Modulation of MUC7 mucin expression by exogenous factors in airway cells in vitro and in vivo

The human MUC7 gene encodes a low-molecular-mass mucin that participates in the maintenance of healthy epithelium in the oral cavity, and possibly in respiratory tracts, by promoting the clearance of various bacteria. We examined whether MUC7 gene is expressed in primary normal human tracheobronchial epithelial cells and whether the expression is modulated by exogenous factors. By assessing MUC7 transcripts, we found that the MUC7 gene was induced by culturing the normal human tracheobronchial epithelial cells at the air-liquid interface, in which the cells were well differentiated. When the cells were treated with a panel of cytolkines (IL-1 beta, IL-4, IL-13, and TNF-alpha), epidermal growth factor, or a bacterial product (Pseudomonas aeruginosa lipopolysaccharide [LPS]), MUC7 transcripts and glycoprotein products were increased 1.7- to 3.2-fold. The effect of LIPS on MUC7 gene expression was also studied in the airway tissues of MUC7 gene transgenic mice. In the in vitro cultured trachea and lung explants, the LIPS-treated tissues showed over 2-fold increased levels of MUC7 mRNA compared with the untreated specimens. These results were confirmed by in vivo studies using the lungs and tracheas harvested from the transgenic mice irritated by LPS through the tracheal instillation. By immunohistochemistry, MUC7 glycoprotein was localized in tracheal submucosa within the serous cells. Upon LIPS stimulation, the overexpressed MUC7 remains confined to the serous glands. In the lungs, MUC7 seems to be expressed within the respiratory epithelium at the level of the bronchioles. Upon stimulation with LIPS, it seems to be overexpressed within the same cells and within the stromal tissue.