Nucleolar stress stimulates the NF-kappaB pathway : mechanism underlying the proapoptotic effects of aspirin

The nucleolus is a multifunctional organelle that, in addition to its primary role in ribosome biogenesis, has emerged as a critical stress sensor and coordinator of stress response. However, the molecular nature of how nucleoli sense stress and coordinate downstream cellular consequence remains poorly understood. NF-κB signalling is a critical regulator of stress response. Many cellular stresses that disrupt nucleolar function also stimulate the NF-κB pathway. However, the role of NF-κB as a downstream effector of nucleolar stress has not yet been examined. Aspirin, a known chemopreventative agent, stimulates the NF-κB pathway to mediate apoptosis but the upstream mechanisms are unclear. In this thesis, I identified a novel nucleolar stress response pathway that culminates in activation of NF-κB signalling, and demonstrated the significance of this nucleolar pathway in the anti-tumour effects of aspirin. Using multiple approaches, I made the novel observations that disruption of the Pol I complex activates the cytoplasmic NF-κB signalling pathway. I show that multiple stress stimuli of NF-κB pathway induce degradation of the crucial Pol I complex component, rDNA transcription initiation factor IA (TIF-IA). I identified the tumour suppressor, p14ARF and the Pol I complex component, upstream binding factor (UBF) as mediators of this degradation. I revealed that inhibition of CDK4 activity lies upstream of UBF/p14ARF-facilitated TIF-IA degradation. Furthermore, using different approaches I show that blocking aspirin/CDK4i-mediated degradation of TIF-IA blocks the effects of these agents on nucleolar morphology and NF-κB signalling. Finally, I show this nucleolar stress response pathway, containing a UBF/p14ARF/TIF-IA axis, is utilized by aspirin to kill colon cancer cells. Taken together, this data presented in this thesis advances understanding of nucleolar stress response, and has therapeutic implications with regard to the anti-tumour effects of aspirin.