Nucleotide depletion reveals the impaired ribosome biogenesis checkpoint as a barrier against DNA damage.

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint (IRBC) and the DNA damage response (DDR) have been implicated in p53 activation following nucleotide depletion. However, it is difficult to reconcile the two checkpoints operating together, as the IRBC induces p21-mediated G1 arrest, whereas the DDR requires that cells enter S phase. Gradual inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme required for de novo GMP synthesis, reveals a hierarchical organization of these two checkpoints. We find that the IRBC is the primary nucleotide sensor, but increased IMPDH inhibition leads to p21 degradation, compromising IRBC-mediated G1 arrest and allowing S phase entry and DDR activation. Disruption of the IRBC alone is sufficient to elicit the DDR, which is strongly enhanced by IMPDH inhibition, suggesting that the IRBC acts as a barrier against genomic instability.

Resveratrol induces antioxidant defence via transcription factor Yap1p.

Resveratrol is a polyphenol suggested to play a protective role against ageing and age-related diseases. We demonstrate that administering low-doses of resveratrol causes ROS accumulation and transcriptional changes in yeast cells and human adipocytes. These changes in gene expression depend on the oxidative transcription factor Yap1p. In particular, resveratrol induces expression of Yap1p gene targets, such as TRX2, TRR1 or AHP1, in a Yap1p-dependent mode. Under resveratrol treatment, Yap1p is phosphorylated and accumulated in the nucleus. Yap1p knockout causes resveratrol sensitivity, which totally depends on the presence of the C-terminal region of Yap1p. Thus, resveratrol may enhance cellular lifespan by hormetic ROS accumulation, which leads to strengthening the cells' antioxidant capacity.

The 40S-LARP1 complex reprograms the cellular translatome upon mTOR inhibition to preserve the protein synthetic capacity.

Ribosomes execute the transcriptional program in every cell. Critical to sustain nearly all cellular activities, ribosome biogenesis requires the translation of ~200 factors of which 80 are ribosomal proteins (RPs). As ribosome synthesis depends on RP mRNA translation, a priority within the translatome architecture should exist to ensure the preservation of ribosome biogenesis capacity, particularly under adverse growth conditions. Here, we show that under critical metabolic constraints characterized by mTOR inhibition, LARP1 complexed with the 40S subunit protects from ribophagy the mRNAs regulon for ribosome biogenesis and protein synthesis, acutely preparing the translatome to promptly resume ribosomes production after growth conditions return permissive. Characterizing the LARP1-protected translatome revealed a set of 5'TOP transcript isoforms other than RPs involved in energy production and in mitochondrial function, among other processes, indicating that the mTOR-LARP1-5'TOP axis acts at the translational level as a primary guardian of the cellular anabolic capacity.

Oncogenic MYC Induces the Impaired Ribosome Biogenesis Checkpoint and Stabilizes p53 Independent of Increased Ribosome Content.

The role of MYC in regulating p53 stability as a function of increased ribosome biogenesis is controversial. On the one hand, it was suggested that MYC drives the overexpression of ribosomal proteins (RP)L5 and RPL11, which bind and inhibit HDM2, stabilizing p53. On the other, it has been proposed that increased ribosome biogenesis leads the consumption of RPL5/RPL11 into nascent ribosomes, reducing p53 levels and enhancing tumorigenesis. Here, we show that the components that make up the recently described impaired ribosome biogenesis checkpoint (IRBC) complex, RPL5, RPL11, and 5S rRNA, are reduced following MYC silencing. This leads to a rapid reduction in p53 protein half-life in an HDM2-dependent manner. In contrast, MYC induction leads to increased ribosome biogenesis and p53 protein stabilization. Unexpectedly, there is no change in free RPL5/RPL11 levels, but there is a striking increase in IRBC complex bound to HDM2. Our data support a cell-intrinsic tumor-suppressor response to MYC expression, which is presently being exploited to treat cancer. SIGNIFICANCE: Oncogenic MYC induces the impaired ribosome biogenesis checkpoint, which could be potentially targeted for cancer treatment.