Vesicle-associated Membrane Protein 2 Plays a Specific Role in the Insulin-dependent Trafficking of the Facilitative Glucose Transporter GLUT4 in 3T3-L1 Adipocytes

Abstract

Vesicle-associated membrane protein 2 (VAMP2) has been implicated in the insulin-regulated trafficking of GLUT4 in adipocytes. It has been proposed that VAMP2 co-localizes with GLUT4 in a postendocytic storage compartment (Martin, S., Tellam, J., Livingstone, C., Slot, J. W., Gould, G. W., and James, D. E. (1996) J. Cell Biol. 134, 625-635), suggesting that it may play a role distinct from endosomal v-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) such as cellubrevin that are also expressed in adipocytes. The present study examines the effects of recombinant glutathione S-transferase (GST) fusion proteins encompassing the entire cytoplasmic tails of VAMP1, VAMP2, and cellubrevin on insulin-stimulated GLUT4 translocation in streptolysin O permeabilized 3T3-L1 adipocytes. GST-VAMP2 inhibited insulin-stimulated GLUT4 translocation by approximately 35%, whereas GST-VAMP1 and GST-cellubrevin were without effect. A synthetic peptide corresponding to the unique N terminus of VAMP2 also inhibited insulin-stimulated GLUT4 translocation in a dose-dependent manner. This peptide had no effect on either guanosine 5'-3-O-(thio)triphosphate-stimulated GLUT4 translocation or on insulin-stimulated GLUT1 translocation. These results imply that GLUT4 and GLUT1 may undergo insulin-stimulated translocation to the cell surface from separate intracellular compartments. To confirm this, adipocytes were incubated with a transferrin-horseradish peroxidase conjugate to fill the itinerant endocytic system after which cells were incubated with H2O2 and diaminobenzidine. This treatment completely blocked insulin-stimulated movement of GLUT1, whereas in the case of GLUT4, movement to the surface was delayed but still reached similar levels to that observed in insulin-stimulated control cells after 30 min. These results suggest that the N terminus of VAMP2 plays a unique role in the insulin-dependent recruitment of GLUT4 from its intracellular storage compartment to the cell surface.