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.

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.