GIP_HUMAN[22-51] is a new proatherogenic peptide identified by native plasma peptidomics

We recently established a new plasma peptidomic technique and comprehensively identified a large number of low-molecular weight and low-abundance native peptides using a single drop of human plasma. To discover a novel polypeptide that potently modulates the cardiovascular system, we performed a bioinformatics analysis of the large-scale identification results, sequentially synthesized the selected peptide sequences, tested their biological activities, and identified a 30-amino-acid proatherogenic peptide, GIP_HUMAN[22-51], as a potent proatherosclerotic peptide hormone. GIP_HUMAN[22-51] has a common precursor with the glucose-dependent insulinotropic polypeptide (GIP) and is located immediately N-terminal to GIP. Chronic infusion of GIP_HUMAN[22-51] into ApoE-/- mice accelerated the development of aortic atherosclerotic lesions, which were inhibited by co-infusions with an anti-GIP_HUMAN[22-51] antibody. GIP_HUMAN[22-51] increased the serum concentrations of many inflammatory and proatherogenic proteins, whereas neutralising antibodies reduced their levels. GIP_HUMAN[22-51] induced IκB-α degradation and nuclear translocation of NF-κB in human vascular endothelial cells and macrophages. Immunoreactive GIP_HUMAN[22-51] was detected in human tissues but there was no colocalization with the GIP. The plasma GIP_HUMAN[22-51] concentration in healthy humans determined using a stable-isotope tagged peptide was approximately 0.6 nM. This study discovered a novel endogenous proatherogenic peptide by using a human plasma native peptidomic resource.

Structural insights into hormone recognition by the human glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone that exerts crucial metabolic functions by binding and activating its cognate receptor, GIPR. As an important therapeutic target, GIPR has been subjected to intensive structural studies without success. Here, we report the cryo-EM structure of the human GIPR in complex with GIP and a Gs heterotrimer at a global resolution of 2.9 Å. GIP adopts a single straight helix with its N terminus dipped into the receptor transmembrane domain (TMD), while the C terminus is closely associated with the extracellular domain and extracellular loop 1. GIPR employs conserved residues in the lower half of the TMD pocket to recognize the common segments shared by GIP homologous peptides, while uses non-conserved residues in the upper half of the TMD pocket to interact with residues specific for GIP. These results provide a structural framework of hormone recognition and GIPR activation.

The glucose-dependent insulinotropic polypeptide (GIP) regulates body weight and food intake via CNS-GIPR signaling

Uncertainty exists as to whether the glucose-dependent insulinotropic polypeptide receptor (GIPR) should be activated or inhibited for the treatment of obesity. Gipr was recently demonstrated in hypothalamic feeding centers, but the physiological relevance of CNS Gipr remains unknown. Here we show that HFD-fed CNS-Gipr KO mice and humanized (h)GIPR knockin mice with CNS-hGIPR deletion show decreased body weight and improved glucose metabolism. In DIO mice, acute central and peripheral administration of acyl-GIP increases cFos neuronal activity in hypothalamic feeding centers, and this coincides with decreased body weight and food intake and improved glucose handling. Chronic central and peripheral administration of acyl-GIP lowers body weight and food intake in wild-type mice, but shows blunted/absent efficacy in CNS-Gipr KO mice. Also, the superior metabolic effect of GLP-1/GIP co-agonism relative to GLP-1 is extinguished in CNS-Gipr KO mice. Our data hence establish a key role of CNS Gipr for control of energy metabolism.

Chronic peptide-based GIP receptor inhibition exhibits modest glucose metabolic changes in mice when administered either alone or combined with GLP-1 agonism

Combinatorial gut hormone therapy is one of the more promising strategies for identifying improved treatments for metabolic disease. Many approaches combine the established benefits of glucagon-like peptide-1 (GLP-1) agonism with one or more additional molecules with the aim of improving metabolic outcomes. Recent attention has been drawn to the glucose-dependent insulinotropic polypeptide (GIP) system due to compelling pre-clinical evidence describing the metabolic benefits of antagonising the GIP receptor (GIPR). We rationalised that benefit might be accrued from combining GIPR antagonism with GLP-1 agonism. Two GIPR peptide antagonists, GIPA-1 (mouse GIP(3–30)NH2) and GIPA-2 (NαAc-K10[γEγE-C16]-Arg18-hGIP(5–42)), were pharmacologically characterised and both exhibited potent antagonist properties. Acute in vivo administration of GIPA-1 during an oral glucose tolerance test (OGTT) had negligible effects on glucose tolerance and insulin in lean mice. In contrast, GIPA-2 impaired glucose tolerance and attenuated circulating insulin levels. A mouse model of diet-induced obesity (DIO) was used to investigate the potential metabolic benefits of chronic dosing of each antagonist, alone or in combination with liraglutide. Chronic administration studies showed expected effects of liraglutide, lowering food intake, body weight, fasting blood glucose and plasma insulin concentrations while improving glucose sensitivity, whereas delivery of either GIPR antagonist alone had negligible effects on these parameters. Interestingly, chronic dual therapy augmented insulin sensitizing effects and lowered plasma triglycerides and free-fatty acids, with more notable effects observed with GIPA-1 compared to GIPA-2. Thus, the co-administration of both a GIPR antagonist with a GLP1 agonist uncovers interesting beneficial effects on measures of insulin sensitivity, circulating lipids and certain adipose stores that seem influenced by the degree or nature of GIP receptor antagonism.

Molecular interactions of full-length and truncated GIP peptides with the GIP receptor - A comprehensive review

Enzymatic cleavage of endogenous peptides is a commonly used principle to initiate, modulate and terminate action for instance among cytokines and peptide hormones. The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and the related hormone glucagon-like peptide-2 (GLP-2) are all rapidly N-terminally truncated with severe loss of intrinsic activity. The most abundant circulating form of full length GIP(1-42) is GIP(3-42) (a dipeptidyl peptidase-4 (DPP-4) product). GIP(1-30)NH₂ is another active form resulting from prohormone convertase 2 (PC2) cleavage of proGIP. Like GIP(1-42), GIP(1-30)NH₂ is a substrate for DPP-4 generating GIP(3-30)NH₂ which, compared to GIP(3-42), binds with higher affinity and very efficiently inhibits GIP receptor (GIPR) activity with no intrinsic activity. Here, we review the action of these four and multiple other N- and C-terminally truncated forms of GIP with an emphasis on molecular pharmacology, i.e. ligand binding, subsequent receptor activation and desensitization. Our overall conclusion is that the N-terminus is essential for receptor activation as GIP N-terminal truncation leads to decreased/lost intrinsic activity and antagonism (similar to GLP-1 and GLP-2), whereas the C-terminal extension of GIP(1-42), as compared to GLP-1, GLP-2 and glucagon (29-33 amino acids), has no apparent impact on the GIPR in vitro, but may play a role for other properties such as stability and tissue distribution. A deeper understanding of the molecular interaction of naturally occurring and designed GIP-based peptides, and their impact in vivo, may contribute to a future therapeutic targeting of the GIP system - either with agonists or with antagonists, or both.

Drug-induced diabetes type 2: In silico study involving class B GPCRs

A disturbance of glucose homeostasis leading to type 2 diabetes mellitus (T2DM) is one of the severe side effects that may occur during a prolonged use of many drugs currently available on the market. In this manuscript we describe the most common cases of drug-induced T2DM, discuss available pharmacotherapies and propose new ones. Among various pharmacotherapies of T2DM, incretin therapies have recently focused attention due to the newly determined crystal structure of incretin hormone receptor GLP1R. Incretin hormone receptors: GLP1R and GIPR together with the glucagon receptor GCGR regulate food intake and insulin and glucose secretion. Our study showed that incretin hormone receptors, named also gut hormone receptors as they are expressed in the gastrointestinal tract, could potentially act as unintended targets (off-targets) for orally administrated drugs. Such off-target interactions, depending on their effect on the receptor (stimulation or inhibition), could be beneficial, like in the case of incretin mimetics, or unwanted if they cause, e.g., decreased insulin secretion. In this in silico study we examined which well-known pharmaceuticals could potentially interact with gut hormone receptors in the off-target way. We observed that drugs with the strongest binding affinity for gut hormone receptors were also reported in the medical information resources as the least disturbing the glucose homeostasis among all drugs in their class. We suggested that those strongly binding molecules could potentially stimulate GIPR and GLP1R and/or inhibit GCGR which could lead to increased insulin secretion and decreased hepatic glucose production. Such positive effect on the glucose homeostasis could compensate for other, adverse effects of pharmacotherapy which lead to drug-induced T2DM. In addition, we also described several top hits as potential substitutes of peptidic incretin mimetics which were discovered in the drug repositioning screen using gut hormone receptors structures against the ZINC15 compounds subset.

Apelin-13 analogues show potent in vitro and in vivo insulinotropic and glucose lowering actions

Nine structurally modified apelin-13 analogues were assessed for their in vitro and acute in vivo antidiabetic potential. Stability was assessed in mouse plasma and insulinotropic efficacy tested in cultured pancreatic BRIN-BD11 cells and isolated mouse pancreatic islets. Intracellular Ca²⁺ and cAMP production in BRIN-BD11 cells was determined, as was glucose uptake in 3T3-L1 adipocytes. Acute antihyperglycemic effects of apelin analogues were assessed following i.p. glucose tolerance tests (ipGGT, 18 mmol/kg) in normal and diet-induced-obese (DIO) mice and on food intake in normal mice. Apelin analogues all showed enhanced in vitro stability (up to 5.8-fold, t½ = 12.8 h) in mouse plasma compared to native apelin-13 (t½ = 2.1 h). Compared to glucose controls, stable analogues exhibited enhanced insulinotropic responses from BRIN-BD11 cells (up to 4.7-fold, p < 0.001) and isolated mouse islets (up to 5.3-fold) for 10^-7 M apelin-13 amide (versus 7.6-fold for 10^-7 M GLP-1). Activation of APJ receptors on BRIN-BD11 cells increased intracellular Ca²⁺ (up to 3.0-fold, p < 0.001) and cAMP (up to 1.7-fold, p < 0.01). Acute ipGTT showed improved insulinotropic and glucose disposal responses in normal and DIO mice (p < 0.05 and p < 0.01, respectively). Apelin-13 amide and (pGlu)apelin-13 amide were the most effective analogues exhibiting acute, dose-dependent and persistent biological actions. Both analogues stimulated insulin-independent glucose uptake by differentiated adipocytes (2.9-3.3-fold, p < 0.05) and inhibited food intake (26-33%, p < 0.001), up to 180 min in mice, versus saline. In contrast, (Ala¹³)apelin-13 and (Val¹³)apelin-13 inhibited insulin secretion, suppressed beta-cell signal transduction and stimulated food intake in mice. Thus, stable analogues of apelin-13 have potential for diabetes/obesity therapy.

A novel GLP-1/xenin hybrid peptide improves glucose homeostasis, circulating lipids and restores GIP sensitivity in high fat fed mice

Combined modulation of peptide hormone receptors including, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and xenin, have established benefits for the treatment of diabetes. The present study has assessed the biological actions and therapeutic efficacy of a novel exendin-4/xenin-8-Gln hybrid peptide, both alone and in combination with the GIP receptor agonist (DAla²)GIP. Exendin-4/xenin-8-Gln was enzymatically stable and exhibited enhanced insulin secretory actions when compared to its parent peptides. Exendin-4/xenin-8-Gln also possessed ability to potentiate the in vitro actions of GIP. Acute administration of exendin-4/xenin-8-Gln in mice induced appetite suppressive effects, as well as significant and protracted glucose-lowering and insulin secretory actions. Twice daily administration of exendin-4/xenin-8-Gln, alone or in combination with (DAla²)GIP, for 21-days significantly reduced non-fasting glucose and increased circulating insulin levels in high fat fed mice. In addition, all exendin-4/xenin-8-Gln treated mice displayed improved glucose tolerance, insulin sensitivity and metabolic responses to GIP. Combination therapy with (DAla²)GIP did not result in any obvious further benefits. Metabolic improvements in all treatment groups were accompanied by reduced pancreatic beta-cell area and insulin content, suggesting reduced insulin demand. Interestingly, body weight, food intake, circulating glucagon, metabolic rate and amylase activity were unaltered by the treatment regimens. However, all treatment groups, barring (DAla²)GIP alone, exhibited marked reductions in total- and LDL-cholesterol. Furthermore, exendin-4 therapy also reduced circulating triacylglycerol. This study highlights the positive antidiabetic effects of exendin-4/xenin-8-Gln, and suggests that combined modulation of GLP-1 and xenin related signalling pathways represents an exciting treatment option for type 2 diabetes.

Thioamide Substitution Selectively Modulates Proteolysis and Receptor Activity of Therapeutic Peptide Hormones

Peptide hormones are attractive as injectable therapeutics and imaging agents, but they often require extensive modification by mutagenesis and/or chemical synthesis to prevent rapid in vivo degradation. Alternatively, the single-atom, O-to-S modification of peptide backbone thioamidation has the potential to selectively perturb interactions with proteases while preserving interactions with other proteins, such as target receptors. Here, we use the validated diabetes therapeutic, glucagon-like peptide-1 (GLP-1), and the target of clinical investigation, gastric inhibitory polypeptide (GIP), as proof-of-principle peptides to demonstrate the value of thioamide substitution. In GLP-1 and GIP, a single thioamide near the scissile bond renders these peptides up to 750-fold more stable than the corresponding oxopeptides toward cleavage by dipeptidyl peptidase 4, the principal regulator of their in vivo stability. These stabilized analogues are nearly equipotent with their parent peptide in cyclic AMP activation assays, but the GLP-1 thiopeptides have much lower β-arrestin potency, making them novel agonists with altered signaling bias. Initial tests show that a thioamide GLP-1 analogue is biologically active in rats, with an in vivo potency for glycemic control surpassing that of native GLP-1. Taken together, these experiments demonstrate the potential for thioamides to modulate specific protein interactions to increase proteolytic stability or tune activation of different signaling pathways.

Glucose-dependent insulinotropic polypeptide (GIP) dose-dependently reduces osteoclast differentiation and resorption

A role for glucose-dependent insulinotropic polypeptide (GIP) in controlling bone resorption has been suspected. However uncertainty remains to identify whether GIP act directly on osteoclasts. The aim of the present study were (i) to identify in different osteoclast differentiation models (human peripheral blood mononuclear cells-PBMC, murine bone marrow macrophage-BMM and murine Raw 264.7 cells) whether GIP was capable of reducing osteoclast formation and resorption; (ii) ascertain whether the highly potent GIP analogue N-AcGIP was capable of inducing a response at lower concentrations and (iii) to decipher the molecular mechanisms responsible for such effects. [d-Ala(2)]-GIP dose-dependently reduced osteoclast formation at concentration as low as 1nM in human PBMC and 10nM in murine BMM cultures. Furthermore, [d-Ala(2)]-GIP also reduced the extent of osteoclast resorption at concentration as low as 1nM in human PBMC and murine BMM cultures. The mechanism of action of [d-Ala(2)]-GIP appeared to be mediated by reduction in intracellular calcium concentration and oscillation that subsequently inhibited calcineurin activity and NFATc1 nuclear translocation. The potency of the highly potent N-AcGIP was determined and highlighted an effect on osteoclast formation and resorption at concentration ten times lower than observed with [d-Ala(2)]-GIP in vitro. Furthermore, N-AcGIP was also capable of reducing the number of osteoclast in ovariectomized mice as well as the circulating level of type I collagen C-telopeptide. Pharmacological concentrations required for reducing osteoclast formation and resorption provide the impetus to design and exploit enzymatically stable GIP analogues for the treatment of bone resorption disorders in humans.

A new stable GIP-Oxyntomodulin hybrid peptide improved bone strength both at the organ and tissue levels in genetically-inherited type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) progress worldwide with detrimental effects on several physiological systems including bone tissue mainly by affecting bone quality. Several gut hormones analogues have been proven potent in ameliorating bone quality. In the present study, we used the leptin receptor-deficient db/db mice as a model of obesity and severe T2DM to assess the extent of bone quality alterations at the organ and tissue levels. We also examined the beneficial effects of gut hormone therapy in this model by using a new triple agonist ([d-Ala(2)]GIP-Oxm) active at the GIP, GLP-1 and glucagon receptors. As expected, db/db mice presented with dramatic alterations of bone strength at the organ level associated with deterioration of trabecular and cortical microarchitectures and an augmentation in osteoclast numbers. At the tissue level, these animals presented also with alterations of bone strength (reduced hardness, indentation modulus and dissipated energy) with modifications of tissue mineral distribution, collagen glycation and collagen maturity. The use of [d-Ala(2)]GIP-Oxm considerably improved bone strength at the organ level with modest effects on trabecular microarchitecture. At the tissue level, [d-Ala(2)]GIP-Oxm ameliorated bone strength reductions with positive effects on collagen glycation and collagen maturity. This study provides support for including gut hormone analogues as possible new therapeutic strategies for improving bone quality in bone complications associated to T2DM.

Pancreatic glucose-dependent insulinotropic polypeptide (GIP) (1-30) expression is upregulated in diabetes and PEGylated GIP(1-30) can suppress the progression of low-dose-STZ-induced hyperglycaemia in mice

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone released from gut K cells. While the predominant form is GIP(1-42), a shorter form, GIP(1-30), is produced by pancreatic alpha cells and promotes insulin secretion in a paracrine manner. Here, we elucidated whether GIP(1-30) expression is modulated in mouse models of diabetes. We then investigated whether PEGylated GIP(1-30) can improve islet function and morphology as well as suppress the progression to hyperglycaemia in mice treated with low-dose streptozotocin (LD-STZ).

METHODS

We examined pancreatic GIP immunoreactivity in rodent diabetic models. We synthesised [D-Ala(2)]GIP(1-30) and modified the C-terminus with polyethylene glycol (PEG) to produce a dipeptidyl peptidase-4 (DPP-4)-resistant long-acting GIP analogue, [D-Ala(2)]GIP(1-30)-PEG. We performed i.p.GTT and immunohistochemical analysis in non-diabetic and LD-STZ diabetic mice, with or without administration of [D-Ala(2)]GIP(1-30)-PEG.

RESULTS

Pancreatic GIP expression was concomitantly enhanced with alpha cell expansion in rodent models of diabetes. Treatment with DPP-4 inhibitor decreased both the GIP- and glucagon-positive areas and preserved the insulin-positive area in LD-STZ diabetic mice. Body weight was not affected by [D-Ala(2)]GIP(1-30)-PEG in LD-STZ or non-diabetic mice. Treatment with GIP significantly ameliorated chronic hyperglycaemia and improved glucose excursions in LD-STZ mice. Treatment with GIP also reduced alpha cell expansion in the islets and suppressed plasma glucagon levels compared with non-treated LD-STZ mice. Additionally, [D-Ala(2)]GIP(1-30)-PEG preserved beta cell area via inhibition of apoptosis in LD-STZ mice.

CONCLUSIONS/INTERPRETATION

Our data suggest that GIP(1-30) expression is upregulated in diabetes, and PEGylated GIP(1-30) can suppress the progression to STZ-induced hyperglycaemia by inhibiting beta cell apoptosis and alpha cell expansion.

Measurement of the incretin hormones: glucagon-like peptide-1 and glucose-dependent insulinotropic peptide

The two incretin hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), are secreted from the gastrointestinal tract in response to meals and contribute to the regulation of glucose homeostasis by increasing insulin secretion. Assessment of plasma concentrations of GLP-1 and GIP is often an important endpoint in both clinical and preclinical studies and, therefore, accurate measurement of these hormones is important. Here, we provide an overview of current approaches for the measurement of the incretin hormones, with particular focus on immunological methods.

Antagonism of gastric inhibitory polypeptide (GIP) by palmitoylation of GIP analogues with N- and C-terminal modifications improves obesity and metabolic control in high fat fed mice

Compromise of gastric inhibitory polypeptide (GIP) receptor signalling represents a possible therapeutic strategy for the treatment of obesity-related diabetes. This study has characterised and evaluated the C-terminally fatty acid derivatised GIP analogues, GIP(3-30)Cex-K(40)[Pal] and Pro(3)GIP(3-30)Cex-K(40)[Pal], as potential GIP inhibitors. Both GIP analogues lack the two N-terminal amino acids cleaved by DPP-4 and have addition of nine amino acids from the C-terminal of exendin(1-39), Cex. GIP(3-30)Cex-K(40)[Pal] and Pro(3)GIP(3-30)Cex-K(40)[Pal] effectively (p < 0.01 to p < 0.001) inhibited GIP-induced cAMP production and insulin secretion in vitro. In normal mice, GIP(3-30)Cex-K(40)[Pal] and Pro(3)GIP(3-30)Cex-K(40)[Pal] displayed a significant (p < 0.05 to p < 0.001) and prolonged inhibitory effect on GIP-induced glucose-lowering and insulin-releasing actions. When injected once daily for 21 days in obese-diabetic high fat fed mice, both GIP(3-30)Cex-K(40)[Pal] and Pro(3)GIP(3-30)Cex-K(40)[Pal] significantly reduced body weight (p < 0.01 to p < 0.001) and lowered circulating glucose (p < 0.001) and insulin (p < 0.01 to p < 0.001) concentrations. The observed beneficial changes were independent of effects on energy intake, locomotor activity or metabolic rate. Oral and intraperitoneal glucose tolerance were significantly (p < 0.05 to p < 0.001) improved in both treatment groups at the end of the study, despite reduced glucose-induced plasma insulin concentrations. This improvement of metabolic control was accompanied by enhanced (p < 0.05 to p < 0.01) insulin sensitivity compared with high fat controls. These data demonstrate the potential offered by GIP(3-30)Cex-K(40)[Pal] and Pro(3)GIP(3-30)Cex-K(40)[Pal] for the treatment of obesity-related diabetes.

Beneficial effects of a N-terminally modified GIP agonist on tissue-level bone material properties

Bone remodeling is under complex regulation from nervous, hormonal and local signals, including gut hormones. Among the gut hormones, a role for the glucose-dependent insulinotropic polypeptide (GIP) has been suggested. However, the rapid degradation of GIP in the bloodstream by the ubiquitous enzyme dipeptidyl peptidase-4 (DPP-4) precludes therapeutic use. To circumvent this problem, a series of N-terminally modified GIP agonists have been developed, with N-AcGIP being the most promising. The aims of the present study were to investigate the effects of N-AcGIP on bone at the micro-level using trabecular and cortical microstructural morphology, and at the tissue-level in rats. Copenhagen rats were randomly assigned into control or N-AcGIP-treated groups and received daily injection for 4 weeks. Bone microstructural morphology was assessed by microCT and dynamic histomorphometry and tissue-level properties by nanoindentation, qBEI and infra-red microscopy. Four week treatment with N-AcGIP did not alter trabecular or cortical microstructural morphology. In addition, no significant modifications of mechanical response and properties at the tissue-level were observed in trabecular bone. However, significant augmentations in maximum load (12%), hardness (14%), indentation modulus (13%) and dissipated energy (16%) were demonstrated in cortical bone. These beneficial modifications of mechanical properties at the tissue-level were associated with increased mineralization (22%) and collagen maturity (13%) of the bone matrix. Taken together, the results support a beneficial role of GIP, and particularly stable analogs such as N-AcGIP, on tissue material properties of bone.

A novel glucagon-related peptide (GCRP) and its receptor GCRPR account for coevolution of their family members in vertebrates

The glucagon (GCG) peptide family consists of GCG, glucagon-like peptide 1 (GLP1), and GLP2, which are derived from a common GCG precursor, and the glucose-dependent insulinotropic polypeptide (GIP). These peptides interact with cognate receptors, GCGR, GLP1R, GLP2R, and GIPR, which belong to the secretin-like G protein-coupled receptor (GPCR) family. We used bioinformatics to identify genes encoding a novel GCG-related peptide (GCRP) and its cognate receptor, GCRPR. The GCRP and GCRPR genes were found in representative tetrapod taxa such as anole lizard, chicken, and Xenopus, and in teleosts including medaka, fugu, tetraodon, and stickleback. However, they were not present in mammals and zebrafish. Phylogenetic and genome synteny analyses showed that GCRP emerged through two rounds of whole genome duplication (2R) during early vertebrate evolution. GCRPR appears to have arisen by local tandem gene duplications from a common ancestor of GCRPR, GCGR, and GLP2R after 2R. Biochemical ligand-receptor interaction analyses revealed that GCRP had the highest affinity for GCRPR in comparison to other GCGR family members. Stimulation of chicken, Xenopus, and medaka GCRPRs activated Gα_s-mediated signaling. In contrast to chicken and Xenopus GCRPRs, medaka GCRPR also induced Gα_q/11-mediated signaling. Chimeric peptides and receptors showed that the K¹⁶M¹⁷K¹⁸ and G¹⁶Q¹⁷A¹⁸ motifs in GCRP and GLP1, respectively, may at least in part contribute to specific recognition of their cognate receptors through interaction with the receptor core domain. In conclusion, we present novel data demonstrating that GCRP and GCRPR evolved through gene/genome duplications followed by specific modifications that conferred selective recognition to this ligand-receptor pair.

Characterization and biological actions of N-terminal truncated forms of glucose-dependent insulinotropic polypeptide

The N-terminal domain of glucose-dependent insulinotropic polypeptide (GIP) plays an important role in regulating biological activity. This study examined biological properties of several N-terminal truncated forms of GIP and two novel forms with substitutions at Phe position-6 with Arg or Val. GIP(6-42), GIP(R6-42), GIP(V6-42), GIP(7-42) and GIP(9-42) stimulated cAMP production in BRIN-BD11 cells similar to native GIP, whereas responses to GIP(3-42), GIP(4-42), GIP(5-42) and GIP(8-42) were reduced (P<0.01 to P<0.001). GIP-induced cyclic AMP production was significantly inhibited by GIP(3-42), GIP(4-42), GIP(5-42), GIP(6-42), GIP(R6-42), GIP(7-42) and GIP(8-42) (P<0.001). Compared with native GIP, in vitro insulinotropic activity of GIP(3-42), GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) was reduced (P<0.05 to P<0.001), with GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) also potently inhibiting GIP-stimulated insulin secretion (P<0.001). In ob/ob mice, GIP(4-42) and GIP(8-42) increased (P<0.05 to P<0.01) plasma glucose concentrations compared to the glucose-lowering action of native GIP. When GIP(8-42) was co-administered with native GIP it countered the ability of the native peptide to lower plasma glucose and increase circulating insulin concentrations. These data confirm the importance of the N-terminal region of GIP in regulating bioactivity and reveal that sequential truncation of the peptide yields novel GIP receptor antagonists which may have functional significance.

Tyr1 and Ile7 of glucose-dependent insulinotropic polypeptide (GIP) confer differential ligand selectivity toward GIP and glucagon-like peptide-1 receptors

Glucagon like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretin hormones released in response to food intake and potentiate insulin secretion from pancreatic beta cells through their distinct yet related G protein-coupled receptors, GLP1R and GIPR. While GLP-1 and GIP exhibit similarity in their N-terminal sequence and overall alpha-helical structure, GLP-1 does not bind to GIPR and vice versa. To determine which amino acid residues of these peptide ligands are responsible for specific interaction with their respective receptors, we generated mutant GIP in which several GLP-1-specific amino acid residues were substituted for the original amino acids. The potency of the mutant ligands was examined using HEK293 cells transfected with GLP1R or GIPR expression plasmids together with a cAMP-responsive element-driven luciferase (CRE-luc) reporter plasmid. A mutated GIP peptide in which Tyr(1), Ile(7), Asp(15), and His(18) were replaced by His, Thr, Glu, and Ala, respectively, was able to activate both GLP1R and GIPR with moderate potency. Replacing the original Tyr(1) and/or Ile(7) in the N-terminal moiety of this mutant peptide allowed full activation of GIPR but not of GLP1R. However, reintroducing Asp(15) and/or His(18) in the central alpha-helical region did not significantly alter the ligand potency. These results suggest that Tyr/His(1) and Ile/Thr(7) of GIP/GLP-1 peptides confer differential ligand selectivity toward GIPR and GLP1R.

Structural basis for ligand recognition of incretin receptors

The glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor are homologous G-protein-coupled receptors (GPCRs). Incretin receptor agonists stimulate the synthesis and secretion of insulin from pancreatic β-cells and are therefore promising agents for the treatment of type 2 diabetes. It is well established that the N-terminal extracellular domain (ECD) of incretin receptors is important for ligand binding and ligand specificity, whereas the transmembrane domain is involved in receptor activation. Structures of the ligand-bound ECD of incretin receptors have been solved recently by X-ray crystallography. The crystal structures reveal a similar fold of the ECD and a similar mechanism of ligand binding, where the ligand adopts an α-helical conformation. Residues in the C-terminal part of the ligand interact directly with the ECD and hydrophobic interactions appear to be the main driving force for ligand binding to the ECD of incretin receptors. Obviously, the-still missing-structures of full-length incretin receptors are required to construct a complete picture of receptor function at the molecular level. However, the progress made recently in structural analysis of the ECDs of incretin receptors and related GPCRs has shed new light on the process of ligand recognition and binding and provided a basis to disclose some of the mechanisms underlying receptor activation at high resolution.

Prolonged GIP receptor activation using stable mini-PEGylated GIP improves glucose homeostasis and beta-cell function in age-related glucose intolerance

In older populations there is significant increase in incidence of impaired glucose tolerance and diabetes. Glucose-dependent insulinotropic polypeptide (GIP) improves glycemic control but its use as a therapeutic is hindered by short biological half-life. The present study examined effects of a longer-acting form of GIP, GIP[mPEG], on glucose homeostasis and beta-cell function in mice with age-related glucose intolerance. GIP[mPEG] decreased glucose and increased insulin concentrations when administered prior to a glucose challenge. Daily administration of GIP[mPEG] for 20 days had no effect on body weight and food intake. However, non-fasting glucose concentrations were decreased and insulin concentrations increased. Glycemic response to intraperitoneal glucose was improved and glucose-mediated insulin secretion enhanced. Insulin sensitivity, circulating triglycerides and resistin levels were unchanged by the treatment regimen, but plasma adiponectin levels increased. These data indicate that prolonged activation of the GIP receptor with GIP[mPEG] counters aspects of impaired beta-cell function and age-related glucose intolerance.

Vaccination against GIP for the treatment of obesity

BACKGROUND

According to the WHO, more than 1 billion people worldwide are overweight and at risk of developing chronic illnesses, including cardiovascular disease, type 2 diabetes, hypertension and stroke. Current therapies show limited efficacy and are often associated with unpleasant side-effect profiles, hence there is a medical need for new therapeutic interventions in the field of obesity. Gastric inhibitory peptide (GIP, also known as glucose-dependent insulinotropic polypeptide) has recently been postulated to link over-nutrition with obesity. In fact GIP receptor-deficient mice (GIPR(-/-)) were shown to be completely protected from diet-induced obesity. Thus, disrupting GIP signaling represents a promising novel therapeutic strategy for the treatment of obesity.

METHODOLOGY/PRINCIPAL FINDINGS

In order to block GIP signaling we chose an active vaccination approach using GIP peptides covalently attached to virus-like particles (VLP-GIP). Vaccination of mice with VLP-GIP induced high titers of specific antibodies and efficiently reduced body weight gain in animals fed a high fat diet. The reduction in body weight gain could be attributed to reduced accumulation of fat. Moreover, increased weight loss was observed in obese mice vaccinated with VLP-GIP. Importantly, despite the incretin action of GIP, VLP-GIP-treated mice did not show signs of glucose intolerance.

CONCLUSIONS/SIGNIFICANCE

This study shows that vaccination against GIP was safe and effective. Thus active vaccination may represent a novel, long-lasting treatment for obesity. However further preclinical safety/toxicology studies will be required before the therapeutic concept can be addressed in humans.

Incretin hormone mimetics and analogues in diabetes therapeutics

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are physiological gut peptides with insulin-releasing and extrapancreatic glucoregulatory actions. Incretin analogues/mimetics activate GLP-1 or GIP receptors whilst avoiding physiological inactivation by dipeptidyl peptidase 4 (DPP-4), and they represent one of the newest classes of antidiabetic drug. The first clinically approved GLP-1 mimetic for the treatment of type-2 diabetes is exenatide (Byetta/exendin) which is administered subcutaneously twice daily. Clinical trials of liraglutide, a GLP-1 analogue suitable for once-daily administration, are ongoing. A number of other incretin molecules are at earlier stages of development. This review discusses the various attributes of GLP-1 and GIP for diabetes treatment and summarises current clinical data. Additionally, it explores the therapeutic possibilities offered by preclinical agents, such as non-peptide GLP-1 mimetics, GLP-1/glucagon hybrid peptides, and specific GIP receptor antagonists.

Association analyses of GIP and GIPR polymorphisms with traits of the metabolic syndrome

Glucose-dependent insulinotropic polypeptide (GIP) stimulates insulin release via interaction with its pancreatic receptor (GIP receptor (GIPR)). GIP also acts as vasoactive protein. To investigate whether variations in GIP and GIPR genes are associated with risk factors of the metabolic syndrome we sequenced gene regions and identified two coding SNPs (GIP Ser103Gly, GIPR Glu354Gln) and one splice site SNP (GIP rs2291726) in 47 subjects. Interestingly, in silico analyses revealed that splice site SNP rs2291726 results in a truncated protein and classified GIPR variant Glu354Gln as a functional amino acid change. Association analyses were performed in a case-cohort study of incident cardiovascular disease (CVD) nested in the EPIC-Potsdam cohort. No significant associations between incident CVD and GIP Ser103Gly and rs2291726 were found. For GIPR Glu354Gln, we obtained a nominal association of heterozygous minor allele carrier with CVD in a codominant model adjusted for BMI, sex, and age (OR: 0.67, CI: 0.50-0.91, p = 0.01) or additional covariates of CVD (OR: 0.72, CI: 0.52-0.97, p = 0.03). In conclusion, we identified a common splice site mutation (rs2291726) of the GIP gene which results in a truncated protein and provide preliminary evidence for an association of the heterozygous GIPR Glu354Gln genotype with CVD.

Chemical gastric inhibitory polypeptide receptor antagonism protects against obesity, insulin resistance, glucose intolerance and associated disturbances in mice fed high-fat and cafeteria diets

AIMS/HYPOTHESIS

Gastric inhibitory polypeptide (GIP) receptor antagonism with (Pro(3))GIP improves glucose tolerance and ameliorates insulin resistance and abnormalities of islet structure/function in ob/ob mice. This study examined the ability of (Pro(3))GIP to counter the development of obesity, insulin resistance and diabetes in mice fed high-fat and cafeteria diets.

MATERIALS AND METHODS

Young Swiss TO mice on standard chow or high-fat, cafeteria or high-carbohydrate diets received daily injections of either saline or (Pro(3))GIP (25 nmol kg(-1)day(-1)) over 16 weeks. Food intake, body weight, and circulating glucose and insulin were measured frequently. At 16 weeks, glucose tolerance, insulin sensitivity, HbA(1c), circulating hormones and plasma lipids were assessed. Adipose tissue, liver and muscle were excised and weighed, and their histology and triacylglycerol content were further examined.

RESULTS

(Pro(3))GIP significantly reduced body weight, enhanced locomotor activity, and improved HbA(1c), glucose tolerance, beta cell responsiveness and insulin sensitivity in mice fed high-fat and cafeteria diets (p < 0.05 to p < 0.01). Similarly, (Pro(3))GIP significantly reduced plasma corticosterone and triacylglycerols (p < 0.05 to p < 0.001), while glucagon, resistin and adiponectin were unchanged. (Pro(3))GIP decreased adipose tissue mass (p < 0.01) and the triacylglycerol content of liver, muscle and adipose tissue (p < 0.01 to p < 0.001). Adipocyte size and liver morphology were partially normalised. (Pro(3))GIP did not significantly affect any of these parameters in mice fed a high-carbohydrate diet.

CONCLUSIONS/INTERPRETATION

(Pro(3))GIP protects against obesity, insulin resistance, glucose intolerance and associated disturbances in mice fed high-fat and cafeteria diets. This highlights chemical GIP receptor antagonism as a new possibility for the treatment of obesity and associated metabolic disturbances.

Understanding interactions of gastric inhibitory polypeptide (GIP) with its G-protein coupled receptor through NMR and molecular modeling

Gastric inhibitory polypeptide (GIP, or glucose-dependent insulinotropic polypeptide) is a 42-amino acid incretin hormone moderating glucose-induced insulin secretion. Antidiabetic therapy based on GIP holds great promise because of the fact that its insulinotropic action is highly dependent on the level of glucose, overcoming the sideeffects of hypoglycemia associated with the current therapy of Type 2 diabetes. The truncated peptide, GIP(1-30)NH2, has the same activity as the full length native peptide. We have studied the structure of GIP(1-30)NH2 and built a model of its G-protein coupled receptor (GPCR). The structure of GIP(1-30)NH2 in DMSO-d6 and H2O has been studied using 2D NMR (total correlation spectroscopy (TOCSY), nuclear overhauser effect spectroscopy (NOESY), double quantum filtered-COSY (DQF-COSY), 13C-heteronuclear single quantum correlation (HSQC) experiments, and its conformation built by MD simulations with the NMR data as constraints. The peptide in DMSO-d6 exhibits an alpha-helix between residues Ile12 and Lys30 with a discontinuity at residues Gln19 and Gln20. In H2O, the alpha-helix starts at Ile7, breaks off at Gln19, and then continues right through to Lys30. GIP(1-30)NH2 has all the structural features of peptides belonging to family B1 GPCRs, which are characterized by a coil at the N-terminal and a long C-terminal alpha-helix with or without a break. A model of the seven transmembrane (TM) helices of the GIP receptor (GIPR) has been built on the principles of comparative protein modeling, using the crystal structure of bovine rhodopsin as a template. The N-terminal domain of GIPR has been constructed from the NMR structure of the N-terminal of corticoptropin releasing factor receptor (CRFR), a family B1 GCPR. The intra and extra cellular loops and the C-terminal have been modeled from fragments retrieved from the PDB. On the basis of the experimental data available for some members of family B1 GPCRs, four pairs of constraints between GIP(1-30)NH2 and its receptor were used in the FTDOCK program, to build the complete model of the GIP(1-30)NH2:GIPR complex. The model can rationalize the various experimental observations including the potency of the truncated GIP peptide. This work is the first complete model at the atomic level of GIP(1-30)NH2 and of the complex with its GPCR.

Characterisation and glucoregulatory actions of a novel acylated form of the (Pro3)GIP receptor antagonist in type 2 diabetes

In this study, we tested the biological activity of a novel acylated form of (Pro3)glucose-dependent insulinotropic polypetide [(Pro3)GIP] prepared by conjugating palmitic acid to Lys16 to enhance its efficacy in vivo by promoting binding to albumin and extending its biological actions. Like the parent molecule (Pro3)GIP, (Pro3)GIPLys16PAL was completely stable to the actions of DPP-IV and significantly (p<0.01 to p<0.001) inhibited GIP-stimulated cAMP production and cellular insulin secretion. Furthermore, acute administration of (Pro3)GIPLys16PAL also significantly (p<0.05 to p<0.001) countered the glucose-lowering and insulin-releasing actions of GIP in ob/ob mice. Daily injection of (Pro3)GIPLys16PAL (25 nmol/kg bw) in 14-18-week-old ob/ob mice over 14 days had no effect on body weight, food intake or non-fasting plasma glucose and insulin concentrations. (Pro3)GIPLys16PAL treatment also failed to significantly alter the glycaemic response to an i.p. glucose load or test meal, but insulin concentrations were significantly reduced (1.5-fold; p<0.05) after the glucose load. Insulin sensitivity was enhanced (1.3-fold; p<0.05) and pancreatic insulin was significantly reduced (p<0.05) in the (Pro3)GIPLys16PAL-treated mice. These data demonstrate that acylation of Lys16 with palmitic acid in (Pro3)GIP does not improve its biological effectiveness as a GIP receptor antagonist.

Suppression of glucagon secretion is lower after oral glucose administration than during intravenous glucose administration in human subjects

AIMS/HYPOTHESIS

The incretin effect describes the augmentation of postprandial insulin secretion by gut hormones. It is not known whether glucagon secretion is also influenced by an incretin effect. A glucagon suppression deficiency has been reported in some patients with type 2 diabetes, but it is unclear whether this abnormality is present prior to diabetes onset. We therefore addressed the questions: (1) Is glucagon secretion different after oral and during intravenous glucose administration? (2) If so, is this related to the secretion of incretin hormones? (3) Is glucagon secretion abnormal in first-degree relatives of patients with type 2 diabetes?

MATERIALS AND METHODS

We examined 16 first-degree relatives of patients with type 2 diabetes and ten matched control subjects with an oral glucose load (75 g) and with an 'isoglycaemic' intravenous glucose infusion.

RESULTS

Glucagon levels were significantly suppressed by both oral and intravenous glucose (p < 0.0001), but glucagon suppression was more pronounced during intravenous glucose administration (76 +/- 2%) than after oral glucose administration (48 +/- 4%; p < 0.001). The differences in the glucagon responses to oral and i.v. glucose were correlated with the increments in gastric inhibitory polypeptide (GIP) (r = 0.60, p = 0.001) and glucagon-like peptide (GLP)-1 (r = 0.46, p < 0.05). There were no differences in glucagon levels between first-degree relatives and control subjects.

CONCLUSIONS/INTERPRETATION

Despite the glucagonostatic actions of GLP-1, the suppression of glucagon secretion by glucose is diminished after oral glucose ingestion, possibly due to the glucagonotropic actions of GIP and GLP-2. Furthermore, in this group of first-degree relatives, abnormalities in glucagon secretion did not precede the development of other defects, such as impaired insulin secretion.

The bioactive conformation of glucose-dependent insulinotropic polypeptide by NMR and CD spectroscopy

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal incretin hormone, which modulates physiological insulin secretion. Because of its glucose-sensitive insulinotropic activity, there has been a considerable interest in utilizing the hormone as a potential treatment for type 2 diabetes. Structural parameters obtained from NMR spectroscopy combined with molecular modeling techniques play a vital role in the design of new therapeutic drugs. Therefore, to understand the structural requirements for the biological activity of GIP, the solution structure of GIP was investigated by circular dichroism (CD) followed by proton nuclear magnetic resonance (NMR) spectroscopy. CD studies showed an increase in the helical character of the peptide with increasing concentration of trifluoroethanol (TFE) up to 50%. Therefore, the solution structure of GIP in 50% TFE was determined. It was found that there was an alpha-helix between residues 6 and 29, which tends to extend further up to residue 36. The implications of the C-terminal extended helical segment in the inhibitory properties of GIP on gastric acid secretion are discussed. It is shown that the adoption by GIP of an alpha-helical secondary structure is a requirement for its biological activity. Knowledge of the solution structure of GIP will help in the understanding of how the peptide interacts with its receptor and aids in the design of new therapeutic agents useful for the treatment of diabetes.

Activation of Lipoprotein Lipase by Glucose-dependent Insulinotropic Polypeptide in Adipocytes

Glucose-dependent insulinotropic polypeptide (GIP) has been mainly studied because of its glucose-dependent insulinotropic action and its ability to regulate beta-cell proliferation and survival. Considerably less is known about the effects of GIP on fat metabolism, and the present study was directed at identifying the mechanisms underlying its stimulatory action on lipoprotein lipase (LPL). In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased LPL activity and triglyceride accumulation through a pathway involving increased phosphorylation of protein kinase B (PKB) and reductions in phosphorylated LKB1 and AMP-activated protein kinase (AMPK). Knockdown of AMPK using RNA interference and application of the AMPK inhibitor, Compound C, supported this conclusion. In contrast, the other major incretin hormone, glucagon-like peptide-1, exhibited no significant effects on LPL activity or PKB, LKB1, or AMPK phosphorylation. Cultured subcutaneous human adipocytes showed similar responses to GIP but with greater sensitivity. Chronic elevation of circulating GIP levels in the Vancouver diabetic fatty Zucker rat in vivo resulted in increased LPL activity and elevated triglyceride accumulation in epididymal fat tissue, combined with a modulation of PKB, LKB1, and AMPK phosphorylation similar to that observed in vitro. This appears to be the first demonstration of a GIP-stimulated signal transduction pathway involved in increasing fat storage in adipocytes.

Comparison of the subchronic antidiabetic effects of DPP IV–resistant GIP and GLP-1 analogues in obese diabetic (ob/ob) mice

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are the two key incretin hormones released from the gastrointestinal tract that regulate blood glucose homeostasis through potent insulin secretion. The rapid degradation of GIP and GLP-1 by the ubiquitous enzyme dipeptidyl peptidase IV (DPP IV) renders both peptides noninsulinotropic. However, DPP IV stable agonists, such as N-AcGIP and (Val8)GLP-1, have now been developed. The present study has examined and compared the metabolic effects of subchronic administration of daily i.p. injections of N-AcGIP, (Val8) GLP-1 and a combination of both peptides (all at 25 nmol/kg bw) in obese diabetic (ob/ob) mice. Initial in vitro experiments confirmed the potent insulinotropic properties of N-AcGIP and (Val8)GLP-1 in the clonal pancreatic BRIN BD11 cell line. Subchronic administration of N-AcGIP, (Val8)GLP-1 or combined peptide administration had no significant effects on the body weight, food intake and plasma insulin concentrations. However, all treatment groups had significantly (p < 0.05) decreased plasma glucose levels and improved glucose tolerance by day 14. The effectiveness of the peptide groups was similar, and glucose concentrations were substantially reduced following injection of insulin to assess insulin sensitivity compared to control. These results provide evidence for an improvement of glucose homeostasis following treatment with enzyme-resistant GIP and GLP-1 analogues.

Evaluation of the antidiabetic activity of DPP IV resistant N-terminally modified versus mid-chain acylated analogues of glucose-dependent insulinotropic polypeptide

Glucose dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone with therapeutic potential for type 2 diabetes due to its insulin-releasing and antihyperglycaemic actions. However, development of GIP-based therapies is limited by N-terminal degradation by DPP IV resulting in a very short circulating half-life. Numerous GIP analogues have now been generated exhibiting DPP IV resistance and extended bioactivity profiles. In this study, we report a direct comparison of the long-term antidiabetic actions of three such GIP molecules, N-AcGIP, GIP(Lys(37)PAL) and N-AcGIP(Lys(37)PAL) in obese diabetic (ob/ob) mice. An extended duration of action of each GIP analogue was demonstrated prior to examining the effects of once daily injections (25nmolkg(-1) body weight) over a 14-day period. Administration of either N-AcGIP, GIP(Lys(37)PAL) or N-AcGIP(Lys(37)PAL) significantly decreased non-fasting plasma glucose and improved glucose tolerance compared to saline treated controls. All three analogues significantly enhanced glucose and nutrient-induced insulin release, and improved insulin sensitivity. The metabolic and insulin secretory responses to native GIP were also enhanced in 14-day analogue treated mice, revealing no evidence of GIP-receptor desensitization. These effects were accompanied by significantly enhanced pancreatic insulin following N-AcGIP(Lys(37)PAL) and increased islet number and islet size in all three groups. Body weight, food intake and circulating glucagon were unchanged. These data demonstrate the therapeutic potential of once daily injection of enzyme resistant GIP analogues and indicate that N-AcGIP is equally as effective as related palmitate derivatised analogues of GIP.

Biological activity and antidiabetic potential of synthetic fragment peptides of glucose-dependent insulinotropic polypeptide, GIP(1-16) and (Pro3)GIP(1-16)

Glucose-dependent insulinotropic polypeptide (GIP) is a key physiological insulin releasing peptide and potential antidiabetic agent. The present study was undertaken in an attempt to develop small molecular weight GIP agonist and antagonist molecules. The bioactivity of two modified C-terminally truncated fragment GIP peptides, GIP(1-16) and (Pro3)GIP(1-16), was examined in terms of insulin secretion and glucose homeostasis using BRIN-BD11 cells and type 2 diabetic mice. In vitro insulin release studies demonstrated that GIP(1-16) and (Pro3)GIP(1-16) possessed weak GIP-receptor agonist and antagonistic properties, respectively. Intraperitoneal administration of GIP(1-16) in combination with glucose to obese diabetic (ob/ob) mice did not effect the glycaemic excursion and had a marginal effect on insulin release. GIP(1-16) was substantially less effective than the native GIP(1-42). (Pro3)GIP(1-16) administration significantly curtailed (P < 0.05) the insulinotropic and glucose lowering effects of native GIP, but was significantly less effective than (Pro3)GIP. Based on the established concept of a therapeutic benefit of GIP receptor antagonism in obesity-diabetes, ob/ob mice received once daily injection of (Pro3)GIP(1-16) for 14 days. No significant effects were observed on food intake, body weight, HbA1c, glucose tolerance, metabolic response to feeding and either insulin secretion or insulin sensitivity following prolonged (Pro3)GIP(1-16) treatment. These data demonstrate that C-terminal truncation of GIP or (Pro3)GIP yields small molecular weight GIP molecules with significantly reduced biological activity that precludes therapeutic utility.

GIP(Lys16PAL) and GIP(Lys37PAL): novel long-acting acylated analogues of glucose-dependent insulinotropic polypeptide with improved antidiabetic potential

Glucose-dependent insulinotropic polypeptide (GIP) is a physiological insulin releasing peptide. We have developed two novel fatty acid derivatized GIP analogues, which bind to serum albumin and demonstrate enhanced duration of action in vivo. GIP(Lys(16)PAL) and GIP(Lys(37)PAL) were resistant to dipeptidyl peptidase IV (DPP IV) degradation. In vitro studies demonstrated that GIP analogues retained their ability to activate the GIP receptor through production of cAMP and to stimulate insulin secretion. Intraperitoneal administration of GIP analogues to obese diabetic (ob/ob) mice significantly decreased the glycemic excursion and elicited increased and prolonged insulin responses compared to native GIP. A protracted glucose-lowering effect was observed 24 h following GIP(Lys(37)PAL) administration. Once a day injection for 14 days decreased nonfasting glucose, improved glucose tolerance, and enhanced the insulin response to glucose. These data demonstrate that fatty acid derivatized GIP peptides represent a novel class of long-acting stable GIP analogues for therapy of type 2 diabetes.

Antidiabetic potential of two novel fatty acid derivatised, N-terminally modified analogues of glucose-dependent insulinotropic polypeptide (GIP): N-AcGIP(LysPAL16) and N-AcGIP(LysPAL37)

Fatty acid derivatisation was used to develop two novel, long-acting, N-terminally modified, glucose-dependent insulinotropic polypeptide (GIP) analogues, N-AcGIP(LysPAL16) and N-AcGIP(LysPAL37). In contrast to GIP, which was rapidly degraded by in vitro incubation with dipeptidylpeptidase IV (DPP IV) (52% intact after 2 h), the analogues remained fully intact for up to 24 h. Both fatty acid-derivatised analogues stimulated cAMP production in GIP receptor Chinese hamster lung (CHL) fibroblasts (EC50 12.1-13.0 nM) and significantly improved in vitro insulin secretion from BRIN-BD11 cells (1.1- to 2.4-fold; p < 0.05 to p < 0.001) compared to control (5.6 mM glucose). Administration of N-AcGIP(LysPAL16) and N-AcGIP(LysPAL37) together with glucose in obese diabetic (ob/ob) mice significantly reduced the glycaemic excursion (1.4- and 1.5-fold, respectively; p < 0.05 to p < 0.01) and improved the insulinotropic response (1.5- and 2.3-fold, respectively; p < 0.01 to p < 0.001) compared to native peptide. Dose-response studies with N-AcGIP(LysPAL37) revealed that even the lowest concentration (6.25 nmol/kg) induced a highly significant decrease (1.4-fold; p < 0.001) in the overall glycaemic excursion, coupled with a significant increase (2.0-fold; p < 0.01) in circulating insulin. Furthermore, basal glucose values remained significantly reduced (p < 0.05) and insulin values increased 24 h following a single injection of N-AcGIP(LysPAL37). The glucose-lowering action of the fatty acid-derivatised peptide was greater than that of N-AcGIP. These data demonstrate that novel fatty acid-derivatised analogues of N-terminally modified AcGIP function as long-acting GIP-receptor agonists, with significant antidiabetic potential.

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

The hormone glucose-dependent insulinotropic polypeptide (GIP) potently stimulates insulin secretion and promotes beta-cell proliferation and cell survival. In the present study we identified Forkhead (Foxo1)-mediated suppression of the bax gene as a critical component of the effects of GIP on cell survival. Treatment of INS-1(832/13) beta-cells with GIP resulted in concentration-dependent activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB)/Foxo1 signaling module. In parallel studies, GIP decreased bax promoter activity. Serial deletion analysis of the bax promoter demonstrated that the region -682 to -320, containing FHRE-II (5AAAACAAACA), was responsible for GIP-mediated effects. Foxo1 bound to FHRE-II in gel mobility shift assays, and Foxo1-FHRE-II interactions conferred GIP responsiveness to the bax promoter. INS-1 cells incubated under proapoptotic and glucolipotoxic conditions demonstrated increased nuclear localization of Foxo1 and bax promoter activity and decreased cytoplasmic phospho-PKB/Foxo1. GIP partially restored expression PKB/Foxo1 and bax promoter activity. Similar protective effects were found with dispersed islet cells from C57BL/6 mice, but not with those from GIP receptor knock-out (GIPR(-/-)) mice. GIP treatment reduced glucolipotoxicity-induced cell death in C57 BL/6 and Bax(-/-) islets, but not GIPR(-/-) mouse islets. Chronic treatment of Vancouver diabetic fatty Zucker rats with GIP resulted in down-regulation of Bax and up-regulation of Bcl-2 in pancreatic beta-cells. The results show that PI3K/PKB/Foxo1 signaling mediates GIP suppression of bax gene expression and that this module is a key pathway by which GIP regulates beta-cell apoptosis in vivo.

Degradation, insulin secretion, and antihyperglycemic actions of two palmitate-derivitized N-terminal pyroglutamyl analogues of glucose-dependent insulinotropic polypeptide

Exploitation of glucose-dependent insulinotropic polypeptide (GIP) is hindered by its short biological half-life and rapid renal clearance. To circumvent these problems, two novel acylated N-terminally modified GIP analogues, N-pGluGIP(LysPAL(16)) and N-pGluGIP(LysPAL(37)), were evaluated. In contrast to native GIP, both analogues were completely resistant to dipeptidyl peptidase IV degradation. In GIP-receptor transfected fibroblasts, N-pGluGIP(LysPAL(16)) and N-pGluGIP(LysPAL(37)) exhibited enhanced stimulation of cAMP production. Insulinotropic responses in clonal beta-cells were similar to native GIP. When administered together with glucose to ob/ob mice, the glycemic excursions were significantly less for both analogues and insulin responses were greater than native GIP. Extended insulinotropic and antihyperglycemic actions were also evident. These data indicate that palmitate-derivitized analogues of N-terminal pyroglutamyl GIP represent a novel class of stable, long-acting, and effective GIP analogues for potential type 2 diabetes therapy.

Glucagon-like peptide 1(GLP-1) in biology and pathology

Post-translational proteolytic processing of the preproglucagon gene in the gut results in the formation of glucagon-like peptide 1 (GLP-1). Owing to its glucose-dependent insulinotropic effect, this hormone was postulated to primarily act as an incretin, i.e. to augment insulin secretion after oral glucose or meal ingestion. In addition, GLP-1 decelerates gastric emptying and suppresses glucagon secretion. Under physiological conditions, GLP-1 acts as a part of the 'ileal brake', meaning that is slows the transition of nutrients into the distal gut. Animal studies suggest a role for GLP-1 in the development and growth of the endocrine pancreas. In light of its multiple actions throughout the body, different therapeutic applications of GLP-1 are possible. Promising results have been obtained with GLP-1 in the treatment of type 2 diabetes, but its potential to reduce appetite and food intake may also allow its use for the treatment of obesity. While rapid in vivo degradation of GLP-1 has yet prevented its broad clinical use, different pharmacological approaches aiming to extend the in vivo half-life of GLP-1 or to inhibit its inactivation are currently being evaluated. Therefore, antidiabetic treatment based on GLP-1 may become available within the next years. This review will summarize the biological effects of GLP-1, characterize its role in human biology and pathology, and discuss potential clinical applications as well as current clinical studies.

Structurally modified analogues of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) as future antidiabetic agents

Glucagon-like peptide-1(7-36)amide (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gastrointestinal insulin-releasing hormones involved in the regulation of postprandial nutrient homeostasis. These two incretin hormones are glucose-dependent stimulators of pancreatic beta-cell function, exhibiting a spectrum of secondary extrapancreatic activities, which favour the efficient control of blood glucose homeostasis. Such actions of GLP-1 and GIP have generated considerable interest in their possible exploitation as novel agents for the treatment of type 2 diabetes. Despite the many attributes of GLP-1 and GIP as possible future antidiabetic agents, their rapid degradation in the circulation by dipeptidyl peptidase IV (DPP IV) to inactive truncated forms GLP-1(9-36)amide and GIP(3-42), severely limits their therapeutic usefulness. This review will consider recent developments in the design and effectiveness of synthetic DPP IV-resistant analogues of GLP-1 and GIP. Consideration will be given to the effects of N-terminal modification and amino acid substitution of GLP-1 and GIP either side of the DPP IV cleavage site on (i) susceptibility to enzymatic degradation, (ii) binding to native hormone receptor, (iii) ability to elevate intracellular cyclic AMP, (iv) potency as insulin secretagogues, and (v) antihyperglycaemic activity in type 2 diabetes. It will be shown that structural modification can produce a varied set of biological activities, ranging from more efficacious analogues to those which antagonise the activity of the native hormone. The antidiabetic properties of the best GLP-1 and GIP analogues indeed promise to provide the basis for novel, effective and long-acting drugs for type 2 diabetes therapy. This approach is currently being pursued actively by the pharmaceutical industry.

NMR structure of the glucose-dependent insulinotropic polypeptide fragment, GIP(1–30)amide

Glucose-dependent insulinotropic polypeptide is an incretin hormone that stimulates insulin secretion and reduces postprandial glycaemic excursions. The glucose-dependent action of GIP on pancreatic beta-cells has attracted attention towards its exploitation as a potential drug for type 2 diabetes. Use of NMR or X-ray crystallography is vital to determine the three-dimensional structure of the peptide. Therefore, to understand the basic structural requirements for the biological activity of GIP, the solution structure of the major biologically active fragment, GIP(1-30)amide, was investigated by proton NMR spectroscopy and molecular modelling. The structure is characterised by a full length alpha-helical conformation between residues F(6) and A(28). This structural information could play an important role in the design of therapeutic agents based upon GIP receptor agonists.

In depth analysis of the N-terminal bioactive domain of gastric inhibitory polypeptide

Gastric inhibitory polypeptide/glucose-dependent insulinotropic polypeptide (GIP) is an important gastrointestinal regulator of insulin release and glucose homeostasis following a meal. Strategies have been undertaken to delineate the bioactive domains of GIP with the intention of developing small molecular weight GIP mimetics. The molecular cloning of receptors for GIP and the related hormone GLP-1 (glucagon-like peptide-1) has allowed examination of the characteristics of incretin analogs in transfected cell models. The current report examines the N-terminal bioactive domain of GIP residing in residues 1-14 by alanine scanning mutagenesis and N-terminal substitution/modification. Further studies examined peptide chimeras of GIP and GLP-1 designed to localize bioactive determinants of the two hormones. The alanine scan of the GIP(1-14) sequence established that the peptide was extremely sensitive to structural perturbations. Only replacement of amino acids 2 and 13 with those found in glucagon failed to dramatically reduce receptor binding and activation. Of four GIP(1-14) peptides modified by the introduction of DP IV-resistant groups, a peptide with a reduced bond between Ala2 and Glu3 demonstrated improved receptor potency compared to native GIP(1-14). The peptide chimera studies supported recent results on the importance of a mid-region helix for bioactivity of GIP, and confirmed existence of two separable regions with independent intrinsic receptor binding and activation properties. Furthermore, peptide chimeras showed that binding of GLP-1 also involves both N- and C-terminal domains, but that it apparently contains only a single bioactive domain in its N-terminus. Together, these results should facilitate development of incretin based therapies using rational drug design for potential treatment of diabetes.

GIP1–39, a novel insulinotropic peptide form and aspects on its mechanism of action

GIP1-39, a novel chain-length form of GIP (gastric inhibitory polypeptide or glucose-dependent insulinotropic polypeptide), has been purified recently from porcine intestine and found to exist abundantly in this tissue. We have characterized that GIP1-39 is an insulinotropic peptide, and demonstrated that GIP1-39 is more potent in stimulating insulin secretion from rat pancreatic islets than GIP1-42, the insulinotropic polypeptide reported originally. Therefore, we have further investigated some aspects on the mechanism behind the insulinotropic effect of GIP1-39 in single rat pancreatic beta cells. GIP1-39 at 100 nM was able to significantly increase intracellular Ca2+ concentration ([Ca2+]i), and capable of enhancing exocytosis assessed by membrane capacitance measurement. The novel GIP1-39 might be a more optimal molecular pattern in stimulating insulin secretion and deserves to be further investigated biologically and clinically.

Comparative effects of GLP-1 and GIP on cAMP production, insulin secretion, and in vivo antidiabetic actions following substitution of Ala8/Ala2 with 2-aminobutyric acid

The two major incretin hormones, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP), are currently being considered as prospective drug candidates for treatment of type 2 diabetes. Interest in these gut hormones was initially spurred by their potent insulinotropic activities, but a number of other antihyperglycaemic actions are now established. One of the foremost barriers in progressing GLP-1 and GIP to the clinic concerns their rapid degradation and inactivation by the ubiquitous enzyme, dipeptidyl peptidase IV (DPP IV). Here, we compare the DPP IV resistance and biological properties of Abu8/Abu2 (2-aminobutyric acid) substituted analogues of GLP-1 and GIP engineered to impart DPP IV resistance. Whereas (Abu8)GLP-1 was completely stable to human plasma (half-life >12 h), GLP-1, GIP, and (Abu2)GIP were rapidly degraded (half-lives: 6.2, 6.0, and 7.1 h, respectively). Native GIP, GLP-1, and particularly (Abu8)GLP-1 elicited significant adenylate cyclase and insulinotropic activity, while (Abu2)GIP was less effective. Similarly, in obese diabetic (ob/ob) mice, GIP, GLP-1, and (Abu8)GLP-1 displayed substantial glucose-lowering and insulin-releasing activities, whereas (Abu2)GIP was only weakly active. These studies illustrate divergent effects of penultimate amino acid Ala8/Ala2 substitution with Abu on the biological properties of GLP-1 and GIP, suggesting that (Abu8)GLP-1 represents a potential candidate for future therapeutic development.

Glucose-dependent insulinotropic polypeptide analogues and their therapeutic potential for the treatment of obesity-diabetes

Glucose-dependent insulinotropic polypeptide (GIP) is a key incretin hormone, released postprandially into the circulation in response to feeding, producing a glucose-dependent stimulation of insulin secretion. It is this glucose-dependency that has attracted attention towards GIP as a potential therapeutic agent for the treatment of type 2 diabetes. A major drawback to achieving this goal has been the rapid degradation of circulating GIP by the ubiquitous enzyme, dipeptidylpeptidase IV (DPP IV). However, recent studies have described a number of novel structurally modified analogues of GIP with enhanced plasma stability, insulinotropic and antihyperglycaemic activity. The purpose of this article was to provide an overview of the biological effects of several GIP modifications and to highlight the potential of such analogues in the treatment of type 2 diabetes and obesity.

Structure-activity relationships of glucose-dependent insulinotropic polypeptide (GIP)

Six GIP(1-NH2) analogs were synthesized with modifications (de-protonation, N-methylation, reversed chirality, and substitution) at positions 1, 3, and 4 of the N-terminus, and additionally, a cyclized GIP derivative was synthesized. The relationship between altered structure to biological activity was assessed by measuring receptor binding affinity and ability to stimulate adenylyl cyclase in CHO-K1 cells transfected with the wild-type GIP receptor (wtGIPR). These structure-activity relationship studies demonstrate the importance of the GIP N-terminus and highlight structural constraints that can be introduced in GIP analogs. These analogs may be useful starting points for design of peptides with enhanced in vivo bioactivity.

Gastric inhibitory polypeptide analogues: do they have a therapeutic role in diabetes mellitus similar to that of glucagon-like Peptide-1?

Gastric inhibitory polypeptide (GIP, also called glucose-dependent insulinotropic polypeptide) and glucagon-like peptide-1 (GLP-1) are peptide hormones from the gut that enhance nutrient-stimulated insulin secretion (the 'incretin' effect). Judging from experiments in mice with targeted deletions of GIP and GLP-1 receptors, the incretin effect is essential for normal glucose tolerance. In patients with type 2 diabetes mellitus it turns out that the incretin effect is severely impaired or abolished. The explanation seems to be that both the secretion of GLP-1 and the effect of GIP are impaired (whereas both the secretion of GIP and the effect of GLP-1 are near normal). The impaired GLP-1 secretion is probably a consequence of diabetic metabolic disturbances. The known genetic variations in the GIP receptor sequence are not associated with type 2 diabetes mellitus, but a defective insulinotropic effect of GIP may be found in first degree relatives of the patients, suggesting a genetic background for the defect. The molecular nature of the defect is not known and given the close similarity of the two receptors and their signalling, the dissociation of their effects is remarkable. Whereas GLP-1 and its analogues are attractive as therapeutic agents for type 2 diabetes mellitus, analogues of GIP are unlikely to be effective. On the other hand, GIP seems to play an important role in lipid metabolism, promoting the disposal of ingested lipids, and mice with a targeted deletion of the GIP receptor do not become obese when exposed to a high-fat diet. Therefore, antagonistic analogues of GIP may be speculated to have a role in the pharmaceutical management of obesity.

Improved stability, insulin-releasing activity and antidiabetic potential of two novel N-terminal analogues of gastric inhibitory polypeptide: N-acetyl-GIP and pGlu-GIP

AIMS/HYPOTHESIS

This study examined the plasma stability, biological activity and antidiabetic potential of two novel N-terminally modified analogues of gastric inhibitory polypeptide (GIP).

METHODS

Degradation studies were carried out on GIP, N-acetyl-GIP (Ac-GIP) and N-pyroglutamyl-GIP (pGlu-GIP) in vitro following incubation with either dipeptidylpeptidase IV or human plasma. Cyclic adenosine 3'5' monophosphate (cAMP) production was assessed in Chinese hamster lung fibroblast cells transfected with the human GIP receptor. Insulin-releasing ability was assessed in vitro in BRIN-BD11 cells and in obese diabetic ( ob/ ob) mice.

RESULTS

GIP was rapidly degraded by dipeptidylpeptidase IV and plasma (t(1/2) 2.3 and 6.2 h, respectively) whereas Ac-GIP and pGlu-GIP remained intact even after 24 h. Both Ac-GIP and pGlu-GIP were extremely potent ( p<0.001) at stimulating cAMP production (EC(50) values 1.9 and 2.7 nmol/l, respectively), almost a tenfold increase compared to native GIP (18.2 nmol/l). Both Ac-GIP and pGlu-GIP (10(-13)-10(-8) mmol/l) were more potent at stimulating insulin release compared to the native GIP ( p<0.001), with 1.3-fold and 1.2-fold increases observed at 10(-8) mol/l, respectively. Administration of GIP analogues (25 nmol/kg body weight, i.p.) together with glucose (18 mmol/kg) in ( ob/ ob) mice lowered ( p<0.001) individual glucose values at 60 min together with the areas under the curve for glucose compared to native GIP. This antihyperglycaemic effect was coupled to a raised ( p<0.001) and more prolonged insulin response after administration of Ac-GIP and pGlu-GIP (AUC, 644+/-54 and 576+/-51 ng.ml(-1) x min, respectively) compared with native GIP (AUC, 257+/-29 ng.ml(-1) x min).

CONCLUSION/INTERPRETATION

Ac-GIP and pGlu-GIP, show resistance to plasma dipeptidylpeptidase IV degradation, resulting in enhanced biological activity and improved antidiabetic potential in vivo, raising the possibility of their use in therapy of Type II (non-insulin-dependent) diabetes mellitus.

Dipeptidyl peptidase IV-resistant [D-Ala(2)]glucose-dependent insulinotropic polypeptide (GIP) improves glucose tolerance in normal and obese diabetic rats

The therapeutic potential of glucose-dependent insulinotropic polypeptide (GIP) for improving glycemic control has largely gone unstudied. A series of synthetic GIP peptides modified at the NH(2)-terminus were screened in vitro for resistance to dipeptidyl peptidase IV (DP IV) degradation and potency to stimulate cyclic AMP and affinity for the transfected rat GIP receptor. In vitro experiments indicated that [D-Ala(2)]GIP possessed the greatest resistance to enzymatic degradation, combined with minimal effects on efficacy at the receptor. Thus, [D-Ala(2)]GIP(1--42) was selected for further testing in the perfused rat pancreas and bioassay in conscious Wistar and Zucker rats. When injected subcutaneously in normal Wistar, Fa/?, or fa/fa Vancouver Diabetic Fatty (VDF) Zucker rats, both GIP and [D-Ala(2)]GIP significantly reduced glycemic excursions during a concurrent oral glucose tolerance test via stimulation of insulin release. The latter peptide displayed greater in vivo effectiveness, likely because of resistance to enzymatic degradation. Hence, despite reduced bioactivity in diabetic models at physiological concentrations, GIP and analogs with improved plasma stability still improve glucose tolerance when given in supraphysiological doses, and thus may prove useful in the treatment of diabetic states.