Am. J. Respir. Cell Mol. Biol.,
Volume 20, Number 4, April, 1999 573-581
Lysozyme Expression during Metaplastic Squamous Differentiation of
Retinoic Acid-Deficient Human Tracheobronchial Epithelial Cells
Joo-Heon
Yoon,
Ja Seok
Koo,
Derek
Norford,
Karen
Guzman,
Thomas
Gray,
and
Paul
Nettesheim
Department of Otorhinolaryngology, Yonsei University College of Medicine, Seoul, Korea; and Laboratory of Pulmonary
Pathobiology, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina
We previously reported (Gray, T. E., K. Guzman, C. W. Davis, L. H. Abdullah, and P. Nettesheim. 1996. Mucociliary differentiation of serially passaged normal human tracheobronchial epithelial cells. Am. J. Respir. Cell Mol. Biol. 14:104-112) that retinoic acid (RA)-deprived cultures of normal human tracheobronchial epithelial (NHTBE) cells became squamous, failed to produce mucin, and instead secreted or released large amounts of lysozyme (LZ). The purpose of the studies reported here was to elucidate the relationship between RA deficiency-induced squamous differentiation and increased LZ, and to determine
what mechanisms were involved. We found that intracellular LZ began to accumulate in RA-deficient
NHTBE cultures early during squamous differentiation. Between Days 10 and 18 of culture, cellular LZ
levels were more than 10 times higher in RA-deficient than in RA-sufficient cultures. On Day 12, large numbers of cells began to exfoliate in RA-deficient cultures and extracellular LZ appeared at the apical
surface, presumably released from the exfoliated cells. Metabolic labeling studies showed that the rate of
LZ synthesis was not increased in RA-deficient cultures over that in RA-sufficient cultures; however, intracellular LZ half-life was much longer in RA-deficient cultures. We concluded that the increased accumulation of both intra- and extracellular LZ in RA-deficient cultures was due to increased LZ stability and
was not the result of increased LZ synthesis. When RA-deficient cultures were treated on Day 7 with
10
6 M RA, intracellular LZ levels did not substantially decrease until 3 d later, coinciding with a marked
increase in mucin secretion. LZ messenger RNA levels were unchanged at 24 h, but were modestly increased (rather than decreased) at all subsequent time points. We concluded that RA does not directly regulate LZ, and that the excessive accumulation of LZ in RA-deprived NHTBE cells is a consequence of
vitamin A deficiency-induced abnormal differentiation.
