title: Ubiquitination is involved in glucose-mediated downregulation of GIP receptors in islets.
abstract: Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone that has a potent stimulatory effect on insulin release under conditions of normal glucose tolerance. However, its insulinotropic effect is reduced or even absent entirely in type 2 diabetic patients. In this study, we addressed the role of glucose concentration in the diabetic range of >or=11 mM, i.e., hyperglycemia per se, as a cause of the lack of response to GIP. Culturing rat and human pancreatic islets in >or=11 mM glucose for up to 24 h resulted in prevention of GIP-mediated intracellular cAMP increase compared with culturing in 5 mM glucose. Western blot analysis revealed a selective 67 +/- 2% (rat) and 60 +/- 8% (human) decrease of GIP-R expression in islets exposed to >or=11 mM glucose compared with 5 mM glucose (P < 0.001). We further immunoprecipitated GIP-R from islets and found that GIP-R was targeted for ubiquitination in a glucose- and time-dependent manner. Downregulation of GIP-R was rescued by treating isolated islets with proteasomal inhibitors lactacystin and MG-132, and the islets were once again capable of increasing intracellular cAMP levels in response to GIP. These results suggest that the GIP-R is ubiquitated, resulting in downregulation of the actions of GIP.
authors.affiliation: Diabetes Section, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhou
authors.forename: Mauren F A
authors.initials: MF
authors.lastname: Livak
authors.forename: Michel
authors.initials: M
authors.lastname: Bernier
authors.forename: Denis C
authors.initials: DC
authors.lastname: Muller
authors.forename: Olga D
authors.initials: OD
authors.lastname: Carlson
authors.forename: Dariush
authors.initials: D
authors.lastname: Elahi
authors.forename: Stuart
authors.initials: S
authors.lastname: Maudsley
authors.forename: Josephine M
authors.initials: JM
authors.lastname: Egan
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000096:biosynthesis; D015536:Down-Regulation / Q000187:drug effects; D000067757:Glucagon-Like Peptide-1 Receptor; D005947:Glucose / Q000494:pharmacology*; D006801:Humans; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D011499:Protein Processing, Post-Translational / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011964:Receptors, Gastrointestinal Hormone / Q000235:genetics / Q000378:metabolism*; D018027:Receptors, Glucagon / Q000235:genetics; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: C000593833:GLP1R protein, human; C000593835:Glp1r protein, rat; D000067757:Glucagon-Like Peptide-1 Receptor; D007328:Insulin; D012333:RNA, Messenger; D011964:Receptors, Gastrointestinal Hormone; D018027:Receptors, Glucagon; D025801:Ubiquitin; C041086:gastric inhibitory polypeptide receptor; D000242:Cyclic AMP; D005947:Glucose