*3.4. Angiotensin-(1–7)*

Ang-(1–7) was first discovered in rat brains in 1983 by Tonnaer and his colleagues [74]. However, at that time, it was thought to be an inactive peptide. The importance of Ang-(1–7) emerged in 1988 when it was found to be the major Ang-I-derived peptide in the presence and absence of ACE inhibition [75]. Ang-(1–7) was initially thought to exert its hypotensive e ffects in a bradykinin-dependent manner [76]. However, it was later demonstrated that Ang-(1–7) opposes the vasoconstrictive and proliferative actions of AT1R-mediated Ang-II actions [2,17]. In fact, the discovery of Ang-(1–7) and its e ffects lead to the belief that RAS local actions are mainly driven by the balance between the vasoconstrictor/proliferative and vasodilator/anti-proliferative actions of Ang-II and Ang-(1–7), respectivley [2].

Ang-(1–7) can be formed by di fferent enzymes and pathways (Figure 1 and Table 1). The most potent and well known Ang-(1–7)-generating enzyme is ACE2 (angiotensin-I converting enzyme 2), which can generate Ang-(1–7) directly from Ang-II, or indirectly from Ang-I through Ang-(1–9) intermediate [77,78]. In fact, the former pathway is more favorable because the a ffinity of ACE2 to Ang-II is 400-folds greater than that to Ang-I [79]. Ang-(1–9) can be generated from Ang-I by the action of ACE2, cathepsin A (CTSA) [80], or carboxypeptidase A3 (CPA3), and then cleaved to form Ang-(1–7) by ACE [77], ACE2, or neprilysin (MME) (Figure 1 and Table 1). Alternatively, Ang-(1–7) can also be formed directly from Ang-I by prolylendopeptidase (PREP), thimet Oligopeptidase 1 (THOP1), [81] and Neurolysin (NLN) or from Ang-II cleavage by ACE2, PREP, and prolylcarboxipeptidase (PRCP) [2,79] (Figure 1 and Table 1).

In fact, ACE2 levels and the ACE/ACE2 ratio is generally considered a reference for Ang-(1–7) production. However, ACE2 is restricted to certain tissues and cells such as endothelial cells of the heart, kidneys, and testes [82]. In addition, the contribution of alternative enzymes in the production of Ang-(1–7) should be considered. For instance, metallopeptidase activity accounts for almost all Ang (1–7) production in atrial homogenate preparations, whereas Ang-II was produced equally by ACE and chymase while cathepsin A was responsible for 65% of the liberated Ang (1–9) [80]. This indicates that local angiotensin peptides production depends as well on the activity of "alternative" enzymes at the tissue level.

Ang-(1–7) exerts its e ffects mainly through the Mas receptor (MasR) (Figure 1 and Table 1). MasR was first described as Ang-(1–7) receptor in 2003, where its deletion abolished the binding of Ang-(1–7) to mouse kidneys, accompanied with the loss of Ang-(1–7)-induced relaxation [83]. By binding to MasR, Ang-(1–7) may induce many e ffects, antagonizing those of Ang-II/AT1R, such as vasodilation, inhibition of cell growth, anti-thrombosis, and anti-arrhythmogenic e ffects [84]. In addition, it was shown that MasR may antagonize AT1R in vitro and in vivo by forming a hetero-dimer with the AT1R, thus blocking the latter's activity [85]. Moreover, Ang-(1–7) can act on the AT2R (Figure 1 and Table 1), which exerts very similar e ffects to those induced by MasR [86]. In addition, emerging evidences raised controversies on the specificity of MasR to Ang-(1–7). Recently, MasR was shown to be stimulated by multiple other molecules such as Neuropeptide FF, Alamandine, Angiotensin III, Angiotensin IV, and Angioprotectin. Similarly, independent studies demonstrated the absence of MAS1 activation after Ang-(1–7) treatment in human mammary arteries from patients undergoing coronary revascularization surgery, splanchnic vessels from cirrhotic liver of human and rats and aorta from Sprague–Dawley rats [87].

Ang-(1–7) is present in the circulation, in addition to several other tissues and organs including the heart, blood vessels, kidney and liver [88], where it exerts local paracrine and autocrine actions. The alteration in circulatory and tissue Ang-(1–7) levels were shown to be associated with several diseases, including hypertension preeclampsia, hypertrophic myocardial disease, cognitive heart disease, myocardial infarction (MI), chronic kidney disease (CKD), and hepatic cirrhosis [2]. For instance, *ACE2*−/− mice developed age-dependent cardiomyopathy with increased oxidative stress, neutrophilic infiltration, inflammatory cytokine, and collagenase levels, mitogen-activated protein kinase (MAPK) activation and pathological hypertrophy [89]. These e ffects were inhibited by irbesartan, an AT1R blocker (ARB), which indicates a critical role for ACE2 in the suppression of Ang-II-mediated heart failure. In addition, a recent study suggested an important role for the ACE/ACE2 imbalance in the pathogenesis of severe acute pancreatitis where the ratio of pancreatic ACE2 to ACE expression was significantly reduced and paralleled the severity of the disease [90]. In another study, a reduction in ACE/ACE2 ratio was shown to be associated with acute respiratory distress syndrome, which was prevented by Ang-(1–7) or ARB treatment [91]. Recent studies have supported a metabolic role for the Ang-(1–7)/MasR arm in the liver and its counter-regulatory action to Ang-II/AT1R that interferes in several steps of intracellular insulin signaling arm in the pathophysiology of liver diseases [92]. Indeed, Ang-(1–7) has been shown to ameliorate glucose tolerance and to enhance insulin sensitivity, while Mas receptor has been described as an essential component of the insulin receptor signaling pathway [93]. Of interest, ACE2 treatment has been shown to ameliorate liver fibrosis through reduction of hepatic Ang-II levels concomitant with increased concentrations of Ang-(1–7) in liver tissue [94–96]. Moreover, Ang-(1–7)/MasR axis agonists may also play a role in the treatment of CKD by controlling the inflammatory response and fibrosis in kidney tissue [97].

Of note, high concentrations of Ang-(1–7) exerts biphasic e ffects on Na<sup>+</sup>-, K<sup>+</sup>-ATPase activity in a dose dependent manner by inducing similar e ffects to those induced by Ang-II at high concentrations, independent of MasR and AT2R, probably through the AT1R [98]. However, in the presence of Ang-II, Ang-(1–7) antagonized the stimulatory e ffects of Ang-II on Na<sup>+</sup>, K<sup>+</sup>-ATPase activity through a A779-sensitive receptor [99]. On the other hand, Ang-(1–7) infusion or MasR deficiency enhanced renal damage in models of renal insu fficiency by aggravating the inflammatory response through NF-κB [100]. In contrast, another study showed that Ang-(1–7) suppressed inflammation by inhibiting the NF-κB pathway in rats with permanent cerebral ischaemia [101].

Taken together, these studies sugges<sup>t</sup> that Ang-(1–7) exerts cell-specific e ffects based on its concentrations, available receptors, angiotensin peptides, and the physiological state of the tissue.
