Involvement of ATP-sensitive Potassium (KATP) Channels in the Loss of Beta-cell Function Induced by Human Islet Amyloid Polypeptide

Abstract

Islet amyloid polypeptide (IAPP) is a major component of amyloid deposition in pancreatic islets of patients with type 2 diabetes. It is known that IAPP can inhibit glucose-stimulated insulin secretion; however, the mechanisms of action have not yet been established. In the present work, using a rat pancreatic beta-cell line, INS1E, we have created an in vitro model that stably expressed human IAPP gene (hIAPP cells). These cells showed intracellular oligomers and a strong alteration of glucose-stimulated insulin and IAPP secretion. Taking advantage of this model, we investigated the mechanism by which IAPP altered beta-cell secretory response and contributed to the development of type 2 diabetes. We have measured the intracellular Ca 2؉ mobilization in response to different secretagogues as well as mitochondrial metabolism. The study of calcium signals in hIAPP cells demonstrated an absence of response to glucose and also to tolbutamide, indicating a defect in ATP-sensitive potassium (K ATP ) channels. Interestingly, hIAPP showed a greater maximal respiratory capacity than control cells. These data were confirmed by an increased mitochondrial membrane potential in hIAPP cells under glucose stimulation, leading to an elevated reactive oxygen species level as compared with control cells. We concluded that the hIAPP overexpression inhibits insulin and IAPP secretion in response to glucose affecting the activity of K ATP channels and that the increased mitochondrial metabolism is a compensatory response to counteract the secretory defect of beta-cells.Type 2 diabetes is characterized by impaired insulin secretion with a progressive decline in beta-cell mass and function (1, 2). A main characteristic of type 2 diabetes is the extracellular accumulation of amyloid fibrils in pancreatic islets (3, 4). The main component of these deposits is the islet amyloid polypeptide (IAPP).2 Specifically, human IAPP (hIAPP) but not rodent IAPP is amyloidogenic (5). Pancreatic beta-cells co-express and co-secrete IAPP and insulin in response to several secretagogue stimuli (6 -8). It is well established that IAPP is implicated in the pathogenesis of diabetes because it forms amyloid deposits leading to beta-cell death (9 -12). In addition, although different studies have demonstrated that IAPP can inhibit glucose-stimulated insulin secretion (GSIS) (13-15), the mechanisms of action have not been established. In pancreatic beta-cells, GSIS depends critically on the activity of ATP-sensitive potassium channels (K ATP ), which serve as a coupling factor between changes in glucose metabolism and membrane electrical activity (16,17). These channels are hetero-octameric complexes comprising four inward-rectifier potassium ion channels (Kir6.2) and four regulatory sulfonylurea receptors (SUR1) (18). Gain-of-function mutations of Kir6.2 are associated with defective GSIS and the development of type 2 diabetes (19,20), whereas loss of function mutations of Kir6.2 and SUR1 are the most common causes of congenital hyperinsulinism of infancy, ...

Chaperones Ameliorate Beta Cell Dysfunction Associated with Human Islet Amyloid Polypeptide Overexpression

“…We have previously seen that hIAPP-INS1E cells showed no change in cell death and no change in ER stress marker CHOP when compared to rIAPP-INS1E control cells [26]. Here, we found that the upstream pathways involved in ER stress, such as sXBP1 or ATF3, were not affected, confirming that hIAPP overexpression does not lead to ER stress under basal conditions (11 mM glucose).…”

“…Furthermore, we have detected toxic intracellular aggregates in a rat pancreatic beta-cell line overexpressing hIAPP, which lead to a defective insulin and IAPP secretion in response to glucose [26]. Nevertheless, the role of hIAPP overexpression and ER stress induction has not been fully clarified.…”

Stress and the inflammatory process: a major cause of pancreatic cell death in type 2 diabetes

“…In addition, Casas et al demonstrated that extracellular hIAPP aggregation is associated with ER stress responses in mouse β-cells, by an intracellular signaling that involves downstream inhibition of the ubiquitin–proteasome pathway, contributing to β-cell apoptosis 77,78. Nevertheless, in a rat pancreatic β-cell line overexpressing hIAPP, the detection of toxic intracellular oligomers, which lead to defective insulin and IAPP secretion levels in response to glucose, did not change the expression of genes involved in ER stress 79. These results agree with other findings with hIAPP transgenic mice, in which the authors demonstrated that amyloid formation was not associated with significant increases in the expression of ER stress markers 80.…”

Human IAPP amyloidogenic properties and pancreatic β-cell death

“…However, the same final results would be achieved indirectly if hIAPP inhibits of the ATP-sensitive potassium channel (K ATP ) closure which would prevent the ␤-cell membrane depolarization required to induce Ca 2+ channel opening. This interesting problem was addressed by Soty et al [94] who, working with an hIAPP transgenic rat pancreatic ␤-cell line, found that membrane depolarization with 30 mM KCl, raises the intracellular Ca 2+ concentration and induces a very strong secretion of insulin, discarding the proposal that the reduction of GSIS in hIAPP-treated islets could be the result of hIAPP inhibition of voltage-gated Ca 2+ channels [93]. However it should be kept in mind that GSIS may be affected differently by hIAPP if it is added externally to islets or if it is overexpressed endogeneously in transgenic cells.…”

“…As is well known ATP-sensitive potassium (K ATP ) channels are constituted by four pore-forming (Kir6.2) subunits and four regulatory sulfonylurea receptors (SUR1) [95]. There is a marked alteration in glucose-stimulated insulin and hIAPP secretion in the transgenic cells expressing hIAPP [94]. The cells, which show intracellular oligomers but no evidence of enhanced apoptosis or ER stress, are not responsive to induction of insulin and hIAPP secretion by glucose or tolbutamide.…”