The lymphatic system plays pivotal roles in mediating tissue fluid homeostasis and immunity, and excessive lymphatic vessel formation is implicated in many pathological conditions, which include inflammation and tumor metastasis. However, the molecular mechanisms that regulate lymphatic vessel formation remain poorly characterized. Sphingosine-1-phosphate (S1P) is a potent bioactive lipid that is implicated in a variety of biologic processes such as inflammatory responses and angiogenesis. Here, we first report that S1P acts as a lymphangiogenic mediator. S1P induced migration, capillarylike tube formation, and intracellular Ca 2؉ mobilization, but not proliferation, in human lymphatic endothelial cells (HLECs) in vitro. Moreover, a Matrigel plug assay demonstrated that S1P promoted the outgrowth of new lymphatic vessels in vivo. HLECs expressed S1P1 and S1P3, and both RNA interference-mediated down-regulation of S1P1 and an S1P1 antagonist significantly blocked S1P-mediated lymphangiogenesis. Furthermore, pertussis toxin, U73122, and BAPTA-AM efficiently blocked S1P-induced in vitro lymphangiogenesis and intracellular Ca 2؉ mobilization of HLECs, indicating that S1P promotes lymphangiogenesis by stimulating S1P1/G i /phospholipase C/Ca 2؉ signaling pathways. Our results suggest that S1P is the first lymphangiogenic bioactive lipid to be identified, and that S1P and its receptors might serve as new therapeutic targets against inflammatory diseases and lymphatic metastasis in tumors. (Blood. 2008; 112:1129-1138)
IntroductionSphingosine-1-phosphate (S1P) has been implicated in a wide spectrum of biologic processes, including the promotion of cell growth and survival, migration and differentiation, platelet aggregation, inflammatory responses, and angiogenesis. 1 S1P is generated by the phosphorylation of sphingosine through a process mediated by sphingosine kinase 1 (SphK1) and SphK2. S1P acts both intracellularly as a second messenger 2 and extracellularly as a ligand for a family of G-protein-coupled S1P receptors. 3 S1P1 couples stringently to the G i protein family, whereas S1P2 and S1P3 couple to the G i , G q , and G 12/13 protein families. Multiple interconnections of S1P signaling through S1P1 and S1P3 induce vascular endothelial cell proliferation, migration, morphogenesis, cytoskeletal reorganization, and adherens junction assembly, whereas signaling via S1P2 negatively regulates S1P-mediated multiple responses of vascular endothelial cells. 4 The defective vascular maturation observed in S1P1-deficient mice highlights a fundamental role for S1P signaling on vasculogenesis. 5 Neutralization of the action of extracellular S1P shows significant inhibition of angiogenesis, tumor growth, metastasis, and lymphocyte transmigration, indicating that S1P is an important pathological regulator of inflammation and angiogenesis. [6][7][8] Lymphatic vessels play important roles in mediating tissue fluid homeostasis and immunity. 9 Although lymphangiogenesis, the formation of new lymphatic vessels from preexisting ves...