Deadly allies: the fatal interplay between platelets and metastasizing cancer cells

Abstract

The general notion that functional platelets are important for successful hematogenous tumor metastasis has been inaugurated more than 4 decades ago and has since been corroborated in numerous experimental settings. Thorough preclinical investigations have, at least in part, clarified some specifics regarding the involvement of platelet adhesion receptors, such as thrombin receptors or integrins, in the metastasis cascade. Pivotal preclinical experiments have demonstrated that hematogenous tumor spread was dramatically diminished when platelets were depleted from the circulation or when functions of platelet surface receptors were inhibited pharmacologically or genetically. Such insight has inspired researchers to devise novel antitumoral therapies based on targeting platelet receptors. However, several mechanistic aspects underlying the impact of platelet receptors on tumor metastasis are not fully understood, and agents directed against platelet receptors have not yet found their way into the clinic. In addition, recent results suggesting that targeted inhibition of certain platelet surface receptors may even result in enhanced experimental tumor metastasis have demonstrated vividly that the role of platelets in tumor metastasis is more complex than has been anticipated previously. This review gives a comprehensive overview on the most important platelet receptors and their putative involvement in hematogenous metastasis of malignant tumors.

Figure 1

Platelet receptors implicated in hematogenous tumor metastasis. Schematic representation of platelet surface molecules whose primary functions contribute to hemostasis and coagulation through binding to ligands expressed by other cells or extracellular matrix (ECM) molecules. Modulation of these molecules, for example, through platelet activation, in genetically engineered mice or by antibody-mediated blockade, has been shown in many cases to impact on experimental tumor metastasis.

Figure 2

Examples for modulation of pulmonary melanoma metastasis through targeted interference with platelet receptors. In all experiments (performed by L.E. in the laboratory of M.P.S.), C57/BL6 mice were intravenously injected with 2.5 × 10⁵ B16F10 melanoma cells, and pulmonary metastasis was evaluated after 10 to 14 days. (A) Antibody-mediated depletion of platelets prevents metastasis formation. (B) Inhibition of platelet GPIbα by function-blocking, monovalent Fab fragments results in a marked increase of pulmonary melanoma metastases. (C) Inhibition of GPIIb/IIIa by specific Fab fragments decreases the number of metastatic melanoma nodules in the lung. (D) Antibody-mediated blockade of GPVI does not significantly influence pulmonary melanoma metastasis. (E) P-selectin deficiency causes marked reduction of pulmonary melanoma metastasis.