Alveolar epithelial cells in idiopathic pulmonary fibrosis display upregulation of TRAIL, DR4 and DR5 expression with simultaneous preferential over-expression of pro-apoptotic marker p53.

Idiopathic pulmonary fibrosis (IPF) is a progressive, debilitating, and fatal lung disease of unknown aetiology with no current cure. The pathogenesis of IPF remains unclear but repeated alveolar epithelial cell (AEC) injuries and subsequent apoptosis are believed to be among the initiating/ongoing triggers. However, the precise mechanism of apoptotic induction is hitherto elusive. In this study, we investigated expression of a panel of pro-apoptotic and cell cycle regulatory proteins in 21 IPF and 19 control lung tissue samples. We reveal significant upregulation of the apoptosis-inducing ligand TRAIL and its cognate receptors DR4 and DR5 in AEC within active lesions of IPF lungs. This upregulation was accompanied by pro-apoptotic protein p53 overexpression. In contrast, myofibroblasts within the fibroblastic foci of IPF lungs exhibited high TRAIL, DR4 and DR5 expression but negligible p53 expression. Similarly, p53 expression was absent or negligible in IPF and control alveolar macrophages and lymphocytes. No significant differences in TRAIL expression were noted in these cell types between IPF and control lungs. However, DR4 and DR5 upregulation was detected in IPF alveolar macrophages and lymphocytes. The marker of cellular senescence p21(WAF1) was upregulated within affected AEC in IPF lungs. Cell cycle regulatory proteins Cyclin D1 and SOCS3 were significantly enhanced in AEC within the remodelled fibrotic areas of IPF lungs but expression was negligible in myofibroblasts. Taken together these findings suggest that, within the remodelled fibrotic areas of IPF, AEC can display markers associated with proliferation, senescence, and apoptotosis, where TRAIL could drive the apoptotic response. Clear understanding of disease processes and identification of therapeutic targets will direct us to develop effective therapies for IPF.

Club cells inhibit alveolar epithelial wound repair via TRAIL-dependent apoptosis.

Club cells (Clara cells) participate in bronchiolar wound repair and regeneration. Located in the bronchioles, they become activated during alveolar injury in idiopathic pulmonary fibrosis (IPF) and migrate into the affected alveoli, a process called alveolar bronchiolisation. The purpose of this migration and the role of club cells in alveolar wound repair is controversial. This study was undertaken to investigate the role of club cells in alveolar epithelial wound repair and pulmonary fibrosis. A direct-contact co-culture in vitro model was used to evaluate the role of club cells (H441 cell line) on alveolar epithelial cell (A549 cell line) and small airway epithelial cell (SAEC) wound repair. Immunohistochemistry was conducted on lung tissue samples from patients with IPF to replicate the in vitro findings ex vivo. Our study demonstrated that club cells induce apoptosis in alveolar epithelial cells and SAECs through a tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-dependent mechanism resulting in significant inhibition of wound repair. Furthermore, in IPF lungs, TRAIL-expressing club cells were detected within the affected alveolar epithelia in areas of established fibrosis, together with widespread alveolar epithelial cell apoptosis. From these findings, we hypothesise that the extensive pro-fibrotic remodelling associated with IPF could be driven by TRAIL-expressing club cells inducing apoptosis in alveolar epithelial cells through a TRAIL-dependent mechanism.

Idiopathic pulmonary fibrosis: immunohistochemical analysis provides fresh insights into lung tissue remodelling with implications for novel prognostic markers.

AIM: This study explored the cellular and biological interrelationships involved in Idiopathic Pulmonary Fibrosis (IPF) lung tissue remodelling using immunohistochemical analysis. METHODS AND RESULTS: IPF and control lung tissues were examined for localisation of Epithelial Mesenchymal Transition (EMT), proliferation and growth factor markers assessing their relationship to key histological aberrations. E-cadherin was expressed in IPF and control (Alveolar type II) ATII cells (>75%). In IPF, mean expression of N-cadherin was scanty (<10%): however 4 cases demonstrated augmented expression in ATII cells correlating to histological disease status (Pearson correlation score 0.557). Twist was expressed within fibroblastic foci but not in ATII cells. Transforming Growth Factor- ß (TGF-ß) protein expression was significantly increased in IPF ATII cells with variable expression within fibroblastic foci. Antigen Ki-67 was observed within hyperplastic ATII cells but not in cells overlying foci. Collagen I and α-smooth muscle actin (α-SMA) were strongly expressed within fibroblastic foci (>75%); cytoplasmic collagen I in ATII cells was present in 3 IPF cases. IPF ATII cells demonstrated variable Surfactant Protein-C (SP-C). CONCLUSIONS: The pathogenesis of IPF is complex and involves multiple factors, possibly including EMT. Histological analysis suggests TGF-ß-stimulated myofib rob lasts initiate a contractile response within established fibroblastic foci while proliferating ATII cells attempt to instigate alveolar epithelium repair. Marker expression (N-cadherin and Ki-67) correlation with histological disease activity (as reflected by fibroblastic foci extent) may emerge as future prognostic indicators for IPF.