Regulation of human breast epithelial stem cells

Breast epithelial stem cells are thought to be the primary targets in the aetiology of breast cancer. As breast cancers are predominantly oestrogen and progesterone receptor-positive (ERα/PR+), we investigated the biology of ERα/PR+ cells and their relationship to stem cells in normal human breast epithelium. Several complementary approaches were used to characterize the stem-cell population and relate it to ERα/PR + cells, including dual label colocalization on tissue sections, isolation of a Hoechst dye-effluxing 'side population' using flow cytometry, and examination of DNA label retention. The intermediate or suprabasal population suggested by others to be breast stem cells comprises ERα/ PR+ cells that coexpress the putative stem-cell markers including cytokeratin 19. Human breast epithelial cells with Hoechst dye-effluxing 'side population' properties characteristic of mammary stem cells in mice were demonstrated by lack of expression of myoepithelial and luminal cell-specific antigens such as CALLA and MUC1 to be undifferentiated cells. Using DNA radiolabelling of human tissue implanted into athymic nude mice, a population of label-retaining putative stem cells (LRC) were shown to be enriched for cells expressing the putative stem-cell markers p21CIP1/WAF1 and Musashi-1, which, interestingly, were expressed in separate subpopulations of ERα/PR+ cells. Finally, expression patterns of Musashi-1 and Notch-1 in relation to ERα/PR+ and adjacent proliferating cells suggest that the evolutionarily conserved Delta/Notch signalling pathway regulates asymmetric division of the putative stem-cell population. The data suggest a model in which ERα/PR+ cells scattered through the epithelium are stem cells that self-renew through asymmetric cell division and generate patches of transit amplifying and differentiated cells. ERα/PR+ breast cancers exhibit loss of the two key regulators of asymmetric cell division, Musashi1 and Notch-1 and thus may arise from symmetric division of the ERα/PR+ stem cell.