The epithelial-derived type II transmembrane serine protease epithin has been shown to be upregulated in a variety of cancer cell lines and tumor tissues, and its upregulation correlates well with tumor progression in many cases. However, little is known regarding the regulation of its expression and the mechanism of its roles in tumor progression. Here, we show that transforming growth factor-beta (TGF-beta), a potent inducer of epithelial-mesenchymal transition (EMT) in tumor progression, upregulates epithin, and that epithin plays a critical role in TGF-beta-induced EMT. Forced overexpression of epithin induced EMT to exhibit characteristic morphological changes, alternations in EMT-related proteins and enhanced cell motility. Conversely, shRNA-mediated knockdown of endogenous epithin inhibited TGF-beta-induced expression of mesenchymal markers and morphological changes. Furthermore. TGF-beta-induced cell migration and invasion were significantly impaired by epithin knockdown. In addition, we demonstrate that TGF-beta upregulates epithin transcriptionally via the Smad2/Smad4-mediated pathway. These results suggest that epithin is a key mediator of TGF-beta-induced EMT in tumor progression. (C) 2010 Elsevier Inc. All rights reserved.