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

Abstract

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

Introduction

APJ, separated by O'Dowd et al. [1], is a member of the seven-transmembrane G protein-coupled receptor (GPCR) family. Its amino acid sequence is highly homologous to angiotensin (Ang) II receptor type-1 receptor (AT₁R). Apelin, which is known as APLN, is an endogenous ligand of APJ [2] and its C-terminal is the specific binding region of the APJ receptor [3]. APLN encodes a 77 amino acid preproprotein (pre-apelin), which contains multiple potential processing enzyme cleavage sites and can be cleaved into several C-terminal bioactive peptides, such as apelin-36, apelin-17, apelin-13, and apelin-12 [2, 4]. Among these apelin isoforms, apelin-13 and apelin-36 account for the primary. In addition to apelin, Elabela/Toddler has recently been proved to be another endogenous ligand for APJ [5, 6]. More importantly, studies show that apelin, but not Ang II, binds APJ. Conversely, Ang II has a high affinity for AT₁R [1, 7].

In the process of signal transduction, apelin triggers intracellular signaling pathway through APJ activation. APJ, as a member of the GPCRs family, is mainly involved in regulating downstream signaling pathway by coupling G proteins. APJ is coupled to Gi/o after stimulation with apelin, which causes an inhibition of forskolin-stimulated cAMP production [8]. Moreover, apelin-13 activates extracellular-regulated kinases 1/2 (ERK1/2) but not p38 mitogen-activated protein kinase (MAPK), which involves the coupling of APJ to the Gi2 protein [9]. MAPK/p70S6 is also activated by apelin through coupling Go/Gi protein, which leads to mammalian cardiomyogenesis [10]. Apelin also stimulates APJ to activate Gαi and triggers a protective effect [11]. Besides, stimulation of APJ with apelin-13 also promotes Gαi2, Gαi3, Gαo, and Gαq activation [12]. Collectively, stimulation of APJ with apelin can elicit the coupling of APJ to G protein, thus subsequently activating downstream effectors.

Recently, accumulating evidence has indicated that apelin/APJ system is widely expressed in many tissues, including heart, blood vessels, lung, liver, brain, kidney, and so on [13–17]. Cumulatively, apelin/APJ system is implicated in regulating physiological processes of cardiovascular system, such as protecting heart from ischemia–reperfusion injury [18], maintaining positive inotropic effects [19], and lowering blood pressure [20, 21]. Moreover, apelin/APJ system plays an important role in the prevention of age-associated diseases [22, 23]. Additionally, apelin signaling aggravates acute liver injury by promoting hepatocellular steatosis [24], but alleviates renal fibrosis through decreasing the deposition of extracellular matrix [25]. Apelin is also strongly correlated with glucose tolerance and insulin sensitivity [26]. Altogether, apelin/APJ system participates in the regulation of pathophysiological processes with pleiotropic actions.

Till date, apelin/APJ system in respiratory diseases has become increasingly important as its roles are being gradually realized. Apelin inhibits vasoconstriction induced by endothelin-1 and AngII in pulmonary arteries from normoxic rats, but not in hypoxic rats [27], suggesting that the inhibitory effect of apelin on vasoconstriction is closely related to oxygen content. More importantly, microinjection of apelin-13 into the nucleus tractus solitarius causes apnea and inhibits phrenic nerve activity. Conversely, phrenic nerve discharge amplitude is notably increased after microinjection of apelin-13 into the rostral ventrolateral medulla [28]. Besides, exogenous administration of apelin-13 inhibits hypoxia-induced proliferation, migration, and autophagy of pulmonary artery vascular smooth muscle cells [29]. Herein, the regulatory role of apelin/APJ system in respiratory diseases is reviewed, and its novel therapeutic strategy for respiratory diseases is required to be explored.

Expression of apelin/APJ system in respiratory system

Particularly, in lung tissue, co-expression of apelin and APJ mRNA is the highest [30, 31]. High concentration of immunoreactive apelin is also detected in the lung [32]. Furthermore, apelin-like immunoreactivity in lung is only expressed in small pulmonary vascular endothelial cells [33], and this phenomenon is also observed by Goetze. Meanwhile, they also found that apelin mRNA is expressed in pulmonary macrovascular endothelial cells [34]. Notably, apelin, as a proinflammatory cytokine, is synthesized and secreted by adipose tissue, and plays an important role in regulating inflammatory and immune responses [35]. A plethora of evidence proves that most respiratory diseases show a strong inflammatory response, suggesting that apelin/APJ system may be a vital regulator in respiratory system.

Indeed, growing studies have emphasized the importance of apelin/APJ system in respiratory system. In addition to the enrichment of apelin/APJ in lung tissue, potent evidence has also confirmed that apelin/APJ signaling is disrupted in pulmonary vasculature of nitrofen-induced congenital diaphragmatic hernia [36]. Additionally, apelin level is significantly elevated in lung tissue and plasma in patients with acute respiratory distress syndrome (ARDS) [37], but reduced in patients with pulmonary arterial hypertension (PAH) [38]. Serum levels of apelin in hemodialysis patients with PAH are dramatically lower than those with normal arterial pressure, and this condition is not affected by hemodialysis [39]. Collectively, these results reveal that apelin/APJ system is responsible for the development of respiratory diseases.

The role of apelin/APJ system in respiratory system

Apelin/APJ system alleviates pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an aggressive disease, which is featured as pulmonary vascular remodeling, pulmonary vasoconstriction, and abnormal angiogenesis. Aberrant proliferation of pulmonary artery endothelial cells (PAECs) and pulmonary artery smooth muscle cells (PASMCs) is the leading cause of vascular remodeling [40]. Importantly, vascular plexiform lesions, as the primary hallmark of severe PAH, are also resulted from the aberrant proliferation of PAECs and PASMCs. Among numerous risk factors of PAH, hypoxia accounts for one [41]. Intriguingly, apelin and APJ expression are upregulated after 1 week of hypoxia [42], and return to baseline levels in response to 3 weeks of hypoxia [43]. In addition, bone morphogenetic protein receptor 2 (BMPR2) is also closely correlated with the development of PAH [44, 45]. Gene mutation of BMPR2 is detected in 70% of patients with heritable PAH and 15–40% of patients with idiopathic PAH [46]. Study shows that BMP-2-mediated EC survival is dependent on the formation of PPARγ/β–catenin complex. Meanwhile, apelin is confirmed as a transcriptional target of the PPARγ/β–catenin complex. Inhibition of BMPR2, PPARγ, or β-catenin reduces apelin expression. Apelin deficiency in PAECs leads to PASMC proliferation. Treatment with apelin attenuates PAH caused by PPARγ deletion [47]. Therefore, BMPR2/PPARγ/β-catenin/apelin might be a pivotal pharmacological target for the treatment of PAH.

As mentioned previously, the occurrence of PAH is attribute to PAECs dysfunction [48]. Dysregulated PAECs is considered to be at least in part relevant with reduced nitric oxide (NO) production [49]. eNOS catalyzes the conversion of l-arginine to citrulline, producing NO. Patients with PAH show a decrease in eNOS [43], which leads to endothelial inflammation [50]. Attractively, apelin/APJ system participates in maintaining endothelial homeostasis through modulating the expression of Kruppel-like factor 2 (KLF2) and eNOS [43, 51]. In addition, KLF2, as an intermediator, regulates the expression of eNOS [52] and can be modulated by AMP-activated kinase (AMPK) [53]. Furthermore, activation of AMPKα1 also enhances eNOS phosphorylation [54]. Interestingly, apelin-13 stimulates vascular smooth muscle cell proliferation and aggravates atherosclerotic lesions via promoting AMPKα phosphorylation and subsequent PINK1/Parkin-dependent mitophagy [55]. Altogether, apelin/APJ signaling activates AMPK-KLF2-eNOS-NO axis to restore PAECs function and maintain pulmonary vascular homeostasis (Fig. 1).

Fig. 1.

Apelin/APJ system attenuates PAH. Apelin inhibits the expression of FGF2 and FGFR1 by promoting miR424/503 expression to attenuate PAH (adapted from Ref. [57]). Apelin induces the production of NO by activating AMPK–KLF2–eNOS signaling pathway to alleviate PAH (adapted from Ref. [43, 51–54]). FGF2 fibroblast growth factor 2, FGFR1 fibroblast growth factor receptor 1, AMPK AMP-activated kinase, KLF2 Kruppel-like factor 2, eNOS endothelial nitric oxide synthase, NO nitric oxide, PAH pulmonary arterial hypertension

Except for the central role of the above axis in PAH, on the other hand, microRNAs (miRNAs) are also strongly associated with the progression of PAH. For example, in monocrotaline-induced PAH rats, miR-371b-5p reduces PAECs apoptosis via PTEN/PI3K/Akt signaling [56]. Importantly, miR424 and miR503, as two key endothelial miRNAs, are both targeted by apelin/APJ signaling, and in turn regulate fibroblast growth factor 2 (FGF2) and fibroblast growth factor receptor 1 (FGFR1) expression. Downregulation of apelin, miR424, and miR503 is observed in PAECs of patients with PAH, which contributes to increased FGF and FGFR1 expression as well as hyperproliferation of PAECs and PASMCs. Restoring expression of miR424 and miR503 impedes FGF and FGFR1 expression and transforms hyperproliferative PAECs into quiescence, thus alleviating the severity of PAH. Accordingly, apelin-miR-424/503-FGF signaling pathway is indispensable for the maintenance of pulmonary vascular homeostasis [57] (Fig. 1). Surprisingly, it was later discovered that miR130/301 regulates apelin-miR424/503-FGF axis by targeting PPARγ. Upregulation of miR130/301 is responsible for hyperproliferation of PAECs in PAH. In detail, PPARγ expression is decreased in response to upregulated miR130/301, which in turn leads to subsequent apelin and miR424/503 reduction as well as FGF elevation [58]. Hence, studies based on apelin–miRNAs axis provide us with another unique perspective to attenuate or even reverse the progression of PAH.

In summary, diverse effectors, including KLF2, eNOS, and miR424/503, targeted by apelin/APJ system, participate in the pathogenesis of PAH. Once the balance of these signaling pathways disrupted, the quiescent PAECs and PASMCs would convert into proliferative, ultimately leading to the deterioration of PAH. Augmentation of apelin can efficiently reverse this adverse effect. Given the beneficial role of apelin/APJ system in PAH, targeting apelin/APJ system might be a potential therapeutic strategy, and provides us with a novel insight into the perturbations of apelin/APJ system in PAH.

Apelin/APJ system may predict pulmonary embolism

Pulmonary embolism (PE) is a pathological process of detached thrombus or other substances, such as tumor and air, which block the pulmonary artery or its branches. PE, is caused by stasis, endothelial dysfunction, and hypercoagulability, which is characterized by abrupt pulmonary hypertension, acute right-ventricular failure and hypoxia [59]. Currently, clearance of mechanical obstruction is an effective method for the treatment of PE. In recent years, apelin has been recognized as a new adipokine and widely applicated in cardiovascular disease research [60]. However, the exact role of apelin in PE remains to be fully elucidated.

Only a few studies have illustrated the relationship between apelin and PE, and in which alteration of apelin level in PE is inconsistent. There is no significant change in plasma apelin levels in acute pulmonary embolism (APE) [61], whereas Selimoglu Sen reported for the first time that the concentration of serum apelin-13 was raised in patients with PE [62]. Feng [63] even depicts the complex effect of apelin on vasomotor tone in a dog model of APE. After the formation of APE, the expression of apelin mRNA is significantly upregulated in lung tissue within several hours and decreased at 24 h. Besides, the expression of apelin protein did not show remarkable alteration in pulmonary arteries within 24 h after APE induction, but obviously elevated in the bronchial epithelial cells as early as 1 h and decreased at 24 h. In dogs with APE, intravenous injection of apelin reduces mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance (PVR), but the enhancement of MPAP and PVR is considered to result from the secretion of vasoactive substances during APE. Importantly, the vasoconstriction effect caused by vasoactive substances can be reversed by apelin-mediated NO production [63]. Indeed, apelin-induced vasodilator effect is associated with NO-dependent mechanism [64]. Mechanistically, NO against pulmonary vasoconstriction by mitigating PE-related hemodynamic changes [65]. Conversely, apelin inhibits NO-dependent relaxation via restraining the activation of large-conductance, calcium-activated K channel [66]. Apelin-induced phosphorylation of myosin light chains in vascular smooth muscle cell also promotes vasoconstriction [67]. Collectively, these results reveal a paradoxical effect of apelin on NO-induced relaxation, which is largely correlated with the complicated etiology of PE and the intricate regulation of apelin on vasomotor tone associated with vascular endothelial cells.

Herein, the function of apelin in PE is weakly clarified. Despite considerable studies which are accessible, more experiments are necessitated about the pathophysiologic effects of apelin. With growing cognition of apelin, we may be capable of developing novel apelinergic-based therapeutic targets, thereby uncovering the essential role of apelin in the pathogenesis of PE.

Apelin/APJ system attenuates acute lung injury/acute respiratory distress syndrome

Acute lung injury (ALI)/ARDS is commonly characterized by pulmonary edema, hypoxia as well as systemic inflammation. Due to the complex pathogenesis of ALI/ARDS, no breakthrough has been achieved in the treatment of ALI/ARDS. The pathogenesis of ALI/ARDS is mainly associated with the injury of pulmonary capillary endothelial cells and alveolar endothelial cells, the increase of vascular permeability, as well as the imbalance of oxidation/antioxidant and coagulation/anticoagulant system.

An abundance of evidence points to a role for apelin/APJ system in maintaining endothelial homeostasis and immune regulation as well as protecting lipids from oxidation [68–71]. Recently, apelin has been identified as one of the dynamical network biomarkers of ALI using high-throughput technologies [72]. Attractively, apelin/APJ system has been reported to be an endogenous anti-injury mechanism that protects against ARDS. Specifically, the expression of apelin and APJ is increased in response to oleic acid (OA)-induced ARDS. Disruption of apelin/APJ signaling exacerbates OA-induced ARDS, and treatment with apelin-13 ameliorates OA-induced lung inflammation and injury. The above study suggests that apelin/APJ system is activated during ARDS to protect lung from inflammation and injury response [37]. However, the specific regulatory mechanism of apelin in ALI/ARDS is not clear. Surprisingly, Zhang et al. have concretely elaborated the mechanism of apelin attenuating ALI induced by LPS. Administration of apelin ameliorates LPS-induced oxidative stress, mitochondrial apoptosis, and inflammatory responses. In detail, LPS triggers mitochondrial reactive oxygen species (ROS) generation. Meanwhile, LPS promotes Bcl-2 X-associated protein (Bax) and cleaved caspase-3 expression, and inhibits B-cell lymphoma 2 (Bcl-2) expression. These effects can be reversed by apelin. Moreover, apelin also suppresses the nuclear translocation of NF-κB and the activation of NF-κB as well as macrophage infiltration triggered by LPS. Furthermore, NLRP3 inflammasome and caspase-1 are activated by LPS. Subsequently, active caspase-1 is involved in the cleavage of the precursors of IL-18 and IL-1β, which activates and matures them and promotes their secretion from cells, leading to pulmonary inflammation. However, treatment with apelin markedly reduces activated NLRP3 inflammasome-induced inflammatory responses. Collectively, apelin protects against LPS-induced ALI by reducing mitochondrial ROS-triggered oxidative damage, mitochondria-mediated cell apoptosis, and inflammatory responses induced by the activation of NF-κB and NLRP3 inflammasome [16] (Fig. 2). Therefore, apelin exerts protective effects on LPS-induced ALI/ARDS. Drugs targeting apelin may be an attractive therapeutic strategy for ALI/ARDS.

Fig. 2.

Apelin/APJ system alleviates ALI/ARDS. LPS induces NLRP3 inflammasome and caspase-1 activation, causing subsequent secretion of IL-18 and IL-1β. This adverse effect can be reversed by apelin. LPS promotes Bax and caspase-3 expression and inhibits Bcl-2 expression, leading to subsequent mitochondrial apoptosis, which is reversed by apelin. Apelin inhibits mitochondrial ROS production induced by LPS. Apelin also suppresses the nuclear translocation of NF-κB triggered by LPS. The above inhibitory effects of apelin can alleviate LPS-induce ALI (adapted from Ref. [16]). LPS lipopolysaccharide, NLRP3 NOD-like receptor protein 3, Caspase-1 cysteinyl aspartate specific proteinase-1, IL-18 interleukin-18, IL-1β interleukin-1β, Bax Bcl-2 X-associated protein, Caspase-3 cysteinyl aspartate specific proteinase-3, Bcl-2 B-cell lymphoma 2, ROS reactive oxygen species, NF-κB nuclear factor kappa-B, IκB inhibitor of NF-κB, ALI/ARDS acute lung injury/acute respiratory distress syndrome

Apelin/APJ system is involved in obstructive sleep apnoea syndrome

Obstructive sleep apnoea syndrome (OSAS) results from intermittent upper airway obstruction, and is featured as hypoxemia [73]. Hypoxia is a well-known inducer of apelin [74]. In addition, microinjection of apelin-13 into the nucleus tractus solitarius can cause apnea and inhibit phrenic nerve activity [28]. Currently, continuous positive airway pressure (CPAP) is considered as an effective method for the treatment of OSAS. Plasma apelin level is elevated in OSAS patients without CPAP treatment, but no significant variation across the 24 h period. After overnight treatment with CPAP, the plasma apelin level is significantly decreased. This may be because changes in apelin secretory dynamics are related to oral glucose and circadian patterns [75]. Similarly, a reduction of plasma apelin level is also noticed following CPAP treatment, but there is no significant difference in OSAS patients and the healthy [76]. Consistent with the above findings, similar serum level of apelin is noticed in OSAS and non-OSAS patients with no significant difference. Nevertheless, serum level of apelin is higher in OSAS patients. Meanwhile, this study confirmed for the first time that salivary apelin levels are related to the severity of OSAS due to it is significantly higher in the severe OSAS group [77]. The expression of tumor necrosis factor-α (TNF-α) is significantly increased in patients with OSAS [78] and decreased after CPAP treatment [79], suggesting that TNF-α is involved in the occurrence and development of OSAS. Besides, TNF-α expression can be inhibited by apelin [80]. Altogether, despite there is no statistically significant difference of plasma apelin level in OSAS and non-OSAS patients, considering increased plasma apelin level in OSAS patients and its reduction after CPAP treatment, apelin may be associated with the occurrence of OSAS.

Given the relatively small sample size and the contribution of complex factors to OSAS, a vague correlation between apelin and OSAS is clarified. Therefore, more studies are needed to explain whether apelin acts as a predictor in the occurrence of OSAS and the specific mechanism by which apelin affects OSAS.

Apelin/APJ system promotes non-small cell lung cancer proliferation and migration

Currently, lung cancer can be divided into two types: small cell lung cancer and non-small cell lung cancer (NSCLC). NSCLC accounts for the majority. Considering the poor prognosis of NSCLC, a new target is urgently wanted for the treatment of NSCLC. Angiogenesis is the basis of tumor proliferation and metastasis [81]. Similar to the function of vascular endothelial growth factor (VEGF), apelin is an emerging angiogenic factor, which plays a vital role in promoting tumor growth, lymphangiogenesis, and metastasis [82–84]. It is generally believed that the tumor microenvironment is highly hypoxic, and both apelin and APJ expression are elevated during hypoxia [85]. Besides, hypoxia activates VEGF-VEGFR2 signaling pathway, which causes subsequent APJ⁺ tumor vessel expansion and tumor growth. Genetic ablation and pharmacological blockade of APJ⁺ tumor vessels obviously eliminate tumor growth [83]. As mentioned, apelin can stimulate tumor growth. Therefore, whether apelin plays a role in the growth and metastasis of NSCLC and what is the specific molecular mechanism? Indeed, apelin is highly expressed in the six studied NSCLC cell lines both at the mRNA and protein levels. Exogenous administration or overexpression of apelin has no effect on NSCLC growth. However, implantation of apelin-overexpressing tumor cells into mice remarkably promotes microvessel densities and perimeters as well as tumor growth. During the follow-up period of 5 years, the overall survival rate of the low apelin expression group is higher than the high apelin expression group [86]. Consistently, a slightly significant correlation between apelin positivity and a shorter survival is observed in stage 4 NSCLC patients [87]. Besides, apelin expression is significantly increased in epidermal growth factor receptor tyrosine kinase inhibitors (EGFR‑TKIs) resistance group compared with the EGFR‑TKIs’ sensitive group. Therefore, it prompts that high apelin expression is related to EGFR‑TKIs resistance by affecting angiogenesis [88]. Collectively, these results indicate that apelin, as a marker of pathological angiogenesis, may become an underlying prognostic protein in patients with NSCLC.

Attractively, apart from the observation of increased plasma apelin concentrations in patients with NSCLC, the precise molecular mechanism by which apelin affects the proliferation of NSCLC cell lines (A549 cells) has also been elaborated. In detail, apelin-13 stimulates the proliferation of NSCLC cells by promoting the conversion of G0/G1 to S phase. Treatment with apelin-13 increases the expression of cyclin D1 by promoting ERK1/2 phosphorylation, which contribute to the subsequent proliferation of NSCLC cells. Administration of apelin-13 also induces autophagy in NSCLC cells by facilitating ERK1/2 phosphorylation, but NSCLC cell proliferation is not affected by impaired autophagy [89]. Furthermore, PAK1/LIMK1/cofilin signaling is involved in promoting the migration and invasion of NSCLC cells [90]. Both apelin and APJ expression are elevated in response to hypoxia, which subsequently induces PAK1 Ser144 and cofilin Ser3 phosphorylation. However, PAK1 inhibitor IPA-3 eliminates the phosphorylation of cofilin Ser3 promoted by apelin. Therefore, apelin promotes the migration of NSCLC cells via phosphorylating PAK1-cofilin signaling pathway (Fig. 3).

Fig. 3.

Apelin/APJ system promotes the proliferation and migration of NSCLC cell. The expression of apelin and APJ are increased during hypoxia (adapted from Ref. [85]). Apelin stimulates the proliferation of NSCLC cell by activating ERK1/2-cyclin D1 signaling (adapted from Ref. [89]). Apelin accelerates the migration of NSCLC cell through triggering PAK1-cofilin pathway (adapted from Ref. [90]). Apelin induces the autophagy of NSCLC cell by promoting ERK1/2 phosphorylation (adapted from Ref. [89]). ERK1/2 extracellular signal-regulated kinase1/2, PAK1 p21-activated kinase 1, NSCLC non-small cell lung cancer

Altogether, apelin is highly expressed in NSCLC. Additionally, apelin facilitates NSCLC cells proliferation by activating ERK1/2-cyclin D1 signaling and promotes NSCLC cells migration via phosphorylating PAK1-cofilin axis. Accordingly, targeting apelin/APJ system may be a potentially effective therapeutic agent for NSCLC. Future studies are also needed to elucidate more detailed molecular mechanisms of NSCLC and explain the role of autophagy in NSCLC proliferation and migration.

The role of apelin/APJ system in other respiratory diseases

In addition to the respiratory diseases mentioned above, limited studies have also clarified the relationship between apelin and pulmonary edema, asthma, and chronic obstructive pulmonary diseases (COPD). High altitude pulmonary edema (HAPE) is characterized by pulmonary vasoconstriction, endothelial dysfunction, and intravascular fluid retention. Hypobaric hypoxia of high altitude decreases the blood arterial oxygen saturation (SaO₂). The activation of apelin signaling mediated by hypoxia-inducible factor (HIF) can reverse this deleterious effect [43]. A recent research has elaborated the genetic and epigenetic regulation of apelin, apelin receptor (APLNR), and endothelial nitric oxide synthase (NOS3) in HAPE. The alleles of apelin, APLNR, and NOS3 are strongly correlated with HAPE. The levels of apelin-13 and nitrite as well as gene expression of apelin and NOS3 are significantly reduced in HAPE. Increased methylation of the apelin CpG island decreases apelin expression and apelin-13 level in HAPE. Specifically, the abnormal methylation of apelin CpG island in HAPE may prevent the binding of HIF to apelin, thereby inhibiting the transcription of apelin and decreasing the expression of apelin-13. Insufficient apelin-13 impedes NO generation by repressing AKT-NOS3 signaling pathway. Therefore, decreased apelin-13 and NO blocks the oxygenation of hemoglobin under hypobaric hypoxia, which exacerbates HAPE [91]. Moreover, after ozone exposure, apelin expression is reduced, but the DNA methylation of apelin gene is increased, which is accompanied by the occurrence of pulmonary edema [92]. In addition, the serum levels of apelin-12 are robustly elevated in asthmatic children, which is independent of obesity, implying that apelin may be a predictor in the occurrence of atopic asthma [93]. Besides, patients with COPD show a significant reduction in apelin levels, suggesting that apelin may also serve as a potential biomarker for predicting the occurrence of COPD. Mutations at the rs198389, rs6668352, and rs198388 sites of BNP lead to a reduction in serum apelin level, which may be one of the causes for the aggravation of COPD [94].

Drugs for apelin/APJ system

To date, a plethora of drugs targeting to apelin/APJ system have been found, including agonists and antagonists [95]. MM07, a biased agonist of APJ, shows a greater potential than apelin to suppress skin fibrosis [96]. Besides, as the earliest nonpeptidic APJ agonist, E339-3D6 exhibits a full agonist activity in APJ internalization [97]. Moreover, analysis of structure–activity relationship uncovers three ligands of E339-3D6 (19, 21, 38), which are more stable than apelin-17 peptide 2 in mouse plasma [98]. ML233 is a small molecule APJ agonist, which is also selective against the AT1 receptor and cell active [99]. In addition to these APJ agonists, ALX40‐4C is demonstrated as the first APJ antagonists. The binding of ALX40-4C to APJ prevents it role as the HIV-1 co-receptor [100]. The mutant apelin-13 (F13A) is also confirmed as an apelin-specific antagonist [101]. In vitro experiments indicate that the proliferation and invasion of trophoblast cells are promoted by Elabela, but repressed by APJ specific antagonist ML221 [102]. Therefore, with the developing understanding of APJ agonists and antagonists, high-affinity, non-side effect drugs targeting APJ are expected to be synthesized, and eventually link the gap between theory and clinical application.

Conclusion

In summary, these results presented highlight a unique role for apelin/APJ system in regulating respiratory diseases. Apelin and APJ have the highest combined expression in lung tissue. As discussed previously, apelin/APJ system is implicated in the occurrence and development of PAH, PE, ALI/ARDS, OSAS, NSCLC, pulmonary edema, asthma, as well as COPD. In detail, apelin/APJ system alleviates the severity of PAH, ALI/ARDS, and may inhibit the occurrence and development of pulmonary edema and asthma. Besides, apelin/APJ system also promotes the proliferation and migration of NSCLC. Given the intricate effect of apelin/APJ system in respiratory diseases, further work is needed to distinguish the pleiotropic actions of apelin/APJ system and make it a promising treatment strategy in respiratory diseases.

Prospection

To date, the correlation between endothelial dysfunction and respiratory diseases has been well documented. Abnormalities in ECs are at least partially related to NO reduction [49]. Notably, apelin plays a bifunctional role in vasomotor tone, which not only exerts vasodilation effect via NO-dependent mechanism [64], but also inhibits NO-induced relaxation of cerebral arteries by repressing large-conductance, calcium-activated K channel [66]. Therefore, it suggests that additional mediators (such as singlet molecular oxygen) may be involved in the vasomotor effect of apelin, and understanding the crosstalk between other mediators and NO is particularly important. Besides, Wnt/β-catenin signaling promotes the production of proinflammatory cytokines IL-17 and IL-23 as well as PMNs infiltration by raising Th17 response, which ultimately exacerbates LPS-induced ALI/ARDS [103]. Conversely, Wnt/β-catenin signaling also inhibits ICAM-1/VCAM-1-mediated the adhesion of PMNs to AECs, which eventually ameliorates ALI/ARDS [104]. Studies have confirmed that apelin as an activator of Wnt/β-catenin signaling [105]. Thus, whether apelin functions in ALI/ARDS via initiating Wnt/β-catenin signaling to regulate distinct downstream signaling? Due to the lack of direct evidence, more studies are needed to focus on the exact role of apelin-Wnt/β-catenin axis in ALI. Additionally, hepcidin deficiency results in iron accumulation, which subsequently elicits excessive ROS production and promotes the development of ALI/ARDS [106]. Interestingly, apelin can induce cardiac hypertrophy via ferritinophagy-promoted sideroflexin1-dependent mitochondrial iron overload [60]. Thus, apelin may promote iron-mediated ROS release by inducing ferritinophagy in alveolar epithelial cells, contributing to the development of ALI/ARDS. This hypothesis requires further experiments to support. There are myriad areas where further investigation is warranted. Perturbations in apelin/APJ system is closely associated with the development of respiratory diseases, and understanding the contribution and influence of apelin/APJ system in respiratory diseases poses an exciting area for scientific discovery.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interests.