INHIBITION OF INSULIN-INDUCED GLUT4 TRANSLOCATION BY MUNC18C THROUGH INTERACTION WITH SYNTAXIN4 IN 3T3-L1 ADIPOCYTES

Insulin induces the translocation of vesicles containing the glucose t ransporter GLUT4 from an intracellular compartment to the plasma membr ane in adipocytes. SNARE proteins have been implicated in the docking and fusion of these vesicles with the cell membrane. The role of Munc1 8c, previously identified as an n-Sec1/Munc18 homolog in 3T3-L1 adipoc ytes, in insulin-regulated GLUT4 trafficking has now been investigated in 3T3-L1 adipocytes. In these cells, Munc18c was predominantly assoc iated with syntaxin4, although it bound both syntaxin2 and syntaxin4 t o similar extents in vitro. In addition, SNAP-23, an adipocyte homolog of SNAP-25, associated with both syntaxins 2 and 4 in 3T3-L1 adipocyt es. Overexpression of Munc18c in 3T3-L1 adipocytes by adenovirus-media ted gene transfer resulted in inhibition of insulin-stimulated glucose transport in a virus dose-dependent manner (maximal effect, similar t o 50%) as well as in inhibition of sorbitol-induced glucose transport (by similar to 35%), which is mediated by a pathway different from tha t used by insulin. In contrast, Munc18b, which is also expressed in ad ipocytes but which did not bind to syntaxin4, had no effect on glucose transport. Furthermore, overexpression of Munc18c resulted in inhibit ion of insulin-induced translocation of GLUT4, but not of that of GLUT 1, to the plasma membrane. These results suggest that Munc18c is invol ved in the insulin-dependent trafficking of GLUT4 from the intracellul ar storage compartment to the plasma membrane in 3T3-L1 adipocytes by modulating the formation of a SNARE complex that includes syntaxin4.