👤 Mathieu Gigoux

GIP is well known as a peptide regulating metabolic functions. In this review paper, we summarize a series of data on GIP receptor (GIPR). First, expression study of GIPR in human neuroendocrine tumours showed a very high incidence (nearly 100%) and a high density in both functional and non functional pancreatic tumours, ileal tumours, bronchial tumours and medullary thyroid carcinomas. Then, data on internalization of GIPR following stimulation by GIP are reported. Rapid and abundant internalization of GIPR also found in tumor pancreatic endocrine cells opens the possibility of tumor imaging and eradication using radiolabeled GIP. Interestingly, internalized GIPR continues to signal in early endosomes stimulating production of cAMP and activation of PKA, thus, supporting the view that GIPR signals from both plasma membrane and vesicles of internalization. At last, we summarize data from studies using in synergy molecular modeling and site-directed mutagenesis, which identified crucial amino acids of transmembrane domains of GIPR involved in GIPR binding site of GIP and/or in its activation and coupling to Gs protein. All together, these last molecular data may help to better understand structure-activity relationship data on GIP and GIPR. Show less

G-protein coupled receptors represent the largest family of membrane receptors. G-protein dependent signal of GPCR is classically thought to originate exclusively from the plasma membrane and, until very recently, internalized GPCRs were considered silent. At present, experimental proofs exist showing that GPCR can continue to signal via G proteins after internalization. We demonstrated that, once internalized in early endosomes, Glucose-dependent Insulinotropic Peptide Receptor (GIPR) continues to stimulate production of cAMP and activate PKA. In addition to indirect proofs showing that kinetics of cAMP production and PKA activation depend on internalization and GIPR trafficking, we identified the active form of Gαs on early endosomes containing GIPR and detected a distinct FRET signal accounting for cAMP production at the surface of endosomes containing GIP, relative to endosomes without GIP. Show less

Until very recently, G-protein dependent signal of GPCRs was thought to originate exclusively from the plasma membrane and internalized GPCRs were considered silent. Here, we demonstrated that, once internalized and located in the membrane of early endosomes, glucose-dependent Insulinotropic receptor (GIPR) continues to trigger production of cAMP and PKA activation. Direct evidence is based on identification of the active form of Gαs in early endosomes containing GIPR using a genetically encoded GFP tagged nanobody, and on detection of a distinct FRET signal accounting for cAMP production at the surface of endosomes containing GIP, compared to endosomes without GIP. Furthermore, decrease of the sustained phase of cAMP production and PKA activation kinetics as well as reversibility of cAMP production and PKA activity following GIP washout in cells treated with a pharmacological inhibitor of GIPR internalization, and continuous increase of cAMP level over time in the presence of dominant-negative Rab7, which causes accumulation of early endosomes in cells, were noticed. Hence the GIPR joins the few GPCRs which signal through G-proteins both at plasma membrane and on endosomes. Show less

How incretins regulate presence of their receptors at the cell surface and their activity is of paramount importance for the development of therapeutic strategies targeting these receptors. We have studied internalization of the human Glucose-Insulinotropic Polypeptide receptor (GIPR). GIP stimulated rapid robust internalization of the GIPR, the major part being directed to lysosomes. GIPR internalization involved mainly clathrin-coated pits, AP-2 and dynamin. However, neither GIPR C-terminal region nor β-arrestin1/2 was required. Finally, N-acetyl-GIP recognized as a dipeptidyl-IV resistant analogue, fully stimulated cAMP production with a ∼15-fold lower potency than GIP and weakly stimulated GIPR internalization and desensitization of cAMP response. Furthermore, docking N-acetyl-GIP in the binding site of modeled GIPR showed slighter interactions with residues of helices 6 and 7 of GIPR compared to GIP. Therefore, incomplete or partial activity of N-acetyl-GIP on signaling involved in GIPR desensitization and internalization contributes to the enhanced incretin activity of this peptide. Show less

The Met receptor tyrosine kinase is activated or genetically amplified in some gastric cancers, but resistance to small-molecule inhibitors of Met often emerges in patients. We found that Met abundance correlated with a proliferation marker in patient gastric tumor sections, and gastric cancer cell lines that have MET amplifications depended on Met for proliferation and anchorage-independent growth in culture. Inhibition of Met induced temporal changes in gene expression in the cell lines, initiated by a rapid decrease in the expression of genes encoding transcription factors, followed by those encoding proteins involved in epithelial-mesenchymal transition, and finally those encoding cell cycle-related proteins. In the gastric cancer cell lines, microarray and chromatin immunoprecipitation analysis revealed considerable overlap between genes regulated in response to Met stimulation and those regulated by signal transducer and activator of transcription 3 (STAT3). The activity of STAT3, extracellular signal-regulated kinase (ERK), and the kinase Akt was decreased by Met inhibition, but only inhibitors of STAT3 were as effective as the Met inhibitor in decreasing tumor cell proliferation in culture and in xenografts, suggesting that STAT3 mediates the pro-proliferative program induced by Met. However, the phosphorylation of ERK increased after prolonged Met inhibition in culture, correlating with decreased abundance of the phosphatases DUSP4 and DUSP6, which inhibit ERK. Combined inhibition of Met and the mitogen-activated protein kinase kinase (MEK)-ERK pathway induced greater cell death in cultured gastric cancer cells than did either inhibitor alone. These findings indicate combination therapies that may counteract resistance to Met inhibitors. Show less