The role of the C-terminal domain of I kappa B alpha in protein degradation and stabilization

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.

Rapid turnover of unspliced Xbp-1 as a factor that modulates the unfolded protein response

The mammalian and yeast unfolded protein responses (UPR) share the characteristic of rapid elimination of unspliced Xbp-1 (Xbp-1u) and unspliced Hac1p, respectively. These polypeptides derive from mRNAs, whose splicing is induced upon onset of the UPR, so as to allow synthesis of transcription factors essential for execution of the UPR itself. Whereas in yeast translation of unspliced Hac1p is blocked, mammalian Xbp-1u is synthesized constitutively and eliminated by rapid proteasomal degradation. Here we show that the rate of Xbp-1u degradation approaches its rate of synthesis. The C terminus of XBP-1u ensures its trafficking to the cytoplasm, and is sufficient to impose rapid degradation. Degradation of XBP-1u involves both ubiquitin-dependent and ubiquitin-independent mechanisms, which might explain its unusually rapid turnover. Xbp-1(-/-) mouse embryonic fibroblasts reconstituted with mutants of XBP-1u that show improved stability differentially activate UPR target genes. Unexpectedly, we found that one of the mutants activates transcription of both Xbp-1-specific and non-Xbp-1-dependent UPR targets in response to tunicamycin treatment, even more potently than does wild type Xbp-1. We suggest that the degradation of Xbp-1u is required to prevent uncontrolled activation of the UPR while allowing short dwell times for initiation of this response.

A requirement for NF-κB induction in the production of replication-competent HHV-8 virions

The gammaherpesvirus human herpesvirus 8 (HHV-8) infects endothelial and B-lymphoid cells and is responsible for the development of Kaposi's sarcoma and primary effusion lymphoma (PEL). In the present study, we demonstrate that the activation of the NF-kappaB pathway during HHV-8 lytic replication is required for the generation of replication-competent virions capable of initiating a de novo infection of endothelial cells. In the HHV-8-positive PEL cell line BCBL-1, tetradecanoyl phorbol acetate (TPA) induction of the lytic cycle activates the NF-kappaB pathway, and this activation requires the induction of the IKKbeta component of the classical IkappaB kinase (IKK) complex. To further investigate the role of NF-kappaB activation in HHV-8 lytic replication, the NF-kappaB super-repressor IkappaBalpha-2NDelta4 was introduced into BCBL-1 cells by retroviral transduction. Expression of IkappaBalpha-2NDelta4 completely abolished NF-kappaB activity, as demonstrated by the loss of NF-kappaB DNA-binding activity and the absence of expression of the endogenous, NF-kappaB-regulated IkappaBalpha gene. NF-kappaB blockade dramatically impaired the ability of HHV-8 to produce infectious particles capable of initiating an effective de novo infection of endothelial EA.hy926 cells, as demonstrated by the lack of viral protein production in the target cells. Diminished infectivity did not appear to be caused by a reduction in virus titer, as demonstrated by equivalent viral DNA content in the supernatant of TPA-stimulated BCBL-1 and BCBL-1/2N4 cells. Although the viral and/or cellular products affected by NF-kappaB inactivation remain to be fully characterized, these data demonstrate an unexpected role for NF-kappaB induction during lytic reactivation in the production of replication-competent HHV-8 virions.

Mechanisms of proinflammatory cytokine-induced biphasic NF-kappaB activation

The transcription factor NF-kappaB regulates genes involved in innate and adaptive immune response, inflammation, apoptosis, and oncogenesis. Proinflammatory cytokines induce the activation of NF-kappaB in both transient and persistent phases. We investigated the mechanism for this biphasic NF-kappaB activation. Our results show that MEKK3 is essential in the regulation of rapid activation of NF-kappaB, whereas MEKK2 is important in controlling the delayed activation of NF-kappaB in response to stimulation with the cytokines TNF-alpha and IL-1alpha. MEKK3 is involved in the formation of the IkappaBalpha:NF-kappaB/IKK complex, whereas MEKK2 participates in assembling the IkappaBbeta:NF-kappaB/IKK complex; these two distinct complexes regulate the proinflammatory cytokine-induced biphasic NF-kappaB activation. Thus, our study reveals a novel mechanism in which different MAP3K and IkappaB isoforms are involved in specific complex formation with IKK and NF-kappaB for regulating the biphasic NF-kappaB activation. These findings provide further insight into the regulation of cytokine-induced specific and temporal gene expression.

Hepatitis C virus NS5A and subgenomic replicon activate NF-kappaB via tyrosine phosphorylation of IkappaBalpha and its degradation by calpain protease

Hepatitis C virus nonstructural protein 5A (NS5A) has been implicated in the HCV antiviral resistance, replication, and transactivation of cellular gene expression. We have recently shown that HCV NS5A activates NF-kappaB via oxidative stress (22). In this study, we investigate the molecular mechanism(s) of NF-kappaB activation in response to oxidative stress induced by NS5A protein. In contrast to the classic Ser32,36 phosphorylation of IkappaBalpha, we report here that tyrosine phosphorylation of IkappaBalpha at Tyr42 and Tyr305 residues is induced by the HCV NS5A and the subgenomic replicons in the NF-kappaB activation process. Use of IkappaBalpha-Tyr42,305 double mutant provided the evidence for their key role in the activation of NF-kappaB. Activation of NF-kappaB was blocked by a series of tyrosine kinase inhibitors but not by IkappaB kinase inhibitor BAY 11-7085. More specifically, a ZAP-70 knock-out cell line expressing NS5A and other nonstructural proteins respectively prevented the NF-kappaB activation, indicating the involvement of ZAP-70 as a probable tyrosine kinase in the activation process. Evidence is also presented for the possible role of calpain proteases in the NS5A-induced IkappaBalpha degradation. These studies collectively define an alternate pathway of NF-kappaB activation by NS5A alone or in the context of the HCV subgenomic replicon. Constitutive activation of NF-kappaB by HCV has implications in the chronic liver disease including hepatocellular carcinoma associated with HCV infection.

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.

Effect of heat shock preconditioning on NF-kappaB/I-kappaB pathway during I/R injury of the rat liver

Hepatic ischemia-reperfusion (I/R) injury continues to be a fatal complication after liver surgery. Heat shock (HS) preconditioning is an effective strategy for protecting the liver from I/R injury, but its exact mechanism is still unclear. Because the activation of nuclear factor-kappaB (NF-kappaB) is an important event in the hepatic I/R-induced inflammatory response, the effect of HS preconditioning on the pathway for NF-kappaB activation was investigated. In the control group, NF-kappaB was activated 60 min after reperfusion, but this activation was suppressed in the HS group. Messenger RNA expressions of proinflammatory mediators during reperfusion were also reduced with HS preconditioning. Concomitant with NF-kappaB activation, NF-kappaB inhibitor I-kappaB proteins were degraded in the control group, but this degradation was suppressed in the HS group. This study shows that HS preconditioning protected the liver from I/R injury by suppressing the activation of NF-kappaB and the subsequent expression of proinflammatory mediators through the stabilization of I-kappaB proteins.

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.

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.

CGP57148B (STI-571) induces differentiation and apoptosis and sensitizes Bcr-Abl–positive human leukemia cells to apoptosis due to antileukemic drugs

The differentiation and apoptosis-sensitizing effects of the Bcr-Abl–specific tyrosine kinase inhibitor CGP57148B, also known as STI-571, were determined in human Bcr-Abl–positive HL-60/Bcr-Abl and K562 cells. First, the results demonstrate that the ectopic expression of the p185 Bcr-Abl fusion protein induced hemoglobin in the acute myeloid leukemia (AML) HL-60 cells. Exposure to low-dose cytosine arabinoside (Ara-C; 10 nmol/L) increased hemoglobin levels in HL-60/Bcr-Abl and in the chronic myeloid leukemia (CML) blast crisis K562 cells, which express the p210 Bcr-Abl protein. As compared with HL-60/neo, HL-60/Bcr-Abl and K562 cells were resistant to apoptosis induced by Ara-C, doxorubicin, or tumor necrosis factor-α (TNF-α), which was associated with reduced processing of caspase-8 and Bid protein and decreased cytosolic accumulation of cytochrome c (cyt c). Exposure to CGP57148B alone increased hemoglobin levels and CD11b expression and induced apoptosis of HL-60/Bcr-Abl and K562 cells. CGP57148B treatment down-regulated antiapoptotic XIAP, cIAP1, and Bcl-xL, without affecting Bcl-2, Bax, Apaf-1, Fas (CD95), Fas ligand, Abl, and Bcr-Abl levels. CGP57148B also inhibited constitutively active Akt kinase and NFκB in Bcr-Abl–positive cells. Attenuation of NFκB activity by ectopic expression of transdominant repressor of IκB sensitized HL-60/Bcr-Abl and K562 cells to TNF-α but not to apoptosis induced by Ara-C or doxorubicin. Importantly, cotreatment with CGP57148B significantly increased Ara-C– or doxorubicin-induced apoptosis of HL-60/Bcr-Abl and K562 cells. This was associated with greater cytosolic accumulation of cyt c and PARP cleavage activity of caspase-3. These in vitro data indicate that combinations of CGP57148B and antileukemic drugs such as Ara-C may have improved in vivo efficacy against Bcr-Abl–positive acute leukemia.

CGP57148B (STI-571) induces differentiation and apoptosis and sensitizes Bcr-Abl–positive human leukemia cells to apoptosis due to antileukemic drugs

The differentiation and apoptosis-sensitizing effects of the Bcr-Abl–specific tyrosine kinase inhibitor CGP57148B, also known as STI-571, were determined in human Bcr-Abl–positive HL-60/Bcr-Abl and K562 cells. First, the results demonstrate that the ectopic expression of the p185 Bcr-Abl fusion protein induced hemoglobin in the acute myeloid leukemia (AML) HL-60 cells. Exposure to low-dose cytosine arabinoside (Ara-C; 10 nmol/L) increased hemoglobin levels in HL-60/Bcr-Abl and in the chronic myeloid leukemia (CML) blast crisis K562 cells, which express the p210 Bcr-Abl protein. As compared with HL-60/neo, HL-60/Bcr-Abl and K562 cells were resistant to apoptosis induced by Ara-C, doxorubicin, or tumor necrosis factor-α (TNF-α), which was associated with reduced processing of caspase-8 and Bid protein and decreased cytosolic accumulation of cytochrome c (cyt c). Exposure to CGP57148B alone increased hemoglobin levels and CD11b expression and induced apoptosis of HL-60/Bcr-Abl and K562 cells. CGP57148B treatment down-regulated antiapoptotic XIAP, cIAP1, and Bcl-xL, without affecting Bcl-2, Bax, Apaf-1, Fas (CD95), Fas ligand, Abl, and Bcr-Abl levels. CGP57148B also inhibited constitutively active Akt kinase and NFκB in Bcr-Abl–positive cells. Attenuation of NFκB activity by ectopic expression of transdominant repressor of IκB sensitized HL-60/Bcr-Abl and K562 cells to TNF-α but not to apoptosis induced by Ara-C or doxorubicin. Importantly, cotreatment with CGP57148B significantly increased Ara-C– or doxorubicin-induced apoptosis of HL-60/Bcr-Abl and K562 cells. This was associated with greater cytosolic accumulation of cyt c and PARP cleavage activity of caspase-3. These in vitro data indicate that combinations of CGP57148B and antileukemic drugs such as Ara-C may have improved in vivo efficacy against Bcr-Abl–positive acute leukemia.

Crucial role of the amino-terminal tyrosine residue 42 and the carboxyl-terminal PEST domain of I kappa B alpha in NF-kappa B activation by an oxidative stress

Activation of transcription factor NF-kappa B involves the signal-dependent degradation of basally phosphorylated inhibitors such as I kappa B alpha. In response to proinflammatory cytokines or mitogens, the transduction machinery has recently been characterized, but the activation mechanism upon oxidative stress remains unknown. In the present work, we provide several lines of evidence that NF-kappa B activation in a T lymphocytic cell line (EL4) by hydrogen peroxide (H2O2) did not involve phosphorylation of the serine residues 32 and 36 in the amino-terminal part of I kappa B alpha. Indeed, mutation of Ser32 and Ser36 blocked IL-1 beta- or PMA-induced NF-kappa B activation, but had no effect on its activation by H2O2. Although I kappa B alpha was phosphorylated upon exposure to H2O2, tyrosine residue 42 and the C-terminal PEST (proline-glutamic acid-serine-threonine) domain played an important role. Indeed, mutation of tyrosine 42 or serine/threonine residues of the PEST domain abolished NF-kappa B activation by H2O2, while it had no effect on activation by IL-1 beta or PMA-ionomycin. This H2O2-inducible phosphorylation was not dependent on I kappa B kinase activation, but could involve casein kinase II, because an inhibitor of this enzyme (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole) blocks NF-kappa B activation. H2O2-induced I kappa B alpha phosphorylation was followed by its degradation by calpain proteases or through the proteasome. Taken together, our findings suggest that NF-kappa B activation by H2O2 involves a new mechanism that is totally distinct from those triggered by proinflammatory cytokines or mitogens.

Peptides corresponding to the N and C termini of IkappaB-alpha, -beta, and -epsilon as probes of the two catalytic subunits of IkappaB kinase, IKK-1 and IKK-2

The signal-inducible phosphorylation of serines 32 and 36 of IkappaB-alpha is the key step in regulating the subsequent ubiquitination and proteolysis of IkappaB-alpha, which then releases NF-kappaB to promote gene transcription. The multisubunit IkappaB kinase (msIKK) responsible for this phosphorylation contains two catalytic subunits, termed IKK-1 and IKK-2. Using recombinant IKK-2, a kinetic pattern consistent with a random, sequential binding mechanism was observed with the use of a peptide corresponding to amino acids 26-42 of IkappaB-alpha. Values of 313 microM, 15.5 microM, and 1.7 min(-1) were obtained for K(peptide), K(ATP), and k(cat), respectively. The value of alpha, a factor by which binding of one substrate changes the dissociation constant for the other substrate, was determined to be 0.2. Interestingly, the recombinant IKK-1 subunit gave similar values for alpha and K(ATP), but values of 1950 microM and 0.016 min(-1) were calculated for K(peptide) and k(cat), respectively. This suggests that the IKK-2 catalytic subunit provides nearly all of the catalytic activity of the msIKK complex with the IKK-1 subunit providing little contribution to catalysis. Using peptides corresponding to different regions of IkappaB-alpha within amino acids 21-47, it was shown that amino acids 31-37 provide most binding interactions (-4.7 kcal/mol of binding free energy) of the full-length IkappaB-alpha (-7.9 kcal/mol) with the IKK-2. This is consistent with the observation that IKK-2 is able to phosphorylate the IkappaB-beta and IkappaB-epsilon proteins, which have consensus phosphorylation sites nearly identical to that of amino acids 31-37 of IkappaB-alpha. A peptide corresponding to amino acids 279-303 in the C-terminal domain of IkappaB-alpha was unable to activate IKK-2 to phosphorylate an N-terminal peptide, which is in contrast to the results observed with the msIKK. Moreover, the IKK-2 catalyzes the phosphorylation of the full-length IkappaB-alpha and the amino acid 26-42 peptide with nearly equal efficiency, while the msIKK catalyzes the phosphorylation of the full-length IkappaB-alpha 25,000 times more efficiently than the 26-42 peptide. Therefore, the C terminus of IkappaB-alpha is important in activating the msIKK through interactions with subunits other than the IKK-2.

NF-kappaB activation and HIV-1 induced apoptosis

HIV infection leads to the progressive loss of CD4+ T cells and the near complete destruction of the immune system in the majority of infected individuals. High levels of viral gene expression and replication result in part from the activation of NF-kappaB transcription factors, which in addition to orchestrating the host inflammatory response also activate the HIV-1 long terminal repeat. NF-kappaB induces the expression of numerous cytokine, chemokine, growth factor and immunoregulatory genes, many of which promote HIV-1 replication. Thus, NF-kappaB activation represents a double edged sword in HIV-1 infected cells, since stimuli that induce an NF-kappaB mediated immune response will also lead to enhanced HIV-1 transcription. NF-kappaB has also been implicated in apoptotic signaling, protecting cells from programmed cell death under most circumstances and accelerating apoptosis in others. Therefore, activation of NF-kappaB can impact upon HIV-1 replication and pathogenesis at many levels, making the relationship between HIV-1 expression and NF-kappaB activation multi-faceted. This review will attempt to analyse the many faces and functions of NF-kappaB in the HIV-1 lifecycle.

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.

Translational homeostasis: eukaryotic translation initiation factor 4E control of 4E-binding protein 1 and p70 S6 kinase activities

Eukaryotic translation initiation factor 4E (eIF4E) is the mRNA 5' cap binding protein, which plays an important role in the control of translation. The activity of eIF4E is regulated by a family of repressor proteins, the 4E-binding proteins (4E-BPs), whose binding to eIF4E is determined by their phosphorylation state. When hyperphosphorylated, 4E-BPs do not bind to eIF4E. Phosphorylation of the 4E-BPs is effected by the phosphatidylinositol (PI) 3-kinase signal transduction pathway and is inhibited by rapamycin through its binding to FRAP/mTOR (FK506 binding protein-rapamycin-associated protein or mammalian target of rapamycin). Phosphorylation of 4E-BPs can also be induced by protein synthesis inhibitors. These observations led to the proposal that FRAP/mTOR functions as a "sensor" of the translational apparatus (E. J. Brown and S. L. Schreiber, Cell 86:517-520, 1996). To test this model, we have employed the tetracycline-inducible system to increase eIF4E expression. Removal of tetracycline induced eIF4E expression up to fivefold over endogenous levels. Strikingly, upon induction of eIF4E, 4E-BP1 became dephosphorylated and the extent of dephosphorylation was proportional to the expression level of eIF4E. Dephosphorylation of p70(S6k) also occurred upon eIF4E induction. In contrast, the phosphorylation of Akt, an upstream effector of both p70(S6k) and 4E-BP phosphorylation, was not affected by eIF4E induction. We conclude that eIF4E engenders a negative feedback loop that targets a component of the PI 3-kinase signalling pathway which lies downstream of PI 3-kinase.

NFkappaB activation is required for interferon regulatory factor-1-mediated interferon beta induction

The interferon regulatory factor 1 (IRF-1) acts as a transcriptional inducer of the interferon beta (IFN-beta) gene and interferon-stimulated genes. Here we report that IRF-1-mediated IFN-beta induction depends on NFkappaB activity. IRF-1 by itself initiates NFkappaB activation by inducing a reduction in cellular MAD3/IkappaBalpha, an inhibitor of NFkappaB. After nuclear translocation, NFkappaB synergizes with IRF-1 on the cis-elements positive regulatory domain (PRD)II and PRDI/III to induce transcription of the IFN-beta gene. In contrast with IFN-beta transcription induced by dsRNA or virus, c-Jun/ATF-2 binding to PRDIV is not involved. Recombinant MAD3/IkappaBalpha is phosphorylated in vitro by extracts from IRF-1-expressing cells. IRF-1-dependent MAD3/IkappaBalpha degradation is not detectable in cells expressing a dominant negative mutant of the protein kinase PKR, suggesting that PKR mediates MAD3/IkappaBalpha degradation.

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.

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.

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.

Lipopolysaccharide-induced NF-kappaB activation in human endothelial cells involves degradation of IkappaBalpha but not IkappaBbeta

We studied the signal transduction pathways involved in NF-kappaB activation and the induction of the cytoprotective A20 gene by lipopolysaccharide (LPS) in human umbilical vein endothelial cells (HUVEC). LPS induced human A20 mRNA expression with a maximum level 2 h after stimulation. The proteasome inhibitor N-acetyl-leucinyl-leucinyl-norleucinal-H (ALLN) and the tyrosine kinase inhibitor herbimycin A (HMA) blocked A20 mRNA expression and partially inhibited NF-kappaB DNA-binding activity induced by LPS treatment. LPS induced IkappaBalpha degradation at 30-60 min after treatment, but did not induce IkappaBbeta degradation up to 120 min. In contrast, TNF-alpha rapidly induced IkappaBalpha degradation within 5 min and IkappaBbeta degradation within 15 min. Cycloheximide did not prevent LPS-induced IkappaBalpha degradation, indicating that newly synthesized proteins induced by LPS were not involved in LPS-stimulated IkappaBalpha degradation. LPS-induced IkappaBalpha degradation was inhibited by ALLN, confirming that ALLN inhibits NF-kappaB activation by preventing IkappaBalpha degradation. Of note, HMA also inhibited LPS-induced IkappaBalpha degradation. However, tyrosine phosphorylation of IkappaBalpha itself was not elicited by LPS stimulation, suggesting that tyrosine phosphorylation of a protein(s) upstream of IkappaBalpha is required for subsequent degradation. We conclude that in HUVEC, LPS induces NF-kappaB-dependent genes through degradation of IkappaBalpha, not IkappaBbeta, and propose that this degradation is induced in part by HMA-sensitive kinase(s) upstream of IkappaBalpha.

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.

The multisubunit IkappaB kinase complex shows random sequential kinetics and is activated by the C-terminal domain of IkappaB alpha

The multisubunit IkappaB kinase (IKK) catalyzes the signal-inducible phosphorylation of N-terminal serines of IkappaB. This phosphorylation is the key step in regulating the subsequent ubiquitination and proteolysis of IkappaB, which then releases NF-kappaB to promote gene transcription. As measured by 33P incorporation into a GST-IkappaB alpha fusion protein, varying both the concentration of GST-IkappaB alpha and [gamma-33P]ATP resulted in a kinetic pattern consistent with a random, sequential binding mechanism. Values of 55 nM and 7 microM were obtained for the dissociation constants of GST-IkappaB alpha and ATP, respectively. The value of alpha, a factor by which binding of one substrate changes the dissociation constant for the other substrate, was determined to be 0.11. This indicates that the two substrates bind in a cooperative fashion. Peptides corresponding to either amino acids 26-42 (N-terminal peptide) or amino acids 279-303 (C-terminal peptide) of IkappaB alpha inhibited the IKK-catalyzed phosphorylation of GST-IkappaB alpha; the C-terminal peptide, unexpectedly, was more potent. The inhibition by the C-terminal peptide was competitive with respect to GST-IkappaB alpha and mixed with respect to ATP, which verified the sequential binding mechanism. The C-terminal peptide was also a substrate for the enzyme, and a dissociation constant of 2.9-6.2 microM was obtained. Additionally, the N-terminal peptide was a substrate (Km = 140 microM). Competitive inhibition of the IKK-catalyzed phosphorylation of the C-terminal peptide by the N-terminal peptide indicated that the peptides are phosphorylated by the same active site. Surprisingly, the presence of the C-terminal peptide greatly accelerated the rate of phosphorylation of the N-terminal peptide as represented by a 160-fold increase in the apparent second-order rate constant (kcat/Km). These results are consistent with an allosteric site present within IKK that recognizes the C terminus of IkappaB alpha and activates the enzyme. This previously unobserved interaction with the C terminus may represent an important mechanism by which the enzyme recognizes and phosphorylates IkappaB.

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.

Distinct domains of IkappaBalpha regulate c-Rel in the cytoplasm and in the nucleus

IkappaBalpha is a critical regulator of Rel/NF-KB-mediated gene activation. It controls the induction of NF-KB factors by retaining them in the cytoplasm and also functions in the nucleus to terminate the induction process. In this study, we show that IkappaBalpha regulates the transcriptional activity of c-Rel in the nuclear compartment. We also demonstrate that discrete functional domains of IkappaBalpha are responsible for the cytoplasmic and nuclear regulation of c-Rel. We show that the determinants for the cytoplasmic regulation of c-Rel reside in the N-terminal and central ankyrin regions of IkappaBalpha and that the N-terminal domain of IkappaBalpha is required to mask the c-Rel nuclear localization signal. Importantly, IkappaBalpha sequences necessary to regulate c-Rel in the nucleus map to its central ankyrin domain and to a few negatively charged amino acids that immediately follow in the C-terminal IkappaBalpha PEST domain. The mapping of the IkappaBalpha determinants that control the cytoplasmic and nuclear activities of c-Rel to specific regions of the molecule suggests that IkappaBalpha inhibitors could be designed to antagonize Rel/NF-kappaB activity in different subcellular compartments or at defined stages of activation.

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.

Cloning and functional characterization of mouse IkappaBepsilon

The biological activity of the transcription factor NF-kappaB is mainly controlled by the IkappaB proteins IkappaBalpha and IkappaBbeta, which restrict NF-kappaB in the cytoplasm and enter the nucleus where they terminate NF-kappaB-dependent transcription. In this paper we describe the cloning and functional characterization of mouse IkappaBepsilon. Mouse IkappaBepsilon contains 6 ankyrin repeats required for its interaction with the Rel proteins and is expressed in different cell types where we found that it is up-regulated by NF-kappaB inducers, as is the case for IkappaBalpha and human IkappaBepsilon. IkappaBepsilon functions as a bona fide IkappaB protein by restricting Rel proteins in the cytoplasm and inhibiting their in vitro DNA binding activity. Surprisingly, IkappaBepsilon did not inhibit transcription of genes regulated by the p50/p65 heterodimer efficiently, such as the human interferon-beta gene. However, IkappaBepsilon was a strong inhibitor of interleukin-8 expression, a gene known to be regulated by p65 homodimers. In addition, IkappaBepsilon appears to function predominantly in the cytoplasm to sequester p65 homodimers, in contrast with the other two members of the family, IkappaBalpha and IkappaBbeta, which also function in the nucleus to terminate NF-kappaB-dependent transcriptional activation.

I kappaB alpha physically interacts with a cytoskeleton-associated protein through its signal response domain

The I kappaB alpha protein is a key molecular target involved in the control of NF-kappaB/Rel transcription factors during viral infection or inflammatory reactions. This NF-kappaB-inhibitory factor is regulated by posttranslational phosphorylation and ubiquitination of its amino-terminal signal response domain that targets I kappaB alpha for rapid proteolysis by the 26S proteasome. In an attempt to identify regulators of the I kappaB alpha inhibitory activity, we undertook a yeast two-hybrid genetic screen, using the amino-terminal end of I kappaB alpha as bait, and identified 12 independent interacting clones. Sequence analysis identified some of these cDNA clones as Dlc-1, a sequence encoding a small, 9-kDa human homolog of the outer-arm dynein light-chain protein. In the two-hybrid assay, Dlc-1 also interacted with full-length I kappaB alpha protein but not with N-terminal-deletion-containing versions of I kappaB alpha. I kappaB alpha interacted in vitro with a glutathione S-transferase-Dlc-1 fusion protein, and RelA(p65) did not displace this association, demonstrating that p65 and Dlc-1 contact different protein motifs of I kappaB alpha. Importantly, in HeLa and 293 cells, endogenous and transfected I kappaB alpha coimmunoprecipitated with Myc-tagged or endogenous Dlc-1. Indirect immunofluorescence analyzed by confocal microscopy indicated that Dlc-1 and I kappaB alpha colocalized with both nuclear and cytoplasmic distribution. Furthermore, Dlc-1 and I kappaB alpha were found to associate with the microtubule organizing center, a perinuclear region from which microtubules radiate. Likewise, I kappaB alpha colocalized with alpha-tubulin filaments. Taken together, these results highlight an intriguing interaction between the I kappaB alpha protein and the human homolog of a member of the dynein family of motor proteins and provide a potential link between cytoskeleton dynamics and gene regulation.

Distinct functional properties of IkappaB alpha and IkappaB beta

The biological activity of the transcription factor NF-kappaB is controlled mainly by the IkappaB alpha and IkappaB beta proteins, which restrict NF-kappaB to the cytoplasm and inhibit its DNA binding activity. Here, we carried out experiments to determine and compare the mechanisms by which IkappaB alpha and IkappaB beta inhibit NF-kappaB-dependent transcriptional activation. First, we found that in vivo IkappaB alpha is a stronger inhibitor of NF-kappaB than is IkappaB beta. This difference is directly correlated with their abilities to inhibit NF-kappaB binding to DNA in vitro and in vivo. Moreover, IkappaB alpha, but not IkappaB beta, can remove NF-kappaB from functional preinitiation complexes in in vitro transcription experiments. Second, we showed that both IkappaBs function in vivo not only in the cytoplasm but also in the nucleus, where they inhibit NF-kappaB binding to DNA. Third, the inhibitory activity of IkappaB beta, but not that of IkappaB alpha, is facilitated by phosphorylation of the C-terminal PEST sequence by casein kinase II and/or by the interaction of NF-kappaB with high-mobility group protein I (HMG I) on selected promoters. The unphosphorylated form of IkappaB beta forms stable ternary complexes with NF-kappaB on the DNA either in vitro or in vivo. These experiments suggest that IkappaB alpha works as a postinduction repressor of NF-kappaB independently of HMG I, whereas IkappaB beta functions preferentially in promoters regulated by the NF-kappaB/HMG I complexes.

A new translational regulator with homology to eukaryotic translation initiation factor 4G

Translation initiation in eukaryotes is facilitated by the cap structure, m7GpppN (where N is any nucleotide). Eukaryotic translation initiation factor 4F (eIF4F) is a cap binding protein complex that consists of three subunits: eIF4A, eIF4E and eIF4G. eIF4G interacts directly with eIF4E and eIF4A. The binding site of eIF4E resides in the N-terminal third of eIF4G, while eIF4A and eIF3 binding sites are present in the C-terminal two-thirds. Here, we describe a new eukaryotic translational regulator (hereafter called p97) which exhibits 28% identity to the C-terminal two-thirds of eIF4G. p97 mRNA has no initiator AUG and translation starts exclusively at a GUG codon. The GUG-initiated open reading frame (907 amino acids) has no canonical eIF4E binding site. p97 binds to eIF4A and eIF3, but not to eIF4E. Transient transfection experiments show that p97 suppresses both cap-dependent and independent translation, while eIF4G supports both translation pathways. Furthermore, inducible expression of p97 reduces overall protein synthesis. These results suggest that p97 functions as a general repressor of translation by forming translationally inactive complexes that include eIF4A and eIF3, but exclude eIF4E.

Transdominant mutants of I kappa B alpha block Tat-tumor necrosis factor synergistic activation of human immunodeficiency virus type 1 gene expression and virus multiplication

The human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) contains two binding sites for the NF-kappa B/Rel family of transcription factors which are required for the transcriptional activation of viral genes by inflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha) and interleukin-1. In the present study, we examined the effect of transdominant mutants of I kappa B alpha on the synergistic activation of the HIV-1 LTR by TNF-alpha and the HIV-1 transactivator, Tat, in Jurkat T cells. The synergistic induction of HIV-1 LTR-driven gene expression represented a 50- to 70-fold stimulation and required both an intact HIV-1 enhancer and Tat-TAR element interaction, since mutations in Tat protein (R52Q, R53Q) or in the bulge region of the TAR element that eliminated Tat binding to TAR were unable to stimulate LTR expression. Coexpression of I kappa B alpha inhibited Tat-TNF-alpha activation of HIV LTR in a dose-dependent manner. Transdominant forms of I kappa B alpha, mutated in critical serine or threonine residues required for inducer-mediated (S32A, S36A) and/or constitutive (S283A, T291A, T299A) phosphorylation of I kappa B alpha were tested for their capacity to block HIV-1 LTR transactivation. I kappa B alpha molecules mutated in the N-terminal sites were not degraded following inducer-mediated stimulation (t1/2, > 4 h) and were able to efficiently block HIV-1 LTR transactivation. Strikingly, the I kappa B alpha (S32A, S36A) transdominant mutant was at least five times as effective as wild-type I kappa B alpha in inhibiting synergistic induction of the HIV-1 LTR. This mutant also effectively inhibited HIV-1 multiplication in a single-cycle infection model in Cos-1 cells, as measured by Northern (RNA) blot analysis of viral mRNA species and viral protein production. These experiments suggest a strategy that may contribute to inhibition of HIV-1 gene expression by interfering with the NF-kappa B/Rel signaling pathway.