Deletion of Both Rab-GTPase–Activating Proteins TBC1D1 and TBC1D4 in Mice Eliminates Insulin- and AICAR-Stimulated Glucose Transport

Abstract

The Rab-GTPase-activating proteins TBC1D1 and TBC1D4 (AS160) were previously shown to regulate GLUT4 translocation in response to activation of AKT and AMPdependent kinase. However, knockout mice lacking either Tbc1d1 or Tbc1d4 displayed only partially impaired insulin-stimulated glucose uptake in fat and muscle tissue. The aim of this study was to determine the impact of the combined inactivation of Tbc1d1 and Tbc1d4 on glucose metabolism in double-deficient (D1/4KO) mice. D1/4KO mice displayed normal fasting glucose concentrations but had reduced tolerance to intraperitoneally administered glucose, insulin, and AICAR. D1/4KO mice showed reduced respiratory quotient, indicating increased use of lipids as fuel. These mice also consistently showed elevated fatty acid oxidation in isolated skeletal muscle, whereas insulin-stimulated glucose uptake in muscle and adipose cells was almost completely abolished. In skeletal muscle and white adipose tissue, the abundance of GLUT4 protein, but not GLUT4 mRNA, was substantially reduced. Cell surface labeling of GLUTs indicated that RabGAP deficiency impairs retention of GLUT4 in intracellular vesicles in the basal state. Our results show that TBC1D1 and TBC1D4 together play essential roles in insulin-stimulated glucose uptake and substrate preference in skeletal muscle and adipose cells.

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.…”