Background/Purpose: Interstitial lung disease (ILD) is one of the most common extraarticular manifestation of rheumatoid arthritis. ILD is characterized by progressive fibrosis of the lung parenchyma and is associated with increased morbidity and mortality. Since TGF-β is thought to play an important role in tissue fibrosis, understanding the molecular details of TGF-β signaling will provide us a novel therapeutic target for fibrotic disorders. Recently, leucine-rich α-2 glycoprotein (LRG) was reported to function as a modulator of TGF-β signaling in angiogenesis in retinal disease and cardiac remodeling. However, the role of LRG in fibrotic diseases including lung fibrosis has not yet been investigated. In this study, we aimed to investigate the involvement of LRG in lung fibrosis using an animal model of lung fibrosis.

Methods: Bleomycin was intratracheally administered to C57BL/6 (WT) mice and LRG knockout (KO) mice, and lung fibrosis was evaluated by Masson’s trichrome staining and hydroxyproline quantification in the lung. TGF-β signaling in the lung was examined by analyzing phosphorylation of Smad proteins and expression of TGF-β downstream genes. Furthermore, in vitro analysis using L929 mouse fibroblast cell line was performed to assess the effect of LRG on TGF-β signaling in fibroblasts.

Results: The amount of LRG in BALF of WT mice was increased after bleomycin treatment. Immunohistochemistry of the lung section showed that LRG was expressed in alveolar epithelial cells, bronchial epithelial cells and infiltrated immune cells in bleomycin-treated WT mouse lung. In LRG KO mice, fibrosis in the lung was less severe as indicated by attenuated Masson’s trichrome staining and lower collagen content in the lung compared with WT mice. In addition, expression of α-SMA was decreased and phosphorylation of Smad2 was reduced in the lung of LRG KO mice compared with WT mice, suggesting that fibroblast activation was inhibited in the absence of LRG. In vitro experiments indicated that LRG enhanced TGF-β-induced phosphorylation of Smad2 and expression of PAI-1 and α-SMA in fibroblasts. On the other hand, Smad1/5/8 signaling was not enhanced by LRG. Although endoglin, an accessory TGF-β receptor, is known to be essential for LRG to modulate TGF-β signaling in endothelial cells, we found that endoglin was not involved in the process of enhancing Smad2 phosphorylation in fibroblasts.

Conclusion: LRG promotes lung fibrosis by enhancing TGF-β-induced phosphorylation of Smad2 in fibroblasts and by accelerating fibroblast differentiation into myofibroblasts.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/leucine-rich-%ce%b1-2-glycoprotein-promotes-lung-fibrosis-by-modulating-tgf-%ce%b2-signaling-in-fibroblasts/