Insulin Rescues Retinal Neurons from Apoptosis by a Phosphatidylinositol 3-Kinase/Akt-mediated Mechanism That Reduces the Activation of Caspase-3

The ability of insulin to protect neurons from apoptosis was examined in differentiated R28 cells, a neural cell line derived from the neonatal rat retina. Apoptosis was induced by serum deprivation, and the number of pyknotic cells was counted. p53 and Akt were examined by immunoblotting after serum deprivation and insulin treatment, and caspase-3 activation was examined by immunocytochemistry. Serum deprivation for 24 h caused ∼20% of R28 cells to undergo apoptosis, detected by both pyknosis and activation of caspase-3. 10 nM insulin maximally reduced the amount of apoptosis with a similar potency as 1.3 nM (10 ng/ml) insulin-like growth factor 1, which acted as a positive control. Insulin induced serine phosphorylation of Akt, through the phosphatidylinositol (PI) 3-kinase pathway. Inhibition of PI 3-kinase with wortmannin or LY294002 blocked the ability of insulin to rescue the cells from apoptosis. SN50, a peptide inhibitor of NF-κB nuclear translocation, blocked the rescue effect of insulin, but neither insulin or serum deprivation induced phosphorylation of IκB. These results suggest that insulin is a survival factor for retinal neurons by activating the PI 3-kinase/Akt pathway and by reducing caspase-3 activation. The rescue effect of insulin does not appear to be mediated by NF-κB or p53. These data suggest that insulin provides trophic support for retinal neurons through a PI 3-kinase/Akt-dependent pathway.