Hydrogen peroxide – production, fate and role in redox signaling of tumor cells
Hydrogen peroxide (H2O2) is involved in various signal transduction pathways and cell fate decisions. The mechanism of the so called “redox signaling” includes the H2O2-mediated reversible oxidation of redox sensitive cysteine residues in enzymes and transcription factors thereby altering their activities. Depending on its intracellular concentration and localization, H2O2 exhibits either pro- or anti-apoptotic activities. In comparison to normal cells, cancer cells are characterized by an increased H2O2 production rate and an impaired redox balance thereby affecting the microenvironment as well as the anti-tumoral immune response. This article reviews the current knowledge about the intracellular production of H2O2 along with redox signaling pathways mediating either the growth or apoptosis of tumor cells. In addition it will be discussed how the targeting of H2O2-linked sources and/or signaling components involved in tumor progression and survival might lead to novel therapeutic targets.
KeywordsReactive Oxygen Species Vascular Endothelial Growth Factor Reactive Oxygen Species Level Increase Reactive Oxygen Species Level Redox Sensor
AMP-activated protein kinase
Protein kinase B
Anti-oxidant response element
Apoptosis signal-regulating kinase 1
Ataxia telangiectasia mutated
Extracellular signal regulated kinase
Tumor necrosis factor receptor superfamily member 6
Glutathione S transferase
Hypoxia inducible factor
High-mobility group box 1 protein
c-Jun amino-terminal kinase
Kelch-like ECH-associated protein 1
Mitogen-activated protein kinase
Mammalian target of rapamycin
Nuclear factor-erythroid 2 p45-related factor 2
cGMP-dependent protein kinase
Pyruvate kinase M2
Phosphatase and tensin homologue deleted on chromosome 10
Protein tyrosine phosphatase
Reactive oxygen species
Sentrin/SUMO-specific protease 3
Tumor necrosis factor alpha
Thioredoxin interacting protein
Vascular endothelial growth factor
Vascular endothelial growth factor receptor 2
Hydrogen peroxide (H2O2) is next to the superoxide anion and hydroxyl radical a key member of the class of reactive oxygen species (ROS), which are in particular generated via the respiratory chain cascade but also as byproducts of the cellular metabolism including protein folding. In contrast to the superoxide anion and hydroxyl radical, the less reactive H2O2 is involved in many physiological processes such as hypoxic signal transduction, cell differentiation and proliferation but also plays a role in mediating immune responses. However, it exerts its effects depending on the cellular context, its local concentration as well as its exposure time [1, 2]. Thus H2O2 is no more considered as an unwanted rather toxic byproduct, but plays an important role in the control of vital cellular processes.
Tumor cells are characterized by an enhanced metabolic activity resulting in changes of the cellular redox state that has to handle the production of high levels of ROS . In many cancer cells persistently upregulated H2O2-dependent signaling pathways are involved in cell differentiation, growth and survival, yet high levels of H2O2 can also induce cell cycle arrest or apoptosis in cells. Due to this dual functionality of H2O2 robust cellular anti-oxidative systems are thought to be essential for maintaining the cellular redox homeostasis. Several defense systems against oxidative stress have been shown to be upregulated in cancer cells via the transcription factor nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) . These include the thioredoxin/thioredoxin reductase (Trx/TrxR) system, peroxiredoxins (Prxs) and several glutathione S-transferases (GSTs), which are involved in mediating the cellular redox homeostasis, but still allow redox modifications of specific redox-sensitive proteins thereby triggering redox signaling events. In this review we will address how (i) cellular H2O2 is produced and how it regulates certain signaling pathways, (ii) tumor cells cope with enhanced H2O2 levels to escape from oxidative stress, (iii) potential redox-sensors might be correlated with tumorigenesis, and how (iv) H2O2-modulated processes/pathways might be used as therapeutic targets.
Sources of H2O2
Transport and subcellular localization of hydrogen peroxide
In comparison to water, H2O2 possesses a reduced membrane permeability, which is influenced by the phosphorylation and glycosylation states of membrane proteins, the lipid composition (lipid rafts) and osmotic stretching of lipid bilayers [10, 11, 12, 13, 14, 15, 16]. Aquaporin (AQP) 8, but not the classical AQP1 facilitates the transport of H2O2 across membranes [17, 18]. Treatment of AQP3-overexpressing HeLa cells with H2O2 resulted in an enhanced phosphorylation of protein kinase B (AKT) , while overexpression of AQP8 increased intracellular H2O2 levels in leukemia cells in the presence of H2O2. Moreover, vascular endothelial growth factor (VEGF) signaling results in increased intracellular H2O2 levels, which can be reduced by silencing AQP8 . Furthermore, silencing of AQP8 can inhibit the epidermal growth factor (EGF) mediated stimulation of tyrosine kinases. . Thus, AQPs not only play important roles in the diffusion of H2O2 across membranes, but also on downstream signaling cascades. Furthermore, H2O2 detoxifying enzymes, such as glutathione peroxidases (GPxs), catalases and Prxs, can lead to rapidly decreasing intracellular H2O2 concentrations  thereby establishing the formation of H2O2 gradients resulting in selective and localized H2O2 signaling events. The inactivation of scavenger enzymes by H2O2 represents a mechanism that allows the selective enrichment (“flooding”) of a cellular area by H2O2 thereby promoting the H2O2-mediated oxidation of specific thiols within target proteins at this site [22, 23].
Features of H2O2 – second messenger like characteristics and principles of redox modifications
The anti-oxidative response – factors that maintain redox signaling
Nrf2 targets and their correlation to cancer
relation to cancer
proto-oncogene expressed in many cancers 
elevated serum levels in patients with ovarian cancer 
overexpression in HCC, CRC, liver metastasis  and
in breast cancer 
overexpressed in many cancer cells and cell lines of distinct histology 
thioredoxin interacting protein
negatively interferes with bladder carcinogenesis 
tumor suppressor in thyroid cancer 
promotion of lung cancer progression 
related to tumor angiogenesis in pancreatic cancer 
oncogenic role in skin tumorigenesis 
overexpression in squamous cell carcinoma 
promotion of lung cancer progression 
Transcription factor Nrf2 as regulator of the anti-oxidative response
Targets of Nrf2
Prxs represent members of the so called thiol-based anti-oxidant system  that act as redox switches to modulate homeostasis . As important H2O2 scavenging enzymes Prxs are involved in the anti-oxidative response and in the regulation of redox-dependent signaling pathways by converting H2O2 into water [52, 53]. In mammals, the family of Prxs consists of 6 members located either in the cytosol (Prx1, Prx2, Prx4, Prx5, Prx6), mitochondria (Prx3, Prx5) or in other cellular compartments (Prx1, nucleus; Prx2, membrane; Prx4, Golgi apparatus, extracellular space, endoplasmic reticulum; Prx5, peroxisomes) [9, 54]. Prxs are upregulated under conditions of oxidative stress [55, 56, 57] and it could be shown that Prx1 and Prx6 are direct targets of Nrf2 [58, 59]. Prx1 – Prx5 are 2-Cys-Prx and utilize Trx as electron donor for their catalytic activity, while Prx6 is a 1-Cys-Prx and depends on GSH instead of Trx for its reduction [54, 60]. The hyper-oxidation of 2-Cys Prx, in particular of Prx1, adds further chaperone function to these Prxs, but depends on certain motif elements downstream of the peroxidatic cysteine residue (GGLG and YF motifs) [23, 61]. The chaperone function is based on the formation of stack like higher molecular weight complexes, thereby preventing the denaturation of proteins from external stresses like heat shock or oxidative stress. This multimeric complex can be subsequently dissolved into low molecular weight species by Srx . Whereas in some species more distant cysteine residues might act as redox sensors, human Prxs are known to gain such a chaperone function only after the peroxidatic cysteine is hyper-oxidized . At the transcriptional level Nrf2 and to some degree also focal adhesion kinase (FAK) have been demonstrated to activate the expression of Prxs [62, 63]. However, there is also evidence that modifications at the post-translational level have an impact on the function of Prxs. For example nitrosylation of the tyrosine residue within the YF motif of Prx2 plays a crucial role in the regulation of disulfide bond formation under oxidative stress conditions resulting in a more active and robust peroxidase . In addition, its glutathionylation may affect its localization to the extracellular compartment, along with Trx, thereby inducing TNFα production leading to an oxidative stress-dependent inflammatory reaction . For Prx3 the complex formation of FoxO3a with the peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1 alpha) is enhanced by sirtuin-1 (SirT1), which is similar to the regulation of other anti-oxidant proteins . The Prx4, which is mainly expressed in the endoplasmic reticulum compartment can be enhanced at the post-transcriptional level by calpain . Due to its high susceptibility to undergo hyperoxidation even at low levels of oxidative stress its chaperone function is frequently involved in the oxidative folding of various ER resident proteins, likely in cooperation with protein disulfide isomerase (PDI) . There is also evidence that Prx4 in addition to Srx plays a crucial role in enhancing RAS-RAF-MEK signaling to control cancer cell proliferation and metastasis formation .
Srxs reduce double oxidized catalytic cysteine (sulfinic acid) residues of 2-Cys-Prxs  thereby restoring their peroxidase function [32, 71]. Based on studies in yeast, the rate constant for the reduction of oxidized Prx by Trx (about 106 M−1 s−1) is much faster than the rate of reduction of hyperoxidized Prx by Srx [72, 73]. Thus, the reduction of hyperoxidized Prx by Srx might be considered as a rate limiting step. Moreover Srxs are involved in deglutathionylation processes  and can regulate the chaperone function of Prx1 by controlling its glutathionylation levels at position cysteine 83 . In contrast to its anti-oxidant function, which is highly specific for Prxs, the deglutathionylation activity of Srx appears much less restricted . The Srx promoter contains a sequence resembling the consensus sequence for ARE, which is important for its regulation . In response to cigarette smoke and under hypoxic conditions, Srx expression is transcriptionally controlled in a Nrf2-dependent manner [77, 78]. By using overexpression and knock out model systems it has been demonstrated that upon treatment with the chemopreventive Nrf2 inducer 3H-1,2-dithiole-3-thione (D3T) the expression of Srx is upregulated and thus prevents the double oxidation of Prx in neurons . Moreover, hyperoxia has been shown to induce the degradation of mitochondrial double oxidized Prx3 in Nrf2-deficient, but not in WT mice. Thus, in the absence of Srx hyperoxidized Prx becomes susceptible to proteolysis . In addition, the disparate resistance of colon carcinoma cells to ROS has been linked to higher basal levels of Nrf2 and Srx as well as to their distinct cellular localizations [56, 80].
Thioredoxin / thioredoxin reductase / TXNIP system
The glutathione (GSH) system is a major thiol-based defense system against oxidative and electrophilic stress in mammals and functions as co-substrate for the GPxs, which efficiently remove H2O2 thereby preventing oxidative insults and influencing together with glutaredoxin (Grx) the redox state of proteins via reversible S-glutathionylation . Thus GSH plays an important role in redox-signaling and in the regulation of protein functions. In addition key enzymes of the GSH biosynthesis can be upregulated by Nrf2 .
The specific role of H2O2 in cancer
Correlation of redox-sensitive proteins with neoplastic transformation
Redox-sensitive proteins involved in the regulation of cell metabolism, angiogenesis and cell death
anti-oxidative defense-related redox sensors
metabolism-related redox sensors
angiogenesis-related redox sensors
C243 and/or C274
cell death-related redox sensors
aggregation, FasL binding 
induction of apoptosis 
Redox control of the cellular energy metabolism with the relation to cellular growth
In addition to oncogenic mutations and signaling pathways  the AMPK activity can be suppressed by oxidation of cysteine residues within the catalytic subunit alpha at positions 130 and 174 promoting its aggregation. In contrast, the reduction of these sites is required for the successful activation of the AMPK complex during energy starvation, which is mediated by Trx thereby providing evidence that oxidative stress and metabolism can be linked via AMPK . Furthermore, AMPK can function as a sensor of genomic stress and interacts/enhances the DNA damage response by interaction with the serine/threonine protein kinase ATM  a redox sensor for the regulation of DNA repair processes. Under physiologic conditions ATM is recruited and activated by DNA double-strand breaks (DSBs) via the formation of MRE11-Rad50-Nibrin (MRN) DNA repair complexes. This results in the phosphorylation of various key proteins involved in DNA repair processes, such as p53, the serine/threonine-protein kinase Chk2 (CHK2) and the histone H2AX (H2AX) [134, 135, 136, 137]. In the presence of H2O2 ATM forms a disulfide-cross-linked dimer resulting in its direct activation independent from the MRN complex formation thereby supporting its redox sensor function . Furthermore, ATM is involved in the regulation of mitochondrial function and metabolic control by interaction with p53, AMPK, mTOR and HIF1α [139, 140, 141], which is independent of DSBs . In addition, the redox status of tumors functions as a major determinant of the ATM-dependent molecular switch of resistance to apoptosis. At low ROS levels apoptosis was blocked, whereas increased cellular ROS levels restored ATM/JNK-mediated apoptotic signaling . There is also evidence that pathological neoangiogenesis requires ATM-mediated oxidative defense, since agents promoting excessive ROS generation have beneficial effects in the treatment of neovascular diseases . Not only AMPK, but also the pyruvate kinase isoform M2 (PKM2), known to be over-expressed in tumors , represents a switch between glycolysis and gluconeogenesis. Inhibition of PKM2 caused by oxidative modification of the cysteine residue at position 358  contributes to maintain cellular anti-oxidant responses by diverting the glucose flux into the pentose phosphate pathway thereby generating sufficient reducing potential for the detoxification of ROS .
Redox control of cellular signaling processes in association with angiogenesis and cell death
ROS, which are generated in response to various stimuli including growth factors, have been shown to modulate cellular growth and angiogenesis. A major source for ROS are NOX enzymes that can be activated by various growth factors, e.g. vascular endothelial growth factor (VEGF) and angiopoietin-1, leading to the induction of genes involved in angiogenesis and thus represent therapeutic targets for the inhibition of tumor angiogenesis . H2O2 derived from NOX activities can affect the vascular endothelial growth factor receptor (VEGFR) 2, which regulates angiogenesis, vascular development, vascular permeability and embryonic hematopoiesis, but also promotes cell proliferation, survival, migration, and differentiation of vascular endothelial cells. Despite VEGFR1 and VEGFR2 can bind VEGFA, VEGFR2 plays the major role in modulating these processes. Its activation depends not only on the autophosphorylation of defined tyrosine residues, but is also regulated by oxidative modifications [147, 148]. Increased cellular H2O2 levels promote the formation of an intracellular disulfide bond thereby blocking the receptor activity, whereas the presence of Prx2 effectively prevents this oxidative modification leaving the receptor responsive to VEGFA stimulation [147, 148]. Furthermore, extracellular H2O2 generated by extracellular SOD promotes VEGFR2 signaling via oxidative inactivation of protein tyrosine phosphatases (PTPs) in mice . Moreover, the expression of TXNIP is required for the VEGF-mediated VEGFR2 activation and angiogenic response in vivo and in vitro by regulating VEGFR2 phosphorylation via S-glutathionylation of the low molecular weight protein tyrosine phosphatase (LMW-PTP) in endothelial cells . In addition the interaction of TXNIP with the poly-ADP-ribose polymerase 1 (PARP1) is a relevant regulator for its translocalization and function leading to the activation of VEFGR2 signaling in human umbilical vein endothelial cells . Furthermore, H2O2 was shown to induce the expression levels of the VEGFR2 ligand VEGF by inducing the transcription factors NFκB or AP-1 . Under hypoxic conditions VEGF expression is upregulated by HIF1α which is over-expressed in many tumors and its activity levels influence angiogenesis as well as tumorigenesis . Under normoxic conditions HIF1α is hydroxylated and subsequently ubiquitinated for proteasomal degradation, whereas under hypoxic conditions its hydroxylation is blocked leading to its accumulation, dimerization with its beta subunit and subsequent translocation into the nucleus, where it regulates the expression of genes linked to cellular transformation, cell proliferation and angiogenesis [154, 155, 156]. The transcriptional activity of HIF1α depends on the translocation of sentrin/SUMO-specific protease 3 (SENP3) from the nucleoli to the nucleoplasm . ROS seem to be involved in limiting its proteasomal degradation. The complex formation with either the heat shock protein 90 (Hsp90) or the co-chaperone/ubiquitin ligase carboxyl terminus of Hsc70-interacting protein (CHIP) leads to the stabilization or degradation of SENP3. Under mild oxidative stress the oxidation of thiol residues favors the recruitment of Hsp90 thereby protecting SENP3 from binding to CHIP, which results in its ubiquitination and subsequent elimination via proteasomal degradation. Thus, the redox status of SENP3 is a decisive factor for its stabilization or degradation  and can regulate the expression of the EMT-inducing transcription factor fork head box C2 (FOXC2) that is de-SUMOylated and thereby activated in response to increased ROS levels. As a result the expression of the mesenchymal marker protein N-cadherin is induced . In HeLa cells ROS levels are involved in the activation of HIF1α by modifying cysteine residues at positions 243 and 532 of SENP3 thereby controlling the interaction of SENP3 with p300, the co-activator of HIF1α. This is accompanied by SUMOylation of p300 resulting in the transcriptional silencing of HIF1α. The shift of HIF1α transactivation by ROS depends on the biphasic redox sensing of SENP3. Whereas low ROS levels lead to SENP3 accumulation and therefore enhanced HIF1α transcriptional activity, high concentrations of ROS inactivated SENP3 resulting in the suppression of HIF1α transcriptional activity. Thus SENP3 is an example for a redox sensitive protein with cysteine residues that can sense different ROS levels [160, 161]. VEGF can also promote endothelial permeability through the activation of the Src family non-receptor tyrosine kinases (SFKs) . Lyn, a member of the SFK family, has been shown to be amplified and upregulated in tumor cells, which is associated with resistance to chemotherapy  and plays an important role in the regulation of both innate and adaptive anti-tumoral immune responses. Since NOX-expressing tumors are able to efficiently produce H2O2, the tumor stroma can mimic features of ‘unhealed’ wounds . Using distinct model systems, extracellular H2O2 levels have been linked to the recruitment of leukocytes, such as neutrophils, representing the first line of innate immune responses [165, 166, 167]. In addition, Lyn serves as a redox sensor for neutrophils monitoring the redox state of wounds. The oxidation-specific modification site was defined as cysteine residue 466, which directly triggered the wound response and calcium signaling [168, 169]. In response to treatment with chromium (V) complexes the formation of ROS and activation of Lyn were found in lymphocytes leading to the activation of caspase-3 and subsequently to the induction of apoptosis . Another kinase with redox-sensor function and involvement in angiogenesis is the cGMP-dependent protein kinase (PKG). PKG represents a member of a serine/threonine-specific protein kinase family that acts as a key mediator of the nitric oxide (NO)/cGMP signaling pathway. GMP binding has been shown to activate PKG resulting in the phosphorylation of serine and threonine residues on many cellular proteins  involved in modulating cellular calcium. Besides this activation mechanism it is also known that PKG can be activated under oxidative stress independent of the respective cGMP or NO levels . PKG controls the regulation of platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression and the feedback of the NO-signaling pathway amongst others. While the expression of PKG in metastatic colon carcinoma blocks tumor angiogenesis by down-regulating the expression level of beta-catenin , PKG signaling can also mediate cytoprotective and anti-apoptotic function in various tissues including non-small-cell lung carcinoma. Thus, PKG inhibitors might be of therapeutic relevance and have been suggest for treatment in combination with cisplatin chemotherapy of solid tumors . PKG-inhibitors limit the migration and invasion capacity of colorectal carcinoma cells . Moreover, pro-apoptotic effects of PKG signaling have been reported for various colon carcinoma as well as breast cancer cell lines, which is in line with the hypothesis that the loss of PKG expression in colon carcinoma cell lines may contribute their resistance to undergo anoikis [176, 177].
Redox control of cellular signaling processes in association with apoptosis
By acting as a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK) ASK1 can activate two distinct sets of MAPKK. Whereas the tumor necrosis factor alpha (TNF-α)-mediated activation of MKK4 (SEK1) via its downstream target JNK leads to the induction of apoptotic cell death, the activation of MKK6 activates p38 subgroups of MAPK, which phosphorylate a wide range of potential targets in response to inflammatory cytokines and cellular stress. A key role in the ASK1-mediated induction of apoptosis via MKK is its dimer formation, known to be induced by exposure to H2O2, but blocked by Trx supporting its role as a redox sensor. Moreover, the interaction of ASK1 and Trx is based on the formation of a disulfide bond at the N-terminal domain of ASK1 leading to its ubiquitination and subsequent proteasomal degradation. However, high levels of H2O2 caused a loss of the protective function of Trx due to the formation of an intramolecular disulfide bond resulting in its release from ASK1, which is accompanied by its activation [178, 179]. Furthermore, the selective inhibition of TrxR by the drug MC3 or by electrophilic pollutants leads to the induction of apoptosis via the Trx-ASK1-p39 signal cascade by blocking the interaction of Trx with ASK1 [180, 181]. In addition, redox alterations induced by selective inhibition of the glucose metabolism leading to massive oxidative stress might serve as a molecular switch that activates the ASK1-JNK/p38 MAPK signaling pathways accompanied by promotion of the radiosensitization of malignant cells . Similar effects have been reported in response to treatment with iron chelators, which also resulted in reduced ASK1-Trx complex formation . The genetic inhibition of ASK1 resulted not only in the inhibition of JNK activation, but also in decreased expression of Fas ligand (FasL) and subsequent apoptosis, whereas the inhibition of p38 did not alter the FasL expression . The activation of Fas upon ligand engagement leads to the formation of a death-inducing signaling complex accompanied by caspase 8-mediated apoptosis . The Fas/FasL interaction results in the S-glutathionylation of Fas at cysteine residue 294 , which not only increases the binding to its ligand, but also its aggregation and recruitment into lipid rafts. This oxidative modification can be linked to the activity of Grx1 , since the depletion of Grx1 results in an increased S-glutathionylation rate along with the induction of apoptosis, while Grx1 overexpression causes opposite effects. The level of oxidative stress mediated by exogenous sources or endogenously generated upon receptor stimulation regulates the sensitivity to Fas-mediated apoptosis . Additionally FOXO4, a TF involved in the regulation of the insulin signaling pathway, can be activated by oxidative stress due to the formation of an intermolecular disulfide bond between cysteine residue 477 and histone acetyltransferase p300 resulting in the formation of a covalently linked heterodimer. The redox modification of FOXO4 is essential for its subsequent CREB-binding protein (CBP)-mediated acetylation . However, the activity of the heterodimeric complex is regulated by the Trx system, which has a strong impact on the turnover of this interaction by reducing the cysteine-dependent heterodimer of FOXO4 and p300 thereby providing evidence that Trx might be a key regulator of ROS-dependent FOXO4 signaling . In addition, the efficient nuclear translocation and subsequent activation of FOXO4 in response to ROS depends on disulfide formation with the nuclear import receptor transportin-1 (TNPO1), whereas its insulin signaling-dependent nuclear shuttling is not dependent on TNPO1 . Although high-mobility group box 1 protein (HMGB1) might act as a redox-sensitive switch between autophagy and apoptosis. HMGB1 is a DNA-binding protein that associates with chromatin, but can also bind single stranded DNA linking the assembly of transcriptional active protein complexes on specific targets. Its reduced form interacts with the receptor for advanced glycation end products (RAGE) thereby inducing beclin1-dependent autophagy . In the presence of higher ROS levels HMGB1 can undergo oxidative modification leading to the formation of a disulfide bond between cysteine residues 23 and 45 , which induces apoptosis via the intrinsic pathway . When released in its partially oxidized status, HMGB1 functions as a pro-inflammatory cytokine , whereas in its fully oxidized form (sulfonylated) all biologic activities are lost. Furthermore, HMGB1 can be released from both activated and dying cells thereby acting as a damage-associated molecular pattern molecule . However, its biochemical and immunological properties depend on both its cellular localization as well as its release mechanism . Due to different intracellular and extracellular functions HMGB1 is a central mediator in inflammation and immunity, but its activity depends on the state of its redox-sensitive cysteine residues at positions 23, 45 and 106 ranging from DNA binding, to induction of chemotaxis and transcription of chemokines [197, 198] suggesting its classification as an “alarmin” for sepsis and cancer . Different diseases, such as cancer, are often accompanied by T cell hyporesponsiveness, which is mediated by ROS. The release of H2O2 produced by tumor-infiltrating macrophages leads to the suppression of potentially tumor reactive T cells . Cofilin (CFL), a member of the actin-depolymerizing factor protein family, binds to F-actin and plays an important role in the regulation of the actin cytoskeleton dynamics as well as in the mitochondrial apoptosis. Its translocation from the cytoplasm into the mitochondria leads to cytochrome c release and activation of caspase signaling, thus representing an early step in the induction of apoptosis [201, 202]. Since CFL is also associated with invasion and metastatic capacity of tumors [203, 204, 205, 206], it is a key therapeutic target for tumors . CFL might function as a redox sensor  and its dephosphorylation-dependent glutathionylation [209, 210] not only leads to a loss of its actin binding affinity, but also blocks its translocation to the mitochondria thereby preventing apoptosis induction. The oxidation-mediated inactivation of CFL can also provoke T cell hyporesponsiveness or the necrotic-like programmed cell death, which modulates the T cell activation processes including the duration of the effectors phase . In contrast, knockdown of CFL could protect T cells from fatal effects of long-term oxidative stress  suggesting that oxidation and mitochondrial localization of CFL represents a check point for necrotic-like cell death. Therefore the oxidation of CFL might provide a molecular explanation for the T cell hyporesponsiveness found in diseases such as cancer under oxidative stress conditions .
Components of redox regulating processes as therapeutic targets
Current therapeutic strategies for the treatment of cancer patients to trigger ROS-mediated cell death
leukemia, solid cancer, melanoma 
inhibition of GSH biosynthesis
lung cancer 
prooxidant, cysteine depletion
metastatic colorectal cancer 
TrxR inhibitor, superoxide formation
glioblastoma multiform 
inhibitor of mitochondrial chain, GPx and TrxR
+ GSH depletion
However, the interaction between different anti-oxidant molecules in distinct tumor models requests further analysis to increase the insights of the underlying molecular mechanisms of these interactions and the identification of additional molecular targets for cancer therapy. In addition, a better understanding of the role of the intracellular redox state balance and the redox-regulated signaling cascades might enhance the therapeutic options for the treatment of various human cancer types.
Many cancer cells are characterized by an increased intrinsic formation of ROS as a result of their malignant transformation process. Yet, they have to adapt to this challenge in order to maintain the capacity for tumor progression. ROS, in particular H2O2, play an important role in facilitating both cell proliferation and cell survival of tumor cells by triggering the redox signaling cascades. New therapeutic approaches are currently developed that aim towards altering the tumor cell redox state, including (i) the selective inhibition of cellular ROS sources [222, 223], e.g. NOX, (ii) the hyperactivation of anti-oxidant enzymes to lower intracellular ROS levels and (iii) the modulation of the anti-oxidant response system towards increasing ROS levels thereby further promoting the induction of apoptosis. So far, the underlying molecular mechanisms of the interactions between different redox signaling compounds and the tumor progression processes are not fully understood. In addition, there is still a need to define additional redox sensors. Therefore, further research is required to gain additional insights into these signaling networks and sensors, which then might lead to the identification and subsequent design of novel targeted therapies for the treatment of cancer patients.
This work was funded/supported by an interdisciplinary DFG grant (grant numbers: MU3275/3-1, WE1467/13-1 and LI1527/3-1).
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