Tumor cells induce platelet aggregation and intraplatelet calcium ion movements

We studied platelet aggregation and changes in cytosolic Ca(++) concentrations induced by cells isolated from 5 human tumor tissues (2 hepatocellular carcinomas, 1 colon carcinoma, 1 gastric carcinoma and 1 pancreatic carcinoma). A Platelet Ionized Calcium Aggregometer was used and washed, aequorin loaded platelets were employed. Tumor cells were able to induce aggregation and an increase in cytoplasmic Ca(++) concentrations in the presence of trace amounts (10 µl) of PPP, while no aggregating response was found after addition of fibrinogen alone to washed platelets. The platelet aggregating activity of tumor cells was maintained in the presence of factor VII deficient plasma or of factor VIII deficient plasma, and disappeared completely when factor X deficient plasma was added to washed platelets. Furthermore, tumor cell induced platelet aggregation and Ca (++) movements were inhibited by hirudin (100 U/ml), a specific thrombin inhibitor, while concanavalin A (100 µg/ml), a tissue factor inhibitor, had no effect. Finally, preincubation of neoplastic cells with HgCl(2) (0.5 mM), a cysteine protease inhibitor, markedly decreased their ability to induce aggregation and Ca(++) movements; on the contrary, incubation of cells with soybean trypsin inhibitor (10 µg/ml), a serine protease inhibitor, or with concanavalin A (100 µg/ml) had no effect. These data suggest that cells isolated from human tumor tissues activate platelet function through the generation of thrombin, due to a cysteine protease which directly activates factor X.