Role of urokinase-type plasminogen activator receptor (uPAR) in senescent lung cells

Rationale: Cellular senescence imposes proliferative arrest on cells in response to various stressors and is implicated in COPD and IPF pathogenesis. Urokinase-type plasminogen activator receptor (uPAR) is elevated in senescent cells in some tissues and has been used as a target to remove these deleterious cells. However, whether uPAR is elevated in senescent lung cells or plays a functional role in lung cell senescence is unknown. Aim: Assess uPAR expression in senescent lung cells. Method: Senescence was induced in human lung fibroblasts (n=4) and small airway epithelial cells (SAEC) (n=2) using hydrogen peroxide (H₂O₂) or etoposide (72h). Senescence was confirmed by expression of senescence-associated β-galactosidase (SA-β-Gal), p21^CIP1 and p16^INK4a. uPAR expression was assessed using immunocytochemistry and Western blotting and its role in SAEC senescence was investigated using siRNA. Results: uPAR was highly expressed in fibroblasts and not elevated with senescence. Western blotting suggested cleavage and/or internalisation of uPAR in senescent fibroblasts. In SAEC, H₂O₂ and etoposide increased the percentage uPAR positive SAEC (+25% and +16%, respectively). However, further analysis suggested uPAR intensity did not correlate with nuclear p21^CIP1 or p16^INK4a staining on an individual cell basis (r² = 0.08). Knock-down of uPAR with siRNA increased p16^INK4a (+26%) and SA-β-Gal expression (+1.8 fold change) in SAEC. Conclusion: uPAR is highly expressed in lung fibroblasts but not increased in senescence. In SAEC, uPAR may be elevated in senescence but does not correlate with the senescence markers examined. This suggests that uPAR is unlikely to be a useful target for senescent lung cell removal.