Platelet agonists enhance the import of phosphatidylethanolamine into human platelets

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 C-14 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 receptor-activating peptide (15 mu M), collagen (10 mu g/ml), and ADP (10 mu M) enhanced PE uptake. High density lipoprotein particles and egg phosphatidylcholine vesicles were also donors for stimulation of platelet PE import. Part of the [C-14]arachidonic acid-labeled PE transferred from low density lipoprotein to platelets activated by thrombin and collagen was metabolized to C-14-eicosanoids, Inhibitors of protein kinase C partially prevented thrombin-induced [C-14]PE uptake, while direct activators of protein kinase C increased incorporation of [C-14]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.