| In this guest post, Kate Whelan looks into the history of cancer stem cells (CSCs) and addresses how over the past decade, a great deal of research into CSCs and the CSC concept has revealed new information that has enabled scientists to explore new potential cancer therapies, and even investigate innovative new approaches to cancer treatments. |
Recent developments in the field, including high throughput screening assays that have been made to test a variety of potential therapies against these cells. These studies are simultaneously providing a wealth of new insights relating to the molecular mechanisms of cancer and cell biology of stem cells.
The idea that cancer originates from adult stem cells was initially put forward in the mid-nineteenth century, and as stem cell research developed during the late twentieth century, scientists realized that stem cells and cancer cells share significant properties [Lapidot et al. 1994; reviewed in Behbod & Rosen, 2004]. This led to development of the ‘cancer stem cell concept’ [Reya et al. 2001], which proposes that a specific subpopulation of highly malignant stem cells exist within a tumor [Al-hajj et al. 2003; Singh et al. 2003; Stingl & Caldas 2007].
These cancer stem cells (CSCs) share features that distinguish stem cells from other types of cells, including the ability for infinite self-renewal, through which they are responsible for the tumor growth and for driving metastasis. CSCs and stem cells also share similar signalling pathways that regulate self-renewal.
Further research over the past decade has characterized CSCs and established some important differences between these cells and stem cells. CSCs behave differently from normal stem cells, particularly with regards to their malignant phenotype, and this is largely due to changes in regulation of CSCs’ self-renewal capability. These include differences in cell division, cell cycle properties, and handling of DNA damage; changes in the activation and inactivation of cancer-specific molecular pathways are also thought to contribute to this phenotype [reviewed by Al-Hajj & Clarke 2004; Falzacappa et al. 2012].
A hierarchy of different sub-populations of genetically distinct cell types has now been identified within tumors and these different sub-types contribute to the complex processes of tumor growth, relapse and metastasis [see Baccelli & Trumpp 2012 for review]. CSCs are also thought to play important roles in tumor initiation, angiogenesis and cancer maintenance [Zhao et al. 2012].
The unique properties of CSCs have been well documented, including their specific ability to generate tumors the ability to recreate any of the cell types from the parent tumour; and the expression of a distinct set of surface bio-markers [reviewed by Maenhaut et al. 2010]. A crucial finding is that CSCs are resistant to current chemo- and radiotherapies, causing cancers to recur after treatment [Dean et al. 2005; reviewed in Gupta et al. 2009]. Significantly, the resistance of CSCs can mean that some treatments actually cause strong selection for CSC survival and expansion [Fillmore & Kuperwasser 2008; Gupta et al. 2009]. It has become critical for scientists to find new therapies that can successfully target CSCs as well as differentiated cancer cells.
Clinical implications and challenges
The CSC concept has been broadly accepted over the past decade or so, however more recent research has identified new information that highlights some challenges which need to be better understood before clinical applications can be developed [reviewed by Visvader & Lindeman 2012; Zhou et al. 2012]. For example, genetically and phenotypically distinct subpopulations of CSCs can exist within a tumour, leading to questions about the behaviour or roles of the different cell types. Furthermore, the phenotype of CSCs between patients is also variable. In several studies, researchers have been able to correlate CSCs and specific patterns of markers with tumour aggressiveness, thus aiding in prognosis [Zhao et al. 2012].
It also seems that tumor cells may undergo reversible phenotypic changes, and it is possible that non-stem cells within the tumor environment can be converted into CSCs [Vermeulen 2012]. This adds considerable confusion and complexity for researchers investigating new therapies to target CSCs and differentiated tumor cells, and underlines the need for a therapy (or more likely, a combination of therapies) that could target all of a tumor sub-populations of cells, across patient groups, with minimal adverse effects to a patient’s normal cells.
Developing new cancer therapies targeting CSCs
Researchers in drug discovery laboratories routinely screen potential new drugs by using high throughput assays that allow thousands of compounds to be tested rapidly and simultaneously for their effects on molecular targets. Frequently, these are carried out in microtitre plates that each contain 1,536 or 3,456 wells, where every well is an individual, miniaturized test. Robotic instruments are usually employed to carry out the assays, since automated liquid handlers can more reliably dispense the very small volumes that are typically required for such assays (often in the nanolitres or few microliters range). Automated platforms can be integrated across an entire laboratory workflow, such that hundreds of microplates are continuously fed through the assay and analysis steps, with minimal manual intervention.
However, this approach has been problematic for researchers trying to screen potential new compounds against CSCs. In general, stem cells are notoriously difficult to work with in many laboratory applications, being quite fragile, requiring specialist growth media and different culturing conditions from more robust cell lines. Furthermore, stem cells can cause blockages in automated dispensers, resulting in inconsistent or inaccurate results and instrument downtime. For CSCs, perhaps the most significant problem is their limited availability. Large-scale screening requires significant supplies of stable and highly enriched populations of cells which are difficult to generate from primary tissue, particularly as CSCs often only make up a small subpopulation within a tumour [Gupta et al. 2009].
Over the past few years, new methods have been developed for identifying potential CSC-targeting compounds. These include the development of a new strategy that identifies chemical compounds that specifically target breast CSCs [Gupta et al. 2009], using immortalized mammary epithelial cells. Salinomcyin was found to be particularly successful in targeting breast CSCs, and while the molecular mechanisms have yet to be fully investigated, this offers valuable insights into CSC development. Furthermore, the approach also demonstrated that searching for agents that target specific states of cancer cell differentiation would also be a viable alternative to screening directly against CSCs.
It has been shown more recently that it is possible to generate CSCs from cancer cell lines, in sufficient quantities for high throughput screening [Mathews et al. 2012]. In addition, innovative microplate dispensers have been developed that can reliably dispense stem cells (eg, Preddator). These advances are allowing researchers to test compounds that might be effective in killing CSCs, and are likely to provide valuable new information about the molecular mechanisms and pathways involved.
Another approach to developing successful new cancer treatments involves targeting functional pathways that CSCs rely on. However, since normal stem cells and CSCs share many of the same pathways, it is important to ensure that normal stem cells would not be adversely affected by treatments targeting CSCs. Researchers have successfully demonstrated that inhibitors of the hedgehog, notch, and Hox family pathways have eliminated CSCs in certain tumour types [reviewed in Zhao et al. 2012], and some of these initial studies have now progressed to phase I and II clinical trials.
Elsewhere, researchers have been investigating ways to target the regulation of CSC behaviour and gene expression, such as through manipulation of microRNA expression. MicroRNAs are known to be important in regulation of normal stem cells’ renewal and differentiation. MicroRNAs have the potential to be able to effectively target CSCs in their diverse stem cell-like states, so having the advantage of being able to target different sub-types of CSCs simultaneously [reviewed in Zhou et al. 2012]. Further research needs to be carried out in order to understand more about how microRNAs regulate stem cells and CSCs, and their wider effects on normal cells.
Current cancer treatments are largely focused on eliminating differentiated tumor cells, and it is likely that the resistance of CSCs to such treatments accounts for many patients’ cancer recurrence and metastasis. Over the past decade, a great deal of research into CSCs and the CSC concept has revealed new information that has enabled scientists to explore new potential cancer therapies, and even investigate innovative new approaches to cancer treatments. A therapy, or combination of therapies, that can successfully eliminate all CSC subtypes from a cancer patient may well lead to dramatic improvements in cancer survival rates.
References
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Al-hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF (2003). Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 100: 3983-3988.
Baccelli I, Trumpp A (2012). The evolving concept of cancer and metastasis stem cells. J Cell Biol 198(3): 281-293.
Behbod F, Rosen JM (2004). Will cancer stem cells provide new therapeutic targets? Carcinogenesis 26(4): 703-711.
Dean M, Fojo T, Bates S (2005). Tumour stem cells and drug resistance. Nature Rev Cancer 5: 275-284.
Falzacappa MV, Ronchini C, Reavie LB, Pelicci PG (2012). Regulation of self-renewal in normal and cancer stem cells. FEBS J 279(19): 3559-3572.
Fillmore CM, Kuperwasser C (2008). Human breast cancer cell lines contain stem-like cells that self-renew, give rise to phenotypically diverse progeny and survive chemotherapy. Breast Cancer Res 10: R25.
Gupta PB, Onder TT, Jiang G, Tao K, Kuperwasser C, Weinberg RA, Lander ES (2009). Identification of selective inhibitors of cancer stem cells by high-throughput screening. Cell 138: 645-659.
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Maenhaut C, Dumont JE, Roger PP, van Staveren WC (2010). Cancer stem cells: a reality, a myth, a fuzzy concept or a misnomer? An analysis. Carcinogenesis 31(2): 149-158.
Mathews LA, Keller JM, Goodwin BL, Guha R, Shinn P, Mull R, Thomas CJ, de Kluyver RL, Sayers TJ, Ferrer M (2012). A 1536-well quantitative high-throughput screen to identify compounds targeting cancer stem cells. J Biomol Screen 17(9): 1231-1242.
Reya T, Morrison SJ, Clarke MF, Weissman IL (2001). Stem cells, cancer, and cancer stem cells. Nature 414(6859): 105-111.
Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, Dirks PB (2003). Identification of a cancer stem cell in human brain tumours. Cancer Res 63: 5821-5828.
Stingl J, Caldas C (2007). Molecular heterogeneity of breast carcinomas and the cancer stem cell hypothesis. Nat Rev Cancer 7: 791-799.
Vermeulen L, de Sousa e Melo F, Richel DJ, Medema JP (2012). The developing cancer stem cell model: clinical challenges and opportunities. Lancet Oncol 13: e83-e89.
Visvader JE, Lindeman GJ (2012). Cancer stem cells: current status and evolving complexities. Cell Stem Cell 10(6): 717-728.
Zhao L, Zhao Y, Bao Q, Niess H, Jauch K-W, Bruns CJ (2012). Clinical implications of targeting of stem cells. Eur Surg Res 49: 8-15.
| By Kate Whelan | Subscribe to 33rd Square |
Author Bio - Kate Whelan (née Rhodes) gained her PhD and three years’ post-doctoral research in spinal cord injury, working with Prof James Fawcett at the Cambridge Centre for Brain Repair, University of Cambridge, UK. She has written this post on behalf of Redd And Whyte - Producers of the Preddator Microplate Dispenser
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