TIF-IA is one major downstream target of extracellular signaling events. Nutrients and growth factors act via mTOR and ERK kinases and lead to activating phosphorylation of TIF-IA. The stress-dependent JNK2 inactivates TIF-IA. | © dkfz.de

The key factor that directs growth-dependent regulation of rDNA transcription is the Transcription Initiation Factor TIF-IA. TIF-IA is the final target of the chain of events by which extracellular signals are transferred into the nucleolus and modulate rRNA synthesis according to the physiological conditions. An important finding is that the mammalian ’target of rapamycin’ (mTOR) kinase pathway is required for TIF-IA activity, linking nutrient availability to rRNA synthesis. A different pathway, but similar scenario, mediates transcription activation by growth factors. After mitogenic stimulation of quiescent cells, pre-rRNA synthesis is increased due to phosphorylation of TIF-IA by ERK and RSK kinases. Finally, TIF-IA is inactivated upon stress by JNK-dependent phosphorylation. Our results identify TIF-IA as a downstream target of the JNK pathway and suggest a critical role of JNK2 to protect rRNA synthesis against the harmful consequences of cellular stress.

A link between nucleolar structure, cell cycle arrest and p53-mediated apoptosis

A model depicting the functional consequences of TIF-IA ablation. In TIF-IA-containing cells the nucleolus is transcriptionally active, and p53 is maintained at low levels. In TIF-IA-deficient cells the nucleolus is disrupted and ribosomal proteins (rPs) are released into the nucleoplasm, leading to upregulation of p53, cell cycle arrest and apoptosis. | © dkfz.de

We have inactivated the murine TIF-IA gene by homologous recombination in mice and embryonic fibroblasts (MEFs). TIF-IA-/- embryos die before/at embryonic day 9.5, displaying retardation of growth and development. In MEFs, Cre-mediated depletion of TIF-IA leads to disruption of nucleoli, cell cycle arrest, upregulation of p53, and induction of apoptosis. Elevated levels of p53 following TIF-IA depletion are due to increased binding of ribosomal proteins, such as L11, to MDM2 and decreased interaction of MDM2 with p53 and p19ARF. RNAi-induced loss of p53 overcomes proliferation arrest and apoptosis in response to TIF-IA ablation. The striking correlation between perturbation of nucleolar function, elevated levels of p53 and induction of cell suicide supports the view that the nucleolus is a stress sensor that regulates p53 activity.