Ascorbate and H2O2 induced oxidative DNA damage in Jurkat cells

Contribution of oxidation reactions to photo-induced damage to cellular DNA

This review article is aimed at providing updated information on the contribution of immediate and delayed oxidative reactions to the photo-induced damage to cellular DNA/skin under exposure to UVB/UVA radiations and visible light. Low-intensity UVC and UVB radiations that operate predominantly through direct excitation of the nucleobases are very poor oxidizing agents giving rise to very low amounts of 8-oxo-7,8-dihydroguanine and DNA strand breaks with respect to the overwhelming bipyrimidine dimeric photoproducts. The importance of these two classes of oxidatively generated damage to DNA significantly increases together with a smaller contribution of oxidized pyrimidine bases upon UVA irradiation. This is rationalized in terms of sensitized photooxidation reactions predominantly mediated by singlet oxygen together with a small contribution of hydroxyl radical that appear to also be implicated in the photodynamic effects of the blue light component of visible light. Chemiexcitation-mediated formation of "dark" cyclobutane pyrimidine dimers in UVA-irradiated melanocytes is a recent major discovery that implicates in the initial stage, a delayed generation of reactive oxygen and nitrogen species giving rise to triplet excited carbonyl intermediate and possibly singlet oxygen. High-intensity UVC nanosecond laser radiation constitutes a suitable source of light to generate pyrimidine and purine radical cations in cellular DNA via efficient biphotonic ionization.

Differential H2O2 Metabolism among Glioblastoma Subtypes Confers Variable Responses to Pharmacological Ascorbate Therapy Combined with Chemoradiation

Glioblastoma (GBM), a highly lethal and aggressive central nervous system malignancy, presents a critical need for targeted therapeutic approaches to improve patient outcomes in conjunction with standard-of-care (SOC) treatment. Molecular subtyping based on genetic profiles and metabolic characteristics has advanced our understanding of GBM to better predict its evolution, mechanisms, and treatment regimens. Pharmacological ascorbate (P-AscH-) has emerged as a promising supplementary cancer therapy, leveraging its pro-oxidant properties to selectively kill malignant cells when combined with SOC. Given the clinical challenges posed by the heterogeneity and resistance of various GBM subtypes to conventional SOC, our study assessed the response of classical, mesenchymal, and proneural GBM to P-AscH-. P-AscH- (20 pmol/cell) combined with SOC (5 µM temozolomide and 4 Gy of radiation) enhanced clonogenic cell killing in classical and mesenchymal GBM subtypes, with limited effects in the proneural subtype. Similarly, following exposure to P-AscH- (20 pmol/cell), single-strand DNA damage significantly increased in classical and mesenchymal but not proneural GBM. Moreover, proneural GBM exhibited increased hydrogen peroxide removal rates, along with increased catalase and glutathione peroxidase activities compared to mesenchymal and classical GBM, demonstrating an altered H2O2 metabolism that potentially drives differential P-AscH- toxicity. Taken together, these data suggest that P-AscH- may hold promise as an approach to improve SOC responsiveness in mesenchymal GBMs that are known for their resistance to SOC.

The comparison of antioxidant properties and nutrigenomic redox-related activities of vitamin C, C-vitamers, and other common ascorbic acid derivatives

The term 'vitamin C' describes a group of compounds with antiscorbutic activity of l-ascorbic acid (AA). Despite AA's omnipresence in plant-derived foods, its derivatives have also been successfully implemented in the food industry as antioxidants, including the D-isomers, which lack vitamin C activity. This study aimed to determine the relationship between redox-related activities for five derivatives of AA using electrochemical, chemical, and biological approaches. Here we report that AA, C-vitamers, and other commonly consumed AA derivatives differ in their redox-related activities. As long as the physiological range of concentrations was maintained, there was no simple relationship between their redox properties and biological activity. Clear distinctions in antioxidant activity were observed mostly at high concentrations, which were strongly correlated with electrochemical and kinetic parameters describing redox-related properties of the studied compounds. Despite obvious similarities in chemical structures and antioxidant activity, we showed that C-vitamers may exhibit different nutrigenomic effects. Together, our findings provide a deeper insight into so far underinvestigated area combining chemical properties with biological activities of commonly applied AA derivatives.

Vitamin C (Ascorbate) and Redox Topics in Cancer

Significance: Vitamin C (ascorbate), in regard to its effectiveness against malignancies, has had a controversial history in cancer treatment. It has been shown that in vitro and in vivo anticancer efficacy of ascorbate relies on its pro-oxidant effect mainly from an increased generation of reactive oxygen species (ROS). A growing understanding of its anticancer activities and pharmacokinetic properties has prompted scientists to re-evaluate the significance of ascorbate in cancer treatment. Recent Advances: A recent resurge in ascorbate research emerged after discovering that, at high doses, ascorbate preferentially kills Kirsten-Ras (K-ras)- and B-raf oncogene (BRAF)-mutant cancer cells. In addition, some of the main hallmarks of cancer cells, such as redox homeostasis and oxygen-sensing regulation (through inhibition of hypoxia-inducible factor-1 alpha [HIF-1α] activity), are affected by vitamin C. Critical Issues: Currently, there is no clear consensus from the literature in regard to the beneficial effects of antioxidants. Results from both human and animal studies provide no clear evidence about the benefit of antioxidant treatment in preventing or suppressing cancer development. Since pro-oxidants may affect both normal and tumor cells, the extremely low toxicity of ascorbate represents a main advantage. This guarantees the safe inclusion of ascorbate in clinical protocols to treat cancer patients. Future Directions: Current research could focus on elucidating the wide array of reactions between ascorbate and reactive species, namely ROS, reactive nitrogen species as well as reactive sulfide species, and their intracellular molecular targets. Unraveling these mechanisms could allow researchers to assess what could be the optimal combination of ascorbate with standard treatments.

High-Dose Vitamin C: Preclinical Evidence for Tailoring Treatment in Cancer Patients

Simple Summary

Vitamin C is an indispensable micronutrient in the human diet due to the multiple functions it carries out in the body. Reports of clinical studies have indicated that, when administered at high dosage by the intravenous route, vitamin C may exert beneficial antitumor effects in patients with advanced stage cancers, including those refractory to previous treatment with chemotherapy. The aim of this article is to provide an overview of the current scientific evidence concerning the different mechanisms of action by which high-dose vitamin C may kill tumor cells. A special focus will be given to those mechanisms that provide the rationale basis for tailoring vitamin C treatment according to specific molecular alterations present in the tumor and for the selection of the most appropriate companion drugs.

Abstract

High-dose vitamin C has been proposed as a potential therapeutic approach for patients with advanced tumors who failed previous treatment with chemotherapy. Due to vitamin C complex pharmacokinetics, only intravenous administration allows reaching sufficiently high plasma concentrations required for most of the antitumor effects observed in preclinical studies (>0.250 mM). Moreover, vitamin C entry into cells is tightly regulated by SVCT and GLUT transporters, and is cell type-dependent. Importantly, besides its well-recognized pro-oxidant effects, vitamin C modulates TET enzymes promoting DNA demethylation and acts as cofactor of HIF hydroxylases, whose activity is required for HIF-1α proteasomal degradation. Furthermore, at pharmacological concentrations lower than those required for its pro-oxidant activity (<1 mM), vitamin C in specific genetic contexts may alter the DNA damage response by increasing 5-hydroxymethylcytosine levels. These more recently described vitamin C mechanisms offer new treatment opportunities for tumors with specific molecular defects (e.g., HIF-1α over-expression or TET2, IDH1/2, and WT1 alterations). Moreover, vitamin C action at DNA levels may provide the rationale basis for combination therapies with PARP inhibitors and hypomethylating agents. This review outlines the pharmacokinetic and pharmacodynamic properties of vitamin C to be taken into account in designing clinical studies that evaluate its potential use as anticancer agent.

High-Dose Vitamin C in Advanced-Stage Cancer Patients

High-dose intravenously administered vitamin C (IVC) is widely used in cancer patients by complementary and alternative medicine practitioners. The most frequent indications for IVC therapy result from the belief in its effectiveness as a potent anti-cancer agent which additionally enhances chemosensitivity of cancer cells and reduces chemotherapy-related toxicities and fatigue intensity. In this narrative review, we decided to deal with this issue, trying to answer the question whether there is any scientific evidence supporting the rationale for application of high-dose IVC therapy in advanced-stage cancer patients. Although results obtained from preclinical studies demonstrated that millimolar ascorbate plasma concentrations achievable only after IVC administration were cytotoxic to fast-growing malignant cells and inhibited tumor growth as well as prolonged the survival of laboratory animals, such positive effects were not found in human studies with advanced-stage cancer patients. We also have not found the rationale for the use of IVC to increase the effectiveness of chemotherapy and to reduce the chemotherapy-induced toxicity in the above mentioned group. Nevertheless, in palliative care, high-dose IVC might be considered as a therapy improving the quality of life and reducing cancer-related symptoms, such as fatigue and bone pain. However, because of the absence of placebo-controlled randomized trials on IVC efficacy in advanced-stage cancer patients, the placebo effect cannot be excluded.

Ascorbate kills breast cancer cells by rewiring metabolism via redox imbalance and energy crisis

The idea to use megadoses of ascorbate (vitamin C) for cancer treatment has recently been revived. Despite clear efficacy in animal experimentation, our understanding of the cellular and molecular mechanisms of this treatment is still limited and suggests a combined oxidative and metabolic mechanism behind the selective cytotoxicity of ascorbate towards cancerous cells. To gain more insight into the cellular effects of high doses of ascorbate, we performed a detailed analysis of metabolic changes and cell survival of both luminal and basal-like breast cancer cells treated with ascorbate and revealed a distinctive metabolic shift virtually reversing the Warburg effect and triggering a severe disruption of redox homeostasis. High doses of ascorbate were cytotoxic against MCF7 and MDA-MB231 cells representing luminal and basal-like breast cancer phenotypes. Cell death was dependent on ascorbate-induced oxidative stress and accumulation of ROS, DNA damage, and depletion of essential intracellular co-factors including NAD⁺/NADH, associated with a multifaceted metabolic rewiring. This included a sharp disruption of glycolysis at the triose phosphate level, a rapid drop in ATP levels, and redirection of metabolites toward lipid droplet accumulation and increased metabolites and enzymatic activity in the pentose phosphate pathway (PPP). High doses of ascorbate also inhibited the TCA cycle and increased oxygen consumption. Together the severe disruptions of the intracellular metabolic homeostasis on multiple levels "redox crisis and energetic catastrophe" consequently trigger a rapid irreversible cell death.

Hydrophobic organic compounds in drinking water reservoirs: Toxic effects of chlorination and protective effects of dietary antioxidants against disinfection by-products

This study investigated formation and genotoxicities of disinfection by-products (DBPs) during chlorination of hydrophobic organic compounds (HOCs) extracted from six drinking water reservoirs around the Pearl River Delta region, China. Chemical analyses firstly showed that after chlorination aromatic HOCs (including polycyclic aromatic hydrocarbons, PAHs) decreased but DBPs (including chlorinated PAHs) increased, while genotoxicity assays revealed that the chlorination increased DNA damage in human Caco-2 cells. Although the link between DBPs and the genotoxicity was weak (insignificant correlations, p ≥ 0.05), we observed that chlorination of HOCs from more contaminated reservoirs in general resulted in higher genotoxicity. Secondly, remedial effects of catalase and dietary antioxidants (i.e. vitamin C and epigallocatechin gallate) in protecting cells against DBPs genotoxicity were detected. After 1 h treatment by the antioxidants, the DNA damage in Caco-2 cells (due to previous exposure to DBPs) significantly decreased (p < 0.05) in 7 out of a total of 18 treatments (38.9%). This is the first study demonstrating that catalase, vitamin C and epigallocatechin gallate protected human cells in vitro against DNA damage upon exposure to chlorinated genotoxic DBPs.

Oxidative Stress-Tolerant Stem Cells from Human Exfoliated Deciduous Teeth Decrease Hydrogen Peroxide-Induced Damage in Organotypic Brain Slice Cultures from Adult Mice

Oxidative stress causes severe tissue injury of the central nervous system in ischemic brain damage (IBD), traumatic brain injury (TBI) and neurodegenerative disorders. In this study, we used hydrogen peroxide (H₂O₂) to induce oxidative stress in organotypic brain slice cultures (OBSCs), and investigated the protective effects of oxidative stress-tolerant (OST) stem cells harvested from human exfoliated deciduous teeth (SHED) which were co-cultivated with OBSCs. Using presto blue assay and immunostaining, we demonstrated that both normal SHED and OST-SHED could prevent H₂O₂-induced cell death, and increase the numbers of mature neuron and neuronal progenitors in the hippocampus of OBSCs. During co-cultivation, OST-SHED, but not normal SHED, exhibited neuronal cell morphology and expressed neuronal markers. Results from ELISA showed that both normal SHED and OST-SHED significantly decreased oxidative DNA damage in H₂O₂-treated OBSCs. SHED could also produce neurotrophic factor BDNF (brain derived neurotrophic factor) and promoted the production of IL-6 in OBSCs. Although OST-SHED had lower cell viability, the neuronal protection of OST-SHED was significantly superior to that of normal SHED. Our findings suggest that SHED, especially OST-SHED, could prevent oxidative stress induced brain damage. OST-SHED can be explored as a new therapeutic tool for IBD, TBI and neurodegenerative disorders.

Oxidative stress and hepatotoxicity of Rana chensinensis exposed to low doses of octylphenol

This study investigated the effects of low doses of octylphenol (OP) on the oxidative stress and hepatotoxicity in amphibian liver. The frog, Rana chensinensis, were exposed to 10^-8, 10^-7, 10^-6 mol/L OP for 10, 20 and 30 days. The activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) in liver were reduced at first, then recovered slightly, and the content of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) were increased significantly. Histopathology showed that in some liver tissues of OP treated frogs, the hematococoel expansion, the fat accumulation, the cytoplasmic vacuolization and even hepatocyte necrosis were present. Ultrastructure revealed that there were lipid droplet accumulation, mitochondria deformation and nuclear condensation in some hepatocytes. These results confirm that low doses OP exposure can give rise to oxidative stress in the liver of frogs, reduce antioxidant enzymes activities, lead to partial organelles damage in hepatocyte and the fat accumulate in hepatic tissue.

The physiological concentration of ferrous iron (II) alters the inhibitory effect of hydrogen peroxide on CD45, LAR and PTP1B phosphatases

Hydrogen peroxide is an important regulator of protein tyrosine phosphatase activity via reversible oxidation. However, the role of iron in this reaction has not been yet elucidated. Here we compare the influence of hydrogen peroxide and the ferrous iron (reagent for Fenton reaction) on the enzymatic activity of recombinant CD45, LAR, PTP1B phosphatases and cellular CD45 in Jurkat cells. The obtained results show that ferrous iron (II) is potent inhibitor of CD45, LAR and PTP1B, but the inhibitory effect is concentration dependent. We found that the higher concentrations of ferrous iron (II) increase the inactivation of CD45, LAR and PTP1B phosphatase caused by hydrogen peroxide, but the addition of the physiological concentration (500 nM) of ferrous iron (II) has even a slightly preventive effect on the phosphatase activity against hydrogen peroxide.

Hydroxyl-radical-induced oxidation of 5-methylcytosine in isolated and cellular DNA

The methylation and oxidative demethylation of cytosine in CpG dinucleotides plays a critical role in the regulation of genes during cell differentiation, embryogenesis and carcinogenesis. Despite its low abundance, 5-methylcytosine (5mC) is a hotspot for mutations in mammalian cells. Here, we measured five oxidation products of 5mC together with the analogous products of cytosine and thymine in DNA exposed to ionizing radiation in oxygenated aqueous solution. The products can be divided into those that arise from hydroxyl radical (•OH) addition at the 5,6-double bond of 5mC (glycol, hydantoin and imidazolidine products) and those that arise from H-atom abstraction from the methyl group of 5mC including 5-hydroxymethylcytosine (5hmC) and 5-formylcytosine (5fC). Based on the analysis of these products, we show that the total damage at 5mC is about 2-fold greater than that at C in identical sequences. The formation of hydantoin products of 5mC is favored, compared to analogous reactions of thymine and cytosine, which favor the formation of glycol products. The distribution of oxidation products is sequence dependent in specific ODN duplexes. In the case of 5mC, the formation of 5hmC and 5fC represents about half of the total of •OH-induced oxidation products of 5mC. Several products of thymine, cytosine, 5mC, as well as 8-oxo-7,8-dihydroguanine (8oxoG), were also estimated in irradiated cells.

A novel biological role of dehydroascorbic acid: Inhibition of Na(+)-dependent transport of ascorbic acid

A U937 cell clone, in which low micromolar concentrations of ascorbic acid (AA) and dehydroascorbic acid (DHA) are taken up at identical rates, was used to investigate possible interactions between transport systems mediating cellular uptake of the two forms of the vitamin. Results obtained with different experimental approaches showed that DHA potently and reversibly inhibits AA uptake through Na(+)-AA cotransporters. Hence, a progressive increase in extracellular DHA concentrations in the presence of a fixed amount of AA caused an initial decrease in the net amount of vitamin C accumulated, and eventually, at higher levels, it caused an accumulation of the vitamin solely based on DHA uptake through hexose transporters. DHA-dependent inhibition of AA uptake was also detected in various other cell types. Taken together, our results provide evidence of a novel biological effect mediated by concentrations of DHA compatible with those produced at inflammatory sites.

Intracellular ascorbate enhances hypoxia-inducible factor (HIF)-hydroxylase activity and preferentially suppresses the HIF-1 transcriptional response

Hypoxia-inducible factor (HIF)-1 drives the transcription of hundreds of genes to support cell survival under conditions of microenvironmental and metabolic stress. HIF-1 is downregulated by iron-containing 2-oxoglutarate-dependent enzymes that require ascorbate as a cofactor. The HIF hydroxylases control both protein stability and the formation of an active transcription complex and, consequently, ascorbate could affect HIF-1α stabilization and/or gene expression, but the relative effect of ascorbate on these separate processes has not been well characterized. In this study we examined the effects of known intracellular ascorbate concentrations on both processes in response to various means of hydroxylase inhibition, including CoCl2, NiCl2, desferrioxamine, dimethyloxalylglycine, and hypoxia. Ascorbate inhibited HIF-1 activity most dramatically with all mechanisms of iron competition. In addition, HIF-1-dependent gene expression was effectively prevented by ascorbate and was inhibited even under conditions that allowed HIF-1α protein stabilization. This suggests that (1) ascorbate acts primarily to stabilize and reduce the iron atom in the hydroxylase active site and (2) the asparagine hydroxylase controlling HIF-1 transcriptional activity is particularly susceptible to fluctuations in intracellular ascorbate. These findings suggest that ascorbate plays a significant role in supporting HIF-hydroxylase function and that it could thereby modulate the cell survival response.

The effect of Msh2 knockdown on toxicity induced by tert-butyl-hydroperoxide, potassium bromate, and hydrogen peroxide in base excision repair proficient and deficient cells

The DNA mismatch repair (MMR) and base excision repair (BER) systems are important determinants of cellular toxicity following exposure to agents that cause oxidative DNA damage. To examine the interactions between these different repair systems, we examined whether toxicity, induced by t-BOOH and KBrO₃, differs in BER proficient (Mpg^+/+, Nth1^+/+) and deficient (Mpg^−/−, Nth1^−/−) mouse embryonic fibroblasts (MEFs) following Msh2 knockdown of between 79 and 88% using an shRNA expression vector. Msh2 knockdown in Nth1^+/+ cells had no effect on t-BOOH and KBrO₃ induced toxicity as assessed by an MTT assay; knockdown in Nth1^−/− cells resulted in increased resistance to t-BOOH and KBrO₃, a result consistent with Nth1 removing oxidised pyrimidines. Msh2 knockdown in Mpg^+/+ cells had no effect on t-BOOH toxicity but increased resistance to KBrO₃; in Mpg^−/− cells, Msh2 knockdown increased cellular sensitivity to KBrO₃ but increased resistance to t-BOOH, suggesting a role for Mpg in removing DNA damage induced by these agents. MSH2 dependent and independent pathways then determine cellular toxicity induced by oxidising agents. A complex interaction between MMR and BER repair systems, that is, exposure dependent, also exists to determine cellular toxicity.

Effects of radiation and vitamin C treatment on metronidazole genotoxicity in mice

The impact of exposure to low dose radiation (LDR) on human health is not clear. Besides, cross adaptation or sensitization with pharmaceutical agents may modify the risk of LDR. In the present study, we analyzed the interaction of radiation and metronidazole (MTZ) in inducing chromosome aberration (CA) and micronucleus (MN) in the bone marrow cells of Balb/C mice in vivo. Further, we evaluated the efficacy of vitamin C to reduce MTZ induced genotoxicity. We found that 10, 20 and 40mg/kg of MTZ induced dose dependent increase in the frequency of CA (r=0.9923, P<0.01) as well as MN (r=0.9823, P<0.05) in polychromatic erythrocytes. However, MTZ did not affect the ratio of polychromatic erythrocytes to normochromatic erythrocytes indicating lack of cytotoxicity. Supplementation with vitamin C prior to MTZ treatment significantly reduced the frequency of CA (P<0.001) as well as MN (P<0.001). Radiation (0.5Gy) exposure prior to MTZ treatment produced a less than additive (for CA) to additive (for MN) effects. However, radiation exposure following MTZ treatment produced additive (for CA) and synergistic (for MN) effects. Further, vitamin C pre-treatment also reduced the genotoxicity indices following the combined treatment of MTZ and radiation. Our findings suggest that MTZ may sensitize bone marrow cells to radiation exposure and enhances genotoxicity. We recommend more studies on the interaction of LDR and marketed pharmaceuticals to minimize possible harmful outcomes through appropriate precautionary measures.

DNA damage and repair of human skin keratinocytes concurrently exposed to pyrene derivatives and UVA light

Polycyclic aromatic hydrocarbons (PAHs), a class of mutagenic environmental contaminants, insert toxicity through both metabolic activation and light irradiation. Pyrene, one of the most widely studied PAHs, along with its mono-substituted derivatives, 1-amino, 1-bromo, 1-hydroxy, and 1-nitropyrene, were chosen to study the effect of substituents on their phototoxicity, DNA damage and repair. Both alkaline Comet assay, which detects direct DNA damages, and Fpg endonuclease Comet assay, which detects oxidative DNA damages, were conducted at 0, 2, 4, 8, and 24 h of incubation of the cells in minimal growth medium after concomitant exposure to pyrene derivatives and UVA light. All these compounds are photocytotoxic and the phototoxicity is both incubation time and PAH dose dependent; whereas, those without light are not toxic. The LC50 obtained are in the range of 3.5 - 9.3 µM. Cellular DNA damages, both direct and oxidative, are observed immediately after the cells are treated with UVA light and the pyrene derivatives at a concentration of 1.0 µM. The amount of DNA damages (both direct and oxidative) increase from 0 to 4 h of incubation. After 4 hours, subsequent damage induction declines, and this is perceived to be mainly through DNA repair. After longer incubation of 8 h, the damaged cellular DNA start to be repaired, resulting in greatly reduced amount of DNA damages, and the DNA damage reaches the minimum at 24 h of incubation. 1-Amopyrene and 1-hydroxypyrene cause more DNA oxidative damages immediately after the exposure (0 h of incubation), and these damages are repaired within the same timeframe as the other tested compounds. The oxidative DNA damages caused by 1-bromopyrene are repaired starting at 2 h of incubation, earlier than the damages caused by all the other compounds.

DNA base damage by reactive oxygen species, oxidizing agents, and UV radiation

Emphasis has been placed in this article dedicated to DNA damage on recent aspects of the formation and measurement of oxidatively generated damage in cellular DNA in order to provide a comprehensive and updated survey. This includes single pyrimidine and purine base lesions, intrastrand cross-links, purine 5',8-cyclonucleosides, DNA-protein adducts and interstrand cross-links formed by the reactions of either the nucleobases or the 2-deoxyribose moiety with the hydroxyl radical, one-electron oxidants, singlet oxygen, and hypochlorous acid. In addition, recent information concerning the mechanisms of formation, individual measurement, and repair-rate assessment of bipyrimidine photoproducts in isolated cells and human skin upon exposure to UVB radiation, UVA photons, or solar simulated light is critically reviewed.

Enhanced antitumor activity of vitamin C via p53 in cancer cells

Ascorbate is an important natural antioxidant that can selectively kill cancer cells at pharmacological concentrations. Despite its benefit, it is quite difficult to predict the antitumor effects of ascorbate, because the relative cytotoxicity of ascorbate differs between cancer cell lines. Therefore, it is essential to examine the basis for this fundamental disagreement. Because p53 is activated by DNA-damaging stress and then regulates various cellular conditions, we hypothesized that p53 can sensitize cancer cells to ascorbate. Using isogenic cancer cells, we observed that the presence of p53 can affect ascorbate cytotoxicity, and also reactivation of p53 can make cancer cells sensitive to ascorbate. p53-dependent enhancement of ascorbate cytotoxicity is caused by increased reactive oxygen species generation via a differentially regulated p53 transcriptional network. We also found that transcriptionally activated p53 was derived from MDM2 ubiquitination by ascorbate and subsequently its signaling network renders cancer cells more susceptible to oxidative stress. Similar to the p53 effect on in vitro ascorbate cytotoxicity, inhibition of tumor growth is also stronger in p53-expressing tumors than in p53-deficient ones in vivo. This is the first observation that ascorbate cytotoxicity is positively related to p53 expression, activating its transcriptional network to worsen intracellular oxidative stress and consequently enhancing its cytotoxicity. Based on our study, reactivation of p53 may help to achieve more consistent cytotoxic effects of ascorbate in cancer therapies.

Melatonin prevents hypochlorous acid-induced alterations in microtubule and chromosomal structure in metaphase-II mouse oocytes

Hypochlorous acid (HOCl) is generated by myeloperoxidase, using chloride and hydrogen peroxide as substrates. Here we demonstrate that HOCl alters metaphase-II mouse oocyte microtubules and chromosomal (CH) alignment which can be prevented by melatonin. Metaphase-II mouse oocytes, obtained commercially, were grouped as: control, melatonin (150, 200nmol/mL), HOCl (10, 20, 50, and 100nmol/mL), and HOCl (50nmol/mL) pretreated with 150 and 200 nmol/mL of melatonin. Microtubule and CH alignment was studied utilizing an indirect immunofluorescence technique and scored by two observers. Pearson chi-square test and Fisher's exact test were used to compare outcomes between controls and treated groups and also among each group. Poor scores for the spindle and chromosomes increased significantly at 50nmol/mL of HOCl (P<0.001). Oocytes treated with melatonin only at 150 and 200 nmol/mL showed no changes; significant differences (P<0.001) were observed when oocytes exposed to 50nmol/mL of HOCl were compared to oocytes pretreated with 200 nmol/mL melatonin. Fifty percent of the oocytes demonstrated good scores, both in microtubule and CH alterations, when pretreated with melatonin at 150 nmol/mL compared to 0% in the HOCl-only group. HOCl alters the metaphase-II mouse oocyte spindle and CH alignment in a dose-dependant manner, which might be a potential cause of poor oocyte quality (e.g., in patients with endometriosis). Melatonin prevented the HOCl-mediated spindle and CH damage, and therefore, may be an attractive therapeutic option to prevent oocyte damage in endometriosis or inflammatory diseases where HOCl levels are known to be elevated.

Induction of DNA damage by deguelin is mediated through reducing DNA repair genes in human non-small cell lung cancer NCI-H460 cells

It has been shown that deguelin, one of the compounds of rotenoids from flavonoid family, induced cytotoxic effects through induction of cell cycle arrest and apoptosis in many types of human cancer cell lines, but deguelin-affected DNA damage and repair gene expression (mRNA) are not clarified yet. We investigated the effects of deguelin on DNA damage and associated gene expression in human lung cancer NCI-H460 cells in vitro. DNA damage was assayed by using the comet assay and DNA gel electrophoresis and the results indicated that NCI-H460 cells treated with 0, 50, 250 and 500 nM deguelin led to a longer DNA migration smear based on the single cell electrophoresis and DNA fragmentation occurred based on the examination of DNA gel electrophoresis. DNA damage and repair gene expression (mRNA) were evaluated by using real-time PCR assay and the results indicated that 50 and 250 nM deguelin for a 24-h exposure in NCI-H460 cells, decreased the gene levels of breast cancer 1, early onset (BRCA1), DNA-dependent serine/threonine protein kinase (DNA-PK), O⁶-methylguanine-DNA methyltransferase (MGMT), p53, ataxia telangiectasia mutated (ATM) and ataxia-telangiectasia and Rad3-related (ATR) mRNA expressions. Collectively, the present study showed that deguelin caused DNA damage and inhibited DNA damage and repair gene expressions, which might be due to deguelin-inhibited cell growth in vitro.

First Steps Towards an Understanding of a Mode ofCarcinogenic Action for Vanadium Pentoxide

Inhalation of vanadium pentoxide clearly increases the incidence of alveolar/bronchiolar neoplasms in male and female B6C3F1 mice at all concentrations tested (1, 2 or 4 mg/m³), whereas responses in F344/N rats was, at most, ambiguous. While vanadium pentoxide is mutagenic in vitro and possibly in vivo in mice, this does not explain the species or site specificity of the neoplastic response. A nose-only inhalation study was conducted in female B6C3F1 mice (0, 0.25, 1 and 4 mg/m³, 6 h/day for 16 days) to explore histopathological, biochemical (α-tocopherol, glutathione and F2-isoprostane) and genetic (comet assays and 9 specific DNA-oxo-adducts) changes in the lungs. No treatment related histopathology was observed at 0.25 mg/m³. At 1 and 4 mg/m³, exposure-dependent increases were observed in lung weight, alveolar histiocytosis, sub-acute alveolitis and/or granulocytic infiltration and a generally time-dependent increased cell proliferation rate of histiocytes. Glutathione was slightly increased, whereas there were no consistent changes in α-tocopherol or 8-isoprostane F2α. There was no evidence for DNA strand breakage in lung or BAL cells, but there was an increase in 8-oxodGuo DNA lesions that could have been due to vanadium pentoxide induction of the lesions or inhibition of repair of spontaneous lesions. Thus, earlier reports of histopathological changes in the lungs after inhalation of vanadium pentoxide were confirmed, but no evidence has yet emerged for a genotoxic mode of action. Evidence is weak for oxidative stress playing any role in lung carcinogenesis at the lowest effective concentrations of vanadium pentoxide.

Carboxymethyl chitin-glucan (CM-CG) protects human HepG2 and HeLa cells against oxidative DNA lesions and stimulates DNA repair of lesions induced by alkylating agents

A large number of functional foods, including those that contain β-d-glucans, have been shown to prevent human DNA against genotoxic effects and associated development of cancer and other chronic diseases. In this paper, carboxymethyl chitin-glucan (CM-CG) isolated from Aspergillus niger was investigated from two standpoints: (1) DNA-protective effects against oxidative DNA damage induced by H(2)O(2) and alkylating DNA damage induced by MMS and MNNG, and (2) a potential effect on rejoining of MMS- and MNNG-induced single strand DNA breaks. The results obtained by the comet assay in human cells cultured in vitro showed that CM-CG reduced significantly the level of oxidative DNA lesions induced by H(2)O(2) but did not change the level of alkylating DNA lesions induced by MMS or MNNG. On the other side, the efficiency of DNA-rejoining of single strand DNA breaks induced by MMS and MNNG was significantly higher in HepG2 cells pre-treated with CM-CG. The antioxidative activity of carboxymethyl chitin-glucan was confirmed by the DPPH assay.

Oxidation reactions of cytosine DNA components by hydroxyl radical and one-electron oxidants in aerated aqueous solutions

Indirect evidence strongly suggests that oxidation reactions of cytosine and its minor derivative 5-methylcytosine play a major role in mutagenesis and cancer. Therefore, there is an emerging necessity to identify the final oxidation products of these reactions, to search for their formation in cellular DNA, and to assess their mutagenic features. In this Account, we report and discuss the main *OH and one-electron-mediated oxidation reactions, two of the most potent sources of DNA damage, of cytosine and 5-methylcytosine nucleosides that have been recently characterized. The addition of *OH to the 5,6-unsaturated double bond of cytosine and 5-methylcytosine generates final degradation products that resemble those observed for uracil and thymine. The main product from the oxidation of cytosine, cytosine glycol, has been shown to undergo dehydration at a much faster rate as a free nucleoside than when inserted into double-stranded DNA. On the other hand, the predominant *OH addition at C5 of cytosine or 5-methylcytosine leads to the formation of 5-hydroxy-5,6-dihydro radicals that give rise to novel products with an imidazolidine structure. The mechanism of the formation of imidazolidine products is accounted for by rearrangement reactions that in the presence of molecular oxygen likely involve an intermediate pyrimidine endoperoxide. The reactions of the radical cations of cytosine and 5-methylcytosine are governed by competitive hydration, mainly at C6 of the pyrimidine ring, and deprotonation from the exocyclic amino and methyl group, leading in most cases to products similar to those generated by *OH. 5-Hydroxypyrimidines, the dehydration products of cytosine and uracil glycols, have a low oxidation potential, and their one-electron oxidation results in a cascade of decomposition reactions involving the formation of isodialuric acid, dialuric acid, 5-hydroxyhydantoin, and its hydroxyketone isomer. In biology, GC --> AT transitions are the most common mutations in the genome of aerobic organisms, including the lacI gene in bacteria, lacI transgenes in rodents, and the HPRT gene in rodents and humans, so a more complete understanding of cytosine oxidation is an essential research goal. The data and insights presented here shed new light on oxidation reactions of cytosine and 5-methylcytosine and should facilitate their validation in cellular DNA.

Effects of low dose radiation and vitamin C treatment on chloroquine-induced genotoxicity in mice

Chloroquine (CHQ) is a commonly used antimalarial agent. We evaluated the genotoxic potential of CHQ using chromosome aberration (CA), micronucleus (MN), and sperm head abnormality (SA) assays in vivo in Swiss albino mice. The interaction between a low dose of radiation and CHQ, as well as the effect of vitamin C on CHQ-induced genotoxicity, was also evaluated. It was observed that CHQ induced CA, as well as MN, in the bone marrow cells under certain treatment conditions. Further, CHQ induced significant increase in the frequency of SA both at 24 hr and 21 days of the treatment. In the present study vitamin C pretreatment apparently reduced the frequency of CA, MN, and SA induced by CHQ. In the combination studies with radiation and CHQ, we found that exposure to low doses of radiation (0.5 Gy) either prior to or following CHQ treatment, in the dose ranges tested, has little or no synergistic effect in the mutagenic evaluations in somatic cells. However, radiation exposure along with CHQ treatment resulted in significant increase in the frequency of SA as compared to the groups receiving CHQ alone at 21 days of the treatment. In summary, CHQ has the potential to induce genotoxicity in mammalian cells. Further, germ cells may be relatively more sensitive as compared to the somatic cells.

Vitamin C modulation of H2O2-induced damage and iron homeostasis in human cells

Vitamin C (ascorbic acid, AA) is an important antioxidant in human plasma. It is clear, however, that AA has other important, nonantioxidant roles in cells. Of particular interest is its involvement in iron metabolism, since AA enhances dietary iron absorption, increases the activity of Fe(2+)-dependent cellular enzymes, promotes Fenton reactions in vitro, and was reported to have deleterious effects in individuals with iron overload. Nevertheless, the ability of AA to modulate iron metabolism and enhance iron-dependent damage in cells, tissues, and organisms has not been fully elucidated. Here we investigated the effect of AA on iron-mediated oxidative stress in normal human fibroblasts. Incubation with physiologically relevant concentrations of AA was not harmful but sensitised cells toward H(2)O(2)-induced, iron-dependent DNA strand breakage and cell death. We also report that AA increased the levels of intracellular catalytic iron and concomitantly modulated the expression of two well-established iron-regulated genes, ferritin and transferrin receptor. In summary, we present evidence of a novel, nonantioxidant role of AA in human cells, where it increases iron availability and enhances ROS-mediated, iron-dependent damage. We suggest that AA may exacerbate the deleterious effects of metals in vivo and promote normal tissue injury in situations associated with elevated ROS production.

Oxidative stress and tumor necrosis factor–α–induced alterations in metaphase II mouse oocyte spindle structure

OBJECTIVE(S)

To examine the effect of exogenous exposure to hydrogen peroxide (H(2)O(2)) and tumor necrosis factor (TNF)-alpha on mouse metaphase II (MII) oocyte spindle structure and to examine the potential benefits of supplementing the culture media with vitamin C.

DESIGN

Prospective study.

SETTING

Research laboratory in a tertiary hospital.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Microtubule changes and alterations in chromosomal alignment.

RESULT(S)

Both concentration- and time-dependent alterations were seen in spindle structure after exposure to H(2)O(2). An H(2)O(2) concentration as low as 12.5 microM increased the odds of an oocyte with altered microtubule and chromosome alignment (score >or=3) by 93%. Significantly increased damage was seen with increasing period of incubation. Higher scores were seen after exposure to both TNF-alpha alone and in combination with H(2)O(2) compared with controls. Changes in chromosomal alignment were comparable among the three groups. Oocytes coincubated with H(2)O(2) and vitamin C at 200 microM demonstrated less damage compared with those with H(2)O(2) alone.

CONCLUSION(S)

Oxidative stress results in concentration and time-dependent alterations in the spindle structure and augments the effects induced by TNF-alpha. Proper oocyte handling in vitro may help reduce oxidative insult, thus improving the oocyte quality. Antioxidants may have a protective effect and need to be further evaluated.