Pharmacological characterization of human incretin receptor missense variants

A Robust Platform for the Molecular Design of Potent, Protease-Stable, Long-Acting GIP Analogues

Glucose-dependent insulinotropic peptide (GIP) is a 42-amino acid peptide hormone that regulates postprandial glucose levels. GIP binds to its cognate receptor, GIPR, and mediates metabolic physiology by improved insulin sensitivity, β-cell proliferation, increased energy consumption, and stimulated glucagon secretion. Dipeptidyl peptidase-4 (DPP4) catalyzes the rapid inactivation of GIP within 6 min in vivo. Here, we report a molecular platform for the design of GIP analogues that are refractory to DPP4 action and exhibit differential activation of the receptor, thus offering potentially hundreds of GIP-based compounds to fine-tune pharmacology. The lead compound from our studies, which harbored a combination of N-terminal alkylation and side-chain lipidation, was equipotent and retained full efficacy at GIPR as the native peptide, while being completely refractory toward DPP4, and was resistant to trypsin. The GIP analogue identified from these studies was further evaluated in vivo and is one of the longest-acting GIPR agonists to date.

Abolishing β-arrestin recruitment is necessary for the full metabolic benefits of G protein-biased glucagon-like peptide-1 receptor agonists

AIM

Earlier studies have shown that peptide glucagon-like peptide-1 receptor (GLP-1R) agonists with reduced β-arrestin recruitment show enhanced anti-hyperglycaemic efficacy through avoidance of GLP-1R desensitization. However, the ligand modifications needed to decrease β-arrestin recruitment usually also reduces GLP-1R affinity, therefore higher doses are needed. Here we aimed to develop new, long-acting, G protein-biased GLP-1R agonists with acute signalling potency comparable with semaglutide, to provide insights into specific experimental and therapeutic scenarios.

MATERIALS AND METHODS

New GLP-1R agonist peptides were assessed using a variety of in vitro and in vivo assays.

RESULTS

First, we show that very substantial reductions in β-arrestin recruitment efficacy are required to realize fully the benefits of GLP-1R agonism on blood glucose lowering in mice, with more moderate reductions being less effective. Secondly, our lead compound (SRB107) performs substantially better than semaglutide for effects on blood glucose and weight loss, which may be jointly attributable to its biased agonist action and protracted pharmacokinetics. Thirdly, we show that biased agonist-specific GLP-1R internalization profiles occur at clinically relevant pharmacological concentrations. Finally, we show that SRB107 cAMP signalling is differentially modulated by single and double GLP1R coding variants seen in human populations, with implications for GLP-1R agonist pharmacogenomics.

CONCLUSIONS

Completely abolishing β-arrestin recruitment improves the anti-hyperglycaemic effects of GLP-1R agonists in mice.

Rare Heterozygous Loss-of-Function Variants in the Human GLP-1 Receptor Are Not Associated With Cardiometabolic Phenotypes

CONTEXT

Lost glucagon-like peptide 1 receptor (GLP-1R) function affects human physiology.

OBJECTIVE

This work aimed to identify coding nonsynonymous GLP1R variants in Danish individuals to link their in vitro phenotypes and clinical phenotypic associations.

METHODS

We sequenced GLP1R in 8642 Danish individuals with type 2 diabetes or normal glucose tolerance and examined the ability of nonsynonymous variants to bind GLP-1 and to signal in transfected cells via cyclic adenosine monophosphate (cAMP) formation and β-arrestin recruitment. We performed a cross-sectional study between the burden of loss-of-signaling (LoS) variants and cardiometabolic phenotypes in 2930 patients with type 2 diabetes and 5712 participants in a population-based cohort. Furthermore, we studied the association between cardiometabolic phenotypes and the burden of the LoS variants and 60 partly overlapping predicted loss-of-function (pLoF) GLP1R variants found in 330 566 unrelated White exome-sequenced participants in the UK Biobank cohort.

RESULTS

We identified 36 nonsynonymous variants in GLP1R, of which 10 had a statistically significant loss in GLP-1-induced cAMP signaling compared to wild-type. However, no association was observed between the LoS variants and type 2 diabetes, although LoS variant carriers had a minor increased fasting plasma glucose level. Moreover, pLoF variants from the UK Biobank also did not reveal substantial cardiometabolic associations, despite a small effect on glycated hemoglobin A1c.

CONCLUSION

Since no homozygous LoS nor pLoF variants were identified and heterozygous carriers had similar cardiometabolic phenotype as noncarriers, we conclude that GLP-1R may be of particular importance in human physiology, due to a potential evolutionary intolerance of harmful homozygous GLP1R variants.

Acute pharmacodynamic responses to exenatide: Drug-induced increases in insulin secretion and glucose effectiveness

AIM

Glucagon-like peptide-1 receptor agonists provide multiple benefits to patients with type 2 diabetes, including improved glycaemic control, weight loss and decreased risk of major adverse cardiovascular events. Because drug responses vary among individuals, we initiated investigations to identify genetic variants associated with the magnitude of drug responses.

METHODS

Exenatide (5 μg, subcutaneously) or saline (0.2 ml, subcutaneously) was administered to 62 healthy volunteers. Frequently sampled intravenous glucose tolerance tests were conducted to assess the impact of exenatide on insulin secretion and insulin action. This pilot study was a crossover design in which participants received exenatide and saline in random order.

RESULTS

Exenatide increased first phase insulin secretion 1.9-fold (p = 1.9 × 10-9 ) and accelerated the rate of glucose disappearance 2.4-fold (p = 2 × 10-10 ). Minimal model analysis showed that exenatide increased glucose effectiveness (Sg ) by 32% (p = .0008) but did not significantly affect insulin sensitivity (Si ). The exenatide-induced increase in insulin secretion made the largest contribution to interindividual variation in exenatide-induced acceleration of glucose disappearance while interindividual variation in the drug effect on Sg contributed to a lesser extent (β = 0.58 or 0.27, respectively).

CONCLUSIONS

This pilot study provides validation for the value of a frequently sampled intravenous glucose tolerance test (including minimal model analysis) to provide primary data for our ongoing pharmacogenomic study of pharmacodynamic effects of semaglutide (NCT05071898). Three endpoints provide quantitative assessments of the effects of glucagon-like peptide-1 receptor agonists on glucose metabolism: first phase insulin secretion, glucose disappearance rates and glucose effectiveness.

Acute pharmacodynamic responses to exenatide: Drug-induced increases in insulin secretion and glucose effectiveness

Background

GLP1R agonists provide multiple benefits to patients with type 2 diabetes - including improved glycemic control, weight loss, and decreased risk of major adverse cardiovascular events. Because drug responses vary among individuals, we initiated investigations to identify genetic variants associated with the magnitude of drug responses.

Methods

Exenatide (5 µg, sc) or saline (0.2 mL, sc) was administered to 62 healthy volunteers. Frequently sampled intravenous glucose tolerance tests were conducted to assess the impact of exenatide on insulin secretion and insulin action. This pilot study was designed as a crossover study in which participants received exenatide and saline in random order.

Results

Exenatide increased first phase insulin secretion 1.9-fold (p=1.9×10 -9 ) and accelerated the rate of glucose disappearance 2.4-fold (p=2×10 -10 ). Minimal model analysis demonstrated that exenatide increased glucose effectiveness (S g ) by 32% (p=0.0008) but did not significantly affect insulin sensitivity (S i ). The exenatide-induced increase in insulin secretion made the largest contribution to inter-individual variation in exenatide-induced acceleration of glucose disappearance while inter-individual variation in the drug effect on S g contributed to a lesser extent (β=0.58 or 0.27, respectively).

Conclusions

This pilot study provides validation for the value of an FSIGT (including minimal model analysis) to provide primary data for our ongoing pharmacogenomic study of pharmacodynamic effects of semaglutide ( NCT05071898 ). Three endpoints provide quantitative assessments of GLP1R agonists' effects on glucose metabolism: first phase insulin secretion, glucose disappearance rates, and glucose effectiveness.

Registration

NCT02462421 (clinicaltrials.gov).

Funding

American Diabetes Association (1-16-ICTS-112); National Institute of Diabetes and Digestive and Kidney Disease (R01DK130238, T32DK098107, P30DK072488).

Association of GLP1R Polymorphisms With the Incretin Response

CONTEXT

Polymorphisms in the gene encoding the glucagon-like peptide-1 receptor (GLP1R) are associated with type 2 diabetes but their effects on incretin levels remain unclear.

OBJECTIVE

We evaluated the physiologic and hormonal effects of GLP1R genotypes before and after interventions that influence glucose physiology.

DESIGN

Pharmacogenetic study conducted at 3 academic centers in Boston, Massachusetts.

PARTICIPANTS

A total of 868 antidiabetic drug-naïve participants with type 2 diabetes or at risk for developing diabetes.

INTERVENTIONS

We analyzed 5 variants within GLP1R (rs761387, rs10305423, rs10305441, rs742762, and rs10305492) and recorded biochemical data during a 5-mg glipizide challenge and a 75-g oral glucose tolerance test (OGTT) following 4 doses of metformin 500 mg over 2 days.

MAIN OUTCOMES

We used an additive mixed-effects model to evaluate the association of these variants with glucose, insulin, and incretin levels over multiple timepoints during the OGTT.

RESULTS

During the OGTT, the G-risk allele at rs761387 was associated with higher total GLP-1 (2.61 pmol/L; 95% CI, 1.0.72-4.50), active GLP-1 (2.61 pmol/L; 95% CI, 0.04-5.18), and a trend toward higher glucose (3.63; 95% CI, -0.16 to 7.42 mg/dL) per allele but was not associated with insulin. During the glipizide challenge, the G allele was associated with higher insulin levels per allele (2.01 IU/mL; 95% CI, 0.26-3.76). The other variants were not associated with any of the outcomes tested.

CONCLUSIONS

GLP1R variation is associated with differences in GLP-1 levels following an OGTT load despite no differences in insulin levels, highlighting altered incretin signaling as a potential mechanism by which GLP1R variation affects T2D risk.

GLP-1R polymorphism (rs1042044) and expression are associated with the risk of papillary thyroid cancer among the Egyptian population

BACKGROUND

Glucagon like peptide-1 receptor (GLP-1R) agonist usage has previously been linked to an elevated incidence of thyroid cell adenomas and carcinomas in animals.

AIM

The goal of this study was to determine if there was an association between GLP-1R gene polymorphism and expression with the risk of papillary thyroid carcinoma (PTC) and its clinical characteristics among the Egyptian population.

MATERIAL AND METHODS

A total of eighty PTC patients and eighty healthy controls were included in the study. Real-time polymerase chain reaction (real-time PCR) and immunohistochemistry were used to determine GLP-1R expression in tumor tissue. The polymorphisms rs1042044 and rs6923761 in the GLP-1R gene were determined using PCR -restriction fragment length polymorphism (PCR-RFLP).

RESULTS

PTC patients exhibited considerably greater frequencies of rs1042044 AA genotypes and A allele than controls (OR (95% CI) = 4.5 (1.75-11.8), P < 0.001; OR (95% CI) = 2.032 (1.301-3.17), P < 0.001 respectively). GLP-1R mRNA and protein expressions were higher in tumor samples than normal thyroid tissues among PTC patients. In addition, high GLP-1R expressions were more common in rs1042044 AA genotype carriers than CC carriers (P < 0.001). GLP-1R mRNA expression showed 95 % sensitivity and 97% specificity for PTC diagnosis. Moreover, GLP-1R expression was closely associated with LN metastasis, tumor size, tumor stage, and multifocality in PTC patients.

CONCLUSION

This research provides new evidence linking the GLP-1R genetic polymorphism and tissue expression to PTC risk and invasiveness among the Egyptian population.

Glucose-Dependent Insulinotropic Polypeptide-A Postprandial Hormone with Unharnessed Metabolic Potential

Glucose-dependent insulinotropic polypeptide (GIP) is released from the upper small intestine in response to food intake and contributes to the postprandial control of nutrient disposition, including of sugars and fats. Long neglected as a potential therapeutic target, the GIPR axis has received increasing interest recently, with the emerging data demonstrating the metabolically favorable outcomes of adding GIPR agonism to GLP-1 receptor agonists in people with type 2 diabetes and obesity. This review examines the physiology of the GIP axis, from the mechanisms underlying GIP secretion from the intestine to its action on target tissues and therapeutic development. Expected final online publication date for the Annual Review of Nutrition, Volume 42 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Therapeutic application of GLP-1 and GIP receptor agonists in Parkinson's disease

INTRODUCTION

Diabetes is a risk factor for Parkinson's disease (PD) and shares similar dysregulated insulin pathways. Glucagon-like peptide-1 (GLP-1) analogs originally designed to treat diabetes have shown potent neuroprotective activity in preclinical studies of PD. They are neuroprotective by inhibiting inflammation, improving neuronal survival, maintenance of synapses, and dopaminergic transmission in the brain. Building on this, three clinical studies have reported impressive effects in patients with PD, testing exendin-4 (Exenatide, Bydureon) or liraglutide (Victoza, Saxenda). Glucose-dependent insulinotropic peptide (GIP) is another peptide hormone that has shown good effects in animal models of PD. Novel dual GLP-1/GIP agonists have been developed that can penetrate the blood-brain barrier (BBB) and show superior effects in animal models compared to GLP-1 drugs.

AREAS COVERED

The review summarizes preclinical and clinical studies testing GLP-1R agonists and dual GLP-1/GIPR agonists in PD and discusses possible mechanisms of action.

EXPERT OPINION

Current strategies to treat PD by lowering the levels of alpha-synuclein have not shown effects in clinical trials. It is time to move on from the 'misfolding protein' hypothesis. Growth factors such as GLP-1 that can cross the BBB have already shown impressive effects in patients and are the future of drug discovery in PD.

Neuroprotective Mechanisms of Glucagon-Like Peptide-1-Based Therapies in Ischemic Stroke: An Update Based on Preclinical Research

The public and social health burdens of ischemic stroke have been increasing worldwide. Hyperglycemia leads to a greater risk of stroke. This increased risk is commonly seen among patients with diabetes and is in connection with worsened clinical conditions and higher mortality in patients with acute ischemic stroke (AIS). Therapy for stroke focuses mainly on restoring cerebral blood flow (CBF) and ameliorating neurological impairment caused by stroke. Although choices of stroke treatment remain limited, much advance have been achieved in assisting patients in recovering from ischemic stroke, along with progress of recanalization therapy through pharmacological and mechanical thrombolysis. However, it is still necessary to develop neuroprotective therapies for AIS to protect the brain against injury before and during reperfusion, prolong the time window for intervention, and consequently improve neurological prognosis. Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are broadly regarded as effective drugs in the treatment of type 2 diabetes mellitus (T2DM). Preclinical data on GLP-1 and GLP-1 RAs have displayed an impressive neuroprotective efficacy in stroke, Parkinson's disease (PD), Alzheimer's disease (AD), Amyotrophic lateral sclerosis (ALS), and other neurodegenerative diseases. Based on the preclinical studies in the past decade, we review recent progress in the biological roles of GLP-1 and GLP-1 RAs in ischemic stroke. Emphasis will be placed on their neuroprotective effects in experimental models of cerebral ischemia stroke at cellular and molecular levels.

GIPR rs10423928 and bone mineral density in postmenopausal women in Shanghai

We demonstrated previously that there is a correlation between glucagon-like peptide-1 (GLP-1) single-nucleotide polymorphism (SNP) and bone mineral density in postmenopausal women. Both GLP-1 and glucose-dependent insulinotropic peptide are incretins. The glucose-dependent insulinotropic peptide receptor (GIPR) SNP rs10423928 has been extensively studied. However, it is not clear whether GIPR gene mutations affect bone metabolism. The aim of this study was to investigate the association between rs10423928 and bone mineral density in postmenopausal women in Shanghai. rs10423928 was detected in 884 postmenopausal women in Shanghai, and the correlation between the GIPR SNP and bone mineral density was assessed. The dominant T/T genotype of rs10423928 was found to be related to the bone mineral density of the femoral neck (P = 0.035). Overall, our findings indicate that the dominant T/T genotype of rs10423928 in postmenopausal women is significantly associated with a higher bone mineral density and that the T/T genotype exerts a bone-protective effect.

Association of GLP1R variants rs2268641 and rs6923761 with obesity and other metabolic parameters in a Polish cohort

INTRODUCTION

Obesity is a complex disease associated with excessive fat accumulation and numerous metabolic complications. So far, many factors leading to the development of this disorder have been identified, including genetic susceptibility. Various studies linked GLP1R variants with anthropometric and metabolic parameters, suggesting the role of the variation in this gene in metabolic health.

OBJECTIVE

The aim of this study is to investigate the association of two single nucleotide variants of GLP1R gene, rs2268641 and rs6923761, with excessive weight, metabolic syndrome, anthropometric measurements and selected metabolic parameters.

METHODS

Normal-weight subjects (n= 340, control group) and subjects with excessive body mass (n = 600, study group) participated in this study. For all participants, anthropometric measurements and metabolic parameters were collected, and genotyping of the two single nucleotide variants of GLP1R gene, rs2268641 and rs6923761, was performed using the high-resolution melting curve analysis.

RESULTS

Significant differences in the genotype distribution of rs2268641 were found, where homozygous TT genotype was significantly less frequent in the study group with excessive body mass (OR=0.66; p=0.0298). For rs6923761, A allele and homozygous AA genotype were significantly more frequent in the study group with excessive weight than in the control group (OR=1.27; p=0.0239 and OR=1.69; p=0.0205, respectively). The association of studied variants with metabolic parameters was found for rs6923761. For this variant, AA carriers had higher body mass in comparison to GG carriers (p=0.0246), and AA carriers had higher glucose concentration in comparison to AG carriers (p=0.0498). We did not find an association of rs2268641 and rs6923761 with metabolic syndrome.

CONCLUSION

In our study, AA carriers of rs6923761 had higher risk of excessive body mass, whereas TT carriers of rs2268641 had lower risk of being overweight. Moreover, homozygous carriers of the minor allele of rs6923761 had higher glucose concentration in comparison to heterozygous subjects. None of the studied variants were associated with metabolic syndrome in the studied population.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Loss of Function Glucose-Dependent Insulinotropic Polypeptide Receptor Variants Are Associated With Alterations in BMI, Bone Strength and Cardiovascular Outcomes

Glucose-dependent insulinotropic polypeptide (GIP) and its receptor (GIPR) are involved in multiple physiological systems related to glucose metabolism, bone homeostasis and fat deposition. Recent research has surprisingly indicated that both agonists and antagonists of GIPR may be useful in the treatment of obesity and type 2 diabetes, as both result in weight loss when combined with GLP-1 receptor activation. To understand the receptor signaling related with weight loss, we examined the pharmacological properties of two rare missense GIPR variants, R190Q (rs139215588) and E288G (rs143430880) linked to lower body mass index (BMI) in carriers. At the molecular and cellular level, both variants displayed reduced G protein coupling, impaired arrestin recruitment and internalization, despite maintained high GIP affinity. The physiological phenotyping revealed an overall impaired bone strength, increased systolic blood pressure, altered lipid profile, altered fat distribution combined with increased body impedance in human carriers, thereby substantiating the role of GIP in these physiological processes.

Association of a glucagon-like peptide-1 receptor gene variant with glucose response to a mixed meal

Dipeptidyl peptidase-4 (DPP-4) inhibitors increase endogenous glucagon-like peptide-1 (GLP-1). We hypothesized that genetic variation in the gene encoding the GLP-1 receptor (GLP1R) could affect the metabolic response to DPP-4 inhibition. To evaluate the relationship between the GLP1R rs6923761 variant (G-to-A nucleic acid substitution) and metabolic responses, we performed mixed meal studies in individuals with type 2 diabetes mellitus and hypertension after 7-day treatment with placebo and the DPP-4 inhibitor sitagliptin. This analysis is a substudy of NCT02130687. The genotype frequency was 13:12:7 GG:GA:AA among individuals of European ancestry. Postprandial glucose excursion was significantly decreased in individuals carrying the rs6923761 variant (GA or AA) as compared with GG individuals during both placebo (P = 0.001) and sitagliptin treatment (P = 0.045), while intact GLP-1 levels were similar among the genotype groups. In contrast, sitagliptin lowered postprandial glucose to a greater degree in GG as compared with GA/AA individuals (P = 0.035). The relationship between GLP1R rs6923761 genotype and therapies that modulate GLP-1 signalling merits study in large populations.

Chronic glucose-dependent insulinotropic polypeptide receptor (GIPR) agonism desensitizes adipocyte GIPR activity mimicking functional GIPR antagonism

Antagonism or agonism of the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) prevents weight gain and leads to dramatic weight loss in combination with glucagon-like peptide-1 receptor agonists in preclinical models. Based on the genetic evidence supporting GIPR antagonism, we previously developed a mouse anti-murine GIPR antibody (muGIPR-Ab) that protected diet-induced obese (DIO) mice against body weight gain and improved multiple metabolic parameters. This work reconciles the similar preclinical body weight effects of GIPR antagonists and agonists in vivo, and here we show that chronic GIPR agonism desensitizes GIPR activity in primary adipocytes, both differentiated in vitro and adipose tissue in vivo, and functions like a GIPR antagonist. Additionally, GIPR activity in adipocytes is partially responsible for muGIPR-Ab to prevent weight gain in DIO mice, demonstrating a role of adipocyte GIPR in the regulation of adiposity in vivo.

GLP1R Single-Nucleotide Polymorphisms rs3765467 and rs10305492 Affect β Cell Insulin Secretory Capacity and Apoptosis Through GLP-1

The increased secretion of glucagon-like peptide-1 (GLP-1) after Roux-en-Y gastric bypass (RYGB) is regarded as the main reason for the improvement of blood glucose. However, the single-nucleotide polymorphisms (SNPs) of GLP-1 Receptor (GLP1R) impair receptor function, subsequently affecting β cell insulin secretion function, ultimately affecting the efficacy of RYGB. In this study, we revealed that two SNPs in GLP1R gene, rs3765467 and rs10305492, could significantly reduce the insulin secreted by β cells and the cyclic AMP concentration, whereas promote β cell apoptosis. Under high glucose exposure, rs3765467 and rs10305492 impaired β cell secretion of insulin and β cell viability in the same way; in other words, GLP1R rs3765467 and rs10305492 exert an effect on pancreatic β cell glucose-stimulated insulin secretion. Moreover, GLP-1 antagonist Exendin (9-39) further enhanced, whereas GLP-1 agonist Exendin-4 partially attenuated the effects of SNPs on the functions and apoptosis of β cells. In conclusion, the rs3765467 and rs10305492 SNPs in GLP1R show to exert a critical effect on regulating insulin secretory capacity of β cells and β cell mass. Through leading to the dysfunction and apoptosis of β cells, GLP1R rs3765467 and rs10305492 might also impair GLP-1 interaction with GLP1R, therefore attenuating the therapeutic effect of RYGB.

Enteroendocrine K Cells Exert Complementary Effects to Control Bone Quality and Mass in Mice

The involvement of a gut-bone axis in controlling bone physiology has been long suspected, although the exact mechanisms are unclear. We explored whether glucose-dependent insulinotropic polypeptide (GIP)-producing enteroendocrine K cells were involved in this process. The bone phenotype of transgenic mouse models lacking GIP secretion (GIP-GFP-KI) or enteroendocrine K cells (GIP-DT) was investigated. Mice deficient in GIP secretion exhibited lower bone strength, trabecular bone mass, trabecular number, and cortical thickness, notably due to higher bone resorption. Alterations of microstructure, modifications of bone compositional parameters, represented by lower collagen cross-linking, were also apparent. None of these alterations were observed in GIP-DT mice lacking enteroendocrine K cells, suggesting that another K-cell secretory product acts to counteract GIP action. To assess this, stable analogues of the known K-cell peptide hormones, xenin and GIP, were administered to mature NIH Swiss male mice. Both were capable of modulating bone strength mostly by altering bone microstructure, bone gene expression, and bone compositional parameters. However, the two molecules exhibited opposite actions on bone physiology, with evidence that xenin effects are mediated indirectly, possibly via neural networks. Our data highlight a previously unknown interaction between GIP and xenin, which both moderate gut-bone connectivity. © 2020 American Society for Bone and Mineral Research.

Recent advances of GIP and future horizons

Recently GIP-GLP-1 co-agonists with powerful effects on glycemic control and body weight in patients with type 2 diabetes have been described. While such effects are the expected ones from a glucagonlike peptide-1 receptor agonist, similar contributions from the GIP component of the co-agonist would be surprising and contrast to the existing literature. Conventionally, GIP is thought of as an important incretin hormone regulating postprandial insulin secretion in glucose tolerant individuals, but such effects are weak or absent in patients with type 2 diabetes, and GIP has been proposed to an obesity-promoting hormone, rather than the opposite. Recent studies with a GIP receptor antagonist suitable for human studies have confirmed these concepts regarding the actions of endogenous GIP and point to potential beneficial metabolic effects of GIP receptor antagonists rather than agonist in the treatment of obesity and type 2 diabetes. So how is it possible that apparently similar results can be obtained with GIP receptor agonists and antagonists? Maybe the explanation should be sought in GIP receptor dynamics, where the agonists clearly elicit beta-arrestin mediated receptor internalization, rendering the target tissues unresponsive, whereas antagonists block the internalization and increase receptor expression on the cell surfaces. This may explain that both antagonists and agonists show efficacy in obesity and type 2 diabetes.

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.

Enhanced agonist residence time, internalization rate and signalling of the GIP receptor variant [E354Q] facilitate receptor desensitization and long-term impairment of the GIP system

In patients with type 2 diabetes mellitus (T2DM), the insulinotropic action of the GIP system is desensitized, whereas this is not the case for the GLP‐1 system. This has raised an interesting discussion of whether GIP agonists or antagonists are most suitable for future treatment of T2DM together with GLP‐1‐based therapies. Homozygous carriers of the GIP receptor (GIPR) variant, [E354Q], display lower bone mineral density, increased bone fracture risk and slightly increased blood glucose. Here, we present an in‐depth molecular pharmacological phenotyping of GIPR‐[E354Q]. In silico modelling suggested similar interaction of the endogenous agonist GIP(1‐42) to [E354Q] as to GIPR wt. This was supported by homologous competition binding in COS‐7 cells revealing GIPR wt‐like affinities of GIP(1‐42) with K_d values of ~2 nmol/L and wt‐like agonist association rates (K_on). In contrast, the dissociation rates (K_off) were slower, resulting in 25% higher agonist residence time for GIPR‐[E354Q]. Moreover, in G_αs signalling (cAMP production) GIP(1‐42) was ~2‐fold more potent and more efficacious on GIPR‐[E354Q] compared to wt with 17.5% higher basal activity. No difference from GIPR wt was found in the recruitment of β‐arrestin 2, whereas the agonist‐induced internalization rate was 2.1‐ to 2.3‐fold faster for [E354Q]. Together with the previously described impaired recycling of [E354Q], our findings with enhanced signalling and internalization rate possibly explained by an altered ligand‐binding kinetics will lead to receptor desensitization and down‐regulation. This could explain the long‐term functional impairment of the GIP system in bone metabolism and blood sugar maintenance for [E354Q] carriers and may shed light on the desensitization of the insulinotropic action of GIP in patients with T2DM.

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.

Pharmacogenetics of type 2 diabetes mellitus, the route toward tailored medicine

Type 2 diabetes mellitus (T2DM) is a chronic disease that has reached the levels of a global epidemic. In order to achieve optimal glucose control, it is often necessary to rely on combination therapy of multiple drugs or insulin because uncontrolled glucose levels result in T2DM progression and enhanced risk of complications and mortality. Several antihyperglycemic agents have been developed over time, and T2DM pharmacotherapy should be prescribed based on suitability for the individual patient's characteristics. Pharmacogenetics is the branch of genetics that investigates how our genome influences individual responses to drugs, therapeutic outcomes, and incidence of adverse effects. In this review, we evaluated the pharmacogenetic evidences currently available in the literature, and we identified the top informative genetic variants associated with response to the most common anti-diabetic drugs: metformin, DPP-4 inhibitors/GLP1R agonists, thiazolidinediones, and sulfonylureas/meglitinides. Overall, we found 40 polymorphisms for each drug class in a total of 71 loci, and we examined the possibility of encouraging genetic screening of these variants/loci in order to critically implement decision-making about the therapeutic approach through precision medicine strategies. It is possible then to anticipate that when the clinical practice will take advantage of the genetic information of the diabetic patients, this will provide a useful resource for the prevention of T2DM progression, enabling the identification of the precise drug that is most likely to be effective and safe for each patient and the reduction of the economic impact on a global scale.

Incretin-based therapy for the treatment of bone fragility in diabetes mellitus

Bone fractures are common comorbidities of type 2 diabetes mellitus (T2DM). Bone fracture incidence seems to develop due to increased risk of falls, poor bone quality and/or anti-diabetic medications. Previously, a relation between gut hormones and bone has been suspected. Most recent evidences suggest indeed that two gut hormones, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), may control bone remodeling and quality. The GIP receptor is expressed in bone cells and knockout of either GIP or its receptor induces severe bone quality alterations. Similar alterations are also encountered in GLP-1 receptor knock-out animals associated with abnormal osteoclast resorption. Some GLP-1 receptor agonist (GLP-1RA) have been approved for the treatment of type 2 diabetes mellitus and although clinical trials may not have been designed to investigate bone fracture, first results suggest that GLP-1RA may not exacerbate abnormal bone quality observed in T2DM. The recent design of double and triple gut hormone agonists may also represent a suitable alternative for restoring compromised bone quality observed in T2DM. However, although most of these new molecules demonstrated weight loss action, little is known on their bone safety. The present review summarizes the most recent findings on peptide-based incretin therapy and bone physiology.

rs6923761 gene variant in glucagon-like peptide 1 receptor: Allelic frequencies and influence on cardiovascular risk factors in a multicenter study of Castilla-Leon

BACKGROUND

Some GLP-1 receptor studies have identified polymorphisms in the GLP-1 receptor gene that might be related to different cardiovascular risk factors.

OBJECTIVE

Our aim was to investigate the allelic distribution of rs6923761 GLP-1 receptor polymorphism in a geographic area of Spain (Community of Castilla y Leon) and to evaluate the influence of this polymorphism on obesity anthropometric parameters and cardiovascular risk factors in the fasted state in obese patients.

DESIGN

A sample of 341 obese subjects (body mass index ≥ 30 kg/m²) was analyzed. Fasting blood glucose, C-reactive protein (CRP), plasma insulin, insulin resistance (HOMA-IR), and lipid profile were determined. Anthropometric parameters, dietary intake and blood pressure were recorded.

RESULTS

One hundred and forty three patients (42.0%) had the genotype GG (wild-type group) and one hundred and ninety eight (58.0%) patients were A carriers: GA (164 patients, 48.1%) or AA (34 patients, 9.9%) (mutant-type group). Valladolid and Segovia health areas had the lowest percentage of wild type genotype and G allelic (than other Health Areas). Burgos Health Area had a higher percentage of wild-type genotype. In wild-type group (GG genotype), BMI (0.9 ± 1.3 kg/m²; p < 0.05), weight (3.3 ± 1.1 kg; p < 0.05), fat mass (2.5 ± 1.1 kg; p < 0.05), waist to hip ratio (0.02 ± 0.005 cm; p < 0.05), waist circumference (2.8 ± 1.1 cm; p < 0.05), triglycerides (14.4 ± 3.3 mg/dl; p < 0.05) insulin (3.1 ± 1.0 mg/dl; p < 0.05) and HOMA-IR (1.2 ± 0.9 mg/dl; p < 0.05) were higher than A allele carriers. In non A allele carriers, lower HDL cholesterol levels than A allele carriers (6.4 ± 2.3 mg/dl; p < 0.05) were found.

CONCLUSION

Data from our study revealed different allelic distribution in this geographic area, with better parameters (Body mass index, weight, fat mass, waist circumference, triglycerides, insulin, HOMA-IR and HDL cholesterol) in A allele carriers than in non A allele carriers.

Gαs regulates Glucagon-Like Peptide 1 Receptor-mediated cyclic AMP generation at Rab5 endosomal compartment

OBJECTIVE

Upon activation, G protein coupled receptors (GPCRs) associate with heterotrimeric G proteins at the plasma membrane to initiate second messenger signaling. Subsequently, the activated receptor experiences desensitization, internalization, and recycling back to the plasma membrane, or it undergoes lysosomal degradation. Recent reports highlight specific cases of persistent cyclic AMP generation by internalized GPCRs, although the functional significance and mechanistic details remain to be defined. Cyclic AMP generation from internalized Glucagon-Like Peptide-1 Receptor (GLP-1R) has previously been reported from our laboratory. This study aimed at deciphering the molecular mechanism by which internalized GLP-R supports sustained cyclic AMP generation upon receptor activation in pancreatic beta cells.

METHODS

We studied the time course of cyclic AMP generation following GLP-1R activation with particular emphasis on defining the location where cyclic AMP is generated. Detection involved a novel GLP-1 conjugate coupled with immunofluorescence using specific endosomal markers. Finally, we employed co-immunoprecipitation as well as immunofluorescence to assess the protein-protein interactions that regulate GLP-1R mediated cyclic AMP generation at endosomes.

RESULTS

Our data reveal that prolonged association of G protein α subunit Gαs with activated GLP-1R contributed to sustained cyclic AMP generation at Rab 5 endosomal compartment.

CONCLUSIONS

The findings provide the mechanism of endosomal cyclic AMP generation following GLP-1R activation. We identified the specific compartment that serves as an organizing center to generate endosomal cyclic AMP by internalized activated receptor complex.

Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes

The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.

Interplay between bone and incretin hormones: A review

Bone is a tissue with multiple functions that is built from the molecular to anatomical levels to resist and adapt to mechanical strains. Among all the factors that might control the bone organization, a role for several gut hormones called "incretins" has been suspected. The present review summarizes the current evidences on the effects of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) in bone physiology.

A missense variant in GLP1R gene is associated with the glycaemic response to treatment with gliptins

Gliptins act by increasing endogenous incretin levels. Glucagon-like peptide-1 receptor (GLP1R) and glucose-dependent insulinotropic peptide receptor (GIPR) are their indirect drug targets. Variants of GLP1R and GIPR have previously been associated with the incretin effect. The aim of the present pilot study was to examine associations of the GLP1R and GIPR gene variants with the glycaemic response to gliptins. A total of 140 consecutive patients with type 2 diabetes were followed-up 6 months after initiation of gliptin treatment. GLP1R rs6923761 (Gly168Ser) and GIPR rs10423928 genotyping was performed using real-time PCR, with subsequent high-resolution melting analysis. The main study outcome was reduction in glycated haemoglobin (HbA1c) after treatment. GLP1R Gly168Ser variant was significantly associated with reduction in HbA1c in an additive model (β = -0.33, p = 0.011). The mean reduction in HbA1c in Ser/Ser homozygotes was significantly lower compared with Gly-allele carriers [0.12 ± 0.23% vs. 0.80 ± 0.09% (1.3 ± 2.5 mmol/mol vs. 8.7 ± 1.0 mmol/mol); p = 0.008]. In conclusion, GLP1R missense variant was associated with a reduced response to gliptin treatment. The genotype-related effect size of ∼0.7% (8 mmol/mol) is equal to an average effect of gliptin treatment and makes this variant a candidate for use in precision medicine.

Restoring Mitochondrial Function: A Small Molecule-mediated Approach to Enhance Glucose Stimulated Insulin Secretion in Cholesterol Accumulated Pancreatic beta cells

Dyslipidemia, particularly the elevated serum cholesterol levels, aggravate the pathophysiology of type 2 diabetes. In the present study we explored the relationship between fasting blood sugar and serum lipid parameters in human volunteers which revealed a significant linear effect of serum cholesterol on fasting blood glucose. Short term feeding of cholesterol enriched diet to rodent model resulted in elevated serum cholesterol levels, cholesterol accumulation in pancreatic islets and hyperinsulinemia with modest increase in plasma glucose level. To explore the mechanism, we treated cultured BRIN-BD11 pancreatic beta cells with soluble cholesterol. Our data shows that cholesterol treatment of cultured pancreatic beta cells enhances total cellular cholesterol. While one hour cholesterol exposure enhances insulin exocytosis, overnight cholesterol accumulation in cultured pancreatic beta cells affects cellular respiration, and inhibits Glucose stimulated insulin secretion. We further report that (E)-4-Chloro-2-(1-(2-(2,4,6-trichlorophenyl) hydrazono) ethyl) phenol (small molecule M1) prevents the cholesterol mediated blunting of cellular respiration and potentiates Glucose stimulated insulin secretion which was abolished in pancreatic beta cells on cholesterol accumulation.

A genomic approach to therapeutic target validation identifies a glucose-lowering GLP1R variant protective for coronary heart disease

Regulatory authorities have indicated that new drugs to treat type 2 diabetes (T2D) should not be associated with an unacceptable increase in cardiovascular risk. Human genetics may be able to guide development of antidiabetic therapies by predicting cardiovascular and other health endpoints. We therefore investigated the association of variants in six genes that encode drug targets for obesity or T2D with a range of metabolic traits in up to 11,806 individuals by targeted exome sequencing and follow-up in 39,979 individuals by targeted genotyping, with additional in silico follow-up in consortia. We used these data to first compare associations of variants in genes encoding drug targets with the effects of pharmacological manipulation of those targets in clinical trials. We then tested the association of those variants with disease outcomes, including coronary heart disease, to predict cardiovascular safety of these agents. A low-frequency missense variant (Ala316Thr; rs10305492) in the gene encoding glucagon-like peptide-1 receptor (GLP1R), the target of GLP1R agonists, was associated with lower fasting glucose and T2D risk, consistent with GLP1R agonist therapies. The minor allele was also associated with protection against heart disease, thus providing evidence that GLP1R agonists are not likely to be associated with an unacceptable increase in cardiovascular risk. Our results provide an encouraging signal that these agents may be associated with benefit, a question currently being addressed in randomized controlled trials. Genetic variants associated with metabolic traits and multiple disease outcomes can be used to validate therapeutic targets at an early stage in the drug development process.

Pharmacogenetic aspects of the treatment of Type 2 diabetes with the incretin effect enhancers

Incretin effect enhancers are drugs used in the treatment of Type 2 diabetes and include GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors (gliptins). Variants in several genes were shown to be involved in the physiology of incretin secretion. Only two gene variants have evidence also from pharmacogenetic studies. TCF7L2 rs7903146 C>T and CTRB1/2 rs7202877 T>G minor allele carriers were both associated with a smaller reduction in HbA1c after gliptin treatment when compared with major allele carriers. After replication in further studies, these observations could be of clinical significance in helping to identify patients with potentially lower or higher response to gliptin treatment.

Molecular Pharmacology of the Incretin Receptors

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important regulators of insulin and glucagon secretion as well as lipid metabolism and appetite. These biological functions make their respective receptors (GIPR and GLP-1R) attractive targets in the treatment of both type 2 diabetes mellitus (T2DM) and obesity. The use of these native peptides in the treatment of these conditions is limited by their short half-lives. However, long-acting GLP-1R agonists and inhibitors of the enzyme that rapidly inactivates GIP and GLP-1 (dipeptidyl peptidase IV) are in clinical use. Although there is a loss of response to both hormones in T2DM, this effect appears to be more pronounced for GIP. This has made targeting GIPR less successful than GLP-1R. Furthermore, results demonstrating that GIPR knockout mice were resistant to diet-induced obesity suggested that GIPR antagonists may prove to be useful therapeutics. More recently, molecules that activate both receptors have shown promise in terms of glycemic and body weight control. This review focused on recent advances in the understanding of the signaling mechanisms and regulation of these two clinically important receptors.

Effects of a high-protein/low-carbohydrate versus a standard hypocaloric diet on adipocytokine levels and cardiovascular risk factors during 9 months, role of rs6923761 gene variant of glucagon-like peptide 1 receptor

BACKGROUND

The role of GLP-1 R variants on body weight response after dietary intervention is unclear.

OBJECTIVE

The aim was to investigate the role of this polymorphism on cardiovascular risk factors, adipokine levels and weight loss secondary to a high-protein/low-carbohydrate vs. standard hypocaloric diets during 9 months.

DESIGN

211 obese subjects were randomly allocated to one of these two diets for a period of 9 months; diet HP (high protein/low carbohydrate) and diet S (standard).

RESULTS

Ninety-four patients (44.5%) had the genotype GG (wild group) and 117 (55.5%) patients had the next genotypes; GA (89 patients, 42.2%) or AA (28 patients, 13.3%) (mutant group). With both diets and in both genotype groups, body mass index, weight, fat mass, waist circumference and systolic blood pressure decreased. Anthropometric parameters were higher in non-A allele carriers than A allele carriers. With diet HP in both genotypes, LDL cholesterol, total cholesterol, leptin, insulin levels and HOMA-R decreased. With the diet S and only in wild genotype, the same parameters decreased, too.

CONCLUSION

Our data showed a lack of association of rs6923761 GLP-1 R polymorphism with weight loss. Better anthropometric parameters in obese subjects with the mutant allele (A) of rs6923761 GLP-1 R polymorphism were observed. Total cholesterol, LDL cholesterol, insulin levels and HOMA-R decreased in all patients with both diets, although A allele carriers treated with standard diet did not show these changes.

Functional elements of the gastric inhibitory polypeptide receptor: Comparison between secretin- and rhodopsin-like G protein-coupled receptors

Innovative crystallographic techniques have resulted in an exponential growth in the number of solved G-protein coupled receptor (GPCR) structures and a better understanding of the mechanisms of class A receptor activation and G protein binding. The recent release of the type 1 receptor for the corticotropin-releasing factor and the glucagon receptor structures, two members of the secretin-like family, gives the opportunity to understand these mechanisms of activation in this family of GPCRs. Here, we addressed the comparison of the functional elements of class A and secretin-like GPCRs, using the glucose-dependent insulinotropic polypeptide receptor (GIPR) as a model receptor. Inactive and active models of GIPR permitted to select, by structural homology with class A GPCRs, several residues that may form key interactions presumably involved in receptor activation and Gs coupling, for pharmacological evaluation. Mutants on these amino acids were expressed in HEKT 293 cells and characterized in terms of GIP-induced cAMP production. We identified various functional domains spanning from the peptide-binding to the G protein pockets: including: a network linking the extracellular part of transmembrane (TM) 6 with TMs 2 and 7; a polar lock that resembles the ionic-lock in class A GPCRs; an interaction between TMs 3 and 7 that favors activation; and two clusters of polar/charged and of hydrophobic residues that interact with the C-terminus of the Gα. The results show that despite the low degree of sequence similarity between rhodopsin- and secretin-like GPCRs, the two families share conserved elements in their mechanisms of activation and G protein binding.

Evaluation of weight loss and metabolic changes in diabetic patients treated with liraglutide, effect of RS 6923761 gene variant of glucagon-like peptide 1 receptor

BACKGROUND

A polymorphism of GLP-1 R (rs6923761) has potential implications in weight loss and metabolic control. We decide to investigate the role of this polymorphism on metabolic changes and weight loss secondary to treatment with liraglutide.

MATERIAL AND METHODS

A population of 90 patients with diabetes mellitus type 2 and overweight, unable to achieve glycemic control (HbA1c>7%) with metformin alone that require initiation of liraglutide treatment in progressive dose to 1.8mg/day subcutaneously, was analyzed.

RESULTS

Fifty one patients (56.7%) had the genotype GG and 39 (43.3%) patients; GA (30 patients, 33.3%) or AA (9 patients, 10%) (A allele carriers). In patients with both genotypes, body mass index (BMI), weight and fat decreased. The proportion of the mentioned reductions was higher in the variant allele carriers; BMI (-0.59±2.5kg/m(2) vs. -1.69±3.9kg/m(2); P<0.05), weight (-2.78±2.8kg vs. -4.52±4.6kg; P<0.05) and fat mass (-0.59±2.5kg vs. -1.69±3.9kg; P<0.05). Weight reduction after liraglutide treatment was greater in the A-allele carriers by 2.9kg (95% CI: 0.27-5.64). The decrease of basal glucose, HOMA-R and HbA1c was similar in both genotypes.

CONCLUSION

Our data showed better anthropometric parameters in overweight diabetic subjects with the variant allele (A) of rs6923761 GLP-1 R polymorphism. A allele carriers had a greater decrease in weight and fat mass after treatment with liraglutide. The present study is a preliminary observation, and its results need to be replicated with a higher number of patients in different populations.

Low-frequency and rare exome chip variants associate with fasting glucose and type 2 diabetes susceptibility

Fasting glucose and insulin are intermediate traits for type 2 diabetes. Here we explore the role of coding variation on these traits by analysis of variants on the HumanExome BeadChip in 60,564 non-diabetic individuals and in 16,491 T2D cases and 81,877 controls. We identify a novel association of a low-frequency nonsynonymous SNV in GLP1R (A316T; rs10305492; MAF=1.4%) with lower FG (β=-0.09±0.01 mmol l(-1), P=3.4 × 10(-12)), T2D risk (OR[95%CI]=0.86[0.76-0.96], P=0.010), early insulin secretion (β=-0.07±0.035 pmolinsulin mmolglucose(-1), P=0.048), but higher 2-h glucose (β=0.16±0.05 mmol l(-1), P=4.3 × 10(-4)). We identify a gene-based association with FG at G6PC2 (pSKAT=6.8 × 10(-6)) driven by four rare protein-coding SNVs (H177Y, Y207S, R283X and S324P). We identify rs651007 (MAF=20%) in the first intron of ABO at the putative promoter of an antisense lncRNA, associating with higher FG (β=0.02±0.004 mmol l(-1), P=1.3 × 10(-8)). Our approach identifies novel coding variant associations and extends the allelic spectrum of variation underlying diabetes-related quantitative traits and T2D susceptibility.

Differential impact of amino acid substitutions on critical residues of the human glucagon-like peptide-1 receptor involved in peptide activity and small-molecule allostery

The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein-coupled receptor that has a critical role in the regulation of glucose homeostasis, principally through the regulation of insulin secretion. The receptor system is highly complex, able to be activated by both endogenous [GLP-1(1-36)NH2, GLP-1(1-37), GLP-1(7-36)NH2, GLP-1(7-37), oxyntomodulin], and exogenous (exendin-4) peptides in addition to small-molecule allosteric agonists (compound 2 [6,7-dichloro-2-methylsulfonyl-3-tert-butylaminoquinoxaline], BETP [4-(3-benzyloxy)phenyl)-2-ethylsulfinyl-6-(trifluoromethyl)pyrimidine]). Furthermore, the GLP-1R is subject to single-nucleotide polymorphic variance, resulting in amino acid changes in the receptor protein. In this study, we investigated two polymorphic variants previously reported to impact peptide-mediated receptor activity (M149) and small-molecule allostery (C333). These residues were mutated to a series of alternate amino acids, and their functionality was monitored across physiologically significant signaling pathways, including cAMP, extracellular signal-regulated kinase 1 and 2 phosphorylation, and intracellular Ca(2+) mobilization, in addition to peptide binding and cell-surface expression. We observed that residue 149 is highly sensitive to mutation, with almost all peptide responses significantly attenuated at mutated receptors. However, most reductions in activity were able to be restored by the small-molecule allosteric agonist compound 2. Conversely, mutation of residue 333 had little impact on peptide-mediated receptor activation, but this activity could not be modulated by compound 2 to the same extent as that observed at the wild-type receptor. These results provide insight into the importance of residues 149 and 333 in peptide function and highlight the complexities of allosteric modulation within this receptor system.

Relation of the rs6923761 gene variant in glucagon-like peptide 1 receptor to metabolic syndrome in obese subjects

BACKGROUND

The role of glucagon-like peptide 1 (GLP-1) variants in metabolic syndrome (MS) and its components remains unclear in obese subjects.

OBJECTIVE

The aim of our study was to evaluate the relationship of rs6923761 with MS and its components in obese subjects.

DESIGN

A population of 1,122 obese subjects was analyzed in a cross-sectional survey. To estimate the prevalence of MS, we considered the definitions of the Adult Treatment Panel III.

RESULTS

Five hundred and forty-eight patients (48.8%) had the GG genotype (wild-type group), whereas 487 patients (43.4%) had the GA genotype and 87 patients (7.8%) the AA genotype. The mean age was 48.9 ± 12.8 years. The prevalence of MS was 47.4% (532 patients), and 52.6% of patients had no MS (n = 590). The odds ratio of MS for the wild-type versus the mutant genotype was 1.02, with a 95% confidence interval of 0.88-1.12. Body mass index, weight, fat mass, waist circumference, and waist to hip ratio were lower in the mutant than in the wild-type group in patients with and without MS.

CONCLUSION

The GLP-1 receptor variant rs6923761 was found to be associated with decreased weight and anthropometric parameters in A allele carriers with and without MS. MS or its components were not associated with this polymorphism in obese adults.

Role of the rs6923761 gene variant in glucagon-like peptide 1 receptor gene on cardiovascular risk factors and weight loss after biliopancreatic diversion surgery

BACKGROUND

Studies of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) have been directed at identifying polymorphisms in the GLP-1R gene that may be a contributing factor in the pathogenesis of obesity.

OBJECTIVE

We decided to investigate the role of the rs6923761 GLP-1R polymorphism on outcomes after biliopancreatic diversion.

DESIGN

A sample of 137 morbidly obese patients was operated. Weight, blood pressure, basal glucose and lipid profile were measured at the basal visit and at each following visit (basal, 3, 9, 12 and 18 months).

RESULTS

Body mass index, weight and waist circumference were lower in non-A allele carriers than A allele carriers 12 and 18 months after surgery. The initial weight percent loss at 12 months (45.6 vs. 39.8%; p < 0.05) or 18 months (49.6 vs. 41.3%; p < 0.05) was higher in patients with GG genotype than A allele carriers.

CONCLUSION

Our study showed a higher weight loss 12 and 18 months after bariatric surgery in GG variant than A allele carriers. The biochemical parameters and cardiovascular comorbidity rates improved similarly in both genotypes.

Effect of rs6923761 gene variant of glucagon-like peptide 1 receptor on metabolic response and weight loss after a 3-month intervention with a hypocaloric diet

BACKGROUND

Studies of the GLP-1 receptor (GLP-1 R) have been directed at identifying polymorphisms in the GLP-1 R gene that may be a contributing factor in the pathogenesis of obesity and cardiovascular risk factors. Nevertheless, the role of GLP-1 R variants on body weight response after dietary intervention has not been evaluated.

OBJECTIVE

We decided to analyze the effects of the rs6923761 GLP-1 R polymorphism on body weight changes and metabolic parameters after 3 months of a hypocaloric diet.

DESIGN

A sample of 91 obese subjects was analyzed in a prospective way. The hypocaloric diet had 1,520 calories per day; 52 % of carbohydrates, 25 % of lipids and 23 % of proteins. Distribution of fats was: 50.7 % of monounsaturated fats, 38.5 % of saturated fats and 11.8 % of polyunsaturated fats.

RESULTS

In both genotype groups (GG vs. GA + AA), weight, body mass index, fat mass, waist circumference, systolic blood pressure, total cholesterol, LDL cholesterol, leptin, insulin and HOMA levels decreased. No statistical differences were detected in these changes between genotypes. In wild group (GG genotype) (pretreatment and posttreatment), BMI, weight, fat mass, waist circumference and triglyceride levels were higher than (GA + AA) group.

CONCLUSION

Our data showed better anthropometric parameters and triglyceride levels in obese subjects with the mutant allele (A) of rs6923761 GLP-1R polymorphism. A lack of association of this polymorphism with weight loss or biochemical changes after a hypocaloric diet was observed.

The GIP receptor displays higher basal activity than the GLP-1 receptor but does not recruit GRK2 or arrestin3 effectively

Background and Objectives

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are important regulators of insulin secretion, and their functional loss is an early characteristic of type 2 diabetes mellitus (T2DM). Pharmacological levels of GLP-1, but not GIP, can overcome this loss. GLP-1 and GIP exert their insulinotropic effects through their respective receptors expressed on pancreatic β-cells. Both the GLP-1 receptor (GLP-1R) and the GIP receptor (GIPR) are members of the secretin family of G protein-coupled receptors (GPCRs) and couple positively to adenylate cyclase. We compared the signalling properties of these two receptors to gain further insight into why GLP-1, but not GIP, remains insulinotropic in T2DM patients.

Methods

GLP-1R and GIPR were transiently expressed in HEK-293 cells, and basal and ligand-induced cAMP production were investigated using a cAMP-responsive luciferase reporter gene assay. Arrestin3 (Arr3) recruitment to the two receptors was investigated using enzyme fragment complementation, confocal microscopy and fluorescence resonance energy transfer (FRET).

Results

GIPR displayed significantly higher (P<0.05) ligand-independent activity than GLP-1R. Arr3 displayed a robust translocation to agonist-stimulated GLP-1R but not to GIPR. These observations were confirmed in FRET experiments, in which GLP-1 stimulated the recruitment of both GPCR kinase 2 (GRK2) and Arr3 to GLP-1R. These interactions were not reversed upon agonist washout. In contrast, GIP did not stimulate recruitment of either GRK2 or Arr3 to its receptor. Interestingly, arrestin remained at the plasma membrane even after prolonged (30 min) stimulation with GLP-1. Although the GLP-1R/arrestin interaction could not be reversed by agonist washout, GLP-1R and arrestin did not co-internalise, suggesting that GLP-1R is a class A receptor with regard to arrestin binding.

Conclusions

GIPR displays higher basal activity than GLP-1R but does not effectively recruit GRK2 or Arr3.

Cardiovascular risk factors and adipocytokines levels after two hypocaloric diets with different fat distribution in obese subjects and rs6923761 gene variant of glucagon-like peptide 1 receptor

BACKGROUND

The role of GLP-1 R variants on body weight response after dietary intervention is unclear.

OBJECTIVE

Our aim was to evaluate the effect of this polymorphism on cardiovascular risk factors, adipokine levels and weight loss secondary to a high monounsaturated fat vs. a high polyunsaturated fat hypocaloric diets in obese subjects.

DESIGN

391 obese subjects were randomly allocated to one of these two diets for a period of 3 months; diet M (high monounsaturated fat hypocaloric diet) and diet P (high polyunsaturated (PUFAs) fat hypocaloric diet).

RESULTS

Two hundred and twelve patients (54.2 %) had the genotype GG (wild group) and 179 (45.8 %) patients had the next genotypes; GA (146 patients, 37.3 %) or AA (33 patients, 8.7 %) (Mutant group). With both diets and in wild-type and mutant-type groups, BMI, weight, fat mass, waist circumference and systolic blood pressure decreased. Anthropometric parameters were higher in non-A-allele carriers than A-allele carriers. With both diets and in both genotypes, leptin, insulin levels and HOMA decreased. With the diet P and in wild genotype, total cholesterol and LDL-cholesterol levels decreased, too.

CONCLUSION

Our data showed a lack of association of rs6923761 GLP-1 R polymorphism with weight loss after a high monounsaturated fat and a high polyunsaturated fat hypocaloric diets. Better anthropometric parameters in obese subjects with the mutant allele (A) of rs6923761 GLP-1 R polymorphism were observed. Insulin levels and HOMA decreased in non-A carriers.

A naturally occurring GIP receptor variant undergoes enhanced agonist-induced desensitization, which impairs GIP control of adipose insulin sensitivity

Glucose-dependent insulinotropic polypeptide (GIP), an incretin hormone secreted from gastrointestinal K cells in response to food intake, has an important role in the control of whole-body metabolism. GIP signals through activation of the GIP receptor (GIPR), a G-protein-coupled receptor (GPCR). Dysregulation of this pathway has been implicated in the development of metabolic disease. Here we demonstrate that GIPR is constitutively trafficked between the plasma membrane and intracellular compartments of both GIP-stimulated and unstimulated adipocytes. GIP induces a downregulation of plasma membrane GIPR by slowing GIPR recycling without affecting internalization kinetics. This transient reduction in the expression of GIPR in the plasma membrane correlates with desensitization to the effects of GIP. A naturally occurring variant of GIPR (E354Q) associated with an increased incidence of insulin resistance, type 2 diabetes, and cardiovascular disease in humans responds to GIP stimulation with an exaggerated downregulation from the plasma membrane and a delayed recovery of GIP sensitivity following cessation of GIP stimulation. This perturbation in the desensitization-resensitization cycle of the GIPR variant, revealed in studies of cultured adipocytes, may contribute to the link of the E354Q variant to metabolic disease.

Relation of the rs6923761 gene variant in glucagon-like peptide 1 receptor with weight, cardiovascular risk factor, and serum adipokine levels in obese female subjects

BACKGROUND

Studies of the glucagon-like peptide 1 (GLP-1) receptor have been directed at identifying polymorphisms in the GLP-1 receptor gene that may be a contributing factor in the pathogenesis of diabetes mellitus and cardiovascular risk factors. Nevertheless, the role of GLP-1 variants on body weight, cardiovascular risk factors, and adipokines remains unclear in obese patients.

OBJECTIVE

Our aim was to analyze the effects of rs6923761 GLP-1 receptor polymorphism on body weight, cardiovascular risk factors, and serum adipokine levels in nondiabetic obese females.

DESIGN

A sample of 645 obese nondiabetic Caucasian females was enrolled in a prospective way. Basal fasting glucose, c-reactive protein (CRP), insulin, insulin resistance (homeostasis model assessment (HOMA)), total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides concentration, and adipokines were measured. Weights, body mass index (BMI), waist circumference, fat mass by bioimpedance, and blood pressure measures were measured.

RESULTS

Three hundred and twenty-seven participants (50.7%) had the genotype GG and 318 (49.3%) study subjects had the next genotypes; GA (270 study subjects, 41.9%) or AA (48 study subjects, 7.4%) (second group). In wild group (GG genotype), BMI (1.8 ± 2.3 kg/m(2) ; P < 0.05), weight (3.1 ± 1.3 kg; P < 0.05), fat mass (2.4 ± 1.1 kg; P < 0.05), waist circumference (2.7 ± 1.9 cm; P < 0.05), triglyceride levels (10.4 ± 5.3 mg/dl; P < 0.05), interleukin 6 (IL-6) (1.5 ± 0.9 ng/dl; P < 0.05), resistin (1.1 ± 0.3 ng/dl; P < 0.05), and leptin (30.1 ± 10.3 ng/dl; P < 0.05) levels were higher than mutant group (GA + AA).

CONCLUSION

Data from our study revealed an association with decreased metabolic and cardiovascular markers in obese females. BMI weight, fat mass, waist circumference, triglycerides, leptin, resistin, and IL-6 serum levels were lower in subjects with A allele than non-A allele subjects.

Unsaturated glycoceramides as molecular carriers for mucosal drug delivery of GLP-1

The incretin hormone Glucagon-like peptide 1 (GLP-1) requires delivery by injection for the treatment of Type 2 diabetes mellitus. Here, we test if the properties of glycosphingolipid trafficking in epithelial cells can be applied to convert GLP-1 into a molecule suitable for mucosal absorption. GLP-1 was coupled to the extracellular oligosaccharide domain of GM1 species containing ceramides with different fatty acids and with minimal loss of incretin bioactivity. When applied to apical surfaces of polarized epithelial cells in monolayer culture, only GLP-1 coupled to GM1-ceramides with short- or cis-unsaturated fatty acids trafficked efficiently across the cell to the basolateral membrane by transcytosis. In vivo studies showed mucosal absorption after nasal administration. The results substantiate our recently reported dependence on ceramide structure for trafficking the GM1 across polarized epithelial cells and support the idea that specific glycosphingolipids can be harnessed as molecular vehicles for mucosal delivery of therapeutic peptides.

Minireview: Signal bias, allosterism, and polymorphic variation at the GLP-1R: implications for drug discovery

The glucagon-like peptide-1 receptor (GLP-1R) controls the physiological responses to the incretin hormone glucagon-like peptide-1 and is a major therapeutic target for the treatment of type 2 diabetes, owing to the broad range of effects that are mediated upon its activation. These include the promotion of glucose-dependent insulin secretion, increased insulin biosynthesis, preservation of β-cell mass, improved peripheral insulin action, and promotion of weight loss. Regulation of GLP-1R function is complex, with multiple endogenous and exogenous peptides that interact with the receptor that result in the activation of numerous downstream signaling cascades. The current understanding of GLP-1R signaling and regulation is limited, with the desired spectrum of signaling required for the ideal therapeutic outcome still to be determined. In addition, there are several single-nucleotide polymorphisms (used in this review as defining a natural change of single nucleotide in the receptor sequence; clinically, this is viewed as a single-nucleotide polymorphism only if the frequency of the mutation occurs in 1% or more of the population) distributed within the coding sequence of the receptor protein that have the potential to produce differential responses for distinct ligands. In this review, we discuss the current understanding of GLP-1R function, in particular highlighting recent advances in the field on ligand-directed signal bias, allosteric modulation, and probe dependence and the implications of these behaviors for drug discovery and development.

Individualized therapy for type 2 diabetes: clinical implications of pharmacogenetic data

Type 2 diabetes mellitus (T2DM) is characterized by insulin resistance, abnormally elevated hepatic glucose production, and reduced glucose-stimulated insulin secretion. Treatment with antihyperglycemic agents is initially successful in type 2 diabetes, but it is often associated with a high secondary failure rate, and the addition of insulin is eventually necessary for many patients, in order to restore acceptable glycemic control and to reduce the risk of development and progression of disease complications. Notably, even patients who appear to have similar requirements of antidiabetic regimens show great variability in drug disposition, glycemic response, tolerability, and incidence of adverse effects during treatment. Pharmacogenomics is a promising area of investigation and involves the search for genetic polymorphisms that may explain the interindividual variability in antidiabetic therapy response. The initial positive results portend that genomic efforts will be able to shed important light on variability in pharmacologic traits. In this review, we summarize the current understanding of genetic polymorphisms that may affect the responses of subjects with T2DM to antidiabetic treatment. These genes belong to three major classes: genes involved in drug metabolism and transporters that influence pharmacokinetics (including the cytochrome P450 [CYP] superfamily, the organic anion transporting polypeptide [OATP] family, and the polyspecific organic cation transporter [OCT] family); genes encoding drug targets and receptors (including peroxisome proliferator-activated receptor gamma [PPARG], the adenosine triphosphate [ATP]-sensitive potassium channel [K(ATP)], and incretin receptors); and genes involved in the causal pathway of T2DM that are able to modify the effects of drugs (including adipokines, transcription factor 7-like 2 (T cell specific, HMG-box) [TCF7L2], insulin receptor substrate 1 [IRS1], nitric oxide synthase 1 (neuronal) adaptor protein [NOS1AP], and solute carrier family 30 (zinc transporter), member 8 [SLC30A8]). In addition to these three major classes, we also review the available evidence on novel genes (CDK5 regulatory subunit associated protein 1-like 1 [CDKAL1], insulin-like growth factor 2 mRNA binding protein 2 [IGF2BP2], potassium voltage-gated channel, KQT-like subfamily, member 1 [KCNQ1], paired box 4 [PAX4] and neuronal differentiation 1 [NEUROD1] transcription factors, ataxia telangiectasia mutated [ATM], and serine racemase [SRR]) that have recently been proposed as possible modulators of therapeutic response in subjects with T2DM.