Fat cells may explain why stress causes health problems, study finds

‘This was a completely unexpected finding,’ say Yale biologists who carried out study

Scientists believe they may have discovered why stress triggers flare-ups of some health conditions (Getty )

Fat cells may hold the secret to understanding why psychological stress can worsen health problems, according to a study which scientists say could lead to new treatments.

Researchers have long been puzzled by the ability of stress to trigger flare-ups of conditions such as diabetes and autoimmune disease, as well as cause depression and anxiety.

The symptom seemed counterintuitive as the hormones associated with the typical “fight or flight” stress response, cortisol and adrenaline, generally suppress inflammation rather than activate it.

“In the clinic, we have all seen super-stressful events that make inflammatory disease worse, and that never made sense to us,” Andrew Wang, assistant professor of internal medicine and immunobiology at Yale University in Connecticut and corresponding author of the study.

But the researchers identified that an immune system protein known as interleukin-6 (IL-6) triggers inflammation in times of stress. They began to study the IL-6 after observing that drawing blood from a mouse – a very stressful procedure for the animal – caused elevated levels of the protein.

In a series of further experiments in Dr Wang’s lab, the team established that IL-6, which is usually secreted in response to infections, was induced by stress alone and worsened inflammatory responses in the animals.

To their surprise, they found that in times of stress IL-6 was secreted in brown fat cells, which are best known for their roles in regulating metabolism and body temperature. When signals from the brain to the fat cells were blocked, stressful events no longer worsened inflammatory responses.

“This was a completely unexpected finding,” said Hua Qing, a postdoctoral associate at Yale’s School of Medicine.

The researchers reasoned that IL-6 must play another role in the stress response besides triggering inflammation. They learned it also helps prepare the body to increase production of glucose in anticipation of threats. The brown fat cell response causes IL-6 levels to peak well after the production of glucose and the release of cortisol and adrenaline, which may explain why stress can trigger inflammation even while immune-suppressing hormones are being released.

Blocking IL-6 production not only protected stressed mice from inflammation but also made them less agitated when placed in a stressful environment, found the study, published in the journal Cell.

Dr Wang’s team suspect IL-6 may play a role in mental health disorders such as depression and anxiety. They noted that many of the symptoms of depression, such as loss of appetite and sex drive, mimic those caused by infectious diseases such as the flu that can be triggered by IL-6.

Existing drugs designed to treat autoimmune diseases such as rheumatoid arthritis block the activity of IL-6. The researchers said their findings suggest these drugs may help alleviate symptoms of depression. There is also preliminary evidence that IL-6 may also play a role in diabetes and obesity.

“There is an ever-growing literature on the role of IL-6 outside of immunity. Our work is exciting because it contributes to shortening that gap of knowledge,” said Reina Desrouleaux, a graduate student in biology and biomedical science.