Changes in intracellular redox status influence multidrug resistance in gastric adenocarcinoma cells

Antioxidants status and physiological responses to early and late heat stress in two rabbit breeds

Early life heat stress negatively affects rabbit production and well-being. However, the physiological response to acute heat stress in later life is not clearly defined. The present study aims to investigate the effects of early and late heat stress at 36°C on some blood constituents, antioxidant enzymes activity in the blood, and muscle in New Zealand white and Baladi Black rabbits. A total of sixty post-weaning rabbits of each breed were randomly divided into two groups; control groups (NZWC and BBC) and early heat-stressed groups for six hours at 36 ± 1°C and 62% relative humidity (RH) (NZWT and BBT groups). After heat stress, six rabbits from each group were slaughtered for blood and muscle tissue collection. The surviving rabbits were kept at 28 ± 1°C and 40% RH till 13 weeks of age. At the end of 13 weeks, all rabbits were exposed to late heat stress as precious described to perform four groups: single late stressed groups; NZWC2, BBC2, and double stressed groups; NZWT2 and BBT2. After late heat stress, six rabbits from each group were slaughtered for blood and muscle tissue collection. The early and late heat stress caused a significant reduction in the blood creatine kinase, lactate dehydrogenase, and high-density lipoprotein and antioxidant enzymes' activity in blood and muscle of both NZW and BB rabbits compared with the control groups. While, the blood total cholesterol, triglycerides, total lipids levels, and lipid peroxidation activity in blood and muscle were significantly increased due to the early and late heat-stressed both breeds compared with the control groups. It could be concluded that the early heat stress at 36°C has negative effects on several physiological indicators and antioxidant activities in the blood and muscle of NZW and BB rabbits.

Oxidative stress in cancer therapy: Friend or enemy?

Excessive cellular oxidative stress is widely perceived as a key factor in pathophysiological conditions and cancer development. Healthy cells use several mechanisms to maintain intracellular levels of reactive oxygen species (ROS) and overall redox homeostasis to avoid damage to DNA, proteins, and lipids. Cancer cells, in contrast, exhibit elevated ROS levels and upregulated protective antioxidant pathways. Counterintuitively, such elevated oxidative stress and enhanced antioxidant defence mechanisms in cancer cells provide a therapeutic opportunity for the development of drugs with different anticancer mechanisms of action (MoA). In this review, oxidative stress and the role of ROS in cells are described. The tumour-suppressive and tumour-promotive functions of ROS are discussed to compare these two different therapeutic strategies (increasing or decreasing ROS to fight cancer). Clinically approved drugs with demonstrated oxidative stress anticancer MoAs are highlighted before describing examples of metal-based anticancer drug candidates causing oxidative stress in cancer cells via novel MoAs.

Metal-Organic Framework Derived Multicomponent Nanoagent as a Reactive Oxygen Species Amplifier for Enhanced Photodynamic Therapy

Intracellular antioxidants such as glutathione (GSH) play a critical role in protecting malignant tumor cells from apoptosis induced by reactive oxygen species (ROS) and in mechanisms of multidrug and radiation resistance. Herein, we rationally design two multicomponent self-assembled photodynamic therapy (PDT) nanoagents, that is, Glup-MFi-c and Glud-MFo-c, which consist of respective GSH-passivation and GSH-depletion linkers in metal-organic frameworks encapsulated with photosensitizers for a deeply comprehensive understanding of GSH-based tumor PDT. Multicomponent coordination, π-π stacking, and electrostatic interactions among metal ions, photosensitizers, and bridging linkers under the protection of a biocompatible polymer generate homogeneous nanoparticles with satisfied size, good colloid stability, and ultrahigh loading capacity. Compared to the GSH-passivated Glup-MFi-c, the GSH-depleted Glud-MFo-c shows pH-responsive release of photosensitizer and [Fe^III(CN)₆] linker in tumor cells to efficiently deplete intracellular GSH, thus amplifying the cell-killing efficiency of ROS and suppressing the tumor growth in vivo. This study demonstrates that Glud-MFo-c acts as a ROS amplifier, providing a useful strategy to deeply understand the role of GSH in combating cancer.

Study of the controversial resveratrol that interact with the endogenous glutathione thiyl radical in cancer cells

Found in various natural food products, many in vitro evidence indicated that resveratrol (RES) has been linked to neuroprotective and cardioprotective effects and prevent cancer development. However, human clinical trials have been conducted with varying results, making the usage of RES controversial. In this paper, we demonstrated that the drug RES could be conjugated with the high levels of endogenous GS^• in cancer cells. 5,5-Dimethyl-1-Pyrroline-N-Oxide (DMPO) was employed to capture the GS^•. The molecular mechanism of the reaction between RES and GS^• was further studied by UV-Vis spectrometry, mass spectrometry and Density Functional Theory (DFT) calculations. Besides, the formation of the adduct GS-RES in cancer cell was obtained when RES was added during incubation. Further study indicated that over 77.6% of the RES was consumed in cancer cells. This study suggested that endogenous GS^• may be one of the important factors to cause the depletion of anti-tumour drugs during chemotherapy, which should be paid special attention in clinical therapeutics and drug development.

EGCG regulates CTR1 expression through its pro-oxidative property in non-small-cell lung cancer cells

Copper transporter 1 (CTR1) plays an important role in increasing cisplatin intake. Our previous studies showed that CTR1 expression was upregulated by (-)-epigallocatechin-3-gallate (EGCG), a green tea polyphenol, therefore enhanced cisplatin sensitivity in ovary cancer and non-small-cell lung cancer (NSCLC) cells. In the current study in the non-small-cell lung cancer cells, we uncovered a potential mechanism of EGCG-induced CTR1 through its pro-oxidative property. We found that EGCG increased reactive oxygen species (ROS) generation, while in the presence of ROS scavenger N-acetyl-cysteine (NAC), ROS production was eliminated. Changes of CTR1 expression were consistent with the ROS level. Simultaneously, EGCG downregulated ERK1/2 while upregulated lncRNA nuclear paraspeckle assembly transcript 1 (NEAT1) through ROS to induce CTR1 expression. Besides, in a nude mouse xenografts model, EGCG treatment raised ROS level, expression of CTR1 and NEAT1 in tumor tissue. Also, ERK1/2 and p-ERK1/2 were suppressed as well. Taken together, these results suggested a novel mechanism that EGCG mediated ROS to regulate CTR1 expression through the ERK1/2/NEAT1 signaling pathway, which provided more possibilities for EGCG as a natural agent in adjuvant therapy of lung cancer.

Dynamic supraparticles for the treatment of age-related diseases

Age-related diseases (ARDs) are arising as a major threat to public health in our fast-aging society. Current development of nanomedicine has sparked much optimism toward ARDs management by improving drug delivery and controlled drug release. However, effective treatments for ARDs, such as cancer and Alzheimer's diseases (AD), are still lacking, due to the complicated pathological features of ARDs including multifactorial pathogenesis, intricate disease microenvironment, and dynamic symptom manifestation. Recently, dynamic supraparticles (DS), which are reversibly self-assembled functional nanoparticles, have provided a novel strategy for combating ARDs. Besides the intrinsic advantages of nanomedicine including multifunctional and multitarget, DS are capable of dynamic structural reconfiguration upon certain stimulation, creating another layer of maneuverability that allows programmed response to the spatiotemporal alterations of ARDs during progression and treatment. In this review, we will overview the challenges faced by ARDs management, and discuss the unique opportunities brought by DS. Then, we will summarize the designed synthesis of DS for ARDs treatment. Finally, we will dissect the therapeutic targets in ARDs that can be exploited by DS, and present the encouraging advances in this field. Hopefully, this review will bridge our knowledge of the design principle of DS and ARDs management, which may inspire the future development of potent theranostic agents to improve the healthcare.

Prognostic significance of 8-hydroxy-2'-deoxyguanosine in solid tumors: a meta-analysis

Background

High level of reactive oxygen species (ROS) has been detected in almost all cancers, which make it become one of the best-characterized phenotypes in cancers. Though ROS plays an important role in tumors, the degree of oxidative stress can be better evaluated by assessing stable metabolites of oxidative reactions because of its high instability. 8-hydroxy-2′-deoxyguanosine (8-OHdG), a product of oxidative damage to 2′-deoxyguanosine, is known as a useful marker for assessing oxidative DNA damage and has been a feature of carcinogenesis in several researches. But the exact prognostic value of 8-OHdG expression in patients with cancer is still unclear.

Methods

A comprehensive search was performed in PubMed, Web of Science, EMBASE. Eligible studies were included based on defined exclusion and inclusion criteria to perform a meta-analysis. STATA 14.0 was used to estimate pooled hazard ratios (HRs) with 95% confidence interval (95% CI), the heterogeneity among studies and publication bias to judge the prognostic value.

Results

A total of 2121 patients from 21 eligible studies were included in the meta-analysis. A significant association was found between elevated 8-OHdG expression and poor OS (overall survival) in cancer patients (pooled HR 1.921, 95% CI: 1.437–2.570); In the subgroup analysis, race of sample, cancer types, detection method of 8-OHdG, sample classification, detection location of 8-OHdG and paper quality (score more or less than 7) did not alter the association between 8-OHdG expression and cancer prognosis. Furthermore, 8-OHdG expression was an independent prognostic marker for overall survival in patients with cancer (pooled HR 2.110, 95% CI: 1.482–3.005) using Cox multivariate analyses.

Conclusions

This meta-analysis found that highly expressed 8-OHdG in tumor tissues may be a predictor of prognosis in most solid tumors. However, especially in breast cancer, low 8-OHdG expression is associated with poor prognosis, which is partly because of the increased antioxidant mechanisms in breast cancer tissues. This study demonstrates for the first time that 8-OHdG expression is associated with the prognosis of cancer patients. In the future, whether the expression level of 8-OHdG can be used as a biomarker for the prognosis of all human cancers requires more research.

Exogenous glutathione contributes to cisplatin resistance in lung cancer A549 cells

BACKGROUND

Recent studies have reported that an elevated intracellular glutathione (GSH) level is associated with resistance of non-small cell lung cancer (NSCLC) cell lines to cisplatin (CDDP). It is well-known that GSH is widely used in the clinic as a hepatoprotective agent. However, whether exogenous GSH can affect the sensitivity of NSCLC cells to CDDP remains unclear. The aim of this study is to evaluate the role of exogenous GSH in the resistance of A549 cells to CDDP.

METHODS

The effect of GSH and CDDP on the proliferation of A549 cells was analyzed by MTT assay. Subsequent experiments were conducted in A549 cells divided into four groups: control group (untreated cells), GSH group (treated with 120 μg/ml GSH for 48 h), CDDP group (treated with 10 μg/ml CDDP for 48 h) and CDDP+GSH group (treated with 10 μg/ml CDDP+120 μg/ml GSH for 48 h). Apoptosis was detected by flow cytometry. Light microscopy, fluorescence microscopy and electron microscopy were performed to study morphologic and ultrastructural differences among the four groups of cells. Intracellular GSH level and γ-GCS expression were determined by immunohistochemistry (IHC). Cellular platinum uptake was assessed by inductively coupled plasma mass spectrometry (ICP-MS). Quantitative RT-PCR analysis was performed to measure the expression of caspase3, caspase9, bax, bcl-2 and MDR-1. Western blot analysis was conducted to examine the protein levels of GST-π, MRP-1 and P-gp.

RESULTS

Growth inhibition and apoptosis were reduced in A549 cells in the CDDP+GSH group compared to those in the CDDP group 48 h post-treatment. Alterations in cellular morphology and ultrastructure, as well as typical characteristics of apoptosis, were observed. Intracellular GSH and γ-GCS levels were elevated by exogenous administration of GSH; in contrast, cellular platinum concentration fell rapidly. Relative to the CDDP group, the CDDP+GSH group exhibited 47.92%, 47.82% and 63.75% downregulation in caspase3, caspase9 and bax mRNA expression, respectively, and a 2.17-fold increase in bcl-2 mRNA level. In addition, there were 1.58-fold and 2.67-fold increases in the level of GST-π and MRP-1, respectively; however, the changes in MDR-1 and P-gp levels were not statistically significant.

CONCLUSIONS

Our data demonstrated that exogenous GSH used as hepatinica in the clinic could induce resistance of A549 cells to CDDP by inhibiting apoptosis, elevating cellular GSH levels, inactivating the mitochondria-mediated signaling pathway, and increasing the expression of GST-π, γ-GCS and MRP1 to increase CDDP efflux.

Impaired pentose phosphate pathway in the development of 3D MCF-7 cells mediated intracellular redox disturbance and multi-cellular resistance without drug induction

Although metabolic reprogramming and redox imbalance are widely reported to be involved in chemo-resistance in cancer treatment, much more attention was paid to anti-cancer drug induced effect. Our previous studies showed that cancer cells can develop P-gp overexpression-mediated intrinsic drug resistance in the formation of 3D MCF-7 multi-cellular layers (MCLs) without any drug induction. However, whether metabolic reprogramming and redox imbalance functioned during this progress remained unrevealed. In our present study, LC-Q/TOF-MS and GC-MS were used in combination for analysing intracellular metabolites. The contribution of pentose phosphate pathway (PPP) and its related redox status were checked by chemical interfering and silencing/over-expression of glucose-6-phosphate dehydrogenase (G6PD). The downstream products of G6PD were assayed by quantitative real-time PCR, western blot and flow cytometry. Results showed that not only G6PD expression but also G6PD activity was significantly lowered along with 3D MCF-7 cells culture time. Impaired PPP disturbed redox-cycling, generated reactive oxygen species (ROS), which triggered cell cycle arrest and caused the switch to Chk2/p53/NF-κB pathway-mediated P-gp induction. Our results provided a new attempt to associate intrinsic small molecule metabolites (impaired PPP) communicating with cell signalling pathways through disturbed intracellular redox status to elucidate multi-cellular resistance (MCR) in 3D MCF-7 cells, which improved the understanding of the mechanisms of P-gp up-regulation in MCR with metabolomic and related redox status support.

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Impaired pentose phosphate pathway (PPP) in the development of 3D MCF-7 cells.

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PPP mediated intracellular redox disturbance and multi-cellular resistance (MCR).

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P-gp up-regulation in MCR with metabolomics and related redox status support.

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Intrinsic small molecule metabolites communicating with cell signalling pathway.

An ultrasensitive fluorogenic probe for revealing the role of glutathione in chemotherapy resistance

Unveiling the detailed roles of glutathione (GSH) in chemoresistance necessitates a reliable assay for its detection in intact live specimens. Herein, by taking advantage of the susceptibility of electron-poor C_sp² -S_sufinyl bond to GSH nucleophilic attack, we developed a naphthalimide-sulfoxide based fluorogenic probe (Na-8) applicable for tracking endogenous GSH fluctuation in live cells. Na-8 features a high degree of sensitivity towards GSH as demonstrated by its utmost 2200-fold fluorogenic response. As a proof of concept, Na-8 has been applied to image GSH in liver cancer HepG2 cells with the normal L02 cell counterparts serving as a control, and elevated GSH level was observed in HepG2 in contrast to L02. Further experiments showed that this elevated GSH level was involved in doxorubicin-resistance but not in cisplatin-resistance. Noteworthy, monitoring GSH change in HepG2 and L02 cells in response to doxorubicin treatment revealed that while normal cells showed a burst of GSH in adaption to doxorubicin treatment, no significant change was detected in HepG2 cells, suggesting that HepG2 cells have been preconditioned by their intrinsic oxidative stress which confers drug-resistance. Given the observed sensitivity and spatiotemporal resolution, Na-8 should hold promise for future study on the detailed roles of GSH in drug-resistance.

Attenuation of Antioxidant Capacity in Human Breast Cancer Cells by Carbon Monoxide through Inhibition of Cystathionine β-Synthase Activity: Implications in Chemotherapeutic Drug Sensitivity

Drug resistance is a major impediment to effective treatment of breast cancer. Compared to normal cells, cancer cells have an increased antioxidant potential due to an increased ratio of reduced to oxidized glutathione (GSH/GSSG). This is known to confer therapeutic resistance. Here, we have identified a mechanism, unique to breast cancer cells, whereby cystathionine β-synthase (CBS) promotes elevated GSH/GSSG. Lentiviral silencing of CBS in human breast cancer cells attenuated GSH/GSSG, total GSH, nuclear factor erythroid 2-related factor 2 (Nrf2), and processes downstream of Nrf2 that promote GSH synthesis and regeneration of GSH from GSSG. Carbon monoxide (CO) reduced GSH/GSSG in three breast cancer cell lines by inhibiting CBS. Furthermore, CO sensitized breast cancer cells to doxorubicin. These results provide insight into mechanism(s) by which CBS increases the antioxidant potential and the ability for CO to inhibit CBS activity to alter redox homeostasis in breast cancer, increasing sensitivity to a chemotherapeutic.

Targeting Free Radicals in Oxidative Stress-Related Human Diseases

Cancer and Alzheimer's disease (AD) are characterized by (i) opposing biological mechanisms, (ii) an inverse correlation between their incidences, and (iii) oxidative stress being a common denominator of both diseases. Increased formation of reactive oxygen species (ROS) in cancer cells from oncogenic signaling and/or metabolic disturbances leads to upregulation of cellular antioxidant capacity to maintain ROS levels below a toxic threshold. Combining drugs that induce high levels of ROS with compounds that suppress cellular antioxidant capacity by depleting antioxidant systems [glutathione (GSH), superoxide dismutase (SOD), and thioredoxin (TRX)] and/or targeting glucose metabolism represents a potential anticancer strategy. In AD, free metals and/or Aβ:metal complexes may cause damage to biomolecules in the brain (via Fenton reaction), including DNA. Metal chelation, based on the application of selective metal chelators or metal delivery, may induce neuroprotective signaling and represents a promising therapeutic strategy. This review examines therapeutic strategies based on the modulation of oxidative stress in cancer and AD.

Identification of the metabolic alterations associated with the multidrug resistant phenotype in cancer and their intercellular transfer mediated by extracellular vesicles

Multidrug resistance (MDR) is a serious obstacle to efficient cancer treatment. Overexpression of P-glycoprotein (P-gp) plays a significant role in MDR. Recent studies proved that targeting cellular metabolism could sensitize MDR cells. In addition, metabolic alterations could affect the extracellular vesicles (EVs) cargo and release. This study aimed to: i) identify metabolic alterations in P-gp overexpressing cells that could be involved in the development of MDR and, ii) identify a potential role for the EVs in the acquisition of the MDR. Two different pairs of MDR and their drug-sensitive counterpart cancer cell lines were used. Our results showed that MDR (P-gp overexpressing) cells have a different metabolic profile from their drug-sensitive counterparts, demonstrating decreases in the pentose phosphate pathway and oxidative phosphorylation rate; increases in glutathione metabolism and glycolysis; and alterations in the methionine/S-adenosylmethionine pathway. Remarkably, EVs from MDR cells were capable of stimulating a metabolic switch in the drug-sensitive cancer cells, towards a MDR phenotype. In conclusion, obtained results contribute to the growing knowledge about metabolic alterations in MDR cells and the role of EVs in the intercellular transfer of MDR. The specific metabolic alterations identified in this study may be further developed as targets for overcoming MDR.

Autophagy inhibitor chloroquine increases sensitivity to cisplatin in QBC939 cholangiocarcinoma cells by mitochondrial ROS

The tumor cells have some metabolic characteristics of the original tissues, and the metabolism of the tumor cells is closely related to autophagy. However, the mechanism of autophagy and metabolism in chemotherapeutic drug resistance is still poorly understood. In this study, we investigated the role and mechanism of autophagy and glucose metabolism in chemotherapeutic drug resistance by using cholangiocarcinoma QBC939 cells with primary cisplatin resistance and hepatocellular carcinoma HepG2 cells. We found that QBC939 cells with cisplatin resistance had a higher capacity for glucose uptake, consumption, and lactic acid generation, and higher activity of the pentose phosphate pathway compared with HepG2 cells, and the activity of PPP was further increased after cisplatin treatment in QBC939 cells. It is suggested that there are some differences in the metabolism of glucose in hepatocellular carcinoma and cholangiocarcinoma cells, and the activation of PPP pathway may be related to the drug resistance. Through the detection of autophagy substrates p62 and LC3, found that QBC939 cells have a higher flow of autophagy, autophagy inhibitor chloroquine can significantly increase the sensitivity of cisplatin in cholangiocarcinoma cells compared with hepatocellular carcinoma HepG2 cells. The mechanism may be related to the inhibition of QBC939 cells with higher activity of the PPP, the key enzyme G6PDH, which reduces the antioxidant capacity of cells and increases intracellular ROS, especially mitochondrial ROS. Therefore, we hypothesized that autophagy and the oxidative stress resistance mediated by glucose metabolism may be one of the causes of cisplatin resistance in cholangiocarcinoma cells. It is suggested that according to the metabolism characteristics of tumor cells, inhibition of autophagy lysosome pathway with chloroquine may be a new route for therapeutic agents against cholangiocarcinoma.

MicroRNA-218 Increases the Sensitivity of Bladder Cancer to Cisplatin by Targeting Glut1

Background/Aims: MicroRNA-218 (miR-218) is down-regulated in many malignancies that have been implicated in the regulation of diverse processes in cancer cells. However, the involvement of miR-218 in chemo-sensitivity to cisplatin and the precise mechanism of this action remained unknown in bladder cancer. Methods: qRT-PCR was used to detect miR-218 and its target Glut1 expression in bladder cancer cell lines T24 and EJ. CCK-8 method was utilized to measure the cell viability. IC 50 was calculated via a probit regression model. Glut1 was detected by western blotting for analysis of potential mechanism. Luciferase reporter assay was utilized to validate Glut1 as a direct target gene of miR-218. The intracellular level of GSH and ROS were determined using a commercial colorimetric assay kit and 2’, 7’-dichlorodihydro-fluorescein diacetate, respectively. Results: Over-expression of miR-218 significantly reduced the rate of glucose uptake and total level of GSH and enhanced the chemo-sensitivity of bladder cancer to cisplatin. Mechanistically, Glut1 was found to be a direct and functional target of miR-218. Up-regulation of Glut1 could restore chemo-resistance in T24 and EJ cells. On the contrary, knockdown of Glut1 could generate a similar effect as up-regulating the expression of miR-218. Conclusions: MiR-218 increases the sensitivity of bladder cancer to cisplatin by targeting Glut1. Restoration of miR-218 and repression of glut1 may provide a potential strategy to restore chemo-sensitivity in bladder cancer.

The Hallmarks of Cancer from a Redox Perspective

SIGNIFICANCE

For a healthy cell to turn into a cancer cell and grow out to become a tumor, it needs to undergo a series of complex changes and acquire certain traits, summarized as "The Hallmarks of Cancer." These hallmarks can all be regarded as the result of altered signal transduction cascades and an understanding of these cascades is essential for cancer treatment.

RECENT ADVANCES

Redox signaling is a long overlooked form of signal transduction that proceeds through the reversible oxidation of cysteines in proteins and that uses hydrogen peroxide as a second messenger.

CRITICAL ISSUES

In this article, we provide examples that show that redox signaling is involved in the regulation of proteins and signaling cascades that play roles in every hallmark of cancer.

FUTURE DIRECTIONS

An understanding of how redox signaling and "classical" signal transduction are intertwined could hold promising strategies for cancer therapy in the future. Antioxid. Redox Signal. 25, 300-325.

Inhibition of glucose-6-phosphate dehydrogenase sensitizes cisplatin-resistant cells to death

The mechanisms of cisplatin resistance, one of the major limitations of current chemotherapy, has only partially been described. We previously demonstrated that cisplatin-resistant ovarian cancer cells (C13), are characterized by reduced mitochondrial activity and higher glucose-dependency when compared to the cisplatin-sensitive counterpart (2008). In this work we further characterized the role of metabolic transformation in cisplatin resistance. By using transmitochondrial hybrids we show that metabolic reprogramming of cisplatin-resistant cell is not caused by inherent mtDNA mutations. We also found that C13 cells not only present an increased glucose-uptake and consumption, but also exhibit increased expression and enzymatic activity of the Pentose Phosphate pathway (PPP) enzyme Glucose-6-Phosphate Dehydrogenase (G6PDH). Moreover, we show that cisplatin-resistant cells are more sensitive to G6PDH inhibition. Even if the metabolomic fingerprint of ovarian cancer cells remains to be further elucidated, these findings indicate that PPP offers innovative potential targets to overcome cisplatin resistance.

Redox Control of Multidrug Resistance and Its Possible Modulation by Antioxidants

Clinical efficacy of anticancer chemotherapies is dramatically hampered by multidrug resistance (MDR) dependent on inherited traits, acquired defence against toxins, and adaptive mechanisms mounting in tumours. There is overwhelming evidence that molecular events leading to MDR are regulated by redox mechanisms. For example, chemotherapeutics which overrun the first obstacle of redox-regulated cellular uptake channels (MDR1, MDR2, and MDR3) induce a concerted action of phase I/II metabolic enzymes with a temporal redox-regulated axis. This results in rapid metabolic transformation and elimination of a toxin. This metabolic axis is tightly interconnected with the inducible Nrf2-linked pathway, a key switch-on mechanism for upregulation of endogenous antioxidant enzymes and detoxifying systems. As a result, chemotherapeutics and cytotoxic by-products of their metabolism (ROS, hydroperoxides, and aldehydes) are inactivated and MDR occurs. On the other hand, tumour cells are capable of mounting an adaptive antioxidant response against ROS produced by chemotherapeutics and host immune cells. The multiple redox-dependent mechanisms involved in MDR prompted suggesting redox-active drugs (antioxidants and prooxidants) or inhibitors of inducible antioxidant defence as a novel approach to diminish MDR. Pitfalls and progress in this direction are discussed.

Increased Oxidative Stress as a Selective Anticancer Therapy

Reactive oxygen species (ROS) are closely related to tumorgenesis. Under hypoxic environment, increased levels of ROS induce the expression of hypoxia inducible factors (HIFs) in cancer stem cells (CSCs), resulting in the promotion of the upregulation of CSC markers, and the reduction of intracellular ROS level, thus facilitating CSCs survival and proliferation. Although the ROS level is regulated by powerful antioxidant defense mechanisms in cancer cells, it is observed to remain higher than that in normal cells. Cancer cells may be more sensitive than normal cells to the accumulation of ROS; consequently, it is supposed that increased oxidative stress by exogenous ROS generation therapy has an effect on selectively killing cancer cells without affecting normal cells. This paper reviews the mechanisms of redox regulation in CSCs and the pivotal role of ROS in anticancer treatment.

Modulation of oxidative stress as an anticancer strategy

The regulation of oxidative stress is an important factor in both tumour development and responses to anticancer therapies. Many signalling pathways that are linked to tumorigenesis can also regulate the metabolism of reactive oxygen species (ROS) through direct or indirect mechanisms. High ROS levels are generally detrimental to cells, and the redox status of cancer cells usually differs from that of normal cells. Because of metabolic and signalling aberrations, cancer cells exhibit elevated ROS levels. The observation that this is balanced by an increased antioxidant capacity suggests that high ROS levels may constitute a barrier to tumorigenesis. However, ROS can also promote tumour formation by inducing DNA mutations and pro-oncogenic signalling pathways. These contradictory effects have important implications for potential anticancer strategies that aim to modulate levels of ROS. In this Review, we address the controversial role of ROS in tumour development and in responses to anticancer therapies, and elaborate on the idea that targeting the antioxidant capacity of tumour cells can have a positive therapeutic impact.