Role for RalA downstream of Rac1 in skeletal muscle insulin signalling.

ABSTRACT

Insulin-stimulated glucose uptake in skeletal muscle is mediated by the translocation of the glucose transporter GLUT4 from intracellular storage sites to the plasma membrane. The small GTPase Rac1 has been implicated in this insulin signalling, but the mechanism whereby Rac1 stimulates GLUT4 translocation remains obscure. In the present study, we examined the role of the small GTPase RalA downstream of Rac1 in skeletal muscle fibres isolated from genetically modified mice. A dominant-negative mutant of RalA, when ectopically overexpressed, significantly reduced GLUT4 translocation in response to insulin or either one of constitutively activated mutants of Rac1 and its upstream regulators, including the guanine-nucleotide-exchange factor FLJ00068, the protein kinase Akt2 and phosphoinositide 3-kinase. Constitutively activated Rac1 also failed to induce GLUT4 translocation in mouse skeletal muscle fibres in which the expression of RalA was abrogated by specific siRNA molecules. Furthermore, we applied a novel approach to detect the activated form of RalA in situ by immunofluorescence microscopy of mouse skeletal muscle fibres, demonstrating that constitutively activated mutants of Rac1 and its upstream regulators as well as insulin indeed cause the activation of RalA. Notably, this RalA activation was remarkably impaired in rac1-deficient skeletal muscle fibres. Taken together, these results provide evidence that RalA is indeed activated and involved in the regulation of GLUT4 translocation in response to insulin downstream of Rac1 in mouse skeletal muscle.