CSP7 Protects Alveolar Epithelial Cells by Targeting <i>p53</i> -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 (<i>Cav1</i> 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 <i>p53</i> in addition to its posttranslational modification with induction of activated <i>caspase-3</i> and β-galactosidase, suggesting a <i>p53</i>-mediated loss of A₂C renewal. Further, we found increased expression of <i>SIAH-1</i>, a <i>p53</i>-inducible E3 ubiquitin ligase known to downregulate <i>TRF2</i> (telomere repeats binding factor 2). Consistent with the loss of <i>TRF2</i> and upregulation of <i>TRF1</i>, <i>TERT</i> (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 <i>p53</i>, <i>PAI-1</i>, <i>SIAH1</i>, and <i>TRF1</i>; as well as loss of <i>TRF2</i> and <i>TERT</i>, which were reversed in wild-type mice after treatment with CSP7. Interestingly, <i>PAI-1^-/-</i> mice, or those lacking microRNA-34a expression in A₂Cs, resisted telomere dysfunction, whereas <i>uPA^-/-</i> mice failed to respond to CSP7 treatment, suggesting <i>p53-microRNA-34a</i> feed-forward induction and that the <i>p53-uPA</i> pathway contributes to telomere dysfunction.