Pulmonary Renin-angiotensin System Research Articles

The pathobiological harmony between the local pulmonary/ bone marrow RAS and its management via tissue-RAS modulating agents in COVID-19

Coronavirus disease 2019 (COVID-19) outbreak, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), poses an unprecedented threat to public health and healthcare systems. It presents unusual pathophysiological effects mainly characterized by immune-inflammatory response and prothrombotic state causing acute respiratory distress syndrome and multiple organ failure. SARS-CoV-2 enters target cells after binding to the angiotensin-converting enzyme 2 (ACE2) receptor and therefore has a direct effect on the renin-angiotensin system (RAS). Apart from affecting numerous organs including lungs, heart, gastrointestinal system, spleen, brain and kidneys, the spike protein of SARS-CoV-2 could attack hematopoietic stem cells and hematopoietic progenitor cells in bone marrow (BM) microenvironment together with the precursor and mature blood cells. Within this hematopoietic viral spread context, it is crucial to search the clinicopathological correlations of COVID-19 in order to develop specific potential therapeutics against pleiotropic SARS-CoV-2 actions. Therefore, pharmacological disruption of the pathological cross-talk of local BM RAS and pulmonary RAS via administration of the tissue-RAS modulating agents such as soluble ACE2, angiotensin (1-7), TXA127 and MAS receptor agonists may prevent the clinical progression of the COVID-19 syndrome via reducing the hematopoietic virus propagation and systemic multi-organ spread.

Sequencing ACE2 in Bos Taurus: A Comparison of Bovine and Human ACE 2’s interaction with SARS‐CoV‐2

The World Health Organization’s (WHO) 2018 blueprint of priority diseases, based upon their potential to cause epidemics and public health emergencies included two respiratory illnesses, severe acute respiratory syndrome (SARS‐CoV) and middle east respiratory syndrome (MERS‐CoV), both caused by members of the Coronaviridae family (BioDefense, 2018). Now three years later, we are amid an ongoing worldwide pandemic caused by another Coronavirus, SARS‐Cov‐2. This virus has rapidly spread across the globe causing over 5.4 million deaths throughout the world. Symptoms of the viral infection include shortness of breath, fever, fatigue, muscle and body aches, headaches and most importantly acute lung injury (ALI) caused by dysregulation of pulmonary RAS (Renin‐Angiotensin System). RAS, the main network controlling blood pressure is regulated by ACE (Angiotensin Converting Enzyme) and its homolog ACE2. It is now known that the SARS‐CoV‐2, uses ACE2 as the cell receptor to enter and infiltrate the host cell. While ACE promotes atrophy, fibrosis, inflammation, oxidants, and vasoconstriction, ACE2 through its metabolism of angiotensin I, restricts excessive atrophy, fibrosis and causes vasodilation. Furthermore, ACE2 with its anti‐inflammatory and antioxidant role protects lung tissues and therefore a downregulation of ACE2 by SARS‐Cov‐2 infection can result in development of acute respiratory distress syndrome (ARDS) (Imai, Kuba, & Penninger, 2008). The Spike (S) protein of SARS‐Cov‐2 binds to ACE2, and recent studies have identified several residues in Exon 1 of ACE‐2, that mediate this binding (Rehman & Tabish, 2020). Our study aims to sequence ACE2 Exon 1 from Bos taurus and then compare it to human ACE2 Exon 1 using a variety of bioinformatics tools to understand and possibly identify polymorphisms that could determine species susceptibility to Covid‐19 infection. This investigation began with genomic DNA extraction from calf thymus. The genomic DNA was then analyzed using spectrophotometry (Biophotometer and Nanodrop) and gel electrophoresis. Thereafter, ACE2 target primers were designed using Primer BLAST and Primer Quest. We are now getting ready to run PCR to amplify our ACE2 Exon 1 target from Bos taurus, and after purifying the PCR products, we will sequence the ACE2 Exon 1 using our SeqStudio genetic analyzer. The sequence will then be analyzed and compared to its human counterpart using a variety of bioinformatics tools to gain a better understanding of polymorphisms that might influence SARS‐Cov‐2’s initial interaction with its host as well as pathogenesis of COVID‐19 in general.ReferencesBioDefense, G. (2018, February 12). https://globalbiodefense.com/2018/02/12/who‐updates‐blueprint‐list‐of‐priority‐diseases/. Retrieved May 14, 2020Imai, Y., Kuba, K., & Penninger, J. M. (2008). The discovery of angiotensin‐converting enzyme 2 and its role in acute lung injury in mice. Exp Physiol 93.5, pp 543–548.Rehman, S. u., & Tabish, M. (2020). Alternative splicing of ACE2 possibly generates variants that may limit the entry of SARS‐CoV‐2: a potential therapeutic approach using SSOs. Clinical Science, 134, 1143–1150.

Pulmonary artery denervation in pulmonary hypertension: physiological and clinical aspects

This article is a review of the findings of experimental and clinical studies of a new method of treatment of pulmonary hypertension - pulmonary artery denervation with the help of radiofrequency ablation, cryodenervation and ultrasonic impact. Pulmonary artery denervation results in decreased neurogenic tonic sympathetic and, probably, increased parasympathetic effects on pulmonary vessels. On models of experimental monocrotaline-induced pulmonary hypertension in various-species animals, it was determined that pulmonary artery denervation is followed by decreased activity of local pulmonary renin-angiotensin system, slowed processes of remodeling of pulmonary vessels, hypertrophy and fibrosis of the right ventricle, with inhibition of progression of pulmonary hypertension by means of suppression of extracellular signal-regulated kinase 1/2 (ERK 1/2) which regulates differentiation, proliferation and migration of smooth muscle cells. However, the problem of the pattern of pulmonary microcirculation changes (pre- and postcapillary resistance, capillary filtration coefficient) after pulmonary artery denervation warrants further study. The findings of clinical studies in patients with pulmonary hypertension suggest that pulmonary artery denervation inducing a decrease of pressure therein, as well as pulmonary vessel resistance did not lead to normalization of pulmonary haemodynamics.The mentioned impact partially removes the neurogenic component of multicircuit and multifactorial regulation of pulmonary circulation. Therefore, along with pulmonary artery denervation, further search for pharmacological agents selectively influencing pulmonary vessels remains a problem of current importance.

Renin-Angiotensin System: An Important Player in the Pathogenesis of Acute Respiratory Distress Syndrome.

Acute respiratory distress syndrome (ARDS) is characterized by massive inflammation, increased vascular permeability and pulmonary edema. Mortality due to ARDS remains very high and even in the case of survival, acute lung injury can lead to pulmonary fibrosis. The renin–angiotensin system (RAS) plays a significant role in these processes. The activities of RAS molecules are subject to dynamic changes in response to an injury. Initially, increased levels of angiotensin (Ang) II and des-Arg9-bradykinin (DABK), are necessary for an effective defense. Later, augmented angiotensin converting enzyme (ACE) 2 activity supposedly helps to attenuate inflammation. Appropriate ACE2 activity might be decisive in preventing immune-induced damage and ensuring tissue repair. ACE2 has been identified as a common target for different pathogens. Some Coronaviruses, including SARS-CoV-2, also use ACE2 to infiltrate the cells. A number of questions remain unresolved. The importance of ACE2 shedding, associated with the release of soluble ACE2 and ADAM17-mediated activation of tumor necrosis factor-α (TNF-α)-signaling is unclear. The roles of other non-classical RAS-associated molecules, e.g., alamandine, Ang A or Ang 1–9, also deserve attention. In addition, the impact of established RAS-inhibiting drugs on the pulmonary RAS is to be elucidated. The unfavorable prognosis of ARDS and the lack of effective treatment urge the search for novel therapeutic strategies. In the context of the ongoing SARS-CoV-2 pandemic and considering the involvement of humoral disbalance in the pathogenesis of ARDS, targeting the renin–angiotensin system and reducing the pathogen’s cell entry could be a promising therapeutic strategy in the struggle against COVID-19.

Disorder of Renin‐Angiotensin System Promoting the Acute Pulmonary Inflammation by Particulate Matter 2.5 (PM2.5) Exposure

Acute or chronic particulate matter 2.5 (PM2.5) exposure can induce oxidative stress, inflammation and pulmonary impairment by traversing the blood‐air barrier of alveolar and then depositing at lung. In the present study, we tested whether the pulmonary inflammation and impairment induced by PM2.5 exposure are related to the disorder of pulmonary renin‐angiotensin system (RAS).In this study, we collected PM2.5 from waste gas which was produced from vehicle engine, and determined the component elements and structures of the PM2.5. The C57BL/6J (wild type, WT) mice and angiotensin converting enzyme II (ACE2) gene knockout (ACE2KO) mice were treated with the PM2.5 by intratracheal instillation once a day for a total of 3 days, and then sacrificed at the 4th day. Lung tissue of the mice was collected for the assays of molecular, biochemical and pathological changes to evaluate the acute pulmonary inflammation and lung injury induced by PM2.5 exposure.The results indicated that the most of collected particulates in this study were aggregated and parts of them formed structure of chains below 2.5 μm diameter. The major compositions of the PM2.5 are carbon and oxide, and contain little silicon, sulfate and copper. The body weight of both WT and ACE2 KO mice treated with PM2.5 was decreased and resting breathing rate was significantly increased. PM2.5 treatment induced pathological leukocyte infiltration in the lung, increased pulmonary immunocytokines concentration, including interleukin‐6 (IL‐6), tumor necrosis factor alpha (TNF‐α) and tumor growth factor beta 1 (TGF‐β1), and also promoted signaling protein ERK1/2 and STAT3 phosphorylation. The quantitative data showed that the levels of physiopathology and inflammation induced by PM2.5 were serious in the ACE2 KO mice compared with those in the WT mice. PM2.5 treatment significantly increased pulmonary ACE expression and activity in both WT and ACE2 KO mice. Interestingly, pulmonary ACE2 activity in the WT mice was also induced. We hypothesized that increased ACE2 activity in the WT mice is a physiological response for increased ACE activity due to PM2.5 exposure. However, the response is lack in the ACE2 KO mice because the mice being ACE2 deficiency. It is one of the reasons that serious physiopathology and inflammation induced by PM2.5 in the ACE2 KO mice compared with those in the WT mice.Our data reveals that acute inflammation and lung injury could be induced by PM2.5 treatment. The disorder of pulmonary RAS may aggravate the disease progressive of pulmonary system due to PM2.5 exposure. Therefore, we propose that it may be helpful for the cardiopulmonary patients by RAS regulation when the patients are exposed to severe particulate pollutants.

Age-Dependent Changes in the Pulmonary Renin-Angiotensin System Are Associated With Severity of Lung Injury in a Model of Acute Lung Injury in Rats.

A growing body of evidence suggests that age affects the main pathophysiologic mechanisms of the acute respiratory distress syndrome. This may imply the need for developing age-tailored therapies for acute respiratory distress syndrome. However, underlying molecular mechanisms governing age-related susceptibility first need to be unraveled. In a rat model of acute lung injury, we investigated whether age affects the balance between the two key enzymes of the pulmonary renin-angiotensin system, angiotensin-converting enzyme, and angiotensin-converting enzyme 2. We hypothesized that aging shifts the balance toward the lung injury-promoting angiotensin-converting enzyme, which may form an explanation for the differences in severity of lung injury between different age groups. Prospective, randomized controlled animal study. University medical research laboratory. Infant (15 ± 2 d), juvenile (37 ± 2 d), adult (4 ± 0.2 mo), and elderly (19.5 ± 0.5 mo) male RCCHan Wistar rats. Lung injury was induced by intratracheal administration of lipopolysaccharide (5 mg/kg) and 4 hours of mechanical ventilation (15 mL/kg). In lipopolysaccharide-exposed and mechanical ventilated rats, angiotensin-converting enzyme activity in bronchoalveolar lavage fluid increased 3.2-fold in elderly when compared with infants. No changes in bronchoalveolar lavage fluid angiotensin-converting enzyme 2 activity were found. In addition, membrane-bound angiotensin-converting enzyme activity decreased. Together with the presence of angiotensin-converting enzyme-sheddase ADAM9 (a disintegrin and metalloproteinase domain-containing protein 9) and an age-dependent increase in tumor necrosis factor-α, an activator of ADAM9, these results indicate increased shedding of angiotensin-converting enzyme in the alveolar compartment, thereby shifting the balance toward the injurious pathway. This imbalance was associated with an increased inflammatory mediator response and more lung injury (wet-to-dry ratio and histology) in elderly rats. Increasing age is associated with an imbalance of the pulmonary renin-angiotensin system, which correlates with aggravated inflammation and more lung injury. These changes might form the ground for new therapeutic strategies in terms of dosing and effectiveness of renin-angiotensin system-modulating agents for treatment of acute respiratory distress syndrome.

Antenatal maternal low protein diet: ACE-2 in the mouse lung and sexually dimorphic programming of hypertension.

Elevated blood pressure is an important global health problem, and in-utero under-nutrition may be an important factor in the pathogenesis of hypertension. In the present study, we tested the hypothesis that antenatal maternal low protein diet (MLPD) leads to sexually dimorphic developmental programming of the components of the pulmonary renin-angiotensin system. This may be important in the antenatal MLPD-associated development of hypertension. In pregnant mice, we administered normal (control) and isocaloric 50 % protein restricted diet, commencing one week before mating and continuing until delivery of the pups. From the 18th to 24th week postnatal, we measured blood pressure in the offspring by use of a non-invasive tail-cuff method. In the same mice, we examined the mRNA and protein expression of the key components of the pulmonary renin-angiotensin system. Also, we examined microRNA complementary to angiotensin converting enzymes (ACE) 2 in the offspring lungs. Our results demonstrate that as a consequence of antenatal MLPD: 1) pup birthweight was significantly reduced in both sexes. 2) female offspring developed hypertension, but males did not. 3) In female offspring, ACE-2 protein expression was significantly reduced without any change in the mRNA levels. 4) miRNA 429, which has a binding site on ACE-2 - 3’ UTR was significantly upregulated in the female antenatal MLPD offspring. 5) In males, ACE-2 mRNA and protein expression were unaltered. We conclude that in the mouse, antenatal MLPD-induced reduction of ACE-2 in the female offspring lung may be an important mechanisms in sexually dimorphic programming of hypertension.Electronic supplementary materialThe online version of this article (doi:10.1186/s12899-015-0016-6) contains supplementary material, which is available to authorized users.

Injurious ventilation has an age-dependent affect on the pulmonary renin-angiotensin system in LPS-challenged rats

The underlying molecular mechanisms for the association between aging and a higher susceptibility to develop ARDS are poorly understood. The pulmonary renin-angiotensin system (RAS), with a lung-protective (angiotensin-converting enzyme (ACE)2-angiotensin (Ang)-1,7-Mas receptor) axis and a lung-injurious (ACE-Ang II-Ang II receptor (AT1)) axis, has been implicated in the pathogenesis of ARDS and changes with age. We hypothesized that injurious ventilation has an age-dependent effect on the pulmonary RAS in LPS-challenged rats that is associated with increased lung injury.

Abstract 19577: Diminazene Aceturate Inhibits Apoptotic Cell Death to Protect from Pulmonary Fibrosis and Associated Pulmonary Hypertension

INTRODUCTION: Studies have demonstrated that Angiotensin-converting enzyme2 (ACE2) plays a protective role against lung fibrosis. In this study, we evaluated the pharmacological actions of a recently identified synthetic activator of ACE2, Diminazene aceturate (DIZE) in the bleomycin model of pulmonary fibrosis (PF). METHODS: PF was induced by a single intratracheal instillation of bleomycin (Bleo, 5U/Kg) in 8 weeks old male Sprague-Dawley rats. Rats were randomized into three groups; Control, Bleo and Bleo+DIZE. DIZE treatment (15mg/Kg, sc) was commenced at the same time as bleomycin administration. After 2-weeks of bleomycin instillation, rats were instrumented to measure right ventricular systolic pressure (RVSP), followed by heart and lung excision to examine cardio-pulmonary remodeling and apoptosis. RESULTS: Bleomycin administration resulted in significant elevation of right ventricular systolic pressure (RVSP; Control: 26+1mmHg; Bleo: 41+2mmHg), signifying development of pulmonary hypertension. In addition, these animals exhibited maladaptive cardiac remodeling in terms of increased myocardial fibrosis (%Fibrosis; Control: 6+0.23; Bleo: 17+0.3) and right ventricular hypertrophy (RVH; Control: 0.26+0.007; Bleo: 0.40+0.03). Co-administration of DIZE significantly attenuated all these parameters (RVSP; 32+0.5mmHg; RVH; 0.30+0.01 and %Fibrosis; 7.2+0.6). A higher Ashcroft score was observed in bleomycin-challenged animals, signifying development of PF, which was prevented by DIZE treatment (Control: 1.2+0.09; Bleo: 6.5+0.06; Bleo+DIZE: 2.8+0.03). The protective effects of DIZE were associated with favorable modulation of the pulmonary renin angiotensin system and decreased pro-inflammatory cytokines. Furthermore, lung sections of bleomycin administered animals showed increased TUNEL staining for apoptosis, which was prevented by DIZE treatment (TUNEL positive index; Con: 0.03+0.005; Bleo: 0.2+0.04; Bleo+DIZE: 0.07+0.003). CONCLUSION: Our studies show that DIZE, an ACE2 activator attenuates bleomycin-induced lung fibrosis and associated cardio-pulmonary pathology, indicating a therapeutic potential of this drug in PF therapy .

Antenatal Maternal Hypoxic Stress: Adaptations in Fetal Lung Renin-Angiotensin System

Antenatal maternal hypoxia (AMH) can lead to intrauterine growth restriction (IUGR), as well as idiopathic pulmonary hypertension of newborn and adult, the latter of which may be a consequence of alterations in the local pulmonary renin-angiotensin system (RAS). Little is known of these adaptations, however. Thus, we tested the hypothesis that antenatal maternal hypoxia is associated with alterations in gene and protein expression of the pulmonary renin-angiotensin system, which may play an important role in pulmonary disorders in the offspring. In FVB/NJ mice, we studied messenger RNA (mRNA) and protein expression, as well as promoter DNA methylation and microRNA (miRNA) levels in response to 48 hours hypoxia (10.5% O(2)) at 15.5 day post coitum (DPC). In response to AMH, the pulmonary mRNA levels of angiotensin-converting enzyme (ACE) 1.2, ACE-2, and angiotensin II type 1b (AT-1b) receptors were increased significantly, as compared to controls (N = 4). In response to antenatal hypoxia, pulmonary protein levels of renin and ACE-2 also were increased significantly, whereas ACE-1 protein expression was reduced. In fetal lungs, we also observed reduced expression of the miRNAs: mmu-mir -199b, -27b, -200b, and -468 that putatively increase the translation of renin, ACE-1, ACE-2, and AT-1 receptors, respectively. In response to AMH, promoter methylation of ACE was unchanged. We conclude that AMH leads to changes in expression of pulmonary RAS of fetal mice. The possible implications of these changes for the regulation of pulmonary vascular contractility in later life remain to be explored.

Angiotensin Receptor Type I Blockade Inhibits Apoptosis in Meconium-Instilled Rabbit Lungs

Background and objectives: In prior studies we demonstrated that meconium-induced lung cell death by apoptosis was associated with activation of the local pulmonary renin-angiotensin system (RASL). Alveolar epithelial apoptosis requires the authocrine synthesis and proteolytic processing of angiotensinogen (AGT) to Angiotensin II (ANG II). Inhibitors of angiotensin converting enzyme (ACE) block meconium-induced apoptosis. ANG II plays an essential role in vascular homeostasis and lung injury. The objectives of this study were to evaluate expression of AGT, ANG II and Caspase 3 in meconium and saline treated newborn lungs and to study mechanisms of its inhibition by a selective antagonist of the AT1 receptor. Methods: Two week old rabbits were studied. Three treatment groups were studied (six rabbit pups in each group): Group 1: rabbits instilled with saline; Group 2: rabbits instilled with 10% meconium; Group 3: Losartan pretreated followed by meconium-instillation. Three groups of A549 human lung epithelial cells were studies as well. Group 4: AGT pretreated and then meconium-exposed cells; Group 5: ANG II pretreated and then meconium exposed cells and Group 6: Caspase 3 inhibitor ZVAD-fmk pretreated and then meconium exposed cells. AGT, ANG II and Caspase 3 were evaluated and compared with and without inhibitors in meconium and control groups. Results: In Situ End Labeling (ISEL) and Caspase 3 assays showed that purified ANG II induced dose dependent apoptosis in rabbit lung lavage cells and the human A549 lung epithelial cell line. Apoptosis also was induced by purified AGT. The increase in apoptotic cells was accompanied by increases in ANG II and Caspase 3 activities. In both airway epithelium and alveolar wall cells, measures of apoptosis were attenuated by Losartan or by the Caspase 3 inhibitor ZVAD-fmk. Conclusions: These data demonstrate the presence of a functional ANG II and Caspase 3 dependent apoptotic pathways in newborn meconium-instilled lungs. They also imply that meconium-induced apoptosis is modulated by the pulmonary RASL system in which ANG II plays a critical role. Both losartan and the Caspase 3 inhibitor ZVAD-fmk pretreatment significantly decreased meconium-induced Caspase-3 activation and lung cell apoptosis.

Angiotensin II receptors in a rat model of hypobaric hypoxic pulmonary hypertension

Hypoxic pulmonary hypertension is characterized by profound remodeling of the walls of small pulmonary arteries: endothelial swelling and proliferation of media and adventitia to a degree that may narrow the lumen. The molecular mechanism responsible for these changes is unknown. Recent studies have demonstrated the existence of a vascular and cardiac renin-angiotensin system. This system can induce hypertrophy and proliferation of aortic smooth muscle cells and cardiac muscle cells in animal models of systemic hypertension. In the present experiments, Northern blot analysis and a receptor binding assay specific to angiotensin II (Ang II) receptors was used to study the pulmonary renin angiotensin system in a rat model of hypobaric hypoxic pulmonary hypertension. mRNA transcripts for the Ang II receptor AT(1A) were constitutively expressed, and specific binding of Ang II to the receptors occurred in lung tissue of control rats that breathed ambient air. After 3 days of hypobaric hypoxia (380 Torr), the AT(1A) mRNA level and the index of Ang II receptors had increased, but right ventricular hypertrophy was not still evident. Ang II receptors were significantly increased after 14 days of hypoxia, when histological evidence of pulmonary hypertension was observed. In addition, the Ang II level in pulmonary tissue was higher after 14 days of hypoxia than in normal controls. These findings suggest that the renin-angiotensin system in pulmonary tissue was activated in these rats, and that it contributes to the morphological changes observed in hypoxic pulmonary hypertension.

Lenti‐viral mediated overexpression of ACE2 or Angiotensin‐(1‐7) prevents bleomycin‐induced pulmonary fibrosis

An activated pulmonary renin angiotensin system (RAS), comprising of ACE‐AngII‐AT1R has been implicated in the pathogenesis of idiopathic pulmonary fibrosis. The recent discovery of a counter‐regulatory arm of the RAS (ACE2‐Ang‐(1‐7)‐Mas), has led us to examine the role of this vasoprotective axis on pulmonary fibrosis with the use of lentiviral vector mediated gene transfer. Lentiviral particles carrying either the ACE2 or Ang‐(1‐7) fusion protein were administered intratracheally into the lungs of 5 weeks old male Sprague‐Dawley rats. Pulmonary fibrosis was induced by intratracheal administration of bleomycin (2.5mg/kg), following 2 weeks of gene transfer. Systemic blood pressure, body weights, right ventricular systolic pressure (RVSP) and right ventricular hypertrophy (RVH) were examined after two weeks of bleomycin administration. Bleomycin administration resulted in significant reduction in body weights, elevated RVSP, RVH and increased fibrotic lesions in the lungs. All these pathophysiological parameters were prevented in lenti‐ACE2 or lenti‐Ang‐(1‐7) treated rats. No significant change in the systemic blood pressure was observed among the different study groups. Collectively, these observations demonstrate that ACE2 or Ang‐(1‐7) gene transfer shifts the balance of the RAS from the vasoconstrictive‐proliferative axis (ACE‐AngII‐AT1R), towards vasoprotective axis (ACE2‐Ang‐(1‐7)‐Mas), resulting in attenuation of bleomycin‐induced pulmonary fibrosis.

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

During several months of 2002, severe acute respiratory syndrome (SARS) caused by SARS‐coronavirus (SARS‐CoV) spread rapidly from China throughout the world, causing more than 800 deaths due to the development of acute respiratory distress syndrome (ARDS), which is the severe form of acute lung injury (ALI). Interestingly, a novel homologue of angiotensin‐converting enzyme, termed angiotensin‐converting enzyme 2 (ACE2), has been identified as a receptor for SARS‐CoV. Angiotensin‐converting enzyme and ACE2 share homology in their catalytic domain and provide different key functions in the renin–angiotensin system (RAS). Angiotensin‐converting enzyme cleaves angiotensin I to generate angiotensin II, which is a key effector peptide of the system and exerts multiple biological functions, whereas ACE2 reduces angiotensin II levels. Importantly, our recent studies using ACE2 knockout mice have demonstrated that ACE2 protects murine lungs from ARDS. Furthermore, SARS‐CoV infections and the Spike protein of the SARS‐CoV reduce ACE2 expression. Notably, injection of SARS‐CoV Spike into mice worsens acute lung failure in vivo, which can be attenuated by blocking the renin–angiotensin pathway, suggesting that the activation of the pulmonary RAS influences the pathogenesis of ALI/ARDS and SARS.

Meconium Increases Type 1 Angiotensin II Receptor Expression and Alveolar Cell Death

The pulmonary renin-angiotensin system (RAS) contributes to inflammation and epithelial apoptosis in meconium aspiration. It is unclear if both angiotensin II receptors (ATR) contribute, where they are expressed and if meconium modifies subtype expression. We examined ATR subtypes in 2 wk rabbit pup lungs before and after meconium exposure and with and without captopril pretreatment or type 1 receptor (AT1R) inhibition with losartan, determining expression and cellular localization with immunoblots, RT-PCR and immunohistochemistry, respectively. Responses of cultured rat alveolar type II pneumocytes were also examined. Type 2 ATR were undetected in newborn lung before and after meconium instillation. AT1R were expressed in pulmonary vascular and bronchial smooth muscle and alveolar and bronchial epithelium. Meconium increased total lung AT1R protein approximately 3-fold (p = 0.006), mRNA 29% (p = 0.006) and immunostaining in bronchial and alveolar epithelium and smooth muscle, which were unaffected by captopril and losartan. Meconium also increased AT1R expression >3-fold in cultured type II pneumocytes and caused concentration-dependent cell death inhibited by losartan. Meconium increases AT1R expression in newborn rabbit lung and cultured type II pneumocytes and induces AT1R-mediated cell death. The pulmonary RAS contributes to the pathogenesis of meconium aspiration through increased receptor expression.

Modification of the pulmonary renin–angiotensin system and lung structural remodelling in congestive heart failure

Lung structural remodelling, characterized by myofibroblast proliferation and collagen deposition, contributes to impaired functional capacity in CHF (congestive heart failure). As the lung is the primary site for the formation of Ang II (angiotensin II), local modifications of this system could contribute to lung remodelling. Rats with CHF, induced following myocardial infarction (MI) via coronary artery ligation, were compared with sham-operated controls. The MI group developed lung remodelling as confirmed by morphometric measurements and immunohistochemistry. Pulmonary Ang II concentrations increased more than 6-fold (P<0.01), and AT1 (Ang II type 1) receptor expression was elevated by 3-fold (P<0.01) with evidence of distribution in myofibroblasts. AT2 (Ang II type 2) receptor expression was unchanged. In isolated lung myofibroblasts, AT1 and AT2 receptors were expressed, and Ang II stimulated proliferation as measured by [3H]thymidine incorporation. In normal rats, chronic intravenous infusion of Ang II (0.5 mg.kg(-1) of body weight.day(-1)) for 28 days significantly increased mean arterial pressure (P<0.05), without pulmonary hypertension, lung remodelling or a change in AT1 receptor expression. We conclude that there is a modification of the pulmonary renin-angiotensin system in CHF, with increased Ang II levels and AT1 receptor expression on myofibroblasts. Although this may contribute to lung remodelling, the lack of effect of increased plasma Ang II levels alone suggests the importance of local pulmonary Ang II levels combined with the effect of other factors activated in CHF.

The pulmonary renin-angiotensin system.

The circulating renin-angiotensin system (RAS) has a well-described role in circulatory homeostasis. Recently, local tissue-based RAS have also been described which appear to play a key role in the injury/repair response. The expression of RAS components and the elevation of angiotensin converting enzyme in a number of interstitial lung diseases suggests the existence of a pulmonary RAS and that angiotensin II could mediate, at least in part, the response to lung injury. Activation of a local RAS within the pulmonary circulation and lung parenchyma could influence the pathogenesis of lung injury via a number of mechanisms including an increase in vascular permeability, vascular tone and fibroblast activity, and by reducing alveolar epithelial cell survival. The ability of both ACE inhibitors and angiotensin II receptor antagonists to attenuate experimental lung injury further supports a role for RAS activation and suggests these agents may be useful in the treatment of diffuse parenchymal lung disease. However, further studies are required to delineate the cell types responsible for RAS component expression in the lung and also to identify the key effector molecules of this system. The presence of common polymorphisms in RAS genes and their study in relation to specific physiological phenotypes will aid both our understanding of the role of RAS in the lung and also aid the targeting of future therapies.