Twist1: A novel target in pulmonary fibrosis that protects against multiple apoptotic pathways.

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive lung disease that is nearly always fatal. Fibroblasts are a major mediator of fibrotic injury and may accumulate in IPF lungs by a variety of mechanisms, including proliferation, recruitment from circulation, and/or epithelial-mesenchymal transformation. Regardless of origin, fibroblasts in IPF lungs persist in a relatively hostile environment, characterized by oxidants, ER stress, and Fas ligand. To identify novel mediators of fibroblast accumulation and survival in IPF, we performed gene expression profiling of IPF lungs. Among the most highly enhanced genes was the basic helix-loop-helix protein, Twist1. Twist1 was localized to the nuclei of cells present in fibroblastic foci. In vitro experiments on fibroblasts isolated from rat lungs (RLFs) revealed that Twist1 mRNA and protein were induced by the pro-fibrotic growth factors bFGF, PDGF, and EGF. Overexpression of Twist1 in RLFs protected the cells against multiple apoptotic stimuli present in IPF lungs. Conversely, short interfering RNA mediated suppression of Twist1 resulted in a decrease in RLF accumulation due to increased apoptosis. Addition of PDGF and other growth factors to Twist1-sufficient fibroblasts led to increased accumulation. However, growth factor stimulation of fibroblasts with suppressed Twist1 expression caused dramatically increased apoptosis. Apoptosis due to Twist1 depletion was mediated, in part, by the pro-apoptotic Bcl-2 family members, Bim and PUMA. These findings implicate Twist1 in the pathogenesis of IPF. We suggest that Twist1 promotes the inappropriate survival and accumulation of fibroblasts in IPF, in part by shaping their responsiveness to growth factors and apoptotic stimuli.