Ral-specific cuanine nucleotide exchange factor activity opposes other Raseffectors in PC12 cells by inhibiting neurite outgrowth

Ras proteins can activate at least three classes of downstream target prote ins: Raf kinases, phosphatidylinositol-3 phosphate (PI3) kinase, and Ral-sp ecific guanine nucleotide exchange factors (Ral-GEFs). In NIH 3T3 cells, ac tivated Ral-GEFs contribute to Ras-induced cell proliferation and oncogenic transformation by complementing the activities of Raf and PI3 kinases. In PC12 cells, activated Raf and PI3 kinases mediate Ras-induced cell cycle ar rest and differentiation into a neuronal phenotype. Here, we show that in P C12 cells, Ral-GEF activity acts opposite to other Ras effecters. Elevation of Ral-GEF activity induced by transfection of a mutant Ras protein that p referentially activates Ral-GEFs, or by transfection of the catalytic domai n of the Ral-GEF Rgr, suppressed cell cycle arrest and neurite outgrowth in duced by nerve growth factor (NGF) treatment. In addition, Rgr reduced neur ite outgrowth induced by a mutant Ras protein that preferentially activates Raf kinases. Furthermore, inhibition of Ral-GEF activity by expression of a dominant negative Ral mutant accelerated cell cycle arrest and enhanced n eurite outgrowth in response to NGF treatment. Ral-GEF activity may functio n, at least in part, through inhibition of the Rho family GTPases, CDC42 an d Rac. In contrast to Ras, which was activated for hours by NGF treatment, Ral was activated for only similar to 20 min. These findings suggest that o ne function of Ral-GEF signaling induced by NGF is to delay the onset of ce ll cycle arrest and neurite outgrowth induced by other Ras effecters. They also demonstrate that Ras has the potential to promote both antidifferentia tion and prodifferentiation signaling pathways through activation of distin ct effector proteins. Thus, in some cell types the ratio of activities amon g Ras effecters and their temporal regulation may be important determinants for cell fate decisions between proliferation and differentiation.