Phosphorylation of IkappaBalpha in the C-terminal PEST domain by casein kinase II affects intrinsic protein stability

The inhibitor of κB kinase β (IKKβ) phosphorylates IκBα twice in a single binding event through a sequential mechanism

Phosphorylation of Inhibitor of κB (IκB) proteins by IκB Kinase β (IKKβ) leads to IκB degradation and subsequent activation of nuclear factor κB transcription factors. Of particular interest is the IKKβ-catalyzed phosphorylation of IκBα residues Ser32 and Ser36 within a conserved destruction box motif. To investigate the catalytic mechanism of IKKβ, we performed pre-steady-state kinetic analysis of the phosphorylation of IκBα protein substrates catalyzed by constitutively active, human IKKβ. Phosphorylation of full-length IκBα catalyzed by IKKβ was characterized by a fast exponential phase followed by a slower linear phase. The maximum observed rate (kp) of IKKβ-catalyzed phosphorylation of IκBα was 0.32 s-1 and the binding affinity of ATP for the IKKβ•IκBα complex (Kd) was 12 μM. Substitution of either Ser32 or Ser36 with Ala, Asp, or Cys reduced the amplitude of the exponential phase by approximately 2-fold. Thus, the exponential phase was attributed to phosphorylation of IκBα at Ser32 and Ser36, whereas the slower linear phase was attributed to phosphorylation of other residues. Interestingly, the exponential rate of phosphorylation of the IκBα(S32D) phosphomimetic amino acid substitution mutant was nearly twice that of WT IκBα and 4-fold faster than any of the other IκBα amino acid substitution mutants, suggesting that phosphorylation of Ser32 increases the phosphorylation rate of Ser36. These conclusions were supported by parallel experiments using GST-IκBα(1-54) fusion protein substrates bearing the first 54 residues of IκBα. Our data suggest a model wherein, IKKβ phosphorylates IκBα at Ser32 followed by Ser36 within a single binding event.

Regulation of stability and inhibitory activity of the tumor suppressor SEF through casein-kinase II-mediated phosphorylation

Inflammation and cancer are intimately linked. A key mediator of inflammation is the transcription-factor NF-κB/RelA:p50. SEF (also known as IL-17RD) is a feedback antagonist of NF-κB/RelA:p50 that is emerging as an important link between inflammation and cancer. SEF acts as a buffer to prevent excessive NF-κB activity by sequestering NF-κB/RelA:p50 in the cytoplasm of unstimulated cells, and consequently attenuating the NF-κB response upon pro-inflammatory cytokine stimulation. SEF contributes to cancer progression also via modulating other signaling pathways, including those triggered by growth-factors. Despite its important role in human physiology and pathology, mechanisms that regulate SEF biochemical properties and inhibitory activity are unknown. Here we show that human SEF is an intrinsically labile protein that is stabilized via CK2-mediated phosphorylation, and identified the residues whom phosphorylation by CK2 stabilizes hSEF. Unlike endogenous SEF, ectopic SEF was rapidly degraded when overexpressed but was stabilized in the presence of excess CK2, suggesting a mechanism for limiting SEF levels depending upon CK2 processivity. Additionally, phosphorylation by CK2 potentiated hSef interaction with NF-κB in cell-free binding assays. Most importantly, we identified a CK2 phosphorylation site that was indispensable for SEF inhibition of pro-inflammatory cytokine signaling but was not required for SEF inhibition of growth-factor signaling. To our knowledge, this is the first demonstration of post-translational modifications that regulate SEF at multiple levels to optimize its inhibitory activity in a specific signaling context. These findings may facilitate the design of SEF variants for treating cytokine-dependent pathologies, including cancer and chronic inflammation.

Co-relation with novel phosphorylation sites of IκBα and necroptosis in breast cancer cells

Background

Phosphorylation of NF-kappaB inhibitor alpha (IκBα) is key to regulation of NF-κB transcription factor activity in the cell. Several sites of IκBα phosphorylation by members of the IκB kinase family have been identified, but phosphorylation of the protein by other kinases remains poorly understood. We investigated a new phosphorylation site on IκBα and identified its biological function in breast cancer cells.

Methods

Previously, we observed that aurora kinase (AURK) binds IκBα in the cell. To identify the domains of IκBα essential for phosphorylation by AURK, we performed kinase assays with a series of IκBα truncation mutants. AURK significantly promoted activation of IκBα at serine 32 but not serine 36; by contrast, IκB kinase (IKK) family proteins activated both of these residues. We also confirmed phosphorylation of IκBα by matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and nano-liquid chromatography hybrid quadrupole orbitrap mass spectrometer (nanoLC-MS/MS; Q-Exactive).

Results

We identified two novel sites of serine phosphorylation, S63 and S262. Alanine substitution of S63 and S262 (S63A and S262A) of IκBα inhibited proliferation and suppressed p65 transcription activity. In addition, S63A and/or S262A of IκBα regulated apoptotic and necroptotic effects in breast cancer cells.

Conclusions

Phosphorylation of IκBα by AURK at novel sites is related to the apoptosis and necroptosis pathways in breast cancer cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08304-7.

Post-translational Modifications of IκBα: The State of the Art

The nuclear factor-kappa B (NF-κB) signaling pathway regulates a variety of biological functions in the body, and its abnormal activation contributes to the pathogenesis of many diseases, such as cardiovascular and respiratory diseases and cancers. Therefore, to ensure physiological homeostasis of body systems, this pathway is strictly regulated by IκBα transcription, IκBα synthesis, and the IκBα-dependent nuclear transport of NF-κB. Particularly, the post-translational modifications of IκBα including phosphorylation, ubiquitination, SUMOylation, glutathionylation and hydroxylation are crucial in the abovementioned regulatory process. Because of the importance of the NF-κB pathway in maintaining body homeostasis, understanding the post-translational modifications of IκBα can not only provide deeper insights into the regulation of NF-κB pathway but also contribute to the development of new drug targets and biomarkers for the diseases.

Activation of Latent HIV-1 T Cell Reservoirs with a Combination of Innate Immune and Epigenetic Regulators

The presence of T cell reservoirs in which human immunodeficiency virus (HIV) establishes latency by integrating into the host genome represents a major obstacle to an HIV cure and has prompted the development of strategies aimed at the eradication of HIV from latently infected cells. The "shock-and-kill" strategy is one of the most pursued approaches to the elimination of viral reservoirs. Although several latency-reversing agents (LRAs) have shown promising reactivation activity, they have failed to eliminate the cellular reservoir. In this study, we evaluated a novel immune system-mediated approach to clearing the HIV reservoir, based on a combination of innate immune stimulation and epigenetic reprogramming. The combination of the STING agonist cGAMP (cyclic GMP-AMP) and the FDA-approved histone deacetylase inhibitor resminostat resulted in a significant increase in HIV proviral reactivation and specific apoptosis in HIV-infected cells in vitro Reductions in the proportion of HIV-harboring cells and the total amount of HIV DNA were also observed in CD4+ central memory T (TCM) cells, a primary cell model of latency, where resminostat alone or together with cGAMP induced high levels of selective cell death. Finally, high levels of cell-associated HIV RNA were detected ex vivo in peripheral blood mononuclear cells (PBMCs) and CD4+ T cells from individuals on suppressive antiretroviral therapy (ART). Although synergism was not detected in PBMCs with the combination, viral RNA expression was significantly increased in CD4+ T cells. Collectively, these results represent a promising step toward HIV eradication by demonstrating the potential of innate immune activation and epigenetic modulation for reducing the viral reservoir and inducing specific death of HIV-infected cells.IMPORTANCE One of the challenges associated with HIV-1 infection is that despite antiretroviral therapies that reduce HIV-1 loads to undetectable levels, proviral DNA remains dormant in a subpopulation of T lymphocytes. Numerous strategies to clear residual virus by reactivating latent virus and eliminating the reservoir of HIV-1 (so-called "shock-and-kill" strategies) have been proposed. In the present study, we use a combination of small molecules that activate the cGAS-STING antiviral innate immune response (the di-cyclic nucleotide cGAMP) and epigenetic modulators (histone deacetylase inhibitors) that induce reactivation and HIV-infected T cell killing in cell lines, primary T lymphocytes, and patient samples. These studies represent a novel strategy for HIV eradication by reducing the viral reservoir and inducing specific death of HIV-infected cells.

Differential Regulation of the Three Eukaryotic mRNA Translation Initiation Factor (eIF) 4Gs by the Proteasome

The 4G family of eukaryotic mRNA translation initiation factors is composed of three members (eIF4GI, eIF4GII, and DAP5). Their specific roles in translation initiation are under intense investigations, but how their respective intracellular amounts are controlled remains poorly understood. Here we show that eIF4GI and eIF4GII exhibit much shorter half-lives than that of DAP5. Both eIF4GI and eIF4GII proteins, but not DAP5, contain computer-predicted PEST motifs in their N-termini conserved across the animal kingdom. They are both sensitive to degradation by the proteasome. Under normal conditions, eIF4GI and eIF4GII are protected from proteasomal destruction through binding to the detoxifying enzyme NQO1 [NAD(P)H:quinone oxidoreductase]. However, when cells are exposed to oxidative stress both eIF4GI and eIF4GII, but not DAP5, are degraded by the proteasome in an N-terminal-dependent manner, and cell viability is more compromised upon silencing of DAP5. These findings indicate that the three eIF4G proteins are differentially regulated by the proteasome and that persistent DAP5 plays a role in cell survival upon oxidative stress.

IL-1β and TNFα Differentially Influence NF-κB Activity and FasL-Induced Apoptosis in Primary Murine Hepatocytes During LPS-Induced Inflammation

Macrophage-derived cytokines largely influence the behavior of hepatocytes during an inflammatory response. We previously reported that both TNFα and IL-1β, which are released by macrophages upon LPS stimulation, affect Fas ligand (FasL)-induced apoptotic signaling. Whereas TNFα preincubation leads to elevated levels of caspase-3 activity and cell death, pretreatment with IL-1β induces increased caspase-3 activity but keeps cells alive. We now report that IL-1β and TNFα differentially influence NF-κB activity resulting in a differential upregulation of target genes, which may contribute to the distinct effects on cell viability. A reduced NF-κB activation model was established to further investigate the molecular mechanisms which determine the distinct cell fate decisions after IL-1β and TNFα stimulation. To study this aspect in a more physiological setting, we used supernatants from LPS-stimulated bone marrow-derived macrophages (BMDMs). The treatment of hepatocytes with the BMDM supernatant, which contains both IL-1β and TNFα, sensitized to FasL-induced caspase-3 activation and cell death. However, when TNFα action was blocked by neutralizing antibodies, cell viability after stimulation with the BMDM supernatant and FasL increased as compared to single FasL stimulation. This indicates the important role of TNFα in the sensitization of apoptosis in hepatocytes. These results give first insights into the complex interplay between macrophages and hepatocytes which may influence life/death decisions of hepatocytes during an inflammatory reaction of the liver in response to a bacterial infection.

IKKβ promotes metabolic adaptation to glutamine deprivation via phosphorylation and inhibition of PFKFB3

In this study, Reid et al. investigate how cancer cells adapt to low glutamine conditions, which is needed for cancer cell proliferation and survival. They show that IKKβ directly interacts with and phosphorylates PFKFB3, a major driver of aerobic glycolysis, at Ser269 upon glutamine deprivation to inhibit its activity, thereby down-regulating aerobic glycolysis when glutamine levels are low and thus providing new insights into cancer cell adaptation.

Glutamine is an essential nutrient for cancer cell survival and proliferation. Enhanced utilization of glutamine often depletes its local supply, yet how cancer cells adapt to low glutamine conditions is largely unknown. Here, we report that IκB kinase β (IKKβ) is activated upon glutamine deprivation and is required for cell survival independently of NF-κB transcription. We demonstrate that IKKβ directly interacts with and phosphorylates 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase isoform 3 (PFKFB3), a major driver of aerobic glycolysis, at Ser269 upon glutamine deprivation to inhibit its activity, thereby down-regulating aerobic glycolysis when glutamine levels are low. Thus, due to lack of inhibition of PFKFB3, IKKβ-deficient cells exhibit elevated aerobic glycolysis and lactate production, leading to less glucose carbons contributing to tricarboxylic acid (TCA) cycle intermediates and the pentose phosphate pathway, which results in increased glutamine dependence for both TCA cycle intermediates and reactive oxygen species suppression. Therefore, coinhibition of IKKβ and glutamine metabolism results in dramatic synergistic killing of cancer cells both in vitro and in vivo. In all, our results uncover a previously unidentified role of IKKβ in regulating glycolysis, sensing low-glutamine-induced metabolic stress, and promoting cellular adaptation to nutrient availability.

The kinase inhibitor D11 induces caspase-mediated cell death in cancer cells resistant to chemotherapeutic treatment

Background

Multi-drug resistance and predisposition to metastasize are major clinical problems in cancer treatment. Malignant primary brain tumor and pancreatic cancer are two well-known examples of malignant tumors resistant to conventional therapies where aberrant EGFR-mediated and NF-κB signal transduction pathways are likely to play an important role. We have recently identified 1,3-Dichloro-6-[(E)-((4-methoxyphenyl)imino)methyl] diben-zo(b,d) furan-2,7-diol (D11) as a potent and selective inhibitor of CK2 a serine/threonine protein kinase that modulates the aforementioned signaling cascades.

Methods

Human cancer cell lines (glioblastoma and pancreatic adenocarcinoma) resistant to conventional chemotherapeutic agents were incubated with increasing concentrations of D11 for variable amounts of time. Cell viability, cell death and effects on major signal transduction pathways deregulated in cancer cells were analyzed by ELISA, FACS and Western blot-based assays, respectively. Moreover, effects on cell migration and in cell protein-protein association were investigated by wound-healing and in situ proximity ligation assays, respectively.

Results

We show here, that D11 treatment leads to i) significant caspase-mediated apoptotic cell death, ii) down-regulation of EGFR expression and iii) inhibition of NF-κB transcriptional activity. Furthermore, cell exposure to D11 results in impaired cell migration and correlates with reduced expression of the ion co-transporter and cell volume regulator Na⁺-K⁺-2Cl⁻ (NKCC1).

Conclusions

Data reported here underline the therapeutic potential of D11 with respect to certain types of cancer that carry aberrant intracellular signaling cascades and/or exhibit sustained cell migration and suggest a new therapeutic strategy against chemotherapy resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-015-0234-6) contains supplementary material, which is available to authorized users.

c-KIT signaling is targeted by pathogenic Yersinia to suppress the host immune response

Background

The pathogenic Yersinia species exhibit a primarily extracellular lifestyle through manipulation of host signaling pathways that regulate pro-inflammatory gene expression and cytokine release. To identify host genes that are targeted by Yersinia during the infection process, we performed an RNA interference (RNAi) screen based on recovery of host NF-κB-mediated gene activation in response to TNF-α stimulation upon Y. enterocolitica infection.

Results

We screened shRNAs against 782 genes in the human kinome and 26 heat shock genes, and identified 19 genes that exhibited ≥40% relative increase in NF-κB reporter gene activity. The identified genes function in multiple cellular processes including MAP and ERK signaling pathways, ion channel activity, and regulation of cell growth. Pre-treatment with small molecule inhibitors specific for the screen hits c-KIT and CKII recovered NF-κB gene activation and/or pro-inflammatory TNF-α cytokine release in multiple cell types, in response to either Y. enterocolitica or Y. pestis infection.

Conclusions

We demonstrate that pathogenic Yersinia exploits c-KIT signaling in a T3SS-dependent manner to downregulate expression of transcription factors EGR1 and RelA/p65, and pro-inflammatory cytokines. This study is the first major functional genomics RNAi screen to elucidate virulence mechanisms of a pathogen that is primarily dependent on extracellular-directed immunomodulation of host signaling pathways for suppression of host immunity.

Identification and characterization of a novel IκB-ε-like gene from lamprey (Lampetra japonica) with a role in immune response

Nuclear factor of kappa B (NF-κB) is a stimuli-activated transcription factor, regulates the expression of a diverse array of genes. Inhibitor of kappa B-epsilon (IκB-ε) is an inhibitor of NF-κB, which retains NF-κB in an inactive state in the cytoplasm. Lampreys (Lampetra japonica) belong to the lowest class of vertebrates with little information about its IκBs. We have identified a cDNA sequence IκB-ε-like in the lamprey and the deduced amino acid sequence of IκB-ε-like. It contains a conserved DSGxxS motif and six consecutive ankyrin repeats, which are necessary for signal-induced degradation of the molecule. Phylogenetic analysis indicated it had high sequence homology with IκB-εs from other vertebrates. FACS analysis showed that IκB-ε-like located in cytoplasm of leukocytes. The degradation of IκB-ε-like could be observed in leukocytes of L. japonica stimulated with lipopolysaccharide. These results indicate that IκB-ε proteins are conserved across vertebrates and the NF-κB-like signaling pathway may exist in the oldest agnatha.

Identification, characterization, and function analysis of the Cactus gene from Litopenaeus vannamei

The nuclear factor-kappa B (NF-κB) pathways play important roles in innate immune responses. IκB is the main cytoplasmic inhibitor of NF-κB. In this study, we identified the LvCactus gene from Litopenaeus vannamei, which is the first cloned IκB homologue in subphylum Crustacea. LvCactus contains six predicted ankyrin repeats, which show similarities to those of Cactus proteins from insects. LvCactus localizes in cytoplasm and interacts with LvDorsal, an L. vannamei homologue to Drosophila melanogaster Dorsal belonging to class II NF-κB family, to prevent its nuclear translocation. Contrary to that of LvDorsal, over-expression of LvCactus down-regulates the activities of shrimp antimicrobial peptides promoters, suggesting LvCactus is an inhibitor of LvDorsal. The promoter of LvCactus was predicted to contain five putative NF-κB binding motifs, among which four were proved to be bound by LvDorsal by chromatin immunoprecipitation assays. Dual-luciferase reporter assays also showed that transcription of LvCactus was promoted by LvDorsal but inhibited by LvCactus itself, indicating a feedback regulatory pathway between LvCactus and LvDorsal. Expression of LvCactus was up-regulated after Lipopolysaccharides, poly (I:C), Vibrio parahaemolyticus, and Staphylococcus aureus injections, suggesting an activation response of LvCactus to bacterial and immune stimulant challenges. Differently, the LvCactus expression levels obviously decreased during white spot syndrome virus (WSSV) infection, indicating the feedback regulatory pathway of LvCactus/LvDorsal could be modified by WSSV.

Nuclear Factor kappa B is central to Marek's disease herpesvirus induced neoplastic transformation of CD30 expressing lymphocytes in-vivo

Background

Marek’s Disease (MD) is a hyperproliferative, lymphomatous, neoplastic disease of chickens caused by the oncogenic Gallid herpesvirus type 2 (GaHV-2; MDV). Like several human lymphomas the neoplastic MD lymphoma cells overexpress the CD30 antigen (CD30^hi) and are in minority, while the non-neoplastic cells (CD30^lo) form the majority of population. MD is a unique natural in-vivo model of human CD30^hi lymphomas with both natural CD30^hi lymphomagenesis and spontaneous regression. The exact mechanism of neoplastic transformation from CD30^lo expressing phenotype to CD30^hi expressing neoplastic phenotype is unknown. Here, using microarray, proteomics and Systems Biology modeling; we compare the global gene expression of CD30^lo and CD30^hi cells to identify key pathways of neoplastic transformation. We propose and test a specific mechanism of neoplastic transformation, and genetic resistance, involving the MDV oncogene Meq, host gene products of the Nuclear Factor Kappa B (NF-κB) family and CD30; we also identify a novel Meq protein interactome.

Results

Our results show that a) CD30^lo lymphocytes are pre-neoplastic precursors and not merely reactive lymphocytes; b) multiple transformation mechanisms exist and are potentially controlled by Meq; c) Meq can drive a feed-forward cycle that induces CD30 transcription, increases CD30 signaling which activates NF-κB, and, in turn, increases Meq transcription; d) Meq transcriptional repression or activation of the CD30 promoter generally correlates with polymorphisms in the CD30 promoter distinguishing MD-lymphoma resistant and susceptible chicken genotypes e) MDV oncoprotein Meq interacts with proteins involved in physiological processes central to lymphomagenesis.

Conclusions

In the context of the MD lymphoma microenvironment (and potentially in other CD30^hi lymphomas as well), our results show that the neoplastic transformation is a continuum and the non-neoplastic cells are actually pre-neoplastic precursor cells and not merely immune bystanders. We also show that NF-κB is a central player in MDV induced neoplastic transformation of CD30-expressing lymphocytes in vivo. Our results provide insights into molecular mechanisms of neoplastic transformation in MD specifically and also herpesvirus induced lymphoma in general.

Hax-1 is rapidly degraded by the proteasome dependent on its PEST sequence

BACKGROUND

HS-1-associated protein X-1 (Hax-1), is a multifunctional protein that has sequence homology to Bcl-2 family members. HAX-1 knockout animals reveal that it plays an essential protective role in the central nervous system against various stresses. Homozygous mutations in the HAX-1 gene are associated with autosomal recessive forms of severe congenital neutropenia along with neurological symptoms. The protein level of Hax-1 has been shown to be regulated by cellular protease cleavage or by transcriptional suppression upon stimulation.

RESULTS

Here, we report a novel post-translational mechanism for regulation of Hax-1 levels in mammalian cells. We identified that PEST sequence, a sequence rich in proline, glutamic acid, serine and threonine, is responsible for its poly-ubiquitination and rapid degradation. Hax-1 is conjugated by K48-linked ubiquitin chains and undergoes a fast turnover by the proteasome system. A deletion mutant of Hax-1 that lacks the PEST sequence is more resistant to the proteasomal degradation and exerts more protective effects against apoptotic stimuli than wild type Hax-1.

CONCLUSION

Our data indicate that Hax-1 is a short-lived protein and that its PEST sequence dependent fast degradation by the proteasome may contribute to the rapid cellular responses upon different stimulations.

It takes two to tango: IκBs, the multifunctional partners of NF-κB

The inhibitory IκB proteins have been discovered as fundamental regulators of the inducible transcription factor nuclear factor-κB (NF-κB). As a generally excepted model, stimulus-dependent destruction of inhibitory IκBs and processing of precursor molecules, both promoted by components of the signal integrating IκB kinase complex, are the key events for the release of various NF-κB/Rel dimers and subsequent transcriptional activation. Intense research of more than 20 years provides evidence that the extending family of IκBs act not simply as reversible inhibitors of NF-κB activation but rather as a complex regulatory module, which assures feedback regulation of the NF-κB system and either can inhibit or promote transcriptional activity in a stimulus-dependent manner. Thus, IκB and NF-κB/Rel family proteins establish a complex interrelationship that allows modulated NF-κB-dependent transcription, tailored to the physiological environment.

Differential inhibition of BmRelish1-dependent transcription in lepidopteran cells by bracovirus ankyrin-repeat proteins

In the tripartite parasitization system of the lepidopteran host Manduca sexta, the endoparasitoid wasp Cotesia congregata and its endosymbiotic virus, C. congregata Bracovirus (CcBV), the expression of viral proteins is necessary for successful parasitization. Here we have examined the in vitro effects of six members of the ankyrin-repeat protein family (Ank) of CcBV, which are thought to interfere with the host's induced innate immune responses, on the transcriptional activity of a heterologous lepidopteran Rel/NFκB transcription factor, Relish1 of Bombyx mori. Using as transcriptional activator BmRelish1-d2 (R1d2), a constitutively active mutant of the major regulator of the Imd pathway, BmRelish1, in conjunction with a reporter gene controlled by a B. mori antimicrobial peptide gene promoter, we have found that 5 of the 6 examined Anks suppress R1d2-dependent transcriptional activity to various degrees. Immunofluorescence studies have also revealed that while some of the Ank proteins have a rather strict cytoplasmic localization, others are detected both in the cytoplasm and the nucleus of the expressing cells and that colocalization with R1d2 occurs exclusively in the nucleus. Thus, our results suggest that functional and spatial differences among the various CcBV Ank family members may be responsible for the observed differential inhibition of R1d2 activity.

KAP degradation by calpain is associated with CK2 phosphorylation and provides a novel mechanism for cyclosporine A-induced proximal tubule injury

The use of cyclosporine A (CsA) is limited by its severe nephrotoxicity that includes reversible vasoconstrictor effects and proximal tubule cell injury, the latter associated whith chronic kidney disease progression. The mechanisms of CsA-induced tubular injury, mainly on the S3 segment, have not been completely elucidated. Kidney androgen-regulated protein (KAP) is exclusively expressed in kidney proximal tubule cells, interacts with the CsA-binding protein cyclophilin B and its expression diminishes in kidneys of CsA-treated mice. Since we reported that KAP protects against CsA toxicity in cultured proximal tubule cells, we hypothesized that low KAP levels found in kidneys of CsA-treated mice might correlate with proximal tubule cell injury. To test this hypothesis, we used KAP Tg mice developed in our laboratory and showed that these mice are more resistant to CsA-induced tubular injury than control littermates. Furthermore, we found that calpain, which was activated by CsA in cell cultures and kidney, is involved in KAP degradation and observed that phosphorylation of serine and threonine residues found in KAP PEST sequences by protein kinase CK2 enhances KAP degradation by calpain. Moreover, we also observed that CK2 inhibition protected against CsA-induced cytotoxicity. These findings point to a novel mechanism for CsA-induced kidney toxicity that might be useful in developing therapeutic strategies aimed at preventing tubular cell damage while maintaining the immunosuppressive effects of CsA.

Autophagosomal IkappaB alpha degradation plays a role in the long term control of tumor necrosis factor-alpha-induced nuclear factor-kappaB (NF-kappaB) activity

Transcription factor NF-κB is persistently activated in many chronic inflammatory diseases and cancers. The short term regulation of NF-κB is well understood, but little is known about the mechanisms of its long term activation. We studied the effect of a single application of TNF-α on NF-κB activity for up to 48 h in intestinal epithelial cells. Results show that NF-κB remained persistently activated up to 48 h after TNF-α and that the long term activation of NF-κB was accompanied by a biphasic degradation of IκBα. The first phase of IκBα degradation was proteasome-dependent, but the second was not. Further investigation showed that TNF-α stimulated formation of autophagosomes in intestinal epithelial cells and that IκBα co-localized with autophagosomal vesicles. Pharmacological or genetic blockade of autophagosome formation or the inhibition of lysosomal proteases decreased TNF-α-induced degradation of IκBα and lowered NF-κB target gene expression. Together, these findings indicate a role of autophagy in the control of long term NF-κB activity. Because abnormalities in autophagy have been linked to ineffective innate immunity, we propose that alterations in NF-κB may mediate this effect.

Fibromodulin suppresses nuclear factor-kappaB activity by inducing the delayed degradation of IKBA via a JNK-dependent pathway coupled to fibroblast apoptosis

Fibulin-5 (FBLN5) belongs to the Fibulin family of secreted extracellular matrix proteins, and our laboratory first established FBLN5 as a novel target for TGF-β in fibroblasts and endothelial cells. To better understand the pathophysiology of FBLN5, we carried out microarray analysis to identify fibroblast genes whose expressions were regulated by FBLN5 and TGF-β. In doing so, we identified fibromodulin (Fmod) as a novel target gene of FBLN5, and we validated the differential expression of Fmod and 12 other FBLN5-regulated genes by semi-quantitative real time PCR. Fmod belongs to the small leucine-rich family of proteoglycans, which are important constituents of mammalian extracellular matrices. Interestingly, parental 3T3-L1 fibroblasts displayed high levels of nuclear factor-κB (NF-κB) activity, although those engineered to express Fmod constitutively exhibited significantly reduced NF-κB activity, suggesting that Fmod functions to inhibit NF-κB signaling. By monitoring alterations in the activation of NF-κB and the degradation of its inhibitor, IκBα, we demonstrate for the first time that Fmod contributes to the constitutive degradation of IκBα protein in 3T3-L1 fibroblasts. Mechanistically, we observed Fmod to delay the degradation of IκBα by promoting the following: (i) activation of c-Jun N-terminal kinase; (ii) inhibition of calpain and casein kinase 2 activity; and (iii) induction of fibroblast apoptosis. Taken together, our study identified a novel function for Fmod in directing extracellular signaling, particularly the regulation of NF-κB activity and cell survival.

Casein kinase 2 dependent phosphorylation of neprilysin regulates receptor tyrosine kinase signaling to Akt

Neprilysin (NEP) is a type II membrane metalloproteinase that cleaves physiologically active peptides at the cell surface thus regulating the local concentration of these peptides available for receptor binding and signal transduction. In addition, the cytoplasmic N-terminal domain of NEP interacts with the phosphatase and tensin homologue deleted on chromosome 10 (PTEN) thereby regulating intracellular signaling via Akt. Thus, NEP serves dual functions in extracellular and intracellular signal transduction. Here, we show that NEP undergoes phosphorylation at serine residue 6 within the N-terminal cytoplasmic domain. In vitro and cell culture experiments demonstrate that Ser 6 is efficiently phosphorylated by protein kinase CK2. The phosphorylation of the cytoplasmic domain of NEP inhibits its interaction with PTEN. Interestingly, expression of a pseudophosphorylated NEP variant (Ser6Asp) abrogates the inhibitory effect of NEP on insulin/insulin-like growth factor-1 (IGF-1) stimulated activation of Akt. Thus, our data demonstrate a regulatory role of CK2 in the interaction of NEP with PTEN and insulin/IGF-1 signaling.

Specification of DNA binding activity of NF-kappaB proteins

Nuclear factor-kappaB (NF-kappaB) is a pleiotropic mediator of inducible and specific gene regulation involving diverse biological activities including immune response, inflammation, cell proliferation, and death. The fine-tuning of the NF-kappaB DNA binding activity is essential for its fundamental function as a transcription factor. An increasing body of literature illustrates that this process can be elegantly and specifically controlled at multiple levels by different protein subsets. In particular, the recent identification of a non-Rel subunit of NF-kappaB itself provides a new way to understand the selective high-affinity DNA binding specificity of NF-kappaB conferred by a synergistic interaction within the whole complex. Here, we review the mechanism of the specification of DNA binding activity of NF-kappaB complexes, one of the most important aspects of NF-kappaB transcriptional control.

Flexible regions within I{kappa}B{alpha} create the ubiquitin-independent degradation signal

Homeostatic regulation of NF-κB requires the continuous synthesis of IκBα and its rapid degradation by the proteasome through a ubiquitin-independent pathway. We previously showed that the ubiquitin-independent degradation signal of unbound IκBα was located in the C-terminal PEST region, and we have now identified a single tyrosine, Tyr-289, and determined that the hydrophobic character of the tyrosine is important for the rapid turnover of IκBα. The sequence composition of the PEST peptide surrounding this Tyr-289 imposes a distinct polyproline II conformation. Enhancing the polyproline II helix formation correlates with slower degradation rates of unbound IκBα. We have further identified a degradation signal located within the 5th ankyrin repeat that is functional once the C terminus is removed. Both the C-terminal and 5th ankyrin repeat degradation signals have inherent flexibility and specific hydrophobic residue(s), which together constitute the ubiquitin-independent degradation signal for IκBα.

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.

The human papillomavirus E7-E2 interaction mechanism in vitro reveals a finely tuned system for modulating available E7 and E2 proteins

Transcription of the human papillomavirus E7 oncoprotein is negatively controlled by the viral E2 protein, and loss of this repression leads to irreversible transformation and carcinogenesis. Here we show that interaction of the HPV16 E7 protein with the DNA binding domain of the E2 protein (E2C) leads to ionic strength-dependent hetero-oligomerization even at the lowest concentrations measurable. Titration experiments followed by light scattering and native gel electrophoresis show insoluble oligomeric complexes with a >or=2000 nm diameter and intermediate soluble complexes 40 and 115 nm in diameter, respectively, formed in excess of E2C. A discrete oligomeric soluble complex formed in excess of E7 displays a diameter of 12 nm. The N-terminal domain of E7 interacts with E2C with a K(D) of 0.1 muM, where the stretch of residues 25-40 of E7, encompassing both a PEST motif and phosphorylation sites, is sufficient for the interaction. Displacement of the soluble E7-E2C complex by an E2 site DNA duplex and site-directed mutagenesis indicate that the protein-protein interface involves the DNA binding helix of E2. The formation of complexes of different sizes and properties in excess of either of the viral proteins reveals a finely tuned mechanism that could regulate the intracellular levels of both proteins as infection and transformation progress. Sequestering E2 into E7-E2 oligomers provides a possible additional route to uncontrolled E7 expression, in addition and prior to the disruption of the E2 gene during viral integration into the host genome.

Control of NF-κB activity by proteolysis

NF-κB transcription factors are critical regulators of many biological processes such as innate and adaptive immune responses, inflammation, cell proliferation and programmed cell death. This versatility necessitates a highly complex and tightly coordinated control of the signaling pathways leading to their activation. Here, we review the role of proteolysis in the regulation of NF-κB activity, more specifically the contribution of the well-known ubiquitin-proteasome system and the involvement of proteolytic activity of caspases and calpains.

The Ca2+-dependent protease Calpain A regulates Cactus/I kappaB levels during Drosophila development in response to maternal Dpp signals

Regulation of NF kappaB activity is central to many processes during development and disease. Activation of NF kappaB family members depends on degradation of inhibitory I kappaB proteins. In Drosophila, a nuclear gradient of the NF kappaB/c-rel protein Dorsal subdivides the embryonic dorsal-ventral axis, defining the extent and location of mesodermal and ectodermal territories. Activation of the Toll pathway directs Dorsal nuclear translocation by inducing proteosomal degradation of the I kappaB homologue Cactus. Another mechanism that impacts on Dorsal activation involves the Toll-independent pathway, which regulates constitutive Cactus degradation. We have shown that the BMP protein Decapentaplegic (Dpp) inhibits Cactus degradation independent of Toll. Here we report on a novel element of this pathway: the calcium-dependent protease Calpain A. Calpain A knockdowns increase Cactus levels, shifting the Dorsal gradient and dorsal-ventral patterning. As shown for mammalian I kappaB, this effect requires PEST sequences in the Cactus C-terminus, implying a conserved role for calpains. Alteration of Calpain A or dpp results in similar effects on Dorsal target genes. Epistatic analysis confirms Calpain A activity is regulated by Dpp, indicating that Dpp signals increase Cactus levels through Calpain A inhibition, thereby interfering with Dorsal activation. This mechanism may allow coordination of Toll, BMP and Ca(2+) signals, conferring precision to Dorsal-target expression domains.

Control of cellular GADD34 levels by the 26S proteasome

GADD34, the product of a growth arrest and DNA damage-inducible gene, is expressed at low levels in unstressed cells. In response to stress, the cellular content of GADD34 protein increases and, on termination of stress, rapidly declines. We investigated the mechanisms that control GADD34 levels in human cells. GADD34 proteins containing either an internal FLAG or a C-terminal green fluorescent protein epitope were degraded at rates similar to endogenous GADD34. However, the addition of epitopes at the N terminus or deletion of N-terminal sequences stabilized GADD34. N-terminal peptides of GADD34, either alone or fused to heterologous proteins, exhibited rapid degradation similar to wild-type GADD34, thereby identifying an N-terminal degron. Deletion of internal PEST repeats had no impact on GADD34 stability but modulated the binding and activity of protein phosphatase 1. Proteasomal but not lysosomal inhibitors enhanced GADD34 stability and eukaryotic initiation factor 2alpha (eIF-2alpha) dephosphorylation, a finding consistent with GADD34's role in assembling an eIF-2alpha phosphatase. GADD34 was polyubiquitinated, and this modification enhanced its turnover in cells. A stabilized form of GADD34 promoted the accumulation and aggregation of the mutant cystic fibrosis transmembrane conductance regulator (CFTRDeltaF508), highlighting the physiological importance of GADD34 turnover in protein processing in the endoplasmic reticulum and the potential impact of prolonged GADD34 expression in human disease.

NF-kappaB dictates the degradation pathway of IkappaBalpha

IkappaB proteins are known as the regulators of NF-kappaB activity. They bind tightly to NF-kappaB dimers, until stimulus-responsive N-terminal phosphorylation by IKK triggers their ubiquitination and proteasomal degradation. It is known that IkappaBalpha is an unstable protein whose rapid degradation is slowed upon binding to NF-kappaB, but it is not known what dynamic mechanisms control the steady-state level of total IkappaBalpha. Here, we show clearly that two degradation pathways control the level of IkappaBalpha. Free IkappaBalpha degradation is not controlled by IKK or ubiquitination but intrinsically, by the C-terminal sequence known as the PEST domain. NF-kappaB binding to IkappaBalpha masks the PEST domain from proteasomal recognition, precluding ubiquitin-independent degradation; bound IkappaBalpha then requires IKK phosphorylation and ubiquitination for slow basal degradation. We show the biological requirement for the fast degradation of the free IkappaBalpha protein; alteration of free IkappaBalpha degradation dampens NF-kappaB activation. In addition, we find that both free and bound IkappaBalpha are similar substrates for IKK, and the preferential phosphorylation of NF-kappaB-bound IkappaBalpha is due to stabilization of IkappaBalpha by NF-kappaB.

Induction of cell death in antiestrogen resistant human breast cancer cells by the protein kinase CK2 inhibitor DMAT

Protein kinase CK2 is involved in cell proliferation and survival, and found overexpressed in virtually all types of human cancer, including breast cancer. We demonstrate that inhibition of CK2 with 2-dimethylamino-4,5,6,7-tetrabromo-benzimidazole (DMAT), a potent and specific CK2 inhibitor, results in caspase-mediated killing of human breast cancer cells with acquired resistance to antiestrogens, while DMAT fails to kill parental MCF-7 cells. The antiestrogen resistant breast cancer cells express reduced levels of Bcl-2 compared to MCF-7 cells. Reduced Bcl-2 protein level is also found in a tamoxifen resistant human breast tumor grown as a xenograft. We show that re-expression of Bcl-2 partially rescues antiestrogen resistant MCF-7 sublines from DMAT-induced cell death. In summary, our data suggest a novel role of CK2 in antiestrogen resistance.

Acute alcohol exposure exerts anti-inflammatory effects by inhibiting IkappaB kinase activity and p65 phosphorylation in human monocytes

Acute alcohol use is associated with impaired immune responses and decreased proinflammatory cytokine production. Our earlier studies have shown that acute alcohol intake inhibits NF-kappaB DNA binding in an IkappaBalpha-independent manner. We report using human peripheral blood monocytes and Chinese hamster ovary cells transfected with CD14 cells that acute alcohol treatment in vitro exerts NF-kappaB inhibition by disrupting phosphorylation of p65. Immunoprecipitation of p65 and IkappaBalpha revealed that acute alcohol exposure for 1 h decreased NF-kappaB-IkappaBalpha complexes in the cytoplasm. Phosphorylation of p65 at Ser(536) is mediated by IkappaB kinase (IKK)beta and is required for NF-kappaB-dependent cellular responses. We show that acute alcohol treatment decreased LPS-induced IKKalpha and IKKbeta activity resulting in decreased phosphorylation of p65 at Ser(536). Furthermore, nuclear expression of IKKalpha increased after alcohol treatment, which may contribute to inhibition of NF-kappaB. Decreased phosphorylation of nuclear p65 at Ser(276) was likely not due to alcohol-induced inhibition of protein kinase A and mitogen- and stress-activated protein kinase-1 activity. Although decreased IkappaBalpha phosphorylation after acute alcohol treatment was attributable to reduced IKKbeta activity, degradation of IkappaBalpha during alcohol exposure was IKKbeta-independent. Alcohol-induced degradation of IkappaBalpha in the presence of a 26S proteasome inhibitor suggested proteasome-independent IkappaBalpha degradation. Collectively, our studies suggest that acute alcohol exposure modulates IkappaBalpha-independent NF-kappaB activity primarily by affecting phosphorylation of p65. These findings further implicate an important role for IKKbeta in the acute effects of alcohol in immune cells.

The role of protein kinase CK2 in intestinal epithelial cell inflammatory signaling

BACKGROUND

The transcription factor NF-kappaB is believed to play a key pathophysiological role in chronic intestinal inflammation. Further characterization of its mechanism of regulation, predominantly through cell signaling pathways, may provide clues as to the means of its intervention. One such potential signaling candidate is the protein kinase CK2. Despite its known ability to influence NF-kappaB activation, it has received no attention in this particular setting.

AIM

To characterize the aspects of its activation in response to IL-1beta in the colonic cell lines Caco2 and HCT116.

MATERIALS AND METHODS

A biochemical analysis of kinase activation was performed using phospho-specific antibodies as well as immune complex kinase assays; transcription factor activity was measured by transient transfection and luciferase-based NF-kappaB reporter assays; pro-inflammatory molecule expression was determined using RT-PCR.

RESULTS

In this report, we show an enhanced activation of CK2 bound to IKKgamma or the p65 subunit of the NF-kappaB in response to IL-1beta stimulation of intestinal epithelial cells. Using two established NF-kappaB reporters, we demonstrate that CK2 is involved in NF-kappaB regulation through the p65 serine 529 site. Using co-immunoprecipitation studies, we also show that p65 is bound to CK2 predominantly in the nucleus. From a functional perspective, two CK2 specific inhibitors were then shown to attenuate IL-8 reporter activation. Finally, the expression of a series of pro-inflammatory molecules including IL-8, GRO-alpha, MCP-1, TNFalpha and iNOS were variably affected in response to CK2 inhibition.

CONCLUSION

CK2 plays an active role in NF-kappaB signaling in intestinal epithelial cell lines and may represent a possible target for intervention.

A facelift for the general transcription factor TFIIA

TFIIA was classified as a general transcription factor when it was first identified. Since then it has been debated to what extent it can actually be regarded as "general". The most notable feature of TFIIA is the proteolytical cleavage of the TFIIAalphabeta into a TFIIAalpha and TFIIAbeta moiety which has long remained a mystery. Recent studies have showed that TFIIA is cleaved by Taspase1 which was initially identified as the protease for the proto-oncogene MLL. Cleavage of TFIIA does not appear to serve as a step required for its activation as the uncleaved TFIIA in the Taspase1 knock-outs adequately support bulk transcription. Instead, cleavage of TFIIA seems to affect its turn-over and may be a part of an intricate degradation mechanism that allows fine-tuning of cellular levels of TFIIA. Cleavage might also be responsible for switching transcription program as the uncleaved and cleaved TFIIA might have distinct promoter specificity during development and differentiation. This review will focus on functional characteristics of TFIIA and discuss novel insights in the role of this elusive transcription factor.

NF-kappa B as a target for cancer therapy

NF-kappaB transcription factors and the signaling pathways that activate them play a critical role in cancer development, progression and therapy, and recently have become a focal point for intense drug discovery and development efforts. This article presents a critical review on the different types of inhibitors targeting the NF-kappaB pathway at several stages.

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

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

Adipocyte enhancer-binding protein-1 promotes macrophage inflammatory responsiveness by up-regulating NF-kappaB via IkappaBalpha negative regulation

Nuclear factor kappaB (NF-kappaB) subunits comprise a family of eukaryotic transcription factors that are critically involved in cell proliferation, inflammation, and apoptosis. Under basal conditions, NF-kappaB subunits are kept under inhibitory regulation by physical interaction with NF-kappaB inhibitors (IkappaB subunits) in the cytosol. Upon stimulation, IkappaB subunits become phosphorylated, ubiquitinated, and subsequently degraded, allowing NF-kappaB subunits to translocate to the nucleus and bind as dimers to kappaB responsive elements of target genes. Previously, we have shown that AEBP1 enhances macrophage inflammatory responsiveness by inducing the expression of various proinflammatory mediators. Herein, we provide evidence suggesting that AEBP1 manifests its proinflammatory function by up-regulating NF-kappaB activity via hampering IkappaBalpha, but not IkappaBbeta, inhibitory function through protein-protein interaction mediated by the discoidin-like domain (DLD) of AEBP1. Such interaction renders IkappaBalpha susceptible to enhanced phosphorylation and degradation, subsequently leading to augmented NF-kappaB activity. Collectively, we propose a novel molecular mechanism whereby NF-kappaB activity is modulated by means of protein-protein interaction involving AEBP1 and IkappaBalpha. Moreover, our study provides a plausible mechanism explaining the differential regulatory functions exhibited by IkappaBalpha and IkappaBbeta in various cell types. We speculate that AEBP1 may serve as a potential therapeutic target for the treatment of various chronic inflammatory diseases and cancer.

Constitutively activated nuclear factor-kappaB, but not induced NF-kappaB, leads to TRAIL resistance by up-regulation of X-linked inhibitor of apoptosis protein in human cancer cells

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in most, but not all, cancer cells. The molecular factors regulating the sensitivity to TRAIL are still incompletely understood. The transcription factor nuclear factor-kappaB (NF-kappaB) has been implicated, but its exact role is controversial. We studied different cell lines displaying varying responses to TRAIL and found that TRAIL can activate NF-kappaB in all our cancer cell lines regardless of their TRAIL sensitivity. Inhibition of NF-kappaB via adenoviral expression of the IkappaB-alpha super-repressor only sensitized the TRAIL-resistant pancreatic cancer cell line Panc-1. Panc-1 cells harbor constitutively activated NF-kappaB, pointing to a possible role of preactivated NF-kappaB in protection from TRAIL. Furthermore, we could reduce X-linked inhibitor of apoptosis protein (XIAP) levels in Panc-1 cells by inhibition of constitutively activated NF-kappaB and sensitize Panc-1 cells to TRAIL by RNA interference against XIAP. These results implicate elevated XIAP levels caused by high basal NF-kappaB activity in TRAIL resistance and suggest that therapeutic strategies involving TRAIL can be abetted by inhibition of NF-kappaB and/or XIAP only in tumor cells with constitutively activated NF-kappaB.

NF-kappaB as a potential molecular target for cancer therapy

Nuclear factor kappaB (NF-kappaB), a transcription factor, plays an important role in carcinogenesis as well as in the regulation of immune and inflammatory responses. NF-kappaB induces the expression of diverse target genes that promote cell proliferation, regulate apoptosis, facilitate angiogenesis and stimulate invasion and metastasis. Furthermore, many cancer cells show aberrant or constitutive NF-kappaB activation which mediates resistance to chemo- and radio-therapy. Therefore, the inhibition of NF-kappaB activation and its signaling pathway offers a potential cancer therapy strategy. In addition, recent studies have shown that NF-kappaB can also play a tumor suppressor role in certain settings. In this review, we focus on the role of NF-kappaB in carcinogenesis and the therapeutic potential of targeting NF-kappaB in cancer therapy.

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.

Down syndrome candidate region 1 increases the stability of the IkappaBalpha protein: implications for its anti-inflammatory effects

Down syndrome candidate region 1 (DSCR1), an endogenous inhibitor of calcineurin, inhibits the expression of genes involved in the inflammatory response. To elucidate the molecular basis of these anti-inflammatory effects, we analyzed the role of DSCR1 in the regulation of NF-kappaB transactivation using glioblastoma cells stably transfected with DSCR1.4 or its truncation mutants (DSCR1.4-(1-133) and DSCR1.4-(134-197)). Overexpression of DSCR1.4 significantly attenuated the induction of cyclooxygenase-2 (COX-2) expression by phorbol 12-myristate 13-acetate (PMA) via a calcineurin-independent mechanism. Experiments using inhibitors of the signaling molecules for NF-kappaB activation showed that NF-kappaB is responsible for the induction of COX-2. Full-length and truncated DSCR1.4 decreased the steady-state activity of NF-kappaB as well as PMA-induced activation of NF-kappaB, which correlated with attenuation of COX-2 induction. DSCR1.4 did not affect the PMA-stimulated phosphorylation or degradation kinetics of IkappaBalpha; however, DSCR1.4 significantly decreased the basal turnover rate of IkappaBalpha and consequently up-regulated its steady-state level. In the same context, knockdown of endogenous DSCR1.4 increased the turnover rate of IkappaBalpha as well as COX-2 induction. These results suggest that DSCR1 attenuates NF-kappaB-mediated transcriptional activation by stabilizing its inhibitory protein, IkappaBalpha.

Dissociation of a MAVS/IPS-1/VISA/Cardif-IKKepsilon molecular complex from the mitochondrial outer membrane by hepatitis C virus NS3-4A proteolytic cleavage

Intracellular RNA virus infection is detected by the cytoplasmic RNA helicase RIG-I that plays an essential role in signaling to the host antiviral response. Recently, the adapter molecule that links RIG-I sensing of incoming viral RNA to downstream signaling and gene activation events was characterized by four different groups; MAVS/IPS-1-1/VISA/Cardif contains an amino-terminal CARD domain and a carboxyl-terminal mitochondrial transmembrane sequence that localizes to the mitochondrial membrane. Furthermore, the hepatitis C virus NS3-4A protease complex specifically targets MAVS/IPS-1/VISA/Cardif for cleavage as part of its immune evasion strategy. With a novel search program written in python, we also identified an uncharacterized protein, KIAA1271 (K1271), containing a single CARD-like domain at the N terminus and a Leu-Val-rich C terminus that is identical to that of MAVS/IPS-1/VISA/Cardif. Using a combination of biochemical analysis, subcellular fractionation, and confocal microscopy, we now demonstrate that NS3-4A cleavage of MAVS/IPS-1/VISA/Cardif/K1271 results in its dissociation from the mitochondrial membrane and disrupts signaling to the antiviral immune response. Furthermore, virus-induced IKKepsilon kinase, but not TBK1, colocalized strongly with MAVS at the mitochondrial membrane, and the localization of both molecules was disrupted by NS3-4A expression. Mutation of the critical cysteine 508 to alanine was sufficient to maintain mitochondrial localization of MAVS/IPS-1/VISA/Cardif and IKKepsilon in the presence of NS3-4A. These observations provide an outline of the mechanism by which hepatitis C virus evades the interferon antiviral response.

Conformational flexibility may explain multiple cellular roles of PEST motifs

PEST sequences are one of the major motifs that serve as signal for the protein degradation and are also involved in various cellular processes such as phosphorylation and protein-protein interaction. In our earlier study, we found that these motifs contribute largely to eukaryotic protein disorder. This observation led us to evaluate their conformational variability in the nonredundant Protein Data Bank (PDB) structures. For this purpose, crystallographic temperature factors, structural alignment of multiple NMR models, and dihedral angle order parameters have been used in this study. The study has revealed the hypermobility of PEST motifs as compared to other regions of the protein. Conformational flexibility may allow them to participate in number of molecular interactions under different conditions. This analysis may explain the role of protein backbone flexibility in bringing about multiple cellular roles of PEST motifs.

Intrinsic unstructuredness and abundance of PEST motifs in eukaryotic proteomes

The study of unfolded protein regions has gained importance because of their prevalence and important roles in various cellular functions. These regions have characteristically high net charge and low hydrophobicity. The amino acid sequence determines the intrinsic unstructuredness of a region and, therefore, efforts are ongoing to delineate the sequence motifs, which might contribute to protein disorder. We find that PEST motifs are enriched in the characterized disordered regions as compared with globular ones. Analysis of representative PDB chains revealed very few structures containing PEST sequences and the majority of them lacked regular secondary structure. A proteome-wide study in completely sequenced eukaryotes with predicted unfolded and folded proteins shows that PEST proteins make up a large fraction of unfolded dataset as compared with the folded proteins. Our data also reveal the prevalence of PEST proteins in eukaryotic proteomes (approximately 25%). Functional classification of the PEST-containing proteins shows an over- and under-representation in proteins involved in regulation and metabolism, respectively. Furthermore, our analysis shows that predicted PEST regions do not exhibit any preference to be localized in the C terminals of proteins, as reported earlier.

The N-terminal domain of the human eIF2beta subunit and the CK2 phosphorylation sites are required for its function

CK2 (protein kinase CK2) is known to phosphorylate eIF2 (eukaryotic translation initiation factor 2) in vitro; however, its implication in this process in living cells has remained to be confirmed. The combined use of chemical inhibitors (emodin and apigenin) of CK2 together with transfection experiments with the wild-type of the K68A kinase-dead mutant form of CK2alpha evidenced the direct involvement of this protein kinase in eIF2beta phosphorylation in cultured HeLa cells. Transfection of HeLa cells with human wild-type eIF2beta or its phosphorylation site mutants showed Ser2 as the main site for constitutive eIF2beta phosphorylation, whereas phosphorylation at Ser67 seems more restricted. In vitro phosphorylation of eIF2beta also pointed to Ser2 as a preferred site for CK2 phosphorylation. Overexpression of the eIF2beta S2/67A mutant slowed down the rate of protein synthesis stimulated by serum, although less markedly than the overexpression of the Delta2-138 N-terminal-truncated form of eIF2beta (eIF2beta-CT). Mutation at Ser2 and Ser67 did not affect eIF2beta integrating into the eIF2 trimer or being able to complex with eIF5 and CK2alpha. The eIF2beta-CT form was also incorporated into the eIF2 trimer but did not bind to eIF5. Overexpression of eIF2beta slightly decreased HeLa cell viability, an effect that was more evident when overexpressing the eIF2beta S2/67A mutant. Cell death was particularly marked when overexpressing the eIF2beta-CT form, being detectable at doses where eIF2beta and eIF2beta S2/67A were ineffective. These results suggest that Ser2 and Ser67 contribute to the important role of the N-terminal region of eIF2beta for its function in mammals.

The proinflammatory actions of angiotensin II are dependent on p65 phosphorylation by the IkappaB kinase complex

The vasoactive hormone angiotensin II (Ang II) probably triggers inflammatory cardiovascular diseases by activating transcription factors such as NF-kappaB. We describe here a novel mode of NF-kappaB activation in cultured vascular smooth muscle cells exposed to Ang II. Ang II treatment resulted in an increase in the phosphotransferase activity of the IKK complex, which was mediated through the AT1 receptor subtype. The typical phosphorylation and proteasome-dependent degradation of the NF-kappaB inhibitor IkappaBalpha were not observed. Rather, Ang II treatment of vascular smooth muscle cells led to the phosphorylation of p65 on serine 536, a signal detected in both the cytoplasm and the nuclear compartments. The use of pharmacological inhibitors that inhibit the activation of MEK by Ang II revealed that phosphorylation of p65 on serine 536 did not require the MEK-ERK-RSK signaling pathway. On the other hand, specifically targeting the IKKbeta subunit of the IKK complex by overexpression of a dominant negative version of IKKbeta (IKKbeta K44A) or silencing RNA technology demonstrated that the IKKbeta subunit of the IKK complex was responsible for the detected phosphoserine 536 signal in Ang II-treated cells. Characterization of the signaling pathway leading to activation of the IKK complex by Ang II revealed that neither epidermal growth factor receptor transactivation nor the phosphatidylinositol 3-kinase-AKT signaling cascade were involved. Collectively, our data demonstrate that the proinflammatory activity of Ang II is independent of the classical pathway leading to IkappaBalpha phosphorylation and degradation but clearly depends on the recruitment of an IKK complex signaling cascade leading to phosphorylation of p65 on serine 536.

Inducible IkappaB kinase/IkappaB kinase epsilon expression is induced by CK2 and promotes aberrant nuclear factor-kappaB activation in breast cancer cells

Aberrant activation of nuclear factor-kappaB (NF-kappaB) transcription factors has been implicated in the pathogenesis of breast cancer. We previously showed elevated activity of IkappaB kinase alpha (IKKalpha), IKKbeta, and protein kinase CK2 in primary human breast cancer specimens and cultured cells. A novel inducible IKK protein termed IKK-i/IKKepsilon has been characterized as a potential NF-kappaB activator. Here, we provide evidence that implicates IKK-i/IKKepsilon in the pathogenesis of breast cancer. We show IKK-i/IKKepsilon expression in primary human breast cancer specimens and carcinogen-induced mouse mammary tumors. Multiple breast cancer cell lines showed higher levels of IKK-i/IKKepsilon and kinase activity compared with untransformed MCF-10F breast epithelial cells. Interestingly, IKK-i/IKKepsilon expression correlated with CK2alpha expression in mammary glands and breast tumors derived from MMTV-CK2alpha transgenic mice. Ectopic CK2 expression in untransformed cells led to increased IKK-i/IKKepsilon mRNA and protein levels. Inhibition of CK2alpha via the pharmacologic inhibitor apigenin or upon transfection of a CK2 kinase-inactive subunit reduced IKK-i/IKKepsilon levels. Expression of a kinase-inactive IKK-i/IKKepsilon mutant in breast cancer cells reduced NF-kappaB activity as judged by transfection assays of reporters driven either by NF-kappaB elements or the promoters of two NF-kappaB target genes, cyclin D1 and relB. Importantly, the kinase-inactive IKK-i/IKKepsilon mutant reduced the endogenous levels of these genes as well as the ability of breast cancer cells to grow in soft agar or form invasive colonies in Matrigel. Thus, CK2 induces functional IKK-i/IKKepsilon, which is an important mediator of the activation of NF-kappaB that plays a critical role in the pathogenesis of breast cancer.

Negative regulation of the retinoic acid-inducible gene I-induced antiviral state by the ubiquitin-editing protein A20

Activation of the interferon regulatory factors (IRFs) 3 and 7 transcription factors is essential for the induction of type I interferon (IFN) and development of the innate antiviral response. Retinoic acid-inducible gene I has been shown to contribute to virus-induced IFN production independent of the Toll-like receptor pathways in response to a variety of RNA viruses and double-stranded RNA. In the present study, we demonstrate that the NF-kappaB-inducible, anti-apoptotic protein A20 efficiently blocks RIG-I-mediated activation of NF-kappaB-, IRF-3-, and IRF-7-dependent promoters but only weakly interferes with TRIF-TLR-3-mediated IFN activation. Expression of A20 completely blocked CARD domain containing DeltaRIG-I-induced IRF-3 Ser-396 phosphorylation, homodimerization, and DNA binding. The level of A20 inhibition was upstream of the TBK1/IKKepsilon kinases that phosphorylate IRF3 and IRF7 and paradoxically, A20 selectively degraded the TRIF protein but not RIG-I. A20 possesses two ubiquitin-editing domains, an N-terminal deubiquitination domain and a C-terminal ubiquitin ligase domain consisting of seven zinc finger domains. Deletion of the N-terminal de-ubiquitination domain had no significant effect on the inhibitory effect of A20, whereas deletion or mutation of zinc finger motif 7 ablated the inhibitory function of A20 on IRF- or NF-kappaB-mediated gene expression. Furthermore, cells stably expressing the active form of RIG-I induced an antiviral state that interfered with replication of vesicular stomatitis virus, an effect that was reversed by stable co-expression of A20. These results suggest that the virus-inducible, NF-kappaB-dependent activation of A20 functions as a negative regulator of RIG-I-mediated induction of the antiviral state.

Role for casein kinase 2 in the regulation of HIF-1 activity

Hypoxia-inducible factor-1 (HIF-1) is a heterodimeric transcription factor that plays a major role in cellular adaptation to hypoxia. The mechanisms regulating HIF-1 activity occurs at multiple levels in vivo. The HIF-1alpha subunit is highly sensible to oxygen and is rapidly degraded by the proteasome 26S in normoxia. Activation in hypoxia occurs through a multistep process including inhibition of HIF-1alpha degradation, but also increase in the transactivation activity of HIF-1. Several data indicate that phosphorylation could play a role in this regulation. In this report, we investigated the role of casein kinase 2 (CK2), an ubiquitous serine/threonine kinase, in the regulation of HIF-1 activity. Hypoxia was capable of increasing the expression of the beta subunit of CK2, of inducing a relocalization of this subunit at the plasma membrane, of inducing nuclear translocation of the alpha subunit and of increasing CK2 activity. Three inhibitors of this kinase, DRB (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole), TBB (4,5,6,7-tetrabromotriazole) and apigenin, as well as overexpression of a partial dominant negative mutant of CK2alpha, were shown to inhibit HIF-1 activity as measured by a reporter assay and through hypoxia-induced VEGF and aldolase expression. This does not occur at the stabilization process since they did not affect HIF-1alpha protein level. DNA-binding activity was also not inhibited. We conclude that CK2 is an important regulator of HIF-1 transcriptional activity but the mechanism of this regulation remains to be determined. Since HIF-1 plays a major role in tumor angiogenesis and since CK2 has been described to be overexpressed in tumor cells, this new pathway of regulation can be one more way for tumor cells to survive.

Independent role of phosphoinositol-3-kinase (PI3K) and casein kinase II (CK-2) in EGFR and Her-2-mediated constitutive NF-kappaB activation in prostate cancer cells

BACKGROUND

Recent research has highlighted the potential role of EGFR and Her-2 in the constitutive activation of NF-kappaB (NF-kappaB) in prostate cancer cells, although the mechanism by which these receptors activate NF-kappaB in these cells remains unclear.

METHODS AND RESULTS

Using pharmacological and genetic approaches we show that in PC-3 cells, EGFR and Her-2 are involved in the constitutive activation of NF-kappaB through two different mechanisms. EGFR activates NF-kappaB through the PI3K/Akt pathway that leads to the phosphorylation of IkappaBalpha on serines 32 and 36, thereby promoting the nuclear translocation of the p65 subunit. In contrast, Her-2 activates NF-kappaB through Casein Kinase II (CK-2) activation independently of IkappaBalpha phosphorylation on serines 32 and 36.

CONCLUSIONS

Our study not only directly clarifies the signaling pathways involved in NF-kappaB activation in prostate cancer cell lines and but also provides a framework for further studies in the clinical characterization and management of prostate cancer.

Protein kinase CK2 phosphorylates and upregulates Akt/PKB

Treatment of Jurkat cells with specific inhibitors of protein kinase CK2 induces apoptosis. Here we provide evidence that the anti-apoptotic effect of CK2 can be at least partially mediated by upregulation of the Akt/PKB pathway. Such a conclusion is based on the following observations: (1) inhibition of CK2 by cell treatment with two structurally unrelated CK2 inhibitors induces downregulation of Akt/PKB, as judged from decreased phosphorylation of its physiological targets, and immunoprecipitate kinase assay; (2) similar results are observed upon reduction of CK2 catalytic subunit by the RNA-interference technique; (3) Akt/PKB Ser129 is phosphorylated by CK2 in vitro and in vivo; (4) such a phosphorylation of activated Akt/PKB correlates with a further increase in catalytic activity. These data disclose an unanticipated mechanism by which constitutive phosphorylation by CK2 may be required for maximal activation of Akt/PKB.