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

cFLIP in the molecular regulation of astroglia-driven neuroinflammation in experimental glaucoma

BACKGROUND

Recent experimental studies of neuroinflammation in glaucoma pointed to cFLIP as a molecular switch for cell fate decisions, mainly regulating cell type-specific caspase-8 functions in cell death and inflammation. This study aimed to determine the importance of cFLIP for regulating astroglia-driven neuroinflammation in experimental glaucoma by analyzing the outcomes of astroglia-targeted transgenic deletion of cFLIP or cFLIPL.

METHODS

Glaucoma was modeled by anterior chamber microbead injections to induce ocular hypertension in mouse lines with or without conditional deletion of cFLIP or cFLIPL in astroglia. Morphological analysis of astroglia responses assessed quantitative parameters in retinal whole mounts immunolabeled for GFAP and inflammatory molecules or assayed for TUNEL. The molecular analysis included 36-plexed immunoassays of the retina and optic nerve cytokines and chemokines, NanoString-based profiling of inflammation-related gene expression, and Western blot analysis of selected proteins in freshly isolated samples of astroglia.

RESULTS

Immunoassays and immunolabeling of retina and optic nerve tissues presented reduced production of various proinflammatory cytokines, including TNFα, in GFAP/cFLIP and GFAP/cFLIPL relative to controls at 12 weeks of ocular hypertension with no detectable alteration in TUNEL. Besides presenting a similar trend of the proinflammatory versus anti-inflammatory molecules displayed by immunoassays, NanoString-based molecular profiling detected downregulated NF-κB/RelA and upregulated RelB expression of astroglia in ocular hypertensive samples of GFAP/cFLIP compared to ocular hypertensive controls. Analysis of protein expression also revealed decreased phospho-RelA and increased phospho-RelB in parallel with an increase in caspase-8 cleavage products.

CONCLUSIONS

A prominent response limiting neuroinflammation in ocular hypertensive eyes with cFLIP-deletion in astroglia values the role of cFLIP in the molecular regulation of glia-driven neuroinflammation during glaucomatous neurodegeneration. The molecular responses accompanying the lessening of neurodegenerative inflammation also seem to maintain astroglia survival despite increased caspase-8 cleavage with cFLIP deletion. A transcriptional autoregulatory response, dampening RelA but boosting RelB for selective expression of NF-κB target genes, might reinforce cell survival in cFLIP-deleted astroglia.

Inflammation in Hypervolemic Hemodialysis Patients: The Roles of RelB and Caspase-4

Hypervolemia is associated with inflammation in hemodialysis (HD) patients. How hypervolemia triggers inflammation is not entirely known. We initiated a cross-sectional study enrolling 40 hemodialysis patients who were categorized into normovolemic (N; 23) and hypervolemic (H; 17) groups by bioimpedance measurement. A caspase activity assay in combination with a specific caspase-4 inhibitor was used to detect caspase-4 activity in isolated peripheral blood mononuclear cells (PBMCs). Transcription factors RelA (pS529) and RelB (pS552) were analyzed by phospho-flow cytometry. Serum endotoxins were detected by an amebocyte lysate-based assay, and IL-6 (interleukin-6) and TNF-α (Tumor necrosis factor-α) gene expression were detected using the ELISA technique. Hypervolemic patients were older, more frequently had diabetes and showed increased CRP and IL-6 levels. Caspase-4 activity, which is linked to intracellular endotoxin detection, was significantly elevated in H patients. While the frequency of RelA-expressing immune cells and the expression density in these cells did not differ, the monocytic frequency of cells positively stained for RelB (pS552) was significantly decreased in H patients. Increased caspase-4 activity in H patients may indicate a cause of inflammation in H patients. The post-translational modification of RelB (pS552) is linked to downregulation of NF-kB activity and may indicate the resolution of inflammation, which is more distinct in N patients compared to H patients. Therefore, both higher inflammatory loads and lower inflammatory resolution capacities are characteristics of H patients.

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.

First Insight into the Modulation of Noncanonical NF-κB Signaling Components by Poxviruses in Established Immune-Derived Cell Lines: An In Vitro Model of Ectromelia Virus Infection

Dendritic cells (DCs) and macrophages are the first line of antiviral immunity. Viral pathogens exploit these cell populations for their efficient replication and dissemination via the modulation of intracellular signaling pathways. Disruption of the noncanonical nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) signaling has frequently been observed in lymphoid cells upon infection with oncogenic viruses. However, several nononcogenic viruses have been shown to manipulate the noncanonical NF-κB signaling in different cell types. This study demonstrates the modulating effect of ectromelia virus (ECTV) on the components of the noncanonical NF-κB signaling pathway in established murine cell lines: JAWS II DCs and RAW 264.7 macrophages. ECTV affected the activation of TRAF2, cIAP1, RelB, and p100 upon cell treatment with both canonical and noncanonical NF-κB stimuli and thus impeded DNA binding by RelB and p52. ECTV also inhibited the expression of numerous genes related to the noncanonical NF-κB pathway and RelB-dependent gene expression in the cells treated with canonical and noncanonical NF-κB activators. Thus, our data strongly suggest that ECTV influenced the noncanonical NF-κB signaling components in the in vitro models. These findings provide new insights into the noncanonical NF-κB signaling components and their manipulation by poxviruses in vitro.

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.

RelB regulates basal and proinflammatory induction of conjunctival CCL2

Purpose: This study investigated the involvement of NF-kB in regulating postoperative conjunctival inflammation.Methods: Experimental surgery was performed as described for the mouse model of conjunctival scarring. Expression of NF-κB in postoperative conjunctival tissues or conjunctival fibroblasts were assessed by real-time PCR, immunoblotting and immunofluorescence analyses. Downregulation of RelB was achieved using small interfering RNA. Cellular cytokine secretion was determined using multiplex cytokine assay.Results: RelB was the most highly induced member of the NF-kB family on day 2 post-surgery. Elevated RelB may be found associated with vimentin-positive cells and fibroblasts in vivo and in vitro. In conjunctival fibroblasts, RelB may be induced by TNF-α but not TGF-β2 while its silencing caused selective induction of CCL2 secretion by both basal and TNF-α-stimulated fibroblasts.Conclusions: High RelB induction in the inflammatory phase and the selective modulation of CCL2 suggest a specific anti-inflammatory role for RelB in the postoperative conjunctiva.

Pulmonary neutrophilia caused by absence of the NF-κB member RelB is dampened by exposure to cigarette smoke

Inflammation is a response to injury and infection. Although protective under physiological conditions, excessive and persistent inflammation is linked to numerous diseases. As the lungs are continuously exposed to the external environment, the respiratory system is particularly liable to damage from inflammation. RelB is a member of the non-canonical NF-κB pathway that may control lung inflammation caused by cigarette smoke (CS), a leading cause of morbidity and mortality worldwide. Our lab has previously shown that RelB protects against CS-induced inflammation in vitro, leading us to hypothesize that RelB would protect against acute CS-induced pulmonary inflammation in vivo. We exposed wild-type (Relb+/+) and RelB-deficient mice (Relb-/-) mice to room air or to CS and found that CS exposure caused a sustained decrease in pulmonary granulocytes in Relb-/- mice that was predominated by a decrease in neutrophils. Pulmonary inflammation caused by other irritants, including chlorine, ovalbumin (OVA; to mimic features of asthma) and lipopolysaccharide (LPS) was not controlled by RelB. Differential cytokine analysis suggests that alterations in chemotactic cytokines do not fully account for the CS-specific decrease in neutrophils in Relb-/- mice. Flow cytometric analysis of the bronchoalveolar lavage and bone marrow cells also reveal that it is unlikely that the sustained decrease is caused by excessive cell death or decreased hematopoiesis from the bone marrow. Overall, our results indicate that RelB regulates acute CS-induced pulmonary inflammation. Understanding how RelB regulates CS-induced inflammation may potentiate the discovery of new therapeutic strategies for many of the inflammatory diseases caused by CS.

PAK4 suppresses RELB to prevent senescence-like growth arrest in breast cancer

Overcoming cellular growth restriction, including the evasion of cellular senescence, is a hallmark of cancer. We report that PAK4 is overexpressed in all human breast cancer subtypes and associated with poor patient outcome. In mice, MMTV-PAK4 overexpression promotes spontaneous mammary cancer, while PAK4 gene depletion delays MMTV-PyMT driven tumors. Importantly, PAK4 prevents senescence-like growth arrest in breast cancer cells in vitro, in vivo and ex vivo, but is not needed in non-immortalized cells, while PAK4 overexpression in untransformed human mammary epithelial cells abrogates H-RAS-V12-induced senescence. Mechanistically, a PAK4 – RELB - C/EBPβ axis controls the senescence-like growth arrest and a PAK4 phosphorylation residue (RELB-Ser151) is critical for RELB-DNA interaction, transcriptional activity and expression of the senescence regulator C/EBPβ. These findings establish PAK4 as a promoter of breast cancer that can overcome oncogene-induced senescence and reveal a selective vulnerability of cancer to PAK4 inhibition.

Oncogene induced senescence protects cells from unrestricted growth and cancer. Here, the authors show that PAK4 overrides this senescence in breast cancer cells through phosphorylation of RELB, thereby inhibiting transcription of the senescence regulator C/EBPβ.

RelB controls adaptive responses of astrocytes during sterile inflammation

In response to brain injury or infections, astrocytes become reactive, undergo striking morphological and functional changes, and secrete and respond to a spectrum of inflammatory mediators. We asked whether reactive astrocytes also display adaptive responses during sterile IL-1β-induced neuroinflammation, which may limit tissue injury associated with many disorders of the central nervous system. We found that astrocytes display days-to-weeks long specific tolerance of cytokine genes, which is coordinated by NF-κB family member, RelB. However, in contrast to innate immune cells, astrocytic tolerance does not involve epigenetic silencing of the cytokine genes. Establishment of tolerance depends on persistent higher levels of RelB in tolerant astrocytes and its phosphorylation on serine 472. Mechanistically, this phosphorylation prevents efficient removal of RelB from cytokine promoters by IκBα and helps to establish tolerance. Importantly, ablation of RelB from astrocytes in mice abolishes tolerance during experimental neuroinflammation in vivo.

NF-κB, IκB, and IKK: Integral Components of Immune System Signaling

The NF-κB (Nuclear Factor kappa B) transcription factor plays crucial roles in the regulation of numerous biological processes including development of the immune system, inflammation, and innate and adaptive immune responses. Control over the immune cell functions of NF-κB results from signaling through one of two different routes: the canonical and noncanonical NF-κB signaling pathways. Present at the end of both pathways are the proteins NF-κB, IκB, and the IκB kinase (IKK). These proteins work together to deliver the myriad outcomes that influence context-dependent transcriptional control in immune cells. In the present chapter, we review the structural information available on NF-κB, IκB, and IKK, the critical terminal components of the NF-κB signaling, in relation to their physiological function.

The Many Roles of Ubiquitin in NF-κB Signaling

The nuclear factor κB (NF-κB) signaling pathway ubiquitously controls cell growth and survival in basic conditions as well as rapid resetting of cellular functions following environment changes or pathogenic insults. Moreover, its deregulation is frequently observed during cell transformation, chronic inflammation or autoimmunity. Understanding how it is properly regulated therefore is a prerequisite to managing these adverse situations. Over the last years evidence has accumulated showing that ubiquitination is a key process in NF-κB activation and its resolution. Here, we examine the various functions of ubiquitin in NF-κB signaling and more specifically, how it controls signal transduction at the molecular level and impacts in vivo on NF-κB regulated cellular processes.

GSK3β modulates NF-κB activation and RelB degradation through site-specific phosphorylation of BCL10

Glycogen synthase kinase 3β (GSK3β) is a ubiquitously expressed serine/threonine kinase involved in the regulation of various cellular functions, such as energy homoeostasis, cell growth and developmental processes. More recently, GSK3β has been identified as a part of a protein complex involved in the regulation of the CARMA1-BCL10-MALT1 complex (CBM complex) formation, which is a key signalling event upon antigen receptor engagement of B and T cells, required for the activation of the NF-κB and JNK pathways. However, conflicting reports have been published regarding the role of GSK3β for the activation of the NF-κB signalling pathways. Therefore, we aimed to determine the impact of GSK3β on the NF-κB signalling induced upon T cell activation. Blocking GSK3β by either pharmacologic inhibitors (SB216763 and SB415286) or by RNAi caused a reduced proteolysis of the MALT1 targets CYLD1, BCL10 and RelB as well as diminished IκBα degradation, NF-κB DNA binding and NF-κB activity. This negative effect on NF-κB appears to be due to a diminished CBM complex formation caused by a reduced BCL10 phosphorylation. Taken together, we provide here evidence for a novel regulatory mechanism by which GSK3β affects NF-κB signalling in activated T cells.

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.

The Ubiquitination of NF-κB Subunits in the Control of Transcription

Nuclear factor (NF)-κB has evolved as a latent, inducible family of transcription factors fundamental in the control of the inflammatory response. The transcription of hundreds of genes involved in inflammation and immune homeostasis require NF-κB, necessitating the need for its strict control. The inducible ubiquitination and proteasomal degradation of the cytoplasmic inhibitor of κB (IκB) proteins promotes the nuclear translocation and transcriptional activity of NF-κB. More recently, an additional role for ubiquitination in the regulation of NF-κB activity has been identified. In this case, the ubiquitination and degradation of the NF-κB subunits themselves plays a critical role in the termination of NF-κB activity and the associated transcriptional response. While there is still much to discover, a number of NF-κB ubiquitin ligases and deubiquitinases have now been identified which coordinate to regulate the NF-κB transcriptional response. This review will focus the regulation of NF-κB subunits by ubiquitination, the key regulatory components and their impact on NF-κB directed transcription.

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.

NF-κB and androgen receptor variant expression correlate with human BPH progression

BACKGROUND

Benign prostatic hyperplasia (BPH) is a common, chronic progressive disease. Inflammation is associated with prostatic enlargement and resistance to 5α-reductase inhibitor (5ARI) therapy. Activation of the nuclear factor-kappa B (NF-κB) pathway is linked to both inflammation and ligand-independent prostate cancer progression.

METHODS

NF-κB activation and androgen receptor variant (AR-V) expression were quantified in transition zone tissue samples from patients with a wide range of AUASS from incidental BPH in patients treated for low grade, localized peripheral zone prostate cancer to advanced disease requiring surgical intervention. To further investigate these pathways, human prostatic stromal and epithelial cell lines were transduced with constitutively active or kinase dead forms of IKK2 to regulate canonical NF-κB activity. The effects on AR full length (AR-FL) and androgen-independent AR-V expression as well as cellular growth and differentiation were assessed.

RESULTS

Canonical NF-κB signaling was found to be upregulated in late versus early stage BPH, and to be strongly associated with non-insulin dependent diabetes mellitus. Elevated expression of AR-variant 7 (AR-V7), but not other AR variants, was found in advanced BPH samples. Expression of AR-V7 significantly correlated with the patient AUASS and TRUS volume. Forced activation of canonical NF-κB in human prostatic epithelial and stromal cells resulted in elevated expression of both AR-FL and AR-V7, with concomitant ligand-independent activation of AR reporters. Activation of NF-κB and over expression of AR-V7 in human prostatic epithelial cells maintained cell viability in the face of 5ARI treatment.

CONCLUSION

Activation of NF-κB and AR-V7 in the prostate is associated with increased disease severity. AR-V7 expression is inducible in human prostate cells by forced activation of NF-κB resulting in resistance to 5ARI treatment, suggesting a potential mechanism by which patients may become resistant to 5ARI therapy.

The Regulation of NF-κB Subunits by Phosphorylation

The NF-κB transcription factor is the master regulator of the inflammatory response and is essential for the homeostasis of the immune system. NF-κB regulates the transcription of genes that control inflammation, immune cell development, cell cycle, proliferation, and cell death. The fundamental role that NF-κB plays in key physiological processes makes it an important factor in determining health and disease. The importance of NF-κB in tissue homeostasis and immunity has frustrated therapeutic approaches aimed at inhibiting NF-κB activation. However, significant research efforts have revealed the crucial contribution of NF-κB phosphorylation to controlling NF-κB directed transactivation. Importantly, NF-κB phosphorylation controls transcription in a gene-specific manner, offering new opportunities to selectively target NF-κB for therapeutic benefit. This review will focus on the phosphorylation of the NF-κB subunits and the impact on NF-κB function.

The paracaspase MALT1: biological function and potential for therapeutic inhibition

The paracaspase MALT1 has a central role in the activation of lymphocytes and other immune cells including myeloid cells, mast cells and NK cells. MALT1 activity is required not only for the immune response, but also for the development of natural Treg cells that keep the immune response in check. Exaggerated MALT1 activity has been associated with the development of lymphoid malignancies, and recently developed MALT1 inhibitors show promising anti-tumor effects in xenograft models of diffuse large B cell lymphoma. In this review, we provide an overview of the present understanding of MALT1's function, and discuss possibilities for its therapeutic targeting based on recently developed inhibitors and animal models.

Transcriptional repression by the HDAC4-RelB-p52 complex regulates multiple myeloma survival and growth

Although transcriptional activation by NF-κB is well appreciated, physiological importance of transcriptional repression by NF-κB in cancer has remained elusive. Here we show that an HDAC4-RelB-p52 complex maintains repressive chromatin around proapoptotic genes Bim and BMF and regulates multiple myeloma (MM) survival and growth. Disruption of RelB-HDAC4 complex by a HDAC4-mimetic polypeptide blocks MM growth. RelB-p52 also represses BMF translation by regulating miR-221 expression. While the NIK-dependent activation of RelB-p52 in MM has been reported, we show that regardless of the activation status of NIK and the oncogenic events that cause plasma cell malignancy, several genetically diverse MM cells including Bortezomib-resistant MM cells are addicted to RelB-p52 for survival. Importantly, RelB is constitutively phosphorylated in MM and ERK1 is a RelB kinase. Phospho-RelB remains largely nuclear and is essential for Bim repression. Thus, ERK1-dependent regulation of nuclear RelB is critical for MM survival and explains the NIK-independent role of RelB in MM.

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.

Lipopolysaccharide directly stimulates Th17 differentiation in vitro modulating phosphorylation of RelB and NF-κB1

Toll-like receptors (TLRs) recognize a wide range of pathogen-associated molecular patterns (PAMP) and are preferentially expressed in innate immune cells. TLR-mediated activation of these cells activates the adaptive immune system. However, it has become clear that TLRs are not only expressed but also functionally active in CD4 T cells. The intestines are continuously exposed to TLR ligands, including lipopolysaccharide (LPS), a TLR4 ligand, and TLR4 is expressed higher in Th17 cells than Th1 and Th2 cells. In addition, development of Th17 cells in the gut mucosa is more dependent on gut microbiota than Th1, Th2, and Treg. Thus, we examined whether LPS directly regulates Th17 differentiation. LPS directly stimulated Th17 differentiation in vitro. In Th17 cells, LPS increased phosphorylation of NF-κB1, resulting in an increase of p50, the processed form of NF-κB1, whereas it decreased phosphorylation of RelB, leading to the up-regulation of RelB. Subcutaneous injection of LPS increased the frequency of IL-17 producing cells in inguinal lymph nodes, worsening experimental autoimmune encephalomyelitis (EAE). Additionally, expression of TLR1, TLR2, TLR4, and TLR5 was reduced upon T cell activation and LPS showed modest effect on TLR4 expression. These findings provide the first evidence that TLR4 activation directly regulate Th17 differentiation.

Breaking the mold: transcription factors in the anucleate platelet and platelet-derived microparticles

Platelets are small anucleate blood cells derived from megakaryocytes. In addition to their pivotal roles in hemostasis, platelets are the smallest, yet most abundant, immune cells and regulate inflammation, immunity, and disease progression. Although platelets lack DNA, and thus no functional transcriptional activities, they are nonetheless rich sources of RNAs, possess an intact spliceosome, and are thus capable of synthesizing proteins. Previously, it was thought that platelet RNAs and translational machinery were remnants from the megakaryocyte. We now know that the initial description of platelets as "cellular fragments" is an antiquated notion, as mounting evidence suggests otherwise. Therefore, it is reasonable to hypothesize that platelet transcription factors are not vestigial remnants from megakaryocytes, but have important, if only partly understood functions. Proteins play multiple cellular roles to minimize energy expenditure for maximum cellular function; thus, the same can be expected for transcription factors. In fact, numerous transcription factors have non-genomic roles, both in platelets and in nucleated cells. Our lab and others have discovered the presence and non-genomic roles of transcription factors in platelets, such as the nuclear factor kappa β (NFκB) family of proteins and peroxisome proliferator-activated receptor gamma (PPARγ). In addition to numerous roles in regulating platelet activation, functional transcription factors can be transferred to vascular and immune cells through platelet microparticles. This method of transcellular delivery of key immune molecules may be a vital mechanism by which platelet transcription factors regulate inflammation and immunity. At the very least, platelets are an ideal model cell to dissect out the non-genomic roles of transcription factors in nucleated cells. There is abundant evidence to suggest that transcription factors in platelets play key roles in regulating inflammatory and hemostatic functions.

Activation of apoptosis by caspase-3-dependent specific RelB cleavage in anticancer agent-treated cancer cells: involvement of positive feedback mechanism

DTCM-glutarimide (DTCM-G) is a newly found anti-inflammatory agent. In the course of experiments with lymphoma cells, we found that DTCM-G induced specific RelB cleavage. Anticancer agent vinblastine also induced the specific RelB cleavage in human fibrosarcoma HT1080 cells. The site-directed mutagenesis analysis revealed that the Asp205 site in RelB was specifically cleaved possibly by caspase-3 in vinblastine-treated HT1080 cells. Moreover, the cells stably overexpressing RelB Asp205Ala were resistant to vinblastine-induced apoptosis. Thus, the specific Asp205 cleavage of RelB by caspase-3 would be involved in the apoptosis induction by anticancer agents, which would provide the positive feedback mechanism.

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.

Monoubiquitination and activity of the paracaspase MALT1 requires glutamate 549 in the dimerization interface

The mucosa-associated lymphoid tissue protein-1 (MALT1, also known as paracaspase) is a protease whose activity is essential for the activation of lymphocytes and the growth of cells derived from human diffuse large B-cell lymphomas of the activated B-cell subtype (ABC DLBCL). Crystallographic approaches have shown that MALT1 can form dimers via its protease domain, but why dimerization is relevant for the biological activity of MALT1 remains largely unknown. Using a molecular modeling approach, we predicted Glu 549 (E549) to be localized within the MALT1 dimer interface and thus potentially relevant. Experimental mutation of this residue into alanine (E549A) led to a complete impairment of MALT1 proteolytic activity. This correlated with an impaired capacity of the mutant to form dimers of the protease domain in vitro, and a reduced capacity to promote NF-κB activation and transcription of the growth-promoting cytokine interleukin-2 in antigen receptor-stimulated lymphocytes. Moreover, this mutant could not rescue the growth of ABC DLBCL cell lines upon MALT1 silencing. Interestingly, the MALT1 mutant E549A was unable to undergo monoubiquitination, which we identified previously as a critical step in MALT1 activation. Collectively, these findings suggest a model in which E549 at the dimerization interface is required for the formation of the enzymatically active, monoubiquitinated form of MALT1.

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.

The NF-κB regulator MALT1 determines the encephalitogenic potential of Th17 cells

Effector functions of inflammatory IL-17-producing Th (Th17) cells have been linked to autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS). However, what determines Th17 cell encephalitogenicity is still unresolved. Here, we show that after EAE induction, mice deficient for the NF-κB regulator MALT1 (Malt1-/- mice) exhibit strong lymphocytic infiltration in the CNS, but do not develop any clinical signs of EAE. Loss of Malt1 interfered with expression of the Th17 effector cytokines IL-17 and GM-CSF both in vitro and in vivo. In line with their impaired GM-CSF secretion, Malt1-/- Th cells failed to recruit myeloid cells to the CNS to sustain neuroinflammation, whereas autoreactive WT Th cells successfully induced EAE in Malt1-/- hosts. In contrast, Malt1 deficiency did not affect Th1 cells. Despite their significantly decreased secretion of Th17 effector cytokines, Malt1-/- Th17 cells showed normal expression of lineage-specific transcription factors. Malt1-/- Th cells failed to cleave RelB, a suppressor of canonical NF-κB, and exhibited altered cellular localization of this protein. Our results indicate that MALT1 is a central, cell-intrinsic factor that determines the encephalitogenic potential of inflammatory Th17 cells in vivo.

Hepatocyte growth factor preferentially activates the anti-inflammatory arm of NF-κB signaling to induce A20 and protect renal proximal tubular epithelial cells from inflammation

Inflammation induces the NF-κB dependent protein A20 in human renal proximal tubular epithelial cells (RPTEC), which secondarily contains inflammation by shutting down NF-κB activation. We surmised that inducing A20 without engaging the pro-inflammatory arm of NF-κB could improve outcomes in kidney disease. We showed that hepatocyte growth factor (HGF) increases A20 mRNA and protein levels in RPTEC without causing inflammation. Upregulation of A20 by HGF was NF-κB/RelA dependent as it was abolished by overexpressing IκBα or silencing p65/RelA. Unlike TNFα, HGF caused minimal IκBα and p65/RelA phosphorylation, with moderate IκBα degradation. Upstream, HGF led to robust and sustained AKT activation, which was required for p65 phosphorylation and A20 upregulation. While HGF treatment of RPTEC significantly increased A20 mRNA, it failed to induce NF-κB dependent, pro-inflammatory MCP-1, VCAM-1, and ICAM-1 mRNA. This indicates that HGF preferentially upregulates protective (A20) over pro-inflammatory NF-κB dependent genes. Upregulation of A20 supported the anti-inflammatory effects of HGF in RPTEC. HGF pretreatment significantly attenuated TNFα-mediated increase of ICAM-1, a finding partially reversed by silencing A20. In conclusion, this is the first demonstration that HGF activates an AKT-p65/RelA pathway to preferentially induce A20 but not inflammatory molecules. This could be highly desirable in acute and chronic renal injury where A20-based anti-inflammatory therapies are beneficial.

Hypercapnia induces cleavage and nuclear localization of RelB protein, giving insight into CO2 sensing and signaling

Carbon dioxide (CO₂) is increasingly being appreciated as an intracellular signaling molecule that affects inflammatory and immune responses. Elevated arterial CO₂ (hypercapnia) is encountered in a range of clinical conditions, including chronic obstructive pulmonary disease, and as a consequence of therapeutic ventilation in acute respiratory distress syndrome. In patients suffering from this syndrome, therapeutic hypoventilation strategy designed to reduce mechanical damage to the lungs is accompanied by systemic hypercapnia and associated acidosis, which are associated with improved patient outcome. However, the molecular mechanisms underlying the beneficial effects of hypercapnia and the relative contribution of elevated CO₂ or associated acidosis to this response remain poorly understood. Recently, a role for the non-canonical NF-κB pathway has been postulated to be important in signaling the cellular transcriptional response to CO₂. In this study, we demonstrate that in cells exposed to elevated CO₂, the NF-κB family member RelB was cleaved to a lower molecular weight form and translocated to the nucleus in both mouse embryonic fibroblasts and human pulmonary epithelial cells (A549). Furthermore, elevated nuclear RelB was observed in vivo and correlated with hypercapnia-induced protection against LPS-induced lung injury. Hypercapnia-induced RelB processing was sensitive to proteasomal inhibition by MG-132 but was independent of the activity of glycogen synthase kinase 3β or MALT-1, both of which have been previously shown to mediate RelB processing. Taken together, these data demonstrate that RelB is a CO₂-sensitive NF-κB family member that may contribute to the beneficial effects of hypercapnia in inflammatory diseases of the lung.

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.

MicroRNA-146a regulates both transcription silencing and translation disruption of TNF-α during TLR4-induced gene reprogramming

Following the TLR-dependent initiation phase of acute systemic proinflammatory responses such as sepsis, an adaptive phase represses or activates a specific pattern of gene expression until the inflammation resolves. Here, we used the THP-1 sepsis cell model of bacterial LPS/endotoxin tolerance to show that TLR4-induced miR-146a supports the feed-forward adaptive processes that silence transcription and disrupt translation of acute proinflammatory genes. First, we found that miR-146a regulates a pathway that promotes the binding of transcription repressor RelB to the TNF-α promoter, a step known to precede histone and DNA modifications, which generate facultative heterochromatin to silence acute proinflammatory genes. However, once RelB binding occurred, miR-146a inhibition could not reverse compacted chromatin, and endotoxin tolerance persisted. Second, we observed that miR-146a regulates a pathway that supports assembly of the translation repressor complex of TNF-α by preventing the interaction of the RNA-binding protein effector Ago2 and RBM4. We also determined that once endotoxin tolerance is established, and specific genes have been reprogrammed, transcription and translation disruption can be reversed only by simultaneously depleting RelB and inhibiting miR-146a. Thus, miR-146a induction supports the TLR4-dependent shift from initiation to gene-specific repression at two levels. Our results also imply that therapies designed to reverse endotoxin tolerance as potential therapies for sepsis should be directed at the transcription and translation pathways of reprogramming.

Glycogen synthase kinase-3β is a crucial mediator of signal-induced RelB degradation

The immediate early transcription factor nuclear factor (IκBs) kappa B (NF-κB) is crucially involved in the regulation of numerous physiological or pathophysiological processes such as inflammation and tumourigenesis. Therefore, the control of NF-κB activity, which is mainly regulated by signal-induced degradation of cytoplasmic inhibitors of NF-κB (IκBs), is of high relevance. One known alternative pathway of NF-κB regulation is the stimulus-induced proteasomal degradation of RelB, a component of the NF-κB dimer. Here, we identified the serine/threonine protein kinase glycogen synthase kinase-3β (GSK-3β) as a critical signalling component leading to RelB degradation. In Jurkat leukaemic T cells as well as in primary human T cells, tetradecanoylphorbolacetate/ionomycin- and CD3/CD28-induced RelB degradation were impaired by a GSK-3β-specific pharmacological inhibitor, an ectopically expressed dominant-negative GSK-3β mutant and by small-interfering RNA-mediated silencing of GSK-3β expression. Furthermore, a physical interaction between RelB and GSK-3β was shown by co-immunoprecipitation, which was already notable in unstimulated cells. Most importantly, as demonstrated by in vitro kinase assays, human RelB is inducibly phosphorylated by GSK-3β, indicating a direct substrate-enzyme relationship. The serine residue 552 is a target of GSK-3β-mediated phosphorylation in vitro and in vivo. We conclude that GSK-3β is a crucial regulator of RelB degradation, stressing the relevant linkage between the NF-κB system and GSK-3β.

A single NFκB system for both canonical and non-canonical signaling

Two distinct nuclear factor κB (NFκB) signaling pathways have been described; the canonical pathway that mediates inflammatory responses, and the non-canonical pathway that is involved in immune cell differentiation and maturation and secondary lymphoid organogenesis. The former is dependent on the IκB kinase adaptor molecule NEMO, the latter is independent of it. Here, we review the molecular mechanisms of regulation in each signaling axis and attempt to relate the apparent regulatory logic to the physiological function. Further, we review the recent evidence for extensive cross-regulation between these two signaling axes and summarize them in a wiring diagram. These observations suggest that NEMO-dependent and -independent signaling should be viewed within the context of a single NFκB signaling system, which mediates signaling from both inflammatory and organogenic stimuli in an integrated manner. As in other regulatory biological systems, a systems approach including mathematical models that include quantitative and kinetic information will be necessary to characterize the network properties that mediate physiological function, and that may break down to cause or contribute to pathology.

A structural guide to proteins of the NF-kappaB signaling module

The prosurvival transcription factor NF-kappaB specifically binds promoter DNA to activate target gene expression. NF-kappaB is regulated through interactions with IkappaB inhibitor proteins. Active proteolysis of these IkappaB proteins is, in turn, under the control of the IkappaB kinase complex (IKK). Together, these three molecules form the NF-kappaB signaling module. Studies aimed at characterizing the molecular mechanisms of NF-kappaB, IkappaB, and IKK in terms of their three-dimensional structures have lead to a greater understanding of this vital transcription factor system.

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.

NF-kappaB regulation: the nuclear response

Nuclear factor κB (NF-κB) is an inducible transcription factor that tightly regulates the expression of a large cohort of genes. As a key component of the cellular machinery NF-κB is involved in a wide range of biological processes including innate and adaptive immunity, inflammation, cellular stress responses, cell adhesion, apoptosis and proliferation. Appropriate regulation of NF-κB is critical for the proper function and survival of the cell. Aberrant NF-κB activity has now been implicated in the pathogenesis of several diseases ranging from inflammatory bowel disease to autoimmune conditions such as rheumatoid arthritis. Systems governing NF-κB activity are complex and there is an increased understanding of the importance of nuclear events in regulating NF-κB's activities as a transcription factor. A number of novel nuclear regulators of NF-κB such as IκB-ζ and PDZ and LIM domain 2 (PDLIM2) have now been identified, adding another layer to the mechanics of NF-κB regulation. Further insight into the functions of these molecules raises the prospect for better understanding and rational design of therapeutics for several important diseases.

Aldosterone activates NF-kappaB in the collecting duct

Besides its classical effects on salt homeostasis in renal epithelial cells, aldosterone promotes inflammation and fibrosis and modulates cell proliferation. The proinflammatory transcription factor NF-kappaB has been implicated in cell proliferation, apoptosis, and regulation of transepithelial sodium transport. The effect of aldosterone on the NF-kappaB pathway in principal cells of the cortical collecting duct, a major physiologic target of aldosterone, is unknown. Here, in both cultured cells and freshly isolated rat cortical collecting duct, aldosterone activated the canonical NF-kappaB signaling pathway, leading to increased expression of several NF-kappaB-targeted genes (IkappaBalpha, plasminogen activator inhibitor 1, monocyte chemoattractant protein 1, IL-1beta, and IL-6). Small interfering RNA-mediated knockdown of the serum and glucocorticoid-inducible kinase SGK1, a gene induced early in the response to aldosterone, but not pharmacologic inhibition of extracellular signal-regulated kinase and p38 kinase, attenuated aldosterone-induced NF-kappaB activation. Pharmacologic antagonism or knockdown of the mineralocorticoid receptor prevented aldosterone-induced NF-kappaB activity. In addition, activation of the glucocorticoid receptor inhibited the transactivation of NF-kappaB by aldosterone. In agreement with these in vitro findings, spironolactone prevented NF-kappaB-induced transcriptional activation observed in cortical collecting ducts of salt-restricted rats. In summary, aldosterone activates the canonical NF-kappaB pathway in principal cells of the cortical collecting duct by activating the mineralocorticoid receptor and by inducing SGK1.

Identification of lysine residues critical for the transcriptional activity and polyubiquitination of the NF-κB family member RelB

RelB is the key component of the alternative NF-κB (nuclear factor κB) signalling pathway. However, RelB exerts also a negative effect via the recruitment of a DNMT1 (DNA methyltransferase 1)–Daxx (death domain-associated protein) complex to NF-κB target genes. Importantly, the molecular mechanisms which determine the functions of RelB are still largely unknown. In the present study, we aimed to analyse whether ubiquitination of RelB might be involved in the regulation of RelB. Indeed, RelB is constitutively polyubiquitinated in the B-cell lines Namalwa and 70Z/3. Although a PMA+ionomycin-induced augmentation of RelB polyubiquitination was linked to its proteasomal degradation in B-cells, the constitutive RelB polyubiquitination seems to affect non-proteasomal functions. Consistently, a significant RelB polyubiquitination in HEK (human embryonic kidney)-293 cells correlated with an augmentation of the transcriptional activity of RelB. Yet, neither nuclear localization nor DNA binding was enhanced by RelB polyubiquitination. Interestingly, basal RelB polyubiquitination depends neither on Lys48 nor on Lys63 conjugates, but might involve unconventional ubiquitin conjugates. Mapping of the ubiquitination target sites in RelB revealed the existence of various lysine residues, which serve as ubiquitination acceptors. However, only the substitution of Lys273/274 and Lys305/308 significantly decreased the basal RelB activity and the ubiquitin-induced augmentation of the RelB activity. Collectively, these results imply a dual role of RelB polyubiquitination for the stability and activity of this transcription factor.

The aryl hydrocarbon receptor attenuates tobacco smoke-induced cyclooxygenase-2 and prostaglandin production in lung fibroblasts through regulation of the NF-kappaB family member RelB

Diseases such as chronic obstructive pulmonary disease and lung cancer caused by cigarette smoke affect millions of people worldwide. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that influences responses to certain environmental pollutants such as tobacco smoke. However, the physiological function(s) of the AhR is unknown. Herein we propose that the physiologic role of the AhR is to limit inflammation. We show that lung fibroblasts from AhR(-/-) mice produce a heightened inflammatory response to cigarette smoke, typified by increased levels of cyclooxygenase-2 (COX-2) and prostaglandins (PGs), when compared with wild type (AhR(+/+)) fibroblasts. This response was dependent on AhR expression as transient transfection of an AhR expression plasmid into AhR(-/-) fibroblasts significantly attenuated the smoke-induced COX-2 and PG production, confirming the anti-inflammatory role of the AhR. The AhR can interact with NF-kappaB. However, the heightened inflammatory response observed in AhR(-/-) fibroblasts was not the result of NF-kappaB (p50/p65) activation. Instead it was coupled with a loss of the NF-kappaB family member RelB in AhR(-/-) fibroblasts. Taken together, these studies provide compelling evidence that AhR expression limits proinflammatory COX-2 and PG production by maintaining RelB expression. The association between RelB and AhR may represent a new therapeutic and more selective target with which to combat inflammation-associated diseases.

RelB is differentially regulated by IkappaB Kinase-alpha in B cells and mouse lung by cigarette smoke

The activation of transcription factor NF-kappaB is controlled by two main pathways: the classical canonical (RelA/p65-p50)- and the alternative noncanonical (RelB/p52)-NF-kappaB pathways. RelB has been shown to play a protective role in RelA/p65-mediated proinflammatory cytokine release in immune-inflammatory lymphoid cells. Increased infiltration of macrophages and lymphoid cells occurs in lungs of patients with chronic obstructive pulmonary disease, leading to abnormal inflammation. We hypothesized that RelB, and its signaling pathway, is differentially regulated in macrophages and B cells and in lung cells, leading to differential regulation of proinflammatory cytokines in response to cigarette smoke (CS). CS exposure increased the levels of RelB and NF-kappaB-inducing kinase associated with recruitment of RelB on promoters of the IL-6 and macrophage inflammatory protein-2 genes in mouse lung. Treatment of macrophage cell line, MonoMac6, with CS extract showed activation of RelB. In contrast, RelB was degraded by a proteasome-dependent mechanism in B lymphocytes (human Ramos, mouse WEHI-231, and primary mouse spleen B cells), suggesting that RelB is differentially regulated in lung inflammatory and lymphoid cells in response to CS exposure. Transient transfection of dominant negative IkappaB-kinase-alpha and double mutants of NF-kappaB-inducing kinase partially attenuated the CS extract-mediated loss of RelB in B cells and normalized the increased RelB level in macrophages. Taken together, these data suggest that RelB is differentially regulated in response to CS exposure in macrophages, B cells, and in lung cells by IkappaB-kinase-alpha-dependent mechanism. Rapid degradation of RelB signals for RelA/p65 activation and loss of its protective ability to suppress the proinflammatory cytokine release in lymphoid B cells.

Stabilization of RelB requires multidomain interactions with p100/p52

The NF-kappaB family member RelB has many properties not shared by other family members such as restricted subunit association and lack of regulation by the classical IkappaB proteins. We show that the protein level of RelB is significantly reduced in the absence of p100 and reduced even more when both p100 and p105 are absent. RelB stabilizes itself by directly interacting with p100, p105, and their processed products. However, RelB forms complexes with its partners using different interaction modes. Although the C-terminal ankyrin repeat domain of p105 is not involved in the RelB-p105 complex formation, all domains and flexible regions of each protein are engaged in the RelB-p100 complex. In several respects the RelB-p52 and RelB-p100 complexes are unique in the NF-kappaB family. The N-terminal domain of p100/p52 interacts with RelB but not RelA. The transcriptional activation domain of RelB, but not RelA, directly interacts with the processing region of p100. These unique protein-protein contacts explain why RelB prefers p52 as its dimeric partner for transcriptional activity and is retained in the cytoplasm as an inhibited complex by p100. This association-mediated stabilization of RelB implies a possible role for RelB in the processing of p100 into p52.

NF-kappaB inhibition reveals differential mechanisms of TNF versus TRAIL-induced apoptosis upstream or at the level of caspase-8 activation independent of cIAP2

Death ligands not only activate a death program but also regulate inflammatory signalling pathways, for example, through NF-kappaB induction. Although tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and TNF both activate NF-kappaB in human keratinocytes, only TRAIL potently induces apoptosis. However, when induction of NF-kappaB was inhibited with a kinase dead IKK2 mutant (IKK2-KD), TNF- but not TRAIL-induced apoptosis was dramatically enhanced. Acquired susceptibility to TNF-induced apoptosis was due to increased caspase-8 activation. To investigate the mechanism of resistance of HaCaT keratinocytes to TNF-induced apoptosis, we analyzed a panel of NF-kappaB-regulated effector molecules. Interestingly, the inhibitor of apoptosis protein (IAP) family member cIAP2, but not cIAP1, X-linked inhibitor of apoptosis, TNF receptor-associated factor (TRAF)-1, or TRAF2, was downregulated in sensitive but not in resistant HaCaT keratinocytes. Surprisingly, however, stable inducible expression of cIAP2 was not sufficient to render IKK2-KD-sensitized keratinocytes resistant to TNF, and reduction of cIAP2 alone did not increase the sensitivity of HaCaT keratinocytes to TNF. In conclusion, we demonstrate that inhibition of NF-kappaB dramatically sensitizes human keratinocytes to TNF- but not to TRAIL-induced apoptosis and that this sensitization for TNF was largely independent of cIAP2. Our data thus clearly exclude the candidates proposed to date to confer TNF apoptosis resistance and suggest the function of an unanticipated effector of NF-kappaB critical for the survival of HaCaT keratinocytes upstream or at the level of caspase-8 activation.

Diversity and regulation in the NF-kappaB system

The nuclear factor (NF)-kappaB family of transcription factors is a key participant in multiple biological processes, most notably in the immune and inflammatory response. Five proteins make up the NF-kappaB family, and these proteins can hetero- and homo-dimerize, giving rise to diversity. Recently, it has been shown that certain members can also interact directly with other transcription factors such as signal transducers of activated transcription, interferon regulatory factor family members and p53, providing further diversity. We propose that this promiscuity might help explain the many of roles of NF-kappaB in specialized cell function and fate. Furthermore, the state of a cell and its cellular background in addition to overall promoter structure and variations in the kappaB target sequence will all define the composition and activity of multimeric NF-kappaB complexes.

Peroxisome proliferator-activated receptor-gamma agonists inhibit respiratory syncytial virus-induced expression of intercellular adhesion molecule-1 in human lung epithelial cells

Respiratory syncytial virus (RSV) is the major causative agent of severe lower respiratory tract disease and death in infants worldwide. The epithelial cells of the airways are the target cells for RSV infection and the site of the majority of the inflammation associated with the disease. However, despite five decades of intensive RSV research there exist neither an effective active vaccine nor a promising antiviral and anti-inflammatory therapy. Recently, peroxisome proliferator-activated receptor-gamma (PPAR-gamma), a member of the nuclear hormone receptor superfamily, has been shown to possess anti-inflammatory properties. Therefore, we hypothesized whether the detrimental increase of intercellular adhesion molecule-1 (ICAM-1) on RSV-infected lung epithelial cells (A549 and primary normal human bronchial epithelial cells (NHBE)) might be modulated by natural and synthetic PPAR-gamma agonists (15d-PGJ2, ciglitazone, troglitazone, Fmoc-Leu). Our data show that all PPAR-gamma agonists under study significantly down-regulated the RSV-induced expression of ICAM-1 on A549- and NHBE cells in a dose-dependent manner resulting in a reduced beta2 integrin-mediated adhesion of monocytic effector cells (U937) to RSV-infected A549 cell monolayers. In contrast, the PPAR-alpha agonist bezafibrate had no impact on the RSV-induced ICAM-1 expression. The reduced ICAM-1 expression was associated with a diminished ICAM-1 mRNA level and binding activity of nuclear factor-kappaB (p65/p50) in A549 cells. These findings suggest that PPARgamma agonists have beneficial effects in the suppression of the inflammatory response during RSV infection and therefore might have clinical efficacy in the course of severe RSV-infection.

Beyond IkappaBs: alternative regulation of NF-kappaB activity

The transcription factor nuclear factor-kappa B (NF-kappaB) is a crucial regulator of many physiological and patho-physiological processes, including control of the adaptive and innate immune responses, inflammation, proliferation, tumorigenesis, and apoptosis. Thus, the tight regulation of NF-kappaB activity within a cell is extremely important. The central mechanism of NF-kappaB regulation is the signal-induced proteolytic degradation of a family of cytoplasmic inhibitors of NF-kappaB, the IkappaBs. However, with the discovery of an IkappaB-independent noncanonical or "alternative" pathway of NF-kappaB activation, the importance of other regulatory mechanisms responsible for the fine-tuning of NF-kappaB became clear. Post-translational modification, especially phosphorylation, of the Rel proteins, of which dimeric NF-kappaB is composed, are such alternative regulatory mechanisms. The best analyzed example is RelA phosphorylation, which takes place at specific amino acids resulting in distinct functional changes of this gene regulatory protein. The interaction of NF-kappaB with other proteins such as glucocorticoid receptors is very important for the regulation of NF-kappaB activity. Recently, exciting new concepts of IkappaB-independent NF-kappaB control like dimer exchange and nucleolar sequestration of RelA have been described, indicating that many aspects of NF-kappaB control are waiting to be discovered.

Post-translational modifications regulating the activity and function of the nuclear factor kappa B pathway

The diverse cellular and biological functions of the nuclear factor kappa B (NF-kappaB) pathway, together with the catastrophic consequences of its aberrant regulation, demand specific and highly regulated control of its activity. As described in this review, regulation of the NF-kappaB pathway is brought about through multiple post-translational modifications that control the activity of the core components of NF-kappaB signaling: the IkappaB kinase (IKK) complex, the IkappaB proteins and the NF-kappaB subunits themselves. These regulatory modifications, which include phosphorylation, ubiquitination, acetylation, sumoylation and nitrosylation, can vary, depending on the nature of the NF-kappaB-inducing stimulus. Moreover, they frequently have distinct, sometimes antagonistic, functional consequences and the same modification can have different effects depending on the context. Given the important role of NF-kappaB in human health and disease, understanding these pathways will not only provide valuable insights into mechanism and function, but could also lead to new drug targets and the development of diagnostic and prognostic biomarkers for many pathological conditions.

NF-κB RelB Forms an Intertwined Homodimer

The X-ray structure of the RelB dimerization domain (DD) reveals that the RelBDD assumes an unexpected intertwined fold topology atypical of other NF-kappaB dimers. All typical NF-kappaB dimers are formed by the association of two independently folded immunoglobulin (Ig) domains. In RelBDD, two polypeptides reconstruct both Ig domains in the dimer with an extra beta sheet connecting the two domains. Residues most critical to NF-kappaB dimer formation are invariant in RelB, and Y300 plays a positive role in RelBDD dimer formation. The presence of RelB-specific nonpolar residues at the surface removes several intradomain surface hydrogen bonds that may render the domain fold unstable. Intertwining may stabilize the RelBDD homodimer by forming the extra beta sheet. We show that, as in the crystal, RelB forms an intertwined homodimer in solution. We suggest that the transiently stable RelB homodimer might prevent its rapid degradation, allowing for heterodimer formation with p50 and p52.