Beige fat cells, pictured on the left in green with red marking expression of UCP1, a gene that indicates their energy-burning capacity, compared with their alternate "white" state, pictured on the right. Photo by University of California San Francisco
SAN FRANCISCO, Aug. 25 (UPI) -- Scientists already know that white fat stores energy and brown fat burns it, but until now they didn't know why brown fat turns into white fat -- which could lead to cellular methods of controlling obesity and diabetes, according to a new study.
Scientists at the University of California San Francisco found beige fat in adults turns into white fat when cells devour their mitochondria, and that preventing this from happening protected mice from obesity and diabetes symptoms in experiments, according to a study published in the journal Cell Metabolism.
Mammals have two types of fat, each of which is designed for a different purpose -- white fat stores energy, and is linked with obesity and diabetes, while brown fat produces heat by burning energy.
Human babies are born with brown fat as a natural defense against cold but over time it turns to white. In adults, beige fat exists inside white fat and can convert white fat cells to the energy-burning state of brown fat but reverts to white fat at some point.
"So we knew beige fat tends to disappear, but we wanted to know why," Svetlana Altshuler-Keylin, a graduate student at UCSF, said in a press release. "We knew that the color of brown and beige fat comes from the amount of pigmented mitochondria they contain, so we wondered whether something was going on with the mitochondria when beige fat turns white."
Mitochondria burn glucose, turning it into energy for cells. Using fluorescent proteins to visualize the mitochondria, the scientists embedded beige fat cells in a gel, stimulated the fat cells with cold or drugs and tracked their transformation for 10 days. The scientists found autophagy, a process of cells digesting their internal components when they are defective or unneeded, caused the death of mitochondria.
Using mice with autophagy genes deleted from their DNA, the scientists then boosted beige fat levels using cold or drugs for a period of time and then stopped. In the mice, more beige fat remained because the cells could not kill off their mitochondria.
Further experiments with mice showed that, after eight weeks of a high-fat diet, those without an autophagy gene gained less weight and showed signs of type 2 diabetes development than mice with the gene.
The theory is that with an effective method to keep beige fat from turning white, it may be easier to control obesity and diabetes in humans -- now the focus of the UCSF scientists.
"For many years our focus has been on learning to convert white fat into beige fat," said Dr. Shingo Kajimura, an associate professor of cell and tissue biology at UCSF. "Now we're realizing we also have to think about how to keep it there for longer time."
