Breast cancer chemoresistance: emerging importance of cancer stem cells

Cancer stem cells (CSCs) have recently been documented in solid tumours. Evidence has suggested that CSCs are involved in carcinogenesis, tumour invasion and metastases, and resistance to various forms of therapies, including chemotherapy. Breast CSCs are characterised by the expression of CD44 but lack of CD24 (CD44(+)/CD24(-) cells). The mechanisms involved in chemoresistance of breast CSCs are complex and not clearly defined. Overexpression of ABC transporters, detoxification enzymes (aldehyde dehydrogenase), low cell turn over rate and the ability to activate the DNA check point response are possibly all involved. Innovative therapies, based on a better understanding of CSCs, should lead to enhanced and long-term cure rates in breast cancer.