['Proliferation of mesangial cells (MCs) is the initial step in glomerulonephritis, and platelet-derived mediators have been shown to play a significant role in this proliferation. Sphingosine 1-phosphate (S1P), one of the sphingolipids, is abundantly stored in platelets and is released upon stimulation. We examined the effects of S1P and related sphingolipids on the cell fate of cultured MCs in order to elucidate potential roles of these lipid mediators in glomerulonephritis.', 'Cell proliferation was evaluated by bromodeoxy uridine (BrdU) incorporation together with MTS assay. Apoptosis of MCs was evaluated by examining annexin V staining and typical morphological changes in nuclei. We also examined the metabolism of [(3)H]sphingosine in MCs in either the presence or absence of platelet-derived growth factor (PDGF). The expression of endothelial differentiation genes (edg), which are the cell surface receptors for S1P in MCs, was examined by RT-PCR.', 'S1P, but not the other sphingolipids, stimulated MC proliferation. In contrast, dimethylsphingosine (DMS) induced apoptosis in the MCs. The amount of sphingosine (Sph) converted into S1P was small and was not affected by PDGF. This observation suggested that Sph kinase activity producing S1P from Sph was low in the MCs. Furthermore, expression of edg-1, -2 and -5 in MCs was confirmed by RT-PCR.'] Does sphingosine 1-phosphate stimulate rat mesangial cell proliferation from outside the cells?