Sphingolipids are metabolized into bioactive products that include ceramide, sphingosine, and sphingosine-1-phosphate (S1P). Sphingosine Kinase (SK) catalyzes the phosphorylation of the lipid sphingosine, creating S1P. S1P subsequently signals through cell surface G protein-coupled receptors, as well as intracellularly, to modulate cell proliferation, survival, motility and differentiation. Two isoforms of SK have been identified, SK1 and SK2. The mRNA for both of these isoforms is widely expressed with SK1 expression highest in brain, heart, kidney, thymus, spleen and lung, while SK2 is highest in kidney and liver. SKs can be activated through growth factor, G protein-coupled, and immunoglobulin receptor signalling. Regulation of SK1 and SK2 activity may occur through phosphorylation. SK1 is phosphorylated at Ser-225 by ERK leading to increased activity and translocation to the plasma membrane. SK2 is phosphorylated in response to EGF, PKC activators, and phorbol esters. ERK1 can phosphorylate both Ser-351 and Thr-578, and non-phosphorylatable mutants of these sites suppress ERK1-mediated chemotaxis.
