Isolation and in vitro propagation of tumorigenic breast cancer cells with stem/progenitor cell properties

Breast cancer-initiating cells have been recently identified in breast carcinoma as CD44⁺/CD24^-/low cells, which exclusively retain tumorigenic activity and display stem cell-like properties. However, at present, direct evidence that breast cancer-initiating cells can be propagated in vitro is still lacking. We report here the isolation and in vitro propagation of breast cancer-initiating cells from three breast cancer lesions and from an established breast carcinoma cell line. Our breast carcinoma-derived cultures encompassed undifferentiated cells capable of self-renewal, extensive proliferation as clonal nonadherent spherical clusters, and differentiation along different mammary epithelial lineages (ductal and myoepithelial). Interestingly, cultured cells were CD44⁺/CD24^- and Cx43^-, overexpressed neoangiogenic and cytoprotective factors, expressed the putative stem cell marker Oct-4, and gave rise to new tumors when as few as 10³ cells were injected into the mammary fat pad of SCID mice. Long-term cultures of breast tumorigenic cells with stem/progenitor cell properties represent a suitable in vitro model to study breast cancer-initiating cells and to develop therapeutic strategies aimed at eradicating the tumorigenic subpopulation within breast cancer.