Inhibition of Inflammatory Responses by Centella asiatica via Suppression of IRAK1-TAK1 in Mouse Macrophages

Centella asiatica (L.) Urb. (C. asiatica) has been widely treated for inflammation-related diseases in China for thousands of years. While C. asiatica showed relevant effects as traditional medicine, the mechanism of C. asiatica suppressing inflammation has not been thoroughly investigated. Therefore, this study was conducted to reveal the anti-inflammatory mechanism of methanol fraction from C. asiatica (MCA) at the molecular level in murine macrophages. Levels of inflammation-related mediators were observed with treatment of MCA. MCA significantly suppressed nitric oxide production and iNOS expression in RAW 264.7 macrophages. Prostaglandin E2 production was alleviated by MCA via the downregulation of cyclooxygenase-2. MCA treatment also reduced pro-inflammatory tumor necrosis factor- α and interleukin (IL)-6 levels. LPS/D-GalN-induced acute hepatitis in mouse was alleviated by MCA treatment. In addition, MCA decreased the phosphorylation of inhibitory κ B α (I κ B α ) at Ser32/36 and thereby blocked I κ B α degradation. TXY motif phosphorylation in the activation loops of mitogen-activated protein kinases (MAPKs) was also suppressed by MCA treatment. Further investigation revealed that MCA inhibited transforming growth factor- β -activated kinase 1 (TAK1) phosphorylation and IL-1 receptor-associated kinase (IRAK1) degradation, the upstream kinases activating nuclear factor κ B and MAPKs. Taken together, MCA exhibited anti-inflammatory properties via the downregulation of IRAK1-TAK1 signaling pathways.