Deletion of Rab GAP AS160 modifies glucose uptake and GLUT4 translocation in primary skeletal muscles and adipocytes and impairs glucose homeostasis

Abstract

Tight control of glucose uptake in skeletal muscles and adipocytes is crucial to glucose homeostasis and is mediated by regulating glucose transporter GLUT4 subcellular distribution. In cultured cells, Rab GAP AS160 controls GLUT4 intracellular retention and release to the cell surface and consequently regulates glucose uptake into cells. To determine AS160 function in GLUT4 trafficking in primary skeletal muscles and adipocytes and investigate its role in glucose homeostasis, we characterized AS160 knockout (AS160(-/-)) mice. We observed increased and normal basal glucose uptake in isolated AS160(-/-) adipocytes and soleus, respectively, while insulin-stimulated glucose uptake was impaired and GLUT4 expression decreased in both. No such abnormalities were found in isolated AS160(-/-) extensor digitorum longus muscles. In plasma membranes isolated from AS160(-/-) adipose tissue and gastrocnemius/quadriceps, relative GLUT4 levels were increased under basal conditions and remained the same after insulin treatment. Concomitantly, relative levels of cell surface-exposed GLUT4, determined with a glucose transporter photoaffinity label, were increased in AS160(-/-) adipocytes and normal in AS160(-/-) soleus under basal conditions. Insulin augmented cell surface-exposed GLUT4 in both. These observations suggest that AS160 is essential for GLUT4 intracellular retention and regulation of glucose uptake in adipocytes and skeletal muscles in which it is normally expressed. In vivo studies revealed impaired insulin tolerance in the presence of normal (male) and impaired (female) glucose tolerance. Concurrently, insulin-elicited increases in glucose disposal were abolished in all AS160(-/-) skeletal muscles and liver but not in AS160(-/-) adipose tissues. This suggests AS160 as a target for differential manipulation of glucose homeostasis.

The Inactivation of RabGAP Function of AS160 Promotes Lysosomal Degradation of GLUT4 and Causes Postprandial Hyperglycemia and Hyperinsulinemia

“…In agreement with previous reports (14,15,17), the AS160 E10KO mice displayed normal glucose tolerance when glucose was intraperitoneally administered (Fig. 1A).…”

“…5E-G). Normal GLUT4 expression and glucose uptake in EDL muscle of the AS160 R917K knockin or AS160 KO mice (14)(15)(16)(17) are most likely a result of the presence of TBC1D1, which regulates GLUT4 expression in white skeletal muscle (32)(33)(34). The AS160 R917K knockin mice displayed significant insulin resistance and exhibited intolerance to glucose administration (Fig.…”

“…1B and C). Previous reports (14,15,17) show that blood glucose levels are normal in the AS160 KO mice after an overnight fast (16 h), but are significantly lower after partial fasting (4-6 h). In agreement with these reports, the basal blood glucose level after an overnight fast was normal in the AS160 E10KO mice in this study ( Fig.…”

“…Surprisingly, deletion of AS160 decreases GLUT4 levels and causes severe insulin resistance in mice (14)(15)(16)(17). More importantly, human patients bearing truncation mutations on AS160 (AS160 Arg363Ter or AS160 Arg684Ter , in which arginine is replaced by a stop codon) have postprandial hyperglycemia and hyperinsulinemia (18,19).…”

“…GLUT4 is also decreased in skeletal muscle of patients harboring the AS160 Arg684Ter mutation (19). Despite the severe insulin resistance, the AS160 knockout (KO) male mice exhibit either normal (14,15,17) or impaired (16) glucose clearance, and have normal plasma insulin levels when intraperitoneally administered with glucose (14,15,17). It is currently unclear whether this difference is due to different administration routes of glucose in human and mice or is inherent between them.…”