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Dysfunctional progenitor and luminal cells with acquired basal cell properties accumulate during human mammary epithelial aging for reasons not understood. Multipotent progenitors from women aged

Summary: Tumor microenvironments are a driver of resistance to anti-cancer drugs. Dissecting cell-microenvironment interactions into tractable units of study presents a challenge. Here, we assess the impact of hundreds of tumor-inspired microenvironments, in parallel, on lapatinib responses in four cancer cell lines. Combinations of ECM and soluble factors were printed on stiffness-tunable substrata to generate a collection of controlled microenvironments in which to explore cell-based functional responses. Proliferation, HER2 protein expression and phosphorylation, and morphology were measured in single cells. Using dimension reduction and linear modeling, the effects of microenvironment constituents were identified and then validated empirically. Each of the cell lines exhibits unique microenvironment-response patterns. Fibronectin, type IV collagen, and matrix rigidity are significant regulators of lapatinib resistance in HER2-amplified breast cancer cells. Small-molecule inhibitors were identified that could attenuate microenvironment-imposed resistance. Thus, we demonstrate a strategy to identify resistance- and sensitivity-driving microenvironments to improve the efficacy of anti-cancer therapeutics. : Tumor microenvironments are a driver of anti-cancer drug resistance. Lin et al. use microenvironment microarrays and a cell-based functional approach to identify microenvironment components that modulate lapatinib responses in isogenic cells. They demonstrate a strategy to identify resistance- and sensitivity-driving microenvironments that may improve the understanding and efficacy of anti-cancer therapeutics. Keywords: microenvironment, microenvironment microarray, MEMA, HER2, lapatinib, verteporfin, AZD0530, YAP, fibronectin, breast cancer, drug resistance