Inhibition of APLN suppresses cell proliferation and migration and promotes cell apoptosis in esophageal cancer cells in vitro, through activating PI3K/mTOR signaling pathway.

Esophageal cancer is the sixth leading cause of cancer mortalities globally with a high incidence rate. Apelin (APLN) plays regulatory roles in different organs. However, its role in esophageal cancer remains unknown. Therefore, our study aims to explore the effect of APLN on esophageal cancer. One hundred and eighty-four (184) esophageal tumor tissues samples from patients with esophageal cancer, and 11 esophageal tissues samples from healthy volunteers were analyzed for the expression of APLN. APLN was highly expressed in the tumor of patients with esophageal cancer and esophageal cancer cells.  Patients with high expressions of APLN had a lower survival rate than the ones with low to medium expressions of APLN.  Human esophageal carcinoma cell lines, TE-1 and ECA-109 cells were transfected with APLN siRNA to knockdown APLN, or transfected with pcDNA-APLN to overexpress APLN. Inhibition of APLN by siRNA-APLN reduced proliferative, migrative, and invasive abilities of esophageal cancer cells and promoted cell apoptosis, which could be all restored by pcDNA-APLN. Moreover, knocking down APLN by siRNA-APLN suppressed the PI3K/mTOR signaling pathway. These findings identify that APLN inhibition might ameliorate esophageal cancer through activating the PI3K/mTOR signaling pathway, thus APLN could be a potential target for esophageal cancer.

Apelin/APJ system: A novel therapeutic target for locomotor system diseases.

Apelin is an endogenous ligand of G protein-coupled receptor APJ. Apelin/APJ system is widely expressed in abundant tissues, especially bone, joint and muscle tissue. This review focus on the effects of apelin/APJ system on locomotor system. An increasing number of evidence suggests that apelin/APJ system plays a crucial role in many physiological and pathological processes of locomotor system. Physiologically, apelin/APJ system promotes bone formation, muscle metabolism and skeletal muscle production. Pathologically, apelin/APJ system exacerbates osteoarthritis pathogenesis, whereas it alleviates osteoporosis. Besides, the level of apelin expression is regulated by different training modes, including continuous aerobic exercise, high-intensity interval training and resistance exercises. More importantly, exercise-induced apelin may be a potent pharmacological agent for the treatment of diseases and the regulation of physiological processes. Considering the pleiotropic effects of apelin on locomotor system, apelin/APJ system may be an important therapeutic target for locomotor system diseases.

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

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

Apelin inhibition prevents resistance and metastasis associated with anti-angiogenic therapy.

Angiogenesis is a hallmark of cancer, promoting growth and metastasis. Anti-angiogenic treatment has limited efficacy due to therapy-induced blood vessel alterations, often followed by local hypoxia, tumor adaptation, progression, and metastasis. It is therefore paramount to overcome therapy-induced resistance. We show that Apelin inhibition potently remodels the tumor microenvironment, reducing angiogenesis, and effectively blunting tumor growth. Functionally, targeting Apelin improves vessel function and reduces polymorphonuclear myeloid-derived suppressor cell infiltration. Importantly, in mammary and lung cancer, Apelin prevents resistance to anti-angiogenic receptor tyrosine kinase (RTK) inhibitor therapy, reducing growth and angiogenesis in lung and breast cancer models without increased hypoxia in the tumor microenvironment. Apelin blockage also prevents RTK inhibitor-induced metastases, and high Apelin levels correlate with poor prognosis of anti-angiogenic therapy patients. These data identify a druggable anti-angiogenic drug target that reduces tumor blood vessel densities and normalizes the tumor vasculature to decrease metastases.

Apelin involved in progression of diabetic nephropathy by inhibiting autophagy in podocytes.

Podocyte autophagy dysfunction has been reported to be responsible for the progression of diabetic nephropathy (DN), however, the factors contributed to autophagy dysfunction in type 2 diabetes are not fully understood. Among promoting factors in DN, an adipokine, apelin, had been showed to trigger podocyte dysfunction. Therefore, it is hypothesized that apelin, which is increased in plasma in type 2 diabetes, lead to podocyte apoptosis through inhibiting podocyte autophagy, which resulted in podocyte dysfunction followed by DN. KkAy mice (diabetic mice) and cultured podocytes (MPC5 cells and native podocytes) were treated with high glucose (HG) and apelin or its antagonist F13A. Renal function, podocyte autophagy, podocyte apoptosis and corresponding cell signaling pathways in podocytes were detected. The results showed that apelin aggravated the renal dysfunction and foot process injuries in kkAy mice, which is positively correlated to podocyte apoptosis and negatively correlated to podocyte autophagy. Apelin induced podocyte apoptosis and inhibited podocyte autophagy in both normal glucose and HG conditions while F13A reversed these effects. Investigations by western blot found that apelin inhibits podocyte autophagy through ERK-, Akt- and mTOR-dependent pathways. In conclusion, increased apelin concentration in plasma inhibited podocyte autophagy, which would lead to podocyte apoptosis and renal dysfunction in diabetes. These effects would contribute to the progression of DN.

Apelin protects against liver X receptor-mediated steatosis through AMPK and PPARα in human and mouse hepatocytes.

Non-alcoholic fatty liver disease is the most commonly occurring chronic liver disease, and hepatic steatosis, a condition defined as extensive lipid accumulation in hepatocytes, is associated with liver dysfunction and metabolic diseases, such as, obesity and type II diabetes. Apelin is an adipokine that acts on a G protein-coupled receptor named APJ, and has been established to play pivotal roles in various physiological conditions. However, the function of apelin in hepatocytes has not been fully investigated. In order to assess the functional roles of apelin and APJ in hepatocytes, we used an in vitro model of liver X receptor (LXR)-mediated hepatocellular steatosis. In Hep3B human hepatoma cells, T0901317 (a specific LXR activator) induced lipid accumulation and this was inhibited by apelin. T0901317 also induced the expression of SREBP-1c, a key transcription factor for lipogenesis. Apelin not only inhibited SREBP-1c induction at the mRNA and protein levels but also induced lipolytic PPARα expression. Furthermore, these protective effects of apelin were inhibited by apelin F13A (a specific APJ antagonist). Furthermore, silencing of APJ by siRNA transfection also inhibited the actions of apelin. Specific inhibitors of cellular signaling components showed inhibition of lipid accumulation by apelin was mediated through Gi/o proteins, AMPK, and SREBP-1c suppression during the early stage and through AMPK, ERKs, and PPARα induction during the late stage. In addition, the protective effect of apelin was confirmed in mouse primary hepatocytes. Our observations suggest apelin-APJ signaling in hepatocytes functions to protect against lipid accumulation in liver through two signaling pathways, that is, via AMPK activation and PPARα induction.

miR-497 accelerates oxidized low-density lipoprotein-induced lipid accumulation in macrophages by repressing the expression of apelin.

microRNAs (miRNAs) play important roles in the pathogenesis of atherosclerosis. A previous study has reported that miR-497 is elevated in advanced atherosclerotic lesions in an apoE-deficient (apoE-/-) mouse model. The purpose of this study is to test whether miR-497 can modulate macrophage foam cell formation, an initiating event in atherosclerosis. We found that miR-497 was upregulated in THP-1 macrophages after treatment with oxidized low-density lipoprotein (oxLDL). Enforced expression of miR-497 promoted lipid accumulation and decreased cholesterol efflux in oxLDL-exposed THP-1 macrophages. In contrast, downregulation of miR-497 suppressed oxLDL-induced lipid accumulation in THP-1 macrophages. Apelin was identified to be a downstream target of miR-497. Overexpression of miR-497 significantly reduced the expression of apelin in THP-1 macrophages. Interestingly, delivery of a miR-497-resistant variant of apelin significantly inhibited lipid accumulation and enhanced cholesterol efflux in miR-497-overexpressing THP-1 macrophages in response to oxLDL. In addition, miR-497 expression was increased and negatively correlated with apelin protein expression in human atherosclerotic lesions. In conclusion, miR-497 contributes to oxLDL-induced lipid deposition in macrophages largely via targeting of apelin and thus represents a potential therapeutic target for atherosclerosis.

Targeting the apelin pathway as a novel therapeutic approach for cardiovascular diseases.

The apelin/apelin receptor system is widely distributed and has a dominant role in cardiovascular homeostasis and disease. The apelin gene is X-linked and is synthesized as a 77 amino acid pre-pro-peptide that is subsequently cleaved to generate a family of apelin peptides that possess similar functions but display different tissue distribution, potency and receptor binding affinity. Loss-of-function experiments using the apelin and the apelin receptor knockout mice and gain-of-function experiments using apelin peptides have delineated a well-defined role of the apelin axis in cardiovascular physiology and diseases. Activation of the apelin receptor by its cognate peptide ligand, apelin, induces a wide range of physiological effects, including vasodilation, increased myocardial contractility, angiogenesis, and balanced energy metabolism and fluid homeostasis. The apelin/apelin receptor pathway is also implicated in atherosclerosis, hypertension, coronary artery disease, heart failure, diabetes and obesity, making it a promising therapeutic target. Hence, research is expanding to develop novel therapies that inhibit degradation of endogenous apelin peptides or their analogues. Chemical synthesis of stable apelin receptor agonists aims to more efficiently enhance the activation of the apelin system. Targeting the apelin/apelin receptor axis has emerged as a novel therapeutic approach against cardiovascular diseases and an increased understanding of cardiovascular actions of the apelin system will help to develop effective interventions.

Apelin/APJ system as a therapeutic target in diabetes and its complications.

The G-protein-coupled receptor APJ and its endogenous ligand apelin are widely expressed in many peripheral tissues and central nervous system, including adipose tissue, skeletal muscles and hypothalamus. Apelin/APJ system, involved in numerous physiological functions like angiogenesis, fluid homeostasis and energy metabolism regulation, is notably implicated in the development of different pathologies such as diabetes and its complications. Increasing evidence suggests that apelin regulates insulin sensitivity, stimulates glucose utilization and enhances brown adipogenesis in different tissues associated with diabetes. Moreover, apelin is also involved in the regulation of diabetic complications via binding to APJ receptor. Apelin improves diabetes-induced kidney hypertrophia, normalizes obesity-associated cardiac hypertrophy and negatively promotes retinal angiogenesis in diabetic retinopathy. In this review, we provide a comprehensive overview about the role of apelin/APJ system in different tissues related with diabetes. Furthermore, we describe the pathogenesis of diabetic complications associated with apelin/APJ system. Finally, agonists and antagonists targeted to APJ receptor are described in the literature. Thus, we highlight apelin/APJ system as a novel therapeutic target for pharmacological intervention in treating diabetes and its complications.