Exploring the functional, protective, and transcriptomic effects of GIP on cytokine-exposed human pancreatic islets and EndoC-βH5 cells
Immune-mediated beta-cell destruction and lack of alpha-cell responsiveness to hypoglycaemia are hallmarks of type 1 diabetes pathology. The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) may hold therapeutic potential for type 1 diabetes due to its insulinotropic and glucagonotropic effects, as well as its beta-cell protective effects shown in rodent islets. Here, we examined the functional, protective, and transcriptomic effects of GIP treatment upon diabetogenic cytokine exposure to interleukin (IL)-1{beta} {+/-} interferon (IFN)-{gamma} in human EndoC-{beta}H5 beta cells and isolated human islets, respectively. GIP dose-dependently augmented glucose-stimulated insulin secretion from EndoC-{beta}H5 cells and increased insulin and glucagon secretion from human islets during high and low glucose concentrations, respectively. The insulinotropic effect of GIP in EndoC-{beta}H5 cells was abrogated by KN-93, an inhibitor of calcium/calmodulin-dependent protein kinase 2 (CaMK2). GIP did not prevent cytokine-induced apoptosis or cytokine-induced functional impairment of human EndoC-{beta}H5 cells. GIP also did not prevent cytokine-induced apoptosis in human islets. GIP treatment of human islets with or without cytokines for 24 hours did not significantly impact the transcriptome. GIP potentiated cytokine-induced secretion of IL-10 and c-c motif chemokine ligand (CCL)-2 from human islets while decreasing the secretion of c-x-c motif chemokine ligand (CXCL)-8. In EndoC-{beta}H5 cells, GIP reduced IFN-{gamma}-induced secretion of tumor necrosis factor (TNF)-, IL-2, IL-6, and IL-10 but increased the secretion of CXCL8, CCL2, CCL4, and CCL11. In conclusion, our results suggest that the insulinotropic effect of GIP is CaMK2-dependent. Furthermore, our results indicate that GIP does not provide substantial cytoprotective effects against diabetogenic cytokine challenge or significantly modulate the transcriptome of human islets when applied at a supraphysiological level. GIP may, however, still exert selective inflammation-modulatory effects upon diabetogenic cytokine exposure.