Role of Leucine Metabolism in Leucine Signaling

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DESCRIPTION (provided by applicant): The long-term goal of these studies is to determine the role of branched chain amino acid (BCAA) metabolism in obesity and nutrient signaling from leucine. Results from a number of studies indicate that increasing dietary protein in the diet has a beneficial effect on insulin sensitivity, satiety, lean body mass and resistance to obesity. Branched chain amino acids (BCAAs) or leucine appear to be the active dietary component. Paradoxically, BCAAs are elevated in obesity and their some of their targets, such PKC-? and the mTOR cell signaling pathway, appear to promote Ser-phosphorylation of IRS-1, a cause of insulin resistance. Further, substrates of mTOR, lipin, S6K1 and 4EBP1&2, have been alternatively implicated in either obesity or resistance to obesity. Since some studies suggest that raising BCAAs in the diet would be beneficial for obesity whereas other findings predict a worsening of obesity co-morbidities; it is not entirely clear what might be expected from raising BCAAs in the diet or preventing peripheral BCAA metabolism. The proposed research will address that and the following questions. What is the role of BCAA metabolism in nutrient signaling leading to the activation of PKC-? and the mTOR pathway? Does defective metabolism play a role in the elevations of BCAAs observed in obesity and can this be reversed by weight loss intervention? In the last funding period, we generated a line of transgenic mice (BCATm KO) deficient in the first step in BCAA metabolism catalyzed by the mitochondrial branched chain aminotransferase isozyme (BCATm) found in most peripheral (non-neuronal) tissues. The block in BCAA metabolism will be exploited to help clarify these inconsistencies and questions related to leucine signaling and metabolism. The BCATm KO has persistently elevated BCAAs, but these can be adjusted over a wide range by dietary manipulation. It also has an obesity- related metabolic phenotype: decreased adipose tissue mass and fat cell size, markedly improved insulin sensitivity and glucose tolerance, increased feeding-related energy expenditure and robust resistance to diet- induced obesity. Remarkably, these changes occur in the context of increased food consumption. The mechanisms underlying the improved insulin sensitivity are not understood and will be investigated here. The resistance to diet-induced obesity and increased energy expenditure found in the BCAT2 KO appear to be due to novel futile cycle that we will elucidate. The specific aims to address these questions are to: 1) Elucidate the mechanisms involved in the increased energy expenditure observed in the BCAT2 KO mouse. 2) Test the alternate hypotheses that the insulin sensitivity and resistance to diet-induced obesity of the BCAT2 KO is due to either elevated BCAAs or the loss of mitochondrial BCAA metabolism. 3) To determine whether leucine metabolism is needed for mTOR and PKC-5 activation in heart and skeletal muscle. 4) Determine the mechanism through which plasma BCAAs are elevated in obesity. PUBLIC HEALTH RELEVANCE: We are examining the role of branched chain amino acid (BCAA) metabolism in nutrient signaling and obesity. BCAAs are believed by some to be the active portion of recently emerging high protein diets (Akins, South Beach), "protein water" as well as BCAA- and Whey-supplemented snack bars for body weight control. The implication of our ongoing research is that blocking the first step in BCAA metabolism with a drug, or to a less extent increasing dietary BCAAs, will promote weight loss.