NEBRASKA REDOX BIOLOGY CENTER EDUCATIONAL PORTAL

Nebraska Redox Biology Center Educational Portal


Redox Processes and Cancer

Cancer is among the major causes of death worldwide. Despite extensive research and efforts for developing anti-tumor therapies, many tumors are still characterized by poor prognosis and high mortality. Recent studies shown that disrupted redox homeostasis is a common hallmarks of cancer cells. Altered redox balance and signaling are implicated in malignant progression and resistance to treatment. The cancer cells may exhibit comparatively high levels of reactive oxygen species (ROS) as a result of metabolic reprogramming and changes in their microenvironment. The redox imbalance can be compensated by increased antioxidant defense. Such redox shift can promote tumor growth by inducing genomic instability and reprogramming of metabolism [ 1, 2, 3, 4, 5 ].

High reactive oxygen species level in cancer cells resulting in development of efficient mechanism for ROS detoxification that for survival under oxidative stress condition. Therefore, strong dependency of cancer cells from their oxidative stress defense systems represents a specific vulnerability which can be used for targeted therapy. Such strategy may include altering redox signaling pathways in tumor cells and disabling key antioxidant systems in the presence of ROS inducers [ 6, 7, 8 ].

                 - Altered redox homeostasis is a general hallmark of cancer cells
                 - Increased ROS levels are able to promote malignant progression and tumor growth
                 - Increased antioxidant capacity is a common feature of malignant cells
                 - Tumors can be sensitized to antitumor therapy by altering of antioxidant defenses


ROS sources and scavengers in the control of redox homeostasis in normal and cancer cells. Normal cells keep constant ROS production and elimination to maintain a favorable redox balance. Disruption of redox homeostasis by co-treatment with ROS inducers and antioxidant inhibitors induces oxidative stress and variable levels of cell death. Cancer cells exhibit higher steady-state levels of ROS counterbalanced by increased antioxidant capacity. The combined use of pro-oxidizing treatment and antioxidant inhibition is expected to cause severe oxidative stress and severe cytotoxicity [ 9 ].

Reactive oxygen species are derived from the oxygen that is consumed in various metabolic reactions occurring mainly in the mitochondria, peroxisomes, and the endoplasmic reticulum. ROS include the superoxide and hydroxyl radicals, and hydrogen peroxide, which is involved in signaling at nanomolar concentration range. ROS have strong interplay with reactive nitrogen and reactive chlorine species production and metabolism. The mitochondrial respiratory chain is a major source of intracellular superoxide, which is converted to H2O2 by superoxide dismutases. In addition, ROS are produced in enzyme-catalyzed reactions involving NADPH oxidases, xanthine oxidases, nitric oxide synthases, and metabolizing enzymes such as the cytochrome P450 enzymes, lipoxygenase, and cyclooxygenase. Endoplasmic reticulum ROS are prodused as byproducts of sulfhydryl oxidases reactions in oxidative protein folding process [ 10, 11, 12 ].

Redox homeostasis is a balance of ROS generation and elimination. Mitochondria, NAPH oxidase (NOX), and endoplasmic reticulum are the three major intracellular sources of ROS [ 1 ].

All known aerobic and anaerobic organisms have evolved complex systems of ROS detoxification to avoid ROS mediated toxicity. Such systems are represented by ROS scavenging enzymes and low molecular weight antioxidants. The most relevant antioxidant enzymes include superoxide dismutases that convert superoxide to less reactive H2O2, catalase that reduces H2O2 to water and molecular oxygen and glutathione peroxidases that eliminate H2O2 using reducing power derived from glutathione/glutaredoxin and peroxiredoxins that uses reducing power of thioredoxin system. Disruption of redox processes that control the ROS balance and the related redox signaling events may lead to malignant progression. Cancer cells adjusts antioxidant enzymes expression to prevent excessive oxidative damage and promote redox signaling. Metabolic pathways reprogramming provides additional supply of small antioxidant molecules like reduced glutathione and NADPH [ 13, 14, 15, 16, 17, 18 ].

Redox control of the eukaryotic cell-cycle

Eukaryotic cells divide when they have appropriate size or when triggered by extracellular stimuli such as growth factors or hormones. Proliferative signals flow through intracellular signaling pathways to activate the cell-cycle. ROS can act as a messenger in signaling cascades involved in cell proliferation and differentiation. Interactions between growth factors and receptors is resulting in ROS generation, which at low concentrations are required to activate proliferative signaling for cell division. In embryonic development, a programmed fluctuation of oxidative state is important to determine cell fate - an hypoxic environment is important for proliferation, whereas mild oxidative conditions leads to differentiation. At higher ROS levels, cell division is stalled, and after prolonged arrest, cells die from apoptosis. Loss of redox control in cell division can result in quiescent cells re-entering cell division leading to cancer, aberrant fetus development, and neurodegenerative disorders. Cell division is highly regulated by the cellular redox environment, and the concentration of ROS determines whether cell division is positively or negatively regulated. Moreover, ROS sensing is also important in redox signaling. ROS production is usually localized; ROS act rapidly and damage biological molecules in close proximity. Oxidative stress signaling that does not always depend on a defined cascade of signal transduction mediated by phosphorylation and triggered by specific recognition of a molecule based on parameters such as shape, charge, or hydrophobicity, but instead on direct oxidation of regulatory molecules or transcription factors. High number of cell-cycle regulators can be modified by cysteine oxidation - a signal that could be almost as important as phosphorylation [ 1, 14, ].

Redox control of the eukaryotic cell-cycle. ROS are produced as a result of oxidative phosphorylation in the mitochondria. Cell-cycle regulators with reactive cysteine residues are highlighted by a red circle labeled with 'ox'. In response to mitogens, signaling pathway kinases convey proliferative signals to activate expression of cyclin D. Cyclin D complexes with Cdk4 or Cdk6 and phosphorylates pRB to release its inhibitory effects on E2F driving cells to re-enter the cell-cycle from quiescence to G1. Once the restrictive point at late G1 (labeled as R) is passed, cells are committed to cell division. Activation of E2F leads to the transcription of cyclin E for transition from G1 to S phase. Subsequent expression of cyclin A and cyclin B leads to transition of S to G2 and G2 to M phases, respectively. The phosphatase Cdc25 activates cyclin A-Cdk1, cyclin B-Cdk1, cyclin E-Cdk2 for entry into M phase by removing the inhibitory phosphorylation on Cdk1 and Cdk2. Opposing the activity of CDKs (cyclin A-Cdk1, cyclin B-Cdk1, cyclin E-Cdk2, cyclin D-Cdk4/6) are CKIs (p21, p27, p57), which sequester CDKs and block their kinase activity. The cell-cycle is synchronized with the metabolic cycle of the cells with S phase and M phase occurring only during the reductive phase of metabolism (blue) and G1 in the oxidative phase (red). [ 14 ].

Energy metabolic pathways and ROS homeostasis in cancer cells

Cancer-cell metabolism can vary depending on influences of the tumour microenvironment and the distance to the vasculature. Cancer cells located closer to the blood supply profit from their access to nutrients and oxygen, and generate ATP aerobically via oxidative phosphorylation and upregulate anabolic pathways, supporting rapid proliferation. The oxidative stress caused by these rapidly proliferating cancer cells induces glycolysis and autophagy in the surrounding stromal cells that generates catabolites, such as lactate or ketones, which in turn are taken up by anabolic cancer cells, and used to fuel mitochondrial metabolism and ATP production (reverse Warburg effect). Similarly, low nutrient availability requires that tumour cells located further from the vasculature and in proximity to anabolic tumour-cell populations commit to alternative catabolic metabolic pathways, such as autophagy, allowing greater adaptability to meet their resources and energy needs.

Cellular metabolic pathway involved in redox homeostasis. Metabolic pathway in the cytosol and mitochondria [ 26 ].

Metabolic reprogramming in cancer cells is linked to specific metabolic pathways responsible for synthesis of amino acids, lipids and nucleotides. These pathways utilizes energetic substrates that can be used to generate antioxidant molecules like NADPH and glutathione, and redox cofactors (NADH and FADH) that are intensively used by cancer cells to maintain redox homeostasis [ 19, 20, 21 ]. Cell energy metabolic pathways have a strong interlink with redox balance of cancer cells. Major pathways include glycolysis, glutaminolysis, fatty acid oxidation, one-carbon metabolism and the pentose phosphate pathway [ 22, 23, 24, 25 ].

Cellular metabolic pathway involved in redox homeostasis. Metabolic pathway in the cytosol and mitochondria [ 26 ].

Glycolysis. Glucose metabolism has an essential role in energy production and control of redox homeostasis in cancer cells. Metabolic intermediates of glucose can be used by another metabolic pathways which directly or indirectly contribute to generate reducing equivalents, mainly PPP-derived NADPH or glutaminolysis-derived reduced glutathione. Glycolysis is a common and essential metabolic pathway occurring in the cytosol and responsible for glucose is transformation to pyruvate, which is used in e tricarboxylic acid (TCA) cycle. Glucose is taken from the extracellular space by specific glucose transporters and converted to glucose-6-phosphate by hexokinase enzymes, then enters into a series of ten enzyme-catalyzed reactions resulting in the generation of pyruvate, ATP and reduced cofactors in the form of NADH. [ 27, 28, 29 ].

In the 1924 Otto Warburg demonstrated that tumor cells exhibit a prevalent use of the glycolytic pathway regardless the presence of sufficient amount of oxygen. This phenomenon was called as Warburg effect. [ 30 ]. Pro-glycolytic shift is likely caused by oncogene activation and loss of tumor suppressors. Normal cells primarily metabolize glucose to pyruvate for growth and survival, followed by complete oxidation of pyruvate to CO2 through the TCA cycle and the oxidative phosphorilation process in the mitochondria, generating 36 ATPs per glucose. Lactate is formed in anaerobic pathway only. Cancer cells convert most glucose to lactate regardless of the availability of oxygen (the Warburg effect), diverting glucose metabolites from energy production to anabolic process to accelerate cell proliferation, at the expense of generating only two ATP molecules per glucose [ 31, 32, 33, 34, 35 ].

Metabolic differences between normal and cancer cells. Normal cells primarily metabolize glucose to pyruvate for growth and survival, followed by complete oxidation of pyruvate to CO2 through the TCA cycle and the oxidative phosphorilation process in the mitochondria, generating 36 ATPs per glucose. O2 is essential once it is required as the final acceptor of electrons. When O2 is limited, pyruvate is metabolized to lactate. Cancer cells convert most glucose to lactate regardless of the availability of O2 (the Warburg effect), diverting glucose metabolites from energy production to anabolic process to accelerate cell proliferation, at the expense of generating only two ATPs per glucose. [ 36 ].

Glutaminolysis. Glutamine is a non-essential amino acid that has a key role in cancer cells metabolism. It serves as a source of carbon and nitrogen for biosynthetic processes, as intermediate for energy production and a precursor for glutathione synthesis [ 1, 37 ]. Increased glutamine catabolism is a common hallmark of cancer cells metabolism reprogramming for support of cell proliferation, signal transduction and redox homeostasis [ N ]. Glutamine utilization and the metabolic profile of different cancer cells is influenced by expression levels of certain oncogenes (Ras and Myc) or tumor suppressors (p53) [ 1, 37 ]. Enzyme glutaminase directly contribute to glutathione synthesis by converting glutamine into glutamate and promoting the uptake of cysteine through the Slc7a11 exchanger [ 38 ]. Metabolic intermediates such as citrate can be diverted from the TCA cycle and exported into the cytosol, where Isocitrate dehydrogenase use them to generate reducing power in the form of NADPH [ 39 ]. This strategy helps tumors to keep the glutathione pool in a reduced state [ 38 ]. In addition, mitochondrial glutamate dehydrogenase 1 is positively regulate the enzymatic activity of the glutathione peroxidase by controlling the intracellular fumarate levels [ 40 ].

Metabolic changes in tumor cells using glycolysis or glutaminolysis. Tumor cells may use both pathways to generate energy and intermediate metabolites for survival and growth. [ 41 ].

Pentose phosphate pathway. + The PPP is a major catabolic pathway of glucose through which cancer cells produce large amounts of ribose-5 phosphate, a precursor of nucleotide synthesis and NADPH, a key molecule that is used to drive anabolic processes and to detoxify harmful ROS [ 1, 42 ]. Activation of the PPP represents a key hallmark of many tumors where this metabolic pathway is found at the crossroad between glycolytic activity, unrestricted proliferation and scavenging of excessive ROS [ 43 ]. The transcriptional regulation of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the PPP, was described in U2OS osteosarcoma cells, wherein its overexpression enhanced the PPP-dependent production of NADPH [ 44, 45 ]. Additional mechanisms of G6PD regulation might directly depend on the availability of glucose: glucose funneling into the oxidative branch of the PPP directly controls the redox homeostasis of human clear cell carcinoma cells [ 46 ].

Fatty acid oxidation Fatty acid oxidation is a set of cyclical series of long-chain fatty acids oxidations that occur in the mitochondria resulting in production of short-chain fatty acids, generating NADH, FADH2 and acetyl-CoA for biosynthetic pathways and produce ATP. In Cancer cells large fraction of the acetyl-CoA enters into the tricarboxylic acid cycle to generates citrate, which is exported into the cytosol and entering into metabolic reactions catalyzed by the malic enzyme and the isocitrate dehydrogenase 1, that produce large amounts of NADPH [ 1, 47 ]. Overexpression of the key FAO regulators, such as the carnitine palmitoyltransferase-1,86 occurs in both solid tumors and leukemia cells [ 48 ].

Schematic overview of the pathways involved in the synthesis of fatty acids (FAs), cholesterol, phosphoglycerides, eicosanoids and sphingolipids. The enzymes involved in catalysing steps in lipid biosynthetic pathways are indicated in red. (a) Glucose- or glutamine-derived citrate is first converted to acetyl-CoA by ACLY. (b) For FA biosynthesis, acetyl-CoA is converted into malonyl-CoA. The repeated condensation of acetyl-CoA and malonyl-CoA by the multifunctional enzyme FASN leads to the generation of palmitic acid, a fully saturated 16-carbon FA. The introduction of a double bond in the ?9 position of the acyl chain by SCD generates mono-unsaturated FAs. (c) Subsequent elongation and further desaturation produces the repertoire of FAs with different saturation levels. (d) Essential FAs (?3 and ?6 FAs) cannot be synthesised by human cells and need to be provided from dietary sources. (e,f) Saturated and unsaturated FAs are combined with glycerol-3-phosphate (glycerol-3-P) to generate (e) phosphoglycerides and (f) phosphoinositides. (g) Arachidonic acid, a long-chain polyunsaturated FA, is used for the synthesis of eicosanoids. (h) Sphingolipids contain acyl chains and polar head groups derived from serine, phosphocholine or phosphoethanolamine. (i) Cholesterol biosynthesis is initiated by the conversion of acetyl-CoA to acetoacetyl-CoA. Addition of another acyl group by HMGCS produces 3-methylglutaryl-3-hydroxy-CoA, which is converted to mevalonate by HMGCR. Subsequent reactions result in the production of farnesyl-pyrophosphate, an essential intermediate for protein prenylation. Cholesterol also forms the structural backbone for steroid hormone biosynthesis. Enzyme abbreviations: ACAT, acetyl-CoA acetyltransferase; ACC, acetyl-CoA carboxylase; ACLY, ATP citrate lyase; AGPAT, 1-acylglycerol-3-phosphate O-acyltransferase; COX1/2, prostaglandin-endoperoxide synthase (PTGS); DGAT, diacylglycerol O-acyltransferase; ELOVL, fatty acid elongase; FADS, fatty acid desaturase; FASN, fatty acid synthase; GPAT, glycerol-3-phosphate acyltransferase; HMGCR, 3-hydroxy-3-methylglutaryl-CoA reductase; HMGCS, 3-hydroxy-3-methylglutaryl-CoA synthase; PPAP, phosphatidic acid phosphatase; SCD, stearoyl-CoA desaturase; SPHK, sphingosine-1-kinase. Metabolite abbreviations: a-KG, a-ketoglutarate; CDP-DAG, cytidine diphosphate-diacylglycerol; CER, ceramide; DAG, diacylglycerol; FA, fatty acid; LPA, lysophosphatidic acid; PA, phosphatidic acid; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PGE2, prostaglandin E2; PGH2, prostaglandin H2; PI, phosphatidylinositol; PIPx, phosphatidylinositol phosphate; PS, phosphatidylserine; S1P, sphingosine-1-phosphate; SPH, sphingosine; TAG, triacylglyceride. [ 49 ].

The serine-glycine one-carbon metabolism The serine glycine one-carbon metabolism is a complex network of biochemical reactions that integrate inputs from amino acids and glucose derivatives (mainly serine and glycine) and generates multiple outputs as carbon units (tetrahydrofolate (THF) and its derivate) that serve different cellular functions. The redistribution of these carbon units from serine and glycine rely on three pathways: the folate cycle, the methionine cycle and the trans-sulfuration pathway. Folate, a vitamin B derivative, is reduced to THF by a series of metabolic reactions and converted into methylenetetrahydrofolate by serine hydroxymethyl transferase (SHMT). This product is either converted to F-THF or reduced by methylenetetrahydrofolate reductase to methylenetetrahydrofolate, whose demethylation completes the folate cycle. The carbon units therefore enter into the methionine cycle with the generation of S-adenosylmethionine by the methionine adenyltransferase, with further conversion by S-adenosyl homocysteine hydrolase into homocysteine. The last modular component of the one-carbon metabolism, the trans-sulfuration pathway, is functionally connected to the methionine cycle through the homocysteine, whose condensation with serine by cystathionine synthase generates cystathione, further metabolized to alpha-ketobutyrate and cysteine by cystathione lyase. The cysteine can therefore be diverted into GSH synthesis. The serine glycine one-carbon metabolism has been associated with cancer cell due to its importance for the regulation of nucleic acid, lipids and protein synthesis of proliferating cells. More recent evidence indicates that this pathway is also crucial for redox balance. THF-derived carbon units are primarily used for nucleotide synthesis in the cytosol, new methods for tracing NAPDH compartmentalization indicate that serine is predominantly utilized in the mitochondria of mammalian cells to generate NADPH [ 1, 50, 51 ].

Serine donates one carbon units to the folate cycle while producing glycine. The folate cycle is essential for synthesis of adenosine, guanosine and thymidylate, and can contribute to mitochondrial NADH, NADPH and ATP regeneration. Glycine also provides one carbon units to the folate cycle, and function as a precursor for the synthesis of purine and glutathione. The folate cycle is coupled to the methionine cycle, which generates methyl groups for cellular biosynthesis and posttranslational modifications. The methionine cycle also provides precursors, such as cysteine for the synthesis of glutathione. dTTP, deoxythymidine triphosphate [ 52 ].

Folate metabolism occurs in both mitochondria and cytosol. In most cells, serine is catabolized primarily in the mitochondria by serine hydroxymethyltransferase 2 (SHMT2) to generate glycine and 5,10-methylene-tetrahydrofolate (5,10-methylene-THF). The glycine cleavage system (of which glycine dehydrogenase (GLDC) is a major component) also contributes to the production of this folate intermediate. Oxidation of 5,10-methylene-THF by methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) or MTHFD2-like (MTHFD2L) produces 10-formyl-THF. Whereas MTHFD2L (which can use NAD+ or NADP+) is expressed in most normal adult tissues, cancer cells predominantly express the NAD+-dependent MTHFD2, and the MTHFD2-catalysed oxidation of 5,10-methylene-THF contributes to mitochondrial NADH regeneration. The one-carbon unit in 10-formyl-THF can be converted into formate in an MTHFD1L-mediated reaction that regenerates ATP from ADP, or metabolized by 10-formyl-THF dehydrogenase (ALDH1L2) and released as CO2 while also generating NADPH. Additionally, 10-formyl-THF is required for the formylation of the mitochondrial initiator N-formylmethionine-tRNA (fMET-tRNA). Formate can be transported to the cytosol and used as substrate for the trifunctional MTHFD1, which produces 10-formyl-THF for de novo purine synthesis and 5,10-methylene-THF for thymidylate synthesis and homocysteine remethylation in the methionine cycle. Cytosolic ALDH1L1 that consumes 10-formyl-THF is commonly downregulated in cancer cells. Most cells default to the mitochondrial-to-cytosol directionality of the folate cycle, but when the mitochondrial pathway is inhibited, the cytosolic pathway can work in reverse to support nucleotide synthesis and proliferation. Dashed lines indicate transport between cytosol and mitochondrion. MTHFR, methylenetetrahydrofolate reductase; TYMS, thymidylate synthase [ 52 ].

Redox homeostasis based cancer therapy

Strategies to manipulate ROS levels as anticancer therapy. Effect of different therapeutic manipulations on the intracellular ROS levels and relative toxicity in both normal and cancer cells [ 53 ].

Methotrexate is a chemotherapy agent widely used for treatement of osteosarcoma as well as active rheumatoid arthritis and other inflammatory disorders. Recent studies shown that methotrexate mechanism is most likely based on ROS generation and resulting oxidative stress mediated apoptosis [ 54, 55 ].

Mitoxantrone is a synthetic antineoplastic cytotoxic drug used for the treatment of cancer and aggressive multiple sclerosis. The anticancer effects is based on triggering of tumor cell apoptosis via cell membrane scrambling. This effect is likely related to increased ROS and ceramide production [ 56 ].

Tamoxifen is anticancer agent that act by blocking tofestrogen receptor in breasts. In addition, tamoxifen tamoxifen can induce apoptosis in an estrogen-independent manner by targeting mitochondria and resulting in ROS production. Another possible mechanism is based on protein kinase CK2 inhibition which is also associated with ROS generation [ 57 ].

Cisplatin is one of the most effective and widely used drugs for the treatment of various cancers. Cisplatin effectively binds to RNA, DNA and proteins forming different types of adducts and have a tendency to accumulate in mitochondria, resulting in mitochondrial damage and ROS generation. Cytotoxicity is likely based on DNA damage and oxidative stress mediated apoptosis [ 58 ].

Paclitaxel is general agent for treatments of lung, ovarian, breast, head and neck cancers. It binds to the beta-subunit of the tubulin heterodimer and accelerate the polymerization of tubulin, resulting in the stabilization of microtubules and inhibition of depolarization. Thus, paclitaxel in inhibiting the cell division. Co-treatment with paclitaxel and lentinan is enhancing apoptosis by ROS generation. The mechanism is based on activations of ROS-TXNIP-NLRP3 inflammasome and ASK1/p38 MAPK signal pathways [ 59 ].

Adriamycin (Doxorubicin) is a anthracycline antibiotic that widely used as chemotherapeutic agent for treatment of metastatic breast cancer, lymphomas and sarcomas, as well as other types of cancer. Proposed mechanisms of anti-cancer effects are related to intercalation into DNA, generation of reactive oxygen species, DNA binding and DNA cross-linking and DNA damage by inhibition of topoisomerase II finaly resulting in apoptosis [ 60, 61 ].

Imatinib mesylate is a potent inhibitor of the Abl kinase and cell growth inhibitor with pro-apoptotic effects. This drugd used for Chronic myeloid leukemia therapy. Imatinib treatments is resulting in increase reactive oxygen species formation [ 62 ].

Camptothecin is a quinolone alkaloid that induces cytotoxicity in a variety of cancer cell lines, likely, by enhancing of apoptosis in a caspase-dependent manner. Recent study shown that camptothecin sensitizes cancer cells to TRAIL-mediated apoptosis via ROS and ERK/p38-dependent death receptor DR5 upregulation [ 63 ].

Flavopiridol is a synthetic flavone that is known to accumulate in mitochondria and induce selective killing of leukemic cells through pathways linked to mitochondrial mediated apoptosis and necrosis. In combination with redox-reactive thalidomide CPS49, flavopiridol were found to induce selective cytotoxicity associated with mitochondrial dysfunction and elevations of ROS production [ 64 ].

Procarbazine was the one of the first developed anti-cancer drugs that generates ROS. The oxidation of procarbazine in aqueous solution leads to the production of hydrogen peroxide which isessential to the cytotoxic effect of the drug. Procarbazine was approved in the late 1960s as a cytotoxic drug is used for the treatment of Hodgkin's lymphoma, non-Hodgkin's lymphoma and brain tumours [ 65 ].

NOV-002 (disodium glutathione disulfide) is a glutathione disulfide mimetic that has been shown to alter intracellular GSH/GSSG ratio by increasing GSSG levels, creating oxidative intracellular signal and inducing S-glutathionylation. NOV-002 modulates signaling pathways involved in tumor cell proliferation and metastasis and enhances anti-tumor immune responsiveness [ 66 ].

Sulfasalazine is anti-inflammatory drug used for therapy of inflammatory bowel disease and rheumatoid arthritis. In addition, sulfasalazine is known as a potent cystine/glutamate antiporter inhibitor. Sulfasalazine teratment leads to cystine starvation of a variety of experimental cancers, including lymphoma, prostate, and breast cancer cell lines, with subsequent reduction of intracellular glutathione levels and growth arrest [ 67 ].

L-Buthionine sulfoximine (BSO) is a specific gamma-glutamylcysteine synthetase inhibitor that blocks the rate-limiting step of glutathionine biosynthesis and depletes the intracellular glutathione pool. In particular, studies have shown that elevated levels of GSH prevent apoptotic cell death whereas depletion of GSH facilitates apoptosis [10,14]. BSO sensitizes tumor cells to oxidative stress and apoptosis induced by various chemotherapeutic agents [ 68 ].

Carboplatin (cis-Diammine(1,1-cyclobutanedicarboxylato)platinum(II)) is a drug used for treatement of different types of tumors, including small-cell lung cancer, ovarian cancer, and carcinomas of head and neck. Carboplatin causes nitric oxide and reactive oxidative species generation, likely resulting in apoptosis [ 69 ].

Etoposide, a topoisomerase II inhibitor commonly used as a chemotherapeutic reagent in the treatment of lymphoma. In these cells, etoposide increased the generation of reactive oxygen species and reduced mitochondria membrane potential, likely resulting in apoptosis [ 70 ].

Bortezomib is a widely used lymphoma treatement agent. It is a reversible inhibitor of the 26 S proteasome. Bortezomib inhibits the ubiquitin-proteasome pathway and alters multiple cellular signaling cascades, thus, inducing ER stress and oxidative stress [ 71 ].

Disulfiram is an acetaldehyde dehydrogenase inhibitor that induces apoptosis by glutathione oxidation and proteasome inhibition. Disulfiram-induced apoptosis is shown to be mediated by JNK activation and Nrf2 and NF-kB inhibition.

Name

Mechanism of action

Effects on ROS

Cancer types

Ionizing radiation

Photons or particles affect chemical bonds and produce highly ROS, which cause damage to DNA and other cellular components

Increases ROS production

Different types of cancer

Methotrexate

Triggers ROS associated cell apoptosis

Increases ROS production

Different types of cancer

Mitoxantrone

Triggers cell membrane scrambling

Significant increases of ROS formation

Different types of cancer

Tamoxifen

Promotes cancer cell senescence

Promotes ROS generation

Breast, colon cancer

Cisplatin

Generation of nuclear DNA adducts

Induces a mitochondrial-dependent ROS generation

Different types of cancer

Paclitaxel (Taxol)

Inhibitor of cell division

Increases ROS production

Different types of cancer

Adriamycin

Reduces cell viability through initiating cell apoptosis and strong G2/M phase cell cycle arrest

Increases ROS production

Different types of cancer

Imatinib

Protein tyrosine kinase inhibitor that induce apoptosis

Increases ROS production

Different types of cancer

Camptothecin

Quinolone alkaloid that induces cytotoxicity

Increases ROS production

Different types of cancer

Flavopiridol

Semisynthetic flavonoid that inhibits cyclin-dependent kinases

Increases ROS production

Leukemia

Procarbazine

Isolated DNA could be degraded by procarbazine in the presence of oxygen

Increases ROS production

Lymphoma, primary brain cancers

NOV-002

Glutathione disulphide mimetic

Alters intracellular GSSG/GSH ratio

Lung, breast and ovarian cancer

Sulphasalazine

Inhibitor of cysteine/glutamate transporter xCT

Reduces intracellular transport of cysteine required for GSH synthesis

Pancreatic and lung cancer

Buthionine sulphoximine (BSO)

Glutamate-cysteine ligase complex inhibitor

Inhibits de novo GSH synthesis

Ovarian and breast cancer, melanoma

Carboplatin

Induction of cell cycle arrest

Induction of ROS owing to ER stress

Different types of cancer

Etoposide

Selective Topo II α inhibitor

Increases ROS production

Neuroblastoma, breast cancer

Bortezomib

Proteasome inhibitor

Induces ROS owing to ER stress

Mantle cell lymphoma, multiple myeloma