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MINI-REVIEW
Journal of
Mitochondria and Mitochondrial
ROS in Cancer: Novel Targets for
Anticancer Therapy
Cellular
Physiology
YUHUI YANG,1,2 SVETLANA KARAKHANOVA,2 WERNER HARTWIG,3 JAN G. D’HAESE,3
PAVEL P. PHILIPPOV,4 JENS WERNER,3 AND ALEXANDR V. BAZHIN3*
1
Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China
2
Department of General Surgery, University of Heidelberg, Heidelberg, Germany
3
Department of General, Visceral, and Transplant Surgery, Ludwig-Maximilians-University, Munich, Germany
4
Department of Cell Signalling, Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, Russia
Mitochondria are indispensable for energy metabolism, apoptosis regulation, and cell signaling. Mitochondria in malignant cells differ
structurally and functionally from those in normal cells and participate actively in metabolic reprogramming. Mitochondria in cancer cells
are characterized by reactive oxygen species (ROS) overproduction, which promotes cancer development by inducing genomic instability,
modifying gene expression, and participating in signaling pathways. Mitochondrial and nuclear DNA mutations caused by oxidative damage
that impair the oxidative phosphorylation process will result in further mitochondrial ROS production, completing the “vicious cycle”
between mitochondria, ROS, genomic instability, and cancer development. The multiple essential roles of mitochondria have been utilized
for designing novel mitochondria-targeted anticancer agents. Selective drug delivery to mitochondria helps to increase specificity and
reduce toxicity of these agents. In order to reduce mitochondrial ROS production, mitochondria-targeted antioxidants can specifically
accumulate in mitochondria by affiliating to a lipophilic penetrating cation and prevent mitochondria from oxidative damage. In consistence
with the oncogenic role of ROS, mitochondria-targeted antioxidants are found to be effective in cancer prevention and anticancer therapy.
A better understanding of the role played by mitochondria in cancer development will help to reveal more therapeutic targets, and will help
to increase the activity and selectivity of mitochondria-targeted anticancer drugs. In this review we summarized the impact of mitochondria
on cancer and gave summary about the possibilities to target mitochondria for anticancer therapies.
J. Cell. Physiol. 231: 2570–2581, 2016. ß 2016 Wiley Periodicals, Inc.
It is well-known that mitochondria play multiple essential roles
in eukaryotic cells. Firstly, mitochondria are the principal site of
ATP production to meet the bioenergetic requirement of cells.
Several carbon sources are utilized to produce ATP, including
pyruvate generated from glycolysis, glutamine, and fatty acids.
These carbon sources enter the tricarboxylic acid (TCA) cycle
in mitochondrial matrix to generate reducing equivalents
NADH and FADH2 which transfer their electrons to the
electron transport chain (ETC) embedded in the inner
mitochondrial membrane (Weinberg and Chandel, 2015).
Energy conserved during this process is used to catalyze the
phosphorylation of ADP to ATP. About 90% of cellular ATP is
generated in mitochondria through this oxidative
phosphorylation (OXPHOS) pathway. Meanwhile,
intermediates in the TCA cycle can be used in macromolecule
synthesis to meet the biosynthetic needs of cell growth and
proliferation. Therefore, mitochondria are indispensable for
energy metabolism and cell survival. Besides ATP production,
mitochondria are also involved in heme and iron sulfur center
biosynthesis, amino acid and nitrogen metabolism, calcium
homeostasis, and cellular redox status regulation (Murphy and
Smith, 2007). On the contrary to cell survival, mitochondria are
crucial participants in cell death processes. Mitochondria
control the intrinsic apoptotic pathway by regulating the
release of proapoptotic factors, such as cytochrome c and
second mitochondria-derived activator of caspase (SMAC),
from the mitochondrial intermembrane space to cytoplasm
(Orrenius et al., 2003). In addition to apoptosis, mitochondria
are also relevant to other forms of cell death, such as necrosis,
or in particular, necroptosis (programmed necrosis), and
autophagy (Gozuacik and Kimchi, 2004; Zong and Thompson,
2006; Galluzzi and Kroemer, 2008).
© 2 0 1 6 W I L E Y P E R I O D I C A L S , I N C .
In order to perform these essential functions accurately,
mitochondria are in constant bidirectional communication with
the rest of the cell, and are therefore regarded as signaling
organelles (Chandel, 2014). Signal transduced from cytosol to
mitochondria is referred as anterograde signaling while signal
transduced in the opposite direction is denoted as retrograde
signaling. Rapid influx of Ca2þ through mitochondrial calcium
uniporter (MCU) increases Ca2þ concentration in
mitochondrial matrix, which regulates the activity of enzymes
of the TCA cycle (Williams et al., 2015). Meanwhile, signals are
Contract grant sponsor: Deutsche Forschungsgemeinschaft.
Contract grant sponsor: Else Kr€
oner-Fresenius Foundation.
Contract grant sponsor: Fundamental Research Funds for the
Central Universities of China.
Contract grant sponsor: National Natural Science Foundation of
China.
Contract grant sponsor: Russian Foundation for Basic Research;
Contract grant numbers: BA 3826/5-1, 2012.A131, 2014QN045,
81402553, 15-04-05171.
*Correspondence to: Alexandr Bazhin, Department of General,
Visceral, and Transplant Surgery, Ludwig-Maximilians-University
Munich, Marchioninistr. 15, 81377 Munich, Germany. E-mail:
[email protected]
Manuscript Received: 21 May 2015
Manuscript Accepted: 16 February 2016
Accepted manuscript online in Wiley Online Library
(wileyonlinelibrary.com): 19 February 2016.
DOI: 10.1002/jcp.25349
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also sent out from mitochondria, such as by the release of
proapoptotic factors as mentioned above. Moreover, the
metabolites of mitochondrial respiration can function as
signals. Citrate is preferentially exported via tricarboxylate
transporter to the cytosol where it is cleaved by ATP citrate
lyase (ACL) to produce oxaloacetate and acetyl-CoA. In
addition to serve as building-blocks of lipid synthesis, acetylCoA is the substrate of histone acetyl transferase (HAT) to
modify histone tails, which profoundly influences the epigenetic
state (Kaelin and McKnight, 2013). Reactive oxygen species
(ROS) produced by mitochondria (see below) are constantly
released and act as secondary messengers in cellular signaling.
Upon recognition of pathogen-associated molecular patterns
(PAMPs) from microbes or damage-associated molecular
patterns (DAMPs) from damaged tissue, ROS released from
mitochondria mediate the activation of inflammasomes (Yang
et al., 2013a). This activation process senses the presence of
danger to the host. Recent studies have also demonstrated that
mitochondria-derived ROS are involved in anti-bacterial and
anti-viral signaling, revealing an essential role of mitochondria in
innate immunity (West et al., 2011; Yang et al., 2013a).
Moreover, mitochondrial outer membrane can serve as
signaling platform by tethering A-kinase-anchoring proteins
(AKAPs), a group of scaffold proteins which facilitate the
localization of signaling enzymes to specific cell area. These
proteins anchor and coordinate cAMP-dependent protein
kinase A (PKA) and other signaling enzymes (Chandel, 2014).
More modes of communication between mitochondria and
other subcellular compartments, such as through dynamics of
mitochondria, expression of kinase and phosphatase,
interaction with endoplasmic reticulum via mitochondriaassociated membranes (MAMs), are still being investigated
(Pagliarini et al., 2005; Campello and Scorrano, 2010;
Acin-Perez et al., 2011; van Vliet et al., 2014).
Due to their irreplaceable roles, mitochondrial malfunction
may give rise to many disorders and diseases, including aging,
neurodegenerative diseases, obesity, and diabetes (Murphy and
Smith, 2007). Mitochondrial malfunction can arise from
mutations of mitochondrial DNA (mtDNA) genes or nuclear
genes, both of which can cause defects in OXPHOS (Murphy
and Smith, 2000). As a consequence, nuclear or mitochondrial
gene defects which lead to impaired ATP production frequently
affect tissues with high energy demand, as in many myopathies
and neural disorders (Wallace, 1999; Leonard and Morris,
2000; Leonard and Schapira, 2000). Mutations of nuclear genes
encoding non-mitochondrial proteins can also lead to
OXPHOS defects, as in Huntington’s disease and autosomal
recessive mitochondrial myopathy (Leonard and Schapira,
2000).
Mitochondria and Cancer Metabolism
Mitochondria in metabolic reprogramming of cancer
cells: more than Warburg effect
Taking into account that mitochondria perform both vital
(energy metabolism) and lethal (cell death) functions in
physiological and pathological settings, it is not surprising that
mitochondria are implicated in cancer initiation and
progression (Ohta, 2006; Fulda et al., 2010). As early as in the
1920s, by meticulously measuring the oxygen uptake and
lactate output of cancer tissue slices, Warburg et al. (1927)
have discovered that cancer cells generate ATP mainly via
glycolysis, even in the presence of normal oxygen pressure
(Warburg, 1928). Glycolysis is highly upregulated in cancer
cells in order to compensate for its low efficiency in ATP
synthesis as compared to OXPHOS. Warburg suggested that
“aerobic glycolysis” might be a general property of cancer cells
to produce sufficient ATP and might reflect impaired
mitochondrial function (Chen et al., 2015). This nowadays
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so-called “Warburg effect” might be the first description of
relevant involvement of mitochondria in cancer. Since then,
more and more evidence has suggested that mitochondria in
malignant cells are structurally and functionally different from
mitochondria in normal cells (Modica-Napolitano and Singh,
2004; Gogvadze et al., 2008). Therefore, defective
mitochondria are previously regarded as the cause of the
universal Warburg phenomenon, and aerobic glycolysis was
thought to be a forced move taken by cancer cells to produce
sufficient ATP (Chen et al., 2015).
However, the original hypothesis for Warburg effect has
been challenged and revisited in recent years. In many cancers,
aerobic glycolysis is greatly enhanced even in the presence of
normal functioning mitochondria. OXPHOS in cancer cells
continues and a similar amount of ATP is produced as in normal
cells (Fantin et al., 2006). Therefore, ATP production is not the
primary objective of the upregulated glycolysis. It is now
generally accepted that cancer cells utilize intermediates of
the glycolysis for anabolic reactions to meet the needs of cell
growth and proliferation. For instance, pyruvate kinase (PK)
dephosphorylates phosphoenolpyruvate (PEP) to pyruvate in
the last step of glycolysis. Interestingly, cancer cells use the M2
isoform (PKM2) instead of the more active M1 isoform (PKM1)
that is predominant in differentiated cells (Ward and
Thompson, 2012). In contrast to PKM1 which is constitutively
active, PKM2 is sensitive to inhibition by tyrosine kinase
downstream of growth factor receptor. PKM2 binds directly
and selectively to phosphorylated tyrosine residues catalyzed
by tyrosine kinase. The binding of PKM2 to phosphotyrosine
leads to the release of allosteric activator fructose-1, 6bisphosphate (FBP), which inhibits the activity of PKM2
(Christofk et al., 2008). In addition, PKM2 can be inhibited by
direct phosphorylation of its tyrosine residues (Hitosugi et al.,
2009). Acetylation of PKM2 suppresses its enzymatic activity
and facilitates its degradation via chaperone-mediated
autophagy (Lv et al., 2011). Collectively, by alternative splicing
and inhibition through multiple mechanisms, the use of less
active PKM2 in cancer cells leads to accumulation of glycolytic
metabolites upstream of pyruvate, diverting them to the
synthesis of amino acids, nucleic acids, and lipids (Ward and
Thompson, 2012).
In spite of the fact that most cancers preserve operational
mitochondria, a subset of cancer cells still possesses mutant
proteins of TCA cycle and subunits of ETC. Due to damaged
function of TCA and/or ETC, these cancer cells rely on the
upregulated glycolysis for ATP supply and on glycolytic
intermediates for macromolecule synthesis (Weinberg and
Chandel, 2015). On the other hand, these cells use isocitrate
dehydrogenase (IDH)-dependent reductive carboxylation of
glutamine-derived a-ketoglutarate (a-KG) to generate citrate
in mitochondria (see below) (Wise et al., 2011; Metallo et al.,
2012; Mullen et al., 2012). Citrate is then exported to
cytoplasm and serves as precursors of macromolecule
synthesis. Therefore, in spite of the presence of impaired
mitochondrial metabolism and bioenergetic dependence on
aerobic glycolysis, part of mitochondrial function are still
required to meet the biosynthetic demand in these cancers
(Weinberg and Chandel, 2015).
Waves of gene expression shape the metabolic phenotype
of cancer
The bioenergetic profile of cancer cells, ranging from
exclusively via glycolysis to mainly via OXPHOS, may be
determined comprehensively by tumor stage, oncogene
activation, nutrient, and oxygen availability (Jose et al., 2011).
Cancer cell metabolic phenotype changes actively and rapidly
to keep in pace with the alterations in cancer
microenvironment. For instance, glucose deprivation leads to
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marked improvement of the mitochondrial biogenesis and
OXPHOS system (Rossignol et al., 2004; Smolkova et al.,
2010), while hypoxia shifts OXPHOS to glycolysis (Zhdanov
et al., 2013; Goto et al., 2014).
Smolkova et al. (2011) have proposed that serial waves of
gene activation promote metabolic changes during cancer
development. Briefly, the first wave of oncogene activation
transforms cancer cells to a partial glycolytic Warburg
phenotype. The hypoxia due to cell proliferation and
deregulated angiogenesis initiates the second wave of
metabolic reprogramming that potentiates the glycolysis,
leading to a classic Warburg phenotype and nearly complete
suppression of OXPHOS. The exacerbating imbalance
between the high energy requirement and nutrient shortage
starts the third wave of gene expression to support cancer cell
survival by glutaminolysis. In this process, a-KG derived from
glutamine either enters the truncated TCA cycle in its normal
forward direction with re-establishment of OXPHOS or
undergoes the abovementioned reductive carboxylation in the
reverse direction of TCA cycle. Both modes of glutaminolysis
may function simultaneously and provide pyruvate, lactate, and
NADPH. The glutaminolysis with a forward-going TCA cycle
may enable at least partial restoration of mitochondrial
biogenesis and reactivation of OXPHOS. The retrograde
signals from these revitalized mitochondria, such as ROS, Ca2þ,
changes in mitochondrial morphology, and dynamics, trigger
the fourth wave of gene reprogramming which may regulate
mitochondrial biogenesis and degradation.
These waves of reprogramming shape the metabolic
phenotype of cancer. This metabolic flexibility enables cancer
cells to adapt to the rapidly changing microenvironment and to
optimally utilize the available metabolic substrates.
Metabolic cooperation between cancer cells and stromal
cells
Cancer metabolic reprogramming also involves the
participation of surrounding stromal cells and their
mitochondria. Breast cancer cells induce oxidative stress and
loss of caveolin 1 (cav-1) in the adjacent cancer associated
fibroblasts (CAFs), which leads to mitophagy in CAFs
(Bonuccelli et al., 2010b; Martinez-Outschoorn et al., 2010). As
a consequence, mitophagy reduces the number of
mitochondria and causes mitochondrial dysfunction, thus
switching the metabolism to glycolysis in CAFs. The increased
production of energy-rich metabolites from glycolysis, such as
lactate and ketones, is secreted into the extracellular space and
is then uptaken by cancer cells to enter the TCA cycle and
undergo oxidative metabolism, fueling cancer growth, and
metastasis (Pavlides et al., 2009; Bonuccelli et al., 2010a). In
consistent with these findings, loss of cav-1 in stromal cells
correlates with early disease recurrence, lymph node
metastasis, advanced tumor stage, tomaxifen-resistance, and
poor clinical outcome in breast cancer patients (Sloan et al.,
2009; Witkiewicz et al., 2009). This metabolic coupling
between cancer cells and CAFs, termed reverse Warburg
effect, shows that cancer cells can extract nutrients from the
surrounding stromal cells (Martinez-Outschoorn et al., 2011).
Another example of metabolic cooperation between cancer
cells and stromal cells is the ability of cancer cells to acquire
mitochondria from surrounding normal cells. Intercellular
mitochondrial transfer was firstly observed in co-culturing
mtDNA depleted lung cancer A549 cells (A549 r0 cells) with
non-hematopoietic stem cells from human bone marrow or
with skin fibroblasts, which showed rescued mitochondrial
functions in A549 r0 cells (Spees et al., 2006). This
mitochondria acquisition was later observed in several in vitro
studies (Pasquier et al., 2013; Antanaviciute et al., 2014;
Caicedo et al., 2015; Wang and Gerdes, 2015). In a recent
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study, when transplanted in syngeneic mice, r0 cancer cells
showed delayed tumor growth and mtDNA acquisition from
host cells. In these mice with tumor derived from r0 cells,
cancer cells isolated from the primary tumor, circulation, and
lung metastasis showed increasing recovery of mitochondrial
function which correlates with increasing tumorigenic capacity
(Tan et al., 2015). This mtDNA acquisition is most probably
accomplished by intercellular transfer of mitochondria, since
there is no known mechanism of mitochondrial genome
transfer across mitochondrial and cellular membranes
(Berridge et al., 2015). Unfortunately, little is known about the
underlying mechanism, although several possible pathways,
such as endocytosis, cell fusion, or exosomes, have been
proposed. In addition to the restoration of mitochondrial
function, studies have also demonstrated that mitochondrial
transfer confers resistance to apoptosis and chemotherapy
(Pasquier et al., 2013; Wang and Gerdes, 2015).
Mitochondrial DNA Alterations
Mitochondria possess their own genome. The human
mitochondrial genome encodes 22 genes for tRNA, 2 for
rRNA, and 13 for components of ETC, with the remaining
mitochondrial proteins encoded by nuclear DNA. It is found
that many chemical carcinogens preferentially bind to mtDNA
rather than to nuclear DNA (Shay and Werbin, 1987). In
addition, mtDNA is under continuous exposure to ROS. As
compared with nuclear genome, the mitochondrial genome is
more susceptible to oxidative attack, therefore it is more
mutable (Yakes and Van Houten, 1997; Zastawny et al., 1998;
Penta et al., 2001). This higher susceptibility can be ascribed to
three factors (Penta et al., 2001). Firstly, mtDNA in
mitochondrial matrix is in proximity to the site of ROS
generation. Secondly, unlike nuclear DNA, mtDNA is under no
protection afforded by histones and chromatin structure.
Thirdly, the DNA repair capability is limited in mitochondria,
for example, lacking nucleotide excision repair completely.
Therefore, it is not surprising that the mutation rate of mtDNA
is reported to be as much as two orders of magnitude higher
than that of nuclear DNA (Penta et al., 2001).
Mammalian mtDNA is present at high copy number
(103–104 copies) in each cell. Since the inheritance of
mtDNA is exclusively maternal, the majority of mtDNA copies
are identical at birth, a state known as homoplasmy. The
occurrence of new mtDNA mutation will lead to coexistence
of mutant and normal mtDNA, a state known as heteroplasmy.
With repetitive cell divisions, the proportion of mutant
mtDNA becomes dominant and homoplasmic. Considering the
large number of mtDNA copies in each cell, the proportion of
mutant mtDNA is critical in phenotype formation. For an
mtDNA mutation to exert impact on cellular function, it needs
to reach a threshold (around 60–90%), at which the wide-type
mtDNA can no longer compensate for the effect of the mutant
(Rossignol et al., 2003; Chatterjee et al., 2011; Mishra and
Chan, 2014; Sobenin et al., 2014). The majority of mtDNA
mutations in human cancer are homoplasmic in nature
(Chatterjee et al., 2006). However, the underlying mechanism
for this enrichment is still not fully understood. It is possible
that a mutation in mtDNA confers growth or survival
advantage to cancer cells which are selected to expand clonally
and become homoplasmic as a consequence (Polyak et al.,
1998; Chatterjee et al., 2011; Verschoor et al., 2013; Yadav and
Chandra, 2013). In this case, some germline mutations may be
actually somatic mutations occurred early in prenatal
development and drifted later towards homoplasmy, and
heteroplasmy is an intermediate state which varies in a wide
range (Chatterjee et al., 2011).
However, some mtDNA mutations are detected as silent
substitutions in amino acids, indicating that mtDNA mutations
ANTIOXIDANTS FOR ANTICANCER THERAPY
may not be necessarily associated with selective advantage
(Fliss et al., 2000). Alternatively, through computer modeling,
Coller et al. (2001) have shown that homoplasmy of mtDNA
mutation can arise entirely by chance through unbiased
mtDNA replication and sorting during cell division.
Nonetheless, other mechanism(s) may also contribute to the
formation of homoplasmic state. For instance, it is possible that
mutant mtDNA may be preferentially replicated in an attempt
to compensate for the functional defects (Hofhaus and
Gattermann, 1999; Hofhaus et al., 2003; Wallace, 2005; Yadav
and Chandra, 2013). Meanwhile, cells with mutant mtDNA may
be relatively protected from apoptosis since the resultant
impaired OXPHOS is generally associated with reduced
sensitivity to apoptosis (Higuchi et al., 1997; Suzuki et al., 2008).
Both these scenarios may favor the progressive displacement
of normal mtDNA.
MtDNA mutations can be inherited as germline mutations
and predispose to cancer, or arise as somatic mutations and
participate in cancer development (Brandon et al., 2006). Both
germline and somatic mtDNA mutations have been associated
with a wide range of cancer types (Wallace, 2012). The roles of
mtDNA alterations in cancer initiation and progression have
been demonstrated by studies using a special cybrid
(cytoplasmic hybrid) system, where cells are generated by
fusion between enucleated cytoplasts containing mutant
mitochondria with cells whose mtDNA has been eliminated (r0
cells). Cybrids carrying mutant ATP synthase subunit 6 gene
showed a growth advantage in nude mice, which may result
from resistance to apoptosis conferred by the mutation
(Shidara et al., 2005). By introducing mtDNA T8993G mutation
into PC3 prostate cancer cells, Petros et al. (2005) showed that
the resultant mutant cybrids generate tumors with increased
growth rate and elevated ROS production as compared to
wide-type cybrids (T8993T). In another study, the metastatic
potential of poorly metastatic cell lines was enhanced by
replacing their mtDNA with mtDNA carrying mutation in
NADH dehydrogenase subunit 6 (ND6) isolated from highly
metastatic cell lines (Ishikawa et al., 2008). This mutation is
associated with increased ROS level. Collectively, mtDNA
mutations may participate in both malignant transformation
and cancer cell behavior determination, probably through ROS
generation (see below).
Quantitative changes in mtDNA, either with increased or
with decreased mtDNA copy number, are also frequently seen
in different types of cancer (Yu, 2011). The mechanism(s) of
these mtDNA content alterations is not fully understood. For
instance, mitochondrial biogenesis and mtDNA can be induced
by oxidative stress in human lung fibroblast (Lee et al., 2000).
Therefore, it is suggested that an increase in mtDNA copy
number may be a feedback response to compensate for the
declined mitochondrial function which generally induces
oxidative stress (Kim et al., 2004; Yu, 2011). On the other hand,
mutations in the D-loop region of mitochondrial genome may
influence the rate of mtDNA transcription/replication and
reduce mtDNA copy number (Yu, 2011). Decreased mtDNA
content is also associated with mutation of p53 and mtDNA
polymerase g (POLG) (Chang et al., 2009; Singh et al., 2009).
These alterations in mtDNA copy number can affect many
aspects of cancer cells, such as cell growth, apoptosis,
chemotherapy resistance, hormone dependence, and
metastasis (Yu, 2011).
Mitochondria-Derived ROS and Cancer
A major contributor to cancer from mitochondria is ROS,
especially from the malfunctioning or dysfunctioning ones. ROS
are a group of highly reactive chemicals under tight control of
intracellular antioxidants (Singh, 2004). Disturbance in the
prooxidation-antioxidation balance due to either elevated ROS
JOURNAL OF CELLULAR PHYSIOLOGY
generation or declined ROS scavenging capacity can lead to
many diseases, including cancer. By causing oxidative DNA
damages and genomic instability, modifying gene expression,
and participating in various signaling pathways, ROS are a
crucial participant in cancer development. The oncogenic
effects of exogenous and endogenous ROS have been
thoroughly reviewed elsewhere (Klaunig and Kamendulis,
2004; Liou and Storz, 2010; Yang et al., 2013b). Here we will
briefly discuss the sources and the central roles of
mitochondrial ROS in cancer.
As the site of cellular respiration, mitochondria are the
primary site of endogenous ROS production. During
mitochondrial respiration, most of the oxygen consumed is
reduced to water. However, approximately 1–2% of molecular
oxygen is converted into ROS in isolated mitochondria,
whereas little is known about the amount of mitochondrial
ROS production in vivo (Murphy, 2009). Mutations of nuclear
DNA and/or mtDNA may lead to ROS overproduction by
impairing the synthesis and function of mitochondrial
respiratory chain.
The TCA cycle and ETC are comprised of serial steps of
electron conduction in which electrons move from one site to
the next. Disruption at any of these locations may lead to
blockage of electrons upstream, which allows the electrons to
react with oxygen to produce superoxide (Sabharwal and
Schumacker, 2014). For instance, the TCA cycle enzyme SDH
catalyzes the oxidation of succinate to fumarate together with
the reduction of ubiquinone to ubiquinol. Within the four
subunits (A–D) of SDH, the A subunit uses a flavin adenine
dinucleotide (FAD) group as the electron carrier, while the
other subunits use three iron-sulfur clusters and a heme group.
Mutations in the B–D subunits impair the electron shuttle to
ubiquinone which is supposed to deliver the electrons to
complex III. This leads to blockage of electrons on the FAD
group in SDHA subunit and generation of superoxide (Guzy
et al., 2008; Owens et al., 2012). The resultant ROS are capable
of inhibiting prolyl hydroxylase (PHD), leading to the
stabilization of HIF-1a. This enhanced HIF-1 activity under
normoxic conditions is described as pseudohypoxia.
Suppressed expression of SDHB, but not SDHA, leads to
HIF-1a stabilization and tumorigenicity (Guzy et al., 2008).
Mutant SDHD also causes genomic instability through
production of superoxide and hydrogen peroxide (Owens
et al., 2012). Mutations in SDHB, SDHC, or SDHD subunits are
observed in paragangliomas and pheochromocytomas in
human (Vicha et al., 2014).
Another TCA cycle enzyme FH converts fumarate to
malate. Germline mutation of FH gene causes hereditary
leiomyomatosis and renal clear cell cancer (HLRCC) (Linehan
and Rouault, 2013). Pseudohypoxic activation of HIF-1 is
detected in FH-deficient cells (Sudarshan et al., 2009). Loss of
FH activity blocks the TCA cycle and may give rise to ROS
production in the abovementioned manner (Sabharwal and
Schumacker, 2014). Further study showed that cells with
mutant FH displayed accumulation of fumarate, which forms
succinated glutathione (GSF) with glutathione. GSF then acts as
an alternative substrate to glutathione reductase to consume
NADPH, the primary reducing equivalent used in ROS
detoxification (Sullivan et al., 2013). Consequently, the overall
antioxidant capacity is compromised, which leads to enhanced
mitochondrial ROS production.
Oncogene hyperactivation has long been associated with
elevated mitochondrial ROS. Induction of KRAS expression
induces mitochondrial dysfunction and ROS production to
promote cancer development (Hu et al., 2012). Another study
showed that ROS produced from complex III are required
for KRAS-mediated tumorigenesis through regulating the
ERK/MAPK signaling pathway (Weinberg et al., 2010). Activity
of other oncogenes also contributes to mitochondrial ROS to
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promote cancer cell proliferation and survival (Anso et al.,
2013). Ectopic MYC over-expression leads to elevated
mitochondrial ROS and concomitant increased oxidative DNA
damage (K et al., 2006). In addition to genomic instability, MYCmediated tumorigenesis may result from HIF-1 stabilization
through PHD inhibition by ROS (Gao et al., 2007).
In addition to mutant nuclear DNA, mtDNA mutations that
impair OXPHOS can increase mitochondrial ROS and
contribute to tumorigenicity (Petros et al., 2005; Park et al.,
2009). By using cybrid technology to exclude the interference
from nuclear DNA, Ishikawa et al. (2008) have shown that
ROS-generating mutations in mtDNA, but not nuclear DNA,
are responsible for the metastatic potential of cancer cells,
which can be effectively suppressed by antioxidant. This mutant
mtDNA contains mutation in the gene encoding ND6, which
causes defective complex I activity and ROS overproduction.
Interestingly, the authors found that the ROS-generating
mtDNA mutations were not related to tumorigenicity in this
study (Ishikawa et al., 2008). Similarly, cybrids containing
mutant ND6 gene-derived from primary lung adenocarcinoma
cells exhibit lower complex I activity, higher ROS level,
increased migration and invasion capacity (Yuan et al., 2015).
However, in another study, this mutant mtDNA-conferred
metastatic potential was not associated with elevated ROS
production (Imanishi et al., 2011), suggesting the involvement
of other pathway(s) in metastasis regulation by mitochondria.
As the genotoxic effects of ROS are well established,
mitochondrial ROS can cause genomic instability (Yang et al.,
2013b). Fibroblasts that are deficient in mitochondrial
superoxide dismutase (SOD2) have increased double strand
breaks and chromosomal translocations (Samper et al., 2003).
Mitochondrial dysfunction induced by uncoupling agent leads
to telomere attrition, telomere loss, and chromosome fusion
and breakage (Liu et al., 2002). Taken together, under oxidative
stress, the resultant mtDNA mutations, nuclear DNA
mutations and genomic instability would lead to further
OXPHOS impairment and cause even higher ROS production
to promote cancer initiation and progression. It is possible that
there is a “vicious cycle” between ROS production, mtDNA
mutations, genomic instability, and cancer development
(Trachootham et al., 2009; Hamanaka and Chandel, 2010;
Klaunig et al., 2011) (Fig. 1).
Targeting Mitochondria for Anticancer Therapy
The fact that mitochondria participate closely in cancer
development makes them a promising target for anticancer
therapy. Through targeting the difference between
mitochondria from cancer cells and those from normal cells,
various agents specifically acting on mitochondria have been
developed. The underlying anticancer mechanisms for these
agents have been thoroughly discussed in several recent
publications (Gogvadze et al., 2009; Fulda et al., 2010; Zhang
et al., 2011).
By potentiating the proapoptotic process and
counteracting the action of antiapoptotic factors, many
compounds can reactivate the impeded apoptosis in cancer
cells. One approach is to increase the conductance of the
permeability transition pore complex (PTPC) of
mitochondria, leading eventually to the rupture of
mitochondrial membrane and the release of proapoptotic
factors from mitochondrial intermembrane space, both of
which are key events in intrinsic (mitochondrial) apoptosis
initiation (Fulda et al., 2010). This can be achieved by
compounds that act directly on PTPC components such as
adenine nucleotide translocator (ANT) and voltagedependent anion channel (VDAC), or by compounds that
deplete the PTPC inhibitors such as glucose, creatine
phosphate and glutathione (Belzacq et al., 2001; Don et al.,
2003; Oudard et al., 2003; Fulda et al., 2010).
Another approach to trigger mitochondrial apoptosis is
the use of Bcl-2 homology domain 3 (BH3) mimetics, which
share structural, and functional similarities with proapoptotic
BH3-only proteins. By antagonizing the antiapoptotic Bcl-2
and/or Bcl-xL, BH3 mimetics facilitate protein permeable
channel formation on the outer mitochondrial membrane
mediated by Bcl-2-associated X protein (BAX) and Bcl-2
antagonist/killer protein (BAK), through which proapoptotic
factors are released (Oltersdorf et al., 2005; Fulda et al.,
2010; Zhang et al., 2011).
Fig. 1. “Vicious cycle” between mitochondria, ROS and cancer development. ROS from malfunctioning or dysfunctioning mitochondria in
cancer cells result in further ROS production by causing mitochondrial and nuclear DNA mutations that impair OXPHOS. On the other hand,
oncogenic ROS promote cancer development by inducing oxidative DNA damages and genomic instability, modifying gene expression, and
participating in various signaling pathways.
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ANTIOXIDANTS FOR ANTICANCER THERAPY
The metabolic reprogramming of malignant cells is emerging
as a novel target for anticancer therapy. Considering the
compromised ability to generate ATP through OXPHOS and
the dependence of ATP production on glycolysis in cancer cells,
drugs that interfere with cancer cell metabolism may possess
therapeutic potential, especially at glycolysis level. Glycolysis
inhibition by glucose analogs or glucose transporter inhibitors
is capable of sensitizing cancer cells, including the resistant
ones, to conventional chemotherapeutic agents, and has
therapeutic effect on cancer growth (Cao et al., 2007; Simons
et al., 2007). Several glycolysis inhibitors are currently under
preclinical and clinical investigation. However, there are also
concerns about their influence on glycolytic metabolism of the
brain and heart, and about its effect on cancers with low
reliance on glycolysis (Fulda et al., 2010; Zhang et al., 2011).
Mitochondrial chaperone heat shock protein 90 (HSP90)directed protein folding helps to maintain energy production in
cancer cells. HSP90 inhibitor disturbs mitochondrial protein
stability and causes impaired energy production, providing a
novel mechanism of targeting bioenergetics of cancer cells
(Chae et al., 2012). Another target of cancer metabolism is the
glutamine catabolism that is essential to replenish the TCA
intermediates for biosynthesis as in the abovementioned
glutaminolysis. Inhibition of conversion from glutamine to
glutamate or from glutamate to a-KG delays cancer growth in
animal models (Thornburg et al., 2008; Qing et al., 2012). Other
therapeutic strategies targeting the aberrant mitochondrial
metabolism include mitochondrial pyruvate dehydrogenase
(PDK) inhibition, lactate dehydrogenase (LDH) inhibition,
and disruption of hexokinase (HK)-VDAC interaction
(Fulda et al., 2010).
Selective Drug Delivery to Mitochondria
Although mitochondria are a promising target in treating
cancer, a challenge to override in the use of mitochondriatargeted agents is to efficiently deliver them to mitochondria in
order to improve efficacy and reduce toxicity. Since many
anticancer drugs act on signaling pathways upstream of and
converge on mitochondria, drug resistance mechanisms that
are also upstream of mitochondria can thus abolish the action
of these agents. Therefore, drugs delivered directly to
mitochondria without engaging the upstream processes may
additionally help to circumvent these resistance mechanisms
(Fulda et al., 2010).
Several strategies of mitochondria-directed drug delivery
have been developed. These systems include: (1) Delocalized
lipophilic cations (DLCs), which target the negative charge of
mitochondrial matrix and readily cross the mitochondrial
membranes (Murphy and Smith, 2007). This method will be
discussed in detail in the next section; (2) Mitochondrial
targeting sequences (MTSs)-containing polypeptides, which
contain 20–40 amino acid residues and can be recognized by
mitochondrial protein import machinery (translocases)
(Mukhopadhyay et al., 2005); (3) Synthetic amino acid- and
peptide-based mitochondrial transporters, which enter cells
via direct uptake and subsequently enter mitochondria in a
charge-driven manner, thus avoiding endosomal and/or
lysosomal sequestration and improving mitochondrial
accumulation (Horton et al., 2008; Zhang et al., 2011);
(4) Vesicle-based transporters, are mitochondria-targeted
liposomes which enter cells by macropinocytosis and then
enter mitochondria by fusion with their outer membranes
(Yamada et al., 2008).
To summarize, the multiple essential roles of mitochondria in
malignant transformation and cancer progression make them a
promising target for anticancer therapy. The involvement of
mitochondria in apoptosis regulation and cancer cell metabolic
reprogramming is useful in designing novel mitochondria-targeted
JOURNAL OF CELLULAR PHYSIOLOGY
anticancer agents. However, in spite of the promising clinical
benefits of mitochondria-targeted therapeutics and drug delivery
systems, there are still concerns and debates on their specificity
and selectivity on malignant cells. For instance, ATP production in
normal cells may be interfered by targeting mitochondrial ATP
production, and mitochondrial ROS-mediated signaling in
immune cells may be disturbed by targeting the mitochondrial
redox status. Therefore, the key to solve this problem is to target
only the mitochondria in cancer cells. For this purpose known
discrepancies between mitochondria in cancer cells and normal
cells could be used. So, the differences in the number, shape,
dynamics, ROS production, apoptotic sensitivity, and metabolic
profile are the base for targeting mitochondria in the treatment of
cancer (Modica-Napolitano and Weissig, 2015). Further studies of
the difference between mitochondria from malignant and normal
cells are needed and will definitely improve the selectivity and
activity of mitochondria-targeted agents.
SkQ1: A Novel Mitochondria-Targeted Antioxidant
In addition to the abovementioned pathways,
mitochondria-targeted agents may kill cancer cells via
ROS-mediated mechanisms. The rationale for targeting the
redox state of cancer cells in anticancer therapy, either to
elevate or to reduce ROS production, has been thoroughly
discussed (Yang et al., 2013b). Several agents induce
mitochondrial membrane damage and subsequent cell
death either by interfering with the OXPHOS process, by
oxidizing the ANT, or by inhibiting the mitochondrial
antioxidant capacity (Costantini et al., 2000; Huang et al.,
2000; Belzacq et al., 2001; Pelicano et al., 2003;
Trachootham et al., 2006).
On the contrary, considering the deleterious effects of
mitochondria-derived ROS in cancers, a decrease in
mitochondrial oxidative damage may be an attractive
therapeutic approach. Since conventional antioxidants
distribute systemically and only a minority fraction is uptaken
by mitochondria, for this purpose, mitochondria-targeted
antioxidants are needed. In addition to specific accumulation in
mitochondria, Murphy and Smith have suggested that the ideal
mitochondria-targeted antioxidants should also possess the
following properties: (1) acceptable oral bioavailability;
(2) selective uptake by mitochondria within organs most
frequently affected by mitochondrial oxidative damage such as
heart, brain, liver, and muscle; (3) efficient protection of
mitochondria from oxidative damage; (4) recyclability to their
active antioxidant state within mitochondria; and (5) the
capacity to act as clinically effective antioxidant at safe
concentrations without causing toxic side effects (Murphy and
Smith, 2007).
As mentioned in the previous section, for selective drug
delivery to mitochondria, one approach is to conjugate the
antioxidant to a lipophilic cation. This class of ions was firstly
described in the late 1960s by Skulachev’s group, and these
penetrating cations were later named as “Skulachev ions”
(Liberman et al., 1969; Green, 1974). Soon after their
discovery, these cations were utilized as “electric locomotive
molecules” to direct non-charged compounds to
mitochondria, taking the advantage that mitochondrial matrix
is the only negatively charged intracellular compartment. The
charge of the ionized atoms in such ions is distributed over a
large hydrophobic molecule, allowing penetration through the
membranes in an electric transmembrane potential (Dc)driven fashion (Skulachev, 2007). According to the Nernst
equation, the lipophilic cation uptake increases approximately
10-fold for every 61.5 mV increase in membrane potential
(Ross et al., 2005). For example, it will be 1,000-fold
accumulation when mitochondrial Dc is 180 mV. Therefore,
through conjugation to lipophilic cations, antioxidants can
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Y A N G E T A L.
enter the cytosol and then accumulate specifically in the
mitochondria, protecting selectively the mitochondria from
oxidative damage.
A variety of antioxidants conjugated to lipophilic penetrating
cations, targeting different species of ROS, have been
developed (Murphy and Smith, 2007). These mitochondriatargeted antioxidants are able to pass readily through
phospholipid bilayers without the facilitation of specific uptake
mechanism and accumulate specifically in mitochondria
because of the large mitochondrial transmembrane potential
(Dcm). To date, the best characterized mitochondria-targeted
antioxidant is 10-(60 -ubiquinonyl) decyltriphenylphosphonium
(MitoQ) developed by Murphy and Smith (Kelso et al., 2001).
MitoQ contains a ubiquinone linked to a triphenylphosphonium
(TPP) cation via a 10-carbon alkyl chain (Fig. 2). Driven by the
plasma transmembrane potential (Dcp: 30–60 mV), MitoQ
enters the cytosol and results in a 5–10-fold accumulation. As
Dcm is around 140–180 mV, MitoQ within the cytosol will
further accumulate in the mitochondrial matrix by several
hundred-fold (Murphy and Smith, 2007). Most of the MitoQ
within mitochondria is distributed to the matrix surface of the
inner mitochondrial membrane, where it is reduced to its
active antioxidant ubiquinol form by complex II of the
respiratory chain (Kelso et al., 2001). As MitoQ is found to
insert into the inner surface of inner mitochondrial membrane
with its antioxidant portion and alkyl chain, its main protective
activity is to prevent lipid peroxidation (James et al., 2005).
Other substrates of MitoQ include peroxynitrite and
superoxide (Kelso et al., 2001; James et al., 2005). However, it
is relatively weak in reacting with hydrogen peroxide (H2O2)
(James et al., 2005). Once oxidized to the ubiquinone form after
detoxifying ROS, MitoQ is readily re-reduced by complex II to
active antioxidant form, which makes it a recyclable antioxidant
(Kelso et al., 2001; James et al., 2005). MitoQ is now under
different stages of investigation in many clinical settings. It is
shown that MitoQ has preventive effects in several animal
models of human diseases, including cardiac ischemiareperfusion injury, hypertension, sepsis, and cardiac toxicities
associated with doxorubicin (adriamycin) (Adlam et al., 2005;
Chandran et al., 2009; Graham et al., 2009; Supinski et al.,
2009). However, in human studies, the results are less fruitful
(Gane et al., 2010; Snow et al., 2010).
Another group of mitochondria-targeted antioxidants was
developed by Skulachev’s group (Izyumov et al., 2010). All
these compounds contain a quinone antioxidant moiety
which is covalently conjugated to a lipophilic cation via alkyl
chains of different length. These compounds are termed
SkQs, with Sk for “Skulachev ions” and Q for quinone.
Among them, 10-(60 -plastoquinonyl) decyltriphenylphosphonium (SkQ1) has shown the highest membrane penetrating
ability and potent antioxidant capability (Antonenko
et al.,2008a). Similar to MitoQ, SkQ1 contains a TPP
penetrating cation, but instead of ubiquinone, SkQ1 contains
plastoquinone as the antioxidant moiety, which is known to
be a lipophilic antioxidant against lipid peroxidation in both
natural and artificial membranes. These two parts are linked
Fig. 2. Chemical structure of MitoQ and SkQ1. Both MitoQ and SkQ1 contain a triphenylphosphonium (TPP) penetrating cation as the
“electric locomotive” to direct the molecule to mitochondrial matrix, the only negatively charged intracellular compartment. For antioxidant
moiety, MitoQ contains a ubiquinone while SkQ1 contains a plastoquinone. The antioxidant moiety and TPP cation are linked together by a
10-carbon alkyl chain in both molecules.
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ANTIOXIDANTS FOR ANTICANCER THERAPY
together by a 10-carbon alkyl chain linker (Antonenko et al.,
2008a) (Fig. 2).
Antioxidant Activity of SkQ1: Higher Efficiency and
Better Safety Profile
SkQ1 is highly permeable through lipid barriers. In various
model systems, including aqueous solutions, lipid micelles,
liposomes, and planar bilayer lipid membranes (BLMs), SkQ1
has shown potent antioxidant property (Antonenko
et al.,2008a). Interestingly, SkQ1 exhibits higher antioxidant
activity than MitoQ in most of these model systems, which is
consistent with the finding that plastoquinone is a more
efficacious antioxidant than ubiquinone (Antonenko et al.,
2008a,b). Therefore, SkQ1 is considered to be efficient in
protecting membrane lipids and membrane-embedded
peptides against oxidative damage (Antonenko et al., 2008a).
This can also be explained by the orientation of SkQ1 in
membrane. Like MitoQ, the positively charged phosphonium
cation of SkQ1 is oriented in the aqueous phase, while the alkyl
chain linker and the plastoquinone antioxidant moiety are
inserted in the membrane (Skulachev, 2007).
In isolated mitochondria, 25 nM SkQ1 efficiently prevents
malondialdehyde (MDA) formation, a product of lipid
peroxidation, at concentration much lower than that of MitoQ
(1,000 nM) (Antonenko et al., 2008a). Moreover, SkQ1 inhibits
the peroxidation of cardiolipin, possibly by interrupting the
chain reaction of lipid peroxidation (Antonenko et al., 2008a).
Cardiolipin is a phospholipid specific for inner mitochondrial
membrane and has a high content of polyunsaturated fatty acid
residues, which makes it extremely sensitive to peroxidation
(Zamzami and Kroemer, 2003; Antonenko et al., 2008a).
Under mitochondrial oxidative stress, cardiolipin oxidation
results in the release of cytochrome c and other proapoptotic
factors, and therefore plays a key role in initiating apoptosis
(Basova et al., 2007; Choi et al., 2007). Consistent with these
findings, SkQ1 is effective in preventing ROS-induced cell death
(Antonenko et al., 2008a). A 7-day SkQ1 pretreatment of
human fibroblasts completely abolished apoptosis induced by
H2O2. In Hela cells, 1-h pre-incubation with SkQ1 prevented
necrosis caused by illumination-induced ROS. It is noteworthy
to point out that these preventive effects of SkQ1 were seen at
extremely low concentration, 0.2–20 nM for antiapoptotic
effect and 0.5–1.0 mM for antinecrotic effect. In addition, in
both types of ROS-induced cell death, SkQ1 was shown to be
more efficient than MitoQ (Antonenko et al., 2008a).
Such extremely high protective efficiency of SkQ1 can be
explained by the following two facts (Skulachev, 2007;
Antonenko et al., 2008a). Firstly, after scavenging ROS,
oxidized SkQ1, like MitoQ, is continuously re-reduced by the
respiratory chain, which makes it a rechargeable antioxidant.
Moreover, the overall rate of re-reduction is higher than that of
oxidation, meaning that SkQ1 is present mainly in its active
reduced form in mitochondria (Fig. 3). Secondly, SkQ1 is
specifically accumulated in mitochondria. As mentioned above,
taken into account both the transmembrane potential of
plasma membrane (Dcp) and mitochondrial membrane (Dcm),
the gradient of SkQ1 can reach 10:1 between cytosol and
extracellular matrix, and 104:1 between mitochondrial matrix
and extracellular matrix. Furthermore, the lipid/water
distribution coefficient of SkQ1 is assumed to be 104:1. Taken
together, the SkQ1 concentration in the inner leaflet of inner
mitochondrial membrane can reach 108-fold increase
comparing to that of extracellular matrix (Fig. 4).
Except for higher efficiency, more importantly, SkQ1 has a
much wider “window” between antioxidant and prooxidant
Fig. 3. SkQ1 is a rechargeable antioxidant. In mitochondrial matrix, SkQ1 in the oxidized form is reduced by complex I and II of the
respiratory chain to its reduced and active form. After detoxifying ROS, oxidized SkQ1 is then re-reduced again, which makes it a
rechargeable antioxidant. Moreover, the overall re-reduction rate is higher than that of oxidation (indicated by thickened arrow). Therefore,
SkQ1 is mainly in its active reduced form in mitochondria.
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Y A N G E T A L.
Fig. 4. Accumulation of SkQ1 in cytoplasm and in mitochondrial matrix. Driven by the cytoplasmic and mitochondrial transmembrane
potential (Dcp and Dcm), SkQ1 accumulates in cytoplasm and mitochondrial matrix with 10-fold and 104-fold increase as compared to the
extracellular concentration, respectively. The lipid/water distribution coefficient of SkQ1 is 104:1, which ultimately results in 108-fold increase
in the concentration of SkQ1 in the inner leaflet of inner mitochondrial membrane compared to that of extracellular matrix. (Fig. is adapted
from Antonenko et al. (2008a)).
concentration than that of MitoQ (Antonenko et al., 2008a).
MitoQ can function as a strong prooxidant when given at high
concentration. In isolated mitochondria from rat heart, MitoQ
starts to show prooxidant effect at concentration only two
times higher than the concentration which shows antioxidant
activity. It has been suggested that the prooxidant activity of
cationic quinones determines their toxic effects (Antonenko
et al., 2008a). Therefore, the narrow “window” of MitoQ may
limit its use in clinical settings. On the contrary, the “window”
between concentrations causing antioxidant and prooxidant
effect is 1,000 for SkQ1. Such a wide “window” implies not
only better “maneuverability,” but also a better safety profile
with lower incidence of side effects.
SkQ1 in Cancer Prevention and Anticancer Therapy
As mentioned in the previous section, mitochondria-targeted
antioxidants may be beneficial in treating cancer. Agapova et al.
showed that SkQ1 possessed both prophylactic and
therapeutic activities against cancer (Agapova et al., 2008).
In p53 knocked out mice (p53/), which showed increased
ROS level and spontaneous tumor development, 5nmol
SkQ1/kg per day supplied in drinking water not only reduced
ROS level, but also delayed tumor appearance and prolonged
their life span. Comparable effect was seen in N-acetylcysteine
(NAC), but at a much higher dose (6 mmol/kg per day), which
implied that SkQ1 was more efficient. Such prophylactic action
of SkQ1 was less effective at lower (0.5 nmol/kg per day) or
higher (50 nmol/kg per day) dose.
As for the therapeutic activity, in athymic mice implanted
with a set of human colon carcinoma HCT116 cells with
JOURNAL OF CELLULAR PHYSIOLOGY
different p53 status, SkQ1 (5 nmol/kg per day) inhibited
significantly the tumor growth of HCT116 cells with a full loss
of p53 expression (p53/). This inhibitory effect was less
prominent in other HCT116 cells expressing wide-type p53. In
another athymic mouse model implanted with human cervical
carcinoma SiHa cells, a higher dose of SkQ1 (50 nmol/kg per
day) significantly increased the lifetime of tumor bearing mice,
but without inhibitory effect on tumor growth.
The authors have also shown that SkQ1 is capable of
inhibiting angiogenesis, stimulating cell differentiation, and
restoring normal epithelial-like morphology of cancer cell lines.
These findings suggest decreased invasiveness of malignant cells
and may explain the abovementioned beneficial anticancer
effect of SkQ1. Reducing the invasiveness by mitochondriatargeted antioxidant SkQ1 is also in consistence with the
finding that ROS-generating mtDNA mutations are responsible
for the metastatic potential of cancer cells (Ishikawa et al.,
2008).
In addition to the direct anticancer effect, SkQ1 may be
useful in combination therapy. Fetisova et al. (2010) have found
that SkQR1, a fluorescent analog of SkQ1 bearing rhodamine19 instead of TPP cation, is a substrate of multidrug resistance
pump P-glycoprotein 170 (Pgp 170). Since Pgp 170 is expressed
in various cancers and correlates with high resistance to
conventional chemotherapy, the accumulation, and ROS
scavenging capacity of SkQ1 in cancer cells positive for Pgp 170
is thus limited. This differential SkQ1 accumulation between
normal cells and cancer cells implies that SkQ1 may be of
potential value in the combination with ROS-elevating
anticancer therapies. It is possible that excessive ROS
generated by these therapies kills cancer cells (Pgp 170
ANTIOXIDANTS FOR ANTICANCER THERAPY
positive) which are not protected (or not protected
sufficiently) by SkQ1; on the contrary, SkQ1 would accumulate
in normal cells (Pgp 170 negative) and provide protection
against ROS-induced damages, thus reducing the side effects.
Concluding Remarks
Mitochondria in malignant cells are different structurally and
functionally from those in normal cells and actively participate
in carcinogenesis and cancer progression. The metabolic
reprogramming of cancer depends largely on the bioenergetic
and biosynthetic functions of mitochondria. Several waves of
gene expression help to define the metabolic profile of cancer
and increase the adaptability of cancer cells to the rapidly
changing microenvironment. Cancer cells can even exploit and
make use of the mitochondria in normal stromal cells. One
major contributor of malfunctioning mitochondria to cancer
development is ROS overproduction, which is most frequently
caused by impaired OXPHOS. The oncogenic mitochondrial
ROS complete the vicious cycle between mitochondria,
genomic instability, and cancer development. These central
roles of mitochondria in cancer make them a promising target
in anticancer therapy. Several strategies have been developed
for selective drug delivery to mitochondria. Among them,
mitochondria-targeted antioxidants are capable of specifically
accumulating in mitochondria and efficiently reducing
mitochondrial oxidative damage. In consistent with the
oncogenic role of ROS, mitochondria-targeted antioxidants,
for example SkQ1, are found to be effective in cancer
prevention and anticancer therapy. A better understanding of
the role of mitochondria in cancer development and the
difference between mitochondria in cancer cells and normal
cells are of great value for developing novel therapeutic targets,
and for improving the activity and selectivity of mitochondriatargeted anticancer agents.
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