Type 2 diabetes (T2D) is characterised by persistent hyperglycemia, and altered metabolism in the liver is a key contributor to T2D through uncontrolled release of glucose. The molecular mechanisms mediating this response in T2D are not completely understood, but include insulin resistance. Recent data in a number of organisms has shown that the majority of metabolic enzymes can be reversibly acetylated at lysine residues. Acetylation of the glycolytic/gluconeogenic enzyme GAPDH controls glucose flux in S.enterica, with GAPDH acetylation favouring glucose consumption though glycolysis, and deacetylation of GAPDH favouring glucose production by gluconeogenesis. Proteomic analysis has also shown that GAPDH is acetylated in mammalian liver. While a number of acetylated lysine residues in GAPDH were identified, their role in the control of glucose metabolism in the mammalian liver is unknown. Furthermore, the enzymes responsible for regulating the deacetylation of mammalian GAPDH remain unidentified. The major aim of this project was to determine the role of GAPDH acetylation in hepatic glucose flux, and its role in T2D.
Expression of an acetylation deficient GAPDH mutant in FAO hepatocytes increased basal glucose production compared with cells expressing WT GAPDH. The increase in glucose production was associated with reduced glycolysis and glucose oxidation. Insulin was able to suppress glucose production in acetylation deficient GAPDH mutant FAO hepatocytes, however they had an increased response to forskolin, an agent that mimics the actions of hormones that increase glucose production, such as glucagon and adrenalin, by increasing cAMP. GAPDH acetylation was also reduced in the liver of diabetic db/db mice.
Our data showed that db/db mice have reduced hepatic GAPDH acetylation in the fed state, and that acetylation deficient GAPDH mutant hepatocytes have increased glucose production. These studies suggest that altered lysine acetylation of GAPDH could contribute to the elevated hepatic glucose output seen in T2D.