Tumor cell adhesion to subendothelial matrix in the presence of platelets and plasma has been examined in vitro using an entirely homologous system of rat Walker 256 carcinosarcoma cells, matrix laid down by rat aortic endothelial cells and rat platelets and plasma. In the presence of platelets or platelets plus plasma, tumor cell adhesion was significantly enhanced when compared to adhesion in the absence of platelets. In the presence of plasma alone (0.1%), we observed no significant increase in tumor cell adhesion.

In order to determine which platelet factors contribute to the enhancement of tumor cell adhesion by platelets, we subjected washed rat platelets to mechanical lysis or thrombin stimulation followed by centrifugation. The membrane fractions and supernatant fractions containing platelet attachment proteins were compared for their abilities to support tumor cell adhesion to subendothelial matrix. Platelet membranes were also recombined with platelet supernatant fractions to determine if platelet attachment proteins or platelet membranes required the presence of the other to enhance tumor cell adhesion. Platelet supernatant fractions which contained release reaction proteins (confirmed by polyacrylamide gel electrophoresis) did not enhance tumor cell adhesion. Purified thrombospondin, fibronectin, β-thromboglobulin, platelet derived growth factor, and serotonin had no effect on tumor cell adhesion. Platelet membrane containing fractions affected tumor cell adhesion to subendothelial matrix as follows: (a) platelets formed an adhesive bridge between tumor cells and the subendothelial matrix as demonstrated by scanning electron microscopy; (b) intact platelets and thrombin stimulated platelets were the most effective at facilitating tumor cell adhesion; (c) preparations containing partially lysed platelet ghosts were more effective in supporting tumor cell adhesion to subendothelial matrix than were preparations containing completely lysed platelet membrane fragments; (d) recombination of platelet supernatant fractions with mechanically lysed platelets did not enhance their ability to support adhesion; (e) fixed platelets, either alone or in combination with platelet supernatant fractions, failed to enhance adhesion. These data indicate that platelet enhanced tumor cell adhesion appears to be dependent on platelet membrane factors including receptor mobility, rather than intraplatelet components.

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This work was supported by USPHS Grants CA-29997 and CA-29405 awarded to K. V. Honn and HL27073 awarded to D. Walz by the NIH, Department of Health and Human Services.

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