Rel-associated pp40: an inhibitor of the rel family of transcription factors

Tyrosine phosphorylation of I kappa B-alpha activates NF-kappa B without proteolytic degradation of I kappa B-alpha

The transcription factor NF-kappa B regulates genes participating in immune and inflammatory responses. In T lymphocytes, NF-kappa B is sequestered in the cytosol by the inhibitor I kappa B-alpha and released after serine phosphorylation of I kappa B-alpha that regulates its ubiquitin-dependent degradation. We report an alternative mechanism of NF-kappa B activation. Stimulation of Jurkat T cells with the protein tyrosine phosphatase inhibitor and T cell activator pervanadate led to NF-kappa B activation through tyrosine phosphorylation but not degradation of I kappa B-alpha. Pervanadate-induced I kappa B-alpha phosphorylation and NF-kappa B activation required expression of the T cell tyrosine kinase p56ick. Reoxygenation of hypoxic cells appeared as a physiological effector of I kappa B-alpha tyrosine phosphorylation. Tyrosine phosphorylation of I kappa B-alpha represents a proteolysis-independent mechanism of NF-kappa B activation that directly couples NF-kappa B to cellular tyrosine kinase.

TGF beta 1 inhibits NF-kappa B/Rel activity inducing apoptosis of B cells: transcriptional activation of I kappa B alpha

TGF beta 1 treatment of B cell lymphomas decreases c-myc gene expression and induces apoptosis. Since we have demonstrated NF-kappa/Rel factors play a key role in transcriptional control of c-myc, we explored the effects of TGF beta1 on WEHI 231 immature B cells. A reduction in NF-kappa B/Rel activity followed TGF beta 1 treatment. In WEHI 231 and CH33 cells, we observed an increase in I kappa B alpha, a specific NF-kappa B/Rel inhibitor, due to transcriptional induction. Engagement of surface CD40 or ectopic c-Rel led to maintenance of NF-kappa B/Rel and c-Myc expression and protection of WEHI 231 cells from TGF beta 1-mediated apoptosis. Ectopic c-Myc expression overrode apoptosis induced by TGF beta 1. Thus, downmodulation of NF-kappa B/Rel reduces c-Myc expression, which leads to apoptosis in these immature B cell models of clonal deletion. The inhibition of NF-kappa B/Rel activity represents a novel TGF beta signaling mechanism.

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.

Immunosuppressant FK506 induces interleukin-6 production through the activation of transcription factor nuclear factor (NF)-kappa(B). Implications for FK506 nephropathy

FK506 is a powerful immunosuppressive drug currently in use that inhibits the activation of several transcription factors (nuclear factor (NF)-AT and NF-kappaB) critical for T cell activation. We show here that, contrary to the situation in T cells, FK506 activates transcription factor NF-kappaB in nonlymphoid cells such as fibroblasts and renal mesangial cells. We further show that FK506 induces NF-kappaB-regulated IL-6 production in vitro and in vivo, in particular in kidney. IL-6 has been shown previously to produce renal abnormalities in vivo, such as mesangioproliferative glomerulonephritis. Similar renal abnormalities were also observed in FK506-treated animals. These results thus suggest a causal relationship between FK506-induced NF-kappaB activation/IL-6 production and some of FK506-induced renal abnormalities.

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

In the present study, the role of the I kappa B alpha C terminus in NF-kappa B/I kappa B alpha regulation was examined in NIH 3T3 cells engineered to inducibly express wild type or mutated human I kappa B alpha proteins under the control of the tetracycline responsive promoter. Deletion studies demonstrated that the last C-terminal 30 amino acids (amino acids (aa) 288 to aa 317, deleted in I kappa B alpha delta 3), including most of the PEST domain, were dispensable for I kappa B alpha function. However, deletions from aa 261 to 317 or aa 269 to 317 (I kappa B alpha delta 1 and I kappa B alpha delta 2 respectively), lacked the ability to dissociate NF-kappa B/DNA complexes in vitro and were unable to inhibit NF-kappa B dependent transcription. Moreover, I kappa B alpha delta 1 and I kappa B alpha delta 2 mutants were resistant to inducer-mediated degradation. Analysis of I kappa B alpha deletions in the presence of protein synthesis inhibitors revealed that, independently of stimulation, I kappa B alpha delta 1 and I kappa B alpha delta 2 had a half-life four times shorter than wild type I kappa B alpha and the interaction of I kappa B alpha delta 1 and I kappa B alpha delta 2 with p65 was dramatically decreased in vivo as measured by co-immunoprecipitation. Interestingly, protease inhibitors which blocked inducer-mediated degradation of I kappa B alpha also stabilized the turnover of I kappa B alpha delta 1 and I kappa B alpha delta 2. Based on these studies, we propose that in the absence of stimulation, the C-terminal domain between aa 269 and 287 may play a role to protect I kappa B alpha from a constitutive protease activity.

T cell leukemia-associated human Notch/translocation-associated Notch homologue has I kappa B-like activity and physically interacts with nuclear factor-kappa B proteins in T cells

Translocation-associated Notch homologue (TAN-1), a gene originally cloned from the translocation breakpoint of a human T cell leukemia carrying a 9:7(q34.3) translocation, encodes a protein belonging to the Notch/Lin-12/Glp-1 receptor family. These receptors mediate the specification of numerous cell fates during development in invertebrates and vertebrates. The intracellular portion of Notch/TAN-1 contains six ankyrin repeats that are similar to those found in cytoplasmic I kappa B proteins. I kappa B proteins are specific inhibitors of nuclear factor (NF)-kappa B/Rel transcription factors. Here we show that TAN-1 has functional properties of an I kappa B-like regulator with specificity for the NF-kappa B p50 subunit. A recombinant polypeptide corresponding to the cytoplasmic portion of TAN-1 (TAN-1C) specifically inhibited the DNA binding of p50-containing NF-kappa B complexes. When overexpressed in an appropriate cell line, TAN-1C prevented kappa B-dependent transactivation in transient reporter gene assays in a fashion similar to the structurally related protein, Bcl-3. TAN-1C could activate kappa B-dependent gene expression by attenuating the inhibitory effect of an excess of p50 homodimers. Immunoprecipitation experiments showed that the TAN-1 from a T cell line is associated with NF-kappa B containing p50 and p65 subunits. These observations indicate that TAN-1C may directly engage NF-kappa B transcription factors and modulate nuclear gene expression.

PEST-dependent cytoplasmic retention of v-Rel by I(kappa)B-alpha: evidence that I(kappa)B-alpha regulates cellular localization of c-Rel and v-Rel by distinct mechanisms

Association of c-Rel with the inhibitor of kappaB-alpha (IkappaB-alpha) protein regulates both cellular localization and DNA binding. The ability of v-Rel, the oncogenic viral counterpart of avian c-Rel, to evade regulation by p40, the avian IkappaB-alpha protein, contributes to v-Rel-mediated oncogenesis. The yeast two-hybrid system was utilized to dissect Rel:IkappaB-alpha interactions in vivo. We find that distinct domains in c-Rel and v-Rel are required for association with p40. Furthermore, while the ankyrin repeat domain of p40 is sufficient for association with c-Rel, both the ankyrin repeat domain and the PEST domain are required for association with v-Rel. Two amino acid differences between c-Rel and v-Rel that are principally responsible for PEST-dependent association of v-Rel with p40 were identified. These same amino acids were principally responsible for PEST-dependent cytoplasmic retention of v-Rel by p40. The presence of mutations in c-Rel that were sufficient to confer PEST-dependent association of the mutant c-Rel protein with p40 did not increase the weak oncogenicity of c-Rel. However, the introduction of these two c-Rel-derived amino acids into v-Rel markedly reduced the oncogenicity of v-Rel. Deletion of the NLS of either c-Rel or v-Rel did not abolish association with p40, but did confer PEST-dependent association of c-Rel with p40. Surprisingly, deletion of the nuclear localization signal in v-Rel did not affect oncogenicity by v-Rel. Analysis of several mutant c-Rel and v-Rel proteins demonstrated that association of Rel proteins with p40 is necessary but not sufficient for cytoplasmic retention. These results are not consistent with the hypothesis that p40 regulates cellular localization of v-Rel and c-Rel by the same mechanism. Rather, these results support the hypothesis that p40 regulates cellular localization of v-Rel and c-Rel by distinct mechanisms.

The NF-kappa B and I kappa B proteins: new discoveries and insights

The transcription factor NF-kappa B has attracted widespread attention among researchers in many fields based on the following: its unusual and rapid regulation, the wide range of genes that it controls, its central role in immunological processes, the complexity of its subunits, and its apparent involvement in several diseases. A primary level of control for NF-kappa B is through interactions with an inhibitor protein called I kappa B. Recent evidence confirms the existence of multiple forms of I kappa B that appear to regulate NF-kappa B by distinct mechanisms. NF-kappa B can be activated by exposure of cells to LPS or inflammatory cytokines such as TNF or IL-1, viral infection or expression of certain viral gene products, UV irradiation, B or T cell activation, and by other physiological and nonphysiological stimuli. Activation of NF-kappa B to move into the nucleus is controlled by the targeted phosphorylation and subsequent degradation of I kappa B. Exciting new research has elaborated several important and unexpected findings that explain mechanisms involved in the activation of NF-kappa B. In the nucleus, NF-kappa B dimers bind to target DNA elements and activate transcription of genes encoding proteins involved with immune or inflammation responses and with cell growth control. Recent data provide evidence that NF-kappa B is constitutively active in several cell types, potentially playing unexpected roles in regulation of gene expression. In addition to advances in describing the mechanisms of NF-kappa B activation, excitement in NF-kappa B research has been generated by the first report of a crystal structure for one form of NF-kappa B, the first gene knockout studies for different forms of NF-kB and of I kappa B, and the implications for therapies of diseases thought to involve the inappropriate activation of NF-kappa B.

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.

Interaction of the v-Rel oncoprotein with NF-kappaB and IkappaB proteins: heterodimers of a transformation-defective v-Rel mutant and NF-2 are functional in vitro and in vivo

The v-Rel oncoprotein of the avian Rev-T retrovirus is a member of the Rel/NF-kappa B family of transcription factors. The mechanism by which v-Rel malignantly transforms chicken spleen cells is not precisely known. To gain a better understanding of functions needed for transformation by v-Rel, we have now characterized the activities of mutant v-Rel proteins that are defective for specific protein-protein interactions. Mutant v-delta NLS, which has a deletion of the primary v-Rel nuclear localizing sequence, does not interact efficiently with I kappa B-alpha but still transforms chicken spleen cells approximately as well as wild-type v-Rel, indicating that interaction with I kappa B-alpha is not essential for the v-Rel transforming function. A second v-Rel mutant, v-SPW, has been shown to be defective for the formation of homodimers, DNA binding, and transformation. However, we now find that v-SPW can form functional DNA-binding heterodimers in vitro and in vivo with the cellular protein NF-kappa B p-52. Most strikingly, coexpression of v-SPW and p52 from a retroviral vector can induce the malignant transformation of chicken spleen cells, whereas expression of either protein alone cannot. Our results are most consistent with a model wherein Rel homodimers or heterodimers must bind DNA and alter gene expression in order to transform lymphoid cells.

Casein kinase II phosphorylates I kappa B alpha at S-283, S-289, S-293, and T-291 and is required for its degradation

The phosphoprotein I kappa B alpha exists in the cytoplasm of resting cells bound to the ubiquitous transcription factor NF-kappa B (p50-p65). In response to specific cellular stimulation, I kappa B alpha is further phosphorylated and subsequently degraded, allowing NF-kappa B to translocate to the nucleus and transactivate target genes. To identify the kinase(s) involved in I kappa B alpha phosphorylation, we first performed an I kappa B alpha in-gel kinase assay. Two kinase activities of 35 and 42 kDa were identified in cellular extracts from Jurkat T and U937 promonocytic cell lines. Specific inhibitors and immunodepletion studies identified the I kappa B alpha kinase activities as those of the alpha and alpha' subunits of casein kinase II (CKII). Immunoprecipitation studies demonstrated that CKII and I kappa B alpha physically associate in vivo. Moreover, phosphopeptide maps of I kappa B alpha phosphorylated in vitro by cellular extracts and in vivo in resting Jurkat T cells contained the same pattern of phosphopeptides as observed in maps of I kappa B alpha phosphorylated in vitro by purified CKII. Sequence analysis revealed that purified CKII and the kinase activity within cell extracts phosphorylated I kappa B alpha at its C terminus at S-283, S-288, S-293, and T-291. The functional role of CKII was tested in an in vitro I kappa B alpha degradation assay with extracts from uninfected and human immunodeficiency virus (HIV)-infected U937 cells. Immunodepletion of CKII from these extracts abrogated both the basal and enhanced HIV-induced degradation of I kappa B alpha. These studies provide new evidence that the protein kinase CKII physically associates with I kappa B alpha in vivo, induces multisite (serine/threonine) phosphorylation, and is required for the basal and HIV-induced degradation of I kappa B alpha in vitro.

Inhibition of endothelial cell activation by adenovirus-mediated expression of I kappa B alpha, an inhibitor of the transcription factor NF-kappa B

During the inflammatory response, endothelial cells (EC) transiently upregulate a set of genes encoding, among others, cell adhesion molecules and chemotactic cytokines that together mediate the interaction of the endothelium with cells of the immune system. Gene upregulation is mediated predominantly at the transcriptional level and in many cases involves the transcription factor nuclear factor (NF) kappa B. We have tested the concept of inhibiting the inflammatory response by overexpression of a specific inhibitor of NF-kappaB, I kappa B alpha. A recombinant adenovirus expressing I kappa B alpha was constructed (rAd.I kappa B alpha) and used to infect EC of human and porcine origin. Ectopic expression of IkappaBalpha resulted in marked, and in some cases complete, reduction of the expression of several markers of EC activation, including vascular cell adhesion molecule 1, interleukins 1, 6, 8, and tissue factor. Overexpressed I kappa B alpha inhibited NF-kappa B specifically since (a) in electrophoretic mobility shift assay, NF-kappa B but not AP-1 binding activity was inhibited, and (b) von Willebrand factor and prostacyclin secretion that occur independently of NF-kappa B, remained unaffected. Functional studies of leukocyte adhesion demonstrated strong inhibition of HL-60 adhesion to I kappa B alpha-expressing EC. These findings suggest that NF-kappa B could be an attractive target for therapeutic intervention in a variety of inflammatory diseases, including xenograft rejection.

Cardiac myotrophin exhibits rel/NF-kappa B interacting activity in vitro

Myotrophin is a soluble-12 kilodalton protein isolated from hypertrophied spontaneously hypertensive rat and dilated cardiomyopathic human hearts. We have recently cloned the gene coding for myotrophin and expressed it in Escherichia coli. In the present study, the expression of myotrophin gene was analyzed, and at least seven transcripts have been detected in rat heart and in other tissues. We have further analyzed the primary structure of myotrophin protein and identified significant new structural and functional domains. Our analysis revealed that one of the ankyrin repeats of myotrophin is highly homologous specifically to those of myotrophin is highly homologous specifically to those of I kappa B alpha/rel ankyrin repeats. In addition, putative consensus phosphorylation sites for protein kinase C and casein kinase II, which were observed in I kappa B alpha proteins, were identified in myotrophin. To verify the significance of these homologies, kappa B gel shift assays were performed with Jurkat T cell nuclear extract proteins and the recombinant myotrophin. Results of these assays indicate that the recombinant myotrophin has the ability to interact with NF-kappa B/rel proteins as revealed by the formation of ternary protein-DNA complexes. While myotrophin-specific antibodies inhibited the formation of these complexes, rel-specific p50 and p65 antibodies supershifted these complexes. Thus, these results clearly indicate that the myotrophin protein to be a unique rel/NF-kappa B interacting protein.

Structure and regulation of the gene encoding avian inhibitor of nuclear factor kappa B-alpha

The Rel/NF-kappa B family of transcription factors exist in the cytoplasm as inactive complexes in association with an inhibitory protein called I kappa B-alpha. We have isolated a clone containing the avian I kappa B-alpha gene from a chicken genomic library. Avian I kappa B-alpha is devoid of any recognizable promoter elements, i.e., TATA and CAAT boxes; however, the 5'-UTR of the gene contains the initiator elements frequently found in TATA-less genes. Avian I kappa B-alpha contains seven putative Rel/NF-kappa B binding sites. A CAT reporter construct containing the 5' upstream region of I kappa B-alpha was expressed when transfected into cells which produce I kappa B-alpha. This construct, however, was not expressed in cells in which I kappa B-alpha activity was not induced, indicating that the regulatory elements which promote I kappa B-alpha expression are contained within 1000 nt of the transcription start site. Southern analysis suggests that I kappa B-alpha is present as a single-copy gene per haploid genome and is expressed in avian hematopoietic tissues, as well as lymphoid cells transformed by avian reticuloendotheliosis virus (REV-T).

The regulation of protein transport to the nucleus by phosphorylation

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Regulation of human immunodeficiency virus type 1 and cytokine gene expression in myeloid cells by NF-kappa B/Rel transcription factors

CD4+ macrophages in tissues such as lung, skin, and lymph nodes, promyelocytic cells in bone marrow, and peripheral blood monocytes serve as important targets and reservoirs for human immunodeficiency virus type 1 (HIV-1) replication. HIV-1-infected myeloid cells are often diminished in their ability to participate in chemotaxis, phagocytosis, and intracellular killing. HIV-1 infection of myeloid cells can lead to the expression of surface receptors associated with cellular activation and/or differentiation that increase the responsiveness of these cells to cytokines secreted by neighboring cells as well as to bacteria or other pathogens. Enhancement of HIV-1 replication is related in part to increased DNA-binding activity of cellular transcription factors such as NF-kappa B. NF-kappa B binds to the HIV-1 enhancer region of the long terminal repeat and contributes to the inducibility of HIV-1 gene expression in response to multiple activating agents. Phosphorylation and degradation of the cytoplasmic inhibitor I kappa B alpha are crucial regulatory events in the activation of NF-kappa B DNA-binding activity. Both N- and C-terminal residues of I kappa B alpha are required for inducer-mediated degradation. Chronic HIV-1 infection of myeloid cells leads to constitutive NF-kappa B DNA-binding activity and provides an intranuclear environment capable of perpetuating HIV-1 replication. Increased intracellular stores of latent NF-kappa B may also result in rapid inducibility of NF-kappa B-dependent cytokine gene expression. In response to secondary pathogenic infections or antigenic challenge, cytokine gene expression is rapidly induced, enhanced, and sustained over prolonged periods in HIV-1-infected myeloid cells compared with uninfected cells. Elevated levels of several inflammatory cytokines have been detected in the sera of HIV-1-infected individuals. Secretion of myeloid cell-derived cytokines may both increase virus production and contribute to AIDS-associated disorders.

Regulation of the DNA binding activity of NF-kappa B

The DNA binding activity of the dimeric sequence-specific transcription factor NF-kappa B can be controlled by a variety of post-translational mechanisms, including interactions with inhibitor proteins and by its redox state. The NF-kappa B family of transcription factors bind to kappa B motif sequences found in promoter and enhancer regions of a wide range of cellular and viral genes. Normally NF-kappa B family proteins are held in the cytoplasm in an inactive, non-DNA binding form by labile I kappa B inhibitor proteins. When the cell is activated by one of a wide range of stimuli, typically those associated with the cellular response to pathogens or stress, proteolytic degradation of I kappa B inhibitor proteins allows active NF-kappa B to translocate to the nucleus where it activates transcription of responsive genes. The initial trigger for I kappa B degradation is a signal-induced site-specific phosphorylation by an as yet unidentified kinase, which appears to target I kappa B for the covalent addition of multiple copies of the ubiquitin polypeptide. This modification subsequently allows the proteolytic degradation of the ubiquitinated I kappa B by the cellular 26S multicatalytic proteinase (proteasome) complex. It was recently shown that increased I kappa B-alpha expression in the cytoplasm leads to I kappa B-alpha accumulating in the nuclear compartment, removing template-bound NF-kappa B, and reducing NF-kappa B-dependent transcription. These NF-kappa B-I kappa B-alpha complexes could then be actively re-exported to the cytoplasm, allowing the cell to respond to further stimuli.

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.

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.

Cytokine induction of nuclear factor kappa B in cycling and growth-arrested cells. Evidence for cell cycle-independent activation

Nuclear factor kappa B (NF-kappa B) is a pleiotropic transcription factor which regulates the expression of a large number of cellular and viral genes. Induction of NF-kappa B has been shown previously to occur during cell cycle transition from G0 to G1, but the relationship of cytokine induction of this transcription factor to cell cycling has not been directly addressed. Here we examine the inductions of NF-kappa B in serum-deprived and cycling cells in response to tumor necrosis factor-alpha (TNF-alpha). In 3T3 fibroblasts deprived of serum, and in the temperature-sensitive G2 phase mutant carcinoma line FT210, we find that NF-kappa B DNA binding activity is rapidly induced upon addition of TNF-alpha. In addition, NF-kappa B induction in cycling cells occurs without a significant change in cell cycle distribution. These data reveal that NF-kappa B is rapidly induced by TNF-alpha in both proliferating and arrested cells and suggest that distinct activation pathways can lead to cell cycle-dependent or -independent induction of NF-kappa B.

Identification of a novel p53 promoter element involved in genotoxic stress-inducible p53 gene expression

p53 is recruited in response to DNA-damaging genotoxic stress and plays an important role in maintaining the integrity of the genome. We show that exposure of cells to various genotoxic agents, including anticancer drugs such as mitomycin and 5-fluorouracil, results in an increase in p53 mRNA levels and in p53 promoter activation, indicating that the p53 genotoxic stress response is partly regulated at the transcriptional level. The results of the p53 promoter analysis show that a novel p53 promoter element, termed a p53 core promoter element (from -70 to -46), is essential for basal p53 promoter activity and promoter activation induced by genotoxic agents such as anticancer drugs and UV. Although a kappa B motif partially overlaps with this element and those genotoxic agents activate NF-kappa B, it does not play a major role in p53 genotoxic stress response: NF-kappa B p65 expression did not induce significant p53 promoter activation, and NF-kappa B inhibitors (N-acetyl cysteine and I kappa B alpha) did not inhibit genotoxic stress-inducible p53 promoter activation. Finally, we characterized nuclear factors, the binding of which to the p53 core promoter element is essential for basal p53 promoter activity and p53 promoter activation induced by genotoxic agents.

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.

Structural and functional characterization of the promoter regions of the NFKB2 gene

In order to clarify the transcriptional regulation of the NFKB2 gene (lyt-10, NF-kappa Bp100), we have characterized the structure and function of its promoter regions. Based on the nucleotide sequence of cDNA clones and the 5' flanking genomic region of the NFKB2 gene, RT-PCR analysis in a number of human cell lines demonstrated the presence of two alternative noncoding first exons (1a and 1b). Two distinct promoter regions, P1 and P2, were identified upstream of each exon, containing multiple sites of transcription initiation, as shown by RNase protection analysis. Sequence analysis of these regions showed a CAAT box upstream of exon 1a and high G-C content regions within both P1 and P2. Consensus binding sites for transcription factors, including SP1, AP1 and putative NF-kappa B (kappa B sites), were found upstream of each exon. In particular, six kappa B sites were identified, all but one of them capable of binding NF-kappa B complexes in vitro. Transfection in HeLa cells of plasmids containing P1 and P2 sequences linked to a chloramphenicol acetyltransferase reporter gene indicated that both P1 and P2 can act independently as promoters. Co-transfection of NF-kappa B effector plasmids (NF-kappa Bp52 and RelA) with a reporter gene linked to P1 and P2 showed that the NFKB2 promoter regions are regulated by NF-kappa B factors. RelA transactivates the NFKB2 promoter in a dose-dependent manner, whereas NF-kappa Bp52 acts as a repressor, indicating that the NFKB2 gene may be under the control of a negative feedback regulatory circuit.

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.

Mutations in the DNA-binding and dimerization domains of v-Rel are responsible for altered kappa B DNA-binding complexes in transformed cells

The c-rel proto-oncogene encodes a member of the Rel/NF-kappa B family of transcription factors. The oncogenic viral form, v-rel, transduced by avian reticuloendotheliosis virus T, induces lymphoid tumors. v-Rel transformation may be mediated directly by binding of v-Rel to cognate DNA sites, resulting in altered gene expression, and/or indirectly by releasing Rel/NF-kappa B transcription factors from cytoplasmic retention molecules, resulting in their translocation to the nucleus and the inappropriate expression of genes under kappa B control. v-Rel-transformed cell lines of different phenotypes contained v-Rel as well as endogenous kappa B DNA-binding proteins in nuclear extracts. Kinetic analysis with avian leukosis virus-transformed B-cell lines expressing v-Rel or c-Rel indicated that the presence of endogenous kappa B DNA-binding proteins in the nucleus is temporally correlated with the relocalization of v-Rel to the cytoplasm. Supershift analysis of these DNA-binding complexes revealed that v-Rel was present in all of the nuclear DNA-binding complexes heterodimerized with c-Rel, NF-kappa B1, and other proteins. In contrast, c-Rel-transformed cells exhibited a less-complex pattern of nuclear kappa B DNA-binding complexes, and the nuclear appearance of these endogenous complexes was not observed. Studies with c-/v-Rel hybrids suggest that the induction of the endogenous kappa B DNA-binding complexes is the result of the mutations in the C-terminal region of the Rel homology (RH) domain of v-Rel. Moreover, v-Rel differed from c-Rel in its DNA-binding specificity. The altered DNA-binding specificity of v-Rel was associated with mutations located in the N-terminal part of the RH domain of v-Rel. These results suggest that two different regions of v-Rel (both located in the RH domain) influence the formation of kappa B DNA-binding complexes differently.

Cloning of a differentially expressed I kappa B-related protein

We have cloned a cDNA corresponding to a novel gene from a human epithelial cell line by subtractive hybridization and polymerase chain reaction techniques. This gene is expressed at the message level and at the protein level in a lung alveolar type II-like epithelial cell line but not in lung fibroblasts. In adult human tissues, the mRNA for this gene was detected only in the heart and the skeletal muscle, but not in the brain, placenta, whole lung, liver, or kidney. We have named this gene I kappa BR (for I kappa B-related) since its 52-kDa protein product has significant homology to the I kappa B family of proteins which function as inhibitory cytoplasmic retention proteins for the vertebrate rel/NF-kappa B transcription factors. Although the important role of NF-kappa B in gene activation in cells of the immune system is now well established, a similar role in other cell types or in vertebrate development is less clear. The deduced amino acid sequence of I kappa BR has the most significant homology to the Drosophila protein Cactus which inhibits the function of the NF-kappa B-like protein Dorsal. In electrophoretic mobility shift experiments, I kappa BR inhibited the ability of the p50:p65 NF-kappa B heterodimer to bind DNA. The DNA binding ability of the p50 homodimer but not the p65 homodimer was drastically inhibited by I kappa BR. In transfection experiments, overexpression of I kappa BR significantly inhibited NF-kappa B-dependent transcription from the Ig kappa enhancer. This new member of the I kappa B family of proteins, I kappa BR, may play an important role in regulation of NF-kappa B function in epithelial cells.

Coupling of a signal response domain in I kappa B alpha to multiple pathways for NF-kappa B activation

The eukaryotic transcription factor NF-kappa B plays a central role in the induced expression of human immunodeficiency virus type 1 and in many aspects of the genetic program mediating normal T-cell activation and growth. The nuclear activity of NF-kappa B is tightly regulated from the cytoplasmic compartment by an inhibitory subunit called I kappa B alpha. This cytoplasmic inhibitor is rapidly phosphorylated and degraded in response to a diverse set of NF-kappa B-inducing agents, including T-cell mitogens, proinflammatory cytokines, and viral transactivators such as the Tax protein of human T-cell leukemia virus type 1. To explore these I kappa B alpha-dependent mechanisms for NF-kappa B induction, we identified novel mutants of I kappa B alpha that uncouple its inhibitory and signal-transducing functions in human T lymphocytes. Specifically, removal of the N-terminal 36 amino acids of I kappa B alpha failed to disrupt its ability to form latent complexes with NF-kappa B in the cytoplasm. However, this deletion mutation prevented the induced phosphorylation, degradative loss, and functional release of I kappa B alpha from NF-kappa B in Tax-expressing cells. Alanine substitutions introduced at two serine residues positioned within this N-terminal regulatory region of I kappa B alpha also yielded constitutive repressors that escaped from Tax-induced turnover and that potently inhibited immune activation pathways for NF-kappa B induction, including those initiated from antigen and cytokine receptors. In contrast, introduction of a phosphoserine mimetic at these sites rectified this functional defect, a finding consistent with a causal linkage between the phosphorylation status and proteolytic stability of this cytoplasmic inhibitor. Together, these in vivo studies define a critical signal response domain in I kappa B alpha that coordinately controls the biologic activities of I kappa B alpha and NF-kappa B in response to viral and immune stimuli.

Identification and characterization of a novel cytokine-inducible nuclear protein from human endothelial cells

Vascular endothelial cells undergo profound changes upon cellular activation including expression of a spectrum of cell activation-associated genes. These changes play important roles in many physiological and pathological events. By differential screening of a cDNA library prepared from interleukin-1 alpha and tumor necrosis factor-alpha-stimulated human dermal microvascular endothelial cells, we have identified a novel cytokine-inducible gene, designated as C-193. The compiled cDNA sequence of C-193 is 1901 base pairs long and shows no significant homology with any known gene sequence. Genomic DNA analysis revealed that C-193 is encoded by a single gene, which is conserved in different mammalian species. The C-193 gene was localized to human chromosome 10 by Southern blot analysis of somatic cell hybrids. Multiple AT-rich mRNA decay elements were identified in the 3'-untranslated region. C-193 mRNA expression was rapidly and transiently induced by treatment with interleukin-1 alpha or tumor necrosis factor-alpha, reached a peak of expression about 16 h post tumor necrosis factor-alpha stimulation, and the induction of C-193 was protein synthesis independent. Lipopolysaccharide and cycloheximide were also potent inducers of C-193 mRNA. Therefore, C-193 represents a new addition to the primary response gene family. In vitro translation of C-193 yielded a 36-kDa protein product, consistent with the predicted open reading frame of 318 amino acids and a calculated molecular mass of 36 kDa for C-193 protein. The predicted protein sequence contains a basic amino acid cluster similar to a nuclear localization signal, four tandem repeats of ankyrin-like sequence, and multiple consensus protein phosphorylation sites. C-193 was engineered with a FLAG tag at its carboxyl terminus and transiently expressed in COS cells. Consistent with the presence of a putative nuclear localization signal, the C-193-FLAG protein was localized to the nucleus of transfected COS cells by indirect immunofluorescence microscopy. C-193-FLAG prepared in vitro was capable of binding DNA cellulose. These results indicate that C-193 protein may play an important role in endothelial cell activation.

Interleukin 1-induced phosphorylation of MAD3, the major inhibitor of nuclear factor kappa B of HeLa cells. Interference in signalling by the proteinase inhibitors 3,4-dichloroisocoumarin and tosylphenylalanyl chloromethylketone

The regulation of the inhibitor of nuclear factor kappa B (I kappa B) by interleukin 1 (IL1) was investigated in HeLa cells. Two forms of I kappa B were resolved by ion-exchange chromatography. The major form (75%) was identified as MAD3 by specific antisera. IL1 generated rapidly (6 min) an electrophoretically retarded form of MAD3 that was stable in acid and was converted into the unmodified form by phosphatase 2A. It thus corresponded to a phosphorylation of the protein on serine or threonine. IL1 also caused the disappearance of MAD3 from the cells, which was complete 15 min after stimulation and coincided with a 46% reduction of cellular I kappa B activity. Newly-synthesized MAD3 accumulated to pre-stimulation levels between 60 and 90 min after stimulation and this coincided with the down-regulation of the phosphorylating activity. The serine proteinase inhibitors 3,4-dichloroisocoumarin (DCI) and tosylphenylalanyl chloromethylketone (TPCK) prevented phosphorylation and disappearance of MAD3. At the same concentrations (10-100 microM), they also increased basal phosphorylation of the small heat shock protein (hsp27) and prevented the IL1- and phorbol 12-myristate 13-acetate-induced increases of its phosphorylation. The inhibitors were thus interfering with protein kinases when blocking degradation of MAD3. Recombinant MAD3 phosphorylated in vitro by protein kinase C was not electrophoretically retarded, suggesting that MAD3 was phosphorylated by another kinase in IL1-stimulated cells. Our results suggest that the IL1-induced phosphorylation of MAD3 on serine or threonine leads to its degradation. DCI and TPCK blocked phosphorylation mechanisms and it could not be concluded that serine proteinases were involved in the breakdown of MAD3.

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.

Shark, a Src homology 2, ankyrin repeat, tyrosine kinase, is expressed on the apical surfaces of ectodermal epithelia

Tyrosine kinases, ankyrin repeats, and Src homology 2 domains play central roles in developmental processes. The cloning of a cDNA for Shark, a single protein that possesses all three domains, is described. During Drosophila embryogenesis, Shark is expressed exclusively by ectodermally derived epithelia and is localized preferentially to the apical surface of these cells. This apical localization persists, even as tissues undergo complex invaginations, moving from the external surface of embryos to form internal structures, but expression is lost when cells lose their polarity. This pattern closely mimics the expression of Crumbs, a protein necessary for proper organization of ectodermal epithelia. Shark's structure and localization pattern suggest that it functions in a signaling pathway for epithelial cell polarity, possibly transducing the Crumbs signal.

Constitutive and inducible kappa B binding activities in the cytosol of v-Rel-transformed lymphoid cells

Constitutive and inducible kapp B binding activities associated with v-Rel and c-Rel in the cytosol of v-Rel-transformed cells have been identified. These activities were resolved by electrophoretic mobility shift assay and chromatographic techniques into a high-molecular-weight protein-DNA complex designated complex I containing v- and c-Rel and lower-molecular-weight complexes II, III and IV which contained only v-Rel and which were stimulated by nucleotides, low pH, and detergent. These experiments suggest that interaction of v-Rel and c-Rel decreases the DNA-binding activity of each.

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.

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.

I kappa B-beta regulates the persistent response in a biphasic activation of NF-kappa B

We have cloned the cDNA encoding I kappa B-beta, one of the two major I kappa B isoforms in mammalian cells. The recombinant I kappa B- beta protein interacts with equal affinity to p65 and c-Rel and does not exhibit a preference between these Rel proteins. Instead the primary difference between I kapp B-alpha and I kappa B-beta is in their response to different inducers of NF-kappa B activity. One class of inducers causes rapid but transient activation of NF-kappa B by primarily affecting I kappa B-alpha complexes, whereas another class of inducers causes persistent activation of NF-kapa B by affecting both I kappa B-alpha and I kappa B-beta complexes. Therefore, the overall activation of NF-kappa B consists of two overlapping phases, a transient phase mediated through I kappa B-alpha and a persistent phase mediated through I kappa B-beta.

The relocalization of v-Rel from the nucleus to the cytoplasm coincides with induction of expression of Ikba and nfkb1 and stabilization of I kappa B-alpha

The v-Rel oncogene induces the expression of major histocompatibility complex class I and II proteins and the interleukin-2 receptor more efficiently than does c-Rel (R. Hrdlicková, J. Nehyba, and E. H. Humphries, J. Virol. 68:308-319, 1994). The kinetics with which these immunoregulatory receptors are induced in B- and T-lymphoid cell lines and chicken embryo fibroblast cultures expressing c-Rel or v-Rel have been examined. v-Rel induced the expression of major histocompatibility complex classes I and II and interleukin-2 receptor more efficiently than did c-Rel at later times after infection. In all three cell types, this increased efficiency was accompanied by a shift in the majority of v-Rel from the nucleus of the cytoplasm. The concomitant relocalization of v-Rel was also demonstrated during the in vitro transformation of spleen cells. The translocation coincided with increased steady-state levels of I kappa B-alpha. Coninfection by retroviral vectors expressing v-Rel, I kappa B-alpha, or NF-kappa B1 demonstrated that either I kappa B-alpha can contribute to the shift of v-Rel to the cytoplasmic compartment. The induction of nfkb1 and Ikba mRNA and the stabilization of I kappa B-alpha by v-Rel were shown to be responsible for these effects. In comparison with c-Rel, the expression of v-Rel was associated with lower levels of transcription of these genes. However, the ability of v-Rel to stabilize I kappa B-alpha remained unchanged. The ability of v-Rel to stabilize I kappa B-alpha but poorly induce Ikba mRNA expression relative to c-Rel may play a role in regulating gene expression, thereby leading to transformation.

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.

NF-kappa B/Rel family members are physically associated phosphoproteins

We performed radioimmunoprecipitation followed by serial immunoblots to show that, in the unstimulated Jurkat T cell line, the NF-kappa B/Rel family proteins, p80-c-Rel, p105-NF-kappa B, p65-NF-kappa B, p50-NF-kappa B and p36-I kappa B alpha, can be detected as complexes using antisera against c-Rel, p105-NF-kappa B or p65-NF-kappa B. p36-I kappa B alpha and p105, both known inhibitors of NF-kappa B function, can physically associate with NF-kappa B/Rel family members, but not with each other. In vivo and in vitro phosphorylation experiments demonstrated that NF-kappa B/Rel family members, including p105, c-Rel, p50, p65 (for the first time for p50 and p65) and p36-I kappa B alpha are also phosphoproteins. Phosphoserine and phosphothreonine residues were identified in these proteins isolated from unstimulated Jurkat cells. Both unphosphorylated and hyperphosphorylated forms of p36-I kappa B alpha were found in the complexes, suggesting that hyperphosphorylated I kappa B alpha is still capable of associating with the NF-kappa B/Rel family members. After stimulation with phorbol 12-myristate 13-acetate and phytohaemagglutinin for 10 min, p105-NF-kappa B and p50-NF-kappa B, but not p36-I kappa B, were highly phosphorylated. Phosphopeptide mapping of p105 showed that phorbol ester/phytohaemagglutinin stimulation may change p105 phosphorylation qualitatively.

BCL3 encodes a nuclear protein which can alter the subcellular location of NF-kappa B proteins

BCL3 is a candidate proto-oncogene involved in the recurring translocation t(14;19) found in some patients with chronic lymphocytic leukemia. BCL3 protein acts as an I kappa B in that it can specifically inhibit the DNA binding of NF-kappa B factors. Here, we demonstrate that BCL3 is predominantly a nuclear protein and provide evidence that its N terminus is necessary to direct the protein into the nucleus. In contrast to I kappa B alpha (MAD3), BCL3 does not cause NF-kappa B p50 to be retained in the cytoplasm; instead, in cotransfection assays, it alters the subnuclear localization of p50. The two proteins colocalize, suggesting that they interact in vivo. Further immunofluorescence experiments showed that a mutant p50, lacking a nuclear localization signal and restricted to the cytoplasm, is brought into the nucleus in the presence of BCL3. Correspondingly, a wild-type p50 directs into the nucleus a truncated BCL3, which, when transfected alone, is found in the cytoplasm. We tested whether BCL3 could overcome the cytoplasmic retention of p50 by I kappa B alpha. Results from triple cotransfection experiments with BCL3, I kappa B alpha, and p50 implied that BCL3 can successfully compete with I kappa B alpha and bring p50 into the nucleus; thus, localization of NF-kappa B factors may be affected by differential expression of I kappa B proteins. These novel properties of BCL3 protein further establish BCL3 as a distinctive member of the I kappa B family.

BCL3 encodes a nuclear protein which can alter the subcellular location of NF-kappa B proteins

BCL3 is a candidate proto-oncogene involved in the recurring translocation t(14;19) found in some patients with chronic lymphocytic leukemia. BCL3 protein acts as an I kappa B in that it can specifically inhibit the DNA binding of NF-kappa B factors. Here, we demonstrate that BCL3 is predominantly a nuclear protein and provide evidence that its N terminus is necessary to direct the protein into the nucleus. In contrast to I kappa B alpha (MAD3), BCL3 does not cause NF-kappa B p50 to be retained in the cytoplasm; instead, in cotransfection assays, it alters the subnuclear localization of p50. The two proteins colocalize, suggesting that they interact in vivo. Further immunofluorescence experiments showed that a mutant p50, lacking a nuclear localization signal and restricted to the cytoplasm, is brought into the nucleus in the presence of BCL3. Correspondingly, a wild-type p50 directs into the nucleus a truncated BCL3, which, when transfected alone, is found in the cytoplasm. We tested whether BCL3 could overcome the cytoplasmic retention of p50 by I kappa B alpha. Results from triple cotransfection experiments with BCL3, I kappa B alpha, and p50 implied that BCL3 can successfully compete with I kappa B alpha and bring p50 into the nucleus; thus, localization of NF-kappa B factors may be affected by differential expression of I kappa B proteins. These novel properties of BCL3 protein further establish BCL3 as a distinctive member of the I kappa B family.

Enhanced I kappa B alpha degradation is responsible for constitutive NF-kappa B activity in mature murine B-cell lines

Nuclear factor kappa B (NF-kappa B) is a ubiquitous transcription factor which binds to decameric DNA sequences (kappa B sites) and regulates transcription of multiple genes. The activity of NF-kappa B is regulated by an inhibitor protein, I kappa B, which sequesters NF-kappa B in the cytoplasm. Release of I kappa B and subsequent nuclear translocation of NF-kappa B generally require activating signals. However, in mature murine B cells, the DNA-binding activity of NF-kappa B is constitutively nuclear and activates the Ig kappa gene, a marker for mature B cells. To understand the basis for the constitutive NF-kappa B activation, we examined the properties of NF-kappa B and I kappa B in both pre-B and mature B cells, the regulated and constitutive states, respectively. We found that expression of I kappa B alpha and p105, members of the I kappa B family, and Rel, a member of the NF-kappa B family, is augmented in mature B cells. Both I kappa B alpha and p 105 are associated with NF-kappa B proteins and sequester most of these proteins in the cytoplasm of mature B cells. However, rapid I kappa B alpha dissociation and degradation lead to continuous nuclear translocation of a small fraction of NF-kappa B proteins, which represent the constitutively active NF-kappa B in mature B cells. We estimate that the protease activity is at least 35-fold greater in mature B cells than in pre-B cells. Rapid degradation of I kappa B alpha is directly involved in constitutive NF-kappa B activation, because stabilization of I kappa B alpha by a protease inhibitor causes loss of NF-kappa B activity in mature B cells. These results provide evidence that continuous and rapid degradation of I kappa B alpha in the absence pf external stimuli is causally involved in the constitutive activation of NF-kappa B in mature murine B cells.

Enhanced I kappa B alpha degradation is responsible for constitutive NF-kappa B activity in mature murine B-cell lines

Nuclear factor kappa B (NF-kappa B) is a ubiquitous transcription factor which binds to decameric DNA sequences (kappa B sites) and regulates transcription of multiple genes. The activity of NF-kappa B is regulated by an inhibitor protein, I kappa B, which sequesters NF-kappa B in the cytoplasm. Release of I kappa B and subsequent nuclear translocation of NF-kappa B generally require activating signals. However, in mature murine B cells, the DNA-binding activity of NF-kappa B is constitutively nuclear and activates the Ig kappa gene, a marker for mature B cells. To understand the basis for the constitutive NF-kappa B activation, we examined the properties of NF-kappa B and I kappa B in both pre-B and mature B cells, the regulated and constitutive states, respectively. We found that expression of I kappa B alpha and p105, members of the I kappa B family, and Rel, a member of the NF-kappa B family, is augmented in mature B cells. Both I kappa B alpha and p 105 are associated with NF-kappa B proteins and sequester most of these proteins in the cytoplasm of mature B cells. However, rapid I kappa B alpha dissociation and degradation lead to continuous nuclear translocation of a small fraction of NF-kappa B proteins, which represent the constitutively active NF-kappa B in mature B cells. We estimate that the protease activity is at least 35-fold greater in mature B cells than in pre-B cells. Rapid degradation of I kappa B alpha is directly involved in constitutive NF-kappa B activation, because stabilization of I kappa B alpha by a protease inhibitor causes loss of NF-kappa B activity in mature B cells. These results provide evidence that continuous and rapid degradation of I kappa B alpha in the absence pf external stimuli is causally involved in the constitutive activation of NF-kappa B in mature murine B cells.

Purification, reconstitution, and I kappa B association of the c-Rel-p65 (RelA) complex, a strong activator of transcription

HeLa cells contain a DNA-binding activity which associates with a kappa B-like DNA element, termed Rel-related protein-binding element (RRBE), localized upstream of the human urokinase promoter. We have purified this activity from the HeLa cell cytosol and have shown that it represents a performed heteromeric complex between p65 (RelA) and c-Rel. Coexpression of c-Rel and p65 (RelA) by in vitro translation formed a DNA-binding complex indistinguishable from purified cellular c-Rel-p65 (RelA) in mobility shift assays. The c-Rel-p65 (RelA) complex was also formed in COS7 cells upon coexpression of c-Rel and p65 (RelA) cDNAs. Cotransfection experiments with COS7 cells, using expression plasmids encoding p50, p65 (RelA), or c-Rel and reporter constructs containing a trimerized RRBE, revealed that c-Rel-p65 (RelA) is a potent activator of the RRBE, giving rise to transcriptional activity higher than that observed with NF-kappa B (p50-p65). In the cytosol, the c-Rel-p65 (RelA) complex existed in a latent, non-DNA-binding form but could be activated by detergent treatment, suggesting that it was associated with an I kappa B protein. Recombinant I kappa B-alpha inhibited the DNA-binding activity of c-Rel-p65 (RelA) via association with either c-Rel or p65 (RelA). Finally, NF-kappa B and c-Rel-p65 (RelA) complexes were found to be differentially expressed and regulated in different cells. The two complexes were present in equimolar amounts in HeLa cells and K562 cells. Stimulation with tetradecanoyl phorbol acetate (TPA) resulted in the nuclear translocation of both NF-kappa B and c-Rel-p65 (RelA) in HeLa cells and of NF-kappa B in HepG2 cells but had no effect on either complex in K562 cells. In addition, TPA stimulation of HepG2 cells induced the expression of a cytosolic latent c-Rel-p65 (RelA) complex which, however, was not translocated to the nucleus. In conclusion, our findings show that c-Rel-p65 (RelA) is an inducible and very potent transcriptional activator which is differentially activated in a cell-type-specific manner.

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.