Autophagy in Idiopathic Pulmonary Fibrosis

Abstract

BackgroundAutophagy is a basic cellular homeostatic process important to cell fate decisions under conditions of stress. Dysregulation of autophagy impacts numerous human diseases including cancer and chronic obstructive lung disease. This study investigates the role of autophagy in idiopathic pulmonary fibrosis.MethodsHuman lung tissues from patients with IPF were analyzed for autophagy markers and modulating proteins using western blotting, confocal microscopy and transmission electron microscopy. To study the effects of TGF-β1 on autophagy, human lung fibroblasts were monitored by fluorescence microscopy and western blotting. In vivo experiments were done using the bleomycin-induced fibrosis mouse model.ResultsLung tissues from IPF patients demonstrate evidence of decreased autophagic activity as assessed by LC3, p62 protein expression and immunofluorescence, and numbers of autophagosomes. TGF-β1 inhibits autophagy in fibroblasts in vitro at least in part via activation of mTORC1; expression of TIGAR is also increased in response to TGF-β1. In the bleomycin model of pulmonary fibrosis, rapamycin treatment is antifibrotic, and rapamycin also decreases expression of á-smooth muscle actin and fibronectin by fibroblasts in vitro. Inhibition of key regulators of autophagy, LC3 and beclin-1, leads to the opposite effect on fibroblast expression of á-smooth muscle actin and fibronectin.ConclusionAutophagy is not induced in pulmonary fibrosis despite activation of pathways known to promote autophagy. Impairment of autophagy by TGF-β1 may represent a mechanism for the promotion of fibrogenesis in IPF.

Autophagy: A Crucial Moderator of Redox Balance, Inflammation, and Apoptosis in Lung Disease

“…Defective fibroblast autophagic processes have been implicated in the pathogenesis of IPF. Lung tissues from IPF patients and human lung fibroblasts treated with transforming growth factor-beta (TGF-b) demonstrate increased cellular senescence and decreased autophagic activity as characterized by decreased LC3B protein expression (4,130). TGF-b1 inhibits autophagy in human lung fibroblasts.…”

“…TGF-b1 inhibits autophagy in human lung fibroblasts. Genetic deletion of the autophagy proteins, LC3 or Beclin 1, potentiated the TGF-b1-induced expression of fibronectin and the myofibroblast marker a-smooth muscle actin in fibroblasts (130). Treatment of mice with the autophagy-inducing drug, rapamycin, partially protected against lung fibrosis (130).…”

“…Genetic deletion of the autophagy proteins, LC3 or Beclin 1, potentiated the TGF-b1-induced expression of fibronectin and the myofibroblast marker a-smooth muscle actin in fibroblasts (130). Treatment of mice with the autophagy-inducing drug, rapamycin, partially protected against lung fibrosis (130). Loss of autophagy in patients with IPF may potentiate the effects of TGF-b1 with respect to ECM production and transformation to a myofibroblast phenotype.…”

“…IPF patients have increased levels of ubiquitinated proteins and p62 (4,130), and type II epithelial cells of patients with sporadic IPF show evidence of severe ER stress (81), a dysfunctional epithelial cell phenotype that facilitates fibrotic remodeling (86). Further experiments are needed to determine the relationships between autophagy, apoptosis, and fibrotic responses as they relate to human disease.…”

Endoglin Regulation of Smad2 Function Mediates Beclin1 Expression and Endothelial Autophagy

“…Similarly, mechanisms governing autophagy appear to be highly context-dependent, as illustrated by the dichotomous effects of transforming growth factor ␤ (TGF-␤). Previous studies have shown that TGF-␤ induces autophagy in certain epithelial and cancer cell types, whereas it counteracts this process in fibroblasts (16,17). Importantly, TGF-␤ can simultaneously activate a number of distinct cellular pathways associated with pro-and anti-autophagic responses, raising the question of how different cell systems integrate these divergent signals to regulate autophagy.…”