Arp2/3- and cofilin-coordinated actin dynamics is required for insulin-mediated GLUT4 translocation to the surface of muscle cells.

ABSTRACT

GLUT4 vesicles are actively recruited to the muscle cell surface upon insulin stimulation. Key to this process is Rac-dependent reorganization of filamentous actin beneath the plasma membrane, but the underlying molecular mechanisms have yet to be elucidated. Using L6 rat skeletal myoblasts stably expressing myc-tagged GLUT4, we found that Arp2/3, acting downstream of Rac GTPase, is responsible for the cortical actin polymerization evoked by insulin. siRNA-mediated silencing of either Arp3 or p34 subunits of the Arp2/3 complex abrogated actin remodeling and impaired GLUT4 translocation. Insulin also led to dephosphorylation of the actin-severing protein cofilin on Ser-3, mediated by the phosphatase slingshot. Cofilin dephosphorylation was prevented by strategies depolymerizing remodeled actin (latrunculin B or p34 silencing), suggesting that accumulation of polymerized actin drives severing to enact a dynamic actin cycling. Cofilin knockdown via siRNA caused overwhelming actin polymerization that subsequently inhibited GLUT4 translocation. This inhibition was relieved by reexpressing Xenopus wild-type cofilin-GFP but not the S3E-cofilin-GFP mutant that emulates permanent phosphorylation. Transferrin recycling was not affected by depleting Arp2/3 or cofilin. These results suggest that cofilin dephosphorylation is required for GLUT4 translocation. We propose that Arp2/3 and cofilin coordinate a dynamic cycle of actin branching and severing at the cell cortex, essential for insulin-mediated GLUT4 translocation in muscle cells.