Involvement of ROS in NF-κB activation by TNFα − Rakyat Biologi

Like IL-1β, TNFα is a potent pro-inflammatory cytokine that plays a crucial role in a series of cellular events such as apoptosis, cell proliferation, differentiation and septic shock [100]. It binds to its cellular TNFR1 receptor, which triggers signalling cascades that activate NF-κB and AP-1 transcription factors. The signalling pathway that leads to NF-κB activation is now well established [21, 101]. The ligation of TNFR1 by trimeric TNFα leads to the aggregation of the receptor and dissociation of Silencer of death domain (SODD), an inhibitor of TNFR1 activity, which allows binding of TRADD protein (TNFR-associated death domain protein) [102]. TRADD subsequently recruits downstream adapters like TRAF proteins (TNF-receptor-associated factor) [103]. Although many members of the TRAF family have been implicated in TNF signalling, it appears that both TRAF2 and TRAF5 have a role in NF-κB activation by TNFα [104]. RIP1 (Receptor interacting protein 1) also plays a crucial role in NF-κB activation by TNFα [105]. RIP1 functions as a scaffold protein notably through its direct binding to NEMO, which allows the recruitment of the IKK complex in TNF signalling [106].

As mentioned above, antioxidants have been reported to inhibit TNF-induced NF-κB activation [19, 51, 83, 107], but the molecular mechanisms underlying this observation are, contrary to IL-1β signalling, still poorly understood and were furthermore recently called into question by Hayakawa et al. [87]. They showed that, whereas NAC and PDTC efficiently blocked TNF-induced IκBα degradation and NF-κB activation, the more potent antioxidants epigallocatechin-gallate (EGCG) and vitamin E analog Trolox failed to inhibit TNF-stimulated NF-κB activation, suggesting that the effect of NAC and PDTC on NF-κB signalling does not rely on their antioxidant capacities, but rather acts by inhibiting a crucial step in TNF signalling. Indeed, they showed that NAC inhibits TNF-stimulated signal transduction by lowering the TNF receptor affinity, and that PDTC is likely to inhibit IkB-ubiquitin ligase activity. These results are reinforced by the observation that, whereas NAC does not inhibit IL-1 or TPA-induced IκBα degradation, PDTC does, suggesting that NAC acts specifically on the early events in TNF signalling, but that PDTC has a larger effect by inhibiting IκBα degradation induced by a broad range of inducers. Finally, they showed that TNF-induced production of ROS only appears after 2h of TNF treatment, which does not explain the NF-κB activation which already takes place after 10 min.

Acetylation and deacetylation events are also implicated in the regulation of NF-κB transcriptional activity upon TNF-induction, which in turn can modify the inflammatory response [108]. The effects of ROS on the modulation of histone acetyltransferases (HAT) and deacetylases (HDAC), the key enzymes responsible for chromatin remodelling, are still poorly understood. The hypothesis that oxidants may play a role in the modulation of HDAC have been recently proposed by Ito et al. and Moodie et al. [59, 109]. They showed that ROS (induced by cigarette smoke or H₂O₂ treatment) reduce HDAC2 expression and activity and increase acetylation of histone H4 in alveolar epithelial cells, which could in turn modify gene transcription an augment inflammatory response, especially in the case of cigarette-induced chronic obstructive pulmonary disease. The readers can obtain more information about that research area in a recent review by Rahman et al. [110]. Finally, it should also be noted that NAC was shown to inhibit p65 ser536 phosphorylation, suggesting that post-translational modification affecting p65 are also redox-sensitive [111].