Citations

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.

Nuclear translocation of mothers against decapentaplegic homolog 2/3 (SMAD2/3), core transcription factors of transforming growth factor β (TGF‐β) signaling, is critical for hepatic stellate cell (HSC) differentiation into metastasis‐promoting myofibroblasts. SMAD2/3 have multiple coactivators, including WW domain‐containing transcription regulator protein 1 (WWTR1 or TAZ) and p300 acetyltransferase. In the nucleus, TAZ binds to SMAD2/3 to prevent SMAD2/3 nuclear export. However, how TAZ and SMAD2/3 enter the nucleus remains poorly understood because neither contains a nuclear localization signal (NLS), an amino acid sequence tagging proteins for nuclear transport. p300 is an NLS‐containing large scaffold protein, so we hypothesized that SMAD2/3 and TAZ may undergo nuclear import through complexing with p300. Coimmunoprecipitation, immunofluorescence, and nuclear fractionation assays revealed that TGF‐β1 promoted binding of SMAD2/3 and TAZ to p300 and that p300 inactivation disrupted TGF‐β1‐mediated SMAD2/3 and TAZ nuclear accumulation. Deleting the p300 NLS blocked TGF‐β1‐induced SMAD2/3 and TAZ nuclear transport. Consistently, p300 inactivation suppressed TGF‐β1‐mediated HSC activation and transcription of genes encoding tumor‐promoting factors, such as connective tissue growth factor, Tenascin C, Periostin, platelet‐derived growth factor C, and fibroblast growth factor 2, as revealed by microarray analysis. Chromatin immunoprecipitation‐real‐time quantitative PCR showed that canonical p300‐mediated acetylation of histones also facilitated transcription in response to TGF‐β1 stimulation. Interestingly, although both TGF‐β1‐mediated and stiffness‐mediated HSC activation require p300, comparison of gene expression data sets revealed that transcriptional targets of TGF‐β1 were distinct from those of stiffness‐p300 mechanosignaling. Lastly, in tumor/HSC coinjection and intrasplenic tumor injection models, targeting p300 of activated‐HSC/myofibroblasts by C646, short hairpin RNA, or cre ‐mediated gene disruption reduced tumor and liver metastatic growth in mice. Conclusion: p300 facilitates TGF‐β1‐stimulated HSC activation by both noncanonical (cytoplasm‐to‐nucleus shuttle for SMAD2/3 and TAZ) and canonical (histone acetylation) mechanisms. p300 is an attractive target for inhibiting HSC activation and the prometastatic liver microenvironment.

Interleukin (IL)-17 is a T helper 17 cytokine implicated in the pathogenesis of many autoimmune diseases, including rheumatoid arthritis (RA). Although IL-17A has a well-established role in murine pulmonary fibrosis models, its role in the tissue remodeling and fibrosis occurring in idiopathic pulmonary fibrosis (IPF) and RA-associated interstitial lung disease (RA-ILD) is not very well defined. To address this question, we utilized complimentary studies to determine responsiveness of human normal and pathogenic lung fibroblasts to IL-17A and used lung biopsies acquired from patients with IPF and RA-ILD to determine IL-17A receptor (IL-17RA) expression. Both normal and pathogenic IPF lung fibroblasts express functional IL-17RA and respond to IL-17A stimulation with cell proliferation, generation of extracellular matrix (ECM) proteins, and induction of myofibroblast transdifferentiation. Small interfering RNA (siRNA) silencing of IL-17RA attenuated this fibroblast response to IL-17A on ECM production. These fibroblast responses to IL-17A are dependent on NF-κB-mediated signaling. In addition, inhibiting Janus activated kinase (JAK) 2 by either siRNA or a selective pharmacological inhibitor, AZD1480—but not a JAK1/JAK3 selective inhibitor, tofacitinib—also significantly reduced this IL-17A-induced fibrogenic response. Lung biopsies of RA-ILD patients demonstrate significantly higher IL-17RA expression in areas of fibroblast accumulation and fibrosis, compared with either IPF or normal lung tissue. These observations support a direct role for IL-17A in lung fibrosis that may be particularly relevant in the context of RA-ILD.

Abstract Supplemental O2 (hyperoxia; 30–90% O2) is a necessary intervention for premature infants, but it contributes to development of neonatal and pediatric asthma, necessitating better understanding of contributory mechanisms in hyperoxia-induced changes to airway structure and function. In adults, environmental stressors promote formation of senescent cells that secrete factors (senescence-associated secretory phenotype), which can be inflammatory and have paracrine effects that enhance chronic lung diseases. Hyperoxia-induced changes in airway structure and function are mediated in part by effects on airway smooth muscle (ASM). In the present study, using human fetal ASM cells as a model of prematurity, we ascertained the effects of clinically relevant moderate hyperoxia (40% O2) on cellular senescence. Fetal ASM exposed to 40% O2 for 7 days exhibited elevated concentrations of senescence-associated markers, including β-galactosidase; cell cycle checkpoint proteins p16, p21, and p-p53; and the DNA damage marker p-γH2A.X (phosphorylated γ-histone family member X). The combination of dasatinib and quercetin, compounds known to eliminate senescent cells (senolytics), reduced the number of hyperoxia-exposed β-galactosidase–, p21-, p16-, and p-γH2A.X–positive ASM cells. The senescence-associated secretory phenotype profile of hyperoxia-exposed cells included both profibrotic and proinflammatory mediators. Naive ASM exposed to media from hyperoxia-exposed senescent cells exhibited increased collagen and fibronectin and higher contractility. Our data show that induction of cellular senescence by hyperoxia leads to secretion of inflammatory factors and has a functional effect on naive ASM. Cellular senescence in the airway may thus contribute to pediatric airway disease in the context of sequelae of preterm birth.

ABSTRACT Myofibroblasts play key roles in wound healing and pathological fibrosis. Here, we used an RNAi screen to characterize myofibroblast regulatory genes, using a high-content imaging approach to quantify α-smooth muscle actin stress fibers in cultured human fibroblasts. Screen hits were validated on physiological compliance hydrogels, and selected hits tested in primary fibroblasts from patients with idiopathic pulmonary fibrosis. Our RNAi screen led to the identification of STAT3 as an essential mediator of myofibroblast activation and function. Strikingly, we found that STAT3 phosphorylation, while responsive to exogenous ligands on both soft and stiff matrices, is innately active on a stiff matrix in a ligand/receptor-independent, but ROCK- and JAK2-dependent fashion. These results demonstrate how a cytokine-inducible signal can become persistently activated by pathological matrix stiffening. Consistent with a pivotal role for this pathway in driving persistent fibrosis, a STAT3 inhibitor attenuated murine pulmonary fibrosis when administered in a therapeutic fashion after bleomycin injury. Our results identify novel genes essential for the myofibroblast phenotype, and point to STAT3 as an important target in pulmonary fibrosis and other fibrotic diseases.

Lung function is inherently mechanical in nature and depends on the capacity to conduct air and blood to and from the gas exchange regions. Variations in the elastic properties of the human lung across anatomical compartments and with aging are likely important determinants of lung function but remain relatively poorly characterized. Here we applied atomic force microscopy microindentation to characterize human lung tissue from subjects ranging in age from 11 to 60 yr old. We observed striking anatomical variations in elastic modulus, with the airways (200- to 350-µm diameter) the stiffest and the parenchymal regions the most compliant. Vessels (diameter < 100 µm) represented an intermediate mechanical environment and displayed diameter-dependent trends in elastic modulus. Binning our samples into younger (11–30 yr old) and older (41–60 yr old) groups, we observed significant age-related increases in stiffness in parenchymal and vessel compartments, with the most pronounced changes in the vessels. To investigate cellular mechanisms that might contribute to vascular stiffening with aging, we studied primary human pulmonary artery smooth muscle cells from subjects ranging in age from 11 to 60 yr old. While we observed no change in the mechanical properties of the cells themselves, we did observe trends toward increases in traction forces and extracellular matrix deposition with aging. These results demonstrate age-related changes in tissue mechanical properties that likely contribute to impaired lung function with aging and underscore the potential to identify mechanisms that contribute to mechanical tissue remodeling through the study of human cells and tissues from across the aging spectrum.

The scaffold protein synectin plays a critical role in the trafficking and regulation of membrane receptor pathways. As platelet-derived growth factor receptor (PDGFR) is essential for hepatic stellate cell (HSC) activation and liver fibrosis, we sought to determine the role of synectin on the PDGFR pathway and development of liver fibrosis. Mice with deletion of synectin from HSC were found to be protected from liver fibrosis. mRNA sequencing revealed that knockdown of synectin in HSC demonstrated reductions in the fibrosis pathway of genes, including PDGFR-β. Chromatin IP assay of the PDGFR-β promoter upon synectin knockdown revealed a pattern of histone marks associated with decreased transcription, dependent on p300 histone acetyltransferase. Synectin knockdown was found to downregulate PDGFR-α protein levels, as well, but through an alternative mechanism: protection from autophagic degradation. Site-directed mutagenesis revealed that ubiquitination of specific PDGFR-α lysine residues was responsible for its autophagic degradation. Furthermore, functional studies showed decreased PDGF-dependent migration and proliferation of HSC after synectin knockdown. Finally, human cirrhotic livers demonstrated increased synectin protein levels. This work provides insight into differential transcriptional and posttranslational mechanisms of synectin regulation of PDGFRs, which are critical to fibrogenesis.