Glucose-Dependent Insulinotropic Polypeptide Receptor (GIPR)
Way back in 1900, duodenal mucous extract was shown to inhibit gastric acid secretion. It was in 1930 that Kosaka and Lim observed that intravenous infusion of intestinal extracts led to inhibition of gastric acid secretion and gastric emptying; the term “enterogastrone” was subsequently floated (Kosaka and Lim 1930; Meier et al. 2002). Brown and coworkers purified this extract to homogeneity and reported the amino acid sequence of this newly found hormone naming it as “Gastric Inhibitory Polypeptide” (GIP) since the latter also inhibited gastric acid secretion. In parallel, Moore and coworkers had for the first time (1906) found antidiabetogenic effect of duodenal mucous membrane extract (Moore 1906). The development of immunoassay for insulin in 1960 revealed that the duodenal extract stimulated secretion of insulin from pancreas in humans. It was also observed that orally administered glucose instigated higher insulin secretion as compared to glucose administered intravenously. This established a hormonal link between intestine and pancreas which was later termed as enteroinsular axis. Subsequent experimentation led to the finding that two peptides (commonly termed as incretins) were released from intestine upon oral food intake that led to the increased insulin secretion from the pancreas. Interestingly, one of the peptides was found to be same as GIP and since it was involved in glucose-mediated insulin secretion, it was renamed by Brown and Pederson as glucose-dependent insulinotropic polypeptide (GIP) maintaining the same acronym; the other peptide being termed as glucagon like polypeptide-1(GLP-1). The insulinotropic effect on pancreatic islet β-cells was then recognized to be the principal physiologic action of GIP. Together with glucagon-like peptide-1, GIP is largely responsible for the secretion of insulin postprandially. In the course of trying to elucidate the molecular mechanism of insulin secretion and GIP effect, functionally relevant and high affinity-binding sites for GIP as GIP receptors were identified on the surface of hamster β-cells by radioisotopic assay using 125I-labeled GIP (Amiranoff et al. 1984, 1985, 1986). Subsequently, the expression of the mRNA for GIP receptor was observed in pancreatic cells followed by its cloning, functional expression, sequencing, and chromosomal localization (Takeda et al. 1987; Gremlich et al. 1995; Yamada et al. 1995).
Physiological Actions of GIP
Glucose-Dependent Insulinotropic Polypeptide Receptor (GIPR)
The human GIPR (hGIPR) is a 466 amino acid, seven transmembrane (heptahelical) receptor belonging to class B of the G-protein-coupled receptor (GPCR) family. It is a glycoprotein, well conserved across all mammalian cells. However, GIP receptors have not been identified in non-mammalian species to date. The gene encoding hGIPR is located on chromosome 19q13.3 and comprises of 14 exons and 12 introns (Gremlich et al. 1995). GIPR is abundantly expressed on the surface of the β-cells of the pancreas and those of the adipose tissue, bone, and nerves. It is also known as a secretin receptor since it belongs to the same family that contains secretin, glucagon, glucagon-like peptide-1 (GLP-1), vasoactive intestinal polypeptide (VIP), growth-hormone-releasing hormone (GHRH), and pituitary adenylate cyclase-activating polypeptide (PACAP). It is involved in the transmission of vital secretory and mitogenic signals to the inside of the cell and activates the intracellular signal transduction pathways (discussed later). GIPR is activated by the binding of GIP, secreted by the K-cells of the duodenum. GIP comes into blood circulation in response to nutrient (fatty acid, glucose) absorption in proportionate amount and is involved in the secretion of insulin from the β-cells of pancreas and thus it is known to be glucose dependent. The insulinotropic action of the peptide is known as incretin effect and due to this GIP-GIPR has immense antidiabetogenic potential (Gault et al. 2003; Ranganath 2008).
Structural Organization and Dynamics of GIP–GIPR Interaction
Like any GPCR, the GIPR assembly on the cell membrane has five domains – the extracellular N-terminus domain (residues 22–138), intracellular C-terminus domain (residues 399–466), seven transmembrane domains (TM) (TM1 residues139–161, TM2 residues 170–189, TM3 residues 218–242, TM4 residues 255–278, TM5 residues 294–319, TM6 residues 342–362, TM7 378–398), three extracellular loops (ECl) (ECl1 residues 190–217, ECl2 residues 279–293, ECl3 residues 363–377) and three intracellular loops (ICl) (ICl1 residues 162–169, ICl2 residues 243–254, ICl3 residues 320–341). The transmembrane domains are linked to each other by intracellular loops on the inside of the cell and by the extracellular loops on the outside. The C-terminus of the receptor is coupled to a heterotrimeric GTP-binding protein also called G-protein, which is made up of three subunits Gα, Gβ, and Gγ. The Gα subunit harbors a catalytic site for binding GTP and in an inactive state of the receptor it is bound to GDP. Upon activation by the receptor, GTP displaces GDP at the Gα subunit, thereby activating it. The GβGγ dimer dissociates from the trimeric form and Gα activates its membrane bound effector, adenylyl cyclase, causing a cascade of reactions in the downstream signaling pathway (Baggio and Drucker 2007; McIntosh et al. 2009).
According to the receptoral dynamics suggested by Hoare, the N-terminus of the receptor is responsible for high affinity binding to the C-terminus of GIP (Hoare 2005). Once bound, the N-terminus of GIP is projected onto the extracellular loops of GIPR for its activation. Upon activation, the signal is transmitted within the cell through one of the transmembrane domains to the intracellular C-terminus domain. A recently solved crystal structure (Parthier et al. 2007) of GIP bound to the extracellular N-terminus domain reveals the N-terminus domain of the receptor as a a-helical structure spanning amino acids Ala32 to Ala52 and the two antiparallel β sheets (β1a: Ser64 to Phe65; β1b: Cys70 to Trp71; β2a: Ala78 to Ser83; β2b: Phe98 to Cys103) and two short helices at the C–terminus end (His91 to Val94 and Thr116 to Cys118). There are three disulfide bonds, a typical characteristic of family B GPCR, these are between the N-terminus and the first β sheet (Cys46 and Cys70), between the two β sheets (Cys61 and Cys103) and between Cys84 and Cys118. GIP is also seen to be composed of an α helix that spans Phe6 to Ala28. The helix is partly amphipathic in nature between Gln20 and Ala28 with all the hydrophobic amino acids (Phe22, Val23, Leu26, and Leu27) aligned on one side and projected towards the N-terminus domain of the receptor; this suggests that the interaction between GIP and its receptor is primarily hydrophobic in nature. Apart from hydrophobic interactions, hydrogen bonding also occurs between the peptide and the receptor domain.
The Signal Transduction Pathways of GIP-GIPR
Insulin Secretory Pathway
GIPR is also known to stimulate insulin secretion by another pathway which leads to increased arachidonic acid production through Group VIA islet Ca2+-independent phospholipase A2 (iPLA2).
β-Cell Growth and Survival Pathways
The PKA/ CREB pathway involves activation of PKA by GIPR signaling and inhibition of phosphorylation of the cytoplasmic enzyme adenosine mono phosphate kinase (AMPK), which is responsible for phosphorylation of TORC2. TORC2 is a coactivator of the nuclear regulating factor CREB, present inside the nucleus. Under the reduced state of phosphorylation, the nuclear localization of TORC2 increases and within the nucleus it interacts with phospho- CREB. The union of the TORC2-phospho- CREB binds to the CRE-1 element of the Bcl-2 promoter and upregulates the expression of the downstream Bcl-2 gene. The Bcl-2 protein so formed prevents the apoptosis of β-cells (Kim et al. 2008).
The growth and cell survival effect of GIPR signaling also occurs via a PKA-independent mechanism. GIPR activation stimulates phosphatidylinositol 3-kinase (PI3K), which activates the downstream protein kinase B ( PKB) by phosphorylation. The so activated PKB in turn phosphorylates various components of the apoptotic machinery such as caspase 9, Bad, glycogen synthase kinase 3B, and a member of the forkhead/winged helix/Foxo family response element (FHRE) in β-cells and downregulates Bax gene. The product of the Bax gene plays an important role as a mediator of apoptotic cell death (Kim et al. 2005).
GIPR is also known to exert a mitogenic effect on β-cells by activating the mitogen-activated protein kinase (MAPK) or the extracellular signal-regulated kinase1/2 ( MEK1/2 – ERK1/2) modules and the PI3K/ PKB pathway. The MEK1/2 and ERK1/2 are activated by phosphorylation and they in turn phosphorylate downstream substrates of ERK1/2, p38MAPK and PKB. This further inactivates caspase-3 and DNA fragmentation and prevents apoptosis (Trumper et al. 2002).
GIPR and Antidiabetic Drug Design
Since the emergence of the incretin concept, there has been increasing efforts directed toward designing novel antidiabetic drugs. The structure of the N-terminus domain of the receptor in complex with the peptide has been solved and the key amino acids involved in this interaction are known; this knowledge can help in designing GIPR agonists. There have been attempts to enhance the action of incretins on the receptor by designing peptide analogs that are resistant to DPP-IV and small molecules that are inhibitors of the enzyme. The design of incretin receptor agonists is in the state of infancy and so far no small molecule has been developed that can activate the GIPR. However, several peptide analogs resistant to DPP-IV and inhibitors of DPP-IV have been introduced into therapeutics (Green and Flatt 2007).
GIPR has emerged as a major signaling molecule on the enteroinsular axis with a key role in glucose homeostasis. As a GPCR, it is predominantly expressed on the surface of β-cells of islets of pancreas, adipose tissue, central nervous system, bone, and to some extent in stomach and liver cells. Along with glucose, it is known to potentiate insulin secretion from pancreas, increase insulin biosynthesis, and enhance β-cell survival by exerting mitogenic effects. It is also known to increase fat deposition in adipose tissue, enhance memory recognition in CNS, and inhibit gastric acid secretion in stomach. The past two decades of research have exhaustively delineated various pathways involved in GIPR signaling in pancreas with PI3K, PKA, PKB, and MAPK being major ones. Due to its role in insulin secretion, GIPR has become an attractive target for studying its role in pathophysiology underlying diabetes and for designing novel antidiabetics. Though GIPR agonists are yet to see the light of the day, inhibitors of DPP-IV, which enhance the action of GIP and prolong GIPR signaling, have been successfully designed and put to practice.
- Amiranoff B, Couvineau A, Vauclin-Jacques N, Laburthe M. Gastric inhibitory polypeptide receptor in hamster pancreatic beta cells. Direct cross-linking, solubilization and characterization as a glycoprotein. Eur J Biochem. 1986;159:353–8. doi:10.1111/j.1432-1033.1986.tb09875.PubMedCrossRefPubMedCentralGoogle Scholar
- Gremlich S, Porret A, Hani EH, Cherif D, Vionnet N, Froguel P, Thorens B. Cloning, functional expression, and chromosomal localization of the human pancreatic islet glucose-dependent insulinotropic polypeptide receptor. Diabetes. 1995;44:1202–8. doi:10.2337/diabetes.44.10.1202.PubMedCrossRefPubMedCentralGoogle Scholar
- Kim SJ, Winter K, Nian C, Tsuneoka M, Koda Y, McIntosh CH. Glucose-dependent insulinotropic polypeptide (GIP) stimulation of pancreatic beta-cell survival is dependent upon phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, inactivation of the forkhead transcription factor Foxo1, and down-regulation of bax expression. J Biol Chem. 2005;280:22297–307. doi:10.1074/jbc.M500540200.PubMedCrossRefPubMedCentralGoogle Scholar
- Kim SJ, Nian C, Widenmaier S, McIntosh CH. Glucose-dependent insulinotropic polypeptide-mediated up-regulation of beta-cell antiapoptotic Bcl-2 gene expression is coordinated by cyclic AMP (cAMP) response element binding protein (CREB) and cAMP-responsive CREB coactivator 2. Mol Cell Biol. 2008;28:1644–56. doi:10.1128/MCB.00325-07.PubMedCrossRefPubMedCentralGoogle Scholar
- Parthier C, Kleinschmidt M, Neumann P, Rudolph R, Manhart S, Schlenzig D, Fanghanel J, Rahfeld JU, Demuth HU, Stubbs MT. Crystal structure of the incretin-bound extracellular domain of a G protein-coupled receptor. Proc Natl Acad Sci USA. 2007;104:13942–7. doi:10.1073/pnas.0706404104.PubMedCrossRefPubMedCentralGoogle Scholar