Tumor‐Tissue Boundaries as Instructive Interfaces in Breast Cancer Cell Invasion

The ability of metastatic cancer cells to invade distant tissues requires them to cross a variety of tissue boundaries, each posing distinct structural and biochemical challenges. In particular, the boundary between dense, extracellular matrix (ECM)‐rich tumor tissue and surrounding stromal tissue is associated with phenotypic changes in MDA‐MB‐231 breast cancer cells following transmigration, including increased invasiveness and aggressiveness. It remains unclear whether this transition arises from selective, permissive filtering of pre‐existing subpopulations, such as cancer stem cells, or an instructive response of the entire cell population. Here, by combining single‐cell migration analysis, heterogeneity analysis of cell proliferation, and computational modeling, it is demonstrated that tumor‐tissue boundaries act as instructive interfaces. Using an established 3D fibrillar collagen I matrix model of interfaces, it is shown that all cells can transmigrate the interface with no evidence of selective filtering of subpopulations. Proliferation heterogeneity remains unchanged between transmigrated and non‐transmigrated cells, further supporting an instructive mechanism. Simulations confirm that the interface instructively modulates cell behavior. These results indicate that tissue boundaries can reprogram cancer cell phenotypes, representing a potentially targetable mechanism in metastatic progression.