Basal phosphorylation of the PEST domain in the I(kappa)B(beta) regulates its functional interaction with the c-rel proto-oncogene product

CD5 dynamically calibrates basal NF-κB signaling in T cells during thymic development and peripheral activation

Immature T cells undergo a process of positive selection in the thymus when their new T cell receptor (TCR) engages and signals in response to self-peptides. As the T cell matures, a slew of negative regulatory molecules, including the inhibitory surface glycoprotein CD5, are up-regulated in proportion to the strength of the self-peptide signal. Together these regulators dampen TCR-proximal signaling and help avoid any subsequent peripheral activation of T cells by self-peptides. Paradoxically, antigen-specific T cells initially expressing more CD5 (CD5^hi) have been found to better persist as effector/memory cells after a peripheral challenge. The molecular mechanisms underlying such a duality in CD5 function is not clear. We found that CD5 alters the basal activity of the NF-κB signaling in resting peripheral T cells. When CD5 was conditionally ablated, T cells were unable to maintain higher expression of the cytoplasmic NF-κB inhibitor IκBα. Consistent with this, resting CD5^hi T cells expressed more of the NF-κB p65 protein than CD5^lo cells, without significant increases in transcript levels, in the absence of TCR signals. This posttranslationally stabilized cellular NF-κB depot potentially confers a survival advantage to CD5^hi T cells over CD5^lo ones. Taken together, these data suggest a two-step model whereby the strength of self-peptide-induced TCR signal lead to the up-regulation of CD5, which subsequently maintains a proportional reserve of NF-κB in peripheral T cells poised for responding to agonistic antigen-driven T cell activation.

The NF-κB regulator IκBβ exhibits different molecular interactivity and phosphorylation status from IκBα in an IKK2-catalysed reaction

Activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) transcription factor, a central player in immune response regulation, is based on phosphorylation of inhibitor of kappaB alpha (IκBα) by the Inhibitor of kappaB kinase (IKK) that triggers IκBα degradation. Although inhibitor of kappaB beta (IκBβ) is structurally similar to IκBα, its precise characteristics remain undefined. Herein, we report that the molecular interactivity of IκBβ with the kinase-active region of IKK subunit 2 (IKK2), as well as its phosphorylation status, differs markedly from those of IκBα. A mass spectrometry analysis revealed that IκBβ phosphorylation sites are distributed in its C-terminal region, whereas IκBα phosphorylation sites are located in the N-terminal region. Furthermore, IKK2 phosphorylation sites in IκBβ are found in a region distinct from typical degradation signals, such as phosphodegron and proline/glutamic acid/serine/threonine-rich sequence (PEST) motifs. Mutation of the IκBβ phosphorylation sites enhances its resistance to homeostatic proteasomal degradation. These findings contribute a novel concept in NF-κB/IKK signalling research.

Prediction of the presence of a seventh ankyrin repeat in IκBε from homology modeling combined with hydrogen-deuterium exchange mass spectrometry (HDX-MS)

The ankyrin repeat (AR) structure is a common protein-protein interaction motif and ankyrin repeat proteins comprise a vast family across a large array of different taxa. Natural AR proteins adopt a conserved fold comprised of several repeats with the N- and C-terminal repeats generally being of more divergent sequences. Obtaining experimental crystal structures for natural ankyrin repeat domains (ARD) can be difficult and often requires complexation with a binding partner. Homology modeling is an attractive method for creating a model of AR proteins due to the highly conserved fold; however, modeling the divergent N- and C-terminal "capping" repeats remains a challenge. We show here that amide hydrogen/deuterium exchange mass spectrometry (HDX-MS), which reports on the presence of secondary structural elements and "foldedness," can aid in the refinement and selection of AR protein homology models when multiple templates are identified with variations between them localizing to these terminal repeats. We report a homology model for the AR protein IκBε from three different templates and use HDX-MS to establish the presence of a seventh AR at the C-terminus identified by only one of the three templates used for modeling.

Therapeutic targeting of CK2 in acute and chronic leukemias

CK2 is a ubiquitously expressed, constitutively active Ser/Thr protein kinase, which is considered the most pleiotropic protein kinase in the human kinome. Such a pleiotropy explains the involvement of CK2 in many cellular events. However, its predominant roles are stimulation of cell growth and prevention of apoptosis. High levels of CK2 messenger RNA and protein are associated with CK2 pathological functions in human cancers. Over the last decade, basic and translational studies have provided evidence of CK2 as a pivotal molecule driving the growth of different blood malignancies. CK2 overexpression has been demonstrated in nearly all the types of hematological cancers, including acute and chronic leukemias, where CK2 is a key regulator of signaling networks critical for cell proliferation, survival and drug resistance. The findings that emerged from these studies suggest that CK2 could be a valuable therapeutic target in leukemias and supported the initiation of clinical trials using CK2 antagonists. In this review, we summarize the recent advances on the understanding of the signaling pathways involved in CK2 inhibition-mediated effects with a particular emphasis on the combinatorial use of CK2 inhibitors as novel therapeutic strategies for treating both acute and chronic leukemia patients.

Overexpression of Hypo-Phosphorylated IκBβ at Ser313 Protects the Heart against Sepsis

IκBβis an inhibitor of nuclear factor kappa B(NF-κB) and participates in the cardiac response to sepsis. However, the role of the hypo-phosphorylated form of IκBβ at Ser313, which can be detected during sepsis, is unknown. Here, we examined the effects of IκBβ with a mutation at Ser313→Ala313 on cardiac damage induced by sepsis. Transgenic (Tg) mice were generated to overexpress IκBβ, in which Ser-313 is replaced with alanine ubiquitously, in order to mimic the hypo-phosphorylated form of IκBβ. Survival analysis showed that Tg mice exhibited decreased inflammatory cytokine levels and decreased rates of mortality in comparison to wild type (WT) mice, after sepsis in a cecal-ligation and puncture model (CLP). Compared to WT septic mice, sepsis in Tg mice resulted in improved cardiac functions, lower levels of troponin I and decreased rates of cardiomyocyte apoptosis, compared to WT mice. The increased formation of autophagicvacuoles detected with electron microscopy demonstrated the enhancement of cardiac autophagy. This phenomenon was further confirmed by the differential expression of genes related to autophagy, such as LC3, Atg5, Beclin-1, and p62. The increased expression of Cathepsin L(Ctsl), a specific marker for mitochondrial stress response, may be associated with the beneficial effects of the hypo-phosphorylated form of IκBβ. Our observations suggest that the hypo-phosphorylated form of IκBβ at Ser313 is beneficial to the heart in sepsis through inhibition of apoptosisand enhancement of autophagy in mutated IκBβ transgenic mice.

Inhibiting IκBβ-NFκB signaling attenuates the expression of select pro-inflammatory genes

Multiple mediators of septic shock are regulated by the transcription factor nuclear factor κB (NFκB). However, complete NFκB inhibition can exacerbate disease, necessitating evaluation of targeted strategies to attenuate the pro-inflammatory response. Here, we demonstrate that in murine macrophages, low-dose NFκB inhibitors specifically attenuates lipopolysaccharide (LPS)-induced IκBβ degradation and the expression of a select subset of target genes (encoding IL1β, IL6, IL12β). Gain- and loss-of-function experiments demonstrate the necessary and sufficient role of inhibitor of NFκB family member IκBβ (also known as NFKBIB) in the expression of these genes. Furthermore, both fibroblasts and macrophages isolated from IκBβ overexpressing mice demonstrate attenuated LPS-induced IκBβ-NFκB signaling and IL1β, IL6 and IL12β expression. Further confirming the role of IκBβ and its NFκB subunit binding partner cRel in LPS-induced gene expression, pre-treatment of wild-type mouse embryonic fibroblasts with a cell-permeable peptide containing the cRel nuclear localization sequence attenuated IL6 expression. We prove that LPS-induced IκBβ-NFκB signaling can be selectively modulated to attenuate the expression of select pro-inflammatory target genes, thus providing therapeutic insights for patients exposed to systemic inflammatory stress.

Fas-associated factor (Faf1) is a novel CD40 interactor that regulates CD40-induced NF-κB activation via a negative feedback loop

CD40-induced signalling through ligation with its natural ligand (CD40L/CD154) is dependent on recruitment of TRAF molecules to the cytoplasmic domain of the receptor. Here, we applied the yeast two-hybrid system to examine whether other proteins can interact with CD40. Fas-Associated Factor 1(FAF1) was isolated from a HeLa cDNA library using the CD40 cytoplasmic tail (216–278 aa) as a bait construct. FAF1 was able to interact with CD40 both in vitro and in vivo. The FAF1 N-terminal domain was sufficient to bind CD40 and required the TRAF6-binding domain within the cytoplasmic tail of CD40 for binding. CD40 ligation induced FAF1 expression in an NFκB-dependent manner. Knockdown of FAF1 prolonged CD40-induced NFκB, whereas overexpression of FAF1 suppressed CD40-induced NFκB activity and this required interaction of FAF1 with the CD40 receptor via its FID domain. Thus, we report a novel role for FAF1in regulating CD40-induced NFκB activation via a negative feedback loop. Loss of FAF1 function in certain human malignancies may contribute to oncogenesis through unchecked NFκB activation, and further understanding of this process may provide a biomarker of NFκB-targeted therapies for such malignancies.

Epigenetic regulation of canonical TNFα pathway by HDAC1 determines survival of cardiac myocytes

Gene transcription is regulated by post-translation modifications. Histone deacetylases (HDACs) remove acetyl groups from histone and non-histone factors inhibiting transcription. Proinflammatory cytokines such as TNFα activate the canonical nuclear factor-κB (NF-κB) pathway. Earlier we established a cytoprotective role for NF-κB in the heart. Though a causal relationship for HDAC1 and NF-κB has been established, the impact of HDAC1 on TNFα signaling is unknown. Herein, we demonstrate that HDAC1 provides a molecular switch for determining cell survival in the TNFα pathway. In contrast to vehicle-treated control cells, TNFα-treated cells displayed a marked increase in NF-κB gene transcription. Notably, cells treated with TNFα were indistinguishable from vehicle controls cells with respect to viability. Interestingly, HDAC activity was reduced in cells treated with TNFα. Conversely, in the presence of HDAC1, NF-κB gene transcription by TNFα was repressed, resulting in mitochondrial perturbations and widespread cell death. Heterologous fusion proteins comprised of yeast Gal4 DNA binding domain fused in frame to the NF-κB p65 transactivation domain were preferentially repressed by HDAC1. Moreover, transcription mediated by Gal4VP16 protein from herpes virus was unaffected by HDAC1 in cardiac myocytes. Mutations that abrogate known catalytic activities of HDAC1, small interference RNA, or pharmacological inhibition of HDAC1 restored NF-κB signaling and suppressed cell death induced by TNFα. These data provide the first evidence for an obligate link between HDAC1 and canonical TNFα pathway for cell survival of cardiac myocytes.

Evolutionary, structural and functional interplay of the IκB family members

A primary level of control for nuclear factor kappa B (NF-κB) is effected through its interactions with the inhibitor protein, inhibitor of kappa B (IκB). Several lines of evidence confirm the existence of multiple forms of IκB that appear to regulate NF-κB by distinct mechanisms. Therefore, we performed a comprehensive bioinformatics analysis to understand the evolutionary history and intrinsic functional diversity of IκB family members. Phylogenetic relationships were constructed to trace the evolution of the IκB family genes. Our phylogenetic analysis revealed 10 IκB subfamily members that clustered into 5 major clades. Since the ankyrin (ANK) domain appears to be more ancient than the Rel homology domain (RHD), our phylogenetic analysis suggests that some undefined ancestral set of ANK repeats acquired an RHD before any duplication and was later duplicated and then diverged into the different IκB subfamilies. Functional analysis identified several functionally divergent sites in the ANK repeat domains (ARDs) and revealed that this region has undergone strong purifying selection, suggesting its functional importance in IκB genes. Structural analysis showed that the major variations in the number of ANK repeats and high conformational changes in the finger loop ARD region contribute to the differing binding partner specificities, thereby leading to distinct IκB functions. In summary, our study has provided useful information about the phylogeny and structural and functional divergence of the IκB family. Additionally, we identified a number of amino acid sites that contribute to the predicted functional divergence of these proteins.

Phosphoproteins in stress-induced disease

The integrated stress response (ISR) is an evolutionarily conserved homeostatic program activated by specific pathological states. These include amino acid deprivation, viral infection, iron deficiency, and the misfolding of proteins within the endoplasmic reticulum (ER), the so-called ER stress. Although apparently disparate, each of these stresses induces phosphorylation of a translation initiation factor, eIF2α, to attenuate new protein translation while simultaneously triggering a transcriptional program. This is achieved by four homologous stress-sensing kinases: GCN2, PKR, HRI, and PERK. In addition to these kinases, mammals possess two specific eIF2α phosphatases, GADD34 and CReP, which play crucial roles in the recovery of protein synthesis following the initial insult. They are not only important in embryonic development but also appear to play important roles in disease, particularly cancer. In this chapter, we discuss each of the eIF2α kinases, in turn, with particular emphasis on their regulation and the new insights provided by recent structural studies. We also discuss the potential for developing novel drug therapies that target the ISR.

Alternative nuclear functions for NF-κB family members

The NF-κB signalling pathway regulates many different biological processes from the cellular level to the whole organism. The majority of these functions are completely dependent on the activation of the cytoplasmic IKK kinase complex that leads to IκB degradation and results in the nuclear translocation of specific NF-κB dimers, which, in general, act as transcription factors. Although this is a well-established mechanism of action, several publications have now demonstrated that some members of this pathway display additional functions in the nucleus as regulators of NF-κB-dependent and independent gene expression. In this review, we compiled and put in context most of the data concerning specific nuclear roles for IKK and IκB proteins.

Specification of DNA binding activity of NF-kappaB proteins

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

IkappaBbeta acts to inhibit and activate gene expression during the inflammatory response

The activation of pro-inflammatory gene programs by nuclear factor-kappaB (NF-kappaB) is primarily regulated through cytoplasmic sequestration of NF-kappaB by the inhibitor of kappaB (IkappaB) family of proteins. IkappaBbeta, a major isoform of IkappaB, can sequester NF-kappaB in the cytoplasm, although its biological role remains unclear. Although cells lacking IkappaBbeta have been reported, in vivo studies have been limited and suggested redundancy between IkappaBalpha and IkappaBbeta. Like IkappaBalpha, IkappaBbeta is also inducibly degraded; however, upon stimulation by lipopolysaccharide (LPS), it is degraded slowly and re-synthesized as a hypophosphorylated form that can be detected in the nucleus. The crystal structure of IkappaBbeta bound to p65 suggested this complex might bind DNA. In vitro, hypophosphorylated IkappaBbeta can bind DNA with p65 and c-Rel, and the DNA-bound NF-kappaB:IkappaBbeta complexes are resistant to IkappaBalpha, suggesting hypophosphorylated, nuclear IkappaBbeta may prolong the expression of certain genes. Here we report that in vivo IkappaBbeta serves both to inhibit and facilitate the inflammatory response. IkappaBbeta degradation releases NF-kappaB dimers which upregulate pro-inflammatory target genes such as tumour necrosis factor-alpha (TNF-alpha). Surprisingly, absence of IkappaBbeta results in a dramatic reduction of TNF-alpha in response to LPS even though activation of NF-kappaB is normal. The inhibition of TNF-alpha messenger RNA (mRNA) expression correlates with the absence of nuclear, hypophosphorylated-IkappaBbeta bound to p65:c-Rel heterodimers at a specific kappaB site on the TNF-alpha promoter. Therefore IkappaBbeta acts through p65:c-Rel dimers to maintain prolonged expression of TNF-alpha. As a result, IkappaBbeta(-/-) mice are resistant to LPS-induced septic shock and collagen-induced arthritis. Blocking IkappaBbeta might be a promising new strategy for selectively inhibiting the chronic phase of TNF-alpha production during the inflammatory response.

Addiction to protein kinase CK2: a common denominator of diverse cancer cells?

At variance with most oncogenic protein kinases whose malignancy is generally due to genetic alterations conferring constitutive activity, CK2 is a highly pleiotropic Ser/Thr protein kinase naturally endowed with constitutive activity and lacking gain-of-function mutants. Nonetheless CK2 is abnormally elevated in a wide variety of tumors and there is strong evidence that it operates as a cancer driver by creating a cellular environment favorable to neoplasia: notably, CK2 plays a global role as an anti-apoptotic and pro-survival agent, it enhances the multi-drug resistance (MDR) phenotype, it assists the chaperone machinery which protects the "onco-kinome" and it promotes neo-angiogenesis. Based on this scenario we propose that the implication of CK2 in neoplasia is an example of "non oncogene addiction", i.e. over reliance of the perturbed cellular signaling network on high CK2 level for its own maintenance. Consistent with this, an ample spectrum of diverse types of cancer cells have been already shown to rely on high CK2 level for their survival, as judged from their response to specific CK2 inhibitors and silencing of endogenous CK2 catalytic subunits. Remarkably, among these are cells whose cancer phenotype arises from the genetic alteration of onco-kinases (e.g. Abl and Alk) different from CK2 and insensitive to the CK2 inhibitors used in those experiments. Based on these premises, CK2 could represent a "multi-purpose" target for the treatment of different kinds of tumors.

Protein kinase CK2 in health and disease: CK2 and its role in Wnt and NF-kappaB signaling: linking development and cancer

CK2 is a highly conserved tetrameric serine/ threonine kinase present in all eukaryotic organisms. It is constitutively active, and appears to be regulated by level of expression and activity, and subcellular localization. In turn, it has been postulated to control the function of many proteins through changes in phosphorylation that affect protein stability, protein-protein interactions, and subcellular localization. Through these mechanisms, CK2 regulates many fundamental cellular properties. An enzyme that carries out such a master regulatory function is likely to be important in organismic development and in cancer. We have shown that overexpression of CK2 catalytic subunits is capable of promoting tumorigenesis, and that loss of CK2 catalytic subunits in development can be lethal. Through studies in cells, mice, and frogs, we and others have identified the Wnt and NF-kappaB pathways as two key signal transduction pathways that are regulated by CK2 activity, in embryonic development and in cancer. These results suggest that inhibiting CK2 could be useful in treating cancer, but dangerous to developing organisms.

A new regulatory mechanism of NF-kappaB activation by I-kappaBbeta in cancer cells

Transglutaminase 2 (TGase 2) catalyzes covalent isopeptide bond formation between glutamine and lysine residues. Recently, we reported that TGase 2 activates nuclear factor-kappa B (NF-kappaB) by depleting inhibitor of NF-kappaBalpha (I-kappaBalpha) levels via polymer formation. Furthermore, TGase 2 expression synergistically increases NF-kappaB activity with canonical pathway. The major I-kappaB proteins such as I-kappaBalpha and I-kappaBbeta resemble each other in both primary sequence and tertiary structure. However, I-kappaBbeta does not degrade fully, while I-kappaBalpha degrades immediately in response to most stimuli. We found that I-kappaBbeta does not contain any of the previously identified TGase 2 target sites. In this study, both an in vitro cross-linking assay and a TGase 2 transfection assay revealed that I-kappaBbeta is independent from TGase 2-mediated polymerization. Furthermore, increased I-kappaBbeta expression reversed NF-kappaB activation in cancer cells, compensating for the loss of I-kappaBalpha via TGase 2 polymerization.

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

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

Requirement of an IkappaB-beta COOH terminal region protein for acidic-adaptation in CHO cells

We previously reported that an IkappaB-beta COOH terminal region protein (designated CTIB) was essential for the proliferation of CHO cells under acidic stress (Lao et al., 2005. J Cell Physiol 203(1):186-192). In order to investigate the mechanisms underlying the requirement of CTIB for acidic adaptation, CTIB was silenced with an RNAi technique in CHO cells. CTIB silencing resulted in those cells completely failing to proliferate and maintain intracellular pH (pHi) homeostasis at an extracellular pH (pHe) of 6.3. An increased activation of p38 MAP kinase was induced by CTIB silencing at the low pH value. CTIB was only present in the cytoplasm and co-immunoprecipitation of the cytoplasmic fraction revealed that the loss of CTIB led to a loss of p65 in the immunoprecipitate complex. CTIB silencing reduced both the decrease in p65 and the increase in p50 in the nucleus when the cells were incubated at pHe 6.3. In cells with CTIB silenced, the transcriptions of p65, p105, and IL1-beta were suppressed, and decreases in both the transcription and activity of MnSOD were observed at pHe 6.3. Suppression of these genes suggested a suppressed NF-kappaB activity since p105, IL1-beta, and MnSOD were target genes of NF-kappaB. Our data demonstrated that CTIB functioned to prevent the over-accumulation of p65 in the nucleus, ensuring the appropriate composition of the NF-kappaB complex in the nucleus to respond to stimuli under acidic conditions.

A comprehensive map of the toll-like receptor signaling network

Recognition of pathogen-associated molecular signatures is critically important in proper activation of the immune system. The toll-like receptor (TLR) signaling network is responsible for innate immune response. In mammalians, there are 11 TLRs that recognize a variety of ligands from pathogens to trigger immunological responses. In this paper, we present a comprehensive map of TLRs and interleukin 1 receptor signaling networks based on papers published so far. The map illustrates the possible existence of a main network subsystem that has a bow-tie structure in which myeloid differentiation primary response gene 88 (MyD88) is a nonredundant core element, two collateral subsystems with small GTPase and phosphatidylinositol signaling, and MyD88-independent pathway. There is extensive crosstalk between the main bow-tie network and subsystems, as well as feedback and feedforward controls. One obvious feature of this network is the fragility against removal of the nonredundant core element, which is MyD88, and involvement of collateral subsystems for generating different reactions and gene expressions for different stimuli.

An I?B-? COOH terminal region protein is essential for the proliferation of CHO cells under acidic stress

CHO-K1 cells were able to proliferate and maintain pHi homeostasis at pH 6.3. A novel acidic sensitive mutant, AS-5B, which proliferated at pH 7.4 but failed to either proliferate or maintain pHi homeostasis at pH 6.3, was derived from CHO-K1 using a replica method. The acidic-sensitivity of AS-5B was not due to deficiencies in sodium proton exchangers, HCO3- (co)transporters or H+-ATPases. A cDNA clone encoding a COOH terminal region of IkappaB-beta conferred partial acidic-resistance on AS-5B, and the encoded protein was present in CHO-K1, but was nearly absent from AS-5B. Our data demonstrated that the expression of this small protein was essential for the proliferation of CHO cells under acidic stress.

Comparative expression of NFkappaB proteins in melanocytes of normal skin vs. benign intradermal naevus and human metastatic melanoma biopsies

Nuclear factor kappa B (NFkappaB) is an essential regulator of gene transcription for hundreds of genes, including many critically involved in apoptosis. NFkappaB complexes containing cRel generally activate pro-apoptotic genes, while those with RelA activate anti-apoptotic genes. We have previously shown that NFkappaB binding by RelA is constitutively elevated in human metastatic melanoma cultures relative to normal melanocytes. Here we extended our investigation to immunohistochemical analysis of human tissue biopsies. We found that RelA expression is significantly elevated in melanocytes of human naevi and melanomas relative to normal skin, but expression of its inhibitor IkappaB-alpha is significantly lower in metastatic melanomas than in intradermal naevi. Antibodies specific for the nuclear localization signal of RelA also showed significantly increased staining in metastatic melanoma biopsies. Notably, in melanomas and in naevi, we also found that RelA is phosphorylated at serine 529, and this activated form accumulates in the nuclei of melanomas. This suggests that increased expression and phosphorylation of RelA occurs at the stage of the benign naevus, but IkappaB-alpha is able to sequester RelA in the cytoplasm and regulate RelA transcriptional transactivation. We also found that antibodies against cRel show a progressive increase in staining from naevi to melanoma. However, staining for IkappaB-epsilon, which primarily inhibits the nuclear localization of cRel was also progressively increased, and cRel expression was predominantly cytoplasmic in melanomas. These results confirm that the altered expression of RelA found in metastatic melanoma cells in tissue culture is relevant to human tumors and offer new insights into the deregulation of NFkappaB signaling.

KappaB-Ras binds to the unique insert within the ankyrin repeat domain of IkappaBbeta and regulates cytoplasmic retention of IkappaBbeta x NF-kappaB complexes

The IkappaBalpha and IkappaBbeta proteins inhibit the transcriptional potential of active NF-kappaB dimers through stable complex formation. It has been shown that inactive IkappaBalpha x NF-kappaB complexes shuttle in and out of the nucleus, whereas IkappaBbeta x NF-kappaB complexes are retained exclusively in the cytoplasm of resting cells. The biochemical mechanism underlying this functional difference and its consequences are unknown. Although the two IkappaB proteins are significantly homologous, IkappaBbeta contains a unique 47-amino acid insertion of unknown function within its ankyrin repeat domain. In this study, we assess the role of the IkappaBbeta insert in regulating cytoplasmic retention of IkappaBbeta.NF-kappaB complexes. Deletion of the IkappaBbeta insert renders IkappaBbeta x NF-kappaB complexes capable of shuttling between the nucleus and cytoplasm, similar to IkappaBalpha x NF-kappaB complexes. A small Ras-like G-protein, kappaB-Ras, participates with the IkappaBbeta insert to effectively mask the NF-kappaB nuclear localization potential. Similarly, a complex between NF-kappaB and a mutant IkappaBbeta protein containing four serine to alanine mutations within its C-terminal proline, glutamic acid, serine, and threonine-rich sequence exhibits nucleocytoplasmic shuttling. This suggests a phosphorylation state-dependent role for the C-terminal proline, glutamic acid, serine, and threonine-rich sequence of IkappaBbeta in proper localization of IkappaBbeta x NF-kappaB complexes. These results are consistent with structural studies, which predicted that binary IkappaBbeta x NF-kappaB complexes should be capable of nuclear translocation, and with previous observations that hypophosphorylated IkappaBbeta.NF-kappaB complexes can reside in the nucleus.

Mitochondria to nucleus stress signaling: a distinctive mechanism of NFkappaB/Rel activation through calcineurin-mediated inactivation of IkappaBbeta

Mitochondrial genetic and metabolic stress causes activation of calcineurin (Cn), NFAT, ATF2, and NFkappaB/Rel factors, which collectively alter the expression of an array of nuclear genes. We demonstrate here that mitochondrial stress-induced activation of NFkappaB/Rel factors involves inactivation of IkappaBbeta through Cn-mediated dephosphorylation. Phosphorylated IkappaBbeta is a substrate for Cn phosphatase, which was inhibited by FK506 and RII peptide. Chemical cross-linking and coimmunoprecipitation show that NFkappaB/Rel factor-bound IkappaBbeta forms a ternary complex with Cn under in vitro and in vivo conditions that was sensitive to FK506. Results show that phosphorylation at S313 and S315 from the COOH-terminal PEST domain of IkappaBbeta is critical for binding to Cn. Mutations at S313/S315 of IkappaBbeta abolished Cn binding, inhibited Cn-mediated increase of Rel proteins in the nucleus, and had a dominant-negative effect on the mitochondrial stress-induced expression of RyR1 and cathepsin L genes. Our results show the distinctive nature of mitochondrial stress-induced NFkappaB/Rel activation, which is independent of IKKalpha and IKKbeta kinases and affects gene target(s) that are different from cytokine and TNFalpha-induced stress signaling. The results provide new insights into the role of Cn as a critical link between Ca2+ signaling and NFkappaB/Rel activation.

One-thousand-and-one substrates of protein kinase CK2?

CK2 (formerly termed "casein kinase 2") is a ubiquitous, highly pleiotropic and constitutively active Ser/Thr protein kinase whose implication in neoplasia, cell survival, and virus infection is supported by an increasing number of arguments. Here an updated inventory of 307 CK2 protein substrates is presented. More than one-third of these are implicated in gene expression and protein synthesis as being either transcriptional factors (60) or effectors of DNA/RNA structure (50) or translational elements. Also numerous are signaling proteins and proteins of viral origin or essential to virus life cycle. In comparison, only a minority of CK2 targets (a dozen or so) are classical metabolic enzymes. An analysis of 308 sites phosphorylated by CK2 highlights the paramount relevance of negatively charged side chains that are (by far) predominant over any other residues at positions n+3 (the most crucial one), n+1, and n+2. Based on this signature, it is predictable that proteins phosphorylated by CK2 are much more numerous than those identified to date, and it is possible that CK2 alone contributes to the generation of the eukaryotic phosphoproteome more so than any other individual protein kinase. The possibility that CK2 phosphosites play some global role, e.g., by destabilizing alpha helices, counteracting caspase cleavage, and generating adhesive motifs, will be discussed.

Acetylation of RelA at discrete sites regulates distinct nuclear functions of NF-kappaB

The nuclear function of the heterodimeric NF-kappaB transcription factor is regulated in part through reversible acetylation of its RelA subunit. We now demonstrate that the p300 and CBP acetyltransferases play a major role in the in vivo acetylation of RelA, principally targeting lysines 218, 221 and 310 for modification. Analysis of the functional properties of hypoacetylated RelA mutants containing lysine-to-arginine substitutions at these sites and of wild-type RelA co-expressed in the presence of a dominantly interfering mutant of p300 reveals that acetylation at lysine 221 in RelA enhances DNA binding and impairs assembly with IkappaBalpha. Conversely, acetylation of lysine 310 is required for full transcriptional activity of RelA in the absence of effects on DNA binding and IkappaBalpha assembly. Together, these findings highlight how site-specific acetylation of RelA differentially regulates distinct biological activities of the NF-kappaB transcription factor complex.

Characterization of the nuclear import and export functions of Ikappa B(epsilon)

Control over the nuclear localization of nuclear factor kappaB/Rel proteins is accomplished in large part through association with members of the inhibitor of kappaB (IkappaB) protein family. For example, the well studied IkappaBalpha protein actively shuttles between the nucleus and the cytoplasm and both inhibits nuclear import and mediates nuclear export of NF-kappaB/Rel proteins. In contrast, the IkappaBbeta protein can inhibit nuclear import of NF-kappaB/Rel proteins but does not remove NF-kappaB/Rel proteins from the nucleus. To further understand how the IkappaB proteins control the nuclear-cytoplasmic distribution of NF-kappaB/Rel proteins, we have characterized the nuclear import and nuclear export functions of IkappaBepsilon. Our results indicate that the IkappaBepsilon protein, like the IkappaBalpha protein, actively shuttles between the nucleus and the cytoplasm. Similar to IkappaBalpha, nuclear import of IkappaBepsilon is mediated by its ankyrin repeat domain and is not blocked by the dominant-negative RanQ69L protein. However, the nuclear import function of the IkappaBepsilon ankyrin repeat domain is markedly less efficient than that of IkappaBalpha, with the result that nuclear shuttling of IkappaBepsilon between the nucleus and the cytoplasm is significantly slower than IkappaBalpha. Nuclear export of IkappaBepsilon is mediated by a short leucine-rich nuclear export sequence (NES)-like sequence ((343)VLLPFDDLKI(352)), located between amino acids 343 and 352. This NES-like sequence is required for RanGTP-dependent binding of IkappaBepsilon to CRM1. Nuclear accumulation of IkappaB(epsilon) is increased by either leptomycin B treatment or alanine substitutions within the IkappaBepsilon-derived NES. A functional NES is required for both efficient cytoplasmic retention and post-induction control of c-Rel by IkappaBepsilon, consistent with the notion that IkappaBepsilon-mediated nuclear export contributes to control over the nucleocytoplasmic distribution of NF-kappaB/Rel proteins.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

IkappaBbeta, but not IkappaBalpha, functions as a classical cytoplasmic inhibitor of NF-kappaB dimers by masking both NF-kappaB nuclear localization sequences in resting cells

NF-kappaB dimers, inhibitor IkappaB proteins, and NF-kappaB.IkappaB complexes exhibit distinct patterns in partitioning between nuclear and cytoplasmic cellular compartments. IkappaB-dependent modulation of NF-kappaB subcellular localization represents one of the more poorly understood processes in the NF-kappaB signaling pathway. In this study, we have combined in vitro biochemical and cell-based methods to elucidate differences in NF-kappaB regulation exhibited by the inhibitors IkappaBbeta and IkappaBalpha. We show that although both IkappaBalpha and IkappaBbeta bind to NF-kappaB with similar global architecture and stability, significant differences exist that contribute to their unique functional roles. IkappaBbeta derives its high affinity toward NF-kappaB dimers by binding to both NF-kappaB subunit nuclear localization signals. In contrast, IkappaBalpha contacts only one NF-kappaB NLS and employs its carboxyl-terminal proline, glutamic acid, serine, and threonine-rich region for high affinity NF-kappaB binding. We show that the presence of one free NLS in the NF-kappaB.IkappaBalpha complex renders it a dynamic nucleocytoplasmic complex, whereas NF-kappaB.IkappaBbeta complexes are localized to the cytoplasm of resting cells.

Inhibition of NF-kappaB-dependent T cell activation abrogates acute allograft rejection

Using a heterotopic model of transplantation, we investigated the role of T cell activation in vivo during allograft rejection in I-kappaB(DeltaN)-transgenic mice that express a transdominant inhibitor of NF-kappaB in T cells. Our results show indefinite prolongation of graft survival in the I-kappaB(DeltaN)-transgenic recipients. Interestingly, at the time of rejection of grafts in wild-type recipients, histology of grafts in the I-kappaB(DeltaN)-transgenic recipients showed moderate rejection; nevertheless, grafts in the I-kappaB(DeltaN) recipients survived >100 days. Analysis of acute phase cytokines, chemokine, chemokine receptors, and immune responses shows that the blockade of NF-kappaB activation in T cells inhibits up-regulation of many of these parameters. Interestingly, our data also suggest that the T cell component of the immune response exerted positive feedback regulation on the expression of multiple chemokines that are produced predominantly by non-T cells. In conclusion, our studies indicate NF-kappaB activation in T cells is necessary for acute allograft rejection.

NF-kappa B-inducible BCL-3 expression is an autoregulatory loop controlling nuclear p50/NF-kappa B1 residence

NF-kappa B is a transcription factor whose nuclear residence is controlled by I kappa B family members. In the NF-kappa B-I kappa B autoregulatory loop, activated (nuclear) Rel A.NF-kappa B1 induces the resynthesis of I kappa B alpha recapturing nuclear Rel A back into the cytoplasm within 1 h of stimulation. In contrast, NF-kappa B1 subunits redistribute more slowly into the cytoplasm (from 6 to 12 h). Here we examine the role of inducible cytoplasmic BCL-3 expression in terminating nuclear NF-kappa B1. Although BCL-3 is a nuclear protein in B lymphocytes, surprisingly, BCL-3 is primarily a cytoplasmic protein in HepG2 cells. Cytoplasmic BCL-3 abundance is induced 6-12 h after tumor necrosis factor-alpha stimulation where it complexes with NF-kappa B1 homodimers. Moreover, BCL-3 mRNA and protein expression are induced by NF-kappa B-activating agents. Two observations are interpreted to indicate that bcl-3 is transactivated by NF-kappa B/Rel A: 1) expression of a dominant negative NF-kappa B inhibitor blocks tumor necrosis factor-alpha-induced BCL-3 expression and 2) expression of constitutively active Rel A is sufficient to induce BCL-3 expression. In gene transfer studies, we identify two high affinity NF-kappa B-binding sites, kappa B1 (located at -872 to -861 nucleotides) and kappa B2 (-106 to -96 nucleotides), and although both bind with high affinity to Rel A, only kappa B2 is required for NF-kappa B-dependent induction of the native BCL-3 promoter. Down-regulation of BCL-3 induction results in prolonged, enhanced NF-kappa B1 binding and increased NF-kappa B-dependent transcription. Together, these data suggest the presence of an NF-kappa B-BCL-3 autoregulatory loop important in terminating NF-kappa B1 action and that individual NF-kappa B isoforms are actively terminated through coordinate induction of inhibitory I kappa B molecules to restore cellular homeostasis.

Enhancement of radiosensitivity by proteasome inhibition: implications for a role of NF-kappaB

PURPOSE

NF-kappaB is activated by tumor necrosis factor, certain chemotherapeutic agents, and ionizing radiation, leading to inhibition of apoptosis. NF-kappaB activation is regulated by phosphorylation of IkappaB inhibitor molecules that are subsequently targeted for degradation by the ubiquitin-proteasome pathway. PS-341 is a specific and selective inhibitor of the proteasome that inhibits NF-kappaB activation and enhances cytotoxic effects of chemotherapy in vitro and in vivo. The objective of this study was to determine if proteasome inhibition leads to enhanced radiation sensitivity.

METHODS AND MATERIALS

Inhibition of NF-kappaB activation in colorectal cancer cells was performed by treatment of LOVO cells with PS-341 or infection with an adenovirus encoding IkappaB super-repressor, a selective NF-kappaB inhibitor. Cells were irradiated at 0, 2, 4, 6, 8, and 10 Gy with or without inhibition of NF-kappaB. NF-kappaB activation was determined by electrophoretic mobility gel shift assay, and apoptosis was evaluated using the TUNEL assay. Growth and clonogenic survival data were obtained to assess effects of treatment on radiosensitization. In vitro results were tested in vivo using a LOVO xenograft model.

RESULTS

NF-kappaB activation was induced by radiation and inhibited by pretreatment with either PS-341 or IkappaBalpha super-repressor in all cell lines. Inhibition of radiation-induced NF-kappaB activation resulted in increased apoptosis and decreased cell growth and clonogenic survival. A 7-41% increase in radiosensitivity was observed for cells treated with PS-341 or IkappaBalpha. An 84% reduction in initial tumor volume was obtained in LOVO xenografts receiving radiation and PS-341.

CONCLUSIONS

Inhibition of NF-kappaB activation increases radiation-induced apoptosis and enhances radiosensitivity in colorectal cancer cells in vitro and in vivo. Results are encouraging for the use of PS-341 as a radiosensitizing agent in the treatment of colorectal cancer.

Interaction of CD163 with the regulatory subunit of casein kinase II (CKII) and dependence of CD163 signaling on CKII and protein kinase C

CD163 is a recently identified member of the scavenger receptor cysteine-rich superfamily, which is expressed on peripheral blood monocytes and most tissue macrophages and is thought to play an important role in the regulation of the inflammatory response of these cells. Cross-linking of CD163 on glucocorticoid-stimulated macrophages results in the secretion of several proinflammatory cytokines, but the precise mechanism of CD163 mediated signal transduction is not understood. The existence of several CD163 isoforms, which differ in the structure of their cytoplasmic domains and putative phosphorylation sites, suggests that these isoforms also differ in their signaling mechanism. Using the Yeast Two-Hybrid system and further in vitro and in vivo studies, we identified the regulatory beta-subunit of casein kinase II (CKII), which specifically binds to the cytoplasmic domain of CD163 and its isoforms. We also found, that in vitro the CD163 isoforms differ in their association with the CKII holoenzyme and in the phosphorylation by CKII. Furthermore, we demonstrated that the cytoplasmic domains of CD163 variants are phosphorylated by PKC-alpha in vitro. Inhibition studies using specific kinase inhibitors reveal that both CKII and PKC are involved in the CD163 signaling mechanism resulting in the secretion of proinflammatory cytokines.

Dynamic shuttling of nuclear factor kappa B between the nucleus and cytoplasm as a consequence of inhibitor dissociation

Activation of the nuclear factor kappaB (NFkappaB) transcription factor is intimately associated with its translocation from the cytoplasm to the nucleus. Using the nuclear export inhibitor leptomycin B, we demonstrate shuttling of the RELA subunit of NFkappaB and the inhibitory subunit IkappaBalpha between these two compartments in unstimulated cells. Determination of the kinetics of nuclear entry shows marked differences for the two components; the entry of IkappaBalpha occurs more rapidly than RELA. The shuttling is suggested to be a consequence of the cytoplasmic dissociation of the NFkappaB.IkappaB complex rather than its direct nuclear import or degradation and resynthesis of IkappaBalpha. Using previously published kinetic data, this proposition is born out by the deduction that 17% of NFkappaB is not complexed to IkappaBalpha in a resting cell. A numerical model is presented to validate the proposed regulation of NFkappaB subcellular localization consequent in part on the nuclear export function and in part on the cytoplasmic retention function of IkappaBalpha. We suggest that the non-saturated interaction of NFkappaB with the inhibitor may enhance the specificity of action of IkappaB proteins on different NFkappaB dimers and allow additional modes of regulation of IkappaB function.

Tiam1 mutations in human renal-cell carcinomas

Tiam1 activates the Rho-like GTPase Rac1, and studies indicate that Tiam1-Rac1 signaling affects invasion in different ways depending on the cell type studied. However, no investigations on Tiam1 in human tumors have been reported. Here, we show that for 4 of 5 human renal-cell carcinoma (RCC) cell lines the expression levels of Tiam1 tended to be inversely correlated with in vitro invasiveness, whereas no obvious correlation could be found between the expression levels of Rac1 and invasion. Subsequent mutation analysis of these cell lines revealed no mutations in Rac1 but up to 5 different point mutations in the Tiam1 gene. Of these, 1 mutation (A441G) was located in the NH2-terminal pleckstrin homology domain, which is essential for membrane localization and functional activity of Tiam1. By analysis of an additional 30 primary human RCCs, mutation A441G was found in 4 of 35 tumors and tumor cell lines (11.5%) but not in the respective normal kidney tissues. By enzymatic digestion, mutation A441G proved to be heterozygous, suggesting a dominant active function. This was supported by showing that stable over-expression of mutated A441G-Tiam1 induced transformation of NIH3T3 cells, as determined in a colony formation assay, whereas empty vector and wild-type Tiam1 failed to do so. In conclusion, a distinct Tiam1 mutation (A441G) was identified in several human RCCs. This mutation induced transformation of NIH3T3 cells and, hence, might play a major role in the progression of human RCCs. Further analyses on Tiam1 mutations in human tumors might give new clues to their role in tumor progression.

Tumor necrosis factor alpha-induced phosphorylation of RelA/p65 on Ser529 is controlled by casein kinase II

Nuclear factor kappaB (NF-kappaB)/Rel transcription factors are key regulators of a variety of genes involved in immune and inflammatory responses, growth, differentiation, apoptosis, and development. In unstimulated cells, NF-kappaB/Rel proteins are sequestered in the cytoplasm by IkappaB inhibitor proteins. Many extracellular stimuli, such as tumor necrosis factor alpha (TNFalpha), cause rapid phosphorylation of IkappaB at N-terminal serine residues leading to ubiquitination and degradation of the inhibitor. Subsequently, NF-kappaB proteins translocate to the nucleus and activate gene expression through kappaB response elements. TNFalpha, as well as certain other stimuli, also induces the phosphorylation of the NF-kappaB proteins. Previously, we have shown that TNFalpha induces RelA/p65 phosphorylation at serine 529 and that this inducible phosphorylation increases NF-kappaB transcriptional activity on an exogenously supplied reporter (). In this report, we demonstrate that casein kinase II (CKII) interacts with p65 in vivo and can phosphorylate p65 at serine 529 in vitro. A CKII inhibitor (PD144795) inhibited TNFalpha-induced p65 phosphorylation in vivo. Furthermore, our results indicate that the association between IkappaBalpha and p65 inhibits p65 phosphorylation by CKII and that degradation of IkappaBalpha allows CKII to phosphorylate p65 to increase NF-kappaB transactivation potential. These data may explain the ability of CKII to modulate cell growth and demonstrate a mechanism whereby CKII can function in an inducible manner.

Inhibitory and stimulatory effects of lactacystin on expression of nitric oxide synthase type 2 in brain glial cells. The role of Ikappa B-beta

Expression of inflammatory nitric oxide synthase (NOS2) is mediated by transcription factor NFkappaB. By using the specific proteasome inhibitor lactacystin to examine IkappaB degradation, we observed a paradoxical increase in lipopolysaccharide- and cytokine-dependent NOS2 expression at low concentrations or when lactacystin was added subsequent to cytokines. Lactacystin reduced the initial accumulation of NOS2 mRNA but reduced its subsequent decrease. Lactacystin increased NOS2 promoter activation after 24 h, but not after 4 h, and similarly prevented initial NFkappaB activation and at later times caused NFkappaB reactivation. Lactacystin reduced initial degradation of IkappaB-alpha and IkappaB-beta, however, at later times selectively increased IkappaB-beta, which was predominantly non-phosphorylated. Expression of full-length rat IkappaB-beta, but not a carboxyl-terminal truncated form, inhibited NOS2 induction and potentiation by lactacystin. Lactacystin increased IkappaB-beta expression in the absence of NOS2 inducers, as well as expression of heat shock protein 70, and the heat shock response due to hyperthermia increased IkappaB-beta expression. These results suggest that IkappaB-beta contributes to persistent NFkappaB activation and NOS2 expression in glial cells, that IkappaB-beta is a stress protein inducible by hyperthermia or proteasome inhibitors, and that delayed addition of proteasome inhibitors can have stimulatory rather than inhibitory actions.

Recent advances towards understanding redox mechanisms in the activation of nuclear factor kappaB

The transcription factor, nuclear factor-kappaB (NF-kappaB) has been studied extensively due to its prominent role in the regulation of immune and inflammatory genes, apoptosis, and cell proliferation. It has been known for more that a decade that NF-kappaB is a redox-sensitive transcription factor. The contribution of redox regulation and the location of potential redox-sensitive sites within the NF-kappaB activation pathway are subject to intense debate due to many conflicting reports. Redox regulation of NF-kappaB has been extensively addressed in this journal and the reader is referred to two comprehensive reviews on the subject [1,2]. With the identification of signaling intermediates proximal to the degradation of the inhibitor, IkappaB, the number of potential redox-sensitive sites is rapidly increasing. The purpose of this review is to address recent insights into the NF-kappaB signaling cascades that are triggered by proinflammatory cytokines such as TNF-alpha and IL-1beta. In addition, the role of nitrogen monoxide (.NO) in the regulation of NF-kappaB will be reviewed. Opportunities for redox regulation that occur upstream of IkappaB-alpha degradation, as well as the potential for redox control of phosphorylation of NF-kappaB subunits, will be discussed. Redox-sensitive steps are likely to depend on the nature of the NF-kappaB activator, the type of reactive oxygen or nitrogen species involved, the selectivity of signaling pathways activated, as well as the cell type under investigation. Lastly, it is discussed how redox regulation of NF-kappaB activation is likely to involve multiple subcellular compartments.

A functional role of I kappa B-epsilon in endothelial cell activation

The NF-kappa B inhibitor I kappa B-epsilon is a new member of the I kappa B protein family, but its functional role in regulating NF-kappa B-mediated induction of adhesion molecule expression is unknown. In vascular endothelial cells, I kappa B-epsilon associates predominantly with the NF-kappa B subunit Rel A and to a lesser extent with c-Rel, whereas I kappa B-alpha and I kappa B-beta associate with Rel A only. Following stimulation with TNF-alpha, pyrrolidine dithiocarbamate (PDTC), N-acetylcysteine, and dexamethasone prevented I kappa B kinase-induced I kappa B-alpha, but not I kappa B-beta or I kappa B-epsilon phosphorylation and degradation. Since the activation of NF-kappa B is required for the induction of adhesion molecule expression, we examined the role of I kappa B-epsilon in the transactivation of promoters from VCAM-1, ICAM-1, and E-selectin. Using reporter gene constructs of adhesion molecule promoters, PDTC inhibited VCAM-1 and E-selectin, but to a lesser extent, ICAM-1 promoter activity. Subcloning of kappa B cis-acting elements of VCAM-1, E-selectin, and ICAM-1 into a heterologous promoter construct revealed that PDTC inhibited VCAM-1 and E-selectin, but to a lesser extent, ICAM-1 kappa B promoter activity. By electrophoretic mobility shift assay, NF-kappa B heterodimers containing c-Rel specifically bind to the kappa B motif in the ICAM-1, but not VCAM-1 or E-selectin promoter. Indeed, overexpression of c-Rel induced ICAM-1 kappa B promoter activity to a greater extent than that of E-selectin and overexpression of I kappa B-epsilon inhibited ICAM-1 and VCAM-1 promoter activity in endothelial cells. These findings indicate that c-Rel-associated I kappa B-epsilon is involved in the induction of ICAM-1 expression.

Inhibition of IkappaB kinase and IkappaB phosphorylation by 15-deoxy-Delta(12,14)-prostaglandin J(2) in activated murine macrophages

Activation of the macrophage cell line RAW 264.7 with lipopolysaccharide (LPS) and gamma interferon (IFN-gamma) induces the expression of gene products involved in host defense, among them type 2 nitric oxide synthase. Treatment of cells with 15-deoxy-Delta(12,14)-prostaglandin J(2) (15dPGJ(2)) inhibited the LPS- and IFN-gamma-dependent synthesis of NO, a process that was not antagonized by similar concentrations of prostaglandin J(2), prostaglandin E(2), or rosiglitazone, a peroxisomal proliferator-activated receptor gamma ligand. Incubation of activated macrophages with 15dPGJ(2) inhibited the degradation of IkappaBalpha and IkappaBbeta and increased their levels in the nuclei. NF-kappaB activity, as well as the transcription of NF-kappaB-dependent genes, such as those encoding type 2 nitric oxide synthase and cyclooxygenase 2, was impaired under these conditions. Analysis of the steps leading to IkappaB phosphorylation showed an inhibition of IkappaB kinase by 15dPGJ(2) in cells treated with LPS and IFN-gamma, resulting in an impaired phosphorylation of IkappaBalpha, at least in the serine 32 residue required for targeting and degradation of this protein. Incubation of partially purified activated IkappaB kinase with 2 microM 15dPGJ(2) reduced by 83% the phosphorylation in serine 32 of IkappaBalpha, suggesting that this prostaglandin exerts direct inhibitory effects on the activity of the IkappaB kinase complex. These results show rapid actions of 15dPGJ(2), independent of peroxisomal proliferator receptor gamma activation, in macrophages challenged with low doses of LPS and IFN-gamma.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

NF-kappaB activation and HIV-1 induced apoptosis

HIV infection leads to the progressive loss of CD4+ T cells and the near complete destruction of the immune system in the majority of infected individuals. High levels of viral gene expression and replication result in part from the activation of NF-kappaB transcription factors, which in addition to orchestrating the host inflammatory response also activate the HIV-1 long terminal repeat. NF-kappaB induces the expression of numerous cytokine, chemokine, growth factor and immunoregulatory genes, many of which promote HIV-1 replication. Thus, NF-kappaB activation represents a double edged sword in HIV-1 infected cells, since stimuli that induce an NF-kappaB mediated immune response will also lead to enhanced HIV-1 transcription. NF-kappaB has also been implicated in apoptotic signaling, protecting cells from programmed cell death under most circumstances and accelerating apoptosis in others. Therefore, activation of NF-kappaB can impact upon HIV-1 replication and pathogenesis at many levels, making the relationship between HIV-1 expression and NF-kappaB activation multi-faceted. This review will attempt to analyse the many faces and functions of NF-kappaB in the HIV-1 lifecycle.

Negative regulation of transactivation function but not DNA binding of NF-kappaB and AP-1 by IkappaBbeta1 in breast cancer cells

The transcription factor NF-kappaB regulates the expression of genes involved in cancer cell invasion, metastasis, angiogenesis, and resistance to chemotherapy. In normal cells NF-kappaB is maintained in the cytoplasm by protein-protein interaction with inhibitor IkappaBs. In contrast, in cancer cells a substantial amount of NF-kappaB is in the nucleus and constitutively activates target genes. To understand the mechanisms of constitutive NF-kappaB activation, we have analyzed the function of IkappaBalpha and IkappaBbeta in breast cancer cells. In most cases, constitutive NF-kappaB DNA binding correlated with reduced levels of either IkappaBalpha or IkappaBbeta isoforms. Overexpression of IkappaBalpha but not IkappaBbeta1 resulted in reduced constitutive DNA binding of NF-kappaB in MDA-MB-231 cells. Unexpectedly, IkappaBbeta1 overexpression moderately increased 12-O-tetradecanoylphorbol-13-acetate- and interleukin-1-inducible NF-kappaB DNA binding. 12-O-Tetradecanoylphorbol-13-acetate- and interleukin-1-induced transactivation by NF-kappaB, however, was lower in IkappaBbeta1-overexpressing cells. Mutants of IkappaBbeta1 lacking the C-terminal casein kinase II phosphorylation sites, which form a stable complex with DNA bound NF-kappaB without inhibiting its transactivation in other cell types, repressed the transactivation by NF-kappaB in MDA-MB-231 cells. Consistent with the results of transient transfections, the expression of urokinase plasminogen activator, an NF-kappaB target gene, was reduced in IkappaBbeta1-overexpressing cells. These results suggest that depending on the cell type, IkappaBbeta1 represses the expression of NF-kappaB-regulated genes by inhibiting either DNA binding or transactivation function of NF-kappaB.

Fibroblast Growth Factor-1 (FGF-1) Enhances IL-2 Production and Nuclear Translocation of NF-κB in FGF Receptor-Bearing Jurkat T Cells

Fibroblast growth factors (FGFs) are heparin-binding proteins crucial to embryogenesis, angiogenesis, and wound healing. FGF-1 is abundantly expressed in the synovium in rheumatoid arthritis and in rejecting allografts, sites of chronic immune-mediated inflammation. The frequency of FGF-1-responsive T cells is increased in the peripheral blood of these disorders, and a high percentage of infiltrating T cells in rheumatoid arthritis synovium express receptors for FGF-1. To understand the action of FGF-1 in T cells, studies were initiated in Jurkat T cells that express the signaling isoform of FGF receptor-1. These experiments show that FGF-1 stimulation of Jurkat T cells provides a second signal that augments TCR-mediated IL-2 production. Analogous to costimulation via CD28, this activity is mediated through activation of Rel/κB, a family of transcription factors known to regulate IL-2 and other activation-inducible proteins. FGF-1 alone induces modest nuclear translocation of κB-binding proteins, and this translocation is enhanced by the combination of anti-CD3 and FGF-1. This NF-κB binding complex is composed of transcriptionally active p65(RelA)/p50 heterodimers and results primarily from the targeted degradation of IκB-α, an inhibitor that sequesters Rel/κB in the cytoplasm. These data are the first to show a connection between FGF-1 signaling and NF-κB activation outside of embryonic development. The signaling events that link FGF receptor-1 engagement and NF-κB activation in Jurkat are probably distinct from the CD28 costimulation pathway, since FGF-1-induced Rel/κB binding proteins do not contain significant levels of c-Rel and are not identical with the CD28 response complex.

Nuclear IkappaBbeta maintains persistent NF-kappaB activation in HIV-1-infected myeloid cells

Monocytic cells exhibit constitutive NF-kappaB activation upon infection with human immunodeficiency virus-1 (HIV-1). Because IkappaBbeta has been implicated in maintaining NF-kappaB.DNA binding, we sought to investigate whether IkappaBbeta was involved in maintaining persistent NF-kappaB activation in HIV-1-infected monocytic cell lines. IkappaBbeta was present in the nucleus of HIV-1-infected cells and participated in the ternary complex formation with NF-kappaB and DNA. In contrast to uninfected cells, the addition of recombinant glutathione S-transferase-IkappaBalpha protein to preformed NF-kappaB.DNA complexes from HIV-1-infected cell extracts did not completely dissociate the complexes, suggesting that IkappaBbeta may protect NF-kappaB complexes from IkappaBalpha-mediated dissociation. Immunodepletion of IkappaBbeta resulted in an NF-kappaB.DNA binding complex that was sensitive to IkappaBalpha-mediated dissociation, thus demonstrating the protective role of IkappaBbeta. In addition, co-transfection studies with an NF-kappaB-dependent reporter construct demonstrated that IkappaBbeta co-expression partially alleviated inhibition of NF-kappaB-mediated gene expression by IkappaBalpha, implying that IkappaBbeta can maintain transcriptionally active NF-kappaB.DNA complexes. Furthermore, constitutive phosphorylation of IkappaBalpha was observed. Immunoprecipitation of the IkappaB kinase (IKK) complex followed by in vitro analysis of kinase activity demonstrated that IKK was constitutively activated in HIV-1-infected myeloid cells. Thus, virus-induced constitutive IKK activation, coupled with the maintenance of a ternary NF-kappaB.DNA complex by IkappaBbeta, maintains persistent NF-kappaB activity in HIV-1-infected myeloid cells.

Mechanisms by which IkappaB proteins control NF-kappaB activity

The biological activity of the transcription factor NF-kappaB is differentially controlled by three IkappaB proteins, Ikappa Balpha, Ikappa Bbeta, and Ikappa Bepsilon. We have examined the molecular basis for the differential inhibitory strengths of IkappaB proteins by constructing hybrid IkappaBs and found that the first ankyrin repeat of Ikappa Balpha is responsible for its strong inhibitory effect. Swapping a putative beta-turn within the first ankyrin repeat between the strong Ikappa Balpha and the weak IkappaBbeta inhibitors switches their in vivo inhibitory activity on NF-kappaB. The qualitatively distinct contacts made by this beta-turn in Ikappa Balpha and Ikappa Bbeta with NF-kappaB determine the efficiency by which IkappaBs sequester NF-kappaB to the cytoplasm, thus explaining their distinct effects on gene activity.

Human CD5 Signaling and Constitutive Phosphorylation of C-Terminal Serine Residues by Casein Kinase II

CD5 is a lymphocyte surface glycoprotein with a long cytoplasmic domain suitable for phosphorylation and signal transduction, which is involved in the modulation of Ag-specific receptor-mediated activation and differentiation signals. In this study, we use Jurkat T cell transfectants of CD5 cytoplasmic tail mutants to reveal phosphorylation sites relevant to signal transduction. Our results show that casein kinase II (CKII) is responsible for the constitutive phosphorylation of CD5 molecules at a cluster of three serine residues located at the extreme C terminus (S458, S459, and S461). Furthermore, the yeast two-hybrid system demonstrates the specific association between the C-terminal regions of the CD5 cytoplasmic tail and the regulatory β subunit of CKII. We demonstrate that CKII associates with and phosphorylates the C-terminal region of CD5, a conserved domain known to be relevant for the generation of second lipid messengers, and thereby enables at least one component of its signaling funcion.

SLP-76 Expression Is Restricted to Hemopoietic Cells of Monocyte, Granulocyte, and T Lymphocyte Lineage and Is Regulated During T Cell Maturation and Activation

The leukocyte-specific adapter protein SLP-76 is known to augment the transcriptional activity of nuclear factor of activated T cells and AP-1 following TCR ligation. A role for SLP-76 in additional receptor-mediated signaling events is less clear. To define the pattern of SLP-76 expression during murine hemopoiesis, we stained cells isolated from various tissues with a combination of surface markers followed by intracellular staining with a fluorochrome-labeled SLP-76-specific Ab. In the bone marrow, SLP-76 expression is largely restricted to cells of granulocyte and monocyte lineage. Heterogeneous SLP-76 expression is first detected in the CD44+CD25− subset within the CD3−CD4−CD8− thymocyte population. Interestingly, SLP-76 expression increases as thymocyte maturation progresses within the CD4−CD8− compartment but decreases as cells mature to a CD4+CD8+ phenotype. SLP-76 expression is then up-regulated following selection and concomitant with maturation to a CD4+ or CD8+ phenotype. In the periphery, SLP-76 is expressed in T lymphocytes with no detectable expression in the B cell compartment. Exposure to the superantigen staphylococcal enterotoxin B augments SLP-76 expression in the reactive T cell subset. Furthermore, in vitro stimulation with TCR-specific Abs augments the existing levels of SLP-76. These data reveal that SLP-76 expression is coordinately regulated with surface expression of a pre-TCR or mature TCR complex during thymocyte development and that TCR ligation elicits signals that result in increased expression of SLP-76.

Functional redundancy of the nuclear factor kappa B inhibitors I kappa B alpha and I kappa B beta

The transcription factor NF-kappaB is sequestered in the cytoplasm by the inhibitor proteins of the IkappaB family. Each member of the IkappaB exhibits structural and biochemical similarities as well as differences. In an effort to address the functional redundancy of two closely related IkappaB molecules, IkappaBalpha and IkappaBbeta, we generated knock-in mice by replacing the IkappaBalpha gene with the IkappaBbeta gene. The knock-in mice do not express IkappaBalpha, but express a T7-tagged IkappaBbeta under the promoter and regulatory sequence of ikba. Unlike the IkappaBalpha-deficient mice, which display severe postnatal developmental defects and die by postnatal day 8, homozygous knock-in mice survive to adulthood, are fertile, and exhibit no apparent abnormalities. Furthermore, thymocytes and embryonic fibroblasts from the knock-in animals exhibit an inducible NF-kappaB response similar to that of wild-type animals. These results indicate that IkappaBalpha and IkappaBbeta share significant similarities in their biochemical activity, and that they acquired their different functions from divergent expression patterns during evolution.