Trafficking of glucose transporters in 3T3-L1 cells. Inhibition of trafficking by phenylarsine oxide implicates a slow dissociation of transporters from trafficking proteins

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Abstract
We have compared the rates of insulin stimulation of cell-surface availability of glucose-transporter isoforms (GLUT1 and GLUT4) and the stimulation of 2-deoxy-D-glucose transport in 3T3-L1 cells. The levels of cell-surface transporters have been assessed by using the bismannose compound 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis(D-mannos -4-yloxy) propyl-2-amine (ATB-BMPA). At 27 degrees C the half-times for the appearance of GLUT1 and GLUT4 at the cell surface were 5.7 and 5.4 min respectively and were slightly shorter than that for the observed stimulation of transport activity (t 1/2 8.6 min). This lag may be due to a slow dissociation of surface transporters from trafficking proteins responsible for translocation. When fully-insulin-stimulated cells were subjected to a low-pH washing procedure to remove insulin at 37 degrees C, the cell-surface levels of GLUT1 and GLUT4 decreased, with half-times of 9.2 and 6.8 min respectively. These times correlated well with decrease in 2-deoxy-D-glucose transport activity that occurred during this washing procedure (t1/2 6.5 min). When fully-insulin-stimulated cells were treated with phenylarsine oxide (PAO), a similar decrease in transport activity occurred (t1/2 9.8 min). However, surface labelling showed that this corresponded with a decrease in GLUT4 only (t1/2 7.8 min). The cell-surface level of GLUT1 remained high throughout the PAO treatment. Light-microsome membranes were isolated from cells which had been cell-surface-labelled with ATB-BMPA. Internalization of both transporter isoforms to this pool occurred when cells were maintained in the presence of insulin for 60 min. In contrast with the surface-labelling results, we have shown that the transfer to the light-microsome pool of both transporters occurred in cells treated with insulin and PAO. These results suggest that both transporters are recycled by fluid-phase endocytosis and exocytosis. PAO may inhibit this recycling at a stage which involves the re-emergence of internalized transporters at the plasma membrane. The GLUT1 transporters that are recycled to the surface in insulin- and PAO-treated cells appear to have low transport activity. This may be because of a failure to dissociate fully from trafficking proteins at the cell surface. GLUT4 transporters appear to have a greater tendency to remain internalized if the normal mechanisms that commit transporters to the cell surface, such as dissociation from trafficking proteins, are uncoupled.