Multiple Ras Effector Pathways Contribute to G(1) Cell Cycle Progression

The involvement of Ras in the activation of multiple early signaling pathways is well understood, but it is less clear how the various Ras effectors interact with the cell cycle machinery to cause G(1) progression. Ras-mediated activation of extracellular-regulated kinase/mitogen-activated protein kinase has been implicated in cyclin D(1) up-regulation, but there is little extracellular-regulated kinase activity during the later stages of G(1), when cyclin D(1) expression becomes maximal, implying that other effector pathways may also be important in cyclin D(1) induction. We have addressed the involvement of Ras effectors from the phosphatidylinositol (PI) 3-kinase and Ral-GDS families in G(1) progression and compared it to that of the Raf/mitogen-activated protein kinase pathway. PI 3-kinase activity is required for the expression of endogenous cyclin D(1) and for S phase entry following serum stimulation of quiescent NIH 3T3 fibroblasts. Activated PI 3-kinase induces cyclin D(1) transcription and E2F activity, at least in part mediated by the serine/threonine kinase Akt/PKB, and to a lesser extent the Rho family GTPase Rac. In addition, both activated Ral-GDS-like factor and Raf stimulate cyclin D(1) transcription and E2F activity and act in synergy with PI 3-kinase. Therefore, multiple cooperating pathways mediate the effects of Ras on progression through the cell cycle.