Extracellular sphingosylphosphorylcholine (SPC) and galactosylsphingosine (psychosine) induced Ca2+ mobilization in a dose-dependent manner in HL60 leukemia cells. The rapid and transient increase in intracellular Ca2+ concentration ([Ca2+](i)) elicited by SPC and psychosine at concentrations lower than 30 mu M was inhibited by treatment of the cells with pertussis toxin (PTX) and U73122, a phospholipase C inhibitor, as was the ease for UTP, a P-2-purinergic agonist. The increase in [Ca2+](i) induced by these lysosphingolipids was associated with inositol phosphate production, which was also sensitive to PTX and U73122. The inositol phosphate response is not secondary to the increase in [Ca2+](i) as evidenced by the observation that thapsigargin and ionomycin, Ca2+ mobilizing agents, never induced inositol phosphate production and, unlike lysosphingolipids, the [Ca2+], rise by these agents was totally insensitive to PTX and U73122, When HL60 cells were differentiated into neutrophil like cells by dibutyryl cyclic AMP, inositol phosphate and Ca2+ responses to AlF4- were enhanced, probably reflecting an increase in the amount of Gi, and Gi, compared with undifferentiated cells. In the neutrophil-like cells, however, the responses to SPC and psychosine were markedly attenuated, This may exclude the possibility that the lysosphingolipids activate rather directly PTX-sensitive GTP-binding proteins or the phospholipase C itself, Other lysosphingolipids including glucosylsphingosine (glucopsychosine) and sphingosylgalactosyl sulfate (lysosulfatides) at 30 mu M or lower concentrations also showed PTX- and U73122-sensitive Ca2+ mobilization and inositol phosphate response in a way similar to SPC and psychosine, However, platelet-activating factor and lysoglycerophospholipids such as lysophosphatidylcholine and lysophosphatidic acid were less effective than these lysosphingolipids in the induction of Ca2+ mobilization. Taken together, the results indicate that a group of lysosphingolipids at appropriate doses induces Ca2+ mobilization through inositol phosphate production by phospholipase C activation. The lysosphingolipids-induced enzyme activation may be mediated by PTX-sensitive GTP-binding protein-coupled receptors, which may be different from previously identified platelet-activating factor receptor or lysophosphatidic acid receptor.
