Growth differentiation factor 15 activates macrophages and fibroblasts

Abstract

Lung fibrosis is a devastating disease characterized by fibroblast accumulation and extracellular matrix deposition in lungs, and bleomycin-induced lung fibrosis is the most widely used experimental model. We found that mRNA expression of growth differentiation factor 15 (GDF15) was elevated in the lungs of bleomycin-treated mice by using comprehensive gene analysis. The protein levels of GDF15 also increased in lung tissue, brochoalveolar lavage fluid and plasma acquired from bleomycin-treated mice compared with those from saline-treated mice. Although GDF15 is believed to be associated with stress responses, the role of GDF15 in lung fibrosis is still unknown. Histological analysis with senescence-associated β-galactosidase staining and anti-p16INK4a antibody showed cellular senescence of alveolar epithelial cells and macrophages in bleomycin-induced fibrotic lungs. From immunohistochemical staining using anti-GDF15 antibody and increased mRNA expression of GDF15 in bleomycin-induced senescent A549 cells, GDF15 appears to be produced from alveolar epithelial cells undergoing bleomycin-induced cellular senescence. Macrophages are broadly classified as M1 phenotype with inflammatory effects and M2 phenotype with anti-inflammatory effects. GDF15 augmented interleukin-4/interleukin-13-induced mRNA expression of M2 markers including arginase 1 and chitinase-like 3. On the other hand, it is known that myofibroblasts that produce extracellular matrix in lung fibrosis are increased by activation of fibroblasts. GDF15 also increased in protein expression of α-smooth muscle actin (a myofibroblast marker) through ALK5-Smad2/3 pathway in WI-38 lung fibroblasts. These results suggested that GDF15 secreted from senescent alveolar epithelial cells acts as a profibrotic factor through activations of M2 macrophages and fibroblasts. This provides that GDF15 could be an attractive therapeutic target for treatment and a predictor of progression of lung fibrosis.