Abstract Epithelial tumor cells often change their epithelial phenotype to mesenchymal one when they invade into surrounding stromal tissue. This change, called epithelial-mesenchymal transition (EMT), is thought to be a crucial event in tumor progression. EMT is known to be regulated by environmental factors. However, it is unclear how EMT contributes to invasive growth of tumor cells. To address this question, we characterized EMT-induced tumor cells under various conditions in vitro. EMT was induced in three human carcinoma cell lines of the lung, pancreas and stomach by treating with TGF-beta and/or TNF-alpha. The EMT treatment of these cell lines induced expression of the invasion marker laminin gamma2 chain and MT1-MMP, in addition to other well-known phenotypes such as the morphological change and vimentin expression. EMT induction enhanced cell motility in monolayer culture. Although EMT cells showed comparable cell growth with the control cells in monolayer culture, their growth rates were extremely suppressed in collagen gel and soft agar culture conditions. Most characteristically, EMT-induced tumor cells commonly and markedly extended invadopodia-like protrusions in collagen gel. Such protrusion formation in collagen gel was effectively blocked by the TGF-beta/Smad pathway inhibitor SB431542 but scarcely suppressed by the synthetic and natural MMP inhibitors, TAPI-1 and TIMP-2, respectively. These data suggest that EMT may enhance interstitial tumor infiltration without cell growth by promoting prominent invadopodia formation and cell motility. The TGF-beta-induced cytoskeletal change seems to play a more critical role than MMP expression in the tumor invasion. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3360. doi:10.1158/1538-7445.AM2011-3360