Radiation Environment and Medicine 2023 Vol.12, No.1 25–36

Review

Radiotherapeutic Review: Acquisition of Radioresistanceand Cancer Stem Cell Properties via Irradiation
Kazuki Hasegawa*, Ryo Saga and Yoichiro Hosokawa
Department of Radiation Science, Graduate School of Health Sciences, Hirosaki University, Hirosaki, Aomori, 036-8564, Japan
Received 18 August 2022; revised 18 November 2022; accepted 22 November 2022

The development of high-precision dose calculation techniques for the radiotherapy of tumors has
enabled treatment with fewer side effects in normal tissues. However, a crucial problem remains
wherein some cancer cells may acquire radioresistance or cancer stem cell (CSC) characteristics
and cause recurrence and metastasis. The acquisition of these phenotypes is associated withcomplex molecular signaling and biological processes that remain unclear. In addition, these
phenotypes are originally inherent within the tumor and are induced by external factors suchas radiation. Therefore, a better understanding of the mechanisms underlying radiosensitizationand how these phenotypes are acquired is essential for improving radiotherapy outcomes. This
review outlines the current findings on the association of radiotherapy with CSC induction,
signaling pathways and radioresistance processes, as well as therapeutic strategies aimed at theradiosensitization of these phenotypes.
Key words: radioresistance, cancer stem cells, epithelial-mesenchymal transition, radiosensitization

1. Introduction
Since Wilhelm Konrad Roentgen discovered X-rays byin 18951) and the first clinical application of radiation in
1896, the field of radiotherapy has undergone tremendous
development 2). With the development of advanced
radiotherapy techniques, it is now possible to concentrate
the dose on tumor tissues and reduce the dose received
by normal tissues, thereby reducing the incidence ofside effects. Hence, either alone or in combination withsurgery and chemotherapy, radiotherapy has become an
extremely efficacious treatment for cancer3, 4). In addition,*Kazuki Hasegawa: Department of Radiation Science Hirosaki University Graduate
School of Health Sciences 66-1 Hon-cho, Hirosaki, Aomori, 036-8564, Japan.
E-mail: *h20gg703@hirosaki-u.ac.jp
https://doi.org/10.51083/radiatenvironmed.12.1_25
Copyright © 2023 by Hirosaki University. All rights reserved.

several factors, including the type of radiation, irradiation(IR) schedule, IR techniques, and biological characteristics
of tumor and adjacent normal tissues, contribute to
the clinical outcomes following radiotherapy. Given its
multifaceted nature, it is essential to understand thevarious aspects of radiotherapy to develop new strategies
to improve radiotherapy outcomes in patients receiving it.
The standard fractionation schedule in radiotherapy
is 1.8–2.0 Gy/day for five days per week. This reducesside effects and allows damaged normal cells to recover
before additional doses are given4, 5). However, treatmentresistant subpopulations called cancer stem cells (CSCs)
are present in cancer tissue and are thought to be
responsible for poor prognosis. Recently, the theory
that radioresistant (RR) cancer cells are CSCs has
attracted considerable attention6-9). Although RR cancer
cells or CSCs continue a tiny population of tumors, theysignificantly impact recurrence and metastasis after

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Fig. 1. Overview of irradiation-induced signaling pathways that promote tumor cell survival and radioresistance.
Activation of ATM, DNA-PKcs, and Rad51 by IR enhances DNA reapair via HR or NHEJ. The activation of thePI3K/Akt pathway, EGFR, STAT, and Ras-MAPK signaling inhibits apoptosis and enhances the proliferationof cancer cells. Activating the inflammatory pathway, including NF- κB, IL, STAT3, and Cox-2 signaling, alsoenhances the inhibition of apoptosis and induces cell proliferation. IR also activates development pathways suchas Hedgehog, Wnt/β-catenin, and Notch signaling to induce EMT. Activating these signaling pathways leads tocancer cell survival and radioresistance.

radiotherapy10-15). Moreover, radiotherapy has been shownto induce RR and CSC phenotypes16-18), indicating theimportance of eliminating cells with these phenotypes
rather than eliminating non-CSCs.
In this review, we present findings on the associationof radiotherapy with CSC induction, signaling pathways,
and radioresistance processes, as well as therapeuticstrategies aimed at the radiosensitization of these
phenotypes. The radiation referred to in this review refers
to X-rays or γ -rays unless otherwise stated.
2. Radioresistance and signaling pathways
Radiotherapy is a ma jor therapeutic modality forcancers19). Although most cancer cells are killed duringlong-term radiotherapy, some develop radioresistanceand remain after treatment, leading to recurrence andmetastasis17). Kuwahara et al. established clinically
relevant resistant (CRR) cell lines using long-term X-ray
IR20) and reported that CRR cells were more tolerant to
X-ray IR than their parental counterparts and also showedradioresistance in vivo20, 21). Todorovic et al. establishedthe radioresistant pharyngeal squamous cell carcinoma
cell line, FaDu-RR, which is approximately 1.6 times
more radioresistant than its parental cell line FaDu,
after long-term treatment with 2 Gy IR22). The enhanced
radioresistance observed in this model was similar to thatobserved in other models23-25). Thus, some reports indicatethat long-term IR induces radioresistance, although themechanism of acquiring radioresistance is complex andis mediated by various pathways. Previous reports haveshown that ionizing radiation activates several signaling
pathways that induce cell senescence, apoptosis, cell cycle
checkpoint activity, and DNA damage repair17, 26-28). This
chapter describes the pathways involved in acquiring
various mechanisms of radioresistance (Fig. 1).
2.1. DNA repair pathways
Nuclear DNA in the main target of IR, which induces
DNA damage (such as SSB, DSB, and base damages)
either directly by ionizing DNA or indirectly bystimulating ROS production29-36). In response to IR-induced
DNA damage, cancer cells rapidly activate ATM, ataxia
telangiectasia and Rad3-related (ATR) protein, and DNAactivated protein kinase (DNA-PK), which are members of
the phosphoinositide-3 kinase-related kinase family. Thesekinases transduce DNA damage signaling, coordinate theassembly of DNA repair apparatuses at damaged sites,
and initiate DNA repair37). DSB repair is mainly performed
via NHEJ or homologous recombination (HR)37.
NHEJ recombines cleaved DNA ends directly without
needing a homologous template; thus, it is an error-prone
process that occurs throughout the cell cycle38, 39). NHEJ
begins with the Ku70/Ku80 heterodimer binding to the
DSB termini, followed by the recruitment and activationof DNA-PK. Polymerase and endonuclease Artemis end-

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processes facilitate ligation by DNA ligase IV using
X-ray repair cross complementing (XRCC) 439). Wei etal. reported that IR under DNA-PK catalytic subunit(DNA-PKcs) inhibition increased γH2AX expression in
the nucleus and enhanced radiosensitivity in the gastriccancer40).
In contrast, HR mainly occurs during the S andG2 phases when a DNA template becomes available
after the DNA replication; thus, it is a non-error-prone
process 39). In the HR pathway, the MNR complex(Mer11, Rad51, and Nbs1) recognizes and binds to
DSB ends, creating 3ʼ sticky ends via exonucleases.
Filaments are created by Rad51 and BRCA1 and 2, which
are synthesized by DNA polymerase after entry into
sister chromatids, guided by Rad54 41). Rad51 acts as animportant promoter in the DSB repair pathway42, 43); its
overexpression has been observed in many tumors andis associated with increased radioresistance and poor
prognosis44). DNA damage is repaired during fractionatedirradiation (FR), a common radiotherapy regimen. This
repair known as SLDR, and is completed within 3–6
hours from IR 45). Therefore, given that NHEJ is thedominant process in SLDR, DNA repair errors may be
responsible for acquiring radioresistance after long-term
FR. Moreover, the expression of DNA-PKcs is increased
in CRR cell lines46) and SLDR between FR was enhanced
in CRR cell lines compared to that in their parent cells47).
2.2. Survival pathway_pi3k/akt signaling pathways
Radiation-induced or constitutive signaling pathways that
act through receptor tyrosine kinases are implicated inmodulating radioresistance in cancer cells. Epidermal
growth factor receptor (EGFR), an ErbB family member,is a membrane-spanning tyrosine kinase receptor whose
expression is correlated with radioresistance and poor
prognosis after radiotherapy in preclinical and clinical
trials48, 49). EGFR promotes tumor proliferation, survival,
invasion and angiogenesis by activating downstream
signaling pathways, including the phosphatidylinositol3-kinase (PI3K)/Akt, signal transducer and activator oftranscription (STAT), and Ras-Raf-mitogen-activatedprotein kinase (MAPK) pathways50-52). The PI3K/Akt
pathway is a key downstream survival pathway activatedin cancer cells that is enhanced by IR53-55). The role of
PI3K/Akt activity in radioresistance has been reported ina variety of cancers, including brain, lung, colon, cervix
and head and neck cancers. For example, the constitutive
phosphorylation of Akt, presumably at Ser473 and Thr308,
mediates radiotherapy resistance in non-small cell lungcancer (NSCLC) cells56), while activating the PI3K/Akt/
cyclooxygenase (Cox)-2 pathway enhances resistance toradiation in human cervical cancer57). The upregulationof P I3K / A k t /ma mma l ia n t a r get of r a pa m y c in(mTOR) induced by IR promotes the suppression of27

apoptosis and activates DNA repair, thereby inducing
radioresistance58-61). Like apoptosis, autophagy has also
been recognized as an essential factor in determining
the response of tumor cells to radiotherapy; it is also
regulated by the PI3K/Akt signaling pathway62, 63). Akt
inhibition has been reported to exert a radiosensitizing
effect on malignant glioma cells by inducing autophagy64).
The pro-survival function brought about by PI3K/Akt
signaling may promote radioresistance in cancer cells.
Inhibition of this signaling pathway has been shownto enhance radiosensitivity in certain types of cancer
cells, accompanied by decreased DNA repair capacity
and increased apoptosis induction65-70). However, several
cancer models have shown that inhibiting PI3K/Akt
does not affect radiosensitivity71-75), suggesting that therelationship between this pathway and radiosensitivity iscell-specific.
2.3. Inflammation pathways
The immune system plays a pivotal role in controlling
tumor development, suppression, or progression.
Radiotherapy can modulate antitumor immune responses,
by modifying the tumor and its microenvironment
through the activation of cytokine cascades. In turn,cytokines modulate inflammation, invasion, and fibrosis
and influence the dose-dependent radiation response,
owing to their multifunctional nature. The balance
between proinf lammatory and anti-inf lammatory
cytokines is critical for determining the outcome ofradiation treatment, adverse reactions, and resistanceto radiation. Interleukin (IL)-6 and IL-8, which areproinflammatory cytokines, have been shown to be
involved in the inflammatory response to IR and enhance
the adaptation of cancer cells 76, 77). In contrast, theinduction of IL-1β by IR enhances cancer cell invasion.
Increased levels of nuclear factor-kappa B (NFκB), a transcription factor that regulates the expressionof inflammatory molecules, are correlated with the
inhibition of apoptosis and induction of radioresistance incancer cells78, 79). The best-validated anti-apoptosis genetargets of NF- κB are B-cell lymphoma (Bcl)-2, Bcl-xL,
and myeloid cell leukemia (Mcl)-1, which are membersof the Bcl-2 family of proteins. Furthermore, IR activates
NF- κB to express cell cycle-specific genes such as cyclin
D1, which is also implicated in radioresistance80). The
radiation-induced transcription factor STAT3 is also
linked to radioresistance due to the production of various
proteins, including cyclin D1, matrix metalloproteinases(MMPs), and proinflammatory cytokines81). The STAT3mediated radioresistant phenotype results from multiple
mechanisms in combination with the radiation-induced
activation of the Janus-activated kinase (JAK)-STAT
pathway81-83). IR also induces inflammation via Cox-276).
Cox-2 is overexpressed in various types of cancer, and28

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its upregulation is associated with higher tumor gradeand distant metastases in breast cancers84). This protein
also plays an important role as a target for anticancer
and anti-inflammatory therapies, while Cox-2-derived
prostaglandins are thought to contribute to tumor growth
and resistance to radiation therapy. Cox-2 activation
via the PI3K/Akt pathway can also induce radiation
resistance in human cancer cells57).
Therefore, it can be expected that the inflammatory
response is closely related to radioresistance and thatusing specific inhibitors or drugs that could regulate thispathway can improve radiotherapy outcomes.
2.4. Developmental pathways
Various developmental pathways contribute to themaintenance of cancer stem cells and cancer cells, whereas
self-renewal pathways contribute to cancer stem cell
resistance to radiation. The Hedgehog signaling pathway
plays a crucial role in mammalian development 85, 86),
and exerts pro-proliferative effects on CSC populations
in various cancers87-89). This pathway contributes toradioresistance in head and neck cancer 90), and the
combination of radiation with the pathway inhibitor
cyclopamine demonstrated improved radiosensitivity.
The Wnt/β -catenin pathway, especially the canonicalpathway, is believed to be important in maintaining thestemness of cancer stem cells and promoting cellularinvasiveness in may cancers by regulating epithelialmesenchymal transition (EMT)91). The Wnt/ β -catenin
pathway mediates the radioresistance of glioblastoma
by promoting invasion92, 93). Hence, invasiveness andradioresistance may be phenotypes associated with Wnt/
β -catenin signaling.
The Notch signaling pathway plays a crucial role in thecontrolling cell-cell communication, cell fate, and patternformation94, 95). This pathway is also involved in stem cellproliferation, differentiation, and apoptosis96) and functions
as an oncoprotein in most human cancers. It also acts
as a tumor suppressor in skin cancer, hepatoma, andsmall-cell lung cancer; thus, it can be either oncogenic ortumor-suppressive97-99). Elevated Notch pathway activity
is correlated with radioresistance and poor prognosis
in patients with NSCLC100), since the inhibition of thispathway elicits the loss of CSC regeneration afterradiotherapy.
T hese developmenta l pa t hwa ys are potent ia ltherapeutic targets because they are involved in the
EMT and play a crucial role in enhancing stemness afterradiotherapy.
3. Radioresistance and epithelial-mesenchymaltransition
EMT is a differentiation process related to embryogenesisthat alters the genetic phenotypes of cancer cells.
When this program is activated, cancer cells lose theirepithelial characteristics and acquire mesenchymal
characteristics such as fibroblast-like morphology,
migration and enhanced invasive ability101, 102). Several
studies have confirmed that radiation can induce EMT or
phenotypic changes similar to EMT28, 103, 104). For example,
the radioresistant esophageal cancer cell line KYSE150R, phenotypic changes similar to EMT are induced byradiation, including decreased E-cadherin and increased
Snail and Twist expression105). These have also been
observed in nasopharyngeal carcinoma106) and colorectal
cancer107). Furthermore, several pathways have beenreported to contribute to the radiation-induced EMT ofcancer cells. For example, radiation increases EMT via
the JAK 2/p21-activated kinase 1/Snail signaling pathway
in lung cancer cells108). In addition, low-dose radiation has
been shown to activate the NF- κB pathway and induce
EMT in cervical cancer cells109).
Interestingly, cancer cells that acquired mesenchymal
phenotypes via EMT induction were more therapeuticallyresistant compared with epithelial phenotypes, suggestingthat radiation-induced EMT may confer radioresistanceto these cancer cells, contributing to cancer relapse
following radiotherapy. Chang et al. revealed thatprostate cancer cells exhibiting EMT after radiotherapy
become more radioresistant 17). Similar results havealso been reported in other types of cancers, including
pancreatic110), colorectal111), breast112), and lung cancer108).
Tahmasebi-Birgani et al. showed that FR induced EMT,
increased expression of cancer stem cell markers clusterof differentiation (CD) 44 and CD133 and cell survival113).
This suggests that the induction of EMT by FR causes
radioresistance. In addition, Chang et al. demonstrated
that FR enhanced radioresistance by activating DNA
damages checkpoint proteins and the PI3K/Akt/mTOR
signaling pathway and inducing the expression of CSC
markers and the EMT phenotype114). Similarly, Escriva et
al. indicated that the Snail protein, an EMT marker, wasstabilized upon IR. Snail also inhibits the upregulationof the p53 target gene phosphatase and tensin homolog
deleted from chromosome 10 (PTEN), a negative
regulator of the PI3K/Akt survival pathway, and activated
the Akt pathway, enhancing radioresistance115).
The phenotypic chances in EMT are accompanied
by increased cell migration and invasive phenotypes116),
and are known to contribute to the acquisition of CSC
characteristics117-119). The EMT markers Twist, Snail, andzinc finger e-box binding homeobox (ZEB), have beenreported to confer resistance to apoptosis and induce
malignant transformation and acquisition of stem cellcharacteristics120-123). Notably, in certain experimental
cancer models, the forced induction of EMT in epithelial
tumor cells significantly increased their tumor-forming

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capacity 124-126). Furthermore, activation of the EMT
program confers resistance to many types of therapeutic
agents in these tumor cells, which is also an importantcharacteristic of CSCs.
4. Radioresistance and cancer stem cells
Several studies have reported that IR promotes cancer
recurrence and metastasis. It was speculated that thisis due to the presence of CSCs and the IR-induced stem
cell transformation of cancer cells107, 127-130). CSCs are a
subpopulation of cells within tumors that exhibit stem
cell characteristics similar to normal stem cells, suchas self-renewal through asymmetric cell division andpluripotency131). CSCs arise from mutations in normal
stem, progenitor, or differentiated cells132, 133), have ahigher tumor-forming potential than to non-CSCs, andare responsible for the biological characteristics ofcancer, such as rapid growth, invasion, and metastasis134).
Additionally, CSCs are resistant to radiotherapy andchemotherapy. For example, non-CSCs within thetumor are radiosensitive and mostly die after treatment;
however, CSCs remain, thus shrinking the tumor itself
but enriching the CSC fraction. Therefore, CSCs cancause recurrence and metastasis135-137). Three typesof markers are utilized to identify CSCs: cell surface
molecules, transcription factors, and signaling pathway
molecules 45, 46, 138). The commonly used cell surface
markers in identifying CSCs are CD24, CD34, CD38,
CD44, CD90, CD133, and aldehyde dehydrogenase(ALDH)47, 139). Clinical trials have indicated that theexpression of CSC markers, including CD44, CD133,
and ALDH, in patients with lung and laryngeal cancers
correlates with poor prognosis after radiotherapy140, 141).
The radioresistance of CSCs is associated with intrinsic
properties such as DNA repair, ROS removal capacity,enhanced inhibition of apoptosis and induction of survival
pathways, and adaptive properties due to IR and themicroenvironment (Fig. 2)12, 13, 142-150). CSCs generally
express high levels of DNA repair genes and contributeto radioresistance12, 144-147). ROS scavengers are also highly
expressed in several CSC models, protecting cells from
ROS-induced DNA damage13, 149). In addition, CD133positive liver CSCs exhibit elevated levels of anti-apoptotic
Bcl-2 and the activation of the PI3K and extracellularsignal-regulated kinase (ERK) pathways compared with
CD133-negative cells149). Thus, the intrinsic properties
associated with therapeutic resistance are enhanced in
CSCs compared to non-CSCs.
More recently, it has been reported that non-CSCs
acquire CSC properties upon IR. Lagadec et al. reported
that breast cancer cells were stained with ALDH, a
CSC marker, and only non-stained cells were isolated
and irradiated. This enhanced tumorigenic potential,

29

radioresistance, and the expression of CSC-related
genes151). Murata et al. suggested that prostate cancer
cell lines had a temporarily decreased expression of CSC
markers CD133 and CD138 during FR and that thesemarkers were re-expressed upon radioresistance152).
Furthermore, crosstalk between cancer cells and theirmicroenvironment is critical for CSC characteristics. The
tumor microenvironment is composed of extracellularmatrix and multiple cell types, including fibroblasts,
vascular endothelial cells, immune cells, and IR induceschanges in these, including fibrosis, hypoxia, andinflammatory responses 153-155). As these IR-inducedchanges in the tumor microenvironment promote
radioresistance, tumor recurrence and metastasis, CSC
marker expression in the cancer cells present there may
be enhanced.
Therefore, it is likely that IR promotes various
factors and processes that confer stemness, which
in turn transform non-CSCs into CSCs. However, themechanism of CSC marker expression is controversial
and its expression is altered by external stresses of IR.
One hypothesis for this mechanism is the reversibility of
CSCs. For example, if cancer cells are constantly exposed
to stress such as IR, they acquire CSC characteristics tosurvive in stressful environments, resulting in increased
marker expression. On the other hand, in a stress-freeenvironment, marker expression may be reduced by ashift to a state closer to non-CSCs, as they are no longer
required to maintain CSC characteristics. However, thismechanism is not yet known. A better understanding ofthese mechanisms may help improve the effectiveness ofradiotherapy.
5. Radiosensitizing strategies
Acquisition of radioresistance or CSC properties isa crucial challenge that causes radiotherapy failure
following recurrence and metastasis. Therefore, novel
sensitizers are required to overcome radioresistanceand improve radiotherapy outcomes. In recent years,
new IR methods and inhibitors targeting specific factorsof radioresistance pathways have been developed as
novel strategies for clinical applications. Some of thesestrategies are discussed below.
5.1. Fractionated irradiation
Fractionated radiotherapy has advantages over single
fraction because it can increase the anticancer therapeutic
effect by decreasing the occurrence of severe side effects
in normal tissues. In most conventional fractionatedradiotherapies, approximately 2.0 Gy of radiation isadministered per day, approximately 10 Gy per week,
up to 60 Gy for a six-week treatment regimen. However,this strategy cannot sufficiently control locally advanced

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Fig. 2. Radiation-induced generation of radioresistant cancer cells contributes to cancer recurrence
and metastasis. CSCs are a small subpopulation of cells within heterogeneous tumors with radioresistantcharacteristics. CSCs survive following IR under radiation-induced stress; however, most non-CSCs are
killed, thus, relatively enriching the CSC fraction. On the other hand, a small number of non-CSCs undergo
dedifferentiation and transform into radioresistant phenotypes via complex mechanisms. The newly generated
radioresistant cancer cells, together with the original CSCs, subsequently contribute to the recurrence andmetastasis of cancer.

cancers. Thus, novel strategies, such as hypofractionationor hyperfractionation, have been proposed to compensate
for the limitations of conventional radiotherapy. The
hypofractionation regimen involves the delivery of daily
dose fraction 3–20 Gy with a small number of fractions
usually over a week, and takes advantage of increased cellkilling per radiotherapy fraction to counteract accelerated
tumor regrowth156, 157). This method may increase thebiological effective dose and improve clinical outcomes158).
In particular, using hypofractionated radiation treatment
to specifically target brain tumors will be beneficial insaving normal brain tissue and preserving the various
brain sub-functions compared to irradiating the entire
brain159). On the other hand, hyperfractionation, a smaller
dose of 0.5–1.8 Gy is delivered multiple fractions per day
for 2-4 weeks157). This method increases the total dose,
resulting in therapeutic benefits and fewer adverse
effects, and has been shown to be superior to conventional
radiotherapy in several cancers160-162).
5.2. Combined irradiation
In an effort to overcome tumor radioresistance, muchresearch has focused on the development of tumorspecific radiosensitizers. Simultaneous treatment withchemotherapeutic agents and radiation is more effective
than each treatment alone in various tumor types.
Moreover, combining therapeutic agents with unrelated
mechanisms of action may result in synergistic, or at least
additive, anti-neoplastic effects.
Studies on chemotherapeutic sensitizers are in
progress, some of which are in the preclinical phase.

Radioresistance is associated with multiple signaling
pathways, and radiosensitizing targets against PI3K/
Akt/mTOR 163), Wnt 164), MAPK 165), and c-MET/PI3K/
Akt166) signaling are now available. For example, the oral
PI3K inhibitor BKM120 decreases PI3K/Akt activity
and increases apoptosis, which causes radiosensitization
by suppressing DNA DSB repair in liver cancer cells167).
BEZ235, a dual PI3K-mTOR inhibitor, has also beenshown to improve the radiosensitivity of colon cancer
cells168). Novel drugs, such as anti-angiogenic/vasculartargeting agents that target genetically stable vascularendothelial cells rather than unstable mutating tumor
cells, have been developed recently. Considerable
preclinical evidence suggest the efficacy of tumor
radiosensitization in combination with radiation andangiogenesis inhibition169-171). Direct evidence has shownthat the downregulation of prostaglandins by Cox-2
inhibitors resulted in a reduction of angiogenesis, also
increasing radiosensitivity172). Curcumin, a polyphenol,is a natural compound with various anticancer activities.
It downregulates STAT3 phosphorylation and NF- κB
expression induced by various inflammatory stimuli173).
Some studies show that curcumin sensitizes cancer cells
to IR by inhibiting NF- κB activation174, 175). Curcumin has
also been shown to inhibit EMT in drug‐resistant lungcancer by inhibiting the Wnt/β‐catenin pathway, thereby
downregulating the expression of the downstream
transcription factor Snail176). Hence, a combination of
curcumin administration and radiotherapy may contributeto improved clinical outcomes.
The inhibition of radioprotective molecules, such as

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glutathione (GSH), is also a radiosensitizing strategy.
GSH inhibition prevents DNA damage repair andenhances the effect of radiotherapy 177). In addition,
the use of halogenated pyrimidine analogs such as
bromodeoxyuridine (BrdUrd) or iododeoxyuridine(IdUrd) as clinical radiosensitizers is a new modality
currently investigated in clinical studies178). Thus, thenovel radiosensitizers being developed and are expectedto eradicate radioresistant cancer cells and CSCs.

31

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6. Conclusion
This review described the signaling pathways involvedin acquiring radioresistance, the mechanisms underlying
cancer cell stemness, side effects of radiotherapy, andvarious radiosensitizing strategies. Several studies haveindicated that radiotherapy may cause undesirable
side effects. Therefore, a better understanding of themechanisms involved in radiation-induced EMT, CSCs,
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Abbreviations
irradiation: IR, cancer stem cells: CSCs, radioresistant: RR,
clinically relevant resistant: CRR, ataxia telangiectasia andRad3-related: ATR, DNA-activated protein kinase: DNAPK, homologous recombination: HR, X-ray repair crosscomplementing 4: XRCC4, DNA-PK catalytic subunit:
DNA-PKcs, breast cancer susceptibility gene: BRCA,
fractionated irradiation: FR, epithelial growth factor
receptor: EGFR, phosphatidylinositol-3-kinase: PI3K,
signal transducer and activator of transcription: STAT,

mitogen-activated protein kinase: MAPK, non-small celllung cancer: NSCLC, cyclooxygenase: Cox, mammaliantarget of rapamycin: mTOR, interleukin: IL, nuclear
factor-kappa B: NF-κB, B-cell lymphoma: Bcl, myeloid cellleukemia: Mcl, matrix metalloproteinases: MMPs, janusactivated kinase: JAK, epithelial-mesenchymal transition:
EMT, cluster of differentiation: CD, phosphatase andtensin homolog deleted from chromosome 10: PTEN,
zinc finger e-box binding homeobox: ZEB, aldehyde
dehydrogenase: ALDH, extracellular signal-regulated
kinase: ERK, glutathione: GSH, bromodeoxyuridine:
BrdUrd, iododeoxyuridine: IdUrd.