Linxi Chen

APELIN-INDUCED VASCULAR SMOOTH MUSCLE CELL PROLIFERATION: THE REGULATION OF CYCLIN D1

Apelin is the endogenous ligand of the G protein-coupled receptor, APJ. Vascular smooth muscle cells express both apelin and APJ, which are important regulatory factors in the cardiovascular and nervous systems. Importantly, APJ is also involved in the pathogenesis if HIV-1 infection. We investigated whether vascular smooth muscle cell proliferation was regulated through an apelin-pERK1/2-cyclin D1 signal transduction pathway. Apelin-13 significantly stimulated vascular smooth muscle cell proliferation and increased cell cycle progression. Apelin-13 a decreased the proportion of cell in the G0/G1 phase while increasing the number of cells in S phase. Apelin-13 also increased the levels of cyclin D1, cyclin E and pERK1/2. Treatment of cells with the MEK inhibitor PD98059 attenuated the apelin-3-induced pERK1/2 activation. Similarly, treatment with PD98059 partially diminished the apelin-13-induced expression of cyclin D1 and vascular smooth muscle cell proliferation. Taken together, these data established that apelin-13 stimulates vascular smooth muscle cell proliferation by promoting the G1-S phase transition, and that this effect is mediated in part by an apelin-pERK1/2-cyclin D1 signal cascade.

Apelin and APJ, a novel critical factor and therapeutic target for atherosclerosis.

Apelin is a bioactive peptide discovered recently that has been proved to be an endogenous ligand of the APJ receptor. Apelin and APJ are widely distributed in the central nervous system and peripheral tissues. Researches have confirmed that apelin/APJ involved in a wide range of physiological and pathological functions in the cardiovascular system. Investigations indicated that apelin is a novel critical factor in the development of atherosclerosis (AS). In this review, we discuss the roles of apelin in the vascular smooth muscle cell proliferation, monocytes-endothelial cell adhesion, and angiogenesis that potentially reveals a new cellular mechanism of AS. Considering these roles, apelin and APJ may be novel therapeutic targets of AS.

Jagged-1/Notch3 signaling transduction pathway is involved in apelin-13-induced vascular smooth muscle cells proliferation

The apelin/apelin receptor (APJ, apelin-angiotensin receptor-like 1) system is a newly deorphanized G protein-coupled receptor system. Both apelin and APJ that are important regulatory factors are expressed in the cardiovascular system. Our previous studies demonstrated that apelin-13 significantly stimulated vascular smooth muscle cell (VSMC) proliferation. In this paper, our data suggested that the Jagged-1/Notch3 signaling transduction pathway is involved in apelin-13-induced VSMC proliferation by promoting the expression of Cyclin D1. Results indicated that apelin-13 stimulates the proliferation of VSMC and the expression of Jagged-1 and Notch3 in concentration- and time-dependent manners. The increased expression of Jagged-1 and Notch3 induced by apelin-13 could be abolished by extracellular signal-regulated protein kinase (ERK) blockade. PD98059 (ERK inhibitor) can inhibit the activation of Jagged-1/Notch3 induced by apelin-13. Down-regulation of Notch3 using small interfering RNA inhibits the expression of Cyclin D1 and prevents apelin-13-induced VSMC proliferation. In conclusion, Jagged-1/Notch3 signaling transduction pathway is involved in VSMC proliferation induced by apelin-13.

ERK1/2 mediates lung adenocarcinoma cell proliferation and autophagy induced by apelin-13

The aim of this study was to investigate the role of apelin in the cell proliferation and autophagy of lung adenocarcinoma. The over-expression of APJ in lung adenocarcinoma was detected by immunohistochemistry, while plasma apelin level in lung cancer patients was measured by enzyme-linked immunosorbent assay. Our findings revealed that apelin-13 significantly increased the phosphorylation of ERK1/2, the expression of cyclin D1, microtubule-associated protein 1 light chain 3A/B (LC3A/B), and beclin1, and confirmed that apelin-13 promoted A549 cell proliferation and induced A549 cell autophagy via ERK1/2 signaling. Moreover, there are pores on the surface of human lung adenocarcinoma cell line A549 and apelin-13 causes cell surface smooth and glossy as observed under atomic force microscopy. These results suggested that ERK1/2 signaling pathway mediates apelin-13-induced lung adenocarcinoma cell proliferation and autophagy. Under our experimental condition, autophagy associated with 3-methyladenine was not involved in cell proliferation.

Apelin/APJ system: a promising therapy target for hypertension

Apelin is a recently described endogenous peptide and its receptor APJ, is a member of the G protein-coupled receptors family. Apelin and APJ are widely distributed in central and peripheral tissues exert important biological effects on cardiovascular system. Recent studies have suggested that apelin/APJ system involves in decreasing the blood pressure and have a close relationship with hypertension, presumably, pathophysiology of hypertension as well. Such as, apelin/APJ system may be concerned in hyperfunction of the sympathetic nervous system, renin–angiotensin–aldosterone system, endothelial injury, excessive endothelin, sodium retention, vascular remodeling, insulin resistance elicit hypertension, as well as in hypertension-induced organ damaged. Meanwhile, on the ground of the variation of apelin level in hypertension therapeutic process and combining with the recently researches on APJ agonist and antagonist, we could infer that apelin/APJ system would be a promising therapeutic target for hypertension and other cardiovascular disease in the future. However, the role of apelin on these pathogenic conditions was not consistent, consequently, the contradictory role of apelin on these pathogenesis of hypertension would be discussed in this article.

Targeting Drugs to APJ Receptor: The Prospect of Treatment of Hypertension and Other Cardiovascular Diseases

The APJ is a class A, rhodopsin-like G protein-coupled receptor (GPCR) with high sequence similarity to the angiotensin receptor AT1. APJ has been shown to be widely expressed in humans tissues, including the central nervous system, cardiovascular system, adipocytes and others. APJ plays an important role in the occurrence and development of cardiovascular and metabolic diseases including atherosclerosis (AS), coronary heart disease (CAD), heart failure(HF), pulmonary arterial hypertension (PAH), myocardial hypertrophy and atrial fibrillation, especially hypertension. Previous researchers found that apelin/APJ could induce vasodilation and then reduce blood pressure. Despite APJ is closely associated with many diseases, there are no drugs that can activate or inhibit APJ directly. In the current review, we have summarized recently reported peptides, small molecule agonists and antagonists targeting APJ. Given the role of apelin/APJ in hypertension and other cardiovascular diseases, we believe that the peptides and compounds based on APJ will be developed for treatment of these diseases.

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

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

Apelin/APJ signaling in hypoxia-related diseases.

The regulatory peptide apelin is the endogenous ligand for the orphan G protein-coupled receptor APJ. Apelin and APJ exist in a variety of tissues, with special status in the heart, lung and tumors. Consequently, the apelin/APJ system exerts a broad range of activities that affect multiple organ systems. Accumulating evidence indicates that the expressions of apelin and APJ are significantly augmented by hypoxia through the hypoxia-inducible factor-1 alpha (HIF-1α) signaling pathway. Increased apelin promotes cellular proliferation, migration and survival, therefore regulating angiogenesis. In addition, the pre-administration of exogenous apelin is involved in the occurrence and development of hypoxia-induced pathological diseases. The purpose of this article is to review the properties of the apelin/APJ system, which is affected by hypoxic conditions, and the regulation of apelin/APJ signaling in hypoxia-associated disorders. Thus, the apelin/APJ system may be a potential therapeutic target in hypoxia-related diseases.

Apelin-13 promotes cardiomyocyte hypertrophy via PI3K-Akt-ERK1/2-p70S6K and PI3K-induced autophagy

Apelin is highly expressed in rat left ventricular hypertrophy Sprague Dawley rat models, and it plays a crucial role in the cardiovascular system. The aim this study was to clarify whether apelin-13 promotes hypertrophy in H9c2 rat cardiomyocytes and to investigate its underlying mechanism. The cardiomyocyte hypertrophy was observed by measuring the diameter, volume, and protein content of H9c2 cells. The activation of autophagy was evaluated by observing the morphology of autophagosomes by transmission electron microscopy, observing the subcellular localization of LC3 by light microscopy, and detecting the membrane-associated form of LC3 by western blot analysis. The phosphatidylinositol 3-kinase (PI3K) signaling pathway was identified and the proteins expression was detected using western blot analysis. The results revealed that apelin-13 increased the diameter, volume, and protein content of H9c2 cells and promoted the phosphorylation of PI3K, Akt, ERK1/2, and p70S6K. Apelin-13 activated the PI3K-Akt-ERK1/2-p70S6K pathway. PI3K inhibitor LY294002, Akt inhibitor 1701-1, ERK1/2 inhibitor PD98059 attenuated the increase of the cell diameter, volume, protein content induced by apelin-13. Apelin-13 increased the autophagosomes and up-regulated the expressions of beclin 1 and LC3-II/I both transiently and stably. The autophagy inhibitor 3MA ameliorated the increase of cell diameter, volume, and protein content that were induced by apelin-13. These results suggested that apelin-13 promotes H9c2 rat cardiomyocyte hypertrophy via PI3K-Akt-ERK1/2-p70S6K and PI3K-induced autophagy.

ELABELA: a novel hormone in cardiac development acting as a new endogenous ligand for the APJ receptor

Human ELA consists of three exons on chromosome 4, which generates a transcript (AK092578) that is annotated as a non-coding RNA. However, Chng et al. [1] has found that this gene contains a conserved open reading frame predicted to express a conserved vertebrate protein of 54 amino acids (aa) consisting of a secretory signal and a mature 32-aa peptide, which was called as ELABELA (ELA). The sequence of human mature ELA is Gln-Arg-Pro-Val-AsnLeu-Thr-Met-Arg-Arg-Lys-Leu-Arg-Lys-His-Asn-Cys-LeuGln-Arg-Arg-Cys-Met-Pro-Leu-His-Ser-Arg-Val-Pro-Phe-Pro. Phylogenetic analysis revealed that the 32-aa mature peptide is evolutionarily highly conserved, with the last 13 residues being nearly invariant in all vertebrate species. ELA has also been previously reported to be highly expressed in undifferentiated human embryonic stem cells (hESCs) and be sharply down-regulated during differentiation [2]. Chng et al. [1] used an allelic series of zebrafish ELA mutants to show that ELA deficiency leads to severe defects in cardiac morphogenesis and often results in the complete absence of a heart. ELA mutant displayed specific defects in the mesendodermal lineage during gastrulation, as observed by the reduction of gata5 and sox17 expression. Taking together, these results suggested that ELA plays a role in the regulation of heart development. Whereas till now, no hormonal peptides has been reported to be involved in early development, particularly in the formation of the three embryonic germ layers. Chng et al. [1] first discovered an endogenous peptide hormone with potent embryonic signaling activity, which has great prospects in therapeutic applications such as heart repair and gene therapy in development. During embryogenesis, six key signaling pathways (Wnt [3], Bmp/Nodal [4], FGF/IGF [5], Notch [6], Hedgehog [7], and Hippo [8]) have been reported to be crucial for embryonic patterning. Chng et al. [1] tried to explain ELA’s functions in cardiac development by activating APJ receptor. Their reasons are as follows: i) ELA is concomitantly expressed with APJ (APLNR) before the onset of gastrulation. ii) The phenotypes of zebrafish ELA mutants stingingly resemble those of the APJ (APLNR) mutants specifically in cardiogenesis. iii) Extracellular ELA binds to APJ in a native cellular context. To our knowledge, APJ is a G protein-coupled receptor and its endogenous ligand is apelin. Apelin activates APJ receptor, and plays an important role in the physiological activities [9–11], especially in cardiovascular system [12]. However, a recent report has shown that APJ has some functions independent of apelin. Moreover, except apelin, APJ has also been activated by stretch in cardiac hypertrophy [13]. Researchers have tried to explore a second ligand for APJ. Chng et al. [1] stated that ELA, not apelin, is hence the long-sought-after alternative and earlier ligand for APJ, functioning in early cardiovascular development [1]. They demonstrated that the expression of ELA happens earlier than apelin and is concomitantly with APJ before the onset of gastrulation. Then, APJ depletion has different effects on cardiac morphogenesis compared with the depletion of apelin in zebrafish [14,15], frog embryos [15], and mice [16,17]. Whereas loss of ELA phenocopies the loss of APLNR (APJ gene). Based on these, they declared that ELA may be the second ligand for APJ in mediating endoderm differentiation and subsequent cardiogenesis. They first confirmed that a second ligand of APJ really exists in vivo, which forms another essential signaling axis in heart development. A further study supported this claim by reporting that a secreted peptide Toddler activates APJ signaling to promote the subsequent zebrafish gastrulation movements [18]. As we all know, apelin activated APJ signals through Gai by increasing the content of phosphorylated extracellular signal-regulated kinase (p-ERK). It was also reported that stretch may activate APJ receptor by recruiting b-arrestin. However, how ELA activates APJ in vivo is still unclear. ELA acts as an endogenous secreted peptide like apelin. It may have the same pathway as apelin in the activation of APJ. But more studies are needed. No matter what the signal will be, ELA’s function in cardiac development suggested that the ELA/APJ axis appears to be exclusive for endoderm development. Therefore, it opens a new field for future research. Acta Biochim Biophys Sin 2014, 46: 620–622 |a The Author 2014. Published by ABBS Editorial Office in association with Oxford University Press on behalf of the Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences. DOI: 10.1093/abbs/gmu032. Advance Access Publication 14 May 2014