Quercetin inhibition of SREBPs and ChREBP expression results in reduced cholesterol and fatty acid synthesis in C6 glioma cells.

Quercetin (Que), a widely distributed flavonoid in the human diet, exerts neuroprotective action because of its property to antagonize oxidative stress. Here, we investigated the effects of Que on lipid synthesis in C6 glioma cells. A rapid Que-induced inhibition of cholesterol and, to a lesser extent, of fatty acid synthesis from [1-14C]acetate was observed. The maximum decrease was detected at the level of palmitate, the end product of de novo fatty acid synthesis. The effect of Que on the enzyme activities of acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS), the two enzymes of this pathway, was investigated directly in situ in permeabilized C6 cells. An inhibitory effect on ACC1 was observed after 4 h of 25 µM Que treatment, while FAS activity was not affected. A reduction of polar lipid biosynthesis was also detected. A remarkable decrease of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) activity, regulatory enzyme of cholesterol synthesis, was evidenced. Expression studies demonstrated that Que acts at transcriptional level, by reducing the mRNA abundance and protein amount of ACC1 and HMGCR. Deepening the molecular mechanism, we found that Que decreased the expression of SREBP-1 and SREBP-2, transcriptional factors representing the main regulators of de novo fatty acid and cholesterol synthesis, respectively. Que also reduced the nuclear content of ChREBP, a glucose-induced transcription factor involved in the regulation of lipogenic genes. Our results represent the first evidence that a direct and rapid downregulatory effect of Que on cholesterol and de novo fatty acid synthesis is elicited in C6 cells.

Disruption of quercetin metabolism by fungicide affects energy production in honey bees (Apis mellifera).

Cytochrome P450 monooxygenases (P450) in the honey bee, Apis mellifera, detoxify phytochemicals in honey and pollen. The flavonol quercetin is found ubiquitously and abundantly in pollen and frequently at lower concentrations in honey. Worker jelly consumed during the first 3 d of larval development typically contains flavonols at very low levels, however. RNA-Seq analysis of gene expression in neonates reared for three days on diets with and without quercetin revealed that, in addition to up-regulating multiple detoxifying P450 genes, quercetin is a negative transcriptional regulator of mitochondrion-related nuclear genes and genes encoding subunits of complexes I, III, IV, and V in the oxidative phosphorylation pathway. Thus, a consequence of inefficient metabolism of this phytochemical may be compromised energy production. Several P450s metabolize quercetin in adult workers. Docking in silico of 121 pesticide contaminants of American hives into the active pocket of CYP9Q1, a broadly substrate-specific P450 with high quercetin-metabolizing activity, identified six triazole fungicides, all fungal P450 inhibitors, that dock in the catalytic site. In adults fed combinations of quercetin and the triazole myclobutanil, the expression of five of six mitochondrion-related nuclear genes was down-regulated. Midgut metabolism assays verified that adult bees consuming quercetin with myclobutanil metabolized less quercetin and produced less thoracic ATP, the energy source for flight muscles. Although fungicides lack acute toxicity, they may influence bee health by interfering with quercetin detoxification, thereby compromising mitochondrial regeneration and ATP production. Thus, agricultural use of triazole fungicides may put bees at risk of being unable to extract sufficient energy from their natural food.

Peltatoside Isolated from Annona crassiflora Induces Peripheral Antinociception by Activation of the Cannabinoid System.

Peltatoside is a natural compound isolated from leaves of Annona crassiflora Mart., a plant widely used in folk medicine. This substance is an analogue of quercetin, a flavonoid extensively studied because of its diverse biological activities, including analgesic effects. Besides, a previous study suggested, by computer structure analyses, a possible quercetin-CB1 cannabinoid receptor interaction. Thus, the aim of this work was to assess the antinociceptive effect of peltatoside and analyze the cannabinoid system involvement in this action. The mouse paw pressure test was used and hyperalgesia was induced by intraplantar injection of carrageenan (200 µg/paw). All used drugs were administered by intraplantar administration in Swiss male mice (n = 6). Peltatoside (100 µg/paw) elicited a local inhibition of hyperalgesia. The peripheral antinociceptive action of peltatoside was antagonized by the CB1 cannabinoid antagonist AM251 (160 µg/paw), but not by CB2 cannabinoid antagonist AM630 (100 µg/paw). In order to assess the role of endocannabinoids in this peripheral antinociceptive effect, we used (i) [5Z,8Z,11Z,14Z]-5,8,11,14-eicosatetraenyl-methyl ester phosphonofluoridic acid, an inhibitor of anandamide amidase; (ii) JZL184, an inhibitor for monoacylglycerol lipase, the primary enzyme responsible for degrading the endocannabinoid 2-arachidonoylglycerol; and (iii) VDM11, an endocannabinoid reuptake inhibitor. MAFP, JZL184, and VDM11 did not induce antinociception, respectively, at the doses 0.5, 3.8, and 2.5 µg/paw, however, these three drugs were able to potentiate the peripheral antinociceptive effect of peltatoside at an intermediary dose (50 µg/paw). Our results suggest that this natural substance is capable of inducing analgesia through the activation of peripheral CB1 receptors, involving endocannabinoids in this process.

Estrogenic effects of flavonoid components in Xiaoyao powder.

The objective of this study was to evaluate the estrogenic effects and mechanisms of three flavonoid components in Xiaoyao powder: quercetin, kaempferol, and isorhamnetin. The drugs were used to treat estrogen receptor (ER)-positive human breast cancer MCF-7 cells, and proliferation was measured using the MTT method. The expression of proteins and mRNA of the ER subtype were measured using western blotting and real time polymerase chain reaction. The quercetin (10(-2) µM, 10(-3) µM), kaempferol (100 µM, 10(-2) µM), and isorhamnetin (10(-3) µM) promoted the proliferation of MCF-7 cells, and the expression of ERα and ERß proteins and mRNA were all increased significantly (P < 0.05). These effects were reversed by treatment with 0.1 µM estrogen antagonist ICI182780. Three flavonoid components in Xiaoyao powder increased the expression of proteins and mRNA of ERα and ERß and promoted the proliferation of MCF-7 cells. These estrogenic effects were mediated by the ER.

Activation of transient receptor potential ankyrin 1 by quercetin and its analogs.

The agonistic activity of quercetin and its analogs towards the transient receptor potential ankyrin 1 (TRPA1) has been experimentally investigated. The human TRPA1 was expressed in HEK293T cells using a tetracycline-inducible system. The activation of TRPA1 was evaluated by a fluo-4 fluorescence assay based on calcium sensing. The results of a structure-activity relationship study led to the selection of six flavonoids, all of which activated the TRPA1 channel in a dose-dependent manner. Notably, the activation of TRPA1 by these flavonoid aglycones was completely inhibited by the co-treatment of the HEK293T cells with the TRPA1-specific antagonist, HC-030031. Several flavonoid glycosides and metabolites were also evaluated, but did not activate the TRPA1 except for methylated quercetin. On the other hand, TRPV1 (vanilloid receptor) did not respond to any of the flavonoids evaluated in this study. Therefore, these data suggest that the flavonoids would be promising ligands for the TRPA1.

Hyperglycemia and Anthocyanin Inhibit Quercetin Metabolism in HepG2 Cells.

A high glucose (Glu) milieu promotes generation of reactive oxygen species, which may not only cause cellular damage, but also modulate phase II enzymes that are responsible for the metabolism of flavonoids. Thus, we examined the effect of a high Glu milieu on quercetin (Q) metabolism in HepG2 cells. HepG2 cells were grown for 3 days in Glu ranging from 5.5 to 50 mmol/L and/or cyanidin-3-glucoside (C3G) ranging from 0 to 25 µmol/L. Subsequently, the capacity of HepG2 cells to metabolize Q was assessed for up to 16 h. Q metabolites were analyzed by high-performance liquid chromatography. Four major Q metabolites were observed in the culture medium and inside the HepG2 cells. Three of these metabolites appear to be sulfated forms of Q or methylated Q, and one was a methylated Q. These metabolites and Q itself were reduced or tended to be reduced in cells grown in a high Glu compared to a normal Glu medium. Addition of C3G or superoxide dismutase plus catalase did not prevent or enhance reduction of Q metabolites. In vitro, a hyperglycemic milieu decreases the production of the principal Q metabolites in HepG2 cells, mediated through mechanisms independent of oxidative stress.

Antidepressant-like effect of quercetin in bulbectomized mice and involvement of the antioxidant defenses, and the glutamatergic and oxidonitrergic pathways.

Olfactory bulbectomy (OB) is an animal model of depression that can mimic symptoms that are characteristic of depressive patients, such as behavioral, neurochemical and neuromorphological changes. Quercetin decreased the immobility time in the forced swimming test and tail suspension test. With the open field test, quercetin did not alter the locomotor activity of mice and in the splash test, quercetin increased the time spent grooming. The repeated treatment with quercetin (25mg/kg, for 14days) reversed the behavioral hyperactivity induced by OB in the open field test and was able to prevent depressant-like effects in the forced swimming test and tail suspension test. Regarding oxidative stress, OB reduced the levels of glutathione and increase the activity of superoxide dismutase and lipid hydroperoxide content (LOOH) in the hippocampus. Only the increase in LOOH levels was reversed by treatment with quercetin. In a further series of experiments with non-bulbectomized mice, the antidepressant effect of quercetin in the tail suspension test was reversed by the pretreatment of mice with NMDA, l-arginine or sildenafil. The administration of methylene blue and 7-nitroindazole, in combination with an underactive dose of quercetin (5mg/kg, p.o.), decreased the immobility time in the tail suspension test compared with the use of drug alone. There was no significant change in locomotor activity in the open field test. Our results suggest that the antidepressant effect of quercetin is dependent on the inhibition of the NMDA receptors and/or synthesis of nitric oxide. In addition, considering the reduction of LOOH levels on the hippocampus, we verify that the antioxidant effects of quercetin also contribute to its antidepressive potential. These data contribute to the understanding of the mechanisms involved in the antidepressant effect of quercetin and reinforce the involvement of the NMDA receptors and the nitric oxide on the pathophysiology of depression.

Estrogen modulation properties of mangiferin and quercetin and the mangiferin metabolite norathyriol.

Mango fruit contain many bioactive compounds, some of which are transcription factor regulators. Estrogen receptor alpha (ERα) and beta (ERß) are two regulators of gene transcription that are important in a variety of physiological processes and also in diseases including breast cancer. We examined the ability of the mango constituents quercetin, mangiferin, and the aglycone form of mangiferin, norathyriol, to activate both isoforms of the estrogen receptor. Quercetin and norathyriol decreased the viability of MCF-7 breast cancer cells whereas mangiferin had no effect on MCF-7 cells. We also determined that quercetin and mangiferin selectively activated ERα whereas norathyriol activated both ERα and ERß. Despite quercetin, mangiferin and norathyriol having similar polyphenolic structural motifs, only norathyriol activated ERß, showing that bioactive agents in mangoes have very specific biological effects. Such specificity may be important given the often-opposing roles of ERα and ERß in breast cancer proliferation and other cellular processes.

Quercetin antagonism of GABAAρ1 receptors is prevented by ascorbic acid through a redox-independent mechanism.

Quercetin is a natural flavonoid widely distributed in plants that acts as a neuroprotective agent and modulates the activity of different synaptic receptors and ion channels, including the ionotropic GABA receptors. GABA(Aρ1) receptors were shown to be antagonized by quercetin, but the mechanisms underlying these antagonistic actions are still unknown. We have analyzed here if the antagonistic action produced by quercetin on GABA(Aρ1) receptors was related to its redox activity or due to alternative mechanism/s. Homomeric GABA(Aρ1) receptors were expressed in frog oocytes and GABA-evoked responses electrophysiologically recorded. Quercetin effects on GABA(Aρ1) receptors were examined in the absence or presence of ascorbic acid. Chemical protection of cysteines by selective sulfhydryl reagents and site directed mutagenesis experiments were also used to determine ρ1 subunit residues involved in quercetin actions. Quercetin antagonized GABA(Aρ1) receptor responses in a dose-dependent, fast and reversible manner. Quercetin inhibition was prevented in the presence of ascorbic acid, but not by thiol reagents that modify the extracellular Cys-loop of these receptors. H141, an aminoacidic residue located near to the ρ1 subunit GABA binding site, was involved in the allosteric modulation of GABA(Aρ1) receptors by several agents including ascorbic acid. Quercetin similarly antagonized GABA-evoked responses mediated by mutant (H141D)GABA(Aρ1) and wild-type receptors, but prevention exerted by ascorbic acid on quercetin effects was impaired in mutant receptors. Taken together the present results suggest that quercetin antagonistic actions on GABA(Aρ1) receptors are mediated through a redox-independent allosteric mechanism.

A subnanomole level photoelectrochemical sensing platform for hexavalent chromium based on its selective inhibition of quercetin oxidation.

An indirect photoelectrochemical sensing platform for toxic hexavalent chromium was for the first time constructed based on its redox reaction with quercetin as both the electron donor and photosensitizer on a TiO(2) photoanode, and thus inhibiting the photocurrent quantitatively and selectively. The presence of even 500-fold coexisting Cr(III) does not interfere in the detection of Cr(VI). Under the optimum conditions, the electrode displayed a linear decrease response as the Cr(VI) concentration increased from 1 to 10 nmol L(-1) and from 20 to 140 nmol L(-1) with a detection limit of 0.24 nmol L(-1). Many possible ions in drinking water did not interfere with the detection, and the real sample detection results agreed well with those obtained by GFAAS. This work provide a novel methodology for the simple, low-cost photoelectrochemical detection of Cr(VI) in drinking water.

Flavonoid binding to human serum albumin.

Dietary flavonoid may have beneficial effects in the prevention of chronic diseases. However, flavonoid bioavailability is often poor probably due to their interaction with plasma proteins. Here, the affinity of daidzein and daidzein metabolites as well as of genistein, naringenin, and quercetin for human serum albumin (HSA) has been assessed in the absence and presence of oleate. Values of the dissociation equilibrium constant (K) for binding of flavonoids and related metabolites to Sudlow's site I range between 3.3x10(-6) and 3.9x10(-5)M, at pH 7.0 and 20.0 degrees C, indicating that these flavonoids are mainly bound to HSA in vivo. Values of K increase (i.e., the flavonoid affinity decreases) in the presence of saturating amounts of oleate by about two folds. Present data indicate a novel role of fatty acids as allosteric inhibitors of flavonoid bioavailability, and appear to be relevant in rationalizing the interference between dietary compounds, food supplements, and drugs.

Binding of arachidonic acid and two flavonoid inhibitors to human 12- and 15-lipoxygenases: a steered molecular dynamics study.

The lipoxygenases (LOX) are a family of non-heme iron-containing dioxygenases which catalyze the stereospecific insertion of molecular oxygen into arachidonic acid, leading to hydroxy derivatives as end products. In this work, we docked arachidonic acid and two of its competitive inhibitors, flavonoids baicalein and quercetin, into the binding pockets of human 12- and 15-lipoxygenase. Steered molecular dynamics (SMD) simulations were employed to study the unbinding processes of the substrate and inhibitors from the two isoforms.

Quercetin-induced PC12 cell death accompanied by caspase-mediated DNA fragmentation.

Flavonoids have been reported to be potent antioxidants and beneficial in oxidative stress related diseases. Quercetin, a major flavonoid in food, deserves much attention because of its antioxidative activity. However, the actions of flavonoids including quercetin are complex and paradoxical. Quercetin caused apoptosis and/or cell death in various cells including cancer cells and normal cells. In this study, we investigated the effects of quercetin with or without hydrogen peroxide (H2O2) on cell death of PC12 cells, a neuronal cell line. We showed that quercetin at 10-30 microM alone caused cell death accompanied by caspase-mediated DNA fragmentation in undifferentiated PC12 cells. Quercetin did not inhibit and rather enhanced 0.1 mM H2O2-induced cell death. The toxic effect of quercetin was not inhibited by antioxidants such as N-acetylcysteine and GSH, although H2O2-induced cell death was inhibited by the antioxidants. Quercetin-induced cell death was reduced by 2 h treatment with nerve growth factor and serum. In addition, quercetin caused cell death in differentiated PC12 cells that were cultured with nerve growth factor for 6 d. Genistein, a soy isoflavone that has the pro-apoptotic activity, also caused cell death with DNA fragmentation. Further evaluation of the potential of dietary flavonoids as neuroprotective reagents is needed.

The flavonoid quercetin induces hypoxia-inducible factor-1alpha (HIF-1alpha) and inhibits cell proliferation by depleting intracellular iron.

Quercetin, a flavonoid with anti-oxidant, metal chelating, kinase modulating and anti-proliferative properties, can induce hypoxia-inducible factor-1alpha (HIF-1alpha) in normoxia, but its mechanism of action has not been determined. In this study we characterized the induction of HIF-1alpha and the inhibition of cell proliferation caused by quercetin in HeLa and ASM (airway smooth muscle) cells and examined the effect of iron on these processes. Furthermore, we investigated the relevance of the intracellular levels of quercetin to HIF-1alpha expression and cell proliferation. Our data demonstrate that quercetin depletes intracellular calcein-chelatable iron and that supplying additional iron from extracellular or intracellular pools abrogates the induction of HIF-1alpha by quercetin. Moreover, addition of iron reverses the quercetin-induced inhibition of DNA synthesis, cell proliferation and cycle progression, but to different extents, depending on cell type. We propose that quercetin stabilises HIF-1alpha and inhibits cell proliferation predominantly by decreasing the concentration of intracellular iron through chelation.

The effect of quercetin on pro-apoptotic activity of cisplatin in HeLa cells.

It is well known that some tumour cells are very resistant to chemotherapy-induced cell death which indicate poor prognosis for patients. Thus the aim of the present study was to investigate the effect of quercetin on pro-apoptotic activity of cisplatin in human cervix carcinoma cells (HeLa). Three variants of experiments were performed. In the first one cells were incubated with studied drugs separately for 8 and 24h. In the second, drugs were added to the culture medium simultaneously. In third cisplatin or quercetin addition was followed by subsequent quercetin or cisplatin treatment, respectively. We observed different apoptotic effects, dependent on the drug succession. Preincubation of cells with quercetin followed by cisplatin treatment appeared to be the most effective and was correlated with strong activation of caspase-3 and inhibition of both heat shock proteins (Hsp72) and multi-drug resistance proteins (MRP) levels. Our results indicate that quercetin pretreatment sensitizes HeLa cells to cisplatin-induced apoptosis in HeLa cells.

Mechanism of heme oxygenase-1 gene induction by quercetin in rat aortic smooth muscle cells.

We previously reported that adenovirus-mediated gene transfer of heme oxygenase-1 (HO-1) inhibits the development of atherosclerosis in apolipoprotein E-deficient mice. This finding implies that HO-1 induction is beneficial for protecting blood vessels. We also found that quercetin, a common polyphenolic compound in foods of plant origin, induces HO-1 expression in RAW264.7 cells. This study was aimed at examining the potency of quercetin as a HO-1 inducer and its regulation in rat aortic smooth muscle cells (RASMCs). We showed that quercetin-induced HO-1 production was in time- and dose-dependent fashions, and that this regulation occurred at both transcription and translation levels. Quercetin increased p38 mitogen-activated protein kinase (p38MAPK), but inhibited extracellular signal-regulated kinase in RASMCs. The level of quercetin-induced HO-1 expression was attenuated by SB202190 (a p38MAPK inhibitor). Taken together from the data in this study, we suggest that quercetin induced HO-1 expression, at least in part, through p38MAPK.

Mechanisms underlying quercetin-induced vasorelaxation in aorta of subchronic diabetic rats: an in vitro study.

In this study, the mechanisms involved in vasorelaxant effect of the flavonoid quercetin was investigated in isolated aortic rings from streptozotocin (STZ)-diabetic rats. After 4 weeks, addition of quercetin (0.1 microM-1 mM) caused a significant dose-dependent relaxation of noradrenaline (NA)- and KCl-preconstricted rings in both control and diabetic groups with a significant inter-group difference of P<0.01. Furthermore, both nitro-L-arginine-methyl ester (L-NAME, 100 microM) and indomethacin (10 microM) markedly attenuated the vasorelaxant responses following quercetin application. Meanwhile, endothelium removal significantly attenuated the quercetin-induced vasorelaxation. It is concluded that the quercetin can relax the preconstricted rings of aorta in subchronic STZ-diabetic rats through nitric oxide- and -prostaglandin-mediated pathways, which themselves could be considered as endothelium-dependent.

3-O-methylquercetin more selectively inhibits phosphodiesterase subtype 3.

Rhamnus nakaharai Hayata (Rhamnaceae), has been used as a folk medicine in Taiwan for treating constipation, inflammation, tumors and asthma. 3-O-methylquercetin (3-MQ), a main constituent of the plant, has been reported to inhibit total cAMP- and cGMP-phosphodiesterase (PDE) of guinea pig trachealis. Therefore we were interested in investigating the inhibitory effect of 3-MQ on various PDE isozymes from guinea pig lungs and hearts. Isolated guinea pig lungs and hearts were homogenized and centrifuged. The supernatant was chromatographed over a column of Q-sepharose, and eluted with various concentrations of NaCl. In the following order, PDE subtypes 1, 5, 2, 4 from lungs, and 3 from hearts were separated. The IC 50 values of 3-MQ on these isozymes were 31.9, 86.9, 18.6, 28.5 and 1.6 microM, respectively. 3-MQ (10-100 microM) non-competitively inhibited PDE2, but competitively inhibited PDE4. 3-MQ (1-10 microM) also competitively inhibited PDE3. However, 3-MQ (10-100 microM) did not competitively inhibit PDE1 and 5, although it had a tendency to competitively inhibit PDE1 at concentrations of 10 - 30 microM. The present results showed that K i value of 3-MQ was similar to that of milrinone in PDE3, and was not significantly different from that of Ro 20 - 1724 in PDE4, respectively. In conclusion, 3-MQ was revealed to be a selective and competitive PDE3/PDE4 inhibitor, although its inhibitory effect on PDE4 was not potent. Therefore, 3-MQ may have a potential in the treatment of asthma beside its antiviral activity.

Mechanisms involved in kaempferol-induced relaxation in rat uterine smooth muscle.

The effect of kaempferol on KCI (60 mM)-induced tonic contraction in isolated rat uterus and its modification by inhibitors of cAMP-dependent protein kinase (PKA) (Rp-cAMPS and TPCK), phosphodiesterase (papaverine), adenylyl cyclase (2',3'-dideoxyadenosine, DDA), transcription (actinomycin D), protein synthesis (cycloheximide) and ornithine decarboxylase (alpha-difluoromethyl-ornithine, DFMO), as well as a polyamine, spermine, have been assayed. Kaempferol (3 to 60 microM) induced concentration-dependent relaxation on KCl-induced tonic contraction (IC50: 10.1 +/- 1.89 microM). This relaxing effect was antagonized (p<0.05) by Rp-cAMPS (10 microM), TPCK (3 microM), DDA (100 microM), actinomycin D (4 and 12 microM), cycloheximide (100 microM), DFMO (10 mM), actinomycin D (12 microM) plus TPCK and actinomycin D (12 microM) plus spermine (1 mM). Furthermore, the displacement obtained with actinomycin D plus DFMO was not statistically significant. Our results suggest that kaempferol through cAMP produces transcriptional events and polyamines are, at least partially, involved in the relaxant effect of kaempferol.

Dietary flavonols quercetin and kaempferol are ligands of the aryl hydrocarbon receptor that affect CYP1A1 transcription differentially.

Transcriptional activation of the human CYP1A1 gene (coding for cytochrome P450 1A1) is mediated by the aryl hydrocarbon receptor (AhR). In the present study we have examined the effect of the common dietary polyphenolic compounds quercetin and kaempferol on the transcription of CYP1A1 and the function of the AhR in MCF-7 human breast cancer cells. Quercetin caused a time- and concentration-dependent increase in the amount of CYP1A1 mRNA and CYP1A1 enzyme activity in MCF-7 cells. The increase in CYP1A1 mRNA caused by quercetin was prevented by the transcription inhibitor actinomycin D. Quercetin also caused an increase in the transcription of a chloramphenicol reporter vector containing the CYP1A1 promoter. Quercetin failed to induce CYP1A1 enzyme activity in AhR-deficient MCF-7 cells. Gel retardation studies demonstrated that quercetin activated the ability of the AhR to bind to an oligonucleotide containing the xenobiotic-responsive element (XRE) of the CYP1A1 promoter. These results indicate that quercetin's effect is mediated by the AhR. Kaempferol did not affect CYP1A1 expression by itself but it inhibited the transcription of CYP1A1 induced by the prototypical AhR ligand 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), as measured by a decrease in TCDD-induced CYP1A1 promoter-driven reporter vector activity, and CYP1A1 mRNA in cells. Kaempferol also abolished TCDD-induced XRE binding in a gel-shift assay. Both compounds were able to compete with TCDD for binding to a cytosolic extract of MCF-7 cells. Known ligands of the AhR are, for the most part, man-made compounds such as halogenated and polycyclic aromatic hydrocarbons. These results demonstrate that the dietary flavonols quercetin and kaempferol are natural, dietary ligands of the AhR that exert different effects on CYP1A1 transcription.