VARIABLE EXPRESSION OF PLATELET-DERIVED GROWTH-FACTOR FAMILY PROTEINS IN ACUTE LUNG INJURY

During acute lung injury, there is an outpouring of growth factors into the alveolar space that drive local repair and fibrosis. During the remodeling that follows the instillation of bleomycin via the trachea into the adult rat, at least two platelet-derived growth factor (PDGF)-like peptides are released sequentially into lung lining fluid. Groups of four to five animals were killed at 3, 6, 15, and 26 days after exposure to bleomycin and lungs lavaged with isotonic saline. PDGF-like peptides in epithelial lining fluid (ELF) were partially purified by cation exchange chromatography and concentrated. Isolated peptides were analyzed by immunoblotting to determine their molecular weight and immunologic identity. Western blots were probed with polyclonal antibodies to PDGF-BB and PDGF-AA. PDGF-Iike peptides of two distinct size classes (38-40 kD and 29 kD) were present in alveolar fluid from all rats with lung injury induced by bleomycin. No PDGF-like peptides were found in comparably prepared ELF from control animals. The 38-40 kD peptide was detected only with anti-PDGF-BB antibody; the 29 kD peptide was detected only with anti-PDGF-AA antibody. The presence of these two peptides varied independently with time after exposure to bleomycin. The 38-40 kD peptide was at peak levels at 3 to 6 days. In contrast, the 29 kD peptide was present at all times following injury but with far less variation over time. In parallel with these immunoassays for PDGF-like molecules, there was abundant growth-promoting activity for fibroblasts present in concentrated ELF during the course of injury. Bioassays of processed lavage fluid were used to determine the mitogenic potency of ELF for lung fibroblasts. The amount of growth-promoting activity paralleled over time the amount of the 38-40 kD peptide detected on immunoblots. At day 3, the mitogenic activity of chromatographically purified PDGF-like peptides was equivalent in potency to 12.1 +/- 1.6 ng/ml of authentic PDGF. Anti-PDGF antibodies blocked the activity of ELF: 64% of the activity was inhibited by anti-PDGF-BB antibodies; 15% of the activity was inhibited by antiPDGF-AA antibodies. The presence of several PDGF-like peptides in ELF during the evolution of tissue repair in this model supports the notion that these important mitogens for mesenchymal cells are central to the remodeling process in lung.