Apoptosis induced by withdrawal of trophic factors is mediated by p38 mitogen-activated protein kinase.

p38 is a member of the mitogen-activated protein (MAP) kinase superfamily activated by stress signals and implicated in cellular processes involving inflammation and apoptosis. Unlike the extracellular signal-regulated kinases (p42 and p44 MAP kinases), which are stimulated by insulin in many cell types, p38 activity is inhibited by insulin in postmitotic fetal neurons for which insulin is a potent survival factor (Heidenreich, K. A., and Kummer, J. L. (1996) J. Biol. Chem. 271, 9891-9894). These data suggested that insulin's effects on neuronal survival are mediated by inhibition of a p38-mediated apoptotic pathway. To better understand the relationship between p38 activity and cell survival, we induced apoptosis in two cell lines and examined the ability of insulin or a specific p38 inhibitor (a pyridinyl imidazole compound PD169316) to block p38 activity and cell death. In Rat-1 fibroblasts grown in the presence of serum, p38 activity was undetectable by immune complex assays, and the number of apoptotic cells was very low (<0.5%). After the removal of serum for 16 h, p38 activity was markedly elevated, and apoptosis increased by 14-15-fold. Insulin (50 ng/ml) inhibited p38 activity by approximately 70% and blocked apoptosis by at least 80%. PD169316 also blocked p38 enzyme activity and apoptosis by approximately 80%. Similar results were obtained in differentiated PC12 cells that were deprived of nerve growth factor (NGF) for 16 h. In the presence of NGF, p38 activity and the number of apoptotic cells was very low (approximately 1.0%). After NGF withdrawal, p38 activity was selectively elevated and apoptosis increased to 15%. Both insulin and PD169316 markedly blocked the increase in p38 activity and apoptosis. The MAP kinase kinase inhibitor, PD98059, had no effect on apoptosis in Rat-1 fibroblasts and only partially blocked apoptosis in PC12 cells. PD98059 did not influence insulin's ability to block apoptosis, indicating that the extracellular signal-regulated kinase pathway does not mediate insulin'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.