Chronic treatment with insulin selectively down-regulates cell-surface GLUT4 glucose transporters in 3T3-L1...

A new method for photoaffinity labeling of glucose transporters has been used to compare the effects of glucose-starvation, acute-insulin, and chronic-insulin treatments on the cell-surface glucose transporters in 3T3-L1 adipocytes. Starvation alone increased the cell-surface levels of GLUT1 and GLUT4 by approximately 4- and approximately 2-fold, respectively. As shown by Calderhead, D, M., Kitagawa, K., Tanner, L.T., Holman, G.D., and Lienhard, G.E. (1990) J. Biol. Chem. 265, 13800-13808) acute-insulin treatment increased cell-surface GLUT1 and GLUT4 by approximately 5- and approximately 15-fold respectively. In contrast to this, chronic-insulin treatment gave a further 3-4-fold increase in both cell-surface and total cellular GLUT1, but availability of GLUT4 at the cell-surface was down-regulated to half the level found in the acute treatment but with no change in the total cellular level. This effect occurred in starved and non-starved cells and suggests that starvation, acute-insulin, and chronic-insulin treatments regulate glucose transporter availability through independent mechanisms. The down-regulation of GLUT4 reached a maximally reduced cell-surface level in 6 h while the rise in GLUT1 reached a maximum after 24-48 h. The rise in GLUT1 appeared to compensate for the decline in cell-surface GLUT4 as glucose transport activity was further increased during the long term treatment with insulin. The down-regulation of GLUT4 due to the chronic-insulin treatment is associated with a marked resistance of the cells to restimulate glucose transport and particularly to recruit further GLUT4 to the cell-surface following an additional insulin treatment. The defect appears to be in the signaling mechanism that is responsible for translocation.