Mutual regulation of the transcriptional activator NF-kappa B and its inhibitor, I kappa B-alpha

The v-abl tyrosine kinase negatively regulates NF-kappa B/Rel factors and blocks kappa gene transcription in pre-B lymphocytes

Transformation of B-lineage precursors by the Abelson murine leukemia virus appears to arrest development at the pre-B stage. Abelson-transformed pre-B cell lines generally retain transcriptionally inactive, unrearranged immunoglobulin kappa alleles. We demonstrate that nontransformed pre-B cells expanded from the mouse bone marrow efficiently transcribe unrearranged kappa alleles. In addition, they contain activated complexes of the NF-kappa B/Rel transcription factor family, in contrast with their Abelson-transformed counterparts. Using conditionally transformed pre-B cell lines, we show that the v-abl viral transforming protein, a tyrosine kinase, blocks germ-line kappa gene transcription and negatively regulates NF-kappa B/Rel activity. An active v-abl kinase specifically inhibits the NF-kappa B/Rel-dependent kappa intron enhancer, which is implicated in promoting both transcription and rearrangement of the kappa locus. v-abl inhibits the activated state of NF-kappa B/Rel complexes in a pre-B cell via a post-translational mechanism that results in increased stability of the inhibitory subunit I kappa B alpha. This analysis suggests a molecular pathway by which v-abl inhibits kappa locus transcription and rearrangement.

Autoregulation of the NF-kappa B transactivator RelA (p65) by multiple cytoplasmic inhibitors containing ankyrin motifs

RelA (p65) functions as the critical transactivating component of the heterodimeric p50-p65 NF-kappa B complex and contains a high-affinity binding site for its cytoplasmic inhibitor, I kappa B alpha. After cellular activation, I kappa B alpha is rapidly degraded in concert with the induced nuclear translocation of NF-kappa B. The present study demonstrates that tumor necrosis factor alpha-induced degradation of I kappa B alpha in human T cells is preceded by its rapid phosphorylation in vivo. However, these effects on I kappa B alpha result in nuclear mobilization of only a fraction of the entire cytoplasmic pool of RelA. Subsequent studies have revealed that (i) cytoplasmic RelA is stably associated not only with I kappa B alpha but also with other ankyrin motif-rich proteins including the products of the NF-kappa B2 (p100) and NF-kappa B1 (p105) genes; (ii) in contrast to RelA-I kappa B alpha, RelA-p100 cytoplasmic complexes are not dissociated following tumor necrosis factor alpha activation; (iii) p100 functions as a potent inhibitor of RelA-mediated transcription in vivo; (iv) the interaction of RelA and p100 involves the conserved Rel homology domain of both proteins but not the nuclear localization signal of RelA, which is required for I kappa B alpha binding; (v) p100 inhibition of RelA function requires the C-terminal ankyrin motif domain, which mediates cytoplasmic retention of RelA; and (vi) as observed with I kappa B alpha, nuclear RelA stimulates p100 mRNA and protein expression. These findings thus reveal the presence of a second inducible autoregulated inhibitory pathway that helps ensure the rapid but transient action of nuclear NF-kappa B.

NF-kappa B and I kappa B alpha: an inducible regulatory system in endothelial activation

Structural analysis of the promoters of several endothelial genes induced at sites of inflammatory or immune responses reveals binding sites for the transcription factor nuclear factor kappa B (NF-kappa B). Endothelial cells express transcripts encoding the p50/p105 and p65 components of NF-kappa B and the rel-related proto-oncogene c-rel; steady state levels of these transcripts are transiently increased by tumor necrosis factor alpha (TNF-alpha). Western blotting revealed that stimulation of endothelial cells with TNF-alpha resulted in nuclear accumulation of the p50 and p65 components of NF-kappa B. Ultraviolet crosslinking and immunoprecipitation demonstrated binding of the p50 and p65 components of NF-kappa B to the E-selectin kappa B site. Endothelial cells express an inhibitor of NF-kappa B activation, I kappa B-alpha (MAD-3). Protein levels of this inhibitor fall rapidly after TNF-alpha stimulation. In parallel, p50 and p65 accumulate in the nucleus and RNA transcript levels for I kappa B-alpha are dramatically upregulated. Recombinant p65 stimulates expression of E-selectin promoter-reporter constructs. I kappa B-alpha inhibits p65 or TNF-alpha-stimulated E-selectin promoter-reporter gene expression in transfected endothelial cells. The NF-kappa B and I kappa B-alpha system may be an inducible regulatory mechanism in endothelial activation.

Autoregulation of I kappa B alpha activity

Transcription factor NF-kappa B regulates the expression of a plethora of genes. The activity of NF-kappa B proteins is regulated by I kappa B proteins. We report that induction of I kappa B alpha, a member of the I kappa B family of proteins, is preceded by activation of NF-kappa B complex. The promoter of the I kappa B alpha gene contains a kappa B site that is directly involved in its induction by the NF-kappa B complex. Degradation of I kappa B alpha protein precedes activation of NF-kappa B DNA binding activity, whereas newly synthesized I kappa B alpha protein inhibits NF-kappa B activity. If the degradation of I kappa B alpha is prevented, the induction of DNA binding activity of NF-kappa B complex is severely curtailed. These data suggest the existence of an autoregulatory loop whereby I kappa B alpha regulates the activity of transcription factor NF-kappa B, which in turn regulates the I kappa B alpha activity.

A novel NF-kappa B complex containing p65 homodimers: implications for transcriptional control at the level of subunit dimerization

The predominant inducible form of the NF-kappa B transcription factor is a heteromeric complex containing two Rel-related DNA-binding subunits, termed p65 and p50. Prior transfection studies have shown that when these p65 and p50 subunits are expressed independently as stable homodimers, p65 stimulates kappa B-directed transcription, whereas p50 functions as a kappa B-specific repressor. While authentic p50 homodimers (previously termed KBF1) have been detected in nuclear extracts from nontransfected cells, experimental evidence supporting the existence of p65 homodimers in vivo was lacking. We now provide direct biochemical evidence for the presence of an endogenous pool of inducible p65 homodimers in intact human T cells. As with the prototypical NF-kappa B p50-p65 heterodimer, this novel p65 homodimeric form of NF-kappa B is functionally sequestered in the cytoplasm but rapidly appears in the nuclear compartment following cellular stimulation. Site-directed mutagenesis studies indicate that the homodimerization function of p65 is dependent upon the presence of cysteine 216 and a conserved recognition motif for protein kinase A (RRPS; amino acids 273 to 276), both of which reside within a 91-amino-acid segment of the Rel homology domain that mediates self-association. In contrast, mutations at these two sites do not affect heterodimerization of p65 with p50 or its functional interaction with I kappa B alpha. These later findings indicate that neither homo- nor heterodimer formation is an absolute prerequisite for I kappa B alpha recognition of p65. Taken together with prior in vivo transcription studies, these results suggest that the biological activities of p65 and p50 homodimers are independently regulated, thereby providing an integrated and flexible control mechanism for the rapid activation and repression of NF-kappa B/Rel-directed gene expression.

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.

Negative regulation of transcription in eukaryotes

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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.

A novel NF-kappa B complex containing p65 homodimers: implications for transcriptional control at the level of subunit dimerization

The predominant inducible form of the NF-kappa B transcription factor is a heteromeric complex containing two Rel-related DNA-binding subunits, termed p65 and p50. Prior transfection studies have shown that when these p65 and p50 subunits are expressed independently as stable homodimers, p65 stimulates kappa B-directed transcription, whereas p50 functions as a kappa B-specific repressor. While authentic p50 homodimers (previously termed KBF1) have been detected in nuclear extracts from nontransfected cells, experimental evidence supporting the existence of p65 homodimers in vivo was lacking. We now provide direct biochemical evidence for the presence of an endogenous pool of inducible p65 homodimers in intact human T cells. As with the prototypical NF-kappa B p50-p65 heterodimer, this novel p65 homodimeric form of NF-kappa B is functionally sequestered in the cytoplasm but rapidly appears in the nuclear compartment following cellular stimulation. Site-directed mutagenesis studies indicate that the homodimerization function of p65 is dependent upon the presence of cysteine 216 and a conserved recognition motif for protein kinase A (RRPS; amino acids 273 to 276), both of which reside within a 91-amino-acid segment of the Rel homology domain that mediates self-association. In contrast, mutations at these two sites do not affect heterodimerization of p65 with p50 or its functional interaction with I kappa B alpha. These later findings indicate that neither homo- nor heterodimer formation is an absolute prerequisite for I kappa B alpha recognition of p65. Taken together with prior in vivo transcription studies, these results suggest that the biological activities of p65 and p50 homodimers are independently regulated, thereby providing an integrated and flexible control mechanism for the rapid activation and repression of NF-kappa B/Rel-directed gene expression.

The I kappa B proteins: multifunctional regulators of Rel/NF-kappa B transcription factors

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Identification of a novel cyclosporin-sensitive element in the human tumor necrosis factor alpha gene promoter

Tumor necrosis factor alpha (TNF-alpha), a cytokine with pleiotropic biological effects, is produced by a variety of cell types in response to induction by diverse stimuli. In this paper, TNF-alpha mRNA is shown to be highly induced in a murine T cell clone by stimulation with T cell receptor (TCR) ligands or by calcium ionophores alone. Induction is rapid, does not require de novo protein synthesis, and is completely blocked by the immunosuppressant cyclosporin A (CsA). We have identified a human TNF-alpha promoter element, kappa 3, which plays a key role in the calcium-mediated inducibility and CsA sensitivity of the gene. In electrophoretic mobility shift assays, an oligonucleotide containing kappa 3 forms two DNA protein complexes with proteins that are present in extracts from unstimulated T cells. These complexes appear in nuclear extracts only after T cell stimulation. Induction of the inducible nuclear complexes is rapid, independent of protein synthesis, and blocked by CsA, and thus, exactly parallels the induction of TNF-alpha mRNA by TCR ligands or by calcium ionophore. Our studies indicate that the kappa 3 binding factor resembles the preexisting component of nuclear factor of activated T cells. Thus, the TNF-alpha gene is an immediate early gene in activated T cells and provides a new model system in which to study CsA-sensitive gene induction in activated T cells.

Rapid proteolysis of I kappa B-alpha is necessary for activation of transcription factor NF-kappa B

Inducible gene expression in eukaryotes is mainly controlled by the activity of transcriptional activator proteins, such as NF-kappa B (refs 1-3), a factor activated upon treatment of cells with phorbol esters, lipopolysaccharide, interleukin-1 and tumour necrosis factor-alpha. Activation of NF-kappa B involves release of the inhibitory subunit I kappa B from a cytoplasmic complex with the DNA-binding subunits Rel-A (formerly p65) and p50 (refs 6, 7). Cell-free experiments have suggested that protein kinase C and other kinases transfer phosphoryl groups onto I kappa B causing release of I kappa B and subsequent activation of NF-kappa B. Here we report that I kappa B-alpha (formerly MAD-3) is degraded in cells after stimulation with phorbol ester, interleukin-1, lipopolysaccharide and tumour necrosis factor-alpha, an event coincident with the appearance of active NF-kappa B. Treatment of cells with various protease inhibitors or an antioxidant completely prevented the inducible decay of I kappa B-alpha as well as the activation of NF-kappa B. Our findings suggest that the activation of NF-kappa B relies on an inducible degradation of I kappa B-alpha through a cytoplasmic, chymotrypsin-like protease. In intact cells, phosphorylation of I kappa B-alpha is apparently not sufficient for activation of NF-kappa B.

Tumor necrosis factor and interleukin-1 lead to phosphorylation and loss of I kappa B alpha: a mechanism for NF-kappa B activation

Nuclear factor kappa B (NF-kappa B) is a critical regulator of several genes which are involved in immune and inflammation responses. NF-kappa B, consisting of a 50-kDa protein (p50) and a 65-kDa protein (p65), is bound to a cytoplasmic retention protein called I kappa B. Stimulation of cells with a variety of inducers, including cytokines such as tumor necrosis factor and interleukin-1, leads to the activation and the translocation of p50/65 NF-kappa B into the nucleus. However, the in vivo mechanism of the activation process remains unknown. Here, we provide the first evidence that the in vivo mechanism of NF-kappa B activation is through the phosphorylation and subsequent loss of its inhibitor, I kappa B alpha. We also show that both I kappa B alpha loss and NF-kappa B activation are inhibited in the presence of antioxidants, demonstrating that the loss of I kappa B alpha is a prerequisite for NF-kappa B activation. Finally, we demonstrate that I kappa B alpha is rapidly resynthesized after loss, indicating that an autoregulatory mechanism is involved in the regulation of NF-kappa B function. We propose a mechanism for the activation of NF-kappa B through the modification and loss of I kappa B alpha, thereby establishing its role as a mediator of NF-kappa B activation.

Tumor necrosis factor and interleukin-1 lead to phosphorylation and loss of I kappa B alpha: a mechanism for NF-kappa B activation

Nuclear factor kappa B (NF-kappa B) is a critical regulator of several genes which are involved in immune and inflammation responses. NF-kappa B, consisting of a 50-kDa protein (p50) and a 65-kDa protein (p65), is bound to a cytoplasmic retention protein called I kappa B. Stimulation of cells with a variety of inducers, including cytokines such as tumor necrosis factor and interleukin-1, leads to the activation and the translocation of p50/65 NF-kappa B into the nucleus. However, the in vivo mechanism of the activation process remains unknown. Here, we provide the first evidence that the in vivo mechanism of NF-kappa B activation is through the phosphorylation and subsequent loss of its inhibitor, I kappa B alpha. We also show that both I kappa B alpha loss and NF-kappa B activation are inhibited in the presence of antioxidants, demonstrating that the loss of I kappa B alpha is a prerequisite for NF-kappa B activation. Finally, we demonstrate that I kappa B alpha is rapidly resynthesized after loss, indicating that an autoregulatory mechanism is involved in the regulation of NF-kappa B function. We propose a mechanism for the activation of NF-kappa B through the modification and loss of I kappa B alpha, thereby establishing its role as a mediator of NF-kappa B activation.

A sense phosphorothioate oligonucleotide directed to the initiation codon of transcription factor NF-kappa B p65 causes sequence-specific immune stimulation

Antisense oligonucleotides have proved effective in achieving targeted inhibition of gene expression. In such experiments, sense oligonucleotides have frequently been used as a control for nonspecific effects, but the results have been variable, raising questions about the reliability of sense oligomers as a control. It is possible that some of the effects of sense oligonucleotides may be specific. We have shown that phosphorothioate antisense oligonucleotides to the p65 subunit of NF-kappa B, a transcription factor, cause a block in cell adhesion. In our efforts to test the efficacy of NF-kappa B p65 oligonucleotides in vivo, we unexpectedly observed that the control p65-sense, but not the p65-antisense, oligonucleotides caused massive splenomegaly in mice. In the current study we demonstrate a sequence-specific stimulation of splenic cell proliferation, both in vivo and in vitro, by treatment with p65-sense oligonucleotides. Cells expanded by this treatment are primarily B-220+, sIg+ B cells. The secretion of immunoglobulins by the p65-sense oligonucleotide-treated splenocytes is also enhanced. In addition, the p65-sense-treated splenocytes, but not several other cell lines, showed an upregulation of NF-kappa B-like activity in the nuclear extracts, an effect not dependent on new protein or RNA synthesis. These results demonstrate that phosphorothioate oligonucleotides can exert sequence-specific effects in vivo, irrespective of sense or antisense orientation.