Autophagy deficiency in beta cells leads to compromised unfolded protein response and progression from obesity to diabetes in mice

Abstract

Aims/hypothesis The unfolded protein response (UPR) in endoplasmic reticulum (ER) and autophagy are known to be related. We investigated the role of autophagy in UPR of pancreatic beta cells and the susceptibility of autophagydeficient beta cells to the ER stress that is implicated in the development of diabetes. Methods Rat insulin promoter (RIP)-Cre + ;autophagy-related 7 (Atg7) F/W mice were bred with ob/w mice to derive RIP-Cre + ;Atg7 F/F -ob/ob mice and to induce ER stress in vivo. GFP-LC3 + -ob/ob mice were generated to examine in vivo autophagic activity. Real-time RT-PCR was performed to study the expression of the genes of the UPR machinery. Proteolysis was assessed by determining release of incorporated radioactive leucine.Results Production of UPR machinery was reduced in autophagy-deficient beta cells, which was associated with diminished production of p85α and p85β regulatory subunits of phosphoinositide 3-kinase. Because of compromised UPR machinery, autophagy-deficient beta cells were susceptible to ER stressors in vitro. When mice with beta cell-specific autophagy deficiency, which have mild hyperglycaemia, were bred with ob/ob mice to induce ER stress in vivo, severe diabetes developed, which was accompanied by an increase in beta cell death and accumulation of reactive oxygen species. The increased demand for UPR present in obesity was unmet in autophagy-deficient beta cells. Autophagy level and autophagic activity were enhanced by lipid, while proteolysis was reduced. Conclusions/interpretation These results suggest that autophagy is important for intact UPR machinery and appropriate UPR in response to lipid injury that increases demand for UPR. Autophagy deficiency in pancreatic beta cells may contribute to the progression from obesity to diabetes.

Systemic autophagy insufficiency compromises adaptation to metabolic stress and facilitates progression from obesity to diabetes

“…Again, we observed increased autophagic flux in the liver of leupeptin-treated ob/ob mice. We have also observed that autophagy level and autophagic activity were increased by lipid injury in other types of tissues such as pancreatic islets 45 . Therefore, our data show that both autophagy level and autophagic activity are increased by lipid or obesity in multiple organs in vivo.…”

“…Degradation of long-lived proteins was measured essentially as previously described 45 with modifications. Briefly, cells were plated in collagencoated plates for 24 h. Cells were then labelled with 0.5 mCi ml À 1 C 14 -leucine (Perkin Elmer) for 16 h. After culture for 2 h in a chase medium to allow degradation of short-lived proteins, cells were incubated with serum-free medium containing 2 mM unlabelled leucine and lipid (PA or OA) in the presence or absence of 10 mg ml À 1 each of E64d/pepstatin A and 20 mM NH 4 Cl at 37°C for 3B6 h. Aliquots of the medium were precipitated with trichloroacetic acid, and proteolysis was measured as the percentage of released radioactivity relative to the initial cellular radioactivity.…”

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

“…This rendered autophagy-impaired beta cells hyper-susceptible to apoptosis in response to ER stress. Furthermore, autophagy deficiency prevented the compensatory increase in beta cell mass in response to HFD and in ob/ob mice, resulting in further deterioration of glucose tolerance [75,76]. These studies convincingly show that basal autophagy is not only indispensable for maintaining beta cell mass and function, but is also required for beta cell compensation to obesity-induced insulin resistance.…”

“…Reminiscent of the situation in adipose tissue, an increased number of autophagosomes and autophagolysosomes was observed in the beta cells of different models of type 2 diabetes, including ob/ob and db/db mice [75,76] and Akita mice [10]. In Zucker diabetic fatty rats and in insulinoma cells, hyperglycaemia and oxidative stress led to the accumulation of polyubiquitinated protein aggregates that were degraded by autophagy [77].…”

“…Beta cell apoptosis was increased and proliferation reduced, resulting in decreased beta cell mass. Surprisingly, the expression of genes involved in cellular protection against ER stress, including anti-oxidants and unfolded protein response (UPR) genes, were all reduced in autophagy-deficient beta cells [76]. This rendered autophagy-impaired beta cells hyper-susceptible to apoptosis in response to ER stress.…”