The liver plays a major role in the uptake of glucose from the bloodstream, its storage, and regulation. Insulin resistance in the liver is a crucial factor in the development of hyperglycemia and hypertriglyceridemia in individuals who suffer type 2-diabetes. Precisely how insulin-initiated signals are modulated in liver cells for glucose uptake and metabolism is unknown.
Gen-Sheng Feng, Ph.D., a Professor in The Burnham Institute's Signal Transduction Program, has focused his efforts on a recently discovered protein called Gab1. Gab1 has a structure that is similar to other proteins in a family known as Insulin Receptor-Signaling, or IRS, proteins. IRS proteins relay signals initiated by insulin receptors and thus play a critical role in insulin regulation inside cells. Biochemical studies on Gab1 in cell cultures suggested that Gab1 is also involved in insulin signaling, but it is not clear how Gab1 acts to control insulin activity in the liver.
To learn how Gab1 functions in the liver, Feng used a highly advanced genetic engineering technology, called tissue-specific gene deletion, to create a mouse in which the Gab1 gene was deleted, or "knocked out", from the liver, and only the liver. Offsprings were termed "LGKO" for liver-Gab1 knockout mice.
Dr. Feng's laboratory, in collaboration with Drs. Andrea Hevener and Jerrold Olefsky at the University of California, San Diego, conducted a thorough investigation of the glucose metabolism and insulin activity in this newly-created mouse strain. Interestingly, the LGKO mice had reduced blood glucose levels and lower levels of serum insulin. The mice retained triglycerides in the liver with a commensurate decrease of circulating triglycerides in the bloodstream. Insulin response to glucose load was diminished in LGKO mice, thus glucose tolerance in the liver was significantly improved in the absence of Gabl protein.
The Feng team conducted biochemical analyses on the LGKO mice, homing in on enzymatic pathways critical to insulin response in cells. Upon stimulation with insulin, they found an elevated level of Akt/PkB kinase, an enzyme needed for insulin signaling, and elevated activity of IRS proteins. There was no activation of another enzyme, Erk, which is elevated in normal liver in reaction to insulin stimulation. Dr. Feng concluded that the function of Gab1 in normal liver cells is to promote signaling in the Erk pathway, which reduces insulin response signals flowing through IRS and Akt proteins.
"We propose that Gab1 acts as a negative regulator on insulin signal strength in the liver," said Dr. Feng. "In this work, by making a new liver-specific gene knockout mouse model, we found a novel balancing mechanism for control of liver insulin signaling. Our observation might be instrumental for understanding better the pathogenesis of type II diabetes and designing anti-diabetes drugs."
Co-authors on this study from Dr. Feng's laboratory were Emilie Bard-Chapeau, Ph.D., and Shinong Long, Ph.D., postdoctoral fellows, and Eric Zhang, graduate student in the Burnham Institute-UCSD's joint graduate training program in Molecular Pathology.
Jerrold Olefsky, Ph.D. and Andrea Hevener, Ph.D. are Professor and Adjunct Associate Professor, respectively, in the Department of Medicine at the University of Calfornia, San Diego.
This project was supported by grants from the National Institutes of Health awarded to Drs. Feng, Olefsky, and Hevener. Dr. Feng acknowledges with appreciation the American Diabetes Foundation for support early in his career that has provided a foundation for much of his current research.
The Burnham Institute, founded in 1976, is an independent not-for-profit biomedical research institution dedicated to advancing the frontiers of scientific knowledge and providing the foundation for tomorrow's medical therapies. The Institute is home to three major centers: the original Cancer Center, the Del E. Webb Neuroscience and Aging Center established in 1999, and the Infectious and Inflammatory Disease Center dedicated in 2004. Since 1981, the Institute's Cancer Center has earned the prestigious designation as a Non-comprehensive Cancer Center by the National Cancer Institute. Discoveries by Burnham scientists have contributed to the development of new drugs for Alzheimer's disease, heart disease and several forms of cancer. Today the Burnham Institute employs over 700, including more than 550 scientists. The majority of the Institute's funding derives from federal sources, but private philanthropic support is essential to continuing bold and innovative research. For additional information about the Institute and ways to support the research efforts of the Institute, visit www.burnham.org.