Raf induces TGF beta production while blocking its apoptotic but not invasive responses: a mechanism leading to increased malignancy in epithelial cells

c-Raf-1 is a major effector of Ras proteins, responsible for activation of the ERK MAP kinase pathway and a critical regulator of both normal growth a nd oncogenic transformation. Using an inducible form of Raf in MDCK cells, we have shown that sustained activation of Raf alone is able to induce the transition from an epithelial to a mesenchymal phenotype. Raf promoted inva sive growth in collagen gels, a characteristic of malignant cells; this was dependent on the operation of an autocrine loop involving TGF beta, whose secretion was induced by Raf. TGF beta induced growth inhibition and apopto sis in normal MDCK cells: Activation of Raf led to inhibition of the abilit y of TGF beta to induce apoptosis but not growth retardation. ERK has been reported previously to inhibit TGF beta signaling via phosphorylation of th e linker region of Smads, which prevents their translocation to the nucleus . However, we found no evidence in this system that ERK can significantly i nfluence the function of Smad2, Smad3, and Smad4 at the level of nuclear tr anslocation, DNA binding, or transcriptional activation. Instead, strong ac tivation of Raf caused a broad protection of these cells from various apopt otic stimuli, allowing them to respond to TGF beta with increased invasiven ess while avoiding cell death. The Raf-MAP kinase pathway thus synergizes w ith TGF beta in promoting malignancy but does not directly impair TGF beta -induced Smad signaling.