DUAL ACTIONS OF SPHINGOSINE-1-PHOSPHATE - EXTRACELLULAR THROUGH THE G(I)-COUPLED RECEPTOR EDG-1 AND INTRACELLULAR TO REGULATE PROLIFERATIONAND SURVIVAL

Sphingosine-l-phosphate (SPP), a bioactive lipid, acts both intracellu larly and extracellularly to cause pleiotropic biological responses. R ecently, we identified SPP as a ligand for the G protein-coupled recep tor Edg-1 (Lee, M.-J., J.R. Van Brocklyn, S. Thangada, C.H. Liu, A.R. Hand, R. Menzeleev, S. Spiegel, and T. Hla. 1998. Science. 279:1552-15 55). Edg-1 binds SPP with remarkable specificity as only sphinganine-1 -phosphate displaced radiolabeled SPP, while other sphingolipids did n ot. Binding of SPP to Edg-1 resulted in inhibition of forskolin-stimul ated cAMP accumulation, in a pertussis toxin-sensitive manner. In cont rast, two well-characterized biological responses of SPP, mitogenesis and prevention of apoptosis, were clearly unrelated to binding to Edg- 1 and correlated with intracellular uptake. SPP also stimulated signal transduction pathways, including calcium mobilization, activation of phospholipase D, and tyrosine phosphorylation of p125(FAK), independen tly of edg-1 expression. Moreover, DNA synthesis in Swiss 3T3 fibrobla sts was significantly and specifically increased by microinjection of SPP. Finally, SPP suppresses apoptosis of HL-60 and pheochromocytoma P C12 cells, which do not have specific SPP binding or expression of Edg -1 mRNA. Conversely, sphinganine-1-phosphate, which binds to and signa ls via Edg-1, does not have any significant cytoprotective effect. Thu s, SPP is a prototype for a novel class of lipid mediators that act bo th extracellularly as ligands for cell surface receptors and intracell ularly as second messengers.