Human Neutrophil Elastase Induces Hypersecretion of Mucin from Well-Differentiated Human Bronchial Epithelial Cells in Vitro via a Protein Kinase Cδ-Mediated Mechanism

The presence of mucus obstruction and neutrophil-predominant inflammation in several lung disorders, such as cystic fibrosis, suggests a relationship between neutrophils and excess mucus production. Mechanisms of human neutrophil elastase (HNE)-induced mucin secretion by well-differentiated normal human bronchial epithelial (NHBE) cells maintained in air/liquid interface culture were investigated. HNE increased mucin secretion in a concentration-dependent manner, with maximal stimulation (more than twofold) occurring within a short (15 minutes) time period. Mucins MUC 5 AC and MUC 5 B, but not MUC 2, were released in response to HNE. Stimulation of mucin secretion required partial elastase enzymatic activity and did not appear to involve a soluble product released by the cells. HNE-stimulated secretion involved activation of protein kinase C (PKC), as HNE exposure rapidly provoked PKC enzymatic activity that was attenuated by the general PKC inhibitors calphostin C and bisindoylmaleimide I. Of the different isoforms, PKCalpha, delta, zeta, lambda, iota, and epsilon were constitutively expressed in NHBE cells while PKCbeta, eta, and mu were PMA-inducible. PKCdelta was the only isoform to translocate from cytoplasm to membrane in response to HNE. Inhibition of PKCdelta attenuated HNE-mediated mucin secretion. The results suggest HNE stimulation of mucin release by human airway epithelial cells involves intracellular activation of PKC, specifically the delta isoform.