TMPRSS6 Cleavage of β-Klotho Modulates FGF19 Signaling

The human liver is a tireless chemical factory, constantly filtering blood, storing energy, and managing the fats that fuel our bodies. When this factory becomes clogged with excess fat, a condition known as metabolic dysfunction-associated steatotic liver disease, it can lead to severe inflammation and scarring. For decades, the primary treatment for this condition has been lifestyle changes, but these are difficult for many people to maintain long-term. Scientists are now searching for new drugs to help, and one promising target has emerged from an unexpected place: a protein called TMPRSS6. This protein is a specialized enzyme, a type of molecular machine that cuts other proteins to regulate them. It is already famous for its role in managing iron levels in the body, but recent clues suggest it also plays a hidden part in how the liver handles fat. The question driving new research is simple yet critical: how does this iron manager influence liver fat, and could stopping it help treat liver disease?

To answer this, researchers at the Université de Sherbrooke in Canada set out to find the specific targets that TMPRSS6 cuts. They knew that because TMPRSS6 sits on the surface of liver cells, it likely chops up other proteins that are also on the surface or floating just outside the cell. To see what it was cutting, the team grew human liver cells in a dish and forced them to produce extra TMPRSS6. They then collected the liquid surrounding these cells and analyzed the proteins floating in it using a highly sensitive technique called mass spectrometry, which acts like a molecular scale to weigh and identify thousands of different proteins at once. By comparing the liquid from cells with extra TMPRSS6 against cells without it, they looked for proteins that appeared or disappeared. Among the many candidates, one stood out: a protein called beta-klotho. This protein is not an enzyme itself but acts as a crucial helper, or co-receptor, for two important signaling molecules called FGF19 and FGF21. These signaling molecules are like messengers that tell the liver to burn fat and stop making new fat. If beta-klotho is damaged or removed, these messages cannot get through, and the liver may begin to accumulate fat.

The researchers then tested whether TMPRSS6 actually interacts with and cuts beta-klotho. They placed both proteins together in a different type of cell, one that does not naturally produce them, to observe their behavior in isolation. They found that when TMPRSS6 was present, it physically grabbed onto beta-klotho and chopped off a large piece of it. This process, known as shedding, releases the cut piece into the surrounding liquid and leaves the remaining part of beta-klotho stuck on the cell surface. Crucially, this cutting only happened when TMPRSS6 was active; when the researchers used a version of the enzyme that had been chemically disabled, the cutting stopped. They also confirmed that this was a direct action by TMPRSS6 and not a side effect caused by other enzymes in the cell. By using specific inhibitors that block TMPRSS6, they showed that the cutting ceased, proving that the enzyme itself was responsible for the damage.

To understand exactly where the cut happened, the team analyzed the fragments of beta-klotho that were released. They identified two specific spots where the enzyme made its incision. One cut occurred in a region that seemed to interfere with how the protein is normally processed by other enzymes in the cell. The second cut happened at a precise location on a specific amino acid, a building block of the protein chain, which is a known favorite target for this type of enzyme. These findings confirmed that TMPRSS6 does not just brush against beta-klotho; it actively dismantles it. The researchers then checked what this meant for the cell's surface. They found that when TMPRSS6 was active, the amount of full, intact beta-klotho sitting on the cell surface dropped dramatically. The cell was left with fewer of these essential helpers available to receive signals.

The final piece of the puzzle was to see if this loss of beta-klotho actually changed how the cell responded to fat-regulating signals. The team set up an experiment where they could measure how strongly the cell reacted when stimulated by the FGF19 messenger. In cells equipped with normal levels of beta-klotho, the signal was strong and clear. However, in cells where TMPRSS6 was active and had chopped up the beta-klotho, the signal was significantly weaker. The cell's ability to respond to the fat-burning command was reduced by about 20 percent. This suggests a direct chain of events: TMPRSS6 cuts beta-klotho, which removes the helper needed for the fat-burning signal, which in turn weakens the liver's ability to manage its fat levels.

This study provides a new and concrete explanation for how TMPRSS6 might contribute to liver disease. While it was already known that this enzyme regulates iron, these findings reveal a second, independent pathway where it disrupts the liver's fat metabolism. The research suggests that by cutting up the beta-klotho helper, TMPRSS6 effectively silences the liver's natural fat-burning instructions. This discovery does not prove that blocking TMPRSS6 will cure liver disease in humans, as the experiments were conducted in cell cultures rather than living organisms. However, it offers a strong mechanistic reason to believe that inhibiting this enzyme could preserve the liver's ability to process fat. For scientists developing new treatments, this provides a clear target: if they can stop TMPRSS6 from cutting beta-klotho, they might be able to keep the liver's fat-burning signals active, offering a potential new strategy to fight the growing global burden of fatty liver disease.