Tamoxifen Directly Inhibits Platelet Activation, Angiogenic Potential and Platelet-Mediated Metastasis

Objective - Platelets, which are mainly known for their role in hemostasis, are now known to play a crucial role in tumor metastasis and neovascularization. Metastatic disease is the cause of roughly 90% of all cancer-related deaths and understanding the mechanisms leading to dissemination of tumor cells to distant sites remains one of the main challenges of cancer research. Platelets carry a plethora of potent angiogenic and metastatic mediators within their alpha-granules and exposure to breast tumor cells induces platelet activation, leading to release of these mediators. Tamoxifen is a selective estrogen receptor modulator that is widely used for the treatment and prevention of breast cancer; its use is associated with a 50% reduction in the risk of invasive and noninvasive breast cancer in women who utilized the drug for at least 5 years. Interestingly, tamoxifen has demonstrated anti-cancer efficacy in estrogen negative breast cancers suggesting that this drug has additional mechanisms of action. Previously tamoxifen and its metabolites have been shown to directly impact platelet function. Because platelets are critical for metastatic spread, we posited that tamoxifen or its metabolites may exert anti-tumor effects through direct platelet inhibition. To test this hypothesis, we examined the impact of tamoxifen on platelet activation, angiogenic potential and metastasis both ex vivoand in breast cancer patients utilizing tamoxifen therapy.

Approach and Results - We found that ex vivo pretreatment with tamoxifen or its metabolite 4-hydroxytamoxifen (4-OH) lead to a significant inhibition of platelet activation in response to TRAP, ADP or the breast tumor cell lines MDA-MB-231 and MCF-7 in platelets from healthy human donors. Platelets, known to promote tumor angiogenesis, are a reservoir for angiogenic proteins that are secreted in a differentially regulated process. Activated platelets exposed to tamoxifen or 4-OH released significantly lower amounts of the pro-angiogenic regulator VEGF in while anti-angiogenic endostatin release is unaffected, thus shifting the balance of angiogenic regulators that are released. To examine the impact of this alteration in angiogenic protein release, we performed functional angiogenesis assays using releasates generated from tamoxifen or 4-OH treated platelets. These in vitroangiogenesis assays confirmed that tamoxifen pretreatment led to dramatically diminished capillary tube formation and decreased endothelial migration. Tamoxifen also significantly inhibited the ability of platelets to promote metastasis in vitro, causing a dramatic decrease in breast tumor cell invasion and transendothelial migration. Next, we utilized membrane-based cytokine arrays to further interrogate the effect of tamoxifen on the release of stored platelet factors. Using this method, we identified several key proteins known to be associated with metastasis that were lower in releasate from tamoxifen treated platelets including TGF-b, IL-6 and IGF-1 while anti-angiogenic angiopoietin-1 was elevated. Platelets isolated from patients on tamoxifen maintenance therapy were also found to have decreased activation responses, diminished VEGF release and lower angiogenic potential.

Conclusions - Overall we demonstrate that tamoxifen directly influences the release of specific stored platelet factors leading to decreased tumor cell support. The mechanism is directly linked to tamoxifen's inhibitory role in platelet activation, causing altered of release of key platelet-derived angiogenic and metastatic factors including VEGF, angiopoietin-1, TGF-b, IL-6 and IGF-1 during tumor cell and platelet crosstalk. Furthermore, translational studies confirmed that platelet activation and angiogenic potential are significantly suppressed in breast cancer patients utilizing tamoxifen therapy. Our work stresses the importance of platelets for successful angiogenesis and metastatic spread and, ultimately, supports the idea of utilizing targeted platelet therapies to inhibit the platelet's role in malignancy.