LS-1-IN-2399 Regulated exocytosis and diabetes

Diabetes is the result of insufficient insulin secretion from pancreatic beta-cells and impaired insulin signaling in the muscle and adipose tissue. At the cellular level, insulin signaling stimulates glucose transporter 4 (GLUT4) to translocate from intracellular stores to the cell surface in muscle and adipose cells to promote glucose clearance from the circulation. Insulin-stimulated GLUT4-storage vesicle (GSV) translocation and fusion with the plasma membrane is mediated by Akt and its downstream effectors. To discover novel Akt substrates in the regulation of GSV exocytosis, we performed GST pulldown experiments and mass spectrometry. We identify actin-capping protein Tropomodulin 3 as a novel Akt2-interacting partner in 3T3-L1 adipocytes. We demonstrate that Tropomodulin 3 is phosphorylated at Ser71 upon insulin-stimulated Akt2 activation, and Ser71 phosphorylation is required for insulin-stimulated GLUT4 insertion into the plasma membrane and glucose uptake. Phosphorylated Tropomodulin 3 regulates insulin-induced actin remodeling, an essential step for GSV fusion with the plasma membrane. Furthermore, interaction of Tropomodulin 3 with its cognate tropomyosin partner, Tm5NM1 is necessary for GSV exocytosis and glucose uptake. Together, these results establish Tropomodulin 3 as a novel Akt2 effector that mediates insulin-induced cortical actin remodeling and subsequent GLUT4 plasma membrane insertion. Our findings suggest that defects in cytoskeletal remodeling may contribute to impaired GLUT4 exocytosis and glucose uptake.