R-RAS CAN ACTIVATE THE PHOSPHOINOSITIDE 3-KINASE BUT NOT THE MAP KINASE ARM OF THE RAS EFFECTOR PATHWAYS

Background: The small GTPase R-Ras displays a less potent transforming activity than the closely related Ras oncogene products. Although R-R as has been reported to interact with c-Raf1 and Ral-GDS in vitro, the pathways by which it exerts its effects on cellular proliferation are not known. Results: Both Pas and R-Ras interact with phosphoinositide (PI) 3-kinase in vitro, and induce elevation of the levels of PI 3-ki nase lipid products in intact cells. Unlike Ras, R-Ras does not activa te Raf or mitogen-activated protein (MAP) kinase in cells. In co-trans fection assays, the serine/threonine protein kinase PKB (or Akt) is ef fectively stimulated by R-Ras, Ras, mutants of Ras that activate PI 3- kinase but not other effecters, and activated forms of PI 3-kinase. Ra s and R-Ras stimulate PKB/Akt through a non-autocrine mechanism that i nvolves PI 3-kinase, The constitutive activation of PI 3-kinase alone is sufficient to activate PKB/Akt, but not the MAP kinase ERK or the s tress-activated protein kinase, Jun N-terminal kinase. Transformation assays in fibroblasts suggest that PKB/Akt and Raf are part of distinc t oncogenic signalling pathways. Conclusion: Both the Raf-MAP kinase a nd PI 3-kinase-PKB/Akt pathways are activated by Ras, but only the PI 3-kinase-PKB/Akt pathway is activated by R-Ras. PI 3-kinase, and downs tream targets such as PKB/Akt, are likely to be essential mediators of transformation induced by R-Ras. PI 3-kinase, as well as Raf, is thus implicated also in Ras transformation.