Effect of Retinoic Acid on the Differentiation and Lysozyme Expression in Human Airway Epithelial Cells

BACKGROUND AND OBJECTIVES: Retinoic acid (RA)-deprived cultures of normal human tracheobronchial epithelial (NHTBE) cells became squamous metaplastic, failed to produce mucin and instead secreted or released large amounts of lysozyme (LZ). The purpose of this study was to elucidate the relationship between RA-deficiency induced squamous metaplasia and increased LZ as a function of time. MATERIALS AND METHOD: The change of lysozyme protein and lysozyme mRNA was investigated over time in cultures using passage-2 normal human tracheobronchial epithelial (NHTBE) cells and passage-2 normal human keratinocytes (NHK). The amount of lysozyme and mucin was measured with dot blot, message of lysozyme with RT-PCR, and cornifin mRNA with Northern blot. RESULTS: Lysozyme message levels were consistently higher in RA-sufficient than RA-deficient cultures. Intracellular and extracellular LZ increased to a peak on the day 16 and thereafter decreased in the RA-deficient cultures. LZ gene expression in the RA-deficient cultures was barely detectable on the day 7 but was clearly expressed between days 10 and 14, but thereafter message levels decreased markedly. On day 12, large numbers of cells began to exfoliate in the RA-deficient cultures. Extracellular LZ appeared simultaneously at the apical surface, presumably released from the exfoliated cells, which contained high concentrations of LZ. Intracellular LZ levels were more than 11 fold less in NHK cells compared to NHTBE cells. CONCLUSION: This study suggests that cellular accumulation of lysozyme protein is a unique feature of metaplastic squamous differentiation. Further studies are needed to find out what mechanisms are involved.ope