Mechanistic Differences in the Inhibition of NF-κB by Turmeric and Its Curcuminoid Constituents

Abstract

Turmeric extract, a mixture of curcumin and its demethoxy (DMC) and bisdemethoxy (BDMC) isomers, is used as an anti-inflammatory preparation in traditional Asian medicine. Curcumin is considered to be the major bioactive compound in turmeric but less is known about the relative anti-inflammatory potency and mechanism of the other components, their mixture, or the reduced in vivo metabolites. We quantified inhibition of the NF-κB pathway in cells, adduction to a peptide mimicking IκB kinase β, and the role of cellular glutathione as a scavenger of electrophilic curcuminoid oxidation products, suggested to be the active metabolites. Turmeric extracts (IC50 14.5 ± 2.9 μM), DMC (IC50 12.1 ± 7.2 μM), and BDMC (IC50 8.3 ± 1.6 μM), but not reduced curcumin, inhibited NF-κB similar to curcumin (IC50 18.2 ± 3.9 μM). Peptide adduction was formed with turmeric and DMC but not with BDMC, and this correlated with their oxidative degradation. Inhibition of glutathione biosynthesis enhanced the activity of DMC but not BDMC in the cellular assay. These findings suggest that NF-κB inhibition by curcumin and DMC involves their oxidation to reactive electrophiles, whereas BDMC does not require oxidation. Because it has not been established whether curcumin undergoes oxidative transformation in vivo, oxidation-independent BDMC may be a promising alternative to test in clinical trials.

Interaction between Gut Microbiota and Curcumin: A New Key of Understanding for the Health Effects of Curcumin

“…The conjugation process typically involves the addition of a single moiety, although double glucuronidation has been reported in isolated liver microsomes [52], and diglutathionylated curcumin has been found in isolated reaction systems [53]. The predominating pathway of conjugation is represented by glucuronidation; indeed the glucuronide of curcumin is usually found as the major metabolite of curcumin in body fluids, organs, and cells [47,54] even though, due to the increase of molecular weight, these metabolites are less active than their substrates [54,55].…”

Identification of G protein-coupled receptor 55 (GPR55) as a target of curcumin

“…Since curcumin can degrade through oxidation 18 – 21 we analyzed its stability in our luciferase reporter assay conditions for 4 h and found that the curcumin-dependent OD 425 value was decreased by 16% (data not shown). Then, we further evaluated the effects of its metabolites (reduced curcumin ( 2 ) 22 , vanillin ( 3 ), ferulic acid ( 4 ), bicyclopentadione (BCP, 5 ), BCP minor diastereomer ( 6 )), as well as moderately stable analogs (5′-methoxycurcumin ( 7 ), 5′,5′′-methoxycurcumin ( 8 )), stable analogs (acetalcurcumin ( 9 ), diacetalcurcumin ( 10 ), 2,6-dimethylcurcumin ( 11 ), 4′,4′′-dimethylcurcumin ( 12 ), stable cyclohexanone derivatives of curcumin ( 13 ), and stable cyclopentanone derivatives of curcumin ( 14 )) and unstable analogs (unstable cyclohexanone derivatives of curcumin ( 15 ), and unstable cyclopentanone derivatives of curcumin ( 16 )) for GPR55 activation (Fig. 3a ).…”

“…Bicyclopentadione isomers were prepared by autoxidation of curcumin 19 . Reduced curcumin was a mixture of tetrahydrocurcumin (70%), hexahydrocurcumin (20%) and octahydrocurcumin (10%) obtained by NaBH 4 reduction of curcumin 22 . Compounds were used as stock solutions dissolved in DMSO.…”

Perspective on Improving the Relevance, Rigor, and Reproducibility of Botanical Clinical Trials: Lessons Learned From Turmeric Trials

“…Similarly, in vivo and in vitro pre-clinical studies from our own laboratories have demonstrated in vivo inhibition of NF-κB activation by curcumin ( 14 ), an effect likely attributable to adduct formation between oxidative curcumin metabolites and IκB kinase β (IKKß), the activating kinase upstream of NF-κB ( 14 , 27 – 29 ). Furthermore, our laboratories have demonstrated that in vivo inhibition of NF-κB activation in a pre-clinical arthritis model is associated with decreased NF-κB-induced cytokines or NF-κB-mediated tissue destructive processes (i.e., formation of bone-resorbing osteoclasts) known to be closely linked with adverse clinical outcome ( Figure 1 ) ( 14 – 16 ).…”

“…This has been extensively described for flavonoids ( 72 – 74 ), and curcumin is also susceptible to reductive as well as enzymatic and non-enzymatic oxidative metabolism ( Figure 5 ) ( 66 , 75 ). Again, in the case of curcumin, our laboratories have demonstrated an important role for oxidative metabolites of curcumin ( 70 , 76 ) in altering protein function via the formation of specific adducts ( 27 , 77 – 80 ). While evidence for protein adduction of curcumin in vivo is yet lacking, in cell-based assays multiple proteins appear to be targeted by reactive oxidative metabolites of curcumin ( 28 , 67 , 77 – 83 ), consistent with curcumin's reported pleiotropic effects.…”