For decades, scientists thought they had hormone-sensitive lipase (HSL) figured out. Known as the body's emergency fuel switch, HSL was believed to hang out on fat droplets inside fat cells, breaking down triglycerides when energy ran low. But researchers at the Institute of Cardiovascular and Metabolic Diseases (I2MC) at the University of Toulouse have discovered that HSL has been moonlighting deep inside the nucleus of those same cells, where it helps regulate genetic activity and maintain healthy fat tissue. The findings, published in Cell Metabolism, solve a long-standing mystery in obesity research and open new directions for understanding diabetes, heart disease, and other metabolic disorders.
Fat cells, or adipocytes, are often dismissed 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 lipid droplets, and when the body needs fuel, hormones like adrenaline trigger HSL to break down triglycerides into fatty acids. Scientists long assumed that removing HSL would prevent fat breakdown and lead to obesity. Instead, studies in both mice and people with HSL gene mutations showed the opposite: they developed lipodystrophy, a rare condition in which the body loses healthy fat tissue. That contradiction puzzled researchers for years.
Although obesity and lipodystrophy seem like opposites, they produce many of the same health complications: insulin resistance, type 2 diabetes, fatty liver disease, inflammation, and cardiovascular problems. This overlap suggested that healthy fat tissue isn't just about how much fat you carry, but how well your fat cells function. The Toulouse team, led by Dominique Langin, wanted to understand why losing HSL caused fat tissue to break down instead of build up. What they found changed the scientific picture of fat metabolism.
The research team discovered HSL in an unexpected place: the nucleus, the cell's control center where DNA is stored and genes are regulated. "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 co-author Jérémy Dufau. Nuclear HSL appears to help regulate mitochondrial activity and the extracellular matrix, both of which are linked to obesity, inflammation, and metabolic disease when they malfunction.
HSL behaves differently depending on its location. On lipid droplets, it acts as an enzyme releasing stored fat during fasting or exercise. In the nucleus, it works more like a regulator maintaining healthy adipose tissue. The amount of nuclear HSL changes with metabolic state: during fasting, adrenaline pushes HSL out of the nucleus to mobilize fat stores, while obese mice fed a high-fat diet showed increased nuclear HSL levels. The protein's movement appears controlled by signaling pathways involving TGF-β and SMAD3, molecules already known to influence inflammation and metabolic disease. Evidence also suggests nuclear HSL interacts with proteins involved in gene expression and RNA processing, directly influencing fat cell function at a genetic level.
The findings explain why complete HSL deficiency causes lipodystrophy instead of obesity: without HSL in the nucleus, fat cells lose their ability to stay healthy. "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 help researchers understand why some obesity treatments succeed while others fail, as many current therapies focus on reducing fat mass without preserving healthy fat tissue function.
As obesity rates continue to rise worldwide - with billions now overweight or obese, increasing risks of diabetes, heart disease, stroke, sleep apnea, and some cancers - researchers hope that understanding how proteins like HSL regulate fat cell health could lead to more targeted therapies. Instead of simply trying to eliminate fat, future treatments may focus on restoring normal adipocyte function and protecting the biological systems that keep fat tissue healthy in the first place.