Abstract

Flavonoids isolated from citrus were evaluated for their ability to affect the inflammation response through suppression of cytokine expression by human monocytes. Several polymethoxylated flavones inhibited lipopolysaccharide-induced monocyte expression of tumor necrosis factor (TNFalpha). Subsequent studies centered on the compound 3,5,6,7,8,3',4'-heptamethoxyflavone (HMF) which produced the highest inhibition (IC50 = 5 microM). HMF was also a potent inhibitor of macrophage inflammatory protein-1alpha (MIP-1alpha) and interleukin-10 (IL-10) production, but not of IL-1beta, IL-6, or IL-8 production. Suppression of TNFalpha production was at the level of mRNA induction as determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). HMF was also a potent inhibitor of human phosphodiesterase activity and was shown to induce a substantial elevation of cAMP levels in monocytes. The similarity of these results to the inhibition profile of the known phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, suggests that the polymethoxylated flavones inhibit cytokine production in part by suppression of phosphodiesterase activity. The ability of HMF to also inhibit IL-10 production suggests the additional existence of a phosphodiesterase-independent mechanism for this compound.