The lipid whisker model of the structure of oxidized cell membranes

The lipid network

Natural cell membranes are composed of a remarkable variety of lipids, which provide specific biophysical properties to support membrane protein function. An improved understanding of this complexity of membrane composition may also allow the design of membrane active drugs. Crafting a relevant model of a cell membrane with controlled composition is becoming an art, with the ability to reveal the molecular mechanisms of biological processes and lead to better treatment of pathologies. By matching physiological observations from in vivo experiments to high-resolution information, more easily obtained from in vitro studies, complex interactions at the lipid interface are determined. The role of the lipid network in biological membranes is, therefore, the subject of increasing attention.

Bioactive oxidatively truncated phospholipids in inflammation and apoptosis: formation, targets, and inactivation

This report reviews structurally related phospholipid oxidation products that are biologically active where molecular mechanisms have been defined. Phospholipids containing polyunsaturated fatty acyl residues are chemically or enzymatically oxidized to phospholipid hydroperoxides, which may fragment on either side of the newly introduced peroxy function to form phospholipids with a truncated sn-2 residue. These truncated phospholipids not subject to biologic control of their production and, depending on the sn-2 residue length and structure, can stimulate the plasma membrane receptor for PAF. Alternatively, these chemically formed products can be internalized by a transport system to either stimulate the lipid activated nuclear transcription factor PPARγ or at higher levels interact with mitochondria to initiate the intrinsic apoptotic cascade. Intracellular PAF acetylhydrolases specifically hydrolyze truncated phospholipids, and not undamaged, biosynthetic phospholipids, to protect cells from oxidative death. Truncated phospholipids are also formed within cells where they couple cytokine stimulation to mitochondrial damage and apoptosis. The relevance of intracellular truncated phospholipids is shown by the complete protection from cytokine induced apoptosis by PAF acetylhydrolase expression. This protection shows truncated phospholipids are the actual effectors of cytokine mediated toxicity. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

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.

Oxidized phospholipids on apoB-100-containing lipoproteins: a biomarker predicting cardiovascular disease and cardiovascular events

Oxidative stress is a well-known etiologic factor in the development of cardiovascular disease. Oxidation of lipoproteins, and in particular of low density lipoprotein, is a necessary if not obligatory mechanism for the generation of macrophage-derived foam cells, the first major initiating factor in the development of an atherosclerotic plaque. Oxidation of lipoproteins does not result in the generation of a single, defined molecular species, but of a variety of oxidation-specific epitopes, such as oxidized phospholipids and malondialdehyde-lysine epitopes. Unique monoclonal antibodies have been developed to bind these well-defined epitopes, and have been used in in vitro assays to detect them on circulating lipoproteins present in plasma. This article will summarize the accumulating clinical data of one oxidation-specific biomarker, oxidized phospholipids (OxPL) on apoB-100 lipoproteins. Elevated levels of OxPL/apoB predict the presence and progression of coronary, femoral and carotid artery disease, are increased following acute coronary syndromes and percutaneous coronary intervention, and predict the development of death, myocardial infarction, stroke and need for revascularization in unselected populations. OxPL/apoB levels are independent of traditional risk factors and the metabolic syndrome, and enhance the risk prediction of the Framingham Risk Score. The OxPLs measured in this assay reflect the biological activity of the most atherogenic lipoprotein(a) (Lp(a)) particles, reflected in patients with high plasma Lp(a) levels with small apo(a) isoforms. The predictive value of OxPL/apoB is amplified by Lp(a) and phospholipases such as lipoprotein-associated phospholipase A(2) and secretory phospholipase A(2), which are targets of therapy in clinical trials. This assay has now been validated in over 10,000 patients and efforts are underway to make it available to the research and clinical communities.

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.

In vivo targeting of inflammation-associated myeloid-related protein 8/14 via gadolinium immunonanoparticles

OBJECTIVE

Myeloid-related protein (Mrp) 8/14 complex (is a highly expressed extracellularly secreted protein, implicated in atherosclerosis. In this study, we evaluated the feasibility of targeting Mrp in vivo through synthetic immuno-nanoprobes.

METHODS AND RESULTS

Anti-Mrp-14 and nonspecific IgG-conjugated gadolinium nanoprobes (aMrp-) were synthesized and characterized. Pharmacokinetics and vascular targeting via MRI of the formulations were assessed in vivo in high fat-fed apolipoprotein E deficient (ApoE(-/-)), ApoE(-/-)/Mrp14(-/-) (double knockout) and chow-fed wild-type (C57BL/6) mice. Bone marrow-derived myeloid progenitor cells were isolated from both ApoE(-/-) and double knockout mice, differentiated to macrophages, and were treated with LPS, with or without Mrp8, Mrp14, or Mrp8/14; conditioned media was used for in vitro studies. Mrp-activated cells secreted significant amounts of proinflammatory cytokines, which was abolished by pretreatment with aMrp-NP. We show in vitro that aMrp-NP binds endothelial cells previously treated with conditioned media containing Mrp8/14. MRI following intravenous delivery of aMrp-NP revealed prolonged and substantial delineation of plaque in ApoE(-/-) but not double knockout or wild-type animals. Nonspecific IgG-conjugated gadolinium nanoprobe-injected animals in all groups did not show vessel wall enhancement. Flow-cytometric analysis of aortic digesta revealed that aMrp-NP present in Ly-6G(+), CD11b(+), CD11c(+), and CD31(+) cells in ApoE(-/-) but not in double knockout animals.

CONCLUSIONS

Targeted imaging with aMrp-NP demonstrates enhancement of plaque with binding to inflammatory cells and reduction in inflammation. This strategy has promise as a theranostic approach for atherosclerosis.

Analysis of oxidized and chlorinated lipids by mass spectrometry and relevance to signalling

Oxidized and chlorinated phospholipids are generated under inflammatory conditions and are increasingly understood to play important roles in diseases involving oxidative stress. MS is a sensitive and informative technique for monitoring phospholipid oxidation that can provide structural information and simultaneously detect a wide variety of oxidation products, including chain-shortened and -chlorinated phospholipids. MSn technologies involve fragmentation of the compounds to yield diagnostic fragment ions and thus assist in identification. Advanced methods such as neutral loss and precursor ion scanning can facilitate the analysis of specific oxidation products in complex biological samples. This is essential for determining the contributions of different phospholipid oxidation products in disease. While many pro-inflammatory signalling effects of oxPLs (oxidized phospholipids) have been reported, it has more recently become clear that they can also have anti-inflammatory effects in conditions such as infection and endotoxaemia. In contrast with free radical-generated oxPLs, the signalling effects of chlorinated lipids are much less well understood, but they appear to demonstrate mainly pro-inflammatory effects. Specific analysis of oxidized and chlorinated lipids and the determination of their molecular effects are crucial to understanding their role in disease pathology.

Aging enhances the production of reactive oxygen species and bactericidal activity in peritoneal macrophages by upregulating classical activation pathways

Maintenance of macrophages in their basal state and their rapid activation in response to pathogen detection are central to the innate immune system, acting to limit nonspecific oxidative damage and promote pathogen killing following infection. To identify possible age-related alterations in macrophage function, we have assayed the function of peritoneal macrophages from young (3-4 months) and aged (14-15 months) Balb/c mice. In agreement with prior suggestions, we observe age-dependent increases in the extent of recruitment of macrophages into the peritoneum, as well as ex vivo functional changes involving enhanced nitric oxide production under resting conditions that contribute to a reduction in the time needed for full activation of senescent macrophages following exposure to lipopolysaccharides (LPS). Further, we observe enhanced bactericidal activity following Salmonella uptake by macrophages isolated from aged Balb/c mice in comparison with those isolated from young animals. Pathways responsible for observed phenotypic changes were interrogated using tandem mass spectrometry, which identified age-dependent increases in levels of proteins linked to immune cell pathways under basal conditions and following LPS activation. Immune pathways upregulated in macrophages isolated from aged mice include proteins critical to the formation of the immunoproteasome. Detection of these latter proteins is dramatically enhanced following LPS exposure for macrophages isolated from aged animals; in comparison, the identification of immunoproteasome subunits is insensitive to LPS exposure for macrophages isolated from young animals. Consistent with observed global changes in the proteome, quantitative proteomic measurements indicate that there are age-dependent abundance changes involving specific proteins linked to immune cell function under basal conditions. LPS exposure selectively increases the levels of many proteins involved in immune cell function in aged Balb/c mice. Collectively, these results indicate that macrophages isolated from old mice are in a preactivated state that enhances their sensitivities to LPS exposure. The hyper-responsive activation of macrophages in aged animals may act to minimize infection by general bacterial threats that arise due to age-dependent declines in adaptive immunity. However, this hypersensitivity and the associated increase in the level of formation of reactive oxygen species are likely to contribute to observed age-dependent increases in the level of oxidative damage that underlie many diseases of the elderly.

Lipid peroxidation modifies the picture of membranes from the "Fluid Mosaic Model" to the "Lipid Whisker Model"

The "Fluid Mosaic Model", described by Singer and Nicolson, explain both how a cell membrane preserves a critical barrier function while it concomitantly facilitates rapid lateral diffusion of proteins and lipids within the planar membrane surface. However, the lipid components of biological plasma membranes are not regularly distributed. They are thought to contain "rafts" - nano-domains enriched in sphingolipids and cholesterol that are distinct from surrounding membranes of unsaturated phospholipids. Cholesterol and fatty acids adjust the transport and diffusion of molecular oxygen in membranes. The presence of cholesterol and saturated phospholipids decreases oxygen permeability across the membrane. Alpha-tocopherol, the main antioxidant in biological membranes, partition into domains that are enriched in polyunsaturated phospholipids increasing the concentration of the vitamin in the place where it is most required. On the basis of these observations, it is possible to assume that non-raft domains enriched in phospholipids containing PUFAs and vitamin E will be more accessible by molecular oxygen than lipid raft domains enriched in sphingolipids and cholesterol. This situation will render some nano-domains more sensitive to lipid peroxidation than others. Phospholipid oxidation products are very likely to alter the properties of biological membranes, because their polarity and shape may differ considerably from the structures of their parent molecules. Addition of a polar oxygen atom to several peroxidized fatty acids reorients the acyl chain whereby it no longer remains buried within the membrane interior, but rather projects into the aqueous environment "Lipid Whisker Model". This exceptional conformational change facilitates direct physical access of the oxidized fatty acid moiety to cell surface scavenger receptors.

Critical insights into cardiovascular disease from basic research on the oxidation of phospholipids: the γ-hydroxyalkenal phospholipid hypothesis

Basic research, exploring the hypothesis that γ-hydroxyalkenal phospholipids are generated in vivo through oxidative cleavage of polyunsaturated phospholipids, is delivering a bonanza of molecular mechanistic insights into cardiovascular disease. Rather than targeting a specific pathology, these studies were predicated on the presumption that a fundamental understanding of lipid oxidation is likely to provide critical insights into disease processes. This investigational approach, from the chemistry of biomolecules to disease phenotype, that complements the more common opposite paradigm, is proving remarkably productive.

The long life of birds: the rat-pigeon comparison revisited

The most studied comparison of aging and maximum lifespan potential (MLSP) among endotherms involves the 7-fold longevity difference between rats (MLSP 5y) and pigeons (MLSP 35y). A widely accepted theory explaining MLSP differences between species is the oxidative stress theory, which purports that reactive oxygen species (ROS) produced during mitochondrial respiration damage bio-molecules and eventually lead to the breakdown of regulatory systems and consequent death. Previous rat-pigeon studies compared only aspects of the oxidative stress theory and most concluded that the lower mitochondrial superoxide production of pigeons compared to rats was responsible for their much greater longevity. This conclusion is based mainly on data from one tissue (the heart) using one mitochondrial substrate (succinate). Studies on heart mitochondria using pyruvate as a mitochondrial substrate gave contradictory results. We believe the conclusion that birds produce less mitochondrial superoxide than mammals is unwarranted. We have revisited the rat-pigeon comparison in the most comprehensive manner to date. We have measured superoxide production (by heart, skeletal muscle and liver mitochondria), five different antioxidants in plasma, three tissues and mitochondria, membrane fatty acid composition (in seven tissues and three mitochondria), and biomarkers of oxidative damage. The only substantial and consistent difference that we have observed between rats and pigeons is their membrane fatty acid composition, with rats having membranes that are more susceptible to damage. This suggests that, although there was no difference in superoxide production, there is likely a much greater production of lipid-based ROS in the rat. We conclude that the differences in superoxide production reported previously were due to the arbitrary selection of heart muscle to source mitochondria and the provision of succinate. Had mitochondria been harvested from other tissues or other relevant mitochondrial metabolic substrates been used, then very different conclusions regarding differences in oxidative stress would have been reached.

Leaf extract of Moringa oleifera prevents ionizing radiation-induced oxidative stress in mice

The present study evaluated the hepatoprotective effect of aqueous ethanolic Moringa oleifera leaf extract (MoLE) against radiation-induced oxidative stress, which is assessed in terms of inflammation and lipid peroxidation. Swiss albino mice were administered MoLE (300 mg/kg of body weight) for 15 consecutive days before exposing them to a single dose of 5 Gy of ⁶⁰Co γ-irradiation. Mice were sacrificed at 4 hours after irradiation. Liver was collected for immunoblotting and biochemical tests for the detection of markers of hepatic oxidative stress. Nuclear translocation of nuclear factor kappa B (NF-κB) and lipid peroxidation were augmented, whereas the superoxide dismutase (SOD), catalase (CAT), reduced glutathione (GSH), and ferric reducing antioxidant power (FRAP) values were decreased by radiation exposure. Translocation of NF-κB from cytoplasm to nucleus and lipid peroxidation were found to be inhibited, whereas increases in SOD, CAT, GSH, and FRAP were observed in the mice treated with MoLE prior to irradiation. Therefore pretreatment with MoLE protected against γ-radiation-induced liver damage. The protection may be attributed to the free radical scavenging activity of MoLE, through which it can ameliorate radiation-induced oxidative stress.

Oxidative Stress and Inflammation in Heart Disease: Do Antioxidants Have a Role in Treatment and/or Prevention?

Inflammation triggered by oxidative stress is the cause of much, perhaps even most, chronic human disease including human aging. The oxidative stress originates mainly in mitochondria from reactive oxygen and reactive nitrogen species (ROS/RNS) and can be identified in most of the key steps in the pathophysiology of atherosclerosis and the consequential clinical manifestations of cardiovascular disease. In addition to the formation of atherosclerosis, it involves lipid metabolism, plaque rupture, thrombosis, myocardial injury, apoptosis, fibrosis and failure. The recognition of the critical importance of oxidative stress has led to the enthusiastic use of antioxidants in the treatment and prevention of heart disease, but the results of prospective, randomized clinical trials have been overall disappointing. Can this contradiction be explained and what are its implications for the discovery/development of future antioxidant therapeutics?

An (1)O2 route to γ-hydroxyalkenal phospholipids by vitamin E-induced fragmentation of hydroperoxydiene-derived endoperoxides

Biologically active phospholipids that incorporate an oxidatively truncated acyl chain terminated by a γ-hydroxyalkenal are generated in vivo. The γ-hydroxyalkenal moiety protrudes from lipid bilayers like whiskers that serve as ligands for the scavenger receptor CD36, fostering endocytosis, e.g., of oxidatively damaged photoreceptor cell outer segments by retinal pigmented endothelial cells. They also covalently modify proteins generating carboxyalkyl pyrroles incorporating the ε-amino group of protein lysyl residues. We postulated that γ-hydroxyalkenals could be generated, e.g., in the eye, through fragmentation of hydroperoxy endoperoxides produced in the retina through reactions of singlet molecular oxygen with polyunsaturated phospholipids. Since phospholipid esters are far more abundant in the retina than free fatty acids, we examined the influence of a membrane environment on the fate of hydroperoxy endoperoxides. We now report that linoleate hydroperoxy endoperoxides in thin films and their phospholipid esters in biomimetic membranes fragment to γ-hydroxyalkenals, and fragmentation is stoichiometrically induced by vitamin E. The product distribution from fragmentation of the free acid in the homogeneous environment of a thin film is remarkably different from that from the corresponding phospholipid in a membrane. In the membrane, further oxidation of the initially formed γ-hydroxyalkenal to a butenolide is disfavored. A conformational preference for the γ-hydroxyalkenal, to protrude from the membrane into the aqueous phase, may protect it from oxidation induced by lipid hydroperoxides that remain buried in the lipophilic membrane core.

Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease

Metabolomics studies hold promise for discovery of pathways linked to disease processes. Cardiovascular disease (CVD) represents the leading cause of death and morbidity worldwide. A metabolomics approach was used to generate unbiased small molecule metabolic profiles in plasma that predict risk for CVD. Three metabolites of the dietary lipid phosphatidylcholine, namely choline, trimethylamine N-oxide (TMAO), and betaine, were identified and then shown to predict risk for CVD in an independent large clinical cohort. Dietary supplementation of mice with choline, TMAO or betaine promoted up-regulation of multiple macrophage scavenger receptors linked to atherosclerosis, and supplementation with choline or TMAO promoted atherosclerosis. Studies using germ-free mice confirmed a critical role for dietary choline and gut flora in TMAO production, augmented macrophage cholesterol accumulation and foam cell formation. Suppression of intestinal microflora in atherosclerosis-prone mice inhibited dietary choline-enhanced atherosclerosis. Genetic variations controlling expression of flavin monooxygenases (FMOs), an enzymatic source of TMAO, segregated with atherosclerosis in hyperlipidemic mice. Discovery of a relationship between gut flora-dependent metabolism of dietary phosphatidylcholine and CVD pathogenesis provides opportunities for development of both novel diagnostic tests and therapeutic approaches for atherosclerotic heart disease.

Cholesterol attenuates and prevents bilayer damage and breakdown in lipoperoxidized model membranes. A spin labeling EPR study

The stabilizing effect of cholesterol on oxidized membranes has been studied in planar phospholipid bilayers and multilamellar 1-palmitoyl-2-linoleoyl-phosphatidylcholine vesicles also containing either 1-palmitoyl-2-glutaroyl-phosphatidylcholine or 1-palmitoyl-2-(13-hydroxy-9,11-octadecanedienoyl)-phosphatidylcholine oxidized phosphatidylcholine in variable ratio. Lipid peroxidation-dependent membrane alterations in the absence and in the presence of cholesterol were analyzed using Electron Paramagnetic Resonance spectroscopy of the model membranes spin labelled with either cholestane spin label (3-DC) or phosphatidylcholine spin label (5-DSPC). Cholesterol, added to lipid mixtures up to 40% final molar ratio, decreased the inner bilayer disorder as compared to cholesterol-free membranes and strongly reduced bilayer alterations brought about by the two oxidized phosphatidylcholine species. Furthermore, Sepharose 4B gel-chromatography and cryo electron microscopy of aqueous suspensions of the lipid mixtures clearly showed that cholesterol is able to counteract the micelle forming tendency of pure 1-palmitoyl-2-glutaroyl-phosphatidylcholine and to sustain multilamellar vesicles formation. It is concluded that membrane cholesterol may exert a beneficial and protective role against bilayer damage caused by oxidized phospholipids formation following reactive oxygen species attack to biomembranes.

Relationship between membrane permeability and specificity of human secretory phospholipase A(2) isoforms during cell death

During apoptosis, a number of physical changes occur in the cell membrane including a gradual increase in permeability to vital stains such as propidium iodide. This study explored the possibility that one consequence of membrane changes concurrent with early modest permeability is vulnerability to degradation by secretory phospholipase A(2). The activity of this hydrolytic enzyme toward mammalian cells depends on the health of the cell; healthy cells are resistant, but they become susceptible early during programmed death. Populations of S49 lymphoma cells during programmed death were classified by flow cytometry based on permeability to propidium iodide and susceptibility to secretory phospholipase A(2). The apoptotic inducers thapsigargin and dexamethasone caused modest permeability to propidium iodide and increased staining by merocyanine 540, a dye sensitive to membrane perturbations. Various secretory phospholipase A(2) isozymes (human groups IIa, V, X, and snake venom) preferentially hydrolyzed the membranes of cells that displayed enhanced permeability. In contrast, cells exposed briefly to a calcium ionophore showed the increase in cell staining intensity by merocyanine 540 without accompanying uptake of propidium iodide. Under that condition, only the snake venom and human group X enzymes hydrolyzed cells that were dying. These results suggested that cells showing modest permeability to propidium iodide during the early phase of apoptosis are substrates for secretory phospholipase A(2) and that specificity among isoforms of the enzyme depends on the degree to which the membrane has been perturbed during the death process. This susceptibility to hydrolysis may be important as part of the signal to attract macrophages toward apoptotic cells.

Oxidation-specific epitopes are danger-associated molecular patterns recognized by pattern recognition receptors of innate immunity

Oxidation reactions are vital parts of metabolism and signal transduction. However, they also produce reactive oxygen species, which damage lipids, proteins and DNA, generating "oxidation-specific" epitopes. In this review, we discuss the hypothesis that such common oxidation-specific epitopes are a major target of innate immunity, recognized by a variety of "pattern recognition receptors" (PRRs). By analogy with microbial "pathogen-associated molecular patterns" (PAMPs), we postulate that host-derived, oxidation-specific epitopes can be considered to represent "danger (or damage)-associated molecular patterns" (DAMPs). We also argue that oxidation-specific epitopes present on apoptotic cells and their cellular debris provided the primary evolutionary pressure for the selection of such PRRs. Furthermore, because many PAMPs on microbes share molecular identity and/or mimicry with oxidation-specific epitopes, such PAMPs provide a strong secondary selecting pressure for the same set of oxidation-specific PRRs as well. Because lipid peroxidation is ubiquitous and a major component of the inflammatory state associated with atherosclerosis, the understanding that oxidation-specific epitopes are DAMPs, and thus the target of multiple arcs of innate immunity, provides novel insights into the pathogenesis of atherosclerosis. As examples, we show that both cellular and soluble PRRs, such as CD36, toll-like receptor-4, natural antibodies, and C-reactive protein recognize common oxidation-specific DAMPs, such as oxidized phospholipids and oxidized cholesteryl esters, and mediate a variety of immune responses, from expression of proinflammatory genes to excessive intracellular lipoprotein accumulation to atheroprotective humoral immunity. These insights may lead to improved understanding of inflammation and atherogenesis and suggest new approaches to diagnosis and therapy.

Detection of macrophages via paramagnetic vesicles incorporating oxidatively tailored cholesterol ester: an approach for atherosclerosis imaging

AIM

Macrophages play a key role in the initiation, progression and complications of atherosclerosis. In this article we describe the synthesis of biocompatible, paramagnetic, fluorescent phosphatidylserine vesicles containing cholesterol ester with a free carboxylic acid function and its use for targeted imaging of macrophages.

METHODS & RESULTS

We synthesized anionic vesicles containing a combination of phosphatidylserine and a novel synthetic oxidized cholesterol ester derivative (cholesterol-9-carboxynonanoate [9-CCN]). In vitro studies to characterize particle size, MRI relaxation times and stability were performed. Vesicles containing 9-CCN demonstrated enhanced ability to bind human low-density lipoprotein and to be internalized by macrophages. Experiments in cultured macrophages with 9-CCN vesicles, alone and in the presence of low-density lipoprotein, indicated uptake of vesicles through scavenger receptor and integrin-dependent pathways. In vivo MRI using 9-CCN vesicles containing gadolinium in a rabbit model of atherosclerosis revealed protracted enhancement of 9-CCN vesicles and colocalization with arterial macrophages not seen with control vesicles. Pharmacokinetic experiments demonstrated prolonged plasma residence time of 9-CCN vesicles, perhaps due to its capacity to bind to low-density lipoprotein.

CONCLUSION

Vesicles containing 9-CCN demonstrate prolonged plasma and plaque retention in experimental atherosclerosis. Such a strategy may represent a simple yet clinically relevant approach for macrophage imaging.

Phosphatidylethanolamines modified by γ-ketoaldehyde (γKA) induce endoplasmic reticulum stress and endothelial activation

Peroxidation of plasma lipoproteins has been implicated in the endothelial cell activation and monocyte adhesion that initiate atherosclerosis, but the exact mechanisms underlying this activation remain unclear. Lipid peroxidation generates lipid aldehydes, including the γ-ketoaldehydes (γKA), also termed isoketals or isolevuglandins, that readily modify the amine headgroup of phosphatidylethanolamine (PE). We hypothesized that aldehyde modification of PE could mediate some of the proinflammatory effects of lipid peroxidation. We found that PE modified by γKA (γKA-PE) induced THP-1 monocyte adhesion to human umbilical cord endothelial cells. γKA-PE also induced expression of adhesion molecules and increased MCP-1 and IL-8 mRNA in human umbilical cord endothelial cells. To determine the structural requirements for γKA-PE activity, we tested several related compounds. PE modified by 4-oxo-pentanal induced THP-1 adhesion, but N-glutaroyl-PE and C(18:0)N-acyl-PE did not, suggesting that an N-pyrrole moiety was essential for cellular activity. As the N-pyrrole headgroup might distort the membrane, we tested the effect of the pyrrole-PEs on membrane parameters. γKA-PE and 4-oxo-pentanal significantly reduced the temperature for the liquid crystalline to hexagonal phase transition in artificial bilayers, suggesting that these pyrrole-PE markedly altered membrane curvature. Additionally, fluorescently labeled γKA-PE rapidly internalized to the endoplasmic reticulum (ER); γKA-PE induced C/EBP homologous protein CHOP and BiP expression and p38 MAPK activity, and inhibitors of ER stress reduced γKA-PE-induced C/EBP homologous protein CHOP and BiP expression as well as EC activation, consistent with γKA-PE inducing ER stress responses that have been previously linked to inflammatory chemokine expression. Thus, γKA-PE is a potential mediator of the inflammation induced by lipid peroxidation.

Structure-activity analysis of diffusible lipid electrophiles associated with phospholipid peroxidation: 4-hydroxynonenal and 4-oxononenal analogues

Electrophile-mediated disruption of cell signal-ing is involved in the pathogenesis of several diseases including atherosclerosis and cancer. Diffusible and membrane bound lipid electrophiles are known to modify DNA and protein substrates and modulate cellular pathways including ER stress, antioxidant response, DNA damage, heat shock, and apoptosis. Herein we report on a structure−activity relationship for several electrophilic analogues of 4-hydroxynonenal (HNE) and 4-oxononenal (ONE) with regard to toxicity and anti-inflammatory activity. The analogues studied were the oxidation products of HNE and ONE, HNEA/ONEA, the in vivo hydrolysis products of oxidized phosphatidylcholine, COOH-HNE/COOH-ONE, and their methyl esters, COOMe-HNE/ONE. The reactivity of each compound toward N-acetylcysteine was determined and compared to the toxicity toward a human colorectal carcinoma cell line (RKO) and a human monocytic leukemia cell line (THP-1). Further analysis was performed in differentiated THP-1 macrophages to assess changes in macrophage activation and pro-inflammatory signaling in response to each lipid electrophile. HNE/ONE analogues inhibited THP-1 macrophage production of the pro-inflammatory cytokines, IL-6, IL-1β, and TNFα, after lipopolysaccharide (LPS)/IFNγ activation. Inhibition of cytokine production was observed at submicromolar concentrations of several analogues with as little as 30 min of exposure. Phagocytosis of fluorescent beads was also inhibited by lipid electrophile treatment. Lipid electrophiles related to HNE/ONE are both toxic and anti-inflammatory, but the anti-inflammatory effects in human macrophages are observed at nontoxic concentrations. Neither toxicity nor anti-inflammatory activity are strongly correlated to the reactivity of the model nucleophile, N-acetylcysteine.

A CD36-dependent pathway enhances macrophage and adipose tissue inflammation and impairs insulin signalling

Aims

Obesity and hyperlipidaemia are associated with insulin resistance (IR); however, the mechanisms responsible remain incompletely understood. Pro-atherogenic hyperlipidaemic states are characterized by inflammation, oxidant stress, and pathophysiologic oxidized lipids, including ligands for the scavenger receptor CD36. Here we tested the hypothesis that the absence of CD36 protects mice from IR associated with diet-induced obesity and hyperlipidaemia.

Methods and results

Adipose tissue from CD36^−/− mice demonstrated a less inflammatory phenotype and improved insulin signalling in vivo and at the level of the adipocyte and macrophage. The pathophysiologic ligand oxidized low-density lipoprotein (oxLDL) activated c-Jun N-terminal kinase (JNK) and disrupted insulin signalling in both adipocytes and macrophages in a CD36-dependent manner. Macrophages isolated from CD36^−/− mice after high-fat diet feeding elicited less JNK activation and inhibition of insulin signalling in adipocytes after co-culture compared with wild-type macrophages.

Conclusion

These data suggest that a CD36-dependent inflammatory paracrine loop between adipocytes and macrophages facilitates chronic inflammation and contributes to IR common in obesity and dyslipidaemia.

Cholesterol slows down the lateral mobility of an oxidized phospholipid in a supported lipid bilayer

We investigated the mobility and phase-partitioning of the fluorescent oxidized phospholipid analogue 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647-ethanolamine (PGPE-Alexa647) in supported lipid bilayers. Compared to the conventional phospholipid dihexadecanoylphosphoethanolamine (DHPE)-Bodipy we found consistently higher diffusion constants. The effect became dramatic when immobile obstacles were inserted into the bilayer, which essentially blocked the diffusion of DHPE-Bodipy but hardly influenced the movements of PGPE-Alexa647. In a supported lipid bilayer made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the differences in probe mobility leveled off with increasing cholesterol content. Using coarse-grained molecular dynamics simulations, we could ascribe this effect to increased interactions between the oxidized phospholipid and the membrane matrix, concomitant with a translation in the headgroup position of the oxidized phospholipid: at zero cholesterol content, its headgroup is shifted to the outside of the DOPC headgroup region, whereas increasing cholesterol concentrations pulls the headgroup into the bilayer plane.

Regulation of platelet function by class B scavenger receptors in hyperlipidemia

Platelets constitutively express class B scavenger receptors CD36 and SR-BI, 2 closely related pattern recognition receptors best known for their roles in lipoprotein and lipid metabolism. The biological role of scavenger receptors in platelets is poorly understood. However, in vitro and in vivo data suggest that class B scavenger receptors modulate platelet function and contribute significantly to thrombosis by sensing pathological or physiological ligands, inducing prothrombotic signaling, and increasing platelet reactivity. Platelet CD36 recognizes a novel family of endogenous oxidized choline phospholipids that accumulate in plasma of hyperlipidemic mice and in plasma of subjects with low high-density lipoprotein levels. This interaction leads to the activation of specific signaling pathways and promotes platelet activation and thrombosis. Platelet SR-BI, on the other hand, plays a critical role in the induction of platelet hyperreactivity and accelerated thrombosis under conditions associated with increased platelet cholesterol content. Intriguingly, oxidized high-density lipoprotein, an SR-BI ligand, can suppress platelet function. These recent findings demonstrate that platelet class B scavenger receptors play roles in thrombosis in dyslipidemia and may contribute to acute cardiovascular events in vivo in hypercholesterolemia.

Atherogenic lipids and lipoproteins trigger CD36-TLR2-dependent apoptosis in macrophages undergoing endoplasmic reticulum stress

Macrophage apoptosis in advanced atheromata, a key process in plaque necrosis, involves the combination of ER stress with other proapoptotic stimuli. We show here that oxidized phospholipids, oxidized LDL, saturated fatty acids (SFAs), and lipoprotein(a) trigger apoptosis in ER-stressed macrophages through a mechanism requiring both CD36 and Toll-like receptor 2 (TLR2). In vivo, macrophage apoptosis was induced in SFA-fed, ER-stressed wild-type but not Cd36⁻(/)⁻ or Tlr2⁻(/)⁻ mice. For atherosclerosis, we combined TLR2 deficiency with that of TLR4, which can also promote apoptosis in ER-stressed macrophages. Advanced lesions of fat-fed Ldlr⁻(/)⁻ mice transplanted with Tlr4⁻(/)⁻Tlr2⁻(/)⁻ bone marrow were markedly protected from macrophage apoptosis and plaque necrosis compared with WT →Ldlr⁻(/)⁻ lesions. These findings provide insight into how atherogenic lipoproteins trigger macrophage apoptosis in the setting of ER stress and how TLR activation might promote macrophage apoptosis and plaque necrosis in advanced atherosclerosis.

Structure and lipid interactions of an anti-inflammatory and anti-atherogenic 10-residue class G(*) apolipoprotein J peptide using solution NMR

The surprising observation that a 10-residue class G(⁎) peptide from apolipoprotein J, [113-122]apoJ, possesses anti-inflammatory and anti-atherogenic properties prompted us to delineate its structural characteristics in the presence of normal and oxidized lipid. Towards this, we have determined high-resolution structure of [113-122]apoJ in solution using nuclear magnetic resonance (NMR) spectroscopy and studied its interaction with lipids, including oxidized lipids, using a number of biophysical methods. Circular dichroism and NMR studies established that in the presence of dodecylphosphocholine (DPC) micelle, this peptide adopts amphipathic α-helical structure. The observed Nuclear Overhauser effects indicate that the amphipathic helical structure of the peptide is stabilized by the N-terminal acetyl and C-terminal amide blocking groups. We used isothermal titration calorimetry to measure binding enthalpy of the peptide with DPC micelle, an oxidized lipid, 1-(palmitoyl)-2-(5-keto-6-octene-dioyl) phosphatidylcholine (KOdiA-PC), and the mixture of these two lipids (5mol% KOdiA-PC in DPC micelle). We find that the peptide binding with DPC micelle is associated with an enthalpy change (-16.75±0.16 Kcal/mol) much larger than that resulting from the binding with KodiA-PC (-3.67±0.13 Kcal/mol). Incorporation of a small amount of KOdiA-PC (5mol%) in DPC micelle also results in the lowering of peptide binding enthalpy (-13.43±0.18 Kcal/mol). These results are consistent with overall negative charge and altered conformational properties of oxidized sn-2 chain of KOdiA-PC. Our results have unambiguously established the amphipathic α-helical structure of [113-122]apoJ peptide in the presence of DPC micelle as well as its ability to bind oxidized lipid. These in vitro results help explain the previously observed anti-inflammatory and anti-atherosclerotic properties of this peptide.

Size-selective uptake of colloidal low density lipoprotein aggregates by cultured white blood cells

This paper illustrates how principles of colloid science are useful in studying atherosclerosis. Accumulation of foam cells in the arterial intima is a key step in atherogenesis. The extent of foam cell formation is enhanced by low density lipoprotein (LDL) aggregates, and we have previously shown that the size of sphingomyelinase (Smase)-hydrolysis-induced aggregates depends directly on the concentration of ceramide generated in the LDL phospholipid monolayer, mediated by the hydrophobic effect. Here, we focus on the effect of LDL aggregate particle sizes on their subsequent uptake by macrophages. Our data show the first direct measurement of uptake as a function of aggregate size and the first direct comparison of uptake after Smase-catalyzed and vortex-mixing-mediated aggregation. Vortex-mixed aggregates with radii 20-77 nm showed maximal uptake approximately 118 microg sterol/mg protein at a 53 nm intermediate size, consistent with a mathematical model describing competition between aggregate surface area and volume. Smase-treated aggregates with radii 25-211 nm also showed maximal uptake at an intermediate size, approximately 58 microg sterol/mg protein for 132 nm particles, and fit a modified model that incorporated ceramide concentration expressed as aggregate size. This study shows that particle size is significant and composition may also be a factor in LDL uptake.

Oxidation changes physical properties of phospholipid bilayers: fluorescence spectroscopy and molecular simulations

Physical properties of oxidized phospholipid (OxPL) membranes consisting of binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 10 mol % of one of two OxPLs, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) or 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC), were investigated experimentally and computationally. Fluorescence solvent relaxation (SR) and fluorescence correlation spectroscopy z-scan (FCS z-scan) show increased headgroup hydration and mobility, and faster lateral diffusion in POPC membrane upon addition of OxPLs. The magnitudes of both effects are distinct for each of the two OxPLs. Molecular dynamics simulations corroborate the experimental findings, providing at the same time a detailed molecular interpretation in terms of changes in bilayer structure and phospholipid orientation.

Generation and biological activities of oxidized phospholipids

Glycerophospholipids represent a common class of lipids critically important for integrity of cellular membranes. Oxidation of esterified unsaturated fatty acids dramatically changes biological activities of phospholipids. Apart from impairment of their structural function, oxidation makes oxidized phospholipids (OxPLs) markers of "modified-self" type that are recognized by soluble and cell-associated receptors of innate immunity, including scavenger receptors, natural (germ line-encoded) antibodies, and C-reactive protein, thus directing removal of senescent and apoptotic cells or oxidized lipoproteins. In addition, OxPLs acquire novel biological activities not characteristic of their unoxidized precursors, including the ability to regulate innate and adaptive immune responses. Effects of OxPLs described in vitro and in vivo suggest their potential relevance in different pathologies, including atherosclerosis, acute inflammation, lung injury, and many other conditions. This review summarizes current knowledge on the mechanisms of formation, structures, and biological activities of OxPLs. Furthermore, potential applications of OxPLs as disease biomarkers, as well as experimental therapies targeting OxPLs, are described, providing a broad overview of an emerging class of lipid mediators.

Markers of oxidative stress in erythrocytes and plasma during aging in humans

Aging is an inevitable universal biological process, which can be characterized by a general decline in physiological function with the accumulation of diverse adverse changes and increased probability of death. Among several theories, oxidative stress/free radical theory offers the best mechanistic elucidation of the aging process and other age -related phenomenon. In the present paper , we discuss the aging process and have focused on the importance of some reliable markers of oxidative stress which may be used as biomarkers of the aging process.

Structural basis for the recognition of oxidized phospholipids in oxidized low density lipoproteins by class B scavenger receptors CD36 and SR-BI

Specific oxidized phospholipids (oxPC(CD36)) accumulate in vivo at sites of oxidative stress and serve as high affinity ligands for scavenger receptors class B (CD36 and SR-BI). Recognition of oxPC(CD36) by scavenger receptors plays a role in several pathophysiological processes. The structural basis for the recognition of oxPC(CD36) by CD36 and SR-BI is poorly understood. A characteristic feature of oxPC(CD36) is an sn-2 acyl group that incorporates a terminal gamma-hydroxy (or oxo)-alpha,beta-unsaturated carbonyl. In the present study, a series of model oxidized phospholipids were designed, synthesized, and tested for their ability to serve as ligands for CD36 and SR-BI. We demonstrated that intact the sn-1 hydrophobic chain, the sn-3 hydrophilic phosphocholine or phosphatidic acid group, and the polar sn-2 tail are absolutely essential for high affinity binding. We further found that a terminal negatively charged carboxylate at the sn-2 position suffices to generate high binding affinity to class B scavenger receptors. In addition, factors such as polarity, rigidity, optimal chain length of sn-2, and sn-3 positions and negative charge at the sn-3 position of phospholipids further modulate the binding affinity. We conclude that all three positions of oxidized phospholipids are essential for the effective recognition by scavenger receptors class B. Furthermore, the structure of residues in these positions controls the affinity of the binding. The present studies suggest that, in addition to oxPC(CD36), other oxidized phospholipids observed in vivo may represent novel ligands for scavenger receptors class B.

Type 2 scavenger receptor CD36 in platelet activation: the role of hyperlipemia and oxidative stress

Platelet hyper-reactivity and a systemic prothrombotic state are associated with atherosclerosis and other inflammatory conditions. CD36, a member of the Type 2 scavenger receptor family, is a multiligand pattern recognition receptor that recognizes specific oxidized phospholipids, molecules expressed on microbial pathogens, apoptotic cells, and cell-derived microparticles. Recent studies have demonstrated that CD36 binding to oxidized LDL or microparticles activates a specific signaling pathway that induces platelet activation. This pathway is activated in vivo in the setting of hyperlipidemia and oxidant stress. Genetic deletion of CD36 protects mice from pathological thrombosis associated with hyperlipidemia without any apparent effect on normal hemostasis. Targeting CD36 or its signaling pathway could potentially lead to the development of novel antithrombotic therapies for patients with atheroinflammatory disorders.

Suppression of mitochondrial function by oxidatively truncated phospholipids is reversible, aided by bid, and suppressed by Bcl-XL

Oxidatively truncated phospholipids are present in atherosclerotic lesions, apoptotic cells, and oxidized low density lipoproteins. Some of these lipids rapidly enter cells to induce apoptosis by the intrinsic pathway, but how such lipids initiate this process is unknown. We show the truncated phospholipid hexadecyl azelaoyl glycerophosphocholine (Az-LPAF), derived from the fragmentation of abundant sn-2 linoleoyl residues, depolarized mitochondria of intact cells. Az-LPAF also depolarized isolated mitochondria and allowed NADH loss, but did not directly interfere with complex I function. Cyclosporin A blockade of the mitochondrial permeability transition pore partially prevented the loss of electrochemical potential. Depolarization of isolated mitochondria by the truncated phospholipid was readily reversed by the addition of albumin that sequestered this lipid. Ectopic expression of the anti-apoptotic protein Bcl-X(L) in HL-60 cells reduced apoptosis by the truncated phospholipid by protecting their mitochondria. Mitochondria isolated from these cells were also protected from Az-LPAF-induced depolarization. Conversely mitochondria isolated from Bid(-/-) animals that lack this pro-apoptotic Bcl-2 family member were resistant to Az-LPAF depolarization. Addition of recombinant full-length Bid, which has phospholipid transfer activity, restored this sensitivity. Thus, phospholipid oxidation products physically interact with mitochondria to continually depolarize this organelle without permanent harm, and Bcl-2 family members modulate this interaction with full-length Bid acting as a co-factor for pro-apoptotic, oxidatively truncated phospholipids.

Synthesis and structural characterization of carboxyethylpyrrole-modified proteins: mediators of age-related macular degeneration

Protein modifications in which the epsilon-amino group of lysyl residues is incorporated into a 2-(omega-carboxyethyl)pyrrole (CEP) are mediators of age-related macular degeneration (AMD). They promote both angiogenesis into the retina ('wet AMD') and geographic retinal atrophy ('dry AMD'). Blood levels of CEPs are biomarkers for clinical prognosis of the disease. To enable mechanistic studies of their role in promoting AMD, for example, through the activation of B- and T-cells, interaction with receptors, or binding with complement proteins, we developed an efficient synthesis of CEP derivatives, that is especially effective for proteins. The structures of tryptic peptides derived from CEP-modified proteins were also determined. A key finding is that 4,7-dioxoheptanoic acid 9-fluorenylmethyl ester reacts with primary amines to provide 9-fluorenylmethyl esters of CEP-modified proteins that can be deprotected in situ with 1,8-diazabicyclo[5.4.0]undec-7-ene without causing protein denaturation. The introduction of multiple CEP-modifications with a wide variety of CEP:protein ratios is readily achieved using this strategy.

Dietary cholesterol plays a role in CD36-mediated atherogenesis in LDLR-knockout mice

OBJECTIVE

CD36 has been shown to play a role in atherosclerosis in the apolipoprotein E-knockout (apoE(o)) mouse. We observed no difference in aortic lesion area between Western diet (WD)-fed LDLR(o) and LDLR(o)/CD36(o) mice. The objective was to understand the mechanism of CD36-dependent atherogenesis.

METHODS AND RESULTS

ApoE(o) mice transplanted with bone marrow from LDLR(o)/CD36(o) mice had significantly less aortic lesion compared with those transplanted with LDLR(o) marrow. Reciprocal macrophage transfer into hyperlipidemic apoE(o) and LDLR(o) animals showed that foam cell formation induced by in vivo modified lipoproteins was dependent on the lipoprotein, not macrophage type. LDLR(o) and LDLR(o)/CD36(o) mice were fed a cholesterol-enriched diet (HC), and we observed significant lesion inhibition in LDLR(o)/CD36(o) mice. LDL/plasma isolated from HC-fed LDLR(o) mice induced significantly greater jnk phosphorylation, cytokine release, and reactive oxygen species secretion than LDL/plasma from WD-fed LDLR(o) mice, and this was CD36-dependent. HC-fed LDLR(o) mice had higher circulating levels of cytokines than WD-fed mice.

CONCLUSIONS

These data support the hypothesis that CD36-dependent atherogenesis is contingent on a proinflammatory milieu that promotes the creation of specific CD36 ligands, not solely hypercholesterolemia, and may explain the greater degree/accelerated rate of atherosclerosis observed in syndromes associated with inflammatory risk.

Anti-inflammatory peptides grab on to the whiskers of atherogenic oxidized lipids

The peptide 4F is known to have potent anti-atherogenic activity. 4F is an 18 residue peptide that has a sequence capable of forming a class A amphipathic helix. Several other class A amphipathic helical, 18 residue peptides with the same polar face but with increasing Phe residues on the nonpolar face have been synthesized with varying degrees of biological activity. In this work we compared the properties of the original 2F peptide, modeled on the consensus sequence of the amphipathic helical segments of the apolipoprotein A-I with the peptide 4F that has two Leu residues replaced with Phe. We demonstrate that the more biologically active 4F peptide has the greatest affinity for binding to several molecular species of oxidized lipids. Lipoprotein particles can be formed by solubilizing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) with peptides. These solubilized lipoprotein particles extract oxidized lipid from liposomes of POPC containing 5 mol% of oxidized lipid. The peptides with the strongest anti-atherogenic activity interact most strongly with the oxidized lipid. The results show that there is a correlation between the biological potency of these peptides and their ability to interact with certain specific cytotoxic lipids, suggesting that this interaction may contribute favourably to their biological properties.

Phosphatidylethanolamine-esterified eicosanoids in the mouse: tissue localization and inflammation-dependent formation in Th-2 disease

In this study, murine peritoneal macrophages from naïve lavage were found to generate four phospholipids that contain 12-hydroxyeicosatetraenoic acid (12-HETE). They comprise three plasmalogen and one diacyl phosphatidylethanolamines (PEs) (16:0p, 18:1p, 18:0p, and 18:0a at sn-1) and are absent in macrophages from 12/15-lipoxygenase (12/15-LOX)-deficient mice. They are generated acutely in response to calcium mobilization, are primarily cell-associated, and are detected on the outside of the plasma membrane. Levels of 12-HETE-PEs in naïve lavage are in a similar range to those of free 12-HETE (5.5 ± 0.2 ng or 18.5 ± 1.03 ng/lavage for esterified versus free, respectively). In healthy mice, 12/15-LOX-derived 12-HETE-PEs are found in the peritoneal cavity, peritoneal membrane, lymph node, and intestine, with a similar distribution to 12/15-LOX-derived 12-HETE. In vivo generation of 12-HETE-PEs occurs in a Th2-dependent model of murine lung inflammation associated with interleukin-4/interleukin-13 expression. In contrast, in Toll receptor-dependent peritonitis mediated either by live bacteria or bacterial products, 12-HETE-PEs are rapidly cleared during the acute phase then reappear during resolution. The human homolog, 18:0a/15-HETE-PE inhibited human monocyte generation of cytokines in response to lipopolysaccharide. In summary, a new family of lipid mediators generated by murine macrophages during Th2 inflammation are identified and structurally characterized. The studies suggest a new paradigm for lipids generated by 12/15-LOX in inflammation involving formation of esterified eicosanoids.

Platelet activation by low concentrations of intact oxidized LDL particles involves the PAF receptor

OBJECTIVE

Mitochondrial depolarization aids platelet activation. Oxidized LDL (oxLDL) contains the medium length oxidatively truncated phospholipid hexadecyl azelaoyl-lysoPAF (HAz-LPAF) that disrupts mitochondrial function in nucleated cells, so oxLDL may augment platelet activation.

METHODS AND RESULTS

Flow cytometry showed intact oxLDL particles synergized with subthreshold amounts of soluble agonists to increase intracellular Ca2+, and initiate platelet aggregation and surface expression of activated gpIIb/IIIa and P-selectin. oxLDL also induced aggregation and increased intracellular Ca2+ in FURA2-labeled cells by itself at low, although not higher, concentrations. HAz-LPAF, alone and in combination with substimulatory amounts of thrombin, rapidly increased cytoplasmic Ca2+ and initiated aggregation. HAz-LPAF depolarized mitochondria in intact platelets, but this required concentrations beyond those that directly activated platelets. An unexpectedly large series of chemically pure truncated phospholipids generated by oxidative fragmentation of arachidonoyl-, docosahexaneoyl-, or linoleoyl alkyl phospholipids were platelet agonists. The PAF receptor, thought to effectively recognize only phospholipids with very short sn-2 residues, was essential for platelet activation because PAF receptor agonists blocked signaling by all these medium length phospholipids and oxLDL.

CONCLUSIONS

Intact oxLDL particles activate platelets through the PAF receptor, and the PAF receptor responds to a far wider range of oxidized phospholipids in oxLDL than anticipated.

The role of oxidized phospholipids in atherosclerosis

There is increasing evidence that oxidized phospholipids (OxPLs) play an important role in atherosclerosis. These phospholipids accumulate in human and mouse lesions. Specific OxPLs have been identified as major regulators of many cell types present in the vessel wall. In endothelial cells, >1,000 genes are regulated. Some of these genes are pro-atherogenic and others anti-atherogenic. The anti-atherogenic effects are likely important in slowing the atherogenic process. Several receptors and signaling pathways associated with OxPL action have been identified and shown to be upregulated in human lesions. A structural model of the mechanism by which specific OxPLs serve as CD36 ligands has been identified. Specific oxidized phospholipids are also present in plasma and associated with Lp(a) particles. In humans, OxPL/apolipoprotein B has been shown to be a prognostic indicator and a separate risk factor for coronary events. Levels of OxPL in plasma have been shown to be correlated with platelet activation. The results of these studies suggest an important role for OxPL in all stages of atherosclerosis.