Mutated protein combination tied to excessive sugar production

Study identifies potential target for diabetes drugs

Researchers at Dana-Farber Cancer Institute have traced runaway sugar production in the liver – an important feature of diabetes – to flaws in a two-protein combination at the heart of a molecular switch that responds to insulin.

The findings, to be posted by the journal Nature on its Web site on May 18, suggest that drugs designed to block the interaction of the two switch proteins might be effective in treating diabetes, and with few side effects.

Building on their discovery of this master switch in fall 2001, scientists led by Dana-Farber’s Bruce Spiegelman, PhD, found that two previously known proteins in mice must “dock,” one on top of the other, to enable the switch to turn on genes that initiate the liver’s sugar-making process. Furthermore, when mutations cause a flaw in one of the proteins, the switch no longer can respond to insulin, the hormone that normally regulates sugar manufacture in the liver.

“The actual molecular connections between the proteins are potential targets for diabetic therapy,” says Spiegelman, the paper’s senior author. It may be possible to design an oral drug that could block the joining of the two proteins – PGC-1alpha and FOXO1 – when the switch is stuck in the “on” position.

The liver’s manufacture of sugar from raw materials, a process called gluconeogenesis, is designed to provide the body (especially the brain) with necessary glucose when the person has been fasting and isn’t obtaining the sugar from food. Glucagon and glucocorticoid hormones initiate the process on by sending signals to liver cells, triggering activity (DNA transcription) in genes that set gluconeogenesis in motion.

Insulin, produced in the pancreas, has the opposite effect, turning off gluconeogenesis when normal feeding resumes. Insulin activates the insulin receptors on liver cells’ surfaces, which send signals into the cells’ nuclei where they are received by the switch made up of the PGC-1alpha and FOXO1 proteins.

FOXO1 protein, known as a transcription factor, binds directly to the DNA molecules of the gluconeogenesis genes, causing them to copy their genetic blueprints into RNA. PGC-1alpha does not directly bind to the DNA, but instead docks onto the FOXO1 protein. Together, “they area a powerful, insulin-sensitive switch” for gluconeogenesis, says Spiegelman. “PGC-1 provides the horsepower, and FOXO1 is the insulin-sensitive receiver” of signals.

In a series of experiments with transgenic mice, Spiegelman and his colleagues showed that if a mutation occurs in the gene producing FOXO1, it results in an abnormal FOXO1 protein that no longer is sensitive to insulin. Consequently, the switch fails and the liver overproduces glucose, which spills into the blood and can damage vital organs and nerves.

In his previous Nature paper [Sept. 13, 2001] Spiegelman demonstrated that the PGC-1alpha protein was the long-sought switch for gluconeogenesis, but how that protein worked with FOXO1 wasn’t clear. At the time, Spiegelman suggested that blocking PGC-1alpha might be a new therapeutic strategy. He now says that targeting just the combination of PGC-1alpha and FOXO1 would be a more finely pointed tool with fewer unwanted effects.

“What’s exciting about this paper is that is unifies two fields,” commented Spiegelman, who is also a professor of cell biology at Harvard Medical School. “One was the discovery of the signaling pathway from the insulin receptor to the FOXO1 protein – and this was found in worms. The other was the work that led to the identification of PGC-1alpha as the switch for gluconeogenesis. Now we know that it is the complex of PGC-1alpha and FOXO1 that is important.”

The research was funded in part by the National Institutes of Health.

Dana-Farber Cancer Institute is a principal teaching affiliate of the Harvard Medical School and is among the leading cancer research and care centers in the United States. It is a founding member of the Dana-Farber/Harvard Cancer Center (DF/HCC), designated a comprehensive cancer center by the National Cancer Institute.