Role of Integrin-α8 in Bleomycin-Induced Lung Injury

Abstract

Integrin-α8 is expressed in lung mesenchymal cells and is increased in several models of fibrosis, including bleomycin-injured murine lungs. However, the role of integrin α8 in lung injury and fibrosis remains unknown. We previously demonstrated that platelet-derived growth factor receptor β (PDGFRβ)+ perivascular mesenchymal cells in the lung constitute a major pool of myofibroblast progenitors. We used α8flox/flox;PDGFRβ-Cre transgenic mice in which integrin α8 is deleted in PDGFRβ+ cells. We administered bleomycin to α8flox/flox;PDGFRβ-Cre mice and α8flox/flox;PDGFRβ-wild type mice (control). We observed significantly increased weight loss starting on Day 8 postinjury and a trend toward increased mortality in α8flox/flox;PDGFRβ-Cre mice compared with controls. On Day 7, we did not see significant differences in bronchoalveolar lavage fluid (BALF) white blood cells, red blood cells, and total protein between the two groups. However, on Day 21, BALF total protein was significantly higher in α8flox/flox;PDGFRβ-Cre mice (2.2 ± 0.4 vs. 0.9 ± 0.1 mg/ml, n ≥ 4). On Day 21, hydroxyproline content was significantly lower in α8flox/flox;PDGFRβ-Cre mice (177 ± 19 vs. 459 ± 39 μg/g, left lung, n ≥ 3). Because we previously showed that α8β1 binds to the latency-associated peptide (LAP) of transforming growth factor (TGF)-β1, we asked whether α8β1 was able to activate latent TGF-β1. NIH 3T3 or mouse lung fibroblasts (MLFs) were cocultured overnight with mink lung epithelial cells stably transfected with a TG-Fβ1-responsive luciferase construct with or without α8β1 blocking antibody. 3T3 cells or MLFs were able to activate TGF-β1, which was blocked by α8β1 antibody. Furthermore, Day 21 lung lysates from α8flox/flox;PDGFRβ-Cre mice had decreased active/total TGF-β1 activity (0.39 ± 0.09) compared with control (0.70 ± 0.2) (P = 0.07). In summary, α8 expression in PDGFRβ+ cells contributes to lung fibrosis, which may be mediated by decreased TGF-β1 activation. Our data also suggest an unsuspected role of α8 in regulating vascular permeability and resolution of lung injury.