Flavonoid metabolites reduce tumor necrosis factor-α secretion to a greater extent than their precursor compounds in human THP-1 monocytes

Abstract

Scope: Flavonoids are generally studied in vitro, in isolation, and as unmetabolized precursor structures. However, in the habitual diet, multiple flavonoids are consumed together and found present in the circulation as complex mixtures of metabolites. Using a unique study design, we investigated the potential for singular or additive anti-inflammatory effects of flavonoid metabolites relative to their precursor structures.

Methods and results: Six flavonoids, 14 flavonoid metabolites, and 29 combinations of flavonoids and their metabolites (0.1-10 μM) were screened for their ability to reduce LPS-induced tumor necrosis factor-α (TNF-α) secretion in THP-1 monocytes. One micromolar peonidin-3-glucoside, cyanidin-3-glucoside, and the metabolites isovanillic acid (IVA), IVA-glucuronide, vanillic acid-glucuronide, protocatechuic acid-3-sulfate, and benzoic acid-sulfate significantly reduced TNF-α secretion when in isolation, while there was no effect on TNF-α mRNA expression. Four combinations of metabolites that included 4-hydroxybenzoic acid (4HBA) and/or protocatechuic acid also significantly reduced TNF-α secretion to a greater extent than the precursors or metabolites alone. The effects on LPS-induced IL-1β and IL-10 secretion and mRNA expression were also examined. 4HBA significantly reduced IL-1β secretion but none of the flavonoids or metabolites significantly modified IL-10 secretion.

Conclusion: This study provides novel evidence suggesting flavonoid bioactivity results from cumulative or additive effects of circulating metabolites.

Keywords: Cytokine; Inflammation; Metabolism; Phase 2 conjugates; Polyphenol.

Figure 1

Structures of the six precursor flavonoids and their metabolites. Details of constituent groups given in inset table. –OGlc represents an oxygen‐linked glucose; –OGlcA represents an oxygen‐linked glucuronide.

Figure 2

Effect of flavonoid treatments on LPS‐induced THP‐1 TNF‐α protein secretion. (A) C3G, (B) P3G, (C) IVA, (D) BA‐sulfate, (E) PCA‐3‐sulfate, (F) IVA‐glucuronide, (G) VA‐glucuronide, (H) Combination 9, (I) Combination 10, (J) Combination 13, (K) Combination 16, (L) PCA, (M) 4HBA, (N) VA, (O) BA‐glucuronide, and (P) PCA‐3‐glucuronide. Data were normalized to an LPS control and then compared to VC. Columns represent mean ± SD (n = 3). Each independent experiment was carried out in technical duplicate. *p < 0.05, **p < 0.01, ***p < 0.001 (one‐way ANOVA and post‐hoc LSD).