INHIBITION OF P38 MITOGEN-ACTIVATED PROTEIN-KINASE BY INSULIN IN CULTURED FETAL NEURONS

Insulin supports the survival and differentiation of many types of fet al neurons. To determine if mitogen-activated protein (MAP) kinases pl ay a role in mediating the neurotrophic actions of insulin, we identif ied the MAP kinases present in fetal chick forebrain neurons and exami ned their regulation by insulin. Cell extracts were fractionated on Mo no Q columns, and phosphotransferase activity was measured using myeli n basic protein as the substrate. In control neurons, four peaks of MA P kinase activity were resolved, Peaks I, II, and IV were identified b y immunoblotting as c-Jun N-terminal kinase (JNK), extracellular signa l-related kinase (ERK), and p38 MAP kinase, respectively. Neurons trea ted with insulin showed a dramatic decrease, 80-90%, in p38 MAP kinase activity without significant changes in the other MAP kinase activiti es. Insulin decreased the phosphotyrosine content of p38 MAP kinase wi th maximal effects observed within 5 min. Pretreatment of neurons with sodium orthovanadate blocked the ability of insulin to inhibit the ty rosine phosphorylation and activity of p38 MAP kinase, suggesting that activation of a tyrosine or dual specific phosphatase is necessary fo r the inhibition of p38 MAP kinase by insulin. Since p38 MAP kinase ha s been recently implicated in neuronal cell apoptosis, negative regula tion of this kinase by insulin may be critical for the neurotrophic ac tions of insulin.