Mechanical stretch induces senescence of alveolar epithelial cells and drives fibroblast activation by paracrine mechanisms II

Severe lung injury requiring mechanical ventilation may lead to secondary fibrosis. Senescence, a cell response characterized by cell cycle arrest and a shift towards a proinflammatory/profibrotic phenotype, is one of the mechanisms involved in lung fibrosis. Here, we explore the contribution of mechanical stretch to senescence of the alveolar epithelium and its link with fibrosis. Human alveolar cells and fibroblasts were exposed in vitro to mechanical stretch, and senescence assessed. In addition, fibroblasts were exposed to culture media preconditioned by senescent epithelial cells and their activation was studied. Transcriptomic profiles from stretched, senescent epithelial cells and activated fibroblasts were combined to identify potential activated pathways. Finally, the senolytic effects of digoxin were tested in these models. Mechanical stretch induced senescence in alveolar epithelial cells, but not in fibroblasts. This stretch-induced senescence has specific features compared to senescence induced by doxorubicin. Fibroblasts were activated after exposure to supernatants conditioned by epithelial senescent cells. Transcriptomic analyses revealed notch signaling as a potential responsible for the epithelial-mesenchymal crosstalk, as fibroblast activation was inhibited by treatment with an inhibitor of g-secretase. Treatment with digoxin reduced the percentage of senescent cells after stretch and ameliorated the fibroblast response to preconditioned media. These results suggest that lung fibrosis in response to mechanical stretch may be caused by the paracrine effects of senescent alveolar cells. This pathogenetic mechanism can be pharmacologically manipulated to improve lung repair. Overall design: Two experimental groups were established. In the first condition A549 were subjected to cyclic stretch (15% elongation, 15 cycles/min) for 24 hours, whereas in the other condition , A549 were treated with doxorubicin (100nM) for 24 hours. Then, ß-galactosidase (Bgal) staining was assessed after 6 days for stretched A549 and A549 treated with doxorubicin. Eventually, positive cells for Bgal were sorted, lysated and RNA was collected for RNAseq analysis.