TLR8-mediated NF-kappaB and JNK activation are TAK1-independent and MEKK3-dependent.
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.
Qin
Jinzhong
J
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Yao
Jianhong
J
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Cui
Grace
G
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Xiao
Hui
H
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Kim
Tae Whan
TW
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Fraczek
Jerzy
J
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
Wightman
Paul
P
3M Center, St. Paul, Minnesota 55144.
Sato
Shintaro
S
Osaka University, Japan Science and Technology Agency, 3-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
Akira
Shizuo
S
Osaka University, Japan Science and Technology Agency, 3-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
Puel
Anne
A
Laboratory of Human Genetics of Infectious Diseases, University of Paris René Descartes INSERM U550, Necker Medical School, Paris 75015, France.
Casanova
Jean-Laurent
JL
Laboratory of Human Genetics of Infectious Diseases, University of Paris René Descartes INSERM U550, Necker Medical School, Paris 75015, France.
Su
Bing
B
University of Texas M. D. Anderson Cancer Center, Houston, Texas 77054.
Li
Xiaoxia
X
Department of Immunology, Cleveland Clinic Foundation, Cleveland, Ohio 44195.
D000818:Animals; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D053592:Interleukin-1 Receptor-Associated Kinases; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D048670:MAP Kinase Kinase 4 / Q000378:metabolism*; D048748:MAP Kinase Kinase Kinase 3 / Q000378:metabolism*; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism*; D051379:Mice; D016328:NF-kappa B / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D015398:Signal Transduction; D051200:Toll-Like Receptor 8 / Q000502:physiology*; D000099151:MAP Kinase Kinase Kinase 7
D053592:Interleukin-1 Receptor-Associated Kinases; D047908:Intracellular Signaling Peptides and Proteins; D048670:MAP Kinase Kinase 4; D048748:MAP Kinase Kinase Kinase 3; D020930:MAP Kinase Kinase Kinases; D016328:NF-kappa B; D017853:Phosphotransferases (Alcohol Group Acceptor); D017346:Protein Serine-Threonine Kinases; D051200:Toll-Like Receptor 8; D000099151:MAP Kinase Kinase Kinase 7; C495349:TLR8 protein, human; C507773:IRAK4 protein, human; C507774:Irak4 protein, mouse