Inducible degradation of I kappa B alpha in vitro and in vivo requires the acidic C-terminal domain of the protein

Development of mKO2 fusion proteins for real-time imaging and mechanistic investigation of the degradation kinetics of human IκBα in living cells

In resting cells, the nuclear factor kappa B (NF-κB) family of transcription factors is stabilized by complexation with the cytoplasmic inhibitor of kappa B alpha (IκBα). Extracellular stimuli, such as tumor necrosis factor alpha (TNFα) or bacterial lipopolysaccharide activate NF-κB through IκBα phosphorylation and ubiquitin-proteasomal degradation. Herein, we developed a novel biosensor, by fusing the monomeric fluorescent protein Kusabira-Orange 2 to IκBα (mKO2-IκBα), to study the dynamics and structure-activity relationship of IκBα degradation. Site-specific deletion studies on the IκBα sequence revealed that the C-terminal PEST domain is required in signal-induced proteasomal degradation of IκBα and functions independently from ankyrin repeats. Using deletion mutants, we show that IκBα ankyrin repeats do not affect IκBα degradability but affect its degradation rate. We demonstrate, by both real-time confocal microscopy and western blot analysis, that the half-life of mKO2-IκBα in response to TNFα is approximately 35 min, which is similar to the half-life of endogenous IκBα. Using this biosensor we also show that selective proteasome inhibitors, such as lactacystin and MG132, inhibit degradation and affect the kinetics of IκBα in a dose-dependent manner. The techniques described here can have a range of possible applications, such as facilitating studies associated with IκBα dynamics and biochemical characteristics, as well as the screening of potential proteasome inhibitors.

EBNA1: Oncogenic Activity, Immune Evasion and Biochemical Functions Provide Targets for Novel Therapeutic Strategies against Epstein-Barr Virus- Associated Cancers

The presence of the Epstein-Barr virus (EBV)-encoded nuclear antigen-1 (EBNA1) protein in all EBV-carrying tumours constitutes a marker that distinguishes the virus-associated cancer cells from normal cells and thereby offers opportunities for targeted therapeutic intervention. EBNA1 is essential for viral genome maintenance and also for controlling viral gene expression and without EBNA1, the virus cannot persist. EBNA1 itself has been linked to cell transformation but the underlying mechanism of its oncogenic activity has been unclear. However, recent data are starting to shed light on its growth-promoting pathways, suggesting that targeting EBNA1 can have a direct growth suppressing effect. In order to carry out its tasks, EBNA1 interacts with cellular factors and these interactions are potential therapeutic targets, where the aim would be to cripple the virus and thereby rid the tumour cells of any oncogenic activity related to the virus. Another strategy to target EBNA1 is to interfere with its expression. Controlling the rate of EBNA1 synthesis is critical for the virus to maintain a sufficient level to support viral functions, while at the same time, restricting expression is equally important to prevent the immune system from detecting and destroying EBNA1-positive cells. To achieve this balance EBNA1 has evolved a unique repeat sequence of glycines and alanines that controls its own rate of mRNA translation. As the underlying molecular mechanisms for how this repeat suppresses its own rate of synthesis in cis are starting to be better understood, new therapeutic strategies are emerging that aim to modulate the translation of the EBNA1 mRNA. If translation is induced, it could increase the amount of EBNA1-derived antigenic peptides that are presented to the major histocompatibility (MHC) class I pathway and thus, make EBV-carrying cancers better targets for the immune system. If translation is further suppressed, this would provide another means to cripple the virus.

Incorporation of Phosphorylated Tyrosine into Proteins: In Vitro Translation and Study of Phosphorylated IκB-α and Its Interaction with NF-κB

Phosphorylated proteins play important roles in the regulation of many different cell networks. However, unlike the preparation of proteins containing unmodified proteinogenic amino acids, which can be altered readily by site-directed mutagenesis and expressed in vitro and in vivo, the preparation of proteins phosphorylated at predetermined sites cannot be done easily and in acceptable yields. To enable the synthesis of phosphorylated proteins for in vitro studies, we have explored the use of phosphorylated amino acids in which the phosphate moiety bears a chemical protecting group, thus eliminating the negative charges that have been shown to have a negative effect on protein translation. Bis-o-nitrobenzyl protection of tyrosine phosphate enabled its incorporation into DHFR and IκB-α using wild-type ribosomes, and the elaborated proteins could subsequently be deprotected by photolysis. Also investigated in parallel was the re-engineering of the 23S rRNA of Escherichia coli, guided by the use of a phosphorylated puromycin, to identify modified ribosomes capable of incorporating unprotected phosphotyrosine into proteins from a phosphotyrosyl-tRNACUA by UAG codon suppression during in vitro translation. Selection of a library of modified ribosomal clones with phosphorylated puromycin identified six modified ribosome variants having mutations in nucleotides 2600-2605 of 23S rRNA; these had enhanced sensitivity to the phosphorylated puromycin. The six clones demonstrated some sequence homology in the region 2600-2605 and incorporated unprotected phosphotyrosine into IκB-α using a modified gene having a TAG codon in the position corresponding to amino acid 42 of the protein. The purified phosphorylated protein bound to a phosphotyrosine specific antibody and permitted NF-κB binding to a DNA duplex sequence corresponding to its binding site in the IL-2 gene promoter. Unexpectedly, phosphorylated IκB-α also mediated the exchange of exogenous DNA into an NF-κB-cellular DNA complex isolated from the nucleus of activated Jurkat cells.

NF-κB and the link between inflammation and cancer

The nuclear factor-κB (NF-κB) transcription factor family has been considered the central mediator of the inflammatory process and a key participant in innate and adaptive immune responses. Coincident with the molecular cloning of NF-κB/RelA and identification of its kinship to the v-Rel oncogene, it was anticipated that NF-κB itself would be involved in cancer development. Oncogenic activating mutations in NF-κB genes are rare and have been identified only in some lymphoid malignancies, while most NF-κB activating mutations in lymphoid malignancies occur in upstream signaling components that feed into NF-κB. NF-κB activation is also prevalent in carcinomas, in which NF-κB activation is mainly driven by inflammatory cytokines within the tumor microenvironment. Importantly, however, in all malignancies, NF-κB acts in a cell type-specific manner: activating survival genes within cancer cells and inflammation-promoting genes in components of the tumor microenvironment. Yet, the complex biological functions of NF-κB have made its therapeutic targeting a challenge.

Regulation of IkappaBalpha function and NF-kappaB signaling: AEBP1 is a novel proinflammatory mediator in macrophages

NF-κB comprises a family of transcription factors that are critically involved in various inflammatory processes. In this paper, the role of NF-κB in inflammation and atherosclerosis and the regulation of the NF-κB signaling pathway are summarized. The structure, function, and regulation of the NF-κB inhibitors, IκBα and IκBβ, are reviewed. The regulation of NF-κB activity by glucocorticoid receptor (GR) signaling and IκBα sumoylation is also discussed. This paper focuses on the recently reported regulatory function that adipocyte enhancer-binding protein 1 (AEBP1) exerts on NF-κB transcriptional activity in macrophages, in which AEBP1 manifests itself as a potent modulator of NF-κB via physical interaction with IκBα and a critical mediator of inflammation. Finally, we summarize the regulatory roles that recently identified IκBα-interacting proteins play in NF-κB signaling. Based on its proinflammatory roles in macrophages, AEBP1 is anticipated to serve as a therapeutic target towards the treatment of various inflammatory conditions and disorders.

Gly-Ala repeats induce position- and substrate-specific regulation of 26 S proteasome-dependent partial processing

Partial degradation or regulated ubiquitin proteasome-dependent processing by the 26 S proteasome has been demonstrated, but the underlying molecular mechanisms and the prevalence of this phenomenon remain obscure. Here we show that the Gly-Ala repeat (GAr) sequence of EBNA1 affects processing of substrates via the ubiquitin-dependent degradation pathway in a substrate- and position-specific fashion. GAr-mediated increase in stability of proteins targeted for degradation via the 26 S proteasome was associated with a fraction of the substrates being partially processed and the release of the free GAr. The GAr did not cause a problem for the proteolytic activity of the proteasome, and its fusion to the N terminus of p53 resulted in an increase in the rate of degradation of the entire chimera. Interestingly the GAr had little effect on the stability of EBNA1 protein itself, and targeting EBNA1 for 26 S proteasome-dependent degradation led to its complete degradation. Taken together, our data suggest a model in which the GAr prevents degradation or promotes endoproteolytic processing of substrates targeted for the 26 S proteasome by interfering with the initiation step of substrate unfolding. These results will help to further understand the underlying mechanisms for partial proteasome-dependent degradation.

Basal shuttle of NF-kappaB/I kappaB alpha in resting T lymphocytes regulates HIV-1 LTR dependent expression

Background

In HIV-infected T lymphocytes, NF-κB/Rel transcription factors are major elements involved in the activation of LTR-dependent transcription from latency. Most NF-κB heterodimer p65/p50 is sequestered as an inactive form in the cytoplasm of resting T lymphocytes via its interaction with IκB inhibitors. In these cells, both absolute HIV latency and low level ongoing HIV replication have been described. These situations could be related to differences in the balance between NF-κB and IκBα ratio. Actually, control of IκBα by cellular factors such as Murr-1 plays a critical role in maintaining HIV latency in unstimulated T lymphocytes. Formerly, our group demonstrated the presence of nuclear IκBα in T cells after PMA activation. Now we attempt to determine the dynamics of NF-κB/IκBα nucleocytosolic transport in absence of activation as a mechanism to explain both the maintenance of latency and the existence of low level ongoing HIV replication in resting CD₄⁺ T lymphocytes.

Results and conclusion

We show that the inhibition of the nuclear export by leptomycin B in resting CD₄⁺ T cells resulted in nuclear accumulation of both IκBα and p65/RelA, as well as formation of NF-κB/IκBα complexes. This proves the existence of a rapid shuttling of IκBα between nucleus and cytosol even in absence of cellular activation. The nuclear accumulation of IκBα in resting CD₄⁺ T lymphocytes results in inhibition of HIV-LTR dependent transcription as well as restrains HIV replication in CD₄⁺ T lymphocytes. On the other hand, basal NF-κB activity detected in resting CD₄⁺ T lymphocytes was related to low level HIV replication in these cells.

A novel ankyrin-repeat membrane protein, IGN1, is required for persistence of nitrogen-fixing symbiosis in root nodules of Lotus japonicus

Nitrogen-fixing symbiosis of legume plants with Rhizobium bacteria is established through complex interactions between two symbiotic partners. Similar to the mutual recognition and interactions at the initial stages of symbiosis, nitrogen fixation activity of rhizobia inside root nodules of the host legume is also controlled by specific interactions during later stages of nodule development. We isolated a novel Fix(-) mutant, ineffective greenish nodules 1 (ign1), of Lotus japonicus, which forms apparently normal nodules containing endosymbiotic bacteria, but does not develop nitrogen fixation activity. Map-based cloning of the mutated gene allowed us to identify the IGN1 gene, which encodes a novel ankyrin-repeat protein with transmembrane regions. IGN1 expression was detected in all organs of L. japonicus and not enhanced in the nodulation process. Immunoanalysis, together with expression analysis of a green fluorescent protein-IGN1 fusion construct, demonstrated localization of the IGN1 protein in the plasma membrane. The ign1 nodules showed extremely rapid premature senescence. Irregularly enlarged symbiosomes with multiple bacteroids were observed at early stages (8-9 d post inoculation) of nodule formation, followed by disruption of the symbiosomes and disintegration of nodule infected cell cytoplasm with aggregation of the bacteroids. Although the exact biochemical functions of the IGN1 gene are still to be elucidated, these results indicate that IGN1 is required for differentiation and/or persistence of bacteroids and symbiosomes, thus being essential for functional symbiosis.

Distinct NF-κB Regulation by Shear Stress Through Ras-Dependent IκBα Oscillations

NF-kappaB, a transcription factor central to inflammatory regulation during development of atherosclerosis, is activated by soluble mediators and through biomechanical inputs such as flow-mediated shear- stress. To investigate the molecular mechanisms underlying shear stress mediated signal transduction in vascular cells we have developed a system that applies flow-mediated shear stress in a controlled manner, while inserted in a confocal microscope. In combination with GFP-based methods, this allows continuous monitoring of flow induced signal transduction in live cells and in real time. Flow-mediated shear stress, induced using the system, caused a successive increase in NF-kappaB-regulated gene activation. Experiments assessing the mechanisms underlying the NF-kappaB induced activity showed time and flow rate dependent effects on the inhibitor, IkappaBalpha, involving nuclear translocation characterized by a biphasic or cyclic pattern. The effect was observed in both endothelial- and smooth muscle cells, demonstrated to impact noncomplexed IkappaBalpha, and to involve mechanisms distinct from those mediating cytokine signals. In contrast, effects on the NF-kappaB subunit relA were similar to those observed during cytokine stimulation. Further experiments showed the flow induced inter-compartmental transport of IkappaBalpha to be regulated through the Ras GTP-ase, demonstrating a pronounced reduction in the effects following blocking of Ras activity. These studies show that flow-mediated shear stress, regulated by the Ras GTP-ase, uses distinct mechanisms of NF-kappaB control at the molecular level. The oscillatory pattern, reflecting inter-compartmental translocation of IkappaBetaalpha, is likely to have fundamental impact on pathway regulation and on development of shear stress-induced distinct vascular cell phenotypes.

Inhibition of NF-κB by a cell permeable form of IκBα induces apoptosis in eosinophils

An 11 amino acid HIV-TAT peptide can deliver target proteins into a variety of cells in a receptor-independent manner. To generate a highly specific inhibitor of the transcription factor NF-kappa B, we have fused the TAT-peptide to a version of I kappa B alpha that is resistant to signal-induced degradation. TAT-I kappa B alpha(S32A, S36A) inhibited NF-kappa B-dependent transcription in HeLa and A549 cells by retaining NF-kappa B p65 in the cytoplasm. Introduction of TAT-I kappa B alpha(S32A, S36A) into human eosinophils inhibited the nuclear translocation of NF-kappa B and induced apoptosis. Thus, continuous NF-kappa B-dependent transcription is important for eosinophil survival. While eosinophils are normally refractive to standard methods of gene delivery, the ability of TAT fusion proteins to be taken up by these cells should enable a detailed molecular analysis of survival pathways in these cells.

RelA control of IkappaBalpha phosphorylation: a positive feedback loop for high affinity NF-kappaB complexes

NF-kappaB-IkappaB complex formation regulates the level and specificity of NF-kappaB activity. Quantitative analyses showed that RelA-NF-kappaB-induced IkappaBalpha binding is regulated through inhibitor retention and phosphorylation. RelA caused an increase in IkappaBalpha phosphorylation and in degradation, which was enhanced monotonically with inhibitor concentration. In vivo analysis demonstrated the RelA-induced IkappaBalpha/RelA interactions to be specific, saturable, and phosphorylation-dependent. In addition, it showed that phosphorylation regulates both the level and affinity of the complexes and demonstrated an increased average affinity to coincide with reduction in the level of complexes during cytokine-induced pathway activation. The data show that RelA regulation of NF-kappaB-IkappaBalpha complex formation is IkappaBalpha phosphorylation-dependent and that IkappaBalpha/NF-kappaB binding is dynamic and determined by concentration of the subunits. In addition, they suggest that regulation of both complex levels and affinities through phosphorylation, with effects on the system steady state, participate in selective activation of the NF-kappaB pathway.

Induced stabilization of IkappaBalpha can facilitate its re-synthesis and prevent sequential degradation

The transcription factor NF-kappaB is responsible for regulating genes that can profoundly impact cell proliferation, apoptosis, inflammation, and immune responses. The NF-kappaB inhibitor IkappaBalpha is rapidly degraded and then re-synthesized after an NF-kappaB stimulus. We have found that the re-synthesis of IkappaBalpha in a human colon-derived cell line (HT-29) includes the post-translational stabilization of newly synthesized IkappaBalpha. The TNF-alpha-induced stabilization of newly synthesized IkappaBalpha involves the C-terminal PEST region of the protein: N-terminal deletion mutants (lacking the IkappaB kinase phosphorylation sites) were readily stabilized by TNF-alpha, whereas deletion of the C-terminus resulted in a constitutively stable protein. The role of the C-terminus in stabilization was further supported by the finding that fusion of the IkappaBalpha C-terminus to GFP generated a protein that could also be stabilized by TNF-alpha. The p38 mitogen-activated protein (MAP) kinase inhibitor SB203580 prevented stabilization of IkappaBalpha and delayed the re-emergence of IkappaBalpha following TNF-alpha-induced degradation. The IkappaBalpha stabilization pathway could prevent sequential rounds of IkappaBalpha degradation without preventing IkappaBalpha phosphorylation. Analysis of two other cell lines (SW480 and THP-1) revealed similarities and cell-specific differences in the regulation of IkappaBalpha stabilization. We propose that cytokine stabilization of newly synthesized IkappaBalpha in some cell types is a critical homeostatic mechanism that limits inflammatory gene expression.

Critical role for CXCR2 and CXCR2 ligands during the pathogenesis of ventilator-induced lung injury

Mortality related to adult respiratory distress syndrome (ARDS) ranges from 35% to 65%. Lung-protective ventilator strategies can reduce mortality during ARDS. The protective strategies limit tidal volumes and peak pressures while maximizing positive end-expiratory pressure. The efficacy of this approach is due to a reduction of shear-stress of the lung and release of inflammatory mediators. Ventilator-induced lung injury (VILI) is characterized by inflammation. The specific mechanism(s) that recruit leukocytes during VILI have not been elucidated. Because the murine CXC chemokines KC/CXCL1 and MIP-2/CXCL2/3, via CXCR2, are potent neutrophil chemoattractants, we investigated their role in a murine model of VILI. We compared two ventilator strategies in C57BL/6 mice: high peak pressure and high stretch (high peak pressure/stretch) versus low peak pressure/stretch for 6 hours. Lung injury and neutrophil sequestration from the high-peak pressure/stretch group were greater than those from the low-peak pressure/stretch group. In addition, lung expression of KC/CXCL1 and MIP-2/CXCL2/3 paralleled lung injury and neutrophil sequestration. Moreover, in vivo inhibition of CXCR2/CXC chemokine ligand interactions led to a marked reduction in neutrophil sequestration and lung injury. These findings were confirmed using CXCR2(-/-) mice. Together these experiments support the notion that increased expression of KC/CXCL1 and MIP-2/CXCL2/3 and their interaction with CXCR2 are important in the pathogeneses of VILI.

Diabetes-associated sustained activation of the transcription factor nuclear factor-kappaB

Activation of the transcription factor nuclear factor-kappaB (NF-kappaB) has been suggested to participate in chronic disorders, such as diabetes and its complications. In contrast to the short and transient activation of NF-kappaB in vitro, we observed a long-lasting sustained activation of NF-kappaB in the absence of decreased IkappaBalpha in mononuclear cells from patients with type 1 diabetes. This was associated with increased transcription of NF-kappaBp65. A comparable increase in NF-kappaBp65 antigen and mRNA was also observed in vascular endothelial cells of diabetic rats. As a mechanism, we propose that binding of ligands such as advanced glycosylation end products (AGEs), members of the S100 family, or amyloid-beta peptide (Abeta) to the transmembrane receptor for AGE (RAGE) results in protein synthesis-dependent sustained activation of NF-kappaB both in vitro and in vivo. Infusion of AGE-albumin into mice bearing a beta-globin reporter transgene under control of NF-kappaB also resulted in prolonged expression of the reporter transgene. In vitro studies showed that RAGE-expressing cells induced sustained translocation of NF-kappaB (p50/p65) from the cytoplasm into the nucleus for >1 week. Sustained NF-kappaB activation by ligands of RAGE was mediated by initial degradation of IkappaB proteins followed by new synthesis of NF-kappaBp65 mRNA and protein in the presence of newly synthesized IkappaBalpha and IkappaBbeta. These data demonstrate that ligands of RAGE can induce sustained activation of NF-kappaB as a result of increased levels of de novo synthesized NF-kappaBp65 overriding endogenous negative feedback mechanisms and thus might contribute to the persistent NF-kappaB activation observed in hyperglycemia and possibly other chronic diseases.

Disruption of NF-kappaB signaling reveals a novel role for NF-kappaB in the regulation of TNF-related apoptosis-inducing ligand expression

The NF-kappaB family of transcription factors functions broadly in the host control of immunoregulatory gene expression, inflammation, and apoptosis. Using Jurkat T cells engineered to inducibly express a transdominant repressor of IkappaBalpha, we examined the role of NF-kappaB in the regulation of cytokine and apoptotic gene expression. In this T cell model, as well as in primary T lymphocytes, expression of TNF-related apoptosis-inducing ligand (TRAIL) apoptotic signaling protein was dramatically down-regulated by inhibition of NF-kappaB binding activity. TRAIL acts through membrane death receptors to induce apoptosis of activated T lymphocytes and can be up-regulated by a variety of physiological and pharmacological inducers. However, regulation of TRAIL gene expression has not been defined. Treatment with TCR mimetics (PMA/ionomycin, PHA, and anti-CD3/CD28 Abs) resulted in a rapid increase in the expression of TRAIL mRNA and cell surface TRAIL protein. Induction of the transdominant repressor of IkappaBalpha dramatically down-regulated surface expression of TRAIL, indicating an essential role for NF-kappaB in the regulation of TRAIL. The induced expression of TRAIL was linked to a c-Rel binding site in the proximal TRAIL promoter at position -256 to -265; mutation of this site or an adjacent kappaB site resulted in a complete loss of the inducibility of the TRAIL promoter. The regulation of TRAIL expression by NF-kappaB may represent a general mechanism that contributes to the control of TRAIL-mediated apoptosis in T lymphocytes.

Degradation of IkappaBalpha is limited by a postphosphorylation/ubiquitination event

Regulation of IkappaBalpha during activation was examined using EGFP. Single cell analysis showed that both localisation- and cytokine-induced degradation of IkappaBalpha are dependent on expression levels. Cells expressing higher levels of the inhibitor demonstrated an increase in nuclear IkappaBalphaEGFP with a pronounced enhancement in the nuclear/cytoplasmic ratio. Enhancing the levels of the endogenous IkappaBalpha by relA transfection caused significant reduction in IL-1-mediated degradation of the fusion protein. Similarly, IkappaBalphaEGFP-transfected cells showed an inverse correlation between the level of the fusion protein and IL-1-mediateddegradation. Comparing absolute levels demonstrated a biphasic response, with reduction in cells expressing over 15-fold that of endogenous levels. Further experiments using Western analysis showed a positive correlation between both phosphorylation and ubiquitination of IkappaBalphaEGFP, and the level the inhibitor. In contrast, and in agreement with the singlecell analysis, while IL-1 stimulation caused the expected degradation at lower levels of the fusion protein,breakdown of IkappaBalphaEGFP was totally inhibited at the higher transfection levels. The data show that turnover of IkappaBalpha is saturable and suggest that limitation of the pathway by enhanced inhibitor expression is regulated through a post phosphorylation/ubiquitination event, at the level of degradation.

Cytokine-induced stabilization of newly synthesized I(kappa)B-alpha

NF-kappaB activation is triggered by the degradation of inhibitory proteins, such as I(kappa)B-alpha. I(kappa)B-alpha levels are only transiently lowered since one gene activated by NF-kappaB is I(kappa)B-alpha. We found that I(kappa)B-alpha was replenished rapidly in a human colon cell line (HT-29), even in the presence of degradation-inducing phosphorylation (at serine-32). This finding lead us to hypothesize that posttranscriptional mechanisms were also in place to facilitate I(kappa)B-alpha replenishment. Expression of I(kappa)B-alpha from the constitutive, non-NF-kappaB regulated cytomegalovirus promoter in HT-29 cells showed that TNF-alpha or IL-1beta treatment increased I(kappa)B-alpha levels in the absence of transcriptional activation. The TNF-alpha-induced increase in transgenic I(kappa)B-alpha appeared to result from the stabilization of newly synthesized I(kappa)B-alpha, since this increase was effectively preempted by a proteasome inhibitor (MG132) or by I(kappa)B-alpha stabilization through the deletion C-terminal destabilizing elements (without additive or synergistic effects). Analysis of a hepatoma cell line (Hepa 1-4C7) indicated that the I(kappa)B-alpha stabilization may be constitutive in these cells. NF-kappaB stimuli therefore appear to trigger negative feedback pathways in some cells that terminate a NF-kappaB response by increasing the stability of newly synthesized I(kappa)B-alpha.

Interaction between hnRNPA1 and IkappaBalpha is required for maximal activation of NF-kappaB-dependent transcription

Transcriptional activation of NF-kappaB is mediated by signal-induced phosphorylation and degradation of its inhibitor, IkappaBalpha. NF-kappaB activation induces a rapid resynthesis of IkappaBalpha which is responsible for postinduction repression of transcription. Following resynthesis, IkappaBalpha translocates to the nucleus, removes template bound NF-kappaB, and exports NF-kappaB to the cytoplasm in a transcriptionally inactive form. Here we demonstrate that IkappaBalpha interacts directly with another nucleocytoplasmic shuttling protein, hnRNPA1, both in vivo and in vitro. This interaction requires one of the N-terminal RNA binding domains of hnRNPA1 and the C-terminal region of IkappaBalpha. Cells lacking hnRNPA1 are defective in NF-kappaB-dependent transcriptional activation, but the defect in these cells is complemented by ectopic expression of hnRNPA1. hnRNPA1 expression in these cells increased the amount of IkappaBalpha degradation, compared to that of the control cells, in response to activation by Epstein-Barr virus latent membrane protein 1. Thus in addition to regulating mRNA processing and transport, hnRNPA1 also contributes to the control of NF-kappaB-dependent transcription.

Lymphocytes lacking I kappa B-alpha develop normally, but have selective defects in proliferation and function

NF-kappaB has been implicated in the development, activation, and function of B and T lymphocytes. We have evaluated the in vivo effects of deletion of IkappaB-alpha, a major inhibitor of NF-kappaB, on lymphocyte development, proliferation, and function. To elucidate the long term role of IkappaB-alpha in lymphocytes, fetal liver cells of 14.5-day-old IkappaB-alpha(-/-) or wild-type embryos were transplanted into irradiated recombinase-activating gene-2-deficient mice. Within 4 wk, the IkappaB-alpha(-/-) fetal liver cells reconstitute mature B and T cell populations in the recipients comparable to those produced by wild-type fetal liver cells. However, the proliferative responses of IkappaB-alpha(-/-) B cells are enhanced, whereas those of IkappaB-alpha(-/-) T cells are reduced. The levels of IgG1, IgG2a, IgA, and IgE produced by IkappaB-alpha(-/-) B cells are elevated relative to those produced by IkappaB-alpha(+/+) or IkappaB-alpha(+/-). Moreover, the specific immune responses to OVA and the generation of germinal centers are impaired in recipients of IkappaB-alpha(-/-) fetal liver cells. These results indicate that IkappaB-alpha plays a vital role in signal transduction pathways regulating lymphocyte proliferation and also in the production of specific Ig isotypes.

Multiple C-terminal lysine residues target p53 for ubiquitin-proteasome-mediated degradation

In normal cells, p53 is maintained at a low level by ubiquitin-mediated proteolysis, but after genotoxic insult this process is inhibited and p53 levels rise dramatically. Ubiquitination of p53 requires the ubiquitin-activating enzyme Ubc5 as a ubiquitin conjugation enzyme and Mdm2, which acts as a ubiquitin protein ligase. In addition to the N-terminal region, which is required for interaction with Mdm2, the C-terminal domain of p53 modulates the susceptibility of p53 to Mdm2-mediated degradation. To analyze the role of the C-terminal domain in p53 ubiquitination, we have generated p53 molecules containing single and multiple lysine-to-arginine changes between residues 370 and 386. Although wild-type (WT) and mutant molecules show similar subcellular distributions, the mutants display a higher transcriptional activity than WT p53. Simultaneous mutation of lysine residues 370, 372, 373, 381, 382, and 386 to arginine residues (6KR p53 mutant) generates a p53 molecule with potent transcriptional activity that is resistant to Mdm2-induced degradation and is refractory to Mdm2-mediated ubiquitination. In contrast to WT p53, transcriptional activity directed by the 6KR p53 mutant fails to be negatively regulated by Mdm2. Those differences are also manifest in HeLa cells which express the human papillomavirus E6 protein, suggesting that p53 C-terminal lysine residues are also implicated in E6-AP-mediated ubiquitination. These data suggest that p53 C-terminal lysine residues are the main sites of ubiquitin ligation, which target p53 for proteasome-mediated degradation.

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.

Signal-dependent and -independent degradation of free and NF-kappa B-bound IkappaBalpha

A family of inhibitory IkappaB molecules regulates the activation of the transcription factor NF-kappaB. One member of the IkappaB family, IkappaBalpha, plays a major role in the rapid signal-induced activation of NF-kappaB. IkappaBalpha itself is transcriptionally regulated by NF-kappaB allowing for a tight autoregulatory loop that is both sensitive to and rapidly influenced by NF-kappaB activating stimuli. For this pathway to remain primed both for rapid activation of NF-kappaB in the presence of signal and then to suppress NF-kappaB activation once that signal is removed, IkappaBalpha must be exquisitely regulated. The regulation of IkappaBalpha is mainly accomplished through phosphorylation, ubiquitination, and subsequent degradation. The mechanism(s) that regulate IkappaBalpha degradation needs to be able to target IkappaBalpha for degradation in both its NF-kappaB bound and free states in the cell. In this study, we utilize a full-length IkappaBalpha mutant that is unable to associate to RelA/p65. We show that the signal-induced IkappaB kinase (IKK) phosphorylation sites on IkappaBalpha can only significantly influence the regulation of signal-dependent but not signal-independent turnover of IkappaBalpha. We also demonstrate that the constitutive carboxyl-terminal casein kinase II phosphorylation sites are necessary for the proper regulation of both signal-dependent and -independent turnover of IkappaBalpha. These findings further elucidate how the phosphorylation of IkappaBalpha influences the complex regulatory mechanisms involved in maintaining a sensitive NF-kappaB pathway.

Activation of nuclear factor kappaB in single living cells. Dependence of nuclear translocation and anti-apoptotic function on EGFPRELA concentration

We have studied the dynamics of nuclear translocation during nuclear factor kappaB activation by using a p65(RELA)-enhanced green fluorescent protein (EGFP) fusion construct. Quantitation of expression levels indicates that EGFPRELA can be detected at physiological concentrations of about 60,000 molecules per cell. Stimulation of transfected fibroblasts with interleukin (IL)-1beta caused nuclear translocation of EGFPRELA, typically resulting in a 30-fold increase in nuclear protein at maximum induction and a concomitant 20% decrease in cytoplasmic levels. The response of individual cells to IL-1beta was graded, and the kinetics of nuclear translocation were dependent on the dose of IL-1beta and the level of EGFPRELA expression. The rate of nuclear uptake was saturable, and the time lag for uptake increased at higher EGFPRELA expression levels. Furthermore, nuclear translocation was reduced at less than saturating doses of IL-1beta suggesting that the pathway is limited by incoming signals. The response to IL-1beta was biphasic, demonstrating a decline in nuclear import rate at expression levels above three to four times endogenous. This correlated with the anti-apoptotic function of EGFPRELA which was more prominent at low expression levels and demonstrated successively less protection at higher levels. In comparison, transfection of p50 had no effect on the level of apoptosis and demonstrated some toxicity in combination with EGFPRELA.

Prolonged activation of NF-kappaB following traumatic brain injury in rats

Activation of transcription factor, nuclear factor kappa B (NF-kappaB), has been shown to play a key role in inflammatory response, neuronal survival and signaling. We investigated the regional and temporal distribution of activated NF-kappaB in rats at 1 h, 2 h, 24 h, 48 h, 1 week, 2 weeks, 1 month, 2 months, 6 months, and 1 year following brain injury in rats. Early after trauma (1-2 h), activated NF-kappaB was detected in axons, and subsequently found in the cytoplasm and nucleus of neurons by 24 h and lasting up to 1 week. In addition, by 24 h posttrauma, activated NF-kappaB was detected in microglia/macrophages and astrocytes in injured cortex. Surprisingly, this activation persisted for at least 1 year following injury in the cortex, primarily at the margins of progressively enlarging ventricle. Activated NF-kappaB was also detected in endothelial cells, as early as 1 h, and persisted for up to 1 year. These results suggest that a neuronal response to brain trauma includes the activation of NF-kappaB first in the axon with subsequent translocation to the nucleus. Furthermore, these results demonstrate that remarkably prolonged activation of NF-kappaB in glia is found in the same regions undergoing persistent atrophy, suggesting NF-kappaB activation may play a role in long-term inflammatory processes following brain trauma.

The NF-kappa B activation pathway: a paradigm in information transfer from membrane to nucleus

Nuclear factor kappa B (NF-kappaB)/Rel proteins are dimeric, sequence-specific transcription factors involved in the activation of an exceptionally large number of genes in response to inflammation, viral and bacterial infections, and other stressful situations requiring rapid reprogramming of gene expression. In unstimulated cells, NF-kappaB is sequestered in an inactive form in the cytoplasm bound to inhibitory IkappaB proteins. Stimulation leads to the rapid phosphorylation, ubiquitinylation, and ultimately proteolytic degradation of IkappaB, which frees NF-kappaB to translocate to the nucleus and activate the transcription of its target genes. The multisubunit IkappaB kinase (IKK) responsible for the inducible phosphorylation of IkappaB appears to be the initial point of convergence for most stimuli that activate NF-kappaB. IKK contains two catalytic subunits, IKKalpha and IKKbeta, both of which phosphorylate IkappaB at sites phosphorylated in vivo. Gene knockout studies indicate that IKKbeta is primarily responsible for the activation of NF-kappaB in response to proinflammatory stimuli, whereas IKKalpha is essential for keratinocyte differentiation. The activity of IKK is regulated by phosphorylation. IKK contains a regulatory subunit, IKKgamma, which is critical for activation of IKK and is postulated to serve as a recognition site for upstream activators. When phosphorylated, the IKK recognition site on IkappaBalpha serves as a specific recognition site for the kappa-TrCP-like component of a Skp1-Cullin-F-box-type E3 ubiquitin-protein ligase. A variety of other signaling events, including phosphorylation of NF-kappaB, phosphorylation of IKK, new synthesis of IkappaBs, and the processing of NF-kappaB precursors provide mechanisms of modulating the amount and duration of NF-kappaB activity.

The PEST domain of IkappaBalpha is necessary and sufficient for in vitro degradation by mu-calpain

Polypeptide sequences enriched in proline (P), glutamate (E), serine (S), and threonine (T), dubbed PEST domains, are proposed to expedite the degradation of proteins. The proteolysis of one PEST-containing protein, IkappaBalpha, is prerequisite to the activation of the transcription factor NF-kappaB. Two mechanisms of IkappaBalpha degradation in vivo have been described, one well characterized through the ubiquitin-proteasome pathway, and another less characterized through calpain. In this report, a mutational analysis was done to identify any regions of IkappaBalpha that facilitate its recognition and proteolysis by calpain in vitro. These studies revealed that the PEST sequence of IkappaBalpha is critical for its calpain-dependent degradation. Furthermore, the IkappaBalpha-PEST domain binds to the calmodulin-like domain of the large subunit of mu-calpain (muCaMLD). Transfer of the IkappaBalpha-PEST domain to a protein incapable of either binding to or being degraded by mu-calpain allowed for the interaction of the chimeric protein with muCaMLD and resulted in its susceptibility to calpain proteolysis. Moreover, the muCaMLD of calpain acts as a competitive inhibitor of calpain-dependent IkappaBalpha degradation. Our data demonstrate that the IkappaBalpha-PEST sequence acts as a modular domain to promote the physical association with and subsequent degradation by mu-calpain and suggest a functional role for PEST sequences in other proteins as potential calpain-targeting units.

Control of NF-kappa B transcriptional activation by signal induced proteolysis of I kappa B alpha

In unstimulated cells the transcription factor NF-kappa B is held in the cytoplasm in an inactive state by I kappa B inhibitor proteins. Ultimately activation of NF-kappa B is achieved by ubiquitination and proteasome-mediated degradation of I kappa B alpha and we have therefore investigated factors which control this proteolysis. Signal-induced degradation of I kappa B alpha exposes the nuclear localization signal of NF-kappa B, thus allowing it to translocate into the nucleus and activate transcription from responsive genes. An autoregulatory loop is established when NF-kappa B induces expression of the I kappa B alpha gene and newly synthesized I kappa B alpha accumulates in the nucleus where it negatively regulates NF-kappa B-dependent transcription. As part of this post-induction repression, the nuclear export signal on I kappa B alpha mediates transport of NF-kappa B-I kappa B alpha complexes from the nucleus to the cytoplasm. As nuclear export of I kappa B alpha is blocked by leptomycin B this drug was used to examine the effect of cellular location on susceptibility of I kappa B alpha to signal-induced degradation. In the presence of leptomycin B, I kappa B alpha is accumulated in the nucleus and in this compartment is resistant to signal-induced degradation. Thus signal-induced degradation of I kappa B alpha is mainly, if not exclusively a cytoplasmic process. An efficient nuclear export of I kappa B alpha is therefore essential for maintaining a low level of I kappa B alpha in the nucleus and allowing NF-kappa B to be transcriptionally active upon cell stimulation. We have detected a modified form of I kappa B alpha, conjugated to the small ubiquitin-like protein SUMO-1, which is resistant to signal-induced degradation. SUMO-1 modified I kappa B alpha remains associated with NF-kappa B and thus overexpression of SUMO-1 inhibits the signal-induced activation of NF-kappa B-dependent transcription. Reconstitution of the conjugation reaction with highly purified proteins demonstrated that in the presence of a novel E1 SUMO-1 activating enzyme, Ubch9 directly conjugated SUMO-1 to I kappa B alpha on residues K21 and K22, which are also used for ubiquitin modification. Thus, while ubiquitination targets proteins for rapid degradation, SUMO-1 modification acts antagonistically to generate proteins resistant to degradation.

Direct association and nuclear import of the hepatitis B virus X protein with the NF-kappaB inhibitor IkappaBalpha

The X protein of hepatitis B virus (HBV) is a transcriptional activator which is required for infection and may play an important role in HBV-associated hepatocarcinogenesis. It has been suggested that X acts as a nuclear coactivator or stimulates several signal transduction pathways by acting in the cytoplasm. One of these pathways leads to the nuclear translocation of NF-kappaB. A recent report indicates that X activates NF-kappaB by acting on two cytoplasmic inhibitors of this family of transcription factors: IkappaBalpha and the precursor/inhibitor p105. We demonstrate here that X directly interacts with IkappaBalpha, which is able to transport it to the nucleus by a piggyback mechanism. This transport requires a region of IkappaBalpha (the second ankyrin repeat) which has been demonstrated to be involved in its nuclear import following NF-kappaB activation. Using deletion mutants, we showed that amino acids 249 to 253 of IkappaBalpha (located in the C-terminal part of the sixth ankyrin repeat) play a critical role in the interaction with X. This small region overlaps one of the domains of IkappaBalpha mediating the interaction with the p50 and p65 subunits of NF-kappaB and is also close to the nuclear export sequence of IkappaBalpha, therefore providing a potential explanation for the nuclear accumulation of IkappaBalpha with X. This association can also be observed upon the induction of endogenous IkappaBalpha by tumor necrosis factor alpha (TNF-alpha) treatment of Chang cells expressing X. In accordance with this observation, band shift analysis indicates that X induces a sustained NF-kappaB activation following TNF-alpha treatment, probably by preventing the reassociation of newly synthesized nuclear IkappaBalpha with DNA-bound NF-kappaB complexes.

Identification by in vivo genomic footprinting of a transcriptional switch containing NF-kappaB and Sp1 that regulates the IkappaBalpha promoter

In unstimulated cells, NF-kappaB transcription factors are retained in the cytoplasm by inhibitory IkappaB proteins. Upon stimulation by multiple inducers including cytokines or viruses, IkappaBalpha is rapidly phosphorylated and degraded, resulting in the release of NF-kappaB and the subsequent increase in NF-kappaB-regulated gene expression. IkappaBalpha gene expression is also regulated by an NF-kappaB autoregulatory mechanism, via NF-kappaB binding sites in the IkappaBalpha promoter. In previous studies, tetracycline-inducible expression of transdominant repressors of IkappaBalpha (TD-IkappaBalpha) progressively decreased endogenous IkappaBalpha protein levels. In the present study, we demonstrate that expression of TD-IkappaBalpha blocked phorbol myristate acetate-phytohemagglutinin or tumor necrosis factor alpha-induced IkappaBalpha gene transcription and abolished NF-kappaB DNA binding activity, due to the continued cytoplasmic sequestration of RelA(p65) by TD-IkappaBalpha. In vivo genomic footprinting revealed stimulus-responsive protein-DNA binding not only to the -63 to -53 kappaB1 site but also to the adjacent -44 to -36 Sp1 site of the IkappaBalpha promoter. In vivo protection of both sites was inhibited by tetracycline-inducible TD-IkappaBalpha expression. Prolonged NF-kappaB binding and a temporal switch in the composition of NF-kappaB complexes bound to the -63 to -53 kappaB1 site of the IkappaBalpha promoter were also observed; with time after induction, decreased levels of transcriptionally active p50-p65 and increased p50-c-Rel heterodimers were detected at the kappaB1 site. Mutation of either the kappaB1 site or the Sp1 site abolished transcription factor binding to the respective sites and the inducibility of the IkappaBalpha promoter in transient transfection studies. These observations provide the first in vivo characterization of a promoter proximal transcriptional switch involving NF-kappaB and Sp1 that is essential for autoregulation of the IkappaBalpha promoter.

A complex containing betaTrCP recruits Cdc34 to catalyse ubiquitination of IkappaBalpha

Activation of transcription factor NF-kappaB is accomplished by degradation of its inhibitor IkappaBalpha. Signal induced phosphorylation of IkappaBalpha on serine 32 and 36 targets the protein for ubiquitination on lysine 21 and 22. Here we use a phosphorylated peptide substrate representing residues 20-43 of IkappaBalpha to investigate requirements for ubiquitination of IkappaBalpha. Phosphorylation dependent polyubiquitination is carried out by a multiprotein complex containing betaTrCP, Skp1 and Cdc53 (Cull). In the presence of ubiquitin activating enzyme and the protein complex containing betaTrCP, polyubiquitination of IkappaBalpha peptide was dependent on the presence of Cdc34, while Ubc5 only stimulated mono- and di-ubiquitination.

Transient nuclear factor kappaB (NF-kappaB) activation stimulated by interleukin-1beta may be partly dependent on proteasome activity, but not phosphorylation and ubiquitination of the IkappaBalpha molecule, in C6 glioma cells. Regulation of NF-kappaB linked to chemokine production

We previously reported that several stresses can induce cytokine-induced neutrophil chemoattractant expression in a nuclear factor kappaB (NF-kappaB)-dependent manner. In this study, we focused further on the regulation of NF-kappaB. The activation of NF-kappaB and the subsequent cytokine-induced neutrophil chemoattractant induction in response to interleukin-1beta (IL-1beta) were inhibited by proteasome inhibitors, MG132 and proteasome inhibitor I. Translocation of NF-kappaB into nuclei occurs by the phosphorylation, multi-ubiquitination, and degradation of IkappaBalpha, a regulatory protein of NF-kappaB. Nascent IkappaBalpha began to degrade 5 min after treatment with IL-1beta and disappeared completely after 15 min. However, IkappaBalpha returned to basal levels after 45-60 min. Interestingly, resynthesized IkappaBalpha was already phosphorylated at Ser-32. These results suggest that 1) the upstream signals are still activated, although the translocation of NF-kappaB peaks at 15 min; and 2) the regulated protein(s) acts downstream of IkappaBalpha phosphorylation. Western blotting showed that the resynthesized and phosphorylated IkappaB molecules were also upward-shifted by multi-ubiquitination in response to IL-1beta treatment. On the other hand, ATP-dependent Leu-Leu-Val-Tyr cleaving activity transiently increased, peaked at 15 min, and then decreased to basal levels at 60 min. Furthermore, the cytosolic fraction that was stimulated by IL-1beta for 15 min, but not for 0 and 60 min, could degrade phosphorylated and multi-ubiquitinated IkappaBalpha. These results indicate that the transient translocation of NF-kappaB in response to IL-1beta may be partly dependent on transient proteasome activation.

HIV-1 reactivation in resting peripheral blood mononuclear cells of infected adults upon in vitro CD4 cross-linking by ligands of the CDR2-loop in extracellular domain 1

HIV-1 infects resting peripheral blood mononuclear cells (PBMCs) but remains inactive state until subsequent cell activation. We have demonstrated that the cross-linking of cell surface CD4 by gp120-anti-gp120 immune complexes or heat-inactivated HIV-1 (iHIV-1) is sufficient to trigger activation signals leading to virus reactivation (9). In this study, we demonstrate that NF-kappaB nuclear translocation and stimulation of virus production by iHIV-1 were strictly linked to the concentrations of viral proteins used as exogenous stimuli. Moreover, we further investigated the physiologic relevance of these observations. When submitted to an in vitro CD4 cross-linking by iHIV-1, PBMCs from HIV-1-infected patients were found to produce virus. This viral reactivation was associated with increased NF-kappaB nuclear translocation in patients' PBMCs. Additionally, virus reactivation in resting PBMCs infected in vitro with HIV-1 was found to be specifically induced by ligands of the CDR2-loop in domain 1 (D1) of CD4 (virus envelope and anti-CD4 monoclonal antibodies). In contrast, virus reactivation was not observed following CD4 oligomerization by antibodies that bind other epitopes in D1, including the D1/CDR3-loop. Finally, soluble CD4 (sCD4) prevented virus reactivation by D1/CDR2-loop ligands. Our results indicate that the signaling events initiated in PBMCs by oligomerization of CD4 at the D1/CDR2-loop can trigger HIV-1 upregulation in infected individuals.

Fine mapping of the binding sites of monoclonal antibodies raised against the Pk tag

The monoclonal antibody (mAb) SV5-Pk is used widely in a variety of procedures to detect recombinant proteins tagged with the Pk tag, a 14 amino acid sequence derived from the P and V proteins of the paramyxovirus Simian Virus 5. Here we report on the isolation and characterisation of four additional SV5-Pk mAbs (termed SV5-Pk2 to 5) that bind the Pk tag. All the SV5-Pk mAbs can detect Pk tagged recombinant proteins in a variety of immunological procedures, including ELISA and immunofluorescence. Using SPOT technology, the minimal binding epitope of each SV5-Pk mAb was defined by one-sided terminal truncation analysis from either the amino- or carboxy-ends of the Pk peptide. Each mAb recognises slightly different epitopes within the Pk tag, ranging from 5 to 9 amino acids in length. The equilibrium dissociation constants (Kd) of the mAbs, as measured by surface plasmon resonance, ranged from approximately 20 to 60 pmol. Cysteine scanner mutations throughout the Pk tag revealed that some amino acids within the minimal binding epitopes were critical for mAb binding, while others could readily be substituted with little or no effect on antibody binding. The development of the Pk tag as a spacer arm for site-directed chemical coupling, and the use of the mAbs to monitor purification and coupling procedures, is discussed.

IkappaB-mediated inhibition of virus-induced beta interferon transcription

We have examined the consequences of overexpression of the IkappaBalpha and IkappaBbeta inhibitory proteins on the regulation of NF-kappaB-dependent beta interferon (IFN-beta) gene transcription in human cells after Sendai virus infection. In transient coexpression studies or in cell lines engineered to express different forms of IkappaB under tetracycline-inducible control, the IFN-beta promoter (-281 to +19) linked to the chloramphenicol acetyltransferase reporter gene was differentially inhibited in response to virus infection. IkappaBalpha exhibited a strong inhibitory effect on virus-induced IFN-beta expression, whereas IkappaBbeta exerted an inhibitory effect only at a high concentration. Despite activation of the IkappaB kinase complex by Sendai virus infection, overexpression of the double-point-mutated (S32A/S36A) dominant repressors of IkappaBalpha (TD-IkappaBalpha) completely blocked IFN-beta gene activation by Sendai virus. Endogenous IFN-beta RNA production was also inhibited in Tet-inducible TD-IkappaBalpha-expressing cells. Inhibition of IFN-beta expression directly correlated with a reduction in the binding of NF-kappaB (p50-RelA) complex to PRDII after Sendai virus infection in IkappaBalpha-expressing cells, whereas IFN-beta expression and NF-kappaB binding were only slightly reduced in IkappaBbeta-expressing cells. These experiments demonstrate a major role for IkappaBalpha in the regulation of NF-kappaB-induced IFN-beta gene activation and a minor role for IkappaBbeta in the activation process.

Characterization of IkappaBalpha nuclear import pathway

IkappaBalpha controls the transcriptional activity of nuclear factor (NF)-kappaB by retaining it in the cytoplasm; but, when expressed in the nucleus, it can also inhibit the interaction of NF-kappaB with DNA and promote the export of NF-kappaB from the nucleus to the cytoplasm. Here, we report that IkappaBalpha, when not bound to NF-kappaB, is constitutively transported to the nucleus, and we confirm that the interaction of IkappaBalpha with NF-kappaB retains IkappaBalpha in the cytoplasm. Nuclear import of IkappaBalpha does not result from passive diffusion but from a specific energy-dependent transport process that requires the ankyrin repeats of IkappaBalpha. Nuclear accumulation of IkappaBalpha is dependent on importins alpha and beta as well as the small GTPase Ran, which are also responsible for the nuclear import mediated by basic nuclear localization sequences (NLS). However, these proteins are not sufficient to promote IkappaBalpha nuclear translocation. Factor(s) can be removed selectively from cell extracts with ankyrin repeats of IkappaBalpha which strongly reduce import of IkappaBalpha but not of proteins containing basic NLS. These findings indicate that IkappaBalpha is imported in the nucleus by a piggy-back mechanism that involves additional protein(s) containing a basic NLS and able to interact with ankyrin repeats of IkappaBalpha.

Molecular mechanisms of constitutive NF-kappaB/Rel activation in Hodgkin/Reed-Sternberg cells

A common characteristic of malignant cells derived from patients with Hodgkin's disease (HD) is a high level of constitutive nuclear NF-kappaB/Rel activity, which stimulates proliferation and confers resistance to apoptosis. We have analysed the mechanisms that account for NF-kappaB activation in a panel of Hodgkin/Reed-Sternberg (H-RS) cell lines. Whereas two cell lines (L428 and KMH-2) expressed inactive IkappaBalpha, no significant changes in NF-kappaB or IkappaB expression were seen in other H-RS cells (L591, L1236 and HDLM-2). Constitutive NF-kappaB was susceptible to inhibition by recombinant IkappaBalpha, suggesting that neither mutations in the NF-kappaB genes nor posttranslational modifications of NF-kappaB were involved. Endogenous IkappaBalpha was bound to p65 and displayed a very short half-life. IkappaBalpha degradation could be blocked by inhibitors of the NF-kappaB activating pathway. Proteasomal inhibition caused an accumulation of phosphorylated IkappaBalpha and a reduction of NF-kappaB activity in HDLM-2 and L1236 cells. By in vitro kinase assays we demonstrate constitutive IkappaB kinase (IKK) activity in H-RS cells, indicating ongoing signal transduction. Furthermore, H-RS cells secrete one or more factor(s) that were able to trigger NF-kappaB activation. We conclude that aberrant activation of IKK's, and in some cases defective IkappaBs, lead to constitutive nuclear NF-kappaB activity, which in turn results in a growth advantage of Hodgkin's disease tumor cells.

A role for casein kinase II phosphorylation in the regulation of IRF-1 transcriptional activity

The Interferon Regulatory Factors (IRFS) play an important role in the transcriptional control of growth regulatory and immunoregulatory genes. The inducibility and availability of IRF-1 and IRF-2 are influenced by external stimuli, such as virus infection or interferon treatment. In the present study, we sought to examine the potential modulatory role of phosphorylation on IRF-1 transcriptional activity. During the purification of IRF recombinant proteins, a kinase activity copurified with IRF-1 (and IRF-2) from baculovirus infected Sf9 insect cell extracts, but not from E. coli extracts. The kinase activity was also identified in Jurkat T cells, specifically interacted with IRF proteins in GST affinity chromatography, and phosphorylated IRF-1 with high specificity in vitro. Using an in gel kinase assay with recombinant IRF-1 as substrate, two molecular weight forms of the kinase (43 and 38 kDa) were identified. Biochemical criteria identified the kinase activity as the alpha catalytic subunit of casein kinase II (CKII). Furthermore, far western analysis of protein-protein interactions demonstrated that casein kinase II directly interacted with IRF-1 protein. Deletion mutation analysis of IRF-1 revealed that IRF-1 was phosphorylated at two clustered sites, one located between amino acids 138-150, the other in the C-terminal acidic activation domain between amino acids 219-231. Cotransfection studies comparing wild type and point mutated forms of IRF-1 demonstrated that mutations of the four phosphoaceptor residues in the C-terminal transactivation domain, significantly decreased transactivation by IRF-1, indicating that casein kinase II may be involved in the regulation of IRF-1 function. Strikingly, the casein kinase II clusters in IRF-1 resemble the sites identified in the C-terminal PEST domain of IkappaBalpha. The present experiments, together with previously published studies with IkappaBalpha, c-Jun and other proteins, indicate a broad role for casein kinase II phosphorylation in the regulation of transcription factor activity.

Triggering the interferon response: the role of IRF-3 transcription factor

The interferon (IFN) regulatory factors (IRF) consist of a growing family of related transcription proteins first identified as regulators of the IFN-alpha/beta gene promoters, as well as the IFN-stimulated response element (ISRE) of some IFN-stimulated genes. IRF-3 was originally identified as a member of the IRF family based on homology with other IRF family members and on binding to the ISRE of the IFN-stimulated gene 15 (ISG15) promoter. Several recent studies have focused attention on the unique molecular properties of IRF-3 and its role in the regulation of IFN gene expression. IRF-3 is expressed constitutively in a variety of tissues, and the relative levels of IRF-3 mRNA do not change in virus-infected or IFN-treated cells. Following virus infection, IRF-3 is posttranslationally modified by protein phosphorylation at multiple serine and threonine residues, located in the carboxy-terminus of IRF-3. Phosphorylation causes the cytoplasmic to nuclear translocation of IRF-3, stimulation of DNA binding, and increased transcriptional activation, mediated through the association of IRF-3 with the CBP/p300 coactivator. The purpose of this review is to summarize recent investigations demonstrating the important role of IRF-3 in cytokine gene transcription. These studies provide the framework for a model in which virus-dependent phosphorylation of IRF-3 alters protein conformation to permit nuclear translocation, association with transcriptional partners, and primary activation of IFN and IFN-responsive genes.

Degradation of proto-oncoprotein c-Rel by the ubiquitin-proteasome pathway

The c-rel proto-oncogene product, c-Rel, belongs to the Rel/NF-kappaB transcription factor family, which regulates a large variety of cellular functions. The activation of NF-kappaB involves the degradation of the inhibitor, IkappaB, through the ubiquitin-proteasome (Ub-Pr)-mediated pathway. Here we report that the turnover of c-Rel is also regulated by the Ub-Pr pathway, thus adding another level of complexity to the regulation of NF-kappaB. High molecular weight ubiquitinated c-Rel conjugates are detected in cells and accumulate in cells treated with proteasome inhibitors. In a cell-free in vitro degradation assay, c-Rel is degraded specifically through the Ub-Pr pathway. N-terminally truncated c-Rel is readily degraded, implying the dispensability of N-terminal sequence; in contrast, a series of deletion mutants missing C-terminal sequences display a reduced susceptibility to the degradation. Interestingly, the sequence between residues 118 and 171 of c-Rel, i.e. the region immediately following the c-Rel/v-Rel homology domain, appears to play an important role in mediating ubiquitin conjugation and the subsequent degradation. Together with our previous study showing an elevated tumorigenic potential for C-terminally truncated mutants, our data suggest that the C-terminal domain of c-Rel plays an important role in mediating c-Rel degradation and growth control.

Association between HTLV-1 Tax and I kappa B alpha is dependent on the I kappa B alpha phosphorylation state

Biological, molecular, and epidemiological data have demonstrated that human T cell leukemia virus type 1 (HTLV-1) encoded Tax protein plays a central role in the initiation of T cell malignancy. The 40-kDa Tax oncoprotein serves as a potent transcriptional activator that induces viral gene expression driven by the HTLV-1 long terminal repeats and also stimulates multiple cellular genes involved in T cell activation, cell cycle regulation, and gene activation. Since Tax has been shown to interact directly and indirectly with the NF-kappa B/I kappa B regulatory proteins, we examined the significance of an in vivo association between Tax and the I kappa B alpha inhibitor. Using GST affinity chromatography, Tax was shown to interact with the I kappa B alpha ankyrin repeats which are essential for interaction with the NF-kappa B/Rel proteins. In vivo, using I kappa B alpha mutants and co-immunoprecipitation, a preferential interaction between HTLV-1 Tax and N-terminally hypophosphorylated I kappa B alpha was detected. Tax also enhanced binding of I kappa B alpha to the proteasome subunit HsN3, resulting in a Tax-enhanced, constitutive degradation of wild-type and mutated forms of I kappa B alpha in the absence of phosphorylation and ubiquitination. Binding of I kappa B alpha to proteasome subunit HC9 was also observed, but this interaction occurred independently of Tax. Taken together, these results suggest a role for Tax as a viral chaperone resulting in the enhanced constitutive turnover of I kappa B alpha. The association of Tax with hypophosphorylated I kappa B alpha may prevent I kappa B alpha from binding to NF-kappa B and also target I kappa B alpha to the proteasome for degradation via a phosphorylation-independent pathway.

Sequential DNA damage-independent and -dependent activation of NF-kappaB by UV

NF-kappaB activation in response to UV irradiation of HeLa cells or of primary human skin fibroblasts occurs with two overlapping kinetics but totally different mechanisms. Although both mechanisms involve induced dissociation of NF-kappaB from IkappaBalpha and degradation of IkappaBalpha, targeting for degradation and signaling are different. Early IkappaBalpha degradation at 30 min to approximately 6 h is not initiated by UV-induced DNA damage. It does not require IkappaB kinase (IKK), as shown by introduction of a dominant-negative kinase subunit, and does not depend on the presence of the phosphorylatable substrate, IkappaBalpha, carrying serines at positions 32 and 36. Induced IkappaBalpha degradation requires, however, intact N- (positions 1-36) and C-terminal (positions 277-287) sequences. IkappaB degradation and NF-kappaB activation at late time points, 15-20 h after UV irradiation, is mediated through DNA damage-induced cleavage of IL-1alpha precursor, release of IL-1alpha and autocrine/paracrine action of IL-1alpha. Late-induced IkappaBalpha requires the presence of Ser32 and Ser36. The late mechanism indicates the existence of signal transfer from photoproducts in the nucleus to the cytoplasm. The release of the 'alarmone' IL-1alpha may account for some of the systemic effects of sunlight exposure.

DNA-dependent protein kinase phosphorylation of IkappaB alpha and IkappaB beta regulates NF-kappaB DNA binding properties

Regulation of the IkappaB alpha and IkappaB beta proteins is critical for modulating NF-kappaB-directed gene expression. Both IkappaB alpha and IkappaB beta are substrates for cellular kinases that phosphorylate the amino and carboxy termini of these proteins and regulate their function. In this study, we utilized a biochemical fractionation scheme to purify a kinase activity which phosphorylates residues in the amino and carboxy termini of both IkappaB alpha and IkappaB beta. Peptide microsequence analysis by capillary high-performance liquid chromatography ion trap mass spectroscopy revealed that this kinase was the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). DNA-PK phosphorylates serine residue 36 but not serine residue 32 in the amino terminus of IkappaB alpha and also phosphorylates threonine residue 273 in the carboxy terminus of this protein. To determine the biological relevance of DNA-PK phosphorylation of IkappaB alpha, murine severe combined immunodeficiency (SCID) cell lines which lack the DNA-PKcs gene were analyzed. Gel retardation analysis using extract prepared from these cells demonstrated constitutive nuclear NF-kappaB DNA binding activity, which was not detected in extracts prepared from SCID cells complemented with the human DNA-PKcs gene. Furthermore, IkappaB alpha that was phosphorylated by DNA-PK was a more potent inhibitor of NF-kappaB binding than nonphosphorylated IkappaB alpha. These results suggest that DNA-PK phosphorylation of IkappaB alpha increases its interaction with NF-kappaB to reduce NF-kappaB DNA binding properties.

Signal-dependent degradation of IkappaBalpha is mediated by an inducible destruction box that can be transferred to NF-kappaB, bcl-3 or p53

Activation of the transcription factor NF-kappaB in response to a variety of stimuli is governed by the signal-induced proteolytic degradation of NF-kappaB inhibitor proteins, the IkappaBs. We have investigated the sequence requirements for signal-induced IkappaBalpha phosphorylation and proteolysis by generating chimeric proteins containing discrete sub-regions of IkappaBalpha fused to the IkappaBalpha homologue Bcl-3, the transcription factor NF-kappaB1/p50 and the tumour suppressor protein p53. Using this approach we show that the N-terminal signal response domain (SRD) of IkappaBalpha directs their signal-dependent phosphorylation and degradation when transferred to heterologous proteins. The C-terminal PEST sequence from IkappaBalpha was not essential for induced proteolysis of the chimeric proteins. A deletion analysis conducted on the SRD identified a 25 amino acid sub-domain of IkappaBalpha that is necessary and sufficient for the degradative response in vivo and for recognition by TNFalpha-dependent IkappaBalpha kinase in vitro . The results obtained should prove instrumental in the further characterization of IkappaB-specific kinases, as well as the E2 and E3 enzymes responsible for IkappaBalpha ubiquitination. Furthermore, they suggest a novel strategy for generating conditional mutants, by targetting heterologous proteins for transient elimination by the IkappaBalpha pathway.

Inducible expression of IkappaBalpha repressor mutants interferes with NF-kappaB activity and HIV-1 replication in Jurkat T cells

Human immunodeficiency virus (HIV-1) utilizes the NF-kappaB/Rel proteins to regulate transcription through NF-kappaB binding sites in the HIV-1 long terminal repeat (LTR). Normally, NF-kappaB is retained in the cytoplasm by inhibitory IkappaB proteins; after stimulation by multiple activators including viruses, IkappaBalpha is phosphorylated and degraded, resulting in NF-kappaB release. In the present study, we examined the effect of tetracycline-inducible expression of transdominant repressors of IkappaBalpha (TD-IkappaBalpha) on HIV-1 multiplication using stably selected Jurkat T cells. TD-IkappaBalpha was inducibly expressed as early as 3 h after doxycycline addition and dramatically reduced both NF-kappaB DNA binding activity and LTR-directed gene activity. Interestingly, induced TD-IkappaBalpha expression also decreased endogenous IkappaBalpha expression to undetectable levels by 24 h after induction, demonstrating that TD-IkappaBalpha repressed endogenous NF-kappaB-dependent gene transcription. TD-IkappaBalpha expression also sensitized Jurkat cells to tumor necrosis factor-induced apoptosis. De novo HIV-1 infection of Jurkat cells was dramatically altered by TD-IkappaBalpha induction, resulting in inhibition of HIV-1 multiplication, as measured by p24 antigen, reverse transcriptase, and viral RNA. Given the multiple functions of the NF-kappaB/IkappaB pathway, TD-IkappaBalpha expression may interfere with HIV-1 multiplication at several levels: LTR-mediated transcription, Rev-mediated export of viral RNA, inhibition of HIV-1-induced pro-inflammatory cytokines, and increased sensitivity of HIV-1-infected cells to apoptosis.

Inhibition of nuclear factor kappaB activation by a virus-encoded IkappaB-like protein

Certain viruses have evolved mechanisms to counteract innate immunity, a host response in which nuclear factor kappaB (NF-kappaB) transcription factors play a central role. African swine fever virus encodes a protein of 28.2 kDa containing ankyrin repeats similar to those of cellular IkappaB proteins, which are inhibitors of NF-kappaB. Transfection of the African swine fever virus IkappaB gene inhibited tumor necrosis factor- or phorbol ester-induced activation of kappaB- but not AP-1-driven reporter genes. Moreover, African swine fever virus IkappaB co-immunoprecipitated with p65 NF-kappaB, and the purified recombinant protein prevented the binding of p65-p50 NF-kappaB proteins to their target sequences in the DNA. NF-kappaB activation induced by tumor necrosis factor, as detected by mobility shift assays or by transfection of kappaB-driven reporter genes, is impaired in African swine fever virus-infected cells. These results indicate that the African swine fever virus IkappaB gene homologue interferes with NF-kappaB activation, likely representing a new mechanism to evade the immune response during viral infection.

Nuclear Targeted Suppression of NF-κB Activity by the Novel Quinone Derivative E3330

The activation of NF-κB consists of at least three steps: degradation of IκBα, translocation of NF-κB into the nucleus, and post-translational modification of NF-κB (e.g., phosphorylation of p65). In the present study, we found that a novel quinone derivative E3330 selectively inhibited NF-κB-mediated gene expression without affecting any of these steps. E3330, when included in the culture medium, suppressed NF-κB DNA-binding activity in PMA-induced Jurkat cell nuclear extracts, suggesting that the inhibition by E3330 of NF-κB-mediated gene expression was due to its ability to suppress NF-κB DNA-binding activity. Fractionation of the nuclear extracts by column chromatography revealed that a nuclear factor enhanced NF-κB DNA-binding activity and that this enhancing activity was interrupted after treatment with E3330. Moreover, a major polypeptide with a molecular mass of 40 kDa was found to be in the highly purified fraction containing the NF-κB-enhancing activity and predominantly bind E3330. Taken together, these results suggest that the NF-κB activity, after dissociation from IκB, is enhanced by a nuclear factor that is active irrespective of PMA treatment, and the nuclear factor-mediated enhancement is selectively inhibited by E3330. Thus, we conclude that E3330 may belong to a novel class of anti-NF-κB drugs.