Islet acid glucan-1,4-alpha-glucosidase: a putative key enzyme in nutrient-stimulated insulin secretion

Little attention has been paid to a possible relationship between lysosomal function and stimulation of secretory processes in endocrine cells. The last few years it has become increasingly evident that the secretion of insulin from the pancreatic beta-cell is the result of a very complex cascade of events, the details of which are far from elucidated and indeed may include the participation of the lysosomal system. We report here, with a combined in vitro and in vivo approach, that selective inhibition of islet lysosomal glycogenolytic acid glucan-1,4-alpha-glucosidase activity by the long-acting 1-deoxynojirimycin derivative emiglitate induces a profound suppression of nutrient-induced insulin release. In islet homogenate emiglitate strongly and dose-dependently inhibited the activity of acid glucan-1,4-alpha-glucosidase (EC50 approximately 10(-6) M) without affecting other classical lysosomal enzyme activities. The emiglitate-induced inhibition curve for glucose-stimulated insulin secretion from isolated islets was remarkably similar to the inhibition curve for acid glucan-1,4-alpha-glucosidase. Moreover, insulin release stimulated by the nonglucose nutrient secretagogues, leucine, and alpha-ketoisocaproic acid (KIC) was totally suppressed by emiglitate. In contrast, receptor activated insulin secretion induced by the insulinotropic hormone cholecystokinin (CCK-8) was unaffected by the drug. Further, parenteral pretreatment of mice with emiglitate markedly suppressed the insulin secretory response to an iv injection of glucose or KIC, whereas the response to an iv injection of CCK-8 was unaffected. In accordance with this, islets isolated from emiglitate-treated mice showed a reduced activity of acid glucan-1,4-alpha-glucosidase and, moreover, such islets incubated in vitro, secreted less insulin in response to glucose than did control islets. Finally, pretreatment of mice with purified fungal acid glucan-1,4-alpha-glucosidase, enzyme replacement, brought about a markedly increased insulin secretory response after an iv injection of KIC, whereas the insulin response after CCK-8 injection was unaffected. Taken together with previous observations, the present data strongly suggest that islet lysosomal acid alpha-glucosidehydrolases are involved in the multifactorial process of nutrient-induced insulin secretion. The existence of hitherto unresolved and complex interactions between different beta-cell organelles in the insulin secretory processes should be thoroughly reevaluated.