Adipocytes from Munc18c-null mice show increased sensitivity to insulin-stimulated GLUT4 externalization

MUNC-ing around with insulin action

Defective uptake of glucose into muscle and fat cells, or insulin resistance, is a central feature of obesity and type 2 diabetes. As we brace ourselves for the diabetes epidemic, it is reassuring to know that real progress is being made in defining the molecular biology of how insulin stimulates glucose uptake and what goes awry in obesity and type 2 diabetes. An understanding of the molecular determinants of insulin-stimulated glucose transport has been one of the holy grails of hormone action research. A major breakthrough was the discovery that insulin stimulates the translocation of a specific glucose transport protein, GLUT4, from intracellular vesicles to the cell surface. Elucidating how this process is regulated has remained a challenge because it represents a convergence of 2 disparate and complex fields of research--namely, vesicle transport and signal transduction. A study reported in this issue of the JCI using mice lacking Munc18c, one of the vesicle-trafficking proteins involved in GLUT4 translocation, has provided new insights into the signaling/trafficking intersection that controls insulin-stimulated GLUT4 movement.

Insulin signaling and the regulation of glucose transport

Gaps remain in our understanding of the precise molecular mechanisms by which insulin regulates glucose uptake in fat and muscle cells. Recent evidence suggests that insulin action involves multiple pathways, each compartmentalized in discrete domains. Upon activation, the receptor catalyzes the tyrosine phosphorylation of a number of substrates. One family of these, the insulin receptor substrate (IRS) proteins, initiates activation of the phosphatidylinositol 3-kinase pathway, resulting in stimulation of protein kinases such as Akt and atypical protein kinase C. The receptor also phosphorylates the adapter protein APS, resulting in the activation of the G protein TC10, which resides in lipid rafts. TC10 can influence a number of cellular processes, including changes in the actin cytoskeleton, recruitment of effectors such as the adapter protein CIP4, and assembly of the exocyst complex. These pathways converge to control the recycling of the facilitative glucose transporter Glut4.