The NF-jB p50 is the N-terminal processed product of the precursor, p105. It has been suggested that p50 is generated not from full-length p105 but cotranslationally from incompletely synthesized molecules by the proteasome. We show that the 20S proteasome endoproteolytically cleaves the fully synthesized p105 and selectively degrades the C-terminus of p105, leading to p50 generation in a ubiquitin-independent manner. As small as 25 residues Cterminus to the site of processing are sufficient to promote processing in vivo. However, any p105 mutant that lacks complete ankyrin repeat domain (ARD) is processed aberrantly, suggesting that native processing must occur from a precursor, which extends beyond the ARD. Remarkably, the mutant p105 that lacks the internal region including the glycine-rich region (GRR) is completely degraded by 20S proteasome in vitro. This suggests that the GRR impedes the complete degradation of the p105 precursor, thus contributing to p50 generation.

Nuclear factor kappa B (NFκB) is a transcription factor that controls inflammation and cell survival. In clinical histology, elevated NFκB activity is a hallmark of poor prognosis in inflammatory disease and cancer, and may be the result of a combination of diverse micro-environmental constituents. While previous quantitative studies of NFκB focused on its signaling dynamics in single cells, we address here how multiple stimuli may combine to control tissue level NFκB activity. We present a novel, simplified model of NFκB (SiMoN) that functions as an NFκB activity calculator. We demonstrate its utility by exploring how type I and type II interferons modulate NFκB activity in macrophages. Whereas, type I IFNs potentiate NFκB activity by inhibiting translation of IκBα and by elevating viral RNA sensor (RIG-I) expression, type II IFN amplifies NFκB activity by increasing the degradation of free IκB through transcriptional induction of proteasomal cap components (PA28). Both cross-regulatory mechanisms amplify NFκB activation in response to weaker (viral) inducers, while responses to stronger (bacterial or cytokine) inducers remain largely unaffected. Our work demonstrates how the NFκB calculator can reveal distinct mechanisms of crosstalk on NFκB activity in interferon-containing microenvironments.

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BackgroundThe pro-survival activity of NF-κB in response to a variety of stimuli has been extensively characterized. Although there have been a few reports addressing the pro-cell death role of NF-κB, the precise mechanism of NF-κB's pro-cell death function still remains elusive.Methodology/Principal FindingsIn the present study, we investigated the role of NF-κB in cell death induced by chronic insult with hydrogen peroxide (H2O2). Here, we show that NF-κB promotes H2O2 induced caspase independent but PARP dependent fibroblast cell death. The pro-death activity of NF-κB is due to the DNA binding activity of RelA, which is induced through IKK- mediated IκBα degradation. NF-κB dependent pro-survival genes, Bcl-2 and XIAP, were significantly repressed, while NF-κB dependent pro-death genes, TNFα and Fas Ligand, were induced in response to H2O2.Conclusions/SignificanceWe discovered an unexpected function of NF-κB, in that it potentiates chronic H2O2 exposure induced cell death, and suggest that NF-κB mediates cell death through the repression of pro-survival genes and induction of pro-death genes. Since unremitting exposure of tissues to H2O2 and other reactive oxygen species can lead to several degenerative disorders and diseases, our results have important implications for the use of NF-κB inhibitors in therapeutic drug design.