Intracellular degradation of insulin and crinophagy are maintained by nitric oxide and cyclo-oxygenase 2 activity in isolated pancreatic islets

Background information. Pancreatic β‐cells require an optimal insulin content to allow instantaneous secretion of insulin. This is maintained by insulin biosynthesis and intracellular degradation of insulin. Degradation may be effected by crinophagy, i.e. the fusion of secretory granules with lysosomes. IL‐1β (interleukin 1β) induces distinct changes of β‐cell lysosomes. To study the mechanisms for intracellular insulin degradation and crinophagy, isolated mouse pancreatic islets were exposed to IL‐1β and known pathways for IL‐1β actions were blocked. Intracellular insulin degradation was determined by following the fate of radioactively labelled insulin. Crinophagy was studied by ultrastructural analysis. The effects of blocking pathways for IL‐1β were monitored by measurements of nitrite and PGE2 (prostaglandin E2). Results. IL‐1β caused an enhancement of islet intracellular insulin degradation and an increase in the lysosomal incorporation of β‐cell secretory granules. The effects of IL‐1β were abolished by aminoguanidine, a selective inhibitor of inducible NOS (nitric oxide synthase), or by rofecoxib, a specific inhibitor of COX‐2 (cyclo‐oxygenase 2). In the absence of IL‐1β, nitroarginine, which is a selective inhibitor of constitutive NOS, caused a decrease in intracellular degradation of insulin in parallel with a decreased production of NO and PGE2 by the islets. Conclusions. The correlation between the enhanced intracellular insulin degradation and lysosomal changes caused by IL‐1β suggests that insulin degradation may be effected by crinophagy. Under physiological conditions, significant β‐cell degradation of insulin may depend on the activity of COX‐2, possibly stimulated by endogenous NO.