Proteins controlling the nuclear uptake of NF-kappa B, Rel and dorsal

Mincle suppresses Toll-like receptor 4 activation

Regulation of Toll-like receptor responses is critical for limiting tissue injury and autoimmunity in both sepsis and sterile inflammation. We found that Mincle, a C-type lectin receptor, regulates proinflammatory Toll-like receptor 4 signaling. Specifically, Mincle ligation diminishes Toll-like receptor 4-mediated inflammation, whereas Mincle deletion or knockdown results in marked hyperresponsiveness to lipopolysaccharide in vitro, as well as overwhelming lipopolysaccharide-mediated inflammation in vivo. Mechanistically, Mincle deletion does not up-regulate Toll-like receptor 4 expression or reduce interleukin 10 production after Toll-like receptor 4 ligation; however, Mincle deletion decreases production of the p38 mitogen-activated protein kinase-dependent inhibitory intermediate suppressor of cytokine signaling 1, A20, and ABIN3 and increases expression of the Toll-like receptor 4 coreceptor CD14. Blockade of CD14 mitigates the increased sensitivity of Mincle(-/-) leukocytes to Toll-like receptor 4 ligation. Collectively, we describe a major role for Mincle in suppressing Toll-like receptor 4 responses and implicate its importance in nonmycobacterial models of inflammation.

Phosphorylation regulates the ferritoid-ferritin interaction and nuclear transport

Ferritin is an iron-sequestering protein that is generally cytoplasmic; however, our previous studies have shown that in avian corneal epithelial (CE) cells ferritin is nuclear. We have also observed that this nuclear localization involves a tissue-specific nuclear transporter that we have termed ferritoid, and that nuclear ferritin protects DNA from oxidative damage. Recently we have determined that ferritoid functions not only as a nuclear transporter, but also, within the nucleus, it remains associated with ferritin as a heteropolymeric complex. This ferritoid-ferritin complex has unique properties such as being half the size of a typical ferritin molecule and showing preferential binding to DNA. It is likely that the association between ferritoid and ferritin is involved both in the nuclear transport of ferritin and in determining certain of the properties of the complex; therefore, we have been examining the mechanisms involved in regulating the association of these two components. As the ferritoid sequence contains six putative phosphorylation sites, we have examined here whether phosphorylation is one such mechanism. We have determined that ferritoid in the nuclear ferritoid-ferritin complexes is phosphorylated, and that inhibition of this phosphorylation, using inhibitors of PKC, prevents its interaction with ferritin. Furthermore, in an experimental model system in which the nuclear transport of ferritin normally occurs (i.e., the co-transfection of COS-1 cells with full length constructs for ferritin and ferritoid), when phosphorylation sites in ferritoid are mutated, the interaction between ferritoid and ferritin is inhibited, as is the nuclear transport of ferritin.

Mechanisms directing the nuclear localization of the CtBP family proteins

The C-terminal binding protein (CtBP) family includes four proteins (CtBP1 [CtBP1-L], CtBP3/BARS [CtBP1-S], CtBP2, and RIBEYE) which are implicated both in transcriptional repression and in intracellular trafficking. However, the precise mechanisms by which different CtBP proteins are targeted to different subcellular regions remains unknown. Here, we report that the nuclear import of the various CtBP proteins and splice isoforms is differentially regulated. We show that CtBP2 contains a unique nuclear localization signal (NLS) located within its N-terminal region, which contributes to its nuclear accumulation. Using heterokaryon assays, we show that CtBP2 is capable of shuttling between the nucleus and cytoplasm of the cell. Moreover, CtBP2 can heterodimerize with CtBP1-L and CtBP1-S and direct them to the nucleus. This effect strongly depends on the CtBP2 NLS. PXDLS motif-containing transcription factors, such as BKLF, that bind CtBP proteins can also direct them to the nucleus. We also report the identification of a splice isoform of CtBP2, CtBP2-S, that lacks the N-terminal NLS and localizes to the cytoplasm. Finally, we show that mutation of the CtBP NADH binding site impairs the ability of the proteins to dimerize and to associate with BKLF. This reduces the nuclear accumulation of CtBP1. Our results suggest a model in which the nuclear localization of CtBP proteins is influenced by the CtBP2 NLS, by binding to PXDLS motif partner proteins, and through the effect of NADH on CtBP dimerization.

A Novel NF-κB-Regulated Site within the Human Iγ1 Promoter Requires p300 for Optimal Transcriptional Activity

Transcriptional activation of germline (GL) promoters occurs through binding of NF-kappaB to three evolutionarily conserved sites within a CD40 response region in the human and mouse GL Igamma and Iepsilon promoters. Here we identify and characterize a novel NF-kappaB binding site (kappaB6) within the human GL Igamma1 promoter that plays an essential role in basal- and CD40-induced transcription. This site is adjacent to identified CREB/activating transcription factor (ATF) sites, present in the Igamma1 but not the Igamma3 promoter, which are important for the amplification of transcription. Our data suggest a cohesive protein complex regulating Igamma1 promoter activity because disruption of any individual NF-kappaB or CREB/ATF site markedly lowers the overall inducible activity of the promoter. In addition, alteration of helical phasing within the promoter indicates spatial orientation of CREB/ATF and NF-kappaB, proteins contributes directly to promoter activity. We found that CREB and p50 transactivators, as well as coactivator p300, interact in vivo with the Igamma1 promoter in the presence and absence of CD40 signaling in Ramos and primary B cells. However, the level of CREB and p300 binding differs as a consequence of activation in primary B cells. Furthermore, overexpression of p300, and not a mutant lacking acetyltransferase activity, significantly increases Igamma1 construct-specific transcription. Together these data support a model whereby CREB and multiple NF-kappaB complexes bind to the Igamma1 promoter and recruit p300. CD40 signals induce p300-dependent changes that result in optimal Igamma1 promoter activity.

The cAMP-dependent protein kinase site (Ser312) enhances dorsal nuclear import through facilitating nuclear localization sequence/importin interaction

Control over the nuclear import of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation and transformation. The Drosophila TF Dorsal shares with other rel TF family members the fact that it contains a phosphorylation site for the cAMP-dependent protein kinase (PKA) 22 amino acids N-terminal to the nuclear localization signal (NLS) at amino acids 335-340. This study examines for the first time the nuclear import kinetics of Dorsal fusion proteins in rat hepatoma cells in vivo and in vitro. Nuclear uptake was found to be not only NLS-dependent, but also strongly dependent on the PKA site, whereby substitution of Ser312 by either Ala or Glu using site-directed mutagenesis severely reduced nuclear accumulation. Exogenous cAMP or PKA catalytic subunit significantly enhanced the nuclear import of wild-type proteins both in vivo and in vitro. Using a direct binding assay, the molecular basis of PKA site enhancement of Dorsal fusion protein nuclear import was determined to be PKA site-mediated modulation of NLS recognition by the importin 58/97 complex. The physiological relevance of these results is supported by the observation that Drosophila embryos expressing PKA site Dorsal mutant variants were impaired in development. We conclude that the Dorsal NLS and PKA site constitute a phosphorylation-regulated NLS essential to Dorsal function and able to function in heterologous mammalian cell systems, where phosphorylation modulates the affinity of NLS recognition by importin.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Role of cysteine and glutathione in signal transduction, immunopathology and cachexia

Abnormally low plasma cystine levels have been found in the late asymptomatic stage of HIV infection and several other diseases associated with progressive loss of skeletal muscle mass. The phenomenon is commonly associated with a low NK cell activity, skeletal muscle wasting or muscle fatigue and increased rates of urea production. In its extreme form, the negative nitrogen balance leads to overt cachexia and is associated with severe debilitation and psychological stress. The low NK cell activity is in most cases not life-threatening but may be disasterous in HIV infection, because it may compromise the initially stable balance between immune system and virus and trigger disease progression. This review summarizes briefly (i) the role of cysteine in the physiological regulation of body cell mass and the development of skeletal muscle wasting, and (ii) the role of glutathione in the immune system.

Reconstitution of the NF-kappa B system in Saccharomyces cerevisiae for isolation of effectors by phenotype modulation

NF-kappa B is a ubiquitous transcription factor that contributes to the induction of many genes playing a central role in immune and inflammatory responses. The NF-kappa B proteins are subject to multiple regulatory influences including post-translational modifications such as phosphorylation and proteolytic processing. A very important component of this regulation is the control of their subcellular localization: cytoplasmic retention of NF-kappa B is achieved through interaction with I kappa B molecules. In response to extracellular signals, these molecules undergo degradation, NF-kappa B translocates to the nucleus and activates its target genes. To investigate novel proteins involved in this dynamic response, we have reconstituted the NF-kappa B/I kappa Beta system in the yeast Saccharomyces cerevisiae. We have successively introduced p65, the main transcriptional activator of the NF-kappa B family, which leads to the activation of two reporter genes controlled by kappa B sites, and the I kappa B alpha inhibitory protein, which abolishes this activation. By transforming such a yeast strain with a cDNA library we have performed a genetic screen for cDNAs encoding proteins capable of either dissociating the p65/I kappa B alpha complex or directly transactivating the expression of the reporter genes. The efficiency of our screen was demonstrated by the isolation of a cDNA encoding the p105 precursor of the p50 subunit of NF-kappa B. We also used this system to test stimuli known to activate signalling pathways in yeast, in order to investigate whether the related mammalian cascades might be involved in NF-kappa B activation. We showed that yeast endogenous kinase cascades activated by pheromone, hypo- or hyperosmotic shock cannot modulate NF-kappa B activity in our system, and that the p38 human MAP kinase does not act directly on the p65/I kappa B alpha complex.

N-alpha-tosyl-L-lysine chloromethylketone prevents expression of iNOS in vascular smooth muscle by blocking activation of NF-kappa B

Certain cytokines and lipopolysaccharide stimulate expression of inducible nitric oxide synthase (iNOS) in vascular smooth muscle, an event that is regulated at the transcriptional level and appears to involve several transcription factors, including nuclear factor kappa B (NF-kappa B). Since proteases play an essential role in NF-kappa B activation, experiments were designed to clarify, in both cultured rat aortic smooth muscle cells (SMCs) and isolated rat aortas, whether protease inhibitors affect the interleukin-1 beta (IL-1 beta)-elicited expression of iNOS. The formation of NO was assessed by nitrite release in cultured SMCs and the attenuation of phenylephrine-induced contraction in aortic rings, the expression of iNOS by Western blot analysis and reverse transcription-polymerase chain reaction, and NF-kappa B activity in nuclear extracts by gel electrophoretic mobility shift assya. Exposure of cultured SMCs to IL-1 beta increased NF-kappa B binding activity within 30 minutes and was associated with nitrite accumulation and the appearance of iNOS protein 24 hours later. These responses were abolished in cells that had been exposed to the cytokine in the presence of the protease inhibitor N-alpha-tosyl-L-lysine chloromethylketone. Aprotinin and p-toluenesulfonyl-L-arginine methyl ester, two other protease inhibitors, also reduced the cytokine-stimulated release of nitrite and the level of iNOS protein. Exposure of rat aortic segments without endothelium to IL-1 beta activated NF-kappa B within 30 minutes and was associated with the appearance of iNOS mRNA and an attenuation of phenylephrine-induced contraction 6 hours later. These responses were blunted when the segments were incubated with the cytokine and N-alpha-tosyl-L-lysine chloromethyl ketone. The present observations indicate that protease inhibitors prevent iNOS expression in both cultured and native vascular SMCs by blocking the activation of NF-kappa B.

Nuclear transport of granzyme B (fragmentin-2). Dependence of perforin in vivo and cytosolic factors in vitro

Cytotoxic T and natural killer cells are able to kill their target cells through synergistic action of the pore-forming protein perforin and the serine protease granzyme B, resulting in very distinctive nuclear changes typical of apoptosis. Whereas perforin acts at the membrane, granzyme B appears to be both capable of entering the cytoplasm of target cells and accumulating in isolated nuclei. In this study we examine nuclear transport of fluoresceinated granzyme B both in vivo in intact cells in the presence of perforin and in vitro in semi-permeabilized cells using confocal laser scanning microscopy. Granzyme B alone was observed to enter the cytoplasm of intact cells but did not accumulate in nuclei. In the presence of sublytic concentrations of perforin, however, it accumulated strongly in intact cell nuclei to levels maximally about 1.5 times those in the cytoplasm after about 2.5 h. In vitro nuclear transport assays showed maximal levels of nuclear and nucleolar accumulation of granzyme B of about 2.5- and 3-fold those in the cytoplasm. In contrast to signal-dependent nuclear accumulation of SV40 large tumor antigen (T-Ag) fusion proteins in vitro, nuclear/nucleolar import of granzyme B was independent of ATP and not inhibitable by the non-hydrolyzable GTP analog GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)). Similar to T-Ag fusion proteins, however, granzyme B nuclear and nucleolar accumulation was dependent on exogenously added cytosol. Specific inhibitors of granzyme B protease activity had no effect on nuclear/nucleolar accumulation, implying that proteolytic activity was not essential for nuclear targeting. The results imply that granzyme B (32 kDa) may be transported from the cytoplasm to the nucleus through passive diffusion and accumulate by binding to nuclear/nucleolar factors in a cytosolic factor-mediated process. Active and passive nuclear transport properties were normal in the presence of unlabeled granzyme B, implying that the nuclear envelope and pore complex are not granzyme B substrates.

The kappa B enhancer of the human interleukin-6 promoter is necessary and sufficient to confer an IL-1 beta and TNF-alpha response in transfected human cell lines: requirement for members of the C/EBP family for activity

The human interleukin-6 (IL-6) promoter contains two regulatory elements, a kappa B enhancer and a NFIL-6 (C/EBP beta) binding site, which have been reported to be essential for inducibility of the IL-6 gene. We show that the kappa B element alone is sufficient to confer inducibility on the IL-6 gene in cells treated with either IL-1 beta or TNF-alpha. Gel-retardation analysis of nuclear extracts from IL-1 beta or TNF-alpha-treated cells using specific antibodies has shown that at least five retarded complexes bind to the IL-6 kappa B element in addition to NF-kappa B. Furthermore, apart from p50 (NF-kappa B1) and p65 (RelA), no other members of the Rel family are present in these complexes. Comparative analysis with the kappa B enhancer of the immunoglobulin kappa chain gene shows that three of these complexes bind specifically to the IL-6 kappa B enhancer: a complex of p50/NFIL6, a p65 homodimer, and a non-Rel-related constitutive protein. Finally, transfection experiments, in which NF-kappa B subunits, NFIL-6, and NFIL-6 beta (C/EBP delta), were overexpressed in cells transfected with mutated IL-6 enhancer elements linked to a reporter gene show that interaction between members of the two families of factors is required for activation of the IL-6 gene in the absence of the NFIL-6 binding site. We conclude that the kappa B enhancer of the IL-6 promoter is the IL-1 beta and TNF-alpha responsive element and that its activity is dependent on the direct interaction of NF-kappa B with non-Rel transcription factors.

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.

A consensus cAMP-dependent protein kinase (PK-A) site in place of the CcN motif casein kinase II site simian virus 40 large T-antigen confers PK-A-mediated regulation of nuclear import

The regulation of nuclear protein transport by phosphorylation plays a central role in gene expression in eukaryotic cells. We previously showed that nuclear import of SV40 large tumor antigen (T-ag) fusion proteins is regulated by the CcN motif, comprising phosphorylation sites for casein kinase II and the cyclin-dependent kinase cdc2, together with the nuclear localization signal. Regulation of nuclear uptake by CcN motif kinase sites also holds true for the yeast transcription factor SWI5 and the Xenopus nuclear phosphoprotein nucleoplasmin. To test directly whether a kinase site other than those of the CcN motif could regulate nuclear import of T-ag, the CcN motif casein kinase II site, which markedly increases the rate of T-ag nuclear import, was replaced by a consensus site for the cAMP-dependent protein kinase (PK-A) using site-directed mutagenesis. The resultant fusion protein could be specifically phosphorylated by PK-A in vitro and in cell extracts. Nuclear import of the fluorescently labeled protein was analyzed in the HTC rat hepatoma cell line both in vivo (microinjected cells) and in vitro (mechanically perforated cells) in the presence and the absence of cAMP and/or PK-A catalytic subunit using confocal laser scanning microscopy. In vitro PK-A-prephosphorylated protein was also tested. All results indicated that the rate of nuclear import was increased by phosphorylation at the PK-A site (2-5-fold), demonstrating that kinases other than those of the CcN motif can regulate nuclear import in response to stimulatory signals. The phosphorylation-regulated nuclear localization signal derived here represents an important first step toward developing a signal conferring inducible nuclear targeting of molecules of interest.

Finely tuned regulation of cytoplasmic retention of Xenopus nuclear factor 7 by phosphorylation of individual threonine residues

Xenopus nuclear factor 7 (xnf7) is a maternal gene product that functi ons in dorsal/ventral patterning of the embryo. The xnf7 protein is stored in the oocyte nucleus germinal vesicle in a hypophosphorylated state. At oocyte maturation, xnf7 is hyperphosphorylated and released into the cytoplasm, where it is anchored until the midblastula stage, where it is dephosphorylated and enters the nucleus. We demonstrated that cytoplasmic anchoring of xnf7 was regulated by changes in the phosphorylation status of four threonines within two sites, site 1 (Thr-103) and site 2 (Thr-209, Thr-212, and Thr-218), which function in an additive manner. A mutant form of xnf7 (xnf7thr-glu) in which the threonines at sites 1 and 2 were mutated to glutamic acids to mimic a permanent state of phosphorylation was retained in the cytoplasm in oocytes and embryos through the gastrula stage. The cytoplasmic form of xnf7 was detected in a large 670-kDa protein complex probably consisting of xnf7 and several other unknown protein components. Anchoring of xnf7 was not dependent on association with either microtubule or microfilament components of the cytoskeleton, since treatment with cytochalasin B and nocodazole did not affect cytoplasmic retention. Both wild-type xnf7 and xnf7thr-glu form dimers in the yeast two-hybrid system; however, homodimerization was not required for cytoplasmic retention. We suggest that the cytoplasmic retention of xnf7 depends on the phosphorylation state of the protein whereas the cytoplasmic anchoring machinery appears to be constitutively present in oocytes and throughout development until the gastrula stage.

Targeting and association of proteins with functional domains in the nucleus: the insoluble solution

The mammalian nucleus is highly organized into distinct functional domains separating different biochemical processes such as transcription, RNA processing, DNA synthesis, and ribosome assembly. A number of proteins known to participate in these processes were found to be specifically localized at their corresponding functional domains. A distinct targeting sequence, necessary and sufficient for the localization to DNA replication foci, was identified in the N-terminal, regulatory domain of DNA methyltransferase and DNA ligase I and might play a role in the coordination of DNA replication and DNA methylation. The fact that the targeting sequence is absent in lower eukaryotic and prokaryotic DNA ligase I homologs suggests that "targeting" is a rather recent development in evolution. Finally, targeting sequences have also been identified in some splicing factors and in viral proteins, which are responsible for their localization to the speckled compartment and to the nucleolus, respectively. These higher levels of organization are likely to contribute to the regulation and coordination of the complex and interdependent biochemical processes in the mammalian nucleus.

Transcriptional roles of nuclear factor kappa B and nuclear factor-interleukin-6 in the tumor necrosis factor alpha-dependent induction of cyclooxygenase-2 in MC3T3-E1 cells

When a mouse osteoblastic cell line MC3T3-E1 was cultured in the presence of tumor necrosis factor alpha (TNF alpha), the release of prostaglandin E2 and the cyclooxygenase activity increased in a dose- and time-dependent manner. The increase of the enzyme activity was attributed mostly to the induction of cyclooxygenase-2 rather than cyclooxygenase-1 as judged by the inhibitory effect of NS398, Western blotting, and Northern blotting. In this system we attempted to elucidate the transcriptional regulation of the cyclooxygenase-2 gene. As examined by the luciferase assay, two positive regulatory regions (-186 to -131 and -512 to -385 base pairs) were found in the 5'-flanking promoter region of the mouse cyclooxygenase-2 gene in the TNF alpha-stimulated cells. The former included putative NF-IL6 (C/EBP beta) and AP2 elements, and the latter contained the NF kappa B motif. A DNA probe including the NF-IL6 and AP2 sites gave positive bands upon electrophoretic mobility shift assay using the nuclear extracts of MC3T3-E1 cells. The bands were supershifted by the addition of anti-NF-IL6 antibody but not by anti-AP2 antibody. A probe including the NF kappa B site also gave positive bands, which were supershifted by anti-NF kappa B p50 and p65 antibodies. Furthermore, when the motif of NF-IL6 or NF kappa B or both was subjected to point mutation, the luciferase activity was markedly reduced. These data suggested a potential role of both NF-IL6 and NF kappa B in the induction of cyclooxygenase-2 by TNF alpha.

Nuclear localization signals of human and Thermoplasma proteasomal alpha subunits are functional in vitro

Proteasomes are located both in the nuclei and in the cytoplasm of eukaryotic cells. Active transport of these complexes through the nuclear pores has been proposed to be mediated by nuclear localization signals (NLS), which have been found in several of the alpha-type proteasomal subunits. We have tested three different putative NLS sequences from human alpha-type proteasomal subunits (Hsc iota, Hsc9, and Hsc3), as well as a putative NLS-type sequence from the archaeon Thermoplasma acidophilum, for their ability to direct non-nuclear proteins to the nucleus. Synthetic peptides containing these putative NLS sequences were generated and conjugated to large fluorescent reporter molecules: allophycocyanin or fluorescein-labeled bovine serum albumin. The conjugates were introduced into digitonin-permeabilized HeLa and 3T3 cells in the presence of cell lysate and ATP, and nuclear import was monitored by fluorescence microscopy. All three putative NLS sequences from human proteasomal subunits were able to direct the reporter molecules to the nucleus in both cell types, although differences in efficiency were observed. Substitution of threonine for the first lysine residue of the eukaryotic NLS motifs inhibited nuclear import completely. Interestingly, the putative NLS sequence found in T. acidophilum was also functional as a nuclear targeting sequence.

Studies into the effect of the tyrosine kinase inhibitor herbimycin A on NF-kappa B activation in T lymphocytes. Evidence for covalent modification of the p50 subunit

The tyrosine kinase inhibitor herbimycin A was found to block NF-kappa B stimulation in response to interleukin-1 and phorbol 12-myristate 13-acetate in EL4.NOB-1 thymoma cells and phorbol 12-myristate 13-acetate in Jurkat T lymphoma cells. The effect appeared not to involve inhibition of tyrosine kinase activation as neither interleukin-1 nor phorbol 12-myristate 13-acetate induced major changes in tyrosine phosphorylation in EL4.NOB-1 or Jurkat cells, respectively. Herbimycin A did not interfere with I kappa B-alpha degradation, and in unstimulated cells, it modified NF-kappa B prior to chemical dissociation with sodium deoxycholate. Because herbimycin A is thiol-reactive, we suspected that the target was the p50 subunit of NF-kappa B, which has a key thiol at cysteine 62. Herbimycin A inhibited DNA binding when added to nuclear extracts prepared from stimulated cells, which were shown to contain high levels of p50. Incubation of herbimycin A with 2-mercaptoethanol attenuated the effect. Herbimycin A was also shown to react directly with p50, blocking its ability to bind to the NF-kappa B consensus sequence. However, a mutant form of p50 in which cysteine 62 was mutated to serine was insensitive to herbimycin A. Finally, we demonstrated that the compound inhibited the expression of interleukin-2 (an NF-kappa B-regulated gene) in EL4.NOB-1 cells. These data therefore suggest that herbimycin A inhibits NF-kappa B by modifying the p50 subunit on cysteine 62 in the NF-kappa B complex, which blocks DNA binding and NF-kappa B-driven gene expression. The results urge caution in the use of herbimycin A as a specific tyrosine kinase inhibitor and suggest that the development of agents that selectively modify p50 may have potential as a means of inhibiting NF-kappa B-dependent gene transcription.

The regulation of protein transport to the nucleus by phosphorylation

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Stimulation of ionotropic glutamate receptors activates transcription factor NF-kappa B in primary neurons

L-Glutamate is the most common excitatory neurotransmitter in the brain and plays a crucial role in neuronal plasticity as well as in neurotoxicity. While a large body of literature describes the induction of immediate-early genes, including c-fos, fosB, c-jun, junB, zif/268, and krox genes by glutamate and agonists in neurons, very little is known about preexisting transcription factors controlling the induction of such genes. This prompted us to investigate whether stimulation of glutamate receptors can activate NF-kappa B, which is present in neurons in either inducible or constitutive form. Here we report that brief treatments with kainate or high potassium strongly activated NF-kappa B in granule cells from rat cerebellum. This was detected at the single cell level by immunostaining with a monoclonal antibody that selectively reacts with the transcriptionally active, nuclear form of NF-kappa B p65. The activation of NF-kappa B could be blocked with the antioxidant pyrrolidine dithiocarbamate, suggesting the involvement of reactive oxygen intermediates. The data may explain the kainate-induced cell surface expression of major histocompatibility complex class I molecules, which are encoded by genes known to be controlled by NF-kappa B. Moreover, NF-kappa B activity was found to change dramatically in neurons during development of the cerebellum between days 5 and 7 after birth.

Selective and rapid nuclear translocation of a c-Myc-containing complex after fertilization of Xenopus laevis eggs

We report here unusual features of c-Myc specific to early embryonic development in Xenopus laevis, a period characterized by generalized transcriptional quiescence and rapid biphasic cell cycles. Two c-Myc protein forms, p61 and p64, are present in large amounts in the oocyte as well as during early development. In contrast, only p64 c-Myc is present in Xenopus somatic cells. p61 c-Myc is the direct translation product from both endogenous c-myc mRNAs and c-myc recombinant DNA. It is converted to the p64 c-Myc form after introduction into an egg extract, in the presence of phosphatase inhibitors. p61 and p64 belong to two distinct complexes localized in the cytoplasm of the oocyte. A 15S complex contains p64 c-Myc, and a 17.4S complex contains p61 c-Myc. Fertilization triggers the selective and total entry of only p64 c-Myc into the nucleus. This translocation occurs in a nonprogressive manner and is completed during the first cell cycles. This phenomenon results in an exceptionally high level of c-Myc in the nucleus, which returns to a somatic cell-like level only at the end of the blastulation period. During early development, when the entire embryonic genome is transcriptionally inactive, c-Myc does not exhibit a DNA binding activity with Max. Moreover, embryonic nuclei not only prevent the formation of c-Myc/Max complexes but also dissociate such preformed complexes. These peculiar aspects of c-Myc behavior suggest a function that could be linked to the rapid DNA replication cycles occurring during the early cell cycles rather than a function involving transcriptional activity.

Cyclin-dependent kinase site-regulated signal-dependent nuclear localization of the SW15 yeast transcription factor in mammalian cells

Control over the nuclear transport of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation, transformation and signal transduction. The Saccharomyces cerevisiae TF SWI5 is excluded from the nucleus in a cell cycle-dependent fashion, mediated by phosphorylation by the cyclin-dependent kinase (cdk) CDC28. Nuclear entry occurs in G1. beta-galactosidase fusion proteins carrying SWI5 amino acids 633-682, including the nuclear localization sequence (NLS: Lys-Lys-Tyr-Glu-Asn-Val-Val-Ile-Lys-Arg-Ser-Pro-Arg-Lys-Arg-Gly-Arg-Pro- Arg-Lys655) were analyzed for subcellular localization in appropriate temperature-sensitive yeast strains blocked in G1 or G2/M using indirect immunofluorescence, and for nuclear import kinetics in living rat hepatoma or Vero African green monkey kidney cells microinjected with fluorescently labeled bacterially expressed protein and quantitative confocal laser microscopy. Cell cycle-dependent nuclear localization in yeast was both NLS and cdk site-dependent, whereby mutation of the cdk site serines (Ser646 and Ser664) to alanine resulted in constitutive nuclear localization. In mammalian cells, the SWI5 fusion proteins were similarly transported to the nucleus in an NLS-dependent fashion, while the mutation to Ala of the cdk site serines increased the maximal level of nuclear accumulation from about 1- to over 8-fold. We suggest that phosphorylation at the cdk sites inhibits nuclear transport of SWI5, consistent with our previous observations for the inhibition of SV40 large tumor antigen nuclear transport by phosphorylation by the cdk cdc2. The results indicate for the first time that a yeast NLS and, fascinatingly, its regulatory mechanisms are functional in higher eukaryotes, implying the universal nature of regulatory signals for protein transport to the nucleus.

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.

Multi-step activation of NF-kappa B/Rel transcription factors

Transcription factors belonging to the NF-kappa B/Rel family are specialized in the transduction of primarily pathogenic signals from the cytoplasm to the cell nucleus. To date, the family comprises five distinct DNA-binding subunits and five regulatory proteins with inhibitory function, called I kappa B proteins. The interaction of dimers of the DNA-binding subunits with the I kappa B proteins leads to the cytoplasmatic retention of the complex and inhibition of its DNA binding. Following stimulation of cells, the I kappa B proteins become phosphorylated and are subsequently degraded, presumably, by the proteasome. The released NF-kappa B/Rel transcription factors can then enter the nucleus, bind to decameric DNA cognate sequences and stimulate transcription of numerous immunologically important target genes. In this article, we discuss several distinct levels at which the NF-kappa B/Rel transcription factors can be regulated.

Phosphorylation of human I kappa B-alpha on serines 32 and 36 controls I kappa B-alpha proteolysis and NF-kappa B activation in response to diverse stimuli

Post-translational activation of the higher eukaryotic transcription factor NF-kappa B requires both phosphorylation and proteolytic degradation of the inhibitory subunit I kappa B-alpha. Inhibition of proteasome activity can stabilize an inducibly phosphorylated form of I kappa B-alpha in intact cells, suggesting that phosphorylation targets the protein for degradation. In this study, we have identified serines 32 and 36 in human I kappa B-alpha as essential for the control of I kappa B-alpha stability and the activation of NF-kappa B in HeLa cells. A point mutant substituting serines 32 and 36 by alanine residues was no longer phosphorylated in response to okadaic acid (OA) stimulation. This and various other Ser32 and Ser36 mutants behaved as potent dominant negative I kappa B proteins attenuating kappa B-dependent transactivation in response to OA, phorbol 12-myristate 13-acetate (PMA) and tumor necrosis factor-alpha (TNF). While both endogenous and transiently expressed wild-type I kappa B-alpha were proteolytically degraded in response to PMA and TNF stimulation of cells, the S32/36A mutant of I kappa B-alpha remained largely intact under these conditions. Our data suggest that such diverse stimuli as OA, TNF and PMA use the same kinase system to phosphorylate and thereby destabilize I kappa B-alpha, leading to NF-kappa B activation.

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The transcriptional regulation of human arachidonate 12-lipoxygenase gene by NF kappa B/Rel

As examined by the luciferase assay, a negative regulatory region including the NF kappa B motif was found in the 5'-flanking region of the 12-lipoxygenase gene in human erythroleukemia cells. The negative control was abolished by a site-specific mutation of the NF kappa B motif. Probes including the NF kappa B region gave positive bands upon a gel-shift assay. The bands were super-shifted by antibodies for NF kappa B p50, NF kappa B p65 and c-Rel, and were lost by a NF kappa B competitor DNA. Furthermore, the NF kappa B sequence was protected in DNase I footprinting. Thus, two kinds of heterodimer (p50 and p65; p50 and c-Rel) seemed to control the over-expression of the human 12-lipoxygenase gene.

Interaction of the COOH-terminal transactivation domain of p65 NF-kappa B with TATA-binding protein, transcription factor IIB, and coactivators

We show that the transactivating COOH terminus of the p65 subunit of human transcription factor NF-kappa B directly binds the general transcription factors TFIIB and TATA-binding protein (TBP) in vitro. Interaction of p65 with TFIIB required the most COOH-terminal sequence repeat within TFIIB. A functional interaction of TFIIB with p65 was evident from assays in yeast cells. Cotransfection experiments in COS cells revealed that only overexpression of TBP was able to further stimulate p65-dependent transactivation of a reporter gene. The coexpression of neither TBP nor TFIIB was able to relieve squelching, indicating the involvement of additional factors in transactivation by p65. A cell-free assay using highly purified factors revealed a specific transcriptional stimulation through the COOH-terminal activation domain of NF-kappa B by at least one cofactor, PC1, isolated from HeLa cells. These data show that the potent acidic transactivation domains in the COOH terminus of p65 are able to functionally recruit various components of the basic transcription machinery as well as coactivators.

Regulation of dorsal in cultured cells by Toll and tube: tube function involves a novel mechanism

We described previously a transient cotransfection assay that allows us to study regulation of the Drosophila Dorsal protein (dl) in cultured cells. For example, we showed that over-expression of the Toll transmembrane receptor was sufficient to cause relocalization of dl from the cytoplasm to the nucleus. Here we present data that the tube protein, shown previously by genetic studies to act downstream of Toll, can function in a novel way to enhance dl activity. In the absence of dl, or when dl is cytoplasmic, tube is also found in the cytoplasm of transfected cells. But when dl is localized to the nucleus, so is tube. tube can then function to enhance reporter gene expression, either by cooperation with dl or as a GAL4-tube fusion protein. tube thus appears capable of acting both as a chaperon or escort for dl as it moves to the nucleus, and then as a transcriptional coactivator. We also show that the intracytoplasmic domain of Toll, and specifically the region sharing homology with the interleukin-1 receptor, is sufficient to induce dl-tube nuclear translocation.

The roles of hydrogen peroxide and superoxide as messengers in the activation of transcription factor NF-kappa B

BACKGROUND

The inducible, higher eukaryotic transcription factor NF-kappa B is activated by a variety of stimuli. Several lines of evidence have suggested that reactive oxygen intermediates (ROIs) serve as messengers for most if not all of these stimuli. To identify the relevant ROI species and to gain more direct evidence for an involvement of ROIs as messengers, we investigated whether changes in the levels of enzymes that control intracellular ROI levels affect the activation of NF-kappa B.

RESULTS

Cell lines stably overexpressing the H2O2-degrading enzyme catalase were deficient in activating NF-kappa B in response to tumor necrosis factor alpha (TNF) or okadaic acid. The catalase inhibitor aminotriazol restored NF-kappa B induction. In contrast, stable overexpression of cytoplasmic Cu/Zn-dependent superoxide dismutase (SOD), which enhances the production of H2O2 from superoxide, potentiated NF-kappa B activation. The level of cytoplasmic NF-kappa B-I kappa B complex was unchanged, indicating that synthesis of NF-kappa B was not affected.

CONCLUSIONS

Our data show that one ROI species, H2O2 acts as a messenger in the TNF- and okadaic acid-induced post-translational activation of NF-kappa B. Superoxide is only indirectly involved, as a source for H2O2. These data explain the inhibitory effects of many antioxidative compounds on the activation of NF-kappa B and its target genes. H2O2 is overproduced in response to various stimuli, and normal levels of catalase appear insufficient to remove it completely. H2O2 can therefore accumulate and act as an intracellular messenger molecule in the response to pathogens.

Interleukin-1 and its inhibitors: implications for disease biology and therapy

IL-1 alpha and IL-1 beta are polypeptide hormones that exhibit a broad spectrum of beneficial and harmful biologic activities. Clinical trials designed to benefit from its stimulatory effects on human hematopoiesis and from its role in improving host defenses, are being currently conducted. Other in vivo studies, using IL-1 inhibitors with an attempts to block the detrimental effects of IL-1, are underway. Because of the multifunctional effects of IL-1 in human physiology and its pathogenetic role in several diseases, the capability to control the effects of IL-1 may prove to be a useful tool in medical practice.

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.

A proteasome inhibitor prevents activation of NF-kappa B and stabilizes a newly phosphorylated form of I kappa B-alpha that is still bound to NF-kappa B

Activation of the inducible transcription factor NF-kappa B involves removal of the inhibitory subunit I kappa B-alpha from a latent cytoplasmic complex. It has been reported that I kappa B-alpha is subject to both phosphorylation and proteolysis in the process of NF-kappa B activation. In this study, we present evidence that the multicatalytic cytosolic protease (proteasome) is involved in the degradation of I kappa B-alpha. Micromolar amounts of the peptide Cbz-Ile-Glu(O-t-Bu)-Ala-leucinal (PSI), a specific inhibitor of the chymotrypsin-like activity of the proteasome, prevented activation of NF-kappa B in response to tumor necrosis factor-alpha (TNF) and okadaic acid (OA) through inhibition of I kappa B-alpha degradation. The m-calpain inhibitor Cbz-Leu-leucinal was ineffective. In the presence of PSI, a newly phosphorylated form of I kappa B-alpha accumulated in TNF- and OA-stimulated cells. However, the covalent modification of I kappa B-alpha was not sufficient for activation of NF-kappa B: no substantial NF-kappa B DNA binding activity appeared in cells because the newly phosphorylated form of I kappa B-alpha was still tightly bound to p65 NF-kappa B. Pyrrolidinedithiocarbamate, an antioxidant inhibitor of NF-kappa B activation which did not interfere with proteasome activities, prevented de novo phosphorylation of I kappa B-alpha as well as its subsequent degradation. This suggests that phosphorylation of I kappa B-alpha is equally necessary for the activation of NF-kappa B.(ABSTRACT TRUNCATED AT 250 WORDS)

Tax protein of human T-cell leukemia virus type I binds to the ankyrin motifs of inhibitory factor kappa B and induces nuclear translocation of transcription factor NF-kappa B proteins for transcriptional activation

Human T-cell leukemia virus type I causes adult T-cell leukemia and tropical spastic paraparesis, and its regulator protein Tax has been implicated in the pathogenic activity of human T-cell leukemia virus type I. Tax activates transcription of viral and cellular genes through specific enhancers: the 21-bp enhancer of human T-cell leukemia virus type I, the nuclear factor kappa B (NF-kappa B)-binding site of the interleukin 2 receptor alpha gene, and the serum-responsive element of c-fos. Tax binds to enhancer-binding proteins including cAMP-responsive element-binding protein, cAMP-responsive element modulator, transcription factor NF-kappa B p50 and p67SRF, and associates with each enhancer DNA indirectly. In addition to this mechanism, we report here that Tax binds to inhibitory factor kappa B gamma (I-kappa B) gamma, which forms a complex with NF-kappa B protein heterodimer p50-p65 or homodimer p50-p50 and retains them in the cytoplasm. Tax binding to I-kappa B gamma induces nuclear translocation of NF-kappa B p65. In association with this nuclear translocation of p65, transcription directed by the kappa B enhancer is strongly activated. Tax binds to the ankyrin motifs of I-kappa B gamma, suggesting its possible interaction with many other proteins carrying ankyrin motifs contributing to various regulatory processes. This is a different mechanism of transcriptional activation by the oncoprotein Tax and seems to be independent from the trans-activation through indirect binding to enhancer DNAs.

1,25-dihydroxyvitamin D3 modulates phosphorylation of serine 205 in the human vitamin D receptor: site-directed mutagenesis of this residue promotes alternative phosphorylation

The vitamin D receptor (VDR) from a variety of animal species is a hormone-modulated substrate for phosphorylation in vivo. In this report, we utilize an expression vector to produce recombinant human VDR (hVDR) in 1,25-dihydroxyvitamin D3-treated COS-1 cells. Immunoprecipitation of the phosphorylated hVDR followed by gel purification and phosphoamino acid analysis revealed modification exclusively on one or more serine residues, consistent with previous studies of the VDR in other species. To identify the region of phosphorylation, immunoprecipitated and gel-purified hVDR from COS-1 cells was first mixed with purified hVDR isolated to homogeneity from Saccharomyces cerevisiae and then digested with trypsin or V8 protease, and the peptides were resolved on HPLC. The single phosphate-containing peptides were recovered and subjected to amino acid sequence analysis, revealing the modification to reside in a region extending from residue 171 to residue 206 common to both the tryptic- and the V8 protease-derived peptides. Sequential cleavage of similar VDR mixtures using trypsin and then CNBr, alpha-chymotrypsin, or thermolysin demonstrated an amino-terminal boundary of the phosphorylated peptide at 202. Selective manual Edman degradation of phosphorylated peptides beginning at 171, 195, and 200 revealed phosphate release only at serine 205. This peptide contained an average of 8-fold less radioactive phosphate in the absence of prior treatment of the culture cells with 1,25(OH)2D3. Site-directed modification of VDR serine 205 to alanine, aspartate, or glutamate each led to fully functional proteins when assessed in a transactivation assay using several VDRE-linked natural promoters. Unexpectedly, evaluation of the serine 205 to alanine hVDR mutant revealed that this protein continued to be phosphorylated in a hormone-dependent manner on an alternative site. These studies show directly that hVDR serine residue 205, a consensus site for casein kinase II, is modified in vivo in response to hormone.

Analysis of cellular phosphoproteins by two-dimensional gel electrophoresis: applications for cell signaling in normal and cancer cells

Two-dimensional (2-D) gel electrophoresis has been used to map proteins from various cell types in an effort to eventually link such maps to the sequencing of the entire human genome. While this analysis indicates the cellular disposition and expression of proteins, another application of 2-D gels, the analysis of phosphoproteins, can provide much information as to the assembly and "wiring" of the signal transduction circuits within cells which appear to be enervated by phosphate exchange. The preparation and separation of 32P-labeled proteins is described, as well as various analytical methods, including: the variety of gel systems available for specialist types of analyses, comparing 33P- and 32P-labeling of proteins, imaging techniques, phosphoamino analysis, phosphopeptide separation, identifying the amino acid groups that are phosphorylated, and the identification of phosphoproteins on 2-D gels by immunoprecipitation, corunning of purified proteins, comparative mapping and microsequencing, and by Western blotting. Examples (in brackets) are given of applications in which 2-D phosphogels can be applied, which offer advantages over other techniques. These include: (i) identifying in vivo substrates for kinases (protein kinase C activated by phorbol myristate acetate), (ii) investigating cytokine signaling pathways (tumor necrosis factor and interleukin-1), (iii) investigating the effects of drugs on signaling pathways (okadaic acid, menadione and cyclooxygenase inhibitors), (iv) characterization of specific phosphoproteins (heat-shock protein Hsp27 and stathmin), (v) comparing normal and transformed cells (MRC-5 human lung fibroblasts and their SV-40-transformed counterparts, MRC-5 SV1 cells), (vi) purifying phosphoproteins, (vii) investigating the relationship of protein phosphorylation to stages in the cell cycle (stathmin), (viii) investigating protein/protein interactions, (ix) mapping in vitro kinase substrates (protein kinase C, protein kinase A, and mitogen activated protein kinase activated protein kinase 2), and (x) locating and identifying cellular phosphatases (Hsp27 phosphatase). It is possible that the mapping of phosphoproteins can be linked to other 2-D gel databases and that information derived from these can be used in the future to better understand the signaling mechanisms of normal and cancerous cells.

The NF-kappa B transcription factor and cancer: high expression of NF-kappa B- and I kappa B-related proteins in tumor cell lines

NF-kappa B is a pleiotropic transcription factor which controls the expression of many genes and viruses. To date, there is good evidence, but no definitive proof, for its role in tumor formation and development of metastasis. To investigate the possibility that members of the NF-kappa B family could participate in the molecular control of the transformed and invasive phenotype, we examined the expression of these proteins in a variety of human tumor cell lines. The expression of p50, p65, p52 and I kappa B was quantified at the protein level using western immunoblot and mobility shift assay and at the RNA level by northern blot. We observed high expression of the NF-kappa B inhibitor I kappa B in the ovarian carcinoma cell line OVCAR-3 together with constitutive nuclear NF-kappa B activity. We also studied the colon carcinoma cell line HT-29 and its metastatic counterpart HTM-29 and we observed specific expression of the p52 NF-kappa B-related protein in the metastatic cells. Our data confirm that NF-kappa B could be involved in the genesis of a variety of cancers including solid tumors and provide us with interesting models to explore the exact role of these transcription factors in cancer.

Cytoplasmic retention of Xenopus nuclear factor 7 before the mid blastula transition uses a unique anchoring mechanism involving a retention domain and several phosphorylation sites

Xenopus nuclear factor 7 (xnf7) is a maternally expressed protein that belongs to the B-box zinc finger gene family consisting of transcription factors, protooncogenes, and ribonucleoproteins. Its function is regulated by retention in the cytoplasm from oocyte maturation until the mid blastula transition (MBT) when it reenters the nucleus. We defined a 22-amino acid cytoplasmic retention domain (CRD) in xnf7 that functioned cooperatively with two phosphorylation sites within the xnf7 molecule to retain the protein in the cytoplasm until the MBT. Deletion of this region or mutations in the phosphorylation sites resulted in the early entry of xnf7 into the nucleus. A mutation changing one of the phosphorylation sites to a glutamic acid resulted in the prolonged retention of the xnf7 protein in the cytoplasm until stages 9-10, well past the MBT. Additionally, a mutant form of xnf7 possessing a second nuclear localization signal at the COOH terminus was retained in the cytoplasm. This suggests that retention of xnf7 was not due to the masking of its NLS as is the case with NFkB and dorsal but was due to a novel anchoring mechanism in which the CRD interacts with an anchor protein. The CRD sequence is also found in another B-box zinc finger protein that is also retained in the cytoplasm until the MBT in the newt. Therefore, we believe that this may be an important mechanism whereby the function of a number of nuclear proteins is regulated during development.

Promoter analysis of the gene encoding the I kappa B-alpha/MAD3 inhibitor of NF-kappa B: positive regulation by members of the rel/NF-kappa B family

In order to characterize the regulation of the gene encoding the I kappa B-alpha/MAD3 inhibitor of the transcription factor NF-kappa B, we have isolated a human genomic clone and sequenced the promoter of this gene. The MAD3 promoter exhibits a potential TATA element upstream of one of the two major transcription sites, and contains several potential NF-kappa B binding sequences, suggesting that the gene is positively regulated by members of this family. Transfection experiments demonstrate that the MAD3 promoter can be activated by various combinations of members of the rel/NF-kappa B family, as well as by phorbol esters and tumor necrosis factor. Specific deletion of one of the kappa B motifs, located 37 bp upstream of the TATA box, abolishes responses to PMA and TNF. This kappa B motif binds NF-kappa B (p50/relA), p50/c-rel and relA/c-rel heterodimers as well as KBF1 (p50 homodimer). These results help to explain the previously observed transient nature of the NF-kappa B response: following NF-kappa B activation, the expression of the inhibitor is increased, therefore the extent of nuclear translocation of the active complex is reduced, resulting in a decreased activation of its target genes.

Relocalization of Drosophila dorsal protein can be induced by a rise in cytoplasmic calcium concentration and the expression of constitutively active but not wild-type Toll receptors

The generation of dorso-ventral polarity in Drosophila relies on the formation of a nuclear gradient of the rel/nuclear factor kappa B transcription factor dorsal in the pre-cellular syncitial embryo by a process of differential nuclear localization. It is thought that the gradient is formed by activation at ventral positions of the membrane receptor Toll that in turn causes the local dissociation of dorsal from the cytoplasmic anchor protein cactus. Although Toll is related in its cytoplasmic domain to the interleukin-1 receptor little is known about the signal transduction pathways that lead from Toll to the relocalization of dorsal. In this paper we have used immunofluorescence microscopy as a direct assay of dorsal protein nuclear localization in the Drosophila cell line Schneider 2. We find that increased cytoplasmic calcium concentration and the expression of constitutively active Toll receptors can induce the relocalization of dorsal. By contrast, we find that activation of endogenous protein kinase A and expression of wild-type Toll receptors, which activate zen-chloramphenicol acetyltransferase reporter genes in this system, have only a marginal effect on the cellular distribution of the dorsal protein. Treatment of cells with activators of protein kinase C and radical oxygen intermediates, both of which activate nuclear factor kappa B, also has little effect on dorsal protein localization. We propose that different threshold levels of dorsal activation can be established by distinctly regulated signal transduction pathways.

The I kappa B proteins: members of a multifunctional family

The I kappa B proteins bind to Rel/NF-kappa B transcription factors and modulate their activities. Although originally described only as cytoplasmic inhibitors of Rel/NF-kappa B transcription complexes, it is now clear that I kappa B proteins also have other functions.

Activation of NF-kappa B by cAMP in human myeloid cells

Nuclear factor kappa B (NF-kappa B) is a DNA-binding regulatory factor that controls the transcription of a number of genes. Various agents are known to activate this factor. We have studied the ability of cAMP to stimulate NF-kappa B activity in human myeloid cells. Electrophoretic mobility assay revealed that structural cAMP analogs and agents elevating intracellular cAMP levels induced NF-kappa B DNA-binding activity. The inducibility was dependent on the maturation stage of myeloid cells. In promyelocytic HL-60 cells cAMP induced higher NF-kappa B activity than in more differential THP-1 cells and in human monocytes. By transfecting HL-60 and THP-1 cells with reporter constructs containing NF-kappa B DNA-binding sites, we observed that cAMP-induced NF-kappa B was transcriptionally active.