Endocrine Abstracts

Islet amyloid deposition is a characteristic feature of Type 2 diabetes. Accumulation of IAPP by abnormal intracellular peptide degradation in beta-cell could promote fibril formation. IAPP is present in high concentrations in beta-cell lysosomes. To determine if fibrils form in lysosomes and the pathway for the accumulation human islets were cultured under conditions of elevated production of insulin (11mM glucose) and inhibition of secretion (0.6mM diazoxide) and/or lysosomal proteolysis (leupeptin 250mM) for 24h. Insulin secretion was measured by RIA. Changes in cellular content of granules and lysosomes were determined by quantitative immuno-electron microscopy. In the presence of 11 mM glucose and 0.6mM diazoxide (n=3), insulin secretion decreased and granule content (granule/cytoplasmic area, n=45 cells/exp), was unchanged. Immunogold labelling for IAPP (gold particles/lysosome, GPL) increased (0.9+/-0.2 to 1.9+/-0.4, p<0.02) without change in lysosomal area. Leupeptin did not affect insulin secretion but increased lysosomal area proportion/beta cell (0.01+/-0.001 to 0.15+/-0.002 area density, p<0.0001) and labelling for IAPP (0.9+/-0.2 to 3.3+/-0.8, GPL, p<0.0001). No intracellular amyloid fibrils were formed. IAPP accumulates, in lysosomes by crinophagy and slow proteolytic degradation. Lysosomal IAPP is not fibrillogenic but could act as a nidus for extracellular fibril formation when beta cells die.