Platelets, primarily known for their role in hemostasis, are now recognized to play an integral role in cancer progression and metastasis. Recent evidence has established that platelets are activated by contact with breast tumor cells, leading to the release of hundreds of growth factors, cytokines, chemokines and angiogenesis mediators that could influence tumor growth and metastasis. Indeed, work from our group has demonstrated that factors released from activated platelets promote both metastasis and angiogenesis. However, little is known about the specific factors and signaling pathways that mediate this critical platelet-tumor cell cross-talk. To address this question, we performed an angiogenesis array (Ray Biotech) to identify specific pro-angiogenic and pro-metastatic factors released by tumor cells during platelet-tumor cell interactions. We identified several factors that were secreted by MCF-7 breast tumor cells in response to activated platelet releasate, including high levels of interleukin 8 (IL-8, CXCL8). IL-8 is a cytokine known to play a critical role in metastasis and angiogenesis and is elevated in the serum and tumor tissue of breast cancer patients. We confirmed that exposure to platelets strongly induced the production of IL-8 in several human breast cancer cell lines (MDA-MB-231, BT-20, SKBR-3 and MCF-7) by ELISA and found that platelets themselves do not contain detectable levels of IL-8. Furthermore, IL-8 production was highest in the more aggressive, triple negative MDA-MB-231 and BT-20 lines, suggesting a link between platelet-induced IL-8 and tumor subtype. To identify the specific component or components of platelet releasate responsible for driving tumor cell IL-8, we first characterized the contents of activated platelet releasate by array (Ray Biotech) and found an abundance of both chemokine (C-C motif) ligand 5 (CCL5, RANTES) and epidermal growth factor (EGF). Next, we treated breast tumor cell lines directly with recombinant CCL5 or EGF and observed an increase in IL-8 production, however sensitivity to CCL5, EGF or the combination varied among the cell lines tested and may depend on receptor expression. To determine if platelet-derived CCL5 or EGF drives tumor cell IL-8, breast tumor cells were pretreated with the CCL5 receptor (CCR5) blocker maraviroc or the EGFR blocker AG-1478 and then exposed to platelets. Blocking CCR5 abrogated the induction of IL-8 in response to platelets in the cell lines that were sensitive to CCL5 while EGFR inhibition diminished induction of IL-8 in response to platelets in the cell lines that were sensitive to EGF. Next we sought to determine the role of platelet-induced IL-8 in metastasis. We performed standard invasion assays using MDA-MB-231 cells transfected with IL-8shRNA or control cells. Platelets were able to increase the invasion of control MDA-MD-231 cells by 5 fold, while IL-8 knockdown reduced the effect of platelets on invasion by 50%. Furthermore, ability of platelets to promote tumor cell migration across an endothelialized membrane was reduced 87% in IL-8 knockdown MDA-MB-231s compared to controls in standard transendothelial migration assays. These results suggest that platelets promote metastasis, in part, by driving tumor cell IL-8. Studies are currently underway to further elucidate the mechanism by which platelets reprogram tumor cells to produce IL-8 and to confirm these findings in vivo. Taken together, these results suggest that platelets, through release of soluble factors, drive tumor cells to produce IL-8 and that blocking this communication can disrupt the pro-metastatic potential of platelets. Ultimately, these studies support targeting specific platelet-tumor cell interaction as a novel means of limiting disease progression in breast cancer.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.