INHIBITION OF PHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY BLOCKS DEPOLARIZATION- AND INSULIN-LIKE GROWTH-FACTOR I-MEDIATED SURVIVAL OF CEREBELLAR GRANULE CELLS

Depolarizing concentrations of potassium promote the survival of many neuronal cell types including cerebellar granule cells. To begin to un derstand the intracellular mediators of neuronal survival, we have tes ted whether the survival-promoting effect of potassium depolarization on cerebellar granule cells is dependent on either mitogen-activated p rotein (MAP) kinase or phosphatidylinositol 3-kinase (PI-3-K) activity . In 7-day cerebellar granule cell cultures, potassium depolarization activated both MAP kinase and PI-3-K. Preventing the activation of MAP kinase with the MEK1 inhibitor PD98059 did not affect potassium savin g. In contrast, the survival-promoting effect of 25 mM potassium was n egated by the addition of 30 mu M LY 294002 or 1 mu M wortmannin, two distinct inhibitors of PI-3-K. The cell death induced by PI-3-K inhibi tion was indistinguishable from the cell death caused by potassium dep rivation; LY 294002-induced death included nuclear condensation, was b locked by cycloheximide, and had the same time course as potassium dep rivation-induced cell death. Cerebellar granule cells can also be main tained in serum free medium containing either 100 ng/ml insulin-like g rowth factor I (IGF-I) or 800 mu M cAMP. PI-3-K inhibition completely blocked the survival-promoting activity of IGF-I, but had no effect on cAMP-mediated survival. These data indicate that the survival-promoti ng effects of depolarization and IGF-I, but not cAMP, require PI-3-K a ctivity.