A new mechanism for tumor‐induced platelet aggregation. Comparison with mechanisms shared by other tumors with possible pharmacologic strategy toward prevention of metastases

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Abstract
Because tumor‐induced platelet aggregation appears to play a role in the development of certain experimental tumor metastases, we examined the mechanism(s) of tumor‐induced platelet aggregation as well as the effect of various anti‐platelets agents. Two mechanisms for tumor‐induced platelet aggregation have been previously described: (1) a mechanism which requires complement, a stable plasma factor, divalent cation and a sialo‐lipo‐pro‐tein vesicular component of the tumor membrane for platelet aggregation; and (2) a mechanism which operates via the generation of thrombin and requires a phos‐pholipid component of the tumor membrane. We now report a new mechanism of tumor‐induced platelet aggregation which is shared by three different tumors: a spontaneously metastatic human melanoma, HM29, a murine melanoma, B16F10, and a carcinogen‐induced metastatic murine colon carcinoma, CT26. These tumors do not require cell‐surface sialic acid or serum complement as does the first mechanism. They do not require cell‐surface phospholipid, as do the tumors representing the other two mechanism. They do not aggregate platelets via the generation of thrombin as do tumors representing the second mechanism. These tumors are unique in that they require a trypsin‐sensitive surface protein for activity. The ability of the thrombin‐generating tumors to aggregate platelets is uniquely sensitive to two highly specific, synthetic thrombin‐competitive inhibitors: DAPA and No. 805. The other two groups of tumors are at least 10 times more sensitive to inhibition of platelet aggregation by elevation of cyclic AMP levels (prostacy‐clin, 6‐keto‐PGE1, dibutyryl cyclic AMP) and at least 10 times more sensitive to inhibition of prostaglandin synthesis (indomethacin, ibuprofen). Thus, tumor‐induced platelet aggregation is heterogeneous with respect to mechanism of action as well as inhibition by anti‐platelet pharmacologic agents. Sensitivity to anti‐platelet agents correlates with the mechanism by which tumor cells aggregate platelets.