Intracellular Degradation of Insulin in Pancreatic Islets

There is a substantial intracellular degradation of insulin in pancreatic islets. This may be a physiological process, which, in correspondence with biosynthesis and secretion of insulin, would optimize the secretory granule content of the pancreatic β-cell. Insulin degradation may be effected by crinophagy, a process where secretory granules fuse with lysosomes. The general aim of this thesis was to investigate possible control mechanisms for intracellular degradation of insulin and crinophagy in isolated pancreatic islets. In islets incubated at low glucose concentrations there was an insulin degradation and this correlated well with the ultrastructural findings, where a lot of secondary lysosomes containing secretory granules were found. In islets incubated at a high glucose concentration there was no insulin degradation and the ultrastructure revealed only a few insulin granules and mostly primary lysosomes, indicating that there was no crinophagic activity. With interleukin-1β the islet insulin degradation, nitric oxide production and prostaglandin E2 production were increased. The effects were abolished either by inhibition of inducible nitric oxide synthetase by aminoguanidine, or by the specific cyclooxygenase-2 inhibitor rofecoxib. These findings indicate that there is a connection between the intracellular degradation of insulin, production of nitric oxide and cyclooxygenase-2 activation. The nitric oxide donor DETA/NO enhanced the intracellular degradation of insulin and cyclooxygenase-2 activation with subsequent production of prostaglandin E2, suggesting that the link between nitric oxide and insulin degradation may be a cyclooxygenase-2 activation and subsequent prostaglandin E2 production. With corticosterone added to islet incubations the insulin degradation decreased, which paralleled with a diminished crinophagy and formation of prostaglandin E2. With progesterone there was instead an increase in insulin degradation and crinophagy and an increased formation of prostaglandin E2. These effects were abolished by mifepristone, an inhibitor of intracellular corticosterone and progesterone receptors. This suggests that the effects from these steroids are exerted via a change in islet gene expression and cyclooxygenase-2 activation. It was also concluded that phospholipase A2 is involved in insulin degradation and that the isoform secretory phospholipase A2 may be involved in triggering this process. This suggests that cyclooxygenase-2 activation with a subsequent production of prostaglandin E2 may provide a control mechanism for intracellular degradation of insulin and crinophagy in pancreatic islets