Intravenous administration of apeling-13 induces a depressor response by releasing an unidentified substance.

Apelin and APJ receptor play an important role in the regulating cardiovascular function; however, conflicting results have been reported regarding the effect of apelin on cardiovascular regulation. In this study, blood pressure and heart rate were measured by femoral arterial catheterization; and cardiac contractility was recorded by left ventricular catheterization through the right carotid artery in rats before and after intravenous administration of [pyr1]-apelin-13. The results show that intravenous administration of apelin-13 caused a dramatic reduction in BP but did not significantly alter heart rate and contractility. To study the mechanism of the apelin-induced depressor response, isometric tension was measured in isolated mesenteric arteries using a myograph approach. Surprisingly, treatment of the arteries with [pyr1]-apelin-13 did not cause relaxation of mesenteric arteries preconstricted with norepinephrine; however, treatment with plasma collected from rats that received intravenous administration of [pyr1]-apelin-13 caused pronounced relaxation of isolated arteries. Incubation with the guanylyl cyclase inhibitor, ODQ, blocked NO-induced relaxation, but did not significantly alter the relaxation response to the plasma from apelin-treated rats. Taken together, these findings demonstrate that intravenous injection of apelin causes a significant depressor response that is mediated by a NO-independent mechanism involving an unidentified substance released into the bloodstream leading to vasodilation.

APELIN IN HEART FAILURE.

Apelin is a biologically active protein encoded by the APLN gene. It was first isolated in 1998 as a ligand for the APJ receptor. It exists in several isoforms differing in polypeptide chain length and biological activity. It is secreted by white adipose tissue, and its expression has been identified in many body tissues, including the cardiovascular system, kidneys, lungs, CNS (especially the hypothalamus, suprachiasmatic and ventricular nuclei), skeletal muscle, mammary glands, adrenal glands, ovaries, stomach, liver cells, placenta, and breast milk. However, the highest concentrations were observed in the endocardium and endothelium of vascular smooth muscle cells. In myocardial tissue, apelin has a positive inotropic effect and exerts an opposing effect to the RAA (renin-angiotensin-aldosterone) system, lowering blood pressure. Therefore, its positive role in early stages of heart failure, in patients with hypertension and ischemic heart disease is emphasized. The synthesis and secretion of apelin by adipocytes makes it possible to classify this peptide as an adipokine. Therefore, its production in adipose tissue is enhanced in obesity. Furthermore, apelin has been shown to increase cellular sensitivity to insulin and improve glucose tolerance in the onset of type 2 diabetes, and therefore appears to play a significant role in the pathogenesis of metabolic disease. An accurate assessment of the importance of apelin in cardiovascular disease requires further studies, which may contribute to the use of apelin in the treatment of heart failure.

Gαi-biased apelin analog protects against isoproterenol-induced myocardial dysfunction in rats.

Apelin receptor (APJ) activation by apelin-13 (APLN-13) engages both Gαi proteins and ß-arrestins, stimulating distinct intracellular pathways and triggering physiological responses like enhanced cardiac contractility. Substituting the C-terminal phenylalanine of APLN-13 with α-methyl-l-phenylalanine [(l-α-Me)Phe] or p-benzoyl-l-phenylalanine (Bpa) generates biased analogs inducing APJ functional selectivity toward Gαi proteins. Using these original analogs, we proposed to investigate how the canonical Gαi signaling of APJ regulates the cardiac function and to assess their therapeutic impact in a rat model of isoproterenol-induced myocardial dysfunction. In vivo and ex vivo infusions of either Bpa or (l-α-Me)Phe analogs failed to enhance rats' left ventricular (LV) contractility compared with APLN-13. Inhibition of Gαi with pertussis toxin injection optimized the cardiotropic effect of APLN-13 and revealed the inotropic impact of Bpa. Moreover, both APLN-13 and Bpa efficiently limited the forskolin-induced and PKA-dependent phosphorylation of phospholamban at the Ser16 in neonatal rat ventricular myocytes. However, only Bpa significantly reduced the inotropic effect of forskolin infusion in isolated-perfused heart, highlighting its efficient bias toward Gαi. Compared with APLN-13, Bpa also markedly improved isoproterenol-induced myocardial systolic and diastolic dysfunctions. Bpa prevented cardiac weight increase, normalized both ANP and BNP mRNA expressions, and decreased LV fibrosis in isoproterenol-treated rats. Our results show that APJ-driven Gαi/adenylyl cyclase signaling is functional in cardiomyocytes and acts as negative feedback of the APLN-APJ-dependent inotropic response. Biased APJ signaling toward Gαi over the ß-arrestin pathway offers a promising strategy in the treatment of cardiovascular diseases related to myocardial hypertrophy and high catecholamine levels.NEW & NOTEWORTHY By using more potent Gαi-biased APJ agonists that strongly inhibit cAMP production, these data point to the negative inotropic effect of APJ-mediated Gαi signaling in the heart and highlight the potential protective impact of APJ-dependent Gαi signaling in cardiovascular diseases associated with left ventricular hypertrophy.

Apelin-13 Protects Corpus Cavernosum Against Fibrosis Induced by High-Fat Diet in an MMP-Dependent Mechanism.

BACKGROUND: An increased fibrosis of the corpora cavernosa is a prevalent process that underlies most cases of erectile dysfunction. Apelin, an endogenous circulating peptide, has been documented as an important effector on cardiovascular homeostasis, controlling vascular function and reducing fibrosis in multiple pathological conditions. Recently, initial studies have shown that Apelin, acting through the APJ receptor, also modulates penile erection, however, the role of this system on penile structure and intracorporal collagen remodeling has not been investigated yet. AIMS: Here we sought to investigate the effect of chronic Apelin treatment on the corpus cavernosum structure of hyperchOlesterolemic mice. METHODS: Apolipoprotein gene-deleted (ApoE-/-) mice were fed with a Western diet for 11 weeks and received Apelin-13 (2 mg/kg/day) or vehicle during the last 3 weeks. Penile samples were obtained for histological and biochemical analyses to assess the intracorporal collagen content and key proteins expression. Furthermore, the effect of Apelin-13 was evaluated in cultured NIH3T3 mouse fibroblasts stimulated with TGF-ß. OUTCOME: Local expression of Apelin-13 in mouse corpus cavernosum and its protective effect against fibrosis. RESULTS: Apelin and APJ receptor were expressed (gene and protein) within the corpus cavernosum of ApoE-/- mice, indicating a local modulation of the Apelin system. Interestingly, 3 weeks of Apelin-13 treatment strongly reduced intracavernosal collagen content. In addition, Apelin-13 enhanced total matrix metalloproteinase (MMP) activity in the mice penis, which was associated with an increased protein expression of MMP-1, MMP-3, MMP-8, and MMP-9, while tissue inhibitor of metalloproteinase were unaltered. These beneficial actions were not associated with changes in nNOS or eNOS protein expression, intracavernosal reactive oxygen species content, or atherosclerotic plaque deposition. Additionally, in cultured fibroblast, Apelin-13 inhibited TGF-ß-induced fibroblast to myofibroblast differentiation and collagen production, possibly through the activation of ERK1/2 kinase. CLINICAL TRANSLATION: These results point out Apelin/APJ system as a potential target to treat intracavernosal fibrosis-related disorders. STRENGTH & LIMITATIONS: These results provide the first evidence of the Apelin system's positive role on erectile tissue structure/remodeling. Nevertheless, additional functional study addressing erectile response would bring extended validation regarding the relevance of such effect. CONCLUSION: These results suggest a local modulation of the Apelin system within the corpus cavernosum. Remarkably, Apelin-13 reduced intracavernosal fibrosis in hypercholesterolemic mice by: (i) enhancing MMPs expression and activity; and (ii) inhibiting fibroblast differentiation into myofibroblast. Altogether, these results suggest an essential protective role of Apelin, indicating Apelin/APJ system as a promising candidate for the development of fibrosis-associated erectile dysfunction treatments. Sturny M, Anguenot L Costa-Fraga FP, et al. Apelin-13 Protects Corpus Cavernosum Against Fibrosis Induced by High-Fat Diet in an MMP-Dependent Mechanism. J Sex Med 2021;18:875-888.

Identification of 6-hydroxy-5-phenyl sulfonylpyrimidin-4(1H)-one APJ receptor agonists.

Heart failure (HF) treatment remains a critical unmet medical need. Studies in normal healthy volunteers and HF patients have shown that [Pyr1]apelin-13, the endogenous ligand for the APJ receptor, improves cardiac function. However, the short half-life of [Pyr1]apelin-13 and the need for intravenous administration have limited the therapeutic potential for chronic use. We sought to identify potent, small-molecule APJ agonists with improved pharmaceutical properties to enable oral dosing in clinical studies. In this manuscript, we describe the identification of a series of pyrimidinone sulfones as a structurally differentiated series to the clinical lead (compound 1). Optimization of the sulfone series for potency, metabolic stability and oral bioavailability led to the identification of compound 22, which showed comparable APJ potency to [Pyr1]apelin-13 and exhibited an acceptable pharmacokinetic profile to advance to the acute hemodynamic rat model.

Enteric apelin enhances the stress-induced stimulation of colonic motor functions.

In response to stress, apelin and corticotropin-releasing factor (CRF) are upregulated in the gastrointestinal (GI) tract. This study was designed to investigate the effect of stress on endogenous apelin in colon and its regulatory role on colonic motor functions. Colon transit (CT) was measured in rats exposed to acute restraint stress (ARS). APJ and CRF receptor antagonists F13A and astressin were administered intraperitoneally 30 min before ARS loading. Colonic muscle contractions were evaluated by in-vivo motility recording and in-vitro organ bath studies. Detection of apelin or CRF was performed using immunohistochemistry in proximal and distal colon of non-stressed (NS) and ARS-loaded rats. Immunoreactivity of CRF1 with apelin or APJ receptor was detected with double-labeled immunofluorescence in colonic myenteric neurons. Compared with NS rats, ARS accelerated the CT which was attenuated significantly by F13A or astressin. Following ARS, the expression of CRF was increased remarkably in distal colon, while the stress-induced change was not prominent in proximal colon. Apelin-positive cells were detected in myenteric ganglia of distal colon, while no apelin immunoreactivity observed in myenteric neurons of proximal colon. Both apelin and APJ receptor are colocalized with CRF1 in myenteric neurons of distal colon. In the in-vivo colonic motility experiments, apelin-13 exhibited a rapid stimulatory effect. CRF administration increased the motility which was abolished by F13A. Apelin-induced contractions in muscle strips were no longer observed with preadministration of F13A. These results suggest that enteric apelin contributes to the action of CRF on colonic motor functions under stressed conditions.LAY SUMMARYIt has been suggested in rodents that acute stress increases the expression of apelin in gastrointestinal tissues. We have found that under stressed conditions, enteric apelin contributes to the CRF-induced alterations in colonic motor functions through APJ receptor.

Half-life extension of peptidic APJ agonists by N-terminal lipid conjugation.

Agonism of the endothelial receptor APJ (putative receptor protein related to AT1; AT1: angiotensin II receptor type 1) has the potential to ameliorate congestive heart failure by increasing cardiac output without inducing hypertrophy. Although the endogenous agonist, pyr-apelin-13 (1), has shown beneficial APJ-mediated inotropic effects in rats and humans, such effects are short-lived given its extremely short half-life. Here, we report the conjugation of 1 to a fatty acid, providing a lipidated peptide (2) with increased stability that retains inotropic activity in an anesthetized rat myocardial infarction (MI) model. We also report the preparation of a library of 15-mer APJ agonist peptide-lipid conjugates, including adipoyl-γGlu-OEG-OEG-hArg-r-Q-hArg-P-r-NMeLeuSHK-G-Oic-pIPhe-P-DBip-OH (17), a potent APJ agonist with high plasma protein binding and a half-life suitable for once-daily subcutaneous dosing in rats. A correlation between subcutaneous absorption rate and lipid length/type of these conjugates is also reported.

Central interaction between the apelinergic and vasopressinergic systems in the regulation of the haemodynamic parameters in rats maintained on a high-fat diet.

A high-fat diet can affect the central activity of the apelinergic and vasopressinergic systems, which can have a significant impact on cardiovascular regulation. The aim of the study was to investigate the role of the central interaction between apelin and vasopressin in the regulation of the cardiovascular system in Sprague Dawley rats maintained on a normal-fat diet (NFD) or on a high-fat diet (HFD). The animals were instrumented with a cannula implanted into the left cerebral ventricle for intracerebroventricular (ICV) infusions of saline (0.9% NaCl), apelin-13 (APLN-13), V1a receptor antagonist (V1aRANT) APJ receptor antagonist (F13A), vasopressin (AVP); and with a catheter placed within the femoral artery for mean arterial blood pressure and heart rate monitoring. Blood, the hypothalamus and the medulla oblongata were collected for biochemical analysis. The hypertensive effect of APLN-13 was blocked by a prior ICV infusion of V1aRANT, only in the NFD rats. However, the hypertensive effect of AVP was blocked by the prior ICV infusion of F13A in both the NFD and HFD rats. A HFD caused an increase in the protein level of APJ and V1a receptors, both in the hypothalamus and the medulla oblongata. This study confirms the presence of an interaction between both peptides in the central regulation of the cardiovascular system in rats on a NFD or a HFD.

Apelin and stem cells: the role played in the cardiovascular system and energy metabolism.

Apelin, a member of the adipokine family, is widely distributed in the body and exerts cytoprotective effects on many organs. Apelin isoforms are involved in different physiological processes, including regulation of the cardiovascular system, cardiac contractility, angiogenesis, and energy metabolism. Several investigations have been performed to study the effect of apelin on stem cell therapy. This review aims to summarize the literature representing the effects of apelin on stem cell properties. Furthermore, this review discusses the therapeutic potential of apelin-treated stem cells for cardiovascular diseases and demonstrates the effect of stem cells overexpressing apelin on energy metabolism. Stem cells with their unique characteristics play a crucial role in the maintenance of tissue integrity. These cells participate in tissue regeneration via multiple mechanisms. Although preclinical and clinical studies have demonstrated the therapeutic potential of stem cells in various diseases, their application in regenerative medicine has not been efficient. A number of strategies such as genetic modification or treatment of stem cells with different factors have been used to improve the efficacy of cell therapy and to increase their survival after transplantation. This article reviews the effect of apelin treatment on the efficacy of cell therapy.

Loss of Apelin Augments Angiotensin II-Induced Cardiac Dysfunction and Pathological Remodeling.

Apelin is an inotropic and cardioprotective peptide that exhibits beneficial effects through activation of the APJ receptor in the pathology of cardiovascular diseases. Apelin induces the expression of angiotensin-converting enzyme 2 (ACE2) in failing hearts, thereby improving heart function in an angiotensin 1⁻7-dependent manner. Whether apelin antagonizes the over-activation of the renin⁻angiotensin system in the heart remains elusive. In this study we show that the detrimental effects of angiotensin II (Ang II) were exacerbated in the hearts of aged apelin-gene-deficient mice. Ang II-mediated cardiac dysfunction and hypertrophy were augmented in apelin knockout mice. The loss of apelin increased the ratio of angiotensin-converting enzyme (ACE) to ACE2 expression in the Ang II-stressed hearts, and Ang II-induced cardiac fibrosis was markedly enhanced in apelin knockout mice. mRNA expression of pro-fibrotic genes, such as transforming growth-factor beta (TGF-ß) signaling, were significantly upregulated in apelin knockout hearts. Consistently, treatment with the ACE-inhibitor Captopril decreased cardiac contractility in apelin knockout mice. In vitro, apelin ameliorated Ang II-induced TGF-ß expression in primary cardiomyocytes, accompanied with reduced hypertrophy. These results provide direct evidence that endogenous apelin plays a crucial role in suppressing Ang II-induced cardiac dysfunction and pathological remodeling.