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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
Review
Cancer Stem Cells in Nasopharyngeal Carcinoma: Current Evidence
Fenggang Yu1, Kwok Seng Loh
2
1Department of Otolaryngology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
2Head & Neck Tumor Group, National Cancer Institute of Singapore, National University Health System (NUHS), Singapore
Corresponding author: Fenggang Yu; E-mail: [email protected]
Citation: Yu FG, Loh KS. Cancer stem cells in nasopharyngeal carcinoma: current evidence. J Nasopharyng
Carcinoma, 2014, 1(6): e6. doi:10.15383/jnpc.6.
Funding: This work was supported by a Grant from the National University Cancer Institute, Singapore (NCIS)
Centre Grant to Dr. Loh and Dr. Yu.
Competing interests: The authors have declared that no competing interests exist.
Conflict of interest: None.
Copyright: 2014 By the Editorial Department of Journal of Nasopharyngeal Carcinoma. This is an open-access
article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use,
distribution, and reproduction in any medium, provided the original author and source are credited.
Abstract: Nasopharyngeal carcinoma (NPC) is highly prevalent in southern China and Southeast Asia. Cancer
resistance to therapy, metastasis and disease recurrence are significant hurdles to successful treatment of NPC.
Identifying mechanisms by which NPC is resistantiscritical to improving patient survival. Evidence gathered in the
last decade suggests that tumor progression and recurrence may befuelled by cancer stem cells (CSCs).
Understanding how CSCs contribute to the pathology of NPC will potentiallyaid the pursuit of novel therapies. In
this review we summarize what major methods are currently used to identify CSCs in NPC and the challenges faced.
Keywords: Nasopharyngeal carcinoma; Cancer stem cells
Introduction
In general,two models have been proposed to explain tumor
growth and heterogeneity[1]. In the first model, all tumor cells are
equipotent and a proportion of tumor cells stochastically
proliferate to fuel tumor growth while other tumor cells
differentiate. In the second model, tumors are hierarchically
organized like normal tissues.Only a discrete fraction of cells with
stem cell features (asymmetric division) is able to indefinitely
sustain the malignant progeny through self-renewaland
differentiation processes. Owing to the analogy to tissue-specific
stem cells , thesesubset of cells are called cancer stem cells
(CSCs) [2]. Thetheory of the CSCshas stirred much confusion
and debate ever since, but it keeps generating excitement and
optimism.
Almost 200 years ago, the father of pathology, Rudolf Virchow,
suggested that cancer cells arise from embryonic-like tissue[3],
but it was not until 1994, in their pioneer work, have John Dick
and colleagues demonstrated the hierarchy of the acute myeloid
leukaemia malignant clone and defined the CSCs for the first
time[4]. Following these papers, many other studies have shown
that populations of cells presenting a higher ability to re-form the
parental tumor on transplantation into immunodeficient mice can
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
be prospectively isolated from a great variety of solid tumors, such
as breast cancer[5], brain tumors[6], colorectal cancer[7,8], skin
squamous cell carcinoma (SCC)[9], head and neck cancer[10],
lung cancer[11], pancreatic cancer[12], prostate cancer[13] and
ovarian cancer[14]. Tumor cells presenting this higher tumor-
repopulating capacity have been referred to as CSCs, or as tumor-
initiating cells, but the best term to describe them is probably
tumor-propagating cells (TPCs).
Nasopharyngeal carcinoma (NPC) is a cancer arising from the
epithelial lining of the nasopharynx. It remains a serious health
problem in many parts of the world,although the worldwide
incidence is low. NPC is particularly endemic to regions in
southern China and South East Asia [15,16].In Singapore, it ranks
as the 3rdmost common cancer in Chinese adult males between
35to 60 years old. Uniquely, Epstein-Barr virus (EBV) is
consistently detected in undifferentiated NPC from these endemic
regions[17].Particularly among head and neck cancers of epithelial
origin, it is associated with the highest rateof locoregional
recurrence and distant metastasis [18,19] , resulting in a great
interest in studying this disease with the intention of developing a
better understanding of its biology and translating these findings
into improved therapeutic approaches. One of the major
mechanisms for recurrence of NPC has been suggested by the
CSCs proposition[20]. While information on CSCs hasbeen
advanced in a variety of cancers, data in NPC is just emerging. In
this paper, we will review the evidence for CSCs in NPC and the
future challenges ahead in elucidating this.
Discovery of NPC CSCs
Historically, the hematopoietic field has led the way in the
identification of stem cells and CSCs[4,21,22].The CSC-theory in
solid tumors was only validated relatively recently. Due to its
distinctive racial/ethnic and geographic distribution, studies of
CSCs in NPC are very scarce[23,24,25,26,27,28,29,30,31] (Table
1). Several important functional assays and surface markers have
been used to investigate the existence of cancer stem-like cells in
various NPC cell lines. Overall, those studies support the evidence
of a subpopulation of NPC cells that are more primitive,
proliferative, therapy resistant and tumorigenic in xenograft than
cells with alternative phenotypes, suggesting CSCs.
Table 1 Markers for CSCs in NPC
Marker Samples % Cells expressing markers Refs
BrdU EBV-NPC cell lines:SUNE-
1(5-8F, 6-10B) and TMNE
approximately 0.3% of label-retaining
cell (LRC) find in 3 kinds of NPC
xenografttumors
[23]
SP EBV- cell lines: CNE-2 about
2.6% of the total cells are SP cells
[24]
CD44
EBV- NPC cell line: SUNE-
1( 5-8F)
CD44+ cells occupied 52.5% of the total
cells
[25]
EBV+
NPC cell line:C666-1 CD44+ cells accounted for 45.3% of the
total cells.
[26]
EBV+ NPC cell line C666-1
cells
And
C666-1 Spheroids Primary
5.2861% of parental C666-1 cells are
CD44+
84.1461% C666-1 spheroids are CD44+
13.06% of NPC xenografts are CD44+
[27]
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
tumors
Xenografts
ALDH1
Tissue sections from NPC
patients
41 (39.0%) of 105 cases were defined as
having high-grade ALDH1 expression
[28]
EBV- NPC cell lines: 5-8F
and CNE2
1.96% of cells are ALDH1 positive [29]
C666-1 8.5% with high ALDH activity [30]
CD133
CNE2
primary culture
3.36±0.35% CD133+ cells
2.17% in primary cells
[31]
Label Retaining Cells (Lrcs)
Dye label–retaining technique can be used to identify normal
tissues that contain quiescent stem cells responsible for tissue
homeostasis. As CSCs can share properties with normal stem cells,
slow-cycling cells might also exist within a tumor. Their dormant
state might account for the relapse in cancer patients that can
occur years to decades after apparently successful treatment. In an
early study by Zhang et al.[23], the authors found there was about
0.3% of label retaining cells (LRC) in NPC cell lines and their
derived xenografttumors, a good indication that NPC contains
stem cells. However, what the lineage relationship of LRCs with
the rest of cells over time and their functions are lacking. This
question could be addressed further by isolation of live LRCs via
fluorescence-activated cell sorting(FACS)and applying them to
functional assays[32,33,34].
Side Population (SP)
The side population (SP) discrimination assay is based on the
differential potential of cells to efflux the Hoechst dye via the
ATP-binding cassette (ABC) family of transporter proteins
expressed within the cell membrane. SP assay has proven to be a
useful approach for the characterization and isolation of putative
stem cell and cancer stem cell populations, particularly in the
absence of specific markers. Wang et al.[24] demonstrated ∼2.6%
SP in CNE-2 line had cancer stem cell characteristics. These cells
were more resistant to chemotherapy and radiotherapy, and were
noted to have increased propensity to form tumors in vivo. The
presence, absence or change in SP has been used loosely as an
indicator of CSC activity across cell lines in some NPC drug
testing studies [35,36,37]. However, whether SP is a robust CSC
marker in all NPC cells should beconfirmedsystematically. Studies
in other cancers even argue that SP is neither necessary nor
sufficient for conferring a CSC phenotype, such as
glioblastomamultiforme (GBM)[38], thyroid cancer[39],
gastrointestinal cancers[40] and adrenocortical carcinoma[41].
Aldehyde Dehydrogenase (ALDH)
Another functional marker is aldehyde dehydrogenase 1 (ALDH1).
ALDH1 is normally responsible for maintaining cellular
homeostasis by detoxifying intracellular aldehydes through the
oxidation and conversion of retinol to retinoic acid. ALDH1 is
highly expressed in hematopoietic stem cells, as well as malignant
CSCs[42,43,44]. It has been used as a prognostic indicator of
metastases and poor survival[45]. Using EBVcell linesSUNE-1(5-
8F) and CNE2,Wu et al.[29] showed that ALDH1positive(1.96%)
cells had faster proliferation, higher clone formation,
migration,tumor formation in mice, greaterstemness gene
expression and SP cells.It correlated with TNM staging and
epithelial-mesenchymal transition (EMT) makers, proposed as
independent prognostic indicators. Using EBV+cell line C666-1,
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
our study[30] demonstrated ALDHHigh
(8.5%) cells possesscancer
stem-like traits: the cellsexhibited significantly greater ability to
proliferate, be clonogenic, resist chemotherapy drugs and radiation,
reconstitute a heterogeneous population,and express pluripotent
markers. Furthermore, subcutaneous injection of these cells into
immunodeficient nude mice resulted in a tendency of tumor
formation at a higher rate (not significant) as compared to cells
with lowALDHactivity. However, we did not find ALDHHigh
cells
are more migratory. Indeedwe showed almost all cells express
ALDH at variablelevels.There is no clear cut distinctionbetween
ALDH‘positive’ and ‘negative’ cells as Wu et al.[29] termed it.
The percentage is arbitrary and really depends on how stringent
one sets the gating. The discrepancy might be due to different
experimental conditions or the EBV status of the cell lines.Further
research by Luo et al. demonstrated that budding cells in the
invasive front of tumors with highlevel expression of ALDH1
correlated with aggressive tumor behaviour and poor patient
survival[28]. The authors speculated that they might possess the
invasive and metastatic properties of CSCs. Like in other
cancers[46,47], ALDHcould be a potential therapeutic target for
NPC CSCs as well.
CD44
CD44 is a cell surface receptor for the extracellular matrix
molecule hyaluronan. It influences cell behaviour by direct
signaling/structural roles or by acting as a co-receptor for receptor
tyrosine kinases[48]. CD44 alone or in combination with other
markers have been used successfully to enrich for CSCs in both
cell line and tumor samples[49]. Su et al.[25]reported that CD44+
cells in SUNE-1(5-8F)weremore proliferative, enriched for
stemness gene expression and more resistant to therapy.But in
vivo tumor imitation, one of the most important criteria for
CSCs,was not functionally addressed. In contrast, Janisiewicz et
al.[26] demonstrated CD44+ C666-1 cells exhibited a more robust
tumor-initiating capacity in the xenograft model. CD44+ cells
differentiated into CD44- cells, indicating a hierarchical
relationship. Patient tumors were heterogeneous for CD44 staining,
and a trend toward an association between CD44 expression and
clinical outcome was observed. Surprisingly, no corresponding
higher proliferation rates were seen in CD44+ population in vitro.
This is consistent with our finding that no difference was detected
for both populations incolony-forming efficiency [30]. This study
raises the question whether CD44- cells cannot survive in vivo or
they intrinsically cannot initiate tumors? In a more sophisticated
study by Lun et al.[28], spheroid culture of C666-1 was used to
enrich for CSCs initially and they found the spheroid cells had at
least 50 times higher tumorigenic potential than the unselected
cells. These cells expressed significantlyhigher level of multiple
stem cell markers (OCT4, NANOG, ALDH1, CD44 and CD133
CKIT, KLF4 and KLF5). Further work on CD44 showed that the
majority of spheroids cells are CD44+ and the CD44
+ cells were
resistant to chemotherapeutic agents and with higher spheroid
formation efficiency and exhibited stronger chemo resistance to
fluorouracil5-FU.CD44+cells could give rise to both CD44
+ and
CD44- cells, suggesting a hierarchical relationship. The
phenotypic heterogeneity also was observed in xenografts and
primary tumors. Serial transplantation is an important
measurement of long-term self-renewal ability of CSCs. The
authors reported spheroid cells could be serially engrafted into
nude mice, but no data has been shown in detail. Although
sphere-forming assays have been extensively used in many
cancers to assess clonogenicity, long-term renewal capacities and
multiline age differentiation, they must be interpreted with caution.
It is important to note that not only stem cells but also their transit-
amplifying progeny are able to form spheres and that, by contrast,
quiescent stem cells cannot form spheres. Thus sphere assays do
not allow for an accurate quantification of stem cell frequency in
vivo[50]. Even using the same protocol for culturing C666-1
spheres, we were unable to form decent passageblespheres from
the primary NPC cells. Does it mean there are actually no CSCs in
primary NPC cells orit is just an artificial adaption for C666-1 line
in long term in vitro cultures? It will be important to define to
what extent the ability of tumor cells to grow as spheres is directly
correlated with their ability to sustain tumor growth in vivo.
CD133
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
CD133 (also known as Prominin 1), a member of pentaspan
transmembrane glycoproteins, is expressed in hematopoietic stem
cells, endothelial progenitor cells, neuronal and glial stem cells. It
specifically localizes to cellular protrusions[51]. CD133 has
previously also been shown to be expressed in subpopulations of
cancer cells from brain, colon, lung, melanoma and other solid
tumors. This led to the assumption that CD133 expressing tumor
cells have stem cell or progenitor like properties and CD133 was
proposed as CSC marker[51]. Lun et al.[27]found that
1.90±0.84% of CD133+in C666-1 cells and completely absent in 2
of the xenografts (xeno-666 and xeno-2117). Consistently, we
only observed very rare C666-1 cells with faint cytoplasmic but
not surface staining of CD133.CD133 was barely detectable in
NPC primary cells or patient biopsies[30]. However, Zhuang et
al.[31] reported 3.36±0.35%CD133+ cells with CSCs
characteristics in CNE cell lines. Overall their study is descriptive.
For example, no significant difference was formed in thecell cycle
distribution between the CD133+ and CD133
- cells, but CD133
+
cells hadsignificantlyhigher proliferative index and had a greater
potential for in vivo tumor formation. The CD133 expression
dropped to zero at 21 days of culture. Whether CD133 is a marker
cannot be concluded from this study. Further extensive studies
with broader spectrum of cell lines, primary cells and
xenograftsareneeded.
Basically, the above studies have demonstrated that NPC cells are
heterogeneous and contain cancer stem-like cells. Based on these
limited publications, it is hard to say which marker works better
than the other to identify NPC CSCs. Even using the same cell
line and same marker [26,27], different results were obtained. The
exact reasons for the reported discrepancies across studies are not
clear. Possible explanations may include differences in techniques,
protocols and reagents such as antibodies. Additional sources of
confusion may mirror the inter/intra-tumor heterogeneity and
colon evolution. These studies highlight the need for
comprehensive analysis by using combinations of different
markers to identify potentially unique functional characteristics of
NPC CSCs. The gold standard of CSC identification continues to
be tumour initiation with serial transplantation in recipient mice,
but this may not be practical to NPC. It is very challenging and
will be discussed in the following section.
Unsolved Issues
Despite the useful data we obtained from the above studies, there
are still many unsolved issues:
Where Do The NPC Cscs Come From?
The stem cell characteristics of CSCs beg the question of the cell
type from which they are derived. Experimental evidence suggests
that CSCs arise either from normal stem cells that have become
cancerous through mutation, or from the transformed somatic cells
that have acquired the ability to self-renew. Lineage tracing in
experimental mouse models has strongly showed that Lgr5+
intestinal stem cells can initiate and maintain murine intestinal
adenomas[52,53]. In mouse models of skin cancer, hair follicle
bulge stem cells[54] can serve as target cells for transformation,
and CD34+ cells resembling their normal bulge stem cell
counterpart are capable of propagating the disease as a cancer
stem cell population[9]. In parallel, mouse models of breast cancer
and recent studies using human tumor samples demonstrate that
tumors can arise and be propagated from the transformation of
more differentiated luminal cells[55].
In NPC, EBV infection is detected in nearly all patients in the
endemic regions.Although the underlying mechanism of how EBV
contributes to cancer is not completely understood, emerging data
indicated that EBV latent membrane protein LMP1 and LMP2a
have transforming properties. Both proteins can activate a number
of signalling pathways such as NF-КB, STAT that trigger
morphological and phenotypic alterations in epithelial cells.
Significantly, they have been shown to induce EMT, increase the
cancer stem cell-like population and contribute to the onset of
metastases in NPC[56,57,58].
A major question surrounding NPC is that it is not known whether
or to what extent epithelial cells become infected when the host
first encounters EBV during primary infection. One hypothesis
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
isthat accumulation of genetic alterations renders cells more
permissive to latent EBV infection[59]. High frequencies of
chromosomal loss at 3p/9p are present in 80̴100% of
NPC[60,61,62], which is similar to the level of EBV infection.
This suggests that EBV infection may not be the initiating event in
NPC pathogenesis, but rather, occurs before the initiation of
invasive growth. Studies[63] indicate that undifferentiated
epithelial cells are more permissive than the terminally
differentiated cells for latent EBV infection and propagation.
Nasopharynx is lined by stratified squamous and respiratory type
epithelium. EBV infection of normal nasopharyngeal cells is rare.
EBV-infected epithelial cells are hardly detectable even in normal
nasopharyngeal biopsies from individuals who are at high risk of
developing NPC[64]. Basal cells in the epithelium of airway act as
stem cells with undifferentiated properties [65,66]. In addition to
their role in epithelial homeostasis, basal cells probably contribute
to disease susceptibility, initiation and progression. For example,
disruption of the normal balance between proliferation and
differentiation in airway basal cells can leadto basal cell
hyperplasia or epithelial hypoplasia[66,67]. Similarly, recent
studies demonstrated many cancers including prostate cancer, skin
basal cell carcinoma and basal-like breast cancer subtype are
originated in basal cells[68,69,70]. Consistently, we found almost
exclusively primary NPC cells are positive both for EBV and
basal cell marker p63(Figure 1). More importantly, like their
normal counterpart, these cells can be differentiated into goblet
cells and ciliated cells(unpublished data). The data suggest that
basal stem cells could bethe initiating and propagating cells of
NPC,although we cannot rule out NPC initiating cells of other
origin such as transformed somatic cells.
Figure 1.Primary cell culture of nasopharyngeal carcinoma (NPC). Primary NPC cells express EBV and basal cell markers revealed by immunohistochemistry with
antibodies against EBNA-1 and p63.
Are NPC Cscs Rare?
According to CSCs model, only a rare of population sits at the top
of the cellular hierarchy to drive the tumor progression. Indeed, in
many types of human tumors, CSCs have been shown to be rare,
with frequencies ranging from 0.0001 to 0.1% determined by the
capability of re-forming secondary tumors on transplantation into
immunodeficient mice[71,72]. By contrast, Morrison and
colleagues [73]demonstrated that the transplantation of melanoma
cells into more severely immunodeficientNOD scid IL2 receptor
gamma chain knockout mice (NSG) mice enhances the frequency
of CSCs by several orders of magnitude as compared to nonobese
diabetic/severe combined immunodeficient (NOD/SCID) mice.
Up to 27% of unselected melanoma cells from four different
patients were able to form xenografttumor, demonstrating that
CSCs are not always rare.Limiting-dilution transplants are
typically used to determine the frequency of CSCs. In available
published NPC studies,nude mice or NOD/SCID mice were
frequently used(Table1).For example, in the study by Lun et al.
injection of at least 1,000 C666-1spheroid cells occasionally
formed one tumor out of six nude mice [27]. In our own study [30],
at least 10000 ALDHHigh
C666-1 cells were necessary for tumor
formation in nude mice.Couldthe frequency of NPC CSCs be
underestimated? Almost all NPC cells wereectopically
transplanted into subcutaneous region, which does not mimic the
native environment and is suboptimal for engrafting[74,75]. This
can be seen from the experience ofxenografting either tissue
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
explants or dissociated primary cells. It is rarely a successful, even
though a large number of cells were transplanted.Orthotropic
models[74,75] have been reported but it is not clear why it has not
been extensively used. On top of that, primary cells may be more
accurate for localising CSCs frequency within primary tumorsthan
cell lines.Strikingly, we found more than 40% of primary NPC
cells highly express CD44 andALDH (unpublished data). More
studies using primary cells and orthotropic modelsinmore severely
immunodeficient NSG mice are required to assess whether the
lowfrequency of CSCs found in NPC is the consequence of
suboptimal assays rather than due to an intrinsic inability to be
propagated in immunodeficient mice.
Are NPC Cscsquiescent Or Fast Proliferating?
In many adult tissues[76,77,78], stem cells show a relative slow
turnover rate at homeostasis. For example, in the central nervous
system, the neural stem cells in the subventricular zone is a
relatively quiescent population with a cell cycle length up to 28
days, whereas the transit amplifying progenitors cells (TA) divide
rapidly with a cell cycle length of approximately 12 h)[79,80].In
melanoma, a subset of slow-cycling cells with doubling times
of >4 weeks within the rapidly proliferating main population is
essential for continuous tumor growth[81].However, in other adult
tissues—such as the small intestine—some cells with bona fide
stem cell activity remain in an actively dividing state[82].
Identification of CSCs has mostly been studied on the basis of
functional assays such as in vitro clonogenic assays, sphere
formation and tumorxenografting. It is important to note that all
these assays are measuresof proliferation. Do we deliberately
select for fast proliferating cells? Are these fast proliferating cells
true CSCs or just TA progenitors?If the original CSCs are
quiescent in vivo but are stimulated to divide in cultures
containingserum and saturated growth factors), we might be able
to capture the cells. Conversely, if they do not respond to in vitro
cultures (conditions are not adequate), we might miss the CSCs.
Mathematical modelling of the clonal fate data suggest that the
tumor is hierarchically organized similarto normal epidermis, but
CSCs divide rapidly instead of being mostly quiescent (like stem
cells) during normal homeostasis[83,84]. In contrast, Parada and
colleagues[85] used a genetic lineage ablation approach in a
mouse model of glioblastoma to identify a subset of glioma CSCs
marked by Nestin. It found that these cells are responsible for
sustaining long-term tumor growth and relapse through the
production of transient populations of highly proliferative cells,
but they themselves are quiescent. Another study demonstrated
that colon CSCs escape 5FU chemotherapy-induced cell death by
entering stemness and quiescence via the c-Yes/YAP axis[86]. As
NPC is a disease which can relapse (15%5̴8%) [87]and the
recurrent NPC is refectory to therapy, it is reasonable to think that
there might be some CSCs which are very quiescent and survive
the primary radiation and chemotherapy. To address this question,
the development of NPC animal models is necessary and lineage
tracing will be helpful to assess the fate of CSCs more directly
within their natural environment.
Is NPC Cscs Status Stable Or Has Plasticity?
CSCs can divide asymmetrically to self-renew and generate
differentiated cells.This forms the basis of a unidirectional
hierarchy of tumor. Like in most studies using cell lines,CSCs are
studied based on the assumption that it is a defined subpopulation
witha marker in a given cancer samples.This may over simplify
the complexity of the heterogeneity of in vivo tumors.Recent
research has identified unexpected plasticity of CSCs[81,88,89].
Chaffer et al.[89] found that certain degree of plasticity exist
within a breast cell population, which allows inter-conversion
between CSC and non-CSC states when driven by selective
pressures (including therapy) or clonal evolution, indicating
hierarchical models is not unidirectional rather bidirectional, not
stable rather dynamic. In intestinal tumors,LGR5- cells can give
rise to LGR5+tumor cells, supporting the idea that, when levels of
active β-catenin are increased, villus cells can reacquire CSC
properties by dedifferentiation[90].It has also been demonstrated
that cell surface markers could be dynamically and reversibly
expressed by tumorigenic cells[91].In Wang’s study[24]it was
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JNPC ★ http://www.journalofnasopharyngealcarcinoma.org/ e-ISSN 2312-0398 Published:2014-03 -20 ★ DOI:10.15383/jnpc.6
found that Non-SP NPC cells can give rise to SP cells.We[30] also
found ALDHLow
NPC cells can regenerate ALDHHigh
cells , which
suggest the possibility of plasticity instead of technical limitation
of FACS.Adding yet another layer of complexity is the notion that
there may exist more than one distinct cancer stem-like state
within a tumor,because CSCs keep accumulatingdriver and
passenger epigenetic and genetic perturbations during theirclone
evolution andbranching[92,93]. As a result, phenotypic plasticity
superimposes on a multiplicity of pre-malignant and malignant
subclones, which makes a single or universal maker for CSCs
seems impossible.
Conclusion
Understanding how NPC CSCs contribute to initiation and
progression in tumors will undoubtedly lead to the identification
of novel targets. However, the complexity of CSCs in terms of
their heterogeneity and plasticity will make any one single marker
and drug unlikely to be efficient. An ideal strategy would be to
target boththe CSC andthe non-stem cells populations of tumor.
More importantly, the CSCs niche in which they are located is
acritical determinant of how they respond to a given treatment[94],
which strongly put forward the niche as an important and
inseparabletarget for novel therapies[95,96,97].Signaling
pathways that potentially kill or differentiate CSCs have been
increasingly identified, and experimentally or clinically
tested[92,98,99].
CSC study in NPC is still in its infancy. Using primary cells and
xenografts may be more disease relevant. Using orthotropic
models in more immunodeficent mice may be more accurate to
investigate CSCs frequency. Establishment of genetic lineage
tracing models may allow more direct trackingof CSCs in vivo.
Therapies targeting NPC CSCs began to emerge [35,36,37,100].
We anticipate that in the near future, successful targeting of CSCs
will significantly improve outcomes in NPC cancer patients and
impact patient management.
Conflict of interest
The authors have no other funding, financial relationships, or
conflicts of interest to disclose.
Declaration of The Source of Funding
This work was supported by a Grant from the National University
Cancer Institute, Singapore (NCIS) Centre Grant to Dr.Loh andDr.
Yu.
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