Using a revolutionary technique to turn off chemical signals inside the cell, scientists at Joslin Diabetes Center have discovered that the different metabolic abnormalities present in type 2 diabetes can be caused by knocking out two key signals in liver cells.
Their findings in mice may someday lead to strategies in humans to boost these two different signals, providing a powerful new way to treat the different metabolic components present in the most common form of diabetes.
"By lowering the level of two key insulin signaling proteins in liver cells, we began to uncover just how complex type 2 diabetes and the related metabolic syndrome are," said principal investigator C. Ronald Kahn, M.D., President of Joslin Diabetes Center and the Mary K. Iacocca Professor of Medicine at Harvard Medical School. "Both protein signals needed to be knocked out at the same time to create the full diabetic syndrome, while depleting just one or the other caused only either the glucose or the lipid abnormalities associated with diabetes. Thus, these two pathways complement each other, each controlling a part of the metabolism that is disrupted in type 2 diabetes or the metabolic syndrome."
Others involved in the study were lead author Cullen Taniguchi, M.D., Ph.D., and former Joslin researcher Kohjiro Ueki, M.D., Ph.D., now at the University of Tokyo. Published Feb. 10 in the online edition of the Journal of Clinical Investigation, the study sheds new light on a complex question: How do cells normally process the hormone insulin and what goes wrong in diabetes?
An estimated 18 million Americans have type 2 diabetes, and about one-third do not even know they have the disease. In this disorder, the body does not make enough insulin or resists its effect, a phenomenon called insulin resistance. Without effective insulin, cells throughout the body are unable to convert sugar in the bloodstream to energy, resulting in chronic fatigue, thirst and other symptoms of high blood sugar. People with diabetes also have abnormalities in lipid metabolism and are two to four times more likely to have cardiovascular disease, and they run a higher risk of damage to nerve, eye, kidney and other body tissues.
Previous discoveries by Dr. Kahn's research team provided insight about the pathway that insulin takes to stimulate cells. Insulin docks at a receptor site at the cell membrane. Once this site is activated, chemical signals pass to other proteins inside the cell, including insulin-receptor substrates (IRS). These spark a chain of other reactions. Ultimately the cell's energy machinery is turned on.
The Joslin study focused on two of these early intracellular signaling proteins, IRS-1 and IRS-2, and especially their role in the liver, which is a key organ for both glucose and lipid metabolism. If turned off, how would that affect the onset of type 2 diabetes? And if an effect occurred, was it causative or something that just happens along the way?
The researchers needed a technique to turn off the two substrates in a living organism in just one tissue at a time. Previous studies had shown that mice bred without the genes for either IRS-1 didn't develop diabetes, while those lacking IRS-2 developed diabetes, but this was primarily because of a defect in the beta cell, so evaluating the role of the liver was impossible.
To solve this dilemma, Dr. Kahn's team used an elegant new genetic tool in a disease study. The technique allows researchers to turn off specific signals with a virus that targets specific cell types with a kind of RNA that would interfere with the liver cells' ability to make IRS-1 or IRS-2. The liver is the focus of considerable diabetes research because it is a major controller of metabolic functions, including those that regulate blood glucose and fat metabolism.
In the study, the effect of the RNA interference lasted one to two weeks, reducing IRS-1 and IRS-2 by up to 80 percent. By designing separate experiments, the researchers found that each substrate acts on a different part of metabolism. Low levels of IRS-1 drive cells to make more glucose, causing blood sugar to rise. Low levels of IRS-2 are linked to higher levels of blood fats such as triglycerides. Acting alone, neither causes diabetes. But when both substrates are low, diabetes results.
"Our findings show what happens when we knock out these two protein signals, causing conditions to worsen," said Dr. Kahn. "The next step is to look for ways to keep their levels up, possibly leading to new ways to prevent and treat diabetes."