Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid (LPL) ligand that binds endothelial differentiation gene (Edg) family G-protein-coupled receptors and has been implicated as an important regulator in endothelial cells during inflammation processes. In this study, we attempt to determine which SIP receptors mediating the inflammatory response in human endothelial cells. Our results indicated that introduction of siRNA against SIP, significantly suppressed SIP-induced ICAM-I mRNA, total protein, and cell surface expressions in human umbilical vein endothelial cells (HUVECs). Moreover, U937 cells adhesion to SIP-treated HUVECs was profoundly reduced by knock-down of S1P, in HUVECs. By knock-down of S1P(3) or S1P(3) in HUVECs, S1P-enhanced IL-8, MCP-1 mRNA expression, and THP-l cell chemotaxis toward SIP-treated HUVEC-conditioned media was profoundly reduced. These results suggested that SIP-induced inflammatory response genes expression is mediated through S1P(1) and S1P(3). Our findings suggest the possible utilization of S1P, or S1P(3) as drug targets to treat severe inflammation. (c) 2007 Elsevier Inc. All rights reserved.