Evidence for and against a pivotal role of PI 3-kinase in a neuronal cell survival pathway

PI 3-kinase has emerged as a key enzyme for regulating neuronal cell survival. However, it has not as yet been demonstrated whether activation of the endogenous pool of the enzyme, that is regulated by the p85 subunit, is sufficient to promote a survival response. It is also not known whether the FGF family of growth factors promote survival via a PI 3-kinase-dependent pathway. We have previously developed a cell permeable p85 binding peptide and shown that it can stimulate a mitogenic response in muscle cells that is dependent on a PI 3-kinase/p70 se kinase pathway. In the present study we show that this peptide can rescue cerebellar granule cells from death induced by serum deprivation and that this response is comparable to a growth factor response (FGF2). Experiments with wortmannin, LY294002, and rapamycin suggest that the peptide survival response is dependent on PI 3-kinase activity, but not p70 S6 kinase activity. The peptide response was correlated with a PI 3-kinase-dependent phosphorylation of Akt, an established downstream effector in the PI 3-kinase survival cascade. In contrast to the survival response stimulated by the p85 binding peptide, the response stimulated by FGF2 was not inhibited by wortmannin or LY294002, nor was it associated with phosphorylation of AM. Thus we can conclude that activation of the endogenous pool of PI 3-kinase that is regulated by p85 is sufficient for cell survival; however, growth factors such as FGF2 can clearly support survival in a PI 3-kinase-independent manner.