By David N. Leff
Born and bred in Texas, a posse of mice should be the envy of the majority of Americans who are severely overweight.
The transgenic animals can eat 40 percent more than normal mice, yet weigh 10 percent to 15 percent less. Their slimming strategy depends on a key enzyme, of which the gene has been knocked out of their genome. That recombinant cDNA sequence, acc2 by name, resides on human chromosome 12.
The acc acronym stands for 'acetyl-CoA carboxylase.' It's a key enzyme that generates a metabolite in the regulation of energy balance in man and beast. The acc1 gene, on chromosome 17, tells a different story. The proteins the two encode - ACC1 and ACC2 - are far apart in function.
"When we made ACC1," recounted molecular biologist Salih Wakil, at Baylor College of Medicine in Houston, "we found that it goes directly into the cell's cytoplasm. When, however, we made ACC2, we saw that it went to the membrane of the mitochondria - the cell's power-plant organelle. That's where it functions.
"Then we decided to differentiate the functions of those two proteins," he continued. "so we knocked out the acc1 gene from the eggs of normal mouse mothers. The embryos unfortunately were dead six or seven days after conception. This demonstrated that ACC1, or fat synthesis, is very important for embryonic development.
"But when we knocked out ACC2," Wakil went on, "we got mice that functioned well, bred well and seemed healthy. We have had them now for over two years and they are fine. We checked for the presence of ACC2 in these mice. Indeed, there was none, showing that knocking out the acc2 gene worked."
To place these two ACC proteins in the scheme of fat metabolism, Wakil went back to first principles - food intake. "Let's say you sit down to a meal," he began. "It contains carbohydrates, proteins, fats, minerals, vitamins and so forth. Carbohydrates are one of the major components of most meals that we eat on a daily basis. It's eventually digested in the body, and makes glucose - sugar.
"Glucose," he continued, "is a 6-carbon compound that goes from the gut to the blood. Blood glucose moves into various cells, liver, heart, muscle, where it gets burned - consumed. Then it goes from the cytoplasm of the cell into the mitochondria. This burns its now 3-carbon to 2-carbon plus some fat and energy, which our bodies use."
Nature's Starring Role For Body Fat
"You need carbohydrate," Wakil observed. "You burn some, you save some and convert it into fat. In the meantime you stop fat burning, so you channel everything to stored fat. This is the way we and all animals evolved - depending on the availability of food. When there is a lot of food, you take it in, use what you need, and store what's left over as a potential resource of fat, for when you don't know where your next meal is coming from.
"When there's no more food," he continued, "you mobilize the fat from the fat tissue into the muscle, heart or liver, and transport it from cytoplasm to mitochondria, where it's burned, and generates energy. So that's how we mammals have evolved."
Wakil, who chairs the departments of biochemistry and molecular biology at Baylor, is senior author of a paper in today's Science, dated March 30, 2001. It's title: "Continuous fatty acid oxidation and reduced fat storage in mice lacking Acetyl-CoA carboxylase 2 [ACC2]."
"That study," Wakil told BioWorld Today, "demonstrates that the ACC2-deficient mice had nearly one-half the fat content of normal mice. The fatty livers of normal animals looked pale tan compared to the bright red, virtually fat-free, livers of our genetically engineered animals.
"In the absence of ACC2," Wakil observed, "burning of fat is continuous. As a result, you accumulate less fat in the body. In these KO mice, there is about half as much fat as in normal ones.
"Malonyl-CoA is an intermediate that the enzyme makes. There's 30 times less of malonyl-CoA metabolite in the muscle, 10 times less in the heart, so the absence of ACC2 in there permits the burning of the fat to continue. That stops the transfer of the fat from the cytoplasm to the mitochondria to burn it. These are mice of the same age, same sex, raised on the same food for 27 weeks or so," Wakil pointed out. "They simply do not accumulate as much fat."
He made the point, "Appetite is highly regulated in animals. Adipose tissue puts out a hormone signal, leptin, which goes to the brain. There, if the leptin level is high, it signals: 'Stop eating.' But if leptin is low, the signal says, 'Hey, we need to put on fat.' So the animal goes out and eats.
"Our mutant mice," he continued, "because they have low fat content, put out low leptin, which tells the brain, 'Hey, go and eat.' And indeed they do. They consume 40 percent more food than the normal mouse. However, most of that food does not go to making fat, because it's oxidized; it's burned."
Svelte Abs Without Benefit Of Jogging
"This work," Wakil observed, "highlights the potential target by which we can regulate fat burning. And if we can do that in people, the couch potato guys can sit on the couch, eat all the potato chips they want, watch TV and have smaller paunches - because they oxidize the fat and don't accumulate it - without having to go out and jog."
Baylor has applied for a patent covering the invention of Wakil and his co-authors. "It highlights," he said, "the significance of the potential target of the system for regulating fat burning." His team is in preliminary contact with pharma and biotech companies.
"Human trials," Wakil observed, "are of course a way down the road. Once there is a potentially successful drug that can be used for the mouse or primate, as a new way to reduce obesity, diabetes, atherosclerosis and other fat-related diseases, then obviously it goes into humans. Within three to five years, somehow, I think something might be there." n