Abstract
The multifunctional adhesive glycoprotein vitronectin (VN) undergoes a unique conformational transition from the plasma form into a multimeric form that represents the reactive heparin-binding form. In this study we investigated the interaction of multimeric vitronectin (VNmult) or VN-gold conjugates (which are equivalent in biochemical properties) with confluent and subconfluent monolayers of porcine endothelial cells. Time-dependent direct binding of radiolabeled VNmult to the luminal face of endothelial cells at 37 degrees C was observed which was competed by heparin, whereas plasma VN showed hardly any binding. At 4 degrees C binding of VNmult remained cell-associated, whereas after 6 hr at 37 degrees C a major portion of the ligand was translocated through cells and was associated with the subcellular matrix. Cytochemical studies with VN-gold conjugates were performed to demonstrate uptake of VNmult. At 4 degrees C only surface decoration of cells with gold label was seen, which was totally reversible in the presence of heparin. Subsequent incubation for various time intervals at 37 degrees C revealed disappearance of gold label from the surface and accumulation of conjugates in a perinuclear distribution inside the cells as judged both by electron microscopy and after silver enhancement by light microscopy. Cross-sections of endothelial cells demonstrated the inclusion of VN-gold conjugates in coated pits, endosomes, and in lysosomal compartments close to the nucleus. Within 2-6 hr a portion of VN-gold conjugates had accumulated with proteoglycans at the matrix face. These data provide strong evidence for specific routing of a portion of VNmult from the circulation into extravascular spaces, where the protein is believed to fulfill major adhesive and regulatory functions particularly as co-factor in plasminogen activation and immune defense.