Endogenous second messenger involved in the control of cell proliferation and motility, and Ca2+ mobilization. Acts as an agonist at sphingosine-1-phosphate receptors (S1P1-5) and as an activator of GPR3, GPR6 and GPR12. Effectors regulated include p38 MAP kinase, PLC, adenylyl cyclase, myosin light chain phosphatase and focal adhesion kinase.