Neutrophil Elastase Induces MUC5AC Mucin Production in Human Airway Epithelial Cells via a Cascade Involving Protein Kinase C, Reactive Oxygen Species, and TNF-α-Converting Enzyme

Mucus hypersecretion is a prominent manifestation in patients with chronic inflammatory airway diseases and contributes to their morbidity and mortality by plugging airways and causing recurrent infections. Human neutrophil elastase (HNE) exists in high concentrations (1-20 microM) in airway secretions of these patients and induces overproduction of MUC5AC mucin, a major component of airway mucus. Previous studies showed that HNE induces MUC5AC mucin production involving reactive oxygen species (ROS) generation and TGF-alpha-dependent epidermal growth factor receptor (EGFR) activation in human airway epithelial cells. However, the molecular mechanisms involved in these responses are not defined. TNF-alpha-converting enzyme (TACE) cleaves pro-TGF-alpha into soluble TGF-alpha and can be activated by ROS. We hypothesize that HNE activates TACE via ROS generation, resulting in cleavage of pro-TGF-alpha, EGFR activation, and MUC5AC mucin expression in airway epithelial cells. Here we show that in human airway epithelial cells HNE increases TGF-alpha release, EGFR phosphorylation, and MUC5AC mucin expression, effects that were attenuated by TACE inhibitor TAPI-1 and by specific knockdown of TACE expression with small interfering RNA, implicating TACE in HNE-induced responses. These responses to HNE were also reduced by pretreatment with ROS scavengers, implicating ROS. Furthermore, we show that HNE causes protein kinase C (PKC) activation and translocation from cytosol to plasma membrane; blockade of this effect by PKC inhibitors reduced HNE-induced ROS generation and other responses, implicating PKC. We conclude that HNE induces MUC5AC mucin expression via a cascade involving PKC-ROS-TACE in human airway epithelial cells.