Pleural
fibrosis is characterized by severe
inflammation of the pleural space and pleural reorganization. Subsequent thickening of the visceral pleura contributes to lung stiffness and impaired lung function. Pleural mesothelial cells (PMCs) can become myofibroblasts via mesothelial-mesenchymal transition (MesoMT) and contribute to pleural organization,
fibrosis, and rind formation. However, the mechanisms that underlie MesoMT remain unclear. Here, we investigated the role of
myocardin in the induction of MesoMT.
Transforming growth factor β (TGF-β) and
thrombin induced MesoMT and markedly upregulated the expression of
myocardin, but not
myocardin-related
transcription factor A (MRTF-A) or MRTF-B, in human PMCs (HPMCs). TGF-β stimulation notably induced the nuclear translocation of
myocardin in HPMCs, whereas nuclear translocation of MRTF-A and MRTF-B was not observed. Several genes under the control of
myocardin were upregulated in cells undergoing MesoMT, an effect that was accompanied by a dramatic cytoskeletal reorganization of HPMCs consistent with a migratory phenotype.
Myocardin gene silencing blocked TGF-β- and
thrombin-induced MesoMT. Although
myocardin upregulation was blocked, MRTF-A and MRTF-B were unchanged.
Myocardin, α-SMA,
calponin, and smooth muscle
myosin were notably upregulated in the thickened pleura of
carbon black/
bleomycin and
empyema mouse models of fibrosing pleural injury. Similar results were observed in human nonspecific
pleuritis. In a TGF-β mouse model of pleural
fibrosis, PMC-specific knockout of
myocardin protected against decrements in lung function. Further, TGF-β-induced pleural thickening was abolished by PMC-specific
myocardin knockout, which was accompanied by a marked reduction of
myocardin,
calponin, and α-SMA expression compared with floxed-
myocardin controls. These novel results show that
myocardin participates in the development of MesoMT in HPMCs and contributes to the pathogenesis of pleural organization and
fibrosis.