Spatial and temporal regulation of GLUT4 translocation by flotillin-1 and caveolin-3 in skeletal muscle cells

Skeletal muscle tissue is one of the main sites where glucose uptake occurs in response to insulin. The glucose transporter type-4 (GLUT4) is primarily responsible for the insulin-stimulated increase in glucose uptake. Upon insulin stimulation, GLUT4 is recruited from intracellular reserves to the plasma membrane. The molecular mechanisms that regulate the translocation of GLUT4 to the sarcolemma remain to be fully identified. Here, we demonstrate that GLUT4 is localized to perinuclear stores that contain flotillin-1, a marker of lipid rafts, in skeletal muscle cells. Stimulation with insulin for 10 min results in the translocation of flotillin1/GLUT4-containing domains to the plasma membrane in a PI3K- and PKC zeta-dependent manner. We also demonstrate that caveolin-3, a marker of caveolae, is required for the insulin receptor-mediated activation of the PI3K- dependent pathway, which occurs 2 min after insulin stimulation. In fact, we demonstrate that lack of caveolin-3 significantly reduces insulin-stimulated glucose uptake in caveolin-3 null myotubes by inhibiting both PI3K and Akt, as well as the movement of GLUT4 to the plasma membrane. Interestingly, caveolin-3 moves away from the plasma membrane toward the cytoplasm 5 min after insulin stimulation and temporarily interacts with flotillin-1/GLUT4-containing domains before they reach the sarcolemma, with the consequent movement of the insulin receptor from caveolin-3-containing domains to flotillin-1-containing domains. Such translocation temporally matches the insulin-stimulated movement of Cbl and CrkII in flotillin-1/GLUT4-containing domains, as well as the activation of the GDP-GTP exchange factor C3G. Disruption of flotillin-1-based domains prevents the activation of C3G, movement of GLUT4 to the sarcolemma, and glucose uptake in response to insulin. Thus, the activation of the Cbl/C3G/TC10-dependent pathway, which occurs before flotillin-1/GLUT4-containing domains reach the plasma membrane, is flotillin-1 mediated and follows the activation of the PI3K- mediated signaling. Taken together, these results indicate that flotillin- 1 and caveolin-3 may regulate muscle energy metabolism through the spatial and temporal segregation of key components of the insulin signaling.