Vaccination against GIP for the treatment of obesity

Duodenal enteroendocrine cells and GIP as treatment targets for obesity and type 2 diabetes

The duodenum is an important source of endocrine and paracrine signals controlling digestion and nutrient disposition, notably including the main incretin hormone glucose-dependent insulinotropic polypeptide (GIP). Bariatric procedures that prevent nutrients from contact with the duodenal mucosa are particularly effective interventions to reduce body weight and improve glycaemic control in obesity and type 2 diabetes. These procedures take advantage of increased nutrient delivery to more distal regions of the intestine which enhances secretion of the other incretin hormone glucagon-like peptide-1 (GLP-1). Preclinical experiments have shown that either an increase or a decrease in the secretion or action of GIP can decrease body weight and blood glucose in obesity and non-insulin dependent hyperglycaemia, but clinical studies involving administration of GIP have been inconclusive. However, a synthetic dual agonist peptide (tirzepatide) that exerts agonism at receptors for GIP and GLP-1 has produced marked weight-lowering and glucose-lowering effects in people with obesity and type 2 diabetes. This appears to result from chronic biased agonism in which the novel conformation of the peptide triggers enhanced signalling by the GLP-1 receptor through reduced internalisation while reducing signalling by the GIP receptor directly or via functional antagonism through increased internalisation and degradation.

What is the pipeline for future medications for obesity?

Obesity is a chronic disease associated with increased risk of obesity-related complications and mortality. Our better understanding of the weight regulation mechanisms and the role of gut-brain axis on appetite has led to the development of safe and effective entero-pancreatic hormone-based treatments for obesity such as glucagon-like peptide-1 (GLP-1) receptor agonists (RA). Semaglutide 2.4 mg once weekly, a subcutaneously administered GLP-1 RA approved for obesity treatment in 2021, results in 15-17% mean weight loss (WL) with evidence of cardioprotection. Oral GLP-1 RA are also under development and early data shows similar WL efficacy to semaglutide 2.4 mg. Looking to the next generation of obesity treatments, combinations of GLP-1 with other entero-pancreatic hormones with complementary actions and/or synergistic potential (such as glucose-dependent insulinotropic polypeptide (GIP), glucagon, and amylin) are under investigation to enhance the WL and cardiometabolic benefits of GLP-1 RA. Tirzepatide, a dual GLP-1/GIP receptor agonist has been approved for glycaemic control in type 2 diabetes as well as for obesity management leading in up to 22.5% WL in phase 3 obesity trials. Other combinations of entero-pancreatic hormones including cagrisema (GLP-1/amylin RA) and the triple agonist retatrutide (GLP-1/GIP/glucagon RA) have also progressed to phase 3 trials as obesity treatments and early data suggests that may lead to even greater WL than tirzepatide. Additionally, agents with different mechanisms of action to entero-pancreatic hormones (e.g. bimagrumab) may improve the body composition during WL and are in early phase clinical trials. We are in a new era for obesity pharmacotherapy where combinations of entero-pancreatic hormones approach the WL achieved with bariatric surgery. In this review, we present the efficacy and safety data for the pipeline of obesity pharmacotherapies with a focus on entero-pancreatic hormone-based treatments and we consider the clinical implications and challenges that the new era in obesity management may bring.

The importance of glucose-dependent insulinotropic polypeptide receptor activation for the effects of tirzepatide

Tirzepatide is a unimolecular co-agonist of the glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors recently approved for the treatment of type 2 diabetes by the US Food and Drug Administration and the European Medicine Agency. Tirzepatide treatment results in an unprecedented improvement of glycaemic control and lowering of body weight, but the contribution of the GIP receptor-activating component of tirzepatide to these effects is uncertain. In this review, we present the current knowledge about the physiological roles of the incretin hormones GLP-1 and GIP, their receptors, and previous results of co-targeting the two incretin hormone receptors in humans. We also analyse the molecular pharmacological, preclinical and clinical effects of tirzepatide to discuss the role of GIP receptor activation for the clinical effects of tirzepatide. Based on the available literature on the combination of GLP-1 and GIP receptor activation, tirzepatide does not seem to have a classical co-activating mode of action in humans. Rather, in vitro studies of the human GLP-1 and GIP receptors reveal a biased GLP-1 receptor activation profile and GIP receptor downregulation. Therefore, we propose three hypotheses for the mode of action of tirzepatide, which can be addressed in future, elaborate clinical trials.

GIP receptor antagonist treatment causes a reduction in weight gain in ovariectomised high fat diet-fed mice

Background and purpose

The incretin hormone, gastric inhibitory peptide/glucose‐dependent insulinotropic polypeptide (GIP), secreted by the enteroendocrine K‐cells in the proximal intestine, may regulate lipid metabolism and adiposity, but its exact role in these processes is unclear.

Experimental approach

We characterized in vitro and in vivo antagonistic properties of a novel GIP analogue, mGIPAnt‐1. We further assessed the in vivo pharmacokinetic profile of this antagonist, as well as its ability to affect high‐fat diet (HFD)‐induced body weight gain in ovariectomised mice during an 8‐week treatment period.

Key results

mGIPAnt‐1 showed competitive antagonistic properties to the GIP receptor in vitro as it inhibited GIP‐induced cAMP accumulation in COS‐7 cells. Furthermore, mGIPAnt‐1 was capable of inhibiting GIP‐induced glucoregulatory and insulinotropic effects in vivo and has a favourable pharmacokinetic profile with a half‐life of 7.2 h in C57Bl6 female mice. Finally, sub‐chronic treatment with mGIPAnt‐1 in ovariectomised HFD mice resulted in a reduction of body weight and fat mass.

Conclusion and Implications

mGIPAnt‐1 successfully inhibited acute GIP‐induced effects in vitro and in vivo and sub‐chronically induces resistance to HFD‐induced weight gain in ovariectomised mice. Our results support the development of GIP antagonists for the therapy of obesity.

The Role of GIP Receptor in the CNS for the Pathogenesis of Obesity

Glucose-dependent insulinotropic polypeptide (GIP) (also known as gastric inhibitory polypeptide) is a hormone produced in the upper gut and secreted to the circulation in response to the ingestion of foods, especially fatty foods. Growing evidence supports the physiological and pharmacological relevance of GIP in obesity. In an obesity setting, inhibition of endogenous GIP or its receptor leads to decreased energy intake, increased energy expenditure, or both, eventually causing weight loss. Further, supraphysiological dosing of exogenous long-lasting GIP agonists alters energy balance and has a marked antiobesity effect. This remarkable yet paradoxical antiobesity effect is suggested to occur primarily via the brain. The brain is capable of regulating both energy intake and expenditure and plays a critical role in human obesity. In addition, the GIP receptor is widely distributed throughout the brain, including areas responsible for energy homeostasis. Recent studies have uncovered previously underappreciated roles of the GIP receptor in the brain in the context of obesity. This article highlights how the GIP receptor expressed by the brain impacts obesity-related pathogenesis.

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

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

Glucagon-like peptide-1 receptor co-agonists for treating metabolic disease

Background

Glucagon-like peptide-1 receptor (GLP-1R) agonists are approved to treat type 2 diabetes and obesity. They elicit robust improvements in glycemic control and weight loss, combined with cardioprotection in individuals at risk of or with pre-existing cardiovascular disease. These attributes make GLP-1 a preferred partner for next-generation therapies exhibiting improved efficacy yet retaining safety to treat diabetes, obesity, non-alcoholic steatohepatitis, and related cardiometabolic disorders. The available clinical data demonstrate that the best GLP-1R agonists are not yet competitive with bariatric surgery, emphasizing the need to further improve the efficacy of current medical therapy.

Scope of review

In this article, we discuss data highlighting the physiological and pharmacological attributes of potential peptide and non-peptide partners, exemplified by amylin, glucose-dependent insulinotropic polypeptide (GIP), and steroid hormones. We review the progress, limitations, and future considerations for translating findings from preclinical experiments to competitive efficacy and safety in humans with type 2 diabetes and obesity.

Major conclusions

Multiple co-agonist combinations exhibit promising clinical efficacy, notably tirzepatide and investigational amylin combinations. Simultaneously, increasing doses of GLP-1R agonists such as semaglutide produces substantial weight loss, raising the bar for the development of new unimolecular co-agonists. Collectively, the available data suggest that new co-agonists with robust efficacy should prove superior to GLP-1R agonists alone to treat metabolic disorders.

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GLP-1 is a preferred partner for co-agonist development.

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Co-agonist combinations must exhibit improved weight loss beyond GLP-1 alone.

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Unimolecular coagonists must exhibit retained or improved cardioprotection.

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Obesity represents an optimal condition for the development of new GLP-1 co-agonists.

Gut Hormone GIP Induces Inflammation and Insulin Resistance in the Hypothalamus

The hypothalamus plays a critical role in controlling energy balance. High-fat diet (HFD) feeding increases the gene expression of proinflammatory mediators and decreases insulin actions in the hypothalamus. Here, we show that a gut-derived hormone, glucose-dependent insulinotropic polypeptide (GIP), whose levels are elevated during diet-induced obesity, promotes and mediates hypothalamic inflammation and insulin resistance during HFD-induced obesity. Unbiased ribonucleic acid sequencing of GIP-stimulated hypothalami revealed that hypothalamic pathways most affected by intracerebroventricular (ICV) GIP stimulation were related to inflammatory-related responses. Subsequent analysis demonstrated that GIP administered either peripherally or centrally, increased proinflammatory-related factors such as Il-6 and Socs3 in the hypothalamus, but not in the cortex of C57BL/6J male mice. Consistently, hypothalamic activation of IκB kinase-β inflammatory signaling was induced by ICV GIP. Further, hypothalamic levels of proinflammatory cytokines and Socs3 were significantly reduced by an antagonistic GIP receptor (GIPR) antibody and by GIPR deficiency. Additionally, centrally administered GIP reduced anorectic actions of insulin in the brain and diminished insulin-induced phosphorylation of Protein kinase B and Glycogen synthase kinase 3β in the hypothalamus. Collectively, these findings reveal a previously unrecognized role for brain GIP signaling in diet-induced inflammation and insulin resistance in the hypothalamus.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

How May GIP Enhance the Therapeutic Efficacy of GLP-1?

Glucagon-like peptide-1 (GLP-1) receptor agonists improve glucose homeostasis, reduce bodyweight, and over time benefit cardiovascular health in type 2 diabetes mellitus (T2DM). However, dose-related gastrointestinal effects limit efficacy, and therefore agents possessing GLP-1 pharmacology that can also target alternative pathways may expand the therapeutic index. One approach is to engineer GLP-1 activity into the sequence of glucose-dependent insulinotropic polypeptide (GIP). Although the therapeutic implications of the lipogenic actions of GIP are debated, its ability to improve lipid and glucose metabolism is especially evident when paired with the anorexigenic mechanism of GLP-1. We review the complexity of GIP in regulating adipose tissue function and energy balance in the context of recent findings in T2DM showing that dual GIP/GLP-1 receptor agonist therapy produces profound weight loss, glycemic control, and lipid lowering.

Insights into incretin-based therapies for treatment of diabetic dyslipidemia

Derangements in triglyceride and cholesterol metabolism (dyslipidemia) are major risk factors for the development of cardiovascular diseases in obese and type-2 diabetic (T2D) patients. An emerging class of glucagon-like peptide-1 (GLP-1) analogues and next generation peptide dual-agonists such as GLP-1/glucagon or GLP-1/GIP could provide effective therapeutic options for T2D patients. In addition to their role in glucose and energy homeostasis, GLP-1, GIP and glucagon serve as regulators of lipid metabolism. This review summarizes the current knowledge in GLP-1, glucagon and GIP effects on lipid and lipoprotein metabolism and frames the emerging therapeutic benefits of GLP-1 analogs and GLP-1-based multiagonists as add-on treatment options for diabetes associated dyslipidemia.

No Acute Effects of Exogenous Glucose-Dependent Insulinotropic Polypeptide on Energy Intake, Appetite, or Energy Expenditure When Added to Treatment With a Long-Acting Glucagon-Like Peptide 1 Receptor Agonist in Men With Type 2 Diabetes

OBJECTIVE

Dual incretin receptor agonists in clinical development have shown reductions in body weight and hemoglobin A_1c (HbA_1c) in patients with type 2 diabetes, but the impact of glucose-dependent insulinotropic polypeptide (GIP) receptor activation remains unclear. Here, we evaluated the effects of high-dose exogenous GIP on energy intake, energy expenditure, plasma glucose, and glucose-regulating hormones in patients with type 2 diabetes treated with a long-acting glucagon-like peptide 1 receptor (GLP-1R) agonist.

RESEARCH DESIGN AND METHODS

In a randomized, double-blind design, men with type 2 diabetes (n = 22, mean ± SEM HbA_1c 6.8 ± 0.1% [51 ± 1.5 mmol/mol]) treated with metformin and long-acting GLP-1R agonists were subjected to two 5-h continuous infusions (separated by a washout period of ≥3 days): one with GIP (6 pmol/kg/min) and another with saline (placebo). After 60 min of infusion, a liquid mixed-meal test was performed, and after 270 min of infusion, an ad libitum meal was served for evaluation of energy intake (primary end point).

RESULTS

Energy intake was similar during GIP and placebo infusion (648 ± 74 kcal vs. 594 ± 55 kcal, respectively; P = 0.480), as were appetite measures and energy expenditure. Plasma glucagon and glucose were higher during GIP infusion compared with placebo infusion (P = 0.026 and P = 0.017) as assessed by area under the curve.

CONCLUSIONS

In patients with type 2 diabetes, GIP infusion on top of treatment with metformin and a long-acting GLP-1R agonist did not affect energy intake, appetite, or energy expenditure but increased plasma glucose compared with placebo. These results indicate no acute beneficial effects of combining GIP and GLP-1.

The pathogenic role of the GIP/GIPR axis in human endocrine tumors: emerging clinical mechanisms beyond diabetes

The glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone produced in the gastrointestinal tract in response to nutrients. GIP has a variety of effects on different systems, including the potentiation of insulin secretion from pancreatic β-cells after food intake (i.e. incretin effect), which is probably the most important. GIP effects are mediated by the GIP receptor (GIPR), a G protein-coupled receptor expressed in several tissues, including islet β-cells, adipocytes, bone cells, and brain. As well as its involvement in metabolic disorders (e.g. it contributes to the impaired postprandial insulin secretion in type 2 diabetes (T2DM), and to the pathogenesis of obesity and associated insulin resistance), an inappropriate GIP/GIPR axis activation of potential diagnostic and prognostic value has been reported in several endocrine tumors in recent years. The ectopic GIPR expression seen in patients with overt Cushing syndrome and primary bilateral macronodular adrenal hyperplasia or unilateral cortisol-producing adenoma has been associated with an inverse rhythm of cortisol secretion, with low fasting morning plasma levels that increase after eating. On the other hand, most acromegalic patients with an unusual GH response to oral glucose suppression have GIPR-positive somatotropinomas, and a milder phenotype, and are more responsive to medical treatment. Neuroendocrine tumors are characterized by a strong GIPR expression that may correlate positively or inversely with the proliferative index MIB-1, and that seems an attractive target for developing novel radioligands. The main purpose of this review is to summarize the role of the GIP/GIPR axis in endocrine neoplasia, in the experimental and the clinical settings.

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.

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

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

Effects of gastric inhibitory polypeptide (GIP) immunoneutralization on mouse motor coordination and memory

Gastric inhibitory polypeptide (GIP) is a regulatory peptide expressed in the mammalian upper small intestine, and both GIP and its receptor (GIPR) are expressed in the cortex and hippocampus regions of the brain as well. While learning and memory deficits have been observed in GIPR^-/- mice, the effects of peripheral GIP immunoneutralization on motor-coordination, learning, and memory have not been examined. In the present study, adult GIPR^-/- mice (KO) and age-matched wild-type C57BL/6 J mice (WT) received weekly vehicle PBS injections for 12 weeks, while a third group of wild-type mice were injected weekly for 12 weeks with 30 mg/kg body weight humanized GIP-mAb (AB) to assess the possibility of long-term effects of peripheral GIP antagonism on rodent memory and behavior. All mice groups then underwent a battery of tests that evaluated motor behavior, body coordination, and memory. Performance deficits in several memory studies after 12 weeks of treatment were demonstrated in KO, but not in AB or WT mice. Body coordination performance showed no significant differences among the 3 groups. A similar short-term study (3 injections over 9 days) was also conducted and the results were similar to those from the long-term study. Thus, short-term and long-term peripheral GIP antagonism by GIP-mAb did not appear to affect learning and memory in mice, consistent with the notion that the GIP-mAb does not cross the blood brain barrier. Furthermore, our studies indicate that GIP signaling in the brain appears to involve local neurocrine pathways.

Glucose-Dependent Insulinotropic Polypeptide Receptor Therapies for the Treatment of Obesity, Do Agonists = Antagonists?

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is associated with obesity in human genome-wide association studies. Similarly, mouse genetic studies indicate that loss of function alleles and glucose-dependent insulinotropic polypeptide overexpression both protect from high-fat diet-induced weight gain. Together, these data provide compelling evidence to develop therapies targeting GIPR for the treatment of obesity. Further, both antagonists and agonists alone prevent weight gain, but result in remarkable weight loss when codosed or molecularly combined with glucagon-like peptide-1 analogs preclinically. Here, we review the current literature on GIPR, including biology, human and mouse genetics, and pharmacology of both agonists and antagonists, discussing the similarities and differences between the 2 approaches. Despite opposite approaches being investigated preclinically and clinically, there may be viability of both agonists and antagonists for the treatment of obesity, and we expect this area to continue to evolve with new clinical data and molecular and pharmacological analyses of GIPR function.

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.

Physiological roles of the GIP receptor in murine brown adipose tissue

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) is secreted from the gut in response to nutrient ingestion and promotes meal-dependent insulin secretion and lipid metabolism. Loss or attenuation of GIP receptor (GIPR) action leads to resistance to diet-induced obesity through incompletely understood mechanisms. The GIPR is expressed in white adipose tissue; however, its putative role in brown adipose tissue (BAT) has not been explored.

METHODS

We investigated the role of the GIPR in BAT cells in vitro and in BAT-specific (GiprBAT-/-) knockout mice with selective elimination of the Gipr within the Myf5+ expression domain. We analyzed body weight, adiposity, glucose homeostasis, insulin and lipid tolerance, energy expenditure, food intake, body temperature, and iBAT oxygen consumption ex vivo. High-fat diet (HFD)-fed GiprBAT-/- mice were studied at room temperature (21 °C), 4 °C, and 30 °C ambient temperatures.

RESULTS

The mouse Gipr gene is expressed in BAT, and GIP directly increased Il6 mRNA and IL-6 secretion in BAT cells. Additionally, levels of thermogenic, lipid and inflammation mRNA transcripts were altered in BAT cells transfected with Gipr siRNA. Body weight gain, energy expenditure, and glucose and insulin tolerance were normal in HFD-fed GiprBAT-/- mice housed at room temperature. However, GiprBAT-/- mice exhibited higher body temperatures during an acute cold challenge and a lower respiratory exchange ratio and impaired lipid tolerance at 21 °C. In contrast, body weight was lower and iBAT oxygen consumption was higher in HFD-fed mice housed at 4 °C but not at 30 °C.

CONCLUSIONS

The BAT GIPR is linked to the control of metabolic gene expression, fuel utilization, and oxygen consumption. However, the selective loss of the GIPR within BAT is insufficient to recapitulate the findings of decreased weight gain and resistance to obesity arising in experimental models with systemic disruption of GIP action.

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.

Incretins in obesity and diabetes

Incretins are hormones secreted from enteroendocrine cells after nutrient intake that stimulate insulin secretion from β cells in a glucose-dependent manner. Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the only two known incretins. Dysregulation of incretin secretion and actions are noted in diseases such as obesity and diabetes. In this review, we first summarize our traditional understanding of the physiology of GIP and GLP-1, and our current knowledge of the relationships between GIP and GLP-1 and obesity and diabetes. Next, we present the results from major randomized controlled trials on the use of GLP-1 receptor agonists for managing type 2 diabetes, and emerging data on treating obesity and prediabetes. We conclude with a glimpse of the future with possible complex interactions between nutrients, gut microbiota, the endocannabinoid system, and enteroendocrine cells.

Evaluation of a rare glucose-dependent insulinotropic polypeptide receptor variant in a patient with diabetes

AIMS

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone that augments insulin secretion in pancreatic β-cells via its glucose-dependent insulinotropic polypeptide receptor (GIPR). Recent genome-wide association studies identified a single nucleotide variant (SNV) in the GIPR encoding gene (GIPR), rs1800437, that is associated with obesity and insulin resistance. In the present study, we tested whether GIPR variants contribute to obesity and disturb glucose homeostasis or diabetes in specific patient populations.

MATERIALS AND METHODS

Exon sequencing of GIPR was performed in 164 children with obesity and insulin resistance and in 80 children with paediatric-onset diabetes of unknown origin. The Study of Health in Pomerania (SHIP) cohort, comprising 8320 adults, was screened for the GIPR variant Arg217Leu. GIPR variants were expressed in COS-7 cells and cAMP production was measured upon stimulation with GIP. Cell surface expression was determined by ELISA. Protein homology modelling of the GIPR variants was performed to extract three-dimensional information of the receptor.

RESULTS

A heterozygous missense GIPR variant Arg217Leu (rs200485112) was identified in a patient of Asian ancestry. Functional characterization of Arg217Leu revealed reduced surface expression and signalling after GIP challenge. The homology model of the GIPR structure supports the observed functional relevance of Arg217Leu.

CONCLUSION

In vitro functional studies and protein homology modelling indicate a potential relevance of the GIPR variant Arg217Leu in receptor function. The heterozygous variant displayed partial co-segregation with diabetes. Based on these findings, we suggest that GIPR variants may play a role in disturbed glucose homeostasis and may be of clinical relevance in homozygous patients.

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.

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.

Protein-altering variants associated with body mass index implicate pathways that control energy intake and expenditure in obesity

Genome-wide association studies (GWAS) have identified >250 loci for body mass index (BMI), implicating pathways related to neuronal biology. Most GWAS loci represent clusters of common, noncoding variants from which pinpointing causal genes remains challenging. Here we combined data from 718,734 individuals to discover rare and low-frequency (minor allele frequency (MAF) < 5%) coding variants associated with BMI. We identified 14 coding variants in 13 genes, of which 8 variants were in genes (ZBTB7B, ACHE, RAPGEF3, RAB21, ZFHX3, ENTPD6, ZFR2 and ZNF169) newly implicated in human obesity, 2 variants were in genes (MC4R and KSR2) previously observed to be mutated in extreme obesity and 2 variants were in GIPR. The effect sizes of rare variants are ~10 times larger than those of common variants, with the largest effect observed in carriers of an MC4R mutation introducing a stop codon (p.Tyr35Ter, MAF = 0.01%), who weighed ~7 kg more than non-carriers. Pathway analyses based on the variants associated with BMI confirm enrichment of neuronal genes and provide new evidence for adipocyte and energy expenditure biology, widening the potential of genetically supported therapeutic targets in obesity.

Dietary steamed wheat bran increases postprandial fat oxidation in association with a reduced blood glucose-dependent insulinotropic polypeptide response in mice

Obesity is a global epidemic associated with a higher risk of cardiovascular disease and metabolic disorders, such as type 2 diabetes. Previous studies demonstrated that chronic feeding of steamed wheat bran (WB) decreases obesity. To clarify the underlying mechanism and the responsible component for the anti-obesity effects of steamed WB, we investigated the effects of dietary steamed WB and arabinoxylan on postprandial energy metabolism and blood variables. Overnight-fasted male C57BL/6J mice were fed an isocaloric diet with or without steamed WB (30%). Energy metabolism was evaluated using an indirect calorimeter, and plasma glucose, insulin, and glucose-dependent insulinotropic polypeptide (GIP) levels were measured for 120 min after feeding. We similarly investigated the effect of arabinoxylan, a major component of steamed WB. Mice fed the WB diet had higher postprandial fat oxidation and a lower blood GIP response compared with mice fed the control diet. Mice fed the arabinoxylan diet exhibited a dose-dependent postprandial blood GIP response; increasing the arabinoxylan content in the diet led to a lower postprandial blood GIP response. The arabinoxylan-fed mice also had higher fat oxidation and energy expenditure compared with the control mice. In conclusion, the findings of the present study revealed that dietary steamed WB increases fat oxidation in mice. Increased fat oxidation may have a significant role in the anti-obesity effects of steamed WB. The postprandial effects of steamed WB are due to arabinoxylan, a major component of WB. The reduction of the postprandial blood GIP response may be responsible for the increase in postprandial fat utilization after feeding on a diet containing steamed WB and arabinoxylan.

Gut: A key player in the pathogenesis of type 2 diabetes?

The gut regulates glucose and energy homeostasis; thus, the presence of ingested nutrients into the gut activates sensing mechanisms that affect both glucose homeostasis and regulate food intake. Increasing evidence suggest that gut may also play a key role in the pathogenesis of type 2 diabetes which may be related to both the intestinal microbiological profile and patterns of gut hormones secretion. Intestinal microbiota includes trillions of microorganisms but its composition and function may be adversely affected in type 2 diabetes. The intestinal microbiota may be responsible of the secretion of molecules that may impair insulin secretion/action. At the same time, intestinal milieu regulates the secretion of hormones such as GLP-1, GIP, ghrelin, gastrin, somatostatin, CCK, serotonin, peptide YY, GLP-2, all of which importantly influence metabolism in general and in particular glucose metabolism. Thus, the aim of this paper is to review the current evidence on the role of the gut in the pathogenesis of type 2 diabetes, taking into account both hormonal and microbiological aspects.

Attenuated secretion of glucose-dependent insulinotropic polypeptide (GIP) does not alleviate hyperphagic obesity and insulin resistance in ob/ob mice

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) is released during meals and promotes nutrient uptake and storage. GIP receptor knockout mice are protected from diet induced weight gain and thus GIP antagonists have been proposed as a treatment for obesity. In this study, we assessed the role of GIP in hyperphagia induced obesity and metabolic abnormalities in leptin deficient (Lep^ob/ob) mice.

METHODS

We crossbred GIP-GFP knock-in homozygous mice (GIP^gfp/gfp) that have complete GIP knockout, and mice heterozygous for the ob mutation (Lep^ob/+) mice to generate Lep^ob/+/GIP^+/+, Lep^ob/ob/GIP^+/+, and Lep^ob/ob/GIP^gfp/gfp mice. Male animals were weighed weekly and both oral glucose and insulin tolerance testing were performed to assess glucose homeostasis and circulating profiles of GIP and insulin. Body composition was evaluated by computerized tomography (CT) scan and analyses of indirect calorimetry and locomotor activity were performed.

RESULTS

Postprandial GIP levels were markedly elevated in Lep^ob/ob/GIP^+/+ mice compared to Lep^ob/+/GIP^+/+ controls and were undetectable in Lep^ob/ob/GIP^gfp/gfp mice. Insulin levels were equivalently elevated in both Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice compared to controls at 8 weeks of age but the hyperinsulinemia was marginally reduced in Lep^ob/ob/GIP^gfp/gfp by 21 weeks, in association with amelioration of glucose intolerance. Both Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice remained equivalently insulin resistant. Body weight gain and subcutaneous and visceral fat volume of both Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice were significantly higher than that of Lep^ob/+/GIP^+/+ mice, while no significant differences were seen between Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice. Locomotor activity and energy expenditure were decreased in both Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice compared to control Lep^ob/+/GIP^+/+ mice, while no significant differences were seen between Lep^ob/ob/GIP^+/+ and Lep^ob/ob/GIP^gfp/gfp mice. There was no significant difference in fat oxidation among the three groups. Fat content in liver was significantly lower in Lep^ob/ob/GIP^gfp/gfp compared to Lep^ob/ob/GIP^+/+ mice, while that of control Lep^ob/+/GIP^+/+ mice was the lowest.

CONCLUSIONS

Our results indicate that GIP knockout does not prevent excess weight gain and metabolic derangement in hyperphagic leptin deficient mice.

Lifestyle Vaccines and Public Health: Exploring Policy Options for a Vaccine to Stop Smoking

Experimental vaccines are being developed for the treatment of 'unhealthy lifestyles' and associated chronic illnesses. Policymakers and other stakeholders will have to deal with the ethical issues that this innovation path raises: are there morally justified reasons to integrate these innovative biotechnologies in future health policies? Should public money be invested in further research? Focusing on the case of an experimental nicotine vaccine, this article explores the ethical aspects of 'lifestyle vaccines' for public health. Based on findings from a qualitative study into a vaccine for smoking cessation, the article articulates possible value conflicts related to nicotine vaccination as an intervention in tobacco control. The 'vaccinization' of lifestyle disease piggybacks on the achievements of classic vaccines. Contrary to expectations of simplicity and success, quitting smoking with a vaccine requires a complex supportive network. Social justice and public trust may become important ethical challenges when deciding whether to use further public funds for research or whether to implement these innovative vaccines in the future.

Virus-based nanoparticles as platform technologies for modern vaccines

Nanoscale engineering is revolutionizing the development of vaccines and immunotherapies. Viruses have played a key role in this field because they can function as prefabricated nanoscaffolds with unique properties that are easy to modify. Viruses are immunogenic via multiple pathways, and antigens displayed naturally or by engineering on the surface can be used to create vaccines against the cognate virus, other pathogens, specific molecules or cellular targets such as tumors. This review focuses on the development of virus-based nanoparticle systems as vaccines indicated for the prevention or treatment of infectious diseases, chronic diseases, cancer, and addiction. WIREs Nanomed Nanobiotechnol 2016, 8:554-578. doi: 10.1002/wnan.1383 For further resources related to this article, please visit the WIREs website.

Mechanisms underlying glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 secretion

The incretin hormones, glucose‐dependent insulinotropic peptide and glucagon‐like peptide‐1, are secreted from intestinal K‐ and L cells, respectively, with the former being most abundant in the proximal small intestine, whereas the latter increase in number towards the distal gut. Although an overlap between K‐ and L cells can be observed immunohistochemically or in murine models expressing fluorescent markers under the control of the two hormone promoters, the majority (>80%) of labeled cells seems to produce only one of these hormones. Transcriptomic analysis showed a close relationship between small intestinal K‐ and L cells, and glucose sensing mechanisms appear similar in both cell types with a predominant role of electrogenic glucose uptake through sodium‐coupled glucose transporter 1. Similarly, both cell types produce the long‐chain fatty acid sensing G‐protein‐coupled receptors, FFAR1 (GPR40) and FFAR4 (GPR120), but differ in the expression/functionality of other lipid sensing receptors. GPR119 and FFAR2/3, for example, have clearly documented roles in glucagon‐like peptide‐1 secretion, whereas agonists for the endocannabinoid receptor type 1 have been found to show largely selective inhibition of glucose‐dependent insulinotropic peptide secretion. In conclusion, although K‐ and L cell populations overlap and share key molecular nutrient‐sensing mechanisms, subtle differences between the responsiveness of the different cell types might be exploited to differentially modulate glucose‐dependent insulinotropic peptide or glucagon‐like peptide‐1 secretion.

Gastric inhibitory polypeptide immunoneutralization attenuates development of obesity in mice

Previous reports have suggested that the abrogation of gastric inhibitory polypeptide (GIP) signaling could be exploited to prevent and treat obesity and obesity-related disorders in humans. This study was designed to determine whether immunoneutralization of GIP, using a newly developed specific monoclonal antibody (mAb), would prevent the development of obesity. Specific mAb directed against the carboxy terminus of mouse GIP was identified, and its effects on the insulin response to oral and to intraperitoneal (ip) glucose and on weight gain were evaluated. Administration of mAb (30 mg/kg body wt, BW) to mice attenuated the insulin response to oral glucose by 70% and completely eliminated the response to ip glucose coadministered with human GIP. Nine-week-old C57BL/6 mice injected with GIP mAbs (60 mg·kg BW(-1)·wk(-1)) for 17 wk gained 46.5% less weight than control mice fed an identical high-fat diet (P < 0.001). No significant differences in the quantity of food consumed were detected between the two treatment groups. Furthermore, magnetic resonance imaging demonstrated that subcutaneous, omental, and hepatic fat were 1.97-, 3.46-, and 2.15-fold, respectively, lower in mAb-treated animals than in controls. Moreover, serum insulin, leptin, total cholesterol (TC), low-density lipoprotein (LDL), and triglycerides were significantly reduced, whereas the high-density lipoprotein (HDL)/TC ratio was 1.25-fold higher in treated animals than in controls. These studies support the hypothesis that a reduction in GIP signaling using a GIP-neutralizing mAb might provide a useful method for the treatment and prevention of obesity and related disorders.

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.

The application of virus-like particles as vaccines and biological vehicles

Virus-like particles (VLPs) can be spontaneously self-assembled by viral structural proteins under appropriate conditions in vitro while excluding the genetic material and potential replication probability. In addition, VLPs possess several features including can be rapidly produced in large quantities through existing expression systems, highly resembling native viruses in terms of conformation and appearance, and displaying repeated cluster of epitopes. Their capsids can be modified via genetic insertion or chemical conjugation which facilitating the multivalent display of a homologous or heterogeneous epitope antigen. Therefore, VLPs are considered as a safe and effective candidate of prophylactic and therapeutic vaccines. VLPs, with a diameter of approximately 20 to 150 nm, also have the characteristics of nanometer materials, such as large surface area, surface-accessible amino acids with reactive moieties (e.g., lysine and glutamic acid residues), inerratic spatial structure, and good biocompatibility. Therefore, assembled VLPs have great potential as a delivery system for specifically carrying a variety of materials. This review summarized recent researches on VLP development as vaccines and biological vehicles, which demonstrated the advantages and potential of VLPs in disease control and prevention and diagnosis. Then, the prospect of VLP biology application in the future is discussed as well.

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

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

Incretins: their physiology and application in the treatment of diabetes mellitus

Therapies targeting the action of incretin hormones have been under close scrutiny in recent years. The incretin effect has been defined as postprandial enhancement of insulin secretion by gut-derived factors. Likewise, incretin mimetics and incretin effect amplifiers are the two different incretin-based treatment strategies developed for the treatment of diabetes. Although, incretin mimetics produce effects very similar to those of natural incretin hormones, incretin effect amplifiers act by inhibiting dipeptidyl peptidase-4 (DPP-4) enzyme to increase plasma concentration of incretins and their biologic effects. Because glucagon-like peptide-1 (GLP-1) is an incretin hormone with various anti-diabetic actions including stimulation of glucose-induced insulin secretion, inhibition of glucagon secretion, hepatic glucose production and gastric emptying, it has been evaluated as a novel therapeutic agent for the treatment of type 2 diabetes mellitus (T2DM). GLP-1 also manifests trophic effects on pancreas such as pancreatic beta cell growth and differentiation. Because DPP-4 is the enzyme responsible for the inactivation of GLP-1, DPP-4 inhibition represents another potential strategy to increase plasma concentration of GLP-1 to enhance the incretin effect. Thus, anti-diabetic properties of these two classes of drugs have stimulated substantial clinical interest in the potential of incretin-based therapeutic agents as a means to control glucose homeostasis in T2DM patients. Despite this fact, clinical use of GLP-1 mimetics and DPP-4 inhibitors have raised substantial concerns owing to possible side effects of the treatments involving increased risk for pancreatitis, and C-cell adenoma/carcinoma. Thus, controversial issues in incretin-based therapies under development are reviewed and discussed in this manuscript.

Glucose-dependent insulinotropic polypeptide signaling in pancreatic β-cells and adipocytes

Glucose‐dependent insulinotropic polypeptide (GIP) was the first incretin to be identified. In addition to stimulating insulin secretion, GIP plays regulatory roles in the maintenance, growth and survival of pancreatic islets, as well as impacting on adipocyte function. The current review focuses on the intracellular signaling pathways by which GIP contributes to the regulation of β‐cell secretion and survival, and adipocyte differentiation and lipogenesis. Studies on signaling underlying the insulinotropic actions of the incretin hormones have largely been carried out with glucagon‐like peptide‐1. They have provided evidence for contributions by both protein kinase A (PKA) and exchange protein directly activated by cyclic adenosine monophosphate (EPAC2), and their probable role in GIP signaling is discussed. Recent studies have shown that inhibition of the kinase apoptosis signal‐regulating kinase 1 (ASK1) by GIP plays a key role in reducing mitochondria‐induced apoptosis in β‐cells through protein kinase B (PKB)‐mediated pathways, and that GIP‐induced post‐translational modification of voltage‐ dependent K⁺ (Kv) channels also contributes to its prosurvival role. Through regulation of gene expression, GIP tips the balance between pro‐ and anti‐apoptotic members of the B‐cell lymphoma‐2 (Bcl‐2) protein family towards β‐cell survival. GIP also plays important roles in the differentiation of pre‐adipocytes to adipocytes, and in the regulation of lipoprotein lipase expression and lipogenesis. These events involve interactions between GIP, insulin and resistin signaling pathways. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00196.x, 2012)

Disruption of GIP/GIPR axis in human adipose tissue is linked to obesity and insulin resistance

CONTEXT

Glucose-dependent insulinotropic peptide (GIP) has a central role in glucose homeostasis through its amplification of insulin secretion; however, its physiological role in adipose tissue is unclear.

OBJECTIVE

Our objective was to define the function of GIP in human adipose tissue in relation to obesity and insulin resistance.

DESIGN

GIP receptor (GIPR) expression was analyzed in human sc adipose tissue (SAT) and visceral adipose (VAT) from lean and obese subjects in 3 independent cohorts. GIPR expression was associated with anthropometric and biochemical variables. GIP responsiveness on insulin sensitivity was analyzed in human adipocyte cell lines in normoxic and hypoxic environments as well as in adipose-derived stem cells obtained from lean and obese patients.

RESULTS

GIPR expression was downregulated in SAT from obese patients and correlated negatively with body mass index, waist circumference, systolic blood pressure, and glucose and triglyceride levels. Furthermore, homeostasis model assessment of insulin resistance, glucose, and G protein-coupled receptor kinase 2 (GRK2) emerged as variables strongly associated with GIPR expression in SAT. Glucose uptake studies and insulin signaling in human adipocytes revealed GIP as an insulin-sensitizer incretin. Immunoprecipitation experiments suggested that GIP promotes the interaction of GRK2 with GIPR and decreases the association of GRK2 to insulin receptor substrate 1. These effects of GIP observed under normoxia were lost in human fat cells cultured in hypoxia. In support of this, GIP increased insulin sensitivity in human adipose-derived stem cells from lean patients. GIP also induced GIPR expression, which was concomitant with a downregulation of the incretin-degrading enzyme dipeptidyl peptidase 4. None of the physiological effects of GIP were detected in human fat cells obtained from an obese environment with reduced levels of GIPR.

CONCLUSIONS

GIP/GIPR signaling is disrupted in insulin-resistant states, such as obesity, and normalizing this function might represent a potential therapy in the treatment of obesity-associated metabolic disorders.

Obesity vaccines

Obesity is one of the largest and fastest growing public health problems in the world. Last century social changes have set an obesogenic milieu that calls for micro and macro environment interventions for disease prevention, while treatment is mandatory for individuals already obese. The cornerstone of overweight and obesity treatment is diet and physical exercise. However, many patients find lifestyle modifications difficult to comply and prone to failure in the long-term; therefore many patients consider anti-obesity drugs an important adjuvant if not a better alternative to behavioral approach or obesity surgery. Since the pharmacological options for obesity treatment remain quite limited, this is an exciting research area, with new treatment targets and strategies on the horizon. This review discusses the development of innovative therapeutic agents, focusing in energy homeostasis regulation and the use of molecular vaccines, targeting hormones such as somatostatin, GIP and ghrelin, to reduce body weight.

Functional analysis of seven genes linked to body mass index and adiposity by genome-wide association studies: a review

Genome-wide association studies (GWAS) have identified a total of about 40 single nucleotide polymorphisms (SNPs) that show significant linkage to body mass index, a widely utilised surrogate measure of adiposity. However, only 8 of these associations have been confirmed by follow-up GWAS using more sophisticated measures of adiposity (computed tomography). Among these 8, there is a SNP close to the gene FTO which has been the subject of considerable work to diagnose its function. The remaining 7 SNPs are adjacent to, or within, the genes NEGR1, TMEM18, ETV5, FLJ35779, LINGO2, SH2B1 and GIPR, most of which are less well studied than FTO, particularly in the context of obesity. This article reviews the available data on the functions of these genes, including information gleaned from studies in humans and animal models. At present, we have virtually no information on the putative mechanism associating the genes FLJ35779 and LINGO2 to obesity. All of these genes are expressed in the brain, and for 2 of them (SH2B1 and GIPR), a direct link to the appetite regulation system is known. SH2B1 is an enhancer of intracellular signalling in the JAK-STAT pathway, and GIPR is the receptor for an appetite-linked hormone (GIP) produced by the alimentary tract. NEGR1, ETV5 and SH2B1 all have suggested roles in neurite outgrowth, and hence SNPs adjacent to these genes may affect development of the energy balance circuitry. Although the genes have central patterns of gene expression, implying a central neuronal connection to energy balance, for at least 4 of them (NEGR1, TMEM18, SH2B1 and GIPR), there are also significant peripheral functions related to adipose tissue biology. These functions may contribute to their effects on the obese phenotype.

Structural and pharmacological characterization of novel potent and selective monoclonal antibody antagonists of glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an endogenous hormonal factor (incretin) that, upon binding to its receptor (GIPr; a class B G-protein-coupled receptor), stimulates insulin secretion by beta cells in the pancreas. There has been a lack of potent inhibitors of the GIPr with prolonged in vivo exposure to support studies on GIP biology. Here we describe the generation of an antagonizing antibody to the GIPr, using phage and ribosome display libraries. Gipg013 is a specific competitive antagonist with equally high potencies to mouse, rat, dog, and human GIP receptors with a K_i of 7 nm for the human GIPr. Gipg013 antagonizes the GIP receptor and inhibits GIP-induced insulin secretion in vitro and in vivo. A crystal structure of Gipg013 Fab in complex with the human GIPr extracellular domain (ECD) shows that the antibody binds through a series of hydrogen bonds from the complementarity-determining regions of Gipg013 Fab to the N-terminal α-helix of GIPr ECD as well as to residues around its highly conserved glucagon receptor subfamily recognition fold. The antibody epitope overlaps with the GIP binding site on the GIPr ECD, ensuring competitive antagonism of the receptor. This well characterized antagonizing antibody to the GIPr will be useful as a tool to further understand the biological roles of GIP.

Glucose metabolism is altered after loss of L cells and α-cells but not influenced by loss of K cells

The enteroendocrine K and L cells are responsible for secretion of glucose-dependent insulinotropic polypeptide (GIP) and glucagon like-peptide 1 (GLP-1), whereas pancreatic α-cells are responsible for secretion of glucagon. In rodents and humans, dysregulation of the secretion of GIP, GLP-1, and glucagon is associated with impaired regulation of metabolism. This study evaluates the consequences of acute removal of Gip- or Gcg-expressing cells on glucose metabolism. Generation of the two diphtheria toxin receptor cellular knockout mice, TgN(GIP.DTR) and TgN(GCG.DTR), allowed us to study effects of acute ablation of K and L cells and α-cells. Diphtheria toxin administration reduced the expression of Gip and content of GIP in the proximal jejunum in TgN(GIP.DTR) and expression of Gcg and content of proglucagon-derived peptides in both proximal jejunum and terminal ileum as well as content of glucagon in pancreas in TgN(GCG.DTR) compared with wild-type mice. GIP response to oral glucose was attenuated following K cell loss, but oral and intraperitoneal glucose tolerances were unaffected. Intraperitoneal glucose tolerance was impaired following combined L cell and α-cell loss and normal following α-cell loss. Oral glucose tolerance was improved following L cell and α-cell loss and supernormal following α-cell loss. We present two mouse models that allow studies of the effects of K cell or L cell and α-cell loss as well as isolated α-cell loss. Our findings show that intraperitoneal glucose tolerance is dependent on an intact L cell mass and underscore the diabetogenic effects of α-cell signaling. Furthermore, the results suggest that K cells are less involved in acute regulation of mouse glucose metabolism than L cells and α-cells.