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

Impaired alveolar epithelial regeneration in patients with idiopathic pulmonary fibrosis (PF) or chronic obstructive pulmonary disease 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 (Cav1 scaffolding domain peptide)-mediated protection of A₂Cs against ongoing senescence and apoptosis during bleomycin-induced PF as well as alveolar injury caused by chronic TS exposure. We found a significant telomere shortening in A₂Cs isolated from idiopathic PF and chronic obstructive pulmonary disease lungs in line with other studies. These cells showed increased p53 in addition to its posttranslational 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 downregulate TRF2 (telomere repeats binding factor 2). Consistent with the loss of TRF2 and upregulation of TRF1, TERT (telomerase reverse transcriptase) was downregulated in A₂Cs. A₂Cs from fibrotic lungs of mice repeatedly instilled with bleomycin or isolated from a chronic TS exposure-induced lung injury model showed reduced telomere length; 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, whereas uPA^-/- mice failed to respond to CSP7 treatment, suggesting p53-microRNA-34a feed-forward induction and that the p53-uPA pathway contributes to telomere dysfunction.