Meridew, Jeffrey A.

Pulmonary fibrosis is a devastating disease characterized by accumulation of activated fibroblasts and scarring in the lung. While fibroblast activation in physiological wound repair reverses spontaneously, fibroblast activation in fibrosis is aberrantly sustained. Here we identified histone 3 lysine 9 methylation (H3K9me) as a critical epigenetic modification that sustains fibroblast activation by repressing the transcription of genes essential to returning lung fibroblasts to an inactive state. We show that the histone methyltransferase G9a (EHMT2) and chromobox homolog 5 (CBX5, also known as HP1α), which deposit H3K9me marks and assemble an associated repressor complex, respectively, are essential to initiation and maintenance of fibroblast activation specifically through epigenetic repression of peroxisome proliferator–activated receptor γ coactivator 1 α gene ( PPARGC1A, encoding PGC1α). Both TGF-β and increased matrix stiffness potently inhibit PGC1α expression in lung fibroblasts through engagement of the CBX5/G9a pathway. Inhibition of the CBX5/G9a pathway in fibroblasts elevates PGC1α, attenuates TGF-β– and matrix stiffness–promoted H3K9 methylation, and reduces collagen accumulation in the lungs following bleomycin injury. Our results demonstrate that epigenetic silencing mediated by H3K9 methylation is essential for both biochemical and biomechanical fibroblast activation and that targeting this epigenetic pathway may provide therapeutic benefit by returning lung fibroblasts to quiescence.

Abstract Reciprocal epithelial–mesenchymal interactions are pivotal in lung development, homeostasis, injury, and repair. Organoids have been used to investigate such interactions, but with a major focus on epithelial responses to mesenchyme and less attention to epithelial effects on mesenchyme. In the present study, we used nascent organoids composed of human and mouse lung epithelial and mesenchymal cells to demonstrate that healthy lung epithelium dramatically represses transcriptional, contractile, and matrix synthetic functions of lung fibroblasts. Repression of fibroblast activation requires signaling via the bone morphogenetic protein (BMP) pathway. BMP signaling is diminished after epithelial injury in vitro and in vivo, and exogenous BMP4 restores fibroblast repression in injured organoids. In contrast, inhibition of BMP signaling in healthy organoids is sufficient to derepress fibroblast matrix synthetic function. Our results reveal potent repression of fibroblast activation by healthy lung epithelium and a novel mechanism by which epithelial loss or injury is intrinsically coupled to mesenchymal activation via loss of repressive BMP signaling.