ERK-MAPK signaling coordinately regulates activity of Rac1 and RhoA for tumor cell motility.

We describe two signaling events downstream of ERK-MAP kinase contributing to cell motility in colon carcinoma cells. The Fos family member Fra-1 is expressed in an ERK-dependent manner. Silencing of Fra-1 expression with short interfering RNAs leads to losses of cell polarization, motility, and invasiveness in vitro. These effects of ablating Fra-1 are a consequence of activation of a RhoA-ROCK pathway by beta1-integrin, leading to an increase in the amount of stress fibers and stabilization of focal adhesions. We propose that Fra-1 promotes cell motility by inactivating beta1-integrin and keeping RhoA activity low. This depression of RhoA activity is necessary to permit a second ERK-dependent signaling event via uPAR, the receptor for urokinase-type plasminogen activator, to activate Rac and to drive motility through polarized lamellipodia extension.