Alveolar type II cell growth on a pulmonary endothelial extracellular matrix

Most of the alveolar epithelium overlies a fused basement membrane produced by epithelial and endothelial cells. To determine how this type of matrix influences type II cell growth and function, we studied the effects of culturing isolated rat alveolar type II cells on an extracellular matrix (ECM) freshly produced by pulmonary vascular endothelial cells grown 5 days in culture. Type II cells from the same rats were cultured on plastic or Matrigel for comparison. A large increase in mitotic activity was seen in type II cells grown on the endothelial ECM at 2 days only; thereafter cells spread rapidly to confluence and lost their lamellar bodies. Cells grown on Matrigel remained cuboidal with lamellar bodies but grew more slowly, as judged by [3H]thymidine uptake and cell numbers. Incorporation of labeled choline into disaturated phosphatidylcholine (DSPC) was used as a marker of surfactant synthesis. After the rapid, brief burst of proliferation, type II cells on endothelial ECM showed a sudden decline in DSPC-DNA by day 4 compared with cells grown on matrigel. Binding of the lectin Bauhinia purpurea (BPA) indicated that after a phase of division, cells on endothelial ECM developed as type I epithelium by 4 days of culture, when > 70% of cells stained positively for BPA binding, whereas few cuboidal cells on Matrigel were stained. The results indicate that type II cells respond briefly to growth factors in pulmonary endothelial ECM; then this type of matrix promotes cell spreading with loss of type II function as cells subsequently resemble type I epithelium.