Glucagon-inducing polypeptide (GIP) plays a key metabolic homeostasis role in islet cells. It also plays an important role in bone metabolism. GIP is synthesized by enteroendocrine K cells in the intestinal epithelium. It acts on a cells and b cells, and promotes lipid metabolism, insulin gene transcription, and b cell proliferation. Its insulinotropic activity is enhanced by postprandial amino acids.
GIP exerts its insulinotropic activity by activating the GIP receptor through cAMP. Studies in T2DM have shown that GIPR-dependent signaling is defective in a cells. This may be behind the loss of homeostasis in metabolic stress conditions.
In the present study, we explored the influence of GIP/GIPR signaling on glucose disposal and body weight in the context of obese diabetes. We injected GIP analogues intraperitoneally and measured insulin and glucose levels. We found that GIP analogues reduced nonfasting glucose and increased the insulin response. The effect was mediated by a central nervous system-mediated reduction in food intake and energy expenditure.
GIP/GIPR signaling promotes insulin gene transcription, b cell proliferation, and a-to-b cell crosstalk. Moreover, GIP and GIPR seem to co-ordinate their activity on islet cells. Increasing studies indicate that the complex b, a, and d islet communication may be partially coordinated by GIP/GIPR signaling.
In the present study, we evaluated the effect of GIP and GIPR agonism on insulin biosynthesis, cAMP activation, and cyclin-dependent kinase-5 expression in human U937 macrophages. We also found that GIP agonism potentiated the incretin effect of GLP-1. These results suggest the potential of GIP and GIPR agonism for the treatment of metabolic disease.