Oncogenic Ras Downregulates Rac Activity, Which Leads to Increased Rho Activity and Epithelial–Mesenchymal Transition

Abstract

Proteins of the Rho family regulate cytoskeletal rearrangements in response to receptor stimulation and are involved in the establishment and maintenance of epithelial cell morphology. We recently showed that Rac is able to downregulate Rho activity and that the reciprocal balance between Rac and Rho activity is a major determinant of cellular morphology and motility in NIH3T3 fibroblasts. Using biochemical pull-down assays, we analyzed the effect of transient and sustained oncogenic Ras signaling on the activation state 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 signaling by oncogenic RasV12 permanently downregulates Rac activity, which leads to upregulation of Rho activity and epithelial–mesenchymal transition. Oncogenic Ras decreases Rac activity through sustained Raf/MAP kinase signaling, which causes transcriptional downregulation of Tiam1, an activator of Rac in epithelial cells. Reconstitution of Rac activity by expression of Tiam1 or RacV12 leads to downregulation of Rho activity and restores an epithelial phenotype in mesenchymal RasV12- or RafCAAX-transformed cells. The present data reveal a novel mechanism by which oncogenic Ras is able to interfere with the balance between Rac and Rho activity to achieve morphological transformation of epithelial cells.

A new superinvasive in vitro phenotype induced by selection of human breast carcinoma cells with the chemotherapeutic drugs paclitaxel and doxorubicin

“…Results show Cdc42, Rac1 and RhoA protein increased in the superinvasive populations. Similar patterns have been observed in Ras-transformed fibroblasts and epithelial cells, and may indicate Ras activation in the selected cells (Zondag et al, 2000;Sahai et al, 2001). RhoA is involved in the generation of contractile force and in moving the body and the tail behind the leading edge.…”

Polarization and Movement of Keratocytes: A Multiscale Modelling Approach

“…Thus, currently the general picture is that Cdc42 activates Rac, which activates Rho (Matozaki et al, 2000). Evidence of mutual inhibition between Cdc42/Rac and Rho has also been found (Van Leeuwen et al, 1997;Sander et al, 1999;Firtel and Chung, 2000;Jiménez et al, 2000;Zondag et al, 2000;Caron, 2003).…”

R-Ras Controls Membrane Protrusion and Cell Migration through the Spatial Regulation of Rac and Rho

“…The regulation of Rho by Ras is not well understood. Oncogenic Ras has been reported to enhance Rho activity, both through the MAP Kinase pathway (Chen et al, 2003) and through the downregulation of Rac (which feeds back to down-regulate Rho; Zondag et al, 2000). In contrast to this, Izawa et al (1998) has also proposed the possible inactivation of Rho and its effector ROCK in Ras-induced transformation.…”

“…For example, activated Rac has been reported to decrease or increase Rho activity in different cell types (Sander et al, 1999;Zondag et al, 2000;Cox et al, 2001;Li et al, 2002). In addition, although activated Rac can lead to increased Rho activity in some cells, this newly active Rho then inhibits Rac activation, suggesting a negative feedback loop by which Rho and Rac activity are precisely regulated (Li et al, 2002).…”