The ribosomal protein L22 binds the MDM4 pre-mRNA and promotes exon skipping to activate p53 upon nucleolar stress

Dependence on Mdm2 for Mdm4 inhibition of p53 activity

Both Mdm2 and Mdm4 inhibit p53 activity by masking of its transcriptional activation domain. In addition, Mdm2 functions as an E3 ubiquitin ligase, targeting p53 for degradation. The amino terminus of Mdm4 binds wild type and mutant p53 while its RING domain, which lacks E3 ligase activity, is required for heterodimerization with Mdm2. To determine how these domains of Mdm4 regulate p53, we generated mouse models with either a deletion of the Mdm4 RING domain (Mdm4ΔR) or all of Mdm4 (Mdm4─) on a hypomorphic (p53neo) background. Mdm4ΔR mice exhibited elevated p53 levels and activity, albeit to a lesser extent than mice with complete Mdm4 loss, indicating that the amino terminus of Mdm4 contributes to p53 inhibition. Moreover, in the absence of Mdm2, neither the deletion of the Mdm4 RING domain nor the complete loss of Mdm4 further increased p53 protein levels on a mutant p53 background, indicating that Mdm4 modulates Mdm2 in its regulation of p53 stability. Collectively, our findings suggest that Mdm4 contributes to p53 inhibition by modulating Mdm2 activity via both its amino terminus and RING domains.

The Central Role of Ribosomal Proteins in p53 Regulation

The tumor suppressor protein p53 prevents the malignant transformation of cells by responding to DNA damage, oncogene activation, and abnormal growth signals including ribosome assembly defects. Under normal conditions, p53 activity is controlled by the regulatory proteins MDM2 and MDM4, which suppress its function through ubiquitin-mediated degradation and transcriptional inhibition. A subset of ribosomal proteins initiates the p53 response to impaired ribosome biogenesis. The ability of some ribosomal proteins to control MDM2 and MDM4 activities, and thereby p53, underscores an intriguing aspect of cell biology: proteins primarily known for their roles in ribosome function can exert extra-ribosomal functions. One notable example is the cellular RNA-protein complex involving RPL5, RPL11, and 5S rRNA (5S RNP) which inhibits MDM2 and stabilizes p53. Another RP, RPL22, is frequently mutated in cancers with microsatellite instability and its paralog RPL22L1 is often amplified. Recent studies have revealed that RPL22 directly modulates the alternative splicing of MDM4 to promote p53 activation, suggesting that the ribosomal protein-p53 relationship is more complex than previously thought. Cellular responses to ribosome biogenesis inhibition extend beyond general alterations in transcription and translation to actively determine cancer cell fate by selectively engaging tumor-suppressor pathways. RPL22's effect on MDM4 and other mRNA splicing events is a striking example. A better understanding of the mechanisms involved could guide the development of improved cancer treatments.

Ribosome-directed cancer therapies: the tip of the iceberg?

Ribosomes and ribosome biogenesis (RiBi) are universally corrupted in cancer, fueling the high rates of translation that sustain malignancy and creating opportunities for discriminating therapeutic intervention. Despite longstanding recognition of the promise of ribosome-directed cancer therapies, only a handful of such agents have been used in the clinic, and with limited success, and the true potential of this approach is unknown. In the past few years, however, understanding of cancer ribosome specialization and the intricacies of RiBi have advanced dramatically, opening opportunities that could not be imagined when existing agents were discovered. Here, we discuss the rationale for targeting ribosomes to treat cancer, review the limitations of current agents, and highlight an important set of recent discoveries we propose could be exploited to discover molecularly-targeted ribosome-directed cancer therapeutics.

MDM4 exon skipping upon dysfunctional ribosome assembly

Recent studies revealed how nucleolar stress enhances MDM4 exon skipping and activates p53 via the ribosomal protein L22 (RPL22; eL22). Tumor-associated L22 mutations lead to full-length MDM4 synthesis, overcoming tumor suppression by p53. This forum article explores how MDM4 splicing patterns integrate stress signaling to take p53-dependent cell fate decisions.

SURF2 is a MDM2 antagonist in triggering the nucleolar stress response

Cancer cells rely on high ribosome production to sustain their proliferation rate. Many chemotherapies impede ribosome production which is perceived by cells as "nucleolar stress" (NS), triggering p53-dependent and independent pathways leading to cell cycle arrest and/or apoptosis. The 5S ribonucleoprotein (RNP) particle, a sub-ribosomal particle, is instrumental to NS response. Upon ribosome assembly defects, the 5S RNP accumulates as free form. This free form is able to sequester and inhibit MDM2, thus promoting p53 stabilization. To investigate how cancer cells can resist to NS, here we purify free 5S RNP and uncover an interaction partner, SURF2. Functional characterization of SURF2 shows that its depletion increases cellular sensitivity to NS, while its overexpression promotes their resistance to it. Consistently, SURF2 is overexpressed in many cancers and its expression level is an independent marker of prognosis for adrenocortical cancer. Our data demonstrate that SURF2 buffers free 5S RNP particles, and can modulate their activity, paving the way for the research of new molecules that can finely tune the response to nucleolar stress in the framework of cancer therapies.