Hypoxia inducible factor regulates the cardiac expression and secretion of apelin

Differences in Oxygen-Induced Retinopathy Susceptibility Between Two Sprague Dawley Rat Vendors: A Comparison of Retinal Transcriptomes

PURPOSE

To determine the retinal transcriptomic differences underlying the oxygen-induced retinopathy phenotypes between Sprague Dawley rat pups from two commonly used commercial vendors. This will allow us to discover genes and pathways that may be related to differences in disease severity in similarly aged premature babies and suggest possible new treatment approaches.

METHODS

We analyzed retinal vascular morphometry and transcriptomes from Sprague Dawley rat pups from Charles River Laboratories and Envigo (previously Harlan). Room air control and oxygen-induced retinopathy groups were compared. Oxygen-induced retinopathy was induced with the rat 50/10 model.

RESULTS

Pups from Charles River Laboratories developed a more severe oxygen-induced retinopathy phenotype, with 3.6-fold larger percent avascular area at P15 and twofold larger % neovascular area at P20 than pups from Envigo. Changes in retinal transcriptomes of rat pups from both vendors were substantial at baseline and in response to oxygen-induced retinopathy. Baseline differences centered on activated pathways of neuronal development in Charles River Laboratories pups. In response to oxygen-induced retinopathy, during the neovascular phase, retinas from Charles River Laboratories pups exhibited activation of pathways regulating necrosis, neuroinflammation, and interferon signaling, supporting the observed increase of neovascularization. Conversely, retinas from Envigo pups showed decreased necrosis and increased focal adhesion kinase signaling, supporting more normal vascular development. Comparing oxygen-induced retinopathy transcriptomes at P15 to those at P20, canonical pathways such as phosphate and tensin homolog, interferon, and coordinated lysosomal expression and regulation element signaling were identified, highlighting potential novel mechanistic targets for future research.

CONCLUSION

Transcriptomic profiles differ substantially between rat pup retinas from Charles River Laboratories and Envigo at baseline and in response to oxygen-induced retinopathy, providing insight into vascular morphologic differences. Comparing transcriptomes identified new pathways for further research in oxygen-induced retinopathy pathogenesis and increased scientific rigor of this model.

From Beats to Metabolism: the Heart at the Core of Interorgan Metabolic Cross Talk

The heart, once considered a mere blood pump, is now recognized as a multifunctional metabolic and endocrine organ. Its function is tightly regulated by various metabolic processes, at the same time it serves as an endocrine organ, secreting bioactive molecules that impact systemic metabolism. In recent years, research has shed light on the intricate interplay between the heart and other metabolic organs, such as adipose tissue, liver, and skeletal muscle. The metabolic flexibility of the heart and its ability to switch between different energy substrates play a crucial role in maintaining cardiac function and overall metabolic homeostasis. Gaining a comprehensive understanding of how metabolic disorders disrupt cardiac metabolism is crucial, as it plays a pivotal role in the development and progression of cardiac diseases. The emerging understanding of the heart as a metabolic and endocrine organ highlights its essential contribution to whole body metabolic regulation and offers new insights into the pathogenesis of metabolic diseases, such as obesity, diabetes, and cardiovascular disorders. In this review, we provide an in-depth exploration of the heart's metabolic and endocrine functions, emphasizing its role in systemic metabolism and the interplay between the heart and other metabolic organs. Furthermore, emerging evidence suggests a correlation between heart disease and other conditions such as aging and cancer, indicating that the metabolic dysfunction observed in these conditions may share common underlying mechanisms. By unraveling the complex mechanisms underlying cardiac metabolism, we aim to contribute to the development of novel therapeutic strategies for metabolic diseases and improve overall cardiovascular health.

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.

Targeting the apelin system for the treatment of cardiovascular diseases

Cardiovascular disease is the leading cause of death worldwide. Its prevalence is rising due to ageing populations and the increasing incidence of diseases such as chronic kidney disease, obesity, and diabetes that are associated with elevated cardiovascular risk. Despite currently available treatments, there remains a huge burden of cardiovascular disease-associated morbidity for patients and healthcare systems, and newer treatments are needed. The apelin system, comprising the apelin receptor and its two endogenous ligands apelin and elabela, is a broad regulator of physiology that opposes the actions of the renin-angiotensin and vasopressin systems. Activation of the apelin receptor promotes endothelium-dependent vasodilatation and inotropy, lowers blood pressure, and promotes angiogenesis. The apelin system appears to protect against arrhythmias, inhibits thrombosis, and has broad anti-inflammatory and anti-fibrotic actions. It also promotes aqueous diuresis through direct and indirect (central) effects in the kidney. Thus, the apelin system offers therapeutic promise for a range of cardiovascular, kidney, and metabolic diseases. This review will discuss current cardiovascular disease targets of the apelin system and future clinical utility of apelin receptor agonism.

Apelin C-Terminal Fragments: Biological Properties and Therapeutic Potential

Creation of bioactive molecules for treatment of cardiovascular diseases based on natural peptides is the focus of intensive experimental research. In the recent years, it has been established that C-terminal fragments of apelin, an endogenous ligand of the APJ receptor, reduce metabolic and functional disorders in experimental heart damage. The review presents literature data and generalized results of our own experiments on the effect of apelin-13, [Pyr]apelin-13, apelin-12, and their chemically modified analogues on the heart under normal and pathophysiological conditions in vitro and in vivo. It has been shown that the spectrum of action of apelin peptides on the damaged myocardium includes decrease in the death of cardiomyocytes from necrosis, reduction of damage to cardiomyocyte membranes, improvement in myocardial metabolic state, and decrease in formation of reactive oxygen species and lipid peroxidation products. The mechanisms of protective action of these peptides associated with activation of the APJ receptor and manifestation of antioxidant properties are discussed. The data presented in the review show promise of the molecular design of APJ receptor peptide agonists, which can serve as the basis for the development of cardioprotectors that affect the processes of free radical oxidation and metabolic adaptation.

APJ as Promising Therapeutic Target of Peptide Analogues in Myocardial Infarction- and Hypertension-Induced Heart Failure

The widely expressed G protein-coupled apelin receptor (APJ) is activated by two bioactive endogenous peptides, apelin and ELABELA (ELA). The apelin/ELA-APJ-related pathway has been found involved in the regulation of many physiological and pathological cardiovascular processes. Increasing studies are deepening the role of the APJ pathway in limiting hypertension and myocardial ischaemia, thus reducing cardiac fibrosis and adverse tissue remodelling, outlining APJ regulation as a potential therapeutic target for heart failure prevention. However, the low plasma half-life of native apelin and ELABELA isoforms lowered their potential for pharmacological applications. In recent years, many research groups focused their attention on studying how APJ ligand modifications could affect receptor structure and dynamics as well as its downstream signalling. This review summarises the novel insights regarding the role of APJ-related pathways in myocardial infarction and hypertension. Furthermore, recent progress in designing synthetic compounds or analogues of APJ ligands able to fully activate the apelinergic pathway is reported. Determining how to exogenously regulate the APJ activation could help to outline a promising therapy for cardiac diseases.

The Yin and Yang Effect of the Apelinergic System in Oxidative Stress

Apelin is an endogenous ligand for the G protein-coupled receptor APJ and has multiple biological activities in human tissues and organs, including the heart, blood vessels, adipose tissue, central nervous system, lungs, kidneys, and liver. This article reviews the crucial role of apelin in regulating oxidative stress-related processes by promoting prooxidant or antioxidant mechanisms. Following the binding of APJ to different active apelin isoforms and the interaction with several G proteins according to cell types, the apelin/APJ system is able to modulate different intracellular signaling pathways and biological functions, such as vascular tone, platelet aggregation and leukocytes adhesion, myocardial activity, ischemia/reperfusion injury, insulin resistance, inflammation, and cell proliferation and invasion. As a consequence of these multifaceted properties, the role of the apelinergic axis in the pathogenesis of degenerative and proliferative conditions (e.g., Alzheimer's and Parkinson's diseases, osteoporosis, and cancer) is currently investigated. In this view, the dual effect of the apelin/APJ system in the regulation of oxidative stress needs to be more extensively clarified, in order to identify new potential strategies and tools able to selectively modulate this axis according to the tissue-specific profile.

The ebb and flow of cardiac lymphatics: a tidal wave of new discoveries

The heart is imbued with a vast lymphatic network that is responsible for fluid homeostasis and immune cell trafficking. Disturbances in the forces that regulate microvascular fluid movement can result in myocardial edema, which has profibrotic and proinflammatory consequences and contributes to cardiovascular dysfunction. This review explores the complex relationship between cardiac lymphatics, myocardial edema, and cardiac disease. It covers the revised paradigm of microvascular forces and fluid movement around the capillary as well as the arsenal of preclinical tools and animal models used to model myocardial edema and cardiac disease. Clinical studies of myocardial edema and their prognostic significance are examined in parallel to the recent elegant animal studies discerning the pathophysiological role and therapeutic potential of cardiac lymphatics in different cardiovascular disease models. This review highlights the outstanding questions of interest to both basic scientists and clinicians regarding the roles of cardiac lymphatics in health and disease.

Chronic Kidney Disease Induces Proarrhythmic Remodeling

BACKGROUND

Patients with chronic kidney disease (CKD) are at increased risk of developing cardiac arrhythmogenesis and sudden cardiac death; however, the basis for this association is incompletely known.

METHODS

Here, using murine models of CKD, we examined interactions between kidney disease progression and structural, electrophysiological, and molecular cardiac remodeling.

RESULTS

C57BL/6 mice with adenine supplemented in their diet developed progressive CKD. Electrocardiographically, CKD mice developed significant QT prolongation and episodes of bradycardia. Optical mapping of isolated-perfused hearts using voltage-sensitive dyes revealed significant prolongation of action potential duration with no change in epicardial conduction velocity. Patch-clamp studies of isolated ventricular cardiomyocytes revealed changes in sodium and potassium currents consistent with action potential duration prolongation. Global transcriptional profiling identified dysregulated expression of cellular stress response proteins RBM3 (RNA-binding motif protein 3) and CIRP (cold-inducible RNA-binding protein) that may underlay the ion channel remodeling. Unexpectedly, we found that female sex is a protective factor in the progression of CKD and its cardiac sequelae.

CONCLUSIONS

Our data provide novel insights into the association between CKD and pathologic proarrhythmic cardiac remodeling. Cardiac cellular stress response pathways represent potential targets for pharmacologic intervention for CKD-induced heart rhythm disorders.

The emerging role of the apelinergic system in kidney physiology and disease

The apelinergic system (AS) is a novel pleiotropic system with an essential role in renal and cardiovascular physiology and disease, including water homeostasis and blood pressure regulation. It consists of two highly conserved peptide ligands, apelin and apela, and a G-protein-coupled apelin receptor. The two ligands have many isoforms and a short half-life and exert both similar and divergent effects. Vasopressin, apelin and their receptors colocalize in hypothalamic regions essential for body fluid homeostasis and interact at the central and renal levels to regulate water homeostasis and diuresis in inverse directions. In addition, the AS and renin-angiotensin system interact both systemically and in the kidney, with implications for the cardiovascular system. A role for the AS in diverse pathological states, including disorders of sodium and water balance, hypertension, heart failure, pre-eclampsia, acute kidney injury, sepsis and diabetic nephropathy, has recently been reported. Furthermore, several metabolically stable apelin analogues have been developed, with potential applications in diverse diseases. We review here what is currently known about the physiological functions of the AS, focusing on renal, cardiovascular and metabolic homeostasis, and the role of the AS in associated diseases. We also describe several hurdles and research opportunities worthy of the attention of the nephrology community.

Distribution, Function, and Expression of the Apelinergic System in the Healthy and Diseased Mammalian Brain

Apelin, a peptide initially isolated from bovine stomach extract, is an endogenous ligand for the Apelin Receptor (APLNR). Subsequently, a second peptide, ELABELA, that can bind to the receptor has been identified. The Apelin receptor and its endogenous ligands are widely distributed in mammalian organs. A growing body of evidence suggests that this system participates in various signaling cascades that can regulate cell proliferation, blood pressure, fluid homeostasis, feeding behavior, and pituitary hormone release. Additional research has been done to elucidate the system's potential role in neurogenesis, the pathophysiology of Glioblastoma multiforme, and the protective effects of apelin peptides on some neurological and psychiatric disorders-ischemic stroke, epilepsy, Parkinson's, and Alzheimer's disease. This review discusses the current knowledge on the apelinergic system's involvement in brain physiology in health and disease.

High-resolution imaging of the osteogenic and angiogenic interface at the site of murine cranial bone defect repair via multiphoton microscopy

The spatiotemporal blood vessel formation and specification at the osteogenic and angiogenic interface of murine cranial bone defect repair were examined utilizing a high-resolution multiphoton-based imaging platform in conjunction with advanced optical techniques that allow interrogation of the oxygen microenvironment and cellular energy metabolism in living animals. Our study demonstrates the dynamic changes of vessel types, that is, arterial, venous, and capillary vessel networks at the superior and dura periosteum of cranial bone defect, suggesting a differential coupling of the vessel type with osteoblast expansion and bone tissue deposition/remodeling during repair. Employing transgenic reporter mouse models that label distinct types of vessels at the site of repair, we further show that oxygen distributions in capillary vessels at the healing site are heterogeneous as well as time- and location-dependent. The endothelial cells coupling to osteoblasts prefer glycolysis and are less sensitive to microenvironmental oxygen changes than osteoblasts. In comparison, osteoblasts utilize relatively more OxPhos and potentially consume more oxygen at the site of repair. Taken together, our study highlights the dynamics and functional significance of blood vessel types at the site of defect repair, opening up opportunities for further delineating the oxygen and metabolic microenvironment at the interface of bone tissue regeneration.

The therapeutic potential of apelin in kidney disease

Chronic kidney disease (CKD) is a leading cause of global morbidity and mortality and is independently associated with cardiovascular disease. The mainstay of treatment for CKD is blockade of the renin-angiotensin-aldosterone system (RAAS), which reduces blood pressure and proteinuria and slows kidney function decline. Despite this treatment, many patients progress to kidney failure, which requires dialysis or kidney transplantation, and/or die as a result of cardiovascular disease. The apelin system is an endogenous physiological regulator that is emerging as a potential therapeutic target for many diseases. This system comprises the apelin receptor and its two families of endogenous ligands, apelin and elabela/toddler. Preclinical and clinical studies show that apelin receptor ligands are endothelium-dependent vasodilators and potent inotropes, and the apelin system has a reciprocal relationship with the RAAS. In preclinical studies, apelin regulates glomerular haemodynamics and acts on the tubule to promote aquaresis. In addition, apelin is protective in several kidney injury models. Although the apelin system has not yet been studied in patients with CKD, the available data suggest that apelin is a promising potential therapeutic target for kidney disease.

Circumventricular Organ Apelin Receptor Knockdown Decreases Blood Pressure and Sympathetic Drive Responses in the Spontaneously Hypertensive Rat

The central site(s) mediating the cardiovascular actions of the apelin-apelin receptor (APJ) system remains a major question. We hypothesized that the sensory circumventricular organs (CVOs), interfacing between the circulation and deeper brain structures, are sites where circulating apelin acts as a signal in the central nervous system to decrease blood pressure (BP). We show that APJ gene (aplnr) expression was elevated in the CVOs of spontaneously hypertensive rats (SHRs) compared to normotensive Wistar Kyoto (WKY) controls, and that there was a greater mean arterial BP (MABP) decrease following microinjection of [Pyr¹]apelin-13 to the CVOs of SHRs compared to WKY rats. Lentiviral APJ-specific-shRNA (LV-APJ-shRNA) was used to knockdown aplnr expression, both collectively in three CVOs and discretely in individual CVOs, of rats implanted with radiotelemeters to measure arterial pressure. LV-APJ-shRNA-injection decreased aplnr expression in the CVOs and abolished MABP responses to microinjection of [Pyr¹]apelin-13. Chronic knockdown of aplnr in any of the CVOs, collectively or individually, did not affect basal MABP in SHR or WKY rats. Moreover, knockdown of aplnr in any of the CVOs individually did not affect the depressor response to systemic [Pyr¹]apelin-13. By contrast, multiple knockdown of aplnr in the three CVOs reduced acute cardiovascular responses to peripheral [Pyr¹]apelin-13 administration in SHR but not WKY rats. These results suggest that endogenous APJ activity in the CVOs has no effect on basal BP but that functional APJ in the CVOs is required for an intact cardiovascular response to peripherally administered apelin in the SHR.

The Zebrafish Cardiac Endothelial Cell-Roles in Development and Regeneration

In zebrafish, the spatiotemporal development of the vascular system is well described due to its stereotypical nature. However, the cellular and molecular mechanisms orchestrating post-embryonic vascular development, the maintenance of vascular homeostasis, or how coronary vessels integrate into the growing heart are less well studied. In the context of cardiac regeneration, the central cellular mechanism by which the heart regenerates a fully functional myocardium relies on the proliferation of pre-existing cardiomyocytes; the epicardium and the endocardium are also known to play key roles in the regenerative process. Remarkably, revascularisation of the injured tissue occurs within a few hours after cardiac damage, thus generating a vascular network acting as a scaffold for the regenerating myocardium. The activation of the endocardium leads to the secretion of cytokines, further supporting the proliferation of the cardiomyocytes. Although epicardium, endocardium, and myocardium interact with each other to orchestrate heart development and regeneration, in this review, we focus on recent advances in the understanding of the development of the endocardium and the coronary vasculature in zebrafish as well as their pivotal roles in the heart regeneration process.

Plasma apelin level in acute myocardial infarction and its relation with prognosis: A prospective study

Objective

Apelin is a novel adipocytokine with a significant role in ischemia/reperfusion injury that is synthesized and secreted in myocardial cells and coronary endothelium. There is debate on its value for the diagnosis and prognosis of myocardial infarction. We aimed to investigate plasma apelin level in patients with acute ST segment elevation (STEMI) and non-ST segment elevation (NSTEMI) myocardial infarction and its relationship with left ventricular function and prognostic parameters.

Methods

Forty-one patients with STEMI, 21 patients with NSTEMI and 10 patients as control group with normal coronary angiograms were included. Plasma apelin level at presentation was investigated regarding its relationship with other diagnostic and prognostic parameters.

Results

Apelin level was significantly higher in acute myocardial infarction (0.31 ± 0.56 ng/mL) compared to control group (0.08 ± 0.05 ng/mL) (p < 0.01). Likewise, it was found to be significantly higher in STEMI group (0.45 ± 0.73 ng/mL) compared to control group (0.08 ± 0.05 ng/mL) (p < 0.01). Although apelin was higher in NSTEMI group (0.13 ± 0.10 ng/mL) compared to control group (0.08 ± 0.05 ng/mL), this difference was not statistically significant (p > 0.05). No correlation was found between apelin and NT-proBNP, hsCRP, troponin, ejection fraction (EF) and Killip score (p > 0.05). A positive correlation was found between apelin and TIMI, GRACE and Gensini scores (p < 0.05). Only GRACE score was found to be correlated with apelin in MI groups.

Conclusion

Apelin level was found to be high in acute myocardial infarction. With its inotropic and vasodilator effects, apelin was thought to have a protective role against severe ischemia.

Association of the serum apelin, but not ghrelin, with the presence and severity of coronary artery disease

Increasing evidence suggests that apelin and ghrelin may participate in atherogenesis. We sought to investigate whether the serum levels of apelin and ghrelin are significantly different in patients with coronary artery disease (CAD) compared to patients with nonsignificant coronary stenosis and determine the correlation between these adipokines and the severity of coronary stenosis. The study population included 31 stable CAD patients, 38 unstable CAD patients, and 39 non-CAD subjects. Serum levels of apelin and ghrelin, fasting blood glucose, lipid parameters, hs-CRP and hematological indices were determined in all groups using routine standard laboratory procedures. Serum apelin levels were significantly lower in patient with unstable CAD (0.354 ± 0.063 ng/mL) compared to stable CAD patients (0.401 ± 0.045 ng/mL, p = 0.003) and non-CAD subjects (0.415 ± 0.055 ng/mL, p<0.001). In addition, serum apelin levels were inversely correlated with the severity of coronary stenosis in CAD patients (p<0.05). However, there was no significant difference in ghrelin levels among the 3 groups. This data may suggest that the presence of unstable CAD may be associated with lower serum apelin which may indicate the potential role of this peptide in the progression and destabilization of coronary plaques.

Cardiovascular response to small-molecule APJ activation

Heart failure (HF) remains a grievous illness with poor prognosis even with optimal care. The apelin receptor (APJ) counteracts the pressor effect of angiotensin II, attenuates ischemic injury, and has the potential to be a novel target to treat HF. Intravenous administration of apelin improves cardiac function acutely in patients with HF. However, its short half-life restricts its use to infusion therapy. To identify a longer acting APJ agonist, we conducted a medicinal chemistry campaign, leading to the discovery of potent small-molecule APJ agonists with comparable activity to apelin by mimicking the C-terminal portion of apelin-13. Acute infusion increased systolic function and reduced systemic vascular resistance in 2 rat models of impaired cardiac function. Similar results were obtained in an anesthetized but not a conscious canine HF model. Chronic oral dosing in a rat myocardial infarction model reduced myocardial collagen content and improved diastolic function to a similar extent as losartan, a RAS antagonist standard-of-care therapy, but lacked additivity with coadministration. Collectively, this work demonstrates the feasibility of developing clinical, viable, potent small-molecule agonists that mimic the endogenous APJ ligand with more favorable drug-like properties and highlights potential limitations for APJ agonism for this indication.

Maternal exercise via exerkine apelin enhances brown adipogenesis and prevents metabolic dysfunction in offspring mice

The obesity rate is rapidly increasing, which has been attributed to lack of exercise and excessive energy intake. Here, we found a previously unidentified explanation, due to lack of maternal exercise. In this study, healthy maternal mice were assigned either to a sedentary lifestyle or to exercise daily, and fetal brown adipose tissue (BAT) development and offspring metabolic health were analyzed. Compared to the sedentary group, maternal exercise enhanced DNA demethylation of Prdm16 promoter and BAT development and prevented obesity of offspring when challenged with a high-energy diet. Apelin, an exerkine, was elevated in both maternal and fetal circulations due to exercise, and maternal administration of apelin mimicked the beneficial effects of exercise on fetal BAT development and offspring metabolic health. Together, maternal exercise enhances thermogenesis and the metabolic health of offspring mice, suggesting that the sedentary lifestyle during pregnancy contributes to the obesity epidemic in modern societies.

The Protective Effects and Mechanisms of Apelin/APJ System on Ischemic Stroke: A Promising Therapeutic Target

The orphan receptor APJ and its endogenous ligand apelin, which are expressed in the brain, are the major components of the apelin/APJ system. Growing evidence shows that the apelin/APJ system plays a vital role in the pathophysiology of cerebral ischemic injury. Targeting the apelin/APJ system may have protective effects on cerebral ischemic injury. In this review, we sum up the latest research progress relating to the actions and therapeutic potential of the apelin/APJ system in ischemic stroke. An in-depth knowledge of the pathophysiological effects of the apelin/APJ system and the underlying mechanisms will help to develop novel therapeutic interventions for ischemic stroke.

Vasohibin1, a new mouse cardiomyocyte IRES trans-acting factor that regulates translation in early hypoxia

Hypoxia, a major inducer of angiogenesis, triggers major changes in gene expression at the transcriptional level. Furthermore, under hypoxia, global protein synthesis is blocked while internal ribosome entry sites (IRES) allow specific mRNAs to be translated. Here, we report the transcriptome and translatome signatures of (lymph)angiogenic genes in hypoxic HL-1 mouse cardiomyocytes: most genes are induced at the translatome level, including all IRES-containing mRNAs. Our data reveal activation of (lymph)angiogenic factor mRNA IRESs in early hypoxia. We identify vasohibin1 (VASH1) as an IRES trans-acting factor (ITAF) that is able to bind RNA and to activate the FGF1 IRES in hypoxia, but which tends to inhibit several IRESs in normoxia. VASH1 depletion has a wide impact on the translatome of (lymph)angiogenesis genes, suggesting that this protein can regulate translation positively or negatively in early hypoxia. Translational control thus appears as a pivotal process triggering new vessel formation in ischemic heart.

Apelin and stem cells: the role played in the cardiovascular system and energy metabolism

Apelin, a member of the adipokine family, is widely distributed in the body and exerts cytoprotective effects on many organs. Apelin isoforms are involved in different physiological processes, including regulation of the cardiovascular system, cardiac contractility, angiogenesis, and energy metabolism. Several investigations have been performed to study the effect of apelin on stem cell therapy. This review aims to summarize the literature representing the effects of apelin on stem cell properties. Furthermore, this review discusses the therapeutic potential of apelin-treated stem cells for cardiovascular diseases and demonstrates the effect of stem cells overexpressing apelin on energy metabolism. Stem cells with their unique characteristics play a crucial role in the maintenance of tissue integrity. These cells participate in tissue regeneration via multiple mechanisms. Although preclinical and clinical studies have demonstrated the therapeutic potential of stem cells in various diseases, their application in regenerative medicine has not been efficient. A number of strategies such as genetic modification or treatment of stem cells with different factors have been used to improve the efficacy of cell therapy and to increase their survival after transplantation. This article reviews the effect of apelin treatment on the efficacy of cell therapy.

The influence of post-infarct heart failure and high fat diet on the expression of apelin APJ and vasopressin V1a and V1b receptors

Vasopressin and apelin are reciprocally regulated hormones which are implicated in the pathophysiology of heart failure and the regulation of metabolism; however, little is known about their interactions under pathological conditions. In this study, we determined how post-infarct heart failure (HF) and a high fat diet (HFD) affect expression of the apelin APJ receptor (APJR) and the V1a (V1aR) and V1b (V1bR) vasopressin receptors in the hypothalamus, the heart, and the retroperitoneal adipose tissue. We performed experiments in male 4-week-old Sprague Dawley rats. The animals received either a normal fat diet (NFD) or a HFD for 8 weeks, then they underwent left coronary artery ligation to induce HF or sham surgery (SO), followed by 4 weeks of NFD or HFD. The HF rats showed higher plasma concentration of NT-proBNP and copeptin. The HF reduced the APJR mRNA expression in the hypothalamus. The APJR and V1aR protein levels in the hypothalamus were regulated both by HF and HFD, while the V1bR protein level in the hypothalamus was mainly influenced by HF. APJR mRNA expression in the heart was significantly higher in rats on HFD, and HFD affected the reduction of the APJR protein level in the right ventricle. The regulation of APJR, V1aR and V1bR expression in the heart and the retroperitoneal adipose tissue were affected by both HF and HFD. Our study demonstrates that HF and HFD cause significant changes in the expression of APJR, V1aR and V1bR, which may have an important influence on the cardiovascular system and metabolism.

The apelinergic system: a perspective on challenges and opportunities in cardiovascular and metabolic disorders

The apelinergic pathway has been generating increasing interest in the past few years for its potential as a therapeutic target in several conditions associated with the cardiovascular and metabolic systems. Indeed, preclinical and, more recently, clinical evidence both point to this G protein-coupled receptor as a target of interest in the treatment of not only cardiovascular disorders such as heart failure, pulmonary arterial hypertension, atherosclerosis, or septic shock, but also of additional conditions such as water retention/hyponatremic disorders, type 2 diabetes, and preeclampsia. While it is a peculiar system with its two classes of endogenous ligand, the apelins and Elabela, its intricacies are a matter of continuing investigation to finely pinpoint its potential and how it enables crosstalk between the vasculature and organ systems of interest. In this perspective article, we first review the current knowledge on the role of the apelinergic pathway in the above systems, as well as the associated therapeutic indications and existing pharmacological tools. We also offer a perspective on the challenges and potential ahead to advance the apelinergic system as a target for therapeutic intervention in several key areas.

Elabela and Apelin actions in healthy and pathological pregnancies

Pregnancy is a dynamic and precisely organized process during which one or more baby develops. Embryonic development relies on the formation of the placenta, allowing nutrient and oxygen exchange between the mother and the fetus. Dysfunction of placental formation lead to pregnancy disorders such as preeclampsia (PE) with serious deleterious consequences for fetal and maternal health. Identifying factors involved in fetoplacental homeostasis could inform better diagnostic and therapeutic strategies for these pathological pregnancies. Here, we summarize actions of elabela, apelin and their common receptor APJ in the fetoplacental unit. Studies indicate that elabela is crucial for embryo cardiovascular system formation and early placental development, while apelin acts in mid/late gestation to modulate fetal angiogenesis and energy homeostasis. Most of these findings, drawn from animal models, indicate a key role of elabela/apelin-APJ system in the fetoplacental unit. This review also provides an overview of clinical studies investigating elabela/apelin-APJ system in pathological complicated pregnancies such as PE and gestational diabetes mellitus (GDM). While elabela-deficient mice display all the features of PE, current clinical studies show no difference in circulating elabela levels between PE and control patients which does not support a role in PE development. Conversely, apelin levels are increased during PE, but the use of apelin as an early PE marker remains to be fully investigated.

Cardioprotective apelin effects and the cardiac-renal axis: review of existing science and potential therapeutic applications of synthetic and native regulated apelin

First described in 1998, apelin is one of the endogenous ligands of the apelinergic receptor. Since its discovery, its possible role in human physiology and disease has been intensively studied. Apelin is a native cardioprotective agent that the body synthesizes to create atheroprotective, antihypertensive, and regenerative effects in the body. By antagonizing the RAA system, apelin could play an important role in heart failure and hypertension. It is also involved in myocardial protection against ischemia/reperfusion injury, post-ischemic remodeling, and myocardial fibrosis. A small number of studies even suggest that serum apelin levels may be involved the development of life-threatening arrhythmias. All this information generated excitement about potential therapeutic effects in patients with heart failure and myocardial infarction. The therapeutic index of apelin is unknown but is anticipated to be favorable based on the small number of studies. In this review, we summarize the mechanisms by which apelin exerts its cardioprotective effects and its connection with the cardiorenal axis. Also, we report the potential therapeutic applications of synthetic and native regulated apelin. If larger studies can be performed, it is possible that apelin-mediated drug treatment may play a major role for a large number of patients worldwide in the future.

The apelin/APJ system as a therapeutic target in metabolic diseases

INTRODUCTION

Apelin, a bioactive peptide, is the endogenous ligand of APJ, a G protein-coupled receptor which is widely expressed in peripheral tissues and in the central nervous system. The apelin/APJ system is involved in the regulation of various physiological functions and is a therapeutic target in different pathologies; the development of APJ agonists and antagonists has thus increased. Area covered: This review focuses on the in vitro and in vivo metabolic effects of apelin in physiological conditions and in the context of metabolic diseases. Expert opinion: In experimental models, novel APJ agonists are efficient in vivo, to treat metabolic diseases and associated complications. However, more clinical trials are necessary to determine whether molecules that target APJ could become an alternative therapeutic strategy in the treatment of metabolic diseases and associated complications.

Elevated Levels of Apelin-36 in Heart Failure Due to Chronic Systemic Hypoxia

Apelin is a novel adipokine identified as an endogenous ligand of the specific orphan receptor APJ. Among the various isoforms of apelin, an increase in the apelin-36 plasma level has been associated with oxidative stress, and this isoform has various biological effects, such as positive inotropic, vasodilatory, and antiatherosclerotic effects. Therefore, apelin-36 may be used as a biomarker of heart failure (HF). Advances in the understanding of the molecular mechanisms underlying HF cannot be achieved without the use of animal models. However, it is unclear whether chronic systemic hypoxia can cause HF in rats. The present study aimed to determine whether chronic systemic hypoxia can cause HF in rats and whether apelin-36 can be used as a biomarker of HF. The study included Sprague-Dawley rats. The rats were randomly divided into seven groups ( n  = 4). One of the groups was a control group, and the six other groups were exposed to hypoxia (8% O2) for different durations (6 hours, 1 day, 3 days, 5 days, 7 days, and 14 days). The exposure groups showed ventricular hypertrophy accompanied by myocardial structural damage, which indicated ventricular remodeling. In addition, the exposure groups showed elevated apelin-36 plasma levels and signs of oxidative stress. Moreover, gel electrophoresis of heart tissue showed five bands that corresponded to apelin isotypes, including apelin-36. In an experimental rat HF model with chronic systemic hypoxia, apelin-36 was elevated along with oxidative stress. Apelin-36 along with oxidative stress may serve as a biomarker of HF in this model.

EFFECTS OF EXERCISE COMBINED WITH APELIN-13 ON CARDIAC FUNCTION IN THE ISOLATED RAT HEART

ABSTRACT Exercise and apelin have been shown to increase cardiac function and elicit tolerance to ischemia/reperfusion (IR) injuries. This study aimed at determining whether the combination of exercise training and apelin pretreatment could integrate the protective effects of each of them in the heart against IR injury. Male rats were divided into four experimental groups: 1: Rats with ischemia/reperfusion (IR), 2: subjected to exercise training for 8 weeks (EX+IR), 3: apelin-13 (10 nmol/kg/day) for 7 days (Apel+IR) in the last week of training, and 4: exercise training plus apelin-13 (EX+Apel+IR). Isolated hearts were perfused using the Langendorff method and subjected to 30 min of regional ischemia followed by 60 min of reperfusion. Treadmill exercise training was conducted for 8 weeks. Hemodynamic parameters were recorded throughout the experiment. Ischemia-induced arrhythmias, myocardial infarct size (IS), creatine kinase-MB (CK-MB) isoenzyme and plasma lactate dehydrogenase (LDH) activity was measured in all animals. Administration of apelin-13 plus exercise increased left ventricular developed pressure (LVDP) at the end of ischemia and reperfusion compared with other groups. After 30 min of ischemia, dP/dtmax was higher in EX+Apel+IR than in Apel+IR and EX+IR groups. During 30 min ischemia, exercise training, apelin-13 and combined treatment produced a significant reduction in the numbers of premature ventricular complexes. A combination of exercise and apelin-13 also reduced infarct size, CK-MB, LDH and severity of arrhythmia. These results suggest that combined therapies with apelin-13 and exercise training may integrate the beneficial effects of each of them alone on cardiac contractility, arrhythmia and limiting of infarct size. Level of evidence I; Therapeutic Studies - Investigating the Results of Treatment.

The mechanism of all-trans retinoic acid in the regulation of apelin expression in vascular endothelial cells

The apelin gene can promote vascular endothelial cell (VEC) proliferation, migration, and angiogenesis. However, the molecular mechanism for regulation of the apelin gene is still unknown. Real-time PCR and Western blotting analysis were employed to detect the effect of all-trans retinoic acid (ATRA) in up-regulating apelin expression in human umbilical vein endothelial cells (HUVECs). Furthermore, the in vivo study also indicated that ATRA could increase apelin expression in balloon-injured arteries of rats, which is consistent with the results from the cultured HUVECs. To ensure whether retinoic acid receptor (RAR) α (RARα) could be induced by ATRA in regulating apelin, the expression of RARα was tested with a siRNA method to knock down RARα or adenovirus vector infection to overexpress RARα. The results showed that ATRA could up-regulate apelin expression time- and dose- dependently in HUVECs. ATRA could induce a RARα increase; however, the expression of RARβ and RARγ were unchanged. The blocking of RARα signaling reduced the response of apelin to ATRA when HUVECs were treated with RARα antagonists (Ro 41-5253) or the use of siRNA against RARα (si-RARα) knockdown RARα expression before using ATRA. In addition, induction of RARα overexpression by infection with pAd-GFP-RARα further increased the induction of apelin by ATRA. These results suggested that ATRA up-regulated apelin expression by promoting RARα signaling.

Predictive value of apelin-12 in patients with ST-elevation myocardial infarction with different renal function: a prospective observational study

OBJECTIVES

To investigate factors predicting the onset of major adverse cardiovascular events (MACEs) after primary percutaneous coronary intervention (pPCI) for patients with ST-segment elevation myocardial infarction (STEMI) .

BACKGROUND

Apelin-12 plays an essential role in cardiovascular homoeostasis. However, current knowledge of its predictive prognostic value is limited.

METHODS

464 patients with STEMI (63.0±11.9 years, 355 men) who underwent successful pPCI were enrolled and followed for 2.5 years. Multivariate cox regression analysis and receiver operating characteristic (ROC) curve analysis were performed to determine the factors predicting MACEs.

RESULTS

118 patients (25.4%) experienced MACEs in the follow-up period. Multivariate cox regression analysis found low apelin-12 (HR=0.132, 95% CI 0.060 to 0.292, P<0.001), low left ventricular ejection fraction (HR=0.965, 95% CI 0.941 to 0.991, P=0.007), low estimated glomerular filtration rate (eGFR) (HR=0.985, 95% CI 0.977 to 0.993, P<0.001), Killip's classification>I (HR=0.610, 95% CI 0.408 to 0.912, P=0.016) and pathological Q-wave (HR=1.536, 95% CI 1.058 to 2.230, P=0.024) were independent predictors of MACEs in the 2.5 year follow-up period. Low apelin-12 also predicted poorer in-hospital prognosis and MACEs in the 2.5 years follow-up period compared with Δapelin-12 (P=0.0115) and eGFR (P=0.0071) among patients with eGFR>90 mL/min×1.73 m². Further analysis showed Δapelin-12 <20% was associated with MACEs in patients whose apelin-12 was below 0.76 ng/mL (P=0.0075) on admission.

CONCLUSIONS

Patients with STEMI receiving pPCI with lower apelin-12 are more likely to suffer MACEs in hospital and 2.5 years postprocedure, particularly in those with normal eGFR levels.

Apelin-APJ system is responsible for stress-induced increase in atrial natriuretic peptide expression in rat heart

BACKGROUND

The cardiovascular system is a primary target of stress and stress is the most important etiologic factor in cardiovascular diseases. Stressors increase expressions of atrial natriuretic peptide (ANP) and apelin in cardiac tissue.

AIM

The aim of the present study was to investigate whether stress-induced apelin has an effect on the expression of ANP in the right atrium of rat heart.

METHODS

The rats were divided into the control, stress and F13A+stress groups. In the stress and F13A+stress groups, the rats were subjected to water immersion and restraint stress (WIRS) for 6h. In the F13A+stress group, apelin receptor antagonist F13A, was injected intravenously immediately before application of WIRS. The plasma samples were obtained for the measurement of corticosterone and atrial natriuretic peptide. The atrial samples were used for immunohistochemistry and western blot analysis.

RESULTS

F13A administration prevented the rise of plasma corticosterone and ANP levels induced by WIRS. While WIRS application increased the expressions of apelin, HIF-1α and ANP in atrial tissue, while F13A prevented the stress-induced increase in the expression of HIF-1α and ANP.

CONCLUSION

Stress-induced apelin induces ANP expression in atrial tissue and may play a role in cardiovascular homeostasis by increasing ANP expression under WIRS conditions.

Thyroid hormone regulates fibronectin expression through the activation of the hypoxia inducible factor 1

Thyroid hormones regulate gene expression via both canonical and non-canonical signaling. Hyperthyroidism is associated with elevated plasma levels of fibronectin (FN): in this study we elucidate the molecular mechanism through which triiodothyronine (T3) regulates FN and demonstrate that T3 induces FN expression via a non-canonical pathway by activating hypoxia-inducible factor-1 (HIF-1). We found that T3 treatment increased cellular and secreted FN in human hepatoma cells (HepG2) and human dermal fibroblasts (HF) via the PI3K/Akt/HIF-1 pathway. The inhibition of either Akt phosphorylation with wortmannin or HIF-1 with YC1 in both cell types prevented HIF-1α synthesis and FN positive regulation upon T3 treatment. We showed that HIF-1α overexpression per se was sufficient to up-regulate FN in both cell lines as demonstrated by the transient transfection of both the constitutively active and wild-type forms of HIF-1α. Our data demonstrate the involvement of the PI3K/Akt/HIF-1 pathway in mediating T3 induced FN up-regulation.

Load-dependent effects of apelin on murine cardiomyocytes

The apelin peptide is described as one of the most potent inotropic agents, produced endogenously in a wide range of cells, including cardiomyocytes. Despite positive effects on cardiac contractility in multicellular preparations, as well as indications of cardio-protective actions in several diseases, its effects and mechanisms of action at the cellular level are incompletely understood.

Here, we report apelin effects on dynamic mechanical characteristics of single ventricular cardiomyocytes, isolated from mouse models (control, apelin-deficient [Apelin-KO], apelin-receptor KO mouse [APJ-KO]), and rat. Dynamic changes in maximal velocity of cell shortening and relaxation were monitored. In addition, more traditional indicators of inotropic effects, such as maximum shortening (in mechanically unloaded cells) or peak force development (in auxotonic contracting cells, preloaded using the carbon fibre technique) were studied.

The key finding is that, using Apelin-KO cardiomyocytes exposed to different preloads with the 2-carbon fibre technique, we observe a lowering of the slope of the end-diastolic stress-length relation in response to 10 nM apelin, an effect that is preload-dependent. This suggests a positive lusitropic effect of apelin, which could explain earlier counter-intuitive findings on an apelin-induced increase in contractility occurring without matching rise in oxygen consumption.

Influence of apelin-12 on troponin levels and the rate of MACE in STEMI patients

BACKGROUND

During acute myocardial infarction, phosphorylated TnI levels, Ca²⁺ sensitivity and ATPase activity are decreased in the myocardium, and the subsequent elevation in Ca²⁺ levels activates protease I (caplain I), leading to the proteolytic degradation of troponins. Concurrently, the levels of apelin and APJ expression are increased by limiting myocardial injury.

METHODS

In this prospective observational study, 100 consecutive patients with ST-elevation acute myocardial infarction were included. Patients meeting the following criteria were included in our study: (1) continuous chest pain lasting for >30 min, (2) observation of ST-segment elevation of more than 2 mm in two adjacent leads by electrocardiography (ECG), (3) increased cardiac troponin I levels, and (4) patients who underwent reperfusion therapy. We evaluated the levels of apelin-12 and troponin I on the first and seventh days after reperfusion therapy in all patients.

RESULTS

Apelin-12 was inversely correlated with troponin I levels (Spearman's correlation = -0.40) with a p value <0.001. There was variability in the apelin values on the seventh day (Kruskal-Wallis test) based on major adverse cardiac events (MACE) (p = 0.012). Using ROC curve analyses, a cut-off value of >2.2 for the association of apelin with MACE was determined, and the AUC was 0.71 (95% CI, 0.58-0.84). Survival analysis using the Kaplan-Meier method showed a lower rate of MACE among patients with apelin levels >2.2 (p = 0.002), and the ROC curve analysis showed a statistically significant difference in the area under the curve (p = 0.004).

CONCLUSION

The influence of apelin levels on troponin levels in the acute phase of STEMI is inversely correlated, whereas in the non-acute phase, low apelin values were associated with a high rate of MACE.

Apelin modulates pathological remodeling of lymphatic endothelium after myocardial infarction

Lymphatic endothelium serves as a barrier to control fluid balance and immune cell trafficking to maintain tissue homeostasis. Long-term alteration of lymphatic vasculature promotes edema and fibrosis, which is an aggravating factor in the onset of cardiovascular diseases such as myocardial infarction. Apelin is a bioactive peptide that plays a central role in angiogenesis and cardiac contractility. Despite an established role of apelin in lymphangiogenesis, little is known about its function in the cardiac lymphatic endothelium. Here, we show that apelin and its receptor APJ were exclusively expressed on newly formed lymphatic vasculature in a pathological model of myocardial infarction. Using an apelin-knockout mouse model, we identified morphological and functional defects in lymphatic vasculature associated with a proinflammatory status. Surprisingly, apelin deficiency increased the expression of lymphangiogenic growth factors VEGF-C and VEGF-D and exacerbated lymphangiogenesis after myocardial infarction. Conversely, the overexpression of apelin in ischemic heart was sufficient to restore a functional lymphatic vasculature and to reduce matrix remodeling and inflammation. In vitro, the expression of apelin prevented the alteration of cellular junctions in lymphatic endothelial cells induced by hypoxia. In addition, we demonstrated that apelin controls the secretion of the lipid mediator sphingosine-1-phosphate in lymphatic endothelial cells by regulating the level of expression of sphingosine kinase 2 and the transporter SPNS2. Taken together, our results show that apelin plays a key role in lymphatic vessel maturation and stability in pathological settings. Thus, apelin may represent a novel candidate to prevent pathological lymphatic remodeling in diseases.

The Role of the Apelin/APJ System in the Regulation of Liver Disease

Apelin is an endogenous peptide that is a ligand for the APJ receptor (angiotensin II receptor like-1, AT-1). The apelin/APJ system is distributed in diverse periphery organ tissues. It has been shown that the apelin/APJ system plays various roles in physiology and pathophysiology of many organs. It regulates cardiovascular development or cardiac disease, glycometabolism and fat metabolism as well as metabolic disease. The apelin/APJ system participates in various cell activities such as proliferation, migration, apoptosis or inflammation. However, apelin/APJ function in the liver is still under investigation. In the liver, the apelin-APJ system could play an inhibitory role in liver regeneration and promote Fas-induced apoptosis. It may participate in the formation of hepatic fibrosis or cirrhosis, and even cancer. In this review, we summarize the role of the apelin/APJ system in liver disease.

Temporal Expression of Apelin/Apelin Receptor in Ischemic Stroke and its Therapeutic Potential

Stroke is one of the leading causes of death and disability worldwide, and ischemic stroke accounts for approximately 87% of cases. Improving post-stroke recovery is a major challenge in stroke treatment. Accumulated evidence indicates that the apelinergic system, consisting of apelin and apelin receptor (APLNR), is temporally dysregulated in ischemic stroke. Moreover, the apelinergic system plays a pivotal role in post-stroke recovery by inhibiting neuronal apoptosis and facilitating angiogenesis through various molecular pathways. In this review article, we summarize the temporal expression of apelin and APLNR in ischemic stroke and the mechanisms of their dysregulation. In addition, the protective role of the apelinergic system in ischemic stroke and the underlying mechanisms of its protective effects are discussed. Furthermore, critical issues in activating the apelinergic system as a potential therapy will also be discussed. The aim of this review article is to shed light on exploiting the activation of the apelinergic system to treat ischemic stroke.

Potassium Channel Interacting Protein 2 (KChIP2) is not a transcriptional regulator of cardiac electrical remodeling

The heart-failure relevant Potassium Channel Interacting Protein 2 (KChIP2) augments Ca_V1.2 and K_V4.3. KChIP3 represses Ca_V1.2 transcription in cardiomyocytes via interaction with regulatory DNA elements. Hence, we tested nuclear presence of KChIP2 and if KChIP2 translocates into the nucleus in a Ca²⁺ dependent manner. Cardiac biopsies from human heart-failure patients and healthy donor controls showed that nuclear KChIP2 abundance was significantly increased in heart failure; however, this was secondary to a large variation of total KChIP2 content. Administration of ouabain did not increase KChIP2 content in nuclear protein fractions in anesthetized mice. KChIP2 was expressed in cell lines, and Ca²⁺ ionophores were applied in a concentration- and time-dependent manner. The cell lines had KChIP2-immunoreactive protein in the nucleus in the absence of treatments to modulate intracellular Ca²⁺ concentration. Neither increasing nor decreasing intracellular Ca²⁺ concentrations caused translocation of KChIP2. Microarray analysis did not identify relief of transcriptional repression in murine KChIP2^−/− heart samples. We conclude that although there is a baseline presence of KChIP2 in the nucleus both in vivo and in vitro, KChIP2 does not directly regulate transcriptional activity. Moreover, the nuclear transport of KChIP2 is not dependent on Ca²⁺. Thus, KChIP2 does not function as a conventional transcription factor in the heart.

The Endothelial Prolyl-4-Hydroxylase Domain 2/Hypoxia-Inducible Factor 2 Axis Regulates Pulmonary Artery Pressure in Mice

Hypoxia-inducible factors 1 and 2 (HIF-1 and -2) control oxygen supply to tissues by regulating erythropoiesis, angiogenesis and vascular homeostasis. HIFs are regulated in response to oxygen availability by prolyl-4-hydroxylase domain (PHD) proteins, with PHD2 being the main oxygen sensor that controls HIF activity under normoxia. In this study, we used a genetic approach to investigate the endothelial PHD2/HIF axis in the regulation of vascular function. We found that inactivation of Phd2 in endothelial cells specifically resulted in severe pulmonary hypertension (∼118% increase in right ventricular systolic pressure) but not polycythemia and was associated with abnormal muscularization of peripheral pulmonary arteries and right ventricular hypertrophy. Concurrent inactivation of either Hif1a or Hif2a in endothelial cell-specific Phd2 mutants demonstrated that the development of pulmonary hypertension was dependent on HIF-2α but not HIF-1α. Furthermore, endothelial HIF-2α was required for the development of increased pulmonary artery pressures in a model of pulmonary hypertension induced by chronic hypoxia. We propose that these HIF-2-dependent effects are partially due to increased expression of vasoconstrictor molecule endothelin 1 and a concomitant decrease in vasodilatory apelin receptor signaling. Taken together, our data identify endothelial HIF-2 as a key transcription factor in the pathogenesis of pulmonary hypertension.

Genetic lineage tracing discloses arteriogenesis as the main mechanism for collateral growth in the mouse heart

Aims

Capillary and arterial endothelial cells share many common molecular markers in both the neonatal and adult hearts. Herein, we aim to establish a genetic tool that distinguishes these two types of vessels in order to determine the cellular mechanism underlying collateral artery formation.

Methods and results

Using Apln-GFP and Apln-LacZ reporter mice, we demonstrate that APLN expression is enriched in coronary vascular endothelial cells. However, APLN expression is reduced in coronary arterial endothelial cells. Genetic lineage tracing, using an Apln-CreER mouse line, robustly labelled capillary endothelial cells, but not arterial endothelial cells. We leveraged this differential activity of Apln-CreER to study collateral artery formation following myocardial infarction (MI). In a neonatal heart MI model, we found that Apln-CreER-labelled capillary endothelial cells do not contribute to the large collateral arteries. Instead, these large collateral arteries mainly arise from pre-existing, infrequently labelled coronary arteries, indicative of arteriogenesis. Furthermore, in an adult heart MI model, Apln-CreER activity also distinguishes large and small diameter arteries from capillaries. Lineage tracing in this setting demonstrated that most large and small coronary arteries in the infarcted myocardium and border region are derived not from capillaries, but from pre-existing arteries.

Conclusion

Apln-CreER-mediated lineage tracing distinguishes capillaries from large arteries, in both the neonatal and adult hearts. Through genetic fate mapping, we demonstrate that pre-existing arteries, but not capillaries, extensively contribute to collateral artery formation following myocardial injury. These results suggest that arteriogenesis is the major mechanism underlying collateral vessel formation.

Hypoxia induces the proliferation of endothelial progenitor cells via upregulation of Apelin/APLNR/MAPK signaling

Endothelial progenitor cells (EPCs) can form new vessels through differentiation into endothelial cells (ECs), thus being important in the prevention of hypoxia/ischemia. Apelin can activate different signaling pathways through its receptor, APLNR, which regulate diverse biological functions, including cardiovascular function. However, the molecular mechanism by which Apelin mediates hypoxia-induced EPCs proliferation remain to be fully elucidated. The present study aimed to determine the role of Apelin/APLNR signaling in hypoxia-induced proliferation of EPCs. MTT assay was used to determine cell proliferation. Reverse transcription-quantitative polymerase chain reaction and western blotting analysis were conducted to examine mRNA and protein expression. It was revealed that hypoxia promoted the proliferation of the EPCs. Further investigation demonstrated that hypoxia promoted the expression levels of hypoxia-inducible factor (HIF)-1α, Apelin and APLNR in the EPCs. In addition, upregulation of Apelin or APLNR promoted the hypoxia-induced proliferation of the EPCs, while knockdown of Apelin or APLNR by small interfering RNA suppressed the hypoxia-induced proliferation of the EPCs, suggesting that the Apelin/APLNR axis is involved in hypoxia-induced proliferation of EPCs. Furthermore, pretreatment of the EPCs with SB-239063 or PD98059, two inhibitors of mitogen-activated protein kinase (MAPK), eliminated the Apelin upregulation-induced EPC proliferation, suggesting that MAPK signaling is a downstream effecter of Apelin/APLNR in EPCs. Therefore, the findings of the present study indicated that the production of HIF-1α, induced by hypoxia, activated the Apelin/APLNR and the downstream MAPK signaling pathways, leading to upregulated proliferation of the EPCs. These findings suggested that Apelin/APLNR signaling may be used as a potential therapeutic target for hypoxic/ischemic injury.