Endothelin-converting enzyme-1 regulates glucagon-like peptide-1 receptor signalling and resensitisation.

Following nutrient ingestion, glucagon-like peptide 1 (GLP-1) is secreted from intestinal L-cells and mediates anti-diabetic effects, most notably stimulating glucose-dependent insulin release from pancreatic β-cells but also inhibiting glucagon release, promoting satiety and weight reduction and potentially enhancing or preserving β-cell mass.  These effects are mediated by the GLP-1 receptor (GLP-1R) which is a therapeutic target in type 2 diabetes.  Although agonism at the GLP-1R has been well-studied, desensitisation and resensitisation are perhaps less well explored.  An understanding of these events is important, particularly in the design and use of novel receptor ligands.  Here, using either HEK-293 cells expressing the recombinant human GLP-1R or the pancreatic β-cell line, INS-1E with endogenous expressesion of the GLP-1R, we demonstrate GLP-1R desensitisation and subsequent resensitisation following removal of extracellular GLP-1 7-36 amide.  Resensitisation is dependent on receptor internalisation, endosomal acidification and receptor recycling.  Resensitisation is also regulated by endothelin converting enzyme-1 (ECE-1) activity, most likely through proteolysis of GLP-1 in endosomes and the facilitation of GLP-1R dephosphorylation and recycling. Inhibition of ECE-1 activity also increases GLP-1-induced activation of extracellular signal regulated kinase (ERK) and generation of cAMP, suggesting processes dependent upon the lifetime of the internalised ligand-receptor complex.