Antioxidative and prooxidative effects of quercetin on A549 cells

Quercetin, a common plant polyphenol, has been reported to show both antioxidant and prooxidant properties. We studied the effects of quercetin on A549 cells in in vitro culture. We found that low concentrations of the flavonoid stimulated cell proliferation and increased total antioxidant capacity (TAC) of the cells; while higher concentrations of the flavonoid decreased cell survival and viability, thiol content, TAC and activities of superoxide dismutase, catalase and glutathione S-transferase. Quercetin decreased production of reactive oxygen species in the cells but produced peroxides in the medium. The cellular effects of quercetin are therefore complex and include both antioxidant effects and induction of oxidative stress due to formation of reactive oxygen species in the extracellular medium.     

Role of flavonoids in intestinal tight junction regulation

The gastrointestinal tract provides a physical barrier to the diffusion of foreign materials from the lumen into the circulatory system. Impairment of the intercellular tight junction (TJ) shield, which is the major determinant of intestinal barrier function, is associated with various diseases. Dietary flavonoids demonstrate various beneficial effects on our health; however, the information regarding their effects on TJ function is quite limited. To date, four flavonoids — epigallocatechin gallate (EGCG), genistein, myricetin and quercetin — have been reported to exhibit promotive and protective effects on intestinal TJ barrier functions. Genistein, a major soybean isoflavone, protects TJ barrier function against oxidative stress, acetaldehyde, enteric bacteria and inflammatory cytokines. Genistein blocks the tyrosine phosphorylation of the TJ proteins induced by oxidative stress and acetaldehyde, which results in the disassembly of the proteins from the junctional complex. Quercetin, a flavonol, enhances intestinal TJ barrier function through the assembly and expression of TJ proteins. The change in phosphorylation status is responsible for the quercetin-mediated assembly of TJ proteins. TJ protein induction has an additional role in this effect. This review presents the recent advances in our understanding of the flavonoid-mediated promotive and protective effects on intestinal TJ barrier function with a particular focus on intracellular molecular mechanisms.     

GC (Guanine‐Cytosine)‐Selective DNA‐Binding and Antitumor Activity of a Quercetin?Manganese(II) Complex

The DNA binding and cleavage properties of quercetin? manganese(II) complexes have been studied, but little attention has been devoted to the relationship between the antitumor activity of these complexes and the DNA‐binding properties. Here, the DNA binding properties of the quercetin? manganese(II) complex [Mn(Que)₂(H₂O)₂] were studied using UV/VIS and fluorescence spectroscopy and viscosity measurements. The results indicate that the complex was preferentially bound to DNA in the GC (guanine? cytosine)‐rich regions via an intercalative mode. Furthermore, the cytotoxicity experiments confirmed its apoptosis‐inducing activity. We also demonstrated that the levels of survivin protein expression in HepG2 cells decreased and that the relative activity of caspase‐3 significantly increased after treatment with the complex. Hence, our results suggest that the antitumor activity of the [Mn(Que)₂(H₂O)₂] complex might be related to its intercalation into DNA and its DNA‐binding selectivity.     

The cardioprotective effects of a combination of quercetin and α-tocopherol on isoproterenol-induced myocardial infarcted rats

The present study aims to evaluate the combined protective effects of quercetin and α-tocopherol on isoproterenol-treated myocardial infarcted rats. Male albino Wistar rats were pretreated with a combination of quercetin (10 mg/kg) and α-tocopherol (10 mg/kg) daily for 14 days. After the pretreatment, rats were injected isoproterenol (100 mg/kg) to induce myocardial infarction. Isoproterenol-treated rats showed increased levels of serum troponins and increased intensities of serum lactate dehydrogenase-1 and -2 isoenzyme bands. Isoproterenol treatment also showed significant decreased levels of antioxidant system and significant increased levels of plasma lipid peroxidation, plasma uric acid, and the heart calcium. Furthermore, isoproterenol-treated rat's electrocardiogram showed elevated ST segments. Combined pretreatment with quercetin and α-tocopherol normalized all the biochemical parameters and minimized the alterations in electrocardiogram. Histopathology of myocardium also confirmed the cardioprotective effects of quercetin and α-tocopherol. In vitro studies confirmed the mechanism of action of quercetin and α-tocopherol. Thus, quercetin and α-tocopherol exhibited cardioprotective effects against isoproterenol-induced cardiotoxicity due to their scavenging free radicals, improving antioxidants and maintaining Ca(2+) levels. Our study also showed that combined pretreatment (quercetin and α-tocopherol) was highly effective than single pretreatment (quercetin or α-tocopherol).     

Modulation of osteoclastogenesis in porcine bone marrow cultures by quercetin and rutin

Flavonols, in contrast to soybean isoflavones, are the most abundant phytoestrogens in western diets, being present in onions, beans, fruits, red wine, and tea. They may protect against atherosclerosis, inhibit certain cancer cell types, and reduce bone resorption. The most widely distributed flavonol is quercetin, which occurs mainly as its glycoside, rutin, but data are very scarce regarding the precise mechanism of action of these compounds on bone-resorbing cells at concentrations similar to those detected in human plasma. We have therefore investigated the effects of nanomolar concentrations of quercetin and rutin on the development and activity of osteoclasts in vitro compared with the effects of 17-estradiol. Nonadherent porcine bone marrow cells were cultured on dentine slices in the presence of 10 nM 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), with or without 10 nM quercetin, 10 nM rutin or 10 nM 17-estradiol for 11 days. Multinuclear TRAP+ cells that resorbed dentine (osteoclasts) developed in the presence of 1,25(OH)₂D₃, but their number was significantly reduced by quercetin, rutin, and 17-estradiol (P < 0.05). Like 17-estradiol, both flavonols also significantly reduced resorption (P<0.05) as assessed by the size of pits resorbed on dentine slices. Osteoclasts and osteoclast progenitors contained estrogen receptor (ER), ER, and RANK proteins. Both flavonols increased nuclear ER protein and decreased ER protein of osteoclast progenitors. Moreover, rutin reduced RANK protein, whereas 17-oestradiol and quercetin promoted apoptosis by cleavage of caspase-8 and caspase-3. All the effects of flavonols were reversed by 1 M ICI 182,780, an estrogen antagonist. Thus, the anti-resorbing properties of flavonols are mainly mediated by ER proteins through the inhibition of RANK protein or the activation of caspases.C.M.R. was supported by a grant from CAPES (Department of Education, Brasilia, Brasil).     

A synthetic approach to the generation of quercetin sulfates and the detection of quercetin 3'-O-sulfate as a urinary metabolite in the rat

To study the biological effects of quercetin, authentic products of quercetin metabolism are required as standards. The synthesis of quercetin sulfate standards is thus described. Quercetin was reacted with a 10-fold molar excess of sulfur trioxide-N-triethylamine, and the products were analyzed by HPLC and mass spectrometry. Four monosulfates and three disulfates were identified, and structural inferences were drawn by ¹H NMR spectrometry of HPLC peak isolates. Analysis of the urine of rats that had received quercetin (1.9 g/kg po) yielded a single peak, which by comparison with the products of the reaction between quercetin and sulfur trioxide-N-triethylamine was identified as quercetin 3′-O-sulfate.     

Lethal quercetin-digoxin interaction in pigs

Digoxin is a popular cardiac glycoside with very narrow therapeutic range. Quercetin is an ubiquitous antioxidant flavonoid. Digoxin is a substrate of P-glycoprotein (P-gp), a multi-drug efflux transporter, and quercetin was reported to be a modulator of P-gp. The aim of this study was to investigate the effect of quercetin on the absorption and disposition of digoxin in pigs. Pigs were orally given digoxin (0.02 mg/kg) with and without quercetin in crossover designs. The blood was collected via jugular vein and fluorescence polarization immunoassay was used to determine the serum concentration of digoxin. The pharmacokinetic parameters were calculated using WINNONLIN. The paired Student's t-test was used for statistical comparison. The coadministration of 50 mg/kg quercetin unexpectedly resulted in sudden death of two among three pigs within 30 min after digoxin administration. The coadministration of 40 mg/kg quercetin significantly elevated the C_max of digoxin by 413% and increased the AUC_0-t by 170%. The results indicated that a very serious pharmacokinetic interaction occurred between quercetin and digoxin. The concomitant administration of digoxin and quercetin or quercetin-containing herbs and dietary supplement should be avoided.     

Identification of linear plasmid pAM1 in the flavonoid degrading strain Actinoplanes missouriensis(T) (DSM 43046)

By pulsed-field gel electrophoresis, a linear DNA element of about 100 kb was identified in Actinoplanes missouriensis(T) DSM 43046, which grows on the flavonoids hesperidin, rutin and quercetin, and which contains a CO forming quercetinase. Among six Actinoplanes species and strains tested, including A. globisporus(T) DSM 43857, A. philippinensis(T) DSM 43019, A. brasiliensis(T) DSM 43805, A. auranticolor(T) DSM 43031, and A. utahensis(T) DSM 43147, only the A. missouriensis strain exhibited such a genetic element. The linear plasmid, named pAM1, has proteins covalently attached to its 5'-ends like other linear replicons of actinomycetes. Attempts to cure pAM1 failed, however a mutant with reduced plasmid content was obtained, which showed reduced ability to degrade the flavonoid rutinosides rutin and hesperidin. Plasmid pAM1 is the first extrachromosomal genetic element identified in an Actinoplanes species and may be useful to develop genetic tools for biotechnologically important Actinoplanes strains.     

Effect of freezing–thawing process and quercetin on human sperm survival and DNA integrity

We aimed in the first part of our work to study the effect of cryopreservation on the human sperm DNA integrity and the activation of caspase 3, the main apoptosis indicator. In the second part, we were interested in testing the effect of quercetin, as an antioxidant, in preventing sperm damage during the freeze–thawing process. Seventeen semen samples were obtained from 17 men recruited for infertility investigations. Liquefied sperm was cryopreserved using spermfreeze®. Nine of the used samples were divided into two aliquots; the first one was cryopreserved with spermfreeze only (control) and the second one was cryopreserved with spermfreeze supplemented with quercetin to a final concentration of 50 μM. Sperm motility and viability were assessed according to WHO criteria. We used TUNEL assay and the Oxy DNA assay to assess sperm DNA integrity. Activated caspase 3 levels were measured in spermatozoa using fluorescein-labeled inhibitor of caspase (FLICA). Cryopreservation led to a significant increase in sperm DNA fragmentation, DNA oxidation and caspase 3 activation (p < 0.01). Supplementation of the cryopreservation medium with quercetrin induced a significant improvement in post thaw sperm parameters, compared to those of control, regarding sperm motility (p = 0.007), viability (p = 0.008) and DNA integrity (p = 0.02); however, it had no effect on caspase 3 activation (p = 0.3). We conclude that oxidative stress plays a major role in inducing sperm cryodamage but implication of apoptosis in this impairment requires further investigations. Quercetin could have protective effect during cryopreservation but further research is needed to confirm this effect.