Akt and Rac1 signaling are jointly required for insulin-stimulated glucose uptake in skeletal muscle and downregulated in insulin resistance

Abstract

Skeletal muscle plays a major role in regulating whole body glucose metabolism. Akt and Rac1 are important regulators of insulin-stimulated glucose uptake in skeletal muscle. However the relative role of each pathway and how they interact are not understood. Here we delineate how Akt and Rac1 pathways signal to increase glucose transport independently of each other and are simultaneously downregulated in insulin resistant muscle. Pharmacological inhibition of Rac1 and Akt signaling was used to determine the contribution of each pathway to insulin-stimulated glucose uptake in mouse muscles. The actin filament-depolymerizing agent LatrunculinB was combined with pharmacological inhibition of Rac1 or Akt, to examine whether either pathway mediates its effect via the actin cytoskeleton. Akt and Rac1 signaling were investigated under each condition, as well as upon Akt2 knockout and in ob/ob mice, to uncover whether Akt and Rac1 signaling are independent and whether they are affected by genetically-induced insulin resistance. While individual inhibition of Rac1 or Akt partially decreased insulin-stimulated glucose transport by ~40% and ~60%, respectively, their simultaneous inhibition completely blocked insulin-stimulated glucose transport. LatrunculinB plus Akt inhibition blocked insulin-stimulated glucose uptake, while LatrunculinB had no additive effect on Rac1 inhibition. In muscles from severely insulin-resistant ob/ob mice, Rac1 and Akt signaling were severely dysregulated and the increment in response to insulin reduced by 100% and 90%, respectively. These findings suggest that Rac1 and Akt regulate insulin-stimulated glucose uptake via distinct parallel pathways, and that insulin-induced Rac1 and Akt signaling are both dysfunctional in insulin resistant muscle. There may thus be multiple treatment targets for improving insulin sensitivity in muscle.

Keywords: 2 deoxy-glucose; 2DG; AS160; Actin cytoskeleton; Akt substrate 160; EDL; FWHM; Full Width at the Half Maximum; GAP; GLUT4; GTPase activating protein; Glucose metabolism; Insulin resistance; Insulin signaling; KO; PAK1; PI3K; Rac1; Rac1 InhibII; Rac1 Inhibitor II; Ras-related C3 botulinum toxin substrate 1; Type 2 diabetes; WT; extensor digitorum longus; knockout; p21 protein-activated kinase 1; phosphatidylinositol 3-kinase; wild-type.