Tamoxifen suppresses platelet activation-supported angiogenesis and metastasis

It has been known for decades that platelets are critical for providing adequate hemostasis. However, more recently, platelets have been implicated in other health and disease related processes such as angiogenesis, wound healing, inflammation, tumor progression and metastasis.^1-4 Platelet interactions with tumor cells occur locally at the tissue level, and also in the blood circulation. These interactions can lead to platelet activation, a process that results in the release of the contents of platelet granules to the tumor microenvironment. This platelet releasate includes large amounts of bioactive molecules which facilitate tumor growth, endothelial transmigration and angiogenesis.⁵ In the bloodstream, platelets are also able to form aggregates with circulating tumor cells. As the platelets coat circulating tumor cells surfaces, this interaction can generate a shielding effect by facilitating immune evasion and preventing clearance of cancer cells. Additionally, the interaction of platelets with circulating tumor cells can also promote epithelial to mesenchymal transition, a process that favors tumor growth and increases metastatic burden.¹ Therefore, it has become evident that by several distinctive mechanisms, platelets promote tumor progression and metastasis.

Tamoxifen is a chemotherapeutic agent that has been widely used over the last 40 years as endocrine therapy for estrogen-receptor positive (ER⁺) breast cancer. Tamoxifen is known to block estrogen receptors therefore disrupting important signaling pathways in tumor cells (process known as estrogen deprivation). In the current issue of ATVB, Johnson et al. characterize an indirect platelet-mediated pathway by which Tamoxifen and its metabolite, 4-hydroxytamoxifen (4-OH) inhibit angiogenesis and metastasis.⁶ The investigators show that platelets from patients taking tamoxifen are less responsive to activation by the breast tumor cell line MCF-7. Moreover, the releasate from these platelets contain less Vascular Endothelial Growth Factor (VEGF) which, in vitro, results in decreased capillary tube formation, a surrogate marker of angiogenesis.

The main conclusions of this study are supported by an extensive and convincing series of experiments that demonstrate the effect of Tamoxifen and its metabolite on platelet activation. The investigators show that platelets incubated with Tamoxifen or 4-OH, exhibit decreased activation responses upon stimulation with ADP or exposure to the cell line MCF-7. Interestingly, exposure to 4-OH also blocks the activation of platelets by the strong agonist TRAP (thrombin related activation peptide) and the triple negative tumor cell line MDA-MB-231. To confirm that this effect has functional consequences, the releasate of platelets exposed in vitro to Tamoxifen or 4-OH did not support the formation of capillary tubes when compared to controls. Furthermore, protein analysis by ELISA and membrane-based protein arrays of the releasates of platelets exposed to Tamoxifen or 4-OH show that the observed differences are associated with the release of significantly lower amounts of key pro-angiogenic molecules such as VEGF, and higher amounts of potent anti-angiogenic mediators like angiopoietin-1. Finally, in vitro invasion assays demonstrated that cancer cells exposed to the releasate of platelets treated with Tamoxifen or 4-OH had decreased migration, indicating that these cancer cells have less metastatic potential.

Although this work is of clear significance and paves the road for further investigations, there are still many unanswered questions. The specific mechanism(s)by which Tamoxifen and 4-OH modulate platelet activity is not completely understood. Although platelets have estrogen receptors, it has previously been shown that inhibition of those receptors does not abrogate platelet responses to Tamoxifen. The mechanism of action of Tamoxifen or 4-OH could alternatively be explained by an effect on thrombopoiesis, perhaps facilitating selective packaging of platelet alpha granules with anti-angiogenic content generating platelet populations with less pro-angiogenic/metastatic potential.

Interestingly, by modulating platelet activation, Tamoxifen could potentially also contribute to decreasing the incidence of cancer-associated thrombosis in patients with breast cancer.

Johnson's findings should however offer a cautionary note to scientists using genetic animal models that rely on the administration of Tamoxifen to express or silence genes in models of metastasis or thrombosis and hemostasis given the proposed effect of Tamoxifen on platelet function, angiogenesis and metastasis.

In summary, Johnson et al. have characterized a platelet-mediated pathway by which Tamoxifen and its metabolite 4-OH, indirectly restrain angiogenesis and metastasis. While these compounds directly affect breast cancer cells by causing estrogen deprivation, platelets exposed to Tamoxifen can potentially limit tumor progression in situ (providing less pro-angiogenic signals) and hinder the metastatic burden of circulating tumor cells.

Figure 1.