Leucine-rich α-2 glycoprotein promotes lung fibrosis by modulating TGF-β signaling in fibroblasts

TGF-beta has an important role in fibrotic diseases, including idiopathic pulmonary fibrosis (IPF). Detailed analysis of TGF-b signaling in pulmonary fibrosis at the molecular level is needed to identify novel therapeutic targets. Recently, leucine-rich alpha-2 glycoprotein (LRG) was reported to function as a modulator of TGF-beta signaling in angiogenesis and tumor progression. However, the involvement of LRG in fibrotic disorders, including IPF, has not yet been investigated. In this study, we investigated the role of LRG in fibrosis by analyzing LRG knockout (KO) mice with bleomycin-induced lung fibrosis, an animal model of pulmonary fibrosis. The amount of LRG in the lungs of wild-type (WT) mice was increased by bleomycin administration prior to fibrosis development. In LRG KO mice, lung fibrosis was significantly suppressed, as indicated by attenuated Masson's trichrome staining and lower collagen content than those in WT mice. Moreover, in the lungs of LRG KO mice, phosphorylation of Smad2 was reduced and expression of alpha-SMA was decreased relative to those in WT mice. In vitro experiments indicated that LRG enhanced the TGF-beta-induced phosphorylation of Smad2 and the expression of Serpine1 and Acta2, the downstream of Smad2, in fibroblasts. Although endoglin, an accessory TGF-beta receptor, is essential for LRG to promote TGF-beta signaling in endothelial cells during angiogenesis, we found that endoglin did not contribute to the ability of LRG to enhance Smad2 phosphorylation in fibroblasts. Taken together, our data suggest that LRG promotes lung fibrosis by modulating TGF-beta-induced Smad2 phosphorylation and activating profibrotic responses in fibroblasts.