Effects of acute intravenous infusion of apelin on left ventricular function in dogs with advanced heart failure

The significance of the apelinergic system in doxorubicin-induced cardiotoxicity

Cancer is the leading cause of death worldwide, and the number of cancer-related deaths is expected to increase. Common types of cancer include skin, breast, lung, prostate, and colorectal cancers. While clinical research has improved cancer therapies, these treatments often come with significant side effects such as chronic fatigue, hair loss, and nausea. In addition, cancer treatments can cause long-term cardiovascular complications. Doxorubicin (DOX) therapy is one example, which can lead to decreased left ventricle (LV) echocardiography (ECHO) parameters, increased oxidative stress in cellular level, and even cardiac fibrosis. The apelinergic system, specifically apelin and its receptor, together, has shown properties that could potentially protect the heart and mitigate the damages caused by DOX anti-cancer treatment. Studies have suggested that stimulating the apelinergic system may have therapeutic benefits for heart damage induced by DOX. Further research in chronic preclinical models is needed to confirm this hypothesis and understand the mechanism of action for the apelinergic system. This review aims to collect and present data on the effects of the apelinergic system on doxorubicin-induced cardiotoxicity.

The Apelin/APJ System: A Potential Therapeutic Target for Sepsis

Apelin is the native ligand for the G protein-coupled receptor APJ. Numerous studies have demonstrated that the Apelin/APJ system has positive inotropic, anti-inflammatory, and anti-apoptotic effects and regulates fluid homeostasis. The Apelin/APJ system has been demonstrated to play a protective role in sepsis and may serve as a promising therapeutic target for the treatment of sepsis. Better understanding of the mechanisms of the effects of the Apelin/APJ system will aid in the development of novel drugs for the treatment of sepsis. In this review, we provide a brief overview of the physiological role of the Apelin/APJ system and its role in sepsis.

Inducible apelin receptor knockdown reduces differentiation efficiency and contractility of hESC-derived cardiomyocytes

AIMS

The apelin receptor, a G protein-coupled receptor, has emerged as a key regulator of cardiovascular development, physiology, and disease. However, there is a lack of suitable human in vitro models to investigate the apelinergic system in cardiovascular cell types. For the first time we have used human embryonic stem cell-derived cardiomyocytes (hESC-CMs) and a novel inducible knockdown system to examine the role of the apelin receptor in both cardiomyocyte development and to determine the consequences of loss of apelin receptor function as a model of disease.

METHODS AND RESULTS

Expression of the apelin receptor and its ligands in hESCs and hESC-CMs was determined. hESCs carrying a tetracycline-inducible short hairpin RNA targeting the apelin receptor were generated using the sOPTiKD system. Phenotypic assays characterized the consequences of either apelin receptor knockdown before hESC-CM differentiation (early knockdown) or in 3D engineered heart tissues as a disease model (late knockdown). hESC-CMs expressed the apelin signalling system at a similar level to the adult heart. Early apelin receptor knockdown decreased cardiomyocyte differentiation efficiency and prolonged voltage sensing, associated with asynchronous contraction. Late apelin receptor knockdown had detrimental consequences on 3D engineered heart tissue contractile properties, decreasing contractility and increasing stiffness.

CONCLUSIONS

We have successfully knocked down the apelin receptor, using an inducible system, to demonstrate a key role in hESC-CM differentiation. Knockdown in 3D engineered heart tissues recapitulated the phenotype of apelin receptor down-regulation in a failing heart, providing a potential platform for modelling heart failure and testing novel therapeutic strategies.

Emerging Therapy for Diabetic Cardiomyopathy: From Molecular Mechanism to Clinical Practice

Diabetic cardiomyopathy is characterized by abnormal myocardial structure or performance in the absence of coronary artery disease or significant valvular heart disease in patients with diabetes mellitus. The spectrum of diabetic cardiomyopathy ranges from subtle myocardial changes to myocardial fibrosis and diastolic function and finally to symptomatic heart failure. Except for sodium–glucose transport protein 2 inhibitors and possibly bariatric and metabolic surgery, there is currently no specific treatment for this distinct disease entity in patients with diabetes. The molecular mechanism of diabetic cardiomyopathy includes impaired nutrient-sensing signaling, dysregulated autophagy, impaired mitochondrial energetics, altered fuel utilization, oxidative stress and lipid peroxidation, advanced glycation end-products, inflammation, impaired calcium homeostasis, abnormal endothelial function and nitric oxide production, aberrant epidermal growth factor receptor signaling, the activation of the renin–angiotensin–aldosterone system and sympathetic hyperactivity, and extracellular matrix accumulation and fibrosis. Here, we summarize several important emerging treatments for diabetic cardiomyopathy targeting specific molecular mechanisms, with evidence from preclinical studies and clinical trials.

Design and preparation of N-linked hydroxypyridine-based APJ agonists

Agonism of the apelin receptor (APJ) has demonstrated beneficial effects in models of heart failure. We have previously disclosed compounds such as 4, which showed good APJ agonist activity but were metabolized to the mono-demethylated, non-interconverting atropisomer metabolites. Herein, we detail the design and optimization of a novel series of N-linked APJ agonists with good potency, metabolic stability, and rat pharmacokinetic profile, which are unable to undergo the same metabolic mono-demethylation cleavage.

Dysregulation of miR-204-5p/APLN axis affects malignant progression and cell stemness of esophageal cancer

OBJECTIVE

This study attempted to investigate the mechanism of miR-204-5p and its downstream gene in regulating bio-functions of esophageal cancer (EC).

METHODS

Bioinformatics analysis was performed to select the mature miRNAs, mRNAs, and clinical data of EC. The miRNA-mRNA regulatory axis was predicted through bioinformatics and used Dual-luciferase analysis to verify the interaction between miR-204-5p and APLN. qRT-PCR was applied to analyze expression of miR-204-5p and APLN mRNA. Western blot was utilized to detect APLN protein expression. Functional assays like CCK-8, wound healing, Transwell, and stem cell sphere formation assays were launched to confirm proliferative, migratory, invasive and stemness of cells in different treatment groups.

RESULTS

MiR-204-5p was lowly expressed while its target gene APLN was highly expressed in tumor tissues. Besides, miR-204-5p overexpression hindered proliferation, invasion, migration, and stemness of EC cells. Additionally, dual-luciferase assay verified the interaction of miR-204-5p and APLN. MiR-204-5p could downregulate APLN level and its overexpression reduced the effect of APLN on EC cell functions.

CONCLUSION

Dysregulation of miR-204-5p/APLN axis was linked with malignant progression of EC. MiR-204-5p/APLN may be an underlying candidate for the design of anticarcinogens.

Significance and Determinants of Plasma Apelin in Patients With Obstructive Hypertrophic Cardiomyopathy

Background

Recent studies suggest apelin has multiple protective effects in some cardiovascular diseases. However, there are few data concerning apelin levels in patients with obstructive hypertrophic cardiomyopathy (OHCM) or the relationship between apelin levels and severity of OHCM.

Methods

We studied 88 patients with OHCM and 32 control subjects with matched age and sex distribution. Complete medical history was collected and related examinations were performed. Cardiac magnetic resonance (CMR) and echocardiography were employed to characterize cardiac morphology and function. Plasma apelin was measured by enzyme-linked immunosorbent assay (ELISA).

Results

Plasma apelin levels were significantly lower in patients with OHCM than those in control subjects (96.6 ± 34.3 vs. 169.4 ± 62.5 μg/L, p < 0.001). When patients with OHCM were divided into two groups according to the mean value of plasma apelin, patients with lower apelin levels (plasma apelin ≤ 96.6 μg/L) had greater septal wall thickness (SWT; 25.6 ± 5.5 vs. 23.2 ± 4.3 mm, p = 0.035) and less right ventricular end-diastolic diameter (RVEDD; 20.4 ± 3.3 vs. 23.0 ± 3.6 mm, p = 0.001). Consistently, plasma apelin levels were inversely correlated with SWT (r = −0.334, p = 0.002) and positively correlated with RVEDD (r = 0.368, p < 0.001). Besides, plasma apelin levels were inversely correlated with Ln (NT-proBNP) (r = −0.307, p = 0.008) and positively correlated with body mass index (BMI; r = 0.287, p = 0.008). On multivariate analysis, the SWT was independently associated with decreasing plasma apelin, while the RVEDD was independently associated with increasing plasma apelin.

Conclusion

Plasma apelin levels are reduced in patients with OHCM. The apelin levels are inversely related to SWT and positively related to RVEDD.

Management of Chronic Congestive Heart Failure Caused by Myxomatous Mitral Valve Disease in Dogs: A Narrative Review from 1970 to 2020

Simple Summary

Myxomatous mitral valve disease (MMVD) is the most common acquired cardiovascular disease in dogs. The progression of the disease and the increasing severity of valvular regurgitation cause a volume overload of the left heart, leading to left atrial and ventricular remodeling and congestive heart failure (CHF). The treatment of chronic CHF secondary to MMVD in dogs has not always been the same over time. In the last fifty years, the drugs utilized have considerably changed, as well as the therapeutic protocols. Some drugs have also changed their intended use. An analysis of the literature concerning the therapy of chronic heart failure in dogs affected by this widespread degenerative disease is not available; a synthesis of the published literature on this topic and a description of its current state of art are needed. To the authors’ knowledge, a review of this topic has never been published in veterinary medicine; therefore, the aim of this study is to overview the treatments of chronic CHF secondary to MMVD in dogs from 1970 to 2020 using the general framework of narrative reviews.

Abstract

The treatment of chronic congestive heart failure (CHF), secondary to myxomatous mitral valve disease (MMVD) in dogs, has considerably changed in the last fifty years. An analysis of the literature concerning the therapy of chronic CHF in dogs affected by MMVD is not available, and it is needed. Narrative reviews (NRs) are aimed at identifying and summarizing what has been previously published, avoiding duplications, and seeking new study areas that have not yet been addressed. The most accessible open-access databases, PubMed, Embase, and Google Scholar, were chosen, and the searching time frame was set in five decades, from 1970 to 2020. The 384 selected studies were classified into categories depending on the aim of the study, the population target, the pathogenesis of MMVD (natural/induced), and the resulting CHF. Over the years, the types of studies have increased considerably in veterinary medicine. In particular, there have been 43 (24.29%) clinical trials, 41 (23.16%) randomized controlled trials, 10 (5.65%) cross-over trials, 40 (22.60%) reviews, 5 (2.82%) comparative studies, 17 (9.60%) case-control studies, 2 (1.13%) cohort studies, 2 (1.13%) experimental studies, 2 (1.13%) questionnaires, 6 (3.40%) case-reports, 7 (3.95%) retrospective studies, and 2 (1.13%) guidelines. The experimental studies on dogs with an induced form of the disease were less numerous (49–27.68%) than the studies on dogs affected by spontaneous MMVD (128–72.32%). The therapy of chronic CHF in dogs has considerably changed in the last fifty years: in the last century, some of the currently prescribed drugs did not exist yet, while others had different indications.

Identification of a Hydroxypyrimidinone Compound () as a Potent APJ Receptor Agonist for the Potential Treatment of Heart Failure

This paper describes our continued efforts in the area of small-molecule apelin receptor agonists. Recently disclosed compound 2 showed an acceptable metabolic stability but demonstrated monodemethylation of the dimethoxyphenyl group to generate atropisomer metabolites in vitro. In this article, we extended the structure-activity relationship at the C2 position that led to the identification of potent pyrazole analogues with excellent metabolic stability. Due to the increased polarity at C2, the permeability for these compounds decreased. Further adjustment of the polarity by replacing the N1 2,6-dimethoxyphenyl group with a 2,6-diethylphenyl group and reoptimization for the potency of the C5 pyrroloamides resulted in potent compounds with improved permeability. Compound 21 displayed excellent pharmacokinetic profiles in rat, monkey, and dog models and robust pharmacodynamic efficacy in the rodent heart failure model. Compound 21 also showed an acceptable safety profile in preclinical toxicology studies and was selected as a backup development candidate for the program.

Identification of 6-Hydroxypyrimidin-4(1H)-one-3-carboxamides as Potent and Orally Active APJ Receptor Agonists

The apelin receptor (APJ) is a significant regulator of cardiovascular function and is involved in heart failure and other cardiovascular diseases. (Pyr¹)apelin-13 is one of the endogenous agonists of the APJ receptor. Administration of (Pyr¹)apelin-13 increases cardiac output in preclinical models and humans. Recently we disclosed clinical lead BMS-986224 (1), a C3 oxadiazole pyridinone APJ receptor agonist with robust pharmacodynamic effects similar to (Pyr¹)apelin-13 in an acute rat pressure-volume loop model. Herein we describe the structure-activity relationship of the carboxamides as oxadiazole bioisosteres at C3 of the pyridinone core and C5 of the respective pyrimidinone core. This study led to the identification of structurally differentiated 6-hydroxypyrimidin-4(1H)-one-3-carboxamide 14a with pharmacodynamic effects comparable to those of compound 1.

Discovery of a Hydroxypyridinone APJ Receptor Agonist as a Clinical Candidate

Apelin-13 is an endogenous peptidic agonist of the apelin receptor (APJ) receptor with the potential for improving cardiac function in heart failure patients. However, the low plasma stability of apelin-13 necessitates continuous intravenous infusion for therapeutic use. There are several approaches to increase the stability of apelin-13 including attachment of pharmacokinetic enhancing groups, stabilized peptides, and Fc-fusion approaches. We sought a small-molecule APJ receptor agonist approach to target a compound with a pharmacokinetic profile amenable for chronic oral administration. This manuscript describes sequential optimization of the pyrimidinone series, leading to pyridinone 14, with in vitro potency equivalent to the endogenous ligand apelin-13 and with an excellent oral bioavailability and PK profile in multiple preclinical species. Compound 14 exhibited robust pharmacodynamic effects similar to apelin-13 in an acute rat pressure-volume loop model and was advanced as a clinical candidate.

In Vitro and In Vivo Evaluation of a Small-Molecule APJ (Apelin Receptor) Agonist, BMS-986224, as a Potential Treatment for Heart Failure

Supplemental Digital Content is available in the text.

New heart failure therapies that safely augment cardiac contractility and output are needed. Previous apelin peptide studies have highlighted the potential for APJ (apelin receptor) agonism to enhance cardiac function in heart failure. However, apelin’s short half-life limits its therapeutic utility. Here, we describe the preclinical characterization of a novel, orally bioavailable APJ agonist, BMS-986224.

Apelin ameliorated acute heart failure via inhibiting endoplasmic reticulum stress in rabbits

This study aimed to investigate whether inhibition of endoplasmic reticulum stress (ERS) mediated the ameliorative effect of apelin on acute heart failure (AHF). Rabbit model of AHF was induced by sodium pentobarbital. Cardiac dysfunction and injury were detected in the rabbit models of AHF, including impaired hemodynamic parameters and increased levels of CK-MB and cTnI. Apelin treatment dramatically improved cardiac impairment caused by AHF. ERS, indexed by increased GRP78, CHOP, and cleaved-caspase12 protein levels, was simultaneously attenuated by apelin. Apelin also could ameliorate increased protein levels of cleaved-caspase3 and Bax, and improved decreased protein levels of Bcl-2. Two common ERS stimulators, tunicamycin (Tm) and dithiothreitol (DTT) blocked the ameliorative effect of apelin on AHF. Phosphorylated Akt levels increased after apelin treatment in the rabbit models of AHF. The Akt signaling inhibitors wortmannin and LY294002 could block the cardioprotective effect of apelin, which could be relieved by ERS inhibitor 4-phenyl butyric acid (4-PBA). The aforementioned beneficial effects of apelin could all be blocked by APJ receptor antagonist F13A. 4-PBA and SC79, an Akt activator, can restore the ameliorative effect of apelin on AHF blocked by F13A. Apelin treatment dramatically ameliorated cardiac impairment caused by AHF, which might be mediated by APJ/Akt/ERS signaling pathway. These results will shed new light on AHF therapy.

Recently Approved and Under Investigation Drugs for Treating Patients with Heart Failure

Heart Failure (HF) represents a leading cause of morbidity and mortality worldwide. Despite the recent advances in the treatment of this condition, patients´ prognosis remains unfavorable in most cases. Sacubitril/valsartan and ivabradine have been recently approved to improve clinical outcomes in patients with HF with reduced ejection fraction. Drugs under investigation for treating patients with HF encompass many novel mechanisms including vasoactive peptides, blocking inflammatory- mediators, natriuretic peptides, selective non-steroidal mineralocorticoid-receptor antagonists, myocardial β3 adrenoreceptor agonists, inhibiting the cytochrome C/cardiolipin peroxidase complex, neuregulin-1/ErbB signaling and inhibiting late inward sodium current. The aim of this manuscript is to review the main drugs under investigation for the treatment of patients with HF and give perspectives for their implementation into clinical practice.

[MeArg1, NLe10]-apelin-12: Optimization of solid-phase synthesis and evaluation of biological properties in vitro and in vivo

Chemically modified peptide apelin-12 ([MeArg¹, NLe¹⁰]-apelin12, peptide M) is able to reduce reactive oxygen species (ROS) formation, cell death, and metabolic and ionic homeostasis disorders in experimental myocardial ischemia-reperfusion injury. These beneficial effects indicate the therapeutic potential of this compound in cardiovascular diseases. The goals of this work were to optimize the synthesis of peptide M, and to study its proteolytic stability and effect on the heart function of rabbits with doxorubicin (Dox) cardiomyopathy. We have developed a rational method of solid-phase synthesis of peptide M using the Fmoc methodology in combination with the temporary protection of the guanidine function of arginine residues by protonation (salt formation) during the formation of the amide bond. It avoids the formation of by-products, and simplifies the post-synthetic procedures, providing an increase in the yield of the final product of higher purity. Comparative evaluation of the proteolytic stability of peptide M and apelin-12 in human blood plasma was carried out using ¹H NMR spectroscopy. It was shown that the half-life of peptide M in plasma is approximately three times longer than that of apelin-12. Intravenous infusion of increasing doses of peptide M caused a gradual increase in left ventricular (LV) fractional shortening and ejection fraction in rabbits after 8 weeks of Dox administration (2 mg/kg weekly). The effect of the modified peptide on LV systolic dysfunction was significantly more pronounced than the effect of apelin-12, which suggests the promise of using this pharmacological agonist of the APJ receptor in patients with heart failure.

Distinct hemodynamic responses to (pyr)apelin-13 in large animal models

This study tested the hypothesis that (pyr)apelin-13 dose-dependently augments myocardial contractility and coronary blood flow, irrespective of changes in systemic hemodynamics. Acute effects of intravenous (pyr)apelin-13 administration (10 to 1,000 nM) on blood pressure, heart rate, left ventricular pressure and volume, and coronary parameters were measured in dogs and pigs. Administration of (pyr)apelin-13 did not influence blood pressure (P = 0.59), dP/dt_max (P = 0.26), or dP/dt_min (P = 0.85) in dogs. However, heart rate dose-dependently increased > 70% (P < 0.01), which was accompanied by a significant increase in coronary blood flow (P < 0.05) and reductions in left ventricular end-diastolic volume and stroke volume (P < 0.001). In contrast, (pyr)apelin-13 did not significantly affect hemodynamics, coronary blood flow, or indexes of contractile function in pigs. Furthermore, swine studies found no effect of intracoronary (pyr)apelin-13 administration on coronary blood flow (P = 0.83) or vasorelaxation in isolated, endothelium-intact (P = 0.89) or denuded (P = 0.38) coronary artery rings. Examination of all data across (pyr)apelin-13 concentrations revealed an exponential increase in cardiac output as peripheral resistance decreased across pigs and dogs (P < 0.001; R² = 0.78). Assessment of the Frank-Starling relationship demonstrated a significant linear relationship between left ventricular end-diastolic volume and stroke volume across species (P < 0.001; R² = 0.70). Taken together, these findings demonstrate that (pyr)apelin-13 does not directly influence myocardial contractility or coronary blood flow in either dogs or pigs.NEW & NOTEWORTHY Our findings provide much needed insight regarding the pharmacological cardiac and coronary effects of (pyr)apelin-13 in larger animal preparations. In particular, data highlight distinct hemodynamic responses of apelin across species, which are independent of any direct effect on myocardial contractility or perfusion.

International Union of Basic and Clinical Pharmacology. CVII. Structure and Pharmacology of the Apelin Receptor with a Recommendation that Elabela/Toddler Is a Second Endogenous Peptide Ligand

The predicted protein encoded by the APJ gene discovered in 1993 was originally classified as a class A G protein-coupled orphan receptor but was subsequently paired with a novel peptide ligand, apelin-36 in 1998. Substantial research identified a family of shorter peptides activating the apelin receptor, including apelin-17, apelin-13, and [Pyr1]apelin-13, with the latter peptide predominating in human plasma and cardiovascular system. A range of pharmacological tools have been developed, including radiolabeled ligands, analogs with improved plasma stability, peptides, and small molecules including biased agonists and antagonists, leading to the recommendation that the APJ gene be renamed APLNR and encode the apelin receptor protein. Recently, a second endogenous ligand has been identified and called Elabela/Toddler, a 54-amino acid peptide originally identified in the genomes of fish and humans but misclassified as noncoding. This precursor is also able to be cleaved to shorter sequences (32, 21, and 11 amino acids), and all are able to activate the apelin receptor and are blocked by apelin receptor antagonists. This review summarizes the pharmacology of these ligands and the apelin receptor, highlights the emerging physiologic and pathophysiological roles in a number of diseases, and recommends that Elabela/Toddler is a second endogenous peptide ligand of the apelin receptor protein.

Effects of iloprost and sildenafil treatment on elabela, apelin-13, nitric oxide, and total antioxidant and total oxidant status in experimental enzyme-positive acute coronary syndrome in rats

Despite significant advances in medicine, mortality due to cardiovascular disease is not yet preventable. We investigated the amounts of elabela (ELA) and apelin, synthesized by cardiomyocytes, and changes of these compounds in cardiac tissue and circulation after administration of iloprost (ILO) and sildenafil (SIL) in rats with induced myocardial ischemia (MI). We also investigated a connection with circulating troponin-I, creatine kinase (CK), creatine kinase-myocardial band (CK-MB) and nitric oxide (NO), and total anti-oxidant (TAS)/total oxidant status (TOS). We established eight study groups of five rats each. Group 1, sham, was given only physiologic serum; group 2, ILO; group 3, SIL; group 4, ILO + SIL; group 5, MI; group 6, MI + ILO; group 7, MI + SIL; group 8, MI + ILO + SIL. Troponin-I, CK, CK-MB and TAS-TOS were investigated using an autoanalyzer. NO, ELA and apelin were analyzed by ELISA. Tissue apelin and ELA expressions and localizations were determined by immunohistochemistry. The MI group compared to the control (sham) group showed that ELA, apelin, troponin-I, CK, CK-MB, NO and TOS levels were elevated significantly. Concentrations of these factors increased in MI, but decreased after ILO and SIL administration. The largest decrease of TOS was identified in the ILO + SIL group. ELA and apelin may be novel indicators of MI and administration of ILO and SIL, individually or together, may be useful for treating MI.

Effects of Apelin on Left Ventricular-Arterial Coupling and Mechanical Efficiency in Rats with Ischemic Heart Failure

Apelin plays important roles in cardiovascular homeostasis. However, its effects on the mechanoenergetics of heart failure (HF) are unavailable. We attempted to investigate the effects of apelin on the left ventricular-arterial coupling (VAC) and mechanical efficiency in rats with HF. HF was induced in rats by the ligation of the left coronary artery. The ischemic HF rats were treated with apelin or saline for 12 weeks. The sham-operated animals served as the control. The left ventricular (LV) afterload and the systolic and diastolic functions, as well as the mechanoenergetic indices were estimated from the pressure-volume loops. Myocardial fibrosis by Masson's trichrome staining, myocardial apoptosis by TUNEL, and collagen content in the aorta as well as media area in the aorta and the mesenteric arteries were determined. Our data indicated that HF rats manifested an increased arterial load (Ea), a declined systolic function (reduced ejection fraction, +dP/dt_max, end-systolic elastance, and stroke work), an abnormal diastolic function (elevated end-diastolic pressure, τ, and declined −dP/dt_max), and decreased mechanical efficiency. Apelin treatment improved those indices. Concomitantly, increased fibrosis in the LV myocardium and the aorta and enhanced apoptosis in the LV were partially restored by apelin treatment. A declined wall-to-lumen ratio in the mesenteric arteries of the untreated HF rats was further reduced in the apelin-treated group. We concluded that the rats with ischemic HF were characterized by deteriorated LV mechanoenergetics. Apelin improved mechanical efficiency, at least in part, due to the inhibiting cardiac fibrosis and apoptosis in the LV myocardium, reducing collagen deposition in the aorta and dilating the resistant artery.

Apelinergic System Structure and Function

Apelin and apela (ELABELA/ELA/Toddler) are two peptide ligands for a class A G-protein-coupled receptor named the apelin receptor (AR/APJ/APLNR). Ligand-AR interactions have been implicated in regulation of the adipoinsular axis, cardiovascular system, and central nervous system alongside pathological processes. Each ligand may be processed into a variety of bioactive isoforms endogenously, with apelin ranging from 13 to 55 amino acids and apela from 11 to 32, typically being cleaved C-terminal to dibasic proprotein convertase cleavage sites. The C-terminal region of the respective precursor protein is retained and is responsible for receptor binding and subsequent activation. Interestingly, both apelin and apela exhibit isoform-dependent variability in potency and efficacy under various physiological and pathological conditions, but most studies focus on a single isoform. Biophysical behavior and structural properties of apelin and apela isoforms show strong correlations with functional studies, with key motifs now well determined for apelin. Unlike its ligands, the AR has been relatively difficult to characterize by biophysical techniques, with most characterization to date being focused on effects of mutagenesis. This situation may improve following a recently reported AR crystal structure, but there are still barriers to overcome in terms of comprehensive biophysical study. In this review, we summarize the three components of the apelinergic system in terms of structure-function correlation, with a particular focus on isoform-dependent properties, underlining the potential for regulation of the system through multiple endogenous ligands and isoforms, isoform-dependent pharmacological properties, and biological membrane-mediated receptor interaction. © 2018 American Physiological Society. Compr Physiol 8:407-450, 2018.

Serum level of apelin-13 negatively correlated with NT-proBNP in heart failure patients

Background: Apelin is a potent endogenous inotropic peptide with a major role in counteracting the aldosterone and angiotensin II and their negative effects on the cardiovascular system. The exact role of apelin in the patho-physiology of this disease is not well understood. We aimed to investigate the possible associations of apelin-13 with clinical and paraclinical characteristics in HF patients as well as studying its dynamics during the course of the heart failure.

Method: We performed a prospective observational cohort single-center study. We compared the baseline serum levels of apelin-13 and NT-proBNP level in 53 heart failure patients (acute heart failure, chronic compensated heart failure and chronic decompensated heart failure). We divided the patients according to the apelin-13 level: above and below the median, and we analyzed the relationship between serum apelin-13 and the clinical, echocardiographic, electrocardiographic and biological parameters. Twenty patients were followed-up (after an average time interval of 9 months), investigating the same parameters.

Results: The median of apelin-13 was 495pg/mL (IQR 276-845pg/mL). We found strong, negative correlation between the serum levels of apelin-13 and NT-proBNP (Spearman rho= −0.83, p<0.001). For the reassessed patients the median apelin level was significantly higher at follow-up (460 pg/mL, IQR 342-871 pg/mL) as compared with the baseline level (395 pg/mL, IQR 270-603 pg/mL), p=0.019, and maintained the negative correlation with NT-proBNP level (Spearman’s rho −0.7, p<0.001. The Low Apelin-13 group have higher NT-proBNP levels and also contains all the patients in NYHA IV class heart failure, 71% of the acute HF patients, and 7 of 8 patients who died before follow-up.

Conclusion: Apelin-13 was negatively correlated with NT-proBNP. The Low Apelin-13 group contained the majority of the patients with a negative outcome (death before follow-up), most of the patients who presented with acute HF and all the patients in NYHA IV class.

Cardiac action of the first G protein biased small molecule apelin agonist

Apelin peptide analogues displaying bias towards G protein signalling pathways have beneficial cardiovascular actions compared with the native peptide in humans in vivo. Our aim was to determine whether small molecule agonists could retain G protein bias. We have identified a biased small molecule, CMF-019, and characterised it in vitro and in vivo. In competition radioligand binding experiments in heart homogenates, CMF-019 bound to the human, rat and mouse apelin receptor with high affinity (pKi=8.58±0.04, 8.49±0.04 and 8.71±0.06 respectively). In cell-based functional assays, whereas, CMF-019 showed similar potency for the Gαi pathway to the endogenous agonist [Pyr(1)]apelin-13 (pD2=10.00±0.13 vs 9.34±0.15), in β-arrestin and internalisation assays it was less potent (pD2=6.65±0.15 vs 8.65±0.10 and pD2=6.16±0.21 vs 9.28±0.10 respectively). Analysis of these data demonstrated a bias of ∼400 for the Gαi over the β-arrestin pathway and ∼6000 over receptor internalisation. CMF-019 was tested for in vivo activity using intravenous injections into anaesthetised male Sprague-Dawley rats fitted with a pressure-volume catheter in the left ventricle. CMF-019 caused a significant increase in cardiac contractility of 606±112mmHg/s (p<0.001) at 500nmol. CMF-019 is the first biased small molecule identified at the apelin receptor and increases cardiac contractility in vivo. We have demonstrated that Gαi over β-arrestin/internalisation bias can be retained in a non-peptide analogue and predict that such bias will have the therapeutic benefit following chronic use. CMF-019 is suitable as a tool compound and provides the basis for design of biased agonists with improved pharmacokinetics for treatment of cardiovascular conditions such as pulmonary arterial hypertension.

Apelin induces vascular smooth muscle cells migration via a PI3K/Akt/FoxO3a/MMP-2 pathway

Apelin is an adipokine that has a critical role in the development of atherosclerosis, which may offer potential for therapy. Because migration of vascular smooth muscle cells (VSMCs) is a key event in the development of atherosclerosis, understanding its effect on the atherosclerotic vasculature is needed. Here we investigated the effect of apelin on VSMC migration and the possible signaling mechanism. In cultured rat VSMCs, apelin dose- and time-dependently promoted VSMC migration. Apelin increased the phosphorylation of Akt, whereas LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), and an Akt1/2 kinase inhibitor blocked the apelin-induced VSMC migration. Apelin dose-dependently induced phosphorylation of Forkhead box O3a (FoxO3a) and promoted its translocation from the nucleus to cytoplasm, which were blocked by LY294002 and Akt1/2 kinase inhibitor. Furthermore, apelin increased matrix metalloproteinase 2 (MMP-2) expression and gelatinolytic activity. Overexpression of a constitutively active, phosphorylation-resistant mutant, TM-FoxO3a, in VSMCs abrogated the effect of apelin on MMP-2 expression and VSMC migration. ARP101, an inhibitor of MMP-2, suppressed apelin-induced VSMC migration. Moreover, the levels of apelin, phosphorylated Akt, FoxO3a, and MMP-2 were higher in human carotid-artery atherosclerotic plaque than in adjacent normal vessels. We demonstrate that PI3K/Akt/FoxO3a signaling may be involved in apelin inducing VSMC migration. Phosphorylation of FoxO3a plays a central role in mediating the apelin-induced MMP-2 activation and VSMC migration.

Apelin, Elabela/Toddler, and biased agonists as novel therapeutic agents in the cardiovascular system

Apelin and its G protein-coupled receptor (GPCR) have emerged as a key signalling pathway in the cardiovascular system. The peptide is a potent inotropic agent and vasodilator. Remarkably, a peptide, Elabela/Toddler, that has little sequence similarity to apelin, has been proposed as a second endogenous apelin receptor ligand and is encoded by a gene from a region of the genome previously classified as 'non-coding'. Apelin is downregulated in pulmonary arterial hypertension and heart failure. To replace the missing endogenous peptide, 'biased' apelin agonists have been designed that preferentially activate G protein pathways, resulting in reduced β-arrestin recruitment and receptor internalisation, with the additional benefit of attenuating detrimental β-arrestin signalling. Proof-of-concept studies support the clinical potential for apelin receptor biased agonists.

Structural apelin analogues: mitochondrial ROS inhibition and cardiometabolic protection in myocardial ischaemia reperfusion injury

BACKGROUND AND PURPOSE

Mitochondria-derived oxidative stress is believed to be crucially involved in cardiac ischaemia reperfusion (I/R) injury, although currently no therapies exist that specifically target mitochondrial reactive oxygen species (ROS) production. The present study was designed to evaluate the potential effects of the structural analogues of apelin-12, an adipocyte-derived peptide, on mitochondrial ROS generation, cardiomyocyte apoptosis, and metabolic and functional recovery to myocardial I/R injury.

EXPERIMENTAL APPROACH

In cultured H9C2 cardiomyoblasts and adult cardiomyocytes, oxidative stress was induced by hypoxia reoxygenation. Isolated rat hearts were subjected to 35 min of global ischaemia and 30 min of reperfusion. Apelin-12, apelin-13 and structural apelin-12 analogues, AI and AII, were infused during 5 min prior to ischaemia.

KEY RESULTS

In cardiac cells, mitochondrial ROS production was inhibited by the structural analogues of apelin, AI and AII, in comparison with the natural peptides, apelin-12 and apelin-13. Treatment of cardiomyocytes with AI and AII decreased cell apoptosis concentration-dependently. In a rat model of I/R injury, pre-ischaemic infusion of AI and AII markedly reduced ROS formation in the myocardial effluent and attenuated cell membrane damage. Prevention of oxidative damage by AI and AII was associated with the improvement of functional and metabolic recovery after I/R in the heart.

CONCLUSIONS AND IMPLICATIONS

These data provide the evidence for the potential of the structural apelin analogues in selective reduction of mitochondrial ROS generation and myocardial apoptosis and form the basis for a promising therapeutic strategy in the treatment of oxidative stress-related heart disease.

The Emerging Potential of the Apelin-APJ System in Heart Failure

The apelin-APJ system is a novel neurohormonal pathway, with studies to date suggesting that it may be of pathophysiologic relevance in heart failure and may indeed be a viable therapeutic target in this syndrome. This interest is driven primarily by the demonstration of its vasodilator, inotropic, and aquaretic actions as well as its apparent antagonistic relationship with the renin-angiotensin system. However, its promise is heightened further by the observation that, unlike other and more established cardioprotective pathways, it appears to be down-regulated in heart failure, suggesting that augmentation of this axis may have a powerful effect on the heart failure syndrome. We review the literature regarding the apelin-APJ system in heart failure and suggest areas requiring further research.

The Apelin-APJ System in the Evolution of Heart Failure

Heart failure is a chronic, progressive disease in which the overexpression of biologically active molecules and neurohomonal activation are the key factors of the evolution and natural history. The apelin-APJ system is a newly discovered molecular pathway and the RAAS counterbalance is its principal effect. The apelin is a potent inotrope, vasodilator and diuretic with crucial cardioprotective effects against angiotensin and aldosterone injuries. Intense and prolonged RAAS induces the downregulation of the apelin and its receptor at myocardial level and cancels their protection. Compared to the vasoactive agents used in the treatment of acute heart failure, exogen apelin has unique intropic and vasodilatory effects without deleterious consequences, being a promising therapeutic option.

The relation between apelin levels, echocardiographic findings and carotid intima media thickness in peritoneal dialysis patients

BACKGROUND

Cardiovascular disease (CVD) is the most important cause of morbidity and mortality in patients with end stage renal disease (ESRD). Apelin expressed in endothelial and other tissues including brain and kidney is an adipocytokine defined recently and is emerging an important mediator of cardiovascular homeostasis. The aim of this study was to test whether apelin levels might be associated with carotid artery atherosclerosis and left ventricular mass index (LVMI) in peritoneal dialysis patients.

PATIENTS AND METHODS

Fifty peritoneal dialysis patients (25 female, mean age 41.4 ± 11.9 years, mean dialysis vintage 65.0 ± 35.4 months) and 18 healthy individuals (9 female, mean age 41.7 ± 6.8 years) were included in this cross-sectional study. Serum apelin 12 levels, echocardiographic findings and carotid intima media thickness (CIMT) were recorded as well as clinical and laboratory data.

RESULTS

There were no differences between the patient and the control groups with regard to demographic characteristics. In patient group, LVMI, CIMT, CRP and apelin levels were elevated compared to control group. However there was no association between apelin, LVMI and CIMT. There was a positive correlation between apelin and CRP, which was not statistically significant. When patients were divided into two groups according to the mean serum apelin levels, LVMI, CIMT and CRP were higher in the high apelin group but this difference did not reach statistical significance.

CONCLUSION

We observed an increased inflammation and CVD risk in peritoneal dialysis patients. However, serum apelin levels seem not to be associated with cardiovascular risk in this group of patients.

Dysregulated microRNA-224/apelin axis associated with aggressive progression and poor prognosis in patients with prostate cancer

Our previous study revealed that microRNA (miR)-224 down-regulation could promote tumor progression of prostate cancer (PCa) and might be associated with poor biochemical recurrence-free survival of patients with this malignancy. However, the underlying mechanisms of miR-224 have not been fully elucidated. In the current study, apelin (APLN) was identified as a target gene of miR-224. Forced expression of miR-224 inhibited PCa cell invasion and migration by suppressing the expression of APLN. In addition, the down-regulation of miR-224 was negatively correlated with the up-regulation of APLN mRNA in PCa tissues. Moreover, miR-224 down-regulation was significantly associated with advanced clinical stage (P = .027) and metastasis (P = .001), whereas APLN up-regulation more frequently occurred in PCa tissues with advanced pathologic stage (P = .003), metastasis (P < .001), and prostate-specific antigen failure (P = .001). Furthermore, patients with PCa in the miR-224-low/APLN-high group more frequently had shorter biochemical recurrence-free survival than those in groups with other expression patterns of the 2 molecules. Taken together, our data strongly confirmed for the first time that the dysregulated miR-224/APLN axis may be associated with tumorigenesis and aggressive progression of PCa. More importantly, miR-224 down-regulation and APLN up-regulation may synergistically predict biochemical recurrence-free survival in patients with PCa.