The orphan receptor APJ and its recently identified
endogenous ligand, apelin, exhibit high levels of mRNA expression in
the heart. However, the functional importance of apelin in the
cardiovascular system is not known. In isolated perfused rat hearts,
infusion of apelin (0.01 to 10 nmol/L) induced a dose-dependent
positive inotropic effect (EC50: 33.1+/-1.5 pmol/L). Moreover,
preload-induced increase in dP/dt(max) was significantly augmented
(P<0.05) in the presence of apelin. Inhibition of phospholipase C
(PLC) with U-73122 and suppression of protein kinase C (PKC) with
stauzrosporine and GF-109203X markedly attenuated the apelin-induced
inotropic effect (P<0.001). In addition, zoniporide, a selective
inhibitor of Na+-H+ exchange (NHE) isoform-1, and KB-R7943, a potent
inhibitor of the reverse mode Na+-Ca2+ exchange (NCX), significantly
suppressed the response to apelin (P<0.001). Perforated
patch-clamp recordings showed that apelin did not modulate L-type
Ca2+ current or voltage-activated K+ currents in isolated adult rat
ventricular myocytes. Apelin mRNA was markedly downregulated in
cultured neonatal rat ventricular myocytes subjected to mechanical
stretch and in vivo in two models of chronic ventricular pressure
overload. The present study provides the first evidence for the
physiological significance of apelin in the heart. Our results show
that apelin is one of the most potent endogenous positive inotropic
substances yet identified and that the inotropic response to apelin
may involve activation of PLC, PKC, and sarcolemmal NHE and
NCX.
Szokodi et al.Circ Res 2002 Sep 6;91(5):434-40
Apelin is
an endogenous ligand of the human orphan receptor APJ. We detected
apelin-like immunoreactivity in the adipocytes, gastric mucosa, and
Kupffer cells in the liver. We also detected apelin-like
immunoreactivity localized within the endothelia of small arteries
in various organs. Further, it was found that mean arterial pressure
after the administration of apelin-12, apelin-13, and apelin-36 at a
dose of 10 nmol/kg in anaesthetized rats was reduced by 26+/-5,
11+/-4, and 5+/-4 mm Hg, respectively. In the presence of a nitric
oxide (NO) synthase inhibitor, the effect of apelin-12 on blood
pressure was abolished. Furthermore, the administration of apelin-12
(10 nmol/kg) in rats produced a transitory elevation of the plasma
nitrite/nitrate concentration from a basal level of 21.4+/-1.6 to
27.0+/-1.5 microM. Thus, apelin may lower blood pressure via a
nitric oxide-dependent mechanism.
Tatemoto et al. Regul Pept 2001 Jun 15;99(2-3):87-92
With the use of an antiserum against human
apelin-36 (Phoenix Pharmaceuticals), apelin-immunoreactivity (irAP)
was detected in neurons and cell processes of the supraoptic nucleus
(SO), paraventricular nucleus (PVH), accessory neurosecretory nuclei
(Acc) and suprachiasmatic nucleus. Strongly labeled cells/processes
were noted in the internal layer of the median eminence,
infundibular stem, anterior and posterior pituitary. Double-labeling
the sections with goat polyclonal neurophysin I-antiserum and rabbit
polyclonal apelin-antiserum revealed a population of magnocellular
neurons in the PVH, SO and Acc expressing both irAP and neurophysin
I-immunoreactivity (irNP), the latter being a marker of
oxytocin-containing neurons. By inference, the AP-positive but
irNP-negative magnocellular neurons could be vasopressin-containing.
The presence of irAP in certain hypothalamic nuclei and pituitary
suggests that the peptide may be a signaling molecule released from
the hypothalamic-hypophysial axis.
Brailoiu et al. Neurosci Lett 2002 Jul
26;327(3):193-7
Apelin is an endogenous ligand of the human
orphan receptor APJ (orphan G protein-coupled receptor). This
peptide is produced through processing from the C-terminal portion
in the pre-proprotein consisting of 77 amino acid residues and
exists in multiple molecular forms. Although the main physiological
functions of apelin have not been clarified yet, it has been
demonstrated that apelin partially suppresses cytokine production
from mouse spleen and, specifically, induces the promotion of
extracellular acidification and inhibition of cAMP production in
Chinese hamster ovary cells. Moreover, it is involved in the
regulation of blood pressure and blood flow. In this study we have
analyzed, by immunohistochemistry, apelin distribution in several
human tissues, demonstrating that apelin has a widespread pattern of
expression. These results seem to confirm that apelin functions
widely in various tissues interacting with its specific receptor
APJ.
De Falco et al. In Vivo 2002 Sep-Oct;16(5):333-6
Apelin is the recently identified endogenous ligand
for the G-protein-coupled receptor, APJ. Preproapelin and APJ mRNA
are found in hypothalamic regions known to be important in the
regulation of food and water intake, and pituitary hormone release.
The effects of intracerebroventricular (ICV) administration of
pyroglutamylated apelin-13 on food and water intake and pituitary
hormone release in rats were investigated. Apelin-13 had little
effect on food intake, but dose-dependently increased drinking
behaviour and water intake at 1 h. Apelin-13 (10 nmol) increased
water intake by up to sixfold compared to saline. Compared to saline
control, apelin-13 (10 nmol) significantly increased plasma ACTH and
corticosterone and decreased plasma prolactin, LH and FSH at 30 min.
In vitro, apelin-13 stimulated the release of CRH and AVP from
hypothalamic explants, but had no effect on NPY release. These
results suggest that apelin may play an important role in the
hypothalamic regulation of water intake and endocrine axes.
Taheri et al. Biochem Biophys Res Commun 2002 Mar 15;291(5):1208-12
Maintenance of human energy homeostasis is
regulated by a complex network. Peptides secreted from the
gastrointestinal tract (GI) are signaling to the brain and other
organs initiating or terminating food intake and energy expenditure.
In the present study we investigated basal plasma levels of apelin,
orexin-A, and leptin in morbid obese patients. In addition, we
measured in a subgroup of these patients in the same individual
orexin-A and leptin plasma levels one year after gastric banding
surgery. METHODS: Basal plasma values were determined in obese
patients (BMI=48+/-1 kg/m2n=32) after an overnight fast and compared
to healthy, normal weighted (BMI=22+/-2 kg/m2n=12) controls. In
addition, blood samples were collected in a subgroup of patients
(BMI=48+/-1 kg/m2n=8) the day before surgery and 1 year after the
operation. Apelin, orexin-A, and leptin levels were analysed using
ELISAs. RESULTS: One year after the operation obese patients
significantly lost weight (from 48+/-2 kg/m2 to 39+/-2 kg/m2;
p<0,001). Apelin, orexin-A and leptin levels in obese patients
were significantly higher compared to control individuals (736+/-50
pg/ml vs. 174+/-14 pg/ml, p<0.0001; 75.3+/-24.1 pg/ml vs.
0.8+/-0.4 pg/ml, p<0.0001; 79.0+/-2.4 ng/ml vs. 5.8+/-0.8 ng/ml,
p<0.0001, respectively). Apelin and leptin plasma concentrations
also correlated significantly with BMI (r=0.769, p<0.0001;
r=0.778; p<0.0001, respectively), while orexin-A correlation was
rather weak (r=0.335, p<0.03). No difference between pre- and
post-operative orexin-A levels was observed, while leptin plasma
levels significantly decreased from 45.1+/-5.4 ng/ml to 27.3+/-6.0
ng/ml (p=0.015). CONCLUSIONS: Apelin, orexin-A, and leptin plasma
levels correlated positively with the BMI. One year after gastric
banding with significant loss in BMI basal plasma levels of leptin
decreased, while orexin-A remained unchanged.
Heinonen et al. Regul Pept. 2005 Aug 15;130(1-2):7-13
Apelin is a recently discovered peptide that is the endogenous ligand for the APJ receptor. The aim of this study
was to characterize apelin expression (mRNA levels) in the rat
gastrointestinal (GI) tract and pancreas, to localize distribution
of apelin peptide containing cells in the stomach by
immunohistochemistry (IHC), to characterize the ontogeny of gastric
apelin expression and peptide, and the influence of apelin on
gastric cell proliferation in vitro. Additionally, the effect of
apelin on cholecystokinin (CCK) secretion, and the involvement of
mitogen-activated protein kinase (MAPK), protein kinase C (PKC) and
changes in [Ca++]i in apelin-induced CCK secretion in
vitro were examined. Northern analysis showed a maximal apelin
expression in the stomach with a lower expression level in the
intestine. Apelin expression was not detected in the pancreas. IHC
revealed abundant apelin positive cells in the glandular epithelium
of the stomach. The ontogeny study showed a higher apelin expression
in the fetal and postnatal rat stomachs when compared to the adult
stomach. In contrast to apelin expression, apelin peptide was not
detected in the rat stomach until 20 days of age and then increased
progressively with age. Apelin was shown to stimulate gastric cell
proliferation in vitro. Apelin also stimulated CCK secretion from a
murine enteroendocrine cell line (STC-1); apelin-stimulated CCK
secretion is mediated through MAPK but not by [Ca++]i
signaling. Together, these data indicate that apelin is an important
new stomach peptide with a potential physiologic role in the GI
tract.
Wang et al. Endocrinology. 2004 Mar;145(3):1342-8.
The results presented herein demonstrate that apelin is
expressed and secreted by both human and mouse adipocytes.
Apelin mRNA levels in isolated adipocytes are close to other
cell types present in white adipose tissue or other organs
known to express apelin such as kidney, heart, and to a lesser
extent brown adipose tissue. Apelin expression is increased
during adipocyte differentiation stage. A comparison of four
different models of obesity in mice showed a large increase in
both apelin expression in fat cells and apelin plasma levels
in all the hyperinsulinemia-associated obesities and clearly
demonstrated that obesity or high fat feeding are not the main
determinants of the rise of apelin expression. The lack of
insulin in streptozotocin-treated mice is associated with a
decreased expression of apelin in adipocytes. Furthermore,
apelin expression in fat cells is strongly inhibited by
fasting and recovered after refeeding, in a similar way to
insulin. A direct regulation of apelin expression by insulin
is observed in both human and mouse adipocytes and clearly
associated with the stimulation of PI3K, PKC and MAPK. These
data provide evidence that insulin exerts a direct control on
apelin gene expression in adipocytes. In obese patients, both
plasma apelin and insulin levels were significantly higher
suggesting that the regulation of apelin by insulin could
influence blood concentrations of apelin. The present work
identifies apelin as a novel adipocyte endocrine secretion and
focuses on its potential link with obesity-associated
variations.
Boucher et al. Endocrinology. 2005 Apr;146(4):1764-71.
The apelin peptide is the endogenous ligand for the apelin G protein-coupled receptor. The distribution of the apelin peptides and receptor are widespread in the central nervous system and periphery, with reported roles in the hypothalamic-pituitary-adrenal axis, blood pressure regulation and as one of the most potent positive inotropic substances yet identified. In this report, we show that in native tissues preproapelin exists as a dimer. Dimeric preproapelin was reduced to monomers by dithiothreitol treatment, indicating disulfide linkages. To evaluate the role of the carboxyl-terminal phenylalanine in the hypotensive action of apelin-13, analogs were generated and tested for their role on blood pressure regulation. Injections of apelin-13 and apelin-12 (15 microg/kg) into spontaneously hypertensive rats lowered systolic and diastolic blood pressure to result in decreases of approximately 60% and 15% in mean arterial blood pressure, respectively. Apelin-13(13[D-Phe]) treatment did not differ from apelin-13 in either efficacy or duration of effect, whereas apelin-13(F13A) revealed a loss of function. However, concomitant administration of apelin-13(F13A) (30 microg/kg) blocked hypotensive effects of apelin-13 (15 microg/kg), which revealed that apelin-13(F13A) behaved as an apelin-specific antagonist.
Lee et al. Endocrinology. 2005 Jan;146(1):231-6.
In the search for an endogenous ligand of the human orphan receptor APJ, we have isolated from bovine stomach extracts a novel 36-amino acid peptide, designated apelin. The APJ receptor is one of the G protein-coupled orphan receptors, many of which have been considered to be specific receptors for unidentified hormones and neuropeptides. We recently found the presence of apelin in the adipocytes and vascular walls as well as in the stomach. We examined biological activities of apelin and found that apelin lowered blood pressure in rats and also released cholecystokinin(CCK) from dispersed intestinal endocrine cells. Since apelin is an endogenous ligand for the HIV entry coreceptor APJ, we tested the effect of apelin on the entry of HIV in association with CD4, and found that apelin blocked the entry of HIV-1 and HIV-2.
Takemoto et al. Nippon Rinsho. 2000 Mar;58(3):737-46.
We have recently identified apelin as the endogenous ligand for human APJ. In rats, the highest expression of APJ mRNA was detected in the lung, suggesting that APJ and its ligand play an important role in the pulmonary system. When apelin-36 and its pyroglutamylated C-terminal peptide, [<Glu(65)]apelin-13, were compared in microphysiometric analyses, the elevation of extracellular acidification induced in cells expressing APJ by [<Glu(65)]apelin-13 was transient, whereas that by apelin-36 was sustained. These responses were almost completely inhibited by a specific inhibitor for G(i) or that for Na(+)/H(+) exchanger. (125)I -Labeled [<Glu(65)]apelin-13 analogue specifically bound to APJ with a high affinity, and [<Glu(65)]apelin-13 was more potent than apelin-36 in competitive inhibition assays. Because pretreatment with apelin-36 but not [<Glu(65)]apelin-13 drastically reduced the binding of the labeled apelin to APJ, the different patterns of acidification induced by these two peptides appeared to reflect their dissociation rather than association with APJ. Apelin elicited the migration of APJ-expressing cells, and [<Glu(65)]apelin-13 was more potent than apelin-36 in this activity. Heterogeneous molecular forms of apelin corresponding to apelin-36 and [<Glu(65)]apelin-13 were produced in bovine colostrum. Apelin-36 and [<Glu(65)]apelin-13 might have different functions in vivo and in vitro.
Hosoya et al. J Biol Chem. 2000 Jul 14;275(28):21061-7.
The orphan G protein-coupled receptor APJ has been shown to be a coreceptor for human and simian immunodeficiency virus (HIV and SIV) strains. We have determined that some HIV and SIV strains use APJ as a coreceptor to infect the brain-derived NP-2/CD4 cells. Because apelin is an endogenous ligand for the APJ receptor, we examined the inhibitory effects of apelin peptides on HIV infection, and found that the apelin peptides inhibit the entry of some HIV-1 and HIV-2 into the NP-2/CD4 cells expressing APJ. The inhibitory efficiency has been found to be in the order of apelin-36>apelin-17>apelin-13>apelin-12.
Zou et al. FEBS Lett 2000 May 4;473(1):15-8.
The apelin peptide was recently discovered and demonstrated to be the endogenous ligand for the G protein-coupled receptor, APJ. A search of the GenBank databases retrieved a rat expressed sequence tag partially encoding the preproapelin sequence. The GenBank search also revealed a human sequence on chromosome Xq25-26.1, containing the gene encoding preproapelin. We have used the rat sequence to screen a rat brain cDNA library to obtain a cDNA encoding the full-length open reading frame of rat preproapelin. This cDNA encoded a protein of 77 amino acids, sharing an identity of 82% with human preproapelin. Northern and in situ hybridization analyses revealed both human and rat apelin and APJ to be expressed in the brain and periphery. Both sequence and mRNA expression distribution analyses revealed similarities between apelin and angiotensin II, suggesting they that share related physiological roles. A synthetic apelin peptide was injected intravenously into male Wistar rats, resulting in immediate lowering of both systolic and diastolic blood pressure, which persisted for several minutes. Intraperitoneal apelin injections induced an increase in drinking behavior within the first 30 min after injection, with a return to baseline within 1 h.
Lee et al. J Neurochem. 2000 Jan;74(1):34-41.
By using a strategy that we have developed to search for the ligands of orphan seven-transmembrane-domain receptors [S. Hinuma et al., Nature 393 (1998) 272-276], we have recently identified a natural ligand, apelin, for the orphan 7TMR, APJ [K. Tatemoto et al., Biochem. Biophys. Res. Commun. 251 (1998) 471-476]. In this paper, we isolated rat and mouse apelin cDNAs, and analyzed the tissue distribution of apelin mRNA in rats. Although apelin mRNA was widely detected in a variety of tissues, the highest expression of apelin mRNA was detected in the mammary gland of pregnant rats. In the mammary gland, biologically active apelin and its mRNA considerably increased during pregnancy and lactation, and reached a maximal level around parturition. Moreover, a large amount of apelin (14-93 pmol/ml) was found to be secreted in the bovine colostrum, and it was still detectable even in commercial bovine milk. Since apelin partially suppressed cytokine production by mouse spleen cells in response to T cell receptor/CD3 cross-linking, the oral intake of apelin in the colostrum and milk might modulate immune responses in neonates.
Habata et al. Biochim Biophys Acta. 1999 Oct 13;1452(1):25-35.
1. We have determined the binding characteristics of [(125)I]-(Pyr(1))Apelin-13, a putative ligand for the APJ orphan receptor in human cardiovascular and rat tissue and investigated the functional properties of (Pyr(1))Apelin-13 in human saphenous vein. 2. The binding of [(125)I]-(Pyr(1))Apelin-13 to sections of human heart tissue was time dependent and rapid at 23 degrees C. Data were fitted to a single site model with an association rate constant (k(obs)) of 0.115 min(-1). [(125)I]-(Pyr(1))Apelin-13 also dissociated from a single site with a dissociation rate constant of 0.0105 min(-1). 3. In saturation binding experiments [(125)I]-(Pyr(1))Apelin-13 bound to human left ventricle with a K(D) value of 0.35+/-0.08 nM, B(max) of 4.3+/-0.9 fmol mg(-1) protein with a Hill slope of 0.97+/-0.04 and to the right atria with a K(D) of 0.33+/-0.09 nM, B(max) of 3.1+/-0.6 fmol mg(-1) protein and a Hill slope of 0.93+/-0.05. 4. [(125)I]-(Pyr(1))Apelin-13 binding sites were localized using autoradiography to human cardiovascular tissue, including coronary artery, aorta and saphenous vein grafts. In rat tissue a high density of receptors were localized to the molecular layer of the rat cerebellum, rat lung, rat heart and low levels in the rat kidney cortex. 2. (Pyr(1))Apelin-13 potently contracted human saphenous vein with a pD(2) value of 8.4+/-0.2 (n=8). The maximum response elicited by the peptide was 22.6+/-6% of 100 mM KCl. 6. We provide the first evidence of APJ receptor expression, relative densities and functional properties of (Pyr(1))Apelin-13 in human cardiovascular tissue.
Katugampola et al. Br J Pharmacol. 2001 Mar;132(6):1255-60.
