Exploring the functional, protective, and transcriptomic effects of GIP on cytokine-exposed human pancreatic islets...

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 cytoprotective effects shown in rodent beta cells. To further increase our understanding of GIP's effects on human beta cells, we here examined the functional, protective, and transcriptomic effects of GIP in human EndoC-βH5 beta cells and isolated human islets in the presence or absence of proinflammatory cytokines (interferon (IFN)-γ ± interleukin (IL)-1β) as a mimic of type 1 diabetes. GIP dose-dependently augmented glucose-stimulated insulin secretion from EndoC-βH5 cells and increased insulin and glucagon secretion from human islets at high and low glucose concentrations, respectively. The insulinotropic effect of GIP in EndoC-βH5 cells was abrogated by KN-93, an inhibitor of calcium/calmodulin-dependent protein kinase 2 (CaMK2). GIP did not prevent cytokine-induced apoptosis in EndoC-βH5 cells or human islets, and GIP did not protect against cytokine-induced functional impairment in EndoC-βH5 cells. GIP treatment of human islets for 24 h had no effects on the transcriptome and did not modulate cytokine-induced transcriptional changes. However, GIP augmented IL-1β + IFNγ-induced secretion of interleukin (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-βH5 cells, GIP reduced IFN-γ-induced secretion of tumor necrosis factor (TNF)-α, 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 findings indicate that GIP neither exerts cytoprotective effects against cytokines nor modulate the transcriptome of human islets. GIP may, however, exert selective modulatory effects on secreted inflammatory factors from cytokine-exposed beta cells and islets.