Peptides originally derived from Chilobrachys jingzhao tarantula venom possess beneficial effects on pancreatic beta cell health and function

Clinical approval of the glucagon-like peptide-1 (GLP-1) mimetic exenatide for the treatment of type 2 diabetes highlights the therapeutic effectiveness of venom-derived peptides. In the present study, we examined and characterised the glucose-lowering potential of synthetic Jingzhaotoxin IX and Jingzhaotoxin XI peptides, which were originally isolated from the venom of the Chinese earth tarantula Chilobrachys jingzhao. Following confirmation of lack of beta-cell toxicity of synthetic peptides, assessment of enzymatic stability and effects on in vitro beta-cell function were studied, alongside putative mechanisms. Glucose homeostatic and appetite suppressive actions of Jingzhaotoxin IX and Jingzhaotoxin XI alone, or in combination with exenatide, were then assessed in normal overnight fasted C57BL/6 mice. Synthetic Jingzhaotoxin peptides were non-toxic and exhibited a decrease in mass of 6 Da in Krebs-Ringer bicarbonate buffer suggesting inhibitor cysteine knot (ICK)-like formation, but interestingly were liable to plasma enzyme degradation. The Jingzhaotoxin peptides evoked prominent insulin secretion from BRIN BD11 beta-cells, with activity somewhat characteristic of Kv2.1 channel binding. In addition, Jingzhaotoxin peptides enhanced beta-cell proliferation and provided significant protection against cytokine-induced apoptosis. When injected co-jointly with glucose, the Jingzhaotoxin peptides slightly decreased blood-glucose levels but had no effect on appetite in overnight fasted mice. Whilst the Jingzhaotoxin peptides did not enhance exenatide-induced benefits on glucose homeostasis, they augmented exenatide-mediated suppression of appetite. Taken together, these data highlight the therapeutic potential of tarantula venom-derived peptides, such as Jingzhaotoxin IX and Jingzhaotoxin XI either alone or in combination with exenatide, for diabetes and related obesity.

A novel peptide isolated from Aphonopelma chalcodes tarantula venom with benefits on pancreatic islet function and appetite control

Proof-of-concept for therapeutic application of venom-derived compounds in diabetes is exemplified by the incretin mimetic, exenatide, originally extracted from the saliva of the venomous Heloderma suspectum lizard. In this regard, we have isolated and sequenced a novel 28 amino acid peptide named Δ-theraphotoxin-Ac1 (Δ-TRTX-AC1) from venom of the Mexican Blond tarantula spider Aphonopelma chalcodes, with potential therapeutic benefits for diabetes. Following confirmation of the structure and safety profile of the synthetic peptide, assessment of enzymatic stability and effects of Δ-TRTX-AC1 on in vitro beta-cell function were studied, alongside potential mechanisms. Glucose homeostatic and satiety actions of Δ-TRTX-AC1 alone, and in combination with exenatide, were then assessed in C57BL/6 mice. Synthetic Δ-TRTX-AC1 was shown to adopt a characteristic inhibitor cysteine knot (ICK)-like structure and was non-toxic to beta-cells. Δ-TRTX-AC1 evoked glucose-dependent insulin secretion from BRIN BD11 cells with bioactivity confirmed in murine islets. Insulin secretory potency was established to be dependent on K_ATP and Ca²⁺ channel beta-cell signalling. In addition, Δ-TRTX-AC1 enhanced beta-cell proliferation and provided significant protection against cytokine-induced apoptosis. When injected co-jointly with glucose in mice at a dose of 250 nmol/kg, Δ-TRTX-AC1 decreased blood-glucose levels and evoked a significant satiating effect. Moreover, whilst Δ-TRTX-AC1 did not enhance exenatide induced benefits on glucose homeostasis, the peptide significantly augmented exenatide mediated suppression of appetite. Together these data highlight the therapeutic potential of tarantula spider venom-derived peptides, such as Δ-TRTX-Ac1, for diabetes and related obesity.

PYY (3-36) protects against high fat feeding induced changes of pancreatic islet and intestinal hormone content and morphometry

BACKGROUND

Prolonged high fat feeding negatively impacts pancreatic and intestinal morphology. In this regard, direct effects of PYY(3-36) on intestinal cell and pancreatic islet morphometry are yet to be fully explored in the setting of obesity.

METHODS

We examined the influence of 21-days twice daily treatment with PYY(3-36) on these parameters in mice fed a high fat diet (HFD).

RESULTS

PYY(3-36) treatment decreased food intake, body weight and circulating glucose in HFD mice. In terms of intestinal morphology, crypt depth was restored to control levels by PYY(3-36), with an additional enlargement of villi length. PYY(3-36) also reversed HFD-induced decreases of ileal PYY, and especially GLP-1, content. HFD increased numbers of PYY and GIP positive ileal cells, with PYY(3-36) fully reversing the effect on PYY cell detection. There were no obvious differences in the overall number of GLP-1 positive ileal cells in all mice, barring PYY(3-36) marginally decreasing GLP-1 villi cell immunoreactivity. Within pancreatic islets, PYY(3-36) significantly decreased alpha-cell area, whilst islet, beta-, PYY- and delta-cell areas remained unchanged. However, PYY(3-36) increased the percentage of beta-cells while also reducing percentage alpha-cell area. This was related to PYY(3-36)-induced reductions of beta-cell proliferation and apoptosis frequencies. Co-localisation of islet PYY with glucagon or somatostatin was elevated by PYY(3-36), with GLP-1/glucagon co-visualisation increased when compared to lean controls.

CONCLUSION

PYY(3-36) exerts protective effects on pancreatic and intestinal morphology in HFD mice linked to elevated ileal GLP-1 content.

GENERAL SIGNIFICANCE

These observations highlight mechanisms linked to the metabolic and weight reducing benefits of PYY(3-36).

Comparison of independent and combined effects of the neurotensin receptor agonist, JMV-449, and incretin mimetics on pancreatic islet function, glucose homeostasis and appetite control

BACKGROUND

Neurotensin receptor activation augments the biosctivity of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). JMV-449, a C-terminal neurotensin-like fragment with a reduced peptide bond, represents a neurotensin receptor agonist.

METHODS

The present study assessed the actions of JMV-449 on pancreatic beta-cells alone, and in combination with GIP and GLP-1. Further studies examined the impact of JMV-449 and incretin mimetics on glucose homeostasis and appetite control in mice.

RESULTS

JMV-449 was resistant to plasma enzyme degradation and induced noticeable dose-dependent insulin-releasing actions in BRIN-BD11 beta-cells. In combination with either GIP or GLP-1, JMV-449 augmented (P < 0.05) the insulinotropic actions of both hormones, as well as enhancing (P < 0.001) insulin secretory activity of both incretin peptides. JMV-449 also increased beta-cell proliferation and induced significant benefits on beta-cell survival in response to cytokine-induced apoptosis. JMV-449 (25 nmol/kg) inhibited (P < 0.05-P < 0.001) food intake in overnight fasted lean mice, and enhanced (P < 0.01) the appetite supressing effects of an enzymatically stable GLP-1 mimetic. When injected co-jointly with glucose, JMV-449 evoked glucose lowering actions, but more interestingly significantly augmented (P < 0.05) the glucose lowering effects of established long-acting GIP and GLP-1 receptor mimetics. In terms of glucose-induced insulin secretion, only GIP receptor signalling was associated with increases in insulin concentrations, and this was not enhanced by JMV-449.

CONCLUSION

JMV-449 is a neurotensin receptor agonist that positively augments key aspects of the biological action profile of GIP and GLP-1.

GENERAL SIGNIFICANCE

These observations emphasise the, yet untapped, therapeutic potential of combined neurotensin and incretin receptor signalling for diabetes.

Effects of an enzymatically stable C-terminal hexapseudopeptide fragment peptide of xenin-25, ψ-xenin-6, on pancreatic islet function and metabolism

Xenin-25 undergoes rapid enzyme metabolism following secretion. Early studies demonstrated bioactivity of a C-terminal hexapeptide fragment of xenin-25, namely xenin-6, which were enhanced through introduction of a reduced N-terminal peptide bond, to yield Ψ-xenin-6. The present study was undertaken to define the biological actions and potential antidiabetic properties of Ψ-xenin-6. In vitro enzymatic stability, insulin and glucagon secretory activity, as well as effects on beta-cell survival were determined. Studies in mice were used to assess the impact of Ψ-xenin-6 on glucose homeostasis and satiety. Ψ-xenin-6 was resistant to murine plasma degradation. In BRIN-BD11 cells and isolated murine islets, Ψ-xenin-6 significantly stimulated insulin secretion, and prominently enhanced the insulinotropic actions of GIP. Xenin-6 and Ψ-xenin-6 had no impact on glucagon secretion, although xenin-6 partially reversed the glucagonotropic action of GIP. Further in vitro investigations revealed that, similar to GLP-1, Ψ-xenin-6 significantly augmented proliferation of human and rodent clonal beta-cells, whilst also fully protecting against cytokine-induced beta-cell cytotoxicity, with greater potency than xenin-25 and xenin-6. When administered to mice in combination with glucose, Ψ-xenin-6 significantly reduced glucose levels and enhanced glucose-induced insulin release, with a duration of biological action beyond 8 h. Ψ-xenin-6 also significantly enhanced the glucose-lowering action of GIP in vivo. In overnight fasted mice, Ψ-xenin-6 exhibited satiety actions at both 25 and 250 nmol/kg. These data demonstrates that Ψ-xenin-6 is a metabolically stable C-terminal fragment analogue of xenin-25, with a metabolic action profile that merits further study as a potential antidiabetic compound.

Characterisation of Glucose-Dependent Insulinotropic Polypeptide Receptor Antagonists in Rodent Pancreatic Beta Cells and Mice

Hypersecretion and alterations in the biological activity of the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), have been postulated as contributing factors in the development of obesity-related diabetes. However, recent studies also point to weight-reducing effects of GIP receptor activation. Therefore, generating precise experimental tools, such as specific and effective GIP receptor (GIPR) antagonists, is of key significance to better understand GIP physiology. Thus, the primary aim of the current study was to uncover improved GIPR antagonists for use in rodent studies, using human and mouse GIP sequences with N- and C-terminal deletions. Initial in vitro studies revealed that the GIPR agonists, human (h) GIP(1-42), hGIP(1-30) and mouse (m) GIP(1-30), stimulated (P < 0.01 to P < 0.001) insulin secretion from rat BRIN-BD11 cells. Analysis of insulin secretory effects of the N- and C-terminally cleaved GIP peptides, including hGIP(3-30), mGIP(3-30), h(Pro³)GIP(3-30), hGIP(5-30), hGIP(3-42) and hGIP(5-42), revealed that these peptides did not modulate insulin secretion. More pertinently, only hGIP(3-30), mGIP(3-30) and h(Pro³)GIP(3-30) were able to significantly (P < 0.01 to P < 0.001) inhibit hGIP(1-42)-stimulated insulin secretion. The human-derived GIPR agonist sequences, hGIP(1-42) and hGIP(1-30), reduced (P < 0.05) glucose levels in mice following conjoint injection with glucose, but mGIP(1-30) was ineffective. None of the N- and C-terminally cleaved GIP peptides affected glucose homeostasis when injected alone with glucose. However, hGIP(5-30) and mGIP(3-30) significantly (P < 0.05 to P < 0.01) impaired the glucose-lowering action of hGIP(1-42). Further evaluation of these most effective sequences demonstrated that mGIP(3-30), but not hGIP(5-30), effectively prevented GIP-induced elevations of plasma insulin concentrations. These data highlight, for the first time, that mGIP(3-30) represents an effective molecule to inhibit GIPR activity in mice.

C-terminal degradation of PYY peptides in plasma abolishes effects on satiety and beta-cell function

The importance of dipeptidyl peptidase-4 mediated N-terminal metabolism of the enteroendocrine-derived hormone, peptide YY (PYY), for receptor binding and subsequent biological action profile is well established. However, an intact C-terminus may be fundamental also for bioactivity of PYY peptides. The current study has demonstrated C-terminal degradation of the major recognised circulating forms of PYY, PYY(1-36) and PYY(3-36), in plasma, resulting in production of PYY(1-34) and PYY(3-34). Interestingly, the angiotensin-converting-enzyme (ACE) inhibitor, captopril, blocked formation of PYY(3-34) from PYY(3-36) in plasma, but did result in the appearance of PYY(3-35). In addition, we were able to evidence C-terminal truncation of PYY(1-35) and PYY(3-35) to PYY(1-34) and PYY(3-34), respectively. As expected, PYY(1-36) and PYY(3-36) inhibited (P < 0.05-P < 0.001) glucose- and alanine-stimulated insulin secretion from BRIN-BD11 beta-cells. In contrast, PYY(1-34), PYY(3-34), PYY(1-35) and PYY(3-35) were devoid of insulinostatic actions. Both PYY(1-36) and PYY(3-36), but not related PYY metabolites, significantly (P < 0.05-P < 0.001) enhanced proliferation of BRIN BD11 and 1.1B4 beta-cell lines, and protected (P < 0.01-P < 0.001) these cell lines against cytokine-induced apoptosis. As expected, PYY(3-36) induced clear (P < 0.05-P < 0.01) appetite suppressive effects in mice, but this action was eliminated by mono- or di-peptide C-terminal truncation. Interestingly, captopril significantly (P < 0.05) augmented the anorexigenic effects of PYY(3-36) in mice. PYY(1-36), PYY(3-36), PYY(1-34) and PYY(3-34) lacked effects on in vivo glucose tolerance or glucose-induced insulin release. Taken together, these data highlight the unrecognised importance of C-terminal integrity of PYY peptides for biological activity and therapeutic usefulness in obesity-diabetes.

Novel dual incretin agonist peptide with antidiabetic and neuroprotective potential

Glucose-dependent insulinotropic hormone (GIP) and glucagon-like peptide-1 (GLP-1) are incretin hormones that exert an array of beneficial actions on metabolism and cognitive function. GLP-1-based therapeutics have been highly successful in terms of obesity and diabetes management, however GIP therapies have found no clinical utility to date. In the present study we describe, for the first time, the therapeutic effectiveness of a novel GIP/GLP-1 hybrid peptide based on the amino acid sequences of GIP, GLP-1 and the clinically approved GLP-1 mimetic, exendin-4. The hybrid peptide, N-ac(d-Ala²)GIP/GLP-1-exe, was enzymatically stable for up to 12 h when incubated with DPP-4. N-ac(d-Ala²)GIP/GLP-1-exe significantly (P < 0.001) stimulated insulin secretion from BRIN-BD11 cells and isolated mouse islets, and evoked dose-dependent increases (P < 0.001) in cAMP production in both GIP-R and GLP-1-R transfected cells. In mice, injection of the hybrid in combination with glucose significantly (P < 0.001) reduced glucose and increased insulin concentrations, with metabolic actions evident (P < 0.05) 8 h post-injection. Twice-daily injection of N-ac(d-Ala²)GIP/GLP-1-exe to high fat fed (HFF) mice for 28 days significantly (P < 0.05-P < 0.001) reduced body weight, HbA_1c, circulating glucose and insulin concentrations. Furthermore, both oral and i.p. glucose tolerance were improved (P < 0.001) and insulin sensitivity enhanced. The hybrid peptide also increased (P < 0.05-P < 0.001) beta cell number, islet area, pancreatic insulin content and islet insulin secretory responsiveness in HFF mice. Finally, N-ac(d-Ala²)GIP/GLP-1-exe treated mice exhibited improved (P < 0.01) recognition memory which was accompanied by enhanced (P < 0.05-P < 0.001) hippocampal neurogenesis, synapse formation and reduced neuronal oxidative stress. These data demonstrate for the first time the beneficial actions of the novel GIP/GLP-1 hybrid, N-ac(d-Ala²)GIP/GLP-1-exe, on glucose homeostasis and memory function in diabetes.

Glucagon receptor antagonist and GIP agonist combination for diet-induced obese mice

Ablation of glucagon receptor signaling represents a potential treatment option for type 2 diabetes (T2DM). Additionally, activation of glucose-dependent insulinotropic polypeptide (GIP) receptor signaling also holds therapeutic promise for T2DM. Therefore, this study examined both independent and combined metabolic actions of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon (glucagon receptor antagonist) and d-Ala(2)GIP (GIP receptor agonist) in diet-induced obese mice. Glucagon receptor binding has been linked to alpha-helical structure and desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon displayed enhanced alpha-helical content compared with native glucagon. In clonal pancreatic BRIN-BD11 beta-cells, desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon was devoid of any insulinotropic or cAMP-generating actions, and did not impede d-Ala(2)GIP-mediated (P<0.01 to P<0.001) effects on insulin and cAMP production. Twice-daily injection of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon or d-Ala(2)GIP alone, and in combination, in high-fat-fed mice failed to affect body weight or energy intake. Circulating blood glucose levels were significantly (P<0.05 to P<0.01) decreased by all treatments regimens, with plasma and pancreatic insulin elevated (P<0.05 to P<0.001) in all mice receiving d-Ala(2)GIP. Interestingly, plasma glucagon concentrations were decreased (P<0.05) by sustained glucagon inhibition (day 28), but increased (P<0.05) by d-Ala(2)GIP therapy, with a combined treatment resulting in glucagon concentration similar to saline controls. All treatments improved (P<0.01) intraperitoneal and oral glucose tolerance, and peripheral insulin sensitivity. d-Ala(2)GIP-treated mice showed increased glucose-induced insulin secretion in response to intraperitoneal and oral glucose. Metabolic rate and ambulatory locomotor activity were increased (P<0.05 to P<0.001) in all desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon-treated mice. These studies highlight the potential of glucagon receptor inhibition alone, and in combination with GIP receptor activation, for T2DM treatment.

Stable oxyntomodulin analogues exert positive effects on hippocampal neurogenesis and gene expression as well as improving glucose homeostasis in high fat fed mice

The weight-lowering and gluco-regulatory actions of oxyntomodulin (Oxm) have been well-documented however potential actions of this peptide in brain regions associated with learning and memory have not yet been evaluated. The present study examined the long-term actions of a stable acylated analogue of Oxm, (dS(2))Oxm(K-γ-glu-Pal), together with parent (dS(2))Oxm peptide, on hippocampal neurogenesis, gene expression and metabolic control in high fat (HF) mice. Groups of HF mice (n = 12) received twice-daily injections of Oxm analogues (both at 25 nmol/kg body weight) or saline vehicle (0.9% wt/vol) over 28 days. Hippocampal gene expression and histology were assessed together with evaluation of energy intake, body weight, non-fasting glucose and insulin, glucose tolerance, insulin sensitivity and lipids. Oxm analogues significantly reduced body weight, improved glucose tolerance, glucose-mediated insulin secretion, insulin sensitivity, islet architecture and lipid profile. Analysis of brain histology revealed significant reduction in hippocampal oxidative damage (8-oxoguanine), enhanced hippocampal neurogenesis (doublecortin) and improved hippocampal and cortical synaptogenesis (synaptophysin) following treatment. Furthermore, Oxm analogues up-regulated hippocampal mRNA expression of MASH1, Synaptophysin, SIRT1, GLUT4 and IRS1, and down-regulated expression of LDL-R and GSK3β. These data demonstrate potential of stable Oxm analogues, and particularly (dS(2))Oxm(K-γ-glu-Pal) to improve metabolic function and enhance neurogenesis, synaptic plasticity, insulin signalling and exert protective effects against oxidative damage in hippocampus and cortex brain regions in HF mice.

Sustained treatment with a stable long-acting oxyntomodulin analogue improves metabolic control and islet morphology in an experimental model of type 1 diabetes

AIM

To assess the therapeutic benefits of regulatory peptides other than insulin, which have to date received limited consideration in the context of type 1 diabetes.

METHODS

We assessed the effects of subchronic administration of the stable, oxyntomodulin (Oxm) analogue, (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], for 28 days in streptozotocin (STZ)-induced insulin-deficient diabetic mice.

RESULTS

Twice-daily injection with (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] significantly countered the excessive food and fluid intake in STZ-induced diabetic mice, and maintained normal body weight. Lean body mass was normalized, whilst fat mass was significantly increased compared with control STZ-induced diabetic mice. In addition, circulating glucose was significantly reduced by the Oxm analogue, whilst plasma and pancreatic insulin concentrations were increased and glucagon decreased by day 28. Plasma lipid profile was normalized by (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] administration and circulating amylase was not significantly altered by induction of diabetes or Oxm analogue therapy. This was associated with significantly improved glucose tolerance and insulin secretion. Peripheral insulin sensitivity was also significantly improved by Oxm analogue treatment. Histological examination of pancreata showed beneficial elevations of total islet and β-cell area, associated with an increase in the number of smaller-sized islets. Further analysis revealed enhanced islet cell proliferation relative to apoptosis in Oxm analogue-treated mice.

CONCLUSION

These studies emphasize the potential of stable Oxm-based peptides, such as (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], as therapeutic agents for insulin-deficient type 1 diabetes.

Pharmacological characterization and antidiabetic activity of a long-acting glucagon-like peptide-1 analogue conjugated to an antithrombin III-binding pentasaccharide

AIMS

To examine the biological characteristics of a novel glucagon-like peptide-1 (GLP-1) conjugate, in which an antithrombin III (ATIII)-binding pentasaccharide is conjugated to d-Ala(8) GLP-1 using a tetraethylene glycol linker.

METHODS

We assessed GLP-1 receptor binding, cAMP generation and insulin secretory activity of the GLP-1 conjugate in vitro. Circulating half-life, glucose homeostatic and subchronic therapeutic effectiveness were then examined in vivo.

RESULTS

The half-life of the GLP-1 conjugate in mice was ∼11 h. In vitro insulin secretion from clonal β cells and islets was increased (p < 0.001) by the conjugate. The conjugate had half maximum effective concentration values of 1.3 × 10(-7) and 9.9 × 10(-8)  M for displacement of (125) I-GLP-1 in competitive GLP-1 receptor binding and cAMP generation, respectively. Glucose tolerance in normal mice, immediately and 4 h after conjugate injection, resulted in significant (p < 0.001) improvements in blood glucose. These effects persisted for >48 h after administration. Daily treatment (21 days) of high-fat-fed and ob/ob mice with 25 nmol/kg conjugate resulted in significant improvement in glucose tolerance (p < 0.001) and reductions in glycated haemoglobin (HbA1c; p < 0.01) equivalent to or better than with exenatide or liraglutide. Treatment of C57BL/KsJ db/db mice for 15 days with 100 nmol/kg conjugate significantly (p < 0.001) reduced glucose and raised plasma insulin. Oral glucose tolerance was significantly (p < 0.001) improved and both 24-h glucose profile (p < 0.001) and HbA1c levels (p < 0.001) were reduced. Islet size (p < 0.001) and pancreatic insulin content were increased without change of islet cell proliferation or apoptosis.

CONCLUSION

These data show that d-Ala(8) GLP-1(Lys(37) ) pentasaccharide exerts significant antidiabetic actions and has a projected pharmacokinetic/pharmacodynamic profile that merits further evaluation in humans for a possible once-weekly dosing regimen.

Xenin-25[Lys13PAL]: a novel long-acting acylated analogue of xenin-25 with promising antidiabetic potential

AIMS

Xenin-25 is co-secreted with glucose-dependent insulinotropic polypeptide (GIP) from intestinal K-cells following a meal. Xenin-25 is believed to play a key role in glucose homoeostasis and potentiate the insulinotropic effect of GIP.

METHODS

This study investigated the effects of sub-chronic administration of the stable and longer-acting xenin-25 analogue, xenin-25[Lys(13)PAL] (25 nmol/kg), in diabetic mice fed with a high-fat diet.

RESULTS

Initial studies confirmed the significant persistent glucose-lowering (p < 0.05) and insulin-releasing (p < 0.05) actions of xenin-25[Lys(13)PAL] compared with native xenin-25. Interestingly, xenin-25 retained significant glucose-lowering activity in GIP receptor knockout mice. Twice-daily intraperitoneal (i.p.) injection of xenin-25[Lys(13)PAL] for 14 days had no significant effect on food intake or body weight in high-fat-fed mice. Non-fasting glucose and insulin levels were also unchanged, but overall glucose levels during an i.p. glucose tolerance and oral nutrient challenge were significantly (p < 0.05) lowered by xenin-25[Lys(13)PAL] treatment. These changes were accompanied by significant improvements in i.p. (p < 0.05) and oral (p < 0.001) nutrient-stimulated insulin concentrations. No appreciable changes in insulin sensitivity were observed between xenin-25[Lys(13)PAL] and saline-treated high-fat mice. However, xenin-25[Lys(13)PAL] treatment restored notable sensitivity to the biological actions of exogenous GIP injection. Consumption of O2, production of CO2, respiratory exchange ratio and energy expenditure were not altered by 14-day twice-daily treatment with xenin-25[Lys(13)PAL]. In contrast, ambulatory activity was significantly (p < 0.05 to p < 0.001) increased during the dark phase in xenin-25[Lys(13)PAL] mice compared with high-fat controls.

CONCLUSIONS

These data indicate that sustained administration of a stable analogue of xenin-25 exerts a spectrum of beneficial metabolic effects in high-fat-fed mice.

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.

Double incretin receptor knock-out (DIRKO) mice present with alterations of trabecular and cortical micromorphology and bone strength

A role for gut hormone in bone physiology has been suspected. We evidenced alterations of microstructural morphology (trabecular and cortical) and bone strength (both at the whole-bone--and tissue-level) in double incretin receptor knock-out (DIRKO) mice as compared to wild-type littermates. These results support a role for gut hormones in bone physiology.

INTRODUCTION

The two incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have been shown to control bone remodeling and strength. However, lessons from single incretin receptor knock-out mice highlighted a compensatory mechanism induced by elevated sensitivity to the other gut hormone. As such, it is unclear whether the bone alterations observed in GIP or GLP-1 receptor deficient animals resulted from the lack of a functional gut hormone receptor, or by higher sensitivity for the other gut hormone. The aims of the present study were to investigate the bone microstructural morphology, as well as bone tissue properties, in double incretin receptor knock-out (DIRKO) mice.

METHODS

Twenty-six-week-old DIRKO mice were age- and sex-matched with wild-type (WT) littermates. Bone microstructural morphology was assessed at the femur by microCT and quantitative X-ray imaging, while tissue properties were investigated by quantitative backscattered electron imaging and Fourier-transformed infrared microscopy. Bone mechanical response was assessed at the whole-bone- and tissue-level by 3-point bending and nanoindentation, respectively.

RESULTS

As compared to WT animals, DIRKO mice presented significant augmentations in trabecular bone mass and trabecular number whereas bone outer diameter, cortical thickness, and cortical area were reduced. At the whole-bone-level, yield stress, ultimate stress, and post-yield work to fracture were significantly reduced in DIRKO animals. At the tissue-level, only collagen maturity was reduced by 9 % in DIRKO mice leading to reductions in maximum load, hardness, and dissipated energy.

CONCLUSIONS

This study demonstrated the critical role of gut hormones in controlling bone microstructural morphology and tissue properties.

Two novel glucagon receptor antagonists prove effective therapeutic agents in high-fat-fed and obese diabetic mice

AIMS

To examine the effect of two novel, enzymatically stable, glucagon receptor peptide antagonists, on metabolic control in two mouse models of obesity/diabetes.

METHOD

The effects of twice daily i.p. administration of desHis(1)Pro(4)Glu(9)-glucagon or desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon for 10 days on metabolic control in high-fat-fed (HFF; 45% fat) and obese diabetic (ob/ob) mice were compared with saline-treated controls.

RESULTS

Neither analogue altered body weight or food intake in either model over 10 days; however, treatment with each peptide restored non-fasting blood glucose towards normal control values in HFF mice. Basal glucose was also reduced (p < 0.01) in desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon treated ob/ob mice by day 10, coinciding with increases (p < 0.001) in circulating insulin. At the end of the treatment period, both analogues significantly (p < 0.05-0.01) improved oral and i.p. glucose tolerance (p < 0.05) and peripheral insulin sensitivity, increased pancreatic insulin and glucagon content (p < 0.05-0.01) and decreased (p < 0.05) cholesterol levels in HFF mice. Similarly beneficial metabolic effects on oral glucose tolerance (p < 0.01) and pancreatic insulin content (p < 0.05) were observed in ob/ob mice, especially after desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon treatment. No significant differences in circulating triglycerides or aspects of indirect calorimetry were noted between peptide treatment groups and respective control HFF and ob/ob mice. Finally, glucagon-mediated elevations of glucose and insulin were significantly (p < 0.05-0.01) annulled after 10 days of desHis(1)Pro(4)Glu(9)-glucagon or desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon treatment in both animal models.

CONCLUSION

These data indicate that peptide-based glucagon receptor antagonists can reverse aspects of genetically and dietary-induced obesity-related diabetes.

Effects of chronic exposure of clonal β-cells to elevated glucose and free fatty acids on incretin receptor gene expression and secretory responses to GIP and GLP-1

AIM

The incretin effect, mediated by glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), is impaired in type 2 diabetes.

METHODS

This study examines the effects of prolonged exposure to elevated glucose and free fatty acids in clonal BRIN BD11 cells on GIP and GLP-1 action.

RESULTS

Glucotoxic conditions (18 h) had no effect on GIP- or GLP-1-mediated insulinotropic responses. In contrast, 48 h glucotoxic culture impaired (p < 0.05 to p < 0.001) insulin release in response to GLP-1, and particularly GIP. Culture under lipotoxic conditions (18 h) impaired (p < 0.05 to p < 0.001) the insulin-releasing effect of GIP, but was without effect on GLP-1. However, 48 h lipotoxic culture compromised both GIP (p < 0.05 to p < 0.001) and GLP-1 (p < 0.05 to p < 0.01) insulin-releasing actions. Glucolipotoxic culture (18 h) completely annulled the insulinotropic action of GIP, whereas GLP-1 effects were similar to control. However, when glucolipotoxic culture was extended to 48 h, both GIP- and GLP-1-mediated effects were (p < 0.05 to p < 0.001) impaired. Assessment of cell viability, number and insulin content revealed detrimental (p < 0.05 to p < 0.001) effects under all culture conditions, barring 18 h glucotoxic and lipotoxic culture. Finally, GIP-R gene and protein expression was increased (p < 0.05 to p < 0.01) under glucotoxic culture, with decreased (p < 0.05 to p < 0.001) expression following glucolipotoxic culture. GLP-1-R gene expression followed a similar trend, but protein levels were generally reduced under all culture conditions.

CONCLUSION

The results indicate that impaired insulinotropic response to GIP and GLP-1 under diabetic milieu involves mechanisms beyond simple expression of respective receptors.

Characterisation of structurally modified analogues of glucagon as potential glucagon receptor antagonists

Acute in vitro and in vivo biological activities of four novel structural analogues of glucagon were tested. desHis(1)Pro(4)-glucagon, desHis(1)Pro(4)Glu(9)-glucagon, desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon and desHis(1)Pro(4)Glu(9)Lys(30)FA-glucagon were stable to DPP-4 degradation and dose-dependently inhibited glucagon-mediated cAMP production (p<0.05 to p<0.001). None stimulated insulin secretion in vitro above basal levels, but all inhibited glucagon-induced insulin secretion (p<0.01 to p<0.001). In normal mice all analogues antagonised acute glucagon-mediated elevations of blood glucose (p<0.05 to p<0.001) and blocked corresponding insulinotropic responses. In high-fat fed mice, glucagon-induced increases in plasma insulin (p<0.05 to p<0.001) and glucagon-induced hyperglycaemia were blocked (p<0.05 to p<0.01) by three analogues. In obese diabetic (ob/ob) mice only desHis(1)Pro(4)Glu(9)-glucagon effectively (p<0.05 to p<0.01) inhibited both glucagon-mediated glycaemic and insulinotropic responses. desHis(1)Pro(4)-glucagon and desHis(1)Pro(4)Glu(9)-glucagon were biologically ineffective when administered 8h prior to glucagon, whereas desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon retained efficacy (p<0.01) for up to 24h. Such peptide-derived glucagon receptor antagonists have potential for type 2 diabetes therapy.

Comparison of independent and combined metabolic effects of chronic treatment with (pGlu-Gln)-CCK-8 and long-acting GLP-1 and GIP mimetics in high fat-fed mice

AIM

The incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) and cholecystokinin (CCK) are gastrointestinal peptides with important physiological effects. However, rapid enzymatic degradation results in short-lived biological actions.

METHODS

This study has examined metabolic actions of exendin-4, GIP[mPEG] and a novel CCK-8 analogue, (pGlu-Gln)-CCK-8 as enzymatically stable forms of GLP-1, GIP and CCK, respectively.

RESULTS

All peptides significantly (p < 0.01-p < 0.001) stimulated insulin secretion from BRIN BD11 cells, and acute in vivo experiments confirmed prominent antihyperglycaemic and insulinotropic responses to GLP-1 or GIP receptor activation in normal mice. Twice daily injection of (pGlu-Gln)-CCK-8 alone and in combination with exendin-4 or GIP[mPEG] in high fat-fed mice significantly decreased accumulated food intake (p < 0.05-p < 0.01), body weight gain (p < 0.05-p < 0.01) and improved (p < 0.05) insulin sensitivity in high fat-fed mice. However, there was no evidence for superior effects compared to (pGlu-Gln)-CCK-8 alone. Combined treatment of (pGlu-Gln)-CCK-8 and exendin-4 resulted in significantly (p < 0.05) lowered circulating glucose levels and improved (p < 0.05) intraperitoneal glucose tolerance. These effects were superior to either treatment regime alone but not associated with altered insulin concentrations. A single injection of (pGlu-Gln)-CCK-8, or combined with exendin-4, significantly (p < 0.05) lowered blood glucose levels 24 h post injection in untreated high fat-fed mice.

CONCLUSION

This study highlights the potential of (pGlu-Gln)-CCK-8 alone and in combination with incretin hormones for the treatment of type 2 diabetes.

Comparison of the metabolic effects of sustained CCK1 receptor activation alone and in combination with upregulated leptin signalling in high-fat-fed mice

AIMS/HYPOTHESIS

Cholecystokinin (CCK) and leptin are important hormones with effects on energy balance. The present study assessed the biological effects of (pGlu-Gln)-CCK-8 and [D-Leu-4]-OB3, smaller isoforms of CCK and leptin, respectively.

METHODS

The actions and overall therapeutic use of (pGlu-Gln)-CCK-8 and [D-Leu-4]-OB3, alone and in combination, were evaluated in normal and high-fat-fed mice.

RESULTS

(pGlu-Gln)-CCK-8 had prominent (p < 0.01 to p < 0.001), acute feeding-suppressive effects, which were significantly augmented (p < 0.05 to p < 0.01) by [D-Leu-4]-OB3. In agreement, the acute dose-dependent glucose-lowering and insulinotropic actions of (pGlu-Gln)-CCK-8 were significantly enhanced by concurrent administration of [D-Leu-4]-OB3. Twice daily injection of (pGlu-Gln)-CCK-8 alone and in combination with [D-Leu-4]-OB3 in high-fat-fed mice for 18 days decreased body weight (p < 0.05 to p < 0.001), energy intake (p < 0.01), circulating triacylglycerol (p < 0.01), non-fasting glucose (p < 0.05 to p < 0.001) and triacylglycerol deposition in liver and adipose tissue (p < 0.001). All treatment regimens improved glucose tolerance (p < 0.05 to p < 0.001) and insulin sensitivity (p < 0.001). Combined treatment with (pGlu-Gln)-CCK-8 and [D-Leu-4]-OB3 resulted in significantly lowered plasma insulin levels, normalisation of circulating LDL-cholesterol and decreased triacylglycerol deposition in muscle. These effects were superior to either treatment regimen alone. There were no changes in overall locomotor activity or respiratory exchange ratio, but treatment with (pGlu-Gln)-CCK-8 significantly reduced (p < 0.001) energy expenditure.

CONCLUSIONS/INTERPRETATION

These studies highlight the potential of (pGlu-Gln)-CCK-8 alone and in combination with [D-Leu-4]-OB3 in the treatment of obesity and diabetes.

Beneficial effects of (pGlu-Gln)-CCK-8 on energy intake and metabolism in high fat fed mice are associated with alterations of hypothalamic gene expression

Cholecystokinin (CCK) is a gastrointestinal hormone with potential therapeutic promise for obesity-diabetes. The present study examined the effects of twice daily administration of the N-terminally modified stable CCK-8 analogue, (pGlu-Gln)-CCK-8, on metabolic control and hypothalamic gene expression in high fat fed mice. Sub-chronic twice daily injection of (pGlu-Gln)-CCK-8 for 16 days significantly decreased body weight (p<0.05), energy intake (p<0.01), circulating blood glucose (p<0.001), and plasma insulin (p<0.001) compared to high fat controls. Furthermore, (pGlu-Gln)-CCK-8 markedly improved glucose tolerance (p<0.05) and insulin sensitivity (p<0.05). Assessment of hypothalamic gene expression on day 16 revealed significantly elevated NPY (p<0.05) and reduced POMC (p<0.05) and MC4R (p<0.05) mRNA expression in (pGlu-Gln)-CCK-8 treated mice. High fat feeding or (pGlu-Gln)-CCK-8 treatment had no significant effects on hypothalamic gene expression of receptors for leptin, CCK₁ and GLP-1. These studies underscore the potential of (pGlu-Gln)-CCK-8 for the treatment of obesity-diabetes and suggest modulation of NPY and melanocortin related pathways may be involved in the observed beneficial effects.

Beneficial effects of the novel cholecystokinin agonist (pGlu-Gln)-CCK-8 in mouse models of obesity/diabetes

AIMS/HYPOTHESIS

Cholecystokinin (CCK) is a rapidly degraded gastrointestinal peptide that stimulates satiety and insulin secretion. We aimed to investigate the beneficial weight-lowering and metabolic effects of the novel N-terminally modified CCK analogue, (pGlu-Gln)-CCK-8.

METHODS

The biological actions of (pGlu-Gln)-CCK-8 were comprehensively evaluated in pancreatic clonal BRIN BD11 cells and in vivo in high-fat-fed and ob/ob mice.

RESULTS

(pGlu-Gln)-CCK-8 was completely resistant to enzymatic degradation and its satiating effects were significantly (p < 0.05 to p < 0.001) more potent than CCK-8. In BRIN-BD11 cells, (pGlu-Gln)-CCK-8 exhibited enhanced (p < 0.01 to p < 0.001) insulinotropic actions compared with CCK-8. When administered acutely to high-fat-fed or ob/ob mice, (pGlu-Gln)-CCK-8 improved glucose homeostasis. Sub-chronic twice daily injections of (pGlu-Gln)-CCK-8 in high-fat-fed mice for 28 days significantly decreased body weight (p < 0.05 to p < 0.001), accumulated food intake (p < 0.05 to p < 0.001), non-fasting glucose (p < 0.05) and triacylglycerol deposition in pancreatic (p < 0.01), adipose (p < 0.05) and liver (p < 0.001) tissue, and improved oral (p < 0.05) and i.p. (p < 0.05) glucose tolerance and insulin sensitivity (p < 0.001). Similar observations were noted in ob/ob mice given twice daily injections of (pGlu-Gln)-CCK-8. In addition, these beneficial effects were not reproduced by simple dietary restriction and were not associated with changes in energy expenditure. There was no evidence for development of tolerance to (pGlu-Gln)-CCK-8, and analysis of histology or blood-borne markers for pancreatic, liver and renal function in mice treated with (pGlu-Gln)-CCK-8 suggested little abnormal pathology.

CONCLUSIONS/INTERPRETATION

These studies emphasise the potential of (pGlu-Gln)-CCK-8 for the alleviation of obesity and insulin resistance.

Dual modulation of GIP and glucagon action by the low molecular weight compound 4-hydroxybenzoic acid 2-bromobenzylidene hydrazide

AIM

The presence of functional gastric inhibitory polypeptide (GIP) receptors on adipocytes and knowledge that GIP plays a key role in fat deposition suggests a beneficial effect of GIP receptor antagonism in obesity and insulin resistance. GIP receptor antagonists studied to date are peptidic GIP analogues that must be administered by injection.

METHODS

The present study has examined in vitro and in vivo metabolic actions of a low molecular weight GIP receptor modulator 4-hydroxybenzoic acid 2-bromobenzylidene hydrazide (4H2BH), suitable for oral administration.

RESULTS

4H2BH alone had no significant effect on cAMP production or insulin secretion from BRIN-BD11 cells. However, 4H2BH significantly inhibited GIP-mediated cAMP production and insulin secretion in vitro. 4H2BH also suppressed (p < 0.05 to p < 0.001) glucagon-induced elevations of cAMP generation and insulin secretion in BRIN-BD11 cells. However, 4H2BH had no effect on glucagon-like peptide-1 (GLP-1) mediated insulinotropic actions. Administration of 4H2BH to mice in combination with glucose and GIP significantly annulled the glucose-lowering actions of GIP. In agreement with this, 4H2BH completely annulled GIP-mediated insulin secretion. Combined injection of 4H2BH with glucagon also partially (p < 0.05 to p < 0.001) impaired glucagon-induced elevations in blood glucose and plasma insulin. 4H2BH had no effect on blood glucose or insulin levels when administered alone.

CONCLUSION

These results indicate that 4H2BH has a dual effect of inhibiting GIP and glucagon-mediated biological actions. Given that hyperglucagonaemia is also a cardinal feature of type 2 diabetes, 4H2BH and related low molecular weight compounds appear worthy of further evaluation for therapeutic potential in obesity diabetes.

Acute and long-term effects of peroxisome proliferator-activated receptor-γ activation on the function and insulin secretory responsiveness of clonal beta-cells

Thiazolidinediones (TZDs) are used as antidiabetic therapy. The purpose of the present study was to examine whether the TZD rosiglitazone has direct actions on pancreatic beta-cells that contribute to its overall effects. Effects of acute and prolonged (48 h) exposure to rosiglitazone, as a model glitazone compound, were assessed in clonal pancreatic BRIN-BD11 beta-cells maintained in standard, glucotoxic and lipotoxic cultures. In acute 20-min incubations, rosiglitazone (0.2-100 μM) did not alter basal or glucose-stimulated insulin secretion. However, rosiglitazone (6.25 μM) enhanced (p<0.001) the acute insulinotropic action of GLP-1. Prolonged exposure to 6.25 μM rosiglitazone in standard media had no effect on cell viability or cellular insulin content, but slightly reduced the insulin secretory response to glucose and alanine (p<0.05). Prolonged (48 h) exposure to glucotoxic or lipotoxic conditions reduced beta-cell viability (p<0.05), cellular insulin content (p<0.001 and p<0.05, respectively), and insulin release in response to glucose and a range of secretagogues. The adverse effect of lipotoxicity on beta-cell viability was prevented by concomitant exposure to 6.25 μM rosiglitazone. Culture with 6.25 μM rosiglitazone further decreased acute insulin release under glucotoxic conditions. However, when insulin secretion was expressed as percentage cellular insulin content, rosiglitazone (6.25 μM) significantly improved many of the adverse effects of gluco- and lipotoxic conditions on insulin secretory responsiveness. The results suggest that despite decrease in cellular insulin content TZDs exert direct beneficial effects on beta-cell viability and function during gluco- or lipotoxicity.

Alterations of glucose-dependent insulinotropic polypeptide (GIP) during cold acclimation

Cold acclimation is initially associated with shivering thermogenesis in skeletal muscle followed by adaptive non-shivering thermogenesis, particularly in brown adipose tissue (BAT). In response, hyperphagia occurs to meet increased metabolic demand and thermoregulation. The present study investigates the effects of cold (4 ± 1 °C) acclimation and hyperphagia on circulating and intestinal levels of gastric inhibitory polypeptide (GIP) in rats. Pair fed animals were used as additional controls in some experiments. Cold acclimation for 42 days significantly (p<0.01) increased daily food intake. There was no corresponding change in body weight. However, body weights of pair fed cold exposed rats were significantly (p<0.01) reduced compared to controls and ad libitum fed cold exposed rats. By day 42, non-fasting plasma glucose was increased (p<0.05) by chronic cold exposure regardless of food intake. Corresponding plasma insulin concentrations were significantly (p<0.01) lower in pair fed cold exposed rats. Circulating GIP levels were elevated (p<0.05) in ad libitum fed cold acclimated rats on days 18 and 24, but returned to normal levels by the end of the study. The glycaemic response to oral glucose was improved (p<0.01) in all cold exposed rats, with significantly (p<0.05) elevated GIP responses in ad libitum fed rats and significantly (p<0.05) reduced insulin responses in pair fed rats. In keeping with this, insulin sensitivity was enhanced (p<0.05) in cold exposed rats compared to controls. By the end of the study, cold acclimated rats had significantly (p<0.01) increased BAT mass and intestinal concentrations of GIP and GLP-1 compared to controls, independent of food intake. These data indicate that changes in the secretion and actions of GIP may be involved in the metabolic adaptations to cold acclimation in rats.

Effects of metformin on BRIN-BD11 beta-cell insulin secretory desensitization induced by prolonged exposure to sulphonylureas

AIMS

Prolonged exposure of pancreatic beta-cells in vitro to the sulphonylureas tolbutamide and glibenclamide induces subsequent desensitization of insulinotropic pathways. Clinically, the insulin-sensitizing biguanide drug metformin is often administered alongside sulphonylurea as antidiabetic therapy. The present study examines the functional effects of metformin (200 µM) on tolbutamide- and glibenclamide-induced desensitisation.

METHODS

Acute and prolonged (18 h) effects of exposure to tolbutamide and glibenclamide alone, or in the presence of metformin, were examined in insulin-secreting BRIN-BD11 cells.

RESULTS

In acute 20 min incubations at 1.1 mM glucose, metformin increased (1.2-1.7-fold; p < 0.001) the insulin-releasing actions of tolbutamide and glibenclamide. At 16.7 mM glucose, metformin significantly enhanced glibenclamide-induced insulin release at all concentrations (50-400 µM) examined, but tolbutamide-stimulated insulin secretion was only augmented at higher concentrations (300-400 µM). Exposure for 18 h to 100 µM tolbutamide or glibenclamide significantly impaired insulin release in response to glucose and a broad range of insulin secretagogues. Concomitant culture with metformin (200 µM) prevented or partially reversed many of the adverse effects on K(ATP) channel dependent and independent insulinotropic pathways. Beneficial effects of metformin were also observed in cells exposed to glibenclamide for 18 h with significant improvements in the insulin secretory responsiveness to alanine, GLP-1 and sulphonylureas. The decrease of viable cell numbers observed with glibenclamide was reversed by co-culture with metformin, but cellular insulin content was depressed.

CONCLUSIONS

The results suggest that metformin can prevent the aspects of sulphonylurea-induced beta-cell desensitization.

Active immunization against (Pro(3))GIP improves metabolic status in high-fat-fed mice

AIM

Ablation of gastric inhibitory polypeptide (GIP) receptor signalling can prevent many of the metabolic abnormalities associated with dietary-induced obesity-diabetes. The present study was designed to assess the ability of active immunization against (Pro(3))GIP to counter metabolic dysfunction associated with diet-induced obesity in high-fat-fed mice.

METHODS

Normal male Swiss NIH mice were injected (s.c.) once every 14 days for 98 days with complexed (Pro(3))GIP peptide, with transfer to a high-fat diet on day 21.

RESULTS

Active immunization against (Pro(3))GIP resulted in circulating GIP antibody production and significantly (p < 0.05 p < 0.01) reduced circulating blood glucose concentrations compared to high-fat control mice from day 84 onwards. Glucose levels were not significantly different from lean controls. The glycaemic response to i.p. glucose was correspondingly improved (p < 0.01) in (Pro(3))GIP-immunized mice. Furthermore, circulating and glucose-stimulated plasma insulin levels were significantly (p < 0.01 to p < 0.001) depressed compared to high-fat control mice. Liver triglyceride, pancreatic insulin and circulating LDL-cholesterol levels were also significantly reduced in (Pro(3))GIP-immunized mice. These changes were independent of any effects on food intake or body weight. The glucose-lowering effect of native GIP was annulled in (Pro(3))GIP-immunized mice consistent with the induction of biologically effective GIP-specific neutralizing antibodies.

CONCLUSION

These results suggest that immunoneutralization of GIP represents an effective means of countering the disruption of metabolic processes induced by high-fat feeding.

Effect of oxygen fluctuations on recombinant Escherichia coli fermentation

Escherichia coli DH5alpha, carrying the pUC19 plasmid for the lacZ fragment of beta-galactosidase and ampicillin resistance, was grown in a batch fermentor under conditions of fluctuating oxygen supply. A Monte Carlo method was used to control the on/off supply of air to simulate circulation of cells in a large fermentor. Rapid changes in oxygen supply reduced the rates of oxygen uptake the carbon dioxide release and prolonged the active second growth phase in batch culture, compared to growth with continuous aeration. Amplification of the plasmid was observed during the stationary phase when air supplied continuously, but not during the Monte Carlo experiments.

Evidence for beneficial effects of compromised gastric inhibitory polypeptide action in obesity-related diabetes and possible therapeutic implications

Gastric inhibitory polypeptide (GIP) is a physiological gut peptide secreted from the intestinal K-cells with well documented insulin-releasing actions. However, the GIP receptor is widely distributed in peripheral organs, including the pancreas, gut, adipose tissue, heart, adrenal cortex and brain, suggesting that it may have other functions. The presence of functional GIP receptors on adipocytes and the key role played by GIP in lipid metabolism and fat deposition suggest a possible beneficial effect of compromised GIP action in obesity and insulin resistance. Several key observations in animal models of obesity-related diabetes with chemically or genetically mediated biological GIP deficiency support this concept. Thus, obese diabetic animals with compromised GIP action due to peptide-based GIP receptor antagonists, small molecular weight GIP receptor antagonists, vaccination against GIP, genetic knockout of GIP receptor or targeted K-cell destruction are protected against obesity and associated metabolic disturbances. In addition, by causing preferential oxidation of fat, blockade of GIP signalling clears triacylglycerol deposits from liver and muscle, thereby restoring mechanisms for suppression of hepatic glucose output and improving insulin sensitivity. Emerging evidence also suggests that rapid cure of diabetes in grossly obese patients undergoing bypass surgery is mediated, in part, by surgical removal of GIP-secreting K-cells in the upper small intestine.

Metabolic effects of sustained activation of the GLP-1 receptor alone and in combination with background GIP receptor antagonism in high fat-fed mice

AIM

Enzyme-resistant glucagon-like peptide-1 (GLP-1) receptor agonists and GIP receptor antagonists have been proposed to have therapeutic potential for the treatment of type 2 diabetes. Such benefits are based on actions mediated primarily through stimulation of insulin secretion or alleviation of insulin resistance respectively. This study examined the long-term actions of the stable GLP-1 receptor agonist (D-Ala(8))GLP-1 and the GIP receptor antagonist (Pro(3))GIP alone and in combination in high fat-fed mice.

METHODS

Mice on high-fat diet for 155 days were injected once daily with (D-Ala(8))GLP-1 or (Pro(3))GIP (25 nmol/kg body weight) for 24 days. In the following 24-day period, half of the (Pro(3))GIP-treated mice were administered an additional dose of (D-Ala(8))GLP-1 (25 nmol/kg body weight), while the remaining mice continued their original treatment regimes.

RESULTS

Daily intraperitoneal injections of (D-Ala(8))GLP-1 or (Pro(3))GIP restored glycaemic control to normal levels and significantly (p < 0.05) improved glucose tolerance compared with high-fat controls by day 24. Food intake and body weights were not affected. On day 48, all treatment groups displayed significantly improved glucose tolerance (p < 0.05) and insulin sensitivity (p < 0.001) compared with high-fat controls on day 48. HDL cholesterol levels were significantly increased in mice treated with (D-Ala(8))GLP-1 alone (p < 0.05) or in combination with (Pro(3))GIP (p < 0.01) compared with normal chow-fed controls.

CONCLUSIONS

These results illustrate efficacy of (Pro(3))GIP and (D-Ala(8))GLP-1 for treatment of glucose intolerance and insulin resistance caused by high-fat feeding. Combination therapy appeared to have little benefit over either treatment alone.

Daily administration of the GIP-R antagonist (Pro3)GIP in streptozotocin-induced diabetes suggests that insulin-dependent mechanisms are critical to anti-obesity-diabetes actions of (Pro3)GIP

AIM

Glucose-dependent insulinotropic polypeptide-receptor (GIP-R) antagonism using (Pro3)GIP improves glucose tolerance and ameliorates insulin resistance and abnormalities of islet structure and function in a commonly used model of obesity-diabetes, namely ob/ob mice. The effect of GIP-R antagonism in a streptozotocin (STZ)-induced model of insulin deficiency has not been evaluated. The present study has investigated the effects of daily administration of (Pro(3))GIP to STZ-treated mice.

METHODS

Swiss TO mice received once-daily injection of (Pro3)GIP (25 nmol/kg body weight) or saline 4 days prior to and 16 days after injection of STZ, and effects on metabolic parameters and islet architecture were assessed.

RESULTS

(Pro3)GIP treatment had no significant effect on hyperphagia or body weight loss. However, hyperglycaemia and glycated haemoglobin were worsened, glucose tolerance further decreased and insulin sensitivity was impaired by (Pro3)GIP. These effects were observed on an STZ-induced background characterized by severe reductions of circulating insulin, beta-cell mass and pancreatic insulin stores.

CONCLUSIONS

These data indicate that the beneficial actions of the GIP-R antagonist, (Pro3)GIP, in obesity-diabetes appear to be largely mediated through insulin-dependent mechanisms that merit further investigation.

Comparison of the metabolic effects of GIP receptor antagonism and PYY(3-36) receptor activation in high fat fed mice

Glucose-dependent insulinotropic polypeptide (GIP) and peptide YY (PYY) are secreted from the intestinal K- and L-cells, respectively, following a meal. Both peptides are believed to play a key role in glucose homeostasis and energy expenditure. This study investigated the effects of daily administration of the stable and specific GIP-R antagonist, (Pro(3))GIP (25 nmol/kg) and the endogenous truncated form of PYY, PYY(3-36) (50 nmol/kg), in mice fed with a high fat diet. Daily i.p. injection of (Pro(3))GIP, PYY(3-36) or combined peptide administration over 24 days significantly (P<0.05-0.01) decreased body weight compared with saline-treated controls without change in food intake. Plasma glucose levels and glucose tolerance were significantly (P<0.05) lowered by (Pro(3))GIP treatment alone, and in combination with PYY(3-36). These changes were accompanied by a slight improvement of insulin sensitivity in all of the treatment groups. (Pro(3))GIP treatment significantly reduced plasma corticosterone (P<0.05), while combined administration with PYY(3-36) significantly lowered serum glucagon (P<0.05). No appreciable changes were observed in either circulating or glucose-stimulated insulin secretion in all treatment groups. (Pro(3))GIP-treated mice had significantly (P<0.01) lowered fasting glucose levels and an improved (P<0.05) glycemic response to feeding. These comparative data indicate that chemical ablation of GIP receptor action using (Pro(3))GIP provides an especially effective means of countering obesity and related abnormalities induced by consumption of high fat energy rich diet.

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.

Early administration of the glucose-dependent insulinotropic polypeptide receptor antagonist (Pro3)GIP prevents the development of diabetes and related metabolic abnormalities associated with genetically inherited obesity in ob/ob mice

AIMS/HYPOTHESIS

Ablation of gastric inhibitory polypeptide (GIP) receptor action is reported to protect against obesity and associated metabolic abnormalities. The aim of this study was to use prediabetic ob/ob mice to examine whether 60 days of chemical GIP receptor ablation with (Pro(3))GIP is able to counter the development of genetic obesity-related diabetes.

MATERIALS AND METHODS

Young (5-7 weeks) ob/ob mice received once daily i.p. injections of either saline vehicle or (Pro(3))GIP (25 nmol kg(-1) day(-1)) over a 60 day period. Food intake, body weight and circulating glucose and insulin were measured at frequent intervals. At 60 days, glucose tolerance, response to native GIP, postprandial responses, insulin sensitivity, HbA(1c), circulating hormones and plasma lipids were assessed.

RESULTS

Body weight and food intake in (Pro(3))GIP-treated mice did not differ from ob/ob controls. GIP receptor blockade significantly improved non-fasting glucose (p < 0.001), HbA(1c) (p < 0.05), glucose tolerance (p < 0.001), meal tolerance (p < 0.001) and insulin sensitivity (p < 0.05). Remarkably, (Pro(3))GIP treatment prevented the age-related development of diabetes, as none of these parameters differed significantly between treated ob/ob mice and normal age-matched lean controls. Circulating levels of glucagon, corticosterone, adiponectin and total cholesterol were unchanged by (Pro(3))GIP, while levels of triacylglycerol, LDL-cholesterol and resistin were decreased (p < 0.05) compared with those in control ob/ob mice. Plasma and pancreatic insulin concentrations were generally lower after (Pro(3))GIP treatment than in control ob/ob mice (p < 0.01), but plasma insulin levels remained substantially raised (p < 0.001) compared with those observed in lean controls.

CONCLUSIONS/INTERPRETATION

These data indicate that sustained GIP receptor antagonism provides an effective means of preventing the development of many of the metabolic abnormalities of obesity-driven diabetes.

Direct and indirect effects of obestatin peptides on food intake and the regulation of glucose homeostasis and insulin secretion in mice

Obestatin is a recently discovered peptide hormone that appears to be involved in reducing food intake, gut motility and body weight. Obestatin is a product of the preproghrelin gene and appears to oppose several physiological actions of ghrelin. This study investigated the acute effects of obestatin (1-23) and the truncated form, obestatin (11-23), on feeding activity, glucose homeostasis or insulin secretion. Mice received either intraperitoneal obestatin (1-23) or (11-23) (1 micromol/kg) 4h prior to an allowed 15 min period of feeding. Glucose excursions and insulin responses were lowered by 64-77% and 39-41%, respectively, compared with saline controls. However this was accompanied by 43% and 53% reductions in food intake, respectively. The effects of obestatin peptides were examined under either basal or glucose (18 mmol/kg) challenge conditions to establish whether effects were independent of changes in feeding. No alterations in plasma glucose or insulin responses were observed. In addition, obestatin peptides had no effect on insulin sensitivity as revealed by hypoglycaemic response when co-administered with insulin. Our observations support a role for obestatin in regulating metabolism through changes of appetite, but indicate no direct actions on glucose homeostasis or insulin secretion.

Mst3b, a purine-sensitive Ste20-like protein kinase, regulates axon outgrowth

The growth of axons is fundamental to the development and repair of brain circuitry. We show here that Mst3b, a neuron-specific homolog of the yeast kinase Ste20, is critical for axon outgrowth. Mst3b is activated in response to trophic factors, and suppressing its expression (via siRNAs) or its function (by a dominant-negative mutant) blocks axon outgrowth. Inosine, a purine nucleoside that stimulates axon outgrowth, activates Mst3b kinase activity, whereas 6-thioguanine, a purine analog that blocks outgrowth, inhibits the activity of this kinase. These findings place Mst3b as a key regulator of axon outgrowth and help explain the purine sensitivity of this process.

Effects of sub-chronic exposure to naturally occurring N-terminally truncated metabolites of glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), GIP(3-42) and GLP-1(9-36)amide, on insulin secretion and glucose homeostasis in ob/ob mice

Glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important enteroendocrine hormones that are rapidly degraded by an ubiquitous enzyme dipeptidyl peptidase IV to yield truncated metabolites GIP(3-42) and GLP-1(9-36)amide. In this study, we investigated the effects of sub-chronic exposure to these major circulating forms of GIP and GLP-1 on blood glucose control and endocrine pancreatic function in obese diabetic (ob/ob) mice. A once daily injection of either peptide for 14 days had no effect on body weight, food intake or pancreatic insulin content or islet morphology. GLP-1(9-36)amide also had no effect on plasma glucose homeostasis or insulin secretion. Mice receiving GIP(3-42) exhibited small but significant improvements in non-fasting plasma glucose, glucose tolerance and glycaemic response to feeding. Accordingly, plasma insulin responses were unchanged suggesting that the observed enhancement of insulin sensitivity was responsible for the improvement in glycaemic control. These data indicate that sub-chronic exposure to GIP and GLP-1 metabolites does not result in physiological impairment of insulin secretion or blood glucose control. GIP(3-42) might exert an overall beneficial effect by improving insulin sensitivity through extrapancreatic action.

Pituitary adenylate cyclase-activating peptide (PACAP): assessment of dipeptidyl peptidase IV degradation, insulin-releasing activity and antidiabetic potential

Pituitary adenylate cyclase-activating peptide (PACAP) is a member of the glucagon family of peptides. Like other members, most notably glucagon-like peptide-1 (GLP-1), PACAP is rapidly degraded by dipeptidylpeptidase IV (DPP IV). This study investigated how degradation by DPP IV affected the insulinotropic activity of PACAP, and whether PACAP exerted acute antihyperglycemic properties in normal or ob/ob mice. DPP IV degradation of PACAP(1-27) over 18 h led to the formation of PACAP(3-27), PACAP(5-27) and ultimately PACAP(6-27). In contrast to 1.4-1.8-fold concentration-dependent stimulation of insulin secretion by PACAP(1-27), these peptide fragments lacked insulinotropic activity. While PACAP(1-27) and PACAP(1-38) generated significant insulin responses when given alone or together with glucose in ob/ob and normal mice, they also elevated plasma glucose. These actions were eliminated following degradation of the peptide by incubation with DPP IV. The hyperglycemic effects may be explained at least partly by a potent glucagon-releasing action in ob/ob and normal mice. In conclusion, PACAP is inactivated by DPP IV and despite insulin-releasing effects, its actions on glucagon secretion and glucose homeostasis do not make it a good therapeutic tool for the treatment of type 2 diabetes.

Chronic treatment with exendin(9-39)amide indicates a minor role for endogenous glucagon-like peptide-1 in metabolic abnormalities of obesity-related diabetes in ob/ob mice

Glucagon-like peptide-1 (GLP-1) is a potent insulinotropic hormone proposed to play a role in both the pathophysiology and treatment of type 2 diabetes. This study has employed the GLP-1 receptor antagonist, exendin-4(9-39)amide (Ex(9-39)) to evaluate the role of endogenous GLP-1 in genetic obesity-related diabetes and related metabolic abnormalities using ob/ob and normal mice. Acute in vivo antagonistic potency of Ex(9-39) was confirmed in ob/ob mice by blockade of the insulin-releasing and anti-hyperglycaemic actions of intraperitoneal GLP-1. In longer term studies, ob/ob mice were given once daily injections of Ex(9-39) or vehicle for 11 days. Feeding activity, body weight, and both basal and glucose-stimulated insulin secretion were not significantly affected by chronic Ex(9-39) treatment. However, significantly elevated basal glucose concentrations and impaired glucose tolerance were evident at 11 days. These disturbances in glucose homeostasis were independent of changes of insulin sensitivity and reversed by discontinuation of the Ex(9-39) for 9 days. Similar treatment of normal mice did not affect any of the parameters measured. These findings illustrate the physiological extrapancreatic glucose-lowering actions of GLP-1 in ob/ob mice and suggest that the endogenous hormone plays a minor role in the metabolic abnormalities associated with obesity-related diabetes.

N-terminal His(7)-modification of glucagon-like peptide-1(7-36) amide generates dipeptidyl peptidase IV-stable analogues with potent antihyperglycaemic activity

Glucagon-like peptide-1(7-36)amide (GLP-1) possesses several unique and beneficial effects for the potential treatment of type 2 diabetes. However, the rapid inactivation of GLP-1 by dipeptidyl peptidase IV (DPP IV) results in a short half-life in vivo (less than 2 min) hindering therapeutic development. In the present study, a novel His(7)-modified analogue of GLP-1, N-pyroglutamyl-GLP-1, as well as N-acetyl-GLP-1 were synthesised and tested for DPP IV stability and biological activity. Incubation of GLP-1 with either DPP IV or human plasma resulted in rapid degradation of native GLP-1 to GLP-1(9-36)amide, while N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 were completely resistant to degradation. N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 bound to the GLP-1 receptor but had reduced affinities (IC(50) values 32.9 and 6.7 nM, respectively) compared with native GLP-1 (IC(50) 0.37 nM). Similarly, both analogues stimulated cAMP production with EC(50) values of 16.3 and 27 nM respectively compared with GLP-1 (EC(50) 4.7 nM). However, N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 exhibited potent insulinotropic activity in vitro at 5.6 mM glucose (P<0.05 to P<0.001) similar to native GLP-1. Both analogues (25 nM/kg body weight) lowered plasma glucose and increased plasma insulin levels when administered in conjunction with glucose (18 nM/kg body weight) to adult obese diabetic (ob/ob) mice. N-pyroglutamyl-GLP-1 was substantially better at lowering plasma glucose compared with the native peptide, while N-acetyl-GLP-1 was significantly more potent at stimulating insulin secretion. These studies indicate that N-terminal modification of GLP-1 results in DPP IV-resistant and biologically potent forms of GLP-1. The particularly powerful antihyperglycaemic action of N-pyroglutamyl-GLP-1 shows potential for the treatment of type 2 diabetes.

Lys9 for Glu9 substitution in glucagon-like peptide-1(7-36)amide confers dipeptidylpeptidase IV resistance with cellular and metabolic actions similar to those of established antagonists glucagon-like peptide-1(9-36)amide and exendin (9-39)

The incretin hormone glucagon-like peptide-1(7-36)amide (GLP-1) has been deemed of considerable importance in the regulation of blood glucose. Its effects, mediated through the regulation of insulin, glucagon, and somatostatin, are glucose-dependent and contribute to the tight control of glucose levels. Much enthusiasm has been assigned to a possible role of GLP-1 in the treatment of type 2 diabetes. GLP-1's action unfortunately is limited through enzymatic inactivation caused by dipeptidylpeptidase IV (DPP IV). It is now well established that modifying GLP-1 at the N-terminal amino acids, His(7) and Ala(8), can greatly improve resistance to this enzyme. Little research has assessed what effect Glu(9)-substitution has on GLP-1 activity and its degradation by DPP IV. Here, we report that the replacement of Glu(9) of GLP-1 with Lys dramatically increased resistance to DPP IV. This analogue, (Lys(9))GLP-1, exhibited a preserved GLP-1 receptor affinity, but the usual stimulatory effects of GLP-1 were completely eliminated, a trait duplicated by the other established GLP-1-antagonists, exendin (9-39) and GLP-1(9-36)amide. We investigated the in vivo antagonistic actions of (Lys(9))GLP-1 in comparison with GLP-1(9-36)amide and exendin (9-39) and revealed that this novel analogue may serve as a functional antagonist of the GLP-1 receptor.

Novel dipeptidyl peptidase IV resistant analogues of glucagon-like peptide-1(7-36)amide have preserved biological activities in vitro conferring improved glucose-lowering action in vivo

Although the incretin hormone glucagon-like peptide-1 (GLP-1) is a potent stimulator of insulin release, its rapid degradation in vivo by the enzyme dipeptidyl peptidase IV (DPP IV) greatly limits its potential for treatment of type 2 diabetes. Here, we report two novel Ala(8)-substituted analogues of GLP-1, (Abu(8))GLP-1 and (Val(8))GLP-1 which were completely resistant to inactivation by DPP IV or human plasma. (Abu(8))GLP-1 and (Val(8))GLP-1 exhibited moderate affinities (IC(50): 4.76 and 81.1 nM, respectively) for the human GLP-1 receptor compared with native GLP-1 (IC(50): 0.37 nM). (Abu(8))GLP-1 and (Val(8))GLP-1 dose-dependently stimulated cAMP in insulin-secreting BRIN BD11 cells with reduced potency compared with native GLP-1 (1.5- and 3.5-fold, respectively). Consistent with other mechanisms of action, the analogues showed similar, or in the case of (Val(8))GLP-1 slightly impaired insulin releasing activity in BRIN BD11 cells. Using adult obese (ob/ob) mice, (Abu(8))GLP-1 had similar glucose-lowering potency to native GLP-1 whereas the action of (Val(8))GLP-1 was enhanced by 37%. The in vivo insulin-releasing activities were similar. These data indicate that substitution of Ala(8) in GLP-1 with Abu or Val confers resistance to DPP IV inactivation and that (Val(8))GLP-1 is a particularly potent N-terminally modified GLP-1 analogue of possible use in type 2 diabetes.

DPP IV resistance and insulin releasing activity of a novel di-substituted analogue of glucose-dependent insulinotropic polypeptide, (Ser2-Asp13)GIP

Structure-function studies suggest that preservation of the N-terminus and secondary structure of glucose-dependent insulinotropic polypeptide (GIP) is important for biological activity. Therefore, a novel di-substituted analogue of GIP, (Ser(2)-Asp(13))GIP, containing a negatively charged Asp residue in place of an Ala in position 13, was synthesised and evaluated for in vitro biological activity. Incubation with dipeptidyl peptidase IV (DPP IV) showed the half-lives of GIP and (Ser(2)-Asp(13))GIP to be 2.3 and >4h, respectively. Insulin releasing studies in clonal pancreatic BRIN-BD11 cells demonstrated that (Ser(2)-Asp(13))GIP (10(-12)to 10(-7)mol/l) was significantly less potent (60-90%; P<0.05 to P<0.001) than native GIP. The peptide failed to display antagonistic properties as it did not significantly alter insulin secretion when incubated in the presence of GIP (10(-7)mol/l). These results demonstrate that despite increased resistance to DPP IV, substituting Ala in position 13 with a negatively charged Asp, thus producing the di-substituted analogue (Ser(2)-Asp(13))GIP, significantly reduces biological activity, most likely due to modifications within the secondary structure.