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Ubiquitination is involved in glucose-mediated downregulation of GIP receptors in islets

¹Diabetes Section, ²Clinical Research Branch, ³Laboratory of Neuroscience, National Institute on Aging, National Institutes of Health; and ⁴Department of Surgery, Johns Hopkins Bayview Medical Center, Baltimore, Maryland

Submitted 29 January 2007 ; accepted in final form 12 May 2007

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 11 mM, i.e., hyperglycemia per se, as a cause of the lack of response to GIP. Culturing rat and human pancreatic islets in 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 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.

pancreatic islet; cyclic adenosine monophosphate; insulin; multivesicular body; glucose-dependent insulinotropic polypeptide