This paper reports studies on the interaction between human platelets, the plasma coagulation system, and two human tumor cell lines grown in tissue culture: Melanoma and breast adenocarcinoma. The interaction was monitored through the use of 125I-labelled fibrinogen, which measures both thrombin activity generated by cell-plasma interaction and fibrin/fibrinogen binding to platelets and tumor cells. Each tumor cell line activates both the platelets and the coagulation system simultaneously resulting in the generation of thrombin or thrombin-like activity. The melanoma cells activate the coagulation system through 'the extrinsic pathway' with a tissue factor-like effect on factor VII, but the breast tumor seems to activate factor X directly. Both tumor cell lines activate platelets to 'make available' a platelet-derived procoagulant material necessary for the conversion of prothrombin to thrombin. The tumor-derived procoagulant activity and the pletelet aggregating potential of cells do not seem to be inter-related, and they are not specific to malignant cells.
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