Discrimination between plasma membrane and intracellular target sites of sphingosylphosphorylcholine

On the background of the emerging concept of G protein-coupled sphingolipid receptors, Ca2+ mobilization by sphingosylphosphoryl-choline (SPPC) in intact cells and SPPC-induced Ca2+ release in permeabilized cells, both occurring at similar, micromolar concentrations, were characterized and compared. In intact human embryonic kidney (HEK-293) cells, SPPC rapidly increased [Ca2+](i) by mobilization of Ca2+ from thapsigargin-sensitive stores. In saponin-permeabilized HEK-293 cells, SPPC released stored Ca2+, in a manner similar to but independent of inositol 1,4,5-trisphosphate. Only the action of SPPC on intact cells, but not that in permeabilized cells, was, at least in part, sensitive to pertussis toxin. In addition and most important, Ca2+ release by SPPC in permeabilized cells was not stereoselective, whereas in intact cells only the naturally occurring D-erythro-SPPC, but not L-threo-SPPC, increased [Ca2+](i). Stereoselectivity of SPPC-induced [Ca2+](i) increase was also demonstrated in bovine aortic endothelial cells. In conclusion, Ca2+ mobilization by SPPC in intact cells is independent of the previously described SPPC-gated Ca2+ channel on endoplasmic reticulum but probably mediated by a membrane sphingolipid receptor. Thus, SPPC can regulate Ca2+ homeostasis by acting apparently at two cellular targets, which exhibit clearly distinct recognition patterns. (C) 1998 Elsevier Science B.V. All rights reserved.