key: cord-0690277-eqk8a34e
authors: Wang, Di; Chai, Xiao-qing; Magnussen, Costan G.; Zosky, Graeme R.; Shu, Shu-hua; Wei, Xin; Hu, Shan-shan
title: Renin-angiotensin-system, a potential pharmacological candidate, in acute respiratory distress syndrome during mechanical ventilation
date: 2019-08-01
journal: Pulm Pharmacol Ther
DOI: 10.1016/j.pupt.2019.101833
sha: 9969c4a278d78d1205c81461f4f3063fc0c7acc8
doc_id: 690277
cord_uid: eqk8a34e

While effective treatments for acute respiratory distress syndrome (ARDS) are lacking, mechanical lung ventilation can sustain adequate gas exchange in critically ill patients with respiratory failure due to ARDS. However, as a result of the phenomenon of ventilator-induced lung injury (VILI), there is an increasing need to seek beneficial pharmacological therapies for ARDS. Recent studies have suggested the renin-angiotensin system (RAS), which consists of the ACE/Ang-II/AT1R axis and ACE2/Ang-(1–7)/MasR axis, plays a dual role in the pathogenesis of ARDS and VILI. This review highlights the deleterious action of ACE/Ang-II/AT1R axis and the beneficial role of ACE2/Ang-(1–7)/MasR axis, as well as AT2R, in VILI and ARDS, and also discusses the possibility of targeting RAS components with pharmacological interventions to improve outcomes in ARDS.

Acute respiratory distress syndrome (ARDS) is a devastating disorder characterized by overwhelming pulmonary inflammation leading to hypoxemia and respiratory failure [1] . ARDS severity can be categorized as mild (200 mm Hg < PaO 2 /FIO 2 ≤ 300 mm Hg), moderate (100 mm Hg < PaO 2 /FIO 2 ≤ 200 mm Hg), or severe (PaO 2 / FIO 2 ≤ 100 mm Hg) [1] . Approximately 10% of patients admitted to intensive care units (ICUs) are diagnosed with ARDS according to the Berlin Definition, representing nearly 3 million patients with ARDS globally per annum [1] [2] [3] [4] . Different from the classification in Berlin definition, recent studies have suggested that ARDS can be stratified into two biological phenotypes, 'hypo-inflammatory' and 'hyper-inflammatory' (also referred to as 'reactive'). The hyper-inflammatory phenotype has more severe clinical features but responds to a range of treatments including PEEP, fluid management and pharmacotherapy [5] [6] [7] . Despite significant insights into the pathophysiology, once ARDS is established, patient mortality is high (up to 45%), and evidencebased therapeutic options are limited [1] [2] [3] [4] . Mechanical ventilation, which supports adequate gas exchange, is a life-saving intervention for critically ill patients with respiratory failure. Unfortunately, mechanical ventilation can also contribute to lung injury via a process known as ventilator-induced lung injury (VILI), also termed as ventilator-associated lung injury (VALI), with a prevalence of 6.2% among mechanically ventilated patients [8] . Historically, the application of potentially injurious mechanical ventilation using large tidal volumes (V t ) and high peak airway pressure (P peak ) increased the risk of development of ARDS both the ICU (odds ratio 2.6 for V t > 700 mL and 1.6 for P peak > 30 cm H2O) and during surgical ventilation (odds ratio 1.56 for each mL/kg increase of intraoperative V t ) [8, 9] . VILI is characterized by inflammatory cell infiltration in the lungs, loss of the epithelial and endothelial integrity, increased capillary permeability, deposition of extracellular matrix and interstitial pulmonary edema and fibrosis [10, 11] . To improve the clinical relevance, a two-hit model, with the LPS challenge (intratracheal or intraperitoneal) followed by injurious ventilation, has been applied in preclinical studies to mimic pre-existing sepsis which is the most common cause of ARDS [12, 13] . VILI, which can occur in previously normal lungs or worsens pre-existing ARDS, can occur via a range of mechanisms (1) including tissue stress caused by over-distension of lung alveoli exposed to increasing transpulmonary pressures (baro-volutrauma), (2) causing high local shearing forces (atelectrauma), (3) these mechanical responses trigger recruitment of leukocytes and the release of chemokines/cytokines leading to pulmonary and systemic inflammatory response (biotrauma) [10, 11] . Accordingly, various artificial ventilation strategies, such as a small V t to avoid lung alveoli over-distension as well as a higher positive end-expiratory pressure (PEEP) at end expiration, are employed to minimize the impact of injurious ventilation [10, 14, 15] . However, there have been few improvements in patient outcomes in recent decades and no pharmacological therapies have been developed that have been shown to be beneficial in large-scale, randomized, controlled clinical trials (RCTs).

However, recent studies have suggested a role for the renin-angiotensin system (RAS) in ARDS and VILI [12, [16] [17] [18] . While the RAS is well known to have a primary role in the control of salt retention and blood pressure, its role in inflammation and injury suggest that is may be a novel therapeutic option for the treatment of ARDS and VILI. The present review highlights the role of angiotensin-II (Ang-II), and other RAS components, in the inflammation and injury associated with VILI.

Renin, released from the juxtaglomerular cells of the kidney, cleaves the macroglobulin precursor angiotensinogen into inactive decapeptide angiotensin-I (Ang-I). Ang-I enters circulation and is further transformed to the active octapeptide Ang-II through enzymatic cleavage by angiotensin converting enzyme (ACE) which is distributed both systemically and locally [19, 20] . Ang-II acts as a prominent mediator in the RAS by binding to angiotensin type 1 receptor (AT1R) or type 2 receptor (AT2R) [19, 20] . Furthermore, Ang-II is cleaved by angiotensin converting enzyme 2 (ACE2) forming angiotensin-(1-7) [Ang-(1-7)], which binds with Mas receptor (MasR) to antagonize AT1R-mediated effects, in most cases [21, 22] . There has been considerable interest in the balance between ACE/Ang-II/AT1R axis and the ACE2/Ang-(1-7)/ MasR axis in blood pressure homeostasis and cardiovascular function (as shown in Figure-1 ). Interestingly, AT2R is normally induced by activation of the ACE2/Ang-(1-7)/MasR axis, while more recent evidence suggests that Ang-(1-7) may activate AT2R to antagonize AT1Rmediated effects [23, 24] .

In addition to the systemic RAS, there is local expression of almost all RAS components in the lung, which is a major source of ACE and, therefore, a major site of systemic Ang II synthesis. Metzger and colleagues found that ACE is expressed abundantly in capillary endothelial cells in the entire capillary network of the alveoli in human lungs [25, 26] . As a result, pulmonary vasoconstriction occurs readily in response to hypoxia which is essential for ventilation-perfusion matching. In contrast, Wiener and colleagues found that ACE2 is primarily located in Clara cells and type II alveolar epithelial cells in murine lungs [27] . Pulmonary ACE2 appears to have a role in regulating Ang II/Ang-(1-7) levels. Recently, studies have shown that local RAS components are distributed in lung-resident immune and inflammatory cells. Thus, there is the potential for the RAS system to have a range of effects in the context of lung injury related to VILI and ARDS.

ACE/Ang-II expression is markedly increased in patients with ARDS and patients with sepsis, the most common cause of ARDS [28, 29] . Clinical epidemiological studies have shown a significant association between polymorphisms in the Ace gene and the susceptibility to ARDS [30] [31] [32] [33] . For example, Marshall and colleagues found that the Ace I/D genotype frequency, but not Ace I/I polymorphism, was increased among patients with ARDS compared with controls, with those homozygous for the D allele conferring higher levels of ACE and Ang-II in tissue and serum [30] . Up-regulation of ACE activity has been broadly associated with a range of ARDS related conditions including pneumonia, aspiration, trauma, and pancreatitis [34] [35] [36] . Notably, during mechanical ventilation, Wosten-van Asperen and colleagues showed that mechanical ventilation up-regulates ACE activity, resulting in increased conversion of Ang-I to Ang-II [13, 17, 18] . In addition, respiratory insults can cause a marked increase in AT1R expression [17, 18, 34, 35] , with potential downstream effects on activation of ACE/ Ang-II signaling. For example, Jerng and colleagues found that AT1R expression, measure at the RNA and protein level, increased with activation of ACE/Ang-II following injurious ventilation in an experimental model of VILI, and further mediated the downstream nuclear translocation of cytosolic fraction of NF-κB [17] which is strongly linked to inflammation.

A number of studies have shown that Ang-II is a critical mediator of the inflammatory cascade and alveolar epithelial injury associated with ARDS. For example, Ang-II induces dose-dependent apoptosis in human and rat alveolar epithelial cells (AECs) through its interaction with AT1R [37] [38] [39] [40] . A number of different signaling pathways are involved in the induction of apoptosis in AECs by Ang-II/AT1R [41, 42] . Wang and colleagues found that activation of pro-oxidant signals and superoxide production by Ang-II/AT1R pathway contributed to hyperoxiainduced lung injury and fibrosis [41] . Li and colleagues found that Ang-II/AT1R pathway caused activation of NF-κB and JAK2/STATs pathways, and induced apoptosis in AECs in response to seawater inhalation-induced lung injury [42] . Similarly, high Ang-II/AT1R levels may also activate the secretion of pro-inflammatory cytokines, promote macrophages and neutrophils chemotaxis and contribute to VILI [43] [44] [45] . For instance, Jiang and colleagues found that Ang-II levels in the lung correlated positively with TNF-α and macrophage inflammatory protein-2 (MIP-2) levels in bronchoalveolar lavage fluid (BALF) in a rodent model of VILI [39] . Critically, TNF-α is necessary for the induction of apoptosis in AECs by the ACE/Ang-II/AT1R axis [46] . This response is likely to be driven by activation of the NF-κB pathway whereby NF-κB activation in sepsis-induced ARDS, associated with an increase of Ang-II, leads to phosphorylation of p38MAPK leading to the production of pro-inflammatory cytokines [47] .

The deleterious role of ACE/Ang-II/AT1R axis activation is supported by the protective effect of ACE inhibitors in experimental studies. Several independent research groups have showed that ACE inhibitors and angiotensin receptor blockers (ARBs) can attenuate the lung edema, lung AECs apoptosis, and microvascular permeability caused by ARDS [13, 43, 44, 48] . For example, Liu and colleagues found that losartan, an ARB, attenuated neutrophilia in LPS-induced respiratory inflammation in mice which was associated with inhibition of dendritic cell maturation and suppression of Th1 and Th17 immune responses [49] . In line with this, another study found that losartan attenuated lung injury induced AECs apoptosis and ROS generation [42] . These observations are likely to be due to inhibition of NF-κB activation and p38MAPK phosphorylation [47] . Similar observations have been reported in models of VILI in response to ACE inhibitors, such as captopril [13, 17] . However, while experimental models using ARBs and ACE inhibitors have been promising, strong evidence from clinical studies about the association between the use of ARBs and ACE inhibitors and outcome of ARDS and VILI are still lacking, especially high quality RCTs (see Table 1 ). 

AT2R is normally only expressed at low levels by lung epithelial cells, endothelial cells, fibroblasts, and activated myofibroblasts in healthy adults [50] . However, pulmonary AT2R, as well as AT1R, are abundantly up-regulated in chronic lung disease such as chronic obstructive pulmonary disease (COPD) as well as idiopathic pulmonary fibrosis (IPF) [50, 51] . For example, as shown in the study by Jerng and colleagues, lung tissue AT2R mRNA, as well as Ang-II and AT1R mRNA levels, were markedly increased in the high-volume ventilation in rats [17] . While AT2R often has a similar increase in ARDS and VILI as AT1R, AT1R opposes AT2R by promoting injury and fibrosis. In line with this, a study by Imai and colleagues showed that Agtr2-knockout mice had more severe pathology in response to acid aspiration induced lung injury than wild-type controls [34] . A number of beneficial actions of AT2R in lung injury have recently been demonstrated [52] [53] [54] . Bruce and colleagues found that treatment with C21, an AT2R non-peptide agonist, ameliorates pulmonary fibrosis and prevents right ventricular fibrosis in pulmonary hypertension, and these beneficial effects are abolished by co-administration of the AT2R antagonist, PD123319 [52] . Furthermore, Rathinasabapathy and colleagues found that stimulation of the AT2R by C21 attenuates bleomycin-induced lung injury by alleviating lung inflammation and fibrosis [53] . A double-blind and placebo-controlled Phase IIa study has been approved by the European Union Clinical Trials Register with EudraCT Number: 2017-004923-63 and begun to evaluate the effect of C21 on fibrotic lung injury and the safety of C21.

ACE2, a homologue of ACE, cleaves a single residue from Ang-II to generate Ang-(1-7) [55, 56] . Thus, ACE2 acts as a negative regulator of RAS by inactivating Ang-II. While originally identified as a potential receptor for SARS corona virus [57] , lung expression of ACE2 has also been shown to be suppressed in ARDS [58, 59] . For example, the acid aspiration-induced lung injury is associated with markedly reduced ACE2 protein expression and elevated Ang-II levels [34] . Decreases of ACE2 activity, and concomitant enhancement of ACE activity occur in response to LPS-induced lung injury [45] . The importance of ACE2 in ARDS has been confirmed in knock-out mouse models whereby inhibition of Ace2 expression resulted in enhanced vascular permeability, increased lung edema, severe inflammatory cell infiltration and impaired lung function [34, 60] . Supplementation with recombinant ACE2 was able to improve these outcome measures [34, 60] , confirming that

Pathologically, ARDS is characterized by inflammatory cell infiltration in the lungs, loss of the epithelial and endothelial integrity, increased capillary permeability, and interstitial pulmonary edema and fibrosis. Ang-II, which locates at a core position in RAS, is originated from upstream Ang-I by ACE. Since ACE is expressed abundantly in the entire pulmonary capillary network, considerable amount of Ang-II enters lung alveoli when ARDS occurs. Ang-II combines with AT1R or AT2R, both of which expressed in AECs and inflammatory cells in alveoli, exerting opposing effects. Overall, Ang-II/ AT1R promotes inflammatory cells activation and recruitment, induces pulmonary AECs and PVMECs apoptosis, leading to increased microvascular permeability and loss of epithelial and endothelial integrity. Conversely, AT2R activation functionally attenuates inflammation, improves AECs and PVMECs survival, reduces lung fibrosis and collagen accumulation, resulting in improved lung function and oxygenation. Besides, Ang-II is further metabolized into downstream Ang-(1-7) by ACE2. Angiotensin-(1-7) combines with MasR to antagonize Ang-II/AT1R effect, thus functionally similar to AT2R. ACE2 plays a beneficial role in lung injury and, potentially, ARDS.

While ACE2 partially blocks Ang-II/AT1R signaling, the beneficial role of this mediator in ARDS is primarily attributed to activation of the Ang-(1-7)/MasR signaling pathway [59] . This has been confirmed in a number of studies where supplementation with Ang-(1-7) alleviated lung edema, myeloperoxidase activity, lung injury, and pulmonary vascular resistance in mouse models of ARDS [59, 61] . Furthermore, Chen and colleagues found that treatment with Ang-(1-7) attenuated the lung fibrosis and collagen deposition in the LPS-induced lung injury through suppression of TGF-β signaling [62] . Blockade of MasR, largely prevents the protective effects of Ang-(1-7) demonstrating the importance of downstream activation of this pathway [61] [62] [63] . The mechanisms of the beneficial effects on outcomes in these models include inhibition of apoptosis in AECs and pulmonary microvascular endothelial cells (PVMECs), as well as a reduction in the proliferation and migration of lung fibroblasts. For instance, studies by Ang-(1-7)/MasR enhances survival of AECs, which normally show excessive apoptosis in ARDS [63, 64] . Gopallawa and colleagues found that Ang-(1-7)/MasR enhanced mitogen-activated protein kinase phosphatase-2 (MKP-2) which drives apoptosis of AECs [65] . A similar effect has been shown in PVMECs whereby activation of the ACE2/Ang-(1-7)/MasR axis reduced apoptosis and cytokine secretion in PMVECs by inhibiting the phosphorylation of JNK/NF-kB which could be restored by MasR blockade [66] .

Pharmacological manipulation of ACE2/ANG-(1-7)/MasR axis, at each level, has been shown to exert a protective effect in various experimental models of ARDS (see Table 1 ). Specifically, supplementation with recombinant human angiotensin-converting enzyme 2 (rhACE2) [60] or Ang-(1-7) [61] can reduce lung injury. Conversely, inhibiting MasR aggravates ARDS [62] . Based on encouraging data from experimental models, GSK2586881, a rhACE2 which has been shown to upregulate Ang-(1-7) in Phase I clinical trials [67] , has progressed to Phase II testing. In these placebo-controlled trials, administration of a broad range of doses of the drug to patients with ARDS resulted in a rapid decrease in Ang-II and up-regulation of Ang-(1-7) without significant haemodynamic effects [68] . While only preliminary (and nonsignificant), there was also some evidence to suggest a decrease in IL-6 levels [68] indicating that this is an intervention that is worth pursuing in the future for the treatment of ARDS.

Multiple components of the RAS have been implicated, directly and indirectly, in the pathogenesis of VILI and ARDS. There is growing evidence that the ACE/Ang-II/AT1R axis, which promotes the injury and fibrosis, plays a dominant role in the pathogenesis of VILI and ARDS, while the ACE2/Ang-(1-7)/MasR axis exerts anti-inflammatory and anti-fibrotic effects. Given the lack of effective pharmacologic treatments for ARDS, the patho-physiological role of RAS in VILI and ARDS is an area that warrants further investigation. It's quite common Table 1 Summary for evidence of renin-angiotensin system (RAS) pharmacological agents in the prevention of acute respiratory distress syndrome (ARDS) and ventilatorinduced lung injury (VILI). that many pharmacological candidates against ARDS have been shown less effective despite identification of potentially promising candidates in preclinical studies. In particular, multiple RAS components have been experimentally assessed, but only two targets and the related compound (rhACE2 and C21) have shown promis and have progressed to further evaluation of clinical trial. For this discrepancy of results between clinical trial and preclinical research, a frequently mentioned reason is heterogeneity of ARDS. Some studies had clearly shown heterogeneity of ARDS and the variability of ACE activity have a potential effect on the outcome of ARDS patients. Importantly, since ARDS is a sort of highly-heterogeneous disease, subgroup analysis based on two biological phenotypes, which indicate different responses to treatment, was increasingly considered to be highly applicable for evaluation of treatment responsiveness. Similarly, Ace I/D genotype offers a possibility to predict the risk of ARDS occurrence. Thus, consideration with an emphasis on biological phenotypes of ARDS and Ace I/D genotype open perspectives to precision medicine with target therapy for ARDS. While some experimental, and a limited number of clinical trials have shown that the RAS is a promising therapeutic target, large scale, welldesigned clinical trials, in parallel with mechanistic studies, are required to determine whether these treatments will benefit these critically ill patients.(see Table 1 )

None.

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Acute respiratory distress syndrome leads to reduced ratio of ACE/ACE2 activities and is prevented by angiotensin-(1-7) or an angiotensin II receptor antagonist

Ventilator-induced inflammatory response in lipopolysaccharide-exposed rat lung is mediated by angiotensin-converting enzyme

Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome

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ACE mediates ventilator-induced lung injury in rats via angiotensin II but not bradykinin

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Angiotensin II, from vasoconstrictor to growth factor: a paradigm shift

Emerging evidence for a functional angiotensin-converting enzyme 2-angiotensin-(1-7)-MAS receptor axis: more than regulation of blood pressure

Angiotensin-converting enzyme 2, angiotensin-(1-7) and Mas: new players of the renin-angiotensin system

Angiotensin-(1-7) acts as a vasodepressor agent via angiotensin II type 2 receptors in conscious rats

Angiotensin II type 2 receptor and receptor mas are colocalized and functionally interdependent in obese zucker rat kidney

Heterogeneous distribution of angiotensin I-converting enzyme (CD143) in the human and rat vascular systems: vessel, organ and species specificity

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Angiotensin converting enzyme 2 is primarily epithelial and is developmentally regulated in the mouse lung

Inhaled nitric oxide does not change transpulmonary angiotensin II formation in patients with acute respiratory distress syndrome

Renin-angiotensin system activation correlates with microvascular dysfunction in a prospective cohort study of clinical sepsis

Angiotensin converting enzyme insertion/deletion polymorphism is associated with susceptibility and outcome in acute respiratory distress syndrome

ACE I/D but not AGT (-6)A/G polymorphism is a risk factor for mortality in ARDS

Polymorphism of the angiotensin-converting enzyme gene affects the outcome of acute respiratory distress syndrome

Effect of angiotensin converting enzyme gene I/D polymorphism and its expression on clinical outcome in acute respiratory distress syndrome

Angiotensin-converting enzyme 2 protects from severe acute lung failure

Angiotensin receptor blockade attenuates cigarette smoke-induced lung injury and rescues lung architecture in mice

Angiotensin-converting enzyme 2 protects from lethal avian influenza A H5N1 infections

Angiotensin II induces apoptosis in human and rat alveolar epithelial cells

Essential roles for angiotensin receptor AT1a in bleomycin-induced apoptosis and lung fibrosis in mice

Angiotensin-converting enzyme inhibitor captopril attenuates ventilator-induced lung injury in rats

Angiotensin receptor subtype AT(1) mediates alveolar epithelial cell apoptosis in response to ANG II

Regulation of the angiotensin II-p22phox-reactive oxygen species signaling pathway, apoptosis and 8-oxoguanine-DNA glycosylase 1 retrieval in hyperoxia-induced lung injury and fibrosis in rats

Losartan, a selective antagonist of AT1 receptor, attenuates seawater inhalation induced lung injury via modulating JAK2/STATs and apoptosis in rat

Losartan attenuates ventilator-induced lung injury

Losartan attenuates microvascular permeability in mechanical ventilator-induced lung injury in diabetic mice

Antagonist of the type-1 ANG II receptor prevents against LPSinduced septic shock in rats

Apoptosis of lung epithelial cells in response to TNF-alpha requires angiotensin II generation de novo

Losartan prevents sepsis-induced acute lung injury and decreases activation of nuclear factor kappaB and mitogen-activated protein kinases

The Angiotensin-converting enzyme inhibitor captopril inhibits poly(adp-ribose) polymerase activation and exerts beneficial effects in an ovine model of burn and smoke injury

Losartan, an antagonist of AT1 receptor for angiotensin II, attenuates lipopolysaccharide-induced acute lung injury in rat

Distribution of type-1 and type-2 angiotensin receptors in the normal human lung and in lungs from patients with chronic obstructive pulmonary disease

The angiotensin II receptor 2 is expressed and mediates angiotensin II signaling in lung fibrosis

Selective activation of angiotensin AT2 receptors attenuates progression of pulmonary hypertension and inhibits cardiopulmonary fibrosis

The selective angiotensin II type 2 receptor agonist, compound 21, attenuates the progression of lung fibrosis and pulmonary hypertension in an experimental model of bleomycin-induced lung injury

Angiotensin II type 2 receptor agonist Compound 21 attenuates pulmonary inflammation in a model of acute lung injury

Cloning and functional expression as a captopril-insensitive carboxypeptidase

The novel angiotensin-converting enzyme (ACE) homolog, ACE2, is selectively expressed by adult Leydig cells of the testis

A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury

The discovery of angiotensin-converting enzyme 2 and its role in acute lung injury in mice

Imbalance between pulmonary angiotensin-converting enzyme and angiotensin-converting enzyme 2 activity in acute respiratory distress syndrome

Angiotensin converting enzyme 2 abrogates bleomycin-induced lung injury

Angiotensin-(1-7) protects from experimental acute lung injury

Angiotensin-(1-7) attenuates lung fibrosis by way of Mas receptor in acute lung injury

Regulation of alveolar epithelial cell survival by the ACE-2/angiotensin 1-7/Mas axis

ACE-2/ANG1-7 ameliorates ER stress-induced apoptosis in seawater aspiration-induced acute lung injury

Angiotensin-(1-7)/mas inhibits apoptosis in alveolar epithelial cells through upregulation of MAP kinase phosphatase-2

Angiotensin-converting enzyme 2/angiotensin-(1-7)/Mas axis prevents lipopolysaccharide-induced apoptosis of pulmonary microvascular endothelial cells by inhibiting JNK/NF-kappaB pathways

Pharmacokinetics and pharmacodynamics of recombinant human angiotensin-converting enzyme 2 in healthy human subjects

A pilot clinical trial of recombinant human angiotensin-converting enzyme 2 in acute respiratory distress syndrome

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pupt.2019.101833.