ACE2, angiotensin-(1-7) and Mas receptor axis in inflammation and fibrosis

Introduction

For years, the renin-angiotensin system (RAS) was described as a linear hormonal system involved in blood pressure regulation and water balance. Within this view, angiotensinogen synthesized in the liver is converted into the inactive peptide angiotensin (Ang) I through the renin action, which is produced by the juxtaglomerular cells of the kidney (Hall et al., 1990; Guyton, 1992). Subsequently, Ang I is cleaved by the ACE generating Ang II (Kokubu et al., 1979) whose actions are mediated by two GPCRs, angiotensin II receptor type 1 (AT₁) and type 2 (AT₂; Inagami, 1998; Touyz and Berry, 2002). In this context, the central players of this system are represented by ACE, Ang II and AT₁ receptor.

In several disease states, activation of this axis, that is, ACE/Ang II/AT₁ receptor, causes deleterious effects, including vasoconstriction, inflammation, fibrosis, cellular growth and migration and fluid retention (Kim and Iwao, 2000; Mehta and Griendling, 2007). Based on this concept, key anti-hypertensive, cardiovascular and renoprotective drugs were developed. Two of them deserve to be mentioned: ACE inhibitors (ACEi) and AT₁ receptor blockers (ARBs). These drugs are widely used with well-documented effectiveness. Nevertheless, important limitations have been reported related to the use of these RAS blockers. For instance, besides their potential side effects, the responses to ACEi treatment are influenced by gender and ethnic diversity and ARBs have limited efficacy in treatment of end-organ damage (Powers et al., 2011).

It is now clear that the RAS is far more complex than previously conceived. In fact, it has been proposed that, in addition to the ACE/Ang II/AT₁ receptor branch, the RAS possesses a counter-regulatory axis composed by ACE2, angiotensin-(1-7) [Ang-(1-7)] and the Mas receptor. It has been suggested that the activity of the RAS and the actions of RAS blockers depend on the balance between the ACE/Ang II/AT₁ and ACE2/Ang-(1-7)/Mas axes (Ferreira and Santos, 2005). Landmark studies were crucial to establish this new concept. At the end of the 1980s, Schiavone and co-workers (Schiavone et al., 1988) and Campagnole-Santos and colleagues (Campagnole-Santos et al., 1989) provided preliminary evidence that Ang-(1-7) was a biologically active peptide of the RAS. Subsequently, several studies showed that Ang-(1-7) exerts relevant cardiovascular and renal effects (Benter et al., 1993; 1995) and two pivotal discoveries clearly established Ang-(1-7) as an active RAS mediator. First, two independent research groups reported simultaneously in 2000 the existence and characterization of an enzyme homologue to ACE, the ACE2, which was established later as the main Ang-(1-7)-forming enzyme (Donoghue et al., 2000; Tipnis et al., 2000). Second, Santos and co-workers discovered that the G-protein coupled Mas is a functional receptor for Ang-(1-7) (Santos et al., 2003a). Thus, Ang-(1-7) is now considered a biologically active member of the RAS, which binds to Mas inducing many beneficial actions, such as vasodilation, inhibition of cell growth, anti-thrombosis and anti-arrhythmogenic effects (le Tran and Forster, 1997; Santos et al., 2003b; 2004; Grobe et al., 2007; Mercure et al., 2008; Nadu et al., 2008; Ferreira et al., 2010; Santiago et al., 2010).

Ang-(1-7) is produced mainly through the action of ACE2, which has approximately 400-fold less affinity to Ang I than to Ang II. Therefore, Ang II is the major substrate for Ang-(1-7) synthesis (Ferrario, 1990; Donoghue et al., 2000; Tipnis et al., 2000; Vickers et al., 2002). ACE2 can also form Ang-(1-7) less efficiently through hydrolysis of Ang I to Ang-(1–9) with subsequent Ang-(1-7) formation (Vickers et al., 2002). It is important to highlight the key role of ACE2 in this new concept of the RAS since it degrades the vasoconstrictive/proliferative peptide Ang II to form the vasodilator/antiproliferative heptapeptide Ang-(1-7). This is a strategic finding that may be exploited for therapeutic purposes (Hernandez Prada et al., 2008).

An important evidence for the relevance of the ACE2/Ang-(1-7)/Mas axis as a potential target to develop new therapeutic approaches comes from observations that administration of ACEi and ARBs causes substantial increases in plasma Ang-(1-7) levels (Iyer et al., 1998b) and increases in ACE2 expression (Sukumaran et al., 2011). These findings led to the assumption that part of the clinical benefits of ACEi and ARBs might be mediated by the ACE2/Ang-(1-7)/Mas axis (Chappell et al., 1998b; Iyer et al., 1998a; Davie and McMurray, 1999). Indeed, some effects of ACEi and ARBs can be blocked or attenuated by the Mas receptor antagonist, compound A-779 (Ang-(1-7) Mas receptor inhibitor), confirming the suggestion that the ACE2/Ang-(1-7)/Mas axis may be relevant for the actions of RAS blockers (Britto et al., 1997).

Here, we review recent findings related to the function of the ACE2/Ang-(1-7)/Mas axis in regulating processes associated with acute and chronic inflammation, including leukocyte influx, fibrogenesis and proliferation of certain cell types. We will also discuss the involvement of the ACE2/Ang-(1-7)/Mas axis in the context of renal disease because of the known relevance of the RAS for the function of this organ and for the regulation of kidney inflammation and fibrosis. This knowledge may help in paving the way for the development of novel treatments for chronic inflammatory and renal diseases.

The ACE2/Ang-(1-7)/Mas axis regulates leukocyte recruitment and activation

Many studies have shown that the RAS plays a relevant role in the pathogenesis of inflammatory diseases (Owen and Campbell, 1998; Ruiz-Ortega et al., 2001; Suzuki et al., 2003; Ma et al., 2010; Sukumaran et al., 2011; 2012; Capettini et al., 2012). In this context, most pro-inflammatory actions of the RAS appear to be due to the effects of Ang II. Indeed, Ang II activates several cell functions and molecular signalling pathways related to tissue injury, inflammation and fibrosis, including calcium mobilization, free radical generation, activation of protein kinases and nuclear transcription factors, recruitment of inflammatory cells, adhesion of monocytes and neutrophils to endothelial and mesangial cells, up-regulation of adhesion molecules and stimulation of expression, synthesis and release of cytokines and chemokines (Ruiz-Ortega et al., 2001; Suzuki et al., 2003; Ma et al., 2010; Sukumaran et al., 2011; 2012; Capettini et al., 2012). Most of these actions of Ang II are mediated by the AT₁ receptor and it has been suggested that blockade of the AT₁ may be useful for the treatment of inflammatory diseases, as reviewed elsewhere (Ruiz-Ortega et al., 2001; 2006; Benicky et al., 2009; Kon et al., 2011; Capettini et al., 2012).

More recently, several studies have shown that the counter-regulatory ACE2/Ang-(1-7)/Mas axis may also influence inflammatory responses. Indeed, there is now much evidence demonstrating that Ang-(1-7) modulates negatively leukocyte migration, cytokine expression and release, and fibrogenic pathways (Grobe et al., 2007; Guo et al., 2008; Ferreira et al., 2009; Yamazato et al., 2009; Shenoy et al., 2010; Silveira et al., 2010a; Thomas et al., 2010; Sriramula et al., 2011; Agarwal et al., 2012; Barroso et al., 2012; El-Hashim et al., 2012; Jiang et al., 2012; Jin et al., 2012; Sukumaran et al., 2012) (Figure 1, Table 1). For example, studies by Sukumaran and co-workers (Sukumaran et al., 2011; 2012) have shown that the ACE2/Ang-(1-7)/Mas axis is activated and is relevant for the anti-inflammatory effects of the ARBs Telmisartan and Olmesartan in a rat model of autoimmune myocarditis. ARBs increased ACE2, Ang-(1-7) and Mas expression in line with reduction of pro-inflammatory cytokines as TNF-α, IFN-γ, IL-1β, IL-6 and increase of the anti-inflammatory cytokine, IL-10 (Sukumaran et al., 2011; 2012). These anti-inflammatory effects were associated with less myocardial fibrosis and down-regulation of PI3K, phospho-Akt, phospho-p38 MAPK, phospho-JNK, phospho-ERK and phospho-MAPK-2, but formal demonstration of the role of the ACE2/Ang-(1-7)/Mas axis was not provided (Sukumaran et al., 2011; 2012).

In the brain, infusion of Ang-(1-7) into the lateral ventricle of Sprague-Dawley rats subjected to permanent middle cerebral artery occlusion was associated with local decrease of oxidative stress, suppression of NF-κB activity and reduction of pro-inflammatory cytokines (Jiang et al., 2012). Accordingly, it has been reported that the overexpression of ACE2 in paraventricular nucleus attenuated Ang II-induced increase in the expression of TNF-α, IL-1β and IL-6 (Sriramula et al., 2011). Up-regulation of IL-10, ACE2 and Mas expression was also reported in spontaneous hypertensive rats subjected to exercise of moderate intensity (Agarwal et al., 2012).

It has been demonstrated that genetic ACE2 deficiency increased vascular inflammation and atherosclerosis in ApoE knockout mouse by increasing gene expression of vascular cells adhesion molecules (VCAM), cytokines, chemokines and MMP (Thomas et al., 2010). In agreement with the latter concept, Jin et al. (2012) also showed that loss of ACE2 resulted in greater increases in Ang II-induced mRNA expression of inflammatory cytokines and chemokines in aorta of ACE2-deficient mice. This was associated with activation of NADPH oxidase activity, production of superoxide, profilin-1 expression and activation of the Akt-ERK-eNOS signalling pathway (Jin et al., 2012). Activation of the ACE2/Ang-(1-7)/Mas axis was also shown to modulate the expression of pro-inflammatory cytokines in a model of pulmonary hypertension. Indeed, there was decreased expression of TNF-α, IL-1β, IL-6, MCP-1 and TGF-β and increased expression of the anti-inflammatory cytokine, IL-10 (Ferreira et al., 2009; Yamazato et al., 2009; Shenoy et al., 2010).

Recruitment of leukocytes into tissues is a coordinated event that involves the timely and precise production and expression of chemokines (and other chemoattractants) and cell adhesion molecules (Petri et al., 2008; Menezes et al., 2011; Williams et al., 2011; Sanz and Kubes, 2012). Several in vitro studies have shown that the RAS modulates the migration and function of leukocytes (Piqueras et al., 2000; Nabah et al., ; Mateo et al., 2006; Nabah et al., 2007; Souza and Costa-Neto, 2012). Ang II contributes significantly to the process of leukocyte migration in vivo by modifying the interaction of leukocytes with endothelial cells (Piqueras et al., 2000; Alvarez et al., 2001; Mateo et al., 2006; Nabah et al., 2007; Silveira et al., 2010a; Company et al., 2011). Consistently with the latter findings, inhibition of ACE or blockade of AT₁ receptors decreases leukocyte endothelial cell interactions in various models of inflammation. Recently, our group demonstrated that administration of Ang-(1-7) or a synthetic analogue, AVE 0991 (Ang-(1-7) Mas receptor agonist), decreased rolling and adhesion of leukocytes to the microvascular endothelium at inflamed joints in a model of antigen-induced arthritis (Silveira et al., 2010a). Blockade of leukocyte adhesion was associated with decreased neutrophil influx into the joints and improvement of joint hypernociception (Silveira et al., 2010a). Altogether, these studies suggest that the ACE2/Ang-(1-7)/Mas axis counter-regulates the actions of the ACE/Ang II/AT₁ axis in the context of leukocyte recruitment.

Macrophages are believed to play a key role in development and progression of atherosclerosis, a pathological process in which there is altered production of many pro-inflammatory cytokines and up-regulation of adhesion molecules within atherosclerotic plaques (Libby, 2009). Few studies have reported that the ACE2/Ang-(1-7)/Mas axis decreases macrophage function (Thomas et al., 2010; Souza and Costa-Neto, 2012). Mas transcripts are up-regulated in macrophages after LPS exposure and Ang-(1-7) was able to regulate mRNA levels of pro-inflammatory cytokines IL-6 and TNF-α and to decrease the phosphorylation levels of Src kinases (Souza and Costa-Neto, 2012). Using macrophages isolated from the bone marrow of ACE2 knockout mice, there was increased expression of the adhesion molecule VCAM-1 and high levels of various cytokines, including TNF-α, CCL2 and IL-6, following LPS induction (Thomas et al., 2010). Moreover, the overexpression of ACE2 inhibited expression of MCP-1 induced by Ang II in macrophages and this effect seemed to be mediated by increased levels of Ang-(1-7) (Guo et al., 2008). Altogether, these studies clearly show that Ang-(1-7) can activate Mas receptor on the surface of leukocytes and usually inhibit their pro-inflammatory function (Figure 1).

The ACE2/Ang-(1-7)/Mas axis regulates fibrogenesis and remodelling

In addition to regulating leukocyte influx and parameters of acute inflammation (Table 1), there is good evidence that Ang II contributes significantly to fibrogenesis and organ remodelling, both of which are features of chronic inflammation (Table 2). Ang II acting on AT₁ receptors promotes hypertrophy (Sadoshima and Izumo, 1993) and stimulates fibroblast proliferation and expression of extracellular matrix proteins (Rosenkranz, 2004). The latter actions of ACE/Ang II/AT₁ axis appear to be blocked by ACEi and ARBs. Indeed, the beneficial effects of these RAS blockers in the context of chronic renal and cardiovascular diseases are, at least in part, attributed to inhibition of tissue fibrosis and remodelling (Lindholm et al., 2002; Weir, 2007; Gerc and Buksa, 2010; Tocci and Volpe, 2011). In contrast to the fibrogenic and proliferative actions of the ACE/Ang II/AT₁ axis, it has been suggested that the ACE2/Ang-(1-7)/Mas axis exerts anti-fibrogenic and anti-proliferative actions (Tallant and Clark, 2003; Gallagher and Tallant, 2004; Iwata et al., 2005; Tallant et al., 2005; Su et al., 2006; Pereira et al., 2007), as detailed below. Thus, imbalances between the two axes of the RAS plays a major role in the pathogenesis of several fibrotic diseases (Santos et al., 2003b; Tallant et al., 2005; Kuba et al., 2006; Lubel et al., 2009; Ferreira et al., 2012).

In models of cardiac fibrosis and remodelling, Grobe et al. (2007) showed that administration of Ang-(1-7) prevented cardiac fibrosis induced by Ang II in Sprague-Dawley rats (Grobe et al., 2007). In trained two-kidney one-clip hypertensive rats, administration of Ang-(1-7) decreased fibrosis and it was accompanied by up-regulation of Mas, AT₂ and endothelial NOS phosphorylation in the heart (Shah et al., 2012). ACE2-deficient mice, which have lower levels of Ang-(1-7), exhibited early cardiac hypertrophy (Oudit et al., 2007) and adverse ventricular remodelling after myocardial infarction (Kassiri et al., 2009). ACE2 deficiency also resulted in progressive cardiac fibrosis in models of aging and cardiac pressure overload (Crackower et al., 2002; Yamamoto et al., 2006; Nakamura et al., 2008). In contrast, ACE2 overexpression reversed cardiac hypertrophy and fibrosis in mice (Huentelman et al., 2005; Der Sarkissian et al., 2008). The anti-proliferative and anti-fibrotic effect of Mas activation in the heart may also involve modulation of several extracellular matrix proteins (Gava et al., 2012). Neonatal and adult Mas-deficient mice showed significantly higher levels of collagen types I and III and fibronectin and reduced levels of collagen IV in both right ventricle and AV valves (Gava et al., 2012).

There is now a large body of evidence suggesting that the presence of Ang II and AT₁ receptors are required for experimental lung fibrosis (Uhal et al., 2012). There is also evidence that the ACE2/Ang-(1-7)/Mas axis may have a regulatory role in the context of pulmonary fibrosis. In this regard, ACE2 mRNA, protein and enzymatic activity were decreased in human idiopathic pulmonary fibrosis and in both rat and mouse models of experimental lung fibrosis induced by bleomycin (Li et al., 2008). Blockade of ACE2 in mice increased pulmonary Ang II levels and enhanced bleomycin-induced lung fibrosis through a mechanism dependent on AT₁ receptors (Li et al., 2008). In the latter study, the role of Ang-(1-7) was not investigated.

More recently, Ang-(1-7) was shown to attenuate ovalbumin-induced leukocyte influx in airways spaces, perivascular and peribronchial inflammation, fibrosis and goblet cell hyper/metaplasia in a murine model of asthma (El-Hashim et al., 2012). Mechanistically, Ang-(1-7) reduced the phosphorylation of ERK1/2 and IκB-α and, consequently, decreased the activation of NF-κB in ovalbumin-challenged mice (El-Hashim et al., 2012). It will be interesting to investigate whether such effects of Ang-(1-7) will be reproduce in other models of chronic pulmonary inflammation in which fibrogenesis is more prominent.

TGF beta (TGF-β1) has been reported to be the most potent pro-fibrotic cytokine (Annes et al., 2003; Goodwin and Jenkins, 2009). Extensive evidence suggests a direct link between the RAS and TGF-β, indicating that TGF-β1 acts downstream of Ang II (Rosenkranz, 2004). Thus, Ang II stimulates TGF-β1 mRNA and protein expression by cardiomyocytes and cardiac fibroblasts (Campbell and Katwa, 1997; Gray et al., 1998) and the treatment with ACEi or ARBs decreased TGF-β1 levels in hypertrophied (Kim et al., 1996) and infarcted hearts (Sun et al., 1998; Yu et al., 2001). Recently, other studies have evaluated the role of ACE2/Ang-(1-7)/Mas axis in modifying expression of TGF-β and key components of the TGF-β pathway (Iwata et al., 2005; Grobe et al., 2007; Zeng et al., 2009; Shenoy et al., 2010; Marques et al., 2012) (Table 2). Ang-(1-7) decreased TGF-β1 mRNA levels in cultured cardiac fibroblasts (Iwata et al., 2005), reduced plasma levels of TGF-β1 in rat model of myocardial infarction (Grobe et al., 2007) and improved vascular remodelling via down-regulation of TGF-β and inhibition of the Smad2 pathway (Zeng et al., 2009). The blockade of Mas with A-779 increased liver tissue levels of TGF-β1 in a rat model of hepatic fibrosis (Pereira et al., 2007). Recently, it has been shown that a p.o. formulation including Ang-(1-7) in hydroxypropyl β-cyclodextrin produced improvement of diastolic and systolic functions and reduction of the expression of fibrosis scar markers (TGF-β and collagen type I) in a rat model of myocardial infarction induced by left coronary artery occlusion (Marques et al., 2012). In another study, intratracheal administration of lentiviruses expressing Ang-(1-7) or ACE2 induced overexpression of Ang-(1-7) in lungs of rats given bleomycin. More importantly, there was reduced expression of TGF-β and significant reduction of pulmonary fibrosis (Shenoy et al., 2010). Altogether, these studies suggest that regulation of TGF-β synthesis appears to contribute significantly to the anti-fibrogenic effects of the ACE2/Ang-(1-7)/Mas axis.

In addition to blocking TGF-β synthesis, several studies have evaluated the capacity of the ACE2/Ang-(1-7)/Mas axis to regulate TGF-β-independent molecular pathways believed to be important for fibrogenesis. For example, treatment of cardiac fibroblasts with Ang-(1-7) reduced Ang II- or ET-1-stimulated increase in phospho-ERK1 and -ERK2 and, in contrast, Ang-(1-7) increased the immunoreactivity and mRNA of dual-specificity phosphatase (McCollum et al., 2012). Ang-(1-7) administration also blocked Ang II-stimulated phosphorylation and activation of ERK1 and ERK2 and MAPK activity in cardiac myocytes (Tallant et al., 2005). Moreover, incubation of cardiac fibroblasts with ET-1 increased COX-2 and PG synthase mRNAs, while Ang-(1-7) blocked the increase of both enzymes, suggesting that the heptapeptide regulates the balance between proliferative and anti-proliferative PGs (McCollum et al., 2012). Treatment of cardiac fibroblasts with recombinant human ACE2 or ACE2 activators reduced collagen production and Ang II-mediated increase in phospho-MAP kinases. Both effects were blocked by the Mas receptor antagonist A-779 (Zhong et al., 2010; Ferreira et al., 2011). These studies suggest that Ang-(1-7) may block key signalling pro-fibrogenic events initiated by Ang II, endothelins and other pro-fibrogenic molecules.

Many studies have reported that Ang-(1-7) exerts antiproliferative actions at different tissues (Tallant and Clark, 2003; Gallagher and Tallant, 2004; Iwata et al., 2005; Tallant et al., 2005; Santos et al., 2006; Su et al., 2006; Pereira et al., 2007). The antiproliferative effects of Ang-(1-7) in vascular smooth muscle cells (Tallant et al., 2005), liver tissue (Pereira et al., 2007) and cardiomyocytes (Iwata et al., 2005) seem to be mediated by Mas. In support of the latter possibility, Mas-deficient mice exhibited impairment of heart function associated with changes in collagen expression towards a pro-fibrotic profile (Santos et al., 2006). The molecular mechanisms that underlie these antiproliferative action of Ang-(1-7) include stimulation of PGs and cAMP production and inhibition of MAP kinases (Tallant and Clark, 2003). For example, Ang-(1-7) caused inhibition of growth of human lung cells via reduction in serum-stimulated phosphorylation of ERK 1 and ERK 2 (Gallagher and Tallant, 2004). In cultured vascular smooth muscle cells, Ang-(1-7) inhibited Ang II-activated protein kinase C and MAPK (Tallant et al., 2005). Similarly, in cultured human endothelial cells, Ang-(1-7) inhibited Ang II-stimulated phosphorylation of c-Src and ERK 1/2, and blunted Ang II-stimulated NADPH oxidase activity (Sampaio et al., 2007). These effects of Ang-(1-7) were associated with phosphorylation of Src homology 2 domain containing protein tyrosine phosphatase-2 and blocked by the Mas antagonist A-779 (Sampaio et al., 2007).

As the ERK cascade is activated in response to different stimuli, such as growth factors, cytokines or DNA-damaging agents, the stimulation of the ACE2/Ang-(1-7)/Mas axis could also be effective in halting glomerulosclerosis. In this regard, Su et al. (2006) showed that Ang-(1-7) inhibits Ang II-stimulated phosphorylation of ERK1/2, p38 MAPKs and JNK in culture rat proximal tubular cells, an effect reversed by pretreatment with A-779 (Su et al., 2006). Ang-(1-7) also prevented Ang II-induced production of TGF-β1 in proximal tubular cells (Su et al., 2006). In cultured pig kidney tubular cells, Ang-(1-7) inhibits glucose-induced phosphorylation of p38 MAPK, an effect attributed to the increased activity of SHP-1 (Gava et al., 2009). Ang-(1-7) attenuates high-glucose-induced TGF-β1 production, but has no effect on enhanced fibronectin or collagen levels in these cells (Gava et al., 2009). Thus, the generation of Ang-(1-7) by proximal tubular ACE2 could counteract the proliferative effects of locally produced Ang II (Su et al., 2006).

The ACE2/Ang-(1-7)/Mas axis in renal physiology

Many studies have shown the role of Ang-(1-7) in regulating renal haemodynamics, glomerular filtration and tubular resorption (see reviews by Simoes e Silva and Flynn, 2012; Zimmerman and Burns, 2012; Figure 2). Renal effects of Ang-(1-7) are complex and may not always be opposite to those elicited by Ang II. While Ang II markedly raises efferent glomerular arteriolar resistance and does not change afferent arteriolar resistance, Ang-(1-7) directly and indirectly vasodilates afferent arterioles and increases renal blood flow by acting via Mas with release of PGs and NO (Ren et al., 2002; Sampaio et al., 2003; Stegbauer et al., 2004; Botelho-Santos et al., 2007). As observed for Ang II acting at the proximal tubule, Ang-(1-7) also exerts differential effects according to peptide concentration and nephron site (Simoes e Silva and Flynn, 2012; Zimmerman and Burns, 2012 for review). In vitro studies and findings in anaesthetized animals have suggested that Ang-(1-7) acts as a natriuretic/diuretic hormone by directly inhibiting sodium reabsorption (DelliPizzi et al., 1994; Vallon et al., 1997; Lopez Ordieres et al., 1998; Handa, 1999; Burgelova et al., 2002) or by modulating Ang II actions (Burgelova et al., 2002; Lara Lda et al., 2006). In contrast, other studies showed that Ang-(1-7) has an antidiuretic effect in water-loaded animals, probably acting at distal nephron sites via a Mas-mediated increase in water transport (Santos and Baracho, 1992; Garcia and Garvin, 1994; Santos et al., 1996; 2001; Simoes e Silva et al., 1997; 1998; Baracho et al., 1998; Magaldi et al., 2003; Pinheiro et al., 2004; Ferreira et al., 2006). In addition, acute and chronic administration of Mas antagonists increased basal urinary volume, water excretion and glomerular filtration rate in normotensive and in spontaneous hypertensive rats (Santos et al., 1996; 2003a; Simoes e Silva et al., 1998; Magaldi et al., 2003). Indeed, differences between species, local and systemic concentrations of Ang-(1-7), nephron segment, level of RAS activation and sodium and water status can be responsible for these divergent effects on renal function (Figure 2).

The intrarenal levels of Ang II and Ang-(1-7) are regulated in accordance with homeostatic needs. Ang-(1-7) is present in the kidney at concentrations that are comparable to Ang II (Chappell et al., 1998a; 2001; 2004; Allred et al., 2000; Ferrario et al., 2002; 2005). Ang-(1-7) is the main product obtained in isolated proximal tubules (Chappell et al., 2001) and it is also present in distal convoluted tubules and collecting ducts (Ferrario et al., 2002). The processing pathways for Ang-(1-7) synthesis are different in the circulation and in the kidney. In the circulation, neutral endopeptidase (NEP) is one of the major enzymes that produce Ang-(1-7) from Ang I or Ang-(1–9) (Chappell et al., 1998b), while in the kidney, NEP may contribute to both the synthesis as well as the degradation of Ang-(1-7) (Allred et al., 2000; Chappell et al., 2001). It appears that ACE2 is the main enzyme responsible for the conversion of Ang II into Ang-(1-7) in the kidney (Chappell et al., 2004; Ferrario et al., 2005). It has been demonstrated that the distribution of ACE2 within renal tubules is similar to that of Ang-(1-7) (Chappell et al., 2004) and rats receiving either lisinopril or losartan increased ACE2 activity and urinary levels of Ang-(1-7) (Ferrario et al., 2005). It was hypothesized that ACE inhibition or AT₁ receptor blockade might increase intrarenal formation of Ang-(1-7) through ACE2 activation (Ferrario et al., 2005). It should also be pointed that there are gender differences in renal activity of ACE2 and in the mRNA expression for this enzyme at renal tissue. Ji and co-workers (Ji et al., 2008) showed that ovariectomy decreased ACE2 protein and mRNA expression in hypertensive rats, while 17-beta-estradiol replacement prevented these effects. In addition, the infusion of Ang-(1-7) attenuated renal injury, which was exacerbated by ovariectomy in this experimental model (Ji et al., 2008). More recently, higher ACE2 activity was detected in the kidney of male mice compared to females, a difference which was linked to the presence of 17-beta-estradiol in the ovarian hormone milieu (Liu et al., 2010). Therefore, changes in 17-beta-estradiol probably lead to fluctuations in renal ACE2 activity in women during puberty, pregnancy and menopause with potential physiological implications.

Immunohistochemical data have shown a similar distribution for Ang-(1-7), ACE2 and Mas within the kidney (Chappell et al., 2004), placing the key components together for activation and activity. Consistent with the latter finding, it has been shown that the biological effects of Ang-(1-7) in the kidney are primarily mediated by Mas (Santos et al., 2003b; Pinheiro et al., 2004; 2009). Mas-deficient mice have fluid retention, glomerular hyperfiltration, microalbuminuria, increased collagen deposition and mRNA overexpression of AT₁ receptor and TGF-β in renal tissues (Pinheiro et al., 2009). These results indicate that the lack of Mas may lead to RAS imbalance with unopposed actions of the ACE/Ang II/AT₁ axis in the kidney.

The ACE2/Ang-(1-7)/Mas axis in renal diseases

Concluding remarks

The discovery that Ang-(1-7) offsets the major biological effects of Ang II has contributed to the realization that the RAS is composed of two opposing axes. The first axis is constituted by the enzyme ACE, with Ang II as the end product, and the AT₁ receptor as the main effector mediating the biological actions of Ang II. The second axis results from ACE2-mediated hydrolysis of Ang II, leading to production of Ang-(1-7), with Mas receptor as the main effector conveying the vasodilator, antiproliferative, anti-inflammatory and anti-fibrotic effects of Ang-(1-7). Activation of the ACE2/Ang-(1-7)/Mas axis decreases inflammatory cell function and fibrogenesis in diverse models of human diseases. In the context of experimental models of renal diseases, most studies suggest that the ACE2/Ang-(1-7)/Mas axis has a protective role. The renoprotective effects of Ang-(1-7) seem to involve the modulation of oxidative stress, leukocyte influx and activation, and fibrosis in renal tissues. The few data provided by human studies also indicate a beneficial role for the activation of this alternative RAS axis in patients with renal diseases. In addition, it has hypothesized that the beneficial effects of ACEi and ARBs in renal diseases might involve, at least in part, the elevation of plasma Ang-(1-7) levels.

Further studies are clearly needed to elucidate the mechanisms by which both RAS axes precisely act in cooperation to modulate inflammation, fibrosis and proliferation in renal disease and other disease states. Nevertheless, current knowledge do support the possibility that drugs which mimic or enhance the function of the ACE2/Ang-(1-7)/Mas axis may be beneficial for the treatment of chronic diseases with inflammatory, fibrotic and proliferative components. Clinical trials with such drugs will eventually demonstrate if this possibility will turn into useful therapeutics.

Conflict of interest

The authors declare no conflict of interest.