Hiroyasu Nakano

ASK1 Is Essential for JNK/SAPK Activation by TRAF2

Tumor necrosis factor (TNF)-induced activation of the c-jun N-terminal kinase (JNK, also known as SAPK; stress-activated protein kinase) requires TNF receptor-associated factor 2 (TRAF2). The apoptosis signal-regulating kinase 1 (ASK1) is activated by TNF and stimulates JNK activation. Here we show that ASK1 interacts with members of the TRAF family and is activated by TRAF2, TRAF5, and TRAF6 overexpression. A truncated derivative of TRAF2, which inhibits JNK activation by TNF, blocks TNF-induced ASK1 activation. A catalytically inactive mutant of ASK1 is a dominant-negative inhibitor of TNF- and TRAF2-induced JNK activation. In untransfected mammalian cells, ASK1 rapidly associates with TRAF2 in a TNF-dependent manner. Thus, ASK1 is a mediator of TRAF2-induced JNK activation.

NF-κB inhibits TNF-induced accumulation of ROS that mediate prolonged MAPK activation and necrotic cell death

NF‐κB downregulates tumor necrosis factor (TNF)‐induced c‐Jun N‐terminal kinase (JNK) activation that promotes cell death, but the mechanism is not yet fully understood. By using murine embryonic fibroblasts (MEFs) that are deficient in TNF receptor‐associated factor (TRAF) 2 and TRAF5 (DKO) or p65 NF‐κB subunit (p65KO), we demonstrate here that TNF stimulation leads to accumulation of reactive oxygen species (ROS), which is essential for prolonged mitogen‐activated protein kinase (MAPK) activation and cell death. Interestingly, dying cells show necrotic as well as apoptotic morphological changes as assessed by electron microscopy and flow cytometry, and necrotic, but not apoptotic, cell death is substantially inhibited by antioxidant. Importantly, TNF does not induce ROS accumulation or prolonged MAPK activation in wild‐type MEFs, indicating that TRAF‐mediated NF‐κB activation normally suppresses the TNF‐induced ROS accumulation that subsequently induces prolonged MAPK activation and necrotic cell death

A critical role of RICK/RIP2 polyubiquitination in Nod-induced NF-κB activation

Nod1 and Nod2 are intracellular proteins that are involved in host recognition of specific bacterial molecules and are genetically associated with several inflammatory diseases. Nod1 and Nod2 stimulation activates NF‐κB through RICK, a caspase‐recruitment domain‐containing kinase. However, the mechanism by which RICK activates NF‐κB in response to Nod1 and Nod2 stimulation is unknown. Here we show that RICK is conjugated with lysine‐63‐linked polyubiquitin chains at lysine 209 (K209) located in its kinase domain upon Nod1 or Nod2 stimulation and by induced oligomerization of RICK. Polyubiquitination of RICK at K209 was essential for RICK‐mediated IKK activation and cytokine/chemokine secretion. However, RICK polyubiquitination did not require the kinase activity of RICK or alter the interaction of RICK with NEMO, a regulatory subunit of IκB kinase (IKK). Instead, polyubiquitination of RICK was found to mediate the recruitment of TAK1, a kinase that was found to be essential for Nod1‐induced signaling. Thus, RICK polyubiquitination links TAK1 to IKK complexes, a critical step in Nod1/Nod2‐mediated NF‐κB activation.

SHARPIN is a component of the NF-κB-activating linear ubiquitin chain assembly complex

Cpdm (chronic proliferative dermatitis) mice develop chronic dermatitis and an immunodeficiency with increased serum IgM, symptoms that resemble those of patients with X-linked hyper-IgM syndrome and hypohydrotic ectodermal dysplasia (XHM-ED), which is caused by mutations in NEMO (NF-κB essential modulator; also known as IKBKG). Spontaneous null mutations in the Sharpin (SHANK-associated RH domain interacting protein in postsynaptic density) gene are responsible for the cpdm phenotype in mice. SHARPIN shows significant similarity to HOIL-1L (also known as RBCK1), a component of linear ubiquitin chain assembly complex (LUBAC), which induces NF-κB activation through conjugation of linear polyubiquitin chains to NEMO. Here, we identify SHARPIN as an additional component of LUBAC. SHARPIN-containing complexes can linearly ubiquitinate NEMO and activated NF-κB. Thus, we re-define LUBAC as a complex containing SHARPIN, HOIL-1L, and HOIP (also known as RNF31). Deletion of SHARPIN drastically reduced the amount of LUBAC, which resulted in attenuated TNF-α- and CD40-mediated activation of NF-κB in mouse embryonic fibroblasts (MEFs) or B cells from cpdm mice. Considering the pleomorphic phenotype of cpdm mice, these results confirm the predicted role of LUBAC-mediated linear polyubiquitination in NF-κB activation induced by various stimuli, and strongly suggest the involvement of LUBAC-induced NF-κB activation in various disorders.

The atypical PKC‐interacting protein p62 channels NF‐κB activation by the IL‐1–TRAF6 pathway

The atypical protein kinase C (aPKC)‐interacting protein, p62, has previously been shown to interact with RIP, linking these kinases to NF‐κB activation by tumor necrosis factor α (TNFα). The aPKCs have been implicated in the activation of IKKβ in TNFα‐stimulated cells and have been shown to be activated in response to interleukin‐1 (IL‐1). Here we demonstrate that the inhibition of the aPKCs or the down‐regulation of p62 severely abrogates NF‐κB activation by IL‐1 and TRAF6, suggesting that both proteins are critical intermediaries in this pathway. Consistent with this we show that p62 selectively interacts with the TRAF domain of TRAF6 but not that of TRAF5 or TRAF2 in co‐transfection experiments. The binding of endogenous p62 to TRAF6 is stimulus dependent, reinforcing the notion that this is a physiologically relevant interaction. Furthermore, we demonstrate that the N‐terminal domain of TRAF6, which is required for signaling, interacts with ζPKC in a dimerization‐dependent manner. Together, these results indicate that p62 is an important intermediary not only in TNFα but also in IL‐1 signaling to NF‐κB through the specific adapters RIP and TRAF6.

NF-κB RelA Phosphorylation Regulates RelA Acetylation

ABSTRACT The nuclear functions of NF-κB p50/RelA heterodimers are regulated in part by posttranslational modifications of its RelA subunit, including phosphorylation and acetylation. Acetylation at lysines 218, 221, and 310 differentially regulates RelAs DNA binding activity, assembly with IκBα, and transcriptional activity. However, it remains unclear whether the acetylation is regulated or simply due to stimulus-coupled nuclear translocation of NF-κB. Using anti-acetylated lysine 310 RelA antibodies, we detected p300-mediated acetylation of RelA in vitro and in vivo after stimulation of cells with tumor necrosis factor alpha (TNF-α). Coexpression of catalytically inactive mutants of the catalytic subunit of protein kinase A/mitogen- and stress-activated kinase 1 or IKK1/IKK2, which phosphorylate RelA on serine 276 or serine 536, respectively, sharply inhibited RelA acetylation on lysine 310. Furthermore, phosphorylation of RelA on serine 276 or serine 536 increased assembly of phospho-RelA with p300, which enhanced acetylation on lysine 310. Reconstitution of RelA-deficient murine embryonic fibroblasts with RelA S276A or RelA S536A decreased TNF-α-induced acetylation of lysine 310 and expression of the endogenous NF-κB-responsive E-selectin gene. These findings indicate that the acetylation of RelA at lysine 310 is importantly regulated by prior phosphorylation of serines 276 and 536. Such phosphorylated and acetylated forms of RelA display enhanced transcriptional activity.

Reactive oxygen species mediate crosstalk between NF-κB and JNK

The activation of NF-κB inhibits apoptosis via a mechanism involving upregulation of various antiapoptotic genes, such as cellular FLICE-inhibitory protein (c-FLIP), Bcl-xL, A1/Bfl-1, and X chromosome-liked inhibitor of apoptosis (XIAP). In contrast, the activation of c-Jun N-terminal kinase (JNK) promotes apoptosis in a manner that is dependent on the cell type and the context of the stimulus. Recent studies have indicated that one of the antiapoptotic functions of NF-κB is to downregulate JNK activation. Further studies have also revealed that NF-κB inhibits JNK activation by suppressing accumulation of reactive oxygen species (ROS). In this review, we will focus on the signaling crosstalk between the NF-κB and JNK cascades via ROS.

Osteoclast differentiation independent of the TRANCE–RANK–TRAF6 axis

Osteoclasts are derived from myeloid lineage cells, and their differentiation is supported by various osteotropic factors, including the tumor necrosis factor (TNF) family member TNF-related activation-induced cytokine (TRANCE). Genetic deletion of TRANCE or its receptor, receptor activator of nuclear factor κB (RANK), results in severely osteopetrotic mice with no osteoclasts in their bones. TNF receptor-associated factor (TRAF) 6 is a key signaling adaptor for RANK, and its deficiency leads to similar osteopetrosis. Hence, the current paradigm holds that TRANCE–RANK interaction and subsequent signaling via TRAF6 are essential for the generation of functional osteoclasts. Surprisingly, we show that hematopoietic precursors from TRANCE-, RANK-, or TRAF6-null mice can become osteoclasts in vitro when they are stimulated with TNF-α in the presence of cofactors such as TGF-β. We provide direct evidence against the current paradigm that the TRANCE–RANK–TRAF6 pathway is essential for osteoclast differentiation and suggest the potential existence of alternative routes for osteoclast differentiation.

CD27, a member of the tumor necrosis factor receptor superfamily, activates NF-kappaB and stress-activated protein kinase/c-Jun N-terminal kinase via TRAF2, TRAF5, and NF-kappaB-inducing kinase.

CD27 is a member of the tumor necrosis factor (TNF) receptor superfamily and is expressed on T, B, and NK cells. The signal via CD27 plays pivotal roles in T-T and T-B cell interactions. Here we demonstrate that overexpression of CD27 activates NF-κB and stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK). Deletion analysis of the cytoplasmic domain of CD27 revealed that the C-terminal PIQEDYR motif was indispensable for both NF-κB and SAPK/JNK activation and was also required for the interaction with TNF receptor-associated factor (TRAF) 2 and TRAF5, both of which have been implicated in NF-κB activation by members of the TNF-R superfamily. Co-transfection of a dominant negative TRAF2 or TRAF5 blocked NF-κB and SAPK/JNK activation induced by CD27. Recently, a TRAF2-interacting kinase has been identified, termed NF-κB-inducing kinase (NIK). A kinase-inactive mutant NIK blocked CD27-, TRAF2-, and TRAF5-mediated NF-κB and SAPK/JNK activation. These results indicate that TRAF2 and TRAF5 are involved in NF-κB and SAPK/JNK activation by CD27, and NIK is a common downstream kinase of TRAF2 and TRAF5 for NF-κB and SAPK/JNK activation.

Transient and Selective NF-κB p65 Serine 536 Phosphorylation Induced by T Cell Costimulation Is Mediated by IκB Kinase β and Controls the Kinetics of p65 Nuclear Import

Full transcriptional activity of the nuclear, DNA-bound form of NF-κB requires additional posttranslational modifications. In this study, we systematically mapped the T cell costimulation-induced phosphorylation sites within the C-terminal half of the strongly trans-activating NF-κB p65 subunit and identified serine 536 as the main phosphorylation site. The transient kinetics of serine 536 phosphorylation paralleled the kinetics of IκBα and IκB kinase (IKK) phosphorylation and also mirrored the principle of T cell costimulation. The TCR-induced pathway leading to serine 536 phosphorylation is regulated by the kinases Cot (Tpl2), receptor interacting protein, protein kinase Cθ, and NF-κB-inducing kinase, but is independent from the phosphatidylinositol 3-kinase/Akt signaling pathway. Loss-of-function and gain-of-function experiments showed phosphorylation of p65 serine 536 by IKKβ, but not by IKKα. Phosphorylation occurs within the cytoplasmic and intact NF-κB/IκBα complex and requires prior phosphorylation of IκBα at serines 32 and 36. Reconstitution of p65−/− cells either with wild-type p65 or a p65 mutant containing a serine to alanine mutation revealed the importance of this phosphorylation site for cytosolic IκBα localization and the kinetics of p65 nuclear import.