Glucagon and Its Receptors in the Mammalian Heart

Cardioprotective benefits of metabolic surgery and GLP-1 receptor agonist-based therapies

Individuals with excessive adipose tissue and type 2 diabetes mellitus (T2DM) face a heightened risk of cardiovascular morbidity and mortality. Metabolic surgery is an effective therapy for people with severe obesity to achieve significant weight loss. Additionally, metabolic surgery improves blood glucose levels and can lead to T2DM remission, reducing major adverse cardiovascular outcomes (MACE). Glucagon-like peptide 1 (GLP-1) receptor agonists (GLP-1RAs) are a class of medication that effectively reduce body weight and MACE in patients with T2DM. This review explores the potential mechanisms underlying the cardioprotective benefits of metabolic surgery and GLP-1RA-based therapies and discusses recent evidence and emerging therapies in this dynamic area of research.

A once-daily GLP-1/GIP/glucagon receptor tri-agonist (NN1706) lowers body weight in rodents, monkeys and humans

Single molecules that combine complementary modes of action with glucagon-like peptide-1 receptor (GLP-1R) agonism are best-in-class therapeutics for obesity treatment. NN1706 (MAR423, RO6883746) is a fatty-acylated tri-agonist designed for balanced activity at GLP-1R and glucose-dependent insulinotropic peptide receptor (GIPR) with lower relative potency at the glucagon receptor (GcgR). Obese mice, rats and non-human primates dosed with NN1706 showed significant body weight reductions and improved glycemic control. In human participants with overweight or obesity, daily subcutaneous NN1706 treatment resulted in substantial body weight loss in a dose-dependent manner without impairing glycemic control (NCT03095807, NCT03661879). However, increased heart rate was observed across NN1706 treatment cohorts, which challenges further clinical development of NN1706.

Ubrogepant, erenumab, and eptinezumab antagonize positive inotropic effects of the calcitonin gene-related peptide in the isolated human atrium

The calcitonin gene-related peptide (CGRP) is an endogenous peptide that is known to be involved in the development of a migraine. CGRP is also present in the human heart, acts via CGRP receptors, and has been shown to increase the force of contraction (FOC) in isolated, electrically driven human atrial preparations (HAP) from adult patients obtained during open-heart surgery. Here, the hypothesis was tested that the positive inotropic effect (PIE) of CGRP could be attenuated by three anti-migraine drugs, namely ubrogepant, erenumab (both CGRP receptor antagonists), and eptinezumab (a CGRP antagonist). CGRP, cumulatively applied at concentrations ranging from 1 to 100 nM, increased the FOC. In the presence of cilostamide, an inhibitor of phosphodiesterase III, CGRP was more potent and effective than in the absence of cilostamide. Furthermore, when 100 nM CGRP was administered, subsequent application of ubrogepant (1 nM), erenumab (2 nM), and eptinezumab (6 nM) led to a reduction of FOC in HAP. In a more effective way, 1 µM carbachol and 1 µM (-)-N6-phenylisopropyladenosine (PIA) attenuated the PIE of CGRP in the presence of cilostamide. Conversely, when we applied first ubrogepant (1 nM), erenumab (2 nM), or eptinezumab (6 nM), then, this pre-incubation attenuated the PIE in HAP of cumulatively applied CGRP compared to CGRP given alone. We conclude that ubrogepant, erenumab, and eptinezumab are functional antagonists of CGRP in HAP at therapeutic concentrations of these anti-migraine drugs. Further investigation is necessary to determine whether this reduction in FOC is beneficial or detrimental for migraine patients.

Advances in Insulin Resistance-Molecular Mechanisms, Therapeutic Targets, and Future Directions

The development of insulin resistance (IR) is characterized by a series of metabolic disturbances, including, but not limited to, impaired glucose uptake, increased blood sugar levels, and disrupted lipid metabolism [...].

Cardiac neurons expressing a glucagon-like receptor mediate cardiac arrhythmia induced by high-fat diet in Drosophila

Cardiac arrhythmia leads to increased risks for stroke, heart failure, and cardiac arrest. Arrhythmic pathology is often rooted in the cardiac conduction system, but the mechanism is complex and not fully understood. For example, how metabolic diseases, like obesity and diabetes, increase the risk for cardiac arrhythmia. Glucagon regulates glucose production, mobilizes lipids from the fat body, and affects cardiac rate and rhythm, attributes of a likely key player. Drosophila is an established model to study metabolic diseases and cardiac arrhythmias. Since glucagon signaling is highly conserved, we used high-fat diet (HFD)-fed flies to study its effect on heart function. HFD led to increased heartbeat and an irregular rhythm. The HFD-fed flies showed increased levels of adipokinetic hormone (Akh), the functional equivalent to human glucagon. Both genetic reduction of Akh and eliminating the Akh producing cells (APC) rescued HFD-induced arrhythmia, whereas heart rhythm was normal in Akh receptor mutants (AkhRnull ). Furthermore, we discovered a pair of cardiac neurons that express high levels of Akh receptor. These are located near the posterior heart, make synaptic connections at the heart muscle, and regulate heart rhythm. Altogether, this Akh signaling pathway provides new understanding of the regulatory mechanisms between metabolic disease and cardiac arrhythmia.

Glucagon Can Increase Force of Contraction via Glucagon Receptors in the Isolated Human Atrium

Glucagon can increase the force of contraction (FOC) in, for example, canine hearts. Currently, whether glucagon can also increase the FOC via cAMP-increasing receptors in the human atrium is controversial discussed. Glucagon alone did not (up to 1 µM) raise the FOC in human right atrial preparations (HAP). Only in the additional presence of the phosphodiesterase (PDE) 3 inhibitor cilostamide (1 µM) or 1 nM isoprenaline did glucagon raise the FOC, starting at 1 µM. The positive inotropic effects of glucagon in HAP were attenuated by a glucagon receptor antagonist (1 µM SC203972), but not by 100 nM exendin(9-39), a glucagon-like peptide-1 receptor (GLP-1R) antagonist. Glucagon (in the presence of cilostamide) demonstrated a reduced efficacy in elevating the FOC in HAP when compared with isoprenaline. In contrast to glucagon, exenatide alone, a GLP-1R agonist, starting at 1 nM, increased the FOC and was more potent and effective than glucagon in raising the FOC in HAP. The effects of exenatide on the FOC were attenuated by exendin(9-39). Hence, glucagon and GLP-1R agonists act functionally via different receptors in the human right atrium. Clinically, these data suggest that endogenous or exogenous glucagon can stimulate glucagon receptors in the human atrium, but only in the presence of PDE inhibitors.

Evaluating sex-specific responses to western diet across the lifespan: impact on cardiac function and transcriptomic signatures in C57BL/6J mice at 530 and 640/750 days of age

BACKGROUND

Long-term consumption of Western Diet (WD) is a well-established risk factor for the development of cardiovascular disease (CVD); however, there is a paucity of studies on the long-term effects of WD on the pathophysiology of CVD and sex-specific responses.

METHODS

Our study aimed to investigate the sex-specific pathophysiological changes in left ventricular (LV) function using transthoracic echocardiography (ECHO) and LV tissue transcriptomics in WD-fed C57BL/6 J mice for 125 days, starting at the age of 300 through 425 days.

RESULTS

In female mice, consumption of the WD diet showed long-term effects on LV structure and possible development of HFpEF-like phenotype with compensatory cardiac structural changes later in life. In male mice, ECHO revealed the development of an HFrEF-like phenotype later in life without detectable structural alterations. The transcriptomic profile revealed a sex-associated dichotomy in LV structure and function. Specifically, at 530-day, WD-fed male mice exhibited differentially expressed genes (DEGs), which were overrepresented in pathways associated with endocrine function, signal transduction, and cardiomyopathies. At 750 days, WD-fed male mice exhibited dysregulation of several genes involved in various lipid, glucagon, and glutathione metabolic pathways. At 530 days, WD-fed female mice exhibited the most distinctive set of DEGs with an abundance of genes related to circadian rhythms. At 640 days, altered DEGs in WD-fed female mice were associated with cardiac energy metabolism and remodeling.

CONCLUSIONS

Our study demonstrated distinct sex-specific and age-associated differences in cardiac structure, function, and transcriptome signature between WD-fed male and female mice.

Survodutide for the Treatment of Obesity: Rationale and Design of the SYNCHRONIZE Cardiovascular Outcomes Trial

Dual agonism of glucagon and glucagon-like peptide-1 (GLP-1) receptors may be more effective than GLP-1 receptor agonism alone in reducing body weight, but the cardiovascular (CV) effects are unknown. The authors describe the rationale and design of SYNCHRONIZE-CVOT, a phase 3, randomized, double-blind, parallel-group, event-driven, CV safety study of survodutide, a dual glucagon and GLP-1 receptor agonist, administered subcutaneously once weekly compared with placebo in adults with a body mass index ≥27 kg/m2 and established CV disease or chronic kidney disease, and/or at least 2 weight-related complications or risk factors for CV disease. The primary endpoint of SYNCHRONIZE-CVOT is time to first occurrence of the composite adjudicated endpoint of 5-point major adverse CV events. This global CV outcomes trial is currently enrolling, with a target recruitment of 4,935 participants. SYNCHRONIZE-CVOT is the first trial that will determine the CV safety and efficacy of survodutide in people with obesity and increased CV risk. (A Study to Test the Effect of Survodutide [BI 456906] on Cardiovascular Safety in People With Overweight or Obesity [SYNCHRONIZE-CVOT]; NCT06077864).

Bisphenol C Induces Cardiac Developmental Defects by Disrupting m6A Homeostasis

Bisphenol A (BPA) is a commonly used plastic additive. Since BPA has been banned in maternal and infant food containers in many countries, BPA substitutes have been widely introduced to replace it. By systematically assessing the potential developmental toxicity of BPA substitutes, we observed that the 41-150 nM in vivo BPC exposure (around the reported concentration detected in infant urine: 6-186 nM) induced cardiac defects in zebrafish. Mechanistically, BPC disrupted m6A homeostasis by downregulation of the key m6A methyltransferase, Mettl3, thereby causing the m6A reader, Igf2bp2b, to fail in recognizing and stabilizing the inefficiently m6A-modified acox1 and tnnt2d mRNA. Then, downregulation of Acox1 (a regulator in cardiac fatty acid metabolism) and Tnnt2d (a component of cardiac troponin for muscle contraction) led to cardiac defects. Indeed, the dual cardiac functional axes regulated by the same m6A reader in response to BPC provided new insight into the regulatory mechanisms of epitranscriptomics and cardiac development. Collectively, our study not only presented evidence showing that the internal exposure levels of BPC in humans could lead to cardiac developmental defects but also demonstrated the underlying mechanism of BPC-mediated defects by disrupting the Mettl3-m6A-Igf2bp2b-Acox1/Tnnt2d pathways, which provided potential molecular markers associated with BPC exposure.

GLP-1 receptor agonist-based therapies and cardiovascular risk: a review of mechanisms

Cardiovascular outcome trials (CVOTs) in people living with type 2 diabetes mellitus and obesity have confirmed the cardiovascular benefits of glucagon-like peptide 1 receptor agonists (GLP-1RAs), including reduced cardiovascular mortality, lower rates of myocardial infarction, and lower rates of stroke. The cardiovascular benefits observed following GLP-1RA treatment could be secondary to improvements in glycemia, blood pressure, postprandial lipidemia, and inflammation. Yet, the GLP-1R is also expressed in the heart and vasculature, suggesting that GLP-1R agonism may impact the cardiovascular system. The emergence of GLP-1RAs combined with glucose-dependent insulinotropic polypeptide and glucagon receptor agonists has shown promising results as new weight loss medications. Dual-agonist and tri-agonist therapies have demonstrated superior outcomes in weight loss, lowered blood sugar and lipid levels, restoration of tissue function, and enhancement of overall substrate metabolism compared to using GLP-1R agonists alone. However, the precise mechanisms underlying their cardiovascular benefits remain to be fully elucidated. This review aims to summarize the findings from CVOTs of GLP-1RAs, explore the latest data on dual and tri-agonist therapies, and delve into potential mechanisms contributing to their cardioprotective effects. It also addresses current gaps in understanding and areas for further research.

The pancreas does not contribute to the non-adrenergic-non-cholinergic stimulation of heart rate in digesting pythons

Vertebrates elevate heart rate when metabolism increases during digestion. Part of this tachycardia is due to a non-adrenergic-non-cholinergic (NANC) stimulation of the cardiac pacemaker, and it has been suggested these NANC factors are circulating hormones that are released from either gastrointestinal or endocrine glands. The NANC stimulation is particularly pronounced in species with large metabolic responses to digestion, such as reptiles. To investigate the possibility that the pancreas may release hormones that exert positive chronotropic effects on the digesting Burmese python heart, a species with very large postprandial changes in heart rate and oxygen uptake, we evaluate how pancreatectomy affects postprandial heart rate before and after autonomic blockade of the muscarinic and the beta-adrenergic receptors. We also measured the rates of oxygen consumption and evaluated the short-term control of the heart using the spectral analysis of heart rate variability and the baroreflex sequence method. Digestion caused the ubiquitous tachycardia, but the intrinsic heart rate (revealed after the combination of atropine and propranolol) was not affected by pancreatectomy and therefore hormones, such as glucagon and insulin, do not appear to contribute to the regulation of heart rate during digestion in Burmese pythons.