Lysosomes and pancreatic islet function: intracellular insulin degradation and lysosomal transformations

Previous observations indicate that substantial amounts of insulin are degraded intracellularly in isolated pancreatic islets exposed to low or physiological glucose concentrations. The intracellular degradation of insulin seems to occur by a crinophagic process. The aim of the present investigation was to elucidate the role of glucose both in the intracellular insulin degradation and in the lysosomal changes associated with crinophagy in the pancreatic islets. Isolated islets were maintained for 1 week in tissue culture at glucose concentrations of either 3.3 or 28 mmol/l. At both glucose concentrations experimental conditions producing both low and high rates of insulin secretion were examined. Differences in intracellular insulin degradation between the low and high insulin secretory conditions were estimated by measuring insulin biosynthesis and the total amounts of intracellular and extracellular insulin in the islet cultures. Primary and secondary lysosomes in the islets were strictly defined by morphological criteria and evaluated by ultrastructural morphometry. The intracellular degradation of insulin was significantly enhanced in the low-secretory conditions, irrespective of the glucose concentration. Also at both glucose concentrations, primary lysosomes occurred with higher frequency in islets with a high insulin secretion, whereas secondary lysosomes predominated in islets with a low rate of secretion. Thus, the degradative process is manifested as a transformation of lysosomes from their primary to their secondary form. Furthermore, it is suggested that glucose does not interfere directly with crinophagy but rather affects intracellular degradation of insulin in the pancreatic B cells by changing retention of secretory material.