Abstract
Transforming growth factor-β (TGF-β) signaling promotes cancer progression. In particular, the epithelial-mesenchymal transition (EMT) induced by TGF-β is considered crucial to the malignant phenotype of cancer cells. Here, we report that the EMT-associated cellular responses induced by TGF-β are mediated by distinct signaling pathways that diverge at Smad3. By expressing chimeric Smad1/Smad3 proteins in SMAD3 knockout A549 cells, we found that the β4 region in the Smad3 MH1 domain is essential for TGF-β-induced cell motility, but is not essential for other EMT-associated responses including epithelial marker downregulation. TGF-β was previously reported to enhance cell motility by activating Rac1 via phosphoinositide 3-kinase. Intriguingly, TGF-β-dependent signaling mediated by Smad3's β4 region causes the downregulation of multiple mRNAs that encode GTPase activating proteins that target Rac1 (ARHGAPs), thereby attenuating Rac1 inactivation. Therefore, two independent pathways downstream of TGF-β type I receptor contribute cooperatively to sustained Rac1 activation, thereby leading to enhanced cell motility.