Year : 2018 | Volume
: 9 | Issue : 4 | Page : 147--154
Intracellular reactive oxygen species determine cancer stem cell radiosensitivity related to predictive biomarker for radiotherapy
Kaushala Prasad Mishra
Ex-Bhabha Atomic Research Center; Foundation for Education and Research, Mumbai, Maharastra, India
Dr. Kaushala Prasad Mishra
Ex-Bhabha Atomic Research Center; Foundation for Education and Research, Mumbai, Maharastra
Cancer cells display a higher level of reactive oxygen species (ROS) mainly due to increased metabolic activities resulting in altered redox balance. Imbalance in redox arises when the generation of ROS exceeds antioxidants defense system. ROS are generated in cells from multiple pathways, but mitochondria contribute significantly to cellular ROS pool by oxidative phosphorylation. Elevated levels of ROS are implicated in cell transformation, proliferation, and tumorigenesis. ROS-mediated signaling pathways activate pro-oncogenes which regulate cancer progression, angiogenesis, and survival. Normal cells maintain intracellular homeostasis by developing an array of enzymatic antioxidant systems such as catalase, superoxide dismutase, and glutathione peroxidase. Chemotherapy and radiotherapy exert their cytotoxic effects on tumor cells by the generation of excessive ROS. The failure of therapies is attributable to a small fraction of core cells in tumor mass called cancer stem cells (CSCs) which have self-renewal property and exhibit proliferation, differentiation, and resistance to treatments. Both normal and CSCs maintain low-ROS level ascribed to stemness. This review describes role and relevance of ROS in CSC with particular emphasis on developing predictive biomarker for outcome of cancer radiotherapy. It is pointed out that CSCs maintain lower ROS homeostasis and evade cell death by increased level of endogenous antioxidants capacity in cancer cells. Search for regulators of ROS and surface markers in CSC may render them sensitive to radiation offering new and effective strategy for cancer treatment.
|How to cite this article:|
Mishra KP. Intracellular reactive oxygen species determine cancer stem cell radiosensitivity related to predictive biomarker for radiotherapy.J Radiat Cancer Res 2018;9:147-154
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Mishra KP. Intracellular reactive oxygen species determine cancer stem cell radiosensitivity related to predictive biomarker for radiotherapy. J Radiat Cancer Res [serial online] 2018 [cited 2019 May 19 ];9:147-154
Available from: http://www.journalrcr.org/text.asp?2018/9/4/147/253997
Most of the cancer cell types have been found to have a higher level of reactive oxygen species (ROS) than their normal counterparts. ROS has been shown to be associated with cancer development, metastasis, progression, and survival.,,, Under normal physiological conditions, the generation of ROS inside cells is tightly regulated by the ROS scavenging system. Low-to-moderate level of ROS is tightly maintained in normal cells essential for their function and survival. To regulate physiologically acceptable level of ROS, cells have evolved endogenous antioxidant enzymes that can neutralize ROS by directly reacting with and accepting electrons from ROS. When ROS production outpaces its scavenging, an excessive accumulation of ROS occurs, generating redox imbalance giving rise to oxidative stress which produces damaging effects on cellular components such as proteins, lipids, and nucleotides. To counteract this, the cells have armed themselves with a large number and types of endogenous antioxidants that are specific to different species of ROS, which help to prevent pathological levels of ROS and repair oxidative damage to cellular structural molecules.,
Researches in the past decades have shown that dietary supplements and antioxidants impart cells both ROS scavenging ability and exhibit anticancer properties. However, antioxidants may not always be safe to use since excessive intake of antioxidants could lead to adverse health issues. This work attempts to evaluate the role of ROS in cell transformation, proliferation, survival, stemness, and sensitivity to therapies with particular reference to acting as biomarker for prognosis of cancer treatments. Results from our laboratory on the potential of the combined effects of several flavonoids and ionizing radiation in cancer radiotherapy have been summarized in this context.,,,, Furthermore, growing importance of ROS in maintaining stemness of normal and cancer stem cells (CSCs) with relevance to self-renewal, differentiation, and resistance to treatments have been described which may serve as marker for predictive outcome in chemotherapy and radiotherapy of cancer.
Over the years, many therapeutic approaches have been employed targeting intracellular ROS levels which have yielded mixed outcomes. Exposure of tumor cells to radiation and drugs has been found to augment the level of ROS beyond barrier as well as express a variety of new proteins.,, Evidently, analyses of expression profiles of ROS and proteins between radiation sensitive and radiation resistant tumor cells may offer a useful tool for predicting the response to radiotherapy. These lines of research may help developing molecular parameters in diagnosis and treatment of cancer with potential for insights into the progress of individualized molecular therapies of patients.
Recent research results on cancer have provided a new direction on tumor induction, progression, and therapies., It is commonly accepted that a small fraction (<1% of tumor mass) of self-renewing and proliferating cells called CSCs are responsible for recurrence, invasion, and prognosis.,, It is noteworthy that CSCs possess lower level of ROS than in normal CSCs. It is believed that the regulation of intracellular ROS is the intrinsic feature of stem cells giving them survival advantage., Researchers envisage developing new strategies for effective therapies by quantifying, monitoring, and relating ROS level with the expression of surface markers on the CSC populations, both embryonic and adult. It is notable that ROS regulate and maintain cell stemness which determine their survival and death. The resistance of CSC to chemo and radiotherapies is believed to be linked to maintaining low level of ROS which opens new possibilities to identify biomarkers for predicting treatment outcome, as well as target for developing radiation and drug therapies. The role of ROS in regulating stem cell dynamics has implications for various diseases, including cancer and age-related illnesses.
With the advances in genomics and proteomics, there is also increasing information about various ways in which ROS are balanced and control cellular processes. Based on microarray analyses, radioresponse-associated gene expression profiles have been reported in several cancers, including lung, head and neck, and colorectal cancers. There is also increasing evidence that ROS are involved in many distinct levels of biological processes, from gene expression and protein translation to protein–protein interactions, etc. They function in cellular signaling, propagating signals from one tissue to another, and in translating environmental cues into cellular responses to balance cellular inputs, such as nutrients and cytokines with appropriate cellular responses. ROS may function as a crucial tool to coordinate numerous cellular processes and adjust cellular activity to the available bioenergy sources. These studies have identified genes involved with DNA repair, apoptosis, cell cycle, metastasis, and hypoxia. During the disease processes, protein biomarkers are proving indicators of physiological states, and therefore, large-scale protein expression arrays are becoming increasingly popular in screening therapeutic strategies. The comparison of the patterns of protein expression in radioresistant and radiosensitive head-and-neck cancer cells, proteome profiles have been performed in various other cancers, including prostate, breast, rectal, and laryngeal cancers.,
It is reported that changes in oxidation state of stem cells,, might be responsible for the communication between mitochondria and the nucleus., Redox-mediated mitochondria-nucleus crosstalk could explain the coordination of cellular metabolism with chromatin remodeling, gene expression, cell cycling, DNA repair, and cell differentiation. ROS have also been implicated in the aging process, but less is known about whether and how ROS might be involved in the aging of stem cells. Since a slight alterations in ROS content may have profound effects on stem cell survival, and elucidating mechanisms whereby ROS metabolism influences stem cell fate could reveal how stem cell aging relates to age-associated diseases. This review endeavors to focuses on addressing the relevance of ROS to CSC fate and usefulness of ROS in association with CSC surface markers as predictive tool for the outcome of radiotherapy and other treatment modalities.
Generation and Regulation of Cellular Reactive Oxygen Species
Under normal physiological conditions, the generation of ROS is tightly regulated by the ROS scavenging system in cells. ROS scavengers are antioxidant enzymes that can neutralize ROS by directly reacting with and/or accepting electrons from ROS. When ROS production outpaces ROS scavenging, an excessive accumulation of ROS occurs, leading to oxidative stress and producing damaging cellular components, including proteins, lipids, and nucleotides. To counteract this, the cell contains multiple types of antioxidants that are specific to different species of ROS, which helps to prevent pathological levels of ROS and to repair oxidative damage to cellular components. These include superoxide dismutase (SOD), catalase, peroxiredoxins, thioredoxin (TRX), glutathione peroxidase (GPX), and glutathione reductase (GR). Glutathione (GSH), a tripeptide, is one of the most abundant antioxidants synthesized by the cell. Oxidized proteins and H2O2 are reduced by GSH through the glutaredoxin and TRX system. Other key antioxidants include SOD and catalase, which reduce O2− and H2O2, respectively. The subcellular localization of antioxidants at areas of high ROS generation, such as within the mitochondria, may further enhance the efficiency of ROS scavenging.
ROS are generated from the reduction of molecular oxygen and originate at multiple locations and distinct chemical processes. Intracellular ROS exist primarily in three forms: superoxide anions (O2−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH−). The superoxide anion contains an unpaired electron that imparts high reactivity and converts rapidly to H2O2 by the antioxidant enzyme SOD. H2O2 can be further reduced to H2O and O2 by various cellular enzymes and antioxidants.
ROS can be detected intracellularly using a variety of methods, but most ROS assays do not distinguish between different ROS species. Although ROS were originally thought to be merely a harmful byproduct of metabolism, accumulating evidence demonstrate the role for ROS in cell fate., H2O2 is thought to be the main ROS species involved with intracellular signaling, and in specific contexts can act directly as a second messenger. This is due mostly to the longer half-life of H2O2 and its ability to diffuse easily through membranes relative to other types of ROS.
Within the cell, ROS is produced in mitochondria electron transport chain of respiration. The primary role of the electron transport chain is to generate the proton motive force, which leads to ATP production through ATP synthase in a process known as oxidative phosphorylation. However, a tiny fraction of O2 consumed by mitochondria is converted into ROS through leakage of electron flow to O2 mainly through electron transport chain complexes.
ROS levels can be maintained by removing substrates away from oxidative phosphorylation resulting in decreased mitochondrial respiration. In addition, ROS is also generated by the membrane-bound protein NADPH oxidase, which consumes NADPH to generate O2− and subsequently, H2O2.
Reactive Oxygen Species and Molecular Targets
ROS were originally shown to have signaling properties when they were found to act as secondary messengers in growth factor, and oncogenic signaling,, However, not all ROS can be employed in signaling events. Only ROS with a substrate specificity that generates reversible oxidation, such as H2O2, are likely to trigger signaling cascade in in vivo physiological settings.
ROS can signal directly to proteins through amino acid oxidation, the most common reaction being oxidation of cysteine residues. ROS signaling to amino acids can cause functional changes in the range of different proteins, and thus these types of modifications have established ROS as crucial regulators of cellular signaling. Such proteins are known as redox sensors, which directly get modified by ROS and undergo conformational changes as a result of the oxidative modification. These changes influence their function, stability, subcellular localization, and interactions with other proteins.
Redox Sensor Molecules
A growing network of proteins has been shown to act as redox sensor and get modulated by ROS. Many of these redox sensor proteins that are directly modulated by ROS in response to oxidative stress are found to be crucial regulators of stem cell survival. Among these proteins are transcription factors that have been connected to the regulation of cellular antioxidant machinery. These include members of the forkhead box O family, nuclear factor erythroid 2 (NRF2), PR domain containing 16, and the p53 (TRP53) tumor suppressor., Modulations of ROS and p53 activity by TRX-interacting protein may be implicated in hematopoietic stem cell (HSC) function specifically during aging. Other transcription factors, such as nuclear factor κB, mediate the transactivation by ROS of hypoxia-inducible factor 1α (HIF1α). Furthermore, other protein types, such as ATM (ataxia-telangiectasia mutated kinase), p38 mitogen-activated protein kinase, mammalian target of rapamycin (mTOR) and protein kinase B protein kinases, as well as the multifunctional apurinic/apyrimidinic endonuclease1/redox factor 1 protein, PTEN (phosphate and tensin homolog), and sirtuins (specifically SIRT1 and SIRT3) are also considered to be redox sensors.
The polycomb group member BMI1 is another protein that regulates stem cell function, modulates ROS levels and is crucial for mitochondrial function.,, However, whether BMI1 is also directly modulated by ROS or whether BMI1 regulates mitochondria in HSCs remains to be determined.
It is noteworthy that the redox-sensing property of many, if not all, of the proteins discussed above was established in somatic cells and often in cultured lines; whether these reactions also occur in primary stem cells or have a similar outcome remains to be established. As well as a role in redox regulation, ROS might also function to alter the epigenetic landscape, which plays a particularly pertinent role in regulating stem cell fate.,, Many metabolic intermediates are necessary substrates for the posttranslational modifications of histones that together establish the epigenetic landscape of stem cells. As the activity of glycolysis and oxidative phosphorylation can directly influence ROS, leading to changes in the concentrations of various metabolic intermediates might represent a potential mechanism of ROS-mediated epigenetic regulation, albeit indirect. For example, acetylation of the lysine tails of histones cannot occur without acetyl CoA, the TCA cycle metabolite, while deacetylation by sirtuin proteins (SIRTs) requires activation by nicotinamide adenine dinucleotide.
Reactive Oxygen Species in Cancer Signaling
ROS are involved in each stage of carcinogenesis such as initiation, promotion, and progression. It is observed that cancer cells produce more ROS than normal cells due to various mechanisms. The increased levels of intracellular ROS, due to its reactivity, can cause damage to DNA, lipids, carbohydrates, and proteins. In fact, researchers have exploited increased ROS in cancer cells as an effective approach to selectively kill cancer cells without causing significant toxicity to normal cells., The intrinsic mechanisms of intracellular ROS increase may arise from accelerated metabolism, oncogene activation, tumor suppressor gene inactivation, and mitochondrial dysfunction. The increase in ROS was shown directly due to Bcr-Abl, as it could be blocked by the small-molecule tyrosine kinase inhibitor ST1571. Increased ROS seems possible through activation of the PI3K/mTOR pathway because inhibition of PI3K or mTOR attenuates the Bcr-Abl-induced ROS. Likewise, the activation of c-Myc was able to increase ROS without the induction of apoptosis, while the treatment with antioxidant NAC decreased the number of c-Myc-induced hMre11 signals and improved cell survival after c-Myc activation. Furthermore, tumor suppressor gene p53 decreases ROS by upregulating the transcription of antioxidant protein TIGAR, p53-induced glycolysis and apoptosis regulator. It is further found that splenocytes and thymocytes of p53−/− mice exhibited elevated level of ROS than in wild-type mice. ROS is involved in signaling mechanism, tumorigenesis process and response to radiation and chemotherapies.
Reactive Oxygen Species Drive Cell Survival/cell Death
Cellular metabolism is the balance of catabolic and anabolic processes that involve the chemical conversion of carbon substrates to generate energy in the form of ATP and to produce macromolecular precursors in the form of nucleotides, lipids, and amino acids (anabolic). The balance of catabolic and anabolic processes can shift cellular process depending on the task being executed. Cellular processes such as growth and proliferation require mostly anabolic processes to generate building blocks for DNA, protein, and membranes.
One of the major pathways by which metabolism can influence signaling pathways is through alterations of ROS levels. In turn, ROS can directly react with various proteins, such as kinases, phosphatases, or transcription factors, to alter processes that regulate cell cycle progression, apoptosis, quiescence, or differentiation., Furthermore, ROS can directly modify metabolic enzymes or proteins that participate in nutrient-sensing pathways to direct the metabolic processes.
Reactive Oxygen Species in Self Renewal and Stemness
After the identification of CSC in leukemia, they have been frequently identified and isolated in a number of solid tumors., The stem cells, whether normal or malignant, display self-renewal, and differentiation properties. The observed heterogeneity in cancer tissue such as morphology, cell surface antigens, and gene expression suggests that tumor treatment may depend on effective CSCs elimination. It is generally recognized that stemness of cancer cells may form a vital target for new treatment strategy. Stemness of cancer cells has low level of ROS and show resistance to therapies which warrants deeper studies for developing new strategy for achieving improved clinical outcome in treatment settings. The focus of research is progressing to manipulate intracellular ROS and antioxidant levels for establishing a useful predictive protocol in cancer chemotherapy and radiotherapy.
Hypoxia, Reactive Oxygen Species, and Surface Markers in Cancer Stem Cell
Solid tumors possess hypoxic niche due to poor vasculature and resulting low oxygen level promotes tumor growth and favors survival of aggressive malignant cells. Hypoxia in neoplastic mass is considered a prognostic indicator of poor treatment outcome and contributes to developing metastasis and cancer progression., In many human cancers, under hypoxic conditions, the hypoxia-inducible factor, (HIF-1α), is found overexpressed which dimerizes with HIF-1 β and translocates into the nucleus. Thus, HIF-1 activates transcription of genes involved in the mechanisms of cancer biology, such as angiogenesis, cell survival, glucose metabolism, and invasiveness offering a target for a selective cancer therapy. Several genes associated with the hypoxic response in normal cells, such as Glut1, Serpin B9, and vascular endothelial growth factor, are upregulated in CSCs. In addition, hypoxia induces the expression of Sox2 and Oct4 genes relevant in stem cell function. Recently, it has been shown that Sox2 with Sox4 play a vital role in the maintenance of stemness in CSCs.
Hypoxia has been demonstrated to induce epithelial-mesenchymal transition (EMT), which promotes invasion and metastasis of cancer cells. During EMT, epithelial cells undergo several biochemical alterations that allow the acquisition of the mesenchymal phenotype enabling cancer cells to evade and colonize at remote locations. EMT-inducers, including transforming growth factor-β and hypoxia, trigger changes in gene expression by complex signaling pathways. It is found that an early event of EMT is the increased expression of the mesenchymal marker, vimentin, and the transcriptional downregulation of E-cadherin. Several cancer reports have demonstrated that EMT is involved in generating cells with properties of stem cells. This suggests that hypoxia-induced EMT may affect CSCs or induce stem-like cells from more differentiated progenitors determining an increase of CSC population responsible for early systemic cancer dissemination and metastasis formation. Studies have shown a functional connection in low-oxygen tension, ROS production, and EMT. ROS can favor EMT but antioxidants can attenuate hypoxia-induced EMT and metastasis dissemination in cancer cells, the maintenance of low ROS levels is crucial to preserve CSC self-renewal and stemness.
It is notable that, in contrast to cancer cells in which ROS levels are increased, CSCs generally maintain low ROS, exhibiting redox patterns that are similar to the corresponding normal stem cells. It is reported that ROS levels are lower in human and murine breast CSCs compared to non-stem breast cancer cells and that the pharmacological depletion of ROS scavengers in CSCs markedly decreases their clonogenicity and shows radiosensitivity. A pioneering study showed that it is possible to use CD34+/CD38− cell surface markers to separate cells into AML CSCs and non-CSCs populations that differ in their self-renewal capacity in vitro and tumorigenicity in vivo. In the CNS, CD133 is believed to be the marker for CSCs in different types of brain tumor. The subpopulation of CD133+ tumor cells is enriched after radiation in gliomas. Gastrointestinal CSCs with a high level of CD44 expression have shown an enhanced capacity of reduced GSH synthesis and defense against ROS by the activation of cystine-glutamate exchange transporter. Therefore, since EMT is a reversible and redox-dependent phenomenon, it is likely that ROS could also stimulate mesenchymal–epithelial transition regulating differentiation of CSCs toward non-stem cancer cells.
Oxidative stress caused by the cellular accumulation of ROS is intrinsically detrimental to CSCs, which have evolved antioxidant systems to protect against ROS increase. Therefore, understanding of redox regulation mechanisms is essential to develop rational therapies that specifically target CSCs.
Cellular Status of Reactive Oxygen Species and Radiosensitivity
Elevated levels of ROS, alteration in redox balance, and deregulated redox signaling are common hallmarks of cancer progression and resistance to treatment. Mitochondria contribute mainly in the generation of ROS during oxidative phosphorylation. Cells maintain intracellular homeostasis by developing an immense antioxidant system including catalase, SOD, and GPX. Besides these enzymes exist an important antioxidant GSH and transcription factor NRF2 which contribute in balancing oxidative stress. ROS–mediated signaling pathways activate pro-oncogenic signaling which eases in cancer progression, angiogenesis, and survival. Concomitantly, to maintain ROS homeostasis and evade cancer cell death, an increased level of antioxidant capacity is associated with cancer cells giving a clue to the importance of antioxidant-based therapies.
Reactive Oxygen Species, Apoptosis, and Signaling
Stem cells are characterized by their ability to self-renew and their multipotent differentiation capacity. Until recently, the focus in stem cell biology has been on the adverse effects of ROS, particularly the damaging effects of ROS accumulation on tissue aging and the development of cancer, and various anti-oxidative and anti-stress mechanisms of stem cells. However, it has become increasingly clear that, in CSCs, redox status plays an important role in stem cell maintenance through regulation of the cell cycle. In contrast to cancer cells, in which ROS levels are increased, most CSCs maintain low ROS levels, exhibiting redox patterns that are similar to the corresponding normal stem cell. To fully elucidate the mechanisms involved in stem cell maintenance and to effectively target CSCs, it remains essential to understand ROS regulatory mechanisms in different cell types, for example, in normal stem cells, cancer cells, and CSCs.
Predictive Biomarker of Radiotherapy
In a variety of human tumors of different histological origins, including glioma, prostate, cervical, breast and pancreatic cancer, Hsp90 and Her-3 proteins have been established as determinants of radiosensitivity. Notably, RR-H460 cell lines exhibited CSC characteristics, displaying several aberrant CSC markers, such as CD44, activated Notch1, Nanog, Sox-2, and β-Catenin. Available results suggest a great potential for careful analysis of ROS level and surface makers in CSC which may prove unique and reliable predictive markers of prognosis of cancer patients in radiotherapy. It appears promising approach to control and enhances intracellular ROS levels in CSCs by oxidative stressors shifting the redox balance from survival advantage to death susceptible frame making them more sensitive to radiation treatment. It remains to be investigated if the expression of CSC surface markers and their quantitative estimation bears relation with the cytosolic ROS. It is seen that overexpression of β-catenin promotes CSC properties and tumorigenesis both in vitro and in vivo, suggesting an important role for the Wnt/β-catenin pathway in the regulation of CSC self-renewal. Therefore, developing specific regulators for inducting CSCs dysfunction may yield highly novel approach to overcome their resistance to chemo- and radiotherapy for providing effective cancer treatment strategy. It is considered a promising strategy to eliminate CSC after radiotherapy for developing effective cancer treatment.
Cancer cells face the challenges of oxidative stress. ROS are known to be involved in initiation and progression of tumors. Cancer cells normally adapt to persistent oxidative stress by regulating redox response. The role of ROS in cancer cell survival, proliferation as well as targets for chemotherapy and radiotherapy have been highlighted. It is clarified that conventional cancer therapies result in a transient reduction in tumor mass by killing non-stem cancer cells but fail to eliminate CSCs. Results have shown role of hypoxia and ROS deregulation in CSC establishment and propagation. CSCs maintain low ROS level and show resistance to chemo and radiotherapy. A general agreement exists on the role of CSCs on metastases and their colonization at secondary sites. In CSCs, such adaptation is potentiated by increased antioxidant mechanisms and resulting in resistance to anticancer therapies. It is suggested that low ROS, surface markers, and sensor proteins in CSC may prove predictive in outcome of cytotoxic drug and cancer radiotherapy.
We apologize to authors whose papers we could not cite due to space limitations. I tried sincerely to refer others work liberally, but I ask for excuse if there are lapses.
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|1||Storz P. Reactive oxygen species in tumor progression. Front Biosci 2005;10:1881-96.|
|2||Glasauer A, Chandel NS. Targeting antioxidants for cancer therapy. Biochem Pharmacol 2014;92:90-101.|
|3||D'autréaux B, Toledano MB. ROS as signalling molecules: mechanisms that generate specificity in ROS homeostasis. Nat Rev Mol Cell Biol 2007;8:813-24.|
|4||Roberts RA, Smith RA, Safe S, Szabo C, Tjalkens RB, Robertson FM, et al. Toxicological and pathophysiological roles of reactive oxygen and nitrogen species. Toxicology 2010;276:85-94.|
|5||Liou GY, Storz P. Reactive oxygen species in cancer. Free Radic Res 2010;44:479-96.|
|6||Kumar B, Kumar A, Pandey BN, Hazra B, Mishra KP. Increased cytotoxicity by the combination of radiation and diospyrin diethylether in fibrosarcoma in culture and in tumor. Int J Radiat Biol 2008;84:429-40.|
|7||Kumar B, Kumar A, Ghosh S, Pandey BN, Mishra KP, Hazra B, et al. Diospyrin derivative, an anticancer quinonoid, regulates apoptosis at endoplasmic reticulum as well as mitochondria by modulating cytosolic calcium in human breast carcinoma cells. Biochem Biophys Res Commun 2012;417:903-9.|
|8||Dayal R, Singh A, Pandey A, Mishra KP. Reactive oxygen species as mediator of tumor radiosensitivity. J Cancer Res Ther 2014;10:811-8.|
|9||Tiwari P, Mishra KP. Silibinin in cancer therapy: A promising prospect. Cancer Res Front 2015;1:303-18.|
|10||Ahire V, Mishra KP, Kulkarni GR. Ellagic acid: A potent radio-sensitizer in cancer radiotherapy. Cancer Res Frontiers 2016;2:141-55.|
|11||Tong L, Chuang CC, Wu S, Zuo L. Reactive oxygen species in redox cancer therapy. Cancer Lett 2015;367:18-25.|
|12||Yang Y, Karakhanova S, Werner J, Bazhin AV. Reactive oxygen species in cancer biology and anticancer therapy. Curr Med Chem 2013;20:3677-92.|
|13||Zhang J, Wang X, Vikash V, Ye Q, Wu D, Liu Y, et al. ROS and ROS-mediated cellular signaling. Oxid Med Cell Longev 2016;2016:4350965.|
|14||Shi X, Zhang Y, Zheng J, Pan J. Reactive oxygen species in cancer stem cells. Antioxid Redox Signal 2012;16:1215-28.|
|15||Diehn M, Cho RW, Lobo NA, Kalisky T, Dorie MJ, Kulp AN, et al. Association of reactive oxygen species levels and radioresistance in cancer stem cells. Nature 2009;458:780-3.|
|16||Bigarella CL, Liang R, Ghaffari S. Stem cells and the impact of ROS signaling. Development 2014;141:4206-18.|
|17||Brieger K, Schiavone S, Miller FJ Jr., Krause KH. Reactive oxygen species: From health to disease. Swiss Med Wkly 2012;142:w13659.|
|18||Dickinson BC, Chang CJ. Chemistry and biology of reactive oxygen species in signaling or stress responses. Nat Chem Biol 2011;7:504-11.|
|19||Circu ML, Aw TY. Reactive oxygen species, cellular redox systems, and apoptosis. Free Radic Biol Med 2010;48:749-62.|
|20||Raza MH, Siraj S, Arshad A, Waheed U, Aldakheel F, Alduraywish S, et al. ROS-modulated therapeutic approaches in cancer treatment. J Cancer Res Clin Oncol 2017;143:1789-809.|
|21||Delaunay-Moisan A, Appenzeller-Herzog C. The antioxidant machinery of the endoplasmic reticulum: Protection and signaling. Free Radic Biol Med 2015;83:341-51.|
|22||Mita AC, Mita MM, Nawrocki ST, Giles FJ. Survivin: Key regulator of mitosis and apoptosis and novel target for cancer therapeutics. Clin Cancer Res 2008;14:5000-5.|
|23||Holmström KM, Finkel T. Cellular mechanisms and physiological consequences of redox-dependent signalling. Nat Rev Mol Cell Biol 2014;15:411-21.|
|24||Liang R, Ghaffari S. Stem cells, redox signaling, and stem cell aging. Antioxid Redox Signal 2014;20:1902-16.|
|25||Hernández-García D, Wood CD, Castro-Obregón S, Covarrubias L. Reactive oxygen species: A radical role in development? Free Radic Biol Med 2010;49:130-43.|
|26||Bao S, Wu Q, McLendon RE, Hao Y, Shi Q, Hjelmeland AB, et al. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 2006;444:756-60.|
|27||Yun HS, Baek JH, Yim JH, Um HD, Park JK, Song JY, et al. Radiotherapy diagnostic biomarkers in radioresistant human H460 lung cancer stem-like cells. Cancer Biol Ther 2016;17:208-18.|
|28||Powell S, McMillan TJ. DNA damage and repair following treatment with ionizing radiation. Radiother Oncol 1990;19:95-108.|
|29||Jang YY, Sharkis SJ. A low level of reactive oxygen species selects for primitive hematopoietic stem cells that may reside in the low-oxygenic niche. Blood 2007;110:3056-63.|
|30||Beachy PA, Karhadkar SS, Berman DM. Tissue repair and stem cell renewal in carcinogenesis. Nature 2004;432:324-31.|
|31||Eyler CE, Rich JN. Survival of the fittest: Cancer stem cells in therapeutic resistance and angiogenesis. J Clin Oncol 2008;26:2839-45.|
|32||Jonathan EC, Bernhard EJ, McKenna WG. How does radiation kill cells? Curr Opin Chem Biol 1999;3:77-83.|
|33||Riley PA. Free radicals in biology: Oxidative stress and the effects of ionizing radiation. Int J Radiat Biol 1994;65:27-33.|
|34||Redza-Dutordoir M, Averill-Bates DA. Activation of apoptosis signalling pathways by reactive oxygen species. Biochim Biophys Acta 2016;1863:2977-92.|
|35||Beckman KB, Ames BN. The free radical theory of aging matures. Physiol Rev 1998;78:547-81.|
|36||Ito K, Hirao A, Arai F, Takubo K, Matsuoka S, Miyamoto K, et al. Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells. Nat Med 2006;12:446-51.|
|37||Huang P, Feng L, Oldham EA, Keating MJ, Plunkett W. Superoxide dismutase as a target for the selective killing of cancer cells. Nature 2000;407:390-5.|
|38||Halliwell B. Free radicals and antioxidants: Updating a personal view. Nutr Rev 2012;70:257-65.|
|39||Murphy MP. How mitochondria produce reactive oxygen species. Biochem J 2009;417:1-3.|
|40||Ghezzi P, Jaquet V, Marcucci F, Schmidt HH. The oxidative stress theory of disease: Levels of evidence and epistemological aspects. Br J Pharmacol 2017;174:1784-96.|
|41||Tahara EB, Navarete FD, Kowaltowski AJ. Tissue-, substrate-, and site-specific characteristics of mitochondrial reactive oxygen species generation. Free Radic Biol Med 2009;46:1283-97.|
|42||Chandel NS, Maltepe E, Goldwasser E, Mathieu CE, Simon MC, Schumacker PT, et al. Mitochondrial reactive oxygen species trigger hypoxia-induced transcription. Proc Natl Acad Sci U S A 1998;95:11715-20.|
|43||Janssen-Heininger YM, Mossman BT, Heintz NH, Forman HJ, Kalyanaraman B, Finkel T, et al. Redox-based regulation of signal transduction: Principles, pitfalls, and promises. Free Radic Biol Med 2008;45:1-7.|
|44||Salmeen A, Andersen JN, Myers MP, Meng TC, Hinks JA, Tonks NK, et al. Redox regulation of protein tyrosine phosphatase 1B involves a sulphenyl-amide intermediate. Nature 2003;423:769-73.|
|45||Lee JM. Inhibition of p53-dependent apoptosis by the KIT tyrosine kinase: Regulation of mitochondrial permeability transition and reactive oxygen species generation. Oncogene 1998;17:1653-62.|
|46||Yalcin S, Zhang X, Luciano JP, Mungamuri SK, Marinkovic D, Vercherat C, et al. Foxo3 is essential for the regulation of ataxia telangiectasia mutated and oxidative stress-mediated homeostasis of hematopoietic stem cells. J Biol Chem 2008;283:25692-705.|
|47||Bonello S, Zähringer C, BelAiba RS, Djordjevic T, Hess J, Michiels C, et al. Reactive oxygen species activate the HIF-1alpha promoter via a functional NFkappaB site. Arterioscler Thromb Vasc Biol 2007;27:755-61.|
|48||Lessard J, Sauvageau G. Bmi-1 determines the proliferative capacity of normal and leukaemic stem cells. Nature 2003;423:255-60.|
|49||Liu J, Cao L, Chen J, Song S, Lee IH, Quijano C, et al. Bmi1 regulates mitochondrial function and the DNA damage response pathway. Nature 2009;459:387-92.|
|50||Brunelle JK, Bell EL, Quesada NM, Vercauteren K, Tiranti V, Zeviani M, et al. Oxygen sensing requires mitochondrial ROS but not oxidative phosphorylation. Cell Metab 2005;1:409-14.|
|51||Challen GA, Sun D, Jeong M, Luo M, Jelinek J, Berg JS, et al. Dnmt3a is essential for hematopoietic stem cell differentiation. Nat Genet 2011;44:23-31.|
|52||Han MK, Song EK, Guo Y, Ou X, Mantel C, Broxmeyer HE, et al. SIRT1 regulates apoptosis and nanog expression in mouse embryonic stem cells by controlling p53 subcellular localization. Cell Stem Cell 2008;2:241-51.|
|53||Chuikov S, Levi BP, Smith ML, Morrison SJ. Prdm16 promotes stem cell maintenance in multiple tissues, partly by regulating oxidative stress. Nat Cell Biol 2010;12:999-1006.|
|54||Valko M, Leibfritz D, Moncol J, Cronin MT, Mazur M, Telser J, et al. Free radicals and antioxidants in normal physiological functions and human disease. Int J Biochem Cell Biol 2007;39:44-84.|
|55||Trachootham D, Alexandre J, Huang P. Targeting cancer cells by ROS-mediated mechanisms: A radical therapeutic approach? Nat Rev Drug Discov 2009;8:579-91.|
|56||Abbott A. Stem cells: The cell division. Nature 2011;480:310-2.|
|57||Dean M, Fojo T, Bates S. Tumour stem cells and drug resistance. Nat Rev Cancer 2005;5:275-84.|
|58||Franco R, Sánchez-Olea R, Reyes-Reyes EM, Panayiotidis MI. Environmental toxicity, oxidative stress and apoptosis: Ménage à trois. Mutat Res 2009;674:3-22.|
|59||Vlashi E, Pajonk F. Cancer stem cells, cancer cell plasticity and radiation therapy. Semin Cancer Biol 2015;31:28-35.|
|60||Velu CS, Niture SK, Doneanu CE, Pattabiraman N, Srivenugopal KS. Human p53 is inhibited by glutathionylation of cysteines present in the proximal DNA-binding domain during oxidative stress. Biochemistry 2007;46:7765-80.|
|61||Dansen TB, Smits LM, van Triest MH, de Keizer PL, van Leenen D, Koerkamp MG, et al. Redox-sensitive cysteines bridge p300/CBP-mediated acetylation and foxO4 activity. Nat Chem Biol 2009;5:664-72.|
|62||Baccelli I, Trumpp A. The evolving concept of cancer and metastasis stem cells. J Cell Biol 2012;198:281-93.|
|63||Lapidot T, Sirard C, Vormoor J, Murdoch B, Hoang T, Caceres-Cortes J, et al. Acell initiating human acute myeloid leukaemia after transplantation into SCID mice. Nature 1994;367:645-8.|
|64||Phillips TM, McBride WH, Pajonk F. The response of CD24(-/low)/CD44+breast cancer-initiating cells to radiation. J Natl Cancer Inst 2006;98:1777-85.|
|65||Gupta SC, Hevia D, Patchva S, Park B, Koh W, Aggarwal BB, et al. Upsides and downsides of reactive oxygen species for cancer: The roles of reactive oxygen species in tumorigenesis, prevention, and therapy. Antioxid Redox Signal 2012;16:1295-322.|
|66||Frijhoff J, Winyard PG, Zarkovic N, Davies SS, Stocker R, Cheng D, et al. Clinical relevance of biomarkers of oxidative stress. Antioxid Redox Signal 2015;23:1144-70.|
|67||Talks KL, Turley H, Gatter KC, Maxwell PH, Pugh CW, Ratcliffe PJ, et al. The expression and distribution of the hypoxia-inducible factors HIF-1alpha and HIF-2alpha in normal human tissues, cancers, and tumor-associated macrophages. Am J Pathol 2000;157:411-21.|
|68||Cannito S, Novo E, Compagnone A, Valfrè di Bonzo L, Busletta C, Zamara E, et al. Redox mechanisms switch on hypoxia-dependent epithelial-mesenchymal transition in cancer cells. Carcinogenesis 2008;29:2267-78.|
|69||Hollier BG, Evans K, Mani SA. The epithelial-to-mesenchymal transition and cancer stem cells: A coalition against cancer therapies. J Mammary Gland Biol Neoplasia 2009;14:29-43.|
|70||Giannoni E, Parri M, Chiarugi P. EMT and oxidative stress: A bidirectional interplay affecting tumor malignancy. Antioxid Redox Signal 2012;16:1248-63.|
|71||Mohyeldin A, Garzón-Muvdi T, Quiñones-Hinojosa A. Oxygen in stem cell biology: A critical component of the stem cell niche. Cell Stem Cell 2010;7:150-61.|
|72||Deacon J, Peckham MJ, Steel GG. The radioresponsiveness of human tumours and the initial slope of the cell survival curve. Radiother Oncol 1984;2:317-23.|
|73||Cheng JX, Liu BL, Zhang X. How powerful is CD133 as a cancer stem cell marker in brain tumors? Cancer Treat Rev 2009;35:403-8.|
|74||McCord AM, Jamal M, Shankavaram UT, Lang FF, Camphausen K, Tofilon PJ, et al. Physiologic oxygen concentration enhances the stem-like properties of CD133+human glioblastoma cells in vitro. Mol Cancer Res 2009;7:489-97.|
|75||Jin L, Hope KJ, Zhai Q, Smadja-Joffe F, Dick JE. Targeting of CD44 eradicates human acute myeloid leukemic stem cells. Nat Med 2006;12:1167-74.|