Abstract
In cancers, there exists a subpopulation of cells which are referred to as cancer stem cells (CSCs) or tumor initiating cells that have enhanced tumor-initiating capacity and metastatic potential, and drive tumor progression. Since the initial identification of acute myeloid leukemia CSCs in 1997, CSCs have been found in many types of cancer and have intrinsic resistance to the current chemotherapeutic strategies. With increased levels of detoxifying enzymes, enhanced DNA repair abilities, impressive efflux capacity, and a slower cell-cycle; CSCs present a formidable obstacle against effective chemotherapy. Several methods of specifically targeting CSCs have been developed in recent years, and these compounds have potential as adjuvant therapies. The following is a review of the mechanisms responsible for chemoresistance in CSCs, with an emphasis on potential strategies to overcome this resistance.
Highlights
Introduction to Cancer Stem CellsFor many years, tumors had been thought of as monoclonal populations of rapidly dividing cells, and that all cells had equivalent cancer-initiating abilities
In breast cancer, cells sorted based on CD44+CD24- are enriched for cancer stem cells (CSCs), and high aldehyde dehydrogenase (ALDH) activity is found in this CSC-enriched population [3,13]
In breast cancer patient tumors, immunohistological staining determined that increased expression of ALDH1A1 and ALDH3A1 was found in tumors that did not respond to cyclophosphamide therapy, and in tumors that had undergone cyclophosphamide therapy [47]
Summary
Tumors had been thought of as monoclonal populations of rapidly dividing cells, and that all cells had equivalent cancer-initiating abilities. First identified by Bonnet & Dick as the tumor-initiating cells of acute myeloid leukemia, CSCs were later isolated from solid tumors by Al-hajj et al in breast cancer, as well as in brain tumours by Singh et al [2,3,4]. Since these seminal publications, CSCs have been isolated from many cancers, including colon, pancreatic, liver, and prostate, lung, head and neck, ovarian, and stomach cancers [5,6,7,8,9,10,11,12]. We will review the literature with regards to chemotherapeutics that CSCs are resistant to, mechanisms of CSC chemotherapeutic resistance and we discuss novel targeted therapies that are being developed which show efficacy towards killing CSCs
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