Glucagon Potentiates Insulin Secretion Via β-Cell GCGR at Physiological Concentrations of Glucose

Incretin-potentiated glucose-stimulated insulin secretion (GSIS) is critical to maintaining euglycemia, of which GLP-1 receptor (GLP-1R) on beta-cells plays an indispensable role. Recently, alpha-cell-derived glucagon but not intestine-derived GLP-1 has been proposed as the critical hormone that potentiates GSIS via GLP-1R. However, the function of glucagon receptors (GCGR) on beta-cells remains elusive. Here, using GCGR or GLP-1R antagonists, in combination with glucagon, to treat single beta-cells, alpha-beta cell clusters and isolated islets, we found that glucagon potentiates insulin secretion via beta-cell GCGR at physiological but not high concentrations of glucose. Furthermore, we transfected primary mouse beta-cells with RAB-ICUE (a genetically encoded cAMP fluorescence indicator) to monitor cAMP level after glucose stimulation and GCGR activation. Using specific inhibitors of different adenylyl cyclase (AC) family members, we revealed that high glucose concentration or GCGR activation independently evoked cAMP elevation via AC5 in beta-cells, thus high glucose stimulation bypassed GCGR in promoting insulin secretion. Additionally, we generated beta-cell-specific GCGR knockout mice which glucose intolerance was more severe when fed a high-fat diet (HFD). We further found that beta-cell GCGR activation promoted GSIS more than GLP-1R in HFD, indicating the critical role of GCGR in maintaining glucose homeostasis during nutrient overload.

Automated summary

The study found that glucagon can enhance insulin secretion through beta-cell GCGR at physiological glucose concentrations, while high glucose bypasses GCGR to promote insulin secretion. In high-fat diet conditions, GCGR plays a more critical role than GLP-1R in maintaining glucose homeostasis.