DNA breakage by tannic acid and Cu(II): sequence specificity of the reaction and involvement of active oxygen species

Interaction of C20-substituted derivative of pregnenolone acetate with copper (II) leads to ROS generation, DNA cleavage and apoptosis in cervical cancer cells: Therapeutic potential of copper chelation for cancer treatment

Cervical cancer is a leading cause of cancer-related deaths among women in developing countries. Therefore, development of new chemotherapeutic agents is required. Unlike normal cells, cancer cells contain elevated copper levels which play an integral role in angiogenesis. Thus, targeting copper via copper-specific chelators in cancer cells can serve as effective anticancer strategy. In this work, a copper chelator pregnenolone acetate nucleus-based tetrazole derivative (ligand-L) was synthesized and characterized by elemental analysis, ESI-MS, ¹H NMR and ¹³C NMR. DNA binding ability of ligand-L was studied using UV-Vis and fluorescence spectroscopy. Fluorescence spectroscopy studies reveal that quenching constant of ligand-l-DNA and ligand-L-Cu(II) were found to be 7.4 × 10³ M^-1 and 8.8 × 10³ M^-1, respectively. In vitro toxicity of ligand-L was studied on human cervical cancer C33A cancer cells. Results showed that ligand-L exhibit significant cytotoxic activity against cervical cancer C33A cells with IC₅₀ value 5.0 ± 1.8 µM. Further, it was found that ligand-L cytotoxicity is due to redox cycling of copper to generate ROS which leads to DNA damage and apoptosis. In conclusion, this is the report where we synthesized pregnenolone acetate-based tetrazole derivative against C33A cells that targets cellular copper to induce pro-oxidant death in cancer cells. These findings will provide significant insights into the development of new chemical molecules with better copper chelating and pro-oxidant properties against cancer cells.

Synthesis of novel coumarin nucleus-based DPA drug-like molecular entity: In vitro DNA/Cu(II) binding, DNA cleavage and pro-oxidant mechanism for anticancer action

Despite substantial research on cancer therapeutics, systemic toxicity and drug-resistance limits the clinical application of many drugs like cisplatin. Therefore, new chemotherapeutic strategies against different malignancies are needed. Targeted cancer therapy is a new paradigm for cancer therapeutics which targets pathways or chemical entities specific to cancer cells than normal ones. Unlike normal cells, cancer cells contain elevated copper which plays an integral role in angiogenesis. Copper is an important metal ion associated with chromatin DNA, particularly with guanine. Thus, targeting copper via copper-specific chelators in cancer cells can serve as an effective anticancer strategy. New pharmacophore di(2-picolyl)amine (DPA)-3(bromoacetyl) coumarin (ligand-L) was synthesized and characterized by IR, ESI-MS, 1H- and 13C-NMR. Binding ability of ligand-L to DNA/Cu(II) was evaluated using a plethora of biophysical techniques which revealed ligand-L-DNA and ligand-L-Cu(II) interaction. Competitive displacement assay and docking confirmed non-intercalative binding mode of ligand-L with ctDNA. Cyclic voltammetry confirmed ligand-L causes quasi reversible Cu(II)/Cu(I) conversion. Further, acute toxicity studies revealed no toxic effects of ligand-L on mice. To evaluate the chemotherapeutic potential and anticancer mechanism of ligand-L, DNA damage via pBR322 cleavage assay and reactive oxygen species (ROS) generation were studied. Results demonstrate that ligand-L causes DNA cleavage involving ROS generation in the presence of Cu(II). In conclusion, ligand-L causes redox cycling of Cu(II) to generate ROS which leads to oxidative DNA damage and pro-oxidant cancer cell death. These findings will establish ligand-L as a lead molecule to synthesize new molecules with better copper chelating and pro-oxidant properties against different malignancies.

Mobilization of Nuclear Copper by Green Tea Polyphenol Epicatechin-3-Gallate and Subsequent Prooxidant Breakage of Cellular DNA: Implications for Cancer Chemotherapy

Epidemiological as well as experimental evidence exists in support of chemopreventive and anticancer properties of green tea and its constituents. The gallocatechin, epicatechin-3-gallate is a major polyphenol present in green tea, shown responsible for these effects. Plant-derived polyphenolic compounds are established natural antioxidants which are capable of catalyzing oxidative DNA degradation of cellular DNA, alone as well as in the presence of transition metal ions, such as copper. Here we present evidence to support that, similar to various other polyphenoic compounds, epicatechin-3-gallate also causes oxidative degradation of cellular DNA. Single cell alkaline gel electrophoresis (Comet assay) was used to assess DNA breakage in lymphocytes that were exposed to various concentrations of epicatechin-3-gallate. Inhibition of DNA breakage in the presence of scavengers of reactive oxygen species (ROS) suggested involvement of ROS generation. Addition of neocuproine (a cell membrane permeable Cu(I) chelator) inhibited DNA degradation, dose-dependently, in intact lymphocytes. In contrast, bathocuproine, which does not permeate cell membrane, was observed to be ineffective. We further show that epicatechin-3-gallate degrades DNA in cell nuclei, which can also be inhibited by neocuproine, suggesting mobilization of nuclear copper in this reaction as well. Our results are indicative of ROS generation, possibly through mobilization of endogenous copper ions, and support our long-standing hypothesis of a prooxidant activity of plant-derived polyphenols as a mechanism for their documented anticancer properties.

Modulatory potential of resveratrol during lung inflammatory disease

Neutrophils are the first cells to achieve the sites of infection or inflammation in the lungs. The massive accumulation of these cells is associated with acute and chronic lung injury. Therefore, they have been implicated in the pathogenesis of many lung diseases through the release of reactive oxygen intermediates, proteolytic enzymes and Neutrophil Extracellular Traps (NETs). The excessive and continuous release of NETs, fibers composed by decondensed chromatin coated with neutrophil proteins, are associated to the impairment of lung function in different pathological settings. Flavonoids inhibit the respiratory burst of neutrophils in mammals. However, one of these flavonoids, resveratrol has a particular chemical property. It reduce Cu(II) to Cu(I) form with concomitant formation of reactive oxygen species, which can produce DNA breakage as reported in several in vitro models. We hypothesize that direct resveratrol administration in lungs can cleave DNA in NETs, improving lung function during acute airway infections or chronic inflammatory lung diseases. If the hypothesis is correct, the control of NET formation can be used to reduce the inflammatory environment in lung after neutrophil stimuli. Additionally, the production of proinflammatory cytokines by neutrophils could be also diminished by resveratrol administration. In this sense, this flavonoid provides a multifaceted opportunity for treatment of lung diseases with strong or chronic neutrophil activation.

Cu(II)-coumestrol interaction leads to ROS-mediated DNA damage and cell death: a putative mechanism for anticancer activity

Phytoestrogens have attracted considerable interest as natural alternatives to hormone replacement therapy and their potential as cancer therapeutic agents. Among phytoestrogens, coumestrol has shown multipharmacological properties such as antiinflammatory, neuroprotective, osteoblastic differentiation and anticancer. Though several studies have described anticancer effects of coumestrol, a clear underlying molecular mechanism has not been elucidated. Unlike normal cells, cancer cells contain elevated copper levels that play an integral role in angiogenesis. Copper is an important metal ion associated with the chromatin DNA, particularly with guanine. Thus, targeting copper in cancer cells can serve as effective anticancer strategy. Using human peripheral lymphocytes, we assessed lipid peroxidation, protein carbonylation, reactive oxygen species (ROS) generation, DNA damage and apoptosis by coumestrol in the presence of exogenously added Cu(II) in cells to simulate malignancy-like condition. Results showed that Cu(II)-coumestrol interaction leads to lipid peroxidation and protein carbonylation (markers of oxidative stress), DNA fragmentation and apoptosis in treated lymphocytes. Further, incubation of lymphocytes with ROS scavengers and membrane-permeant copper chelator, neocuproine, resulted in inhibition of DNA damage and apoptosis. This suggests that coumestrol engages in redox cycling of Cu(II) to generate ROS that leads to DNA fragmentation and apoptosis. In conclusion, this is the first report showing that coumestrol targets cellular copper to induce prooxidant death in malignant cells. We believe that such a prooxidant cytotoxic mechanism better explains the anticancer activity of coumestrol. These findings will provide significant insights into the development of new chemical molecules with better copper-chelating and prooxidant properties against cancer cells.

Mobilization of Copper ions by Flavonoids in Human Peripheral Lymphocytes Leads to Oxidative DNA Breakage: A Structure Activity Study

Epidemiological studies have linked dietary consumption of plant polyphenols with lower incidence of various cancers. In particular, flavonoids (present in onion, tomato and other plant sources) induce apoptosis and cytotoxicity in cancer cells. These can therefore be used as lead compounds for the synthesis of novel anticancer drugs with greater bioavailability. In the present study, we examined the chemical basis of cytotoxicity of flavonoids by studying the structure–activity relationship of myricetin (MN), fisetin (FN), quercetin (QN), kaempferol (KL) and galangin (GN). Using single cell alkaline gel electrophoresis (comet assay), we established the relative efficiency of cellular DNA breakage as MN > FN > QN > KL > GN. Also, we determined that the cellular DNA breakage was the result of mobilization of chromatin-bound copper ions and the generation of reactive oxygen species. The relative DNA binding affinity order was further confirmed using molecular docking and thermodynamic studies through the interaction of flavonoids with calf thymus DNA. Our results suggest that novel anti-cancer molecules should have ortho-dihydroxy groups in B-ring and hydroxyl groups at positions 3 and 5 in the A-ring system. Additional hydroxyl groups at other positions further enhance the cellular cytotoxicity of the flavonoids.

Mobilization of copper ions in human peripheral lymphocytes by catechins leading to oxidative DNA breakage: A structure activity study

Epidemiological studies suggest that dietary consumption of plant polyphenols is related to a lower incidence of various cancers. Among these compounds catechins (present in green tea and other beverages) are considered to be potent inducers of apoptosis and cytotoxicity to cancer cells. Thus these compounds can be used as leads to synthesize novel anticancer drugs with greater bioavailability. In view of this in this paper we have examined the chemical basis of cytotoxicity of catechins by studying the structure-activity relationship between catechin (C), epicatechin (EC), epigallocatechin (EGC) and epigallocatechin-3-gallate (EGCG). Using single cell alkaline gel electrophoresis (comet assay) we have established the relative efficiency of cellular DNA breakage as EGCG>EGC>EC>C. We also show that cellular DNA breakage is the result of mobilization of copper ions bound to chromatin and the generation of reactive oxygen species. Further the relative DNA binding affinity order was confirmed using molecular docking and thermodynamic studies by studying the interaction of catechins with calf thymus DNA. The results suggest that the synthesis of any novel anti cancer molecule based on the structure of catechins should have as many galloyl moieties as possible resulting in an increased number of hydroxyl groups that may facilitate the binding of the molecule to cellular DNA.

Redox cycling of endogenous copper by thymoquinone leads to ROS-mediated DNA breakage and consequent cell death: putative anticancer mechanism of antioxidants

Plant-derived dietary antioxidants have attracted considerable interest in recent past for their chemopreventive and cancer therapeutic abilities in animal models. Thymoquinone (TQ) is the major bioactive constituent of volatile oil of Nigella sativa and has been shown to exert various pharmacological properties, such as anti-inflammatory, cardiovascular, analgesic, anti-neoplastic, anticancer and chemopreventive. Although several mechanisms have been suggested for the chemopreventive and anticancer activity of TQ, a clear mechanism of action of TQ has not been elucidated. TQ is a known antioxidant at lower concentrations and most of the studies elucidating the mechanism have centered on the antioxidant property. However, recent publications have shown that TQ may act as a prooxidant at higher concentrations. It is well known that plant-derived antioxidants can switch to prooxidants even at low concentrations in the presence of transition metal ions such as copper. It is well established that tissue, cellular and serum copper levels are considerably elevated in various malignancies. Copper is an important metal ion present in the chromatin and is closely associated with DNA bases, particularly guanine. Using human peripheral lymphocytes and comet assay, we first show that TQ is able to cause oxidative cellular DNA breakage. Such a DNA breakage can be inhibited by copper-chelating agents, neocuproine and bathocuproine, and scavengers of reactive oxygen species. Further, it is seen that TQ targets cellular copper in prostate cancer cell lines leading to a prooxidant cell death. We believe that such a prooxidant cytotoxic mechanism better explains the anticancer activity of plant-derived antioxidants.

The antioxidant ascorbic acid mobilizes nuclear copper leading to a prooxidant breakage of cellular DNA: implications for chemotherapeutic action against cancer

PURPOSE

Ascorbic acid is an essential micronutrient and is considered to have an antioxidant function in living systems. For the past several decades, ascorbic acid has been the subject of considerable interest as an anticancer agent. Several studies have shown that ascorbic acid is cytotoxic to a variety of cancer cells, whereas normal cells are relatively resistant to such cytotoxic action. In this study, we propose a putative molecular mechanism that accounts for the preferential cytotoxicity of ascorbic acid against cancer cells.

METHODS

Standard and lysed version of alkaline single-cell gel electrophoresis (Comet assay); ferrous oxidation-xylenol orange (FOX) assay.

RESULTS

We show that ascorbic acid acts as a prooxidant and leads to oxidative DNA breakage in lymphocytes and lymphocyte nuclei. Scavengers of reactive oxygen species were able to inhibit ascorbic acid-induced DNA breakage, suggesting the involvement of reactive oxygen species in this reaction. We further show that such DNA breakage is inhibited by both iron and copper chelators in cells, whereas in nuclei, similar inhibition was achieved only by copper chelators, indicating an important role of chromatin-bound copper in the prooxidant cellular DNA breakage by ascorbic acid.

CONCLUSION

We propose that the copper-dependent cellular redox status is an important element in the cytotoxic action of ascorbic acid against cancer cells. It is well established that cellular copper levels are considerably elevated in various malignancies. Therefore, cancer cells may be more subject to electron transfer between copper and ascorbate to generate reactive oxygen species. In light of these observations and those in literature, in this paper we explain that the preferential cytotoxicity of ascorbic acid against cancer cells is the result of elevated copper levels in such cells. Further, this study identifies nuclear copper as a novel molecular target for cytotoxic action of ascorbic acid, which has implications for its chemotherapeutic properties against cancer.

Plumbagin induces cell death through a copper-redox cycle mechanism in human cancer cells

Plumbagin, a naphthoquinone derived from the medicinal plant Plumbago zeylanica has been shown to exert anticancer and anti-proliferative activities in cells in culture as well as animal tumor models. In our previous paper, we have reported the cytotoxic action of plumbagin in plasmid pBR322 DNA as well as human peripheral blood lymphocytes through a redox mechanism involving copper. Copper has been shown to be capable of mediating the action of several plant-derived compounds through production of reactive oxygen species (ROS). The objective of the present study was to determine whether plumbagin induces apoptosis in human cancer cells through the same mechanism which we proposed earlier. Using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt assay, 3-(4,5-B-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay for cell growth inhibition, histone/DNA ELISA, homogeneous caspase-3/7 assay for apoptosis as well as alkaline comet assay for DNA single-strand breaks detection in this report, we confirm that plumbagin causes effective cell growth inhibition, induces apoptosis and generates single-strand breaks in cancer cells. Incubation of cancer cells with scavengers of ROS and neocuproine inhibited the cytotoxic action of plumbagin proving that generation of ROS and Cu(I) are the critical mediators in plumbagin-induced cell growth inhibition. This study is the first to investigate the copper-mediated anticancer mechanism of plumbagin in human cancer cells and these properties of plumbagin could be further explored for the development of anticancer agents with higher therapeutic indices, especially for skin cancer.

The efficacy of protective effects of tannic acid, gallic acid, ellagic acid, and propyl gallate against hydrogen peroxide-induced oxidative stress and DNA damages in IMR-90 cells

There is increasing evidence that reactive oxygen species (ROS) are intimately involved in the oxidative damage of tissues for a wide variety of pulmonary diseases. Thus, it is desirable to search for chemopreventive agents that can counteract ROS-mediated injury to the pulmonary tissues. Using a human lung fibroblast IMR-90 cells as the experimental model, we first demonstrated that nearly 90% of intracellular ROS could be removed when H(2)O(2)-treated cells (200 microM) simultaneously incubated with 10 microg/mL of tannic acid (TA), gallic acid (GA), ellagic acid (EA), and propyl gallate (PA). Using C(11)-BODIPY(581/591 )as a lipid peroxidation probe, we also attested that all these compounds examined (10 microg/mL) could alleviate H(2)O(2)-evoked lipid peroxidation phenomena. Next, we examined the protective effects of these compounds on the depletion of intracellular glutathione (iGSH) in H(2)O(2)-treated cells using CMF-DA probe. Interestingly, PA was demonstrated to be the only compound that could effectively protect the integrity of iGSH from being depleted by this system. Finally, the protective effects of these compounds against oxidative DNA damage were evaluated using 8-oxoguanine formation as a marker. Our data indicated that all four compounds suppressed the formation of 8-oxoguanine effectively. Taken together, our data suggested that TA, GA, EA, and PA can protect cells from oxidative stress.

Prooxidant DNA breakage induced by caffeic acid in human peripheral lymphocytes: Involvement of endogenous copper and a putative mechanism for anticancer properties

Plant-derived dietary material contains several classes of polyphenols such as flavonoids, curcuminoids, stilbenes and hydroxycinnamic acids. They are recognized as naturally occurring antioxidants but also act as prooxidants catalyzing cellular DNA degradation in the presence of transition metal ions such as copper. Earlier we have shown that the stilbene resveratrol is able to mobilize endogenous copper ions leading to oxidative breakage of cellular DNA. In this paper, we show that caffeic acid (a hydroxycinnamic acid), which is a major constituent of coffee, is also capable of DNA breakage in human peripheral lymphocytes. Incubation of lymphocytes with neocuproine inhibited the DNA degradation confirming that Cu(I) is an intermediate in the DNA cleavage reaction. Further, we have also shown that caffeic acid generates oxidative stress in lymphocytes, which is inhibited by scavengers of reactive oxygen species and neocuproine. These results are in further support of our hypothesis that anticancer mechanism of plant polyphenols involves mobilization of endogenous copper, possibly chromatin bound copper, and the consequent prooxidant action.

Ascorbic acid mobilizes endogenous copper in human peripheral lymphocytes leading to oxidative DNA breakage: a putative mechanism for anticancer properties

Several decades back ascorbic acid was proposed as an effective anticancer agent. However, this idea remained controversial and the mechanism of action unclear. In this paper, we show that ascorbic acid at a concentration reported to be achievable through high doses of oral consumption is capable of cytotoxic action against normal cells. Several antioxidants of both animal as well as plant origin including ascorbic acid also possess prooxidant properties. Copper is an essential component of chromatin and can take part in redox reactions. Previously we have proposed a mechanism for the cytotoxic action of plant antioxidants against cancer cells that involves mobilization of endogenous copper ions and the consequent generation of reactive oxygen species. Using human peripheral lymphocytes and Comet assay we show here that ascorbic acid is able to cause oxidative DNA breakage in normal cells at a concentration of 100-200 microM. Neocuproine, a Cu(I) specific sequestering agent inhibited DNA breakage in a dose dependent manner indicating that Cu(I) is an intermediate in the DNA cleavage reaction. The results are in support of our above hypothesis that involves events that lead to a prooxidant action by antioxidants. The results would support the idea that even a plasma concentration of around 200 microM. would be sufficient to cause pharmacological tumor cell death particularly when copper levels are elevated. This would account for the observation of several decades back by Pauling and co-workers where oral doses of ascorbic acid in gram quantities were found to be effective in treating some cancers.

Resveratrol-Cu(II) induced DNA breakage in human peripheral lymphocytes: implications for anticancer properties

Resveratrol (3,4',5-trihydroxy stilbene), a plant derived polyphenol found in mulberries, grapes and red wine is considered to possess chemopreventive properties against cancer. It is recognized as a naturally occurring antioxidant but also catalyzes oxidative DNA degradation in vitro in the presence of transition metal ions such as copper. Using a cellular system of lymphocytes isolated from human peripheral blood and Comet assay, we have confirmed that resveratrol-Cu(II) system is indeed capable of causing DNA degradation in cells such as lymphocytes. Also, trans-stilbene, which does not have any hydroxyl groups, is inactive in the lymphocyte system. Pre-incubation of lymphocytes with resveratrol indicates that it is capable of either traversing the cell membrane or binding to it. Our results are in partial support of our hypothesis that anticancer properties of various plant derived polyphenols may involve mobilization of endogenous copper and the consequent prooxidant action.

The antioxidant effect of tannic acid on the in vitro copper-mediated formation of free radicals

Tannic acid (TA) has well-described antimutagenic and antioxidant activities. The antioxidant activity of TA has been previously attributed to its capacity to form a complex with iron ions, interfering with the Fenton reaction [Biochim. Biophys. Acta 1472, 1999, 142]. In this work, we observed that TA inhibits, in the micromolar range, in vitro Cu(II) plus ascorbate-mediated hydroxyl radical (*OH) formation (determined as 2-deoxyribose degradation) and oxygen uptake, as well as copper-mediated ascorbate oxidation and ascorbate radical formation (quantified in EPR studies). The effect of TA against 2-deoxyribose degradation was three orders of magnitude higher than classic *OH scavengers, but was similar to several other metal chelators. Moreover, the inhibitory effectiveness of TA, by the four techniques used herein, was inversely proportional to the Cu(II) concentration in the media. These results and the observation of copper-induced changes in the UV spectra of TA are indications that the antioxidant activity of TA relates to its copper chelating ability. Thus, copper ions complexed to TA are less capable of inducing ascorbate oxidation, inhibiting the sequence of reactions that lead to 2-deoxyribose degradation. On the other hand, the efficiency of TA against 2-deoxyribose degradation declined considerably with increasing concentrations of the *OH detector molecule, 2-deoxyribose, suggesting that the copper-TA complex also possesses an *OH trapping activity.

Prooxidant property of green tea polyphenols epicatechin and epigallocatechin-3-gallate: implications for anticancer properties

It is believed that anticancer and apoptosis inducing properties of green tea are mediated by it's polyphenolic constituents particularly catechins. A number of reports have shown that green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) is among the most effective chemopreventive and apoptosis-inducing agents present in the beverage. Plant polyphenols are naturally occurring antioxidants but they also exhibit prooxidant properties. Over the last several years we have shown that various classes of plant polyphenols including flavonoids, curcuminoids and tannins are capable of catalyzing oxidative DNA cleavage particularly in the presence of transition metal ions such as copper and iron. With a view to understand the chemical basis of various pharmacological properties of green tea, in this paper we have compared the prooxidant properties of green tea polyphenols--EGCG and EC ((-)-epicatechin). The rate of oxidative DNA degradation as well as hydroxyl radical and superoxide anion formation was found to be greater in the case of EGCG as compared with EC. It was also shown that copper mediated oxidation of EC and EGCG possibly leads to the formation of polymerized polyphenols. Further, it was indicated that copper oxidized catechins were more efficient prooxidants as compared with their unoxidized forms. These results correlate with the observation by others that EGCG is the most effective apoptosis inducing polyphenol present in green tea. They are also in support of our hypothesis that prooxidant action of plant polyphenols may be an important mechanism of their anticancer properties. A model for binding of Cu(II) to EC has been presented where the formation of quinone and a quinone methide has been proposed.

Effects of tannic acid and its related compounds on food mutagens or hydrogen peroxide-induced DNA strands breaks in human lymphocytes

The effect of tannic acid (TA), gallic acid (GA), propyl gallate (PA) and ellagic acid (EA) on DNA damage in human lymphocytes induced by food mutagens [3-amino-1-methyl-5H-pyrido (4,3-b) indole (Trp-P-2) and 2-amino-1-methyl-6-phenylimadazo (4,5-b) pyridine (PhIP) or H2O2 was evaluated by using single-cell electrophoresis (comet assay). The toxicity of these tested compounds (0.1-100 microg/ml) on lymphocytes was not found. These compounds did not cause DNA strand breaks at lower concentrations of 0.1-10 microg/ml. At a concentration of 100 microg/ml, TA and GA exhibited slight DNA damage, whereas PA and EA showed no DNA strand breaks. TA and its related compounds decreased the DNA strand breaks induced by Trp-P-2, PhIP or H2O2 at concentrations of 0.1-10 microg/ml. DNA repair enzymes endonuclease III (Endo III) and formamidopyrimidine-DNA glycoslase (FPG)] were used to examine the levels of oxidised pyrimidines and purines in human lymphocytes induced by H2O2. All the compounds at 10 microg/ml can reduce the level of FPG sensitive sites. However, only EA inhibited the formation of EndoIII sensitive sites. The results indicated that these compounds can enhance lymphocytes resistance towards DNA strand breaks induced by food mutagens or H2O2 in vitro.

The biflavonoid, amentoflavone degrades DNA in the presence of copper ions

Previous reports from this laboratory have shown that flavonoids including apigenin are capable of inducing oxidative DNA cleavage in the presence of copper ions. In the present report, we have examined the ability of amentoflavone, a biflavonoid which is a dimer of apigenin, to catalyze the degradation of DNA. Amentoflavone was found to degrade calf thymus DNA in the presence of Cu(II) at a rate almost twice that of apigenin. Amentoflavone was also shown to reduce Cu(II) to Cu(I) and to generate hydroxyl radicals in the presence of copper ions. In the presence of Cu(II), the absorption spectrum of amentoflavone undergoes a shift and a quenching effect indicating that the biflavonoid is capable of binding to copper ions. Amentoflavone and apigenin were isolated from Cycas rumphii and Trifolium alexandrinum, respectively. The results are discussed in relation to the putative chemopreventive mechanism of amentoflavone.

Oxidative DNA damage by capsaicin and dihydrocapsaicin in the presence of Cu(II)

Capsaicin is the pungent phenolic principle of the Capsicum species, and has shown a wide range of pharmacological properties, including antigenotoxic, antimutagenic, and anticarcinogenic effects. Other studies have, however, shown it to be a tumor promoter and potential mutagen, and a carcinogen, resulting in capsaicin being termed a 'double edged sword'. In the present study, we show that capsaicin is capable of causing strand scission in calf thymus and plasmid DNA in the presence of Cu(II) and that this breakage is mediated by reactive oxygen species, especially the hydroxyl radical. Our results further show that capsaicin can directly generate hydroxyl radicals in the presence of Cu(II). To explore the chemical basis of the DNA breakage reaction by capsaicin, we have compared these properties of capsaicin with its saturated structural analog dihydrocapsaicin (DHC). The rate of DNA degradation, as well as hydroxyl radical formation, was found to be greater in the case of capsaicin. Both capsaicin and DHC are able to reduce Cu(II) to Cu(I), which was shown to be an essential intermediate in the DNA cleavage reaction. Stoichiometric analysis indicated that whereas 1 mol of capsaicin reduced 3 mol of Cu(II), 1 mol of DHC reduced only 2 mol of Cu(II). This explains the greater activity of capsaicin and also leads to a model for copper binding to the capsaicins.

DNA breakage by resveratrol and Cu(II): reaction mechanism and bacteriophage inactivation

Resveratrol (3,4',5-trihydroxy stilbene) is a phytoalexin and a polyphenolic compound present in human dietary material such as peanuts, mulberries, grapes and red wine. It is widely considered to possess cardiovascular protective properties and has also been shown to be chemopreventive against various stages of chemically induced carcinogenesis. It has recently been shown that resveratrol induces strand breakage in DNA in the presence of copper ions. In this paper, we have shown that resveratrol catalyzes the reduction of Cu(II) to Cu(I), which is accompanied by the formation of 'oxidized product(s)' of resveratrol, which in turn also appear to catalyze the reduction of Cu(II). Strand scission by the resveratrol-Cu(II) system was found to be biologically active as assayed by bacteriophage inactivation. The results are discussed in relation to the putative chemopreventive mechanism of resveratrol.

Anti-oxidant, pro-oxidant properties of tannic acid and its binding to DNA

Tannic acid has numerous food and pharmacological applications. It is an additive in medicinal products, and is used as a flavouring agent and as an anti-oxidant in various foods and beverages. We have previously shown that tannic acid in the presence of Cu(II) causes DNA degradation through generation of reactive oxygen species. On the other hand, it exhibits antimutagenic and anticarcinogenic activities, and induces apoptosis in animal cells. It is known that most plant-derived polyphenolic anti-oxidants also act as pro-oxidants under certain conditions. In this paper, we compare the anti-oxidant and pro-oxidant properties of tannic acid and its structural component gallic acid. It is shown that tannic acid is the most efficient generator of the hydroxyl radical in the presence of Cu(II), as compared with gallic acid and its analogues syringic acid and pyrogallol. The anti-oxidant activity of tannic acid was studied by its effect on hydroxyl radical and singlet oxygen mediated cleavage of plasmid DNA. Again, tannic acid provided the maximum protection against cleavage, while gallic acid and its structural analogues were found to be non-inhibitory or partially inhibitory. The results suggest that the structural features of tannic acid that are important for its anti-oxidant action are also those that contribute to the generation of hydroxyl radicals in the presence of Cu(II). Restriction analysis of treated phage DNA and thermal melting profiles of calf thymus DNA indicated that tannic acid strongly binds to DNA. Indirect evidence indicates that modification of DNA bases may also occur.

Bilirubin-Cu(II) complex degrades DNA

It has recently been reported that bilirubin forms a complex with Cu(II). In this paper we show that the formation of the complex results in the reduction of Cu(II) to Cu(I) and the redox cycling of the metal gives rise to the formation of reactive oxygen species, particularly hydroxyl radical. The bilirubin-Cu(II) complex causes strand breakage in calf thymus DNA and supercoiled plasmid DNA. Cu(I) was shown to be an essential intermediate in the DNA cleavage reaction by using the Cu(I) specific sequestering reagent neocuproine. Bilirubin-Cu(II) produced hydroxyl radical and the involvement of active oxygen species was established by the inhibition of DNA breakage by various oxygen radical quenchers.

Pro-oxidant, anti-oxidant and cleavage activities on DNA of curcumin and its derivatives demethoxycurcumin and bisdemethoxycurcumin

Curcumin, a naturally occurring phytochemical responsible for the colour of turmeric shows a wide range of pharmacological properties including antioxidant, anti-inflammatory and anti-cancer effects. We have earlier shown that curcumin in the presence of Cu(II) causes strand cleavage in DNA through generation of reactive oxygen species, particularly the hydroxyl radical. Thus, curcumin shows both antioxidant as well as pro-oxidant effects. In order to understand the chemical basis of various biological properties of curcumin, we have studied the structure-activity relationship between curcumin and its two naturally occurring derivatives namely demethoxycurcumin (dmC) and bisdemethoxycurcumin (bdmC). Curcumin was found to be the most effective in the DNA cleavage reaction and a reducer of Cu(II) followed by dmC and bdmC. The rate of formation of hydroxyl radicals by the three curcuminoids also showed a similar pattern. The relative antioxidant activity was examined by studying the effect of these curcuminoids on cleavage of plasmid DNA by Fe(II)-EDTA system (hydroxyl radicals) and the generation of singlet oxygen by riboflavin. The results indicate that curcumin is considerably more active both as an antioxidant as well as an oxidative DNA cleaving agent. The DNA cleavage activity is the consequence of binding of Cu(II) to various sites on the curcumin molecule. Based on the present results, we propose three binding sites for Cu(II). Two of the sites are provided by the phenolic and methoxy groups on the two benzene rings and the third site is due to the presence of 1,3-diketone system between the rings. Furthermore, both the antioxidant as well as pro-oxidant effects of curcuminoids are determined by the same structural moieties.

Mechanisms involved in the protective effect of estradiol-17beta on lipid peroxidation and DNA damage

Previous studies from our laboratory have shown that estrogens can protect against lipoprotein peroxidation and DNA damage. In this study, the mechanism of estradiol-17beta (E2) action was investigated by comparing E2 with selective scavengers of reactive oxygen species (ROS) in terms of inhibition of 1) human low-density lipoprotein (LDL) peroxidation (measured by the diene conjugation method) and 2) DNA damage (measured by the formation of strand breaks in supercoiled OX-174 RFI DNA). In addition, the direct effect of E2 on the generation of individual ROS was also measured. By use of ROS scavengers, it was determined that lipoprotein peroxidation was predominantly due to superoxide (39%), with some contributions from hydrogen peroxide (23%) and peroxy (38%) radicals. E2 was a more effective inhibitor of peroxidation than all the ROS scavengers combined. In DNA damage, scavengers of hydrogen peroxide, hydroxyl, and superoxide radical offered significant protection (49-65%). E2 alone offered a similar degree of protection, and no additional effect was evident when it was combined with ROS scavengers. E2 caused a significant reduction (37%) in the production of superoxide radical by bovine heart endothelial cells in culture but had no effect on the formation of either hydrogen peroxide or hydroxyl radicals. These studies show that 1) the protection offered by E2 in terms of lipid peroxidation could be due to its ability to inhibit generation of superoxide radical and prevent further chain propagation, and 2) in DNA damage protection, E2 mainly appears to inhibit chain propagation.

Strand scission in DNA induced by curcumin in the presence of Cu(II)

Curcumin, a naturally occurring phytochemical responsible for the colour of turmeric, has shown a wide range of pharmacological properties including anti-inflammatory, anti-tumour promoter and anti-oxidant effects. In this paper we show that in the presence of Cu(II), curcumin caused breakage of calf thymus and supercoiled plasmid pBR322 DNA. The products were relaxed circles with no detectable linear forms. Other metal ions tested (Mg(II), Ca(II), Fe(II) and Ni(II)) were ineffective or less effective in the DNA breakage reaction. Cu(I) was shown to be an essential intermediate by using the Cu(I)-specific sequestering reagent neocuproine. The involvement of active oxygen species, such as H2O2 and (1)O2 was established by the inhibition of DNA breakage by catalase and azide. Curcumin is also able to directly produce O2- and H2O2 and in the presence of Cu(II), OH is generated. Absorption spectra of curcumin in the presence of DNA indicated that a complex is formed between the two. The results are discussed in relation to the established pro-oxidant activities of other known anti-oxidants.

DNA breakage by L-DOPA and Cu(II): breakage by melanin and bacteriophage inactivation

We have previously shown that L-DOPA in the presence of Cu(II) caused DNA cleavage through the generation of reactive oxygen species such as the hydroxyl radical. Since L-DOPA is the precursor for the synthesis of melanin, we have studied the action of melanin on DNA in a similar reaction. In this paper, we show that melanin in the presence of Cu(II) also causes DNA strand breakage. However, the rate of such strand breakage is considerably less than l-DOPA. Melanin and L-DOPA are both capable of generating superoxide anion. Furthermore, the action of L-DOPA and Cu(II) on bacteriophage lambda reduces its viability. The results are discussed in relation to the putative role of L-DOPA-Cu(II) system as a source of endogenous generation of reactive oxygen species.

DNA breakage by tannic acid and Cu(II): generation of active oxygen species and biological activity of the reaction

Tannic acid was shown to reduce oxygen to superoxide anion. In the presence of Cu(II), the hydroxyl radical and hydrogen peroxide were formed. Strand scission reaction was shown to account for the biological activity of tannic acid as assayed by bacteriophage inactivation. The inactivating activity occurs through the Fenton pathway for free radical production and subsequent DNA cleavage by these radicals.