Lysosomes and Pancreatic Islet Function: Time Course of Insulin Biosynthesis, Insulin Secretion, and Lysosomal Transformation After Rapid Changes in Glucose Concentration

The aim of this study was to estimate the time course of lysosomal transformations associated with crinophagy, i.e., the degradation of insulin within lysosomes, in the beta-cells of pancreatic islets. Primary and secondary lysosomes were identified in mouse islet β-cells and subjected to ultrastructural morphometry. Islets from an in situ preparation were compared with isolated islets incubated in vitro. Under the in vitro conditions, the islets were initially exposed to 28 or 3.3 mM glucose for 24 h. Then the glucose concentration was rapidly changed to 3.3 and 28 mM glucose, respectively, and the islets were incubated for up to an additional 24 h. The β-cell lysosomes were analyzed and related to alterations in insulin biosynthesis and secretion and islet insulin content after the rapid change in glucose concentration. In vivo, the β-cell lysosomal population was predominantly composed of secondary lysosomes, which frequently contained secretory granule cores. After the initial 24-h period at 3.3 mM glucose, the volume density and the average volume of the secondary β-cell lysosomes were increased, suggesting increased crinophagic activity. The mean diameter of the primary β-cell lysosomes was decreased after 24 h at either 28 or 3.3 mM glucose. The change in glucose from 28 to 3.3 mM resulted in alterations in insulin biosynthesis and secretion, leading to an accumulation of insulin within the β-cells. Lysosomal transformations suggestive of increased crinophagy were observed 24 h after the alteration in glucose concentration. The change from 3.3 to 28 mM glucose resulted in a parallel increase in insulin biosynthesis and secretion without a change in islet insulin content. Twenty-four hours after the change in glucose concentration, the lysosomal population was changed from one composed predominantly of secondary lysosomes to one composed predominantly of primary lysosomes. The results lend support to the notion that there is significant crinophagic activity in the pancreatic β-cell in vivo. It is further suggested that an increase in β-cell secretory granule content, resulting from differences in the glucose-stimulated response of insulin biosynthesis and secretion, stimulates crinophagy. The time course of the lysosomal alterations after rapid changes in glucose concentration suggests that the lysosomes are mainly involved in the long-term adaptation of the pancreatic β-cell to varying functional demands.