Fibronectin assembly regulates lumen formation in breast acini

Fibronectin (FN) is an extracellular matrix (ECM) glycoprotein that self‐assembles into FN fibrils, forming a FN matrix contributing to the stiffness of the ECM. Stromal FN stiffness in cancer has been shown to impact epithelial functions such as migration, cancer metastasis, and epithelial‐to‐mesenchymal transition. The role of the FN matrix of epithelial cells in driving such processes remains less well understood and is the focus of this study. Hypoxia, defined by low oxygen tension (<5%) is one of the hallmarks of tumor microenvironments impacting fibril reorganization in stromal and epithelial cells. Here, using the MCF10 breast epithelial progression series of cell lines encompassing normal, preinvasive, and invasive states, we show that FN fibril formation decreases during hypoxia, coinciding with a decrease in migratory potential of these cells. Conversely, we find that FN fibril disruption during three‐dimensional acinar growth of normal breast cells resulted in acinar luminal filling. Our data also demonstrates that the luminal filling upon fibril disruption in untransformed MCF10A cells results in a loss of apicobasal polarity, characteristic of pre‐invasive and invasive breast cell lines MCF10AT and MCF10 DCIS.com. Overall this is the first study that relates fibril‐mediated changes in epithelial cells as critical players in lumen clearing of breast acini and maintenance of the untransformed growth characteristic.