Nuclear factor-κB (NF-κB) is a transcription factor that is essential in the regulation of immune and inflammatory responses.¹ It influences a diverse target of gene expressions that regulate apoptosis, facilitate cell survival, proliferation, and differentiation.^1,2 Before cell stimulation, NF-κB dimers that are located in the cytoplasm are inactive.³ Prior to activation, NF-κB dimers consisting of RelA, c-REL, and p50 are held in the cytoplasm by inhibitory κB (IκB) proteins.^3,8 The IκB kinase (IKK) complex is activated by various extracellular signals such as proinflammatory cytokines and viral infections.^3,4 This IKK complex phosphorylates two conserved serine residues and targets NF-κB-bound IκBs, which results in ubiquitin-mediated dissociation of IκB from NF-κB, thus leading to translocation of activated NF-κB into the nucleus.^2,7 The activation of NF-κB promotes tumor invasion, metastasis, and allows malignant cells to escape apoptosis. Consequently, many chemotherapeutic drugs have been found to activate NF-κB, thus contributing to chemoresistance and chemotherapy failure.³ Increasing evidence suggests that, the inhibition of NF-κB activation can reduce chemoresistance and improve the effectiveness of chemotherapeutic agents.³

Among the compounds that have been reported, curcumin was found to inhibit the activation of NF-κB and thus, induce apoptosis in tumor cells.⁶ Unfortunately, its clinical applications remains limited due to its poor bioavailability and low potency⁶, these prompted researchers to chemically modify curcumin in order to increase its potency against NF-κB and cancerous cells.⁸

In this issue, Qui et al.⁸ reports progress in the synthesis and identification of new 4-arylidene curcumin analogues as a potential chemotherapeutic agent. Different kinds of 4-arylidene curcumin analogues were synthesized by coupling 1, 3-diketones curcumin analogues with various aromatic aldehydes in toluene with acetic acid, using piperidine as a catalyst (figure 1).

The chemotherapeutic activities of the synthesized compounds were tested on the growth of A549 lung adenocarcinoma cells with curcumin used as control. The authors reported that majority of the 4-arylidene curcumin analogues exhibited potent anticancer activities against A549 growth with GI₅₀ in the range of 0.23 – 0.93 μM, while very poor antiproliferation activities of curcumin was observed at 15.23 μM. This shows a 10- to 60-fold increase in the potency of 4-arylidene curcumin analogues over the parent compound, curcumin. Remarkably, the cytotoxic activities of these newly designed curcumin analogues were not limited to A549 cells. The growth of other carcinoma cells H1944, squamous cells H157, and large carcinoma cells H460, were effectively inhibited by selected 4-acrylidene curcumin analogues, with GI₅₀ values at micromolar concentrations low to 0.07 μM. Likewise, in a related study, Zambre et al.⁹ reported that copper(II) conjugates of Knoevenagel condensates of curcumin analogue showed inhibitory activities against human leukemic KBM-5 cells. Taken together, these two forms of curcumin analogues offer new possibilities at both ends as potential anticancer agents.

One of the key curcumin targets that is important for the survival of cancer is IκB kinase (IKK), which regulates NF-κB activation.⁶ Activated NF-κB is situated in the nucleus to promote transcription that is triggered by tumor-necrosis factor (TNFα).^1,5 Thus, Qiu et al.⁸ used nuclear translocation of NF-κB in response to TNFα as the main indicator to examine the mode of action of curcumin in comparison to 4-arylidene curcumin analogue. A549 cells were treated in a 384-well plate format with curcumin and its new analogue respectively, before the addition of TNFα to trigger nuclear translocation of NF-κB p65 subunit. As a result, curcumin inhibited TNFα-induced nuclear translocation of NF-κB with a mean IC₅₀ of 9.5 μM, which is consistent with the work of Kasinski et al.⁴ Interestingly, most of the synthesized 4-arylidene curcumin analogue showed improved inhibitory activities against NF-κB translocation with mean IC₅₀ values in the range of 1.0 – 4.9 μΜ. This finding proved the superiority of the newly designed curcumin analogue over curcumin in blocking nuclear translocation of NF-κB. Consequently, in a related paper, Zambre et al.⁹ developed novel curcumin analogues that were synthesized using Knoevenagel condensation to convert enolic diketones of curcumin into non-enolizable ones. The synthesized compounds were examined for their potential in blocking TNFα-induced NF-κB activation. It was reported that copper(II) conjugates of Knoevenagel condensates of curcumin showed greater potentials in blocking TNFα-induced NF-κB activation than curcumin, confirming the potency superiority of curcumin analogues over the parental curcumin.

NF-κB is principally activated by IKKβ in its well organized signaling pathways.^2,7 As a result, Qiu et al.⁸ choose three potent newly synthesized 4-arylidene curcumin analogues A, B and C to directly investigate their effect on IKK enzymatic activity, with curcumin used as control. Report was given that upon stimulation of A549 cells with TNFα, TNFα induced considerable IκB phosphorylation followed by degradation of the phosphorylated IκB. However, following the treatment of the cells with curcumin and its newly designed analogue, respectively, curcumin inhibited IκB phosphorylation and degradation at high concentrations, while the selected potent newly synthesized curcumin analogues significantly inhibited IκB phosphorylation and degradation at a lower concentration with IC₅₀ values in micromolar range of 2.2 – 5.0 μΜ. Again, this showed that the 4-arylidene curcumin analogues exhibited greater inhibitory activities against IκB than curcumin. In another study, Kasinski et al.⁴ proposed that the inhibitory activities of curcumin and its analogues may be a result of direct inhibition of IKKβ kinase. To examine this model, the authors performed a reconstituted IKK inhibition assay with recombinant IKKβ, report showed that the addition of curcumin in various tested concentrations had no significant effect in inhibiting IKKβ.⁴ However, in this issue⁸, the selected newly synthesized curcumin analogues induced a dose dependent inhibition of IKKβ. Thus, the structural modification of these curcumin analogues results to improved inhibitory activities over curcumin in the in vitro IKK kinase assay.

Furthermore, they compared the anticancer efficacy of curcumin and its newly designed analogues. The authors reported that the synthesized 4-arylidene curcumin analogues inhibited colony formation of lung cancer cells at low concentrations in the micromolar range of less than 0.2 – 0.4 μM.⁸ However, curcumin also inhibited colony formation of cancer cells at higher concentration (4 μM). Again, this finding showed the superiority in potency of 4-arylidene curcumin analogues over the parent compound as an anticancer agent. Thus, the chemical modification of the parental curcumin has led to identification of new 4-arylidene curcumin analogues as potential anticancer agents targeting NF-κB signaling pathway.

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