Characterization of a mutant cell line that does not activate NF-kappaB in response to multiple stimuli

The signaling pathways underlying BDNF-induced Nrf2 hippocampal nuclear translocation involve ROS, RyR-Mediated Ca2+ signals, ERK and PI3K

The neurotrophin Brain-Derived Neurotrophic Factor (BDNF) induces complex neuronal signaling cascades that are critical for the cellular changes underlying synaptic plasticity. These pathways include activation of Ca²⁺ entry via N-methyl-D-aspartate receptors and sequential activation of nitric oxide synthase and NADPH oxidase, which via generation of reactive nitrogen/oxygen species stimulate Ca²⁺-induced Ca²⁺ release mediated by Ryanodine Receptor (RyR) channels. These sequential events underlie BDNF-induced spine remodeling and type-2 RyR up-regulation. In addition, BDNF induces the nuclear translocation of the transcription factor Nrf2, a master regulator of antioxidant protein expression that protects cells against the oxidative damage caused by injury and inflammation. To investigate the possible BDNF-induced signaling cascades that mediate Nrf2 nuclear translocation in primary hippocampal cultures, we tested here whether reactive oxygen species, RyR-mediated Ca²⁺ release, ERK or PI3K contribute to this response. We found that pre-incubation of cultures with inhibitory ryanodine to suppress RyR-mediated Ca²⁺ release, with the reducing agent N-acetylcysteine or with inhibitors of ERK or PI3K activity, prevented the nuclear translocation of Nrf2 induced by incubation for 6 h with BFNF. Based on these combined results, we propose that the key role played by BDNF as an inducer of neuronal antioxidant responses, characterized by BDNF-induced Nfr2 nuclear translocation, entails crosstalk between reactive oxygen species and RyR-mediated Ca²⁺ release, and the participation of ERK and PI3K activities.

Transcription Factor NF-κB: An Update on Intervention Strategies

The nuclear factor (NF)-κB family of transcription factors are ubiquitous and pleiotropic molecules that regulate the expression of more than 150 genes involved in a broad range of processes including inflammation, immunity, cell proliferation, differentiation, and survival. The chronic activation or dysregulation of NF-κB signaling is the central cause of pathogenesis in many disease conditions and, therefore, NF-κB is a major focus of therapeutic intervention. Because of this, understanding the relationship between NF-κB and the induction of various downstream signaling molecules is imperative. In this review, we provide an updated synopsis of the role of NF-κB in DNA repair and in various ailments including cardiovascular diseases, HIV infection, asthma, herpes simplex virus infection, chronic obstructive pulmonary disease, and cancer. Furthermore, we also discuss the specific targets for selective inhibitors and future therapeutic strategies.

Distinct Activation Mechanisms of NF-κB Regulator Inhibitor of NF-κB Kinase (IKK) by Isoforms of the Cell Death Regulator Cellular FLICE-like Inhibitory Protein (cFLIP)

The viral FLICE-like inhibitory protein (FLIP) protein from Kaposi sarcoma-associated herpesvirus activates the NF-κB pathway by forming a stable complex with a central region (amino acids 150-272) of the inhibitor of NF-κB kinase (IKK) γ subunits, thereby activating IKK. Cellular FLIP (cFLIP) forms are also known to activate the NF-κB pathway via IKK activation. Here we demonstrate that cFLIPL, cFLIPS, and their proteolytic product p22-FLIP all require the C-terminal region of NEMO/IKKγ (amino acids 272-419) and its ubiquitin binding function for activation of the IKK kinase (or kinase complex), but none form a stable complex with IKKγ. Our results further reveal that cFLIPLrequires the linear ubiquitin chain assembly complex and the kinase TAK1 for activation of the IKK kinase. Similarly, cFLIPSand p22-FLIP also require TAK1 but do not require LUBAC. In contrast, these isoforms are both components of complexes that incorporate Fas-associated death domain and RIP1, which appear essential for kinase activation. This conservation of IKK activation among the cFLIP family using different mechanisms suggests that the mechanism plays a critical role in their function.

Identifying post-translational modifications of NEMO by tandem mass spectrometry after high affinity purification

An integral component of NF-κB signalling is NEMO, NF-κB essential modulator, a regulatory protein of the IκB kinase (IKK) complex. Post-translational modifications of NEMO, including phosphorylation, SUMOylation, and ubiquitination are critical events during stimuli induced NF-κB activation. Here we demonstrate a method to detect post-translational modifications of NEMO using cells stably expressing polyhistidine tagged NEMO which allows for high-affinity purification of NEMO following rapid denaturing lysis and characterization by MS/MS. We identified a previously uncharacterized basal phosphorylation of NEMO at Serine 387 and tested the biological significance of this phosphorylation through a somatic genetic complementation analysis using the NEMO mutants S387A, S388D, and P388I in 1.3E2 NEMO-deficient murine pre-B cells. NF-κB signalling induced by bacterial lipopolysaccharide, Interleukin-1ß or the DNA damaging agent etoposide was not perturbed by these mutations of NEMO. Thus, S387 phosphorylation of NEMO is not a general requirement to mediate efficient NF-κB signalling and therefore may have cell type and/or stimulus-specific activity in vivo.

The anti-inflammatory effect of melatonin on methamphetamine-induced proinflammatory mediators in human neuroblastoma dopamine SH-SY5Y cell lines

Methamphetamine (METH) is a highly addictive drug that is commonly abused worldwide. This psychostimulant drug causes the disturbances in the dopaminergic and serotonergic neurons of several brain areas. Exposure to METH has been shown to induce oxidative stress, reactive oxygen species, reactive nitrogen species, and neuroinflammation. However, the mechanism underlying METH-induced inflammation in neurons is still unclear. In this study, we investigated whether METH caused inflammatory effects in human dopaminergic neuroblastoma SH-SY5Y cells and whether this effect involved the nuclear factor-κB (NF-κB) transcription factor pathway. The present results showed that METH significantly increased inducible nitric oxide synthase (iNOS) expression in a concentration-dependent manner and significantly increased the levels of tumor necrosis factor (TNF)-α mRNA and phosphorylated NF-κB, which is translocated into the nucleus. Moreover, our results also show that METH downregulated another transcription factor, the nuclear factor erythroid 2-related factor (Nrf2), a transcription factor implicated in the expression of several antioxidant/detoxificant enzymes. Furthermore, we also examined the anti-inflammatory effect of melatonin against these METH-induced neuroinflammatory functions. The results show that melatonin significantly decreases the iNOS protein expression and TNF-α mRNA levels caused by METH. The activation and the level of pNF-κB were decreased while Nrf2 expression was increased when cells were pre-incubated with 100 nM of melatonin. In order to show the relationship between cell death and the increase of iNOS, 100 μM of L-NAME, an iNOS inhibitor pretreatment significantly prevented cell death caused by METH. These results demonstrate, for the first time, that METH directly induces inflammation in neurons via an NF-κB-dependent pathway and that the anti-neuroinflammatory effects of melatonin result from the inhibition of activated NF-κB in parallel with potentiated antioxidant/detoxificant defense by activated Nrf2 pathway.

DNA damage-dependent NF-κB activation: NEMO turns nuclear signaling inside out

The dimeric transcription factor nuclear factor κB (NF-κB) functions broadly in coordinating cellular responses during inflammation and immune reactions, and its importance in the pathogenesis of cancer is increasingly recognized. Many of the signal transduction pathways that trigger activation of cytoplasmic NF-κB in response to a broad array of immune and inflammatory stimuli have been elaborated in great detail. NF-κB can also be activated by DNA damage, though relatively less is known about the signal transduction mechanisms that link DNA damage in the nucleus with activation of NF-κB in the cytoplasm. Here, we focus on the conserved signaling pathway that has emerged that promotes NF-κB activation following DNA damage. Post-translational modification of NF-κB essential modulator (NEMO) plays a central role in linking the cellular DNA damage response to NF-κB via the ataxia telangiectasia mutated (ATM) kinase. Accumulating evidence suggests that DNA damage-dependent NF-κB activation may play significant biological roles, particularly during lymphocyte differentiation and progression of human malignancies.

Direct inhibition of NF-κB activation by peptide targeting the NOA ubiquitin binding domain of NEMO

Aberrant and constitutive NF-κB activation are frequently reported in numerous tumor types, making its inhibition an attractive target for the treatment of certain cancers. NEMO (NF-κB essential modulator) is the crucial component of the canonical NF-κB pathway that mediates IκB kinase (IKK) complex activation. IKK activation resides in the ability of the C-terminal domain of NEMO to properly dimerize and interact with linear and K63-linked polyubiquitin chains. Here, we have identified a new NEMO peptide inhibitor, termed UBI (ubiquitin binding inhibitor) that derives from the NOA/NUB/UBAN ubiquitin binding site located in the CC2-LZ domain of NEMO. UBI specifically inhibits the NF-κB pathway at the IKK level in different cell types stimulated by a variety of NF-κB signals. Circular dichroïsm and fluorescence studies showed that UBI exhibits an increased α-helix character and direct, good-affinity binding to the NOA-LZ region of NEMO. We also showed that UBI targets NEMO in cells but its mode of inhibition is completely different from the previously reported LZ peptide (herein denoted NOA-LZ). UBI does not promote dissociation of NEMO subunits in cells but impairs the interaction between the NOA UBD of NEMO and polyubiquitin chains. Importantly, we showed that UBI efficiently competes with the in vitro binding of K63-linked chains, but not with linear chains. The identification of this new NEMO inhibitor emphasizes the important contribution of K63-linked chains for IKK activation in NF-κB signaling and would provide a new tool for studying the complex role of NF-κB in inflammation and cancer.

Kaposi's sarcoma-associated herpesvirus vFLIP and human T cell lymphotropic virus type 1 Tax oncogenic proteins activate IkappaB kinase subunit gamma by different mechanisms independent of the physiological cytokine-induced pathways

Activation of IκB kinase subunit γ (IKKγ), a key regulator of the classical NF-κB pathway, by the vFLIP protein of Kaposi's sarcoma-associated herpesvirus (KSHV) and the Tax protein of human T cell lymphotropic virus type 1 (HTLV1) is essential for virus-associated cancer. We show that vFLIP and Tax activate this pathway by different interactions with IKKγ and independently of the ubiquitin-mediated signaling pathways induced by cytokines. Our data provide new insights into the mechanisms by which IKKγ can be activated and show that NF-κB activation by oncogenic viruses can be targeted without affecting physiologically important pathways.

Analysis of nondegradative protein ubiquitylation with a monoclonal antibody specific for lysine-63-linked polyubiquitin

Modification of proteins by the addition of lysine (K)-63-linked polyubiquitin (polyUb) chains is suggested to play important roles in a variety of cellular events, including DNA repair, signal transduction, and receptor endocytosis. However, identifying such modifications in living cells is complex and cumbersome. We have generated a monoclonal antibody (mAb) that specifically recognizes K63-linked polyUb, but not any other isopeptide-linked (K6, K11, K27, K29, K33, or K48) polyUb or monoubiquitin. We demonstrate the sensitivity and specificity of this K63Ub-specific mAb to detect K63Ub-modified proteins in cell lysates by Western blotting and in cells by immunofluorescence, and K63Ub-modified TRAF6 and MEKK1 in vitro and ex vivo. This unique mAb will facilitate the analysis of K63-linked polyubiquitylation ex vivo and presents a strategy for the generation of similar reagents against other forms of polyUb.

Osmotically-induced genes are controlled by the transcription factor TonEBP in cultured cardiomyocytes

Changes in cardiac osmolarity occur in myocardial infarction. Osmoregulatory mechanisms may, therefore, play a crucial role in cardiomyocyte survival. Tonicity-responsive enhancer binding protein (TonEBP) is a key transcription factor participating in the adaptation of cells to increases in tonicity. However, it is unknown whether cardiac TonEBP is activated by tonicity. Hypertonicity activated transcriptional activity of TonEBP, increased the amounts of both TonEBP mRNA and protein, and induced both the mRNA and protein of TonEBP target genes (aldose reductase and heat shock protein-70). Hypotonicity decreased the amount of TonEBP protein indicating bidirectional osmoregulation of this transcription factor. Adenoviral expression of a dominant negative TonEBP suppressed the hypertonicity-dependent increase of aldose reductase protein. These results indicated that TonEBP controls osmoregulatory mechanisms in cardiomyocytes.

Osmotic stress blocks NF-kappaB-dependent inflammatory responses by inhibiting ubiquitination of IkappaB

The inhibitory effects of hypertonic conditions on immune responses have been described in clinical studies; however, the molecular mechanism underlying this phenomenon has yet to be defined. Here we investigate osmotic stress-mediated modification of the NF-kappaB pathway, a central signaling pathway in inflammation. We unexpectedly found that osmotic stress could activate IkappaBalpha kinase but did not activate NF-kappaB. Osmotic stress-induced phosphorylated IkappaBalpha was not ubiquitinated, and osmotic stress inhibited interleukin 1-induced ubiquitination of IkappaBalpha and ultimately blocked expression of cytokine/chemokines. Thus, blockage of IkappaBalpha ubiquitination is likely to be a major mechanism for inhibition of inflammation by hypertonic conditions.

Identification of TRAF6-dependent NEMO polyubiquitination sites through analysis of a new NEMO mutation causing incontinentia pigmenti

The regulatory subunit NEMO is involved in the mechanism of activation of IkappaB kinase (IKK), the kinase complex that controls the NF-kappaB signaling pathway. During this process, NEMO is modified post-translationally through K63-linked polyubiquitination. We report the molecular characterization of a new missense mutation of NEMO (A323P) which causes a severe form of incontinentia pigmenti (OMIM#308300), an inherited disease characterized predominantly by skin inflammation. The A323P mutation was found to impair TNF-, IL-1-, LPS- and PMA/ionomycin-induced NF-kappaB activation, as well as to disrupt TRAF6-dependent NEMO polyubiquitination, due to a defective NEMO/TRAF6 interaction. Mutagenesis identified the affected ubiquitination sites as three lysine residues located in the vicinity of A323. Unexpectedly, these lysines were ubiquitinated together with two previously identified lysines not connected to TRAF6. Mutation of all these ubiquitination sites severely impaired NF-kappaB activation induced by stimulation with IL-1, LPS, Nod2/RICK or serum/LPA. In contrast, mutation at all of these sites had only a limited effect on stimulation by TNF. These findings indicate that post-translational modification of NEMO through K63-linked polyubiquitination is a key event in IKK activation and that perturbation of this step may cause human pathophysiology.

Post-induction, Stimulus-specific Regulation of Tumor Necrosis Factor mRNA Expression

The tumor necrosis factor (TNF) gene is activated by multiple extracellular signals in a stimulus- and cell type-specific fashion. Based on the presence of kappaB-like DNA motifs in the region upstream of the TNF gene, some have proposed a direct role for NF-kappaB in lipopolysaccharide (LPS)-induced TNF gene transcription in cells of the monocyte/macrophage lineage. However, we have previously demonstrated a general and critical role for a minimal TNF promoter region bearing only one of the kappaB-like motifs, kappa3, which is bound by nuclear factor of activated T cell proteins in lymphocytes and fibroblasts in response to multiple stimuli and Ets proteins in LPS-stimulated macrophages. Here, in an effort to resolve these contrasting findings, we used a combination of site-directed mutagenesis of the TNF promoter, quantitative DNase I footprinting, and analysis of endogenous TNF mRNA production in response to multiple stimuli under conditions that inhibit NF-kappaB activation (using the proteasome inhibitor lactacystin and using cells lacking either functional NF-kappaB essential modulator, which is the IkappaB kinase regulatory subunit, or the Nemo gene itself). We find that TNF mRNA production in response to ionophore is NF-kappaB-independent, but inhibition of NF-kappaB activation attenuates virus- and LPS-induced TNF mRNA levels after initial induction. We conclude that induction of TNF gene transcription by virus or LPS does not depend upon NF-kappaB binding to the proximal promoter; rather, a stimulus-specific post-induction mechanism involving NF-kappaB, yet to be characterized, is involved in the maintenance of maximal TNF mRNA levels.

Key function for the Ubc13 E2 ubiquitin-conjugating enzyme in immune receptor signaling

The Ubc13 E2 ubiquitin-conjugating enzyme is key in the process of 'tagging' target proteins with lysine 63-linked polyubiquitin chains, which are essential for the transmission of immune receptor signals culminating in activation of the transcription factor NF-kappaB. Here we demonstrate that conditional ablation of Ubc13 resulted in defective B cell development and in impaired B cell and macrophage activation. In response to all tested stimuli except tumor necrosis factor, Ubc13-deficient cells showed almost normal NF-kappaB activation but considerably impaired activation of mitogen-activated protein kinase. Ubc13-induced activation of mitogen-activated protein kinase required, at least in part, ubiquitination of the adaptor protein IKKgamma. These results show that Ubc13 is key in the mammalian immune response.

Growth, differentiation, and malignant transformation of pre-B cells mediated by inducible activation of v-Abl oncogene

The nonreceptor tyrosine kinase, encoded by the v-Abl oncogene of Abelson murine leukemia virus induces transformation of progenitor B cells. The v-Abl oncogene promotes cell cycle progression and inhibits pre-B cell differentiation. The temperature-sensitive form of Abelson murine leukemia virus offers a reversible model to study the role of v-Abl in regulating growth and differentiation. Inactivation of v-Abl elevates p27 and Foxo3a levels and activates NF-kappaB/Rel, which leads to G1 arrest and induction of Ig L chain gene rearrangement, respectively. In turn, v-Abl reactivation reduces p27 and Foxo3a levels, thus permitting G1-arrested cells to reenter the cell cycle. However, the cell lines derived from SCID mice that are defective in the catalytic subunit of DNA-dependent protein kinase retain elevated levels of p27 and Foxo3a proteins despite reactivation of v-Abl. Consequently, these cells are locked in the G1 phase for an extended period of time. The few cells that manage to bypass the G1 arrest become tumorigenic and fail to undergo pre-B cell differentiation induced by v-Abl inactivation. Deregulation of p27, Foxo3a, c-myc, and NF-kappaB/Rel was found to be associated with the malignant transformation of SCID temperature-sensitive form of Abelson murine leukemia virus pre-B cells.

Hyperosmotic stress activates p65/RelB NFkappaB in cultured cardiomyocytes with dichotomic actions on caspase activation and cell death

NFkappaB is a participant in the process whereby cells adapt to stress. We have evaluated the activation of NFkappaB pathway by hyperosmotic stress in cultured cardiomyocytes and its role in the activation of caspase and cell death. Exposure of cultured rat cardiomyocytes to hyperosmotic conditions induced phosphorylation of IKKalpha/beta as well as degradation of IkappaBalpha. All five members of the NFkappaB family were identified in cardiomyocytes. Analysis of the subcellular distribution of NFkappaB isoforms in response to hyperosmotic stress showed parallel migration of p65 and RelB from the cytosol to the nucleus. Measurement of the binding of NFkappaB to the consensus DNA kappaB-site binding by EMSA revealed an oscillatory profile with maximum binding 1, 2 and 6h after initiation of the hyperosmotic stress. Supershift analysis revealed that p65 and RelB (but not p50, p52 or cRel) were involved in the binding of NFkappaB to DNA. Hyperosmotic stress also resulted in activation of the NFkappaB-lux reporter gene, transient activation of caspases 9 and 3 and phosphatidylserine externalization. The effect on cell viability was not prevented by ZVAD (a general caspase inhibitor). Blockade of NFkappaB with AdIkappaBalpha, an IkappaBalpha dominant negative overexpressing adenovirus, prevented activation of caspase 9 (more than that caspase 3) but did not affect cell death in hyperosmotically stressed cardiomyocytes. We conclude that hyperosmotic stress activates p65 and RelB NFkappaB isoforms and NFkappaB mediates caspase 9 activation in cardiomyocytes. However cell death triggered by hyperosmotic stress was caspase- and NFkappaB-independent.

The N-terminus of PKR is responsible for the activation of the NF-kappaB signaling pathway by interacting with the IKK complex

The interferon-induced double-stranded RNA (dsRNA)-activated protein kinase (PKR) has been shown to activate NF-kappaB independently of its kinase function after interaction with the IKK complex. In order to investigate the mechanism of NF-kappaB activation by PKR, we identified the domain of PKR responsible for stimulating the NF-kappaB pathway in PKR-deficient fibroblasts using an NF-kappaB dependent reporter assay. The N-terminal 1-265 AA of PKR activates NF-kappaB, whereas the 1-180 AA N-terminus restricted to the two dsRNA Binding Domains (DRBD), the third basic domain alone (AA 181-265), or the C-terminus of PKR (AA 266-550) were unable to stimulate the expression of the NF-kappaB dependent reporter gene. Using confocal microscopy, we confirmed that PKR full length as well as PKR N-terminus colocalized with IKKbeta. By GST-pulldown analysis, using different PKR domains, we then revealed the specific ability of the PKR N-terminus 1-265 to bind to and activate IKK and showed that this activation requires the integrity of the IKK complex. This activation is not only due to DRBDs since the DRBD fragment 1-180 failed to inhibit PKR 1-265 induced NF-kappaB activation. Our results therefore demonstrate that the ability of PKR to mediate NF-kappaB activation resides in its full N-terminus, and requires both DRBDs and the third basic domain.

Inhibition of NF-κB Activation by Peptides Targeting NF-κB Essential Modulator (NEMO) Oligomerization

NF-kappa B essential modulator/IKK-gamma (NEMO/IKK-gamma) plays a key role in the activation of the NF-kappa B pathway in response to proinflammatory stimuli. Previous studies suggested that the signal-dependent activation of the IKK complex involves the trimerization of NEMO. The minimal oligomerization domain of this protein consists of two coiled-coil subdomains named Coiled-coil 2 (CC2) and leucine zipper (LZ) (Agou, F., Traincard, F., Vinolo, E., Courtois, G., Yamaoka, S., Israel, A., and Veron, M. (2004) J. Biol. Chem. 279, 27861-27869). To search for drugs inhibiting NF-kappa B activation, we have rationally designed cell-permeable peptides corresponding to the CC2 and LZ subdomains that mimic the contact areas between NEMO subunits. The peptides were tagged with the Antennapedia/Penetratin motif and delivered to cells prior to stimulation with lipopolysaccharide. Peptide transduction was monitored by fluorescence-activated cell sorter, and their effect on lipopolysaccharide-induced NF-kappa B activation was quantified using an NF-kappa B-dependent beta-galactosidase assay in stably transfected pre-B 70Z/3 lymphocytes. We show that the peptides corresponding to the LZ and CC2 subdomains inhibit NF-kappa B activation with an IC(50) in the mum range. Control peptides, including mutated CC2 and LZ peptides and a heterologous coiled-coil peptide, had no inhibitory effect. The designed peptides are able to induce cell death in human retinoblastoma Y79 cells exhibiting constitutive NF-kappa B activity. Our results provide the "proof of concept" for a new and promising strategy for the inhibition of NF-kappa B pathway activation through targeting the oligomerization state of the NEMO protein.

In vitro and ex vivo activation of the TLR5 signaling pathway in intestinal epithelial cells by a commensal Escherichia coli strain

The capacity of non-pathogenic enteric bacteria to induce a pro-inflammatory response is under debate in terms of its effect on the symbiosis between the mammalian host and its commensal gut microflora. Activation of NF-kappaB and induction of interleukin-8 (IL-8) and CCL-20 by the commensal Escherichia coli strain MG1655 were first studied in vitro in the human intestinal epithelial cell (IECs) lines HT29-19A and Caco-2, transfected or not with plasmids encoding dominant negative Toll-like receptor (TLR) 5 and myeloid differentiation factor-88 (MyD88) adaptor protein. The response of enterocytes in situ was then assessed using murine ileal biopsies mounted in Ussing chambers. Commensal E. coli induced NF-kappaB DNA binding, NF-kappaB transcriptional activity, CCL-20 expression, and IL-8 secretion in the human IEC lines. E. coli MG1655 flagellin was necessary and sufficient to trigger this pro-inflammatory pathway via its interaction with TLR5 and the subsequent recruitment of the adaptor protein MyD88. Following epithelial cell polarization, signaling could be induced by live E. coli and flagellin on the apical side of HT29-19A. The in vivo relevance of our findings was confirmed, because immunohistochemical staining of murine ileum demonstrated expression of TLR5 in the apical part of enterocytes in situ. Furthermore, flagellin added on the mucosal side of murine ileal biopsies mounted in Ussing chambers induced a basolateral production of KC, a functional murine homolog of human IL-8. These findings provide strong evidence that flagellin released by flagellated commensal bacteria in the intestinal lumen can induce a pro-inflammatory response in enterocytes in vivo.

Lactic acid bacteria secrete metabolites retaining anti-inflammatory properties after intestinal transport

BACKGROUND

Probiotic bacteria have a beneficial effect on intestinal inflammation. In this study, we have examined the effect of lactic acid and commensal Gram positive (+) bacteria conditioned media (CM) on tumour necrosis factor alpha (TNF-alpha) release and the mechanisms involved.

METHODS

Lipopolysaccharide (LPS) induced TNF-alpha secretion by peripheral blood mononuclear cells or the THP-1 cell line was monitored in the presence or absence of bacteria CM obtained from two probiotic strains, Bifidobacterium breve (Bb) and Streptococcus thermophilus (St), and three commensal bacterial strains (Bifidobacterium bifidum, Ruminococcus gnavus, and unidentified Streptococcus). Bb and St bacteria CM were allowed to cross filter grown intestinal epithelial cell monolayers (HT29-19A) to assess intestinal transport of active bacterial products. These products were characterised and their effect on LPS binding to THP-1 cells and nuclear factor kappa B (NF kappa B) activation assessed.

RESULTS

Dose dependent inhibition of LPS induced TNF-alpha secretion was noted for both probiotic bacteria CM (64% and 71% inhibition for Bb and St, respectively) and to a lesser extent commensal bacteria CM (21-32% inhibition). Active products from Bb and St were resistant to digestive enzymes and had a molecular mass <3000 Da. Their inhibitory effect was preserved after transepithelial transport across intestinal cell monolayers, mainly in inflammatory conditions. LPS-FITC binding to THP-1 cells and NF kappa B activation were significantly inhibited by Bb and St CM.

CONCLUSION

B breve and S thermophilus release metabolites exerting an anti-TNF-alpha effect capable of crossing the intestinal barrier. Commensal bacteria also display a TNF-alpha inhibitory capacity but to a lesser extent. These results underline the beneficial effect of commensal bacteria in intestinal homeostasis and may explain the role of some probiotic bacteria in alleviating digestive inflammation.

The trimerization domain of NEMO is composed of the interacting C-terminal CC2 and LZ coiled-coil subdomains

NEMO (NF-kappaB essential modulator) plays a key role in the canonical NF-kappaB pathway as the scaffold/regulatory component of the IkappaB kinase (IKK) complex. The self-association of NEMO involves the C-terminal halves of the polypeptide chains containing two putative coiled-coil motifs (a CC2 and a LZ leucine zipper), a proline-rich region, and a ZF zinc finger motif. Using purified truncation mutants, we showed that the minimal oligomerization domain of NEMO is the CC2-LZ segment and that both CC2 and LZ subdomains are necessary to restore the LPS-dependent activation of the NF-kappaB pathway in a NEMO-deficient cell line. We confirmed the association of the oligomerization domain in a trimer and investigated the specific role of CC2 and LZ subdomains in the building of the oligomer. Whereas a recombinant CC2-LZ polypeptide self-associated into a trimer with an association constant close to that of the wild-type protein, the isolated CC2 and LZ peptides, respectively, formed trimers and dimers with weaker association constants. Upon mixing, isolated CC2 and LZ peptides associated to form a stable hetero-hexamer as shown by gel filtration and fluorescence anisotropy experiments. We propose a structural model for the organization of the oligomerization domain of activated NEMO in which three C-terminal domains associate into a pseudo-hexamer forming a six-helix bundle. This model is discussed in relation to the mechanism of activation of the IKK complex by upstream activators.

Segregation of NF-kappaB activation through NEMO/IKKgamma by Tax and TNFalpha: implications for stimulus-specific interruption of oncogenic signaling

Nuclear factor-kappaB essential modulator (NEMO), also called IKKgamma, has been proposed as a 'universal' adaptor of the I-kappaB kinase (IKK) complex for stimuli such as proinflammatory cytokines, microbes, and the HTLV-I Tax oncoprotein. Currently, it remains unclear whether the many signals that activate NF-kappaB through NEMO converge identically or differently. We have adopted two approaches to answer this question. First, we generated and targeted intracellularly three NEMO-specific monoclonal antibodies (mAbs). These mAbs produced two distinct intracellular NF-kappaB inhibition profiles segregating TNFalpha from Tax activation. Second, using NEMO knockout mouse fibroblasts and 10 NEMO mutants, we found that different regions function in trans either to complement or to inhibit dominantly TNFalpha, IL-1beta, or Tax activation of NF-kappaB. For instance, NEMO (1-245 amino acids) supported Tax-mediated NF-kappaB activation, but did not serve TNFalpha- or IL-1beta signaling. Altogether, our findings indicate that while NEMO 'universally' adapts numerous NF-kappaB activators, it may do so through separable domains. We provide the first evidence that selective targeting of NEMO can abrogate oncogenic Tax signaling without affecting signals used for normal cellular metabolism.

Nuclear Role of IκB Kinase-γ/NF-κB Essential Modulator (IKKγ/NEMO) in NF-κB-dependent Gene Expression

The I kappa B kinase (IKK) complex, which is composed of the two kinases IKK alpha and IKK beta and the regulatory subunit IKK gamma/nuclear factor-kappa B (NF-kappa B) essential modulator (NEMO), is important in the cytokine-induced activation of the NF-kappa B pathway. In addition to modulation of IKK activity, the NF-kappa B pathway is also regulated by other processes, including the nucleocytoplasmic shuttling of various components of this pathway and the post-translational modification of factors bound to NF-kappa B-dependent promoters. In this study, we explored the role of the nucleocytoplasmic shuttling of components of the IKK complex in the regulation of the NF-kappa B pathway. IKK gamma/NEMO was demonstrated to shuttle between the cytoplasm and the nucleus and to interact with the nuclear coactivator cAMP-responsive element-binding protein-binding protein (CBP). Using both in vitro and in vivo analysis, we demonstrated that IKK gamma/NEMO competed with p65 and IKK alpha for binding to the N terminus of CBP, inhibiting CBP-dependent transcriptional activation. These results indicate that, in addition to the key role of IKK gamma/NEMO in regulating cytokine-induced IKK activity, its ability to shuttle between the cytoplasm and the nucleus and to bind to CBP can lead to transcriptional repression of the NF-kappa B pathway.

Latent Membrane Protein 1 of Epstein-Barr Virus Stimulates Processing of NF-κB2 p100 to p52

Recent studies have identified a limited number of cellular receptors that can stimulate an alternative NF-kappa B activation pathway that depends upon the inducible processing of NF-kappa B2 p100 to p52. Here it is shown that the latent membrane protein (LMP)-1 of Epstein-Barr virus can trigger this signaling pathway in both B cells and epithelial cells. LMP1-induced p100 processing, which is mediated by the proteasome and is dependent upon de novo protein synthesis, results in the nuclear translocation of p52.RelB dimers. Previous studies have established that LMP1 also stimulates the canonical NF-kappa B-signaling pathway that triggers phosphorylation and degradation of I kappa B alpha. Interestingly, LMP1 activation of these two NF-kappa B pathways is shown here to require distinct regions of the LMP1 C-terminal cytoplasmic tail. Thus, C-terminal-activating region 1 is required for maximal triggering of p100 processing but is largely dispensable for stimulation of I kappa B alpha phosphorylation. In contrast, C-terminal-activating region 2 is critical for maximal LMP1 triggering of I kappa B alpha phosphorylation and up-regulation of p100 levels but does not contribute to activation of p100 processing. Because p100 deletion mutants that constitutively produce p52 oncogenically transform fibroblasts in vitro, it is likely that stimulation of p100 processing by LMP1 will play an important role in its transforming function.

Two carboxyl-terminal activation regions of Epstein-Barr virus latent membrane protein 1 activate NF-kappaB through distinct signaling pathways in fibroblast cell lines

Latent membrane protein 1 (LMP1), an Epstein-Barr virus transforming protein, is able to activate NF-kappaB through its carboxyl-terminal activation region 1 (CTAR1) and 2 (CTAR2), but the exact role of each domain is not fully understood. Here we show that LMP1 activates NF-kappaB in different NF-kappaB essential modulator (NEMO)-defective cell lines, but not in cells lacking both IkappaB kinase 1 (IKK1) and 2 (IKK2). Mutational studies reveal that CTAR1, but not CTAR2, mediates NEMO-independent NF-kappaB activation and that this process largely depends on IKK1. Retroviral expression of LMP1 mutants in cells lacking either functional NF-kappaB inducing kinase (NIK), NEMO, IKK1, or IKK2 further illustrates distinct signals from the two activation regions of LMP1 for persistent NF-kappaB activation. One originates in CTAR2, operates through the canonical NEMO-dependent pathway, and induces NFKB2 p100 production; the second signal originates in CTAR1, utilizes NIK and IKK1, and induces the processing of p100. Our results thus help clarify how two functional domains of LMP1 persistently activate NF-kappaB through distinct signaling pathways.

A hypermorphic IkappaBalpha mutation is associated with autosomal dominant anhidrotic ectodermal dysplasia and T cell immunodeficiency

X-linked anhidrotic ectodermal dysplasia with immunodeficiency (XL-EDA-ID) is caused by hypomorphic mutations in the gene encoding NEMO/IKKgamma, the regulatory subunit of the IkappaB kinase (IKK) complex. IKK normally phosphorylates the IkappaB-inhibitors of NF-kappaB at specific serine residues, thereby promoting their ubiquitination and degradation by the proteasome. This allows NF-kappaB complexes to translocate into the nucleus where they activate their target genes. Here, we describe an autosomal-dominant (AD) form of EDA-ID associated with a heterozygous missense mutation at serine 32 of IkappaBalpha. This mutation is gain-of-function, as it enhances the inhibitory capacity of IkappaBalpha by preventing its phosphorylation and degradation, and results in impaired NF-kappaB activation. The developmental, immunologic, and infectious phenotypes associated with hypomorphic NEMO and hypermorphic IKBA mutations largely overlap and include EDA, impaired cellular responses to ligands of TIR (TLR-ligands, IL-1beta, and IL-18), and TNFR (TNF-alpha, LTalpha1/beta2, and CD154) superfamily members and severe bacterial diseases. However, AD-EDA-ID but not XL-EDA-ID is associated with a severe and unique T cell immunodeficiency. Despite a marked blood lymphocytosis, there are no detectable memory T cells in vivo, and naive T cells do not respond to CD3-TCR activation in vitro. Our report highlights both the diversity of genotypes associated with EDA-ID and the diversity of immunologic phenotypes associated with mutations in different components of the NF-kappaB signaling pathway.

NF-kappa B activation in vivo in both host and tumour cells by the antivascular agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA)

5,6-Dimethylxanthenone-4-acetic acid (DMXAA), a new anticancer agent developed in this centre, has an antivascular action and causes regression of transplantable murine tumours that is mediated partially by the intratumoral production of tumour necrosis factor (TNF). DMXAA activates the nuclear factor-kappaB (NF-kappaB) transcription factor, which is involved in TNF synthesis and has also been suggested to mediate resistance to TNF. We wished to determine whether tumour cell NF-kappaB activation modulated the in vitro and in vivo effects of DMXAA. We compared the response of the 70Z/3 pre-B lymphoma cell line with that of its mutant 1.3E2 sub-line, which has a defective gamma-subunit of IKK, the kinase that phosphorylates IkappaB leading to NF-kappaB activation. As shown by electrophoretic mobility shift assays (EMSAs), DMXAA induced in vitro translocation of NF-kappaB (p50 and p65 subunits) into the nucleus of 70Z/3 cells, but not of 1.3E2 cells. However, when the cell lines were then grown as subcutaneous tumours in mice and treated with DMXAA (25 mg/kg), activation of NF-kappaB was found in nuclear extracts prepared from both 70/Z3 and 1.3E2 tumours, as well as from Colon 38 tumours that were used for comparison. This suggests that DMXAA induces NF-kappaB responses in host components of the tumour. Tumours grown from both 70Z/3 and 1.3E2 cells were found to regress completely following DMXAA treatment. Thus, the antitumour action of DMXAA appears to be independent of the ability of the target tumour cell population to induce NF-kappaB expression. Moreover, activation of NF-kappaB in the tumour cell did not confer resistance to DMXAA-induced therapy.

Lymphotoxin-beta receptor mediates NEMO-independent NF-kappaB activation

Lymphotoxin-beta receptor (LTbetaR) is a member of the tumor necrosis factor receptor (TNFR) superfamily that activates nuclear factor-kappaB (NF-kappaB) through the IkappaB kinase (IKK) complex, the core of which is comprised of IKK1, IKK2 and NF-kappaB essential modulator (NEMO). We demonstrate here that the LTbetaR signaling to NF-kappaB activation does not necessarily require NEMO, which is essential for TNFR signaling. In the absence of NEMO, the p50 and RelB, but not RelA subunits of NF-kappaB are found in the nuclear DNA binding complexes induced by the LTbetaR signaling. Our results thus disclose NEMO-independent NF-kappaB activation by LTbetaR.

The human herpes virus 8-encoded viral FLICE inhibitory protein physically associates with and persistently activates the Ikappa B kinase complex

The human herpesvirus 8 (HHV8, also called Kaposi's sarcoma-associated herpesvirus) has been linked to Kaposi's sarcoma and primary effusion lymphoma (PEL) in immunocompromised individuals. We demonstrate that PEL cell lines have a constitutively active NF-kappaB pathway, which is associated with persistent phosphorylation of IkappaBalpha. To elucidate the mechanism of NF-kappaB activation in PEL cell lines, we have investigated the role of viral FLICE inhibitory protein (vFLIP) in this process. We report that stable expression of HHV8 vFLIP in a variety of cell lines is associated with persistent NF-kappaB activation caused by constitutive phosphorylation of IkappaBalpha. HHV8 vFLIP gets recruited to a approximately 700-kDa IkappaB kinase (IKK) complex and physically associates with IKKalpha, IKKbeta, NEMO/IKKgamma, and RIP. HHV8 vFLIP is incapable of activating NF-kappaB in cells deficient in NEMO/IKKgamma, thereby suggesting an essential role of an intact IKK complex in this process. Our results suggest that HHV8 vFLIP might contribute to the persistent NF-kappaB activation observed in PEL cells by associating with and stimulating the activity of the cellular IKK complex.

B-cell receptor- and phorbol ester-induced NF-kappaB and c-Jun N-terminal kinase activation in B cells requires novel protein kinase C's

Antigen receptor signaling is known to activate NF-kappaB in lymphocytes. While T-cell-receptor-induced NF-kappaB activation critically depends on novel protein kinase C theta (PKCtheta), the role of novel PKCs in B-cell stimulation has not been elucidated. In primary murine splenic B cells, we found high expression of the novel PKCs delta and epsilon but only weak expression of the theta isoform. Rottlerin blocks phorbol ester (phorbol myristate acetate [PMA])- or B-cell receptor (BCR)-mediated NF-kappaB and c-Jun N-terminal kinase (JNK) activation in primary B and T cells to a similar extent, suggesting that novel PKCs are positive regulators of signaling in hematopoietic cells. Mouse 70Z/3 pre-B cells have been widely used as a model for NF-kappaB activation in B cells. Similar to the situation in splenic B cells, rottlerin inhibits BCR and PMA stimulation of NF-kappaB in 70Z/3 cells. A derivative of 70Z/3 cells, 1.3E2 cells, are defective in NF-kappaB activation due to the lack of the IkappaB kinase (IKKgamma) protein. Ectopic expression of IKKgamma can rescue NF-kappaB activation in response to lipopolysaccharides (LPS) and interleukin-1beta (IL-1beta), but not to PMA. In addition, PMA-induced activation of the mitogen-activated protein kinase JNK is blocked in 1.3E2 cells, suggesting that an upstream component common to both pathways is either missing or mutated. Analysis of various PKC isoforms revealed that exclusively PKCtheta was absent in 1.3E2 cells while it was expressed in 70Z/3 cells. Stable expression of either novel PKCtheta or -delta but not classical PKCbetaII in 1.3E2 IKKgamma-expressing cells rescues PMA activation of NF-kappaB and JNK signaling, demonstrating a critical role of novel PKCs for B-cell activation.

IKKgamma /NEMO facilitates the recruitment of the IkappaB proteins into the IkappaB kinase complex

IKKgamma/NEMO is an essential regulatory component of the IkappaB kinase complex that is required for NF-kappaB activation in response to various stimuli including tumor necrosis factor-alpha and interleukin-1beta. To investigate the mechanism by which IKKgamma/NEMO regulates the IKK complex, we examined the ability of IKKgamma/NEMO to recruit the IkappaB proteins into this complex. IKKgamma/NEMO binding to wild-type, but not to a kinase-deficient IKKbeta protein, facilitated the association of IkappaBalpha and IkappaBbeta with the high molecular weight IKK complex. Following tumor necrosis factor-alpha treatment of HeLa cells, the majority of the phosphorylated form of endogenous IkappaBalpha was associated with the high molecular weight IKK complex in HeLa cells and parental mouse embryo fibroblasts but not in IKKgamma/NEMO-deficient cells. Finally, we demonstrate that IKKgamma/NEMO facilitates the association of the IkappaB proteins and IKKbeta and leads to increases in IKKbeta kinase activity. These results suggest that an important function of IKKgamma/NEMO is to facilitate the association of both IKKbeta and IkappaB in the high molecular weight IKK complex to increase IkappaB phosphorylation.

Toxoplasma gondii: selective killing of extracellular parasites by oxidation using pyrrolidine dithiocarbamate

Extracellular Toxoplasma parasites are sensitive to pyrrolidine dithiocarbamate (PDTC) at low micromolar concentrations. Loss of parasite viability following PDTC treatment is shown to be mediated by oxidation, which is reminiscent of PDTC killing in mammalian cells. Intracellular parasites, by contrast, are resistant to PDTC killing, although treatment does cause reversible growth arrest. In addition to the possible implications relative to the biology of the parasite, these observations suggest that PDTC could be of use in eliminating undesired extracellular parasites during assays and selections in vitro.

Functional activities of the human T-cell leukemia virus type I Tax oncoprotein: cellular signaling through NF-kappa B

Human T-cell leukemia virus type I (HTLV-I) is the etiological agent for adult T-cell leukemia (ATL), as well as for tropical spastic paraparesis (TSP) and HTLV-I associate myelopathy (HAM). A biological understanding of the involvement of HTLV-I and in ATL has focused significantly on the workings of the virally-encoded 40 kDa phospho-oncoprotein, Tax. Tax is a transcriptional activator. Its ability to modulate the expression and function of many cellular genes has been reasoned to be a major contributory mechanism explaining HTLV-I-mediated transformation of cells. In activating cellular gene expression, Tax impinges upon several cellular signal-transduction pathways, including those for CREB/ATF and NF-kappa B. In this paper, we review aspects of Tax's transcriptional potential with particular focus on recent evidence linking Tax to IKK (I kappa B-kinase)-complex and MAP3Ks (mitogen-activated protein kinase kinase kinases).

Novel NEMO/IkappaB kinase and NF-kappa B target genes at the pre-B to immature B cell transition

The IkappaB kinase (IKK) signaling complex is responsible for activating NF-kappaB-dependent gene expression programs. Even though NF-kappaB-responsive genes are known to orchestrate stress-like responses, critical gaps in our knowledge remain about the global effects of NF-kappaB activation on cellular physiology. DNA microarrays were used to compare gene expression programs in a model system of 70Z/3 murine pre-B cells versus their IKK signaling-defective 1.3E2 variant with lipopolysaccharide (LPS), interleukin-1 (IL-1), or a combination of LPS + phorbol 12-myristate 13-acetate under brief (2 h) or long term (12 h) stimulation. 70Z/3-1.3E2 cells lack expression of NEMO/IKKgamma/IKKAP-1/FIP-3, an essential positive effector of the IKK complex. Some stimulated hits were known NF-kappaB target genes, but remarkably, the vast majority of the up-modulated genes and an unexpected class of repressed genes were all novel targets of this signaling pathway, encoding transcription factors, receptors, extracellular ligands, and intracellular signaling factors. Thirteen stimulated (B-ATF, Pim-2, MyD118, Pea-15/MAT1, CD82, CD40L, Wnt10a, Notch 1, R-ras, Rgs-16, PAC-1, ISG15, and CD36) and five repressed (CCR2, VpreB, lambda5, SLPI, and CMAP/Cystatin7) genes, respectively, were bona fide NF-kappaB targets by virtue of their response to a transdominant IkappaBalphaSR (super repressor). MyD118 and ISG15, although directly induced by LPS stimulation, were unaffected by IL-1, revealing the existence of direct NF-kappaB target genes, which are not co-induced by the LPS and IL-1 Toll-like receptors.

X-linked anhidrotic ectodermal dysplasia with immunodeficiency is caused by impaired NF-kappaB signaling

The molecular basis of X-linked recessive anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID) has remained elusive. Here we report hypomorphic mutations in the gene IKBKG in 12 males with EDA-ID from 8 kindreds, and 2 patients with a related and hitherto unrecognized syndrome of EDA-ID with osteopetrosis and lymphoedema (OL-EDA-ID). Mutations in the coding region of IKBKG are associated with EDA-ID, and stop codon mutations, with OL-EDA-ID. IKBKG encodes NEMO, the regulatory subunit of the IKK (IkappaB kinase) complex, which is essential for NF-kappaB signaling. Germline loss-of-function mutations in IKBKG are lethal in male fetuses. We show that IKBKG mutations causing OL-EDA-ID and EDA-ID impair but do not abolish NF-kappaB signaling. We also show that the ectodysplasin receptor, DL, triggers NF-kappaB through the NEMO protein, indicating that EDA results from impaired NF-kappaB signaling. Finally, we show that abnormal immunity in OL-EDA-ID patients results from impaired cell responses to lipopolysaccharide, interleukin (IL)-1beta, IL-18, TNFalpha and CD154. We thus report for the first time that impaired but not abolished NF-kappaB signaling in humans results in two related syndromes that associate specific developmental and immunological defects.

Atypical forms of incontinentia pigmenti in male individuals result from mutations of a cytosine tract in exon 10 of NEMO (IKK-gamma)

Familial incontinentia pigmenti (IP [MIM 308310]), or Bloch-Sulzberger syndrome, is an X-linked dominant and male-lethal disorder. We recently demonstrated that mutations in NEMO (IKK-gamma), which encodes a critical component of the NF-kappaB signaling pathway, were responsible for IP. Virtually all mutations eliminate the production of NEMO, causing the typical skewing of X inactivation in female individuals and lethality in male individuals, possibly through enhanced sensitivity to apoptosis. Most mutations also give rise to classic signs of IP, but, in this report, we describe two mutations in families with atypical phenotypes. Remarkably, each family included a male individual with unusual signs, including postnatal survival and either immune dysfunction or hematopoietic disturbance. We found two duplication mutations in these families, at a cytosine tract in exon 10 of NEMO, both of which remove the zinc (Zn) finger at the C-terminus of the protein. Two deletion mutations were also identified in the same tract in additional families. However, only the duplication mutations allowed male individuals to survive, and affected female individuals with duplication mutations demonstrated random or slight skewing of X inactivation. Similarly, NF-kappaB activation was diminished in the presence of duplication mutations and was completely absent in cells with deletion mutations. These results strongly indicate that male individuals can also suffer from IP caused by NEMO mutations, and we therefore urge a reevaluation of the diagnostic criteria.

Redox-modulated pathways in inflammatory skin diseases

Inflammatory skin diseases account for a large proportion of all skin disorders and constitute a major health problem worldwide. Contact dermatitis, atopic dermatitis, and psoriasis represent the most prevalent inflammatory skin disorders and share a common efferent T-lymphocyte mediated response. Oxidative stress and inflammation have recently been linked to cutaneous damage in T-lymphocyte mediated skin diseases, particularly in contact dermatitis. Insights into the pathophysiology responsible for contact dermatitis can be used to better understand the mechanism of other T-lymphocyte mediated inflammatory skin diseases, and may help to develop novel therapeutic approaches. This review focuses on redox sensitive events in the inflammatory scenario of contact dermatitis, which comprise for example, several kinases, transcription factors, cytokines, adhesion molecules, dendritic cell surface markers, the T-lymphocyte receptor, and the cutaneous lymphocyte-associated antigen (CLA). In vitro and animal studies clearly point to a central role of several distinct but interconnected redox-sensitive pathways in the pathogenesis of contact dermatitis. However, clinical evidence that modulation of the skin's redox state can be used therapeutically to modulate the inflammatory response in contact dermatitis is presently not convincing. The rational for this discrepancy seems to be multi-faceted and complex and will be discussed.

Role of IKKgamma/nemo in assembly of the Ikappa B kinase complex

IKKgamma/NEMO is a protein that is critical for the assembly of the high molecular weight IkappaB kinase (IKK) complex. To investigate the role of IKKgamma/NEMO in the assembly of the IKK complex, we conducted a series of experiments in which the chromatographic distribution of extracts prepared from cells transiently expressing epitope-tagged IKKgamma/NEMO and the IKKs were examined. When expressed alone following transfection, IKKalpha and IKKbeta were present in low molecular weight complexes migrating between 200 and 400 kDa. However, when coexpressed with IKKgamma/NEMO, both IKKalpha and IKKbeta migrated at approximately 600 kDa which was similar to the previously described IKK complex that is activated by cytokines such as tumor necrosis factor-alpha. When either IKKalpha or IKKbeta was expressed alone with IKKgamma/NEMO, IKKbeta but not IKKalpha migrated in the higher molecular weight IKK complex. Constitutively active or inactive forms of IKKbeta were both incorporated into the high molecular weight IKK complex in the presence of IKKgamma/NEMO. The amino-terminal region of IKKgamma/NEMO, which interacts directly with IKKbeta, was required for formation of the high molecular weight IKK complex and for stimulation of IKKbeta kinase activity. These results suggest that recruitment of the IKKs into a high molecular complex by IKKgamma/NEMO is a crucial step involved in IKK function.

Human T-lymphotropic virus type I Tax protein utilizes distinct pathways for p53 inhibition that are cell type-dependent

p53 plays a pivotal role in transmitting signals from many forms of genotoxic stress to genes and factors that control the cell cycle and apoptosis. We have previously shown that the human T-lymphotropic virus type I Tax protein can inhibit p53 function. Recently we reported that Tax inhibits p53 function in Jurkat cells and mouse embryo fibroblasts through a mechanism involving the nuclear factor kappa B pathway and correlates with phosphorylation on serines 15 and 392 of p53. However, several groups have also observed a mechanism that correlates with p300 binding of Tax. To address this controversy and to determine the mechanism by which Tax inhibits p53 function, we examined the activation functions of Tax required for p53 inhibition. In HeLa and H1299 cells the cAMP-response element-binding protein/activating transcription factor activation function is essential, as demonstrated by the Tax mutants M47 and K88A. In addition, expression of exogenous p300 in H1299 cells allows full recovery of p53 transactivation in the presence of Tax. Consistent with p300 being a limiting factor in H1299, Saos-2, and HeLa cells, we found that the level of endogenous p300 is relatively low in these cells compared with Jurkat cells or the human T-lymphotropic virus type I-infected C81 and MT2 cells. Thus our data suggests that Tax utilizes distinct mechanisms to inhibit p53 function that are cell type-dependent.

Activation of I-kappaB kinase by the HTLV type 1 Tax protein: mechanistic insights into the adaptor function of IKKgamma

The Tax protein encoded by human T cell leukemia virus type 1 (HTLV-1) induces constitutive nuclear expression of the transcription factor NF-kappaB, causing aberrant expression of a large array of cellular genes. Tax activates NF-kappaB by stimulating the activity of the I-kappaB kinase (IKK), which in turn leads to phosphorylation and degradation of the NF-kappaB inhibitor I-kappaBalpha. In normal T cells, IKK activation occurs transiently on cellular stimulation through the T cell receptor (TCR) and the CD28 costimulatory molecule. However, this inducible kinase is constitutively activated in Tax-expressing and HTLV-1-infected T cells, which contributes to the deregulated nuclear expression of NF-kappaB. As a genetic approach to dissect the pathways mediating IKK activation by Tax and T cell activation signals, somatic cell mutagenesis was performed to isolate signaling-defective mutant Jurkat T cell lines. One of the mutant cell lines was shown to have a defect in NF-kappaB activation by both T cell mitogens and Tax. Interestingly, this mutant cell line lacks expression of the IKK regulatory protein, IKKgamma. Expression of exogenous IKKgamma in the mutant cells restored NF-kappaB activation, thus confirming the essential role of this regulatory factor in IKK activation by the cellular and viral stimuli. Mechanistic studies have shown that Tax physically interacts with IKKgamma via specific domains, including two homologous leucine zipper motifs present in IKKgamma. The Tax/IKKgamma interaction serves to recruit Tax to the IKK catalytic subunits, IKKalpha and IKKbeta, and this recruitment appears to be an essential mechanism by which Tax stimulates the activity of IKK.

Mechanisms of NF-kappaB activation by the HTLV type 1 tax protein

The Tax protein encoded by the human T cell leukemia virus type I virus (HTLV-1) activates the expression of both viral genes and cellular genes involved in T lymphocyte growth and proliferation. One of the critical cellular pathways activated by Tax is NF-kappaB. NF-kappaB is normally sequestered in the cytoplasm, bound to a family of inhibitory proteins known as I-kappaB. In contrast to the transient activation of the NF-kappaB pathway seen in response to cytokines, Tax results in constitutive nuclear levels of NF-kappaB. Tax activation of the NF-kappaB pathway is mediated by its ability to enhance the phosphorylation and subsequent degradation of I-kappaB. The persistent activation of the NF-kappaB pathway by Tax is believed to be one of the major events involved in HTLV-1-mediated cellular transformation of T lymphocytes. This review summarizes data exploring the role of Tax in activating the NF-kappaB pathway and discusses our studies to determine the mechanism by which Tax activates the NF-kappaB pathway.

Activation of the IkappaB kinase complex by TRAF6 requires a dimeric ubiquitin-conjugating enzyme complex and a unique polyubiquitin chain

TRAF6 is a signal transducer in the NF-kappaB pathway that activates IkappaB kinase (IKK) in response to proinflammatory cytokines. We have purified a heterodimeric protein complex that links TRAF6 to IKK activation. Peptide mass fingerprinting analysis reveals that this complex is composed of the ubiquitin conjugating enzyme Ubc13 and the Ubc-like protein Uev1A. We find that TRAF6, a RING domain protein, functions together with Ubc13/Uev1A to catalyze the synthesis of unique polyubiquitin chains linked through lysine-63 (K63) of ubiquitin. Blockade of this polyubiquitin chain synthesis, but not inhibition of the proteasome, prevents the activation of IKK by TRAF6. These results unveil a new regulatory function for ubiquitin, in which IKK is activated through the assembly of K63-linked polyubiquitin chains.

The I kappa B kinase (IKK) complex is tripartite and contains IKK gamma but not IKAP as a regular component

A critical step in the activation of NF-kappa B is the phosphorylation of I kappa Bs by the I kappa B kinase (IKK) complex. IKK alpha and IKK beta are the two catalytic subunits of the IKK complex and two additional molecules, IKK gamma/NEMO and IKAP, have been described as further integral members. We have analyzed the function of both proteins for IKK complex composition and NF-kappa B signaling. IKAP and IKK gamma belong to distinct cellular complexes. Quantitative association of IKK gamma was observed with IKK alpha and IKK beta. In contrast IKAP was complexed with several distinct polypeptides. Overexpression of either IKK gamma or IKAP blocked tumor necrosis factor alpha induction of an NF-kappa B-dependent reporter construct, but IKAP in addition affected several NF-kappa B-independent promoters. Whereas specific down-regulation of IKK gamma protein levels by antisense oligonucleotides significantly reduced cytokine-mediated activation of the IKK complex and subsequent NF-kappa B activation, a similar reduction of IKAP protein levels had no effect on NF-kappa B signaling. Using solely IKK alpha, IKK beta, and IKK gamma, we could reconstitute a complex whose apparent molecular weight is comparable to that of the endogenous IKK complex. We conclude that while IKK gamma is a stoichiometric component of the IKK complex, obligatory for NF-kappa B signaling, IKAP is not associated with IKKs and plays no specific role in cytokine-induced NF-kappa B activation.

CD40 induces apoptosis in carcinoma cells through activation of cytotoxic ligands of the tumor necrosis factor superfamily

CD40, a tumor necrosis factor (TNF) receptor (TNFR) family member, conveys signals regulating diverse cellular responses, ranging from proliferation and differentiation to growth suppression and cell death. The ability of CD40 to mediate apoptosis in carcinoma cells is intriguing given the fact that the CD40 cytoplasmic C terminus lacks a death domain homology with the cytotoxic members of the TNFR superfamily, such as Fas, TNFR1, and TNF-related apoptosis-inducing ligand (TRAIL) receptors. In this study, we have probed the mechanism by which CD40 transduces death signals. Using a trimeric recombinant soluble CD40 ligand to activate CD40, we have found that this phenomenon critically depends on the membrane proximal domain (amino acids 216 to 239) but not the TNFR-associated factor-interacting PXQXT motif in the CD40 cytoplasmic tail. CD40-mediated cytotoxicity is blocked by caspase inhibitors, such as zVAD-fmk and crmA, and involves activation of caspase 8 and caspase 3. Interestingly, CD40 ligation was found to induce functional Fas ligand, TRAIL (Apo-2L) and TNF in apoptosis-susceptible carcinoma cells and to up-regulate expression of Fas. These findings identify a novel proapoptotic mechanism which is induced by CD40 in carcinoma cells and depends on the endogenous production of cytotoxic cytokines and autocrine or paracrine induction of cell death.

Phorbol esters and cytokines regulate the expression of the NEMO-related protein, a molecule involved in a NF-kappa B-independent pathway

The NF-kappaB signaling pathway plays a crucial role in the immune, inflammatory, and apoptotic responses. Recently, we identified the NF-kappaB Essential Modulator (NEMO) as an essential component of this pathway. NEMO is a structural and regulatory subunit of the high molecular kinase complex (IKK) responsible for the phosphorylation of NF-kappaB inhibitors. Data base searching led to the isolation of a cDNA encoding a protein we called NRP (NEMO-related protein), which shows a strong homology to NEMO. Here we show that NRP is present in a novel high molecular weight complex, that contains none of the known members of the IKK complex. Consistently, we could not observe any effect of NRP on NF-kappaB signaling. Nonetheless, we could demonstrate that treatment with phorbol esters induces NRP phosphorylation and decreases its half-life. This phosphorylation event could only be inhibited by K-252a and stauroporin. We also show that de novo expression of NRP can be induced by interferon and tumor necrosis factor alpha and that these two stimuli have a synergistic effect on NRP expression. In addition, we observed that endogenous NRP is associated with the Golgi apparatus. Analogous to NEMO, we find that NRP is associated in a complex with two kinases, suggesting that NRP could play a similar role in another signaling pathway.

Invasive Shigella flexneri activates NF-kappa B through a lipopolysaccharide-dependent innate intracellular response and leads to IL-8 expression in epithelial cells

The pathogenesis of Shigella flexneri infection centers on the ability of this organism to invade epithelial cells and initiate an intense inflammatory reaction. Because NF-kappa B is an important transcriptional regulator of genes involved in inflammation, we investigated the role of this transcription factor during S. flexneri infection of epithelial cells. Infection of HeLa cells with invasive S. flexneri induced NF-kappa B DNA-binding activity; noninvasive S. flexneri strains did not lead to this activation. The pathway leading to NF-kappa B activation by invasive S. flexneri involved the kinases, NF-kappa B-inducing kinase, I kappa B kinase-1, and I kappa B kinase-2. NF-kappa B activation was linked to inflammation, because invasive S. flexneri activated an IL-8 promoter-driven reporter gene, and the kappa B site within this promoter was indispensable for its induction. Microinjection of bacterial culture supernatants into HeLa cells suggested that LPS is responsible for NF-kappa B activation by S. flexneri infection. In conclusion, the eukaryotic transcription factor NF-kappa B was activated during S. flexneri infection of epithelial cells, which suggests a role for this transcriptional regulator in modulating the immune response during infection in vivo.

PKR stimulates NF-kappaB irrespective of its kinase function by interacting with the IkappaB kinase complex

The interferon (IFN)-induced double-stranded RNA-activated protein kinase PKR mediates inhibition of protein synthesis through phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) and is also involved in the induction of the IFN gene through the activation of the transcription factor NF-kappaB. NF-kappaB is retained in the cytoplasm through binding to its inhibitor IkappaBalpha. The critical step in NF-kappaB activation is the phosphorylation of IkappaBalpha by the IkappaB kinase (IKK) complex. This activity releases NF-kappaB from IkappaBalpha and allows its translocation to the nucleus. Here, we have studied the ability of PKR to activate NF-kappaB in a reporter assay and have shown for the first time that two catalytically inactive PKR mutants, PKR/KR296 and a deletion mutant (PKR/Del42) which lacks the potential eIF2alpha-binding domain, can also activate NF-kappaB. This result indicated that NF-kappaB activation by PKR does not require its kinase activity and that it is independent of the PKR-eIF2alpha relationship. Transfection of either wild-type PKR or catalytically inactive PKR in PKR(0/0) mouse embryo fibroblasts resulted in the activation of the IKK complex. By using a glutathione S-transferase pull-down assay, we showed that PKR interacts with the IKKbeta subunit of the IKK complex. This interaction apparently does not require the integrity of the IKK complex, as it was found to occur with extracts from cells deficient in the NF-kappaB essential modulator, one of the components of the IKK complex. Therefore, our results reveal a novel pathway by which PKR can modulate the NF-kappaB signaling pathway without using its kinase activity.