The effect of inhibition of nucleotide synthesis on ribosome biogenesis and the induction of p53

Abstract

Ribosome biogenesis is one of the most energy consuming anabolic processes in a cell, required for the generation of the translational machinery to grow and proliferate. Moreover, this process necessitates the coordination of protein and nucleotide synthesis to generate ribosomal proteins (RPs) and ribosomal RNA (rRNA). Critically, increased rates of ribosome biogenesis are a hallmark of c-Myc driven CRC required to sustain exacerbated growth and proliferation, with recent studies showing that drugs that target ribosome biogenesis are clinically efficacious. We have previously shown that upon ribosome biogenesis impairment, a pre‐ribosomal complex formed by RPL11 and RPL5 and noncoding 5S rRNA is re‐directed from the incorporation into the pre-60S ribosome, to bind and inhibit HDM2, leading to p53 stabilization and cell cycle arrest. We have termed this response the Impaired Ribosome Biogenesis Checkpoint (IRBC). In this study I set out to analyze the effect of nucleotide depletion on ribosome biogenesis in c-Myc-driven CRC cell lines, addressing the role of the IRBC. Nucleotide depletion inhibited rRNA synthesis and elicited the IRBC, p53 stabilization, but failed to induce G1 cell cycle arrest as previously reported. I found that this was due to the loss of 5S RNA production, the limiting factor in triggering the IRBC, causing a disruption of the IRBC complex. Moreover, this allowed cells to escape G1 arrest and enter S phase, where they encountered replicative stress. These data support the hypothesis that in nucleotide deprived conditions the IRBC acts to hold cells in G1 to prevent them from replicating their DNA cells and eventually encountering genomic instability.