In a series of experiments in mouse cells, the researchers showed that SIRT6-deficiency hypoglycemia is caused by increased cellular uptake of glucose and not by elevated insulin levels or defects in the absorption of glucose from food. They then found increased levels of glycolysis and reduced mitochondrial respiration in SIRT6-knockout cells, something usually seen when cells are starved for oxygen or glucose, and showed that activation of the switch from cellular respiration to glycolysis is controlled through SIRT6's regulation of a protein called HIF1alpha. Normally, SIRT6 represses glycolytic genes through its role as a compactor of chromatin - the tightly wound combination of DNA and a protein backbone that makes up chromosomes. In the absence of SIRT6, this structure is opened, causing activation of these glycolytic genes. The investigators' finding increased expression of glycolytic genes in living SIRT6-knockout mice - which also had elevated levels of lactic acid, characteristic of a switch to glycolytic glucose processing - supported their cellular findings.

Studies in yeast, worms and flies have suggested a role for sirtuins in aging and longevity, and while much of the enzymes' activity in mammals is unclear, SIRT6's control of critical glucose-metabolic pathways could signify a contribution to lifespan regulation. Elevated glycolysis also is commonly found in tumor cells, suggesting that a lack of SIRT6 could contribute to tumor growth. Conversely, since knocking out SIRT6 causes blood sugar to drop, limited SIRT6 inhibition could be a novel strategy for treating type 2 diabetes.

"There's a lot we still don't know about SIRT6," adds Mostoslavsky, who is an assistant professor of Medicine at Harvard Medical School. "We need to identify the factors that interact with SIRT6 and determine how it is regulated; investigate whether it acts as a tumor suppressor and how it might help lower glucose levels in diabetes; and determine its target organs in living animals, all of which we are investigating."

Source: Massachusetts General Hospital