CSP7 Protects Alveolar Epithelial Cells by Targeting p53-Fibrinolytic Pathways During Lung Injuries.

Impaired alveolar epithelial regeneration in patients with idiopathic pulmonary fibrosis (IPF) and chronic obstructive pulmonary disease (COPD) is attributed to telomere dysfunction in type II alveolar epithelial cells (A2Cs). Genetic susceptibility, aging, and toxicant exposures, including tobacco smoke (TS), contribute to telomere dysfunction in A2Cs. Here we investigated whether improvement of telomere function plays a role in CSP7-mediated protection of A2Cs against ongoing senescence and apoptosis during bleomycin (BLM)-induced pulmonary fibrosis (PF) as well as alveolar injury caused by chronic TS exposure. We found a significant telomere shortening in A2Cs isolated from IPF and COPD lungs in line with other studies. These cells showed increased p53 in addition to its post-translational modification with induction of activated caspase-3 and β-galactosidase, suggesting a p53-mediated loss of A2C renewal. Further, we found increased expression of SIAH-1, a p53-inducible E3 ubiquitin ligase known to down-regulate telomere repeats binding factor 2 (TRF2). Consistent with the loss of TRF2 and upregulation of TRF1, telomerase reverse transcriptase (TERT) was downregulated in A2Cs. A2Cs from fibrotic lungs of mice either repeatedly instilled with BLM or isolated from chronic TS exposure-induced lung injury model showed reduced telomere length along with induction of p53, PAI-1, SIAH1 and TRF1 as well as loss of TRF2 and TERT, which were reversed in wild-type mice after treatment with CSP7. Interestingly, PAI-1-/- mice, or those lacking microRNA-34a expression in A2Cs, resisted telomere dysfunction, while uPA-/- mice failed to respond to CSP7 treatment, suggesting p53-microRNA-34a feed-forward induction and p53-uPA pathway contributes to telomere dysfunction.