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GEN News Highlights : Jul 13, 2012

Is Beige Fat the New Brown Fat?

Scientists have identified a new type of energy-burning fat cell in mice and humans, termed beige fat, which they claim could provide the basis for new approaches to treating obesity. The existence of a type of fat that is distinct from both calorie-burning brown fat and calorie-storing white fat was proposed by Dana-Farber Cancer Institute Bruce Spiegelman, Ph.D., back in 2008. In a new series of studies reported in Cell, Dr. Spiegelman’s team reports on the isolation of beige fat cells from inguinal subcutaneous fat deposits in mice, and the identification of a similar population of cells in human brown fat samples.

The researchers claim beige fat has characteristics of both white fat and brown fat, despite the finding that beige fat cells have gene-expression profiles that distinguish it from either of the other types of fat. In one respect beige fat more closely resembles white fat as it exhibits low basal levels of the protein UCP1 needed by mitochondria to respire at high rates. High expression of UCP1 is characteristic of brown fat cells. In addition, beige fat also appears to be derived from a specialized type of stem cell, and not from the myf-5 lineage that gives rise to brown fat cells. However, in other respects beige fat is more similar to brown fat in that the cells can be switched on by a recently identified peptide hormone known as irisin to upregulate UCP1 and rev up energy expenditure. As the team writes, “the beige cells have the capability to switch between an energy storage and energy dissipation phenotype in a manner that other fat cells lack.”

Importantly, genetic analysis of samples of brown fat that are found under the collarbone and along the spine in adult humans (the presence of brown fat in adults has only recently been recognized) indicated that the cells display the same markers as those of the rodent beige fat, rather than those of classical brown fat.

Research is already ongoing to try harness therapeutic potential of activating brown-fat-mediated thermogenesis, but trials of drugs that increase the activity of brown adipose fat haven’t been successful either because of lack of efficacy or intolerable side effects resulting from the activation of beta-adrenergic receptors in other tissues, the researchers note. The discovery that beige fat can in fact be switched on and off may provide new avenues for development. “Going forward, it means that what you want to study for potential therapies are the beige fat cells in these hotspots we’re walking around with,” Dr. Spiegelman concludes. The researchers report their findings in a paper titled “Beige Adipocytes Are a Distinct Type of Thermogenic Fat Cell in Mouse and Human.”