Strand scission in DNA induced by dietary flavonoids: role of Cu(I) and oxygen free radicals and biological consequences of scission

Ptaquiloside from bracken (Pteridium spp.) promotes oral carcinogenesis initiated by HPV16 in transgenic mice

Bracken (Pteridium spp.) is a common weed that is consumed as food especially in Asia, and is suspected of promoting carcinogenesis induced by papillomaviruses in the digestive and urinary systems. This is particularly worrying because the incidence of head-and-neck cancers associated with the human papillomavirus (HPV) is rapidly increasing, and HPV co-carcinogens urgently need to be identified. This study tested the hypothesis that two bracken compounds, ptaquiloside and rutin, are able to promote head-and-neck and bladder carcinogenesis in HPV16-transgenic mice. Expression of HPV16 E6 and E7 in oral and bladder tissues was confirmed using quantitative real-time PCR. Mice were exposed orally to ptaquiloside (0.5 mg per animal per week for 10 weeks from 20 weeks-old) or rutin (413 mg kg-1 day-1 for 24 weeks from 6 weeks-old), sacrificed at 30 weeks-old and studied histologically. HPV16 E6 and E7 expression was higher in oral mucosa compared with the bladder (p 0.001). Importantly, ptaquiloside, but not rutin, increased the incidence of oral squamous cell carcinomas (p = 1.2 × 10-8) in HPV16-transgenic mice. Also, cancers of unexposed transgenic mice were restricted to the tongue base, while ptaquiloside-exposed mice showed multifocal lesions throughout the oral cavity. Wild-type controls showed no oral lesions. No bladder lesions were observed in any treated or untreated group. These results indicate that ptaquiloside from bracken is able to promote oral carcinogenesis initiated by HPV16. Rutin did not show any carcinogenic effects in this model. The absence of bladder lesions may reflect an insufficient incubation period or factors related to the specific viral oncogenes present in this model.

Effects of N-Acetylcysteine, Quercetin, and Phytic Acid on Spontaneous Hepatic and Renal Lesions in LEC Rats

The effects of anti-oxidants were examined in Long-Evans Cinnamon (LEC) rats, which develop acute hepatic injury, and subsequent hepatic and renal tumors due to accumulation of excess Cu. The rats, at the age of 15 weeks, were supplied a diet containing either 1% of N-acetylcysteine (NAC), quercetin (QC), or phytic acid (PA), or basal diet alone. At weeks 2 and 6 posttreatment, animals were sacrificed for collection of blood and tissue samples. In the NAC-treated group, the development of hepatic and renal lesions was dramatically reduced. In addition, accumulation of Cu and Fe in the liver was suppressed. Acrolein-modified protein, a new marker for lipid peroxidation, was not detected in the liver or kidney of NAC treated rats, even though deposition was evident in control. Neither QC nor PA affected the development of spontaneous hepatic lesions. These results indicate that oxidative stress was reduced by NAC in the liver and kidney, and suggest that Cu and Fe may be involved in the generation of oxidative stress in the liver. In addition, it was suggested that the different effects of the anti-oxidants on lesion development in LEC rats might be related to different mechanisms of action with regard to oxidative stress.

Quercetin-induced changes in femoral bone microstructure of adult male rabbits

Flavonoids are a group of plant metabolites with antioxidant effects. One of the most abundant flavonoids in the human diet is quercetin. It is found widely in fruits, vegetables and has a lot of beneficial effects on human health. Quercetin has a positive pharmacological effect on bone metabolism and it prevents the organism against bone loss. However, its impact on the size of basic structural units of the compact bone is still unknown. Therefore, the aim of present study was to investigate the impact of the quercetin on femoral bone microstructure in 5-month-old male rabbits. Five rabbits of Californian broiler line were randomly divided into two groups. In the experimental group (E group; n=3), animals were intramuscularly injected with quercetin at dose 1000 μg.kg-1 body weight (bw) for 90 days, 3 times per week. Two rabbits without quercetin administration served as a control group (C group). According to our results, intramuscular application of quercetin had an insignificant effect on cortical bone thickness in male rabbits. In these rabbits, changes in qualitative histological characteristics were present in the middle part of the compacta, where primary vascular longitudinal bone tissue was present and expanded there from the periosteum. Also, a lower number of secondary osteons was found in these animals. From the histomorphometrical point of view, significantly decreased sizes of primary osteons' vascular canals and secondary osteons (p <0.05) were found in rabbits administered by quercetin. Our findings indicate that subchronic administration of quercetin at the dose used in our study had considerable impact on both qualitative and quantitative histological characteristics of the compact bone in adult male rabbits.

Oxidative stress, prooxidants, and antioxidants: the interplay

Oxidative stress is a normal phenomenon in the body. Under normal conditions, the physiologically important intracellular levels of reactive oxygen species (ROS) are maintained at low levels by various enzyme systems participating in the in vivo redox homeostasis. Therefore, oxidative stress can also be viewed as an imbalance between the prooxidants and antioxidants in the body. For the last two decades, oxidative stress has been one of the most burning topics among the biological researchers all over the world. Several reasons can be assigned to justify its importance: knowledge about reactive oxygen and nitrogen species production and metabolism; identification of biomarkers for oxidative damage; evidence relating manifestation of chronic and some acute health problems to oxidative stress; identification of various dietary antioxidants present in plant foods as bioactive molecules; and so on. This review discusses the importance of oxidative stress in the body growth and development as well as proteomic and genomic evidences of its relationship with disease development, incidence of malignancies and autoimmune disorders, increased susceptibility to bacterial, viral, and parasitic diseases, and an interplay with prooxidants and antioxidants for maintaining a sound health, which would be helpful in enhancing the knowledge of any biochemist, pathophysiologist, or medical personnel regarding this important issue.

Apigenin induces DNA damage through the PKCδ-dependent activation of ATM and H2AX causing down-regulation of genes involved in cell cycle control and DNA repair

Apigenin, an abundant plant flavonoid, exhibits anti-proliferative and anti-carcinogenic activities through mechanisms yet not fully defined. In the present study, we show that the treatment of leukemia cells with apigenin resulted in the induction of DNA damage preceding the activation of the apoptotic program. Apigenin-induced DNA damage was mediated by p38 and protein kinase C-delta (PKCδ), yet was independent of reactive oxygen species or caspase activity. Treatment of monocytic leukemia cells with apigenin induced the phosphorylation of the ataxia-telangiectasia mutated (ATM) kinase and histone H2AX, two key regulators of the DNA damage response, without affecting the ataxia-telangiectasia mutated and Rad-3-related (ATR) kinase. Silencing and pharmacological inhibition of PKCδ abrogated ATM and H2AX phosphorylation, whereas inhibition of p38 reduced H2AX phosphorylation independently of ATM. We established that apigenin delayed cell cycle progression at G1/S and increased the number of apoptotic cells. In addition, genome-wide mRNA analyses showed that apigenin-induced DNA damage led to down-regulation of genes involved in cell-cycle control and DNA repair. Taken together, the present results show that the PKCδ-dependent activation of ATM and H2AX define the signaling networks responsible for the regulation of DNA damage promoting genome-wide mRNA alterations that result in cell cycle arrest, hence contributing to the anti-carcinogenic activities of this flavonoid.

Resveratrol-induced apoptosis is enhanced in low pH environments associated with cancer

Many critical factors such as hypoxia, nutrient deficiency, activation of glycolytic pathway/Warburg effect contribute to the observed low pH in tumors compared to normal tissue. Studies suggest that such tumor specific acidic environment can be exploited for the development of therapeutic strategies against cancer. Independent observations show reduction in pH of mammalian cells undergoing internucleosomal DNA fragmentation and apoptosis. As such, our group has extensively demonstrated that anticancer mechanisms of different plant polyphenols involve mobilization of endogenous copper and consequent internucleosomal DNA breakage. Copper is redox active metal, an essential component of chromatin and is sensitive to subtle pH changes in its microenvironment. Here we explored whether, acidic pH promotes growth inhibition, apoptosis, and DNA damaging capacity of chemopreventive agent resveratrol. Our results reveal that growth inhibition and internucleosomal DNA fragmentation induced apoptosis in Capan-2 and Panc-28 pancreatic cancer cell lines (and not in normal HPDE cells) by resveratrol is enhanced at lower pH. Using comet assay, we further demonstrate that DNA breakage by resveratrol is enhanced with acidification. Membrane permeable copper specific chelator neocuproine (and not iron chelator orthophenanthroline) abrogated growth inhibition and apoptosis by resveratrol. Western blot results show enhanced activation of DNA laddering marker H2.aX by resveratrol at acidic pH that was reversed by neocuproine and not by orthophenanthroline. Our findings provide irrevocable proof that low pH environment can be turned into tumor weakness and assist in eradication of cancer cells by resveratrol.

Bracken-associated human and animal health hazards: chemical, biological and pathological evidence

Bracken (Pteridium aquilinum) is a widely distributed carcinogenic fern, to whose toxins human populations are exposed through multiple routes. Animals are also affected by bracken toxins, leading to serious production losses yearly. Accordingly, several governmental reports regarding the safeguard of public health against bracken carcinogens have been recently issued. This review describes the main bioactive compounds identified in bracken and their biological effects at the molecular, cellular, pathological and populational levels, with particular emphasis on ptaquiloside, the main bracken carcinogen. Recent biopathological studies shedding further light on the genotoxicity immunotoxicity and carcinogenicity of ptaquiloside are discussed. Key steps on the long effort to understand bracken toxicology are also reviewed, along with the latest findings on new bracken toxins and human exposures routes. The presence of ptaquiloside and related terpene glycosides in milk, meat and water are of particular concern from the viewpoints of both human and animal health.

Potent antioxidant dendrimers lacking pro-oxidant activity

It is well known that antioxidants have protective effects against oxidative stress. Unfortunately, in the presence of transition metals, antioxidants, including polyphenols with potent antioxidant activities, may also exhibit pro-oxidant effects, which may irreversibly damage DNA. Therefore, antioxidants with strong free radical-scavenging abilities and devoid of pro-oxidant effects would be of immense biological importance. We report two antioxidant dendrimers with a surface rich in multiple phenolic hydroxyl groups, benzylic hydrogens, and electron-donating ring substituents that contribute to their potent free radical-quenching properties. To minimize their pro-oxidant effects, the dendrimers were designed with a metal-chelating tris(2-aminoethyl)amine (TREN) core. The dendritic antioxidants were prepared by attachment of six syringaldehyde or vanillin molecules to TREN by reductive amination. They exhibited potent radical-scavenging properties: 5 times stronger than quercetin and 15 times more potent than Trolox according to the 1,1-diphenyl-2-picrylhydrazyl assay. The antioxidant dendrimers also protected low-density lipoprotein, lysozyme, and DNA against 2,2'-azobis(2-amidinopropane) dihydrochloride-induced free radical damage. More importantly, unlike quercetin and Trolox, the two TREN antioxidant dendrimers did not damage DNA via their pro-oxidant effects when incubated with physiological amounts of copper ions. The dendrimers also showed no cytotoxicity toward Chinese hamster ovary cells.

Evaluation of tolerable levels of dietary quercetin for exerting its antioxidative effect in high cholesterol-fed rats

The tolerable level of dietary quercetin for exerting its antioxidative effect was evaluated in high cholesterol-fed rats, using quercetin-containing diets (31-1260 mg quercetin/kg body weight/day) and onion diets (19-94 mg quercetin aglycone equivalent/kg body weight/day), from the viewpoint of a safety assessment. After feeding for 4 weeks, the urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) levels of the quercetin-containing diet groups fed more than 157 mg quercetin/kg body weight/day were higher than the group fed a quercetin-free diet, although the plasma quercetin metabolite levels and plasma antioxidative activity were elevated depending on the amounts of quercetin or onion diet intake. No significant effect on body weight gain by quercetin-containing diets or onion diets was observed. However, ratios of the liver and kidney weights to the body weight were significantly increased in the quercetin-containing diet groups fed more than 314 mg and 157 mg quercetin/kg body weight/day, respectively, and in the onion diet groups fed more than 47 mg quercetin aglycone equivalent/kg body weight/day. These results indicated that the tolerable level for dietary quercetin for exerting its antioxidative effect was between 126 and 157 mg/kg/day for the quercetin diet and between 19 and 34 mg/kg/day for the onion diet.

Plant polyphenols in cancer and heart disease: implications as nutritional antioxidants

Certain dietary antioxidants such as vitamin E and vitamin C are important for maintaining optimum health. There is now much interest in polyphenolic products of the plant phenylpropanoid pathway as they have considerable antioxidant activity in vitro and are ubiquitous in our diet. Rich sources include tea, wine, fruits and vegetables although levels are affected by species, light, degree of ripeness, processing and storage. This confounds the formulation of databases for the estimation of dietary intakes. Most attention to date has focused on the flavonoids, a generic term which includes chalcones, flavones, flavanones, flavanols and anthocyanins. There is little convincing epidemiological evidence that intakes of polyphenols are inversely related to the incidence of cancer whereas a number of studies suggest that high intakes of flavonoids may be protective against CHD. In contrast, numerous cell culture and animal models indicate potent anticarcinogenic activity by certain polyphenols mediated through a range of mechanisms including antioxidant activity, enzyme modulation, gene expression, apoptosis, upregulation of gap junction communication and P-glycoprotein activation. Possible protective effects against heart disease may be due to the ability of some polyphenols to prevent the oxidation of LDL to an atherogenic form although anti-platelet aggregation activity and vasodilatory properties are also reported. However, some polyphenols are toxic in mammalian cells. Thus, until more is known about their bioavailability, metabolism and intracellular location, increasing intakes of polyphenols by supplements or food fortification may be unwise.

Synthesis and antioxidant properties of dendritic polyphenols

Three dendritic polyphenols (generation 1) were synthesized: a syringaldehyde-based dendrimer (1), a vanillin-based dendrimer (2), and an iodinated vanillin-based dendrimer (3). They all showed strong antioxidant activity according to the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical assay. The syringaldehyde dendrimer was twice and 10 times stronger than quercetin and Trolox, respectively. The vanillin-based dendrimer and its more hydrophobic iodinated derivative were also more potent antioxidants than quercetin and Trolox. The DPPH order of potency was 1>2, 3>quercetin>Trolox. All three dendrimers also protected human LDL from free radical attack in a dose-dependent manner. Their order of free radical scavenging was 1>3>2>quercetin>Trolox. The increased hydrophobic nature of the iodinated derivative may have contributed to its better LDL protection than 2. Protection of linoleic acid oxidation was studied by the beta-carotene-linoleate assay. Dendrimer 1 was clearly superior to the other antioxidants in protecting the fatty acid. In case of DNA protection against free radical damage, the order of activity was 1>quercetin>2>3, Trolox. Pro-oxidant effect on copper-induced DNA oxidation showed the following order: quercetin, Trolox>1>2>3. Results of the study show that dendritic antioxidants, even at the generation 1 level, provide promising antioxidant properties for their potential use as drug candidates for diseases associated with oxidative stress.

Flavonoids with DNA strand-scission activity from Rhus javanica var. roxburghiana

The flavonoids isolated from the stems of Rhus javanica var. roxburghiana, taxifolin (1), fisetin (2), fustin (3), 3,7,4'-trihydroxyflavanone (4) and 3,7,4'-trihydroxyflavone (5) caused breakage of supercoiled plasmid pBR322 DNA in the presence of Cu(II). Cu(I) was shown to be an essential intermediate by using the Cu(I)-specific sequestering reagent neocuproine. The Cu(II)-mediated DNA scissions induced by 1, 2, 3 and 5 were inhibited by the addition of catalase and exhibited DNA strand break by the addition of KI and superoxide dimutase (SOD), while in the Cu(II)-mediated DNA scissions induced by 4 was inhibited by the addition of KI, SOD, and catalase. It is concluded that 1, 2, 3, and 5 can induce H2O2 and superoxide anion, while 4 can induce OH* and H2O2 and subsequent oxidative damage of DNA in the presence of Cu(II).

A real-time QCM-D approach to monitoring mammalian DNA damage using DNA adsorbed to a polyelectrolyte surface

We have successfully demonstrated that the quartz crystal microbalance with dissipation monitoring (QCM-D) can be used to monitor real-time damage to genomic mammalian DNA adsorbed to a polyelectrolyte surface. To reveal the capabilities of this technique, we exposed DNA surfaces to quercetin, an agent that has been implicated in causing DNA strand breaks in a Cu(II)-dependent fashion in vitro. We show that the QCM-D frequency and dissipation patterns that result from exposure of the DNA surfaces to quercetin-Cu(II) are consistent with the induction of DNA strand scission. We use QCM-D to furthermore demonstrate that this process is dependent on Cu(II) and that the DNA damage induced by quercetin can still be detected if Cu(II) is in situ with the DNA surface and not in solution phase.

Base excision repair pathway is involved in the repair of lesions generated by flavonoid-enriched fractions of pepper tree (Schinus terebinthifolius, Raddi) stem bark

Cell-free and bacterial assays indicate that flavonoid-enriched fractions and the flavonoids of pepper tree stem bark from Schinus terebinthifolius Raddi have genotoxic rather than antigenotoxic properties. In the present report, we have examined the ability of flavonoid-enriched fractions to damage plasmid DNA and the repair pathways involved in the recognition of these DNA lesions. High concentrations of two flavonoid-enriched fractions were able to break phosphodiester bonds in DNA. In addition, studies using bacterial strains deficient in nucleotide excision repair and base excision repair (BER) enzymes indicated that the flavonoid-enriched fractions generated lesions that were substrates for enzymes belonging to the BER pathway. In addition, in vitro studies indicated that the DNA damage produced by the flavonoid-enriched fractions was also a substrate for exonuclease III and that the phosphodiester breakage was amplified by copper ions. These results indicate that flavonoids from the pepper tree (Schinus terebinthifolius, Raddi) generate lesions on DNA that are potential targets of FPG and MutY glycosylase from the BER pathway. Chromatographic and spectral analyses helped to support the hypothesis that the flavonoids of the Brazilian pepper tree bark are the main factors involved in the fraction's damage potential. The isolated flavonoids from Fraction II were also tested in vitro and support the oxidative damage potential of these flavonoids.

Evaluation of the ability of antioxidants to counteract lipid oxidation: Existing methods, new trends and challenges

Oxidative degradation of lipids, especially that induced by reactive oxygen species (ROS), leads to quality deterioration of foods and cosmetics and could have harmful effects on health. Currently, a very promising way to overcome this is to use vegetable antioxidants for nutritional, therapeutic or food quality preservation purposes. A major challenge is to develop tools to assess the antioxidant capacity and real efficacy of these molecules. Many rapid in vitro tests are now available, but they are often performed in dissimilar conditions and different properties are thus frequently measured. The so-called 'direct' methods, which use oxidizable substrates, seem to be the only ones capable of measuring real antioxidant power. Some oxidizable substrates correspond to molecules or natural extracts exhibiting biological activity, such as lipids, proteins or nucleic acids, while others are model substrates that are not encountered in biological systems or foods. Only lipid oxidation and direct methods using lipid-like substrates will be discussed in this review. The main mechanisms of autoxidation and antioxidation are recapitulated, then the four components of a standard test (oxidizable substrate, medium, oxidation conditions and antioxidant) applied to a single antioxidant or complex mixtures are dealt with successively. The study is focused particularly on model lipids, but also on dietary and biological lipids isolated from their natural environment, including lipoproteins and phospholipidic membranes. Then the advantages and drawbacks of existing methods and new approaches are compared according to the context. Finally, recent trends based on the chemometric strategy are introduced as a highly promising prospect for harmonizing in vitro methods.

A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity, including lack of genotoxic/carcinogenic properties

Quercetin is a naturally-occurring flavonol (a member of the flavonoid family of compounds) that has a long history of consumption as part of the normal human diet. Because a number of biological properties of quercetin may be beneficial to human health, interest in the addition of this flavonol to various traditional food products has been increasing. Prior to the use of quercetin in food applications that would increase intake beyond that from naturally-occurring levels of the flavonol in the typical Western diet, its safety needs to be established or confirmed. This review provides a critical examination of the scientific literature associated with the safety of quercetin. Results of numerous genotoxicity and mutagenicity, short- and long-term animal, and human studies are reviewed in the context of quercetin exposure in vivo. To reconcile results of in vitro studies, which consistently demonstrated quercetin-related mutagenicity to the absence of carcinogenicity in vivo, the mechanisms that lead to the apparent in vitro mutagenicity, and those that ensure absence of quercetin toxicity in vivo are discussed. The weight of the available evidence supports the safety of quercetin for addition to food.

High-throughput evaluation of antioxidant and pro-oxidant activities of polyphenols with thymidine protection assays

A recently reported high-throughput screening strategy has been applied to the rapid selection of new water-soluble antioxidants that display strong protective activities. Based on a competitive immunoassay, a triple-screening procedure was used to evaluate the ability of different compounds to protect thymidine under different oxidative stresses. The pro-oxidant effect of norbadione A in the presence of iron was observed, while some pulvinic acid derivatives proved strongly protective during gamma radiolysis, UV irradiation, and Fenton-like oxidation.

A dihydroflavonol glucoside from Commiphora africana that mediates DNA strand scission

A crude CH(2)Cl(2)-MeOH extract prepared from Commiphora africana was found to mediate Cu(2+)-dependent relaxation of supercoiled plasmid DNA. Bioassay-guided fractionation of this extract was carried out and was monitored by the use of an in vitro DNA strand scission assay. The dihydroflavonol glucoside phellamurin (1) was identified as the active principle responsible for the DNA cleavage activity of the crude extract.

The modulating effects of quercetin and rutin on the mitomycin C induced DNA damage

The present study was carried out to investigate the modulating effects of the two flavonoids quercetin and rutin on the mutagenic anticancer drug mitomycin C by single cell gel electrophoresis (Comet assay) in human lymphocytes. Lymphocytes were incubated with different concentrations of quercetin and rutin, with or without mitomycin C, and DNA damage was evaluated. Concentrations of 0.03, 0.15, 0.3, 0.6, 1.5 and 3mM quercetin significantly reduced the DNA strand breakage induced by mitomycin C (P<0.001) but the highest concentration of 6mM quercetin did not show a protective effect. The frequency of damaged cells induced by mitomycin C was not changed at 0.02 mM, and also at the highest concentrations of 1.64 and 3.28 mM rutin. However, at concentrations of 0.08, 0.16, 0.33 and 0.82 mM rutin cells were protected from DNA damage. Thus, in human lymphocytes quercetin and rutin displayed protective effects on DNA damage induced by mitomycin C, in a concentration-dependent manner.

Enhancement of glutathione and g-glutamylcysteine synthetase, the rate limiting enzyme of glutathione synthesis, by chemoprotective plant-derived food and beverage components in the human hepatoma cell line HepG2

Glutathione (GSH) is an important antioxidant and cofactor of detoxifying metabolism. Therefore, elevation of GSH as achieved by inducing g-glutamylcysteine synthetase (GCS), the limiting enzyme of GSH synthesis, may contribute to chemoprevention against cancer. In previous animal studies, increases in GCS were mainly found in liver and other organs that are not easily accessible in humans. Thus, employment and evaluation of alternative systems such as human-derived cell lines are encouraged. In the present experiment, we used the hepatoma cell line HepG2 to investigate the response of GCS and GSH to five plant-derived chemoprotectants contained in regularly consumed foodstuffs and beverages (kahweol/cafestol [K/C] [15.5-62.0 mM], a-angelicalactone [100-400 mM], benzyl isothiocyanate [1.7-5.0 mM], diallyl sulfide [175-700 mM], and quercetin [10-50 mM]). All treatments led to dose-dependent increases in both GCS activity and GSH concentration. Time course studies with K/C indicated that the enhancement of GCS preceded that of GSH, suggesting a causal relationship. K/C did not enhance g-glutamyl transpeptidase, a further enzyme that assists GSH-related chemoprotection. Although GCS induction has been suggested to require an initial short-lived GSH depletion, we did not find any decrease in GSH after 3 h of incubation with K/C. In summary, HepG2 cells were shown to be a useful model to investigate the capacity of potential chemoprotectants to enhance GCS and GSH. To our knowledge, the present study is also the first to show increases in GCS by K/C and a-angelicalactone in vitro and by diallyl sulfide and quercetin in any system.

Prooxidant action of gallic acid compounds: copper-dependent strand breaks and the formation of 8-hydroxy-2'-deoxyguanosine in DNA

Gallic acid and its alkylesters, polyphenolic compounds with antioxidative activity, acted as a prooxidant causing a copper-dependent DNA damage. Treatment of DNA from plasmid pBR322 and calf thymus with gallic acid plus copper ion caused strand scission and the formation of 8-hydroxy-2'-deoxyguanosine in DNA. Addition of catalase protected DNA from the gallic acid/copper-dependent strand breaks and the formation of 8-hydroxy-2'-deoxyguanosine, indicating that hydroxyl radical may participate in the DNA damage. Ethyl-, propyl- and butylgallates showed only a little DNA damage. Octyl- and laurylgallates caused negligible damage of DNA. DNA strand breaks and formation of 8-hydroxy-2'-deoxyguanosine were closely related to the reduction of copper by gallate compounds. These results imply that cuprous ion reduced by gallate derivatives may play a key role in the oxidative cleavage of DNA and the formation of base adduct. The cytotoxic effect of gallate compounds can be explained by their prooxidant action dependent on the reducing activity.

Wogonin and fisetin induce apoptosis in human promyeloleukemic cells, accompanied by a decrease of reactive oxygen species, and activation of caspase 3 and Ca(2+)-dependent endonuclease

Seven structurally related flavonoids including luteolin, nobiletin, wogonin, baicalein, apigenin, myricetin and fisetin were used to study their biological activities on the human leukemia cell line, HL-60. On MTT assay, wogonin, baicalein, apigenin, myricetin and fisetin showed obvious cytotoxic effects on HL-60 cells, with wogonin and fisetin being the most-potent apoptotic inducers among them. The cytotoxic effects of wogonin and fisetin were accompanied by the dose- and time-dependent appearance of characteristics of apoptosis including DNA fragmentation, apoptotic bodies and the sub-G1 ratio. Treatment with an apoptosis-inducing concentration of wogonin or fisetin causes rapid and transient induction of caspase 3/CPP32 activity, but not caspase 1 activity. Further, cleavage of poly(ADP-ribose) polymerase (PARP) and decrease of pro-caspase 3 protein were detected in wogonin- and fisetin-treated HL-60 cells. An increase in the pro-apoptotic protein, bax, and a decrease in the anti-apoptotic protein, Mcl-1, were detected in fisetin- and wogonin-treated HL-60 cells. However, Bcl-2, Bcl-XL, and Bad all remained unchanged in wogonin- and fisetin-treated HL-60 cells. In vitro chromatin digestion revealed that endonuclease activity was profoundly enhanced in wogonin- and fisetin-treated HL-60 cells, and the addition of ethylenediaminetetraacetic acid (EDTA) or ethyleneglycoltetraacetic acid (EGTA) into the reaction blocked endonuclease activation and at an optimum pH of 7.5. The caspase 3 inhibitor, Ac-DEVD-CHO, but not the caspase 1 inhibitor, Ac-YVAD-CHO, attenuated wogonin- and fisetin-induced DNA ladders, PARP cleavage, and endonuclease activation. Pretreatment of HL-60 cells with N-acetyl-cysteine or catalase efficiently inhibited H(2)O(2) (200 microM)-induced apoptosis, but showed no inhibitory effect on wogonin- and fisetin-induced DNA ladders, caspase 3 activation, or bax protein induction. Decrease in endogenous ROS production was detected in wogonin- and fisetin-treated HL-60 cells by DCHF-DA assay. In conclusion, our experiments indicate that a decrease in intracellular peroxide level was involved in wogonin- and fisetin-induced apoptosis; activation of caspase 3 and endonuclease, induction of bax protein and suppression of Mcl-1 protein were detected in the process.

Effects of the flavonoid baicalin and its metabolite baicalein on androgen receptor expression, cell cycle progression and apoptosis of prostate cancer cell lines

Recent studies on the Chinese herbal medicine PC SPES showed biological activities against prostate cancer in vitro, in vivo and in patients with advanced stages of the disease. In investigating its mode of action, we have isolated a few of the active compounds. Among them, baicalin was the most abundant (about 6%) in the ethanol extract of PC SPES, as determined by HPLC. Baicalin is known to be converted in vivo to baicalein by the cleavage of the glycoside moiety. Therefore, it is useful to compare their activities in vitro. The effects of baicalin and baicalein were studied in androgen-positive and -negative human prostate cancer lines LNCaP and JCA-1, respectively. Inhibition of cell growth by 50% (ED(50)) in LNCaP cells was seen at concentrations of 60.8 +/- 3.2 and 29.8 +/- 2.2 microM baicalin and baicalein, respectively. More potent growth inhibitory effects were observed in androgen-negative JCA-1 cells, for which the ED(50) values for baicalin and baicalein were 46.8 +/- 0.7 and 17.7 +/- 3.4, respectively. Thus, it appears that cell growth inhibition by these flavonoids is independent of androgen receptor status. Both agents (1) caused an apparent accumulation of cells in G(1) at the ED(50) concentration, (2) induced apoptosis at higher concentrations, and (3) decreased expression of the androgen receptor in LNCaP cells.

Prooxidant activity of flavonoids: copper-dependent strand breaks and the formation of 8-hydroxy-2'-deoxyguanosine in DNA

The naturally occurring flavonoids caused strand scission of DNA in the presence of copper ion. Flavonoids such as myricetin, baicalein, and quercetin as well as ascorbic acid cleaved plasmid pBR322 DNA and calf thymus DNA potently. Addition of catalase protected DNA from the strand breaks caused by flavonoids. Treatment of calf thymus DNA with these flavonoids or ascorbate plus copper produced 8-hydroxy-2'-deoxyguanosine. Cuprous ion reduced by flavonoids and ascorbic acid may play a key role in the oxidative cleavage of DNA and the formation of base adduct. Mutagenic and carcinogenic action of flavonoids may be explained by the prooxidant effects of the compounds.

Flavonoids as inhibitors or enhancers of the cytotoxicity of tumor necrosis factor-alpha in L-929 tumor cells

The effects of some selected flavonoids on tumor necrosis factor-alpha (TNF)-induced cytotoxicity in murine fibroblast L-929 cells were studied. All of the flavanones tested as well as a flavan, epicatechin, protected L-929 cells from TNF-induced cell death of the flavanones tested, hesperetin, isosakuranetin, and pinocembrin failed to modify TNF cytotoxicity, but the 3',4'-dihydroxyflavanones, eriodictyol and taxifolin, showed a protective effect. Eriodictyol was the most potent protective agent of all the flavonoids tested, while a 4'-hydroxyflavanone, naringenin, rather showed enhancement of TNF cytotoxicity. Of the flavones tested, chrysin and apigenin markedly augmented the cytotoxicity of TNF, while luteolin showed a weak protective effect. The magnitude of protection and potentiation by these flavonoids were concentration-dependent and these effects were not altered when the flavonoids were added as much as 2 h after TNF treatment.

Effect of antiproliferative flavonoids on ascorbic acid accumulation in human colon adenocarcinoma cells

Dietary flavonoids were found to be antiproliferative for human colon cancer cells, Caco-2 and HT-29, and rat nontransformed intestinal crypt cells, IEC-6. The antiproliferative potency was found to be structure-dependent. We report here a correlation between the antiproliferative potency of these flavonoids and their ability to inhibit cellular accumulation of ascorbic acid (vitamin C). Caco-2, HT-29 and IEC-6 cells were found to accumulate ascorbic acid in a sodium-dependent fashion although some ascorbic acid may also enter the cells through sodium-independent mechanisms. Flavonoids that have been found to be antiproliferative, quercetin and genistein, inhibited the accumulation of ascorbic acid. The inhibition was dose-dependent and could be observed after as short as 10-min of incubation. The degree of inhibition of accumulation was more during rapid cell division as compared to post-confluency Caco-2 cells. Flavonoids that were found to show little antiproliferative effect, naringenin and catechin, also had little effect on ascorbic acid accumulation. The antiproliferative property of flavonoids could be linked to their ascorbic acid deprivation property.

The effect of dietary flavonoids on DNA damage (strand breaks and oxidised pyrimdines) and growth in human cells

The effects of the flavonoids quercetin, myricetin and silymarin on DNA damage and cytotoxicity in human cells were investigated. DNA strand breaks and oxidised pyrimidines were determined using alkaline single cell gel electrophoresis (the comet assay). Inhibition of cell growth was also measured. Caco-2 (colon), HepG2 (liver), HeLa (epithelial) cells and normal human lymphocytes showed different, dose-dependent susceptibilities (in terms of strand breakage) to the various flavonoids, quercetin being the most damaging. This agreed well with the ability of the flavonoids to inhibit cell growth. None of the flavonoids induced DNA base oxidation above background levels. All of the flavonoids under investigation caused depletion of reduced glutathione, which, in the case of quercetin, occurred prior to cell death. Neither cytotoxicity nor genotoxicity was associated with the antioxidant enzyme capacity (glutathione, glutathione reductase, glutathione peroxidase and catalase) of the cells.

Antioxidant and prooxidant behavior of flavonoids: structure-activity relationships

The antioxidant and prooxidant behavior of flavonoids and the related activity-structure relationships were investigated in this study using the oxygen radical absorbance capacity assay. Three different reactive species were used in the assay: 2,2'-azobis(2-amidino-propane) dihydrochloride, a peroxyl radical generator; Cu(2+)-H2O2, mainly a hydroxyl radical generator; and Cu2+, a transition metal. Flavonoids including flavones, isoflavones, and flavanones acted as antioxidants against peroxyl and hydroxyl radicals and served as prooxidants in the presence of Cu2+. Both the antioxidant and the copper-initiated prooxidant activities of a flavonoid depend upon the number of hydroxyl substitutions in its backbone structure, which has neither antioxidant nor prooxidant action. In general, the more hydroxyl substitutions, the stronger the antioxidant and prooxidant activities. The flavonoids that contain multiple hydroxyl substitutions showed antiperoxyl radical activities several times stronger than Trolox, an alpha-to copherol analogue. The single hydroxyl substitution at position 5 provides no activity, whereas the di-OH substitution at 3' and 4' is particularly important to the peroxyl radical absorbing activity of a flavonoid. The conjugation between rings A and B does not affect the antioxidant activity but is very important for the copper-initiated prooxidant action of a flavonoid. The O-methylation of the hydroxyl substitutions inactivates both the antioxidant and the prooxidant activities of the flavonoids.

Antiproliferative potency of structurally distinct dietary flavonoids on human colon cancer cells

Dietary flavonoids are known to be antiproliferative and may play an important role in cancer chemoprevention, especially cancers of the gastrointestinal tract, because of a direct contact with food. This study was designed to compare the antiproliferative potency of several structurally distinct dietary flavonoids in colon cancer cells, Caco-2 and HT-29, and in rat non-transformed intestinal crypt cells, IEC-6. Flavonoids varied significantly in their antiproliferative potency depending on the structural features but the observations were consistent among the three cell lines studied. Of the two most potent flavonoids, quercetin and genistein, the effect was found to be dose-dependent and chromatin condensation, an indication of apoptosis, was noticed. Quercetin was found to distribute throughout the cell with higher amounts in the perinuclear and nucleoli areas. The lack of specific cell membrane enrichment by quercetin was consistent with its lack of effect on the transepithelial resistance. While several flavonoids including quercetin were found to be unstable, the chemical instability did not correlate with the antiproliferative potency, although it may contribute to the antiproliferative effect.