The Adaptor Protein APPL2 Inhibits Insulin-Stimulated Glucose Uptake by Interacting With TBC1D1 in Skeletal Muscle

Abstract

Insulin stimulates glucose uptake by promoting the trafficking of GLUT4 to the plasma membrane in muscle cells, and impairment of this insulin action contributes to hyperglycemia in type 2 diabetes. The adaptor protein APPL1 potentiates insulin-stimulated Akt activation and downstream actions. However, the physiological functions of APPL2, a close homolog of APPL1, in regulating glucose metabolism remain elusive. We show that insulin-evoked plasma membrane recruitment of GLUT4 and glucose uptake are impaired by APPL2 overexpression but enhanced by APPL2 knockdown. Likewise, conditional deletion of APPL2 in skeletal muscles enhances insulin sensitivity, leading to an improvement in glucose tolerance. We identified the Rab-GTPaseactivating protein TBC1D1 as an interacting partner of APPL2. Insulin stimulates TBC1D1 phosphorylation on serine 235, leading to enhanced interaction with the BAR domain of APPL2, which in turn suppresses insulinevoked TBC1D1 phosphorylation on threonine 596 in cultured myotubes and skeletal muscle. Substitution of serine 235 with alanine diminishes APPL2-mediated inhibition on insulin-dependent TBC1D1 phosphorylation on threonine 596 and the suppressive effects of TBC1D1 on insulin-induced glucose uptake and GLUT4 translocation to the plasma membrane in cultured myotubes. Therefore, the APPL2-TBC1D1 interaction is a key step to fine tune insulin-stimulated glucose uptake by regulating the membrane recruitment of GLUT4 in skeletal muscle.

Roles of TBC1D1 and TBC1D4 in insulin- and exercise-stimulated glucose transport of skeletal muscle

“…Although pS237-TBC1D1 and pT596-TBC1D1 are 14-3-3 binding sites, insulin did not increase the ability of TBC1D1 to bind 14-3-3 [17]. Insulin increased pS235-TBC1D1 and binding of the adaptor protein containing PH domain, PTB domain and leucine zipper motif 2 (APPL2) to TBC1D1 in myocytes and mouse EDL [44]. In myocytes overexpressing APPL2, the binding of TBC1D1 to APPL2 required pS235-TBC1D1 and attenuated insulin-stimulated pT596-TBC1D1 and glucose uptake [44].…”

“…Insulin increased pS235-TBC1D1 and binding of the adaptor protein containing PH domain, PTB domain and leucine zipper motif 2 (APPL2) to TBC1D1 in myocytes and mouse EDL [44]. In myocytes overexpressing APPL2, the binding of TBC1D1 to APPL2 required pS235-TBC1D1 and attenuated insulin-stimulated pT596-TBC1D1 and glucose uptake [44]. Cells expressing a T596D-TBC1D1 phospho-mimic mutant were protected from APPL2 overexpression-induced insulin resistance.…”

Distinct Roles for APPL1 and APPL2 in Regulating Toll‐like Receptor 4 Signaling in Macrophages

“…As adaptors, APPLs contain multiple protein and membrane‐binding modules, allowing docking of target proteins on specific membrane domains in the cell . APPLs can bind directly to membrane lipids, receptors and signaling molecules to coalesce the players and generate responses at specific membrane domains or cell compartments . In the context of the more well‐studied isoform APPL1, the N‐terminal membrane‐curvature BAR (Bin‐amphiphysin‐Rvs) domain linked to a PH domain and a C‐terminal phosphotyrosine‐binding domain bind to the signaling kinases Akt and PI3K, recruiting them for a variety of activated receptors .…”

Nonfunctional overreaching and hepatic adaptations of APPL1 and APPL2

“…On the other hand, adiponectin modulates the dissociation of the APPL1/APPL2 heterodimers, which can also be triggered by insulin. Interestingly, Cheng et al 22 demonstrated that APPL2 inhibits insulin-stimulated glucose uptake in skeletal muscle. In this study, only the OTR/up group increased the hepatic levels of APPL2, which may be linked to the increased phosphorylation of the insulin receptor substrate 1 (IRS-1) at serine 307 3 , a molecular marker directly related to insulin signaling pathway impairment 33 .…”

“…It is known that APPL1 can limit the interaction between Akt and TRB3 in mouse liver tissue, which is accompanied by an increase in the membrane translocation of APPL1, Akt activation, and enhancement in response to insulin stimulation 22,23 . Also, adiponectin also stimulates APPL1 in mouse hepatocyte cells with activation of another class of proteins, the p38 mitogen-activated protein kinases (p38MAPK), which activates glucose transporters protein.…”