A resorcylic acid lactone, 5Z-7-oxozeaenol, prevents inflammation by inhibiting the catalytic activity of TAK1 MAPK kinase kinase

Epoxyquinol B, a naturally occurring pentaketide dimer, inhibits NF-kappaB signaling by crosslinking TAK1

Several epoxyquinoids interfere with NF-kappaB signaling by targeting IKKbeta or NF-kappaB. We report that epoxyquinol B (EPQB), classified as an epoxyquiniod, inhibits NF-kappaB signaling through inhibition of the TAK1 complex, a factor upstream of IKKbeta and NF-kappaB. cDNA microarray analysis revealed that EPQB decreased TNF-alpha-induced expression of NF-kappaB target genes. EPQB covalently bound to a recombinant TAK1-TAB1 fusion protein in vitro, and inhibited its kinase activity. Furthermore, in vitro/in situ treatment with EPQB resulted in a ladder-like hypershift of TAK1 protein bands. We reported recently that EPQB crosslinks proteins via cysteine residues by opening its two epoxides, and our current results suggest that EPQB inhibits NF-kappaB signaling by crosslinking TAK1 itself or TAK1 through other proteins.

Genes for the biosynthesis of the fungal polyketides hypothemycin from Hypomyces subiculosus and radicicol from Pochonia chlamydosporia

Gene clusters for biosynthesis of the fungal polyketides hypothemycin and radicicol from Hypomyces subiculosus and Pochonia chlamydosporia, respectively, were sequenced. Both clusters encode a reducing polyketide synthase (PKS) and a nonreducing PKS like those in the zearalenone cluster of Gibberella zeae, plus enzymes with putative post-PKS functions. Introduction of an O-methyltransferase (OMT) knockout construct into H. subiculosus resulted in a strain with increased production of 4-O-desmethylhypothemycin, but because transformation of H. subiculosus was very difficult, we opted to characterize hypothemycin biosynthesis using heterologous gene expression. In vitro, the OMT could methylate various substrates lacking a 4-O-methyl group, and the flavin-dependent monooxygenase (FMO) could epoxidate substrates with a 1',2' double bond. The glutathione S-transferase catalyzed cis-trans isomerization of the 7',8' double bond of hypothemycin. Expression of both hypothemycin PKS genes (but neither gene alone) in yeast resulted in production of trans-7',8'-dehydrozearalenol (DHZ). Adding expression of OMT, expression of FMO, and expression of cytochrome P450 to the strain resulted in methylation, 1',2'-epoxidation, and hydroxylation of DHZ, respectively. The radicicol gene cluster encodes halogenase and cytochrome P450 homologues that are presumed to catalyze chlorination and epoxidation, respectively. Schemes for biosynthesis of hypothemycin and radicicol are proposed. The PKSs encoded by the two clusters described above and those encoded by the zearalenone cluster all synthesize different products, yet they have significant sequence identity. These PKSs may provide a useful system for probing the mechanisms of fungal PKS programming.

A polyketide macrolactone synthase from the filamentous fungus Gibberella zeae

Resorcylic acid lactones represent a unique class of fungal polyketides and display a wide range of biological activities, such as nanomolar inhibitors of Hsp90 and MAP kinase. The biosynthesis of these compounds is proposed to involve two fungal polyketide synthases (PKS) that function collaboratively to yield a 14-membered macrolactone with a resorcylate core. We report here the reconstitution of Gibberella zeae PKS13, which is the nonreducing PKS associated with zearalenone biosynthesis. Using a small molecule mimic of the natural hexaketide starter unit, we reconstituted the entire repertoire of PKS13 activities in vitro, including starter-unit selection, iterative condensation, regioselective C2-C7 cyclization, and macrolactone formation. PKS13 synthesized both natural 14-membered and previously uncharacterized 16-membered resorcylic acid lactones, indicating relaxed control in both iterative elongation and macrocyclization. PKS13 exhibited broad starter-unit specificities toward fatty acyl-CoAs ranging in sizes between C6 and C16 and displayed the highest activity toward decanoyl-CoA. PKS13 was shown to be active in Escherichia coli and synthesized numerous alkyl pyrones and alkyl resorcylic esters without exogenously supplied precursors. We demonstrated that PKS13 can interact with E. coli fatty acid biosynthetic machinery and can be primed with fatty-acyl ACPp at low-micromolar concentrations. PKS13 synthesized new polyketides in E. coli when the culture was supplemented with synthetic precursors, showcasing its utility in precursor-directed biosynthesis. PKS13 is therefore a highly versatile polyketide macrolactone synthase that is useful in the engineered biosynthesis of polyketides, including resorcylic acid lactones that are not found in nature.

Altered TAB1:I kappaB kinase interaction promotes transforming growth factor beta-mediated nuclear factor-kappaB activation during breast cancer progression

The conversion of transforming growth factor beta (TGF-beta) from a tumor suppressor to a tumor promoter occurs frequently during mammary tumorigenesis, yet the molecular mechanisms underlying this phenomenon remain undefined. We show herein that TGF-beta repressed nuclear factor-kappaB (NF-kappaB) activity in normal NMuMG cells, but activated this transcription factor in their malignant counterparts, 4T1 cells, by inducing assembly of TGF-beta-activated kinase 1 (TAK1)-binding protein 1 (TAB1):I kappaB kinase beta (IKK beta) complexes, which led to the stimulation of a TAK1:IKK beta:p65 pathway. TAB1:IKK beta complexes could only be detected in NMuMG cells following their induction of epithelial-mesenchymal transition (EMT), which, on TGF-beta treatment, activated NF-kappaB. Expression of a truncated TAB1 mutant [i.e., TAB1(411)] reduced basal and TGF-beta-mediated NF-kappaB activation in NMuMG cells driven to undergo EMT by TGF-beta and in 4T1 cells stimulated by TGF-beta. TAB1(411) expression also inhibited TGF-beta-stimulated tumor necrosis factor-alpha and cyclooxygenase-2 expression in 4T1 cells. Additionally, the ability of human MCF10A-CA1a breast cancer cells to undergo invasion in response to TGF-beta absolutely required the activities of TAK1 and NF-kappaB. Moreover, small interfering RNA-mediated TAK1 deficiency restored the cytostatic activity of TGF-beta in MCF10A-CA1a cells. Finally, expression of truncated TAB1(411) dramatically reduced the growth of 4T1 breast cancers in syngeneic BALB/c, as well as in nude mice, suggesting a potentially important role of NF-kappaB in regulating innate immunity by TGF-beta. Collectively, our findings have defined a novel TAB1:TAK1:IKK beta:NF-kappaB signaling axis that forms aberrantly in breast cancer cells and, consequently, enables oncogenic signaling by TGF-beta.

The role of RIP2 in p38 MAPK activation in the stressed heart

The activation of p38 MAPK by dual phosphorylation aggravates myocardial ischemic injury and depresses cardiac contractile function. SB203580, an ATP-competitive inhibitor of p38 MAPK and other kinases, prevents this dual phosphorylation during ischemia. Studies in non-cardiac tissue have shown receptor-interacting protein 2 (RIP2) lies upstream of p38 MAPK, is SB203580-sensitive and ischemia-responsive, and aggravates ischemic injury. We therefore examined the RIP2-p38 MAPK signaling axis in the heart. Adenovirus-driven expression of wild-type RIP2 in adult rat ventricular myocytes caused robust, SB203580-sensitive dual phosphorylation of p38 MAPK associated with activation of p38 MAPK kinases MKK3, MKK4, and MKK6. The effect of SB203580 was recapitulated by unrelated inhibitors of RIP2 or the downstream MAPK kinase kinase, TAK1. However, overexpression of wild-type, kinase-dead, caspase recruitment domain-deleted, or kinase-dead and caspase recruitment domain-deleted forms of RIP2 had no effect on the activating dual phosphorylation of p38 MAPK during simulated ischemia. Similarly, p38 MAPK activation and myocardial infarction size in response to true ischemia did not differ between hearts from wild-type and RIP2 null mice. However, both p38 MAPK activation and the contractile depression caused by the endotoxin component muramyl dipeptide were attenuated by SB203580 and in RIP2 null hearts. Although RIP2 can cause myocardial p38 MAPK dual phosphorylation in the heart under some circumstances, it is not responsible for the SB203580-sensitive pattern of activation during ischemia.

Total synthesis of aigialomycin D: surprising chemoselectivity dependence on alkyne structure in nickel-catalyzed cyclizations

The total synthesis of aigialomycin D was carried out using a nickel-catalyzed ynal macrocyclization as a key step. This key step allowed macrocycle assembly and formation of a disubstituted alkene and a secondary hydroxyl stereocenter in a single step, although the stereocenter was formed unselectively. An interesting side reaction involving five-membered-ring synthesis by an aldehyde/styrene cyclization was observed when macrocyclization of an alkynyl silane was attempted. A mechanistic basis for this surprising process is provided.

The effect of the fungal metabolite radicicol analog A on mRNA degradation

The AU-rich element (ARE) is a stability determinant found in the 3' UTR of a number of short-lived mRNAs. The best characterized ARE is the Shaw-Kamen (SK) box or AUUUA motif. Previously, a fungal metabolite, radicicol analog A (RAA), was shown to destabilize SK box-containing mRNAs based on 16 mRNAs examined [T. Kastelic et al., Cytokine 8 (1996) 751-761]. Using serial analysis of gene expression (SAGE) to examine the global effect of RAA on mRNA expression in interferon-gamma/lipopolysaccharide-stimulated THP-1 human monocytes, we observed that the expression level of greater than 99% of the SAGE tags was unchanged by RAA treatment and only 34 of the 17,608 unique tags annotated were reduced (p< or =0.0001). RAA destabilized approximately half of the down-regulated transcripts. Whereas all the destabilized mRNAs possessed at least one SK box, for transcripts not destabilized but nonetheless down-regulated, RAA appears to function by a SK box-independent mechanism not currently understood.

Constitutive activation of TAK1 by HTLV-1 tax-dependent overexpression of TAB2 induces activation of JNK-ATF2 but not IKK-NF-kappaB

HTLV-1 Tax oncoprotein induces persistent activation of the transcription factor NF-kappaB and CREB (cAMP-response element-binding protein)/ATF. Transforming growth factor-beta-activated kinase 1 (TAK1) has been shown to play a critical role in these transcription factors. Here, we found that TAK1 was constitutively activated in Tax-positive HTLV-1-transformed T cells. Tax induced persistent overexpression of TAK1-binding protein 2 (TAB2), but not TAB3, which is essential for TAK1 activation. Surprisingly, TAK1 was not involved in the activation of NF-kappaB. On the other hand, JNK and p38 mitogen-activated protein kinases were activated by TAK1. In addition, ATF2, but not CREB, was a target for the TAK1-JNK pathway, and p38 negatively regulated TAK1 activity through TAB1 phosphorylation. These results indicate that Tax-mediated TAK1 activation is important for the activation of ATF2 rather than NF-kappaB.

YopJ targets TRAF proteins to inhibit TLR-mediated NF-kappaB, MAPK and IRF3 signal transduction

The Yersinia pestis virulence factor YopJ is a potent inhibitor of the NF-kappaB and MAPK signalling pathways, however, its molecular mechanism and relevance to pathogenesis are the subject of much debate. In this report, we characterize the effects of this type III effector protein on bone fide signalling events downstream of Toll-like receptors (TLRs), critical sensors in innate immunity. YopJ inhibited TLR-mediated NF-kappaB and MAP kinase activation, as suggested by previous studies. In addition, induction of the TLR-mediated interferon response was blocked by YopJ, indicating that YopJ also inhibits IRF3 signalling. Examination of the NF-kappaB signalling pathway in detail suggested that YopJ acts at the level of TAK1 (MAP3K7) activation. Further studies revealed a YopJ-dependent decrease in the ubiquitination of TRAF3 and TRAF6. These data support the hypothesis that YopJ is a deubiquitinating protease that acts on TRAF proteins to prevent or remove the K63-polymerized ubiquitin conjugates required for signal transduction. Our data do not directly address the alternative hypothesis that YopJ is an acetyltransferase that acts on the activation loop of IKK and MKK proteins, but support the conclusion that the critical function of YopJ is to deubiquinate TRAF proteins.

Molecular mechanisms involved in the regulation of cytokine production by muramyl dipeptide

MDP (muramyl dipeptide), a component of peptidoglycan, interacts with NOD2 (nucleotide-binding oligomerization domain 2) stimulating the NOD2-RIP2 (receptor-interacting protein 2) complex to activate signalling pathways important for antibacterial defence. Here we demonstrate that the protein kinase activity of RIP2 has two functions, namely to limit the strength of downstream signalling and to stabilize the active enzyme. Thus pharmacological inhibition of RIP2 kinase with either SB 203580 [a p38 MAPK (mitogen-activated protein kinase) inhibitor] or the Src family kinase inhibitor PP2 induces a rapid and drastic decrease in the level of the RIP2 protein, which may explain why these RIP2 inhibitors block MDP-stimulated downstream signalling and the production of IL-1beta (interleukin-1beta) and TNFalpha (tumour necrosis factor-alpha). We also show that RIP2 induces the activation of the protein kinase TAK1 (transforming-growth-factor-beta-activated kinase-1), that a dominant-negative mutant of TAK1 inhibits RIP2-induced activation of JNK (c-Jun N-terminal kinase) and p38alpha MAPK, and that signalling downstream of NOD2 or RIP2 is reduced by the TAK1 inhibitor (5Z)-7-oxozeaenol or in TAK1-deficient cells. We also show that MDP activates ERK1 (extracellular-signal-regulated kinase 1)/ERK2 and p38alpha MAPK in human peripheral-blood mononuclear cells and that the activity of both MAPKs and TAK1 are required for MDP-induced signalling and production of IL-1beta and TNFalpha in these cells. Taken together, our results indicate that the MDP-NOD2/RIP2 and LPS (lipopolysaccharide)-TLR4 (Toll-like receptor 4) signalling pathways converge at the level of TAK1 and that many subsequent events that lead to the production of pro-inflammatory cytokines are common to both pathways.

Phosphorylation and ubiquitination of the IkappaB kinase complex by two distinct signaling pathways

The IkappaB kinase (IKK) complex serves as the master regulator for the activation of NF-kappaB by various stimuli. It contains two catalytic subunits, IKKalpha and IKKbeta, and a regulatory subunit, IKKgamma/NEMO. The activation of IKK complex is dependent on the phosphorylation of IKKalpha/beta at its activation loop and the K63-linked ubiquitination of NEMO. However, the molecular mechanism by which these inducible modifications occur remains undefined. Here, we demonstrate that CARMA1, a key scaffold molecule, is essential to regulate NEMO ubiquitination upon T-cell receptor (TCR) stimulation. However, the phosphorylation of IKKalpha/beta activation loop is independent of CARMA1 or NEMO ubiquitination. Further, we provide evidence that TAK1 is activated and recruited to the synapses in a CARMA1-independent manner and mediate IKKalpha/beta phosphorylation. Thus, our study provides the biochemical and genetic evidence that phosphorylation of IKKalpha/beta and ubiquitination of NEMO are regulated by two distinct pathways upon TCR stimulation.

Synthesis of Macrolide−Saccharide Hybrids by Ring-Closing Metathesis of Precursors Derived from Glycitols and Benzoic Acids

The benzomacrolactone structural motif is a privileged or evolutionarily selected scaffold that codes properties required for binding to proteins and novel analogues thereof may provide a source of new bioactive compounds. Saccharides are also privileged structures, with (amino)sugars, iminosugars, and sugar amino acids being applied as scaffolds for the development of nonpeptidal peptidomimetics. The syntheses of novel polyhydroxylated oxamacrolides, structural analogues of natural polyketide derived macrolides, are described herein, providing a basis for their development as scaffolds. The syntheses were carried out from benzoic acids and appropriately protected D-mannitol or D-sorbitol (D-glucitol). Ring-closing metathesis was applied in the macrocyclization step with high E-alkene selectivities being observed. X-ray crystal structures, for two polyhydroxylated derivatives, show that the macrocyclic rings display similar conformations. In addition, intermolecular hydrogen-bonding networks are observed in the lattices.

TAK1 MAPK kinase kinase mediates transforming growth factor-beta signaling by targeting SnoN oncoprotein for degradation

Transforming growth factor-beta (TGF-beta) regulates a variety of physiologic processes through essential intracellular mediators Smads. The SnoN oncoprotein is an inhibitor of TGF-beta signaling. SnoN recruits transcriptional repressor complex to block Smad-dependent transcriptional activation of TGF-beta-responsive genes. Following TGF-beta stimulation, SnoN is rapidly degraded, thereby allowing the activation of TGF-beta target genes. Here, we report the role of TAK1 as a SnoN protein kinase. TAK1 interacted with and phosphorylated SnoN, and this phosphorylation regulated the stability of SnoN. Inactivation of TAK1 prevented TGF-beta-induced SnoN degradation and impaired induction of the TGF-beta-responsive genes. These data suggest that TAK1 modulates TGF-beta-dependent cellular responses by targeting SnoN for degradation.

Interleukin-1 (IL-1)-induced TAK1-dependent Versus MEKK3-dependent NFkappaB activation pathways bifurcate at IL-1 receptor-associated kinase modification

Interleukin-1 (IL-1) receptor-associated kinase (IRAK) is phosphorylated after it is recruited to the receptor, subsequently ubiquitinated, and eventually degraded upon IL-1 stimulation. Although a point mutation changing lysine 134 to arginine (K134R) in IRAK abolished IL-1-induced IRAK ubiquitination and degradation, mutations of serines and threonines adjacent to lysine 134 to alanines ((S/T)A (131-144)) reduced IL-1-induced IRAK phosphorylation and abolished IRAK ubiquitination. Through the study of these IRAK modification mutants, we uncovered two parallel IL-1-mediated signaling pathways for NFkappaB activation, TAK1-dependent and MEKK3-dependent, respectively. These two pathways bifurcate at the level of IRAK modification. The TAK1-dependent pathway leads to IKKalpha/beta phosphorylation and IKKbeta activation, resulting in classical NFkappaB activation through IkappaBalpha phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKgamma phosphorylation and IKKalpha activation, resulting in NFkappaB activation through IkappaBalpha phosphorylation and subsequent dissociation from NFkappaB but without IkappaBalpha degradation. These results provide significant insight to our further understanding of NFkappaB activation pathways.

Establishment of ex vivo systems to identify compounds acting on innate immune responses and to determine their target molecules using transgenic Drosophila

Innate immunity is an evolutionarily conserved self-defense mechanism against microbial infections. In Drosophila, induction of antimicrobial peptides is a major immune response that is regulated by two distinct signaling pathways called the IMD pathway and the Toll pathway, similar to the tumor necrosis factor-alpha signaling and Toll-like receptor/interleukin-1 signaling pathways, respectively, in mammals. In mammals, innate immunity interacts with adaptive immunity and has a key role in the regulated immune response. Therefore, innate immunity is a pharmaceutical target for the development of immune regulators. Previously, based on the striking conservation between the mechanisms that regulate Drosophila immunity and human innate immunity, we established an ex vivo culture in which compounds acting on innate immunity can be evaluated using a reporter gene that reflects activation of the IMD pathway [Yajima et al. [Yajima, M., Takada, M., Takahashi, N., Kikuchi, H., Natori, S., Oshima, Y., Kurata, S., 2003. A newly established in vitro culture using transgenic Drosophila reveals functional coupling between the phospholipase A2-generated fatty acid cascade and lipopolysaccharide-dependent activation of the immune deficiency (imd) pathway in insect immunity. The Biochemical Journal 371(Pt 1), 205-210] Biochem J 371, 205-210]. Here, we combined the ex vivo culture with a reporter gene that reflects the heat shock response and demonstrated that the resulting systems are useful for screening compounds that act specifically on innate immunity, including mammalian innate immune responses. Identification of target molecules is essential for the development of more potent medicines with fewer side effects. In this study, we also established ex vivo systems capable of identifying target molecules of the identified compounds using targeted activation of the IMD pathway.

Diversity-Oriented Synthesis of Pochonins and Biological Evaluation against a Panel of Kinases

Pochonins A-F were recently characterized as six new members of the naturally occurring family of 14-membered resorcylic acid lactones. As there are a high number of ATPase and kinase inhibitors among natural resorcylic lactones, a library based on the pochonin scaffold, with five points of diversity, was prepared which includes diversity beyond that of the natural analogues. The library was synthesized by using solid-supported reagents amenable to automation. Testing the library for its inhibition against a panel of 24 kinases at 10 microM afforded a >14 % hit rate. These results demonstrate the potential of the resorcylides towards the inhibition of therapeutically relevant kinases.

TAB1 modulates IL-1alpha mediated cytokine secretion but is dispensable for TAK1 activation

Biochemical evidence indicates that TGF-beta-activated kinase 1 (TAK1), a key modulator of the inflammatory response, exists in a complex with various adaptor proteins including the TAK1 binding protein 1 (TAB1). However, the physiological importance of TAB1 in TAK1 activation, and in the subsequent induction of proinflammatory mediators, remains unclear. In this study, a critical role for TAK1 in IL-1alpha or TNFalpha stimulated MAPK and NFkappaB activation was confirmed by inhibition of the nuclear accumulation of NFkappaB p65 and phosphorylated forms of c-Jun and p38 following siRNA mediated TAK1 silencing. These effects were associated with significant reductions in IL-1alpha stimulated levels of secreted IL-6, IL-8, MCP-1 and GM-CSF. In contrast, IL-1alpha or TNFalpha dependent cellular redistribution of NFkappaB p65 and phosphorylated c-Jun and p38 was not affected by 80% siRNA mediated knockdown of TAB1 protein levels. Interestingly, IL-6, IL-8 and GM-CSF release from TAB1 siRNA transfected cells was significantly reduced following IL-1alpha treatment, but was unchanged after TNFalpha stimulation, suggesting differential roles for TAB1 in IL-1alpha and TNFalpha signalling pathways. These findings may imply an as yet unidentified role for TAB1 in the inflammatory response, which is independent of the activation of classical TAK1 associated signalling cascades.

Chemistry and biology of resorcylic acid lactones

While resorcylic acid lactones (RALs) have been known for a long time, the more recent discoveries that radicicol is a potent and selective HSP90 inhibitor while other members such as hypothemycin, LL-Z1640-2 and LL-783,277 are potent kinase inhibitors have stimulated a renewed interest in this family of natural products. The recent developments regarding the chemistry and biology of RALs are reviewed.

The Yersinia enterocolitica effector YopP inhibits host cell signalling by inactivating the protein kinase TAK1 in the IL-1 signalling pathway

The mechanism by which YopP simultaneously inhibits mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB pathways has been elusive. Ectopic expression of YopP inhibits the activity and ubiquitination of a complex consisting of overexpressed TGF-beta-activated kinase 1 (TAK1) and its subunit TAK1-binding protein (TAB)1, but not of MEK kinase 1. YopP, but not the catalytically inactive mutant YopP(C172A), also suppresses basal and interleukin-1-inducible activation of endogenous TAK1, TAB1 and TAB2. YopP does not affect the interaction of TAK1, TAB1 and TAB2 but inhibits autophosphorylation of TAK1 at Thr 187 and phosphorylation of TAB1 at Ser 438. Glutathione S-transferase-tagged YopP (GST-YopP) binds to MAPK kinase (MAPKK)4 and TAB1 but not to TAK1 or TAB2 in vitro. Furthermore, YopP in synergy with a previously described negative regulatory feedback loop inhibits TAK1 by MAPKK6-p38-mediated TAB1 phosphorylation. Taken together, these data strongly suggest that YopP binds to TAB1 and directly inhibits TAK1 activity by affecting constitutive TAK1 and TAB1 ubiquitination that is required for autoactivation of TAK1.

Solution- and solid-phase synthesis of radicicol (monorden) and pochonin C

A modular synthesis for pochonin C and radicicol is reported. The two natural products were prepared in seven and eight steps, respectively, from three readily available fragments. Alternative syntheses of these compounds were achieved using a combination of polymer-bound reagents and solid phase reactions. The conformation of the two natural products was studied and compared by using 2D NMR spectroscopy.

Targeted covalent inactivation of protein kinases by resorcylic acid lactone polyketides

Resorcylic acid lactones containing a cis-enone are susceptible to Michael addition reactions and are potent inhibitors of several protein kinases. A structural-bioinformatics analysis identified a conserved Cys residue in the ATP-binding site of the kinases reported to be inhibited by cis-enone resorcylic acid lactones but absent in those that are not. Mining of the kinome database revealed that a subset of some 46 kinases contained this Cys residue. Screening a panel of 124 kinases with the resorcylic acid lactone hypothemycin showed that 18 of 19 targets containing the conserved Cys were inhibited. Kinetic analyses showed time-dependent inhibition, a hallmark of covalent inactivation, and biochemical studies of the interaction of extracellular signal-regulated kinase (ERK)2 with hypothemycin confirmed covalent adduct formation. Resorcylic acid lactones are unique among kinase inhibitors in that they target mitogen-activated protein (MAP) kinase pathways at four levels: mitogen receptors, MAP kinase kinase (MEK)1/2 and ERK1/2, and certain downstream ERK substrates. Cell lines dependent on the activation of Tyr kinase mitogen receptor targets of the resorcylic acid lactones were unusually sensitive toward hypothemycin and showed the expected inhibition of kinase phosphorylation due to inhibition of the mitogen receptors and/or MEK1/2 and ERK1/2. Among cells without mitogen receptor targets, those harboring an ERK pathway-activating B-RAF V600E mutation were selectively and potently inhibited by hypothemycin. Hypothemycin also prevented stimulated activation of the p38 cascade through inhibition of the Cys-containing targets MEK3/6 and TGF-beta-activated kinase 1 and of the JNK/SAPK (c-Jun N-terminal kinase/stress-activated protein kinase) cascade through inhibition of MEK4/7.

TAK1 is a component of the Epstein-Barr virus LMP1 complex and is essential for activation of JNK but not of NF-kappaB

Epstein-Barr virus latent membrane protein 1 (LMP1) activates NF-kappaB and c-Jun N-terminal kinase (JNK), which is essential for LMP1 oncogenic activity. Genetic analysis has revealed that tumor necrosis factor receptor-associated factor 6 (TRAF6) is an indispensable intermediate of LMP1 signaling leading to activation of both NF-kappaB and JNK. However, the mechanism by which LMP1 engages TRAF6 for activation of NF-kappaB and JNK is not well understood. Here we demonstrate that TAK1 mitogen-activated protein kinase kinase kinase and TAK1-binding protein 2 (TAB2), together with TRAF6, are recruited to LMP1 through its N-terminal transmembrane region. The C-terminal cytoplasmic region of LMP1 facilitates the assembly of this complex and enhances activation of JNK. In contrast, IkappaB kinase gamma is recruited through the C-terminal cytoplasmic region and this is essential for activation of NF-kappaB. Furthermore, we found that ablation of TAK1 resulted in the loss of LMP1-induced activation of JNK but not of NF-kappaB. These results suggest that an LMP1-associated complex containing TRAF6, TAB2, and TAK1 plays an essential role in the activation of JNK. However, TAK1 is not an exclusive intermediate for NF-kappaB activation in LMP1 signaling.

Structural Basis for the Interaction of TAK1 Kinase with its Activating Protein TAB1

Transforming growth factor-beta (TGF-beta)-activated kinase 1 (TAK1) is a member of the MAPKKK family of protein kinases, and is involved in intracellular signalling pathways stimulated by transforming growth factor beta, interleukin-1 and tumour necrosis factor-alpha. TAK1 is known to rely upon an additional protein, TAK1-binding protein 1 (TAB1), for complete activation. However, the molecular basis for this activation has yet to be elucidated. We have solved the crystal structure of a novel TAK1 chimeric protein and these data give insight into how TAK1 is activated by TAB1. Our results reveal a novel binding pocket on the TAK1 kinase domain whose shape complements that of a unique alpha-helix in the TAK1 binding domain of TAB1, providing the basis for an intimate hydrophobic association between the protein activator and its target.

TAK1-mediated transcriptional activation of CD28-responsive element and AP-1-binding site within the IL-2 promoter in Jurkat T cells

We focused on the functional involvement of transforming growth factor-beta-activated kinase 1 (TAK1) in transcriptional regulation of interleukin-2 (IL-2) in T cells. Costimulation of Jurkat cells with 12-O-tetradecanoylphorbol-13-acetate and A23187 leads to a rapid phosphorylation of TAK1 and TAK1-binding protein 1 (TAB1), critical for TAK1 activation. A specific inhibitor of TAK1 blocked production of IL-2. In addition, overexpression of TAK1 and TAB1 induced secretion of IL-2. CD28-responsive element/activator protein-1-binding site (RE/AP) within the IL-2 promoter was a functional target for TAK1. The RE/AP-driven transcription was regulated by TAK1-mediated activation of the c-Jun NH2-terminal kinase, p38 and IkappaB kinase. These results indicate that TAK1 plays a critical role in T cell activation by controlling production of IL-2.

Transforming growth factor beta-activated kinase 1 is a key mediator of ovine follicle-stimulating hormone beta-subunit expression

FSH, a key regulator of gonadal function, contains a beta-subunit (FSHbeta) that is transcriptionally induced by activin, a member of the TGFbeta-superfamily. This study used 4.7 kb of the ovine FSHbeta-promoter linked to luciferase (oFSHbetaLuc) plus a well-characterized activin-responsive construct, p3TPLuc, to investigate the hypothesis that Smad3, TGFbeta-activated kinase 1 (TAK1), or both cause activin-mediated induction of FSH. Overexpression of either Smad3 or TAK1 induced oFSHbetaLuc in gonadotrope-derived LbetaT2 cells as much as activin itself. Induction of p3TPLuc by activin is known to require Smad3 activation in many cell types, and this was true in LbetaT2 cells, where 10-fold induction by activin (2-8 h after activin treatment) was blocked more than 90% by two dominant negative (DN) inhibitors of Smad3 [DN-Smad3 (3SA) and DN-Smad3 (D407E)]. By contrast, 6.5-fold induction of oFSHbetaLuc by activin (10-24 h after activin treatment) was not blocked by either DN-Smad inhibitor, suggesting that activation of Smad3 did not trigger induction of oFSHbetaLuc. By contrast, inhibition of TAK1 by a DN-TAK1 construct led to a 50% decrease in activin-mediated induction of oFSHbetaLuc, and a specific inhibitor of TAK1 (5Z-7-Oxozeanol) blocked induction by 100%, indicating that TAK1 is necessary for activin induction of oFSHbetaLuc. Finally, inhibiting p38-MAPK (often activated by TAK1) blocked induction of oFSHbetaLuc by 60%. In conclusion, the data presented here indicate that activation of TAK1 (and probably p38-MAPK), but not Smad3, is necessary for triggering induction of oFSHbeta by activin.

Ubiquitin signalling in the NF-kappaB pathway

The transcription factor NF-kappaB (nuclear factor kappa enhancer binding protein) controls many processes, including immunity, inflammation and apoptosis. Ubiquitination regulates at least three steps in the NF-kappaB pathway: degradation of IkappaB (inhibitor of NF-kappaB), processing of NF-kappaB precursors, and activation of the IkappaB kinase (IKK). Recent studies have revealed several enzymes involved in the ubiquitination and deubiquitination of signalling proteins that mediate IKK activation through a degradation-independent mechanism.

TAK1-binding protein 2 facilitates ubiquitination of TRAF6 and assembly of TRAF6 with IKK in the IL-1 signaling pathway

TAK1 mitogen-activated protein kinase kinase kinase participates in the Interleukin-1 (IL-1) signaling pathway by mediating activation of JNK, p38, and NF-kappaB. 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, IkappaB 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.

Simultaneous blockade of NFkappaB, JNK, and p38 MAPK by a kinase-inactive mutant of the protein kinase TAK1 sensitizes cells to apoptosis and affects a distinct spectrum of tumor necrosis factor [corrected] target genes

The inflammatory response is characterized by the induction (or repression) of hundreds of genes. The activity of many of these genes is controlled by MAPKs and the IkappaB kinase-NFkappaB pathway. To reveal the effects of blocking these pathways simultaneously, fibroblasts were infected with retroviruses encoding TAK1K63W, an inactive mutant of the protein kinase TAK1. Expression of this protein inhibited tumor necrosis factor (TNF)-induced activation of NFkappaB, JNK, and p38 MAPK and sensitized the cells to TNF-induced apoptosis. 23 different microarray experiments were used to analyze the expression of >7000 genes in these cells. We identified 518 genes that were regulated by TNF in both TAK1K63W-expressing cells and control cells, 37 genes induced by TNF only when TAK1K63W was present, and 48 TNF-induced genes that were suppressed by TAK1K63W. The TNF-inducible genes that were most strongly suppressed by TAK1K63W, ccl2, ccl7, ccl5, cxcl1, cxcl5, cxcl10, saa3, and slpi also had much lower basal levels of expression, indicating that TAK1 also played a role in their normal expression. Chromatin immunoprecipitation studies on four of these genes suggested that inactivation of TAK1 activity led to direct suppression of expression at the transcriptional level because of impaired recruitment of RNA polymerase II to their promoters. ccl2 induction by TNF or interleukin-1 was also suppressed in cells that expressed TAK1 antisense RNA or that were genetically deficient in JNK1/2 or p65 NFkappaB. These data suggest that regulation of the expression of a selected group of inflammation-related genes is funneled through TAK1, making it a potentially useful target for more specific anti-inflammatory drug development.

TAB2 and TAB3 activate the NF-kappaB pathway through binding to polyubiquitin chains

The activation of NF-kappaB and IKK requires an upstream kinase complex consisting of TAK1 and adaptor proteins such as TAB1, TAB2, or TAB3. TAK1 is in turn activated by TRAF6, a RING domain ubiquitin ligase that facilitates the synthesis of lysine 63-linked polyubiquitin chains. Here we present evidence that TAB2 and TAB3 are receptors that bind preferentially to lysine 63-linked polyubiquitin chains through a highly conserved zinc finger (ZnF) domain. Mutations of the ZnF domain abolish the ability of TAB2 and TAB3 to bind polyubiquitin chains, as well as their ability to activate TAK1 and IKK. Significantly, replacement of the ZnF domain with a heterologous ubiquitin binding domain restored the ability of TAB2 and TAB3 to activate TAK1 and IKK. We also show that TAB2 binds to polyubiquitinated RIP following TNFalpha stimulation. These results indicate that polyubiquitin binding domains represent a new class of signaling domains that regulate protein kinase activity through a nonproteolytic mechanism.

The TRAF6 ubiquitin ligase and TAK1 kinase mediate IKK activation by BCL10 and MALT1 in T lymphocytes

The CARD domain protein BCL10 and paracaspase MALT1 are essential for the activation of IkappaB kinase (IKK) and NF-kappaB in response to T cell receptor (TCR) stimulation. Here we present evidence that TRAF6 ubiquitin ligase and TAK1 protein kinase mediate IKK activation by BCL10 and MALT1. RNAi-mediated silencing of MALT1, TAK1, TRAF6, and TRAF2 suppressed TCR-dependent IKK activation and interleukin-2 production in T cells. Furthermore, we have reconstituted the pathway from BCL10 to IKK activation in vitro with purified proteins of MALT1, TRAF6, TAK1, and ubiquitination enzymes including Ubc13/Uev1A. We find that a small fraction of BCL10 and MALT1 proteins form high molecular weight oligomers. Strikingly, only these oligomeric forms of BCL10 and MALT1 can activate IKK in vitro. The MALT1 oligomers bind to TRAF6, induce TRAF6 oligomerization, and activate the ligase activity of TRAF6 to polyubiquitinate NEMO. These results reveal an oligomerization --> ubiquitination --> phosphorylation cascade that culminates in NF-kappaB activation in T lymphocytes.

A dominant negative TAK1 inhibits cellular fibrotic responses induced by TGF-beta

Transforming growth factor-beta (TGF-beta) is crucially virulent in the progression of fibrotic disorders. TAK1 (TGF-beta activated kinase 1) is one of the mitogen-activated kinase kinase kinase (MAPKKK) that is involved in TGF-beta signal transduction. To elucidate the importance of TAK1 in TGF-beta-induced fibrotic marker expression, we investigated whether dominant negative TAK1 could suppress TGF-beta signaling. Based on the finding that TAB1 (TAK1 binding protein 1) binding to TAK1 is required for TAK1 activation, a minimal portion of TAK1 lacking kinase activity that binds to TAB1 was designed as a TAK1 dominant negative inhibitor (TAK1-DN). The effect of TAK1-DN was assessed in the cells that respond to TGF-beta stimulation and that lead to the increase in production of extracellular matrix (ECM) proteins. TAK1-DN, indeed, decreased the ECM protein production, indicating that TAK1-DN retains the ability to intercept the TGF-beta signaling effectively.

Lack of high affinity fiber receptor activity explains the resistance of ciliated airway epithelia to adenovirus infection

Although recombinant adenoviruses are attractive vectors for gene transfer to airway epithelia, they have proven to be relatively inefficient. To investigate the mechanisms of adenovirus-mediated gene transfer to airway epithelia, we examined the role of adenovirus fiber and penton base, the two proteins involved in attachment to and entry of virus into the cell. We used human airway epithelia grown under conditions that allow differentiation and development of a ciliated apical surface that closely resembles the in vivo condition. We found that addition of fiber protein inhibited virus binding and vector-mediated gene transfer to immature airway epithelia, as well as to primary cultures of rat hepatocytes and HeLa cells. However, fiber protein had no effect on vector binding and gene transfer to ciliated airway epithelia. We obtained similar results with addition of penton base protein: the protein inhibited gene transfer to immature epithelia, whereas there was no effect with ciliated epithelia. Moreover, infection was not attenuated with an adenovirus containing a mutation in penton base that prevents the interaction with cell surface integrins. These data suggest that the receptors required for efficient infection by adenovirus are either not present or not available on the apical surface of ciliated human airway epithelia. The results explain the reason for inefficient gene transfer and suggest approaches for improvement.