Eat Less, Weigh More? Enzyme Makes Lean Mice susceptible To Dietary Fat

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Eat Less, Weigh More? Enzyme Makes Lean Mice " susceptible " To Dietary

Fat

http://www.medicalnewstoday.com/medicalnews.php?newsid=42709

Working with genetically engineered mice, s Hopkins scientists

have interfered with the brain's ability to control an animal's

response to a high-fat diet. The report, to be published in the

latest issue of the Proceedings of the National Academy of Sciences

online in May, is based on the identification of a gene - CPT1c - the

brain needs to manage body weight.

According to the researchers, the CPT1c gene protects against weight

gain caused by a high-fat diet. So-called knockout mice lacking the

CPT1c gene gain more weight than their littermates carrying normal

copies of the gene.

" We think our study reveals a direct weight management pathway, " says

Wolfgang, Ph.D., a postdoctoral fellow in the Department of

Biological Chemistry at The s Hopkins University School of

Medicine and an author of the report. " CPT1c seems to allow the body

to respond immediately to the level of nutrients and fat in the

bloodstream. "

Hopeful that the discovery has broad implications for understanding

the genetic underpinnings of obesity and weight management, the

Hopkins investigators say the work affirms the central role of the

brain in managing hunger and satiety and offers up new targets for

drugs that manipulate CPT1c. But none have been developed so far,

says Wolfgang.

The newly discovered gene makes a protein found only in the brain,

notably in the region that controls hunger, thirst and metabolism -

the hypothalamus. Proteins similar to CPT1c are known to help break

down fat to release energy to feed cells. Mice lacking the CPT1c gene

are the same length as their littermates who carry normal copies of

the gene but on average weigh 15 percent less when fed a low-fat

diet.

Further analysis revealed that when deprived of food for four hours

prior to feeding with standard laboratory mouse chow, the knockout

mutant mice ate about 25 percent less food than their normal

siblings. Therefore, the researchers concluded, CPT1c must play a

role in feeding behavior and appetite control.

And when fed a high-fat diet (mouse chow laced with lard) for 10

weeks, mice lacking CPT1c still ate less than their normal

littermates, but they were much heavier.

What scientists already know about the regulation of body weight

helps explain why the absence of CPT1c may have its seemingly

paradoxical effect.

Under normal circumstances, says Wolfgang, body weight is maintained

by a combination of food intake and energy expenditure, how hungry

the body is, and how much energy cells need. Many cells in the body

use a sugar called glucose as a source of energy. When the body is

starved, the body literally feeds on itself, breaking down fat to

form fatty acids that fuel energy needs. When the cells of the body

are well fed and energy is in ample supply, molecular signals from

the brain tell cells in the body to store the excess energy by

converting it to fat. Weight gain results when food intake greatly

exceeds energy expenditure. But when the brain's appetite/energy

regulator is out of whack, so are the rules for gaining and losing

weight.

" How do you know when to stop eating? " asks M. Lane, Ph.D.,

senior author of the study and a professor of biological chemistry in

the Institute for Basic Biomedical Sciences at Hopkins. " The liver

sure isn't going to tell you, it just keeps storing fat as long as

the body is well fed. " Instead, he notes, it is the control regions

of the brain, namely the hypothalamus, that governs eating behavior.

Previously, the same researchers showed that a molecule called

malonyl-CoA is critical for fat metabolism. And as it turns out,

malonyl-CoA interacts with CPT1c, according to Lane.

Increasing the amount of malonyl-CoA in the liver causes those cells

to synthesize fat, which is stored. Increasing malonyl-CoA in the

hypothalamus somehow tells the cells in the body to break down fats

for energy and the muscle cells to use more energy. Therefore,

identifying molecules that interact with malonyl-CoA will help

scientists understand how energy balance and body weight is

controlled.

" We are beginning to understand what the hypothalamus inputs are, but

unlike the liver, where nearly the whole organ is involved in the

same thing, the brain is very specialized and only a few neurons do

very specific things, " says Lane. The researchers hope to further

understand how malonyl-CoA and CPT1c function to control body weight

and appetite. ###

Funding for this study was provided by Astellas Pharma Inc. Tsukuba.

Authors on the paper are Wolfgang, Yun Dai, Seung Hun Cha and Lane,

all of Hopkins, and Takeshi Kurama, Akira Suwa, Makoto Asaumi, Shun-

ichiro Matsumoto and Teruhiko Shimokawa, of Astellas Pharma Inc.

Tsukuba of Japan.