Quercetin disrupts tyrosine-phosphorylated phosphatidylinositol 3-kinase and myeloid differentiation factor-88 association, and inhibits MAPK/AP-1 and IKK/NF-κB-induced inflammatory mediators production in RAW 264.7 cells

Quercetin is a major bioflavonoid widely present in fruits and vegetables. It exhibits anti-inflammatory, anti-tumor, antioxidant properties and reduces cardiovascular disease risks. However, the molecular mechanism of action against inflammation in RAW 264.7 cells is only partially explored. Quercetin effect on LPS-induced gene and protein expressions of inflammatory mediators and cytokines were determined. Moreover, involvement of heme-oxygenase-1, protein kinases, adaptor proteins and transcription factors in molecular mechanism of quercetin action against inflammation were examined. Quercetin inhibited LPS-induced NO, PGE(2), iNOS, COX-2, TNF-alpha, IL-1 beta, IL-6 and GM-CSF mRNA and protein expressions while it promoted HO-1 induction in a dose- and time-dependent manner. It also suppressed I-kappa B-phosphorylation, NF-kappa B translocation, AP-1 and NF-kappa B-DNA-binding and reporter gene transcription. Quercetin attenuated p38(MAPK) and JNK1/2 but not ERK1/2 activations and this effect was further confirmed by SB203580 and SP600125-mediated suppressions of HO-1, iNOS, and COX-2 protein expressions. Moreover, quercetin arrested Src, PI3K, PDK1 and Akt activation in a time- and dose-dependent manner, which was comparable to PP2 and LY294002 inhibition of Src, PI3K/Akt and iNOS expressions. Quercetin further arrested Src and Syk tyrosine phosphorylations and their kinase activities followed by inhibition of PI3K tyrosine phosphorylation. Moreover, quercetin disrupted LPS-induced p85 association to TLR4/MyD88 complex and it then limited activation of IRAK1, TRAF6 and TAK1 with a subsequent reduction in p38(MAPK) and JNK activations, and suppression in IKK alpha/beta-mediated I-kappa B phosphorylation. Quercetin limits LPS-induced inflammation via inhibition of Src- and Syk-mediated PI3K- (p85) tyrosine phosphorylation and subsequent TLR4/MyD88/PI3K complex formation that limits activation of downstream signaling pathways. (C) 2013 Elsevier GmbH. All rights reserved.