Kinetic analysis of glucose transporter trafficking in fibroblasts and adipocytes

Insulin regulates hexose uptake by the redistribution of glucose transport proteins from intracellular compartments to the cell surface. We have submitted the trafficking of GLUT1, GLUT4, and GLUT1/GLUT4 chimeras to a mathematical analysis in the context of different models. Our data suggest that a model with one intracellular and one cell surface compartment can describe the glucose transporter-trafficking kinetics in fibroblasts. Moreover, the difference in cellular distribution between GLUT1 and GLUT4 overexpressed in fibroblasts is best explained by a slower rate of movement of GLUT4 to the plasma membrane. In 3T3-L1 adipocytes, glucose transporter-trafficking kinetics is adequately described by a three-pool model which includes flow of transporters from the endosomal compartment to cell surface. The kinetic roles of previously identified motifs in GLUT4 trafficking were defined in proposed fibroblast and adipocyte glucose transporter-trafficking models. The C-terminus is important in reducing the exocytosis rate from the endosomal compartment to the cell surface in both fibroblasts and adipocytes, and the N-terminus behaves similarly in adipocytes. The C-terminus has an additional signal(s) that allows GLUT4 to be sequestered more efficiently into the insulin responsive vesicle compartment. Mutation of the dileucine motif in the C-terminus significantly reduces the endocytosis of GLUT4 in both fibroblasts and adipocytes, but these amino acids appear not to be primarily responsible for the different kinetics of wild-type GLUT1 and GLUT4.