Scientists have discovered a new tumor-suppressing mechanism by examining condensates and ribosome formation.
The body has natural processes that can hinder the growth and spread of cancer cells. Researchers have thoroughly investigated these processes, particularly those involving the tumor suppressor protein p53, because they play a vital role in fighting disease. Scientists at St. Jude Children’s Research Hospital have revealed a previously unknown tumor suppression mechanism by studying proteins that regulate p53. The p14 Alternative Reading Frame protein (p14ARF), typically present in low amounts in cells, shows increased levels when cells face oncogenic stress and activates p53. The team uncovered an alternative tumor suppression pathway for p14ARF, demonstrating how the formation of condensates and the disruption of ribosome production contribute to this process. Their work was published in Nature Communications.
The researchers found that when p14ARF is expressed more, it leads to phase separation with various cellular elements. This happens in the nucleolus, a compartment in the cell’s nucleus responsible for making ribosomes and lacking defined membranes. In the nucleolus, p14ARF interacts with nucleophosmin, a crucial ribosome production protein, and they together form a gel-like mixture. This results in reduced nucleophosmin activity, disrupted ribosome production, and potential cell toxicity. The initial action of p14ARF highlights a probable new pathway for tumor suppression.
p14ARF causes disruptions in the nucleolus
“Typically, p14ARF is only found in low quantities in healthy cells, but its levels increase when oncogenic stress occurs, like the upregulation of MYC or other oncogenes,” said Richard Kriwacki, PhD, a lead author from St. Jude Department of Structural Biology.
Kriwacki’s research into p14ARF began when they noticed that with increased expression, p14ARF would target the nucleolus. By binding to nucleophosmin, p14ARF disrupts the normal functions of the revered ribosome factory in the nucleolus.
“When overexpressed, p14ARF can localize to the nucleolus and impede nucleophosmin from functioning as a chaperone, preventing the transport of pre-ribosomal particles,” said Eric Gibbs, the first author from the Department of Structural Biology. “As a result, nucleophosmin remains stuck in the nucleolus, causing ribosomal particles to become trapped as well.”
Gibbs and Kriwacki previously demonstrated that p14ARF and nucleophosmin form biomolecular condensates in isolation. These condensates, often appearing as small droplets under the microscope, regulate numerous biological processes, from gene transcription to cellular signaling, by concentrating certain biomolecules like DNA, RNA, and proteins temporarily. However, participating in phase separation often requires specific sequence characteristics, or a “ticket to ride.”
Networking by p14ARF is crucial for tumor suppression
To further explore the nature of p14ARF-nucleophosmin condensates, Gibbs and Kriwacki utilized advanced biophysical techniques like small-angle neutron scattering at Oak Ridge National Laboratory and nuclear magnetic resonance spectroscopy. “We discovered that p14ARF not only phase separates with nucleophosmin but also builds a larger, symmetrical network structure,” Gibbs explained. “This structure limits the movement dynamics of both p14ARF and nucleophosmin within the condensates.”
Interestingly, although p14ARF is typically considered an intrinsic disordered protein with no stable structure, the researchers found underlying organized features within the condensates that support network formulation. “We discovered that hydrophobic regions in the secondary structure create different crosslinks, establishing the network and giving the condensate its distinctive properties,” Gibbs said. This structured network contributes to the gel-like state of the condensates, essentially freezing nucleophosmin in place.
This research uncovers a new pathway through which p14ARF aids in tumor suppression. Also, since condensates usually operate based on fluid dynamics, with solidification being linked to disease, this study offers a unique viewpoint on biomolecular condensation.
“It’s intriguing to see that p14ARF acts as a tumor suppressor by effectively immobilizing the liquid-like nucleolus and seemingly hindering vital stages in ribosome production,” Kriwacki commented. “Now we can understand p14ARF‘s role as a tumor suppressor from the perspective of what is usually viewed as harmful within the nucleolus.”
