Rho proteins induce metastatic properties in vivo

Rho proteins have been implicated in the regulation of multiple signal transduction processes. Some of the members of this family, including the rho gene from Aplysia californica and the human genes (rhoA, rhoB and rac-1), are proto-oncogenes since when properly mutated they can induce cell transformation, and the generated rho-transformed cells are tumorigenic when inoculated into mice. In addition to their tumorigenic activity, there is evidence suggesting that Rho proteins may contribute to the metastatic phenotype. However, all the experiments implicating Rho proteins or Rho-regulating proteins in the induction of metastatic potential are either indirect or have been performed in vitro. In this study we investigated whether cells transformed by rho oncogenes do have metastatic potential in vivo. We present evidence that cells transformed by the Aplysia californica rho gene, when injected directly into the blood stream are able to efficiently colonize lungs and secondary organs, consistent with the acquisition of the metastatic potential. Moreover, tumors derived from subcutaneous injections of these rho-transformed cells are also able to metastasize in distant organs, a strong support to the hypothesis that Rho proteins play a role in the metastatic phenotype. Finally, cells transformed by the human oncogenes dbl, vav and ost, three well-known guanine exchange factors for members of the Rho family, or cells transformed by the activated human rac-1 or rhoA genes do also have metastatic potential when injected into the blood stream. These results demonstrate that signaling pathways regulated by Rho proteins play an important role in the acquisition of the metastatic phenotype in vivo.