Abstract
In breast cancer (BC), radial alignment of collagen fibers at the tumor-matrix interface facilitates collective invasion of cancer cells into the surrounding stromal matrix, a critical step toward metastasis. Collagen remodeling is driven by proteases and cellular forces, mediated by matrix mechanical plasticity, or irreversible matrix deformation in response to force. However, the specific mechanisms causing collagen radial alignment remain unclear. Here, we study collective invasion of BC tumor spheroids in collagen-rich matrices. Increasing plasticity to BC-relevant ranges facilitates invasion, with increasing stiffness potentiating a transition from single cell to collective invasion. At enhanced plasticity, cells radially align collagen at the tumor-matrix interface prior to invasion. Surprisingly, cells migrate tangentially to the tumor-matrix interface in a swirling-like motion, perpendicular to the direction of alignment. Mechanistically, swirling generates local shear stresses, leading to distally propagating contractile radial stresses due to negative normal stress, an underappreciated property of collagen-rich matrices. These contractile stresses align collagen fibers radially, facilitating collective invasion. The basement membrane (BM), which separates epithelia from stroma in healthy tissues, acts as a mechanical insulator by preventing swirling cells from aligning collagen. Thus, after breaching the BM, swirling of BC cells at the tumor-stroma interface radially aligns collagen to facilitate invasion.