Transforming growth factor-beta signaling during epithelial-mesenchymal transformation: implications for embryogenesis and tumor metastasis.

The molecular mechanisms of epithelial-mesenchymal transformation (EMT) have long been studied to gain a greater understanding of this distinct change in cellular morphology. Early studies of the developing embryo have designated the involvement of Wnt signaling in EMT, through an activated complex of the lymphoid-enhancing factor-1 (LEF-1) transcription factor and the cell adhesion molecule beta-catenin. However, more recent studies have implicated a significant role of the transforming growth factor-beta (TGF-beta) in causing EMT in both development and pathology. The ability of TGF-beta isoforms to signal through a variety of molecules such as Smads, phosphatidylinositol 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK) creates an incredible complexity as to their role in this transition. Here we assess the biochemical signaling pathways of TGF-beta and their potential cross-interaction with traditional Wnt signaling molecules to bring about EMT during embryogenesis and tumor metastasis.