Apelin attenuates postburn sepsis via a phosphatidylinositol 3-kinase/protein kinase B dependent mechanism: A randomized animal study

Kinetics of Inflammatory Mediators in the Immune Response to Burn Injury: Systematic Review and Meta-Analysis of Animal Studies

Burns are often accompanied by a dysfunctional immune response, which can lead to systemic inflammation, shock, and excessive scarring. The objective of this study was to provide insight into inflammatory pathways associated with burn-related complications. Because detailed information on the various inflammatory mediators is scattered over individual studies, we systematically reviewed animal experimental data for all reported inflammatory mediators. Meta-analyses of 352 studies revealed a strong increase in cytokines, chemokines, and growth factors, particularly 19 mediators in blood and 12 in burn tissue. Temporal kinetics showed long-lasting surges of proinflammatory cytokines in blood and burn tissue. Significant time-dependent effects were seen for IL-1β, IL-6, TGF-β1, and CCL2. The response of anti-inflammatory mediators was limited. Burn technique had a profound impact on systemic response levels. Large burn size and scalds further increased systemic, but not local inflammation. Animal characteristics greatly affected inflammation, for example, IL-1β, IL-6, and TNF-α levels were highest in young, male rats. Time-dependent effects and dissimilarities in response demonstrate the importance of appropriate study design. Collectively, this review presents a general overview of the burn-induced immune response exposing inflammatory pathways that could be targeted through immunotherapy for burn patients and provides guidance for experimental set-ups to advance burn research.

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.

The Protective Role of Apelin in the Early Stages of Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the most common and serious microvascular complications of diabetes. Although current treatments can control the progression of DR to a certain extent, there is no effective treatment for early DR. Apart from vascular endothelial growth factor, it has been noted that the apelin/APJ system contributes to the pathogenesis of DR. We used a high-fat diet/streptozotocin-induced type 2 diabetic mouse model. The mice were divided into a lentivirus control group (LV-EGFP), an apelin-overexpression group (LV-Apelin+), and an apelin-knockdown group (LV-Apelin-), all of which were administrated intravitreal injections. LV-Apelin+ ameliorated the loss of pericytes in DR mice, whereas LV-Apelin- aggravated the loss of pericytes. Similarly, LV-Apelin+ reduced the leakage of retinal vessels, whereas LV-Apelin- exacerbated it. The genes and signaling pathway related to cell adhesion molecules were downregulated, whereas the cell-cell tight junctions and anti-apoptotic genes were upregulated in response to apelin overexpression. However, the alterations of these same genes and signaling pathways were reversed in the case of apelin knockdown. Additionally, LV-Apelin+ increased ZO-1 and occludin levels, whereas LV-Apelin- decreased them. Our results suggest that apelin can reduce vascular leakage by protecting pericytes, which offers a promising new direction for the early treatment of DR.

Fc-Elabela fusion protein attenuates lipopolysaccharide-induced kidney injury in mice

Endotoxemia-induced acute kidney injury (AKI) is a common clinical condition that lacks effective treatments. Elabela (ELA) is a recently discovered kidney peptide hormone, encoded by the gene apela, and has been reported to improve cardio-renal outcomes in sepsis. However, ELA is a small peptide and is largely unsuitable for clinical use because of its short in vivo half-life. In the present study, we evaluated the potential renoprotective effects of a long-acting constant fragment (Fc)-ELA fusion protein in liposaccharide (LPS)-induced AKI in mice. LPS administration in mice for 5 days greatly lowered the gene expression of apela and impaired kidney function, as evidenced by elevated serum creatinine and the ratio of urine protein to creatinine. In addition, renal inflammation and macrophage infiltration were apparent in LPS-challenged mice. Treatment with the Fc-ELA fusion protein partially restored apela expression and attenuated the kidney inflammation. Moreover, LPS treatment induced reactive oxygen species (ROS) production and apoptosis in kidney HK-2 cells as well as in the mouse kidney, which were mitigated by ELA or Fc-ELA treatment. Finally, we found that ELA promoted the survival of HK-2 cells treated with LPS, and this action was abolished by LY204002, a PI3K/Akt inhibitor. Collectively, we have demonstrated that the Fc-ELA fusion protein has significant renoprotective activities against LPS-induced AKI in mice.

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.

H2S attenuates sepsis-induced cardiac dysfunction via a PI3K/Akt-dependent mechanism

The heart is the most vulnerable target organ in sepsis, and it has been previously reported that hydrogen sulfide (H₂S) has a protective role in heart dysfunction caused by sepsis. Additionally, studies have demonstrated that the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt) signaling pathway has a protective function during sepsis. However, the potential association between H₂S and PI3K/Akt in sepsis-induced cardiac dysfunction is unclear. Therefore, the PI3K inhibitor LY294002 was used to investigate the role of PI3K/Akt signaling in the protective effects of H₂S during sepsis-induced myocardial injury. A rat sepsis model was established using cecal ligation and puncture (CLP) surgery. Sodium hydrosulfide, a H₂S donor, was administered intraperitoneally (8.9 µmol/kg), and serum myocardial enzyme levels, inflammatory cytokine levels, cardiac histology and cardiomyocyte apoptosis were assessed to determine the extent of myocardial damage. The results demonstrated that exogenous H₂S reduced serum myocardial enzyme levels, decreased the levels of the inflammatory factors tumor necrosis factor (TNF)-α and interleukin (IL)-6, and increased the level of anti-inflammatory IL-10 following CLP. Staining of histological sections demonstrated that myocardial damage and cardiomyocyte apoptosis were alleviated by the administration of exogenous H₂S. Western blot analysis was used to detect phosphorylated and total PI3K and Akt levels, as well as NF-κB, B-cell lymphoma-2, Bcl-2-associated X protein (Bax) and caspase levels, and the results demonstrated that H₂S significantly increased PI3K and Akt phosphorylation. This indicated that the PI3K/Akt signaling pathway was activated by H₂S. Additionally, H₂S reduced Bax and caspase expression, indicating that apoptosis was inhibited, and decreased NF-κB levels, indicating that inflammation was reduced. Furthermore, the PI3K inhibitor LY294002 eliminated the protective effects of H₂S. In conclusion, the results of the current study suggest that exogenous H₂S activates PI3K/Akt signaling to attenuate myocardial damage in sepsis.

Fc-apelin fusion protein attenuates lipopolysaccharide-induced liver injury in mice

Apelin is a peptide hormone with anti-oxidative and anti-inflammatory activities and is proposed to be a potential therapeutic for many disease conditions, including sepsis. However, short in vivo half-life of the apelin peptide would limit its potential clinical applications. This study aims to investigate the effects of Fc-apelin, a novel long-acting apelin fusion protein, on lipopolysaccharide (LPS)-induced liver injury. Liver injury was induced by systemic injection of LPS in mice. Hepatoprotective activities of Fc-apelin against inflammation were evaluated in LPS mice and/or hepatoma Huh-7 cells with respect to serum ALT, apoptosis, oxidative stress, macrophage infiltration and gene expression. We found that LPS induced systemic inflammation and liver damage. Co-administration of Fc-apelin significantly attenuated serum ALT elevation, diminished LPS-induced apoptosis and ROS production in the liver and in Huh-7 cells, mitigated hepatic macrophage infiltration, and reduced TNFα and IL-6 gene expression. Collectively, Fc-apelin fusion protein exerts protective effects against LPS-induced liver damage and may serve as a potential therapeutic for endotoxin-induced liver injury.

Vascular effects of apelin: Mechanisms and therapeutic potential

Apelin is a vasoactive peptide and is an endogenous ligand for APJ receptors, which are widely expressed in blood vessels, heart, and cardiovascular regulatory regions of the brain. A growing body of evidence now demonstrates a regulatory role for the apelin/APJ receptor system in cardiovascular physiology and pathophysiology, thus making it a potential target for cardiovascular drug discovery and development. Indeed, ongoing studies are investigating the potential benefits of apelin and apelin-mimetics for disorders such as heart failure and pulmonary arterial hypertension. Apelin causes relaxation of isolated arteries, and systemic administration of apelin typically results in a reduction in systolic and diastolic blood pressure and an increase in blood flow. Nonetheless, vasopressor responses and contraction of vascular smooth muscle in response to apelin have also been observed under certain conditions. The goal of the current review is to summarize major findings regarding the apelin/APJ receptor system in blood vessels, with an emphasis on regulation of vascular tone, and to identify areas of investigation that may provide guidance for the development of novel therapeutic agents that target this system.

The apelinergic system as an alternative to catecholamines in low-output septic shock

Catecholamines, in concert with fluid resuscitation, have long been recommended in the management of septic shock. However, not all patients respond positively and controversy surrounding the efficacy-to-safety profile of catecholamines has emerged, trending toward decatecholaminization. Contextually, it is time to re-examine the “maintaining blood pressure” paradigm by identifying safer and life-saving alternatives. We put in perspective the emerging and growing knowledge on a promising alternative avenue: the apelinergic system. This target exhibits invaluable pleiotropic properties, including inodilator activity, cardio-renal protection, and control of fluid homeostasis. Taken together, its effects are expected to be greatly beneficial for patients in septic shock.

Apelinergic System Structure and Function

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

ELABELA Improves Cardio-Renal Outcome in Fatal Experimental Septic Shock

OBJECTIVES

Apelin-13 was recently proposed as an alternative to the recommended β-adrenergic drugs for supporting endotoxin-induced myocardial dysfunction. Since Apelin-13 signals through its receptor (Apelin peptide jejunum) to exert singular inotropic/vasotropic actions and to optimize body fluid balance, this candidate pathway might benefit septic shock management. Whether the newly discovered ELABELA (ELA), a second endogenous ligand of the Apelin peptide jejunum receptor highly expressed in the kidney, further improves cardio-renal impairment remains unknown.

DESIGN, SETTING, AND SUBJECTS

Interventional study in a rat model of septic shock (128 adult males) to assess the effects of ELA and Apelin-13 on vascular and cardio-renal function. Experiments were performed in a tertiary care University-based research institute.

INTERVENTIONS

Polymicrobial sepsis-induced cardiac dysfunction was produced by cecal ligation puncture to assess hemodynamic efficacy, cardioprotection, and biomechanics under acute or continuous infusions of the apelinergic agonists ELA or Apelin-13 (39 and 15 µg/kg/hr, respectively) versus normal saline.

MEASUREMENTS AND MAIN RESULTS

Apelinergic agonists improved 72-hour survival after sepsis induction, with ELA providing the best clinical outcome after 24 hours. Apelinergic agonist infusion counteracted cecal ligation puncture-induced myocardial dysfunction by improving left ventricular pressure-volume relationship. ELA-treated cecal ligation puncture rats were the only group to 1) display a significant improvement in left ventricular filling as shown by increased E-wave velocity and left ventricular end-diastolic volume, 2) exhibit a higher plasma volume, and 3) limit kidney injury and free-water clearance. These beneficial renal effects were superior to Apelin-13, likely because full-length ELA enabled a distinctive regulation of pituitary vasopressin release.

CONCLUSIONS

Activation of the apelinergic system by exogenous ELA or Apelin-13 infusion improves cardiovascular function and survival after cecal ligation puncture-induced sepsis. However, ELA proved better than Apelin-13 by improving fluid homeostasis, cardiovascular hemodynamics recovery, and limiting kidney dysfunction in a vasopressinergic-dependent manner.

Apelin Compared With Dobutamine Exerts Cardioprotection and Extends Survival in a Rat Model of Endotoxin-Induced Myocardial Dysfunction

OBJECTIVE

Dobutamine is the currently recommended β-adrenergic inotropic drug for supporting sepsis-induced myocardial dysfunction when cardiac output index remains low after preload correction. Better and safer therapies are nonetheless mandatory because responsiveness to dobutamine is limited with numerous side effects. Apelin-13 is a powerful inotropic candidate that could be considered as an alternative noncatecholaminergic support in the setting of inflammatory cardiovascular dysfunction.

DESIGN

Interventional controlled experimental animal study.

SETTING

Tertiary care university-based research institute.

SUBJECTS

One hundred ninety-eight adult male rats.

INTERVENTIONS

Using a rat model of "systemic inflammation-induced cardiac dysfunction" induced by intraperitoneal lipopolysaccharide injection (10 mg/kg), hemodynamic efficacy, cardioprotection, and biomechanics were assessed under IV osmotic pump infusions of apelin-13 (0.25 μg/kg/min) or dobutamine (7.5 μg/kg/min).

MEASUREMENTS AND MAIN RESULTS

In this model and in both in vivo and ex vivo studies, apelin-13 compared with dobutamine provoked distinctive effects on cardiac function: 1) optimized cardiac energy-dependent workload with improved cardiac index and lower vascular resistance, 2) upgraded hearts' apelinergic responsiveness, and 3) consecutive downstream advantages, including increased urine output, enhanced plasma volume, reduced weight loss, and substantially improved overall outcomes. In vitro studies confirmed that these apelin-13-driven processes encompassed a significant and rapid reduction in systemic cytokine release with dampening of myocardial inflammation, injury, and apoptosis and resolution of associated molecular pathways.

CONCLUSIONS

In this inflammatory cardiovascular dysfunction, apelin-13 infusion delivers distinct and optimized hemodynamic support (including positive fluid balance), along with cardioprotective effects, modulation of circulatory inflammation and extended survival.