Cancer stem cells (CSCs) have been identified in a variety of tumour types, including breast tumours, and have been proposed to be responsible for recurrence, resistance to chemotherapy and, perhaps, metastasis of cancers. Targeting of these CSCs in the treatment of cancer is therefore of great interest. The ability of the ionophore antibiotic, salinomycin, to kill breast tumour CSCs was recently reported and now collaborators from the University of Michigan Comprehensive Cancer Center and the Centre de Recherche en Cancérologie de Marseille have identified a new potential drug target.
In this latest study the team identified overexpression of CXCR1, the receptor for interleukin-8 (IL-8), by the CSC subpopulation in a breast cancer cell line. Furthermore, addition of recombinant IL-8 increased the CSC population and enhanced the propensity for invasion. Conversely, use of CXCR1-blocking antibodies or repertaxin, a small-molecule CXCR1 antagonist, selectively depleted the breast CSCs both in vitro and in murine xenograft models.
CXCR1 blockade also induced massive apoptosis in bulk tumour cells, mediated by FASL/FAS signalling. The effects on CSC viability as well as FASL production were mediated by the focal adhesion kinase/AKT/forkhead transcription factor FKHRL1 (FAK/AKT/FOXO3A) pathway. Importantly, administration of repertaxin reduced tumour growth and the development of systemic breast cancer metastasis in NOD/SCID mice.
The authors of the study, published in the Journal of Clinical Investigation, suggest blockade of CXCR1 as a novel target for depletion of CSCs, potentially enhancing the efficacy of chemotherapeutic regimes.
There is growing evidence for a subpopulation of cells in breast cancer, known as breast cancer stem cells, that may be responsible for disease recurrence, resistance to conventional treatments, and perhaps metastasis. A team of US scientists has now shown that it is possible to selectively kill breast cancer stem cells (CSCs) using the ionophore antibiotic, salinomycin.
Patient-derived CSCs are difficult to collect and maintain in culture and a key aspect of the work was an ability to generate relatively homogenous and stable populations of CSC-like cells that could then be used for screening. Induction of epithelial-mesenchymal transition (EMT) in normal or neoplastic mammary epithelial cell populations had previously been shown to result in the enrichment of cells with stem cell-like properties and the researchers showed that normal and cancer cell populations experimentally induced into EMT also show increased resistance to chemotherapy drug treatment. Using two populations of mammary epithelial cells – one that had been induced to undergo EMT and one which had not – the team screened around 16,000 compounds and found that just 32 of them showed selective toxicity towards the cells that had undergone EMT. On the basis of potency and selectivity, salinomycin was chosen for further studies and was also found to decrease the proportion of CSCs that occur naturally as a subpopulation of breast cancer cells. Pre-treatment with salinomycin was found to decrease the tumour-seeding ability of cancer cell lines more than 100-fold compared with paclitaxel pre-treatment, and salinomycin also reduced mammary tumour size in mice to a greater extent than the chemotherapy drug, paclitaxel, or vehicle. Similar methodology could potentially be used to identify compounds that selectively kill CSCs from other types of cancer including leukemia, brain cancer, lung cancer, prostate cancer and melanoma, but it will be important to extend the findings to primary tumour cells from patients.
Salinomycin is used in farming to prevent coccidiodomycosis in poultry, and to alter gut flora in order to improve nutrient absorption in ruminants. The compound interferes with potassium transport across mitochondrial membranes, reducing intracellular energy production. It may also disrupt Na+/Ca2+ exchange in skeletal, and in some cases, cardiac muscle, allowing a fatal accumulation of intracellular calcium. Cases of animal poisoning with ionophore antibiotics have been widely described and the symptoms, which include progressive muscle weakening, appear to be similar to those seen following accidental exposure in man. Although the current study focussed largely on salinomycin, around 30% of the primary screen hits were confirmed on retesting, and expanding the screen may provide active compounds that may be more suitable for further development. The mechanism(s) by which salinomycin exhibits selective toxicity for CSCs is not known, but less toxic compounds that selectively target CSCs – but not normal stem cells – could find a place in future cancer therapies.
The study was published in the August 13th advance online issue of Cell.
Cancer stem cells are slowly dividing tumourigenic cells that possess characteristics of normal stem cells. It has been proposed that these cells persist in treated tumours and are responsible for tumour re-growth and metastasis. As a consequence, more effective chemotherapy may be achieved by targeting these cells in addition to the rapidly proliferating tumour cells. So far, the difficulty has been the lack of understanding of cancer stem cells and how they might be selectively targeted over normal stem cells.
In research carried out at The Jackson Laboratory in mice, Alox5 (5-Lipoxygenase) has been shown to be essential for development and maintenance of cancer stem cells in bcr-abl dependent chronic myeloid leukemia (CML). CML did not develop in mice without the Alox5 gene as a result of impaired function of the leukemia stem cells. Importantly, the lack of Alox5 did not affect normal stem cells, indicating distinct pathways in normal and leukemic stem cells for differentiation and self-renewal.
Alox5 is known to be essential for processing fatty acids to leukotrienes, key components of the inflammatory response, and inhibitors of Alox5 have been developed for the treatment of asthma. Using the approved (for asthma) Alox5 inhibitor, Zileuton, the scientists at The Jackson Laboratory were able to demonstrate a greater therapeutic effect in the CML model than the gold standard, Gleevec. Combining the two therapeutics provided an even better response.
Alox5 has also been shown to be over-expressed in certain cancers, for example in colon cancer. The availability of a clinically approved Alox5 inhibitor should enable a rapid progression to clinical studies in cancer.