The role of mitochondrial apoptotic pathway in islet amyloid-induced ß-cell death.

Islet amyloid, formed by aggregation of human islet amyloid polypeptide (hIAPP), contributes to ß-cell death in type 2 diabetes. We previously showed that extracellular hIAPP aggregates promote Fas-mediated ß-cell apoptosis. Here, we tested if hIAPP aggregates can trigger the mitochondrial apoptotic pathway (MAP). hIAPP aggregation in Ad-hIAPP transduced INS-1 and human islet ß-cells promoted cytochrome c release, caspase-9 activation and apoptosis, which were reduced by Bax inhibitor. Amyloid formation in hIAPP-expressing mouse islets during culture increased caspase-9 activation in ß-cells. Ad-hIAPP transduced islets from CytcKA/KA and BaxBak ßDKO mice (models of blocked MAP), had lower caspase-9-positive and apoptotic ß-cells than transduced wild-type islets, despite comparable amyloid formation. Blocking Fas (markedly) and Bax or caspase-9 (modestly) reduced ß-cell death induced by extracellular hIAPP aggregates. These findings suggest a role for MAP in amyloid-induced ß-cell death and a potential strategy to reduce intracellular amyloid ß-cell toxicity by blocking cytochrome c apoptotic function.

Early overnutrition in male mice negates metabolic benefits of a diet high in monounsaturated and omega-3 fats.

Overconsumption of saturated fats promotes obesity and type 2 diabetes. Excess weight gain in early life may be particularly detrimental by promoting earlier diabetes onset and potentially by adversely affecting normal development. In the present study we investigated the effects of dietary fat composition on early overnutrition-induced body weight and glucose regulation in Swiss Webster mice, which show susceptibility to high-fat diet-induced diabetes. We compared glucose homeostasis between a high-fat lard-based (HFL) diet, high in saturated fats, and a high-fat olive oil/fish oil-based (HFO) diet, high in monounsaturated and omega-3 fats. We hypothesized that the healthier fat profile of the latter diet would improve early overnutrition-induced glucose dysregulation. However, early overnutrition HFO pups gained more weight and adiposity and had higher diabetes incidence compared to HFL. In contrast, control pups had less weight gain, adiposity, and lower diabetes incidence. Plasma metabolomics revealed reductions in various phosphatidylcholine species in early overnutrition HFO mice as well as with diabetes. These findings suggest that early overnutrition may negate any beneficial effects of a high-fat diet that favours monounsaturated and omega-3 fats over saturated fats. Thus, quantity, quality, and timing of fat intake throughout life should be considered with respect to metabolic health outcomes.

Early overnutrition reduces Pdx1 expression and induces ß cell failure in Swiss Webster mice.

Childhood obesity and early rapid growth increase the risk for type 2 diabetes. Such early overnutrition can be modeled in mice by reducing litter size. We investigated the effects of early overnutrition and increased dietary fat intake on ß cell function in Swiss Webster mice. On a moderate-fat diet, early overnutrition accelerated weight gain and induced hyperinsulinemia in pups. Early overnutrition males exhibited higher ß cell mass but reduced islet insulin content and Pdx1 expression. Males had a high diabetes incidence that was increased by early overnutrition, characterized by a progressive increase in insulin secretion as well as ß cell death, indicated by histological analysis and increased circulating miR-375 levels. Females maintained normoglycemia throughout life. High-fat diet (HFD) increased diabetes incidence in males, whereas low-fat diet was completely protective. This protective effect was abolished in early overnutrition males transiently exposed to HFD in early life. Although Swiss Webster mice are not known to be diabetes-prone, the high diabetes incidence suggests an underlying genetic susceptibility that can be induced by overnutrition and increased dietary fat intake in early life. Thus, the nutritional environment in early life may impact long-term ß cell function and increase diabetes risk, particularly in genetically susceptible individuals.

Amyloid formation disrupts the balance between interleukin-1ß and interleukin-1 receptor antagonist in human islets.

OBJECTIVES: ß-cell dysfunction and apoptosis associated with islet inflammation play a key role in the pathogenesis of type 2 diabetes (T2D). Growing evidence suggests that islet amyloid, formed by aggregation of human islet amyloid polypeptide (hIAPP), contributes to islet inflammation and ß-cell death in T2D. We recently showed the role of interleukin-1ß (IL-1ß)/Fas/caspase-8 apoptotic pathway in amyloid-induced ß-cell death. In this study, we used human islets in culture as an ex vivo model of amyloid formation to: (1) investigate the effects of amyloid on islet levels of the natural IL-1 receptor antagonist (IL-1Ra); (2) examine if modulating the IL-1ß/IL-1Ra balance can prevent amyloid-induced ß-cell Fas upregulation and apoptosis. METHODS: Isolated human islets (n = 10 donors) were cultured in elevated glucose (to form amyloid) with or without a neutralizing human IL-1ß antibody for up to 7 days. Parallel studies were performed with human islets in which amyloid formation was prevented by adeno-siRNA-mediated suppression of hIAPP expression (as control). ß-cell levels of IL-1Ra, Fas, apoptosis as well as islet function, insulin- and amyloid-positive areas, and IL-1Ra release were assessed. RESULTS: Progressive amyloid formation in human islets during culture was associated with alterations in IL-1Ra. Islet IL-1Ra levels were higher at early stages but were markedly reduced at later stages of amyloid formation. Furthermore, IL-1Ra release from human islets was reduced during 7-day culture in a time-dependent manner. These changes in IL-1Ra production and release from human islets during amyloid formation adversely correlated with islet IL-1ß levels, ß-cell Fas expression and apoptosis. Treatment with IL-1ß neutralizing antibody markedly reduced amyloid-induced ß-cell Fas expression and apoptosis, thereby improving islet ß-cell survival and function during culture. CONCLUSIONS: These data suggest that amyloid formation impairs the balance between IL-1ß and IL-1Ra in islets by increasing IL-1ß production and reducing IL-1Ra levels thereby promoting ß-cell dysfunction and death. Restoring the IL-1ß/IL-1Ra ratio may provide an effective strategy to protect islet ß-cells from amyloid toxicity in T2D.