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 mediate protective effects of the IGFs against apoptosis. To determine whether ceramide-mediated induction of apoptosis involved a decreased survival effect 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 fibroblasts (P< 0.05 at ceramide doses of 2 microM or greater), not only corroborating evidence that the IGF-I receptor functions as a survival receptor, but also suggesting that ceramide is not inducing apoptosis by suppressing a survival effect of the IGF-IR. Ceramide has been reported to induce death through suppression of MAP kinase, and activation of JUN kinase signalling; since our initial data suggested that ceramide had not affected an anti-apoptotic signalling event of the IGF-IR, we monitored the activation of these enzymes. To our surprise, in the presence of ceramide, not only was JUN kinase activity increased, but so too was MAP kinase. Inhibition of MAP kinase, using the MEKK inhibitor, PD98059, significantly reduced ceramide-induced cell death (P< 0. 001). Ceramide also enhanced IGF-induced tyrosine phosphorylation 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 contradictory 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.
Stewart
C E
CE
University Dept of Surgery, Bristol Royal Infirmary, Bristol, BS2 8HW, UK. c.stewart-paton@bristol.ac.uk
Newcomb
P V
PV
Savage
P B
PB
Dickens
M
M
Tavare
J
J
Holly
J M
JM
D000818:Animals; D017209:Apoptosis; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002453:Cell Cycle; D002455:Cell Division; D002470:Cell Survival; D002518:Ceramides / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005347:Fibroblasts; D005434:Flow Cytometry; D020287:In Situ Nick-End Labeling; D048031:JNK Mitogen-Activated Protein Kinases; D051379:Mice; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000378:metabolism*; D015398:Signal Transduction; D013110:Sphingosine / Q000031:analogs & derivatives / Q000494:pharmacology; D013997:Time Factors
D002518:Ceramides; C064769:N-acetylsphingosine; D019869:Phosphatidylinositol 3-Kinases; D017526:Receptor, IGF Type 1; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D013110:Sphingosine