The Hopkins team discovered that adding a single protein, dubbed BMP4, induced mouse stem cells to become fat cells. A very similar signal is likely to be involved in humans, too, say the scientists.
Fat cells, or adipocytes, store the body's excess energy both by increasing their size by "stuffing" themselves full of fat, and by increasing their number. Stem cells are cellular "reserves" that hang around until told to change into another, more specialized type of cell.
The stem cells that the Hopkins team studied have the ability to become fat, muscle, bone or cartilage, but how they commit to these fates is pretty murky. For the past 15 years, researchers have known about signals -- those for muscle and bone -- that direct this type of stem cell to a particular fate.
"Apart from the muscle- and bone-inducing signals, not much is known about the initial switch from stem cell to labeled 'pre-fat' cell," says Daniel Lane, Ph.D., professor of biological chemistry. "BMP4 is the first proven fat-cell producing signal for these stem cells."
To see if BMP4 could direct stem cells to become fat in culture dishes, the researchers treated cells with this suspected, but not yet proven fat-commitment protein. Qi-Qun Tang, M.D., Ph.D., assistant professor of pediatrics and biological chemistry, and postdoctoral fellow Tamara Otto, Ph.D., discovered that these treated stem cells, when pushed a little more, uniformly became fat cells. Stem cells not exposed to BMP4 didn't become fat cells when pushed with a"cocktail" of differentiation-inducers that the researchers had previously shown makes "pre-fat" cells convert into fat cells.