Sphingosine 1-phosphate (SPP) is a lipid second messenger that also acts as a first messenger through the G protein-coupled receptor Edg-1. Here we show that SPP also binds to the related receptors H218 and Edg-3 with high affinity and specificity. SPP and sphinganine 1-phosphate bind to these receptors, whereas neither sphingosylphosphorylcholine nor lysophosphatidic acid compete with SPP for binding to either receptor. Transfection of HEK293 cells with H218 or edg-3, but not edg-1, induces rounded cell morphology in the presence of serum, which contains high levels of SPP. SPP treatment of cells overexpressing H218 cultured in delipidated serum causes cell rounding. A similar but less dramatic effect was observed in cells overexpressing Edg-3 but not with Edg-1. Cell rounding was correlated with apoptotic cell death, probably as a result of loss of attachment. Nerve growth factor-induced neuritogenesis in PC12 cells was inhibited by overexpression of H218 and to a lesser extent Edg-3. SPP treatment rapidly enhanced neurite retraction in PC12 cells overexpressing Edg-1, Edg-3, or H218. Thus, H218, and possibly Edg-3, may be the cell surface receptors responsible for cell rounding and neurite retraction induced by SPP. Moreover, the identification of these two additional SPP receptors indicates that a family of highly specific receptors exists that mediate different responses to SPP.The sphingolipid metabolite sphingosine 1-phosphate (SPP) 1 is emerging as a member of a new class of lipid second messengers (1, 2). SPP is mitogenic in diverse cell types (3-6) and suppresses programmed cell death or apoptosis (7-10). Various stimuli, including platelet-derived growth factor and serum (6, 11), nerve growth factor (NGF) (8, 12), vitamin D 3 (13), activation of protein kinase C (14, 15) or protein kinase A (10), cross-linking of the Fc⑀R1 (16) or Fc␥R1 (17) receptor by antigens, and binding of carbachol to m2 and m3 muscarinic acetylcholine receptors (18), increase cellular levels of SPP by activation of sphingosine kinase. Moreover, competitive inhibitors of sphingosine kinase eliminate the formation of SPP and selectively block cellular proliferation induced by platelet-derived growth factor and serum (11,19), the cytoprotective effects of protein kinase C and cAMP activators (7, 10), NGF (8), and vitamin D 3 (13) as well as Fc⑀R1-, Fc␥R1-, and muscarinic acetylcholine receptor-mediated calcium signaling (16, 17), further supporting a role for endogenous SPP in cell growth, survival, and calcium mobilization. In addition, microinjected SPP mobilizes calcium from internal sources (18) and is mitogenic for Swiss 3T3 fibroblasts (20), indicating that SPP acts intracellularly to regulate calcium homeostasis and proliferation.Several other responses to SPP are mediated through cell surface receptors, including platelet activation (21), inhibition of melanoma cell motility (22), activation of G i protein-gated inward rectifying K ϩ channels in atrial myocytes (23), and Rho-dependent neurite retraction and cell rounding o...