Abstract

The sphingolipids, sphingosylphosphorylcholine (SPPC) and sphingosine-1-phosphate (SPP), induce a rapid and transient rise in intracellular free calcium concentration ([Ca2+]i) in a variety of cell lines via activation of pertussis toxin-sensitive G protein-coupled receptors. We investigated whether these sphingolipids act on different receptors by testing the effect of varying concentrations of SPPC on [Ca2+]i in human leukemia HL-60 cells, which have been found to be nonresponsive to SPP. SPPC potently (EC50 = 1.5 microM) and rapidly increased [Ca2+]i in HL-60 cells in a pertussis toxin-sensitive manner. Differentiation of HL-60 cells through treatment with dibutyryl cAMP into granulocyte-like cells did not change the magnitude or the pertussis toxin sensitivity of the SPPC-induced [Ca2+]i rise, indicating that the receptor for SPPC is constitutively expressed in HL-60 cells. SPPC did not activate phospholipase C or D in HL-60 cells. However, SPPC, but not SPP, stimulated the generation of superoxide anions in dibutyryl cAMP-differentiated HL-60 cells as well as in human neutrophils, suggesting that the SPPC receptor may play a role in the inflammatory defense against invading microorganisms. On the basis of these results, we conclude that there apparently is a heterogeneity of G protein-coupled receptors for sphingolipids in mammalian cells.