Antiproliferative properties of sphingosine-1-phosphate in human hepatic myofibroblasts

Sphingosine-1-phosphate (S1P) is a potent lysophospholipid mediator mostly released by activated platelets. It is involved in several functions in per ipheral tissues. but its effects in the central nervous system are poorly d ocumented. Therefore, we have examined the effects of S1P on the proliferat ion of striatal astrocytes from the mouse embryo. These cells have been fou nd to express mRNAs for the S1P receptors, Edg-1 and Edg-3. S1P stimulated thymidine incorporation and induced activation of extracellular signal-regu lated kinases (Erks). Both effects were prevented by U0126, an Erk kinase i nhibitor. The S1P-evoked activation of Erk1 was totally blocked in astrocyt es pretreated with a combination of either phorbol eater (24 h) and LY29400 2, or phorbol ester (24 h) and pertussis toxin (PTX). Each individual treat ment only partially inhibited Erk1 activation. This suggests that several s eparate mechanisms mediate this process, one involving protein kinase C and another involving Gi/Go proteins and phosphatidylinositol 3-kinase. In con trast, the stimulatory effect of SIP on astrocyte proliferation was totally blocked by either PTX or LY294002, but not by a downregulation of protein kinase C. S1P dramatically inhibited the evoked production of cyclic AMP, a response that was impaired by PTX. Finally, S1P stimulated the production of inositol phosphates and increased intracellular calcium by mobilization from thapsigargin-sensitive stores. These latter effects were mainly insens itive to PTX. Probably, Gi/Go protein activation and phosphoinositide hydro lysis are early events that regulate the activation of Erks by S1P. Altoget her, these observations show that astrocytes are targets for S1P. Their pro liferation in response to S1P could have physiopathological consequences at sites of brain lesions and alterations of the blood-brain barrier.