Lysosomal acid lipase and lipophagy are constitutive negative regulators of glucose-stimulated insulin secretion from pancreatic beta cells

Abstract

Aims/hypothesis Lipolytic breakdown of endogenous lipid pools in pancreatic beta cells contributes to glucosestimulated insulin secretion (GSIS) and is thought to be mediated by acute activation of neutral lipases in the amplification pathway. Recently it has been shown in other cell types that endogenous lipid can be metabolised by autophagy, and this lipophagy is catalysed by lysosomal acid lipase (LAL). This study aimed to elucidate a role for LAL and lipophagy in pancreatic beta cells. Methods We employed pharmacological and/or genetic inhibition of autophagy and LAL in MIN6 cells and primary islets. Insulin secretion following inhibition was measured using RIA. Lipid accumulation was assessed by MS and confocal microscopy (to visualise lipid droplets) and autophagic flux was analysed by western blot. Results Insulin secretion was increased following chronic (≥8 h) inhibition of LAL. This was more pronounced with glucose than with non-nutrient stimuli and was accompanied by augmentation of neutral lipid species. Similarly, following inhibition of autophagy in MIN6 cells, the number of lipid droplets was increased and GSIS was potentiated. Inhibition of LAL or autophagy in primary islets also increased insulin secretion. This augmentation of GSIS following LAL or autophagy inhibition was dependent on the acute activation of neutral lipases. Conclusions/interpretation Our data suggest that lysosomal lipid degradation, using LAL and potentially lipophagy, contributes to neutral lipid turnover in beta cells. It also serves as a constitutive negative regulator of GSIS by depletion of substrate for the non-lysosomal neutral lipases that are activated acutely by glucose.

High-fat diet increases autophagic flux in pancreatic beta cells in vivo and ex vivo in mice

“…Whether CHOP accumulation under these conditions arises downstream of some (eIF2α), but not all (IRE1α), arms of the unfolded protein response, or as a consequence of the ISR, was not resolved in our studies. Regardless, our data now suggest that suppression of CHOP would represent a beneficial adaptation to high-fat feeding, albeit one that might come at the cost of reduced insulin secretion due to disruption of lipid signalling pathways [9]. Although it is commonly assumed, based on genetic models [12,13], that modulation of ER stress explains the reciprocal relationship between autophagy and beta cell apoptosis [2,4,14], we believe our CHOP data are the first to confirm this mechanistic link in the context of high-fat feeding.…”

“…Islet isolation, treatment and immunoblotting Mouse islets were isolated as described previously [9] and then incubated with or without chloroquine (50 μmol/l) for 2 h immediately after isolation. Immunoblotting of islet lysates was as recently described [9].…”

“…Immunoblotting of islet lysates was as recently described [9]. For further details and sources of antibodies please refer to the electronic supplementary material (ESM) Methods.…”

ATGL Promotes Autophagy/Lipophagy via SIRT1 to Control Hepatic Lipid Droplet Catabolism

“…Consistent with previous studies (Ong et al 2011; Sapiro et al 2009), overexpressing ATGL increased TAG turnover, but this effect was partially or completely abrogated when autophagy was blunted via exposure of hepatocytes to the autophagy inhibitor chloroquine or knockdown of ATG5 (Figures 3A–B). Moreover, blocking lysosomal lipid hydrolysis, via the lysosomal acid lipase inhibitor LAListat (Pearson et al 2014), prevented the increase in TAG turnover in response to ATGL overexpression (Figure 3C). Consistent with the TAG turnover data, blocking autophagy or lysosomal lipid degradation attenuated the oxidation of hydrolyzed FAs following ATGL overexpression (Figures 3D–F).…”

Autophagy in adipose tissue and the beta cell: implications for obesity and diabetes

“…It is likely that autophagy functions as a quality control process by eliminating aged secretory granules, and is probably required for the maintenance of beta cell homeostasis. Recently, it was suggested that lysosomal lipid degradation (lipophagy) negatively regulates glucose-stimulated insulin secretion by depletion of substrate for non-lysosomal neutral lipases that regulate insulin secretion [72]. Thus, autophagy-lysosomal degradation may affect both insulin content and secretion.…”