Characterization of genetic variants of GIPR reveals a contribution of ß-arrestin to metabolic phenotypes.

Incretin-based therapies are highly successful in combatting obesity and type 2 diabetes1. Yet both activation and inhibition of the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) in combination with glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) activation have resulted in similar clinical outcomes, as demonstrated by the GIPR-GLP-1R co-agonist tirzepatide2 and AMG-133 (ref. 3) combining GIPR antagonism with GLP-1R agonism. This underlines the importance of a better understanding of the GIP system. Here we show the necessity of ß-arrestin recruitment for GIPR function, by combining in vitro pharmacological characterization of 47 GIPR variants with burden testing of clinical phenotypes and in vivo studies. Burden testing of variants with distinct ligand-binding capacity, Gs activation (cyclic adenosine monophosphate production) and ß-arrestin 2 recruitment and internalization shows that unlike variants solely impaired in Gs signalling, variants impaired in both Gs and ß-arrestin 2 recruitment contribute to lower adiposity-related traits. Endosomal Gs-mediated signalling of the variants shows a ß-arrestin dependency and genetic ablation of ß-arrestin 2 impairs cyclic adenosine monophosphate production and decreases GIP efficacy on glucose control in male mice. This study highlights a crucial impact of ß-arrestins in regulating GIPR signalling and overall preservation of biological activity that may facilitate new developments in therapeutic targeting of the GIPR system.

A look at duodenal mucosal resurfacing: Rationale for targeting the duodenum in type 2 diabetes.

Affecting 5%-10% of the world population, type 2 diabetes (T2DM) is firmly established as one of the major health burdens of modern society. People with T2DM require long-term therapies to reduce blood glucose, an approach that can mitigate the vascular complications. However, fewer than half of those living with T2DM reach their glycaemic targets despite the availability of multiple oral and injectable medications. Adherence and access to medications are major barriers contributing to suboptimal diabetes treatment. The gastrointestinal tract has recently emerged as a target for treating T2DM and altering the underlying disease course. Preclinical and clinical analyses have elucidated changes in the mucosal layer of the duodenum potentially caused by dietary excess and obesity, which seem to be prevalent among individuals with metabolic disease. Supporting these findings, gastric bypass, a surgical procedure which removes the duodenum from the intestinal nutrient flow, has remarkable effects that improve, and often cause remission of, diabetes. From this perspective, we explore the rationale for targeting the duodenum with duodenal mucosal resurfacing (DMR). We examine the underlying physiology of the duodenum and its emerging role in T2DM pathogenesis, the rationale for targeting the duodenum by DMR as a potential treatment for T2DM, and current data surrounding DMR. Importantly, DMR has been demonstrated to change mucosal abnormalities common in those with obesity and diabetes. Given the multifactorial aetiology of T2DM, understanding proximate contributors to disease pathogenesis opens the door to rethinking therapeutic approaches to T2DM, from symptom management toward disease modification.

Reduction of prevalence of patients meeting the criteria for metabolic syndrome with tirzepatide: a post hoc analysis from the SURPASS Clinical Trial Program.

BACKGROUND: Metabolic syndrome is characterized as the co-occurrence of interrelated cardiovascular risk factors, including insulin resistance, hyperinsulinemia, abdominal obesity, dyslipidemia and hypertension. Once weekly tirzepatide is approved in the US and EU for the treatment of type 2 diabetes (T2D) and obesity. In the SURPASS clinical trial program for T2D, tirzepatide demonstrated greater improvements in glycemic control, body weight reduction and other cardiometabolic risk factors versus placebo, subcutaneous semaglutide 1 mg, insulin degludec, and insulin glargine. This post hoc analysis assessed the effect of tirzepatide use on the prevalence of patients meeting the criteria for metabolic syndrome across SURPASS 1-5. METHODS: Metabolic syndrome was defined as having ≥ 3 of 5 criteria according to the US National Cholesterol Education Program: Adult Treatment Panel III. Analyses were based on on-treatment data at the primary endpoint from patients adherent to treatment (taking ≥ 75% study drug). A logistic regression model with metabolic syndrome status as the response variable, metabolic syndrome status at the baseline visit as an adjustment, and randomized treatment as fixed explanatory effect was used. The effect of tirzepatide use on the prevalence of patients meeting the criteria for metabolic syndrome by categorical weight loss, background medication and gender were assessed. RESULTS: In SURPASS, the prevalence of patients meeting the criteria for metabolic syndrome at baseline was 67-88% across treatment groups with reductions at the primary endpoint to 38-64% with tirzepatide versus 64-82% with comparators. Reductions in the prevalence of patients meeting the criteria for metabolic syndrome was significantly greater with all tirzepatide doses versus placebo, semaglutide 1 mg, insulin glargine, and insulin degludec (p < 0.001). Individual components of metabolic syndrome were also reduced to a greater extent with tirzepatide vs comparators. Greater reductions in body weight were associated with greater reductions in the prevalence of patients meeting the criteria for metabolic syndrome and its individual components. Background SGLT2i or sulfonylurea use or gender did not impact the change in prevalence of patients meeting the criteria for metabolic syndrome. CONCLUSIONS: In this post hoc analysis, tirzepatide at all doses studied was associated with a greater reduction in the prevalence of patients meeting the criteria for metabolic syndrome compared to placebo, semaglutide 1 mg, insulin degludec, and insulin glargine. Although more evidence is needed, these data would support greater potential improvement in cardiovascular risk factor profile with tirzepatide treatment in people across the continuum of T2D.

High Doses of Exogenous Glucagon Stimulate Insulin Secretion and Reduce Insulin Clearance in Healthy Humans.

Glucagon is generally defined as a counterregulatory hormone with a primary role to raise blood glucose concentrations by increasing endogenous glucose production (EGP) in response to hypoglycemia. However, glucagon has long been known to stimulate insulin release, and recent preclinical findings have supported a paracrine action of glucagon directly on islet ß-cells that augments their secretion. In mice, the insulinotropic effect of glucagon is glucose dependent and not present during basal euglycemia. To test the hypothesis that the relative effects of glucagon on hepatic and islet function also vary with blood glucose, a group of healthy subjects received glucagon (100 ng/kg) during fasting glycemia or experimental hyperglycemia (∼150 mg/dL) on 2 separate days. During fasting euglycemia, administration of glucagon caused blood glucose to rise due to increased EGP, with a delayed increase of insulin secretion. When given during experimental hyperglycemia, glucagon caused a rapid, threefold increase in insulin secretion, as well as a more gradual increase in EGP. Under both conditions, insulin clearance was decreased in response to glucagon infusion. The insulinotropic action of glucagon, which is proportional to the degree of blood glucose elevation, suggests distinct physiologic roles in the fasting and prandial states.

Assessment of the incretin effect in healthy subjects: concordance between clamp and OGTT methods.

The incretin effect describes the insulin response to nutrient ingestion that exceeds the response to glycemia per se. It is mediated by gastrointestinal factors and is necessary to maintain postprandial glucose homeostasis. The incretin effect results in a more than twofold increase of the insulin response to a meal in healthy people and two different techniques have been used in the past to measure its magnitude. Most studies employ an OGTT on 1 day, followed by a matching glucose infusion on a separate day. Another study design employs a hyperglycemic glucose clamp that is maintained after oral ingestion of glucose. Both protocols allow quantification of the incretin effect by comparing the insulin response to an identical glycemic stimulus. Here we performed a within-subject comparison of both techniques to quantify the incretin effect and suggest different calculation methods to interpret the results derived from the clamp experiment in a cohort of healthy young adults (n = 10, age 33 ± 4 yr). All subjects participated on four different study days: 1) OGTT, 2) isoglycemic glucose infusion (Iso-IV), 3) hyperglycemic clamp with oral glucose ingestion (clamp-OGTT), and 4) hyperglycemic clamp (clamp). With the classic OGTT/Iso-IV method, the insulin response to glucose ingestion increased more than twofold and was 60 ± 6% and 49 ± 5% for insulin and c-peptide. Different estimates of the incretin effect based on the clamp method ranged from 58% to 79% for insulin and 38% to 61% for c-peptide, both significantly higher than values derived from the OGTT/isoglycemic infusion method. However, when the effect of continuous hyperglycemia on insulin secretion was accounted for, using extrapolation from early time points of the clamp, good agreement was noted between the two methods. Based on these results, both techniques seem to be equally suited to measure the incretin effect and should be employed according to the scientific questions, experimental contingencies, and investigator experience.NEW & NOTEWORTHY This proof-of-concept study shows that the incretin effect can be reliably assessed by two different methods with similar quantitative results. A single-day hyperglycemic clamp with oral glucose ingestion allows the determination of the incretin effect with fewer study days and less day-to-day variability.

GIPR/GLP-1R dual agonist therapies for diabetes and weight loss-chemistry, physiology, and clinical applications.

The incretin system is an essential metabolic axis that regulates postprandial metabolism. The two incretin peptides that enable this effect are the glucose-dependent insulinotropic polypeptide (GIP) and the glucagon-like peptide 1 (GLP-1), which have cognate receptors (GIPR and GLP-1R) on islet ß cells as well as in other tissues. Pharmacologic engagement of the GLP-1R is a proven strategy for treating hyperglycemia in diabetes and reducing body weight. Tirzepatide is the first monomeric peptide with dual activity at both incretin receptors now available for clinical use, and in clinical trials it has shown unprecedented effects to reduce blood glucose and body weight. Here, we discuss the foundational science that led to the development of monomeric multi-incretin receptor agonists, culminating in the development of tirzepatide. We also look to the future of this field and comment on how the concept of multi-receptor agonists will continue to progress for the treatment of metabolic disease.

The incretin co-agonist tirzepatide requires GIPR for hormone secretion from human islets.

The incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) mediate insulin responses that are proportionate to nutrient intake to facilitate glucose tolerance1. The GLP-1 receptor (GLP-1R) is an established drug target for the treatment of diabetes and obesity2, whereas the therapeutic potential of the GIP receptor (GIPR) is a subject of debate. Tirzepatide is an agonist at both the GIPR and GLP-1R and is a highly effective treatment for type 2 diabetes and obesity3,4. However, although tirzepatide activates GIPR in cell lines and mouse models, it is not clear whether or how dual agonism contributes to its therapeutic benefit. Islet beta cells express both the GLP-1R and the GIPR, and insulin secretion is an established mechanism by which incretin agonists improve glycemic control5. Here, we show that in mouse islets, tirzepatide stimulates insulin secretion predominantly through the GLP-1R, owing to reduced potency at the mouse GIPR. However, in human islets, antagonizing GIPR activity consistently decreases the insulin response to tirzepatide. Moreover, tirzepatide enhances glucagon secretion and somatostatin secretion in human islets. These data demonstrate that tirzepatide stimulates islet hormone secretion from human islets through both incretin receptors.

The ubiquitination status of the glucagon receptor determines signal bias.

The pancreatic hormone glucagon activates the glucagon receptor (GCGR), a class B seven-transmembrane G protein-coupled receptor that couples to the stimulatory heterotrimeric G protein and provokes PKA-dependent signaling cascades vital to hepatic glucose metabolism and islet insulin secretion. Glucagon-stimulation also initiates recruitment of the endocytic adaptors, ßarrestin1 and ßarrestin2, which regulate desensitization and internalization of the GCGR. Unlike many other G protein-coupled receptors, the GCGR expressed at the plasma membrane is constitutively ubiquitinated and upon agonist-activation, internalized GCGRs are deubiquitinated at early endosomes and recycled via Rab4-containing vesicles. Herein we report a novel link between the ubiquitination status and signal transduction mechanism of the GCGR. In the deubiquitinated state, coupling of the GCGR to Gs is diminished, while binding to ßarrestin is enhanced with signaling biased to a ßarrestin1-dependent p38 mitogen activated protein kinase (MAPK) pathway. This ubiquitin-dependent signaling bias arises through the modification of lysine333 (K333) on the cytoplasmic face of transmembrane helix V. Compared with the GCGR-WT, the mutant GCGR-K333R has impaired ubiquitination, diminished G protein coupling, and PKA signaling but unimpaired potentiation of glucose-stimulated-insulin secretion in response to agonist-stimulation, which involves p38 MAPK signaling. Both WT and GCGR-K333R promote the formation of glucagon-induced ßarrestin1-dependent p38 signaling scaffold that requires canonical upstream MAPK-Kinase3, but is independent of Gs, Gi, and ßarrestin2. Thus, ubiquitination/deubiquitination at K333 in the GCGR defines the activation of distinct transducers with the potential to influence various facets of glucagon signaling in health and disease.

An injectable PEG-like conjugate forms a subcutaneous depot and enables sustained delivery of a peptide drug.

Many biologics have a short plasma half-life, and their conjugation to polyethylene glycol (PEG) is commonly used to solve this problem. However, the improvement in the plasma half-life of PEGylated drugs' is at an asymptote because the development of branched PEG has only had a modest impact on pharmacokinetics and pharmacodynamics. Here, we developed an injectable PEG-like conjugate that forms a subcutaneous depot for the sustained delivery of biologics. The PEG-like conjugate consists of poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) conjugated to exendin, a peptide drug used in the clinic to treat type 2 diabetes. The depot-forming exendin-POEGMA conjugate showed greater efficacy than a PEG conjugate of exendin as well as Bydureon, a clinically approved sustained-release formulation of exendin. The injectable depot-forming exendin-POEGMA conjugate did not elicit an immune response against the polymer, so that it remained effective and safe for long-term management of type 2 diabetes upon chronic administration. In contrast, the PEG conjugate induced an anti-PEG immune response, leading to early clearance and loss of efficacy upon repeat dosing. The exendin-POEGMA depot also showed superior long-term efficacy compared to Bydureon. Collectively, these results suggest that an injectable POEGMA conjugate of biologic drugs that forms a drug depot under the skin, providing favorable pharmacokinetic properties and sustained efficacy while remaining non-immunogenic, offers significant advantages over other commonly used drug delivery technologies.

LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial.

BACKGROUND: Treating hyperglycaemia and obesity in individuals with type 2 diabetes using multi-receptor agonists can improve short-term and long-term outcomes. LY3437943 is a single peptide with agonist activity for glucagon, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide 1 (GLP-1) receptors that is currently in development for the treatment of type 2 diabetes and for the treatment of obesity and associated comorbidities. We investigated the safety, pharmacokinetics, and pharmacodynamics of multiple weekly doses of LY3437943 in people with type 2 diabetes in a 12-week study. METHODS: In this phase 1b, proof-of-concept, double-blind, placebo-controlled, randomised, multiple-ascending dose trial, adults (aged 20-70 years) with type 2 diabetes for at least 3 months, a glycated haemoglobin A1c (HbA1c) value of 7·0-10·5%, body-mass index of 23-50 kg/m2, and stable bodyweight (<5% change in previous 3 months) were recruited at four centres in the USA. Using an interactive web-response system, participants were randomly assigned to receive once-weekly subcutaneous injections of LY3437943, placebo, or dulaglutide 1·5 mg over a 12-week period. Five ascending dose cohorts were studied, with randomisation in each cohort such that a minimum of nine participants received LY3437943, three received placebo, and one received dulaglutide 1·5 mg within each cohort. The top doses in the two highest dose cohorts were attained via stepwise dose escalations. The primary outcome was to investigate the safety and tolerability of LY3437943, and characterising the pharmacodynamics and pharmacokinetics were secondary outcomes. Safety was analysed in all participants who received at least one dose of study drug, and pharmacodynamics and pharmacokinetics in all participants who received at least one dose of study drug and had evaluable data. This trial is registered at ClinicalTrials.gov, NCT04143802. FINDINGS: Between Dec 18, 2019, and Dec 28, 2020, 210 people were screened, of whom 72 were enrolled, received at least one dose of study drug, and were included in safety analyses. 15 participants had placebo, five had dulaglutide 1·5 mg and, for LY3437943, nine had 0·5 mg, nine had 1·5 mg, 11 had 3 mg, 11 had 3/6 mg, and 12 had 3/6/9/12 mg. 29 participants discontinued the study prematurely. Treatment-emergent adverse events were reported by 33 (63%), three (60%), and eight (54%) participants who received LY3437943, dulaglutide 1·5 mg, and placebo, respectively, with gastrointestinal disorders being the most frequently reported treatment-emergent adverse events. The pharmacokinetics of LY3437943 were dose proportional and its half-life was approximately 6 days. At week 12, placebo-adjusted mean daily plasma glucose significantly decreased from baseline at the three highest dose LY3437943 groups (least-squares mean difference -2·8 mmol/L [90% CI -4·63 to -0·94] for 3 mg; -3·1 mmol/L [-4·91 to -1·22] for 3/6 mg; and -2·9 mmol/L [-4·70 to -1·01] for 3/6/9/12 mg). Placebo-adjusted sHbA1c also decreased significantly in the three highest dose groups (-1·4% [90% CI -2·17 to -0·56] for 3 mg; -1·6% [-2·37 to -0·75] for 3/6 mg; and -1·2% [-2·05 to -0·45] for 3/6/9/12 mg). Placebo-adjusted bodyweight reduction with LY3437943 appeared to be dose dependent (up to -8·96 kg [90% CI -11·16 to -6·75] in the 3/6/9/12 mg group). INTERPRETATION: In this early phase study, LY3437943 showed an acceptable safety profile, and its pharmacokinetics suggest suitability for once-weekly dosing. This finding, together with the pharmacodynamic findings of robust reductions in glucose and bodyweight, provides support for phase 2 development. FUNDING: Eli Lilly and Company.

The past, present, and future physiology and pharmacology of glucagon.

The evolution of glucagon has seen the transition from an impurity in the preparation of insulin to the development of glucagon receptor agonists for use in type 1 diabetes. In type 2 diabetes, glucagon receptor antagonists have been explored to reduce glycemia thought to be induced by hyperglucagonemia. However, the catabolic actions of glucagon are currently being leveraged to target the rise in obesity that paralleled that of diabetes, bringing the pharmacology of glucagon full circle. During this evolution, the physiological importance of glucagon advanced beyond the control of hepatic glucose production, incorporating critical roles for glucagon to regulate both lipid and amino acid metabolism. Thus, it is unsurprising that the study of glucagon has left several paradoxes that make it difficult to distill this hormone down to a simplified action. Here, we describe the history of glucagon from the past to the present and suggest some direction to the future of this field.

Tirzepatide, a dual GIP/GLP-1 receptor co-agonist for the treatment of type 2 diabetes with unmatched effectiveness regrading glycaemic control and body weight reduction.

Tirzepatide is the first dual GIP/GLP-1 receptor co-agonist approved for the treatment of type 2 diabetes in the USA, Europe, and the UAE. Tirzepatide is an acylated peptide engineered to activate the GIP and GLP-1 receptors, key mediators of insulin secretion that are also expressed in regions of the brain that regulate food intake. Five clinical trials in type 2-diabetic subjects (SURPASS 1-5) have shown that tirzepatide at 5-15 mg per week reduces both HbA1c (1.24 to 2.58%) and body weight (5.4-11.7 kg) by amounts unprecedented for a single agent. A sizable proportion of patients (23.0 to 62.4%) reached an HbA1c of < 5.7% (which is the upper limit of the normal range indicating normoglycaemia), and 20.7 to 68.4% lost more than 10% of their baseline body weight. Tirzepatide was significantly more effective in reducing HbA1c and body weight than the selective GLP-1 RA semaglutide (1.0 mg per week), and titrated basal insulin. Adverse events related to tirzepatide were similar to what has been reported for selective GLP-1RA, mainly nausea, vomiting, diarrhoea, and constipation, that were more common at higher doses. Cardiovascular events have been adjudicated across the whole study program, and MACE-4 (nonfatal myocardial infarction, non-fatal stroke, cardiovascular death and hospital admission for angina) events tended to be reduced over up to a 2 year-period, albeit with low numbers of events. For none of the cardiovascular events analysed (MACE-4, or its components) was a hazard ratio > 1.0 vs. pooled comparators found in a meta-analysis covering the whole clinical trial program, and the upper bounds of the confidence intervals for MACE were < 1.3, fulfilling conventional definitions of cardiovascular safety. Tirzepatide was found to improve insulin sensitivity and insulin secretory responses to a greater extent than semaglutide, and this was associated with lower prandial insulin and glucagon concentrations. Both drugs caused similar reductions in appetite, although tirzepatide caused greater weight loss. While the clinical effects of tirzepatide have been very encouraging, important questions remain as to the mechanism of action. While GIP reduces food intake and body weight in rodents, these effects have not been demonstrated in humans. Moreover, it remains to be shown that GIPR agonism can improve insulin secretion in type 2 diabetic patients who have been noted in previous studies to be unresponsive to GIP. Certainly, the apparent advantage of tirzepatide, a dual incretin agonist, over GLP-1RA will spark renewed interest in the therapeutic potential of GIP in type 2 diabetes, obesity and related co-morbidities.

Effects of GLP-1 and GIP on Islet Function in Glucose-Intolerant, Pancreatic-Insufficient Cystic Fibrosis.

Impaired insulin and incretin secretion underlie abnormal glucose tolerance (AGT) in pancreatic insufficient cystic fibrosis (PI-CF). Whether the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) can enhance pancreatic islet function in cystic fibrosis (CF) is not known. We studied 32 adults with PI-CF and AGT randomized to receive either GLP-1 (n = 16) or GIP (n = 16) during glucose-potentiated arginine (GPA) testing of islet function on two occasions, with either incretin or placebo infused, in a randomized, double-blind, cross-over fashion. Another four adults with PI-CF and normal glucose tolerance (NGT) and four matched control participants without CF underwent similar assessment with GIP. In PI-CF with AGT, GLP-1 substantially augmented second-phase insulin secretion but without effect on the acute insulin response to GPA or the proinsulin secretory ratio (PISR), while GIP infusion did not enhance second-phase or GPA-induced insulin secretion but increased the PISR. GIP also did not enhance second-phase insulin in PI-CF with NGT but did so markedly in control participants without CF controls. These data indicate that GLP-1, but not GIP, augments glucose-dependent insulin secretion in PI-CF, supporting the likelihood that GLP-1 agonists could have therapeutic benefit in this population. Understanding loss of GIP's insulinotropic action in PI-CF may lead to novel insights into diabetes pathogenesis.

GLP-1 Receptor Blockade Reduces Stimulated Insulin Secretion in Fasted Subjects With Low Circulating GLP-1.

CONTEXT: Glucagon-like peptide 1 (GLP-1), an insulinotropic peptide released into the circulation from intestinal enteroendocrine cells, is considered a hormonal mediator of insulin secretion. However, the physiological actions of circulating GLP-1 have been questioned because of the short half-life of the active peptide. Moreover, there is mounting evidence for localized, intra-islet mediation of GLP-1 receptor (GLP-1r) signaling including a role for islet dipeptidyl-peptidase 4 (DPP4). OBJECTIVE: To determine whether GLP-1r signaling contributes to insulin secretion in the absence of enteral stimulation and increased plasma levels, and whether this is affected by DPP4. METHODS: Single-site study conducted at an academic medical center of 20 nondiabetic subjects and 13 subjects with type 2 diabetes. This was a crossover study in which subjects received either a DPP4 inhibitor (DPP4i; sitagliptin) or placebo on 2 separate days. On each day they received a bolus of intravenous (IV) arginine during sequential 60-minute infusions of the GLP-1r blocker exendin[9-39] (Ex-9) and saline. The main outcome measures were arginine-stimulated secretion of C-Peptide (C-PArg) and insulin (InsArg). RESULTS: Plasma GLP-1 remained at fasting levels throughout the experiments and IV arginine stimulated both α- and ß-cell secretion in all subjects. Ex-9 infusion reduced C-PArg in both the diabetic and nondiabetic groups by ~14% (P < .03 for both groups). Sitagliptin lowered baseline glycemia but did not affect the primary measures of insulin secretion. However, a significant interaction between sitagliptin and Ex-9 suggested more GLP-1r activation with DPP4i treatment in subjects with diabetes. CONCLUSION: GLP-1r activation contributes to ß-cell secretion in diabetic and nondiabetic people during α-cell activation, but in the absence of increased circulating GLP-1. These results are compatible with regulation of ß-cells by paracrine signals from α-cells. This process may be affected by DPP4 inhibition.