Activation of phospholipase A2 by 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine in vitro

Destruction of polyunsaturated alkyl/acyl and alkenyl/acyl glycerophosphocholine of plasma lipoproteins during incubation with group V and X secretory phospholipase A2 s

Plasma lipoproteins are carriers of various glycerophospholipids including diacyl, alkenyl/acyl, and alkyl/acyl glycerophosphocholines (GPCs), which become distributed among cells and tissues during metabolism. For metabolic function, these phospholipids require hydrolysis by phospholipases, but the responsible enzymes have not been identified. We had previously shown that after complete digestion of lipoprotein diacyl- and oxo-diacyl-GPCs, degradation of residual alkyl/acyl and alkenyl/acyl GPCs continues, despite the fact that ether lipids are resistant to hydrolysis by Ca²⁺ -activated secretory PLA₂ s and require the presence of the Ca²⁺ -independent PLA₂ . In the course of further investigation, we came across a report by Khaselev and Murphy in which the autoxidative degradation of plasmalogens in the presence of 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) proceeded beyond the formation of dihydroperoxides, hydroxides and epoxides, and led to an attack on the enyl bond of the plasmalogen, resulting in formation of 1-OH/2-20:4-GPC and 1-formyl/2-20:4-GPC. Our preliminary investigation indicated that lipoprotein 16:0p/20:4ω6-GPC yielded the same autoxidation products as those reported for synthetic 16:0p/20:4ω6-GPC in the presence of AAPH. Such autoxidative degradation of lipoprotein plasmalogens had not been previously reported with or without AAPH. Subsequent study led to the conclusion that this reaction was not limited to arachidonates, but extended to other polyunsaturated eicosanoids, docosanoids, and tetracosanoids, as well as oligounsaturated octadecanoids. These observations led to a hypothesis that the autoxidative cleavage of the lipoprotein plasmalogens proceeded under the influence of apo-protein-derived free radicals as intermediates of oxidative processes.

Enhanced gene expression by a novel designed leucine zipper endosomolytic peptide

An endosomal trap is a major barrier in gene therapy. We have designed an endosomolytic peptide based on the leucine zipper sequence and characterized it both structurally and functionally. The results illustrated that leucine zipper endosomolytic peptide (LZEP) exhibited appreciable hemolysis of human red blood cells (hRBCs) at pH 5.0, but negligible hemolysis at pH 7.4. Calcein release experiment indicated that only at pH 5.0 but not at pH 7.4, LZEP was able to permeabilize hRBCs. LZEP revealed significant self-assembly as well as peptide induced α-helical structure at pH 5.0. Unlike at pH 5.0, LZEP failed to self-assemble and showed a random coil structure at pH 7.4. Transfection data depicted that lipoplexes modified with LZEP resulted in significantly higher gene expression as compared to lipoplexes without LZEP. Interestingly, the transfection efficacy of LZEP modified lipid nanoparticles reached the levels of Lipofectamine 2000 (LF 2000), without any cellular toxicity as observed by MTT assay. The results suggest a novel approach for designing endosomolytic peptides by employing the leucine zipper sequence and simultaneously the designed peptides could be utilized for enhancing gene delivery into mammalian cells.

Activation of phospholipase A2 by prostaglandin in vitro

Prostaglandins are a diverse family of biological active molecules that are synthesized after liberation of arachnidonic or linolenic acid from the plasma membrane by phospholipase A2 (PLA2). Specific prostaglandins may be pro-inflammatory or anti-inflammatory due to a poorly understood biochemical equilibrium. Some of the anti-inflammatory prostaglandins namely, prostaglandin A1 (PGA1) and prostaglandin E1 (PGE1) have a cyclopentenone moiety that can react and modify a protein's activity. These two prostaglandins are electrophilic reactive lipid species and are formed as a result of the reaction cascade initiated by PLA2. It was of interest to study the interaction with these prostaglandins as they could either amplify or block this enzyme's activity. We found that the former is true initially as there is a shorter time to activate the protein on the lipid membrane and an overall increase in hydrolysis was observed when PGA1 and PGE1 prostaglandin was added with PLA2 and liposomes. The interfacial activation model was further explored in which there is a modification of the enzyme rather than a modifcation of the substrate. However, after a time the protein was shown to form amyloid like fibrils thereby blocking further hydrolysis. The fibrillization kinetics in the presence of the one of the prostaglandins was monitored using thioflavin T (ThT) and the resulting fibrils were characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Modification of PLA2 by these prostaglandins leading to amyloid like fibrils gives an additional perspective of control of the interfacial activation mechanism of this enzyme.

Oxidized lipids: not just another brick in the wall 1

Over the past decade, there has been intense investigation in trying to understand the pathological role that oxidized phospholipids play in cardiovascular disease. Phospholipids are targets for oxidation, particularly during conditions of excess free radical generation. Once oxidized, they acquire novel roles uncharacteristic of their precursors. Oxidized phosphatidylcholines have an important role in multiple physiological and pathophysiological conditions including atherosclerosis, neurodegenerative diseases, lung disease, inflammation, and chronic alcohol consumption. Circulating oxidized phosphatidylcholine may also serve as a clinical biomarker. The focus of this review, therefore, will be to summarize existing evidence that oxidized phosphatidylcholine molecules play an important role in cardiovascular pathology.

Oxidative Phospholipidomics in health and disease: Achievements, challenges and hopes

Phospholipid peroxidation products are recognized as important bioactive lipid mediators playing an active role as modulators in signalling events in inflammation, immunity and infection. The biochemical responses are determined by the oxidation structural features present in oxPL modulating biophysical and biological properties in model membranes and lipoproteins. In spite of the extensive work conducted with model systems over the last 20 years, the study of oxPL in biological systems has virtually stagnated. In fact, very little is known concerning the predominant oxPL in fluids and tissues, their basal levels, and any variations introduced with age, gender and ethnicity in health and disease. In consequence, knowledge on oxPL has not yet translated into clinical diagnostic, in the early and timely diagnosis of "silent" diseases such as atherosclerosis and cardiovascular diseases, or as prognosis tools in disease stratification and particularly useful in the context of multimorbidities. Their use as therapeutic solutions or the development of innovative functional biomaterials remains to be explored. This review summarizes the achievements made in the identification of oxPL revealing an enormous structural diversity. A brief overview of the challenges associated with the analysis of such diverse array of products is given and a critical evaluation on key aspects in the analysis pipeline that need to be addressed. Once these issues are addressed, Oxidative Phospholipidomics will hopefully lead to major breakthrough discoveries in biochemistry, pharmaceutical, and clinical areas for the upcoming 20 years. This article is part of Special Issue entitled 4-Hydroxynonenal and Related Lipid Oxidation Products.

Chlorinated Phospholipids and Fatty Acids: (Patho)physiological Relevance, Potential Toxicity, and Analysis of Lipid Chlorohydrins

Chlorinated phospholipids are formed by the reaction of hypochlorous acid (HOCl), generated by the enzyme myeloperoxidase under inflammatory conditions, and the unsaturated fatty acyl residues or the head group. In the first case the generated chlorohydrins are both proinflammatory and cytotoxic, thus having a significant impact on the structures of biomembranes. The latter case leads to chloramines, the properties of which are by far less well understood. Since HOCl is also widely used as a disinfecting and antibacterial agent in medicinal, industrial, and domestic applications, it may represent an additional source of danger in the case of abuse or mishandling. This review discusses the reaction behavior of in vivo generated HOCl and biomolecules like DNA, proteins, and carbohydrates but will focus on phospholipids. Not only the beneficial and pathological (toxic) effects of chlorinated lipids but also the importance of these chlorinated species is discussed. Some selected cleavage products of (chlorinated) phospholipids and plasmalogens such as lysophospholipids, (chlorinated) free fatty acids and α-chloro fatty aldehydes, which are all well known to massively contribute to inflammatory diseases associated with oxidative stress, will be also discussed. Finally, common analytical methods to study these compounds will be reviewed with focus on mass spectrometric techniques.

Trp53 deficient mice predisposed to preterm birth display region-specific lipid alterations at the embryo implantation site

Here we demonstrate that conditional deletion of mouse uterine Trp53 (p53^d/d), molecularly linked to mTORC1 activation and causally linked to premature uterine senescence and preterm birth, results in aberrant lipid signatures within the heterogeneous cell types of embryo implantation sites on day 8 of pregnancy. In situ nanospray desorption electrospray ionization mass spectrometry imaging (nano-DESI MSI) was used to characterize the molecular speciation of free fatty acids, monoacylglycerol species, unmodified and oxidized phosphatidylcholine (PC/Ox-PC), and diacylglycerol (DG) species within implantation sites of p53^d/d mice and floxed littermates. Implantation sites from p53^d/d mice exhibited distinct spatially resolved changes demonstrating accumulation of DG species, depletion of Ox-PC species, and increase in species with more unsaturated acyl chains, including arachidonic and docosahexaenoic acid. Understanding abnormal changes in the abundance and localization of individual lipid species early in the progression to premature birth is an important step toward discovering novel targets for treatments and diagnosis.

Phospholipid Ozonation Products Activate the 5-Lipoxygenase Pathway in Macrophages

Ozone is a highly reactive environmental toxicant that can react with the double bonds of lipids in pulmonary surfactant. This study was undertaken to investigate the proinflammatory properties of the major lipid-ozone product in pulmonary surfactant, 1-palmitoyl-2-(9'-oxo-nonanoyl)-glycerophosphocholine (16:0/9al-PC), with respect to eicosanoid production. A dose-dependent increase in the formation of 5-lipoxygenase (5-LO) products was observed in murine resident peritoneal macrophages (RPM) and alveolar macrophages (AM) upon treatment with 16:0/9al-PC. In contrast, the production of cyclooxygenase (COX) derived eicosanoids did not change from basal levels in the presence of 16:0/9al-PC. When 16:0/9al-PC and the TLR2 ligand, zymosan, were added to RPM or AM, an enhancement of 5-LO product formation along with a concomitant decrease in COX product formation was observed. Neither intracellular calcium levels nor arachidonic acid release was influenced by the addition of 16:0/9al-PC to RPM. Results from mitogen-activated protein kinase (MAPK) inhibitor studies and direct measurement of phosphorylation of MAPKs revealed that 16:0/9al-PC activates the p38 MAPK pathway in RPM, which results in the activation of 5-LO. Our results indicate that 16:0/9al-PC has a profound effect on the eicosanoid pathway, which may have implications in inflammatory pulmonary disease states where eicosanoids have been shown to play a role.

Formation of Aldehydic Phosphatidylcholines during the Anaerobic Decomposition of a Phosphatidylcholine Bearing the 9-Hydroperoxide of Linoleic Acid

Lipid oxidation-derived carbonyl compounds are associated with the development of various physiological disorders. Formation of most of these products has recently been suggested to require further reactions of oxygen with lipid hydroperoxides. However, in rat and human tissues, the formation of 4-hydroxy-2-nonenal is greatly elevated during hypoxic/ischemic conditions. Furthermore, a previous study found an unexpected result that the decomposition of a phosphatidylcholine (PC) bearing the 13-hydroperoxide of linoleic acid under a nitrogen atmosphere afforded 9-oxononanoyl-PC rather than 13-oxo-9,11-tridecadienoyl-PC as the main aldehydic PC. In the present study, products of the anaerobic decomposition of a PC bearing the 9-hydroperoxide of linoleic acid were analysed by electrospray ionization mass spectrometry. 9-Oxononanoyl-PC (ONA-PC) and several well-known bioactive aldehydes including 12-oxo-9-hydroperoxy-(or oxo or hydroxy)-10-dodecenoyl-PCs were detected. Hydrolysis of the oxidized PC products, methylation of the acids obtained thereby, and subsequent gas chromatography-mass spectroscopy with electron impact ionization further confirmed structures of some of the key aldehydic PCs. Novel, hydroxyl radical-dependent mechanisms of formation of ONA-PC and peroxyl-radical dependent mechanisms of formation of the rest of the aldehydes are proposed. The latter mechanisms will mainly be relevant to tissue injury under hypoxic/anoxic conditions, while the former are relevant under both normoxia and hypoxia/anoxia.

Interactions and dynamics of two extended conformation adapting phosphatidylcholines in model biomembranes

In order to obtain molecular level insight into the biophysics of the apoptosis promoting phospholipid 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC) we studied its partitioning into different lipid phases by isothermal titration calorimetry (ITC). To aid the interpretation of these data for PazePC, we additionally characterized by both ITC and fluorescence spectroscopy the fluorescent phospholipid analog 1-palmitoyl-2-{6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]hexanoyl}-sn-glycero-3-phosphocholine (NBD-C6-PC), which similarly to PazePC can adopt extended conformation in lipid bilayers. With the NBD-hexanoyl chain reversing its direction and extending into the aqueous space out of the bilayer, 7-nitro-2,1,3-benzoxadiazol-4-yl (NBD) becomes accessible to the water soluble dithionite, which reduces to non-fluorescent product. Our results suggest that these phospholipid derivatives first partition and penetrate into the outer bilayer leaflet of liquid disordered phase liposomes composed of unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Upon increase up to 2 mol% PazePC and NBD-C6-PC of the overall content, flip-flop from the outer into the inner bilayer leaflet commences. Interestingly, the presence of 40 mol% cholesterol in POPC liposomes did not abrogate the partitioning of PazePC into the liquid ordered phase. In contrast, only insignificant partitioning of PazePC and NBD-C6-PC into sphingomyelin/cholesterol liposomes was evident, highlighting a specific membrane permeability barrier function of this particular lipid composition against oxidatively truncated PazePC, thus emphasizing the importance of detailed characterization of the biophysical properties of membranes found in different cellular organelles, in terms of providing barriers for lipid-mediated cellular signals in processes such as apoptosis. Our data suggest NBD-C6-PC to represent useful fluorescent probe to study the cellular dynamics of oxidized phospholipid species, such as PazePC.

Separation and characterization of oxidized isomeric lipid-peptide adducts by ion mobility mass spectrometry

Phospholipids are major components of cell membranes and lipoprotein complexes. They are prone to oxidation by endogenous and exogenous reactive oxygen species yielding a large variety of modified lipids including small aliphatic and phospholipid bound aldehydes and ketones. These carbonyls are strong electrophiles that can modify proteins and, thereby, alter their structures and functions triggering various pathophysiological conditions. The analysis of lipid-protein adducts by liquid chromatography-MS is challenged by their mixed chemical nature (polar peptide and hydrophobic lipid), low abundance in biological samples, and formation of multiple isomers. Thus, we investigated traveling wave ion mobility mass spectrometry (TWIMS) to analyze lipid-peptide adducts generated by incubating model peptides corresponding to the amphipathic β1 sheet sequence of apolipoprotein B-100 with 1-palmitoyl-2-(oxo-nonanoyl)-sn-glycerophosphatidylcholine (PONPC). The complex mixture of peptides, lipids, and peptide-lipid adducts was separated by TWIMS, which was especially important for the identification of two mono-PONPC-peptide isomers containing Schiff bases at different lysine residues. Moreover, TWIMS separated structural conformers of one peptide-lipid adduct possessing most likely different orientations of the hydrophobic sn-1 fatty acyl residue and head group of PONPC, relative to the peptide backbone.

Class specific peptide inhibitors for secretory phospholipases A2

Phospholipases A2 (PLA2) catalyze the hydrolytic cleavage of free fatty acids from the sn-2 OH-moiety of glycerophospholipids. These enzymes have a number of functions, from digestion to signaling and toxicity of several venoms. They have also been implicated in inflammation and are connected to diverse diseases, such as cancer, ischemia, atherosclerosis, and schizophrenia. Accordingly, there is a keen interest to develop selective inhibitors for therapeutic use. We recently proposed a novel mechanism for the control of PLA2 activity with highly active protofibrils of PLA2 existing transiently before conversion to inactive amyloid fibrils [19]. In keeping with the above mechanism several algorithms identified (85)KMYFNLI(91) and (17)AALSYGFYG(25) in bee venom (bv) and human lacrimal fluid (Lf) PLA2, respectively, as a regions potentially forming amyloid type aggregates. Interestingly, in keeping with the proposed role of these sequences in the control of the activity of these enzymes, preincubation of 2nM bvPLA2 with (85)KMYFNLI(91) caused complete inhibition of PLA2 activity while the scrambled control peptide YNFLIMK had no effect. Approximately 36% attenuation of the hydrolytic activity of LfPLA2 present in human lacrimal fluid was observed in the presence of 80nM (17)AALSYGFYG(25).

Fusogenicity of Naja naja atra cardiotoxin-like basic protein on sphingomyelin vesicles containing oxidized phosphatidylcholine and cholesterol

This study investigated the effect of oxidized phosphatidylcholine (oxPC) and cholesterol (Chol) on Naja naja atra cardiotoxin-like basic protein (CLBP)-induced fusion and leakage in sphingomyelin (SM) vesicles. Compared with those on PC/SM/Chol vesicles, CLBP showed a lower activity to induce membrane permeability but a higher fusogenicity on oxPC/SM/Chol vesicles. A reduction in inner-leaflet fusion elucidated that CLBP fusogenicity was not in parallel to its membrane-leakage activity on oxPC/SM/Chol vesicles. The lipid domain formed by Chol and SM supported CLBP fusogenicity on oxPC/SM/Chol vesicles, while oxPC altered the interacted mode of CLBP with oxPC/SM/Chol vesicles as evidenced by Fourier transform infrared spectra analyses and colorimetric phospholipid/polydiacetylene membrane assay. Although CLBP showed similar binding affinity with PC/SM/Chol and oxPC/SM/Chol vesicles, the binding capability of CLBP with PC/SM/Chol and oxPC/SM/Chol vesicles was affected differently by NaCl. This emphasized that CLBP adopted different membrane interaction modes upon binding with PC/SM/Chol and oxPC/SM/Chol vesicles. CLBP induced fusion in vesicles containing oxPC bearing the aldehyde group, and aldehyde scavenger methoxyamine abrogated the CLBP ability to induce oxPC/SM/Chol fusion. Taken together, our data indicate that Chol and oxPC bearing aldehyde group alter the CLBP membrane-binding mode, leading to fusogenicity promotion while reducing the membrane-damaging activity of CLBP.

Protein modification by aldehydophospholipids and its functional consequences

Phospholipid aldehydes represent a particular subclass of lipid oxidation products. They are chemically reactive and can form Schiff bases with proteins and aminophospholipids. As chemically bound molecular entities they modulate the functional properties of biomolecules in solution and the surface of supramolecular systems including plasma lipoproteins and cell membranes. The lipid-protein and lipid-lipid conjugates may be considered the active primary platforms that are responsible for the biological effects of aldehydophospholipids, e.g. receptor binding, cell signaling, and recognition by the immune system. Despite the fact that aldehydophospholipids are covalently associated, they are subject to exchange between nucleophiles since their imine conjugates are not stable. As a consequence, aldehydophospholipids exist in a dynamic equilibrium between different "states" depending on the lipid and protein environment. Aldehydophospholipids may also contribute to the systemic administration and activity of oxidized phospholipids by inducing release of microparticles by cells. These effects are lipid-specific. Future studies should help clarify the mechanisms and consequences of these membrane-associated effects of "phospholipid stress". This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Chemistry of phospholipid oxidation

The oxidation of lipids has long been a topic of interest in biological and food sciences, and the fundamental principles of non-enzymatic free radical attack on phospholipids are well established, although questions about detail of the mechanisms remain. The number of end products that are formed following the initiation of phospholipid peroxidation is large, and is continually growing as new structures of oxidized phospholipids are elucidated. Common products are phospholipids with esterified isoprostane-like structures and chain-shortened products containing hydroxy, carbonyl or carboxylic acid groups; the carbonyl-containing compounds are reactive and readily form adducts with proteins and other biomolecules. Phospholipids can also be attacked by reactive nitrogen and chlorine species, further expanding the range of products to nitrated and chlorinated phospholipids. Key to understanding the mechanisms of oxidation is the development of advanced and sensitive technologies that enable structural elucidation. Tandem mass spectrometry has proved invaluable in this respect and is generally the method of choice for structural work. A number of studies have investigated whether individual oxidized phospholipid products occur in vivo, and mass spectrometry techniques have been instrumental in detecting a variety of oxidation products in biological samples such as atherosclerotic plaque material, brain tissue, intestinal tissue and plasma, although relatively few have achieved an absolute quantitative analysis. The levels of oxidized phospholipids in vivo is a critical question, as there is now substantial evidence that many of these compounds are bioactive and could contribute to pathology. The challenges for the future will be to adopt lipidomic approaches to map the profile of oxidized phospholipid formation in different biological conditions, and relate this to their effects in vivo. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Uptake and protein targeting of fluorescent oxidized phospholipids in cultured RAW 264.7 macrophages

The truncated phospholipids 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) are oxidation products of 1-palmitoyl-2-arachidonoyl phosphatidylcholine. Depending on concentration and the extent of modification, these compounds induce growth and death, differentiation and inflammation of vascular cells thus playing a role in the development of atherosclerosis. Here we describe the import of fluorescent POVPC and PGPC analogs into cultured RAW 264.7 macrophages and the identification of their primary protein targets. We found that the fluorescent oxidized phospholipids were rapidly taken up by the cells. The cellular target sites depended on the chemical reactivity of these compounds but not on the donor (aqueous lipid suspension, albumin or LDL). The great differences in cellular uptake of PGPC and POVPC are a direct consequence of the subtle structural differences between both molecules. The former compound (carboxyl lipid) can only physically interact with the molecules in its immediate vicinity. In contrast, the aldehydo-lipid covalently reacts with free amino groups of proteins by forming covalent Schiff bases, and thus becomes trapped in the cell surface. Despite covalent binding, POVPC is exchangeable between (lipo)proteins and cells, since imines are subject to proton-catalyzed base exchange. Protein targeting by POVPC is a selective process since only a limited subfraction of the total proteome was labeled by the fluorescent aldehydo-phospholipid. Chemically stabilized lipid–protein conjugates were identified by MS/MS. The respective proteins are involved in apoptosis, stress response, lipid metabolism and transport. The identified target proteins may be considered primary signaling platforms of the oxidized phospholipid.

Physiological effects of oxidized phospholipids and their cellular signaling mechanisms in inflammation

Oxidized phospholipids, such as the products of the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine by nonenzymatic radical attack, are known to be formed in a number of inflammatory diseases. Interest in the bioactivity and signaling functions of these compounds has increased enormously, with many studies using cultured immortalized and primary cells, tissues, and animals to understand their roles in disease pathology. Initially, oxidized phospholipids were viewed largely as culprits, in line with observations that they have proinflammatory effects, enhancing inflammatory cytokine production, cell adhesion and migration, proliferation, apoptosis, and necrosis, especially in vascular endothelial cells, macrophages, and smooth muscle cells. However, evidence has emerged that these compounds also have protective effects in some situations and cell types; a notable example is their ability to interfere with signaling by certain Toll-like receptors (TLRs) induced by microbial products that normally leads to inflammation. They also have protective effects via the stimulation of small GTPases and induce up-regulation of antioxidant enzymes and cytoskeletal rearrangements that improve endothelial barrier function. Oxidized phospholipids interact with several cellular receptors, including scavenger receptors, platelet-activating factor receptors, peroxisome proliferator-activated receptors, and TLRs. The various and sometimes contradictory effects that have been observed for oxidized phospholipids depend on their concentration, their specific structure, and the cell type investigated. Nevertheless, the underlying molecular mechanisms by which oxidized phospholipids exert their effects in various pathologies are similar. Although our understanding of the actions and mechanisms of these mediators has advanced substantially, many questions do remain about their precise interactions with components of cell signaling pathways.