β1‐integrin‐mediated adhesion of melanoma cells to the propolypeptide of von Willebrand factor

Cell‐adhesion activity of the bovine propolypeptide of von Willebrand factor (pp‐vWF) was assessed by means of an in vitro assay with several cell lines of both normal and tumor‐cell origin. pp‐vWF promoted adhesion and spreading of B16 mouse melanoma cells and G‐361 human melanoma cells. However, it could not induce adhesion of any other cell lines tested including endothelial cells, normal fibroblasts, and tumor cells of sarcoma, carcinoma, neuroblastoma and leukemia origin. A monospecific polyclonal antibody against pp‐vWF, but not against fibronectin, laminin, and von Willebrand factor (vWF), completely blocked the pp‐vWF‐mediated adhesion, indicating that the cell adhesion was due to the pp‐vWF molecule and not due to possible contamination of these three well‐known adhesive proteins. The cell‐adhesion activity was also observed with human pp‐vWF and, furthermore, the adhesion to both bovine and human pp‐vWF was not affected by a peptide containing the Arg‐Gly‐Asp sequence while the peptide abolished the cell adhesion to vWF. The adhesion was completely dependent on Mg 2+ and inhibited by Ca 2+ . Inhibition by an anti‐(β1 integrin) mAb (4 B4) indicates that the receptor for this protein belongs to the β1‐integrin family. A monoclonal antibody (TC4) among several antibodies directed against bovine pp‐vWF inhibited the B16 adhesion to immobilized pp‐vWF. The epitope for this monoclonal antibody lies in a central 8‐kDa portion of pp‐vWF, suggesting that this region is important for the cell‐adhesion activity. This idea was supported by the finding that purified 8‐kDa fragment promoted adhesion of B16 cells in a concentration‐dependent manner. As pp‐vWF shows unique cell‐type specificity in its adhesion activity, which is completely different from that of fibronectin, laminin, vWF and collagen, it may be a novel type of adhesive glycoprotein that utilizes a β1‐integrin receptor.