Role of Leucine Metabolism in Leucine Signaling

描述(申请人提供)：这些研究的长期目标是确定支链氨基酸(BCAA)代谢在肥胖和亮氨酸营养信号中的作用。多项研究的结果表明，在饮食中增加膳食蛋白质对胰岛素敏感性、饱腹感、瘦体重和对肥胖的抵抗力有好处。支链氨基酸(BCAA)或亮氨酸似乎是活跃的饮食成分。矛盾的是，支链氨基酸在肥胖和他们的一些目标，如PKC-？和mTOR细胞信号通路，似乎促进了IRS-1的丝氨酸磷酸化，这是导致胰岛素抵抗的原因之一。此外，mTOR的底物，脂类，S6K1和4EBP1&2，已经被交替地与肥胖或抵抗肥胖有关。由于一些研究表明在饮食中增加支链氨基酸对肥胖有益，而另一些研究结果则预测肥胖并存的情况会恶化；目前还不完全清楚在饮食中增加支链氨基酸或阻止外周支链氨基酸代谢会带来什么。拟议的研究将解决这一问题和以下问题。支链氨基酸代谢在导致PKC-激活的营养信号中起什么作用？那么mTOR途径呢？在肥胖患者中观察到的支链氨基酸水平升高是否与代谢缺陷有关？减肥干预能否逆转这一现象？在上一次资助期间，我们培育了一批在大多数外周(非神经性)组织中发现的线粒体支链氨基转移酶同工酶(BCATm)催化BCAA代谢第一步缺陷的转基因小鼠(BCATm KO)。支链氨基酸代谢的阻断将被用来帮助澄清这些与亮氨酸信号和代谢相关的不一致和问题。BCATm KO持续升高支链氨基酸，但这些可以通过饮食控制在很大范围内进行调整。它还具有与肥胖相关的代谢表型：脂肪组织质量和脂肪细胞尺寸减少，显著改善胰岛素敏感性和葡萄糖耐量，增加与摄食相关的能量消耗，并对饮食诱导的肥胖产生强大的抵抗力。值得注意的是，这些变化是在食品消费增加的背景下发生的。胰岛素敏感性改善的机制尚不清楚，将在这里进行研究。在BCAT2 KO中发现的对饮食诱导的肥胖和能量消耗增加的抵抗力似乎是由于我们将阐明的新的无效循环。解决这些问题的具体目的是：1)阐明在BCAT2 KO小鼠中观察到的能量消耗增加的机制。2)验证另一种假设，即BCAT2 KO的胰岛素敏感性和对饮食诱导的肥胖的抵抗力是由于支链氨基酸的升高或线粒体支链氨基酸代谢的丧失。3)确定心肌和骨骼肌中mTOR和PKC-5的激活是否需要亮氨酸代谢。4)确定肥胖患者血浆支链氨基酸水平升高的机制。 公共卫生相关性：我们正在研究支链氨基酸(BCAA)代谢在营养信号和肥胖中的作用。一些人认为支链氨基酸是最近出现的高蛋白饮食(Akins，South海滩)、“蛋白质水”以及添加支链氨基酸和乳清的小吃店中用于控制体重的活跃部分。我们正在进行的研究表明，用药物阻断支链氨基酸代谢的第一步，或者在较小程度上增加饮食中的支链氨基酸，将促进减肥。

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.