Newer pharmacological interventions directed at gut hormones for obesity

The objective is to review the newer pharmacological interventions for obesity, specifically single, dual and triple incretin receptor agonists that are either available or in the pipeline for treatment of obesity. The three incretin receptor targets are glucagon like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) and glucagon. There are several approved single or dual incretin agonists which can be administered subcutaneously daily (e.g., liraglutide) or weekly (e.g., semaglutide, dulaglutide, and exenatide QW), and other experimental dual or triple incretin agonists. Analogues of amylin, peptide YY and oxyntomodulin, as well as the combination of a GLP1R agonist and GIPR antagonist also are in development. Oral semaglutide (administered daily) is approved for type 2 diabetes mellitus and is on track for regulatory review for obesity. The review includes specifically perspectives on the effects of these mechanisms and pharmacological agents on gastric emptying, which contribute to satiation and weight loss, in addition to the established evidence on effects on central mechanisms controlling appetite. In the future, it is anticipated that small molecule GLP-1 receptor agonists (e.g., oral danuglipron) will be developed for treating obesity. These pharmacological agents are having significant impact on glycaemic control and obesity and on their co-morbidities.

A Review of Incretin Therapies Approved and in Late-Stage Development for Overweight and Obesity Management

OBJECTIVE

To review clinical trial data for incretin therapies that are approved or in late-stage development for overweight or obesity management, along with clinical implications of these therapies and future directions.

METHODS

We searched for clinical trials involving incretin therapies studied specifically for overweight or obesity management in ClinicalTrials.gov and PubMed from registry inception through December 2023.

RESULTS

Glucagon-like peptide-1 (GLP-1) receptor agonism, alone and in combination with glucose-dependent insulinotropic polypeptide (GIP) receptor agonism or glucagon agonism, leads to significant weight reduction in people with overweight or obesity. Newer incretin therapies have demonstrated weight reduction between 15% to 25%, far outpacing non-incretin therapies for weight management and achieving levels of weight loss that may prevent weight-related complications. However, the discontinuation of incretin therapies is associated with weight regain. The main side effects of incretin therapies are transient, mild-to-moderate gastrointestinal side effects - nausea, diarrhea, constipation, and vomiting - that commonly occur in the first 4 to 8 weeks of treatment. There is a rich late-stage pipeline of incretin therapies for weight management, consisting of oral GLP-1 receptor agonists, dual GLP-1/GIP receptor agonists, dual GLP-1/glucagon receptor agonists, triple GLP-1/GIP/glucagon receptor agonists, and combination therapies with nonincretin drugs.

CONCLUSION

Newer incretin therapies for weight management have the potential to improve the treatment for overweight and obesity, the treatment and prevention of weight-related complications, and the individualization of weight management. Ensuring that these therapies are accessible - and that treatment with them is consistent and sustainable - is necessary to translate findings from trials into the real world.

The Road towards Triple Agonists: Glucagon-Like Peptide 1, Glucose-Dependent Insulinotropic Polypeptide and Glucagon Receptor - An Update

Obesity is the fifth leading risk factor for global deaths with numbers continuing to increase worldwide. In the last 20 years, the emergence of pharmacological treatments for obesity based on gastrointestinal hormones has transformed the therapeutic landscape. The successful development of glucagon-like peptide-1 (GLP-1) receptor agonists, followed by the synergistic combined effect of glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 receptor agonists achieved remarkable weight loss and glycemic control in those with the diseases of obesity and type 2 diabetes. The multiple cardiometabolic benefits include improving glycemic control, lipid profiles, blood pressure, inflammation, and hepatic steatosis. The 2023 phase 2 double-blind, randomized controlled trial evaluating a GLP-1/GIP/glucagon receptor triagonist (retatrutide) in patients with the disease of obesity reported 24.2% weight loss at 48 weeks with 12 mg retatrutide. This review evaluates the current available evidence for GLP-1 receptor agonists, dual GLP-1/GIP receptor co-agonists with a focus on GLP-1/GIP/glucagon receptor triagonists and discusses the potential future benefits and research directions.

GIPR Agonism Enhances TZD-Induced Insulin Sensitivity in Obese IR Mice

Recent studies have found that glucose-dependent insulinotropic polypeptide receptor (GIPR) agonism can enhance the metabolic efficacy of glucagon-like peptide-1 receptor agonist treatment by promoting both weight-dependent and -independent improvements on systemic insulin sensitivity. These findings have prompted new investigations aimed at better understanding the broad metabolic benefit of GIPR activation. Herein, we determined whether GIPR agonism favorably influenced the pharmacologic efficacy of the insulin-sensitizing thiazolidinedione (TZD) rosiglitazone in obese insulin-resistant (IR) mice. Genetic and pharmacological approaches were used to examine the role of GIPR signaling on rosiglitazone-induced weight gain, hyperphagia, and glycemic control. RNA sequencing was conducted to uncover potential mechanisms by which GIPR activation influences energy balance and insulin sensitivity. In line with previous findings, treatment with rosiglitazone induced the mRNA expression of the GIPR in white and brown fat. However, obese GIPR-null mice dosed with rosiglitazone had equivalent weight gain to that of wild-type (WT) animals. Strikingly, chronic treatment of obese IR WT animals with a long-acting GIPR agonist prevented rosiglitazone-induced weight-gain and hyperphagia, and it enhanced the insulin-sensitivity effect of this TZD. The systemic insulin sensitization was accompanied by increased glucose disposal in brown adipose tissue, which was underlined by the recruitment of metabolic and thermogenic genes. These findings suggest that GIPR agonism can counter the negative consequences of rosiglitazone treatment on body weight and adiposity, while improving its insulin-sensitizing efficacy at the same time.

ARTICLE HIGHLIGHTS

Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial

Importance

The effect of continued treatment with tirzepatide on maintaining initial weight reduction is unknown.

Objective

To assess the effect of tirzepatide, with diet and physical activity, on the maintenance of weight reduction.

Design, Setting, and Participants

This phase 3, randomized withdrawal clinical trial conducted at 70 sites in 4 countries with a 36-week, open-label tirzepatide lead-in period followed by a 52-week, double-blind, placebo-controlled period included adults with a body mass index greater than or equal to 30 or greater than or equal to 27 and a weight-related complication, excluding diabetes.

Interventions

Participants (n = 783) enrolled in an open-label lead-in period received once-weekly subcutaneous maximum tolerated dose (10 or 15 mg) of tirzepatide for 36 weeks. At week 36, a total of 670 participants were randomized (1:1) to continue receiving tirzepatide (n = 335) or switch to placebo (n = 335) for 52 weeks.

Main Outcomes and Measures

The primary end point was the mean percent change in weight from week 36 (randomization) to week 88. Key secondary end points included the proportion of participants at week 88 who maintained at least 80% of the weight loss during the lead-in period.

Results

Participants (n = 670; mean age, 48 years; 473 [71%] women; mean weight, 107.3 kg) who completed the 36-week lead-in period experienced a mean weight reduction of 20.9%. The mean percent weight change from week 36 to week 88 was -5.5% with tirzepatide vs 14.0% with placebo (difference, -19.4% [95% CI, -21.2% to -17.7%]; P < .001). Overall, 300 participants (89.5%) receiving tirzepatide at 88 weeks maintained at least 80% of the weight loss during the lead-in period compared with 16.6% receiving placebo (P < .001). The overall mean weight reduction from week 0 to 88 was 25.3% for tirzepatide and 9.9% for placebo. The most common adverse events were mostly mild to moderate gastrointestinal events, which occurred more commonly with tirzepatide vs placebo.

Conclusions and Relevance

In participants with obesity or overweight, withdrawing tirzepatide led to substantial regain of lost weight, whereas continued treatment maintained and augmented initial weight reduction.

Trial Registration

ClinicalTrials.gov Identifier: NCT04660643.

The dual incretin co-agonist tirzepatide increases both insulin secretion and glucose effectiveness in model experiments in mice

Tirzepatide is a dual GIP and GLP-1 receptor co-agonist which is approved for glucose-lowering therapy in type 2 diabetes. Here, we explored its effects on beta cell function, insulin sensitivity and insulin-independent glucose elimination (glucose effectiveness) in normal mice. Anesthetized female C57/BL/6 J mice were injected intravenously with saline or glucose (0.125, 0.35 or 0.75 g/kg) with or without simultaneous administration of synthetic tirzepatide (3 nmol/kg). Samples were taken at 0, 1, 5, 10, 20 and 50 min. Glucose elimination rate was estimated by the percentage reduction in glucose from min 5 to min 20 (KG). The 50 min areas under the curve (AUC) for insulin and glucose were determined. Beta cell function was assessed as AUCinsulin divided by AUCglucose. Insulin sensitivity (SI) and glucose effectiveness (SG) were determined by minimal model analysis of the insulin and glucose data. Tirzepatide glucose-dependently reduced glucose levels and increased insulin levels. The slope for the regression of AUCinsulin versus AUCglucose was increased 7-fold by tirzepatide from 0.014 ± 0.004 with glucose only to 0.099 ± 0.016 (P < 0.001). SI was not affected by tirzepatide, whereas SG was increased by 78% (P < 0.001). The increase in SG contributed to an increase in KG by 74 ± 4% after glucose alone and by 67 ± 8% after glucose+ tirzepatide, whereas contribution by SI times AUCinsulin insulin (i.e., disposition index) was 26 ± 4% and 33 ± 8%, respectively. In conclusion, tirzepatide stimulates both insulin secretion and glucose effectiveness, with stimulation of glucose effectiveness being the prominent process to reduce glucose.

Mechanisms of action of incretin receptor based dual- and tri-agonists in pancreatic islets

Simultaneous activation of the incretin G-protein-coupled receptors (GPCRs) via unimolecular dual-receptor agonists (UDRA) has emerged as a new therapeutic approach for type 2 diabetes. Recent studies also advocate triple agonism with molecules also capable of binding the glucagon receptor. In this scoping review, we discuss the cellular mechanisms of action (MOA) underlying the actions of these novel and therapeutically important classes of peptide receptor agonists. Clinical efficacy studies of several UDRAs have demonstrated favorable results both as monotherapies and when combined with approved hypoglycemics. Although the additive insulinotropic effects of dual glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic peptide receptor (GIPR) agonism were anticipated based on the known actions of either glucagon-like peptide-1 (GLP-1) or glucose-dependent insulinotropic peptide (GIP) alone, the additional benefits from GCGR were largely unexpected. Whether additional synergistic or antagonistic interactions among these G-protein receptor signaling pathways arise from simultaneous stimulation is not known. The signaling pathways affected by dual- and tri-agonism require more trenchant investigation before a comprehensive understanding of the cellular MOA. This knowledge will be essential for understanding the chronic efficacy and safety of these treatments.

GLP-1 receptor agonism and GIP receptor antagonism induce substantial alterations in enteroendocrine and islet cell populations in obese high fat fed mice

Effects of sustained activation of glucagon-like peptide-1 (GLP-1) receptors (GLP-1R) as well as antagonism of receptors for glucose-dependent insulinotropic peptide (GIP) on intestinal morphology and related gut hormone populations have not been fully investigated. The present study assesses the impact of 21-days twice daily treatment with the GLP-1R agonist exendin-4 (Ex-4), or the GIP receptor (GIPR) antagonist mGIP(3-30), on these features in obese mice fed a high fat diet (HFD). HFD mice presented with reduced crypt depth when compared to normal diet (ND) controls, which was reversed by Ex-4 treatment. Both regimens lead to an enlargement of villi length in HFD mice. HFD mice had increased numbers of GIP and PYY positive ileal cells, with both treatment interventions reversing the effect on PYY positive cells, but only Ex-4 restoring GIP ileal cell populations to ND levels. Ex-4 and mGIP (3-30) marginally decreased GLP-1 villi immunoreactivity and countered the reduction of ileal GLP-1 content caused by HFD. As expected, HFD mice presented with elevated pancreatic islet area. Interestingly, mGIP(3-30), but not Ex-4, enhanced islet and beta-cell areas in HFD mice despite lack of effect of beta-cell turnover, whilst Ex-4 increased delta-cell area. Co-localisation of islet PYY or GLP-1 with glucagon was increased by Ex-4, whilst islet PYY co-immunoreactivity with somatostatin was enhanced by mGIP(3-30) treatment. These observations highlight potential new mechanisms linked to the metabolic benefits of GLP-1R agonism and GIPR antagonism in obesity.

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

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

Molecular basis of signal transduction mediated by the human GIPR splice variants

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is a potential drug target for metabolic disorders. It works with glucagon-like peptide-1 receptor and glucagon receptor in humans to maintain glucose homeostasis. Unlike the other two receptors, GIPR has at least 13 reported splice variants (SVs), more than half of which have sequence variations at either C or N terminus. To explore their roles in endogenous peptide-mediated GIPR signaling, we determined the cryoelectron microscopy (cryo-EM) structures of the two N terminus-altered SVs (referred as GIPR-202 and GIPR-209 in the Ensembl database, SV1 and SV2 here, respectively) and investigated the outcome of coexpressing each of them in question with GIPR in HEK293T cells with respect to ligand binding, receptor expression, cAMP (adenosine 3,5-cyclic monophosphate) accumulation, β-arrestin recruitment, and cell surface localization. It was found that while both N terminus-altered SVs of GIPR neither bound to the hormone nor elicited signal transduction per se, they suppressed ligand binding and cAMP accumulation of GIPR. Meanwhile, SV1 reduced GIPR-mediated β-arrestin 2 responses. The cryo-EM structures of SV1 and SV2 showed that they reorganized the extracellular halves of transmembrane helices 1, 6, and 7 and extracellular loops 2 and 3 to adopt a ligand-binding pocket-occupied conformation, thereby losing binding ability to the peptide. The results suggest a form of signal bias that is constitutive and ligand-independent, thus expanding our knowledge of biased signaling beyond pharmacological manipulation (i.e., ligand specific) as well as constitutive and ligand-independent (e.g., SV1 of the growth hormone-releasing hormone receptor).

Acarbose diminishes postprandial suppression of bone resorption in patients with type 2 diabetes

AIMS

The alpha-glucosidase inhibitor acarbose is an antidiabetic drug delaying assimilation of carbohydrates and, thus, increasing the amount of carbohydrates in the distal parts of the intestines, which in turn increases circulating levels of the gut-derived incretin hormone glucagon-like peptide 1 (GLP-1). As GLP-1 may suppress bone resorption, acarbose has been proposed to potentiate meal-induced suppression of bone resorption. We investigated the effect of acarbose treatment on postprandial bone resorption in patients with type 2 diabetes and used the GLP-1 receptor antagonist exendin(9-39)NH₂ to disclose contributory effect of acarbose-induced GLP-1 secretion.

METHODS

In a randomised, placebo-controlled, double-blind, crossover study, 15 participants with metformin-treated type 2 diabetes (2 women/13 men, age 71 (57-85 years), BMI 29.7 (23.6-34.6 kg/m²), HbA1c 48 (40-74 mmol/mol)/6.5 (5.8-11.6 %) (median and range)) were subjected to two 14-day treatment periods with acarbose and placebo, respectively, separated by a six-week wash-out period. At the end of each period, circulating bone formation and resorption markers were assessed during two randomised 4-h liquid mixed meal tests (MMT) with infusions of exendin(9-39)NH₂ and saline, respectively. Glucagon-like peptide 2 (GLP-2) was also assessed.

RESULTS

Compared to placebo, acarbose impaired the MMT-induced suppression of CTX as assessed by baseline-subtracted area under curve (P = 0.0037) and nadir of CTX (P = 0.0128). During acarbose treatment, exendin(9-39)NH₂ infusion lowered nadir of CTX compared to saline (P = 0.0344). Neither parathyroid hormone or the bone formation marker procollagen 1 intact N-terminal propeptide were affected by acarbose or GLP-1 receptor antagonism. Acarbose treatment induced a greater postprandial GLP-2 response than placebo treatment (P = 0.0479) and exendin(9-39)NH₂ infusion exacerbated this (P = 0.0002).

CONCLUSIONS

In patients with type 2 diabetes, treatment with acarbose reduced postprandial suppression of bone resorption. Acarbose-induced GLP-1 secretion may contribute to this phenomenon as the impairment was partially reversed by GLP-1 receptor antagonism. Also, acarbose-induced reductions in other factors reducing bone resorption, e.g. glucose-dependent insulinotropic polypeptide, may contribute.

Beyond the pancreas: contrasting cardiometabolic actions of GIP and GLP1

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP1) exhibit incretin activity, meaning that they potentiate glucose-dependent insulin secretion. The emergence of GIP receptor (GIPR)-GLP1 receptor (GLP1R) co-agonists has fostered growing interest in the actions of GIP and GLP1 in metabolically relevant tissues. Here, we update concepts of how these hormones act beyond the pancreas. The actions of GIP and GLP1 on liver, muscle and adipose tissue, in the control of glucose and lipid homeostasis, are discussed in the context of plausible mechanisms of action. Both the GIPR and GLP1R are expressed in the central nervous system, wherein receptor activation produces anorectic effects enabling weight loss. In preclinical studies, GIP and GLP1 reduce atherosclerosis. Furthermore, GIPR and GLP1R are expressed within the heart and immune system, and GLP1R within the kidney, revealing putative mechanisms linking GIP and GLP1R agonism to cardiorenal protection. We interpret the clinical and mechanistic data obtained for different agents that enable weight loss and glucose control for the treatment of obesity and type 2 diabetes mellitus, respectively, by activating or blocking GIPR signalling, including the GIPR-GLP1R co-agonist tirzepatide, as well as the GIPR antagonist-GLP1R agonist AMG-133. Collectively, we update translational concepts of GIP and GLP1 action, while highlighting gaps, areas of uncertainty and controversies meriting ongoing investigation.

Enhanced Endosomal Signaling and Desensitization of GLP-1R vs GIPR in Pancreatic Beta Cells

The incretin receptors, glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR), are prime therapeutic targets for the treatment of type 2 diabetes (T2D) and obesity. They are expressed in pancreatic beta cells where they potentiate insulin release in response to food intake. Despite GIP being the main incretin in healthy individuals, GLP-1R has been favored as a therapeutic target due to blunted GIPR responses in T2D patients and conflicting effects of GIPR agonists and antagonists in improving glucose tolerance and preventing weight gain. There is, however, a recently renewed interest in GIPR biology, following the realization that GIPR responses can be restored after an initial period of blood glucose normalization and the recent development of dual GLP-1R/GIPR agonists with superior capacity for controlling blood glucose levels and weight. The importance of GLP-1R trafficking and subcellular signaling in the control of receptor outputs is well established, but little is known about the pattern of spatiotemporal signaling from the GIPR in beta cells. Here, we have directly compared surface expression, trafficking, and signaling characteristics of both incretin receptors in pancreatic beta cells to identify potential differences that might underlie distinct pharmacological responses associated with each receptor. Our results indicate increased cell surface levels, internalization, degradation, and endosomal vs plasma membrane activity for the GLP-1R, while the GIPR is instead associated with increased plasma membrane recycling, reduced desensitization, and enhanced downstream signal amplification. These differences might have potential implications for the capacity of each incretin receptor to control beta cell function.

An update on peptide-based therapies for type 2 diabetes and obesity

Long-acting analogues of the naturally occurring incretin, glucagon-like peptide-1 (GLP-1) and those modified to interact also with receptors for glucose-dependent insulinotropic polypeptide (GIP) have shown high glucose-lowering and weight-lowering efficacy when administered by once-weekly subcutaneous injection. These analogues herald an exciting new era in peptide-based therapy for type 2 diabetes (T2D) and obesity. Of note is the GLP-1R agonist semaglutide, available in oral and injectable formulations and in clinical trials combined with the long-acting amylin analogue, cagrilintide. Particularly high efficacy in both glucose- and weight lowering capacities has also been observed with the GLP-1R/GIP-R unimolecular dual agonist, tirzepatide. In addition, a number of long-acting unimolecular GLP-1R/GCGR dual agonist peptides and GLP-1R/GCGR/GIPR triagonist peptides have entered clinical trials. Other pharmacological approaches to chronic weight management include the human monoclonal antibody, bimagrumab which blocks activin type II receptors and is associated with growth of skeletal muscle, an antibody blocking activation of GIPR to which are conjugated GLP-1R peptide agonists (AMG-133), and the melanocortin-4 receptor agonist, setmelanotide for use in certain inherited obesity conditions. The high global demand for the GLP-1R agonists liraglutide and semaglutide as anti-obesity agents has led to shortage so that their use in T2D therapy is currently being prioritized.

Glucose metabolism, gut-brain hormones, and acromegaly treatment: an explorative single centre descriptive analysis

PURPOSE

Active acromegaly is associated with impaired glucose metabolism, which improves upon treatment. Treatment options include surgery, medical therapy with somatostatin analogues (SSA) and Pegvisomant (PEG), and irradiation. The objective of the study was to describe the differential effect of various treatment regimens on the secretion of glucose, insulin, glucagon, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) in patients with acromegaly.

METHODS

23 surgically treated, non-diabetic patients with acromegaly and 12 healthy controls underwent an oral glucose tolerance test (OGTT) and subsequently isoglycaemic intravenous glucose infusion on a separate day. Baseline hormone concentrations, time-to-peak and area under the curve (AUC) on the OGTT-day and incretin effect were compared according to treatment regimens.

RESULTS

The patients treated with SSA (N = 15) had impaired GIP-response (AUC, P = 0.001), and numerical impairment of all other hormone responses (P > 0.3). Patients co-treated with PEG (SSA + PEG, N = 4) had increased secretion of insulin and glucagon compared to patients only treated with SSA (SSA ÷ PEG, N = 11) (insulin_AUC mean ± SEM, SSA + PEG 49 ± 8.3 nmol/l*min vs SSA ÷ PEG 25 ± 3.4, P = 0.007; glucagon_AUC, SSA + PEG 823 ± 194 pmol/l*min vs SSA ÷ PEG 332 ± 69, P = 0.009). GIP secretion remained significantly impaired, whereas GLP-1 secretion was numerically increased with PEG (SSA + PEG 3088 ± 366 pmol/l*min vs SSA ÷ PEG 2401 ± 239, P = 0.3). No difference was found in patients treated with/without radiotherapy nor substituted or not with hydrocortisone.

CONCLUSION

SSA impaired the insulin, glucagon, and incretin hormone secretions. Co-treatment with PEG seemed to counteract the somatostatinergic inhibition of the glucagon and insulin response to OGTT. We speculate that PEG may exert its action through GH-receptors on pancreatic δ-cells. Clinical trial registration NCT02005978.

Tirzepatide: A New Generation Therapeutic for Diabetes Type 2

Tirzepatide (mounjaro®) is a derivative of the human glucose-dependent insulinotropic polypeptide (GIP) hormone with a position-20 being modified with 1,20- eicosanedioic acid via a chemical linker. It acts as a glucagon-like peptide-1 (GLP-1) receptor and GIP receptor agonist. It has recently been approved by FDA as an adjunct therapy to exercise and diet to improve glycemic control in patients with type II diabetes mellitus (T2DM). It represents a new transforming paradigm in the management of T2DM. This mini-review will shed light on its different pharmacokinetic and pharmacodynamic aspects.

New Horizons: Emerging Antidiabetic Medications

Over the past century, since the discovery of insulin, the therapeutic offer for diabetes has grown exponentially, in particular for type 2 diabetes (T2D). However, the drugs in the diabetes pipeline are even more promising because of their impressive antihyperglycemic effects coupled with remarkable weight loss. An ideal medication for T2D should target not only hyperglycemia but also insulin resistance and obesity. Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) and the new class of GLP1 and gastric inhibitory polypeptide dual RAs counteract 2 of these metabolic defects of T2D, hyperglycemia and obesity, with stunning results that are similar to the effects of metabolic surgery. An important role of antidiabetic medications is to reduce the risk and improve the outcome of cardiovascular diseases, including coronary artery disease and heart failure with reduced or preserved ejection fraction, as well as diabetic nephropathy, as shown by SGLT2 inhibitors. This review summarizes the main drugs currently under development for the treatment of type 1 diabetes and T2D, highlighting their strengths and side effects.

GIP Affects Hepatic Fat and Brown Adipose Tissue Thermogenesis but Not White Adipose Tissue Transcriptome in Type 1 Diabetes

CONTEXT

Glucose-dependent insulinotropic polypeptide (GIP) has been proposed to exert insulin-independent effects on lipid and bone metabolism.

OBJECTIVE

We investigated the effects of a 6-day subcutaneous GIP infusion on circulating lipids, white adipose tissue (WAT), brown adipose tissue (BAT), hepatic fat content, inflammatory markers, respiratory exchange ratio (RER), and bone homeostasis in patients with type 1 diabetes.

METHODS

In a randomized, placebo-controlled, double-blind, crossover study, 20 men with type 1 diabetes underwent a 6-day continuous subcutaneous infusion with GIP (6 pmol/kg/min) and placebo (saline), with an interposed 7-day washout period.

RESULTS

During GIP infusion, participants (26 ± 8 years [mean ± SD]; BMI 23.8 ± 1.8 kg/m2; glycated hemoglobin A1c 51 ± 10 mmol/mol [6.8 ± 3.1%]) experienced transiently increased circulating concentrations of nonesterified fatty acid (NEFA) (P = 0.0005), decreased RER (P = 0.009), indication of increased fatty acid β-oxidation, and decreased levels of the bone resorption marker C-terminal telopeptide (P = 0.000072) compared with placebo. After 6 days of GIP infusion, hepatic fat content was increased by 12.6% (P = 0.007) and supraclavicular skin temperature, a surrogate indicator of BAT activity, was increased by 0.29 °C (P < 0.000001) compared with placebo infusion. WAT transcriptomic profile as well as circulating lipid species, proteome, markers of inflammation, and bone homeostasis were unaffected.

CONCLUSION

Six days of subcutaneous GIP infusion in men with type 1 diabetes transiently decreased bone resorption and increased NEFA and β-oxidation. Further, hepatic fat content, and supraclavicular skin temperature were increased without affecting WAT transcriptomics, the circulating proteome, lipids, or inflammatory markers.

Evidence for Involvement of GIP and GLP-1 Receptors and the Gut-Gonadal Axis in Regulating Female Reproductive Function in Mice

Substantial evidence suggests crosstalk between reproductive and gut-axis but mechanisms linking metabolism and reproduction are still unclear. The present study evaluated the possible role of glucose-dependent-insulinotropic-polypeptide (GIP) and glucagon-like-peptide-1 (GLP-1) in reproductive function by examining receptor distribution and the effects of global GIPR and GLP-1R deletion on estrous cycling and reproductive outcomes in mice. GIPR and GLP-1R gene expression were readily detected by PCR in female reproductive tissues including pituitary, ovaries and uterine horn. Protein expression was confirmed with histological visualisation of incretin receptors using GIPR-Cre and GLP1R-Cre mice in which the incretin receptor expressing cells were fluorescently tagged. Functional studies revealed that female GIPR^-/- and GLP-1R^-/- null mice exhibited significantly (p < 0.05 and p < 0.01) deranged estrous cycling compared to wild-type controls, indicative of reduced fertility. Furthermore, only 50% and 16% of female GIPR^-/- and GLP-1R^-/- mice, respectively produced litters with wild-type males across three breeding cycles. Consistent with a physiological role of incretin receptors in pregnancy outcome, litter size was significantly (p < 0.001-p < 0.05) decreased in GIPR^-/- and GLP-1R^-/- mice. Treatment with oral metformin (300 mg/kg body-weight), an agent used clinically for treatment of PCOS, for a further two breeding periods showed no amelioration of pregnancy outcome except that litter size in the GIPR^-/- group was approximately 2 times greater in the second breeding cycle. These data highlight the significance of incretin receptors in modulation of female reproductive function which may provide future targets for pharmacological intervention in reproductive disorders.

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.

Establishment of an enzyme-linked immunosorbent assay for mouse pancreatic polypeptide clarifies the regulatory mechanism of its secretion from pancreatic γ cells

Pancreatic polypeptide (PP), secreted from γ cells of the islets of Langerhans, is a 36 amino-acid peptide encoded by the Ppy gene. Although previous studies have reported that PP causes a decrease in appetite, the molecular mechanism that regulates PP secretion has not been fully elucidated. Lack of understanding of the regulatory mechanism of PP secretion may be partially owing to the lack of assay systems that can specifically detect PP. We recently developed the mouse monoclonal antibody 23-2D3 that specifically recognizes PP. In the present study, we developed a sandwich enzyme-linked immunosorbent assay for the measurement of mouse PP, and directly monitored intracellular Ca²⁺ concentrations in Ppy-expressing cells from a newly developed reporter mouse. Using these systems, we identified agonists, such as carbachol and glucose-dependent insulinotropic polypeptide (GIP), which stimulate PP secretion. We further demonstrated that, unlike the case of GIP-induced insulin secretion from β cells, there is a unique mechanism by which PP secretion is triggered by an increase in intracellular Ca²⁺ concentrations via voltage-dependent calcium channels even in low-glucose conditions.

Obesity-associated Blunted Subcutaneous Adipose Tissue Blood Flow After Meal Improves After Bariatric Surgery

CONTEXT

Glucose-dependent insulinotropic peptide (GIP) and meal ingestion increase subcutaneous adipose tissue (SAT) perfusion in healthy individuals. The effects of GIP and a meal on visceral adipose tissue (VAT) perfusion are unclear.

OBJECTIVE

Our aim was to investigate the effects of meal and GIP on VAT and SAT perfusion in obese individuals with type 2 diabetes mellitus (T2DM) before and after bariatric surgery.

METHODS

We recruited 10 obese individuals with T2DM scheduled for bariatric surgery and 10 control individuals. Participants were studied under 2 stimulations: meal ingestion and GIP infusion. SAT and VAT perfusion was measured using 15O-H2O positron emission tomography-magnetic resonance imaging at 3 time points: baseline, 20 minutes, and 50 minutes after the start of stimulation. Obese individuals were studied before and after bariatric surgery.

RESULTS

Before bariatric surgery the responses of SAT perfusion to meal (P = .04) and GIP-infusion (P = .002) were blunted in the obese participants compared to controls. VAT perfusion response did not differ between obese and control individuals after a meal or GIP infusion. After bariatric surgery SAT perfusion response to a meal was similar to that of controls. SAT perfusion response to GIP administration remained lower in the operated-on than control participants. There was no change in VAT perfusion response after bariatric surgery.

CONCLUSION

The vasodilating effects of GIP and meal are blunted in SAT but not in VAT in obese individuals with T2DM. Bariatric surgery improves the effects of a meal on SAT perfusion, but not the effects of GIP. Postprandial increase in SAT perfusion after bariatric surgery seems to be regulated in a GIP-independent manner.

The GIP/GIPR axis in medullary thyroid cancer: clinical and molecular findings

The improper expression of glucose-dependent insulinotropic polypeptide receptor (GIPR) and the GIP/GIPR axis activation has been increasingly recognized in endocrine tumors, with a potential diagnostic and prognostic value. A high tumor-to-normal tissue ratio (T/N ratio) of GIPR was reported both in humans' and in rats' medullary thyroid cancer (MTC), suggesting a direct link between the neoplastic transformation and the mechanism of receptor overexpression. In this study, we evaluated the potential diagnostic and prognostic significance of GIPR expression in a large cohort of MTC patients by correlating GIPR mRNA steady-state levels to clinical phenotypes. The molecular effect of GIP/GIPR axis stimulation in MTC-derived cells was also determined. We detected GIPR expression in ~80% of tumor specimens, especially in sporadic, larger, advanced-stage cancers with higher Ki-67 values. GIPR stimulation induced cAMP elevation in MTC-derived cells and a small but significant fluctuation in Ca2+, both likely associated with increased calcitonin secretion. On the contrary, the effects on PI3K-Akt and MAPK-ERK1/2 signaling pathways were marginal. To conclude, our data confirm the high T/N GIPR ratio in MTC tumors and suggest that it may represent an index for the degree of advancement of the malignant process. We have also observed a functional coupling between GIP/GIPR axis and calcitonin secretion in MTC models. However, the molecular mechanisms underlying this process and the possible implication of GIP/GIPR axis activation in MTC diagnosis and prognosis need further evaluation.

Role and mechanism(s) of incretin-dependent therapies for treating diabetes mellitus

Diabetes mellitus (DM) is a worldwide ailment which leads to chronic complications like cardiac disorders, renal perturbations, limb amputation and blindness. Type one diabetes (T1DM), Type two diabetes (T2DM), Another types of diabetes, such as genetic errors in function of β-cell and action of insulin, cystic fibrosis, chemical-instigated diabetes or following tissue transplantation), and pregnancy DM (GDM). In response to nutritional ingestion, the gut may release a pancreatic stimulant that affects carbohydrate metabolism. The duodenum produces a 'chemical excitant' that stimulates pancreatic output, and researchers have sought to cure diabetes using gut extract injections, coining the word 'incretin' to describe the phenomena. Incretins include GIP and GLP-1. The 'enteroinsular axis' is the link between pancreas and intestine. Nutrient, neuronal and hormonal impulses from intestine to cells secreting insulin were thought to be part of this axis. In addition, the hormonal component, incretin, must meet two requirements: (1) it secreted by foods, mainly carbohydrates, and (2) it must induce an insulinotropic effect which is glucose-dependent. In this review, we clarify the ability of using incretin-dependent treatments for treating DM.

GIP and GLP-2 together improve bone turnover in humans supporting GIPR-GLP-2R co-agonists as future osteoporosis treatment

The intestinal hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are key regulators of postprandial bone turnover in humans. We hypothesized that GIP and GLP-2 co-administration would provide stronger effect on bone turnover than administration of the hormones separately, and tested this using subcutaneous injections of GIP and GLP-2 alone or in combination in humans. Guided by these findings, we designed series of GIPR-GLP-2R co-agonists as template for new osteoporosis treatment. The clinical experiment was a randomized cross-over design including 10 healthy men administered subcutaneous injections of GIP and GLP-2 alone or in combination. The GIPR-GLP-2R co-agonists were characterized in terms of binding and activation profiles on human and rodent GIP and GLP-2 receptors, and their pharmacokinetic (PK) profiles were improved by dipeptidyl peptidase-4 protection and site-directed lipidation. Co-administration of GIP and GLP-2 in humans resulted in an additive reduction in bone resorption superior to each hormone individually. The GIPR-GLP-2R co-agonists, designed by combining regions of importance for cognate receptor activation, obtained similar efficacies as the two native hormones and nanomolar potencies on both human receptors. The PK-improved co-agonists maintained receptor activity along with their prolonged half-lives. Finally, we found that the GIPR-GLP-2R co-agonists optimized toward the human receptors for bone remodeling are not feasible for use in rodent models. The successful development of potent and efficacious GIPR-GLP-2R co-agonists, combined with the improved effect on bone metabolism in humans by co-administration, support these co-agonists as a future osteoporosis treatment.

Incretins Enhance PGF2α-Induced Synthesis of IL-6 and Osteoprotegerin in Osteoblasts

Incretins including glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), which are secreted from the small intestine after oral food ingestion, are currently well-known to stimulate insulin secretion from pancreatic β-cells and used for the treatment of type 2 diabetes mellitus. We have previously reported that prostaglandin F_2α (PGF_2α) stimulates the synthesis of interleukin-6 (IL-6) and osteoprotegerin in osteoblast-like MC3T3-E1 cells, and that IL-6 and osteoprotegerin release are mediated through the p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase or stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) pathways. In the present study, we investigated the effects of incretins including GLP-1 and GIP, on the PGF_2α-induced synthesis of IL-6 and osteoprotegerin and examined the detailed mechanism in osteoblast-like MC3T3-E1 cells. We found that GIP and GLP-1 significantly stimulated the PGF_2α-induced synthesis of IL-6 in osteoblast-like MC3T3-E1 cells. In addition, GIP and GLP-1 significantly enhanced the PGF_2α-induced mRNA expression levels of IL-6. On the other hand, GIP and GLP-1 markedly stimulated the PGF_2α-induced synthesis of osteoprotegerin. However, the phosphorylation of p44/p42 MAP kinase, p38 MAP kinase, or JNK induced by PGF_2α was not affected by GIP or GLP-1. Therefore, these results strongly suggest that incretins enhance the PGF_2α-induced synthesis of IL-6 and osteoprotegerin in osteoblast-like MC3T3-E1 cells. However, these syntheses are not mediated through p44/p42 MAP kinase, p38 MAP kinase, or JNK pathways.

Noninvasive Evaluation of GIP Effects on β-Cell Mass Under High-Fat Diet

Pancreatic β-cell mass (BCM) has an importance in the pathophysiology of diabetes mellitus. Recently, glucagon-like peptide-1 receptor (GLP-1R)-targeted imaging has emerged as a promising tool for BCM evaluation. While glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide (GIP) is known to be involved in high-fat diet (HFD)-induced obesity, the effect of GIP on BCM is still controversial. In this study, we investigated indium 111 (¹¹¹In)-labeled exendin-4 derivative ([Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4) single-photon emission computed tomography/computed tomography (SPECT/CT) as a tool for evaluation of longitudinal BCM changes in HFD-induced obese mice, at the same time we also investigated the effects of GIP on BCM in response to HFD using GIP-knockout (GIP^-/-) mice. ¹¹¹In-exendin-4 SPECT/CT was able to distinguish control-fat diet (CFD)-fed mice from HFD-fed mice and the pancreatic uptake values replicated the BCM measured by conventional histological methods. Furthermore, BCM expansions in HFD-fed mice were demonstrated by time-course changes of the pancreatic uptake values. Additionally, ¹¹¹In-exendin-4 SPECT/CT demonstrated the distinct changes in BCM between HFD-fed GIP^-/- (GIP^-/-+HFD) and wild-type (WT+HFD) mice; the pancreatic uptake values of GIP^-/-+HFD mice became significantly lower than those of WT+HFD mice. The different changes in the pancreatic uptake values between the two groups preceded those in fat accumulation and insulin resistance. Taken together with the finding of increased β-cell apoptosis in GIP^-/-+HFD mice compared with WT+HFD mice, these data indicated that GIP has preferable effects on BCM under HFD. Therefore, ¹¹¹In-exendin-4 SPECT/CT can be useful for evaluating increasing BCM and the role of GIP in BCM changes under HFD conditions.

The new dual gastric inhibitory peptide/glucagon-like peptide 1 agonist tirzepatide in type 2 diabetes: Is the future bright?

Tirzepatide is a dual gastric inhibitory peptide/glucagon-like peptide 1 (GIP/GLP-1) receptor agonist formulated as a synthetic linear peptide, based on the native GIP sequence. It has a prolonged half-life of 5 days, which enables once-weekly dosing. Studies have hitherto demonstrated its superiority in achieving optimal glycaemic control and body weight management, as compared with various agents used in the treatment of type 2 diabetes mellitus (T2DM), including GLP-1 receptor agonists. Thus, it is expected to enrich our therapeutic armamentarium in T2DM. However, further experience, notably longer follow-up data and information on cardiovascular effects, is still needed.

GIPR agonism mediates weight-independent insulin sensitization by tirzepatide in obese mice

Tirzepatide (LY3298176), a dual GIP and GLP-1 receptor (GLP-1R) agonist, delivered superior glycemic control and weight loss compared with GLP-1R agonism in patients with type 2 diabetes. However, the mechanism by which tirzepatide improves efficacy and how GIP receptor (GIPR) agonism contributes is not fully understood. Here, we show that tirzepatide is an effective insulin sensitizer, improving insulin sensitivity in obese mice to a greater extent than GLP-1R agonism. To determine whether GIPR agonism contributes, we compared the effect of tirzepatide in obese WT and Glp-1r-null mice. In the absence of GLP-1R-induced weight loss, tirzepatide improved insulin sensitivity by enhancing glucose disposal in white adipose tissue (WAT). In support of this, a long-acting GIPR agonist (LAGIPRA) was found to enhance insulin sensitivity by augmenting glucose disposal in WAT. Interestingly, the effect of tirzepatide and LAGIPRA on insulin sensitivity was associated with reduced branched-chain amino acids (BCAAs) and ketoacids in the circulation. Insulin sensitization was associated with upregulation of genes associated with the catabolism of glucose, lipid, and BCAAs in brown adipose tissue. Together, our studies show that tirzepatide improved insulin sensitivity in a weight-dependent and -independent manner. These results highlight how GIPR agonism contributes to the therapeutic profile of dual-receptor agonism, offering mechanistic insights into the clinical efficacy of tirzepatide.

Effects of endogenous GIP in patients with type 2 diabetes

OBJECTIVE

The insulinotropic effect of exogenous, intravenously infused glucose-dependent insulinotropic polypeptide (GIP) is impaired in patients with type 2 diabetes. We evaluated the effects of endogenous GIP in relation to glucose and bone metabolism in patients with type 2 diabetes using a selective GIP receptor antagonist and hypothesized that the effects of endogenous GIP were preserved.

DESIGN

A randomized, double-blinded, placebo-controlled, crossover study.

METHODS

Ten patients with overweight/obesity and type 2 diabetes (mean±s.d.; HbA1c 52 ± 11 mmol/mol; BMI 32.5 ± 4.8 kg/m2) were included. We infused a selective GIP receptor antagonist, GIP(3-30)NH2 (1200 pmol/kg/min), or placebo (saline) during two separate, 230-min, standardized, liquid mixed meal tests followed by a meal ad libitum. Subcutaneous adipose tissue biopsies were analyzed.

RESULTS

Compared with placebo, GIP(3-30)NH2 reduced postprandial insulin secretion (Δbaseline-subtracted area under the curve (bsAUC)C-peptide% ± s.e.m.; -14 ± 6%, P = 0.021) and peak glucagon (Δ% ± s.e.m.; -11 ± 6%, P = 0.046) but had no effect on plasma glucose (P = 0.692). Suppression of bone resorption (assessed by circulating carboxy-terminal collagen crosslinks (CTX)) was impaired during GIP(3-30)NH2 infusion compared with placebo (ΔbsAUCCTX; ±s.e.m.; -4.9 ± 2 ng/mL × min, P = 0.005) corresponding to a ~50% reduction. Compared with placebo, GIP(3-30)NH2 did not affect plasma lipids, meal consumption ad libitum or adipose tissue triglyceride content.

CONCLUSIONS

Using a selective GIP receptor antagonist during a meal, we show that endogenous GIP increases postprandial insulin secretion with little effect on postprandial glycaemia but is important for postprandial bone homeostasis in patients with type 2 diabetes.

The glucose-dependent insulinotropic polypeptide (GIP) regulates body weight and food intake via CNS-GIPR signaling

Uncertainty exists as to whether the glucose-dependent insulinotropic polypeptide receptor (GIPR) should be activated or inhibited for the treatment of obesity. Gipr was recently demonstrated in hypothalamic feeding centers, but the physiological relevance of CNS Gipr remains unknown. Here we show that HFD-fed CNS-Gipr KO mice and humanized (h)GIPR knockin mice with CNS-hGIPR deletion show decreased body weight and improved glucose metabolism. In DIO mice, acute central and peripheral administration of acyl-GIP increases cFos neuronal activity in hypothalamic feeding centers, and this coincides with decreased body weight and food intake and improved glucose handling. Chronic central and peripheral administration of acyl-GIP lowers body weight and food intake in wild-type mice, but shows blunted/absent efficacy in CNS-Gipr KO mice. Also, the superior metabolic effect of GLP-1/GIP co-agonism relative to GLP-1 is extinguished in CNS-Gipr KO mice. Our data hence establish a key role of CNS Gipr for control of energy metabolism.

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

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

Pharmacokinetics of exogenous GIP(1-42) in C57Bl/6 mice; Extremely rapid degradation but marked variation between available assays

Like other peptide hormones, glucose-dependent insulinotropic polypeptide (GIP) is rapidly cleared from the circulation. Dipeptidyl peptidase-4 (DPP-4) is known to be involved. Information on the overall pharmacokinetics of GIP in rodents is, however, lacking. We investigated the pharmacokinetics of exogenous GIP after intravenous, subcutaneous and intraperitoneal injection with and without DPP-4 inhibition in conscious female C57Bl/6 mice. Secondly, we compared total and intact GIP levels measured by an in-house RIA and commercially available ELISA kits to determine the suitability of these methods for in vivo and in vitro measurements. GIP half-life following intravenous injection amounted to 93 ± 2 s, which was extended to 5 ± 0.6 min by inhibition of DPP-4. Intact GIP levels following subcutaneous and intraperitoneal GIP administration were approximately 15 % of total GIP. The area under the curve of intact GIP (GIP exposure) following GIP injection was significantly increased by DPP-4 inhibition, whereas total GIP levels remained unchanged. We found significant variation between measurements of total, but not intact GIP performed with our in-house RIA and ELISAs in samples obtained after in vivo administration of GIP. Different preanalytical sample preparation (EDTA plasma, heparin plasma, assay buffer and PBS) significantly influenced results for all ELISA kits used. Thus, in experiments involving exogenous GIP(1-42) administration in mice, it is important to consider that this will result in a very low ratio of intact:total peptide but co-administration of a DPP-4 inhibitor greatly elevates this ratio. Furthermore, for comparison of GIP levels, it is essential to maintain uniformity concerning assay methodology and sample preparation.

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

Background

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

Scope of review

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

Major conclusions

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

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

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

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Unimolecular coagonists must exhibit retained or improved cardioprotection.

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

Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist

Tirzepatide (LY3298176) is a dual GIP and GLP-1 receptor agonist under development for the treatment of type 2 diabetes mellitus (T2DM), obesity, and nonalcoholic steatohepatitis. Early phase trials in T2DM indicate that tirzepatide improves clinical outcomes beyond those achieved by a selective GLP-1 receptor agonist. Therefore, we hypothesized that the integrated potency and signaling properties of tirzepatide provide a unique pharmacological profile tailored for improving broad metabolic control. Here, we establish methodology for calculating occupancy of each receptor for clinically efficacious doses of the drug. This analysis reveals a greater degree of engagement of tirzepatide for the GIP receptor than the GLP-1 receptor, corroborating an imbalanced mechanism of action. Pharmacologically, signaling studies demonstrate that tirzepatide mimics the actions of native GIP at the GIP receptor but shows bias at the GLP-1 receptor to favor cAMP generation over β-arrestin recruitment, coincident with a weaker ability to drive GLP-1 receptor internalization compared with GLP-1. Experiments in primary islets reveal β-arrestin1 limits the insulin response to GLP-1, but not GIP or tirzepatide, suggesting that the biased agonism of tirzepatide enhances insulin secretion. Imbalance toward GIP receptor, combined with distinct signaling properties at the GLP-1 receptor, together may account for the promising efficacy of this investigational agent.

GIP and GLP-1 Potentiate Sulfonylurea-Induced Insulin Secretion in Hepatocyte Nuclear Factor 1α Mutation Carriers

Sulfonylureas (SUs) provide an efficacious first-line treatment in patients with hepatocyte nuclear factor 1α (HNF1A) diabetes, but SUs have limitations due to risk of hypoglycemia. Treatment based on the incretin hormones glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) is characterized by their glucose-dependent insulinotropic actions without risk of hypoglycemia. The effect of SUs together with GIP or GLP-1, respectively, on insulin and glucagon secretion in patients with HNF1A diabetes is currently unknown. To investigate this, 10 HNF1A mutation carriers and 10 control subjects without diabetes were recruited for a double-blinded, placebo-controlled, crossover study including 6 experimental days in a randomized order involving 2-h euglycemic-hyperglycemic clamps with coadministration of: 1) SU (glimepiride 1 mg) or placebo, combined with 2) infusions of GIP (1.5 pmol/kg/min), GLP-1 (0.5 pmol/kg/min), or saline (NaCl). In HNF1A mutation carriers, we observed: 1) hypoinsulinemia, 2) insulinotropic effects of both GIP and GLP-1, 3) additive to supra-additive effects on insulin secretion when combining SU+GIP and SU+GLP-1, respectively, and 4) increased fasting and arginine-induced glucagon levels compared with control subjects without diabetes. Our study suggests that a combination of SU and incretin-based treatment may be efficacious in patients with HNF1A diabetes via potentiation of glucose-stimulated insulin secretion.

Gut Hormone GIP Induces Inflammation and Insulin Resistance in the Hypothalamus

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

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

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

Glucose-Dependent Insulinotropic Polypeptide Is a Pancreatic Polypeptide Secretagogue in Humans

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) has been suggested to stimulate the secretion of pancreatic polypeptide (PP), an islet hormone thought to regulate gut motility, appetite, and glycemia.

OBJECTIVE

To determine whether human GIP1-42 (hGIP) stimulates PP secretion.

METHOD

As glycemia modulates the secretion of PP, we measured plasma PP concentrations from 2 studies in healthy men (n = 10) and in patients with type 2 diabetes (T2D) (n = 12), where hGIP1-42 had been administered intravenously during fasting glycemia, hyperglycemia (12 mmol/L), and insulin-induced hypoglycemia (targets: 2.5 mmol/L [healthy]; 3.5 mmol/L [T2D]). Porcine GIP1-42 (pGIP) was also infused intra-arterially in isolated porcine pancreata (n = 4).

RESULTS

Mean fasting plasma glucose concentrations were approximately 5 mmol/L (healthy) and approximately 8 mmol/L (T2D). At fasting glycemia, PP concentrations were higher during intravenous hGIP1-42 infusion compared with saline in healthy men (mean [standard error of the mean, SEM], net incremental areas under the curves (iAUCs)[0-30min], 403 [116] vs -6 [57] pmol/L × min; P = 0.004) and in patients with T2D (905 [177] vs -96 [86] pmol/L × min; P = 0.009). During hyperglycemic clamping, mean [SEM] PP concentrations were significantly higher during hGIP1-42 infusion compared with saline in patients with T2D (771 [160] vs -183 [117] pmol/L × min; P = 0.001), but not in healthy individuals (-8 [86] vs -57 [53] pmol/L × min; P = 0.69). When plasma glucose levels were declining in response to exogenous insulin, mean [SEM] PP concentrations were higher during hGIP1-42 infusion compared with saline in healthy individuals (294 [88] vs -82 [53] pmol/L × min; P = 0.0025), but not significantly higher in patients with T2D (586 [314] vs -120 [53]; P = 0.070). At target hypoglycemia, PP levels surged in both groups during both hGIP1-42 and saline infusions. In isolated pancreata, pGIP1-42 increased mean [SEM] PP output in the pancreatic venous effluent (baseline vs infusion, 24[5] vs 79 [16] pmol/min x min; P = 0.044).

CONCLUSION

GIP1-42 increases plasma PP secretion in healthy individuals, patients with T2D, and isolated porcine pancreata. Hyperglycemia blunts the stimulatory effect of hGIP1-42 in healthy individuals, but not in patients with T2D.

GIP has neuroprotective effects in Alzheimer and Parkinson's disease models

Glucose-dependent Insulinotropic polypeptide (GIP) is a peptide hormone of the incretin family. It has growth factor properties and can re-activate energy utilization. In progressive neurodegenerative disorders such as Alzheimer's and Parkinson's disease, energy utilization is much reduced, and GIP has the potential to reverse this. Furthermore, GIP can reduce the inflammation response in the brain and reduce levels of pro-inflammatory cytokines. Tests in animal models of Alzheimer's and Parkinson's disease show good neuroprotective effects. In Parkinson's disease models, motor activity is normalized, dopaminergic neurons are protected, synapse numbers and dopamine levels are maintained. Levels of growth factors that are essential for neuronal and synaptic function are increased and alpha-synuclein levels are reduced. The chronic inflammation response and mitochondrial damage is reduced. In Alzheimer's disease models, memory is rescued, synapse numbers and synaptic plasticity in the hippocampus is normalized, amyloid plaque load and the chronic inflammation is reduced. Similar protective effects have been previously reported with analogues of glucagon-like peptide 1 (GLP-1), the sister incretin hormone. First clinical trials show good protective effects in both diseases. Recently, novel dual GLP-1/GIP receptor agonists have been developed. The ability to cross the blood-brain barrier (BBB) is key to their neuroprotective effects. We have developed two dual GLP-1/GIP receptor agonist that have cell penetrating sequences added for better BBB penetration. In direct comparisons, these dual agonists show improved neuroprotection in a mouse model of Parkinson's disease. Therefore, such novel multiple receptor agonists hold great promise as potential treatments for Alzheimer's and Parkinson's disease.

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.

Endogenously released GIP reduces and GLP-1 increases hepatic insulin extraction

GIP was proposed to play a key role in the development of non- alcoholic fatty liver disease (NAFLD) in response to sugar intake. Isomaltulose, is a 1,6-linked glucose-fructose dimer which improves glucose homeostasis and prevents NAFLD compared to 1,2-linked sucrose by reducing glucose-dependent insulinotropic peptide (GIP) in mice. We compared effects of sucrose vs. isomaltulose on GIP and glucagon-like peptide-1 (GLP-1) secretion, hepatic insulin clearance (HIC) and insulin sensitivity in normal (NGT), impaired glucose tolerant (IGT) and Type 2 diabetes mellitus (T2DM) participants. A randomized crossover study was performed in 15 NGT, 10 IGT and 10 T2DM subjects. In comparison to sucrose, peak glucose concentrations were reduced by 2.3, 2.1 and 2.5 mmol/l (all p < 0.05) and insulin levels were 88% (p < 0.01, NGT), 32% (p < 0.05, IGT) and 55% (T2DM) lower after the isomaltulose load. Postprandial GIP_iAUC concentrations were decreased (56%, p < 0.01 in NGT; 42%, p < 0.05 in IGT and 40%,p < 0.001 in T2DM) whereas GLP-1_iAUC was 77%, 85% and 85% higher compared to sucrose (p < 0.01), respectively. This resulted in ∼35 - 50% improved insulin sensitivity and reduced insulinogenic index after isomaltulose, which correlated closely with improved HIC, respectively (r = 0.62, r=-0.70; p < 0.001). HIC was inversely related to GIP (r=-0.44, p < 0.001) and positively related to GLP-1 levels (r = 0.40, p = 0.001). CONCLUSION: Endogenously released GIP correlated with reduced, and GLP-1 with increased hepatic insulin extraction. Increased peripheral insulin levels may contribute to insulin resistance and obesity. We propose that the unfavorable effects of high glycemic index Western diets are related to increased GIP-release and reduced HIC.

GIP analogues augment bone strength by modulating bone composition in diet-induced obesity in mice

Receptors to glucose-dependent insulinotropic polypeptide (GIP), have been identified on bone and GIP receptor (GIPr) knockout mice exhibit reduced bone strength and quality. Despite this, little is known on the potential beneficial bone effects of exogenous GIP on bone physiology. The aim of the present study was to assess whether stable GIP analogues were capable of ameliorating bone strength in mice with diet-induced obesity. The stable GIP analogue (D-Ala²)-GIP, and (D-Ala²)-GIP-Tag, a specific GIP analogue homing exclusively to bone, were employed. In vitro studies were used to assess effects of (D-Ala²)-GIP and (D-Ala²)-GIP-Tag on bone mineralization, lysyl oxidase activity, collagen maturity as well as osteoclast formation and activity. Subsequent in vivo studies employed obese-prediabetic Swiss NIH mice subjected to a 42-day period of daily administration of saline, (D-Ala²)-GIP or (D-Ala²)-GIP-Tag. In vitro studies confirmed that (D-Ala²)-GIP and (D-Ala²)-GIP-Tag had similar beneficial biological effects on bone cells. Administration of (D-Ala²)-GIP and (D-Ala²)-GIP-Tag resulted in lower blood glucose levels without any effects on body weight. Both GIP analogues augmented bone strength to a similar extent. Trabecular or cortical bone microarchitecture were not changed over the time course of the study. However, (D-Ala²)-GIP and (D-Ala²)-GIP-Tag augmented enzymatic collagen crosslinking as well as the heterogeneity of enzymatic collagen crosslinking, mineral-to-matrix ratio and significantly reduced the heterogeneity in mineral bone crystallite size. This study demonstrates that activation of skeletal GIPr by stable GIP analogues enhance bone strength in prediabetes and suggest that these analogues may be beneficial in the treatment of bone disease.

The role of GIP in α-cells and glucagon secretion

Glucose-dependent insulinotropic polypeptide (GIP) is an intestinally derived peptide that is secreted in response to feeding. The GIP receptor (GIPR) is expressed in many cell types involved in the regulation of metabolism, including α- and β-cells. Glucagon and insulin exert tremendous control over glucose metabolism. Thus, GIP action in islets strongly dictates metabolic control in the postprandial state. Loss of GIPR activity in β-cells is a characteristic of type 2 diabetes (T2D) which associates with reduced postprandial insulin secretion and hyperglycemia. Less is known about GIPR activity in α-cells or the control of glucagon secretion. GIP stimulates glucagon secretion in a glucose-dependent manner in healthy people, with enhanced activity at lower glycemia. However, GIP stimulates glucagon secretion even at hyperglycemia in people with T2D, suggesting that inappropriate GIPR activity in α-cells contributes to the pathogenesis of T2D. Here, we review the literature describing GIP action and GIPR activity in the α-cell, detailing the basic science that has shaped the view of how GIP regulates glucagon secretion. We also contrast the effects of GIP on glucagon secretion in healthy and T2D people. Finally, we contextualize these observations in light of recent work that redefines the role of glucagon in glucose homeostasis, suggesting that hyperglucagonemia per se does not drive hyperglycemia. As new medications for T2D that incorporate GIPR activity are being developed, it is clear that a better understanding of GIPR activity beyond the β-cell is necessary. This work highlights the importance of focusing on the GIPR in α-cells.

The glucose-dependent insulinotropic polypeptide signaling axis in the central nervous system

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone released from the epithelium of the upper small intestine. While GIP shares common actions on the pancreatic beta cell with glucagon-like peptide-1 (GLP-1), unlike GLP-1, GIP presents a complex target for the development of diabetes and obesity therapies due to its extra-pancreatic effects on fat mass. Recent pharmacological developments, however, have provided insight into a previously unrecognized role for GIP receptor (GIPR) signaling in regulating appetite. Additionally, GIP-based therapeutics have demonstrated promising neuroprotective properties. Together these observations identify an important central component of the GIP/GIPR signaling axis, and have triggered a resurgence of research interest into the central actions of GIP. In this review, we discuss what is currently known about where GIP may act in the central nervous system (CNS), the characteristics of its target cell populations, and the physiological effects of manipulating the activity Gipr-expressing cells in the brain.

Glucose-dependent insulinotropic polypeptide (GIP) and cardiovascular disease

Accumulating evidence suggests that glucose-dependent insulinotropic polypeptide (GIP) in addition to its involvement in type 2 diabetic pathophysiology may be involved in the development of obesity and the pathogenesis of cardiovascular disease. In this review, we outline recent preclinical and clinical cardiovascular-related discoveries about GIP. These include chronotropic and blood pressure-lowering effects of GIP. Furthermore, GIP has been suggested to control vasodilation via secretion of nitric oxide, and vascular leukocyte adhesion and inflammation via expression and secretion of endothelin 1. Also, GIP seems to regulate circulating lipids via effects on adipose tissue uptake and metabolism of lipids. Lastly, we discuss how dysmetabolic conditions such as obesity and type 2 diabetes may shift the actions of GIP in an atherogenic direction, and we provide a perspective on the therapeutic potential of GIP receptor agonism and antagonism in cardiovascular diseases. We conclude that GIP actions may have implications for the development of cardiovascular disease, but also that the potential of GIP-based drugs for the treatment of cardiovascular disease currently is uncertain.

Cardiovascular biology of the GIP receptor

Glucose-dependent insulinotropic polypeptide (GIP) is a gut hormone secreted primarily from enteroendocrine K cells in the duodenum and proximal jejunum following nutrient ingestion, primarily acting on islet β-cells to potentiate insulin secretion in a glucose-dependent manner. New discoveries of GIP receptor (GIPR) biology in adipose tissue, as well as findings that co-agonists for the glucagon-like peptide-1 receptor (GLP-1R) and GIPR induce greater weight loss than that seen with GLP-1R agonists alone, has led to continued interest in manipulating GIPR activity for the treatment of obesity/type 2 diabetes mellitus (T2DM). As cardiovascular diseases represent the number one cause of death in people with T2DM, there has also been growing interest in understanding the cardiovascular actions of the GIP/GIPR axis. Although controversy surrounds whether GIPR agonism or antagonism will induce weight loss in obesity/T2DM, such actions undoubtedly will impact cardiovascular function and outcomes since obesity is a major risk factor for cardiovascular disease. Furthermore, GIPR agonism is associated with reduced progression of atherosclerotic lesions in preclinical studies. Conversely, genetic deletion of GIPR activity within cardiac myocytes of the heart results in robust protection against experimental myocardial infarction. Nonetheless, interrogation of the GIP/GIPR axis on cardiac function in humans will involve the systemic actions of the GIPR within the myocardium and other systems (e.g. adipose tissue, vasculature), which will influence the long-term future of GIPR modification for the treatment of obesity/T2DM.