Role of ion channels in ionizing radiation-induced cell death


Biochimica et Biophysica Acta 1848 (2015) 2657–2664

Contents lists available at ScienceDirect

Biochimica et Biophysica Acta

journal homepage: www.elsevier.com/locate/bbamem
Review
Role of ion channels in ionizing radiation-induced cell death☆
Stephan M. Huber a,⁎, Lena Butz a,b, Benjamin Stegen a, Lukas Klumpp a, Dominik Klumpp a, Franziska Eckert a
a Department of Radiation Oncology, University of Tübingen, Germany
b Department of Pharmacology, Toxicology and Clinical Pharmacy, Institute of Pharmacy, University of Tübingen, Germany
☆ This article is part of a Special Issue entitled: Membra
cancers.
⁎ Corresponding author at: Department of Radiation On

Hoppe-Seyler-Str. 3, 72076 Tübingen, Germany. Tel.: +49
E-mail address: stephan.huber@uni-tuebingen.de (S.M

http://dx.doi.org/10.1016/j.bbamem.2014.11.004
0005-2736/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 31 July 2014
Received in revised form 30 October 2014
Accepted 5 November 2014
Available online 15 November 2014

Keywords:
Ion transport
Radiation
Cancer
Cell death
Therapy resistance
Ca2+-activated K+ channels
Neoadjuvant, adjuvant or definitive fractionated radiation therapy are implemented in first line anti-cancer
treatment regimens of many tumor entities. Ionizing radiation kills the tumor cells mainly by causing double
strand breaks of their DNA through formation of intermediate radicals. Survival of the tumor cells depends on
both, their capacity of oxidative defense and their efficacy of DNA repair. By damaging the targeted cells, ionizing
radiation triggers a plethora of stress responses. Among those is the modulation of ion channels such as Ca2+-
activated K+ channels or Ca2+-permeable nonselective cation channels belonging to the super-family of tran-
sient receptor potential channels. Radiogenic activation of these channels may contribute to radiogenic cell
death as well as to DNA repair, glucose fueling, radiogenic hypermigration or lowering of the oxidative stress bur-
den. The present review article introduces these channels and summarizes our current knowledge on the mech-
anisms underlying radiogenic ion channel modulation. This article is part of a Special Issue entitled: Membrane
channels and transporters in cancers.

© 2014 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2657
2. Radiotherapy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2658
3. Radiosensitizing ion channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2658
4. Ion channels conferring intrinsic radioresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2659
5. Ion channels in acquired radioresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2660
6. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2661
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2662
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2662
1. Introduction

Ionizing radiation kills or inactivates cells mostly by damaging the
nuclear DNA and cell survival critically depends on successful repair of
the DNA damage [1]. Ionizing radiation may lead to necrotic as well as
apoptotic cell death depending on cell type, dose and fractionation pro-
tocols [2]. The major death pathway in this scenario in normal tissue
cells is apoptosis. However, cancer cells which often have developed
strategies to evade apoptosis [3] may either undergo (regulated) necro-
sis or reenter the cell cycle with accumulated DNA damages. During the
ne channels and transporters in

cology, University of Tübingen,
7071 29 82183.
. Huber).
subsequent cell divisions those cells will not be able to segregate the
chromosomes and end up as multinucleated giant cells in mitotic catas-
trophe. Mitotic catastrophe again leads either to apoptotic or necrotic
cell death. Another possible mechanism of radiation-induced death in
cells with disturbed apoptosis machinery is excess autophagy. While
autophagy is a survival strategy [4] excess autophagy overdigests the
cytoplasm and cell organelles forcing the cell into apoptosis or necrosis
[5].

Meanwhile, the evidence is overwhelming that ion channels fulfill
pivotal functions in cell death mechanisms such as apoptosis (for
review see the article by Annarosa Arcangeli in this special issue on
“Membrane channels and transporters in cancers”) as well as in stress
response and survival strategies. Notably, tumor cells have been dem-
onstrated to express a set of ion channels which is different to that of
the parental normal cells. These channels may fulfill specific oncogenic
functions in neoplastic transformation, malignant progression or tissue

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2658 S.M. Huber et al. / Biochimica et Biophysica Acta 1848 (2015) 2657–2664
invasion and metastasis (for review see [1]). In addition, they may con-
tribute to the cellular stress response for instance during fractionated
radiation therapy and may confer radioresistance.

The present review intends to sum up data on ion channel function
in the stress response to ionizing radiation. In particular, ion channels
that may induce cell death in tumor cells and facilitate radiogenic cell
killing are introduced. In addition, data on ion channels which, in con-
trast to the before mentioned, confer radioresistance are reviewed.
Finally, ion channels of tumor cells that might contribute to acquired
radioresistance, e.g. by promoting radiogenic hypermigration or transi-
tion into relatively radioresistant cancer stem (cell)-like cells (CSCs) are
described. Prior to that, a brief introduction into radiotherapy and its
radiobiological principles is given in the next paragraphs.

2. Radiotherapy

Radiation therapy together with surgery and systemic chemothera-
py is the main pillar of anti-cancer treatment. About half of all cancer
patients receive radiation therapy, half of all cures from cancer include
radiotherapy [6]. Despite modern radiation techniques and advanced
multimodal treatments, local failures and distant metastases often
limit the prognosis of the patients, especially due to limited salvage
treatments [7].

Ionizing radiation impairs the clonogenic survival of tumor cells
mainly by causing double strand breaks in the DNA backbone. The num-
ber of double strand breaks increases linearly with the absorbed radia-
tion dose. The intrinsic capacity to detoxify radicals formed during
transfer of radiation energy to cellular molecules such as H2O (giving
rise to hydroxyl radicals, ˙OH) and the ability to efficiently repair DNA
double strand breaks by non-homologous end joining or homologous
recombination determines the radiosensitivity of a given tumor cell.
Irradiated tumor cells which leave residual DNA double strand breaks
un-repaired lose their clonogenicity meaning that these cells can not
restore tumor mass (for review see [8]).

In addition to these intrinsic resistance factors, the microenviron-
ment may lower the radiosensitivity of tumor cells. Hypoxic areas are
frequent in solid tumors reaching a certain mass. Tumor hypoxia, how-
ever, decreases the efficacy of radiation therapy [9]. Ionizing radiation
directly or indirectly generates radicals in the deoxyribose moiety of
the DNA backbone. In a hypoxic atmosphere, cellular thiols can react
with those DNA radicals resulting in chemical DNA repair. At higher
oxygen partial pressure, in sharp contrast, radicals of the deoxyribose
moiety are chemically transformed to strand break precursors [10]. By
this mechanism, hypoxia increases radioresistance by a factor of two
to three (oxygen enhancement ratio) [11].

Fractionated treatment regimens which improve recovery of the
normal tissue after irradiation but not of the tumor have been
established in radiotherapy [12]. In addition to limit normal tissue tox-
icity, killing of tumor mass by initial radiation fractions has been dem-
onstrated to reoxygenate and thereby radiosensitize solid tumors
during further fractionated radiotherapy. Beyond that, fractionated
radiation regimens aim to redistribute tumor cells in a more vulnerable
phase of the cell cycle in the time intervals between two fractions [13].
Accelerated repopulation of the tumor after irradiation is a frequently
reported phenomenon. Possible mechanisms of accelerated repopula-
tion include induction of CSCs: It has been proposed that radiation ther-
apy induces CSCs to switch from an asymmetrical into a symmetrical
mode of cell division; i.e., a CSC which is thought to normally divide
into a daughter CSC and a lineage-committed progenitor cells is induced
by the radiotherapy to divide symmetrically into two proliferative CSC
daughter cells. This is thought to accelerate repopulation of the tumor
after end of radiotherapy. Importantly, CSCs are thought be relatively
radioresistant possibly due to i) high oxidative defense and, therefore,
low radiation-induced insults, ii) activated DNA checkpoints resulting
in fast DNA repair, and iii) an attenuated radiation-induced cell cycle
redistribution [14].
Finally, fractionated radiation therapy, which applies fractions of
sublethal radiation doses (usually 2 Gy per fraction), has been demon-
strated in a variety of tumor entities in vitro and in animal models to
stimulate hypermigration and hypermetastasis of tumor cells as well
as infiltration of the tumor by CD11b-positive myeloid cells and subse-
quent vasculogenesis. It is tempting to speculate that radiogenic
hypermigration boosts cellular interaction of tumor cells with non-
tumor cells, e.g. endothelial cells. It has been proposed that CSCs lodge
within perivascular niches where a complex regulatory network
supports CSC survival [15]. As a matter of fact, CSCs but not non-CSCs
gain radioresistance when transplanted orthotopically in mice [16]
supporting the idea of a tumor microenvironment-dependent acquired
radioresistance. Ion channels contribute to both, intrinsic and acquired
radioresistance of tumor cells as discussed in the next paragraphs
3. Radiosensitizing ion channels

Member 2 of the melastatin family of transient receptor potential
channel (TRPM2) is a Ca2+-permeable nonselective cation channel.
Heterologous expression of TRPM2 in human embryonic kidney cells
[17] or A172 human glioblastoma cells [18] facilitates oxidative stress-
induced cell death. Reactive oxygen species (ROS) have been demon-
strated to trigger TRPM2 activation [19,20]. The principal activator,
however, of TRPM2 is ADP-ribose (ADPR) that binds to a special domain
located at the C-terminus of the channel [21,22]. Sources of ADPR are
the mitochondria [23] or ADPR polymers. The latter are formed,
e.g., during DNA repair by poly (ADP-ribose) polymerases (PARPs).
ADPR is released from the ADPR polymers by glycohydrolases [21,24].

Expression of TRPM2 has been demonstrated in several tumor enti-
ties such as insulinoma [25], hepatocellular carcinoma [25], prostate
cancer [26], lymphoma [27], leukemia [28] and lung cancer cell lines
[29]. TRPM2 activity increases the susceptibility to cell death [30] prob-
ably by overloading cells with Ca2+ (Fig. 1A).

Remarkably, cancer cells may evade TRPM2-mediated cell death. In
lung cancer cells, de-methylation of a GpC island within the TRPM2
gene gives rise to new promoters that regulate transcription of a non-
functional truncated TRPM2 channel [29] and to a TRPM2 specific anti-
sense RNA. This antisense RNA inhibits TRPM2 translation. Moreover,
the truncated channel is non-functional and acts dominant negative,
thus switching off the tumor-suppressing function of the full-length
TRPM2 protein [29] (Fig. 1B).

The initially described member of the vanilloid family of TRP chan-
nels, the nociceptive and heat receptor TRPV1, is reportedly expressed
in several tumor entities such as uveal melanoma [31], pancreatic [32]
and prostatic neuroendocrine tumors [33], glioblastoma [34] and
urothelial cancer of human bladder [35]. At least in the latter two
tumor entities, TRPV1 exerts anti-oncogenic effects [35,36]. TRPV1
expression inversely correlates with glioma grading [34]. Remarkably,
neural precursor cells have been demonstrated to induce ER stress-
mediated cell death of glioblastoma cells by activating glioblastoma
TRPV1 channels through secretion of endogenous vanilloids [37].
Along those lines is the observation that a TRPV1 antagonist promotes
tumorigenesis in mouse skin [38].

Notably, targeting of TRPM2 and TRPV1 by RNA interference has
been demonstrated to decrease gamma irradiation-induced formation
of nuclear γH2AX foci and further DNA damage response in A549 lung
adenocarcinoma cells [39]. Since γH2AX foci are used as a surrogate
for DNA double strand breaks, one might speculate that TRPM2 or
TRPV1 may amplify ionizing radiation-induced insults (Fig. 1). Another
interpretation which has been favored by the author of the study [39]
would be that activity of TRPM2 and TRPV1 is required for the formation
of DNA repair complexes. In combination, the data hint to the possibility
of radiosensitizing cancer cells by pharmacologically activating TRPM2
or TRPV1 channels. Whether this might become a promising new strat-
egy of tumor radiosensitization has to await animal studies.



TRPM2

TRPM2

TRPM2

ADP-ribose

cell death
DSB

radiation

Ca2+

Ca2+overload

ER

nucleus

mito.
oxidative
stress

TRPM2 TRPM2-TE

TRPM2

TRPM2-TE

TRPM2

TE

AS

CpG island

ADP-ribose

TRPM2-AS

DSB

radiation

Ca2+

survival

Ca2+

ER

nucleus

mito.

survival

BA

Fig. 1. Speculative mechanism of a putative TRPM2-mediated radiosensitization (A) and reported strategy [29] of lung cancer cells to avoid TRPM2-mediated susceptibility to cell death
(B), for details see text. TRPM2-TE (TE): truncated TRPM2, TRPM2-AS (AS): TRPM2 antisense RNA, mito.: mitochondrion).

2659S.M. Huber et al. / Biochimica et Biophysica Acta 1848 (2015) 2657–2664
4. Ion channels conferring intrinsic radioresistance

DNA repair involves cell cycle arrest, chromatin relaxation and for-
mation of repair complexes at the site of DNA damage. Moreover,
radiation-induced formation of radicals requires activated radical
detoxification pathways and increased oxidative defense to constrain
the radiation-induced insults. All these processes of stress response
lead to elevated ATP consumption which requires intensified energy
supply. Recent in vitro observations suggest that these processes
depend at least partially on radiation-induced ion channel activation.

Studies of our laboratory indicate that survival of irradiated human
leukemia cells critically depends on Ca2+ signaling involving radiogenic
activation of TRPV5/6-like nonselective cation and Kv3.4 voltage-gated
K+ channels [40,41]. The nonselective cation channels in concert with
Kv3.4 generate radiogenic Ca

2+ signals that contribute to G2/M cell
cycle arrest by CaMKII-mediated inhibition of the phosphatase cdc25B.
Activity of the latter is required in these cells for release from
radiation-induced G2/M arrest via dephosphorylation and thereby
activation of cdc2, a component of the mitosis promoting factor. Exper-
imental interference with the radiogenic Ca2+ signals, e.g. by pharma-
cological inhibition or knock-down of Kv3.4 overrides cell cycle arrest
resulting in increased apoptosis and decreased clonogenic survival of
irradiated leukemia cells [40,41]. This radiosensitization by Kv3.4
targeting demonstrates the pivotal role of radiogenic Kv3.4 channel
activation for cell cycle arrest and DNA repair.

Similar to leukemia cells, A549 lung adenocarcinoma cells reported-
ly respond to ionizing radiation with activation of Kv K

+ channels [42]
and transient hyperpolarization of the plasma membrane. Later on,
the membrane potential of the irradiated A549 cells strongly depolar-
izes. This depolarization is dependent on external glucose and inhibited
by phlorizin, a sodium glucose cotransporter (SGLT) blocker. In parallel,
irradiation induces phlorizin-sensitive 3H-glucose uptake within few
minutes after irradiation [43]. Combined, these data suggest that radio-
genic activation of SGLT transporters and Kv K

+ channels cooperate in
glucose fuelling of the irradiated A549 cells, the former by generating
the glucose entry routes, the latter by increasing and maintaining the
driving force for Na+-coupled glucose entry. Glucose uptake by SGLTs
is mainly driven by the inwardly directed electrochemical driving
force for Na+ which in turn is highly dependent on the K+ channel-
regulated membrane potential. SGLTs allow efficient glucose uptake
even from a glucose-depleted microenvironment which is typical for
malperfused solid tumors [44]. It is therefore not surprising that several
tumor entities such as colorectal, pancreatic, lung, head and neck, pros-
tate, kidney, cervical, breast, bladder and prostate cancer as well as
chondrosarcomas and leukemia upregulate SGLTs [45–53].
SGLT has been shown to be in complex with the EGFR [50,53] and
radiogenic SGLT activation depends on EGFR tyrosine kinase activi-
ty [43]. Importantly, radiogenic increase in glucose fuelling seems to
be required for cell survival since the SGLT inhibitor phlorizin
radiosensitizes A549 lung adenocarcinoma and FaDu head and neck
squamous carcinoma cells [43].

Intracellular ATP concentration has been reported to drop in irra-
diated A549 cells indicative of an irradiation-caused energy crisis.
Notably, recovery from radiation-induced ATP decline is EGFR/SGLT-
dependent and associated with improved DNA-repair leading to
increased clonogenic cell survival. This is evident from the fact that
EGFR or SGLT blockade delays recovery of intracellular ATP concentra-
tion and histone modifications necessary for chromatin remodeling
during DNA repair. Vice versa, inhibition of the histone H3 modification
prevents chromatin remodeling as well as energy crisis [8]. Together,
these data suggest that irradiation-associated interactions between
SGLT1 and EGFR result in increased glucose uptake, which counteracts
the energy crisis in tumor cells caused by chromatin remodeling
required for DNA repair (Fig. 2) [8,43].

Besides plasma membrane ion channels, mitochondrial transport
pathways have been shown to contribute to cellular stress response.
Stress-induced upregulation of uncoupling proteins (UCPs) conveys
hyperpolarization of the membrane potential across the inner mito-
chondrial membrane (ΔΨm,) and thereby formation of reactive oxygen
species [54]. UCPs are reportedly upregulated in a number of aggressive
human tumors (leukemia, breast, colorectal, ovarian, bladder, esopha-
gus, testicular, kidney, pancreatic, lung, and prostate cancer) in which
they are proposed to contribute to malignant progression (for review
see [54]).

In addition to malignant progression, UCPs may alter the therapy
sensitivity of tumor cells. UCP-2 expression has been associated with
paclitaxel resistance of p53 wildtype lung cancer, CPT-11 resistance of
colon cancer and gemcitabine resistance of pancreatic adenocarcinoma,
lung adenocarcinoma, or bladder carcinoma. Accordingly, experimental
targeting of UCPs has been demonstrated to sensitize tumor cells to
chemotherapy in vitro (for review see [54]).

Notably, ionizing radiation induces up-regulation of UCP-2 expres-
sion in colon carcinoma cells [55] and in a radiosensitive subclone of B
cell lymphoma [56], as well as UCP-3 expression in rat retina [57].
Radioprotection might result from lowering the radiation-induced bur-
den of reactive oxygen species. As a matter of fact, multi-resistant
subclones of leukemia cells reportedly show higher UCP-2 protein
expression, lower ΔΨm, lower radiation induced formation of reactive
oxygen species, and decreased DNA damage as compared to their
parental sensitive cells [58].



radiation

DSB

histone phosphorylation 
(γ

recruitment of repair enzymes

oxidative insults

Na+

glucose

SGLT

energy crisis ↓

phlorizin

oxidative defense ↑  

EGFR
P

K+

driving force
erlotinibcell death

radiation radiation

DSB

histone phosphorylation 

histone acetylation

BA

survival

DNA decondensation

energy crisis

histone acetylation

(γH2AX foci) H2AX foci)

Fig. 2. Radiation-caused energy crisis (A) and functional significance of SGLT1-mediated glucose fueling for DNA repair and cell survival (B) of irradiated A549 lung adenocarcinoma cells
(DSB: double strand breaks).

2660 S.M. Huber et al. / Biochimica et Biophysica Acta 1848 (2015) 2657–2664
In summary, these data indicate that ion transports through chan-
nels may regulate processes that mediate intrinsic radioresistance.
Only few laboratories worldwide including ours are working on the
radiophysiology of tumor cells. The investigation of ion transports in
irradiated cells therefore is at its very beginning and the few data
available are mostly phenomenological in nature. The molecular mech-
anisms that underlie e.g. radiogenic channel activation are still ill-
defined. Nevertheless, the data prove functional significance of ion
transports and electrosignaling for the survival of irradiated tumor
cells and might have translational implications for radiotherapy in the
future.

5. Ion channels in acquired radioresistance

Microenvironmental stress such as hypoxia, interstitial nutrient
depletion or low pH has been proposed to switch tumor cells from a
“Grow” into a “Go” phenotype. By migration and tissue invasion “Go”
tumor cells may evade the locally confined stress burden and resettle
in distant and less hostile regions. Once resettled, tumor cells may
readapt the “Grow” phenotype by reentering cell cycling and may
establish tumor satellites in more or less close vicinity of the primary
focus (for review see [54]).

In accordance with this hypothesis, sublethal ionizing irradiation as
applied in single fractions of fractionated radiotherapy has been
BK

CXCR4

SDF-1

K+

CaMKII

volume d
H2O

macrophag
vascular da

Ca2+

S-nitrosylation

HIF-1α

Fig. 3. Hypothetical signaling underlying radiogenic hypermigration of glioblastoma cells. SDF-
damage of the tumor vasculature and resultant tumor hypoxia has been proposed to induce SDF
NO in tumor-associated macrophages leading to HIF-1α stabilization by S-nitrosylation [100]. F
unpublished results). SDF-1 induces Ca2+ signals through CXCR4 chemokine receptor that in t
possibly invasion, e.g., via calpain-dependent [112] activation of matrix metalloproteinases [11
demonstrated in vitro and/or in rodent tumor models to induce migra-
tion, invasion and metastasis or spreading of cervix carcinoma [59],
head and neck squamous cell carcinoma [60], lung adenocarinoma
[61,62], colorectal carcinoma [62], breast cancer [62–64], meningioma
[65], medulloblastoma [66] and glioblastoma. In particular, in glioblas-
toma the experimental evidence for such radiogenic hypermigration is
meanwhile overwhelming [67–80]. Glioblastoma cells show a highly
migrative phenotype that may “travel” large distances through the
brain [81]. At least in theory, radiogenic hypermigration might, there-
fore, contribute to locoregional treatment failure by promoting emigra-
tion of tumor cells from the target volume during fractionated radiation
therapy.

Migration and radiogenic hypermigration are well documented in
glioma cells. They invade the surrounding brain parenchyma primarily
by moving along axon bundles and the vasculature. During brain inva-
sion along those tracks cells have to squeeze between very narrow
interstitial spaces which requires effective local cell volume decrease
and reincrease. Glioblastoma cells are capable of losing all unbound
cell water [82]. The electrochemical driving force for this tremendous
cell volume decrease is provided by an unusually high cytosolic Cl−

concentration (100 mM) [83,84] which is utilized as an osmolyte. Dur-
ing local regulatory volume decrease, extrusion of Cl− and K+ along
their electrochemical gradients involves ClC-3 Cl−- channels [85,86],
Ca2+-activated high conductance BK- [74,87,88] and intermediate
invasion

ClC-3Cl-

ynamics

ionizing
radiation

es, endothelial cells, NO
mage, hypoxia, ROS

matrix metalloproteinases

migration

actin dynamics

1 is a HIF-1α target gene and hypoxia is a strong inductor of SDF-1 expression. IR-caused
-1 expression [111]. Beyond that, ionizing radiation reportedly stimulates the generation of
inally, radiation may directly stabilize HIF-1α as deduced from in vitro experiments (own
urn contribute to the programming and mechanics of migration (for details see text) and
3,114].



CXCR4

SDF-1

TGFBR

CD133+

K+

MMP-2

SDF-1

“homing“

endothelial cell
pericyte

v 3

nutrition

VEGF

VEGF

CD144+

TGF-ß

transition

3
1

CD133- IK

CXCR4

SDF-1

TGFBR

CD133+K+

radiation

SDF-1

radiation

K+

“niche“perivascular 

MMP-2

CXCR4

IK/BK

transition

signaling network

A

B

C

CD133+

VEGF

β α

α β

Fig. 4. Synopsis of the signaling network in glioblastoma conferring radioresistance and
speculative role of ionizing radiation herein. A. Irradiation induces secretion of SDF-1
[80,95–97] and transition of CD133− “differentiated” glioblastoma cells to CD133+ GSCs
with up-regulated CXCR4 [106], β1/α3 integrins [108], TGFB2 receptor [115], TGF-β
responsiveness [115], and IK Ca2+-activated K+ channel-dependent highly migratory
and invasive phenotype [109] B. Irradiation promotes “homing” of GSCs to perivascular
niches by stimulating cell migration. C. The reciprocal interaction between glioblastoma
and endothelial cells strongly depends on matrix metalloproteinase-2 (MMP-2) expres-
sion by glioblastoma [114] and SDF-1 signaling of endothelial cells [110]. Importantly,
irradiation induces upregulation of MMP-2 in glioblastoma cells (B) which is required
for tissue invasion [67,71,72,79,114] and VEGF secretion (B) [71,75] which reportedly
may promote angiogenesis [116]. In addition, transition of glioblastoma cells into endo-
thelial cells [117] and pericytes [15] reconstruct the glioblastoma vasculature which sup-
ports both, vessel function and tumor growth.

2661S.M. Huber et al. / Biochimica et Biophysica Acta 1848 (2015) 2657–2664
conductance IK K+ channels [86,89]. Inhibition of either of these chan-
nels attenuates glioblastoma cell migration or invasion [83,90–94]
confirming their pivotal function in these processes.

Ionizing radiation has been demonstrated in our laboratory to acti-
vate BK K+ channels in glioblastoma cells in vitro [74]. Radiogenic BK
channel activity, in turn, is required for Ca2+/calmodulin kinase II
(CaMKII)- [74] and consecutive CaMKII-dependent ClC-3 channel acti-
vation (own unpublished observation and [85]). Inhibition of BK or
CaMKII abolishes radiogenic hypermigration [74] indicating BK channel
activation as key event of radiogenic hypermigration of glioblastoma
cells. Radiogenic hypermigration is paralleled by radiogenic expression
of the chemokine SDF-1 (stromal cell-derived factor-1, CXCL12) in
different tumor entities including glioblastoma [80,95–97]. Glioblasto-
ma cells reportedly express CXCR4 chemokine receptors and SDF-1
stimulates glioblastoma cell migration via CXCR4-mediated Ca2+ sig-
naling [93]. CXCR4 receptors reportedly signal through phospholipase
C and BK channels have been shown to be functionally coupled with
IP3 receptors in the ER [98] suggesting (and confirmed by own unpub-
lished observations) that radiogenic SDF1/CXCR4 signaling is upstream
of BK channel activation. SDF-1, in turn, is a target gene of the transcrip-
tion factor HIF-1α which reportedly becomes stabilized, e.g. by S-
nitrosylation, upon irradiation [95,99–101] (Fig. 3). Together, this
gives a good example of radiogenic signaling which integrates biochem-
ical signaling, electrosignaling (i.e., BK-dependent regulation of mem-
brane potential) and Ca2+ signaling modules (more details are given
in the legend to Fig. 3).

Ionizing radiation has been demonstrated to select stem (cell)-like
glioblastoma cells (GSCs) or even induce transition of “differentiated”
cancer cells to GSCs/CSCs in glioblastoma [102–104] and other tumor
entities [14]. Notably, “stemness” is associated with SDF-1 secretion
[105] and markedly increased CXCR4 chemokine expression [106]. Im-
portantly, CXCR4 upregulation is required to maintain “stemness” of
non-small cell lung cancer [107] and glioblastoma cells [105]. In accor-
dance to CXCR4 upregulation, GSCs show a highly migratory/invasive
phenotype [108,109]. Most importantly, this phenotype is highly
dependent on the Ca2+-activated IK K+ channel [89,109]. Furthermore,
IK channels have been demonstrated to be overexpressed in about one
third of the glioma patients with IK protein expression correlating with
poor patient survival [89].

Unexpectedly, a previous report demonstrated that xenografted
CD133+ stem-like subpopulations of glioblastoma exhibit a higher
radioresistance than xenografted CD133− cells while radiosensitivity
of both subpopulations does not differ in vitro [16]. This clearly indicates
a function of the brain microenvironment for radioresistance. In partic-
ular, endothelial cells have been postulated to promote glioblastoma
therapy resistance [110]. Part of the reported reciprocal interaction
between glioblastoma cells and endothelial cells as well as of the com-
plex signaling network in perivascular “niches” is schematically sum-
marized in Fig. 4.

Albeit merely speculative, the idea that radiogenic hypermigration
might promote “homing” of (CXCR4-highly-expressing stem-like) glio-
blastoma cells to perivascular niches is highly attractive. The subse-
quent reciprocal modifications of glioblastoma and endothelial cells
might eventually induce radioresistance of glioblastoma cells. Together,
these data suggest that radiogenic hypermigration might contribute to
the apparently high radioresistance of glioblastoma cells either by pro-
moting evasion from the radiation target volume or by stimulating the
chemotaxis of glioblastoma cells to “radioprotective” perivascular
niches.

6. Concluding remarks

The radiation physiology of cancer cells is yet a neglected research
field. While the number of reports on ion channel function in neoplastic
transformation, malignant progression or metastasis of cancer cells in-
creases constantly only little is known about the role of ion channels
in radiotherapy. The few data available strongly suggest that ionizing
radiation-induced ion channel modifications are a common phenome-
non. Importantly, these modifications impact on the stress response
and survival of irradiated tumor cells. By modulating intracellular
Ca2+ signals radiosensitive ion channels may directly crosstalk with
the biochemical signaling of the DNA damage response. By driving
local cell volume changes radiogenic ion channel modifications may
promote cell migration and stress evasion of irradiated tumor cells. By
stabilizing the membrane potential ionizing radiation-induced K+

channel activity might facilitate Na+-coupled glucose uptake providing
the energy for DNA-repair. Finally, mitochondrial channels upregulated



2662 S.M. Huber et al. / Biochimica et Biophysica Acta 1848 (2015) 2657–2664
by ionizing radiation might lower the oxidative insults associated
with ionizing radiation. Given the aberrant and partly specific ion
channel expression of tumor cells, a more profound understanding
of the mechanisms underlying radiogenic ion channel modifications
might be harnessed in the future to develop new strategies for the
radiosensitization of tumors.
Acknowledgment

This work was supported by a grant from the Wilhelm-Sander-
Stiftung awarded to SH (2011.083.1). BS and DK were supported by
the DFG International Graduate School 1302 (TP T9 SH).
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	Role of ion channels in ionizing radiation-�induced cell death
	1. Introduction
	2. Radiotherapy
	3. Radiosensitizing ion channels
	4. Ion channels conferring intrinsic radioresistance
	5. Ion channels in acquired radioresistance
	6. Concluding remarks
	Acknowledgment
	References