Acertannin attenuates LPS-induced inflammation by interrupting the binding of LPS to the TLR4/MD2 complex and activating Nrf2-mediated HO-1 activation

Acertannin (ACTN) is a polyphenol known for its powerful anticancer and antioxidant effects. However, its anti-inflammatory effects have not been investigated at the molecular levels. Therefore, to evaluate anti-inflammatory effects of ACTN and its signaling pathway, the expression of proinflammatory markers was measured in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophages. Molecular docking predicted the binding site of ACTN to the TLR4/MD2 complex. Moreover, in LPS-microinjected zebrafish, we investigated whether ACTN reduces nitric oxide and reactive oxygen species (ROS) production. ACTN significantly attenuated LPS-induced proinflammatory cytokines and mediators by inhibiting nuclear factor-kappa B (NF-κB) activation. ACTN also reduced LPS-induced ROS production and activated nuclear factor E2-related factor 2 and heme oxygenase-1 (HO-1). In addition, zinc protoporphyrin, an HO-1 inhibitor, markedly abolished the anti-inflammatory and antioxidant effects of ACTN in LPS-stimulated zebrafish larvae. Moreover, molecular docking predictions verified that ACTN forms a conventional hydrogen bond with LYS91 in myeloid differentiation factor-2 (MD2) and interrupts LPS binding to the Toll-like receptor 4 (TLR4)/MD2 complex. In addition, ACTN forms many non-covalent bonds, such as π-π stacking, π-alkyl, unfavorable donor–donor, and van der Waals interactions, with the TLR4/MD2 complex. Furthermore, the binding of ACTN to the TLR4/MD2 complex inhibited the recruitment of intracellular adaptor proteins, including myeloid differentiation primary response 88 and interleukin-1 receptor-associated kinase 4, and consequently attenuated NF-κB-mediated inflammatory responses. The conclusion of this study is that ACTN is a potent anti-inflammatory agent in LPS-mediated inflammation, such as endotoxemia.