Granular pneumocytes in primary culture secrete several major components of the extracellular matrix

Primary cultures of rat alveolar type II epithelial cells (granular pneumocytes) produced several components of the pulmonary extracellular matrix. Fractionation by ion-exchange chromatography of radiolabeled protein secreted into the culture medium resulted in the partial purification of two of these components: fibronectin and type IV procollagen. Identification of these proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was confirmed by radioimmune-precipitation studies with affinity-purified antibodies. Thrombospondin, a platelet alpha-granule protein that was recently shown to be secreted by endothelial and other mesenchymally derived cells and may be involved in platelet aggregation, was, in addition, purified by elution from diethylaminoethylcellulose with 0.5 M NaCl. The levels of these secreted proteins were measured by radioimmune precipitation. Of the total radiolabeled culture medium protein secreted during a 24-h period by the granular pneumocytes, fibronectin, type IV procollagen, and thrombospondin represented 3-15%, 2%, and 3%, respectively. The biosynthesis, by alveolar epithelial cells, of proteins that constitute or are closely associated with the alveolar basement membrane implies that this structure is at least partially derived from the cells themselves. Furthermore, it suggests that the type II epithelial cell is involved in pulmonary cytodifferentiation, in lung morphogenesis and repair, and in certain interstitial lung disorders in which derangement of the extracellular matrix occurs.