Insulin-like growth factor and potassium depolarization maintain neuronal survival by distinct pathways: possible involvement of PI 3-kinase in IGF-1 signaling.
Cultured cerebellar granule neurons die by apoptosis when switched from a medium containing an elevated level of potassium (K+) to one with lower K+ (5 mM). Death resulting from the lowering of K+ can be prevented by insulin-like growth factor (IGF-1). To understand how IGF-1 inhibits apoptosis and maintains neuronal survival, we examined the role of phosphoinositide 3-kinase (PI 3-kinase). Activation of PI 3-kinase has been shown previously to be required for NGF-mediated survival in the PC12 pheochromocytoma cell line. We find that in primary neurons, IGF-1 treatment leads to a robust activation of PI 3-kinase, as judged by lipid kinase assays and Western blot analysis. Activation of PI 3-kinase is likely to occur via tyrosine phosphorylation of the insulin receptor substrate protein. Treatment with two chemically distinct inhibitors of PI 3-kinase, wortmannin and LY294002, reduces PI 3-kinase activation by IGF-1 and inhibits its survival-promoting activity, suggesting that PI 3-kinase is necessary for IGF-1-mediated survival. Death resulting from PI 3-kinase blockade is accompanied by DNA fragmentation, a hallmark of apoptosis. Furthermore, neurons subjected to PI 3-kinase blockade can be rescued by transcriptional and translation inhibitors, suggesting that IGF-1-mediated activation of PI 3-kinase leads to a suppression of "killer gene" expression. In sharp contrast to IGF-1, elevated K+ does not activate PI 3-kinase and can maintain neuronal survival in the presence of PI 3-kinase inhibitors. Therefore, survival of granule neurons can be maintained by PI 3-kinase dependent (IGF-1-activated) and independent (elevated K+-activated) pathways.
D'Mello
S R
SR
Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut 06269, USA.
Borodezt
K
K
Soltoff
S P
SP
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002525:Cerebellar Cortex / Q000166:cytology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D009474:Neurons / Q000166:cytology / Q000187:drug effects*; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000037:antagonists & inhibitors / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000502:physiology*; D000077191:Wortmannin
D000730:Androstadienes; D004791:Enzyme Inhibitors; D009419:Nerve Tissue Proteins; D007334:Insulin-Like Growth Factor I; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D011188:Potassium; D000077191:Wortmannin