Cellular plasticity is essential for healthy function during development and healing. However, it is the dysregulated plasticity of neoplastic cells that gives cancer its advantageous heterogeneity and resistance to therapy. Plasticity also endows cancer cells with the ability to mimic and aberrantly activate fundamental developmental processes such as collective cell migration (CCM) in the form of collective invasion, which initiates the metastatic cascade that is the primary cause for cancer-related deaths. Remarkably, the initiation of this process is strongly associated with changes in the extracellular matrix (ECM) environment of the tumor, which functions as both a physical barrier and dynamic signaling cue for cell migration. Namely, simultaneous increases in the density, stiffness, and alignment of Collagen I fibers in the tumor ECM mark a key transition. However, challenges associated with modeling this process limit our capacity for identifying new approaches that target these plastic cancer cells. In this dissertation, we develop ECM engineering and dynamic measurement techniques to enable study of the biophysical cell-cell and cell-matrix interactions that drive collective cancer cell invasion. We then use these newfound insights to develop a phenotypic screening model that successfully identifies a cytodifferentiation therapy of plastic Triple Negative Breast Cancer (TNBC) cells.
In chapter I, we advance a matrix engineering approach to tune Collagen I fibrillar architecture independently of known modulators of collective cell behavior such as ECM density and stiffness. We find that tighter fibril networks are less degradable and adhesive, inducing cellular confinement that prompts a switch from single cell migration to CCM. Chapter II integrates multiple modalities of biophysical imaging with in silico modeling to dissect the underlying cell-cell and cell-matrix interactions of the two distinct CCM behaviors that emerge from single populations of cancer cells in confining matrices: collective rotational migration and collective invasive migration. We find that the predominant phenotype of invasive CCM relies on a combination of proteolytic-based matrix remodeling and physical remodeling while the less common rotational CCM relies solely on mechanical-based matrix remodeling. Contrary to the current paradigm, we also find that epithelial characteristics such as cadherin-based cell-cell junctions or establishment of a Laminin-rich basement membrane are non-essential for either CCM behavior. In chapter III, we conduct a phenotypic screen of TNBC cells within confining Collagen I matrices to identify cytodifferentiation therapies targeting CCM, a trait that is associated with enrichment for Cancer Stem Cells (CSCs). Phenotypic screening identified two structurally similar leads that abrogate collective invasion, induce CSC cytodifferentiation and Mesenchymal-to-Epithelial Transition, and increase sensitivity to traditional chemotherapies. Importantly, this cytodifferentiation response is not observed in 2D-treated cells or in non-confining 3D matrices, highlighting our simple 3D phenotypic screening assay as an improved method for identifying relevant cytodifferentiation therapies for TNBC. Taken together, we provide a bottom-up description of how understanding and designing cell-matrix interactions can give rise to physiologically relevant CCM behaviors that provide a novel paradigm for identifying therapeutics that target cancer plasticity.
