Covalent regulation of branched chain amino acid metabolism

PROJECT SUMMARY/ABSTRACT Type 2 diabetes is associated with derangements in a variety of pathways of intermediary metabolism. In addition to the widely-known effects on glucose metabolism, diabetes is also associated with abnormalities in lipid and amino acid metabolism including elevations in branched chain amino acids (BCAA). BCAAs are increased in people with obesity and insulin resistance and high plasma BCAA concentrations are predictive of future development of diabetes. Dietary restriction of BCAAs in rats improves insulin sensitivity and genome- wide association studies in humans have demonstrated that genetic variations in enzymes controlling BCAA metabolism are associated with increased BCAA concentrations and are also linked to the development of insulin resistance. These data suggest that high BCAA concentrations play a causative role in the development of insulin resistance and that strategies that correct accumulation of these bioactive amino acids may have value as therapeutic avenues for treating diabetes. Circulating BCAA concentrations are regulated by dietary intake, rates of protein synthesis and proteolysis, and BCAA catabolism. To be oxidized, BCAAs are first reversibly converted to branched chain keto acids (BCKA) by branched chain aminotransferases. Next, BCKA undergo oxidation that is catalyzed by the BCKA dehydrogenase (BCKDH) enzyme complex in the mitochondrial matrix. BCKDH is regulated by inhibitory phosphorylation and is activated by a protein phosphatase (PPM1K) that removes this covalent modification. Genetic or pharmacologic approaches to reduce BCKDH phosphorylation lead to enhanced BCKA catabolism and improved insulin sensitivity in rodent models of obesity. This application is designed to test the novel hypothesis that PPM1K phosphorylation at a conserved serine residue suppresses the activity of this phosphatase leading to increased BCKDH phosphorylation. Our studies are designed to identify mechanisms by which this phosphorylation is regulated, define the effects of this covalent modification on intrinsic PPM1K activity, and determine the effects of PPM1K phosphorylation on BCAA metabolism in liver of diabetic rodents.