INHIBITION OF MAP KINASE KINASE BLOCKS THE DIFFERENTIATION OF PC-12 CELLS INDUCED BY NERVE GROWTH-FACTOR

The mitogen-activated protein kinase (MAP kinase) pathway is thought t o play an important role in the actions of neurotrophins. A small mole cule inhibitor of the upstream kinase activator of MAP kinase, MAP kin ase kinase (MER) was examined for its effect on the cellular action of nerve growth factor (NGF) in PC-12 pheochromocytoma cells. PD98059 se lectively blocks the activity of MEK, inhibiting both the phosphorylat ion and activation of MAP kinases in vitro, Pretreatment of PC-12 cell s with the compound completely blocked the 4-fold increase in MAP kina se activity produced by NGF. Half-maximal inhibition was observed at 2 mu m PD98059, with maximal effects at 10-100 mu M The tyrosine phosph orylation of immunoprecipitated MAP kinase was also completely blocked by the compound. In contrast, the compound was without effect on NGF- dependent tyrosine phosphorylation of the pp140(trk) receptor or its s ubstrate She and did not block NGF-dependent activation of phosphatidy linositol 3'-kinase. However, PD98059 completely blocked NGF-induced n eurite formation in these cells without altering cell viability. These data indicate that the MAP kinase pathway is absolutely required for NGF-induced neuronal differentiation in PC-12 cells.