Platelet surface receptors and melanoma metastasis

Abstract

The aim of this thesis was to investigate the roles of the platelet surface receptors GPVI and GPIbα in the process of hematogenous melanoma metastasis. In the last decades, there has been accumulating evidence for a contribution of platelets and platelet receptors to hematogenous tumor metastasis. It is known today that functional platelets are a prerequisite to hematogenous dissemination of tumor cells and that the blockade of different platelet surface receptors or other factors of primary hemostasis can diminish metastasis in vivo. Therefore, the two important platelet receptors GPVI and GPIbα, which both play a central role in hemostasis, were studied in the context of experimental pulmonary metastasis. GPVI as well as GPIbα were inhibited by monoclonal, monovalent antibodies (p0p/B Fab fragments) in a syngeneic murine model of experimental pulmonary metastasis using C57BL/6 mice and B16F10 melanoma cells. Antibody treatment of the mice did not induce thrombocytopenia, excluding an influence of platelet numbers on experimental metastasis. While blockade of GPVI showed no effect on in vivo melanoma metastasis in our experimental system, GPIbα-blockade led to an unexpected and significant increase in the number of metastatic foci. This effect could only be seen when GPIbα was blocked either before or together with tumor cell injection, pointing to a role of GPIbα in the early steps of the metastatic cascade. This observation was confirmed by the assessment of early fate of melanoma cells transfected with green fluorescent protein (GFP) under GPIbα blockade. GFP-transfected cells showed improved survival and pulmonary arrest of tumor cells shortly after GPIbα inhibition. In contrast, GPIbα blockade in P-selectin-deficient mice had no enhancing effect on metastasis, suggesting the involvement of GPIbα in the initial, P-selectin-dependent steps of metastasis. In conclusion, these findings show that GPIbα contributes to the control of early steps in tumor metastasis, in addition to its role in hemostasis. It could be demonstrated for the first time, that blockade of a platelet surface receptor does not always lead to a reduction of metastasis but can also have a pro-metastatic effect.