Table 1.
Phenotypic characteristics of breast cancer stem cells
| Factor | Characteristics | Reference(s) |
| Cell surface markers that are expressed by putative breast cancer stem cells | ||
| ABCG2 | ABCG2 (ATP-binding cassette G2) is a class of drug transporters capable of pumping cytotoxic drugs out of the cell. | [6] |
| CD44 | CD44 is involved in cellular adhesion, motility, and metastases. | [34,35] |
| CD10 | CD10 is a common acute lymphoblastic leukaemia antigen that is overexpressed on many tumours. | [39] |
| EpCAM/ESA | Epithelial cell adhesion molecule/Epithelial surface antigen is expressed on mammary tissue and tumours. | [39] |
| CD29 (β1-integrin) | CD29 is a membrane receptor involved in cell adhesion and metastatic diffusion of tumour cells. | [9] |
| CD49f (α6-integrin) | CD49f is involved in basal and endothelial cell distribution and is a candidate stem cell marker. | [39] |
| CD133 (prominin-1) | CD133 is a cell surface glyoprotein with an unknown function in cancer stem cells and its expression is documented for various types of cancer. | [116,117] |
| ALDH1 | Aldehyde dehydrogenase-1 plays a role in the differentiation of stem cells and its activity predicts poorer clinical outcomes. | [112] |
| CXCR4 | CXCR4 is a chemokine receptor involved in metastasis and its expression is increased in mammospheres. | [44,45] |
| ER | Oestrogen receptor is expressed on breast cancer cells, mammary progenitors, and breast cancer stem cells. | [54,59] |
| Signalling pathways that play a role in cancer stem cells | ||
| Delta/Notch pathway | This pathway is involved in cell fate development and is expressed in stem cells and early progenitor cells. | [60] |
| Notch-4 | Notch-4 plays a role in mammary development and its overexpression has been shown to promote mammary tumours. | [45,60,61] |
| Wnt signalling pathway | This pathway is involved in stem cell self-renewal and its overexpression can lead to epithelial and mammary tumours. | [27,63] |
| β-catenin | β-catenin is a downstream target of the Wnt pathway. A pro-oncogenic role has been described. | [63] |
| Hedgehog/Patched pathway | This pathway is involved in embryonic growth and cell fate determination. | [64] |
| PITCH | A receptor for the Hedgehog signalling family, PITCH has been connected to early embryonic tumourigenesis. | [64] |
| EGFR | Epidermal growth factor receptor signalling has been found to be upregulated in breast cancer stem cells and may be required for mammosphere formation. | [77,111] |