title: Regulation of GIP and GLP1 receptor cell surface expression by N-glycosylation and receptor heteromerization.
abstract: In response to a meal, Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-like Peptide-1 (GLP-1) are released from gut endocrine cells into the circulation and interact with their cognate G-protein coupled receptors (GPCRs). Receptor activation results in tissue-selective pleiotropic responses that include augmentation of glucose-induced insulin secretion from pancreatic beta cells. N-glycosylation and receptor oligomerization are co-translational processes that are thought to regulate the exit of functional GPCRs from the ER and their maintenance at the plasma membrane. Despite the importance of these regulatory processes, their impact on functional expression of GIP and GLP-1 receptors has not been well studied. Like many family B GPCRs, both the GIP and GLP-1 receptors possess a large extracellular N-terminus with multiple consensus sites for Asn-linked (N)-glycosylation. Here, we show that each of these Asn residues is glycosylated when either human receptor is expressed in Chinese hamster ovary cells. N-glycosylation enhances cell surface expression and function in parallel but exerts stronger control over the GIP receptor than the GLP-1 receptor. N-glycosylation mainly lengthens receptor half-life by reducing degradation in the endoplasmic reticulum. N-glycosylation is also required for expression of the GIP receptor at the plasma membrane and efficient GIP potentiation of glucose-induced insulin secretion from the INS-1 pancreatic beta cell line. Functional expression of a GIP receptor mutant lacking N-glycosylation is rescued by co-expressed wild type GLP1 receptor, which, together with data obtained using Bioluminescence Resonance Energy Transfer, suggests formation of a GIP-GLP1 receptor heteromer.
authors.affiliation: Cardiovascular Research Group, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Gina M
authors.initials: GM
authors.lastname: Whitaker
authors.forename: Francis C
authors.initials: FC
authors.lastname: Lynn
authors.forename: Christopher H S
authors.initials: CH
authors.lastname: McIntosh
authors.forename: Eric A
authors.initials: EA
authors.lastname: Accili
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002522:Chlorocebus aethiops; D016384:Consensus Sequence; D000242:Cyclic AMP / Q000096:biosynthesis; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005786:Gene Expression Regulation; D000067757:Glucagon-Like Peptide-1 Receptor; D006031:Glycosylation; D006801:Humans; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D008024:Ligands; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D020033:Protein Isoforms; D055503:Protein Multimerization; D011964:Receptors, Gastrointestinal Hormone / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018027:Receptors, Glucagon / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment
chemical_list: C000593833:GLP1R protein, human; D000067757:Glucagon-Like Peptide-1 Receptor; D007328:Insulin; D008024:Ligands; D008562:Membrane Glycoproteins; D020033:Protein Isoforms; D011964:Receptors, Gastrointestinal Hormone; D018027:Receptors, Glucagon; C041086:gastric inhibitory polypeptide receptor; D000242:Cyclic AMP