Ras regulates sympathetic neuron survival by suppressing the p53-mediated cell death pathway

In this report, we examine how the Ras protein regulates neuronal survival, focusing on sympathetic neurons. Adenovirus-expressed constitutively activ ated Ras (RasV12) enhanced survival and the phosphorylation of Akt (protein kinase B) and MAP kinase (MAPK), two targets of Ras activity. Functional i nhibition of endogenous Ras by adenovirus-expressed dominant-inhibitory Ras (N17Ras) decreased nerve growth factor (NGF) dependent survival and both A kt and MAPK phosphorylation as well. To determine the signaling pathways th rough which Ras mediates survival, we used Ras effector mutants and pharmac ological inhibitors that selectively suppress phosphatidylinositol 3-kinase (PI3-K)/Akt or MAP kinase kinase (MEK)/MAPK pathways. The Ras effector mut ant Ras V(12)Y40C, which selectively stimulates PI3-K and Akt, rescued surv ival in the absence of NGF, and the PI3-K inhibitor LY 294002 inhibited bot h Ras- and NGF-dependent survival. Ras(V12)T(35)S, which activates MEK/MAPK but not PI3-K/Akt, was less effective at rescuing survival, whereas the ME K inhibitor PD 098059 also partially suppressed Ras-dependent survival. To investigate the mechanisms by which Ras suppresses neuronal death, we exami ned whether Ras functions by inhibiting the proapoptotic p53 pathway (Jun-N -terminal kinase/p53/BAX) that is necessary for neuronal death after NGF wi thdrawal and p75NTR activation. We found that RasV12 suppressed c-jun, BAX, and p53 levels, whereas inhibition of NGF-induced Ras-survival activity vi a N17Ras increased the levels of these proteins. Furthermore, the E1B55K pr otein, which suppresses p53 activity, blocked N17Ras-induced neuronal death . Together, these results indicate that Ras is, in part, both necessary and sufficient for survival of sympathetic neurons and that this effect is med iated by activation of both the PI3-K- and MEK-signaling cascades, which in turn suppress a proapoptotic p53 pathway.