Platelet-adhesive protein-tumor cell interaction was studied in vitro and in vivo. Monoclonal antibody 1OE5, which inhibits binding of fibronectin and von Willebrand factor to the platelet membrane glycoprotein GPIIb-GPIIIa complex, inhibited the binding of mouse CT26 and human HCI8 colon carcinoma cells to platelets by 63-65%, whereas an irrelevant monoclonal antibody, 3B2, had no effect. Monoclonal antibody 6D1, which inhibits binding of von Willebrand factor to GPIb, also had no effect. RGDS, a tetrapeptide that represents the adhesive domain of fibronectin and von Willebrand factor inhibited binding of the tumors to platelets by 64-69%. Monospecific polyclonal antifibronectin antibody inhibited binding by 60-82%; anti-von Willebrand factor antibody inhibited binding by 75-81%.In vivo, polyclonal monospecific anti-mouse von Willebrand factor antibody inhibited pulmonary metastases induced by CT26 tumor cells by 53-64%, B16a amelanotic melanoma cells by 45% and T241 Lewis bladder cells by 46% without induction of thrombocytopenia. Pulmonary metastases with CT26 cells could be inhibited by induction of thrombocytopenia, and reconstituted by infusion of either murine or human platelets. Reconstitution of pulmonary metastases with human platelets could be inhibited 77% by preincubation of human platelets with monoclonal antibody 1OE5 before infusion of platelets into mice.Thus, platelets appear to contribute to metastases by their adhesive interaction with tumor cells via the adhesive proteins fibronectin and von Willebrand factor.

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 platelets aggregation have been previously described: (1) a mechanism which requires complement, a stable plasma factor, divalent cation and a sialo-lipo-protein vesicular component of the tumor membrane for platelet aggregation; and (2) a mechanism which operates via the generation of thrombin and requires a phospholipid 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 (prostacyclin, 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.

No abstract

No abstract