Insulin-like growth factor I prevents the development of sensitivity to kainate neurotoxicity in cerebellar granule cells

This study reports that insulin-like growth factor I (IGF-I) prevents cereb ellar granule cells from developing sensitivity to kainate neurotoxicity, S ensitivity to kainate neurotoxicity normally develops 5-6 days after switch ing cultures to a serum-free medium containing 25 mM K+. Addition of either IGF-I or insulin to the serum free medium at the time of the switch preven ted the development of sensitivity to kainate, whereas brain-derived neurot rophic factor, neurotrophin-3, neurotrophin-4, and nerve growth factor did not. The dose-response curves indicated IGF-I was more potent than insulin, favoring the assignment of the former as the physiological protective agen t. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (10-100 nM) and LY 294002 (0.3-1 mu M) abolished the protection afforded by IGF-I. The p70 S6 kinase (p70(S6k)) inhibitor rapamycin (5-50 nM) also abolished the protection afforded by IGF-I, The activities of both enzymes decreased in cultures switched to serum-free medium but increased when IGF-I was incl uded; wortmannin (100 nM) lowered the activity of PI 3-K from 2 to 5 days a fter medium switch, whereas rapamycin (50 nM) prevented the increase observ ed for p70(S6k) activity over the same interval. The mitogen-activated prot ein kinase kinase inhibitor U 0126 and the mitogen-activated protein kinase inhibitor SE 203580 did not abolish IGF-I protection. Kainate neurotoxicit y was not prevented by Joro spider toxin; therefore, the development of kai nate neurotoxicity could not be explained by the formation of calcium-perme able alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors, These results indicate that IGF-I functions through a signal transduction pathway involving PI 3-K and p70(S6k) to prevent the development of sensitivity to kainate neurotoxicity in cerebellar granule cells.