TUMOR-CELLS INDUCE PLATELET-AGGREGATION AND INTRAPLATELET CALCIUM-IONMOVEMENTS

We studied platelet aggregation and changes in cytosolic Ca++ concentr ations induced by cells isolated from 5 human tumor tissues (2 hepatoc ellular carcinomas, 1 colon carcinoma, 1 gastric carcinoma and 1 pancr eatic carcinoma). A Platelet Ionized Calcium Aggregometer was used and washed, aequorin loaded platelets were employed. Tumor cells were abl e to induce aggregation and an increase in cytoplasmic Ca++ concentrat ions in the presence of trace amounts (10 mul) of PPP, while no aggreg ating response was found after addition of fibrinogen alone to washed platelets. The platelet aggregating activity of tumor cells was mainta ined in the presence of factor VII deficient plasma or of factor VIII deficient plasma, and disappeared completely when factor X deficient p lasma was added to washed platelets. Furthermore, tumor cell induced p latelet aggregation and Ca++ movements were inhibited by hirudin (100 U/ml), a specific thrombin inhibitor, while concanavalin A (100 mug/ml ), a tissue factor inhibitor, had no effect. Finally, preincubation of neoplastic cells with HgCl2 (0.5 mM), a cysteine protease inhibitor, markedly decreased their ability to induce aggregation and Ca++ moveme nts; on the contrary, incubation of cells with soybean trypsin inhibit or (10 mug/ml), a serine protease inhibitor, or with concanavalin A (1 00 mug/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 facto r X.