Inducible Activation of RelB in Fibroblasts

Targeting signaling factors for degradation, an emerging mechanism for TRAF functions

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) form a family of proteins that are best known as signaling adapters of TNFRs. However, emerging evidence suggests that TRAF proteins, particularly TRAF2 and TRAF3, also regulate signal transduction by controlling the fate of intracellular signaling factors. A well-recognized function of TRAF2 and TRAF3 in this aspect is to mediate ubiquitin-dependent degradation of nuclear factor-κB (NF-κB)-inducing kinase (NIK), an action required for the control of NIK-regulated non-canonical NF-κB signaling pathway. TRAF2 and TRAF3 form a complex with the E3 ubiquitin ligase cIAP (cIAP1 or cIAP2), in which TRAF3 serves as the NIK-binding adapter. Recent evidence suggests that the cIAP-TRAF2-TRAF3 E3 complex also targets additional signaling factors for ubiquitin-dependent degradation, thereby regulating important aspects of immune and inflammatory responses. This review provides both historical aspects and new insights into the signaling functions of this ubiquitination system.

Decreased expression of the NF-κB family member RelB in lung fibroblasts from Smokers with and without COPD potentiates cigarette smoke-induced COX-2 expression

BACKGROUND

Heightened inflammation, including expression of COX-2, is associated with COPD pathogenesis. RelB is an NF-κB family member that attenuates COX-2 in response to cigarette smoke by a mechanism that may involve the miRNA miR-146a. There is no information on the expression of RelB in COPD or if RelB prevents COX-2 expression through miR-146a.

METHODS

RelB, Cox-2 and miR-146a levels were evaluated in lung fibroblasts and blood samples derived from non-smokers (Normal) and smokers (At Risk) with and without COPD by qRT-PCR. RelB and COX-2 protein levels were evaluated by western blot. Human lung fibroblasts from Normal subjects and smokers with and without COPD, along with RelB knock-down (siRNA) in Normal cells, were exposed to cigarette smoke extract (CSE) in vitro and COX-2 mRNA/protein and miR-146a levels assessed.

RESULTS

Basal expression of RelB mRNA and protein were significantly lower in lung cells derived from smokers with and without COPD, the latter of which expressed more Cox-2 mRNA and protein in response to CSE. Knock-down of RelB in Normal fibroblasts increased Cox-2 mRNA and protein induction by CSE. Basal miR-146a levels were not different between the three groups, and only Normal fibroblasts increased miR-146a expression in response to smoke. There was a positive correlation between systemic RelB and Cox-2 mRNA levels and circulating miR-146a levels were higher only in GOLD stage I subjects.

CONCLUSIONS

Our data indicate that RelB attenuates COX-2 expression in lung structural cells, such that loss of pulmonary RelB may be an important determinant in the aberrant, heightened inflammation associated with COPD pathogenesis.

IKK phosphorylates RelB to modulate its promoter specificity and promote fibroblast migration downstream of TNF receptors

TNFα is a potent cytokine that plays a critical role in numerous cellular processes, particularly immune and inflammatory responses, programmed cell death, angiogenesis, and cell migration. Thus, understanding the molecular mechanisms that mediate TNFα-induced cellular responses is a crucial issue. It is generally accepted that global DNA binding activity of the NF-κB avian reticuloendotheliosis viral (v-rel) oncogene related B (RelB) subunit is not induced upon TNFα treatment in fibroblasts, despite its TNFα-induced nuclear accumulation. Here, we demonstrate that RelB plays a critical role in promoting fibroblast migration upon prolonged TNFα treatment. We identified the two kinases IκB kinase α (IKKα) and IκB kinase β (IKKβ) as RelB interacting partners whose activation by TNFα promotes RelB phosphorylation at serine 472. Once phosphorylated on serine 472, nuclear RelB dissociates from its interaction with the inhibitory protein IκBα and binds to the promoter of critical migration-associated genes, such as the matrix metallopeptidase 3 (MMP3). Further, we show that RelB serine 472 phosphorylation status controls MMP3 expression and promigration activity downstream of TNF receptors. Our findings provide new insights into the regulation of RelB activity and reveal a novel link between selective NF-κB target gene expression and cellular response in response to TNFα.

Regulation of naive T cell function by the NF-kappaB2 pathway

T cell activation involves the orchestration of several signaling pathways, including that of the 'classical' transcription factor NF-kappaB components NF-kappaB1-RelA. The function of the 'nonclassical' NF-kappaB2-RelB pathway is less clear, although T cells lacking components of this pathway have activation defects. Here we show that mice deficient in NF-kappaB-inducing kinase have a complex phenotype consisting of immunosuppression mediated by CD25(-)Foxp3(-) memory CD4(+) cells and, in the absence of those cells, hyper-responsive naive CD4(+) T cells, which caused autoimmune lesions after adoptive transfer into hosts deficient in recombination-activating genes. Biochemical studies indicated involvement of a cell-intrinsic mechanism in which NF-kappaB2 (p100) limits nuclear translocation of NF-kappaB1-RelA and thereby functions as a regulatory 'brake' for the activation of naive T cells.

RelB regulates human dendritic cell subset development by promoting monocyte intermediates

In humans, epithelial Langerhans cells (LCs) and monocyte-derived/interstitial dendritic cells (DCs) constitute 2 myeloid DC sublineages. Molecular mechanisms involved in their development from common myeloid progenitors remain poorly defined. Here we demonstrate that the nuclear factor-κB (NF-κB) transcription factor RelB regulates the generation of monocytic CD14+CD11b+ precursors of interstitial DCs from human hematopoietic progenitors. RelB overexpression promoted, whereas endogenous RelB inhibition (by p100ΔN) blocked, precursor cell development along this DC subset pathway. RelB inhibition specifically arrested precursor progression from CD14loCD11b- to CD14+CD11b+ stages. Precursors were still capable of LC and granulocyte differentiation but were defective in macrophage–colony-stimulating factor (M-CSF)–dependent monocyte/macrophage differentiation. RelB inhibition markedly differed from classical NF-κB signaling inhibition because IκBα superrepressor (IκBα-SR), but not p100ΔN, impaired LC/DC differentiation, DC adhesion, and progenitor cell proliferation. Although RelB up-regulation and nuclear translocation are regarded as hallmarks of human myeloid DC maturation, ectopic RelB overexpression failed to promote DC maturation. Our results suggest that RelB regulates human monopoiesis and monocyte-derived DC subset development.

RelB/p50 dimers are differentially regulated by tumor necrosis factor-alpha and lymphotoxin-beta receptor activation: critical roles for p100

Tumor necrosis factor-alpha (TNF-alpha) and lymphotoxin-beta receptor (LTbetaR) signaling both play important roles in inflammatory and immune responses through activation of NF-kappaB. Using various deficient mouse embryonic fibroblast cells, we have compared the signaling pathways leading to NF-kappaB induction in response to TNF-alpha and LTbetaR activation. We demonstrate that LTbetaR ligation induces not only RelA/p50 dimers but also RelB/p50 dimers, whereas TNF-alpha induces only RelA/p50 dimers. LTbetaR-induced binding of RelB/p50 requires processing of p100 that is mediated by IKKalpha but is independent of IKKbeta, NEMO/IKKgamma, and RelA. Moreover, we show that RelB, p50, and p100 can associate in the same complex and that TNF-alpha but not LTbeta signaling increases the association of p100 with RelB/p50 dimers in the nucleus, leading to the specific inhibition of RelB DNA binding. These results suggest that the alternative NF-kappaB pathway based on p100 processing may account not only for the activation of RelB/p52 dimers but also for that of RelB/p50 dimers and that p100 regulates the binding activity of RelB/p50 dimers via at least two distinct mechanisms depending on the signaling pathway involved.

RelB cellular regulation and transcriptional activity are regulated by p100

RelB mediates the constitutive nuclear pool of NF-kappaB transcriptional activity in myeloid and lymphoid cells, which is believed to be secondary to its weak interaction with the classical NF-kappaB inhibitor proteins, the IkappaBs. In other cell types, RelB is located in the cytosol, thus suggesting that RelB is also regulated by an inhibitory protein(s). In this study, it is demonstrated that RelB is associated in the cytosol with p100 but not with IkappaBalpha, IkappaBbeta, IkappaBepsilon, nor p105. Its cytosolic control is not affected by stimuli that lead to RelA nuclear translocation, and RelB nuclear localization is prevented by p100, but not by p105 or IkappaBalpha. Structure function analysis p100-RelB interactions indicates that p100 amino acids 623-900 are required for effective interaction and repression of nuclear translocation and RelB driven NF-kappaB-dependent transcription. Moreover, this carboxyl-portion of p100 contains a nuclear export signal(s), which is required for effective retrieval of RelB from the nucleus. Finally, overexpression of NF-kappaB-inducing kinase, a kinase that has recently been shown to induce p100 processing, possibly through IKKalpha activation, causes nuclear translocation of RelB protein. Thus, these studies indicate that p100 is a bone fide inhibitor of RelB and that this transcription factor may be regulated by NF-kappaB-inducing kinase and/or IKKalpha.

Transcription of the RelB gene is regulated by NF-kappaB

RelA and RelB are two members of the NF-kappaB family that differ structurally and functionally. While RelA is regulated through its cytosolic localization by inhibitor proteins or IkappaB and not through transcriptional mechanisms, the regulation of RelB is poorly understood. In this study we demonstrate that stimuli (TNF or LPS) lead within minutes to the nuclear translocation of RelA, but require hours to result in the nuclear translocation of RelB. The delayed nuclear translocation of RelB correlates with increases in its protein synthesis which are secondary to increases in RelB gene transcription. RelA is alone sufficient to induce RelB gene transcription and to mediate the stimuli-driven increase in RelB transcription. Cloning and characterization of the RelB 5' untranslated gene region indicates that RelB transcription is dependent on a TATA-less promoter containing two NF-kappaB binding sites. One of the NF-kappaB sites is primarily involved in the binding of p50 while the other one in the binding and transactivation by RelA and also RelB. Lastly, it is observed that p21, a protein involved in cell cycle control and oncogenesis known to be regulated by NF-kappaB, is upregulated at the transcriptional level by RelB. Thus, RelB is regulated at least at the level of transcription in a RelA and RelB dependent manner and may exert an important role in p21 regulation.

Hypoxia induces expression of the chemokines monocyte chemoattractant protein-1 (MCP-1) and IL-8 in human dermal fibroblasts

Hypoxia is an important factor in the pathophysiology of vascular and inflammatory diseases. Leucocyte infiltration, as a consequence of adhesion molecule up-regulation and chemokine release, is a prominent feature of these diseases. The objective of our study was to investigate the potential role of resident fibroblasts in hypoxia-induced chemotactic responses. We show that MCP-1 and IL-8 mRNA are specifically induced by hypoxia in dermal fibroblasts. This response is paralleled by increased NF-kappaB p65/p50 binding activity, and it is inhibited by pretreatment with N-acetyl-L-cysteine. MCP-1 secreted by fibroblasts is chemotactic for monocytic cells and this activity is significantly increased by hypoxia. Chemotactic index correlates with MCP-1 protein levels and is significantly decreased by neutralizing anti-MCP-1 MoAb. These findings demonstrate the ability of resident fibroblasts to mediate chemotaxis of leucocytes through the release of chemokines in response to hypoxia. Our data point to MCP-1 as an important component in this response, and therefore it may be a potential target in inflammatory responses associated with hypoxia.

Distribution of IkappaB proteins in gastric mucosa and other organs of mouse and gerbil

The NF-kappaB/IkappaB complex is a major transcription regulator of inflammatory and immune responses. Helicobacter pylori infection causes chronic inflammation in gastric mucosa by inducing dissociation of the inhibitory IkappaB protein from the complex with a resulting increased expression of interleukin (IL)-8. To clarify which of several known IkappaB proteins could be involved in this inflammatory response, we undertook immunohistochemical examination of normal mouse stomach as well as other murine tissues for comparison, using polyclonal antibodies specific for alpha-, beta-, gamma-, and in-isoforms of IkappaB. The results showed strong immunoreactivity for the alpha-isoform in parietal cells and for the beta-isoform in pit cells of the stomach, along with the presence of these proteins in various other sites. Comparative staining revealed a similar but not identical distribution of IkappaB proteins in the Mongolian gerbil, a rodent model for H. pylori infection. The findings suggest that the alpha- and beta-isoforms are dominant IkappaB proteins in gastric parietal and foveolar cells, respectively, and point to a role for these transcription regulators in modulating pathological responses in stomach and other organs. (J Histochem Cytochem 48:191-199, 2000)

Tumor necrosis factor-alpha-induced proliferation of human Mo7e leukemic cells occurs via activation of nuclear factor kappaB transcription factor

Tumor necrosis factor-alpha (TNF-alpha) stimulates proliferation of Mo7e, CMK, HU-3, and M-MOK human leukemic cell lines. We report here the signal transduction pathway involved in TNF-alpha-induced Mo7e cell proliferation. Mo7e cells spontaneously die in the absence of growth factors, but treating the cells with interleukin (IL)-3, IL-6, thrombopoietin, granulocyte/macrophage colony-stimulating factor, or TNF-alpha promotes their survival and proliferation. Although most of these factors activate MAP kinase and Jun NH2-terminal kinase/signal transducer and activators of transcription signaling pathways, TNF-alpha fails to activate either pathway. When Mo7e cells were treated with TNF-alpha, nuclear factor kappaB (NF-kappaB) was activated transiently. The activated NF-kappaB consisted of heterodimers of p65 and p50 subunits. The degradation of IkappaBalpha coincided with activation of NF-kappaB in TNF-alpha-treated cells. To investigate the role of activated NF-kappaB in TNF-alpha-induced Mo7e proliferation, a cell-permeable peptide (SN50) carrying the nuclear localization sequence of p50 NF-kappaB was used to block nuclear translocation of activated NF-kappaB. Pretreating Mo7e cells with SN50 blocked TNF-alpha-induced nuclear translocation of NF-kappaB and inhibited TNF-alpha-induced Mo7e cell survival and proliferation. A mutant SN50 peptide did not affect TNF-alpha-induced Mo7e cell growth. SN50 had no effects on IL-3- or granulocyte/macrophage colony-stimulating factor-induced Mo7e cell proliferation. The results indicate that activation of NF-kappaB is involved in TNF-alpha-induced Mo7e cell survival and proliferation.

Evidence for Distinct Intracellular Signaling Pathways in CD34+ Progenitor to Dendritic Cell Differentiation from a Human Cell Line Model

Intracellular signals that mediate differentiation of pluripotent hemopoietic progenitors to dendritic cells (DC) are largely undefined. We have previously shown that protein kinase C (PKC) activation (with phorbol ester (PMA) alone) specifically induces differentiation of primary human CD34+ hemopoietic progenitor cells (HPC) to mature DC. We now find that cytokine-driven (granulocyte-macrophage CSF and TNF-α) CD34+ HPC→DC differentiation is preferentially blocked by inhibitors of PKC activation. To further identify intracellular signals and downstream events important in CD34+ HPC→DC differentiation we have characterized a human leukemic cell line model of this process. The CD34+ myelomonocytic cell line KG1 differentiates into dendritic-like cells in response to granulocyte-macrophage CSF plus TNF-α, or PMA (with or without the calcium ionophore ionomycin, or TNF-α), with different stimuli mediating different aspects of the process. Phenotypic DC characteristics of KG1 dendritic-like cells include morphology (loosely adherent cells with long neurite processes), MHC I+/MHC IIbright/CD83+/CD86+/CD14− surface Ag expression, and RelB and DC-CK1 gene expression. Functional DC characteristics include fluid phase macromolecule uptake (FITC-dextran) and activation of resting T cells. Comparison of KG1 to the PMA-unresponsive subline KG1a reveals differences in expression of TNF receptors 1 and 2; PKC isoforms α, βI, βII, and μ; and RelB, suggesting that these components/pathways are important for DC differentiation. Together, these findings demonstrate that cytokine or phorbol ester stimulation of KG1 is a model of human CD34+ HPC to DC differentiation and suggest that specific intracellular signaling pathways mediate specific events in DC lineage commitment.

Phorbol Esters Induce Differentiation of Human CD34+ Hemopoietic Progenitors to Dendritic Cells: Evidence for Protein Kinase C-Mediated Signaling

The intracellular signals that mediate the differentiation of pluripotent hemopoietic progenitors to dendritic cells (DC) are largely undefined. We have found that the phorbol ester PMA by itself induced 47% ± 8.7% of input human CD34+ hemopoietic progenitors to differentiate into cells with morphology and surface Ag phenotype characteristic of DC by day 7 of culture. Functionally, PMA-generated DC processed and presented whole soluble Ag and also induced resting T cell proliferation and Ag-specific CTL effector function. Unlike cytokine-driven DC differentiation, PMA suppressed proliferation and induced cell death (in part via apoptosis) in cells that did not differentiate to DC. The effects of PMA were blocked by inhibitors of protein kinase C activation, suggesting a central role for this signaling molecule. PMA-mediated signaling also induced expression of the RelB transcription factor, an NF-κB family member implicated in DC differentiation. These findings suggest that phorbol esters activate protein kinase C, which then initiates the terminal component of an intracellular signaling pathway(s) involved in the DC differentiation of CD34+ hemopoietic progenitors.