The Inactivation Of RabGAP Function Of AS160 Promotes Lysosomal Degradation Of GLUT4 And Causes Postprandial Hyperglycemia And Hyperinsulinemia In Mice

Type 2 diabetes mellitus(T2DM)is a widespread metabolic disease that seriously affects human health by its diverse complications.In patients with T2DM,the incidence of postprandial hyperglycemia is particularly high because peripheral organs cannot effectively absorb glucose from the blood after a meal.Chronic hyperglycemia causes damages to blood vessels,which consequently leads to dysfunction of multiple organs and results in diabetic complications.Insulin,a hormone secreted from pancreatic islets,can lower blood glucose via promoting translocation of glucose transporter(GLUT)from intracellular storage vesicles onto plasma membrane to facilitate glucose uptake.The principal insulin-regulated GLUT to mediate glucose uptake is GLUT4(Glucose transporter type 4),which is highly expressed in skeletal muscle and adipose and plays a key role in whole-body glucose homeostasis.Due to insulin resistance in T2DM,insulin-stimulated GLUT4 translocation is impaired in skeletal muscle and adipose tissues,and glucose uptake in these tissues decreases,which consequently leads to hyperglycemia.Given the important role of GLUT4 in regulating glucose homeostasis,elucidation of molecular mechanisms controlling its expression and translocation will help to identify novel therapeutic targets for drug development to combat T2DM.Upon insulin stimulation,the phosphatidylinositol 3-kinase(PI 3-kinase)-protein kinase B(PKB,also known as Akt)pathway becomes activated in skeletal muscle and adipose tissues.Downstream of PKB,AS 160(Akt substrate of 160 KDa)plays a critical role in regulating GLUT4 translocation.AS 160 is a Rab GTPase-activating protein(RabGAP),and its GAP activity is inhibited due to phosphorylation by PKB upon insulin stimulation.Consequently,Rab small G proteins become GTP-loaded and promote GLUT4 translocation.Therefore,AS 160 has been considered as a negative regulator for GLUT4 translocation for a long time.It had also been proposed that inactivation of AS 160 might be a new therapeutic approach to combat T2DM.However,contrast to such a proposal,deletion of AS 160 in mice decreased GLUT4 expression and caused whole-body insulin resistance.Moreover,an AS 160 truncation mutation was found to be widespread in Greenlandic,Canadian and Alaskan Inuit.The premature stop mutation locates in front of the Rab-GAP domain of AS 160,and renders the protein undetectable.The patients bearing this mutation also have lower GLUT4 expression and exhibit whole-body insulin resistance.Therefore,in this study I aimed to investigate how AS 160 regulates GLUT4 expression and how its deletion causes insulin resistance.To this end,I generated a number of genetically-modified mice including whole-body AS 160 knockout mouse,tissue-specific AS 160 knockout mouse and AS 160 Rab-GAP domain inactivation mouse.With those mouse models and their primary cells,I found loss of AS 160 or inactivation of its GAP function promotes lysosomal degradation of GLUT4 in a tissue-autonomous manner.Importantly,loss of AS 160 in skeletal muscle inhibits insulin-stimulated glucose uptake due to diminished GLUT4 proteins,and causes postprandial hyperglycemia and hyperinsulinemia in mice.Interestingly,lysosomal inhibitors could restore GLUT4 expression and mitigate insulin resistance in cultured AS 160-deletion cells.In summary,my studies demonstrate that AS 160 plays a key role in regulation of lysosome-dependent GLUT4 degradation,and its deficiency causes insulin resistance at least partially due to GLUT4 degradation.These findings have implications for discovery of drugs to combat T2DM.