Molecular determinants of NF-kappaB-inducing kinase action

The Therapeutic Potential of Targeting NIK in B Cell Malignancies

NF-κB-inducing kinase (NIK) is a key player in non-canonical NF-κB signaling, involved in several fundamental cellular processes, and is crucial for B cell function and development. In response to certain signals and ligands, such as CD40, BAFF and lymphotoxin-β activation, NIK protein stabilization and subsequent NF-κB activation is achieved. Overexpression or overactivation of NIK is associated with several malignancies, including activating mutations in multiple myeloma (MM) and gain-of-function in MALT lymphoma as a result of post-translational modifications. Consequently, drug discovery studies are devoted to pharmacologic modulation of NIK and development of specific novel small molecule inhibitors. However, disease-specific in vitro and in vivo studies investigating NIK inhibition are as of yet lacking, and clinical trials with NIK inhibitors remain to be initiated. In order to bridge the gap between bench and bedside, this review first briefly summarizes our current knowledge on NIK activation, functional activity and stability. Secondly, we compare current inhibitors targeting NIK based on efficacy and specificity, and provide a future perspective on the therapeutic potential of NIK inhibition in B cell malignancies.

The Tumor Suppressor, p53, Negatively Regulates Non-Canonical NF-κB Signaling through miRNAInduced Silencing of NF-κB-Inducing Kinase

NF-κB signaling through both canonical and non-canonical pathways plays a central role in immune responses and inflammation. NF-κB-inducing kinase (NIK) stabilization is a key step in activation of the non-canonical pathway and its dysregulation implicated in various hematologic malignancies. The tumor suppressor, p53, is an established cellular gatekeeper of proliferation. Abnormalities of the TP53 gene have been detected in more than half of all human cancers. While the non-canonical NF-κB and p53 pathways have been explored for several decades, no studies to date have documented potential cross-talk between these two cancer-related mechanisms. Here, we demonstrate that p53 negatively regulates NIK in an miRNA-dependent manner. Overexpression of p53 decreased the levels of NIK, leading to inhibition of the non-canonical NF-κB pathway. Conversely, its knockdown led to increased levels of NIK, IKKα phosphorylation, and p100 processing. Additionally, miR-34b induced by nutlin-3 directly targeted the coding sequences (CDS) of NIK. Treatment with anti-miR-34b-5p augmented NIK levels and subsequent non-canonical NF-κB signaling. Our collective findings support a novel cross-talk mechanism between non-canonical NF-κB and p53.

NF-κB/mTOR-mediated autophagy can regulate diquat-induced apoptosis

Autophagy and apoptosis are the major types of cell death in pesticide-induced neurotoxicity, and autophagy is known to play a role in cell protection by inhibiting apoptosis. In this study, we characterized the relationship between autophagy and apoptosis in diquat (DQ)-induced cell death and explored a novel pharmacotherapeutic approach involving autophagy regulation to prevent DQ neurotoxicity. DQ was cytotoxic to PC12 cells in a concentration-dependent manner, as shown by decreased cell viability and decreased dopamine (DA) levels. DQ-induced apoptosis was found in PC12 cells, as demonstrated by activation of caspase-3 and -9 and by nuclear condensation. By monitoring expression of microtubule-associated protein 1A/1B light chain 3B (LC3-II) and p62, DQ was found to induce autophagy. Exposure of PC12 cells to DQ led to the production of reactive oxygen species (ROS), and N-acetyl-cysteine (NAC) antioxidant effectively blocked both apoptosis and autophagy. Interestingly, DQ in PC12 cells showed increased p53 and NF-κB in a time-dependent manner; furthermore, pifithrin-α (PFT-α), a p53 inhibitor, downregulates the cytotoxicity of DQ, as shown by decreased LC3-II and cleaved caspase-3. SN50, an NF-κB inhibitor, results in diminished LC3-II, cleaved caspase-3, and p53. DQ induces mitogen-activated protein kinase (MAPK) signaling including ERK, JNK, and p38, which inhibit regulated apoptosis and autophagic cell death by controlling mTOR signaling. In addition, modulation of DQ-induced apoptosis in response to autophagy regulation was investigated. Pretreatment with rapamycin, an autophagy inducer, significantly enhanced the viability of DQ-exposed cells by alleviating DQ-induced apoptosis. Conversely, cell pretreatment with 3-methyladenine (3MA), an autophagy inhibitor increased DQ toxicity. Our results suggest that DQ-induced cytotoxicity is modified by autophagy regulation. Pharmacologic induction of autophagy may be a useful treatment strategy in neurodegenerative disorders.

NF-kappaB-inducing kinase in cancer

Dysregulation of the alternative NF-κB signaling has severe developmental consequences that can ultimately lead to oncogenesis. Pivotal for the activation of the alternative NF-κB pathway is the stabilization of the NF-κB-inducing kinase (NIK). The aim of this review is to focus on the emerging role of NIK in cancer. The documented subversion of NIK in cancers highlights NIK as a possible therapeutic target. Recent studies show that the alterations of NIK or the components of its regulatory complex are manifold including regulation on the transcript level, copy number changes, mutations as well as protein modifications. High NIK activity is associated with different human malignancies and has adverse effects on tumor patient survival. We discuss here research focusing on deciphering the contribution of NIK towards cancer development and progression. We also report that it is possible to engineer inhibitors with high specificity for NIK and describe developments in this area.

A TRAF3-NIK module differentially regulates DNA vs RNA pathways in innate immune signaling

Detection of viral genomes by the innate immune system elicits an antiviral gene program mediated by type I interferons (IFNs). While viral RNA and DNA species induce IFN via separate pathways, the mechanisms by which these pathways are differentially modulated are unknown. Here we show that the positive regulator of IFN in the RNA pathway, TRAF3, has an inhibitory function in the DNA pathway. Loss of TRAF3 coincides with increased expression of the alternative NF-κB-inducing molecule, NIK, which interacts with the DNA pathway adaptor, STING, to enhance IFN induction. Cells lacking NIK display defective IFN activation in the DNA pathway due to impaired STING signaling, and NIK-deficient mice are more susceptible to DNA virus infection. Mechanistically, NIK operates independently from alternative NF-κB signaling components and instead requires autophosphorylation and oligomerization to activate STING. Thus a previously undescribed pathway for NIK exists in activating IFN in the DNA pathway.

NF-κB Activation in Lymphoid Malignancies: Genetics, Signaling, and Targeted Therapy

The NF-κB transcription factor family plays a crucial role in lymphocyte proliferation and survival. Consequently, aberrant NF-κB activation has been described in a variety of lymphoid malignancies, including diffuse large B-cell lymphoma, Hodgkin lymphoma, and adult T-cell leukemia. Several factors, such as persistent infections (e.g., with Helicobacter pylori), the pro-inflammatory microenvironment of the cancer, self-reactive immune receptors as well as genetic lesions altering the function of key signaling effectors, contribute to constitutive NF-κB activity in these malignancies. In this review, we will discuss the molecular consequences of recurrent genetic lesions affecting key regulators of NF-κB signaling. We will particularly focus on the oncogenic mechanisms by which these alterations drive deregulated NF-κB activity and thus promote the growth and survival of the malignant cells. As the concept of a targeted therapy based on the mutational status of the malignancy has been supported by several recent preclinical and clinical studies, further insight in the function of NF-κB modulators and in the molecular mechanisms governing aberrant NF-κB activation observed in lymphoid malignancies might lead to the development of additional treatment strategies and thus improve lymphoma therapy.

NIK/MAP3K14 Regulates Mitochondrial Dynamics and Trafficking to Promote Cell Invasion

Although the role of NF-κB-inducing kinase (NIK) in immunity is well established, its relevance in cancer is just emerging. Here we describe novel functions for NIK in regulating mitochondrial dynamics and motility to promote cell invasion. We show that NIK is localized to mitochondria in cancer cell lines, ex vivo tumor tissue, and mouse embryonic fibroblasts (MEFs). NIK promotes mitochondrial fission, velocity, and directional migration, resulting in subcellular distribution of mitochondria to the periphery of migrating cells. Moreover, NIK is required for recruitment of Drp1 to mitochondria, forms a complex with Drp1, and regulates Drp1 phosphorylation at Ser-616 and dephosphorylation at Ser-637. Consistent with a role for NIK in regulating mitochondrial dynamics, we demonstrate that Drp1 is required for NIK-dependent, cytokine-induced invasion. Importantly, using MEFs, we demonstrate that the established downstream mediators of NIK signaling, IκB kinase α/β (IKKα/β) and NF-κB, are not required for NIK to regulate cell invasion, Drp1 mitochondrial localization, or mitochondrial fission. Our results establish a new paradigm for IKK-independent NIK signaling and significantly expand the current dogma that NIK is predominantly cytosolic and exclusively regulates NF-κB activity. Overall, these findings highlight the importance of NIK in tumor pathogenesis and invite new therapeutic strategies that attenuate mitochondrial dysfunction through inhibition of NIK and Drp1.

Glucocorticoids increase skeletal muscle NF-κB inducing kinase (NIK): links to muscle atrophy

Glucocorticoids (GC) are a frontline therapy for numerous acute and chronic diseases because of their demonstrated efficacy at reducing systemic inflammation. An unintended side effect of GC therapy is the stimulation of skeletal muscle atrophy. Pathophysiological mechanisms responsible for GC-induced skeletal muscle atrophy have been extensively investigated, and the ability to treat patients with GC without unintended muscle atrophy has yet to be realized. We have reported that a single, standard-of-care dose of Methylprednisolone increases in vivo expression of NF-κB-inducing kinase (NIK), an important upstream regulatory kinase controlling NF-κB activation, along with other key muscle catabolic regulators such as Atrogin-1 and MuRF1 that induce skeletal muscle proteolysis. Here, we provide experimental evidence that overexpressing NIK by intramuscular injection of recombinant human NIK via adenoviral vector in mouse tibialis anterior muscle induces a 30% decrease in the average fiber cross-sectional area that is associated with increases in mRNA expression of skeletal muscle atrophy biomarkers MuRF1, Atrogin-1, myostatin and Gadd45. A single injection of GC induced NIK mRNA and protein within 2 h, with the increased NIK localized to nuclear and sarcolemmal locations within muscle fibers. Daily GC injections induced skeletal muscle fore limb weakness as early as 3 days with similar atrophy of muscle fibers as observed with NIK overexpression. NIK overexpression in primary human skeletal muscle myotubes increased skeletal muscle atrophy biomarkers, while NIK knockdown significantly attenuated GC-induced increases in NIK and Atrogin-1. These results suggest that NIK may be a novel, previously unrecognized mediator of GC-induced skeletal muscle atrophy.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

NFκB-inducing kinase inhibits NFκB activity specifically in neurons of the CNS

The control of NFκB in CNS neurons appears to differ from that in other cell types. Studies have reported induction of NFκB in neuronal cultures and immunostaining in vivo, but others have consistently detected little or no transcriptional activation by NFκB in brain neurons. To test if neurons lack some component of the signal transduction system for NFκB activation, we transfected cortical neurons with several members of this signaling system along with a luciferase-based NFκB-reporter plasmid; RelA was cotransfected in some conditions. No component of the NFκB pathway was permissive for endogenous NFκB activity, and none stimulated the activity of exogenous RelA. Surprisingly, however, the latter was inhibited by cotransfection of NFκB-inducing kinase (NIK). Fluorescence imaging of RelA indicated that co-expression of NIK sequestered RelA in the cytoplasm, similar to the effect of IκBα. NIK-knockout mice showed elevated expression of an NFκB-reporter construct in neurons in vivo. Cortical neurons cultured from NIK-knockout mice showed elevated expression of an NFκB-reporter transgene. Consistent with data from other cell types, a C-terminal fragment of NIK suppressed RelA activity in astrocytes as well as neurons. Therefore, the inhibitory ability of the NIK C-terminus was unbiased with regard to cell type. However, inhibition of NFκB by full-length NIK is a novel outcome that appears to be specific to CNS neurons. This has implications for unique aspects of transcription in the CNS, perhaps relevant to aspects of development, neuroplasticity, and neuroinflammation. Full-length NIK was found to inhibit (down arrow) transcriptional activation of NFκB in neurons, while it elevated (up arrow) activity in astrocytes. Deletion constructs corresponding to the N-terminus or C-terminus also inhibited NFκB in neurons, while only the C-terminus did so in astrocytes. One possible explanation is that the inhibition in neurons occurs via two different mechanisms, including the potential for a neuron-specific protein (e.g., one of the 14-3-3 class) to create a novel complex in neurons, whereas the C-terminus may interact directly with NFκB. [Structure of NIK is based on Liu J., Sudom A., Min X., Cao Z., Gao X., Ayres M., Lee F., Cao P., Johnstone S., Plotnikova O., Walker N., Chen G., and Wang Z. (2012) Structure of the nuclear factor κB-inducing kinase (NIK) kinase domain reveals a constitutively active conformation. J Biol Chem. 287, 27326-27334); N-terminal lobe is oriented at top].

Mono-(2-Ethylhexyl) Phthalate Promotes Pro-Labor Gene Expression in the Human Placenta

Women exposed to phthalates during pregnancy are at increased risk for delivering preterm, but the mechanism behind this relationship is unknown. Placental corticotropin-releasing hormone (CRH) and cyclooxygenase-2 (COX-2) are key mediators of parturition and are regulated by the non-canonical NF-kB (RelB/p52) signaling pathway. In this study, we demonstrate that one of the major phthalate metabolites, mono-(2-ethylhexyl)-phthalate (MEHP), increased CRH and COX-2 mRNA and protein abundance in a dose-dependent manner in primary cultures of cytotrophoblast. This was coupled with an increase in nuclear import of RelB/p52 and its association with the CRH and COX-2 promoters. Silencing of NF-kB inducing kinase, a central signaling component of the non-canonical NF-kB pathway, blocked MEHP-induced upregulation of CRH and COX-2. These results suggest a potential mechanism mediated by RelB/p52 by which phthalates could prematurely induce pro-labor gene activity and lead to preterm birth.

Loss of NDRG2 enhanced activation of the NF-κB pathway by PTEN and NIK phosphorylation for ATL and other cancer development

The activation of nuclear factor kappa B (NF-κB) signaling has a central role in the development of adult T-cell leukemia/lymphoma (ATL) and many other cancers. However, the activation mechanism of the NF-κB pathways remains poorly understood. Recently, we reported that N-myc downstream-regulated gene 2 (NDRG2) is a negative regulator of the phosphoinositide 3-kinase (PI3K)/AKT pathway by promoting the active dephosphorylated form of PTEN at its C-terminus via the recruitment of PP2A. Additionally, the down-regulation of NDRG2 expression promotes the inactive phosphorylated form of PTEN, which results in constitutively active PI3K/AKT signaling in various cancer cell types. Here, we investigated the involvement of NDRG2 in modulating NF-κB signaling. The forced expression of NDRG2 in ATL cells down-regulates not only the canonical pathway by inhibiting AKT signaling but also the non-canonical pathway by inducing NF-κB-inducing kinase (NIK) dephosphorylation via the recruitment of PP2A. Therefore, NDRG2 works as a PP2A recruiter to suppress not only PI3K/AKT signaling but also NF-κB signaling, which is particularly important in host defenses or immune responses to Human T-cell leukemia virus type 1 (HTLV-1) infection. Furthermore, the loss of NDRG2 expression might play an important role in the progression of tumor development after HTLV-1 infection.

Catalytic subunits of the phosphatase calcineurin interact with NF-κB-inducing kinase (NIK) and attenuate NIK-dependent gene expression

Nuclear factor (NF)-κB-inducing kinase (NIK) is a serine/threonine kinase that activates NF-κB pathways, thereby regulating a wide variety of immune systems. Aberrant NIK activation causes tumor malignancy, suggesting a requirement for precise regulation of NIK activity. To explore novel interacting proteins of NIK, we performed in vitro virus screening and identified the catalytic subunit Aα isoform of serine/threonine phosphatase calcineurin (CnAα) as a novel NIK-interacting protein. The interaction of NIK with CnAα in living cells was confirmed by co-immunoprecipitation. Calcineurin catalytic subunit Aβ isoform (CnAβ) also bound to NIK. Experiments using domain deletion mutants suggested that CnAα and CnAβ interact with both the kinase domain and C-terminal region of NIK. Moreover, the phosphatase domain of CnAα is responsible for the interaction with NIK. Intriguingly, we found that TRAF3, a critical regulator of NIK activity, also binds to CnAα and CnAβ. Depletion of CnAα and CnAβ significantly enhanced lymphotoxin-β receptor (LtβR)-mediated expression of the NIK-dependent gene Spi-B and activation of RelA and RelB, suggesting that CnAα and CnAβ attenuate NF-κB activation mediated by LtβR-NIK signaling. Overall, these findings suggest a possible role of CnAα and CnAβ in modifying NIK functions.

Survey of phosphorylation near drug binding sites in the Protein Data Bank (PDB) and their effects

While it is currently estimated that 40 to 50% of eukaryotic proteins are phosphorylated, little is known about the frequency and local effects of phosphorylation near pharmaceutical inhibitor binding sites. In this study, we investigated how frequently phosphorylation may affect the binding of drug inhibitors to target proteins. We examined the 453 non-redundant structures of soluble mammalian drug target proteins bound to inhibitors currently available in the Protein Data Bank (PDB). We cross-referenced these structures with phosphorylation data available from the PhosphoSitePlus database. Three hundred twenty-two of 453 (71%) of drug targets have evidence of phosphorylation that has been validated by multiple methods or labs. For 132 of 453 (29%) of those, the phosphorylation site is within 12 Å of the small molecule-binding site, where it would likely alter small molecule binding affinity. We propose a framework for distinguishing between drug-phosphorylation site interactions that are likely to alter the efficacy of drugs versus those that are not. In addition we highlight examples of well-established drug targets, such as estrogen receptor alpha, for which phosphorylation may affect drug affinity and clinical efficacy. Our data suggest that phosphorylation may affect drug binding and efficacy for a significant fraction of drug target proteins.

Regulation of NF-κB by TNF family cytokines

The NF-κB family of inducible transcription factors is activated in response to a variety of stimuli. Amongst the best-characterized inducers of NF-κB are members of the TNF family of cytokines. Research on NF-κB and TNF have been tightly intertwined for more than 25 years. Perhaps the most compelling examples of the interconnectedness of NF-κB and the TNF have come from analysis of knock-out mice that are unable to activate NF-κB. Such mice die embryonically, however, deletion of TNF or TNFR1 can rescue the lethality thereby illustrating the important role of NF-κB as the key regulator of transcriptional responses to TNF. The physiological connections between NF-κB and TNF cytokines are numerous and best explored in articles focusing on a single TNF family member. Instead, in this review, we explore general mechanisms of TNF cytokine signaling, with a focus on the upstream signaling events leading to activation of the so-called canonical and noncanonical NF-κB pathways by TNFR1 and CD40, respectively.

Upregulation of glucose metabolism by NF-κB2/p52 mediates enzalutamide resistance in castration-resistant prostate cancer cells

Cancer cells reprogram their metabolic pathways to facilitate fast proliferation. Previous studies have shown that overexpression of NF-κB2/p52 (p52) in prostate cancer cells promotes cell growth and leads to castration resistance through aberrant activation of androgen receptor (AR). In addition, these cells become resistant to enzalutamide. In this study, we investigated the effects of p52 activation on glucose metabolism and on response to enzalutamide therapy. Data analysis of gene expression arrays showed that genes including GLUT1 (SLC2A1), PKM2, G6PD, and ME1 involved in the regulation of glucose metabolism were altered in LNCaP cells overexpressing p52 compared with the parental LNCaP cells. We demonstrated an increased amount of glucose flux in the glycolysis pathway, as well as the pentose phosphate pathway (PPP) upon p52 activation. The p52-overexpressing cells increase glucose uptake and are capable of higher ATP and lactate production compared with the parental LNCaP cells. The growth of p52-overexpressing cells depends on glucose in the culture media and is sensitive to glucose deprivation compared with the parental LNCaP cells. Targeting glucose metabolism by the glucose analog 2-deoxy-d-glucose synergistically inhibits cell growth when combined with enzalutamide, and resensitizes p52-overexpressing cells to enzalutamide treatment. These results suggest that p52 modulates glucose metabolism, enhances glucose flux to glycolysis and PPPs, thus facilitating fast proliferation of the cells. Co-targeting glucose metabolism together with AR axis synergistically inhibits cell growth and restores enzalutamide-resistant cells to enzalutamide treatment.

Caffeic acid regulates LPS-induced NF-κB activation through NIK/IKK and c-Src/ERK signaling pathways in endothelial cells

The redox sensitive, proinflammatory nuclear transcription factor NF-κB plays a key role in inflammation. In a redox state disrupted by oxidative stress, pro-inflammatory genes are upregulated by the activation of NF-κB via diverse kinases. Thus, the search and characterization of new substances that modulate NF-κB are topics of considerable research interest. Caffeic acid is a component of garlic, some fruits, and coffee, and is widely used as a phenolic agent in beverages. In the present study, caffeic acid was examined with respect to the modulation of inflammatory NF-κB activation via the redox-related c-Src/ERK and NIK/IKK pathways via the reduction of oxidative stress. YPEN-1 cells (an endothelial cell line) were used to explore the molecular mechanism underlying the anti-inflammatory effect of caffeic acid by examining its modulation of NF-κB signaling pathway by LPS. Our results show that LPS-induced oxidative stress-related NF-κB activation upregulated pro-inflammatory COX-2, NF-κB targeting gene which were all inhibited effectively by caffeic acid. Our study shows that caffeic acid inhibits the activation of NF-κB via the c-Src/ERK and NIK/IKK signal transduction pathways. Our results indicate that antioxidative effect of caffeic acid and its restoration of redox balance are responsible for its anti-inflammatory action. Thus, the study provides new information regarding the anti-inflammatory properties of caffeic acid and the roles in the regulation of LPS-induced oxidative stress induces alterations in signal transduction pathways.

Non-Canonical NF-κB Signaling Initiated by BAFF Influences B Cell Biology at Multiple Junctures

It has been more than a decade since it was recognized that the nuclear factor of kappa light polypeptide gene enhancer in B cells (NF-κB) transcription factor family was activated by two distinct pathways: the canonical pathway involving NF-κB1 and the non-canonical pathway involving NF-κB2. During this time a great deal of evidence has been amassed on the ligands and receptors that activate these pathways, the cytoplasmic adapter molecules involved in transducing the signals from receptors to nucleus, and the resulting physiological outcomes within body tissues. In contrast to NF-κB1 signaling, which can be activated by a wide variety of receptors, the NF-κB2 pathway is typically only activated by a subset of receptor and ligand pairs belonging to the tumor necrosis factor (TNF) family. Amongst these is B cell activating factor of the TNF family (BAFF) and its receptor BAFFR. Whilst BAFF is produced by many cell types throughout the body, BAFFR expression appears to be restricted to the hematopoietic lineage and B cells in particular. For this reason, the main physiological outcomes of BAFF mediated NF-κB2 activation are confined to B cells. Indeed BAFF mediated NF-κB2 signaling contributes to peripheral B cell survival and maturation as well as playing a role in antibody responses and long term maintenance plasma cells. Thus the importance BAFF and NF-κB2 permeates the entire B cell lifespan and impacts on this important component of the immune system in a variety of ways.

Inducible tumor necrosis factor (TNF) receptor-associated factor-1 expression couples the canonical to the non-canonical NF-κB pathway in TNF stimulation

The NF-κB transcription factor mediates the inflammatory response through distinct (canonical and non-canonical) signaling pathways. The mechanisms controlling utilization of either of these pathways are largely unknown. Here we observe that TNF stimulation induces delayed NF-κB2/p100 processing and investigate the coupling mechanism. TNF stimulation induces TNF-associated factor-1 (TRAF-1) that directly binds NF-κB-inducing kinase (NIK) and stabilizes it from degradation by disrupting its interaction with TRAF2·cIAP2 ubiquitin ligase complex. We show that TRAF1 depletion prevents TNF-induced NIK stabilization and reduces p52 production. To further examine the interactions of TRAF1 and NIK with NF-κB2/p100 processing, we mathematically modeled TRAF1·NIK as a coupling signaling complex and validated computational inference by siRNA knockdown to show non-canonical pathway activation is dependent not only on TRAF1 induction but also NIK stabilization by forming TRAF1·NIK complex. Thus, these integrated computational-experimental studies of TNF-induced TRAF1 expression identified TRAF1·NIK as a central complex linking canonical and non-canonical pathways by disrupting the TRAF2-cIAP2 ubiquitin ligase complex. This feed-forward kinase pathway is essential for the activation of non-canonical pathway.

Understanding NIK regulation from its structure

In this issue of Structure, de Leon-Boenig and colleagues bring to light several key characteristics of NF-κB inducing kinase (NIK), which is at the center of many pathological disorders, using an impressive body of structural and functional studies. Structures of NIK described here provide deep insights into the mechanism of NIK activity regulation and may facilitate the design of new generation inhibitors for therapeutic use.

The crystal structure of the catalytic domain of the NF-κB inducing kinase reveals a narrow but flexible active site

The NF-κB inducing kinase (NIK) regulates the non-canonical NF-κB pathway downstream of important clinical targets including BAFF, RANKL, and LTβ. Despite numerous genetic studies associating dysregulation of this pathway with autoimmune diseases and hematological cancers, detailed molecular characterization of this central signaling node has been lacking. We undertook a systematic cloning and expression effort to generate soluble, well-behaved proteins encompassing the kinase domains of human and murine NIK. Structures of the apo NIK kinase domain from both species reveal an active-like conformation in the absence of phosphorylation. ATP consumption and peptide phosphorylation assays confirm that phosphorylation of NIK does not increase enzymatic activity. Structures of murine NIK bound to inhibitors possessing two different chemotypes reveal conformational flexibility in the gatekeeper residue controlling access to a hydrophobic pocket. Finally, a single amino acid difference affects the ability of some inhibitors to bind murine and human NIK with the same affinity.

Structure of the nuclear factor κB-inducing kinase (NIK) kinase domain reveals a constitutively active conformation

NF-κB-inducing kinase (NIK) is a central component in the non-canonical NF-κB signaling pathway. Excessive NIK activity is implicated in various disorders, such as autoimmune conditions and cancers. Here, we report the first crystal structure of truncated human NIK in complex with adenosine 5'-O-(thiotriphosphate) at a resolution of 2.5 Å. This truncated protein is a catalytically active construct, including an N-terminal extension of 60 residues prior to the kinase domain, the kinase domain, and 20 residues afterward. The structure reveals that the NIK kinase domain assumes an active conformation in the absence of any phosphorylation. Analysis of the structure uncovers a unique role for the N-terminal extension sequence, which stabilizes helix αC in the active orientation and keeps the kinase domain in the catalytically competent conformation. Our findings shed light on the long-standing debate over whether NIK is a constitutively active kinase. They also provide a molecular basis for the recent observation of gain-of-function activity for an N-terminal deletion mutant (ΔN324) of NIK, leading to constitutive non-canonical NF-κB signaling with enhanced B-cell adhesion and apoptosis resistance.

NF-κB, the first quarter-century: remarkable progress and outstanding questions

The ability to sense and adjust to the environment is crucial to life. For multicellular organisms, the ability to respond to external changes is essential not only for survival but also for normal development and physiology. Although signaling events can directly modify cellular function, typically signaling acts to alter transcriptional responses to generate both transient and sustained changes. Rapid, but transient, changes in gene expression are mediated by inducible transcription factors such as NF-κB. For the past 25 years, NF-κB has served as a paradigm for inducible transcription factors and has provided numerous insights into how signaling events influence gene expression and physiology. Since its discovery as a regulator of expression of the κ light chain gene in B cells, research on NF-κB continues to yield new insights into fundamental cellular processes. Advances in understanding the mechanisms that regulate NF-κB have been accompanied by progress in elucidating the biological significance of this transcription factor in various physiological processes. NF-κB likely plays the most prominent role in the development and function of the immune system and, not surprisingly, when dysregulated, contributes to the pathophysiology of inflammatory disease. As our appreciation of the fundamental role of inflammation in disease pathogenesis has increased, so too has the importance of NF-κB as a key regulatory molecule gained progressively greater significance. However, despite the tremendous progress that has been made in understanding the regulation of NF-κB, there is much that remains to be understood. In this review, we highlight both the progress that has been made and the fundamental questions that remain unanswered after 25 years of study.

The noncanonical NF-κB pathway

The noncanonical nuclear factor-κB (NF-κB) signaling pathway mediates activation of the p52/RelB NF-κB complex and, thereby, regulates specific immunological processes. This NF-κB pathway relies on the inducible processing of NF-κB2 precursor protein, p100, as opposed to the degradation of IκBα in the canonical NF-κB pathway. A central signaling component of the noncanonical NF-κB pathway is NF-κB-inducing kinase (NIK), which functions together with a downstream kinase, IKKα (inhibitor of NF-κB kinase α), to induce phosphorylation-dependent ubiquitination and processing of p100. Under normal conditions, NIK is targeted for continuous degradation by a tumor necrosis factor (TNF) receptor-associated factor-3 (TRAF3)-dependent E3 ubiquitin ligase. In response to signals mediated by a subset of TNF receptor superfamily members, NIK becomes stabilized as a result of TRAF3 degradation, leading to the activation of noncanonical NF-κB. This review discusses both the historical perspectives and the recent progress in the regulation and biological function of the noncanonical NF-κB pathway.

Non-canonical NF-κB signaling activation and regulation: principles and perspectives

Nuclear-factor κB (NF-κB) transcription factors are activated by a wide variety of stimuli in diverse cell types and control key aspects of immune function and development. Receptor-mediated activation of NF-κB appears to occur through two distinct signaling pathways termed as the canonical and non-canonical NF-κB pathways. Although much work has demonstrated the physiological importance of non-canonical NF-κB signaling to immunity and its involvement in diverse pathologies, such as cancers and autoimmune disease, the architecture and regulation of the pathway is only beginning to be understood. The non-canonical pathway appears to be activated by a select set of receptors within the tumor necrosis factor superfamily, and we discuss the molecular mechanisms that connect ligation of these receptors to pathway activation. It has become increasingly clear that the key regulatory step of the pathway involves modulation of the post-translational degradation of NF-κB-inducing kinase (NIK), the central activating kinase of non-canonical NF-κB signaling. How NIK post-translational stability is controlled before and after receptor ligation is an important aspect of understanding non-canonical NF-κB signaling. Furthermore, how release of NF-κB dimers downstream of the pathway's activation is actually connected to its identified physiological and pathological roles is a key remaining question in the field.

NF-κB-inducing kinase increases renal tubule epithelial inflammation associated with diabetes

The impact of increased NF-κB-inducing kinase (NIK), a key component of the NF-κB activation pathways, on diabetes-induced renal inflammation remains unknown. We overexpressed NIK wild type (NIKwt) or kinase-dead dominant negative mutants (NIKdn) in HK-2 cells and demonstrated that RelB and p52, but not RelA, abundance and DNA binding increased in nuclei of NIKwt but not NIKdn overexpressed cells, and this corresponded with increases in multiple proinflammatory cytokines. Since TRAF3 negatively regulates NIK expression, we silenced TRAF3 by >50%; this increased nuclear levels of p52 and RelB, and transcript levels of proinflammatory cytokines and transcription factors. In HK-2 cells and mouse primary proximal tubule epithelial cells treated with methylglyoxal-modified albumin, multiple proinflammatory cytokines and NIK were increased in association with increased nuclear RelB and p52. These observations indicate that NIK regulates proinflammatory responses of renal proximal tubular epithelial cells via mechanisms involving TRAF3 and suggest a role for NF-κB noncanonical pathway activation in modulating diabetes-induced inflammation in renal tubular epithelium.

The kinase NIK as a therapeutic target in multiple myeloma

INTRODUCTION

Multiple myeloma (MM) is a neoplasm derived from B lymphocytes and often results in uncontrolled clonal expansion of antibody-secreting cells. While current treatments are able to prolong survival, MM remains incurable. Excessive NF-κB activity in MM contributes to tumor progression and survival.

AREAS COVERED

The contribution of NF-κB-inducing kinase (NIK) to alternative NF-κB signaling, where it is the key kinase, and classical NF-κB signaling. Modulation of NIK by natural and chemical factors and current and potential therapies for MM that target NIK.

EXPERT OPINION

Mutations affecting the activation of NIK have been identified in MM samples and cell lines, suggesting that NIK may be an important target for therapy of MM. NIK contributes to activation of both NF-κB pathways in MM, giving us the opportunity to limit two pathways contributing to oncogenic survival with a single therapeutic. Many of the mutations identified in MM cells result in the same outcome, hyperactive NIK, thus a single therapeutic may be effective in many patients even though they carry differing mutations. As NIK appears only to activate classical NF-κB when overexpressed, and in normal cells NIK levels are usually low, it is possible that therapeutics designed to limit the amount of NIK may not produce serious side effects in healthy cells.

Non-canonical NF-κB signaling pathway

The non-canonical NF-κB pathway is an important arm of NF-κB signaling that predominantly targets activation of the p52/RelB NF-κB complex. This pathway depends on the inducible processing of p100, a molecule functioning as both the precursor of p52 and a RelB-specific inhibitor. A central signaling component of the non-canonical pathway is NF-κB-inducing kinase (NIK), which integrates signals from a subset of TNF receptor family members and activates a downstream kinase, IκB kinase-α (IKKα), for triggering p100 phosphorylation and processing. A unique mechanism of NIK regulation is through its fate control: the basal level of NIK is kept low by a TRAF-cIAP destruction complex and signal-induced non-canonical NF-κB signaling involves NIK stabilization. Tight control of the fate of NIK is important, since deregulated NIK accumulation is associated with lymphoid malignancies.

An atypical E3 ligase zinc finger protein 91 stabilizes and activates NF-kappaB-inducing kinase via Lys63-linked ubiquitination

The NF-κB transcription factors control many physiological processes, including inflammation, immunity, and apoptosis. Its activity contributes to the development of various cell malignancies. NF-κB-inducing kinase (NIK) plays a pivotal role in NF-κB activation. However, the molecular mechanism to stabilize and activate NIK remains elusive, although it is known that cIAP1/2 (cellular inhibitor of apoptosis 1 and 2) ubiquitinate NIK for degradation. Here, we report a novel NF-κB-related zinc finger protein 91 (ZFP91) that stabilizes and activates NIK in a ubiquitination-dependent manner. We show that ZFP91 interacts with and promotes the Lys(63)-linked ubiquitination of NIK and subsequent processing of p100 to p52. The results of in vitro biochemical assays indicate that ZFP91 functions as an E3 ligase directly to NIK. Remarkably, the ubiquitination of NIK coincides with its Thr(559) phosphorylation. Furthermore, knockdown of ZFP91 expression by RNA interference inhibits the CD40 ligation-induced activation of NIK and p100 processing as well as the expression of noncanonical NF-κB target genes. These data clearly indicate that ZFP91 is an important regulator of the noncanonical NF-κB pathway.

NF-κB inducing kinase: a key regulator in the immune system and in cancer

NF-κB inducing kinase (NIK) is a kinase that activates the canonical and non-canonical NF-κB pathways to control transcriptional expression of certain proteins such as cytokines, chemokines and NF-κB signaling molecules. Many advances have been made in understanding the molecular mechanisms by which the stability of NIK is regulated to affect downstream signaling. Genetic mouse models suggest that NIK plays an essential role in the regulation of the immune system as well as in the bone microenvironment. Increasing evidence links NIK to the tumorigenesis of hematological cancers, such as multiple myeloma, and solid tumors, such as pancreatic carcinoma and melanoma. Understanding the mechanism by which NIK is de-regulated will potentially provide therapeutic options for certain diseases such as autoimmunity and cancer.

Controlling the fate of NIK: a central stage in noncanonical NF-kappaB signaling

The noncanonical nuclear factor kappaB (NF-kappaB) pathway is a specific arm of NF-kappaB signaling that regulates important aspects of immune function. Activation of this pathway centers on the modulation of a pivotal signaling component: NF-kappaB-inducing kinase (NIK). Under normal conditions, NIK undergoes constitutive degradation, which keeps its abundance below the threshold required for its function, and signal-induced activation of the noncanonical NF-kappaB pathway is coupled with the stabilization and accumulation of NIK. A study now shows that signal-induced accumulation of NIK is subject to feedback control, which involves its phosphorylation by a downstream kinase, inhibitor of kappaB (IkappaB) kinase alpha (IKKalpha), and degradation. Thus, controlling the fate of NIK is emerging as a central mechanism in noncanonical NF-kappaB signaling.

The nuclear factor-kappaB-interleukin-6 signalling pathway mediating vascular inflammation

Vascular inflammation is a common pathophysiological response to diverse cardiovascular disease processes, including atherosclerosis, myocardial infarction, congestive heart failure, and aortic aneurysms/dissection. Inflammation is an ordered process initiated by vascular injury that produces enhanced leucocyte adherence, chemotaxis, and finally activation in situ. This process is coordinated by local secretion of adhesion molecules, chemotactic factors, and cytokines whose expression is the result of vascular injury-induced signal transduction networks. A wide variety of mediators of the vascular injury response have been identified; these factors include vasoactive peptides (angiotensin II, Ang II), CD40 ligands, oxidized cholesterol, and advanced glycation end-products. Downstream, the nuclear factor-kappaB (NF-kappaB) transcription factor performs an important signal integration step, responding to mediators of vascular injury in a stimulus-dependent and cell type-specific manner. The ultimate consequence of NF-kappaB signalling is the activation of inflammatory genes including adhesion molecules and chemotaxins. However, clinically, the hallmark of vascular NF-kappaB activation is the production of interleukin-6 (IL-6), whose local role in vascular inflammation is relatively unknown. The recent elucidation for the role of the IL-6 signalling pathway in Ang II-induced vascular inflammation as one that controls monocyte activation as well as its diverse signalling mechanism will be reviewed. These new discoveries further our understanding for the important role of the NF-kappaB-IL-6 signalling pathway in the process of vascular inflammation.

Comparison of peptide array substrate phosphorylation of c-Raf and mitogen activated protein kinase kinase kinase 8

Kinases are pivotal regulators of cellular physiology. The human genome contains more than 500 putative kinases, which exert their action via the phosphorylation of specific substrates. The determinants of this specificity are still only partly understood and as a consequence it is difficult to predict kinase substrate preferences from the primary structure, hampering the understanding of kinase function in physiology and prompting the development of technologies that allow easy assessment of kinase substrate consensus sequences. Hence, we decided to explore the usefulness of phosphorylation of peptide arrays comprising of 1176 different peptide substrates with recombinant kinases for determining kinase substrate preferences, based on the contribution of individual amino acids to total array phosphorylation. Employing this technology, we were able to determine the consensus peptide sequences for substrates of both c-Raf and Mitogen Activated Protein Kinase Kinase Kinase 8, two highly homologous kinases with distinct signalling roles in cellular physiology. The results show that although consensus sequences for these two kinases identified through our analysis share important chemical similarities, there is still some sequence specificity that could explain the different biological action of the two enzymes. Thus peptide arrays are a useful instrument for deducing substrate consensus sequences and highly homologous kinases can differ in their requirement for phosphorylation events.

Development of an insect-cell-based assay for detection of kinase inhibition using NF-kappaB-inducing kinase as a paradigm

Identification of small-molecule inhibitors by high-throughput screening necessitates the development of robust, reproducible and cost-effective assays. The assay approach adopted may utilize isolated proteins or whole cells containing the target of interest. To enable protein-based assays, the baculovirus expression system is commonly used for generation and isolation of recombinant proteins. We have applied the baculovirus system into a cell-based assay format using NIK [NF-kappaB (nuclear factor kappaB)-inducing kinase] as a paradigm. We illustrate the use of the insect-cell-based assay in monitoring the activity of NIK against its physiological downstream substrate IkappaB (inhibitor of NF-kappaB) kinase-1. The assay was robust, yielding a signal/background ratio of 2:1 and an average Z' value of >0.65 when used to screen a focused compound set. Using secondary assays to validate a selection of the hits, we identified a compound that (i) was non-cytotoxic, (ii) interacted directly with NIK, and (iii) inhibited lymphotoxin-induced NF-kappaB p52 translocation to the nucleus. The insect cell assay represents a novel approach to monitoring kinase inhibition, with major advantages over other cell-based systems including ease of use, amenability to scale-up, protein expression levels and the flexibility to express a number of proteins by infecting with numerous baculoviruses.

NF-kappaB2/p52 enhances androgen-independent growth of human LNCaP cells via protection from apoptotic cell death and cell cycle arrest induced by androgen-deprivation

PURPOSE

Androgen-deprivation therapy only causes a temporary regression of prostate cancer, as all tumors will eventually progress to refractory to hormonal therapy after 1-3 years of treatment. The underlying mechanisms of prostate cancer androgen refractory progression are incompletely understood. In this study, we employed in vitro as well as in vivo models to examine the role of NF-kappaB2/p52 in prostate cancer growth and androgen independent progression.

EXPERIMENTAL DESIGN

The effects of NF-kappaB2/p52 on cell growth, androgen responsiveness, cell cycle and apoptosis were examined in androgen sensitive LNCaP cells. The effect of NF-kappaB2/p52 on tumor growth was examined in intact and castrated male mice.

RESULTS

Overexpression of NF-kappaB2/p52 enhances androgen-sensitive LNCaP human prostate cancer cell growth and clonogenic ability in androgen-deprived condition in vitro. NF-kappaB2/p52 induced androgen-independent growth is through protecting LNCaP cells from apoptotic cell death and cell cycle arrest induced by androgen-deprivation. In addition, NF-kappaB2/p52 stimulates Cyclin D1 expression and knock down of Cyclin D1 expression by siRNA abolished NF-kappaB2/p52-induced cell growth in vitro. Adenoviral mediated NF-kappaB2/p52 expression in LNCaP cells enhances tumor growth in intact male nude mice and induces tumor growth in castrated male nude mice, suggesting that overexpression of NF-kappaB2/p52 induces androgen-independent growth of androgen-sensitive LNCaP cells.

CONCLUSIONS

Overexpression of NF-kappaB2/p52 protects androgen sensitive LNCaP cells from apoptotic cell death and cell cycle arrest induced by androgen-deprivation. NF-kappaB2/p52 activation induces androgen-independent growth in vitro and in vivo.

p21-activated kinase signaling regulates oxidant-dependent NF-kappa B activation by flow

Disturbed blood flow induces inflammatory gene expression in endothelial cells, which promotes atherosclerosis. Flow stimulates the proinflammatory transcription factor nuclear factor (NF)-kappaB through integrin- and Rac-dependent production of reactive oxygen species (ROS). Previous work demonstrated that NF-kappaB activation by flow is matrix-specific, occurring in cells on fibronectin but not collagen. Activation of p21-activated kinase (PAK) followed the same matrix-dependent pattern. We now show that inhibiting PAK in cells on fibronectin blocked NF-kappaB activation by both laminar and oscillatory flow in vitro and at sites of disturbed flow in vivo. Constitutively active PAK rescued flow-induced NF-kappaB activation in cells on collagen. Surprisingly, PAK was not required for flow-induced ROS production. Instead, PAK modulated the ability of ROS to activate the NF-kappaB pathway. These data demonstrate that PAK controls NF-kappaB activation by modulating the sensitivity of cells to ROS.

Differential regulation of IKK alpha-mediated activation of IRF3/7 by NIK

Type I interferons (IFNs) are critical mediators of the innate immune system to defend viral infection. Interferon regulatory factor (IRF) 3 and IRF7 are transcription factors that play critical roles in type I IFN production in response to viral infection. It has been shown that the protein kinase I kappaB kinase alpha (IKK alpha) is critically involved in IRF7 activation and IFN-alpha production in Toll-like receptor 7/9 (TLR7/9) signaling cascades. However, overexpression of IKK alpha does not activate the IFN-alpha promoters. Here we show that the protein kinase nuclear factor kappaB-inducing kinase (NIK) confers IKK alpha the ability to activate IRF3/7. Previous studies have shown that NIK phosphorylates IKK alpha at Ser-176 and Ser-180 residues, and mutation of each of the two residues to glutamate, which mimics its phosphorylation, caused constitutive activation of NF-kappaB. However, mutation of the two serine residues has differential effects on IKK alpha-mediated activation of IRF3/7. While IKK alpha(S176E) constitutively activates IRF3/7, IKK alpha(S180E) losses its ability to activate IRF3/7. These findings suggest that IKK alpha-mediated activation of NF-kappaB and IRF3/7 are differentially regulated by NIK, and NIK plays an important role in TLR7/9-mediated IFN-alpha production.

The LT beta R signaling pathway

The lymphotoxin-beta receptor (LTbetaR, TNFRSF3) signaling pathway activates gene transcription programs and cell death important in immune development and host defense. The TNF receptor associated factors (TRAF)-2, 3 and 5 function as adaptors linking LTbetaR signaling targets. Interestingly, TRAF deficient mice do not phenocopy mice deficient in components of the LTbetaR pathway, presenting a conundrum. Here, an update of our understanding and models of the LTbetaR signaling pathway are reviewed, with a focus on this conundrum.

Physiological roles and mechanisms of signaling by TRAF2 and TRAF5

RAF2 and TRAF5 are closely related members of the TRAF family of proteins. They are important signal transducers for a wide range of TNF receptor superfamily members, including TNFR1, TNFR2, CD40 and other lymphocyte costimulatory receptors, RANK/TRANCE-R, EDAR, LTbetaR, LMP-1 and IRE1. TRAF2 andTRAF5 therefore regulate diverse physiological roles, ranging from T and B cell signaling and inflammatory responses to organogenesis and cell survival. The major pathways mediated by TRAF2 and TRAF5 are the classical and alternative pathways of NF-kappaB activation, and MAPK and JNK activation. TRAF2 is heavily regulated by ubiquitin signals, and many of the signaling functions of TRAF2 are mediated through its RING domain and likely its own role as an E3 ubiquitin ligase.

Monarch-1 suppresses non-canonical NF-kappaB activation and p52-dependent chemokine expression in monocytes

CATERPILLER (NOD, NBD-LRR) proteins are rapidly emerging as important mediators of innate and adaptive immunity. Among these, Monarch-1 operates as a novel attenuating factor of inflammation by suppressing inflammatory responses in activated monocytes. However, the molecular mechanisms by which Monarch-1 performs this important function are not well understood. In this report, we show that Monarch-1 inhibits CD40-mediated activation of NF-kappaB via the non-canonical pathway in human monocytes. This inhibition stems from the ability of Monarch-1 to associate with and induce proteasome-mediated degradation of NF-kappaB inducing kinase. Congruently, silencing Monarch-1 with shRNA enhances the expression of p52-dependent chemokines.

Embelin, an Inhibitor of X Chromosome-Linked Inhibitor-of-Apoptosis Protein, Blocks Nuclear Factor-κB (NF-κB) Signaling Pathway Leading to Suppression of NF-κB-Regulated Antiapoptotic and Metastatic Gene Products

Identifying the active chemical ingredients of ancient medicines and the molecular targets of those ingredients is an attractive therapeutic objective. Embelin, identified primarily from the Embelia ribes plant, is one such compound shown to exhibit chemopreventive, anti-inflammatory, and apoptotic activities through an unknown mechanism. Because nuclear factor-kappaB (NF-kappaB) regulates several genes associated with inflammation, proliferation, carcinogenesis, and apoptosis, we postulated that embelin might mediate its activity through modulation of NF-kappaB activation. We found that embelin inhibited tumor necrosis factor (TNF) alpha-induced NF-kappaB activation. Both inducible and constitutive NF-kappaB activation were abrogated by embelin. In addition, NF-kappaB activated by diverse stimuli such as interleukin-1beta, lipopolysaccharide, phorbol myristate acetate, okadaic acid, hydrogen peroxide, and cigarette smoke condensate also was suppressed. We found that embelin inhibited sequentially the TNFalpha-induced activation of the inhibitory subunit of NF-kappaBalpha (IkappaBalpha) kinase, IkappaBalpha phosphorylation, IkappaBalpha degradation, and p65 phosphorylation and nuclear translocation. Embelin also suppressed NF-kappaB-dependent reporter gene transcription induced by TNFalpha, TNF receptor-1 (TNFR1), TNFR1-associated death domain protein, TNFR-associated factor-2, NF-kappaB-inducing kinase, and IkappaBalpha kinase but not by p65. Furthermore, we found that embelin down-regulated gene products involved in cell survival, proliferation, invasion, and metastasis of the tumor. This down-regulation was associated with enhanced apoptosis by cytokine and chemotherapeutic agents. Together, our results indicate that embelin is a novel NF-kappaB blocker and potential suppressor of tumorigenesis.

Short-term feeding of baicalin inhibits age-associated NF-kappaB activation

Baicalin is a flavonoid isolated from Scutellaria baicalensis and is known to affect multiple biological functions, including the inhibition of aldose reductase, HIV infection, and nitric oxide producing activity. Oxidative stress is considered a major cause of aging and various age-related diseases, and among the key cellular components exquisitely sensitive to oxidative stress is the transcription factor, nuclear factor-kappaB (NF-kappaB). In the present study, we attempted to elucidate the mechanisms underlying the suppression of age-related NF-kappaB activation by baicalin in kidney tissue from old rats. Results showed NF-kappaB activation and the upregulation of NF-kappaB targeting genes, hemoxygenase-1, inducible nitric oxide synthase (iNOS), and COX-2 with age. In contrast, the increased expression of these NF-kappaB targeting genes was effectively inhibited by baicalin. Baicalin was shown to inhibit the NF-kappaB cascade via three signal transduction pathways, NIK/IKK, extracellular signal-regulated kinase (ERK), and p38 mitogen-activated protein kinase (MAPK). Our results clearly indicated the anti-oxidative effects of baicalin on age-related redox imbalance. Thus, the significance of the current study is the new information revealing the anti-oxidative properties of baicalin and the role it plays in the regulation of age-related alterations.

Molecular mediators of alphavbeta3-induced endothelial cell survival

The alphavbeta3 integrin interaction with the extracellular matrix (ECM) plays an essential role in inhibiting apoptosis in endothelial cells. We have recently shown that alphavbeta3 ligation on rat aortic endothelial cells (RAECs) specifically activates the transcription factor nuclear factor kappaB (NF-kappaB) and promotes cell survival. Inhibiting NF-kappaB nuclear translocation abolished the protective effect of alphavbeta3 ligands. Here, we report that ligation of alphavbeta3 by its ligand, osteopontin (OPN), induces the phosphorylation and activation of inhibitory kappa B kinase beta IKKbeta and promotes the specific degradation of inhibitory kappa Balpha (IkappaBalpha) in RAECs. Overexpression of a dominant negative (DN) IKKbeta protein prevents IkappaBalpha phosphorylation, NF-kappaB activation, and inhibits the protective effects of OPN. The NF-kappaB-inducing kinase (NIK) has been shown to be one of the upstream kinases involved in IKK activation. OPN-mediated NF-kappaB activity is increased upon NIK wild-type (WT) overexpression and blocked following NIK DN overexpression. In addition, NIK-/-mouse embryonic fibroblasts (MEFs) plated on OPN display reduced NF-kappaB activity and decreased IkappaBalpha phosphorylation compared to NIK+/+MEFs. Finally, functional inhibition of integrin beta3-dependent NF-kappaB signaling decreases OPN-induced IkappaBalpha, IKKbeta and NIK phosphorylation. These studies for the first time show that the alphavbeta3-NF-kappaB-dependent endothelial survival pathway is dependent on IkappaBalpha, IKKbeta, and NIK.

An alternative splice product of IkappaB kinase (IKKgamma), IKKgamma-delta, differentially mediates cytokine and human T-cell leukemia virus type 1 tax-induced NF-kappaB activation

NF-kappaB is an inducible transcription factor mediating innate immune responses whose activity is controlled by the multiprotein IkappaB kinase (IKK) "signalsome". The core IKK consists of two catalytic serine kinases, IKKalpha and IKKbeta, and a noncatalytic subunit, IKKgamma. IKKgamma is required for IKK activity by mediating kinase oligomerization and serving to couple the core catalytic subunits to upstream mitogen-activated protein 3-kinase cascades. We have discovered an alternatively spliced IKKgamma mRNA isoform, encoding an in-frame deletion of exon 5, termed IKKgamma-delta. Using a specific reverse transcription-PCR assay, we find that IKKgamma-delta is widely expressed in cultured human cells and normal human tissues. Because IKKgamma-Delta protein is lacking a critical coiled-coil domain important in protein-protein interactions, we sought to determine its signaling properties by examining its ability to self associate, couple to activators of the canonical pathway, and mediate human T-cell leukemia virus type 1 (HTLV-1) Tax-induced NF-kappaB activity. Coimmunoprecipitation and confocal colocalization assays indicate IKKgamma-delta has strong homo- and heterotypic association with wild-type (WT) IKKgamma and, like IKKgamma WT, associates with the IKKbeta kinase. Similarly, IKKgamma-delta mediates IKK kinase activity and downstream NF-kappaB-dependent transcription in response to tumor necrosis factor (TNF) and the NF-kappaB-inducing kinase-IKKalpha signaling pathway. Surprisingly, however, in contrast to IKKgamma WT, IKKgamma-delta is not able to mediate HTLV-1 Tax-induced NF-kappaB-dependent transcription, even though IKKgamma-delta binds and colocalizes with Tax. These observations suggest that IKKgamma-delta is a functionally distinct alternatively spliced mRNA product differentially mediating TNF-induced, but not Tax-induced, signals converging on the IKK signalsome. Differing levels of IKKgamma-delta expression, therefore, may affect signal transduction cascades coupling to IKK.

Stat3 activation of NF-{kappa}B p100 processing involves CBP/p300-mediated acetylation

Activation of the noncanonical NF-kappaB signaling pathway involved in the proteolytic processing of NF-kappaB p100 to p52 is tightly regulated, and overproduction of p52 leads to lymphocyte hyperplasia and transformation. We have demonstrated that active but not latent Stat3, expressed in many types of human cancers involved in cell proliferation and survival, induces p100 processing to p52 by activation of IKKalpha and subsequent phosphorylation of p100. The Stat3-mediated p100 processing to p52 requires activation of Stat3 by the acetyltransferase activity of cAMP-response element-binding protein (CREB)-binding protein (CBP)/p300. A mutant of Stat3 defective in acetylation blocked Stat3-mediated p100 processing to p52 and acted as a dominant negative in blocking the production of p52. Furthermore, overexpression of p52 protected cells from apoptotic cell death. Thus, activation of the processing of p100 to p52 by Stat3 may represent one of the common pathways used by cancer cells to survive and escape therapy.

Binding of Manumycin A Inhibits IκB Kinase β Activity

IkappaB kinase (IKK) catalytic subunits play a key role in cytokinemediated nuclear factor (NF)-kappaB signaling, and a loss of NF-kappaB function appears to inhibit inflammation and oncogenesis. Manumycin A is a potent and selective farnesyltransferase inhibitor with antitumor activity. We found that manumycin A caused a rapid and potent inhibition of IKK activity induced by tumor necrosis factor alpha in a number of cell types. Most unexpectedly, other classes of farnesyltransferase inhibitors had no inhibitory effect. To identify the molecular mechanisms of manumycin A action, cultured human HepG2 hepatoma cells were transiently transfected with various IKKalpha and IKKbeta constructs, and a striking difference in manumycin A sensitivity was observed. Furthermore, cells expressing wild-type IKKbeta and IKKbeta mutated in the activation loop at Cys-179 exhibited covalent homotypic dimerization of IKKbeta in response to manumycin A, whereas substitution of Cys-662 and -716 conferred protection against dimer formation. Direct inhibition of IKK activity and formation of stable IKKbeta dimers were observed in the presence of manumycin A that could be blocked by dithiothreitol. IKK interaction with the adaptor protein IKKgamma/NEMO was disrupted in manumycin A-treated cells. Most importantly, administration of manumycin A to mice xenografted with murine B16F10 tumors caused potent IKK-suppressive effects. Thus, manumycin A with its epoxyquinoid moieties plays an important regulatory function in IKK signaling through pathways distinct from its role as a protein farnesylation inhibitor.

RelB/p52 NF-kappaB complexes rescue an early delay in mammary gland development in transgenic mice with targeted superrepressor IkappaB-alpha expression and promote carcinogenesis of the mammary gland

Classical NF-kappaB (p65/p50) transcription factors display dynamic induction in the mammary gland during pregnancy. To further elucidate the role of NF-kappaB factors in breast development, we generated a transgenic mouse expressing the IkappaB-alpha S32/36A superrepressor (SR) protein under control of the mouse mammary tumor virus (MMTV) long terminal repeat promoter. A transient delay in mammary ductal branching was observed in MMTV-SR-IkappaB-alpha mice early during pregnancy at day 5.5 (d5.5) and d7.5; however, development recovered by mid- to late pregnancy (d14.5). Recovery correlated with induction of nuclear cyclin D1 and RelB/p52 NF-kappaB complexes. RelB/p52 complexes induced cyclin D1 and c-myc promoter activities and failed in electrophoretic mobility shift assay to interact with IkappaB-alpha-glutathione S-transferase, indicating that their weak interaction with IkappaB-alpha can account for the observed recovery of mammary gland development. Activation of IKKalpha and NF-kappaB-inducing kinase was detected by d5.5, implicating the alternative NF-kappaB signaling pathway in RelB/p52 induction. Constitutively active IKKalpha induced p52, RelB, and cyclin D1 in untransformed mammary epithelial cells. Moreover, mouse mammary tumors induced by 7,12-dimethylbenz(a)anthracene treatment displayed increased RelB/p52 activity. Inhibition of RelB in breast cancer cells repressed cyclin D1 and c-Myc levels and growth in soft agar. These results implicate RelB/p52 complexes in mammary gland development and carcinogenesis.

The inhibitory effect of alendronate, a nitrogen-containing bisphosphonate on the PI3K-Akt-NFkappaB pathway in osteosarcoma cells

1 Bisphosphonates are inhibitors of tumor cell growth as well as of bone resorption by inducing cell apoptosis. However, little is known regarding the mechanisms by which the drug induces cell apoptosis. The aim of the present study was to determine the effect of alendronate, one of the nitrogen-containing bisphosphonates on the phoshoinositide 3-kinase (PI3K)-Akt-NFkappaB pathway, the major cell survival pathway. 2 The PI3K-Akt-NFkappaB pathway was activated in the osteosarcoma cell line MG-63 treated with tumor necrosis factor-alpha or insulin. Saos-2 was also used in some experiments. This was assessed by the production of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), increased PI3K activity, phosphorylation of Akt at serine 473 and threonine 308, increase in activity of the inhibitor of nuclear factor kappaB (IkappaB) kinase (IKK) and finally phosphorylation of IkappaB and its subsequent degradation. 3 Pretreatment with alendronate at 100 microM for 24 h prior to the stimulation with tumor necrosis factor-alpha or insulin partially inhibited the IkappaB phosphorylation and degradation. These events were more clearly observed in the presence of inhibitors of proteasomes, which are responsible for the degradation of IkappaB. The drug also partially inhibited the activity of IKK, but almost fully inhibited the phosphorylation of Akt and the production of PtdIns(3,4,5)P(3). 4 The inhibitory effect of alendronate on IkappaB phosphorylation and degradation was not attenuated by the exogenous addition of geranylgeraniol to replenish the cytosolic isoprenyl lipid substrate. 5 The present findings demonstrate that alendronate inhibited the PI3K-Akt-NFkappaB cell survival pathway at the point of PI3K activation, thus indicating the presence of new targets of alendronate.