Endothelial cell barrier dysfunction results in the increased vascular permeability observed in inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Sphingosine 1-phosphate (S1P), a biologically active phosphorylated lipid growth factor released from activated platelets, enhances the endothelial cell barrier integrity in vitro and in vivo. To begin to identify the molecular mechanisms mediating S1P induced endothelial barrier enhancement, quantitative proteomics analysis (iTRAQ TM ) was performed on membrane rafts isolated from human pulmonary artery endothelial cells in the absence or presence of S1P stimulation. Our results demonstrated that S1P mediates rapid and specific recruitment (1 M, 5 min) of myristoylated alanine-rich protein kinase C substrate (MARCKS) and MARCKS-related protein (MRP) to membrane rafts. Western blot experiments confirmed these findings with both MARCKS and MRP. Finally, small interfering RNA-mediated silencing of MARCKS or MRP or both attenuates S1P-mediated endothelial cell barrier enhancement. These data suggest the regulation of S1P-mediated endothelial cell barrier enhancement via the cell specific localization of MARCKS and MRP and validate the utility of proteomics approaches in the identification of novel molecular targets. Molecular & Cellular Proteomics 6:689 -696,
2007.The pulmonary endothelium serves as a semipermeable cellular barrier between blood and the interstitium and airspaces of the lung. Endothelial cell (EC) 1 barrier dysfunction results in increased vascular permeability, which is a cardinal feature of inflammation and an essential component of tumor metastasis, angiogenesis, and atherosclerosis. Proteins and lipids released after platelet activation have long been appreciated as enhancing the integrity of the microcirculation (1, 2). Sphingosine 1-phosphate (S1P), a biologically active phosphorylated lipid growth factor released from platelet, has multiple EC effects, including promoting barrier integrity in vivo and in vitro across both human and bovine pulmonary artery and lung microvascular ECs (3, 4). Our previous studies demonstrated S1P mediated barrier enhancement to be dependent on S1P binding to its major surface receptor, S1P 1 ; activation of the small GTPase, Rac1; and rearrangement of the cortical actin cytoskeleton (3, 5, 6). Very recently, we further detailed the essential involvement of PI 3-kinase, Tiam1, and âŁ-actinin in specialized membrane domains (i.e. membrane rafts) in S1P-treated EC barrier regulation (7). However, the underlying signaling mechanisms by which S1P increases vascular integrity via signaling to the endothelial cytoskeleton remain poorly understood.As defined by the recent Keystone Symposium on lipid rafts and cell function (March 23-28, 2006, in Steamboat Springs, CO) (8), "Membrane rafts are small (10 -200 nm), heterogeneous, highly dynamic, sterol-and sphingolipid-enriched domains that compartmentalize cellular processes. Small rafts can sometimes be stabilized to form larger platforms through protein-protei...