Reemergence of hedgehog mediates epithelial-mesenchymal crosstalk in pulmonary fibrosis.

Hedgehog signaling plays important roles in cell development and differentiation. In this study, the ability of Sonic Hedgehog (SHH) to induce myofibroblast differentiation was analyzed in isolated human lung fibroblasts, and its in vivo significance was evaluated in rodent bleomycin-induced pulmonary fibrosis. The results showed that SHH could induce myofibroblast differentiation in human lung fibroblasts in a Smo- and Gli1-dependent manner. Gel shift analysis, chromatin immunoprecipitation assay, and site-directed mutagenesis revealed that a Gli1 binding consensus in the α-SMA gene promoter was important for mediating SHH-induced myofibroblast differentiation. Analysis of Hedgehog reemergence in vivo revealed that of all three Hedgehog isoforms, only SHH was significantly induced in bleomycin-injured lung along with Gli1. The induction of SHH was only noted in epithelial cells, and its expression was undetectable in lung fibroblasts or macrophages. transforming growth factor (TGF)-β induced SHH significantly in cultured alveolar epithelial cells, whereas SHH induced TGF-β in lung fibroblasts. Pulmonary fibrosis and α-smooth muscle actin (α-SMA) expression were significantly reduced in mice that were Smo deficient only in type I collagen-expressing cells. Thus, the reemergence of SHH in epithelial cells could result in induction of myofibroblast differentiation in a Smo-dependent manner and subsequent Gli1 activation of the α-SMA promoter.