Scientists Discover Key To Manipulating Fat; Pathway Also Explains Stress-induced Weight Gain
In the paper, the Georgetown researchers describe a mechanism they found by which stress activates weight gain in mice, and they say this pathway -- which they were able to manipulate -- may explain why people who are chronically stressed gain more weight than they should based on the calories they consume.
The key to the process that was found is a peptide (small protein) neurotransmitter called neuropeptide Y (NPY). NPY is produced under conditions of stress, as a by-product of the "fight-or-flight" response mediated by the sympathetic nervous system. An animal under conditions of chronic stress will have higher levels of NPY. The hormone was discovered 25 years ago, and earlier research indicates that it acts in the brain to increase appetite. (Although other aspects of the response may suppress appetite, which is why some overly stressed people may be emaciated.) The appetite-suppressing hormone leptin acts by inhibiting the activity of neurons that contain NPY.
What the new research found, however, is that, more importantly, NPY, acting outside the brain in adipose tissue, also alters an animal's metabolism to increase storage of fat:
As part of the study, Zukowska and her team examined the effect of several forms of chronic stress that mice in the wilderness can encounter, such as exposure for an hour a day over a two-week period to standing in a puddle of cold water or to an aggressive alpha mouse, and they conducted the experiments in combination with a regular diet or with a high-fat, high-sugar diet. Stressed animals fed a normal diet did not gain weight, but stressed mice given a high-fat diet did. In fact, the researchers found these mice put on more weight than expected given the calories they were consuming.
"They gained twice as much fat as would be expected, and it was all in their belly area," Kuo said. Stressed versus non-stressed animals ate the same amount of food, but the stressed animals processed it differently, she said, explaining, "the novel finding here is that NPY works on fat tissue, not in the brain."
In part, the research showed that NPY experimentally delivered in a mouse activated a G-protein coupled receptor called (naturally) a neuropeptide Y2 receptor (Y2R). (This is just one of 5 known NPY receptors.) Activation of Y2R was observed to promote storage of fat in adipose tissue:
[The] pathway involves two players -- a neurotransmitter (neuropeptide Y, or NPY) and the receptor (neuropeptide Y2 receptor, or Y2R) it activates in two types of cells in the fat tissue: endothelial cells lining blood vessels and fat cells themselves. In order to add fat selectively to the mice they tested, researchers injected NPY into a specific area. The researchers found that both NPY and Y2R are activated during stress, leading to apple-shape obesity and metabolic syndrome.
So NPY can lead to increased fat storage. But the converse is, happily, also true: blocking the NPY receptor shrinks fat:
Both the weight gain and metabolic syndrome, however, were prevented by administration of Y2R blocker into the abdominal fat.
Metabolic syndrome, you recall, comprises several undesirable elements, such as hyperglycemia, high blood pressure, central obesity, decreased HDL cholesterol, and elevated triglycerides. All of these can lead to more serious health problems, such as diabetes and cardiovascular disease. So there is the possibility that blocking Y2R could be beneficial to humans:
"We are hopeful that these findings might eventually lead to control of metabolic syndrome, which is a huge health issue for many Americans," [the study's senior author, Zofia Zukowska] said. "Decreasing fat in the abdomen of the mice we studied reduced the fat in their liver and skeletal muscles, and also helped to control insulin resistance, glucose intolerance, blood pressure and inflammation. Blocking Y2R might work the same way in humans, but much study will be needed to prove that."
Another account of this research reports expressions of optimism for development of human drugs to control stress-induced health problems:
Stress can be fattening, study finds
Mary F. Dallman of UC San Francisco said in an editorial in the same journal: "A large gap in our understanding of how chronic stressors lead to abdominal obesity has been filled…. Their results were remarkable and have profound implications for new drug development."
But it's not a sure thing. There is another hormone, called PYY, Pancreatic Peptide YY, or Pancreatic Peptide YY3-36. PYY is structurally similar to NPY and in fact can activate some NPY receptors. It has been found to decrease appetite when it activates NPY receptors in the brain. At least two biotech companies (Amylin and Nastech) have investigated using PYY directly as a drug to induce weight loss. So far this effort has had only mixed results.
Here's another report on the research discussed above: How we can stop stress from making us obese. And here's a blog post that raises some good questions about this research: Scientists Stressed About Weight Loss.
Apart from the effects of NPY, chronic stress can cause a variety of health problems besides weight gain, obesity, and their knock-on effects. Robert Sapolsky of Stanford has done copious research into the ill effects of chronic stress. He points out that the fight-or-flight response of animals in the wild, which is activated during periods of acute danger (predators), and is adaptive in those circumstances, can turn harmful when stress is chronic, as happens frequently with primates such as humans:
Why Do Humans And Primates Get More Stress-related Diseases Than Other Animals?
Why do humans and their primate cousins get more stress-related diseases than any other member of the animal kingdom? The answer, says Stanford University neuroscientist Robert Sapolsky, is that people, apes and monkeys are highly intelligent, social creatures with far too much spare time on their hands.
"Primates are super smart and organized just enough to devote their free time to being miserable to each other and stressing each other out," he said. "But if you get chronically, psychosocially stressed, you're going to compromise your health. So, essentially, we've evolved to be smart enough to make ourselves sick."
Tags: stress, obesity, weight gain, neuropeptide Y