Identification of short-chain oxidized phosphatidylcholine in human plasma

A game-theoretic approach to deciphering the dynamics of amyloid-β aggregation along competing pathways

Aggregation of amyloid-β (Aβ) peptides is a significant event that underpins Alzheimer's disease (AD). Aβ aggregates, especially the low-molecular weight oligomers, are the primary toxic agents in AD pathogenesis. Therefore, there is increasing interest in understanding their formation and behaviour. In this paper, we use our previously established results on heterotypic interactions between Aβ and fatty acids (FAs) to investigate off-pathway aggregation under the control of FA concentrations to develop a mathematical framework that captures the mechanism. Our framework to define and simulate the competing on- and off-pathways of Aβ aggregation is based on the principles of game theory. Together with detailed simulations and biophysical experiments, our models describe the dynamics involved in the mechanisms of Aβ aggregation in the presence of FAs to adopt multiple pathways. Specifically, our reduced-order computations indicate that the emergence of off- or on-pathway aggregates are tightly controlled by a narrow set of rate constants, and one could alter such parameters to populate a particular oligomeric species. These models agree with the detailed simulations and experimental data on using FA as a heterotypic partner to modulate the temporal parameters. Predicting spatio-temporal landscape along competing pathways for a given heterotypic partner such as lipids is a first step towards simulating scenarios in which the generation of specific 'conformer strains' of Aβ could be predicted. This approach could be significant in deciphering the mechanisms of amyloid aggregation and strain generation, which are ubiquitously observed in many neurodegenerative diseases.

Fatty Acid Concentration and Phase Transitions Modulate Aβ Aggregation Pathways

Aggregation of amyloid β (Aβ) peptides is a significant event that underpins Alzheimer disease (AD) pathology. Aβ aggregates, especially the low-molecular weight oligomers, are the primary toxic agents in AD and hence, there is increasing interest in understanding their formation and behavior. Aggregation is a nucleation-dependent process in which the pre-nucleation events are dominated by Aβ homotypic interactions. Dynamic flux and stochasticity during pre-nucleation renders the reactions susceptible to perturbations by other molecules. In this context, we investigate the heterotypic interactions between Aβ and fatty acids (FAs) by two independent tool-sets such as reduced order modelling (ROM) and ensemble kinetic simulation (EKS). We observe that FAs influence Aβ dynamics distinctively in three broadly-defined FA concentration regimes containing non-micellar, pseudo-micellar or micellar phases. While the non-micellar phase promotes on-pathway fibrils, pseudo-micellar and micellar phases promote predominantly off-pathway oligomers, albeit via subtly different mechanisms. Importantly off-pathway oligomers saturate within a limited molecular size, and likely with a different overall conformation than those formed along the on-pathway, suggesting the generation of distinct conformeric strains of Aβ, which may have profound phenotypic outcomes. Our results validate previous experimental observations and provide insights into potential influence of biological interfaces in modulating Aβ aggregation pathways.

Non-esterified fatty acids generate distinct low-molecular weight amyloid-β (Aβ42) oligomers along pathway different from fibril formation

Amyloid-β (Aβ) peptide aggregation is known to play a central role in the etiology of Alzheimer’s disease (AD). Among various aggregates, low-molecular weight soluble oligomers of Aβ are increasingly believed to be the primary neurotoxic agents responsible for memory impairment. Anionic interfaces are known to influence the Aβ aggregation process significantly. Here, we report the effects of interfaces formed by medium-chain (C9–C12), saturated non-esterified fatty acids (NEFAs) on Aβ42 aggregation. NEFAs uniquely affected Aβ42 aggregation rates that depended on both the ratio of Aβ:NEFA as well the critical micelle concentration (CMC) of the NEFAs. More importantly, irrespective of the kind of NEFA used, we observed that two distinct oligomers, 12–18 mers and 4–5 mers were formed via different pathway of aggregation under specific experimental conditions: (i) 12–18 mers were generated near the CMC in which NEFAs augment the rate of Aβ42 aggregation towards fibril formation, and, (ii) 4–5 mers were formed above the CMC, where NEFAs inhibit fibril formation. The data indicated that both 12–18 mers and 4–5 mers are formed along an alternate pathway called ‘off-pathway’ that did not result in fibril formation and yet have subtle structural and morphological differences that distinguish their bulk molecular behavior. These observations, (i) reflect the possible mechanism of Aβ aggregation in physiological lipid-rich environments, and (ii) reiterate the fact that all oligomeric forms of Aβ need not be obligatory intermediates of the fibril formation pathway.

Spin labeling EPR studies of the properties of oxidized phospholipid-containing lipid vesicles

This study aims at characterizing the structure and some properties of phospholipid multi-lamellar vesicles (MLVs) containing the oxidized species gamma-palmitoyl-beta-(9-hydroperoxy-10,12-octadecanedienoyl)-lecithin (HPPLPC), gamma-palmitoyl-beta-(9-hydroxy-10,12-octadecanedienoyl)-lecithin (HOPLPC), gamma-palmitoyl-beta-glutaroyl-lecithin (GlPPC) and gamma-palmitoyl-beta-azelaoyl-lecithin (AzPPC). Sepharose 4B gel-chromatography was used to ensure and check that only MLVs are used in EPR measurements. Gel-solid to gel-liquid transition temperature (Tm), lateral phase separation, fluidity gradient and polarity profile were studied by use of EPR spectroscopy of enclosed n-doxylstearoyl lecithin spin labels. Contrarily to conjugate dienes and normal phospholipids, pure carboxyacyl species yielded aqueous suspensions showing gel-chromatography elution profile resembling that of lysolecithin micelles. Conjugate dienes/DPPC MLVs showed lateral phase separation at room temperature and Tm value lower than pure DPPC MLVs. Pure conjugate dienes MLVs resembled more PLPC MLVs and displayed free miscibility with PLPC in mixed MLVs. Pure HPPLPC MLV bilayer appeared to be slightly more rigid, while that of HOPLPC and the polarity profile of MLVs made of the pure conjugate dienes species were similar to those of normal PLPC. It is concluded that carboxyacyl lecithins in MLVs tend to disrupt vesicle structure, while conjugated dienes lecithins are more able to affect some physical properties of the bilayer, and that DPPC in MLVs enhances these effects while PLPC shows a better compatibility with the lipoperoxides.

Fluorescence detection of a lipid-induced tetrameric intermediate in amyloid fibril formation by apolipoprotein C-II

The misfolding and self-assembly of proteins into amyloid fibrils that occurs in several debilitating and age-related diseases is affected by common components of amyloid deposits, notably lipids and lipid complexes. We have examined the effect of the short-chain phospholipids, dihexanoylphosphatidylcholine (DHPC) and dihexanoylphosphatidylserine (DHPS), on amyloid fibril formation by human apolipoprotein C-II (apoC-II). Micellar DHPC and DHPS strongly inhibited apoC-II fibril formation, whereas submicellar levels of these lipids accelerated apoC-II fibril formation to a similar degree. These results indicate that the net negative charge on DHPS, compared with the neutrally charged DHPC, is not critical for either the inhibition or activation process. We also investigated the mechanism for the submicellar, lipid-induced activation of fibril formation. Emission data for fluorescently labeled apoC-II indicated that DHPC and DHPS stimulate the early formation and accumulation of oligomeric species. Sedimentation velocity and equilibrium experiments using a new fluorescence detection system identified a discrete lipid-induced tetramer formed at low apoC-II concentrations in the absence of significant fibril formation. Seeding experiments showed that this tetramer was on the fibril-forming pathway. Fluorescence resonance energy transfer experiments established that this tetramer forms rapidly and is stabilized by submicellar, but not micellar, concentrations of DHPC and DHPS. Several recent studies show that oligomeric intermediates in amyloid fibril formation are toxic. Our results indicate that lipids promote on-pathway intermediates of apoC-II fibril assembly and that the accumulation of a discrete tetrameric intermediate depends on the molecular state of the lipid.

Different oxidized phospholipid molecules unequally affect bilayer packing

The aim of this study was to gain more detailed knowledge about the effect of the presence of defined oxidized phospholipid molecules in phospholipid bilayers. After chromatographic and mass spectrometry analysis, the previously used product of the Fenton reaction with unsaturated lecithins proved to consist of a plethora of oxidatively modified lecithins, useless either for the detailed study of the effects brought about in the bilayer or as the source of defined oxidized phospholipid molecules. The latter, particularly 2-(omega-carboxyacyl)- and 2-(n-hydroperoxyacyl)-lecithins, can be more conveniently prepared by chemical or enzymatic synthesis rather than by chemical or physical oxidation. The effect of those molecules and of commercially available 12-hydroxy-stearic and dodecanedioic acid was studied in planar supported phospholipid bilayers (SPBs) by use of EPR spectrometry. The SPBs also contained 2-(5-doxylstearoyl)-lecithin as the spin probe, and the EPR spectral anisotropy loss, indicative of bilayer disordering, was measured as a function of the molar percentage of oxidized lipid. Most oxidized lipid molecules examined in this study were able to induce bilayer disordering, while hydroperoxyl group-bearing acyl chains appeared to be much less effective. It is concluded that the effects of different oxidized phospholipids on phospholipid bilayer structure cannot be generalized, as happens with batch-oxidized phospholipids, and that the use of defined oxidized phospholipid molecular species for membrane oxidative stress guarantees a more reliable and detailed response.

Separation and determination of in vitro oxidized phospholipids by capillary zone electrophoresis

A simple capillary zone electrophoresis (CZE) method was used to determine in vitro oxidized phosphatidyl choline (ox-PC). To optimize the capillary electrophoresis (CE) conditions, organic buffer additives, buffer ionic strength, buffer pH, and applied voltage were examined. The optimal CE separation buffer chosen was an aqueous-organic solvent system containing 10% sodium phosphate buffer (5 mM, pH 7.40), 80% methanol, and 10% acetonitrile. One major peak with a small shoulder was found for phosphatidyl choline (PC), whereas one major peak and a complex region containing several lower-mobility peaks were found for ox-PC. The lower-mobility species of ox-PC has high levels of conjugated dienes characterized by strong absorbance at 234 nm. The electropherograms of PC and ox-PC were significantly different and highly reproducible. The intensities of lower-mobility species decreased significantly when the antioxidant vitamin C concentration was increased from 6 to 600 microM. This study provides a simple CZE method to differentiate in vitro oxidized from nonoxidized PC molecular species.

Separation of peroxidation products of diacyl-phosphatidylcholines by reversed-phase liquid chromatography-mass spectrometry

Lipid peroxidation process has attracted much attention due to the growing evidence of its involvement in the pathogenesis of age-related diseases. The monitoring of the lipid peroxidation products in phospholipids, formed under oxidative stress conditions, may provide new markers for oxidative stress signaling and for disease states, giving new insights in the pathogenesis process. Reversed-phase liquid chromatographic method coupled to mass spectrometry was developed for the separation of oxidized glycero-phosphatidylcholine (GPC) peroxidation products formed by the Fenton reaction that mimic in vivo oxidative stress conditions. The LC-MS conditions were applied for the separation of peroxidation products of oleoyl- (POPC), lineloyl- (PLPC) and arachidonoyl-palmitoyl phosphatidylcholine (PAPC). The peroxidation products separated included products resulting from the insertion of oxygen atoms in the sn-2 chain (long-chain), and products with the sn-2 chain shortened resulting from cleavage of oxygen-centered radicals (short-chain). Among long-chain products were the keto, hydroxy, hydroperoxide and poly-hydroxy derivatives, while short-chain products included dicarboxylic acids, aldehydes and hydroxy-aldehydes. Separation of long-chain products formed in each phosphatidylcholine was observed, and the reconstructed ion chromatogram of each ion showed an increase in the number of peaks with the increase in the number of oxygen atoms inserted into the phospholipid. Separation of short-chain products took place according to the functional group present at the sn-2 moiety that allowed the elution of dicarboxylic acids distinct from aldehydes. Separation between isomeric structures that were present in short- and long-chain products was also achieved.

Vitamin E differentially regulates the expression of peroxiredoxin-1 and -6 in alveolar type II cells

Vitamin E is the primary lipophilic antioxidant in mammals. Lack of vitamin E may lead to an increase of cytotoxic phospholipid-peroxidation products (PL-Ox). However, we could previously show that alimentary vitamin E-depletion in rats did not change the concentrations of dienes, hydroperoxides, and platelet-activating factor-related oxidation products in alveolar type II cells (TII cells). We hypothesized that vitamin E deficiency increases the activity of enzymes involved in the degradation of PL-Ox. Degradation of PL-Ox may be catalyzed by phospholipase A2, PAF-acetylhydrolase, or peroxiredoxins (Prx's). Alimentary vitamin E deficiency in rats increased the expression of Prx-1 at the mRNA and protein levels and the formation of Prx-SO3, but it did not change the expression of Prx-6 or the activity of phospholipase A2 and PAF-acetylhydrolase in TII cells. H2O2-induced oxidative stress in isolated TII cells activated protein kinase Calpha (PKCalpha) and increased the expression of Prx-1 and Prx-6. Inhibition of PKCalpha in isolated TII cells by long-time incubation with PMA inhibited PKCalpha and Prx-1 but not Prx-6. We concluded that the expression of Prx-1 and -6 is selectively regulated in TII cells; PKCalpha regulates the expression of Prx-1 but not Prx-6. Prx-6 expression may be closely linked to lipid peroxidation.

Identification and analysis of products formed from phospholipids in the free radical oxidation of human low density lipoproteins

Phospholipids reside in the surface layer of LDLs and constitute approximately 20-25% of the particle by weight. We report a study of the primary products generated from the most abundant molecular species of phosphatidylcholines present in LDL during in vitro free radical oxidations. The 13-hydroperoxides of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC) and 1-stearoyl-2-linoleoyl-sn-glycero-phosphocholine (SLPC) and the 15-hydroperoxides of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (PAPC) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphocholine (SAPC) were found to increase in a time-dependent manner and in significant amounts even in the presence of alpha-tocopherol. Phospholipid alcohols also formed during the course of the oxidations. Early in the LDL oxidations, while alpha-tocopherol was still present, the thermodynamically favored trans,trans products of PLPC and SLPC were found to form in significantly larger quantities than those formed from cholesteryl linoleate. Additionally, quantities of PAPC 11-hydroperoxide (11-OOH) decreased over time relative to PAPC 15-OOH, even while alpha-tocopherol was still present in the oxidation, presumably as a result of further oxidation of PAPC 11-OOH to form cyclic peroxide oxidation products. These results suggest that alpha-tocopherol is more closely associated with the inner cholesteryl ester-rich hydrophobic core of an LDL particle and is not as effective as an antioxidant in the outer phospholipid layer as it is in the lipid core.

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.

Atherogenic autoantigen: oxidized LDL complexes with beta2-glycoprotein I

Beta2-Glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies present in patients with antiphospholipid syndrome (APS). In 1997, we demonstrated that beta2-GPI specifically binds to Cu2+-oxidized low-density lipoprotein (oxLDL) and that the beta2-GPI-oxLDL complex is subsequently targeted by anti-beta2-GPI antibodies in vitro. Then ligands for beta2-GPI were purified from oxLDL and characterized as omega-carboxylated 7-ketocholesteryl esters, such as 7-ketocholesteryl-9-carboxynonanoate (oxLig-1) and 7-ketocholesteryl-12-carboxy (keto) dodecanoate (oxLig-2). These ligands mediate to form oxLDL-beta2-GPI complexes, and the complexes are taken up avidly by macrophages via anti-beta2-GPI autoantibody-mediated phagocytosis. We recently demonstrated that appearance of autoantibodies against a complex of beta2-GPI and oxLig-1 are highly associated with a history of arterial thrombosis. Serum oxLDL-beta2-GPI complex and their IgG immune complexes are also risk factors arterial thrombosis in APS patients. There is increasing circumstantial evidence of autoimmune mechanism involving beta2-GPI and oxLDL in the atherogenesis in APS.

Pulmonary surfactant protein-A (SP-A) restores the surface properties of surfactant after oxidation by a mechanism that requires the Cys6 interchain disulfide bond and the phospholipid binding domain

Reactive oxygen species produced by activated leuko-cytes in the alveolar epithelial lining fluid have been implicated in the inactivation of pulmonary surfactant and the impairment of lung function. Oxidation of bovine lipid extract surfactant (BLES), a therapeutic surfactant, with hypochlorous acid (H-BLES) or the Fenton reaction (F-BLES) led to temporary increases in conjugated dienes and formation of malondialdehyde and 4-hydroxy-2-nonenal. Electrospray ionization mass spectrometry revealed the appearance of lipid hydroperoxides, peroxides, lysophospholipids, and free fatty acids. Captive bubble tensiometer studies of H-BLES demonstrated prolonged adsorption times, film instability at low surface tensions during film compression, and reduced respreadability during film expansion. F-BLES exhibited prolonged adsorption times, a marked effect on increasing compressibility during compression, and a lesser effect on reducing respreadability on expansion. Addition of native bovine or rat surfactant-associated protein A (SP-A) reversed the effects of oxidation on surfactant biophysical properties. Studies using mutant recombinant rat SP-As indicated that an intact carbohydrate recognition domain and disulfide-dependent oligomeric assembly are critical for these effects, but the collagen-like region is not required. We conclude that SP-A can reverse the detrimental effects of surfactant oxidation on the biophysical properties of surfactant, by a mechanism that is dependent on interchain disulfide bond formation and the C-terminal domains of the protein.

Circulating oxidized LDL forms complexes with beta2-glycoprotein I: implication as an atherogenic autoantigen

Beta2-glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies (Abs, aPL) present in patients with antiphospholipid syndrome (APS). We recently reported (J. Lipid Res., 42: 697, 2001; J. Lipid Res., 43: 1486, 2002) that beta2-GPI specifically binds to Cu2+-oxidized LDL (oxLDL) and that the beta2-GPI ligands are omega-carboxylated 7-ketocholesteryl esters. In the present study, we demonstrate that oxLDL forms stable and nondissociable complexes with beta2-GPI in serum, and that high serum levels of the complexes are associated with arterial thrombosis in APS. A conjugated ketone function at the 7-position of cholesterol as well as the omega-carboxyl function of the beta2-GPI ligands was necessary for beta2-GPI binding. The ligand-mediated noncovalent interaction of beta2-GPI and oxLDL undergoes a temperature- and time-dependent conversion to much more stable but readily dissociable complexes in vitro at neutral pH. In contrast, stable and nondissociable beta2-GPI-oxLDL complexes were frequently detected in sera from patients with APS and/or systemic lupus erythematodes. Both the presence of beta2-GPI-oxLDL complexes and IgG Abs recognizing these complexes were strongly associated with arterial thrombosis. Further, these same Abs correlated with IgG immune complexes containing beta2-GPI or LDL. Thus, the beta2-GPI-oxLDL complexes acting as an autoantigen are closely associated with autoimmune-mediated atherogenesis.

Structural transitions in short-chain lipid assemblies studied by (31)P-NMR spectroscopy

The self-assembled supramolecular structures of diacylphosphatidylcholine (diC(n)PC), diacylphosphatidylethanolamine (diC(n)PE), diacylphosphatidyglycerol (diC(n)PG), and diacylphosphatidylserine (diC(n)PS) were investigated by (31)P nuclear magnetic resonance (NMR) spectroscopy as a function of the hydrophobic acyl chain length. Short-chain homologs of these lipids formed micelles, and longer-chain homologs formed bilayers. The shortest acyl chain lengths that supported bilayer structures depended on the headgroup of the lipids. They increased in the order PE (C(6)) < PC (C(9)) < or = PS (C(9) or C(10)) < PG (C(11) or C(12)). This order correlated with the effective headgroup area, which is a function of the physical size, charge, hydration, and hydrogen-bonding capacity of the four headgroups. Electrostatic screening of the headgroup charge with NaCl reduced the effective headgroup area of PS and PG and thereby decreased the micelle-to-bilayer transition of these lipid classes to shorter chain lengths. The experimentally determined supramolecular structures were compared to the assembly states predicted by packing constraints that were calculated from the hydrocarbon-chain volume and effective headgroup area of each lipid. The model accurately predicted the chain-length threshold for bilayer formation if the relative displacement of the acyl chains of the phospholipid were taken into account. The model also predicted cylindrical rather than spherical micelles for all four diacylphospholipid classes and the (31)P-NMR spectra provided evidence for a tubular network that appeared as an intermediate phase at the micelle-to-bilayer transition. The free energy of micellization per methylene group was independent of the structure of the supramolecular assembly, but was -0.95 kJ/mol (-0.23 kcal/mol) for the PGs compared to -2.5 kJ/mol (-0.60 kcal/mol) for the PCs. The integral membrane protein OmpA did not change the bilayer structure of thin (diC(10)PC) bilayers.

Bioactive oxidized lipids in the plasma of cardiac surgical intensive care patients

Critical illness is associated with increased oxidative stress that may give rise to the formation of lipid hydroperoxides (LOOH) and various secondary degradation products such as fragmented phosphatidylcholine (FPC) and lipids related to the platelet-activating factor (PAF). Because some oxidized phospholipids are potent proinflammatory agents, we measured the concentration of LOOH, FPC, and PAF-like activity in blood plasma of 36 patients who had undergone cardiac surgery and developed postoperative complications associated with systemic inflammatory response syndrome (SIRS) or multiple organ failure (MOF). These patients were compared to two control groups, namely preoperative patients scheduled for cardiac surgery (n = 13), and postoperative patients without complications (n = 19). Postoperative patents had higher concentrations of LOOH and lower concentrations of FPC than preoperative patients (P < 0.01). However, SIRS and MOF had no significant effect on the concentration of oxidatively modified lipids. This is despite the fact that MOF patients showed evidence of increased lipid peroxidation (7-fold higher ratio of alpha-tocoquinone/alpha-tocopherol compared to control). LOOH correlated positively with the white blood cell count. Postoperative patients had 4-fold higher plasma activities of phospholipase A2 and this activity was further increased in patients with SIRS (P < 0.04). Phospholipase A2 activity correlated negatively with the concentration of FPC. The data suggest that oxidatively modified lipids do not accumulate in patients with SIRS and MOF, perhaps because enhanced peroxidation of lipids is offset by enhanced lipolytic activity.

Cyclooxygenase-2 is induced in monocytes by peroxisome proliferator activated receptor gamma and oxidized alkyl phospholipids from oxidized low density lipoprotein

Low density lipoprotein (LDL) oxidation and monocyte infiltration of the vessel wall underlie atherogenesis. These cells express cyclooxygenase-2, but the way oxidized LDL stimulates cyclooxygenase-2 transcription is unknown. Oxidized LDL, oxidatively fragmented phospholipids isolated from oxidized LDL, a synthetic oxidized alkylphospholipid (azPC) that is a potent peroxisome proliferator activated receptor (PPAR) gamma agonist, or the PPARgamma agonist rosiglitazone all induced cyclooxygenase-2 expression and enhanced prostaglandin E(2) (PGE(2)) secretion in primary human monocytes. The cyclooxygenase-2 inhibitor NS398 blocked PPARgamma-induced PGE(2) secretion. Phospholipase A(1) and A(2) digestion shows that oxidized alkylphospholipids, and not oxidized fatty acids, were the relevant agonists. The upstream PPAR-responsive element (PPRE) of cyclooxygenase-2 was required for induction of a luciferase reporter by oxidized phospholipids, azPC, and rosiglitazone, and a (COX-2 PPRE)(3)-luciferase reporter was responsive to these PPARgamma agonists. Circulating human monocytes do not contain PPARgamma, but PPARgamma was induced rapidly (<4 h) in monocytes upon ligation of surface ICAM-3, but not P-selectin glycoprotein-1 even though both interactions prime cytokine secretion. Cyclooxygenase-2 induction by oxidized phospholipids only occurred in monocytes containing PPARgamma. Thus PPARgamma was induced rapidly in primary monocytes by appropriate outside-in signaling, sensitizing them to previously undetectable agonists in oxidized LDL. Cyclooxygenase-2 and PGE(2) secretion are induced, not inhibited, by selective PPARgamma agonists that include oxidatively fragmented phospholipids in oxidized LDL.

Detoxification of oxidized LDL by transferring its oxidation product(s) to lecithin:cholesterol acyltransferase

In the present study, we isolated modified LCAT (m-LCAT) by hydroxyapatite column chromatography after incubation of crude LCAT (after DEAE SephadexA-50 column chromatography, penultimate step of LCAT purification) with oxidized LDL (oxLDL) at 37 degrees C for 1 h. The activity was found to be about 30% lower than that of native LCAT (n-LCAT). When activity was determined in the presence of oxLDL, m-LCAT was less inhibited than n-LCAT by oxLDL. Treatments of purified LCAT either at 56 degrees C for 30 min, at 100 degrees C for 10 min, or with 6 mM 5-5' -dithiobis-2-nitrobenzoic acid or 9 mM diisopropyl fluorophosphates (each at 37 degrees C for 30 min) resulted in the loss of its cholesterol-esterifying activity. When examined for their ability to detoxify oxLDL, native LCAT and LCAT treated at 56 degrees C for 30 min were found to detoxify oxLDL. These results indicate that oxidation product(s) of LDL is transferred and bound to LCAT in a way that does not depend on its cholesterol-esterifying activity, but rather on the availability of the sulfhydryl group of cysteine residue and the hydroxyl group of serine residue.

Correlation of antiphospholipid antibody recognition with the structure of synthetic oxidized phospholipids. Importance of Schiff base formation and aldol condensation

The oxidation of low density lipoproteins (LDL) has been correlated with atherogenesis through a variety of pathways. The process involves nonspecific fragmentation, oxidative breakdown, and modification of the lipids and protein of LDL. The process yields a variety of bioactive products, including aldehyde-containing phospholipids, which can cross-react with primary amines (i.e. peptides or phospholipid head groups) to yield Schiff base products. We also demonstrate that such oxidized phospholipid products may further react through a post-oxidation chemical pathway involving aldol condensation. EO6, an IgM monoclonal autoantibody to oxidized phospholipids, blocks the uptake of oxidized LDL (OxLDL) by macrophages. Because the epitope(s) of EO6 also blocks the uptake of OxLDL, a series of oxidized phospholipids, their peptide complexes, and their aldol condensates have been synthesized and characterized, and their antigenicity has been determined. This study defines structural motifs of oxidized phospholipids responsible for antigenicity for EO6. Certain monomeric phospholipids containing short chain fatty acids were antigenic whether oxidized or not in the sn-2 position. However, oxidized phospholipids containing sn-1 long chain fatty acids were not antigenic unless the sn-2 oxidized fatty acid contained an aldehyde that first reacted with a peptide yielding a Schiff base or the sn-2 oxidized fatty acid underwent an aldol type self-condensation. Our data indicate that the phosphorylcholine head group is essential for antigenicity, but its availability depends on the oxidized phospholipid conformation. We suggest that upon oxidation, similar reactions occur in phospholipids on the surface of LDL, generating ligands for macrophage recognition. Synthetic imine adducts of oxidized phospholipids of this type are capable of blocking the uptake of OxLDL.

Oxidized alkyl phospholipids are specific, high affinity peroxisome proliferator-activated receptor gamma ligands and agonists

Synthetic high affinity peroxisome proliferator-activated receptor (PPAR) agonists are known, but biologic ligands are of low affinity. Oxidized low density lipoprotein (oxLDL) is inflammatory and signals through PPARs. We showed, by phospholipase A(1) digestion, that PPARgamma agonists in oxLDL arise from the small pool of alkyl phosphatidylcholines in LDL. We identified an abundant oxidatively fragmented alkyl phospholipid in oxLDL, hexadecyl azelaoyl phosphatidylcholine (azPC), as a high affinity ligand and agonist for PPARgamma. [(3)H]azPC bound recombinant PPARgamma with an affinity (K(d)((app)) approximately 40 nm) that was equivalent to rosiglitazone (BRL49653), and competition with rosiglitazone showed that binding occurred in the ligand-binding pocket. azPC induced PPRE reporter gene expression, as did rosiglitazone, with a half-maximal effect at 100 nm. Overexpression of PPARalpha or PPARgamma revealed that azPC was a specific PPARgamma agonist. The scavenger receptor CD36 is encoded by a PPRE-responsive gene, and azPC enhanced expression of CD36 in primary human monocytes. We found that anti-CD36 inhibited azPC uptake, and it inhibited PPRE reporter induction. Results with a small molecule phospholipid flippase mimetic suggest azPC acts intracellularly and that cellular azPC accumulation was efficient. Thus, certain alkyl phospholipid oxidation products in oxLDL are specific, high affinity extracellular ligands and agonists for PPARgamma that induce PPAR-responsive genes.

Importance of the free sulfhydryl groups of lecithin-cholesterol acyltransferase for its sensitivity to oxidative inactivation

Lecithin-cholesterol acyltransferase (LCAT) of human plasma is known to be highly susceptible to oxidative inactivation, although the mechanism of this inactivation is unknown. We tested the hypothesis that the high sensitivity of the enzyme is due to the derivatization of its two free SH groups flanking the active site pocket. Modification of the SH groups with a reversible inhibitor protected the enzyme against oxidative inactivation. Mutagenesis of either of the cysteines to glycine increased the resistance of the enzyme, which retained 46% of activity in presence of 150 microM Cu(2+), compared to only 27% of the activity retained by the wild type enzyme (WT). Replacement of both the cysteines with glycines resulted in retention of over 65% activity. Cysteine replacement similarly protected the enzyme from inactivation by the oxidized substrate. Chicken LCAT, which has only one cysteine (Cys(26)), was more resistant than the human enzyme. Introduction of an additional cysteine corresponding to the second cysteine in human LCAT (N184C) resulted in increased susceptibility of chicken enzyme (87% loss of activity in presence of 150 microM Cu(2+), compared to 55% loss in WT). Substitution of the lone cysteine with glycine (C26G) resulted in a more resistant enzyme, which lost <40% activity under the same conditions. These results show that the primary targets of the oxidizing agents or the products of oxidation are the SH groups of the enzyme, whose derivatization leads to steric inhibition of the activity.

Update on solid-phase extraction for the analysis of lipid classes and related compounds

This article provides information on the different procedures and methodologies developed when solid-phase extraction (SPE) is used for lipid component separation. The analytical systematics, established by different authors and designed to separate groups of compounds and also specific components by using a combination of chromatographic supports and solvents are presented. The review has been divided into three parts, which we consider well defined: edible fats and oils, fatty foods and biological samples. Separations of non-polar and polar lipids is the most extensive systematic, although many other published methods have been established to isolate specific components or a reduced number of components from edible fats and oils, fatty foods or biological samples susceptible to further analysis by other quantitative techniques.

Bioactive phospholipid oxidation products

Oxidation of phospholipids results in chain-shortened fragments and oxygenated derivatives of polyunsaturated sn-2 fatty acyl residues, generating a myriad of phospholipid products. Certain oxidation products of phosphatidylcholine bind to and activate the human receptor for PAF, and these PAF-like lipids are potent, selective inflammatory mediators. Formation of PAF-like lipids is nonenzymatic and so their accumulation is unregulated. PAF-like lipids are produced in vivo in response to oxidative stresses and are responsible for attendant acute inflammatory responses. PAF-like lipids almost exclusively contain an ether-linked alkyl residue at the sn-1 position of the phosphatidylcholine backbone and molecular identification of these is facilitated by phospholipase A(1) treatment to remove the bulk of the inactive phospholipids. The identity of biologically active species generated by oxidative fragmentation and oxidation can be elucidated by understanding relevant reactions leading to the formation of PAF-like lipids, and then their structure can be established by tandem mass spectrometry and chemical synthesis.

Inflammatory platelet-activating factor-like phospholipids in oxidized low density lipoproteins are fragmented alkyl phosphatidylcholines

Oxidation of human low density lipoprotein (LDL) generates proinflammatory mediators and underlies early events in atherogenesis. We identified mediators in oxidized LDL that induced an inflammatory reaction in vivo, and activated polymorphonuclear leukocytes and cells ectopically expressing human platelet-activating factor (PAF) receptors. Oxidation of a synthetic phosphatidylcholine showed that an sn-1 ether bond confers an 800-fold increase in potency. This suggests that rare ether-linked phospholipids in LDL are the likely source of PAF-like activity in oxidized LDL. Accordingly, treatment of oxidized LDL with phospholipase A(1) greatly reduced phospholipid mass, but did not decrease its PAF-like activity. Tandem mass spectrometry identified traces of PAF, and more abundant levels of 1-O-hexadecyl-2-(butanoyl or butenoyl)-sn-glycero-3-phosphocholines (C(4)-PAF analogs) in oxidized LDL that comigrated with PAF-like activity. Synthesis showed that either C(4)-PAF was just 10-fold less potent than PAF as a PAF receptor ligand and agonist. Quantitation by gas chromatography-mass spectrometry of pentafluorobenzoyl derivatives shows the C(4)-PAF analogs were 100-fold more abundant in oxidized LDL than PAF. Oxidation of synthetic alkyl arachidonoyl phosphatidylcholine generated these C(4)-PAFs in abundance. These results show that quite minor constituents of the LDL phosphatidylcholine pool are the exclusive precursors for PAF-like bioactivity in oxidized LDL.

Stimulation of monocytes and platelets by short-chain phosphatidylcholines with and without terminal carboxyl group

Oxidation of unsaturated phosphatidylcholine (PC) produces fragmented phospholipids which have similar bioactivities as the platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-PC). Since a large number of molecular species are produced upon PC oxidation, the active ingredients have not been identified. We synthesized several short-chain PCs which are known to be characteristic PC oxidation products to test their PAF-like activity. The synthetic PCs contained palmitoyl or hexadecyl residues (both C16) in sn-1 position, and propionyl (C3), valeroyl (C5), succinyl (C4 with omega-carboxyl), glutaroyl (C5 with omega-carboxyl), or suberoyl (C8 with omega-carboxyl) residues in sn-2 position. Biological activity was measured by: (1) increase of intracellular calcium in human monocytes; (2) [3H]serotonin release from rabbit platelets; and (3) aggregation of human platelets. Specificity of the cellular response was tested by inhibition with the PAF-receptor antagonists BN 52021 and WEB 2086. Synthetic PC oxidation products activated both monocytes and platelets in a PAF-specific manner. The effective concentration varied with respect to assay system and chemical structure. In general, 1-hexadecyl-PCs were more effective than 1-palmitoyl-PCs, while increasing chain length in sn-2 position lowered biological activity. However, several 1-palmitoyl-PCs activated monocytes in concentrations between 10-8 and 10-6 M. In contrast, platelets were less susceptible to 1-palmitoyl-PCs. No significant difference was found between 2-valeroyl-PC (C5 with omega-methyl) and 2-glutaroyl-PC (C5 with omega-carboxyl). The data suggest that typical products of PC oxidation, containing propionyl, succinyl, or glutaroyl residues in sn-2 position, display PAF-like activity at micromolar concentrations.

Novel function of lecithin-cholesterol acyltransferase. Hydrolysis of oxidized polar phospholipids generated during lipoprotein oxidation

Although the major function of lecithin-cholesterol acyltransferase (LCAT) is cholesterol esterification, our previous studies showed that it can also hydrolyze platelet-activating factor (PAF). Because of the structural similarities between PAF and the truncated phosphatidylcholines (polar PCs) generated during lipoprotein oxidation, we investigated the possibility that LCAT may also hydrolyze polar PCs to lyso-PC during the oxidation of plasma. PAF acetylhydrolase (PAF-AH), which is known to hydrolyze polar PCs in human plasma, was completely inhibited by 0.2 mM p-aminoethyl benzenesulfonyl fluoride (Pefabloc), a new serine esterase inhibitor, which had no effect on LCAT at this concentration. On the other hand, 1 mM diisopropylfluorophosphate (DFP) completely inhibited LCAT but had no effect on PAF-AH. Polar PC accumulation during the oxidation of plasma increased by 44% in the presence of 0.2 mM Pefabloc and by 30% in the presence of 1 mM DFP. The formation of lyso-PC was concomitantly inhibited by both of the inhibitors. The combination of the two inhibitors resulted in the maximum accumulation of polar PCs, suggesting that both PAF-AH and LCAT are involved in their breakdown. Oxidation of chicken plasma, which has no PAF-AH activity, also resulted in the formation of lyso-PC from the hydrolysis of polar PC, which was inhibited by DFP. Polar PCs, either isolated from oxidized plasma or by oxidation of labeled synthetic PCs, were hydrolyzed by purified LCAT, which had no detectable PAF-AH activity. These results demonstrate a novel function for LCAT in the detoxification of polar PCs generated during lipoprotein oxidation, especially when the PAF-AH is absent or inactivated.