One-carbon metabolism controls colorectal cancer cell state and chemosensitivity through chromatin remodeling

Colorectal cancer is a disease where the lining of the colon grows out of control, and for decades, doctors have fought it with a drug called 5-fluorouracil. This medicine works by stopping cells from making the building blocks they need to copy their DNA, effectively halting their ability to multiply. To make this drug work better, it is almost always given alongside a form of folic acid, a common vitamin. The traditional explanation for this pairing is simple: the vitamin acts like a chemical wedge, helping the drug lock onto its target and stay there longer. However, this approach has always been a bit of a gamble. While it helps some patients, it does not help everyone, and scientists have long wondered why the same treatment works so differently in different people. The missing piece of the puzzle may not be the drug itself, but rather the internal environment of the cancer cells before the treatment even begins.

A new study from researchers in Hungary reveals that folic acid does more than just help a drug stick to its target; it fundamentally changes the identity of the cancer cells. In the human body, cells are not static; they can shift between different states, much like a person changing their role in a family. Some cells act like stem cells, which are young, flexible, and capable of becoming many different things, while others are fully grown and specialized, performing a specific job. The researchers found that the amount of folic acid available to a cancer cell determines which of these two identities it adopts. When folic acid is plentiful, the cancer cells tend to mature into a specialized, differentiated state. When folic acid is scarce, the cells remain in a younger, stem-like state that is more resistant to change.

The team tested this idea using human colorectal cancer cells in the lab. They created a special system where stem-like cells glowed one color and mature cells glowed another, allowing them to watch the population shift in real time. When they added folic acid to the cells, the population rapidly changed. Within hours, the cells began turning off the genes that keep them in a stem-like state and turning on the genes that make them mature. By the time two days had passed, the balance had tipped significantly: there were far fewer stem-like cells and many more mature ones. This shift happened not just in a single type of cell line, but also in patient-derived organoids, which are tiny, three-dimensional models of tumors grown from actual patient tissue.

This change in identity had a direct and powerful effect on how the cells responded to chemotherapy. The researchers treated the cells with 5-fluorouracil under different conditions. They found that the cells which had been exposed to folic acid and had become mature were much more sensitive to the drug. In fact, the amount of drug needed to kill half of the cells dropped dramatically when folic acid was present. Conversely, when the researchers deprived the cells of folic acid, the cells stayed in their stem-like state and became much harder to kill, requiring significantly higher doses of the drug to achieve the same result. This proved that the vitamin's effect was not just about helping the drug bind to a target; it was about preparing the battlefield by changing the nature of the enemy.

To understand how this happened, the scientists looked inside the cells at their genetic machinery. They discovered that folic acid acts as a fuel for a process called methylation, which is a chemical tag that cells place on their DNA to control which genes are turned on or off. When folic acid was available, the cells had a higher capacity to add these tags. Specifically, the researchers saw a rapid increase in a type of tag that acts as a "stop" signal for genes associated with stemness. This process required a specific enzyme, which the team confirmed by blocking it and seeing that the cells could no longer respond to folic acid. The result was a reorganization of the cell's internal architecture, closing down the pathways that keep the cell in a flexible, resistant state and opening up the pathways for maturation.

The study also traced how these changes affected the cell's behavior after treatment. When cells were treated with the drug, those that had been primed with folic acid were more likely to die or stop dividing permanently, a state known as senescence. In contrast, cells that were starved of folic acid were more likely to survive the initial attack and eventually grow back. The researchers measured this by counting how many new colonies formed from the survivors. They found that folic-deprived cells produced many small, stubborn colonies, while folic-replete cells produced fewer, larger colonies that were less likely to cause a relapse. This suggests that the vitamin influences not just whether a cell dies immediately, but also the long-term fate of the cells that survive the treatment.

Perhaps most importantly, the researchers connected these laboratory findings to human disease. They analyzed data from actual patients, comparing the genetic activity in normal colon tissue, pre-cancerous growths called adenomas, and full-blown cancer. They found that the pattern seen in the lab was mirrored in the patients: cancerous tissue showed high activity of the stem-like program and low activity of the mature program, while normal tissue showed the opposite. The pre-cancerous adenomas sat in the middle, showing a mix of both. This alignment suggests that the shift from a mature state to a stem-like state is a key part of how colorectal cancer develops and becomes resistant to treatment.

The findings challenge the long-held view that folic acid is merely a helper for chemotherapy. Instead, it appears to be an instruction manual for the cell, telling it whether to stay in a flexible, resistant state or to mature into a vulnerable one. The study shows that the timing and availability of this vitamin can reshape the tumor's identity before the drug is even administered. While the researchers do not claim that changing folic acid levels will immediately cure cancer, their work provides a clear explanation for why the same treatment works differently in different contexts. It suggests that the success of chemotherapy may depend heavily on the metabolic state of the tumor at the moment treatment begins, offering a new way to think about how we might tailor these therapies to be more effective for every patient.