The biological efficacy of Apelin against focal transient cerebral ischemia-reperfusion injury. A systematic review and meta-analysis of animal studies

BACKGROUND

Apelin has been extensively studied, and emerging experimental evidence suggests that Apelin may have effects on stroke by reducing infarct volume and neurological deficits, inhibiting the apoptosis process and reducing brain water content. However, the credibility of the evidence is uncertain. Thus, we aimed to perform a systematic review and meta-analysis to evaluate preclinical studies that used Apelin for the treatment of transient focal cerebral ischemia.

METHODS

Electronic bibliographic databases including PubMed, EMBASE, Scopus, and Google Scholar were searched for finding relevant studies from January 2000 to July 2023. The methodological quality and risk of bias scores for animal studies were calculated based on the CAMARADES and the SYRCLE's RoB tools, respectively. The effect sizes were assessed using Comprehensive Meta-Analysis (CMA) software.

RESULTS

A total of twelve eligible studies were used for the systematic review and meta-analysis. The median scores of study quality and risk of bias were 7.5 out of 10, and 5 out of 10, respectively. Apelin treatment effectively decreased infarct volume (primary outcome) [Hedges' g = 2.72, 95 % CI (1.93, 3.51), p < 0.001], neurological deficit [Hedges' g = 1.76, 95 % CI (0.96, 2.55), p < 0.001], cleaved caspase 3 [Hedges' g = 2.16, 95 % CI (0.87, 3.44), p = 0.001], and apoptotic cell number [Hedges' g = 4.07, 95 % CI (1.25,6.89), p = 0.005] compared with the control group. According to subgroup analysis, more notable neuroprotective effects were observed with intravenous administration than with intracerebroventricular (ICV) administration. Moreover, we determined that effect size of infarct volume was markedly related to the species. The combined measurement of two studies demonstrated that Apelin could reduce BCL2 and TNF-α levels as well as brain water content compared with the control group. However, pooled measurement of two studies showed that no relevancy was discovered between CHOP and altering infarct volume.

CONCLUSION

The present meta-analysis was conducted to assess preclinical studies related to Apelin treatment in rodent ischemic stroke. Apelin can exert promising neuroprotective effects by reducing infarct volume, neurological deficit, caspase 3, apoptotic cell number, TNF- α and brain water content and increasing BCL2. The current evidence supports the anti-apoptotic and anti-inflammatory properties of Apelin, but its effectiveness in decreasing CHOP level in animal models of ischemic stroke needs further elucidation. This study was registered within the International Prospective Register of Systematic Reviews (PROSPERO) as number CRD42023460926.

A novel insight into the neuroprotective effects of cannabidiol: maintained apelin/dopamine synthesis, NRF2 signaling, and AKT/CREB/BDNF gene expressions

Neuroinflammation is a process associated with degeneration and loss of neurons in different parts of the brain. The most important damage mechanisms in its formation are oxidative stress and inflammation. This study aimed to investigate the protective effects of cannabidiol (CBD) against neuroinflammation through various mechanisms. Thirty‑two female rats were randomly divided into 4 groups as control, lipopolysaccharide (LPS), LPS + CBD and CBD groups. After six hours following LPS administration, rats were sacrificed, brain and cerebellum tissues were obtained. Tissues were stained with hematoxylin‑eosin for histopathological analysis. Apelin and tyrosine hydroxylase synthesis were determined immunohistochemically. Total oxidant status and total antioxidant status levels were measured, and an oxidative stress index was calculated. Protein kinase B (AKT), brain-derived neurotrophic factor (BDNF), cyclic‑AMP response element‑binding protein (CREB) and nuclear factor erythroid 2‑related factor 2 (NRF2) mRNA expression levels were also determined. In the LPS group, hyperemia, degeneration, loss of neurons and gliosis were seen in all three tissues. Additionally, Purkinje cell loss in the cerebellum, as well as neuronal loss in the cerebral cortex and hippocampus, were found throughout the LPS group. The expressions of AKT, BDNF, CREB and NRF2, apelin and tyrosine hydroxylase synthesis all decreased significantly. CBD treatment reversed these changes and ameliorated oxidative stress parameters. CBD showed protective effects against neuroinflammation via regulating AKT, CREB, BDNF expressions, NRF2 signaling, apelin and tyrosine hydroxylase synthesis.

Protective effect of apelin-13 on ventilator-induced acute lung injury

BACKGROUND

Mechanical Ventilation (MV) is an essential mechanism of life support in the clinic. It may also lead to ventilator-induced acute lung injury (VILI) due to local alveolar overstretching and/or repeated alveolar collapse. However, the pathogenesis of VILI is not completely understood, and its occurrence and development may be related to physiological processes such as the inflammatory response, oxidative stress, and apoptosis. Some studies have found that the the apelin/APJ axis is an endogenous antagonistic mechanism activated during acute respiratory distress syndrome(ARDS), that can counteract the injury response and prevent uncontrolled lung injury. To indicate that apelin-13 plays a protective role in VILI, an animal model of VILI was established in this study to explore whether apelin-13 can alleviate VILI in rats by inhibiting inflammation, apoptosis and oxidative stress.

METHODS

SD rats were divided into four groups: control, high tidal volume, high tidal volume + normal saline and high tidal volume + apelin-13. After tracheotomy, the rats in control maintained spontaneous breathing, and the other rats were connected to the small animal ventilator for 4 h to establish the rat VILI model. The mRNA expression of apelin was measured by real-time quantitative polymerase chain reaction(qRT-PCR), immunofluorescence and Western blotting(WB) were used to detect the expression level of APJ, and WB was used to detect the expression of the apoptotic proteins Bax and bcl-2. The degree of lung injury was evaluated by pathological staining of lung tissue,W/D ratio, and BALF total protein concentration. The expression of inflammatory factors(IL-1β, IL-6, TNF-α) in alveolar lavage fluid was measured using ELISA. The activities of MPO and cat and the content of MDA, an oxidative product, in lung tissue were measured to evaluate the degree of oxidative stress in the lung.

RESULTS

After treatment with apelin-13, the apelin/APJ axis in the lung tissue of VILI model rats was activated, and the effect was further enhanced. The pathological damage of lung tissue was alleviated, the expression of the antiapoptotic protein Bcl-2 and the proapoptotic protein Bax was reversed, and the levels of the inflammatory cytokines IL-1β, IL-6, TNF-α levels were all decreased. MPO activity and MDA content decreased, while CAT activity increased.

CONCLUSION

The apelin/apj axis is activated in VILI. Overexpression of apelin-13 further plays a protective role in VILI, mainly by including reducing pathological damage, the inflammatory response, apoptosis and antioxidant stress in lung tissue, thus delaying the occurrence and development of VILI.

Effects of Apelin on the fibrosis of retinal tissues and Müller cells in diabetes retinopathy through the JAK2/STAT3 signalling pathway

Retinal fibrosis was a key characteristic of diabetes retinopathy (DR). Apelin was found to be a candidate for tissue fibrosis. Nevertheless, the role of Apelin in the Müller cells in DR remains unclear. This study identified the function and mechanism of Apelin in Müller cells and the fibrosis of retinal tissue. Western blot was carried out to detect the Apelin, GFAP, Collagen I, α-SMA, JAK2 and STAT3 protein levels. Masson staining was performed to display the histopathological changes in retinal tissue of diabetic mellitus (DM) rats. The immunofluorescence staining was conducted to evaluate the Apelin levels in the retinal tissue. The levels of GFAP, Collagen I and α-SMA in the retinal tissue of DM rats was visualised by the immunohistochemistry staining. The results showed that Apelin, GFAP, Collagen I andα-SMA expression was prominently elevated in the retinal tissue of DM rats and high glucose (HG)-exposed Müller cells. The results of Masson staining showed that the epiretinal fibrotic membrane was observed in DM rats. Apelin knockdown declined the GFAP, Collagen I andα-SMA levels. Besides, the protein levels of p-JAK2 and p-STAT3 were elevated in the HG-treated Müller cells, while Apelin knockdown declined them. FLLL32 treatment neutralised the role of Apelin. In conclusion, Apelin facilitated the fibrogenic activity of Müller cells through activating the JAK2/STAT3 signalling pathway, and thus inducing the retinal fibrosis in DR.

Apelin-13 alleviates contrast-induced acute kidney injury by inhibiting endoplasmic reticulum stress

Contrast-induced acute kidney injury (CI-AKI) is a severe complication associated with significant morbidity and mortality, and effective therapeutic strategies are still lacking. Apelin is an endogenous physiological regulator with antioxidative, anti-inflammatory and antiapoptotic properties. However, the role of apelin-13 in CI-AKI remains unclear. In our study, we found that the protein expression levels of apelin were significantly downregulated in rat kidney tissues and HK-2 cells during contrast media treatment. Moreover, we explored the protective effect of apelin-13 on renal tubule damage using in vitro and in vivo models of CI-AKI. Exogenous apelin-13 ameliorated endoplasmic reticulum stress, reactive oxygen species and apoptosis protein expression in contrast media-treated cells and rat kidney tissues. Mechanistically, the downregulation of endoplasmic reticulum stress contributed critically to the antiapoptotic effect of apelin-13. Collectively, our findings reveal the inherent mechanisms by which apelin-13 regulates CI-AKI and provide a prospective target for the prevention of CI-AKI.

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.

Apelin/APJ system protects placental trophoblasts from hypoxia-induced oxidative stress through activating PI3K/Akt signaling pathway in preeclampsia

BACKGROUND

Preeclampsia is a placentally induced syndrome with diverse clinical presentation that currently has no cure. Oxidative stress is a potent inducer of placental dysfunction. The apelin receptor (APJ) system is a pleiotropic pathway with a potential for therapeutic targeting in preeclampsia. This study examines the alteration of circulating apelin levels and placental APJ expression in preeclampsia and investigates whether apelin/APJ system can protect placental trophoblast from hypoxia-induced oxidative stress injury through PI3K/AKT signaling pathway.

RESULTS

Our results confirmed that maternal apelin concentration was increased in women with preeclampsia, but APJ expression was reduced in the preeclamptic placentas. Apelin-13 treatment not only specifically attenuated CoCl2-induced superoxide production, but also prevented CoCl2-induced reduction of SOD activity and SOD1 expression. In addition, apelin-13 suppressed CoCl2-induced apoptosis by increasing the expression of bcl-2/bax ratio and by decreasing the expression of active caspase-3 in placental trophoblasts. Furthermore, we found that apelin-13 binding APJ activated the PI3K and AKT kinases and inhibition of PI3K kinase significantly blocked the anti-oxidative effects of apelin-13 in placental trophoblasts.

CONCLUSIONS

Decrease of placental APJ expression is associated with oxidative stress-induced placental dysfunction in preeclampsia, and increased circulating apelin could be a moderately successful marker to differentiate subjects with preeclampsia from healthy pregnant women. Inhibition of superoxide production and caspase-3 cleavage, together with upregulation of SOD activity/expression and bcl-2/bax ratio, could be the potential molecular mechanisms by which apelin-13/APJ protects placental trophoblasts from oxidative stress injury.

The effects of apelin on IGF1/FSH-induced steroidogenesis, proliferation, Bax expression, and total antioxidant capacity in granulosa cells of buffalo ovarian follicles

Apelin (APLN) was believed to be an adipokine secreted from adipose tissue. However, studies demonstrate that it is a pleiotropic peptide and has several effects on the female reproductive system. In this study, We examined the effects of different doses of IGF1 and FSH in the presence of APLN-13 on the production of progesterone in buffalo ovary granulosa cells. Furthermore, different doses of APLN isoforms (APLN-13 and APLN-17) were tested on proliferation, Bax protein expression, and antioxidant capacity in the same cells. Granulosa cells of buffalo ovaries were cultured in the presence of different doses of IGF1 and FSH with or without APLN-13 (10-9 M) to evaluate its effect on the secretion of progesterone tested by ELISA assay. The WST-1 method was used to survey the effect of APLN on granulosa cell proliferation and cytotoxicity. In addition, the antioxidant capacity of the cells in the presence of APLN was assessed using the FRAP method. mRNA and Bax protein levels were measured in granulosa cells treated with APLN using real-time PCR and western blot techniques. APLN-13 (10-9) stimulated the effect of IGF1 on the production of progesterone, and its levels were affected by APLN-13 dose-dependently. However, it did not significantly stimulate the effect of FSH on the secretion of progesterone. APLN-13 (all doses) and APLN-17 (10-8 and 10-9 M) improved the proliferation of granulosa cells. Moreover, preincubation of the cells for an hour by APLN receptor antagonist (ML221, 10 µM) did not significantly affect the proliferation of cells induced by APLN. Neither APLN-13 nor APLN-17 were not cytotoxic for the cells compared to the control treatment. APLN-13 at the doses of 10-6 and 10-8 M substantially up and down-regulated Bax protein expression; however, such effects were not observed when the cells were preincubated with ML221. In addition, APLN-17 did not influence the expression amount of Bax. Furthermore, both APLN-13 and -17 improved the total antioxidant capacity of the ovarian granulosa cells, but such effects were not seen when the cells were preincubated with ML221. According to these results, APLN enhanced the steroidogenesis induced by IGF1 but did not affect the steroidogenesis induced by FSH. APLN also enhanced the cell proliferation and antioxidant capacity of buffalo ovaries follicular granulosa cells; however, its effect on Bax expression was different.

7β-Hydroxycholesterol and 7-ketocholesterol: New oxidative stress biomarkers of sarcopenia inducing cytotoxic effects on myoblasts and myotubes

Aging is a complex biological process which can be associated with skeletal muscle degradation leading to sarcopenia. The aim of this study consisted i) to determine the oxidative and inflammatory status of sarcopenic patients and ii) to clarify the impact of oxidative stress on myoblasts and myotubes. To this end, various biomarkers of inflammation (C-reactive protein (CRP), TNF-α, IL-6, IL-8, leukotriene B4 (LTB4)) and oxidative stress (malondialdehyde, conjugated dienes, carbonylated proteins and antioxidant enzymes: catalase, superoxide dismutase, glutathione peroxidase) as well as oxidized derivatives of cholesterol formed by cholesterol autoxidation (7-ketocholesterol, 7β-hydroxycholesterol), were analyzed. Apelin, a myokine which contributes to muscle strength, was also quantified. To this end, a case-control study was conducted to evaluate the RedOx and inflammatory status in 45 elderly subjects (23 non-sarcopenic; 22 sarcopenic) from 65 years old and higher. SARCopenia-Formular (SARC-F) and Timed Up and Go (TUG) tests were used to distinguish between sarcopenic and non-sarcopenic subjects. By using red blood cells, plasma and/or serum, we observed in sarcopenic patients an increased activity of major antioxidant enzymes (superoxide dismutase, glutathione peroxidase, catalase) associated with lipid peroxidation and protein carbonylation (increased level of malondialdehyde, conjugated dienes and carbonylated proteins). Higher levels of 7-ketocholesterol and 7β-hydroxycholesterol were also observed in the plasma of sarcopenic patients. Significant differences were only observed with 7β-hydroxycholesterol. In sarcopenic patients comparatively to non-sarcopenic subjects, significant increase of CRP, LTB4 and apelin were observed whereas similar levels of TNF-α, IL-6 and IL-8 were found. The increased plasma level of 7-ketocholesterol and 7β-hydroxycholesterol in sarcopenic patients led us to study the cytotoxic effect of these oxysterols on undifferentiated (myoblasts) and differentiated (myotubes) murine C2C12 cells. With the fluorescein diacetate and sulforhodamine 101 assays, an induction of cell death was observed both on undifferentiated and differentiated cells: the cytotoxic effects were less pronounced with 7-ketocholesterol. In addition, IL-6 secretion was never detected whatever the culture conditions, TNF-α secretion was significantly increased on undifferentiated and differentiated C2C12 cells treated with 7-ketocholesterol- and 7β-hydroxycholesterol, and IL-8 secretion was increased on differentiated cells. 7-ketocholesterol- and 7β-hydroxycholesterol-induced cell death was strongly attenuated by α-tocopherol and Pistacia lentiscus L. seed oil both on myoblasts and/or myotubes. TNF-α and/or IL-8 secretions were reduced by α-tocopherol and Pistacia lentiscus L. seed oil. Our data support the hypothesis that the enhancement of oxidative stress observed in sarcopenic patients could contribute, especially via 7β-hydroxycholesterol, to skeletal muscle atrophy and inflammation via cytotoxic effects on myoblasts and myotubes. These data bring new elements to understand the pathophysiology of sarcopenia and open new perspectives for the treatment of this frequent age-related disease.

Hypophagia induced by intracerebroventricular injection of apelin-13 is mediated via CRF1/CRF2 and MC3/MC4 receptors in neonatal broiler chicken

Previous studies have shown the role of apelin and its receptors in the regulation of food intake. In the present study, we investigate the mediating role of melanocortin, corticotropin, and neuropeptide Y systems in apelin-13- induced food intake in broilers. Eight trials were run in the current investigation to ascertain the relationships between the aforementioned systems and apelin-13 on food intake and behavioral changes after apelin-13 administration. In experiment 1, hens were given an intracerebroventricular administration of a solution for control in addition to apelin-13 (0.25, 0.5, and 1 µg). Astressin-B (a CRF1/CRF2 receptor antagonist, 30 µg), apelin-13 (1 µg), and administration of astressin-B and apelin-13 concurrently, were all injected into the birds in experiment 2. Experiments 3 through 8 were quite similar to experiment 2, with the exception of astressin2-B (CRF2 receptor antagonist, 30 µg), SHU9119 (MC3/MC4 receptor antagonist, 0.5 nmol), MCL0020 (MC4 receptor antagonist, 0.5 nmol), BIBP-3226 (NPY1 receptor antagonist, 1.25 nmol), BIIE 0246 (NPY2 receptor antagonist, 1.25 nmol), and CGP71683A (NPY5 receptor antagonist, 1.25 nmol) were injected instead of astressin-B. After then, total food consumption was monitored for 6 h. Apelin-13 injections of 0.5 and 1 µg decreased feeding (P < 0.05). The hypophagic effects of apelin were attenuated following the simultaneous administration of Astressin-B and Astressin2-B with apelin-13 (P > 0.05). Co-infusion of SHU9119 and apelin-13 reduced the appetite-decreasing effects of apelin-13 (P > 0.05). When MCL0020 and apelin-13 were injected at the same time, the hypophagia that apelin-13 induced was eliminated (P > 0.05). BIBP-3226, BIIE 0246, and CGP71683A had no effect on the hypophagia brought on by apelin-13 (P > 0.05). Also, apelin-13 significantly increased number of steps, jumps, exploratory food, pecks and standing time while decreased siting time (P < 0.05). These findings suggest that apelin-13-induced hypophagia in hens may involve the CRF1/CRF2 and MC3/MC4 receptors.

Potential role of the apelin-APJ pathway in sex-related differential cardiotoxicity induced by doxorubicin in mice

Preclinical and clinical findings suggest sexual dimorphism in cardiotoxicity induced by a chemotherapeutic drug, doxorubicin (DOX). However, molecular alterations leading to sex-related differential vulnerability of heart to DOX toxicity are not fully explored. In the present study, RNA sequencing in hearts of B6C3F₁ mice indicated more differentially expressed genes in males than females (224 vs. 19; ≥1.5-fold, False Discovery Rate [FDR] < 0.05) at 1 week after receiving 24 mg/kg total cumulative DOX dose that induced cardiac lesions only in males. Pathway analysis further revealed probable inactivation of cardiac apelin fibroblast signaling pathway (p = 0.00004) only in DOX-treated male mice that showed ≥1.25-fold downregulation in the transcript and protein levels of the apelin receptor, APJ. In hearts of DOX-treated females, the transcript levels of apelin (1.24-fold) and APJ (1.47-fold) were significantly (p < 0.05) increased compared to saline-treated controls. Sex-related differential DOX effect was also observed on molecular targets downstream of the apelin-APJ pathway in cardiac fibroblasts and cardiomyocytes. In cardiac fibroblasts, upregulation of Tgf-β2, Ctgf, Sphk1, Serpine1, and Timp1 (fibrosis; FDR < 0.05) in DOX-treated males and upregulation of only Tgf-β2 and Timp1 (p < 0.05) in females suggested a greater DOX toxicity in hearts of males than females. Additionally, Ryr2 and Serca2 (calcium handling; FDR < 0.05) were downregulated in conjunction with 1.35-fold upregulation of Casp12 (sarcoplasmic reticulum-mediated apoptosis; FDR < 0.05) in DOX-treated male mice. Drug effect on the transcript level of these genes was less severe in female hearts. Collectively, these data suggest a likely role of the apelin-APJ axis in sex-related differential DOX-induced cardiotoxicity in our mouse model.

Apelin as a new therapeutic target for COVID-19 treatment

BACKGROUND

Apelin is an endogenous neuropeptide that binds to the G-protein-coupled receptor (APJ) and participates in a variety of physiological processes in the heart, lungs and other peripheral organs. Intriguingly, [Pyr1]-Apelin-13, a highly potent pyroglutamic form of apelin, has the potential to bind to and be degraded by angiotensin-converting enzyme 2 (ACE2). ACE2 is known to operate as a viral receptor in the early stages of severe acute respiratory coronavirus (SARS-CoV-2) infection.

AIM

This study aimed to determine if apelin protects against SARS-CoV-2 infection by inhibiting ACE2 binding to SARS-CoV-2 spike protein.

DESIGN AND METHODS

To determine whether [Pyr1]-Apelin-13 inhibits ACE2 binding to the SARS-CoV-2 spike protein (S protein), we performed a cell-to-cell fusion assay using ACE2-expressing cells and S protein-expressing cells and a pseudovirus-based inhibition assay. We then analyzed publicly available transcriptome data while focusing on the beneficial effects of apelin on the lungs.

RESULTS

We found that [Pyr1]-Apelin-13 inhibits cell-to-cell fusion mediated by ACE2 binding to the S protein. In this experiment, [Pyr1]-Apelin-13 protected human bronchial epithelial cells, infected with pseudo-typed lentivirus-producing S protein, against viral infection. In the presence of [Pyr1]-Apelin-13, the level of viral spike protein expression was also reduced in a concentration-dependent manner. Transcriptome analysis revealed that apelin may control inflammatory responses to viral infection by inhibiting the nuclear factor kappa B pathway.

CONCLUSION

Apelin is a potential therapeutic candidate against SARS-CoV-2 infection.

Proteomic Analysis of Effects of Spironolactone in Heart Failure With Preserved Ejection Fraction

BACKGROUND

The TOPCAT trial (Treatment of Preserved Cardiac Function Heart Failure With an Aldosterone Antagonist Trial) suggested clinical benefits of spironolactone treatment among patients with heart failure with preserved ejection fraction enrolled in the Americas. However, a comprehensive assessment of biologic pathways impacted by spironolactone therapy in heart failure with preserved ejection fraction has not been performed.

METHODS

We conducted aptamer-based proteomic analysis utilizing 5284 modified aptamers to 4928 unique proteins on plasma samples from TOPCAT participants from the Americas (n=164 subjects with paired samples at baseline and 1 year) to identify proteins and pathways impacted by spironolactone therapy in heart failure with preserved ejection fraction. Mean percentage change from baseline was calculated for each protein. Additionally, we conducted pathway analysis of proteins altered by spironolactone.

RESULTS

Spironolactone therapy was associated with proteome-wide significant changes in 7 proteins. Among these, CARD18 (caspase recruitment domain-containing protein 18), PKD2 (polycystin 2), and PSG2 (pregnancy-specific glycoprotein 2) were upregulated, whereas HGF (hepatic growth factor), PLTP (phospholipid transfer protein), IGF2R (insulin growth factor 2 receptor), and SWP70 (switch-associated protein 70) were downregulated. CARD18, a caspase-1 inhibitor, was the most upregulated protein by spironolactone (-0.5% with placebo versus +66.5% with spironolactone, P<0.0001). The top canonical pathways that were significantly associated with spironolactone were apelin signaling, stellate cell activation, glycoprotein 6 signaling, atherosclerosis signaling, liver X receptor activation, and farnesoid X receptor activation. Among the top pathways, collagens were a consistent theme that increased in patients receiving placebo but decreased in patients randomized to spironolactone.

CONCLUSIONS

Proteomic analysis in the TOPCAT trial revealed proteins and pathways altered by spironolactone, including the caspase inhibitor CARD18 and multiple pathways that involved collagens. In addition to effects on fibrosis, our studies suggest potential antiapoptotic effects of spironolactone in heart failure with preserved ejection fraction, a hypothesis that merits further exploration.

Peripheral apelin mediates visceral hypersensitivity and impaired gut barrier in a rat irritable bowel syndrome model

Growing evidence indicates that visceral hypersensitivity and impaired gut barrier play an important role in the pathophysiology of irritable bowel syndrome (IBS). In animal models, these changes are known to be mediated via corticotropin-releasing factor (CRF)-Toll like receptor 4 (TLR4)-proinflammatory cytokine signaling. Apelin, an endogenous ligand of APJ, was reported to modulate CRF-induced enhanced colonic motility. In this context, we hypothesized that apelin also modulates visceral sensation and gut barrier, and tested this hypothesis. We measured visceral pain threshold in response to colonic balloon distention by abdominal muscle contractions assessed by electromyogram in rats. Colonic permeability was estimated by quantifying the absorbed Evans blue in colonic tissue. Intraperitoneal (ip) administration of [Ala13]-apelin-13, an APJ antagonist, blocked lipopolysaccharide (LPS)- or CRF-induced visceral hypersensitivity and colonic hyperpermeability (IBS model) in a dose-response manner. These inhibitory effects were blocked by compound C, an AMPK inhibitor, N^G-nitro-L-arginine methyl ester, a nitric oxide (NO) synthesis inhibitor or naloxone in the LPS model. On the other hand, ip [Pyr1]-apelin-13, an APJ agonist, caused visceral hypersensitivity and colonic hyperpermeability, and these effects were reversed by astressin, a CRF receptor antagonist, TAK-242, a TLR4 antagonist or anakinra, an interleukin-1 receptor antagonist. APJ system modulated CRF-TLR4-proinflammatory cytokine signaling to cause visceral hypersensitivity and colonic hyperpermeability. APJ antagonist blocked these GI changes in IBS models, which were mediated via AMPK, NO and opioid signaling. Apelin may contribute to the IBS pathophysiology, and the inhibition of apelinergic signaling may be a promising therapeutic option for IBS.

Exerkine apelin reverses obesity-associated placental dysfunction by accelerating mitochondrial biogenesis in mice

Maternal exercise (ME) protects against adverse effects of maternal obesity (MO) on fetal development. As a cytokine stimulated by exercise, apelin (APN) is elevated due to ME, but its roles in mediating the effects of ME on placental development remain to be defined. Two studies were conducted. In the first study, 18 female mice were assigned to control (CON), obesogenic diet (OB), or OB with exercise (OB/Ex) groups (n = 6); in the second study, the same number of female mice were assigned to three groups; CON with PBS injection (CD/PBS), OB/PBS, or OB with apelin injection (OB/APN). In the exercise study, daily treadmill exercise during pregnancy significantly elevated the expression of PR domain 16 (PRDM16; P < 0.001), which correlated with enhanced oxidative metabolism and mitochondrial biogenesis in the placenta (P < 0.05). More importantly, these changes were partially mirrored in the apelin study. Apelin administration upregulated PRDM16 protein level (P < 0.001), mitochondrial biogenesis (P < 0.05), placental nutrient transporter expression (P < 0.001), and placental vascularization (P < 0.01), which were impaired due to MO (P < 0.05). In summary, MO impairs oxidative phosphorylation in the placenta, which is improved by ME; apelin administration partially mimics the beneficial effects of exercise on improving placental function, which prevents placental dysfunction due to MO.NEW & NOTEWORTHY Maternal exercise prevents metabolic disorders of mothers and offspring induced by high-fat diet. Exercise intervention enhances PRDM16 activation, oxidative metabolism, and vascularization of placenta, which are inhibited due to maternal obesity. Similar to maternal exercise, apelin administration improves placental function of obese dams.

Apelin Alleviates Meniscus Endothelial Cell Apoptosis in Osteoarthritis

Osteoarthritis (OA) is a degenerative disease characterized by articular cartilage and/or chondrocyte destruction, and although it has long been considered as a primary disease, the importance of meniscus endothelial cell modulation in the subchondral microenvironment has recently drawn attention. Previous studies have shown that apelin could potentially inhibit cellular apoptosis; however, it remains unclear whether apelin could play a protective role in protecting the endothelium in the OA meniscus. In this study, with the advantages of single-cell RNA sequencing (scRNA-seq) data, in combination with flow cytometry, we identified two endothelial subclusters in the meniscus, featured by high expression of Homeobox A13 (HOXA13) and Ras Protein-Specific Guanine Nucleotide Releasing Factor 2 (RASGRF2), respectively. Compared with control patients, both subclusters decreased in absolute cell numbers and exhibited downregulated APJ endogenous ligand (APLN, coding for apelin) and upregulated apelin receptor (APLNR, coding apelin receptor). Furthermore, we confirmed that in OA, decreased endothelial cell numbers, including both subclusters, were related to intrinsic apoptosis factors: one more relevant to caspase 3 (CASP3) and the other to BH3-Interacting Domain Death agonist (BID). In vitro culturing of meniscal endothelial cells purified from patients proved that apelin could significantly inhibit apoptosis by downregulating these two factors in endothelial cell subclusters, suggesting that apelin could potentially serve as a therapeutic target for patients with OA.

Apelin and Apelin Receptor in Follicular Granulosa Cells of Buffalo Ovaries: Expression and Regulation of Steroidogenesis

Apelin (APLN), as a ligand for APJ (an orphan G-protein-coupled receptor), is an adipokine with pleiotropic effects in many physiological processes of the body. It has an important role in the control of reproduction particularly in females (mainly in control of ovarian function). This study was carried out to investigate the mRNA and protein amounts of APLN/APJ in granulose cells (GCs) of ovarian follicles with small (SF), medium (MF), and large (LF) sizes of buffalo (Bubalus bubalis) and the effect of IGF1 and follicle-stimulating hormone (FSH) on the expression levels of APLN/APJ. In addition, we evaluated the effect of various doses of APLN (isoforms -13 and -17) singly or in combination with IGF1 and FSH on estradiol (E2) and progesterone (P4) secretion in GCs. The mRNA and protein abundance of APLN was the highest in GCs of LF while the APJ expression enhanced with follicle enlargement in GCs (p-value <0.01). IGF1 and FSH elevated the mRNA and protein amounts of APLN and FSH, and IGF1 increased the expression of APJ in buffalo GCs (p-value <0.01). Both isoforms of APLN (-13/-17) singly or in the presence of IGF1 or FSH increased the secretion of E2 and P4 with or without preincubation of cells with APJ antagonist (ML221 10 µM), although we had some variation in the effects. Concurrently, APLN-13/-17 significantly increased the mRNA and protein expression of CYP19A1 and StAR (p-value <0.01). ML221 substantially diminished the secretion of E2 and P4 and also the expression of CY19A1 and StAR in buffalo GCs (p-value <0.01). We also revealed that APLN-13/-17 (10^-9 M), singly or in response to IGF1 and FSH, increased the production of E2 and P4 in different times of stimulation. In conclusion, APLN may play a crucial role in steroidogenesis and follicular development in ovarian GCs of buffalo.

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 improves cardiac function mainly through peripheral vasodilation in a mouse model of dilated cardiomyopathy

There is growing evidence that apelin plays a role in the regulation of the cardiovascular system by increasing myocardial contractility and acting as a vasodilator. However, it remains unclear whether apelin improves cardiac contractility in a load-dependent or independent manner in pathological conditions. For this purpose we investigated the cardiovascular effects of apelin in α-actin transgenic mice (mActin-Tg mice), a model of cardiomyopathy. [Pyr¹]apelin-13 was administered by continuous infusion at 2 mg/kg/d for 3 weeks. Effects on cardiac function were determined by echocardiography and a Pressure-Volume (PV) analysis. mActin-Tg mice showed a dilated cardiomyopathy (DCM) phenotype similar to that encountered in patients expressing the same mutation. Compared to WT animals, mActin-Tg mice displayed cardiac systolic impairment [significant decrease in ejection fraction (EF), cardiac output (CO), and stroke volume (SV)] associated with cardiac ventricular dilation and diastolic dysfunction, characterized by an impairment in mitral flow velocity (E/A) and in deceleration time (DT). Load-independent myocardial contractility was strongly decreased in mActin-Tg mice while total peripheral vascular resistance (TPR) was significantly increased. As compared to vehicle-treated animals, a 3-week treatment with [Pyr¹]apelin-13 significantly improved EF%, SV, E/A, DT and corrected TPR, with no significant effect on load-independent indices of myocardial contractility, blood pressure and heart rate. In conclusion [Pyr¹]apelin-13 displayed no intrinsic contractile effect but improved cardiac function in dilated cardiomyopathy mainly by reducing peripheral vascular resistance, with no change in blood pressure.

Gαi-biased apelin analog protects against isoproterenol-induced myocardial dysfunction in rats

Apelin receptor (APJ) activation by apelin-13 (APLN-13) engages both Gαi proteins and β-arrestins, stimulating distinct intracellular pathways and triggering physiological responses like enhanced cardiac contractility. Substituting the C-terminal phenylalanine of APLN-13 with α-methyl-l-phenylalanine [(l-α-Me)Phe] or p-benzoyl-l-phenylalanine (Bpa) generates biased analogs inducing APJ functional selectivity toward Gαi proteins. Using these original analogs, we proposed to investigate how the canonical Gαi signaling of APJ regulates the cardiac function and to assess their therapeutic impact in a rat model of isoproterenol-induced myocardial dysfunction. In vivo and ex vivo infusions of either Bpa or (l-α-Me)Phe analogs failed to enhance rats' left ventricular (LV) contractility compared with APLN-13. Inhibition of Gαi with pertussis toxin injection optimized the cardiotropic effect of APLN-13 and revealed the inotropic impact of Bpa. Moreover, both APLN-13 and Bpa efficiently limited the forskolin-induced and PKA-dependent phosphorylation of phospholamban at the Ser16 in neonatal rat ventricular myocytes. However, only Bpa significantly reduced the inotropic effect of forskolin infusion in isolated-perfused heart, highlighting its efficient bias toward Gαi. Compared with APLN-13, Bpa also markedly improved isoproterenol-induced myocardial systolic and diastolic dysfunctions. Bpa prevented cardiac weight increase, normalized both ANP and BNP mRNA expressions, and decreased LV fibrosis in isoproterenol-treated rats. Our results show that APJ-driven Gαi/adenylyl cyclase signaling is functional in cardiomyocytes and acts as negative feedback of the APLN-APJ-dependent inotropic response. Biased APJ signaling toward Gαi over the β-arrestin pathway offers a promising strategy in the treatment of cardiovascular diseases related to myocardial hypertrophy and high catecholamine levels.NEW & NOTEWORTHY By using more potent Gαi-biased APJ agonists that strongly inhibit cAMP production, these data point to the negative inotropic effect of APJ-mediated Gαi signaling in the heart and highlight the potential protective impact of APJ-dependent Gαi signaling in cardiovascular diseases associated with left ventricular hypertrophy.

Apelin/APJ-Manipulated CaMKK/AMPK/GSK3β Signaling Works as an Endogenous Counterinjury Mechanism in Promoting the Vitality of Random-Pattern Skin Flaps

A random-pattern skin flap plays an important role in the field of wound repair; the mechanisms that influence the survival of random-pattern skin flaps have been extensively studied but little attention has been paid to endogenous counterinjury substances and mechanism. Previous reports reveal that the apelin-APJ axis is an endogenous counterinjury mechanism that has considerable function in protecting against infection, inflammation, oxidative stress, necrosis, and apoptosis in various organs. As an in vivo study, our study proved that the apelin/APJ axis protected the skin flap by alleviating vascular oxidative stress and the apelin/APJ axis works as an antioxidant stress factor dependent on CaMKK/AMPK/GSK3β signaling. In addition, the apelin/APJ-manipulated CaMKK/AMPK/GSK3β-dependent mechanism improves HUVECs' resistance to oxygen and glucose deprivation/reperfusion (OGD/R), reduces ROS production and accumulation, maintained the normal mitochondrial membrane potential, and suppresses oxidative stress in vitro. Besides, activation of the apelin/APJ axis promotes vascular migration and angiogenesis under relative hypoxia condition through CaMKK/AMPK/GSK3β signaling. In a word, we provide new evidence that the apelin/APJ axis is an effective antioxidant and can significantly improve the vitality of random flaps, so it has potential be a promising clinical treatment.

Maternal Inactivity Programs Skeletal Muscle Dysfunction in Offspring Mice by Attenuating Apelin Signaling and Mitochondrial Biogenesis

Although maternal exercise (ME) becomes increasingly uncommon, the effects of ME on offspring muscle metabolic health remain largely undefined. Maternal mice are subject to daily exercise during pregnancy, which enhances mitochondrial biogenesis during fetal muscle development; this is correlated with higher mitochondrial content and oxidative muscle fibers in offspring muscle and improved endurance capacity. Apelin, an exerkine, is elevated due to ME, and maternal apelin administration mirrors the effect of ME on mitochondrial biogenesis in fetal muscle. Importantly, both ME and apelin induce DNA demethylation of the peroxisome proliferator-activated receptor γ coactivator-1α (Ppargc1a) promoter and enhance its expression and mitochondrial biogenesis in fetal muscle. Such changes in DNA methylation were maintained in offspring, with ME offspring muscle expressing higher levels of PGC-1α1/4 isoforms, explaining improved muscle function. In summary, ME enhances DNA demethylation of the Ppargc1a promoter in fetal muscle, which has positive programming effects on the exercise endurance capacity and protects offspring muscle against metabolic dysfunction.

Son et al. demonstrate that maternal exercise facilitates fetal muscle development, which improves muscle function and exercise endurance in offspring. Maternal administration of apelin, an exerkine, mirrors the beneficial effects of maternal exercise on mitochondrial biogenesis and fetal muscle development. These findings suggest apelin and its receptor as potential drug targets for improving fetal muscle development of sedentary mothers.

Apelin inhibits an endothelium-derived hyperpolarizing factor-like pathway in rat cerebral arteries

Apelin has complex vasomotor actions inasmuch as the peptide may cause either vasodilation or vasoconstriction depending on the vascular bed and experimental conditions. In cerebral arteries, apelin inhibits endothelium-dependent relaxations mediated by nitric oxide (NO); however, its effects on relaxation to other endothelium-derived substances (e.g. prostacyclin, endothelium-derived hyperpolarizing factors(s) (EDHF)) are unknown. The present study was designed to determine effects of apelin on endothelium-dependent relaxations that are independent of NO in rat cerebral arteries. In arterial rings contracted with 5-HT, A23187 caused endothelium-dependent relaxation that was unaffected by inhibitors of eNOS, guanylyl cyclase or cyclooxygenase, but was attenuated by MS-PPOH, a selective inhibitor of cytochrome P450 catalyzed synthesis of epoxyeicosatrienoic acids (EETs) and by 14,15-EE(Z)E, an EET-receptor antagonist. Apelin inhibited A23187-induced relaxation, as well as relaxations evoked by exogenous 11,12- and 14,15-EET. These effects of apelin were mimicked by the selective BK_Ca channel blocker, iberiotoxin. The APJ receptor antagonist, F13A abolished the effects of apelin on A23187-induced relaxations. Both 11,12- and 14,15-EET also increased BK_Ca channel current density in isolated cerebral artery smooth muscle cells, effects that were inhibited in a similar manner by apelin and iberiotoxin. These findings provide evidence that apelin impairs endothelium-dependent relaxation of cerebral arteries by inhibiting an NO-independent pathway (i.e. "EDHF-like") involving activation of smooth muscle cell BK_Ca channels by endothelium-derived EETs. Inhibition of such pathway may create an environment favoring vasoconstriction in cerebral arteries.

Apelin attenuates streptozotocin-induced learning and memory impairment by modulating necroptosis signaling pathway

Apelin is a neuropeptide that plays an important role in neuronal protection. In this study, we investigated the effects of apelin intracerebroventricular administration on spatial learning and memory-related behaviors, and necroptosis signaling pathways in the hippocampus of streptozotocin (STZ) -injected rats. Apelin treatment was implemented following STZ-induced dementia for 15 days. After conducting a behavioral test (Morris Water Maze), the cellular and molecular aspects were examined to detect the apelin effect on the necroptosis signaling pathway. We demonstrated that STZ administration significantly slowed down the learning capability. However apelin treatment notably reversed this neuroinflammation induced behavioral impairment. Furthermore, molecular investigations showed that apelin treatment reduced the hippocampal RIP1, RIP3, and TNF-α level. Our results suggest that apelin treatment attenuates STZ-induced dementia. This effect may be mediated by inhibition of the necroptosis signaling pathway which seems to be associated with the ability of apelin to reduce central TNF-α level. This data provides evidence of the neuroprotective effect of apelin on STZ-induced learning and memory impairment and characterize some of the underlying mechanisms.

Apelin peptides linked to anti-serum albumin domain antibodies retain affinity in vitro and are efficacious receptor agonists in vivo

The apelin receptor is a potential target in the treatment of heart failure and pulmonary arterial hypertension where levels of endogenous apelin peptides are reduced but significant receptor levels remain. Our aim was to characterise the pharmacology of a modified peptide agonist, MM202, designed to have high affinity for the apelin receptor and resistance to peptidase degradation and linked to an anti-serum albumin domain antibody (AlbudAb) to extend half-life in the blood. In competition, binding experiments in human heart MM202-AlbudAb (pKi  = 9.39 ± 0.09) bound with similar high affinity as the endogenous peptides [Pyr1 ]apelin-13 (pKi  = 8.83 ± 0.06) and apelin-17 (pKi  = 9.57 ± 0.08). [Pyr1 ]apelin-13 was tenfold more potent in the cAMP (pD2  = 9.52 ± 0.05) compared to the β-arrestin (pD2  = 8.53 ± 0.03) assay, whereas apelin-17 (pD2  = 10.31 ± 0.28; pD2  = 10.15 ± 0.13, respectively) and MM202-AlbudAb (pD2  = 9.15 ± 0.12; pD2  = 9.26 ± 0.03, respectively) were equipotent in both assays, with MM202-AlbudAb tenfold less potent than apelin-17. MM202-AlbudAb bound to immobilised human serum albumin with high affinity (pKD  = 9.02). In anaesthetised, male Sprague Dawley rats, MM202-AlbudAb (5 nmol, n = 15) significantly reduced left ventricular systolic pressure by 6.61 ± 1.46 mm Hg and systolic arterial pressure by 14.12 ± 3.35 mm Hg and significantly increased cardiac contractility by 533 ± 170 mm Hg/s, cardiac output by 1277 ± 190 RVU/min, stroke volume by 3.09 ± 0.47 RVU and heart rate by 4.64 ± 2.24 bpm. This study demonstrates that conjugating an apelin mimetic peptide to the AlbudAb structure retains receptor and in vivo activity and may be a new strategy for development of apelin peptides as therapeutic agents.

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.

Apelin Affects the Progression of Osteoarthritis by Regulating VEGF-Dependent Angiogenesis and miR-150-5p Expression in Human Synovial Fibroblasts

Synovium-induced angiogenesis is central to osteoarthritis (OA) pathogenesis and thus a promising therapeutic target. The adipokine apelin (APLN) is involved in both OA pathogenesis and angiogenesis. We examined the role of APLN in synovium-induced angiogenesis by investigating the crosstalk between APLN and vascular endothelial growth factor (VEGF) expression in human OA synovial fibroblasts (OASFs). We found higher levels of APLN and VEGF expression in OA samples compared with normal samples. APLN-induced stimulation of VEGF expression and VEGF-dependent angiogenesis in OASFs was mitigated by FAK/Src/Akt signaling. APLN also inhibited levels of microRNA-150-5p (miR-150-5p), which represses VEGF production and angiogenesis. Analyses of an OA animal model showed that shAPLN transfection of OASFs rescued pathologic changes in OA cartilage and histology. Here, we found APLN enhances VEGF expression and angiogenesis via FAK/Src/Akt cascade and via downstream suppression of miR-150-5p expression. These findings help to clarify the pathogenesis of adipokine-induced angiogenesis in OA synovium.

[Preventive and Therapeutic Effects of Exogenous Apelin Regulating Autophagy on the Formation of Pulmonary Artery Hhypertension in Rats]

OBJECTIVE

To investigate the effect of exogenous Apelin on pulmonary artery hypertension (PAH) and its related mechanism.

METHODS

26 male SD rats were randomly divided into Control group ( n=6), Model group ( n=10) and Intervention group ( n=10). The rat model of PAH was established by left pneumonectomy combined with monocrotaline injection (PE+MCT) in the Model group and the Intervention group, while the Control group rats were opened chest cavity and injected the same amount of normal saline. From the 2nd week after operation, the Intervention group was intraperitoneally injected with 10 nmol/(kg·d) Apelin-13 for 3 weeks, while the Control group and Model group were injected the same volume of normal saline. The mean pulmonary arterial pressure (mPAP) was measured and the right ventricular hypertrophy index ( RVHI) was calculated in all three groups of rats at the 5th week after operation. The pulmonary tissue HE staining was performed to observe the pulmonary tissue and pulmonary vascular morphology. Protein LC3 was detected by immunofluorescence staining of lung tissues, the mRNA expression level of P62 and Beclin-1 in lung tissues was measured by RT-PCR, and the protein expressions of LC3, LC3-Ⅱ/LC3-Ⅰ, P62 and Beclin-1 in lung tissues were measured by Western blot.

RESULTS

Compared with the Control group, the Model group showed increased mPAP and RVHI ( P<0.05), disordered pulmonary tissue structure and thicker pulmonary vascular wall. In Model group rats, expression of LC3 protein and LC3-Ⅱ/LC3-Ⅰ increased in lung tissues, and the expression of Beclin-1 mRNA and the Beclin-1 protein also increased in lung tissues, while the level of P62 mRNA and the expression of P62 protein decreased ( P<0.05). After Apelin-13 intervention, the above indexes were all improved ( P<0.05, compared with the Model group).

CONCLUSION

Exogenous Apelin has a certain preventive and therapeutic effect on the formation of PAH, and the mechanism may be related to its inhibition effect on autophagy.

Using Apelin and exercise to protect the cardiac cells: synergic effect in ischemia reperfusion injuries treatment in rats

AIM

Apelin is an active endogenous peptide, which affects blood vessels. Also exercise increases angiogenesis after myocardial infarction and exerts cardio protective effects. The aim of the present study was to investigate the effect of Apelin and aerobic exercise on reducing the severity of Ischemia-reperfusion injury in rats.

METHODS

The rats were divided into the following 4 groups 8 weeks before surgery (Langendorff model of perfusion): I) Ischemia-reperfusion (I/R), II: Exercise Ischemia- reperfusion (EX+I/R), III: Apelin+Ischemia-reperfusion (APE+I/R) (Apelin 10 nmol/kg/day, i.p), and IV: Exercise+Apelin+Ischemia-reperfusion (EX+APE+I/R). Exercise was performed on a treadmill 8 weeks before the surgery at a speed of 17 m/min for 10 to 50 min/day. The ventricular function was evaluated after I/R injury, histopathological and immunohistopathology indices were then measured at the scar tissue.

RESULTS

The results of H(et)E, Masson's trichrome staining indicated that APE+EX pre-treatment reduced cardiac fibrosis and the percentage of collagen deposition. It also enhanced the microvessels density (MVD) and decreased the number of inflammatory cells and apoptosis rate.

CONCLUSION

According to our study, Apelin and exercise preconditioning had anti-fibrotic and anti-apoptotic effects on the ischemia-reperfusion myocardium cells, which could lead to the protection of cardiac cells (Tab. 5, Fig. 3, Ref. 33).

Using Apelin and exercise to protect the cardiac cells: synergic effect in ischemia reperfusion injuries treatment in rats

AIM

Apelin is an active endogenous peptide, which affects blood vessels. Also exercise increases angiogenesis after myocardial infarction and exerts cardio protective effects. The aim of the present study was to investigate the effect of Apelin and aerobic exercise on reducing the severity of Ischemia-reperfusion injury in rats.

METHODS

The rats were divided into the following 4 groups 8 weeks before surgery (Langendorff model of perfusion): I) Ischemia-reperfusion (I/R), II: Exercise Ischemia- reperfusion (EX+I/R), III: Apelin+Ischemia-reperfusion (APE+I/R) (Apelin 10 nmol/kg/day, i.p), and IV: Exercise+Apelin+Ischemia-reperfusion (EX+APE+I/R). Exercise was performed on a treadmill 8 weeks before the surgery at a speed of 17 m/min for 10 to 50 min/day. The ventricular function was evaluated after I/R injury, histopathological and immunohistopathology indices were then measured at the scar tissue.

RESULTS

The results of H(et)E, Masson's trichrome staining indicated that APE+EX pre-treatment reduced cardiac fibrosis and the percentage of collagen deposition. It also enhanced the microvessels density (MVD) and decreased the number of inflammatory cells and apoptosis rate.

CONCLUSION

According to our study, Apelin and exercise preconditioning had anti-fibrotic and anti-apoptotic effects on the ischemia-reperfusion myocardium cells, which could lead to the protection of cardiac cells (Tab. 5, Fig. 3, Ref. 33).

Superoxide anions modulate the performance of apelin in the paraventricular nucleus on sympathetic activity and blood pressure in spontaneously hypertensive rats

The present study was designed to determine how apelin in paraventricular nucleus (PVN) modulates the renal sympathetic nerve activity (RSNA), arterial blood pressure (ABP), mean arterial pressure (MAP), and heart rate (HR), and whether superoxide anions regulate the performance of PVN apelin in spontaneously hypertensive rats (SHRs). Acute experiment was carried out with 13-week-old male Wistar-Kyoto rats (WKY) and SHRs under anaesthesia. RSNA, ABP, MAP and HR after PVN microinjection were measured. Apelin microinjection into PVN increased RSNA, ABP, MAP and HR in WKY rats and SHRs, more obviously in SHRs. APJ antagonist F13A decreased the RSNA, ABP, MAP and HR in SHRs, and inhibited the effects of apelin. Apelin and APJ mRNA levels were higher in the PVN in SHRs. PVN microinjection of superoxide anion scavengers tempol and tiron, or NAD(P)H oxidase inhibitor apocynin, decreased the RSNA, ABP, MAP and HR in SHRs, and inhibited the effects of apelin, but the superoxide dismutase (SOD) inhibitor diethyldithiocarbamic acid (DETC) potentiated the effects of apelin. NAD(P)H oxidase activity and superoxide anion levels in PVN were increased by apelin, but decreased by APJ antagonist F13A. The apelin-induced increases in NAD(P)H oxidase activity and superoxide anion level were abolished by pre-treatment with F13A. These results indicate that apelin in PVN increases the sympathetic outflow and blood pressure via activating APJ receptor. The enhanced activity of endogenous apelin and APJ receptor in PVN contributes to sympathetic activation in hypertension, and the superoxide anion is involved in these apelin-mediated processes in PVN.

Apelin-36-[L28A] and Apelin-36-[L28C(30kDa-PEG)] peptides that improve diet induced obesity are G protein biased ligands at the apelin receptor

BACKGROUND

Apelin signalling pathways have important cardiovascular and metabolic functions. Recently, apelin-36-[L28A] and apelin-36-[L28C(30kDa-PEG)], were reported to function independent of the apelin receptor in vivo to produce beneficial metabolic effects without modulating blood pressure. We aimed to show that these peptides bound to the apelin receptor and to further characterise their pharmacology in vitro at the human apelin receptor.

METHODS

[Pyr¹]apelin-13 saturation binding experiments and competition binding experiments were performed in rat and human heart homogenates using [¹²⁵I]apelin-13 (0.1 nM), and/or increasing concentrations of apelin-36, apelin-36-[L28A] and apelin-36-[L28C(30kDa-PEG)] (50pM-100μM). Apelin-36 and its analogues apelin-36-[F36A], apelin-36-[L28A], apelin-36-[L28C(30kDa-PEG)], apelin-36-[A28 A13] and [40kDa-PEG]-apelin-36 were tested in forskolin-induced cAMP inhibition and β-arrestin assays in CHO-K1 cells heterologously expressing the human apelin receptor. Bias signaling was quantified using the operational model for bias.

RESULTS

In both species, [Pyr¹]apelin-13 had comparable subnanomolar affinity and the apelin receptor density was similar. Apelin-36, apelin-36-[L28A] and apelin-36-[L28C(30kDa-PEG)] competed for binding of [¹²⁵I]apelin-13 with nanomolar affinities. Apelin-36-[L28A] and apelin-36-[L28C(30kDa-PEG)] inhibited forskolin-induced cAMP release, with nanomolar potencies but they were less potent compared to apelin-36 at recruiting β-arrestin. Bias analysis suggested that these peptides were G protein biased. Additionally, [40kDa-PEG]-apelin-36 and apelin-36-[F36A] retained nanomolar potencies in both cAMP and β-arrestin assays whilst apelin-36-[A13 A28] exhibited a similar profile to apelin-36-[L28C(30kDa-PEG)] in the β-arrestin assay but was more potent in the cAMP assay.

CONCLUSIONS

Apelin-36-[L28A] and apelin-36-[L28C(30kDa-PEG)] are G protein biased ligands of the apelin receptor, suggesting that the apelin receptor is an important therapeutic target in metabolic diseases.

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

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

Apelin attenuates depressive-like behavior and neuroinflammation in rats co-treated with chronic stress and lipopolysaccharide

Several experimental studies have proved that activation of neuroinflammation pathways may contribute to the development of depression, a neuropsychiatric disorder disease. Our previous studies have shown the antidepressant properties of apelin, but the mechanism was unkown. This study was performed to verify whether the antidepressant effect of apelin was related to its anti-inflammation effect in the central nervous system. To achieve our aim, we selected the co-treatment of chronic stress and LPS to induced an inflammatory process in rats. The effect of this co-treatment was evaluated through the expression of inflammatory markers and glial cell activation. LPS injection co-treated with unpredictable chronic mild stress resulted in the activation of microglial cell and astrocyte, expression of inflammatory markers and depressive behaviors. Treatment with apelin significantly attenuates the deleterious effects in these rats. Our results showed that apelin improved depressive phenotype and decreased the activation of glial cells in stress co-treatment group. The down-regulations of p-NF-κB and p-IKKβ suggested that the effects are possibly mediated by inhibition of the NF-κB-mediated inflammatory response. These findings speculated that intracerebroventricular injection of apelin could be a therapeutic approach for the treatment of depression, and the antidepressant function of apelin may closely associated with its alleviation in neuroinflammation.

The effects of apelin on myometrium contractions in pregnant rats

Apelin, which is a new hormone, is secreted especially in the brain by hypothalamus as well as by many other organs like the stomach, fat tissue, and the heart. For apelin, whose possible effects on many bodily functions like the endocrine system, cardiovascular system and metabolic activities are still under investigation, the reproductive system is also an important target area. The purpose of the present study was to investigate the effects of plasma apelin levels in rats that were in diestrus, pregnancy and lactation periods, and to examine its possible effects on myometrium contractions of pregnant rats and non-pregnant rats that were in diestrus period.  The plasma apelin concentrations in female adult Wistar rats were determined with the ELISA method in the diestrus period, and on the 12th, 18th, and 21st days of the pregnancy, and on the 2nd and 10th days of lactation (n=7 for each group). In addition, the effect of apelin at 0.01, 0.1, 1 and 10 μM doses on isometric contractions in the rat uterus on the 21st day of pregnancy and in diestrus period was tested by using isolated organ bath. This protocol was repeated under conditions that were pre-treated with protein kinase C inhibitor in calcium-free medium, and the possible effect of apelin on contractions and the mechanisms that might mediate this effect were investigated. When plasma apelin levels were compared with the rats in diestrus period, the apelin concentrations in the 21-day pregnancy group were high at a significant level (p&lt;0.05); and low at a significant level in the 2-day lactation group (p&lt;0.05). In myometrium contraction trials, it was observed that apelin induced the contractions. Apelin increased the frequency of the myometrium contractions at a significant level when applied at 1 μM and 10 μM concentrations (p&lt;0.05 and p&lt;0.001). Only after the apelin application at 10 μM concentration did the amplitude of the contractions increase at a significant level (p&lt;0.01). In the myometrium of the rats that were on the 21st day of pregnancy, the frequency of the contractions was statistically significant at 0.1 μM, 1 μM and 10 μM doses (p&lt;0.01). In addition, the amplitude of the contractions increased at a statistically significant level at 1 μM and 10 μM dose application (p&lt;0.05 and p&lt;0.01, respectively). The apelin application induced the contractions in calcium-free medium. When apelin was applied after the pre-application with protein kinase C inhibitor, no contractions were observed. The present study showed that apelin levels were increased at the end of pregnancy in rats, and the hormone induced the uterus contractions. This effect does not occur with protein kinase C inhibitor and in calcium-free medium, which shows that protein kinase C pathway might play a role in these mechanism.  These findings show that apelin might be an endogenous peptide that plays a role on uterine contractions at birth in rats.

Apelin and APJ orchestrate complex tissue-specific control of cardiomyocyte hypertrophy and contractility in the hypertrophy-heart failure transition

The G protein-coupled receptor APJ is a promising therapeutic target for heart failure. Constitutive deletion of APJ in the mouse is protective against the hypertrophy-heart failure transition via elimination of ligand-independent, β-arrestin-dependent stretch transduction. However, the cellular origin of this stretch transduction and the details of its interaction with apelin signaling remain unknown. We generated mice with conditional elimination of APJ in the endothelium (APJendo-/-) and myocardium (APJmyo-/-). No baseline difference was observed in left ventricular function in APJendo-/-, APJmyo-/-, or control (APJendo+/+, APJmyo+/+) mice. After exposure to transaortic constriction, APJendo-/- mice displayed decreased left ventricular systolic function and increased wall thickness, whereas APJmyo-/- mice were protected. At the cellular level, carbon fiber stretch of freshly isolated single cardiomyocytes demonstrated decreased contractile responses to stretch in APJ-/- cardiomyocytes compared with APJ+/+ cardiomyocytes. Ca2+ transients did not change with stretch in either APJ-/- or APJ+/+ cardiomyocytes. Application of apelin to APJ+/+ cardiomyocytes resulted in decreased Ca2+ transients. Furthermore, hearts of mice treated with apelin exhibited decreased phosphorylation in cardiac troponin I NH2-terminal residues (Ser22 and Ser23) consistent with increased Ca2+ sensitivity. These data establish that APJ stretch transduction is mediated specifically by myocardial APJ, that APJ is necessary for stretch-induced increases in contractility, and that apelin opposes APJ's stretch-mediated hypertrophy signaling by lowering Ca2+ transients while maintaining contractility through myofilament Ca2+ sensitization. These findings underscore apelin's unique potential as a therapeutic agent that can simultaneously support cardiac function and protect against the hypertrophy-heart failure transition. NEW & NOTEWORTHY These data address fundamental gaps in our understanding of apelin-APJ signaling in heart failure by localizing APJ's ligand-independent stretch sensing to the myocardium, identifying a novel mechanism of apelin-APJ inotropy via myofilament Ca2+ sensitization, and identifying potential mitigating effects of apelin in APJ stretch-induced hypertrophic signaling.

Activation of Large Conductance, Calcium-Activated Potassium Channels by Nitric Oxide Mediates Apelin-Induced Relaxation of Isolated Rat Coronary Arteries

Apelin increases coronary blood flow, cardiac contractility, and cardiac output. Based on these favorable hemodynamic effects, apelin and apelin-like analogs are being developed for treating heart failure and related disorders; however, the molecular mechanisms underlying apelin-induced coronary vasodilation are unknown. This study aimed to elucidate the signaling pathways by which apelin causes smooth muscle relaxation in coronary arteries. Receptors for apelin (APJ receptors) were expressed in coronary arteries, as determined by Western blot and polymerase chain reaction analyses. Immunofluorescence imaging studies identified APJ receptors on endothelial and smooth muscle cells. In isolated endothelial cells, apelin caused an increase in 4,5-diaminofluorescein fluorescence that was abolished by nitro-l-arginine (NLA) and F13A (H-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Ala-OH), an APJ receptor antagonist, consistent with increased nitric oxide (NO) production. In arterial rings, apelin caused endothelium-dependent relaxations that were abolished by NLA, F13A, and iberiotoxin. Neither oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) nor DT-2, a protein kinase G inhibitor, had any effect on apelin-induced relaxations, and apelin itself had no effect on intracellular cGMP accumulation in coronary arteries. Patch-clamp studies in isolated smooth muscle cells demonstrated that the NO donors, diethyl amine NONOate and sodium nitroprusside, caused increases in large conductance, calcium-activated potassium channel (BKCa) currents, which were inhibited by iberiotoxin but not ODQ. Thus, apelin causes endothelium-dependent relaxation of coronary arteries by stimulating endothelial APJ receptors and releasing NO, which acts in a cGMP-independent manner and increases BKCa activity in the underlying smooth muscle cells. These results provide a mechanistic basis for apelin-induced coronary vasodilation and may provide guidance for the future development of novel apelin-like therapeutic agents.

Apelin, a promising target for Alzheimer disease prevention and treatment

Alzheimer's disease (AD) is a progressive neurodegenerative disease with high outbreak rates. It is estimated that about 35 million individuals around the world suffered from dementia in 2010. AD is expected to increase twofold every 20 years and, by 2030, approximately 65 million people could suffer from this illness. AD is determined clinically by a cognitive impairment and pathologically by the production of amyloid beta (Aβ), neurofibrillary tangles, toxic free radicals and inflammatory mediators in the brain. There is still no treatment to cure or even alter the progressive course of this disease; however, many new therapies are being investigated and are at various stages of clinical trials. Neuropeptides are signaling molecules used by neurons to communicate with each other. One of the important neuropeptides is apelin, which can be isolated from bovine stomach. Apelin and its receptor APJ have been shown to broadly disseminate in the neurons and oligodendrocytes of the central nervous system. Apelin-13 is known to be the predominant neuropeptide in neuroprotection. It is involved in the processes of memory and learning as well as the prevention of neuronal damage. Studies have shown that apelin can directly or indirectly prevent the production of Aβ and reduce its amounts by increasing its degradation. Phosphorylation and accumulation of tau protein may also be inhibited by apelin. Apelin is considered as an anti-inflammatory agent by preventing the production of inflammatory mediators such as interleukin-1β and tumor necrosis factor alpha. It has been shown that in vivo and in vitro anti-apoptotic effects of apelin have prevented the death of neurons. In this review, we describe the various functions of apelin associated with AD and present an integrated overview of recent findings that, in general, recommend apelin as a promising therapeutic agent in the treatment of this ailment.

Chronic apelin treatment improves hepatic lipid metabolism in obese and insulin-resistant mice by an indirect mechanism

PURPOSE

Apelin treatment has been shown to improve insulin sensitivity in insulin resistant mice by acting in skeletal muscles. However, the effects of systemic apelin on the hepatic energy metabolism have not been addressed. We thus aimed to determine the effect of chronic apelin treatment on the hepatic lipid metabolism in insulin resistant mice. The apelin receptor (APJ) expression was also studied in this context since its regulation has only been reported in severe liver pathologies.

METHODS

Mice were fed a high-fat diet (HFD) in order to become obese and insulin resistant compared to chow fed mice (CD). HFD mice then received a daily intraperitoneal injection of apelin (0.1 µmol/kg) or PBS during 28 days.

RESULTS

Triglycerides content and the expression of different lipogenesis-related genes were significantly decreased in the liver of HFD apelin-treated compared to PBS-treated mice. Moreover, at this stage of insulin resistance, the beta-oxidation was increased in liver homogenates of HFD PBS-treated mice compared to CD mice and reduced in HFD apelin-treated mice. Finally, APJ expression was not up-regulated in the liver of insulin resistant mice. In isolated hepatocytes from chow and HFD fed mice, apelin did not induce significant effect.

CONCLUSIONS

Altogether, these results suggest that systemic apelin treatment decreases steatosis in insulin resistant mice without directly targeting hepatocytes.

Apelin attenuates TGF-β1-induced epithelial to mesenchymal transition via activation of PKC-ε in human renal tubular epithelial cells

Epithelial to mesenchymal transition (EMT), a process whereby fully differentiated epithelial cells transition to a mesenchymal phenotype, has been implicated in the pathogenesis of renal fibrosis. Apelin, a bioactive peptide, has recently been recognized to protect against renal profibrotic activity, but the underlying mechanism has not yet been elucidated. In this study, we investigated the regulation of EMT in the presence of apelin-13 in vitro. Expression of the mesenchymal marker alpha-smooth muscle actin (α-SMA) and the epithelial marker E-cadherin was examined by immunofluorescence and western blotting in transforming growth factor beta 1 (TGF-β1)-stimulated human proximal tubular epithelial cells. Expression of extracellular matrix, fibronectin and collagen-I was examined by quantitative real-time PCR and ELISA. F13A, an antagonist of the apelin receptor APJ, and small interfering RNA targeting protein kinase C epsilon (PKC-ε) were used to explore the relevant signaling pathways. Apelin attenuated TGF-β1-induced EMT, and inhibited the EMT-associated increase in α-SMA, loss of E-cadherin, and secretion of extracellular matrix. Moreover, apelin activated PKC-ε in tubular epithelial cells, which in turn decreased phospho-Smad2/3 levels and increased Smad-7 levels. APJ inhibition or PKC-ε deletion diminished apelin-induced modulation of Smad signaling and suppression of tubular EMT. Our findings identify a novel PKC-ε-dependent mechanism in which apelin suppresses TGF-β1-mediated activation of Smad signaling pathways and thereby inhibits tubular EMT. These results suggest that apelin may be a new agent that can suppress renal fibrosis and retard chronic kidney disease progression.

[Apelin attenuates hypoxia induced pulmonary hypertension of mice through regulation of lipid metabolism]

OBJECTIVE

To observe the role of apelin in the prevention of pulmonary hypertension induced by hypoxia in mice.

METHODS

Adult male apoE gene knockout (apoE-KO) mice were exposed to isobaric hypoxic chamber (9%~11% O₂, regular chow feed, 23 h/d)for 3 weeks to establish hypoxia-induced pulmonary hypertension. Thirty apoE-KO mice were randomly divided into normoxia group, hypoxia group and hypoxic with apelin (10 nmol/(kg·d), ip) group. The concentrations of high density lipoprotein (HDL), low density lipoprotein (LDL)and total cholesterol in plasma were detected by Elisa method. The mRNA levels of ATP-binding cassette transporter A1(ABCA1), low density lipoprotein receptor (LDLR), scavenger receptor class B1 (SR-B1), and HMG-CoA reductase (HMGCR)in liver were measured by real-time PCR. The protein level of peroxisome proliferators-activated receptor gamma (PPARγ) in lung was measured by Western blot.

RESULTS

①The right ventricular systolic pressure (RVSP) and the weight ratio of right ventricle (RV) to left ventricle plus septum (LV+S) of hypoxia group were significantly higher than those of normoxia group by 87% and 85% (P<0.05), respectively. RVSP and RV/(LV+S) of apelin group were significantly lower than those of hypoxia group by 39% and 33%(P<0.05), respectively. ②The plasma concentration of HDL and HDL/LDL of apelin group were significantly higher than those of hypoxia group by 21% and 20%(P<0.05), respectively. ③The mRNA levels of LDLR, SR-B1 and ABCA1 in liver of apelin group were significantly up-regulated than those of hypoxia group by 241%, 112% and 69% (P<0.05), respectively, while the mRNA level of HMGCR was down-regulated by 45% (P<0.05). ④The protein level of PPARγ in lung of apelin group was significantly up-regulated than that of hypoxia group by 47% (P<0.05).

CONCLUSIONS

Apelin attenuates hypoxia-induced pulmonary hypertension of mice through regulation of lipid metabolism.