Role of cyclic AMP signaling in the production and function of the incretin hormone glucagon-like peptide-1

Pancreatic cells express the proglucagon gene (gcg) and thereby produce the peptide hormone glucagon,which stimulates hepatic glucose production and thereby increases blood glucose levels.The same gcg gene is also expressed in the intestinal endocrine L cells and certain neural cells in the brain.In the gut,gcg expression leads to the production of glucagon-like peptide-1 (GLP-1).This incretin hormone stimulates insulin secretion when blood glucose,level is high.In addition,GLP-1 stimulates pancreatic cell proliferation,inhibits cell apoptosis,and has been utilized in the trans-differentiation of insulin producing cells.Today,a longterm effective GLP-1 receptor agonist has been developed as a drug in treating diabetes and potentially other metabolic disorders.Extensive investigations have shown that the expression of gcg and the production of GLP-1 can be activated by the elevation of the second messenger cyclic AMP (cAMP).Recent studies suggest that in addition to protein kinase A (PKA),exchange protein activated by cAMP (Epac),another effector of cAMP signaling,and the crosstalk between PKA and Wnt signaling pathway,are also involved in cAMP-stimulated gcg expression and GLP-1 production.Furthermore,functions of GLP-1 in pancreatic cells are mainly mediated by cAMP-PKA,cAMP-Epac and Wnt signaling pathways as well.