Diabetes News
September 21, 2005 (Eurekalert) — Chronically high levels of insulin, as is found in people with type 2 diabetes, may block a specific hormone that is a trigger for releasing energy into the body, according to researchers at the University of California, San Diego (UCSD) School of Medicine. Their findings, which may point to improved treatment options for the disease, will be published in the September 22 issue of the journal Nature.
The research team, led by Roger Y. Tsien, Ph.D., professor of Chemistry and Biochemistry in UCSD's Department of Pharmacology and Howard Hughes Medical Institute investigator, found that high levels of insulin can block the stress hormone catecholomine, which normally causes the release of cellular energy. For normal metabolism to occur, the body needs a balanced input of two types of hormones: insulin and catacholomines. Insulin, the main energy storage hormone, works by blocking activation of the protein kinease A (PKA) enzyme. After a meal, insulin levels go up, and the body stores energy to be used later. When energy is needed, the catacholomine triggers activation of the PKA, and energy is released. But in people with type 2 diabetes, the hormonal balance has been thrown off, because the body continues to produce and store more and more insulin instead of releasing the energy.
"Somehow, insulin knows how to specifically block catecholomine-induced PKA, but not other molecules," said Christopher Hupfeld, assistant professor of Medicine in the UCSD Division of Endocrinology and Metabolism and a co-author of the paper. "When the body has a constantly high level of insulin, this energy- release stimulus is lost."
The team's findings provide new understanding to the cause and effect occurring when insulin levels are too high. It also underscores the goal of physicians to bring down insulin levels in type 2 diabetes using medicines called insulin sensitizers, so that the body becomes more sensitive to using its own insulin, rather than compensating for insulin resistance by making more.
In order to understand the mechanisms of insulin resistance present in type 2 diabetes, the researchers used a new breed of cellular enzyme reporter to track PKA. The reporter is a "marker" protein, created with a special fluorescent tag so that scientists can physically view the protein under a microscope and watch how the live cell works in real time.
The PKA is activated inside the adipocyte cell, the major site of energy storage in the body where many aspects of metabolism are controlled. There, energy is stored in the form of triglycerides, commonly known as fat. If a person is obese, excess triglycerides are stored in the adipocytes.
"If insulin levels get too high for too long a time – which happens in type 2 diabetes – the normal catecholamine signal that triggers energy release can be drowned out. This can lead to excessive energy storage in the adipocyte," said Hupfeld. "This may be one reason why chronic obesity and Type II diabetes are often seen together."
"By correcting this hormonal imbalance, we may at some point improve treatment options," said Hupfeld.
This work was supported by the Howard Hughes Medical Institute, the Alliance for Cell Signaling, the National Institutes of Health, the Johns Hopkins University School of Medicine and the W.M. Keck Foundation.
Contributors to the paper include Jin Zhang, UCSD Department of Pharmacology; Susan S. Taylor, UCSD Department of Chemistry and Biochemistry and the Howard Hughes Medical Institute; and Jerrold M. Olefsky, UCSD Division of Endocrinology and Metabolism.