Sphingosine kinase expression increases intracellular sphingosine-1-phosphate and promotes cell growth and survival

Sphingosine-1-phosphate (SPP) is a bioactive lipid that has recently been i dentified as the ligand for the EDG family of G protein-coupled cell surfac e receptors. However, the mitogenic and survival effects of exogenous SPP m ay not correlate with binding to cell-surface receptors (Van Brocklyn, J.R. , M.J. Lee, R. Menzeleev; A. Olivera, L. Edsall, O. Cuvillier, D.M. Thomas, P.J.P. Coopman, S, Thangada, T. Hla, and S. Spiegel. 1998. J. Cell Biol. 1 42:229-240). The recent cloning of sphingosine kinase. a unique lipid kinas e responsible for the formation of SPP, has provided a new tool to investig ate the role of intracellular SPP. Expression of sphingosine kinase markedl y increased SPP levels in NIH 3T3 fibroblasts and HEK293 cells, but no dete ctable secretion of SPP into the medium was observed. The increased sphingo sine kinase activity in NIH 3T3 fibroblasts was sufficient to promote growt h in low-serum media, expedite the G(1)/S transition, and increase DNA synt hesis and the proportion of cells in the S phase of the cell cycle with a c oncomitant increase in cell numbers. Transient or stable overexpression of sphingosine kinase in NIH 3T3 fibroblasts or HEK293 cells protected against apoptosis induced by serum deprivation or ceramide elevation. N,N-Dimethyl sphingosine, a competitive inhibitor of sphingosine kinase, blocked the eff ects of sphingosine kinase overexpression on cell proliferation and suppres sion of apoptosis, In contrast, pertussis toxin did not abrogate these biol ogical responses. In Jurkat T cells, overexpression of sphingosine kinase a lso suppressed serum deprivation- and ceramide-induced apoptosis and, to a lesser extent, Fas-induced apoptosis, which correlated with inhibition of D EVDase activity, as well as inhibition of the executionary caspase-3. Taken together with ample evidence showing that growth and survival factors acti vate sphingosine kinase, our results indicate that SPP functions as a secon d messenger important for growth and survival of cells. Hence, SPP belongs to a novel class of lipid mediators that can function inside and outside ce lls.