Sustained postconfluent culture of human mammary epithelial cells enriches for luminal and c-Kit+ subtypes

A challenge in human mammary epithelial cell (HMEC) culture is sustaining the representation of competing luminal, myoepithelial, and progenitor lineages over time. As cells replicate in culture, myoepithelial cells come to dominate the composition of the culture with serial passaging. This drift in composition presents a challenge for studying luminal and progenitor cells, which are prospective cells of origin for most breast cancer subtypes. We demonstrate the use of postconfluent culture on HMECs. Postconfluent culture entails culturing HMECs for 2-5 weeks without passaging but maintaining frequent feedings in low-stress M87A culture medium. In contrast, standard HMEC culture entails enzymatic subculturing every 3-5 days to maintain subconfluent density. When compared to standard HMEC culture, postconfluent culture yields increased proportions of luminal cells and c-Kit+ progenitor cells. Postconfluent cultures develop a distinct multilayered morphology with individual cells showing decreased physical deformability as compared to cells in standard culture. Gene expression analysis of postconfluent cells shows increased expression of lineage-specific markers and extracellular matrix components. Postconfluent culture is a novel, useful strategy for altering the lineage composition of HMECs, by increasing the proportional representation of luminal and progenitor cells. We speculate that postconfluent culture creates a microenvironment with cellular composition closer to the physiological state and eases the isolation of scarce cell subtypes. As such, postconfluent culture is a valuable tool for researchers using HMECs for breast cancer research.