Increased expression of senescence markers in cystic fibrosis airways

Cystic Fibrosis (CF) is a chronic lung disease characterized by chronic neutrophilic airway inflammation and increased levels of neutrophil elastase (NE) in the airways. We have previously reported that NE treatment triggers cell cycle arrest. Cell cycle arrest can lead to senescence, a complete loss of replicative capacity. Importantly, senescent cells can be proinflammatory and would perpetuate CF chronic inflammation. By immunohistochemistry, we evaluated whether airway sections from CF and control subjects expressed markers of senescence, including p16(INK4a) (p16), a cyclin-dependent kinase inhibitor, phospho-Histone H2A.X (γH2A.X), and phospho-checkpoint 2 kinase (phospho-Chk2), which are also DNA damage response markers. Compared with airway epithelium from control subjects, CF airway epithelium had increased levels of expression of all three senescence markers. We hypothesized that the high load of NE in the CF airway triggers epithelial senescence by upregulating expression of p16, which inhibits cyclin-dependent kinase 4 (CDK4). Normal human bronchial epithelial (NHBE) cells, cultured in air-liquid interface were treated with NE (0, 200, and 500 nM) to induce visible injury. Total cell lysates were collected and evaluated by Western analysis for p16 protein expression and CDK4 kinase activity. NE significantly increased p16 expression and decreased CDK4 kinase activity in NHBE cells. These results support the concept that NE triggers expression of senescence markers in CF airway epithelial cells.