The spectrum of tumors associated with oncogenic Ras in humans often differs from those in mice either treated with carcinogens or engineered to sporadically express oncogenic Ras, suggesting that the mechanism of Ras transformation may be different in humans. Ras stimulates primarily three main classes of effector proteins, Rafs, PI3-kinase, and RalGEFs, with Raf generally being the most potent at transforming murine cells. Using oncogenic Ras mutants that activate single effectors as well as constitutively active effectors, we find that the RalGEF, and not the Raf or PI3-kinase pathway, is sufficient for Ras transformation in human cells. Thus, oncogenic Ras may transform murine and human cells by distinct mechanisms, and the RalGEF pathway-previously deemed to play a secondary role in Ras transformation-could represent a new target for anti-cancer therapy. Extracellular signals detected by cell-surface receptors can be transmitted to the cell by stimulating the conversion of the Ras oncoprotein from an inactive GDP-bound to an active GTP-bound state. In the GTP-bound state, Ras stimulates downstream targets, or effectors, which, in turn, affect numerous activities of the cell, such as proliferation, apoptosis, and differentiation (Shields et al. 2000). Mutated and constitutively activated forms of Ras are found in 30% of all human cancers (Bos 1989). Significant experimental and epidemiological evidence supports that such aberrant Ras activation plays a critical role in human oncogenesis. Consequently, there is considerable effort to develop inhibitors of the Ras-signaling pathway for use as novel anticancer drugs (Kloog and Cox 2000;Shields et al. 2000).Ras transformation is mediated by interactions with multiple downstream effectors whose contribution to Ras transformation has been evaluated primarily in rodent fibroblast model cell systems. Substantial evidence from these studies supports the role of Raf serine/threonine kinases (c-Raf-1, A-Raf, and B-Raf) as key effectors and mediators of Ras-transforming activity. Raf kinases phosphorylate and activate the MEK1 and MEK2 dual specificity kinases, which, in turn, phosphorylate and activate the ERK1/p44 and ERK2/p42 mitogen-activated protein kinases (MAPKs) (Kyriakis et al. 1992). The critical contribution of the Raf/MEK/ERK pathway to Ras transformation is shown by the ability of constitutively activated mutants of Raf or MEK to cause tumorigenic transformation of NIH 3T3 cells (Bonner et al. 1985;Stanton et al. 1989;Leevers et al. 1994;Stokoe et al. 1994). Additionally, dominant-negative mutants of Raf-1, MEK, and ERK, as well as pharmacologic inhibitors of MEK, have been shown to effectively block Ras transformation in vitro or suppress tumorigenic growth of cancer cell lines in vivo (Kolch et al. 1991;Schaap et al. 1993;Cowley et al. 1994;Westwick et al. 1994;Khosravi-Far et al. 1995;Qiu et al. 1995;Monia et al. 1996;Sebolt-Leopold et al. 1999).Perhaps the second best-characterized effector important for Ras transformation is phosphatidylinositol 3-kina...

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