Abstract
The contribution of epithelial–mesenchymal transition (EMT) to human lung fibrogenesis is controversial. Here we provide evidence that ZEB1-mediated EMT in human alveolar epithelial type II (ATII) cells contributes to the development of lung fibrosis by paracrine signalling to underlying fibroblasts. Activation of EGFR–RAS–ERK signalling in ATII cells induced EMT via ZEB1. ATII cells had extremely low extracellular matrix gene expression even after induction of EMT, however conditioned media from ATII cells undergoing RAS-induced EMT augmented TGFβ-induced profibrogenic responses in lung fibroblasts. This epithelial–mesenchymal crosstalk was controlled by ZEB1 via the expression of tissue plasminogen activator (tPA). In human fibrotic lung tissue, nuclear ZEB1 expression was detected in alveolar epithelium adjacent to sites of extracellular matrix (ECM) deposition, suggesting that ZEB1-mediated paracrine signalling has the potential to contribute to early fibrotic changes in the lung interstitium. Targeting this novel ZEB1 regulatory axis may be a viable strategy for the treatment of pulmonary fibrosis.
Highlights
Epithelial–mesenchymal transition (EMT), a dynamic and reversible biological process by which epithelial cells lose their cell polarity and down-regulate cadherin-mediated cell–cell adhesion to gain migratory properties, is involved in embryonic development, wound healing, fibrosis and cancerSciences Unit, University of Southampton, Southampton SO16 6YD, UK 7 Oncogene Biology, The Francis Crick Institute, London NW1 1AT, UK 8 Unità Operativa Complessa di Pneumologia, Università Cattolica del Sacro Cuore, Fondazione Policlinico A
Our results showed that treatment of ATIIER:KRASV12 cells with epidermal growth factor (EGF) for 24 h induced EMT, reflected by a change in their morphology from typical cuboidal epithelial cells to a more elongated mesenchymal cell phenotype with a reorganisation of the actin cytoskeleton as demonstrated using Phalloidin staining of filamentous actin (F-actin) (Fig. 1b)
This phenotypic switch was accompanied by a significant increase in mRNA expression of ZEB1 and VIM (Vimentin), and a reduction in CDH1 (E-cadherin); mRNA levels of other EMT-transcription factors (TFs), such as SNAI1, SNAI2, TWIST and ZEB2 were not increased by activation of epidermal growth factor receptor (EGFR) signalling (Fig. 1c)
Summary
Epithelial–mesenchymal transition (EMT), a dynamic and reversible biological process by which epithelial cells lose their cell polarity and down-regulate cadherin-mediated cell–cell adhesion to gain migratory properties, is involved in embryonic development, wound healing, fibrosis and cancerSciences Unit, University of Southampton, Southampton SO16 6YD, UK 7 Oncogene Biology, The Francis Crick Institute, London NW1 1AT, UK 8 Unità Operativa Complessa di Pneumologia, Università Cattolica del Sacro Cuore, Fondazione Policlinico A. EMT is executed in response to pleiotropic signalling factors, including the transforming growth factor β (TGFβ) superfamily, Sonic Hedgehog (Shh), Wnt/β-catenin, fibroblast growth factor (FGF) and epidermal growth factor (EGF) These factors regulate the expression of specific transcription factors (TFs) called EMT-TFs (e.g. Snail, ZEB, Twist and others) that promote repression of epithelial features and induction of mesenchymal characteristics [2, 3]. IPF is generally regarded as a consequence of multiple interacting genetic and environmental risk factors, with repetitive local micro-injuries to ageing alveolar epithelium playing a central role [5]. These micro-injuries initiate the progressive accumulation of extracellular matrix (ECM) deposited by myofibroblasts. Lineage tracing in transgenic mice indicates that the contribution of those cells to the population of myofibroblasts is negligible [11,12,13,14]
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