Oncogenically transformed cells (upper left) degrade a fluorescently-labeled substrate (green).

Oncogenically transformed cells (upper left) degrade a fluorescently-labeled substrate (green) at sites of F-actin condensation into podosomes (red). Inhibition of Rho GTPase function (lower right) impairs podosome assembly and function. Nuclei are stained with DAPI (blue).

Transformation of fibroblasts by oncogenic Src causes disruption of actin stress fibers and formation of invasive adhesions called podosomes. Since the small GTPase Rho stimulates stress fiber formation, Rho inactivation by Src has been thought to be necessary for stress fiber disruption. However, the Martin lab has found that Rho[GTP] levels do not decrease following transformation by activated Src. They oobserved that inactivation of Rho in Src-transformed fibroblasts by dominant negative RhoA or the Rho specific inhibitor C3 exoenzyme disrupted podosome structure as judged by localization of podosome components F-actin, cortactin, and Fish. Inhibition of Rho strongly inhibited Src-induced proteolytic degradation of the extracellular matrix. Furthermore, development of an in situ Rho[GTP] affinity assay allowed the detection of endogenous Rho[GTP] at podosomes, where it colocalized with F-actin, cortactin, and Fish. Therefore, Rho is not globally inactivated in Src-transformed fibroblasts, but is necessary for the assembly and function of structures implicated in tumor cell invasion.

Berdeaux, R., B. Díaz, L. Kim, and G. S. Martin. 2004. Active Rho is localized to podosomes induced by oncogenic Src and is required for their assembly and function. J Cell Biol 166:317-323.