Glucagon-Like Peptide 1 Protects Pancreatic β-Cells From Death by Increasing Autophagic Flux and Restoring Lysosomal Function

Abstract

Studies in animal models of type 2 diabetes have shown that glucagon-like peptide 1 (GLP-1) receptor agonists prevent β-cell loss. Whether GLP-1 mediates β-cell survival via the key lysosomal-mediated process of autophagy is unknown. In this study, we report that treatment of INS-1E β-cells and primary islets with glucolipotoxicity (0.5 mmol/L palmitate and 25 mmol/L glucose) increases LC3 II, a marker of autophagy. Further analysis indicates a blockage in autophagic flux associated with lysosomal dysfunction. Accumulation of defective lysosomes leads to lysosomal membrane permeabilization and release of cathepsin D, which contributes to cell death. Our data further demonstrated defects in autophagic flux and lysosomal staining in human samples of type 2 diabetes. Cotreatment with the GLP-1 receptor agonist exendin-4 reversed the lysosomal dysfunction, relieving the impairment in autophagic flux and further stimulated autophagy. Small interfering RNA knockdown showed the restoration of autophagic flux is also essential for the protective effects of exendin-4. Collectively, our data highlight lysosomal dysfunction as a critical mediator of β-cell loss and shows that exendin-4 improves cell survival via restoration of lysosomal function and autophagic flux. Modulation of autophagy/lysosomal homeostasis may thus define a novel therapeutic strategy for type 2 diabetes, with the GLP-1 signaling pathway as a potential focus.

Oleate disrupts cAMP signaling, contributing to potent stimulation of pancreatic β-cell autophagy

“…4B). The results confirm previous findings that ER stress contributes to Oleate enhances ␤-cell autophagy the stimulation of autophagic flux due to palmitate (20,25), but now show that this mechanism does not account for the response due to oleate.…”

“…One interpretation of this variability is that palmitate exerts multiple and conflicting effects on autophagy, the balance of which determines the overall outcome. One positive input is almost certainly ER stress (20,25). Thus we found that PBA inhibited autophagic flux in the presence of palmitate, reducing LC3II to levels below those of CQ alone (Fig.…”

“…In vitro studies on ␤-cell autophagy have thus focused predominately on palmitate and have generally (10, 20 -25) shown an enhancement of autophagy when conducted in conjunction with lysosomal inhibitors to allow quantification of flux. Mechanistically, this increase is thought to be secondary to the stimulation of ER stress (20,25) that occurs in response to chronic palmitate treatment of ␤-cells (26). But there are also indications of an impairment of fusion between autophagosomes and lysosomes potentially explained by changes in lumenal pH or lipid remodelling (22)(23)(24)(25).…”

“…Mechanistically, this increase is thought to be secondary to the stimulation of ER stress (20,25) that occurs in response to chronic palmitate treatment of ␤-cells (26). But there are also indications of an impairment of fusion between autophagosomes and lysosomes potentially explained by changes in lumenal pH or lipid remodelling (22)(23)(24)(25). More limited evidence suggests that oleate might enhance LC3II accumulation under steady-state conditions (10,21,22).…”

Type 2 diabetes is associated with suppression of autophagy and lipid accumulation in β‐cells

“…Hyperlipidaemia was induced by 500 μmol/L OA, 500 μmol/L PA or 250 μmol/L oleic acid +250 μmol/L palmitic acid (OA+PA), as used by others. 22,31 This was the maximal dose which did not significantly reduce cell number or induce apoptosis ( Figure S3A…”