The Pseudomonas aeruginosa Secretory Product Pyocyanin Inactivates α(1) Protease Inhibitor: Implications for the Pathogenesis of Cystic Fibrosis Lung Disease

α(1) Protease inhibitor (α(1)PI) modulates serine protease activity in the lung. Reactive oxygen species inactivate α(1)PI, and this process has been implicated in the pathogenesis of a variety of forms of lung injury. An imbalance of protease-antiprotease activity is also detected in the airways of patients with cystic fibrosis-associated lung disease who are infected with Pseudomonas aeruginosa. P. aeruginosa secretes pyocyanin, which, through its ability to redox cycle, induces cells to generate reactive oxygen species. We tested the hypothesis that redox cycling of pyocyanin could lead to inactivation of α(1)PI. When α(1)PI was exposed to NADH and pyocyanin, a combination that results in superoxide production, α(1)PI lost its ability to form an inhibitory complex with both porcine pancreatic elastase (PPE) and trypsin. Similarly, addition of pyocyanin to cultures of human airway epithelial cells to which α(1)PI was also added resulted in a loss of the ability of α(1)PI to form a complex with PPE or trypsin. Neither superoxide dismutase, catalase, nor dimethylthiourea nor depletion of the media of O(2) to prevent formation of reactive oxygen species blocked pyocyanin-mediated inactivation of α(1)PI. These data raise the possibility that a direct interaction between reduced pyocyanin and α(1)PI is involved in the process. Consistent with this possibility, pretreatment of α(1)PI with the reducing agent β-mercaptoethanol also inhibited binding of trypsin to α(1)PI. These data suggest that pyocyanin could contribute to lung injury in the P. aeruginosa-infected airway of cystic fibrosis patients by decreasing the ability of α(1)PI to control the local activity of serine proteases.