Sphingosine and phorbol ester modulate protein kinase C activity and modify ATP-evoked calcium mobilization in glioma C6 cells

The effect of sphingosine and 12-O-tetradecanyl-phorbol-13-acetate (TPA) on ATP-evoked Ca2+ mobilization in glioma C6 cells was studied with the Fura-2 video-imaging technique. Treatment of the cells with TPA, an activator of protein kinase C, reduced the ATP-evoked release of Ca2+ from the intracellular stores, whereas sphingosine, known from in vitro studies as a protein kinase C inhibitor, potentiated Ca2+ release synergistically with ATP. ATP-induced Ca2+ mobilization was also enhanced by a specific protein kinase C inhibitor, GF 109203X. Pretreatment of the cells with GF 109203X prevented TPA action, whereas TPA diminished the stimulatory effect of sphingosine. However, this sphingosine effect was only observed after a short (1 min) treatment, whereas a longer treatment (5 min) reduced ATP-evoked Ca2+ release. It is therefore concluded that sphingosine has two apparent actions: it inhibits protein kinase C providing a positive feedback regulation of receptor signals and it releases Ca2+ from intracellular stores by an unknown mechanism, possibly independent of protein kinase C. (C) 1999 Academic Press.