 Intracellularly, it regulates proliferation and survival, and extracellularly, it is a ligand for EDG1.Various stimuli increase cellular levels of S1P by activation of sphingosine kinase (SPHK), the enzyme that catalyzes the phosphorylation of sphingosine.Competitive inhibitors of SPHK block formation of S1P and selectively inhibit cellular proliferation induced by a variety of factors, including platelet-derived growth factor and serum.