The TAK1 MAPKKK mediates activation of JNK and NF-KB in the IL-1-activated signaling pathway. Here we report the identification of TAB2, a novel intermediate in the IL-1 pathway that functionally links TAK1 to TRAF6. Expression of TAB2 induces JNK and NF-kappaB activation, whereas a dominant-negative mutant TAB2 impairs their activation by IL-1. IL-1 stimulates translocation of TAB2 from the membrane to the cytosol where it mediates the IL-1-dependent association of TAK1 with TRAF6. These results define TAB2 as an adaptor linking TAK1 and TRAF6 and as a mediator of TAK1 activation in the IL-1 signaling pathway.

Wnt signaling controls a variety of developmental processes. The canonical Wnt/β-catenin pathway functions to stabilize β-catenin, and the noncanonical Wnt/Ca2+ pathway activates Ca2+/calmodulin-dependent protein kinase II (CaMKII). In addition, the Wnt/Ca2+ pathway activated by Wnt-5a antagonizes the Wnt/β-catenin pathway via an unknown mechanism. The mitogen-activated protein kinase (MAPK) pathway composed of TAK1 MAPK kinase kinase and NLK MAPK also negatively regulates the canonical Wnt/β-catenin signaling pathway. Here we show that activation of CaMKII induces stimulation of the TAK1-NLK pathway. Overexpression of Wnt-5a in HEK293 cells activates NLK through TAK1. Furthermore, by using a chimeric receptor (β2AR-Rfz-2) containing the ligand-binding and transmembrane segments from the β2-adrenergic receptor (β2AR) and the cytoplasmic domains from rat Frizzled-2 (Rfz-2), stimulation with the β-adrenergic agonist isoproterenol activates activities of endogenous CaMKII, TAK1, and NLK and inhibits β-catenin-induced transcriptional activation. These results suggest that the TAK1-NLK MAPK cascade is activated by the noncanonical Wnt-5a/Ca2+ pathway and antagonizes canonical Wnt/β-catenin signaling.

[Keywords: Myb; HIPK2; NLK; TAK1; phosphorylation; degradation] Supplemental material is available at http://www.genesdev.org.

Interleukin-1 (IL-1) is a proinflammatory cytokine that recognizes a surface receptor complex and generates multiple cellular responses. IL-1 stimulation activates the mitogen-activated protein kinase kinase kinase TAK1, which in turn mediates activation of c-Jun N-terminal kinase and NF-B. TAB2 has previously been shown to interact with both TAK1 and TRAF6 and promote their association, thereby triggering subsequent IL-1 signaling events. The serine/threonine kinase IL-1 receptor-associated kinase (IRAK) also plays a role in IL-1 signaling, being recruited to the IL-1 receptor complex early in the signal cascade. In this report, we investigate the role of IRAK in the activation of TAK1. Genetic analysis reveals that IRAK is required for IL-1-induced activation of TAK1. We show that IL-1 stimulation induces the rapid but transient association of IRAK, TRAF6, TAB2, and TAK1. TAB2 is recruited to this complex following translocation from the membrane to the cytosol upon IL-1 stimulation. In IRAK-deficient cells, TAB2 translocation and its association with TRAF6 are abolished. These results suggest that IRAK regulates the redistribution of TAB2 upon IL-1 stimulation and facilitates the formation of a TRAF6-TAB2-TAK1 complex. Formation of this complex is an essential step in the activation of TAK1 in the IL-1 signaling pathway.Interleukin-1 (IL-1) plays a central role in eliciting a variety of inflammatory responses. These responses to IL-1 are mediated by a cascade of intracellular signaling events, including activation of c-Jun N-terminal kinase (JNK) and nuclear transcription factor B (NF-B) (6). IL-1 signaling is initiated by the formation of a high-affinity complex composed of IL-1, the IL-1 receptor (IL-1RI), and the IL-1 receptor accessory protein (IL-1RAcP) ( Fig. 1) (8,12,42). Formation of this complex causes the intracellular adapter molecule MyD88 to be recruited to the complex, which in turn facilitates the association of the serine/threonine IL-1 receptor-associated kinase (IRAK) (2, 3, 29, 41). Next, IRAK dissociates from the receptor complex and interacts with TRAF6, a factor required for IL-1-induced JNK and NF-B activation (4, 25). The mechanism by which TRAF6 is then able to activate the JNK and NF-B pathways is not understood.In unstimulated cells, NF-B resides in the cytoplasm in an inactive form, due to its association with the inhibitory IB proteins. Following stimulation with IL-1, the IB proteins are specifically phosphorylated and degraded through a ubiquitin proteasome-dependent mechanism. Proteolysis of IB releases NF-B and allows it to translocate to the nucleus, where it activates transcription of specific target genes (1, 37, 39). The kinases responsible for phosphorylating IB are known as the IB kinases (IKKs) IKK␣/IKK1 and IKK␤/IKK2 (5,14,28,31,43,47), and they form a large multiprotein complex that also contains NEMO/IKK␥ (32, 45). Although the IKKs themselves can be activated by members of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, including MEKK1 (19,20) a...

The mammalian mitogen-activated protein (MAP) kinase kinase kinase apoptosis signal-regulating kinase 1 (ASK1) is a pivotal component in cytokine-and stress-induced apoptosis. It also regulates cell differentiation and survival through p38 MAP kinase activation. Here we show that Ca 2 þ signalling regulates the ASK1-p38 MAP kinase cascade. Ca 2 þ influx evoked by membrane depolarization in primary neurons and synaptosomes induced activation of p38, which was impaired in those derived from ASK1-deficient mice. Ca 2 þ /calmodulin-dependent protein kinase type II (CaMKII) activated ASK1 by phosphorylation. Moreover, p38 activation induced by the expression of constitutively active CaMKII required endogenous ASK1. Thus, ASK1 is a critical intermediate of Ca 2 þ signalling between CaMKII and p38 MAP kinase.

The heparin-binding growth factor midkine (MK) comprises a family with pleiotrophin/heparin-binding growth-associated molecule. The biological phenomena in which MK is involved can be categorized into five areas: (i) cancer, (ii) inflammation/immunity, (iii) blood pressure, (iv) development and (v) tissue protection. The phenotypes are clear in vivo, but the mechanisms by which MK exerts these actions are not fully understood. Candidate receptors for MK include anaplastic lymphoma kinase, protein tyrosine phosphatase ζ, Notch2, LDL receptor-related protein 1, integrins and proteoglycans. Some physical associations between these candidate receptors are also known. Because of the striking in vivo phenotypes after manipulation of MK, MK could be an important molecular target for the treatment of various diseases. To this end, it will be important to pursue studies to fully understand the mechanisms of MK action.

TAK1 mitogen-activated protein kinase kinase kinase participates in the Interleukin-1 (IL-1) signaling pathway by mediating activation of JNK, p38, and NF-κ κ κ κ B. TAK1-binding protein 2 (TAB2) was previously identified as an adaptor that links TAK1 to an upstream signaling intermediate, tumor necrosis factor receptor-associated factor 6 (TRAF6). Recently, ubiquitination of TRAF6 was shown to play an essential role in the activation of TAK1. However, the mechanism by which IL-1 induces TRAF6 ubiquitination remains to be elucidated. Here we report that TAB2 functions to facilitate TRAF6 ubiquitination and thereby mediates IL-1-induced cellular events. A conserved ubiquitin binding domain in TAB2, the CUE domain, is important for this function. We also found that TAB2 promotes the assembly of TRAF6 with a downstream kinase, Iκ κ κ κ B kinase (IKK). These results show that TAB2 acts as a multifunctional signaling molecule, facilitating both IL-1-dependent TRAF6 ubiquitination and assembly of the IL-1 signaling complex.

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