Apelin pathway in cardiovascular, kidney, and metabolic diseases: Therapeutic role of apelin analogs and apelin receptor agonists

The apelin/apelin receptor (ApelinR) signal transduction pathway exerts essential biological roles, particularly in the cardiovascular system. Disturbances in the apelin/ApelinR axis are linked to vascular, heart, kidney, and metabolic disorders. Therefore, the apelinergic system has surfaced as a critical therapeutic strategy for cardiovascular diseases (including pulmonary arterial hypertension), kidney disease, insulin resistance, hyponatremia, preeclampsia, and erectile dysfunction. However, apelin peptides are susceptible to rapid degradation through endogenous peptidases, limiting their use as therapeutic tools and translational potential. These proteases include angiotensin converting enzyme 2, neutral endopeptidase, and kallikrein thereby linking the apelin pathway with other peptide systems. In this context, apelin analogs with enhanced proteolytic stability and synthetic ApelinR agonists emerged as promising pharmacological alternatives. In this review, we focus on discussing the putative roles of the apelin pathway in various physiological systems from function to dysfunction, and emphasizing the therapeutic potential of newly generated metabolically stable apelin analogs and non-peptide ApelinR agonists.

Effective Use of Empirical Data for Virtual Screening against APJR GPCR Receptor

Alzheimer's disease is a neurodegenerative disorder incompatible with normal daily activity, affecting one in nine people. One of its potential targets is the apelin receptor (APJR), a G-protein coupled receptor, which presents considerably high expression levels in the central nervous system. In silico studies of APJR drug-like molecule binding are in small numbers while high throughput screenings (HTS) are already sufficiently many to devise efficient drug design strategies. This presents itself as an opportunity to optimize different steps in future large scale virtual screening endeavours. Here, we ran a first stage docking simulation against a library of 95 known binders and 3829 generated decoys in an effort to improve the rescoring stage. We then analyzed receptor binding site structure and ligands binding poses to describe their interactions. As a result, we devised a simple and straightforward virtual screening Stage II filtering score based on search space extension followed by a geometric estimation of the ligand-binding site fitness. Having this score, we used an ensemble of receptors generated by Hamiltonian Monte Carlo simulation and reported the results. The improvements shown herein prove that our ensemble docking protocol is suited for APJR and can be easily extrapolated to other GPCRs.

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 β-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 β-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.

A metabolically stable apelin-17 analog decreases AVP-induced antidiuresis and improves hyponatremia

Apelin and arginine-vasopressin (AVP) are conversely regulated by osmotic stimuli. We therefore hypothesized that activating the apelin receptor (apelin-R) with LIT01-196, a metabolically stable apelin-17 analog, may be beneficial for treating the Syndrome of Inappropriate Antidiuresis, in which AVP hypersecretion leads to hyponatremia. We show that LIT01-196, which behaves as a potent full agonist for the apelin-R, has an in vivo half-life of 156 minutes in the bloodstream after subcutaneous administration in control rats. In collecting ducts, LIT01-196 decreases dDAVP-induced cAMP production and apical cell surface expression of phosphorylated aquaporin 2 via AVP type 2 receptors, leading to an increase in aqueous diuresis. In a rat experimental model of AVP-induced hyponatremia, LIT01-196 subcutaneously administered blocks the antidiuretic effect of AVP and the AVP-induced increase in urinary osmolality and induces a progressive improvement of hyponatremia. Our data suggest that apelin-R activation constitutes an original approach for hyponatremia treatment.

Chronic apelin analogue administration is more effective than established incretin therapies for alleviating metabolic dysfunction in diabetic db/db mice

Stable apelin-13 peptide analogues have shown promising acute antidiabetic effects in mice with diet-induced obesity diabetes. Here the efficacy of (pGlu)apelin-13 amide (apelin amide) and the acylated analogue (pGlu)(Lys⁸GluPAL)apelin-13 amide (apelin FA), were examined following chronic administration in db/db mice, a genetic model of degenerative diabetes. Groups of 9-week old male db/db mice (n = 8) received twice daily injections (09:00 and 17:00 h; i.p.) or saline vehicle, apelin amide, apelin FA, or the established incretin therapies, exendin-4(1-39) or liraglutide, all at 25 nmol/kg body weight for 21 days. Control C57BL/6J mice were given saline twice daily. No changes in body weight or food intake were observed with either apelin or liraglutide treatments, but exendin-4 showed a reduction in cumulative food intake (p < 0.01) compared with saline-treated db/db mice. Apelin analogues and incretin mimetics induced sustained improvements of glycaemia (p < 0.05 to p < 0.001, from day 9-21), lowered HbA_1c at 21 days (p < 0.05) and raised plasma insulin concentrations. The treatments also improved OGTT and ipGTT with enhanced insulin responses compared with saline-treated control db/db mice (p < 0.05 to p < 0.001). Apelin amide was superior to incretin mimetics in lowering plasma triglycerides by 34% (p < 0.05). Apelin analogues unlike both incretin mimetics reduced pancreatic α-cell area (p < 0.05 to p < 0.01) and all peptide treatments enhanced pancreatic insulin content (p < 0.05 to p < 0.01). In conclusion, longer-term administration of apelin-13 analogues, induced similar and in some respects more effective metabolic improvements than incretin mimetics in db/db mice, providing a viable alternative approach for counteracting metabolic dysfunction for mild and more degenerative forms of the disease.

Apelin administration improves insulin sensitivity in overweight men during hyperinsulinaemic-euglycaemic clamp

AIMS

Apelin is a recently identified adipokine known to improve glucose tolerance and insulin sensitivity in murine models. This study was dedicated to the proof of concept that apelin administration also enhances insulin sensitivity in humans.

MATERIALS AND METHODS

Healthy overweight men were enrolled in this randomized, double-blind, placebo-controlled, cross-over study that successively considered the efficacy and the tolerance of 2 doses of (pyr1)-Apelin-13. A first group of subjects received 9 nmol/kg (n = 8) of (pyr1)-Apelin-13 and, after examination of safety data, a second group received 30 nmol/kg (n = 8). Each volunteer underwent 2 hyperinsulinaemic-euglycaemic clamps where the basal level of glucose infusion rate (GIR) was measured from the 90th to the 120th minute (level 1). Continuous intravenous administration of apelin or placebo was ongoing for 2 hours and GIR was finally evaluated from the 210th to the 240th minute (level 2). Primary evaluation endpoint was the difference in GIR between level 2 and level 1 (ΔGIR).

RESULTS

A slight increase in ΔGIR was observed with the low apelin dose (0.65 ± 0.71 mg/kg/min, P = .055) whereas the highest dose significantly improved insulin sensitivity (0.82 ± 0.71 mg/kg/min, P = .033). Cardiovascular monitoring and safety reports did not reveal any side effect of apelin administration.

CONCLUSION

As the first demonstration of the insulin-sensitizing action of apelin in humans, alongside numerous studies in rodents, this trial confirms that the apelin/APJ pathway should be considered as a new target to design alternative therapeutic strategies to control insulin resistance in type 2 diabetic patients.