Expression of aminopeptidase N in fetal rat lung during development.

The ectopeptidase, aminopeptidase N, serves as a cell surface marker of the apical surface of the alveolar type II epithelial cell in adult lung. It is also present in fetal lung before differentiation of morphologically mature type II alveolar epithelial cells, suggesting that it is expressed by precursors of the type II cells. We have examined the mRNA coding for the aminopeptidase in adult and fetal lung and in mature type II cells and determined levels of mRNA and immunoreactive protein during fetal lung development. Comparison of the temporal patterns of steady-state levels of aminopeptidase mRNA and immunoreactive protein during development show that the expression of the protein is developmentally regulated and that expression is regulated, at least in part, at a pretranslational level. Both mRNA and immunoreactive protein levels increase severalfold on the final gestational day, suggesting that the function of the aminopeptidase may be associated with air breathing.