Species-specific action of (Pro3)GIP - a full agonist at human GIP receptors, but a partial agonist and competitive antagonist at rat and mouse GIP receptors

Glucose-Dependent Insulinotropic Polypeptide Receptor-Expressing Cells in the Hypothalamus Regulate Food Intake

Ambiguity regarding the role of glucose-dependent insulinotropic polypeptide (GIP) in obesity arises from conflicting reports asserting that both GIP receptor (GIPR) agonism and antagonism are effective strategies for inhibiting weight gain. To enable identification and manipulation of Gipr-expressing (Gipr) cells, we created Gipr-Cre knockin mice. As GIPR-agonists have recently been reported to suppress food intake, we aimed to identify central mediators of this effect. Gipr cells were identified in the arcuate, dorsomedial, and paraventricular nuclei of the hypothalamus, as confirmed by RNAscope in mouse and human. Single-cell RNA-seq identified clusters of hypothalamic Gipr cells exhibiting transcriptomic signatures for vascular, glial, and neuronal cells, the latter expressing somatostatin but little pro-opiomelanocortin or agouti-related peptide. Activation of G_q-DREADDs in hypothalamic Gipr cells suppressed food intake in vivo, which was not obviously additive with concomitant GLP1R activation. These data identify hypothalamic GIPR as a target for the regulation of energy balance.

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.

A Dual GLP-1/GIP Receptor Agonist Does Not Antagonize Glucagon at Its Receptor but May Act as a Biased Agonist at the GLP-1 Receptor

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are important regulators of metabolism, making their receptors (GLP-1R and GIPR) attractive targets in the treatment of type 2 diabetes mellitus (T2DM). GLP-1R agonists are used clinically to treat T2DM but the use of GIPR agonists remains controversial. Recent studies suggest that simultaneous activation of GLP-1R and GIPR with a single peptide provides superior glycemic control with fewer adverse effects than activation of GLP-1R alone. We investigated the signaling properties of a recently reported dual-incretin receptor agonist (P18). GLP-1R, GIPR, and the closely related glucagon receptor (GCGR) were expressed in HEK-293 cells. Activation of adenylate cyclase via Gα_s was monitored using a luciferase-linked reporter gene (CRE-Luc) assay. Arrestin recruitment was monitored using a bioluminescence resonance energy transfer (BRET) assay. GLP-1, GIP, and glucagon displayed exquisite selectivity for their receptors in the CRE-Luc assay. P18 activated GLP-1R with similar potency to GLP-1 and GIPR with higher potency than GIP. Interestingly, P18 was less effective than GLP-1 at recruiting arrestin to GLP-1R and was inactive at GCGR. These data suggest that P18 can act as both a dual-incretin receptor agonist, and as a G protein-biased agonist at GLP-1R.

Effects of combined GIP and GLP-1 infusion on energy intake, appetite and energy expenditure in overweight/obese individuals: a randomised, crossover study

AIMS/HYPOTHESIS

Glucagon-like peptide 1 (GLP-1) reduces appetite and energy intake in humans, whereas the other incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), seems to have no effect on eating behaviour. Interestingly, studies in rodents have shown that concomitant activation of GIP and GLP-1 receptors may potentiate the satiety-promoting effect of GLP-1, and a novel dual GLP-1/GIP receptor agonist was recently shown to trigger greater weight losses compared with a GLP-1 receptor agonist in individuals with type 2 diabetes. The aim of this study was to delineate the effects of combined GIP and GLP-1 receptor activation on energy intake, appetite and resting energy expenditure in humans.

METHODS

We examined 17 overweight/obese men in a crossover design with 5 study days. On day 1, a 50 g OGTT was performed; on the following 4 study days, the men received an isoglycaemic i.v. glucose infusion (IIGI) plus saline (154 mmol/l NaCl; placebo), GIP (4 pmol kg^-1 min^-1), GLP-1 (1 pmol kg^-1 min^-1) or GIP+GLP-1 (4 and 1 pmol kg^-1 min^-1, respectively). All IIGIs were performed in a randomised order blinded for the participant and the investigators. The primary endpoint was energy intake as measured by an ad libitum meal after 240 min. Secondary endpoints included appetite ratings and resting energy expenditure, as well as insulin, C-peptide and glucagon responses.

RESULTS

Energy intake was significantly reduced during IIGI+GLP-1 compared with IIGI+saline infusion (2715 ± 409 vs 4483 ± 568 kJ [mean ± SEM, n = 17], p = 0.014), whereas there were no significant differences in energy intake during IIGI+GIP (4062 ± 520 kJ) or IIGI+GIP+GLP-1 (3875 ± 451 kJ) infusion compared with IIGI+saline (p = 0.590 and p = 0.364, respectively). Energy intake was higher during IIGI+GIP+GLP-1 compared with IIGI+GLP-1 infusion (p = 0.039).

CONCLUSIONS/INTERPRETATION

While GLP-1 infusion lowered energy intake in overweight/obese men, simultaneous GIP infusion did not potentiate this GLP-1-mediated effect.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02598791 FUNDING: This study was supported by grants from the Innovation Fund Denmark and the Vissing Foundation.

Exciting advances in GPCR-based drugs discovery for treating metabolic disease and future perspectives

INTRODUCTION

Current pharmacological therapies that target single receptors have limited efficacy for the treatment of diabetes and obesity. Novel approaches with hybrid peptides that activate more than one receptor at once to generate beneficial effects through synergistic effects have shown promising results. Several unimolecular dual and tri-agonists, mainly associated with GPCR like GLP-1/GCG/GIP receptors, have shown exceptional efficacy in preclinical models, and are currently being evaluated in clinical trials to investigate their safety and beneficial effects in humans. Areas covered: Herein, the authors review the development of drugs used in the treatment of metabolic disease, from single agonists to the new generation of tri-agonist peptides and compile the latest knowledge available on GPCR-based drug discovery. The authors also provide the reader with their expert perspectives on this exciting area of drug development. Expert opinion: The co-agonists that have been clinically tested so far have been well tolerated and reduce body weight as well as fasting glucose levels in patients with Type 2 Diabetes Mellitus to a higher degree than single agonists alone. The promising data collected so far now warrant large scale randomized clinical trials to assess whether a unimolecular polypharmacy-based approach could translate into safe and efficacious treatments for obesity and its comorbidities.

Gut Hormones and Their Effect on Bone Metabolism. Potential Drug Therapies in Future Osteoporosis Treatment

Bone homeostasis displays a circadian rhythm with increased resorption during the night time as compared to day time, a difference that seems-at least partly-to be caused by food intake during the day. Thus, ingestion of a meal results in a decrease in bone resorption, but people suffering from short bowel syndrome lack this response. Gut hormones, released in response to a meal, contribute to this link between the gut and bone metabolism. The responsible hormones appear to include glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), known as incretin hormones due to their role in regulating glucose homeostasis by enhancing insulin release in response to food intake. They interact with their cognate receptors (GIPR and GLP-1R), which are both members of the class B G protein-coupled receptors (GPCRs), and already recognized as targets for treatment of metabolic diseases, such as type 2 diabetes mellitus (T2DM) and obesity. Glucagon-like peptide-2 (GLP-2), secreted concomitantly with GLP-1, acting via another class B receptor (GLP-2R), is also part of this gut-bone axis. Several studies, including human studies, have indicated that these three hormones inhibit bone resorption and, moreover, that GIP increases bone formation. Another hormone, peptide YY (PYY), is also secreted from the enteroendocrine L-cells (together with GLP-1 and GLP-2), and acts mainly via interaction with the class A GPCR NPY-R2. PYY is best known for its effect on appetite regulation, but recent studies have also shown an effect of PYY on bone metabolism. The aim of this review is to summarize the current knowledge of the actions of GIP, GLP-1, GLP-2, and PYY on bone metabolism, and to discuss future therapies targeting these receptors for the treatment of osteoporosis.

Emerging hormonal-based combination pharmacotherapies for the treatment of metabolic diseases

Obesity and its comorbidities, such as type 2 diabetes mellitus and cardiovascular disease, constitute growing challenges for public health and economies globally. The available treatment options for these metabolic disorders cannot reverse the disease in most individuals and have not substantially reduced disease prevalence, which underscores the unmet need for more efficacious interventions. Neurobiological resilience to energy homeostatic perturbations, combined with the heterogeneous pathophysiology of human metabolic disorders, has limited the sustainability and efficacy of current pharmacological options. Emerging insights into the molecular origins of eating behaviour, energy expenditure, dyslipidaemia and insulin resistance suggest that coordinated targeting of multiple signalling pathways is probably necessary for sizeable improvements to reverse the progression of these diseases. Accordingly, a broad set of combinatorial approaches targeting feeding circuits, energy expenditure and glucose metabolism in concert are currently being explored and developed. Notably, several classes of peptide-based multi-agonists and peptide-small molecule conjugates with superior preclinical efficacy have emerged and are currently undergoing clinical evaluation. Here, we summarize advances over the past decade in combination pharmacotherapy for the management of obesity and type 2 diabetes mellitus, exclusively focusing on large-molecule formats (notably enteroendocrine peptides and proteins) and discuss the associated therapeutic opportunities and challenges.

Optimized GIP analogs promote body weight lowering in mice through GIPR agonism not antagonism

OBJECTIVE

Structurally-improved GIP analogs were developed to determine precisely whether GIP receptor (GIPR) agonism or antagonism lowers body weight in obese mice.

METHODS

A series of peptide-based GIP analogs, including structurally diverse agonists and a long-acting antagonist, were generated and characterized in vitro using functional assays in cell systems overexpressing human and mouse derived receptors. These analogs were characterized in vivo in DIO mice following acute dosing for effects on glycemic control, and following chronic dosing for effects on body weight and food intake. Pair-feeding studies and indirect calorimetry were used to survey the mechanism for body weight lowering. Congenital Gipr-/- and Glp1r-/- DIO mice were used to investigate the selectivity of the agonists and to ascribe the pharmacology to effects mediated by the GIPR.

RESULTS

Non-acylated, Aib2 substituted analogs derived from human GIP sequence showed full in vitro potency at human GIPR and subtly reduced in vitro potency at mouse GIPR without cross-reactivity at GLP-1R. These GIPR agonists lowered acute blood glucose in wild-type and Glp1r-/- mice, and this effect was absent in Gipr-/- mice, which confirmed selectivity towards GIPR. Chronic treatment of DIO mice resulted in modest yet consistent, dose-dependent decreased body weight across many studies with diverse analogs. The mechanism for body weight lowering is due to reductions in food intake, not energy expenditure, as suggested by pair-feeding studies and indirect calorimetry assessment. The weight lowering effect was preserved in DIO Glp-1r-/- mice and absent in DIO Gipr-/- mice. The body weight lowering efficacy of GIPR agonists was enhanced with analogs that exhibit higher mouse GIPR potency, with increased frequency of administration, and with fatty-acylated peptides of extended duration of action. Additionally, a fatty-acylated, N-terminally truncated GIP analog was shown to have high in vitro antagonism potency for human and mouse GIPR without cross-reactive activity at mouse GLP-1R or mouse glucagon receptor (GcgR). This acylated antagonist sufficiently inhibited the acute effects of GIP to improve glucose tolerance in DIO mice. Chronic treatment of DIO mice with high doses of this acylated GIPR antagonist did not result in body weight change. Further, co-treatment of this acylated GIPR antagonist with liraglutide, an acylated GLP-1R agonist, to DIO mice did not result in increased body weight lowering relative to liraglutide-treated mice. Enhanced body weight lowering in DIO mice was evident however following co-treatment of long-acting selective individual agonists for GLP-1R and GIPR, consistent with previous data.

CONCLUSIONS

We conclude that peptide-based GIPR agonists, not peptide-based GIPR antagonists, that are suitably optimized for receptor selectivity, cross-species activity, and duration of action consistently lower body weight in DIO mice, although with moderate efficacy relative to GLP-1R agonists. These preclinical rodent pharmacology results, in accordance with recent clinical results, provide definitive proof that systemic GIPR agonism, not antagonism, is beneficial for body weight loss.

Increased Body Weight and Fat Mass After Subchronic GIP Receptor Antagonist, but Not GLP-2 Receptor Antagonist, Administration in Rats

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are hormones secreted from the enteroendocrine cells after a meal. They exert their actions through activation of G protein-coupled receptors (R), the GIPR and GLP-2R, respectively. Both have been reported to influence metabolism. The purpose of the study was to investigate the role of the hormones in the regulation of lipid and bone homeostasis by subchronic treatment with novel GIPR and GLP-2R antagonists. Rats were injected once daily with vehicle, GIPR, or GLP-2R antagonists for 3 weeks. Body weight, food intake, body composition, plasma lipoprotein lipase (LPL), adipokines, triglycerides and the marker of bone resorption carboxy-terminal collagen crosslinks (CTX), were examined. In rats, subchronic treatment with GIPR antagonist, rat GIP (3-30)NH₂, did not modify food intake and bone resorption, but significantly increased body weight, body fat mass, triglycerides, LPL, and leptin levels compared with vehicle treated rats. Subchronic (Pro3)GIP (a partial GIPR agonist), GLP-2(11-33), and GLP-2(3-33) (GLP-2R antagonists) treatment did not affect any parameter. The present results would be consistent with a role for GIP, but not GLP-2, in the maintenance of lipid homeostasis in rats, while neither GIPR nor GLP-2R antagonism appeared to influence bone resorption in rats.

Gastric inhibitory polypeptide/glucose-dependent insulinotropic polypeptide signaling in adipose tissue

GIPR signaling in adipose tissue plays an important role in HFD‐induced insulin resistance and hepatic steatosis in vivo, with no direct effect on fat accumulation, through IL‐6 signaling

Targeting the Incretin/Glucagon System With Triagonists to Treat Diabetes

Glucagonlike peptide 1 (GLP-1) receptor agonists have been efficacious for the treatment of type 2 diabetes due to their ability to reduce weight and attenuate hyperglycemia. However, the activity of glucagonlike peptide 1 receptor-directed strategies is submaximal, and the only potent, sustainable treatment of metabolic dysfunction is bariatric surgery, necessitating the development of unique therapeutics. GLP-1 is structurally related to glucagon and glucose-dependent insulinotropic peptide (GIP), allowing for the development of intermixed, unimolecular peptides with activity at each of their respective receptors. In this review, we discuss the range of tissue targets and added benefits afforded by the inclusion of each of GIP and glucagon. We discuss considerations for the development of sequence-intermixed dual agonists and triagonists, highlighting the importance of evaluating balanced signaling at the targeted receptors. Several multireceptor agonist peptides have been developed and evaluated, and the key preclinical and clinical findings are reviewed in detail. The biological activity of these multireceptor agonists are founded in the success of GLP-1-directed strategies; by including GIP and glucagon components, these multireceptor agonists are thought to enhance GLP-1's activities by broadening the tissue targets and synergizing at tissues that express multiple receptors, such at the brain and pancreatic islet β cells. The development and utility of balanced, unimolecular multireceptor agonists provide both a useful tool for querying the actions of incretins and glucagon during metabolic disease and a unique drug class to treat type 2 diabetes with unprecedented efficacy.

Human GIP(3-30)NH2 inhibits G protein-dependent as well as G protein-independent signaling and is selective for the GIP receptor with high-affinity binding to primate but not rodent GIP receptors

GIP(3-30)NH₂ is a high affinity antagonist of the GIP receptor (GIPR) in humans inhibiting insulin secretion via G protein-dependent pathways. However, its ability to inhibit G protein-independent signaling is unknown. Here we determine its action on arrestin-recruitment and receptor internalization in recombinant cells. As GIP is adipogenic, we evaluate the inhibitory actions of GIP(3-30)NH₂ in human adipocytes. Finally, we determine the receptor selectivity of GIP(3-30)NH₂ among other human and animal GPCRs. cAMP accumulation and β-arrestin 1 and 2 recruitment were studied in transiently transfected HEK293 cells and real-time internalization in transiently transfected HEK293A and in HEK293A β-arrestin 1 and 2 knockout cells. Furthermore, human subcutaneous adipocytes were assessed for cAMP accumulation following ligand stimulation. Competition binding was examined in transiently transfected COS-7 cells using human ¹²⁵I-GIP(3-30)NH₂. The selectivity of human GIP(3-30)NH₂ was examined by testing for agonistic and antagonistic properties on 62 human GPCRs. Human GIP(3-30)NH₂ inhibited GIP(1-42)-induced cAMP and β-arrestin 1 and 2 recruitment on the human GIPR and Schild plot analysis showed competitive antagonism with a pA₂ and Hill slope of 16.8 nM and 1.11 ± 0.02 in cAMP, 10.6 nM and 1.15 ± 0.05 in β-arrestin 1 recruitment, and 10.2 nM and 1.06 ± 0.05 in β-arrestin 2 recruitment. Efficient internalization of the GIPR was dependent on the presence of either β-arrestin 1 or 2. Moreover, GIP(3-30)NH₂ inhibited GIP(1-42)-induced internalization in a concentration-dependent manner and notably also inhibited GIP-mediated signaling in human subcutaneous adipocytes. Finally, the antagonist was established as GIPR selective among 62 human GPCRs being species-specific with high affinity binding to the human and non-human primate (Macaca fascicularis) GIPRs, and low affinity binding to the rat and mouse GIPRs (K_d values of 2.0, 2.5, 31.6 and 100 nM, respectively). In conclusion, human GIP(3-30)NH₂ is a selective and species-specific GIPR antagonist with broad inhibition of signaling and internalization in transfected cells as well as in human adipocytes.

Glucose-dependent insulinotropic polypeptide (GIP) receptor antagonists as anti-diabetic agents

Glucose-dependent insulinotropic polypeptide (GIP) is an intestinal hormone with a broad range of physiological actions. In the postprandial state, the hormone stimulates insulin secretion and during eu- and hypoglycemia, it stimulates glucagon secretion. In addition, GIP increases triacylglycerol (TAG) uptake in adipose tissue and decreases bone resorption. However, the importance of these actions in humans are not clearly understood as a specific GIP receptor (GIPR) antagonist - an essential tool to study GIP physiology - has been missing. Several different GIPR antagonists have been identified comprising both peptides, vaccines against GIP, GIP antibodies or antibodies against the GIPR. However, most of these have only been tested in rodents. In vitro, N- and C-terminally truncated GIP variants are potent and efficacious GIPR antagonists. Recently, GIP(3-30)NH₂, a naturally occurring peptide, was shown to block the GIPR in humans and decrease GIP-induced insulin secretion as well as adipose tissue blood flow and TAG uptake. So far, there are no studies with a GIPR antagonist in patients with type 2 diabetes (T2D), but because the elevations in fasting plasma glucagon and paradoxical postprandial glucagon excursions, seen in patients with T2D, are aggravated by GIP, a GIPR antagonist could partly alleviate this and possibly improve the fasting and postprandial glycemia. Since the majority of patients with T2D are overweight, inhibition of GIP-induced fat deposition may be beneficial as well. Here we summarize the studies of GIPR antagonists and discuss the therapeutic potential of the GIP system in humans.

Gut hormone polyagonists for the treatment of type 2 diabetes

Chemical derivatives of the gut-derived peptide hormone glucagon-like peptide 1 (GLP-1) are among the best-in-class pharmacotherapies to treat obesity and type 2 diabetes. However, GLP-1 analogs have modest weight lowering capacity, in the range of 5-10%, and the therapeutic window is hampered by dose-dependent side effects. Over the last few years, a new concept has emerged: combining the beneficial effects of several key metabolic hormones into a single molecular entity. Several unimolecular GLP-1-based polyagonists have shown superior metabolic action compared to GLP-1 monotherapies. In this review article, we highlight the history of polyagonists targeting the receptors for GLP-1, GIP and glucagon, and discuss recent progress in expanding of this concept to now allow targeted delivery of nuclear hormones via GLP-1 and other gut hormones, as a novel approach towards more personalized pharmacotherapies.

GIP(3-30)NH2 is an efficacious GIP receptor antagonist in humans: a randomised, double-blinded, placebo-controlled, crossover study

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone secreted postprandially from enteroendocrine K cells, but despite therapeutically interesting effects, GIP physiology in humans remains incompletely understood. Progress in this field could be facilitated by a suitable GIP receptor antagonist. For the first time in humans, we investigated the antagonistic properties of the naturally occurring GIP(3-30)NH₂ in in vivo and in in vitro receptor studies.

METHODS

In transiently transfected COS-7 cells, GIP(3-30)NH₂ was evaluated with homologous receptor binding and receptor activation (cAMP accumulation) studies at the glucagon-like peptide 1 (GLP-1), glucagon-like peptide-2 (GLP-2), glucagon, secretin and growth hormone-releasing hormone (GHRH) receptors. Ten healthy men (eligibility criteria: age 20-30 years, HbA_1c less than 6.5% [48 mmol/mol] and fasting plasma glucose [FPG] less than 7 mmol/l) were included in the clinical study. Data were collected as plasma and serum samples from a cubital vein cannula. As primary outcome, insulin secretion and glucose requirements were evaluated together with in a randomised, four-period, crossover design by infusing GIP(3-30)NH₂ (800 pmol kg^-1 min^-1), GIP (1.5 pmol kg^-1 min^-1), a combination of these or placebo during hyperglycaemic clamp experiments. The content of the infusions were blinded to the study participants and experimental personnel. No study participants dropped out.

RESULTS

GIP(3-30)NH₂ neither bound, stimulated nor antagonised a series of related receptors in vitro. The elimination plasma half-life of GIP(3-30)NH₂ in humans was 7.6 ± 1.4 min. Markedly larger amounts of glucose were required to maintain the clamp during GIP infusion compared with the other days. GIP-induced insulin secretion was reduced by 82% (p < 0.0001) during co-infusion with GIP(3-30)NH₂, and the need for glucose was reduced to placebo levels. There were no effects of GIP(3-30)NH₂ alone or of GIP with or without GIP(3-30)NH₂ on plasma glucagon, GLP-1, somatostatin, triacylglycerols, cholesterol, glycerol or NEFA. GIP(3-30)NH₂ administration was well tolerated and without side effects.

CONCLUSIONS/INTERPRETATION

We conclude that GIP(3-30)NH₂ is an efficacious and specific GIP receptor antagonist in humans suitable for studies of GIP physiology and pathophysiology.

TRIAL REGISTRATION

ClinicalTrials.gov registration no. NCT02747472.

FUNDING

The study was funded by Gangstedfonden, the European Foundation for the Study of Diabetes, and Aase og Ejnar Danielsens fond.

The equine glucose-dependent insulinotropic polypeptide receptor: A potential therapeutic target for insulin dysregulation

Metabolic disease is a significant problem that causes a range of species-specific comorbidities. Recently, a better understanding of glucose-dependent insulinotropic polypeptide (GIP) biology has led to the suggestion that inhibiting its action may attenuate obesity in several species. In horses, antagonism of GIP may also reduce hyperinsulinemia, which leads to insulin-associated laminitis, a painful comorbidity unique to this species. However, little is known about GIP in horses. The aims of this study were to examine the tissue distribution of equine GIP receptors (eGIPR), to determine whether eGIPR can be blocked using a GIP antagonist not tested previously in horses, and to establish whether there is any association between GIP concentrations and body mass in this species. Archived tissues from healthy horses were used to establish that eGIPR gene expression was strong in pancreas, heart, liver, kidney, and duodenum and absent in gluteal muscle. Pancreatic islets were isolated from fresh horse pancreas using collagenase digestion and layering through a density gradient. Islet viability was confirmed microscopically and by demonstrating that insulin production was stimulated by glucose in a concentration-dependent manner. Insulin release was also shown to be concentration-dependent with GIP up to 0.1µM, and the response to GIP was decreased ( = 0.037) by the antagonist (Pro3)GIP. As for the relationship between body mass and GIP in vivo postprandial GIP concentrations in archived plasma samples were positively correlated with body condition and cresty neck scores ( < 0.05). Thus, the eGIPR is a potential therapeutic target for insulin dysregulation and obesity in horses.

Incretin-based therapy for type 2 diabetes mellitus is promising for treating neurodegenerative diseases

Incretin hormones include glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Due to their promising action on insulinotropic secretion and improving insulin resistance (IR), incretin-based therapies have become a new class of antidiabetic agents for the treatment of type 2 diabetes mellitus (T2DM). Recently, the links between neurodegenerative diseases and T2DM have been identified in a number of studies, which suggested that shared mechanisms, such as insulin dysregulation or IR, may underlie these conditions. Therefore, the effects of incretins in neurodegenerative diseases have been extensively investigated. Protease-resistant long-lasting GLP-1 mimetics such as lixisenatide, liraglutide, and exenatide not only have demonstrated promising effects for treating neurodegenerative diseases in preclinical studies but also have shown first positive results in Alzheimer's disease (AD) and Parkinson's disease (PD) patients in clinical trials. Furthermore, the effects of other related incretin-based therapies such as GIP agonists, dipeptidyl peptidase-IV (DPP-IV) inhibitors, oxyntomodulin (OXM), dual GLP-1/GIP, and triple GLP-1/GIP/glucagon receptor agonists on neurodegenerative diseases have been tested in preclinical studies. Incretin-based therapies are a promising approach for treating neurodegenerative diseases.

A novel GIP analogue, ZP4165, enhances glucagon-like peptide-1-induced body weight loss and improves glycaemic control in rodents

AIM

To investigate the effects of the novel glucose-dependent insulinotropic polypeptide (GIP) analogue, ZP4165, on body weight and glycaemic control in rodents, and to investigate if ZP4165 modulates the anti-obesity and anti-hyperglycaemic effects of a glucagon-like peptide-1 (GLP-1) agonist (liraglutide).

METHODS

The acute insulinotropic effect of ZP4165 was investigated in rats during an oral glucose tolerance test. The long-term effects of ZP4165 on body weight and glycaemic control, either alone or in combination with liraglutide, were assessed in diet-induced obese mice and diabetic db/db mice.

RESULTS

ZP4165 showed insulinotropic action in rats. The GIP analogue did not alter the body weight of obese mice but enhanced GLP-1-induced weight loss. In diabetic mice, 4 weeks' dosing with ZP4165 reduced glycated haemoglobin levels vs vehicle by an extent similar to the GLP-1 agonist.

CONCLUSIONS

ZP4165 potentiated the anti-obesity effect of a GLP-1 agonist in obese mice and improved glycaemic control in diabetic mice. These studies support further investigation of dual-incretin therapy as a more effective treatment option than mono GLP-1 medication for type 2 diabetes mellitus and obesity.

The role of somatostatin in GLP-1-induced inhibition of glucagon secretion in mice

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) receptor agonists are currently used for the treatment of type 2 diabetes. Their main mechanism of action is enhancement of glucose-induced insulin secretion (from increased beta cell glucose sensitivity) and inhibition of glucagon secretion. The latter has been demonstrated to account for about half of their blood glucose-lowering activity. Whereas the effect of GLP-1 on insulin secretion is clearly dependent on ambient glucose concentrations and has been described in detail, the mechanism responsible for the inhibitory effect of GLP-1 on glucagon secretion is heavily debated. Glucagon inhibition is also said to be glucose-dependent, although it is unclear what is meant by this. We hypothesise here that GLP-1 does not inhibit glucagon secretion during hypoglycaemia because the inhibition depends on somatostatin secretion, which in turn is dependent on glucose levels.

METHODS

We used the perfused mouse pancreas model to investigate this hypothesis.

RESULTS

We found that, in this model, GLP-1 was able to significantly inhibit glucagon secretion from pancreatic alpha cells at all glucose levels tested: 6.0, 1.5 and 0.5 mmol/l (-27.0%, -37.1%, and -23.6%, respectively), and the decrease in glucagon secretion was invariably accompanied by an increase in somatostatin secretion (+286.8%, +158.7%, and +118.8%, respectively). Specific blockade of somatostatin receptor 2 increased glucagon secretion (+118.8% at 1.5 mmol/l glucose and +162.9% at 6.0 mmol/l glucose) and completely eliminated the inhibitory effect of GLP-1.

CONCLUSIONS/INTERPRETATION

We have shown here that the glucagon-lowering effect of GLP-1 is entirely mediated through the paracrine actions of somatostatin in the perfused mouse pancreas. However, in this model, the inhibitory effect of GLP-1 was preserved at hypoglycaemic levels, leaving unanswered the question of how this is avoided in vivo in individuals treated with GLP-1 receptor agonists.

The Gluco- and Liporegulatory and Vasodilatory Effects of Glucose-Dependent Insulinotropic Polypeptide (GIP) Are Abolished by an Antagonist of the Human GIP Receptor

A truncated form of human glucose-dependent insulinotropic polypeptide (GIP), GIP(3-30)NH₂, was recently identified as an antagonist of the human GIP receptor. This study examined the ability of GIP(3-30)NH₂ to antagonize the physiological actions of GIP in glucose metabolism, subcutaneous abdominal adipose tissue blood flow (ATBF), and lipid metabolism in humans. Eight lean subjects were studied by measuring arteriovenous concentrations of metabolites and ATBF on three different occasions during hyperglycemic-hyperinsulinemic clamps with concomitant infusions of GIP, GIP(3-30)NH₂, or both GIP and GIP(3-30)NH₂ During infusion of GIP(3-30)NH₂ alone and in combination with GIP, insulin levels and the total glucose amount infused to maintain the clamp were lower than during GIP alone. In addition, ATBF remained constant during the antagonist and increased only slightly in combination with GIP, whereas it increased fivefold during GIP alone. Adipose tissue triacylglyceride (TAG) and glucose uptake decreased, and the free fatty acid/glycerol ratio increased during the antagonist alone and in combination with GIP. The changes in glucose infusion rates and plasma insulin levels demonstrate an inhibitory effect of the antagonist on the incretin effect of GIP. In addition, the antagonist inhibited GIP-induced increase in ATBF and decreased the adipose tissue TAG uptake, indicating that GIP also plays a crucial role in lipid metabolism.

Effects of gastric inhibitory polypeptide, glucagon-like peptide-1 and glucagon-like peptide-1 receptor agonists on Bone Cell Metabolism

The relationship between gut and skeleton is increasingly recognized as part of the integrated physiology of the whole organism. The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted from the intestine in response to nutrient intake and exhibit several physiological functions including regulation of islet hormone secretion and glucose levels. A number of GLP-1 receptor agonists (GLP-1RAs) are currently used in treatment of type 2 diabetes and obesity. However, GIP and GLP-1 cognate receptors are widely expressed suggesting that incretin hormones mediate effects beyond control of glucose homeostasis, and reports on associations between incretin hormones and bone metabolism have emerged. The aim of this MiniReview was to provide an overview of current knowledge regarding the in vivo and in vitro effects of GIP and GLP-1 on bone metabolism. We identified a total of 30 pre-clinical and clinical investigations of the effects of GIP, GLP-1 and GLP-1RAs on bone turnover markers, bone mineral density (BMD), bone microarchitecture and fracture risk. Studies conducted in cell cultures and rodents demonstrated that GIP and GLP-1 play a role in regulating skeletal homeostasis, with pre-clinical data suggesting that GIP inhibits bone resorption whereas GLP-1 may promote bone formation and enhance bone material properties. These effects are not corroborated by clinical studies. While there is evidence of effects of GIP and GLP-1 on bone metabolism in pre-clinical investigations, clinical trials are needed to clarify whether similar effects are present and clinically relevant in humans.

Novel GLP-1R/GIPR co-agonist "twincretin" is neuroprotective in cell and rodent models of mild traumatic brain injury

Several single incretin receptor agonists that are approved for the treatment of type 2 diabetes mellitus (T2DM) have been shown to be neuroprotective in cell and animal models of neurodegeneration. Recently, a synthetic dual incretin receptor agonist, nicknamed "twincretin," was shown to improve upon the metabolic benefits of single receptor agonists in mouse and monkey models of T2DM. In the current study, the neuroprotective effects of twincretin are probed in cell and mouse models of mild traumatic brain injury (mTBI), a prevalent cause of neurodegeneration in toddlers, teenagers and the elderly. Twincretin is herein shown to have activity at two different receptors, dose-dependently increase levels of intermediates in the neurotrophic CREB pathway and enhance viability of human neuroblastoma cells exposed to toxic concentrations of glutamate and hydrogen peroxide, insults mimicking the inflammatory conditions in the brain post-mTBI. Additionally, twincretin is shown to improve upon the neurotrophic effects of single incretin receptor agonists in these same cells. Finally, a clinically translatable dose of twincretin, when administered post-mTBI, is shown to fully restore the visual and spatial memory deficits induced by mTBI, as evaluated in a mouse model of weight drop close head injury. These results establish twincretin as a novel neuroprotective agent and suggest that it may improve upon the effects of the single incretin receptor agonists via dual agonism.

Transcriptomic profiling of pancreatic alpha, beta and delta cell populations identifies delta cells as a principal target for ghrelin in mouse islets

AIMS/HYPOTHESIS

Intra-islet and gut-islet crosstalk are critical in orchestrating basal and postprandial metabolism. The aim of this study was to identify regulatory proteins and receptors underlying somatostatin secretion though the use of transcriptomic comparison of purified murine alpha, beta and delta cells.

METHODS

Sst-Cre mice crossed with fluorescent reporters were used to identify delta cells, while Glu-Venus (with Venus reported under the control of the Glu [also known as Gcg] promoter) mice were used to identify alpha and beta cells. Alpha, beta and delta cells were purified using flow cytometry and analysed by RNA sequencing. The role of the ghrelin receptor was validated by imaging delta cell calcium concentrations using islets with delta cell restricted expression of the calcium reporter GCaMP3, and in perfused mouse pancreases.

RESULTS

A database was constructed of all genes expressed in alpha, beta and delta cells. The gene encoding the ghrelin receptor, Ghsr, was highlighted as being highly expressed and enriched in delta cells. Activation of the ghrelin receptor raised cytosolic calcium levels in primary pancreatic delta cells and enhanced somatostatin secretion in perfused pancreases, correlating with a decrease in insulin and glucagon release. The inhibition of insulin secretion by ghrelin was prevented by somatostatin receptor antagonism.

CONCLUSIONS/INTERPRETATION

Our transcriptomic database of genes expressed in the principal islet cell populations will facilitate rational drug design to target specific islet cell types. The present study indicates that ghrelin acts specifically on delta cells within pancreatic islets to elicit somatostatin secretion, which in turn inhibits insulin and glucagon release. This highlights a potential role for ghrelin in the control of glucose metabolism.

Diabetes and obesity treatment based on dual incretin receptor activation: 'twincretins'

The gut incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted after meal ingestion and work in concert to promote postprandial insulin secretion and regulate glucagon secretion. GLP-1 also slows gastric emptying and suppresses appetite, whereas GIP seems to affect lipid metabolism. The introduction of selective GLP-1 receptor (GLP-1R) agonists for the treatment of type 2 diabetes and obesity has increased the scientific and clinical interest in incretins. Combining the body weight-lowering and glucose-lowering effects of GLP-1 with a more potent improvement of β cell function through additional GIP action could potentially offer a more effective treatment of diabetes and obesity, with fewer adverse effects than selective GLP-1R agonists; therefore, new drugs designed to co-activate both the GIP receptor (GIPR) and the GLP-1R simultaneously are under development. In the present review, we address advances in the field of GIPR and GLP-1R co-agonism and review in vitro studies, animal studies and human trials involving co-administration of the two incretins, as well as results from a recently developed GIPR/GLP-1R co-agonist, and highlight promising areas and challenges within the field of incretin dual agonists.

Dynamics of glucagon secretion in mice and rats revealed using a validated sandwich ELISA for small sample volumes

Glucagon is a metabolically important hormone, but many aspects of its physiology remain obscure, because glucagon secretion is difficult to measure in mice and rats due to methodological inadequacies. Here, we introduce and validate a low-volume, enzyme-linked immunosorbent glucagon assay according to current analytical guidelines, including tests of sensitivity, specificity, and accuracy, and compare it, using the Bland-Altman algorithm and size-exclusion chromatography, with three other widely cited assays. After demonstrating adequate performance of the assay, we measured glucagon secretion in response to intravenous glucose and arginine in anesthetized mice (isoflurane) and rats (Hypnorm/midazolam). Glucose caused a long-lasting suppression to very low values (1-2 pmol/l) within 2 min in both species. Arginine stimulated secretion 8- to 10-fold in both species, peaking at 1-2 min and returning to basal levels at 6 min (mice) and 12 min (rats). d-Mannitol (osmotic control) was without effect. Ketamine/xylazine anesthesia in mice strongly attenuated (P < 0.01) α-cell responses. Chromatography of pooled plasma samples confirmed the accuracy of the assay. In conclusion, dynamic analysis of glucagon secretion in rats and mice with the novel accurate sandwich enzyme-linked immunosorbent assay revealed extremely rapid and short-lived responses to arginine and rapid and profound suppression by glucose.

Unimolecular Polypharmacy for Treatment of Diabetes and Obesity

Many complex diseases have historically proven to be defiant to the best mono-therapeutic approaches. Several examples of combination therapies have largely overcome such challenges, notably for the treatment of severe hypertension and tuberculosis. Obesity and its consequences, such as type 2 diabetes, have proven to be equally resistant to therapeutic approaches based on single medicines. Proper management of type 2 diabetes often requires adjunctive medications, and the recent registration of a few compound mixtures has set the precedent for combinatorial treatment of obesity. On the other hand, double or triple therapeutic combinations are more difficult to advance to regulatory approval than single molecules. More recently, several classes of novel unimolecular combination therapeutics have emerged with superior efficacy than currently prescribed options and pose the potential to reverse obesity and type 2 diabetes. Here, we summarize the discovery, pre-clinical validation, and first clinical test of such peptide hormone poly-agonist drug candidates.

N-terminally and C-terminally truncated forms of glucose-dependent insulinotropic polypeptide are high-affinity competitive antagonists of the human GIP receptor

Background and Purpose

Glucose‐dependent insulinotropic polypeptide (GIP) affects lipid, bone and glucose homeostasis. High‐affinity ligands for the GIP receptor are needed to elucidate the physiological functions and pharmacological potential of GIP in vivo. GIP(1–30)NH₂ is a naturally occurring truncation of GIP(1–42). Here, we have characterized eight N‐terminal truncations of human GIP(1–30)NH₂.

Experimental Approach

COS‐7 cells were transiently transfected with human GIP receptors and assessed for cAMP accumulation upon ligand stimulation or competition binding with ¹²⁵I‐labelled GIP(1–42), GIP(1–30)NH₂, GIP(2–30)NH₂ or GIP(3–30)NH₂.

Key Results

GIP(1–30)NH₂ displaced ¹²⁵I‐GIP(1–42) as effectively as GIP(1–42) (K_i 0.75 nM), whereas the eight truncations displayed lower affinities (K_i 2.3–347 nM) with highest affinities for GIP(3–30)NH₂ and GIP(5–30)NH₂ (5–30)NH₂. Only GIP(1–30)NH₂ (E_max 100% of GIP(1–42)) and GIP(2–30)NH₂ (E_max 20%) were agonists. GIP(2‐ to 9–30)NH₂ displayed antagonism (IC₅₀ 12–450 nM) and Schild plot analyses identified GIP(3–30)NH₂ and GIP(5–30)NH₂ as competitive antagonists (K_i 15 nM). GIP(3–30) NH₂ was a 26‐fold more potent antagonist than GIP(3–42). Binding studies with agonist (¹²⁵I‐GIP(1–30)NH₂), partial agonist (¹²⁵I‐GIP(2–30)NH₂) and competitive antagonist (¹²⁵I‐GIP(3–30)NH₂) revealed distinct receptor conformations for these three ligand classes.

Conclusions and Implications

The N‐terminus is crucial for GIP agonist activity. Removal of the C‐terminus of the endogenous GIP(3–42) creates another naturally occurring, more potent, antagonist GIP(3–30)NH₂, which like GIP(5–30)NH₂, was a high‐affinity competitive antagonist. These peptides may be suitable tools for basic GIP research and future pharmacological interventions.

Species-specific action of (Pro3)GIP - a full agonist at human GIP receptors, but a partial agonist and competitive antagonist at rat and mouse GIP receptors

Background and Purpose

Specific, high potency receptor antagonists are valuable tools when evaluating animal and human physiology. Within the glucose‐dependent, insulinotropic polypeptide (GIP) system, considerable attention has been given to the presumed GIP receptor antagonist, (Pro3)GIP, and its effect in murine studies. We conducted a pharmacological analysis of this ligand including interspecies differences between the rodent and human GIP system.

Experimental Approach

Transiently transfected COS‐7 cells were assessed for cAMP accumulation upon ligand stimulation and assayed in competition binding using ¹²⁵I‐human GIP. Using isolated perfused pancreata both from wild type and GIP receptor‐deficient rodents, insulin‐releasing, glucagon‐releasing and somatostatin‐releasing properties in response to species‐specific GIP and (Pro3)GIP analogues were evaluated.

Key Results

Human (Pro3)GIP is a full agonist at human GIP receptors with similar efficacy (E_max) for cAMP production as human GIP, while both rat and mouse(Pro3)GIP were partial agonists on their corresponding receptors. Rodent GIPs are more potent and efficacious at their receptors than human GIP. In perfused pancreata in the presence of 7 mM glucose, both rodent (Pro3)GIP analogues induced modest insulin, glucagon and somatostatin secretion, corresponding to the partial agonist activities observed in cAMP production.

Conclusions and Implications

When evaluating new compounds, it is important to consider interspecies differences both at the receptor and ligand level. Thus, in rodent models, human GIP is a comparatively weak partial agonist. Human (Pro3)GIP was not an antagonist at human GIP receptors, so there is still a need for a potent antagonist in order to elucidate the physiology of human GIP.

Investigating the Glucagon Receptor and Glucagon-Like Peptide 1 Receptor Activity of Oxyntomodulin-Like Analogues in Male Wistar Rats

Aims

To investigate the effect of Glu-3 OXM-like analogues on food intake and bodyweight in male rats.

Background

Oxyntomodulin (OXM) is a natural agonist at both the glucagon receptor (GCGr) and the glucagon-like peptide 1 receptor (GLP-1r), and peripheral administration reduces food intake and increases energy expenditure in rodents and humans. Substituting the native glutamine (Gln) at amino acid position 3 of OXM for glutamate (Glu) has previously been shown to diminish GCGr activity without affecting GLP-1r activity. The effects of Glu-3 OXM analogues have not been investigated in rats.

Methods

The effect of 2 Glu-3-substituted OXM-like analogues (eg, OXM14E3 and OXM15E3) on food intake and body weight was investigated in male Wistar rats during 6 days of daily subcutaneous (SC) administration. The effects of Glu-3 substitution on analogue binding and activity at the rat GCGr and rat GLP-1 receptor were investigated in vitro using Chinese hamster ovary or Chinese hamster lung cells.

Results

We report the novel finding that 2 5-nmol/kg Glu-3 OXM-like analogues (OXM14E3 and OXM15E3) significantly increased rat body weight by up to 4% compared with the equivalent non-Glu-3 analogues (OXM14 and OXM15), without affecting food intake. The effect of OXM15E3 on body weight was dose–dependent. Glu-3 analogues, including Glu-3 OXM, decreased glucagon-mediated cyclic adenosine monophosphate accumulation in Chinese hamster ovary cells expressing the rat GCGr, suggesting they may be acting as antagonists.

Conclusions

The results indicate Glu-3 OXM-like analogues might not be suitable tools to investigate the mechanism of OXM analogue action in a rat model because they significantly increase body weight independent of food intake. Glu-3 OXM analogues are partial agonists at the rat GCGr and may also act as antagonists, possibly resulting in the observed increase in body weight.