Discrimination between plasma membrane and intracellular target sites of sphingosylphosphorylcholine

Abstract

On the background of the emerging concept of G protein-coupled sphingolipid receptors, Ca 2+ mobilization by sphingosylphosphorylcholine (SPPC) in intact cells and SPPC-induced Ca 2+ 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 [Ca 2+] i by mobilization of Ca 2+ from thapsigargin-sensitive stores. In saponin-permeabilized HEK-293 cells, SPPC released stored Ca 2+, 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, Ca 2+ 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 [Ca 2+] i. Stereoselectivity of SPPC-induced [Ca 2+] i increase was also demonstrated in bovine aortic endothelial cells. In conclusion, Ca 2+ mobilization by SPPC in intact cells is independent of the previously described SPPC-gated Ca 2+ channel on endoplasmic reticulum but probably mediated by a membrane sphingolipid receptor. Thus, SPPC can regulate Ca 2+ homeostasis by acting apparently at two cellular targets, which exhibit clearly distinct recognition patterns.