Apoptosis resistance plays an important role in breast cancer progression and metastasis and is critical for the pathogenesis of therapeutic resistance. Research aimed at understanding the molecular basis for chemotherapy and radiation resistance has traditionally viewed breast cancer as a cell autonomous disease. Yet, epigenetic phenomena such as cell shape and the organizational features of a tissue, as induced by the extracellular matrix, are known to affect gene expression to regulate the malignant phenotype and to promote tumor behavior. The chemical composition and the three-dimensional spatial organization of the matrix dictate its mechanical properties and apoptosis responsiveness of mammary epithelial cells to exogenous death cues (Weaver et al., Cancer Cell 2002). Moreover, breast cancers often display heterogeneity in tissue stiffness and response to therapy at the primary site, and breast cancers metastasize to multiple tissues that each has distinct mechanical properties. Therefore, an investigation was conducted to determine whether matrix stiffness could modulate apoptosis resistance of mammary epithelial cells (MECs). In a compliant three-dimensional matrix, we showed that mammary tumors have enhanced autocrine laminin-5 and α6β4 integrin expression, which stimulate Rac and NFκB activity to drive apoptosis resistance. Moreover, we showed that mammary tumorigenesis is coincident with an increase in tissue stiffness that is associated with enhanced collagen deposition. We then determined that tumor-associated mammary tissue stiffness could promote β1 integrin-associated focal adhesions to alter cell signaling through ERK and Rho to enhance cell growth and disrupt MEC morphogenesis. We finally demonstrated that matrix stiffness could promote JNK activity and apoptosis induction in response to chemotherapeutics and radiation. We could show that inhibiting JNK increases survival of MECs on a stiff matrix treated with various chemotherapies and that activating JNK enhances apoptosis of MECs on a soft matrix. Collectively, these data illustrate the underlying mechanisms by which ECM stiffening during tumor progression could promote the malignant phenotype, and how the mechanical properties of pathological breast tissue or different sites of metastasis could affect therapeutic efficacy.
