Transforming growth factor-beta 1 (TGF-(beta 1) is a multifunctional growth factor that plays a role in cell proliferation, differentiation, extracellular matrix production, apoptosis, and cell motility. We show here that TGF-beta 1 increased the invasiveness of MM1 cells, which are a highly invasive clone of rat ascites hepatoma cells. Both mRNA and protein levels of RhoC but not RhoA in TGF-beta 1-treated MM1 cells increased. In parallel with this increase in expression, RhoC activity was induced by TGF-beta 1 treatment. When RhoC was overexpressed in MM1 cells, the invasive capacity increased. The RhoC-overexpressing cells formed more nodules than did mock cells when injected into rat peritoneum. Furthermore, when RhoC expression was reduced by transfection with shRNA/RhoC, the invasiveness of MM1 cells decreased with concomitant suppression of RhoC expression. Thus, the induced expression of RhoC by TGF-beta 1 in MM1 cells plays a critical role in TGF-beta 1-induced cell migration. (c) 2006 Elsevier Inc. All rights reserved.