Andrographolide suppresses TRIF-dependent signaling of toll-like receptors by targeting TBK1

Toll-like receptors (TLRs) play a crucial role in danger recognition and induction of innate immune response against bacterial and viral infections. The TLR adaptor molecule, toll-interleuidn-1 receptor domain-containing adapter inducing interferon-beta (TRW), facilitates TLR3 and TLR4 signaling, leading to the activation of the transcription factor, NF-kappa B and interferon regulatory factor 3 (IRF3). Andrographolide, the active component of Andivgraphis paniculata, exerts anti-inflammatory effects; however, the principal molecular mechanisms remain unclear. The objective of this study was to investigate the role of andrographolide in TLR signaling pathways. Andrographolide suppressed NF-kappa B activation as well as COX-2 expression induced by TLR3 or TLR4 agonists. Andrographolide also suppressed the activation of IRF3 and the expression of interferon inducible protein-10 (IP-10) induced by TLR3 or TLR4 agonists. Andrographolide attenuated ligand-independent activation of IRF3 following overexpression of TRIP, TBK1, or IRF3. Furthermore, andrographolide inhibited TBK1 ldnase activity in vitro. These results indicate that andrographolide modulates the TRIP-dependent pathway of TLR5 by targeting TBK1 and represents a potential new anti-inflammatory candidate.