COMPARISON OF GLUT4 AND GLUT1 SUBCELLULAR TRAFFICKING IN BASAL AND INSULIN-STIMULATED 3T3-L1 CELLS

The two glucose transporter isoforms GLUT4 and GLUT1 present in 3T3-L1 cells were labeled in the insulin-stimulated and basal states with the impermeant bis-mannose photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl- 1,3-bis-(D-mannos-4-yloxy)-2-propylamine. The redistributions of these labeled transporters from the plasma membrane to the low density microsome membrane fraction were followed while cells were maintained at either insulin-stimulated or basal steady states. In both these steady states GLUT4 and GLUT1 were continuously recycled. Analysis of the time courses for tracer-tagged GLUT4 and GLUT1 redistribution showed that the endocytosis rate constants were only almost-equal-to 30% slower in the insulin-stimulated (0.08 and 0.093 min-1) compared with the basal (0.116 and 0.121 min-1) state. In the insulin-stimulated state, the rate constants for GLUT4 and GLUT1 exocytosis (0.086 and 0.096 min-1) were similar to those of endocytosis. In contrast, the exocytosis rate constants of GLUT4 and GLUT1 in the basal state were 0.01 and 0.035 min-1. We therefore conclude that the main effect of insulin is to increase GLUT4 and GLUT1 exocytosis rate constants by almost-equal-to 9- and 3-fold, respectively, and that the unique feature of the GLUT4 isoform is the very slow rate of exocytosis in the basal state.