Jun-ichiro Inoue

TAK1 is a ubiquitin-dependent kinase of MKK and IKK

TRAF6 is a signal transducer that activates IκB kinase (IKK) and Jun amino-terminal kinase (JNK) in response to pro-inflammatory mediators such as interleukin-1 (IL-1) and lipopolysaccharides (LPS). IKK activation by TRAF6 requires two intermediary factors, TRAF6-regulated IKK activator 1 (TRIKA1) and TRIKA2 (ref. 5). TRIKA1 is a dimeric ubiquitin-conjugating enzyme complex composed of Ubc13 and Uev1A (or the functionally equivalent Mms2). This Ubc complex, together with TRAF6, catalyses the formation of a Lys 63 (K63)-linked polyubiquitin chain that mediates IKK activation through a unique proteasome-independent mechanism. Here we report the purification and identification of TRIKA2, which is composed of TAK1, TAB1 and TAB2, a protein kinase complex previously implicated in IKK activation through an unknown mechanism. We find that the TAK1 kinase complex phosphorylates and activates IKK in a manner that depends on TRAF6 and Ubc13–Uev1A. Moreover, the activity of TAK1 to phosphorylate MKK6, which activates the JNK–p38 kinase pathway, is directly regulated by K63-linked polyubiquitination. We also provide evidence that TRAF6 is conjugated by the K63 polyubiquitin chains. These results indicate that ubiquitination has an important regulatory role in stress response pathways, including those of IKK and JNK.

Induction and activation of the transcription factor NFATc1 (NFAT2) integrate RANKL signaling in terminal differentiation of osteoclasts.

Signaling by RANKL is essential for terminal differentiation of monocytes/macrophages into osteoclasts. The TRAF6 and c-Fos signaling pathways both play important roles downstream of RANKL. We show here that RANKL selectively induces NFATc1 expression via these two pathways. RANKL also evokes Ca(2+) oscillations that lead to calcineurin-mediated activation of NFATc1, and therefore triggers a sustained NFATc1-dependent transcriptional program during osteoclast differentiation. We also show that NFATc1-deficient embryonic stem cells fail to differentiate into osteoclasts in response to RANKL stimulation, and that ectopic expression of NFATc1 causes precursor cells to undergo efficient differentiation without RANKL signaling. Thus, NFATc1 may represent a master switch for regulating terminal differentiation of osteoclasts, functioning downstream of RANKL.

The kinase TAK1 can activate the NIK-IκB as well as the MAP kinase cascade in the IL-1 signalling pathway

Interleukin-1 (IL-1) is a proinflammatory cytokine that has several effects in the inflammation process. When it binds to its cell-surface receptor, IL-1 initiates a signalling cascade that leads to activation of the transcription factor NF-κB and is relayed through the protein TRAF6 and a succession of kinase enzymes, including NF-κB-inducing kinase (NIK) and IκB kinases (IKKs). However, the molecular mechanism by which NIK is activated is not understood. Here we show that the MAPKK kinase TAK1 (ref. 8) acts upstream of NIK in the IL-1-activated signalling pathway and that TAK1 associates with TRAF6 during IL-1 signalling. Stimulation of TAK1 causes activation of NF-κB, which is blocked by dominant-negative mutants of NIK, and an inactive TAK1 mutant prevents activation of NF-κB that is mediated by IL-1 but not by NIK. Activated TAK1 phosphorylates NIK, which stimulates IKK-α activity. Our results indicate that TAK1 links TRAF6 to the NIK–IKK cascade in the IL-1 signalling pathway.

Lipopolysaccharide stimulates the MyD88-independent pathway and results in activation of IFN-regulatory factor 3 and the expression of a subset of lipopolysaccharide-inducible genes

Bacterial lipopolysaccharide (LPS) triggers innate immune responses through Toll-like receptor (TLR) 4, a member of the TLR family that participates in pathogen recognition. TLRs recruit a cytoplasmic protein, MyD88, upon pathogen recognition, mediating its function for immune responses. Two major pathways for LPS have been suggested in recent studies, which are referred to as MyD88-dependent and -independent pathways. We report in this study the characterization of the MyD88-independent pathway via TLR4. MyD88-deficient cells failed to produce inflammatory cytokines in response to LPS, whereas they responded to LPS by activating IFN-regulatory factor 3 as well as inducing the genes containing IFN-stimulated regulatory elements such as IP-10. In contrast, a lipopeptide that activates TLR2 had no ability to activate IFN-regulatory factor 3. The MyD88-independent pathway was also activated in cells lacking both MyD88 and TNFR-associated factor 6. Thus, TLR4 signaling is composed of at least two distinct pathways, a MyD88-dependent pathway that is critical to the induction of inflammatory cytokines and a MyD88/TNFR-associated factor 6-independent pathway that regulates induction of IP-10.

Interferon-α induction through Toll-like receptors involves a direct interaction of IRF7 with MyD88 and TRAF6

Toll-like receptors (TLRs) are involved in the recognition of microbial pathogens. A subset of TLRs, TLR7, TLR8 and TLR9, induces antiviral responses by producing interferon-α (IFN-α). Production of IFN-α is dependent on the Toll–interleukin-1 receptor domain–containing adaptor MyD88. Here we show that MyD88 formed a complex with the transcription factor IRF7 but not with IRF3. The death domain of MyD88 interacted with an inhibitory domain of IRF7, and this interaction resulted in activation of the IFN-α-dependent promoters. Furthermore, the adaptor molecule TRAF6 also bound and activated IRF7. Ubiquitin ligase activity of TRAF6 was required for IRF7 activation. These results indicate that TLR-mediated IFN-α induction requires the formation of a complex consisting of MyD88, TRAF6 and IRF7 as well as TRAF6-dependent ubiquitination.

Severe osteopetrosis, defective interleukin-1 signalling and lymph node organogenesis in TRAF6-deficient mice.

TRAF6, a member of the tumour necrosis factor receptor‐associated factor family, was first identified as a transducer of CD40 and interleukin‐1 receptor (IL‐1R) signals based on the interaction of TRAF6 with the cytoplasmic tail of CD40 and with the IL‐1R associated kinase in vitro. However, the functions of TRAF6 in vivo remain unidentified.

Identification of TRAF6, a Novel Tumor Necrosis Factor Receptor-associated Factor Protein That Mediates Signaling from an Amino-terminal Domain of the CD40 Cytoplasmic Region

CD40 signalings play crucial roles in B-cell function. To identify molecules which transduce CD40 signalings, we have utilized the yeast two-hybrid system to clone cDNAs encoding proteins that bind the cytoplasmic tail of CD40. A cDNA encoding a putative signal transducer, designated TRAF6, has been molecularly cloned. TRAF6 has a tumor necrosis factor receptor (TNFR)-associated factor (TRAF) domain in its carboxyl terminus and has a RING finger domain, a cluster of zinc fingers and a coiled-coil domain, which are also present in other TRAF family proteins. TRAF6 does not associate with the cytoplasmic tails of TNFR2, CD30, lymphotoxin-β receptor, and LMP1 of Epstein-Barr virus. Deletion analysis showed that residues 246-269 of CD40 which are required for its association with TRAF2, TRAF3, and TRAF5 are dispensable for its interaction with TRAF6, whereas residues 230-245 were required. Overexpression of TRAF6 activates transcription factor NFκB, and its TRAF-C domain suppresses NFκB activation triggered by CD40 lacking residues 246-277. These results suggest that TRAF6 could mediate the CD40 signal that is transduced by the amino-terminal domain (230-245) of the CD40 cytoplasmic region and appears to be independent of other known TRAF family proteins.

Induction of interleukin 2 receptor gene expression by p40x encoded by human T-cell leukemia virus type 1.

Human T‐cell leukemia virus type 1 (HTLV‐1) is an etiologic agent of adult T‐cell leukemia (ATL). A viral product, p40x, encoded by the pX sequence of HTLV‐1 is a trans‐acting transcriptional activator of the long terminal repeat (LTR) and has been suspected of involvement in leukemogenesis, activating the cellular genes. The cellular interleukin‐2 (IL‐2) and its receptor (IL‐2R), the latter of which is expressed on ATL leukemic cells, were shown to be transiently induced by transfection of plasmid pMTPX expressing pX in two T‐cell lines, Jurkat and HSB‐2, but not in other human T‐ or B‐cell lines. The cell type specificity of IL‐2R induction by pX expression was the same as that by phytohaemagglutinin/phorbol ester activation, indicating the requirement for some specific cellular factors or a certain state of cellular differentiation. Induction of IL‐2 and IL‐2R at mRNA level was also demonstrated in transfected cells. Transfections with mutants of pMTPX in which the open reading frames for p40x, p27x‐III and p21x‐III were inactivated indicated that p40x alone was sufficient for induction of the IL‐2R in inducible cells. This induction of the IL‐2R by p40x of HTLV‐1 may contribute to preferential proliferation of HTLV‐1 infected cells at an early stage of ATL development and eventually increase the number of putative target cells for malignant transformation.

Segregation of TRAF6‐mediated signaling pathways clarifies its role in osteoclastogenesis

Signals emanating from the receptor for interleukin‐1 (IL‐1), lipopolysaccharide (LPS) or osteoclast differentiation factor/receptor activator of NFκB ligand (ODF/RANKL) stimulate transcription factors AP‐1 through mitogen‐activated protein kinase (MAPK) activation and NFκB through IκB kinase (IKK) activation. These kinases are thought to be activated by tumor necrosis factor receptor‐associated factor 6 (TRAF6). However, molecular mechanisms by which TRAF6 activates various downstream kinases remain to be elucidated. We identified functional domains of TRAF6 under physiological conditions established by appropriate expression of TRAF6 mutants in TRAF6‐deficient cells. In IL‐1 and LPS signaling pathways, the RING finger and first zinc finger domains are not required for NFκB activation but are required for full activation of MAPK. However, IL‐1 and LPS signals utilize distinct regions within the zinc finger domains of TRAF6 to activate NFκB. Furthermore, the RING finger domain is not required for differentiation of splenocytes to multinuclear osteoclasts, but is essential for osteoclast maturation. Thus, TRAF6 plays essential roles in both the differentiation and maturation of osteoclasts by activating various kinases via its multiple domains.

The Tumor Necrosis Factor Family Receptors RANK and CD40 Cooperatively Establish the Thymic Medullary Microenvironment and Self-Tolerance

Medullary thymic epithelial cells (mTECs) establish T cell self-tolerance through the expression of autoimmune regulator (Aire) and peripheral tissue-specific self-antigens. However, signals underlying mTEC development remain largely unclear. Here, we demonstrate crucial regulation of mTEC development by receptor activator of NF-kappaB (RANK) and CD40 signals. Whereas only RANK signaling was essential for mTEC development during embryogenesis, in postnatal mice, cooperation between CD40 and RANK signals was required for mTEC development to successfully establish the medullary microenvironment. Ligation of RANK or CD40 on fetal thymic stroma in vitro induced mTEC development in a tumor necrosis factor-associated factor 6 (TRAF6)-, NF-kappaB inducing kinase (NIK)-, and IkappaB kinase beta (IKKbeta)-dependent manner. These results show that developmental-stage-dependent cooperation between RANK and CD40 promotes mTEC development, thereby establishing self-tolerance.