Neurochem Res
DOI 10.1007/s11064-008-9723-8
REVIEW ARTICLE
Stem Cell Markers in Gliomas
Paola Dell’Albani
Accepted: 21 April 2008
Ó Springer Science+Business Media, LLC 2008
Abstract Gliomas are the most common tumours of the
central nervous system (CNS) and a frequent cause of
mental impairment and death. Treatment of malignant
gliomas is often palliative because of their infiltrating
nature and high recurrence. Genetic events that lead to
brain tumours are mostly unknown. A growing body of
evidence suggests that gliomas may rise from cancer stem
cells (CSC) sharing with neural stem cells (NSC) the
capacity of cell renewal and multipotency. Accordingly, a
population of cells called ‘‘side population’’ (SP), which
has been isolated from gliomas on the basis of their ability
to extrude fluorescent dyes, behaves as stem cells and is
resistant to chemotherapeutic treatments. This review will
focus on the expression of the stem cell markers nestin and
CD133 in glioma cancer stem cells. In addition, the possible role of Platelet Derived Growth Factor receptor type a
(PDGFR-a) and Notch signalling in normal development
and tumourigenesis of gliomas are also discussed. Future
work elucidating the mechanisms that control normal
development will help to identify new cancer stem cellrelated genes. The identification of important markers and
the elucidation of signalling pathways involved in survival,
proliferation and differentiation of CSCs appear to be
fundamental for developing an effective therapy of brain
tumours.
Special issue article in honor of Dr. Anna Maria Giuffrida-Stella.
P. Dell’Albani (&)
Institute of Neurological Sciences, National Research Council
(CNR), V.le Regina Margherita, 6, 95123 Catania, Italy
e-mail: [email protected]
Keywords Gliomas Stem cell markers CD133 Nestin Notch PDGF
Gliomas represent the most frequent primary tumours of
the central nervous system (CNS) and an important cause
of mental impairment and death. Gliomas are histologically
classified according to their hypothesized line of differentiation (e.g. astrocytes, oligodendrocytes or ependymal
cells) and fall into four clinical grades according to their
degree of malignancy. Grade I tumours are biologically
‘‘benign’’, while grade II tumours are low-grade malignancies with long clinical courses. Grade III and IV are
malignant gliomas and are lethal within a few years and
9–12 months, respectively. Furthermore, more than fifty
percent of grade II gliomas transform into grade III and IV
tumours within 5–10 years of diagnosis. With the exception of grade I tumours, which are surgically curable if
resectable at the time of diagnosis, all other grade gliomas
are not curable with surgery because of their tendency to
affect the cerebral hemispheres in a diffuse manner.
Malignant gliomas are highly recurrent tumours even after
surgery, chemotherapy, radiation and immunotherapy.
Ionizing radiation (IR) represents the most effective therapy for glioblastoma but radiotherapy remains only
palliative [1] because of radioresistance. The treatment
strategies for gliomas have not changed appreciably for
many years and most are based on a limited understanding
of the biology of the disease. New insights into the causes
and potential treatment of CNS tumours have come from
discovering connections with genes that control cell
growth, proliferation, differentiation, and death during
normal development. This review will focus on the neural
stem cell markers CD133 and nestin in gliomas. The
123
Neurochem Res
possible role of developmental pathways Notch and PDGF/
PDGFR in brain tumours will also be discussed in the
context of the recent theory of cancer stem cells.
Neural stem cells (NSCs) are the CNS tissue-specific stem
cells. NSCs cells have proliferative and self-renewal
capacities, i.e. they are able to maintain the number of
quiescent stem cells in a given brain region or to increase it
in particular situations, and multi-potentiality, i.e. they are
able to generate neurons, astrocytes and oligodendrocytes
[2–4]. During an early embryonic phase of development
NSCs divide symmetrically maintaining stemness and
expanding the cellular population. In a second neurogenic
phase, NSCs undergo to asymmetric division giving rise to
new stem cells and to proliferating precursors belonging to
the neuronal lineage. After this phase a glial progeny will
develop with the progressive decline of stem cells, even
though small numbers of stem cells persist in specific
regions of the adult brain, primarily in the subventricular
zone (SVZ) of the forebrain lateral ventricles and in the
dentate gyrus of the hippocampus. Persistent neurogenesis
in the adult brain has been described in rodents, monkeys
[5] and humans [6], but the identity and functional organization of adult NSCs has been clearly defined in rodents.
Cells in the SVZ of adult rodent brain could be classified
essentially in four types: multipotent astrocytes (type B
cells), immature precursors (type C cells), migrating neuroblasts (type A cells) and ependymal cells (for a scheme
of adult rodent neurogenesis see Fig. 1). The type B cells
are believed to be the primary neural precursors. They are
slowly dividing cells and exhibit morphological and neurochemical properties of astrocytes because they express
the glial fibrillary acidic protein (GFAP), an intermediate
filament (IF) protein considered a marker of mature and
reactive astrocytes in the CNS [7]. In addition to GFAP,
type B cells express other IFs such as vimentin and nestin,
while they are immuno-negative for neuronal markers such
as PSA-NCAM and TuJ1 [8]. The PDGFR-a is also present
in type B cells. PDGFR-a signalling, occurring early in the
adult stem cell lineage, is believed to regulate the balance
between oligodendrocyte and neuron production [9]. Type
Fig. 1 Adult rodent neurogenesis in SVZ and neural stem cells
markers. Adult SVZ neural stem cells (NSC) differentiate into
progressive stages. SVZ astrocytes (Type B cells) and a small fraction
of ependymal cells (CD133-positive) have self-renewing capacity and
multipotency. Type B and CD133-positive ependymal cells are
considered putative stem cells, while types C and A are possible
progenitor or committed precursor cells. Dashed arrows indicate
possible NSC differentiation pathways. Markers of NSC, mature glia
and neurons are listed. Abbreviations: SVZ, sub ventricular zone;
NSC, neural stem cell; O-2A, oligodendrocyte-type-2 astrocyte
lineage; GFAP, glial fibrillary acidic protein; PSA-NCAM, polysialylated neural cell adhesion molecule; PDGFR-a, Platelet Derived
Growth Factor receptor a; A2B5, monoclonal antibody that recognizes oligodendroglial precursor cells; O4, monoclonal antibody that
recognizes the sulfatides; MBP, myelin basic protein; SMI32,
monoclonal antibody that recognizes neurofilament H in neuron
Neurogenesis and Carcinogenesis
123
Neurochem Res
B cells give rise to clusters of a transit-amplifying cell
population of immature precursors (type C cells), which in
turn generate migrating neuroblasts type A cells. Type A
cells migrate to the olfactory bulb where they mature
continuing to produce neurons [10–12]. Recently, data
suggest that ependymal cells contribute to the lineage of
the postnatal SVZ. A subpopulation of CD133 positive (+)
cells among ependymal cells of the SVZ fulfil the criteria
of NSCs [13]. The existence of a migratory pathway in the
adult human brain is controversial. In human adult SVZ, a
‘‘ribbon’’of SVZ astrocytes both able to proliferate in vivo
and behaving as multipotent progenitor cells when placed
in vitro, have been described [14]. However, these authors
did not observe any chains of migrating neuroblast in the
SVZ or to the olfactory bulb.
In the past, tumour cells in the brain were hypothesized
to derive mostly from the transformation of mature neural
cells such as astrocytes, oligodendrocytes or neuronal
precursors. Recently, this theory has changed since the
concept of cancer stem cells has been extended to brain
tumours. To date, two models have been proposed to
explain cellular cancer proliferation. In the classic stocastic
model all the cells in a tumour have similar tumourigenic
potential that is activated asynchronously and at a low
frequency. In contrast, the hierarchical model proposes that
only a rare subset of cells within the tumour have significant proliferation capacity and the ability to generate new
tumours resembling the primary tumour, while the other
tumour cells are terminally differentiated and cell-death
committed [15]. The hierarchical hypothesis correlates
with the cancer-stem-cell theory now supported by accumulating experimental data showing that cancers, like
normal organs, may be maintained by a hierarchical
organization that includes stem cells, transient amplifying
cells (precursor cells), and differentiated cells [15, 16].
Malignant gliomas contain both proliferating and differentiating cells, which express either neuronal or glial
markers, and can be generated from both NSCs and glial
lineage cells, such as oligodendrocyte precursor cells or
astrocytes, which can behave as NSCs in appropriate
conditions. [10, 17–19]. These observations raise the possibility that they may contain multipotent neural-stem-cell
(NSC)-like cells. [20].
Neural and Cancer Stem Cells
Stem cells can be identified by the expression of specific
markers, although they do not appear to be organ-specific.
Normal and cancer stem cells share the expression of
several markers, the ability for self-renewal and differentiation, and signalling pathways involved in the regulation
of cellular survival, proliferation. Furthermore, they show
telomerase activity [21], resistance to apoptosis and
increased membrane transporter activity. Recently, a small
population of stem cells, also termed ‘‘side population’’
(SP), has been identified in several normal tissues and
tumours on the basis of the ability to extrude fluorescent
dyes, by the ‘‘flow cytometry-based side population technique’’ [22–25]. SP-stem cells have several fundamental
properties such as (i) they are generally very rare (about
0.01–5%), (ii) they rarely divide, even though they have an
elevated proliferative potential, (iii) they can self-renew.
SP cells are capable of sustained expansion ex vivo and are
able to generate, through asymmetric division, both SP and
non-SP progeny [26]. SP cancer stem cells obtained from
brain tumours form neurospheres, which have the capacity
for self-renewal and are able to differentiate into phenotypically diverse populations including neuronal, astrocytic
and oligodendroglial cells when dissociated in single cell
suspension [22]. Several articles have recently reported the
presence of stem cell-enriched SP in long-term cultured
glioma cell lines such as rat C6 [27] and human U87-MG,
U373-MG [28]. SP stem cells demonstrate elevated
chemioresistence. Malignant SP cells readily export many
cytotoxic drugs, because of the high expression levels of
drug-transporter proteins belonging to the ABC family,
such as MDR-1 (i.e., ABCB1 or P-glycoprotein), MRP-1
(ABCC1), ABCA2, ABCA3 and ABCG2. A recent report
suggests that SP cells are heterogeneous with respect to the
expression of drug transporter proteins [28] ABCG2 was
present in proliferating cells preferentially. These cells can
undergo to asymmetrical division giving rise to ABCG2
positive cancer cells identified as tumour progenitor cells
and to ABCG2 negative. Among the ABCG2 negative
cells authors distinguish between primitive cancer stem
cells, which show high self-renewal, proliferative potential
and high expression levels of ‘‘stemness’’ genes such as
Notch-1, and differentiated tumour cells, which are partially or fully differentiated cells that constitute the bulk of
tumour mass (Fig. 2).
Markers of Stem Cells in Brain Tumours
CD133 and nestin are currently the most accredited
markers for the identification of NSCs. Their use in CSCs
research has been fundamental to reveal the biological
properties of glioma stem cells, such as tumour progression
and resistance to IR or chemotherapy.
CD133 (prominin-1) is a cell membrane glycoprotein,
with five transmembrane domains, originally found on
neuroepithelial stem cells in mice [29]. In humans, prominin-1 has been isolated from hematopoietic stem cells by
an antibody recognizing AC133, a specific protein epitope.
In general, CD133 is present in different types of stem cells
123
Neurochem Res
Fig. 2 Simplified scheme of gliomas cell populations composition.
Essentially two cellular populations could be observed in gliomas: a
very small cell population (about 0.01–5%) called cancer stem cells
(Side Population, SP) responsible of cancer establishment and
persistence, and a population of differentiated, death-committed
cells, non SP cells, that represent the tumour mass (about 99.9–95%).
Cancer stem cells (SP) show up-regulated levels of proteins (ABCG2,
CXCR4, IAPs, ChK1-2, RAD17, ATM, MGMT and HES1) that are
involved in various mechanisms used to escape cell-death and to
invade neighbouring tissues
and several cancers, and is down-regulated in differentiated
cells [30]. Five alternative promoters, three of which are
partially regulated by methylation, drive the transcription
of several mRNA isoforms of prominin-1 [31]. Despite
knowledge on regulation of CD133 transcription, its
function remains unclear. CD133 localization in membrane
protrusions suggests an involvement in the dynamic organization of membrane protrusions and therefore in the
mechanisms influencing cell polarity, migration and interaction of stem cells with neighbouring cells and/or
extracellular matrix, but experimental data are currently
lacking. In addition, it is not known whether CD133 has a
role in self-renewal and differentiation of stem cells, which
has important implication in cancerogenesis.
CD133+ cells isolated from human brain tumours
exhibit stem cell properties in vitro [22] and are able to
initiate and drive tumour progression in vivo [23, 32],
strongly suggesting that CD133+ cells might be the brain
tumour initiating cells. This notion has been recently
challenged by studies demonstrating that glioblastoma
CD133 negative (-) cells have also properties of stem
cells and are tumourigenic when engrafted intracerebrally
into nude mice [33]. Interestingly, glioma human biopsy
engrafted intracerebrally into nude mice were initially
CD133-, but up-regulated CD133 after serial passages in
vivo along with increasing angiogenesis [34]. CD133+
cells exhibit resistance to drugs and toxins through the
123
expression of several ABC transporters, active DNA repair
capacity and resistance to apoptosis [35]. A recent paper
suggests that a preferential activation of the DNA damage
checkpoints in response to radiation and a more efficient
repair radiation-induced DNA damage in CD133+ cells
than CD133- cells, might underlie resistance of CD133+
subpopulation to IR [36]. Thus, a therapy targeting
checkpoints response in CD133+ tumour cells might
overcome gliomas radio-resistence and provide a therapeutic model for malignant brain cancers. However, other
key stem cell pathways, such as Wnt/b-catenin, Notch,
sonic hedgehog (SHH), PTEN, epidermal growth factor
receptor (EGFR) and Bmi-1 are activated by IR in CSCs
and might be involved in IR resistance [37]. CD133+ cells
are also characterized by an intrinsic resistance to chemotherapeutic agents such as temozolomide, carboplatin,
paclitaxel (Taxol) and etoposide (VP16). A gene expression study revealed a higher expression of drug
transporters BCRP1/ABCG2 and MGMT in CD133+ than
in CD133-, but also higher levels of apoptosis suppressors
such as Bcl-2, FLIP, BCL-XL and several inhibitor of
apoptosis proteins (IAPs), such as XIAP, cIAP1, cIAP2,
NAIP and survivin [38]. IAPs bind and inhibit caspases
3, 7 and 9, preventing apoptosis [39] and modulate cell
division, cell cycle progression and signal transduction
pathways. Interestingly, CD133 expression was significantly higher in recurrent GBM tissue obtained from
patients as compared to their respective newly diagnosed
tumours.
CD133+ cancer stem cells obtained from glioblastoma
biopsies showed also an elevated increase of the expression
levels of the cell surface chemokine receptor CXCR4
versus CD133- cells [38]. This receptor is also expressed
in human NSCs and may have a significant role in directing
NSC migration during CNS development [40]. Interestingly an over-expression of CXCR4 has been related to an
highly invasive potential of gliomas [41]. Altogether these
results suggest that CD133+ cancer stem cells may play an
important role not only in recurrence after chemo- and
radio-therapies in glioma invasion, but also in brain
invasion.
Nestin is a protein belonging to class VI of IFs, that is
produced in stem/progenitor cells in the mammalian CNS
during development [42] and is a marker of proliferating and
migrating cells. IFs are highly diverse intra-cytoplasmic
proteins including vimentin, GFAP and neurofilaments and
exhibit cell type specificity of expression. IFs are cytoskeleton constituents and are involved in the control of cell
morphology, adhesion and proliferation. When differentiation starts, cells that exit the cell cycle down-regulate nestin
and subsequently up-regulate alternative IFs such as neurofilaments in committed neurons [43], and GFAP in glial
precursors [44, 45]. In the adult CNS, nestin is expressed in
Neurochem Res
stem cells of the SVZ and to a lesser extent in the choroid
plexus [46] even though several morphological types of
nestin-positive cells (neuron-like, astrocyte-like, cells with
smaller cell bodies and fewer processes) are detectable in
different areas of forebrain of normal adult human brain [47].
Down-regulated nestin may be re-expressed in the adult
organism under certain pathological conditions such as brain
injury, ischemia, inflammation and neoplastic transformation [48]. Nestin has been detected in brain tumours such as
pilocytic astrocytomas and malignant gliomas including
glioblastoma multiforme [49–51]. IF subtypes has been
linked to enhanced motility and invasion in a number of
different cancer subtypes. The co-expression of nestin and
vimentin in different astrocytoma cell lines has been related
to a migratory cell phenotype with increased motility and
invasiveness (metastatic potential) of different astrocytoma
cell lines [52]. Moreover, Dahlstrand and colleagues [49]
showed high nestin expression in high malignant tumours
such as glioblastoma multiforme when compared to less
anaplastic glial tumours. This study assigns to nestin a role as
new potential prognostic marker for glioblastomas. Thus,
nestin expression in tumour cells may be related to their
dedifferentiated status, enhanced cell motility, invasive
potential and increased malignancy. In addition, nestin has
been also identified in the cell nucleus of tumour cell lines
obtained from glioblastoma patients [53]. Authors hypothesize that nuclear nestin may affect the organization of
chromatin or may serve as specific regulator of gene
expression [53, 54]. These data give new insights for further
studies on the relationship between nestin re-expression and
tumour malignancy.
Notch and PDGF-Mediated Signalling in Gliomas
Some of the signalling pathways that are involved in differentiation and proliferation of glial progenitors are
altered in gliomas. Notch signalling is essential for the
maintenance of NSCs, by enhancing the NSC self-renewal
and by inhibiting its differentiation into neuronal and glial
progenitors [55, 56]. PDGF signalling is implicated in
oligodendrocyte proliferation and differentiation [57].
The Notch family of transmembrane receptors comprises
Notch-1, -2, -3 and -4. Mature Notch receptors are heterodimers derived from the cleavage of Notch pre-proteins
into an extracellular and a trans-membrane subunits
including the intracellular region. Mammalian Notch genes
are widely expressed during embryonic development.
Notch signalling is a ligand-receptor initiated pathway. The
interaction between a ligand (DLL-1, -3,-4 and JAG-1, -2)
and Notch receptors triggers two successive cleavages: the
first mediated by the Tumor Necrosis Factor-a-converting
enzyme (TACE) and the second by c-secretase, originating
an intra-cytoplasmic fragment of Notch (NICD). NICD
translocates to the nucleus where it binds to the transcription factor CBF1/Su(H)/LAG1 (CSL). This interaction
results in the displacement of the co-repressor (CoR) and
recruitment of the co-activator (CoA) leading to transcriptional activation of target genes. Notch signalling activates
a diverse repertoire of genes, the products of which can
activate or inhibit many different cellular functions. In
most cases, Notch signalling blocks differentiation towards
a primary differentiation fate in a cell and instead directs
the cell to a second, alternative differentiation program
or forces the cell to remain in an undifferentiated state.
Therefore, Notch signalling has been referred to as a
‘‘gatekeeper against differentiation’’ [58]. In the nervous
system, Notch is essential for the maintenance of the neural stem cell [55, 59] and promotes differentiation of
various glial cell types, including astrocytes [60], Müller
glial cells [61] and radial glial cells [62]. Interestingly,
recent data show that Notch signalling pathway prevents
nestin degradation during stem cell differentiation, by a
mechanism that possibly involves ubiquitin-proteosome
pathways [63].
Altered Notch expression and signalling have been
observed in spontaneous human tumours and in tumour
models [64]. Recently, a growing body of evidence suggests that Notch-1 signalling might be critical for
tumourigenesis and might represent an important target in
the treatment of gliomas. Notch-1 and its ligands are overexpressed in many glioma cell lines and primary human
gliomas as well as in a KRAS-induced glioblastoma mouse
model [65–67]. Interestingly, knockdown of Notch-1 and
its ligands induces apoptosis and inhibits proliferation of
cultured glioma cell lines and prolonged survival in murine
orthotopic brain tumour model [65]. Recent data suggest
that Notch signalling promotes the formation of cancer
stem cells in gliomas. Notch signalling can directly upregulate nestin expression in gliomas and cooperate with
KRAS to generate periventricular lesions characterized by
continued proliferation of stem cells in the SVZ [66]. In
addition, a constitutive activation of Notch signalling in
glioma cell lines promotes growth and increases the formation of neurosphere-like colonies in the presence of
growth factors [68]. A link between Notch signalling and
cancer stem cells can also be hypothesized for medulloblastomas. In medulloblastoma cell cultures a blockade of
Notch signalling through inhibition of c-secretase drastically reduced the number of CD133+ cells, totally
abolished the SP cells and inhibited the ability of forming
tumours in vivo. These data suggest that the loss of
tumour forming capacity could be due to the depletion of
stem-like cells [69]. Accordingly, Hes1 mRNA, a marker
of Notch pathway activity, is substantially up-regulated in
the CD133-enriched fraction of medulloblastoma cell line
123
Neurochem Res
cultures. This suggests that Notch signalling is especially
active in stem-like cancer cells and supports the possibility that Notch pathway inhibition may target this
population.
PDGF was originally identified in platelets and in serum
as a mitogen for fibroblasts, smooth muscle cells (SMC)
[70] and glial cells in culture. To date four PDGF ligands
PDGFA-D are known. The four PDGF polypeptide chains
form five dimeric PDGF isoforms: PDGF-AA, -AB, -BB,
-CC, and -DD [71]. PDGF isoforms exert their cellular
effects through tyrosine kinase a- and b-receptors.
During embryogenesis, glial and neuronal progenitors
express the PDGFR-a, whereas neurons and astrocytes
express PDGF [72]. The PDGF a-receptor is constantly
expressed during differentiation of neural stem cells, but is
phosphorylated only after PDGF-AA treatment, while the
PDGFR-b is very low or not detectable in uncommitted
cells, but its expression increases with differentiation [73].
During the post-natal period, as glial progenitors differentiate into oligodendrocytes, PDGFR-a expression is
down-regulated [72]. In adult brain, PDGFR-a is present in
the ventricular and sub-ventricular zone of the lateral
ventricles possibly restricted to neural stem cells, whereas
PDGF is widely expressed by neurons and astrocytes [74].
Ablation of the PDGFR-a in a subpopulation of post-natal
neural stem cells shows that this receptor is required for
oligodendrogenesis, but not for neurogenesis. Interestingly, the infusion of PDGF-AA alone into mice SVZ
arrests neuroblast production and induces SVZ B cell
proliferation contributing to the generation of large hyperplasias with some features of gliomas [9]. Thus
activation of PDGF signalling in SVZ B stem cells might
represent an event contributing to initiate tumourigenesis.
Numerous studies have demonstrated co-expression of the
PDGF-A, PDGF-B, and of the PDGFRs in glioblastomas,
suggesting that both autocrine and paracrine stimulation
could play an important role in glial tumourigenesis [75].
Lokker and colleagues [76] observed a decrease in cellular
survival and proliferation of glioma cell lines by blocking
the PDGF autocrine signalling providing evidence for a
critical role of the autocrine loop in maintaining cell
transformation. Furthermore, amplification of the PDGFRa gene has been observed in low grade and in a subset of
high-grade gliomas [77]. In neural progenitors and in
more mature astrocytes of newborn mice the overexpression of the PDGFR-b determines the formation of
oligodendrogliomas and oligoastrogliomas respectively
[78]. Data present in literature on PDGF/PDGFRs
expression in gliomas show how important will be to
understand the diverse molecular events that play a role in
PDGF/PDGFRs expression, signalling activation and cellular responses in gliomagenesis (for a recent review see
[79]).
123
Conclusion
To date gliomagenesis might take place in undifferentiated
precursor cells both in germinal zone of developing CNS or
in regions of mature brain where neurogenesis persists
throughout adult life. In undifferentiated precursor cells
mutations can dysregulate normal self-renewal signalling
pathways affecting proliferation, moreover hyper-activation of specific growth factor receptors in autocrine or
paracrine loops can determine cell proliferation and
hyperplasias having features of gliomas. Both nestin and
CD133 are expressed in NSC and are down-regulated when
cell differentiate. However, their expression could be
re-gained in pathological conditions such as neoplasm
formations. In this review it has been highlighted the
concept that a glioma is a mix of cell populations and more
importantly that only a small and rare population of cells is
really responsible of tumour growth, of tumour survival to
radio- and chemio-therapies and of tumour recurrence,
while the bulk of cells are cell-death committed and differentiated cells. The CD133+/ABCG2+ cell population
could be a good candidate to be the real cancer stem cell
population. These cancer stem cells seem to be able to
activate a series of mechanisms responsible for tumour
growth and recurrence such as: (i) expression of several
ABC transporters; (ii) activation of DNA damage checkpoints; (iii) activation of DNA repair system; (iv) high
expression of IAPs and apoptosis suppressors; (v) expression of elevated levels of CXCR4 related to high invasive
potential. Nestin re-expression in tumours might indicate a
dedifferentiated status in which an increased cell motility
correlates with augmented invasive potential and malignancy. Furthermore, the activation of signalling pathways
such as Notch and PDGF/PDGFRs collaborates in the
maintenance of cancer stem cells. It is now starting to
appear the possible draft, even nebulous, of the cancer
stem cell involved in the initiation and progression of
tumour growth, and recurrence. More work needs to be
done to clearly define new cancer stem cell markers and to
recognize known markers as important in neoplastic
transformation to find out therapies that specifically target
the tumour mass and especially the cancer stem cells
responsible of tumour recurrence.
Future Directions
The identification of brain cancer stem cells will provide a
powerful tool for the investigation of the tumourigenic
process in the central nervous system, and will be fundamental in developing novel therapeutic strategies to target
these cells, that are insignificant within the population of
tumour cells, but relevant cells to be destroyed. In the
Neurochem Res
future therapeutic protocols should be able to target both
stem-like and better-differentiated cells in the tumoural
mass. Drugs blocking specific signalling pathway such as
Notch signalling, or other pathways required in stem cells
such as Wnt and Hedgehog, should be used to deplete the
cancer stem cell population, while traditional chemotherapeutic agents could be used at the same time to de-bulk
the larger mass of tumour cells. This will result in a rapid
removal of both subpopulations preventing the possibility
that some tumour cells could give rise to tumour
recurrence.
Furthermore, since it is becoming evident a significant
heterogeneity within specific subtypes of solid tumours like
GBMs, it will be rational to develop specific molecular
targeted drugs, which could be personalized for individual
tumours.
Finally, taking into account all the emerging findings in
the field of cancer stem cells, we can just say that actually
what it is known is only the tip of a huge iceberg. Scientists
need to figure out the entire iceberg architecture, with the
unique goal to find out the resolutive therapy of tumours.
Acknowledgements I would like to thank Mr. Francesco Marino
for his helpful work for figure editing. I thank Dr. Maria Vincenza
Catania for encouraging and help me to write this review and for
useful discussion.
References
1. Garden AS, Maor MH, Yung WK et al (1991) Outcome and
patterns of failure following limited-volume irradiation for
malignant astrocytomas. Radiother Oncol 20:99–110
2. Gage FH, Ray J, Fisher LJ (1995) Isolation, characterization, and
use of stem cells from CNS. Annu Rev Neurosci 18:159–192
3. Loeffler M, Potten CS (1997) Stem-like cells and cellular pedigrees, a conceptual introduction. In: Potten CS (ed) Stem cells.
London, Academic Press, Inc., pp 1–27
4. Gritti A, Vescovi AL, Galli R (2002) Adult neural stem cells:
plasticity and developmental potential. J Physiol 96:81–90
5. Gould E, Tanapat P, McEwen BS et al (1998) Proliferation of
granule cell precursors in the dentate gyrus of adult monkeys is
diminished by stress. Proc Natl Acad Sci USA 95:3168–3171
6. Eriksson PS, Perfilieva E, Björk-Eriksson T et al. (1998) Neurogenesis in the adult human hippocampus. Nat Med 4:1313–
1317
7. Ridet JL, Malhotra SK, Privat A et al (1997) Reactive astrocytes:
cellular and molecular cues to biological function. Trends Neurosci 20:570–577
8. Doetsch F, Garcia-Verdugo JM, Alvarez-Buylla A (1997) Cellular composition and three-dimensional organization of the
subventricular germinal zone in the adult mammalian brain. J
Neurosci 17(13):5046–5061
9. Jackson EL, Garcia-Verdugo JM, Gil-Perotin S et al (2006)
PDGFRa-positive B cells are neural stem cells in the adult SVZ
that form glioma-like growths in response to increased PDGF
signalling. Neuron 51:187–199
10. Doetsch F, Caille I, Lim DA et al (1999) Subventricular zone
astrocytes are neural stem cells in the adult mammalian brain.
Cell 97:1–20
11. Lim DA, Alvarez-Buylla A (2001) Glial characteristics of adult
subventricular zone stem cells. In: Rao MS (ed) Stem cells in
CNS development. Humana Press, Towton, NJ, pp 71–92
12. Parras CM, Galli R, Britz O et al (2004) Mash1 specifies neurons
and oligodendrocytes in the postnatal brain. EMBO J 23:4495–
4505
13. Coskum V, Wu H, Blanchi B et al (2008) CD133 neural stem
cells in the ependymal of mammalian postnatal forebrain. Proc
Natl Acad Sci USA 105(3):1026–1031
14. Sanai N, Tramontin AD, Quinones-Hinojosa A et al (2004)
Unique astrocyte ribbon in adult human brain contains neural
stem cells but lacks chain migration. Nature 427:740–744
15. Reya T, Morrison SJ, Clarke MF, Weissman IL (2001) Stem
cells, cancer, and cancer stem cells. Nature 414:105–111
16. Tu SM, Lin SH, Logothetis CJ (2002) Stem-cell origin of
metastasis and heterogeneity in solid tumours. Lancet Oncol
3:508–513
17. Kondo T, Raff M (2000) Oligodendrocyte precursor cells
reprogrammed to become multipotential CNS stem cells. Science
289:1754–1757
18. Laywell ED, Rakic P, Kukekov VG et al (2000) Identification of
a multipotent astrocytic stem cell in the immature and adult
mouse brain. Proc Natl Acad Sci USA 97:13889–13894
19. Nunes MC, Roy NS, Keyoung HM et al (2003) Identification and
isolation of multipotential neural progenitor cells from the subcortical white matter of the adult human brain. Nat Med 9:439–447
20. Ignatova TN, Kukekov VG, Laywell ED et al (2002) Human
cortical glial tumors contain neural stem-like cells expressing
astroglial and neuronal markers in vitro. Glia 39:193–206
21. Zhenju J, Lenhard R (2006) Telomeres and telomerase in cancer
stem cell. Eur J Cancer 42:1197–1203
22. Singh SK, Clarke ID, Terasaki M et al (2003) Identification of a
cancer stem cell in human brain tumors. Cancer Res 63:5821–
5828
23. Singh SK, Hawkins C, Clarke ID et al (2004) Identification of
human brain tumour initiating cells. Nature 432:396–401
24. Yuan X, Curtin J, Xiong Y et al (2004) Isolation of cancer stem
cells from adult glioblastoma multiforme. Oncogene 23:9392–
9400
25. Hemmati HD, Nakano I, Lazareff JA et al (2003) Cancerous stem
cells can arise from pediatric brain tumors. Proc Natl Acad Sci
USA 100:15178–15183
26. Hirschmann-Jax C, Foster AE, Wulf GG et al (2004) A distinct
‘‘side population’’ of cells with high drug efflux capacity in
human tumor cells. Proc Natl Acad Sci USA 101:14228–14233
27. Kondo T, Setoguchi T, Taga T (2004) Persistence of a small
population of cancer stem-like cells in the C6 rat glioma cell line.
Proc Natl Acad Sci USA 101:781–786
28. Patrawala L, Calhoun T, Schneider-Broussard R et al (2005) Side
population is enriched in tumorigenic, stem-like cancer cells,
whereas ABCG2+ and ABCG2- cancer cells are similarly
tumorigenic. Cancer Res 65(14):6208–6219
29. Weigmann A, Corbeil D, Hellwig A et al (1997) Prominin, a
novel microvilli-specific polytopic membrane protein of the
apical surface of epithelial cells, is targeted to plasmalemmal
protrusions of non-epithelial cells. Proc Natl Acad Sci USA
94(23):12425–12430
30. Kania G, Corbeil D, Fuchs J et al (2005) Somatic stem cell
marker prominin-1/CD133 is expressed in embryonic stem cellderived progenitors. Stem Cells 23(6):791–804
31. Shmelkov SV, Jun L, St. Clair R et al (2004) Alternative promoters regulate transcription of the gene that encodes stem cell
surface protein AC133. Blood 103(6):2055–2061
32. Galli R, Binda E, Orfanelli U et al (2004) Isolation and characterization of tumorigenic, stem-like neural precursors from
human glioblastoma. Cancer Res 64:7011–7021
123
Neurochem Res
33. Beier D, Hau P, Proescholdt M et al (2007) CD133+ and
CD133- glioblastoma-derived cancer stem cells show differential growth characteristics and molecular profiles. Cancer Res
67(9):4010–4015
34. Wang J, Sakariassen PO, Tsinkalovsky O et al (2008) CD133
negative glioma cells form tumors in nude rats and give rise to
CD133+ cells. Int J Cancer 122:761–768
35. Dean M, Fojo T, Bates S (2005) Tumour stem cells and drug
resistance. Nat Rev Cancer 5:275–284
36. Bao S, Wu Q, McLendon RE et al (2006) Glioma stem cells
promote radioresistance by preferential activation of the DNA
damage response. Nature 444:756–760
37. Rich JN (2007) Cancer stem cells in radiation resistance. Cancer
Res 67(19):8980–8984
38. Liu G, Yuan X, Zeng Z et al (2006) Analysis of gene expression
and chemoresistance of CD133+ cancer stem cells in glioblastoma. Mol Cancer 5:67–78
39. Schimmer AD (2004) Inhibitor of apoptosis proteins: translating
basic knowledge into clinical practice. Cancer Res 64:7183–7190
40. Ehtesham M, Yuan X, Kabos P et al (2004) Glioma tropic neural
stem cells consist of astrocytic precursors and their migratory
capacity is mediated by CXCR4. Neoplasia 6:287–293
41. Ehtesham M, Winston JA, Kabos P, Thompson RC (2006)
CXCR4 expression mediates glioma cell invasiveness. Oncogene
25(19):2801–2806
42. Zimmerman L, Parr B, Lendahl U et al (1994) Independent
regulatory elements in the nestin gene direct transgene expression
to neural stem cells or muscle precursors. Neuron 12:11–24
43. Cattaneo E, McKay R (1990) Proliferation and differentiation of
neuronal stem cells regulated by nerve growth factor. Nature
347:762–765
44. Reynolds BA, Weiss S (1992) Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous
system. Science 255:1707–1710
45. Vescovi AL, Reynolds BA, Fraser DD et al (1993) bFGF regulates the proliferative fate of unipotent (neuronal) and bipotent
(neuronal/astroglial) EGF-generated CNS progenitor cells. Neuron 11:951–966
46. Morshead CM, Reynolds BA, Craig CG et al (1994) Neural stem
cells in the adult mammalian forebrain: a relatively quiescent
subpopulation of subependymal cells. Neuron 13:1071–1082
47. Gu H, Wang S, Messam CA, Yao Z (2002) Distribution of nestin
immunoreactivity in the normal adult human forebrain. Brain Res
943:174–180
48. Holmin S, Almquist P, Lendahl U et al (1997) Adult nestinexpressing subependymal cells differentiate to astrocytes in
response to brain injury. Eur J NeuroSci 9:65–75
49. Dahlstrand J, Collins VP, Lendahl U (1992) Expression of the
class VI intermediate filament nestin in human central nervous
system tumors. Cancer Res 52:5334–5341
50. Almqvist PM, Mah R, Lendahl U et al (2002) Immunohistochemical detection of nestin in pediatric brain tumors. J
Histochem Cytochem 50:147–158
51. Strojnik T, Røsland GV, Sakariassen PO et al (2007) Neural stem
cell markers, nestin and musashi proteins, in the progression of
human glioma: correlation of nestin with prognosis of patient
survival. Surg Neurol 68(2):133–143
52. Rutka JT, Ivanchuk S, Mondal S et al (1999) Co-expression of
nestin and vimentin intermediate filaments in invasive human
astrocytoma cells. Int J Dev Neurosci 17:503–515
53. Veselska R, Kuglik P, Cejpek P et al (2006) Nestin expression in
the cell lines derived from glioblastoma multiforme. BMC Cancer 6:32–43
54. Thomas SK, Messam CA, Spengler BA et al (2004) Nestin is a
potential mediator of malignancy in human neuroblastoma cells. J
Biol Chem 279:27994–27999
123
55. Nakamura Y, S-I Sakakibara, Miyata T et al (2000) The bHLH
gene Hes1 as a repressor of the neuronal commitment of CNS
stem cells. J Neurosci 20:283–293
56. Hitoshi S, Alexson T, Tropepe V et al (2002) Notch pathway
molecules are essential for the maintenance, but not the generation, of mammalian neural stem cells. Genes Dev 16:846–858
57. Pringle NP, Mudhar HS, Collarini EJ et al (1992) PDGF receptors
in the rat CNS: during late neurogenesis, PDGF alpha-receptor
expression appears to be restricted to glial cells of the oligodendrocyte lineage. Development 115:535–551
58. Artavanis-Tsakonas S, Rand MD, Lake RJ (1999) Notch signalling: cell fate control and signal integration in development.
Science 284:770–776
59. Hitoshi S, Seaberg RM, Koscik C et al (2004) Primitive neural
stem cells from the mammalian epiblast differentiate to definitive
neural stem cells under the control of Notch signalling. Genes
Dev 18:1806–1811
60. Tanigaki K, Nogaki F, Takahashi J et al (2001) Notch1 and
Notch3 instructively restrict bFGF-responsive multipotent neural
progenitor cells to an astroglial fate. Neuron 29:45–55
61. Hojo M, Ohtsuka T, Hashimoto N et al (2000) Glial cell fate
specification modulated by the bHLH gene Hes5 in mouse retina.
Development 127:2515–2522
62. Gaiano N, Nye JS, Fishell G (2000) Radial glial identity is promoted by Notch1 signalling in the murine forebrain. Neuron
26:395–404
63. Mellodew K, Suhr R, Uwanogho DA et al (2004) Nestin
expression is lost in a neural stem cell line through a mechanism
involving the proteasome and Notch signalling. Devel Brain Res
151:13–23
64. Jang MS, Zlobin A, Kast WM, Miele L (2000) Notch signalling
as a target in multimodality cancer therapy. Curr Opin Mol Ther
2(1):55–65
65. Purow BW, Haque RM, Noel MW et al (2005) Expression of
Notch-1 and its ligands, Delta-like-1 and Jagged-1, is critical for
glioma cell survival and proliferation. Cancer Res 65(6):2353–
2363
66. Shih AH, Holland EC (2006) Notch signalling enhances nestin
expression in gliomas. Neoplasia 8(12):1072–1082
67. Kanamori M, Kawaguchi T, Nigro JM et al (2007) Contribution
of Notch signalling activation to human glioblastoma multiforme.
J Neurosurg 106(3):417–427
68. Zhang XP, Zheng G, Zou L et al (2008) Notch activation promotes
cell proliferation and the formation of neural stem-like colonies in
human glioma cells. Mol Cell Bioch 307(1–2):101–108
69. Fan X, Matsui W, Khaki L et al (2006) Notch pathway inhibition
depletes stem-like cells and blocks engraftment in embryonal
brain tumors. Cancer Res 66(15):7445–7452
70. Ross R, Glomset J, KariYa L et al (1974) A platelet-dependent
serum factor that stimulates the proliferation of arterial smooth
muscle cells in vitro. Proc Natl Acad Sci USA 71:1207–1210
71. Tallquist M, Kazlauskas A (2004) PDGF signaling in cells and
mice. Cytokine Growth Factor Rev 15(4):205–213
72. Yeh HJ, Silos-Santiago I, Wang YX et al (1993) Developmental
expression of the platelet-derived growth factor alpha-receptor
gene in mammalian central nervous system. Proc Natl Acad Sci
USA 90(5):1952–1956
73. Erlandsson A, Enarsson M, Forsberg-Nilsson K. (2001) Immature
neurons from CNS stem cells proliferate in response to PlateletDerived Growth Factor. J Neurosci 21(10):3483–3491
74. Oumesmar B, Vignais L, Baron-Van Evercooren A (1997)
Developmental expression of platelet-derived growth factorreceptor in neurons and in glial cells of the mouse CNS. J Neurosci 17:125–139
75. Hermanson M, Funa K, Hartman M et al (1992) Platelet-derived
growth factor and its receptors in human glioma tissue:
Neurochem Res
expression of messenger RNA and protein suggests the presence
of autocrine and paracrine loops. Cancer Res 52(11):3213–3219
76. Lokker NA, Sullivan CM, Hollenbach SJ et al (2002) Plateletderived growth factor (PDGF) autocrine signalling regulates
survival and mitogenic pathways in glioblastoma cells: evidence
that the novel PDGF-C and PDGF-D ligands may play a role in
the development of brain tumors. Cancer Res 62(13):3729–3735
77. Puputti M, Tynninen O, Sihto H et al (2006) Amplification of
KIT, PDGFRA, VEGFR », and EGFR in gliomas. Mol Cancer
Res 4(12):927–934
78. Dai C, Celestino JC, Okada Y et al (2001) PDGF autocrine
stimulation dedifferentiates cultured astrocytes and induces oligodendrogliomas and oligoastrocytomas from neural progenitors
and astrocytes in vivo. Genes Dev 15:1913–1925
79. Shih AH, Holland EC (2006) Platelet-derived growth factor
(PDGF) and glial tumorigenesis. Cancer Lett 232(2):139–147
123