Thrombospondin, a potentiator of tumor cell metastasis.

The platelet-secreted protein thrombospondin (TSP) potentiates tumor cell metastasis. Human TSP injected i.v. into mice 5 min prior to i.v. injection of T241 sarcoma cells potentiates lung tumor colony formation. Several lines of evidence suggest that the TSP-enhancing effect involves both TSP-mediated tumor cell adhesion and the host's hemostatic system: TSP potentiates the initial, rapid sequestering of tumor cells in the lung; TSP promotes the adhesion of tumor cells in vitro; the effect of TSP on tumor cell metastasis is dependent on the presence of platelets and a normal plasma clotting system, since TSP does not potentiate lung tumor colony formation in either thrombocytopenic mice or mice anticoagulated with Coumadin. Our results suggest a central role for TSP in the metastatic process.