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Nutrition Division, Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, UK;
* School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK; and
Nutrient Bioavailability, Nestlé Research Center, CH-1000 Lausanne 26, Switzerland
3To whom correspondence should be addressed. E-mail: yongping.bao{at}bbsrc.ac.uk.
Flavonoids have the potential to modulate inflammation by inhibition of cyclooxygenase-2 (COX-2) transcription. In this study, we compared the effect of the human flavonoid plasma metabolites (quercetin 3'-sulfate, quercetin 3-glucuronide and 3'-methylquercetin 3-glucuronide) on expression of COX-2 mRNA in human lymphocytes ex vivo using TaqMan real-time RT-PCR. We show that the flavonoid quercetin metabolites as detected in human plasma at physiologically significant concentrations inhibit COX-2 expression in human lymphocytes ex vivo. To examine the effect in vivo, we measured COX-2 mRNA levels in 8 subjects (5 men and 3 women) participating in a 3-way, single-blind, randomized crossover study after consumption of a single meal of white, low-quercetin onions, compared with yellow, high-quercetin onions. After consumption of high-quercetin onions, quercetin conjugates were detected in plasma (up to a maximum concentration of 4 µmol/L at 
1 h). However, the expression of COX-2 mRNA in lymphocytes was unchanged by the consumption of high-quercetin onions compared with the low-quercetin group. The results show that a single high dose of the flavonoid quercetin from onions does not change COX-2 mRNA expression in human lymphocytes in vivo even though this change occurred in vitro and ex vivo.
KEY WORDS: • flavonoids • quercetin metabolites • cyclooxgenase-2 • lymphocyte • gene expression
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