Signal-specific and phosphorylation-dependent RelB degradation: a potential mechanism of NF-kappaB control

Phosphorylation of NF-kappaB and IkappaB proteins: implications in cancer and inflammation

Nuclear factor-kappaB (NF-kappaB) is a transcription factor that has crucial roles in inflammation, immunity, cell proliferation and apoptosis. Activation of NF-kappaB mainly occurs via IkappaB kinase (IKK)-mediated phosphorylation of inhibitory molecules, including IkappaBalpha. Optimal induction of NF-kappaB target genes also requires phosphorylation of NF-kappaB proteins, such as p65, within their transactivation domain by a variety of kinases in response to distinct stimuli. Whether, and how, phosphorylation modulates the function of other NF-kappaB and IkappaB proteins, such as B-cell lymphoma 3, remains unclear. The identification and characterization of all the kinases known to phosphorylate NF-kappaB and IkappaB proteins are described here. Because deregulation of NF-kappaB and IkappaB phosphorylations is a hallmark of chronic inflammatory diseases and cancer, newly designed drugs targeting these constitutively activated signalling pathways represent promising therapeutic tools.

Rheumatoid arthritis synovium contains plasmacytoid dendritic cells

We have previously described enrichment of antigen-presenting HLA-DR⁺ nuclear RelB⁺ dendritic cells (DCs) in rheumatoid arthritis (RA) synovium. CD123⁺HLA-DR⁺ plasmacytoid DCs (pDCs) and their precursors have been identified in human peripheral blood (PB), lymphoid tissue, and some inflamed tissues. We hypothesized recruitment of pDCs into the inflamed RA synovial environment and their contribution as antigen-presenting cells (APCs) and inflammatory cells in RA. CD11c⁺ myeloid DCs and CD123⁺ pDCs were compared in normal and RA PB, synovial fluid (SF), and synovial tissue by flow cytometry, immunohistochemistry, and electron microscopy and were sorted for functional studies. Nuclear RelB^-CD123⁺ DCs were located in perivascular regions of RA, in a similar frequency to nuclear RelB⁺CD123^- DCs, but not normal synovial tissue sublining. Apart from higher expression of HLA-DR, the numbers and phenotypes of SF pDCs were similar to those of normal PB pDCs. While the APC function of PB pDCs was less efficient than that of PB myeloid DCs, RA SF pDCs efficiently activated resting allogeneic PB T cells, and high levels of IFN-γ, IL-10, and tumor necrosis factor α were produced in response to incubation of allogeneic T cells with either type of SF DCs. Thus, pDCs are recruited to RA synovial tissue and comprise an APC population distinct from the previously described nuclear RelB⁺ synovial DCs. pDCs may contribute significantly to the local inflammatory environment.

cFLIPL inhibits tumor necrosis factor-related apoptosis-inducing ligand-mediated NF-kappaB activation at the death-inducing signaling complex in human keratinocytes

Human keratinocytes undergo apoptosis following treatment with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) via surface-expressed TRAIL receptors 1 and 2. In addition, TRAIL triggers nonapoptotic signaling pathways including activation of the transcription factor NF-kappaB, in particular when TRAIL-induced apoptosis is blocked. The intracellular protein cFLIP(L) interferes with TRAIL-induced apoptosis at the death-inducing signaling complex (DISC) in many cell types. To study the role of cFLIP(L) in TRAIL signaling, we established stable HaCaT keratinocyte cell lines expressing varying levels of cFLIP(L). Functional analysis revealed that relative cFLIP(L) levels correlated with apoptosis resistance to TRAIL. Surprisingly, cFLIP(L) specifically blocked TRAIL-induced NF-kappaB activation and TRAIL-dependent induction of the proinflammatory target gene interleukin-8. Biochemical characterization of the signaling pathways involved showed that apoptosis signaling was inhibited at the DISC in cFLIP(L)-overexpressing keratinocytes, although cFLIP(L) did not significantly impair enzymatic activity of the receptor complex. In contrast, recruitment and modification of receptor-interacting protein was blocked in cFLIP(L)-overexpressing cells. Taken together, our data demonstrate that cFLIP(L) is not only a central antiapoptotic modulator of TRAIL-mediated apoptosis but also an inhibitor of TRAIL-induced NF-kappaB activation and subsequent proinflammatory target gene expression. Hence, cFLIP(L) modulation in keratinocytes may not only influence apoptosis sensitivity but may also lead to altered death receptor-dependent skin inflammation.

T Cells Signaled by NF-κB−Dendritic Cells Are Sensitized Not Anergic to Subsequent Activation

Paradoxically, while peripheral self-tolerance exists for constitutively presented somatic self Ag, self-peptide recognized in the context of MHC class II has been shown to sensitize T cells for subsequent activation. We have shown that MHC class II(+)CD86(+)CD40(-) DC, which can be generated from bone marrow in the presence of an NF-kappa B inhibitor, and which constitutively populate peripheral tissues and lymphoid organs in naive animals, can induce Ag-specific tolerance. In this study, we show that CD40(-) human monocyte-derived dendritic cells (DC), generated in the presence of an NF-kappa B inhibitor, signal phosphorylation of TCR zeta, but little proliferation or IFN-gamma in vitro. Proliferation is arrested in the G(1)/G(0) phase of the cell cycle. Surprisingly, responding T cells are neither anergic nor regulatory, but are sensitized for subsequent IFN-gamma production. The data indicate that signaling through NF-kappa B determines the capacity of DC to stimulate T cell proliferation. Functionally, NF-kappa B(-)CD40(-)class II(+) DC may either tolerize or sensitize T cells. Thus, while CD40(-) DC appear to "prime" or prepare T cells, the data imply that signals derived from other cells drive the generation either of Ag-specific regulatory or effector cells in vivo.

NF-κB and Oct-2 Synergize to Activate the Human 3′ Igh hs4 Enhancer in B Cells

In B cells, the Igh gene locus contains several DNase I-hypersensitive (hs) sites with enhancer activity. These include the 3' Igh enhancers, which are located downstream of the Calpha gene(s) in both mouse and human. In vivo experiments have implicated murine 3' enhancers, hs3B and/or hs4, in class switching and somatic hypermutation. We previously reported that murine hs4 was regulated by NF-kappaB, octamer binding proteins, and Pax5 (B cell-specific activator protein). In this study we report that human hs4 is regulated differently. EMSAs and Western analysis of normal B cells before and after stimulation with anti-IgM plus anti-CD40 showed the same complex binding pattern formed by NF-kappaB, Oct-1, and Oct-2 (but not by Pax5). A similar EMSA pattern was detected in mature human B cell lines (BL-2, Ramos, and HS-Sultan) and in diffuse large B cell lymphoma cell lines, although yin yang 1 protein (YY1) binding was also observed. We have confirmed the in vivo association of these transcription factors with hs4 in B cells by chromatin immunoprecipitation assays. The diffuse large B cell lymphoma cell lines had a distinctive slow-migrating complex containing YY1 associated with Rel-B. We have confirmed by endogenous coimmunoprecipitation an association of YY1 with Rel-B, but not with other NF-kappaB family members. Transient transfection assays showed robust hs4 enhancer activity in the mature B cell lines, which was dependent on synergistic interactions between NF-kappaB and octamer binding proteins. In addition, human hs4 enhancer activity required Oct-2 and correlated with expression of Oct coactivator from B cells (OCA-B).

Critical role of RelB serine 368 for dimerization and p100 stabilization

In mature B cells RelB-containing complexes are constitutively present in the nucleus, and they are less susceptible to inhibitory kappaB proteins. In most other cell types inhibitory kappaB proteins prevent nuclear translocation and activation of NFkappaB. We reasoned that this characteristic might be because of post-translational modifications of RelB. In Drosophila, signal-dependent phosphorylation of the Rel homologue Dorsal at serine 317 has been shown to be critical for nuclear import. The evolutionary conservation of this serine prompted us to analyze the function of the corresponding site in RelB. As a model system we used the murine S107 plasmacytoma cell line, which lacks endogenous RelB expression. Analysis of S107 cells expressing wild type RelB and serine 368 mutants reveals that serine 368 is not required for nuclear import but that it is critical for RelB dimerization with other members of the NFkappaB family. Similar effects were obtained when the conserved serine in RelA was mutated. We further demonstrate that expression of functional RelB, but not of serine 368 mutants, severely reduces p52 generation and strongly increases expression of the p52 precursor, p100. Wild type RelB, but not mutant RelB, prolonged p100 half-life. We therefore suggest an inhibitory effect of RelB on p100 processing, which is possibly regulated in a signal-dependent manner.

TRAIL-induced apoptosis and gene induction in HaCaT keratinocytes: differential contribution of TRAIL receptors 1 and 2

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) exerts a potent cytotoxic activity especially against many tumor cell types such as transformed keratinocytes. The specific role of the different TRAIL receptors in this process, however, is unknown. In this report we examine the role the TRAIL receptors play in both the apoptotic and nonapoptotic responses of HaCaT keratinocytes to leucine zipper TRAIL (LZ-TRAIL). By employing receptor-specific blocking antibodies we demonstrate that TRAIL receptor 1 plays the primary role in mediating caspase activation and apoptosis in HaCaT cells. Furthermore, we show that this receptor mainly mediates nuclear factor kappaB activation and expression of the pro-inflammatory cytokine interleukin-8 and that nuclear factor kappaB activation is critically required for the induction of pro-inflammatory cytokines in response to LZ-TRAIL. Taken together, our data suggest that beside its potent pro-apoptotic role, LZ-TRAIL leads to pro-inflammatory responses that are mainly mediated by TRAIL receptor 1 in HaCaT keratinocytes.

RelB forms transcriptionally inactive complexes with RelA/p65

RelB is an unusual member of the NF-kappaB transcription factor family that acts as both a transcriptional activator as well as a repressor of NF-kappaB-dependent gene expression. Although RelB promotes gene expression when it associates with p50/NF-kappaB1 or p52/NF-kappaB2, the precise molecular mechanisms through which it represses NF-kappaB remain unclear. To examine this inhibitory function in more detail, we employed reporter gene assays and found that RelB represses at the level of RelA. Furthermore, electrophoretic mobility shift analysis revealed that in vitro translated RelB impaired the DNA binding activity of RelA and that overexpressed RelB significantly reduced tumor necrosis factor-alpha-induced RelA activity in murine embryonic fibroblasts. Intriguingly, this inhibitory effect was due to the formation of RelA.RelB heterodimers that were unable to bind to kappaB sites in vitro strongly suggesting that these newly described NF-kappaB dimers cannot bind DNA. Expression pattern analysis revealed that RelA.RelB heterodimers appeared at relatively low levels in both lymphoid and non-lymphoid cells. However, the presence of these complexes increased following stimulation with phorbolesters or lipopolysaccharide or by overexpression of constitutively active IKKbeta. Functional characterization of RelA.RelB heterodimers in NIH3T3 murine embryonic fibroblasts revealed that they are not regulated by IkappaB proteins and are located in both the cytoplasm and the nucleus. Taken together, our findings demonstrate that sequestration of RelA in transcriptionally inactive RelA.RelB complexes provides a molecular mechanism that may explain the repressive role of RelB on NF-kappaB-dependent gene expression.

Proteasome inhibition results in TRAIL sensitization of primary keratinocytes by removing the resistance-mediating block of effector caspase maturation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exerts potent cytotoxic activity against transformed keratinocytes, whereas primary keratinocytes are relatively resistant. In several cell types, inhibition of the proteasome sensitizes for TRAIL-induced apoptosis by interference with NF-kappaB activation. Here we describe a novel intracellular mechanism of TRAIL resistance in primary cells and how this resistance is removed by proteasome inhibitors independent of NF-kappaB in primary human keratinocytes. This sensitization was not mediated at the receptor-proximal level of TRAIL DISC formation or caspase 8 activation but further downstream. Activation of caspase 3 was critical, as it only occurred when mitochondrial apoptotic pathways were activated, as reflected by Smac/DIABLO, HtrA2, and cytochrome c release. Smac/DIABLO and HtrA2 are needed to release the X-linked inhibitor-of-apoptosis protein (XIAP)-mediated block of full caspase 3 maturation. XIAP can effectively block caspase 3 maturation and, intriguingly, is highly expressed in primary but not in transformed keratinocytes. Ectopic XIAP expression in transformed keratinocytes resulted in increased resistance to TRAIL. Our data suggest that breaking of this resistance via proteasome inhibitors, which are potential anticancer drugs, may sensitize certain primary cells to TRAIL-induced apoptosis and could thereby complicate the clinical applicability of a combination of TRAIL receptor agonists with proteasome inhibitors.

Nitric oxide induces degradation of the neutral ceramidase in rat renal mesangial cells and is counterregulated by protein kinase C

Ceramide levels are strongly increased by stimulation of renal mesangial cells with nitric oxide (NO). This effect was shown previously to be due to a dual action of NO, comprising an activation of sphingomyelinases and an inhibition of ceramidase activity. In this study we show that the NO-triggered inhibition of neutral ceramidase activity is paralleled by a down-regulation at the protein level. A complete loss of neutral ceramidase protein is obtained after 24 h of stimulation. Whereas the selective proteasome inhibitor lactacystin blocked NO-evoked ceramidase degradation, several caspase inhibitors were ineffective. Moreover, the NO-induced degradation is reversed by the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), and also by the physiological PKC activators platelet-derived growth factor-BB (PDGF), angiotensin II and ATP, resulting in a normalization of neutral ceramidase protein as well as activity. In vivo phosphorylation studies using (32)P(i)-labeled mesangial cells revealed that TPA, PDGF, angiotensin II, and ATP trigger an increased phosphorylation of the neutral ceramidase, which is blocked by the broad spectrum PKC inhibitor Ro-31 8220 but not by CGP 41251, which has a preferential action on Ca(2+)-dependent isoforms, thus suggesting the involvement of a Ca(2+)-independent PKC isoform. In vitro phosphorylation assays using recombinant PKC isoenzymes and neutral ceramidase immunoprecipitated from unstimulated mesangial cells show that particularly the PKC-delta isoform and to a lesser extent the PKC-alpha isoform are efficient in directly phosphorylating neutral ceramidase. In summary, our data show that NO is able to induce degradation of neutral ceramidase, thereby promoting accumulation of ceramide in the cell. This effect is reversed by PKC activation, most probably by the PKC-delta isoenzyme, which can directly phosphorylate and thereby prevent neutral ceramidase degradation. These novel regulatory interactions will provide therapeutically valuable information to target neutral ceramidase stability and subsequent ceramide accumulation.

A ubiquitin-proteasome pathway represses the Drosophila immune deficiency signaling cascade

BACKGROUND

The inducible production of antimicrobial peptides is a major immune response in Drosophila. The genes encoding these peptides are activated by NF-kappaB transcription factors that are controlled by two independent signaling cascades: the Toll pathway that regulates the NF-kappaB homologs, Dorsal and DIF; and the IMD pathway that regulates the compound NF-kappaB-like protein, Relish. Although numerous components of each pathway that are required to induce antimicrobial gene expression have been identified, less is known about the mechanisms that either repress antimicrobial genes in the absence of infection or that downregulate these genes after infection.

RESULTS

In a screen for factors that negatively regulate the IMD pathway, we isolated two partial loss-of-function mutations in the SkpA gene that constitutively induce the antibacterial peptide gene, Diptericin, a target of the IMD pathway. These mutations do not affect the systemic expression of the antifungal peptide gene, Drosomycin, a target of the Toll pathway. SkpA encodes a homolog of the yeast and human Skp1 proteins. Skp1 proteins function as subunits of SCF-E3 ubiquitin ligases that target substrates to the 26S proteasome, and mutations affecting either the Drosophila SCF components, Slimb and dCullin1, or the proteasome also induce Diptericin expression. In cultured cells, inhibition of SkpA and Slimb via RNAi increases levels of both the full-length Relish protein and the processed Rel-homology domain.

CONCLUSIONS

In contrast to other NF-kappaB activation pathways, the Drosophila IMD pathway is repressed by the ubiquitin-proteasome system. A possible target of this proteolytic activity is the Relish transcription factor, suggesting a mechanism for NF-kappaB downregulation in Drosophila.