Recombinant IkappaB kinases alpha and beta are direct kinases of Ikappa Balpha

Quantitative Determination of AZD3264, a Selective Ikb-Kinase IKK2 Inhibitor, in Dog Plasma by Solvent-Induced Phase Transition Extraction Coupled with HPLC-MS/MS and its Application to Pharmacokinetic Study in Dogs

BACKGROUND

AZD3264 is a small molecule inhibitor of selective IkB-kinase IKK2 currently in preclinical development for the potential treatment of asthma and chronic pulmonary obstructive disorder.

OBJECTIVE

A method for the quantitative analysis of AZD3264 was established and optimized by using HPLC tandem mass spectrometry in dog plasma.

METHOD

Plasma samples were pretreated using a solvent-induced phase transition extraction method with a methanol solution of omeprazole as the internal standard. Chromatographic separation was performed using a Thermo Hypersil GOLD-C18 (50 mm × 4.6 mm, 3 μm) column with the temperature maintained at 25°C. Mobile phase consisted of 0.1% formic acid in water and acetonitrile in a gradient mode at a flow rate of 0.6 mL/min. Mass spectrometric detection was carried out in selected reaction monitoring mode with positive electrospray ionization, and the mass transitions of AZD3264 and omeprazole were m/z 442.1 → 425.0 and m/z 346.0 → 198.0, respectively.

RESULTS

The intra-batch accuracy was within 95.11-105.06% and the precision was within 6.50-9.98%. The inter-batch accuracy was within 96.83-102.80% with a precision of 7.62-9.50%. The selectivity, sensitivity, linearity, dilution linearity, extraction recovery and matrix effect, stability, and carry-over met all requirements of the guidelines for bioanalytical method validation. AZD3264 showed linear pharmacokinetic characteristics following intravenous administration to dogs at 0.3-2.7 mg/kg.

CONCLUSIONS

The developed and validated method was successfully employed in pharmacokinetic studies in dogs following intravenous administration at the doses of 0.3, 0.9, and 2.7 mg/kg.

HIGHLIGHTS

This was the first investigation of the in vivo pharmacokinetic characteristics of AZD3264 in dogs by LC-MS/MS with SIPTE method for plasma sample preparation.

Gamabufotalin Inhibits Osteoclastgenesis and Counteracts Estrogen-Deficient Bone Loss in Mice by Suppressing RANKL-Induced NF-κB and ERK/MAPK Pathways

Osteolytic bone disease is a condition of imbalanced bone homeostasis, characterized mainly by excessive bone-resorptive activity, which could predispose these populations, such as the old and postmenopausal women, to developing high risk of skeletal fragility and fracture. The nature of bone homeostasis is the coordination between the osteoblasts (OBs) and osteoclasts (OCs). Abnormal activation of osteoclasts (OCs) could compromise the bone homeostasis, constantly followed by a clutch of osteolytic diseases, including postmenopausal osteoporosis, osteoarthritis, and rheumatoid arthritis. Thus, it is imperatively urgent to explore effective medical interventions for patients. The traditional Chinese medicine (TCM) gamabufotalin (CS-6) is a newly identified natural product from Chansu and has been utilized for oncologic therapies owing to its good clinical efficacy with less adverse events. Previous study suggested that CS-6 could be a novel anti-osteoporotic agent. Nevertheless, whether CS-6 suppresses RANK-(receptor activator of nuclear factor-κ B ligand)/TRAF6 (TNF receptor-associated factor 6)-mediated downstream signaling activation in OCs, as well as the effects of CS-6 on OC differentiation in vivo, remains elusive. Therefore, in this present study, we aimed to explore the biological effects of CS-6 on osteoclastogenesis and RANKL-induced activation of related signaling pathways, and further to examine the potential therapeutic application in estrogen-deficient bone loss in the mice model. The results of in vitro experiment showed that CS-6 can inhibit RANKL-induced OC formation and the ability of bone resorption in a dose-dependent manner at both the early and late stages of osteoclastogenesis. The gene expression of OC-related key genes such as tartrate-resistant acid phosphatase (TRAP), CTSK, DC-STAMP, MMP9, and β3 integrin was evidently reduced. In addition, CS-6 could mitigate the systemic estrogen-dependent bone loss and pro-inframammary cytokines in mice in vivo. The molecular mechanism analysis suggested that CS-6 can suppress RANKL/TRAF6-induced early activation of NF-κB and ERK/MAPK signaling pathways, which consequently suppressed the transcription activity of c-Fos and NFATc1. Taken together, this present study provided ample evidence that CS-6 has the promise to become a therapeutic candidate in treating osteolytic conditions mediated by elevated OC formation and bone resorption.

Arenobufagin, isolated from toad venom, inhibited epithelial-to-mesenchymal transition and suppressed migration and invasion of lung cancer cells via targeting IKKβ/NFκB signal cascade

ETHNOPHARMACOLOGICAL RELEVANCE

Lung cancer is the leading cause of cancer incidence and mortality worldwide. Arenobufagin (Arg), a representative natural bufadienolide compound, is one of the major bioactive components isolated from toad venom ("Chan Su"named in Chinese to treat multifarious clinical neoplasms in China). However, the underlying molecular mechanisms that Arg inhibited the metastasis of lung cancer cells remain poorly understood.

MATERIALS AND METHODS

The mobility capacities of lung cancer cells treated with Arg were evaluated using wound healing assay. The anti-migratory and anti-invasive effects of Arg on lung cancer cells were investigated by transwell invasion assay and matrigel invasion assay. iTRAQ-labeled LC-MS proteomics was used to analyze the potential proteins related to metastasis in lung cancer cells treated with Arg and differentially-expressed proteins related to EMT and NFκB signaling cascade were further confirmed by Western blotting assay. The changed subcellular localization of p65 in lung cancer A549 and H1299 cells treated with Arg was detected by immunofluorescence staining. Molecular docking and molecular dynamic (MD) simulation assay were performed to verify the binding between Arg and IKKα/IKKβ. siRNA knockdown was used to check whether Arg inhibited EMT of lung cancer cells via targeting NFκB signaling cascade, which was further verified by in vivo study of lung cancer cell xenograft mice model and pulmonary metastasis mice model accompanying with immunohistochemical and hematoxylin-eosin (HE) staining.

RESULTS

Arg suppressed the wound closure of lung cancer cells using wound healing assay. Moreover, Arg significantly inhibited the migration and invasion of lung cancer cells by transwell invasion assay and matrigel invasion assay. 24 unique differentially-expressed proteins related to metastasis in lung cancer cells treated with Arg were identified using iTRAQ-labeled LC-MS proteomics and 14 differentially-expressed proteins related to EMT were further confirmed by Western blotting assay. Arg significantly decreased the phosphorylation of IKKβ, IκBα and p65 in the cytoplasm of lung cancer cells by Western blotting assay, and remarkably reduced the release of p65 from the cytoplasm to the nucleus. Arg could be bound in the ATP binding pocket of IKKα and IKKβ by molecular docking assay, and MD simulation assay further demonstrated that Arg binding to the ATP-binding pocket of IKKβ was very stable in 300 ns MD simulation, compared with the binding of Arg and IKKα. IKKβ/NFκB signaling cascade was also involved in the inhibitory effect of Arg on EMT of lung cancer cells by siRNA knockdown assay. The study of lung cancer cell xenograft mice model and pulmonary metastasis mice model in vivo indicated that Arg inhibited EMT and suppressed migration and invasion of lung cancer cells via downregulating IKKβ/NFκB signaling cascade.

CONCLUSION

In the present study, we explored the molecular mechanism of Arg prohibiting the metastasis of lung cancer cells in vitro and in vivo, which displayed Arg could target IKKβ to inactive NFκB signaling cascade and further change the expression of proteins related to EMT. These results highlight the potential of toad venom as a potential chemotherapeutic agent and warrant its development as the clinical therapy for lung cancer.

NIK-IKK complex interaction controls NF-κB-dependent inflammatory activation of endothelium in response to LTβR ligation

NF-κB-inducing kinase (NIK; also known as MAP3K14) is a central regulator of non-canonical NF-κB signaling in response to stimulation of TNF receptor superfamily members, such as the lymphotoxin-β receptor (LTβR), and is implicated in pathological angiogenesis associated with chronic inflammation and cancer. Here, we identify a previously unrecognized role of the LTβR-NIK axis during inflammatory activation of human endothelial cells (ECs). Engagement of LTβR-triggered canonical and non-canonical NF-κB signaling promoted expression of inflammatory mediators and adhesion molecules, and increased immune cell adhesion to ECs. Sustained LTβR-induced inflammatory activation of ECs was NIK dependent, but independent of p100, indicating that the non-canonical arm of NF-κB is not involved. Instead, prolonged activation of canonical NF-κB signaling, through the interaction of NIK with IκB kinase α and β (also known as CHUK and IKBKB, respectively), was required for the inflammatory response. Endothelial inflammatory activation induced by synovial fluid from rheumatoid arthritis patients was significantly reduced by NIK knockdown, suggesting that NIK-mediated alternative activation of canonical NF-κB signaling is a key driver of pathological inflammatory activation of ECs. Targeting NIK could thus provide a novel approach for treating chronic inflammatory diseases.

Gamabufotalin, a bufadienolide compound from toad venom, suppresses COX-2 expression through targeting IKKβ/NF-κB signaling pathway in lung cancer cells

BACKGROUND

Gamabufotalin (CS-6), a major bufadienolide of Chansu, has been used for cancer therapy due to its desirable metabolic stability and less adverse effect. However, the underlying mechanism of CS-6 involved in anti-tumor activity remains poorly understood.

METHODS

The biological functions of gamabufotalin (CS-6) were investigated by migration, colony formation and apoptosis assays in NSCLC cells. The nuclear localization and interaction between transcriptional co-activator p300 and NF-κB p50/p65 and their binding to COX-2 promoter were analyzed after treatment with CS-6. Molecular docking study was used to simulate the interaction of CS-6 with IKKβ. The in vivo anti-tumor efficacy of CS-6 was also analyzed in xenografts nude mice. Western blot was used to detect the protein expression level.

RESULTS

Gamabufotalin (CS-6) strongly suppressed COX-2 expression by inhibiting the phosphorylation of IKKβ via targeting the ATP-binding site, thereby abrogating NF-κB binding and p300 recruitment to COX-2 promoter. In addition, CS-6 induced apoptosis by activating the cytochrome c and caspase-dependent apoptotic pathway. Moreover, CS-6 markedly down-regulated the protein levels of COX-2 and phosphorylated p65 NF-κB in tumor tissues of the xenograft mice, and inhibited tumor weight and size.

CONCLUSIONS

Our study provides pharmacological evidence that CS-6 exhibits potential use in the treatment of COX-2-mediated diseases such as lung cancer.

Platelet IκB kinase-β deficiency increases mouse arterial neointima formation via delayed glycoprotein Ibα shedding

OBJECTIVE

On the luminal surface of injured arteries, platelet activation and leukocyte-platelet interactions are critical for the initiation and progression of arterial restenosis. The transcription factor nuclear factor-κB is a critical molecule in platelet activation. Here, we investigated the role of the platelet nuclear factor-κB pathway in forming arterial neointima after arterial injury.

METHODS AND RESULTS

We performed carotid artery wire injuries in low-density lipoprotein receptor-deficient (LDLR(-/-)) mice with a platelet-specific deletion of IκB kinase-β (IKKβ) (IKKβ(fl/fl)/PF4(cre)/LDLR(-/-)) and in control mice (IKKβ(fl/fl)/LDLR(-/-)). The size of the arterial neointima was 61% larger in the IKKβ(fl/fl)/PF4(cre)/LDLR(-/-) mice compared with the littermate control IKKβ(fl/fl)/LDLR(-/-) mice. Compared with the control mice, the IKKβ(fl/fl)/PF4(cre)/LDLR(-/-) mice exhibited more leukocyte adhesion at the injured area. The extent of glycoprotein Ibα shedding after platelet activation was compromised in the IKKβ-deficient platelets. This effect was associated with a low level of the active form of A Disintegrin And Metalloproteinase 17, the key enzyme involved in mediating glycoprotein Ibα shedding in activated IKKβ-deficient platelets.

CONCLUSIONS

Platelet IKKβ deficiency increases the formation of injury-induced arterial neointima formation. Thus, nuclear factor-κB-related inhibitors should be carefully evaluated for use in patients after an arterial intervention.

Piceatannol inhibits phorbol ester-induced NF-kappa B activation and COX-2 expression in cultured human mammary epithelial cells

There are multiple lines of evidence supporting that inflammation is causally linked to carcinogenesis. Abnormal upregulation of cyclooxygenase-2 (COX-2), a rate-limiting enzyme in the prostaglandin biosynthesis, has been implicated in carcinogenesis. Trans-3,4,3',5'-tetrahydroxystilbene (piceatannol), a naturally occurring hydroxylated stilbene with potent anti-inflammatory and antioxidative activities, has been shown to inhibit the proliferation of several cancer cells by inducing apoptosis or blocking cell cycle progression. In this study, we examined the effect of piceatannol on activation of the nuclear transcription factor NF-kappa B, one of the major transcription factors that regulate proinflammatory COX-2 gene transcription, in human mammary epithelial (MCF-10A) cells treated with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). When pretreated to MCF-10A cells, piceatannol markedly inhibited TPA-induced NF-kappa B DNA binding to a greater extent than resveratrol and oxyresveratrol, stilbene analogs structurally related to piceatannol. Piceatannol also inhibited TPA-induced phosphorylation and degradation of Ikappa Balpha as well as nuclear translocation of the phosphorylated form of p65, the functionally active subunit of NF-kappa B. Likewise, TPA-induced expression of COX-2 was abrogated by piceatannol pretreatment. The thiol reducing agent dithiothreitol abolished the inhibitory effects of piceatannol on NF-kappa B DNA binding activity, suggesting that piceatannol may directly modify NF-kappa B or its regulator through reaction with the cysteine thiol(s).

4-Phenyl-7-azaindoles as potent and selective IKK2 inhibitors

The synthesis and SAR of a novel series of IKK2 inhibitors are described. Modification around the hinge binding region of the 7-azaindole led to a series of potent and selective inhibitors with good cellular activity.

Phosphorylation and stabilization of TAp63gamma by IkappaB kinase-beta

Post-translational modification of the p53 family members is key to their regulation. Here we report the phosphorylation of TAp63gamma, but not DeltaNp63gamma, by IkappaB kinase beta (IKKbeta). Activation of IKKbeta by gamma radiation or tumor necrosis factor-alpha led to increased TAp63gamma protein levels in cells. IKKbeta, but not its kinase-defective mutant IKKbeta-K44A, led to this observed stabilization of TAp63gamma. This stabilization of TAp63gamma in response to gamma radiation was significantly decreased in the absence of IKKbeta. Phosphorylation of TAp63gamma blocks ubiquitylation and possible degradation of this protein. We postulate that phosphorylation of TAp63gamma by IKKbeta stabilizes the TAp63gamma protein by blocking ubiquitylation-dependent degradation of this protein.

Bridging the gap between in silico and cell-based analysis of the nuclear factor-kappaB signaling pathway by in vitro studies of IKK2

Previously, we have shown by sensitivity analysis, that the oscillatory behavior of nuclear factor (NF-kappaB) is coupled to free IkappaB kinase-2 (IKK2) and IkappaBalpha(IkappaBalpha), and that the phosphorylation of IkappaBalpha by IKK influences the amplitude of NF-kappaB oscillations. We have performed further analyses of the behavior of NF-kappaB and its signal transduction network to understand the dynamics of this system. A time lapse study of NF-kappaB translocation in 10,000 cells showed discernible oscillations in levels of nuclear NF-kappaB amongst cells when stimulated with interleukin (IL-1alpha), which suggests a small degree of synchronization amongst the cell population. When the kinetics for the phosphorylation of IkappaBalpha by IKK were measured, we found that the values for the affinity and catalytic efficiency of IKK2 for IkappaBalpha were dependent on assay conditions. The application of these kinetic parameters in our computational model of the NF-kappaB pathway resulted in significant differences in the oscillatory patterns of NF-kappaB depending on the rate constant value used. Hence, interpretation of in silico models should be made in the context of this uncertainty.

Cell-based assay strategy for identification of motif-specific RANK signaling pathway inhibitors

Osteoclasts, the principal bone-resorbing cells, not only play a pivotal role in skeletal development and maintenance but are also implicated in the pathogenesis of various bone disorders such as postmenopausal osteoporosis, bone erosion in inflammatory conditions, and tumor-induced osteolysis. As a result, several antiresorptive drugs (agents capable of inhibiting osteoclast formation and/or function) have been developed and are widely used to prevent and treat these bone diseases. However, current antiresorptive agents either lack satisfactory efficacy or cause serious side effects in clinical management of these bone disorders. Almost a decade ago, the receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL) was identified as an essential factor required for osteoclast formation. RANKL exerts the effect by binding to its receptor RANK on osteoclast precursors. RANKL also has a decoy receptor, osteoprotegerin (OPG), which inhibits RANKL function by competing with RANK for RANKL. The unraveling of the critical role for the RANKL/RANK/OPG system in osteoclast biology provides an unprecedented opportunity to develop more effective antiresorptive drugs. Unfortunately, the agents currently under development, such as OPG, RANK-Fc, and anti-RANKL antibodies, all inherit a serious drawback--lack of specificity, due to the involvement of the RANKL/RANK/OPG system in other biological processes such as immune response and mammary gland development. Thus, future efforts may need to shift to explore RANK signaling pathways as more effective therapeutic targets. Here, we review our current understanding of RANK signaling in osteoclasts and then discuss the potential of RANK signaling pathways as therapeutic pathways. Moreover, we further describe a strategy for constructing novel cell-based systems for identifying compounds inhibiting signaling from two recently identified RANK motifs through high throughput screening. We hope that this review will not only provide readers with an update on progress in this area of research but, more importantly, will also serve as a starting point for further discussion and eventual development of new strategies for harnessing the ultimate potential of the RANKL/RANK/OPG system as antiresorptive therapeutic targets.

Synthesis and biological evaluation of 4-amino derivatives of benzimidazoquinoxaline, benzimidazoquinoline, and benzopyrazoloquinazoline as potent IKK inhibitors

We have recently identified BMS-345541 (1) as a highly selective and potent inhibitor of IKK-2 (IC50 = 0.30 microM), which however was considerably less potent against IKK-1 (IC50 = 4.0 microM). In order to further explore the SAR around the imidazoquinoxaline tricyclic structure of 1, we prepared a series of tetracyclic analogues (7, 13, and 18). The synthesis and biological activities of these potent IKK inhibitors are described.

IkappaB kinase subunits alpha and gamma are required for activation of NF-kappaB and induction of apoptosis by mammalian reovirus

Reoviruses induce apoptosis both in cultured cells and in vivo. Apoptosis plays a major role in the pathogenesis of reovirus encephalitis and myocarditis in infected mice. Reovirus-induced apoptosis is dependent on the activation of transcription factor NF-kappaB and downstream cellular genes. To better understand the mechanism of NF-kappaB activation by reovirus, NF-kappaB signaling intermediates under reovirus control were investigated at the level of Rel, IkappaB, and IkappaB kinase (IKK) proteins. We found that reovirus infection leads initially to nuclear translocation of p50 and RelA, followed by delayed mobilization of c-Rel and p52. This biphasic pattern of Rel protein activation is associated with the degradation of the NF-kappaB inhibitor IkappaBalpha but not the structurally related inhibitors IkappaBbeta or IkappaBepsilon. Using IKK subunit-specific small interfering RNAs and cells deficient in individual IKK subunits, we demonstrate that IKKalpha but not IKKbeta is required for reovirus-induced NF-kappaB activation and apoptosis. Despite the preferential usage of IKKalpha, both NF-kappaB activation and apoptosis were attenuated in cells lacking IKKgamma/Nemo, an essential regulatory subunit of IKKbeta. Moreover, deletion of the gene encoding NF-kappaB-inducing kinase, which is known to modulate IKKalpha function, had no inhibitory effect on either response in reovirus-infected cells. Collectively, these findings indicate a novel pathway of NF-kappaB/Rel activation involving IKKalpha and IKKgamma/Nemo, which together mediate the expression of downstream proapoptotic genes in reovirus-infected cells.

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

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

Akt regulates basal and induced processing of NF-kappaB2 (p100) to p52

NF-kappaB is a family of transcription factors important for innate and adaptive immunity. NF-kappaB is restricted to the cytoplasm by inhibitory proteins that are degraded when specifically phosphorylated, permitting NF-kappaB to enter the nucleus and activate target genes. Phosphorylation of the inhibitory proteins is mediated by an IkappaB kinase (IKK) complex, which can be composed of two subunits with enzymatic activity, IKKalpha and IKKbeta. The preferred substrate for IKKbeta is IkappaBalpha, degradation of which liberates p65 (RelA) to enter the nucleus where it induces genes important to innate immunity. IKKalpha activates a non-canonical NF-kappaB pathway in which p100 (NF-kappaB2) is processed to p52. Once produced, p52 can enter the nucleus and induce genes important to adaptive immunity. This study shows that Akt binds to and increases the activity of IKKalpha and thereby increases p52 production in cells. Constitutively active Akt augments non-canonical NF-kappaB activity, whereas kinase dead Akt or inhibition of phosphatidylinositol 3-kinase have the opposite effect. Basal and ligand-induced p52 production is reduced in mouse embryo fibroblasts deficient in Akt1 and Akt2 compared with parental cells. These observations show that Akt plays a role in activation of basal and induced non-canonical NF-kappaB activity.

A selective novel low-molecular-weight inhibitor of IkappaB kinase-beta (IKK-beta) prevents pulmonary inflammation and shows broad anti-inflammatory activity

1 Pulmonary inflammatory diseases such as asthma are characterized by chronic, cell-mediated inflammation of the bronchial mucosa. 2 Recruitment and activation of inflammatory cells is orchestrated by a variety of mediators such as cytokines, chemokines, or adhesion molecules, the expression of which is regulated via the transcription factor nuclear factor kappa B (NF-kappaB). 3 NF-kappaB signaling is controlled by the inhibitor of kappa B kinase complex (IKK), a critical catalytic subunit of which is IKK-beta. 4 We identified COMPOUND A as a small-molecule, ATP-competitive inhibitor selectively targeting IKK-beta kinase activity with a K(i) value of 2 nM. 5 COMPOUND A inhibited stress-induced NF-kappaB transactivation, chemokine-, cytokine-, and adhesion molecule expression, and T- and B-cell proliferation. 6 COMPOUND A is orally bioavailable and inhibited the release of LPS-induced TNF-alpha in rodents. 7 In mice COMPOUND A inhibited cockroach allergen-induced airway inflammation and hyperreactivity and efficiently abrogated leukocyte trafficking induced by carrageenan in mice or by ovalbumin in a rat model of airway inflammation. 8 COMPOUND A was well tolerated by rodents over 3 weeks without affecting weight gain. 9 Furthermore, in mice COMPOUND A suppressed edema formation in response to arachidonic acid, phorbol ester, or edema induced by delayed-type hypersensitivity. 10 These data suggest that IKK-beta inhibitors offer an effective therapeutic approach for inhibiting chronic pulmonary inflammation.

Features of Selective Kinase Inhibitors

Small-molecule inhibitors of protein and lipid kinases have emerged as indispensable tools for studying signal transduction. Despite the widespread use of these reagents, there is little consensus about the biochemical criteria that define their potency and selectivity in cells. We discuss some of the features that determine the cellular activity of kinase inhibitors and propose a framework for interpreting inhibitor selectivity.

Evidence for a phosphorylation-independent role for Ser 32 and 36 in proteasome inhibitor-resistant (PIR) IkappaBalpha degradation in B cells

Constitutive NF-kappaB activity has emerged as an important cell survival regulator. Canonical inducible NF-kappaB activation involves IkappaB kinase (IKK)-dependent dual phosphorylation of Ser 32 and 36 of IkappaBalpha to cause its beta-TrCP-dependent ubiquitylation and proteasomal degradation. We recently reported that constitutive NF-kappaB (p50/c-Rel) activity in WEHI231 B cells is maintained through proteasome inhibitor-resistant (PIR) IkappaBalpha degradation in a manner that requires Ser 32 and 36, without the requirement of a direct interaction with beta-TrCP. Here we specifically examined whether dual phosphorylation of Ser 32 and 36 was required for PIR degradation. Through mutagenesis studies, we found that dual replacement of Ser 32 and 36 with Glu permitted beta-TrCP and proteasome-dependent, but not PIR, degradation. Moreover, single replacement of either Ser residue with Leu permitted PIR degradation in WEHI231 B cells. These results indicate that PIR degradation occurs in the absence of dual phosphorylation, thereby explaining the beta-TrCP-independent nature of the PIR pathway. Additionally, we found evidence that PIR IkappaBalpha degradation controls constitutive NF-kappaB activation in certain multiple myeloma cells. These results suggest that B lineage cells can differentiate between PIR and canonical IkappaBalpha degradation through the absence or presence of dually phosphorylated IkappaBalpha.

Molecular mechanism of hTid-1, the human homolog of Drosophila tumor suppressor l(2)Tid, in the regulation of NF-kappaB activity and suppression of tumor growth

hTid-1, a human homolog of the Drosophila tumor suppressor l(2)Tid and a novel DnaJ protein, regulates the activity of nuclear factor kappaB (NF-kappaB), but its mechanism is not established. We report here that hTid-1 strongly associated with the cytoplasmic protein complex of NF-kappaB-IkappaB through direct interaction with IkappaBalpha/beta and the IKKalpha/beta subunits of the IkappaB kinase complex. These interactions resulted in suppression of the IKK activity in a J-domain-dependent fashion and led to the cytoplasmic retention and enhanced stability of IkappaB. Overexpression of hTid-1 by using recombinant baculovirus or adenovirus led to inhibition of cell proliferation and induction of apoptosis of human osteosarcoma cells regardless of the p53 expression status. Adherent cultured cells transduced with Ad.hTid-1 detached from the dish surface. Morphological changes consistent with apoptosis and cell death were evident 48 h after Ad.EGFP-hTid-1 transduction. In contrast, cells transduced with Ad.EGFP or Ad.EGFP-hTd-1DeltaN100, a mutant that has the N-terminal J domain deletion and that lost suppressive activity on IKK, continued to proliferate. Similar data were obtained with A375 human melanoma cells. Ad.EGFP or Ad.EGFP-hTd-1DeltaN100 ex vivo-transduced A375 cells injected subcutaneously into nude mice produced growing tumors, whereas Ad.EGFP-hTid-1-transduced cells did not. Collectively, the data suggest that hTid-1 represses the activity of NF-kappaB through physical and functional interactions with the IKK complex and IkappaB and, in doing so, it modulates cell growth and death.

Development and comparison of two nonradioactive kinase assays for I kappa B kinase

In response to diverse stimuli, the transcription factor NF-kappaB is activated by the IKK kinase complex containing two kinases (IKKalpha and IKKbeta) that phosphorylate IkappaB, an inhibitory protein of NF-kappaB. The phosphorylation of IkappaB results in ubiquitination and degradation of IkappaB, allowing NF-kappaB to translocate to the nucleus where it regulates its target genes. To elucidate the role of IKK in the NF-kappaB signaling pathway, we have developed and characterized two quantitative, sensitive, and nonradioactive assays for evaluating IKKbeta activity: a dissociation-enhanced lanthanide fluorescence immunoassay called DELFIA and a homogeneous time-resolved fluorescence resonance energy transfer assay called LANCE. We show that the two assays have similar sensitivity and Michaelis constants (Km) for adenosine 5'-triphosphate and substrate; however, the LANCE format was far more efficient and easier to perform. Additionally, the assays were validated with the known kinase inhibitor K252a and several other kinase inhibitors, which showed that the IC(50) values of the two assays were comparable. In summary, both assays are quantitative, sensitive, reproducible, and amenable to high-throughput screening with improved waste management over radioactive assays.

Phosphorylation of serine 337 of NF-kappaB p50 is critical for DNA binding

It has been demonstrated that phosphorylation of the p50 subunit of NF-kappaB is required for efficient DNA binding, yet the specific phospho-residues of p50 have not been determined. In this study, we substituted all of the serine and conserved threonine residues in the p50 Rel homology domain and identified three serine residues, Ser65, Ser337, and Ser342, as critical for DNA binding without affecting dimerization. Although substitution with negatively charged aspartic acid at each of these positions failed to restore DNA binding, substitution with threonine, a potential phospho-acceptor, retained DNA binding for residues 65 and 337. In particular, Ser337, in a consensus site for protein kinase A (PKA) and other kinases, was shown to be phosphorylated both in vitro and in vivo. Importantly, phosphorylation of Ser337 by PKA in vitro dramatically increased DNA binding of p50. This study shows for the first time that the DNA binding ability of NF-kappaB p50 subunit is regulated through phosphorylation of residue Ser337, which has implications for both positive and negative control of NF-kappaB transcription.

Cell type-specific expression of the IkappaB kinases determines the significance of phosphatidylinositol 3-kinase/Akt signaling to NF-kappa B activation

Phosphatidylinositol (PI) 3-kinase/Akt signaling activates NF-kappa B through pleiotropic, cell type-specific mechanisms. This study investigated the significance of PI 3-kinase/Akt signaling to tumor necrosis factor (TNF)-induced NF-kappa B activation in transformed, immortalized, and primary cells. Pharmacological inhibition of PI 3-kinase blocked TNF-induced NF-kappa B DNA binding in the 293 line of embryonic kidney cells, partially affected binding in MCF-7 breast cancer cells, HeLa and ME-180 cervical carcinoma cells, and NIH 3T3 cells but was without significant effect in H1299 and human umbilical vein endothelial cells, cell types in which TNF activated Akt. NF-kappa B is retained in the cytoplasm by inhibitory proteins, I kappa Bs, which are phosphorylated and targeted for degradation by I kappa B kinases (IKK alpha and IKK beta). Expression and the ratios of IKK alpha and IKK beta, which homo- and heterodimerize, varied among cell types. Cells with a high proportion of IKK alpha (the IKK kinase activated by Akt) to IKK beta were most sensitive to PI 3-kinase inhibitors. Consequently, transient expression of IKK beta diminished the capacity of the inhibitors to block NF-kappa B DNA binding in 293 cells. Also, inhibitors of PI 3-kinase blocked NF-kappa B DNA binding in Ikk beta-/- but not Ikk alpha-/- or wild-type cells in which the ratio of IKK alpha to IKK beta is low. Thus, noncoordinate expression of I kappa B kinases plays a role in determining the cell type-specific role of Akt in NF-kappa B activation.

Tumor necrosis factor-alpha inhibits endothelial nitric-oxide synthase gene promoter activity in bovine aortic endothelial cells

Tumor necrosis factor-alpha (TNF-alpha) has been shown to reduce endothelial nitric-oxide synthase (eNOS) gene expression through post-transcriptional regulation of mRNA stability. The current study documented an independent effect of the cytokine on the eNOS gene promoter. TNF-alpha effected a time- and dose-dependent reduction in activity of a transiently transfected human -1197 eNOS-luciferase reporter. This reduction was inhibited by co-transfection of dominant negative IKKbeta as well as a nonphosphorylatable constitutively suppressive mutant of IkappaB implying involvement of the NFkappaB cascade in the inhibitory effect. The locus of the TNF-alpha-dependent inhibition was traced to two Sp1-binding sites positioned between -109 and -95 and -81 and -67 relative to the transcription start site. Electrophoretic mobility shift analysis and immunoperturbation studies showed evidence for Sp1 and Sp3 binding to each element. TNF-alpha treatment had no effect on the binding pattern to the downstream (-81 to -67) site but did suppress association of Sp1 and Sp3 to the upstream (-109 to -95) site. Collectively, these data indicate that TNF-alpha exerts transcriptional, as well as post-transcriptional, effects on eNOS gene expression and suggest a potential mechanism to account for the endothelial dysfunction that accompanies disorders such as diabetes mellitus and heart failure.

Induction of IkappaB-kinase by cholecystokinin is mediated by trypsinogen activation in rat pancreatic lobules

BACKGROUND AND AIMS

Supramaximal concentrations of cholecystokinin (CCK) or cerulein induce the intracellular activation of trypsinogen and the transcription factor NF-kappaB, a key regulator of inflammatory gene expression. Both events occur early in the development of an acute pancreatitis. The aim of this study was to examine the relationship between intracellular trypsinogen and NF-kappaB activation.

METHODS

We detected NF-kappaB-binding activity in electromobility shift assays, IkappaB proteolysis in Western analysis and endogenous IkappaB-kinase (IKKalpha and beta) activation using immune complex kinase assays following treatment with CCK in rat pancreatic lobules. To block intrapancreatic trypsinogen activation, a potent and cell-permeable serine-protease inhibitor, Pefabloc, was used.

RESULTS

CCK-induced IkappaBalpha degradation and subsequent NF-kappaB activation correlated closely with the catalytic activity of IKKs to phosphorylate IkappaBalpha in vitro. Activation is dose-dependent and peaked at 30 min. Doses of Pefabloc sufficient to inhibit trypsin activation reduced CCK-induced activation of NF-kappaB whereas TNF-alpha-induced NF-kappaB activation was not blocked but slightly increased. Moreover, treatment with Pefabloc as well as another serine protease inhibitor, FUT175, inhibited CCK-induced IKK activation.

CONCLUSION

These results suggest that intrapancreatic activation of trypsinogen may contribute to NF-kappaB signaling via IKK activation in cerulein pancreatitis. This also explains the fact that only doses of CCK which activate trypsinogen induce NF-kappaB activation in pancreatic acinar cells. Thus, trypsinogen activation is likely to modulate signaling events in acinar cells in the initial phase of acute pancreatitis.

BMS-345541 is a highly selective inhibitor of I kappa B kinase that binds at an allosteric site of the enzyme and blocks NF-kappa B-dependent transcription in mice

The signal-inducible phosphorylation of serines 32 and 36 of I kappa B alpha is critical in regulating the subsequent ubiquitination and proteolysis of I kappa B alpha, which then releases NF-kappa B to promote gene transcription. The multisubunit I kappa B kinase responsible for this phosphorylation contains two catalytic subunits, termed I kappa B kinase (IKK)-1 and IKK-2. BMS-345541 (4(2'-aminoethyl)amino-1,8-dimethylimidazo(1,2-a)quinoxaline) was identified as a selective inhibitor of the catalytic subunits of IKK (IKK-2 IC(50) = 0.3 microm, IKK-1 IC(50) = 4 microm). The compound failed to inhibit a panel of 15 other kinases and selectively inhibited the stimulated phosphorylation of I kappa B alpha in cells (IC(50) = 4 microm) while failing to affect c-Jun and STAT3 phosphorylation, as well as mitogen-activated protein kinase-activated protein kinase 2 activation in cells. Consistent with the role of IKK/NF-kappa B in the regulation of cytokine transcription, BMS-345541 inhibited lipopolysaccharide-stimulated tumor necrosis factor alpha, interleukin-1 beta, interleukin-8, and interleukin-6 in THP-1 cells with IC(50) values in the 1- to 5-microm range. Although a Dixon plot of the inhibition of IKK-2 by BMS-345541 showed a non-linear relationship indicating non-Michaelis-Menten kinetic binding, the use of multiple inhibition analyses indicated that BMS-345541 binds in a mutually exclusive manner with respect to a peptide inhibitor corresponding to amino acids 26-42 of I kappa B alpha with Ser-32 and Ser-36 changed to aspartates and in a non-mutually exclusive manner with respect to ADP. The opposite results were obtained when studying the binding to IKK-1. A binding model is proposed in which BMS-345541 binds to similar allosteric sites on IKK-1 and IKK-2, which then affects the active sites of the subunits differently. BMS-345541 was also shown to have excellent pharmacokinetics in mice, and peroral administration showed the compound to dose-dependently inhibit the production of serum tumor necrosis factor alpha following intraperitoneal challenge with lipopolysaccharide. Thus, the compound is effective against NF-kappa B activation in mice and represents an important tool for investigating the role of IKK in disease models.

IKK-i and TBK-1 are enzymatically distinct from the homologous enzyme IKK-2: comparative analysis of recombinant human IKK-i, TBK-1, and IKK-2

NF-kappaB is sequestered in the cytoplasm by the inhibitory IkappaB proteins. Stimulation of cells by agonists leads to the rapid phosphorylation of IkappaBs leading to their degradation that results in NF-kappaB activation. IKK-1 and IKK-2 are two direct IkappaB kinases. Two recently identified novel IKKs are IKK-i and TBK-1. We have cloned, expressed, and purified to homogeneity recombinant human (rh)IKK-i and rhTBK-1 and compared their enzymatic properties with those of rhIKK-2. We show that rhIKK-i and rhTBK-1 are enzymatically similar to each other. We demonstrate by phosphopeptide mapping and site-specific mutagenesis that rhIKK-i and rhTBK-1 are phosphorylated on serine 172 in the mitogen-activated protein kinase kinase activation loop and that this phosphorylation is necessary for kinase activity. Also, rhIKK-i and rhTBK-1 have differential peptide substrate specificities compared with rhIKK-2, the mitogen-activated protein kinase kinase activation loop of IKK-2 being a more favorable substrate than the IkappaBalpha peptide. Finally, using analogs of ATP, we demonstrate unique differences in the ATP-binding sites of rhIKK-i, rhTBK-1, and rhIKK-2. Thus, although these IKKs are structurally similar, their enzymatic properties may provide insights into their unique functions.

I-kappa B kinases alpha and beta have distinct roles in regulating murine T cell function

NF-kappaB is a transcription factor that regulates a variety of genes involved in the control of the immune and inflammatory responses. Activation of NF-kappaB is mediated by an inducible I-kappaB kinase (IKK) complex comprised of two catalytic subunits, IKKalpha and IKKbeta. In this study, the role of these kinases in the development and function of T lymphocytes was explored using transgenic mice expressing the dominant-negative forms of one or both kinases under the control of a T cell-specific promoter. Activation of the NF-kappaB pathway in thymocytes isolated from these transgenic mice following treatment with either PMA and ionomycin or anti-CD3 was markedly inhibited. Although inhibition of IKKalpha and/or IKKbeta function did not alter T cell development in these transgenic mice, the proliferative response to anti-CD3 was reduced in thymocytes isolated from mice expressing dominant-negative IKKbeta. However, inhibition of both IKKalpha and IKKbeta was required to markedly reduce cytokine production in thymocytes isolated from these transgenic mice. Finally, we demonstrated that IKKalpha and IKKbeta have opposite roles on the regulation of anti-CD3-induced apoptosis of double-positive thymocytes. These results suggest that IKKalpha and IKKbeta have distinct roles in regulating thymocyte function.

Kinetic mechanisms of IkappaB-related kinases (IKK) inducible IKK and TBK-1 differ from IKK-1/IKK-2 heterodimer

Nuclear factor-kappaB activation depends on phosphorylation and degradation of its inhibitor protein, IkappaB. The phosphorylation of IkappaBalpha on Ser(32) and Ser(36) is initiated by an IkappaB kinase (IKK) complex that includes a catalytic heterodimer composed of IkappaB kinase 1 (IKK-1) and IkappaB kinase 2 (IKK-2) as well as a regulatory adaptor subunit, NF-kappaB essential modulator. Recently, two related IkappaB kinases, TBK-1 and IKK-i, have been described. TBK-1 and IKK-i show sequence and structural homology to IKK-1 and IKK-2. TBK-1 and IKK-i phosphorylate Ser(36) of IkappaBalpha. We describe the kinetic mechanisms in terms of substrate and product inhibition of the recombinant human (rh) proteins, rhTBK-1, rhIKK-I, and rhIKK-1/rhIKK-2 heterodimers. The results indicate that although each of these enzymes exhibits a random sequential kinetic mechanism, the effect of the binding of one substrate on the affinity of the other substrate is significantly different. ATP has no effect on the binding of an IkappaBalpha peptide for the rhIKK-1/rhIKK-2 heterodimer (alpha = 0.99), whereas the binding of ATP decreased the affinity of the IkappaBalpha peptide for both rhTBK-1 (alpha = 10.16) and rhIKK-i (alpha = 62.28). Furthermore, the dissociation constants of ATP for rhTBK-1 and rhIKK-i are between the expected values for kinases, whereas the dissociation constants of the IkappaBalpha peptide for each IKK isoforms is unique with rhTBK-1 being the highest (K(IkappaBalpha) = 69.87 microm), followed by rhIKK-i (K(IkappaBalpha) = 5.47 microm) and rhIKK-1/rhIKK-2 heterodimers (K(IkappaBalpha) = 0.12 microm). Thus this family of IkappaB kinases has very unique kinetic properties.

Nuclear factor-kappaB activation in alveolar macrophages requires IkappaB kinase-beta, but not nuclear factor-kappaB inducing kinase

Cytokine mediated activation of alveolar macrophages (AMs) is an important event in the pathogenesis of fibrosing alveolitis (FA). Through membrane-associated antigens, cytokines (e.g., tumor necrosis-factor-alpha and interleukin-1) are believed to activate a common kinase cascade that initiates the cytoplasmic degradation of IkappaB and nuclear translocation of "nuclear factor-kappaB" (NF-kappaB). In the nucleus, NF-kappaB promotes the transcription of genes encoding chemokines and cytokines involved in chronic inflammation. Preventing cytokine-mediated NF-kappaB activation is a potential strategy for attenuating the lung injury that occurs in FA. Previously, we have demonstrated that, unlike AMs from healthy volunteers, AMs from patients with inflammatory lung diseases express the coxsackie/adenovirus receptor and the alphav integrins required for adenovirus (Adv) infection. This property allows Adv-mediated transgene delivery to diseased, but not normal, AMs and analysis of molecular pathways involved in gene transcription. In this study, AMs were infected with Adv constructs expressing a defective beta subunit of IkappaB kinase (AdvIKKbetakd) and a defective NF-kappaB inducing kinase (AdvNIKkd) to investigate the contribution of these molecules to NF-kappaB activation. We observed that IKKbeta, but not NIK, was required for NF-kappaB activation. The results of this study identify IKKbeta, but not NIK, as a potential therapeutic target in diseases that involve NF-kappaB-dependent gene transcription.

1alpha,25-dihydroxyvitamin D3 stimulates phosphorylation of IkappaBalpha and synergizes with TPA to induce nuclear translocation of NFkappaB during monocytic differentiation of NB4 leukemia cells

Treatment of NB4 acute promyelocytic leukemia cells with 1,25-dihydroxyvitamin D3 (1,25D3) or analogs 20-epi-22-oxa-24a,26a,27a-trihomo-1alpha,25-dihydroxyvitamin D3, 1,24-dihydroxy-22-ene-24-cyclopropylvitamin D3, 1alpha,25-dihydroxylumisterol3, or 1alpha,25(OH)2-d5-previtamin D3 in combination with TPA induces monocytic differentiation. The role of 1,25D3 in the induction of maturation has been shown to be a priming effect. Differentiation in response to these agents requires VDR-independent signaling of 1,25D3, PKC signaling, intracellular calcium, and calpain activity. In this study we identify the NFkappaB/IkappaB signaling pathway as a target of 1,25D3 and TPA action. One of the priming effects of 1,25D3 appears to be the rapid phosphorylation of serine residues on IkappaBalpha. On their own, 1,25D3, its analogs, and TPA do not alter IkappaBalpha expression; however, combinations of analogs with TPA result in a synergistic decrease in IkappaBalpha expression. Decreased expression of IkappaBalpha likely results from enhanced degradation, which allows the observed subsequent nuclear translocation of NFkappaB subunit p65. Since nuclear-localized NFkappaB was observed only in combination-treated cells, it is proposed that nuclear targets of NFkappaB are required for monocytic differentiation. Intracellular calcium and proteolytic activity are both necessary for the induction of IkappaB regulation and translocation of NFkappaB and are critical components of the nongenomic signaling cascades of the 1,25D3-induced differentiation pathway.

Regulation of beta-catenin function by the IkappaB kinases

Both the beta-catenin and the nuclear factor kappaB (NF-kappaB) proteins are important regulators of gene expression and cellular proliferation. Two kinases, IKKalpha and IKKbeta, are critical activators of the NF-kappaB pathway. Here we present evidence that these kinases are also important in the regulation of beta-catenin function. IKKalpha- and IKKbeta-deficient mouse embryo fibroblasts exhibited different patterns of beta-catenin cellular localization. IKKbeta decreases beta-catenin-dependent transcriptional activation, while IKKalpha increases beta-catenin-dependent transcriptional activity. IKKalpha and IKKbeta interact with and phosphorylate beta-catenin using both in vitro and in vivo assays. Our results suggest that differential interactions of beta-catenin with IKKalpha and IKKbeta may in part be responsible for regulating beta-catenin protein levels and cellular localization and integrating signaling events between the NF-kappaB and Wingless pathways.

Plasmin-induced expression of cytokines and tissue factor in human monocytes involves AP-1 and IKKbeta-mediated NF-kappaB activation

It was previously shown that plasmin activates human peripheral monocytes in terms of lipid mediator release and chemotactic migration. Here it is demonstrated that plasmin induces proinflammatory cytokine release and tissue factor (TF) expression by monocytes. Plasmin 0.043 to 1.43 CTA U/mL, but not active site-blocked plasmin, triggered concentration-dependent expression of mRNA for interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), and TF with maximum responses after 4 hours. Plasmin-mediated mRNA expression was inhibited in a concentration-dependent manner by the lysine analogue trans-4-(aminomethyl)cyclohexane-1-carboxylic acid (t-AMCA). Increases in mRNA levels were followed by concentration- and time-dependent release of IL-1alpha, IL-1beta and TNF-alpha and by TF expression on monocyte surfaces. Neither cytokines nor TF could be detected when monocytes were preincubated with actinomycin D or cycloheximide. Electrophoretic mobility shift assays indicated plasmin-induced activation of NF-kappaB; DNA-binding complexes were composed of p50, p65, and c-Rel, as shown by supershift experiments. Nuclear translocation of NF-kappaB/Rel proteins coincided with IkappaBalpha degradation. At variance with endotoxic lipopolysaccharide, plasmin elicited the rapid degradation of another cytoplasmic NF-kappaB inhibitor, p105. Proteolysis of NF-kappaB inhibitors was apparently due to transient activation of IkappaB kinase (IKK) beta that reached maximum activity at 1 hour after plasmin stimulation. In addition, AP-1 binding was increased in plasmin-treated monocytes, with most complexes composed of JunD, c-Fos, and FosB. These findings further substantiate the role of plasmin as a proinflammatory activator of human monocytes and reveal an important new link between the plasminogen-plasmin system and inflammation. (Blood. 2001;97:3941-3950)

Shared pathways of IkappaB kinase-induced SCF(betaTrCP)-mediated ubiquitination and degradation for the NF-kappaB precursor p105 and IkappaBalpha

p105 (NFKB1) acts in a dual way as a cytoplasmic IkappaB molecule and as the source of the NF-kappaB p50 subunit upon processing. p105 can form various heterodimers with other NF-kappaB subunits, including its own processing product, p50, and these complexes are signal responsive. Signaling through the IkappaB kinase (IKK) complex invokes p105 degradation and p50 homodimer formation, involving p105 phosphorylation at a C-terminal destruction box. We show here that IKKbeta phosphorylation of p105 is direct and does not require kinases downstream of IKK. p105 contains an IKK docking site located in a death domain, which is separate from the substrate site. The substrate residues were identified as serines 923 and 927, the latter of which was previously assumed to be a threonine. S927 is part of a conserved DSGPsi motif and is functionally most critical. The region containing both serines is homologous to the N-terminal destruction box of IkappaBalpha, -beta, and -epsilon. Upon phosphorylation by IKK, p105 attracts the SCF E3 ubiquitin ligase substrate recognition molecules betaTrCP1 and betaTrCP2, resulting in polyubiquitination and complete degradation by the proteasome. However, processing of p105 is independent of IKK signaling. In line with this and as a physiologically relevant model, lipopolysaccharide (LPS) induced degradation of endogenous p105 and p50 homodimer formation, but not processing in pre-B cells. In mutant pre-B cells lacking IKKgamma, processing was unaffected, but LPS-induced p105 degradation was abolished. Thus, a functional endogenous IKK complex is required for signal-induced p105 degradation but not for processing.

Domain-specific interaction with the I kappa B kinase (IKK)regulatory subunit IKK gamma is an essential step in tax-mediated activation of IKK

The human T-cell leukemia virus type 1 Tax oncoprotein deregulates the NF-kappa B signaling pathway by persistently stimulating a key signal transducer, the I kappa B kinase (IKK). Tax physically associates with the IKK regulatory subunit, IKK gamma, although the underlying biochemical mechanism and functional significance remain unclear. We show that the Tax-IKK gamma interaction requires two homologous leucine zipper domains located within IKK gamma. These leucine zipper domains are unique for the presence of a conserved upstream region that is essential for Tax binding. Site-directed mutagenesis analysis revealed that a leucine-repeat region of Tax is important for IKK gamma binding. Interestingly, all the Tax mutants defective in IKK gamma binding failed to engage the IKK complex or stimulate IKK activity, and these functional defects can be rescued by fusing the Tax mutants to IKK gamma. These results provide mechanistic insights into how Tax specifically targets and functionally activates the cellular kinase IKK.

Suppression of NF-kappaB activity by sulfasalazine is mediated by direct inhibition of IkappaB kinases alpha and beta

BACKGROUND & AIMS

Activation of NF-kappaB/Rel has been implicated in the pathogenesis of inflammatory bowel disease (IBD). Various drugs used in the treatment of IBD, such as glucocorticoids, 5-aminosalicylic acid, and sulfasalazine, interfere with NF-kappaB/Rel signaling. The aim of this study was to define the molecular mechanism by which sulfasalazine inhibits NF-kappaB activation.

METHODS

The effects of sulfasalazine and its moieties on NF-kappaB signaling were evaluated using electromobility shift, transfection, and immune complex kinase assays. The direct effect of sulfasalazine on IkappaB kinase (IKK) activity was investigated using purified recombinant IKK-alpha and -beta proteins.

RESULTS

NF-kappaB/Rel activity induced by tumor necrosis factor alpha, 12-O-tetradecanoylphorbol-13-acetate, or overexpression of NF-kappaB-inducing kinase, IKK-alpha, IKK-beta, or constitutively active IKK-alpha and IKK-beta mutants was inhibited dose dependently by sulfasalazine. Sulfasalazine inhibited tumor necrosis factor alpha-induced activation of endogenous IKK in Jurkat T cells and SW620 colon cells, as well as the catalytic activity of purified IKK-alpha and IKK-beta in vitro. In contrast, the moieties of sulfasalazine, 5-aminosalicylic acid, and sulfapyridine or 4-aminosalicylic acid had no effect. Activation of extracellular signal-related kinase (ERK) 1 and 2, c-Jun-N-terminal kinase (JNK) 1, and p38 was unaffected by sulfasalazine. The decrease in substrate phosphorylation by IKK-alpha and -beta is associated with a decrease in autophosphorylation of IKKs and can be antagonized by excess adenosine triphosphate.

CONCLUSIONS

These data identify sulfasalazine as a direct inhibitor of IKK-alpha and -beta by antagonizing adenosine triphosphate binding. The suppression of NF-kappaB activation by inhibition of the IKKs contributes to the well-known anti-inflammatory and immunosuppressive effects of sulfasalazine.

Activation of I-kappaB kinase by the HTLV type 1 Tax protein: mechanistic insights into the adaptor function of IKKgamma

The Tax protein encoded by human T cell leukemia virus type 1 (HTLV-1) induces constitutive nuclear expression of the transcription factor NF-kappaB, causing aberrant expression of a large array of cellular genes. Tax activates NF-kappaB by stimulating the activity of the I-kappaB kinase (IKK), which in turn leads to phosphorylation and degradation of the NF-kappaB inhibitor I-kappaBalpha. In normal T cells, IKK activation occurs transiently on cellular stimulation through the T cell receptor (TCR) and the CD28 costimulatory molecule. However, this inducible kinase is constitutively activated in Tax-expressing and HTLV-1-infected T cells, which contributes to the deregulated nuclear expression of NF-kappaB. As a genetic approach to dissect the pathways mediating IKK activation by Tax and T cell activation signals, somatic cell mutagenesis was performed to isolate signaling-defective mutant Jurkat T cell lines. One of the mutant cell lines was shown to have a defect in NF-kappaB activation by both T cell mitogens and Tax. Interestingly, this mutant cell line lacks expression of the IKK regulatory protein, IKKgamma. Expression of exogenous IKKgamma in the mutant cells restored NF-kappaB activation, thus confirming the essential role of this regulatory factor in IKK activation by the cellular and viral stimuli. Mechanistic studies have shown that Tax physically interacts with IKKgamma via specific domains, including two homologous leucine zipper motifs present in IKKgamma. The Tax/IKKgamma interaction serves to recruit Tax to the IKK catalytic subunits, IKKalpha and IKKbeta, and this recruitment appears to be an essential mechanism by which Tax stimulates the activity of IKK.

Activation of IKKalpha and IKKbeta through their fusion with HTLV-I tax protein

Human T-cell leukemia virus type I (HTLV-I) Tax protein persistently stimulates the activity of IkappaB kinase (IKK), resulting in constitutive activation of the transcription factor NF-kappaB. Tax activation of IKK requires physical interaction of this viral protein with the IKK regulatory subunit, IKKgamma. The Tax/IKKgamma interaction allows Tax to engage the IKK catalytic subunits, IKKalpha and IKKbeta, although it remains unclear whether this linker function of IKKgamma is sufficient for supporting the Tax-specific IKK activation. To address this question, we have examined the sequences of IKKgamma required for modulating the Tax/IKK signaling. We demonstrate that when fused to Tax, a small N-terminal fragment of IKKgamma, containing its minimal IKKalpha/beta-binding domain, is sufficient for bringing Tax to and activating the IKK catalytic subunits. Disruption of the IKKalpha/beta-binding activity of this domain abolishes its function in modulating the Tax/IKK signaling. We further demonstrate that direct fusion of Tax to IKKalpha and IKKbeta leads to activation of these kinases. These findings suggest that the IKKgamma-directed Tax/IKK association serves as a molecular trigger for IKK activation.

The I kappa B kinase (IKK) complex is tripartite and contains IKK gamma but not IKAP as a regular component

A critical step in the activation of NF-kappa B is the phosphorylation of I kappa Bs by the I kappa B kinase (IKK) complex. IKK alpha and IKK beta are the two catalytic subunits of the IKK complex and two additional molecules, IKK gamma/NEMO and IKAP, have been described as further integral members. We have analyzed the function of both proteins for IKK complex composition and NF-kappa B signaling. IKAP and IKK gamma belong to distinct cellular complexes. Quantitative association of IKK gamma was observed with IKK alpha and IKK beta. In contrast IKAP was complexed with several distinct polypeptides. Overexpression of either IKK gamma or IKAP blocked tumor necrosis factor alpha induction of an NF-kappa B-dependent reporter construct, but IKAP in addition affected several NF-kappa B-independent promoters. Whereas specific down-regulation of IKK gamma protein levels by antisense oligonucleotides significantly reduced cytokine-mediated activation of the IKK complex and subsequent NF-kappa B activation, a similar reduction of IKAP protein levels had no effect on NF-kappa B signaling. Using solely IKK alpha, IKK beta, and IKK gamma, we could reconstitute a complex whose apparent molecular weight is comparable to that of the endogenous IKK complex. We conclude that while IKK gamma is a stoichiometric component of the IKK complex, obligatory for NF-kappa B signaling, IKAP is not associated with IKKs and plays no specific role in cytokine-induced NF-kappa B activation.

Characterization of the recombinant IKK1/IKK2 heterodimer. Mechanisms regulating kinase activity

Nuclear factor kappa B (NF-kappaB) is a ubiquitous, inducible transcription factor that regulates the initiation and progression of immune and inflammatory stress responses. NF-kappaB activation depends on phosphorylation and degradation of its inhibitor protein, IkappaB, initiated by an IkappaB kinase (IKK) complex. This IKK complex includes a catalytic heterodimer composed of IkappaB kinase 1 (IKK1) and IkappaB kinase 2 (IKK2) as well as a regulatory adaptor subunit, NF-kappaB essential modulator. To better understand the role of IKKs in NF-kappaB activation, we have cloned, expressed, purified, and characterized the physiological isoform, the rhIKK1/rhIKK2 heterodimer. We compared its kinetic properties with those of the homodimers rhIKK1 and rhIKK2 and a constitutively active rhIKK2 (S177E, S181E) mutant. We demonstrate activation of these recombinantly expressed IKKs by phosphorylation during expression in a baculoviral system. The K(m) values for ATP and IkappaBalpha peptide for the rhIKK1/rhIKK2 heterodimer are 0.63 and 0.60 micrometer, respectively, which are comparable to those of the IKK2 homodimer. However, the purified rhIKK1/rhIKK2 heterodimer exhibits the highest catalytic efficiency (k(cat)/K(m)) of 47.50 h(-1) micrometer(-1) using an IkappaBalpha peptide substrate compared with any of the other IKK isoforms, including rhIKK2 (17.44 h(-1) micrometer(-1)), its mutant rhIKK2 (S177E, S181E, 1.18 h(-1) micrometer(-1)), or rhIKK1 (0.02 h(-1) micrometer(-1)). Kinetic analysis also indicates that, although both products of the kinase reaction, ADP and a phosphorylated IkappaBalpha peptide, exhibited competitive inhibitory kinetics, only ADP with the low K(i) of 0.77 micrometer may play a physiological role in regulation of the enzyme activity.

The NF-kappa B signaling pathway is not required for Fas ligand gene induction but mediates protection from activation-induced cell death

Stimulation of T cells by antigens or mitogens triggers multiple signaling pathways leading to activation of genes encoding interleukin-2 and other growth-regulatory cytokines. The same stimuli also activate the gene encoding an apoptosis-inducing molecule, Fas ligand (FasL), which contributes to activation-induced cell death. It has been proposed that the signaling pathways involved in cytokine gene induction also contribute to activation-induced FasL expression; however, genetic evidence for this proposal is lacking. In the present study, the role of the NF-kappaB signaling pathway in FasL gene expression was examined using a mutant T cell line deficient in an essential NF-kappaB signaling component, IkappaB kinase gamma. These mutant cells have a blockade in signal-induced activation of NF-kappaB but remained normal in the activation of NF-AT and AP-1 transcription factors. Interestingly, the NF-kappaB signaling defect has no effect on mitogen-stimulated FasL gene expression, although it completely blocks the interleukin-2 gene induction. We further demonstrate that NF-kappaB activation is required for protecting T cells from apoptosis induction by mitogens and an agonistic anti-Fas antibody. These genetic results suggest that the NF-kappaB signaling pathway is not required for activation-induced FasL expression but rather mediates cell growth and protection from activation-induced cell death.

Phosphorylation meets ubiquitination: the control of NF-[kappa]B activity

NF-kappaB (nuclear factor-kappaB) is a collective name for inducible dimeric transcription factors composed of members of the Rel family of DNA-binding proteins that recognize a common sequence motif. NF-kappaB is found in essentially all cell types and is involved in activation of an exceptionally large number of genes in response to infections, inflammation, and other stressful situations requiring rapid reprogramming of gene expression. NF-kappaB is normally sequestered in the cytoplasm of nonstimulated cells and consequently must be translocated into the nucleus to function. The subcellular location of NF-kappaB is controlled by a family of inhibitory proteins, IkappaBs, which bind NF-kappaB and mask its nuclear localization signal, thereby preventing nuclear uptake. Exposure of cells to a variety of extracellular stimuli leads to the rapid phosphorylation, ubiquitination, and ultimately proteolytic degradation of IkappaB, which frees NF-kappaB to translocate to the nucleus where it regulates gene transcription. NF-kappaB activation represents a paradigm for controlling the function of a regulatory protein via ubiquitination-dependent proteolysis, as an integral part of a phosphorylationbased signaling cascade. Recently, considerable progress has been made in understanding the details of the signaling pathways that regulate NF-kappaB activity, particularly those responding to the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-1. The multisubunit IkappaB kinase (IKK) responsible for inducible IkappaB phosphorylation is the point of convergence for most NF-kappaB-activating stimuli. IKK contains two catalytic subunits, IKKalpha and IKKbeta, both of which are able to correctly phosphorylate IkappaB. Gene knockout studies have shed light on the very different physiological functions of IKKalpha and IKKbeta. After phosphorylation, the IKK phosphoacceptor sites on IkappaB serve as an essential part of a specific recognition site for E3RS(IkappaB/beta-TrCP), an SCF-type E3 ubiquitin ligase, thereby explaining how IKK controls IkappaB ubiquitination and degradation. A variety of other signaling events, including phosphorylation of NF-kappaB, hyperphosphorylation of IKK, induction of IkappaB synthesis, and the processing of NF-kappaB precursors, provide additional mechanisms that modulate the level and duration of NF-kappaB activity.

Raf induces NF-kappaB by membrane shuttle kinase MEKK1, a signaling pathway critical for transformation

NF-kappaB is regulated by inhibitor proteins (IkappaBs), which retain NF-kappaB in the cytoplasm. Signal-induced phosphorylation by the IkappaB-kinase complex containing the IkappaB-kinases 1 and 2 (IKK-1/2 or IKK-alpha/beta) and subsequent degradation of the IkappaB proteins are prerequisites for NF-kappaB activation. Many signals induce NF-kappaB, one of them being oncogenic Raf kinase. We investigated whether NF-kappaB induction is critical for Raf-mediated transformation. Here, we demonstrate that inhibition of NF-kappaB interferes with transformation by the Raf-oncogene, and we characterized the mechanism of NF-kappaB induction by activated Raf kinase and the tumor promoter phorbol 12-myristate 13-acetate (PMA). NF-kappaB activation by PMA and Raf critically depends on the IkappaB-kinase complex, most notably on IKK-2. A major signaling pathway induced by Raf is the mitogenic cytoplasmic kinase cascade. However, different inhibitors of this cascade do not affect PMA- and Raf-mediated NF-kappaB activation. Raf does not phosphorylate the IkappaB-kinase proteins directly. Raf rather synergizes with another membrane shuttle kinase MEKK1, and Raf-mediated activation of NF-kappaB is blocked by a dominant negative form of MEKK1. These results suggest that Raf induction of NF-kappaB is relayed by MEKK1, but not by the classical mitogenic cytoplasmic kinase cascade.

Functional isoforms of IkappaB kinase alpha (IKKalpha) lacking leucine zipper and helix-loop-helix domains reveal that IKKalpha and IKKbeta have different activation requirements

The activity of the NF-kappaB family of transcription factors is regulated principally by phosphorylation and subsequent degradation of their inhibitory IkappaB subunits. Site-specific serine phosphorylation of IkappaBs by two IkappaB kinases (IKKalpha [also known as CHUK] and IKKbeta) targets them for proteolysis. IKKalpha and -beta have a unique structure, with an amino-terminal serine-threonine kinase catalytic domain and carboxy-proximal helix-loop-helix (HLH) and leucine zipper-like (LZip) amphipathic alpha-helical domains. Here, we describe the properties of two novel cellular isoforms of IKKalpha: IKKalpha-DeltaH and IKKalpha-DeltaLH. IKKalpha-DeltaH and IKKalpha-DeltaLH are differentially spliced isoforms of the IKKalpha mRNA lacking its HLH domain and both its LZip and HLH domains, respectively. IKKalpha is the major RNA species in most murine cells and tissues, except for activated T lymphocytes and the brain, where the alternatively spliced isoforms predominate. Remarkably, IKKalpha-DeltaH and IKKalpha-DeltaLH, like IKKalpha, respond to tumor necrosis factor alpha stimulation to potentiate NF-kappaB activation in HEK293 cells. A mutant, catalytically inactive form of IKKalpha blocked IKKalpha-, IKKalpha-DeltaH-, and IKKalpha-DeltaLH-mediated NF-kappaB activation. Akin to IKKalpha, its carboxy-terminally truncated isoforms associated with the upstream activator NIK (NF-kappaB-inducing kinase). In contrast to IKKalpha, IKKalpha-DeltaLH failed to associate with either itself, IKKalpha, IKKbeta, or NEMO-IKKgamma-IKKAP1, while IKKalpha-DeltaH complexed with IKKbeta and IKKalpha but not with NEMO. Interestingly, each IKKalpha isoform rescued HEK293 cells from the inhibitory effects of a dominant-negative NEMO mutant, while IKKalpha could not. IKKalpha-DeltaCm, a recombinant mutant of IKKalpha structurally akin to IKKalpha-DeltaLH, was equally functional in these assays, but in sharp contrast, IKKbeta-DeltaCm, a structurally analogous mutant of IKKbeta, was inactive. Our results demonstrate that the functional roles of seemingly analogous domains in IKKalpha and IKKbeta need not be equivalent and can also exhibit different contextual dependencies. The existence of cytokine-inducible IKKalpha-DeltaH and IKKalpha-DeltaLH isoforms illustrates potential modes of NF-kappaB activation, which are not subject to the same in vivo regulatory constraints as either IKKalpha or IKKbeta.