Set against the backdrop of an increasingly overweight population, the 1994 discovery of the fat-regulating protein leptin was widely heralded as a boon for obesity research. The hormone continues to be a focus of investigation. Findings published online this week by the Proceedings of the National Academy of Sciences indicate that increasing leptin levels in the body can fundamentally change the nature of fat cells¿from idle storage containers to fat-burning machines.

Roger Unger of the Touchstone Center for Diabetes Research at the University of Texas and his colleagues studied rats injected with the leptin gene. Two weeks after the insertion--which raised the creatures¿ leptin levels to 50 times that of control animals--the researchers determined that their fat cells had changed substantially. "This is the first careful examination of the fat cells after leptin therapy," Unger remarks. "The structure of the cells change from the normal appearance of a fat cell to a very novel cell that¿s really never been seen before." (The image above shows cells from a control rat (left) and those from a leptin-treated animal (right).

The team determined that the change resulted from an increase in activity in the mitochondria, the cell¿s energy-producing organelles. This increase enabled the cells to burn fat and, as a result, the animals lost about 26 percent of their body weight on average. Although current attempts to use leptin to treat obese people are still very experimental, Unger remarks that "the ability to convert fat cells into fat-burning cells may suggest novel therapeutic strategies for obesity."