INTRACELLULAR CALCIUM MOBILIZATION AND PHOSPHOLIPID DEGRADATION IN SPHINGOSYLPHOSPHORYLCHOLINE-STIMULATED HUMAN AIRWAY EPITHELIAL-CELLS

Extracellular sphingosylphosphorylcholine (SPC) caused a remarkable el evation in the intracellular Ca2+ concentration ([Ca2+](i)) in immorta lized human airway epithelial cells (CFNP9o(-)). An increase in total inositol phosphates formation was determined: however, the dose respon ses for [Ca2+](i) elevation and inositol phosphates production were sl ightly different and, furthermore, PMA and pertussis toxin almost comp letely inhibited [Ca2+](i) mobilization by SPC, whereas inositol phosp hates production was only partially reduced. The possible direct inter action of SPC with Ca2+ channels of intracellular stores was determine d by experiments with permeabilized cells, where SPC failed to evoke C a2+ release, whereas lysophosphatidic acid was shown to be effective. The level of phosphatidic acid was increased by SPC only in the presen ce of AACOCF(3), a specific inhibitor of phospholipase A(2) (PLA(2)) a nd blocked by both pertussis toxin and R59022, an inhibitor of diacylg lycerol kinase. R59022 enhanced diacylglycerol production by SPC and a lso significantly reduced [Ca2+](i) mobilization. Only polyunsaturated diacylglycerol and phosphatidic acid were generated by SPC. Lastly, S PC caused stimulation of arachidonic acid release, indicating the invo lvement of PLA(2). Taken together, these data suggest that, after SPC stimulation, phospholipase C-derived diacylglycerol is phosphorylated by a diacylglycerol kinase to phosphatidic acid, which is further hydr olysed by PLA(2) activity to arachidonic and lysophosphatidic acids. W e propose that lysophosphatidic acid might be the intracellular messen ger able to release Ca2+ from internal stores.