Angiotensin converting enzyme 2 (ACE2) is a negative regulator of the renin-angiotensin system (RAS), catalyzing the conversion of Angiotensin II to Angiotensin 1-7. Apelin is a second catalytic substrate for ACE2 and functions as an inotropic and cardioprotective peptide. While an antagonistic relationship between the RAS and apelin has been proposed, such functional interplay remains elusive. Here we found that ACE2 was downregulated in apelin-deficient mice. Pharmacological or genetic inhibition of angiotensin II type 1 receptor (AT1R) rescued the impaired contractility and hypertrophy of apelin mutant mice, which was accompanied by restored ACE2 levels. Importantly, treatment with angiotensin 1-7 rescued hypertrophy and heart dysfunctions of apelin-knockout mice. Moreover, apelin, via activation of its receptor, APJ, increased ACE2 promoter activity in vitro and upregulated ACE2 expression in failing hearts in vivo. Apelin treatment also increased cardiac contractility and ACE2 levels in AT1R-deficient mice. These data demonstrate that ACE2 couples the RAS to the apelin system, adding a conceptual framework for the apelin-ACE2–angiotensin 1-7 axis as a therapeutic target for cardiovascular diseases.

Angiotensin-converting enzyme 2 (ACE2) is a negative regulator of the renin-angiotensin system, and functions as the key SARS coronavirus receptor and stabilizer of neutral amino acid transporters. ACE2 catalyzes the conversion of angiotensin II to angiotensin 1–7, thereby counterbalancing ACE activity. Accumulating evidence indicates that the enzymatic activity of ACE2 has a protective role in cardiovascular diseases. Loss of ACE2 can be detrimental, as it leads to functional deterioration of the heart and progression of cardiac, renal, and vascular pathologies. Recombinant soluble human ACE2 protein has been demonstrated to exhibit beneficial effects in various animal models, including cardiovascular diseases. ACE2 is a multifunctional enzyme and thus potentially acts on other vasoactive peptides, such as Apelin, a vital regulator of blood pressure and myocardium contractility. In addition, ACE2 is structurally a chimeric protein that has emerged from the duplication of 2 genes: homology with ACE at the carboxypeptidase domain and homology with Collectrin in the transmembrane C-terminal domain. ACE2 has been implicated in the pathology of Hartnup’s disease, a disorder of amino acid homeostasis, and, via its function in amino acid transport, it has been recently revealed that ACE2 controls intestinal inflammation and diarrhea, thus regulating the gut microbiome. This review summarizes and discusses the structure and multiple functions of ACE2 and the relevance of this key enzyme in disease pathogenesis. (Circ J 2013; 77: 301–308)

Elabela/Toddler/Apela (ELA) has been identified as a novel endogenous peptide ligand for APJ/Apelin receptor/Aplnr. ELA plays a crucial role in early cardiac development of zebrafish as well as in maintenance of self-renewal of human embryonic stem cells. Apelin was the first identified APJ ligand, and exerts positive inotropic heart effects and regulates the renin–angiotensin system. The aim of this study was to investigate the biological effects of ELA in the cardiovascular system. Continuous infusion of ELA peptide significantly suppressed pressure overload-induced cardiac hypertrophy, fibrosis and impaired contractility in mice. ELA treatment reduced mRNA expression levels of genes associated with heart failure and fibrosis. The cardioprotective effects of ELA were diminished in APJ knockout mice, indicating that APJ is the key receptor for ELA in the adult heart. Mechanistically, ELA downregulated angiotensin-converting enzyme (ACE) expression in the stressed hearts, whereas it showed little effects on angiotensin-converting enzyme 2 (ACE2) expression, which are distinct from the effects of Apelin. FoxM1 transcription factor, which induces ACE expression in the stressed hearts, was downregulated by ELA but not by Apelin. ELA antagonized angiotensin II-induced hypertension, cardiac hypertrophy, and fibrosis in mice. The ELA-APJ axis protects from pressure overload-induced heart failure possibly via suppression of ACE expression and pathogenic angiotensin II signalling. The different effects of ELA and Apelin on the expression of ACE and ACE2 implicate fine-tuned mechanisms for a ligand-induced APJ activation and downstream signalling.