Abstract 4951: Platelet cloaking of CTCs is a universal phenomenon, which drives metastasis
Authors: Spillane Cathy D. , Cooke Niamh M. , Egan Karl , O'Toole Sharon , Finn Stephen , Kenny Dermot , Sheils Orla , O'Leary John J. 
Published: May 2015

Abstract
Disseminated malignancy is responsible for the majority of cancer-related deaths and circulating tumour cells (CTCs) play a central role in metastasis. We and others have described the phenomenon of platelet cloaking of CTCs, where there is a potent dynamic interaction between cancer cells and platelets, resulting in pro-survival, pro-angiogenic and epithelial mesenchymal transition (EMT) signals in the cancer cells. In this study we sought to further our understanding of this phenomenon by examining the universal nature of it and its contribution to the metastatic process.
The interaction between platelets and 14 in vitro human cancer cell lines of different origin and metastatic potential was assessed by flow cytometry. This work demonstrated that platelet cloaking of cancer cells is a universal phenomenon occurring across all tumour types examined (breast, cervix, lung, melanoma, ovary, prostate & thyroid). However, the process is heterogeneous with the extent of platelet adhesion varying both across and within tumour types, ranging from 34% (C33a, primary cervical carcinoma) to 83% (SK-MES-1, metastatic lung squamous cell carcinoma).
We next examined whether EMT changes previously attributed to the interaction were a constant result across all cancer types. This was achieved by morphology and RT-PCR analysis of an EMT marker panel. The effect of platelets on the epithelial phenotype of the cancer cells again showed the universal but heterogeneous nature of the interaction, with morphology changes akin to EMT observed at varying degrees across all cancer types. Analysis of the EMT markers supported these results, with all cell lines having EMT-like changes but with no consistent pattern to the alterations. One exception was plasminogen activator inhibitor 1 (PAI-1) where a significant and consistent increase was observed.
As cellular invasion is essential for metastasis and EMT and invasion are interlinked, we investigated the invasive capacity of cancer cells cloaked with platelets or platelets pre-treated with an anti-platelet agent (aspirin). This was done using the metastatic ovarian cancer cell lines, 59M and SKOV3, and matrigel invasion assays. The invasion of SKOV3 cells incubated with platelets was significantly greater (p≤0.0001) than SKOV3 cells alone. In contrast, platelets did not enhance invasion of the already highly invasive 59M cells. Aspirin pre-treatment of platelets significantly abrogated the platelet-mediated enhancement of invasion in SKOV3 cells (p&lt;0.05).
For the first time we describe the universal nature of the platelet cloaking phenomenon and the role it plays in driving metastasis, through induction of EMT and invasion. We also show for the first time that aspirin can attenuate the invasion induced by this interaction. Ultimately these studies could allow the establishment of therapies tailored to inhibiting metastasis, thus significantly reducing cancer mortality and morbidity rates.
Citation Format: Cathy D. Spillane, Niamh M. Cooke, Karl Egan, Sharon O'Toole, Stephen Finn, Dermot Kenny, Orla Sheils, John J. O'Leary. Platelet cloaking of CTCs is a universal phenomenon, which drives metastasis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4951. doi:10.1158/1538-7445.AM2014-4951