Transdifferentiation of epithelial cells into cells with mesenchymal properties and appearance, that is, epithelial-mesenchymal transition (EMT), is essential during development, and occurs in pathological contexts, such as in fibrosis and cancer progression. Although EMT can be induced by many extracellular ligands, TGF-β and TGF-β-related proteins have emerged as major inducers of this transdifferentiation process in development and cancer. Additionally, it is increasingly apparent that signaling pathways cooperate in the execution of EMT. This update summarizes the current knowledge of the coordination of TGF-β-induced Smad and non-Smad signaling pathways in EMT, and the remarkable ability of Smads to cooperate with other transcription-directed signaling pathways in the control of gene reprogramming during EMT.

Increased activity of transforming growth factor-β (TGF-β), which binds to and stimulates cell surface receptors, contributes to cancer progression and fibrosis by driving epithelial cells toward a migratory mesenchymal phenotype and increasing the abundance of extracellular matrix proteins. The abundance of TGF-β receptors at the cell surface determines cellular responsiveness to TGF-β, which is often produced by the same cells that have the receptors, and thus serves as an autocrine signal. We found that Akt-mediated phosphorylation of AS160, a RabGAP [guanosine triphosphatase (GTPase)-activating protein], promoted the translocation of TGF-β receptors from intracellular stores to the plasma membrane of mouse embryonic fibroblasts and NMuMG epithelial cells. Consequently, insulin, which is commonly used to treat hyperglycemia and activates Akt signaling, increased the amount of TGF-β receptors at the cell surface, thereby enhancing TGF-β responsiveness. This insulin-induced increase in autocrine TGF-β signaling contributed to insulin-induced gene expression responses, attenuated the epithelial phenotype, and promoted the migration of NMuMG cells. Furthermore, the enhanced delivery of TGF-β receptors at the cell surface enabled insulin to increase TGF-β-induced gene responses. The enhancement of TGF-β responsiveness in response to Akt activation may help to explain the biological effects of insulin, the progression of cancers in which Akt is activated, and the increased incidence of fibroses in diabetes.