PHOSPHORYLATION OF TROPOMODULIN3 BY AMPK REGULATES GLUT4 TRANSLOCATION AND GLUCOSE UPTAKE IN L6 MYOBLASTS

Abstract

Insulin and muscle contractions mediate translocation of glucose transporter 4 (GLUT4) to the plasma membrane for glucose uptake in skeletal muscles. Tropomodulin3 (Tmod3), an actin-capping protein that blocks the elongation and de-polymerization of actin filaments at the pointed end, is an Akt2 substrate and plays a vital role in GLUT4 fusion with the plasma membrane and glucose uptake under insulin-stimulated condition. It is well understood that insulin promotes GLUT4 translocation and glucose uptake in muscles and adipocytes by PI3K/Akt signaling pathway. Although it is believed that AMPK (5’ adenosine monophosphate-activated protein kinase) signaling regulates GLUT4 translocation during muscle contraction, the exact regulating mechanisms are not clearly elucidated. Bioinformatics and biochemical analysis identified Ser25 residue of Tmod3 as a potential site of phosphorylation. Ser25 phosphorylation was required for activated-AMPK-induced GLUT4 translocation and glucose uptake in L6 myoblasts, providing new mechanistic insights in the regulation of glucose homeostasis. Furthermore, Tmod3 played a key role in AMPK-induced actin re-modeling and in activated-AMPK-stimulated GLUT4 fusion with the plasma membrane and glucose uptake. Tmod3 is identified as a key downstream effector of AMPK in the regulation of GLUT4 translocation and glucose homeostasis in muscle cells.