mTORC1 signaling is a central regulator of autophagy and altered autophagic activity has been implicated in the beta-cells of patients with type 2 diabetes, and in the beta-cells of obese mice. The importance of mTORC1 signaling and Raptor in beta-cells has been recently demonstrated using raptor knock-out models (βraKO mice). These studies have shown that complete deletion of Raptor impairs beta-cell proliferation and survival of the beta-cell, and both in function and insulin processing. However, nothing is known whether reduced mTORC1 levels impair beta-cell adaptation to diabetogenic conditions. Therefore, we decided to study mice with haploinsufficiency of Raptor in beta-cells (βraHET). Mice with heterozygous deletion of Raptor in beta-cells were generated by crossing raptorf/f with Rip-Cre mice (βraHET). βraHET and control mice were fed regular chow (RC) or high-fat diet (HFD). Mice fed with RC showed no difference at the metabolic level, islets morphology, or beta-cell function. βraHET mice fed HFD for eight weeks showed an impaired Intraperitoneal Glucose Tolerance Test, fed insulin secretion and a decreased beta-cell mass, compared to control mice. Isolated islets from βraHET mice fed HFD for eight weeks showed impaired insulin secretion either measured by perifusion or static incubation compared to control islets. Next, we tested whether isolated islets from βraHET mice are more susceptible to beta-cell stressors. Beta-cell death determined by TUNEL and cleavage Caspase 3 was increased (50 and 60%) in the βraHET islets after 24 h exposure to palmitate. In addition, autophagy markers such as LC3 and p62 were increased in islets from βraHET treated with palmitate. The increase in cell death and autophagy is specific to lipotoxicity since there βraHET islets exhibited no increase in cleavage Caspase 3 in TUNEL when they were treated with proinflammatory cytokines or ER Stress inducers.
In conclusion, we demonstrate that adequate levels of Raptor are critical in the adaptation of the beta-cell to HFD.
M. Blandino-Rosano: None. E. Bernal-Mizrachi: None.