The "Janus Face" of Platelets in Cancer

Abstract

Besides their vital role in hemostasis and thrombosis, platelets are also recognized to be involved in cancer, where they play an unexpected central role: They actively influence cancer cell behavior, but, on the other hand, platelet physiology and phenotype are impacted by tumor cells. The existence of this platelet-cancer loop is supported by a large number of experimental and human studies reporting an association between alterations in platelet number and functions and cancer, often in a way dependent on patient, cancer type and treatment. Herein, we shall report on an update on platelet-cancer relationships, with a particular emphasis on how platelets might exert either a protective or a deleterious action in all steps of cancer progression. To this end, we will describe the impact of (i) platelet count, (ii) bioactive molecules secreted upon platelet activation, and (iii) microvesicle-derived miRNAs on cancer behavior. Potential explanations of conflicting results are also reported: Both intrinsic (heterogeneity in platelet-derived bioactive molecules with either inhibitory or stimulatory properties; features of cancer cell types, such as aggressiveness and/or tumour stage) and extrinsic (heterogeneous characteristics of cancer patients, study design and sample preparation) factors, together with other confounding elements, contribute to "the Janus face" of platelets in cancer. Given the difficulty to establish the univocal role of platelets in a tumor, a better understanding of their exact contribution is warranted, in order to identify an efficient therapeutic strategy for cancer management, as well as for better prevention, screening and risk assessment protocols.

Keywords: miRNAs; microvesicles; paraneoplastic thrombocytosis and thrombocytopenia; platelet activation; platelet-derived bioactive molecules; platelet-tumor crosstalk.

Figure 1

Schematic representation of the main platelet effects on tumor biology. See text for details. In black: platelet-derived bioactive molecules with positive effects. In blue: platelet-derived compounds with negative effects. In red: platelet-derived compounds with both positive and negative effects. Lines with dot indicate either stimulation or inhibition, depending on the platelet-derived bioactive molecule. 12/15 HETEs: 12 and 15 hydroxyeicosatraenoic acid; Ang-1: Angiopoietin; LPA: lysophosphatidic acid; EGF: Endothelial Growth Factor; P-sel: P-selectin; PDGF: Platelet-Derived Growth Factor; TGF-β: tumor growth factor-β; VEGF: vascular endothelial growth factor.