Membrane-bound TGF-beta on tumor-associated mesenchymal stem-like cells initiates epithelial-to-mesenchymal transition in human colorectal cancer

Growing evidence indicates that tumor-associated stroma plays a negative role in human colorectal cancer (CRC). Nature of specific stromal cell populations involved and mechanisms underlying their negative impact remain to be fully understood. In this study we describe the expansion from human primary CRCs of a mesenchymal cell population, referred to as tumor-associated stromal cells (TASCs), resembling bone marrow-derived mesenchymal stem cells (BM-MSCs) in morphology, phenotypes and differentiation potential. We found that, upon co-culture with tumor cells, TASCs acquire membrane-bound TGF-mbTGF-expression, a phenomenon mediated by v6 integrin. MbTGF-expression proved to be critical for triggering epithelial-to-mesenchymal transition (EMT) in tumor cells, eventually leading to enhanced dissemination of circulating tumor cells and increased metastasis formation, in an orthotopic mouse model. Our data identify CRC-associated mesenchymal stem-like cells as critical EMT initiators and suggest mbTGF- as potential novel therapeutic target.