Sphingosine-1-phosphate, a platelet-derived lysophospholipid mediator, negatively regulates cellular Rac activity and cell migration in vascular smooth muscle cells

Previous studies demonstrated that sphingosine-1-phosphate (SIP) induced migration of human umbilical vein endothelial cells (HUVECs) whereas it inhibited that of vascular smooth muscle cells (SMCs). This study explored the molecular mechanisms underlying the contrasting S I P actions on vascular cell motility. In rat and human aortic SMCs, the chemoattractant platelet-derived growth factor B-chain (PDGF) induced rapid 5- to 6-fold increases in the cellular amount of GTP-bound, active form of Rac. SIP did not affect PDGF-stimulated tyrosine phosphorylation of PDGF-P receptor, but strongly inhibited PDGF-induced Rac activation, with a dose-response relationship similar to that for inhibition of PDGF-elicited chemotaxis. Dihydrosphingosine-1-phosphate, which is a weaker agonist for the SIP receptors, but not an inactive ligand sphingosine, also inhibited PDGF-stimulated chemotaxis and Rac activation although to lesser extents compared with S I P, suggesting that negative regulation by S I P of both chemotaxis and Rac was a receptor-mediated process. In contrast, SIP by itself stimulated Rac activity in HUVECs. Among the five SIP receptor isoforms, SMCs prominently expressed Edg-5 mRNA, whereas HUVECs expressed abundant Edg-1 mRNA but lacked detectable expression of Edg-5 mRNA. Adenovirus-mediated expression of a dominant-negative form of either Rac or Cdc42, but not RhoA, markedly attenuated chernotaxis of SMCs and HUVECs toward PDGF and S I P, respectively. Overexpression of Edg-1 in SMCs and Edg-5 in HUVECs reduced S I P-induced inhibition and stimulation, respectively, of Rac activity and migration. These results together indicate that Edg isoform-specific, negative or positive regulation of cellular Rac activity is critically involved in SIP-mediated bimodal regulation of cell motility in SMCs and HUVECs.