The adaptor protein APPL1 increases glycogen accumulation in rat skeletal muscle through activation of the PI3-kinase signalling pathway

Abstract

APPL1 is an adaptor protein that binds to both AKT and adiponectin receptors and is hypothesised to mediate the effects of adiponectin in activating downstream effectors such as AMP-activated protein kinase (AMPK). We aimed to establish whether APPL1 plays a physiological role in mediating glycogen accumulation and insulin sensitivity in muscle and the signalling pathways involved. In vivo electrotransfer of cDNA- and shRNA-expressing constructs was used to over-express or silence APPL1 for 1 week in single tibialis cranialis muscles of rats. Resulting changes in glucose and lipid metabolism and signalling pathway activation were investigated under basal conditions and in high-fat diet (HFD)- or chow-fed rats under hyperinsulinaemic–euglycaemic clamp conditions. APPL1 over-expression (OE) caused an increase in glycogen storage and insulin-stimulated glycogen synthesis in muscle, accompanied by a modest increase in glucose uptake. Glycogen synthesis during the clamp was reduced by HFD but normalised by APPL1 OE. These effects are likely explained by APPL1 OE-induced increase in basal and insulin-stimulated phosphorylation of IRS1, AKT, GSK3β and TBC1D4. On the contrary, APPL1 OE, such as HFD, reduced AMPK and acetyl-CoA carboxylase phosphorylation and PPARγ coactivator-1α and uncoupling protein 3 expression. Furthermore, APPL1 silencing caused complementary changes in glycogen storage and phosphorylation of AMPK and PI3-kinase pathway intermediates. Thus, APPL1 may provide a means for crosstalk between adiponectin and insulin signalling pathways, mediating the insulin-sensitising effects of adiponectin on muscle glucose disposal. These effects do not appear to require AMPK. Activation of signalling mediated via APPL1 may be beneficial in overcoming muscle insulin resistance.

APPL1 potentiates insulin secretion in pancreatic β cells by enhancing protein kinase Akt-dependent expression of SNARE proteins in mice

“…However, because insulin secretion is a highly dynamic process regulated by complex mechanisms, the pathological pathways leading to β-cell dysfunction remain poorly characterized. In the present study, we provide both in vitro and in vivo evidence showing that APPL1, a multidomain adapter protein involved in the peripheral actions of insulin (14)(15)(16), is a physiological regulator of insulin secretion in pancreatic β cells. GSIS is blunted in APPL1 KO mice, but is augmented by transgenic expression of APPL1.…”

“…APPL1 has been reported to modulate both substrate specificity and activity of Akt (14)(15)(16)(17)32), a protein kinase that plays a central role in mediating the peripheral actions of insulin. In addition, the PI3K/Akt signaling pathway has been shown to control insulin secretion at the step of exocytosis, partly via enhancing the expression of SNARE proteins (4,7).…”

“…In the liver, APPL1 potentiates the inhibitory effects of insulin on hepatic gluconeogenesis via Akt activation, and overexpression of APPL1 in liver alleviates hyperglycemia in db/db diabetic mice (14). In skeletal muscles and adipocytes, APPL1 mediates insulinstimulated glucose uptake via controlling the translocation of cytosolic glucose transporter 4 to the plasma membrane (15,16). In the blood vessels, APPL1 counteracts obesity-induced vascular insulin resistance by shifting the vasoconstrictor effect of insulin to nitric oxide-dependent vasodilatation (17).…”

Mouse strain-dependent variation in obesity and glucose homeostasis in response to high-fat feeding

“…Immunoblotting Whole-tissue lysates were prepared from powdered muscle and liver by manual homogenisation in RIPA buffer [23]. Proteins were resolved by SDS-PAGE electrophoresis, and immunoblot analysis was conducted as described elsewhere [19,24,25].…”

Adipose tissue in control of metabolism

“…There is accumulating evidence showing that adiponectin exhibits insulin sensitizing effect through multiple signalling pathways downstream of adiponectin receptors, such as AMPK, Ca 2+ , PPARα, ceramide and S1P 231:3 , Zhou et al 2009, Iwabu et al 2010, Holland et al 2011. Moreover, as an AdipoR1 and AdipoR2 interactive protein, APPL1 (adaptor protein containing pleckstrin homology domain, phosphotyrosine-binding domain and leucine zipper motif) positively mediates adiponectin signalling and its insulin sensitizing effect in muscle cells (Mao et al 2006, Zhou et al 2009, Fang et al 2010, Cleasby et al 2011, Xin et al 2011. AMPK pathway, downstream of adiponectin receptor signalling, is critical for adiponectin action in the liver, muscle and other organs (Yamauchi et al 2002).…”