When cell division (mitosis) takes too long, it can be a sign that something is wrong with the cells, for example DNA damage or chromosomal instability. That's why our cells come with an innate ability to tell the time, with a stress response known as the mitotic stopwatch pathway activating after prolonged mitosis, and triggering cell cycle arrest, or even cell death. Now, researchers at the Okinawa Institute of Science and Technology (OIST) have found that certain cancers can 'lose their sense of time' to avoid cellular stress responses, revealing new insights that could shape future anti-cancer therapeutics.
The research published in Nature Communications focuses on a protein called ubiquitin-specific protease 28 (USP28), which is involved in the mitotic stopwatch pathway. USP28 forms part of a complex stabilizing another protein, p53, a known tumor suppressor. The team identified the molecular regions of USP28 that form the essential interactions which bind the complex together, and certain mutations that can help cancers to evade this stopwatch effect.
Professor Franz Meitinger, head of the Cell Proliferation and Gene Editing Unit at OIST and corresponding author on the study, explains, "Cells always need to be ready to respond to damage. They constantly produce p53, a protein pivotal to many stress response pathways, responsible for triggering cell cycle arrest or cell death when present in sufficient concentration. It degrades very quickly, so it only reaches sufficient levels when stabilized, for example by being bound into a complex. If this stabilization is blocked, cells can continue dividing, as is the case for cancer. Our goal is to understand the molecular foundations of cancer, to support future treatment efforts."
Putting USP28 under the microscope
Using a combination of experimental methods including live cell imaging and single-cell tracking, alongside theoretical methods such as AlphaFold protein structure prediction, the team revealed the molecular basis of USP28 activity.
"We found that the C-terminus of USP28 is involved in binding," says Dr. Hazrat Belal, first author on the study. The C-terminus is one of the ends of a protein chain, characterized by a particular molecular group called a carboxyl. "In many cancers, USP28 has mutations in this region. Our investigations found that these mutations disrupted the protein complex formation, enabling continued cell division and cancer cell proliferation."
Understanding the Mitotic Stopwatch
This study is the latest a decade of research from the authors, who have previously reported on several aspects of the mitotic stopwatch pathway mechanisms . However, many mysteries remain.
"We don't yet know how cells can tell the time. How do they know when 30 or 60 minutes have passed, and when to trigger the mitotic stopwatch?" says Prof. Meitinger. "And what exactly does the complex look like? We have some structural information, but still lack the complete picture."
Developing anti-mitotic drugs
By shining new light on how cancer avoids the mitotic stopwatch pathway, the researchers hope that this information can help inform anti-cancer therapeutics. "Anti-mitotic agents such as paclitaxel are widely used in the treatment of many solid tumors. By uncovering the molecular mechanisms that control mitotic stress responses, we can inform the development of next-generation anti-mitotic therapies and identify which agents are most effective in specific biological or clinical contexts."
