ASSEMBLY OF EXOGENOUS FIBRONECTIN INTO TYPE-II CELL-EXTRACELLULAR MATRIX

Type II pulmonary epithelial cells (T2P) in primary culture assemble a biologically active extracellular matrix (ECM) from endogenously synt hesized components, including fibronectin. Fibronectin is a well-recog nized attachment protein that mediates cell adhesion, migration, and c ytodifferentiation. In some cell types, exogenous fibronectin also is incorporated into ECM. The latter pathway of ECM assembly was thus inv estigated in T2P. Cells were cultured for 3 days in Dulbecco's modifie d Eagle's medium (DMEM) with or without 10% fetal calf serum (FCS), a source of exogenous fibronectin. Cell and matrix fractions were harves ted on culture days 1, 2, and 3 to determine synthesis of cell and mat rix proteins and matrix fibronectin content. During 3 days in DMEM con taining 10% FCS, T2P flattened and spread to confluence more rapidly t han cells in DMEM; they also produced ECM with higher fibronectin cont ent than did cells in DMEM alone. On culture days 2 and 3, 10% FCS dou bled (on average) synthesis of ECM fibronectin; in contrast, ECM fibro nectin content increased nearly 10-fold. These observations suggest th at cultured type II cells incorporate exogenous fibronectin into newly assembled ECM to a greater extent than the newly synthesized glycopro tein. Components of both endogenous and exogenous origin may therefore contribute to T2P assembly of a biologically active ECM.