Synthesis of fibronectin by cultured human endothelial cells.

Abstract

Plasma fibronectin is probably the major nonimmune particulate opsonin in blood and is cross-linked to fibrin during the final stage of blood coagulation. Fibronectin also occurs in an insoluble form in basement membranes especially those underlying endothelial cells and in loose connective tissue. Fibronectin was demonstrated in cultured human endothelial cells and in the surrounding extracellular matrix by immunofluorescence microscopy by using antibody to human plasma fibronectin. Cultured human endothelial cells released fibronectin into the culture medium which was immunologically identical to the fibronectin in human plasma. Cultured human endothelial cells were labeled with [3H] leucine. The radioactive fibronectin in the endothelial postculture medium and in urea extracts of cellular monolayers was isolated with anti-fibronectin coupled to Protein A-Sepharose or double antibody immunoprecipitation and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When reduced, the [3H] fibronectin synthesized by cultured endothelial cells had the same MW (.apprx. 200,000) as plasma fibronectin. Unreduced, the [3H] fibronectin synthesized by endothelial cells migrated as a dimer, as did plasma fibronectin. Fibronectin accounted for .apprx. 15% of the protein synthesized and released by endothelial cells into the culture medium. Cultured endothelial cells synthesized fibronectin, secreted it into the culture medium, and incorporated it into extracellular matrix. The endothelial cell may be a major site of synthesis of circulating plasma fibronectin. Fibronectin derived from endothelial cells may be an important structural component of the subendothelium.