Perturbation of dynamin II with an amphiphysin SH3 domain increases GLUT4 glucose transporters at the plasma membrane in 3T3-L1 adipocytes. Dynamin II participates in GLUT4 endocytosis.

The GLUT4 glucose transporter continuously recycles between the cell surface and an endosomal compartment in adipocytes. Insulin decreases the rate of GLUT4 endocytosis in addition to increasing its exocytosis. Endocytosis of the transporter is thought to occur at least in part via the clathrin-mediated endocytic system. The protein dynamin is involved in the final stages of clathrin-coated vesicle formation. Here we show that the dynamin II isoform is expressed in 3T3-L1 adipocytes and is present in isolated plasma membrane and low density microsomal fractions. Insulin reduced the levels of dynamin II associated with the plasma membrane by about half, raising the possibility that the hormone may reduce GLUT4 endocytosis by removing dynamin from the cell surface. A fusion protein containing the amphiphysin SH3 domain selectively bound dynamin II from 3T3-L1 adipocyte cell lysates. Microinjection of the fusion protein into these cells inhibited transferrin endocytosis and increased the levels of GLUT4 at the cell surface. Glutathione S-transferase alone, the SH3 domains of spectrin and Crk, and a mutated amphiphysin SH3 domain unable to bind dynamin II did not affect GLUT4 distribution. However, a peptide containing the dynamin II sequence that binds amphiphysin increased the surface presence of GLUT4. Moreover, in cells first treated with insulin to externalize GLUT4, the dynamin peptide, but not an unrelated control peptide, inhibited GLUT4 internalization upon insulin removal. These results suggest that interactions of dynamin II with amphiphysin may play an important role in GLUT4 endocytosis. We hypothesize that insulin may reduce GLUT4 endocytosis by regulating the function of dynamin II at the cell surface, as part of the mechanism to increase glucose uptake.