Platelet Agonists Enhance the Import of Phosphatidylethanolamine into Human Platelets

Abstract

It is unknown whether the endocytosis-independent transfer of phospholipids from lipoproteins to platelets is regulated by platelet agonists such as thrombin. The movements of the choline phospholipids phosphatidylcholine and sphingomyelin (labeled with either 14 C or the fluorescent pyrenedecanoic acid) between low density lipoproteins and platelets were unaffected by thrombin (0.5 unit/ml). In contrast, thrombin accelerated the import of diacyl phosphatidylethanolamine (PE) and alkenylacyl phosphatidylethanolamine into platelets by about 4-fold. Similarly, thrombin receptoractivating peptide (15 M), collagen (10 g/ml), and ADP (10 M) enhanced PE uptake. High density lipoprotein particles and egg phosphatidylcholine vesicles were also donors for stimulation of platelet PE import. Part of the [ 14 C]arachidonic acid-labeled PE transferred from low density lipoprotein to platelets activated by thrombin and collagen was metabolized to 14 C-eicosanoids. Inhibitors of protein kinase C partially prevented thrombin-induced [14 C]PE uptake, while direct activators of protein kinase C increased incorporation of [ 14 C]PE into platelets. Proteinaceous factor(s) recovered in the extracellular medium from ADP-and thrombin-activated platelet suspensions were found to accelerate the transfer of pyrenedecanoic acid-labeled PE between donor and acceptor lipid vesicles. The stimulation of import of ethanolamine phospholipids led to a 2-fold enhancement of the prothrombinase activity of thrombin-activated platelets. Our study demonstrates that physiological platelet stimuli increase specifically the transfer of ethanolamine phospholipids from lipoproteins to platelets through a secretion-dependent mechanism. This might contribute to the increase of procoagulant activity of stimulated platelets.

Transfer of very low density lipoprotein-associated phospholipids to activated human platelets

“…Both LPL and thrombin stimulated the import by platelets of VLDL-derived PAPC, PAPE, and DPPC with similar efficiencies. These results are in marked contrast with those of a previously published work (11). Those authors compared the trans- fers of various PLs from LDL to platelets.…”

“…In addition, the uptake of PC and PE species by platelets appeared to be regulated by different mechanisms. The transfer of LDL-or HDL-derived PE into platelets, but not that of PC or sphingomyelin, was stimulated by platelet activators, including thrombin, collagen, and ADP, and was dependent on the secretion of an unidentified cellular protein factor (11). Although LDL and HDL transport the major part of lipoprotein PL, they do not necessarily represent the sole source of PL for platelets.…”

Developmental and Metabolic Effects of Disruption of the Mouse CTP:Phosphoethanolamine Cytidylyltransferase Gene (Pcyt2)

“…Phosphatidylethanolamine (PE) is a dominant inner-leaflet phospholipid in cell membranes (9), where it plays a role in membrane function by structurally stabilizing membrane-anchored proteins (10,13,26), and participates in important cellular processes such as cell division (10), cell fusion (24,25), blood coagulation (7,11,12), and apoptosis (11). In Drosophila, where PE is the major membrane phospholipid, the release of sterol regulatory element-binding proteins is controlled by PE, as opposed to sterols in mammalian cells (8).…”

The transfer of VLDL-associated phospholipids to activated platelets depends upon cytosolic phospholipase A2 activity

“…However, major differences emerge when comparing the PL transfers obtained from the different lipoprotein fractions. The transfer of LDL-or HDL-derived PE into platelets, but not that of PC or sphingomyelin, was stimulated by platelet activators, including thrombin, collagen, and ADP, and was dependent upon the secretion of an unidentified cellular protein factor (14). In contrast, there was no apparent specificity of the PL species transferred from VLDL to platelets (9).…”