Identification of cancer stem cells: from leukemia to solid cancers

Abstract

Cancer stem cells (CSCs) are widely considered to be a small cell population in leukemia and many solid cancers with the properties including self-renewal and differentiation to non-tumorigenic cancer cells. Identification and isolation of CSCs significantly depend on the special surface markers of CSCs. Aberrant gene expression and signal transduction contribute to malignancies of CSCs, which result in cancer initiation, progression and recurrence. The inefficient therapy of cancers is mainly attributed to the failure of elimination of the malignant CSCs. However, CSCs have not been detected in all cancers and hierarchical organization of tumors might challenge cancer stem cell models. Additionally, opinions about the validity of the CSC hypothesis, the biological properties of CSCs, and the relevance of CSCs to cancer therapy differ widely. In this review, we discuss the debate of cancer stem cell model, the parameters by which CSCs can or cannot be defined, and the advances in the therapy of CSCs.

Keywords

Cancer stem cell / leukemia / solid tumor / signal transduction / therapy

Introduction

Cancer stem cells (CSCs) are defined as a rare cell population in many cancers, and are capable of self-renewal and differentiation to various types of cancer cells. CSCs share many similarities with normal stem cells, except that they are also responsible for tumor growth, metastasis and invasion (Reya et al., 2001). In 1963, Bruce et al. discovered that not all cancer cells have the similar ability of proliferation, and only 1%-4% of cancer cells can form colony in vitro or turn into tumor in animal spleen (Bruce and Van Der Gaag, 1963). However, the first direct evidence of the presence of CSCs is provided by Bonnet in 1997. They demonstrated that CD34⁺CD38^- cells from acute myeloid leukemia (AML) patients could initiate AML in non-obese diabetic mice with severe combined immunodeficiency disease (NOD/SCID mice) and these cells possessed the capacities of self-renewal, proliferation and differentiation (Bonnet and Dick, 1997). Consequently, Al-Hajj identified CSCs in solid cancer for the first time through special molecular markers expressed on the surface of cells in 2003 (Al-Hajj et al., 2003). Hitherto, CSCs have been proved to exist in many other solid cancers like brain cancer (Hemmati et al., 2003; Piccirillo et al., 2006), breast cancer (Al-Hajj et al., 2003; Ponti et al., 2005), melanomas (Fang et al., 2005), lung cancer (Kim et al., 2005), colon cancer (O’Brien et al., 2007), ovarian cancer (Szotek et al., 2006) and prostate cancer (Collins et al., 2005), which have important values in clinical research.

Tumor growth model

Two distinct models have been proposed to account for tumor growth and the heterogeneity within a tumor (Fig. 1). In the cancer stem cell model, tumor growth depends exclusively on rare stem cells within it, and the tumor cells derived by differentiation from the cancer stem cells lack self-renewal potential and thus do not contribute significantly to tumor growth (Wang and Dick, 2005; Clarke et al., 2006). Heterogeneity within the tumor is ascribed to somewhat aberrant differentiation from the cancer stem cell. However, an alternative view, named as “clonal evolution model,” has been proposed (Campbell and Polyak, 2007). This model pointed out that most of the tumor cells contribute to tumor maintenance, albeit perhaps to varying degrees. This model ascribes tumor heterogeneity not only to differentiation, but also to genetic and epigenetic variation plus microenvironmental influences. It assumes that a tumor is composed of subclones at different stages of neoplastic progression, each having a variable growth and survival advantage over normal cells.

The cancer stem cell model is thus highly hierarchical with a unique self-renewing cell type at the top, whereas the clonal evolution model attributes much of the intratumor variation to subclonal differences in the mutational profile during the developmental stages of cancer, due to genetic and/or epigenetic changes plus environmental influences.

Origin of CSCs

Although CSCs have been positively identified in leukemia and other solid tumors, their originality is still unclear. Current opinion is that CSCs are mainly derived from normal stem cells. Stem cells self renew through asymmetric division to produce the progenitor cells and maintain their own pool cells. Stem cells and CSCs share many common properties including self-renewal, proliferation and differentiation, while the latter have the ability to proliferate unlimitedly (Fig. 2). In addition, they have similar signal pathways to regulate self-renewal such as RAS/mitogen-activated protein kinase (MAPK), Wnt/β-catenin, phosphatidylinositol 3-kinase (PI3K)/AKT and Nuclear factor kappa B (NF-κB) signaling. The same marker is usually expressed on the surface of stem cells and CSCs. For instance, hematopoietic stem cells (HSCs) and leukemia stem cells (LSCs) both express CD34 and CD90 (Wuchter et al., 2001; van der Pol et al., 2003).

However, CSCs in primary tumors do not always display the same properties as normal stem cells. The frequency of functionally defined CSCs can vary significantly among different patients. In some cases, CSCs are rather rare, whereas in others CSCs can constitute a substantial proportion of the tumor mass (Quintana et al., 2008). In normal steady-state systems, one can expect well-conserved development behavior, but upon any kind of substantial genetic or epigenetic influences, the rules that define cell and tissue behavior are not easily predicted. A genetic factor that might account partially for the diversity of abundance of CSCs in solid tumors is the ability of cells to undergo an epithelial-to-mesenchymal transition (EMT). Recent studies have suggested that induction of EMT in immortalized human mammary epithelial cells results in cells with stem-like properties (Mani et al., 2008). A number of other studies have found that cells at the leading invasive edge of solid cancers exhibit more mesenchymal features and are characterized by the expression of CSC markers (Ginestier et al., 2007; Hermann et al., 2007; Burk et al., 2008). Thus, there has been a convergence of the well-established concept that EMT is associated with tumor progression and with the cancer stem cells (Thiery, 2002; Brabletz et al., 2005). Changes in the surrounding microenvironment that influence expression of transforming growth factor beta (TGFβ) family members and other cytokines expressed by mesenchymal stem cells or other cells in the microenvironment may influence EMT, which is critical for metastasis and colonization at distant sites (Karnoub et al., 2007; Burk et al., 2008; Gregory et al., 2008; Wong et al., 2008). Therefore, the CSC state may be transitory in certain circumstances, with tumor cells acquiring a stem-like phenotype upon stimulation with the appropriate environmental factors.

Thus, in the context of inherently unstable conditions such as cancer, it is no surprise that CSCs display varying behaviors. Moreover, the properties of CSCs appear to be influenced by both the specific genetic alterations in a given tumor as well as the stage of neoplastic progression. Consequently, for any specific type of cancer the patient-to-patient variability of CSCs may be quite substantial. Together, these issues make any consistent definition of CSC properties very difficult, and the mechanisms of originality of CSCs are still not fully understood.

The surface molecular markers of CSCs

Most cancers including solid cancers and leukemia have been proved to consist of tumorigenic cells and nontumorigenic cells, and the tumorigenic cells which are capable of self-renewal and differentiation usually constitute only a small proportion of the whole. Recently, there are various methods used for enrichment and isolation of very small populations of CSCs, such as fluorescence-activated cell sorting (FACS) and Hoechst dye efflux technique.

CSCs are identified and obtained based on their characteristic of expressing special surface markers. However, somatic stem cells and their malignant compartment often share the same intrinsic and extrinsic factors for modulating proliferation and differentiation, and they also share the common surface markers. Indeed, surface markers are shared between CSCs and HSCs, neural stem cells, granulocyte-macrophage progenitors and these markers are specially related to tumorigenesis, progression, metastasis and recurrence in many malignant tumors (Yang and Chang, 2008). LCS is the earliest studied and most understood in the field of CSCs with the classic surface marker CD34⁺CD38^-. Initially, there were difficulties to distinguish between HSCs and CSCs as both kinds of cells express the same surface marker CD34 until it is confirmed by Bonnet that the molecular marker CD38 is absent on the surface of LSCs (Bonnet and Dick, 1997). In their experiment, only a few subpopulations of cells from AML patients can transfer disease upon translation in immunocompromised NOD/SCID mice. Experiments from others further demonstrated that cells with CD34⁺CD38^-Thy-1^- were able to proliferate in mice or in vitro (Blair et al., 1997). Jordan et al. showed that the interleukin-3 receptor alpha chain (IL-3 alpha or CD123) was strongly expressed in CD34⁺CD38^- cells (about 98% positive) from 16 of 18 primary specimens by flow cytometric analysis using primary AML tissue (Jordan et al., 2000). Furthermore, it was revealed that markers with CD34⁺CD38^-CD71^-HLA-DR^-CD90^-CD117^-CD123⁺ and CD90^-CD117^-CD123⁺ were uniquely present on the surface of LSCs but not of normal HSCs (Guzman and Jordan, 2004). In AML, apart from the CD34⁺CD38^- compartment, the side population (SP) compartment contains LSCs. Moshaver et al. showed that AML SP cells are distinguished from normal SP cells based on the expression of CLL-1 and lineage markers, and SP fraction contains different subpopulations. Therefore, the presence of a low fraction (median 0.0016%) SP subsets could be as a candidate for the enrichment of LSCs (Moshaver et al., 2008).

Identifying and isolating CSCs from solid cancers are useful to better understand surface markers. Al-Hajj et al. identified breast cancer cells with surface marker Lin^-ESA⁺CD44⁺CD24^-/low as special breast cancer initiating cells. They proved that this kind of breast cancer stem cells is more capable of transforming into tumor in NOD/SCID mice than Lin^-EAS^-CD44⁺CD24⁺ breast cancer cells (Al-Hajj et al., 2003). CD44⁺CD24^-Cx43^- cells isolated from tissues and cell lines of breast cancer were cultured in vitro. As a result, it was observed that these cells exhibited globular growth and expressed Oct-4, which is putatively deemed to be the marker of stem cell (Ponti et al., 2005). Although breast cancer with a high proportion of CD44⁺CD24^-/low cancer cells was more apt to osseous metastasis, there was no necessary correlation between the fraction of CD44⁺CD24^-/low breast cancer cells and tumor progression (Abraham et al., 2005). On the other hand, Singh et al. found that injection of a very small dose of CD133⁺ brain cancer cells can form tumor in NOD/SCID mice, indicating that CD133 is a good marker of CSC (Singh et al., 2004). In addition, bronchoalveolar stem cells with the surface marker Sca-1⁺CD45^-Pecam^-CD34⁺ and prostate cancer cells with CD44⁺α₂β₁^hiCD133⁺ are considered to be lung cancer stem cells and prostate cancer stem cells respectively (Collins et al., 2005; Kim et al., 2005).

Although progress has been made in understanding the surface molecules of CSCs, the surface markers are still not identified and isolated in all cancers. Even if there are some molecular markers identified on the surface of CSCs, the types of them are not enough to effectively discriminate CSCs in many other tumors.

Abnormal signal transduction maintains the tumorigenesis of CSCs

Wnt/β-catenin signaling involved in regulating HoxB4 and Notch1 is responsible for the self-renewal and proliferation of stem cells (Reya et al., 2003). The activation of the β-catenin pathway leads to the translocation of β-catenin into the nucleus, where it interacts with lymphoid enhancer factor/T-cell factor (LEF/TCF) transcription factors and regulates the expression of transcriptional genes such as c-myc (Fig. 3) (van Noort et al., 2002; Nusse, 2003; Tian et al., 2003). Nuclear β-catenin was expressed at high levels in granulocyte-macrophage progenitor pool from patients with chronic myelogenous (or myeloid) leukemia (CML) in accelerated phase and from those with CML in blast crisis. In addition, granulocyte-macrophage progenitors with a high level of β-catenin in CML have the ability of self-renewal and retain the ability to form myeloid colonies in colony-forming cell assays, whereas β-catenin-mediated renewal of granulocyte-macrophage progenitors in CML is inhibited by the expression of axin (Fig. 3), an inhibitor of β-catenin signaling, which reduces the self-renewal capacity of leukemia cells (Jamieson et al., 2004). Loss of β-catenin leads to a significant decrease in the capacity of mice to develop CML induced by BCR-ABL (Zhao et al., 2007). It has been revealed that transformation and maintenance of established leukemia in mouse models induced by co-expression of the Hoxa9 and Meis1a oncogenes depend on activation of the β-catenin pathway (Wang et al., 2010). Therefore, the activation of β-catenin signaling is necessary for the transformation of progenitors by certain oncogenes such as Hoxa9.

Aberrant PI3K/AKT pathway has been implicated in many human cancers, including AML and CML (Fig. 3). Carpten et al. discovered AKT1 with E17K mutation, which leads to constitutive activation of AKT1 (Carpten et al., 2007). We have previously observed AKT1 (E17K) mutation in v-Abl-transformed cells (Guo et al., 2010). Compared to wild-type AKT1, AKT1 (E17K) mutant greatly increased the efficiency of transformation induced by v-Abl oncogene and highly resisted to apoptosis by enhancing the expression levels of antiapoptotic protein BCL2 and phosphorylation levels of proapoptotic BAD. Furthermore, there is reciprocal signaling between AKT and Pim in v-Abl transformation because AKT1 (E17K) can rescue the inactivation of v-Abl after deletion of Pim-1 and Pim-2 protein levels in cells. Based on these findings, the relationship between AKT and Pim kinases and the detailed mechanism of the transformation by AKT mutant need to be fully understood, especially in LSCs. Kharas elucidated that transplantation of myristoylated AKT1(myr-AKT) to murine bone marrow led to the development of myeloproliferative disease, T-cell lymphoma, or AML in recipients and HSCs in myr-AKT mice experienced transient expansion and increased cycling correlated with impaired engraftment (Kharas et al., 2010). Therefore, the activation of AKT including mutations or overexpression of AKT itself has a key role in oncogene-induced transformation and maintaining tumorigenesis. Besides, several members of PI3K pathway, such as phosphatase and tensin homolog (Pten) (Fig. 3), mammalian target of rapamycin (mTOR), have functions in proliferation of HSCs and LSCs. The silencing or inactivating mutations of Pten have been identified widely in various human neoplasias, including prostate cancer, endometrial carcinomas, glioblastomas, melanoma and T-cell acute lymphoblastic leukemia (Gutierrez et al., 2009). The aberrations of downstream of PI3K/AKT pathway result in upregulation of AKT signal transduction and progression of cancers. The functional involvement of aberrant PI3K/AKT pathway in CSC development remains to be further determined.

Another important signal transduction in tumor initiation is the Janus kinase/signal transducers and activators of transcription (JAK/STAT)/Pim pathway linked to survival of cancer cells. JAK/STAT holds the model that cytokine-receptor compound stimulates the phosphorylation of JAK associated to the receptor cytoplasmic domain allowing recruitment of STAT, which is phosphorylated, dimerizes and moves to the nucleus to bind special DNA sequences and initiates gene expression related to cell survival (Fig.3) (Murray, 2007). The aberrant JAK/STAT pathway has been detected in many cancers, especially in leukemia. JAK2-V617F is increasingly activated by IL-27R, a component of a heterodimeric type I cytokine receptor which has been identified in patients with AML and can induce the transformation of hematopoietic cells (Pradhan et al., 2007). Haan discovered that suppressor of cytokine signaling (SOCS) proteins were bound to JAK2 (V617F, T875N and K539L) on membranes to make the JAK2 mutations ubiquitinated and degraded through proteasome. Therefore, SOCS proteins inhibit the activation of JAK2 mutants from patients with myeloproliferative disorders (Fig. 3) (Haan et al., 2009). In addition, Pim kinases exert essential functions in v-Abl transformation possibly through regulation of SOCS-1 and apoptotic signaling (Chen et al., 2008). The particular connection among ABL, JAK/STAT, SOCS, and Pim is not fully understood and how they play a role in forming LSCs and keeping tumorigenesis is under-investigated.

In brief, many signaling pathways are potentially involved in the development of CSCs. The balance between survival and apoptosis signaling is necessary for cells to protect themselves from malignancy. Profound elucidation of the network constituted by various pathways in cancer cells has significant sense to study CSCs and design medicament against CSCs.

CSCs debate

Studies that initially documented the CSC concept were carried out with acute myelogenous leukemia (Bonnet and Dick, 1997) and solid tumors, which have all been found to be sustained by a minority of malignant cells with a capacity for self-renewal. However, these studies, in particular those involving solid tumors, are controversial, because they are based in large part on xenotransplatation assays in which human tumor cells were grown in immunocompromised mice (Clarke et al., 2006). The possibility exists that CSCs preferentially adapt and proliferate when grown in a foreign species.

Although studies on different cancers have revealed that only rare human cancer cells (0.1% to 0.0001%) have tumorigenic potential when transplanted into NOD/SCID mice, Morrison’s group found that at least 25 percent of melanoma cells had the capacity of unlimited proliferation and tumor formation in NOD/SCID \mathrm{I}\mathrm{L}\mathrm{2}{\mathrm{R}}_{\mathrm{\gamma }}^{\mathrm{n}\mathrm{u}\mathrm{l}\mathrm{l}} mice (Quintana et al., 2008). The inconsistent results were due to the variance between different mice used in animal experiment. Although NOD/SCID mice have deficiencies in immune system that lacks B cells and T cells, natural killer (NK) cells still exist in them to exert biological functions of attacking the cancer cells from human, whereas NOD/SCID \mathrm{I}\mathrm{L}\mathrm{2}{\mathrm{R}}_{\mathrm{\gamma }}^{\mathrm{n}\mathrm{u}\mathrm{l}\mathrm{l}} mice lack NK cells in addition to B and T cells. Thereby, the percentage of melanoma cells that form tumor in NOD/SCID \mathrm{I}\mathrm{L}\mathrm{2}{\mathrm{R}}_{\mathrm{\gamma }}^{\mathrm{n}\mathrm{u}\mathrm{l}\mathrm{l}} mice is orders of magnitude higher than the percentage that form tumor in NOD/SCID mice. This suggests that NOD/SCID mice transplantation assay has probably underestimated the frequency of cancer cells with tumorigenesis in some cancers. Even if the result from NOD/SCID \mathrm{I}\mathrm{L}\mathrm{2}{\mathrm{R}}_{\mathrm{\gamma }}^{\mathrm{n}\mathrm{u}\mathrm{l}\mathrm{l}} mice transplantation assay shows a significant increase in the fraction of human cancer cells with tumorigenic potential, it is inaccurate for the differences between mouse and human tissue microenvironments and immune responses. Furthermore, in some cancers, many tumor cells were found to have tumorigenic potential (Kelly et al., 2007; Williams et al., 2007), implying that the frequency of tumorigenic cells might be higher in certain cancers.

The concept that a specific subpopulation of tumor cells possesses distinct stem cell properties implies that CSCs arise as an intrinsic property of tumor biology and development. However, the surrounding microenvironment and the immune system are known to play important roles in cancer progression (Bissell and Labarge, 2005; Mantovani, 2009). Therefore, the animal model might be problematic because a xenograft lacks of an appropriate microenvironment and lacks of an intact immune system when evaluating the tumor-initiating capacity of these human cancer cells. Thus, it is likely that the subpopulation of cells that appeared nontumorigenic might actually be tumorigenic in the presence of the appropriate microenvironment. Therefore, to re-estimate cancer stem cell model is necessary for its limitations, and to find an ideal animal model is critical.

The therapy of CSCs

The traditional cancer treatments aiming at killing most of cancer cells have been developed rapidly. However, the standard treatments omit CSCs and therefore enhance the resistance of tumors to certain medicine, chemotherapy and radiotherapy, leading to an extremely low curative ratio and a potential source of relapse.

CML begins in chronic phase (CP), and then it progresses to accelerated phase, finally to blast crisis (BC). As a tyrosine kinase, BCR-ABL, which is present during the whole progression of CML, is targeted by tyrosine kinase inhibitors (TKIs), such as imatinib mesylate (IM), nilotinib and dasatinib. IM is the first inhibitor launched to the market owing to its remarkable specificity, safety and efficacy in CP CML (Druker et al., 2001). However, with the proceeding of CML, the therapeutic efficiency of IM has remarkably decreased, and CML relapses after years in the absence of IM. It can be explained by the theory that the surviving cells are TKI-refractory LSCs. There might be two explanations to interpret TKI-insensitivity of LSCs. One is BCR-ABL-dependent way, that is, overexpression of BCR-ABL or mutations in the kinase domain of BCR-ABL (Deininger et al., 2000) including T315I, V304D (Jiang et al., 2010) leads to resistance to TKIs. The self-renewal of LSCs still depends on BCR-ABL oncoprotein, while the drug is inefficient in inhibiting BCR-ABL. The other is BCR-ABL-independent pathways in which anti-apoptotic, pro-survival signals are provided by alternative pathways via genetic and epigenetic mutations even when the activity of BCR-ABL is actually inhibited by TKIs (Jørgensen and Holyoake, 2007; Savona and Talpaz, 2008).

Recently, a number of treatments of CSCs including CML stem cells have been advanced. BMS-214662, a putative farnesyl transferase inhibitor (FTI) with little effect on normal stem cells, is effective against CML stem sells in combination with IM, dasatinib or MAPK/extracellular signal regulated kinase (ERK) kinase 1/2 (MEK1/2) inhibitors (Jørgensen and Holyoake, 2007). Moreover, compared to monotreatment, IM with Bortezomib or proteasome inhibitor is more efficient to decrease BCR-ABL kinase activity (Hu et al., 2009). Clinical investigations demonstrate that nilotinib is more effective than imatinib in treatment of CML (Jabbour et al., 2010). The synergy between histone deacetylase inhibitors (HDACis) and IM more effectively induces apoptosis in quiescent CML progenitors than IM does alone. This combination arrests CML stem cells to engraft immunodeficient mice and significantly leads to a decrease in the number of LSCs in vivo (Zhang et al., 2010). It is important to explore other inhibitors that work through ABL-kinase-independent mechanisms in combination with TKIs. CD44, which functions in homing and engraftment, is highly expressed on mouse stem-progenitor cells within BCR-ABL. Since LSCs rely on CD44 more than normal HSCs do, CD44 is a possible target for treating CML (Krause et al., 2006). The activity of nuclear factor of activated T cells (NFAT) is impaired by the inhibition of the Wnt/Ca²⁺/NFAT pathway, leading to increased sensitivity to BCR-ABL inhibitor dasatinib. Besides, NFAT inhibitor with cyclosporin A actually promoted the apoptosis of leukemia cells (Gregory et al., 2010). Pten has a pivotal function in suppressing the progression of BCR-ABL-induced leukemias such as CML and B-cell acute lymphoblastic leukemia (B-ALL), which provides a potential treatment of Ph⁺ leukemia (Peng et al., 2010).

In addition, acute promyelocytic leukemia (APL) induced by the fusion oncoprotein promyelocytic leukemia-retinoic acid receptor-α (PML-RARA) is alleviated by the treatment with combination of retinoic acid and arsenic trioxide, which results in degradation of PML-RARA and inhibits the proliferation of leukemia-initiating cells (LICs). Since eradication of LICs caused by retinoic acid is increased via activation of cAMP pathway, cAMP is thought to be another potential target to treat APL (Nasr et al., 2008). Herman et al. demonstrated that CAL-101, as a PI3K-delta selective inhibitor, facilitated apoptosis in primary chronic lymphocytic leukemia (CLL) cells lacking abnormal fusion protein kinase in vitro. Compared to malignant cells, CAL-101 has little positive effect on apoptosis in normal T cells or NK cells (Herman et al., 2010). Targeting Wnt/β-catenin pathway which is essential for the self-renewal of CSCs, but not for normal epidermal homeostasis (Malanchi et al., 2008) might afford a new therapeutic opportunity in many cancers like AML (Wang et al., 2010) or malignant human squamous cell carcinoma (Malanchi et al., 2008). Genetically engineered stem cells (GESTECs) expressing cytosine deaminase or carbosyl esterase have the capacity of migrating to ovarian cancer cells and inhibiting the growth of them with the prodrugs 5-fluorocytosine or camptothecin-11 and these results provide a possible treatment of ovarian cancer (Kim et al., 2010). This potential treatment might also be applied to other cancers such as leukemia.

To date, curative potential of immunotherapy of tumor cells including CSCs has been demonstrated clearly, and newer approaches focus on immune targeting of antigens which are the special markers of CSCs. A recombinant yeast-based vaccine expressing the BCR-ABL(T315I) antigen has the ability to stimulate the anti-BCR-ABL(T315I) immune response, leading to diminished BCR-ABL(T315I) leukemia cells in immunized animals (Bui etal., 2010). Thereby, yeast-based immunotherapy might overcome the cancer drug resistance in treatment of tumors. Furthermore, non-coding RNAs play a critical role in regulating the expression of genes involving proliferation, differentiation and apoptosis of normal cells and cancer cells including CSCs. Infecting breast tumor-initiating cells (BT-IC) with let-7 miRNAs-lentivirus inhibited proliferation in vitro and tumor formation and metastasis in NOD/SCID mice, while downregulation of let-7 facilitated survival of cancer cells (Yu et al., 2007). Lin 28 is shown to be essential for blocking cleavage of pri-let-7 miRNAs (Viswanathan et al., 2008); thus Lin28, which probably promotes the progression of certain cancers, might be targeted for treatment of these cancers. Besides, overexpression of polycistronic miRNAs in K562 cells enhanced proliferation, and upregulation of ploycistronic pri-miRNA transcripts has been detected in CML. The results demonstrate that these miRNAs might be potential therapeutic targets (Venturini et al., 2007). However, the mechanism that long non-coding RNAs affect tumorigenesis of cancer cells is still elusive.

Nonetheless, the therapies including chemotherapy, targeting therapy and immunotherapy are partially effective. Especially, to eliminate CSCs from cancers remains to be a challenging task for researchers all over the world.

Future directions

In summary, compelling data have suggested that CSCs exist in leukemia and many solid cancers to maintain tumorigenesis, while the originality of CSCs is still in dispute as well as cancer stem cell models. Identification and isolation of CSCs are based on the surface markers of CSCs through certain biological techniques such as FACS. However, the identified surface markers are insufficient and more special molecular markers should be found in order to isolate CSCs more accurately. It is important to understand genetic and epigenetic differences between tumorigenic and nontumorigenic cancer cells. Besides, precise studies are required to examine the relationships among signaling pathways in CSCs. Nevertheless, there exist differences between LSCs and solid cancer stem cells in properties including surface markers. Moreover, the molecular mechanisms regarding initiation and progression of leukemia and solid tumors are not completely identical. Thus, different therapeutic options should be developed against leukemia and solid cancers. For example, promyelocytic leukemia (PML) is often absent in many solid cancers, while it exhibits high level expression in CML blasts and is necessary for maintenance of LICs. Therefore, targeting PML in CML patients might eliminate LICs and lead to regression of CML (Ito et al., 2008). Novel treatments targeting CSCs in different cancers have been increased over the past several years and the better understanding of the biological properties of CSCs will provide potential targets for therapeutic strategies (Fig. 2).

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