Smad7 is required for TGF-beta-induced activation of the small GTPase Cdc42.

Transforming growth factor beta (TGF-beta) is a potent regulator of cell growth and differentiation in many cell types. The Smad signaling pathway constitutes a main signal transduction route downstream of TGF-beta receptors. The inhibitory Smads, Smad6 and Smad7, are considered to function as negative regulators of the TGF-beta/Smad signaling cascade. In a previous study, we found that TGF-beta induces rearrangements of the actin filament system in human prostate carcinoma cells and that this response requires the small GTPases Cdc42 and RhoA. On the basis of the current view on the function of Smad7 in TGF-beta signaling, we hypothesized that Smad7 would function as a negative regulator of the TGF-beta-induced activation of Cdc42 and RhoA, but instead we found that the reverse is the case; Smad7 is required for the TGF-beta-induced activation of Cdc42 and the concomitant reorganization of the actin filament system. These observations propose a novel role for Smad7 in TGF-beta-dependent activation of Rho GTPases.