MicroRNA-326 Regulates Profibrotic Functions of Transforming Growth Factor-β in Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal disorder resulting from the progressive remodeling of lungs, with no known effective treatment. Although transforming growth factor (TGF)-beta has a well-established role in lung fibrosis, clinical experience with neutralizing antibodies to TGF-b has been disappointing, and strategies to directly suppress TGF-beta 1 secretion are needed. In this study we used a combination of in silico, in vitro, and in vivo approaches to identify microRNAs involved in TGF-beta 1 regulation and to validate the role of miR-326 in pulmonary fibrosis. We show that hsa-miR-326 regulates TGF-beta 1 expression and that hsa-miR-326 levels are inversely correlated to TGF-beta 1 protein levels in multiple human cell lines. The increase in TGF-beta 1 expression during the progression of bleomycin-induced lung fibrosis in mice was associated with loss of mmu-miR-326. Restoration of mmu-miR-326 levels by intranasal delivery of miR-326 mimics was sufficient to inhibit TGF-beta 1 expression and attenuate the fibrotic response. Moreover, human IPF lung specimens had markedly diminished miR-326 expression as compared with nonfibrotic lungs. Additional targets of miR-326 controlling TGF-beta signaling and fibrosis-related pathways were identified, and miR-326 was found to down-regulate profibrotic genes, such as Ets1, Smad3, and matrix metalloproteinase 9, whereas it up-regulates antifibrotic genes, such as Smad7. Our results suggest for the first time that miR-326 plays a key role in regulating TGF-beta 1 expression and other profibrotic genes and could be useful in developing better therapeutic strategies for alleviating lung fibrosis.