REGULATION OF THE SECRETORY PHENOTYPE OF HUMAN AIRWAY EPITHELIUM BY RETINOIC ACID, TRIIODOTHYRONINE, AND EXTRACELLULAR-MATRIX

The purpose of our studies was to identify factors which regulate the composition of airway secretions produced by normal human tracheobronc hial epithelial (NHTBE) cells. Individual factors were removed from th e culture media of NHTBE cells grown in air-liquid interface (ALI) cul tures (which support mucociliary differentiation) and the effects on m ucin. lysozyme (LZ), and secretory leukocyte protease inhibitor (SLPI) secretion and gene expression were examined. Deletion of hydrocortiso ne, epinephrine, transferrin, or gentamycin-amphotericin from the medi a had no reproducible effects, deletion of insulin was incompatible wi th culture growth. We identified 3 factors, namely retinoic acid (RA) triiodothyronine (T3) and collagen gel substratum, which had a major i mpact on the profile of NHTBE secretions. Removal of Ra from the media caused a drastic decrease in mucin secretion and a decrease in expres sion of the mucin genes MUC2 and MUC5AC. LZ and SLPI secretions were i ncreased in these cultures. Paradoxically LZ mRNA was decreased, while SLPI mRNA levels were increased. Removal of T3 selectively increased mucin secretion, MUC2 gene expression was not affected, but MUC5AC mRN A levels reproducibly increased, suggesting that the expression of the se two mucin genes is differentially regulated. LZ and SLPI secretion levels were not significantly affected by deletion of T3 from the cult ure media: however, LZ mRNA levels were increased in the absence of T3 while SLPI transcript levels were not affected. Omission of the attac hment substratum, type I collagen gel, resulted in significant increas es in all 3 secretory Products. MUC2 and MUC5AC steady state mRNA leve ls were not consistently affected. In contrast LZ and SLPI gene expres sion were reproducibly increased. Our studies show that individual fac tors in the epithelial environment can regulate expression of specific secretory cell gene products in a highly selective manner.