Ang I is a decapeptide that is converted into the octapeptide Ang II by ACE. Unlike ACE, Ang I can be converted to Ang1-9 by ACE2, and, more importantly, Ang II is converted to Ang 1-7 through ACE2 (17). Ang1-7 has a range of anti-inflammatory, antioxidant, vasodilatory, and natriuretic effects that are mediated by the G protein coupled receptor (GPCR) Mas receptor (11,26,27). Ang1-7 may be produced directly from Ang I through the alternative pathways involving a zinc metallopeptidase neprilysin or conversion of Ang1-9 to Ang1-7 via ACE, although at a significantly lower efficiency (17). Genetic deletion studies have established ACE2 as an essential regulator of cardiovascular function (28). Studies focused on the regulation of ACE2 in cardiac myocytes and cardiac fibroblasts have demonstrated that although Ang II significantly reduced ACE2 activity and downregulated ACE2 mRNA in cardiac myocytes, it only reduced ACE2 activity in fibroblasts (29). In myocytes, endothelin (ET)-1 also significantly decreased ACE2 mRNA production (29). This reduction in ACE2 mRNA by Ang II or ET-1 was blocked by inhibitors of mitogen-activated protein kinase 1 (MAPK1), which suggested that Ang II and ET-1 activate extracellular signal-regulated kinase (ERK)1/ERK2 to reduce ACE2 (29). Furthermore, in vivo murine studies showed Ang II−mediated loss of membrane-bound cardiomyocyte ACE2 correlated with the upregulation of TACE/ADAM17 activity, which was prevented with AT1 receptor blockade (30). Cardiac fibroblasts and coronary endothelial cells also express ACE2 and TACE, and this reciprocal relationship extends to these cell types as well (31,32). Ang II activates several other signaling cascades, such as the PKC and JAK2-STAT3 signaling pathways, which results in myocardial hypertrophy and increased fibrosis (33). The binding of Ang1-7 to the C-terminal domain also inhibits the proteolytic function of the ACE enzyme and promotes bradykinin function (34). Studies in human vascular and cardiac tissue and plasma showed Ang1-7 has a higher affinity to ACE than Ang I, which suggests the inhibitory effects of Ang1-7 on ACE may contribute to its protective effects (35). The treatment of ACE2 knockout mice with Ang II infusion and recombinant ACE2 (rhACE2) eliminated ERK1/2, JAK2-STAT3, and PKC signaling by rhACE2 and was at least partially responsible for attenuation of Ang II−induced myocardial hypertrophy and fibrosis and improvement in diastolic dysfunction (33). Other studies highlighted the role of the ACE2/Ang1-7/Mas axis in modulating the expression of pro-inflammatory cytokines, such as TNF-α, interleukin (IL)-1β, IL-6, monocyte chemoattractant protein-1, and transforming growth factor-β in cardiac and/or lung fibrosis, pulmonary hypertension, and vascular remodeling (36, 37, 38, 39, 40, 41) (Figure 1 ).
Figure 1 RAS and ACE2/Ang1-7/Mas Axis Regulation
Angiotensinogen is converted to angiotensin I (Ang I) via renin. Ang I is converted to Ang II via angiotensin-converting enzyme (ACE), which also hydrolyzes bradykinin into its inactive metabolites, promoting inflammation. The pro-inflammatory effects of Ang II are mediated by Ang II type I receptor (AT1), which stimulates aldosterone secretion from the adrenal medulla and antidiuretic hormone from the posterior pituitary. Aldosterone decreases membrane ACE2 expression. Endothelin-1 inhibits angiotensin 1-7 (Ang1-7) via extracellular signal-regulated kinase (ERK)1/ERK2 pathways. Ang II, under favorable conditions (dashed line), can be converted to Ang1-7 via ACE2, whose counter regulatory effects are mediated by the Mas receptor. Ang1-7 can also be formed via conversion of Ang I to an intermediate Ang1-9 or directly via zinc metallopeptidase neprilysin/prolyl endopeptidase (PEP). RAS = renin-angiotensin system.