EVIDENCE THAT P21RAS MEDIATES THE NEUROTROPHIC EFFECTS OF INSULIN ANDINSULIN-LIKE GROWTH-FACTOR-I IN CHICK FOREBRAIN NEURONS

Insulin and insulin-like growth factors (IGF-I and IGF-II) support the survival and differentiation of many types of neurons, including thos e from fetal chick forebrain. The mechanisms by which these peptides e xert their neurotrophic actions are poorly understood. The aims of thi s study were to determine if insulin and IGF-I activate p21ras in feta l chick forebrain neurons and if activation of p21ras mediates the neu rotrophic actions of these peptides. Activation of neuronal p21ras was examined by measuring the amount of GTP bound to p21ras before and af ter growth factor treatment. Insulin and IGF-I increased the ratio of GTP/GTP + GDP by 31 +/- 9.0% and 36 +/- 8.0%, respectively. p21Ras act ivation by insulin and IGF-I was maximal within 5 min. In the presence of insulin the response was sustained out to 180 min, whereas the res ponse to IGF-I decreased significantly by 180 min. Both peptides stimu lated p21ras at low concentrations with a maximal response obtained at 10 ng/ml for each peptide, indicating that insulin and IGF-I activate ras by interacting with their homologous receptor. Pretreatment of ne urons with lovastatin (2 mu g/ml), an inhibitor of ras isoprenylation, completely blocked the activation of p21ras by insulin and IGF-I. Lov astatin also blocked the ability of these growth factors to support th e survival and differentiation of fetal chick neurons in culture. We c onclude that insulin and IGF-I activate p21ras in fetal chick forebrai n neurons by increasing the amount of GTP bound to p21ras. The activat ion of neuronal p21ras is necessary for insulin and IGF-I to promote s urvival and differentiation in these neurons.