Activation of c-Ha-Ras by nitric oxide modulates survival responsiveness in neuronal PC12 cells

p21(c-Ha-Ras) (Bas) can be activated by the guanine nucleotide exchange fac tor mSOS1 or by S-nitrosylation of cysteine 118 via nitric oxide (NO), To d etermine whether these two Ras-activating mechanisms modulate distinct biol ogical effects, a NO-nonresponsive Ras mutant (Ras(C118S)) was stably expre ssed in the PC12 cells, a cell line that generates NO upon nerve growth-fac tor treatment, We report here that RasC118S functions indistinguishably fro m wild type Ras in activating and maintaining the mSOS1- and Raf-1-dependen t mitogen-activated protein kinase cascade necessary for neuronal different iation. However, continuous (>5 days) exposure to nerve growth factor revea ls that, in contrast to parental or wild-type Ras-overexpressing PC12 cells , Ras(C118S)-expressing PC12 cells cannot sustain the basal interaction bet ween Ras and phosphatidylinositol 3-kinase, This results in spontaneous apo ptosis of these cells despite the presence of nerve growth factor and serum . Thus unique downstream effector interactions and biological outcomes can be differentially modulated by distinct modes of Bas activation.