Bi-directional epithelial-mesenchymal crosstalk provides self-sustaining pro-fibrotic signals in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is the prototypic progressive fibrotic lung disease with a median survival of 2-4 years. Injury to and/or dysfunction of alveolar epithelium are strongly implicated in IPF disease initiation, but what factors determine why fibrosis progresses rather than normal tissue repair occurs remain poorly understood. We previously demonstrated that ZEB1-mediated epithelial-mesenchymal transition (EMT) in human alveolar epithelial type II (ATII) cells augments TGF-β-induced profibrogenic responses in underlying lung fibroblasts by paracrine signalling. Here we investigated bi-directional epithelial-mesenchymal crosstalk and its potential to drive fibrosis progression. RNA sequencing (RNA-seq) of lung fibroblasts exposed to conditioned media from ATII cells undergoing RAS-induced EMT identified many differentially expressed genes including those involved in cell migration and extracellular matrix (ECM) regulation. We confirmed that paracrine signalling between AS-activated ATII cells and fibroblasts augmented fibroblast recruitment and demonstrated that this involved a ZEB1-tissue plasminogen activator (tPA) axis. In a reciprocal fashion, paracrine signalling from TGF-β-activated lung fibroblasts or IPF fibroblasts induced RAS activation in ATII cells, at least partially via the secreted protein, SPARC. Together these data identify that aberrant bi-directional epithelial-mesenchymal crosstalk in IPF drives a chronic feedback loop that maintains a wound-healing phenotype and provides self-sustaining pro-fibrotic signals.

特发性肺纤维化（Idiopathic Pulmonary Fibrosis, IPF）是最典型的进行性纤维化性肺病，患者中位生存期仅2~4年。肺泡上皮损伤及/或功能异常已被证实与IPF的发病密切相关，但目前仍不清楚究竟是哪些因素决定了纤维化进展而非正常组织修复。本团队既往研究证实，人肺泡Ⅱ型上皮细胞（Alveolar Epithelial Type II, ATII）中由ZEB1介导的上皮间质转化（Epithelial-Mesenchymal Transition, EMT），可通过旁分泌信号通路增强转化生长因子-β（Transforming Growth Factor-β, TGF-β）诱导的下方肺成纤维细胞的促纤维化应答。本研究旨在探讨双向上皮-间质细胞串扰及其驱动纤维化进展的潜在机制。对暴露于经RAS诱导EMT的ATII细胞条件培养基的肺成纤维细胞进行RNA测序（RNA Sequencing, RNA-seq），结果筛选得到大量差异表达基因，其中包括参与细胞迁移及细胞外基质（Extracellular Matrix, ECM）调控的相关基因。本研究证实，经AS激活的ATII细胞与成纤维细胞之间的旁分泌信号可增强成纤维细胞的募集，且该过程依赖于ZEB1-组织型纤溶酶原激活剂（Tissue Plasminogen Activator, tPA）轴。反之，由TGF-β激活的肺成纤维细胞或IPF患者肺成纤维细胞分泌的旁分泌信号，可诱导ATII细胞发生RAS激活，该过程至少部分依赖于分泌蛋白SPARC（Secreted Protein Acidic and Rich in Cysteine, SPARC）。综上，本研究数据证实，IPF中异常的双向上皮-间质细胞串扰可形成慢性反馈环路，该环路既能维持创伤修复表型，又能提供自我维持的促纤维化信号。