Ligand-induced regulation of glucagon-like peptide-I receptor function and expression in insulin-secreting beta cells

Glucagon-like peptide-I (GLP-I) is a potent incretin hormone and mediates its actions via the cyclic AMP (cAMP) pathway. The GLP-I receptor belongs to the family of seven-transmembrane domain receptors coupled to G proteins, We have analyzed the regulation of GLP-I receptor function and expression by its own ligand and the cAMP-dependent pathway in rat insulinoma-derived beta cells (RINm5F). The GLP-I receptor underwent rapid homologous desensitization, which occurred at the receptor level. This was characterized by a reduced binding capacity not mediated by protein kinase A (PKA), GLP-I receptor mRNA levels were downregulated during incubation of cells by agents increasing cAMP levels including GLP-I itself, This effect was dependent upon time and concentration. Forskolin, the PKA activator 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole-3 ,5-monophosphorothiotate, and GLP-I stabilized the GLP-I receptor mRNA, All induced downregulation of the GLP-I receptor number within 3 h, a time point at which GLP-I receptor mRNA levels were not decreased, This effect was not influenced by cycloheximide. Therefore, in addition to transcriptional effects, posttranslational mechanisms exist to regulate GLP-I receptor numbers in insulin-secreting cells.