Kalirin, a GEF for Rac1, plays an important role in FSTL-1-mediated glucose uptake in skeletal muscle cells

Follistatin-like 1 (FSTL-1) is a novel myokine; however, little is known about its metabolic role. Here, FSTL-1 stimulated glucose uptake in an AMP-activated protein kinase (AMPK)-dependent manner in L6 rat skeletal muscle cells. FSTL-1 increased intracellular calcium concentration. Calcium/calmodulin-dependent protein kinase kinase (CaMKK) inhibition blocked FSTL-1-induced AMPK phosphorylation and glucose uptake. In addition, FSTL-1 stimulated the phosphorylation of p21-activated kinase 1 (PAK1), a small GTPase Rac1 downstream protein. PAK1 knockdown or inhibition of Rac1 blocked FSTL-1-induced glucose uptake; moreover, kalirin, a Rac1 guanine nucleotide exchange factor (GEF), was induced by FSTL-1. Kalirin knockdown with siRNA blocked FSTL-1-induced PAK1 phosphorylation and glucose uptake. Consistent with the induction of Rac1 GEF kalirin, the GTP-bound form of Rac1 was increased by FSTL-1. FSTL-1 increased the production of glucose transporter type 4 (GLUT4) protein and also stimulated the translocation of GLUT4 to the plasma membrane. Translocation of GLUT4 was not observed in cells pre-treated with AMPK inhibitor, Rac1 inhibitor, or kalirin siRNA. In primary myoblast cell culture, FSTL-1 increased glucose uptake in an AMPK-dependent manner. A CaMKK inhibitor or kalirin knockdown blocked FSTL-1-induced glucose uptake. These results suggest that kalirin and Rac1 GEF play important roles in FSTL-1-mediated glucose regulation in skeletal muscle cells.