Characterization of the 5'‐regulatory regions of the rat and human apelin genes and regulation of breast apelin by USF

rs2736098, a synonymous polymorphism, is associated with carcinogenesis and cell count in multiple tissue types by regulating TERT expression

rs2736098 is a synonymous polymorphism in TERT (telomerase reverse transcriptase), an enzyme involved in tumor onset of multiple tissues, and should play no roles in carcinogenesis. However, a search in cancer somatic mutation database indicated that the mutation frequency at rs2736098 is much higher than the average one for TERT. Moreover, there are significant H3K4me1 and H3K27Ac signals, two universal histone modifications for active enhancers, surrounding rs2736098. Therefore, we hypothesized that rs2736098 might be within an enhancer region, regulate TERT expression and influence cancer risk. Through luciferase assay, it was verified that the enhancer activity of rs2736098C allele is significantly higher than that of T in multiple tissues. Transfection of plasmids containing TERT coding region with two different alleles indicated that rs2736098C allele can induce a significantly higher TERT expression than T. By chromatin immunoprecipitation, it was observed that the fragment spanning rs2736098 can interact with USF1 (upstream transcription factor 1). The two alleles of rs2736098 present evidently different binding affinity with nuclear proteins. Database and literature search indicated that rs2736098 is significantly associated with carcinogenesis in multiple tissues and count of multiple cell types. All these facts indicated that rs2736098 is also an oncogenic polymorphism and plays important role in cell proliferation.

Distribution, Function, and Expression of the Apelinergic System in the Healthy and Diseased Mammalian Brain

Apelin, a peptide initially isolated from bovine stomach extract, is an endogenous ligand for the Apelin Receptor (APLNR). Subsequently, a second peptide, ELABELA, that can bind to the receptor has been identified. The Apelin receptor and its endogenous ligands are widely distributed in mammalian organs. A growing body of evidence suggests that this system participates in various signaling cascades that can regulate cell proliferation, blood pressure, fluid homeostasis, feeding behavior, and pituitary hormone release. Additional research has been done to elucidate the system's potential role in neurogenesis, the pathophysiology of Glioblastoma multiforme, and the protective effects of apelin peptides on some neurological and psychiatric disorders-ischemic stroke, epilepsy, Parkinson's, and Alzheimer's disease. This review discusses the current knowledge on the apelinergic system's involvement in brain physiology in health and disease.

The Apelin/APLNR system modulates tumor immune response by reshaping the tumor microenvironment

Apelin is an endogenous ligand of the Apelin receptor (APLNR), a seven-transmembrane G protein-coupled receptor, which is widely distributed in human tissue. The Apelin/APLNR system is involved in regulating several physiological and pathological processes. The Apelin expression is increased in a variety of cancer and the Apelin/APLNR system could regulate the development of tumors through mediating autophagy, apoptosis, pyroptosis, and other biological processes to regulate tumor cell proliferation, migration, and invasion. The Apelin/APLNR system also participates in immune response and immune regulation through PI3K-Akt, ERK-MAPK, and other signal pathways. The latest research points out that there is a negative regulatory relationship between APLNR and immune checkpoint PD-L1. In this review, we outline the significance of the Apelin/APLNR signaling pathway in tumorigenesis and its immune regulation. These endeavors provide new insights into the translational application of Apelin/APLNR in cancer and may contribute to the promotion of more effective treatments for cancers.

A Tead1-Apelin axis directs paracrine communication from myogenic to endothelial cells in skeletal muscle

Apelin (Apln) is a myokine that regulates skeletal muscle plasticity and metabolism and declines during aging. Through a yeast one-hybrid transcription factor binding screen, we identified the TEA domain transcription factor 1 (Tead1) as a novel regulator of the Apln promoter. Single-cell analysis of regenerating muscle revealed that the apelin receptor (Aplnr) is enriched in endothelial cells, whereas Tead1 is enriched in myogenic cells. Knock-down of Tead1 stimulates Apln secretion from muscle cells in vitro and myofiber-specific overexpression of Tead1 suppresses Apln secretion in vivo. Apln secretion via Tead1 knock-down in muscle cells stimulates endothelial cell expansion via endothelial Aplnr. In vivo, Apln peptide supplementation enhances endothelial cell expansion while Tead1 muscle overexpression delays endothelial remodeling following muscle injury. Our work describes a novel paracrine crosstalk in which Apln secretion is controlled by Tead1 in myogenic cells and influences endothelial remodeling during muscle repair.

Potential Therapeutic Role for Apelin and Related Peptides in Diabetes: An Update

Type 2 diabetes mellitus (T2DM) is an epidemic with an ever-increasing global prevalence. Current treatment strategies, although plentiful and somewhat effective, often fail to achieve desired glycaemic goals in many people, leading ultimately to disease complications. The lack of sustained efficacy of clinically-approved drugs has led to a heightened interest in the development of novel alternative efficacious antidiabetic therapies. One potential option in this regard is the peptide apelin, an adipokine that acts as an endogenous ligand of the APJ receptor. Apelin exists in various molecular isoforms and was initially studied for its cardiovascular benefits, however recent research suggests that it also plays a key role in glycaemic control. As such, apelin peptides have been shown to improve insulin sensitivity, glucose tolerance and lower circulating blood glucose. Nevertheless, native apelin has a short biological half-life that limits its therapeutic potential. More recently, analogues of apelin, particularly apelin-13, have been developed that possess a significantly extended biological half-life. These analogues may represent a promising target for future development of therapies for metabolic disease including diabetes and obesity.

Pioglitazone protects blood vessels through inhibition of the apelin signaling pathway by promoting KLF4 expression in rat models of T2DM

Apelin, identified as the endogenous ligand of APJ, exerts various cardiovascular effects. However, the molecular mechanism underlying the regulation of apelin expression in vascular cells is poorly described. Pioglitazone (PIO) and Krüppel-like factor 4 (KLF4) exhibit specific biological functions on vascular physiology and pathophysiology by regulating differentiation- and proliferation-related genes. The present study aimed to investigate the roles of PIO and KLF4 in the transcriptional regulation of apelin in a high-fat diet/streptozotocin rat model of diabetes and in PIO-stimulated vascular smooth muscle cells (VSMCs). Immunohistochemistry, qRT-PCR, and Western blotting assays revealed that the aorta of the Type 2 diabetes mellitus (T2DM) rat models had a high expression of apelin, PIO could decrease the expression of apelin in the PIO-treated rats. In vitro, Western blotting assays and immunofluorescent staining results showed that the basal expression of apelin was decreased but that of KLF4 was increased when VSMCs were stimulated by PIO treatment. Luciferase and chromatin immunoprecipitation assay results suggested that KLF4 bound to the GKLF-binding site of the apelin promoter and negatively regulated the transcription activity of apelin in VSMCs under PIO stimulation. Furthermore, qRT-PCR and Western blotting assay results showed that the overexpression of KLF4 markedly decreased the basal expression of apelin, but the knockdown of KLF4 restored the PIO-induced expression of apelin. In conclusion, PIO inhibited the expression of apelin in T2DM rat models to prevent diabetic macroangiopathy, and negatively regulated the gene transcription of apelin by promoting transcription of KLF4 in the apelin promoter.

The effects of apoptosis and apelin on lymph node metastasis in invasive breast carcinomas

This study aimed to evaluate the biological and clinical significance of apelin-36 in breast cancer and to compare apelin-36 expression and apoptotic index in both breast tissue and metastatic lymph nodes in patients with invasive breast carcinoma. In this study, both tumor tissue and metastatic lymph nodes of the same patient were collected from 60 cases of invasive breast carcinoma patients (IDC, ILC) and 20 cases of normal breast tissue with no tumor from mammoplasty were used as the control group. The expression of apelin was examined with immunohistochemically, and the apoptotic index was examined with TUNEL methods. According to Kruskal-Wallis analysis, there was a significant difference between IDC and the control group when the apelin expression was compared between the breast tissues (p = 0.001). There were significant differences between the three groups when comparing relationships with apoptotic index (p < 0.001). According to the Mann-Whitney U test, both tumor size and expression of apelin in lymph nodes in ILCs were significantly higher than IDCs. (p = 0.026, p = 0.024, respectively). According to correlation analysis, there was a good correlation between the expression of apelin in breast tissue and apelin expression in lymph nodes (p = 0.000). It is also found a similar relationship in terms of the apoptotic index (p = 0.000). In addition, the negative correlation was found between apelin expression and the apoptotic index in breast tissues (p = 0.003). Based on these results, apelin-36 can be used as a marker for determining the metastasis potential in invasive breast cancer.

Role of Apelin/APJ axis in cancer development and progression

Apelin is an endogenous peptide, which is expressed in a vast board of organs such as the brain, placenta, heart, lungs, kidneys, pancreas, testis, prostate and adipose tissues. The apelin receptor, called angiotensin-like-receptor 1 (APJ), is also expressed in the brain, spleen, placenta, heart, liver, intestine, prostate, thymus, testis, ovary, lungs, kidneys, stomach, and adipose tissue. The apelin/APJ axis is involved in a number of physiological and pathological processes. The apelin expression is increased in various kinds of cancer and the apelin/APJ axis plays a key role in the development of tumors through enhancing angiogenesis, metastasis, cell proliferation and also through the development of cancer stem cells and drug resistance. The apelin also stops the apoptosis of cancer cells. The apelin/APJ axis was considered in this review as an attractive therapeutic target for cancer treatment.

The Role of the Apelin/APJ System in the Regulation of Liver Disease

Apelin is an endogenous peptide that is a ligand for the APJ receptor (angiotensin II receptor like-1, AT-1). The apelin/APJ system is distributed in diverse periphery organ tissues. It has been shown that the apelin/APJ system plays various roles in physiology and pathophysiology of many organs. It regulates cardiovascular development or cardiac disease, glycometabolism and fat metabolism as well as metabolic disease. The apelin/APJ system participates in various cell activities such as proliferation, migration, apoptosis or inflammation. However, apelin/APJ function in the liver is still under investigation. In the liver, the apelin-APJ system could play an inhibitory role in liver regeneration and promote Fas-induced apoptosis. It may participate in the formation of hepatic fibrosis or cirrhosis, and even cancer. In this review, we summarize the role of the apelin/APJ system in liver disease.

Temporal Expression of Apelin/Apelin Receptor in Ischemic Stroke and its Therapeutic Potential

Stroke is one of the leading causes of death and disability worldwide, and ischemic stroke accounts for approximately 87% of cases. Improving post-stroke recovery is a major challenge in stroke treatment. Accumulated evidence indicates that the apelinergic system, consisting of apelin and apelin receptor (APLNR), is temporally dysregulated in ischemic stroke. Moreover, the apelinergic system plays a pivotal role in post-stroke recovery by inhibiting neuronal apoptosis and facilitating angiogenesis through various molecular pathways. In this review article, we summarize the temporal expression of apelin and APLNR in ischemic stroke and the mechanisms of their dysregulation. In addition, the protective role of the apelinergic system in ischemic stroke and the underlying mechanisms of its protective effects are discussed. Furthermore, critical issues in activating the apelinergic system as a potential therapy will also be discussed. The aim of this review article is to shed light on exploiting the activation of the apelinergic system to treat ischemic stroke.

Synthetic retinoid Am80 up-regulates apelin expression by promoting interaction of RARα with KLF5 and Sp1 in vascular smooth muscle cells

Previous studies have demonstrated that both retinoids and apelin possess potent cardiovascular properties and that retinoids can mediate the expression of many genes in the cardiovascular system. However, it is not clear whether and how retinoids regulate apelin expression in rat VSMCs (vascular smooth muscle cells). In the present study, we investigated the molecular mechanism of apelin expression regulation by the synthetic retinoid Am80 in VSMCs. The results showed that Am80 markedly up-regulated apelin mRNA and protein levels in VSMCs. Furthermore, KLF5 (Krüppel-like factor 5) and Sp1 (stimulating protein-1) co-operatively mediated Am80-induced apelin expression through their direct binding to the TCE (transforming growth factor-β control element) on the apelin promoter. Interestingly, upon Am80 stimulation, the RARα (retinoic acid receptor α) was recruited to the apelin promoter by interacting with KLF5 and Sp1 prebound to the TCE site of the apelin promoter to form a transcriptional activation complex, subsequently leading to the up-regulation of apelin expression in VSMCs. An in vivo study indicated that Am80 increased apelin expression in balloon-injured arteries of rats, consistent with the results from the cultured VSMCs. Thus the results of the present study describe a novel mechanism of apelin regulation by Am80 and further expand the network of RARα in the retinoid pathway.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

The apelin receptor (APJ; gene symbol APLNR) is a member of the G protein-coupled receptor gene family. Neural gene expression patterns of APJ, and its cognate ligand apelin, in the brain implicate the apelinergic system in the regulation of a number of physiological processes. APJ and apelin are highly expressed in the hypothalamo-neurohypophysial system, which regulates fluid homeostasis, in the hypothalamic-pituitary-adrenal axis, which controls the neuroendocrine response to stress, and in the forebrain and lower brainstem regions, which are involved in cardiovascular function. Recently, apelin, synthesised and secreted by adipocytes, has been described as a beneficial adipokine related to obesity, and there is growing awareness of a potential role for apelin and APJ in glucose and energy metabolism. In this review we provide a comprehensive overview of the structure, expression pattern and regulation of apelin and its receptor, as well as the main second messengers and signalling proteins activated by apelin. We also highlight the physiological and pathological roles that support this system as a novel therapeutic target for pharmacological intervention in treating conditions related to altered water balance, stress-induced disorders such as anxiety and depression, and cardiovascular and metabolic disorders.

Biology of the apelin-APJ axis in vascular formation

Apelin is a bioactive peptide with diverse physiological actions on many tissues mediated by its interaction with its specific receptor APJ. Since the identification of apelin and APJ in 1998, pleiotropic roles of the apelin/APJ system have been elucidated in different tissues and organs, including modulation of the cardiovascular system, fluid homeostasis, metabolic pathway and vascular formation. In blood vessels, apelin and APJ expression are spatiotemporally regulated in endothelial cells (ECs) during angiogenesis. In vitro analysis revealed that the apelin/APJ system regulates angiogenesis by the induction of proliferation, migration and cord formation of cultured ECs. Moreover, apelin seems to stabilize cell-cell junctions of ECs. In addition, genetically engineered mouse models suggest that apelin/APJ regulates vascular stabilization and maturation in physiological and pathological angiogenesis. In this review, we summarize the current understanding of the apelin/APJ system for vascular formation and maturation.

Fat intake leads to differential response of rat adipocytes to glucose, insulin and ascorbic acid

Antioxidant-based treatments have emerged as novel and interesting approaches to counteract fat accumulation in obesity and associated metabolic disturbances. Adipocytes from rats that were fed on chow or high-fat diet (HFD) for 50 d were isolated (primary adipocytes) and incubated (72 h) on low (LG; 5.6 mmol/L) or high (HG; 25 mmol/L) glucose levels, in the presence or absence of 1.6 nmol/L insulin and 200 μmol/L vitamin C (VC). Adipocytes from HFD-fed animals presented lower insulin-induced glucose uptake, lower lactate and glycerol release, and lower insulin-induced secretion of some adipokines as compared with controls. HG treatment restored the blunted response to insulin regarding apelin secretion in adipocytes from HFD-fed rats. VC treatment inhibited the levels of nearly all variables, irrespective of the adipocytes’ dietary origin. The HG treatment reduced adipocyte viability, and VC protected from this toxic effect, although more drastically in control adipocytes. Summing up, in vivo chow or HFD intake determines a differential response to insulin and glucose treatments that appears to be dependent on the insulin-resistance status of the adipocytes, while VC modifies some responses from adipocytes independently of the previous dietary intake of the animals.

Apelin gene polymorphism influences apelin expression and obesity phenotypes in Chinese women

BACKGROUND

Apelin, which is a newly identified adipokine, is related to obesity and insulin resistance. A positive correlation between plasma apelin concentrations and obesity traits was reported.

OBJECTIVE

We tested associations between apelin gene (APLN) polymorphisms, BMI, and waist circumference (WC) and compared APLN expression levels in cells of different genotypes.

DESIGN

Four tagging single nucleotide polymorphisms (SNPs) and one promoter SNP were genotyped in 1627 Chinese subjects. Because APLN was located on the chromosome X, statistical analyses were conducted in a sex-specific manner. Adipocytes of different genotypes were derived from the omental fat tissue of 10 women. We treated the primary adipocytes with high glucose plus insulin because of a close relation between insulin resistance and obesity.

RESULTS

SNP rs3115757 was significantly associated with BMI and WC in women. Compared with the CG or GG genotype, the CC genotype had an OR of 2.07 (95% CI: 1.23, 3.49) for having a high WC (P = 0.006) and an OR of 2.29 (95% CI: 1.25, 4.19) for having a BMI (in kg/m(2)) ≥27 (P = 0.007). None of the SNPs was associated with BMI or WC in men. In adipocytes that carried the CC genotype of rs3115757, APLN messenger RNA levels and protein concentrations were higher in cells treated with high glucose plus insulin than in those with normal glucose. There was no difference between the 2 conditions in adipocytes of the CG or GG genotype.

CONCLUSION

Both association and functional studies suggested that APLN polymorphisms were associated with risks of obesity phenotypes.

Hypoxia induces apelin expression in human adipocytes

Adipokines play a central role in the development of diseases associated with insulin resistance and obesity. Hypoxia in adipose tissue leads to a dysregulation of the expression of adipokines. The effect of hypoxia on the more recently identified adipokine apelin in human adipocytes is unclear. Therefore, we aimed at investigating the role of hypoxia on the expression of the adipokine apelin. Differentiated human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes were cultured under hypoxic conditions for varying time periods. A modular incubator chamber was used to create a hypoxic tissue culture environment (defined as 1% O(2), 94% N, and 5% CO(2)). In addition, hypoxic conditions were mimicked by using CoCl(2). The effect of hypoxia on the expression of the investigated adipokines was measured by real-time PCR and the secretion of apelin was quantified by ELISA. Induction of hypoxia significantly induced mRNA expression of leptin and apelin in differentiated SGBS adipocytes compared with the normoxic control condition. Expression of adiponectin was significantly decreased by hypoxia. In addition, the amount of secreted apelin protein in response to hypoxia was elevated compared to untreated cells. Furthermore, we could demonstrate that the observed hypoxia-induced induction of apelin mRNA expression is in the first phase dependent on HIF-1α. In our study, we could demonstrate for the first time that apelin expression and secretion by human adipocytes are strongly induced under hypoxic conditions and that the early response on hypoxia with apelin induction is dependent on HIF-1α.

Apelin expression in human non-small cell lung cancer: role in angiogenesis and prognosis

INTRODUCTION

The recently discovered bioactive peptide, apelin, has been demonstrated to stimulate angiogenesis in various experimental systems. However, its clinical significance and role in tumor vascularization have not yet been investigated in a human malignancy. Therefore, our aim was to study whether apelin expression is associated with angiogenesis and/or tumor growth/behavior in human non-small cell lung cancer (NSCLC).

METHODS

A total of 94 patients with stage I-IIIA NSCLC and complete follow-up information were included. Apelin expression in human NSCLC samples and cell lines was measured by quantitative reverse-transcriptase polymerase chain reaction, enzyme-linked immunosorbent assay, and immunohistochemistry. Effects of exogenous apelin and apelin transfection were studied on NSCLC cell lines in vitro. In vivo growth of tumors expressing apelin or control vectors were also assessed. Morphometric variables of human and mouse tumor capillaries were determined by anti-CD31 labeling.

RESULTS

Apelin was expressed in all of the six investigated NSCLC cell lines both at the mRNA and protein levels. Although apelin overexpression or apelin treatments did not increase NSCLC cell proliferation in vitro, increasing apelin levels by gene transfer to NSCLC cells significantly stimulated tumor growth and microvessel densities and perimeters in vivo. Apelin mRNA levels were significantly increased in human NSCLC samples compared with normal lung tissue, and high apelin protein levels were associated with elevated microvessel densities and poor overall survival.

CONCLUSIONS

This study reveals apelin as a novel angiogenic factor in human NSCLC. Moreover, it also provides the first evidence for a direct association of apelin expression with clinical outcome in a human cancer.

High regulatability favors genetic selection in SLC18A2, a vesicular monoamine transporter essential for life

SLC18A2 encodes the vesicular monoamine transporter 2 protein that regulates neurotransmission and reduces cytosolic toxicity of monoamines. Deletion of this gene causes lethality in mice, and DNA sequence variation in this gene is associated with alcoholism and Parkinson's disease, among other disorders. The Caucasian SLC18A2 promoter has at least 20 haplotypes (A-T), with A representing two-thirds of 1460 chromosomes. It is not known why A is selected in the human lineage. To understand the selection, here we took a functional approach by investigating the regulations of 4 representative haplotypes (A, C, G, and T) by 17 agents. We show that 76.5% of the agents were able to regulate A but only 11.8-23.5% of them regulated the 3 other infrequent ones, observing a positive correlation between haplotype frequency and regulatability. Pathway and molecular analyses revealed five signaling hubs that regulate the four haplotypes differentially, probably through targeting the polymorphic core promoter region. These findings suggest that greater diversity of transcriptional regulations is the driving force for the haplotype selection in SLC18A2.

Ontogeny of apelin and its receptor in the rodent gastrointestinal tract

Apelin is the endogenous ligand for the APJ receptor and both apelin and APJ are expressed in the gastrointestinal (GI) tract. The aim of this study was to define ontogeny of apelin and APJ in the developing rodent GI tract by measuring expression levels and characterizing abundance and cellular localization at an embryonic stage (E18.5 or E21), two postnatal stages (P4, P16) and in the adult. Apelin and APJ mRNA levels were measured by real time RT-PCR, apelin and APJ-containing cells were identified by immunohistochemical (IHC) staining. Gastric, duodenal and colonic apelin and APJ mRNA levels were highest at birth and declined postnatally. In the postnatal rat stomach, few apelin peptide-containing cells were identified, the density of gastric apelin-containing cells increased progressively after weaning and into adulthood. A robust APJ immunostaining was observed postnatally in the epithelium, intestinal goblet cells and in smooth muscle cells. In the adult rat, APJ immunostaining in the surface epithelium and goblet cells decreased markedly. During the early postnatal period, in an apelin-deficient mouse, APJ expression and immunostaining in the gut were reduced suggesting that apelin regulates APJ. Together, our data support a role for the apelin-APJ system in the regulation of smooth muscle, epithelial and goblet cell function in the GI tract.

Involvement of a Stat3 binding site in inflammation-induced enteric apelin expression

Apelin is the endogenous ligand for the APJ receptor; both are expressed in the gastrointestinal tract. Experimental colitis in rodents and inflammatory bowel disease in humans are associated with increased intestinal apelin production. Our aim was to use LPS and proinflammatory cytokine-treated (IL-6 and IFN-gamma) rodents or enteric cells to identify signaling mechanisms underlying inflammation-induced enteric apelin expression. LPS, IL-6, or IFN-gamma treatment of rodents increased enteric apelin expression. Pharmacological blockade of Jak/Stat signaling or IL-6 antibody administration inhibited elevations in enteric apelin expression. Transient transfection experiments showed that LPS, IL-6, or IFN-gamma increased apelin expression by stimulation of apelin promoter activity, and blockade of Jak/Stat signaling abolished elevations in apelin promoter activity. A chromatin immunoprecipitation assay showed that IL-6 induced binding of phospho-Stat3 to a putative Stat3 site in the apelin promoter; mutation of this site abrogated the LPS-induced elevation in apelin promoter activity. Together, our findings indicate that binding of phospho-Stat3 to the apelin promoter is the final step underlying proinflammatory cytokine-induced enteric apelin expression during intestinal inflammation.

A possible role for hypoxia-induced apelin expression in enteric cell proliferation

Apelin is the endogenous ligand for the APJ receptor, and apelin and APJ are expressed in the gastrointestinal (GI) tract. Intestinal inflammation increases intestinal hypoxia-inducible factor (HIF) and apelin expression. Hypoxia and inflammation are closely linked cellular insults. The purpose of these studies was to investigate the influence of hypoxia on enteric apelin expression. Exposure of rat pups to acute hypoxia increased hepatic, stomach-duodenal, and colonic apelin mRNA levels 10-, 2-, and 2-fold, respectively (P < 0.05 vs. controls). Hypoxia also increased colonic APJ mRNA levels, and apelin treatment during hypoxia exposure enhanced colonic APJ mRNA levels further. In vitro hypoxia also increased apelin and APJ mRNA levels. The hypoxia-induced elevation in apelin expression is most likely mediated by HIF, since HIF-activated apelin transcriptional activity is dependent on an intact, putative HIF binding site in the rat apelin promoter. Acute exposure of rat pups to hypoxia lowered gastric and colonic epithelial cell proliferation; hypoxia in combination with apelin treatment increased epithelial proliferation by 50%. In vitro apelin treatment of enteric cells exposed to hypoxia increased cell proliferation. Apelin treatment during normoxia was ineffective. Our studies imply that the elevation in apelin expression during hypoxia and inflammation in the GI tract functions in part to stimulate epithelial cell proliferation.

Apelin, an endogenous neuronal peptide, protects hippocampal neurons against excitotoxic injury

Several G protein-coupled receptors (GPCRs) mediate neuronal cell migration and survival upon activation by their native peptide ligands but activate death-signaling pathways when activated by certain non-native ligands. In cultured neurons, we recently described expression of the unique seven-transmembrane (7TM) -G protein-coupled receptor, APJ, which is also strongly expressed in neurons in the brain and various cell types in other tissues. We now demonstrate that the endogenous APJ peptide ligand apelin activates signaling pathways in rat hippocampal neurons and modulates neuronal survival. We found that (i) both APJ and apelin are expressed in hippocampal neurons; (ii) apelin peptides induce phosphorylation of the cell survival kinases AKT and Raf/ERK-1/2 in hippocampal neurons; and (iii) apelin peptides protect hippocampal neurons against NMDA receptor-mediated excitotoxicity, including that induced by human immunodeficiency virus type 1. Thus, apelin/APJ signaling likely represents an endogenous hippocampal neuronal survival response, and therefore apelin should be further investigated as a potential neuroprotectant against hippocampal injury.

Immunohistochemical localization of apelin in human normal breast and breast carcinoma

The peptide apelin is a high-affinity ligand for the G-protein coupled receptor APJ. Apelin/APJ signaling plays important roles in blood pressure regulation, body fluid homeostasis, and cardiovascular development. More recently, it has been recognized that apelin/APJ signaling may also be involved in tumor angiogenesis. Studies in experimental animals have shown that apelin is abundantly secreted in the milk, and the mammary gland contains high level of pre-proapelin mRNAs and apelin protein. High level of apelin mRNA is expressed in cultured human breast carcinoma cell line (Hs 578T). However, the status of apelin expression and localization in human breast carcinoma has not been studied. In the present study immunohistochemistry was performed to investigate the expression and localization of apelin in normal human breast tissue and breast carcinoma. Cytoplasmic apelin immunoreactivity was detected in the ductal and lobular epithelial cells and vascular endothelial cells of the normal breast tissue. The myoepithelial cells were negative. The malignant tumor cells of invasive ductal or lobular carcinoma also expressed similar level of immunoreactive apelin. The fuctional significance of apelin expression in normal nonlactating breast and breast carcinoma warrants further investigation.

Increased colonic apelin production in rodents with experimental colitis and in humans with IBD

Apelin and its receptor, the APJ receptor, are expressed in the gastrointestinal tract. The aims of this study were to examine the effects of sodium dextran sulfate (DSS)-induced experimental colitis in rats and mice and inflammatory bowel disease (IBD) in humans on intestinal apelin production, and the influence of exogenous apelin on colonic epithelial cell proliferation in mice. In rodents with experimental colitis, colonic apelin mRNA levels were elevated during the inflammatory reaction as well as during the tissue repair phase that ensues after DSS withdrawal. Fluctuations in colonic apelin expression were paralleled by similar changes in apelin immunostaining. Apelin immunostaining was increased in the surface epithelium, in epithelial cells along the length of the tubular gland and in the stem cell region at the gland base. In ulcerative colitis (UC) and Crohn's disease patients, apelin immunostaining revealed a pattern of increased intestinal apelin content similar to that observed in rodents with experimental colitis. Administration of synthetic apelin to mice during the recovery phase of DSS-induced colitis stimulated colonic epithelial cell proliferation significantly. Our observations that colonic apelin production is increased during and after DSS exposure indicate that apelin plays multiple roles during the different stages of colitis. Additionally, the stimulatory action of exogenous apelin on colonic epithelial proliferation suggests that the increased apelin production during intestinal recovery stage may contribute to the repair of the intestinal epithelium in experimental rodent models of colitis and in IBD patients.