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

Blockade of gastric inhibitory polypeptide (GIP) action as a novel means of countering insulin resistance in the treatment of obesity-diabetes

Gastric inhibitory polypeptide (GIP) is a 42 amino acid hormone secreted from intestinal K-cells in response to nutrient ingestion. Despite a recognised physiological role for GIP as an insulin secretagogue to control postprandial blood glucose levels, growing evidence reveals important actions of GIP on adipocytes and promotion of fat deposition in tissues. As such, blockade of GIP receptor (GIPR) action has been proposed as a means to counter insulin resistance, and improve metabolic status in obesity and related diabetes. In agreement with this, numerous independent observations in animal models support important therapeutic applications of GIPR antagonists in obesity-diabetes. Sustained administration of peptide-based GIPR inhibitors, low molecular weight GIPR antagonists, GIPR neutralising antibodies as well as genetic knockout of GIPR's or vaccination against GIP all demonstrate amelioration of insulin resistance and reduced body weight gain in response to high fat feeding. These observations were consistently associated with decreased accumulation of lipids in peripheral tissues, thereby alleviating insulin resistance. Although the impact of prolonged GIPR inhibition on bone turnover still needs to be determined, evidence to date indicates that GIPR antagonists represent an exciting novel treatment option for obesity-diabetes.

Effects of 2 Novel PYY(1-36) Analogues, (P3L31P34)PYY(1-36) and PYY(1-36)(Lys12PAL), on Pancreatic Beta-Cell Function, Growth, and Survival

Recent studies have identified a beneficial role for peptide tyrosine tyrosine (PYY) on pancreatic beta-cell function and survival. These effects are linked to the activation of neuropeptide Y1 receptors (NPYR1s) by PYY(1-36). However, PYY(1-36) is subject to rapid degradation by dipeptidyl peptidase-4 (DPP-4), resulting is the loss of NPYR1 activity. Therefore, the aim of this study was to develop 2 enzymatically stable PYY(1-36) analogues, namely, (P³L³¹P³⁴)PYY(1-36) and PYY(1-36)(Lys¹²PAL), with further structural modifications to enhance NPYR1 specificity. As expected, (P³L³¹P³⁴)PYY(1-36) was fully resistant to DPP-4-mediated degradation in vitro, whereas PYY(1-36) and PYY(1-36)(Lys¹²PAL) were both liable to DPP-4 breakdown. PYY(1-36) and (P³L³¹P³⁴)PYY(1-36) induced significant reductions in glucose-stimulated insulin secretion (GSIS) from BRIN BD11 cells, but only PYY(1-36) diminished alanine-stimulated insulin secretion. In contrast, PYY(1-36)(Lys¹²PAL) had no impact on GSIS or alanine-induced insulin release. All 3 PYY peptides significantly enhanced proliferation in BRIN BD11 and 1.1B4 beta-cell lines, albeit only at the highest concentration examined, 10^-6 M, for (P³L³¹P³⁴)PYY(1-36) and PYY(1-36)(Lys¹²PAL) in BRIN BD11 cells. Regarding the protection of beta-cells against cytokine-induced apoptosis, PYY(1-36) induced clear protective effects. Both (P³L³¹P³⁴)PYY(1-36) and PYY(1-36)(Lys¹²PAL) offered some protection against apoptosis in BRIN BD11 cells, but were significantly less efficacious than PYY(1-36). Similarly, in 1.1B4 cells, both PYY analogues (10^-6 M) protected against cytokine-induced apoptosis, but (P³L³¹P³⁴)PYY(1-36) was significantly less effective than PYY(1-36). All 3 PYY peptides had no impact on refeeding in overnight fasted mice. These data underline the beta-cell benefits of PYY(1-36) and highlight the challenges of synthesising stable, bioactive, NPYR1-specific, PYY(1-36) analogues.

Exendin-4(Lys27 PAL)/gastrin/xenin-8-Gln: A novel acylated GLP-1/gastrin/xenin hybrid peptide that improves metabolic status in obese-diabetic (ob/ob) mice

BACKGROUND

Therapeutic benefits of peptide-based drugs is limited by rapid renal elimination.

METHODS

Therefore, to prolong the biological action profile of the recently characterized triple-acting hybrid peptide, exendin-4/gastrin/xenin-8-Gln, a fatty acid (C-16) has been covalently attached, creating exendin-4(Lys²⁷ PAL)/gastrin/xenin-8-Gln. Exendin-4/gastrin and liraglutide/gastrin/xenin-8-Gln were also synthesized as direct comparator peptides.

RESULTS

All hybrid peptides evoked significant concentration-dependent increases of insulin secretion from isolated murine islets and BRIN-BD11 cells. Following administration of peptides with glucose to mice, all hybrids significantly reduced the overall glycaemic excursion and increased insulin concentrations. In contrast to other treatments, exendin-4(Lys²⁷ PAL)/gastrin/xenin-8-Gln displayed impressive antihyperglycaemic actions even 12 hours after administration, highlighting protracted duration of effects. Exendin-4/gastrin/xenin-8-Gln, exendin-4/gastrin, and exendin-4(Lys²⁷ PAL)/gastrin/xenin-8-Gln were then progressed to a 31-day twice-daily treatment regimen in obese-diabetic ob/ob mice. All treatments decreased nonfasting glucose and HbA_1c concentrations, as well as enhancing circulating and pancreatic insulin levels. Exendin-4/gastrin and exendin-4/gastrin/xenin-8-Gln also decreased food intake. Glucose tolerance was improved by all treatments, but only exendin-4(Lys²⁷ PAL)/gastrin/xenin-8-Gln augmented glucose-induced insulin secretion. Interestingly, treatment regimens that included a xenin component induced clear advantages on the metabolic response to glucose-dependent insulinotropic polypeptide (GIP) and the glucose-lowering actions of insulin.

CONCLUSION

This study emphasizes the therapeutic promise of long-acting, multi-targeting hybrid gut peptides for type 2 diabetes.

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.

Genetic Variability of the Glucose-Dependent Insulinotropic Peptide Gene Is Involved in the Premature Coronary Artery Disease in a Chinese Population with Type 2 Diabetes

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) is closely related to diabetes and obesity, both of which are confirmed to increase the risk of coronary artery disease (CAD). Our study aimed to investigate whether the polymorphisms in GIP genes could affect the risk of cardiovascular disease in type 2 diabetic patients in the Chinese Han population.

METHODS

We selected and genotyped two haplotype-tagging single nucleotide polymorphisms (tag-SNPs) (rs2291725 C>T, rs8078510 G>A) of GIP gene based on CHB data in HapMap Phase II database (r² < 0.8). The case-control study of Chinese Han population involved 390 diabetic patients with CAD as positive group and 276 diabetic patients without CAD as control group. Allele and genotype frequencies were compared between the two groups.

RESULTS

In dominant inheritance model, the carriers of T/T or T/C had a lower risk of CAD (OR = 0.635, 95% CI = 0.463-0.872, p = 0.005), even after adjustment other CAD risk factors (gender, age, BMI, smoking status, dyslipidemia, hypertension history, and diabetic duration) (OR' = 0.769, 95% CI' = 0.626-0.945, p' = 0.013). The allele A at rs8078510 was associated with decreased risk of CAD (OR = 0.732, p = 0.039). p = 0.018 in subgroup analysis, individuals with higher BMI (≥24 kg/m²) had increased risk for CAD when carrying C/C at rs2291725 (OR' = 1.291, 95% CI' = 1.017-1.639, p' = 0.036). In age < 55 men and age < 65 women, the carriers of allele C at rs2291725 had a higher risk of CAD than noncarriers (OR = 1.627, p = 0.015). Carriers of allele G in rs8078510 had higher susceptibility to CAD (OR = 2.049, 95% = CI 1.213-3.463, p = 0.007). p = 0.004; in addition, allele G in rs8078510 would bring higher CAD risk to the carriers who ever smoked (OR = 1.695, 95% CI = 1.080-2.660, p = 0.021).

CONCLUSION

The genetic variability of GIP gene is associated with CAD and it may play a role in the premature CAD in the Chinese Han population with type 2 diabetes.

Sequential induction of beta cell rest and stimulation using stable GIP inhibitor and GLP-1 mimetic peptides improves metabolic control in C57BL/KsJ db/db mice

AIMS/HYPOTHESIS

GIP(6-30)Cex-K(40)[Pal] has been characterised as a fatty-acid-derived gastric inhibitory polypeptide (GIP) inhibitor that can induce pancreatic beta cell rest by diminishing the incretin effect. We investigated its therapeutic efficacy with and without the glucagon-like peptide-1 (GLP-1) beta cell cytotropic agent liraglutide.

METHODS

The therapeutic efficacy of GIP(6-30)Cex-K(40)[Pal] alone, and in combination with liraglutide, was determined in C57BL/KsJ db/db mice using a sequential 12 h administration schedule.

RESULTS

GIP(6-30)Cex-K(40)[Pal] was devoid of cAMP-generating or insulin-secretory activity, and inhibited GIP-induced cAMP production and insulin secretion. GIP(6-30)Cex-K(40)[Pal] also inhibited GIP-induced glucose-lowering and insulin-releasing actions in mice. Dose- and time-dependent studies in mice revealed that 2.5 nmol/kg GIP(6-30)Cex-K(40)[Pal], and 0.25 nmol/kg liraglutide, imparted distinct biological effects for 8-12 h post administration. When GIP(6-30)Cex-K(40)[Pal] (2.5 nmol/kg) and liraglutide (0.25 nmol/kg) were administered sequentially at 12 h intervals (at 08:00 and 20:00 hours) to db/db mice for 28 days, mice treated with GIP(6-30)Cex-K(40)[Pal] (08:00 hours) and liraglutide (20:00 hours) displayed pronounced reductions in circulating glucose and insulin. Both oral and intraperitoneal glucose tolerance and glucose-stimulated plasma insulin concentrations were improved together with enhanced insulin sensitivity. The expression of genes involved in adipocyte lipid deposition was generally decreased. The other treatment modalities, including GIP(6-30)Cex-K(40)[Pal] (08:00 and 20:00 hours), liraglutide (08:00 and 20:00 hours) and liraglutide (08:00 hours) combined with GIP(6-30)Cex-K(40)[Pal] (20:00 hours), also imparted beneficial effects but these were not as prominent as those of GIP(6-30)Cex-K(40)[Pal] (08:00 hours) and liraglutide (20:00 hours).

CONCLUSION/INTERPRETATION

These data demonstrate that periods of beta cell rest combined with intervals of beta cell stimulation benefit diabetes control and should be further evaluated as a potential treatment option for type 2 diabetes.

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.

Short peptide motifs for long-lasting anchoring to the cell surface

A rational design strategy has been developed for the construction of stable peptide-based anchors for the efficient modification of cell surfaces. Six types of peptide composed of five residues with divalent hydrophobic groups have been designed using this new strategy. Among them, a peptide with a sequence of NBD-Lys-Lys(X)-Lys-Lys-Lys(X)-NH2 (NBD: fluorophore, Lys(X): N-ε-palmitoyl-l-lysine) was found to show the highest modification efficacy and longevity in culture medium. The good performance of this peptide was attributed to (1) its high aqueous solubility, which allowed it to partition from the medium to the cell surface, and (2) the high binding affinity of the saturated palmitoyl groups to the cell membrane. We found that the distribution of the peptide was affected by recycling endosome, which enabled the representation of the peptide following its endocytotic disappearance from the cell membrane. Biotin was also presented on the cell surface using this peptide-based anchor to examine its recognition by streptavidin. The efficacy of the recognition process increased as the length of the oligoethylene glycol spacer increased, indicating that it was necessary for the biotin tag to move away from the membrane glycoproteins on the cell surface to facilitate its efficient recognition by streptavidin.

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.

Effects of short-term chemical ablation of glucagon signalling by peptide-based glucagon receptor antagonists on insulin secretion and glucose homeostasis in mice

Glucagon is a hormone with important effects on blood glucose regulation. This study has utilized the stable glucagon receptor antagonists, desHis1Pro4Glu9-glucagon and desHis1Pro4Glu9(Lys12PAL)-glucagon, to evaluate the effects of sustained inhibition of glucagon receptor signalling in normal mice. Twice-daily injection of either analogue for 10 days had no effect on food intake, body weight and non-fasting plasma glucose concentrations. However, insulin levels were significantly raised (p<0.05 to p<0.01) from day 3 onwards in desHis1Pro4Glu9-glucagon mice. After 10 days, glucose tolerance was improved (p<0.05) in desHis1Pro4Glu9-glucagon treated mice. Glucose-mediated insulin secretion and circulating cholesterol levels were significantly (p<0.05 to p<0.01) decreased in both treatment groups. Importantly, the effects of glucagon to increase blood glucose and insulin concentrations were still annulled on day 10. Insulin sensitivity was almost identical in all groups of mice at the end of the study. In addition, no changes in pancreatic insulin and glucagon content or islet morphology were observed in either treatment group. Finally, acute injection of desHis1Pro4Glu9-glucagon followed by a 24-h fast in treatment naïve mice was not associated with any hypoglycaemic episodes. These data indicate that peptide-based glucagon receptor antagonists represent safe and effective treatment options for type 2 diabetes.

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.

A simple matter of life and death-the trials of postnatal Beta-cell mass regulation

Pancreatic beta-cells, which secrete the hormone insulin, are the key arbiters of glucose homeostasis. Defective beta-cell numbers and/or function underlie essentially all major forms of diabetes and must be restored if diabetes is to be cured. Thus, the identification of the molecular regulators of beta-cell mass and a better understanding of the processes of beta-cell differentiation and proliferation may provide further insight for the development of new therapeutic targets for diabetes. This review will focus on the principal hormones and nutrients, as well as downstream signalling pathways regulating beta-cell mass in the adult. Furthermore, we will also address more recently appreciated regulators of beta-cell mass, such as microRNAs.

Administration of an acylated GLP-1 and GIP preparation provides added beneficial glucose-lowering and insulinotropic actions over single incretins in mice with Type 2 diabetes and obesity. Clin Sci (Lond). 2011;121:107-117. [PMID: 21332446 DOI: 10.1042/cs20110006]

The present study examined the glucose-lowering and insulinotropic properties of acylated GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) peptides in Type 2 diabetes and obesity. GLP-1, GIP, Liraglutide, N-AcGIP(Lys(37)Myr) (N-acetylGIP with myristic acid conjugated at Lys(37)), a simple combination of both peptides and a Lira-AcGIP preparation [overnight preparation of Liraglutide and N-AcGIP(Lys(37)Myr)] were incubated with DPP-IV (dipeptidyl peptidase-IV) to assess peptide stability, and BRIN-BD11 cells were used to evaluate cAMP production and insulin secretion. Acute glucose-lowering and insulinotropic actions were evaluated in Swiss TO mice. Subchronic studies on glucose homoeostasis, insulin secretion, food intake and bodyweight were evaluated in ob/ob mice. Liraglutide, N-AcGIP(Lys(37)Myr), a simple combination of both peptides and the Lira-AcGIP preparation demonstrated improved DPP-IV resistance (P<0.001), while stimulating cAMP production and insulin secretion (1.4-2-fold; P<0.001). The Lira-AcGIP preparation was more potent at lowering plasma glucose (20-51% reduction; P<0.05-P<0.001) and stimulating insulin secretion (1.5-1.8-fold; P<0.05-P<0.001) compared with Liraglutide and N-AcGIP(Lys(37)Myr) or a simple peptide combination. Daily administration of the Lira-AcGIP preparation to ob/ob mice lowered bodyweight (7-9%; P<0.05), food intake (23%; P<0.05) and plasma glucose (46% reduction; P<0.001), while increasing plasma insulin (1.5-1.6-fold; P<0.001). The Lira-AcGIP preparation enhanced glucose tolerance, insulin response to glucose and insulin content (P<0.05-P<0.001). These findings demonstrate that a combined preparation of the acylated GLP-1 and GIP peptides Liraglutide and N-AcGIP(Lys(37)Myr) markedly improved glucose-lowering and insulinotropic properties in diabetic obesity compared with either incretin mimetic given individually.

Neuroprotective properties of GLP-1: theoretical and practical applications

In the past few years, the development of pharmaceutical agents that enhance the physiological effects of glucagon-like peptide-1 (GLP-1), either through GLP-1 receptor agonism (GLP-1 agonists) or by inhibiting GLP-1 degradation (dipeptidylpeptidase-4 inhibitors) has broadened the range of treatment options for individuals with type 2 diabetes. It has been recognized for some time that GLP-1 also has extra-pancreatic effects, notably targeting the brain, where it regulates appetite and satiety, as well as peripheral functions highly controlled by the autonomic nervous system, such as gastric emptying. Furthermore, data are beginning to emerge that indicate a potential role for GLP-1 in neuroprotection. The increased risk of Alzheimer's disease, Parkinson's disease and stroke in people with type 2 diabetes suggests that shared mechanisms/pathways of cell death, possibly related to insulin dysregulation, may underlie all of these disorders. Although the disease anatomy varies with each disorder, a wide range of genetic and environmental triggers result in activation of similar biochemical pathways in all of them, suggesting a complex network of biochemical events that feed in to a final common path towards cellular dysfunction and death. This article summarizes the evidence for neuronal activity of GLP-1 and examines the limited data that currently exist on the therapeutic potential of GLP-1 in specific neurological and neurodegenerative conditions, namely Alzheimer's disease, Parkinson's disease, Huntingdon's disease, stroke and peripheral sensory neuropathy.

Gastrointestinal hormones and the regulation of β-cell mass

Type 2 diabetes occurs due to a relative deficit in β-cell mass or function. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), and gastrin are gastrointestinal hormones that are secreted in response to nutrient intake, regulating digestion, insulin secretion, satiety, and β-cell mass. In this review, we focus upon β-cell mass regulation. β-cell mass expands through β-cell proliferation and islet neogenesis; β-cell mass is lost via apoptosis. GLP-1 and GIP are well-studied gastrointestinal hormones and influence β-cell proliferation, apoptosis, and islet neogenesis. CCK regulates β-cell apoptosis and mitogenesis, and gastrin stimulates islet neogenesis. GLP-1 and GIP bind to G protein-coupled receptors and regulate β-cell mass via multiple signaling pathways. The protein kinase A pathway is central to this process because it directly regulates proliferative and anti-apoptotic genes and transactivates several signaling cascades, including Akt and mitogen-activated protein kinases. However, the signaling pathways downstream of G protein-coupled CCK receptors that influence β-cell mass remain unidentified. Gastrointestinal hormones integrate nutrient signals from the gut to the β-cell, regulating insulin secretion and β-cell mass adaptation.

A GIP receptor agonist exhibits beta-cell anti-apoptotic actions in rat models of diabetes resulting in improved beta-cell function and glycemic control

AIMS

The gastrointestinal hormone GIP promotes pancreatic islet function and exerts pro-survival actions on cultured beta-cells. However, GIP also promotes lipogenesis, thus potentially restricting its therapeutic use. The current studies evaluated the effects of a truncated GIP analog, D-Ala(2)-GIP(1-30) (D-GIP(1-30)), on glucose homeostasis and beta-cell mass in rat models of diabetes.

MATERIALS AND METHODS

The insulinotropic and pro-survival potency of D-GIP(1-30) was evaluated in perfused pancreas preparations and cultured INS-1 beta-cells, respectively, and receptor selectivity evaluated using wild type and GIP receptor knockout mice. Effects of D-GIP(1-30) on beta-cell function and glucose homeostasis, in vivo, were determined using Lean Zucker rats, obese Vancouver diabetic fatty rats, streptozotocin treated rats, and obese Zucker diabetic fatty rats, with effects on beta-cell mass determined in histological studies of pancreatic tissue. Lipogenic effects of D-GIP(1-30) were evaluated on cultured 3T3-L1 adipocytes.

RESULTS

Acutely, D-GIP(1-30) improved glucose tolerance and insulin secretion. Chronic treatment with D-GIP(1-30) reduced levels of islet pro-apoptotic proteins in Vancouver diabetic fatty rats and preserved beta-cell mass in streptozotocin treated rats and Zucker diabetic fatty rats, resulting in improved insulin responses and glycemic control in each animal model, with no change in body weight. In in vitro studies, D-GIP(1-30) exhibited equivalent potency to GIP(1-42) on beta-cell function and survival, but greatly reduced action on lipoprotein lipase activity in 3T3-L1 adipocytes.

CONCLUSIONS

These findings demonstrate that truncated forms of GIP exhibit potent anti-diabetic actions, without pro-obesity effects, and that the C-terminus contributes to the lipogenic actions of GIP.

Glucagon-like peptide-1 analogues enhance synaptic plasticity in the brain: a link between diabetes and Alzheimer’s disease. Eur J Pharmacol. 2010;630:158-162. [PMID: 20035739 DOI: 10.1016/j.ejphar.2009.12.023]

Type 2 diabetes has been identified as a risk factor for patients with Alzheimer's disease. Insulin signalling is often impaired in Alzheimer's disease, contributing to the neurodegenerative process. One potential strategy to help prevent this is the normalisation of insulin signalling in the brain. Therefore, the present study was designed to test the effects of novel enzyme-resistant analogues of the insulin-releasing incretin hormone, glucagon-like peptide 1 (GLP-1). The effects of Liraglutide (Victoza) and other novel GLP-1 analogues were tested on synaptic plasticity (LTP) in area CA1 of the hippocampus. At a dose of 15nmol in 5microl i.c.v., Liraglutide (P<0.005), Asp(7)GLP-1 (P<0.001), N-glyc-GLP-1 (P<0.01), and Pro(9)GLP-1 (P<0.001). In contrast, the GLP-1 receptor antagonist exendin(9-39)amide impaired LTP (P<0.001). Co-injection of exendin(9-39) and Liraglutide showed no effect on LTP. These results clearly demonstrate that Liraglutide and other GLP-1 analogues elicit effects on neurotransmission in the brain. Furthermore, GLP-1 peptides are not only effective in modulating insulin-release and achieving glycaemic control in type 2 diabetes, but are also effective in modulating synaptic plasticity. These findings are consistent with our previous observations that the novel analogue (Val(8))GLP-1 enhances LTP and reverses the impairments of LTP induced by beta-amyoid fragments. Therefore, the drug effects seen here could potentially ameliorate the impairments in neuronal communication and cognitive processes observed in Alzheimer's disease.

Therapeutic potential for GIP receptor agonists and antagonists

Glucose-dependent insulinotropic polypeptide (GIP or gastric inhibitory polypeptide) is a 42-amino-acid hormone, secreted from the enteroendocrine K cells, which has insulin-releasing and extrapancreatic glucoregulatory actions. However, the unfavourable pharmacokinetic profile and the weak biological effects of native GIP limit its effectiveness for the treatment of type 2 diabetes. To overcome this, longer-acting GIP agonists exhibiting enzymatic stability and enhanced bioactivity have been generated and successfully tested in animal models of diabetes. Thus, GIP receptor agonists offer one of the newest classes of potential antidiabetic drug. GIP is also known to play a role in lipid metabolism and fat deposition. Accordingly, both genetic and chemical ablation of GIP signalling in mice with obesity-diabetes can protect against, or even reverse many of the obesity-associated metabolic disturbances. Strong parallels exist with the beneficial metabolic effects of Roux-en-Y gastric bypass in obese, insulin-resistant humans that surgically ablates GIP-secreting K cells. The purpose of this article is to highlight the therapeutic potential of GIP-based therapeutics in the treatment of type 2 diabetes and obesity.

Fatty acid derivatised analogues of glucose-dependent insulinotropic polypeptide with improved antihyperglycaemic and insulinotropic properties

C-terminal acylation of Lys(37) with myristic (MYR; tetradecanoic acid), palmitic (PAL; hexadecanoic acid) and stearic (octadecanoic acid) fatty acids with or without N-terminal acetylation was employed to develop long-acting analogues of the glucoregulatory hormone, glucose-dependent insulinotropic polypeptide (GIP). All GIP analogues exhibited resistance to dipeptidylpeptidase-IV (DPP-IV) and significantly improved in vitro cAMP production and insulin secretion. Administration of GIP analogues to ob/ob mice significantly lowered plasma glucose-GIP(Lys(37)MYR), N-AcGIP(Lys(37)MYR) and GIP(Lys(37)PAL) increased plasma insulin concentrations. GIP(Lys(37)MYR) and N-AcGIP(Lys(37)MYR) elicited protracted glucose-lowering effects when administered 24h prior to an intraperitoneal glucose load. Daily administration of GIP(Lys(37)MYR) and N-AcGIP(Lys(37)MYR) to ob/ob mice for 24 days decreased glucose and significantly improved plasma insulin, glucose tolerance and beta-cell glucose responsiveness. Insulin sensitivity, pancreatic insulin content and triglyceride levels were not changed. These data demonstrate that C-terminal acylation particularly with myristic acid provides a class of stable, longer-acting forms of GIP for further evaluation in diabetes therapy.

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.

Effects of gastric inhibitory polypeptide (GIP) and related analogues on glucagon release at normo- and hyperglycaemia in Wistar rats and isolated islets

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone secreted by endocrine K-cells in response to nutrient absorption. This study has utilised numerous well-characterised dipeptidyl peptidase IV-resistant GIP analogues to evaluate the glucagonotropic actions of GIP in Wistar rats and isolated rat islets. Intraperitoneal administration of GIP analogues (25 nmol/kg body weight) in combination with glucose had no effect on circulating glucagon concentrations compared to controls in Wistar rats. However, plasma glucose concentrations were significantly (p<0.05 to p<0.001) lowered by the GIP-receptor agonists, N-AcGIP, GIP(Lys37)PAL and N-AcGIP(Lys37)PAL. The GIP antagonist, (Pro3)GIP, caused a significant (p<0.05) reduction in glucagon levels following concurrent administration with saline in Wistar rats. In isolated rat islets native GIP induced a significant (p<0.01) enhancement of glucagon release at basal glucose concentrations, which was completely annulled by (Pro3)GIP. Furthermore, glucagon release in the presence of GLP-1, GIP(Lys37)PAL, N-AcGIP(Lys37)PAL and (Pro3)GIP was significantly (p<0.05 to p<0.001) decreased compared to native GIP in isolated rat islets. These data indicate a modest effect of GIP on glucagon secretion from isolated rat islets, which was not observed in vivo. However, the GIP agonists N-AcGIP, GIP(Lys37)PAL and N-AcGIP(Lys37)PAL had no effect on glucagon release demonstrating an improved therapeutic potential for the treatment of type 2 diabetes.

C-terminal mini-PEGylation of glucose-dependent insulinotropic polypeptide exhibits metabolic stability and improved glucose homeostasis in dietary-induced diabetes

Glucose-dependent insulinotropic polypeptide has been proposed as a potential therapeutic for type 2 diabetes, however, efforts to bring forward this drug have been hindered due to its short circulating half-life. We have adopted a novel strategy to increase potency and prolong GIP action through C-terminal mini-PEGylation (GIP[mPEG]). In contrast to GIP, GIP[mPEG] was resistant to dipeptidylpeptidase-IV (DPP-IV) up to and including 24h. Both GIP[mPEG] and GIP concentration-dependently stimulated cAMP production (EC50 6.6 and 0.7 nM, respectively) and insulin secretion (p < 0.01 to p < 0.001) in pancreatic BRIN-BD11 cells. Acute injection of GIP[mPEG] together with glucose to high fat fed mice significantly lowered plasma glucose (p < 0.05) and increased plasma insulin responses (p < 0.05). Furthermore, GIP[mPEG] markedly lowered plasma glucose when administered 4-24h prior to a glucose load (p < 0.05). Daily administration of GIP[mPEG] for 20 days in high fat mice did not alter body weight, food intake or non-fasting plasma insulin, however, non-fasting plasma glucose concentrations were significantly lowered (p < 0.05). Moreover, glucose tolerance was significantly improved (p < 0.05) together with glucose-mediated plasma insulin responses (p < 0.05). Insulin sensitivity, pancreatic insulin content, triglyceride and adiponectin levels were not changed. In summary, these data demonstrate that C-terminal mini-PEGylation of GIP is a useful strategy to prolong metabolic stability and improve biological action thus representing a novel therapeutic option for type 2 diabetes.

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.

Comparison of the anti-diabetic effects of GIP- and GLP-1-receptor activation in obese diabetic (ob/ob) mice: studies with DPP IV resistant N-AcGIP and exendin(1-39)amide

BACKGROUND

The two major incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are being actively explored as anti-diabetic agents because they lower blood glucose through multiple mechanisms. The rapid inactivation of GIP and GLP-1 by the ubiquitous enzyme, dipeptidyl peptidase IV (DPP IV) makes their biological actions short-lived, but stable agonists such as N-acetylated GIP (N-AcGIP) and exendin(1-39)amide have been advocated as stable and specific GIP and GLP-1 analogues.

METHODS

The present study examined the sub-chronic (14 days) anti-diabetic actions of single daily doses of N-AcGIP and exendin(1-39)amide given alone or in combination to obese diabetic (ob/ob) mice over a 14-day period.

RESULTS

Initial experiments confirmed the potent anti-hyperglycaemic and insulinotropic properties of N-AcGIP and exendin(1-39)amide. Sub-chronic administration of N-AcGIP alone or in combination with exendin(1-39)amide significantly decreased non-fasting plasma glucose and improved glucose tolerance compared to control ob/ob mice. This was associated with a significant enhancement of the insulin response to glucose and a notable improvement of insulin sensitivity. Combined treatment with N-AcGIP and exendin(1-39)amide also significantly decreased glycated haemoglobin. Exendin(1-39)amide alone had no significant effect on any of the metabolic parameters monitored. In addition, no significant effects were observed on body weight and food intake in any of the treatment groups.

CONCLUSIONS

The results illustrate significant anti-diabetic potential of N-AcGIP alone and in combination with exendin(1-39)amide.

Effects of subchronic treatment with the long-acting glucose-dependent insulinotropic polypeptide receptor agonist, N-AcGIP, on glucose homeostasis in streptozotocin-induced diabetes

OBJECTIVES

N-AcGIP is a potent and dipeptidylpeptidase IV-resistant analogue of glucose-dependent insulinotropic polypeptide with significantly improved antidiabetic actions in type 2 diabetes. The present study investigated the effects of subchronic treatment with N-AcGIP on glucose homeostasis in a type 1 model, namely, streptozotocin (STZ)-induced diabetic mice.

METHODS

Swiss TO mice given a single intraperitoneal injection of STZ (150 mg/kg body weight) received once-daily injection of N-AcGIP (25 nmol/kg body weight) or saline for 20 days and effects on metabolic parameters and islet architecture assessed.

RESULTS

Daily injection of N-AcGIP for 20 days did not significantly alter the characteristic STZ-induced changes of pancreatic insulin content, body weight, food intake, glucose, and glycated hemoglobin levels. Glucose tolerance and insulin sensitivity were also unchanged by N-AcGIP treatment. Circulating insulin was undetectable, and the number of intact islets and insulin expression was greatly reduced in both groups. Some proliferative activity was identified by 5-bromo-2-deoxyuridine staining in the pancreas, but this and expression of glucagon and somatostatin were similar in the 2 groups.

CONCLUSIONS

These data indicate that subchronic treatment with the long-acting glucose-dependent insulinotropic polypeptide receptor agonist, N-AcGIP, does not have beneficial effects in insulin-deficient STZ-diabetic mice. This supports the primary antidiabetic action of this analogue in type 2 diabetes as stimulation of beta-cell function and insulin secretion.