|LEE, JUN KYUNG|
|KIM, SO YOUNG|
|KIM, YOON SUN - Kyungpook National University|
|LEE, WON-HA - Kyungpook National University|
|LEE, JOO YOUNG - Gwangju Institute Of Science And Technology|
Submitted to: Biochemical Pharmacology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2009
Publication Date: 4/15/2009
Citation: Lee, J., Kim, S., Kim, Y., Lee, W., Hwang, D.H., Lee, J. 2009. Suppression of the TRIF-dependent signaling pathway of Toll-like receptors by luteolin. Biochemical Pharmacology. Vol.77:1391-1400.
Interpretive Summary: In this study, we attempted to identify a novel molecular target of luteolin, one of the plant flavonoid compounds, in MyD88-independent signaling pathway of Toll-like receptors. Luteolin suppressed MyD88-independent transcription factor activation (IRF3) and target genes including IFN' induced by LPS (a TLR4 agonist) or poly(I:C) (a TLR3 agonist). In addition, luteolin inhibited ligand-independent IRF3 activation induced by overexpression of constitutively active TLR4, TRIF, or TBK1. Luteolin suppressed kinase activity of TBK1 as demonstrated by in vitro kinase assay and consequently the expression of TBK1-dependent genes was decreased. Other structural analogs such as quercetin, chrysin, and eriodictyol also inhibited TBK1 kinase activity and TBK1-target gene expression. These results suggest a novel mechanism as to how the dietary flavonoids exert anti-inflammatory activity and that the dietary regulation of TLR activation can be an attractive strategy for reducing the risk of the inflammatory diseases.
Technical Abstract: Toll-like receptors (TLRs) play an important role in induction of immune and inflammatory responses by recognizing invading pathogens. TLRs have two major downstream signaling pathways activated through the interaction with adaptor molecules, MyD88 and TRIF, leading to the expression of proinflammatory cytokines and chemokines. Flavonoids have been reported to have anti-inflammatory properties. However, the underlying mechanism has not been entirely elucidated. In this study, we attempted to identify the molecular target of luteolin, one of the flavonoid compounds,in TLR pathway. Luteolin suppressed IRF3 activation induced by LPS (a TLR4 agonist) or poly(I:C) (a TLR3 agonist) in macrophage cell line (RAW264.7) resulting in the subsequent reduction in IFNß production. In addition, luteolin inhibited ligandindependent IRF3 activation induced by overexpression of constitutively active TLR4, TRIF, or TBK1. While luteolin did not prevent TLR4 oligomerization, in vitro kinase assay demonstrated that luteolin suppressed kinase activity of TBK1 and consequently decreased the expression of TBK1-dependent genes such as IP-10 and iNOS suggesting that TBK1 is a novel target of luteolin. The inhibitory effects of structural analogs such as quercetin, chrysin, and eriodictyol on TBK1 kinase activity were well correlated with their suppressive effects on IRF3 activation and TBK1-target gene expression. These results showed that luteolin suppresses TRIF-dependent signaling pathways and inflammatory genes of TLRs through the inhibition of TBK1. These further suggest that beneficial effects of phytochemicals on chronic inflammatory diseases are elicited by modulation of TLRs and their downstream signaling components.