Activation of hepatic USP5 as a novel strategy to ameliorate metabolic dysfunction-associated steatotic liver disease by deubiquitinating CPT-1A

The liver is the body's central processing plant, a resilient organ that filters toxins, stores energy, and manages the complex chemistry of digestion. When this factory becomes overwhelmed by excess fat, a condition known as metabolic dysfunction-associated steatotic liver disease, or MASLD, can take hold. This is not merely a matter of having too much fat in the organ; it is a state where the liver's ability to burn that fat for energy breaks down, leading to inflammation, scarring, and potentially liver failure. For decades, the medical community has struggled to find effective treatments for this growing epidemic, which affects a significant portion of the global population. The search for a solution has turned toward the microscopic machinery inside our cells, specifically looking for the switches that control how the liver handles fat.

In a recent study, researchers have identified a specific protein that acts as a crucial guardian for liver health, and they have discovered a natural compound that can turn this guardian back on. The team focused on a protein called USP5, which functions as a cellular editor. In the complex world of cell biology, proteins are often tagged with small molecular markers called ubiquitin. These tags can act like a "destroy me" sign, sending the protein to the cell's recycling center to be broken down. USP5 is a specialized tool that can snip off these tags, effectively saving the protein from destruction. The researchers found that in livers affected by MASLD, the levels of this protective USP5 protein drop significantly. Without enough USP5, a vital enzyme responsible for burning fat is tagged for destruction and disappears, causing fat to pile up in the liver cells.

To understand the role of this protein, the scientists first looked at human liver samples from patients with the disease. They found that the more severe the liver condition, the lower the amount of USP5 present. To confirm this was a cause rather than just a symptom, they created mice that lacked the gene for USP5 specifically in their liver cells. When these mice were fed a diet high in fat and sugar, they developed much worse liver disease than normal mice, with severe fat accumulation, inflammation, and early signs of scarring. Conversely, when the researchers boosted the levels of USP5 in the livers of other mice, the animals were protected from these harmful effects, even when fed the same unhealthy diet. This proved that USP5 is essential for keeping the liver healthy and that its absence drives the disease forward.

The team then investigated how USP5 performs this protective work. They discovered that USP5 directly binds to a key enzyme called CPT-1A, which acts as a gatekeeper for the mitochondria, the power plants inside cells. CPT-1A is responsible for shuttling fatty acids into the mitochondria so they can be burned for energy. In a healthy liver, USP5 keeps CPT-1A safe by removing the "destroy me" tags that would otherwise lead to its breakdown. However, in the diseased state where USP5 is missing, CPT-1A is rapidly destroyed. Without this gatekeeper, fatty acids cannot enter the power plants to be burned, and they instead accumulate as toxic fat droplets within the liver cells. The researchers confirmed this link by showing that if they forced the liver cells to produce more CPT-1A, it could rescue the liver even when USP5 was missing, proving that CPT-1A is the primary target of USP5's protection.

Having established this mechanism, the researchers set out to find a way to reactivate USP5 in patients. They screened thousands of natural compounds to see which ones could bind to USP5 and boost its activity. One compound, a flavonoid called ligustroflavone, stood out. This substance, found in certain plants, was able to bind directly to USP5 and increase its ability to remove the destructive tags from CPT-1A. When the researchers tested this compound in mice with established liver disease, they saw remarkable results. The mice treated with ligustroflavone showed a significant reduction in liver fat, inflammation, and scarring. The treatment worked by restoring the levels of the CPT-1A enzyme, allowing the liver to resume burning fat effectively. Importantly, the compound appeared to be safe, causing no damage to the liver or other organs in the animals tested.

This work offers a new perspective on how to treat a disease that currently has very few effective drug options. By identifying USP5 as a critical regulator of liver fat metabolism and finding a natural molecule that can activate it, the study provides a clear path forward. The findings suggest that the key to unlocking the liver's ability to heal itself may lie in stabilizing the very enzymes that burn fat. While more research is needed to see if these results translate directly to humans, the discovery of ligustroflavone as a potential activator of this protective pathway represents a promising new direction in the fight against liver disease. It moves the focus from simply managing symptoms to restoring the fundamental biological processes that keep the liver functioning.