One year of sitagliptin treatment protects against islet amyloid-associated ß-cell loss and does not induce pancreatitis or pancreatic neoplasia in mice.

The dipeptidyl peptidase-4 (DPP-4) inhibitor sitagliptin is an attractive therapy for diabetes, as it increases insulin release and may preserve ß-cell mass. However, sitagliptin also increases ß-cell release of human islet amyloid polypeptide (hIAPP), the peptide component of islet amyloid, which is cosecreted with insulin. Thus, sitagliptin treatment may promote islet amyloid formation and its associated ß-cell toxicity. Conversely, metformin treatment decreases islet amyloid formation by decreasing ß-cell secretory demand and could therefore offset sitagliptin's potential proamyloidogenic effects. Sitagliptin treatment has also been reported to be detrimental to the exocrine pancreas. We investigated whether long-term sitagliptin treatment, alone or with metformin, increased islet amyloid deposition and ß-cell toxicity and induced pancreatic ductal proliferation, pancreatitis, and/or pancreatic metaplasia/neoplasia. hIAPP transgenic and nontransgenic littermates were followed for 1 yr on no treatment, sitagliptin, metformin, or the combination. Islet amyloid deposition, ß-cell mass, insulin release, and measures of exocrine pancreas pathology were determined. Relative to untreated mice, sitagliptin treatment did not increase amyloid deposition, despite increasing hIAPP release, and prevented amyloid-induced ß-cell loss. Metformin treatment alone or with sitagliptin decreased islet amyloid deposition to a similar extent vs untreated mice. Ductal proliferation was not altered among treatment groups, and no evidence of pancreatitis, ductal metaplasia, or neoplasia were observed. Therefore, long-term sitagliptin treatment stimulates ß-cell secretion without increasing amyloid formation and protects against amyloid-induced ß-cell loss. This suggests a novel effect of sitagliptin to protect the ß-cell in type 2 diabetes that appears to occur without adverse effects on the exocrine pancreas.

Matrix metalloproteinase-9 reduces islet amyloid formation by degrading islet amyloid polypeptide.

Deposition of islet amyloid polypeptide (IAPP) as amyloid is a pathological hallmark of the islet in type 2 diabetes, which is toxic to ß-cells. We previously showed that the enzyme neprilysin reduces islet amyloid deposition and thereby reduces ß-cell apoptosis, by inhibiting fibril formation. Two other enzymes, matrix metalloproteinase (MMP)-2 and MMP-9, are extracellular gelatinases capable of degrading another amyloidogenic peptide, Aß, the constituent of amyloid deposits in Alzheimer disease. We therefore investigated whether MMP-2 and MMP-9 play a role in reducing islet amyloid deposition. MMP-2 and MMP-9 mRNA were present in mouse islets but only MMP-9 activity was detectable. In an islet culture model where human IAPP (hIAPP) transgenic mouse islets develop amyloid but nontransgenic islets do not, a broad spectrum MMP inhibitor (GM6001) and an MMP-2/9 inhibitor increased amyloid formation and the resultant ß-cell apoptosis. In contrast, a specific MMP-2 inhibitor had no effect on either amyloid deposition or ß-cell apoptosis. Mass spectrometry demonstrated that MMP-9 degraded amyloidogenic hIAPP but not nonamyloidogenic mouse IAPP. Thus, MMP-9 constitutes an endogenous islet protease that limits islet amyloid deposition and its toxic effects via degradation of hIAPP. Because islet MMP-9 mRNA levels are decreased in type 2 diabetic subjects, islet MMP-9 activity may also be decreased in human type 2 diabetes, thereby contributing to increased islet amyloid deposition and ß-cell loss. Approaches to increase islet MMP-9 activity could reduce or prevent amyloid deposition and its toxic effects in type 2 diabetes.

ß-cell loss and ß-cell apoptosis in human type 2 diabetes are related to islet amyloid deposition.

Amyloid deposition and reduced ß-cell mass are pathological hallmarks of the pancreatic islet in type 2 diabetes; however, whether the extent of amyloid deposition is associated with decreased ß-cell mass is debated. We investigated the possible relationship and, for the first time, determined whether increased islet amyloid and/or decreased ß-cell area quantified on histological sections is correlated with increased ß-cell apoptosis. Formalin-fixed, paraffin-embedded human pancreas sections from subjects with (n = 29) and without (n = 39) diabetes were obtained at autopsy (64 ± 2 and 70 ± 4 islets/subject, respectively). Amyloid and ß cells were visualized by thioflavin S and insulin immunolabeling. Apoptotic ß cells were detected by colabeling for insulin and by TUNEL. Diabetes was associated with increased amyloid deposition, decreased ß-cell area, and increased ß-cell apoptosis, as expected. There was a strong inverse correlation between ß-cell area and amyloid deposition (r = -0.42, P < 0.001). ß-Cell area was selectively reduced in individual amyloid-containing islets from diabetic subjects, compared with control subjects, but amyloid-free islets had ß-cell area equivalent to islets from control subjects. Increased amyloid deposition was associated with ß-cell apoptosis (r = 0.56, P < 0.01). Thus, islet amyloid is associated with decreased ß-cell area and increased ß-cell apoptosis, suggesting that islet amyloid deposition contributes to the decreased ß-cell mass that characterizes type 2 diabetes.

Activation of the NLRP3 inflammasome by islet amyloid polypeptide provides a mechanism for enhanced IL-1ß in type 2 diabetes.

Interleukin 1ß (IL-1ß) is an important inflammatory mediator of type 2 diabetes. Here we show that oligomers of islet amyloid polypeptide (IAPP), a protein that forms amyloid deposits in the pancreas during type 2 diabetes, triggered the NLRP3 inflammasome and generated mature IL-1ß. One therapy for type 2 diabetes, glyburide, suppressed IAPP-mediated IL-1ß production in vitro. Processing of IL-1ß initiated by IAPP first required priming, a process that involved glucose metabolism and was facilitated by minimally oxidized low-density lipoprotein. Finally, mice transgenic for human IAPP had more IL-1ß in pancreatic islets, which localized together with amyloid and macrophages. Our findings identify previously unknown mechanisms in the pathogenesis of type 2 diabetes and treatment of pathology caused by IAPP.

Neprilysin impedes islet amyloid formation by inhibition of fibril formation rather than peptide degradation.

Deposition of islet amyloid polypeptide (IAPP) as islet amyloid in type 2 diabetes contributes to loss of beta-cell function and mass, yet the mechanism for its occurrence is unclear. Neprilysin is a metallopeptidase known to degrade amyloid in Alzheimer disease. We previously demonstrated neprilysin to be present in pancreatic islets and now sought to determine whether it plays a role in degrading islet amyloid. We used an in vitro model where cultured human IAPP (hIAPP) transgenic mouse islets develop amyloid and thereby have increased beta-cell apoptosis. Islet neprilysin activity was inhibited or up-regulated using a specific inhibitor or adenovirus encoding neprilysin, respectively. Following neprilysin inhibition, islet amyloid deposition and beta-cell apoptosis increased by 54 and 75%, respectively, whereas when neprilysin was up-regulated islet amyloid deposition and beta-cell apoptosis both decreased by 79%. To determine if neprilysin modulated amyloid deposition by cleaving hIAPP, analysis of hIAPP incubated with neprilysin was performed by mass spectrometry, which failed to demonstrate neprilysin-induced cleavage. Rather, neprilysin may act by reducing hIAPP fibrillogenesis, which we showed to be the case by fluorescence-based thioflavin T binding studies and electron microscopy. In summary, neprilysin decreases islet amyloid deposition by inhibiting hIAPP fibril formation, rather than degrading hIAPP. These findings suggest that targeting the role of neprilysin in IAPP fibril assembly, in addition to IAPP cleavage by other peptidases, may provide a novel approach to reduce and/or prevent islet amyloid deposition in type 2 diabetes.