Protein May Determine Fat, Muscle

Friday, August 11th 2000, 12:00 am

By: News On 6

WASHINGTON (AP) â€” Researchers probing the inner workings of the cell have found a protein that acts as a ``fat switch,'' causing some developing cells to become either muscle or fat.

But don't look for the ``fat switch'' to be the answer to a bulging waistline and a groaning bathroom scale. University of Michigan Medical School researchers say the protein could not be used as a diet pill.

``If you could see my midriff, you would know that breaks my heart,'' said Ormond A. MacDougald, one of the scientists.

In a study appearing Friday in the journal Science, co-authors MacDougald and Sarah E. Ross reported that a protein called Wnt 10b acts as a molecular switch to determine which developing cells become fat and which become muscle.

MacDougald said that both fat and muscle cells evolve from what are called precursor cells. The ultimate destiny in the body of some of these precursor cells is determined by a family of 18 proteins called Wnts.

In laboratory studies, MacDougald and Ross found that Wnt 10b appears to regulate production of muscle cells. When Wnt 10b links with certain precursor cells, they develop into muscle. But when the Wnt protein is missing, the same cells will turn into fat.

``We used muscle precursor cells and showed that if you inhibit the Wnt signal then they turn into fat cells,'' said MacDougald. ``Wnt has to be suppressed in those cells for adipocytes (cells that turn into fat) to form.''

In effect, said MacDougald, Wnt 10b appears to act as ``a molecular switch to turn on either fat or muscle.''

The action of Wnt 10b was discovered in test tube experiments, but the role of the protein was proven when precursor cells with or without Wnt 10b were implanted in lab mice.

Precursor cells with the Wnt protein continued to evolve toward mature muscle cells, while those lacking Wnt ``are redirected from muscle cells toward fat cells,'' said MacDougald.

MacDougald said the action of the Wnt protein is very local, affecting only a few cells in its vicinity. It does not work like a hormone, such as insulin, that travels through the blood stream and affects billions of cells.

As a result, he said, Wnt 10b is unlikely to be developed into an anti-fat pill.

``Our findings will be important in learning how obesity develops, but the Wnt protein will not be a target for an anti-obesity drug,'' said MacDougald.

Instead, he predicted that the best target for a magic slimness pill would be one that controls appetite â€” not one that controls how the body makes fat or muscle.

That means that Americans who want to lose weight still have to do it the old-fashioned way: eat less food and get more exercise.