Apelin

Overview

Apelin is a bioactive peptide ligand for the APJ orphan G protein-coupled receptor, originally identified in 1998 by Tatemoto and colleagues from bovine stomach extracts using receptor-binding assays. The apelin precursor (preproapelin) comprises 77 amino acids and undergoes proteolytic processing to generate multiple biologically active isoforms including apelin-36, apelin-17, apelin-13, and apelin-8. Apelin-13 (pGlu-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro), a 13-amino-acid pyroglutamylated peptide, represents the most potent and prevalent circulating form.

Gene and Tissue Distribution

The human APJ gene (APLN) maps to chromosome Xq25-26 and encodes a 380-amino-acid G protein-coupled receptor. Apelin is expressed in multiple tissues including adipocytes, cardiomyocytes, endothelial cells, stomach, and brain. Adipose tissue represents a major source of circulating apelin, establishing it as an adipokine. Expression is regulated by nutritional status, with fasting increasing and refeeding decreasing plasma apelin levels.

Receptor Signaling Mechanisms

Apelin activates APJ, which couples to both Gq/11 and Gi/o proteins, enabling pleiotropic signaling. Gq/11 coupling stimulates phospholipase C and intracellular calcium mobilization, while Gi/o coupling inhibits adenylyl cyclase and activates PI3K/Akt pathways. APJ also forms heterodimers with angiotensin II type 1 receptor (AT1R), where apelin binding to APJ inhibits AT1R signaling, counterbalancing the renin-angiotensin system.

Cardiovascular Actions

Apelin produces potent endothelium-dependent and -independent vasodilation through nitric oxide production. In the heart, apelin exerts positive inotropic effects and enhances cardiac contractility without increasing myocardial oxygen demand. It promotes angiogenesis and protects against ischemia-reperfusion injury through Akt-dependent survival signaling.

Fluid and Metabolic Homeostasis

Apelin regulates body fluid balance by acting on hypothalamic circuits controlling water intake and by modulating renal sodium excretion. In metabolism, apelin enhances insulin sensitivity, promotes glucose uptake in skeletal muscle, and inhibits adipocyte differentiation. Circulating apelin levels are elevated in obesity and early-stage type 2 diabetes, suggesting a compensatory role in metabolic stress.

Pathophysiological Significance

Plasma apelin is paradoxically elevated in early heart failure but declines in advanced disease, suggesting a counterregulatory role. Apelin-APJ signaling is disrupted in hypertension, atherosclerosis, and pulmonary arterial hypertension, making it a potential therapeutic target.

References

1. Tatemoto, K., et al. (1998). Isolation and characterization of APJ, an orphan receptor. Biochemical and Biophysical Research Communications, 251(2), 471-476.
2. Castan-Laurell, I., et al. (2011). Apelin, a novel adipokine. Clinical Science, 120(3), 101-109.
3. Koeppen, M., et al. (2009). Apelin and the cardiovascular system. Cardiovascular Research, 82(1), 33-42.