Sphingosine-1-phosphate induces mitogenesis and alterations in intracellular calcium (Ca2+) in vascular smooth muscle cells

Sphingosine-1-phosphate (SPP) is a putative second messenger involved in cell growth and differentiation. The precise role of SPP in signal transduction, however, is unclear. The mitogenic activity of Platelet-derived Growth Factor (PDGF) has been correlated with increases in cellular levels of sphingosine, SPP, and activation of sphingosine kinase in Swiss 3T3 fibroblasts (Olivera and Spiegel, Nature, 365 :557,1993). Addition of exogenous SPP results in intracellular Co2+ transients and mitogenesis in these cells. Recent studies in human vascular smooth muscle cells (VSMC) indicate that SPP is not mitogenic but is involved in the chemotactic signaling pathway (Bornfeldt et al, J. Cell BioL, 130 :193,1995). We have measured the effect of exogenous SPP on Ca2+ and mitogenesis in porcine aortic VSMC to examine the possibility of cell-type specific sphingolipid signaling pathways. SPP addition (0.1-10/M) resulted in a dose-dependent induction of DNA synthesis, with a maximum 3.4-fold increase in BrdU-positive cells compared to controls. BB-PDGF produced a 4.6-fold stimulation. SPP also elicited Ca2+ transients in VSMC that were not blocked by EGTA but were inhibited by thapsigargin, consistent with the release of intraceUular Ca2+ stores by SPP. At the single cell level, low-dose SPP (0.1/iM) stimulation produced a biphasic Ca2+ response with secondary oscillations following an initial Ca2+ spike. Our data demonstrate that SPP is mitogenic in porcine VSMC and we are currently examining the role of this second messenger and associated Ca2+ alterations in PDGF signal transduction. Supported by NIH GM49304.