The role of miR-29 in pulmonary fibrosis
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Idiopathic Pulmonary Fibrosis is a progressive, relentless disorder and despite years of extensive research, no effective treatments exist. miRNAs are small regulatory RNAs that control gene expression by translational suppression and destabilization of target mRNAs. There is increasing evidence that miRNAs regulate genes associated with fibrosis in organs such as the heart, kidney, liver and lung. The hypothesis of this study is that miR-29 plays a central role in pulmonary fibrosis through regulation of multiple extracellular matrix and smooth muscle related genes. In a genome-wide screening for miRNAs potentially involved in bleomycin-induced fibrotic lung injury, miR-29 family members were significantly reduced in fibrotic lungs, inversely correlating with expression levels of profibrotic target genes such as multiple collagens, and the severity of fibrosis. miR-29 expression was also significantly reduced in lung tissue of IPF patients and was suppressed by TGF-β and other profibrotic cytokines in vitro. miR-29 increases throughout lung development and was preferentially expressed in cells of mesenchymal origin, in subsets of interstitial cells of the alveolar wall, pleura and at the entrance of alveolar ducts, known sites of pulmonary fibrosis. miR-29 also directly controlled a large number of genes associated with pulmonary fibrosis in vitro. High miR-29 expression was also found in smooth muscle cells of conducting airways and blood vessels in the lung. Significant down-regulation of multiple smooth muscle cell markers, including CNN1, CNN2, TGLN, SMTN, ACTA2, ACTG2 and MYOCD was observed with reduced miR-29 in lung myofibroblast and smooth muscle cells. Further, KLF4 was identified as a direct target of miR-29 that is up-regulated at the mRNA and protein level in miR-29 KD cells and co-localizes with miR-29 in the lung. Reduced expression of smooth muscle marker genes in miR-29 KD cells were accompanied by increased collagen, suggesting a role for miR-29 in regulating ECM gene expression as well as modulating smooth muscle cell phenotype. These findings suggest a role for miR-29 in remodeling of airway and vasculature smooth muscle cells, a common feature of fibrotic lung diseases. Together, these studies defined a central role for miR-29 in fibrosis, and identified it as an attractive therapeutic target.
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