Oncogenic Ras downregulates Rac activity, which leads to increased Rho activity and epithelial-mesenchymal transition

Proteins of the Rho family regulate cytoskeletal rearrangements in response to receptor stimulation and are involved in the establishment and maintena nce of epithelial cell morphology. We recently showed that Rac is able to d ownregulate Rho activity and that the reciprocal balance between Rac and Rh o activity is a major determinant of cellular morphology and motility in NI H3T3 fibroblasts. Using biochemical pull-down assays, we analyzed the effec t of transient and sustained oncogenic Ras signaling on the activation stat e of Rac and Rho in epithelial MDCK cells. In contrast to the activation of Rac by growth factor-induced Ras signaling, we found that sustained signal ing by oncogenic RasV12 permanently downregulates Rac activity, which leads to upregulation of Rho activity and epithelial-mesenchymal transition. Onc ogenic Ras decreases Rac activity through sustained Raf/MAP kinase signalin g, which causes transcriptional downregulation of Tiam1, an activator of Ra c in epithelial cells. Reconstitution of Rac activity by expression of Tiam 1 or RacV12 leads to downregulation of Rho activity and restores an epithel ial phenotype in mesenchymal RasV12- or RafCAAX-transformed cells. The pres ent data reveal a novel mechanism by which oncogenic Ras is able to interfe re with the balance between Rac and Rho activity to achieve morphological t ransformation of epithelial cells.