Platelets as crucial partners for tumor metastasis: from mechanistic aspects to pharmacological targeting

Abstract

Platelets are anucleated cells that circulate in the blood as sentinels of tissue integrity. In fact, they are rich in a plethora of proteins and other factors stored in different granules which they selectively release upon stimulation. Moreover, platelets synthesize a vast number of lipids and release various types of vesicles, including exosomes which are rich in genetic material. Platelets possess a central function to interact with other cell types, including inflammatory cells and cancer cells. Recent findings have enlightened the capacity of platelets to induce changes in the phenotype of cancer cells which acquire invasiveness thus enhancing their metastatic potential. Thus, it has been hypothesized that targeting the platelet may represent a novel strategy to prevent the development and progression of cancer. This is supported by the efficacy of the antiplatelet agent low-dose aspirin. Studies are ongoing to verify whether other antiplatelet agents share the anticancer effectiveness of aspirin.

Keywords: Antiplatelet agents; Aspirin; Cancer cells; Epithelial–mesenchymal transitions; Metastasis; Platelets; Prostaglandin E2.

Fig. 1

Platelet structure, primary receptors, eicosanoid machinery, and targeted therapeutics. Platelets are involved in the release of many mediators that are stored in different granules in the cytoplasm and microvesicles. A plethora of proteins are contained in α-granules; there are cell adhesion proteins, blood clotting factors, growth and angiogenic factors. The plasma membrane of platelets expresses several transmembrane receptors, involved in the crosstalk with other platelets and different cell types. Platelets adhere to the damaged vascular endothelium through the binding of integrin receptors (GP, glycoproteins) to the extracellular matrix proteins, such as collagen and VWF (von Willebrand factor). Possible strategies to inhibit the adhesion of platelets to a damaged endothelium involve the use of agents that interact with collagen binding sites, such as revacept, thus preventing the activation of the collagen receptors GPIa/IIa and GPVI expressed on the plasma membrane of platelets. Platelet aggregation, mediated by the binding of fibrinogen or fibronectin to GPIIb/IIIa on agonist-stimulated platelets, is inhibited by different antagonists of the GPIIb/IIIa receptor (abciximab, eptifibatide, and tirofiban). The activation of platelets by ADP (adenosine diphosphate) is mainly affected by antagonists of the P2Y12 receptor, among them, there is the pro-drug clopidogrel. An important antiplatelet agent is aspirin, which irreversibly inhibits the activity of cyclooxygenase (COX)-1 involved in the production of prostaglandin (PG) H₂ that is then converted to the potent pro-aggregatory agent thromboxane (TX)A₂ by the activity of TXA₂ synthase (TXAS). A secondary product of COX-1-dependent pathway is PGE₂ produced by the activity of different PGE synthases (PGES). TXA₂ and PGE₂ cause different platelet responses by the interaction with specific receptors. Various receptor antagonists in clinical development are reported. Picotamide and ridrogrel act by a dual mechanism involving the blockage of TP receptors and the inhibition of TXAS. There are four different receptors for PGE₂ on the platelet surface: EP1, EP2, EP3 and EP4. The stimulation of EP3 leads to platelet activation, and specific antagonists are under clinical development, including DG-041. EP4 and EP2 signaling may increase intraplatelet cAMP (cyclic adenosine monophosphate) levels and thus possibly counteracting the platelet activation by EP3. Some agonists and antagonists of these receptors have been synthesized. An important receptor present on the platelet plasma membrane is the IP receptor for prostacyclin (PGI₂), for which different commercially available agonists have been developed (Iloprost, Treprostinil, Selexipag). Another abundant eicosanoid produced by platelets is 12(S)-HETE [12(S)-hydroxyeicosatetraenoic acid] via the activity of the platelet-type lipoxygenase (p12-LOX). The mechanism of action is 12-(S)HETE has not been entirely understood. Recently, it has been proposed the activation of the orphan receptor GPR31 by 12-(S)HETE. The discovery of selective inhibitors of 12-LOX, such as ML355, will allow enhancing our knowledge of the role played by 12-LOX in health and disease. Protease-activated receptors (PARs) are involved in platelet activation by thrombin. There are two receptors, known as PAR-1 and PAR-4. However, PAR-1 possesses a higher affinity for thrombin. The PAR-1 antagonist vorapaxar was approved for clinical use in 2014. New antiplatelet agents include serotonin receptor antagonists (5-HT2A antagonists)

Fig. 2

Major pathways for the biosynthesis of eicosanoids in platelets. Arachidonic acid (AA), esterified in membrane phospholipids, can be released upon platelet activation by different stimuli via the action of phospholipases (PLs), including the cytosolic phospholipase A₂ (cPLA₂). In platelets, AA is transformed to PGH₂ by the activity of COX-1; then, PGH₂ is the substrate of different synthases, thus leading to the formation of the prostanoids: TXA₂, PGE₂, PGD₂, PGF_2α. Thromboxane synthase (TXAS), cytosolic PGE synthase (cPGES), microsomal PGE synthase-2 (mPGES-2), lipocalin-type prostaglandin D synthase (L-PGDS), hematopoietic prostaglandin D synthase (H-PGDS) and PGF synthase (PGFS) are the downstream synthases involved in the production of prostanoids. AA is also transformed to 12(S)-HETE by the activity of p12-LOX. The enzyme produces 12(S)-hydroperoxy-eicosatetraenoic acid [12(S)-HPETE] which is, then, converted to 12(S)-HETE by glutathione reductase (GR). PGE₂, PGD₂ and 12(S)-HETE can be esterified into membrane phospholipids by the action of fatty acid CoA ligase (FACL) to produce new lipid mediators, i.e., the phospholipid esterified eicosanoids

Fig. 3

Molecular determinants involved in the crosstalk between platelets and cancer cells. Platelets interact with tumor cells through different receptors expressed on platelet surface (i.e., collagen receptor GPVI, P-selectin and the integrins α6β1 or αIIbβ3, and GPIIb/IIIa). The direct interaction between the two cell types triggers platelet activation, the secretion of platelet α-granule content, i.e., growth and angiogenic factors (such as PDGF, TGF-β, and VEGF). Moreover, platelets release ADP from dense granules and synthesize prostanoids, including TXA₂ and PGE₂. The interaction of platelets with cancer cells causes the induction of COX-2 in cancer cells, which contributes to a further increase in PGE₂ production. Aberrant PGE₂ generation is a hallmark of cancer. The overexpression of COX-2 in cancer cells involves both transcriptional and posttranscriptional mechanisms mediated by the release of PDGF. Among the numerous events triggered by platelet–cancer cell crosstalk, there is the induction of the epithelial–mesenchymal transition (EMT) phenomenon in cancer cells, which can be mediated by soluble mediators(proteins and lipids) (modified from Dovizio et al. [97] and Guillem-Llobat et al. [98])