Regulation of NF-kappa B activity by I kappa B alpha and I kappa B beta stability

A Novel Quinoline Inhibitor of the Canonical NF-κB Transcription Factor Pathway

The NF-κB family of transcription factors is a master regulator of cellular responses during inflammation, and its dysregulation has been linked to chronic inflammatory diseases, such as inflammatory bowel disease. It is therefore of vital importance to design and test new effective NF-κB inhibitors that have the potential to be utilized in clinical practice. In this study, we used a commercial transgenic HeLa cell line as an NF-κB activation reporter to test a novel quinoline molecule, Q3, as a potential inhibitor of the canonical NF-κB pathway. Q3 inhibited NF-κB-induced luciferase in concentrations as low as 5 μM and did not interfere with cell survival or induced cell death. A real-time PCR analysis revealed that Q3 could inhibit the TNF-induced transcription of the luciferase gene, as well as the TNF gene, a known downstream target gene. Immunocytochemistry studies revealed that Q3 moderately interferes with TNF-induced NF-κB nuclear translocation. Moreover, docking and molecular dynamics analyses confirmed that Q3 could potentially modulate transcriptional activity by inhibiting the interaction of NF-κB and DNA. Therefore, Q3 could be potentially developed for further in vivo studies as an NF-κB inhibitor.

Therapeutic targeting of nuclear export and import receptors in cancer and their potential in combination chemotherapy

Systemic modalities are crucial in the management of disseminated malignancies and liquid tumours. However, patient responses and tolerability to treatment are generally poor and those that enter remission often return with refractory disease. Combination therapies provide a methodology to overcome chemoresistance mechanisms and address dose-limiting toxicities. A deeper understanding of tumorigenic processes at the molecular level has brought a targeted therapy approach to the forefront of cancer research, and novel cancer biomarkers are being identified at a rapid rate, with some showing potential therapeutic benefits. The Karyopherin superfamily of proteins is soluble receptors that mediate nucleocytoplasmic shuttling of proteins and RNAs, and recently, nuclear transport receptors have been recognized as novel anticancer targets. Inhibitors against nuclear export have been approved for clinical use against certain cancer types, whereas inhibitors against nuclear import are in preclinical stages of investigation. Mechanistically, targeting nucleocytoplasmic shuttling has shown to abrogate oncogenic signalling and restore tumour suppressor functions through nuclear sequestration of relevant proteins and mRNAs. Hence, nuclear transport inhibitors display broad spectrum anticancer activity and harbour potential to engage in synergistic interactions with a wide array of cytotoxic agents and other targeted agents. This review is focussed on the most researched nuclear transport receptors in the context of cancer, XPO1 and KPNB1, and highlights how inhibitors targeting these receptors can enhance the therapeutic efficacy of standard of care therapies and novel targeted agents in a combination therapy approach. Furthermore, an updated review on the therapeutic targeting of lesser characterized karyopherin proteins is provided and resistance to clinically approved nuclear export inhibitors is discussed.

Preclinical activity of selinexor in combination with eribulin in uterine leiomyosarcoma

Leiomyosarcoma (LMS) is a rare soft tissue sarcoma (STS) that begins in smooth muscle tissue and most often initiates in the abdomen or uterus. Compared with other uterine cancers, uterine LMS (ULMS) is an aggressive tumor with poor prognosis and a high risk of recurrence and death, regardless of the stage at presentation. Selinexor is a first-in-class selective inhibitor of nuclear export (SINE) compound that reversibly binds to exportin 1 (XPO1), thereby reactivating tumor suppressor proteins and downregulating the expression of oncogenes and DNA damage repair (DDR) proteins. In this study, we evaluated the effects of selinexor in combination with doxorubicin and eribulin in the LMS tumor model in vitro and in vivo. Treatment of selinexor combined with eribulin showed synergistic effects on tumor growth inhibition in SK-UT1 LMS-derived xenografts. Immunohistochemical assessment of the tumor tissues showed a significantly reduced expression of proliferation (Ki67) and XPO1 markers following combination therapy compared to the control group. Global transcriptome analyses on tumor tissue revealed that the combination therapy regulates genes from several key cancer-related pathways that are differentially expressed in ULMS tumors. To our knowledge, this is the first preclinical study demonstrating the anti-cancer therapeutic potential of using a combination of selinexor and eribulin in vivo. Results from this study further warrant clinical testing a combination of chemotherapy agents with selinexor to reduce the morbidity and mortality from ULMS.

Monitoring the Levels of Cellular NF-κB Activation States

The NF-κB signaling system plays an important regulatory role in the control of many biological processes. The activities of NF-κB signaling networks and the expression of their target genes are frequently elevated in pathophysiological situations including inflammation, infection, and cancer. In these conditions, the outcome of NF-κB activity can vary according to (i) differential activation states, (ii) the pattern of genomic recruitment of the NF-κB subunits, and (iii) cellular heterogeneity. Additionally, the cytosolic NF-κB activation steps leading to the liberation of DNA-binding dimers need to be distinguished from the less understood nuclear pathways that are ultimately responsible for NF-κB target gene specificity. This raises the need to more precisely determine the NF-κB activation status not only for the purpose of basic research, but also in (future) clinical applications. Here we review a compendium of different methods that have been developed to assess the NF-κB activation status in vitro and in vivo. We also discuss recent advances that allow the assessment of several NF-κB features simultaneously at the single cell level.

Co-expression analysis identifies neuro-inflammation as a driver of sensory neuron aging in Aplysia californica

Aging of the nervous system is typified by depressed metabolism, compromised proteostasis, and increased inflammation that results in cognitive impairment. Differential expression analysis is a popular technique for exploring the molecular underpinnings of neural aging, but technical drawbacks of the methodology often obscure larger expression patterns. Co-expression analysis offers a robust alternative that allows for identification of networks of genes and their putative central regulators. In an effort to expand upon previous work exploring neural aging in the marine model Aplysia californica, we used weighted gene correlation network analysis to identify co-expression networks in a targeted set of aging sensory neurons in these animals. We identified twelve modules, six of which were strongly positively or negatively associated with aging. Kyoto Encyclopedia of Genes analysis and investigation of central module transcripts identified signatures of metabolic impairment, increased reactive oxygen species, compromised proteostasis, disrupted signaling, and increased inflammation. Although modules with immune character were identified, there was no correlation between genes in Aplysia that increased in expression with aging and the orthologous genes in oyster displaying long-term increases in expression after a virus-like challenge. This suggests anti-viral response is not a driver of Aplysia sensory neuron aging.

Maturation of the Acute Hepatic TLR4/NF-κB Mediated Innate Immune Response Is p65 Dependent in Mice

Compared to adults, neonates are at increased risk of infection. There is a growing recognition that dynamic qualitative and quantitative differences in immunity over development contribute to these observations. The liver plays a key role as an immunologic organ, but whether its contribution to the acute innate immune response changes over lifetime is unknown. We hypothesized that the liver would activate a developmentally-regulated acute innate immune response to intraperitoneal lipopolysaccharide (LPS). We first assessed the hepatic expression and activity of the NF-κB, a key regulator of the innate immune response, at different developmental ages (p0, p3, p7, p35, and adult). Ontogeny of the NF-κB subunits (p65/p50) revealed a reduction in Rela (p65) and Nfkb1 (p105, precursor to p50) gene expression (p0) and p65 subunit protein levels (p0 and p3) vs. older ages. The acute hepatic innate immune response to LPS was associated by the degradation of the NF-κB inhibitory proteins (IκBα and IκBβ), and nuclear translocation of the NF-κB subunit p50 in all ages, whereas nuclear translocation of the NF-κB subunit p65 was only observed in the p35 and adult mouse. Consistent with these findings, we detected NF-κB subunit p65 nuclear staining exclusively in the LPS-exposed adult liver compared with p7 mouse. We next interrogated the LPS-induced hepatic expression of pro-inflammatory genes (Tnf, Icam1, Ccl3, and Traf1), and observed a gradually increase in gene expression starting from p0. Confirming our results, hepatic NF-κB subunit p65 nuclear translocation was associated with up-regulation of the Icam1 gene in the adult, and was not detected in the p7 mouse. Thus, an inflammatory challenge induces an NF-κB-mediated hepatic innate immune response activation across all developmental ages, but nuclear translocation of the NF-κB subunit p65 and associated induction of pro-inflammatory genes occurred only after the first month of life. Our results demonstrate that the LPS-induced hepatic innate immune response is developmentally regulated by the NF-κB subunit p65 in the mouse.

Synergistic Effects of Bortezomib-OV Therapy and Anti-Invasive Strategies in Glioblastoma: A Mathematical Model

It is well-known that the tumor microenvironment (TME) plays an important role in the regulation of tumor growth and the efficacy of anti-tumor therapies. Recent studies have demonstrated the potential of combination therapies, using oncolytic viruses (OVs) in conjunction with proteosome inhibitors for the treatment of glioblastoma, but the role of the TME in such therapies has not been studied. In this paper, we develop a mathematical model for combination therapies based on the proteosome inhibitor bortezomib and the oncolytic herpes simplex virus (oHSV), with the goal of understanding their roles in bortezomib-induced endoplasmic reticulum (ER) stress, and how the balance between apoptosis and necroptosis is affected by the treatment protocol. We show that the TME plays a significant role in anti-tumor efficacy in OV combination therapy, and illustrate the effect of different spatial patterns of OV injection. The results illustrate a possible phenotypic switch within tumor populations in a given microenvironment, and suggest new anti-invasion therapies.

The Crosstalk of Endoplasmic Reticulum (ER) Stress Pathways with NF-κB: Complex Mechanisms Relevant for Cancer, Inflammation and Infection

Stressful conditions occuring during cancer, inflammation or infection activate adaptive responses that are controlled by the unfolded protein response (UPR) and the nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) signaling pathway. These systems can be triggered by chemical compounds but also by cytokines, toll-like receptor ligands, nucleic acids, lipids, bacteria and viruses. Despite representing unique signaling cascades, new data indicate that the UPR and NF-κB pathways converge within the nucleus through ten major transcription factors (TFs), namely activating transcription factor (ATF)4, ATF3, CCAAT/enhancer-binding protein (CEBP) homologous protein (CHOP), X-box-binding protein (XBP)1, ATF6α and the five NF-κB subunits. The combinatorial occupancy of numerous genomic regions (enhancers and promoters) coordinates the transcriptional activation or repression of hundreds of genes that collectively determine the balance between metabolic and inflammatory phenotypes and the extent of apoptosis and autophagy or repair of cell damage and survival. Here, we also discuss results from genetic experiments and chemical activators of endoplasmic reticulum (ER) stress that suggest a link to the cytosolic inhibitor of NF-κB (IκB)α degradation pathway. These data show that the UPR affects this major control point of NF-κB activation through several mechanisms. Taken together, available evidence indicates that the UPR and NF-κB interact at multiple levels. This crosstalk provides ample opportunities to fine-tune cellular stress responses and could also be exploited therapeutically in the future.

Discovery of novel potent nuclear factor kappa-B inhibitors (IKK-β) via extensive ligand-based modeling and virtual screening

Inhibitor kappa-B kinase-beta (IKK-β) controls the activation of nuclear transcription factor kappa-B and has been linked to inflammation and cancer. Therefore, inhibitors of this kinase should have potent anti-inflammatory and anticancer properties. Accordingly, we explored the pharmacophoric space of 218 IKK-β inhibitors to identify high-quality binding models. Subsequently, genetic algorithm-based quantitative structure activity relationship (QSAR) analysis was employed to select the best possible combination of pharmacophoric models and physicochemical descriptors that explain bioactivity variation among training compounds. Three successful pharmacophores emerged in 2 optimal QSAR equations (r12175  = 0.733, r12LOO  = 0.52, F1 = 65.62, r12PRESS against 43 test inhibitors = 0.63 and r22175  = 0.683, r22LOO  = 0.52, F2 = 72.66, r22PRESS against 43 test inhibitors = 0.65). Two pharmacophores were merged in a single binding model. Receiver operating characteristic curve validation proved the excellent qualities of this model. The merged pharmacophore and the associated QSAR equations were applied to screen the National Cancer Institute list of compounds. Ten hits were found to exhibit potent anti-IKK-β bioactivity, out of which, one illustrates IC50 of 11.0nM.

Selinexor, a Selective Inhibitor of Nuclear Export (SINE) compound, acts through NF-κB deactivation and combines with proteasome inhibitors to synergistically induce tumor cell death

The nuclear export protein, exportin-1 (XPO1/CRM1), is overexpressed in many cancers and correlates with poor prognosis. Selinexor, a first-in-class Selective Inhibitor of Nuclear Export (SINE) compound, binds covalently to XPO1 and blocks its function. Treatment of cancer cells with selinexor results in nuclear retention of major tumor suppressor proteins and cell cycle regulators, leading to growth arrest and apoptosis. Recently, we described the selection of SINE compound resistant cells and reported elevated expression of inflammation-related genes in these cells. Here, we demonstrated that NF-κB transcriptional activity is up-regulated in cells that are naturally resistant or have acquired resistance to SINE compounds. Resistance to SINE compounds was created by knockdown of the cellular NF-κB inhibitor, IκB-α. Combination treatment of selinexor with proteasome inhibitors decreased NF-κB activity, sensitized SINE compound resistant cells and showed synergistic cytotoxicity in vitro and in vivo. Furthermore, we showed that selinexor inhibited NF-κB activity by blocking phosphorylation of the IκB-α and the NF-κB p65 subunits, protecting IκB-α from proteasome degradation and trapping IκB-α in the nucleus to suppress NF-κB activity. Therefore, combination treatment of selinexor with a proteasome inhibitor may be beneficial to patients with resistance to either single-agent.

Pseudomonas aeruginosa quorum-sensing molecule homoserine lactone modulates inflammatory signaling through PERK and eI-F2α

Pseudomonas aeruginosa secrete N-(3-oxododecanoyl)-homoserine lactone (HSL-C12) as a quorum-sensing molecule to regulate bacterial gene expression. Because HSL-C12 is membrane permeant, multiple cell types in P. aeruginosa-infected airways may be exposed to HSL-C12, especially adjacent to biofilms where local (HSL-C12) may be high. Previous reports showed that HSL-C12 causes both pro- and anti-inflammatory effects. To characterize HSL-C12's pro- and anti-inflammatory effects in host cells, we measured protein synthesis, NF-κB activation, and KC (mouse IL-8) and IL-6 mRNA and protein secretion in wild-type mouse embryonic fibroblasts (MEF). To test the role of the endoplasmic reticulum stress inducer, PERK we compared these responses in PERK(-/-) and PERK-corrected PERK(-/-) MEF. During 4-h treatments of wild-type MEF, HSL-C12 potentially activated NF-κB p65 by preventing the resynthesis of IκB and increased transcription of KC and IL-6 genes (quantitative PCR). HSL-C12 also inhibited secretion of KC and/or IL-6 into the media (ELISA) both in control conditions and also during stimulation by TNF-α. HSL-C12 also activated PERK (as shown by increased phosphorylation of eI-F2α) and inhibited protein synthesis (as measured by incorporation of [(35)S]methionine by MEF). Comparisons of PERK(-/-) and PERK-corrected MEF showed that HSL-C12's effects were explained in part by activation of PERK→phosphorylation of eI-F2α→inhibition of protein synthesis→reduced IκBα production→activation of NF-κB→increased transcription of the KC gene but reduced translation and secretion of KC. HSL-C12 may be an important modulator of early (up to 4 h) inflammatory signaling in P. aeruginosa infections.

A novel mechanism of crosstalk between the p53 and NFκB pathways: MDM2 binds and inhibits p65RelA

The inflammation regulating transcription factor NFκB and the tumor-suppressing transcription factor p53 can act as functional antagonists. Chronic inflammation (NFκB activity) may contribute to the development of cancer through the inhibition of p53 function, while, conversely, p53 activity may dampen inflammation. Here we report that the E3 ubiquitin ligase MDM2, whose gene is transcriptionally activated by both NFκB and p53, can bind and inhibit the p65RelA subunit of NFκB. The interaction is mediated through the N-terminal and the acidic/zinc finger domains of MDM2 on the one hand and through the N-terminal Rel homology domain of p65RelA on the other hand. Co-expression of MDM2 and p65RelA caused ubiquitination of the latter in the nucleus, and this modification was dependent of a functional MDM2 RING domain. Conversely, inhibition of endogenous MDM2 by small-molecule inhibitors or siRNA significantly reduced the ubiquitination of ectopic and endogenous p65RelA. MDM2 was able to equip p65RelA with mutated ubiquitin moieties capable of multiple monoubiquitination but incapable of polyubiquitination; moreover, MDM2 failed to destabilize p65RelA detectably, suggesting that the ubiquitin modification of p65RelA by MDM2 was mostly regulatory rather than stability-determining. MDM2 inhibited the NFκB-mediated transactivation of a reporter gene and the binding of NFκB to its DNA binding motif in vitro. Finally, knockdown of endogenous MDM2 increased the activity of endogenous NFκB as a transactivator. Thus, MDM2 can act as a direct negative regulator of NFκB by binding and inhibiting p65RelA.

Monoubiquitination of nuclear RelA negatively regulates NF-κB activity independent of proteasomal degradation

Termination and resolution of inflammation are tightly linked to the inactivation of one of its strongest inducers, NF-κB. While canonical post-stimulus inactivation is achieved by upregulation of inhibitory molecules that relocate NF-κB complexes to the cytoplasm, termination of the NF-κB response can also be accomplished directly in the nucleus by posttranslational modifications, e.g., ubiquitination of the RelA subunit. Here we reveal a functional role for RelA monoubiquitination in regulating NF-κB activity. By employing serine-to-alanine mutants, we found that hypo-phosphorylated nuclear RelA is monoubiquitinated on multiple lysine residues. Ubiquitination was reversed by IκBα expression and was reduced when nuclear translocation was inhibited. RelA monoubiquitination decreased NF-κB transcriptional activity despite prolonged nuclear presence and independently of RelA degradation, possibly through decreased CREB-binding protein (CBP) co-activator binding. Polyubiquitin-triggered proteasomal degradation has been proposed as a model for RelA inactivation. However, here we show that proteasomal inhibition, similar to RelA hypo-phosphorylation, resulted in nuclear translocation and monoubiquitination of RelA. These findings indicate a degradation-independent mechanism for regulating the activity of nuclear RelA by ubiquitination.

Expression of classical NF-kappaB pathway effectors in human ovarian carcinoma

AIMS

Functional studies have demonstrated that nuclear factor (NF)-kappaB promotes tumour progression in ovarian cancer cells. However, surprisingly little is known of the expression of effectors of the NF-kappaB pathway in human ovarian cancer in vivo.

METHODS AND RESULTS

Immunohistochemistry and in situ hybridization revealed that in a cohort of 85 primary ovarian carcinomas, total p65 expression was inversely correlated to nuclear and cytoplasmic phospho-IkappaBalpha (P = 0.002 and P = 0.05, respectively), and IkappaBalpha mRNA expression (P = 0.032). In contrast, phospho-p65 expression was paralleled by the expression of nuclear (P = 0.027) and cytoplasmic phospho-IkappaBalpha (P = 0.01). Total p65 expression was an adverse prognostic factor for overall survival (P = 0.018). In contrast, total IkappaBalpha and phosphorylated nuclear and cytoplasmic IkappaBalpha expression were favourable prognostic markers (P = 0.001, P = 0.031, P = 0.001, respectively). Cytoplasmic phospho-IkappaBalpha expression remained a significant prognostic factor on multivariate analysis (P = 0.010). In cultured, stimulated OVCAR-3 ovarian cancer cells the cytoplasmic retranslocation of p65 was delayed by inhibition of the nuclear membrane transporter chromosomal region maintenance/exportin1 protein (CRM1). A positive association of p65 and CRM1 expression was demonstrated in ovarian cancer tissue (P < 0.0001).

CONCLUSIONS

Total and phosphorylated IkappaBalpha protein expression might serve as markers for NF-kappaB activation in human ovarian carcinoma. Cytoplasmic localization of p65 may be related to deregulated nucleocytoplasmic transport in carcinomas overexpressing CRM1.

Ubiquitin-independent degradation of proteins by the proteasome

The proteasome is the main proteolytic machinery of the cell and constitutes a recognized drugable target, in particular for treating cancer. It is involved in the elimination of misfolded, altered or aged proteins as well as in the generation of antigenic peptides presented by MHC class I molecules. It is also responsible for the proteolytic maturation of diverse polypeptide precursors and for the spatial and temporal regulation of the degradation of many key cell regulators whose destruction is necessary for progression through essential processes, such as cell division, differentiation and, more generally, adaptation to environmental signals. It is generally believed that proteins must undergo prior modification by polyubiquitin chains to be addressed to, and recognized by, the proteasome. In reality, however, there is accumulating evidence that ubiquitin-independent proteasomal degradation may have been largely underestimated. In particular, a number of proto-oncoproteins and oncosuppressive proteins are privileged ubiquitin-independent proteasomal substrates, the altered degradation of which may have tumorigenic consequences. The identification of ubiquitin-independent mechanisms for proteasomal degradation also poses the paramount question of the multiplicity of catabolic pathways targeting each protein substrate. As this may help design novel therapeutic strategies, the underlying mechanisms are critically reviewed here.

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.

World Health Organization and beyond: new aspects in the pathology of an old disease

Hodgkin's lymphoma is a highly enigmatic lymphoma disease that still covers most of its secrets up to now. Much effort has been made to successfully wrest at least some of the pathogenetic particularities. The current diagnostic criteria are well established allowing hemato-pathologists to make a clear-cut distinction from other lymphomas in almost all cases. Although classic Hodgkin's lymphoma is curable in the vast majority of cases by treatment with highly aggressive drugs with or without radiotherapy, further molecular studies may lead to the identification of therapeutic targets that enable a more tailored treatment with fewer side effects.

Monocyte-astrocyte networks regulate matrix metalloproteinase gene expression and secretion in central nervous system tuberculosis in vitro and in vivo

CNS tuberculosis (CNS-TB) is the most deadly form of tuberculous disease accounting for 10% of clinical cases. CNS-TB is characterized by extensive tissue destruction, in which matrix metalloproteinases (MMPs) may play a critical role. We investigated the hypothesis that Mycobacterium tuberculosis activates monocyte-astrocyte networks increasing the activity of key MMPs. We examined the expression of all human MMPs and the tissue inhibitors of metalloproteinases (TIMPs) in human astrocytes stimulated by conditioned medium from M. tuberculosis-infected monocytes (CoMTB). Real-time RT-PCR showed that gene expression of MMP-1, -2, -3, -7, and -9 was increased (p < 0.05). MMP-9 secretion was significantly up-regulated at 24 h and increased over 120 h (p < 0.01). MMP-1, -3, and -7 secretion was not detected. Secretion of MMP-2 was constitutive and unaffected by CoMTB. Astrocyte gene expression and secretion of TIMP-1 was not affected by CoMTB although TIMP-2 secretion increased 3-fold at 120 h. Immunohistochemical analysis of human brain biopsies confirmed that astrocyte MMP-9 secretion is a predominant feature in CNS-TB in vivo. Dexamethasone inhibited astrocyte MMP-9, but not TIMP-1/2 secretion in response to CoMTB. CoMTB stimulated the nuclear translocation of NF-kappaB, inducing a 6-fold increase in nuclear p65 and a 2-fold increase in nuclear p50. This was associated with degradation of IkappaBalpha and beta within 30 min, persisting for 24 h. In summary, networks active between monocytes and astrocytes regulate MMP-9 activity in tuberculosis and astrocytes are a major source of MMP-9 in CNS-TB. Astrocytes may contribute to a matrix degrading environment within the CNS and subsequent morbidity and mortality.

Hepatitis C virus nonstructural 5B protein regulates tumor necrosis factor alpha signaling through effects on cellular IkappaB kinase

Hepatitis C virus (HCV) NS5B protein is a membrane-associated phosphoprotein that possesses an RNA-dependent RNA polymerase activity. We recently reported that NS5A protein interacts with TRAF2 and modulates tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB and Jun N-terminal protein kinase (JNK). Since NS5A and NS5B are the essential components of the HCV replication complex, we examined whether NS5B could modulate TNF-alpha-induced NF-kappaB and JNK activation. In this study, we have demonstrated that TNF-alpha-induced NF-kappaB activation is inhibited by NS5B protein in HEK293 and hepatic cells. Furthermore, NS5B protein inhibited both TRAF2- and IKK-induced NF-kappaB activation. Using coimmunoprecipitation assays, we show that NS5B interacts with IKKalpha. Most importantly, NS5B protein in HCV subgenomic replicon cells interacted with endogenous IKKalpha, and then TNF-alpha-mediated IKKalpha kinase activation was significantly decreased by NS5B. Using in vitro kinase assay, we have further found that NS5B protein synergistically activated TNF-alpha-mediated JNK activity in HEK293 and hepatic cells. These data suggest that NS5B protein modulates TNF-alpha signaling pathways and may contribute to HCV pathogenesis.

Inflammatory Cytokines Induce Production of CHI3L1 by Articular Chondrocytes

Elevated levels of CHI3L1 (chitinase-3-like protein 1) are associated with disorders exhibiting increased connective tissue turnover, such as rheumatoid arthritis, osteoarthritis, scleroderma, and cirrhosis of the liver. This secreted protein is not synthesized in young healthy cartilage, but is produced in cartilage from old donors or patients with osteoarthritis. The molecular processes governing the induction of CHI3L1 are currently unknown. To elucidate the molecular events involved in CHI3L1 synthesis, we investigated two models of articular chondrocytes: neonatal rat chondrocytes, which do not express CHI3L1, and human chondrocytes, which express CHI3L1 constitutively. In neonatal rat chondrocytes, the inflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 potently induced steady-state levels of CHI3L1 mRNA and protein secretion. Treatment of chondrocytes with TNF-alpha for as little as 1 h was sufficient for sustained induction up to 72 h afterward. Using inhibitors selective for the major signaling pathways implicated in mediating the effects of TNF-alpha and interleukin-1, only inhibition of NF-kappaB activation was effective in curtailing cytokine-induced expression, including after removal of the cytokine, indicating that induction and continued production of CHI3L1 are controlled mainly by this transcription factor. Inhibition of NF-kappaB signaling also abolished constitutive expression by human chondrocytes. Thus, induction and continued secretion of CHI3L1 in chondrocytes require sustained activation of NF-kappaB. Selective induction of CHI3L1 by cytokines acting through NF-kappaB coupled with the known restriction of the catabolic responses by CHI3L1 in response to these inflammatory cytokines represents a key regulatory feedback process in controlling connective tissue turnover.

Downregulation of NF-kappaB activity associated with alteration in proliferative response in the spleen after burn injury

Alterations in proliferation status and cellular composition of immune organs are among key events in the modulation of immune function after burn injury. Nuclear factor (NF)-kappaB is a transcription factor that plays a pivotal role in the response to injury as well as immune cell differentiation and proliferation. In this study, we investigated the effects of burn injury on the activity of NF-kappaB and its association with cellular proliferation in the spleen. Western analysis of whole spleen tissues of mice after 18% burn injury revealed a marked reduction in nuclear NF-kappaB rel A protein expression 3 to 21 days after injury when there was an increase in proliferative activity in the red pulp of the spleen after injury as indicated by an increase in proliferating cell nuclear antigen (PCNA). In the splenic B cells, however, the down-regulation of NF-kappaB rel A was associated with decreased PCNA expression as well as IkappaBalpha and phosphorylated IkappaBalpha. In contrast, no significant change in NF-kappaB rel A or PCNA expression was observed for splenic T cells. These data suggest that there is a differential regulation of NF-kappaB and proliferative activity in the splenic cell subsets after burn injury. Furthermore, the regulation of NF-kappaB may be linked to the proliferative changes seen in the spleen after burn injury.

Translational repression mediates activation of nuclear factor kappa B by phosphorylated translation initiation factor 2

Numerous stressful conditions activate kinases that phosphorylate the alpha subunit of translation initiation factor 2 (eIF2alpha), thus attenuating mRNA translation and activating a gene expression program known as the integrated stress response. It has been noted that conditions associated with eIF2alpha phosphorylation, notably accumulation of unfolded proteins in the endoplasmic reticulum (ER), or ER stress, are also associated with activation of nuclear factor kappa B (NF-kappaB) and that eIF2alpha phosphorylation is required for NF-kappaB activation by ER stress. We have used a pharmacologically activable version of pancreatic ER kinase (PERK, an ER stress-responsive eIF2alpha kinase) to uncouple eIF2alpha phosphorylation from stress and found that phosphorylation of eIF2alpha is both necessary and sufficient to activate both NF-kappaB DNA binding and an NF-kappaB reporter gene. eIF2alpha phosphorylation-dependent NF-kappaB activation correlated with decreased levels of the inhibitor IkappaBalpha protein. Unlike canonical signaling pathways that promote IkappaBalpha phosphorylation and degradation, eIF2alpha phosphorylation did not increase phosphorylated IkappaBalpha levels or affect the stability of the protein. Pulse-chase labeling experiments indicate instead that repression of IkappaBalpha translation plays an important role in NF-kappaB activation in cells experiencing high levels of eIF2alpha phosphorylation. These studies suggest a direct role for eIF2alpha phosphorylation-dependent translational control in activating NF-kappaB during ER stress.

A mechanistic insight into a proteasome-independent constitutive inhibitor kappaBalpha (IkappaBalpha) degradation and nuclear factor kappaB (NF-kappaB) activation pathway in WEHI-231 B-cells

Inducible activation of the transcription factor NF-kappaB (nuclear factor kappaB) is classically mediated by proteasomal degradation of its associated inhibitors, IkappaBalpha (inhibitory kappaBalpha) and IkappaBbeta. However, certain B-lymphocytes maintain constitutively nuclear NF-kappaB activity (a p50-c-Rel heterodimer) which is resistant to inhibition by proteasome inhibitors. This activity in the WEHI-231 B-cell line is associated with continual and preferential degradation of IkappaBalpha, which is also unaffected by proteasome inhibitors. Pharmacological studies indicated that there was a correlation between inhibition of IkappaBalpha degradation and constitutive p50-c-Rel activity. Domain analysis of IkappaBalpha by deletion mutagenesis demonstrated that an N-terminal 36-amino-acid sequence of IkappaBalpha represented an instability determinant for constitutive degradation. Moreover, domain grafting studies indicated that this sequence was sufficient to cause IkappaBbeta, but not chloramphenicol acetyltransferase, to be rapidly degraded in WEHI-231 B-cells. However, this sequence was insufficient to target IkappaBbeta to the non-proteasome degradation pathway, suggesting that there was an additional cis-element(s) in IkappaBalpha that was required for complete targeting. Nevertheless, the NF-kappaB pool associated with IkappaBbeta now became constitutively active by virtue of IkappaBbeta instability in these cells. These findings further support the notion that IkappaB instability governs the maintenance of constitutive p50-c-Rel activity in certain B-cells via a unique degradation pathway.

The role of Toll-like receptor 2 in microbial disease and immunity

Cells expressing Toll-like receptor (TLR), TLR2 in association with TLR1, TLR6 or some other unknown co-receptor can respond upon interaction with a large variety of microbial ligands. The variety of TLR2 ligands is the greatest among all the TLRs and this is due to the heterodimerization needed for TLR2 mediated responses. Like other TLRs, TLR2 signaling induces antigen presenting cell activation, pro-inflammatory cytokine production and increased expression of co-stimulatory ligand expression. These events are important for induction of innate immune responses and improved acquired immunity. There is strong suggestive evidence that alteration or lack of TLR2 function in vivo may correlate to decreased immune protection from pathogens that contain TLR2 ligands, but more work needs to be performed to strengthen this correlation.

Distinct pathways for NF-kappa B regulation are associated with aberrant macrophage IL-12 production in lupus- and diabetes-prone mouse strains

One characteristic of mice prone to a variety of autoimmune diseases is the aberrant regulation of cytokine production by macrophages (Mphi), noted in cells isolated well before the onset of disease. Strikingly, the pattern of IL-12 dysregulation, in particular, is consistent with the nature of the autoimmune disease that will develop in each strain, i.e., elevated in mice prone to Th1-mediated organ-specific disease (nonobese diabetic (NOD) and SJL mice) and reduced in lupus-prone strains (MRL/+ and NZB/W). Mechanistically, the abnormal regulation of IL-12 in these strains was found to be strictly associated with novel patterns of Rel binding in vitro to the unique NF-kappaB site in the IL-12 p40 promoter. In this study, we report several new findings related to these Rel-kappaB interactions. Evaluation of the p40 NF-kappaB site in vivo, assessed by chromatin immunoprecipitation, revealed Rel usage patterns similar to those found in vitro using EMSA, with preferential association of the p40 kappaB site with c-Rel in NOD Mphi but with p50 in NZB/W Mphi. Moreover, blocking c-Rel in primary Mphi, using short interfering RNA, selectively blocked IL-12 production and normalized the minimal, residual IL-12 levels. Nuclear extracts from NOD Mphi were characterized by c-Rel hyperphosphorylation, and dephosphorylation of nuclear proteins completely blocked binding to the kappaB site. In contrast, elevated IkappaB appears to be a likely mechanism accounting for the reduced nuclear c-Rel levels noted in NZB/W Mphi. Alterations in NF-kappaB metabolism thus appear to define a pathway regulating intrinsic IL-12 defects in both diabetes- and lupus-prone strains.

Lipopolysaccharide: neutralization by polymyxin B shuts down the signaling pathway of nuclear factor kappaB in peripheral blood mononuclear cells, even during activation

BACKGROUND

There have been many studies on anti-lipopolysaccharide (LPS) agents and LPS-neutralizing agents; however, there have been no reports on the changes in clinical status and mediators that occur when these agents are used. Polymyxin (PMX) (treatment using a column containing polymyxin B-immobilized fiber) removed circulating endotoxin, and reduced various cytokines within 120 min, even in patients with high levels of plasma cytokines. Our purpose was examine the mechanisms of PMX treatment by which plasma cytokines are reduced by endotoxin neutralization with polymyxin B, even during therapy for sepsis and/or endotoxin shock.

METHODS

We studied the interaction between nuclear factor kappaB (NF-kappaB) binding activity and tumor necrosis factor alpha (TNF-alpha) secretion in an experimental system using LPS-stimulated human peripheral blood mononuclear cells (PBMCs), after neutralization of LPS with polymyxin B. PBMCs were incubated with LPS in vitro, and TNF-alpha secretion and NF-kappaB activation were assessed. We then studied the changes in NF-kappaB activation and TNF-alpha secretion when both polymyxin B and LPS were added simultaneously and when polymyxin B was added after 30 or 120 min of incubation with LPS.

RESULTS

Immediate inhibition of NF-kappaB binding activity and suppression of TNF-alpha secretion were observed after LPS neutralization with polymyxin B regardless of whether PBMCs were already producing TNF-alpha.

CONCLUSIONS

These findings may indicate one of the mechanisms operating in the clinical changes that occur after circulating endotoxin removal, and are likely to have therapeutic value, even for patients with high proinflammatory cytokine levels.

Cell-specific association and shuttling of IkappaBalpha provides a mechanism for nuclear NF-kappaB in B lymphocytes

Mature B lymphocytes are unique in containing nuclear Rel proteins prior to cell stimulation. This activity consists largely of p50-c-Rel heterodimers, and its importance for B-cell function is exemplified by reduced B-cell viability in several genetically altered mouse strains. Here we suggest a mechanism for the cell specificity and the subunit composition of constitutive B-cell NF-kappaB based on the observed properties of Rel homo- and heterodimers and IkappaBalpha. We show that c-Rel lacks a nuclear export sequence, making the removal of c-Rel-containing complexes from the nucleus less efficient than removal of p65-containing complexes. Second, the nuclear import potential of p65 and c-Rel homodimers but not p50-associated heterodimers was attenuated when they were complexed to IkappaBalpha, leading to a greater propensity of heterodimers to be nuclear. We propose that subunit composition of B-cell NF-kappaB reflects the inefficient retrieval of p50-c-Rel heterodimers from the nucleus. Cell specificity may be a consequence of c-Rel-IkappaBalpha complexes being present only in mature B cells, which leads to nuclear c-Rel due to IkappaBalpha turnover and shuttling of the complex.

In vitro susceptibility to TRAIL-induced apoptosis of acute leukemia cells in the context of TRAIL receptor gene expression and constitutive NF-kappa B activity

The TNF-related apoptosis-inducing ligand (TRAIL) is currently under evaluation as a possible (co-)therapeutic in cancer treatment. We therefore examined 129 cell samples from patients with de novo acute leukemia as to their constitutive susceptibility to TRAIL-induced apoptosis In vitro. Only 21 (16%) cell samples revealed at least 10% TRAIL-susceptible cells/sample as detected by flow cytometric annexinV staining after 24 h culture compared with medium control. Precursor B cell ALL samples (11 (27%) of 41) were more TRAIL-susceptible compared with AML (5 (9%) of 54; P < 0.05) but not compared with precursor T cell ALL (5 (15%) of 34; P = 0.20). Furthermore, we examined constitutive mRNA expression levels of TRAIL receptors R1-R4 by semi-quantitative RT-PCR (n = 58). Expression levels were heterogeneous, however, there was no significant correlation between the expression of the signal-transducing receptors (R1, R2) as well as of the decoy receptors (R3, R4) and TRAIL sensitivity in this series. Constitutive NF-kappa B activity has been shown to influence TRAIL susceptibility of leukemic cells. In 39 leukemic cell samples examined, we found a generally high NF-kappa B activity as detected by electrophoretic mobility shift assay which did not differ between TRAIL-susceptible and TRAIL-resistant cases. Finally, 49 acute leukemic cell samples were coincubated with doxorubicin in vitro. Doxorubicin sensitized four of 35 initially TRAIL-resistant samples and augmented TRAIL-induced apoptosis in two of 14 TRAIL-susceptible samples. In summary, constitutive TRAIL susceptibility differs between leukemia subtypes and does not correlate with mRNA expression levels of the TRAIL receptors R1-R4 as well as constitutive NF-kappa B activation status. The observed sensitization of leukemic cells to TRAIL by doxorubicin in vitro indicates that TRAIL should be further evaluated as to its possible role as an in vivo cotherapeutic in acute leukemia.

Differential regulation of myocardial NF kappa B following acute or chronic TNF-alpha exposure

Tumor necrosis factor alpha (TNF-alpha) is a critical mediator of myocardial dysfunction during acute inflammatory states. TNF-alpha is also present in the serum of patients with chronic cardiac diseases. In monocytes, TNF-alpha stimulates cells by activating distinct signaling pathways that involve nuclear translocation of NF kappa B. Since NF kappa B may also regulate the expression of genes that could contribute to myocardial dysfunction, the cardiomyocyte NF kappa B activation following acute or chronic TNF-alpha challenges was investigated. To accomplish this, the authors either acutely administered TNF-alpha to healthy mice, or used transgenic mice which chronically overexpress TNF-alpha exclusively in cardiac myocytes. Following acute administration of TNF-alpha, cardiac NF kappa B translocation was detected from 15 min to 2 h post-challenge. The time course of I kappa B alpha degradation was consistent with the kinetics of NF kappa B translocation. I kappa B beta degradation was slower and less dramatic. In transgenic mice chronically overexpressing TNF-alpha, myocardial NF kappa B activation was detected at all ages tested (21, 40, and 75 days). In contrast to acutely challenged animals, two distinct NF kappa B proteins were activated in chronically challenged animals, p50--65 heterodimers as well as p50 homodimers. Activation of both could be transiently blocked by administration of a recombinant chimeric TNF-alpha receptor antagonist (rhTNFR:Fc). I kappa B alpha, but not I kappa B beta, levels were elevated in transgenics when compared to wild-type animals. These data indicate that following acute TNF-alpha administration, which simulates bacterial sepsis, myocardial p50-p65 translocates within minutes. Chronic TNF-alpha exposure, which is thought to occur in long-standing congestive heart failure, results in translocation of transcriptionally inactive p50 homodimers in addition to transcriptionally active p50--65 heterodimers. It is speculated that activation of p50 homodimers constitutes an adaptive response to minimize the inflammatory consequences of chronic cardiac TNF-alpha exposure.

Nuclear factor-kappaB activation is not involved in a MPTP model of Parkinson's disease

In the present study the involvement of hydroxyl free radicals and nuclear factor-kappaB (NF-kappaB) activation was investigated in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model of Parkinson's disease. MPTP (30 mg/kg, s.c.) produced a significant 2-fold increase in hydroxyl free radicals in the striatum of C57BL/6 mice determined by microdialysis in combination with the salicylate hydroxylation assay. Electrophoretic mobility shift assays did not detect NF-kappaB activation after MPTP treatment. Furthermore, p50-deficient mice showed only minor differences in striatal dopamine and metabolite levels as well as tyrosine hydroxylase immunoreactivity after MPTP administration in comparison to wildtype mice. We postulate that, although hydroxyl radical production was enhanced, NF-kappaB plays only a minor role in the MPTP model because neither neurochemical nor immunocytochemical parameters were altered in p50-deficient mice in comparison to controls.

Regulation of estrogenic and nuclear factor kappa B functions by polyamines and their role in polyamine analog-induced apoptosis of breast cancer cells

The natural polyamines -putrescine, spermidine, and spermine- are essential for cell growth and differentiation. Polyamines are involved in several gene regulatory functions, although their mechanism(s) of action has not been elucidated. We investigated the role of polyamines in the function of NF-kappa B and estrogen receptor-alpha (ER alpha), two transcription factors implicated in breast cancer cell proliferation and cell survival, using MCF-7 breast cancer cells. We found that spermine facilitated the binding of ER alpha and NF-kappa B to estrogen response element (ERE)- and NF-kappa B response element (NRE), respectively, and enhanced ER alpha-mediated transcriptional activation in transient transfection experiments. We also found that the association of the co-regulatory protein CBP/p300 with ER alpha and NF-kappa B was increased by spermine treatment of MCF-7 cells. Spermine also increased the nuclear translocation of NF-kappa B compared to the control. In contrast, treatment of MCF-7 cells with polyamine analogs, BE-3-4-3 and BE-3-3-3, resulted in transcriptional inhibition of both ERE- and NRE-driven reporter plasmids. In addition, polyamine analogs inhibited the association of ER alpha and NF-kappa B with CBP/p300 and were unable to facilitate nuclear translocation of NF-kappa B. APO-BRDU assay demonstrated that polyamine analogs induced apoptosis, with a loss of the anti-apoptotic protein Bcl-2. These data show a gene regulatory function of polyamines involving transcriptional activation of ER alpha and NF-kappa B, potentially leading to the up-regulation of genes involved in breast cancer cell proliferation. Our results with BE-3-4-3 and BE-3-3-3 suggest that down-regulation of ER alpha- and NF-kappa B-regulated genes is a possible mechanism for the action of polyamine analogs in inducing apoptosis of breast cancer cells.

Cardiac-specific abrogation of NF- kappa B activation in mice by transdominant expression of a mutant I kappa B alpha

Nuclear factor-kappaB (NF-kappa B) is a pleiotropic oxidant-sensitive transcription factor that is present in the cytosol in an inactive form complexed to an inhibitory kappaB (I kappa B) monomer. Various stimuli, including ischemia, hypoxia, free radicals, cytokines, and lipopolysaccharide (LPS), activate NF-kappa B by inducing phosphorylation of I kappa B. Phosphorylation of serine residues at positions 32 and 36 is critical for ubiquitination and degradation of I kappa B alpha with consequent migration of NF-kappa B to the nucleus. Although NF-kappa B is thought to contribute to numerous pathophysiologic processes, definitive assessment of its role has been hindered by the inability to achieve specific inhibition in vivo. Pharmacologic inhibitors of NF-kappa B are available, but their utility for in vivo studies is limited by their relative lack of specificity. Targeted ablation of genes encoding NF-kappa B subunits has not been productive in this regard because of fetal lethality in the case of p65 and functional redundancy in the Rel family of proteins. To overcome these limitations, we have created a viable transgenic mouse that expresses a phosphorylation-resistant mutant of I kappa B alpha (I kappa B alpha(S32A,S36A)) under the direction of a cardiac-specific promoter. Several transgenic lines were obtained with copy numbers ranging from one to seven. The mice exhibit normal cardiac morphology and histology. Total myocardial I kappa B alpha protein level is elevated 3.5- to 6.5-fold with a concomitant 50-60% decrease in the level of I kappa B beta. Importantly, expression of I kappa B(S32A,S36A) results in complete abrogation of myocardial NF-kappa B activation in response to tumor necrosis factor- alpha (TNF-alpha) and LPS stimulation. Thus, novel transgenic mice have been created that make it possible to achieve cardiac-specific and selective inhibition of NF-kappa B in vivo. These transgenic mice should be useful in studies of various cardiac pathophysiological phenomena that involve NF-kappa B activation, including ischemic preconditioning, heart failure, septic shock, acute coronary syndromes, cardiac allograft rejection, and apoptosis.

Activation of a rel-A/CEBP-beta-related transcription factor heteromer by PGG-glucan in a murine monocytic cell line

PGG-Glucan is a soluble beta-glucan immunomodulator that enhances a variety of leukocyte microbicidal activities without activating inflammatory cytokines. Although several different cell surface receptors for soluble (and particulate) beta-glucans have been described, the signal transduction pathway(s) used by these soluble ligands have not been elucidated. Previously we reported that PGG-Glucan treatment of mouse BMC2.3 macrophage cells activates a nuclear factor kappa-B-like (NF-kappaB) transcription factor complex containing subunit p65 (rel-A) attached to an unidentified cohort. In this study, we identify the cohort to be a non-rel family member: a CCAAT enhancer-binding protein-beta (C/EBP-beta)-related molecule with an apparent size of 48 kDa, which is a different protein than the previously identified C/EBP-beta p34 also present in these cells. C/EBP-beta is a member of the bZIP family whose members have previously been shown to interact with rel family members. This rel/bZIP heteromer complex activated by PGG-Glucan is different from the p65/p50 rel/rel complex induced in these cells by lipopolysaccharide (LPS). Thus, our data demonstrate that PGG-Glucan uses signal transduction pathways different from those used by LPS, which activates leukocyte microbicidal activities and inflammatory cytokines. We further show that heteromer activation appears to use protein kinase C (PKC) and protein tyrosine kinase (PTK) pathways, but not mitogen-activated protein kinase p38. Inhibitor kappa-B-alpha (IkappaB-alpha) is associated with the heteromer; this association decreases after PGG-Glucan treatment. These data are consistent with a model whereby treatment of BMC2.3 cells with PGG-Glucan activates IkappaB-alpha via PKC and/or PTK pathways, permitting translocation of the rel-A/CEBP-beta heteromer complex to the nucleus and increases its DNA-binding affinity.

The kinetics of association and phosphorylation of IkappaB isoforms by IkappaB kinase 2 correlate with their cellular regulation in human endothelial cells

Activation of the transcription factor NF-kappaB depends on the specific dual phosphorylation of its inhibitor protein IkappaB by the homologous cytokine-inducible IkappaB kinases 1 and 2 (IKK1/2). Various IkappaB isoforms exist: IkappaBalpha, IkappaBbeta1/2 (two alternative splice variants), and IkappaBepsilon. However, the individual relevance and the specific regulation of these isoforms is not well-understood. We have studied the direct interaction of recombinant IkappaBalpha, IkappaBbeta1, IkappaBbeta2, and IkappaBepsilon with the recombinant homodimeric IKK2. Fluorescence-based active site titration revealed that each IKK2 dimer contains two binding sites for IkappaB. By using surface plasmon resonance analysis, we found that all IkappaB proteins interact with the IKK2 dimer following a noncooperative binding mechanism. Further, the four IkappaB proteins bind to the kinase with equilibrium dissociation constants (KD) in the range of 50-300 nM; the association rate constants for all IkappaB isoforms with IKK2 were between 6.0 x 10(3) and 22.5 x 10(3) M-1 s-1, and the dissociation rate constants were between 1.25 x 10(-3) and 1.75 x 10(-3) s-1. This high-affinity binding suggests that the previously observed preassociation of all analyzed IkappaB proteins with the biochemically purified 700 kDa IkappaB kinase (IKK) complex is based on a direct enzyme-substrate association between the various IkappaB isoforms and the IKK proteins. The apparent catalytic efficiencies (kcat/KM) of IKK2 for IkappaBalpha, IkappaBbeta1, IkappaBbeta2, and IkappaBepsilon were 22 x 10(3), 10 x 10(3), 5.4 x 10(3), and 8.5 x 10(3) s-1 M-1, respectively, with KM values ranging between 1.7 x 10(-6) and 3.2 x 10(-6) M and kcat values ranging between 1.5 x 10(-2) and 3.7 x 10(-2) s-1. The relative affinities and catalytic efficiencies of IKK2 for the IkappaB isoforms were also reflected by the kinetics observed for the TNF-induced, phosphorylation-dependent degradation of the alpha, beta1, beta2, and epsilon isoforms of IkappaB in human umbilical vein endothelial cells. Therefore, differential regulation of the IkappaB isoforms in some cell types is not a direct result of the IKK activity, but appears to be due to parallel events.

Molecular mechanisms of constitutive NF-kappaB/Rel activation in Hodgkin/Reed-Sternberg cells

A common characteristic of malignant cells derived from patients with Hodgkin's disease (HD) is a high level of constitutive nuclear NF-kappaB/Rel activity, which stimulates proliferation and confers resistance to apoptosis. We have analysed the mechanisms that account for NF-kappaB activation in a panel of Hodgkin/Reed-Sternberg (H-RS) cell lines. Whereas two cell lines (L428 and KMH-2) expressed inactive IkappaBalpha, no significant changes in NF-kappaB or IkappaB expression were seen in other H-RS cells (L591, L1236 and HDLM-2). Constitutive NF-kappaB was susceptible to inhibition by recombinant IkappaBalpha, suggesting that neither mutations in the NF-kappaB genes nor posttranslational modifications of NF-kappaB were involved. Endogenous IkappaBalpha was bound to p65 and displayed a very short half-life. IkappaBalpha degradation could be blocked by inhibitors of the NF-kappaB activating pathway. Proteasomal inhibition caused an accumulation of phosphorylated IkappaBalpha and a reduction of NF-kappaB activity in HDLM-2 and L1236 cells. By in vitro kinase assays we demonstrate constitutive IkappaB kinase (IKK) activity in H-RS cells, indicating ongoing signal transduction. Furthermore, H-RS cells secrete one or more factor(s) that were able to trigger NF-kappaB activation. We conclude that aberrant activation of IKK's, and in some cases defective IkappaBs, lead to constitutive nuclear NF-kappaB activity, which in turn results in a growth advantage of Hodgkin's disease tumor cells.