Chronic WNT/β-catenin signaling induces cellular senescence in lung epithelial cells

The rapid expansion of the elderly population has led to the recent epidemic of age-related diseases, including increased incidence and mortality of chronic lung diseases, such as Idiopathic Pulmonary Fibrosis (IPF). Cellular senescence is a major hallmark of aging and has a higher occurrence in IPF. The lung epithelium represents a major site of tissue injury, cellular senescence and aberrant activity of developmental pathways such as the WNT/beta-catenin pathway in IPF. The potential impact of WNT/beta-catenin signaling on alveolar epithelial senescence in general as well as in IPF, however, remains elusive. Here, we characterized alveolar epithelial cells of aged mice and assessed the contribution of chronic WNT/beta-catenin signaling on alveolar epithelial type (AT) II cell senescence. Whole lungs from old (16-24 months) versus young (3 months) mice had relatively less epithelial (EpCAM(+)) but more inflammatory (CD45(+)) cells, as assessed by flow cytometry. Compared to young ATII cells, old ATII cells showed decreased expression of the ATII cell marker Surfactant Protein C along with increased expression of the ATI cell marker Hopx, accompanied by increased WNT/beta-catenin activity. Notably, when placed in an organoid assay, old ATII cells exhibited decreased progenitor cell potential. Chronic canonical WNT/beta-catenin activation for up to 7 days in primary ATII cells as well as alveolar epithelial cell lines induced a robust cellular senescence, whereas the non-canonical ligand WNT5A was not able to induce cellular senescence. Moreover, chronic WNT3A treatment of precision-cut lung slices (PCLS) further confirmed ATII cell senescence. Simultaneously, chronic but not acute WNT/beta-catenin activation induced a profibrotic state with increased expression of the impaired ATII cell marker Keratin 8. These results suggest that chronic WNT/beta-catenin activity in the IPF lung contributes to increased ATII cell senescence and reprogramming. In the fibrotic environment, WNT/beta-catenin signaling thus might lead to further progenitor cell dysfunction and impaired lung repair.