Sphingosine 1-phosphate as a major bioactive lysophospholipid that is released from platelets and interacts with endothelial cells

The serum-borne lysophospholipid mediators sphingosine 1-phosphate (Sph-1-P ) and lysophosphatidic acid (LPA) have been shown to be released from activ ated platelets and to act on endothelial cells. In this study, we employed the repeated lipid extraction (under alkaline and acidic conditions), capab le of detecting Sph-1-P, LPA, and possibly structurally similar lysophospho lipids, whereby a marked formation of [P-32]Sph-1-P, but not [P-32]LPA, was observed In [P-32]orthophosphate-labeled platelets. Platelet Sph-1-P relea se, possibly mediated by protein kinase C, was greatly enhanced in the pres ence of albumin, which formed a complex with Sph-1-P. This finding suggests that platelet Sph-1-P may become accessible to depletion by albumin when i ts transbilayer movement (flipping) across the plasma membrane is enhanced by protein kinase C. Although human umbilical vein endothelial cells expres sed receptors for both Sph-1-P and LPA, Sph-1-P acted much more potently th an LPA on the cells in terms of intracellular Ca++ mobilization, cytoskelet al reorganization, and migration. The results suggest that Sph-1-P, rather than LPA, is a major bioactive lysophospholipid that is released from plate lets and interacts with endothelial cells, under the conditions in which cr itical platelet-endothelial interactions (including thrombosis, angiogenesi s, and atherosclerosis) occur. Furthermore, albumin-bound Sph-1-P may accou nt for at least some of the serum biological activities on endothelial cell s, which have-been ascribed to the effects of albumin-bound LPA, based on t he similarities between LPA and serum effects. (C) 2000 by The American Soc iety of Hematology.