Considering that vitronectin (VN) can promote both cell adhesion and matrix degradation, it is likely to play a dual role at the cell-biomaterial interface. In this paper we therefore describe details of the dynamic interplay between matrix adhesion and pericellular proteolysis in endothelial cells adhered to glass model substratum. Initially we show that coating concentration determines protein organization at the surface. When the protein coating density approached saturation (63 ng cm-2), VN spontaneously organized itself in multimeric aggregates at the surface (30-50 nm in diameter). At subsaturation protein density (17 ng cm-2) VN molecules were present predominantly as single entities, indicating that a minimum coating density was required for VN multimerization. By fluorescent visualization of surface-associated VN in different ways, we provide the first evidence of significant proteolytic remodelling of VN by endothelial cells (HUVECs) at the sites of αv integrin clusters. The degree of proteolysis was estimated using a novel approach relying on dequenching of FITC-labeled VN upon proteolytic activity, showing that about one-third of the surface-associated VN was proteolytically altered by adhering HUVECs. In addition, we demonstrate that HUVECs can internalize surface-associated VN and deposit it in a linear pattern along longitudinal actin filaments. Deposited VN was partly colocalized with urokinase receptors. Taken altogether, we elucidate the complex and dynamic behavior of VN during initial cell-biomaterials interactions, the equilibrium if which could have a significant impact on the biocompatibility of any blood contacting implants.
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