Auranofin Downregulates Nuclear Factor-κB Activation via Nrf2-Independent Mechanism

Abstract

Transcription factors Nrf2 and NF-kB are important regulators of the innate immune response, and their cross-talks in inflammation have been reported. Previously, we demonstrated that gold(I)-compound auranofin, an inhibitor of NF-kB signal, induced Nrf2 activation in human synovial cells and monocytic cells. To investigate whether the Nrf2 activation is involved in the mechanism of the auranofin-attenuated NF-kB signaling, we examined the effects of Nrf2 knockdown on NF-kB activation using rheumatic synovial cells. When the cells were transfected with a specific siRNA for Nrf2, the gene expression was perfectly blocked. However, the Nrf2 knockdown did not cancel the suppressive effect of auranofin on TNF-α-induced IkB-a degradation. Treatment with a specific siRNA for HO-1, which is a target of Nrf2 and plays a role in anti-inflammation, also did not affect the blocking activity of auranofin on IkB-a degradation. In addition, auranofin-inhibited ICAM-1 expression was not restored by Nrf2 knockdown. These findings indicate that the activated Nrf2 and HO-1 are not associated with the suppressive action of auranofin on the pro-inflammatory cytokines-stimulated NF-kB activation. This suggests that Nrf2/HO-1 and NF-kB signals, which are regulated by auranofin, participate in the anti-inflammatory action of auranofin via independent pathways in rheumatic synovial cells.Key words : Auranofin, nuclear factor-erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), nuclear factor-κB (NF-κB), intercellular adhesion molecule-1 (ICAM-1) † Present address Introduction Auranofin (2, 3, 4, 6-tetra-O-acetyl-1-thio-β-D-glucopyranosato-S-[triethylphosphine] gold) is a thiol-reactive gold(I) compound that has been used for the treatment of rheumatoid arthritis for a long time [1]. The drug possesses the inhibitory activity for nuclear factor κB (NF-κB), signal transducer and activator of transcription 3 (STAT3), and toll-like receptor 4 [6,12,26]. Recently, we reported a novel anticancer activity of auranofin by demonstrating that auranofin induced apoptosis and promoted all-trans retinoic acid-mediated differentiation of acute promyelocytic leukemia cells [11,20,21]. Therefore, the gold(I) compounds including auranofin have been studied for therapeutic application to inflammation and cancer [3,4,19,22,24].In the mechanisms on anti-inflammatory and antirheumatic actions of auranofin, the inhibitory effect on NF-κB signal pathway has been well known. The thiol-reactive auranofin modifies Cys-179 in beta subunit of inhibitory κB kinase (IKK), which results in the blockade of inhibitory κB-α (IκB-α) phosphorylation and degradation [6,7]. Consequently, auranofin attenuates the transcriptional activity of NF-κB and downregulates the expression of pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β).Heme oxygenase-1 (HO-1), which suppresses an inflammatory response, is also associated with the pharmacological property of auranofin in rheumatoid arthritis [14,16]. In the previous study, we ...