Both N- and C-terminal domains of RelB are required for full transactivation: role of the N-terminal leucine zipper-like motif

Rel/NF-κB Transcription Factors Emerged at the Onset of Opisthokonts

The Rel/NF-κB transcription factor family has myriad roles in immunity, development, and differentiation in animals, and was considered a key innovation for animal multicellularity. Rel homology domain-containing proteins were previously hypothesized to have originated in a last common ancestor of animals and some of their closest unicellular relatives. However, key taxa were missing from previous analyses, necessitating a systematic investigation into the distribution and evolution of these proteins. Here, we address this knowledge gap by surveying taxonomically broad data from eukaryotes, with a special emphasis on lineages closely related to animals. We report an earlier origin for Rel/NF-κB proteins than previously described, in the last common ancestor of animals and fungi, and show that even in the sister group to fungi, these proteins contain elements that in animals are necessary for the subcellular regulation of Rel/NF-κB.

RelB and Neuroinflammation

Neuroinflammation within the central nervous system involves multiple cell types that coordinate their responses by secreting and responding to a plethora of inflammatory mediators. These factors activate multiple signaling cascades to orchestrate initial inflammatory response and subsequent resolution. Activation of NF-κB pathways in several cell types is critical during neuroinflammation. In contrast to the well-studied role of p65 NF-κB during neuroinflammation, the mechanisms of RelB activation in specific cell types and its roles during neuroinflammatory response are less understood. In this review, we summarize the mechanisms of RelB activation in specific cell types of the CNS and the specialized effects this transcription factor exerts during neuroinflammation.

Biological characteristics of transcription factor RelB in different immune cell types: implications for the treatment of multiple sclerosis

Transcription factor RelB is a member of the nuclear factror-kappa B (NF-κB) family, which plays a crucial role in mediating immune responses. Plenty of studies have demonstrated that RelB actively contributes to lymphoid organ development, dendritic cells maturation and function and T cells differentiation, as well as B cell development and survival. RelB deficiency may cause a variety of immunological disorders in both mice and humans. Multiple sclerosis (MS) is an inflammatory and demyelinating disease of the central nervous system which involves a board of immune cell populations. Thereby, RelB may exert an impact on MS by modulating the functions of dendritic cells and the differentiation of T cells and B cells. Despite intensive research, the role of RelB in MS and its animal model, experimental autoimmune encephalomyelitis, is still unclear. Herein, we give an overview of the biological characters of RelB, summarize the updated knowledge regarding the role of RelB in different cell types that contribute to MS pathogenesis and discuss the potential RelB-targeted therapeutic implications for MS.

Hypoxia induces rapid, STAT3 and ROS dependent, mitochondrial translocation of RelA(p65) and IκBα

The nuclear factor-κB (NF-κB) family of transcription factors can directly or indirectly regulate many important areas of biology, including immunity, inflammation and cell survival. One intriguing aspect of NF-κB crosstalk with other cell signalling pathways is its regulation of mitochondrial biology, including biogenesis, metabolism and apoptosis. In addition to regulating the expression of mitochondrial genes encoded in the nucleus, NF-κB signalling components are also found within mitochondria themselves and associated with mitochondrial DNA. However, complete biochemical analysis of mitochondrial and sub-mitochondrial localisation of all NF-κB subunits has not been undertaken. Here, we show that only the RelA NF-κB subunit and its inhibitor IκBα reside within mitochondria, whilst p50 is found in the endoplasmic reticulum (ER). Fractionation of mitochondria revealed that only RelA was found in the mitoplast, the location of the mtDNA. We demonstrate that hypoxia leads to a very rapid but transient accumulation of RelA and IκBα in mitochondria. This effect required reactive oxygen species (ROS) but was not dependent on the hypoxia sensing transcription factor subunit HIF1α or intracellular Ca2+ release. We also observed rapid mitochondrial localisation of transcription factor STAT3 following hypoxia. Inhibition of STAT3 blocked RelA and IκBα mitochondrial localisation revealing a previously unknown aspect of crosstalk between these key cellular regulators.

Nuclear Factor-kappaB in Autoimmunity: Man and Mouse

NF-κB (nuclear factor-kappa B) is a transcription complex crucial for host defense mediated by innate and adaptive immunity, where canonical NF-κB signaling, mediated by nuclear translocation of RelA, c-Rel, and p50, is important for immune cell activation, differentiation, and survival. Non-canonical signaling mediated by nuclear translocation of p52 and RelB contributes to lymphocyte maturation and survival and is also crucial for lymphoid organogenesis. We outline NF-κB signaling and regulation, then summarize important molecular contributions of NF-κB to mechanisms of self-tolerance. We relate these mechanisms to autoimmune phenotypes described in what is now a substantial catalog of immune defects conferred by mutations in NF-κB pathways in mouse models. Finally, we describe Mendelian autoimmune syndromes arising from human NF-κB mutations, and speculate on implications for understanding sporadic autoimmune disease.

Carbon dioxide-dependent regulation of NF-κB family members RelB and p100 gives molecular insight into CO2-dependent immune regulation

CO₂ is a physiological gas normally produced in the body during aerobic respiration. Hypercapnia (elevated blood pCO₂ >≈50 mm Hg) is a feature of several lung pathologies, e.g. chronic obstructive pulmonary disease. Hypercapnia is associated with increased susceptibility to bacterial infections and suppression of inflammatory signaling. The NF-κB pathway has been implicated in these effects; however, the molecular mechanisms underpinning cellular sensitivity of the NF-κB pathway to CO₂ are not fully elucidated. Here, we identify several novel CO₂-dependent changes in the NF-κB pathway. NF-κB family members p100 and RelB translocate to the nucleus in response to CO₂ A cohort of RelB protein-protein interactions (e.g. with Raf-1 and IκBα) are altered by CO₂ exposure, although others are maintained (e.g. with p100). RelB is processed by CO₂ in a manner dependent on a key C-terminal domain located in its transactivation domain. Loss of the RelB transactivation domain alters NF-κB-dependent transcriptional activity, and loss of p100 alters sensitivity of RelB to CO₂ Thus, we provide molecular insight into the CO₂ sensitivity of the NF-κB pathway and implicate altered RelB/p100-dependent signaling in the CO₂-dependent regulation of inflammatory signaling.

Post-Translational Modifications of RelB NF-κB Subunit and Associated Functions

The family of NF-κB transcription factors plays a key role in diverse biological processes, such as inflammatory and immune responses, cell survival and tumor development. Beyond the classical NF-κB activation pathway, a second NF-κB pathway has more recently been uncovered, the so-called alternative NF-κB activation pathway. It has been shown that this pathway mainly controls the activity of RelB, a member of the NF-κB family. Post-translational modifications, such as phosphorylation, acetylation, methylation, ubiquitination and SUMOylation, have recently emerged as a strategy for the fine-tuned regulation of NF-κB. Our review discusses recent progress in the understanding of RelB regulation by post-translational modifications and the associated functions in normal and pathological conditions.

RelB: an outlier in leukocyte biology

RelB is one of the more unusual members of the NF-κB family. This family, arguably the best known group of transcription regulators, regulates an astonishing array of cell types and biological processes. This includes regulation of cell growth, differentiation and death by apoptosis, and the development and function of the innate and adaptive-immune system. RelB is best known for its roles in lymphoid development, DC biology, and noncanonical signaling. Within the last few years, however, surprising functions of RelB have emerged. The N-terminal leucine zipper motif of RelB, a motif unique among the NF-κB family, may associate with more diverse DNA sequences than other NF-κB members. RelB is capable of direct binding to the AhR that supports the xenobiotic-detoxifying pathway. RelB can regulate the circadian rhythm by directly binding to the BMAL partner of CLOCK. Finally, RelB also couples with bioenergy NAD(+) sensor SIRT1 to integrate acute inflammation with changes in metabolism and mitochondrial bioenergetics. In this review, we will explore these unique aspects of RelB, specifically with regard to its role in immunity.

Malt1-dependent RelB cleavage promotes canonical NF-kappaB activation in lymphocytes and lymphoma cell lines

The protease activity of the paracaspase Malt1 contributes to antigen receptor-mediated lymphocyte activation and lymphomagenesis. Malt1 activity is required for optimal NF-κB activation, but little is known about the responsible substrate(s). Here we report that Malt1 cleaved the NF-κB family member RelB after Arg-85. RelB cleavage induced its proteasomal degradation and specifically controlled DNA binding of RelA- or c-Rel-containing NF-κB complexes. Overexpression of RelB inhibited expression of canonical NF-κB target genes and led to impaired survival of diffuse large B-cell lymphoma cell lines characterized by constitutive Malt1 activity. These findings identify a central role for Malt1-dependent RelB cleavage in canonical NF-κB activation and thereby provide a rationale for the targeting of Malt1 in immunomodulation and cancer treatment.

The NF-kappaB family of transcription factors and its regulation

Nuclear factor-kappaB (NF-kappaB) consists of a family of transcription factors that play critical roles in inflammation, immunity, cell proliferation, differentiation, and survival. Inducible NF-kappaB activation depends on phosphorylation-induced proteosomal degradation of the inhibitor of NF-kappaB proteins (IkappaBs), which retain inactive NF-kappaB dimers in the cytosol in unstimulated cells. The majority of the diverse signaling pathways that lead to NF-kappaB activation converge on the IkappaB kinase (IKK) complex, which is responsible for IkappaB phosphorylation and is essential for signal transduction to NF-kappaB. Additional regulation of NF-kappaB activity is achieved through various post-translational modifications of the core components of the NF-kappaB signaling pathways. In addition to cytosolic modifications of IKK and IkappaB proteins, as well as other pathway-specific mediators, the transcription factors are themselves extensively modified. Tremendous progress has been made over the last two decades in unraveling the elaborate regulatory networks that control the NF-kappaB response. This has made the NF-kappaB pathway a paradigm for understanding general principles of signal transduction and gene regulation.

Nuclear factor-kappa B family member RelB inhibits human immunodeficiency virus-1 Tat-induced tumor necrosis factor-alpha production

Human Immunodeficiency Virus-1 (HIV-1)-associated neurocognitive disorder (HAND) is likely neuroinflammatory in origin, believed to be triggered by inflammatory and oxidative stress responses to cytokines and HIV protein gene products such as the HIV transactivator of transcription (Tat). Here we demonstrate increased messenger RNA for nuclear factor-kappa B (NF-kappaB) family member, transcription factor RelB, in the brain of doxycycline-induced Tat transgenic mice, and increased RelB synthesis in Tat-exposed microglial cells. Since genetic ablation of RelB in mice leads to multi-organ inflammation, we hypothesized that Tat-induced, newly synthesized RelB inhibits cytokine production by microglial cells, possibly through the formation of transcriptionally inactive RelB/RelA complexes. Indeed, tumor necrosis factor-alpha (TNFalpha) production in monocytes isolated from RelB deficient mice was significantly higher than in monocytes isolated from RelB expressing controls. Moreover, RelB overexpression in microglial cells inhibited Tat-induced TNFalpha synthesis in a manner that involved transcriptional repression of the TNFalpha promoter, and increased phosphorylation of RelA at serine 276, a prerequisite for increased RelB/RelA protein interactions. The Rel-homology-domain within RelB was necessary for this interaction. Overexpression of RelA itself, in turn, significantly increased TNFalpha promoter activity, an effect that was completely blocked by RelB overexpression. We conclude that RelB regulates TNFalpha cytokine synthesis by competitive interference binding with RelA, which leads to downregulation of TNFalpha production. Moreover, because Tat activates both RelB and TNFalpha in microglia, and because Tat induces inflammatory TNFalpha synthesis via NF-kappaB, we posit that RelB serves as a cryoprotective, anti-inflammatory, counter-regulatory mechanism for pathogenic NF-kappaB activation. These findings identify a novel regulatory pathway for controlling HIV-induced microglial activation and cytokine production that may have important therapeutic implications for the management of HAND.

KappaB-Ras is a nuclear-cytoplasmic small GTPase that inhibits NF-kappaB activation through the suppression of transcriptional activation of p65/RelA

NF-κB is an important transcription factor involved in various biological responses, including inflammation, cell differentiation, and tumorigenesis. κB-Ras was identified as an IκB-interacting small GTPase and is reported to disturb cytokine-induced NF-κB activation. In this study, we established that κB-Ras is a novel type of nuclear-cytoplasmic small GTPase that mainly binds to GTP, and its localization seemed to be regulated by its GTP/GDP-binding state. Unexpectedly, the GDP-binding form of the κB-Ras mutant exhibited a more potent inhibitory effect on NF-κB activation, and this inhibitory effect seemed to be due to suppression of the transactivation of a p65/RelA NF-κB subunit. κB-Ras suppressed phosphorylation at serine 276 on the p65/RelA subunit, resulting in decreased interaction between p65/RelA and the transcriptional coactivator p300. Interestingly, the GDP-bound κB-Ras mutant exhibited higher interactive affinity with p65/RelA and inhibited the phosphorylation of p65/RelA more potently than wild-type κB-Ras. Taken together, these findings suggest that the GDP-bound form of κB-Ras in cytoplasm suppresses NF-κB activation by inhibiting its transcriptional activation.

Regulation of IkappaBalpha function and NF-kappaB signaling: AEBP1 is a novel proinflammatory mediator in macrophages

NF-κB comprises a family of transcription factors that are critically involved in various inflammatory processes. In this paper, the role of NF-κB in inflammation and atherosclerosis and the regulation of the NF-κB signaling pathway are summarized. The structure, function, and regulation of the NF-κB inhibitors, IκBα and IκBβ, are reviewed. The regulation of NF-κB activity by glucocorticoid receptor (GR) signaling and IκBα sumoylation is also discussed. This paper focuses on the recently reported regulatory function that adipocyte enhancer-binding protein 1 (AEBP1) exerts on NF-κB transcriptional activity in macrophages, in which AEBP1 manifests itself as a potent modulator of NF-κB via physical interaction with IκBα and a critical mediator of inflammation. Finally, we summarize the regulatory roles that recently identified IκBα-interacting proteins play in NF-κB signaling. Based on its proinflammatory roles in macrophages, AEBP1 is anticipated to serve as a therapeutic target towards the treatment of various inflammatory conditions and disorders.

Protein partners of deubiquitinating enzymes

Protein modification by ubiquitin and ubiquitin-like molecules is a critical regulatory process. Like most regulated protein modifications, ubiquitination is reversible. Deubiquitination, the reversal of ubiquitination, is quickly being recognized as an important regulatory strategy. Nearly one hundred human DUBs (deubiquitinating enzymes) in five different gene families oppose the action of several hundred ubiquitin ligases, suggesting that both ubiquitination and its reversal are highly regulated and specific processes. It has long been recognized that ubiquitin ligases are modular enzyme systems that often depend on scaffolds and adaptors to deliver substrates to the catalytically active macromolecular complex. Although many DUBs bind ubiquitin with reasonable affinities (in the nM to microM range), a larger number have little affinity but exhibit robust catalytic capability. Thus it is apparent that these DUBs must acquire their substrates by binding the target protein in a conjugate or by associating with other macromolecular complexes. We would then expect that a study of protein partners of DUBs would reveal a variety of substrates, scaffolds, adaptors and ubiquitin receptors. In the present review we suggest that, like ligases, much of the regulation and specificity of deubiquitination arises from the association of DUBs with these protein partners.

RelB is the NF-kappaB subunit downstream of NIK responsible for osteoclast differentiation

NF-kappaB inducing kinase (NIK) is required for osteoclastogenesis in response to pathologic stimuli, and its loss leads to functional blockade of both alternative and classical NF-kappaB caused by cytoplasmic retention by p100. We now show that deletion of p100 restores the capacity of NIK-deficient osteoclast (OC) precursors to differentiate and normalizes RelB and p65 signaling. Differentiation of NIK-/- precursors is also restored by overexpression of RelB, but not p65. Additionally, RelB-/- precursors fail to form OCs in culture, and this defect is rescued by re-expression of RelB, but not by overexpression of p65. To further support the role of RelB in OCs, we challenged RelB-/- mice with TNF-alpha in vivo and found a diminished osteoclastogenic response. We then examined tumor-induced osteolysis in both RelB-/- and NIK-/- mice by using the B16 melanoma model. Growth of tumor cells in the bone marrow was similar to WT controls, but the absence of either RelB or NIK completely blocked the tumor-induced loss of trabecular bone. Thus, the alternative NF-kappaB pathway, culminating in activation of RelB, has a key and specific role in the differentiation of OCs that cannot be compensated for by p65.

X-ray structure of a NF-kappaB p50/RelB/DNA complex reveals assembly of multiple dimers on tandem kappaB sites

We describe here the X-ray crystal structure of NF-kappaB p50/RelB heterodimer bound to a kappaB DNA. Although the global modes of subunit association and kappaB DNA recognition are similar to other NF-kappaB/DNA complexes, this complex reveals distinctive features not observed for non-RelB complexes. For example, Lys274 of RelB is removed from the protein-DNA interface whereas the corresponding residues in all other subunits make base-specific contacts. This mode of binding suggests that RelB may allow the recognition of more diverse kappaB sequences. Complementary surfaces on RelB and p50, as revealed by the crystal contacts, are highly suggestive of assembly of multiple p50/RelB heterodimers on tandem kappaB sites in solution. Consistent with this model our in vitro binding experiments reveal optimal assembly of two wild-type p50/RelB heterodimers on tandem HIV kappaB DNA with 2 bp spacing but not by a mutant heterodimer where one of the RelB packing surface is altered. We suggest that multiple NF-kappaB dimers assemble at diverse kappaB promoters through direct interactions utilizing unique protein-protein interaction surfaces.

The IkappaB function of NF-kappaB2 p100 controls stimulated osteoclastogenesis

The prototranscription factor p100 represents an intersection of the NF-kappaB and IkappaB families, potentially serving as both the precursor for the active NF-kappaB subunit p52 and as an IkappaB capable of retaining NF-kappaB in the cytoplasm. NF-kappaB-inducing kinase (NIK) controls processing of p100 to generate p52, and thus NIK-deficient mice can be used to examine the biological effects of a failure in such processing. We demonstrate that treatment of wild-type osteoclast precursors with the osteoclastogenic cytokine receptor activator of NF-kappaB ligand (RANKL) increases both expression of p100 and its conversion to p52, resulting in unchanged net levels of p100. In the absence of NIK, p100 expression is increased by RANKL, but its conversion to p52 is blocked, leading to cytosolic accumulation of p100, which, acting as an IkappaB protein, binds NF-kappaB complexes and prevents their nuclear translocation. High levels of unprocessed p100 in osteoclast precursors from NIK-/- mice or a nonprocessable form of the protein in wild-type cells impair RANKL-mediated osteoclastogenesis. Conversely, p100-deficient osteoclast precursors show enhanced sensitivity to RANKL. These data demonstrate a novel, biologically relevant means of regulating NF-kappaB signaling, with upstream control and kinetics distinct from the classical IkappaBalpha pathway.

RelB nuclear translocation regulates B cell MHC molecule, CD40 expression, and antigen-presenting cell function

Mice with targeted RelB mutations demonstrated an essential role for RelB in immune responses and in myeloid dendritic cell differentiation. Human studies suggested a more global transcriptional role in antigen presentation. Burkitt lymphoma cell lines were used as a model to examine the role of RelB in antigen presentation. After transient transfection of BJAB with RelB, strong nuclear expression of RelB-p50 heterodimers was associated with increased APC function and expression of CD40 and MHC class I. Antisense RelB in DG75 reduced antigen-presenting capacity and CD40-mediated up-regulation of MHC molecules. The data indicate that RelB transcriptional activity directly affects antigen presentation and CD40 synthesis. Stimulation of RelB transcriptional activity may provide a positive feedback loop for facilitating productive APC/T cell interactions.

MEK kinase 1 gene disruption alters cell migration and c-Jun NH2-terminal kinase regulation but does not cause a measurable defect in NF-kappa B activation

MEK kinase 1 (MEKK1) is a 196-kDa mitogen-activated protein kinase (MAPK) kinase kinase that, in addition to regulating the c-Jun NH(2)-terminal kinase (JNK) pathway, is involved in the control of cell motility. MEKK1(-/-) mice are defective in eyelid closure, a TGFalpha-directed process involving the migration of epithelial cells. MEKK1 expression in epithelial cells stimulates lamellipodia formation, a process required for cell movement. In addition, mouse embryo fibroblasts derived from MEKK1(-/-) mice are inhibited in their migration relative to MEKK1(+/+) fibroblasts. MEKK1 is required for JNK but not NF-kappaB activation in response to virus infection, microtubule disruption, and stimulation of embryonic stem cells with lysophosphatidic acid. MEKK1 is not required for TNFalpha or IL-1 regulation of JNK or NF-kappaB activation in macrophages or fibroblasts. Thus, MEKK1 senses microtubule integrity, contributes to the regulation of fibroblast and epithelial cell migration, and is required for activation of JNK but not NF-kappaB in response to selected stress stimuli.

RelB modulation of IkappaBalpha stability as a mechanism of transcription suppression of interleukin-1alpha (IL-1alpha), IL-1beta, and tumor necrosis factor alpha in fibroblasts

Members of the NF-kappaB/RelB family of transcription factors play important roles in the regulation of inflammatory and immune responses. RelB, a member of this family, has been characterized as a transcription activator and is involved in the constitutive NF-kappaB activity in lymphoid tissues. However, in a previous study we observed an overexpression of chemokines in RelB-deficient fibroblasts. Here we show that RelB is an important transcription suppressor in fibroblasts which limits the expression of proinflammatory mediators and may exert its function by modulating the stability of IkappaBalpha protein. Fibroblasts from relb(-/-) mice overexpress interleukin-1alpha (IL-1alpha), IL-1beta, and tumor necrosis factor alpha in response to lipopolysaccharide (LPS) stimulation. These cells have an augmented and prolonged LPS-inducible IKK activity and an accelerated degradation which results in a diminished level of IkappaBalpha protein, despite an upregulated IkappaBalpha mRNA expression. Consequently, NF-kappaB activity was augmented and postinduction repression of NF-kappaB activity was impaired in these cells. The increased kappaB-binding activity and cytokine overexpression was suppressed by introducing RelB cDNA or a dominant negative IkappaBalpha into relb(-/-) fibroblasts. Our findings suggest a novel transcription suppression function of RelB in fibroblasts.

Functional redundancy of the nuclear factor kappa B inhibitors I kappa B alpha and I kappa B beta

The transcription factor NF-kappaB is sequestered in the cytoplasm by the inhibitor proteins of the IkappaB family. Each member of the IkappaB exhibits structural and biochemical similarities as well as differences. In an effort to address the functional redundancy of two closely related IkappaB molecules, IkappaBalpha and IkappaBbeta, we generated knock-in mice by replacing the IkappaBalpha gene with the IkappaBbeta gene. The knock-in mice do not express IkappaBalpha, but express a T7-tagged IkappaBbeta under the promoter and regulatory sequence of ikba. Unlike the IkappaBalpha-deficient mice, which display severe postnatal developmental defects and die by postnatal day 8, homozygous knock-in mice survive to adulthood, are fertile, and exhibit no apparent abnormalities. Furthermore, thymocytes and embryonic fibroblasts from the knock-in animals exhibit an inducible NF-kappaB response similar to that of wild-type animals. These results indicate that IkappaBalpha and IkappaBbeta share significant similarities in their biochemical activity, and that they acquired their different functions from divergent expression patterns during evolution.

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.

Expression of constitutively active IkappaB beta in T cells of transgenic mice: persistent NF-kappaB activity is required for T-cell immune responses

The transcription factor NF-kappaB is normally sequestered in the cytoplasm by members of the IkappaB family, including IkappaB alpha, IkappaB beta, and the recently cloned IkappaB epsilon. Upon cellular activation, these inhibitors are rapidly phosphorylated on two amino-terminal serines, ubiquitinated, and degraded by the 26S proteasome, releasing a functional NF-kappaB. To determine the importance of IkappaB beta in NF-kappaB regulation in T cells, we generated transgenic mice expressing a constitutively active IkappaB beta mutant (mIkappaB beta) under the control of the lck promoter. The transgene contains the two critical N-terminal serine residues mutated to alanines and therefore no longer susceptible to degradation upon cell activation. mIkappaB beta is unable to totally displace IkappaB alpha from RelA-containing complexes, thus allowing a transient activation of NF-kappaB upon T-cell stimulation. However, mIkappaB beta completely blocks NF-kappaB activity after IkappaB alpha degradation. In addition, as a consequence of this inhibition, ikba expression is down regulated, along with that of other NF-kappaB-regulated genes. These transgenic mice have a significant reduction in the peripheral T-cell population, especially CD8+ cells. The remaining T cells have impaired proliferation in response to phorbol 12-myristate 13-acetate plus phytohemagglutinin or calcium ionophore but not to anti-CD3/anti-CD28 costimulation. As a result of these alterations, transgenic animals present defects in immune responses such as delayed-type hypersensitivity and the generation of specific antibodies against T-cell-dependent antigens. These results show that in nonstimulated T cells, IkappaB beta cannot efficiently displace IkappaB alpha bound to RelA-containing complexes and that persistent NF-kappaB activity is required for proper T-cell responses in vivo.

p50-NF-kappaB complexes partially compensate for the absence of RelB: severely increased pathology in p50(-/-)relB(-/-) double-knockout mice

RelB-deficient mice (relB(-/-)) have a complex phenotype including multiorgan inflammation and hematopoietic abnormalities. To examine whether other NF-kappaB/Rel family members are required for the development of this phenotype or have a compensatory role, we have initiated a program to generate double-mutant mice that are deficient in more than one family member. Here we report the phenotypic changes in relB(-/-) mice that also lack the p50 subunit of NF-kappaB (p50(-/-)). The inflammatory phenotype of p50(-/-)relB(-/-) double-mutant mice was markedly increased in both severity and extent of organ involvement, leading to premature death within three to four weeks after birth. Double-knockout mice also had strongly increased myeloid hyperplasia and thymic atrophy. Moreover, B cell development was impaired and, in contrast to relB(-/-) single knockouts, B cells were absent from inflammatory infiltrates. Both p50(-/-) and heterozygous relB(-/+) animals are disease-free. In the absence of the p50, however, relB(-/+) mice (p50(-/-)relB(-/+)) had a mild inflammatory phenotype and moderate myeloid hyperplasia. Neither elevated mRNA levels of other family members, nor increased kappaB-binding activities of NF-kappaB/Rel complexes could be detected in single- or double-mutant mice compared to control animals. These results indicate that the lack of RelB is, in part, compensated by other p50-containing complexes and that the "classical" p50-RelA-NF-kappaB activity is not required for the development of the inflammatory phenotype.

Activation of NF-kappaB/Rel by CD40 engagement induces the mouse germ line immunoglobulin Cgamma1 promoter

Interaction between CD40 on B cells and CD40 ligand (CD40L) on T cells has been shown to mediate T-cell contact help for B-cell proliferation, differentiation, and immunoglobulin isotype switching. It has recently been shown that cross-linking CD40 on mouse B cells induces germ line gamma1 and epsilon transcripts and that interleukin-4 synergizes with CD40 signaling to further induce these germ line transcripts. Germ line transcripts have been shown to be required for class switch recombination. Here we show that signaling via CD40 increases expression of a transiently transfected luciferase reporter plasmid driven by the germ line Cgamma1 promoter in M12.4.1 B-lymphoma cells. By linker-scanning mutation analysis of the promoter, we have identified a CD40-responsive region (CD40RR) which is able to confer inducibility by CD40L to a minimal c-fos promoter. The CD40RR contains three binding sites for NF-kappaB/Rel proteins which are each required for maximal induction of CD40RR activity by CD40L. Binding of the NF-kappaB/Rel proteins p50, p65, c-Rel, and RelB to the CD40RR is induced by CD40 signaling in M12.4.1 cells and in splenic B cells. Cotransfection of expression plasmids for p50 and p65 or p50 and RelB, but not c-Rel, into M12.4.1 cells transactivates the CD40RR and the germ line gamma1 promoter. These data demonstrate that NF-kappaB Rel proteins activated by CD40 ligation play an important role in induction of the germ line Cgamma1 promoter.

Recombinant NFAT1 (NFATp) is regulated by calcineurin in T cells and mediates transcription of several cytokine genes

Transcription factors of the NFAT family play a key role in the transcription of cytokine genes and other genes during the immune response. We have identified two new isoforms of the transcription factor NFAT1 (previously termed NFATp) that are the predominant isoforms expressed in murine and human T cells. When expressed in Jurkat T cells, recombinant NFAT1 is regulated, as expected, by the calmodulin-dependent phosphatase calcineurin, and its function is inhibited by the immunosuppressive agent cyclosporin A (CsA). Transactivation by recombinant NFAT1 in Jurkat T cells requires dual stimulation with ionomycin and phorbol 12-myristate 13-acetate; this activity is potentiated by coexpression of constitutively active calcineurin and is inhibited by CsA. Immunocytochemical analysis indicates that recombinant NFAT1 localizes in the cytoplasm of transiently transfected T cells and translocates into the nucleus in a CsA-sensitive manner following ionomycin stimulation. When expressed in COS cells, however, NFAT1 is capable of transactivation, but it is not regulated correctly: its subcellular localization and transcriptional function are not affected by stimulation of the COS cells with ionomycin and phorbol 12-myristate 13-acetate. Recombinant NFAT1 can mediate transcription of the interleukin-2, interleukin-4, tumor necrosis factor alpha, and granulocyte-macrophage colony-stimulating factor promoters in T cells, suggesting that NFAT1 contributes to the CsA-sensitive transcription of these genes during the immune response.

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

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

Characterization of elements determining the dimerization properties of RelB and p50

Members of the Rel/NF-kappa B family of transcription factors share a region of approximately 300 amino acids which mediates dimerization and sequence-specific binding to DNA. Here we report a detailed characterization of the dimerization domain of RelB. The structural core sufficient to form stable Rel/NF-kappa B dimeric complexes consists of about 110 residues. The dimerization and DNA binding properties of more than 50 RelB mutants were analyzed by using p50 and p52 as partners. We present evidence that amino acids of a conserved element in the dimerization domain play a role in the recognition of a kappa B DNA target sequence. The analysis of hybrid molecules with dimerization domains containing different parts of p50 and RelB allowed us to identify some important structural elements determining homo- and heterodimerization properties. Furthermore, we were able to rescue the dimerization-defective mutant RelB-N287D by the introduction of a counteracting mutation intramolecularly (cis), and also intermolecularly (trans) by a mutation in the NF-kappa B dimerization partner p50. Correspondingly, a dimerization defective p50 mutant was effectively rescued by RelB-N287D.

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.

Identification of the rel family members required for virus induction of the human beta interferon gene

We have carried out experiments to determine which members of the rel family of transcription factors are involved in virus induction of the beta interferon (IFN-beta) gene. First, we examined the inducibility of artificial DNA binding sites that preferentially interact with different homo- or heterodimeric combinations of rel proteins in vitro. We found that only those sites capable of binding the p50/p65 heterodimer are virus inducible. Second, we analyzed a series of mutant rel DNA-binding sites in the context of the intact IFN-beta promoter. We found a correlation between (i) sites capable of binding both the p50/p65 heterodimer and the high-mobility-group protein HMG I(Y) and (ii) virus inducibility. Third, cotransfection of the IFN-beta gene enhancer/promoter with plasmids capable of expressing several different rel proteins revealed that only the combination of p50 and p65 efficiently activated transcription. Finally, we have used antibodies directed against different rel proteins to show that virus-inducible protein-DNA complexes assembled on the IFN-beta enhancer in vitro contain both p50 and p65. We conclude that the p50/p65 heterodimer is responsible for the NF-kappa B-dependent activation of the IFN-beta gene promoter in response to virus infection.

Alternate RNA splicing of murine nfkb1 generates a nuclear isoform of the p50 precursor NF-kappa B1 that can function as a transactivator of NF-kappa B-regulated transcription

The NF-kappa B1 subunit of the transcription factor NF-kappa B is derived by proteolytic cleavage from the N terminus of a 105-kDa precursor protein. The C terminus of p105NF-kappa B1, like those of I kappa B proteins, contains ankyrin-related repeats that inhibit DNA binding and nuclear localization of the precursor and confer I kappa B-like properties upon p105NF-kappa B1. Here we report the characterization of two novel NF-kappa B1 precursor isoforms, p84NF-kappa B1 and p98NF-kappa B1, that arise by alternate splicing within the C-terminal coding region of murine nfkb1. p98NF-kappa B1, which lacks the 111 C-terminal amino acids (aa) of p105NF-kappa B1, has a novel 35-aa C terminus encoded by an alternate reading frame of the gene. p84NF-kappa B1 lacks the C-terminal 190 aa of p105NF-kappa B1, including part of ankyrin repeat 7. RNA and protein analyses indicated that the expression of p84NF-kappa B1 and p98NF-kappa B1 is restricted to certain tissues and that the phorbol myristate acetate-mediated induction of p84NF-kappa B1 and p105NF-kappa B1 differs in a cell-type-specific manner. Both p84NF-kappa B1 and p98NF-kappa B1 are found in the nuclei of transfected cells. Transient transfection analysis revealed that p98NF-kappa B1, but not p105NF-kappa B1 or p84NF-kappa B1, acts as a transactivator of NF-kappa B-regulated gene expression and that this is dependent on sequences in the Rel homology domain required for DNA binding and on the novel 35 C-terminal aa of this isoform. In contrast to previous findings, which indicated that p105NF-kappa B1 does not bind DNA, all of the NF-kappa B1 precursors were found to specifically bind with low affinity to a highly restricted set of NF-kappa B sites in vitro, thereby raising the possibility that certain of the NF-kappa B1 precursor isoforms may directly modulate gene expression.

Alternate RNA splicing of murine nfkb1 generates a nuclear isoform of the p50 precursor NF-kappa B1 that can function as a transactivator of NF-kappa B-regulated transcription

The NF-kappa B1 subunit of the transcription factor NF-kappa B is derived by proteolytic cleavage from the N terminus of a 105-kDa precursor protein. The C terminus of p105NF-kappa B1, like those of I kappa B proteins, contains ankyrin-related repeats that inhibit DNA binding and nuclear localization of the precursor and confer I kappa B-like properties upon p105NF-kappa B1. Here we report the characterization of two novel NF-kappa B1 precursor isoforms, p84NF-kappa B1 and p98NF-kappa B1, that arise by alternate splicing within the C-terminal coding region of murine nfkb1. p98NF-kappa B1, which lacks the 111 C-terminal amino acids (aa) of p105NF-kappa B1, has a novel 35-aa C terminus encoded by an alternate reading frame of the gene. p84NF-kappa B1 lacks the C-terminal 190 aa of p105NF-kappa B1, including part of ankyrin repeat 7. RNA and protein analyses indicated that the expression of p84NF-kappa B1 and p98NF-kappa B1 is restricted to certain tissues and that the phorbol myristate acetate-mediated induction of p84NF-kappa B1 and p105NF-kappa B1 differs in a cell-type-specific manner. Both p84NF-kappa B1 and p98NF-kappa B1 are found in the nuclei of transfected cells. Transient transfection analysis revealed that p98NF-kappa B1, but not p105NF-kappa B1 or p84NF-kappa B1, acts as a transactivator of NF-kappa B-regulated gene expression and that this is dependent on sequences in the Rel homology domain required for DNA binding and on the novel 35 C-terminal aa of this isoform. In contrast to previous findings, which indicated that p105NF-kappa B1 does not bind DNA, all of the NF-kappa B1 precursors were found to specifically bind with low affinity to a highly restricted set of NF-kappa B sites in vitro, thereby raising the possibility that certain of the NF-kappa B1 precursor isoforms may directly modulate gene expression.

NF-kappa B subunit-specific regulation of the interleukin-8 promoter

Interleukin-8 (IL-8), a chemotactic cytokine for T lymphocytes and neutrophils, is induced in several cell types by a variety of stimuli including the inflammatory cytokines IL-1 and tumor necrosis factor alpha TNF-alpha. Several cis elements, including a binding site for the inducible transcription factor NF-kappa B, have been identified in the regulatory region of the IL-8 gene. We have examined the ability of various NF-kappa B subunits to bind to, and activate transcription from, the IL-8 promoter. A nuclear complex was induced in phorbol myristate acetate-treated Jurkat T cells which bound specifically to the kappa B site of the IL-8 promoter and was inhibited by addition of purified I kappa B alpha to the reaction mixture. Only antibody to RelA (p65), but not to NFKB1 (p50), NFKB2 (p50B), c-Rel, or RelB was able to abolish binding, suggesting that RelA is a major component in these kappa B binding complexes. Gel mobility shift analysis with in vitro-translated and purified proteins indicated that whereas the kappa B element in the human immunodeficiency virus type 1 long terminal repeat bound to all members of the kappa B/Rel family examined, the IL-8 kappa B site bound only to RelA and to c-Rel and NFKB2 homodimers, but not to NFKB1 homodimers or heterodimers of NFKB1-RelA. Transient transfection analysis demonstrated a kappa B-dependent expression of the IL-8 promoter in a human fibrosarcoma cell line (8387) and in Jurkat T lymphocytes. Cotransfection with various NF-kappa B subunits indicated that RelA and c-Rel, but neither NFKB1 nor heterodimeric NFKB1-RelA, was able to activate transcription from the IL-8 promoter. Furthermore, cotransfection of NFKB1 and RelA, although able to support activation from the human immunodeficiency virus type 1 long terminal repeat, failed to activate expression from the IL-8 promoter. Antisense oligonucleotides to RelA, but not NFKB1, inhibited phorbol myristate acetate-induced IL-8 production in Jurkat T lymphocytes. These data demonstrate the differential ability of members of the kappa B/Rel family to bind to, and activate transcription from, the IL-8 promoter. Furthermore, while providing a novel example of a kappa B-regulated promoter in which the classical NF-kappa B complex is unable to activate transcription from the kappa B element, these data provide direct evidence for the role of RelA in regulation of IL-8 gene expression.

Cross-coupling of the NF-kappa B p65 and Fos/Jun transcription factors produces potentiated biological function

NF-kappa B and AP-1 represent distinct mammalian transcription factors that target unique DNA enhancer elements. The heterodimeric NF-kappa B complex is typically composed of two DNA binding subunits, NF-kappa B p50 and NF-kappa B p65, which share structural homology with the c-rel proto-oncogene product. Similarly, the AP-1 transcription factor complex is comprised of dimers of the c-fos and c-jun proto-oncogene products or of closely related proteins. We now demonstrate that the bZIP regions of c-Fos and c-Jun are capable of physically interacting with NF-kappa B p65 through the Rel homology domain. This complex of NF-kappa B p65 and Jun or Fos exhibits enhanced DNA binding and biological function via both the kappa B and AP-1 response elements including synergistic activation of the 5' long terminal repeat of the human immunodeficiency virus type 1. These findings support a combinatorial mechanism of gene regulation involving the unexpected cross-coupling of two different classes of transcription factors to form novel protein complexes exhibiting potentiated biological activity.

NF-kappa B subunit-specific regulation of the interleukin-8 promoter

Interleukin-8 (IL-8), a chemotactic cytokine for T lymphocytes and neutrophils, is induced in several cell types by a variety of stimuli including the inflammatory cytokines IL-1 and tumor necrosis factor alpha TNF-alpha. Several cis elements, including a binding site for the inducible transcription factor NF-kappa B, have been identified in the regulatory region of the IL-8 gene. We have examined the ability of various NF-kappa B subunits to bind to, and activate transcription from, the IL-8 promoter. A nuclear complex was induced in phorbol myristate acetate-treated Jurkat T cells which bound specifically to the kappa B site of the IL-8 promoter and was inhibited by addition of purified I kappa B alpha to the reaction mixture. Only antibody to RelA (p65), but not to NFKB1 (p50), NFKB2 (p50B), c-Rel, or RelB was able to abolish binding, suggesting that RelA is a major component in these kappa B binding complexes. Gel mobility shift analysis with in vitro-translated and purified proteins indicated that whereas the kappa B element in the human immunodeficiency virus type 1 long terminal repeat bound to all members of the kappa B/Rel family examined, the IL-8 kappa B site bound only to RelA and to c-Rel and NFKB2 homodimers, but not to NFKB1 homodimers or heterodimers of NFKB1-RelA. Transient transfection analysis demonstrated a kappa B-dependent expression of the IL-8 promoter in a human fibrosarcoma cell line (8387) and in Jurkat T lymphocytes. Cotransfection with various NF-kappa B subunits indicated that RelA and c-Rel, but neither NFKB1 nor heterodimeric NFKB1-RelA, was able to activate transcription from the IL-8 promoter. Furthermore, cotransfection of NFKB1 and RelA, although able to support activation from the human immunodeficiency virus type 1 long terminal repeat, failed to activate expression from the IL-8 promoter. Antisense oligonucleotides to RelA, but not NFKB1, inhibited phorbol myristate acetate-induced IL-8 production in Jurkat T lymphocytes. These data demonstrate the differential ability of members of the kappa B/Rel family to bind to, and activate transcription from, the IL-8 promoter. Furthermore, while providing a novel example of a kappa B-regulated promoter in which the classical NF-kappa B complex is unable to activate transcription from the kappa B element, these data provide direct evidence for the role of RelA in regulation of IL-8 gene expression.