APPL is highly expressed in

APPL1 is highly expressed in pancreatic β cells, but its levels are significantly decreased in several mouse models of obesity and diabetes, including HFD-induced obese mice and db/db mice [6], [73], suggesting that the dysregulation of APPL1 may be associated with malfunction of the pancreas in obesity. Under normal physiological conditions, APPL1 expression in pancreatic beta Z-YVAD-FMK receptor is positively correlated with glucose-induced insulin secretion [6], [73]. It is vital in maintaining mitochondrial function in the beta cells as knockdown of APPL1 in INS-1(832/13) cells leads to the significantly down-regulated expression of several genes involved in mitochondrial biogenesis, such as mitochondrial transcription factor A (Tfam) and peroxisome proliferator-activated receptor-γ coactivator-1α (Pgc-1α) [6]. Consequently, the oxygen consumption rate (OCR), maximal mitochondrial respiration capacity, ATP production, and mitochondrial membrane potential (MMP) are all significantly decreased in the APPL1 knockdown cells [6]. In addition, deletion of the APPL1 gene leads to impairment of both the first and second phases of insulin secretion in hyperglycemic clamp tests [6]. In line with these findings, glucose-stimulated insulin secretion (GSIS) and glucose intolerance are significantly decreased in APPL1 knockout mice. Conversely, overexpression of APPL1 prevents HFD-induced glucose intolerance and enhances GSIS [73]. The effects of APPL1 on insulin secretion are associated with its actions on the expression of the exocytotic machinery SNARE proteins (including syntaxin-1, synaptosomal-associated protein 25, and vesicle-associated membrane protein 2) and related exocytosis and insulin-stimulated AKT activation [73]. These data demonstrate that APPL1 may couple insulin-stimulated AKT activation to GSIS by promoting the expression of the core exocytotic machinery during exocytosis. Therefore, it is reasonable to speculate that the obesity-associated reduction of APPL1 expression in pancreatic islets may serve as a pathological link coupling insulin resistance to β-cell dysfunction in obesity and T2D. Nevertheless, the exact mechanism to explain obesity-induced APPL1 reduction remains unclear. As a critical player in both insulin and adiponectin signaling, APPL1 serves as an important mediator in the cross-talk between these two signaling pathways. On the one hand, APPL1 directly interacts with adiponectin receptors and acts as a positive regulator of adiponectin signaling through the activation of AMPK and p38 MAPK [4], [59], leading to increased insulin sensitivity. On the other hand, APPL1 potentiates insulin sensitivity by enhancing insulin-stimulated AKT phosphorylation [4], [24], [71] and promoting IRS1/2-IR interaction [29]. Interestingly, treatment of C2C12 cells with adiponectin alone may have no effect on AKT phosphorylation, while a notable synergistic effect on AKT activation is observed when the cells are treated with both adiponectin and insulin [4]. Furthermore, down-regulation of APPL1 expression by siRNA reduces the synergistic effect of adiponectin on insulin-stimulated AKT phosphorylation [4]. Hence, APPL1-mediated cross-talk between insulin and adiponectin-signaling pathways could be a critical mechanism for the insulin-sensitizing effect of adiponectin. Regarding insulin-stimulated glucose uptake in muscle cells, APPL2 plays an opposite role compared to APPL1 [42]. Over-expression of APPL2 impairs, whereas deletion of it enhances, the insulin-induced plasma membrane recruitment of GLUT4 and glucose uptake. This process requires the Rab-GTPase-activating protein Tre-2/Bub2/Cdc16 domain family, member 1 (TBC1D1), an interacting partner and downstream effector of APPL2 [42]. The APPL2–TBC1D1 interaction may prevent APPL1/AKT-mediated phosphorylation of TBC1D1 at Thr596, thereby impairing insulin-evoked GLUT4 translation to the plasma membrane [42].