AGONIST-INDUCED INTERNALIZATION AND RECYCLING OF THE GLUCAGON-LIKE PEPTIDE-1 RECEPTOR IN TRANSFECTED FIBROBLASTS AND IN INSULINOMAS

Glucagon-like peptide-1 (GLP-1) is the most potent stimulator of gluco se-induced insulin secretion and its pancreatic beta-cell receptor is a member of a new subfamily of G-protein-coupled receptors which inclu des the receptors for vasoactive intestinal polypeptide, secretin and glucagon. Here we studied agonist-induced GLP-1 receptor internalizati on in receptor-transfected Chinese hamster lung fibroblasts using thre e different approaches. First, iodinated GLP-1 bound at 4 degrees C to transfected cells was internalized with a t(1/2) of 2-3 min following warming up of the cells to 37 degrees C. Secondly, exposure to GLP-1 induced a shift in the distribution of the receptors from plasma membr ane-enriched to endosomes-enriched membrane fractions, as assessed by Western blot detection of the receptors using specific antibodies. Thi rdly, continuous exposure of GLP-1 receptor-expressing cells to iodina ted GLP-1 led to a linear accumulation of peptide degradation products in the medium following a lag time of 20-30 min, indicating a continu ous cycling of the receptor between the plasma membrane and endosomal compartments. Potassium depletion and hypertonicity inhibited transfer rin endocytosis, a process known to occur via coated pit formation, as well as GLP-1 receptor endocytosis. In contrast to GLP-1, the antagon ist exendin-(9-39) did not lead to receptor endocytosis. Surface re-ex pression following one round of GLP-1 receptor endocytosis occurred wi th a half-time of about 15 min. The difference in internalization and surface re-expression rates led to a progressive redistribution of the receptor in intracellular compartments upon continuous exposure to GL P-1. Finally, endogenous GLP-1 receptors expressed by insulinoma cells were also found to be internalized upon agonist binding. Together our data demonstrate that the GLP-1 receptor is internalized upon agonist binding by a route similar to that taken by single transmembrane segm ent receptors. The characterization of the pathway and kinetics of GLP -1-induced receptor endocytosis will be helpful towards understanding the role of internalization and recycling in the control of signal tra nsduction by this receptor.