Signal transduction through NF-kappa B

The ATM protein is required for sustained activation of NF-kappaB following DNA damage

Cells lacking an intact ATM gene are hypersensitive to ionizing radiation and show multiple defects in the cell cycle-coupled checkpoints. DNA damage usually triggers cell cycle arrest through, among other things, the activation of p53. Another DNA-damage responsive factor is NF-kappaB. It is activated by various stress situations, including oxidative stress, and by DNA-damaging compounds such as topoisomerase poisons. We found that cells from Ataxia Telangiectasia patients exhibit a defect in NF-kappaB activation in response to treatment with camptothecin, a topoisomerase I poison. In AT cells, this activation is shortened or suppressed, compared to that observed in normal cells. Ectopic expression of the ATM protein in AT cells increases the activation of NF-kappaB in response to camptothecin. MO59J glioblastoma cells that do not express the DNA-PK catalytic subunit respond normally to camptothecin. These results support the hypothesis that NF-kappaB is a DNA damage-responsive transcription factor and that its activation pathway by DNA damage shares some components with the one leading to p53 activation.

Ubiquitin-dependent degradation of IkappaBalpha is mediated by a ubiquitin ligase Skp1/Cul 1/F-box protein FWD1

Activation of the transcription factor nuclear factor kappa B (NF-kappaB) is controlled by proteolysis of its inhibitory subunit (IkappaB) via the ubiquitin-proteasome pathway. Signal-induced phosphorylation of IkappaBalpha by a large multisubunit complex containing IkappaB kinases is a prerequisite for ubiquitination. Here, we show that FWD1 (a mouse homologue of Slimb/betaTrCP), a member of the F-box/WD40-repeat proteins, is associated specifically with IkappaBalpha only when IkappaBalpha is phosphorylated. The introduction of FWD1 into cells significantly promotes ubiquitination and degradation of IkappaBalpha in concert with IkappaB kinases, resulting in nuclear translocation of NF-kappaB. In addition, FWD1 strikingly evoked the ubiquitination of IkappaBalpha in the in vitro system. In contrast, a dominant-negative form of FWD1 inhibits the ubiquitination, leading to stabilization of IkappaBalpha. These results suggest that the substrate-specific degradation of IkappaBalpha is mediated by a Skp1/Cull 1/F-box protein (SCF) FWD1 ubiquitin-ligase complex and that FWD1 serves as an intracellular receptor for phosphorylated IkappaBalpha. Skp1/Cullin/F-box protein FWD1 might play a critical role in transcriptional regulation of NF-kappaB through control of IkappaB protein stability.

Mitogen-activated protein kinase/ERK kinase kinases 2 and 3 activate nuclear factor-kappaB through IkappaB kinase-alpha and IkappaB kinase-beta

Recent evidence indicates that nuclear factor-kappaB (NF-kappaB), a transcription factor critically important for immune and inflammatory responses, is activated by a protein kinase cascade. The essential features of this cascade are that a mitogen-activated protein kinase kinase kinase (MAP3K) activates an IkappaB kinase (IKK) that site-specifically phosphorylates IkappaB. The IkappaB protein, which ordinarily sequesters NF-kappaB in the cytoplasm, is subsequently degraded by the ubiquitin-proteasome pathway, thereby allowing the nuclear translocation of NF-kappaB. Thus far, only two MAP3Ks, NIK and MEKK1, have been identified that can activate this pathway. We now show that MEKK2 and MEKK3 can in vivo activate IKK-alpha and IKK-beta, induce site-specific IkappaBalpha phosphorylation, and, relatively modestly, activate an NF-kappaB reporter gene. In addition, dominant negative versions of either IKK-alpha or IKK-beta abolish NF-kappaB activation induced by MEKK2 or MEKK3, thereby providing evidence that these IKKs mediate the NF-kappaB-inducing activities of these MEKKs. In contrast, other MAP3Ks, including MEKK4, ASK1, and MLK3, fail to show evidence of activation of the NF-kappaB pathway. We conclude that a distinct subset of MAP3Ks can activate NF-kappaB.

Nuclear retention of IkappaBalpha protects it from signal-induced degradation and inhibits nuclear factor kappaB transcriptional activation

Transcriptional activation of nuclear factor kappaB (NF-kappaB) is mediated by signal-induced phosphorylation and degradation of its inhibitor, IkappaBalpha. However, NF-kappaB activation induces rapid resynthesis of IkappaBalpha, which is responsible for post-induction repression of transcription. Newly synthesized IkappaBalpha translocates to the nucleus, where it dissociates NF-kappaB from DNA and transports NF-kappaB from the nucleus to the cytoplasm in a nuclear export sequence-dependent process that is sensitive to leptomycin B (LMB). In the present study, LMB was used as a tool to inhibit nuclear export sequence-mediated nuclear protein export and evaluate the consequences for regulation of NF-kappaB-dependent transcriptional activity. Pretreatment of cells with LMB inhibits NF-kappaB-dependent transcriptional activation mediated by interleukin 1beta or tumor necrosis factor alpha. This is a consequence of the inhibition of signal-induced degradation of IkappaBalpha. Although LMB treatment does not affect the signal transduction pathway leading to IkappaBalpha degradation, it blocks IkappaBalpha nuclear export. IkappaBalpha is thus accumulated in the nucleus, and in this compartment it is resistant to signal-induced degradation. These results indicate that the signal-induced degradation of IkappaBalpha is mainly, if not exclusively, a cytoplasmic process. An efficient nuclear export of IkappaBalpha is therefore essential for maintaining a low level of IkappaBalpha in the nucleus and allowing NF-kappaB to be transcriptionally active upon cell stimulation.

The Rho-family GTP exchange factor Vav is a critical transducer of T cell receptor signals to the calcium, ERK, and NF-kappaB pathways

Vav is a GTP/GDP exchange factor (GEF) for members of the Rho-family of GTPases that is rapidly tyrosine-phosphorylated after engagement of the T cell receptor (TCR), suggesting that it may transduce signals from the receptor. T cells from mice made Vav-deficient by gene targeting (Vav-/-) fail to proliferate in response to TCR stimulation because they fail to secrete IL-2. We now show that this is due at least in part to the failure to initiate IL-2 gene transcription. Furthermore, we analyze TCR-proximal signaling pathways in Vav-/- T cells and show that despite normal activation of the Lck and ZAP-70 tyrosine kinases, the mutant cells have specific defects in TCR-induced intracellular calcium fluxes, in the activation of extracellular signal-regulated mitogen-activated protein kinases and in the activation of the NF-kappaB transcription factor. Finally, we show that the greatly reduced TCR-induced calcium flux of Vav-deficient T cells is an important cause of their proliferative defect, because restoration of the calcium flux with a calcium ionophore reverses the phenotype.

The ubiquitin-proteasome pathway and pathogenesis of human diseases

The ubiquitin-proteasome pathway plays a pivotal role in the degradation of short-lived and regulatory proteins important in a variety of basic cellular processes, including regulation of the cell cycle, modulation of cell surface receptors and ion channels, and antigen presentation. The pathway involves an enzymatic cascade through which multiple ubiquitin molecules are covalently attached to the protein substrate, which is then degraded by the 26S proteasome complex. The pathway has been implicated in several forms of malignancy, in the pathogenesis of several genetic diseases (including cystic fibrosis, Angelman's syndrome, and Liddle syndrome), in immune surveillance/viral pathogenesis, and in the pathology of muscle wasting. The molecular mechanisms that underlie these processes are being unraveled at present.

IkappaBalpha ubiquitination is catalyzed by an SCF-like complex containing Skp1, cullin-1, and two F-box/WD40-repeat proteins, betaTrCP1 and betaTrCP2

Destruction of the transcriptional inhibitor IkappaB by the ubiquitin (Ub) system is required for signal-dependent activation of the multifunctional transcriptional factor NF-kappaB, but details of this ubiquitination are largely unknown. We report here that the IkappaBalpha-ubiquitin ligase (IkappaBalpha-E3) is an SCF-like complex containing Skp1, cullin-1, and two homologous F-box/WD40-repeat proteins, betaTrCP1 and betaTrCP2. Intriguingly, all these components are cooperatively recruited to bind to a phosphorylated IkappaBalpha (pIkappaBalpha) produced by tumor necrosis factor-alpha (TNF-alpha) stimulation. IkappaBalpha-E3 bound to pIkappaBalpha catalyzed in vitro ubiquitination of pIkappaBalpha in the presence of ATP, Ub, and E1-activating and E2-conjugating enzymes. Forced expression of betaTrCP1 and betaTrCP2 resulted in dramatic augmentation of the in vitro polyubiquitination activity of IkappaBalpha-E3. These results indicate that the long-sought IkappaBalpha-E3 is an SCF-like complex consisting of multiple proteins which are coordinately assembled during phosphorylation of IkappaBalpha in response to external signals.

In vivo and in vitro recruitment of an IkappaBalpha-ubiquitin ligase to IkappaBalpha phosphorylated by IKK, leading to ubiquitination

Activation of the transcriptional factor NF-kappaB is triggered by signal-dependent degradation of its inhibitor protein IkappaB through the ubiquitin (Ub)-proteasome pathway. We found here that a phosphorylated IkappaBalpha immunoprecipitated (IP-pIkappaBalpha) from the crude extract of HeLa cells which had been treated with tumor necrosis factor-alpha (TNFalpha) caused a dramatic ubiquitination of itself, termed autoubiquitination, when incubated with ATP, Ub, and E1-activating and E2-conjugating enzymes. IP-pIkappaBalpha also catalyzed ubiquitination of an in vitro synthesized 35S-IkappaBalpha previously phosphorylated by IkappaB-kinase (IKK) which is referred to as transubiquitination. No appreciable activity of auto- and transubiquitination was observed in an unphosphorylated IP-IkappaBalpha. Moreover, the putative IkappaBalpha-Ub ligase (IkappaBalpha-E3) present in HeLa cell cytosol associated in vitro with an IKK-phosphorylated recombinant IkappaBalpha, a process independent of NF-kappaB binding to IkappaBalpha or TNFalpha stimulation. Replacement of the two Ser residues at positions 32 and 36 corresponding to IKK phosphorylation sites by Ala resulted in almost complete prevention of binding of an IkappaBalpha-E3 to IkappaBalpha. These results indicate that phosphorylation of IkappaBalpha is necessary and sufficient for recruitment of this IkappaBalpha-E3 to associate with IkappaBalpha.

Listeriolysin O-dependent activation of endothelial cells during infection with Listeria monocytogenes: activation of NF-kappa B and upregulation of adhesion molecules and chemokines

The facultative intracellular bacterium Listeria monocytogenes is an invasive pathogen that crosses the vascular endothelium and disseminates to the placenta and the central nervous system. Its interaction with endothelial cells is crucial for the pathogenesis of listeriosis. By infecting in vitro human umbilical vein endothelial cells (HUVEC) with L. monocytogenes, we found that wild-type bacteria induced the expression of the adhesion molecules (ICAM-1 and E-selectin), chemokine secretion (IL-8 and monocyte chemotactic protein-1) and NF-kappa B nuclear translocation. The activation of HUVEC required viable bacteria and was abolished in prfA-deficient mutants of L. monocytogenes, suggesting that virulence genes are associated with endothelial cell activation. Using a genetic approach with mutants of virulence genes, we found that listeriolysin O (LLO)-deficient mutants inactivated in the hly gene did not induce HUVEC activation, as opposed to mutants inactivated in the other virulence genes. Adhesion molecule expression, chemokine secretion and NF-kappa B activation were fully restored by a strain of Listeria innocua transformed with the hly gene encoding LLO. The relevance in vivo of endothelial cell activation for listerial pathogenesis was investigated in transgenic mice carrying an NF-kappa B-responsive lacZ reporter gene. NF-kappa B activation was visualized by a strong lacZ expression in endothelial cells of capillaries of mice infected with a virulent haemolytic strain, but was not seen in those infected with a non-haemolytic isogenic mutant. Direct evidence that LLO is involved in NF-kappa B activation in transgenic mice was provided by injecting intravenously purified LLO, thus inducing stimulation of NF-kappa B in endothelial cells of blood capillaries. Our results demonstrate that functional listeriolysin O secreted by bacteria contributes as a potent inflammatory stimulus to inducing endothelial cell activation during the infectious process.

Clearance of Chlamydia trachomatis from the murine genital mucosa does not require perforin-mediated cytolysis or Fas-mediated apoptosis

The molecular mechanisms of resistance to genital infection with the mouse pneumonitis (MoPn) strain of Chlamydia trachomatis are unknown. A role for major histocompatibility complex class II-restricted, interleukin-12-dependent CD4(+) T cells has been established, but the functional activity of these cells does not depend on secretion of gamma interferon. Here we examined the potential contribution of T-cell-mediated cytotoxicity and apoptosis to mucosal clearance of MoPn by using mice deficient in the molecular mediators of target cell lysis. Animals lacking perforin, Fas, Fas ligand, or both perforin and Fas ligand were infected genitally with C. trachomatis MoPn and monitored for expression of immunity to chlamydial antigens and clearance of MoPn from the genital mucosa. In each case, the profile of spleen cytokine production, the magnitude of the host antibody response, and the kinetics of chlamydial clearance were similar to those of genetically intact controls. Compensatory overproduction of tumor necrosis factor alpha, an alternate mediator of apoptosis in certain cell types, did not appear to account for the ability of mutant mice to resolve Chlamydia infections. These results fail to support CD4(+) T-cell-mediated apoptosis or CD8(+) T-cell-mediated cytotoxicity as being critical to the clearance of C. trachomatis MoPn urogenital infections.

TNF receptor associated factors in cytokine signaling

Just four years ago the first two members of a new family of molecules involved in signal transduction by members of the TNF receptor superfamily were described and designated TNF Receptor Associated Factors (TRAFs). In the meantime six human and murine TRAFs as well as a TRAF protein from C. elegans have been molecularly cloned. From our current point of view, TRAF proteins appear to represent multifunctional signal adaptors, tightly embedded in a network of signals culminating in the activation of kinase cascades that finally lead to the activation of c-Jun N-terminal kinase. p38 mitogen activated protein kinase, and the transcription factor NF-kappaB, thereby also affecting the balance between survival and cell death. Some of the activities of the individual TRAF family members may be redundant although transgenic knockout animal models have already shown that crucial signaling pathways for single TRAF molecules in vivo can be defined.

Requirement for Nuclear Factor-κB Activation by a Distinct Subset of CD40-Mediated Effector Functions in B Lymphocytes

CD40 stimulation, which is crucial for generating an effective T-dependent humoral response, leads to the activation of transcription factors NF-AT (nuclear factor of activated T cells), AP-1 (activator protein-1), and NF-κB (nuclear factor-κB). However, which CD40-mediated B cell functions actually require activation of specific transcription factors is unknown. We examined the causal relationship between NF-κB activation and CD40 effector functions by evaluating CD40 functions in the presence of an inducible mutant inhibitory κBα (IκBα) superrepressor. IκBαAA inhibited nuclear translocation of multiple NF-κB dimers without the complicating effect of depriving cells of NF-κB during development. This approach complements studies that use mice genetically deficient in single or multiple NF-κB subunits. Interestingly, only a subset of CD40 effector functions was found to require NF-κB activation. Both CD40-induced Ab secretion and B7-1 up-regulation were completely abrogated by expression of IκBαAA. Surprisingly, up-regulation of Fas, CD23, and ICAM-1 was partially independent, and up-regulation of LFA-1 was completely independent, of CD40-induced NF-κB activation. For the first time, it is clear that distinct transcription factors are required for the dynamic regulation of CD40 functions.

Chlamydial and human heat shock protein 60s activate human vascular endothelium, smooth muscle cells, and macrophages

Both chlamydial and human heat shock protein 60s (HSP 60), which colocalize in human atheroma, may contribute to inflammation during atherogenesis. We tested the hypothesis that chlamydial or human HSP 60 activates human endothelial cells (ECs), smooth muscle cells (SMCs), and monocyte-derived macrophages. We examined the expression of adhesion molecules such as endothelial-leukocyte adhesion molecule-1 (E-selectin), intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1), and the production of the proinflammatory cytokine interleukin-6 (IL-6). We also tested whether either HSP 60 induces nuclear factor-kappaB (NF-kappaB), which contributes to the gene expression of these molecules. Either chlamydial or human HSP 60 induced E-selectin, ICAM-1, and VCAM-1 expression on ECs similar to levels induced by Escherichia coli lipopolysaccharide (LPS). Each HSP 60 also significantly induced IL-6 production by ECs, SMCs, and macrophages to an extent similar to that induced by E. coli LPS, as assessed by enzyme-linked immunosorbent assay (ELISA). In ECs, either HSP 60 triggered activation of NF-kappaB complexes containing p65 and p50 Rel proteins. Heat treatment abolished all these effects, but did not alter the ability of E. coli LPS to induce these functions. Chlamydial and human HSP 60s therefore activate human vascular cell functions relevant to atherogenesis and lesional complications. These findings help to elucidate the mechanisms by which a chronic asymptomatic chlamydial infection might contribute to the pathophysiology of atheroma.

NF-kappaB activation is a critical regulator of human granulocyte apoptosis in vitro

During beneficial inflammation, potentially tissue-damaging granulocytes undergo apoptosis before being cleared by phagocytes in a non-phlogistic manner. Here we show that the rate of constitutive apoptosis in human neutrophils and eosinophils is greatly accelerated in both a rapid and concentration-dependent manner by the fungal metabolite gliotoxin, but not by its inactive analog methylthiogliotoxin. This induction of apoptosis was abolished by the caspase inhibitor zVAD-fmk, correlated with the inhibition of nuclear factor-kappa B (NF-kappaB), and was mimicked by a cell permeable inhibitory peptide of NF-kappaB, SN-50; other NF-kappaB inhibitors, curcumin and pyrrolidine dithiocarbamate; and the proteasome inhibitor, MG-132. Gliotoxin also augmented dramatically the early (2-6 h) pro-apoptotic effects of tumor necrosis factor-alpha (TNF-alpha) in neutrophils and unmasked the ability of TNF-alpha to induce eosinophil apoptosis. In neutrophils, TNF-alpha caused a gliotoxin-inhibitable activation of an inducible form of NF-kappaB, a response that may underlie the ability of TNF-alpha to delay apoptosis at later times (12-24 h) and limit its early killing effect. Furthermore, cycloheximide displayed a similar capacity to enhance TNF-alpha induced neutrophil apoptosis even at time points when cycloheximide alone had no pro-apoptotic effect, suggesting that NF-kappaB may regulate the production of protein(s) which protect neutrophils from the cytotoxic effects of TNF-alpha. These data shed light on the biochemical and molecular mechanisms regulating human granulocyte apoptosis and, in particular, indicate that the transcription factor NF-kappaB plays a crucial role in regulating the physiological cell death pathway in granulocytes.

TPL-2 kinase regulates the proteolysis of the NF-kappaB-inhibitory protein NF-kappaB1 p105

The transcription factor NF-kappaB is composed of homodimeric and heterodimeric complexes of Rel/NF-kappaB-family polypeptides, which include Rel-A, c-Rel, Rel-B, NF-kappaB/p50 and NF-kappaB2/p52 . The NF-kappaB1 gene encodes a larger precursor protein, p105, from which p50 is produced constitutively by proteasome-mediated removal of the p105 carboxy terminus. The p105 precursor also acts as an NFkappaB-inhibitory protein, retaining associated p50, c-Rel and Rel-A proteins in the cytoplasm through its carboxy terminus. Following cell stimulation by agonists, p105 is proteolysed more rapidly and released Rel subunits translocate into the nucleus. Here we show that TPL-2 , which is homologous to MAP-kinase-kinase kinases in its catalytic domain, forms a complex with the carboxy terminus of p105. TPL-2 was originally identified, in a carboxy-terminal-deleted form, as an oncoprotein in rats and is more than 90% identical to the human oncoprotein COT. Expression of TPL-2 results in phosphorylation and increased degradation of p105 while maintaining p50 production. This releases associated Rel subunits or p50-Rel heterodimers to generate active nuclear NF-kappaB. Furthermore, kinase-inactive TPL-2 blocks the degradation of p105 induced by tumour-necrosis factor-alpha. TPL-2 is therefore a component of a new signalling pathway that controls proteolysis of NF-kappaB1 p105.

Mitochondria mediate tumor necrosis factor-alpha/NF-kappaB signaling in skeletal muscle myotubes

Tumor necrosis factor-alpha (TNF-alpha) is implicated in muscle atrophy and weakness associated with a variety of chronic diseases. Recently, we reported that TNF-alpha directly induces muscle protein degradation in differentiated skeletal muscle myotubes, where it rapidly activates nuclear factor kappaB (NF-kappaB). We also have found that protein loss induced by TNF-alpha is NF-kappaB dependent. In the present study, we analyzed the signaling pathway by which TNF-alpha activates NF-kappaB in myotubes differentiated from C2C12 and rat primary myoblasts. We found that activation of NF-kappaB by TNF-alpha was blocked by rotenone or amytal, inhibitors of complex I of the mitochondrial respiratory chain. On the other hand, antimycin A, an inhibitor of complex III, enhanced TNF-alpha activation of NK-kappaB. These results suggest a key role of mitochondria-derived reactive oxygen species (ROS) in mediating NF-kappaB activation in muscle. In addition, we found that TNF-alpha stimulated protein kinase C (PKC) activity. However, other signal transduction mediators including ceramide, Ca2+, phospholipase A2 (PLA2), and nitric oxide (NO) do not appear to be involved in the activation of NF-kappaB.

Soluble CD14 acts as a negative regulator of human T cell activation and function

T cell activation is controlled by the coordination of stimulatory and negative regulatory signals which are not completely defined. In this study we tested for a possible direct effect of CD14 on the regulation of T cell activation and function. We show that soluble CD14 (sCD14) induces inhibition of antigen-mediated peripheral blood mononuclear cells (PBMC) proliferation and anti-CD3-mediated proliferation of CD4+CD8+, CD4+CD8+ and CD4+CD8+ Tcell clones. This effect is not due to cell death, but results from a marked inhibition of IL-2 production. Proliferation of T cell clones due to exogenous IL-2 is not affected by sCD14. We also found that sCD14 inhibits production of another Th1-like cytokine, IFN-gamma and a Th2-like cytokine, IL-4. Importantly, sCD14 induces a progressive accumulation of the inhibitory protein IkappaB-alpha. We show that sCD14 binds to activated T cells. Following cell activation, biotinylated sCD14 stains CD3+ PBMC, as well as human T cell clones with varying intensity. The binding is saturable, can be inhibited by excess of unlabeled sCD14 and, following binding, sCD14 is internalized. Collectively, these findings reveal a previously unrecognized function of sCD14, namely its capacity to negatively regulate T lymphocyte activation and function by interacting directly with activated T cells.

Peroxisome proliferator-activated receptor (PPAR) alpha-regulated growth responses and their importance to hepatocarcinogenesis

Peroxisome proliferators (PPs) are a class of non-genotoxic rodent hepatocarcinogens that act by perturbing liver growth regulation. We have demonstrated previously that PPs suppress both spontaneous rat hepatocyte apoptosis and that induced by exogenous stimuli such as transforming growth factor-beta1 (TGF beta1). More recently, we have demonstrated that PPs can suppress apoptosis induced by more diverse stimuli such as DNA damage or ligation of Fas, a receptor related to the tumour necrosis factor alpha (TNF alpha) family of cell surface receptors. PPs transcriptionally activate the peroxisome proliferator activated receptor-alpha, PPAR alpha, a member of the nuclear hormone receptor superfamily. We investigated whether activation of PPAR alpha mediates the suppression of rat hepatocyte apoptosis induced by PPs. We isolated a naturally occurring variant form of PPAR alpha (hPPAR alpha-6/29) from human liver by PCR cloning. hPPAR alpha-6/29 shared the ability of mPPAR alpha to bind to DNA but, unlike mPPAR alpha, could not be activated by PPs. Furthermore, hPPAR alpha-6/29 could act as a dominant negative regulator of PPAR-mediated gene transcription. When introduced into primary rat liver cell cultures by transient transfection, hPPAR alpha-6/29 prevented the suppression of hepatocyte apoptosis by the PP nafenopin, but not that seen in response to phenobarbitone (PB), a non-genotoxic carcinogen whose action does not involve PPAR alpha. The suppression of hepatocyte apoptosis was abrogated completely even though only 30% of hepatocytes were transfected, suggesting the involvement of a soluble factor. Recent data have suggested that TNF alpha, perhaps released by liver Kupffer cells in response to PPs, may play a key role in mediating the effects of PPs on hepatocyte growth regulation.

Identification of the receptor component of the IkappaBalpha-ubiquitin ligase

NF-kappaB, a ubiquitous, inducible transcription factor involved in immune, inflammatory, stress and developmental processes, is retained in a latent form in the cytoplasm of non-stimulated cells by inhibitory molecules, IkappaBs. Its activation is a paradigm for a signal-transduction cascade that integrates an inducible kinase and the ubiquitin-proteasome system to eliminate inhibitory regulators. Here we isolate the pIkappaBalpha-ubiquitin ligase (pIkappaBalpha-E3) that attaches ubiquitin, a small protein which marks other proteins for degradation by the proteasome system, to the phosphorylated NF-kappaB inhibitor pIkappaBalpha. Taking advantage of its high affinity to pIkappaBalpha, we isolate this ligase from HeLa cells by single-step immunoaffinity purification. Using nanoelectrospray mass spectrometry, we identify the specific component of the ligase that recognizes the pIkappaBalpha degradation motif as an F-box/WD-domain protein belonging to a recently distinguished family of beta-TrCP/Slimb proteins. This component, which we denote E3RSIkappaB (pIkappaBalpha-E3 receptor subunit), binds specifically to pIkappaBalpha and promotes its in vitro ubiquitination in the presence of two other ubiquitin-system enzymes, E1 and UBC5C, one of many known E2 enzymes. An F-box-deletion mutant of E3RS(IkappaB), which tightly binds pIkappaBalpha but does not support its ubiquitination, acts in vivo as a dominant-negative molecule, inhibiting the degradation of pIkappaBalpha and consequently NF-kappaB activation. E3RS(IkappaB) represents a family of receptor proteins that are core components of a class of ubiquitin ligases. When these receptor components recognize their specific ligand, which is a conserved, phosphorylation-based sequence motif, they target regulatory proteins containing this motif for proteasomal degradation.

Dendritic cell survival and maturation are regulated by different signaling pathways

Although dendritic cell (DC) activation is a critical event for the induction of immune responses, the signaling pathways involved in this process have not been characterized. In this report, we show that DC activation induced by lipopolysaccharide (LPS) can be separated into two distinct processes: first, maturation, leading to upregulation of MHC and costimulatory molecules, and second, rescue from immediate apoptosis after withdrawal of growth factors (survival). Using a DC culture system that allowed us to propagate immature growth factor-dependent DCs, we have investigated the signaling pathways activated by LPS. We found that LPS induced nuclear translocation of the nuclear factor (NF)-kappaB transcription factor. Inhibition of NF-kappaB activation blocked maturation of DCs in terms of upregulation of major histocompatibility complex and costimulatory molecules. In addition, we found that LPS activated the extracellular signal-regulated kinase (ERK), and that specific inhibition of MEK1, the kinase which activates ERK, abrogated the ability of LPS to prevent apoptosis but did not inhibit DC maturation or NF-kappaB nuclear translocation. These results indicate that ERK and NF-kappaB regulate different aspects of LPS-induced DC activation: ERK regulates DC survival whereas NF-kappaB is responsible for DC maturation.

Human toll-like receptor 2 confers responsiveness to bacterial lipopolysaccharide

Bacterial lipopolysaccharide (LPS) induces activation of the transcription factor nuclear factor kappaB (NF-kappaB) in host cells upon infection. LPS binds to the glycosylphosphatidylinositol (GPI)- anchored membrane protein CD14, which lacks an intracellular signaling domain. Here we investigated the role of mammalian Toll-like receptors (TLRs) as signal transducers for LPS. Overexpression of TLR2, but not TLR1, TLR4, or CD14 conferred LPS inducibility of NF-kappaB activation in mammalian 293 cells. Mutational analysis demonstrated that this LPS response requires the intracellular domain of TLR2. LPS signaling through TLR2 was dependent on serum which contains soluble CD14 (sCD14). Coexpression of CD14 synergistically enhanced LPS signal transmission through TLR2. In addition, purified recombinant sCD14 could substitute for serum to support LPS-induced TLR2 activation. LPS stimulation of TLR2 initiated an interleukin 1 receptor-like NF-kappaB signaling cascade. These findings suggest that TLR2 may be a signaling component of a cellular receptor for LPS.

[Nuclear transcription factor kappa B (NF-kappa B)]

INTRODUCTION

The transcription factor NF-kappa B is a key regulator of genes involved in responses to infection, inflammation and stress. It was first identified as a protein with ADN-binding activity specific for the 10-base-pair kappa B site in the immunoglobulin k light-chain enhancer of B lymphocytes.

CURRENT KNOWLEDGE AND KEY POINTS

NF-kappa B is normally present in the cell cytoplasm bound to an inhibitory I kappa B protein. The nuclear localization signal (NLS) in NF-kappa B is a short amino acid signal sequence involved in nuclear transport. Inducers of NF-kappa activation trigger dissociation and degradation of I kappa B from the NF-kappa B complex. This allows NF-kappa B to translocate to the nucleus and bind to kappa B DNA sites. Repression of NF-kappa B dependent gene expression is one of the major elements of immunosuppression by glucocorticoids. FUTURES PROSPECTS AND PROJECTS: Endothelial cells at sites of inflammatory responses express a variety of genes that are under the control of nuclear factor NF-kappa B. This transcriptional factor with its inhibitors may be linked in an autoregulatory system that can be activated by multiple signals relevant to vascular pathophysiology. NF-kappa B is therefore an obvious target for new types of anti-inflammatory treatment.

Hypoxia-mediated regulation of gene expression in mammalian cells

The molecular mechanism underlying oxygen sensing in mammalian cells has been extensively investigated in the areas of glucose transport, glycolysis, erythropoiesis, angiogenesis and catecholamine metabolism. Expression of functionally operative representative proteins in these specific areas, such as the glucose transporter 1, glycolytic enzymes, erythropoietin, vascular endothelial growth factor and tyrosine hydroxylase are all induced by hypoxia. Recent studies demonstrated that both transcriptional activation and post-transcriptional mechanisms are important to the hypoxia-mediated regulation of gene expression. In this article, the cis-acting elements and trans-acting factors involved in the transcriptional activation of gene expression will be reviewed. In addition, the mechanisms of post-transcriptional mRNA stabilization will also be addressed. We will discuss whether these two processes of regulation of hypoxia-responsive genes are mechanistically linked and co-operative in nature.

Activation of human macrophages by mechanical ventilation in vitro

Positive-pressure mechanical ventilation supports gas exchange in patients with respiratory failure but is also responsible for significant lung injury. In this study, we have developed an in vitro model in which isolated lung cells can be submitted to a prolonged cyclic pressure-stretching strain resembling that of conventional mechanical ventilation. In this model, cells cultured on a Silastic membrane were elongated up to 7% of their initial diameter, corresponding to a 12% increase in cell surface. The lung macrophage was identified as the main cellular source for critical inflammatory mediators such as tumor necrosis factor-alpha, the chemokines interleukin (IL)-8 and -6, and matrix metalloproteinase-9 in this model system of mechanical ventilation. These mediators were measured in supernatants from ventilated alveolar macrophages, monocyte-derived macrophages, and promonocytic THP-1 cells. Nuclear factor-kappaB was found to be activated in ventilated macrophages. Synergistic proinflammatory effects of mechanical stress and molecules such as bacterial endotoxin were observed, suggesting that mechanical ventilation might be particularly deleterious in preinjured or infected lungs. Dexamethasone prevented IL-8 and tumor necrosis factor-alpha secretion in ventilated macrophages. Mechanical ventilation induced low levels of IL-8 secretion by alveolar type II-like cells. Other lung cell types such as endothelial cells, bronchial cells, and fibroblasts failed to produce IL-8 in response to a prolonged cyclic pressure-stretching load. This model is of particular value for exploring physical stress-induced signaling pathways, as well as for testing the effects of novel ventilatory strategies or adjunctive substances aimed at modulating cell activation induced by mechanical ventilation.

Selective inhibition of NF-kappaB activation by the flavonoid hepatoprotector silymarin in HepG2. Evidence for different activating pathways

The bioflavonoid silymarin is found to potently suppress both nuclear factor kappa-B (NF-kappaB)-DNA binding activity and its dependent gene expression induced by okadaic acid in the hepatoma cell line HepG2. Surprisingly, tumor necrosis factor-alpha-induced NF-kappaB activation was not affected by silymarin, thus demonstrating a pathway-dependent inhibition by silymarin. Many genes encoding the proteins of the hepatic acute phase response are under the control of the transcription factor NF-kappaB, a key regulator in the inflammatory and immune reactions. Thus, the inhibitory effect of silymarin on NF-kappaB activation could be involved in its hepatoprotective property.

DNA and a CpG oligonucleotide derived from Babesia bovis are mitogenic for bovine B cells

DNAs from bacteria and variety of nonvertebrate organisms, including nematodes, mollusks, yeasts, and insects, cause polyclonal activation of murine B lymphocytes. Similar studies have not been reported for bovine B cells, and to date no studies have reported mitogenic properties of protozoal DNA for any species. However, we and others have observed that protozoal parasite antigens can induce the proliferation of lymphocytes from nonexposed donors. Extending these studies, we now show that the mitogenic property of protozoal antigen preparations is in part attributable to parasite DNA and that Babesia bovis DNA is directly mitogenic for bovine B cells. DNase treatment of B. bovis extracts abrogated B. bovis-induced proliferation of peripheral blood mononuclear cells from nonexposed cattle. Like DNAs from other organisms that were mitogenic for murine B cells, B. bovis DNA is largely nonmethylated and induced a dose-dependent proliferation of bovine B cells, which was reduced upon methylation. Furthermore, B. bovis and E. coli DNAs enhanced immunoglobulin secretion by cultured B cells, inducing moderate increases in immunoglobulin G1 and stronger increases in immunoglobulin G2. Because certain nonmethylated CpG motifs present in bacterial DNA are known to stimulate proliferation of murine and human B cells, an 11-kb fragment of B. bovis DNA was analyzed for CG dinucleotide content and for the presence of known immunostimulatory sequences (ISS) centered on a CG motif. The frequency of CG dinucleotides was approximately one-half of the expected frequency, and several CpG hexameric sequences with known activity for murine B cells were identified. An oligodeoxynucleotide containing one of these ISS (AACGTT), which is present within the rhoptry-associated protein-1 (rap-1) open reading frame, was shown to stimulate B-cell proliferation. These ISS may be involved in host immune modulation during protozoal infection and may be useful as vaccine adjuvants.

Nuclear factor kappa B: important transcription factor and therapeutic target

Nuclear factor kappa B (NF-kappa B) is an ubiquitous rapid response transcription factor in cells involved in immune and inflammatory reactions, and exerts its effect by expressing cytokines, chemokines, cell adhesion molecules, growth factors, and immunoreceptors. In this manner, NF-kappa B contributes to immunologically mediated diseases such as allograft rejection, rheumatoid arthritis, and bronchial asthma. The prototypic inducible form of NF-kappa B is a heterodimer composed of NF-kB1 and RelA, which both belong to the NF-kappa B/Rel family of proteins. Inactive NF-kappa B is present in the cytoplasm complexed with an inhibitory protein, I kappa B. NF-kappa B is activated by a number of incoming signals from the cell surface. Released from I kappa B inhibition, NF-kappa B translocates into the nucleus and binds to the kappa B motif of the target gene. The NF-kappa B activation process can be inhibited by pharmacologic agents at each activation step. Glucocorticoids inhibit NF-kappa B by directly associating with NF-kappa B or by upregulating I kappa B expression. Cyclosporine and tacrolimus prevent NF-kappa B activation by inhibiting the action of calcineurin, a phosphatase that indirectly induces I kappa B degradation. Deoxyspergualin inhibits NF-kappa B by blocking its nuclear translocation. Aspirin and salicylates inhibit upstream events inducing I kappa B phosphorylation. Tepoxalin and antioxidants inhibit NF-kappa B activation by influencing the redox state of the cell. Further research is required to develop more specific inhibitors to treat diseases mediated by NF-kappa B.

Characterization and quantitation of NF-kappaB nuclear translocation induced by interleukin-1 and tumor necrosis factor-alpha. Development and use of a high capacity fluorescence cytometric system

A new quantitative cytometric technique, termed the ArrayScanTM, is described and used to measure NF-kappaB nuclear translocation induced by interleukin (IL)-1 and tumor necrosis factor-alpha (TNFalpha). The amount of p65 staining is measured in both the nuclei defined by Hoechst 33342 labeling and in the surrounding cytoplasmic area within a preselected number of cells/well in 96-well plates. Using this technique in synchronously activated human chondrocytes or HeLa cells, NF-kappaB was found to move to the nucleus with a half-time of 7-8 min for HeLa and 12-13 min for chondrocytes, a rate in each case about 4-5 min slower than that of Ikappa Balpha degradation. IL-1 receptor antagonist and anti-TypeI IL-1 receptor antiserum on the one hand and anti-TNFalpha and monoclonal anti-TNF receptor 1 antibodies on the other hand could be shown to respectively inhibit IL-1 and TNFalpha stimulation in both cell types. In contrast, a polyclonal anti-TNF receptor 1 antiserum exhibited both a 50% agonism and a 50% antagonism to a TNFalpha stimulation in a dose-dependent fashion, indicating that subtle functional responses to complex agonist and antagonist stimuli could be measured. The effects of different proteasome inhibitors to prevent Ikappa Balpha degradation and subsequent NF-kappaB translocation could also be discriminated; Leu-Leu-Leu aldehyde was only a partial inhibitor with an IC50 of 2 microM, while clastolactacystin beta-lactone was a complete inhibitor with an IC50 of 10 microM. The nonselective kinase inhibitor K252a completely inhibited both IL-1 and TNFalpha stimulation in both cell types with an IC50 of 0.4 microM. This concentration, determined after a 20-min stimulation, was shown to be comparable with that obtained for inhibition of IL-6 production induced by a 100-fold lower IL-1 and TNFalpha concentration measured after 17 h of stimulation. These results suggest that the ArrayScanTM technology provides a rapid, sensitive, quantitative technique for measuring early events in the signal transduction of NF-kappaB.

The TRAF family of signal transducers mediates NF-kappaB activation by the TRANCE receptor

Tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE), a member of the TNF family expressed on activated T-cells, bone marrow stromal cells, and osteoblasts, regulates the function of dendritic cells (DC) and osteoclasts. The TRANCE receptor (TRANCE-R), recently identified as receptor activator of NF-kappabeta (RANK), activates NF-kappaB, a transcription factor critical in the differentiation and activation of those cells. In this report we identify the TNF receptor-associated factor (TRAF) family of signal transducers as important components of TRANCE-R-mediated NF-kappaB activation. Coimmunoprecipitation experiments suggested potential interactions between the cytoplasmic tail of TRANCE-R with TRAF1, TRAF2, TRAF3, TRAF5, and TRAF6. Dominant negative forms of TRAF2, TRAF5, and TRAF6 and an endogenous inhibitor of TRAF2, TRAF-interacting protein (TRIP), substantially inhibited TRANCE-R-mediated NF-kappaB activation, suggesting a role of TRAFs in regulating DC and osteoclast function. Overexpression of combinations of TRAF dominant negative proteins revealed competition between TRAF proteins for the TRANCE-R and the possibility of a TRAF-independent NF-kappaB pathway. Analysis of TRANCE-R deletion mutants suggested that the TRAF2 and TRAF5 interaction sites were restricted to the C-terminal 93 amino acids (C-region). TRAF6 also complexed to the C-region in addition to several regions N-terminal to the TRAF2 and TRAF5 association sites. Furthermore, transfection experiments with TRANCE-R deletion mutants revealed that multiple regions of the TRANCE-R can mediate NF-kappaB activation.

Subtherapeutic corticosteroids potentiate the ability of interleukin 10 to prevent chronic inflammation in rats

BACKGROUND & AIMS

Interleukin (IL)-10, which inhibits macrophages and T-helper lymphocyte type 1 (TH1) lymphocytes, attenuates chronic granulomatous inflammation induced by bacterial cell wall polymers. This study determines whether corticosteroids enhance the protective effects of IL-10 in cultured peripheral blood mononuclear cells (PBMNCs) and in vivo when started before or after the onset of experimental chronic granulomatous inflammation.

METHODS

Intestines of Lewis rats were injected intramurally with streptococcal peptidoglycan-polysaccharide (PG-APS) polymers. Daily murine recombinant IL-10 and/or dexamethasone (DEX) therapy was started 12 hours before or at several intervals after PG-APS injection.

RESULTS

IL-10 plus corticosteroids additively inhibited IL-1beta secretion in human PBMNCs but preserved the beneficial IL-1RA/IL-1beta ratio induced by IL-10. IL-10 started before PG-APS injection significantly attenuated intestinal and extraintestinal inflammation, with even more pronounced effects in combination with subtherapeutic doses of DEX. The combination of DEX decreased the effective dose of IL-10 by at least one half. After onset of systemic inflammation using doses effective for prevention, IL-10 monotherapy had nearly no benefit and DEX plus IL-10 was similar to the mild therapeutic effect of DEX alone.

CONCLUSIONS

The combination of IL-10 and corticosteroids allows lower doses of both agents in preventing chronic intestinal and systemic inflammation. However, timing of IL-10 administration is a critical variable in regulating inflammation.

The role of tumour necrosis factor, interleukin 6, interferon-gamma and inducible nitric oxide synthase in the development and pathology of the nervous system

Proinflammatory cytokines, tumour necrosis factor (TNF)-alpha, interferon (IFN)-gamma and interleukin (IL)-6, have multiple effects in the central nervous system (CNS) not strictly cytotoxic being involved in controlling neuronal and glial activation, proliferation, differentiation and survival, thus influencing neuronal and glial plasticity, degeneration as well as development and regeneration of the nervous system. Moreover, they can contribute to CNS disorders, including multiple sclerosis. Alzheimer's disease and human immunodeficiency virus-associated dementia complex. Recent results with deficient mice in the expression of those cytokines indicate that they are in general more sensible to insults resulting in neural damage. Some of the actions induced by TNF-alpha, and IFN-gamma, including both beneficial and detrimental, are mediated by inducible nitric oxide synthase (iNOS)-derived nitric oxide (NO) production. NO produced by iNOS may be beneficial by promoting the differentiation and survival of neurons. IL-6 does not induce iNOS, explaining why this cytokine is less often involved in this dual role protection pathology. Some of the proinflammatory as well as the neurotrophic effects of those cytokines also involve upregulation of cell adhesion molecules (CAM). Those apparently conflicting results may be reconciled considering that proinflammatory cytokines are involved in promoting the disease, mostly by inducing expression of CAM leading to alteration of the blood-brain barrier integrity, whereas they have a protective role once disease is established due to its immunosuppressive or neurotrophic role. Understanding the dichotomy pathogenesis/neuroprotection of those cytokines may provide a rationale for better therapeutic strategies.

NF-kappaB antiapoptosis: induction of TRAF1 and TRAF2 and c-IAP1 and c-IAP2 to suppress caspase-8 activation

Tumor necrosis factor alpha (TNF-alpha) binding to the TNF receptor (TNFR) potentially initiates apoptosis and activates the transcription factor nuclear factor kappa B (NF-kappaB), which suppresses apoptosis by an unknown mechanism. The activation of NF-kappaB was found to block the activation of caspase-8. TRAF1 (TNFR-associated factor 1), TRAF2, and the inhibitor-of-apoptosis (IAP) proteins c-IAP1 and c-IAP2 were identified as gene targets of NF-kappaB transcriptional activity. In cells in which NF-kappaB was inactive, all of these proteins were required to fully suppress TNF-induced apoptosis, whereas c-IAP1 and c-IAP2 were sufficient to suppress etoposide-induced apoptosis. Thus, NF-kappaB activates a group of gene products that function cooperatively at the earliest checkpoint to suppress TNF-alpha-mediated apoptosis and that function more distally to suppress genotoxic agent-mediated apoptosis.

Nf-kappa B: an important transcription factor in photobiology

Increased gene expression as a consequence of environmental stress is typically observed in mammalian cells. In the past few years the cis- and trans-acting genetic elements responsible for gene induction by radiation (from UV-C to visible light) started to be well characterized. The molecular mechanisms involved in the cell response to radiation reveal that an important control occurs at the transcriptional level and is coordinated by various transcription factors. Among these transcription factors, the well-known Rel/NF-kappa B family of vertebrate transcription factors plays a pivotal role as it controls both the inflammatory and immune responses. The NF-kappa B family comprises a number of structurally related, interacting proteins that bind DNA as dimers and whose activity is regulated by subcellular location. This family includes many members (p50, p52, RelA, RelB, c-Rel, ...), most of which can form DNA-binding homo- or heterodimers. Nuclear expression and consequent biological action of the eukaryotic NF-kappa B transcription factor complex are tightly regulated through its cytoplasmic retention by ankyrin-rich inhibitory proteins known as I kappa B. In the best-characterized example, I kappa B-alpha interacts with a p50/RelA (NF-kappa B) heterodimer to retain the complex in the cytoplasm and inhibit its DNA-binding activity. Upon receiving a variety of signals, many of which are probably mediated by the generation of reactive oxygen species (ROS), I kappa B-alpha undergoes phosphorylation, is then ubiquitinated at nearby lysine residues and finally degraded by the proteasome, while still complexed with NF- kappa B. Removal of I kappa B-alpha uncovers the nuclear localization signals on subunits of NF-kappa B, allowing the complex to enter the nucleus, bind to DNA and affect gene expression. In this paper, we shall show that molecular mechanisms leading to NF-kappa B activation by UV or by photosensitization are initiated by oxidative damage at the membrane level or by the induction of DNA alterations. While the exact nature of the transduction intermediates is still unknown, we shall show that NF-kappa B activation by radiation follows different pathways from those used by pro-inflammatory cytokines.

MEKK1 activates both IkappaB kinase alpha and IkappaB kinase beta

A critical step in the signal-induced activation of the transcription factor NF-kappaB is the site-specific phosphorylation of its inhibitor, IkappaB, that targets the latter for degradation by the ubiquitin-proteasome pathway. We have previously shown that mitogen-activated protein kinase/ERK kinase kinase 1 (MEKK1) can induce both this site-specific phosphorylation of IkappaB alpha at Ser-32 and Ser-36 in vivo and the activity of a high molecular weight IkappaB kinase complex in vitro. Subsequently, others have identified two proteins, IkappaB kinase alpha (IKK-alpha) and IkappaB kinase beta (IKK-beta), that are present in a tumor necrosis factor alpha-inducible, high molecular weight IkappaB kinase complex. These kinases are believed to directly phosphorylate IkappaB based on the examination of the kinase activities of IKK immunoprecipitates, but more rigorous proof of this has yet to be demonstrated. We show herein that recombinant IKK-alpha and IKK-beta can, in fact, directly phosphorylate IkappaB alpha at Ser-32 and Ser-36, as well as homologous residues in IkappaB beta in vitro, and thus are bona fide IkappaB kinases. We also show that MEKK1 can induce the activation of both IKK-alpha and IKK-beta in vivo. Finally, we show that IKK-alpha is present in the MEKK1-inducible, high molecular weight IkappaB kinase complex and treatment of this complex with MEKK1 induces phosphorylation of IKK-alpha in vitro. We conclude that IKK-alpha and IKK-beta can mediate the NF-kappaB-inducing activity of MEKK1.

NF-kappaB in Th2 cells: delayed and long-lasting induction through the TCR complex

Nucelar NFkappaB was analyzed in murine Th2 cells after stimulation via the TCR pathway. Signals delivered through the TCR/CD3 complex induced active NFkappaB translocation to the nucleus of Th2 cells after a late phase (24 h) of the activation process, which is in contrast to the rapid appearance of nuclear NFkappaB (3 h) in Th1 cells after the same stimulation. The slow kinetic of NFkappaB nuclear uptake in Th2 cells was not accelerated by CD28 triggering or under stimulation with antigen plus antigen-presenting cells. Th1 and Th2 cells were also different in the composition of NFkappaB complexes induced. Whereas in Th1 cells TCR triggering induced the presence of nuclear p50.p65 heterodimers, in Th2 cells the complexes induced were shown to be composed of p65 plus another NFkappaB protein distinct from p50. The delayed NFkappaB induction in Th2 cells was dependent on protein synthesis and the significance of this is discussed.

Complementation cloning of NEMO, a component of the IkappaB kinase complex essential for NF-kappaB activation

We have characterized a flat cellular variant of HTLV-1 Tax-transformed rat fibroblasts, 5R, which is unresponsive to all tested NF-kappaB activating stimuli, and we report here its genetic complementation. The recovered full-length cDNA encodes a 48 kDa protein, NEMO (NF-kappaB Essential MOdulator), which contains a putative leucine zipper motif. This protein is absent from 5R cells, is part of the high molecular weight IkappaB kinase complex, and is required for its formation. In vitro, NEMO can homodimerize and directly interacts with IKK-2. The NEMO cDNA was also able to complement another NF-kappaB-unresponsive cell line, 1.3E2, in which the protein is also absent, allowing us to demonstrate that this factor is required not only for Tax but also for LPS, PMA, and IL-1 stimulation of NF-kappaB activity.

The tax oncoprotein of human T-cell leukemia virus type 1 associates with and persistently activates IkappaB kinases containing IKKalpha and IKKbeta

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV1) chronically activates transcription factor NF-kappaB by a mechanism involving degradation of IkappaBalpha, an NF-kappaB-associated cytoplasmic inhibitor. Tax-induced breakdown of IkappaBalpha requires phosphorylation of the inhibitor at Ser-32 and Ser-36, which is also a prerequisite for the transient activation of NF-kappaB in cytokine-treated T lymphocytes. However, it remained unclear how Tax interfaces with the cellular NF-kappaB/IkappaB signaling machinery to generate a chronic rather than a transient NF-kappaB response. We now demonstrate that Tax associates with cytokine-inducible IkappaB kinase (IKK) complexes containing catalytic subunits IKKalpha and IKKbeta, which mediate phosphorylation of IkappaBalpha at Ser-32 and Ser-36. Unlike their transiently activated counterparts in cytokine-treated cells, Tax-associated forms of IKK are constitutively active in either Tax transfectants or HTLV1-infected T lymphocytes. Moreover, point mutations in Tax that ablate its IKK-binding function also prevent Tax-mediated activation of IKK and NF-kappaB. Together, these findings suggest that the persistent activation of NF-kappaB in HTLV1-infected T-cells is mediated by a direct Tax/IKK coupling mechanism.