Inflammatory Platelet-activating Factor-like Phospholipids in Oxidized Low Density Lipoproteins Are Fragmented Alkyl Phosphatidylcholines

Abstract

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-Ohexadecyl-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 chromatographymass 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. Platelet-activating factor (PAF)1 is a phospholipid autacoid with a wide variety of actions, primarily on cells and events that comprise the inflammatory system. PAF initiates the rapid inflammatory response as it is the leukocyte activating molecule produced and displayed by stimulated endothelial cells (1). PAF does not induce the bactericidal effector functions of leukocytes, but rather stimulates their adhesive and migratory behavior that allows them to transit the endothelial barrier. Leukocytes (polymorphonuclear leukocytes or PMN), monocytes, and eosinophils, as well as platelets, express the PAF receptor and accordingly are activated by PAF in concentrations ranging from picomolar to nanomolar levels. The potency of PAF, its broad actions, and the potentially deleterious events it invokes rationalize the tight regulation of PAF synthesis (2).PAF is recognized by a single, specific receptor that is a member of the family of seven-transmembrane-spanning, Gprotein-linked receptors (3, 4). Alone among this large family of receptors and related orphan sequences, the PAF receptor recognizes an intact phospholipid, and does so with a marked specificity. The PAF receptor shows a several hundredfold selectivity for the sn-1 ether bond of PAF, and complete specificity for the sn-2 acetyl residue compared with the long chain fatty acyl residue of most alkyl phosphatidylcholines (5, 6). The choline headgroup confers a several thousandfold advantage over the related phosphatidylethanolamine analog (7). Thus, compared with Edg-2 and Edg-4 receptors for lysophosphatidic...

Emigration of monocyte-derived cells from atherosclerotic lesions characterizes regressive, but not progressive, plaques

“…One of the lipids studied was the stable PAF analog carbamyl PAF (Fig. 1a), included in these experiments to model PAF-like signals that are major products generated during oxidation of phosphatidylcholine in low density lipoprotein particles (14,15). These products of oxidation mimic PAF closely enough to bind to the PAF receptor (14).…”

Signaling pathways involved in OxPAPC-induced pulmonary endothelial barrier protection

“…Upon oxidative fragmentation of sn-2 unsaturated fatty acid residues, OxPL may become agonists for the platelet-activating factor (PAF) receptor (Marathe, Davies et al 1999). These "PAF-like lipids" are formed in vivo and induce many effects typical of PAF (Marathe, Prescott et al 2001).…”

“…These "PAF-like lipids" are formed in vivo and induce many effects typical of PAF (Marathe, Prescott et al 2001). In the case of fragmented acyl-oxidized phospholipids, however, many biological actions can not be explained solely by the ability to stimulate PAF receptors, because many effects of these acyl-oxidized phospholipids are not reproduced by PAF (Marathe, Davies et al 1999Subbanagounder, Leitinger et al 1999;Subbanagounder, Watson et al 2000). In this study we examined potential involvement of PAF receptor signaling in the barrier-protective effects of OxPAPC.…”

Oxidized glycerophosphocholines as biologically active mediators for ultraviolet radiation-mediated effects

“…Among the phospholipid reaction products are those that are ligands for the PAF-receptor. The most potent of the nonenzymatically generated PAF analogs are native PAF, as well as the butanoyl (C 4 -PAF) and butenoyl (C 4:1 -PAF) species that are one tenth as potent as PAF 16,17 . In addition, alkyl GPCs found in oxidized LDL with small chain sn-2 groups ending in an ω-oxy function, such as a 4-carbon ω-carboxylic acid (succinoyl-PAF) and 5 carbon ω-aldehydes (5-oxovaleroyl-PAF), have also been shown to serve as PAF-R ligands [17][18][19][20] .…”