Sphingosylphosphorylcholine, a naturally occurring lipid mediator, inhibits human platelet function

The lysophospholipids, lysophosphatidic acid and sphingosine 1-phosphate, have been reported to activate platelets. Here we examined effects of the naturally occurring related sphingosylphosphorylcholine (SPC) on human platelet function. Platelet activation was determined as aggregation, elevation of intracellular Ca2+ concentrations, surface expression of P-selectin, GP 53, and GP IIb/IIIa neoepitope PAC-1, and of fibrinogen binding to the platelet surface. Platelets were activated by ADP (5 and 20 μM), the thrombin receptor-activating peptide TRAP-6 (5 and 20 μM), the thromboxane A2 mimetic U-46619 (1 μM) and collagen (20 and 50 μg ml−1) but not by SPC (up to 20 μM). SPC concentration-dependently (IC50 approximately 1–10 μM) inhibited activation of washed human platelets in response to all of the above agonists, with almost complete inhibition occurring at 20 μM SPC. The SPC stereoisomers, D-erythro SPC and L-threo SPC, exhibited similar concentration–response curves in inhibiting 20 μM ADP-induced platelet aggregation, suggesting that SPC did not act via specific lysophospholipid receptors. Although SPC slightly activated platelet protein kinase A (as assessed by VASP phosphorylation), this effect could not explain the marked platelet inhibition. Possible protein kinase C inhibition also did not explain the inhibition of platelet activation by SPC. On the other hand, SPC suppressed agonist-induced Ca2+ mobilization and phospholipase C stimulation. These results indicate that the lysophospholipid SPC is an effective inhibitor of human platelet activation, apparently primarily by uncoupling agonist-activated receptors from their effectors. British Journal of Pharmacology (2003) 138, 435–444. doi:10.1038/sj.bjp.0705063