Interfacial Stress Regulates Plasticity and Drug Resistance at the Breast Cancer-Host Interface.

The confinement of breast cancer cells at the interface of the mammary gland lumen and its surrounding extracellular matrix is thought to be a key physical driver of cellular plasticity. The relationship between confinement-induced solid stress and drug resistance is not well understood due to a scarcity of models that faithfully isolate the contribution of confinement on cancer cell behavior. Herein, drop-on-demand printing is used to uniquely replicate the spatial compartmentalization of the native cancer-host interface: MCF-7 breast cancer cells are dispensed within bioinert cup-shaped alginate-based hydrogels in high-throughput to yield "confined" spheroids. Hydrogel confinement affects the emergence of CD44+-CD133+ cells at the spheroid interface that drives a two-fold increase in doxorubicin/tamoxifen resistance compared to control "unconfined" spheroids. Whilst the peripheral drug-resistant phenotype is observed clinically, the model is unique in facilitating the emergence of this population in an in vitro setting. Pharmacological modulation of mechanotransduction proteins (YAP, myosin), abrogated the emergence of this peripheral phenotype, implicating mechanotransduction pathways as an effective way to target solid stress-induced drug resistance. Together, this supports an "interfacial stress-stemness-drug resistance" relationship that sheds new light on the existing paradigm of spatial emergence of drug resistance in breast cancer.