In Simple Terms
Researchers have found a process in the liver that may make it harder for the body to remove bad cholesterol from the blood when food contains a lot of cholesterol. In mice, blocking one part of that process left more cholesterol-grabbing receptors in place and lowered cholesterol in the blood. A drug that might block the process has been tested for safety in people, but has not yet been shown to lower cholesterol in humans.
The Liver Is a Key Filter for the Blood
Low-density lipoprotein, or LDL, travels through the bloodstream. Liver cells can take it up and process it. Special receptors on the surface of these cells act like docking points: they bind LDL particles and help bring them into the cell. The more of these receptors the liver has available, the more effectively it can remove LDL from circulation.
That means LDL levels depend not only on how much cholesterol is in the blood, but also on how well the liver’s uptake system is working. If the number of receptors falls, or if they do not remain available for as long, the liver becomes less efficient at clearing LDL. This helps explain how drugs such as statins and PCSK9 inhibitors work: they help preserve or increase the availability of LDL receptors, supporting the removal of cholesterol from the blood.
Still, the treatments currently available do not bring everyone’s cholesterol down to the desired level, the researchers note. Some people may need additional options because existing treatments do not work well enough for them or because other barriers prevent them from reaching safer levels. That makes it valuable to investigate other processes that affect the number of LDL receptors, rather than focusing only on mechanisms that are already well known.
The Chain of Events Begins with a Protein Called Ral
In a study published in Nature, a team from the University of California San Diego School of Medicine investigated a previously unknown mechanism using experiments in mice and human cells. The researchers found that a high intake of dietary cholesterol activates a protein called Ral. The study’s lead researcher was Alan Saltiel, a professor of medicine at the school and director of the Joint Diabetes Research Center at the University of California San Diego and the University of California, Los Angeles.
Ral was not a new subject for Saltiel and his colleagues; the team had previously studied the protein in fat cells. In the new work, they found that it was connected to a chain of events affecting the liver’s ability to handle LDL. According to the results, higher Ral activity is associated with fewer available LDL receptors, weakening the cells’ ability to take cholesterol out of the blood.
The central idea is that dietary cholesterol may do more than increase the amount of cholesterol entering the body. It may also change how liver cells regulate the machinery they use to remove LDL. Over time, this process could reduce the number of receptors the liver needs to capture LDL particles. The findings do not mean that every cholesterol-rich meal immediately causes a noticeable change. Rather, they describe a biological mechanism observed in the researchers’ experiments—one whose implications for human health still need to be tested in clinical trials.
An Enzyme Helps Drive the Loss of Receptors
After tracing signals linked to Ral, the team identified another part of the process: an enzyme called cathepsin A, or CTSA. The researchers found that CTSA contributes to the chain of events that ends with fewer LDL receptors. This makes CTSA a possible point of intervention. If its activity can be blocked, the receptors might remain available for longer, allowing liver cells to regain some of their ability to clear cholesterol.
The scientists tested that possibility with a small molecule that inhibits CTSA. In mice, blocking the enzyme made LDL receptors more stable and led to a substantial drop in circulating LDL. This result gives the pathway significance beyond a theoretical explanation: it suggests that affecting one of its components can change measures related to cholesterol removal in an animal model.
But a successful experiment in mice is not the same as proving that a treatment works in people. The study does not show that the drug lowered cholesterol in patients, nor does it provide clinical evidence that inhibiting CTSA is safe and effective for that purpose. It offers a promising biological basis and a preclinical result worth following up—not a treatment recommendation that can be put into practice today.
A Different Target from Those Used by Current Drugs
The discovery also matters because the Ral–CTSA pathway is separate, according to the study, from the mechanisms targeted by established cholesterol-lowering treatments. A distinct target could be important for people who do not respond well enough to available drugs. In theory, it could offer an additional treatment strategy rather than repeating the same approach.
But calling a target new does not mean a treatment is ready. Showing that a pathway contributes to the loss of LDL receptors is not enough to determine the right dose, how long a treatment should be used, or which groups of people might benefit. The results also do not establish whether CTSA inhibition would help on its own or in combination with other treatments. Those questions require studies designed to assess its effects in people and monitor potential side effects.
It is also important to distinguish the discovery of a new mechanism from an explanation for every case of high cholesterol. LDL levels can be influenced by many factors, while this study focuses on one pathway linked to the effects of dietary cholesterol on the liver’s ability to remove LDL. The discovery may add one piece to a broader picture of how cholesterol is regulated, but it does not reduce the whole process to a single protein or enzyme.
An Existing Experimental Drug Could Shorten the Road
Moving from the discovery of a biological target to a suitable medicine often takes years. Researchers need to find a compound that can affect the target, then assess its safety and other properties before testing it more widely. In this case, the team had reason for cautious optimism: an experimental drug that inhibits CTSA had already been developed, although it was being investigated as a possible treatment for heart failure. That development program was later halted for strategic reasons.
Before the program stopped, the drug had reached a Phase 1 clinical trial, a stage designed primarily to assess safety in humans. The researchers say the drug passed the safety evaluation at that stage. Previous testing in people could provide a practical starting point and mean that some parts of development do not have to begin from scratch. But it does not show that the drug lowers LDL or that it is suitable for this use.
The team believes the compound could be a candidate for a Phase 2 trial to investigate its potential effectiveness against high cholesterol. Phase 2 testing differs from an initial safety assessment: it examines whether a treatment has an effect in patients while continuing to evaluate its safety. However, the source does not say that such a trial has begun, and it gives no expected start date.
Nor does a drug’s safety in an earlier trial for a different condition automatically mean that its balance of benefits and risks will be appropriate when used to lower cholesterol. The dose, length of treatment, or characteristics of participants in future trials may differ. Any move to the next stage would therefore require a clinical trial designed to answer questions about effectiveness, tolerability, and dosage—not an assumption that earlier results have already answered them.
What We Know—and What Remains Uncertain
The study’s evidence comes from two connected sets of findings. Experiments in human cells and mice revealed a link between dietary cholesterol, Ral and CTSA activity, and the fate of LDL receptors. The researchers then tested an enzyme inhibitor and found that receptors became more stable and LDL fell noticeably in mice. Together, these results give researchers a pathway to investigate, but they do not amount to proof that the treatment will work in people.
The next question for the field is whether the effect seen in experimental models can translate into a measurable benefit for patients. Future studies will need to establish who might benefit, whether the effect lasts, and how the treatment compares with or could complement existing therapies. The published material does not report the number of participants or results from a human trial testing whether the drug lowers cholesterol, because, according to the available information, the drug has not yet been tested for that purpose.
The research involved scientists from the University of California San Diego, the University of California San Francisco, UT Health San Antonio, and the University of Utah. The contributors included Xue Feng, Xu Chang, Yuchi Wang, Twisha Kurlagonda, Alyssa Set, Sadatsugu Sakani, Se Young Park, Choraiboon Wisesaowapak, Kylie Nguyen, Jimmy Yan, Himani Bothulu, Catherine Dinh, Felicia Chu, Yuyao Ren, Bishen Zhang, Patrick Secrest, Zhao-Wei Hong, Preethi Veeragandham, Yulia Skorobogatko, Tatiana Kisileva, and Amit R. Majithia from the University of California San Diego. Priya Jaishankar, Bosu Yang, and Adam Renslo were from the University of California San Francisco; Linming Han and Peng Zhao were from UT Health San Antonio; and Philip L. S. M. Gordts was from the University of Utah.
The research was partially funded by the National Institutes of Health through grants P30DK063491, R01DK117551, R01DK128796, and R01DK135289, as well as by an American Diabetes Association postdoctoral fellowship, number 1-25-PDF-76. These details place the work within a multi-institution research effort, but they do not change the central conclusion: the pathway is promising, but a treatment for people has yet to be tested.
The Bottom Line
The study published in Nature describes a possible way dietary cholesterol can weaken the liver’s ability to clear LDL. Activation of the protein Ral is linked to a process involving the enzyme CTSA, which ends with fewer LDL receptors. When the researchers blocked CTSA in experiments, the receptors remained more stable and LDL levels fell substantially in mice. An experimental drug that has already undergone a Phase 1 safety assessment might help move research toward testing whether the approach works, but it has not yet been shown to lower cholesterol in people. For now, this is a potential new treatment avenue, not a ready-to-use therapy. Clinical trials will determine whether this biological target can lead to another option for patients.