Detection and possible role of proteasomes in the bronchoalveolar space of the injured lung

Recent observations suggest the presence of 20S proteasomes(20S) in the lung epithelial lining fluid. However, the physiologicalrelevance of 20S in the alveolar space and possible contributionto disease processes are unknown. Thus, we evaluated whetherextracellular proteasomes could have a pathophysiological role inthe injured lung using a rat model of lung contusion (LC).Bronchoalveolar lavage fluids (BALF) were obtained at varioustime points for up to 168 h after LC or sham procedure. Enzymeactivities, ELISA and Western blots indicated enzymatically active20S, the 19S subunit Rpt5 and ubiquitin in BALF. 20S andubiquitin increased significantly after LC, peaked at 24 h andnormalized within 168 h. Mg2+/ATP-dependent peptidaseactivities were detectable 6-24 h after LC. BALF after LC alsocontained ubiquitin-protein-ligase activity. Addition of Mg2+/ATPto BALF after LC led to significant proteolysis and could beprevented with epoxomicin and EDTA. These data suggest forthe first time that the Mg2+/ATP-dependent 26S proteasomecomplex exists outside the cell, is released into the lung epitheliallining fluid after LC and contributes to the proteolysis of the bulkof protein in the alveolar space of the injured lung. We infer thatproteasome complexes may have a pathophysiological roleduring lung edema clearance.