Expanded nuclear roles for IkappaBs

Phosphorylation-induced flexibility of proto-oncogenic Bcl3 regulates transcriptional activation by NF-κB p52 homodimer

B cell lymphoma 3 (Bcl3), a member of the IκB family proteins, modulates transcription by primarily associating with NF-κB p50 and p52 homodimers. Bcl3 undergoes extensive phosphorylation, though the functions of many of these modifications remain unclear. We previously described that phosphorylation at Ser33, Ser114 and Ser446 partially switches Bcl3 from acting as an IκB-like inhibitor to a transcription regulator by associating with the (p52:p52):DNA binary complex. Here, we identified another critical phosphorylation site, Ser366. Substituting at all four residues to phospho-mimetic glutamate further enhances Bcl3's transcriptional activity. Phospho-modifications retain Bcl3's ability to stably bind p52 but induces reciprocal structural changes as revealed by HDX-MS experiments; the N-terminal region stiffens, while the C-terminus becomes more flexible. The increased flexibility allowed the Bcl3:(p52p52) binary complex to better accommodate DNA. The removal of the C-terminal 28-residues transformed Bcl3 into a transcriptional activator independent of phosphorylation. Notably, most identified mutations in Bcl3 from various cancers map to its C-terminus, suggesting the functional relevance of Bcl3 C-terminal structural flexibility and enhanced interaction with (p52p52):DNA complex to transcriptional potential and disease. Overall, this study uncovers the mechanistic basis by which phosphorylation-driven structural changes convert Bcl3 from an inhibitor to a transcriptional cofactor of NF-κB, and how deregulation of its activity through altered phosphorylation or mutation can lead to cancer.

X-ray Crystallographic Study of Preferred Spacing by the NF-κB p50 Homodimer on κB DNA

Though originally characterized as an inactive or transcriptionally repressive factor, the NF-κB p50 homodimer has become appreciated as a physiologically relevant driver of specific target gene expression. By virtue of its low affinity for cytoplasmic IκB protein inhibitors, p50 accumulates in the nucleus of resting cells, where it is a binding target for the transcriptional co-activator IκBζ. In this study, we employed X-ray crystallography to analyze the structure of the p50 homodimer on κB DNA from the promoters of human interleukin-6 (IL-6) and neutrophil-gelatinase-associated lipocalin (NGAL) genes, both of which respond to IκBζ. The NF-κB p50 homodimer binds 11-bp on IL-6 κB DNA, while, on NGAL κB DNA, the spacing is 12-bp. This begs the question: what DNA binding mode is preferred by NF-κB p50 homodimer? To address this, we engineered a "Test" κB-like DNA containing the core sequence 5'-GGGGAATTCCCC-3' and determined its X-ray crystal structure in complex with p50. This revealed that, when presented with multiple options, NF-κB p50 homodimer prefers to bind 11-bp, which necessarily imposes asymmetry on the complex despite the symmetry inherent in both the protein and its target DNA, and that the p50 dimerization domain can contact DNA via distinct modes.

Genome reading by the NF-κB transcription factors

The NF-κB family of dimeric transcription factors regulates transcription by selectively binding to DNA response elements present within promoters or enhancers of target genes. The DNA response elements, collectively known as κB sites or κB DNA, share the consensus 5'-GGGRNNNYCC-3' (where R, Y and N are purine, pyrimidine and any nucleotide base, respectively). In addition, several DNA sequences that deviate significantly from the consensus have been shown to accommodate binding by NF-κB dimers. X-ray crystal structures of NF-κB in complex with diverse κB DNA have helped elucidate the chemical principles that underlie target selection in vitro. However, NF-κB dimers encounter additional impediments to selective DNA binding in vivo. Work carried out during the past decades has identified some of the barriers to sequence selective DNA target binding within the context of chromatin and suggests possible mechanisms by which NF-κB might overcome these obstacles. In this review, we first highlight structural features of NF-κB:DNA complexes and how distinctive features of NF-κB proteins and DNA sequences contribute to specific complex formation. We then discuss how native NF-κB dimers identify DNA binding targets in the nucleus with support from additional factors and how post-translational modifications enable NF-κB to selectively bind κB sites in vivo.

Alert cell strategy in SIRS-induced vasculitis: sepsis and endothelial cells

Sepsis refers to systemic inflammatory response syndrome and organ failure resulting from infection. Inflammatory receptors (e.g., Toll-like receptors and nucleotide oligomerization domain) recognize bacterial components as inflammatory ligands. These are expressed not only in leukocytes but also in major organs and vascular endothelial cells. "Alert cell" is defined as the cell that expresses the inflammatory receptor and intracellular signaling system to produce inflammatory mediators such as inflammatory cytokines, chemokines, nitric oxide, and prostanoids in organs and the vasculature. NF-κB and AP-1, which are the transcriptional factors of these inflammatory molecules, are important regulators of multiple organ failure in sepsis and systemic inflammation. The vascular endothelial injury would induce multiple organ failure as tissue ischemia and organ death. Drug discovery targeted at alert cells holds a promise for therapy of inflammation including sepsis.

The TGFβ1 pathway is required for NFκB dependent gene expression in mouse keratinocytes

The transforming growth factor-beta 1 (TGFβ1) and NFκB pathways are important regulators of epidermal homeostasis, inflammatory responses and carcinogenesis. Previous studies have shown extensive crosstalk between these pathways that is cell type and context dependent, but this has not been well-characterized in epidermal keratinocytes. Here we show that in primary mouse keratinocytes, TGFβ1 induces NFκB-luciferase reporter activity that is dependent on both NFκB and Smad3. TGFβ1-induced NFκB-luciferase activity was blocked by the IκB inhibitor parthenolide, the IκB super-repressor, a dominant negative TGFβ1-activated kinase 1 (TAK1) and genetic deletion of NFκB1. Coexpression of NFκB p50 or p65 subunits enhanced NFκB-luciferase activity. Similarly, inhibition of the TGFβ1 type I receptor with SB431542 or genetic deletion of Smad3 blocked TGFβ1 induction of NFκB-luciferase. TGFβ1 rapidly induced IKK phosphorylation but did not cause a detectable decrease in cytoplasmic IκB levels or nuclear translocation of NFκB subunits, although EMSA showed rapid NFκB nuclear binding activity that could be blocked by SB431542 treatment. TNFα, a well characterized NFκB target gene was also induced by TGFβ1 and this was blocked in NFκB+/- and -/- keratinocytes and by the IκB super-repressor. To test the effects of the TGFβ1 pathway on a biologically relevant activator of NFκB, we exposed mice and primary keratinocytes in culture to UVB irradiation. In primary keratinocytes UVB caused a detectable increase in levels of Smad2 phosphorylation that was dependent on ALK5, but no significant increase in SBE-dependent gene expression. Inhibition of TGFβ1 signaling in primary keratinocytes with SB431542 or genetic deletion of Tgfb1 or Smad3 suppressed UVB induction of TNFα message. Similarly, UVB induction of TNFα mRNA was blocked in skin of Tgfb1+/- mice. These studies demonstrate that intact TGFβ1 signaling is required for NFκB-dependent gene expression in mouse keratinocytes and skin and suggest that a convergence of these pathways in the nucleus rather than the cytoplasm may be critical for regulation of inflammatory pathways in skin by TGFβ1.

Regulation of TNF-α with a focus on rheumatoid arthritis

Cytokines and chemokines represent two important groups of proteins that control the human immune system. Dysregulation of the network in which these immunomodulators function can result in uncontrolled inflammation, leading to various diseases including rheumatoid arthritis (RA), characterized by chronic inflammation and bone erosion. Potential triggers of RA include autoantibodies, cytokines and chemokines. The tight regulation of cytokine and chemokine production, and biological activity is important. Tumor necrosis factor-α (TNF-α) is abundantly present in RA patients' serum and the arthritic synovium. This review, therefore, discusses first the role and regulation of the major proinflammatory cytokine TNF-α, in particular the regulation of TNF-α production, post-translational processing and signaling of TNF-α and its receptors. Owing to the important role of TNF-α in RA, the TNF-α-producing cells and the dynamics of its expression, the direct and indirect action of this cytokine and possible biological therapy for RA are described.

IkappaBbeta is an essential co-activator for LPS-induced IL-1beta transcription in vivo

IkBβ forms a complex with the NF-κB subunits RelA and c-Rel that inhibits the transcription of IL-1β and other genes. Mice lacking IkBβ are protected against LPS-induced shock.

Inhibitor of κB (IκB) β (IκBβ) represents one of the major primary regulators of NF-κB in mammals. In contrast to the defined regulatory interplay between NF-κB and IκBα, much less is known about the biological function of IκBβ. To elucidate the physiological role of IκBβ in NF-κB signaling in vivo, we generated IκBβ-deficient mice. These animals proved to be highly refractory to LPS-induced lethality, accompanied by a strong reduction in sepsis-associated cytokine production. In response to LPS, IκBβ is recruited to the IL-1β promoter forming a complex with the NF-κB subunits RelA/c-Rel required for IL-1β transcription. Further transcriptome analysis of LPS-stimulated wild-type and IκBβ-deficient BM-derived macrophages revealed several other genes with known regulatory functions in innate immunity arguing that a subset of NF-κB target genes is under control of IκBβ. Collectively, these findings provide an essential proinflammatory role for IκBβ in vivo, and establish a critical function for IκBβ as a transcriptional coactivator under inflammatory conditions.

Roles of Bcl-3 in the pathogenesis of murine type 1 diabetes

OBJECTIVE

It has long been recognized that autoimmunity is often associated with immunodeficiency. The mechanism underlying this paradox is not well understood. Bcl-3 (B-cell lymphoma 3) is an atypical member of the IκB (inhibitor of the nuclear factor-κB) family that is required for lymphoid organogenesis and germinal center responses. Mice deficient in Bcl-3 are immunodeficient because of the microarchitectural defects of their lymphoid organs. The goal of this study is to define the potential roles of Bcl-3 in type 1 diabetes.

RESEARCH DESIGN AND METHODS

Bcl-3-deficient NOD mice were generated by backcrossing Bcl-3-deficient C57BL/6 mice to NOD mice. Spontaneous and induced type 1 diabetes were studied in these mice by both pathologic and immunologic means. The effect of Bcl-3 on inflammatory gene transcription was evaluated in a promoter reporter assay.

RESULTS

We found that Bcl-3-deficient NOD and C57BL/6 mice were, paradoxically, more susceptible to autoimmune diabetes than wild-type mice. The increase in diabetes susceptibility was caused by Bcl-3 deficiency in hematopoietic cells but not nonhematopoietic cells. Bcl-3 deficiency did not significantly affect anti-islet Th1 or Th2 autoimmune responses, but markedly increased inflammatory chemokine and T helper 17 (Th17)-type cytokine expression. Upon transfection, Bcl-3 significantly inhibited the promoter activities of inflammatory chemokine and cytokine genes.

CONCLUSIONS

These results indicate that in addition to mediating lymphoid organogenesis, Bcl-3 prevents autoimmune diabetes by inhibiting inflammatory chemokine and cytokine gene transcription. Thus, a single Bcl3 gene mutation leads to both autoimmunity and immunodeficiency.

Telomere-independent Rap1 is an IKK adaptor and regulates NF-kappaB-dependent gene expression

We describe a genome-wide gain-of-function screen for regulators of NF-kappaB, and identify Rap1 (Trf2IP), as an essential modulator of NF-kappaB-mediated pathways. NF-kappaB is induced by ectopic expression of Rap1, whereas its activity is inhibited by Rap1 depletion. In addition to localizing on telomeres, mammalian Rap1 forms a complex with IKKs (IkappaB kinases), and is crucial for the ability of IKKs to be recruited to, and phosphorylate, the p65 subunit of NF-kappaB to make it transcriptionally competent. Rap1-mutant mice display defective NF-kappaB activation and are resistant to endotoxic shock. Furthermore, levels of Rap1 are positively regulated by NF-kappaB, and human breast cancers with NF-kappaB hyperactivity show elevated levels of cytoplasmic Rap1. Similar to inhibiting NF-kappaB, knockdown of Rap1 sensitizes breast cancer cells to apoptosis. These results identify the first cytoplasmic role of Rap1 and provide a mechanism through which it regulates an important signalling cascade in mammals, independent of its ability to regulate telomere function.

The complicated role of NF-kappaB in T-cell selection

The nuclear factor (NF)-kappaB transcription factor family plays important roles in the immune system. Aberrant NF-kappaB signaling is frequently associated with inflammation and autoimmune diseases but the underlying mechanisms are not fully understood. Recent studies show that NF-kappaB plays a critical role in T-cell central tolerance. Two NF-kappaB signaling pathways have been identified: the canonical pathway and the alternative pathway. In the establishment of T-cell central tolerance, the alternative pathway appears to be the key signaling component in thymic stromal cells for their development and function, while the canonical pathway exerts its function more in autonomous T-cell selection. This review intends to summarize the current understanding of the role of NF-kappaB in establishing T-cell central tolerance and highlight unsolved intriguing questions for future work.

Transcriptional control of the inflammatory response

Inflammation is a multicomponent response to tissue stress, injury and infection, and a crucial point of its control is at the level of gene transcription. The inducible inflammatory gene expression programme--such as that triggered by Toll-like receptor signalling in macrophages--is comprised of several coordinately regulated sets of genes that encode key functional programmes; these are controlled by three classes of transcription factors, as well as various transcriptional co-regulators and chromatin modifications. Here, we discuss the mechanisms of and the emerging principles in the transcriptional regulation of inflammatory responses in diverse physiological settings.

RelB sustains IkappaBalpha expression during endotoxin tolerance

Transcription factors and chromatin structural modifiers induce clinically relevant epigenetic modifications of blood leukocytes during severe systemic inflammation (SSI) in humans and animals. These changes affect genes with distinct functions, as exemplified by the silencing of a set of acute proinflammatory genes and the sustained expression of a group of antimicrobial and anti-inflammatory genes. This paradigm is closely mimicked in the THP-1 human promonocyte cell model of lipopolysaccharide (LPS) endotoxin tolerance. We previously reported that LPS-induced de novo expression of RelB is required for generating tolerance to interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha) expression. RelB represses transcription by binding with heterochromatic protein 1 alpha (HP1alpha) to the proximal promoters of IL-1beta and TNF-alpha. In contrast, we report herein that RelB is required for sustained expression of anti-inflammatory IkappaBalpha in LPS-tolerant THP-1 cells. RelB transcription activation requires binding to the IkappaBalpha proximal promoter along with NF-kappaB p50 and is associated with an apparent dimer exchange with p65. We also observed that RelB induced during human SSI binds to the IkappaBalpha proximal promoter of circulating leukocytes. We conclude that RelB functions as a dual transcription regulator during LPS tolerance and human SSI by activating and repressing innate immunity genes.

FADD and the NF-kappaB family member Bcl-3 regulate complementary pathways to control T-cell survival and proliferation

Fas-associated protein with death domain/mediator of receptor induced toxicity (FADD/MORT1) was first described as a transducer of death receptor signalling but was later recognized also to be important for proliferation of T cells. B-cell lymphoma 3 (Bcl-3) is a relatively little understood member of the nuclear factor (NF)-kappaB family of transcription factors. We recently found that Bcl-3 is up-regulated in T cells from mice where FADD function is blocked by a dominant negative transgene (FADD-DN). To understand the importance of this, we generated FADD-DN/bcl-3(-/-) mice. Here, we report that T cells from these mice show massive cell death and severely reduced proliferation in response to T-cell receptor (TCR) stimulation in vitro. Transgenic co-expression of Bcl-2 (FADD-DN/bcl-3(-/-)/vav-bcl-2 mice) rescued the survival but not the proliferation of T cells. FADD-DN/bcl-3(-/-) mice had normal thymocyte numbers but reduced numbers of peripheral T cells despite an increase in cycling T cells in vivo. However, activation of the classical NF-kappaB and extracellular regulated kinase (ERK) pathways and expression of interleukin (IL)-2 mRNA upon stimulation were normal in T cells from FADD-DN/bcl-3(-/-) mice. These data suggest that FADD and Bcl-3 regulate separate pathways that both contribute to survival and proliferation in mouse T cells.

The nuclear I kappaB protein I kappaB zeta specifically binds NF-kappaB p50 homodimers and forms a ternary complex on kappaB DNA

Although they share sequence homology to classical cytoplasmic I kappaB inhibitors of transcription factor NF-kappaB, the proteins I kappaB zeta, Bcl-3, and I kappa BNS function in the nucleus as factors that influence NF-kappaB-dependent gene expression profiles. Through the use of purified recombinant proteins and by comparison with the classical I kappaB protein I kappaB alpha, we have discovered mechanistic details of the interaction between I kappaB zeta and functional NF-kappaB dimers. Whereas I kappaB alpha and other classical I kappaB proteins bind tightly to NF-kappaB dimers that possess the p65 subunit, I kappaB zeta binds preferentially to NF-kappaB p50 homodimers. This altered specificity is particularly interesting in light of the fact that both NF-kappaB subunits exhibit high sequence and structural homology, while the I kappaB alpha and I kappaB zeta proteins are also conserved in primary amino acid sequence. We further show that I kappaB zeta is capable of forming a stable ternary complex with the NF-kappaB p50 homodimer and kappaB DNA. Again, this is a stark contrast from I kappaB alpha, which inhibits NF-kappaB p65 homodimer binding to NF-kappaB target DNA sequences. Removal of the DNA sequences flanking the NF-kappaB binding site does not directly affect the interaction of p50 and I kappaB zeta. Rather, we have discovered that the carboxy-terminal glycine-rich region of the NF-kappaB p50 homodimer is involved in mediating high-affinity binding of I kappaB zeta and NF-kappaB p50. We conclude that the NF-kappaB p50 homodimer functions as a legitimate activator of gene expression through formation of a ternary complex between itself, I kappaB zeta, and DNA. The requirement for formation of this complex could explain why the nuclear I kappaB protein I kappaB zeta is absolutely required for expression of the pluripotent pro-inflammatory cytokine interleukin-6 in peritoneal macrophages.

Transcription factors in muscle atrophy caused by blocked neuromuscular transmission and muscle unloading in rats

The muscle wasting associated with long-term intensive care unit (ICU) treatment has a negative effect on muscle function resulting in prolonged periods of rehabilitation and a decreased quality of life. To identify mechanisms behind this form of muscle wasting, we have used a rat model designed to mimic the conditions in an ICU. Rats were pharmacologically paralyzed with a postsynaptic blocker of neuromuscular transmission, and mechanically ventilated for one to two weeks, thereby unloading the limb muscles. Transcription factors were analyzed for cellular localization and nuclear concentration in the fast-twitch muscle extensor digitorum longus (EDL) and in the slow-twitch soleus. Significant muscle wasting and upregulation of mRNA for the ubiquitin ligases MAFbx and MuRF1 followed the treatment. The IkappaB family-member Bcl-3 displayed a concomitant decrease in concentration, suggesting altered kappaB controlled gene expression, although NFkappaB p65 was not significantly affected. The nuclear levels of the glucocorticoid receptor (GR) and the thyroid receptor alpha1 (TRalpha1) were altered and also suggested as potential mediators of the MAFbx- and MuRF1-induction in the absence of induced Foxo1. We believe that this model, and the strategy of quantifying nuclear proteins, will provide a valuable tool for further, more detailed, analyses of the muscle wasting occurring in patients kept on a mechanical ventilator.

TLR8: an innate immune receptor in brain, neurons and axons

Toll-like receptors (TLRs) play essential roles in generating innate immune responses, and are evolutionarily conserved across species. In mammals, TLRs specifically recognize the conserved microbial structural motifs referred to as pathogen-associated molecular patterns (PAMPs). Ligand recognition by TLRs activates signaling cascades that culminate in proinflammatory cytokine production and eventual elimination of invading pathogens. Although TLRs in mammals are expressed predominantly in the immune system, certain TLRs with poorly characterized function are also found in neurons. We recently profiled TLR8 expression during mouse brain development and established its localization in neurons and axons. We uncovered a novel role for TLR8 as a suppressor of neurite outgrowth as well as an inducer of neuronal apoptosis, and found that TLR8 functions in neurons through an NF-kappaB-independent mechanism. These findings add a new layer of complexity for TLR signaling, and expand the realm of mammalian TLR function to the CNS beyond the originally discovered immune context. Herein, we complement our earlier report with additional data, discuss their biological and mechanistic implications in central nervous system (CNS) developmental and pathological processes, and thus further our perspective on TLR signaling and potential physiological roles in mammals.

Postshock mesenteric lymph induces endothelial NF-kappaB activation

BACKGROUND

Posthemorrhagic shock mesenteric lymph (PSML) has been shown to activate pulmonary endothelial cells and cause lung injury. Although multiple mediators may be involved, most of these effects are mediated by nuclear factor-kappa B (NF-kappaB) activation. Degradation of the inhibitor of kappa B (IkappaB) is a key regulatory step in the activation of NF-kappaB. We therefore hypothesized that PSML would cause IkappaB degradation with subsequent NF-kappaB phosphorylation and nuclear translocation.

METHODS

Mesenteric lymph was collected from male rats before shock and each hour after shock for up to 3 h (n = 5). Buffer (control), buffer + 10% (v/v) lymph, or buffer + tumor necrosis factor (10 ng/mL) were incubated with human pulmonary endothelial cells for 30 min and then lysed. Immunoblots of lysates were probed for IkappaB and phospho-p65. Immunohistochemistry was performed on cells grown on glass slides and then treated as above with the third PSML sample. Cells were fixed and then probed for p65. Statistical analysis was performed with Student's t-test and analysis of variance with significance was set at P < 0.05.

RESULTS

Western blots of cell lysates for IkappaB demonstrated a steady decrease in total IkappaB with each lymph sample. Phosphorylation of NF-kappaB , p65 component, steadily increased with each PSML sample, with a maximum reached during the third PSML sample, which also significantly increased translocation of NF-kappaB to the nucleus.

CONCLUSION

Postshock mesenteric lymph bioactivity is mediated by pathways which involved IkappaB degradation. These pathways offer novel off targets for clinical intervention to prevent the distal organ injury caused by postinjury hemorrhagic shock.

Identification of conserved domains in the promoter regions of nitric oxide synthase 2: implications for the species-specific transcription and evolutionary differences

BACKGROUND

The majority of the genes involved in the inflammatory response are highly conserved in mammals. These genes are not significantly expressed under normal conditions and are mainly regulated at the transcription and prost-transcriptional level. Transcription from the promoters of these genes is very dependent on NF-kappaB activation, which integrates the response to diverse extracellular stresses. However, in spite of the high conservation of the pattern of promoter regulation in kappaB-regulated genes, there is inter-species diversity in some genes. One example is nitric oxide synthase 2 (NOS-2), which exhibits a species-specific pattern of expression in response to infection or pro-inflammatory challenge.

RESULTS

We have conducted a comparative genomic analysis of NOS-2 with different bioinformatic approaches. This analysis shows that in the NOS-2 gene promoter the position and the evolutionary divergence of some conserved regions are different in rodents and non-rodent mammals, and in particular in primates. Two not previously described distal regions in rodents that are similar to the unique upstream region responsible of the NF-kappaB activation of NOS-2 in humans are fragmented and translocated to different locations in the rodent promoters. The rodent sequences moreover lack the functional kappaB sites and IFN-gamma response sites present in the homologous human, rhesus monkey and chimpanzee regions. The absence of kappaB binding in these regions was confirmed by electrophoretic mobility shift assays.

CONCLUSION

The data presented reveal divergence between rodents and other mammals in the location and functionality of conserved regions of the NOS-2 promoter containing NF-kappaB and IFN-gamma response elements.

NF-kappaB regulation of YY1 inhibits skeletal myogenesis through transcriptional silencing of myofibrillar genes

NF-kappaB signaling is implicated as an important regulator of skeletal muscle homeostasis, but the mechanisms by which this transcription factor contributes to muscle maturation and turnover remain unclear. To gain insight into these mechanisms, gene expression profiling was examined in C2C12 myoblasts devoid of NF-kappaB activity. Interestingly, even in proliferating myoblasts, the absence of NF-kappaB caused the pronounced induction of several myofibrillar genes, suggesting that NF-kappaB functions as a negative regulator of late-stage muscle differentiation. Although several myofibrillar promoters contain predicted NF-kappaB binding sites, functional analysis using the troponin-I2 gene as a model revealed that NF-kappaB-mediated repression does not occur through direct DNA binding. In the search for an indirect mediator, the transcriptional repressor YinYang1 (YY1) was identified. While inducers of NF-kappaB stimulated YY1 expression in multiple cell types, genetic ablation of the RelA/p65 subunit of NF-kappaB in both cultured cells and adult skeletal muscle correlated with reduced YY1 transcripts and protein. NF-kappaB regulation of YY1 occurred at the transcriptional level, mediated by direct binding of the p50/p65 heterodimer complex to the YY1 promoter. Furthermore, YY1 was found associated with multiple myofibrillar promoters in C2C12 myoblasts containing NF-kappaB activity. Based on these results, we propose that NF-kappaB regulation of YY1 and transcriptional silencing of myofibrillar genes represent a new mechanism by which NF-kappaB functions in myoblasts to modulate skeletal muscle differentiation.

Myxoma virus in the European rabbit: interactions between the virus and its susceptible host

Myxoma virus (MV) is a poxvirus that evolved in Sylvilagus lagomorphs, and is the causative agent of myxomatosis in European rabbits (Oryctolagus cuniculus). This virus is not a natural pathogen of O. cuniculus, yet is able to subvert the host rabbit immune system defenses and cause a highly lethal systemic infection. The interaction of MV proteins and the rabbit immune system has been an ideal model to help elucidate host/poxvirus interactions, and has led to a greater understanding of how other poxvirus pathogens are able to cause disease in their respective hosts. This review will examine how MV causes myxomatosis, by examining a selection of the identified immunomodulatory proteins that this virus expresses to subvert the immune and inflammatory pathways of infected rabbit hosts.

The human cytomegalovirus virion possesses an activated casein kinase II that allows for the rapid phosphorylation of the inhibitor of NF-kappaB, IkappaBalpha

We documented that the NF-kappaB signaling pathway was rapidly induced following human cytomegalovirus (HCMV) infection of human fibroblasts and that this induced NF-kappaB activity promoted efficient transactivation of the major immediate-early promoter (MIEP). Previously, we showed that the major HCMV envelope glycoproteins, gB and gH, initiated this NF-kappaB signaling event. However, we also hypothesized that there were additional mechanisms utilized by the virus to rapidly upregulate NF-kappaB. In this light, we specifically hypothesized that the HCMV virion contained IkappaBalpha kinase activity, allowing for direct phosphorylation of IkappaBalpha following virion entry into infected cells. In vitro kinase assays performed on purified HCMV virion extract identified bona fide IkappaBalpha kinase activity in the virion. The enzyme responsible for this kinase activity was identified as casein kinase II (CKII), a cellular serine-threonine protein kinase. CKII activity was necessary for efficient transactivation of the MIEP and IE gene expression. CKII is generally considered to be a constitutively active kinase. We suggest that this molecular characteristic of CKII represents the biologic rationale for the viral capture and utilization of this kinase early after infection. The packaging of CKII into the HCMV virion identifies that diverse molecular mechanisms are utilized by HCMV for rapid NF-kappaB activation. We propose that HCMV possesses multiple pathways to increase NF-kappaB activity to ensure that the correct temporal regulation of NF-kappaB occurs following infection and that sufficient threshold levels of NF-kappaB are reached in the diverse array of cells, including monocytes and endothelial cells, infected in vivo.

Intranasal administration of E-selectin to induce immunological tolerization can suppress subarachnoid hemorrhage-induced vasospasm implicating immune and inflammatory mechanisms in its genesis

Evidence that inflammatory and immune mechanisms may have a critical role in the development of vasospasm after subarachnoid hemorrhage is accumulating. We examined, therefore, whether induction of immunological tolerance to the adhesion molecule that is uniquely expressed on activated endothelium, E-selectin, could inhibit the vasospasm provoked by subarachnoid blood in a rat subarachnoid hemorrhage model. We found that intranasal instillation of E-selectin every other day for 10 days on a mucosal tolerization schedule suppressed delayed type hypersensitivity to E-selectin confirming tolerance to that molecule and markedly suppressed basilar artery spasm after subarachnoid hemorrhage. The results of this proof-of-concept study suggest that agents that can mimic the local effects of the mediators of mucosal tolerance could have therapeutic potential for the management of post-subarachnoid hemorrhage vasospasm.

Role for IκBα, but not c-Rel, in skeletal muscle atrophy

Skeletal muscle atrophy is associated with a marked and sustained activation of nuclear factor-κB (NF-κB) activity. Previous work showed that p50 is one of the NF-κB family members required for this activation and for muscle atrophy. In this work, we tested whether another NF-κB family member, c-Rel, is required for atrophy. Because endogenous inhibitory factor κBα (IκBα) was activated (i.e., decreased) at 3 and 7 days of muscle disuse (i.e., hindlimb unloading), we also tested if IκBα, which binds and retains Rel proteins in the cytosol, is required for atrophy and intermediates of the atrophy process. To do this, we electrotransferred a dominant negative IκBα (IκBαΔN) in soleus muscles, which were either unloaded or weight bearing. IκBαΔN expression abolished the unloading-induced increase in both NF-κB activation and total ubiquitinated protein. IκBαΔN inhibited unloading-induced fiber atrophy by 40%. The expression of certain genes known to be upregulated with atrophy were significantly inhibited by IκBαΔN expression during unloading, including MAFbx/atrogin-1, Nedd4, IEX, 4E-BP1, FOXO3a, and cathepsin L, suggesting these genes may be targets of NF-κB transcription factors. In contrast, c-Rel was not required for atrophy because the unloading-induced markers of atrophy were the same in c-rel−/−and wild-type mice. Thus IκBα degradation is required for the unloading-induced decrease in fiber size, the increase in protein ubiquitination, activation of NF-κB signaling, and the expression of specific atrophy genes, but c-Rel is not. These data represent a significant advance in our understanding of the role of NF-κB/IκB family members in skeletal muscle atrophy, and they provide new candidate NF-κB target genes for further study.

The NF-kappaB regulator Bcl-3 and the BH3-only proteins Bim and Puma control the death of activated T cells

Apoptosis of activated T cells is critical for the termination of immune responses. Here we show that adjuvant-stimulated dendritic cells secrete cytokines that prime activated T cells for survival and analyze the roles of the NF-kappaB regulator Bcl-3 and the proapoptotic Bcl-2 family members Bim and Puma. Bcl-3 overexpression increased survival, and activated bcl-3-/- T cells died abnormally rapidly. Cytokines from adjuvant-stimulated dendritic cells induced Bcl-3, but survival through cytokine priming was Bcl-3-independent. Apoptosis inhibition by Bcl-3 involved blockade of Bim activation, because Bim was overactivated in Bcl-3-deficient cells, and Bcl-3 failed to increase survival of bim-/- T cells. However, adjuvants increased survival also in Bim-deficient T cells. This Bim-independent death pathway is at least in part regulated by Puma, as shown by analysis of puma-/- and noxa-/- T cells. IL-1, IL-7, and IL-15 primed T cells for survival even in the absence of Bim or Puma. Our data define interrelations and a Bim-independent pathway to activated T cell death.

RelA/p65 regulation of IkappaBbeta

IkappaB inhibitor proteins are the primary regulators of NF-kappaB. In contrast to the defined regulatory interplay between NF-kappaB and IkappaBalpha, much less is known regarding the regulation of IkappaBbeta by NF-kappaB. Here, we describe in detail the regulation of IkappaBbeta by RelA/p65. Using p65(-/-) fibroblasts, we show that IkappaBbeta is profoundly reduced in these cells, but not in other NF-kappaB subunit knockouts. This regulation prevails during embryonic and postnatal development in a tissue-specific manner. Significantly, in both p65(-/-) cells and tissues, IkappaBalpha is also reduced, but not nearly to the same extent as IkappaBbeta, thus highlighting the degree to which IkappaBbeta is dependent on p65. This dependence is based on the ability of p65 to stabilize IkappaBbeta protein from the 26S proteasome, a process mediated in large part through the p65 carboxyl terminus. Furthermore, IkappaBbeta was found to exist in both a basally phosphorylated and a hyperphosphorylated form. While the hyperphosphorylated form is less abundant, it is also more stable and less dependent on p65 and its carboxyl domain. Finally, we show that in p65(-/-) fibroblasts, expression of a proteolysis-resistant form of IkappaBbeta, but not IkappaBalpha, causes a severe growth defect associated with apoptosis. Based on these findings, we propose that tight control of IkappaBbeta protein by p65 is necessary for the maintenance of cellular homeostasis.