In Vivo Regulation of TGF-β by R-Ras2 Revealed through Loss of the RasGAP Protein NF1

Ras superfamily proteins participate in TGF-beta-mediated developmental pathways that promote either tumor suppression or progression. However, the specific Ras proteins, which integrate in vivo with TGF-beta signaling pathways, are unknown. As a general approach to this question, we activated all Ras proteins in vivo by genetic deletion of the RasGAP protein Nf1 and examined mice doubly deficient in a Ras protein to determine its requirement in formation of TGF-beta-dependent neurofibromas that arise in Nf1-deficient mice. Animals lacking Nf1 and the Ras-related protein R-Ras2/TC21 displayed a delay in formation of neurofibromas but an acceleration in formation of brain tumors and sarcomas. Loss of R-Ras2 was associated with elevated expression of TGF-beta in Nf1-deficient Schwann cell precursors, blockade of a Nf1/TGF beta RII/AKT-dependent autocrine survival loop in tumor precursor cells, and decreased precursor cell numbers. Furthermore, the increase in size of sarcomas from xenografts doubly deficient in these genes was also found to be TGF-beta-dependent, in this case resulting from cell nonautonomous effects on endothelial cells and myofibroblasts. Extending these findings in clinical specimens, we documented an increase in TGF-beta ligands and an absence of TGF-beta receptor II in malignant peripheral nerve sheath tumors, which correspond to tumors in the Nf1-deficient mouse model. Together, our findings reveal R-Ras2 as a critical regulator of TGF-beta signaling in vivo. Cancer Res; 72(20); 5317-27. (C) 2012 AACR.