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Brown fat tissue, which burns energy stores to generate heat, is relatively scarce in adult humans. By contrast, white fat cells are abundant, and store energy for use elsewhere in the body. Last year, Bruce Spiegelman of the Dana-Farber Cancer Institute in Boston, Massachusetts, and his colleagues showed that the protein PRDM16 can induce muscle precursor cells to differentiate into brown fat cells (Nature 454, 961–967; 2008). On page 1154, Spiegelman and his colleagues now report that connective tissue cells called fibroblasts can also be so induced with a PRDM16 complex. When transplanted into mice, the cells produce fat pads that look and behave like brown fat tissue. Spiegelman spoke to Nature about the cells and their potential for treating metabolic disorders such as type-2 diabetes and obesity.

What did you hope to learn from transplanting induced fibroblasts?

We wanted to know whether the transplants had the physiological characteristics of brown fat in terms of functioning as hot spots for glucose uptake, which they did. Going forwards, we plan to look at questions such as: how many brown fat cells does it take to significantly affect metabolism? What optimizes the cells' survival times? And where should they be implanted?

Why is the location of transplantation important?

To survive and function well, fat pads need blood vessels, and probably nerves. One day, we hope to use the pads therapeutically, to soak up glucose and convert it into heat instead of stored fat. We would want that to work optimally — and anatomy probably matters for optimum function.

How did you get started on this line of research?

It goes back to my postdoctoral work at the Massachusetts Institute of Technology with Howard Green, who pioneered the study of fat-cell differentiation. In the 1980s and 1990s my group was working exclusively on white-fat-cell development, and identified the master gene, PPARG. But there was this other kind of fat cell — the brown one — that I was also interested in. It wasn't until 2004 that we started to identify what controls brown-fat function and identity.

Are you using fat to combat fat?

Brown fat is a kind of fat, but we now appreciate that it comes from muscle-like precursors. It provides a way to control energy expenditure. The popular press loves this idea of 'fat fighting fat' — among all the silly phrases out there, at least this one is fairly accurate.