A protein long understood as a fat-burning workhorse has been caught doing something entirely different inside fat cells, a finding that challenges fundamental assumptions about obesity and metabolism.
The protein, called HSL, has been known since the 1960s for its role breaking down stored fat when the body needs energy. Adrenaline activates it, triggering the release of fuel from lipid droplets within adipocytes. That much seemed settled science. But researchers at the University of Toulouse discovered HSL also operates in the nucleus of fat cells, where it regulates genes that keep adipose tissue healthy and functioning properly.
The implications are striking. People born without functional HSL do not gain weight as logic might suggest. Instead, they develop lipodystrophy, a condition of dangerously low fat mass. The same protein dysfunction appears in obesity, yet produces opposite symptoms. Both conditions damage metabolic health and increase cardiovascular risk, suggesting the real culprit is not fat itself but broken fat cell regulation.
"HSL has been known since the 1960s as a fat-mobilizing enzyme," said Dominique Langin, who led the research. "But we now know that it also plays an essential role in the nucleus of adipocytes, where it helps maintain healthy adipose tissue."
The research team mapped where HSL travels within the cell. When adrenaline spikes during fasting, it triggers HSL to leave the nucleus and work on lipid droplets instead. In obese mice, however, HSL lingers in the nucleus at higher levels, suggesting this balance goes awry in disease. Co-author Jérémy Dufau explained the nuclear function allows HSL to "associate with many other proteins and take part in a program that maintains an optimal amount of adipose tissue and keeps adipocytes healthy."
The discovery reframes how scientists should approach obesity treatment. Rather than targeting fat storage capacity, therapies might focus on restoring proper fat cell function through nuclear regulation. With roughly one in two French adults overweight or obese, and 2.5 billion people affected globally, the pressure to find new prevention and treatment angles is intense.
Author Jessica Williams: "This is a reminder that decades of research into a single enzyme can still miss the whole picture, and that sometimes the breakthrough comes from asking what else a familiar protein might be doing."
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