Activation of myocardial cAMP-dependent protein kinase by lysoplasmenylcholine

Adipocyte lysoplasmalogenase TMEM86A regulates plasmalogen homeostasis and protein kinase A-dependent energy metabolism

Dysregulation of adipose tissue plasmalogen metabolism is associated with obesity-related metabolic diseases. We report that feeding mice a high-fat diet reduces adipose tissue lysoplasmalogen levels and increases transmembrane protein 86 A (TMEM86A), a putative lysoplasmalogenase. Untargeted lipidomic analysis demonstrates that adipocyte-specific TMEM86A-knockout (AKO) increases lysoplasmalogen content in adipose tissue, including plasmenyl lysophosphatidylethanolamine 18:0 (LPE P-18:0). Surprisingly, TMEM86A AKO increases protein kinase A signalling pathways owing to inhibition of phosphodiesterase 3B and elevation of cyclic adenosine monophosphate. TMEM86A AKO upregulates mitochondrial oxidative metabolism, elevates energy expenditure, and protects mice from metabolic dysfunction induced by high-fat feeding. Importantly, the effects of TMEM86A AKO are largely reproduced in vitro and in vivo by LPE P-18:0 supplementation. LPE P-18:0 levels are significantly lower in adipose tissue of human patients with obesity, suggesting that TMEM86A inhibition or lysoplasmalogen supplementation might be therapeutic approaches for preventing or treating obesity-related metabolic diseases.

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.

Roles of endogenous ether lipids and associated PUFAs in the regulation of ion channels and their relevance for disease

Ether lipids (ELs) are lipids characterized by the presence of either an ether linkage (alkyl lipids) or a vinyl ether linkage [i.e., plasmalogens (Pls)] at the sn1 position of the glycerol backbone, and they are enriched in PUFAs at the sn2 position. In this review, we highlight that ELs have various biological functions, act as a reservoir for second messengers (such as PUFAs) and have roles in many diseases. Some of the biological effects of ELs may be associated with their ability to regulate ion channels that control excitation-contraction/secretion/mobility coupling and therefore cell physiology. These channels are embedded in lipid membranes, and lipids can regulate their activities directly or indirectly as second messengers or by incorporating into membranes. Interestingly, ELs and EL-derived PUFAs have been reported to play a key role in several pathologies, including neurological disorders, cardiovascular diseases, and cancers. Investigations leading to a better understanding of their mechanisms of action in pathologies have opened a new field in cancer research. In summary, newly identified lipid regulators of ion channels, such as ELs and PUFAs, may represent valuable targets to improve disease diagnosis and advance the development of new therapeutic strategies for managing a range of diseases and conditions.

From peroxisomal disorders to common neurodegenerative diseases - the role of ether phospholipids in the nervous system

The emerging diverse roles of ether (phospho)lipids in nervous system development and function in health and disease are currently attracting growing interest. Plasmalogens, a subgroup of ether lipids, are important membrane components involved in vesicle fusion and membrane raft composition. They store polyunsaturated fatty acids and may serve as antioxidants. Ether lipid metabolites act as precursors for the formation of glycosyl-phosphatidyl-inositol anchors; others, like platelet-activating factor, are implicated in signaling functions. Consolidating the available information, we attempt to provide molecular explanations for the dramatic neurological phenotype in ether lipid-deficient human patients and mice by linking individual functional properties of ether lipids with pathological features. Furthermore, recent publications have identified altered ether lipid levels in the context of many acquired neurological disorders including Alzheimer's disease (AD) and autism. Finally, current efforts to restore ether lipids in peroxisomal disorders as well as AD are critically reviewed.

Limonin, a Component of Dictamni Radicis Cortex, Inhibits Eugenol-Induced Calcium and cAMP Levels and PKA/CREB Signaling Pathway in Non-Neuronal 3T3-L1 Cells

Limonin, one of the major components in dictamni radicis cortex (DRC), has been shown to play various biological roles in cancer, inflammation, and obesity in many different cell types and tissues. Recently, the odorant-induced signal transduction pathway (OST) has gained attention not only because of its function in the perception of smell but also because of its numerous physiological functions in non-neuronal cells. However, little is known about the effects of limonin and DRC on the OST pathway in non-neuronal cells. We investigated odorant-stimulated increases in Ca²⁺ and cAMP, major second messengers in the OST pathway, in non-neuronal 3T3-L1 cells pretreated with limonin and ethanol extracts of DRC. Limonin and the extracts significantly decreased eugenol-induced Ca²⁺ and cAMP levels and upregulated phosphorylation of CREB and PKA. Our results demonstrated that limonin and DRC extract inhibit the OST pathway in non-neuronal cells by modulating Ca²⁺ and cAMP levels and phosphorylation of CREB.

The YhhN protein of Legionella pneumophila is a Lysoplasmalogenase

Lysoplasmalogenase catalyzes hydrolytic cleavage of the vinyl-ether bond of lysoplasmalogen to yield fatty aldehyde and glycerophospho-ethanolamine or glycerophospho-choline. We recently purified lysoplasmalogenase from rat liver microsomes and identified the protein as TMEM86B, an integral membrane protein that is a member of the YhhN family found in numerous species of eukaryotes and bacteria. To test the hypothesis that bacterial YhhN proteins also function as lysoplasmalogenase enzymes, we cloned the Lpg1991 gene of Legionella pneumophila, which encodes a 216 amino acid YhhN protein (LpYhhN), and expressed it in Escherichia coli as a C-terminal-GFP-His8-fusion. Membranes were solubilized and the fusion protein was purified by nickel-affinity chromatography, cleaved with Tobacco Etch Virus protease, and subjected to a reverse nickel column to purify the un-tagged LpYhhN. Both the fusion protein and un-tagged LpYhhN exhibit robust lysoplasmalogenase activity, cleaving the vinyl-ether bond of lysoplasmalogen with a Vmax of 12 µmol/min/mg protein and a Km of 45 μM. LpYhhN has no activity on diradyl plasmalogen, 1-alkenyl-glycerol, and monoacylglycerophospho-ethanolamine or monoacylglycerophospho-choline; the pH optimum is 6.5-7.0. These properties are very similar to mammalian TMEM86B. Sequence analysis suggests that YhhN proteins contain eight transmembrane helices, an N-in/C-in topology, and about 5 highly conserved amino acid residues that may form an active site. This work is the first to demonstrate a function for a bacterial YhhN protein, as a vinyl ether bond hydrolase specific for lysoplasmalogen. Since L. pneumophila does not contain endogenous plasmalogens, we hypothesize that LpYhhN may serve to protect the bacterium from lysis by lysoplasmalogen derived from plasmalogens of the host.

Identification of a cAMP-response element in the regulator of G-protein signaling-2 (RGS2) promoter as a key cis-regulatory element for RGS2 transcriptional regulation by angiotensin II in cultured vascular smooth muscles

Mice deficient in regulator of G-protein signaling-2 (RGS2) have severe hypertension, and RGS2 genetic variations occur in hypertensive humans. A potentially important negative feedback loop in blood pressure homeostasis is that angiotensin II (Ang II) increases vascular smooth muscle cell (VSMC) RGS2 expression. We reported that Group VIA phospholipase A(2) (iPLA(2)β) is required for this response (Xie, Z., Gong, M. C., Su, W., Turk, J., and Guo, Z. (2007) J. Biol. Chem. 282, 25278-25289), but the specific molecular causes and consequences of iPLA(2)β activation are not known. Here we demonstrate that both protein kinases C (PKC) and A (PKA) participate in Ang II-induced VSMC RGS2 mRNA up-regulation, and that actions of PKC and PKA precede and follow iPLA(2)β activation, respectively. Moreover, we identified a conserved cAMP-response element (CRE) in the murine RGS2 promoter that is critical for cAMP-response element-binding protein (CREB) binding and RGS2 promoter activation. Forskolin-stimulated RGS2 mRNA up-regulation is inhibited by CREB sequestration or specific disruption of the CREB-RGS2 promoter interaction, and Ang II-induced CREB phosphorylation and nuclear localization are blocked by iPLA(2)β pharmacologic inhibition or genetic ablation. Ang II-induced intracellular cyclic AMP accumulation precedes CREB phosphorylation and is diminished by inhibiting iPLA(2), cyclooxygenase, or lipoxygenase. Moreover, three single nucleotide polymorphisms identified in hypertensive patients are located in the human RGS2 promoter CREB binding site. Point mutations corresponding to these single nucleotide polymorphisms interfere with stimulation of human RGS2 promoter activity by forskolin. Our studies thus delineate a negative feedback loop to attenuate Ang II signaling in VSMC with potential importance in blood pressure homeostasis and the pathogenesis of human essential hypertension.

Plasmalogens the neglected regulatory and scavenging lipid species

Plasmalogens are a class of phospholipids carrying a vinyl ether bond in sn-1 and an ester bond in sn-2 position of the glycerol backbone. Although they are widespread in all tissues and represent up to 18% of the total phospholipid mass in humans, their physiological function is still poorly understood. The aim of this review is to give an overview over the current knowledge in plasmalogen biology and pathology with an emphasis on neglected aspects of their involvement in neurological and metabolic diseases. Furthermore a better understanding of plasmalogen biology in health and disease could also lead to the development of better diagnostic and prognostic biomarkers for vascular and metabolic diseases such as obesity and diabetes mellitus, inflammation, neuro-degeneration and cancer.

Purification, identification, and cloning of lysoplasmalogenase, the enzyme that catalyzes hydrolysis of the vinyl ether bond of lysoplasmalogen

Lysoplasmalogenase (EC 3.3.2.2 and EC 3.3.2.5) is an enzyme that catalyzes hydrolytic cleavage of the vinyl ether bond of lysoplasmalogen, forming fatty aldehyde and glycerophosphoethanolamine or glycerophosphocholine and is specific for the sn-2-deacylated form of plasmalogen. Here we report the purification, characterization, identification, and cloning of lysoplasmalogenase. Rat liver microsomal lysoplasmalogenase was solubilized with octyl glucoside and purified 500-fold to near homogeneity using four chromatography steps. The purified enzyme has apparent K(m) values of ∼50 μm for both lysoplasmenylcholine and lysoplasmenylethanolamine and apparent V(m) values of 24.5 and 17.5 μmol/min/mg protein for the two substrates, respectively. The pH optimum was 7.0. Lysoplasmalogenase was competitively inhibited by lysophosphatidic acid (K(i) ∼20 μm). The predominant band on a gel at ∼19 kDa was subjected to trypsinolysis, and the peptides were identified by mass spectrometry as Tmem86b, a protein of unknown function. Transient transfection of human embryonic kidney (HEK) 293T cells showed that TMEM86b cDNA yielded lysoplasmalogenase activity, and Western blot analyses confirmed the synthesis of TMEM86b protein. The protein was localized in the membrane fractions. The TMEM86b gene was also transformed into Escherichia coli, and its expression was verified by Western blot and activity analyses. Tmem86b is a hydrophobic transmembrane protein of the YhhN family. Northern blot analyses demonstrated that liver expressed the highest level of Tmem86b, which agreed with tissue distribution of activity. Overexpression of TMEM86b in HEK 293T cells resulted in decreased levels of plasmalogens, suggesting that the enzyme may be important in regulating plasmalogen levels in animal cells.

Mice deficient in group VIB phospholipase A2 (iPLA2gamma) exhibit relative resistance to obesity and metabolic abnormalities induced by a Western diet

Phospholipases A(2) (PLA(2)) play important roles in metabolic processes, and the Group VI PLA(2) family is comprised of intracellular enzymes that do not require Ca(2+) for catalysis. Mice deficient in Group VIA PLA(2) (iPLA(2)beta) develop more severe glucose intolerance than wild-type (WT) mice in response to dietary stress. Group VIB PLA(2) (iPLA(2)gamma) is a related enzyme distributed in membranous organelles, including mitochondria, and iPLA(2)gamma knockout (KO) mice exhibit altered mitochondrial morphology and function. We have compared metabolic responses of iPLA(2)gamma-KO and WT mice fed a Western diet (WD) with a high fat content. We find that KO mice are resistant to WD-induced increases in body weight and adiposity and in blood levels of cholesterol, glucose, and insulin, even though WT and KO mice exhibit similar food consumption and dietary fat digestion and absorption. KO mice are also relatively resistant to WD-induced insulin resistance, glucose intolerance, and altered patterns of fat vs. carbohydrate fuel utilization. KO skeletal muscle exhibits impaired mitochondrial beta-oxidation of fatty acids, as reflected by accumulation of larger amounts of long-chain acylcarnitine (LCAC) species in KO muscle and liver compared with WT in response to WD feeding. This is associated with increased urinary excretion of LCAC and much reduced deposition of triacylglycerols in liver by WD-fed KO compared with WT mice. The iPLA(2)gamma-deficient genotype thus results in a phenotype characterized by impaired mitochondrial oxidation of fatty acids and relative resistance to the metabolic abnormalities induced by WD.

Coupled calcium and zinc dyshomeostasis and oxidative stress in cardiac myocytes and mitochondria of rats with chronic aldosteronism

A dyshomeostasis of extra- and intracellular Ca(2+) and Zn(2+) occurs in rats receiving chronic aldosterone/salt treatment (ALDOST). Herein, we hypothesized that the dyshomeostasis of intracellular Ca(2+) and Zn(2+) is intrinsically coupled that alters the redox state of cardiac myocytes and mitochondria, with Ca(2+) serving as a pro-oxidant and Zn(2+) as an antioxidant. Toward this end, we harvested hearts from rats receiving 4 weeks of ALDOST alone or cotreatment with either spironolactone (Spiro), an aldosterone receptor antagonist, or amlodipine (Amlod), an L-type Ca(2+) channel blocker, and from age/sex-matched untreated controls. In each group, we monitored cardiomyocyte [Ca(2+)]i and [Zn(2+)]i and mitochondrial [Ca(2+)]m and [Zn(2+)]m; biomarkers of oxidative stress and antioxidant defenses; expression of Zn transporters, Zip1 and ZnT-1; metallothionein-1, a Zn(2+)-binding protein; and metal response element transcription factor-1, a [Zn(2+)]i sensor and regulator of antioxidant defenses. Compared with controls, at 4-week ALDOST, we found the following: (a) increased [Ca(2+)]i and [Zn(2+)]i, together with increased [Ca(2+)]m and [Zn(2+)]m, each of which could be prevented by Spiro and attenuated with Amlod; (b) increased levels of 3-nitrotyrosine and 4-hydroxy-2-nonenal in cardiomyocytes, together with increased H(2)O(2) production, malondialdehyde, and oxidized glutathione in mitochondria that were coincident with increased activities of Cu/Zn superoxide dismutase and glutathione peroxidase; and (c) increased expression of metallothionein-1, Zip1 and ZnT-1, and metal response element transcription factor-1, attenuated by Spiro. Thus, an intrinsically coupled dyshomeostasis of intracellular Ca(2+) and Zn(2+) occurs in cardiac myocytes and mitochondria in rats receiving ALDOST, where it serves to alter their redox state through a respective induction of oxidative stress and generation of antioxidant defenses. The importance of therapeutic strategies that can uncouple these two divalent cations and modulate their ratio in favor of sustained antioxidant defenses is therefore suggested.

Group VIA phospholipase A2 (iPLA2beta) participates in angiotensin II-induced transcriptional up-regulation of regulator of g-protein signaling-2 in vascular smooth muscle cells

Rgs2 (regulator of G-protein signaling-2)-deficient mice exhibit severe hypertension, and genetic variations of RGS2 occur in hypertensive patients. RGS2 mRNA up-regulation by angiotensin II (Ang II) in vascular smooth muscle cells (VSMC) is a potentially important negative feedback mechanism in blood pressure homeostasis, but how it occurs is unknown. Here we demonstrate that group VIA phospholipase A2 (iPLA2beta) plays a pivotal role in Ang II-induced RGS2 mRNA up-regulation in VSMC by three independent approaches, including pharmacologic inhibition with a bromoenol lactone suicide substrate, suppression of iPLA2beta expression with antisense oligonucleotides, and genetic deletion in iPLA2beta-null mice. Selective inhibition of iPLA2beta by each of these approaches abolishes Ang II-induced RGS2 mRNA up-regulation. Furthermore, using adenovirus-mediated gene transfer, we demonstrate that restoration of iPLA2beta-expression in iPLA2beta-null VSMC reconstitutes the ability of Ang II to up-regulate RGS2 mRNA expression. In contrast, Ang II-induced vasodilator-stimulated phosphoprotein phosphorylation and Ang II receptor expression are unaffected. Moreover, in wild-type but not iPLA2beta-null VSMC, Ang II stimulates iPLA2 enzymatic activity significantly. Both arachidonic acid and lysophosphatidylcholine, products of iPLA2beta action, induce RGS2 mRNA up-regulation. Inhibition of lipoxygenases, particularly 15-lipoxygenase, and cyclooxygenases, but not cytochrome P450-dependent epoxygenases inhibits Ang II- or AA-induced RGS2 mRNA expression. Moreover, RGS2 protein expression is also up-regulated by Ang II, and this is attenuated by bromoenol lactone. Disruption of the Ang II/iPLA2beta/RGS2 feedback pathway in iPLA2beta-null cells potentiates Ang II-induced vasodilator-stimulated phosphoprotein and Akt phosphorylation in a time-dependent manner. Collectively, our results demonstrate that iPLA2beta participates in Ang II-induced transcriptional up-regulation of RGS2 in VSMC.

Lysoplasmenylcholine increases neutrophil adherence to human coronary artery endothelial cells

We demonstrated previously that thrombin stimulation of human coronary artery endothelial cells (HCAEC) results in release of choline lysophospholipids [lysophosphatidylcholine (lysoPtdCho) and lysoplasmenylcholine (lysoPlsCho)]. These amphiphilic metabolites have been implicated in arrhythmogenesis following the onset of myocardial ischemia, but studies examining their direct effects on the vasculature remain limited. We and others have shown that thrombin and lysoPtdCho can increase cell surface adhesion molecules and adherence of circulating inflammatory cells to the endothelium. This study supports our hypothesis that these changes may be mediated, at least in part, by lysoPlsCho, thus implicating this metabolite as an inflammatory mediator in the coronary vasculature and a modulator of the progression of atherosclerosis. Apical stimulation of HCAEC with thrombin resulted in the production and release of choline lysophospholipids from the apical surface of the HCAEC monolayer. Basolateral stimulation had no effect on choline lysophospholipid production or release from either the apical or basolateral surface of the HCAEC monolayer. Incubation of HCAEC with lysoPlsCho or lysoPtdCho resulted in similar increases in HCAEC surface expression of P-selectin and E-selectin. Furthermore, lysoPlsCho increased cell surface expression of P-selectin, E-selectin, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 with a time course similar to that of thrombin stimulation. Increased presence of cell surface adhesion molecules may contribute to the significant increase in adherence of neutrophils to either thrombin- or lysoPlsCho-stimulated HCAEC. These results demonstrate that the presence of thrombin at sites of vascular injury in the coronary circulation, resulting in increased choline lysophospholipid release from the HCAEC apical surface, has the potential to propagate vascular inflammation by upregulation of adhesion molecules and recruitment of circulating inflammatory cells to the endothelium.

RanGAP-mediated nuclear protein import in vascular smooth muscle cells is augmented by lysophosphatidylcholine

The intracellular mechanism responsible for the mitogenic effects of lysophosphatidylcholine (LPC) is unclear. Import of proteins from the cytoplasm into the cell nucleus is integral to the regulation of gene expression and cell growth. We hypothesized that LPC exerts its intracellular effects through alterations in nuclear protein import. Rabbit aortic smooth muscle cells incubated with LPC induced a significant increase in cell proliferation in both quiescent cells (63.2+/-6.48% of control) and cells grown in 1% fetal bovine serum (FBS) (28.3+/-7.35% of control). Vascular smooth muscle cells were preincubated with LPC then microinjected with a marker protein for nuclear import. A significant stimulation of nuclear protein transport was observed. Using a conventional nuclear protein import assay in permeabilized cells, a significant stimulation of import (72.3+/-5.2% of control) was again observed when the cytosolic nuclear import cocktail was treated with LPC. This effect was not observed with other lysophosphatidyl species. LPC also activated the extracellular signal-regulated kinase (ERK) 1/2 mitogen-activated protein kinase (MAPK) pathway, and this was blocked by 2'-amino-3'-methoxyflavone (PD98059), which inhibits the activation of ERK 1/2. The stimulation of nuclear import was also blocked by PD98059. LPC-induced MAPK activation augmented GTP hydrolysis by RanGAP, a RanGTPase activating protein and a critical regulatory component of nuclear protein import, and this stimulation was again blocked by PD98059. We conclude that LPC alters gene expression and cell proliferation through striking effects on nuclear protein import via a MAP kinase-induced activation of RanGAP. This may play an important role in cancer and atherosclerosis and other disorders involving accelerated cell growth/proliferation.

Activation of MAPKs in thrombin-stimulated ventricular myocytes is dependent on Ca2+-independent PLA2

Thrombin stimulation of isolated rabbit ventricular myocytes activates a membrane-associated, Ca2+-independent PLA2(iPLA2) that selectively hydrolyzes plasmalogen phospholipids and results in increased production of arachidonic acid and lysoplasmenylcholine. To determine whether MAPK regulates myocardial iPLA2activity, we isolated ventricular myocytes from rabbit heart by collagenase digestion and pretreated them with MAPK inhibitors before stimulating them with thrombin. Pretreatment with PD-98059 to inhibit p42/44 MAPK or SB-203580 to inhibit p38 MAPK had no significant effect on thrombin-stimulated, membrane-associated iPLA2activity. Thrombin stimulation resulted in significant increases in both p42/44 and p38 MAPK activity after 2 min. Pretreatment with the iPLA2-selective inhibitor bromoenol lactone completely inhibited thrombin-stimulated MAPK activity, suggesting that activation of MAPKs was dependent on iPLA2activation. Ventricular myocyte MAPK activity was increased by incubation of the myocytes with lysoplasmenylcholine, a metabolite produced by iPLA2-catalyzed membrane plasmalogen phospholipid hydrolysis. Altogether, these data suggest that activation of MAPKs occurs downstream of and is dependent on iPLA2activation in thrombin-stimulated rabbit ventricular myocytes.

The role and regulation of COX-2 during viral infection

Prostaglandins are lipid mediators, generated by cyclooxygenase (COX), that have been shown to participate in the regulation of virus replication and the modulation of inflammatory responses following infection. A number of studies support a role for PGE2 in the modulation of virus replication and virulence in a cell type and virus selective manner. Virus infection also stimulates the expression of a number of proinflammatory gene products, including COX-2, inducible nitric oxide synthase (iNOS) as well as proinflammatory cytokines. This review will focus on the mechanisms by which proinflammatory prostaglandin production regulates virus replication and virulence. In addition, the signaling pathways that are activated during a virus infection, and that regulate proinflammatory gene expression in macrophages will be reviewed. Specific attention will be placed on the ability of virus infection to activate multiple signaling cascades (such as PKR, MAPK, iPLA2, NF-kappaB) and how these pathways are integrated in the regulation of individual target gene expression.

Functions and biosynthesis of plasmalogens in health and disease

Plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) constitute a special class of phospholipids characterized by the presence of a vinyl-ether bond at the sn-1 position. Although long considered as biological peculiarities, interest in this group of phospholipids has grown in recent years, thanks to the realization that plasmalogens are involved in different human diseases. In this review, we summarize the current state of knowledge with respect to the enzymatic synthesis of plasmalogens, the characteristic topology of the enzymes involved and the biological roles that have been assigned to plasmalogens.

Purification of recombinant human cPLA2 gamma and identification of C-terminal farnesylation, proteolytic processing, and carboxymethylation by MALDI-TOF-TOF analysis

Cytosolic phospholipase A(2)gamma (cPLA(2)gamma) is a calcium-independent, membrane-associated phospholipase A(2) that possesses a C-terminal prenylation motif (-CCLA) whose covalent structure cannot be deduced from the primary sequence alone. Accordingly, we overexpressed human cPLA(2)gamma containing an N-terminal His tag ((His)(6)cPLA(2)gamma) in Sf9 cells and quantitatively solubilized and purified the enzyme by sequential immobilized metal affinity and Mono Q column chromatographies. The final preparation appeared as a single 61 kDa band after SDS-PAGE/silver-staining, possessed high lysophospholipase activity (50 micromol min(-1) mg(-1)), and was inhibited by, but did not hydrolyze, palmitoyl-CoA. Radiolabeling of recombinant human cPLA(2)gamma with [(3)H]-mevalonolactone in the absence of statins and subsequent cleavage of prenyl groups with Raney nickel revealed that the enzyme is only farnesylated and is not geranylgeranylated. Analysis of CNBr-digested cPLA(2)gamma by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) mass spectrometry demonstrated the presence of a farnesyl moiety at Cys-538, cleavage of the Cys(538)-Cys(539) bond, and carboxymethylation of the resultant C-terminal prenylated cysteine. Collectively, these results describe the solubilization and purification of recombinant cPLA(2)gamma to homogeneity and identify cPLA(2)gamma as a farnesylated protein that undergoes at least three sequential posttranslational modifications that likely facilitate its targeting and interactions with its membrane substrates.

Myeloperoxidase-derived reactive chlorinating species from human monocytes target plasmalogens in low density lipoprotein

A role for myeloperoxidase (MPO) in atherosclerosis has received considerable attention recently. To identify potential chlorinated lipid products in human low density lipoprotein (LDL), studies were designed to demonstrate that MPO-derived reactive chlorinating species (RCS) target the plasmalogen pool of LDL isolated from peripheral human blood in vitro. The vinyl ether bond of LDL plasmalogens was targeted by MPO-derived RCS, resulting in the release of the 16- and 18-carbon-containing alpha-chloro fatty aldehydes, 2-chlorohexadecanal and 2-chlorooctadecanal, respectively, from the plasmalogen glycerol backbone. Targeting of the LDL plasmalogen vinyl ether bond was dependent on the presence of MPO-derived RCS. Electrospray ionization mass spectrometric analysis of MPO-treated LDL demonstrated that a novel population of unsaturated lysophosphatidylcholine molecular species was produced by a phospholipase A2-independent mechanism. Unsaturated lysophosphatidylcholine molecular species elicited cyclic AMP response element binding protein phosphorylation in RAW 264.7 cells. Additionally, MPO-mediated targeting of both monocyte and LDL plasmalogen pools was demonstrated in phorbol myristate acetate-stimulated human monocytes, resulting in the production of both 2-chlorohexadecanal and 2-chlorooctadecanal. In contrast, alpha-chloro fatty aldehydes were not produced in phorbol myristate acetate-stimulated mouse monocytes. Collectively, the present studies demonstrate a novel MPO-specific mechanism that mediates the production of a novel group of unsaturated lysophosphatidylcholine molecular species and chlorinated aldehydes from both LDL and monocyte plasmalogen pools that may have important effects during inflammatory reactions mediated by monocytes, most notably atherosclerosis.

Calcium-independent phospholipase A2 mediates CREB phosphorylation in double-stranded RNA-stimulated endothelial cells

One of the products of a calcium-independent phospholipase A2 (iPLA2) attack of plasmenylcholine, lysoplasmenylcholine, has previously been shown to activate cAMP-dependent protein kinase (PKA). Because endothelial cells respond to some agonists in part by the activation of iPLA2, the present study was designed to determine whether double-stranded RNA (dsRNA), the primary activator of the antiviral response in endothelial cells, elicits cAMP response element binding protein (CREB) phosphorylation through a mechanism mediated by iPLA2. dsRNA stimulated CREB phosphorylation in bovine pulmonary artery endothelial cells that was inhibited by the iPLA2 inhibitor, bromoenol lactone, and the PKA inhibitor, H-89. Additionally, the product of iPLA2 hydrolysis of plasmenylcholine and lysoplasmenylcholine elicited CREB phosphorylation in bovine pulmonary endothelial cells. Taken together, the present studies suggest that dsRNA as well as other agonists of endothelial cells elicit signaling mechanisms that include in part CREB phosphorylation mediated by iPLA2.

Eosinophil peroxidase-derived reactive brominating species target the vinyl ether bond of plasmalogens generating a novel chemoattractant, alpha-bromo fatty aldehyde

Plasmalogens are a subclass of glycerophospholipids that are enriched in the plasma membrane of many mammalian cells. The vinyl ether bond of plasmalogens renders them susceptible to oxidation. Accordingly, it was hypothesized that reactive brominating species, a unique oxidant formed at the sites of eosinophil activation, such as in asthma, might selectively target plasmalogens for oxidation. Here we show that reactive brominating species produced by the eosinophil peroxidase system of activated eosinophils attack the vinyl ether bond of plasmalogens. Reactive brominating species produced by eosinophil peroxidase target the vinyl ether bond of plasmalogens resulting in the production of a neutral lipid and lysophosphatidylcholine. Chromatographic and mass spectrometric analyses of this neutral lipid demonstrated that it was 2-bromohexadecanal (2-BrHDA). Reactive brominating species produced by eosinophil peroxidase attacked the plasmalogen vinyl ether bond at acidic pH. Bromide was the preferred substrate for eosinophil peroxidase, and chloride was not appreciably used even at a 1000-fold molar excess. Furthermore, 2-BrHDA production elicited by eosinophil peroxidase-derived reactive brominating species in the presence of 100 microM NaBr doubled with the addition of 100 mM NaCl. The potential physiological significance of this pathway was suggested by the demonstration that 2-BrHDA was produced by phorbol myristate acetate-stimulated eosinophils and by the demonstration that 2-BrHDA is a phagocyte chemoattractant. Taken together, the present studies demonstrate the targeting of the vinyl ether bond of plasmalogens by the reactive brominating species produced by eosinophil peroxidase and by activated eosinophils, resulting in the production of brominated fatty aldehydes.

Novel role for calcium-independent phospholipase A(2) in the macrophage antiviral response of inducible nitric-oxide synthase expression

The double-stranded (ds) RNA-dependent protein kinase (PKR) is a primary regulator of antiviral responses; however, the ability of dsRNA to activate nuclear factor-kappa B (NF-kappa B) and dsRNA + interferon gamma (IFN-gamma) to stimulate inducible nitric-oxide synthase (iNOS) expression by macrophages isolated from PKR(-/-) mice suggests that signaling pathways in addition to PKR participate in antiviral activities. We have identified a novel phospholipid-signaling cascade that mediates macrophage activation by dsRNA and viral infection. Bromoenol lactone (BEL), a selective inhibitor of the calcium-independent phospholipase A(2) (iPLA(2)), prevents dsRNA- and virus-induced iNOS expression by RAW 264.7 cells and mouse macrophages. BEL does not modulate dsRNA-induced interleukin 1 expression, nor does it affect dsRNA-induced NF-kappa B activation. Protein kinase A (PKA) and the cAMP response element binding protein (CREB) are downstream targets of iPLA(2), because selective PKA inhibition prevents dsRNA-induced iNOS expression, and the inhibitory actions of BEL on dsRNA-induced iNOS expression are overcome by the direct activation of PKA. In addition, BEL inhibits dsRNA-induced CREB phosphorylation and CRE reporter activation. PKR does not participate in iPLA(2) activation or iNOS expression, because dsRNA stimulates iPLA(2) activity and dsRNA + IFN-gamma induces iNOS expression and nitric oxide production to similar levels by macrophages isolated from PKR(+/+) and PKR(-/-) mice. These findings support a PKR-independent signaling role for iPLA(2) in the antiviral response of macrophages.

Increasing plasmalogen levels protects human endothelial cells during hypoxia

Supplementation of cultured human pulmonary arterial endothelial cells (PAEC) with sn-1-O-hexadecylglycerol (HG) resulted in an approximately twofold increase in cellular levels of plasmalogens, a subclass of phospholipids known to have antioxidant properties; this was due, primarily, to a fourfold increase in the choline plasmalogens. Exposure of unsupplemented human PAEC to hypoxia (PO(2) = 20-25 mmHg) caused an increase in cellular reactive oxygen species (ROS) over a period of 5 days with a coincident decrease in viability. In contrast, HG-supplemented cells survived for at least 2 wk under these conditions with no evidence of increased ROS. Hypoxia resulted in a selective increase in the turnover of the plasmalogen plasmenylethanolamine. Human PAEC with elevated plasmalogen levels were also more resistant to H(2)O(2), hyperoxia, and the superoxide generator plumbagin. This protection was seemingly specific to cellular stresses in which significant ROS were generated because the sensitivity to lethal heat shock or glucose deprivation was not altered in HG-treated human PAEC. HG, by itself, was not sufficient for protection; HG supplementation of bovine PAEC had no effect upon plasmalogen levels and did not rescue these cells from the cytotoxic effects of hypoxia. This is the initial demonstration that plasmalogen content can be substantially enhanced in a normal cell. These data also demonstrate that HG can protect cells during hypoxia and other ROS-mediated stress, likely due to the resulting increase in these antioxidant phospholipids.

Reactive chlorinating species produced during neutrophil activation target tissue plasmalogens: production of the chemoattractant, 2-chlorohexadecanal

Recently alpha-chloro fatty aldehydes have been shown to be products of reactive chlorinating species targeting the vinyl ether bond of plasmalogens utilizing a cell-free system. Accordingly, the present experiments were designed to show that alpha-chloro fatty aldehydes are produced by activated neutrophils and to determine their physiologic effects. A sensitive gas chromatography-mass spectrometry technique was developed to detect pentafluorobenzyl oximes of alpha-chloro fatty aldehydes utilizing negative ion chemical ionization. Phorbol 12-myristate 13-acetate activation of neutrophils resulted in the production of both 2-chlorohexadecanal and 2- chlorooctadecanal through a myeloperoxidase-dependent mechanism that likely involved the targeting of both 16 and 18 carbon vinyl ether-linked aliphatic groups present in the sn-1 position of neutrophil plasmalogens. 2-Chlorohexadecanal was also produced by fMLP-treated neutrophils. Additionally, reactive chlorinating species released from activated neutrophils targeted endothelial cell plasmalogens resulting in 2-chlorohexadecanal production. Physiologically relevant concentrations of 2-chlorohexadecanal induced neutrophil chemotaxis in vitro suggesting that alpha-chloro fatty aldehydes may have a role in neutrophil recruitment. Taken together, these studies demonstrate for the first time a novel biochemical mechanism that targets the vinyl ether bond of plasmalogens during neutrophil activation resulting in the production of alpha-chloro fatty aldehydes that may enhance the recruitment of neutrophils to areas of active inflammation.

Calcium-independent phospholipase A(2) mediates CREB phosphorylation and c-fos expression during ischemia

In isolated, perfused adult rat hearts, global ischemia increased the phosphorylation of cAMP response element-binding protein (CREB) relative to control levels, and this phosphorylation was reversed with reperfusion. CREB phosphorylation elicited by 5 min of global ischemia was sensitive to treatments with the calcium-independent phospholipase A(2) (iPLA(2)) inhibitor bromoenol lactone (BEL) and occurred in the absence of increases in myocardial cAMP content. In contrast, CREB phosphorylation elicited by 15 min of global ischemia was likely mediated by elevated cAMP levels. The expression of c-fos, in response to brief myocardial ischemia, was also sensitive to BEL treatment. The induction of iPLA(2)-mediated CREB phosphorylation was further substantiated by the observations that lysoplasmenylcholine increased both the phosphorylation of CREB and the induction of c-fos expression in the absence and presence of BEL. CREB phosphorylation in both ischemic hearts and lysoplasmenylcholine-perfused hearts was inhibited by pretreatment of hearts with the specific cAMP-dependent protein kinase (PKA) inhibitor H-89. Taken together, these data demonstrate that iPLA(2) mediates CREB phosphorylation through a PKA-dependent pathway during brief periods of myocardial ischemia, possibly through the formation of lysophospholipids.

Selective plasmalogen substrate utilization by thrombin-stimulated Ca(2+)-independent PLA(2) in cardiomyocytes

Thrombin stimulation of rabbit ventricular myocytes activates a membrane-associated, Ca(2+)-independent phospholipase A(2) (PLA(2)) capable of hydrolyzing plasmenylcholine (choline plasmalogen), plasmanylcholine (alkylacyl choline phospholipid), and phosphatidylcholine substrates. To identify the endogenous phospholipid substrates, we quantified the effects of thrombin stimulation on diradyl phospholipid mass and arachidonic acid and lysophospholipid production. Thrombin stimulation resulted in a selective decrease in arachidonylated plasmenylcholine, with no change in arachidonylated phosphatidylcholine. The decrease in arachidonylated plasmenylcholine was accompanied by an increase in plasmenylcholine species containing linoleic and linolenic acids at the sn-2 position. A decrease in arachidonylated plasmenylethanolamine was also observed after thrombin stimulation, with no concomitant change in arachidonylated phosphatidylethanolamine. Thrombin stimulation resulted in the selective production of lysoplasmenylcholine, with no increase in lysophosphatidylcholine content. There was no evidence for significant acetylation of lysophospholipids to form platelet-activating factor. Arachidonic acid released after thrombin stimulation was rapidly oxidized to prostacyclin. Thus thrombin-stimulated Ca(2+)-independent PLA(2) selectively hydrolyzes arachidonylated plasmalogen substrates, resulting in production of lysoplasmalogens and prostacyclin as the principal bioactive products.

Identification and characterization of alkenyl hydrolase (lysoplasmalogenase) in microsomes and identification of a plasmalogen-active phospholipase A2 in cytosol of small intestinal epithelium

A lysoplasmalogenase (EC 3.3.2.2; EC 3.3.2.5) that liberates free aldehyde from 1-alk-1'-enyl-sn-glycero-3-phospho-ethanolamine or -choline (lysoplasmalogen) was identified and characterized in rat gastrointestinal tract epithelial cells. Glycerophosphoethanolamine was produced in the reaction in equimolar amounts with the free aldehyde. The microsomal membrane associated enzyme was present throughout the length of the small intestines, with the highest activity in the jejunum and proximal ileum. The rate of alkenyl ether bond hydrolysis was dependent on the concentrations of microsomal protein and substrate, and was linear with respect to time. The enzyme hydrolyzed both ethanolamine- and choline-lysoplasmalogens with similar affinities; the Km values were 40 and 66 microM, respectively. The enzyme had no activity with 1-alk-1'-enyl-2-acyl-sn-glycero-3-phospho-ethanolamine or -choline (intact plasmalogen), thus indicating enzyme specificity for a free hydroxyl group at the sn-2 position. The specific activities were 70 nmol/min/mg protein and 57 nmol/min/mg protein, respectively, for ethanolamine- and choline-lysoplasmalogen. The pH optimum was between 6.8 and 7.4. The enzyme required no known cofactors and was not affected by low mM levels of Ca2+, Mg2+, EDTA, or EGTA. The detergents, Triton X-100, deoxycholate, and octyl glucoside inhibited the enzyme. The chemical and physical properties of the lysoplasmalogenase were very similar to those of the enzyme in liver and brain microsomes. In developmental studies the specific activities of the small intestinal and liver enzymes increased markedly, 11.1- and 3.4-fold, respectively, in the first approximately 40 days of postnatal life. A plasmalogen-active phospholipase A2 activity was identified in the cytosol of the small intestines (3.3 nmol/min/mg protein) and liver (0.3 nmol/min/mg protein) using a novel coupled enzyme assay with microsomal lysoplasmalogenase as the coupling enzyme.