Two different signal transduction pathways are implicated in the regulation of initiation factor 2B activity in insulin-like growth factor-1-stimulated neuronal cells

Eukaryotic initiation factor eIF-2B plays an important role in translation regulation and has been suggested to be implicated in the increased protein synthesis promoted in response to growth factors. We have used primary cul tured neurons to delineate the signaling pathways by which insulin-like gro wth factor-1 (IGF-1), which plays a critical role in the survival of neuron al cells, promotes eIF-2B and protein synthesis activation. Treatment of co rtical neurons with IGF-1 (100 ng/ml) for 30 min stimulates [H-3]methionine incorporation, and a parallel increase in eIF-2B activity was observed. Wo rtmannin and LY294002 reversed both effects, indicating that phosphatidylin ositol 3-kinase mediates IGF-1-induced protein synthesis and eIF-2B activat ion. IGF-1 induced glycogen synthase kinase-3 (GSK-3) inactivation in a pho sphatidylinositol 3-kinase-dependent fashion because it is inhibited by wor tmannin and LY294002. By using GSK-3 immunoprecipitated from untreated and IGF-1-treated cells, we demonstrate the phosphorylation of eIF-2B coinciden t with its inactivation. The treatment of cortical neurons with IGF-1 also promoted the activation of mitogen-activated protein kinase (MAPK). The MAP K-activating kinase (MEK) inhibitor PD98059 inhibited MAPK activation and r eversed IGF-1-induced protein synthesis and eIF-2B activation. These findin gs suggest that IGF-1-induced eIF-2B activation on neurons is promoted thro ugh phosphatidylinositol S-kinase and GSK-3 kinase, and we report an IGF-1- induced MEK/MAPK activation pathway implicated in eIF-2B activation.