Stability of the Rel homology domain is critical for generation of NF-kappa B p50 subunit

Case Report: A Novel Mutation in NFKB1 Associated With Pyoderma Gangrenosum

Pyoderma gangrenosum (PG) is a rare, destructive inflammatory skin disease of which a painful nodule or pustule breaks down to form a progressively enlarging ulcer. Ulcerations associated with PG may occur after trauma or injury to the skin. The etiology has not been clearly elucidated. Our report described a PG patient with a heterozygous splice-donor-site mutation in NFKB1 (c.730+5G>A) causing the absence of exon 8 and the formation of truncated p105 (p.Asp191_Lys244delinsGlu; p105delEx8), which led to distinct symptoms of high fever and excessive inflammation in wound area after routine surgical procedures. The functional analysis showed that the variant caused reduced phosphorylation of p105 and resulted in the decreased processing of p105 to p50. We conclude that the patient's symptoms were caused by dysregulation of the NF-κB signaling pathway.

HDAC1 interacts with the p50 NF-?B subunit via its nuclear localization sequence to constrain inflammatory gene expression

The NF-?B p50 subunit is an important regulator of inflammation, with recent experimental evidence to support it also having a tumor suppressor role. Classically, p50 functions in heterodimeric form with the RelA (p65) NF-?B subunit to activate inflammatory genes. However, p50 also forms homodimers which actively repress NF-?B-dependent inflammatory gene expression and exert an important brake on the inflammatory process. This repressive activity of p50:p50 is thought to be in part mediated by an interaction with the epigenetic repressor protein Histone Deacetylase 1 (HDAC1). However, neither the interaction of p50 with HDAC1 nor the requirement of HDAC1 for the repressive activities of p50 has been well defined. Here we employed in silico prediction with in vitro assays to map sites of interaction of HDAC1 on the p50 protein. Directed mutagenesis of one such region resulted in almost complete loss of HDAC1 binding to p50. Transfected mutant p50 protein lacking the putative HDAC1 docking motif resulted in enhanced cytokine and chemokine expression when compared with cells expressing a transfected wild type p50. In addition, expression of this mutant p50 was associated with enhanced chemoattraction of neutrophils and acetylation of known inflammatory genes demonstrating the likely importance of the p50:HDAC1 interaction for controlling inflammation. These new insights provide an advance on current knowledge of the mechanisms by which NF-?B-dependent gene transcription are regulated and highlight the potential for manipulation of p50:HDAC1 interactions to bring about experimental modulation of chronic inflammation and pathologies associated with dysregulated neutrophil accumulation and activation.

miR-15b-5p resensitizes colon cancer cells to 5-fluorouracil by promoting apoptosis via the NF-κB/XIAP axis

Drug resistance, which is closely correlated with an imbalance in apoptosis, endows colorectal cancer (CRC) with enhanced progression capacity irrespective of the treatment with therapeutics. We report that miR-15b-5p is a tumor suppressor whose level is globally decreased in CRC cells and tissues. Over-expression of miR-15b-5p not only promoted 5-fluorouracil (5-FU)-induced cellular apoptosis but also reversed the chemoresistance of 5-FU in vitro and in vivo. As a key mediator of inflammation-induced cancer, miR-15b-5p enhances these therapeutic effects are mainly attributed to targeting of the NF-κB signaling pathway through negative regulation of NF-κB1 and one of its kinase complexes IKK-α. miR-15b-5p mediates NF-ĸB regulation by targeting the anti-apoptosis protein XIAP in vitro. Together, these results establish an axis of miR-15b-mediated apoptosis regulation, which reverses chemoresistance and suppresses CRC progression. These findings suggest that miR-15b-5p may be a potential agent for CRC treatment, particularly for 5-FU-resistant CRC.

Targeting IKK and NF-κB for Therapy

In addition to regulating immune responses, the NF-κB family of transcription factors also promotes cellular proliferation and survival. NF-κB and its activating kinase, IKK, have become appealing therapeutic targets because of their critical roles in the progression of many diseases including chronic inflammation and cancer. Here, we discuss the conditions that lead to pathway activation, the effects of constitutive activation, and some of the strategies used to inhibit NF-κB signaling.

NFKB1: a suppressor of inflammation, ageing and cancer

The pleiotropic consequences of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) pathway activation result from the combinatorial effects of the five subunits that form the homo- and heterodimeric NF-κB complexes. Although biochemical and gene knockout studies have demonstrated overlapping and distinct functions for these proteins, much is still not known about the mechanisms determining context-dependent functions, the formation of different dimer complexes and transcriptional control in response to diverse stimuli. Here we discuss recent results that reveal that the nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (NFKB1) (p105/p50) subunit is an important regulator of NF-κB activity in vivo. These effects are not restricted to being a dimer partner for other NF-κB subunits. Rather p50 homodimers have a critical role as suppressors of the NF-κB response, while the p105 precursor has a variety of NF-κB-independent functions. The importance of Nfkb1 function can be seen in mouse models, where Nfkb1(-/-) mice display increased inflammation and susceptibility to certain forms of DNA damage, leading to cancer, and a rapid ageing phenotype. In humans, low expression of Kip1 ubiquitination-promoting complex 1 (KPC1), a ubiquitin ligase required for p105 to p50 processing, was shown to correlate with a reduction in p50 and glioblastoma incidence. Therefore, while the majority of research in this field has focused on the upstream signalling pathways leading to NF-κB activation or the function of other NF-κB subunits, such as RelA (p65), these data demonstrate a critical role for NFKB1, potentially revealing new strategies for targeting this pathway in inflammatory diseases and cancer.

Multitasking with ubiquitin through multivalent interactions

Ubiquitylation - the post-translational modification of proteins with ubiquitin - serves powerful regulatory roles in eukaryotes. It can label proteins for destruction or activate gene transcription. Despite its versatility, ubiquitin is used to signal for cellular events with exquisite specificity. To achieve both versatility and specificity, ubiquitin signaling pathways use multivalency, namely the coordinated use of multiple interaction surfaces. Multivalent interactions regulate each stage of ubiquitin signaling pathways, and appear within the ubiquitin signal, the ubiquitylated substrate, ubiquitin processing enzymes and ubiquitin recognition proteins.

5-Androstenediol Promotes Survival of γ-Irradiated Human Hematopoietic Progenitors through Induction of Nuclear Factor-κB Activation and Granulocyte Colony-Stimulating Factor Expression

5-Androstenediol (5-AED) stimulates hematopoiesis and enhances survival in animals exposed to ionizing radiation (IR), suggesting that this steroid may act on hematopoietic progenitor cells. We used gamma-irradiated primary human CD34(+) hematopoietic progenitor cells to show that 5-AED protects hematopoietic cells from IR damage, as shown by enhanced cell survival, clonogenicity, proliferation, and differentiation. Unlike in tumor cells, IR did not induce nuclear factor-kappaB (NFkappaB) activation in primary progenitors. However, IR stimulated IkappaB(beta) release from NFkappaB/IkappaB complexes and caused NFkappaB1 (p50) degradation. 5-AED stabilized NFkappaB1 in irradiated cells and induced NFkappaB gene expression and NFkappaB activation (DNA binding). 5-AED stimulated interleukin-6 and granulocyte colony-stimulating factor (G-CSF) secretion. The survival-enhancing effects of 5-AED on clonogenic cells were abrogated by small interfering RNA inhibition of NFkappaB gene expression and by neutralization of G-CSF with antibody. The effects of 5-AED on survival and G-CSF secretion were blocked by the NFkappaB inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132). 5-AED had no effect on accumulation of the proapoptotic factor p53 after IR, as determined by Western blot. The results indicate that NFkappaB1 degradation after IR may be responsible for the radiation sensitivity of CD34+ cells compared with tumor cells. 5-AED exerts survival-enhancing effects on irradiated human hematopoietic progenitor cells via induction, stabilization, and activation of NFkappaB, which results in increased secretion of hematopoietic growth factor G-CSF.

Using Ciona to study developmental programmed cell death

Ciona intestinalis, a member of Tunicates, the closest group to vertebrates, has emerged as an appropriate organism for the study of developmentally regulated programmed cell death. First, because massive phases of apoptosis occur all along embryogenesis. Second, because the lecithotrophic mode of development is associated with autophagic process occurring during juvenile formation. Third, because the biochemical cell death machinery is close to that found in mammals. Altogether, the Ciona system contributes to identify new specific regulatory pathways and to explain how molecular mechanisms of programmed cell death evolved from invertebrates to vertebrates.

Two distinct ubiquitin-dependent mechanisms are involved in NF-kappaB p105 proteolysis

Generation of the p50 subunit of NF-kappaB is a rare case in which the ubiquitin system processes a longer precursor, p105, into a shorter active subunit: in the vast majority of cases, the target protein is completely degraded. The mechanisms involved in this process have remained elusive. It appears that a Gly rich region (GRR) in the middle of the molecule serves as a "processing stop signal", though under certain conditions, such as after stimulation, p105 can be completely degraded. Since NF-kappaB plays critical roles in a broad array of basic cellular processes, it is important to dissect the mechanisms that regulate its proteolysis-both destruction and processing. We have previously shown that signal-induced degradation of p105 requires ubiquitination on multiple lysines. Here we describe a novel region, a Processing Inhibitory Domain-PID, that upon its removal, the molecule is processed in high efficiency, which requires ubiquitination on a single, though non-specific, lysine.

Fertilization regulates apoptosis of Ciona intestinalis extra-embryonic cells through thyroxine (T4)-dependent NF-κB pathway activation during early embryonic development

In Ciona intestinalis, the elimination of extra-embryonic test cells during early stage of development is delayed by a fertilization signal. Test cells undergo a caspase-dependent apoptosis event repressed by thyroxine (T4)-activated NF-kappaB. When apoptosis was experimentally blocked, the hatching stage was delayed. The incubation of unfertilized eggs with a 1-h-fertilized egg extract or purified T4 restored apoptosis in test cells at a similar timing than found in fertilized eggs. Ciona expresses specific genes forming a functional IkappaB/NF-kappaB pathway. One, Ci-p65, was transiently induced upon fertilization via T4 and found to exert its anti-apoptotic role in test cells nuclei as well as in a reconstituted cell system. Blocking NF-kappaB activity by dexamethasone-induced overexpression of Ci-IkappaB abrogated the repression of apoptosis in test cells. Overall, the data are consistent for defining a central coupling role of both T4 and NF-kappaB during early embryo development.

Partitioning between unfolding and release of native domains during ClpXP degradation determines substrate selectivity and partial processing

Energy-dependent proteases, such as ClpXP, are responsible for the regulated destruction of proteins in all cells. AAA+ ATPases in these proteases bind protein substrates and power their mechanical denaturation and subsequent translocation into a secluded degradation chamber where polypeptide cleavage occurs. Here, we show that model unfolded substrates are engaged rapidly by ClpXP and are then spooled into the degradation chamber at a rate proportional to their length. Degradation and competition studies indicate that ClpXP initially binds native and unfolded substrates similarly. However, stable native substrates then partition between frequent release and infrequent denaturation, with only the latter step resulting in committed degradation. During degradation of a fusion protein with three tandem native domains, partially degraded species with one and two intact domains accumulated. These processed proteins were not bound to the enzyme, showing that release can occur even after translocation and degradation of a substrate have commenced. The release of stable substrates and committed engagement of denatured or unstable native molecules ensures that ClpXP degrades less stable substrates in a population preferentially. This mechanism prevents trapping of the enzyme in futile degradation attempts and ensures that the energy of ATP hydrolysis is used efficiently for protein degradation.

A Single PXY Motif Located within the Carboxyl Terminus of Spt23p and Mga2p Mediates a Physical and Functional Interaction with Ubiquitin Ligase Rsp5p

Proteasome-dependent processing of the endoplasmic reticulum localized transcription factor Spt23p of Saccharomyces cerevisiae generates its transcriptionally competent form and requires the WW domain containing Rsp5p ubiquitin ligase. Although previous studies documented an Rsp5p-Spt23p association in cells, very little is known about the nature of this interaction. We report here the identification of an imperfect type I WW domain-binding site (LPKY) within the carboxyl-terminal region of Spt23p that is required for Rsp5p binding in vitro and in vivo. Deletion of this motif abrogates Rsp5p-induced ubiquitination of Spt23p in vitro and reduces ubiquitination of the Spt23p precursor in yeast. In addition, the Spt23pDeltaLPKY mutant is inefficiently processed and is defective at up-regulating target gene (OLE1) expression in cells. Deletion of the corresponding LPKY site within Mga2p, an Spt23p homologue, also abrogates Rsp5p binding and Rsp5p-dependent ubiquitination in vitro as well as Rsp5p binding and Mga2p polyubiquitination in cells. However, the Mga2pDeltaLPKY mutant undergoes efficient proteasome-dependent processing. These experiments indicate that the LPKY motif of Spt23p is required for Rsp5p binding, Rsp5-induced ubiquitination, proteasome-dependent processing, and its OLE1 inducing function. They also suggest that the LPKY motif of Mga2p is required for Rsp5p binding and ubiquitination, and Rsp5p regulates Mga2p function by a mechanism that is independent of providing the partial degradation signal.

Ub on the move

Ubiquitination is an increasingly common post-translation modification that controls both the expression and activity of numerous proteins in the eukaryotic cell. One frequent target of the ubiquitin (Ub) modification machinery is transcription factors. Although ubiquitination generally modulates their function by inducing proteasome-dependent degradation, past and recent studies indicate that ubiquitination also regulates nuclear-cytoplasmic trafficking of transcriptional regulators. Ubiquitination is known to modulate transcription factor localization by destroying sequestering proteins and by directly promoting nuclear import and export of modified substrates. This review discusses old and new paradigms relating Ub modification and the control of transcription factor shuttling in and out of the nucleus.