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Abstract

Cancer stem cells can generate tumors from only a small number of cells, whereas differentiated cancer cells cannot. The prominent feature of cancer stem cells is its ability to self-renew and differentiate into multiple types of cancer cells. Cancer stem cells have several distinct tumorigenic abilities, including stem cell signal transduction, tumorigenicity, metastasis, and resistance to anticancer drugs, which are regulated by genetic or epigenetic changes. Like normal adult stem cells involved in various developmental processes and tissue homeostasis, cancer stem cells maintain their self-renewal capacity by activating multiple stem cell signaling pathways and inhibiting differentiation signaling pathways during cancer initiation and progression. Recently, many studies have focused on targeting cancer stem cells to eradicate malignancies by regulating stem cell signaling pathways, and products of some of these strategies are in preclinical and clinical trials. In this review, we describe the crucial features of cancer stem cells related to tumor relapse and drug resistance, as well as the new therapeutic strategy to target cancer stem cells named “differentiation therapy.”

Keywords

Cancer stem cells stemness signaling pathway differentiation therapy

Introduction

Studies have revealed that cancer stem cells (CSCs) are a rare cell population in most cancers, including blood cancers and solid tumors. CSCs, also known as tumor-initiating cells, have the abilities to self-renew and maintain their stemness traits, to differentiate into various non-stem cancer cells, and to play a crucial role in cancer propagation, metastasis, and recurrence.¹ CSCs were first identified in acute myeloid leukemia (AML) in 1997;² the first CSCs from solid tumors were identified in breast cancer in 2003.³ So far, CSCs have been isolated from almost all solid tumors, including brain cancer,⁴ colon cancer,⁵ pancreatic cancer,⁶ prostate cancer,⁷ melanoma,⁸ and ovarian cancer.⁹ CSCs are the smallest population at the apex state of the cancer cell hierarchy, but they play the most important role in tumorigenesis.^10–12 They are also highly resistant to traditional radiotherapy and chemotherapy; therefore, the tumor can recur within short periods.^13,14 In addition, CSCs are considered a source of local invasion and metastasis, demonstrating the recurrence of tumors and the difficulty of achieving a complete cure.^15–17 However, lower in the hierarchy, differentiated cancer progenitor cells and terminally differentiated cancer cells comprise most of the cancer cell population and do not generate tumors.¹⁰ Therefore, this indicates the importance of CSCs and the need to target CSCs to eliminate cancer.

Cell origin and markers of CSCs

There are two hypotheses regarding the “cell origin” of CSCs. One is that CSCs are mutated adult stem cells (ASCs). This hypothesis states that the accumulation of genetic mutations in ASCs during cell division drives cancer initiation. This is most to tissues such as skin or intestine that have a high cell turnover rate.¹⁸ The other is that CSCs are de-differentiated mutated cells. This hypothesis states that mutations in differentiated cells can cause de-differentiation, with these cells acquiring stem cell–like characteristics. Sox2 has been reported to promote de-differentiation of pancreatic cancer cells, causing them to express stem cell–like traits.¹⁹ In the case of glioblastomas, ID4 is the key factor that de-differentiates cancer cells into glioma stem cells.²⁰ This hypothesis suggests that every single differentiated cancer cell has the potential to be converted to a CSC.

It becomes important to ask how could we isolate and target CSCs? CSCs can be isolated by identifying their specific cell surface biomarkers. The first identified CSCs from AML had a CD34⁺CD38⁻ phenotype.² Since then, more specific surface biomarkers for CSCs in AML have been discovered. For instance, CD34⁺CD38⁻HLA⁻DR⁻CD71⁻CD90⁻CD117⁻CD123⁺ cells are significantly different from normal hematopoietic stem cells (HSCs).²¹ In the case of solid tumors, the first CSCs isolated from breast cancer were ESA⁺CD44⁺CD24^−/low cells.³ Moreover, CD133 is a common cell surface biomarker found in brain,⁴ liver,²² lung,²³ colon,⁵ and pancreatic²⁴ cancers. Cells in all of these solid tumors have other specific cell surface markers, such as SSEA1 for gliomas,²⁵ CD44 for colon cancers,²⁶ α2β1 for prostate cancers,²⁷ and ABCG2 for lung cancers.²⁸ Some studies have also shown that CD133 is not a specific biomarker for CSCs in gliomas. CD133⁻ glioma cells also have CSC properties with tumor-initiating ability.^29,30 However, cell surface biomarkers are still critically important for isolating CSCs, and additional therapies that target CSCs using CSC biomarkers should be developed to effectively eradicate cancer.

Signaling pathways in CSCs

To regulate adult tissue homeostasis, ASCs maintain their self-renewal properties by controlling multiple stemness signaling pathways (Wnt/β-catenin, Hedgehog/Gli, Jagged/Notch, Janus kinase/signal transducer and activator of transcription (JAK/STAT), and nuclear factor kappa beta (NF-κB)) and differentiation signaling pathways (bone morphogenetic protein (BMP) and retinoic acid (RA); Figure 1). Similar to ASCs, CSCs possess same stemness and differentiation signaling pathway networks to retain their capacity, which are frequently dysregulated.

Figure 1.

Representative signaling pathways in cancer stem cells. Multiple stemness signaling pathways (light red area) such as Wnt, Shh, Notch, JAK/STAT, and NF-κB and differentiation signaling pathways (light blue area) such as BMP and RA regulate self-renewal and differentiation of cancer stem cells as well as in adult stem cells. Wnt signaling pathway needs the interaction of ligand and receptor to disrupt the “destruction complex” so that β-catenin translocates into nucleus to activate transcription of various stemness factors. Shh signaling pathway also needs the interaction of ligand and receptor and leads to translocation of Gli into nucleus and transactivation of stemness factors. Notch receptor generates Notch intracellular domain (NICD) through two-time proteolysis after binding with its ligand, and NICD translocates into nucleus to activate transcription of stemness factors. The Janus kinase (JAK) activated by interaction of cytokines phosphorylates signal transducer and activator of transcription (STAT), leads to translocation of STATs into nucleus and activate transcription of target stemness factors. Canonical NF-κB signaling is activated by the IKK complex. Loss of IκBα leads to the accumulation of p65-p50 dimers in the nucleus and induces transcription of target genes. As a representative differentiation signaling pathway, BMP signaling needs phosphorylation and activation of receptor by BMP ligands. The activated receptor further phosphorylates receptor-regulated Smads, and the Smads translocate into nucleus to activate transcription of differentiation factors. After interaction with retinoic acid (RA), retinoic acid receptor binds to retinoic acid response element and activates transcription of differentiation factors. Each number in this figure represents the targeted therapeutic points listed in Table 1.

A canonical Wnt/β-catenin signaling pathway requires nuclear translocation of β-catenin to sustain normal neural stem cell status.³¹ When a Wnt ligand binds to and activates its Frizzled family receptor, the receptor protein disrupts the Axin-GSK3-APC “destruction complex,” leading to stabilization of the transcription factor β-catenin by preventing its degradation. Then, β-catenin translocates to the nucleus and activates transcription of downstream genes involved in stemness. However, aberrant activation of the Wnt signaling pathway, including mutations in adenomatous polyposis coli (APC), which eliminate its ability to form the “destruction complex” and constitutively activate β-catenin, is the most well-known cause of human colon carcinoma.³² Such mutations in APCs have also been found in other malignancies such as leukemia and breast and cutaneous carcinomas.^33,34 Besides APC, β-catenin mutation is the other most well-known cause that constitutively activates Wnt signaling pathway in gastric cancers and colorectal cancers.^35,36 Also, R-spondin mutations and its fusion proteins and deleterious RNF43 mutations were indicated as a driver of tumor growth in the Wnt-dependent manner in colorectal cancers.^37–39 In some chronic lymphocytic leukemia (CLL) cases, frequent silencing of DKK1/2, Wnt inhibiting factors, and somatic mutations in FZD5 and BCL9 were found.^40,41 Mutation or deletion of Axin1, one of the intracellular components of “destruction complex” in Wnt/β-catenin signaling pathway, has been also reported to promote malignancies of several cancers, such as hepatocellular carcinomas, sporadic medulloblastomas, and colorectal cancers.^42–44

The Hedgehog/Gli signaling pathway is activated by nuclear translocation of Gli in NSCs.⁴⁵ When a Hedgehog ligand binds to its cell surface transmembrane receptor, patched (PTCH) and smoothened (SMO) protein, which is suppressed by PTCH, leads to translocation of the Gli2 transcription factor to the nucleus and regulation of transcription of downstream genes relative to stemness. However, three different mechanisms of aberrant activation of hedgehog signaling pathway were found in multiple types of cancers.⁴⁶ The first one is a ligand-independent signaling pathway activation by mutations. The patients with basal cell carcinomas showed mutations in PTCH1 or SMO so that the hedgehog signaling pathway is constitutively activated at a ligand-independent manner.^47–50 The second one is a ligand-dependent signaling pathway activation by autocrine manner. Overexpression or mutations in intracellular hedgehog signaling pathway components, such as SUFU, Gli1, and Gli3, are associated with tumorigenesis of medulloblastoma or pancreatic cancers.^51–54 The third one is a ligand-dependent signaling pathway activation by paracrine manner. Hedgehog ligand secreted from tumor cells binds to receptor and activates hedgehog signaling pathway in stromal cells, which provides vascular endothelial growth factor (VEGF) and insulin-like growth factor (IGF) reversely to tumor cells.^55,56 Furthermore, there is a reverse paracrine manner found in some B-cell lymphomas and multiple leukemia. Hedgehog ligand secreted from stromal cells activates Hedgehog signaling pathway in tumor cells, in turn leading to activation of proliferation and survival.^57,58

Notch is a receptor that interacts with the Jagged ligand. After Jagged/Notch interactions, two proteolysis reactions (including γ-secretase) generate the Notch intracellular domain (NICD) that translocates to the nucleus and regulates transcription of downstream genes involved in stemness.⁵⁹ In mammals, there are four Notch receptors (Notch1, Notch2, Notch3, and Notch4) and five ligands (DLL1, DLL3, DLL4, JAG1, and JAG2). In gliomas, Jagged/Notch signaling plays an important role in crosstalk between CSCs and endothelial cells. The signaling activated by PDGF-NO-ID4 axis promotes tumor progression through increase in CSC self-renewal and tumor vasculature.⁶⁰ Mutations in Notch receptor activate the Notch downstream signaling pathway and transactivate gene expressions involved in stem cell signaling in a Jagged ligand-independent manner. Notch1 mutations activate Jagged/Notch signaling pathway and promote tumorigenesis in breast cancer or T-lineage acute lymphoblastic leukemia (T-ALL).^59,61,62 Besides, loss-of-function mutations in FBXW7, which decrease turnover of the NICD, were found in some cases of T-ALL patients.^63,64 Gain-of-function mutations in Notch2 were reported in diffused large B-cell lymphomas.^65,66 Moreover, accelerated Notch4 receptor activity and loss or downregulation of Notch signaling regulator Numb have been reported in CSCs in breast cancer.^67,68

JAK/STAT signaling also plays an important role in both adult tissue homeostasis and tumorigenesis. JAK is activated by interactions between ligands such as cytokines and growth factors and their cell surface receptors. Activated JAK phosphorylates STAT, and phosphorylated STATs translocate to the nucleus as heterodimers or homodimers where the downstream genes are transactivated.⁶⁹ However, 9% of patients with hepatocellular carcinoma showed missense mutations in JAK1, resulting in activation of JAK/STAT signaling pathway and cytokine- and receptor-independent growth.⁷⁰ In addition, JAK2-V617F mutations have been found responsible for hematopoietic malignancies, and this point mutation overactivates the JAK/STAT signaling pathway to promote cytokine-independent growth.^71–74 Furthermore, there are other types of mutations in JAK2, such as K539L,^75,76 T875N,⁷⁷ and deletion of IREED.⁷⁸ In acute megakaryoblastic myeloid leukemia, JAK3 mutations on A572V, V722I, or P132T have been identified.⁷⁹ Although, rare amplifications in STAT5A/B locus have been identified in prostate cancer. These amplifications increase translocation of STAT5 into nucleus and messenger RNA (mRNA) expression, leading to promotion of tumor growth.⁸⁰

NF-κB family contains five members, such as p65 (RelA), NF-κB1 (p105/p50), NF-κB2 (p100/p52), RelB, and c-Rel. The precursors p105 and p100 cleave to active subunits p50 and p52.⁸¹ NF-κB canonical signaling pathway is generally activated by various stimuli, including lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-α), or interleukin-1 (IL-1). Without stimulation, p65-p50 dimers bind to the inhibitor of κB proteins (IκBs), retaining the localization of NF-κB at the cytoplasmic at its steady state. While the receptors are activated by stimuli, IκBα is phosphorylated and induces its ubiquitination proteasomal degradation by the IκB kinase (IKK) complex, which comprised one non-catalytic regulatory subunit IKKγ (a.k.a NF-κB essential modifier (NEMO)) and two kinase subunits IKKα and IKKβ. After loss of IκBα, NF-κB is accumulated in nucleus, further binds to DNA and promotes transcription of target genes which involve in inflammation, anti-apoptosis, survival, and proliferation.^82–86 Furthermore, aberrant and constitutive activation of NF-κB signaling pathway also regulates cancer promotion, metastasis, and angiogenesis.^87–90 Next-generation sequencing analysis revealed that amplifications of c-Rel, IKKβ, and NEMO and mutations on upstream factors Carma1, Bcl10, and Malt1 genes have been found in some lymphomas and breast cancers.^91,92 Besides, overexpression of growth factor receptors, such as epidermal growth factor receptor (EGFR) and Her2, also could activate NF-κB signaling pathway in cancers.^93,94 In case of glioblastoma, NF-κB-dependent transition of proneural to mesenchymal subtype promotes radiotherapy resistance and poor prognosis.⁹⁵ NF-κB also maintains tumor growth–promoting macrophage phenotype (M2) instead of tumor growth–repressing macrophage phenotype (M1).^96,97

To date, many types of inhibitors of stemness signaling pathways have been developed, such as β-catenin inhibitors and ligand antagonists targeting Wnt/β-catenin signaling, SMO and Gli inhibitors targeting Hedgehog signaling, γ-secretase inhibitors and ligand antagonists targeting Notch signaling, JAK and STAT inhibitors targeting JAK/STAT signaling, and NF-κB signaling inhibitors (Table 1). Most of these inhibitors and antagonists are in the preclinical and clinical trial phase I/II/III, and one of them has already been approved by the FDA.

Table 1.

Inhibitors or antagonists used for targeting stemness signaling pathways.

Signaling	Name	Target	Number in Figure 1	Trials	References
Notch	DAPT	γ-Secretase	1	Preclinical	Fan et al.⁹⁸
	MRK-003	γ-Secretase	1	Preclinical	Kondratyev et al.⁹⁹
	MK0752	γ-Secretase	1	Clinical I	Zhang et al.¹⁰⁰
	R04929097	γ-Secretase	1	Clinical II	Richter et al.¹⁰¹
	OMP-21M18	DLL4	2	Clinical I	Smith et al.¹⁰²
Wnt	ICG-001	CBP/β-catenin	3	Clinical I	Takahashi-Yanaga and Kahn¹⁰³
	PRI-724	CBP/β-catenin	3	Clinical II	Gang et al.¹⁰⁴
	JW67	β-catenin	3	Preclinical	Waaler et al.¹⁰⁵
	OMP-54F28	Frizzled	4	Clinical I	Le et al.¹⁰⁶
	OMP18R5	Frizzled	4	Clinical I	Gurney et al.¹⁰⁷
Hedgehog	Cyclopamine	Smoothened	5	Preclinical	Xia et al.¹⁰⁸
	IPI-269609	Smoothened	5	Clinical I	Feldmann et al.¹⁰⁹
	GDC-0449 (Vismodegib)	Smoothened	5	Clinical II	Singh et al.¹¹⁰
	As2O3 (Arsenic trioxide)	Gli	6	FDA approved	Beauchamp et al.¹¹¹
	Gant61	Gli	6	Preclinical	Lauth et al.¹¹²
JAK/STAT	CHZ868	JAK2	7	Preclinical	Wu et al.¹¹³
	Tofacitinib	JAK2/3	7	Preclinical	Elliott et al.¹¹⁴
	Ruxolitinib	JAK1/2	7	Preclinical	Dechow et al.¹¹⁵
	Niclosamide	STAT3	8	Clinical I	Shi et al.¹¹⁶
	OPB-31121	STAT3	8	Clinical I	Bendell et al.¹¹⁷
NF-κB	PDTC	NF-κB	9	Preclinical	Fan et al.¹¹⁸
	Paclitaxel	NF-κB	9	Preclinical	Ganta and Amiji¹¹⁹
	Sulforaphane	NF-κB	9	Preclinical	Rausch et al.¹²⁰
	Curcumin	NF-κB	9	Preclinical	Bentires-Alj et al.¹²¹

JAK/STAT: Janus kinase/signal transducer and activator of transcription; NF-κB: nuclear factor kappa beta.

Induction of CSC differentiation

ASCs and CSCs have the potential to differentiate into multiple types of cells, including lineage-differentiated cells and non-tumorigenic-differentiated cancer cells upon exposure to specific differentiation signals (Figure 1).

RA, a small lipophilic derivative of vitamin A, is a well-known inducer of differentiation and proliferation in normal stem cells. By interacting with RA, retinoic acid receptor (RAR) binds to the retinoic acid response element (RAREs) on DNA as a heterodimer with retinoid X receptor (RXR), leading to transcription of genes that regulate differentiation.¹²² During early vertebrate organogenesis, RA functions as an essential factor for intercellular signaling.¹²³ A pharmaceutical form of RA named tretinoin was developed and is already used as an inducer in differentiation therapy for patients with acute promyelocytic leukemia (APL).¹²⁴ It is also known as all-trans retinoic acid (ATRA), a carboxylic acid form of vitamin A. Induction of differentiation by treatment with ATRA can increase sensitivity to therapies and reduce CSC motility and tumorigenicity by blocking angiogenic cytokines in glioblastomas.¹²⁵ ATRA treatment to breast cancer stem-like cells, MCF7/C6, leads to cell differentiation, reduced invasion and migration, and increased sensitivity to anticancer treatment.¹²⁶ However, in some glioma study, ATRA induces differentiation of CSC, but their differentiation was not induced perfectly, thereby glioma CSC appears again.¹²⁷

BMP signaling is the most powerful inducer of differentiation. BMP is a secreted glycoprotein belonging to the transforming growth factor beta (TGF-β) superfamily. It is a ligand that binds to receptor BMPR2, which phosphorylates the neighbor receptor BMPR1. Then, phosphorylated BMPR1 induces phosphorylation of receptor-regulated Smads (R-Smads: including Smad1, Smad5, and Smad8). Furthermore, the phosphorylated R-Smads form heterodimers with co-Smad (Smad4) and translocate to the nucleus, where downstream genes are transcriptionally regulated.¹²⁸ Most genes transcriptionally activated or suppressed by BMP signaling are regulators of terminal differentiation.¹²⁹ BMP signaling has two side functions in cellular differentiation in adult tissues: BMP signaling functions as both a suppressor of Wnt/β-catenin signaling pathway and a cell fate determinant by inducing differentiation. These phenomena often occur in the skin, central nervous system, and intestine.^130–132 At an early phase in embryonic development, BMP signaling promotes the growth of neural stem cells. In a later phase, BMP signaling blocks proliferation of neural precursors and induces their differentiation.¹³³ In the case of cancers, impaired BMP signaling or overexpression of BMP antagonists promotes tumorigenesis in primary tumor sites or cancer cell colonization in metastatic sites. For example, the tumor stromal cells expressing high level of Gremlin 1 (an antagonist of BMP) create a CSC niche that allows glioma CSCs to maintain their self-renewal ability by inhibiting BMP signaling.¹³⁴ BMP7 secreted from endogenous neural precursor cells inhibits proliferation, self-renewal, and tumorigenicity of glioma CSCs.¹³⁵ A recent study also demonstrated that a variant type of BMP7 decreases tumor growth, angiogenesis, and invasion by inducing differentiation of glioma CSCs.¹³⁶ Thus, induction of differentiation by activating BMP-associated signaling is a promising strategy for cancer treatment. A previous study showed that chromatin modifications of the BMPR2 promoter inhibit the expression of BMPR2 and that restoration of chromatin in the BMPR2 promoter or ectopic transduction of BMPR1B induces differentiation of CSCs and decreases tumorigenicity in glioblastomas.¹³⁷

Inhibition of CSC differentiation

During early embryonic development, differentiation signaling is less effective than stemness signaling. Hence, decreased differentiation signaling is as important as maintaining self-renewal capacity of ASCs and CSCs.

How do ASCs and CSCs inhibit differentiation? Inhibition of differentiation/DNA-binding (ID) family genes is the most well-described function of the proteins that inhibit lineage differentiation.¹³⁸ There are four members of the ID family, all of which are basic helix-loop-helix (bHLH) transcription factors without a DNA-binding domain. ASC and CSC traits are often maintained by ID proteins. When ID proteins are downregulated, a homodimer or heterodimer of the E protein binds to E-boxes and regulates transcription of genes involved in lineage differentiation.¹³⁸ Highly expressed ID proteins in normal stem/progenitor cells are known to negatively regulate transcription of genes involved in lineage differentiation and cell-cycle inhibition so that cells can maintain their stemness properties. In contrast, low levels of ID protein expression induced cell differentiation. When ID proteins were absent, mice showed lethality, with defects in immune and vascular cells and a reduction in self-renewal and proliferation of neural stem/progenitor cells. Similarly, decreased ID proteins were associated with the loss of CSC properties, along with a reduction in angiogenesis and tumorigenesis.¹³⁸ In case of malignant high-grade gliomas, ID proteins are involved in various phenotype and capacities of glioma CSCs. For example, ID1 activates Wnt/β-catenin and Hedgehog/Gli stemness signaling pathways, leading to maintenance of self-renewal and tumorigenesis of glioma CSCs.¹³⁹ EGFR amplification or oncogenic EGFR variant III mutant gene activates glioma CSC generation and angiogenesis through ID1 and ID3 upregulation.¹⁴⁰ ID4 is a forceful reprogramming factor that convert glioma cells or immortalized astrocytes to glioma CSCs by dedifferentiation.²⁰

MicroRNAs (miRNAs) are known to negatively regulate gene expression by pairing perfect or imperfect complementary sequences within the 3′-untranslated region (UTR) of mRNA. They are noncoding RNA molecules ~22 nucleotides in length that are abnormally expressed in various cancers. There are two kinds of miRNAs in cancers: oncogenic miRNAs (oncomiRs) and tumor-suppressive miRNAs. OncomiRs are generally elevated in cancers and are reported to increase cancer cell progression, survival, and metastasis. MiR-148a is a well-known oncomiR that inhibits tumor suppressor genes, MIG6 and BIM, and indirectly increases EGFR expression and phosphorylation in CSCs of glioblastomas.¹⁴¹ In addition, a recent study showed that delivery with synthesized spherical nucleic acids from miR-182 (182-SNAs) resulted in reduction of tumor burden and increase in mice survival by inducting differentiation of glioma CSCs.¹⁴²

Strategies to target CSCs

In the last few decades, numerous cancer therapies have been developed. However, malignancies are still a threat because of resistance to drug therapy and irradiation. The major cause of this resistance is CSCs.

Several therapeutic strategies to eliminate CSCs have been identified. Cell surface biomarkers such as CD44, CD47, and CD133 are considered the primary targets for CSC treatment. CD44 is specifically expressed in leukemia stem cells; it is not present in normal HSCs.¹⁴³ Treatment with antibodies against CD44 significantly decreased CSCs in AML-bearing mice. CD47 is also highly expressed in acute lymphoblastic leukemia (ALL) and thus could be a specific target for treatment of CSCs in ALL.¹⁴⁴ As described above, CD133 is a cell surface biomarker for many kinds of cancers, including glioblastomas and colon and prostate cancers. Patients with high levels of CD133 expression have a poor prognosis.¹⁴⁵ Thus, several studies have been conducted to target CSCs expressing high levels of CD133.^146,147

In addition, stemness signaling pathways, stemness biomarkers, epigenetic alterations, and even the microenvironment could be potential therapeutic or predictive targets in CSCs. Stemness signaling pathways such as Wnt/β-catenin, Hedgehog, Jagged/Notch, JAK/STAT, and NF-κB have been shown to be abnormally activated in various malignancies. Thus, inhibiting or reversing these abnormal stemness signaling pathways might be a promising strategy for targeting CSCs (Table 1). Blockade of Notch signaling was shown to significantly reduce the CD44⁺CD24^−/low CSC subpopulation and brain metastasis in breast cancers.¹⁴⁸ Pharmaceutic or antibody treatment that targets Wnt/β-catenin signaling shows strong anti-tumor effects.^149,150 Decoying Myc, the transcriptional target of Wnt/β-catenin signaling, reduced the growth of CSCs and induced their differentiation.¹⁵¹ In addition, modification of the aberrant Wnt signaling pathway has been proposed as a potential method for targeting CSCs.¹⁵² Targeting of Hedgehog signaling in CSCs has also been accomplished with an Hedgehog antagonist and SMO inhibitor.^153,154 Impaired NF-κB signaling leads to polarization of M2 to M1 phenotype, which in turn increases inflammatory cells and reduces tumor progression.^95,96 Nestin and CD146 have been shown to play important roles in tumor metastasis, suggesting the possibility of predicting early recurrence.¹⁵⁵ In addition, some natural products can affect the epigenetic alterations in CSCs and inhibit cancer initiation and progression.¹⁵⁶ Besides, a new therapeutic strategy was recently proposed to target the microenvironment of CSCs for cancer treatment. A novel antagonist of CXCR4, a receptor for CXCL12 that increases in tumors, induced CSC differentiation and reduced tumor cell growth in a preclinical glioblastoma multiforme (GBM) model.¹⁵⁷

Conclusion

Recently, targeting CSCs is emerging as an important therapeutic strategy.^158–160 Because targeting cell surface biomarkers of CSCs or key stemness signaling pathways might impair normal stem cells, inducing CSC differentiation offers promise as a relatively safe cancer treatment. BMP-mediated differentiation of CSCs is a promising strategy. Although the BMP signaling cascade is abnormal in glioblastomas because of promoter hypermethylation in CSCs, demethylation of the promoter or forced expression of BMPR2 can induce CSC differentiation and reduce tumorigenicity.¹³⁶ Leukemia was the first tumor subjected to anticancer research for developing a differentiation therapy.¹⁶¹ As mentioned earlier, ATRA has been used for the treatment of APL, in which differentiation is inhibited by fusion genes with the RA receptors (promyelocytic leukemia-RA receptor α (PML-RARα)).^162,163 Therefore, whether other cancers show similar effects is worth examining. Although alkylating antineoplastic agents (e.g. cyclophosphamide and busulfan) are the most popular anticancer drugs, these agents only kill fast-growing cancer cells, but not relatively quiescent CSCs. Differentiation therapy might induce CSCs into terminally differentiated cancer cells, and then, these cells could be treated with alkylating agents or a combination of the two therapies. In recent years, many studies have shown that cells that undergo epithelial-to-mesenchymal transition (EMT) acquire stemness features from several tumor types.^164–168 Therefore, differentiation therapies that induce mesenchymal-to-epithelial transition (MET) have been proposed as a new alternative.¹⁶⁹ Although induction of MET has a high risk of metastatic colonization at distant sites, this method can still be considered an attractive therapeutic strategy, as it reduces aggressive tumorigenic CSCs. To eradicate cancer and prevent recurrence, differentiation strategies require additional research and development.

Footnotes

Acknowledgements

We are grateful to all members of the Cell Growth Regulation Laboratory for their helpful discussions.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grants from the National Research Foundation (NRF), funded by the Ministry of Science, ICT and Future Planning (2015R1A5A1009024); from the Next-Generation Biogreen21 Program (PJ01107701); and from Korea University. X.J. (Xun) was supported by grants from the General Program of the National Natural Science Foundation of China (No. 81572891).

References

Reya

Morrison

Clarke

et al . Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105–111.

Bonnet

Dick

JE.

Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997; 3: 730–737.

Al-Hajj

Wicha

Benito-Hernandez

et al . Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 2003; 100: 3983–3988.

Singh

Clarke

Terasaki

et al . Identification of a cancer stem cell in human brain tumors. Cancer Res 2003; 63: 5821–5828.

O’Brien

Pollett

Gallinger

et al . A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 2007; 445: 106–110.

Heidt

Dalerba

et al . Identification of pancreatic cancer stem cells. Cancer Res 2007; 67: 1030–1037.

Maitland

Collins

AT.

Prostate cancer stem cells: a new target for therapy. J Clin Oncol 2008; 26: 2862–2870.

Schatton

Murphy

Frank

et al . Identification of cells initiating human melanomas. Nature 2008; 451: 345–349.

Zhang

Balch

Chan

et al . Identification and characterization of ovarian cancer-initiating cells from primary human tumors. Cancer Res 2008; 68: 4311–4320.

10.

Meacham

Morrison

SJ.

Tumour heterogeneity and cancer cell plasticity. Nature 2013; 501(7467): 328–337.

11.

Lacoste

Raymond

Cassim

et al . Highly tumorigenic hepatocellular carcinoma cell line with cancer stem cell-like properties. PLoS ONE 2017; 12: e0171215.

12.

Nguyen

Giraud

Chambonnier

et al . Characterization of biomarkers of tumorigenic and chemoresistant cancer stem cells in human gastric carcinoma. Clin Cancer Res 2017; 23: 1586–1597.

13.

Carnero

Garcia-Mayea

Mir

et al . The cancer stem-cell signaling network and resistance to therapy. Cancer Treat Rev 2016; 49: 25–36.

14.

Lathia

Mack

Mulkearns-Hubert

et al . Cancer stem cells in glioblastoma. Genes Dev 2015; 29: 1203–1217.

15.

Wang

Plukker

JTM

Coppes

RP.

Cancer stem cells with increased metastatic potential as a therapeutic target for esophageal cancer. Semin Cancer Biol 2017; 44: 60–66.

16.

Peitzsch

Tyutyunnykova

Pantel

et al . Cancer stem cells: the root of tumor recurrence and metastases. Semin Cancer Biol 2017; 44: 10–24.

17.

Lee

Jeong

et al . Induction of metastasis, cancer stem cell phenotype, and oncogenic metabolism in cancer cells by ionizing radiation. Mol Cancer 2017; 16: 10.

18.

López-Lázaro

Stem cell division theory of cancer. Cell Cycle 2015; 14: 2547–2548.

19.

Herreros-Villanueva

Zhang

Koenig

et al . SOX2 promotes dedifferentiation and imparts stem cell-like features to pancreatic cancer cells. Oncogenesis 2013; 2: e61.

20.

Jeon

Jin

Lee

et al . Inhibitor of differentiation 4 drives brain tumor-initiating cell genesis through cyclin E and notch signaling. Genes Dev 2008; 22: 2028–2033.

21.

Guzman

Jordan

CT.

Considerations for targeting malignant stem cells in leukemia. Cancer Control 2004; 11: 97–104.

22.

Yang

et al . Significance of CD90+ cancer stem cells in human liver cancer. Cancer Cell 2008; 13: 153–166.

23.

Eramo

Lotti

Sette

et al . Identification and expansion of the tumorigenic lung cancer stem cell population. Cell Death Differ 2008; 15: 504–514.

24.

Simeone

DM.

Pancreatic cancer stem cells: implications for the treatment of pancreatic cancer. Clin Cancer Res 2008; 14: 5646–5648.

25.

Son

Woolard

Nam

et al . SSEA-1 is an enrichment marker for tumor-initiating cells in human glioblastoma. Cell Stem Cell 2009; 4: 440–452.

26.

Dalerba

Dylla

Park

et al . Phenotypic characterization of human colorectal cancer stem cells. Proc Natl Acad Sci U S A 2007; 104: 10158–10163.

27.

Dubrovska

Kim

Salamone

et al . The role of PTEN/Akt/PI3K signaling in the maintenance and viability of prostate cancer stem-like cell populations. Proc Natl Acad Sci U S A 2009; 106: 268–273.

28.

Lam

et al . Side population in human lung cancer cell lines and tumors is enriched with stem-like cancer cells. Cancer Res 2007; 67: 4827–4833.

29.

Joo

Kim

Jin

et al . Clinical and biological implications of CD133-positive and CD133-negative cells in glioblastomas. Lab Invest 2008; 88: 808–815.

30.

Mazzoleni

Politi

Pala

et al . Epidermal growth factor receptor expression identifies functionally and molecularly distinct tumor-initiating cells in human glioblastoma multiforme and is required for gliomagenesis. Cancer Res 2010; 70: 7500–7513.

31.

Staal

Clevers

HC.

WNT signalling and haematopoiesis: a WNT-WNT situation. Nat Rev Immunol 2005; 5: 21–30.

32.

Reya

Clevers

Wnt signalling in stem cells and cancer. Nature 2005; 434: 843–850.

33.

Malanchi

Peinado

Kassen

et al . Cutaneous cancer stem cell maintenance is dependent on beta-catenin signalling. Nature 2008; 452: 650–653.

34.

Zeng

Nusse

Wnt proteins are self-renewal factors for mammary stem cells and promote their long-term expansion in culture. Cell Stem Cell 2010; 6: 568–577.

35.

Clements

Wang

Sarnaik

et al . Beta-Catenin mutation is a frequent cause of Wnt pathway activation in gastric cancer. Cancer Res 2002; 62: 3503–3506.

36.

Ilyas

Tomlinson

Rowan

et al . Beta-Catenin mutations in cell lines established from human colorectal cancers. Proc Natl Acad Sci U S A 1997; 94: 10330–10334.

37.

Giannakis

Hodis

Jasmine Mu

et al . RNF43 is frequently mutated in colorectal and endometrial cancers. Nat Genet 2014; 46: 1264–1266.

38.

Seshagiri

Stawiski

Durinck

et al . Recurrent R-spondin fusions in colon cancer. Nature 2012; 488: 660–664.

39.

Van de Wetering

Francies

Francis

et al . Prospective derivation of a living organoid biobank of colorectal cancer patients. Cell 2015; 161: 933–945.

40.

Hawkes

Hamilton

White

et al . A randomised controlled trial of a theory-based intervention to improve sun protective behaviour in adolescents (“you can still be HOT in the shade”): study protocol. BMC Cancer 2012; 12: 1.

41.

Wang

Shalek

Lawrence

et al . Somatic mutation as a mechanism of Wnt/β-catenin pathway activation in CLL. Blood 2014; 124: 1089–1098.

42.

Satoh

Daigo

Furukawa

et al . AXIN1 mutations in hepatocellular carcinomas, and growth suppression in cancer cells by virus-mediated transfer of AXIN1. Nat Genet 2000; 24: 245–250.

43.

Dahmen

RP1

Koch

Denkhaus

et al . Deletions of AXIN1, a component of the WNT/wingless pathway, in sporadic medulloblastomas. Cancer Res 2001; 61: 7039–7043.

44.

Segditsas

Tomlinson

Colorectal cancer and genetic alterations in the Wnt pathway. Oncogene 2006; 25: 7531–7537.

45.

Manoranjan

Venugopal

McFarlane

et al . Medulloblastoma stem cells: where development and cancer cross pathways. Pediatr Res 2012; 71: 516–522.

46.

Rubin

De Sauvage

FJ.

Targeting the Hedgehog pathway in cancer. Nat Rev Drug Discov 2006; 5: 1026–1033.

47.

Reifenberger

Wolter

Knobbe

et al . Somatic mutations in the PTCH, SMOH, SUFUH and TP53 genes in sporadic basal cell carcinomas. Br J Dermatol 2005; 152: 43–51.

48.

Raffel

Jenkins

Frederick

et al . Sporadic medulloblastomas contain PTCH mutations. Cancer Res 1997; 57: 842–845.

49.

Reifenberger

Wolter

Weber

et al . Missense mutations in SMOH in sporadic basal cell carcinomas of the skin and primitive neuroectodermal tumors of the central nervous system. Cancer Res 1998; 58: 1798–1803.

50.

Xie

Murone

Luoh

et al . Activating Smoothened mutations in sporadic basal-cell carcinoma. Nature 1998; 391: 90–92.

51.

Taylor

Liu

Raffel

et al . Mutations in SUFU predispose to medulloblastoma. Nat Genet 2002; 31: 306–310.

52.

Jones

Zhang

Parsons

et al . Core signaling pathways in human pancreatic cancers revealed by global genomic analyses. Science 2008; 321: 1801–1806.

53.

Watkins

Berman

Burkholder

et al . Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer. Nature 2003; 422: 313–317.

54.

Berman

Karhadkar

Maitra

et al . Widespread requirement for Hedgehog ligand stimulation in growth of digestive tract tumours. Nature 2003; 425: 846–851.

55.

Yauch

Gould

Scales

et al . A paracrine requirement for hedgehog signalling in cancer. Nature 2008; 455: 406–410.

56.

Theunissen

de Sauvage

FJ.

Paracrine Hedgehog signaling in cancer. Cancer Res 2009; 69: 6007–6010.

57.

Hegde

Peterson

Emanuel

et al . Hedgehog-induced survival of B-cell chronic lymphocytic leukemia cells in a stromal cell microenvironment: a potential new therapeutic target. Mol Cancer Res 2008; 6: 1928–1936.

58.

Xie

Johnson

Zhang

et al . Mutations of the PATCHED gene in several types of sporadic extracutaneous tumors. Cancer Res 1997; 57: 2369–2372.

59.

Andersson

Lendahl

Therapeutic modulation of Notch signalling–are we there yet?

Nat Rev Drug Discov 2014; 13: 357–378.

60.

Jeon

Kim

Jin

et al . Crosstalk between glioma-initiating cells and endothelial cells drives tumor progression. Cancer Res 2014; 74: 4482–4492.

61.

Robinson

Kalyana-Sundaram

et al . Functionally recurrent rearrangements of the MAST kinase and Notch gene families in breast cancer. Nat Med 2011; 17: 1646–1651.

62.

Weng

Ferrando

Lee

et al . Activating mutations of NOTCH1 in human T cell acute lymphoblastic leukemia. Science 2004; 306: 269–271.

63.

King

Trimarchi

Reavie

et al . The ubiquitin ligase FBXW7 modulates leukemia-initiating cell activity by regulating MYC stability. Cell 2013; 153: 1552–1566.

64.

Malyukova

Dohda

Von der Lehr

et al . The tumor suppressor gene hCDC4 is frequently mutated in human T-cell acute lymphoblastic leukemia with functional consequences for Notch signaling. Cancer Res 2007; 67: 5611–5616.

65.

Lee

Kumano

Nakazaki

et al . Gain-of-function mutations and copy number increases of Notch2 in diffuse large B-cell lymphoma. Cancer Sci 2009; 100: 920–926.

66.

Stephens

Davies

Mitani

et al . Whole exome sequencing of adenoid cystic carcinoma. J Clin Invest 2013; 123: 2965–2968.

67.

Stylianou

Clarke

Brennan

Aberrant activation of notch signaling in human breast cancer. Cancer Res 2006; 66: 1517–1525.

68.

Harrison

Farnie

Howell

et al . Regulation of breast cancer stem cell activity by signaling through the Notch4 receptor. Cancer Res 2010; 70: 709–718.

69.

Hebenstreit

Horejs-Hoeck

Duschl

JAK/STAT-dependent gene regulation by cytokines. Drug News Perspect 2005; 18: 243–249.

70.

Kan

Zheng

Liu

et al . Whole-genome sequencing identifies recurrent mutations in hepatocellular carcinoma. Genome Res 2013; 23: 1422–1433.

71.

James

Ugo

Le Couédic

et al . A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera. Nature 2005; 434: 1144–1148.

72.

Levine

Wadleigh

Cools

et al . Activating mutation in the tyrosine kinase JAK2 in polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. Cancer Cell 2005; 7: 387–397.

73.

Kralovics

Passamonti

Buser

et al . A gain-of-function mutation of JAK2 in myeloproliferative disorders. N Engl J Med 2005; 352: 1779–1790.

74.

Pradhan

Lambert

Reuther

GW.

Transformation of hematopoietic cells and activation of JAK2-V617F by IL-27R, a component of a heterodimeric type I cytokine receptor. Proc Natl Acad Sci U S A 2007; 104: 18502–18507.

75.

Scott

Tong

Levine

et al . JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis. N Engl J Med 2007; 356: 459–468.

76.

Colaizzo

Amitrano

Tiscia

et al . A new JAK2 gene mutation in patients with polycythemia vera and splanchnic vein thrombosis. Blood 2007; 110: 2768–2769.

77.

Mercher

Wernig

Moore

et al . JAK2T875N is a novel activating mutation that results in myeloproliferative disease with features of megakaryoblastic leukemia in a murine bone marrow transplantation model. Blood 2006; 108: 2770–2779.

78.

Malinge

Ben-Abdelali

Settegrana

et al . Novel activating JAK2 mutation in a patient with Down syndrome and B-cell precursor acute lymphoblastic leukemia. Blood 2007; 109: 2202–2204.

79.

Walters

Mercher

et al . Activating alleles of JAK3 in acute megakaryoblastic leukemia. Cancer Cell 2006; 10: 65–75.

80.

Haddad

Mirtti

et al . STAT5A/B gene locus undergoes amplification during human prostate cancer progression. Am J Pathol 2013; 182: 2264–2275.

81.

Hayden

Ghosh

Shared principles in NF-kappaB signaling. Cell 2008; 132: 344–362.

82.

Huang

Kudo

Yoshida

et al . A nuclear export signal in the N-terminal regulatory domain of IkappaBalpha controls cytoplasmic localization of inactive NF-kappaB/IkappaBalpha complexes. Proc Natl Acad Sci U S A 2000; 97: 1014–1019.

83.

Mayo

Wang

Cogswell

et al . Requirement of NF-kappaB activation to suppress p53-independent apoptosis induced by oncogenic Ras. Science 1997; 278: 1812–1815.

84.

Baldwin

AS.

Regulation of cell death and autophagy by IKK and NF-κB: critical mechanisms in immune function and cancer. Immunol Rev 2012; 246: 327–345.

85.

Erez

Truitt

Olson

et al . Cancer-associated fibroblasts are activated in incipient neoplasia to orchestrate tumor-promoting inflammation in an NF-kappaB-dependent manner. Cancer Cell 2010; 17: 135–147.

86.

Koliaraki

Pasparakis

Kollias

IKKβ in intestinal mesenchymal cells promotes initiation of colitis-associated cancer. J Exp Med 2015; 212: 2235–2251.

87.

Stein

Baldwin

AS.

NF-κB suppresses ROS levels in BCR-ABL(+) cells to prevent activation of JNK and cell death. Oncogene 2011; 30: 4557–4566.

88.

Huber

Azoitei

Baumann

et al . NF-kappaB is essential for epithelial-mesenchymal transition and metastasis in a model of breast cancer progression. J Clin Invest 2004; 114: 569–581.

89.

Huang

Robinson

Deguzman

et al . Blockade of nuclear factor-kappaB signaling inhibits angiogenesis and tumorigenicity of human ovarian cancer cells by suppressing expression of vascular endothelial growth factor and interleukin 8. Cancer Res 2000; 60: 5334–5339.

90.

Cooks

Pateras

Tarcic

et al . Mutant p53 prolongs NF-κB activation and promotes chronic inflammation and inflammation-associated colorectal cancer. Cancer Cell 2013; 23: 634–646.

91.

Beroukhim

Mermel

Porter

et al . The landscape of somatic copy-number alteration across human cancers. Nature 2010; 463: 899–905.

92.

Boehm

Zhao

Yao

et al . Integrative genomic approaches identify IKBKE as a breast cancer oncogene. Cell 2007; 129: 1065–1079.

93.

Merkhofer

Cogswell

Baldwin

AS.

Her2 activates NF-kappaB and induces invasion through the canonical pathway involving IKKalpha. Oncogene 2010; 29: 1238–1248.

94.

Tanaka

Babic

Nathanson

et al . Oncogenic EGFR signaling activates an mTORC2-NF-κB pathway that promotes chemotherapy resistance. Cancer Discov 2011; 1: 524–538.

95.

Bhat

Balasubramaniyan

Vaillant

et al . Mesenchymal differentiation mediated by NF-κB promotes radiation resistance in glioblastoma. Cancer Cell 2013; 24: 331–346.

96.

Saccani

Schioppa

Porta

et al . p50 nuclear factor-kappaB overexpression in tumor-associated macrophages inhibits M1 inflammatory responses and antitumor resistance. Cancer Res 2006; 66: 11432–11440.

97.

Hagemann

Lawrence

McNeish

et al . “Re-educating” tumor-associated macrophages by targeting NF-kappaB. J Exp Med 2008; 205: 1261–1268.

98.

Fan

Khaki

Zhu

et al . NOTCH pathway blockade depletes CD133-positive glioblastoma cells and inhibits growth of tumor neurospheres and xenografts. Stem Cells 2010; 28: 5–16.

99.

Kondratyev

Kreso

Hallett

et al . Gamma-secretase inhibitors target tumor-initiating cells in a mouse model of ERBB2 breast cancer. Oncogene 2012; 31: 93–103.

100.

Zhang

Chung

Lunatic Fringe is a potent tumor suppressor in Kras-initiated pancreatic cancer. Oncogene 2016; 35: 2485–2495.

101.

Richter

Bedard

Chen

et al . A phase I study of the oral gamma secretase inhibitor R04929097 in combination with gemcitabine in patients with advanced solid tumors (PHL-078/CTEP 8575). Invest New Drugs 2014; 32: 243–249.

102.

Smith

Eisenberg

Manikhas

et al . A phase I dose escalation and expansion study of the anticancer stem cell agent demcizumab (anti-DLL4) in patients with previously treated solid tumors. Clin Cancer Res 2014; 20: 6295–6303.

103.

Takahashi-Yanaga

Kahn

Targeting Wnt signaling: can we safely eradicate cancer stem cells?

Clin Cancer Res 2010; 16: 3153–3162.

104.

Gang

Hsieh

Pham

et al . Small-molecule inhibition of CBP/catenin interactions eliminates drug-resistant clones in acute lymphoblastic leukemia. Oncogene 2014; 33: 2169–2178.

105.

Waaler

Machon

Von Kries

et al . Novel synthetic antagonists of canonical Wnt signaling inhibit colorectal cancer cell growth. Cancer Res 2011; 71: 197–205.

106.

McDermott

Jimeno

Targeting the Wnt pathway in human cancers: therapeutic targeting with a focus on OMP-54F28. Pharmacol Ther 2015; 146: 1–11.

107.

Gurney

Axelrod

Bond

et al . Wnt pathway inhibition via the targeting of Frizzled receptors results in decreased growth and tumorigenicity of human tumors. Proc Natl Acad Sci U S A 2012; 109: 11717–11722.

108.

Xia

Chen

et al . Targeting pancreatic cancer stem cells for cancer therapy. Biochim Biophys Acta 2012; 1826: 385–399.

109.

Feldmann

Fendrich

McGovern

et al . An orally bioavailable small-molecule inhibitor of Hedgehog signaling inhibits tumor initiation and metastasis in pancreatic cancer. Mol Cancer Ther 2008; 7: 2725–2735.

110.

Singh

Srivastava

et al . Hedgehog signaling antagonist GDC-0449 (Vismodegib) inhibits pancreatic cancer stem cell characteristics: molecular mechanisms. PLoS ONE 2011; 6: e27306.

111.

Beauchamp

Ringer

Bulut

et al . Arsenic trioxide inhibits human cancer cell growth and tumor development in mice by blocking Hedgehog/GLI pathway. J Clin Invest 2011; 121: 148–160.

112.

Lauth

Bergström

Shimokawa

et al . Inhibition of GLI-mediated transcription and tumor cell growth by small-molecule antagonists. Proc Natl Acad Sci U S A 2007; 104: 8455–8460.

113.

Kopp

et al . Activity of the Type II JAK2 Inhibitor CHZ868 in B Cell Acute Lymphoblastic Leukemia. Cancer Cell 2015; 28: 29–41.

114.

Elliott

Cleveland

Grann

et al . FERM domain mutations induce gain of function in JAK3 in adult T-cell leukemia/lymphoma. Blood 2011; 118: 3911–3921.

115.

Dechow

Steidle

Götze

et al . GP130 activation induces myeloma and collaborates with MYC. J Clin Invest 2014; 124: 5263–5274.

116.

Shi

Zheng

et al . Niclosamide inhibition of STAT3 synergizes with erlotinib in human colon cancer. Onco Targets Ther 2017; 10: 1767–1776.

117.

Bendell

Hong

Burris

III et al . Phase 1, open-label, dose-escalation, and pharmacokinetic study of STAT3 inhibitor OPB-31121 in subjects with advanced solid tumors. Cancer Chemother Pharmacol 2014; 74: 125–130.

118.

Fan

Zhang

et al . Co-delivery of PDTC and doxorubicin by multifunctional micellar nanoparticles to achieve active targeted drug delivery and overcome multidrug resistance. Biomaterials 2010; 31: 5634–5642.

119.

Ganta

Amiji

Coadministration of Paclitaxel and curcumin in nanoemulsion formulations to overcome multidrug resistance in tumor cells. Mol Pharm 2009; 6: 928–939.

120.

Rausch

Liu

Kallifatidis

et al . Synergistic activity of sorafenib and sulforaphane abolishes pancreatic cancer stem cell characteristics. Cancer Res 2010; 70: 5004–5013.

121.

Bentires-Alj

Barbu

Fillet

et al . NF-kappaB transcription factor induces drug resistance through MDR1 expression in cancer cells. Oncogene 2003; 22: 90–97.

122.

Germain

Iyer

Zechel

et al . Co-regulator recruitment and the mechanism of retinoic acid receptor synergy. Nature 2002; 415: 187–192.

123.

Marshall

Morrison

Studer

et al . Retinoids and Hox genes. FASEB J 1996; 10: 969–978.

124.

Castaigne

Chomienne

Daniel

et al . All-trans retinoic acid as a differentiation therapy for acute promyelocytic leukemia. I. Clinical results. Blood 1990; 76: 1704–1709.

125.

Campos

Wan

Farhadi

et al . Differentiation therapy exerts antitumor effects on stem-like glioma cells. Clin Cancer Res 2010; 16: 2715–2728.

126.

Yan

et al . All-trans retinoic acids induce differentiation and sensitize a radioresistant breast cancer cells to chemotherapy. BMC Complement Altern Med 2016; 16: 113.

127.

Niu

et al . Effect of all-trans retinoic acid on the proliferation and differentiation of brain tumor stem cells. J Exp Clin Cancer Res 2010; 29: 113.

128.

Wakefield

Hill

CS.

Beyond TGFβ: roles of other TGFβ superfamily members in cancer. Nat Rev Cancer 2013; 13: 328–341.

129.

Oshimori

Fuchs

The harmonies played by TGF-β in stem cell biology. Cell Stem Cell 2012; 11: 751–764.

130.

Blanpain

Fuchs

Epidermal homeostasis: a balancing act of stem cells in the skin. Nat Rev Mol Cell Biol 2009; 10: 207–217.

131.

Bond

Bhalala

Kessler

JA.

The dynamic role of bone morphogenetic proteins in neural stem cell fate and maturation. Dev Neurobiol 2012; 72: 1068–1084.

132.

Zhang

Tong

et al . BMP signaling inhibits intestinal stem cell self-renewal through suppression of Wnt-beta-catenin signaling. Nat Genet 2004; 36: 1117–1121.

133.

Panchision

McKay

RD.

The control of neural stem cells by morphogenic signals. Curr Opin Genet Dev 2002; 12: 478–487.

134.

Sneddon

Zhen

Montgomery

et al . Bone morphogenetic protein antagonist gremlin 1 is widely expressed by cancer-associated stromal cells and can promote tumor cell proliferation. Proc Natl Acad Sci U S A 2006; 103: 14842–14847.

135.

Chirasani

Sternjak

Wend

et al . Bone morphogenetic protein-7 release from endogenous neural precursor cells suppresses the tumourigenicity of stem-like glioblastoma cells. Brain 2010; 133: 1961–1972.

136.

Tate

Pallini

Ricci-Vitiani

et al . A BMP7 variant inhibits the tumorigenic potential of glioblastoma stem-like cells. Cell Death Differ 2012; 19: 1644–1654.

137.

Lee

Son

Woolard

et al . Epigenetic-mediated dysfunction of the bone morphogenetic protein pathway inhibits differentiation of glioblastoma-initiating cells. Cancer Cell 2008; 13: 69–80.

138.

Lasorella

Benezra

Iavarone

The ID proteins: master regulators of cancer stem cells and tumour aggressiveness. Nat Rev Cancer 2014; 14: 77–91.

139.

Jin

Jeon

Jin

et al . The ID1-CULLIN3 axis regulates intracellular SHH and WNT signaling in glioblastoma stem cells. Cell Rep 2016; 16: 1629–1641.

140.

Jin

Yin

Kim

et al . EGFR-AKT-Smad signaling promotes formation of glioma stem-like cells and tumor angiogenesis by ID3-driven cytokine induction. Cancer Res 2011; 71: 7125–7134.

141.

Kim

Zhang

Skalski

et al . microRNA-148a is a prognostic oncomiR that targets MIG6 and BIM to regulate EGFR and apoptosis in glioblastoma. Cancer Res 2014; 74: 1541–1553.

142.

Kouri

Hurley

Daniel

et al . miR-182 integrates apoptosis, growth, and differentiation programs in glioblastoma. Genes Dev 2015; 29: 732–745.

143.

Jin

Hope

Zhai

et al . Targeting of CD44 eradicates human acute myeloid leukemic stem cells. Nat Med 2006; 12: 1167–1174.

144.

Chao

Alizadeh

Tang

et al . Therapeutic antibody targeting of CD47 eliminates human acute lymphoblastic leukemia. Cancer Res 2011; 71: 1374–1384.

145.

Lin

Hassan

et al . Elevated circulating endothelial progenitor marker CD133 messenger RNA levels predict colon cancer recurrence. Cancer 2007; 110: 534–542.

146.

Wang

Chiou

Chou

et al . Photothermolysis of glioblastoma stem-like cells targeted by carbon nanotubes conjugated with CD133 monoclonal antibody. Nanomedicine 2011; 7: 69–79.

147.

Brescia

Ortensi

Fornasari

et al . CD133 is essential for glioblastoma stem cell maintenance. Stem Cells 2013; 31: 857–869.

148.

McGowan

Simedrea

Ribot

et al . Notch1 inhibition alters the CD44hi/CD24lo population and reduces the formation of brain metastases from breast cancer. Mol Cancer Res 2011; 9: 834–844.

149.

Chen

Dodge

Tang

et al . Small molecule-mediated disruption of Wnt-dependent signaling in tissue regeneration and cancer. Nat Chem Biol 2009; 5: 100–107.

150.

Reguart

You

et al . Blockade of Wnt-1 signaling induces apoptosis in human colorectal cancer cells containing downstream mutations. Oncogene 2005; 24: 3054–3058.

151.

Johari

Ebrahimi-Rad

Maghsood

et al . Myc decoy oligodeoxynucleotide inhibits growth and modulates differentiation of mouse embryonic stem cells as a model of cancer stem cells. Anticancer Agents Med Chem. Epub ahead of print 12 April 2017. DOI: 10.2174/1871521409666170412142507.

152.

Yang

Chen

et al . Wnt signaling as potential therapeutic target in lung cancer. Expert Opin Ther Targets 2016; 20: 999–1015.

153.

Feldmann

Fendrich

McGovern

et al . An orally bioavailable small-molecule inhibitor of Hedehog signaling inhibits tumor initiation and metastasis in pancreatic cancer. Mol Cancer Ther 2008; 7: 2725–2735.

154.

Ramaswamy

Teng

et al . Hedgehog signaling is a novel therapeutic target in tamoxifen-resistant breast cancer aberrantly activated by PI3K/AKT pathway. Cancer Res 2012; 72: 5048–5059.

155.

Tampaki

Tampakis

Nonni

et al . Nestin and cluster of differentiation 146 expression in breast cancer: predicting early recurrence by targeting metastasis? Tumour Biol 2017; 39: 1010428317691181.

156.

Wang

Liu

et al . Genetic and epigenetic studies for determining molecular targets of natural product anticancer agents. Curr Cancer Drug Targets 2013; 13: 506–518.

157.

Gravina

Mancini

Colapietro

et al . The novel CXCR4 antagonist, PRX177561, reduces tumor cell proliferation and accelerates cancer stem cell differentiation in glioblastoma preclinical models. Tumour Biol 2017; 39: 1010428317695528.

158.

Atkinson

Zhang

Combination of chemotherapy and cancer stem cell targeting agents: preclinical and clinical studies. Cancer Lett 2017; 396: 103–109.

159.

Yang

Jin

Tong

et al . Therapeutic potential of cancer stem cells. Med Oncol 2015; 32: 619.

160.

Jiang

Peng

Zhang

et al . The implications of cancer stem cells for cancer therapy. Int J Mol Sci 2012; 13: 16636–16657.

161.

Greaves

Leukaemia “firsts” in cancer research and treatment. Nat Rev Cancer 2016; 16: 163–172.

162.

Wang

Chen

Acute promyelocytic leukemia: from highly fatal to highly curable. Blood 2008; 111: 2505–2515.

163.

Schenk

Chen

Göllner

et al . Inhibition of the LSD1 (KDM1A) demethylase reactivates the all-trans-retinoic acid differentiation pathway in acute myeloid leukemia. Nat Med 2012; 18: 605–611.

164.

Rasheed

Yang

Wang

et al . Prognostic significance of tumorigenic cells with mesenchymal features in pancreatic adenocarcinoma. J Natl Cancer Inst 2010; 102: 340–351.

165.

Kong

Banerjee

Ahmad

et al . Epithelial to mesenchymal transition is mechanistically linked with stem cell signatures in prostate cancer cells. PLoS ONE 2010; 5: e12445.

166.

Fan

Samuel

Evans

et al . Overexpression of snail induces epithelial-mesenchymal transition and a cancer stem cell-like phenotype in human colorectal cancer cells. Cancer Med 2012; 1: 5–16.

167.

Long

Xiang

et al . CD133+ ovarian cancer stem-like cells promote non-stem cancer cell metastasis via CCL5 induced epithelial-mesenchymal transition. Oncotarget 2015; 6: 5846–5859.

168.

Zhou

Kannappan

Chen

et al . RBP2 induces stem-like cancer cells by promoting EMT and is a prognostic marker for renal cell carcinoma. Exp Mol Med 2016; 48: e238.

169.

Pattabiraman

Weinberg

RA.

Targeting the epithelial-to-mesenchymal transition: the case for differentiation-based therapy. Cold Spring Harb Symp Quant Biol 2016; 81: 11–19.

Cancer stem cells and differentiation therapy

Abstract

Keywords

Introduction

Cell origin and markers of CSCs

Signaling pathways in CSCs

Induction of CSC differentiation

Inhibition of CSC differentiation

Strategies to target CSCs

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References