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CUTTING EDGE |
B-Activating Kinase-Associated Protein 1 Participates in TLR3/Toll-IL-1 Homology Domain-Containing Adapter Molecule-1-Mediated IFN Regulatory Factor 3 Activation1





Departments of
*
Immunology and
Molecular Biology, Osaka Medical Center for Cancer and Cardiovascular Diseases, Osaka, Japan;
Division of Molecular Biology, Nara Institute for Science and Technology, Nara, Japan;
Department of Biochemistry and Cell Biology, Graduate School of Medicine, Nagoya City University, Nagoya, Japan; and
¶ Department of Microbiology and Immunology, Graduate School of Medicine, Hokkaido University, Sapporo, Japan
| Abstract |
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(TRIF)) mediates IFN regulatory factor 3 (IRF3) phosphorylation followed by IFN-
production. The regulatory subunit TNFR-associated factor family member-associated NF-
B activator (TANK) couples with the kinase complex I
B kinase-related kinase
/NF-
B-activating kinase (NAK) (TANK-binding kinase 1 (TBK1)) that involveTICAM-1-dependent IFN-
induction. There are several TANK-homologous proteins. We tested whether TICAM-1 binds and coprecipitates with TANK or its family proteins. The results are: 1) the TANK family protein NAK-associated protein 1 (NAP1), but not TANK, coprecipitates withTICAM-1; 2) NAP1 overexpression markedly enhances TBK1-mediated IFN-
promoter activation; 3) a dominant-negative form, NAP (158270), suppresses IRF3 activation in response to poly(I:C) or LPS; 4) RNA interference targeting of the NAP1 message results in a failure of poly(I:C)-mediated IRF3 polymerization and IFN-
production. Thus, NAP1 is the kinase subunit responsible for TLR3/4-mediated IFN-
induction in the TICAM-1 pathway. | Introduction |
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B activation is known to be MyD88-dependent. TLR3 and TLR4 induce activation of IFN regulatory factor 3 (IRF3) followed by induction of IFN-
independent of MyD88 (1, 2).
Recently, a novel IFN-inducible adapter was identified downstream of TLR3 (2, 3). By yeast two-hybrid and functional analyses, we have identified TLR adapters that participate in TLR-mediated IRF3 activation and IFN-
induction (4, 5). In TLR3, TIR-containing adapter molecule (TICAM)-1 is the adapter directed to IRF3 activation in myeloid dendritic cells and epithelial cells (4, 6), while in TLR4 the TICAM-2-TICAM-1 complex is the adapter (5, 7, 8). Meanwhile, two groups reported that I
B kinase-related kinase
(IKK
) and TNFR-associated factor (TRAF) family member-associated NF-
B activator (TANK)-binding kinase 1 (TBK1), kinases which activate IRF3, localize downstream of TICAM-1 (9, 10). The current working concept is that these kinases form a complex with TANK (11, 12). Thus, the pathway converging to IRF3 activation in dendritic cells appeared to have been identified as TLR3/4-TICAM-TANK-IKK
/TBK1-IRF3 and the IFN-
promoter. Recent reports, however, demonstrated that NF-
B-activating kinase (NAK)-associated protein 1 (NAP1) is a subunit for the kinase complex IKK
/TBK1 in the pathway for NF-
B activation (13).
In this study, we tested the virtual molecular association in this pathway, namely the TICAM-1 pathway, particularly focusing on the physical/functional association in the TICAM-1 and NAP1 interaction.
| Materials and Methods |
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HEK293 and HeLa cells were cultured and used as described (4). Poly(I:C) (Amersham Biosciences), LPS (Escherichia coli serotype 0111:B4; Sigma-Aldrich), and anti-IRF3 Ab (IBL) were purchased. The p-125 luc reporter was a gift from Dr. T. Taniguchi (Tokyo University, Tokyo, Japan) and contained the human IFN-
promoter region (125 to +19). The Gal4-IRF3, Gal4-DBD, and p55 UASG-Luc for IRF3 activation were gifts form Dr. T. Fujita (Tokyo Metropolitan Institute of Medical Science, Tokyo, Japan) (14). Expression vectors for human TLR3, TLR4, MD-2, and hemagglutinin (HA)-tagged human TICAM-1 in pEF-BOS (5) and those for NAP1 and NAK (TBK1) in pcDNA3.1 (13, 15) were prepared as described previously. Methods for immunoprecipitation and immunoblotting were described previously (4, 5). The monospecific properties of the polyclonal Abs against NAP1 (13) and TICAM-1 (5) were shown in previous reports.
Assay for IRF3 and IFN-
promoter activation
Activation of the IFN-
promoter was measured by reporter assay. HEK293 cells (2 x 105 cells) were transfected in 24-well plates using LipofectAMINE 2000 (Invitrogen Life Technologies) with a p-125 luc (IFN-
) reporter plasmid (0.1 µg) together with the indicated amounts of the TICAM-1, NAK (TBK1), NAP1 full-length and/or NAP1 158270 plasmids.
Two methods, reporter assay (14) and native gel assay (16) were used to determine the degree of IRF3 activation. In some experiments, cells were stimulated with medium alone, LPS (100 ng/ml), or poly(I:C) (10 µg/ml). Signal intensity was measured by NIH Image Scan. Data were expressed as the means ± SD. Experiments were performed at least three times in triplicate. The primer sets for RT-PCR were described previously (5, 13, 15).
RNA interference
The gene silencing vector pH1 was constructed from the pH1' vector (a gift from Drs. M. Okabe and H. Hasuwa, Osaka University, Osaka, Japan) (17). Oligonucleotides were cloned into the pH1 vector to express the small interference hairpin-loop (sih) GFP, sihNAP1-A, and sihNAP1-B hairpins downstream of the human H1 RNA promoter as described (18). The following sequences were targeted: for sihNAP1-A, 5'-AAGCTAATAGCTCGATTTGAAGA-3'; sihNAP1-B, 5'-AAGTGATAATATGCAGCATGCAT-3'. The sequence for sihGFP has been described (18). HeLa cells were transfected with pH1-empty, pH1-GFP, or pH1-NAP1-A/B using PolyFect (Qiagen) and selected bulk populations in 1 µg/ml Puromycin and picked up as single colonies. mRNA was isolated from the cells and silencing efficiency assessed by RT-PCR.
| Results and Discussion |
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promoter activation in the TICAM-1 pathway.
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promoter. First, TICAM-1 expression induced activation of the IFN-
promoter and this activation level was slightly enhanced if full-length NAP1 was simultaneously expressed (Fig. 2A). Previous reports showed that NAP1 interacts with TBK1 and IKK
to facilitate their oligomerization and the phosphorylation of the p65/RelA subunit of NF-
B. NAP1 binds TBK1 in the 158270 region (14). We then tested whether NAP1 (158270) affects TICAM-1-dependent IFN-
promoter activation (Fig. 2B). NAP1 (158270) blocked TICAM-1-mediated IFN-
promoter activation in a dose-dependent manner although NAP1 (158270) per se had no effect (Fig. 2B). Thus, we designated NAP1 (158270) to be dominant-negative. Because the NAP1-TBK1 complex activates NF-
B (13), we next checked the synergistic effect of NAP1 and TBK1 on IFN-
promoter activation (Fig. 2C). In a dose-dependent manner, NAP1 enhanced IFN-
promoter activation by the functional effect of TBK1. Under similar conditions, TICAM-1-mediated activation of NF-
B, either via TLR3 or TLR4, was suppressed by NAP1 (158270) (data not shown). Hence, the NAP1-TBK1 complex emerges a pathway to IFN-
promoter activation in addition to NF-
B. The dominant-negative form of NAP1 exhibited no suppressive effect on IFN-
promoter potentiation secondary to TBK1 overexpression (Fig. 2D), suggesting that NAP1 localizes upstream of TBK1 in the TICAM-1 pathway.
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promoter (14, 18), and TLR3 stimulation as well as TICAM-1 transfection results in IRF3 activation (4, 6). Using GAL4-IRF3 (13, 16), we measured the level of IRF3 activation in cells expressing variable levels of NAP1 (158270) (Fig. 3A). Spontaneous IRF3 activation detected in intact cells was inhibited by NAP1 (158270). The NAP1 (158270)-dependent IRF3 inhibition was prominent if TICAM-1 cotransfected so as to potentiate IRF3 activation. These results were confirmed by gel-shift assay for the determination of IRF3 activation (16). TICAM-1-mediated IRF3 dimer formation was decreased (13.4 vs 8.5) by coexpression of NAP1 (158270) (Fig. 3B). IRF3 activation followed by poly(I:C) stimulation of TLR3-expressing HEK293 cells was monitored with GAL4-IRF3 (Fig. 3C). IRF3 activation was up-regulated in response to poly(I:C) stimulation, and expression of NAP1 (158270) in this cell system led to the suppression of IRF3 activation (Fig. 3C). LPS-mediated IRF3 activation was next tested with cells expressing TLR4/CD14/MD-2 in the GAL4-IRF3 assay system. When HEK293 cells were transfected with an appropriate ratio of TLR4, CD14, and MD-2 (5), IRF3 was sufficiently activatable in response to LPS (Fig. 3D). NAP1 (158270) again inhibited the IRF3 activation induced by LPS and TLR4 (Fig. 3D). Thus, IRF3 is evidently activated through the NAP1-TBK1 complex in the TICAM-1 pathway.
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mRNA production (Fig. 4D) were impaired. These results were confirmed by RT-PCR for IFN-
mRNA (data not shown). Thus, the functional loss of NAP1 abolishes IRF3-mediated IFN-
induction. Taken together, NAP1 physically and functionally associates with TICAM-1 and in conjunction with TBK1, activates IRF3 to stimulate IFN-
promoter activation.
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B and IRF3 (4, 6). Receptor-interacting protein 1 binds the C-terminal domain out of the TIR of TICAM-1 and participates in TICAM-1-mediated NF-
B activation (19). Because IL-1R-associated kinase 1/4 and TRAF6 are responsible for activation of the kinase complex for liberation of NF-
B in the MyD88-dependent pathway (1), TICAM-1 and MyD88 adapters allow the selection of different pathways for NF-
B activation. In this study, our findings suggest that receptor-interacting protein 1 and NAP1 differentially interact with TICAM-1 to induce NF-
B and IRF3 activation, respectively, downstream of TICAM-1. This accounts for the bimodal function of TICAM-1 (1, 3, 20). TANK cannot be replaced for NAP1 in this pathway.
IKK
and IKK
are the kinases for I
B degradation. These kinases form a complex with IKK
that is known to be associated with TANK (21). This NF-
B kinase complex cannot activate IRF3, but topologically resembles our current model of the NAP1 kinase complex (13). In fact, the NAP1-TBK1 complex activates NF-
B along with IRF3 (data not shown).
Sharma et al. (10) report that IKK
and TBK1 are targets for viral components that are generated by infected viruses. They call these kinases "virus-activated kinases" (22). We favor the interpretation that viral factors target the NAP1 kinase complex. The virus-mediated IRF3-activating pathway may merge with the TLR3-TICAM-1 pathway at the NAP1-TBK1 region. Our present findings would be a key to elucidate the specific relationship between viral infection and TICAM-1-mediated signaling.
| Footnotes |
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1 This work was supported in part by Core Research for Evolutional Science, and Technology (CREST), Japan Science and Technology Agency (JST), grants-in-aid from the Ministry of Education, Science, and Culture (Specified Project for Advanced Research), and the Naito Memorial Foundation (to M.M.). ![]()
2 Address correspondence and reprint requests to Dr. Tsukasa Seya, Department of Microbiology and Immunology, Graduate School of Medicine, Hokkaido University, Kita-15, Nishi-7, Kita-ku, Sapporo 060-8638 Japan. E-mail address: seya-tu{at}med.hokudai.ac.jp ![]()
3 Abbreviations used in this paper: TIR, Toll-IL-1R homology domain; IRF, IFN regulatory factor; TICAM, TIR-containing adapter molecule; IKK
, I
B kinase-related kinase
; TRAF, TNFR-associated factor; TANK, TRAF family member-associated NF-
B activator; TBK, TANK-binding kinase; NAK, NF-
B-activating kinase; NAP, NAK-associated protein; HA, hemagglutinin; sih, small interference hairpin-loop; siRNA, small interference RNA. ![]()
Received for publication August 9, 2004. Accepted for publication October 13, 2004.
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