The KRAS G12C Inhibitor Divarasib Stabilizes RBM39 and Antagonizes Aryl-Sulfonamide Degraders
This study reveals that the KRAS G12C inhibitor divarasib noncovalently binds to and stabilizes the splicing factor RBM39, thereby antagonizing RBM39 degraders and modulating cellular cytotoxicity through an RBM39-INSR signaling axis.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Cancer often begins when a cell's internal switch gets stuck in the "on" position, driving it to grow and divide uncontrollably. One of the most common switches to get stuck involves a protein called KRAS, which acts as a molecular signal handler for the cell. For decades, scientists considered this protein impossible to target with drugs because it is smooth and slippery, offering no obvious place for a medicine to grab onto. However, researchers recently discovered a specific mutation in the KRAS gene that creates a tiny, sticky spot on the protein's surface. This breakthrough allowed for the development of drugs designed to lock onto that spot and turn the signal off. While these new drugs have shown great promise in treating lung cancer, patients respond to them in different ways, and some experience unexpected side effects. This variation suggests that these drugs might be doing more than just turning off the stuck switch; they may be interacting with other parts of the cell in ways that scientists have not yet fully understood.
In a new study, researchers set out to uncover these hidden interactions using a method that looks at how drugs change the physical stability of proteins inside a cell. They focused on a specific drug called divarasib, which is designed to target the mutated KRAS protein in lung cancer. While standard tests that look for chemical bonds between a drug and a protein only found the intended target, the researchers used a different approach that measures how well proteins hold their shape when exposed to stress. By treating cell samples with the drug and then gradually increasing the stress, they could see which proteins became more stable and resistant to breaking down. This technique revealed that divarasib was not just sticking to the KRAS protein; it was also binding to a completely different protein called RBM39. This finding was significant because standard tests had missed this interaction entirely, suggesting that the drug was engaging with the cell in a more complex way than previously thought.
The researchers confirmed that divarasib binds directly to RBM39 without forming a permanent chemical bond, acting more like a stabilizing hand that holds the protein together. In the cells, this binding had a clear effect: it increased the amount of RBM39 present and made the protein last longer. This discovery became even more important when the team looked at how divarasib interacted with a different class of cancer drugs known as degraders. These degrader drugs work by tagging RBM39 for destruction, effectively removing it from the cell to stop cancer growth. The study showed that divarasib acted as a shield against these degraders. When cells were treated with both the KRAS drug and a degrader, the KRAS drug prevented the degrader from destroying RBM39. This mutual interference meant that the two types of drugs canceled each other out, reducing the ability of either one to kill the cancer cells effectively.
Beyond simply protecting the protein, the researchers traced the consequences of this stabilization to a specific signaling pathway involving the insulin receptor. RBM39 is known to help control how genetic instructions are read and edited, and the study found that when divarasib stabilized RBM39, it changed how the insulin receptor gene was processed. Specifically, it influenced which version of the insulin receptor was produced, shifting the balance toward a form that is more common in healthy cells. When the researchers blocked this effect by reducing the levels of RBM39 or the insulin receptor, the cancer cells became less sensitive to the killing power of divarasib. This suggests that the drug's ability to stabilize RBM39 and alter insulin receptor signaling is a key part of how it works, and perhaps why it causes certain side effects or varies in effectiveness between patients.
The study highlights a critical limitation in how scientists currently search for drug targets. Traditional methods are excellent at finding where a drug forms a chemical bond, but they often miss interactions where a drug simply holds a protein in place without bonding to it. By using stability-based profiling, the researchers were able to see a hidden layer of drug activity that conventional tests overlooked. This work does not just identify a new target for divarasib; it provides a new way of thinking about how these powerful drugs function. It suggests that the success or failure of a treatment might depend on a delicate balance of multiple proteins, not just the one the drug was originally designed to hit. For patients and doctors, this means that understanding these secondary interactions could be essential for predicting who will benefit from a treatment and for designing better combinations of drugs that work together rather than against each other.
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