Distinctive signaling pathways in the induction of airway MUC5B and -AC expression by phorbol 12-myristate 13-acetate and their implication in lung diseases

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 gene activity were specifically sensitive to inhibition of protein kinase C (PKC)-δ, which was further confirmed by the forced expression of the dominant negative mutant of PKC-δ. With regards to downstream transduction, PMA-induced MUC5B expression was sensitive to inhibitors and dominant negative expression of signaling molecules involved in the Ras/mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase-1-mediated c-Jun N-terminal kinase and p38 pathways. This is in contrast to the inhibition of PMA-induced MUC5AC expression by inhibitors of the Ras/epidermal growth factor receptor (EGFR)/extracellular regulated kinase (ERK) signaling pathway. These results demonstrated for the first time that PMA-stimulated expression of MUC5AC and -5B is regulated through distinctive EGFR/ERK-dependent and -independent signaling pathways.