Islet amyloid polypeptide in pancreatic islets from type 2 diabetic subjects

Aims/hypothesis: Islet amyloid polypeptide (IAPP) is a chief constituent of amyloid deposits in pancreatic islets, characteristic histopathology for type 2 diabetes. The goal of this study was to analyze islet cell composition in diabetic islets for the process of transforming water-soluble IAPP in beta-cells to water-insoluble amyloid deposits by Immunocytochemical staining using different dilutions of anti-IAPP antibody. IAPP in beta-cell granules may initiate beta-cell necrosis through apoptosis to form interstitial amyloid deposits in type 2 diabetic islets. Results: Control islets revealed twice as much beta-cells as alpha-cells whereas 15 of 18 type 2 diabetic cases (83%) revealed alpha-cells as major cells in larger islets. Diabetic islets consisted of more larger islets with more sigma-cells than beta-cells, which contribute to hyperglucagonemia. In control islets, percentage of IAPP-positive cells against beta-cells was 40-50% whereas percentage for type 2 diabetic islets was about 25%. Amyloid deposits in diabetic islets were not readily immunostained for IAPP using 1:800 diluted antibody, however, 1:400 and 1:200 diluted solutions provided stronger immunostaining in early stages of islet amyloidogenesis after treating the deparaffinized sections with formic acid. Methods: Using commercially available rabbit antihuman IAPP antibody, immunocytochemical staining was performed on 18 cases of pancreatic tissues from type 2 diabetic subjects by systematically immunostaining for insulin, glucagon, somatostatin (SRIF) and IAPP compared with controls. Sizes of islets were measured by 1 cm scale, mounted in 10x eye piece. Conclusions/Interpretation: alpha cells were major islet cells in majority of diabetic pancreas (83%) and all diabetic islets contained less IAPP-positive cells than controls, indicating that IAPP deficiency in pancreatic islets is responsible for decreased IAPP in blood. In diabetic islets, water-soluble IAPP disappeared in beta-cell granules, which transformed to water-insoluble amyloid deposits. Amyloid deposits were not readily immunostained using IAPP 1:800 diluted antibody but were stronger immunostained for IAPP in early stages of amyloid deposited islets using less diluted solutions after formic acid treatment. In early islet amyloidogenesis, dying beta-cell cytoplasm was adjacently located to fine amyloid fibrils, supporting that IAPP in secretary granules from dying beta-cells served as nidus for islet beta-sheet formation.