Cigarette Smoke Extract Promotes Human Lung Myofibroblast Differentiation by the Induction of Endoplasmic Reticulum Stress

Background: Idiopathic pulmonary fibrosis (IPF) is a progressive and lethal fibrotic lung disease with an unknown aetiology. Persistent myofibroblast differentiation is a prominent feature of IPF. Cigarette smoking is a risk factor for IPF and an indicator of poor prognosis. Cigarette smoking induces endoplasmic reticulum (ER) stress, and it has been shown that ER stress promotes fibroblast-to-myofibroblast differentiation in lung fibrosis. In this study, we investigated whether cigarette smoke extract (CSE) promotes lung myofibroblast differentiation via the induction of ER stress. Objectives: Our study concentrates on exploring the relationship between smoking and ER stress in the differentiation of lung fibroblasts to myofibroblasts. Methods: Human embryonic lung fibroblasts (MRC-5 fibroblasts) were stimulated with various doses of CSE. Levels of alpha-smooth muscle actin (alpha-SMA) protein were evaluated by immunofluorescence and western blot analyses. ER stress was induced by thapsigargin (TG) and inhibited by 4-phenyl butyric acid (4-PBA). Protein levels of glucose-regulated protein-78 (GRP78), inositol-requiring enzyme 1 (IRE1), X box-binding protein-1 (XBP-1) and activating transcription factor 6 (ATF6) were determined by western blotting. GRP78 siRNA was transfected into MRC-5 cells using Lipofectamine RNAiMAX Reagent. Results: CSE at a concentration of 1.0% significantly increased alpha-SMA expression in MRC-5 cells. There was no significant cell apoptosis after cells were exposed to CSE. CSE treatment significantly increased the expression of GRP78, IRE1, XBP-1 and ATF6 at the protein level at 48 h. Pretreatment with TG enhanced, whereas pretreatment with 4-PBA inhibited, the CSE-induced expression of alpha-SMA, GRP78 and XBP-1. Furthermore, knockdown of GRP78 blocked alpha-SMA expression in MRC-5 cells exposed to CSE. Conclusion: Our data suggested that CSE promotes lung fibroblast-to-myofibroblast differentiation by the induction of ER stress.