Pertussis toxin- and PMA-insensitive calcium mobilization by sphingosine in CFPAC-1 cells: evidence for a phosphatidic acid-dependent mechanism

In a pancreatic duct adenocarcinoma cell line (CFPAC-1) sphingosine (10 μM) induced both mobilization of intracellular Ca2+ and stimulation of inositol phosphates accumulation. Whereas this latter effect was significantly inhibited by treatment with pertussis toxin or by short-term incubation with phorbol 12-myristate 13-acetate, Ca2+ mobilization was completely insensitive to both treatments. Experiments with permeabilized cells showed that sphingosine or the sphingosine metabolites sphingosine-1-phosphate and sphingosylphosphorylcholine were unable to directly release Ca2+ from internal stores, whereas phosphatidic acid, but not arachidonic acid, was effective. Phosphatidic acid formation was markedly enhanced (2.9-fold over control) by sphingosine, this effect being significantly reduced by preincubation with the diacylglycerol kinase inhibitor R59022. Ca2+ mobilization by sphingosine was also cut down by preincubation with R59022. In conclusion, the results suggest that sphingosine activates phospholipase C through a mechanism functionally coupled through a G protein and under control of PKC. Mobilization of [Ca2+]i by sphingosine is independent of phospholipase C stimulation and likely due to elevation of phosphatidic acid generated by stimulation of diacylglycerol kinase activity.