It turns out that a key protein involved in fat metabolism does more than scientists originally thought. Instead of just releasing fat, it helps to maintain healthy adipose tissue and balance in the body. If it is missing or its function is disrupted, this can have surprisingly harmful consequences. This finding changes the way researchers look at obesity and metabolic diseases.
Why the Loss of HSL Does Not Lead to Weight Gain
Fat cells, also known as adipocytes, are not just passive stores of excess weight. They play an active role in controlling how the body uses and stores energy. Inside these cells, fat is packed into structures called lipid droplets, which serve as energy reserves that the body can draw on when needed, for example during the fasting phase between meals. To release this stored energy, the body relies on a protein called HSL. This protein acts like a switch. When energy levels drop, hormones such as adrenaline activate HSL and trigger the release of fat that can be used by organs throughout the body. At first glance, it may seem logical that without HSL, fat would accumulate as the body would have difficulty accessing its stored energy. In fact, HSL (hormone-sensitive lipase) is a key enzyme in lipolysis, the breakdown of stored triglycerides into free fatty acids. However, studies on mice and humans with mutations in the HSL gene show a surprising result: instead of gaining fat, these individuals actually lose fat.
The reason for this lies deeper in the biology of fat cells. Without functioning HSL, not only is the breakdown of fat impaired, but the normal development and maturation of adipocytes is also impaired. Fat cells can then no longer efficiently absorb lipids, store them and release them again when required. This impaired “lipid turnover” means that fat is not stored correctly in the adipose tissue. Instead, there is a general loss of fat mass, which ultimately leads to lipodystrophy – a disease in which the body lacks functioning adipose tissue. In addition, excess lipids that cannot be stored in adipose tissue are deposited in other organs such as the liver or muscles. This so-called ectopic fat deposition is metabolically unfavorable and can interfere with insulin action. This creates a paradoxical situation: despite low fat mass, those affected develop similar metabolic problems to people who are severely overweight.
Although obesity and lipodystrophy appear to be opposing diseases, they have one important thing in common: In both cases, fat cells do not function properly. While in obesity there are often too many but functionally impaired fat cells, in lipodystrophy there is a lack of sufficiently functional fat tissue. In both situations, the body’s ability to store lipids safely and release them in a controlled manner is limited. This dysfunction can lead to similar health problems, including insulin resistance, increased blood lipid levels and an increased risk of cardiovascular disease. So it’s not just the amount of adipose tissue that matters, but its quality and function. This finding fundamentally changes the way we look at metabolic diseases.
A Surprising Discovery Inside the Fat Cells
To better understand this unexpected behavior, researchers led by Dominique Langin at the University of Toulouse as part of the Institut des Maladies Métaboliques et Cardiovasculaires investigated where HSL acts inside fat cells. Until now, it was known that HSL is mainly located on the surface of lipid droplets, where it helps to break down fat. However, the new study brought something unexpected to light: HSL is also found in the nucleus of adipocytes, the part of the cell that controls gene activity. There, HSL does not appear to primarily break down fat, but is involved in the regulation of genes that are important for the function and development of fat cells.
“In the nucleus of adipocytes, HSL can interact with many other proteins and participate in a program that maintains an optimal amount of adipose tissue and keeps the adipocytes ‘healthy’,” explains Jérémy Dufau, co-author of the study. This discovery shows that HSL not only controls fat breakdown, but also plays an important role in how well fat cells function overall.
How HSL Moves Within the Cell
The researchers also discovered that the amount of HSL in the cell nucleus is carefully regulated and reacts dynamically to the body’s energy state. Adrenaline, which activates HSL to release fat, fulfills a dual function: it not only activates lipolysis (the breakdown of fat) in the cytoplasm, but also signals the protein to leave the cell nucleus. This export from the cell nucleus takes place via specific transport mechanisms of the cell and is closely linked to signaling pathways that are activated by a lack of energy, for example during fasting or physical activity.
When HSL leaves the cell nucleus, the regulation of certain genes changes, particularly those involved in the differentiation and function of adipocytes. In contrast, studies in obese mice show that larger amounts of HSL remain in the nucleus. This indicates that the fine-tuned balance between nuclear and cytoplasmic HSL is disturbed in metabolic diseases. Such a maldistribution could lead to genes not being regulated correctly, which in the long term impairs the function of fat cells and contributes to the development of insulin resistance or other metabolic disorders.
A New Role for a Known Fat Enzyme
“HSL has been known as a fat-mobilizing enzyme since the 1960s. But we now know that it also plays an essential role in the nucleus of fat cells, where it contributes to the maintenance of healthy adipose tissue,” summarizes Dominique Langin. This newly identified function goes beyond pure fat degradation: in the cell nucleus, HSL appears to be involved in the regulation of genes that determine how fat cells grow, differentiate and respond to hormonal signals.
This helps to explain why people with a deficiency of HSL develop lipodystrophy. Without the nuclear function of HSL, a crucial control mechanism that ensures that fat cells can mature properly and fulfill their storage function is apparently missing. This not only leads to a lack of adipose tissue, but also to an incorrect distribution of lipids in the body, for example in the liver or muscles, where they can have a harmful effect.
These findings open up new ways of understanding metabolic diseases, including obesity and its complications. In particular, future therapies could aim to specifically influence the nuclear function of HSL instead of just modulating the breakdown of fat. This brings into focus the idea of not only reducing adipose tissue, but also specifically “repairing” or optimizing its function – an approach that could be more effective in the long term than pure weight reduction.
Why this Discovery is Important Right Now
The timing of this discovery is significant as metabolic diseases are on the rise worldwide. In France, around one in two adults is already overweight or obese, and globally it is estimated that around 2.5 billion people are affected. This development poses enormous challenges for healthcare systems, as obesity significantly increases the risk of serious diseases such as type 2 diabetes, cardiovascular disease and fatty liver, and can also affect quality of life and life expectancy.
Against this background, the new findings on the role of HSL are particularly relevant because they support a fundamental rethink: Obesity is increasingly understood not only as a problem of energy quantity (i.e. “too many calories”), but also as a disorder of adipose tissue function. The discovery that HSL is involved in the regulation of genes in the cell nucleus indicates that the “quality” and functionality of fat cells is just as important as their quantity.
This opens up new perspectives for medicine. Current therapies often focus on weight reduction or inhibiting fat absorption. In future, treatment approaches could focus more specifically on improving or restoring the function of fat cells. This could be particularly important for patients for whom traditional measures such as dieting or exercise alone are not sufficient or whose metabolism is already severely impaired.
The research also provides important information for prevention. If it is better understood how fat cells remain “healthy” and which molecular processes play a role in this, strategies could be developed at an early stage to prevent malfunctions – for example through targeted medication or personalized lifestyle interventions. Overall, this discovery shows that progress in basic research is crucial in order to combat complex widespread diseases such as obesity more effectively in the long term. It shifts the focus from pure weight loss to a deeper understanding of the biological mechanisms that determine our metabolic health.