Type II pneumocytes as a protective factor against interstitial lung injury

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Interstitial lung diseases involve a group showing poor prognosis including idiopathic pulmonary fibrosis (IPF). A common pathophysiological change is irreversible fibrosis.In lung tissues injured in alveolar interstitum, to make alveolar epithelial cells regenerate is very important for repair of the injured lung. Several recent studies showed that rennin-angiotensin system induces lung fibrosis and that apoptosis of type II pneumocytes is a key factor of its mechanism. However, precise manner of this system is unknown. Aims of this study were to clarify a mechanism of interstital lung injury and following fibrosis via angiotensin II receptor 1 (AT1) and to estimate an efficacy of AT1 selective antagonist (Candesartan) as a therapeutic agent against bleomycin-induced lung injury prepared in rats. Distribution of expression of AT1 receptors in normal lungs was nonspecific for many types of cells including alveolar macrophages and type II pneumocytes. Their expression in injured lungs was more extensive in neutrophils, alveolar macrophages and fibroblasts than in other types of cells. Administration of Candesartan significantly inhibited an increased content of hydroxyproline as a quantitative indicator of fibrosis, and an increased cell number of neutrophils and alveolar macrophages, whereas it did not inhibit an increased expression of AT1. Thus, this AT1 antagonist may provide an ability to modulate a process of fibrosis in the lung. A speculated mechanism by the AT1 antagonist is based on prevention of apoptosis of type II pneumocytes.