Clinically, increased breast tumor stiffness is associated with metastasis and poorer outcomes. Yet, in vitro studies of tumor cells in 3D scaffolds have found decreased invasion in stiffer environments. To resolve this apparent contradiction, MDA-MB-231 breast tumor spheroids were embedded in 'low' (2 kPa) and 'high' (12 kPa) stiffness 3D hydrogels comprised of methacrylated gelatin/collagen I, a material that allows for physiologically-relevant changes in stiffness while matrix density is held constant. Cells in high stiffness materials exhibited delayed invasion, but more abundant actin-enriched protrusions, compared to those in low stiffness. We find that cells in high stiffness had increased expression of Mena, an invadopodia protein associated with metastasis in breast cancer, as a result of EGFR and PLCγ1 activation. As invadopodia promote invasion *

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