Isolation and partial characterization of endothelial cell extracellular complexes

Human endothelial cells release components into the growth medium that stimulate cell‐substratum adhesion. Several macromolecular components were isolated by ultracentrifugation of the endothelial cell conditioned medium. The components were heterogeneous, consisting of several sizes when examined by sedimentation velocity and gel filtration. When the extracellular components were evaluated by electron microscopy, structurally discrete particles were observed. The extracellular components and the complexes mediated cell‐substratum adhesion to both human umbilical and arterial endothelial cells. The majority of the extracellular components that promote endothelial cell adhesion were pelleted by ultracentrifugation. Although the complexes contained fibronectin, antibodies to fibronectin did not inhibit cell adhesion to the complexes. Significant inhibition of endothelial cell adhesion was observed in the presence of heparin and heparan sulfate. The supernatant fraction following ultracentrifugation of the growth medium contained a component that suppressed endothelial cell adhesion to culture dishes coated with fibronectin, type I collagen, and endothelial cell complexes. SDS‐polyacrylamide gel electrophoresis indicated that the complexes contained several components, and the majority of the large‐molecular‐weight components were pelleted by ultracentrifugation. The conditioned medium from human endothelial cells contains specific complexes that promote cell‐substratum adhesion and components that suppress cell‐substratum adhesion.