A Naturally Occurring GIP Receptor Variant Undergoes Enhanced Agonist-Induced Desensitization, Which Impairs GIP Control of Adipose Insulin Sensitivity

Abstract

d Glucose-dependent insulinotropic polypeptide (GIP), an incretin hormone secreted from gastrointestinal K cells in response to food intake, has an important role in the control of whole-body metabolism. GIP signals through activation of the GIP receptor (GIPR), a G-protein-coupled receptor (GPCR). Dysregulation of this pathway has been implicated in the development of meta-bolic disease. Here we demonstrate that GIPR is constitutively trafficked between the plasma membrane and intracellular com-partments of both GIP-stimulated and unstimulated adipocytes. GIP induces a downregulation of plasma membrane GIPR by slowing GIPR recycling without affecting internalization kinetics. This transient reduction in the expression of GIPR in the plasma membrane correlates with desensitization to the effects of GIP. A naturally occurring variant of GIPR (E354Q) associated with an increased incidence of insulin resistance, type 2 diabetes, and cardiovascular disease in humans responds to GIP stimula-tion with an exaggerated downregulation from the plasma membrane and a delayed recovery of GIP sensitivity following cessa-tion of GIP stimulation. This perturbation in the desensitization-resensitization cycle of the GIPR variant, revealed in studies of cultured adipocytes, may contribute to the link of the E354Q variant to metabolic disease. G lucose-dependent insulinotropic polypeptide (GIP) is se-creted by K cells of the gastrointestinal tract in response to food (1, 2). GIP together with the other incretin hormone, gluca-gon-like peptide 1, have prominent roles in the control of whole-body energy metabolism. A primary function of these hormones is to stimulate glucose-dependent insulin release from pancreatic beta cells (3–5). In addition to its effect on the pancreas, GIP functions to regulate several aspects of adipocyte metabolism, in-cluding increasing the sensitivity of adipocytes to insulin, thereby setting the tone for an optimal insulin response (e.g., see refer-ences 6–13). GIP signals through the GIP receptor (GIPR), a G-protein-coupled receptor (GPCR) coupled to the stimulatory G alpha subunit and elevated cyclic AMP (cAMP) levels (4, 14, 15). In individuals with type 2 diabetes mellitus (T2DM), GIP-me-diated insulinotropic effects are attenuated despite normal to ele-vated levels of blood GIP (16–18). This GIP resistance potentially contributes to the pathophysiology of T2DM. The importance of GIP function in metabolic homeostasis is highlighted by the dis-covery in genome-wide association studies of a number of single nucleotide polymorphisms in the GIPR gene linked to an in-creased risk of metabolic diseases, including insulin resistance, T2DM, and cardiovascular diseases (19–21). One of these variants results in the substitution of glutamine for glutamic acid at posi-tion 354 (E354Q) of GIPR, which has been shown in various stud-ies to be associated with insulin resistance (22), cardiovascular disease (21), and defects in beta-cell function (23). Despite extensive characterization of GIP's effects on metabo-lism, little is known about the behavior of GIPR. Because the traf-ficking behaviors of GPCRs are critical for their signal transduc-tion, we embarked on a study of GIPR trafficking. Here we report that GIPR constitutively cycles between the trans-Golgi network (TGN) and the plasma membrane in both GIP-stimulated and unstimulated adipocytes. GIP induces a rapid, reversible down-regulation of plasma membrane GIPR by promoting a slowing of GIPR recycling without an effect on internalization kinetics. The E354Q substitution results in more pronounced GIP-stimulated downregulation and a prolonged desensitization period. Our data suggest that the link between the E354Q substitution and meta-bolic disease might result from a disruption of the GIPR desensi-tization-resensitization cycle.