Neutrophil-derived elastase induces TGF-β1 secretion in human airway smooth muscle via NF-κB pathway

Neutrophils are infiltrated in airways of individuals with more severe and chronic asthma, with uncertain significance. Airway smooth muscle (ASM), apart from its contractile properties, is critically involved in the pathogenesis of asthma by producing inflammatory mediators. In the present study, we investigated the impact of neutrophil-derived elastase (NE) on ASM in terms of TGF-β1 release, and we explored the underlying mechanisms. Primary ASM cells were serum starved for 24 h before stimulation with NE (0.01-0.5 μg/ml). TGF-β1 in supernatant was determined by ELISA and mRNA quantified by real-time RT-QPCR. NF-κB nuclear translocation and activation was examined by Western blotting and κB-2 dEGFP reporter gene assay. Association of IL-1 receptor-associated kinase (IRAK) with MyD88 was studied by co-immunoprecipitation and Toll-like receptor 4 (TLR4) determined by FACS scan and Western blotting. We demonstrated that NE enhanced TGF-β1 release in a time-dependent manner. This induction was inhibited by actinomycin D (5 mM), cycloheximide (5 mM), and NF-κB inhibitors, including pyrrolidine dithiocarbamate (PDTC, 1 mM), aspirin (2.5 mM), and sodium salyicylate (2.5 mM). Stimulation with NE was rapidly followed by association of IRAK with MyD88, phosphorylation of IκBα, and nuclear translocation of p65 with increased transactivation activity. We also found that TLR4 levels were reduced upon NE treatment. These data suggest that NE upregulates TGF-β1 gene expression and release via My88/IRAK/NF-κB pathway, possibly through activation of TLR4, and shed light on a potential role of neutrophils in the pathogenesis of asthma.