Physiological roles of the GIP receptor in murine brown adipose tissue

The interplay of glucose-dependent insulinotropic polypeptide in adipose tissue

Adipose tissue was once known as a reservoir for energy storage but is now considered a crucial organ for hormone and energy flux with important effects on health and disease. Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone secreted from the small intestinal K cells, responsible for augmenting insulin release, and has gained attention for its independent and amicable effects with glucagon-like peptide 1 (GLP-1), another incretin hormone secreted from the small intestinal L cells. The GIP receptor (GIPR) is found in whole adipose tissue, whereas the GLP-1 receptor (GLP-1R) is not, and some studies suggest that GIPR action lowers body weight and plays a role in lipolysis, glucose/lipid uptake/disposal, adipose tissue blood flow, lipid oxidation, and free-fatty acid (FFA) re-esterification, which may or may not be influenced by other hormones such as insulin. This review summarizes the research on the effects of GIP in adipose tissue (distinct depots of white and brown) using cellular, rodent, and human models. In doing so, we explore the mechanisms of GIPR-based medications for treating metabolic disorders, such as type 2 diabetes and obesity, and how GIPR agonism and antagonism contribute to improvements in metabolic health outcomes, potentially through actions in adipose tissues.

Evaluating the efficacy of GIPR agonists on non-alcoholic fatty liver disease: A Mediation Mendelian Randomization Study

OBJECTIVE

Non-alcoholic fatty liver disease (NAFLD) is becoming the most common chronic liver disease worldwide while still lacks drugs for treatment or prevention. We aimed to investigate the causal role of glucose-dependent insulinotropic polypeptide receptor agonists (GIPRAs) on NAFLD and identify the mediated risk factors by which GIPRAs exert their therapeutic effects.

METHODS

Genetic proxies of GIPRAs were identified as cis-SNPs of GIPR associated with both the gene expression level and HbA1c and analyses including colocalization and linkage disequilibrium (LD) were performed for validation. We then performed two-sample two-step mendelian randomization to determine the causal effect of GIPRAs on NAFLD.

RESULTS

The MR analysis suggested genetic proxies of GIPRAs were causally associated with reduced risk of NAFLD (Odds ratio (OR): 0.46, 95 % confidence interval (95 % CI): 0.24-0.88, P = 0.02) and T2DM (OR: 0.10, 95 % CI: 0.07-0.13, P < 0.01). In addition, Mediation analysis showed evidence of indirect effect of GIPRAs on NAFLD via TRIG (0.88, [0.85-0.92], P < 0.01) and HDL-C (0.85, [0.80-0.90], P < 0.01).

CONCLUSIONS

Our study provided strong evidence to support the causal role of GIPRAs on reducing the risk of NAFLD probably through improving lipid metabolism, especially TG and HDL-C, providing guidance for future clinical trials.

Advances in the Management of Diabetes and Overweight using Incretin-based Pharmacotherapies

Throughout the previous three decades, the secretion of glucagon-like peptide-1 hormone has attracted much attention to attain possible therapy goals for the treatment of both hypoglycaemic along type II diabetes militates and overweight. The pharmaceutical generation of peptides similar to hypoglycaemia-based medicines is exemplified by agonists of the GLP- 1R (Glucagon-like peptide-1 receptors). Pharmacokinetic profiles are continuously being improved, beginning with the native hormone with a two- to three-minute quarter and progressing through growth every day with once-drug combinations. Due to contradictory data that indicate stimulation or inhibition of the Glucagon-like peptide receptor, the Glucose-dependent insulin tropic peptide receptor offers favorable effects on systemic metabolism. The recent Glp-1R (Glucagon-like peptide-1 receptor-) targeting monomolecular drugs has demonstrated therapeutic effectiveness and has stoked interest in Glucose-dependent insulin tropic polypeptide antagonism as a treatment for overweight and diabetes mellitus. These drugs have been shown to dramatically improve carbohydrates with body weight management in sick people who have obesity and type II diabetes mellitus. In this study, recent breakthroughs in compelling therapeutic interventions are discussed, and the biology and pharmacology of the glucose-like peptide are reviewed.

The intestine as an endocrine organ and the role of gut hormones in metabolic regulation

Gut hormones orchestrate pivotal physiological processes in multiple metabolically active tissues, including the pancreas, liver, adipose tissue, gut and central nervous system, making them attractive therapeutic targets in the treatment of obesity and type 2 diabetes mellitus. Most gut hormones are derived from enteroendocrine cells, but bioactive peptides that are derived from other intestinal epithelial cell types have also been implicated in metabolic regulation and can be considered gut hormones. A deeper understanding of the complex inter-organ crosstalk mediated by the intestinal endocrine system is a prerequisite for designing more effective drugs that are based on or target gut hormones and their receptors, and extending their therapeutic potential beyond obesity and diabetes mellitus. In this Review, we present an overview of gut hormones that are involved in the regulation of metabolism and discuss their action in the gastrointestinal system and beyond.

Relevance and consequence of chronic inflammation for obesity development

BACKGROUND

Increasing prevalence of morbid obesity accompanied by comorbidities like type 2 diabetes mellitus (T2DM) led to a demand for improving therapeutic strategies and pharmacological intervention options. Apart from genetics, inflammation processes have been hypothesized to be of importance for the development of obesity and related aspects like insulin resistance.

MAIN TEXT

Within this review, we provide an overview of the intricate interplay between chronic inflammation of the adipose tissue and the hypothalamus and the development of obesity. Further understanding of this relationship might improve the understanding of the underlying mechanism and may be of relevance for the establishment of new treatment strategies.

Glucagon-like peptide-1/glucose-dependent insulinotropic polypeptide receptor co-agonists for cardioprotection, type 2 diabetes and obesity: a review of mechanisms and clinical data

PURPOSE OF REVIEW

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are approved for the management of type 2 diabetes (T2D) and obesity, and some are recommended for cardiorenal risk reduction in T2D. To enhance the benefits with GLP-RA mono-agonist therapy, GLP-1/glucose-dependent insulinotropic polypeptide (GIP) receptor co-agonists are in development to capitalize on the synergism of GLP-1 and GIP agonism. We review the mechanisms of action and clinical data for GLP-1/GIP receptor co-agonists in T2D and obesity and their potential role in cardiovascular protection.

RECENT FINDINGS

Tirzepatide, a first-in-class unimolecular GLP-1/GIP receptor co-agonist, is approved for T2D and is awaiting approval for obesity management. Phase 3 trials in T2D cohorts revealed significant reductions in glycemia and body weight and superiority compared with GLP-1R mono-agonism with semaglutide. Tirzepatide has demonstrated significant body weight reductions in individuals with obesity but not diabetes. It enhances lipid metabolism, reduces blood pressure, and lowers liver fat content. Pooled phase 2/3 data showed cardiovascular safety in T2D while a post hoc analysis suggested tirzepatide slows the decline of kidney function in T2D.

SUMMARY

GLP-1/GIP receptor co-agonists are a novel addition to the diabetes and obesity armamentarium. The cardiorenal-metabolic benefits position them as promising multiprong tools for metabolically complex individuals with chronic vascular complications.

Myoglobin in Brown Adipose Tissue: A Multifaceted Player in Thermogenesis

Brown adipose tissue (BAT) plays an important role in energy homeostasis by generating heat from chemical energy via uncoupled oxidative phosphorylation. Besides its high mitochondrial content and its exclusive expression of the uncoupling protein 1, another key feature of BAT is the high expression of myoglobin (MB), a heme-containing protein that typically binds oxygen, thereby facilitating the diffusion of the gas from cell membranes to mitochondria of muscle cells. In addition, MB also modulates nitric oxide (NO•) pools and can bind C16 and C18 fatty acids, which indicates a role in lipid metabolism. Recent studies in humans and mice implicated MB present in BAT in the regulation of lipid droplet morphology and fatty acid shuttling and composition, as well as mitochondrial oxidative metabolism. These functions suggest that MB plays an essential role in BAT energy metabolism and thermogenesis. In this review, we will discuss in detail the possible physiological roles played by MB in BAT thermogenesis along with the potential underlying molecular mechanisms and focus on the question of how BAT-MB expression is regulated and, in turn, how this globin regulates mitochondrial, lipid, and NO• metabolism. Finally, we present potential MB-mediated approaches to augment energy metabolism, which ultimately could help tackle different metabolic disorders.

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 β 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.

Adipocyte G Protein-Coupled Receptors as Potential Targets for Novel Antidiabetic Drugs

The functional state of adipocytes plays a central role in regulating numerous important metabolic functions, including energy and glucose homeostasis. While white adipocytes store excess calories as fat (triglycerides) and release free fatty acids as a fuel source in times of need, brown and beige adipocytes (so-called thermogenic adipocytes) convert chemical energy stored in substrates (e.g., fatty acids or glucose) into heat, thus promoting energy expenditure. Like all other cell types, adipocytes express many G protein-coupled receptors (GPCRs) that are linked to four major functional classes of heterotrimeric G proteins (Gs, Gi/o, Gq/11, and G12/13). During the past few years, novel experimental approaches, including the use of chemogenetic strategies, have led to a series of important new findings regarding the metabolic consequences of activating or inhibiting distinct GPCR/G protein signaling pathways in white, brown, and beige adipocytes. This novel information should guide the development of novel drugs capable of modulating the activity of specific adipocyte GPCR signaling pathways for the treatment of obesity, type 2 diabetes, and related metabolic disorders.

Perspectives on weight control in diabetes - Tirzepatide

Tirzepatide, a once-weekly glucose-dependent insulinotropic polypeptide (GIP)/glucagon-like peptide-1 (GLP-1) receptor agonist (GIP/GLP-1 RA) improves glycemic control. Besides improvement of glycemic control, tirzepatide treatment is associated with significantly more weight loss as compared to potent selective GLP-1 receptor agonists as well as other beneficial changes in cardio-metabolic parameters, such as reduced fat mass, blood pressure, improved insulin sensitivity, lipoprotein concentrations, and circulating metabolic profile in individuals with type 2 diabetes (T2D). Some of these changes are partially associated with weight reduction. We review here the putative mechanisms of GIP receptor agonism contributing to GLP-1 receptor agonism-induced weight loss and respective findings with GIP/GLP-1 RAs, including tirzepatide in T2D preclinical models and clinical studies. Subsequently, we summarize the clinical data on weight loss and related non-glycemic metabolic changes of tirzepatide in T2D. These findings suggest that the robust weight loss and associated changes are important contributors to the clinical profile of tirzepatide for the treatment of T2D diabetes and serve as the basis for further investigations including clinical outcomes.

The activation of gastric inhibitory peptide/gastric inhibitory peptide receptor axis via sonic hedgehog signaling promotes the bridging of gapped nerves in sciatic nerve injury

Schwann cells play an essential role in peripheral nerve regeneration by generating a favorable microenvironment. Gastric inhibitory peptide/gastric inhibitory peptide receptor (GIP/GIPR) axis deficiency leads to failure of sciatic nerve repair. However, the underlying mechanism remains elusive. In this study, we surprisingly found that GIP treatment significantly enhances the migration of Schwann cells and the formation of Schwann cell cords during recovery from sciatic nerve injury in rats. We further revealed that GIP and GIPR levels in Schwann cells were low under normal conditions, and significantly increased after injury demonstrated by real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Wound healing and Transwell assays showed that GIP stimulation and GIPR silencing could affect Schwann cell migration. In vitro and in vivo mechanistic studies based on interference experiment revealed that GIP/GIPR might promote mechanistic target of rapamycin complex 2 (mTORC2) activity, thus facilitating cell migration; Rap1 activation might be involved in this process. Finally, we retrieved the stimulatory factors responsible for GIPR induction after injury. The results indicate that sonic hedgehog (SHH) is a potential candidate whose expression increased upon injury. Luciferase and chromatin immunoprecipitation (ChIP) assays showed that Gli3, the target transcription factor of the SHH pathway, dramatically augmented GIPR expression. Additionally, in vivo inhibition of SHH could effectively reduce GIPR expression after sciatic nerve injury. Collectively, our study reveals the importance of GIP/GIPR signaling in Schwann cell migration, providing a therapeutic avenue toward peripheral nerve injury.

A chemical signal that promotes insect survival via thermogenesis

Cold-activated thermogenesis of brown adipose tissues (BAT) is vital for the survival of animals under cold stress and also inhibits the development of tumours. The development of small-molecule tools that target thermogenesis pathways could lead to novel therapies against cold, obesity, and even cancer. Here, we identify a chemical signal that is produced in beetles in the winter to activate fat thermogenesis. This hormone elevates the basal body temperature by increasing cellular mitochondrial density and uncoupling in order to promote beetle survival. We demonstrate that this hormone activates UCP4- mediated uncoupled respiration through adipokinetic hormone receptor (AKHR). This signal serves as a novel fat-burning activator that utilizes a conserved mechanism to promote thermogenesis not only in beetles, nematode and flies, but also in mice, protecting the mice against cold and tumor growth. This hormone represents a new strategy to manipulate fat thermogenesis.

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.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

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.

Crosstalk between incretin hormones, Th17 and Treg cells in inflammatory diseases

Intestinal epithelial cells constantly crosstalk with the gut microbiota and immune cells of the gut lamina propria. Enteroendocrine cells, secrete hormones, such as incretin hormones, which participate in host physiological events, such as stimulating insulin secretion, satiety, and glucose homeostasis. Interestingly, evidence suggests that the incretin pathway may influence immune cell activation. Consequently, drugs targeting the incretin hormone signaling pathway may ameliorate inflammatory diseases such as inflammatory bowel diseases, cancer, and autoimmune diseases. In this review, we discuss how these hormones may modulate two subsets of CD4 + T cells, the regulatory T cells (Treg)/Th17 axis important for gut homeostasis: thus, preventing the development and progression of inflammatory diseases. We also summarize the main experimental and clinical findings using drugs targeting the glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP-1) signaling pathways and their great impact on conditions in which the Treg/Th17 axis is disturbed such as inflammatory diseases and cancer. Understanding the role of incretin stimulation in immune cell activation and function, might contribute to new therapeutic designs for the treatment of inflammatory diseases, autoimmunity, and tumors.

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

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

GIPR Is Predominantly Localized to Nonadipocyte Cell Types Within White Adipose Tissue

The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) augments glucose-dependent insulin secretion through its receptor expressed on islet β-cells. GIP also acts on adipose tissue; yet paradoxically, both enhanced and reduced GIP receptor (GIPR) signaling reduce adipose tissue mass and attenuate weight gain in response to nutrient excess. Moreover, the precise cellular localization of GIPR expression within white adipose tissue (WAT) remains uncertain. We used mouse genetics to target Gipr expression within adipocytes. Surprisingly, targeting Cre expression to adipocytes using the adiponectin (Adipoq) promoter did not produce meaningful reduction of WAT Gipr expression in Adipoq-Cre:Giprflx/flx mice. In contrast, adenoviral expression of Cre under the control of the cytomegalovirus promoter, or transgenic expression of Cre using nonadipocyte-selective promoters (Ap2/Fabp4 and Ubc) markedly attenuated WAT Gipr expression. Analysis of single-nucleus RNA-sequencing, adipose tissue data sets localized Gipr/GIPR expression predominantly to pericytes and mesothelial cells rather than to adipocytes. Together, these observations reveal that adipocytes are not the major GIPR+ cell type within WAT-findings with mechanistic implications for understanding how GIP and GIP-based co-agonists control adipose tissue biology.

Roles of Glucose-Dependent Insulinotropic Polypeptide in Diet-Induced Obesity

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are incretins that play an important role in glucose metabolism, by increasing glucose‐induced insulin secretion from pancreatic β‐cells and help regulate bodyweight. Although they show a similar action on glucose‐induced insulin secretion, two incretins are distinct in various aspects. GIP is secreted from enteroendocrine K cell mainly expressed in the upper small intestine, and GLP‐1 is secreted from enteroendocrine L cells mainly expressed in the lower small intestine and colon by the stimulation of various nutrients. The mechanism of GIP secretion induced by nutrients, especially carbohydrates, is different from that of GLP‐1 secretion. GIP promotes fat deposition in adipose tissue, and contributes to fat‐induced obesity. In contrast, GLP‐1 participates in reducing bodyweight by suppressing food consumption and/or slowing gastric emptying. There is substantial evidence that GIP and GLP‐1 might differently contribute to bodyweight control. Although meal contents influence both glycemic and weight control, we do not fully understand whether incretin actions differ depending on the contents of the meal and what kind of signaling is involved in its context. We focus on the molecular mechanism of GIP secretion induced by nutrients, as well as the roles of GIP in weight changes caused by various diets.

Leveraging GPCR signaling in thermogenic fat to counteract metabolic diseases

Background

Thermogenic brown and beige adipocytes are recognized for their unique capacity to consume extraordinary levels of metabolites and lipids from the blood to fuel heat-producing catabolic processes [[1], [2], [3], [4], [5], [6], [7]]. In humans, the functions of thermogenic adipocytes are associated with cardiometabolic protection and improved glycemic control [[8], [9], [10], [11], [12], [13]]. Consequently, engaging these macronutrient-consuming and energy-dissipating activities has gained attention as a promising therapeutic strategy for counteracting metabolic diseases, such as obesity and diabetes.

Scope of review

In this review, we highlight new advances in our understanding of the physiological role of G protein-coupled receptors (GPCRs) in controlling thermogenic adipocyte biology. We further extend our discussion to the opportunities and challenges posed by pharmacologically targeting different elements of GPCR signaling in these highly specialized fat cells.

Major conclusions

GPCRs represent appealing candidates through which to harness adipose thermogenesis. Yet safely and effectively targeting these druggable receptors on brown and beige adipocytes has thus far proven challenging. Therefore, continued interrogation across the GPCR landscape is necessary for future leaps within the field of thermogenic fat biology to unlock the therapeutic potential of adipocyte catabolism.

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Brown and beige thermogenic adipocytes robustly consume and catabolize macronutrients.

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The catabolic activity of thermogenic adipocytes promotes organismal energy balance.

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Thermogenic adipocyte functions are tightly controlled by G protein-coupled receptors (GPCRs).

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GPCRs can be potentially targeted at multiple levels to therapeutically harness thermogenic activity.

Glucose-dependent insulinotropic polypeptide and tissue inflammation: Implications for atherogenic cardiovascular disease

Glucose-dependent insulinotropic polypeptide (GIP) has pleiotropic actions on pancreatic endocrine function, adipose tissue lipid metabolism, and skeletal calcium metabolism. Recent data indicate a potential new role for GIP in the pathogenesis of cardiovascular disease. This review focuses on the emerging literature that highlights GIP’s role in inflammation—an established process in the initiation and progression of atherosclerosis. In vasculature tissue, GIP may reduce concentrations of circulating inflammatory cytokines, attenuate vascular endothelial inflammation, and directly limit atherosclerotic vascular damage. Important to recognize is that evidence exists to support both pro- and anti-inflammatory effects of GIP even within the same tissue/cell type. Therefore, future study designs must account for factors such as model heterogeneity, physiological relevance of doses/exposures, potential indirect effects on inflammatory pathways, and the glucose-dependent insulinotropic polypeptide receptor (GIPR) agonist form. Elucidating the specific effects of enhanced GIP signaling in vascular inflammation and atherosclerosis is crucial given the existing widespread use of DPP4 inhibitors and the emergence of dual-incretin receptor agonists for type 2 diabetes treatment.

The transcription factor hepatocyte nuclear factor 4A acts in the intestine to promote white adipose tissue energy storage

The transcription factor hepatocyte nuclear factor 4 A (HNF4A) controls the metabolic features of several endodermal epithelia. Both HNF4A and HNF4G are redundant in the intestine and it remains unclear whether HNF4A alone controls intestinal lipid metabolism. Here we show that intestinal HNF4A is not required for intestinal lipid metabolism per se, but unexpectedly influences whole-body energy expenditure in diet-induced obesity (DIO). Deletion of intestinal HNF4A caused mice to become DIO-resistant with a preference for fat as an energy substrate and energetic changes in association with white adipose tissue (WAT) beiging. Intestinal HNF4A is crucial for the fat-induced release of glucose-dependent insulinotropic polypeptide (GIP), while the reintroduction of a stabilized GIP analog rescues the DIO resistance phenotype of the mutant mice. Our study provides evidence that intestinal HNF4A plays a non-redundant role in whole-body lipid homeostasis and points to a non-cell-autonomous regulatory circuit for body-fat management.

HNF4A is a nuclear receptor that regulates liver lipid homeostasis. Here the authors show that HNF4A is not required for intestinal lipid metabolism but controls energy expenditure under diet induced obesity through the fat-induced release of glucose-dependent insulinotropic polypeptide.

Recent Advances in Incretin-Based Pharmacotherapies for the Treatment of Obesity and Diabetes

The incretin hormone glucagon-like peptide-1 (GLP-1) has received enormous attention during the past three decades as a therapeutic target for the treatment of obesity and type 2 diabetes. Continuous improvement of the pharmacokinetic profile of GLP-1R agonists, starting from native hormone with a half-life of ~2-3 min to the development of twice daily, daily and even once-weekly drugs highlight the pharmaceutical evolution of GLP-1-based medicines. In contrast to GLP-1, the incretin hormone glucose-dependent insulinotropic polypeptide (GIP) received little attention as a pharmacological target, because of conflicting observations that argue activation or inhibition of the GIP receptor (GIPR) provides beneficial effects on systemic metabolism. Interest in GIPR agonism for the treatment of obesity and diabetes was recently propelled by the clinical success of unimolecular dual-agonists targeting the receptors for GIP and GLP-1, with reported significantly improved body weight and glucose control in patients with obesity and type II diabetes. Here we review the biology and pharmacology of GLP-1 and GIP and discuss recent advances in incretin-based pharmacotherapies.

Food phenolics stimulate adipocyte browning via regulating gut microecology

Fat browning has piqued the interest of researchers as a potential target for treating obesity and related metabolic disorders. Recruitment of brown adipocytes leads to enhanced energy dissipation and reduced adiposity, thus facilitating the maintenance of metabolic homeostasis. Evidence is increasing to support the crucial roles of polyphenols and gut microecology in turning fat "brown". However, it is not clear whether the intestinal microecology is involved in polyphenol-mediated regulation of adipose browning, so this concept is worthy of exploration. In this review, we summarize the current knowledge, mostly from studies with murine models, supporting the concept that the effects of food phenolics on brown fat activation and white fat browning can be attributed to their regulatory actions on gut microecology, including microbial community profile, gut metabolites, and gut-derived hormones. Furthermore, the potential underlying pathways involved are also discussed. Basically, understanding gut microecology paves the way to determine the underlying roles and mechanisms of food phenolics in adipose browning.

Distinct Identity of GLP-1R, GLP-2R, and GIPR Expressing Cells and Signaling Circuits Within the Gastrointestinal Tract

Enteroendocrine cells directly integrate signals of nutrient content within the gut lumen with distant hormonal responses and nutrient disposal via the production and secretion of peptides, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Given their direct and indirect control of post-prandial nutrient uptake and demonstrated translational relevance for the treatment of type 2 diabetes, malabsorption and cardiometabolic disease, there is significant interest in the locally engaged circuits mediating these metabolic effects. Although several specific populations of cells in the intestine have been identified to express endocrine receptors, including intraepithelial lymphocytes (IELs) and αβ and γδ T-cells (Glp1r+) and smooth muscle cells (Glp2r+), the definitive cellular localization and co-expression, particularly in regards to the Gipr remain elusive. Here we review the current state of the literature and evaluate the identity of Glp1r, Glp2r, and Gipr expressing cells within preclinical and clinical models. Further elaboration of our understanding of the initiating G-protein coupled receptor (GPCR) circuits engaged locally within the intestine and how they become altered with high-fat diet feeding can offer insight into the dysregulation observed in obesity and diabetes.

The Role of GIP in the Regulation of GLP-1 Satiety and Nausea

Gastric inhibitory peptide (GIP) is best known for its role as an incretin hormone in control of blood glucose concentrations. As a classic satiation signal, however, the literature illustrates a mixed picture of GIP involvement with an at best weak anorectic response profile being reported for GIP receptor (GIPR) signaling. Not surprisingly, the pursuit of exploiting the GIP system as a therapeutic target for diabetes and obesity has fallen behind that of the other gastrointestinal-derived incretin, glucagon-like peptide 1 (GLP-1). However, recent discoveries highlighted here support potential therapeutic advantages of combinatorial therapies targeting GIP and GLP-1 systems together, with perhaps the most surprising finding that GIPR agonism may have antiemetic properties. As nausea and vomiting are the most common side effects of all existing GLP-1 pharmacotherapies, the ability for GIP agonism to reduce GLP-1-induced illness behaviors but retain (if not enhance) weight loss and glycemic control may offer a new era in the treatment of obesity and diabetes.

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

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

GIPR Function in the Central Nervous System: Implications and Novel Perspectives for GIP-Based Therapies in Treating Metabolic Disorders

During the past decade, pharmaceutical engineering of unimolecular agents has revealed the therapeutic potential of glucose-dependent insulinotropic polypeptide receptor (GIPR) agonism. From this work, one of the most intriguing findings is that engagement of GIPR enhances the weight loss profile of glucagon-like peptide 1 (GLP-1)-based therapeutics. Consequently, this pharmacological approach, in combination with novel Gipr mouse models, has provided evidence indicating that activation of GIPR in certain areas of the brain that regulate energy balance is required for the synergistic weight loss of dual GIPR and GLP-1 receptor (GLP-1R) agonism. This has led to significant interest in understanding how GIPR activity in the brain functions to reduce caloric intake, induce negative energy balance, and drive weight loss. Herein, we review key findings in this field and provide a novel perspective explaining how GIP may act in the brain to affect energy balance both alone and in concert with GLP-1R agonism.

Glucose-dependent insulinotropic polypeptide modifies adipose plasticity and promotes beige adipogenesis of human omental adipose-derived stem cells

The adipocyte precursors (APs) located in white adipose tissue (WAT) are functionally significant in adipose plasticity and browning. Modifying adipogenesis or WAT browning targeted on APs is a promising mechanism for anti-obesity drug. We herein explored the in vitro actions and mechanisms of glucose-dependent insulinotropic polypeptide (GIP), a gut-derived peptide, in human adipose-derived mesenchymal stem cells (hADSCs) isolated from omentum. The hADSCs were cotreated with 100 nM GIP with or without equimolar concentration of GIP3-42 (a GIP receptor antagonist), and subsequently examined in vitro. CCK-8, EdU incorporation, and flow cytometry assays were used to assess cellular proliferation. Annexin V FTIC/PI double stain, TUNEL staining, and Western blot were applied for apoptosis evaluation. Adipogenesis was reflected by Western blot, real-time PCR, Oil Red O staining, mitochondrial staining, and mitochondrial DNA analysis. Results showed that GIP promoted proliferation and inhibited apoptosis of hADSCs via pleiotropic effects. Besides, GIP facilitated de novo beige adipogenesis, by accelerating mitotic clonal expansion (MCE), upregulating core adipogenic regulators (C/EBPα and PPARγ), augmenting beige-related genes (UCP1, PGC1α, and PRDM16), increasing mitochondrial content and improving beige adipocyte functionalities. Above all, our study expands knowledge on the mechanisms of GIP modifying adipogenesis especially in inducing beige adipogenesis, and thus provides a theoretical support for clinical usage of GIP on obesity treatment.

Lipolysis drives expression of the constitutively active receptor GPR3 to induce adipose thermogenesis

Thermogenic adipocytes possess a therapeutically appealing, energy-expending capacity, which is canonically cold-induced by ligand-dependent activation of β-adrenergic G protein-coupled receptors (GPCRs). Here, we uncover an alternate paradigm of GPCR-mediated adipose thermogenesis through the constitutively active receptor, GPR3. We show that the N terminus of GPR3 confers intrinsic signaling activity, resulting in continuous Gs-coupling and cAMP production without an exogenous ligand. Thus, transcriptional induction of Gpr3 represents the regulatory parallel to ligand-binding of conventional GPCRs. Consequently, increasing Gpr3 expression in thermogenic adipocytes is alone sufficient to drive energy expenditure and counteract metabolic disease in mice. Gpr3 transcription is cold-stimulated by a lipolytic signal, and dietary fat potentiates GPR3-dependent thermogenesis to amplify the response to caloric excess. Moreover, we find GPR3 to be an essential, adrenergic-independent regulator of human brown adipocytes. Taken together, our findings reveal a noncanonical mechanism of GPCR control and thermogenic activation through the lipolysis-induced expression of constitutively active GPR3.

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.

Incretin Hormones in Obesity and Related Cardiometabolic Disorders: The Clinical Perspective

The prevalence of obesity continues to grow rapidly worldwide, posing many public health challenges of the 21st century. Obese subjects are at major risk for serious diet-related noncommunicable diseases, including type 2 diabetes mellitus, cardiovascular disease, and non-alcoholic fatty liver disease. Understanding the mechanisms underlying obesity pathogenesis is needed for the development of effective treatment strategies. Dysregulation of incretin secretion and actions has been observed in obesity and related metabolic disorders; therefore, incretin-based therapies have been developed to provide new therapeutic options. Incretin mimetics present glucose-lowering properties, together with a reduction of appetite and food intake, resulting in weight loss. In this review, we describe the physiology of two known incretins—glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and their role in obesity and related cardiometabolic disorders. We also focus on the available and incoming incretin-based medications that can be used in the treatment of the above-mentioned conditions.

Targeting the GIPR for obesity: To agonize or antagonize? Potential mechanisms

BACKGROUND

Glucose-dependent insulinotropic peptide (GIP) is one of two incretin hormones that communicate nutrient intake with systemic metabolism. Although GIP was the first incretin hormone to be discovered, the understanding of GIP's biology was quickly outpaced by research focusing on the other incretin hormone, glucagon-like peptide 1 (GLP-1). Early work on GIP produced the theory that GIP is obesogenic, limiting interest in developing GIPR agonists to treat type 2 diabetes. A resurgence of GIP research has occurred in the last five years, reinvigorating interest in this peptide. Two independent approaches have emerged for treating obesity, one promoting GIPR agonism and the other antagonism. In this report, evidence supporting both cases is discussed and hypotheses are presented to reconcile this apparent paradox.

SCOPE OF THE REVIEW

This review presents evidence to support targeting GIPR to reduce obesity. Most of the focus is on the effect of singly targeting the GIPR using both a gain- and loss-of-function approach, with additional sections that discuss co-targeting of the GIPR and GLP-1R.

MAJOR CONCLUSIONS

There is substantial evidence to support that GIPR agonism and antagonism can positively impact body weight. The long-standing theory that GIP drives weight gain is exclusively derived from loss-of-function studies, with no evidence to support that GIPR agonisms increases adiposity or body weight. There is insufficient evidence to reconcile the paradoxical observations that both GIPR agonism and antagonism can reduce body weight; however, two independent hypotheses centered on GIPR antagonism are presented based on new data in an effort to address this question. The first discusses the compensatory relationship between incretin receptors and how antagonism of the GIPR may enhance GLP-1R activity. The second discusses how chronic GIPR agonism may produce desensitization and ultimately loss of GIPR activity that mimics antagonism. Overall, it is clear that a deeper understanding of GIP biology is required to understand how modulating this system impacts metabolic homeostasis.

Gut Hormone GIP Induces Inflammation and Insulin Resistance in the Hypothalamus

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

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

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

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

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

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.

Proglucagon-Derived Peptides, Glucose-Dependent Insulinotropic Polypeptide, and Dipeptidyl Peptidase-4-Mechanisms of Action in Adipose Tissue

Proglucagon-derived peptides (PGDPs) and related gut hormones exemplified by glucose-dependent insulinotropic polypeptide (GIP) regulate energy disposal and storage through actions on metabolically sensitive organs, including adipose tissue. The actions of glucagon, glucagon-like peptide (GLP)-1, GLP-2, GIP, and their rate-limiting enzyme dipeptidyl peptidase-4, include direct and indirect regulation of islet hormone secretion, food intake, body weight, all contributing to control of white and brown adipose tissue activity. Moreover, agents mimicking actions of these peptides are in use for the therapy of metabolic disorders with disordered energy homeostasis such as diabetes, obesity, and intestinal failure. Here we highlight current concepts and mechanisms for direct and indirect actions of these peptides on adipose tissue depots. The available data highlight the importance of indirect peptide actions for control of adipose tissue biology, consistent with the very low level of endogenous peptide receptor expression within white and brown adipose tissue depots. Finally, we discuss limitations and challenges for the interpretation of available experimental observations, coupled to identification of enduring concepts supported by more robust evidence.

Pharmacological antagonism of the incretin system protects against diet-induced obesity

Objective

Glucose-dependent insulinotropic polypeptide is an intestinally derived hormone that is essential for normal metabolic regulation. Loss of the GIP receptor (GIPR) through genetic elimination or pharmacological antagonism reduces body weight and adiposity in the context of nutrient excess. Interrupting GIPR signaling also enhances the sensitivity of the receptor for the other incretin peptide, glucagon-like peptide 1 (GLP-1). The role of GLP-1 compensation in loss of GIPR signaling to protect against obesity has not been directly tested.

Methods

We blocked the GIPR and GLP-1R with specific antibodies, alone and in combination, in healthy and diet-induced obese (DIO) mice. The primary outcome measure of these interventions was the effect on body weight and composition.

Results

Antagonism of either the GIPR or GLP-1R system reduced food intake and weight gain during high-fat feeding and enhanced sensitivity to the alternative incretin signaling system. Combined antagonism of both GIPR and GLP-1R produced additive effects to mitigate DIO. Acute pharmacological studies using GIPR and GLP-1R agonists demonstrated both peptides reduced food intake, which was prevented by co-administration of the respective antagonists.

Conclusions

Disruption of either axis of the incretin system protects against diet-induced obesity in mice. However, combined antagonism of both GIPR and GLP-1R produced additional protection against diet-induced obesity, suggesting additional factors beyond compensation by the complementary incretin axis. While antagonizing the GLP-1 system decreases weight gain, GLP-1R agonists are used clinically to target obesity. Hence, the phenotype arising from loss of function of GLP-1R does not implicate GLP-1 as an obesogenic hormone. By extension, caution is warranted in labeling GIP as an obesogenic hormone based on loss-of-function studies.

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Acute administration of either GIP or GLP-1 reduces food intake inmice, which is blocked by antagonizing antibodies.

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Chronic antagonism of the GIPR limits weight gain, improves glucose tolerance, and enhances sensitivity to GLP-1R agonists.

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Chronic antagonism of the GLP-1R reduces weight gain and enhances sensitivity to GIPR agonists.

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Chronic antagonism of both GIPR and GLP-1R provides additive protections against weight gain when mice are fed a HFD.

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Incretin receptor antagonism reduces food intake but does not change energy expenditure.