Protein missing in fat tissue ‘favours bad over good fat’

Something more than overeating and a lack of exercise causes the body to store more fat and burn less energy — a protein called p62, researchers have claimed.

According to a study by Sanford-Burnham researchers, when p62 is missing in fat tissue, the body’s metabolic balance shifts-inhibiting “good” brown fat, while favouring “bad” white fat. These findings indicate that p62 might make a promising target for new therapies aimed at curbing obesity.

"Without p62 you’re making lots of fat but not burning energy, and the body thinks it needs to store energy,” Jorge Moscat, lead author of the study, said.

"It’s a double whammy,” he said.

Mr. Moscat, who led the study with collaborators at Helmholtz Zentrum Munchen in Germany and the University of Cincinnati, had previously produced mice that completely lack the p62 protein in them. As a result, the animals were obese. They also had metabolic syndrome. In other words, as compared to mice with p62, mice lacking p62 weighed more, exhausted less energy, had diabetes and had a hyper-inflammatory response that’s characteristic of obesity.

While those results showed that the lack of p62 leads to obesity, “We didn’t know which tissue was responsible for these effects, because p62 was missing in all of them,” Mr. Moscat said.

Some researchers believe that muscle tissue, where energy is expended, controls obesity. Others suspect the liver is a key player, or that the brain’s appetite control centre is most responsible for obesity.

But then there’s fat itself-both white fat and brown fat. White fat is the type we think of as unwanted body fat. Brown fat, on the other hand, is beneficial because it burns calories. Many researchers now believe that brown fat somehow malfunctions in obesity, but the details are unclear.

In their latest study, Mr. Moscat and colleagues set out to pinpoint the specific tissue responsible for obesity when p62 is missing. They made several different mouse models, each missing p62 in just one specific organ system, such as the central nervous system, the liver, or muscle. In every case, the mice were normal. They weren’t obese like the mice lacking p62 everywhere.

Then they made a mouse model lacking p62 only in their fat tissue. These mice were obese, just like the mice missing p62 in all tissues. Upon further study, the researchers found that p62 blocks the action of an enzyme called ERK while activating another enzyme called p38. When p62 is missing, the enzyme p38 is less active in brown fat, while ERK is more active in white fat. As a result, Mr. Moscat said, p62 is “a master regulator” in normal fat metabolism.

According to Mr. Moscat, the discovery of p62’s role in brown fat tissue is encouraging, because fat tissue is much more accessible than other parts of the body-the brain. "This makes it easier to think about new strategies to control obesity,” he added.

The study has been published in the Journal of Clinical Investigation.