Structural identification of phosphatidylcholines having an oxidatively shortened linoleate residue generated through its oxygenation with soybean or rabbit reticulocyte lipoxygenase

Characterization of phosphatidylcholine oxidation products by MALDI MS(n)

Phospholipid oxidation has been implicated in the pathogenesis and progression of numerous age-related and neurodegenerative diseases. Despite these implications, this broad class of biomolecules remains poorly characterized. In this work, the fragmentation patterns of [M + H](+) and [M + Na](+) ions of intact phosphatidylcholine oxidation products (OxPCs) were characterized by matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI MS(n), n = 2, 3, and 4). MS(2) of both the [M + H](+) and [M + Na](+) ions of short-chain OxPCs yielded product ions related to the PC headgroup and the fatty acid substituents. MS(3) of the [M + Na - N(CH3)3](+) ions yielded fragmentation indicative of the OxPC modification; specifically, a product ion corresponding to the neutral loss of CO2 (NL of 44) was observed for OxPCs containing a terminal carboxylic acid rather than an aldehyde. Furthermore, MS(4) of the [M + Na - HPO4(CH2)2N(CH3)3](+) ions resulted in fragmentation pathways dependent on the sn-2 fatty acid chain length and type of functional group(s). Specifically, CHO-containing OxPCs with palmitic acid esterified to the sn-1 position of the glycerol backbone yielded a NL of 254, 2 u less than the nominal mass of palmitic acid, whereas the analogous terminal COOH-containing OxPCs demonstrated a NL of 256. Finally, the presence of a γ-ketone relative to the terminal carboxyl group resulted in C-C bond cleavages along the sn-2 substituent, providing diagnostic product ions for keto-containing OxPCs. This work illustrates the enhanced selectivity afforded by MS(n) on the linear ion trap and develops a method for the identification of individual products of PC oxidation.

Oxidatively truncated phospholipids are required agents of tumor necrosis factor α (TNFα)-induced apoptosis

TNFα generates reactive oxygen species (ROS) at the cell surface that induce cell death, but how ROS communicate to mitochondria and their specific apoptotic action(s) are both undefined. ROS oxidize phospholipids to hydroperoxides that are friable and fragment adjacent to the (hydro)peroxide function, forming truncated phospholipids, such as azelaoyl phosphatidylcholine (Az-PC). Az-PC is relatively soluble, and exogenous Az-PC rapidly enters cells to damage mitochondrial integrity and initiate intrinsic apoptosis. We determined whether this toxic phospholipid is formed within cells during TNFα stimulation in sufficient quantities to induce apoptosis and if they are essential in TNFα-induced cytotoxicity. We found that TNFα induced ROS formation and phospholipid peroxidation in Jurkat cells, and either chemical interference with NADPH oxidase activity or siRNA suppression of the NADPH oxidase-4 subunit blocked ROS accumulation and phospholipid peroxidation. Mass spectrometry showed that phospholipid peroxides and then Az-PC increased after TNFα exposure, whereas ROS inhibition abolished Az-PC accumulation and TNFα-induced cell death. Glutathione peroxidase-4 (GPx4), which specifically metabolizes lipid hydroperoxides, fell in TNFα-stimulated cells prior to death. Ectopic GPx4 overcame this, reduced peroxidized phospholipid accumulation, blocked Az-PC accumulation, and prevented death. Conversely, GPx4 siRNA knockdown enhanced phospholipid peroxidation, increasing TNFα-stimulated Az-PC formation and apoptosis. Truncated phospholipids were essential elements of TNFα-induced apoptosis because overexpression of PAFAH2 (a phospholipase A(2) that selectively hydrolyzes truncated phospholipids) blocked TNFα-induced Az-PC accumulation without affecting phospholipid peroxidation. PAFAH2 also abolished apoptosis. Thus, phospholipid oxidation and truncation to apoptotic phospholipids comprise an essential element connecting TNFα receptor signaling to mitochondrial damage and apoptotic death.

Mass spectrometry analysis of oxidized phosphatidylcholine and phosphatidylethanolamine

Oxidized phospholipids (OxPLs) are rapidly becoming recognized as important mediators of cellular and immune signaling. They are generated either enzymatically or non-enzymatically and 100s of structures exist of which only a small fraction have been analyzed to date. Pleiotropic activities, including regulation of adhesion molecule expression, pro-coagulant activity and inhibition of Toll-like receptor signaling have been observed and some are detected in models of human and animal disease, including atherosclerosis and infection. More recently, the acute generation of specific oxidized phospholipids by cellular enzymes in immune cells was reported. Assays for analysis and quantification of OxPLs were first developed approx 15years ago, primarily for hydro(pero)xy-species. Many were based on monitoring a single precursor ion with/without LC separation, based on the PL headgroup. Others combined LC with monitoring precursor to product transitions, but were unable to provide information regarding position of oxidation on unsaturated sn-2 fatty acid due to sensitivity issues. More recently, LC/MS/MS methods for specific OxPLs have been reported that enable high sensitivity quantitation in biological samples. In this review, widely used methods for detecting and quantifying various classes of OxPL will be summarized, along with practical advice for their use. In particular, the focus will be on LC/MS/MS, which today is almost universally the method of choice.

Lyso-phosphatidylcholine induces osteogenic gene expression and phenotype in vascular smooth muscle cells

OBJECTIVE

Calcifying vascular cells in human atherosclerotic plaques actively contribute to ectopic vascular mineralization. Lyso-phosphatidylcholine (LPC), a product of oxidized phosphatidylcholine hydrolysis, is found at concentrations of 1-12 microg/g tissue throughout the atheroma. The objective of this study was to determine if LPC induces an osteogenic phenotype in vascular smooth muscle cells.

METHODS AND RESULTS

Proliferating human aortic smooth muscle cells were treated with a wide-range of LPC concentrations (0.1 nM to 100 microM) over 14 days. Von Kossa, Alizarin Red S, and alkaline phosphatase staining were used to identify mineralizations. RT-PCR, ELISA, alkaline phosphatase activity, and 45Ca incorporation assays were used to evaluate the osteo-inductive effect of LPC on smooth muscle phenotype. Histology and morphometry revealed that cells treated with as little as 10 nM LPC produced calcium phosphate deposits in culture. LPC-treated vascular smooth muscle cells showed a significant increase in 45Ca incorporation and alkaline phosphatase activity. Furthermore, LPC treatment induced a significant loss of Schnurri 3 protein, a key repressor of Runt-related transcription factor 2 stability. Genomic studies revealed that osteogenic gene expression was significantly up-regulated in LPC-treated cells, which is attributed to increased Runt-related transcription factor 2 expression and transcriptional activity.

CONCLUSION

LPC induces osteogenic morphology, physiology, gene expression, and phenotype in vascular smooth muscle cells. The present study suggests that localized concentrations of LPC in human atherosclerotic plaques may be a contributing factor to the generation of calcifying vascular cells.

Generating and maintaining jasmonic acid in Arabidopsis

Jasmonic acid (JA) is a lipid-derived plant hormone that mediates diverse biological phenomena. It is one of major goals in JA research field to elucidate the regulatory mechanism of JA level. Recently we have demonstrated cooperative and differentiated roles of two chloroplast localized galactolipases, DGL (DONGLE) and DAD1 (DEFECTIVE IN ANTHER DEHISCENCE 1), for the regulation of JA content. The DGL maintains a basal level of JA in unwounded vegetative tissues, while the DAD1 is involved in JA production in floral tissues. The JA in vegetative tissues regulates cell expansion while the JA produced in flowers regulates pollen maturation. After wounding, the cooperative function of DGL and DAD1 causes drastic increase of JA. The analysis of induction kinetics showed that the two enzymes have temporally separated roles in wound response; DGL in early phase and DAD1 in late phase of JA production. In this addendum, we discuss the implications of our recent findings and extend our working model for JA homeostasis in plants.

Measurement of phosphatidylcholine hydroperoxides in solution and in intact membranes by the ferric-xylenol orange assay

Formation of a colored complex between ferric iron and xylenol orange (XO) has been used for the determination of hydroperoxides (FOX method). Original or modified FOX methods were performed on aqueous or organic solutions consisting of a single phase. However, for lipid peroxides in heterogeneous samples, such as biological materials, much of the lipid is sequestered in a separate phase. Organic solvent extraction of these lipids is often incomplete and may result in additional peroxidation during the extraction procedure. In this study, we applied the FOX assay for measurement of the membrane phosphatidylcholine hydroperoxides (PC-OOH) in separated phases. The presence of membranous egg yolk phosphatidylcholine (EYPC) in 60% MeOH shifted the broad peak at 560 nm of Fe(3+)-XO complex to 610 nm with a sharp peak associated with the increased intensity of the absorbance. The shift of the peak is useful to measure the unknown amounts of Fe(3+) because the uncomplexed XO considerably contributed to the absorbance of the peak at 560 nm but did not affect the absorbance at 610 nm. EYPC was required to form the membranes to shift the peak because the shift occurred in 60% MeOH but did not by the treatment with detergents or in 90% MeOH in which EYPC did not form the membranes. The molar absorption coefficient (epsilon 610) was 32,700 M(-1) cm(-1), which was about twice the molar absorption co efficient (epsilon 560) reported. We applied this method to the assay of PC-OOH prepared from EYPC and obtained the molar absorption coefficients (epsilon 610), which were 79,100 and 115,700 M(-1) cm(-1) in the presence and absence of BHT, respectively. This finding allows the determination of PC-OOH concentration even in chemically complex systems.

Wounding stimulates the accumulation of glycerolipids containing oxophytodienoic acid and dinor-oxophytodienoic acid in Arabidopsis leaves

Although oxylipins can be synthesized from free fatty acids, recent evidence suggests that oxylipins are components of plastid-localized polar complex lipids in Arabidopsis (Arabidopsis thaliana). Using a combination of electrospray ionization (ESI) collisionally induced dissociation time-of-flight mass spectrometry (MS) to identify acyl chains, ESI triple-quadrupole (Q) MS in the precursor mode to identify the nominal masses of complex polar lipids containing each acyl chain, and ESI Q-time-of-flight MS to confirm the identifications of the complex polar lipid species, 17 species of oxylipin-containing phosphatidylglycerols, monogalactosyldiacylglycerols (MGDG), and digalactosyldiacylglycerols (DGDG) were identified. The oxylipins of these polar complex lipid species include oxophytodienoic acid (OPDA), dinor-OPDA (dnOPDA), 18-carbon ketol acids, and 16-carbon ketol acids. Using ESI triple-Q MS in the precursor mode, the accumulation of five OPDA- and/or dnOPDA-containing MGDG and two OPDA-containing DGDG species were monitored as a function of time in mechanically wounded leaves. In unwounded leaves, the levels of these oxylipin-containing complex lipid species were low, between 0.001 and 0.023 nmol/mg dry weight. However, within the first 15 min after wounding, the levels of OPDA-dnOPDA MGDG, OPDA-OPDA MGDG, and OPDA-OPDA DGDG, each containing two oxylipin chains, increased 200- to 1,000-fold. In contrast, levels of OPDA-hexadecatrienoic acid MGDG, linolenic acid (18:3)-dnOPDA MGDG, OPDA-18:3 MGDG, and OPDA-18:3 DGDG, each containing a single oxylipin chain, rose 2- to 9-fold. The rapid accumulation of high levels of galactolipid species containing OPDA-OPDA and OPDA-dnOPDA in wounded leaves is consistent with these lipids being the primary products of plastidic oxylipin biosynthesis.

Oxidized phospholipid: POVPC binds to platelet-activating-factor receptor on human macrophages. Implications in atherosclerosis

Atherosclerosis as a chronic inflammatory disease resulting from the imbalance of the pro- and anti-inflammatory factors in the vessel wall. PAF and PAF-like oxidized phospholipids generated upon LDL oxidation in the intima of the arteries may interact with infiltrated monocytes/macrophages and lead to the alteration of gene expression patterns accompanied by an impaired production of chemokines, interleukins and proteolytic and lipolytic enzymes. The aim of this study was to evaluate the binding capacity of the major component of PAF-like oxidized phospholipids, namely the 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine (POVPC) to PAF-receptor (PAF-R) on the surface of human monocytes/macrophages and to further characterize the gene expression induced by such binding. We show that, POVPC binds to cultured human macrophages via PAF-R and transduces the signals leading to the intracellular Ca(2+) fluxes and modifies the transcription levels of numerous pro-inflammatory and pro-atherogenic genes. Although a some similarity of the gene expression patterns was observed when macrophages were activated with POVPC versus PAF, we observed that only POVPC treatment induced a several-fold activation of IL-8 gene. In turn, only PAF activated PAF-R, matrix metalloproteinase-13 and 15-lipoxygenase mRNA accumulation. Thus, we suggest, that POVPC signals in mature macrophages only in part through the PAF-R, a part of its effects may involve other receptors.

Molecular and mechanistic characterization of platelet-activating factor-like bioactivity produced upon LDL oxidation

Oxidation of LDL is thought to be involved in both initiating and sustaining atherogenesis through the formation of proinflammatory lipids and the covalent modification of LDL particles. Platelet-activating factor (PAF; 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a potent phospholipid mediator involved in inflammation. Upon oxidation of LDL, oxidized phospholipids with PAF-like structure are generated, and some of them may act via the PAF receptor. We evaluated the contribution of 1-0-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (C16:0 PAF) and of other PAF analogs on the PAF-like bioactivity formed upon Cu2+-initiated oxidation of LDL. Reverse-phase HPLC purification and electrospray ionization-MS analyses showed that upon oxidation of LDL with inactivated PAF-acetylhydrolase (PAF-AH), C16:0 PAF accounted for >30% of PAF-like biological activity and its sn-2 butenoyl analog accounted for >50%. However, upon LDL oxidation in the presence of exogenous 1-0-alkyl-sn-glycero-3-phosphocholine (lyso-PAF) without PAF-AH inactivation, C16:0 PAF formation accounted for >90% of the biological activity recovered. We suggest that the C16:0 PAF, despite being a minor constituent of the LDL peroxidation products, may contribute substantially to the bioactivity formed in oxidized LDL. The higher bioactivity of C16:0 PAF, and the higher selectivity of the LDL-attached lyso-PAF transacetylase toward very short acyl chains [acetate (C2) vs. butanate (C4)], may explain the contribution described above.

The rapid oxidative degradation of a phosphatidylcholine bearing an oxidatively modified acyl chain with a 2,4-dienal terminal

Phosphatidylcholines (PCs) having an acyl chain with a 2,4-dienal terminal are expected to be important bioactive compounds formed during lipid peroxidation in vivo. However, they have not been isolated from biological tissues. Here we used electrospray mass spectroscopy to investigate whether a high autoxidative instability may contribute to the difficulty in detecting one such compound, 13-oxo-9,11-tridecadienoyl PC (OTDA-PC, 1). Although we found that pure, synthetic OTDA-PC was very stable, OTDA-PC formed during the decomposition of a PC bearing the 13-hydroperoxide of alpha-linolenic acid (PC-LNA-OOH, 2) was readily converted (i) anaerobically to its corresponding acid PC, 13-carboxy-9,11-tridecadienoyl PC, 3; (ii) aerobically to other bioactive aldehyde (or acid) PC species that have been detected in atherosclerotic tissues. We attribute the high oxidative instability of OTDA-PC to a high vulnerability of its carbonyl hydrogen [H-C(=O)R] to abstraction by lipid-derived radicals, and propose mechanisms for its conversion to the other oxidised PC species (vide supra).

Fragmentation study of short-chain products derived from oxidation of diacylphosphatidylcholines by electrospray tandem mass spectrometry: identification of novel short-chain products

Lineloyl-palmitoyl (PLPC) and arachidonoyl-palmitoyl (PAPC) phosphatidylcholine were oxidized under Fenton reaction conditions (H2O2 and Fe2+), and the short-chain products formed were identified by electrospray ionization mass spectrometry (ESI-MS). The short-chain products resulted from beta-cleavage of oxygen-centered radicals and comprised aldehydes, hydroxyaldehydes and dicarboxylic acids that yielded both [MH]+ and [MNa]+ ions. The fragmentation of the [MH]+ and [MNa]+ ions of the peroxidation products was studied by tandem mass spectrometry (MS/MS). The MS/MS spectra of both ions showed ions resulting from characteristic losses of glycerophosphatidylcholine. Other product ions, resulting from C-C cleavages occurring in the vicinity of the functional group, and fragmentations involving the hydroxy groups, were the most informative since they allowed us to obtain structural information relating to the sn-2 acyl residue. Both fragmentation pathways are due to charge-remote fragmentation occurring by a 1,4-hydrogen elimination mechanism and/or by homolytic cleavage. Furthermore, the fragmentation pathway of some ions observed in the ESI-MS spectrum was not consistent with the fragmentation behavior expected for some of the short-chain species identified in the literature and allowed the reassignment of the ions as different structures. Isobaric ions were observed in the ESI-MS spectra of both oxidized phospholipids, and were differentiated based on distinct fragmentation. The detailed knowledge of lipid peroxidation degradation products is of major importance and should be very valuable in providing new markers for oxidative stress signaling and for disease states monitoring.

In situ detection and localization of lipid peroxidation in individual bovine sperm cells

Reactive oxygen species (ROS) have been implicated in many pathologies, including sub- and infertility. Freeze/thawing of sperm samples is routinely performed in the cattle breeding industries in order to perform artificial insemination. This freeze/thaw procedure is known to induce ROS in sperm samples. Lipid peroxidation in fresh and frozen/thawed sperm cells was assessed by mass spectrometric analysis of the main endogenous phospholipid class, phosphatidylcholine, and by fluorescence techniques using the lipid peroxidation reporter probe C11-BODIPY(581/591). Peroxidation as reported by the fluorescent probe, clearly corresponded with the presence of hydroxy- and hydroperoxyphosphatidylcholine in the sperm membranes, which are early stage products of lipid peroxidation. This allowed us, for the first time, to correlate endogenous lipid peroxidation with localization of this process in living sperm cells. Lipid peroxidation was particularly strong in the midpiece and tail of frozen/thawed spermatozoa and significantly less intense in the head. Induction of peroxidation in fresh sperm cells with the lipid soluble ROS tert-butylhydroperoxide gave an even more pronounced effect, demonstrating antioxidant activity in the head of fresh sperm cells. Furthermore, we were able to show that spontaneous peroxidation was not a result of cell death, as only a pronounced subpopulation of living cells showed peroxidation after freeze/thawing.

Temporary membrane distortion of vascular smooth muscle cells is responsible for their apoptosis induced by platelet-activating factor-like oxidized phospholipids and their degradation product, lysophosphatidylcholine

To obtain information about the mechanism of apoptosis induced by oxidized low density lipoproteins (oxLDL) in atherosclerotic plaques, we examined the effects of lysophosphatidylcholine (LPC) and platelet-activating factor (PAF)-like lipids (PAF-LL), which can be derived from oxLDL, on rat vascular smooth muscle cells (VSMC). All the lipids with different structures examined induced apoptosis of VSMC, so we studied the mechanism of induction of apoptosis by LPC. LPC-induced apoptosis was inhibited by alpha-tocopherol (alpha-T) and cholesterol (Chol), but not by other antioxidants such as palmitoyl ascorbic acid and PAF receptor antagonist. The cells temporarily became spherical and highly permeable before induction of apoptosis, and their change in shape was prevented by alpha-T and Chol. From these results, we suggest that the apoptosis induced by oxLDL-derived phospholipids in VSMC is caused by temporary membrane distortion, not through specific receptors.

Platelet-activating factor acetylhydrolase, and not paraoxonase-1, is the oxidized phospholipid hydrolase of high density lipoprotein particles

Paraoxonase-1 (PON1), an high density lipoprotein (HDL)-associated organophosphate triesterase, suppresses atherosclerosis in an unknown way. Purified PON1 protects lipoprotein particles from oxidative modification and hydrolyzes pro-atherogenic oxidized phospholipids and the inflammatory mediator platelet-activating factor (PAF). We find human PON1 acted as a phospholipase A(2) but not as a phospholipase C or D through cleavage of phosphodiester bonds as expected. PON1 requires divalent cations, but EDTA did not block the phospholipase A(2) activity of PON1. In contrast, a serine esterase inhibitor abolished phospholipase activity even though PON1 has no active-site serine residues. PAF acetylhydrolase, an oxidized phospholipid phospholipase A(2), is a serine esterase associated with specific HDL particles. Western blotting did not reveal detectable amounts of PAF acetylhydrolase in PON1 preparations, although very low amounts of PAF acetylhydrolase might still account for PON1 phospholipase A(2) activity. We revised the standard PON1 purification by first depleting HDL of PAF acetylhydrolase to find PON1 purified in this way no longer hydrolyzed oxidized phospholipids or PAF. Serum from a donor with an inactivating mutation in the PAF acetylhydrolase gene did not hydrolyze oxidized phospholipids or PAF, yet displayed full paraoxonase activity. We conclude that PAF acetylhydrolase is the sole phospholipase A(2) of HDL and that PON1 has no phospholipase activity toward PAF or pro-atherogenic oxidized phospholipids.

Oxidant stress and endothelial cell dysfunction

Reactive oxygen species (ROS) are generated at sites of inflammation and injury, and at low levels, ROS can function as signaling molecules participating as signaling intermediates in regulation of fundamental cell activities such as cell growth and cell adaptation responses, whereas at higher concentrations, ROS can cause cellular injury and death. The vascular endothelium, which regulates the passage of macromolecules and circulating cells from blood to tissues, is a major target of oxidant stress, playing a critical role in the pathophysiology of several vascular diseases and disorders. Specifically, oxidant stress increases vascular endothelial permeability and promotes leukocyte adhesion, which are coupled with alterations in endothelial signal transduction and redox-regulated transcription factors such as activator protein-1 and nuclear factor-kappaB. This review discusses recent findings on the cellular and molecular mechanisms by which ROS signal events leading to impairment of endothelial barrier function and promotion of leukocyte adhesion. Particular emphasis is placed on the regulation of cell-cell and cell-surface adhesion molecules, the actin cytoskeleton, key protein kinases, and signal transduction events.

Phospholipase A(2)s and lipid peroxidation

Lipid peroxidation of membrane phospholipids can proceed both enzymatically via the mammalian 15-lipoxygenase-1 or the NADPH-cytochrome P-450 reductase system and non-enzymatically. In some cells, such as reticulocytes, this process is biologically programmed, whereas in the majority of biological systems lipid peroxidation is a deleterious process that has to be repaired via a deacylation-reacylation cycle of phospholipid metabolism. Several reports in the literature pinpoint a stimulation by lipid peroxidation of the activity of secretory phospholipase A(2)s (mainly pancreatic and snake venom enzymes) which was originally interpreted as a repair function. However, recent experiments from our laboratory have demonstrated that in mixtures of lipoxygenated and native phospholipids the former are not preferably cleaved by either secretory or cytosolic phospholipase A(2)s. We propose that the platelet activating factor (PAF) acetylhydrolases of type II, which cleave preferentially peroxidised or lipoxygenated phospholipids, are competent for the phospholipid repair, irrespective of their role in PAF metabolism. A corresponding role of Ca(2+)-independent phospholipase A(2), which has been proposed to be involved in phospholipid remodelling in biomembranes, has not been addressed so far. Direct and indirect 15-lipoxygenation of phospholipids in biomembranes modulates cell signalling by several ways. The stimulation of phospholipase A(2)-mediated arachidonic acid release may constitute an alternative route of the arachidonic acid cascade. Thus, 15-lipoxygenase-mediated oxygenation of membrane phospholipids and its interaction with phospholipase A(2)s may play a crucial role in the pathogenesis of diseases, such as bronchial asthma and atherosclerosis.