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Curr Biol. 1997 Jan 1;7(1):63-70.

R-Ras can activate the phosphoinositide 3-kinase but not the MAP kinase arm of the Ras effector pathways.

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Imperial Cancer Research Fund, 44 Lincoln's Inn Fields, London WC2A 3PX, UK.

Erratum in

  • Curr Biol 1997 Mar 1;7(3):197.



The small GTPase R-Ras displays a less potent transforming activity than the closely related Ras oncogene products. Although R-Ras 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.


Both Ras and R-Ras interact with phosphoinositide (PI) 3-kinase in vitro, and induce elevation of the levels of PI 3-kinase lipid products in intact cells. Unlike Ras, R-Ras does not activate Raf or mitogen-activated protein (MAP) kinase in cells. In co-transfection assays, the serine/threonine protein kinase PKB (or Akt) is effectively stimulated by R-Ras, Ras, mutants of Ras that activate PI 3-kinase but not other effectors, and activated forms of PI 3-kinase. Ras and R-Ras stimulate PKB/Akt through a non-autocrine mechanism that involves 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 stress-activated protein kinase, Jun N-terminal kinase. Transformation assays in fibroblasts suggest that PKB/Akt and Raf are part of distinct oncogenic signalling pathways.


Both the Raf-MAP kinase and 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 downstream 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.

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