Metformin Blocks Downregulation of Cell Surface GLUT4 Caused by Chronic Insulin Treatment of Rat Adipocytes

Large decreases in insulin-responsive glucose transport occur in rat adipocytes maintained in culture for 24 h in the continuous presence of insulin. After 24 h in culture, an acute treatment with insulin increased 3-O-methyl-D-glucose transport by only approximately fivefold. In chronically insulin-treated cells, the transport activity was more severely reduced. The transport activity was only approximately twofold higher than in basal cells. To attribute changes in transport to alterations in cell surface transporters, we labeled the cell surface GLUT4 and GLUT1 transporters with the impermeant photoaffinity label 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis(D-mannos -4-yloxy)-2-propylamine. Cell surface labeling was compared with the labeling obtained in digitonin-permeabilized cells where the normally impermeant reagent had access to the total cellular pool of transporters. Labeling showed that in basal cells the proportions of GLUT4 and GLUT1 at the cell surface were 20 and 22% of the total. After an acute treatment with insulin, the proportions of GLUT4 and GLUT1 at the cell surface were increased to 49 and 37% of the total, respectively. The chronic insulin treatment was associated with a very low proportion of GLUT4 (25% of the total) at the cell surface. The downregulation of GLUT4 observed after chronic insulin treatment was alleviated by metformin, and the proportion of GLUT4 at the cell surface was maintained at 60% of the total. Furthermore, cells that were chronically treated with insulin showed severe resistance to subsequent acute insulin restimulation of transport and GLUT4 recruitment to the cell surface. This effect was also alleviated by inclusion of metformin during the chronic insulin treatment.