Insights into the NF-κB-DNA Interaction through NMR Spectroscopy

Identification and mechanistic exploration of structural and conformational dynamics of NF-kB inhibitors: rationale insights from in silico and in vitro studies

Increased expression of target genes that code for proinflammatory chemical mediators results from a series of intracellular cascades triggered by activation of dysregulated NF-κB signaling pathway. Dysfunctional NF-kB signaling amplifies and perpetuates autoimmune responses in inflammatory diseases, including psoriasis. This study aimed to identify therapeutically relevant NF-kB inhibitors and elucidate the mechanistic aspects behind NF-kB inhibition. After virtual screening and molecular docking, five hit NF-kB inhibitors opted, and their therapeutic efficacy was examined using cell-based assays in TNF-α stimulated human keratinocyte cells. To investigate the conformational changes of target protein and inhibitor-protein interaction mechanisms, molecular dynamics (MD) simulations, binding free energy calculations together with principal component (PC) analysis, dynamics cross-correlation matrix analysis (DCCM), free energy landscape (FEL) analysis and quantum mechanical calculations were carried out. Among identified NF-kB inhibitors, myricetin and hesperidin significantly scavenged intracellular ROS and inhibited NF-kB activation. Analysis of the MD simulation trajectories of ligand-protein complexes revealed that myricetin and hesperidin formed energetically stabilized complexes with the target protein and were able to lock NF-kB in a closed conformation. Myricetin and hesperidin binding to the target protein significantly impacted conformational changes and internal dynamics of amino acid residues in protein domains. Tyr57, Glu60, Lys144 and Asp239 residues majorly contributed to locking the NF-kB in a closed conformation. The combinatorial approach employing in silico tools integrated with cell-based approaches substantiated the binding mechanism and NF-kB active site inhibition by the lead molecule myricetin, which can be explored as a viable antipsoriatic drug candidate associated with dysregulated NF-kB.Communicated by Ramaswamy H. Sarma.

Structural and dynamic studies of DNA recognition by NF-κB p50 RHR homodimer using methyl NMR spectroscopy

Protein dynamics involving higher-energy sparsely populated conformational substates are frequently critical for protein function. This study describes the dynamics of the homodimer (p50)2 of the p50 Rel homology region (RHR) of the transcription factor NF-κB, using 13C relaxation dispersion experiments with specifically (13C, 1H)-labeled methyl groups of Ile (δ), Leu and Val. Free (p50)2 is highly dynamic in solution, showing μs-ms relaxation dispersion consistent with exchange between the ground state and higher energy substates. These fluctuations propagate from the DNA-binding loops through the core of the domain. The motions are damped in the presence of κB DNA, but the NMR spectra of the DNA complexes reveal multiple local conformations of the p50 RHR homodimer bound to certain κB DNA sequences. Varying the length and sequence of κB DNA revealed two factors that promote a single bound conformation for the complex: the length of the κB site in the duplex and a symmetrical sequence of guanine nucleotides at both ends of the recognition motif. The dynamic nature of the DNA-binding loops, together with the multiple bound conformations of p50 RHR with certain κB sites, is consistent with variations in the transcriptional activity of the p50 homodimer with different κB sequences.

Interactions of a Long Noncoding RNA with Domains of NF-κB and IκBα: Implications for the Inhibition of Non-Signal-Related Phosphorylation

The transcription factor NF-κB is one of the central mediators of cellular signaling pathways. Under resting conditions, the canonical RelA-p50 (p65-p50) heterodimer NF-κB remains sequestered in the cytoplasm in complex with its inhibitor IκBα. Signal-mediated activation of NF-κB involves phosphorylation, ubiquitination and degradation of IκBα, and translocation of NF-κB to the nucleus. It was recently shown that a long noncoding RNA (termed NKILA) can modulate the NF-κB signaling circuit by interacting with the NF-κB-IκBα complex in the cytoplasm. In the current study, we investigated the interaction of RNA sequences derived from NKILA with domains of NF-κB and IκBα using NMR spectroscopy and native gel electrophoresis. Our results indicate that two RNA hairpin sequences interact with the DNA-binding domains of the Rel homology regions of RelA (p65) and p50 and that the same RNA sequences can affect the phosphorylation of the N-terminus of IκBα under low-salt conditions. We also observe that full-length RHR dimers (heterodimer of p65 and p50 and homodimer of p50) show a stronger interaction with the RNA hairpins than the individual domains of NF-κB. All of the interactions we observe between fragments of NKILA and domains of NF-κB are weak and nonspecific, consistent with the proposed function of the NKILA-NF-κB-IκBα interaction in protecting the NFκB-IκBα complex from aberrant activation of the NF-κB signaling pathway.