Dexamethasone silence MUC5AC by mediating deacetylation of its proximal promoter in airway epithelial cells

Abstract

MUC5AC, a major mucin of airway epithelium, is exaggeratedly produced in response to various stimuli, including EGF, and is a key component for mucus plugging in obstructive airway diseases, such as asthma. Dexamethasone (Dex), a synthetic glucocorticoid, is widely used in the treatment of asthma. Dex exerts its effect by binding the glucocorticoid receptor (GR), a ligand-activated transcription factor which controls target genes by binding glucocorticoid responsive elements (GREs). The MUC5AC gene promoter has multiple GREs to some of which GR binding is reportedly prerequisite for Dex-mediated repression of MUC5AC transcription in airway epithelial cells. It is conceivable that there might be an EGFR signal that influences activation of GR in the pathway for Dex-mediated repression of EGF-induced MUC5AC and that Dex-activated GR somehow links to repression of MUC5AC, which are the specific aims of this study. As expected, Dex abrogated MUC5AC expression that was highly induced in NCI-H292 airway epithelial cells following EGF treatment for 24 h. Dex did not alter the patterns of EGFR or ERK phosphorylation that predominantly occurred within 1 h following EGF treatment, suggesting that its repressive function is not exerted at the early phase of EGF stimulation. Interestingly, the Dex-mediated repression of MUC5AC did not require a new protein synthesis. Kinetic experiments, in which Dex was added to or removed from the cultures at different time points following EGF stimulation, demonstrated that Dex’s repressive effect came into play preferentially at the late phase of EGF stimulation. Chromatin immunoprecipitation analysis revealed that histone H3K27 acetylation increased in the MUC5AC promoter at the early phase in the presence of EGF and Dex, but the acetylation levels were reduced to baseline at the late phase, which precisely corresponded to kinetics of GR nuclear translocation. Collectively, the timing of GR nuclear translocation and the entailing reorganization of chromatin play a crucial role for Dex- mediated repression of MUC5AC. Running Title: Silencing of MUC5AC by dexamethasone