Platelet Activation by Low Concentrations of Intact Oxidized LDL Particles Involves the PAF Receptor

Abstract

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 Ca 2ϩ , and initiate platelet aggregation and surface expression of activated gpIIb/IIIa and P-selectin. oxLDL also induced aggregation and increased intracellular Ca 2ϩ 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 Ca 2ϩ 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. Key Words: oxidized LDL Ⅲ PAF Ⅲ platelet Ⅲ phospholipid Ⅲ PAF acetylhydrolase P latelet mitochondria maintain cellular energetics and viability, 1 but these organelles also affect platelet activation. Thrombin produces a rapid decrease in mitochondrial transmembrane potential, thereby increasing reactive oxygen specie formation and caspase 3 activation needed for maximal aggregation. 2 Combining maximal amounts of thrombin with collagen 3 or with activators of Bax, 4 a proapoptotic protein that physically targets mitochondria, generates a subpopulation of highly activated platelets, and blockade of the mitochondrial permeability transition pore suppresses this form of platelet activation. 5,6 Oxidized low density lipoprotein (oxLDL) has a fundamental role in thrombotic disease and atherogenesis through activation of inflammatory cells that includes platelets, 7,8 although the identity of the agonist(s) is unclear because oxLDL contains numerous bioactive compounds. 9 Oxidation of lipoprotein particles, either chemically or enzymatically, 10,11 fragments phospholipids that contain polyunsaturated fatty acyl residues. One such oxidatively-truncated phospholipid, hexadecyl azelaoyl lysoPAF (HAz-LPAF), derived from oxidation of common linoleoyl residues, rapidly enters nucleated cells, traffics to mitochondria, and initiates the mitochondrial-dependent pathway to apoptotic cell death. 12 We determined whether oxidized LDL contains agents that disrupt platelet mitochondrial function, thereby increasing cellular responsiveness. We prepared pure samples of a series of fragmented alkyl glycerophosphocholines-acylated lysoPAFs-found in oxidized LD...

Platelet-activating Factor Receptor Agonists Mediate Xeroderma Pigmentosum A Photosensitivity

“…Of importance, recent studies have demonstrated that the non-PAF Ox-GPCs we measured and other Ox-GPCs chemically synthesized have significant PAF-R agonistic activity in vitro (48). It should be noted that the low amount of UVB irradiation used in the in vitro studies (600 J/m 2 ) that generates ϳ2-fold increased levels of these compounds does not generate Ox-GPCs in other cell types such as epithelial cells (28,40), which fits with the concept that the increased responsiveness of XPA-deficient cells to UVB irradiation is mediated at least in part by the resulting Ox-GPCs.…”

“…The MRM transition pairs were monitored for species previously identified as PAF receptor agonists (28,47,48) (5)oxovaleroyl-GPC (16a 5al), and 1-palmitoyl-2-glutaroyl-GPC (16a 5COOH).…”

Platelets Mediate Oxidized Low-Density Lipoprotein–Induced Monocyte Extravasation and Foam Cell Formation

“…21,22 Because monocytes constitutively express P-selectin glycoprotein ligand-1 (PSGL-1), OxLDL-stimulated platelets also interacted with monocytes within minutes of activation ( Figure 1A). As depicted in Figure 1B, the percentage of platelet-monocyte complexes significantly increased with the degree of oxidative modification of LDL, whereas native LDL had no effect on PMA formation.…”

To hydrolyze or not to hydrolyze: the dilemma of platelet-activating factor acetylhydrolase

“…Such modifi ed phospholipids have been identifi ed in oxidized LDLs (11)(12)(13); in models of oxidative insult such as alcoholic blood ( 14 ), smokers blood ( 15 ), and electronegative LDLs ( 16 ); and in models of cutaneous infl ammation ( 17 ). Notable among these lipids are the oxidatively truncated phospholipids butanoyl/butenoyl PAF/PC ( 11 ), azelaoyl PAF/PC ( 18,19 ), palmitoyl glutaroyl PC, palmitoyl oxovaleryl PC ( 12 ), kodia PC ( 13 ), and many more ( 19 ) ( Table 1 ). Although PAF is recognized by PAF receptor (PAF-R) at subnanomolar concentrations ( 11,20 ), some of the above mentioned molecules also bind to PAF-R with barrier through juxtacrine signaling ( 34,35 ).…”

“…varying affi nities to activate PAF-R, hence popularly referred as PAF mimetics/PAF-like lipids ( 11,(19)(20)(21). However, these oxidized phospholipids accumulate during oxidative insult, as they are formed nonenzymatically, leading to exaggerated infl ammatory responses that are also curtailed by the action of PAF-AH ( 6,7,11 ).…”