Friction-induced budding of a cancer cell monolayer

The environment surrounding a tumor plays a crucial role in cancer cell dissemination. Within this microenvironment, cancer-associated fibroblasts (CAFs) generate compressive forces and actively remodel tumors. Using in vitro circular clusters of cancer cell monolayers surrounded by CAFs, we generate structures that are reminiscent of multicellular buds observed in vivo for colo-rectal cancer. A supracellular contractile ring spontaneously assembles at the inner edge of the CAF monolayer and drives its closure on top of the cancer cells through a purse-string mechanism. The frictional shear stress exerted by CAFs triggers multilayering of cancer cells, followed by the emergence of a multicellular bud constricted by the CAF ring. To explain this observation, we developed a theoretical model based on continuum mechanics. This model outlines the early transformations in the shape of cancer cell monolayer and links the layering of cells to a general criterion involving height deformation. It identifies the specific physical conditions that favors budding, and reproduces the observed dependence of the budding probability and bud sizes with the diameter of the cancer cell cluster. Our findings highlight the importance of active mechanical interactions between the tumor and its micro-environment on aggressive modes of cancer invasion.