Elevated expression of gel-forming MUC5AC and -5B genes is a major pathological feature of various airway diseases. Normally, MUC5B is expressed predominantly by submucosal gland mucous cells. However, trans-differentiation of MUC5B expression by surface airway epithelial cells occurs in various lung-diseased tissues. The nature of this phenomenon is not known. Phorbol 12-myristate 13-acetate (PMA) was found to be a potent stimulator of MUC5B gene expression under air–liquid interface conditions in three airway epithelial cell systems: primary cultures of normal human bronchial epithelial cells; an immortalized normal human bronchial epithelial cell line, HBE1; and a human lung adenocarcinoma cell line, A549. Stimulation was time- and dose-dependent, could be demonstrated by promoter–reporter gene transfection, and was sensitive to mithramycin A, suggesting the involvement of a specificity protein 1-based transcriptional mechanism in the stimulation. Both PMA-induced MUC5B message and promoter–reporter ge...