The Roles Of Lipid Rafts And Caveolae In The Regulation Of GLUT4 Translocation In 3T3-L1 Adipocyte

Caveolae and non-caveolar lipid rafts are two types of membrane lipid microdomains that play an important role in insulin-stimulated glucose uptake in adipocytes. However, they cannot be discriminated from each other because of the limitation of the current methods and are often equated. In order to ascertain their specific functions in the process, caveolae were ablated by caveolin-1 RNA interference. In Cav-1 RNAi adipocyte, neither insulin stimulated glucose uptake nor GLUT4 (glucose transporter 4) translocation to membrane lipid microdomains were affected by the ablation of caveolae. With a modified sucrose density gradient, caveolae and non-caveolar lipid rafts could be separated. Interestingly, the similar lipid microdomain corresponding to caveolae still existed in RNAi cells, which shed light on the speciality of the lipid and the protein compostions. In the wild type 3T3-L1 adipocyte, GLUT4 was found to be translocated into both caveolae and non-caveolar lipid rafts. However, in Cav-1 RNAi adipocyte, GLUT4 was localized predominantly in non-caveolar lipid rafts. After the removal of insulin, caveolae-localized GLUT4 was internalized faster than non-caveolar lipid rafts-associated GLUT4. The internalization of GLUT4 from plasma membrane was significantly decreased in Cav-1 RNAi adipocyte. These results suggested that insulin stimulated GLUT4 translocation and glucose uptake is a caveolae-independent event. Lipid rafts and caveolae mostly differently functioned in the regulation of GLUT4 trafficking. Lipid rafts took charge of insulin signaling and caveolae played a function in the internalization of GLUT4 from plasma membrane after the removal of insulin.