Apelin Peptides

* Please kindly note that our products and services can only be used to support research purposes (Not for clinical use).

Online Inquiry

What is the Apelin?

In humans, obesity and insulin cause a dramatic increase in the plasma levels of apelin, a bioactive peptide that appears to function as both a circulating and paracrine hormone. This peptide is produced and secreted by adipocytes, the stroma-vascular fraction, and various other organs, including the cardiovascular system and the heart. There has been extensive investigation into the potential cardiovascular effects of apelin because its encoding gene has the highest sequence identity with the angiotensin AT1 receptor. By blocking angiotensin type 1 receptors, the cardiac apelin system is restored in animal models of heart failure, where it is down-regulated by angiotensin II. In both healthy and diseased rat hearts, as well as in isolated cardiomyocytes, apelin exhibits a favorable hemodynamic profile and produces beneficial inotropic effects. Patients with chronic heart failure and lone atrial fibrillation often have decreased plasma apelin levels, which can be improved with cardiac resynchronization therapy. Hypoxia, ischemic cardiomyopathy, and exercise training all cause apelin production to rise in cardiac tissues, suggesting that this is a compensatory mechanism in rats.

Apelin structure

The majority of the protein apelin is made by white adipose tissue, and it has important biological functions. It was first discovered that this adipokine hormone existed as an endogenous ligand of the Apelin receptor (APJ) in bovine stomach extracts. The apelin-encoding gene, APLN, is situated on the X chromosome's long arm, specifically at positions Xq 25-26. The signal sequences and ligand-receptor interaction are located at the protein's N-terminus. The ligand's biological action relies heavily on the C-terminus. The 77-amino-acid pre-propeptide is the common ancestor of all of apelin's many final forms. It is possible to convert pre-proapelin into endogenous isoforms such apelin-36, apelin-17, apelin-13, apelin-16, or even exogenous apelin-12 by post-translational modification. It is interesting to note that the length of the polypeptide chain distinguishes each of the apelin variants listed above. It is transformed into short-chain versions since researchers have proved that the longer chains of this protein have reduced biological activity.

The peptide sequences of apelin and ELA isoforms. (Chapman F A., et al., 2021)

Apelin peptide

The receptor-activating potencies of apelin isoforms vary significantly. Size seems to have a role in the function of these peptides, however the exact role depends on the circumstance. Cell acidification experiments in CHO cells showed that the most powerful isoforms are apelin-13 and apelin-17, which are shorter variants. While all isoforms had similar potencies when monitoring cell migration or activation of the ERK pathway tests, apelin-13 and apelin-17 were found to be more effective in stimulating cell proliferation than apelin-36.

To activate the receptor, all apelin isoforms must include the same twelve amino acid residues at the C-terminus, which are highly conserved across several species. Apelinase-13 has a longer half-life in the plasma because its N-terminal glutamine is post-translationally changed to pyro-glutamate, which makes the peptide resistant to destruction by N-terminal exo-peptidases.It is possible that the various isoforms regulate separate signaling pathways in the heart, brain, and endocrine glands due to their widespread expression and varied distribution patterns throughout these organs. The shortest active isoform, apelin-12, was discovered to lower blood pressure in rats. Apelin-55, the longest isoform, binds to APJR and activates the ERK pathway; it was previously thought to be a pre-peptide in apelin's cleavage route.

Amino acid sequence of apelin peptide isoforms. (Murali S., et al., 2023)

(1) Apelin 13

Apelin-13 is generated by the hydrolysis of the apelin prepeptide, which is abundant in arginine and lysine residues, by Bacillus subtilisin protein convertase subtilisin/kexin 3 (PCSK3/furin), and may subsequently be degraded by angiotensin-converting enzyme 2 (ACE2) into an inactive form. Apelin-13 undergoes post-translational modification to provide a more stable active variant, pyroglutamylated apelin-13 [(Pyr1)-apelin-13]. Apelin-13 and (Pyr1)-apelin-13 are the predominant apelin isoforms present in human blood. Moreover, research indicates that apelin-13 is extensively expressed throughout several systems, including the neurological, cardiovascular, respiratory, digestive, and endocrine systems. Apelin-13 exerts a substantial regulatory influence on the cardiovascular system, modulating blood pressure bidirectionally based on the site of action; elevated apelin-13 levels in the periphery result in increased blood pressure, whereas heightened levels in the central nervous system produce the contrary effect. The process may include apelin-13's capacity to counteract the vasoconstrictive effects of angiotensin II via APJ receptor binding and to stimulate vascular smooth muscle cell proliferation via the activation of PI3K/AKT signaling pathway. Moreover, apelin-13 suppresses GSK-3β expression and mitochondrial permeability by activating the reperfusion injury salvage kinase (RISK) during myocardial ischemia and the opening of the ischemia-reperfusion transition pore (mPTP), thereby providing significant protection to cardiomyocytes. In mice with acute lung injury, apelin-13 mitigated oxidative stress and nuclear factor (NF-κB) signaling pathways, decreasing the synthesis of inflammatory mediators and thereby lessening structural damage to lung tissue.

Overview of the main mechanisms and signaling path ways of apelin-13 in regulating stroke. (Zhang Y., et al., 2023)

(2) Apelin 36

Apelin-36, a 36-amino acid peptide matching to sequence 42–77, is one of numerous active fragments produced by the proteolytic processing of the apelin proprotein after translation and cleavage. Patients diagnosed with myocardial ischemia/reperfusion (I/R) damage had a lower concentration of apelin in their plasma. Furthermore, there is a strong correlation between low apelin levels and an increased risk of significant adverse cardiovascular events after myocardial infarction. According to the results, apelin might be an important factor in I/R damage of the brain. In animal models of cerebral I/R damage, apelin-13 has been shown to protect neurons and astrocytes against apoptosis, but apelin-36 has only been shown to have a protective effect in one research. Most investigations only showed that apelin-13 or apelin-36 had a preventive impact on I/R injury-induced apoptosis, but this is important since it indicates that these compounds were administered preventatively. After an ischemic stroke, mice given a low dosage of apelin-36 (rather than apelin-13) had a much smaller infarct volume. Additionally, apelin-36 reduced caspase-3 activation and apoptosis caused by cerebral I/R damage. In addition, apelin-36 prevented the increase of CHOP and GRP78 caused by I/R damage, suggesting that it hindered the activation of ERS/UPR. Due to its ability to decrease ERS/UPR activation, low-dose apelin-36 administered post-stroke may reduce infarct and apoptosis caused by cerebral I/R damage.

The schematic diagram of apelin-36's effect. (Qiu J., et al., 2017)

(3) Pyr apelin 13

The most prevalent cardiovascular isoform of the apelin peptide, [Pyr1]apelin-13, is found in human vascular and cardiac endothelial cells and plasma. In vitro, apelins mediate three main activities. Apelin is one hundred times more powerful than endothelin-1 at increasing cardiac contractility and inotropic action when it interacts with the apelin receptor on myocytes of the heart. Apelin releases vasodilators in endothelium-protected arteries, which may counteract the effects of vasoconstrictors. In both healthy volunteers and heart failure patients, apelin infusion into the forearm had a nitric oxide dependent arterial dilatation as its primary in vivo impact. Intracoronary [Pyr1]apelin-13 induced coronary vasodilatation and enhanced cardiac contractility in individuals with heart failure. The cardiac index, mean arterial blood pressure, and peripheral vascular resistance were all improved when patients and volunteers received systemic infusions of [Pyr1]apelin-13.

The interactions between apelin/apelin receptor with ACE2 of the renin angiotensin system. (Yang P., et al., 2017)

Apelin function

Both the central and peripheral control of the cardiovascular system, including blood pressure and blood flow, as well as water and food intake, energy metabolism, and even immunological function, are impacted by the apelin signaling pathway. By stimulating the production of nitric oxide (NO), apelin promotes myocardial contractility and produces endothelium-dependent vasorelaxation. Additionally, an in vivo investigation found that apelin is a powerful angiogenic factor that induces the migration, proliferation, and formation of blood vessels by endothelial cells. Apelin mRNA expression was found in brain regions important for fluid homeostasis regulation, suggesting that apelin may have a function in this process. Obese people had higher apelin and APJ mRNA levels in their white fat and plasma compared to lean controls. The increase in apelin expression, however, may be primarily caused by obesity, which is known to be related with hyperinsulinemia. Conversely, apelin inhibits the secretion of insulin. According to the results compiled by Heinonen's group, there is a positive relationship between apelin plasma levels and BMI. Obese individuals also had the greatest levels of apelin, according to studies conducted on young women with eating problems. Similar to insulin plasma levels in humans and rats, apelin serum levels are correlated with dietary status. In addition, hyperinsulinemic obese mice, people with type 2 diabetes, and those who are overweight all have elevated apelin plasma concentrations. There may be a connection between glucose homeostasis and apelin's effect on insulin release in pancreatic islets in mice. Recent evidence suggests apelin may have a function in energy metabolism; a 14-day apelin therapy regulated obesity and increased uncoupling protein expression in mice. There is evidence in the literature that apelin may reduce inflammation by inhibiting the production of inflammatory mediators. The production of antioxidant enzymes is enhanced, and it also prevents adipocytes from releasing reactive oxygen species (ROS). Overexpression of apelin in rat cancer cells further suggests a potential function for apelin in lymphatic tumor growth.

Apelin expression and function in the organism. (Kurowska P., et al., 2018)

(1) Regulate hypothalamus-pituitary axis

The hypothalamus and pituitary glands are the principal locations of apelin activity in the central nervous system. The immunohistochemistry method was initially used to detect apelinergic neurons in the rat central nervous system. These neurons were found to have a topographical distribution, which suggests that apelin has multiple roles in controlling behaviors, pituitary hormone release, and circadian rhythms. The paraventricular nucleus (PVN) and its magnocellular and parvocellular regions, as well as the hypothalamic supraoptic nucleus, were shown to express the Apelin and APJ genes in rats. There were apelinergic nerve fibers found in many regions of the hypothalamus, including the periventricular, suprachiasmatic, ventromedial, dorsomedial, nucleic, and retrochiasmatic regions. Using immunofluorescence, we can see that the apelin-immunoreactive neuronal cell bodies were distributed throughout the whole rostrocaudal breadth of the Arc protein, which is connected with the mouse cytoskeleton. In addition, apelin was shown to be marginally associated with neuropeptide Y (NPY) and proopiomelanocortin (POMC). Arc POMC neurons have APJ, according to immunohistochemistry and in situ hybridization. The intermediate lobes, anterior and posterior pituitaries, and rat pituitary also showed Apelin/APJ mRNA. Additionally, Reaux's group found apelin co-expressed with corticotrophs and somatotrophs in the anterior pituitary using rats as a model, employing immunofluorescence labeling.

An role of apelin in the regulation of hormone release is suggested by the hypothalamic distribution of apelin fibers and receptors. The release of basal adrenocorticotropic hormone (ACTH) was shown to be considerably enhanced by apelin-17 in an ex vivo perifusion system of rat anterior pituitaries. It has been shown that apelin-17 increased the release of α-melanocyte-stimulating hormone (α-MSH) in hypothalamic explants through the perifusion technique. This indicates that apelin, whether released somatodendritically or axonally from POMC neurons, may stimulate the release of α-MSH in an autocrine function. It is possible that apelin plays a role in food intake in the hypothalamus as well. In rats, injecting apelin-13 intracerebroventricularly (icv) increased food intake through hypothalamic mechanisms such as increased orexin mRNA expression, inhibited cocaine- and amphetamine-regulated transcript (CART) mRNA expression, and serotonin secretion. Elevated plasma levels of interleukin-1 beta were shown to be related with an upregulation of proinflammatory markers in mice whose hypothalamus had been chronically infused with apelin intravenously. Increased expression of c-Fos and secretion of plasma ACTH and corticosterone (CORT) were seen in the PVN when Apelin-13 was present. In addition, apelin-13 was shown to decrease levels of prolactin (PRL), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) when administered intracerebroventricularly (icv), revealing the site of the postranslation change. It has been shown in an in vitro study that apelin-13 enhances the release of corticotropin-releasing hormone (CRH) and vasopressin (AVP) from hypothalamic explants, but has no effect on the release of neuropeptide Y (NPY). This finding provides more evidence that apelin may be involved in the hypothalamic regulation of water intake and endocrine axes. A gender-specific function of apelin in peripheral immune activation of the hypothalamus-pituitary-adrenal axis was proven by Newson's team using APJ KO mice, who had previously shown that APJ is involved in the integration of neuroendocrine responses to acute stress. Furthermore, Tobin's group found that adding apelin-13 to the hypothalamic supraoptic nucleus boosted the activity of vasopressin cells but had no influence on oxytocin neurons, indicating that apelin acts as a local autocrine feed-back on magnocellular vasopressin neurons.

(2) Regulate Ovary's physiology and pathology

The apelin/APJ complex is found in different ovarian cells and changes as the corpus luteum (CL) and ovarian follicles develop. This suggests that apelin may regulate apoptosis, proliferation, secretion of steroid hormones, and folliculogenesis, among other ovarian cell functions. Researchers have shown that apelin can control steroidogenesis in ovarian cells in vitro. The secretion of P4 and E2, as well as the amount of the 3β-hydroxysteroid dehydrogenase/Δ5-4 isomerase (3βHSD) protein, are both significantly increased in primary cell cultures and in IGFI-induced human and porcine ovarian cells when the APJ receptor is activated by apelin. The scientists speculated that apelin activates the mitogen-activated protein kinase 3 (MAPK3), AMPK kinase pathways, and serine-threonine kinase as molecular mechanisms for its effect on steroid production. Analogous findings have been made in in vitro investigations with bovine ovarian cells, demonstrating that apelin activates Akt kinase to promote P4 synthesis and cell proliferation. Furthermore, the scientists proved that apelin inhibits bovine oocyte development in vitro and P4 release by cumulus cells, suggesting that this adipokine plays a direct function in oocyte maturation. The study conducted by Shuang's group demonstrated that apelin activates the Akt kinase pathway in rat granulosa cells (Gc), promoting proliferation and inhibiting apoptosis. Furthermore, as a result of their findings of elevated APJ receptor expression in atretic bovine follicles, Shimizu's group hypothesizes that apelin is involved in follicular atresia caused by Gc apoptosis during bovine follicular

Apelin also has a role in controlling CL luteolysis. In the middle CL, which is sensitive to PGF2α, there was a brief rise in blood flow linked to the activation of endothelial nitric oxide (eNOS), the first signal that starts luteolysis. The eNOS pathway is activated by apelin, which causes the blood arteries to dilate because it stimulates the synthesis of nitric oxide [3]. The luteolytic action of apelin may also be explained via CL apoptosis. One of the components that reduces the rate of ovarian apoptosis is apelin. Conversely, apelin suppresses apoptosis in osteoblast cells by increasing expression of the antiapoptotic B-cell lymphoma 2 (Bcl-2) protein and mitigating proapoptotic Bax production.

(3) Regulate cardiovascular system

The role of apelin in heart development was first recognized in amphibians. The influence of apelin on the development of embryonic stem cells (ESCs) into cardiac lineage was subsequently examined in both mice and humans. Over fifty percent of APJ-null mouse embryos perished in utero owing to cardiovascular developmental anomalies, which included inadequate maturation of yolk sac and embryonic vasculature, malformed right ventricles, and reduced atrioventricular cushion formation. It was shown that apelin, when paired with mesodermal differentiation factors, enhances the proportion of contractile embryonic bodies produced from ESCs and upregulates many particular markers associated with differentiation into cardiomyocytes.

Apelin is essential for proper vascular development. APJ expression was observed in the endothelium of primary blood arteries and developing hearts in frogs and mice. Furthermore, during angiogenesis, apelin plays a role in adjusting blood vessel dimensions to meet the tissue's requirements for oxygen and nutrients in mouse embryos. In mice, the development of retinal capillaries throughout the fetal stage and the first two postnatal weeks is marked by a temporary increase in apelin/APJ mRNAs inside retinal endothelial cells. The involvement of the apelin/APJ system was validated in apelin-deficient animals, where a deficiency in retinal vascularization during the early postnatal phase was seen.

In vitro investigations validated the function of apelin in vascular formation. Apelin enhances the proliferation, migration, and capillary-like development of the retinal endothelial RF/6A cell line, which exhibits elevated levels of apelin and APJ transcripts. Apelin-13 elicited same effects in myocardial microvascular endothelial cells. Apelin not only aids in cardiovascular development but also facilitates post-ischemic vascular regeneration. In ischemic hind limbs of mice, transgenic overexpression of apelin, in conjunction with vascular endothelial growth factor (VEGF), facilitates the formation of comparatively big, non-leaky capillaries. This does not imply that a synergy between apelin and VEGF is essential for apelin's function, since this peptide is recognized for its capacity to suppress VEGF's effects, hence preventing pro-edemigenic hyper-permeability. Conversely, a decrease in capillary density and vascular integrity was seen in the hearts of apelin-knockout mice after myocardial infarction.

Antagonism between apelin/APJ system. (Folino A., et al., 2015)

Apelin in diabetes

When compared to healthy individuals, the apelin levels in obese patients with type 2 diabetes are much higher. In a similar vein, apelin levels are higher in type 1 diabetics compared to healthy controls. Put another way, a rise in apelin concentration is caused by a lack of insulin secretion. Insulin may have an effect on apelin. A rise in apelin levels may be caused by hyperinsulinemia and conditions like diabetes or obesity. In this case, insulin may upregulate apelin. Therefore, it was stage-dependent as to whether obesity caused apelin levels to rise. According to the scientific community, hyperapelinemia is a compensatory mechanism that inhibits pancreatic secretion, increases insulin sensitivity, and causes glucose absorption in muscle tissues of mice that is not reliant on insulin. Reducing apelin levels, which is a byproduct of weight reduction, lends credence to this theory. Conversely, hyperapelinemia occurs due to the absence of insulin in type 1 diabetes. The extent to which apelin affects glucose is digestion-dependent. At the start of digestion, apelin stimulates GluT2 translocation, which leads to increased intestinal glucose absorption; at the conclusion of digestion, it promotes muscular glucose use.

Apelin has the ability to activate many signaling pathways, leading to a reduction in cAMP content in β-cells and the stimulation of NO-synthases (eNOS) via phosphodiesterase 3B and AMP-activated protein kinase (AMPK) activation. Also, it has the ability to exert its effects via a route that is reliant on AKT. Apelin may have a role in glucose control by inhibiting cAMP via Gi. Gi trigger the activation of AMPK, an enzyme critical to energy consumption during ATP depletion. When apelin was given orally, it was found by Bertrand's team that enterocytes had less SGLT1 and more Glut2 due to AMPK activation. When histones are acetylated, inflammatory cytokines and chemokines may be amplified. One of the potential causes of DN, however, is chronic inflammation. In their study, Chen's team demonstrated that cultured retinal cells were able to suppress high glucose-induced inflammation by inhibiting histone acetylation. Subsequently, they proved that both healthy individuals and those with type 2 diabetes may enhance their glucose tolerance and glucose utilization after receiving apelin-13. The researchers found that Akita mice prevented DN with modest doses of apelin-13 by preventing renal hypotrophy and glomerular enlargement and by inhibiting inflammation caused by diabetes. Diabetes or worsening intensity may be caused by inflammatory cytokines and endothelial stress (ER stress). The control of cell death and survival pathways is influenced by proteins that may begin endoplasmic reticulum stress, including PKR-like eukaryotic initiation factor 2α kinase (PERK) and increase of inositol-requiring enzyme 1α (IRE1α). The inhibitory effects of apelin-13 on IRE1α activation and JNK signaling triggered by IRE1α were shown by Chen's group in their study of Akita mice. This proved that apelin-13 is involved in the cell death and endoplasmic reticulum stress pathways.

Decreased efficiency of mitochondrial oxidative phosphorylation may be caused, in part, by consuming a diet high in fat. The result was a decrease in fatty acid oxidation and the accumulation of FAs as diacylglycerol rather than oxidation. Insulin resistance developed due to endoplasmic reticulum stress caused by an excess of fatty acids in the mitochondria. A major contributor to insulin resistance, endothelial dysfunction (ER stress) stimulates the inflammatory process and disrupts insulin's ability to reach its target tissues. In addition to improving insulin sensitivity, Attané's group demonstrated that apelin therapy of insulin-resistant mice boosted mitochondrial biogenesis, oxidative phosphorylation, and fatty acid oxidation. Lenin's group showed in their own investigation that mice given a high-fat diet treated with apelin had less weight gain and glucose intolerance.

Treatment of mouse with apelin-13. (Alipour F G., et al., 2017)

References

1. Chapman F A., et al., The therapeutic potential of apelin in kidney disease, Nature Reviews Nephrology, 2021, 17(12): 840-853.
2. Murali S., et al., Structure–function relationship and physiological role of apelin and its G protein coupled receptor, Biophysical reviews, 2023, 15(1): 127-143.
3. Zhang Y., et al., Neuroprotective roles of Apelin-13 in neurological diseases, Neurochemical Research, 2023, 48(6): 1648-1662.
4. Qiu J., et al., Low dose of apelin-36 attenuates ER stress-associated apoptosis in rats with ischemic stroke, Frontiers in neurology, 2017, 8: 556.
5. Yang P., et al., [Pyr1] Apelin-13 (1–12) is a biologically active ACE2 metabolite of the endogenous cardiovascular peptide [Pyr1] Apelin-13, Frontiers in Neuroscience, 2017, 11: 92.
6. Kurowska P., et al., Apelin in reproductive physiology and pathology of different species: a critical review, International journal of endocrinology, 2018, 2018(1): 9170480.
7. Folino A., et al., Effects of apelin on the cardiovascular system, Heart failure reviews, 2015, 20: 505-518.
8. Alipour F G., et al., An overview on biological functions and emerging therapeutic roles of apelin in diabetes mellitus, Diabetes & Metabolic Syndrome: Clinical Research & Reviews, 2017, 11: S919-S923.