Mechanism of anti-inflammatory effect of tricin, a flavonoid isolated from Njavara rice bran in LPS induced hPBMCs and carrageenan induced rats

Abstract

Njavara is an indigenous medicinal rice variety traditionally used in Ayurvedic system of medicine practiced in Kerala, India. Tricin is a bioflavonoid present in significantly higher levels in rice bran of Njavara. Present study attempted to identify the molecular target of tricin in TLR mediated signaling pathways by using lipopolysaccharide (LPS) induced human peripheral blood mononuclear cells (hPBMCs) and carrageenan induced paw edema in rats as experimental models. Tricin acted upstream in the activation of inflammation cascade by interfering with TLR4 activation, preferably by blocking the LPS induced activation of TLR4, MYD88 and TRIF proteins in hPBMCs. Subsequently, tricin significantly blocked the activation of downstream kinases like p38MAPK, JNK1/2 and IRF3. Thus the inhibitory effect of tricin on NF-κB and IRF3 together confirms the specific inhibition of both MYD88 dependent and TRIF dependent pathways. Tricin treatment also inhibited the pro-inflammatory effect of LPS by blocking the TLR4 signaling mediated activation of cytosolic phospholipase A2 (cPLA2), which is confirmed by specific inhibition of COX-2. Results demonstrated that in addition to NF-κB, tricin can prevent the activation of STAT proteins by significantly inhibiting the activation of both STAT1 and STAT3 via the down regulation of upstream phosphorylating enzymes like JAK1 and JAK2. The protective anti-inflammatory effect of tricin was also confirmed by in vivo experiments. Thus, this study provides strong evidence that tricin exerts its anti-inflammatory effect via a mechanism involving the TLR4/NF-κB/STAT signaling cascade.