Cross-talk between Ras and Rho signalling pathways in transformation favours proliferation and increased motility

Transformation by oncogenic Ras requires the function of the Rho family GTPases. We find that Ras‐transformed cells have elevated levels of RhoA‐GTP, which functions to inhibit the expression of the cell cycle inhibitor p21/Waf1. These high levels of Rho‐GTP are not a direct consequence of Ras signalling but are selected for in response to sustained ERK–MAP kinase signalling. While the elevated levels of Rho‐GTP control the level of p21/Waf, they no longer regulate the formation of actin stress fibres in transformed cells. We show that the sustained ERK–MAP kinase signalling resulting from transformation by oncogenic Ras down‐regulates ROCK1 and Rho‐kinase, two Rho effectors required for actin stress fibre formation. The repression of Rho‐ dependent stress fibre formation by ERK–MAP kinase signalling contributes to the increased motility of Ras‐transformed fibroblasts. Overexpression of the ROCK target LIM kinase restores actin stress fibres and inhibits the motility of Ras‐transformed fibroblasts. We propose a model in which Ras and Rho signalling pathways cross‐talk to promote signalling pathways favouring transformation.