Transforming growth factor-beta (TGF-beta) cooperates with oncogenic members of the Ras superfamily to promote cellular transformation and tumor progression. Apart from the classic (H-, K-, and N-) Ras GTPases, only the R-Ras subfamily (R-Ras, R-Ras2/TC21, and R-Ras3/M-Ras) has significant oncogenic potential. In this study, we show that oncogenic R-Ras transformation of EpH4 cells requires TGF-beta signaling. When murine EpH4 cells were stably transfected with a constitutively active R-Ras(G38V) mutant, they were no longer sensitive to TGF-beta-mediated growth inhibition and showed increased proliferation and transformation in response to exogenous TGF-beta. R-Ras/EpH4 cells require TGF-beta signaling for transformation to occur and they produce significantly elevated levels of endogenous TGF-beta, which signals in an autocrine fashion. The effects of TGF-beta are independent of Smad2/3 activity and require activation of TGF-beta-associated kinase 1 (TAK1) and its downstream effectors c-Jun NH(2)-terminal kinase and p38 mitogen-activated protein kinase as well as the phosphoinositide 3-kinase/Akt and mammalian target of rapamycin pathways. Thus, TAK1 is a novel link between TGF-beta signaling and oncogenic R-Ras in the promotion of tumorigenesis.