Apelin/APJ signaling is a critical regulator of statin effects in vascular endothelial cells--brief report

Dysregulation of the Long Noncoding RNA X-Inactive-Specific Transcript Expression in Male Patients with Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a sex-biased disease with female sex as a significant risk factor. Increased expression of the long noncoding RNA X-inactive-specific transcript (Xist), as induced by an intersectin-1s protein fragment with proliferative potential (EHITSN), may explain the sexual dimorphism of female pulmonary artery endothelial cells (ECs) and at least in part, the imbalance sex/ratio of PAH. Xist is essential for X-chromosome inactivation and dosage compensation of X-linked genes. Herein, increased Xist expression was detected in a subset of ECs and lung tissue samples of male patients with PAH. The role of different Xist expression levels in ECs of male patients with PAH (ECPAH) was studied in several lines of male ECPAH in conjunction with molecular, biochemical, morphologic, and functional approaches. Male ECPAH showed on average 10.3-fold increase in high Xist versus low Xist, a significant association between Xist levels and their proliferative potential, and a heterogeneous methylation of the Xist/XIST antisense RNA (Tsix) locus. Interestingly, Xist up-regulation in male ECPAH decreased the expression of Krueppel-like factor 2 (Klf2), via EHITSN interaction with enhancer of zeste polycomb repressive complex 2 subunit (EZH2), the catalytic subunit of the polycomb repressive complex 2. Moreover, the studies demonstrate that EHITSN-triggered p38/ETS domain-containing protein Elk1/AP-1 transcription factor subunit (c-Fos) signaling is a pathologic mechanism central to ECPAH proliferation and the dynamic crosstalk with cell cycle regulatory proteins cyclin A1/cyclin D2 and Xist-EZH2-Klf2 interaction participate directly and differentially in establishing the proliferative profile of male ECPAH.

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.

The Impact of Sex Chromosomes in the Sexual Dimorphism of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a sex-biased disease with a poorly understood female prevalence. Emerging research suggests that nonhormonal factors, such as the XX or XY sex chromosome complement and sex bias in gene expression, may also lead to sex-based differences in PAH incidence, penetrance, and progression. Typically, one of females' two X chromosomes is epigenetically silenced to offer a gender-balanced gene expression. Recent data demonstrate that the long noncoding RNA X-inactive specific transcript, essential for X chromosome inactivation and dosage compensation of X-linked gene expression, shows elevated levels in female PAH lung specimens compared with controls. This molecular event leads to incomplete inactivation of the females' second X chromosome, abnormal expression of X-linked gene(s) involved in PAH pathophysiology, and a pulmonary artery endothelial cell (PAEC) proliferative phenotype. Moreover, the pathogenic proliferative p38 mitogen-activated protein kinase/ETS transcription factor ELK1 (Elk1)/cFos signaling is mechanistically linked to the sexually dimorphic proliferative response of PAECs in PAH. Apprehending the complicated relationship between long noncoding RNA X-inactive specific transcript and X-linked genes and how this relationship integrates into a sexually dimorphic proliferation of PAECs and PAH sex paradox remain challenging. We highlight herein new findings related to how the sex chromosome complement and sex-differentiated epigenetic mechanisms to control gene expression are decisive players in the sexual dimorphism of PAH. Pharmacologic interventions in the light of the newly elucidated mechanisms are discussed.

Apelin pathway in cardiovascular, kidney, and metabolic diseases: Therapeutic role of apelin analogs and apelin receptor agonists

The apelin/apelin receptor (ApelinR) signal transduction pathway exerts essential biological roles, particularly in the cardiovascular system. Disturbances in the apelin/ApelinR axis are linked to vascular, heart, kidney, and metabolic disorders. Therefore, the apelinergic system has surfaced as a critical therapeutic strategy for cardiovascular diseases (including pulmonary arterial hypertension), kidney disease, insulin resistance, hyponatremia, preeclampsia, and erectile dysfunction. However, apelin peptides are susceptible to rapid degradation through endogenous peptidases, limiting their use as therapeutic tools and translational potential. These proteases include angiotensin converting enzyme 2, neutral endopeptidase, and kallikrein thereby linking the apelin pathway with other peptide systems. In this context, apelin analogs with enhanced proteolytic stability and synthetic ApelinR agonists emerged as promising pharmacological alternatives. In this review, we focus on discussing the putative roles of the apelin pathway in various physiological systems from function to dysfunction, and emphasizing the therapeutic potential of newly generated metabolically stable apelin analogs and non-peptide ApelinR agonists.

Apelin/APJ system: an emerging therapeutic target for respiratory diseases

Apelin is an endogenous ligand of G protein-coupled receptor APJ. It is extensively expressed in many tissues such as heart, liver, and kidney, especially in lung tissue. A growing body of evidence suggests that apelin/APJ system is closely related to the development of respiratory diseases. Therefore, in this review, we focus on the role of apelin/APJ system in respiratory diseases, including pulmonary arterial hypertension (PAH), pulmonary embolism (PE), acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), obstructive sleep apnoea syndrome (OSAS), non-small cell lung cancer (NSCLC), pulmonary edema, asthma, and chronic obstructive pulmonary diseases. In detail, apelin/APJ system attenuates PAH by activating AMPK-KLF2-eNOS-NO signaling and miR424/503-FGF axis. Also, apelin protects against ALI/ARDS by reducing mitochondrial ROS-triggered oxidative damage, mitochondria apoptosis, and inflammatory responses induced by the activation of NF-κB and NLRP3 inflammasome. Apelin/APJ system also prevents the occurrence of pulmonary edema via activating AKT-NOS3-NO pathway. Moreover, apelin/APJ system accelerates NSCLC cells' proliferation and migration via triggering ERK1/2-cyclin D1 and PAK1-cofilin signaling, respectively. Additionally, apelin/APJ system may act as a predictor in the development of OSAS and PE. Considering the pleiotropic actions of apelin/APJ system, targeting apelin/APJ system may be a potent therapeutic avenue for respiratory diseases.

Sex differences in the proliferation of pulmonary artery endothelial cells: implications for plexiform arteriopathy

The sex-biased disease pulmonary arterial hypertension (PAH) is characterized by the proliferation and overgrowth of dysfunctional pulmonary artery endothelial cells (PAECs). During inflammation associated with PAH, granzyme B cleaves intersectin-1 to produce N-terminal (EH_ITSN) and C-terminal (SH3A-E_ITSN) protein fragments. In a murine model of PAH, EH_ITSN triggers plexiform arteriopathy via p38-ELK1-c-Fos signaling. The SH3A-E_ITSN fragment also influences signaling, having dominant-negative effects on ERK1 and ERK2 (also known as MAPK3 and MAPK1, respectively). Using PAECs engineered to express tagged versions of EH_ITSN and SH3A-E_ITSN, we demonstrate that the two ITSN fragments increase both p38-ELK1 activation and the ratio of p38 to ERK1 and ERK2 activity, leading to PAEC proliferation, with female cells being more responsive than male cells. Furthermore, expression of EH_ITSN substantially upregulates the expression and activity of the long non-coding RNA Xist in female PAECs, which in turn upregulates the X-linked gene ELK1 and represses expression of krüppel-like factor 2 (KLF2). These events are recapitulated by the PAECs of female idiopathic PAH patients, and may account for their proliferative phenotype. Thus, upregulation of Xist could be an important factor in explaining sexual dimorphism in the proliferative response of PAECs and the imbalanced sex ratio of PAH.

Repurposing antimalarial aminoquinolines and related compounds for treatment of retinal neovascularization

Neovascularization is the pathological driver of blinding eye diseases such as retinopathy of prematurity, proliferative diabetic retinopathy, and wet age-related macular degeneration. The loss of vision resulting from these diseases significantly impacts the productivity and quality of life of patients, and represents a substantial burden on the health care system. Current standard of care includes biologics that target vascular endothelial growth factor (VEGF), a key mediator of neovascularization. While anti-VGEF therapies have been successful, up to 30% of patients are non-responsive. Therefore, there is a need for new therapeutic targets, and small molecule inhibitors of angiogenesis to complement existing treatments. Apelin and its receptor have recently been shown to play a key role in both developmental and pathological angiogenesis in the eye. Through a cell-based high-throughput screen, we identified 4-aminoquinoline antimalarial drugs as potent selective antagonists of APJ. The prototypical 4-aminoquinoline, amodiaquine was found to be a selective, non-competitive APJ antagonist that inhibited apelin signaling in a concentration-dependent manner. Additionally, amodiaquine suppressed both apelin-and VGEF-induced endothelial tube formation. Intravitreal amodaiquine significantly reduced choroidal neovascularization (CNV) lesion volume in the laser-induced CNV mouse model, and showed no signs of ocular toxicity at the highest doses tested. This work firmly establishes APJ as a novel, chemically tractable therapeutic target for the treatment of ocular neovascularization, and that amodiaquine is a potential candidate for repurposing and further toxicological, and pharmacokinetic evaluation in the clinic.

Downregulation of the Apelinergic Axis Accelerates Aging, whereas Its Systemic Restoration Improves the Mammalian Healthspan

Aging drives the occurrence of numerous diseases, including cardiovascular disease (CVD). Recent studies indicate that blood from young mice reduces age-associated pathologies. However, the "anti-aging" factors in juvenile circulation remain poorly identified. Here, we characterize the role of the apelinergic axis in mammalian aging and identify apelin as an anti-aging factor. The expression of apelin (apln) and its receptor (aplnr) exhibits an age-dependent decline in multiple organs. Reduced apln signaling perturbs organismal homeostasis; mice harboring genetic deficiency of aplnr or apln exhibit enhanced cardiovascular, renal, and reproductive aging. Genetic or pharmacological abrogation of apln signaling also induces cellular senescence mediated, in part, by the activation of senescence-promoting transcription factors. Conversely, restoration of apln in 15-month-old wild-type mice reduces cardiac hypertrophy and exercise-induced hypertensive response. Additionally, apln-restored mice exhibit enhanced vigor and rejuvenated behavioral and circadian phenotypes. Hence, a declining apelinergic axis promotes aging, whereas its restoration extends the murine healthspan.

Stabilization of symptomatic carotid atherosclerotic plaques by statins: a clinico-pathological analysis

Human and animal studies have revealed a stabilization of atherosclerotic plaques by statins. However, the stabilization of human carotid plaques has not been thoroughly described pathologically. This analysis explored the relationship between statin therapy and plaque stability in carotid endarterectomy (CEA) specimens. We analyzed specimens harvested between May 2015 and February 2017, from 79 consecutive patients presenting with > 70% carotid artery stenoses, of whom 66 were untreated (group 1) and 13 treated (group 2) with a statin. Immunohistochemistry was performed, using an endothelial specific antibody to CD31, CD34 and platelet derived growth factor receptor-β. The prevalence of plaque ruptures (P = 0.009), lumen thrombi (P = 0.009), inflammatory cells (P = 0.008), intraplaque hemorrhages (P = 0.030) and intraplaque microvessels (P < 0.001) was significantly lower in group 2 than in group 1. Among 66 patients presenting with strokes and infarct sizes > 1.0 cm³ on magnetic resonance imaging, the mean infarct volume was significantly smaller (P = 0.031) in group 2 (4.2 ± 2.5 cm³) than in group 1 (8.2 ± 7.1 cm³). The difference in mean concentration of low-density lipoprotein cholesterol between group 1 (121 ± 32 mg/dl) and group 2 (105 ± 37 mg/dl) was non-significant (P = 0.118). This analysis of plaques harvested from patients undergoing CEA suggests that statin therapy mitigates the plaque instability, which, in patients presenting with strokes, might decrease infarct volume.

Endothelial APLNR regulates tissue fatty acid uptake and is essential for apelin's glucose-lowering effects

Treatment of type 2 diabetes mellitus continues to pose an important clinical challenge, with most existing therapies lacking demonstrable ability to improve cardiovascular outcomes. The atheroprotective peptide apelin (APLN) enhances glucose utilization and improves insulin sensitivity. However, the mechanism of these effects remains poorly defined. We demonstrate that the expression of APLNR (APJ/AGTRL1), the only known receptor for apelin, is predominantly restricted to the endothelial cells (ECs) of multiple adult metabolic organs, including skeletal muscle and adipose tissue. Conditional endothelial-specific deletion of Aplnr (Aplnr^ECKO ) resulted in markedly impaired glucose utilization and abrogation of apelin-induced glucose lowering. Furthermore, we identified inactivation of Forkhead box protein O1 (FOXO1) and inhibition of endothelial expression of fatty acid (FA) binding protein 4 (FABP4) as key downstream signaling targets of apelin/APLNR signaling. Both the Apln^-/- and Aplnr^ECKO mice demonstrated increased endothelial FABP4 expression and excess tissue FA accumulation, whereas concurrent endothelial Foxo1 deletion or pharmacologic FABP4 inhibition rescued the excess FA accumulation phenotype of the Apln^-/- mice. The impaired glucose utilization in the Aplnr^ECKO mice was associated with excess FA accumulation in the skeletal muscle. Treatment of these mice with an FABP4 inhibitor abrogated these metabolic phenotypes. These findings provide mechanistic insights that could greatly expand the therapeutic repertoire for type 2 diabetes and related metabolic disorders.

The Vitamin E Analog Gamma-Tocotrienol (GT3) and Statins Synergistically Up-Regulate Endothelial Thrombomodulin (TM)

Statins; a class of routinely prescribed cholesterol-lowering drugs; inhibit 3-hydroxy-3-methylglutaryl-coenzymeA reductase (HMGCR) and strongly induce endothelial thrombomodulin (TM); which is known to have anti-inflammatory; anti-coagulation; anti-oxidant; and radioprotective properties. However; high-dose toxicity limits the clinical use of statins. The vitamin E family member gamma-tocotrienol (GT3) also suppresses HMGCR activity and induces TM expression without causing significant adverse side effects; even at high concentrations. To investigate the synergistic effect of statins and GT3 on TM; a low dose of atorvastatin and GT3 was used to treat human primary endothelial cells. Protein-level TM expression was measured by flow cytometry. TM functional activity was determined by activated protein C (APC) generation assay. Expression of Kruppel-like factor 2 (KLF2), one of the key transcription factors of TM, was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR). TM expression increased in a dose-dependent manner after both atorvastatin and GT3 treatment. A combined treatment of a low-dose of atorvastatin and GT3 synergistically up-regulated TM expression and functional activity. Finally; atorvastatin and GT3 synergistically increased KLF2 expression. These findings suggest that combined treatment of statins with GT3 may provide significant health benefits in treating a number of pathophysiological conditions; including inflammatory and cardiovascular diseases.

MicroRNA 139-5p coordinates APLNR-CXCR4 crosstalk during vascular maturation

G protein-coupled receptor (GPCR) signalling, including that involving apelin (APLN) and its receptor APLNR, is known to be important in vascular development. How this ligand–receptor pair regulates the downstream signalling cascades in this context remains poorly understood. Here, we show that mice with Apln, Aplnr or endothelial-specific Aplnr deletion develop profound retinal vascular defects, which are at least in part due to dysregulated increase in endothelial CXCR4 expression. Endothelial CXCR4 is negatively regulated by miR-139-5p, whose transcription is in turn induced by laminar flow and APLN/APLNR signalling. Inhibition of miR-139-5p in vivo partially phenocopies the retinal vascular defects of APLN/APLNR deficiency. Pharmacological inhibition of CXCR4 signalling or augmentation of the miR-139-5p-CXCR4 axis can ameliorate the vascular phenotype of APLN/APLNR deficient state. Overall, we identify an important microRNA-mediated GPCR crosstalk, which plays a key role in vascular development.

G protein-coupled receptors APLNR and CXCR4 are crucial for vascular development. Here, the authors show that these two signaling pathways communicate and that in response to blood flow APLNR signaling induces a decrease in CXCR4 expression via miR-139-5p, thereby restricting CXCR4 expression to the non-flow exposed tip cells in the retinal vasculature.

Epigenetic modulation as a therapeutic approach for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare but progressive and currently incurable disease, which is characterized by vascular remodeling in association with muscularization of the arterioles, medial thickening and plexiform lesion formation. Despite our advanced understanding of the pathogenesis of PAH and the recent therapeutic advances, PAH still remains a fatal disease. In addition, the susceptibility to PAH has not yet been adequately explained. Much evidence points to the involvement of epigenetic changes in the pathogenesis of a number of human diseases including cancer, peripheral hypertension and asthma. The knowledge gained from the epigenetic study of various human diseases can also be applied to PAH. Thus, the pursuit of novel therapeutic targets via understanding the epigenetic alterations involved in the pathogenesis of PAH, such as DNA methylation, histone modification and microRNA, might be an attractive therapeutic avenue for the development of a novel and more effective treatment. This review provides a general overview of the current advances in epigenetics associated with PAH, and discusses the potential for improved treatment through understanding the role of epigenetics in the development of PAH.

A static pressure sensitive receptor APJ promote H9c2 cardiomyocyte hypertrophy via PI3K-autophagy pathway

This study is designed to investigate whether APJ receptor acts as a sensor in static pressure-induced cardiomyocyte hypertrophy and to investigate the mechanism of PI3K-autophagy pathway. The left ventricular hypertrophy rat model was established by coarctation of abdominal aorta. H9c2 rat cardiomyocytes were cultured in the presence of static pressure which was given by a custom-made pressure incubator. The results revealed that the expression of apelin/APJ system, PI3K, Akt and their phosphorylation were significantly increased in the operation group. Static pressure up-regulated the APJ expression, PI3K phosphorylation, Akt phosphorylation, LC3-II/I and beclin-1 expression in cardiomyocytes. APJ shRNA pGPU6/Neo-rat-399, PI3K inhibitor LY294002, Akt inhibitor 1701-1 blocked the up-regulation of APJ, PI3K phosphorylation, Akt phosphorylation, LC3-II/I and beclin-1 expression, respectively. Moreover, static pressure increased the diameter, volume, protein content of cells, and these could be reversed when the cells were treated with pGPU6/Neo-rat-399, LY294002, and autophagy inhibitor 3-methyladenine, respectively. These results suggested that static pressure up-regulates APJ expression to promote cardiomyocyte hypertrophy by a PI3K-autophagy pathway.

Targeting inflammation and oxidative stress in atrial fibrillation: role of 3-hydroxy-3-methylglutaryl-coenzyme a reductase inhibition with statins

SIGNIFICANCE

Atrial fibrillation (AF) is a burgeoning health-care problem, and the currently available therapeutic armamentarium is barely efficient. Experimental and clinical evidence implicates inflammation and myocardial oxidative stress in the pathogenesis of AF.

RECENT ADVANCES

Local and systemic inflammation has been found to both precede and follow the new onset of AF, and NOX2-dependent generation of reactive oxygen species in human right atrial samples has been independently associated with the occurrence of AF in the postoperative period in patients undergoing cardiac surgery. Anti-inflammatory and antioxidant agents can prevent atrial electrical remodeling in animal models of atrial tachypacing and the new onset of AF after cardiac surgery, suggesting a causal relationship between inflammation/oxidative stress and the atrial substrate that supports AF.

CRITICAL ISSUES

Statin therapy, by redressing the myocardial nitroso-redox balance and reducing inflammation, has emerged as a potentially effective strategy for the prevention of AF. Evidence indicates that statins prevent AF-induced electrical remodeling in animal models of atrial tachypacing and may reduce the new onset of AF after cardiac surgery. However, whether statins have antiarrhythmic properties in humans has yet to be conclusively demonstrated, as data from randomized controlled trials specifically addressing the relevance of statin therapy for the primary and secondary prevention of AF remain scanty.

FUTURE DIRECTIONS

A better understanding of the mechanisms underpinning the putative antiarrhythmic effects of statins may afford tailoring AF treatment to specific clinical settings and patient's subgroups. Large-scale randomized clinical trials are needed to support the indication of statin therapy solely on the basis of AF prevention.

Apelin-APJ signaling: a potential therapeutic target for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by the vascular remodeling of the pulmonary arterioles, including formation of plexiform and concentric lesions comprised of proliferative vascular cells. Clinically, PAH leads to increased pulmonary arterial pressure and subsequent right ventricular failure. Existing therapies have improved the outcome but mortality still remains exceedingly high. There is emerging evidence that the seven-transmembrane G-protein coupled receptor APJ and its cognate endogenous ligand apelin are important in the maintenance of pulmonary vascular homeostasis through the targeting of critical mediators, such as Krűppel-like factor 2 (KLF2), endothelial nitric oxide synthase (eNOS), and microRNAs (miRNAs). Disruption of this pathway plays a major part in the pathogenesis of PAH. Given its role in the maintenance of pulmonary vascular homeostasis, the apelin-APJ pathway is a potential target for PAH therapy. This review highlights the current state in the understanding of the apelin-APJ axis related to PAH and discusses the therapeutic potential of this signaling pathway as a novel paradigm of PAH therapy.

Endothelium as a gatekeeper of fatty acid transport

The endothelium transcends all clinical disciplines and is crucial to the function of every organ system. A critical, but poorly understood, role of the endothelium is its ability to control the transport of energy supply according to organ needs. Fatty acids (FAs) in particular represent a key energy source that is utilized by a number of tissues, but utilization must be tightly regulated to avoid potentially deleterious consequences of excess accumulation, including insulin resistance. Recent studies have identified important endothelial signaling mechanisms, involving vascular endothelial growth factor-B, peroxisome proliferator-activated receptor-γ, and apelin, that mediate endothelial regulation of FA transport. In this review, we discuss the mechanisms by which these signaling pathways regulate this key endothelial function.

Essential role of Apelin signaling during lymphatic development in zebrafish

OBJECTIVE

Apelin and its cognate receptor Aplnr/Apj are essential for diverse biological processes. However, the function of Apelin signaling in lymphatic development remains to be identified, despite the preferential expression of Apelin and Aplnr within developing blood and lymphatic endothelial cells in vertebrates. In this report, we aim to delineate the functions of Apelin signaling during lymphatic development.

APPROACH AND RESULTS

We investigated the functions of Apelin signaling during lymphatic development using zebrafish embryos and found that attenuation of Apelin signaling substantially decreased the formation of the parachordal vessel and the number of lymphatic endothelial cells within the developing thoracic duct, indicating an essential role of Apelin signaling during the early phase of lymphatic development. Mechanistically, we found that abrogation of Apelin signaling selectively attenuates lymphatic endothelial serine-threonine kinase Akt 1/2 phosphorylation without affecting the phosphorylation status of extracellular signal-regulated kinase 1/2. Moreover, lymphatic abnormalities caused by the reduction of Apelin signaling were significantly exacerbated by the concomitant partial inhibition of serine-threonine kinase Akt/protein kinase B signaling. Apelin and vascular endothelial growth factor-C (VEGF-C) signaling provide a nonredundant activation of serine-threonine kinase Akt/protein kinase B during lymphatic development because overexpression of VEGF-C or apelin was unable to rescue the lymphatic defects caused by the lack of Apelin or VEGF-C, respectively.

CONCLUSIONS

Taken together, our data present compelling evidence suggesting that Apelin signaling regulates lymphatic development by promoting serine-threonine kinase Akt/protein kinase B activity in a VEGF-C/VEGF receptor 3-independent manner during zebrafish embryogenesis.

Mechanisms of thrombosis in obesity

PURPOSE OF REVIEW

Obesity has become a worldwide epidemic that is driving increased morbidity and mortality from thrombotic disorders such as myocardial infarction, stroke, and venous thromboembolism. Effective prevention and treatment of thrombosis in obese patients is limited by an incomplete understanding of the underlying prothrombotic mechanisms and by uncertainties about risks, benefits, and dosing of anticoagulant drugs in this patient population.

RECENT FINDINGS

This review summarizes our current understanding of established and emerging mechanisms contributing to the obesity-induced prothrombotic state. The mechanistic impact of chronic inflammation and impaired fibrinolysis in mediating obesity-associated thrombosis is highlighted. Recent data demonstrating the aberrant expression of adipokines and microRNAs, which appear to function as key modulators of proinflammatory and prothrombotic pathways in obesity, are also reviewed. Finally, some challenges and new approaches to the prevention and management of thrombotic disorders in obese and overweight patients are discussed.

SUMMARY

Obesity-driven chronic inflammation and impaired fibrinolysis appear to be major effector mechanisms of thrombosis in obesity. The proinflammatory and hypofibrinolytic effects of obesity may be exacerbated by dysregulated expression and secretion of adipokines and microRNAs, which further increase the risk of thrombosis and suggest new potential targets for therapy.

Apelin is a positive regulator of ACE2 in failing hearts

Angiotensin converting enzyme 2 (ACE2) is a negative regulator of the renin-angiotensin system (RAS), catalyzing the conversion of Angiotensin II to Angiotensin 1-7. Apelin is a second catalytic substrate for ACE2 and functions as an inotropic and cardioprotective peptide. While an antagonistic relationship between the RAS and apelin has been proposed, such functional interplay remains elusive. Here we found that ACE2 was downregulated in apelin-deficient mice. Pharmacological or genetic inhibition of angiotensin II type 1 receptor (AT1R) rescued the impaired contractility and hypertrophy of apelin mutant mice, which was accompanied by restored ACE2 levels. Importantly, treatment with angiotensin 1-7 rescued hypertrophy and heart dysfunctions of apelin-knockout mice. Moreover, apelin, via activation of its receptor, APJ, increased ACE2 promoter activity in vitro and upregulated ACE2 expression in failing hearts in vivo. Apelin treatment also increased cardiac contractility and ACE2 levels in AT1R-deficient mice. These data demonstrate that ACE2 couples the RAS to the apelin system, adding a conceptual framework for the apelin-ACE2-angiotensin 1-7 axis as a therapeutic target for cardiovascular diseases.

Apelin-APJ signaling is a critical regulator of endothelial MEF2 activation in cardiovascular development

RATIONALE

The peptide ligand apelin and its receptor APJ constitute a signaling pathway with numerous effects on the cardiovascular system, including cardiovascular development in model organisms such as xenopus and zebrafish.

OBJECTIVE

This study aimed to characterize the embryonic lethal phenotype of the Apj-/- mice and to define the involved downstream signaling targets.

METHODS AND RESULTS

We report the first characterization of the embryonic lethality of the Apj-/- mice. More than half of the expected Apj-/- embryos died in utero because of cardiovascular developmental defects. Those succumbing to early embryonic death had markedly deformed vasculature of the yolk sac and the embryo, as well as poorly looped hearts with aberrantly formed right ventricles and defective atrioventricular cushion formation. Apj-/- embryos surviving to later stages demonstrated incomplete vascular maturation because of a deficiency of vascular smooth muscle cells and impaired myocardial trabeculation and ventricular wall development. The molecular mechanism implicates a novel, noncanonical signaling pathway downstream of apelin-APJ involving Gα13, which induces histone deacetylase (HDAC) 4 and HDAC5 phosphorylation and cytoplasmic translocation, resulting in activation of myocyte enhancer factor 2. Apj-/- mice have greater endocardial Hdac4 and Hdac5 nuclear localization and reduced expression of the myocyte enhancer factor 2 (MEF2) transcriptional target Krüppel-like factor 2. We identify a number of commonly shared transcriptional targets among apelin-APJ, Gα13, and MEF2 in endothelial cells, which are significantly decreased in the Apj-/- embryos and endothelial cells.

CONCLUSIONS

Our results demonstrate a novel role for apelin-APJ signaling as a potent regulator of endothelial MEF2 function in the developing cardiovascular system.