APPL is highly expressed in pancreatic cells but its levels

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 Alfacalcidol sale 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].