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Acta Biochim
Biophys Sin 2009, 41: 341–351 |
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doi: 10.1093/abbs/gmp028. |
The role of autophagy in sensitizing malignant glioma
cells to radiation therapy
Wenzhuo Zhuang1, Zhenghong
Qin2, and Zhongqin Liang1,2*
1Department of Pharmacology,
2Laboratory of Aging and Nervous Diseases,
*Correspondence address. Tel: +86-512-65880119;
Fax: +86-512-65190599; E-mail: [email protected]
Malignant gliomas
represent the majority of primary brain tumors. The current standard treatments
for malignant gliomas include surgical resection,
radiation therapy, and chemotherapy. Radiotherapy, a standard adjuvant therapy,
confers some survival advantages, but resistance of the glioma cells to the
efficacy of radiation limits the success of the treatment. The mechanisms
underlying glioma cell radioresistance have remained
elusive. Autophagy is a protein degradation system characterized by a prominent
formation of double-membrane vesicles in the cytoplasm. Recent studies suggest
that autophagy may be important in the regulation of cancer development and
progression and in determining the response of tumor cells to anticancer
therapy. Also, autophagy is a novel response of glioma cells to ionizing
radiation. Autophagic cell death is considered
programmed cell death type II, whereas apoptosis is programmed cell death type
I. These two types of cell death are predominantly distinctive, but many
studies demonstrate a cross-talk between them. Whether autophagy in cancer
cells causes death or protects cells is controversial. The regulatory pathways
of autophagy share several molecules. PI3K/Akt/mTOR, DNA-PK, tumor suppressor
genes, mitochondrial damage, and lysosome may play
important roles in radiation- induced autophagy in glioma cells. Recently, a
highly tumorigenic glioma tumor subpopulation, termed
cancer stem cell or tumor-initiating cell, has been shown to promote
therapeutic resistance. This review summarizes the main mediators associated
with radiation-induced autophagy in malignant glioma cells and discusses the
implications of the cancer stem cell hypothesis for the development of future
therapies for brain tumors.
Keywords autophagy;
glioma cell; radiation; PI3K/Akt/mTOR; DNA-PK
Received: November 1, 2008 Accepted: February 25, 2008
Introduction
Malignant gliomas are the most
common primary central nervous system (CNS) tumors in adults accounting for 78%
of all primary malignant CNS tumors [1]. The current standard treatments for
malignant gliomas include surgical resection,
radiation therapy, and chemotherapy. Despite this multimodality treatment,
clinical recurrence or progression is nearly universal. The median survival of
patients with gliomas is only 9–12 months [2]. Ionizing radiation (IR) is the
gold-standard adjuvant treatment for malignant gliomas.
Although radiation-induced apoptosis has extensively been studied over the past
decade, it is not a major form of cell death (believed to account for 20%) [3].
Other forms of non-apoptotic cell death have been described, which include
mitotic catastrophe, necrosis, autophagy, and senescence. Malignant glioma
cells are less resistant to autophagy-related cell death than to apoptosis
[4,5].
Autophagy is a so-called ‘self-eating’
system responsible for degrading long-lived proteins and cytoplasmic
organelles, the products of which are recycled to generate macromolecules and
ATP so as to maintain cellular homeostasis [6]. This ability makes autophagy a
good candidate for a survival mechanism in response to several stresses, such
as damaged mitochondria, protein aggregation, pathogens, and nutrient
starvation [6,7]. However, several recent studies
suggest that autophagy also functions as a pro-death mechanism at the cellular
level [7–10].
Autophagy has also been noted in neurodegenerative diseases such as Parkinson’s, Alzheimer’s, and Huntington’s diseases [7,11], and in
dying cells during development and tissue remodeling [12]. Recently, interest
in autophagy has been renewed among oncologists, because different types of
cancer cells undergo autophagy in response to anticancer therapies. IR also
induces autophagy in some types of cancer cells including malignant glioma
cells [13–15].
Whether autophagy represents a survival mechanism or
rather contributes to cell death remains uncertain. The role of autophagy in
cancers treated with chemotherapy or irradiation is a topic of intense debate
and may, depending on the circumstances, have diametrically opposite
consequences for the tumor. The outcome of autophagy observed in malignant
glioma cells after IR is not straightforward. On some occasions, autophagy
induces death of damaged cells; in others, autophagy plays a protective role
rather than a death program. For example, loss of DNA-associated protein kinase radiosensitizes malignant
glioma cells by inducing autophagic cell death [16].
In contrast, however, inhibition of autophagy radio-sensitized malignant glioma
cells, which are very resistant to radiation [15]. Autophagy is a dynamic
multistep process that can be modulated at several points, both positively and
negatively [17]. Some findings suggest that inhibition of autophagy at
different stages may yield different outcomes. 3-Methyladenine (3-MA) [18] is a
phosphatidylinositol 3-kinase (PI3K) inhibitor
whereas bafilomycin A1 is an inhibitor of H+-ATPase. 3-MA inhibits the formation of autophagy at its
early stage whereas bafilomycin A1 attenuates
acidification of vacuoles, resulting in the inhibition of the fusion of autophagosomes and lysosomes at
the late stage [19]. Inhibition of an early stage of autophagy by 3-MA rescues
cancer cells from death, whereas inhibition of a late stage of autophagy by bafilomycin A1 induces apoptosis in the same malignant
glioma cell types treated with temozolomide (TMZ)
[20]. However, both 3-MA and bafilomycin A1 radio-sensitize
malignant glioma cells (U373-MG) [15].
Whether radiation-induced autophagy in malignant glioma
cells causes death or protects cells is controversial. In multiple studies,
autophagy has been inhibited pharmacologically or genetically, resulting in contrasting
outcomes—survival or death—depending on the specific contexts. This fact may
encourage us to better understand the nuances of how autophagy in response to
radiation affects malignant glioma development, progression, and treatment so
that we can use this information to prevent and more effectively treat
malignant glioma. Recent studies have demonstrated the existence of a small
fraction of glioma cells endowed with features of primitive neural progenitor
cells and tumor-initiating function. Such cells have been defined as glioma
stem cells [21–25]. Taken
together, these accumulating data may lead to develop a new therapy to
radio-sensitize malignant glioma cells by modulating autophagy.
Cells Death/Survival Signal Pathways in IR-Induced
Autophagy
The signaling pathway composed of PI3K, protein kinase B (Akt), and mammalian
target of rapamycin (mTOR)
is a cell survival pathway that is important for normal cell growth and
proliferation [26]. This pathway has also been implicated in tumorigenesis [27] and is becoming an important target for
cancer treatment [28, 29]. The PI3K/Akt pathway is known to be activated by
radiation. It is widely known that mTOR, a downstream
effector of Akt, plays a
critical role in regulating autophagy in cells from yeast to mammalian cells
[30,31]. mTOR inhibits autophagy predominantly by
activating a downstream molecule, p70S6 kinase
(p70S6K).
Phosphatidylinositol 3-kinase/Protein Kinase B
Some findings strongly support inhibition of the PI3K/ Akt/mTOR pathway as a promising strategy for treatment of
malignant gliomas both as a single agent and as a
radio-sensitizer [32,33]. Using clonogenic assays,
the PI3K inhibitor LY294002 has been shown to sensitize prostate cancer cells,
breast cancer cells, and malignant glioma cells to radiation [34–36]. The LY294002 and UCN-01 (7-hydroxystaurosporine, Akt inhibitor) synergistically augmented the effect of rapamycin (an inducer of autophagy, inactivating mTOR [37]) in all of the three malignant glioma cell lines,
U87-MG, T
Mammalian Target of Rapamycin
In the presence of Rad001 (everolimus,
an mTOR inhibitor), both autophagy and the
sensitization to radiation were enhanced in Bax/Bak-/- DKO MEF cells, demonstrating
that inhibition of pro-apoptotic proteins and induction of autophagy sensitizes
cancer cells to radiation therapy [41]. In MCF-7 cells, radiation leads to
inhibition of the mTOR pathway with the consequent
development of autophagy, mitochondria hyperpolarization
and decreased the level of the translation initiation factor eIF
Inhibition of the DNA-PK Radio-Sensitizes Glioma Cells
(By Inducing Autophagy)
DNA repair is one of the main reasons for the resistance
to IR. The main deleterious damage induced by IR is DNA-double-strand breaks (Dsbs). They can lead to fragmentation, translocation, misrepair, and loss of chromosomes. Such genotoxic events activate a number of signaling pathways
that serve to activate DNA repair process and cell cycle arrest, or trigger
cells into apoptosis. The major mechanism underlying the repair of DNA-Dsbs in mammalian cells is non-homologous end-joining
[43,44] and requires the DNA-PK (DNA-dependent protein kinase).
DNA-PK is a serine–threonine protein kinase consisting of
three subunits: a 450,000-Da catalytic subunit (DNA-PKcs),
a heterodimeric complex composed of the proteins Ku70
(70,000 Da) and Ku80 (86,000 Da).
Ku binds to both ends of a double-strand break and recruits DNA-PKcs to the DNA end.
Cells lacking DNA-PK activity as a result of mutation in
any of the subunits are deficient in the rejoining of radiation-induced DNA-Dsbs and are radiosensitive in the clonogenic
assay [45,46]. In general, IR induces apoptosis and cell cycle arrest. The
human glioma cell line M059J lacking the catalytic subunit of DNA-PK and its
DNA-PKcs proficient counterpart M059K both display
radiation-induced apoptosis. The apoptotic course differs between the two cell
lines and is dependent on the quality of IR [47]. The cell cycle distribution
was investigated after exposure to 60Co photons or accelerated
nitrogen ions (14N) to elucidate the further different responses of
M059J and M059K cells with regard to cell cycle perturbations [48]. Typically,
IR induces DNA damage, which generates a complex cascade of events leading to
cell cycle arrest, transcriptional and posttranscriptional activation of a
subset of genes including those associated with DNA repair, and triggering of
apoptosis. On the other hand, non-apoptotic cell death, autophagy, has recently
attracted attention as a novel response of cancer cells to chemotherapy and IR.
DNA damage does not induce apoptosis in DNA-PKcs-/- cells [49,50]. Low-dose IR
induced massive autophagic cell death in M059J cells.
Most M059K cells survived, and proliferated although a small portion of the
cells underwent apoptosis. The treatment of M059K cells with antisense oligonucleotides against DNA-PKcs
caused radiation-induced autophagy and radio-sensitized the cells. Furthermore,
antisense oligonucleotides against DNA-PKcs radio-sensitized other malignant glioma cell lines
with DNA-PK activity, U373-MG and T
DNA-PKcs is a member of the
PI3K-like family [45], which has a catalytic domain homologue to PI3K. Other
members of the PI3K-like family are ataxiatelangiectasia
mutated (ATM), ATM- and Rad3-related (ATR), and mTOR.
ATM and ATR are associated with the control of cell cycle checkpoints in
response to DNA damage [51]. mTOR is a modulator of
autophagy. The targets that DNA-PKcs phosphorylates include DNA-PKcs
[45], two Ku subunits [52], XRCC4 [53], p53 [54], MDM2 [55], and c-Abl [56]. The phosphorylation of
DNA-PKcs, Ku subunits, and XRCC4 is associated with
DNA repair, whereas that of p53, MDM2, and c-Abl
induces apoptosis. The different roles played by these targets are, therefore,
consistent with the notion that DNA-PK has dual roles in DNA damage: one is to
sense DNA damage and repair it and the other is to induce apoptosis [16].
Specifically, in response to DNA damage, the cell first tries to repair the
damage and survive. However, if the cell cannot repair the damage, it undergoes
apoptosis and avoids passing damaged DNA to its progeny cells. Furthermore, the
mTOR/p70S6K pathway was suppressed by IR and autophagy was induced in M059J
cells [16].
Thus, it is tempting to speculate that DNA-PKcs plays a key role not only in the induction of
apoptosis but also in the inhibition of autophagy. The specific inhibition of
DNA-PKcs may be promising as a new therapy to radiosensitize malignant glioma cells by inducing
autophagy. This information is important for the selection of treatment for glioblastoma resistant to conventional radiation therapy.
Tumor Suppressor Genes Modulate the Glioma Cellular
Sensitivity to Radiation
p53 gene
Some studies imply that the tumor suppressor genes
contribute to autophagy, like apoptosis and cell cycle arrest. A tumor
suppressor gene, p53, is mutated in ~50% of all tumors [57] and is known as an important
determinant of DNA-damage-induced apoptosis. Recently, a direct link between
p53 and autophagy has been suggested [58]. p53 is a central signal integrator
of stress, such as DNA damage, hypoxia, and oncogenic
activation. These types of stresses could activate p53. As depicted in Fig.
1, DNA
damage following IR leads to activation of p53. p53 was demonstrated to inhibit
the mTOR pathway via activation of AMP-activated kinase (AMPK) [59]. AMPK in turn inhibits mTOR via upregulation of the
PTEN and TSC2 genes
[59].
It has been shown that autophagy can be modulated by p53
[59]. Furthermore, damage-regulated autophagy modulator (DRAM), a p53 target
gene encoding a lysosomal protein that induces
autophagy, is an effector of p53-mediated death. The
discovery of DRAM suggests that induction of autophagy by p53 via DRAM
contributes to apoptotic cell death [58]. However, apoptosis upon irradiation
contributes only minor to the therapeutic effect in solid tumor cells [60, 61].
The level of p53 does not influence the formation of autophagic
vesicles upon irradiation because there is no difference in accumulation of autophagosomes among HTB43 pharyngeal cancer, MDA-MB-231
breast cancer, and HTB35 cervical squamous cell
carcinoma cells with mutated p53, or A549 lung cancer cells with wild-type p53
and A549 cells in which p53 function was blocked by activation of an Ecdysone-inducible mutated p53 (mtp53) construct [62].
PTEN gene
Another tumor suppressor gene PTEN (phosphatase and tensin homologue
on chromosome 10) also induces autophagy [63]. PTEN modulates the cell cycle,
cell survival, and cell growth by inhibiting the PI3K/Akt pathway. PTEN dephosphorylates the second messenger PIP3, interrupting
PI3K activation of Akt and decreasing overall flux
through the PI3K pathway [64]. PTEN has been shown to promote autophagy in
HT-29 colon cancer cells [63]. Moreover, the prostate cancer cell lines PTEN-/- PC-3 and PTEN+/+ DU145 became more vulnerable to
irradiation after treatment with RAD001 (mTOR
inhibitor), with the PTEN-deficient PC-3 cell line showing the greater
sensitivity. This increased susceptibility to radiation is associated with
induction of autophagy [65]. Mutation of PTEN has also been observed in
malignant glioma cells [66,67]. These data implicate that PTEN may play a role
in radiation-induced autophagy in malignant glioma cells.
Beclin 1 gene
Beclin 1, the mammalian orthologue of the yeast Apg6/ Vps30
gene,
plays a role in two fundamentally important cell biological pathways: autophagy
and apoptosis. Beclin 1 is a major determinant in the
initiation of autophagy [68–70].
Beclin-1 interacts with Bcl-2/Bcl-xL and forms part of a class III PI3K complex
playing an important role in production of PI-3-phosphate. The Beclin-PI3K
complex is located at the cytosol and the trans-Golgi
network. This may be essential for sorting autophagosomal
components and lysosomal proteins [68,71–73]. In fact, Beclin 1 is mono-allelically deleted in human breast, ovarian, gastric, and
prostate cancers and is expressed at reduced levels in those tumors
[13,68,74,75]. The expression of Beclin 1 protein and
its mRNA were found decreased also in a series of human brain tumors including glial and non-glial neoplasms, which had previously been demonstrated in a few
experimental studies, both in spontaneous and in therapy-induced autophagy
[76]. Further studies are needed to highlight Beclin
1 function in radiationinduced autophagy in malignant
glioma cells.
Other tumor suppressor molecules associated with
autophagy include BNIP3 and death-associated protein kinase
(DAPK). The pro-cell death Bcl-2 family member BNIP3 (Bcl-2/adenovirus E1B 19 kDa interacting protein 3) is known to induce autophagy and
cell death. It was shown that BNIP3 plays a central role in As2O3-induced autophagic cell death
in malignant glioma cells [77]. Recently, BNIP3 was reported to induce
autophagy, while expression of BNIP3 siRNA or a
dominant-negative form of BNIP3 reduced hypoxiainduced
autophagy in two glioma cell lines (U87 and U373) [78]. DAPK and DAPK-related
protein kinase-1 proteins are calcium/calmodulin-related
serine/threonine death kinases
that induce both apoptotic and autophagic cell death
in cancer cells [79].
The interrelationship between these tumor suppressors and
their differential role in modulating the glioma cellular sensitivity to
radiation remain to be determined. More studies will be necessary to clarify
how to best manipulate these pathways before such new therapies can be developed.
The Glioma Genome
Cancer is a disease of genome alterations. A statistical
approach called Genomic Identification of Significant Targets in Cancer
(GISTIC) is applied to a newly generated, high-resolution data set of
chromosomal aberrations in 141 gliomas. Focusing on
chromosome 7 (chr7), focal high-level amplification at the epidermal growth
factor receptor (EGFR) gene is associated with the activation of EGFR itself
whereas broad lower-level amplification of the whole chromosome often activates
the MET axis by increasing the dosage of both MET and its ligand
hepatocyte growth factor (HGF) [80]. Co-expression of
EGFR deletion mutant variant III (EGFRvIII) and PTEN
by glioblastoma cells are associated with
responsiveness to EGFR kinase inhibitors [81]. To
identify the genetic alterations in gliomas, Parsons et
al. [82]
sequenced 20,661 protein coding genes, determined the presence of
amplifications and deletions using high-density oligonucleotide
arrays, and performed gene expression analyses using next-generation sequencing
technologies in 22 human tumor samples. This comprehensive analysis led to the
discovery of a variety of genes that were not known to be altered in gliomas. McLendon et
al. [83]
reported the interim integrative analysis of DNA copy number, gene expression,
DNA methylation aberrations in 206 glioblastomas and nucleotide sequence aberrations in 91 of
the 206 glioblastomas. They provide new insights into
the roles of ERBB2, NF1, and TP53, uncovers frequent mutations of the
phosphatidylinositol-3-OH kinase regulatory subunit
gene PIK3R1. These
analyses provide a network view of the pathways altered in the development of glioblastoma. The remaining events likely point to
cancer-related genes and other functional elements that remain to be
discovered. This may substantially increase the sensitivity to radiation in
glioma.
Mitochondrial Damage in IR-Induced Autophagy
Mitochondria can perform multiple cellular functions
including energy production, cell proliferation, and apoptosis. However, the
role of mitochondrial damage in autophagy is not clear. In fact, low levels of
mitochondrial membrane permeabilization and
depolarization can trigger sequestration and autophagy of damaged mitochondria
[84,85]. A post-mitochondrial caspase cascade is
delayed as a result of early disposal of damaged mitochondria within autophagosomes [86]. Mitochondrial transmembrane
potential dissipate to some extent in autophagy induced by radiation, TMZ, and
arsenic trioxide in malignant glioma cells [15,20,77]. Another study clearly
shows that superoxide anion generated by selenite
triggers mitochondrial damage and subsequent autophagy, leading to irreversible
cell death in glioma cells [87]. Further investigation is indicated to examine
the mechanism of mitochondrial damage in IR-induced autophagy in malignant
glioma cells.
Lysosome in IR-Induced Autophagy
Autophagy is a lysosome-dependent
degradative pathway frequently activated in tumor
cells treated with chemotherapy or radiation. IR-mediated cell damage is, in
vitro and perhaps in vivo, a consequence of intralysosomal iron-catalyzed oxidative processes leading
to lysosomal rupture with release of hydrolytic
enzymes and redox-active iron [88]. Lysosomes have an important role in injury of cells and
tissues. In radiation response, autophagy is increased in some cases and lysosomes are responsible for regulating the degradation of
the phagocytotic vacuoles. Tumor invasion and
metastasis are associated with altered lysosomal
trafficking and increased expression of the lysosomal
proteases termed cathepsins. Genetic suppression of cathepsin B and matrix metalloprotease-9 expression
significantly reduced tumor cell invasion, tumor growth, and angiogenesis in a
mouse glioblastoma model [89]. Further studies are,
however, required to determine the function of lysosome
in radiation-induced autophagy in malignant glioma cells.
Switch between Apoptosis and Autophagy
There is ample evidence that radiation-induced cell death
is affected by various intertwined biochemical processes in the autophagic and apoptotic pathways. Bax
and Bak act as a gateway for caspase-mediated
cell death. They play critical roles in mediating the mechanism of cell death
following irradiation. The inhibition of apoptosis resulted in an increase in
radio-sensitivity of the cancer cells. Irradiation up-regulates autophagic programmed cell death in cells that are unable
to undergo Bax/Bak-mediated apoptotic cell death
[90]. Activation of PI3K/Akt/mTOR biochemical cascade confers survival
advantage in neoplastic cells by both inhibitory
effects of mTOR on autophagy and the inhibitory
effect of Akt on apoptosis. Some results showed that
both apoptosis and autophagy pathways are intertwined through PI3K/Akt/mTOR
regulation under irradiation. By blocking apoptosis with the pan-caspase inhibitor zVAD, autophagy
was effectively increased in both the PC-3 and DU145 prostate cancer cell
lines. Furthermore, both of the cell lines exhibited overall decreased cell
survival when zVAD was combined with RAD001. The zVAD-induced inhibition of apoptosis or the RAD001-induced
autophagy resulted in increased radio-sensitivity when employed singularly,
while combination of zVAD and RAD001 led to additive,
rather than synergistic, effects on cell death [65]. The cytotoxicity
of radiation is increased in the situations of autophagy upregulation,
possibly because of the synergistic and redundant mechanisms that can amplify
the death trigger signaling through the endoplasmic reticulum (ER) stress. ER
is an organelle present in eukaryotic cells for key functions, such as calcium
sequestration, protein translation and folding, and maturation. Although ER
stress has primarily been associated with cell survival under cellular stress,
insurmountable cell stress triggers programmed death pathway, usually via
apoptosis. However, it has been shown recently that ER stress can also induce
cell death through activation of alternative pathway in autophagy [91].
Whether autophagy observed in treated cancer cells
represents a mechanism that contributes to tumor cell resistance to
therapy-induced apoptosis or a mechanism for initiating a non-apoptotic form of
programmed cell death remains controversial. The ability of radiation or
chemotherapy to induce cell death in cancer cell lines that display resistance
to apoptosis depends on type II programmed cell death executed by autophagy
[92]. Some data demonstrate that inhibitors of autophagy enhance the efficacy
of therapeutic strategies designed to induce tumor cell apoptosis [93]. Another
report provides evidences that besides apoptosis induction 4-HPR can also
induce autophagy in glioma cell. 4-HPR-induced autophagy may provide survival
advantage and inhibition of autophagy may enhance the cytotoxicity
to 4-HPR [94]. There are many important questions to be addressed in future
investigations in trying to determine the relative influences on apoptosis and
autophagy in glioma cells. It will be important to identify the presence of
biochemical switches that direct glioma cells towards apoptosis or autophagy.
Glioma Stem Cells in Radiation Resistance
The brain tumor stem cell hypothesis proposes the
existence of original multipotent glioma cells that
are characterized by the expression of stem cell markers and by the capacity
for self-renewal, multi-lineage differentiation, and re-establishment of tumors
after transplantation [21–25]. An
implication of the brain tumor stem cell model is that brain tumor stem cells
are resistant to radiation and chemotherapy and may therefore be responsible
for tumor recurrence [95,96]. The molecular signatures underline the need for
development of multimodality treatments targeting not only the tumor cells, but
also including strategies aimed at the glioma stem-like cell compartment, and
interfering with tumor–host
interaction that provides the specialized microenvironment relevant for the
maintenance of tumor stem-like cells (the stem cell niche), angiogenesis, and
immune response [97]. Targeting L1CAM, a neuronal cell adhesion molecule, using
lentiviral-mediated short hairpin RNA (shRNA) interference in CD133+ glioma cells potently disrupted
neurosphere formation, induced apoptosis, and
inhibited growth specifically in glioma stem cells [98]. Malignant glioma cells
expressing CD133, which was recently identified as a potential brain tumor stem
cell marker in brain cancer [21] and in other solid tumors [99,100], are
resistant to IR because they are more efficient at inducing the repair of
damaged DNA than is the bulk of the tumor cells. Radiation treatment fails in
the long run because it cannot kill the subpopulation of CD133+ tumor-initiating cells [96].
Cancer stem cells (CSCs) contribute to glioma radioresistance through preferential activation of the DNA
damage checkpoint response and an increase in DNA repair capacity [96,101].
Although IR damages tumor cells through several mechanisms, IR kills cancer
cells primarily through DNA damage. The ability to repair DNA damage is
essential to cellular survival because maintaining DNA breaks induces
apoptosis, senescence or autophagy [16,102–104]. Delta-24-RGD, an oncolytic
adenovirus with enhanced tropism to glioma cells and selective replication in
cancer cells with an abnormal Rb pathway [51,105],
efficiently eliminates glioma CSCs. Glioma CSCs are susceptible to adenovirus-mediated cell death via autophaghy in vitro and in vivo [106].
These results imply that therapeutically targeting the glioma stem cells may
yield significant benefits for glioma patients.
However, Kelly et al. [107] questioned xenotransplant experiments supporting the CSC hypothesis
because they found a high frequency of leukemia-initiating cells (L-IC) in some
transgenic mouse models. Questions remain as to whether the currently
identified glioma stem cells are the cell-of-origin for glioma initiation and
progression, or the results of such processes. CD133+ CSC maintain only a subset of
primary glioblastomas. The remainder stems from
previously unknown CD133- tumor cells with apparent stem cell-like properties but
distinct molecular profiles and growth characteristics in vitro and in
vivo [108].
Ogden et al. [109] characterize the expression of a putative CSC
marker (CD133) and a glial progenitor marker (A2B5)
in a diverse set of human gliomas. They tested the tumorigenic potential of three different populations of
glioma cells (A2B5+CD133+, A2B5+CD1332, and A2B52CD1332) from six different tumors and found that in some cases
(four of six), CD133- cells could give rise to tumors. The implication of such
findings in cancer development is so far unclear. Taken together, determining
whether the growth of gliomas is sustained by most of
the tumor cells or by a rare subpopulation has important ramifications for the
design of novel therapies. Therefore, the brain tumor stem cell hypothesis
merits more rigorous tests.
Summary
New therapies that provide highly specific tumor cell
killing and complete eradication of cancer cells are urgently needed.
Pretreatment of glioma cells with locked nucleic acid (LNA)-antimiR-21 oligonucleotides leads to synergistic anti-tumor efficacy
both in
vitro and in vivo [110]. The induced cytotoxic T
lymphocyte (CTL) specific for interleukin-13 receptor alpha2 (IL-13Ralpha2)
peptide could be a potential target of specific immunotherapy for human
leukocyte antigen (HLA)-A2 patients with malignant glioma [111]. Given that
various malignant glioma cells undergo autophagy after radiation, we propose to
use autophagy to our benefit to kill malignant glioma cells. Enhancement of
autophagy may promote radiationinduced autophagic cell death, and its inhibition may lead to
apoptosis, thus resulting in a greater degree of malignant glioma cell death
than currently available therapies do. In recent studies, we found out that
malignant glioma U251 cells undergo autophagy instead of apoptosis when
irradiated. We detected increases in the mRNA and protein levels of LC3-II, the
protein levels of cathepsin L and the formation of autophagosomes (data not shown). By manipulating the
pathways of autophagy, we may be able to develop more effective adjunctive
treatment strategies to increase the sensitivity of malignant glioma to
radiation. However, the role of autophagy in radiation sensitivity has not
previously been established. More studies will be necessary to clarify how to
best manipulate these pathways before such new therapies can be developed.
Funding
This work was supported by grants from the Natural
Science Foundation of Jiangsu Province (BK20066051).
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