Cascading Samll GTPases in Insulin Action.

Abstract

Insulin stimulates glucose uptake into adipocytes and muscle cells by stimulating the translocation of the glucose transporter 4 Glut4 from intracellular storage vesicles to the plasma membrane. This process is tightly regulated by insulin signaling cascades in concert with vesicle transport machineries. Small GTPases such as Rab10 and RalA play important roles in insulin-stimulated Glut4 translocation by functioning at the intersection of insulin signaling and vesicle trafficking. A novel effector pull-down assay to evaluate the intracellular activity of Rab10 was developed and used to characterize Rab10 as a functional target of the Akt substrate protein AS160. AS160 contains a GAP (GTPase-activating protein) domain that directly stimulates guanosine triphosphate hydrolysis of Rab10, leading to inactivation of the small GTPase. We show that Akt-catalyzed phosphorylation of AS160 inhibits the GAP activity, as activated Akt relieves the inhibitory effect of AS160 on Rab10 activity.

Activated Rab10 promotes Glut4 translocation to the plasma membrane by recruiting downstream effector proteins. One potential downstream target of Rab10 is the RalA GTPase, which has been reported to mobilize the exocyst complex, targeting complex that guides the Glut4 vesicle to the plasma membrane. Overexpression of constitutively active Rab10 increases RalA activity, while depletion of Rab10 with siRNA-mediated knockdown significantly reduces RalA activation by insulin in adipocytes. Rab10 potentially regulates RalA activity through an as-yet-unidentified guanine-nucleotide exchange factor for RalA, which facilitates guanosine triphosphate binding to RalA, leading to activation of the small GTPase and mobilization of the exocyst complex. Taken together, these data suggest that a small GTPase cascade of Rab10 and RalA, together with their regulatory GAP and GEF proteins, connects the insulin/Akt signaling pathway with the transport machineries in Glut4 translocation.