Assistant Professor, Interdisciplinary Division of Biomedical Engineering, Hong Kong Polytechnic University, China.

Speech Title: Blood shear stress selects circulating tumor cells with metastatic advantages
INTRODUCTION: Metastasis is a multi-step process, including detachment of cancer cells from primary and/or metastatic lesions, intravasation, survival in circulation, extravasation into distant organs, and formation of metastatic tumors, which accounts for over 90% of cancer-related deaths 1 . During hematogenous metastasis, survival of tumor cells in blood circulation is one critical step. The presence and frequency of circulating tumor cells (CTCs) are correlated with poor prognosis in many cancer types. Although a primary tumor releases thousands of cancer cells every day into the circulation, less than 0.01% of CTCs eventually grow into metastatic tumors and most patients develop very few metastases 1 , suggesting that metastasis is highly inefficient. In particular, the half-life of CTCs in circulation is limited and the majority die within 24 hr. Therefore, survival of CTCs in blood circulation is one major rate- limiting step during metastasis. Although its influence on CTC biology has been extensively studied, the roles of blood shear flow in metastatic potential of CTCs is unclear. In particular, whether blood circulation selects a minor subpopulation of CTCs with metastatic advantages remains elusive.
METHODS: In this study, human breast cancer cells with differential metastatic potential are adopted. The corresponding cancer stem cells (CSCs) are selected by culturing cancer cells in 3D soft fibrin gels as demonstrated by our previous research 2,3 . These cancer cells and the derived CSCs are used for circulation in a circulatory microfluidic system under various shear stresses and circulation times. The viability of these treated and non-treated cells is examined by MTT. The migratory and invasive abilities of both cells are measured by wound healing analysis and transwell assay, respectively. To explore the underlying mechanisms, the genes related to metastasis and stemness are quantified.
RESULTS AND DISCUSSION: Our data show that breast cancer cells with high metastatic potential show higher survival rate than cells with weak metastatic ability in blood shear flow. Highly metastatic CSCs selected by fibrin gels survive better in the circulation than non-selected cancer cells. These data suggest that metastatic CTCs have the ability to survive blood circulation, which is necessary for the subsequent extravasation and the development of metastases. Moreover, cell viability is related to the magnitude of shear stress and circulation time. Importantly, those survived cancer cells after blood circulation exhibit enhanced migration and invasion abilities, which are supported by the upregulation of the genes related to metastasis and stemness.
These data suggest that blood shear stress not only eliminates CTCs with weak metastatic ability but also selects a subpopulation with metastatic advantages, highlighting the importance of blood shear circulation in metastasis.
ACKNOWLEDGEMENT: The study was financially supported by the NSFC Grant (Ref. No. 11672255) and the Internal Research Fund of the Hong Kong Polytechnic University (Ref. No. 1-ZE4Q).