Cell softness regulates tumorigenicity and stemness of cancer cells

Identifying and sorting highly tumorigenic and metastatic tumor cells from a heterogeneous cell population is a daunting challenge. Here, we show that microfluidic devices can be used to sort marker-based heterogeneous cancer stem cells (CSC) into mechanically stiff and soft sub-populations. The isolated soft tumor cells (<400 Pa) but not the stiff ones (?700 Pa) can form a tumor in immunocompetent mice with 100 cells per inoculation. Notably, only the soft, but not the stiff cells, isolated from CD133+, ALDH+ or side population CSCs, are able to form a tumor with only 100 cells in NOD-SCID or immunocompetent mice. The Wnt signaling protein BCL9L is upregulated in soft tumor cells and regulates their stemness and tumorigenicity. Clinically, BCL9L expression is correlated with a worse prognosis. Our findings suggest that the intrinsic softness is a unique marker of highly tumorigenic and metastatic tumor cells.