcriptome analysis revealed that deletion of NAPE-PLD activates a transcriptional program for nutrient transportation, including lipids and lipoproteins, and inactivates cell-cycle or mitosis-related genes in Caco-2 cells. In addition, the basolateral secretion of lipoproteins was increased in NAPE-PLD-deleted cells although lipoprotein size was not affected. By contrast, cellular lipid levels were reduced in NAPE-PLD-deleted cells. Overall, these results indicate that NAPE-PLD plays important roles in OEA synthesis and fat absorption by regulating lipoprotein production in the intestinal epithelial cells.-Igarashi, M., Watanabe, K., Tsuduki, T., Kimura, I., Kubota, N. NAPE-PLD controls OEA synthesis and fat absorption by regulating lipoprotein synthesis in an in vitro model of intestinal epithelial cells.

Endocannabinoids and related N-acylethanolamines: biological activities and metabolism

The plant Cannabis sativa contains cannabinoids represented by Δ⁹-tetrahydrocannabinol, which exert psychoactivity and immunomodulation through cannabinoid CB1 and CB2 receptors, respectively, in animal tissues. Arachidonoylethanolamide (also referred to as anandamide) and 2-arachidonoylglycerol (2-AG) are well known as two major endogenous agonists of these receptors (termed "endocannabinoids") and show various cannabimimetic bioactivities. However, only 2-AG is a full agonist for CB1 and CB2 and mediates retrograde signals at the synapse, strongly suggesting that 2-AG is physiologically more important than anandamide. The metabolic pathways of these two endocannabinoids are completely different. 2-AG is mostly produced from inositol phospholipids via diacylglycerol by phospholipase C and diacylglycerol lipase and then degraded by monoacylglycerol lipase. On the other hand, anandamide is concomitantly produced with larger amounts of other N-acylethanolamines via N-acyl-phosphatidylethanolamines (NAPEs). Although this pathway consists of calcium-dependent N-acyltransferase and NAPE-hydrolyzing phospholipase D, recent studies revealed the involvement of several new enzymes. Quantitatively major N-acylethanolamines include palmitoylethanolamide and oleoylethanolamide, which do not bind to cannabinoid receptors but exert anti-inflammatory, analgesic, and anorexic effects through receptors such as peroxisome proliferator-activated receptor α. The biosynthesis of these non-endocannabinoid N-acylethanolamines rather than anandamide may be the primary significance of this pathway. Here, we provide an overview of the biological activities and metabolisms of endocannabinoids (2-AG and anandamide) and non-endocannabinoid N-acylethanolamines.

Development of a Multiplexed Activity-Based Protein Profiling Assay to Evaluate Activity of Endocannabinoid Hydrolase Inhibitors

Endocannabinoids, an important class of signaling lipids involved in health and disease, are predominantly synthesized and metabolized by enzymes of the serine hydrolase superfamily. Activity-based protein profiling (ABPP) using fluorescent probes, such as fluorophosphonate (FP)-TAMRA and β-lactone-based MB064, enables drug discovery activities for serine hydrolases. FP-TAMRA and MB064 have distinct, albeit partially overlapping, target profiles but cannot be used in conjunction due to overlapping excitation/emission spectra. We therefore synthesized a novel FP-probe with a green BODIPY as a fluorescent tag and studied its labeling profile in mouse proteomes. Surprisingly, we found that the reporter tag plays an important role in the binding potency and selectivity of the probe. A multiplexed ABPP assay was developed in which a probe cocktail of FP-BODIPY and MB064 visualized most endocannabinoid serine hydrolases in mouse brain proteomes in a single experiment. The multiplexed ABPP assay was employed to profile endocannabinoid hydrolase inhibitor activity and selectivity in the mouse brain.

Lipidomics Suggests a New Role for Ceramide Synthase in Phagocytosis

Phagocytosis is an evolutionarily conserved biological process where pathogens or cellular debris are cleared by engulfing them in a membrane-enclosed cellular compartment called the phagosome. The formation, maturation, and subsequent degradation of a phagosome is an important immune response essential for protection against many pathogens. Yet, the global lipid profile of phagosomes remains unknown, especially as a function of their maturation in immune cells. Here, we show using mass spectrometry based quantitative lipidomics that the ceramide class of lipids, especially very long chain ceramides, are enriched on maturing phagosomes with a concomitant decrease in the biosynthetic precursors of ceramides. We thus posit a new function for the enzyme ceramide synthase during phagocytosis in mammalian macrophages. Biochemical assays, cellular lipid feeding experiments, and pharmacological blockade of ceramide synthase together show that this enzyme indeed controls the flux of ceramides on maturing phagosomes. We also find similar results in the primitive eukaryote Dictyostelium discoideum, suggesting that ceramide enrichment may be evolutionarily conserved and likely an indispensible step in phagosome maturation.

Lipid-metabolizing serine hydrolases in the mammalian central nervous system: endocannabinoids and beyond

The metabolic serine hydrolases hydrolyze ester, amide, or thioester bonds found in broad small molecule substrates using a conserved activated serine nucleophile. The mammalian central nervous system (CNS) express a diverse repertoire of serine hydrolases that act as (phospho)lipases or lipid amidases to regulate lipid metabolism and signaling vital for normal neurocognitive function and CNS integrity. Advances in genomic DNA sequencing have provided evidence for the role of these lipid-metabolizing serine hydrolases in neurologic, psychiatric, and neurodegenerative disorders. This review briefly summarizes recent progress in understanding the biochemical and (patho)physiological roles of these lipid-metabolizing serine hydrolases in the mammalian CNS with a focus on serine hydrolases involved in the endocannabinoid system. The development and application of specific inhibitors for an individual serine hydrolase, if available, are also described. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

Chemical Proteomic Analysis of Serine Hydrolase Activity in Niemann-Pick Type C Mouse Brain

The endocannabinoid system (ECS) is considered to be an endogenous protective system in various neurodegenerative diseases. Niemann-Pick type C (NPC) is a neurodegenerative disease in which the role of the ECS has not been studied yet. Most of the endocannabinoid enzymes are serine hydrolases, which can be studied using activity-based protein profiling (ABPP). Here, we report the serine hydrolase activity in brain proteomes of a NPC mouse model as measured by ABPP. Two ABPP methods are used: a gel-based method and a chemical proteomics method. The activities of the following endocannabinoid enzymes were quantified: diacylglycerol lipase (DAGL) α, α/β-hydrolase domain-containing protein 4, α/β-hydrolase domain-containing protein 6, α/β-hydrolase domain-containing protein 12, fatty acid amide hydrolase, and monoacylglycerol lipase. Using the gel-based method, two bands were observed for DAGL α. Only the upper band corresponding to this enzyme was significantly decreased in the NPC mouse model. Chemical proteomics showed that three lysosomal serine hydrolase activities (retinoid-inducible serine carboxypeptidase, cathepsin A, and palmitoyl-protein thioesterase 1) were increased in Niemann-Pick C1 protein knockout mouse brain compared to wild-type brain, whereas no difference in endocannabinoid hydrolase activity was observed. We conclude that these targets might be interesting therapeutic targets for future validation studies.

Non-endocannabinoid N-acylethanolamines and 2-monoacylglycerols in the intestine

This review focuses on recent findings of the physiological and pharmacological role of non-endocannabinoid N-acylethanolamines (NAEs) and 2-monoacylglycerols (2-MAGs) in the intestine and their involvement in the gut-brain signalling. Dietary fat suppresses food intake, and much research concerns the known gut peptides, for example, glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK). NAEs and 2-MAGs represent another class of local gut signals most probably involved in the regulation of food intake. We discuss the putative biosynthetic pathways and targets of NAEs in the intestine as well as their anorectic role and changes in intestinal levels depending on the dietary status. NAEs can activate the transcription factor PPARα, but studies to evaluate the role of endogenous NAEs are generally lacking. Finally, we review the role of diet-derived 2-MAGs in the secretion of anorectic gut peptides via activation of GPR119. Both PPARα and GPR119 have potential as pharmacological targets for the treatment of obesity and the former for treatment of intestinal inflammation. LINKED ARTICLES: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

N-acylethanolamine hydrolyzing acid amidase inhibition: tools and potential therapeutic opportunities

N-acylethanolamines (NAEs) (e.g., N-palmitoylethanolamine, N-arachidonoylethanolamine, N-oleoylethanolamine) are bioactive lipids involved in many physiological processes including pain, inflammation, anxiety, cognition and food intake. Two enzymes are responsible for the hydrolysis of NAEs and therefore regulate their endogenous levels and effects: fatty acid amide hydrolase (FAAH) and N-acylethanolamine-hydrolyzing acid amidase (NAAA). As discussed here, extensive biochemical characterization of NAAA was carried out over the years that contributed to a better understanding of NAAA enzymology. An increasing number of studies describe the synthesis and pharmacological characterization of NAAA inhibitors. Recent medicinal chemistry efforts have led to the development of potent and stable inhibitors that enable studying the effects of NAAA inhibition in preclinical disease models, notably in the context of pain and inflammation.

Cassaine diterpenoids from the seeds of Erythrophleum fordii and their cytotoxic activities

Six new cassaine diterpenoids (1, 3-7), along with three known ones (2, 8-9) were isolated from the seeds of Erythrophleum fordii. Their structures were elucidated by extensive spectroscopic methods and acid hydrolysis. Compound 2 was tested to be the most potent one and showed more sensitive activities on MCF-7 and A549 cancer cells with IC₅₀ values of 3.66 ± 1.20 and 2.87 ± 0.46 μM, respectively. Furthermore, compound 2 reduced the number of cell colonies significantly in a dose-dependent manner in the colony formation assay and triggered apoptosis of MCF-7 cell.

Phosphatidylserine-stimulated production of N-acyl-phosphatidylethanolamines by Ca2+-dependent N-acyltransferase

N-acyl-phosphatidylethanolamine (NAPE) is known to be a precursor for various bioactive N-acylethanolamines including the endocannabinoid anandamide. NAPE is produced in mammals through the transfer of an acyl chain from certain glycerophospholipids to phosphatidylethanolamine (PE) by Ca²⁺-dependent or -independent N-acyltransferases. The ε isoform of mouse cytosolic phospholipase A₂ (cPLA₂ε) was recently identified as a Ca²⁺-dependent N-acyltransferase (Ca-NAT). In the present study, we first showed that two isoforms of human cPLA₂ε function as Ca-NAT. We next purified both mouse recombinant cPLA₂ε and its two human orthologues to examine their catalytic properties. The enzyme absolutely required Ca²⁺ for its activity and the activity was enhanced by phosphatidylserine (PS). PS enhanced the activity 25-fold in the presence of 1 mM CaCl₂ and lowered the EC₅₀ value of Ca²⁺ >8-fold. Using a PS probe, we showed that cPLA₂ε largely co-localizes with PS in plasma membrane and organelles involved in the endocytic pathway, further supporting the interaction of cPLA₂ε with PS in living cells. Finally, we found that the Ca²⁺-ionophore ionomycin increased [¹⁴C]NAPE levels >10-fold in [¹⁴C]ethanolamine-labeled cPLA₂ε-expressing cells while phospholipase A/acyltransferase-1, acting as a Ca²⁺-independent N-acyltransferase, was insensitive to ionomycin for full activity. In conclusion, PS potently stimulated the Ca²⁺-dependent activity and human cPLA₂ε isoforms also functioned as Ca-NAT.

Whole-exome sequencing and gene-based rare variant association tests suggest that PLA2G4E might be a risk gene for panic disorder

Panic disorder (PD) is characterized by recurrent and unexpected panic attacks, subsequent anticipatory anxiety, and phobic avoidance. Recent epidemiological and genetic studies have revealed that genetic factors contribute to the pathogenesis of PD. We performed whole-exome sequencing on one Japanese family, including multiple patients with panic disorder, which identified seven rare protein-altering variants. We then screened these genes in a Japanese PD case-control group (384 sporadic PD patients and 571 controls), resulting in the detection of three novel single nucleotide variants as potential candidates for PD (chr15: 42631993, T>C in GANC; chr15: 42342861, G>T in PLA2G4E; chr20: 3641457, G>C in GFRA4). Statistical analyses of these three genes showed that PLA2G4E yielded the lowest p value in gene-based rare variant association tests by Efficient and Parallelizable Association Container Toolbox algorithms; however, the p value did not reach the significance threshold in the Japanese. Likewise, in a German case-control study (96 sporadic PD patients and 96 controls), PLA2G4E showed the lowest p value but again did not reach the significance threshold. In conclusion, we failed to find any significant variants or genes responsible for the development of PD. Nonetheless, our results still leave open the possibility that rare protein-altering variants in PLA2G4E contribute to the risk of PD, considering the function of this gene.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Fever and hypothermia in systemic inflammation

Systemic inflammation-associated syndromes (e.g., sepsis and septic shock) often have high mortality and remain a challenge in emergency medicine. Systemic inflammation is usually accompanied by changes in body temperature: fever or hypothermia. In animal studies, systemic inflammation is often modeled by administering bacterial lipopolysaccharide, which triggers autonomic and behavioral thermoeffector responses and causes either fever or hypothermia, depending on the dose and ambient temperature. Fever and hypothermia are regulated changes of body temperature, which correspond to mild and severe forms of systemic inflammation, respectively. Mediators of fever and hypothermia are called endogenous pyrogens and cryogens; they are produced when the innate immune system recognizes an infectious pathogen. Upon an inflammatory challenge, hepatic and pulmonary macrophages (and later brain endothelial cells) start to release lipid mediators, of which prostaglandin (PG) E₂ plays the key role, and cytokines. Blood PGE₂ enters the brain and triggers fever. At later stages of fever, PGE₂ synthesized within the blood-brain barrier maintains fever. In both cases, PGE₂ is synthesized by cyclooxygenase-2 and microsomal PGE₂synthase-1. Mediators of hypothermia are not well established. Both fever and hypothermia are beneficial host defense responses. Based on evidence from studies in laboratory animals and clinical trials in humans, fever is beneficial for fighting mild infection. Based mainly on animal studies, hypothermia is beneficial in severe systemic inflammation and infection.

Confronting the catalytic dark matter encoded by sequenced genomes

The post-genomic era has provided researchers with a deluge of protein sequences. However, a significant fraction of the proteins encoded by sequenced genomes remains without an identified function. Here, we aim at determining how many enzymes of uncertain or unknown function are still present in the Saccharomyces cerevisiae and human proteomes. Using information available in the Swiss-Prot, BRENDA and KEGG databases in combination with a Hidden Markov Model-based method, we estimate that >600 yeast and 2000 human proteins (>30% of their proteins of unknown function) are enzymes whose precise function(s) remain(s) to be determined. This illustrates the impressive scale of the ‘unknown enzyme problem’. We extensively review classical biochemical as well as more recent systematic experimental and computational approaches that can be used to support enzyme function discovery research. Finally, we discuss the possible roles of the elusive catalysts in light of recent developments in the fields of enzymology and metabolism as well as the significance of the unknown enzyme problem in the context of metabolic modeling, metabolic engineering and rare disease research.

An involvement of phospholipase A/acyltransferase family proteins in peroxisome regulation and plasmalogen metabolism

The H-Ras-like suppressor (HRASLS) is a protein family consisting of five members in humans. Despite their discovery as tumor suppressors, we demonstrated that all these proteins are phospholipid-metabolizing enzymes, such as phospholipase (PL) A₁ /A₂ and acyltransferase. We thus proposed to rename HRASLS1-5 as PLA/acyltransferase (PLAAT)-1-5. Notably, PLAATs exhibit N-acyltransferase activity to biosynthesize N-acylated ethanolamine phospholipids, including N-acyl-plasmalogen, which serve as precursors of bioactive N-acylethanolamines. Furthermore, the overexpression of PLAAT-3 in animal cells causes disappearance of peroxisomes and a remarkable reduction in plasmalogen levels. This finding might be related to the inhibitory effect of PLAAT-3 on the chaperone activity of the peroxin PEX19. In this article, we will review our recent findings about PLAAT proteins, with special reference to their roles in peroxisome biogenesis and plasmalogen metabolism.

Mammalian enzymes responsible for the biosynthesis of N-acylethanolamines

Bioactive N-acylethanolamines (NAEs) are ethanolamides of long-chain fatty acids, including palmitoylethanolamide, oleoylethanolamide and anandamide. In animal tissues, NAEs are biosynthesized from membrane phospholipids. The classical "transacylation-phosphodiesterase" pathway proceeds via N-acyl-phosphatidylethanolamine (NAPE), which involves the actions of two enzymes, NAPE-generating Ca²⁺-dependent N-acyltransferase (Ca-NAT) and NAPE-hydrolyzing phospholipase D (NAPE-PLD). Recent identification of Ca-NAT as Ɛ isoform of cytosolic phospholipase A₂ enabled the further molecular biological approaches toward this enzyme. In addition, Ca²⁺-independent NAPE formation was shown to occur by N-acyltransferase activity of a group of proteins named phospholipase A/acyltransferases (PLAAT)-1-5. The analysis of NAPE-PLD-deficient mice confirmed that NAEs can be produced through multi-step pathways bypassing NAPE-PLD. The NAPE-PLD-independent pathways involved three members of the glycerophosphodiesterase (GDE) family (GDE1, GDE4 and GDE7) as well as α/β-hydrolase domain-containing protein (ABHD)4. In this review article, we will focus on recent progress made and latest insights in the enzymes involved in NAE synthesis and their further characterization.

Metabolism of the Endocannabinoid Anandamide: Open Questions after 25 Years

Cannabis extracts have been used for centuries, but its main active principle ∆⁹-tetrahydrocannabinol (THC) was identified about 50 years ago. Yet, it is only 25 years ago that the first endogenous ligand of the same receptors engaged by the cannabis agents was discovered. This “endocannabinoid (eCB)” was identified as N-arachidonoylethanolamine (or anandamide (AEA)), and was shown to have several receptors, metabolic enzymes and transporters that altogether drive its biological activity. Here I report on the latest advances about AEA metabolism, with the aim of focusing open questions still awaiting an answer for a deeper understanding of AEA activity, and for translating AEA-based drugs into novel therapeutics for human diseases.

Endocannabinoid Turnover

In this review, we consider the biosynthetic, hydrolytic, and oxidative metabolism of the endocannabinoids anandamide and 2-arachidonoylglycerol. We describe the enzymes associated with these events and their characterization. We identify the inhibitor profile for these enzymes and the status of therapeutic exploitation, which to date has been limited to clinical trials for fatty acid amide hydrolase inhibitors. To bring the review to a close, we consider whether point block of a single enzyme is likely to be the most successful approach for therapeutic exploitation of the endocannabinoid system.

Oleic acid-derived oleoylethanolamide: A nutritional science perspective

The fatty acid ethanolamide oleoylethanolamide (OEA) is an endogenous lipid mediator derived from the monounsaturated fatty acid, oleic acid. OEA is synthesized from membrane glycerophospholipids and is a high-affinity agonist of the nuclear transcription factor peroxisome proliferator-activated receptor α (PPAR-α). Dietary intake of oleic acid elevates circulating levels of OEA in humans by increasing substrate availability for OEA biosynthesis. Numerous clinical studies demonstrate a beneficial relationship between high-oleic acid diets and body composition, with emerging evidence to suggest OEA may mediate this response through modulation of lipid metabolism and energy intake. OEA exposure has been shown to stimulate fatty acid uptake, lipolysis, and β-oxidation, and also promote food intake control. Future research on high-oleic acid diets and body composition is warranted to confirm these outcomes and elucidate the underlying mechanisms by which oleic acid exerts its biological effects. These findings have significant practical implications, as the oleic acid-derived OEA molecule may be a promising therapeutic agent for weight management and obesity treatment.

Coenzyme-A-Independent Transacylation System; Possible Involvement of Phospholipase A2 in Transacylation

The coenzyme A (CoA)-independent transacylation system catalyzes fatty acid transfer from phospholipids to lysophospholipids in the absence of cofactors such as CoA. It prefers to use C20 and C22 polyunsaturated fatty acids such as arachidonic acid, which are esterified in the glycerophospholipid at the sn-2 position. This system can also acylate alkyl ether-linked lysophospholipids, is involved in the enrichment of arachidonic acid in alkyl ether-linked glycerophospholipids, and is critical for the metabolism of eicosanoids and platelet-activating factor. Despite their importance, the enzymes responsible for these reactions have yet to be identified. In this review, we describe the features of the Ca²⁺-independent, membrane-bound CoA-independent transacylation system and its selectivity for arachidonic acid. We also speculate on the involvement of phospholipase A2 in the CoA-independent transacylation reaction.

Lipoquality control by phospholipase A2 enzymes

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

Dysfunctional oleoylethanolamide signaling in a mouse model of Prader-Willi syndrome

Prader-Willi syndrome (PWS), the leading genetic cause of obesity, is characterized by a striking hyperphagic behavior that can lead to obesity, type-2 diabetes, cardiovascular disease and death. The molecular mechanism underlying impaired satiety in PWS is unknown. Oleoylethanolamide (OEA) is a lipid mediator involved in the control of feeding, body weight and energy metabolism. OEA produced by small-intestinal enterocytes during dietary fat digestion activates type-α peroxisome proliferator-activated receptors (PPAR-α) to trigger an afferent signal that causes satiety. Emerging evidence from genetic and human laboratory studies suggests that deficits in OEA-mediated signaling might be implicated in human obesity. In the present study, we investigated whether OEA contributes to feeding dysregulation in Magel2^m+/p- (Magel2 KO) mice, an animal model of PWS. Fasted/refed male Magel2 KO mice eat more than do their wild-type littermates and become overweight with age. Meal pattern analyses show that hyperphagia in Magel2 KO is due to increased meal size and meal duration rather than to lengthening of the intermeal interval, which is suggestive of a defect in mechanisms underlying satiation. Food-dependent OEA accumulation in jejunum and fasting OEA levels in plasma are significantly greater in Magel2 KO mice than in wild-type controls. Together, these findings indicate that deletion of the Magel2 gene is accompanied by marked changes in OEA signaling. Importantly, intraperitoneal administration of OEA (10mg/kg) significantly reduces food intake in fasted/refed Magel2 KO mice, pointing to a possible use of this natural compound to control hunger in PWS.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Comparative analyses of isoforms of the calcium-independent phosphatidylethanolamine N-acyltransferase PLAAT-1 in humans and mice

N-Acylphosphatidylethanolamines (NAPEs) are a class of glycerophospholipids, which are known as precursors for different bioactive N-acylethanolamines. We previously reported that phospholipase A/acyltransferase-1 (PLAAT-1), which was originally found in mammals as a tumor suppressor, catalyzes N-acylation of phosphatidylethanolamines to form NAPEs. However, recent online database suggested the presence of an uncharacterized isoform of PLAAT-1 with an extra sequence at the N terminus. In the present study, we examined the occurrence, intracellular localization, and catalytic properties of this longer isoform, as well as the original shorter isoform from humans and mice. Our results showed that human tissues express the longer isoform but not the short isoform at all. In contrast, mice expressed both isoforms with different tissue distribution. Unlike the cytoplasmic localization of the shorter isoform, the long isoform was found in both cytoplasm and nucleus, inferring that the extra sequence harbors a nuclear localization signal. As assayed with purified proteins, neither isoform required calcium for full activity. Moreover, the overexpression of each isoform remarkably increased cellular NAPE levels. These results conclude that the new long isoform of PLAAT-1 is a calcium-independent N-acyltransferase existing in both cytoplasm and nucleus and suggest a possible formation of NAPEs in various membrane structures including nuclear membrane. J. Lipid Res 2016. 57: 2051-2060.