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The role of cancer stem cells in cancer metastasis: New perspective and progress
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Cancer Epidemiology The International Journal of Cancer Epidemiology, Detection, and Prevention journal homepage: www.cancerepidemiology.net
The role of cancer stem cells in cancer metastasis: New perspective and progress Xiaofeng Wang a,1, Yuzhen Zhu b,1, Yushui Ma a,1, Jin Wang c, Feng Zhang a, Qing Xia a,*, Da Fu a,c,** a
Department of Orthopaedics, Zhongshan Hospital, Fudan University, Shanghai 200032, China Department of Gastroenterology, Tongji Hospital, Tongji University, Shanghai 200065, China c The Key Laboratory of Stem Cell Biology, Institute of Health Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200025, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 May 2012 Received in revised form 15 July 2012 Accepted 19 July 2012 Available online xxx
Recent studies have identified the important role of cancer stem cells (CSCs) in carcinogenesis and relapse. However, with respect to multistage cancer metastasis, the role of CSCs has not been welldefined. In several human cancers, data showed that some phenotypic subsets of CSCs were responsible for cancer metastasis. In this review, we surveyed recent advances in the role and mechanism of metastatic CSCs. ß 2012 Elsevier LtdPublished by Elsevier Ltd. All rights reserved.
Keywords: CSCs Metastasis EMT MicroRNAs Angiogenesis
1. The basic procedure of metastatic cascade Metastasis, frequently a final and fatal step in the progression of solid malignancies, encompasses several fundamental biological processes: cancer initiation, epithelial–mesenchymal transition (EMT), breach of the basement membrane barrier, neighbor invasion, intravasation, mesenchymal–epithelial transition (MET), extravasation, colonization and outgrowth of micrometastases and secondary cancer [1]. In fact, the evolution of metastatic cascade is a dynamic process influenced by unique cellular lineages, altered microenvironments, distinct anatomical restrictions and multiple genetic and epigenetic alterations [2]. A number of studies have attempted to address the question of cellular heterogeneity within cancer tissues and its implications for the selection of metastatic cancer cells. Luzzi et al. [3] traced the
Abbreviations: CSCs, cancer stem cells; ALDH, aldehyde dehydrogenases; AdCC, adenoid cystic carcinoma; HCC, hepatocellular carcinoma; EMT, epithelial– mesenchymal transition; SDF-1, stromal cell derived factor 1; CXCR-4, chemokine C-X-C motif receptor 4; VEGF, vascular endothelial growth factor. * Corresponding author. ** Corresponding author at: The Key Laboratory of Stem Cell Biology, Institute of Health Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200025, China. Tel.: +86 21 63846925; fax: +86 21 63846925. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Zhu), [email protected] (Y. Ma), [email protected] (J. Wang), [email protected] (F. Zhang), [email protected] (Q. Xia), [email protected] (D. Fu). 1 These authors contributed equally to this work.
fate of B16F1 melanoma cells injected intraportally into murine livers. Although 80% of the injected cells survived and extravasated at day 3, only a small number of these cells (1 in 40) formed micrometastases, and only 1 in 100 micrometastases continued to grow into macrometastases, which suggested that the metastatic action of caner appears inefficiently. 2. Migrating cancer stem cells (CSCs) Why so few cancer cells successfully navigate the multistep metastatic process? The precise nature of these cancer cells remains undefined. However, some properties of CSCs, a subpopulation of cells that display stem cell properties, mediate metastasis, and contribute to treatment resistance, suggest them as candidates for mediating metastatic progression. First, it is theoretically possible that only CSCs within cancer tissues have the ability to initiate and sustain cancer growth. Therefore, even if nonCSCs migrate (which is likely, given the number of cancer cells that can be detected in the blood), they would not be able to propagate into heterogeneously diverse metastatic lesions [4]. Furthermore, the inherent plasticity of stem cells makes CSCs more adaptable to survive in a foreign environment where growth factors and other signaling molecules are different from those in the primary caner site [4]. Also cellular plasticity in stem cells may facilitate EMT, which has been postulated as a key event during the early phase of cancer metastasis [5]. Brabletz et al. [6] firstly establish the hypothesis of migrating CSCs, which possess both an element of stemness and mobility. It is postulated that these cells undergo EMT at the invasive front of the
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Please cite this article in press as: Wang X, et al. The role of cancer stem cells in cancer metastasis: New perspective and progress. Cancer Epidemiology (2012), http://dx.doi.org/10.1016/j.canep.2012.07.007
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primary cancer and migrate to colonize new tissues, where its acquired stemness to facilitate production of the cancer heterogeneity commonly observed in metastatic colonies [6]. The migrating cancer stem cell hypothesis is attractive owing to its integration of primary cancer initiation, progression and metastasis. 3. Metastatic capacity of CSC subpopulations characterized by different markers Although CSCs have been identified in many types of cancer and the migrating cancer stem cell hypothesis has gained more attention in recent years, the issues that whether metastatic competence is inherent to all CSCs is still outstanding. Several studies have defined rare CSCs subpopulations which have distinct metastatic competence. Roberta et al. [7] identified a subpopulation of CD26+ CSCs uniformly present in both the primary and metastatic tumors in colorectal cancer patients with liver metastasis. Furthermore, circulating CD26+ CSCs were capable of forming metastatic growth in the liver after injected into mouse cecal wall. In patients without distant metastasis at the time of presentation, the presence of CD26+ cells in their primary tumors predicted distant metastasis on follow-up [7]. Using a molecular tracking strategy, Dieter et al. [8] identified a types of extensively self-renewing longterm tumor-initiating cell (LT-TICs) with specific roles in cancer metastasis, which suggested the existence of a subpopulation of cancer cells with stem cell properties endowed with metastatic capacity on malignant progression in colorectal cancer. Aldehyde dehydrogenases (ALDHs) are a group of enzymes that catalyze the oxidation of aldehydes [9]. Recent studies demonstrated that high level of ALDH activity could serve as a putative CSCs maker and its presence strongly correlates with tumor malignancy as well as self-renewal properties of CSCs in various human cancers. Intriguingly, evidences show that ALDH+ human breast cancer MDA-MB-231 cells with stem-like properties are responsible for the bone metastasis [10]. Sun and Wang [11] found that a subpopulation of cells expressing high level of ALDH activity residing in human adenoid cystic carcinoma (AdCC) possessed enhanced invasive potential in vitro and highly metastatic capability in vivo, which suggesting ALDH+ CSCs are responsible for mediating AdCC metastasis. CD133 is one of the most representative cancer stem cell markers. Woo et al. [12] found that CD133 high expressers in stage I lung adenocarcinomas (ADC) show a significantly higher risk of recurrence than CD133 low expressers. CD133 high expressers having strong proliferating activity and/or with vessel invasion show a higher risk of recurrence, which suggested the level of CD133 expression is an independent prognostic marker and its combination with proliferating activity and/or vessel invasion could have excellent prognostic value to predict postoperative recurrence in patients with stage I lung ADC. Lingala et al. [13] co-stained hepatocellular carcinoma (HCC) specimens with anti-CD133 and anti-ALDH monoclonal antibodies, and found that doubly positive HCC cell clusters were colocalized in the areas adjacent to connective tissue and within invaded vessels, which suggested that CD133+/ALDH+ CSCs are highly metastatic. 4. EMT and MET in migrating CSCs EMT and MET are recognized as critical events for metastasis of carcinomas. The former are responsible for degrading the surrounding matrix to lead the way of invasion and intravasation and the latter is important for cancer cells then enter the blood stream and reestablish colonies in the secondary sites [14]. Recent studies suggested that CSCs normally have characteristics
associated with mesenchymal cells and also play a critical role in tumor initiation, growth, metastasis [15]. Migrating CSCs are located predominantly at the tumor-host interface and are derived from stationary CSCs through the acquisition of a transient EMT phenotype in addition to stemness [6]. EMT endows human mammary epithelial cells with CSCs-like properties which characterized by their CD44high/CD24low phenotype [16] through up-regulating Mena, a member of the Ena/VASP family which plays a role in cell migration [17]. Intriguingly, many of circulating tumor cells expressing Mena display a mesenchymal phenotype which indicates EMT, and also acquired a breast cancer stem cell CD44+/ CD24 /Lin phenotype [18]. Vimentin (VIM) is related to EMT and also associated with cancer metastatic potential. Li et al. [19] demonstrated that human colorectal cancer CD133+ SW480 single cell progenies (SCPs) are heterogeneous in invasion and metastasis. A few CD133+ SW480 cells expressed VIM in the metastatic group SCP17 and SCP26, but the non-metastatic group SCP24 and SCP40 cells showed no VIM staining. Also, studies showed that in invasive CSCs, an epithelial marker E-cadherin was down-regulated, mesenchymal markers were up-regulated, and Transgelin which regulates EMT associated genes were over-expressed [20], which indicated some link exit between EMT and CSCs in tumor invasion and metastasis. 5. MicroRNAs, EMT and CSCs Accumulating data suggested that upregulation of some microRNAs and downregulation or absence of some of them have been found in metastatic CSCs. Reduction of microRNA let-7 in breast CSCs increased in vivo tumorigenic and metastatic capability [21]. An additional example of potential role of microRNAs in metastatic CSCs was represented by miR-30. Over-expression of miR-30 in breast CSCs xenograft reduced lung metastasis, whereas blocking miR-30 expression enhanced metastasis in vivo [22]. Multiple microRNAs have been reported to be involved in EMT, which suggested that microRNAs might connect CSCs and metastasis through regulation of EMT [23]. EMT-inducer zinc finger E-box binding homeobox 1 (ZEB1) could enhance metastasis not only by promoting tumor cell mobility and dissemination, but also by maintaining a stem cell phenotype through inhibition of miR-200 family members, which is necessary for the formation of metastases from disseminated tumor cells. 6. Chemokines, CSCs and metastasis To date, chemokines and their receptors have been known to play important roles in inflammation, infection, tissue injury, cardiovascular diseases, allergy, and malignant tumors [24]. Studies showed that chemokine receptors may potentially facilitate tumor dissemination at each of the key steps of metastasis [25]. The CXCR4/SDF-1 axis could mediate metastasis of the distinct subpopulation of CSCs. In the invasive front of human pancreatic cancer tissue, a distinct subpopulation of CD133+/CXCR4+ CSCs was identified to determine the metastatic phenotype of the individual tumor. Depletion of the cancer stem cell pool for these migrating CSCs significantly reduces the metastatic phenotype of pancreatic tumors, which indicates that a subpopulation of migrating CD133+/CXCR4+ CSCs is essential for tumor metastasis [26]. In the CD133+ SW480 SCPs, which bear heterogeneous invasive and metastatic ability, CXCR4 expression was associated with stronger infiltrating and metastatic abilities, indicating that CD133+ SCPs of SW480 cell line are prone to produce invasive and metastatic phenotypes and chemokines and their receptors are responsible for the recurrence and metastases of solid tumors [20].
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7. CSCs and angiogenesis Tumor cell invasion alone is not sufficient to produce distant metastases; it requires also the transport of malignant cells through blood and/or lymph vessels. Evidences showed that CD133+ glioblastoma stem cells secrete increased levels of vascular endothelial growth factor (VEGF) compared to CD133 counterparts, implicating CSCs as contributing to tumor angiogenesis and VEGF promotes tumorigenesis and angiogenesis of human glioblastoma stem cells [27]. Furthermore, intratumor hypoxic conditions not only promote sustained angiogenesis but also induce and select for an invasive and metastatic phenotype [28]. Recent studies have shown that hypoxia may enhance tumor stemness by increasing the invasive and tumorigenic ability of tumor side populations [29]. It is therefore possible that hypoxia may also keep cancer stem-like cells reside in hypoxic niches within a cancer. Further studies should be aimed at isolating these potential subtypes within the tumors and also determining to what extent epigenetic mechanisms regulate their adaptability and plasticity. 8. Therapeutic interventions against metastatic CSCs The separation and identification of metastatic CSCs as a subpopulation could enhance the understanding of the initiation, progression, metastasis or relapse of cancer at the molecular, genomic or epigenetic levels, as well as lead to the identification of novel molecules for the development of therapeutic agents targeting cancer. A novel telomerase inhibitor MST312 was found to have a strong antiproliferative effect on ALDH+ lung CSCs and induce apoptosis in the whole tumor population [30]. Treatment with MST312 could significantly reduce the number of ALDH+ CSCs and their telomeric length in vivo, indicating that antitelomeric therapy using MST312 mainly targets lung CSCs and may represent a novel approach for effective treatment of lung cancer [30]. CD44, an adhesion receptor involved in metastasis, is also a marker of CSCs and drug-resistant phenotypes. Piotrowicz et al. [31] found an A6 peptide (acetyl-KPSSPPEE-amino) has antitumor activity. A6 inhibited the migration of a subset of ovarian and breast cancer cell lines, which in vitro correlated with CD44 expression. Importantly, A6 potentiated the CD44-dependent adhesion of cancer cells to hyaluronic acid and activated CD44mediated signaling, as evidenced by focal adhesion kinase and MAP/ERK kinase phosphorylation. A6 potently blocked the migration of CD44-positive CSCs in vitro through an interaction with CD44 that altered its structure and activated CD44 to enhance ligand binding and downstream signaling. The concurrent ability of A6 to agonize the CD44 receptor suggested that CD44 activation in CSCs may represent a novel strategy for inhibiting metastatic disease. 9. Perspectives In the metastatic cascade, tumor cell leave home through EMT. Do pre-metastatic CSCs undergo EMT to invade the basement membrane barrier and leave home? EMT is a reversible shift and it is regulated by numerous signaling pathways, including Wnt, Notch and Hedgehog [32]. Researches about these signaling pathways have not been directly investigated. How CSCs enter vessels which is an important step for cancer metastasis. Do tumor-associated macrophages orientate CSCs toward vessels? Or some other cells that play a crucial role in this process. Although the niche of CSCs is comparatively less known, well-characterized evidence has elucidated the role that the niche plays in regulating normal stem cell migration. Evidences
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show that normal stem cell niche derived factors, such as matrix metalloproteinase-9 and calcium sensing receptor, play key roles in cancer metastasis [33]. The links between normal stem cells and metastatic cancer cells invoke further study into the metastatic CSCs. 10. Conclusion Recently, emerging studies have focus on the roles of CSCs on metastasis. While knowledge of the identification and characterization of CSCs are quickly expanding, little is known about the cellular and molecular mechanisms underlying their distinct functions, such as the metastatic mechanism. Elucidation of the specific signaling pathway and mechanism underlying the action of CSCs might improve the efficacy of cancer treatments. Expanding our knowledge on the metastatic mechanism of CSCs will help to discover the pertinent prognostic markers and develop novel therapeutic targets. Funding China Postdoctoral Science Foundation Funded Project (20100480542 & 201104227), Nature Science Foundation of Shanghai (12ZR1436000) and Youth Innovation Promotion Association, Chinese Academy of Sciences. Conflict of interest statement The authors declare that there are no conflicts of interest. Acknowledgments The authors would like to thank the team members of Prof. Fu’s laboratory for their helpful discussion and critical reading of the manuscript. References [1] Eccles SA, Welch DR. Metastasis: recent discoveries and novel treatment strategies. Lancet 2007;369:1742–57. [2] Gupta GP, Massague J. Cancer metastasis: building a framework. Cell 2006;127:679–85. [3] Luzzi KJ, MacDonald IC, Schmidt EE, Kerkvliet N, Morris VL, Chambers AF, et al. Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and limited survival of earlymicrometastasis. American Journal of Pathology 1998;153:865–73. [4] Li F, Tiede B. Beyond tumorigenesis: cancer stem cells in metastasis. Cell Research 2007;17:3–14. [5] Kang Y, Massague´ J. Epithelial–mesenchymal transitions: twist in development and metastasis. Cell 2004;118:277–9. [6] Brabletz T, Jung A, Spaderna S, Hlubek F, Kirchner T. Opinion: migrating cancer stem cells-an integrated concept of malignant tumour progression. Nature Reviews Cancer 2005;5:744–9. [7] Pang R, Law WL, Chu AC, Poon JT, Lam CS, Chow AK, et al. A subpopulation of CD26+ cancer stem cells with metastatic capacity in human colorectal cancer. Cell Stem Cell 2010;6:603–15. [8] Dieter SM, Ball CR, Hoffmann CM, Nowrouzi A, Herbst F, Zavidij O, et al. Distinct types of tumor-initiating cells form human colon cancer tumors and metastases. Cell Stem Cell 2011;9:357–65. [9] Hess DA, Wirthlin L, Craft TP, Herrbrich PE, Hohm SA, Lahey R, et al. Selection based on CD133 and high aldehyde dehydrogenase activity isolates long-term reconstituting human hematopoietic stem cells. Blood 2006;107:2162–9. [10] Patel SA, Dave MA, Murthy RG, Helmy KY, Rameshwar P. Metastatic breast cancer cells in the bone marrow microenvironment: novel insights into oncoprotection. Oncology Reviews 2011;5:93–102. [11] Sun SY, Wang ZL. ALDH high adenoid cystic carcinoma cells display cancer stem cell properties and are responsible for mediating metastasis. Biochemical and Biophysical Research Communications 2010;396:843–8. [12] Woo T, Okudela K, Mitsui H, Yazawa T, Ogawa N, Tajiri M, et al. Prognostic value of CD133 expression in stage I lung adenocarcinomas. International Journal of Clinical and Experimental Pathology 2010;4:32–42. [13] Lingala S, Cui YY, Chen X, Ruebner BH, Qian XF, Zern MA, et al. Immunohistochemical staining of cancer stem cell markers in hepatocellular carcinoma. Experimental and Molecular Pathology 2010;89:27–35.
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[14] Yao D, Dai C, Peng S. Mechanism of the mesenchymal–epithelial transition and its relationship with metastatic tumor formation. Molecular Cancer Research 2011;9:1608–20. [15] Visvader JE, Lindeman GJ. Cancer stem cells in solid tumours: accumulating evidence and unresolved questions. Nature Reviews Cancer 2008;8:755– 68. [16] Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelialmesenchymal transition generates cells with properties of stem cells. Cell 2008;133:704–15. [17] Philippar U, Roussos ET, Oser M, Yamaguchi H, Kim HD, Giampieri S, et al. A Mena invasion isoform potentiates EGF-induced carcinoma cell invasion and metastasis. Developmental Cell 2008;15:813–28. [18] Goswami S, Philippar U, Sun D, Patsialou A, Avraham J, Wang W, et al. Identification of invasion specific splice variants of the cytoskeletal protein Mena present in mammary tumor cells during invasion in vivo. Clinical and Experimental Metastasis 2009;26:153–9. [19] Li GQ, Liu C, Yuan J. CD133+ single cell-derived progenies of colorectal cancer cell line SW480 with different invasive and metastatic potential. Clinical and Experimental Metastasis 2010;27:517–27. [20] Lin Y, Buckhaults PJ, Lee JR. Association of the actin-binding protein transgelin with lymph node metastasis in human colorectal cancer. Neoplasia 2009;11:864–73. [21] Yu F. let-7 regulates self renewal and tumorigenicity of breast cancer cells. Cell 2007;131:1109–23. [22] Yu F, Deng H, Yao H. Mir-30 reduction maintains self-renewal and inhibits apoptosis in breast tumor-initiating cells. Oncogene 2010;29: 4194–204. [23] Gregory PA, Bracken CP, Bert AG, Goodall GJ. MicroRNAs as regulators of epithelial mesenchymal transition. Cell Cycle 2008;7:3112–7.
[24] Rot A, von Andrian UH. Chemokines in innate andadaptive host defense: basic chemokinese grammar for immune cells. Annual Review of Immunology 2004;22:891–928. [25] Keeley EC, Mehrad B, Strieter RM. Cxc chemokines in cancer angiogenesis and metastases. Advances in Cancer Research 2010;106:91–111. [26] Hermann PC, Huber SL, Herrler T, Aicher A, Ellwart JW, Guba M, et al. Distinct populations of cancer stem cells determine tumor growth and metastatic activity in human pancreatic cancer. Cell Stem Cell 2007;1:313–23. [27] Bao S, Wu Q, Sathornsumetee S, Hao Y, Li Z, Hjelmeland AB, et al. Stem cell-like glioma cells promote tumor angiogenesis through vascular endothelial growth factor. Cancer Research 2006;66:7843–8. [28] Sullivan R, Graham CH. Hypoxia-driven selection of the metastatic phenotype. Cancer and Metastasis Reviews 2007;26:319–31. [29] Das B, Tsuchida R, Malkin D, Koren G, Baruchel S, Yeger H. Hypoxia enhances tumor stemness by increasing the invasive and tumorigenic side population fraction. Stem Cells 2008;26:1818–30. [30] Serrano D, Bleau AM, Fernandez-Garcia I, Fernandez-Marcelo T, Iniesta P, Ortiz-de-Solorzano C, et al. Inhibition of telomerase activity preferentially targets aldehyde dehydrogenase-positive cancer stem-like cells in lung cancer. Molecular Cancer 2011;10:96. [31] Piotrowicz RS, Damaj BB, Hachicha M, Incardona F, Howell SB, Finlayson M. A6 peptide activates CD44 adhesive activity, induces FAK and MEK phosphorylation, and inhibits the migration and metastasis of CD44-expressing cells. Molecular Cancer Therapeutics 2011;10:2072–82. [32] Yang J, Weinberg RA. Epithelial–mesenchymal transition: at the crossroads of development and tumor metastasis. Developmental Cell 2008;14:818–29. [33] Adams GB, Chabner KT, Alley IR, Olson DP, Szczepiorkowski ZM, Poznansky MC, et al. Stem cell engraftment at the endosteal niche is specified by the calcium-sensing receptor. Nature 2006;439:599–603.
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CANEP-472; No. of Pages 4 Cancer Epidemiology xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Cancer Epidemiology The International Journal of Cancer Epidemiology, Detection, and Prevention journal homepage: www.cancerepidemiology.net
The role of cancer stem cells in cancer metastasis: New perspective and progress Xiaofeng Wang a,1, Yuzhen Zhu b,1, Yushui Ma a,1, Jin Wang c, Feng Zhang a, Qing Xia a,*, Da Fu a,c,** a
Department of Orthopaedics, Zhongshan Hospital, Fudan University, Shanghai 200032, China Department of Gastroenterology, Tongji Hospital, Tongji University, Shanghai 200065, China c The Key Laboratory of Stem Cell Biology, Institute of Health Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200025, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 May 2012 Received in revised form 15 July 2012 Accepted 19 July 2012 Available online xxx
Recent studies have identified the important role of cancer stem cells (CSCs) in carcinogenesis and relapse. However, with respect to multistage cancer metastasis, the role of CSCs has not been welldefined. In several human cancers, data showed that some phenotypic subsets of CSCs were responsible for cancer metastasis. In this review, we surveyed recent advances in the role and mechanism of metastatic CSCs. ß 2012 Elsevier LtdPublished by Elsevier Ltd. All rights reserved.
Keywords: CSCs Metastasis EMT MicroRNAs Angiogenesis
1. The basic procedure of metastatic cascade Metastasis, frequently a final and fatal step in the progression of solid malignancies, encompasses several fundamental biological processes: cancer initiation, epithelial–mesenchymal transition (EMT), breach of the basement membrane barrier, neighbor invasion, intravasation, mesenchymal–epithelial transition (MET), extravasation, colonization and outgrowth of micrometastases and secondary cancer [1]. In fact, the evolution of metastatic cascade is a dynamic process influenced by unique cellular lineages, altered microenvironments, distinct anatomical restrictions and multiple genetic and epigenetic alterations [2]. A number of studies have attempted to address the question of cellular heterogeneity within cancer tissues and its implications for the selection of metastatic cancer cells. Luzzi et al. [3] traced the
Abbreviations: CSCs, cancer stem cells; ALDH, aldehyde dehydrogenases; AdCC, adenoid cystic carcinoma; HCC, hepatocellular carcinoma; EMT, epithelial– mesenchymal transition; SDF-1, stromal cell derived factor 1; CXCR-4, chemokine C-X-C motif receptor 4; VEGF, vascular endothelial growth factor. * Corresponding author. ** Corresponding author at: The Key Laboratory of Stem Cell Biology, Institute of Health Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200025, China. Tel.: +86 21 63846925; fax: +86 21 63846925. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Zhu), [email protected] (Y. Ma), [email protected] (J. Wang), [email protected] (F. Zhang), [email protected] (Q. Xia), [email protected] (D. Fu). 1 These authors contributed equally to this work.
fate of B16F1 melanoma cells injected intraportally into murine livers. Although 80% of the injected cells survived and extravasated at day 3, only a small number of these cells (1 in 40) formed micrometastases, and only 1 in 100 micrometastases continued to grow into macrometastases, which suggested that the metastatic action of caner appears inefficiently. 2. Migrating cancer stem cells (CSCs) Why so few cancer cells successfully navigate the multistep metastatic process? The precise nature of these cancer cells remains undefined. However, some properties of CSCs, a subpopulation of cells that display stem cell properties, mediate metastasis, and contribute to treatment resistance, suggest them as candidates for mediating metastatic progression. First, it is theoretically possible that only CSCs within cancer tissues have the ability to initiate and sustain cancer growth. Therefore, even if nonCSCs migrate (which is likely, given the number of cancer cells that can be detected in the blood), they would not be able to propagate into heterogeneously diverse metastatic lesions [4]. Furthermore, the inherent plasticity of stem cells makes CSCs more adaptable to survive in a foreign environment where growth factors and other signaling molecules are different from those in the primary caner site [4]. Also cellular plasticity in stem cells may facilitate EMT, which has been postulated as a key event during the early phase of cancer metastasis [5]. Brabletz et al. [6] firstly establish the hypothesis of migrating CSCs, which possess both an element of stemness and mobility. It is postulated that these cells undergo EMT at the invasive front of the
1877-7821/$ – see front matter ß 2012 Elsevier LtdPublished by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.canep.2012.07.007
Please cite this article in press as: Wang X, et al. The role of cancer stem cells in cancer metastasis: New perspective and progress. Cancer Epidemiology (2012), http://dx.doi.org/10.1016/j.canep.2012.07.007
G Model
CANEP-472; No. of Pages 4 2
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primary cancer and migrate to colonize new tissues, where its acquired stemness to facilitate production of the cancer heterogeneity commonly observed in metastatic colonies [6]. The migrating cancer stem cell hypothesis is attractive owing to its integration of primary cancer initiation, progression and metastasis. 3. Metastatic capacity of CSC subpopulations characterized by different markers Although CSCs have been identified in many types of cancer and the migrating cancer stem cell hypothesis has gained more attention in recent years, the issues that whether metastatic competence is inherent to all CSCs is still outstanding. Several studies have defined rare CSCs subpopulations which have distinct metastatic competence. Roberta et al. [7] identified a subpopulation of CD26+ CSCs uniformly present in both the primary and metastatic tumors in colorectal cancer patients with liver metastasis. Furthermore, circulating CD26+ CSCs were capable of forming metastatic growth in the liver after injected into mouse cecal wall. In patients without distant metastasis at the time of presentation, the presence of CD26+ cells in their primary tumors predicted distant metastasis on follow-up [7]. Using a molecular tracking strategy, Dieter et al. [8] identified a types of extensively self-renewing longterm tumor-initiating cell (LT-TICs) with specific roles in cancer metastasis, which suggested the existence of a subpopulation of cancer cells with stem cell properties endowed with metastatic capacity on malignant progression in colorectal cancer. Aldehyde dehydrogenases (ALDHs) are a group of enzymes that catalyze the oxidation of aldehydes [9]. Recent studies demonstrated that high level of ALDH activity could serve as a putative CSCs maker and its presence strongly correlates with tumor malignancy as well as self-renewal properties of CSCs in various human cancers. Intriguingly, evidences show that ALDH+ human breast cancer MDA-MB-231 cells with stem-like properties are responsible for the bone metastasis [10]. Sun and Wang [11] found that a subpopulation of cells expressing high level of ALDH activity residing in human adenoid cystic carcinoma (AdCC) possessed enhanced invasive potential in vitro and highly metastatic capability in vivo, which suggesting ALDH+ CSCs are responsible for mediating AdCC metastasis. CD133 is one of the most representative cancer stem cell markers. Woo et al. [12] found that CD133 high expressers in stage I lung adenocarcinomas (ADC) show a significantly higher risk of recurrence than CD133 low expressers. CD133 high expressers having strong proliferating activity and/or with vessel invasion show a higher risk of recurrence, which suggested the level of CD133 expression is an independent prognostic marker and its combination with proliferating activity and/or vessel invasion could have excellent prognostic value to predict postoperative recurrence in patients with stage I lung ADC. Lingala et al. [13] co-stained hepatocellular carcinoma (HCC) specimens with anti-CD133 and anti-ALDH monoclonal antibodies, and found that doubly positive HCC cell clusters were colocalized in the areas adjacent to connective tissue and within invaded vessels, which suggested that CD133+/ALDH+ CSCs are highly metastatic. 4. EMT and MET in migrating CSCs EMT and MET are recognized as critical events for metastasis of carcinomas. The former are responsible for degrading the surrounding matrix to lead the way of invasion and intravasation and the latter is important for cancer cells then enter the blood stream and reestablish colonies in the secondary sites [14]. Recent studies suggested that CSCs normally have characteristics
associated with mesenchymal cells and also play a critical role in tumor initiation, growth, metastasis [15]. Migrating CSCs are located predominantly at the tumor-host interface and are derived from stationary CSCs through the acquisition of a transient EMT phenotype in addition to stemness [6]. EMT endows human mammary epithelial cells with CSCs-like properties which characterized by their CD44high/CD24low phenotype [16] through up-regulating Mena, a member of the Ena/VASP family which plays a role in cell migration [17]. Intriguingly, many of circulating tumor cells expressing Mena display a mesenchymal phenotype which indicates EMT, and also acquired a breast cancer stem cell CD44+/ CD24 /Lin phenotype [18]. Vimentin (VIM) is related to EMT and also associated with cancer metastatic potential. Li et al. [19] demonstrated that human colorectal cancer CD133+ SW480 single cell progenies (SCPs) are heterogeneous in invasion and metastasis. A few CD133+ SW480 cells expressed VIM in the metastatic group SCP17 and SCP26, but the non-metastatic group SCP24 and SCP40 cells showed no VIM staining. Also, studies showed that in invasive CSCs, an epithelial marker E-cadherin was down-regulated, mesenchymal markers were up-regulated, and Transgelin which regulates EMT associated genes were over-expressed [20], which indicated some link exit between EMT and CSCs in tumor invasion and metastasis. 5. MicroRNAs, EMT and CSCs Accumulating data suggested that upregulation of some microRNAs and downregulation or absence of some of them have been found in metastatic CSCs. Reduction of microRNA let-7 in breast CSCs increased in vivo tumorigenic and metastatic capability [21]. An additional example of potential role of microRNAs in metastatic CSCs was represented by miR-30. Over-expression of miR-30 in breast CSCs xenograft reduced lung metastasis, whereas blocking miR-30 expression enhanced metastasis in vivo [22]. Multiple microRNAs have been reported to be involved in EMT, which suggested that microRNAs might connect CSCs and metastasis through regulation of EMT [23]. EMT-inducer zinc finger E-box binding homeobox 1 (ZEB1) could enhance metastasis not only by promoting tumor cell mobility and dissemination, but also by maintaining a stem cell phenotype through inhibition of miR-200 family members, which is necessary for the formation of metastases from disseminated tumor cells. 6. Chemokines, CSCs and metastasis To date, chemokines and their receptors have been known to play important roles in inflammation, infection, tissue injury, cardiovascular diseases, allergy, and malignant tumors [24]. Studies showed that chemokine receptors may potentially facilitate tumor dissemination at each of the key steps of metastasis [25]. The CXCR4/SDF-1 axis could mediate metastasis of the distinct subpopulation of CSCs. In the invasive front of human pancreatic cancer tissue, a distinct subpopulation of CD133+/CXCR4+ CSCs was identified to determine the metastatic phenotype of the individual tumor. Depletion of the cancer stem cell pool for these migrating CSCs significantly reduces the metastatic phenotype of pancreatic tumors, which indicates that a subpopulation of migrating CD133+/CXCR4+ CSCs is essential for tumor metastasis [26]. In the CD133+ SW480 SCPs, which bear heterogeneous invasive and metastatic ability, CXCR4 expression was associated with stronger infiltrating and metastatic abilities, indicating that CD133+ SCPs of SW480 cell line are prone to produce invasive and metastatic phenotypes and chemokines and their receptors are responsible for the recurrence and metastases of solid tumors [20].
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7. CSCs and angiogenesis Tumor cell invasion alone is not sufficient to produce distant metastases; it requires also the transport of malignant cells through blood and/or lymph vessels. Evidences showed that CD133+ glioblastoma stem cells secrete increased levels of vascular endothelial growth factor (VEGF) compared to CD133 counterparts, implicating CSCs as contributing to tumor angiogenesis and VEGF promotes tumorigenesis and angiogenesis of human glioblastoma stem cells [27]. Furthermore, intratumor hypoxic conditions not only promote sustained angiogenesis but also induce and select for an invasive and metastatic phenotype [28]. Recent studies have shown that hypoxia may enhance tumor stemness by increasing the invasive and tumorigenic ability of tumor side populations [29]. It is therefore possible that hypoxia may also keep cancer stem-like cells reside in hypoxic niches within a cancer. Further studies should be aimed at isolating these potential subtypes within the tumors and also determining to what extent epigenetic mechanisms regulate their adaptability and plasticity. 8. Therapeutic interventions against metastatic CSCs The separation and identification of metastatic CSCs as a subpopulation could enhance the understanding of the initiation, progression, metastasis or relapse of cancer at the molecular, genomic or epigenetic levels, as well as lead to the identification of novel molecules for the development of therapeutic agents targeting cancer. A novel telomerase inhibitor MST312 was found to have a strong antiproliferative effect on ALDH+ lung CSCs and induce apoptosis in the whole tumor population [30]. Treatment with MST312 could significantly reduce the number of ALDH+ CSCs and their telomeric length in vivo, indicating that antitelomeric therapy using MST312 mainly targets lung CSCs and may represent a novel approach for effective treatment of lung cancer [30]. CD44, an adhesion receptor involved in metastasis, is also a marker of CSCs and drug-resistant phenotypes. Piotrowicz et al. [31] found an A6 peptide (acetyl-KPSSPPEE-amino) has antitumor activity. A6 inhibited the migration of a subset of ovarian and breast cancer cell lines, which in vitro correlated with CD44 expression. Importantly, A6 potentiated the CD44-dependent adhesion of cancer cells to hyaluronic acid and activated CD44mediated signaling, as evidenced by focal adhesion kinase and MAP/ERK kinase phosphorylation. A6 potently blocked the migration of CD44-positive CSCs in vitro through an interaction with CD44 that altered its structure and activated CD44 to enhance ligand binding and downstream signaling. The concurrent ability of A6 to agonize the CD44 receptor suggested that CD44 activation in CSCs may represent a novel strategy for inhibiting metastatic disease. 9. Perspectives In the metastatic cascade, tumor cell leave home through EMT. Do pre-metastatic CSCs undergo EMT to invade the basement membrane barrier and leave home? EMT is a reversible shift and it is regulated by numerous signaling pathways, including Wnt, Notch and Hedgehog [32]. Researches about these signaling pathways have not been directly investigated. How CSCs enter vessels which is an important step for cancer metastasis. Do tumor-associated macrophages orientate CSCs toward vessels? Or some other cells that play a crucial role in this process. Although the niche of CSCs is comparatively less known, well-characterized evidence has elucidated the role that the niche plays in regulating normal stem cell migration. Evidences
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show that normal stem cell niche derived factors, such as matrix metalloproteinase-9 and calcium sensing receptor, play key roles in cancer metastasis [33]. The links between normal stem cells and metastatic cancer cells invoke further study into the metastatic CSCs. 10. Conclusion Recently, emerging studies have focus on the roles of CSCs on metastasis. While knowledge of the identification and characterization of CSCs are quickly expanding, little is known about the cellular and molecular mechanisms underlying their distinct functions, such as the metastatic mechanism. Elucidation of the specific signaling pathway and mechanism underlying the action of CSCs might improve the efficacy of cancer treatments. Expanding our knowledge on the metastatic mechanism of CSCs will help to discover the pertinent prognostic markers and develop novel therapeutic targets. Funding China Postdoctoral Science Foundation Funded Project (20100480542 & 201104227), Nature Science Foundation of Shanghai (12ZR1436000) and Youth Innovation Promotion Association, Chinese Academy of Sciences. Conflict of interest statement The authors declare that there are no conflicts of interest. Acknowledgments The authors would like to thank the team members of Prof. Fu’s laboratory for their helpful discussion and critical reading of the manuscript. References [1] Eccles SA, Welch DR. Metastasis: recent discoveries and novel treatment strategies. Lancet 2007;369:1742–57. [2] Gupta GP, Massague J. Cancer metastasis: building a framework. Cell 2006;127:679–85. [3] Luzzi KJ, MacDonald IC, Schmidt EE, Kerkvliet N, Morris VL, Chambers AF, et al. Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and limited survival of earlymicrometastasis. American Journal of Pathology 1998;153:865–73. [4] Li F, Tiede B. Beyond tumorigenesis: cancer stem cells in metastasis. Cell Research 2007;17:3–14. [5] Kang Y, Massague´ J. Epithelial–mesenchymal transitions: twist in development and metastasis. Cell 2004;118:277–9. [6] Brabletz T, Jung A, Spaderna S, Hlubek F, Kirchner T. Opinion: migrating cancer stem cells-an integrated concept of malignant tumour progression. Nature Reviews Cancer 2005;5:744–9. [7] Pang R, Law WL, Chu AC, Poon JT, Lam CS, Chow AK, et al. A subpopulation of CD26+ cancer stem cells with metastatic capacity in human colorectal cancer. Cell Stem Cell 2010;6:603–15. [8] Dieter SM, Ball CR, Hoffmann CM, Nowrouzi A, Herbst F, Zavidij O, et al. Distinct types of tumor-initiating cells form human colon cancer tumors and metastases. Cell Stem Cell 2011;9:357–65. [9] Hess DA, Wirthlin L, Craft TP, Herrbrich PE, Hohm SA, Lahey R, et al. Selection based on CD133 and high aldehyde dehydrogenase activity isolates long-term reconstituting human hematopoietic stem cells. Blood 2006;107:2162–9. [10] Patel SA, Dave MA, Murthy RG, Helmy KY, Rameshwar P. Metastatic breast cancer cells in the bone marrow microenvironment: novel insights into oncoprotection. Oncology Reviews 2011;5:93–102. [11] Sun SY, Wang ZL. ALDH high adenoid cystic carcinoma cells display cancer stem cell properties and are responsible for mediating metastasis. Biochemical and Biophysical Research Communications 2010;396:843–8. [12] Woo T, Okudela K, Mitsui H, Yazawa T, Ogawa N, Tajiri M, et al. Prognostic value of CD133 expression in stage I lung adenocarcinomas. International Journal of Clinical and Experimental Pathology 2010;4:32–42. [13] Lingala S, Cui YY, Chen X, Ruebner BH, Qian XF, Zern MA, et al. Immunohistochemical staining of cancer stem cell markers in hepatocellular carcinoma. Experimental and Molecular Pathology 2010;89:27–35.
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Please cite this article in press as: Wang X, et al. The role of cancer stem cells in cancer metastasis: New perspective and progress. Cancer Epidemiology (2012), http://dx.doi.org/10.1016/j.canep.2012.07.007