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Increasing the response to TGF-β signals
TGF-β is a cytokine that binds to cell surface receptors and triggers epithelial-to-mesenchymal transition, a process that promotes the pathological scarring process known as fibrosis and contributes to cancer progression and metastasis. Budi et al. found that insulin, which is released in response to high glucose and used to treat diabetes, enhanced the trafficking of TGF-β receptors to the surface of mouse embryonic fibroblasts and epithelial cells. These results may help to explain why diabetics are prone to fibrosis and how hyperglycemia and insulin could enhance cancer progression and metastasis.
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.