Effects of retinoic acid, triiodothyronine and hydrocortisone on mucin and lysozyme expression in cultured human middle ear epithelial cells

Mucous hypersecretion is a major complication of otitis media and can prolong the disease course and increase morbidity. Mucin, a major component of mucus, is a macromolecular complex of glycoprotein and makes mucus viscous. Lysozyme is a secretory element of the middle ear mucosa, which has a non-specific and innate antibacterial function. We attempted to identify factors that regulate these secretory products and their morphological phenotype using cultured human middle ear epithelial cells. Cellular differentiation was induced by creating an air-liquid interface on culture day 9 in serum-free conditioned media. Omission of retinoic acid (RA) caused decrease in the secretion of mucin and lysozyme, and in the cellular expression of MUC 2, MUC 5AC and MUC 5B mRNA. In contrast, removal of triiodothyronine (T3) caused an increase in the secretion of mucin and the level of MUC5AC mRNA. When hydrocortisone (HC) was removed from the media, the secretion of mucin was decreased without an apparent change of message level. The expression of MUC 1 mRNA was not changed by the respective deficiency of RA, T3 or HC. The effect of T3 or HC on lysozyme was not significant. This study shows that RA, T3 and HC influence the morphological phenotype and the secretory function of mucin and lysozyme in cultured human middle ear epithelial cells. This culture system can serve as an in vitro model for study of the regulation of various cellular secretions in human middle ear epithelium.