Activation of the small GTPase Rac1 by a specific guanine-nucleotide-exchange factor suffices to induce glucose uptake into skeletal-muscle cells

Background information. Insulin-stimulated glucose uptake into skeletal muscle is crucial for glucose homoeostasis, and depends on the recruitment of GLUT4 (glucose transporter 4) to the plasma membrane. Mechanisms underlying insulin-dependent GLUT4 translocation, particularly the role of Rho family GTPases, remain controversial. Results. In the present study, we show that constitutively active Rac1, but not other Rho family GTPases tested, induced GLUT4 translocation in the absence of insulin, suggesting that Rac1 activation is sufficient for GLUT4 translocation in muscle cells. Rac1 activation occurred in dorsal membrane ruffles of insulin-stimulated cells as revealed by a novel method to visualize activated Rac1 in situ. We further identified FLJ00068 as a GEF (guanine-nucleotide-exchange factor) responsible for this Rac1 activation. Indeed, constitutively active FLJ00068 caused Rac1 activation in dorsal membrane ruffles and GLUT4 translocation without insulin stimulation. Down-regulation of Rac1 or FLJ00068 by RNA interference, on the other hand, abrogated insulin-induced GLUT4 translocation. Basal, but not insulin-stimulated, activity of the serine/threonine kinase Akt was required for the induction of GLUT4 translocation by constitutively active Rac1 or FLJ00068. Conclusion. Collectively, Rac1 activation specifically in membrane ruffles by the GEF FLJ00068 is sufficient for insulin induction of glucose uptake into skeletal-muscle cells.