Comparative Study on Inhibitory Activity of Zerumbone and Zerumbone 2,3-epoxide on Nf-κb Activation and No Production

In the present study the significant role of the α,β-unsaturated carbonyl structure in the anti-inflammatory activity of the natural humulane sesqui-terpenoids zerumbone and zerumbone 2,3-epoxide was evidenced from a comparative study of the ability of zerumbone and zerumbone 2,3-epoxide to inhibit NF-κB activation and NO production in LPS (lipopolysaccharide)-stimulated RAW 264.7 cells. The IC 50 of these compounds were 1.97 μM ± 0.18 and 30.11 μM ± 4.10 in the NF-κB activation assay and 3.58 μM ± 0.46 and 34.7 μM ± 3.72 in the nitric oxide production assay, respectively.


Introduction
Nuclear factor-κB (NF-κB) consists of homodimers and heterodimers of Rel/NF-κB proteins that bind to DNA target sites, where they directly regulate gene transcription.Activation of NF-κB has been showed to be involved in the pathogenesis of chronic inflammatory diseases, such as inflammatory bowel diseases and rheumatoid arthritis, and a variety of human cancers such as leukemias, lymphomas, and solid tumors [1].Therefore, pharmacological intervention of NF-κB could be a valuable strategy to modulate Sci Pharm.2009; 77; 589-595.inflammatory processes as well as cancer.Several small molecular natural compounds have been reported to inhibit NF-κB at one or multiple activation steps of the signaling NF-κB pathway [2][3][4].The natural humulane sesquiterpenoid zerumbone (1) (Fig. 1) was shown to modulate proinflammatory genes and cancer cell proliferation all of which are associated with the activation of NF-κB [5][6][7].Zerumbone possesses a monocyclic elevenmembered ring containing a cross-conjugated dienone moiety, which is closely resembled to the α,β-unsaturated carbonyl group in the well-known NF-κB inhibitors such as ent-kaurane diterpenoids [8] or sesquiterpene lactones [9].High biological activity seems to be dependent on the α,β-unsaturated carbonyl group of zerumbone [5] since this group can react with the cysteine sulfhydryl groups (Cys 38 and Cys 120) in the p65 subunit of NF-κB.Zerumbone and its natural analog zerumbone 2,3-epoxide (2) were isolated by us from the rhizomes of the Vietnamese Curcuma zedoaria species (Zingiberaceae), which is commercially available for the treatment of gastrointestinal disorders and ulcers [10,11].The significant role of the α,β-unsaturated carbonyl structure in the antiinflammatory activity of these humulane sesquiterpenoids can be further evidenced from a comparative study of the ability of zerumbone and zerumbone 2,3-epoxide to inhibit NF-κB activation and NO production in LPS (lipopolysaccharide)-stimulated RAW 264.7 cells.

Results and Discussion
Zerumbone (1) and zerumbone 2,3-epoxide (2) were examined for their dose-dependent effect on LPS (lipopolysaccharide)-induced NF-κB activation in RAW 264.7 cells, which were stably transfected with a plasmid containing κB elements linked to SEAP (secreted alkaline phosphatase) gene as NF-κB reporter gene.SEAP activity in the culture medium was measured after treatment without LPS or with LPS (1 μg/mL) in the presence of various concentrations of compounds 1 (0.3-30 μM) and 2 (1-100 μM).As shown in Fig. 2, zerumbone potently inhibited LPS-induced SEAP activity dosedependently with an IC 50 of 1.97 μM ± 0.18.Meanwhile, zerumbone 2,3-epoxide did weakly inhibit NF-κB activity with an IC 50 value of 30.11 μM ± 4.10 (Fig. 2), i.e. 15-fold greater than that of zerumbone.High-output production of nitric oxide (NO) produced by inducible form of nitric oxide synthase (iNOS) is considered as a promoter of tissue injury in inflammation.Since NF-κB is the major transcription factor involved in iNOS gene expression after exposure to various stimuli including LPS, the effect of zerumbone and zerumbone 2,3-epoxide on the NO production was measured in LPS-stimulated RAW 264.7 cells.RAW 264.7 cells were treated with zerumbone (0.3-30 μM) and zerumbone 2,3-epoxide (1-100 μM) in the culture medium, followed by stimulation with LPS (1 μg/mL).As a parameter of NO synthesis, nitrite concentration was measured using the supernatant of RAW 264.7 cells by the Griess reaction.Consistent with the NF-κB inhibitory activity, both compounds inhibited nitrite production in a concentration-dependent manner with an IC 50 of 3.58 μM ± 0.46 for zerumbone and 34.7 μM ± 3.72 for zerumbone 2,3-epoxide (Fig. 3).This is the first report on the NF-κB inhibitory activity of zerumbone 2,3-epoxide.It is noticeable that in both NF-κB activity and NO production assays almost no effect on cell viability was observed at concentrations up to 30 μM of zerumbone and 100 μM of zerumbone 2,3-epoxide in MTT-reduction assay (data not shown).The presence of the cross-conjugated dienone is assumed to be the structural requirement for the inhibition of NF-κB activity of zerumbone [5] and its analog zerumbone 2,3-epoxide.However, the inhibitory effects of zerumbone 2,3-epoxide on both NF-κB activation and NO production were markedly weaker than those of zerumbone although the former possesses an additional epoxide moiety, which is considered as an electrophilic site for potential nucleophilic attack.This profound decrease in the inhibitory activity of zerumbone 2,3-epoxide on NF-κB activation and NO production may be associated with the change in the molecular geometry of zerumbone 2,3-epoxide due to the presence of the epoxide ring.A single-crystal X-ray analysis of the structure of zerumbone 2,3-epoxide [12] demonstrated a conformation in which the two methyl groups at C-3 and C-7 were located at one face of the ring while the epoxide oxygen lay on the opposite face, minimizing steric repulsion among these groups.No effective conjugation of the double bond with the carbonyl group can be formed since in comparison with the structure of zerumbone, the C-6/C-7 double bond of zerumbone 2,3-epoxide is significantly distorted from the plane of the dienone [12].Therefore, C-6 of zerumbone 2,3-epoxide becomes less favorable Michael acceptor than that of zerumbone, resulting in the weaker inhibition of NF-κB activity and NO production.

NF-κB inhibitory activity assay
RAW 264.7 cells were transfected with a plasmid containing eight copies of κB elements linked to SEAP (secreted alkaline phosphatase) gene.Transfected cells were seeded in a 96-well plate at a density of 5 × 10 4 cells/well.After 3 h incubation at 37° C, the cells were treated with various concentrations of compounds tested and LPS (1 μg/mL) for 24 h.Then 100 μL of each culture supernatant was transferred to a new 96-well plate and heated at 65° C for 5 min.An additional 100 μL of 2 × SEAP assay buffer (2 M diethanolamine, 1 mM MgCl 2 , 20 mM L-homoarginine) was added to each well and incubated at 37° C for 10 min.The reaction was initiated by the addition of 20 μL of 120 mM p-nitrophenyl phosphate dissolved in 1 × SEAP assay buffer and incubated at 37° C. The absorbance of the reaction mixture was measured at 405 nm with a microplate reader (Molecular Devices Co., Menlo Park, CA, USA).Data represent mean ± SD of three independent experiments performed in triplicate.

Cell viability assay
Cytotoxicity of compounds was evaluated by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (Sigma Chemical Co., St. Louis, MO, USA)]-based colorimetric assay.Then 10 μL of MTT solution (5 mg/μL) was added to the remaining cells after SEAP assays.After incubation for 3 h at 37° C, the formed formazan crystals in viable cells were solubilized with 100 μL of DMSO and the absorbance of each well was read at 570 nm using a microplate reader (Molecular Devices Co., Menlo Park, CA, USA).Cell numbers were obtained by standard curve.

Determination of NO production
RAW 264.7 cells were transferred in 96-well plates at a density of 1 × 10 5 cells/well.After 3 h incubation at 37° C, the cells were stimulated with LPS (1 μg/mL) for 24 h in the absence or presence of the compound tested.As a parameter of NO synthesis, nitrite concentration was measured in the supernatant of RAW 264.7 cells by the Griess reaction.Briefly, 100 μL of cell culture supernatant was reacted with 100 μL of Griess reagent (1:1 mixture of 0.1% N-(1-naphthyl)ethylenediamine in H 2 O and 1% sulfanilamide in 5% phosphoric acid) in a 96-well plate, and absorbance was read with a microplate reader at 570 nm.The nitrite concentration in the supernatants was calculated by comparison with a sodium nitrite standard curve.Data represent mean ± SD of three independent experiments performed in triplicate.

Fig. 2 .
Fig. 2.Effect of zerumbone (1) and zerumbone 2,3-epoxide (2) on the activation of NF-κB.RAW 264.7 cells transfected with a plasmid containing eight copies of κB elements linked to SEAP gene were treated with the indicated concentrations of 1 and 2 and then stimulated with LPS.SEAP activity in the culture medium was measured as described in Experimental.Data represent mean ± SD of three independent experiments performed in triplicate.

Fig. 3 .
Fig. 3. Effect of zerumbone (1) and zerumbone 2,3-epoxide (2) on nitrite accumulation in LPS-stimulated RAW 264.7 cells.RAW 264.7 cells were treated with the indicated concentrations of 1 and 2 and then stimulated with LPS.NO in the culture medium was measured as described in Experimental.Data represent mean ± SD of three independent experiments performed in triplicate.