Effects of synthetic and naturally occurring flavonoids on Na+,K+-ATPase: aspects of the structure-activity relationship and action mechanism

A previously undescribed phenylethanoid glycoside from Callicarpa kwangtungensis Chun acts as an agonist of the Na/K-ATPase signal transduction pathway

The new concept that Na/K-ATPase acts as a receptor prompted us to look for new ligands from Callicarpa kwangtungensis Chun. Using column chromatography, an undescribed phenethyl alcohol glycoside, callicarpanoside A, and an undescribed benzyl alcohol glycoside, callicarpanoside B, along with twelve known polyphenols were isolated from Callicarpa kwangtungensis Chun. All the isolated compounds were evaluated for their Na/K-ATPase (NKA) inhibitory activities. Using our NKA technology platform-based screening assay protocols, callicarpanoside B was identified as an undescribed Na/K-ATPase agonist. In particular, the newly identified benzyl alcohol glycoside was found to bind NKA and activate the receptor NKA/Src complex, resulting in the activation of protein kinase cascades. These cascades included extracellular signal-regulated kinases and protein kinase C epsilon, as well as NKA α1 endocytosis at nanomolar concentrations. Unlike the class of cardiotonic steroids, callicarpanoside B showed less inhibition of NKA activity and caused less cellular toxicity. Moreover, callicarpanoside B was found to bind NKA at a different site other than the cardiotonic steroids binding site. Thus, we have identified an undescribed NKA α1 agonist that may be used to enhance the physiological processes of NKA α1 signaling.

Metabolite profiles of Pulicaria crispa and P. incisa in relation to their in-vitro/ in-vivo antioxidant activity and hepatoprotective effect: A comparative mass spectrometry-based metabolomics

Plants of the genus Pulicaria (family: Asteraceae) are widely used in central Asia and the Middle East for treatment of different human diseases. Ultra performance liquid chromatography coupled to high resolution mass spectrometry (UPLC/MS) was utilized to establish the metabolic profiles of two Pulicaria species: P. crispa and P. incisa. 122 metabolites were identified including flavonoids (37), phenolic acids (22), sesquiterpenes (17), diterpenes (7), and fatty acids (27), with enrichment in methoxylated flavonoids (20), caffeoylquinic acid conjugates (14) xanthane sesquiterpenes (9) and hydroxylated fatty acids (20) in both Pulicaria species. The metabolite profile of P. incisa was characterized by the presence of tri- and tetra-methoxylated flavonoids while xanthane sesquiterpenes were the main chemical markers of P. crispa. Additionally, a novel sesquiterpene acid (dihydropulicaric acid) was annotated in both species based on its MS fragments. Antioxidant activity for P. crispa and P. incisa methanol extracts was assessed in vitro based on DPPH and ABTS assays and further in vivo using chlorpromazine intoxicated rat model. Results revealed that P. incisa extract was more effective in inhibiting both DPPH and ABTS free radicals (IC₅₀ 0.36 and 0.52 mg/mL, respectively) than P. crispa (IC₅₀ 0.51 and 0.73 mg/mL). In the animal model, antioxidant activity of P. incisa (20 mg/kg/day) was also slightly higher causing a 55 % reduction in MDA levels and 65 % increase in GSH activity compared to untreated animals. Furthermore, both extracts showed a hepatoprotective effect as revealed by improvement in levels of serum biomarkers of liver functions: total bilirubin, alanine transaminase (ALT) and aspartate transaminase (AST) comparable to silymarin at 25 mg/kg/day. These findings were also supported by the preserved integrity of the hepatic tissues of animals receiving either extracts at a dose of 20 mg/kg b.wt. The present study reveals for the potential antioxidant and hepatoprotective effects for Pulicaria in relation to its bioactive metabolites.

Natural flavonoids inhibit the plasma membrane Ca2+-ATPase

Research on flavonoids from plant sources has recently sparked increasing interest because of their beneficial health properties. Different studies have shown that flavonoids change the intracellular Ca²⁺ homeostasis linked to alterations in the function of mitochondria, Ca²⁺ channels and Ca²⁺ pumps. These findings hint at plasma membrane Ca²⁺-ATPase (PMCA) involvement, as it transports Ca²⁺ actively to the extracellular medium coupled to ATP hydrolysis, thus maintaining ion cellular homeostasis. The present study aims to investigate the effect of several natural flavonoids on PMCA both in isolated protein systems and in living cells, and to establish the relationship between flavonoid structure and inhibitory activity on PMCA. Our results show that natural flavonoids inhibited purified and membranous PMCA with different effectiveness: quercetin and gossypin were the most potent and their inhibition mechanisms seem to be different, as quercetin does not prevent ATP binding whereas gossypin does. Moreover, PMCA activity was inhibited in human embryonic kidney cells which transiently overexpress PMCA, suggesting that the effects observed on isolated systems could occur in a complex structure like a living cell. In conclusion, this work reveals a novel molecular mechanism through which flavonoids inhibit PMCA, which leads to Ca²⁺ homeostasis and signaling alterations in the cell.

Enterodiol is Actively Transported by Rat Liver Cell Membranes

The interaction of enterodiol and the well-described polyphenol epigallocatechin gallate (EGCG) with hepatic membranes has been matter of interest in the last few years. On one hand, EGCG is only able to bind to the phospholipid polar head groups, as it has been already described in synthetic lipid bilayers and erythrocyte membranes but cannot get inserted into the hydrophobic core or be transported into the lumen of membrane vesicles. On the other, enterodiol has no interaction with non-energized membranes either, but it is able to interact and even be transported upon addition of ATP. In fact, the ATPase activity undergoes a twofold increase in the presence of enterodiol but not in the presence of EGCG. This is the first report on the transport of enterodiol by liver membranes, and it may help explain the rather high blood concentrations of this estrogenic enterolignan compared to EGCG, which is extensively metabolized by the intestine and the liver. The present results suggest that a fraction of enterodiol may escape the liver inactivation by being pumped out from the hepatocytes to the bloodstream.

Molecular Mechanisms of Inhibition of Streptococcus Species by Phytochemicals

This review paper summarizes the antibacterial effects of phytochemicals of various medicinal plants against pathogenic and cariogenic streptococcal species. The information suggests that these phytochemicals have potential as alternatives to the classical antibiotics currently used for the treatment of streptococcal infections. The phytochemicals demonstrate direct bactericidal or bacteriostatic effects, such as: (i) prevention of bacterial adherence to mucosal surfaces of the pharynx, skin, and teeth surface; (ii) inhibition of glycolytic enzymes and pH drop; (iii) reduction of biofilm and plaque formation; and (iv) cell surface hydrophobicity. Collectively, findings from numerous studies suggest that phytochemicals could be used as drugs for elimination of infections with minimal side effects.

Phytochemical regulation of Fyn and AMPK signaling circuitry

During the past decades, phytochemical terpenoids, polyphenols, lignans, flavonoids, and alkaloids have been identified as antioxidative and cytoprotective agents. Adenosine monophosphate-activated protein kinase (AMPK) is a kinase that controls redox-state and oxidative stress in the cell, and serves as a key molecule regulating energy metabolism. Many phytochemicals directly or indirectly alter the AMPK pathway in distinct manners, exerting catabolic metabolism. Some of them are considered promising in the treatment of metabolic diseases such as type II diabetes, obesity, and hyperlipidemia. Another important kinase that regulates energy metabolism is Fyn kinase, a member of the Src family kinases that plays a role in various cellular responses such as insulin signaling, cell growth, oxidative stress and apoptosis. Phytochemical inhibition of Fyn leads to AMPK-mediated protection of the cell in association with increased antioxidative capacity and mitochondrial biogenesis. The kinases may work together to form a signaling circuitry for the homeostasis of energy conservation and expenditure, and may serve as targets of phytochemicals. This review is intended as a compilation of recent advancements in the pharmacological research of phytochemicals targeting Fyn and AMPK circuitry, providing information for the prevention and treatment of metabolic diseases and the accompanying tissue injuries.

Water-soluble and cleavable quercetin-amino acid conjugates as safe modulators for P-glycoprotein-based multidrug resistance

Quercetin-amino acid conjugates with alanine or glutamic acid moiety attached at 7-O and/or 3-O position of quercetin were prepared, and their multidrug resistance (MDR)-modulatory effects were evaluated. A quercetin-glutamic acid conjugate, 7-O-Glu-Q (3a), was as potent as verapamil in reversing MDR and sensitized MDR MES-SA/Dx5 cells to various anticancer drugs with EC50 values of 0.8-0.9 μM. Analysis on Rh-123 accumulation confirmed that 3a inhibits drug efflux by Pgp, and Pgp ATPase assay showed that 3a interacts with the drug-binding site of Pgp to stimulate its ATPase activity. Physicochemical analysis of 3a revealed that solubility, stability, and cellular uptake of quercetin were significantly improved by the glutamic acid promoiety, which eventually dissociates from 3a to produce quercetin and quercetin metabolites in intracellular milieu. Taken together, potent MDR-modulating activity along with intracellular conversion into the natural flavonoid quercetin warrants development of the quercetin-amino acid conjugates as safe MDR modulators.

Effect of polyphenols extracts from Brassica vegetables on erythrocyte membranes (in vitro study)

The aim of this work was to estimate the in vitro effects of polyphenol extracts from Brassica vegetables (Brussels sprouts and red cabbage) on erythrocyte membranes with normal and high concentration of cholesterol. To determine the effect of phenolic compounds we prospectively studied cholesterol concentration, lipid peroxidation, membrane fluidity and ATPase activity. Polyphenol extracts from Brassica vegetables resulted in statistically significant reductions in cholesterol concentrations in hypercholesterolemic erythrocytes. For control erythrocytes, no significant reduction of cholesterol levels was observed for both extracts. Decreases in lipid peroxidation intensity were observed after incubation of hypercholesterolemic erythrocytes with the extracts. No changes in membrane fluidity for both extracts were noted for normal and hypercholesterolemic erythrocytes. The activity of ATPase decreased after incubation of normal and hypercholesterolemic erythrocytes with extract from Brassica vegetables. Our results indicate that polyphenols from red cabbage and Brussels sprout may directly influence erythrocyte membrane properties.

Mechanisms underlying the diuretic effects of Tropaeolum majus L. extracts and its main component isoquercitrin

ETHNOPHARMACOLOGICAL RELEVANCE

Previous studies have shown that the extracts obtained from Tropaeolum majus L., and its main compound isoquercitrin (ISQ), exhibit pronounced diuretic effects, supporting the ethnopharmacological use of this plant. The aim of this study was to evaluate the efficacy and mechanisms underlying the diuretic action of an ethanolic extract of Tropaeolum majus (HETM), its purified fraction (TMLR), and its main compound ISQ, in spontaneously hypertensive rats (SHR).

MATERIALS AND METHODS

The diuretic effects of HETM (300mg/kg; p.o.), TMLR (100mg/kg; p.o.), and ISQ (10mg/kg; p.o.), were compared with classical diuretics in 7days repeated-dose treatment. The urinary volume, sodium, potassium, chloride, bicarbonate, conductivity, pH and density were estimated in the sample collected for 15h. The plasmatic concentration of sodium, potassium, urea, creatinine, aldosterone, vasopressin, nitrite and angiotensin converting enzyme (ACE) activity were measured in samples collected at the end of the experiment (seventh day). Using pharmacological antagonists or inhibitors, we determine the involvement of bradykinin, prostaglandin and nitric oxide (NO) in ISQ-induced diuresis. In addition, reactive oxygen species (ROS) and the activity of erythrocytary carbonic anhydrase and renal Na(+)/K(+)/ATPase were evaluated in vitro.

RESULTS

HETM, TMLR and ISQ increased diuresis similarly to spironolactone and also presented K(+)-sparing effects. All groups presented both plasmatic aldosterone levels and ACE activity reduced. Previous treatment with HOE-140 (a B2-bradykinin receptor antagonist), or indomethacin (a cyclooxygenase inhibitor), or L-NAME (a NO synthase inhibitor), fully avoided the diuretic effect of ISQ. In addition, the 7days treatment with ISQ resulted in increased plasmatic levels of nitrite and reducing ROS production. Moreover, the renal Na(+)/K(+)/ATPase activity was significantly decreased by ISQ.

CONCLUSION

Our results suggest that the mechanisms through ISQ and extracts of Tropaeolum majus increase diuresis in SHR rats are mainly related to ACE inhibition, increased bioavailability of bradykinin, PGI2, and nitric oxide, besides an inhibitory effect on Na(+)/K(+)-ATPase.

Flavonoid dimers as bivalent modulators for pentamidine and sodium stiboglucanate resistance in leishmania

Drug resistance by overexpression of ATP-binding cassette (ABC) transporters is an impediment in the treatment of leishmaniasis. Flavonoids are known to reverse multidrug resistance (MDR) in Leishmania and mammalian cancers by inhibiting ABC transporters. Here, we found that synthetic flavonoid dimers with three (compound 9c) or four (compound 9d) ethylene glycol units exhibited a significantly higher reversing activity than other shorter or longer ethylene glycol-ligated dimers, with approximately 3-fold sensitization of pentamidine and sodium stibogluconate (SSG) resistance in Leishmania, respectively. This modulatory effect was dosage dependent and not observed in apigenin monomers with the linker, suggesting that the modulatory effect is due to its bivalent nature. The mechanism of reversal activity was due to increased intracellular accumulation of pentamidine and total antimony in Leishmania. Compared to other MDR modulators such as verapamil, reserpine, quinine, quinacrine, and quinidine, compounds 9c and 9d were the only agents that can reverse SSG resistance. In terms of reversing pentamidine resistance, 9c and 9d have activities comparable to those of reserpine and quinacrine. Modulators 9c and 9d exhibited reversal activity on pentamidine resistance among LeMDR1(-/-), LeMDR1(+/+), and LeMDR1-overexpressed mutants, suggesting that these modulators are specific to a non-LeMDR1 pentamidine transporter. The LeMDR1 copy number is inversely related to pentamidine resistance, suggesting that it might be involved in importing pentamidine into the mitochondria. In summary, bivalency could be a useful strategy for the development of more potent ABC transporter modulators and flavonoid dimers represent a promising reversal agent for overcoming pentamidine and SSG resistance in parasite Leishmania.

Flavonoid Dimers as Bivalent Modulators for P-Glycoprotein-Based Multidrug Resistance: Synthetic Apigenin Homodimers Linked with Defined-Length Poly(ethylene glycol) Spacers Increase Drug Retention and Enhance Chemosensitivity in Resistant Cancer Cells

Much effort has been spent on searching for better P-glycoprotein- (P-gp-) based multidrug resistance (MDR) modulators. Our approach was to target the binding sites of P-gp using dimers of dietary flavonoids. A series of apigenin-based flavonoid dimers, linked by poly(ethylene glycol) chains of various lengths, have been synthesized. These flavonoid dimers modulate drug chemosensitivity and retention in breast and leukemic MDR cells with the optimal number of ethylene glycol units equal to 2-4. Compound 9d bearing four ethylene glycol units increased drug accumulation in drug-resistant cells and enhanced cytotoxicity of paclitaxel, doxorubicin, daunomycin, vincristine, and vinblastine in drug-resistant breast cancer and leukemia cells in vitro, resulting in reduction of IC50 by 5-50 times. This compound also stimulated P-gp's ATPase activity by 3.3-fold. Its modulating activity was presumably by binding to the substrate binding sites of P-gp and disrupting drug efflux.

2-Methoxy-3,8,9-trihydroxy coumestan: a new synthetic inhibitor of Na+,K+-ATPase with an original mechanism of action

The aim of the present work was to analyse the interaction between Na(+),K(+)-ATPase and one of our recent synthesized coumestans, namely the original molecule 2-methoxy-3,8,9-trihydroxy coumestan (PCALC36). Rat brain (mainly alpha 2 and alpha 3 Na(+),K(+)-ATPase isoforms) and kidney (alpha 1 isoform) fractions enriched with Na(+),K(+)-ATPase were utilized to compare the inhibition promoted by PCALC36 with that of classical inhibitors like ouabain and vanadate. Analysis of inhibition curves revealed that unlike ouabain, which was about a thousand times more potent to inhibit brain isoforms than kidney isoform, PCALC36 had a similar affinity for brain (IC(50)=4.33+/-0.90 microM) and kidney (IC(50)=11.04+/-0.86 microM) isoforms. The inhibitory effect of PCALC36 was not antagonized by 1-10 mM K(+), as observed with ouabain. Whereas vanadate was more potent in ionic conditions promoting the E2 conformation of the enzyme, the inhibitory effect of PCALC36 was equal in ionic conditions favouring either the E1 or E2 conformations. Equilibrium binding assays with [3H]ouabain revealed that the addition of 2-10 microM PCALC36 did not change the K(d) of ouabain but decreased its maximal binding (B(max)) in a concentration-dependent manner (from 76.6 to 44.0 pmol/mg protein). This inhibitory effect of PCALC36 was not reverted after an extensive washing procedure indicating that it forms a very stable complex with Na(+),K(+)-ATPase. We conclude that PCALC36, a new molecule with a non-steroidal skeleton, inhibits the Na(+),K(+)-ATPase by a mechanism of action different from the cardiac glycosides and could thus serve as a structural paradigm to develop new inotropic drugs.

Effect of quercetin on hypoxic injury in freshly isolated rat proximal tubules

The bioflavonoid quercetin, which has antioxidant properties, protects renal tubular epithelial cells from oxidant-induced injury by inhibiting lipid peroxidation. We examined the effect of quercetin on hypoxia-induced injury in freshly isolated rat renal proximal tubules. Hypoxia induced rapid loss of cellular ATP, followed by functional and structural alterations measured as a decrease in tubular potassium content and sequentially by an increase in lactate dehydrogenase release. Furthermore, hypoxia increased lipid peroxidation, measured as thiobarbituric acid-reactive substances. Quercetin significantly inhibited hypoxia-induced functional and structural tubular injury in addition to lipid peroxidation but did not alter hypoxia-induced ATP depletion. These results demonstrate the potency of the bioflavonoid quercetin in protecting proximal tubules from hypoxic injury, which is independent of tubular energy metabolism and may be related to the inhibition of lipid peroxidation.

Recent advances in the discovery of flavonoids and analogs with high-affinity binding to P-glycoprotein responsible for cancer cell multidrug resistance

P-glycoprotein (P-gp) is a plasma membrane glycoprotein that confers multidrug resistance on cells by virtue of its ability to exclude cytotoxic drugs in an ATP-dependent manner. The most commonly considered hypothesis is that P-gp acts as an ATP-driven drug-export pump, the mechanism of which is not understood in detail. Therefore, a tremendous effort is being made to find out modulator molecules to inhibit P-gp. We have been developing flavonoid derivatives as a new class of promising modulators using a new in vitro rational-screening assay based on measurements of the binding-affinity toward the C-terminal nucleotide-binding domain (NBD2) of P-gp. This review is focused on our results obtained with a variety of flavonoids. Structure-activity relationships of flavonoids as potential MDR modulators are reported.

Inhibition of mitochondrial proton F0F1-ATPase/ATP synthase by polyphenolic phytochemicals

Mitochondrial proton F0F1-ATPase/ATP synthase synthesizes ATP during oxidative phosphorylation. In this study, we examined the effects of several groups of polyphenolic phytochemicals on the activity of the enzyme. Resveratrol, a stilbene phytoalexin that is present in grapes and red wine, concentration-dependently inhibited the enzymatic activity of both rat brain and liver F0F1-ATPase/ATP synthase (IC(50) of 12 - 28 microM). Screening of other polyphenolic phytochemicals using rat brain F0F1-ATPase activity resulted in the following ranking potency (IC(50) in parenthesis): piceatannol (8 microM)>resveratrol (19 microM)=(-)epigallocatechin gallate (17 microM)>(-)epicatechin gallate, curcumin (45 microM)>genistein=biochanin A=quercetin=kaempferol=morin (55 - 65 microM)>phloretin=apigenin=daidzein (approx. 100 microM). Genistin, quercitrin, phloridzin, (+)catechin, (+)epicatechin, (-)epicatechin and (-)epigallocatechin had little effect at similar concentrations. Tannic acid, theaflavins (tea extract) and grape seed proanthocyanidin extract (GSPE) had IC(50) values of 5, 20 and 30 microg ml(-1), respectively. Several monophenolic antioxidants and non-phenolic compounds were ineffective at concentrations of 210 microM or higher. The inhibition of F0F1-ATPase by resveratrol and genistein was non-competitive in nature. The effects of polyphenolic phytochemicals were additive. Both resveratrol and genistein had little effect on the Na(+)/K(+)-ATPase activity of porcine cerebral cortex, whereas quercetin had similar inhibitory potency as for F0F1-ATPase. In conclusion, the ATP synthase is a target for dietary phytochemicals. This pharmacological property of these phytochemicals should be included in the examination of their health benefits as well as potential cytotoxicity.

Flavonoids: a class of modulators with bifunctional interactions at vicinal ATP- and steroid-binding sites on mouse P-glycoprotein

A hexahistidine-tagged C-terminal nucleotide-binding domain (H6-NBD2) from mouse P-glycoprotein was designed, overexpressed, and purified as a highly soluble recombinant protein. Intrinsic fluorescence of its single tryptophan residue allowed monitoring of high-affinity binding of 2'(3')-N-methylanthraniloyl-ATP (MANT-ATP), a fluorescent ATP derivative that induces a marked quenching correlated to fluorescence resonance-energy transfer. H6-NBD2 also bound all flavonoids known to modulate the multidrug resistance phenotype of P-glycoprotein-positive cancer cells, with similar affinities and relative efficiencies. Flavones (like quercetin or apigenin) bound more strongly than flavanones (naringenin), isoflavones (genistein), or glycosylated derivatives (rutin). Kaempferide, a 4'-methoxy 3,5,7-trihydroxy flavone, was even more reactive and induced a complete quenching of H6-NBD2 intrinsic fluorescence. Kaempferide binding was partly prevented by preincubation with ATP, or partly displaced upon ATP addition. Interestingly, kaempferide was also able to partly prevent the binding of the antiprogestin RU 486 to a hydrophobic region similar to that recently found, close to the ATP site, in the N-terminal cytosolic domain. Conversely, RU 486 partly prevented kaempferide binding, the effect being additive to the partial prevention by ATP. Furthermore, MANT-ATP binding, which occurred at the ATP site and extended to the vicinal steroid-interacting hydrophobic region, was completely prevented or displaced by kaempferide. All results indicate that flavonoids constitute a new class of modulators with bifunctional interactions at vicinal ATP-binding site and steroid-interacting region within a cytosolic domain of P-glycoprotein.

Inhibition of 7-ethoxycoumarin O-deethylase activity in rat liver microsomes by naturally occurring flavonoids: structure-activity relationships

1. The inhibitory effects of several naturally occurring flavonoids and related compounds on cytochrome P450-dependent 7-ethoxycoumarin O-deethylase (ECOD) and the structure-activity relationships were studied in liver microsomes from rats treated with 3-methylcholanthrene (MC). 2. All the flavonoids (flavone, apigenin, chrysin, flavonol, fisetin, kaempferol, morin, myrisetin, quercetin, flavanone, hesperetin and naringenin) studied inhibited microsomal ECOD activity in the following order: flavones > flavonols > flavanones, were mixed type inhibitors and had Ki in the range of 0.17-4.5 microM. (+/-)-Catechin had no effect. 3. The double bond between C2 and C3 of the C ring, the keto group and hydroxyl group of this ring in the flavonoids seem to play major roles in inhibiting the ECOD activity. 4. The hydroxyl groups in the C5 and C7 positions of A ring in the flavone and the hydroxyl group in the C3 position of C ring in the flavonol classes, respectively, were important factors for the inhibition of the enzyme. 5. In a series of 3, 5, 7-trihydroxyflavones, the hydroxyl group at the C4 in the B ring was also an important factor for the inhibition of ECOD activity, but hydroxyl groups in other positions of the B ring had little effect on the inhibition. 6. We conclude that all the flavonoids studied inhibit ECOD activity by interfering with the binding of substrate to the active site and other site(s) of the enzyme and that their structural differences lead to different binding affinities at the active site and possibly to binding at other site(s) of the enzyme for the flavonoids.

Comparative effects of flavonoids on the growth, viability and metabolism of a colonic adenocarcinoma cell line (HT29 cells)

The aim of the present study was to compare the effect of five structural classes of flavonoids on the viability and metabolism of a colonic adenocarcinoma cell line (HT29 cells). The most prominent structural features of flavonoids favoring both their cytotoxic activity and their capacity to inhibit lactate release appear to be the desaturation of the 2, 3 bond and the position of attachment of the B ring. Indeed, flavonol and flavone are the most potent and, in both classes, the order of potency can be modulated by hydroxyl or methoxyl substituents. On the other hand, in our model, we did not find any correlation between flavonoid structure and their capacity to modulate cAMP level. This last point is discussed.

Quercetin exerts a preferential cytotoxic effect on active dividing colon carcinoma HT29 and Caco-2 cells

The effect of the naturally occurring flavonol, quercetin, was investigated on cell growth and metabolism of two human carcinoma cell lines, HT29 and Caco-2 cells, both during the exponentially growing phase and after confluence. Our results show clearly that, after a 48-h period of treatment, quercetin (in the range of concentration from 15 microM to 120 microM) exerted a preferential cytotoxic effect on active proliferating cells. This effect was dose dependent and was accompanied by a simultaneous inhibition of lactate release and a dramatic decrease of total cellular ATP content. In contrast, in confluent cells, quercetin failed to affect cell viability or lactate release, but led nevertheless to a depletion of cellular ATP level. In conclusion, the cytotoxicity of quercetin is markedly higher in actively growing cells in comparison with confluent cells.