Role of Leucine Metabolism in Leucine Signaling

描述（由申请人提供）：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传导中的作用。许多研究结果表明，增加饮食中的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有益的影响。支链氨基酸（BCAAs）或亮氨酸似乎是有效的膳食成分。矛盾的是，BCAAs在肥胖和它们的一些靶标(如PKC-？和mTOR细胞信号通路，似乎促进了IRS-1的ser磷酸化，这是胰岛素抵抗的一个原因。此外，mTOR的底物、脂素、S6K1和4EBP1&2也与肥胖或对肥胖的抵抗有关。由于一些研究表明，提高饮食中的支链氨基酸对肥胖有益，而另一些研究结果预测肥胖合并症会恶化；目前还不完全清楚提高饮食中的支链氨基酸或阻止外周支链氨基酸代谢可能会带来什么。拟议的研究将解决这个问题以及以下问题。BCAA代谢在导致PKC-激活的营养信号传导中的作用是什么？mTOR通路呢？代谢缺陷是否在肥胖中观察到的支链氨基酸升高中起作用，这是否可以通过减肥干预来逆转？在上一个资助期，我们培育了一种转基因小鼠（BCATm KO），该小鼠在大多数外周（非神经元）组织中发现的线粒体支链转氨酶同工酶（BCATm）催化的BCAA代谢的第一步中存在缺陷。BCAA代谢的阻滞将被用来帮助澄清这些不一致以及与亮氨酸信号和代谢相关的问题。BCATm KO持续升高BCAAs，但这些可以通过饮食控制在大范围内调整。它还具有肥胖相关的代谢表型：脂肪组织质量和脂肪细胞大小减少，胰岛素敏感性和葡萄糖耐量显著改善，饲养相关的能量消耗增加，对饮食诱导的肥胖具有强大的抵抗力。值得注意的是，这些变化发生在食品消费增加的背景下。改善胰岛素敏感性的机制尚不清楚，将在这里进行研究。在BCAT2 KO中发现的对饮食引起的肥胖的抵抗和能量消耗的增加似乎是由于一种新的无效循环，我们将对此进行解释。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BCAT2 KO的胰岛素敏感性和对饮食性肥胖的抵抗是由于BCAAs升高或线粒体BCAA代谢丧失的替代假设。3)确定心脏和骨骼肌中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升高的机制。

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.