Ratio of miRNA-29 to miRNA-199 expression coordinates mesenchymal stem cell repair of bleomycin-induced pulmonary injury

Our previous work demonstrated the anti-fibrotic effects of infusion of adipose-derived mesenchymal stem cells (ASCs) to prevent or repair bleomycin (BLM)-induced lung injury. The present study investigates mechanisms driving these anti-fibrotic effects. Pulmonary fibrosis developed at day 12 in 22-month-old C57BL/6 male mice after intratracheal BLM instillation. There was a decrease in indices of pulmonary fibrosis, including collagen content, AKT activation, collagen types I and III, α_V-integrin, tumor necrosis factor alpha, and transforming growth factor β mRNA after infusion of ASCs 12 days post-BLM treatment compared to BLM alone. Infusion of ASCs increased the population of alveolar types I and II epithelial cells that had been reduced after BLM treatment. miRNAscope technology and reverse-transcription polymerase chain reaction revealed that ASC-treated mice demonstrated increased miR-29a, decreased miR-199, and increased telomere length, telomerase RNA component, and telomerase reverse transcriptase compared to BLM alone. In vitro and ex vivo experiments using double-transfected mouse or human myofibroblasts (miR-29 mimic, and miR-199 inhibitor) confirmed that alterations of these miRNAs regulate downstream effectors of fibrosis. These data suggest that alteration of the ratio of anti-fibrotic to fibrotic miRNAs and increase in telomere length are critical mechanisms of ASC-mediated repair of BLM-induced pulmonary fibrosis.