Phosphatidylinositol 3-kinase and Akt effectors mediate insulin-like growth factor-I neuroprotection in dorsal root ganglia neurons.
Insulin-like growth factor-I (IGF-I) protects neurons of the peripheral nervous system from apoptosis, but the underlying signaling pathways are not well understood. We studied IGF-I mediated signaling in embryonic dorsal root ganglia (DRG) neurons. DRG neurons express IGF-I receptors (IGF-IR), and IGF-I activates the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. High glucose exposure induces apoptosis, which is inhibited by IGF-I through the PI3K/Akt pathway. IGF-I stimulation of the PI3K/Akt pathway phosphorylates three known Akt effectors: the survival transcription factor cyclic AMP response element binding protein (CREB) and the pro-apoptotic effector proteins glycogen synthase kinase-3beta (GSK-3beta) and forkhead (FKHR). IGF-I regulates survival at the nuclear level through accumulation of phospho-Akt in DRG neuronal nuclei, increased CREB-mediated transcription, and nuclear exclusion of FKHR. High glucose increases expression of the pro-apoptotic Bcl protein Bim (a transcriptional target of FKHR). However, IGF-I does not regulate Bim or anti-apoptotic Bcl-xL protein expression levels, which suggests that IGF-I neuroprotection is not through regulation of their expression. High glucose also induces loss of the initiator caspase-9 and increases caspase-3 cleavage, effects blocked by IGF-I. These data suggest that IGF-I prevents apoptosis in DRG neurons by regulating PI3K/Akt pathway effectors, including GSK-3beta, CREB, and FKHR, and by blocking caspase activation.
Neuroscience Program, University of Michigan, Ann Arbor, Michigan 48109, USA.
Gina M
GM
Leinninger
Carey
C
Backus
Michael D
MD
Uhler
Stephen I
SI
Lentz
Eva L
EL
Feldman
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D051858:Forkhead Transcription Factors; D005727:Ganglia, Spinal / Q000166:cytology* / Q000196:embryology; D005947:Glucose / Q000037:antagonists & inhibitors / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D018696:Neuroprotective Agents / Q000494:pharmacology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D021381:Protein Transport / Q000187:drug effects; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects; D000071161:Forkhead Box Protein O1
D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases; D004268:DNA-Binding Proteins; D051858:Forkhead Transcription Factors; D005947:Glucose; D007334:Insulin-Like Growth Factor I; D020928:Mitogen-Activated Protein Kinases; D009419:Nerve Tissue Proteins; D018696:Neuroprotective Agents; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; C074639:Foxo1 protein, rat; D000071161:Forkhead Box Protein O1; C494920:Akt1 protein, rat; C505410:Casp3 protein, rat; C506435:Casp9 protein, rat