Glucagon-Like Peptide-1 Receptor Agonist and Glucagon Increase Glucose-Stimulated Insulin Secretion in Beta Cells via Distinct Adenylyl Cyclases

Diabetes mellitus is a chronic disease in which the pancreas no longer produces enough insulin. Pancreatic alpha cell mass increases in response to insufficient insulin secretion. However, the reason for this increase is not clear. It is possible that the increased alpha-cells may stimulate compensatory insulin release in response to the insufficient insulin such as insulin resistance. In this study, we investigated whether glucagon and glucagon-like peptide-1 (GLP-1), hormones produced by alpha cells, contribute to insulin secretion in INS-1 cells, a beta cell line. We confirmed that alpha cell area in the pancreatic islets and glucagon secretion were increased in HFD-induced obese mice. Co-treatment with glucagon and exendin-4 (Ex-4), a GLP-1 receptor agonist, additively increased glucose-stimulated insulin secretion in INS-1 cells. In parallel, cAMP production was also additively increased by co-treatment with these hormones. The increase of insulin secretion by Ex-4 in the presence of high glucose was inhibited by 2'5'-dideoxyadenosine, a transmembrane adenylyl cyclase inhibitor, but not by KH-7, a soluble adenylyl cyclase inhibitor. The increase of insulin secretion by glucagon in INS-1 cells was inhibited by both 2'5'-dideoxyadenosine and KH-7. We suggest that glucagon and GLP-1 produced from alpha cells additively increase cAMP and insulin secretion in the presence of high glucose via distinct adenylyl cyclases in INS-1 cells, and this may contribute to the compensatory increase of insulin secretion by an increase of pancreatic alpha cell mass under conditions of insulin resistance.