Raf-MEK-Erk Cascade in Anoikis Is Controlled by Rac1 and Cdc42 via Akt

Abstract

Signals from the extracellular matrix are essential for the survival of many cell types. Dominant-negative mutants of two members of Rho family GTPases, Rac1 and Cdc42, mimic the loss of anchorage in primary mouse fibroblasts and are potent inducers of apoptosis. This pathway of cell death requires the activation of both the p53 tumor suppressor and the extracellular signal-regulated mitogen-activated protein kinases (Erks). Here we characterize the proapoptotic Erk signal and show that it differs from the classically observed survival-promoting one by the intensity of the kinase activation. The disappearance of the GTP-bound forms of Rac1 and Cdc42 gives rise to proapoptotic, moderate activation of the Raf-MEK-Erk cascade via a signaling pathway involving the kinases phosphatidlyinositol 3-kinase and Akt. Moreover, concomitant activation of p53 and inhibition of Akt are both necessary and sufficient to signal anoikis in primary fibroblasts. Our data demonstrate that the GTPases of the Rho family control three major components of cellular signal transduction, namely, p53, Akt, and Erks, which collaborate in the induction of apoptosis due to the loss of anchorage.

ACKNOWLEDGMENTS

We are grateful to A. Brunet, P. Chavrier, A. Hall, P. Lenormand, C. Marshall, C. Norbury, S. Roche, M. Vandromme, M. White, and E. Yonish-Rouach for various plasmids used in this work and to P. Boquet for the generous gift of C. difficile toxin B. We thank Damien Gregoire for help with the anoikis experiments, Pierre Travo for help with immunofluorescence microscopy, and Bob Hipskind for invaluable comments on the manuscript.

This work was supported by INSERM, CNRS, and Association pour la Recherche contre le Cancer (support given to U.H.). T.F.F. is the recipient of Career Development Award DAMD17-00-1-0214 and O.Z. is the recipient of a fellowship from Association pour la Recherche contre le Cancer.