Inhibition of transcription by B Cell Leukaemia 3 (Bcl-3) requires interaction with Nuclear Factor (NF)-{kappa}B p50 [Immunology]

January 23rd, 2014 by Collins, P. E., Kiely, P. A., Carmody, R. J.

B cell leukaemia (Bcl)-3 is an essential negative regulator of Nuclear Factor (NF)-κB during Toll-Like Receptor (TLR) and TNF Receptor signalling. Bcl-3 also interacts with a number of transcriptional regulators including homodimers of the NF-κB p50 subunit. Deletion of Bcl-3 results in increased NF-κB p50 ubiquitination and proteasomal degradation, and increased inflammatory gene expression. We have employed immobilised peptide array technology to define a region of p50 required for the formation of a Bcl-3:p50 homodimer immunosuppressor complex. Our data demonstrates that amino acids 359-361 and 363 of p50 are critical for interaction with Bcl-3 and are essential for Bcl-3 mediated inhibition of inflammatory gene expression. Bcl-3 is unable to interact with p50 when these amino acids are mutated, rendering it incapable of inhibiting the transcriptional activity of NF-κB. Bcl-3-interaction defective p50 is hyper-ubiquitinated and has a significantly reduced half-life relative to wild type p50. Nfkb1-/- cells reconstituted with mutated p50 precursor p105 are hyper-responsive to TNFα stimulation relative to wild type p105 as measured by inflammatory gene expression. Mutant p105 recapitulates a Bcl3-/- phenotype. This study demonstrates that interaction with p50 is necessary and sufficient for the anti-inflammatory properties of Bcl-3 and further highlights the importance of p50 homodimer stability in the control of NF-κB target gene expression.