Endothelial cell differentiation of human breast tumour stem/progenitor cells

Breast tumor stem cells have been reported to differentiate in the epithelial lineage but a cross-lineage potential has not been investigated. We aimed to evaluate whether breast tumor stem cells were able to differentiate also into the endothelial lineage. We isolated and cloned a population of breast tumor stem cells, cultured as mammospheres that expressed the stem markers nestin and Oct-4 and not epithelial and endothelial differentiation markers, and formed serially transplantable tumors in SCID mice. When cultured in the presence of serum, mammosphere-derived clones differentiated in the epithelial lineage. When cultured in the presence of VEGF, the same clones were also able to differentiate in the endothelial lineage acquiring endothelial markers and properties, such as the ability to organize in Matrigel into capillary-like structures. In the transplanted tumors, originated from mammospheres, we demonstrate that some of the intratumor vessels were of human origin, suggesting an in vivo endothelial differentiation of mammosphere-derived cells. Finally, endothelial cell clones originated from mammospheres were able, when implanted in Matrigel in SCID mice, to form after 7 days a human vessel network and, after 3-4 weeks, an epithelial tumor suggesting that in the endothelial-differentiated cells a tumorigenic stem cell population is maintained. In conclusion, the results of the present study demonstrate that stem cells of breast cancer have the ability to differentiate not only in epithelial but also in endothelial lineage, further supporting the hypothesis that the tumor-initiating population possesses stem cell characteristics relevant for tumor growth and vascularization.