Role of tumor proteinases in tumor cell-induced platelet aggregation.

In a previous work we found a correlation between in vivo metastatic potential of cancer cells and their platelet aggregating activity in sublines of the mFS6 murine fibrosarcoma. In the present study the effects of different proteinase inhibitors on platelet aggregation induced by these cells were investigated. When the platelets were incubated with the inhibitors, only those effective against cysteine proteinases strongly reduced platelet aggregation by cancer cells; serine protease inhibitors, including hirudin, had no effect on platelet response. Incubation of neoplastic cells with the same inhibitors gave similar, though less evident results. Addition of neoplastic cells to platelet-rich plasma also caused significant production of fibrinopeptide A, more by the less malignant cells. Thus, in this experimental model a cysteine proteinase of the neoplastic cells appears to play an important role in the platelet aggregation induced by them, and this property was detected in the M4 cells with high metastatic in vivo activity.