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

Abstract

dGlucose-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 metabolic disease. Here we demonstrate that GIPR is constitutively trafficked between the plasma membrane and intracellular compartments 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 stimulation with an exaggerated downregulation from the plasma membrane and a delayed recovery of GIP sensitivity following cessation 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 secreted by K cells of the gastrointestinal tract in response to food (1, 2). GIP together with the other incretin hormone, glucagon-like peptide 1, have prominent roles in the control of wholebody 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, including increasing the sensitivity of adipocytes to insulin, thereby setting the tone for an optimal insulin response (e.g., see references 6-13). GIP signals through the GIP receptor (GIPR), a Gprotein-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-mediated insulinotropic effects are attenuated despite normal to elevated levels of blood . This GIP resistance potentially contributes to the pathophysiology of T2DM. The importance of GIP function in metabolic homeostasis is highlighted by the discovery in genome-wide association studies of a number of single nucleotide polymorphisms in the GIPR gene linked to an increased 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 position 354 (E354Q) of GIPR, which has been shown in various studies 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 metabolism, little is known about the beha...

Internalization and desensitization of the human glucose-dependent-insulinotropic receptor is affected by N-terminal acetylation of the agonist

“…Our results on GIPR internalization in HEK cells differ from those recently reported in transfected 3T3L1 adipocytes (Mohammad et al, 2014). In the last study, it was shown that GIPR constitutively internalizes and recycles to the cell surface and GIP regulates down-regulation of plasma membrane GIPR by slowing GIPR recycling without affecting kinetics of GIPR internalization (Mohammad et al, 2014).…”

“…In the last study, it was shown that GIPR constitutively internalizes and recycles to the cell surface and GIP regulates down-regulation of plasma membrane GIPR by slowing GIPR recycling without affecting kinetics of GIPR internalization (Mohammad et al, 2014). We have no explanation for such different findings excepted that GIPR, like other G-protein coupled receptors, could behave differently according to the cell context (Tobin et al, 2008).…”

“…First, although exposure of pancreatic islet cells to GIP has been shown to produce homologous desensitization of the GIP receptor, the impact of GIPR internalization and trafficking on GIPR-dependent response and the underlying molecular mechanisms have not been investigated in detail yet (Tseng et al, 1996;Hinke et al, 2000). Recently, it has been reported that in 3T3-L1 adipocytes, GIPR constitutively internalizes and recycles to the cell surface and that GIP induces a down-regulation of plasma membrane GIPR by slowing GIPR recycling without affecting kinetics of GIPR internalization (Mohammad et al, 2014). Accordingly, GIPR does not conform to the typical behavior of G-protein coupled receptors which has been essentially studied for members of rhodopsin-related GPCRs (sub-family 1) and much less frequently for members of sub-family 2 of GPCRs such as the GIPR (Marchese et al, 2008;Ferguson, 2001).…”

Glucose-Dependent Insulinotropic Polypeptide Stimulates Osteopontin Expression in the Vasculature via Endothelin-1 and CREB

“…Using PolyPhen-2, a tool to predict the possible impact of an amino acid substitution on the structure and function of a human protein (http://genetics.bwh.harvard .edu/pph2/), this amino acid substitution is predicted to be "probably damaging," with a score of 1.000. This variant, also designated E354Q, was recently shown to cause GIP-induced desensitization by increasing internalization of the GIPR (66). However, based on the impact of the variant studied here (rs10423928) on OPN concentrations after GIP infusion, it seems to be associated with a gain of function, but final proof will require additional studies to elucidate the effects of GIP on different human vessels in relation to genotype.…”

Downregulation of a GPCR by β-Arrestin2-Mediated Switch from an Endosomal to a TGN Recycling Pathway

“…The GIP receptor (GIPR) is coupled to trimeric Gα s signal transduction. Trafficking of GIPR does not conform to the typical GPCR behavior (Mohammad et al, 2014). In adipocytes, a cell type that natively expresses GIPR, the receptor is constitutively internalized and recycled to the PM independent of GIP stimulation.…”