Activation of NF-κB by a novel, CDK4-regulated, nucleolar stress response pathway

Abstract

p53 as an effector of nucleolar stress is well defined, but p53 independent mechanisms are largely unknown. Like p53, the NF-{kappa}B transcription factor plays a critical role in maintaining cellular homeostasis under stress. Many stresses that stimulate NF-{kappa}B also disrupt nucleoli. However, the link between nucleolar function and activation of the NF-{kappa}B pathway is as yet unknown. Here we demonstrate that siRNA silencing of PolI complex components stimulates NF-{kappa}B signalling. Unlike p53 nucleolar stress response, this effect does not appear to be linked to inhibition of rDNA transcription. We show that specific stress stimuli of NF-{kappa}B induce degradation of a critical component of the PolI complex, TIF-IA. This degradation precedes activation of the NF-{kappa}B pathway and is associated with an atypical nucleolar architecture. It is mimicked by CDK4 inhibition and is dependent upon upstream binding factor (UBF) and p14ARF. We show that blocking stress effects on TIF-IA blocks their ability to activate the NF-{kappa}B pathway. Finally, using ex vivo culture, we show a strong correlation between degradation of TIF-IA and activation of NF-{kappa}B in freshly resected, human colorectal tumours exposed to the chemopreventative agent, aspirin. Together, our study provides compelling evidence for a new, NF-{kappa}B nucleolar stress response pathway that has in vivo relevance and therapeutic implications.