Pressor effect of apelin-13 in the rostral ventrolateral medulla: role of NAD(P)H oxidase-derived superoxide

Targeting the elabela/apelin-apelin receptor axis as a novel therapeutic approach for hypertension

ABSTRACT

Hypertension is the leading risk factor for global mortality and morbidity and those with hypertension are more likely to develop severe symptoms in cardiovascular and cerebrovascular system, which is closely related to abnormal renin-angiotensin system and elabela/apelin-apelin receptor (APJ) axis. The elabela/apelin-APJ axis exerts essential roles in regulating blood pressure levels, vascular tone, and cardiovascular dysfunction in hypertension by counterbalancing the action of the angiotensin II/angiotensin II type 1 receptor axis and enhancing the endothelial nitric oxide (NO) synthase/NO signaling. Furthermore, the elabela/apelin-APJ axis demonstrates beneficial effects in cardiovascular physiology and pathophysiology, including angiogenesis, cellular proliferation, fibrosis, apoptosis, oxidative stress, and cardiovascular remodeling and dysfunction during hypertension. More importantly, effects of the elabela/apelin-APJ axis on vascular tone may depend upon blood vessel type or various pathological conditions. Intriguingly, the broad distribution of elabela/apelin and alternative isoforms implicated its distinct functions in diverse cardiac and vascular cells and tissue types. Finally, both loss-of-function and gain-of-function approaches have defined critical roles of the elabela/apelin-APJ axis in reducing the development and severity of hypertensive diseases. Thus, targeting the elabela/apelin-APJ axis has emerged as a pre-warning biomarker and a novel therapeutic approach against progression of hypertension, and an increased understanding of cardiovascular actions of the elabela/apelin-APJ axis will help to develop effective interventions for hypertension. In this review, we focus on the physiology and biochemistry, diverse actions, and underlying mechanisms of the elabela/apelin-APJ axis, highlighting its role in hypertension and hypertensive cardiovascular injury and dysfunction, with a view to provide a prospective strategy for hypertensive disease therapy.

The role of autonomic pathways in peripheral apelin-induced gastrointestinal dysmotility: involvement of the circumventricular organs

NEW FINDINGS

What is the central question of this study? Are central autonomic pathways and circumventricular organs involved in apelin-induced inhibition of gut motility? What is the main finding and its importance? Peripherally administered apelin-13 inhibits gastric and colonic motor functions through sympathetic and parasympathetic autonomic pathways, which seems to be partly mediated by the apelin receptor in circumventricular organs.

ABSTRACT

Peripheral administration of apelin-13 has been shown to inhibit gastrointestinal (GI) motility, but the relevant mechanisms are incompletely understood. This study aimed to investigate (i) whether the apelin receptor (APJ) is expressed in circumventricular structures involved in autonomic functions, (ii) whether they are activated by peripherally administered apelin, (iii) the role of autonomic pathways in peripheral exogenous apelin-induced GI dysmotility, and (iv) the changes in apelin levels in the extracellular environment of the brain following its peripheral application. Ninety minutes after apelin-13 administration (300 μg kg^-1 , i.p.), gastric emptying (GE) and colon transit (CT) were measured in rats that underwent parasympathectomy and/or sympathectomy. Plasma and cerebrospinal fluid (CSF) samples were also collected from another group of rats that received apelin-13 or vehicle injection. The immunoreactivities for APJ and c-Fos in circumventricular organs (CVOs) were evaluated by immunohistochemistry. Compared with vehicle-treated rats, GE and CT were inhibited significantly by apelin-13 treatment, and were completely restored in animals that underwent the combination of parasympathectomy and sympathectomy and sympathectomy alone, respectively. Apelin concentrations were elevated in both plasma and CSF following peripheral administration of apelin-13. APJ expression was detected in area postrema (AP), subfornical organ and organum vasculosum of lamina terminalis, and c-Fos expression was observed in response to apelin injection. Apelin-induced c-Fos expression in AP was partially attenuated by pretreatment with the cholecystokinin-1 receptor antagonist lorglumide, whereas it was completely abolished in vagotomized rats. The present data suggest that APJ in CVOs could indirectly contribute to the inhibitory action of peripheral apelin on GI motor functions.

Targeting Hypertension: Superoxide Anions are Involved in Apelininduced Long-term High Blood Pressure and Sympathetic Activity in the Paraventricular Nucleus

AIM

To determine whether apelin in paraventricular nucleus (PVN) can be a therapeutic target for hypertension.

BACKGROUND

Apelin is a specific endogenous ligand of orphan G protein-coupled receptor APJ.

OBJECTIVE

This study was designed to determine how apelin chronically regulates sympathetic nerve activity and blood pressure in PVN of rats.

METHODS

Apelin and APJ antagonist F13A were infused into PVN with osmotic minipumps. The NAD(P)H oxidase activity and superoxide anions levels in PVN of rats were determined by chemiluminescence.

RESULTS

Infusion of apelin into PVN of Wistar-Kyoto (WKY) rats induced chronic increases in systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), plasma norepinephrine (NE) level, maximal depressor response to hexamethonium (Hex), NAD(P)H oxidase activity, superoxide anions levels, and Nox4 expression. Infusion of F13A into PVN of spontaneously hypertensive rats (SHRs) caused chronic decreases in SBP, DBP, MAP, plasma NE level, maximal depressor response to Hex, NAD(P)H oxidase activity, and superoxide anions levels. Hex, a sympathetic ganglion blocker, inhibited apelin-induced increases in SBP, DBP and MAP. SOD overexpression in PVN of SHRs inhibited the apelin-induced increase in SBP, DBP, MAP, plasma NE level, and maximal depressor response to Hex. PVN Nox4 knockdown also attenuated the apelin-induced increase in SBP, DBP, MAP, plasma NE level, and maximal depressor response to Hex. Chronic injection of F13A into PVN reduced fibrosis of renal artery, thoracic aorta, and heart in SHRs.

CONCLUSION

These results demonstrated that in PVN apelin induced long-term high blood pressure and sympathetic activity via increasing oxidative stress.

Superoxide anions modulate the performance of apelin in the paraventricular nucleus on sympathetic activity and blood pressure in spontaneously hypertensive rats

The present study was designed to determine how apelin in paraventricular nucleus (PVN) modulates the renal sympathetic nerve activity (RSNA), arterial blood pressure (ABP), mean arterial pressure (MAP), and heart rate (HR), and whether superoxide anions regulate the performance of PVN apelin in spontaneously hypertensive rats (SHRs). Acute experiment was carried out with 13-week-old male Wistar-Kyoto rats (WKY) and SHRs under anaesthesia. RSNA, ABP, MAP and HR after PVN microinjection were measured. Apelin microinjection into PVN increased RSNA, ABP, MAP and HR in WKY rats and SHRs, more obviously in SHRs. APJ antagonist F13A decreased the RSNA, ABP, MAP and HR in SHRs, and inhibited the effects of apelin. Apelin and APJ mRNA levels were higher in the PVN in SHRs. PVN microinjection of superoxide anion scavengers tempol and tiron, or NAD(P)H oxidase inhibitor apocynin, decreased the RSNA, ABP, MAP and HR in SHRs, and inhibited the effects of apelin, but the superoxide dismutase (SOD) inhibitor diethyldithiocarbamic acid (DETC) potentiated the effects of apelin. NAD(P)H oxidase activity and superoxide anion levels in PVN were increased by apelin, but decreased by APJ antagonist F13A. The apelin-induced increases in NAD(P)H oxidase activity and superoxide anion level were abolished by pre-treatment with F13A. These results indicate that apelin in PVN increases the sympathetic outflow and blood pressure via activating APJ receptor. The enhanced activity of endogenous apelin and APJ receptor in PVN contributes to sympathetic activation in hypertension, and the superoxide anion is involved in these apelin-mediated processes in PVN.

Novel transmitters in brain stem vagal neurocircuitry: new players on the pitch

The last few decades have seen a major increase in the number of neurotransmitters and neuropeptides recognized as playing a role in brain stem neurocircuits, including those involved in homeostatic functions such as stress responsiveness, gastrointestinal motility, feeding, and/or arousal/wakefulness. This minireview will focus on the known physiological role of three of these novel neuropeptides, i.e., apelin, nesfatin-1, and neuropeptide-S, with a special emphasis on their hypothetical roles in vagal signaling related to gastrointestinal motor functions.

Vascular effects of apelin: Mechanisms and therapeutic potential

Apelin is a vasoactive peptide and is an endogenous ligand for APJ receptors, which are widely expressed in blood vessels, heart, and cardiovascular regulatory regions of the brain. A growing body of evidence now demonstrates a regulatory role for the apelin/APJ receptor system in cardiovascular physiology and pathophysiology, thus making it a potential target for cardiovascular drug discovery and development. Indeed, ongoing studies are investigating the potential benefits of apelin and apelin-mimetics for disorders such as heart failure and pulmonary arterial hypertension. Apelin causes relaxation of isolated arteries, and systemic administration of apelin typically results in a reduction in systolic and diastolic blood pressure and an increase in blood flow. Nonetheless, vasopressor responses and contraction of vascular smooth muscle in response to apelin have also been observed under certain conditions. The goal of the current review is to summarize major findings regarding the apelin/APJ receptor system in blood vessels, with an emphasis on regulation of vascular tone, and to identify areas of investigation that may provide guidance for the development of novel therapeutic agents that target this system.

Apelin-13 inhibits gastric motility through vagal cholinergic pathway in rats

The expression of apelin and its receptors (APJ) in central autonomic networks suggests that apelin may regulate gastrointestinal motor functions. In rodents, central administration of apelin-13 has been shown to inhibit gastric emptying; however, the mechanisms involved remain to be determined. Using male adult Sprague-Dawley rats, the aims of the present study were 1) to determine the expression of APJ receptor in the dorsal vagal complex (DVC), 2) to assess the effects of central application of apelin-13 into the DVC on gastric tone and motility, and 3) to investigate the neuronal pathways responsible for apelin-induced alterations. APJ receptor immunoreactivity was detected in gastric-projecting and choline acetyltransferase-positive neurons of the DVC. Microinjection of apelin-13 into the DVC significantly decreased gastric tone and motility in both corpus and antrum. The apelin-induced reduction in gastric tone and motility was prevented by surgical vagotomy or fourth ventricular application of the APJ receptor antagonist, [Ala13]apelin-13 (F13A). Systemic administration of the muscarinic receptor antagonist atropine, but not the nitric oxide synthase inhibitor nitro-l-arginine methyl ester (l-NAME), abolished the apelin-induced inhibitory responses. The present results indicate a central modulatory role of apelin in the vagal neurocircuitry that controls gastric motor functions via withdrawal of the tonically active cholinergic pathway. NEW & NOTEWORTHY This is the first study investigating the effects induced by brain stem application of apelin-13 while monitoring gastric tone and motility in rats. We have found that gastric-projecting neurons of the dorsal vagal complex express apelin receptors (APJ), which mediate the inhibitory actions of apelin-13. The inhibitory effects of apelin were abolished by systemic preadministration of atropine, but not nitro-l-arginine methyl ester (l-NAME). Apelin seems to modulate gastric motility via withdrawal of the tonically active vagal cholinergic pathway.

Apelinergic System Structure and Function

Apelin and apela (ELABELA/ELA/Toddler) are two peptide ligands for a class A G-protein-coupled receptor named the apelin receptor (AR/APJ/APLNR). Ligand-AR interactions have been implicated in regulation of the adipoinsular axis, cardiovascular system, and central nervous system alongside pathological processes. Each ligand may be processed into a variety of bioactive isoforms endogenously, with apelin ranging from 13 to 55 amino acids and apela from 11 to 32, typically being cleaved C-terminal to dibasic proprotein convertase cleavage sites. The C-terminal region of the respective precursor protein is retained and is responsible for receptor binding and subsequent activation. Interestingly, both apelin and apela exhibit isoform-dependent variability in potency and efficacy under various physiological and pathological conditions, but most studies focus on a single isoform. Biophysical behavior and structural properties of apelin and apela isoforms show strong correlations with functional studies, with key motifs now well determined for apelin. Unlike its ligands, the AR has been relatively difficult to characterize by biophysical techniques, with most characterization to date being focused on effects of mutagenesis. This situation may improve following a recently reported AR crystal structure, but there are still barriers to overcome in terms of comprehensive biophysical study. In this review, we summarize the three components of the apelinergic system in terms of structure-function correlation, with a particular focus on isoform-dependent properties, underlining the potential for regulation of the system through multiple endogenous ligands and isoforms, isoform-dependent pharmacological properties, and biological membrane-mediated receptor interaction. © 2018 American Physiological Society. Compr Physiol 8:407-450, 2018.

Angiotensin-(1–7) attenuates angiotensin II-induced cardiac hypertrophy via a Sirt3-dependent mechanism

The objectives of the present study were to investigate the effect of ANG-(1–7) on the development of cardiac hypertrophy and to identify the intracellular mechanism underlying this action of ANG-(1–7). Blood pressure and heart rate were recorded using radiotelemetry before and after chronic subcutaneous infusion of control (PBS), ANG II, ANG-(1–7), or ANG II + ANG-(1–7) for 4 wk in normotensive rats. Chronic administration of ANG-(1–7) did not affect either basal blood pressure or the ANG II-induced elevation in blood pressure. However, ANG-(1–7) significantly attenuated ANG II-induced cardiac hypertrophy and perivascular fibrosis in these rats. These effects of ANG-(1–7) were confirmed in cultured cardiomyocytes, in which ANG-(1–7) significantly attenuated ANG II-induced increases in cell size. This protective effect of ANG-(1–7) was significantly attenuated by pretreatment with A779 (a Mas receptor antagonist) or Mito-TEMPO (a mitochondria-targeting superoxide scavenger) as well as blockade of Sirt3 (a deacetylation-acting protein) by viral vector-mediated overexpression of sirtuin (Sirt)3 short hairpin (sh)RNA. Western blot analysis demonstrated that treatment with ANG-(1–7) dramatically increased Sirt3 expression. In addition, ANG-(1–7) attenuated the ANG II-induced increase in mitochondrial ROS generation, an effect that was abolished by A779 or Sirt3 shRNA. Moreover, ANG-(1–7) increased FoxO3a deacetylation and SOD2 expression, and these effects were blocked by Sirt3 shRNA. In summary, the protective effects of ANG-(1–7) on ANG II-induced cardiac hypertrophy and increased mitochondrial ROS production are mediated by elevated SOD2 expression via stimulation of Sirt3-dependent deacetylation of FoxO3a in cardiomyocytes. Thus, activation of the ANG-(1–7)/Sirt3 signaling pathway could be a novel therapeutic strategy in the management of cardiac hypertrophy and associated complications.

NEW & NOTEWORTHY Chronic subcutaneous ANG-(1–7) has no effect on ANG II-induced elevations in blood pressure but significantly attenuates ANG II-induced cardiac hypertrophy and fibrosis by a mitochondrial ROS-dependent mechanism. This protective effect of ANG-(1–7) against the action of ANG II action is mediated by stimulation of sirtuin-3-mediated deacetylation of FoxO3a, which triggers SOD2 expression.

Vasopressin V1a receptors mediate the hypertensive effects of [Pyr1 ]apelin-13 in the rat rostral ventrolateral medulla

Key points

Dysfunctions in CNS regulation of arterial blood pressure lead to an increase in sympathetic nerve activity that participates in the pathogenesis of hypertension.

The apelin‐apelin receptor system affects arterial blood pressure homeostasis; however, the central mechanisms underlying apelin‐mediated changes in sympathetic nerve activity and blood pressure have not been clarified.

We explored the mechanisms involved in the regulation of [Pyr¹]apelin‐13‐mediated cardiovascular control within the rostral ventrolateral medulla (RVLM) using selective receptor antagonists.

We show that [Pyr¹]apelin‐13 acts as a modulating neurotransmitter in the normotensive RVLM to affect vascular tone through interaction with the vasopressin V1a receptor but that [Pyr¹]apelin‐13‐induced sympathoexcitation is independent of angiotensin II receptor type 1, oxytocin, ionotropic glutamate and GABA_A receptors.

Our data confirm a role for the apelin peptide system in cardiovascular regulation at the level of the RVLM and highlight that this system is a possible potential therapeutic target for the treatment of hypertension.

Abstract

Apelin is a ubiquitous peptide that can elevate arterial blood pressure (ABP) yet understanding of the mechanisms involved remain incomplete. Bilateral microinjection of [Pyr¹]apelin‐13 into the rostral ventrolateral medulla (RVLM), a major source of sympathoexcitatory neurones, increases ABP and sympathetic nerve activity. We aimed to investigate the potential involvement of neurotransmitter systems through which the apelin pressor response may occur within the RVLM. Adult male Wistar rats were anaesthetized and ABP was monitored via a femoral arterial catheter. Bilateral RVLM microinjection of [Pyr¹]apelin‐13 significantly increased ABP (9 ± 1 mmHg) compared to saline (−1 ± 2mmHg; P < 0.001), which was blocked by pretreatment with the apelin receptor antagonist, F13A (0 ± 1 mmHg; P < 0.01). The rise in ABP was associated with an increase in the low frequency spectra of systolic BP (13.9 ± 4.3% total power; P < 0.001), indicative of sympathetic vasomotor activation. The [Pyr¹]apelin‐13‐mediated pressor response and the increased low frequency spectra of systolic BP response were fully maintained despite RVLM pretreatment with the angiotensin II type 1 receptor antagonist losartan, the oxytocin receptor antagonist desGly‐NH₂, d(CH₂)₅[D‐Tyr²,Thr⁴]OVT, the ionotropic glutamate receptor antagonist kynurenate or the GABA_A antagonist bicuculline (P > 0.05). By contrast, the [Pyr¹]apelin‐13 induced pressor and sympathoexcitatory effects were abolished by pretreatment of the RVLM with the vasopressin V1a receptor antagonist, SR 49059 (−1 ± 1 mmHg; 1.1 ± 1.1% total power, respectively; P < 0.001). These findings suggest that the pressor action of [Pyr¹]apelin‐13 in the RVLM of normotensive rats is not mediated via angiotensin II type 1 receptor, oxytocin, ionotropic glutamate or GABA_A receptors but instead involves a close relationship with the neuropeptide modulator vasopressin.

Dysfunctions in CNS regulation of arterial blood pressure lead to an increase in sympathetic nerve activity that participates in the pathogenesis of hypertension.

The apelin‐apelin receptor system affects arterial blood pressure homeostasis; however, the central mechanisms underlying apelin‐mediated changes in sympathetic nerve activity and blood pressure have not been clarified.

We explored the mechanisms involved in the regulation of [Pyr¹]apelin‐13‐mediated cardiovascular control within the rostral ventrolateral medulla (RVLM) using selective receptor antagonists.

We show that [Pyr¹]apelin‐13 acts as a modulating neurotransmitter in the normotensive RVLM to affect vascular tone through interaction with the vasopressin V1a receptor but that [Pyr¹]apelin‐13‐induced sympathoexcitation is independent of angiotensin II receptor type 1, oxytocin, ionotropic glutamate and GABA_A receptors.

Our data confirm a role for the apelin peptide system in cardiovascular regulation at the level of the RVLM and highlight that this system is a possible potential therapeutic target for the treatment of hypertension.

High-fat diet and chronic stress reduce central pressor and tachycardic effects of apelin in Sprague-Dawley rats

Central application of apelin elevates blood pressure and influences neuroendocrine responses to stress and food consumption. However, it is not known whether the central cardiovascular effects of apelin depend also on caloric intake or chronic stress. The purpose of the present study was to determine the effects of intracerebroventricular administration of apelin on blood pressure (mean arterial blood pressure) and heart rate in conscious Sprague-Dawley rats consuming either a normal-fat diet (NFD) or high-fat diet (HFD) for 12 weeks. During the last 4 weeks of the food regime, the rats were exposed (NFDS and HFDS groups) or not exposed (NFDNS and HFDNS groups) to chronic stress. Each group was divided into two subgroups receiving intracerebroventricular infusions of either vehicle or apelin. Apelin elicited significant increase of mean arterial blood pressure and heart rate in the NFDNS rats. This effect was abolished in the HFDNS, HFDS and NFDS groups. HFD resulted in a significant elevation of blood concentrations of total cholesterol, triglycerides glucose and insulin. Chronic stress reduced plasma concentration of total and high-density lipoprotein cholesterol, and increased plasma corticosterone concentration and APJ receptor mRNA expression in the hypothalamus, whereas a combination of a HFD with chronic stress resulted in the elevation of plasma triglycerides, total cholesterol and low-density lipoprotein cholesterol, and in increased plasma corticosterone concentration, apelin concentration and APJ receptor mRNA expression in the hypothalamus. It is concluded that a HFD and chronic stress result in significant suppression of the central pressor action of apelin, and cause significant though not unidirectional changes of metabolic and endocrine parameters.

Hypothalamic apelin/reactive oxygen species signaling controls hepatic glucose metabolism in the onset of diabetes

AIMS

We have previously demonstrated that central apelin is implicated in the control of peripheral glycemia, and its action depends on nutritional (fast versus fed) and physiological (normal versus diabetic) states. An intracerebroventricular (icv) injection of a high dose of apelin, similar to that observed in obese/diabetic mice, increase fasted glycemia, suggesting (i) that apelin contributes to the establishment of a diabetic state, and (ii) the existence of a hypothalamic to liver axis. Using pharmacological, genetic, and nutritional approaches, we aim at unraveling this system of regulation by identifying the hypothalamic molecular actors that trigger the apelin effect on liver glucose metabolism and glycemia.

RESULTS

We show that icv apelin injection stimulates liver glycogenolysis and gluconeogenesis via an over-activation of the sympathetic nervous system (SNS), leading to fasted hyperglycemia. The effect of central apelin on liver function is dependent of an increased production of hypothalamic reactive oxygen species (ROS). These data are strengthened by experiments using lentiviral vector-mediated over-expression of apelin in hypothalamus of mice that present over-activation of SNS associated to an increase in hepatic glucose production. Finally, we report that mice fed a high-fat diet present major alterations of hypothalamic apelin/ROS signaling, leading to activation of glycogenolysis. INNOVATION/CONCLUSION: These data bring compelling evidence that hypothalamic apelin is one master switch that participates in the onset of diabetes by directly acting on liver function. Our data support the idea that hypothalamic apelin is a new potential therapeutic target to treat diabetes.

Apelin-13 inhibits large-conductance Ca2+-activated K+ channels in cerebral artery smooth muscle cells via a PI3-kinase dependent mechanism

Apelin-13 causes vasoconstriction by acting directly on APJ receptors in vascular smooth muscle (VSM) cells; however, the ionic mechanisms underlying this action at the cellular level remain unclear. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in VSM cells are critical regulators of membrane potential and vascular tone. In the present study, we examined the effect of apelin-13 on BK_Ca channel activity in VSM cells, freshly isolated from rat middle cerebral arteries. In whole-cell patch clamp mode, apelin-13 (0.001-1 μM) caused concentration-dependent inhibition of BK_Ca in VSM cells. Apelin-13 (0.1 µM) significantly decreased BK_Ca current density from 71.25±8.14 pA/pF to 44.52±7.10 pA/pF (n=14 cells, P<0.05). This inhibitory effect of apelin-13 was confirmed by single channel recording in cell-attached patches, in which extracellular application of apelin-13 (0.1 µM) decreased the open-state probability (NP_o) of BK_Ca channels in freshly isolated VSM cells. However, in inside-out patches, extracellular application of apelin-13 (0.1µM) did not alter the NP_o of BK_Ca channels, suggesting that the inhibitory effect of apelin-13 on BK_Ca is not mediated by a direct action on BK_Ca. In whole cell patches, pretreatment of VSM cells with LY-294002, a PI3-kinase inhibitor, markedly attenuated the apelin-13-induced decrease in BK_Ca current density. In addition, treatment of arteries with apelin-13 (0.1 µM) significantly increased the ratio of phosphorylated-Akt/total Akt, indicating that apelin-13 significantly increases PI3-kinase activity. Taken together, the data suggest that apelin-13 inhibits BK_Ca channel via a PI3-kinase-dependent signaling pathway in cerebral artery VSM cells, which may contribute to its regulatory action in the control of vascular tone.

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.

Post-infarct treatment with [Pyr1]-apelin-13 reduces myocardial damage through reduction of oxidative injury and nitric oxide enhancement in the rat model of myocardial infarction

Apelin is a newly discovered peptide that has been recently shown to have cardioprotective effects in the animal model of myocardial infarction (MI) and ischemia/reperfusion (I/R) injuries. The aim of the present study was to investigate the long term cardioprotective effect of [Pyr1]-apelin-13 in the rat model of MI. Male Wistar rats (n=22) were randomly divided into three groups: (1) sham operated group (2) control MI group and (3) MI treated with apelin (MI-AP group). MI animals were subjected to 30 min of left anterior descending coronary artery (LAD) ligation and 14 days of reperfusion. 24h after LAD ligation, apelin (10 mol/kg/day) was administered i.p. for 5 days. Blood sampling was performed at days 1, 3, 5 and 7 after MI for determination of serum changes of lactate dehydrogenase (LDH), creatine kinase-MB (CK-MB), malondialdehyde (MDA) and nitric oxide (NO). Myocardial infarct size (IS) and hemodynamic function were also measured at the end of the study at day 14. We found out that post infarct treatment with apelin decreases infarct size, serum levels of LDH, CK-MB and MDA and increases heart rate and serum level of NO in the consecutive days, but there were no significant differences in blood pressure in the MI-AP group in comparison with MI. In conclusion, apelin has long term cardioprotective effects against myocardial infarction through attenuation of cardiac tissue injury and lipid peroxidation and enhancement of NO production.

Apelin and pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a devastating disease characterized by pulmonary vasoconstriction, pulmonary arterial remodeling, abnormal angiogenesis and impaired right ventricular function. Despite progress in pharmacological therapy, there is still no cure for PAH. The peptide apelin and the G-protein coupled apelin receptor (APLNR) are expressed in several tissues throughout the organism. Apelin is localized in vascular endothelial cells while the APLNR is localized in both endothelial and smooth muscle cells in vessels and in the heart. Apelin is regulated by hypoxia inducible factor -1α and bone morphogenetic protein receptor-2. Patients with PAH have lower levels of plasma-apelin, and decreased apelin expression in pulmonary endothelial cells. Apelin has therefore been proposed as a potential biomarker for PAH. Furthermore, apelin plays a role in angiogenesis and regulates endothelial and smooth muscle cell apoptosis and proliferation complementary and opposite to vascular endothelial growth factor. In the systemic circulation, apelin modulates endothelial nitric oxide synthase (eNOS) expression, induces eNOS-dependent vasodilatation, counteracts angiotensin-II mediated vasoconstriction, and has positive inotropic and cardioprotective effects. Apelin attenuates vasoconstriction in isolated rat pulmonary arteries, and chronic treatment with apelin attenuates the development of pulmonary hypertension in animal models. The existing literature thus renders APLNR an interesting potential new therapeutic target for PH.

Overexpression of klotho protein modulates uninephrectomy-induced compensatory renal hypertrophy by suppressing IGF-I signals

The klotho gene is highly expressed in the distal convoluted tubule of the kidney, while its encoded protein has many physiological and pathophysiological renal roles. We investigated the effect of klotho protein on physiological compensatory renal hypertrophy after nephrectomy in klotho transgenic (KLTG) mice. Renal hypertrophy was suppressed in KLTG mice compared with wild-type mice, and this was associated with suppression of insulin growth factor-1 (IGF-1) signaling by klotho protein. In vitro, IGF-1 signaling was suppressed in human proximal tubular cells transfected with the klotho plasmid. Our data suggest that klotho modulates compensatory renal hypertrophy after nephrectomy via suppression of the IGF-1 signaling pathway, indicating a novel physiological role for klotho protein in the kidney.