Photolysis of intracellular caged sphingosine-1-phosphate causes Ca2+ mobilization independently of G-protein-coupled receptors

Sphingosine-1-phosphate (S1P), the product of sphingosine kinase, activates several widely expressed G-protein-coupled receptors (GPCR). S1P might also play a role as second messenger, but this hypothesis has been challenged by recent findings. Here we demonstrate that intracellular S1P can mobilize Ca2+ in intact cells independently of S1P-GPCR. Within seconds, S1P generated by the photolysis of caged S1P raised the intracellular free Ca2+ concentration in HEK-293, SKNMC and HepG2 cells, in which the response to extracellularly applied S1P was either blocked or absent. Ca2+ transients induced by photolysis of caged S1P were caused by Ca2+ mobilization from thapsigargin-sensitive stores. These results provide direct evidence for a true intracellular action of S1P.