Increased, not decreased activation of the insulin-like growth factor (IGF) receptor signalling pathway during ceramide-induced apoptosis

The insulin-like growth factors (IGFs) are capable of blocking apoptosis in many cell lines in vitro. The IGF-I receptor (IGF-IR) is believed to media te protective effects of the IGFs against apoptosis. To determine whether c eramide-mediated induction of apoptosis involved a decreased survival effec t of the IGF-IR, apoptosis was induced in IGF-I receptor positive (R+) and negative (R-) murine fibroblasts by incubation with increasing doses of the sphingolipid analogue, C2 ceramide. Lower ceramide doses were required to induce death in receptor negative compared with receptor positive fibroblas ts (P < 0.05 at ceramide doses of 2 mu M or greater), not only corroboratin g evidence that the IGF-I receptor functions as a survival receptor, but al so suggesting that ceramide is not inducing apoptosis by suppressing a surv ival effect of the IGF-IR. Ceramide has been reported to induce death throu gh suppression of MAP kinase, and activation of JUN kinase signalling(1,2); since our initial data suggested that ceramide had not affected an anti-ap optotic signalling event of the IGF-IR, we monitored the activation of thes e enzymes. To our surprise, in the presence of ceramide, not only was JUN k inase activity increased, but so too was MAP kinase. Inhibition of MAP kina se, using the MEKK inhibitor, PD98059, significantly reduced ceramide-induc ed cell death (P < 0.001). Ceramide also enhanced IGF-induced tyrosine phos phorylation of the IGF-I receptor and activated PI-3 kinase. The cumulative effects of these events resulted in increased progression to the G2 phase of the cell cycle, arrest without subsequent mitosis, and apoptosis, These results indicate that ceramide is capable of eliciting apparently contradic tory events within a single cell type, and suggest that in the presence of an IGF-IR, survival is enhanced because ceramide can activate PI-3 kinase, believed to be an anti-apoptotic enzyme. (C) 1999 Churchill Livingstone.