Search

Idiopathic pulmonary fibrosis (IPF) is a chronic and fatal lung disease, and fibroblasts derived from patients with IPF are resistant to type I collagen matrix-induced cell death. The alteration of the PTEN-Akt axis permits IPF fibroblasts to maintain a pathological phenotype on collagen by suppressing autophagy. However, the precise underlying mechanism by which the Akt downstream molecule suppresses autophagic activity remains elusive. FoxO3a is a direct target of Akt and is implicated with the transcriptional activation of autophagy. Therefore, we investigated whether reduced FoxO3a expression causes abnormally low autophagy in IPF fibroblasts on collagen. We found that FoxO3a mRNA and protein levels are low in IPF fibroblasts, which subsequently suppresses the autophagosomal marker LC3B expression on collagen matrix. In contrast, the majority of control fibroblasts showed an increase in FoxO3a and LC3B expression at both the mRNA and protein levels. The luciferase assay confirmed that FoxO3a binds to the promoter region of LC3B and transcriptionally activates LC3B. The overexpression of wild-type FoxO3a increased LC3B mRNA and protein expression in IPF fibroblasts, whereas the dominant negative FoxO3a decreased the LC3B level in control fibroblasts. The inhibition of autophagic activity sensitized control fibroblasts to collagen matrix-induced cell death. In contrast, enhanced viability was found when autophagic function was inhibited in IPF fibroblasts. Our study showed that aberrantly low FoxO3a expression participates in reducing autophagic activity via transcriptional suppression of LC3B in IPF fibroblasts on collagen. This suggests that low autophagic activity by the alteration of FoxO3a may contribute to IPF progression., (Copyright © 2015 the American Physiological Society.)

Idiopathic pulmonary fibrosis (IPF) is a lethal and progressive lung disease characterized by persistent (myo)fibroblasts and the relentless accumulation of collagen matrix. Unlike normal lung fibroblasts, IPF lung fibroblasts have suppressed forkhead box O3a (FoxO3a) activity, which allows them to expand in this diseased environment. microRNA-96 (miR-96) has recently been found to directly bind to the 3'-untranslated region of FoxO3a mRNA, which subsequently inhibits its function. We examined whether aberrantly low FoxO3a expression is in part due to increased miR-96 levels in IPF fibroblasts on polymerized collagen, thereby causing IPF fibroblasts to maintain their pathological properties. miR-96 expression was upregulated in IPF fibroblasts compared with control fibroblasts when cultured on collagen. In contrast, FoxO3a mRNA levels were reduced in most IPF fibroblasts. However, when miR-96 function was inhibited, FoxO3a mRNA and protein expression were increased, suppressing IPF fibroblast proliferation and promoting their cell death in a dose-dependent fashion. Likewise, FoxO3a and its target proteins p21, p27, and Bim expression was also increased in the presence of a miR-96 inhibitor in IPF fibroblasts. However, when control fibroblasts were treated with miR-96 mimic, FoxO3a, p27, p21, and Bim mRNA and protein levels were decreased. In situ hybridization analysis further revealed the presence of enhanced miR-96 expression in cells within the fibroblastic foci of IPF lung tissue. Our results suggest that when IPF fibroblasts interact with collagen-rich matrix, pathologically altered miR-96 expression inhibits FoxO3a function, causing IPF fibroblasts to maintain their pathological phenotype, which may contribute to the progression of IPF., (Copyright © 2014 the American Physiological Society.)