Regulation of cytokeratin expression in rat lung alveolar epithelial cells in vitro

Type II pneumocytes are the stem cells for the alveolar epithelium, proliferating and differentiating into type I cells during lung growth or after injury. The pattern of cytokeratin expression by type II cells in vitro has been linked to the state of differentiation of these cells. In particular, cytokeratin 19 expression has been associated with the type II cell phenotype. We now examine the roles of cell shape and cell-cell interactions in the regulation of cytokeratin expression by rat type II cells in vitro. Type II cell spreading and intercellular contacts were modulated by seeding cultures at high or low density (3.5 or 0.5 x 10(5) cells/cm2). When cultured at high density, cells demonstrated increased cytokeratin 19 protein synthesis and diminished cytokeratin 18 protein synthesis compared with highly spread cells at low density. This effect was a reflection of changes in the abundance of mRNAs for the individual cytokeratins. Alveolar epithelial cells at high density also formed extensive desmosomes between the cells. Desmosome formation was significantly decreased when cells were seeded at low density or in reduced (0.05 mM) calcium medium. Cytokeratin 19 mRNA and protein expression were also significantly decreased when desmosome formation was inhibited in reduced calcium medium, while calcium content of the medium had little effect on cytokeratin 18. These studies suggest that type II cell expression of cytokeratin 19, a differentiation-related cytokeratin, is regulated by factors influencing cell shape and intercellular contacts between epithelial cells. They further suggest that cell-cell interactions between epithelial cells may play a role in the modulation of epithelial cell phenotype.