Crucial role of the small GTPase Rac1 in insulin‐stimulated translocation of glucose transporter 4 to the mouse skeletal muscle sarcolemma

Abstract

The Rho family GTPase Rac1 has been implicated in the regulation of glucose uptake in myoblast cell lines. However, no evidence for the role of Rac1 has been provided by a mouse model. The purpose of this study is to test the involvement of Rac1 in insulin action in mouse skeletal muscle. Intravenous administration of insulin indeed elicited Rac1 activation in gastrocnemius muscle, suggesting the involvement of Rac1 in this signaling pathway. We then examined whether insulin-stimulated translocation of the facilitative glucose transporter GLUT4 from its storage sites to the skeletal muscle sarcolemma depends on Rac1. We show that ectopic expression of constitutively activated Rac1, as well as intravenous administration of insulin, caused translocation of GLUT4 to the gastrocnemius muscle sarcolemma, as revealed by immunofluorescent staining of a transiently expressed exofacial epitope-tagged GLUT4 reporter. Of particular note, insulin-dependent, but not constitutively activated Rac1-induced, GLUT4 translocation was markedly suppressed in skeletal muscle-specific rac1-knockout mice compared to control mice. Immunogold electron microscopic analysis of endogenous GLUT4 gave similar results. Collectively, we propose a critical role of Rac1 in insulin-dependent GLUT4 translocation to the skeletal muscle sarcolemma, which has heretofore been predicted solely by cell culture studies.

Inhibition or Ablation of p21-activated Kinase (PAK1) Disrupts Glucose Homeostatic Mechanisms in Vivo

“…Consistent with our data showing normal AKT activation in insulin-stimulated PAK1 KO skeletal muscle extracts, Klip and colleagues showed PAK1 (termed PAK65) to be downstream of PI-3 kinase in the insulin signaling cascade (21). More recent data extend this by showing PAK1 to be downstream of Rac1, and a Rac1 knock-out mouse exhibits defective insulin stimulated GLUT4 translocation (22). Because there is no evidence for the participation of Cdc42 in insulin action in skeletal muscle, it is very likely that Rac1 is the Rho family GTPase that signals to PAK1 in this tissue (20,22).…”

“…More recent data extend this by showing PAK1 to be downstream of Rac1, and a Rac1 knock-out mouse exhibits defective insulin stimulated GLUT4 translocation (22). Because there is no evidence for the participation of Cdc42 in insulin action in skeletal muscle, it is very likely that Rac1 is the Rho family GTPase that signals to PAK1 in this tissue (20,22). Insulin was recently shown to reduce the amount of phosphocofilin in L6 rat skeletal myoblasts (19).…”

Chicken heat shock protein HSPB1 increases and interacts with αB-crystallin in aged skeletal muscle

“…Immunofluorescence staining was performed with specific antibodies (Hsp25/27; 1:100, DHPR1; 1:400) diluted with Can Get Signal immunostain solution A (Toyobo) using paraffin sections as described by Ueda et al 10,13) The bound antibodies were visualized using anti-mouse antibody labeled with Dylight-488 or anti-rabbit antibody labeled with Alexa-546. Images were obtained using a confocal laser-scanning microscope (LSM 700; Carl Zeiss, Jena, Germany) and processed by the Zeiss LSM Image Browser.…”

Identification of P-Rex1 as a Novel Rac1-Guanine Nucleotide Exchange Factor (GEF) That Promotes Actin Remodeling and GLUT4 Protein Trafficking in Adipocytes

“…Muscle expression of dominantnegative Rac1 blocks insulin-induced actin remodeling and GLUT4 translocation to the cell surface (11). Following intravenous insulin injection, Rac1 is activated in mouse gastrocnemius (21) and human muscle (22). Muscle-specific Rac1 knockout mice exhibit significantly impaired insulin-stimulated GLUT4 translocation, independent of Akt or AS160 phosphorylation (21), but whether this results in altered glucose uptake is unknown.…”

“…Following intravenous insulin injection, Rac1 is activated in mouse gastrocnemius (21) and human muscle (22). Muscle-specific Rac1 knockout mice exhibit significantly impaired insulin-stimulated GLUT4 translocation, independent of Akt or AS160 phosphorylation (21), but whether this results in altered glucose uptake is unknown. In contrast, overexpression of constitutively active Rac1 has no significant effect on insulin stimulation of GLUT4 trafficking in 3T3-L1 adipocytes (23) or glucose uptake (24,25).…”