Up-regulation of trypsin and mesenchymal MMP-8 during development of hyperoxic lung injury in the rat

Acute lung injury is marked by damage to alveolar-capillary barrier. High pulmonary levels of matrix-degrading serine proteinase trypsin and matrix metalloproteinases (MMP)-2, -8, and -9 have been shown in preterm infants with respiratory distress syndrome (RDS). We studied expression of trypsin and MMP-2, -8, and -9 in rats exposed to >95% oxygen for 24, 48, or 60 h. As demonstrated by zymography and Western immunoblotting, levels of trypsin and MMP-2, -8, and -9 in bronchoalveolar lavage fluid (BALF) sharply increased after 48 h of hyperoxia relative to normoxia controls. This coincided with increase in alveolar-capillary permeability, as indicated by increased protein concentration in BALF. Both neutrophil-derived 80-kD and mesenchymal cell-derived 60-kD MMP-8 isoforms were detected in BALF. Of them, mesenchymal-type MMP-8 predominated. In immunohistochemistry, alveolar epithelium showed strong trypsin expression at 48 and 60 h of hyperoxia, whereas it was predominantly negative in controls. MMP-8 was mostly expressed in macrophages. Marked up-regulation of trypsin and MMP-8 early during hyperoxic lung injury suggests that these enzymes play a role in the pathogenesis of acute lung injury and may therefore be potential targets for therapy of lung injury.