Autoregulation of I kappa B alpha activity

Tosylphenylalanine chloromethyl ketone inhibits TNF-alpha mRNA synthesis in the presence of activated NF-kappa B in RAW 264.7 macrophages

Serine proteinase inhibitors such as N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) and N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) were shown to inhibit production of tumour necrosis factor-alpha (TNF-alpha) in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages. The proteinase inhibitors were also reported to inhibit activation of the transcription factor nuclear factor-kappa B (NF-kappa B) by blocking the signalling pathway for stimuli-induced phosphorylation of the inhibitory subunit (I kappa B alpha) and thus preventing its degradation. In RAW 264.7 cells TPCK and TLCK significantly suppressed LPS-induced increase in TNF-alpha mRNA, induction of nuclear kappa B-binding activity and degradation of I kappa B alpha. TPCK and TLCK effectively blocked TNF-alpha mRNA synthesis even when they were added after LPS stimulation. In these cells, however, the inhibitory modes of the two inhibitors were found to be different: while addition of TLCK suppressed I kappa B alpha degradation and reduced NF-kappa B activity, a comparable decrease in the nuclear kappa B-binding activity or in I kappa B alpha degradation was not observed in cells treated with TPCK. Our results show that TPCK inhibits LPS-induced TNF-alpha mRNA synthesis in the presence of activated NF-kappa B and suggests that mechanisms other than NF-kappa B activation are involved in the transcriptional regulation of the TNF-alpha gene.

Regulation of IkappaB beta in WEHI 231 mature B cells

Constitutive activation of NF-kappaB in WEHI 231 early mature B cells resembles the persistent activation of NF-kappaB that is observed upon prolonged stimulation of other cells. In both cases, NF-kappaB DNA binding complexes are found in the nucleus, despite the abundance of cytosolic IkappaB alpha. Recently, we have shown that prolonged activation of 70Z/3 cells with lipopolysaccharide results in the degradation of IkappaB beta, followed by its subsequent resynthesis as a hypophosphorylated protein. This protein was shown to facilitate transport of a portion of NF-kappaB to the nucleus in a manner that protects it from cytosolic IkappaB alpha. We now demonstrate that the most abundant form of IkappaB beta in WEHI 231 cells is a hypophosphorylated protein. This hypophosphorylated IkappaB beta is found in a stable complex with NF-kappaB in the cytosol and is also detected in NF-kappaB DNA binding complexes in the nucleus. It is likely that hypophosphorylated IkappaB beta in WEHI 231 cells also protects NF-kappaB from IkappaB alpha, thus leading to the continuous nuclear import of this transcription factor.

The serine/threonine phosphatase inhibitor calyculin A induces rapid degradation of IkappaBbeta. Requirement of both the N- and C-terminal sequences

Signal-initiated activation of the transcription factor NF-kappaB is mediated through proteolysis of its cytoplasmic inhibitory proteins IkappaBalpha and IkappaBbeta. While most NF-kappaB inducers trigger the degradation of IkappaBalpha, only certain stimuli are able to induce the degradation of IkappaBbeta. The degradation of IkappaBalpha is targeted by its site-specific phosphorylations, although the mechanism underlying the degradation of IkappaBbeta remains elusive. In the present study, we have analyzed the effect of phosphatase inhibitors on the proteolysis of IkappaBbeta. We show that the serine/threonine phosphatase inhibitor calyculin A induces the hyperphosphorylation and subsequent degradation of IkappaBbeta in both human Jurkat T cells and the murine 70Z-3 preB cells, which is associated with the nuclear expression of active NF-kappaB. The calyculin A-mediated degradation of IkappaBbeta is further enhanced by the cytokine tumor necrosis factor-alpha (TNF-alpha), although TNF-alpha alone is unable to induce the degradation of IkappaBbeta. Mutational analyses have revealed that the inducible degradation of IkappaBbeta induced by calyculin A, and TNF-alpha requires two N-terminal serines (serines 19 and 23) that are homologous to the inducible phosphorylation sites present in IkappaBalpha. Furthermore, the C-terminal 51 amino acid residues, which are rich in serines and aspartic acids, are also required for the inducible degradation of IkappaBbeta. These results suggest that the degradation signal of IkappaBbeta may be controlled by the opposing actions of protein kinases and phosphatases and that both the N- and C-terminal sequences of IkappaBbeta are required for the inducible degradation of this NF-kappaB inhibitor.

Nuclear localization of I kappa B alpha promotes active transport of NF-kappa B from the nucleus to the cytoplasm

I kappa B alpha tightly regulates the transcriptional activity of NF-kappa B by retaining it in the cytoplasm in an inactive form. In the present work, we report that I kappa B alpha, when expressed in the nuclear compartment, not only abrogates NF-kappa B/DNA interactions and NF-kappa B-dependent transcription, but also transports NF-kappa B back to the cytoplasm. This function of I kappa B alpha is insured by a nuclear export sequence located in the C-terminal domain of I kappa B alpha and homologous to the previously described export signal found in HIV-1 Rev protein as well as in PKI (the inhibitor of the catalytic subunit of protein kinase A). Thus, inhibition of NF-kappa B/DNA binding and the consecutive efficient nuclear export of the transcription factor of I kappa B alpha could represent an important mechanism for the control of the expression of NF-kappa B-dependent genes.

Immunological defects in mice with a targeted disruption in Bcl-3

The proto-oncogene bcl-3 is a member of the IkappaB family. The Bcl-3 protein is known to interact specifically with the p50 and p52 subunits of NFkappaB. However, the function of this interaction is not well understood. To determine the in vivo role of Bcl-3, mice were generated that lack the bcl-3 gene, Bcl 3(-/-). Here we report that Bcl 3(-/-) mice appear developmentally normal, but exhibit severe defects in humoral immune responses and protection from in vivo pathogenic challenges. Relative to wild-type mice, Bcl 3(-/-) mice are unable to clear L. monocytogenes and are more susceptible to infection with S. pneumoniae. This phenotype is similar to that observed in the p50(-/-) mice and the cross between the Bcl-3(-/-) and p50(-/-) mice generates animals with an enhanced phenotype. In accordance with the observed defects in their immune response, the Bcl 3(-/-) mice have normal immunoglobulin levels before and after immunization, but fail to produce antigen-specific antibodies. Additionally, spleens from Bcl-3(-/-) mice are abnormal and void of germinal centers. In contrast, the p50(-/-) mice have normal germinal centers. We propose that in in vivo, Bcl-3 can function independently of p50.

IkappaB alpha overexpression in human breast carcinoma MCF7 cells inhibits nuclear factor-kappaB activation but not tumor necrosis factor-alpha-induced apoptosis

Nuclear factor-kappaB (NF-kappaB) is one of major component induced by tumor necrosis factor-alpha (TNF), and its role in the signaling of TNF-induced cell death remains controversial. In order to delineate whether the involvement of NF-kappaB activation is required for triggering of the apoptotic signal of TNF, we inhibited the nuclear translocation of this transcription factor in TNF-sensitive MCF7 cells by introducing a human MAD-3 mutant cDNA coding for a mutated IkappaB alpha that is resistant to both phosphorylation and proteolytic degradation and that behaves as a potent dominant negative IkappaB alpha protein. Our results demonstrated that the mutated IkappaB alpha was stably expressed in the transfected MCF7 cells and blocked the TNF-induced NF-kappaB nuclear translocation. Indeed, TNF treatment of these cells induced the proteolysis of only the endogenous IkappaB alpha but not the mutated IkappaB alpha. The nuclear NF-kappaB released from the endogenous IkappaB alpha within 30 min of TNF treatment was rapidly inhibited by the mutated IkappaB alpha. There was no significant difference either in cell viability or in the kinetics of cell death between control cells and the mutated IkappaB alpha transfected cells. Furthermore, electron microscopic analysis showed that the cell death induced by TNF in both control and mutated IkappaB alpha transfected cells was apoptotic. The inhibition of NF-kappaB translocation in mutated IkappaB alpha-transfected cells persisted throughout the same time course that apoptosis was occurring. Our data provide direct evidence that the inhibition of NF-kappaB did not alter TNF-induced apoptosis in MCF7 cells and support the view that TNF-mediated apoptosis is NF-kappaB independent.

Suppression of TNF-alpha-induced apoptosis by NF-kappaB

Tumor necrosis factor alpha (TNF-alpha) signaling gives rise to a number of events, including activation of transcription factor NF-kappaB and programmed cell death (apoptosis). Previous studies of TNF-alpha signaling have suggested that these two events occur independently. The sensitivity and kinetics of TNF-alpha-induced apoptosis are shown to be enhanced in a number of cell types expressing a dominant-negative IkappaBalpha (IkappaBalphaM). These findings suggest that a negative feedback mechanism results from TNF-alpha signaling in which NF-kappaB activation suppresses the signals for cell death.

Bradykinin stimulates NF-kappaB activation and interleukin 1beta gene expression in cultured human fibroblasts

Bradykinin (BK), a pluripotent nonameric peptide, is known for its proinflammatory functions in both tissue injury and allergic inflammation of the airway mucosa and submucosa. To understand the mechanisms by which BK serves as an inflammatory mediator, the human lung fibroblast cell line WI-38 was stimulated with BK and the expression of IL-1beta gene was examined. BK at nanomolar concentrations induced a marked increase in immunoreactive IL-1beta, detectable within 2 h in both secreted and cell-associated forms. BK-induced IL-1beta synthesis was inhibited by a B2-type BK receptor antagonist and by treatment of the cells with pertussis toxin, indicating the involvement of a BK receptor that couples to the G(i)/G(o) class of heterotrimeric G proteins. Whereas cycloheximide and actinomycin D both inhibited BK-induced IL-1beta synthesis, results from Northern blot and nuclear run-on assays suggested that BK acted primarily at the transcription level which led to the accumulation of IL-1beta message in stimulated cells. Gel mobility shift assays were used with nuclear extracts from stimulated WI-38 cells to examine the transcription mechanism for BK-induced IL-1beta expression. A DNA binding activity specific for the decameric kappaB enhancer was detected and was found to contain the p50 and p65 subunits of the NF-kappaB/rel protein family. BK-induced NF-kappaB activation correlated with IL-1beta message upregulation with respect to agonist concentration, time course, sensitivity to bacterial toxins, and blockade by the B2 receptor antagonist. After BK stimulation, a significant increase in the activity of chloramphenicol acetyltransferase was observed in WI-38 cells transfected with a reporter plasmid bearing the kappaB enhancers from the IL-1beta gene. Deletion of the kappaB enhancer sequence significantly reduced BK-induced chloramphenicol acetyltransferase activity. These findings suggests a novel function of BK in the activation of NF-kappaB and the induction of cytokine gene expression.

Signal-induced degradation of I(kappa)B(alpha): association with NF-kappaB and the PEST sequence in I(kappa)B(alpha) are not required

Signal-induced degradation of I(kappa)B(alpha) via the ubiquitin-proteasome pathway requires phosphorylation on residues serine 32 and serine 36 followed by ubiquitination on lysines 21 and 22. We investigated the role of other regions of I(kappa)B(alpha) which may be involved in its degradation. Here we report that the carboxy-terminal PEST sequence is not required for I(kappa)B(alpha) signal-induced degradation. However, removal of the PEST sequence stabilizes free I(kappa)B(alpha) in unstimulated cells. We further report that a PEST deletion mutant does not associate well with NF-(kappa)B proteins but is degraded in response to signal. Therefore, we conclude that both association with NF-(kappa)B and a PEST sequence are not required for signal-induced I(kappa)B(alpha) degradation. Additionally, the PEST sequence may be required for constitutive turnover of free I(kappa)B(alpha).

I(kappa)B(gamma) inhibits DNA binding of NF-kappaB p50 homodimers by interacting with residues that contact DNA

NF-(kappa)B is an inducible transcription factor that activates many cellular genes involved in stress and immune response and whose DNA binding activity and cellular distribution are regulated by I(kappa)B inhibitor proteins. The interaction between NF-(kappa)B p50 and DNA was investigated by protein footprinting using chemical modification and partial proteolysis. Both methods confirmed lysine-DNA contacts already found in the crystal structure (K-147, K-149, K-244, K-275, and K-278) but also revealed an additional contact in the lysine cluster K-77-K-78-K-80 which was made on an extended DNA. Molecular modelling of such a DNA-protein complex revealed that lysine 80 is ideally placed to make phosphate backbone contacts in the extended DNA. Thus, it seems likely that the entire AB loop, containing lysines 77, 78, and 80, forms a C-shaped clamp that closes around the DNA recognition site. The same protein footprinting approaches were used to probe the interaction of p50 with the ankyrin repeat containing proteins I(kappa)B(gamma) and I(kappa)B(alpha). Lysine residues in p50 that were protected from modification by DNA were also protected from modification by I(kappa)B(gamma) but not I(kappa)B(alpha). Similarly, proteolytic cleavage at p50 residues which contact DNA was inhibited by bound I(kappa)B(gamma) but was enhanced by the presence of I(kappa)B(alpha). Thus, I(kappa)B(gamma) inhibits the DNA binding activity of p50 by direct interactions with residues contacting DNA, whereas the same residues remain exposed in the presence of I(kappa)B(alpha), which binds to p50 but does not block DNA binding.

Mutual transcriptional interference between RelA and androgen receptor

Cross-modulation between androgen receptor (AR) and NF-kappaB/Rel proteins was studied using various androgen- and NF-kappaB-regulated reporter genes under transient transfection conditions. In COS-1 cells, elevated expression of RelA (p65) repressed AR-mediated transactivation in a dose-dependent manner, whereas NFkappaB1 (p50), another major member of the NF-kappaB family, did not influence transactivation. The repression of AR appeared to involve the N-terminal region of the protein between residue 297 and the DNA-binding domain. RelA-mediated transrepression could not be overcome by increasing the amount of AR. Transcriptional interference between RelA and AR was mutual in that cotransfected AR was able to attenuate transactivation by RelA in a dose- and steroid-dependent fashion. An excess of RelA was able to rescue the repression to some extent. Immunological analyses of RelA and AR protein levels indicated that transrepression was not due to reciprocal decrease in their amounts. Neither did AR increase the concentration of IkappaBalpha, which can sequester and inactivate RelA. Electrophoretic mobility shift assays using extracts from cotransfected cells and purified recombinant proteins showed that AR and RelA did not significantly influence each other's DNA binding activity. Nevertheless, protein-protein interaction experiments demonstrated a weak association between AR and RelA. Collectively, these data suggest that the mutual repression in intact cells is due to formation of AR-RelA complexes that are held together by another partner or to competition for a coactivator required for transcription.

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.

Hepatitis B virus HBx protein activates transcription factor NF-kappaB by acting on multiple cytoplasmic inhibitors of rel-related proteins

The HBx protein is a small polypeptide encoded by mammalian hepadnaviruses that is essential for viral infectivity and is thought to play a role in development of hepatocellular carcinoma during chronic hepatitis B virus infection. HBx is a transactivator that stimulates Ras signal transduction pathways in the cytoplasm and certain transcription elements in the nucleus. To better understand the activities of HBx protein and its mechanism of action, we have explored the manner by which HBx activates the transcription factor NF-kappaB during transient expression. We show that HBx induces prolonged formation, in a Ras-dependent manner, of transcriptionally active NF-kappaB DNA-binding complexes, which make up the family of Rel-related proteins, p50, p52, RelA, and c-Rel. HBx was found to activate NF-kappaB through two distinct cytoplasmic pathways by acting on both the 37-kDa IkappaBalpha inhibitor and the 105-kappaDa NF-kappaB1 precursor inhibitor protein, known as p105. HBx induces phosphorylation of IkappaBalpha, a three- to fourfold reduction in IKBalpha stability, and concomitant nuclear accumulation of NF-kappaB DNA-binding complexes, similar to that reported for human T-cell leukemia virus type 1 Tax protein. In addition, HBx mediates a striking reduction in cytoplasmic p105 NF-kappaB1 inhibitor and p50 protein levels and release of RelA protein that was sequestered by the p105 inhibitor, concomitant with nuclear accumulation of NF-kappaB complexes. HBx mediated only a slight reduction in the cytoplasmic levels of NF-kappaB2 p100 protein, an additional precursor inhibitor of NF-kappaB, which is thought to be less efficiently processed or less responsive to release of NF-kappaB. No evidence was found for HBx activation of NF-kappaB by targeting acidic sphingomyelinase- controlled pathways. Studies also suggest that stimulation of NF-kappaB by HBx does not involve activation of Ras via the neutral sphingomyelin-ceramide pathway. Thus, HBx protein is shown to activate the NF-kappaB family of Rel-related proteins by acting on two distinct NF-kappaB cytoplasmic inhibitors.

Constitutive phosphorylation of IkappaBalpha by casein kinase II occurs preferentially at serine 293: requirement for degradation of free IkappaBalpha

IkappaBalpha is a phosphoprotein that sequesters the NF-kappaB/Rel transcription factors in the cytoplasm by physical association. Following induction by a wide variety of agents, IkappaBalpha is further phosphorylated and degraded, allowing NF-kappaB/Rel proteins to translocate to the nucleus and induce transcription. We have previously reported that the constitutive phosphorylation site resides in the C-terminal PEST region of IkappaBalpha and is phosphorylated by casein kinase II (CKII). Here we show that serine 293 is the preferred CKII phosphorylation site. Additionally, we show compensatory phosphorylation by CKII at neighboring serine and threonine residues. Thus, only when all five of the serine and threonine residues in the C-terminal region of IkappaBalpha are converted to alanine (MutF), is constitutive phosphorylation abolished. Finally, we show that constitutive phosphorylation is required for efficient degradation of free IkappaBalpha, in that unassociated Mutf has a half-life two times longer than wild-type IkappaBalpha. A serine residue alone at position 293, as well as aspartic acid at this position, can revert the Mutf phenotype. Therefore, the constitutive CKII phosphorylation site is an integral part of the PEST region of IkappaBalpha, and this phosphorylation is required for rapid proteolysis of the unassociated protein.

Inhibition of NF-kappaB DNA binding by nitric oxide

It has been suggested that the NF-kappaB transcription factor family may mediate expression of the gene encoding the cytokine-inducible form of nitric oxide synthase (iNOS). To establish if nitric oxide (NO) could in turn affect activity of NF-kappaB, the ability of NO-donor compounds to influence NF-kappaB DNA binding activity in vitro was investigated. NO-donor compounds sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) both inhibited the DNA binding activity of recombinant NF-kappaB p50 and p65 homodimers and of p50-p65 heterodimers. Inhibition of NF-kappaB p50 DNA binding by NO-donor compounds involved modification of the conserved redox-sensitive C62 residue, as a C62S p50 mutant was significantly more resistant to SNP-mediated inactivation. Non-reducing SDS-polyacrylamide gel electrophoresis demonstrated that SNP could inhibit p50 DNA binding by mechanisms other than the formation of intersubunit disulphide bonds involving p50 residue C62. Electrospray ionization mass spectrometry of a synthetic NF-kappaB p5O peptide containing the C62 residue suggested that NO gas can modify C62 by S-nitrosylation. This study indicates that NO-donors can directly inhibit the DNA binding activity of NF-kappaB family proteins, suggesting that cellular NO provides another control mechanism for modulating the expression of NF-kappaB-responsive genes.

Inhibition of I kappa B-alpha phosphorylation and degradation and subsequent NF-kappa B activation by glutathione peroxidase overexpression

We report here that both kappa B-dependent transactivation of a reporter gene and NF-kappa B activation in response to tumor necrosis factor (TNF alpha) or H2O2 treatments are deficient in human T47D cell transfectants that overexpress seleno-glutathione peroxidase (GSHPx). These cells feature low reactive oxygen species (ROS) levels and decreased intracellular ROS burst in response to TNF alpha treatment. Decreased ROS levels and NF-kappa B activation were likely to result from GSHPx increment since these phenomena were no longer observed when GSHPx activity was reduced by selenium depletion. The cellular contents of the two NF-kappa B subunits (p65 and p50) and of the inhibitory subunit I kappa B-alpha were unaffected by GSHPx overexpression, suggesting that increased GSHPx activity interfered with the activation, but not the synthesis or stability, of Nf-kappa B. Nuclear translocation of NF-kappa B as well as I kappa B-alpha degradation were inhabited in GSHPx-overexpressing cells exposed to oxidative stress. Moreover, in control T47D cells exposed to TNF alpha, a time correlation was observed between elevated ROS levels and I kappa B-alpha degradation. We also show that, in growing T47D cells, GSHPx overexpression altered the isoform composition of I kappa B-alpha, leading to the accumulation of the more basic isoform of this protein. GSHPx overexpression also abolished the TNF alpha-mediated transient accumulation of the acidic and highly phosphorylated I kappa B-alpha isoform. These results suggest that intracellular ROS are key elements that regulate the phosphorylation of I kappa B-alpha, a phenomenon that precedes and controls the degradation of this protein, and then NF-kappa B activation.

Permanent occupancy of the human immunodeficiency virus type 1 enhancer by NF-kappa B is needed for persistent viral replication in monocytes

This work aimed to ascertain the role of kappaB-responsive elements of the human immunodeficiency virus type 1 (HIV-1) enhancer not only in early initiation but also in long-term maintenance of proviral transcription in cells of the monocytic lineage. For this purpose, we used three main approaches. The first was to abruptly terminate tumor necrosis factor-induced NF-kappaB binding to the enhancer sequences in U1 monocytic cells, using a short pulse of exogenous tumor necrosis factor. This resulted in concomitant decrease in nuclear NF-kappaB DNA-binding activity and endogenous long terminal repeat transcriptional activity. The second was to suppress the permanent NF-kappaB translocation induced by HIV-1 replication itself in chronically infected U937 cells, using a specific proteasome inhibitor (Z-LLL-H). As early as 2 h after addition of the inhibitor to the culture medium, there was an inhibition of both constitutive activation of NF-kappaB and HIV-1 genome expression. The third approach was to monitor the replication competence in U937 cells of an infectious HIV-1 provirus carrying point mutations in the kappaB-responsive elements of both long terminal repeats. Compared with its wild-type counterpart, this mutated provirus showed a profoundly decreased, Z-LLL-H-insensitive transcriptional and replicative activity in U937 monocytes. Together, our results indicate that occupancy of the viral enhancer by NF-kappaB (p50/p65) heterodimers is required for ongoing transcription of integrated HIV provirus in monocytes, even in cells chronically infected and permanently producing functional HIV Tat protein. Thus, the ability of HIV-1 replication to activate NF-kappaB is crucial to the intense self-perpetuated viral transcription observed in cells of the monocytic lineage.

IkappaBalpha deficiency results in a sustained NF-kappaB response and severe widespread dermatitis in mice

The ubiquitous transcription factor NF-kappaB is an essential component in signal transduction pathways, in inflammation, and in the immune response. NF-kappaB is maintained in an inactive state in the cytoplasm by protein-protein interaction with IkappaBalpha. Upon stimulation, rapid degradation of IkappaBalpha allows nuclear translocation of NF-kappaB. To study the importance of IkappaBalpha in signal transduction, IkappaBalpha-deficient mice were derived by gene targeting. Cultured fibroblasts derived from IkappaBalpha-deficient embryos exhibit levels of NF-kappaB1, NF-kappaB2, RelA, c-Rel, and IkappaBbeta similar to those of wild-type fibroblasts. A failure to increase nuclear levels of NF-kappaB indicates that cytoplasmic retention of NF-kappaB may be compensated for by other IkappaB proteins. Treatment of wild-type cells with tumor necrosis factor alpha (TNF-alpha) resulted in rapid, transient nuclear localization of NF-kappaB. IkappaBalpha-deficient fibroblasts are also TNF-alpha responsive, but nuclear localization of NF-kappaB is prolonged, thus demonstrating that a major irreplaceable function Of IkappaBalpha is termination of the NF-kappaB response. Consistent with these observations, and with IkappaBalpha and NF-kappaB's role in regulating inflammatory and immune responses, is the normal development Of IkappaBalpha-deficient mice. However, growth ceases 3 days after birth and death usually occurs at 7 to 10 days of age. An increased percentage of monocytes/macrophages was detected in spleen cells taken from 5-, 7-, and 9-day-old pups. Death is accompanied by severe widespread dermatitis and increased levels of TNF-alpha mRNA in the skin.

Inactivation of IkappaBbeta by the tax protein of human T-cell leukemia virus type 1: a potential mechanism for constitutive induction of NF-kappaB

In resting T lymphocytes, the transcription factor NF-kappaB is sequestered in the cytoplasm via interactions with members of the I kappa B family of inhibitors, including IkappaBalpha and IkappaBbeta. During normal T-cell activation, IkappaBalpha is rapidly phosphorylated, ubiquitinated, and degraded by the 26S proteasome, thus permitting the release of functional NF-kappaB. In contrast to its transient pattern of nuclear induction during an immune response, NF-kappaB is constitutively activated in cells expressing the Tax transforming protein of human T-cell leukemia virus type I (HTLV-1). Recent studies indicate that HTLV-1 Tax targets IkappaBalpha to the ubiquitin-proteasome pathway. However, it remains unclear how this viral protein induces a persistent rather than transient NF-kappaB response. In this report, we provide evidence that in addition to acting on IkappaBalpha, Tax stimulates the turnover Of IkappaBbeta via a related targeting mechanism. Like IkappaBalpha, Tax-mediated breakdown of IkappaBbeta in transfected T lymphocytes is blocked either by cell-permeable proteasome inhibitors or by mutation Of IkappaBbeta at two serine residues present within its N-terminal region. Despite the dual specificity of HTLV-1 Tax for IkappaBalpha and IkappaBbeta at the protein level, Tax selectively stimulates NF-kappaB-directed transcription of the IkappaBalpha gene. Consequently, IkappaBbeta protein expression is chronically downregulated in HTLV-1-infected T lymphocytes. These findings with IkappaBbeta provide a potential mechanism for the constitutive activation of NF-kappaB in Tax-expressing cells.

Hyaluronan fragments activate an NF-kappa B/I-kappa B alpha autoregulatory loop in murine macrophages

Macrophages play an important role in the acute tissue inflammatory response through the release of cytokines and growth factors in response to stimuli such as lipopolysaccharide (LPS). Macrophage inflammatory effector functions are also influenced by interactions with the extracellular matrix (ECM). Such macrophage-ECM interactions may be important in regulating chronic inflammatory responses. Recent evidence has suggested that hyaluronan (HA), a glycosaminoglycan (GAG) component of ECM can induce inflammatory gene expression in murine macrophages. HA exists in its native form as a large polymer, but is found as smaller fragments under inflammatory conditions. The NF-kappa B/I-kappa B transcriptional regulatory system has been shown to be a critical component of the host inflammatory response. We examined the effects of high molecular weight HA and lower molecular weight HA fragments on NF-kappa B activation in mouse macrophages. Only the smaller HA fragments were found to activate NF-kappa B DNA binding activity. After HA stimulation, I-kappa B alpha mRNA was induced and I-kappa B alpha protein levels, which initially decreased, were restored. The induction of I-kappa Balpha expression was not observed for other GAGs. The time course of I-kappa B alpha protein regeneration in response to HA fragments was consistent with an autoregulatory mechanism. In support of this mechanism, in vitro translated murine I-kappa B alpha inhibited HA fragment-induced NF-kappa B DNA binding activity. The NF-kappa B DNA binding complex in HA-stimulated extracts was found to contain p50 and p65 subunits. Activation of the NF-kappa B/I-kappa B system in macrophages by ECM fragments may be an important mechanism for propagating the tissue inflammatory response.

Proteasome inhibitors block VCAM-1 and ICAM-1 gene expression in endothelial cells without affecting nuclear translocation of nuclear factor-kappa B

Endothelial cells play a major role in recruiting leukocytes to sites of inflammation. This is accomplished, at least in part, by up-regulation of cell surface adhesion molecules, including VCAM-1 and ICAM-1, in response to cytokines. In this report, we investigated the role of the proteasome complex in mediating the interleukin (IL)- 1 beta induction of VCAM-1 and ICAM-1 gene expression in human endothelial cells. We present evidence that a proteasome inhibitor, n-acetyl-leucinyl-leucinyl-norleucinal (norLEU), as well as specific protease inhibitors, n-tosyl-Lys-chloromethylketone and N-tosyl-Phe-chloromethylketone, blocked IL-1 beta induction of VCAM-1 and ICAM-1 promoter-driven reporter gene expression in stably transfected endothelial cells. These inhibitors also blocked cytokine induced cell surface expression of VCAM-1 and ICAM-1 by human umbilical vein endothelial cells. As expected, the protease inhibitors blocked the activation of nuclear factor (NF)-kappa B in response to IL-1 beta stimulation. In contrast, norLEU did not prevent IL-1 beta-induced nuclear translocation of NF-kappa B. The effects of norLEU were specific because it did not inhibit the IL-1 beta induction of plasminogen activator inhibitor type 1 gene expression. This study demonstrates that inhibition of the proteolytic activity of the proteasome blocks IL-1 beta induction of VCAM-1 and ICAM-1 gene expression in human endothelial cells.

Role of IkappaBalpha ubiquitination in signal-induced activation of NFkappaB in vivo

In unstimulated cells, the transcription factor NF-kappaB is held in the cytoplasm in an inactive state by the inhibitor protein IkappaBalpha. Stimulation of cells results in rapid phosphorylation and degradation of IkappaBalpha, thus releasing NF-kappaB, which translocates to the nucleus and activates transcription of responsive genes. Here we demonstrate that in cells where proteasomal degradation is inhibited, signal induction by tumor necrosis factor alpha results in the rapid accumulation of higher molecular weight forms of IkappaBalpha that dissociate from NF-kappaB and are consistent with ubiquitin conjugation. Removal of the high molecular weight forms of IkappaBalpha by a recombinant ubiquitin carboxyl-terminal hydrolase and reactivity of the immunopurified material with a monoclonal antibody specific for ubiquitin indicated that IkappaBalpha was conjugated to multiple copies of ubiquitin. Western blot analysis of immunopurified IkappaBalpha from cells expressing epitope-tagged versions of IkappaBalpha and ubiquitin revealed the presence of multiple copies of covalently bound tagged ubiquitin. An S32A/S36A mutant of IkappaBalpha that is neither phosphorylated nor degraded in response to signal induction fails to undergo inducible ubiquitination in vivo. Thus signal-induced activation of NF-kappaB involves phosphorylation-dependent ubiquitination of IkappaBalpha, which targets the protein for rapid degradation by the proteasome and releases NF-kappaB for translocation to the nucleus.

Phosphorylation of p105 PEST sequence via a redox-insensitive pathway up-regulates processing of p50 NF-kappaB

The p105 Rel protein has dual functions; it is the precursor of the p5O subunit of NF-kappaB, and it acts as an IkappaB-like inhibitor to retain other Rel subunits in the cytoplasm. We have investigated the posttranslational regulation of p105 following activation of Jurkat T cells and find that a rapid and sustained phosphorylation of p105 is induced. The inducible phosphorylation occurs on multiple serines in the C-terminal-most 150 amino acids of the molecule, a region rich in Pro, Glu, Ser, and Thr residues. Phosphorylation of p105 in Jurkat cells treated with phorbol 12-myristate 13-acetate/ionomycin or with okadaic acid, another activator of NF-kappaB, is correlated with an increase in proteolytic processing to p5O. Intact PEST sequences are required for the phorbol 12-myristate 13-acetate/ionomycin-induced p105 processing, as a 68-amino acid C-terminal deletion abolishes the response to stimulation. When compounds that block Ikappa B alpha phosphorylation and degradation were tested, the serine protease inhibitors L-1-tosylamido-2-phenylethyl chloromethyl ketone and 1-chloro-3-tosyl-amido-7-amino-2-heptanone blocked inducible p105 phosphorylation, but the antioxidants pyrrolidine dithiocarbamate and butylated hydroxyanisol did not. Thus, while regulation of the p105 IkappaB resembles that of lkappaBa, involving inducible serine phosphorylation and proteolysis of the inhibitory ankyrin repeat domain, it depends on a different, redox-insensitive, signaling pathway.

Critical role for lysines 21 and 22 in signal-induced, ubiquitin-mediated proteolysis of I kappa B-alpha

The NF-kappa B transcription factor induces rapid transcription of many genes in response to a variety of extracellular signals. NF-kappa B is readily activated from normally inhibited cytoplasmic stores by induced proteolytic degradation of I kappa B-alpha, a principal inhibitor of this transcription factor. Following the inhibitor's degradation, NF-kappa B is free to translocate to the nucleus and induce gene transcription. The I kappa B-alpha inhibitor is targeted for degradation by signal-induced phosphorylation of two closely spaced serines in its NH2 terminus (Ser32 and Ser36). Proteolytic degradation appears to be carried out by proteasomes which can recognize ubiquitinated intermediates of the I kappa B-alpha inhibitor. We provide evidence which supports a ubiquitin-mediated mechanism. Amino acid substitutions of two adjacent potential ubiquitination sites in the NH2 terminus of I kappa B-alpha (Lys21 and Lys22) almost completely block the rapid, signal-induced degradation of the mutant protein, while they do not interfere with induced phosphorylation. The mutant I kappa B-alpha also does not permit signal-induced activation of NF-kappa B bound to it. The data suggest that ubiquitination at either of the two adjacent lysines (21 and 22) is required for degradation following induced phosphorylation at nearby serines 32 and 36. Such dependence on ubiquitination of specific sites for protein degradation is unusual. This mechanism of degradation may also apply to I kappa B-beta, an inhibitor related to and functionally overlapping with I kappa B-alpha, as well as to cactus, an I kappa B homolog of Drosophila.

Stimulation of NF-kappa B activation and gene expression by platelet-activating factor

PAF stimulation of NF-kappa B activation and transcription of immediate-early genes have been investigated in peripheral blood mononuclear cells and in transfected Chinese hamster ovary cells expressing the cloned PAF receptor. These studies identified a G protein-coupled pathway for PAF induction of gene expression and transcription factor activation, which differs from the mechanisms employed by other immediate-early gene inducers. Potential significance of PAF induced NF-kappa B activation is discussed.

Mechanism of the tumor necrosis factor alpha-mediated induction of endothelial tissue factor

This study examines the regulation of the human tissue factor (TF) promotor in vitro and in vivo. Transient transfections were performed in bovine aortic endothelial cells to investigate the role of two fundamentally different AP-1 sites and a closely located NF-kappa B site in the human TF promoter. The NF-kappa B site is functionally active, since overexpression of NF-kappa B(p65) resulted in induction of TF mRNA and activity. Promoter analysis showed that NF-kappa B induction was dependent on the integrity of the region from base pair -188 to -181. Over-expression of Jun/Fos resulted in TF induction of transcription and protein/activity. Functional studies revealed that the proximal AP-1 site, but not the distal, was inducible by Jun/Fos heterodimers. The distal AP-1 site, which has a G-->A switch at position 4, was inductible by Jun homodimers. Electrophoretic mobility shift assays, using extracts of tumor necrosis factor alpha (TNF alpha)-stimulated bovine aortic endothelial cells, demonstrated TNF alpha-inducible binding to the proximal AP-1 site, comprising JunD/Fos heterodimers. At the distal AP-1 site, only minor induction of binding activity, characterized as proteins of the Jun and ATF family, was observed. Consistently, this site only marginally participates in TNF alpha induction. Functional studies with TF promotor plasmids confirmed that deletion of the proximal AP-1 or the NF-kappa B site decreased TNF alpha-mediated TF induction to a higher extend than loss of the distal AP-1 site. However, integrity of both AP-1 sites and the NF-kappa B site was required for optimal TNF alpha stimulation. The relevance of these in vitro data was confirmed in vivo in a mouse tumor model. Expression plasmids for a dominant negative Jun mutant or I-kappa B were packaged in liposomes. When either mutated Jun or I-kappa B were injected intravenously 48 h before TNF alpha, a reduction in TNF alpha-mediated TF expression in the tumor endothelial cells was observed. Simultaneously, fibrin/fibrinogen deposition decreased and free blood flow could be restored. Thus, TNF alpha-induced up-regulation of endothelial cell TF depends on a concerted action of members of the bZIP and NF-kappa B family.

Avian I kappa B alpha is transcriptionally induced by c-Rel and v-Rel with different kinetics

The Rel/NF-kappa B family of transcription factors participates in the regulation of genes involved in defense responses, inflammation, healing and regeneration processes, and embryogenesis. The control of the transcriptional activation potential of the Rel/NF-kappa B proteins is mediated, in part, by their association with inhibitory proteins of the I kappa B family. This association results in the cytoplasmic retention of these factors until the cell receives a proper stimulatory signal. The I kappa B alpha gene is a target for regulation by the Rel/NF-kappa B proteins and is in fact upregulated in response to Rel/NF-kappa B activation. A naturally occurring oncogenic variant of the Rel/NF-kappa B family, v-rel, transforms avian lymphocytes, bone marrow cells, monocytes, and fibroblasts. Avian I kappa B alpha expression is upregulated in cells transformed by v-Rel. Avian I kappa B alpha is also upregulated in fibroblasts overexpressing c-Rel and oncogenic variants of c-Rel. c-Rel, a carboxy-terminally truncated variant of c-Rel, and v-Rel are all able to directly transactivate the expression of the avian I kappa B alpha gene. However, c-Rel was the most potent activator of this gene, and the induction of I kappa B alpha expression showed faster kinetics in cells overexpressing c-Rel than in those overexpressing v-Rel. The regulation of I kappa B alpha induction by the Rel proteins was shown to be dependent on a 362-bp region of the I kappa B alpha promoter that contains two potential NF-kappa B binding sites and one AP-1-like binding site. Results of electrophoretic mobility shift assays using these NF-kappa B binding sites indicate that major changes in the profile of DNA binding complexes in fibroblasts overexpressing v-Rel correlated temporally with the kinetic changes in v-Rel's ability to activate the expression of the I kappa B alpha gene.

Mice lacking the c-rel proto-oncogene exhibit defects in lymphocyte proliferation, humoral immunity, and interleukin-2 expression

The c-rel proto-oncogene, which is expressed predominantly in hemopoietic cells encodes a subunit of the NF-kappa B-like family of transcription factors. In mice with an inactivated c-rel gene, whereas development of cells from all hemopoietic lineages appeared normal, humoral immunity was impaired and mature B and T cells were found to be unresponsive to most mitogenic stimuli. Phorbol ester and calcium ionophore costimulation, in contrast to certain membrane receptor-mediated signals, overcame the T cell-proliferative defect, demonstrating that T cell proliferation occurs by Rel-dependent and -independent mechanisms. The ability of exogenous interleukin-2 to restore T Cell, but not B cell, proliferation indicates that Rel regulates the expression of different genes in B and T cells that are crucial for cell division and immune function.

Constitutive phosphorylation of I kappa B alpha by casein kinase II

The NF-kappa B/Rel proteins are sequestered in the cytoplasm in association with the phosphorylated form of I kappa B alpha. Upon induction with a wide variety of agents, the activity of NF-kappa B/Rel proteins is preceded by the rapid degradation of I kappa B alpha protein. We report the identification and partial purification of a cellular kinase from unstimulated or stimulated murine cells, which specifically phosphorylates the C terminus of I kappa B alpha. There are several consensus sites for casein kinase II (CKII) in the C-terminal region of I kappa B alpha. Additionally, the activity of the cellular kinase is blocked by antibodies against the alpha subunit of CKII. No phosphorylation of the C-terminal region of I kappa B alpha can be detected if the five possible serine and threonine residues that can be phosphorylated by CKII are mutated to alanine. A two-dimensional tryptic phosphopeptide map of I kappa B alpha from unstimulated cells was identical to that obtained by in vitro phosphorylation of I kappa B alpha with the partially purified cellular kinase. We propose that constitutive phosphorylation of I kappa B alpha is carried out by CKII.

Activation of NF-kappa B by phosphatase inhibitors involves the phosphorylation of I kappa B alpha at phosphatase 2A-sensitive sites

Activation of NF-kappa B by various cellular stimuli involves the phosphorylation and subsequent degradation of its inhibitor, I kappa B alpha, although the underlying mechanism remains unclear. In the present study, the role of serine/threonine phosphatases in the regulation of I kappa B alpha phosphorylation was investigated. Our studies demonstrate that incubation of human T cells with low concentrations (approximately 1-5 nM) of calyculin A or okadaic acid, potent inhibitors of protein phosphatase type 1 (PP-1) and type 2A (PP-2A), induces the phosphorylation of I kappa B alpha even in the absence of any cellular stimulus. This action of the phosphatase inhibitors, which is associated with the activation of the RelA.p50 NF-kappa B heterodimer, is not affected by agents that block the induction of I kappa B alpha phosphorylation by tumor necrosis factor alpha (TNF-alpha). Furthermore, the phosphorylated I kappa B alpha from calyculin A-treated cells, but not that from TNF-alpha-stimulated cells, is sensitive to PP-2A in vitro, suggesting the existence of fundamental differences in the phosphorylation of I kappa B alpha induced by the two different NF-kappa B inducers. However, induction of I kappa B alpha phosphorylation by both TNF-alpha and the phosphatase inhibitors is associated with the subsequent degradation of I kappa B alpha. We further demonstrate that TNF-alpha- and calyculin A-induced I kappa B alpha degradation exhibits similar but not identical sensitivities to a proteasome inhibitor. Together, these results suggest that phosphorylation of I kappa B alpha, mediated through both the TNF-alpha-inducible and the PP-2A-opposing kinases, may serve to target I kappa B alpha for proteasome-mediated degradation.

Activation of NF-kappa B by the Tax protein of HTLV-1

The Tax protein, encoded by the human T cell leukemia virus HTLV-1, is responsible for transcriptional activation of the viral genome through conserved 21bp repeats located in its promoter. Tax also activates the transcription of cellular genes such as interleukin 2, interleukin 2 receptor (IL2R), GM-CSF, vimentin, c-fos, c-jun as well as the major histocompatibility complex class I genes. Tax does not bind DNA directly, but seems to activate transcription indirectly by enhancing the activity of the transcription factors that recognize responsive elements located in the promoters of the Tax-responsive genes, or by forming ternary complexes with these factors and DNA. One class of target sites for Tax are the kappa B sequences which are bound by members of the rel/NF-kappa B family. It has been previously shown that Tax is able to induce nuclear translocation of NF-kappa B. The activity of the NF-kappa B transcription factor is normally controlled through cytoplasmic retention by either of two types of molecules: the inhibitor I kappa B alpha/MAD3 or the p105 and p100 precursors of the p50 and p52 DNA-binding subunits. Treatment of cells with classical NF-kappa B inducers like TNF, IL-1, PMA or LPS results in MAD-3 degradation followed by nuclear translocation of NF-kappa B. On the other hand, the mechanisms involved in the dissociation of the cytoplasmic p105/p100-containing complexes are largely unknown. We demonstrate here that Tax can induce translocation of members of the NF-kappa B family retained in the cytoplasm through interaction with either p105 or p100. On the other hand Tax induces no apparent degradation of MAD-3. These results suggest that Tax activates NF-kappa B essentially through the p105/p100-retention pathway.

Overexpression of RelA in transgenic mouse thymocytes: specific increase in levels of the inhibitor protein I kappa B alpha

RelA (p65) is one of the strongest activators of the Rel/NF-kappa B family. As a first step to elucidate the mechanisms that regulate its activity in vivo, we have generated transgenic mice overexpressing RelA in the thymus. Although the levels of RelA were significantly increased in thymocytes of transgenic mice, the overall NF-kappa B-binding activity in unstimulated cells was not augmented compared with that in control thymocytes. This could be explained by the dramatic increase of endogenous I kappa B alpha levels observed in RelA-overexpressing cells in both cytoplasmic and nuclear compartments. The ikba mRNA levels were not augmented by overexpressed RelA, but I kappa B alpha inhibitor was found to be stabilized through association with RelA. Although a fraction of RelA was associated with cytoplasmic p105, no changes in the precursor levels were observed. Upon stimulation of RelA-overexpressing thymocytes with phorbol 12-myristate 13-acetate and lectin (phytohemaglutinin), different kappa B-binding complexes, including RelA homodimers, were partially released from I kappa B alpha. Association of RelA with I kappa B alpha prevented complete degradation of the inhibitor. No effect of phorbol 12-myristate 13-acetate-lectin treatment was detected on RelA associated with p105. Our data indicate that cytoplasmic retention of overexpressed RelA by I kappa B alpha is the major in vivo mechanism controlling the potential excess of NF-kappa B activity in long-term RelA-overexpressing thymocytes.

Platelet-activating factor induces NF-kappa B activation through a G protein-coupled pathway

The capability of platelet-activating factor (PAF) to induce transcription factor activation was examined. In stably transfected Chinese hamster ovary cells expressing the PAF receptor (CHO-PAFR), PAF stimulation resulted in the nuclear expression of a DNA binding activity with specificity to the kappa B sequence. The p50 and p65 proteins, constituents of the prototypic nuclear factor kappa B (NF-kappa B), were identified as components of the DNA protein complexes by antipeptide antibodies in gel supershift as well as UV cross-linking experiments. PAF induced an initial decrease and subsequent increase of cytoplasmic I kappa B alpha levels, accompanied by up-regulation of the I kappa B alpha messenger RNA, a feature of NF-kappa B activation. PAF-induced kappa B binding activity was detected within 15 min after agonist stimulation, peaked at 30-40 min, and remained detectable by 2.5 h. SR 27417, a PAF receptor antagonist, blocked PAF-induced kappa B binding activity but not that induced by tumor necrosis factor-alpha (TNF alpha). Cholera toxin treatment markedly reduced PAF-induced kappa B binding activity, whereas pertussis toxin had no significant inhibitory effect. Neither of the two toxins affected the kappa B binding activity induced by TNF alpha in the same cells. In addition to the CHO-PAFR cells, PAF stimulated kappa B binding activity in the murine P388D1 macrophage and the human ASK.0 B cell lines that express endogenous PAF receptors. These results imply a potential role of PAF in the regulation of gene expression through a G protein-coupled transcription factor activation pathway.

Induction and stabilization of I kappa B alpha by nitric oxide mediates inhibition of NF-kappa B

To determine the mechanism(s) by which the endogenous mediator nitric oxide (NO) inhibits the activation of transcription factor NF-kappa B, we stimulated human vascular endothelial cells with tumor necrosis factor-alpha in the presence of two NO donors, sodium nitroprusside and S-nitrosoglutathione. Electrophoretic mobility shift assays demonstrated that both NO donors inhibited NF-kappa B activation by tumor necrosis factor-alpha. This effect was not mediated by guanylyl cyclase activation since the cGMP analogue 8-bromo-cGMP had no similar effect. Inhibition of endogenous constitutive NO production by L-N-monomethylarginine, however, activated NF-kappa B, suggesting tonic inhibition of NF-kappa B under basal conditions. NO had little or no effects on other nuclear binding proteins such as AP-1 and GATA. Immunoprecipitation studies showed that NO stabilized the NF-kappa B inhibitor, I kappa B alpha, by preventing its degradation from NF-kappa B. NO also increased the mRNA expression of I kappa B alpha, but not NF-kappa B subunits, p65 or p50, and transfection experiments with a chloramphenicol acetyltransferase reporter gene linked to the I kappa B alpha promoter suggested transcriptional induction of I kappa B alpha by NO. We propose that the induction and stabilization of I kappa B alpha by NO are important mechanisms by which NO inhibits NF-kappa B and attenuate atherogenesis.

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

After exposure of cells to tumor necrosis factor (TNF), I kappa B alpha is rapidly degraded by a proteolytic activity that is required for nuclear localization and activation of transcription factor NF-kappa B. To investigate this problem, we have developed a cell-free system to study the degradation of I kappa B alpha initiated in vivo. In this in vitro system, characteristics of endogenous I kappa B alpha degradation were comparable to those observed in vivo. Recombinant I kappa B alpha, when added to lysates from cells exposed to TNF, was specifically degraded by a cellular proteolytic activity; however, it was stable in extracts from unstimulated cells. Inhibition characteristics of the proteolytic activity responsible for I kappa B alpha degradation suggest the involvement of a serine protease. Analysis of mutated forms of I kappa B alpha in the in vitro system demonstrated that an I kappa B alpha species which was unable to interact with NF-kappa B was still efficiently degraded. In contrast, deletion of the C-terminal 61 amino acids from I kappa B alpha rendered the protein resistant to proteolytic degradation. Expression of I kappa B alpha mutated forms in COS-7 cells confirmed the importance of the C-terminal domain for the degradation of the protein in vivo following cell activation. Thus, it is likely that the acidic, negatively charged region represented by the C-terminal 61 amino acids of the protein contains residues critical for TNF-inducible degradation of I kappa B alpha.

Inducible nuclear expression of newly synthesized I kappa B alpha negatively regulates DNA-binding and transcriptional activities of NF-kappa B

The transcription factor NF-kappa B is exploited by many viruses, including the human immunodeficiency virus, for expression of viral genes, but its primary role appears to be in the rapid induction of cellular genes during immune and inflammatory responses. The inhibitor protein I kappa B alpha maintains NF-kappa B in an inactive form in the cytoplasms of unstimulated cells, but upon cell activation, I kappa B alpha is rapidly degraded, leading to nuclear translocation of free NF-kappa B. However, NF-kappa B-dependent transcription of the I kappa B alpha gene leads to rapid resynthesis of the I kappa B alpha protein and inhibition of NF-kappa B-dependent transcription. Here we demonstrate a new regulatory function of I kappa B alpha exerted on NF-kappa B in the nuclear compartment. Although normally found in the cytoplasm, I kappa B alpha, newly synthesized in response to tumor necrosis factor or interleukin I, is transported to the nucleus. In the nucleus I kappa B alpha associates with the p50 and p65 subunits of NF-kappa B, inhibiting DNA binding of the transcription factor. Furthermore, nuclear expression of I kappa B alpha correlates with transcription termination of transfected NF-kappa B-dependent luciferase genes. Following the appearance of I kappa B alpha in the nuclei of activated cells, a dramatic reduction in the amount of nuclear p50 occurs, suggesting that NF-kappa B-I kappa B alpha complexes are cleared from the nucleus.

Domain organization of I kappa B alpha and sites of interaction with NF-kappa B p65

The DNA-binding activity and cellular distribution of the transcription factor NF-kappa B are regulated by the inhibitor protein I kappa B alpha. I kappa B alpha belongs to a family of proteins that contain multiple repeats of a 30- to 35-amino-acid sequence that was initially recognized in the erythrocyte protein ankyrin. Partial proteolysis has been used to study the domain structure of I kappa B alpha and to determine the sites at which it interacts with NF-kappa B. The data reveal a tripartite structure for I kappa B alpha in which a central, protease-resistant domain composed of five ankyrin repeats is flanked by an unstructured N-terminal extension and a compact, highly acidic C-terminal domain that is connected to the core of the protein by a flexible linker. Functional analysis of V8 cleavage products indicates that I kappa B alpha molecules lacking the N-terminal region can interact with and inhibit the DNA-binding activity of the p65 subunit of NF-kappa B, whereas I kappa B alpha molecules which lack both the N- and C-terminal regions are incapable of doing so. Protease cleavage of the N terminus of I kappa B alpha was unaffected by the presence of the p65 subunit of NF-kappa B, whereas bound p65 blocked cleavage of the flexible linker connecting the C-terminal domain to the ankyrin repeat-containing core of the protein. This linker region is highly conserved within the human, rat, pig, and chicken homologs of I kappa B alpha, and while it has been suggested that it represents a sixth ankyrin repeat, it is also likely that this is a flexible region of the protein that interacts with NF-kappa B.

Phosphorylation of I kappa B alpha precedes but is not sufficient for its dissociation from NF-kappa B

NF-kappa B is an important activator of immune and inflammatory response genes. NF-kappa B is sequestered in the cytoplasm of nonstimulated cells through interaction with the I kappa B inhibitors. These inactive complexes are dissociated in response to a variety of extracellular signals, thereby allowing free NF-kappa B dimers to translocate to the nucleus and active transcription of specific target genes. The current dogma is that phosphorylation of the I kappa Bs is responsible for dissociation of the inactive complexes, an event that is rendered irreversible by rapid I kappa B degradation. Here, we show that inducers of NF-kappa B activity stimulate the hyperphosphorylation of one of the I kappa Bs, I kappa B alpha. However, contrary to the present dogma the hyperphosphorylated form of I kappa B alpha remains associated with NF-kappa B components such as RelA (p65). Thus, phosphorylation of I kappa B alpha is not sufficient to cause dissociation of the inactive NF-kappa B:I kappa B alpha complex. However, that complex is disrupted through the selective degradation of phosphorylated I kappa B alpha in response to extracellular signals. Using a variety of protease inhibitors, some of which have specificity towards the multicatalytic proteinase complex, we demonstrate that degradation of I kappa B alpha is required for NF-kappa B activation. The results of these experiments are more consistent with a new model according to which phosphorylation of I kappa B alpha associated with NF-kappa B marks it for proteolytic degradation. I kappa B alpha is degraded while bound to NF-kappa B. The selective degradation of I kappa B alpha releases active NF-kappa B dimers which can translocate to the nucleus to activate specific target genes.

Transcription-independent turnover of I kappa B alpha during monocyte adherence: implications for a translational component regulating I kappa B alpha/MAD-3 mRNA levels

We identified I kappa B alpha/MAD-3 as an immediate-early gene in human monocytes that is expressed in response to a variety of signals, including adhesion, lipopolysaccharide, and phorbol myristate acetate. Within 5 min of monocyte adhesion, the level of the I kappa B alpha protein is markedly diminished but is rapidly replaced in a cycloheximide-sensitive manner within 20 min. Accompanying the rapid turnover of the I kappa B alpha protein is simultaneous translocation of NF-kappa B-related transcription factors to nuclei of adhered monocytes. The demonstration that NF-kappa B can regulate I kappa B alpha/MAD-3 gene transcription in other cell types suggested that the rapid increase in steady-state I kappa B alpha/MAD-3 mRNA levels we observed within 30 min of monocyte adherence would result from NF-kappa B-dependent transcriptional stimulation of the I kappa B alpha/MAD-3 gene. Nuclear run-on analyses indicated that, instead, while several immediate-early cytokine genes, such as the interleukin 1 beta (IL-1 beta) gene, were transcriptionally activated during monocyte adhesion, the rate of I kappa B alpha/MAD-3 gene transcription remained constant. The adherence-dependent increase in I kappa B alpha/MAD-3 mRNA levels was also not a consequence of mRNA stabilization events. Interestingly, while increases in both IL-1 beta and I kappa B alpha/MAD-3 mRNA levels were detected in nuclei of adherent monocytes, cytoplasmic levels of IL-1 beta mRNA increased during adherence whereas those of I kappa B alpha/MAD-3 mRNA did not. Taken together, our data suggest that two interactive mechanisms regulate monocytic I kappa B alpha/MAD-3 mRNA levels. We propose that adherent monocytes regulate nuclear processing (or decay) of I kappa B alpha/MAD-3 mRNA, thereby increasing mRNA levels without stimulating I kappa B alpha/MAD-3 gene transcription. Moreover, since inhibition of protein synthesis leads to accumulation of I kappa B alpha/MAD-3 mRNA without stimulating I kappa B alpha/MAD-3 gene transcription, we suggest that low cytoplasmic levels of I kappa B alpha/MAD-3 mRNA are maintained by a translation-dependent degradation mechanism.

In vivo stimulation of I kappa B phosphorylation is not sufficient to activate NF-kappa B

NF-kappa B is a major inducible transcription factor in many immune and inflammatory reactions. Its activation involves the dissociation of the inhibitory subunit I kappa B from cytoplasmic NF-kappa B/Rel complexes, following which the Rel proteins are translocated to the nucleus, where they bind to DNA and activate transcription. Phosphorylation of I kappa B in cell-free experiments results in its inactivation and release from the Rel complex, but in vivo NF-kappa B activation is associated with I kappa B degradation. In vivo phosphorylation of I kappa B alpha was demonstrated in several recent studies, but its role is unknown. Our study shows that the T-cell activation results in rapid phosphorylation of I kappa B alpha and that this event is a physiological one, dependent on appropriate lymphocyte costimulation. Inducible I kappa B alpha phosphorylation was abolished by several distinct NF-kappa B blocking reagents, suggesting that it plays an essential role in the activation process. However, the in vivo induction of I kappa B alpha phosphorylation did not cause the inhibitory subunit to dissociate from the Rel complex. We identified several protease inhibitors which allow phosphorylation of I kappa B alpha but prevent its degradation upon cell stimulation, presumably through inhibition of the cytoplasmic proteasome. In the presence of these inhibitors, phosphorylated I kappa B alpha remained bound to the Rel complex in the cytoplasm for an extended period of time, whereas NF-kappa B activation was abolished. It appears that activation of NF-kappa B requires degradation of I kappa B alpha while it is a part of the Rel cytoplasmic complex, with inducible phosphorylation of the inhibitory subunit influencing the rate of degradation.

Regulation of I kappa B alpha and p105 in monocytes and macrophages persistently infected with human immunodeficiency virus

The mechanisms regulating human immunodeficiency virus (HIV) persistence in human monocytes/macrophages are partially understood. Persistent HIV infection of U937 monocytic cells results in NF-kappa B activation. Whether virus-induced NF-kappa B activation is a mechanism that favors continuous viral replication in macrophages remains unknown. To further delineate the molecular mechanisms involved in the activation of NF-kappa B in HIV-infected monocytes and macrophages, we have focused on the regulation of the I kappa B molecules. First, we show that persistent HIV infection results in the activation of NF-kappa B not only in monocytic cells but also in macrophages. In HIV-infected cells, I kappa B alpha protein levels are decreased secondary to enhanced protein degradation. This parallels the increased I kappa B alpha synthesis secondary to increased I kappa B alpha gene transcription, i.e., increased RNA and transcriptional activity of its promoter-enhancer. Another protein with I kappa B function, p105, is also modified in HIV-infected cells: p105 and p50 steady-state protein levels are increased as a result of increased synthesis and proteolytic processing of p105. Transcriptional activity of p105 is also increased in infected cells and is also mediated by NF-kappa B through a specific kappa B motif. These results demonstrate the existence of a triple autoregulatory loop in monocytes and macrophages involving HIV, p105 and p50, and MAD3, with the end result of persistent NF-kappa B activation and viral persistence. Furthermore, persistent HIV infection of monocytes and macrophages provides a useful model with which to study concomitant modifications of different I kappa B molecules.

Tumor necrosis factor alpha-induced phosphorylation of I kappa B alpha is a signal for its degradation but not dissociation from NF-kappa B

Activation of the NF-kappa B/Rel family of transcription factors is regulated by a cytoplasmic inhibitor, I kappa B alpha. Activity of I kappa B alpha is in turn modulated by phosphorylation and proteolysis. It has been postulated that phosphorylation of I kappa B alpha leads to its dissociation from NF-kappa B, and free I kappa B alpha is targeted for rapid degradation. However, this phosphorylation-mediated dissociation event has not been demonstrated in vivo. We demonstrate that, contrary to this hypothesis, phosphorylation of I kappa B alpha induced by tumor necrosis factor alpha in HeLa cells does not induce dissociation. We propose a model in which (i) induced phosphorylation of I kappa B alpha does not result in its dissociation from NF-kappa B, (ii) phosphorylation of I kappa B alpha serves as a signal for degradation, and (iii) degradation of I kappa B alpha occurs while it is still complexed with NF-kappa B.

Transcriptional regulation of NF-kappa B2: evidence for kappa B-mediated positive and negative autoregulation

NF-kappa B is an inducible transcription factor complex which regulates the expression of a variety of genes which are involved in the immune, inflammatory, and acute-phase responses. The maintenance of NF-kappa B activity in stimulated cells requires ongoing protein synthesis, suggesting several modes of regulation. In this report, we have characterized the transcriptional regulation of one family member, NF-kappa B2. The genomic structure and sequence of NF-kappa B2 revealed the presence of two promoters and at least four kappa B regulatory elements, which mediate responsiveness to phorbol myristate acetate and tumor necrosis factor alpha. Similar to other NF-kappa B family members, NF-kappa B2 is positively autoregulated. In contrast to other family members, we find that kappa B elements in the NFKB2 promoter can also mediate transcriptional repression in the absence of NF-kappa B. We identified a nuclear complex which binds specifically to a subset of kappa B-related sites but not to the canonical kappa B element. Because of its putative inhibitory or repressive effect, this binding activity has been termed Rep-kappa B. This mechanism of repressing basal NF-kappa B2 transcription in an inactivated state enables the cell to tightly control NF-kappa B2 activity. These data demonstrate that a novel mode of kappa B-dependent regulation is mediated by specific kappa B sites in the NFKB2 promoter.

Inducible phosphorylation of I kappa B alpha is not sufficient for its dissociation from NF-kappa B and is inhibited by protease inhibitors

The ubiquitous transcription factor NF-kappa B is regulated by its cytoplasmic inhibitor I kappa B. A variety of cellular stimuli cause the dissociation of NF-kappa B from I kappa B, allowing NF-kappa B to translocate to the nucleus and regulate gene expression. Although the activation of NF-kappa B in vivo is associated with the phosphorylation and degradation of I kappa B alpha, it has remained unclear how each of these events contributes to this process. Recently, studies utilizing protease inhibitors have suggested that the proteolysis of I kappa B alpha is a necessary event in the activation of NF-kappa B. We demonstrate in this study that these and an additional protease inhibitor also completely repress inducible phosphorylation of I kappa B alpha. This surprising result suggests a more complex role of proteases in NF-kappa B activation. In addition, data presented here indicate that many of these inhibitors also directly modify NF-kappa B and inhibit its DNA binding activity. Due to the pleiotropic effects of these protease inhibitors, it is difficult to conclude from their use how I kappa B alpha phosphorylation and degradation contribute to NF-kappa B activation. In the present study, a more direct approach demonstrates that phosphorylation of I kappa B alpha alone is not sufficient for NF-kappa B activation.

Transcriptional regulation of NF-kappa B2: evidence for kappa B-mediated positive and negative autoregulation

NF-kappa B is an inducible transcription factor complex which regulates the expression of a variety of genes which are involved in the immune, inflammatory, and acute-phase responses. The maintenance of NF-kappa B activity in stimulated cells requires ongoing protein synthesis, suggesting several modes of regulation. In this report, we have characterized the transcriptional regulation of one family member, NF-kappa B2. The genomic structure and sequence of NF-kappa B2 revealed the presence of two promoters and at least four kappa B regulatory elements, which mediate responsiveness to phorbol myristate acetate and tumor necrosis factor alpha. Similar to other NF-kappa B family members, NF-kappa B2 is positively autoregulated. In contrast to other family members, we find that kappa B elements in the NFKB2 promoter can also mediate transcriptional repression in the absence of NF-kappa B. We identified a nuclear complex which binds specifically to a subset of kappa B-related sites but not to the canonical kappa B element. Because of its putative inhibitory or repressive effect, this binding activity has been termed Rep-kappa B. This mechanism of repressing basal NF-kappa B2 transcription in an inactivated state enables the cell to tightly control NF-kappa B2 activity. These data demonstrate that a novel mode of kappa B-dependent regulation is mediated by specific kappa B sites in the NFKB2 promoter.

Tax induces nuclear translocation of NF-kappa B through dissociation of cytoplasmic complexes containing p105 or p100 but does not induce degradation of I kappa B alpha/MAD3

The activity of the NF-kappa B transcription factor is controlled through cytoplasmic retention by either of two types of molecules: the inhibitor I kappa B alpha/MAD3 or the p105 and p100 precursors of the p50 and p52 DNA-binding subunits. Treatment of cells with classical NF-kappa B inducers such as tumor necrosis factor, interleukin-1, phorbol myristate acetate, and lipopolysaccharide results in MAD3 degradation followed by nuclear translocation of NF-kappa B. On the other hand, the mechanisms involved in the dissociation of the cytoplasmic p105/p100-containing complexes are largely unknown. The Tax protein encoded by human T-cell leukemia virus type 1 is a potent activator of viral and cellular gene transcription. It does not bind DNA directly but seems to activate transcription indirectly either by enhancing the activities of the transcription factors that recognize responsive elements located in the promoters of the Tax-responsive genes or by forming ternary complexes with these factors and DNA. It has been previously shown that Tax is able to induce nuclear translocation of NF-kappa B. We demonstrate here that Tax can induce translocation of members of the NF-kappa B family retained in the cytoplasm through their interaction with either p105 or p100. On the other hand, Tax induces no apparent degradation of MAD3, although experiments using cycloheximide indicate that it decreases the half-life of MAD3. However, this activity is shared by a mutant of Tax which is unable to activate NF-kappa B. These results suggest that Tax activates NF-kappa B essentially through the p105/p100 retention pathway.

Pentoxifylline and other protein kinase C inhibitors down-regulate HIV-LTR NF-kappa B induced gene expression

BACKGROUND

This investigation deals with the molecular mechanism of anti-human immunodeficiency virus type 1 (HIV-1) action of pentoxifylline (PTX) [1-(5'-oxohexyl)-3, 7-dimethylxanthine] a drug widely used for the treatment of conditions involving defective regional microcirculation.

MATERIALS AND METHODS

The inhibition by PTX of protein kinase C (PKC) or cAMP-dependent protein kinase (PKA)-mediated activation by phorbol ester (PMA) and tumor necrosis factor alpha (TNF-alpha) of HIV-1-LTR-regulated reporter gene expression was studied in human CD4+ T lymphocytes (Jurkat) and human embryo kidney cells (293-27-2). A protein kinase C is involved in activation of NF-kappa B in whole cells, identified by using inhibitors specific for PKC- or PKA-catalyzed NF-kappa B activation in whole cell and cell-free systems.

RESULTS

PTX inhibited PKC- or PKA-catalyzed activation of NF-kappa B in cytoplasmic extracts from unstimulated Jurkat or 293-27-2 cells, but not interaction of preactivated NF-kappa B with its motifs. Calphostin C, a specific inhibitor of PKC, inhibited NF-kappa B activation and HIV-1 LTR-driven reporter gene expression in both PMA- and TNF-alpha-treated cells. In contrast, although H88 specifically inhibited PKA activity in the cell-free extract, it did not affect NF-kappa B action in PMA- or TNF-alpha-treated cells.

CONCLUSIONS

The mechanism of inhibitory action of PTX on virus replication and NF-kappa B-induced transactivation of HIV-1 gene expression has been elucidated as due to blocking PKC-dependent PMA- or TNF-alpha-induced activation of NF-kappa B in Jurkat and 293-27-2 cells. Other protein kinase inhibitors may be useful in down regulating transcription of HIV-1 provirus and thereby virus replication in HIV-infected patients.