The Flavonoid Quercetin Inhibits Proinflammatory Cytokine (Tumor Necrosis Factor Alpha) Gene Expression in Normal Peripheral Blood Mononuclear Cells via Modulation of the NF-κβ System

Abstract

The flavonoids comprise a large class of low-molecular-weight plant metabolites ubiquitously distributed in food plants. These dietary antioxidants exert significant antitumor, antiallergic, and anti-inflammatory effects. The molecular mechanisms of their biological effects remain to be clearly understood. We investigated the anti-inflammatory potentials of a safe, common dietary flavonoid component, quercetin, for its ability to modulate the production and gene expression of the proinflammatory cytokine tumor necrosis factor alpha (TNF-α) by human peripheral blood mononuclear cells (PBMC). Our results showed that quercetin significantly inhibited TNF-α production and gene expression in a dose-dependent manner. Our results provide direct evidence of the anti-inflammatory effects of quercetin by PBMC, which are mediated by the inhibition of the proinflammatory cytokine TNF-α via modulation of NF-κβ1 and Iκβ.

The natural antioxidant flavonoids constitute significant components of the diet and display a diverse array of biological effects (16, 19, 21, 26). Polyphenolic compounds, including a large class of flavonoids, are enriched in certain vegetables, fruits, seeds, and beverages (e.g., tea and wine) and are regarded as a class of semiessential nutrients for humans. Dietary intake rich in these compounds has been suggested to improve the health of individuals and decrease the risk of cardiovascular disease. The beneficial effects of flavonoids have been attributed to their antioxidant and anti-inflammatory properties (18, 19, 31). The effects of flavonoids, including quercetin, on a variety of inflammatory processes and immune functions have been extensively reviewed (4, 6, 11, 17, 25, 27, 28, 40). Tumor necrosis factor alpha (TNF-α) is one of the major proinflammatory cytokines involved in the pathogenesis of chronic inflammatory diseases and is modulated by oxidative stress (5, 35). TNF-α is a multifunctional cytokine that regulates the growth, proliferation, differentiation, and viability of activated leukocytes. TNF-α also triggers the cellular release of other cytokines, chemokines, or inflammatory mediators and displays antiviral and antimicrobial effects (1, 2, 39).

Numerous signaling cascades have been elucidated in promotion of proinflammatory conditions by proinflammatory cytokines, such as TNF-α, which involves the activation of inducible transcriptional factors (1, 12, 13, 14, 29, 39). NF-κβ is one of the principal inducible transcription factors whose modulation triggers a cascade of signaling events involving an integrated sequence of protein-regulated steps, some of which are potential key targets for intervention in treating inflammatory conditions (3, 7, 20, 29, 33, 34). Previous studies have shown that quercetin inhibits lipopolysaccharide (LPS)-stimulated NF-κβ activation in RAW 264.7 macrophage (8, 37) and also inhibits LPS-induced Iκβ phosphorylation in bone marrow-derived macrophage (11). Although quercetin exhibits several biological effects, the molecular mechanisms of its anti-inflammatory effects by peripheral blood mononuclear cells (PBMCs) have not been clearly elucidated. We hypothesize that flavonoids exert anti-inflammatory effects by PBMCs by inhibiting the endogenous production of the proinflammatory cytokine TNF-α and that these effects are mediated through the regulation of NF-κβ and Iκβ. Therefore, the present study was undertaken to investigate the direct effect of quercetin on the gene expression and protein secretion of the proinflammatory cytokine TNF-α. We further investigated whether the transcription factor NF-κβ was involved in the regulation of TNF-α by quercetin by normal PBMCs.

MATERIALS AND METHODS

Cell culture.

Total PBMCs were separated by Ficoll Hypaque centrifugation. PBMCs (3 × 10⁶ cells/ml) were cultured in RPMI 1640 medium (Invitrogen, Grand Island, NY) containing 10% fetal bovine serum (complete medium) with quercetin (Sigma-Aldrich, St. Louis, MO) at concentrations ranging between 1 to 50 μM for 24, 48, 72, and 96 h at 37°C in a 5% CO₂ incubator. These concentrations selected for the in vitro studies are similar to levels found in plasma in human subjects that have ingested 150 mg or 300 mg of quercetin (15). For phosphorylation studies, PBMCs were treated with quercetin at 50 μM for 30 and 60 min at 37°C in a 5% CO₂ incubator.

Stimulated TNF-α production.

PBMCs (3 × 10⁶ cells/ml) were treated with quercetin and additionally stimulated with phorbol myristate acetate (PMA) (5 ng/ml) and Ca²⁺ ionophore (50 ng/ml) for 24 h.

RNA extraction.

After cell stimulation, cytoplasmic RNA was extracted by an acid guanidinium thiocyanate-phenol chloroform method as described by Chomczynski and Sacchi (9). Cultured cells were pelleted by centrifugation and resuspended in a 4 M solution of guanidinium thiocyanate. Cells were lysed by repeat pipetting and phenol-chloroform extracted in the presence of sodium acetate. After centrifugation, RNA was precipitated from the aqueous layer by the addition of an equal volume of isopropanol. The mixture was kept at −20°C for 1 h and then centrifuged to pellet the RNA. The RNA pellet was washed with 75% ethanol to remove any remaining traces of guanidium. The final pellet was dried and resuspended in diethyl pyrocarbonate water, and the concentration of RNA was determined using a spectrophotometer at 260 nm. DNA contamination in the RNA preparation was removed by treating the RNA preparation with DNase (1 IU/μg of RNA; Promega, Madison, WI) for 30 min at 37°C, followed by proteinase K digestion at 37°C for 15 min and subsequent extraction with phenol-chloroform-isoamyl alcohol and precipitation with ammonium acetate-ethanol. The isolated RNA was stored at −70°C until use. The DNA contamination in the RNA preparation was checked by including a control in which the reverse transcriptase enzyme was not included in the PCR amplification procedure. RNA preparation, which was devoid of any DNA contamination, was used in the subsequent experiments in semiquantitative real-time Q-PCR.

Real-time Q-PCR.

TNF-α and NF-κβ1 gene expressions were quantitated using real-time PCR. The relative abundance of each mRNA species was assessed using the SYBR green master mix from Stratagene (La Jolla, CA) to perform quantitative PCR (Q-PCR) using the ABI Prism 5700 instrument that detects and plots the increase in fluorescence versus PCR cycle number to produce a continuous measure of PCR amplification. To provide precise quantification of the initial target in each PCR, the amplification plot is examined at a point during the early log phase of product accumulation. This is accomplished by assigning a fluorescence threshold above the background and determining the time point at which each sample's amplification plot reaches the threshold (defined as the threshold cycle number, or C_T). Differences in threshold cycle number are used to quantify the relative amount of PCR target contained within each tube (32). Relative mRNA species expression was quantitated and expressed as the transcript accumulation index (TAI = 2^−ΔΔCT), calculated using the comparative C_T method (22). All data were controlled for quantity of RNA input by performing measurements on an endogenous reference gene, β-actin. In addition, results on RNA from treated samples were normalized to results obtained on RNA from the control, untreated sample.

Fluorescence-activated cell sorter (FACS) analysis. (i) Detection of the surface markers CD4 and CD14 and intracellular TNF-α.

Immunofluorescent staining was used to identify and quantitate the number of cells that express the intracellular cytokine TNF-α as described previously (22, 28). To determine which cell population of PBMCs predominantly contributes to TNF-α production, cells were stained for the surface markers, CD4 (T cells) and CD14 (monocytes). Golgi stop (BD Pharmingen, San Diego, CA), an intracellular protein transport inhibitor, was used to enhance the ability to detect cytokine-producing cells. The CD4 (R&D Systems Minneapolis, MN) and CD14 (eBiocience, San Diego, CA) antibodies were fluorescein isothiocyanate-conjugated antibodies. The TNF-α monoclonal antibody (R&D Systems) was conjugated to phycoerythrin. After stimulation, cells were harvested, washed, suspended in staining buffer, and stained for TNF-α alone, CD4 alone, CD14 alone, TNF-α and CD4, and TNF-α and CD14. When staining for both the surface marker and the intracellular cytokine, cells were stained for surface markers prior to fixation. Cells were fixed with cytofix/cytoperm buffer (BD Pharmingen) and permeabilized by washing twice in 1× Perm solution (BD Pharmingen). Cells were stained for intracellular TNF-α and resuspended in staining buffer prior to flow cytometric analysis. Stained cells were subjected to a light scatter analysis. A fixed population of cells were gated and represented as side scatter on the y axis and forward scatter on the x axis. Cells positive for TNF-α were expressed as a percentage of the total cells gated.

(ii) Detection of the phosphorylated forms of Iκβα and Iκββ.

After stimulation, cells were harvested, washed, and suspended in staining buffer. Cells were fixed with cytofix/cytoperm buffer (BD Pharmingen) and were permeabilized by washing twice in 1× Perm solution (BD Pharmingen). Cells were stained with antibodies for Iκβα, Iκββ, phospho-Iκβα, and phospho-Iκββ (rabbit polyclonal antibodies; Cell Signaling), followed by detection using a goat anti-rabbit immunoglobulin G (IgG) fluorescein isothiocyanate-conjugated secondary antibody (Sigma-Aldrich). Cells were resuspended in staining buffer prior to flow cytometric analysis.

Western blot analysis.

PBMCs were cultured with quercetin (1 to 50 μM) for 48 h, and protein was extracted for Western blot analyses (10), using mammalian protein extraction reagent (Pierce, Rockford, IL). Protein concentrations were determined using Coomassie protein reagent (Bio-Rad, Hercules, CA). Thirty micrograms of total protein was loaded per lane and separated by 7.5% sodium dodecyl sulfate-Tris-glycine polyacrylamide gel electrophoresis (ISC Bioexpress, Kaysville, UT). Proteins were transferred to nitrocellulose membranes and blocked overnight in 1× Tris-buffered saline (TBS) containing 0.1% Tween and 5% nonfat dry milk. Membranes were probed with the rabbit polyclonal antibody directed against TNF-α (Cell Signaling, Beverly, MA) and a goat polyclonal β-actin antibody (Santa Cruz Biotech, Santa Cruz, CA) per the manufacturer's instructions. After an overnight incubation with primary antibodies, the membranes were washed three times in 1× TBS with 0.5% Tween 20 prior to incubation with secondary antibodies. The membranes were incubated for 2 h at room temperature with secondary antibodies (biotin-conjugated goat anti-rabbit IgG, conjugated donkey anti-goat IgG; Santa Cruz Biotech) per the manufacturer's instructions. After secondary antibody incubations, the membranes were washed three times, for 10 min each, in 1× TBS with 0.5% Tween 20 and then incubated for another 30 min with a streptavidin-alkaline phosphatase conjugate (Invitrogen) followed by colorimetric detection using nitroblue tetrazolium/5-bromo-4-chloro-3-indolylphosphate reagent (Roche, Indianapolis, IN). Densitometry analyses were done using a Syngene image analyzer with Gene Tools Analysis software, version 3.02.00 (Syngene, Frederick, MD). Data were normalized to levels of β-actin.

ELISA.

After 96 h of stimulation with quercetin, supernatants were harvested and stored at −70°C until further analysis. TNF-α protein secretion in culture supernatants was quantitated using an enzyme-linked immunosorbent assay (ELISA) kit obtained from BioSource International (Camarillo, CA) and used as described by the manufacturer. The sensitivity of the TNF-α ELISA was 1.7 pg/ml.

RESULTS

Effect of quercetin on viability of PBMCs.

Data presented in Table 1 show the effect of quercetin on the viability of PBMCs at 24, 48, 72, and 96 h in culture. PBMCs (3 × 10⁶ cells/ml) were cultured with and without quercetin at different concentrations for 24, 48, 72, and 96 h, and the viability was determined using the trypan blue dye exclusion technique. Results show that at 96 h of incubation, the viability of the PBMCs remained greater than 90% at the highest concentration of quercetin (i.e., 50 μM). Additionally, the MTT assay, a cell toxicity assay, also shows comparable viability results (data not shown). Previous studies have also shown long-term viability (up to 120 h) of PBMCs in media supplemented with 10% fetal calf serum alone (41).

TABLE 1.

Effect of quercetin on viability of PBMCs

Treatmenta	% Viability after incubation period (h) ofb:
	24	48	72	96
Control	95.4 ± 2.5	94.14 ± 4.5	95.4 ± 4.5	93.1 ± 3.6
Quercetin, 1 μM	96.9 ± 3.4	97.39 ± 2.4	96.1 ± 3.3	96.8 ± 2.1
Quercetin, 5 μM	98.1 ± 1.2	97.9 ± 1.7	98.8 ± 5.2	97.6 ± 4.9
Quercetin, 10 μM	97.8 ± 1.9	95.4 ± 2.8	96.4 ± 3.7	96.7 ± 7.8
Quercetin, 25 μM	93.9 ± 3.3	93.4 ± 2.7	95.3 ± 7.3	92.7 ± 4.1
Quercetin, 50 μM	92.4 ± 2.3	93.6 ± 4.9	94.1 ± 1.8	90.4 ± 3.8

^aPBMCs (3 × 10⁶ cells/ml) were cultured with and without quercetin at different concentrations for 24, 48, and 72 h. Viability was determined using the trypan blue dye exclusion technique.

^bValues represent mean ± SD of results from three separate experiments done in triplicate.

Quercetin downregulates TNF-α gene expression.

Data presented in Fig. 1 show the effects of different concentrations of quercetin on TNF-α gene expression in PBMCs at 24, 48, and 72 h of incubation as quantitated by real-time Q-PCR. At 24 h, quercetin at 10 to 50 μM concentrations produced a significant dose-dependent decrease in TNF-α gene expression, while at 48 and 72 h of incubation, quercetin at all concentrations, including the lowest concentration of 1 μM, produced a significant dose-dependent decrease in TNF-α gene expression. Heat-inactivated quercetin had no effect on TNF-α gene expression (data not shown). These data suggest that quercetin significantly modulates TNF-α gene expression.

FIG. 1.

Effect of quercetin on TNF-α gene expression as quantitated by real-time Q-PCR. PBMCs were cultured for 24, 48, and 72 h with quercetin at 1, 5, 10, 25, and 50 μM, and TNF-α gene expression was quantitated. Data represented are means ± SD of results from three separate experiments. Statistical significance was determined by Student's t test. TAI, transcript accumulation index.

Quercetin modulates TNF-α-positive phenotypes.

Data presented in Fig. 2 show the effect of quercetin (48 h) on the intracellular marker TNF-α as determined by FACS analysis. Data shown in Fig. 2a to f are representative histograms showing TNF-α-positive cells. The percentage of TNF-α-positive cells at 1, 5, 10, and 50 μM quercetin were 3.4 (Fig. 2c), 2.9 (Fig. 2d), 1.2 (Fig. 2e), and 0.7 (Fig. 2f), respectively, with the control value being 5.9% (Fig. 2b). Figure 2g shows the mean percentage ± standard deviation (SD) of TNF-α-positive cells, from three separate experiments compared to control cultures. PBMCs treated with 5 μM (2.4%, P = 0.007), 10 μM (1.03%, P = 0.001), and 50 μM (0.66%, P = 0.0001) quercetin significantly decrease the percentage of TNF-α-positive cells compared to the untreated control (5.2%). Quercetin at 1 μM (3.2%, P = 0.06) had no effect of the percentage of TNF-α-positive cells compared to the control. These results confirm gene expression data as analyzed by real-time Q-PCR for TNF-α (Fig. 1).

FIG. 2.

Effect of quercetin on the phenotypic expression of intracellular TNF-α by PBMCs. PBMCs were cultured with 1, 5, 10, and 50 μM quercetin for 48 h and subjected to FACS analysis. (a) Isotype control (mouse IgG₁); (b) control; (c) 1 μM quercetin; (d) 5 μM quercetin; (e) 10 μM quercetin; (f) 50 μM quercetin; (g) graph showing the mean percentage ± SD of TNF-α-positive cells. Statistical significance was determined by analysis of variance followed by Tukey's test (n = 3).

CD14⁺ monocytes and CD4⁺ T cells release TNF-α.

Since PBMCs are a heterogeneous population of cells, we sought to determine which cell populations produce TNF-α. Data presented in Fig. 3a to e are representative dot plots showing that both CD4⁺ and CD14⁺ cell populations produce TNF-α. Data shown in Fig. 3b demonstrate that 89.7% of CD4⁺ T cells express TNF-α, which is decreased to 62.9% (Fig. 3c) following 48 h of 50 μM quercetin treatment. Figure 3d demonstrates that 44.3% of CD14⁺ monocytes express TNF-α, which is decreased to 23.1% (Fig. 3e.) following 48 h of 50 μM quercetin treatment. Data shown in Fig. 3f show the mean percentage ± SD of positive cells from three separate experiments compared to control cultures. These data suggest that CD4⁺ and CD14⁺ cells are the major cell populations in PBMCs in which quercetin modulates TNF-α expression.

FIG. 3.

FACS analyses to determine the cell population in PBMCs producing TNF-α. (a) Isotype control; (b) TNF-α and CD4⁺ cells, control; (c) TNF-α and CD4⁺, quercetin (50 μM, 48 h); (d) TNF-α and CD14⁺ cells, control; (e) TNF-α and CD14⁺ cells, quercetin (50 μM, 48 h); (f) graph showing the mean percentage ± SD of positive cells. Statistical significance was determined by Student's t test (n = 3). con, control.

Quercetin modulates stimulated TNF-α production.

Data presented in Fig. 4 show the effect of quercetin on stimulated (PMA/Ca²⁺ ionophore) TNF-α production as determined by FACS analysis. Data shown in Fig. 4a to g are representative histograms showing TNF-α-positive cells. The percentage of TNF-α-positive cells in the unstimulated and stimulated controls and in the presence of stimulation (1, 5, 10, and 50 μM quercetin) were 2.8 (Fig. 4b), 15.9 (Fig. 4c), 11.5 (Fig. 4d), 5.8 (Fig. 4e), 3.1 (Fig. 4f), and 1.8 (Fig. 4g), respectively. Figure 4g shows the mean percentage ± SD of TNF-α-positive cells from three separate experiments compared to respective control cultures. The percentage of TNF-α-positive cells was significantly increased in PMA/Ca²⁺ ionophore-stimulated PBMCs (17.6%, P = 0.0001) compared to the unstimulated control (4.5%). Quercetin at 1 μM (11.8%, P = 0.001), 5 μM (5.2%, P = 0.001), 10 μM (2.7%, P = 0.001), and 50 μM (1.4%, P = 0.0001) significantly decreased the percentage of TNF-α-positive cells induced by PMA/Ca²⁺ ionophore stimulation compared to stimulation alone (17.6%).

FIG. 4.

Effect of quercetin on stimulated intracellular TNF-α production by PBMCs. PBMCs were cultured with 1, 5, 10, and 50 μM quercetin for 24 h, stimulated with PMA/Ca²⁺ ionophore for an additional 24 h, and then subjected to FACS analysis. (a) Isotype control (mouse IgG₁); (b) unstimulated control; (c) PMA/Ca²⁺ ionophore alone (stimulated control); (d) 1 μM quercetin plus PMA/Ca²⁺ ionophore; (e) 5 μM quercetin plus PMA/Ca²⁺ ionophore; (f) 10 μM quercetin plus PMA/Ca²⁺ ionophore; (g) 50 μM quercetin plus PMA/Ca²⁺ionophore; (h) graph showing the mean percentage ± SD of TNF-α-positive cells. Statistical significance was determined by analysis of variance followed by Tukey's test (n = 3); *, compared to unstimulated control; **, compared to stimulated control.

Quercetin downregulates TNF-α production.

Data presented in Fig. 5 show the effects of quercetin (48 h) on TNF-α protein expression by PBMCs as detected by Western blot analysis. Figure 5a shows the β-actin loading control with no change in protein expression by 1, 5, 10, or 50 μM quercetin treatment compared to the untreated control. Data demonstrate (Fig. 5b) that quercetin at 5, 10, and 50 μM downregulated TNF-α protein (26 kDa) expression by PBMCs. Figure 5c shows the densitometric analysis (% change in optical density units) of TNF-α protein ± SD from four separate experiments compared to control cultures. PBMCs treated with 5 μM (21.3% inhibition, P = 0.05), 10 μM (26.3.3% inhibition, P = 0.044), and 50 μM (39.30% inhibition, P = 0.001) quercetin showed a significant suppression in TNF-α protein expression. These results confirm gene expression data as analyzed by real-time Q-PCR for TNF-α (Fig. 1) as well as FACS analysis data (Fig. 2).

FIG. 5.

Effect of quercetin (48 h) on TNF-α protein expression as quantitated by Western blot analysis. (a) β-Actin loading control in the untreated control and 1, 5, 10 and 50 μM quercetin-treated samples; (b) TNF-α expression in the untreated control and 1 5, 10, and 50 μM quercetin-treated samples; (c) densitometric analysis (% change in optical density [OD] units) of TNF-α protein in the quercetin treated samples compared to control cultures. Data represented are means ± SD of results from four separate experiments and are normalized to levels of β-actin. Statistical significance was determined by Student's t test (n = 4).

ELISA.

Data presented in Fig. 6 show the effects of quercetin on the endogenous production of TNF-α by PBMCs as quantitated by ELISA. Levels of TNF-α were measured in the culture supernatants at 96 h of treatment with various concentrations of quercetin. Quercetin (5 to 50 μM) significantly downregulated TNF-α production by PBMCs. Thus, the quantitation of TNF-α through ELISA is consistent with the gene expression data as analyzed by real-time Q-PCR (Fig. 1) and TNF-α-positive phenotypic analysis by FACS (Fig. 2).

FIG. 6.

Effect of quercetin (96 h) on TNF-α protein expression as quantitated by ELISA (BioSource International). The sensitivity of the TNF-α ELISA was 1.7 pg/ml. Data represented are means ± SD of results from three separate experiments. Statistical significance was determined by Student's t test (n = 3). Conc, concentration.

Quercetin modulates NF-κβ gene expression.

The transcription factor NF-κβ is a significant mediator of various immune and inflammatory responses and is negatively modulated by Iκβ. Cellular activation induces the phosphorylation of Iκβ proteins (Iκβα and Iκββ). Phosphorylation of Iκβα and Iκββ targets them for ubiquitination and degradation. Degradation of Iκβ and Iκβ results in the translocation of NF-κβ to the nucleus where it binds to specific promoter regions of various genes encoding for inflammatory cytokines (3, 7, 14, 20, 29, 33, 34). Since the NF-κβ complex regulates inflammatory cytokine production, we examined the effects of quercetin on NF-κβ1 gene modulation. Data presented in Fig. 7 show the effect of quercetin on NF-κβ1 gene expression by real-time PCR by PBMCs. As shown at 24 and 48 h of incubation, quercetin significantly downregulated NF-κβ1 gene expression at concentrations of 5 to 50 μM. At 72 h, quercetin significantly downregulated NF-κβ1 gene expression at 10 to 50 μM concentrations. These data suggest that the inhibitory effects of quercetin on TNF-α production may be mediated by downregulation of NF-κβ1.

FIG. 7.

Effect of quercetin on NF-κβ1 gene expression as quantitated by real-time PCR. PBMCs were cultured for 24, 48, and 72 h with quercetin at (1 to 50 μM); NF-κβ1 gene expression was quantitated. Data represented are means ± SD of results from three separate experiments. Statistical significance was determined by Student's t test (n = 3). TAI, transcript accumulation index.

Quercetin modulates phosphorylation of Ikβα and Ikββ.

Phosphorylation of Iκβα and Iκββ targets these proteins for ubiquitination and degradation, which results in activation of NF-κβ; therefore, it was of interest to investigate the effects of quercetin on phosphorylation of both Iκβα and Iκββ. Data shown in Fig. 8 and 9 demonstrate the effects of quercetin (50 μM) on the phosphorylation state of Iκβα and Iκββ as determined by FACS analyses. Data shown in Fig. 8a to d are representative histograms showing that the percentage of phospho-Iκβα-positive cells were 31.7 (Fig. 8c) and 18.7 (Fig. 8d) following 30 and 60 min of exposure to quercetin (50 μM), respectively, compared to the control (44.9%, Fig. 8b). Figure 8e shows the mean percentage ± SD of phospho-Iκβα-positive cells from two separate experiments compared to control cultures. PBMCs treated with 50 μM quercetin for 60 min showed a significant decrease in the percentage of phospho-Iκβα-positive cells (18.55%, P = 0.05), while a 30-min exposure to quercetin had no effect on the percentage of phospho- Iκβα-positive cells (32.55%, P = 0.171) compared to 42.1% in the untreated population. Quercetin (50 μM, 30 and 60 min) had no effect on total Iκβα-positive cells (data not shown).

FIG. 8.

Effect of quercetin on phosphorylation of Iκβα. PBMCs were cultured with quercetin (50 μM) for 30 and 60 min and then subjected to FACS analyses. (a) Isotype control; (b) control; (c) 50 μM quercetin, 30 min; (d) 50 μM quercetin, 60 min; (e) graph showing the mean percentages ± SD of phospho-Iκβα-positive cells. Statistical significance was determined by Student's t test (n = 3).

FIG. 9.

Effect of quercetin on phosphorylation of Iκββ. PBMCs were cultured with quercetin (50 μM) for 30 and 60 min and then subjected to FACS analyses. (a) Isotype control; (b) control; (c) 50 μM quercetin, 30 min; (d) 50 μM quercetin, 60 min; (e) graph showing the mean percentages ± SD of phospho-Iκββ-positive cells. Statistical significance was determined by Student's t test (n = 3).

As shown in the representative histograms (Fig. 9a to d), the percentages of phospho-Iκββ-positive cells were 22.9% (30 min of quercetin) (Fig. 9c) and 18.2% (60 min of quercetin) (Fig. 9d) compared to the control (34.1%) (Fig. 9b). Figure 9e shows the mean percentage ± SD of phospho-Iκββ-positive cells from two separate experiments compared to control cultures. PBMCs treated with 50 μM quercetin for 30 and 60 min showed a significant decrease in phospho-Iκββ-positive cells, 24.3% (P = 0.022) and 17.86% (P = 0.023), respectively, compared to 48.7% in the untreated population. Data are represented as the mean percentages of positive cells from two separate experiments. Quercetin (50 μM, 30 and 60 min) had no effect on total Iκββ-positive cells (data not shown).

DISCUSSION

Many polyphenolic compounds from plants, including a large class of bioflavonoids, are known to offer health benefits to humans. Flavonoids represent a group of phytochemicals exhibiting a wide range of biological activities arising mainly from their antioxidant properties and ability to modulate several enzymes or cell receptors (16, 19, 21, 26). Flavonoids have been recognized to exert antibacterial and antiviral activity, anti-inflammatory, antiangiogenic and antiallergic effects, analgesic, hepatoprotective, cytostatic, apoptotic, estrogenic, and antiestrogenic properties (4, 6, 11, 16, 19, 21, 25-28, 40). The beneficial effects have been attributed to their antioxidant and anti-inflammatory properties. The current study shows that the anti-inflammatory effects of quercetin may be mediated by downregulating endogenous and PMA/Ca²⁺-stimulated TNF-α production by PBMCs. The findings from this study further support data demonstrating that quercetin inhibits LPS-induced TNF-α production (11, 17, 23, 24, 30, 36-38, 40). Previous studies from this laboratory demonstrate that quercetin inhibits interleukin 4 expression and increases gamma interferon production in PBMCs (28). These data and the findings from the present study suggest that quercetin has the ability to modulate the immune response.

An impressive variety of stimuli (TNF-α, interleukin-1, T-cell activation signals, bacterial endotoxins, viral transforming proteins, certain growth factors, and reactive oxygen intermediates) lead to the rapid nuclear accumulation of the transcription factor NF-κβ induced by phosphorylation and degradation of Iκβ. NF-κβ is widely recognized as a critical mediator of immune and inflammatory responses (3, 7, 14, 20, 29, 33, 34). In most cell types, NF-κβ is found in the cytoplasm, where it is associated with an inhibitory protein known as Iκβ (Iκβα and Iκββ). Iκβ negatively modulates NF-κβ by preventing its translocation to the nucleus. Upon cellular activation, Iκβα and Iκββ are phosphorylated by the cellular kinase complex IKK. This complex is composed of two kinases: IKKα and IKKβ. Phosphorylation of Iκβα and Iκββ results in their ubiquitination and degradation, resulting in the translocation of NF-κβ to the nucleus, where it binds to specific promoter regions of various genes encoding for inflammatory cytokines (3, 7, 14, 20, 29, 33, 34). Previous studies demonstrate that quercetin inhibits LPS-stimulated NF-κβ activation in RAW 264.7 macrophage (8, 37) and also inhibits LPS-induced Iκβα phosphorylation in bone marrow-derived macrophage (11). Our studies show that a possible mechanism of quercetin-mediated suppression of TNF-α gene and protein expression is mediated by downregulating gene expression for NF-κβ1. Furthermore, our FACS analyses show that quercetin decreases the phosphorylation of Iκβα and Iκββ, suggesting that quercetin decreases the activation of NF-κβ. This decrease in phosphorylation of Iκβα and Iκββ may be a direct mechanism by which quercetin inhibits the activity of NF-κβ, thereby decreasing endogenous TNF-α expression. Further studies are necessary to elucidate the intricate roles that NF-κβ and Iκβ play in the inhibition of TNF-α expression.

These findings suggest that the cytokine TNF-α can be inhibited by quercetin, which may be of clinical significance in host defense mechanisms against various infections. Our data suggest that the major population of cells in which quercetin modulates TNF-α expression are predominantly CD4⁺ T cells and CD14⁺ monocytes. A decrease in endogenous TNF-α production in the presence of quercetin indicates that flavonoids have the capacity to modulate the immune response and have potential anti-inflammatory activity. In addition to its well-known proinflammatory role, TNF-α has complex effects on the growth, differentiation, and death of immune cells. TNF-α inhibition is a validated approach to treating several inflammatory diseases (5). Although the results of this study are preliminary, we believe that quercetin-induced suppression of TNF-α can result in the stimulation of anti-inflammatory cytokines via inhibiting the activation of NF-κβ, and therefore, we anticipate that quercetin can be widely used as an anti-TNF-α therapy. Evaluation of the molecular mechanisms of quercetin-induced anti-inflammatory effects may be a promising area for the development of new flavonoid-based neutrapharmaceutical agents for the treatment of various inflammatory diseases.