For decades, scientists believed they understood one of the body’s key fat-burning proteins. Known as hormone-sensitive lipase, or HSL, the enzyme was thought to work mainly as the body’s emergency fuel switch, helping release stored fat when energy runs low.
But researchers uncovered something unexpected. HSL was not just working on the surface of fat droplets inside fat cells. It was also operating deep inside the nucleus of those cells, where DNA is stored and important genetic activity is controlled. The discovery revealed an entirely different side to a protein scientists had studied since the 1960s.
The findings, published in Cell Metabolism, helped solve a long-standing mystery in obesity research and opened new directions for understanding diabetes, heart disease, and other metabolic disorders.
Fat Cells Do Far More Than Store Calories
Fat cells, also called adipocytes, are often viewed as passive storage containers for excess calories. In reality, they are highly active cells that help regulate the body’s entire energy system.
Inside adipocytes, fat is stored in structures called lipid droplets. When the body needs fuel between meals or during fasting, hormones such as adrenaline trigger the release of that stored energy. HSL plays a central role in this process by breaking down triglycerides into fatty acids that other organs can use for fuel.
Scientists long assumed that removing HSL would prevent fat breakdown and lead to obesity. Surprisingly, that is not what happened.
Studies in both mice and people with mutations in the HSL gene showed the opposite effect. Instead of accumulating extra fat, they developed lipodystrophy, a rare condition in which the body loses healthy fat tissue.
That contradiction puzzled researchers for years.
Obesity and Dangerous Fat Loss Share Similar Problems
Although obesity and lipodystrophy seem completely different, they can produce many of the same health complications.
In obesity, fat tissue becomes enlarged and dysfunctional. In lipodystrophy, the body lacks enough properly functioning fat tissue. In both cases, adipocytes fail to regulate energy normally, which can contribute to insulin resistance, type 2 diabetes, fatty liver disease, inflammation, and cardiovascular problems.
This overlap suggested that healthy fat tissue is not simply about how much fat the body carries. The quality and function of fat cells may be just as important.
Researchers at the Institute of Cardiovascular and Metabolic Diseases (I2MC) at the University of Toulouse wanted to understand why the loss of HSL caused fat tissue to break down instead of build up. What they found changed the scientific picture of fat metabolism.
Scientists Discover HSL Inside the Cell Nucleus
The research team, led by Dominique Langin, discovered that HSL was located in an unexpected place inside adipocytes: the nucleus.
The nucleus acts as the cell’s control center. It contains DNA and regulates which genes are switched on or off. Proteins found in the nucleus often help control cell growth, repair, metabolism, and communication.
“In the nucleus of adipocytes, HSL is able to associate with many other proteins and take part in a program that maintains an optimal amount of adipose tissue and keeps adipocytes ‘healthy’,” explained Jérémy Dufau, co-author of the study.
Researchers found that nuclear HSL appears to help regulate important cellular systems, including mitochondrial activity and the extracellular matrix, which provides structural support for tissues.
Mitochondria are often called the power plants of cells because they generate energy. The extracellular matrix helps maintain the shape and integrity of tissues. Problems in either system have been linked to obesity, inflammation, and metabolic disease.
A Protein With Two Very Different Jobs
The study showed that HSL behaves differently depending on where it is located inside the cell.
On lipid droplets, HSL acts as an enzyme that helps release stored fat during fasting or exercise. In the nucleus, however, it appears to work more like a regulator that helps maintain healthy adipose tissue.
Researchers also discovered that the amount of HSL inside the nucleus changes in response to the body’s metabolic state.
During fasting, adrenaline activates HSL and pushes it out of the nucleus so it can help mobilize fat stores. In obese mice fed a high-fat diet, nuclear HSL levels increased.
The protein’s movement appears to be controlled by signaling pathways involving TGF-β and SMAD3, molecules already known to influence inflammation, tissue remodeling, and metabolic disease.
Scientists also found evidence that nuclear HSL interacts with proteins involved in gene expression and RNA processing, suggesting it may directly influence how fat cells function at a genetic level.
Why the Discovery Matters
The findings helped explain why complete HSL deficiency causes lipodystrophy instead of obesity. Without HSL in the nucleus, fat cells may lose their ability to stay healthy and properly maintain adipose tissue.
“HSL has been known since the 1960s as a fat-mobilizing enzyme. But we now know that it also plays an essential role in the nucleus of adipocytes, where it helps maintain healthy adipose tissue,” Langin said.
The discovery may also help researchers better understand why some obesity treatments succeed while others fail. Many current therapies focus mainly on reducing fat mass. But the study suggests preserving healthy fat tissue function could be equally important.
Scientists are increasingly recognizing that adipose tissue acts as a complex endocrine organ that communicates with the brain, liver, muscles, and immune system through hormones and signaling molecules. Dysfunctional fat tissue can disrupt the body far beyond weight gain alone.
Obesity Remains a Global Health Challenge
The research arrives as obesity rates continue to rise worldwide. According to global estimates, billions of people are now overweight or obese, increasing the risk of diabetes, heart disease, stroke, sleep apnea, and some cancers.
Researchers hope that understanding how proteins like HSL regulate fat cell health could eventually lead to more targeted therapies for metabolic disease.
Instead of simply trying to eliminate fat, future treatments may focus on restoring the normal function of adipocytes and protecting the biological systems that keep fat tissue healthy in the first place.
