Stem cells in human breast development and cancer

The epithelial components of the breast are thought to arise from a stem cell population that is capable of both self-renewal and lineage-specific differentiation. We and others have hypothesized that mammary stem cells or their immediate progeny are targets for transformation during carcinogenesis. Normal stem cells and carcinoma cells share many characteristics including self-renewal capacity, telomerase expression, ability to differentiate, resistance to apoptosis, and ability to home to specific sites. Mammary transformation may require dysregulation of pathways that control normal stem cell self-renewal such as Notch, Wnt, Hedgehog, and Bmi-1. In order to study these pathways in normal mammary development, we have developed an in vitro culture system in which primary human epithelial cells isolated from reduction mammoplasties are cultured as 'mammospheres' on non-adherent surfaces. Cells within mammospheres are able to self-renew, as well as to differentiate into all the lineages found in the mammary gland. Utilizing this system, we demonstrate bidirectional interaction between Notch and Hedgehog signaling and Bmi-1 in the regulation of stem cell self-renewal. When mammospheres are admixed with irradiated human mammary fibroblasts and implanted into the cleared fatpads of NOD/SCID mice, they are able to reconstitute the ductal alveolar structures found in the human mammary gland.

The stem cell model of carcinogenesis may also provide a partial explanation for the generation of cellular heterogeneity seen within mammary tumors. Using flow cytometry, we have identified a small population of cells within primary or metastatic breast cancers that bear the cell surface phenotype ESA⁺CD44⁺CD24^-/lowLineage^- that have the properties of human tumor stem cells. As few as 200 of these cells are able to reproducibly generate tumors in NOD/SCID mice, while the vast majority of cells in these tumors that lack this phenotype are incapable of tumor formation even when tens of thousands of cells are injected. Consistent with a stem cell model, tumorigenic cells generate tumors that recapitulate the phenotypic heterogeneity found in the original tumors. We have demonstrated that pathways that control normal stem cell self-renewal, such as Hedgehog, are activated in mammary tumor stem cells, compared with their differentiated progeny. Despite progress in breast cancer therapeutics, metastatic breast cancer remains an incurable disease. Current therapies that have been developed by virtue of their ability to induce tumor regression may selectively target more differentiated cells in tumors, while leaving the tumor stem cell population intact, accounting for treatment resistance and relapse. Multiple mechanisms may account for this resistance to apoptosis, including increased expression of anti-apoptotic genes, increased DNA repair mechanisms, and transporter proteins such as BCRP found in the tumor stem cell population. The targeting of stem cell self-renewal pathways such as Hedgehog or Notch may thus provide a novel and more effective approach for the treatment of advanced breast cancer.