Shock/Sepsis/Trauma/Critical CareAngiotensin-(1-7) attenuates lung fibrosis by way of Mas receptor in acute lung injury
Introduction
Acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), are devastating clinical syndromes with high morbidity, mortality, and disability, and approximately 60% of patients with ARDS will develop pulmonary fibrosis [1], [2]. Pulmonary fibrosis occurs early in the course of ARDS, significantly correlating with a poor outcome [3], [4]. Compared with patients without fibrosis, the patients with ARDS who develop pulmonary fibrosis exhibit increased mortality (57%) [5]. However, the mechanism underlying dysfunctional repair or fibrosis in ALI is poorly understood. Despite numerous preclinical and clinical trials, none of the tested pharmacologic interventions have been significantly proved to have clinical benefits.
The renin-angiotensin system (RAS) plays an important role in the pathophysiology of ALI and has been shown to be involved in the process of fibrosis in many chronic lung diseases [6], [7]. The concentration of angiotensin (Ang) II, one of the key effector peptides of the RAS, increased significantly in lung tissue and plasma after ALI [8]. Ang II is a potent promoter of fibrosis and induces the epithelial–mesenchymal transition by classic stimulation of transforming growth factor (TGF)-β and a TGF-β–independent pathway. In a bleomycin-induced pulmonary fibrosis model, the levels of Ang II, Ang II receptor type 1 (AT1 receptor), and TGF-β increased; treatment with losartan attenuated TGF-β expression, lung collagen deposition, and development of pulmonary fibrosis [9]. Thus, Ang II might play a vital role in the fibrosis of ALI by way of the AT1 receptor.
Angiotensin-converting enzyme (ACE)2 cleaves a single residue from Ang II to generate Ang-(1-7) and thus plays a key role in controlling the balance between the ACE-Ang II-AT1 receptor axis and the ACE2-Ang-(1-7)-Mas axis of the RAS; however, it is expensive to produce. Ang-(1-7), another bioactive peptide within the RAS, acting by way of the G protein-coupled receptor Mas, opposes most of the actions of Ang II. Ang-(1-7) has a relatively low molecular mass and is derived from human protein. It appears to be a good candidate for use in vivo. Ang-(1-7) blocked Ang II-stimulated lung fibroblast proliferation [10]. Liver fibrosis was aggravated significantly in bile duct-ligated rats by treatment with A-779, which is used as a Mas receptor antagonist [11]. Therefore, Ang-(1-7) might play an important role in regulating tissue fibrosis.
We assumed that Ang-(1-7) by way of the Mas receptor attenuates lung fibrosis at an early phase of LPS-induced ALI. The objective of the present study was to clarify whether, during LPS-induced ALI, Ang-(1-7) alters the development lung fibrosis and how these responses might be modulated by a Mas receptor antagonist in vivo.
Section snippets
Experimental animals
Male C57BL/6 mice, aged 8–14 wk and weighing 20–25 g, were obtained from the Institute of Experimental Animals (Nanjing Jinling Hospital, Nanjing University, Nanjing, China). They were allowed free access to standard rodent chow and drinking water. The local institutional animal care and use committee approved the present study.
ALI model
The mice received a single dose of LPS 5 mg/kg from Escherichia coli serotype 0111:B4 (Sigma-Aldrich, St Louis, MO) in 50 μL of sterile normal saline intratracheally [12]
LPS-induced ALI and pulmonary fibrosis in mice
A single intratracheal injection of LPS into the mice resulted in an increase in pulmonary edema within 3 d (wet to dry weight ratio, 4.6 ± 0.3 in control versus 5.4 ± 0.2 in LPS). The histopathologic examination showed a normal alveolar structure in the control group. In contrast, after LPS stimulation, the alveolar structure was damaged, the alveolar wall thickness, edema, bleeding, and inflammatory cell infiltrates were increased, and the formation of hyaline membranes was enhanced. Compared
Discussion
We observed that the LPS intratracheal injection–induced ALI resulted in exaggerated inflammatory mediator production and lung fibrosis. Ang-(1-7) treatment improved the lung edema and inflammation and attenuated the lung fibrosis in the LPS-induced ALI model. In contrast, the A779 Mas receptor inhibitor aggravated the LPS-induced lung fibrosis, which might indicate that Ang-(1-7) by way of the Mas receptor attenuated the lung injury and fibrosis in ALI. Losartan, a blocker of the Ang II type 1
Conclusions
The results of our study have shown that Ang-(1-7) exerts a protective effect in LPS-induced lung injury and lung fibrosis that might be, at least partially, mediated through the Mas receptor. Additionally, the present study has provided evidence of the therapeutic potential of Ang-(1-7) in the prevention and treatment of ALI.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (project 81000828) and Scientific Research Projects for Postgraduate Students of the Jiangsu Province (CX08B_165Z).
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