AS160 deficiency causes whole-body insulin resistance via composite effects in multiple tissues.

Hong Yu Wang,Chao Quan,Carol Mackintosh,Bingxian Xie,Shuai Chen,Min Li,David H Wasserman,Serge Ducommun,Kei Sakamoto

doi:10.1042/bj20120702

Abstract

AS160 (Akt substrate of 160 kDa) is a Rab GTPase-activating protein implicated in insulin control of GLUT4 (glucose transporter 4) trafficking. In humans, a truncation mutation (R363X) in one allele of AS160 decreased the expression of the protein and caused severe postprandial hyperinsulinaemia during puberty. To complement the limited studies possible in humans, we generated an AS160-knockout mouse. In wild-type mice, AS160 expression is relatively high in adipose tissue and soleus muscle, low in EDL (extensor digitorum longus) muscle and detectable in liver only after enrichment. Despite having lower blood glucose levels under both fasted and random-fed conditions, the AS160-knockout mice exhibited insulin resistance in both muscle and liver in a euglycaemic clamp study. Consistent with this paradoxical phenotype, basal glucose uptake was higher in AS160-knockout primary adipocytes and normal in isolated soleus muscle, but their insulin-stimulated glucose uptake and overall GLUT4 levels were markedly decreased. In contrast, insulin-stimulated glucose uptake and GLUT4 levels were normal in EDL muscle. The liver also contributes to the AS160-knockout phenotype via hepatic insulin resistance, elevated hepatic expression of phosphoenolpyruvate carboxykinase isoforms and pyruvate intolerance, which are indicative of increased gluconeogenesis. Overall, as well as its catalytic function, AS160 influences expression of other proteins, and its loss deregulates basal and insulin-regulated glucose homoeostasis, not only in tissues that normally express AS160, but also by influencing liver function.

Highlights

Insulin-regulated disposal of glucose from the bloodstream into skeletal muscle and fat tissues is mediated by the glucose transporter GLUT4, which undergoes translocation from intracellular storage sites on to the cell surface upon insulin stimulation [1]
We generated an Akt substrate of 160 kDa (AS160)-knockout mouse model to study the effects of AS160 deletion on glucose homoeostasis in mouse, which reveals similarities and differences between the AS160-knockout mouse and the human patients bearing the AS160 truncation mutation [19]
A peptide of AS160, corresponding to the truncated protein that might be expressed in the human patients with the AS160-R363X mutation, was able to inhibit insulin-stimulated GLUT4 translocation when overexpressed in 3T3-L1 adipocytes [19]

Summary

Introduction

Insulin-regulated disposal of glucose from the bloodstream into skeletal muscle and fat tissues is mediated by the glucose transporter GLUT4, which undergoes translocation from intracellular storage sites on to the cell surface upon insulin stimulation [1]. The insulin-activated PI3K (phosphoinositide 3kinase)/PKB (protein kinase B, known as Akt) signalling axis plays a key role in mediating insulin-stimulated GLUT4 trafficking (reviewed in [2]). Hunting for substrate(s) downstream of PKB in controlling GLUT4 trafficking has been the focus of research in the field for decades. Insulin can promote phosphorylation of AS160 on multiple sites, including Ser318, Ser341, Ser570, Ser588, Thr642, Ser666 and Ser751 [3,4]. Phosphorylated AS160 interacts with the 14-3-3 family of phosphoprotein-binding proteins, which is mainly mediated by phosphorylated Thr642 [4,5]

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Results

Conclusion