Researchers Find Potential Mechanism Linking Obesity To Type 2 Diabetes

The experiments in mice suggest a potential new approach to therapies in humans. The study is published in the June 15 issue of the journal Cell.

“The ultimate question is to understand if the mechanism that works in mice also operates in humans,” says senior author Brad Lowell, M.D., Ph.D., an endocrinology researcher at Beth Israel Deaconess. “We’ve done the kind of research we call proof of principle. It will stimulate new research to find out how broadly the mechanism operates and if it accounts for beta cell dysfunction in type 2 diabetes in humans.”

Diabetes is one of the leading causes of death and disability in this country, affecting an estimated 16 million Americans, about one-third of whom don’t know they have the disease. Type 2 diabetes accounts for about 90 percent of diabetes cases, and it is clear that obesity promotes the development of type 2 diabetes.

Normally, a meal triggers the pancreas to release the hormone insulin, which prompts muscle and fat cells to take in necessary glucose from the blood. Insulin also turns off glucose production by the liver. In the process leading up to type 2 diabetes, the body’s cells somehow become less responsive to insulin, known as insulin resistance. Glucose builds up in the blood, which triggers the pancreas to produce more insulin. For months or years, the number of insulin-producing pancreatic cells increase, and each pancreatic beta cell also produces more insulin, temporarily overriding the resistance. Eventually, the pancreas exhausts its ability to make enough insulin to compensate for the resistance, causing the chronic high blood sugar and symptoms of diabetes.

The new study suggests that a protein known as uncoupling protein-2, or UCP2, may play a pivotal role in the transition from insulin resistance to diabetes. Normally, glucose generates a signal within pancreatic beta cells, which triggers them to secrete insulin. UCP2 interferes with the glucose signal inside beta cells, which limits insulin secretion, according to a series of studies in mouse models lead by Beth Israel Deaconess researcher Chen-Yu Zhang, M.D., Ph.D., also an instructor in medicine at Harvard Medical School.

"If they have UCP2, the mice secrete less insulin at any given level of blood glucose,” says Lowell who is also an associate professor of medicine at Harvard Medical School. “If they don’t have UCP2, the mice secrete more insulin.”

Several lines of evidence suggest that UCP2 is a key component of glucose sensing by beta cells, and that the protein links obesity, beta cell dysfunction and type 2 diabetes. In one experiment, mice genetically engineered to lack the UCP2 gene released more insulin in response to the same amounts of glucose compared to normal mice, showing UCP2 works by reducing insulin secretion in response to blood glucose levels. Another mouse model of obesity-induced diabetes showed high levels of protein in their beta cells, leading to the hypothesis that increased UCP2 in obesity promotes the development of diabetes by impairing insulin secretion. Finally, when the researchers genetically engineered these obese, diabetic mice to lack UCP2, their insulin secretion was improved and their blood sugar levels were greatly reduced.

“The suggestion that pancreatic beta cell UCP2 is a useful target for the therapy of type 2 diabetes is exciting,” write Kenneth Polonsky and Clay Semenkovich, Washington University School of Medicine, St. Louis, in an accompanying review article in the same issue of Cell. “Additional studies are needed to confirm the hypothesis that an increase in UCP2 expression is an important trigger in the failure of beta cell compensation for insulin resistance, a poorly understood event that is critical to the development of type 2 diabetes.”

The researchers were supported by the National Institutes of Health, Eli Lilly, the Canadian Diabetes Association and the Human Frontier Science Program.

Co-authors on the paper include postdoctoral fellow Gyorgy Baffy, postdoctoral fellow Stefan Krauss, postdoctoral fellow Odile Peroni, postdoctoral fellow Danica Grujic, postdoctoral fellow Thilo Hagen, postdoctoral fellow Antonio Vidal-Puig, postdoctoral fellow Olivier Boss, HMS instructor Young-Bum Kim, and HMS assistant professor Xin Xiao Zheng, all researchers at Beth Israel Deaconess; postdoctoral fellow Pascale Perret and Howard Hughes Medical Investigator Gerald Shulman at Yale University School of Medicine; researcher Michael Wheeler, University of Toronto; and researcher Catherine Chan, University of Prince Edward Island.

Beth Israel Deaconess Medical Center is a major patient care, research and teaching affiliate of Harvard Medical School and a founding member of CareGroup Healthcare System. Beth Israel Deaconess is the third largest recipient of National Institutes of Health research funding among independent U.S. teaching hospitals.