The mouse c-rel protein has an N-terminal regulatory domain and a C-terminal transcriptional transactivation domain

IKK biology

The inhibitor of nuclear factor-κB (IκB) kinase (IKK) complex is the master regulator of the NF-κB signaling pathway. The activation of the IKK complex is a tightly regulated, highly stimulus-specific, and target-specific event that is essential for the plethora of functions attributed to NF-κB. More recently, NF-κB-independent roles of IKK members have brought increased complexity to its biological function. This review highlights some of the major advances in the studies of the process of IKK activation and the biological roles of IKK family members, with a focus on NF-κB-independent functions. Understanding these complex processes is essential for targeting IKK for therapeutics.

Nuclear factor-κB in immunity and inflammation: the Treg and Th17 connection

Although nuclear factor-kB (NF-kB) is generally considered to be a pro-inflammatory transcription factor, recent studies indicate that it also plays a critical role in the development of an anti-inflammatory T cell subset called regulatory T (Treg) cells. Two NF-kB proteins, c-Rel and p65, drive the development of Treg cells by promoting the formation of a Foxp3-specific enhanceosome. Consequently, c-Rel-deficient mice have marked reductions in Treg cells, and c-Rel-deficient T cells are compromised in Treg cell differentiation. However, with the exception of Foxp3, most NF-kB target genes in immune cells are pro-inflammatory. These include several Th17-related cytokine genes and the retinoid-related orphan receptor-g (Rorg or Rorc) that specifies Th17 differentiation and lineage-specific function. T cells deficient in c-Rel or p65 are significantly compromised in Th17 differentiation, and c-Rel -deficient mice are defective in Th17 responses. Thus, NF-kB is required for the development of both anti-inflammatory Treg and pro-inflammatory Th17 cells.

The suppression of SH3BGRL is important for v-Rel-mediated transformation

The v-rel oncogene is the most efficient transforming member of the Rel/NF-kappaB family of transcription factors. v-Rel induces avian and mammalian lymphoid cell tumors and transforms chicken embryo fibroblasts in culture by the aberrant regulation of genes under the control of Rel/NF-kappaB proteins. Here we report that the expression of SH3BGRL, a member of the SH3BGR (SH3 domain-binding glutamic acid-rich) family of proteins, is downregulated in v-Rel-expressing fibroblasts, lymphoid cells, and splenic tumor cells. Chromatin immunoprecipitation experiments demonstrated that v-Rel binds to the sh3bgrl promoter in transformed cells. Coexpression of SH3BGRL with v-Rel in primary splenic lymphocytes reduced the number of colonies formed by 76%. Mutations in the predicted SH3-binding domain of SH3BGRL abolished the suppressive effect on v-Rel transformation and resulted in colony numbers comparable to those formed by v-Rel alone. However, mutations in the predicted EVH1-binding domain of SH3BGRL only had a modest effect on suppression of v-Rel transformation. This study provides the first example of a gene that is downregulated in v-Rel-expressing cells that also plays a role in v-Rel transformation.

Genes and genomics of autoimmune inflammation: from Rel to TRAIL

Development of autoimmune diseases requires coordinated expression of a myriad of genes. To explore the spectrum and global patterns of gene expression during autoimmune inflammation, we have recently performed functional genomic studies of autoimmune inflammation in the central nervous system (CNS). Inflammation in the CNS not only induced the expression of many immune-related genes, but also significantly altered the gene expression profile of neural cells. A number of unique clusters of genes were identified, which represent putative immune and nervous responses in autoimmune inflammation. This review will focus on two clusters of genes that we have been studying during the past few years: the Rel/nuclear factor (NF)-kappaB family and the tumor necrosis factor (TNF) family.

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

The two DNA-binding subunits of the transcription factor NF-kappa B, the products of the rel oncogene family and the product of the developmental control gene dorsal of Drosophila are homologous within a 300 amino acid region. This sequence represents a novel DNA-binding and dimerization domain. The access of the NF-kappa B/Rel/dorsal (NRD) transcription factor family to the cell nucleus is regulated. There is now evidence that functionally and structurally related accessory proteins of NF-kappa B, Rel and dorsal control the nuclear entry as well as DNA-binding activity of the transcription factors. This review summarizes current knowledge about the nuclear-uptake regulatory proteins (NURPs) I kappa B-alpha, I kappa B-beta/pp40 and cactus.

Deletion of either C-terminal transactivation subdomain enhances the in vitro transforming activity of human transcription factor REL in chicken spleen cells

The REL gene is amplified in many human B-cell lymphomas and we have previously shown that expression of REL from a retroviral vector can malignantly transform chicken spleen cells in vitro. To identify REL protein functions necessary for malignant transformation, we have performed deletion analysis on REL sequences encoding residues of two C-terminal subdomains that are involved in transcriptional activation. We find that deletion of both C-terminal transactivation subdomains abolishes the ability of REL to transform chicken spleen cells in vitro. In contrast, deletion of either transactivation subdomain alone, which reduces the transactivation ability of REL, enhances the transforming activity of REL. Transforming REL mutants missing C-terminal sequences can also be selected at a low frequency in vitro. The REL transactivation domain can be functionally replaced in transformation assays by a portion of the VP16 transactivation domain that activates at a level similar to REL-transforming mutants. We also find that deletion of 29 C-terminal amino acids causes the subcellular localization of REL to change from cytoplasmic to nuclear in chicken embryo fibroblasts. In contrast, wild-type REL and all transforming REL mutants are located primarily in the cytoplasm of transformed spleen cells. Nevertheless, treatment of transformed spleen cells with leptomycin B causes wild-type REL and two REL mutants to relocalize to the nucleus, and nuclear extracts from these transformed cells contain REL DNA-binding activity. Taken together, these results suggest the following: (1) that REL must activate transcription to transform cells in vitro; (2) that a reduced level of transactivation enhances the oncogenicity of REL; (3) that REL shuttles from the cytoplasm to the nucleus in transformed chicken spleen cells; and (4) that mutations in REL, in addition to amplifications, could activate its oncogenicity in human lymphomas.

Kinetic perspectives of T cell antigen receptor signaling. A two-tier model for T cell full activation

T-cell activation consists of multiple layers of signaling events. Interleukin-2 production is of interest for many, since its expression determines a critical difference between partial and full T-cell activation. To achieve full activation of T cells, it is necessary for the T-cell antigen receptor (TCR) to be engaged for an extended period of time. However, why extended stimulation is required for full T-cell activation is not understood at the molecular level. In this review, orchestrated events of TCR signal transduction will be analyzed in a kinetic manner and connected toward the understanding of the mechanism of T-cell activation. Based on recent results, a model of the mechanism that dictates the threshold between partial and full T-cell activation is proposed.

The chicken RelB transcription factor has transactivation sequences and a tissue-specific expression pattern that are distinct from mammalian RelB

Rel/NF-kappaB proteins are eukaryotic transcription factors that control the expression of genes involved in a large variety of cellular processes. Rel proteins share a highly conserved DNA-binding/dimerization domain called the Rel Homology (RH) domain. We have constructed and characterized a composite cDNA encoding most of the chicken RelB transcription factor. The predicted chicken RelB protein has a high degree of sequence similarity to other vertebrate RelB proteins within the RH domain, but is much less conserved outside this domain. Chicken RelB does not bind DNA as a homodimer, but forms DNA-binding heterodimers with NF-kappaB p50 or p52. Overexpressed chicken RelB localizes to the nucleus in chicken embryo fibroblasts, and the nonconserved C-terminal sequences of chicken RelB contain a transactivation domain that functions in chicken and mouse fibroblasts. Thus, chicken RelB has functional properties similar to other vertebrate RelB proteins. However, Western blotting of diverse chicken tissues indicates that chicken RelB is more widely expressed than mammalian RelB.

Regulation of nuclear factor kappa B transactivation. Implication of phosphatidylinositol 3-kinase and protein kinase C zeta in c-Rel activation by tumor necrosis factor alpha

Transactivation by c-Rel (nuclear factor kappaB) was dependent on phosphorylation of several serines in the transactivation domain, indicating that it is a phosphorylation-dependent Ser-rich domain. By Ser --> Ala mutational and deletion analysis, we have identified two regions in this domain: 1) a C-terminal region (amino acids 540-588), which is required for basal activity; and 2) the 422-540 region, which responds to external stimuli as tumor necrosis factor (TNF) alpha or phorbol myristate acetate plus ionomycin. Ser from 454 to 473 were shown to be required for TNFalpha-induced activation, whereas Ser between 492 and 519 were required for phorbol myristate acetate plus ionomycin activation. Phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) zeta were identified as downstream signaling molecules of TNFalpha-activation of c-Rel transactivating activity. Interestingly, dominant negative forms of PI3K inhibited PKCzeta activation and dominant negative PKCzeta inhibited PI3K-mediated activation of c-Rel transactivating activity, indicating a cross-talk between both enzymes. We have identified the critical role of different Ser for PKCzeta- and PI3K-mediated responses. Interestingly, those c-Rel mutants not only did not respond to TNFalpha but also acted as dominant negative forms of nuclear factor kappaB activation.

LPS induction of gene expression in human monocytes

Lipopolysaccharide (LPS [endotoxin]) is the principal component of the outer membrane of Gram-negative bacteria. Recent studies have elucidated how LPS is recognized by monocytes and macrophages of the innate immune system. Human monocytes are exquisitely sensitive to LPS and respond by expressing many inflammatory cytokines. LPS binds to LPS-binding protein (LBP) in plasma and is delivered to the cell surface receptor CD14. Next, LPS is transferred to the transmembrane signaling receptor toll-like receptor 4 (TLR4) and its accessory protein MD2. LPS stimulation of human monocytes activates several intracellular signaling pathways that include the IkappaB kinase (IKK)-NF-kappaB pathway and three mitogen-activated protein kinase (MAPK) pathways: extracellular signal-regulated kinases (ERK) 1 and 2, c-Jun N-terminal kinase (JNK) and p38. These signaling pathways in turn activate a variety of transcription factors that include NF-kappaB (p50/p65) and AP-1 (c-Fos/c-Jun), which coordinate the induction of many genes encoding inflammatory mediators.

The conserved redox-sensitive cysteine residue of the DNA-binding region in the c-Rel protein is involved in the regulation of the phosphorylation of the protein

The DNA-binding activity of the transcription nuclear factor kappaB (NF-kappaB) is regulated by a redox-control mechanism involving the reduction of a disulphide bond from a specific cysteine residue conserved in all members of the NF-kappaB family. Thioredoxin is involved in this redox control. DNA binding and transactivating capacity of NF-kappaB are up-regulated by inducible phosphorylation. Here we demonstrate that the conserved redox cysteine in the c-Rel protein is involved in the phosphorylation regulation of the protein. When this cysteine residue is mutated to an aspartic acid residue, the mutant protein loses its capacity to be phosphorylated and its DNA-binding activity. In addition, our results suggest that, when the conserved redox cysteine is chemically modified by N-ethylmaleimide and 2-chloro-1,3-dinitrobenzene, the protein c-Rel cannot be phosphorylated. In contrast, the protein in which the cysteine residue was replaced by a serine residue, creating a potential phosphorylation site, is highly phosphorylated and binds kappaB sequences. The protein could loose the redox regulation of the phosphorylation when the residue replacing the cysteine can be itself phosphorylated. We also show that specific inhibitors of thioredoxin reductases impair the phosphorylation of the c-Rel protein, suggesting that the redox regulation of the protein controls its phosphorylation.

Oxidized low density lipoprotein inhibits interleukin-12 production in lipopolysaccharide-activated mouse macrophages via direct interactions between peroxisome proliferator-activated receptor-gamma and nuclear factor-kappa B

Lipopolysaccharide (LPS) increases the production of interleukin-12 (IL-12) from mouse macrophages via a kappaB site within the IL-12 p40 promoter. In this study, we found that oxidized low density lipoprotein (oxLDL) inhibited this LPS-stimulated production of IL-12 in a dose-dependent manner while native LDL did not. OxLDL inhibited p40 promoter activation in monocytic RAW264.7 cells transiently transfected with p40 promoter/reporter constructs, and the repressive effect mapped to a region in the p40 promoter containing a binding site for nuclear factor-kappaB (NF-kappaB) (p40-kappaB). Activation of macrophages by LPS in the presence of oxLDL resulted in markedly reduced binding to the kappaB site, as demonstrated by the electrophoretic mobility shift assays. In contrast, native LDL did not inhibit the IL-12 p40 promoter activation and NF-kappaB binding to the kappaB sites, suggesting that oxidative modification of LDL was crucial for the inhibition of NF-kappaB-mediated IL-12 production. 9-Hydroxyoctadecadienoic acid, a major oxidized lipid component of oxLDL, significantly inhibited IL-12 production in LPS-stimulated mouse macrophages and also suppressed NF-kappaB-mediated activation in IL-12 p40 promoter. The NF-kappaB components p50 and p65 directly bound peroxisome proliferator-activated receptor-gamma (PPAR-gamma) in vitro. In cotransfections of CV-1 and HeLa cells, PPAR-gamma inhibited the NF-kappaB transactivation in an oxLDL-dependent manner. From these results, we propose that oxLDL-mediated suppression of the IL-12 production from LPS-activated mouse macrophages may, at least in part, involve both inhibition of the NF-kappaB-DNA interactions and physical interactions between NF-kappaB and PPAR-gamma.

Differential activation of murine macrophages by angelan and LPS

In our previous studies, we showed that angelan, a polysaccharide purified from Angelica gigas Nakai, is a potent LPS-mimetic in murine macrophages [Jeon, Y.J., Han, S.B., Ahn, K.S., Kim, H.M., 1999. Activation of NF-kB/Rel in angelan-stimulated macrophages. Immunopharmacology 43, 1-9]. Angelan stimulates murine macrophage to produce cytokines including iNOS and activate NF-kappaB/Rel. In the present study, we investigated the role of CD14 and complement receptor type 3 (CR3) in mediating NO production and NF-kappaB/Rel activation induced by angelan and LPS. Three major differences between angelan and LPS were observed. First, angelan does not require serum proteins for NO response and NF-kappaB/Rel activation, while the activation by LPS requires serum proteins. Second, blocking of either CD14 or CR3 decreased angelan-induced NO response, while LPS-mediated NO production was inhibited by anti-CD14 mAb only. Third, angelan induced strong NF-kappaB/Rel and slight AP-1 DNA binding, whereas LPS potently activated both NF-kappaB/Rel and AP-1. Both angelan and LPS degraded IkappaB proteins and subsequently induced the mobilization of NF-kappaB/Rel proteins (p65, c-rel and p50) into nucleus. This suggests that macrophages display a common signaling machinery leading to the NF-kappaB/Rel activation in response to different stimulants. In conclusion, angelan and LPS use the membrane receptor CD14 and CR3 differentially for signaling NF-kappaB/Rel activation and NO production.

Tumor necrosis factor-alpha activation of NF-kappa B requires the phosphorylation of Ser-471 in the transactivation domain of c-Rel

Activation of the transcription factor NF-kappaB is controlled at two levels in resting T cells: an initial activation induced by the triggering of the TcR.CD3 complex and a second phase controlled by paracrine- or autocrine-secreted TNFalpha. The initial phase is regulated by p65 (RelA), whereas the second one is mainly dependent on c-Rel. We describe here a mutant clone, D6, derived from the parental T lymphoblastic line Jurkat that fails to activate NF-kappaB upon TNFalpha stimulation. This clone had no alteration in tumor necrosis factor-alpha (TNFalpha) signaling pathways nor in IkappaBalpha, -beta, or -epsilon expression and degradation. However, TNFalpha induced an exacerbated apoptotic response in this clone compared with Jurkat cells. This mutant clone showed a defect in the intermediate-late translocation of c-Rel to the nucleus promoted by TNFalpha stimulation, whereas early translocation is not affected. Activation or translocation of p65-containing complexes was not altered in this mutant clone. Sequencing of the c-Rel gene from this clone revealed a mutation of Ser-471 to Asn in the transactivation domain. The mutant S471N transactivation domain fused to the Gal4 DNA binding domain could not be activated by TNFalpha, unlike the wild type. Moreover, the overexpression of the mutant protein c-Rel S471N into Jurkat cells abolished TNFalpha-induced NF-kappaB activity, thus demonstrating that this mutation is responsible for the failure of TNFalpha stimulation of NF-kappaB. Moreover, extracts from TNFalpha-stimulated Jurkat cells phosphorylated in vitro recombinant wild type GST-c-Rel 464-481 but not the GST-c-Rel mutant. Thus, TNFalpha-induced phosphorylation of Ser-471 seems to be absolutely necessary for TNFalpha activation of c-Rel.

Dexamethasone inhibits IL-1 beta gene expression in LPS-stimulated RAW 264.7 cells by blocking NF-kappa B/Rel and AP-1 activation

In the present study, the mechanism by which dexamethasone (DEX) inhibited IL-1beta gene expression in bacterial lipopolysaccharide (LPS)-activated RAW 264.7 cells was investigated. The decrease in LPS-induced IL-1beta mRNA expression was demonstrated by quantitative reverse transcription polymerase chain reaction (RT-PCR). Since the promoter in IL-1beta gene contains binding motifs for NF-kappaB/Rel, AP-1, NF-IL6, and CREB/ATF, which appear to be important in LPS-mediated IL-1beta induction, the effects of DEX on the activation of these transcription factors were examined. Treatment of DEX to RAW 264.7 cells induced a dose-related inhibition of NF-kappaB/Rel and AP-1 in chloramphenicol acetyltransferase activity, while neither NF-IL6 nor CREB/ATF activation was affected by DEX. Treatment of RAW 264.7 cells with DEX inhibited DNA binding of NF-kappaB/Rel and AP-1 proteins to their cognate DNA sites as measured by electrophoretic mobility shift assay (EMSA). DEX treatment caused a significant reduction in nuclear c-rel, p65, and p50 protein contents, and these decreases were paralleled by the accumulation of cytoplasmic c-rel, p65, and p50. DEX treatment of RAW 264.7 cells did not inhibit the nuclear translocation of c-jun and c-fos. We found that the inhibition of IL-1beta production by DEX is not related to p38, which is important in the IL-1beta induction. These results suggest that DEX may inhibit IL-1beta gene expression by a mechanism involving the blocking of LPS-induced NF-kappaB/Rel and AP-1 activation.

cRel-TD kinase: a serine/threonine kinase binding in vivo and in vitro c-Rel and phosphorylating its transactivation domain

The activity of transcription factors is often modulated by signal responsive protein kinases. Rel/NF-kappaB transcription factors are regulated by IkappaB inhibitors, the phosphorylation of which causes ubiquitination and degradation, resulting in nuclear translocation of NF-kappaB and activation of target genes. Here we report pulldown and immunoprecipitation experiments showing that a mammalian 66 kDa protein kinase binds murine c-Rel, both in vitro and in vivo. This kinase appears to have at least two binding sites on c-Rel, a proline-directed serine/ threonine substrate specificity similar to MAP kinases and to specifically phosphorylate the C-terminal domain of murine c-Rel at an ERK consensus site.

Activation of the ubiquitin proteolytic system in murine acquired immunodeficiency syndrome affects IkappaBalpha turnover

Murine acquired immunodeficiency syndrome (MAIDS) is a complex immunopathology caused by a defective murine leukemia virus (LP-BM5) that mainly targets B-lymphocytes. Lymphadenophathy, splenomegaly, hypergammaglobulinemia and progressive immunodeficiency are prominent features of MAIDS. Previously, we showed that the ubiquitin proteolytic system was upregulated in infected lymph nodes [Crinelli, R., Fraternale, A., Casabianca, A. & Magnani, M. (1997) Eur. J. Biochem. 247, 91-97]. In this report, we demonstrate that increased 26S proteasome activity is responsible for accelerated turnover of the IkappaBalpha inhibitor in lymph node extracts derived from animals with MAIDS. The molecular mechanisms mediating IkappaBalpha proteolysis involved constitutive phosphorylation of IkappaBalpha at Ser32 and Ser36 and subsequent ubiquitination, suggesting persistent activation of an NF-kappaB inducing pathway. Interestingly, enhanced IkappaBalpha degradation did not result in enhanced NF-kappaB DNA binding activity, but rather in a different subunit composition. The modulation of NF-kappaB/IkappaB system may affect multiple immunoregulatory pathways and may in part explain the mechanisms leading to the profound immune dysregulation involved in MAIDS pathogenesis.

Activation of NF-kappaB/Rel in angelan-stimulated macrophages

In our previous studies we showed that the primary target cell of angelan, a polysaccharide purified from Angelica gigas Nakai, is a macrophage (Han et al., 1998). In the present study we examined the effect of angelan on iNOS, IL-1beta, and TNF-alpha transcription in mouse macrophage line RAW 264.7. We show that angelan produces a marked induction of iNOS, IL-1beta, and TNF-alpha transcription by RAW 264.7 cells. Since these gene transcriptions have been recently shown to be under the control of NF-kappaB/Rel family of transcription factors, we assessed the effect of angelan on NF-kappaB/Rel using a electrophoretic mobility shift assay. Treatment of RAW 264.7 cells with angelan produced strong induction of NF-kappaB/Rel binding. Treatment of RAW 264.7 cells with angelan slightly induced AP-1 binding activity, whereas Oct binding was not affected by angelan. Angelan stimulated macrophages to activate NF-kappaB/Rel, whereas neither B-cells nor T-cells were affected by the angelan. In conclusion, we demonstrate that the stimulation effect of angelan on macrophage is mediated by specific activation of NF-kappaB/Rel.

Overexpression of RelA causes G1 arrest and apoptosis in a pro-B cell line

NF-kappaB/Rel family proteins form a network of post-translationally regulated transcription factors that respond to a variety of extracellular stimuli and mediate distinct cellular responses. These responses include cytokine gene expression, regulated cell cycle activation, and both the protection from and induction of the cell death program. To examine the function of individual Rel family proteins in B cell development and resolve their role in the signaling of apoptosis, we used a tetracycline-regulated gene expression system to overexpress either c-Rel or RelA in the transformed pro-B cell line 220-8. Elevated levels of RelA, but not c-Rel, induced a G1 cell cycle arrest followed by apoptosis. Both the DNA binding and transactivation domains of RelA were required for this effect. When RelA was overexpressed in the immature B cell line WEHI 231 or the mature B cell line M12, neither cell cycle arrest nor apoptosis was evident. The differential effects of elevated RelA levels in these cell lines suggests that susceptibility to NF-kappaB-induced apoptosis may reflect a relevant selection event during B cell development.

Retinoids inhibit interleukin-12 production in macrophages through physical associations of retinoid X receptor and NFkappaB

Lipopolysaccharide (LPS) increases the production of interleukin-12 (IL-12) from mouse macrophages via a kappaB site within the IL-12 p40 promoter. In this study, we found that retinoids inhibit this LPS-stimulated production of IL-12 in a dose-dependent manner. The NFkappaB components p50 and p65 bound retinoid X receptor (RXR) in a ligand-independent manner in vitro, and the interaction interfaces involved the p50 residues 1-245, the p65 residues 194-441, and the N-terminal A/B/C domains of RXR. Activation of macrophages by LPS resulted in markedly enhanced binding activities to the kappaB site, which significantly decreased upon addition of retinoids, as demonstrated by the electrophoretic mobility shift assays. In cotransfections of CV-1 and HeLa cells, RXR also inhibited the NFkappaB transactivation in a ligand-dependent manner, whereas a mutant RXR lacking the AF2 transactivation domain, which serves as ligand-dependent binding sites for transcription integrators SRC-1 and p300, was without any effect. In addition, coexpression of increasing amounts of SRC-1 or p300 relieved the retinoid-mediated inhibition of the NFkappaB transactivation. From these results, we propose that retinoid-mediated suppression of the IL-12 production from LPS-activated macrophages may involve both inhibition of the NFkappaB-DNA interactions and competitive recruitment of transcription integrators between NFkappaB and RXR.

Multiple hemopoietic defects and lymphoid hyperplasia in mice lacking the transcriptional activation domain of the c-Rel protein

The c-rel protooncogene encodes a member of the Rel/nuclear factor (NF)-kappaB family of transcriptional factors. To assess the role of the transcriptional activation domain of c-Rel in vivo, we generated mice expressing a truncated c-Rel (Deltac-Rel) that lacks the COOH-terminal region, but retains a functional Rel homology domain. Mice with an homozygous mutation in the c-rel region encoding the COOH terminus of c-Rel (c-relDeltaCT/DeltaCT) display marked defects in proliferative and immune functions. c-relDeltaCT/DeltaCT animals present histopathological alterations of hemopoietic tissues, such as an enlarged spleen due to lymphoid hyperplasia, extramedullary hematopoiesis, and bone marrow hypoplasia. In older c-relDeltaCT/DeltaCT mice, lymphoid hyperplasia was also detected in lymph nodes, liver, lung, and stomach. These animals present a more severe phenotype than mice lacking the entire c-Rel protein. Thus, in c-relDeltaCT/DeltaCT mice, the lack of c-Rel activity is less efficiently compensated by other NF-kappaB proteins.

Nuclear Targeted Suppression of NF-κB Activity by the Novel Quinone Derivative E3330

The activation of NF-κB consists of at least three steps: degradation of IκBα, translocation of NF-κB into the nucleus, and post-translational modification of NF-κB (e.g., phosphorylation of p65). In the present study, we found that a novel quinone derivative E3330 selectively inhibited NF-κB-mediated gene expression without affecting any of these steps. E3330, when included in the culture medium, suppressed NF-κB DNA-binding activity in PMA-induced Jurkat cell nuclear extracts, suggesting that the inhibition by E3330 of NF-κB-mediated gene expression was due to its ability to suppress NF-κB DNA-binding activity. Fractionation of the nuclear extracts by column chromatography revealed that a nuclear factor enhanced NF-κB DNA-binding activity and that this enhancing activity was interrupted after treatment with E3330. Moreover, a major polypeptide with a molecular mass of 40 kDa was found to be in the highly purified fraction containing the NF-κB-enhancing activity and predominantly bind E3330. Taken together, these results suggest that the NF-κB activity, after dissociation from IκB, is enhanced by a nuclear factor that is active irrespective of PMA treatment, and the nuclear factor-mediated enhancement is selectively inhibited by E3330. Thus, we conclude that E3330 may belong to a novel class of anti-NF-κB drugs.

Identification and expression of the Xenopus homolog of mammalian p100-NFkappaB2

We have molecularly cloned a cDNA encoding a new Rel-related protein in Xenopus laevis. The product is most homologous to mammalian p100-NFkappaB2. Furthermore, the putative protein kinase A-phosphorylation site (RRPS), which is found in most of the Rel family proteins and is replaced by KRKR in mammalian p100, is also replaced by KRKK in our clone, indicating that our cDNA most likely encodes the Xenopus p100 (Xp100). Like mammalian p52, a processed product of p100, Xp52 alone binds to the kappaB site but does not activate transcription, while the XRelB/Xp52 heterodimer activates transcription, which is inhibited by the carboxyl-terminal half of Xp100 (XIkappaBdelta). Xp100 transcripts are present at all stages of oocyte maturation and in all adult tissues examined. Xp100 transcripts decrease at the gastrula stage and resume their expression at the neurula stage, which is different from other Xenopus rel family. Xp100 is highly expressed in somitogenic mesoderm at the neurula stage, while in the gastrula and tailbud stages, Xp100 transcripts are not localized to restricted regions. These results suggest that Xp100 could be involved in the late-stage development of Xenopus laevis, especially in the maturation of somites.

NF-kappa B/Rel family members regulating the ICAM-1 promoter in monocytic THP-1 cells

A kappa B-site was identified in the promoter of the intercellular adhesion molecule-1 (ICAM-1) gene, which is involved in regulation of ICAM-1 expression by tumor necrosis factor alpha (TNF-alpha) and glucocorticoids. We now report on the transcription factors which bind and transactivate this enhancer sequence. In vitro, the ICAM-1 kappa B site appeared to bind RelA and c-Rel homodimers as well as heterodimers with NF-kappa B1, but weakly NF-kappa B1 homodimers. In addition, both RelA and c-Rel, but not NF-kappa B1, were shown to transactivate an ICAM-1 kappa B-reporter construct. In monocytic THP-1 cells TNF-alpha induced two nuclear complexes which in vitro bound to the ICAM-1 kappa B site. Using antibodies in an electrophoretic mobility supershift assay, one of these complexes was shown to contain NF-kappa B1 and RelA, and to bind with higher affinity to the consensus kappa B site in the HIV long terminal repeat. The second complex contained RelA, and exhibited higher affinity towards the ICAM-1 kappa B than to the HIV kappa B site. The glucocorticoid receptor was shown to repress activity of both the RelA homodimer and the NF-kappa B1/RelA heterodimer. We argue that in vivo RelA homodimers are likely to play a dominant role in TNF-alpha-induced ICAM-1 transcription in monocytic cells.

Distinct domains of the RelA NF-kappaB subunit are required for negative cross-talk and direct interaction with the glucocorticoid receptor

The RelA subunit of NF-kappaB and the glucocorticoid receptor mutually repress each others transcriptional activity, thus providing a mechanism for immunosuppression. Deletion analysis of the glucocorticoid receptor has shown that the DNA binding domain and the ligand binding domain are essential components for repression. Here, we show by deletions and point mutations that both the Rel homology domain and the transactivation domains of RelA are required for repression of the transcriptional activity of the glucocorticoid receptor in intact cells. However, only the Rel homology domain of RelA was found to associate with the glucocorticoid receptor in vitro. RelA mutants, not able to repress glucocorticoid receptor activity, but still able to dimerize, behaved as transdominant inhibitors of the repressive activity of wild type RelA. Furthermore, we show that the 13 S E1A protein is able to interfere with the transrepressive activity of RelA. We propose that negative cross-talk between the glucocorticoid receptor and RelA is due to direct interaction via the Rel homology domain of RelA and the DNA binding domain of the glucocorticoid receptor in combination with interference by the transactivation domains of RelA with the transcriptional activity of the glucocorticoid receptor.

Determination of functional domains of the human transcription factor PAX8 responsible for its nuclear localization and transactivating potential

The conserved structure of the transcription factors of the Pax gene family may reflect functional conservation. We have demonstrated that the human Pax8 transcription factor is organized in several functional domains and contains two regions responsible for its nuclear localization, in addition to an activating region at the carboxy terminus of the protein and an inhibitory region encoded by the exon 9 present only in a splice variant PAX8a. Regions of PAX8 determining the nuclear localization of the PAX8A/lacZ fusions contain short amino acid sequences similar to several described nuclear localization sites (NLS). These NLS were identified in the paired domain and between the octapeptide and the residual homeodomain, respectively. The activating domain is encoded by the exons 10 and 11 and its function is modulated by the adjacent domains encoded by the exons 9 and 12. The domain encoded by exon 9 significantly inhibits the function of the activating domain. Pax8 is expressed in thyroid cells and its product binds promoters of the thyroglobulin and thyroperoxidase genes through its paired domain. Thyroid cell growth and differentiation depend on thyrotropin which, by stimulating cAMP synthesis, activates the cAMP-dependent protein kinase A (PKA). We have investigated a link between thyrotropin stimulation and gene activation by Pax8. Stimulation of cAMP synthesis augments Pax8-specific transcription in thyroid cells, indicating that PKA is involved in Pax8 activation. Cotransfection of GAL4/PAX8 fusions and the catalytic subunit of PKA in A126, a PKA-deficient derivative of the PC12 pheochromocytoma cell line, synergistically activates the GAL4-specific reporter, suggesting the activating domain of PAX8 is dependent upon the catalytic subunit of the PKA. We propose that this dependence is due to a hypothetical adaptor which forms a target for PKA and interacts with the activating domain of PAX8. We show that PAX8 isolated from the thyroid cell line FTRL5 is a phosphoprotein in which phosphorylation is not dependant on cAMP pathway activation. Our results suggest that Pax8 is part of the cAMP signaling pathway and mediates thyrotropin-dependent gene activation in thyroid cells. Investigation of the PAX8 expression in a panel of Wilms' tumors shows a striking correlation between the expression of PAX8 and another transcription factor, WT1, indicating that these two genes may interact in vivo.

NF-kappa B RelA-deficient lymphocytes: normal development of T cells and B cells, impaired production of IgA and IgG1 and reduced proliferative responses

To investigate the function of NF-kappa B RelA (p65), we generated mice deficient in this NF-kappa B family member by homologous recombination. Mice lacking RelA showed liver degeneration and died around embryonic day 14.5. To elucidate the role of RelA in lymphocyte development and function, we transplanted fetal liver cells of 13.5-day embryos from heterozygote matings into irradiated SCID mice. Within 4 weeks, both T and B cells had developed in the SCID mice receiving relA-/- fetal liver transplants, similar to the relA+/+ and +/- cases. T cells were found to mature to Thy-1+/TCR alpha beta +/CD3+/CD4+ or CD8+, while B cells had the ability to differentiate to IgM+/B220+ and to secrete immunoglobulins. However, the secretion of IgG1 and IgA was reduced in RelA-deficient B cells. Furthermore, both T and B cells lacking RelA showed marked reduction in proliferative responses to stimulation with Con A, anti-CD3, anti-CD3 + anti-CD28, LPS, anti-IgM, and PMA + calcium ionophore. The results indicate that RelA plays a critical role in production of specific Ig isotypes and also in signal transduction pathways for lymphocyte proliferation.

Biological and biochemical inhibitors of the NF-kappa B/Rel proteins and cytokine synthesis

The NF-kappa B/Rel family of transcription factors participates in the activation of a diverse range of genes involved in inflammation, immune response, lymphoid differentiation, growth control and development. The present review provides a brief overview of NF-kappa B/Rel activation and a detailed analysis of important biological and biochemical inhibitors of the NF-kappa B/Rel pathway. Given the pleiotropic role of NF-kappa B in controlling cytokines and other immunoregulatory genes, the inhibition of NF-kappa B activation by steroid hormones, antioxidants, protease inhibitors and other compounds may provide a pharmacological basis for interfering with pathological inflammatory conditions, cancer and AIDS.

The v-rel oncogene promotes malignant T-cell leukemia/lymphoma in transgenic mice

The oncogene product from the avian reticuloendotheliosis virus strain T, v-Rel, is a member of the Rel/ NF-kappa B family of transcription factors. The mechanism by which v-Rel induces oncogenic transformation remains unclear. Several attempts to transform mammalian cells with v-Rel have failed, suggesting that v-Rel transformation may be a species-specific event. However, here we demonstrate that v-Rel, but not a truncated c-Rel, expressed under the control of the lck promoter, efficiently induced malignancies in transgenic mice. Most of the animals died before 10 months of age and developed immature, multicentric aggressive T-cell leukemia/lymphomas. Most tumors contain CD4+CD8+ cells or CD4-CD8+ cells, which have an immature rather than a mature peripheral phenotype. No tumor development was observed in control littermates and transgenic mice expressing a truncated form of c-Rel. Tumor formation was correlated with the presence of constitutive p50/v-Rel DNA binding activity and overexpression of several kappa B-regulated genes in v-rel transgenic thymocytes. However, v-Rel is also transforming in transgenic thymocytes lacking p50, indicating that p50/v-Rel heterodimer formation is not essential for the transforming activity of v-Rel. The transforming activity of v-Rel in p50 null mice has been identified as v-Rel/v-Rel homodimers. Since tumors represent immature T-lymphocytes, constitutive v-Rel expression appears to be leukemogenic at earlier stages of T-cell development. These v-Rel mice should aid in the study of lymphoma development, T-cell development and NF-kappa B regulation.

Redox regulation of transcriptional activators

Transcription factors/activators are a group of proteins that bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector genes and transactivate or repress effector gene expression. The up- or downregulation of effector genes will ultimately lead to many biological changes such as proliferation, growth suppression, differentiation, or senescence. Transcription factors are subject to transcriptional and posttranslational regulation. This review will focus on the redox (reduction/oxidation) regulation of transcription factors/activators with emphasis on p53, AP-1, and NF-kappa B. The redox regulation of transcriptional activators occurs through highly conserved cysteine residues in the DNA binding domains of these proteins. In vitro studies have shown that reducing environments increase, while oxidizing conditions inhibit sequence-specific DNA binding of these transcriptional activators. When intact cells have been used for study, a more complex regulation has been observed. Reduction/oxidation can either up- or downregulate DNA binding and/or transactivation activities in transcriptional activator-dependent as well as cell type-dependent manners. In general, reductants decrease p53 and NF-kappa B activities but dramatically activate AP-1 activity. Oxidants, on the other hand, greatly activate NF-kappa B activity. Furthermore, redox-induced biochemical alterations sometimes lead to change in the biological functions of these proteins. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism by which small antioxidant molecules play protective roles in anticancer and antiaging processes. Better understanding of the mechanism of redox regulation, particularly in vivo, will have an important impact on drug discovery for chemoprevention and therapy of human disease such as cancer.

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.

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

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

Transactivation domain 2 (TA2) of p65 NF-kappa B. Similarity to TA1 and phorbol ester-stimulated activity and phosphorylation in intact cells

The p65 subunit of the inducible transcription factor NF-kappa B contains at least two strong transactivation domains (TADs) within its C terminus. The first domain, TA1, is contained within the last 30 amino acids of p65, whereas TA2 comprises the adjacent 90 amino acids. In this study, squelching experiments revealed that both TADs of p65, as well as the related subunit c-Rel, compete for the same cofactor(s) mediating transactivation. Both TADs of p65 share a common sequence motif, which is evolutionarily conserved and displays a remarkable degree of spatial organization when aligned on an alpha-helical surface. The functional importance of the common sequence motif was confirmed by deletion analysis of TA2. Within the conserved sequence motif, a 7-amino-acid repeat was noted. Idealized heptad repeats fused to the DNA binding domain of Gal4 were transcriptionally active, but only as multimers. Phosphorylation and transcriptional activity of a defined region within the TA2 domain was found to be stimulated by phorbol ester treatment of cells. In contrast, TA1 was constitutively phosphorylated, and its activity did not significantly respond to phorbol ester stimulation. The stimulatory effect of phorbol ester on transcription of the TA2 domain was completely blocked by the protein kinase C inhibitor. These data suggest that protein kinase C has a dual effect on NF-kappa B activity. It not only causes removal of I kappa B-alpha from cytoplasmic NF-kappa B but also augments the transactivation potential of activated nuclear NF-kappa B.

Members of the nuclear factor kappa B family transactivate the murine c-myb gene

Expression of the c-myb proto-oncogene is primarily detected in normal tissue and tumor cell lines of immature hematopoietic origin, and the down-regulation of c-myb expression is associated with hematopoietic maturation. Cell lines that represent mature, differentiated hematopoietic cell types contain 10-100-fold less c-myb mRNA than immature hematopoietic cell types. Differences in steady-state c-myb mRNA levels appear to be primarily maintained by a conditional block to transcription elongation that occurs in the first intron of the gene. The block to transcription elongation has been mapped, using nuclear run-on analysis, to a region of DNA sequence that is highly conserved between mouse and man. Two sets of DNA-protein interactions, flanking the site of the block to transcription elongation, were detected that exhibited DNA-binding activities that strongly correlated with low steady-state c-myb mRNA levels. Several criteria demonstrated that members of the nuclear factor kappa B (NF-kappa B) family of transcription factors were involved in the DNA-protein interactions identified in these two sets. Surprisingly, cotransfection experiments demonstrated that coexpression of members of the NF-kappa B family, specifically p50 with p65 and p65 with c-Rel, transactivated a c-myb/chloramphenicol acetyltransferase reporter construct that contained 5'-flanking sequences, exon I, intron I, and exon II of the c-myb gene. Transactivation by these heterodimer combinations was dependent on regions of the c-myb first intron containing the NF-kappa B-binding sites. These findings suggest that NF-kappa B family members may be involved in either modifying the efficiency of transcription attenuation or acting as an enhancer-like activity to increase transcription initiation. Thus, the regulation of c-myb transcription may be quite complex, and members of the NF-kappa B family likely play an important role in this regulation.

Transcriptional regulation of the intercellular adhesion molecule-1 gene by inflammatory cytokines in human endothelial cells. Essential roles of a variant NF-kappa B site and p65 homodimers

Intercellular adhesion molecule-1 (ICAM-1) is greatly up-regulated on endothelial cells at sites of inflammation and is involved in leukocyte attachment and extravasation. Previously, we had shown that the ICAM-1 gene expression in human umbilical vein endothelial cells (HUVECs) was transcriptionally regulated by tumor necrosis factor-alpha (TNF-alpha) (Wertheimer, S. J., Myers, C. L., Wallace, R. W., and Parks, T. P. (1992) J. Biol. Chem. 267, 12030-12035). In the present investigation, TNF-alpha-induced transcription was found to be initiated exclusively at two sites, 40 and 41 base pairs upstream of the translation start site. Deletion analysis of the 5' regulatory region of the ICAM-1 gene revealed a 92-base pair sequence which was both necessary and sufficient to confer TNF-alpha responsiveness to a linked luciferase reporter gene in transient transfection assays. This TNF-alpha-responsive region contained a variant NF-kappa B site at -187 to -178, which when mutated, completely abolished ICAM-1 promoter activation by TNF-alpha, interleukin-1 beta, and lipopolysaccharide. Two inducible nuclear protein complexes bound to the ICAM-1 kappa B and were identified as the NF-kappa B p65 homodimer and p65/p50 heterodimer. Overexpression of p65, but not p50, transactivated the ICAM-1 promoter in a kappa B site-dependent manner in HUVECs. In addition, p65-mediated transactivation was suppressed by co-expression of p50. Our results suggest that cytokine activation of the ICAM-1 promoter in HUVECs may critically depend on p65 homodimers binding to a variant kappa B site.

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.

Evolution of the oncogenic potential of v-rel: rel-induced expression of immunoregulatory receptors correlates with tumor development and in vitro transformation

v-rel is a viral oncogene that evolved from turkey c-rel, an NF-kappa B-related transcription factor. Numerous structural alterations record the evolutionary selection of v-rel and distinguish it from c-rel. To evaluate the biological significance of these alterations, we constructed a set of five c/v-rel hybrids in which three mutation clusters (c-Rel amino acids 1 to 97,222 to 302, and 328 to 598) were differentially distributed. These constructs, in addition to parental v-rel and c-rel and two C-terminal deletion mutants of c-rel, were expressed from a retroviral vector. An analysis of cells infected with each of the nine viruses revealed that mutations in all three domains contributed to the ability of v-rel to induce two endogenous c-rel target genes, major histocompatibility complex (MHC) class I and class II, in the B-cell line DT95 as well as MHC class II in normal splenocytes. The analysis revealed a strong nonlinear correlation between the ability of a Rel protein to induce expression of MHC proteins and its capacity to produce splenic tumors and establish in vitro transformation. This correlation is consistent with the hypothesis that v-rel transforms by constitutively altering expression of genes regulated by c-rel and in this way simulates events associated with immune response-linked proliferation of cells of hematopoietic origin. Further, the 16 carboxy-terminal amino acids of c-Rel were identified as a domain responsible for producing a cytotoxic and/or cytostatic effect in DT95. Because this effect is likely to differentially influence induction of MHC expression and tumorigenesis/transformation, it may represent one factor that contributes to the nonlinearity of their correlation.

Transformation of avian fibroblasts overexpressing the c-rel proto-oncogene and a variant of c-rel lacking 40 C-terminal amino acids

The v-rel oncogene was derived from the c-rel proto-oncogene, which encodes a transcriptional activator. Expression of v-rel transforms avian hematopoietic cells and fibroblasts. Here we report that overexpression (via a replication-competent retroviral vector) of full-length c-Rel as well as a 40-amino-acid, carboxy-terminal deletion construct of c-Rel (c-Rel delta) resulted in the morphological transformation of chicken embryo fibroblasts (CEFs). Subcellular localization of Rel polypeptides in these transformed cells as determined by immunofluorescence and immunoprecipitation revealed their presence in both the nucleus and the cytoplasm, with the majority of Rel polypeptides showing cytoplasmic localization. Cytoplasmic localization could be due to interaction with I kappa B molecules, and in fact, the overexpression of c-Rel or the C-terminal deletion construct of c-Rel resulted in an increase in the levels of mRNA encoding the avian I kappa B protein pp40 and the avian homolog of the NF-kappa B protein, p105. However, expression of v-Rel resulted in the induction of pp40 mRNA only. While c-Rel was a weak activator of kappa B-mediated transcription of a reporter construct in transformed CEFs, v-Rel and c-Rel delta were transcriptional repressors. However, in spite of these differences, all of these proteins resulted in the transformation of CEFs.

The Bel1 protein of human foamy virus contains one positive and two negative control regions which regulate a distinct activation domain of 30 amino acids

The Bel1 transactivator is essential for the replication of human foamy virus (HFV). To define the functional domains of HFV Bel1, we generated random missense mutations throughout the entire coding sequence of Bel1. Functional analyses of 24 missense mutations have revealed the presence of at least two functional domains in Bel1. One domain corresponds to a basic amino acid-rich motif which acts as a bipartite nuclear targeting sequence. A second, central domain corresponds to a presumed effector region which, when mutated, leads to dominant-negative mutants and/or lacks transactivating ability. In addition, deletion analyses and domain-swapping experiments further showed that Bel1 protein contains a strong carboxy-terminal activation domain. The activating region is also capable of functioning as a transcription-activating domain in yeast cells, although it does not bear any significant sequence homology to the well-characterized acidic activation domain which is known to function only in yeast and mammalian cells. We also demonstrated that the regions of Bel1 from residues 1 to 76 and from residues 153 to 225 repressed transcriptional activation exerted by the Bel1 activation domain. In contrast, the region from residues 82 to 150 appears to overcome an inhibitory effect. These results indicate that Bel1 contains one positive and two negative regulatory domains that modulate a distinct activation domain of Bel1. These regulatory domains of Bel1 cannot affect the function of the VP16 activation domain, suggesting that these domains specifically regulate the activation domain of Bel1. Furthermore, in vivo competition experiments showed that the positive regulatory domain acts in trans. Thus, our results demonstrate that Bel1-mediated transactivation appears to undergo a complex regulatory pathway which provides a novel mode of regulation for a transcriptional activation domain.

In vivo evolution of c-rel oncogenic potential

The c-rel proto-oncogene belongs to the NF-kappa B/rel and I kappa B gene families, which regulate several inducible processes, including self-defense/repair and embryogenesis. Transduction of the c-rel transcription factor by the avian retrovirus resulted in the formation of a highly oncogenic virus, reticuloendotheliosis virus strain T (REV-T), that encodes the oncogene v-rel. To examine the oncogenic potential of c-rel, we inserted it into a REV-T-based retroviral vector, rescued virus [REV-C(CSV)], and infected 1-day-old chicks. All birds developed tumors, and all cell lines established from REV-C-induced tumors expressed c-rel proteins that lacked C-terminal sequences. These proteins, responsible for both in vivo and in vitro cell proliferation, were apparently selected for their oncogenic potential. In order to examine the cooperation of C-terminal deletions with other oncogenic alterations in vivo, point mutations present in the N-terminal and middle regions of v-rel were analyzed by a similar protocol. The data obtained support four conclusions. (i) c-rel proteins bearing any of three single-amino-acid mutations present in the N-terminal portion of v-rel were sufficiently oncogenic to induce tumor development in the absence of additional mutations. (ii) Combining a mutation from the N-terminal region of v-rel with a deletion of the C-terminal sequences of c-rel increases the oncogenicity of the protein in an additive manner. (iii) Mutations present in the middle of v-rel cooperated synergistically with C-terminal deletions to produce highly transforming viruses. (iv) Deletion of c-rel produced a variety of transforming rel proteins with sizes that extended from 42 to 65 kDa. The most frequently isolated rel deletion was 62 kDa in size. To examine the basis for the selection of different rel mutants, their ability to induce immunoregulatory surface receptors was analyzed. The data revealed a correlation between the induction capacity of these mutants and their corresponding contribution to in vivo tumorigenic potential. Moreover, an analysis of the subcellular localization of different rel proteins revealed an inverse correlation between the size of the protein and the proportion in the nucleus of lymphoid cells.

NF-kappa B-like factors in the murine brain. Developmentally-regulated and tissue-specific expression

NF-kappa B and related factors are important transducers of external signals to the cell nucleus. They are abundant in the brain, where they may be significant for the regulation of gene transcription in plasticity-related processes for instance, via activation of protein kinase C. The subunit composition and levels of these factors in the mouse and rat brain and other tissues, using an assay based on gel retardation of the oligonucleotides corresponding to the kappa B DNA-element, are reported here. Three major kappa B-binding factors were observed. Factors I and II were activated by the dissociating agent deoxycholate. DNA protein cross-linking and antibody neutralization experiments suggest that factor I is a heterodimer of c-Rel and p65; factor II is a heterodimer of p50 and p65 (authentic NF-kappa B), and of p50 and c-Rel; factor III is the p50 homodimer (KBF1). All three factors were generally expressed in the 17-day-old rat embryo and 5-day-old pup, whereas in the adult rat, expression was more limited and showed certain tissue specificity. Factor II was the most generally expressed and the only factor observed in adult brain. Factor I was only detected in the adult testis whereas factor III was observed in the adult spleen and, in small amounts, in the liver and lung. Two minor kappa B-specific factors (A and B), distinctive to the brain and spleen, respectively, showed very slow gel mobility. Their estimated molecular weights were about 125 kDa and 95 kDa, respectively. Expression of factor A was stable in the rat brain during development. Factor A may be identical to a previously described brain-specific factor, BETA (Korner et al., Neuron, 3 (1989) 563-572). Thus, the expression pattern of kappa B-binding activities is apparently developmentally regulated and tissue-specific particularly in the adult. In the adult mouse and rat brain, only factors II (probably NF-kappa B and p50/c-Rel heterodimer) and A (probably BETA) could be observed.

Association between proto-oncoprotein Rel and TATA-binding protein mediates transcriptional activation by NF-kappa B

The c-Rel protein is able to associate in vitro and in vivo with the TATA-binding protein (TBP) of the TFIID complex. Coexpression of TBP with c-Rel augments transactivation from the kappa B site in Drosophila Schneider cells. DNA-binding mutants of TBP not only fail to cooperate, but they repress transactivation by c-Rel. There may be a direct communication between kappa B enhancer binding proteins and basal transcription factors which leads to enhanced transcription.