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

Abstract

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, PGE₂, iNOS, COX-2, TNF-α, IL-1β, 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-κB-phosphorylation, NF-κB translocation, AP-1 and NF-κB-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 and JNK activations, and suppression in IKKα/β-mediated I-κB 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.

Keywords: (4-amino-5-(4-chlorophenyl)-7-(dimethylethyl) pyrazolo [3,4-d]pyrimidine; 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-1-hydro-chloride; AP-1; Adaptor-protein; COX-2; ERK1/2; GM-CSF; IKK; IL-1 receptor-associated kinase 1; IL-1 receptor-associated kinase 4; IL-1β; IL-6; IRAK-1; IRAK4; Innate-immunity; IκB; JNK1/2; LY-294002; MAPK; MyD88; NF-κB; NO; PDK1; PGE(2); PI3K; PKB (Akt); PP2; Protein-kinases; TAB1 and 2; TAK1; TAK1-binding protein-1 and -2; TNF receptor-associated factor 6; TNF-α; TRAF6; Transcription-factor; activator protein-1; c-jun N-terminal kinase; cyclooxygenase-2; extracellular signal regulated protein kinase; granulocyte-macrophage colony stimulating factor; iNOS; inducible nitric oxide synthase; inhibitor of NF-κB; inhibitory NF-κB kinase; interleukin-1β; interleukin-6; mitogen-activated protein kinase; myeloid differentiation primary-response protein 88; nitric oxide; nuclear factor κB; phosphatidylinositol 3-kinase; phosphoinositide-dependent protein kinase-1; prostaglandin E(2); protein kinase B; transforming-growth-factor-β-activated kinase 1; tumor necrosis factor-α.