Review Article
Cancer stem cells and tumor metastasis ABSTRACT Cancer stem cells (CSCs) are endowed with an inherent resistance to cytotoxic drugs, and are closely related to the migration, invasiveness, and anti-apoptotic ability of the cancer cells. Epithelial-mesenchymal transition (EMT) is a process where epithelial cells acquire the highly invasive and metastatic characteristics of mesenchymal cells, and has a close connection with CSCs. In this paper, the authors address the recent progress made in the relationship between the EMT status of CSCs and tumor metastasis and between the localization and chemotaxis of CSCs and the formation of the metastatic colonies, as well as the relation of CSCs with the therapeutic strategy for cancer treatment. KEY WORDS: Cancer stem cells, tumor metastasis, epithelial-mesenchymal transition, review
INTRODUCTION Stem cells (SCs) have the ability to self-renew and generate diverse cells. In brief, after cell division, some cells keep the characteristics of SCs, while others commit to mature cells with limited life span. In vivo, SCs can not only compensate for normal apoptosis, but also repair damaged tissues to help maintain homeostasis. A study[1] has shown that active SCs mainly compensate for normal apoptosis and quiescent SCs repair damaged tissues. Moreover, it is assumed that SCs play a key role in the onset, development, invasion, and metastasis of tumors. Cancer stem cells (CSCs) refer to a group of tumor cells that are able to self-renew and produce diverse cells.[2] Conceptually, the major difference between SCs and CSCs lies in the fact that SCs can regulate and keep the state of their microenvironment, but CSCs cannot. SCs regulate cell proliferation, differentiation, and apoptosis, which coordinate with each other. The generation of a new cell implies the apoptosis of another cell. The error at any stage may cause an imbalanced state and finally the formation of tumors. EPITHELIAL–MESENCHYMAL TRANSITION STATUS OF CSCS AND TUMOR METASTASIS CSCs were originally detected in the malignant tumors of hematopoietic system,[3] and then were proved to exist in solid tumors such as tumors of the breast, colon, liver, etc.[4-7] The tumorigenic ability of CSCs is stronger than that of tumor cells. The vaccination of a few CSCs in immunodeficient mice may cause tumors. CSCs are involved not only S148
in the onset of tumors but also in the proliferation of tumors. A small amount of CSCs in tumors can induce the formation of the same tumor tissue with the primary tumor, which illustrates CSCs are able to generate diverse cells.
Shaoqiang Sun, Xue Shan Qiu1 Departments of Clinical Medicine and 1Pathology, China Medical University, Shenyang, China For correspondence: Dr. Xue Shan Qiu, Department of Pathology, China Medical University, Shenyang, China. E-mail: [email protected]
Mani et al. [8] have found that epithelial– mesenchymal transition (EMT) generates tumor cells with the properties of CSCs. Their study reveals a close relationship between CSCs and EMT. It implies that CSCs might promote the invasion and metastasis of tumors by acquiring some properties of the mesenchyma. When tumor cells interact with fibroblasts, granulocytes, macrophages, and other cells in niches, these cells may release the signal to induce EMT, resulting in increased invasion of tumor cells.[9] EMT occurs in many malignant tumors. In certain microenvironment, some epithelial cells acquire the properties of mesenchymal cells and thus invade the adjacent tissues or spread to remote organs. EMT and mesenchymal-to-epithelial transition (MET) play a certain role in embryo development, tissue regeneration, and injury repair. [10] EMT is also crucial in invasion and metastasis of malignant tumors. It has been proved that EMT in patients with tumors indicates poor prognosis.[11] Brabletz et al.[12] have found that cells induced by EMT spread unevenly at the invasion front of colorectal cancers, and the expression of Wnt signal transduction pathway increases. In addition, they have found that in the extracellular matrix, single cells express nuclear -catenin as well as salient mesenchymal characteristics such as the expression of vimentin and non-expression of E-cadherin. It is believed that EMT is irreversible
Access this article online Website: www.cancerjournal.net DOI: 10.4103/0973-1482.122510 PMID: *** Quick Response Code:
Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
Sun and Qiu: Functions of cancer stem cells in tumor metastasis and cancer treatment
during embryonic development; transitional mesenchymal cells can restore to their original epithelial state through MET. Similarly, the MET of CSCs may occur when the cells are induced by some factors during metastasis and lead to the formation of metastatic sites. Patient with tumors might have a better prognosis if CSCs in circulation could be detected and target killed. In recent years, most researchers[13] use associated specific antigens and epithelial cell surface makers such as epithelial cell adhesion molecule (EpCAM) to detect CSCs in peripheral circulation, so some CSCs which do not express associated proteins are difficult to detect. CSCS AND TUMOR METASTASIS Localization and chemotaxis of CSCs Hermann et al.[14] have found among CD133-positive pancreatic CSCs, some cells in circulation express C-X-C chemokine receptor type 4 (CXCR4) and these cells cause tumor metastasis. A study[15] shows CXCR4 is expressed at the surface of highly malignant and metastatic tumor cells. Hermann et al.[14] have also found CXCR4 receptor antagonists could reduce the circulating CSCs and tumor metastasis. Based on these results, it can be concluded that circulating CSCs play an important role in tumor metastasis. However, the specific metastatic mechanisms of different cancers are still unclear. Some people[16] think chemotaxis occurs between CSCs that can express CXCR4 and stromal cell-derived factor-1 (SDF-1). The concentration gradient of SDF-1 between the metastatic site and primary site induces the migration of CSCs. Because of chemotaxis, circulating CSCs migrate to certain organs or tissues and commit to the corresponding primary cancer. At present, no reliable evidence can prove that the circulating CSCs still retain the properties of SCs.[17] CSCs and the formation of the metastatic colonies In recent years, Pang et al.[18] have discovered that CD26 could be used as a CSC marker to predict the invasion and metastasis of colon cancer. Among 27 cases of colon cancer without hepatic metastasis, only 8 cases were CD26 positive. However, CD26-positive tumor cells were found in the primary and metastatic sites in all the patients with hepatic metastasis. The experiment on tumor formation in Nonobese Diabetic/ Severe Combined Immunodeficiency (NOD/SCID) mice shows that the vaccination of CD26-positive and -negative cells causes the formation of tumor, but metastatic sites only form in mice vaccinated with CD26-positive cells and only CD26-positive circulation cancer cells can be detected in portal veins. Al-Hajj et al.[4] vaccinated immunodeficient mice by limiting dilution analysis (LDA) and found that in breast cancer, the tumor cells which expressed CD44 and did not express CD24 or expressed CD24 to a lesser extent had the characteristics of SCs. It is reported that aldehyde dehydrogenase (ALDH) 1 can be used as a CSC marker for breast cancer to enrich CSCs.[19] ALDH can also be used as a marker for breast cancer to enrich CSCs. The vaccination Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
experiment in NOD/SCID mice showed that compared with cells with ALDH (low expression) CD44 (high expression/−), cells with ALDH (high expression) CD44 (+) CD24 (−) or ALDH (high expression) CD44 (+) CD133 (+) have advanced malignant and metastatic ability.[20] Yet, the latest study[21] suggests the expression of ALDH1 in liver cancer is not related to liver cancer cells and might be an index of differentiation in liver cancer. CSCS AND THE THERAPEUTIC STRATEGY FOR CANCER TREATMENT CSCs have been found in many types of tumors, but their relationship with clinical treatment and prognosis still deserves further study. As mentioned above, SCs are composed of two types: Active and quiescent. Quiescent SCs are similar to dormant cancer cells. Stimulated by some external factors, dormant cancer cells, which are a small part of cancer cells in normal condition, may acquire energy and proliferate, leading to relapse and metastasis of tumors.[22] Cancer cells are thought to be dormant before relapse when the primary cancer reoccurs 5 years after the malignant tumor is removed. Breast cancer reoccurs even 10 years after the surgery.[23] Based on the evidence, a question might arise whether dormant cancer cells are quiescent CSCs that spread from the primary site before the surgery. If so, it is not difficult to understand that their resistance to cytotoxic drugs results from their properties of SCs. Therefore, it is significant for cancer treatment if quiescent SCs could be target killed or killed when they proliferate again after induction. It has been proved that quiescent SCs exist in cancers such as melanoma, leukemia, breast cancer, pancreatic cancer, and liver cancer.[24-28] Quiescent CSCs are resistant to cytotoxic drugs because such drugs (e.g., drugs that induce DNA damage) can only affect proliferating cells. Besides, CSCs are resistant to radiotherapy or chemotherapy because of other mechanisms. There mechanisms are related to the fact that CSCs have the properties of SCs. For instance, the efflux mechanism of CSCs results in cancer resistance to several drugs, repair reaction after DNA damage or low concentration of reactive oxygen species (ROS).[29-30] Therefore, it is crucial to the treatment and prevention of tumor relapse that quiescent CSCs are killed, their resistance to radiotherapy or chemotherapy is damaged, or they are prevented from proliferation. PROBLEMS AND PROSPECTS Direct evidence that CSCs cause tumor metastasis is still lacking. With more and more reports[31,32] on the relationship between CSCs and tumor metastasis, it becomes clear that not all the tumor cells are involved in tumor metastasis. It is possible that only a small part of tumor cells that have the characteristics of SCs and the ability to invade play an important role in tumor metastasis. There cells can escape from autoimmune system and resist external damage, so they can induce new metastatic sites in certain microenvironment. S149
Sun and Qiu: Functions of cancer stem cells in tumor metastasis and cancer treatment
In sum, various molecular mechanisms regulate the differentiation, self-renewal, proliferation, chemotaxis, and resistance to cytotoxic drugs of CSCs. The mechanisms and individualized therapies should be further explored so that the invasion, metastasis, and resistance to radiotherapy or chemotherapy of CSCs could be reduced and the prognosis after surgery might be improved. As for solid tumors, they can be confined to the primary sites if the relationship between CSCs and tumor invasion or metastasis is clarified, CSCs are killed, or CSCs are prevented from invading the blood vessels from tumor stroma, or from invading or moving at metastatic sites within the blood vessels or lymphatic vessels. If so, the tumor can be removed completely, thus increasing the survival rate and life quality of patients. REFERENCES 1. 2.
3.
4.
5.
6.
7.
8.
9.
10. 11.
12.
13.
14.
Li L. Clevers H. Coexistence of quiescent and active adult stem cells in mammals. Science 2010;327:542-5. Visvader JE, Lindeman GJ. Cancer stem cells in solid tumours: Accumulating evidence and unresolved questions. Nat Rev Cancer 2008;8:755-68. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997;3:730-7. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 2003;100:3983-8. O’Brien CA, Pollett A, Gallinger S, Dick JE. A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 2007;445:106-10. Ricci-Vitiani L, Lombardi DG, Pilozzi E, Biffoni M, Todaro M, Peschle C, et al. Identification and expansion of human colon-cancer-initiating cells. Nature 2007;445:111-5. Ma S, Chan KW, Hu L, Lee TK, Wo JY, Ng IO, et al. Identification and characterization of tumorigenic liver cancer stem/progenitor cells. Gastroenterology 2007;132:2542-56. Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelial-mesenchymal transition generates cells with properties of stem cells. Cell 2008;133:704-15. Le NH, Franken P, Fodde R. Tumour-stroma interactions in colorectal cancer: Converging on beta-catenin activation and cancer stemness. Br J Cancer 2008;98:1886-93. Thiery JP, Acloque H, Huang RY, Nieto MA. Epithelial-mesenchymal transitions in development and disease. Cell 2009;139:871-90. Yang J, Weinberg RA. Epithelial-mesenchymal transition: At the crossroads of development and tumor metastasis. Dev Cell 2008;14:818-29. Brabletz T, Jung A, Reu S, Porzner M, Hlubek F, Kunz-Schughart LA, et al. Variable beta-catenin expression in colorectal cancers indicates tumor progression driven by the tumor environment. Proc Natl Acad Sci U S A 2001;98:10356-61. Pantel K, Brakenhoff RH, Brandt B. Detection, clinical relevance and specific biological properties of disseminating tumour cells. Nat Rev Cancer 2008;8:329-40. 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.
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15. Gassmann P, Haier J, Schlüter K, Domikowsky B, Wendel C, Wiesner U, et al. CXCR4 regulates the early extravasation of metastatic tumor cells in vivo. Neoplasia 2009;11:651-61. 16. Vagima Y, Lapid K, Kollet O, Goichberg P, Alon R, Lapidot T. Pathways implicated in stem cell migration: The SDF-1/CXCR4 axis. Methods Mol Biol 2011;750:277-89. 17. Pantel K, Alix-Panabières C, Riethdorf S. Cancer micrometastases. Nat Rev Clin Oncol 2009;6:339-51. 18. 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. 19. Ginestier C, Hur MH, Charafe-Jauffret E, Monville F, Dutcher J, Brown M, et al. ALDH1 is a marker of normal and malignant human mammary stem cells and a predictor of poor clinical outcome. Cell Stem Cell 2007;1:555-67. 20. Croker AK, Goodale D, Chu J, Postenka C, Hedley BD, Hess DA, et al. High aldehyde dehydrogenase and expression of cancer stem cell markers selects for breast cancer cells with enhanced malignant and metastatic ability. J Cell Mol Med 2009;13:2236-52. 21. Suzuki E, Chiba T, Zen Y, Miyaqi S, Tada M, Kanai F, et al. Aldehyde dehydrogenase 1 is associated with recurrence-free survival but not stem cell-like properties in hepatocellular carcinoma. Hepatol Res, 2012;42:1100-11. 22. Aguirre-Ghiso JA. Models, mechanisms and clinical evidence for cancer dormancy. Nat Rev Cancer 2007;7:834-46. 23. Hanrahan EO, Gonzalez-Angulo AM, Giordano SH, Rouzier R, Broglio KR, Hortobagyi GN, et al. Overall survival and cause-specific mortality of patients with stage T1a, bN0M0 breast carcinoma. J Clin Oncol 2007;25:4952-60. 24. Roesch A, Fukunaga-Kalabis M, Schmidt EC, Zabierowski SE, Brafford PA, Vultur A, et al. A temporarily distinct subpopulation of slow-cycling melanoma cells is required for continuous tumor growth. Cell 2010;41:583-94. 25. Saito Y, Uchida N, Tanaka S, Suzuki N, Tomizawa-Murasawa M, Sone A, et al. Induction of cell cycle entry eliminates human leukemia stem cells in a mouse model of AML. Nat Biotechnol 2010;28:275-80. 26. Pece S, Tosoni D, Confalonieri S, Mazzarol G, Vecchi M, Ronzoni S, et al. Biological and molecular heterogeneity of breast cancers correlates with their cancer stem cell content. Cell 2010;140:62-73. 27. Dembinski JL, Krauss S. Characterization and functional analysis of a slow cycling stem cell-like subpopulation in pancreas adenocarcinoma. Clin Exp Metastasis 2009;26:611-23. 28. Haraguchi N, Ishii H, Mimori K, Tanaka F, Ohkuma M, Kim HM, et al. CD13 is a therapeutic target in human liver cancer stem cells. J Clin Invest 2010;120:3326-39. 29. Bao S, Wu Q, McLendon RE, Hao Y, Shi Q, Hjelmeland AB, et al. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 2006;444:756-60. 30. Diehn M, Cho RW, Lobo NA, Kalisky T, Dorie MJ, Kulp AN, et al. Association of reactive oxygen species levels and radioresistance in cancer stem cells. Nature 2009;458:780-3. 31. Singh S, Chellappan S. Lung cancer stem cells: Molecular features and therapeutic targets. Mol Aspects Med 2013; In Press. doi: 10.1016/j. mam. 2013.08.03 32. Long H, Zhang S, Liu C, Shi J, Tao L, Situ D, et al. Characterization of a stem cell population in lung cancer cell line Glc-82. Thorac Cancer 2012;3:8-18.
Cite this article as: Sun S, Qiu XS. Cancer stem cells and tumor metastasis. J Can Res Ther 2013;9:S148-50. Source of Support: Nil, Conflict of Interest: No.
Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
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Cancer stem cells and tumor metastasis ABSTRACT Cancer stem cells (CSCs) are endowed with an inherent resistance to cytotoxic drugs, and are closely related to the migration, invasiveness, and anti-apoptotic ability of the cancer cells. Epithelial-mesenchymal transition (EMT) is a process where epithelial cells acquire the highly invasive and metastatic characteristics of mesenchymal cells, and has a close connection with CSCs. In this paper, the authors address the recent progress made in the relationship between the EMT status of CSCs and tumor metastasis and between the localization and chemotaxis of CSCs and the formation of the metastatic colonies, as well as the relation of CSCs with the therapeutic strategy for cancer treatment. KEY WORDS: Cancer stem cells, tumor metastasis, epithelial-mesenchymal transition, review
INTRODUCTION Stem cells (SCs) have the ability to self-renew and generate diverse cells. In brief, after cell division, some cells keep the characteristics of SCs, while others commit to mature cells with limited life span. In vivo, SCs can not only compensate for normal apoptosis, but also repair damaged tissues to help maintain homeostasis. A study[1] has shown that active SCs mainly compensate for normal apoptosis and quiescent SCs repair damaged tissues. Moreover, it is assumed that SCs play a key role in the onset, development, invasion, and metastasis of tumors. Cancer stem cells (CSCs) refer to a group of tumor cells that are able to self-renew and produce diverse cells.[2] Conceptually, the major difference between SCs and CSCs lies in the fact that SCs can regulate and keep the state of their microenvironment, but CSCs cannot. SCs regulate cell proliferation, differentiation, and apoptosis, which coordinate with each other. The generation of a new cell implies the apoptosis of another cell. The error at any stage may cause an imbalanced state and finally the formation of tumors. EPITHELIAL–MESENCHYMAL TRANSITION STATUS OF CSCS AND TUMOR METASTASIS CSCs were originally detected in the malignant tumors of hematopoietic system,[3] and then were proved to exist in solid tumors such as tumors of the breast, colon, liver, etc.[4-7] The tumorigenic ability of CSCs is stronger than that of tumor cells. The vaccination of a few CSCs in immunodeficient mice may cause tumors. CSCs are involved not only S148
in the onset of tumors but also in the proliferation of tumors. A small amount of CSCs in tumors can induce the formation of the same tumor tissue with the primary tumor, which illustrates CSCs are able to generate diverse cells.
Shaoqiang Sun, Xue Shan Qiu1 Departments of Clinical Medicine and 1Pathology, China Medical University, Shenyang, China For correspondence: Dr. Xue Shan Qiu, Department of Pathology, China Medical University, Shenyang, China. E-mail: [email protected]
Mani et al. [8] have found that epithelial– mesenchymal transition (EMT) generates tumor cells with the properties of CSCs. Their study reveals a close relationship between CSCs and EMT. It implies that CSCs might promote the invasion and metastasis of tumors by acquiring some properties of the mesenchyma. When tumor cells interact with fibroblasts, granulocytes, macrophages, and other cells in niches, these cells may release the signal to induce EMT, resulting in increased invasion of tumor cells.[9] EMT occurs in many malignant tumors. In certain microenvironment, some epithelial cells acquire the properties of mesenchymal cells and thus invade the adjacent tissues or spread to remote organs. EMT and mesenchymal-to-epithelial transition (MET) play a certain role in embryo development, tissue regeneration, and injury repair. [10] EMT is also crucial in invasion and metastasis of malignant tumors. It has been proved that EMT in patients with tumors indicates poor prognosis.[11] Brabletz et al.[12] have found that cells induced by EMT spread unevenly at the invasion front of colorectal cancers, and the expression of Wnt signal transduction pathway increases. In addition, they have found that in the extracellular matrix, single cells express nuclear -catenin as well as salient mesenchymal characteristics such as the expression of vimentin and non-expression of E-cadherin. It is believed that EMT is irreversible
Access this article online Website: www.cancerjournal.net DOI: 10.4103/0973-1482.122510 PMID: *** Quick Response Code:
Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
Sun and Qiu: Functions of cancer stem cells in tumor metastasis and cancer treatment
during embryonic development; transitional mesenchymal cells can restore to their original epithelial state through MET. Similarly, the MET of CSCs may occur when the cells are induced by some factors during metastasis and lead to the formation of metastatic sites. Patient with tumors might have a better prognosis if CSCs in circulation could be detected and target killed. In recent years, most researchers[13] use associated specific antigens and epithelial cell surface makers such as epithelial cell adhesion molecule (EpCAM) to detect CSCs in peripheral circulation, so some CSCs which do not express associated proteins are difficult to detect. CSCS AND TUMOR METASTASIS Localization and chemotaxis of CSCs Hermann et al.[14] have found among CD133-positive pancreatic CSCs, some cells in circulation express C-X-C chemokine receptor type 4 (CXCR4) and these cells cause tumor metastasis. A study[15] shows CXCR4 is expressed at the surface of highly malignant and metastatic tumor cells. Hermann et al.[14] have also found CXCR4 receptor antagonists could reduce the circulating CSCs and tumor metastasis. Based on these results, it can be concluded that circulating CSCs play an important role in tumor metastasis. However, the specific metastatic mechanisms of different cancers are still unclear. Some people[16] think chemotaxis occurs between CSCs that can express CXCR4 and stromal cell-derived factor-1 (SDF-1). The concentration gradient of SDF-1 between the metastatic site and primary site induces the migration of CSCs. Because of chemotaxis, circulating CSCs migrate to certain organs or tissues and commit to the corresponding primary cancer. At present, no reliable evidence can prove that the circulating CSCs still retain the properties of SCs.[17] CSCs and the formation of the metastatic colonies In recent years, Pang et al.[18] have discovered that CD26 could be used as a CSC marker to predict the invasion and metastasis of colon cancer. Among 27 cases of colon cancer without hepatic metastasis, only 8 cases were CD26 positive. However, CD26-positive tumor cells were found in the primary and metastatic sites in all the patients with hepatic metastasis. The experiment on tumor formation in Nonobese Diabetic/ Severe Combined Immunodeficiency (NOD/SCID) mice shows that the vaccination of CD26-positive and -negative cells causes the formation of tumor, but metastatic sites only form in mice vaccinated with CD26-positive cells and only CD26-positive circulation cancer cells can be detected in portal veins. Al-Hajj et al.[4] vaccinated immunodeficient mice by limiting dilution analysis (LDA) and found that in breast cancer, the tumor cells which expressed CD44 and did not express CD24 or expressed CD24 to a lesser extent had the characteristics of SCs. It is reported that aldehyde dehydrogenase (ALDH) 1 can be used as a CSC marker for breast cancer to enrich CSCs.[19] ALDH can also be used as a marker for breast cancer to enrich CSCs. The vaccination Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
experiment in NOD/SCID mice showed that compared with cells with ALDH (low expression) CD44 (high expression/−), cells with ALDH (high expression) CD44 (+) CD24 (−) or ALDH (high expression) CD44 (+) CD133 (+) have advanced malignant and metastatic ability.[20] Yet, the latest study[21] suggests the expression of ALDH1 in liver cancer is not related to liver cancer cells and might be an index of differentiation in liver cancer. CSCS AND THE THERAPEUTIC STRATEGY FOR CANCER TREATMENT CSCs have been found in many types of tumors, but their relationship with clinical treatment and prognosis still deserves further study. As mentioned above, SCs are composed of two types: Active and quiescent. Quiescent SCs are similar to dormant cancer cells. Stimulated by some external factors, dormant cancer cells, which are a small part of cancer cells in normal condition, may acquire energy and proliferate, leading to relapse and metastasis of tumors.[22] Cancer cells are thought to be dormant before relapse when the primary cancer reoccurs 5 years after the malignant tumor is removed. Breast cancer reoccurs even 10 years after the surgery.[23] Based on the evidence, a question might arise whether dormant cancer cells are quiescent CSCs that spread from the primary site before the surgery. If so, it is not difficult to understand that their resistance to cytotoxic drugs results from their properties of SCs. Therefore, it is significant for cancer treatment if quiescent SCs could be target killed or killed when they proliferate again after induction. It has been proved that quiescent SCs exist in cancers such as melanoma, leukemia, breast cancer, pancreatic cancer, and liver cancer.[24-28] Quiescent CSCs are resistant to cytotoxic drugs because such drugs (e.g., drugs that induce DNA damage) can only affect proliferating cells. Besides, CSCs are resistant to radiotherapy or chemotherapy because of other mechanisms. There mechanisms are related to the fact that CSCs have the properties of SCs. For instance, the efflux mechanism of CSCs results in cancer resistance to several drugs, repair reaction after DNA damage or low concentration of reactive oxygen species (ROS).[29-30] Therefore, it is crucial to the treatment and prevention of tumor relapse that quiescent CSCs are killed, their resistance to radiotherapy or chemotherapy is damaged, or they are prevented from proliferation. PROBLEMS AND PROSPECTS Direct evidence that CSCs cause tumor metastasis is still lacking. With more and more reports[31,32] on the relationship between CSCs and tumor metastasis, it becomes clear that not all the tumor cells are involved in tumor metastasis. It is possible that only a small part of tumor cells that have the characteristics of SCs and the ability to invade play an important role in tumor metastasis. There cells can escape from autoimmune system and resist external damage, so they can induce new metastatic sites in certain microenvironment. S149
Sun and Qiu: Functions of cancer stem cells in tumor metastasis and cancer treatment
In sum, various molecular mechanisms regulate the differentiation, self-renewal, proliferation, chemotaxis, and resistance to cytotoxic drugs of CSCs. The mechanisms and individualized therapies should be further explored so that the invasion, metastasis, and resistance to radiotherapy or chemotherapy of CSCs could be reduced and the prognosis after surgery might be improved. As for solid tumors, they can be confined to the primary sites if the relationship between CSCs and tumor invasion or metastasis is clarified, CSCs are killed, or CSCs are prevented from invading the blood vessels from tumor stroma, or from invading or moving at metastatic sites within the blood vessels or lymphatic vessels. If so, the tumor can be removed completely, thus increasing the survival rate and life quality of patients. REFERENCES 1. 2.
3.
4.
5.
6.
7.
8.
9.
10. 11.
12.
13.
14.
Li L. Clevers H. Coexistence of quiescent and active adult stem cells in mammals. Science 2010;327:542-5. Visvader JE, Lindeman GJ. Cancer stem cells in solid tumours: Accumulating evidence and unresolved questions. Nat Rev Cancer 2008;8:755-68. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997;3:730-7. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 2003;100:3983-8. O’Brien CA, Pollett A, Gallinger S, Dick JE. A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 2007;445:106-10. Ricci-Vitiani L, Lombardi DG, Pilozzi E, Biffoni M, Todaro M, Peschle C, et al. Identification and expansion of human colon-cancer-initiating cells. Nature 2007;445:111-5. Ma S, Chan KW, Hu L, Lee TK, Wo JY, Ng IO, et al. Identification and characterization of tumorigenic liver cancer stem/progenitor cells. Gastroenterology 2007;132:2542-56. Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelial-mesenchymal transition generates cells with properties of stem cells. Cell 2008;133:704-15. Le NH, Franken P, Fodde R. Tumour-stroma interactions in colorectal cancer: Converging on beta-catenin activation and cancer stemness. Br J Cancer 2008;98:1886-93. Thiery JP, Acloque H, Huang RY, Nieto MA. Epithelial-mesenchymal transitions in development and disease. Cell 2009;139:871-90. Yang J, Weinberg RA. Epithelial-mesenchymal transition: At the crossroads of development and tumor metastasis. Dev Cell 2008;14:818-29. Brabletz T, Jung A, Reu S, Porzner M, Hlubek F, Kunz-Schughart LA, et al. Variable beta-catenin expression in colorectal cancers indicates tumor progression driven by the tumor environment. Proc Natl Acad Sci U S A 2001;98:10356-61. Pantel K, Brakenhoff RH, Brandt B. Detection, clinical relevance and specific biological properties of disseminating tumour cells. Nat Rev Cancer 2008;8:329-40. 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.
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15. Gassmann P, Haier J, Schlüter K, Domikowsky B, Wendel C, Wiesner U, et al. CXCR4 regulates the early extravasation of metastatic tumor cells in vivo. Neoplasia 2009;11:651-61. 16. Vagima Y, Lapid K, Kollet O, Goichberg P, Alon R, Lapidot T. Pathways implicated in stem cell migration: The SDF-1/CXCR4 axis. Methods Mol Biol 2011;750:277-89. 17. Pantel K, Alix-Panabières C, Riethdorf S. Cancer micrometastases. Nat Rev Clin Oncol 2009;6:339-51. 18. 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. 19. Ginestier C, Hur MH, Charafe-Jauffret E, Monville F, Dutcher J, Brown M, et al. ALDH1 is a marker of normal and malignant human mammary stem cells and a predictor of poor clinical outcome. Cell Stem Cell 2007;1:555-67. 20. Croker AK, Goodale D, Chu J, Postenka C, Hedley BD, Hess DA, et al. High aldehyde dehydrogenase and expression of cancer stem cell markers selects for breast cancer cells with enhanced malignant and metastatic ability. J Cell Mol Med 2009;13:2236-52. 21. Suzuki E, Chiba T, Zen Y, Miyaqi S, Tada M, Kanai F, et al. Aldehyde dehydrogenase 1 is associated with recurrence-free survival but not stem cell-like properties in hepatocellular carcinoma. Hepatol Res, 2012;42:1100-11. 22. Aguirre-Ghiso JA. Models, mechanisms and clinical evidence for cancer dormancy. Nat Rev Cancer 2007;7:834-46. 23. Hanrahan EO, Gonzalez-Angulo AM, Giordano SH, Rouzier R, Broglio KR, Hortobagyi GN, et al. Overall survival and cause-specific mortality of patients with stage T1a, bN0M0 breast carcinoma. J Clin Oncol 2007;25:4952-60. 24. Roesch A, Fukunaga-Kalabis M, Schmidt EC, Zabierowski SE, Brafford PA, Vultur A, et al. A temporarily distinct subpopulation of slow-cycling melanoma cells is required for continuous tumor growth. Cell 2010;41:583-94. 25. Saito Y, Uchida N, Tanaka S, Suzuki N, Tomizawa-Murasawa M, Sone A, et al. Induction of cell cycle entry eliminates human leukemia stem cells in a mouse model of AML. Nat Biotechnol 2010;28:275-80. 26. Pece S, Tosoni D, Confalonieri S, Mazzarol G, Vecchi M, Ronzoni S, et al. Biological and molecular heterogeneity of breast cancers correlates with their cancer stem cell content. Cell 2010;140:62-73. 27. Dembinski JL, Krauss S. Characterization and functional analysis of a slow cycling stem cell-like subpopulation in pancreas adenocarcinoma. Clin Exp Metastasis 2009;26:611-23. 28. Haraguchi N, Ishii H, Mimori K, Tanaka F, Ohkuma M, Kim HM, et al. CD13 is a therapeutic target in human liver cancer stem cells. J Clin Invest 2010;120:3326-39. 29. Bao S, Wu Q, McLendon RE, Hao Y, Shi Q, Hjelmeland AB, et al. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 2006;444:756-60. 30. Diehn M, Cho RW, Lobo NA, Kalisky T, Dorie MJ, Kulp AN, et al. Association of reactive oxygen species levels and radioresistance in cancer stem cells. Nature 2009;458:780-3. 31. Singh S, Chellappan S. Lung cancer stem cells: Molecular features and therapeutic targets. Mol Aspects Med 2013; In Press. doi: 10.1016/j. mam. 2013.08.03 32. Long H, Zhang S, Liu C, Shi J, Tao L, Situ D, et al. Characterization of a stem cell population in lung cancer cell line Glc-82. Thorac Cancer 2012;3:8-18.
Cite this article as: Sun S, Qiu XS. Cancer stem cells and tumor metastasis. J Can Res Ther 2013;9:S148-50. Source of Support: Nil, Conflict of Interest: No.
Journal of Cancer Research and Therapeutics - Supplement 3 - 2013 - Volume 9
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