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* This work was supported by NCI, National Institutes of Health Grants CA42802, CA100707, and CA98628. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The novel oleanane triterpenoid 2-cyano-3,12-dioxooleana-1,9,-dien-28-oic acid (CDDO) and the C-28 methyl ester (CDDO-Me) induce apoptosis of human tumor cells by disruption of redox balance and are currently in clinical trials. The present studies show that CDDO and CDDO-Me block tumor necrosis factorα-induced targeting of NF-κB p65 to the nucleus. CDDO-Me also blocked tumor necrosis factor α-induced phosphorylation of IκBα. In concert with these results, we found that CDDO-Me inhibits IκBα kinaseβ (IKKβ) activity in cells. In support of a direct mechanism, CDDO-Me inhibited recombinant IKKβ activity in vitro. The results also demonstrate that (i) CDDO and CDDO-Me form adducts with IKKβ, but not IKKβ with mutation of Cys-179 to Ala, and (ii) CDDO-Me inhibits IKKβ by a mechanism dependent on oxidation of Cys-179. These findings indicate that CDDO and CDDO-Me directly block IKKβ activity and thereby the NF-κB pathway by interacting with Cys-179 in the IKKβ activation loop.
). How the CDDO triterpenoids disrupt redox balance is not known. However, the A-ring of these triterpenoids contains an α,β-unsaturated carbonyl moiety that can form reversible adducts with reactive thiol groups in dithiothreitol (DTT) (
). Phosphorylation of IκBα induces ubiquitination and degradation of IκBα and release of NF-κB p65 to the nucleus. In the classical NF-κB pathway, the IκB kinase β (IKKβ) in a complex with the regulatory IKKγ subunit is the major kinase responsible for phosphorylation of IκBα (
). The present results demonstrate that CDDO and CDDO-Me block the NF-κB pathway by inhibiting IKKβ. The results also indicate that CDDO and CDDO-Me directly inhibit IKKβ by interacting with Cys-179 in the IKKβ activation loop.
Cell Culture—Human U-937 myeloid leukemia cells were grown in RPMI 1640 medium containing 10% heat-inactivated fetal bovine serum, 100 units/ml penicillin, 100 μg/ml streptomycin, and 2 mml-glutamine. 293 cells were grown in Dulbecco's modified Eagle's medium containing 10% heat-inactivated fetal bovine serum, antibiotics, and l-glutamine. Cells were treated with CDDO or CDDO-Me (provided by Reata Pharmaceuticals), human TNF-α (20 ng/ml; BD Biosciences), the proteasome inhibitor MG-132 (25 μm; Calbiochem), or DTT (300 μm; Sigma).
Subcellular Fractionation—Nuclear and cytosolic fractions were prepared as described (
). Soluble proteins were incubated with anti-IKKβ (Cell Signaling Technology) or anti-FLAG (Sigma) and precipitated with protein A/G beads. Immune complexes or cell lysates were subjected to immunoblotting with anti-NF-κB p65 (Santa Cruz Biotechnology), anti-lamin B (Calbiochem), anti-IκBα (Santa Cruz Biotechnology), anti-α-tubulin (Santa Cruz Biotechnology), anti-phospho-IκBα (Cell Signaling Technology), anti-β-actin (Sigma), anti-IKKβ, anti-phospho-IKKβ (Cell Signaling Technology), anti-Bcl-2, anti-Bcl-xL (Santa Cruz Biotechnology), or anti-FLAG (Sigma). The immune complexes were detected with horseradish peroxidase-conjugated second antibodies and enhanced chemiluminescence (ECL; Amersham Biosciences).
Luciferase Assays—Cells were transfected with pNF-κB-Luc (Stratagene) and SV-40-Renilla-Luc (Promega) in the presence of Lipofectamine 2000 (Invitrogen). After 24 h, lysates prepared in passive lysis buffer were analyzed using the dual luciferase assay kit (Promega).
IKKβ Kinase Assays—Anti-IKKβ precipitates or recombinant His-IKKβ (Upstate Cell Signaling Solutions) were incubated in kinase buffer (50 mm HEPES, pH 7.4, 10 mm MgCl2, 10 mm MnCl2, 0.1 mm sodium vanadate, 10 μm ATP, and 1 mm DTT) with GST-IκBα and [γ-32P]ATP (PerkinElmer Life Sciences) for 30 min at 30 °C. DTT was omitted from the reactions where indicated. The reaction products were analyzed by SDS-PAGE and autoradiography.
Generation of IKKβ(C179A) Mutant—Mutation of IKKβ Cys-179 to Ala was generated by site-directed mutagenesis (Stratagene) using pGEX-IKKβ as the template and confirmed by DNA sequencing. IKKβ and IKKβ(C179A) were purified after cleavage with thrombin to remove the GST moiety.
Binding of CDDO-Me-biotin and CDDO-biotin to IKKβ—CDDO and CDDO-Me were biotinylated as described (
). For in vitro binding studies, (i) anti-FLAG precipitates from 293 cells expressing FLAG-IKKβ or FLAG-IKKβ(C179A) were incubated with 5 μm CDDO-biotin, and (ii) recombinant IKKβ or IKKβ(C179A) was incubated with 1 μm CDDO-Me-biotin or 5 μm CDDO-biotin. For in vivo studies, 293 cells expressing FLAG-IKKβ or FLAG-IKKβ(C179A) were cultured with 5 μm CDDO-biotin. Lysates were then precipitated with anti-FLAG. Proteins were separated by SDS-PAGE and transferred to nitrocellulose membranes. After washing, the membranes were incubated with streptavidin horseradish peroxidase (Amersham Biosciences) and developed with enhanced chemiluminescence (ECL; Amersham Biosciences).
RESULTS AND DISCUSSION
CDDO-Me Inhibits NF-κB p65 Activation by Blocking IκBα Phosphorylation—To assess the effects of CDDO-Me on regulation of the NF-κB pathway, we stimulated human U-937 myeloid leukemia cells with TNF-α to induce translocation of NF-κB p65 to the nucleus (Fig. 1A). Treatment of the TNF-α-stimulated cells with CDDO-Me was associated with a concentration-dependent decrease in nuclear translocation of p65 (Fig. 1A). Equal loading and purity of the nuclear lysates was confirmed by immunoblotting with antibodies against nuclear lamin B, cytosolic IκBα, and cytosolicα-tubulin (Fig. 1A). In concert with these results, TNF-α-induced expression of Bcl-2 and Bcl-xL, which is activated by NF-κB (
), was attenuated by CDDO-Me treatment, a response delayed compared with that for nuclear translocation of p65 (Fig. 1B). Similar findings were obtained when the TNF-α-stimulated cells were treated with the parent compound, CDDO (data not shown), indicating that this effect is not selective for the methyl ester. NF-κB p65 is released from cytosolic IκBα and targeted to the nucleus in response to phosphorylation and ubiquitination of IκBα (
). To determine whether CDDO-Me affects IκBα phosphorylation, cytosolic lysates from TNF-α-stimulated cells were immunoblotted with anti-phospho-IκBα. The results demonstrate that CDDO-Me inhibits TNF-α-induced phosphorylation of IκBα (Fig. 1C). In concert with these results, CDDO and CDDO-Me also inhibited TNF-α-induced degradation of IκBα (Fig. 1C). These findings indicate that CDDO and CDDO-Me act upstream to IκBα in the NF-κB pathway.
CDDO-Me Directly Inhibits IKKβ—The IKKβ kinase function is necessary and sufficient for phosphorylation of IκBα (
). To determine whether CDDO-Me inhibits IKKβ activity, anti-IKKβ immunoprecipitates were prepared from cells pretreated with CDDO-Me and then stimulated with TNF-α. Incubation of the precipitates in kinase reactions with GST-IκBα and [γ-32P]ATP demonstrated that CDDO-Me treatment is associated with inhibition of IKKβ activity (Fig. 2A). Consistent with these results, CDDO-Me inhibited TNF-α-induced autophosphorylation of IKKβ on Ser-181 (Fig. 2B). To determine whether CDDO-Me inhibits IKKβ activity in vitro, anti-IKKβ precipitates from TNF-α-stimulated cells were incubated with GST-IκBα in the absence and presence of CDDO-Me. The results show that IKKβ activity is also inhibited by CDDO-Me in vitro (Fig. 2C, left). Notably, addition of DTT to the kinase reactions blocked CDDO-Me-mediated inhibition of IKKβ activity (Fig. 2C, right). In this regard, DTT contains thiol groups that form reversible adducts with the CDDO α,β-unsaturated carbonyl moiety (
). To determine whether the effects of CDDO-Me are direct, we preincubated recombinant kinase-active His-IKKβ with CDDO-Me and then assayed for phosphorylation of GST-IκBα. His-IKKβ activity was inhibited by CDDO-Me (Fig. 2D, left). By contrast, the inhibitory effect of CDDO-Me was blocked in the presence of DTT (Fig. 2D, right). Taken together with the finding that DTT abolishes CDDO-Me-mediated inhibition of IKKβ, these results indicate that CDDO-Me directly inhibits IKKβ activity.
CDDO-Me Inhibition of IKKβ Is Reversed by Mutation of Cys-179—IKKβ contains a cysteine at position 179 in its activation loop. To determine whether this cysteine is involved in inhibition by CDDO-Me, we transfected 293 cells to express wild-type FLAG-IKKβ or FLAG-IKKβ with a C179A mutation. Analysis of anti-FLAG precipitates for phosphorylation of GST-IκBα demonstrated similar levels of activity for FLAG-IKKβ and FLAG-IKKβ(C179A) (Fig. 3A). CDDO-Me treatment was associated with inhibition of wild-type IKKβ (Fig. 3A). By contrast, CDDO-Me had no apparent effect on IKKβ(C179A) activity (Fig. 3A). In concert with these results, CDDO-Me also had little effect on IKKβ(C179) activity when added directly to in vitro kinase assays (Fig. 3B). Moreover, CDDO-Me-induced inhibition of NF-κB-mediated transcription was substantially attenuated in cells expressing IKKβ(C179A) as compared with that obtained with wild-type IKKβ (Fig. 3C). These findings indicate that CDDO-Me inhibits IKKβ by reacting with Cys-179.
CDDO-Me Inhibits IKKβ by Oxidizing Cys-179—CDDO forms reversible adducts with DTT and cysteine-rich protein targets (
). To determine whether CDDO-Me interacts directly with IKKβ in vitro, recombinant IKKβ was incubated with CDDO-Me conjugated to biotin (CDDO-Me-biotin). Analysis of the reaction products demonstrated the formation of IKKβ-CDDO adducts (Fig. 4A). By contrast, the interaction was substantially blocked when recombinant IKKβ(C179A) was incubated with CDDO-Me-biotin (Fig. 4A). Unlabeled CDDO-Me competed with CDDO-Me-biotin for binding to IKKβ, indicating that the interaction with IKKβ is not due to the biotin moiety (Fig. 4B, left). In addition, unlabeled CDDO-Me and CDDO-Me-biotin were similarly effective in inhibiting IKKβ (Fig. 4B, right). Previous studies of certain direct chemical inhibitors of IKKβ have demonstrated the induction of IKKβ dimerization (
). Immunoblot analysis of lysates from cells expressing FLAG-IKKβ demonstrated that CDDO-Me treatment is associated with the induction of a higher molecular mass species that reacts with anti-IKKβ (Fig. 4C). The absence of this high molecular mass species in cells expressing FLAG-IKKβ(C179A) (Fig. 4C) suggests that CDDO-Me may induce the formation of IKKβ dimers by a mechanism dependent on the interaction with Cys-179. Cells were also pretreated with DTT to block the interaction between CDDO-Me and IKKβ. The results demonstrate that DTT reverses CDDO-Me-induced inhibition of IκBα phosphorylation and degradation (Fig. 4D). Consistent with these results, DTT also reversed CDDO-Me-induced inhibition of NF-κB p65 targeting to the nucleus (Fig. 4E). These findings indicate that CDDO-Me inhibits IKKβ by direct oxidation of Cys-179.
CDDO Also Forms Adducts with IKKβ Cys-179—To determine whether CDDO also interacts with IKKβ by oxidation of Cys-179, FLAG-IKKβ and FLAG-IKKβ(C179A) immunoprecipitated from 293 cells were incubated with CDDO conjugated to biotin (CDDO-biotin). The results demonstrate that CDDO forms adducts with IKKβ and not IKKβ(C179A) (Fig. 5A). To determine whether CDDO forms adducts with IKKβ in vivo, cells expressing FLAG-IKKβ or FLAG-IKKβ(C179A) were cultured with CDDO-biotin. In concert with the in vitro results, analysis of anti-FLAG precipitates demonstrated that CDDO binds to IKKβ and not IKKβ(C179A) (Fig. 5B). Moreover, incubation of recombinant IKKβ and IKKβ(C179A) with CDDObiotin confirmed that CDDO directly forms adducts with the IKKβ Cys-179 residue (Fig. 5C). These findings demonstrate that, like CDDO-Me, CDDO interacts with IKKβ by oxidizing Cys-179.
CDDO-Me Functions as an Electrophile in Inhibiting IKKβ—CDDO contains an α,β-unsaturated carbonyl in the A-ring that forms reversible adducts with thiol nucleophiles (
). The present studies demonstrate that CDDO-Me directly inhibits IKKβ activity and thereby the NF-κB pathway (Fig. 5D). Previous work has indicated that CDDO inhibits the NF-κB pathway following nuclear translocation of p65 (
). By contrast, our results clearly demonstrate that CDDO and CDDO-Me interact directly with IKKβ. CDDO-Me-induced inhibition of IKKβ in vitro and in cells was reversed by DTT, which forms reversible adducts with CDDO, indicating that, like the cyclopentenone prostaglandins, CDDO-Me inhibits IKKβ by oxidation of a reactive cysteine moiety. Similar results were obtained with CDDO, indicating that the presence of the methyl ester is not required for direct interaction with the IKKβ Cys-179 residue. Moreover, the findings that (i) IKKβ with a C179A mutation is insensitive to the effects of CDDO-Me and (ii) CDDO-Me and CDDO bind directly to IKKβ, but not IKKβ(C179A), in vitro and in cells support oxidation of the thiol group on Cys-179 as the inhibitory mechanism. Thus, the anti-inflammatory effects of CDDO and its derivatives (