APOPTOSIS INDUCED BY WITHDRAWAL OF TROPHIC FACTORS IS MEDIATED BY P38MITOGEN-ACTIVATED PROTEIN-KINASE

p38 is a member of the mitogen-activated protein (MAP) kinase superfam ily activated by stress signals and implicated in cellular processes i nvolving inflammation and apoptosis, Unlike the extracellular signal-r egulated kinases (p42 and p44 MAP kinases), which are stimulated by in sulin in many cell types, p38 activity is inhibited by insulin in post mitotic fetal neurons for which insulin is a potent survival factor (H eidenreich, K. A., and Kummer, J, L, (1996) J, Biol. Chem. 271, 9891-9 894), These data suggested that insulin's effects on neuronal survival are mediated by inhibition of a p38-mediated apoptotic pathway, To be tter understand the relationship between p38 activity and cell surviva l, we induced apoptosis in two cell lines and examined the ability of insulin or a specific p38 inhibitor (a pyridinyl imidazole compound PD 169316) to block p38 activity and cell death, In Rat-1 fibroblasts gro wn in the presence of serum, p38 activity was undetectable by immune c omplex assays, and the number of apoptotic cells was very low (< 0.5%) , After the removal of serum for 16 h, p38 activity was markedly eleva ted, and apoptosis increased by 14-15-fold, Insulin (50 ng/ml) inhibit ed p38 activity by similar to 70% and blocked apoptosis by at least 80 %, PD169316 also blocked p38 enzyme activity and apoptosis by approxim ately 80%, Similar results were obtained in differentiated PC12 cells that were deprived of nerve growth factor (NGF) for 16 h, In the prese nce of NGF, p38 activity and the number of apoptotic cells was very lo w (similar to 1.0%). After NGF withdrawal, p38 activity was selectivel y elevated and apoptosis increased to 15%, Both insulin and PD169316 m arkedly blocked the increase in p38 activity and apoptosis. The MAP ki nase kinase inhibitor, PD98059, had no effect on apoptosis in Rat-1 fi broblasts and only partially blocked apoptosis in PC12 cells, PD98059 did not influence insulin's ability to block apoptosis, indicating tha t the extracellular signal-regulated kinase pathway does not mediate i nsulin's survival effects, These data further support the role of p38 in cellular apoptosis and support the hypothesis that insulin promotes cell survival, at least in part, by inhibiting the p38 pathway.