TLR8-mediated NF-kappaB and JNK activation are TAK1-independent and MEKK3-dependent

J Biol Chem. 2006 Jul 28;281(30):21013-21021. doi: 10.1074/jbc.M512908200. Epub 2006 May 31.


TLR8-mediated NF-kappaB and IRF7 activation are abolished in human IRAK-deficient 293 cells and IRAK4-deficient fibroblast cells. Both wild-type and kinase-inactive mutants of IRAK and IRAK4, respectively, restored TLR8-mediated NF-kappaB and IRF7 activation in the IRAK- and IRAK4-deficient cells, indicating that the kinase activity of IRAK and IRAK4 is probably redundant for TLR8-mediated signaling. We recently found that TLR8 mediates a unique NF-kappaB activation pathway in human 293 cells and mouse embryonic fibroblasts, accompanied only by IkappaBalpha phosphorylation and not IkappaBalpha degradation, whereas interleukin (IL)-1 stimulation causes both IkappaBalpha phosphorylation and degradation. The intermediate signaling events mediated by IL-1 (including IRAK modifications and degradation and TAK1 activation) were not detected in cells stimulated by TLR8 ligands. TLR8 ligands trigger similar levels of IkappaBalpha phosphorylation and NF-kappaB and JNK activation in TAK1(-/-) mouse embryo fibroblasts (MEFs) as compared with wild-type MEFs, whereas lack of TAK1 results in reduced IL-1-mediated NF-kappaB activation and abolished IL-1-induced JNK activation. The above results indicate that although TLR8-mediated NF-kappaB and JNK activation are IRAK-dependent, they do not require IRAK modification and are TAK1-independent. On the other hand, TLR8-mediated IkappaBalpha phosphorylation, NF-kappaB, and JNK activation are completely abolished in MEKK3(-/-) MEFs, whereas IL-1-mediated signaling was only moderately reduced in these deficient MEFs as compared with wild-type cells. The differences between IL-1R- and TLR8-mediated NF-kappaB activation are also reflected at the level of IkappaB kinase (IKK) complex. TLR8 ligands induced IKKgamma phosphorylation, whereas IKKalpha/beta phosphorylation and IKKgamma ubiquitination that can be induced by IL-1 were not detected in cells treated with TLR8 ligands. We postulate that TLR8-mediated MEKK3-dependent IKKgamma phosphorylation might play an important role in the activation of IKK complex, leading to IkappaBalpha phosphorylation.

MeSH terms

  • Animals
  • Fibroblasts / metabolism
  • Humans
  • Interleukin-1 Receptor-Associated Kinases
  • Intracellular Signaling Peptides and Proteins / metabolism
  • MAP Kinase Kinase 4 / metabolism*
  • MAP Kinase Kinase Kinase 3 / metabolism*
  • MAP Kinase Kinase Kinases / metabolism*
  • Mice
  • NF-kappa B / metabolism*
  • Phosphotransferases (Alcohol Group Acceptor) / metabolism
  • Protein Serine-Threonine Kinases / metabolism
  • Signal Transduction
  • Toll-Like Receptor 8 / physiology*


  • Intracellular Signaling Peptides and Proteins
  • NF-kappa B
  • TLR8 protein, human
  • Toll-Like Receptor 8
  • Phosphotransferases (Alcohol Group Acceptor)
  • IRAK4 protein, human
  • Interleukin-1 Receptor-Associated Kinases
  • Irak4 protein, mouse
  • Protein Serine-Threonine Kinases
  • MAP Kinase Kinase Kinase 3
  • MAP Kinase Kinase Kinases
  • MAP kinase kinase kinase 7
  • MAP Kinase Kinase 4